Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

CsFeBr3 is an S = 1 hexagonal antiferromagnet that has a singlet ground state owing to its large easy-plane single-ion anisotropy. The critical behavior of the magnetic-field-induced phase transition for a magnetic field parallel to the c axis, which can be described by the Bose-Einstein condensation (BEC) of magnons under the U(1) symmetry, was investigated via magnetization and specific heat measurements down to 0.1 K. For the specific heat measurement, we have developed a method of effectively suppressing the torque acting on a sample with strong anisotropy that uses the spin dimer compound Ba2CoSi2O6Cl2 with large and anisotropic Van Vleck paramagnetism. The temperature dependence of the transition field H-c(T) was found to follow the power-law H-c(T)-H-c alpha T-phi with a critical exponent of phi = 1.50 +/- 0.02 and critical field of H-c = 2.60 T. This result verifies the universality of the three-dimensional BEC of magnons described by phi(BEC) = 3/ 2.

DOI： 10.1103/PhysRevB.96.144404

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We report the results of multifrequency high-magnetic-field electron-spin resonance (ESR) measurements on the highly frustrated antiferromagnet Ba2CoTeO6. This compound is magnetically composed of two subsystems A and B, which are described as a spin-1/2 triangular-lattice Heisenberg antiferromagnet and a honeycomb-lattice J1-J2 Ising antiferromagnet, respectively. Ba2CoTeO6 undergoes successive magnetic phase transitions at T-N1 = 12.0 K and T-N2 = 3.0 K. For a magnetic field H parallel to the c axis, subsystem B exhibits successive metamagnetic transitions with magnetization plateaus at one-third and one-half of the saturation magnetization. Below TN2, we observed collective ESR modes for H parallel to c, which are characteristic of a triangular-lattice Heisenberg antiferromagnet with weak easy-plane anisotropy. We also observed a local excitation mode, which can be assigned as a single flip of the Ising-like spin of subsystem B. From a detailed analysis of the collective and local ESR modes, combined with the magnetization process, we determined the magnetic parameters of subsystems A and B, and confirmed that the two subsystems are almost decoupled.

DOI： 10.1103/PhysRevB.96.064419

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A spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) is a prototypical frustrated quantum magnet, which exhibits remarkable quantum many-body effects that arise from the synergy between spin frustration and quantum fluctuation. The ground-state properties of a spin-1/2 TLHAF are theoretically well understood. However, the theoretical consensus regarding the magnetic excitations is limited. The experimental study of the magnetic excitations in spin-1/2 TLHAFs has also been limited. Here we show the structure of magnetic excitations in the spin-1/2 TLHAF Ba3CoSb2O9 investigated by inelastic neutron scattering. Significantly different from theoretical expectations, the excitation spectrum has a three-stage energy structure. The lowest-energy first stage is composed of dispersion branches of single-magnon excitations. The second and third stages are dispersive continua accompanied by a columnar continuum extending above 10 meV, which is six times larger than the exchange interaction J = 1.67 meV. Our results indicate the shortcomings of the current theoretical framework.

DOI： 10.1038/s41467-017-00316-x

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High-field magnetization of the spin-1/2 antiferromagnet alpha-Cu2V2O7 was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the a axis), two distinct transitions were observed at H-c1 = 6.5 T and H-c2 = 18.0 T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for H-c1 < H < H-c2 due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the a axis. Above H-c2, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. Density functional theory was employed to compute the exchange interactions, which were used as inputs for quantum Monte Carlo calculations and then further refined by fitting to the magnetic susceptibility data. Contrary to our previous report in Phys. Rev. B 92, 024423 (2015), the dominant exchange interaction is between the third nearest-neighbor spins, which form zigzag spin chains that are coupled with one another through an intertwining network of the nonnegligible nearest and second nearest-neighbor interactions. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state.

DOI： 10.1103/PhysRevB.95.245119

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K2CuF4 is magnetically described as a spin-1/2, quasi-two-dimensional (2D), square-lattice XXZ ferromagnet with weak easy-plane anisotropy. The magnetic ordering for an applied magnetic field H parallel to the c axis is equivalent to the Bose-Einstein condensation (BEC) of lattice bosons, as discussed by Matsubara and Matsuda [T. Matsubara and H. Matsuda, Prog. Theor. Phys. 16, 569 (1956)]. Magnetization and specific-heat measurements were performed to obtain the temperature versus magnetic field phase diagram for H parallel to c. The phase boundary between polarized and ordered phases was found to be expressed by the power law H-c(T) - H-c(0) proportional to T-phi with exponent phi approximate to 1.0 in a wide temperature range, in agreement with the theory of quasi-2D BEC.

DOI： 10.1103/PhysRevB.95.174406

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DOI： 10.1088/1742-6596/755/1/011001

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We synthesized single crystals of composition Ba2CuSi2O6Cl2 and investigated their quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi2O6 also known as Han purple. Ba2CuSi2O6Cl2 has a singlet ground state with an excitation gap of Delta/k(B) = 20.8 K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the g factor except for a small jump anomaly for a magnetic field perpendicular to the c axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. The magnetic-field-induced spin ordering in Ba2CuSi2O6Cl2 is described as the quasi-2D Bose-Einstein condensation of triplets.

DOI： 10.1103/PhysRevB.94.094421

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We have performed magnetization measurements of the gapped quantum magnet CsFeCl3 at temperatures (T) down to 0.5 K at ambient pressure and down to 1.8 K at hydrostatic pressures (P) of up to 1.5 GPa. The lower-field (H) phase boundary of the field-induced ordered phase at ambient pressure is found to follow the power-law behavior expressed by the formula H-N(T) - H-c alpha T-N(phi) . The application of pressure extends the phase boundary to both a lower field and higher temperature. Above the critical pressure P-c similar to 0.9 GPa, the transition field HN associated with the excitation gap becomes zero, and a signature of the magnetic phase transition is found in the T dependence of magnetization in a very low applied field. This suggests that CsFeCl3 exhibits a pressure-induced magnetic phase transition at Pc.

DOI： 10.1103/PhysRevB.94.104409

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We report the results of magnetization and specific heat measurements of Ba2CoTeO6 composed of two subsystems A and B, which are magnetically described as an S = 1/2 triangular-lattice Heisenberg-like antiferromagnet and a J(1)-J(2) honeycomb-lattice Ising-like antiferromagnet, respectively. These two subsystems were found to be approximately decoupled. Ba2CoTeO6 undergoes magnetic phase transitions at T-N1 = 12.0 K and T-N2 = 3.0 K, which can be interpreted as the orderings of subsystems B and A, respectively. Subsystem A exhibits a magnetization plateau at one-third of the saturation magnetization for the magnetic field H perpendicular to the c axis owing to the quantum order-by-disorder, whereas for H parallel to c, subsystem B shows three-step metamagnetic transitions with magnetization plateaus at zero, one-third, and one-half of the saturation magnetization. The analysis of the magnetization process for subsystem B shows that the classical ground states at these plateaus are infinitely degenerate within the Ising model.

DOI： 10.1103/PhysRevB.93.094420

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The magnetic structure of the double perovskite compound Sr2CuTeO6 was determined from neutron powder diffraction data. This material is magnetically described as an S = 1/2 quasi-two-dimensional square-lattice Heisenberg model with antiferromagnetic nearest-neighbor and next-nearest-neighbor interactions. Sr2CuTeO6 undergoes a magnetic phase transition at T-N similar or equal to 29 K. The spin structure below T-N is Neel antiferromagnetic on the square lattice, which means that the nearest-neighbor interaction (J(1)) is stronger than the next-nearest-neighbor interaction (J(2)), in contrast to other isostructural compounds such as Ba2CuWO6 and Sr2CuWO6, for which vertical bar J(1)vertical bar < vertical bar J(2)vertical bar is realized.

DOI： 10.1103/PhysRevB.93.054426

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Elementary excitations of the S = 1/2 one-dimensional antiferromagnet KCuGaF6 were investigated by inelastic neutron scattering in zero and finite magnetic fields perpendicular to the (1,1,0) plane combined with specific heat measurements. KCuGaF6 exhibits no long-range magnetic ordering down to 50 mK despite the large exchange interaction J/k(B) = 103 K. At zero magnetic field, well-defined spinon excitations were observed. The energy of the des Cloizeaux and Pearson [J. des Cloizeaux and J. J. Pearson, Phys. Rev. 128, 2131 (1962)] mode of the spinon excitations is somewhat larger than that calculated with the above exchange constant. This discrepancy is mostly ascribed to the effective XY anisotropy arising from the large Dzyaloshinsky-Moriya [T. Moriya, Phys. Rev. 120, 91 (1960)] interaction with an alternating D vector. KCuGaF6 in a magnetic field is represented by the quantum sine-Gordon model, for which low-energy elementary excitations are composed of solitons and antisolitons and their bound states called breathers. Unlike the theoretical prediction, it was found that the energy of a soliton is smaller than that of the first breather, although the energy of the first breather coincides with that observed in a previous electron spin resonance measurement.

DOI： 10.1103/PhysRevB.92.174412

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Resistance and magnetic torque measurements have been performed for a two dimensional organic superconductor lambda-(BETS)(2)GaCl4, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. In magnetic fields along the c-axis (parallel to the conducting layers), the interlayer resistance shows a peak at similar to 7T for large currents in the superconducting state. The peak disappears when the magnetic field is tilted from the layers only by 0.2 degrees, which shows that the peak is caused by the dynamics of the Josephson vortices penetrating in the insulating layers. The current-voltage characteristics show that the depinning current of the Josephson vortices is consistently suppressed at around 7T. The diamagnetic signal, irreversibility field, and depinning current rapidly decrease with increasing field above similar to 10T at low temperatures. The results suggest that there exists a phase boundary in the superconducting state, likely a transition to a Fulde-Ferrell-Larkin-Ovchinnikov phase.

DOI： 10.7566/JPSJ.84.104709

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We have systematically investigated the variation of the exchange parameters and the ground state in the S = 1/2 kagome-lattice antiferromagnet (Rb1-xCsx)(2)Cu3SnF12 via magnetic measurements using single crystals. One of the parent compounds, Rb2Cu3SnF12, which has a distorted kagome lattice accompanied by four sorts of nearest-neighbor exchange interaction, has a disordered ground state described by a pinwheel valence-bond-solid state. The other parent compound, Cs2Cu3SnF12, which has a uniform kagome lattice at room temperature, has an ordered ground state with the q = 0 spin structure. The analysis of magnetic susceptibilities shows that with increasing cesium concentration x, the exchange parameters increase with the tendency to be uniform. It was found that the ground state is disordered for x < 0.53 and ordered for x > 0.53. The pseudogap observed for x < 0.53 and the Neel temperature for x > 0.53 approach zero at x(c) similar or equal to 0.53. This is indicative of the occurrence of a quantum phase transition at x(c).

DOI： 10.1103/PhysRevB.91.214429

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DOI： 10.7566/JPSJ.83.115001

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We determined the crystal structure of Ba2CoSi2O6Cl2, which was synthesized in this work, and investigated its quantum magnetic properties using single crystals. This compound should be described as a two-dimensionally coupled spin-1/2 XY-like spin dimer system. Ba2CoSi2O6Cl2 exhibits a stepwise magnetization process with a plateau at half of the saturation magnetization, irrespective of the field direction, although all the Co2+ sites are equivalent. This indicates that spin triplets are localized owing to the almost perfect frustration of interdimer exchange interactions. Thus, the spin states for the zero and 1/2 magnetization-plateau states are almost exactly given by the simple product of singlet dimers and the alternate product of singlet and triplet dimers, respectively.

DOI： 10.7566/JPSJ.83.103701

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We report the results of low-temperature thermal and magnetic measurements on Ba3CoNb2O9 powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. Ba3CoNb2O9 is found to undergo two-step antiferromagnetic transitions at T-N1 = 1.39 K and T-N2 = 1.13 K. As the magnetic field is increased, both T-N1 and T-N2 monotonically decrease. The magnetic field vs temperature phase diagram indicates that the exchange interactions are nearly of the Heisenberg type with weak easy-axis anisotropy and that the exchange interaction between triangular lattices is crucial, in contrast to the case of the quasi-two-dimensional TLAF Ba3CoSb2O9

DOI： 10.1103/PhysRevB.90.014403

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DOI： 10.1103/PhysRevB.87.224512

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We have performed high-field magnetization and electronic spin resonance (ESR) measurements on Ba3CoSb2O9 single crystals, which approximates the two-dimensional (2D) S=1/2 triangular-lattice Heisenberg antiferromagnet. For an applied magnetic field H parallel to the ab plane, the entire magnetization curve including the plateau at one-third of the saturation magnetization (Ms) is in excellent agreement with the results of theoretical calculations except a small step anomaly near (3/5)M s, indicative of a theoretically undiscovered quantum phase transition. However, for Hâ̂\c, the magnetization curve exhibits a cusp near Ms/3 owing to the weak easy-plane anisotropy and the 2D quantum fluctuation. From a detailed analysis of the collective ESR modes observed in the ordered state, combined with the magnetization process, we have determined all the magnetic parameters including the interlayer and anisotropic exchange interactions. © 2013 American Physical Society.

DOI： 10.1103/PhysRevLett.110.267201

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We report the magnetoresistance and magnetic torque measurements for a highly correlated two-dimensional organic superconductor κ-(BEDT-TTF) 2Cu[N(CN)2]Br (κ-Br), which has an effectively larger electron correlation than the isostructural superconductor κ-(BEDT-TTF)2Cu(NCS)2 (κ-NCS). In a magnetic field applied parallel to the conducting layers, we found that the fluctuating superconductivity (FSC) appears in a wide temperature and field range, in which the magnetic torque shows diamagnetism whereas the resistance shows no appreciable reduction in value. The FSC phase diagram of κ-Br is similar to that of κ-NCS, which is inconsistent with previous Nernst measurements. © 2013 The Physical Society of Japan.

DOI： 10.7566/JPSJ.82.064711

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The current-voltage characteristics in the charge order state of the two-dimensional organic conductor alpha-(BEDT-TTF)(2)I-3 exhibit power law behavior at low temperatures. The power law is understood in terms of the electric-field-dependent potential between electrons and holes, which are thermally excited from the charge order state. The power law exponent steeply changes from 1 to 3 in the range from 30 to 45 K with decreasing temperature, thereby suggesting the occurrence of a Kosterlitz-Thouless-type transition; many (few) unbound electron-hole pairs are thermally excited above (below) the transition. The effects of the finite size and interlayer coupling on the power law behavior are discussed.

DOI： 10.1103/PhysRevLett.110.196602

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Resistance measurements have been performed for a two-dimensional magnetic-field-induced superconductor lambda-(BETS)(2)FeCl4, where BETS denotes bis(ethylenedithio)tetraselenafulvalene. In magnetic fields along the c-axis (parallel to the conducting layers), characteristic dip structures in the interlayer resistance are observed up to 24.2 T, ascribed to the commensurability effect between the Josephson vortex lattice and periodic oscillation of the order parameter of the Fulde and Ferrell, and Larkin and Ovchinnikov (FFLO) phase. The dip structures disappear when the magnetic field is tilted from the layers only by 1 degrees, showing that the FFLO phase is removed by the orbital effect. When the field is nearly parallel to the layers, the resistance in the superconducting state becomes significantly larger than the normal state value, showing the presence of peculiar vortex dynamics.

DOI： 10.7566/JPSJ.82.034715

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Resistivity and Hall effect measurements of EuFe2As2 up to 3.2 GPa indicate no divergence of quasiparticle effective mass at the pressure Pc where the magnetic and structural transition disappears. This is corroborated by analysis of the temperature (T) dependence of the upper critical field. T-linear resistivity is observed at pressures slightly above Pc. The scattering rates for both electrons and holes are shown to be approximately T-linear. When a field is applied, a T2 dependence is recovered, indicating that the origin of the T-linear dependence is spin fluctuations. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.88.224510

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We present results of Shubnikov-de Haas oscillation measurements on detwinned BaFe2As2 and de Haas-van Alphen oscillation measurements on KFe2As2. The Fermi surface of BaFe2As2 in the antiferromagnetic phase is found to consist of one hole and two electron pockets, all of which are three-dimensional and closed, and can reasonably be accounted for by LSDA band calculations. We find only moderate mass enhancements m*/m(band) of 2-3. In the case of KFe2As2, four quasi-two-dimensional Fermi surface cylinders epsilon, alpha, zeta, and beta are observed in qualitative agreement with previous ARPES data. In sharp contrast to BaFe2As2, agreement between the observed and LDA-calculated Fermi surface is poor: LDA calculations seem to predict wrong crystal-field splitting of Fe 3d states. Large effective masses up to 20 m(e), m(e) being the free electron mass, are found. The Sommerfeld coefficient estimated from the observed Fermi surface and effective masses is consistent with the measured value of 93 mJ/K(2)mol [H. Fukazawa et al., J. Phys. Soc. Jpn. 80, SA118 (2011)] and is 8-9 times larger than the band value, indicating strong electronic correlations in KFe2As2. d

DOI： 10.1088/1742-6596/449/1/012022

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We report magnetoresistance and magnetic torque measurements for the highly correlated 2D organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). Under magnetic field parallel to the conducting layers, we obtain convincing evidence of fluctuating superconductivity in a wide temperature and field range, where the magnetic torque shows diamagnetism but the resistance shows no appreciable decrease.

DOI： 10.1103/PhysRevB.85.220506

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We have determined the Fermi surface in KOs2O6 (T-c = 9.6 K and B-c2 similar to 32 T) via de Haas-van Alphen (dHvA) oscillation measurements and a band structure calculation. We find effective masses up to 26(1)m(e) (m(e) is the free electron mass), which are unusually heavy for compounds where the mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved mass enhancement parameters lambda(dHvA) are large but fairly homogeneous, concentrated in the range 5-8. We discuss origins of the large homogeneous mass enhancement in terms of rattling motion of the K ions.

DOI： 10.1103/PhysRevB.85.180503

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Magnetic torque measurements have been carried out for two-dimensional magnetic-field-induced organic superconductor lambda-(BETS)(2)FeCl4, where BETS stands for bis(ethylenedithio) tetraselenafulvalene, to investigate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. The in-plane upper critical field of the field-induced-superconducting phase steeply decreases with decreasing temperature below 2.2 K. The in-plane field dependence of the diamagnetic susceptibility shows a significant decrease below 25 T at low temperatures, showing that magnetic fluxes are less excluded from the sample. These results show the presence of the FFLO phase with the tricritical point between the FFLO, homogeneous superconducting, and paramagnetic metallic phases at 2.2 K and 23 T. The stability of the FFLO phase is also investigated as a function of the magnetic field angle and compared with theories.

DOI： 10.1103/PhysRevB.85.174530

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Electric conductance and dielectric constant have been measured in the charge-ordered states of the quasi two-dimensional organic conductors alpha-(BEDT-TTF)(2)I-3 and alpha'-(BEDT-TTF)(2)IBr2 to understand the charge transport mechanism. The current-voltage (I-V) characteristics show a strong nonlinear behavior, which is qualitatively explained by the transport of individual carriers thermally excited from the charge-ordered states in BEDT-TTF layers. The discrepancy with the simulation based on the thermal activation model at low temperatures is likely due to quantum tunneling through the potential barrier. The dielectric constants show a large difference between the in-plane and out-of-plane directions of the samples, showing a highly two-dimensional nature of Coulomb potential. The temperature dependence of dielectric constant is ascribed to the polarization effect of the thermally excited electron-hole pairs, consistent with the model of nonlinear I-V characteristics. All these features are qualitatively the same between alpha-(BEDT-TTF)(2)I-3 and alpha'-(BEDT-TTF)(2)IBr2, despite the different structural and magnetic properties.

DOI： 10.1143/JPSJ.81.044703

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We report cyclotron resonance (CR) measurements for the Fe-based superconductor KFe2As2. Only one series of CR with an effective mass of 3.4m(e) (m(e) is the free electron mass) is observed. The CR signal is attributed to the quasi-two-dimensional alpha Fermi surface at the Gamma point. Our main finding is that the effective mass obtained from the CR is significantly smaller than that obtained from the de-Haas van Alphen (dHvA) measurements. This is the direct evidence of the mass enhancement by electronic correlation. A comparison between the CR and dHvA measurements shows that both intra- and interband electronic correlations contribute to the mass enhancement in KFe2As2.

DOI： 10.1088/1742-6596/400/2/022054

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We have investigated the upper critical field B-c2 of the stoichiometric Fe-based superconductor LiFeAs, via the magnetic torque measurements in dc-magnetic fields up to 35T and at temperatures down to 0.3 K. B-c2 at 0.3 K is obtained to be 26.4T and 15.5T for the applied field B-a parallel to ab and B-a parallel to c, respectively, indicating nearly-isotropic superconductivity. The detailed analyses show that, while B-c2 for Ba parallel to c is limited by the orbital effect, the spin-paramagnetic effect is evident in the temperature dependence of B-c2 for B-a parallel to ab.

DOI： 10.1088/1742-6596/391/1/012133

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We have performed electrical resistivity measurements of EuFe2As2 under hydrostatic pressures (P) up to 3.2GPa and magnetic fields up to 15 T. In zero field, the superconducting (SC) ground state with a sharp transition to zero resistivity, indicative of bulk superconductivity, emerges at T-c similar to 30K in a pressure range from 2.5GPa to similar to 3.0GPa. Under applied field of 15 T, the SC transition is suppressed to lower temperatures with increasing pressure, suggesting that the application of pressure reduces the upper critical field Bc(2) for the P-induced superconductivity. Meanwhile, the antiferromagnetic (AF) order of the Eu2+ moments survives up to 3.2GPa, the highest pressure in the experiments, with no significant change in the AF ordering temperature and the magnetic field effect.

DOI： 10.1088/1742-6596/391/1/012132

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We report the first observation of the continuous Fermi surface (FS) variation from La compound (LaRu2Si2) with no f electron to Ce compound (CeRu2Si2) with itinerant f electron via the de Haas-van Alphen (dHvA) effect. The dHvA frequency smoothly varies with Ce concentration and there is no discontinuous change with Ce concentration. It is found that the effective mass and signal amplitude with Ce concentration depends strongly on the Fermi surface sheet, and the effective mass is enhanced toward x(c) (= 0.91) and the signal amplitude reduces around x(c) or somewhere between x(c) and x . .0.8.

DOI： 10.1088/1742-6596/391/1/012042

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

Current-voltage (I-V) characteristics and magnetoresistance have been measured to investigate the magnetic field effect on the charge transport in the pi-d organic conductor (EDT-TTFVO)(2)FeBr4. The I-V characteristics show nonlinear behavior at low temperatures, which is qualitatively explained by the transport of electrons and holes thermally excited from the charge ordered states in the EDT-TTFVO molecule layers.
At low temperatures, the magnetoresistance has a shape change at similar to 0.3 T, likely due to the spin flop transition of the 3d spins, and then a broad minimum at 7.6 T. The absence of the field angle dependence shows that the broad minimum arises mainly from the Zeeman effect, but not from the orbital effect. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201100725

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We show that the Fermi surface (FS) in the antiferromagnetic phase of BaFe2As2 is composed of one hole and two electron pockets, all of which are three dimensional and closed, in sharp contrast to the FS observed by angle-resolved photoemission spectroscopy. Considerations on the carrier compensation and Sommerfeld coefficient rule out existence of unobserved FS pockets of significant sizes. A standard band structure calculation reasonably accounts for the observed FS, despite the overestimated ordered moment. The mass enhancement, the ratio of the effective mass to the band mass, is 2-3.

DOI： 10.1103/PhysRevLett.107.176402

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Cyclotron resonance (CR) measurements for the Fe-based superconductor KFe2As2 are performed. One signal for CR is observed, and is attributed to the two-dimensional alpha Fermi surface at the Gamma point. We found a large discrepancy in the effective masses of CR [(3.4 +/- 0.05)m(e) (m(e) is the free-electron mass)] and de Haas-van Alphen results, a direct evidence of mass enhancement due to electronic correlation. A comparison of the CR and de Haas-van Alphen results shows that both intra- and interband electronic correlations contribute to the mass enhancement in KFe2As2.

DOI： 10.1103/PhysRevLett.107.166402

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We report extensive measurements on a new compound (Yb0.24Sn0.76) Ru that crystallizes in the cubic CsCl structure. Valence-band photoemission (PES) and L-3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a Gamma(7)-doublet ground state and a Gamma(8) quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Gamma(6)-excited state resides at 32 meV; however, the Gamma(8)/Gamma(6) transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch-Gruneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K there is a peak in specific heat that is too broad to represent a magnetic-phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand this peak also is too narrow to represent the Kondo effect in the Gamma(7)-doublet ground state. On the basis of the field dependence of the specific heat, we argue that antiferromagnetic (AF) short-range order (SRO) (possibly coexisting with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

DOI： 10.1103/PhysRevB.84.075152

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We report the first observation of a single-vortex flow in a mesoscopic superconductor. A flow of a single vortex is successfully controlled by an rf current superimposed on a dc current, evidence of which is provided by voltage steps in current-voltage (I-V) characteristics. Irrespective of the number of vortices confined to the disk, we unambiguously observe that when a single vortex inside the disk is driven out of the disk, another vortex enters the disk similarly to two balls colliding in billiards: only one vortex passes through the Al disk at the same time in mesoscopic systems.

DOI： 10.1103/PhysRevLett.107.077002

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The effect of dilute La substitution in Ce1-xLaxRhIn5 (0 <= x <= 0.07) has been investigated macroscopically by specific heat measurement and microscopically by nuclear magnetic/quadrupole resonance (NMR/NQR). The Neel temperature decreases gradually by dilute La substitutions from 3.8 K (x = 0) to 2.9 K (x = 0.07). The specific heat for Ce1-xLaxRhIn5 exhibits nuclear Schottky contributions below 1 K, which are compatible with the microscopic parameters obtained by NMR/NQR. Below the Neel temperature, the residual Sommerfeld coefficient of similar to 50 mJ K-2/ mol is nearly independent of increasing La substitution, while the entropy in the ordered state is slightly enhanced. Microscopically, 1/T-1 in the AF ordered state can be explained by spin-wave term of 4f moments, spin fluctuation term by uncompensated localized Ce moments near the La ions, and a small Korringa contribution by non-4f electrons, which suggests a localized 4f nature in CeRhIn5.

DOI： 10.1143/JPSJS.80SA.SA059

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We have constructed a pressure-temperature (P-T) phase diagram of P-induced superconductivity in EuFe2As2 single crystals via resistivity (rho) measurements up to 3.2 GPa. As hydrostatic pressure is applied, the temperature T-0 where an antiferromagnetic (AF) transition of the Fe moments and a structural phase transition occur shifts to lower temperatures, and the corresponding resistive anomaly becomes undetectable for P >= 2.5 GPa. This suggests that the critical pressure P-c where T-0 becomes zero is about 2.5 GPa. We have found that the AF order of the Eu2+ moments survives up to 3.2 GPa, the highest pressure in the experiments, without significant changes in the AF ordering temperature T-N. The superconducting (SC) ground state with a sharp transition to zero resistivity at T-c similar to 30 K, indicative of bulk superconductivity, emerges in a pressure range from P-c similar to 2.5 to similar to 3.0 GPa. At pressures close to but outside the SC phase, the rho(T) curve shows a partial SC transition (i.e., zero resistivity is not attained) followed by a reentrant-like hump at approximately T-N with decreasing temperature. When nonhydrostatic pressure with a uniaxial-like strain component is applied using a solid pressure medium, the partial superconductivity is continuously observed in a wide pressure range from 1.1 to 3.2 GPa.

DOI： 10.1103/PhysRevB.83.214513

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We investigated the superconducting gap structure of SrNi2P2 (T-c = 1.4 K) via low-temperature magnetothermal conductivity kappa(T, H) measurements. Zero field thermal conductivity kappa decreases exponentially kappa proportional to exp(-aT(c)/T) with a = 1.5, in accord with the BCS theory, and rolls over to a phonon-like kappa proportional to T-3 behavior at low temperature, similar to a number of conventional s-wave superconductors. In addition, we observed a "concave" field dependence of the residual linear term kappa(0)(H)/T. These facts strongly rule out the presence of nodes in the superconducting energy gap of SrNi2P2. Together with a fully gapped Fermi surface in the superconducting state of BaNi2As2 (T-c = 0.6-0.7 K), demonstrated in our recent work, these results lead us to stipulate that fully gapped superconductivity is likely to be a universal feature of Ni-based pnictide superconductors.

DOI： 10.1103/PhysRevB.83.094527

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We have carried out high-field resistivity measurements up to 27 T in EuFe2As2 at P = 2.5 GPa, a virtually optimal pressure for the P-induced superconductivity, where T-c = 30 K. The B-c2-T-c phase diagram has been constructed in a wide temperature range with a minimum temperature of 1.6 K (approximate to 0.05 x T-c), for both B parallel to ab (B-c2(ab)) and B parallel to c (B-c2(c)). The upper critical fields B-c2(ab)(0) and B-c2(c) (0), determined by the onset of resistive transitions, are 25 and 22 T, respectively, which are significantly smaller than those of other Fe-based superconductors with similar values of T-c. The small B-c2(0) values and the B-c2(T) curves with positive curvature around 20 K can be explained by a multiple pair-breaking model that includes the exchange field due to the magnetic Eu2+ moments. The anisotropy parameter, Gamma = B-c2(ab)/B-c2(c), in EuFe2As2 at low temperatures is comparable to that of other "122" Fe-based systems.

DOI： 10.1103/PhysRevB.83.100501

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We report on the upper critical field B-c2 of a superconducting LiFeAs single crystal with T-c similar to 16 K, determined from magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. B-c2 at 0.3 K is obtained to be 26.4 and 15.5 T for the applied field B-a parallel to ab and B-a parallel to c, respectively. The anisotropy parameter Gamma = B-c2(ab)/B-c2(c) is similar to 3 at T-c and decreases to 1.7 as T -> 0, showing rather isotropic superconductivity. While B-c2 is orbitally-limited for Ba parallel to c, the spin-paramagnetic effect is evident in the temperature dependence of B-c2 for B-a parallel to ab.

DOI： 10.1143/JPSJ.80.013706

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We report specific heat (C) and magnetization (M) of single crystalline Ce4Pt12Sn25 at temperature down to approximate to 50 mK and in fields up to 3 T. C/T exhibits a sharp anomaly at 180 mK, with a large jump in a Sommerfeld coefficient gamma = C/T of Delta gamma = 30 J/molK(2)-Ce, which, together with corresponding cusp-like magnetization anomaly, indicate antiferromagnetic (AFM) ground state with Nel temperature T-N = 0.18 K. Numerical calculations based on Heisenberg model reproduce well zero field specific heat data, and point to a very small Kondo scale T-K clearly placing Ce4Pt12Sn25 in the weak exchange coupling J < J(c) limit of the Doniac diagram. Magnetic field suppresses AFM state at H* approximate to 0.7 T, much more rapidly than indicated by theoretical calculations.

DOI： 10.1088/1742-6596/273/1/012045

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High-pressure electrical resistivity measurements up to 3.0 GPa have been performed on EuFe2As2 single crystals with residual resistivity ratios RRR = 7 and 15. At ambient pressure, a magnetic / structural transition related to FeAs-layers is observed at T-0 = 190 K and 194 K for samples with RRR = 7 and 15, respectively. Application of hydrostatic pressure suppresses T-0, and then induces similar superconducting behavior in the samples with different RRR values. However, the critical pressure similar to 2.7 GPa, where T-0 -&gt; 0, for the samples with RRR = 15 is slightly but distinctly larger than similar to 2.5 GPa for the samples with RRR = 7.

DOI： 10.1088/1742-6596/273/1/012098

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We report on low-temperature thermal conductivity of the noncentrosymmetric superconductor LaRhSi3 (T-c = 2.3 K), which is a paramagnetic analog of the pressure-induced superconductor CeRhSi3. In the normal state (either in zero field above T-c, or in magnetic field above H-c2), the thermal conductivity kappa is mostly due to electrons, and shows a nearly T-linear dependence. In the superconducting state, kappa decreases exponentially below T-c, and crosses over to kappa proportional to T-2 behavior at T &lt;&lt; T-c. The temperature dependence of thermal conductivity of LaRhSi3 is similar to that of a number of conventional s-wave superconductors. The field dependence of the residual linear term kappa(0)/T as a function of magnetic field suggests that LaRhSi3 has no nodes in the superconducting gap.

DOI： 10.1088/1742-6596/273/1/012077

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We have performed low-temperature specific heat C and thermal conductivity kappa measurements on the Ni-pnictide superconductors BaNi2As2 (T-c = 0.7 K) and SrNi2P2 (T-c = 1.4 K). The temperature dependences C(T) and kappa(T) of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual kappa for BaNi2As2 rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba0.6La0.4Ni2As2 shows a mild suppression of T-c and H-c2 relative to BaNi2As2.

DOI： 10.1088/1742-6596/273/1/012097

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We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces (FSs) in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe2As2 with T-c approximate to 3: 7 K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, correspond to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs.

DOI： 10.1103/PhysRevLett.105.246403

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We report specific heat (C) and magnetization (M) of single crystalline Ce4Pt12Sn25 at temperature down to similar to 50 mK and in fields up to 3 T. C/T exhibits a sharp anomaly at 180 mK, with a large Delta C/T similar to 30 J/mol Ce K-2, which, together with the corresponding cusplike magnetization anomaly, indicates an antiferromagnetic (AFM) ground state with a Neel temperature T-N = 180 m K. Numerical calculations based on a Heisenberg model reproduce both zero-field C and M data, thus placing Ce4Pt12Sn25 in the weak exchange coupling J < J(c) limit of the Doniach diagram, with a very small Kondo scale T-K << T-N. Magnetic field suppresses the AFM state at H* approximate to 0.7 T, much more effectively than expected from the Heisenberg model, indicating additional effects possibly due to frustration or residual Kondo screening.

DOI： 10.1103/PhysRevB.82.174426

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We report resistivity rho and Hall effect measurements on EuFe2As2 at ambient pressure and 28 kbar and magnetization measurements at ambient pressure. We analyze the temperature and magnetic-field dependence of rho and the Hall effect using a molecular-field theory for magnetoresistance and an empirical formula for the anomalous Hall effect and find that electron scattering due to the Eu2+ local moments plays only a minor role in determining electronic transport properties of EuFe2As2.

DOI： 10.1143/JPSJ.79.103706

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Magnetic torque measurements have been performed on a KOs2O6 single crystal in magnetic fields up to 35.3 T and at temperatures down to 0.6 K. The upper critical field is determined to be similar to 30 T. De Haas-van Alphen oscillations are observed. A large mass enhancement of (1 + lambda) = m*/m(band) = 7.6 is found. It is suggested that, for the large upper critical field to be reconciled with Pauli paramagnetic limiting, the observed mass enhancement must be of electron-phonon origin, including electron-rattling-mode interactions, for the most part.

DOI： 10.1143/JPSJ.79.083703

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DOI： 10.1103/PhysRevLett.104.259701

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We report on a band structure calculation and de Haas-van Alphen measurements of KFe2As2. Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18m(e), m(e) being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (greater than or similar to 3) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors.

DOI： 10.1143/JPSJ.79.053702

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The heavy-fermion antiferromagnet CePt2In7 is a new, structurally more two-dimensional member of the CemMnIn3m+2n family. Applying pressure to CePt2In7 induces a broad dome of superconductivity that coexists with magnetic order for 1 &lt;= P &lt;= 3 GPa. The maximum T-c=2.1 K appears near the critical pressure P-c = 3.5 GPa where the Neel temperature extrapolates to zero temperature. An analysis of the initial slope of the upper critical field, the T-2 coefficient of the electrical resistivity, and specific heat indicates an enhancement of the effective mass m* as P-c is approached, suggesting that critical fluctuations may mediate superconductivity. Electronic-structure calculations reveal a delicate balance between structural anisotropy and f-d hybridization, which may account for comparable T-c's in CePt2In7 and more three-dimensional CeRhIn5.

DOI： 10.1103/PhysRevB.81.180507

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Magnetic susceptibility, magnetization, specific heat, and electrical resistivity studies on single crystals of Ce4Pt12Sn25 reveal an antiferromagnetic transition at T-N = 0.19 K, which develops from a paramagnetic state with a very large specific heat coefficient (C/T) of 14 J mol(-1) K-2-Ce just above T-N. On the basis of its crystal structure and these measurements, we argue that a weak magnetic exchange interaction in Ce4Pt12Sn25 is responsible for its low ordering temperature and a negligible Kondo-derived contribution to physical properties above T-N. The anomalous enhancement of specific heat above T-N is suggested to be related, in part, to weak geometric frustration of f - moments in this compound.

DOI： 10.1088/0953-8984/22/6/065601

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The physical properties of CePt2In7 are presented at pressures up to 3.12 Gpa. Antiferromagnetic order occurs at T-N = 505 K at ambient pressure and first increases with pressure up to P similar to 1.5 GPa, then decreases with further applied pressure up to 3.12 GPa. Another feature, attributed to superconductivity, is observed at 1 K at 1 GPa in the specific heat that grows in magnitude and increases to 2.1 K when the magnetism is weak at 3.12 GPa. Therefore, CePt2In7 displays an evolution with pressure and a coexistence of magnetism and superconductivity that is remarkably similar to that of the heavy fermion superconductor CeRhIn5

DOI： 10.1088/1742-6596/200/1/012011

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We report low-temperature thermal conductivity down to 40 mK of the antiferromagnet BaFe2As2, which is the parent compound of recently discovered iron-based superconductors. In the investigated temperature range below 4 K, the thermal conductivity kappa is well described by the expression kappa=aT+bT(2.22). We attribute the "aT"-term to an electronic contribution which is found to satisfy the Wiedemann-Franz law in the T -&gt; 0 K limit and the remaining thermal conductivity, similar to T-2.22, is attributed to phonon conductivity. A small influence on thermal conductivity by magnetic fields up to 8 T is well accounted by the observed magnetoresistance. The result is consistent with a fully gapped magnon spectrum, inferred previously from inelastic neutron scattering measurements.

DOI： 10.1103/PhysRevB.79.214439

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We review the properties of Ni-based superconductors which contain Ni2X2 (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi2As2. Coupled with the lack of magnetism. the majority of evidence suggests that the Ni-based compounds are conventional elect ron-phonon mediated superconductors. However, the increase in T-c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure-property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.03.031

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CeCoIn5 undergoes a phase transition within the superconducting (SC) state. A new superconducting state occupies a high field-low temperature (HFLT) corner of the SC state in the H-T plane, and is a strong candidate for realization of the spatially inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state predicted theoretically in 1960s. We present specific heat data on Cd- and Hg-doped CeCoIn5 that shows that the HFLT phase is suppressed by minute amount of impurities, few hundred parts per million. At this concentration, the average distance between impurities is close to twice the superconducting coherence length xi, suggesting that xi is the characteristic length of the HFLT state. We conclude that our investigations support the superconducting, posibly FFLO, origin of the HFLT state.

DOI： 10.1007/s10948-008-0419-8

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We have performed low-temperature specific heat and thermal conductivity measurements of the Ni-based superconductor BaNi2As2 (T-c=0.7 K) in a magnetic field. In a zero field, thermal conductivity shows T-linear behavior in the normal state and exhibits a BCS-like exponential decrease below T-c. The field dependence of the residual thermal conductivity extrapolated to zero temperature is indicative of a fully gapped superconductor. This conclusion is supported by the analysis of the specific heat data, which are well fit by the BCS temperature dependence from T-c down to the lowest temperature of 0.1 K.

DOI： 10.1103/PhysRevLett.102.147004

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We report electrical transport and x-ray studies under high pressure of the filled skutterudite compound CeRu4Sb12. Electrical resistivity measurements in the temperature range 2-300 K and under hydrostatic pressures up to 10.0 GPa have revealed a pressure-induced metal-semiconductor transition above 5.0 GPa in CeRu4Sb12. The energy gap in the semiconducting state of CeRu4Sb12 estimated from an activation law increases with a pressure coefficient of similar to 12 K/GPa. On the other hand, a high-pressure x-ray study has determined the relationship between lattice constant and energy gap of CeRu4Sb12 under high pressure. We suggest that an electronic mechanism of a semiconducting state in CeRu4Sb12 under high pressure could originate from an enhanced c-f hybridization and that the P-induced state could be categorized as a Kondo semiconductor.

DOI： 10.1103/PhysRevB.79.014441

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We report the synthesis and physical properties of single crystals of stoichiometric BaNi(2)As(2) that crystallizes in the ThCr(2)Si(2) structure with lattice parameters a = 4.112(4) angstrom and c = 11.54(2) angstrom. The resistivity and heat capacity show a first order phase transition at T(0) = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4 x 10(-10) Omega cm Oe(-1). The resistivity, ac susceptibility, and heat capacity provide evidence for bulk superconductivity at T(c) = 0.7 K. The Sommerfeld coefficient at Tc is 11.6 +/- 0.9 mJ mol(-1) K(-2). The upper critical field is anisotropic with initial slopes of dH(c2)(c)/ dT = -0.19 T K(-1) and dH(c2)(ab)/ dT = -0.40 T K(-1), as determined from the resistivity.

DOI： 10.1088/0953-8984/20/34/342203

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We report the properties of two new isostructural compounds, U(3)Bi(4)Ni(3) and U(3)Bi(4)Rh(3). The first of these compounds is nonmetallic, and the second is a nearly ferromagnetic metal, both as anticipated from their electron count relative to other U-based members of the larger 3-4-3 family. For U(3)Bi(4)Rh(3), a logarithmic increase in C/T below 3 K, a resistivity proportional to T(4/3), and the recovery of Fermi-liquid behavior in both properties with applied fields greater than 3 T, suggest that U(3)Bi(4)Rh(3) may be a new example of a material displaying ferromagnetic quantum criticality.

DOI： 10.1103/PhysRevB.77.245111

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We have performed pressure-dependent electrical resistivity measurements of a single crystal Ce2Pd3Si5, which is an antiferromagnetic Kondo lattice compound, at pressures up to 9.5 GPa and at temperatures down to similar to 0.4 K. As the pressure is increased, the Neel temperature T-N deduced from the temperature dependence of resistivity decreases steeply above 4.8 GPa and disappears at P &gt;= 8.6 GPa. Both A and rho(0) values in Fermi liquid law rho = rho(0) + AT(2) at low temperature region increase abruptly above 4.8 GPa and exhibit a plateau at similar to 9 GPa. These results suggest that non-magnetic state could be formed at P &gt;= 8.6 GPa and that possible quantum critical point might exist at pressures around 9 GPa in this compound. However, there is no sign of superconductivity in resistivity data at the lowest temperature we studied around the possible QCP. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.10.171

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We have investigated the transport properties of the quasi-one-dimensional (I-D) organic conductor (TMTTF)(2)AsF6 at high pressures. A superconducting transition was observed, for the first time, at 2.38 K from a resistance measurement at 4.5 GPa. The presence of zero resistance was also confirmed at similar to 2.3 K in the pressure range of 4.65 &lt;= P &lt;= 5 GPa. We discuss the electron correlation in (TMTTF)(2)AsF6 based on its pressure-temperature phase diagram.

DOI： 10.1143/JPSJ.76.053703

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The ferromagnetic compound Fe2P shows a first-order paramagnetic to ferromagnetic transition at T-C = 209 K. To investigate the magnetic properties of this compound under high pressures above 3 GPa, it has been tried to measure the magnetization using a diamond anvil type high-pressure apparatus by MPMS (quantum design). As the results of trial and error, we have succeeded to obtain the data with sufficient precision to understand the substantial behavior of the compound even under considerable high pressures above 5.0 GPa. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.1014

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temperature region below 100 K, temperature dependence of the resistivity shows significant change between 2.0 and 4.0 GPa. Under 4.0 GPa, anomalies of the resistivity were observed at (T-t3 similar to) 21.6 K and (T-t4 similar to) 7.1 K. There is a fair possible that they originated from novel magnetic phase transitions of Yb or Mn sublattice. With increasing pressure, T-t4 increases linearly. In contrast to the pressure dependence of T-t4, T-t3 decreases linearly. (c) 0 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.674

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Electrical resistivity measurements have been performed on the heavy fermion superconductor PrOs4Sb12 between 0.3 and 300K at pressures up to 9.8 GPa. Zero resistivity due to superconducting state was observed at all pressures up to P = 9.8 GPa. With increasing pressure up to 2.0 GPa, superconducting transition temperature T-c was linearly shifted to lower temperature at a rate of - 0.16K/GPa. Effect of pressure on T-c became smaller on further pressurization, following that the value of T-c was saturated to a constant value of 1.3K at high pressure. While, the upper critical. field H-c2 was strongly suppressed by increasing pressure. H-c2 at 9.8 GPa estimated from the onset temperature of superconducting transition was 0.91T at 0.5 K. (c) 2006 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.255

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PrPb3 shows antiferroquadrupolar (AFQ) ordering below T-Q = 0.4 K. Recently, Onimaru et al. revealed the existence of spatially modulated AFQ structure by neutron diffraction technique. Suggestively, an indirect RKKY-type quadrupolar interaction should play an important role in this system. In the present work, in order to examine stability of the AFQ ordered state in pressure, we carried out electric resistivity measurements under high pressures up to 7.3 GPa and at temperatures down to 0.3 K. We found that a kink observed at T-Q in rho(T) gradually rose with increasing pressures up to 5 GPa, indicating that the AFQ ordered state would be stabilized by applying pressure. However, further increases in pressure above 5 GPa result in disappearance of the kink which should be observed at T-Q.

DOI： 10.1143/JPSJS.76SA.56

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We have performed magnetic measurements on the single crystal TmTe at extremely high pressure using a novel diamond anvil cell. Between 2 K and 80 K, the magnetic susceptibility traces are unchanged for pressures up to 2.6 GPa. At 3.6 GPa, a paramagnet-ferromagnet transition appeared at T-C = 12.2 K, but disappears above 6.1 GPa. We postulate that the origin of ferromagnetic phase is related to the pressure induced valence change of Tm.

DOI： 10.1143/JPSJS.76SA.58

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Electrical resistivity measurements of a ferromagnetic Kondo compound SmFe4P12 have been carried out under pressures up to similar to 8 GPa and at temperatures down to similar to 0.3 K. With increasing pressure, shoulder-like behavior in electrical resistivity below 100 K attributed to the Kondo coherent scattering was strongly suppressed and was shifted to higher temperature region, which was often observed in Ce-based Kondo lattice compounds under applied pressure. Meanwhile, anomalous drops in resistivity ascribed to the ferromagnetic ordering have been observed at all investigated pressures up to 7.8 GPa and the values of transition temperature were hardly changed under applied pressures up to 7.8 GPa.

DOI： 10.1143/JPSJS.76SA.64

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We have performed an electrical resistivity measurement of the heavy fermion superconductor PrOs4Sb12 under hydrostatic pressures up to 10 GPa and at temperature down to similar to 2 K. As pressure was increased to 8 GPa, a roll-off behavior in rho(T) around 5 K attributed to the CEF effect was slightly shifted to higher temperature region with remaining its shape almost the same. At 9 and 10 GPa, T-2-dependences of resistivity were observed below similar to 5 K without showing a roll-off in rho(T). The obtained A values in rho = rho(0) + AT(2) at 9 and 10 GPa were 0.32 and 0.20 mu Omega cm/K-2, respectively. This fact suggests that the CEF state of PrOs4Sb12 might be drastically changed at P &gt; 8 GPa.

DOI： 10.1143/JPSJS.76SA.150

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Electrical resistivity of Y1-xGdxCo2 alloy system has been measured at temperatures from 2 to 300 K in magnetic field up to 15T and under pressure up to 10 GPa. The compounds with the composition near to phase boundary between paramagnetic and ferromagnetic ground state (x(c) approximate to 0.12) show strong enhancement of electrical resistivity at low temperatures. Large positive magnetoresistance was observed in ferromagnetic alloys in composition range 0.15 &lt; x &lt; 0.3. The anomalous magneto resistance as well as the composition dependence of the low-temperature resistivity were explained by an interplay of metamagnetic instability of Co 3d itinerant electrons and structural disorder within rare-earth sublattice. The pressure effect on electrical resistivity for Y1-xGdxCo2 at low temperatures is in agreement with the variation of magnetoresistance with the composition. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Electrical resistivity measurements of YbInCu4 were carried out under several pressures up to P = 7.0 GPa and temperatures down to 0.3 K. At 2.9 GPa, YbInCu4 undergoes a valence transition with hysteresis. Above 3.3 GPa, the valence transition completely disappears, as found in our previous work. Fits of the data at low temperature to rho = rho(0) + AT(2) show that the resistivity coefficient A and the residual resistivity rho(0) as a function of pressure have a broad peak around 4 GPa. This fact implies that YbInCu4 around 4 GPa might be in the vicinity of a quantum critical point, or that carrier density and ground-state degeneracy of YbInCu4 might be changed around 4 GPa. Moreover, slight decreases of the resistivities are observed at T-M similar to 2.4 K independently of pressures from 3.3 GPa up to 7.0 GPa. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.042

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

We investigated ground state properties of a heavy-fermion antiferromagnet Ce7Ni3 under hydrostatic pressures and magnetic fields. This compound undergoes two antiferromagnetic phase transitions at T-N1 = 1.9 K and T-N2 = 0.7 K. Below T-NI, a spin-density-wave (SDW) develops. Upon applying rather weak pressure 0.39 GPa = P-c, both T-N1 and T-N2 vanish, and non-Fermi liquid behavior appears in the specific heat and magnetic susceptibility. The enhancement of residual resistivity along the a axis near P-c is attributed to the increased spin fluctuations along the a axis. By applying fields B along the c axis, T-NI is suppressed and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements revealed another field-induced magnetic (FIM) phase in the region B parallel to c &gt; 0.7 T and T &lt; 0.5 K. Neutron diffraction experiments indicate that the magnetic unit cell in the c-plane for the FIM phase is treble that of the chemical unit cell. Moreover, this magnetic reflection intensity remains even in the region between the FIM phase and SDW phase. This observation indicates the presence of large spin fluctuations in the c-plane associated with the magnetic frustration, which should be responsible for the magnetic instability of Ce7Ni3. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2005.04.024

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

High pressure magnetotransport studies to 8.3 GPa using a diamond anvil cell are reported for the organic Mott-insulator, beta '-(ET)(2)ICl2. Previous research shows that, at a pressure of 8.2 GPa, this material possesses the highest T-C among the organic conductors. A sharp drop observed in p(T) at 8.3GPa which we associate with the superconducting phase is completely suppressed in a magnetic field (H = 18T).

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Transport, thermal, and magnetic measurements have been carried out on (Pr1-xSmx)(1-x)CaxCo3. The system exhibits a structural phase transition accompanied by the spin state change from the intermediate spin (IS) state to the: low spin (LS) state with decreasing temperature T. We have constructed a T-y phase diagram for x = 0.3 and T-x ones for y = 0.2 and 0.3. By analyzing their magnetic susceptibilities, the number of Co ions excited to the IS state (or the electron number in the e(x) orbitals). n(IS) is roughly estimated. With increasing y or with decreasing x, n(IS) decreases, and the phase transition changes gradually to the (IS -&gt; LS) crossover-like one. We discuss the possible role of the Pr atoms in realizing the transition.

DOI： 10.1143/JPSJ.74.2294

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The electrical resistivity of the filled skutterudite compound CeRu4Sb12 was measured in the temperature range 2 - 300 K and under hydrostatic pressures up to 8 GPa: Above 6 GPa; we observed metal-semiconductor transition in CeRu4Sb12 at low temperature and a energy gap estimated from activation law was enhanced with increasing pressure at a rate of 12.2 K/GPa: Semiconductive behavior of CeRu4Sb12 under high pressure may have the same origin as other Ce-based skutterudite compounds which show semiconducting behavior. (C) 2003 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.173

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We have performed low-temperature specific heat and electrical resistivity measurements on a single-crystalline Ce2CoIn8. We observed a distinct jump at 0.47 K in specific heat, providing clear evidence of bulk superconductivity. H-c2(0) is 1.1 T for Hparallel toa and 0.9 T for Hparallel toc, respectively. The unknown steps observed resistivity, T-a1 and T-a2, shifted to low-temperature with increasing field and exhibits large anisotropy by field directions. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.052

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We constructed the piston cylinder type hybrid high pressure cell using a home-made Ni-40Cr-3.5Al-0.005B alloy. It was designed in the hydrostatic pressure cell; 67 mm in length and 25 mm in outer diameter for using it with dilution refrigerator. We have succeeded in the resistivity measurement under hydrostatic pressure up to about 3.8 GPa at temperatures down to T = 33 mK. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(02)02449-3

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