Nakamura Ai

Split Fermi Surface Properties in Ullmannite NiSbS and PdBiSe with the Cubic Chiral Crystal Structure

We grew single crystals of ullmannite NiSbS and PbBiSe with the cubic chiral structure and carried out electrical resistivity, specific heat, and de Haas–van Alphen (dHvA) experiments to clarify their Fermi surface properties. The Fermi surfaces were found to split into two, reflecting the non-centrosymmetric crystal structure. The splitting energies between the two nearly spherical electron Fermi surfaces named α and α′ were determined as 220 K in NiSbS and 1050 K in PdBiSe for H || [100] or [001]. This difference in splitting energies between the two compounds originates mainly from the fact that the spin–orbit interactions of Ni-3d, Sb-5p, and S-3p electrons in NiSbS are smaller in magnitude than those of Pd-4d, Bi-6p, and Se-4p electrons in PdBiSe, respectively.

Pressure-Induced Valence Transition and Characteristic Electronic States in EuRh2Si2

EuRh2Si2 is well known to show a valence transition at a pressure of about 1 GPa from a Eu-divalent (antiferromagnetic) state to a nearly Eu-trivalent (paramagnetic) state, which was clarified using polycrystalline samples. We have succeeded in growing single crystals of EuRh2Si2 by the Bridgman method and studied their electronic properties measuring the electrical resistivity under pressure. EuRh2Si2 indicates a first-order valence transition in the pressure range from 1 to 2 GPa, distinguished by a sharp transition and a prominent hysteresis in the temperature dependence of the electrical resistivity. A critical end point in the valence transition is estimated as \(P_{\text{CEP}} \simeq 2.05\) GPa and \(T_{\text{CEP}} \simeq 170\) K. In the pressure range from 2 to 3 GPa, the electrical resistivity is found to exhibit a characteristic behavior for a moderately heavy-fermion compound such as EuIr2Si2. At pressures higher than 3 GPa, the resistivity reveals a normal metallic behavior in a nearly trivalen...

Calorimetry Study of the Phase Diagrams of EuNi2Ge2 and Eu2Ni3Ge5 under Pressure

We report here the phase diagrams of EuNi2Ge2 and Eu2Ni3Ge5 studied by ac calorimetry under pressure using a diamond anvil cell. We follow the antiferromagnetic transition for EuNi2Ge2 up to 1.5 GPa. The sudden disappearance of magnetic order at around 2 GPa is confirmed, consistent with the probable occurrence of a first-order valence transition near that pressure. The ac calorimetry results on Eu2Ni3Ge5 clearly show two antiferromagnetic transitions, and suggest that magnetic order persists up to higher pressure than previously expected. At high pressure, where heavy-fermion behavior has been reported, the Neel temperature is decreasing, and magnetic order is expected to disappear at an extrapolated pressure of 12–14 GPa. A semi quantitative analysis of the pressure dependence of the specific heat does not show any large changes, but is compatible with a moderate enhancement of γ. The phase diagrams of Yb and Ce heavy fermion systems are compared and discussed with our system.

Transport and Magnetic Properties of EuAl4 and EuGa4

We succeeded in growing a single crystal of the Eu-divalent compound EuAl4 with the BaAl4-type tetragonal structure by the Al self-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization, specific heat, and thermoelectric power. EuAl4 orders antiferromagnetically below TN1 = 15.4 K, with three successive antiferromagnetic transitions at TN2 = 13.2 K, TN3 = 12.2 K, and TN4 = 10.0 K. The latter two transitions are of the first-order. The corresponding magnetization curve indicates three successive metamagnetic transitions with hystereses and saturates above 16 kOe. We observed a plausible characteristic feature of the charge density wave (CDW) below TCDW = 140 K. We also studied an effect of pressure on the electronic state by measuring the electrical resistivity and thermoelectric power. TCDW is found to decrease with increasing pressure at a rate of dTCDW/dP = −54.7 K/GPa and becomes zero at about 2.5 GPa. The present antiferromagnetic ordering is, however, found to be ...

Electronic Structure of EuAl4 Studied by Photoelectron Spectroscopy

The electronic structure of the divalent Eu compound EuAl4, which shows a charge density wave transition at TCDW = 140 K, was studied by hard X-ray angle-integrated photoelectron spectroscopy (HAXPES) and soft X-ray angle-resolved photoelectron spectroscopy (ARPES). The valence band and core-level spectra obtained by HAXPES are consistent with the divalent nature of Eu atoms in EuAl4. From the ARPES results, the Fermi surface as well as band structure in the vicinity of the Fermi energy (EF) of EuAl4 are very similar to those of its isostructural divalent Sr compound SrAl4, which has no 4f electrons. This suggests that the Eu atoms are divalent in EuAl4, and the 4f electrons are localized below 1.8 eV with the Eu 4f7 electronic configuration in the ground state. The ARPES spectra measured along the Γ–(Σ)–Z high-symmetry line did not show significant temperature dependences above and below TCDW within the energy resolution of 80–90 meV. Moreover, the Fermi surface mapping along the kz direction showed that...

Resistivity and Thermopower of Ho(Co1-xAlx)2-Effects of Pressure and Magnetic Field

The electrical resistivity ρ and thermopower S of the cubic Laves phase compounds Ho(Co 1- x Al x ) 2 ( x =0, 0.15 and 0.20) have been measured at temperatures from 2 to 300 K in magnetic fields up to 10 T. The measurements of ρ under pressures up to 3 GPa have been performed in the temperature range between 2 K and 300 K. Both ρ( T ) and S ( T ) of the pure HoCo 2 reveal abrupt changes at the magnetic ordering temperature T C , indicating the first order transition. ρ( T ) of the Al substituted samples ( x =0.15 and 0.20) show anomalous behavior in the form of an increase below T C . The variations of ρ and S at T C clearly indicate that the type of the magnetic transition changes under substitution of Al for Co from the first order to the second order. The residual resistivity of Ho(Co 1- x Al x ) 2 shows a rapid increase with increasing x . T C increases linearly for x < 0.15, having a maximum at x ≈0.15 and decreases with further increase of x . The peculiar x dependence of T C and the anomalous behav...

Giant Hall Resistivity and Magnetoresistance in Cubic Chiral Antiferromagnet EuPtSi

EuPtSi crystallizes in the cubic chiral structure (P213, No. 198), which is the same as the non-centrosymmetric space group of MnSi with the skyrmion structure, and orders antiferromagnetically below a Neel temperature TN = 4.05 K. The magnetization at 2 K for the [111] direction indicates two metamagnetic transitions at the magnetic fields HA1 = 9.2 kOe and HA2 = 13.8 kOe and saturates above Hc = 26.6 kOe. The present magnetic phase between HA1 and HA2 is most likely closed in the \((H,T)\) phase and is observed in a wide temperature range from 3.6 to 0.5 K. In this magnetic phase known as the A-phase, we found giant additional Hall resistivity ΔρH(H) and magnetoresistance Δρ(H), reaching ΔρH(H) = 0.12 µΩ·cm and Δρ(H) = 1.4 µΩ·cm, respectively. These findings are obtained for H || [111] and [100], but not for H || [110] and [112], revealing an anisotropic behavior in the new material EuPtSi.

De Haas-van Alphen Effect in Rh2Ga9 and Ir2Ga9 without Inversion Symmetry in the Crystal Structure and Related Compounds T2Al9 (T: Co, Rh, Ir) with Inversion Symmetry

We succeeded in growing high-quality single crystals of Rh2Ga9 and Ir2Ga9 with the non-centrosymmetric (distorted Co2Al9-type) monoclinic structure by the Ga-self flux method, and carried out the de Haas–van Alphen (dHvA) experiments. The Fermi surface is found to be split into two different Fermi surfaces, reflecting the non-centrosymmetric crystal structure. A magnitude of the antisymmetric spin–orbit interaction or a splitting energy between the two Fermi surfaces are determined to be 56 K for dHvA branch α and 45 K for branch β in Rh2Ga9, where these dHvA branches correspond to main Fermi surfaces. The present splitting values are compared with 290 and 130 K in Ir2Ga9, respectively. The splitting energy is found to be larger in the Ir-5d conduction electrons than in the Rh-4d conduction electrons. The split dHvA branches in Rh2Ga9 and Ir2Ga9 are also compared with the single dHvA branch in Co2Al9, Rh2Al9, and Ir2Al9 with inversion symmetry in the crystal structure.

Magnetic Properties of Heavy Fermion Compound Ce5Si4 with Chiral Structure

The low-temperature magnetic properties of Ce5Si4 with a chiral structure have been studied by electrical resistivity, heat capacity, and magnetization measurements using single-crystalline samples...

Superconducting Properties of CeIr3 Single Crystal

Superconducting properties of CeIr3 single crystal with rhombohedral structure were examined for the first time using DC magnetization, specific heat, and electrical resistivity measurements. A bul...