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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 Hc2. Hc2 for H b-axis (H b c2 ) 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 a c2 shows upward curvature with a low temperature value exceeding 30 T, while H c c2 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 b c2 curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.The coexistence of superconductivity (SC) and ferromagnetism (FM) has attracted much attention, since the exotic SC state based on spin-triplet pairing mediated by longitudinal spin fluctuations is expected. 1 The first example was discovered in UGe 2 under pressure, where T sc is much lower than T Curie . 2 The SC phase exists only in the FM phase, and SC disappears in the paramagnetic (PM) phase above the critical pressure P c . Soon after that, SC was found at ambient pressure in the weak ferromagnet URhGe. 3 T sc (= 0.25 K) is much lower than T Curie (= 9.5 K). The upper critical field H c2 exceeds the Pauli paramagnetic limit, thus it is believed that the spin-triplet state with equal-spin pairing is realized. Recently, reentrant superconductivity (RSC) was found between 8 and 13 T when the field is applied along the b-axis of the orthorhombic TiNiSi-type structure (space group: Pnma). 4 With increasing field, the magnetic moment gradually tilts from the c-axis (easy-magnetization axis) to b-axis. Finally the moment is suddenly aligned to the b-axis at the spin reorientation field H R . The fieldinduced critical magnetic fluctuations induce RSC. Recently, we have observed the enhancement of effective mass in the RSC phase, and explained the emergence of RSC. 5,6 A newcomer, UCoGe, which crystallizes in the same structure as URhGe, was recently reported. 7 UCoGe is a weak ferromagnet with T Curie ∼ 3 K and the ordered moment µ 0 = 0.07 µ B /U. T sc (∼ 0.6 K) is larger than that in URhGe. Since T Curie is low, one can naively consider that UCoGe is close to the quantum critical point. Indeed, our previous measurement shows that T Curie is immediately suppressed by applying a small pressure (P c ∼ 1 GPa). 8 Contrary to the case of UGe 2 where SC exists only in the FM domain, SC survives even in the PM phase with the maximum T sc (≃ 0.75 K) around P c . A new theory * E-mail address: aokidai@gmail.com from symmetry considerations was developed in order to explain the temperature-pressure phase diagram. 9 H c2 at ambient pressure shows strong anisotropy. H c2 for H a (H a c2 ) and b-axis (H b c2 ) reveal almost the same temperature depend...

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...

By performing combined resistivity and calorimetric experiments under pressure, we have determined a precise temperature-pressure ͑T , P͒ phase diagram of the heavy fermion compound URu 2 Si 2 . It will be compared with previous diagrams determined by elastic neutron diffraction and strain gauge techniques. At first glance, the low-pressure ordered phase referred to as hidden order is dominated by Fermi-surface nesting, which has strong consequences on the localized spin dynamics. The high-pressure phase is dominated by large moment antiferromagnetism ͑LMAF͒ coexisting with at least dynamical nesting needed to restore on cooling a local moment behavior. ac calorimetry confirms unambiguously that bulk superconductivity does not coexist with LMAF. URu 2 Si 2 is one of the most spectacular examples of the dual itinerant and local character of uranium-based heavy fermion compounds.

We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline SmB 6 in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures 5 KϽTϽ20 K the electrodynamic response and the dc conductivity of SmB 6 are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of SmB 6 are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.

Precise resistivity measurements on the ferromagnetic superconductor UGe2 under pressure p and magnetic field H reveal a previously unobserved change of the anomaly at the Curie temperature. Therefore, the tricritical point (TCP) where the paramagnetic-to-ferromagnetic transition changes from a second order to a first order transition is located in the p-T phase diagram. Moreover, the evolution of the TCP can be followed under the magnetic field in the same way. It is the first report of the boundary of the first order plane which appears in the p-T-H phase diagram of weak itinerant ferromagnets. This line of critical points starts from the TCP and will terminate at a quantum critical point. These measurements provide the first estimation of the location of the quantum critical point in the p-H plane and will inspire similar studies of the other weak itinerant ferromagnets.

We present a study of the upper critical field of the newly discovered heavy fermion superconductor UTe 2 by magnetoresistivity measurements in pulsed magnetic fields up to 60 T and static magnetic fields up to 35 T. We show that superconductivity survives up to the metamagnetic transition at H m ≈ 35 T at low temperature. Above H m superconductivity is suppressed. At higher temperature superconductivity is enhanced under magnetic field leading to reentrance of superconductivity or an almost temperature independent increase of H c2 . By studying the angular dependence of the upper critical field close to the b axis (hard magnetization axis) we show that the maximum of the reentrant superconductivity temperature is depinned from the metamagnetic field. A key ingredient for the field-reinforcement of superconductivity on approaching H m appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction. 1 arXiv:1905.05181v1 [cond-mat.str-el]

The pressure-temperature phase diagram of CeRhIn5 has been studied under high magnetic field by resistivity measurements. Clear signatures of a quantum critical point has been found at a critical pressure of pc ≈ 2.5 GPa. The field induced magnetic state in the superconducting state is stable up to the highest field. At pc the antiferromagnetic ground-state under high magnetic field collapses very rapidly. Clear signatures of pc are the strong enhancement of the resistivity in the normal state and of the inelastic scattering term. No clear T 2 temperature dependence could be found for pressures above Tc. From the analysis of the upper critical field within a strong coupling model we present the pressure dependence of the coupling parameter λ and the gyromagnetic ratio g. No signatures of a spatially modulated order parameter could be evidenced. A detailed comparison with the magnetic field-temperature phase diagram of CeCoIn5 is given. The comparison between CeRhIn5 and CeCoIn5 points out the importance to take into account the field dependence of the effective mass in the calculation of the superconducting upper critical field Hc2. It suggests also that when the magnetic critical field HM(0) becomes lower than Hc2(0), the persistence of a superconducting pseudo-gap may stick the antiferromagnetism to Hc2(0).