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We review recent progress in single crystal growth and study of electronic properties in. Czocharalski pulling, using purified uranium metal and subsequent annealing under ultra-high vacuum, is successfully applied to this compound, and it yields the highest residual resistivity ratio. These high-quality single crystals allow us to investigate Fermi surfaces using quantum oscillation and to make detailed transport measurements at low temperature.

Temperature (T) and magnetic field (H) dependence of the magnetic properties in metamagnetic UCoAl have been investigated using a soft x-ray magnetic circular dichroism (XMCD). In order to extract element-specific magnetic properties at the U and Co sites, the XMCD experiment has been performed at the U 4d-5f (N4,5) and Co 2p-3d (L2,3) absorption edges, respectively. Directions of magnetic moments at the U and Co sites have been determined from shapes of the XMCD spectra. The directions of the total magnetic moments at the U and Co sites are parallel to the H direction (c axis), but the direction of the spin magnetic moment at the U site is opposite to that at the Co site. The XMCD intensities at both the U and Co sites at T=5.5 K increase steeply at H=0.77 T (Hm), corresponding to the metamagnetic transition. The XMCD intensities do not saturate, even in the field-induced ferromagnetic state above Hm. In addition, the ratio of the increase of the XMCD intensity at the Co site is smaller than that at the U site. From comparison of the H dependence of the XMCD intensities at T=25 and 5.5 K, we found that the magnetic behavior of the Co atom has a stronger T dependence than that of the U atom. © 2013 American Physical Society.

We report precision c-axis magnetic measurements on a high-quality single crystal of the heavy fermion metamagnet UCoAl. The metamagnetic transition at H M changes from 1st order at low temperature to a crossover at high temperature. H M is nearly linearly increasing with increasing temperature up to a critical temperature T 0 . The critical temperature T 0 is determined from both the field and the temperature dependences of magnetization to be ∼ 11 K. The field dependence of the Sommerfeld coefficient γ is estimated from M(T) by using a Maxwell relation. γ(H) shows a step-like decrease at H M . This behavior is consistent with the previous reports of specific heat and resistivity measurements at low temperatures. © 2013 The Korean Physical Society.

We have performed NMR measurements under pressure in UCoAl with a quantum critical endpoint of the metamagnetic transition from the paramagnetic phase to the ferromagnetic (FM) phase. 59 Co-NMR sensitively detects the evolution of the internal field caused by applying the external field. The metamagnetic field H m increases with increasing pressure consistently with other experimental methods, accompanied by the suppression of the magnetization in the field-induced FM phase and the magnetization jump at H m . The loss of the NMR signal on approaching the QCEP indicates the development of the spin fluctuations. © 2013 The Korean Physical Society.

We have studied the high-pressure magnetic property in UGe 2 where ferromagnetic superconductivity appears under high pressure. In this study, we focus on the magnetic property at pressures above the ferromagnetic critical pressure P c =1.6 GPa. The temperature and magnetic field dependences of the dc-magnetization have been measured under high pressures up to 5.1 GPa by using a ceramic anvil high pressure cell. At pressures above P c , the magnetic susceptibility x shows a broad maximum around T χmax and the magnetization at 2.0 K shows an abrupt increase (metamagnetic transition) at H c . With increasing pressure, the peak structure in x becomes broader, and the peak position T χmax moves to the higher temperature region. The metamagnetic field H c increases rapidly with increasing pressure. At pressures above 4.1 GPa, x shows a simple temperature dependence, and the magnetization increases linearly with increasing field. These phenomena in UGe 2 resemble to those in the intermetallic compounds of 3d transition metals such as Co(S 1-x Se x ) and YCo 2 . We discuss the experimental results by using the phenomenological spin-fluctuation theory. © 2013 The Korean Physical Society.

The pressure dependent electrical resistivity of URu2Si 2 has been studied at high pressure across the first order phase boundary of Px where the ground state switches under pressure from "hidden order" (HO) to large moment antiferromagnetic (LAFM) states. We have measured an ultra-clean single crystal whose quality is the highest among those used in previous studies. We have previously analyzed the resistivity data with the generalized power law ρ = ρ0 + A nTn. It was found that the electric transport property deviates from Fermi liquid theory in the HO phase but obeys the theory well above Px. In this paper, we re-analyze the data using the polynomial in T expression ρ = ρ0 + α1T + α 2T2. The analysis finds the relation α1/ α2 α Tsc in the HO phase. While the pressure dependence of α2 is very weak, α1 is roughly proportional to Tsc. This suggests a strong correlation between the anomalous quasiparticle scattering and the superconductivity and that both have a common origin. The present study clarifies a universality of the HO phase inherent in strongly correlated electron superconductors near quantum criticality. © Published under licence by IOP Publishing Ltd.

The pressure dependent electrical resistivity of URu 2Si 2 has been studied at high pressure across the first order phase boundary of P x where the ground state switches under pressure from "hidden order" (HO) to large moment antiferromagnetic (LAFM) states. The electrical transport in URu 2Si 2 at low temperatures shows a strong sample dependence. We have measured an ultraclean single crystal whose quality is the highest among those used in previous studies. The generalized power law ρ=ρ 0+A nTn analysis finds that the electric transport property deviates from Fermi liquid theory in the HO phase but obeys the theory well above P x. The analysis using the polynomial in T expression ρ= ρ 0+α 1T+α 2T2 reveals the relation α 1/α 2 T sc in the HO phase. While the pressure dependence of α 2 is very weak, α 1 is roughly proportional to T sc. This suggests a strong correlation between the anomalous quasiparticle scattering and the superconductivity and that both have a common origin. The present study clarifies a universality of the HO phase inherent in strongly correlated electron superconductors near quantum criticality. © 2012 American Physical Society.

Resistivity and specific heat measurements were performed in the low carrier unconventional superconductor URu2Si2 on various samples with very different qualities. The superconducting transition temperature (TSC) and the hidden order transition temperature (T HO) of these crystals were evaluated as a function of the residual resistivity ratio (RRR). In high quality single crystals the resistivity does not seem to follow a T2 dependence above TSC, indicating that the Fermi liquid regime is restricted to low temperatures. However, an analysis of the isothermal longitudinal magnetoresistivity points out that the T2 dependence may be "spoiled" by residual inhomogeneous superconducting contribution. We discuss a possible scenario concerning the distribution of TSC related with the fact that the hidden order phase is very sensitive to the pressure inhomogeneity. © 2011 The Physical Society of Japan.

We report 59Co-NMR measurements in UCoAl where metamagnetism occurs owing to the enhancement of ferromagnetism by a magnetic field. The metamagnetic transition from a paramagnetic (PM) state to a ferromagnetic state is a first-order transition at low temperatures, but it changes to a crossover at high temperatures on crossing the critical end point (CEP) at Tcep ∼ 12 K. The contrasting behavior between the relaxation rates 1/T 1 and 1/T2 suggests that the longitudinal magnetic fluctuation of the U moments is strongly enhanced, especially near the CEP. A wide diffusion of the fluctuation from the CEP can be confirmed even in the PM state where the magnetic transition does not occur. © 2011 The Physical Society of Japan.

A miniature opposed-anvil high-pressure cell has been developed for magnetic measurement in a commercial superconducting quantum interference device magnetometer. Non-magnetic anvils made of composite ceramic material were used to generate high-pressure with a Cu-Be gasket. We have examined anvils with different culet sizes (1.8, 1.6, 1.4, 1.2, 1.0, 0.8, and 0.6 mm). The pressure generated at low temperature was determined by the pressure dependence of the superconducting transition of lead (Pb). The maximum pressure P(max) depends on the culet size of the anvil: the values of P(max) are 2.4 and 7.6 GPa for 1.8 and 0.6 mm culet anvils, respectively. We revealed that the composite ceramic anvil has potential to generate high-pressure above 5 GPa. The background magnetization of the Cu-Be gasket is generally two orders of magnitude smaller than the Ni-Cr-Al gasket for the indenter cell. The present cell can be used not only with ferromagnetic and superconducting materials with large magnetization but also with antiferromagnetic compounds with smaller magnetization. The production cost of the present pressure cell is about one tenth of that of a diamond anvil cell. The anvil alignment mechanism is not necessary in the present pressure cell because of the strong fracture toughness (6.5 MPa m(1∕2)) of the composite ceramic anvil. The simplified pressure cell is easy-to-use for researchers who are not familiar with high-pressure technology. Representative results on the magnetization of superconducting MgB(2) and antiferromagnet CePd(5)Al(2) are reported.