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Shubnikov-de Haas measurements of high quality URu2Si2 single crystals reveal two previously unobserved Fermi surface branches in the so-called hidden order phase. Therefore about 55 % of the enhanced mass is now detected. Under pressure in the antiferromagnetic state, the Shubnikov-de Haas frequencies for magnetic fields applied along the crystalline c axis show little change compared with the zero pressure data. This implies a similar Fermi surface in both the hidden order and antiferromagnetic states, which strongly suggests that the lattice doubling in the antiferromagnetic phase due to the ordering vector QAF = (0 0 1) already occurs in the hidden order. These measurements provide a good test for existing or future theories of the hidden order parameter. PACS numbers:The electronic properties of uranium compounds are determined by the tenuous balance between the localized and itinerant character of the 5f electrons which may lead to the formation of enigmatic ground states [1]. One famous example is the heavy fermion compound URu 2 Si 2 which shows a second order phase transition to a "hidden order" (HO) state at T 0 = 17.5 K. The transition to the HO state is associated with a huge entropy loss of 0.2R ln 2 [2]. Despite intense research for 25 years, the order parameter has not yet been identified. The possible proximity to a 5f 2 configuration of the uranium atoms leads to the possibility of multipolar ordering which is highly debated in Pr 3+ systems in the 4f 2 configuration [3]. Thus the resolution of the HO parameter will have a deep impact on the understanding of heavy fermion materials. A large diversity of theoretical proposals have been given. The most recent ones include multipolar orders [4-6]), dynamical spin density wave [7] or hybridization wave [8].The Fermi surface (FS) properties are directly linked to the itineracy of the 5f electrons and to the change of the symmetry entering into the HO phase. Changes of the FS at T 0 have been observed in various experiments. Optical conductivity [9] and transport measurements [10,11] indicate a gap opening and a drop in the number of charge carriers at T 0 . Recent STM measurements show that a hybridization gap opens suddenly at T 0 [12, 13] while in ARPES measurements abrupt changes of the electronic spectrum are detected [14,15]. Here we focus on the FS determination via Shubnikov-de Haas (SdH) measurements on a new generation of high quality crystals. SdH measurements under pressure provide the great opportunity to study the difference of quantum oscillations between the low pressure HO phase and the high pressure antiferromagnetic (AF) phase with propagation vector Q AF = (0 0 1) and ordered moment m 0 = 0.3 µ B /U. A small pressure of P x ≈ 0.8 GPa is enough to switch the ground state from HO to AF [16][17][18]. The AF phase has been well characterized, notably the change from body centered tetragonal to simple tetragonal crystal structure below T 0 [7,19]. Inelastic neutron scattering experiments under pressure suggest that, due to the disappear...

Resistivity and magnetostriction measurements were performed at high magnetic fields and under pressure on UCoAl. At ambient pressure, the 1st order metamagnetic transition at H_m ~ 0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T_0 ~11 K. With increasing pressure, H_m linearly increases, while T_0 decreases and is suppressed at the quantum critical endpoint (QCEP, P_QCEP ~ 1.5 GPa, H_m ~ 7 T). At higher pressure, the value of H_m identified as a crossover continuously increases, while a new anomaly appears above P_QCEP at higher field H* in resistivity measurements. The field dependence of the effective mass (m*) obtained by resistivity and specific heat measurements exhibits a step-like drop at H_m at ambient pressure. With increasing pressure, it gradually changes into a peak structure and a sharp enhancement of m* is observed near the QCEP. Above P_QCEP, the enhancement of m* is reduced, and a broad plateau is found between H_m and H*. We compare our results on UCoAl with those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic ruthenate Sr3Ru2O7.Comment: 10 pages, 14 figures, accepted for publication in J. Phys. Soc. Jp

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We performed the Shubnikov-de Haas (SdH) experiments of the low carrier heavy fermion compound URu 2 Si 2 at high fields up to 34 T and at low temperatures down to 30 mK. All main SdH branches named α, β and γ were observed for all the measured field-directions (H [001] → [100], [100] → [110] and [001] → [110]), indicating that these are attributed to the closed Fermi surfaces with nearly spherical shapes. Anomalous split of branch α was detected for the field along the basal plane, and the split immediately disappears by tilting the field to [001] direction, implying a fingerprint of the hidden order state. High field experiments reveal the complicated field-dependence of the SdH frequencies and the cyclotron masses due to the Zeeman spin-splitting associated with the Fermi surface reconstruction in the hidden order state with small carrier numbers. A new SdH branch named ω with large cyclotron mass of 25 m 0 was detected at high fields above 23 T close to the hidden order instabilities.KEYWORDS: Shubnikov-de Haas effect, de Haas-van Alphen effect, Fermi surface, cyclotron effective mass, spin split, hidden order, URu 2 Si 2

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We grew single crystals of non-centrosymmetric compounds CeTSi 3 and CeTGe 3 (T: transition metal), and studied the magnetic properties by measuring the electrical resistivity, magnetic susceptibility and magnetization. We also studied the effect of pressure on the electronic states in antiferromagnets CeTGe 3 (T: Co, Rh, Ir) by measuring the resistivity under pressure. No noticeable change of the Néel temperature was observed up to 8 GPa in CeRhGe 3 and CeIrGe 3 , which are far from the magnetic quantum critical point. On the other hand, the Néel temperature in CeCoGe 3 was strongly decreased as a function of pressure, and pressure-induced superconductivity was observed in the pressure region from 5.4 GPa to about 7.5 GPa. The slope of upper critical field H c2 at 6.5 GPa is found to be extremely large, with an upturn curvature of H c2 with decreasing temperature: ÀdH c2 =dT ¼ 200 kOe/K at the superconducting transition temperature T sc ¼ 0:69 K, revealing unconventional superconductivity.

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 (THO) 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 T 2 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 T 2 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.