Takashi Tayama

Superconductivity at 1 K in Cd2Re2O7

We report the first pyrochlore oxide superconductor Cd2Re2O7. Resistivity,magnetic susceptibility,and specific heat measurements on single crystals evidence a bulk superconductivity at 1 K. Another phase transition found at 200 K suggests that a peculiar electronic structure lies behind the superconductivity.

Faraday force magnetometer for high-sensitivity magnetization measurements at very low temperatures and high fields

A Faraday force magnetometer has been developed for static magnetization measurements at very low temperatures down to 100 mK and in fields up to 9 T. The magnetic force acting on a specimen located in the adiabatic vacuum chamber of a dilution refrigerator is detected by a newly designed load-sensing variable capacitor. Use of a superconducting magnet with gradient coils enables accurate measurement of magnetic moment with a resolution of better than 1×10-7 Am2 ( 1×10-4 emu). An application to magnetization measurements on the heavy-electron antiferromagnet CeB6 is presented.

Magnetic Phase Diagram of CexLa1-xB6 Studied by Static Magnetization Measurement at Very Low Temperatures.

DC magnetization of single crystals of Ce x La 1- x B 6 , 0.5≤ x ≤1, have been measured at low temperatures ( T ) down to 40 mK in magnetic field H //[100], and the H - T - x phase diagrams were obtained. At zero field, the antiferro-quadrupolar (AFQ) transition temperature T Q rapidly decreases with decreasing x and vanishes abruptly between x =0.75 and 0.7. The Neel temperature T N , which is lower than T Q in CeB 6 , varies more slowly with x and exceeds T Q for \(x \gtrsim 0.8\). A clear peak observed in the susceptibility strongly suggests that a new antiferromagnetic (AF) state having no AFQ moment (phase IV) exists for \(0.8\gtrsim x\gtrsim 0.5\), in addition to the two ordered phases II (AFQ) and III (AF+AFQ). As H increases, a first-order phase IV-III transition is observed with a metamagnetic jump in the magnetization. Successive I (paramagnetic)-IV-III phase transitions are observable in a very narrow region of the H - T - x space.

Magnetic phase diagram of the heavy fermion superconductor PrOs4Sb12

We investigated the magnetic phase diagram of the first Pr-based heavy fermion superconductor PrOs 4 Sb 12 by means of high-resolution dc magnetization measurements in low temperatures down to 0.06 K. The temperature dependence of the magnetization M ( T ) at 0.1 kOe exhibits two distinct anomalies at T c1 =1.83 K and T c2 =1.65 K, in agreement with the specific heat measurements at zero field. Increasing magnetic field H , both T c1 ( H ) and T c2 ( H ) move toward lower temperatures without showing a tendency of intersecting to each other. Above 10 kOe, the transition at T c2 ( H ) appears to merge into a line of the peak effect which is observed near the upper critical field H c2 in the isothermal M ( H ) curves, suggesting a common origin for these two phenomena. The presence of the field-induced ordered phase (called phase A here) is confirmed for three principal directions above 40 kOe, with the anisotropic A-phase transition temperature T A : T A [100] > T A [111] > T A [110] . The present results ...

Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order

Spin liquids are magnetically frustrated systems, in which spins are prevented from ordering or freezing, owing to quantum or thermal fluctuations among degenerate states induced by the frustration. Chiral spin liquids are a hypothetical class of spin liquids in which the time-reversal symmetry is macroscopically broken in the absence of an applied magnetic field or any magnetic dipole long-range order. Even though such chiral spin-liquid states were proposed more than two decades ago, an experimental realization and observation of such states has remained a challenge. One of the characteristic order parameters in such systems is a macroscopic average of the scalar spin chirality, a solid angle subtended by three nearby spins. In previous experimental reports, however, the spin chirality was only parasitic to the non-coplanar spin structure associated with a magnetic dipole long-range order or induced by the applied magnetic field, and thus the chiral spin-liquid state has never been found. Here, we report empirical evidence that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order. In particular, we employ the anomalous Hall effect to directly probe the broken time-reversal symmetry for the metallic frustrated magnet Pr2Ir2O7. An onset of the Hall effect is observed at zero field in the absence of uniform magnetization, within the experimental accuracy, suggesting an emergence of a chiral spin liquid. The origin of this spontaneous Hall effect is ascribed to chiral spin textures, which are inferred from the magnetic measurements indicating the spin ice-rule formation.

Evolution of 4 f electron states in the metal-insulator transition of Pr Ru 4 P 12

Magnetic excitations of the filled skutterudite

A new macroscopically degenerate ground state in the spin ice compound Dy2Ti2O7 under a magnetic field

\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}

New High-Field Ordered State in PrFe4P12

exhibiting a metal-insulator (M-I) transition at

Specific Heat of Kagomé Ice in the Pyrochlore Oxide Dy2Ti2O7

{T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}=63\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Magnetization ''Steps'' on a Kagome Lattice in Volborthite

were studied by inelastic neutron scattering experiment. The spectra at temperatures much lower than