Magnetic measurements at pressures above 10 GPa in a miniature ceramic anvil cell for a superconducting quantum interference device magnetometer

A miniature ceramic anvil high pressure cell (mCAC) was earlier designed by us for magnetic measurements at pressures up to 7.6 GPa in a commercial superconducting quantum interference (SQUID) magnetometer [N. Tateiwa et al., Rev. Sci. Instrum. 82, 053906 (2011)]. Here, we describe methods to generate pressures above 10 GPa in the mCAC. The efficiency of the pressure generation is sharply improved when the Cu-Be gasket is sufficiently preindented. The maximum pressure for the 0.6 mm culet anvils is 12.6 GPa when the Cu-Be gasket is preindented from the initial thickness of 0.30 to 0.06 mm. The 0.5 mm culet anvils were also tested with a rhenium gasket. The maximum pressure attainable in the mCAC is about 13 GPa. The present cell was used to study YbCu2Si2 which shows a pressure induced transition from the non-magnetic to magnetic phases at 8 GPa. We confirm a ferromagnetic transition from the dc magnetization measurement at high pressure. The mCAC can detect the ferromagnetic ordered state whose spontaneous magnetic moment is smaller than 1 mB per unit cell. The high sensitivity for magnetic measurements in the mCAC may result from the the simplicity of cell structure. The present study shows the availability of the mCAC for precise magnetic measurements at pressures above 10 GPa

Improved sensitivity of magnetic measurements under high pressure in miniature ceramic anvil cell for a commercial SQUID magnetometer

Two modifications have been made to a miniature ceramic anvil high pressure cell (mCAC) designed for magnetic measurements at pressures up to 12.6 GPa in a commercial superconducting quantum interference (SQUID) magnetometer [N. Tateiwa et al., Rev. Sci. Instrum. 82, 053906 (2011)., ibid. 83, 053906 (2012)]. Replacing the Cu-Be piston in the former mCAC with a composite piston composed of the Cu-Be and ceramic cylinders reduces the background magnetization significantly smaller at low temperatures, enabling more precise magnetic measurements at low temperatures. A second modification to the mCAC is the utilization of a ceramic anvil with a hollow in the center of the culet surface. High pressures up to 5 GPa were generated with the "cupped ceramic anvil" with the culet size of 1.0 mm.Comment: Rev. Sci. Instrum. 84, 046105 (2013

Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe

Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field H_c2. H_c2 for H // b-axis (H_c2^b) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of H_c2^a shows upward curvature with a low temperature value exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H // b-axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H_c2^b curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp

Spin relaxation of conduction electrons in bulk III-V semiconductors

Spin relaxation time of conduction electrons through the Elliot-Yafet, D'yakonov-Perel and Bir-Aronov-Pikus mechanisms is calculated theoretically for bulk GaAs, GaSb, InAs and InSb of both $n$- and $p$-type. Relative importance of each spin relaxation mechanism is compared and the diagrams showing the dominant mechanism are constructed as a function of temperature and impurity concentrations. Our approach is based upon theoretical calculation of the momentum relaxation rate and allows understanding of the interplay between various factors affecting the spin relaxation over a broad range of temperature and impurity concentration.Comment: an error in earlier version correcte

Evidence for Uniform Coexistence of Ferromagnetism and Unconventional Superconductivity in UGe_2: A ^73Ge-NQR Study under Pressure

We report on the itinerant ferromagnetic superconductor UGe_2 through ^73Ge-NQR measurements under pressure (P). The P dependence of the NQR spectrum signals a first-order transition from the low-temperature (T) and low-P ferromagnetic phase (FM2) to high-T and high-P one (FM1) around a critical pressure of P_x ~ 1.2 GPa. The superconductivity exhibiting a maximum value of T_sc=0.7 K at P_x ~ 1.2 GPa, was found to take place in connection with the P-induced first-order transition. The nuclear spin-lattice relaxation rate 1/T_1 has probed the ferromagnetic transition, exhibiting a peak at the Curie temperature as well as a decrease without the coherence peak below T_sc. These results reveal the uniformly coexistent phase of ferromagnetism and unconventional superconductivity with a line-node gap. We remark on an intimate interplay between the onset of superconductivity and the underlying electronic state for the ferromagnetic phases.Comment: 8 pages, 9 figures. to appear in J. Phys. Soc. JPN, 74 No.2 (2005

Response of the Heavy-Fermion Superconductor CeCoIn5_5 to Pressure: Roles of Dimensionality and Proximity to a Quantum-Critical Point

We report measurements of the pressure-dependent superconducting transition temperature $T_c$ and electrical resistivity of the heavy-fermion compound CeCoIn$_5$. Pressure moves CeCoIn$_5$ away from its proximity to a quantum-critical point at atmospheric pressure. Experimental results are qualitatively consistent with theoretical predictions for strong-coupled, d-wave superconductivity in an anisotropic 3D superconductor.Comment: 9 pages, 5 figure

Possible Phase Transition Deep Inside the Hidden Order Phase of Ultraclean URu2Si2

To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu2Si2, we measure electrical transport properties of ultraclean crystals in a high field/low temperature regime. Unlike previous studies, the present system with much less impurity scattering resolves a distinct anomaly of the Hall resistivity at H*=22.5 T well below the destruction field of the HO phase ~36 T. In addition, a novel quantum oscillation appears above a magnetic field slightly below H*. These results indicate an abrupt reconstruction of the Fermi surface, which implies a possible phase transition well within the HO phase caused by a band-dependent destruction of the HO parameter. The present results definitely indicate that the HO transition should be described by an itinerant electron picture.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter

Ferromagnetism and Superconductivity in Uranium Compounds

Recent advances on ferromagnetic superconductors, UGe2, URhGe and UCoGe are presented. The superconductivity (SC) peacefully coexists with the ferromagnetism (FM), forming the spin-triplet state of Cooper pairs. The striking new phenomena, such as SC reinforced by the magnetic field, are associated with Ising-type ferromagnetic fluctuations. A variety of ferromagnetic ordered moments between UGe2, URhGe and UCoGe affords to understand the relation between FM, tricriticality and SC.Comment: 11 pages, 16 figures, accepted for publication in J. Phys. Soc. Jpn. as a review article of Special Topics of "Recent developments in superconductivity