Fermi-Surface Instability in the Heavy-Fermion Superconductor UTe_{2}.

Transport measurements are presented up to fields of 29 T in the recently discovered heavy-fermion superconductor UTe_{2} with magnetic field H applied along the easy magnetization a axis of the body-centered orthorhombic structure. The thermoelectric power varies linearly with temperature above the superconducting transition, T_{SC}=1.5 K, indicating that superconductivity develops in a Fermi liquid regime. As a function of field the thermoelectric power shows successive anomalies which appear at critical values of the magnetic polarization. Remarkably, the lowest magnetic field instability for H∥a occurs for the same critical value of the magnetization (0.4 µ_{B}) than the first order metamagnetic transition at 35 T for field applied along the b axis. It can be clearly identified as a Lifshitz transition. The estimated number of charge carriers at low temperature reveals a metallic ground state distinct from LDA calculations indicating that strong electronic correlations are a major issue.

Thermal Conductivity through the Quantum Critical Point in YbRh_{2}Si_{2} at Very Low Temperature.

The thermal conductivity of YbRh_{2}Si_{2} has been measured down to very low temperatures under field in the basal plane. An additional channel for heat transport appears below 30 mK, both in the antiferromagnetic and paramagnetic states, respectively, below and above the critical field suppressing the magnetic order. This excludes antiferromagnetic magnons as the origin of this additional contribution to thermal conductivity. Moreover, this low temperature contribution prevails a definite conclusion on the validity or violation of the Wiedemann-Franz law at the field-induced quantum critical point.

Verification of the Wiedemann-Franz law in YbRh2Si2 at a quantum critical point.

The thermal conductivity measurements are performed on the heavy-fermion compound YbRh(2)Si(2) down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at B(c)=0.66 T∥c axis. In the limit as T→0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of the unconventional quantum criticality of YbRh(2)Si(2).

Strong pressure dependence of the magnetic penetration depth in single crystals of the heavy-fermion superconductor CeCoIn5 studied by muon spin rotation.

In the tetragonal heavy fermion system CeCoIn(5) the unconventional superconducting state is probed by means of muon spin rotation. The pressure dependence (0-1 GPa) of the basal-plane magnetic penetration depth (λ(a)), the penetration depth anisotropy (γ = λ(c)/λ(a)) and the temperature dependence of 1/λ(i)(2) (i = a, c) were studied in single crystals. A strong decrease of λ(a) with pressure was observed, while γ and λ(i)(2)(0)/λ(i)(2)(T) are pressure independent. A linear relationship between 1/λ(a)(2)(270 mK) and T(c) was also found. The large decrease of λ(a) with pressure is the signature of an increase of the number of superconducting quasiparticles by a factor of about 2.

Thermoelectric response near a quantum critical point of ß-YbAlB4 and YbRh2Si2: a comparative study.

The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds ß-YbAlB4 and YbRh2Si2, often considered as model systems for the local quantum criticality case. We observe a striking difference in the behavior of the Seebeck coefficient S in the vicinity of their respective quantum critical point (QCP). Approaching the critical field, S/T is enhanced in ß-YbAlB4, but drastically reduced in YbRh2Si2. The ratio of thermopower to specific heat remains constant for ß-YbAlB4, but it is significantly reduced near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The interplay between valence and magnetic quantum criticality and the additional possibility of a Lifshitz transition crossing the critical field under magnetic field are discussed as the origin of the different behaviors of these compounds.

Unconventional superconductivity in UTe2.

The novel spin-triplet superconductor candidate UTe2was discovered only recently at the end of 2018 and already attracted enormous attention. We review key experimental and theoretical progress which has been achieved in different laboratories. UTe2is a heavy-fermion paramagnet, but following the discovery of superconductivity, it has been expected to be close to a ferromagnetic instability, showing many similarities to the U-based ferromagnetic superconductors, URhGe and UCoGe. This view might be too simplistic. The competition between different types of magnetic interactions and the duality between the local and itinerant character of the 5fUranium electrons, as well as the shift of the U valence appear as key parameters in the rich phase diagrams discovered recently under extreme conditions like low temperature, high magnetic field, and pressure. We discuss macroscopic and microscopic experiments at low temperature to clarify the normal phase properties at ambient pressure for field applied along the three axis of this orthorhombic structure. Special attention will be given to the occurrence of a metamagnetic transition atHm= 35 T for a magnetic field applied along the hard magnetic axisb. Adding external pressure leads to strong changes in the magnetic and electronic properties with a direct feedback on superconductivity. Attention is paid on the possible evolution of the Fermi surface as a function of magnetic field and pressure. Superconductivity in UTe2is extremely rich, exhibiting various unconventional behaviors which will be highlighted. It shows an exceptionally huge superconducting upper critical field with a re-entrant behavior under magnetic field and the occurrence of multiple superconducting phases in the temperature-field-pressure phase diagrams. There is evidence for spin-triplet pairing. Experimental indications exist for chiral superconductivity and spontaneous time reversal symmetry breaking in the superconducting state. Different theoretical approaches will be described. Notably we discuss that UTe2is a possible example for the realization of a fascinating topological superconductor. Exploring superconductivity in UTe2reemphasizes that U-based heavy fermion compounds give unique examples to study and understand the strong interplay between the normal and superconducting properties in strongly correlated electron systems.

Probing insulators under pressure.

Applying pressure on a material can reveal many physical properties and is a very efficient tool to understand its physics. Resistivity measurements have been the ideal probe to study metals under pressure. However, in the case of insulators, resistivity, or conductivity, it is often not the appropriate quantity characterizing the material. In this work, we present a newly developed in situ pressure tuning system that can be used over a wide temperature range (2 K-300 K) and allows changing the pressure at any temperature. We also present AC calorimetry and capacitance/loss measurements under pressure and demonstrate how this combination can be used to characterize a material that is too insulating for standard resistivity techniques.

Multigap superconductivity in the heavy-Fermion system CeCoIn5.

New thermal conductivity experiments on the heavy-fermion superconductor CeCoIn5 down to 10 mK rule out the suggested existence of unpaired electrons. Moreover, they reveal strong multigap effects with a remarkably low "critical" field Hc2S for the small gap band, showing that the complexity of heavy-fermion band structure has a direct impact on their response under magnetic field.

Penetration depth study of superconducting gap structure of 2H-NbSe2.

We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths (lambda(a) and lambda(c), respectively) in 2H-NbSe2. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast with some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.

Superconducting PrOs4Sb12: a thermal conductivity study.

The superconducting state of the heavy fermion PrOs4Sb12 is studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The field and temperature dependence of the thermal conductivity confirm multiband superconductivity and point to fully open gaps on the whole Fermi surface.

Pressure induced effects on the Fermi surface of superconducting 2H-NbSe2.

The pressure dependence of the critical temperature T(c) and upper critical field H(c2)(T) has been measured up to 19 GPa in the layered superconducting material 2H-NbSe2. T(c)(P) has a maximum at 10.5 GPa, well above the pressure for the suppression of the charge density wave (CDW) order. Using an effective two-band model to fit H(c2)(T), we obtain the pressure dependence of the anisotropy in the electron-phonon coupling and Fermi velocities, which reveals the peculiar interplay between CDW order, Fermi surface complexity, and superconductivity in this system.

Multiband superconductivity in the heavy fermion compound PrOs4Sb12.

The thermal conductivity of the heavy fermion superconductor Pr(Os(4)Sb(12) was measured down to T(c)/40 throughout the vortex state. At lowest temperatures and for magnetic fields H approximately 0.07H(c2), already 40% of the normal state thermal conductivity is restored. This behavior (similar to that observed in MgB2) is a clear signature of multiband superconductivity in this compound.

Coexistence of superconductivity and ferromagnetism in URhGe.

The discovery of superconductivity at high pressure (albeit over a restricted range) in the ferromagnetic material UGe2 raised the possibility that bulk superconductivity might be found in other ferromagnets. The exact symmetry of the paired state and the dominant mechanism responsible for the pairing, however, remain unidentified. Meanwhile, the conjecture that superconductivity could occur more generally in ferromagnets has been fuelled by the recent observation of a low-temperature transition that suggests an onset of superconductivity in high-quality crystals of the itinerant-ferromagnet ZrZn2 (ref. 2), although the thermodynamic signature of this transition could not be detected. Here we show that the ferromagnet URhGe is superconducting at ambient pressure. In this case, we find the thermodynamic signature of the transition-its form is consistent with a superconducting pairing of a spin-triplet type, although further testing with cleaner samples is needed to confirm this. The combination of superconductivity and ferromagnetism may thus be more common and consequently more important than hitherto realized.