Kiichi Amaya

Superconductivity in oxygen

Among the simple diatomic molecules, oxygen is of particular interest because it shows magnetism at low temperatures. Moreover, at pressures exceeding 95 GPa (∼0.95 Mbar), solid molecular oxygen becomes metallic, accompanied by a structural transition. The metallization process is characterized by an increase in optical reflectivity, and a change in the slope of the resistance–temperature curve. Here we report that at pressures of around 100 GPa, solid oxygen becomes superconducting, with a transition temperature of 0.6 K. The transition is revealed by both resistivity measurements and a Meissner demagnetization signal.

Superconductivity of CeRhIn5 under High Pressure

The pressure dependence of transport properties in CeRhIn5 was studied up to 8.5 GPa by using a diamond-anvil cell. The electrical resistivity does not follow the T 2 -dependecne of a Fermi liquid nature under any pressure. In this non-Fermi liquid state, superconductivity was observed in a wide pressure range of 1:5 GPa <P <7:6 GPa. The superconducting temperature TC has a double-maximum structure with a minimum at 5.2 GPa. A maximum of TC ¼ 2:2 K was observed at P ¼ 2:5 GPa, where the corresponding electrical resistivity just above TC has the maximum. The upper critical ;eld HC2 and the magnetoresistances were also investigated under pressure.

Orbital Frustration and Resonating Valence Bond State in the Spin-1/2 Triangular Lattice LiNiO2

From the 7 Li nuclear magnetic resonance (NMR) and specific heat measurements of LiNiO 2 , consisting of a spin-1/2 triangular lattice, it is demonstrated that the previously suggested spin-glass-like freezing below T s g =8 K, which is caused by defects of Li ions, is not the true ground state, but rather a quantum disordered state is realized without a spin gap. This state is characterized by power-law behavior of the temperature dependence of the specific heat below 0.5 K. A spin-liquid state with short-range ferromagnetic correlations is suggested from new insight into the frustration of orbital ordering for doubly degenerate e g shells in the triangular lattice.

Magnetic properties of a pressure-induced superconductor UGe2

We performed DC-magnetization and neutron scattering experiments under pressure P for a pressure-induced superconductor UGe 2 . We found that the magnetic moment is enhanced at a characteristic temperature T * in the ferromagnetic state, where T * is smaller than a Curie temperature T C . This enhancement becomes remarkable in the vicinity of P C * =1.20 GPa, where T * becomes 0 K and the superconducting transition temperature T SC shows a maximum. The characteristic temperature T * , which decreases with increasing pressure, also depends on the magnetic field.

Superconductivity of Calcium under High Pressures

Electrical resistance of Ca is measured under pressures up to 150 GPa and at temperatures down to 0.05 K. Superconductivity is observed at temperatures below 3 K under pressures above 85 GPa. The superconducting transition temperature increases linearly with increasing pressure and reaches about 15 K at 150 GPa.

Heat capacities of Haldane-Gap antiferromagnet NENP in magnetic field

Heat capacity measurements of the quasi one-dimensional S =1 antiferromagnet Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ) are performed in static magnetic fields up to 13 T. Applying a magnetic field, a broad anomaly appears below 5 K, which reflects the gap energy between the nonmagnetic ground state singlet and the excited states. The field-dependence of the energy gap estimated from the measured heat capacities gives the critical field values close to those obtained in high-field-magnetization measurements and non-zero gap remaining at the critical field. Within our experimental range of temperature and magnetic field, we have not observed any sharp peaks corresponding to the expected three dimensional magnetic long range ordering.

Magnetic properties of 1,5-dimethyl-6-oxoverdazyl radical crystals observation of a spin-peierls transition in 3-(4- cyanophenyl)-1,5-dimethyl-6-oxoverdazyl radical, p-CyDOV

Abstract Magnetic susceptibilities of 5 kinds of 3-(4-R-phenyl)-1,5-dimethyl-6-oxoverdazyl radicals (PDOV (R  H), p-MDOV (R  CH3O), p-MeDOV (R  CH3), p-CyDOV (R  CN), and p-NDOV (R  NO2)) have been measured in the temperature range of 4.2–300 K. The susceptibilities of PDOV, p-MDOV, and p-MeDOV can be explained by an antiferromagnetic Heisenberg alternating chain model. The susceptibility of p-CyDOV shows a broad maximum at 54 K, and decreases abruptly at 15 K, suggesting a spin-Peierls transition. High-field magnetization data (H = 0−35 T) which provide a clear evidence for a spin-Peierls transition were reported.

Observation of pressure-induced superconductivity of sulfur

The electrical resistance of sulfur has been measured under pressures up to 100 GPa and temperatures down to 70 mK. Under pressures approaching 100 GPa, sulfur shows metallic reflection comparable to that of used Pt electrodes. The temperature dependence of the resistance reveals metallic behavior, and the sharp drop of the resistance at around 15 K and 100 GPa is due to superconducting transition as the transition temperature shows typical magnetic field dependence.

Pressure-induced Superconductivity in Elemental Materials

Both experimental and theoretical approaches in our search for superconductivity under pressure are reviewed. We have developed a double extreme condition of very low-temperature down to 50 mK using a 3 He/ 4 He dilution refrigerator and ultrahigh pressure up to 200 GPa using a diamond-anvil cell (DAC). Experimental techniques producing ultrahigh pressure as well as measuring electrical resistance, magnetization and optical response are developed especially for studying the pressure-induced superconductivity of simple systems such as elemental materials. The details of these techniques are described in §1. In §2, we review several examples of pressure-induced superconductivity observed for the first time by our group. Comparisons between experimental results and theories based on the first principles band calculations are given in §3. Finally, we point out remaining problems to be solved in further studies in the search for pressure-induced superconductivity.

Low temperature magnetic properties of organic radical magnet, MOTMP

The magnetic properties of MOTMP 4-methacryloyloxy-2,2,6,6-tetramethyl-piperidin-1-oxyl are studied at tempratures below 1 K. The dominant magnetic coupling between the radicals is ferromagnetic. The one-dimensional behavior observed in the temperature dependence of susceptibility is consistent with previous result of heat capacity measurement in which the intrachain ferromagnetic interaction is estimated to be J / k =0.9 K. The susceptibility in the ordered state, however, shows spin-flip transition, which reveals antiferromagnetic long range order. From the analysis of ordering temperature T N and critical field H C , the interchain interactions are also estimated by the mean field theory.