Unconventional Superconductivity in Heavy Fermion UTe2 (original) (raw)
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Spin-Triplet Superconductivity in UTe2 and Ferromagnetic Superconductors
Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2019), 2020
The spin-triplet state is most likely realized in uranium ferromagnetic superconductors, UGe 2 , URhGe, UCoGe. The microscopic coexistence of ferromagnetism and superconductivity means that the Cooper pair should be realized under the strong internal field due the ferromagnetism. leading to the spin-triplet state with equal spin pairing. The field-reinforced superconductivity, which is observed in all three materials when the ferromagnetic fluctuations are enhanced, is one of the strong evidences for the spin-triplet superconductivity. We present here the results of a newly discovered spin-triplet superconductor, UTe 2 , and compare those with the results of ferromagnetic superconductors. Although no magnetic order is found in UTe 2 , there are similarities between UTe 2 and ferromagnetic superconductors. For example, the huge upper critical field exceeding the Pauli limit and the field-reentrant superconductivity for H b-axis are observed in UTe 2 , URhGe and UCoGe. We also show the specific heat results on UTe 2 in different quality samples, focusing on the residual density of states in the superconducting phase.
Topological band and superconductivity in UTe2
Physical Review B
UTe 2 is a likely spin-triplet superconductor that also exhibits evidence for chiral Majorana edge states. A characteristic structural feature of UTe 2 is inversion-symmetry related pairs of U atoms, forming rungs of ladders. Here we show how each rung's two sublattice degrees of freedom play a key role in understanding the electronic structure and the origin of superconductivity. In particular, DFT+U calculations generically reveal a topological band near the chemical potential originating from a band inversion associated with 5 f electrons residing on these rungs, necessitating a microscopic description that includes these rung degrees of freedom. Furthermore, we show that a previously identified strong ferromagnetic interaction within a U-U rung leads to a pseudospin-triplet superconducting state that accounts for a nonzero polar Kerr angle, the observed magnetic field-temperature phase diagrams, and nodal Weyl fermions. Our analysis may also be relevant for other U-based superconductors.
Unconventional superconductivity in heavy fermion compounds
Journal of Magnetism and Magnetic Materials, 1988
Theoretical aspects of unconventional superconductivity in heavy fermion metals are discussed and confronted with experiment• Theoretical predictions on the detectability of collective modes in the electromagnetic absorption, the dependence of thermodynamic and transport properties on the concentration and scattering strength of impurities and a two-phase model of U 1 _.~ThxB%3 are presented.
Superconductivity in Heavy Fermion Materials
IJEAT, 2020
The heavy fermion materials have small superconducting transition temperature and large specific heat corresponding to large effective masses. In these materials the superconductivity co-exists with ferromagnetic or antiferromagnetic order at low temperature. It shows phenomena like magnetic instabilities, quantum critical points (QCP), non-fermi liquid (NFL) and unconventional superconductivity. By comparing the superconducting properties, phase diagram and effect of magnetic field and pressure of heavy fermions based on uranium, cerium, and praseodymium, the basic physics behind pairing mechanism can be imagined. This paper aims to present remarkable findings in superconductivity of various heavy fermion materials.
Philosophical Magazine, 2018
In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu 2 Si 2 , fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu 2 Si 2. In YbRh 2 Si 2 , superconductivity appears to be suppressed at T ≳ 10 mK by AF order (T N = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at T A slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4felectronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at T c = 2 mK. Like the pressure-induced QCP in CeRhIn 5 , the magnetic field-induced one in YbRh 2 Si 2 is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavyfermion superconductors and other families of unconventional superconductors with higher T c s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon. Heavy-fermion metals, superconductivity, quantum critical phenomena 1. Quantum criticality in antiferromagnetic heavy-fermion metals Unconventional superconductivity, i.e., superconductivity which is not driven by lattice vibrations, frequently develops in strongly correlated metals on the brink of antiferromagnetic (AF) order [1, 2]. The continuous suppression of AF order by non-thermal control parameters, such as external/chemical pressure and magnetic field gives rise to a quantum critical point (QCP) which determines the physical properties in a wide range of parameters. Strong deviations from the predictions of Landau's Fermi-liquid theory [3], socalled non-Fermi-liquid (NFL) phenomena, are commonly observed in the normal metallic state out of which superconductivity develops. The interplay between quantum criticality and superconductivity in strongly correlated electron systems is a timely, controversial and much debated topic which has been studied over the last two decades, most intensively with AF heavy-fermion metals [4, 5]. These are intermetallic compounds of certain lanthanides, such as Ce and Yb, or actinides, such as U and Pu. The lanthanide-based heavyfermion metals are model systems for the Kondo lattice, where at the QCP the on-site Kondo screening, characterized by k B T K , with T K being the Kondo temperature of the crystalfield (CF)-derived lowest-lying Kramers doublet of the localized 4f-shell, exactly cancels the intersite magnetic Ruderman Kittel Kasuya Yoshida (RKKY) interaction, characterized by k B T RKKY. So far, two different types of AF QCPs have been established for heavy-fermion metals. Some of them exhibit a "conventional" QCP, which means that in this scenario the AF order is of itinerant nature [6-8]. This kind of spin-density wave (SDW) order, with three-dimensional (3D) quantum critical fluctuations of the AF order parameter, is common to transition-metal compounds where d-electrons contribute to the conduction band. A 2. CeCu 2 Si 2 : Fully gapped d-wave superconductivity in the vicinity of a threedimensional spin-density-wave quantum critical point Heavy-fermion superconductivity was first discovered in the Kondo lattice system CeCu 2 Si 2 with almost trivalent Ce (T K ≃ 20 K) [21]. It has been considered an unconventional superconductor from early on: (i) The non-f-electron reference compound LaCu 2 Si 2 does not superconduct (at T ≥ 20 mK) [21], which implies that superconductivity in the Ce homologue should be ascribed to the periodic lattice of 100 % magnetic Ce 3+ ions. (ii) The reduced jump in the Sommerfeld coefficient of the electronic specific heat [γ(T) = C(T)/T] at the superconducting transition temperature, ΔC/γ 0 T c [γ 0 ≃1 J/(K 2 mol)], is of order unity, which implies that the Cooper pairs are formed by heavy-mass quasiparticles, i.e., slowly propagating Kondo singlets. As their Fermi velocity v F * is only of the order of the velocity of sound, the electron-phonon interaction is not retarded, i.e., the direct Coulomb repulsion among the charge carriers cannot be avoided. (iii) Therefore, an alternative pairing mechanism must be at work which, in analogy to superfluidity in 3 He [22], was early on assumed to be magnetic in origin [23-25]. (iv) Already a tiny amount of nonmagnetic impurities was found to fully suppress superconductivity in CeCu 2 Si 2 [26], similar to the
Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials
The crystal structure of a new superconductor UTe2 has been investigated using single-crystal neutron diffraction for the first time at the low temperature (LT) of 2.7 K, just above the superconducting transition temperature of ∼1.6 K, in order to clarify whether the orthorhombic structure of type Immm (No. 71), reported for the room-temperature (RT) structure persists down to the superconducting phase and can be considered as a parent symmetry for the development of spin-triplet superconductivity. In contrast to the previously reported phase transition at about 100 K [Stöwe (1996). J. Solid State Chem. 127, 202–210], our high-precision LT neutron diffraction data show that the body-centred RT symmetry is indeed maintained down to 2.7 K. No sign of a structural change from RT down to 2.7 K was observed. The most significant change depending on temperature was observed for the U ion position and the U–U distance along the c direction, implying its potential importance as a magnetic i...
Journal of the Physical Society of Japan
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 fieldreinforcement of superconductivity on approaching H m appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction.
Magnetic-Field-Induced Phenomena in the Paramagnetic Superconductor UTe2
Journal of the Physical Society of Japan
We present magnetoresistivity measurements on the heavy-fermion superconductor UTe 2 in pulsed magnetic fields μ 0 H up to 68 T and temperatures T from 1.4 to 80 K. Magnetic fields applied along the three crystallographic directions a (easy magnetic axis), b, and c (hard magnetic axes), are found to induce different phenomenadepending on the field directionbeyond the low-field suppression of the superconducting state. For H ∥ a, a broad anomaly in the resistivity is observed at 0 H Ã ' 10 T and T = 1.4 K. For H ∥ c, no magnetic transition nor crossover are observed. For H ∥ b, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field 0 H m ' 35 T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at T CEP ' 7 K. At higher temperatures a crossover persists up to 28 K, i.e., below the temperature T max ¼ 35 K where the magnetic susceptibility is maximal. A sharp maximum in the Fermi-liquid quadratic coefficient A of the low-temperature resistivity is found at H m. It indicates an enhanced effective mass associated with critical magnetic fluctuations, possibly coupled with a Fermi surface instability. Similarly to the URhGe case, we show that UTe 2 is a candidate for fieldinduced reentrant superconductivity in the proximity of H m .
Superconductivity from fractionalized excitations in an enigmatic heavy fermion material
In the heavy fermion material, URu 2 Si 2 (URS), 1 the identity of the order parameter in an enigmatic phase, known as the hidden order (HO) phase, is not known despite a quarter of a century of its existence. Buried deep inside this phase lies a much less explored unconventional superconducting state that is of our central interest here. 1,18-21 First, we identify a mixed singlet-triplet density wave (st-DDW) 22,23 to be the hidden order state. It has no net charge or spin modulations and does not break time reversal symmetry (TRS). It does have topological order with quantized spin Hall effect. 22 Thus, it is naturally impervious to common experimental probes and is an excellent candidate for HO. We then construct, including st-DDW, a global phase diagram in which there is a deconfined quantum critical point, 24 which is ultimately responsible for the basic mechanism of superconductivity. We argue that the skyrmionic spin texture in st-DDW 23 ultimately fractionalizes into fermonic merons and anti-merons, 24 which results in two copies of unconventional chiral d-wave BCS superconductors, consistent with experiments. The superconducting state breaks TRS, which can be directly detected by polar Kerr effect (PKE) measurements; 27 in contrast HO, identified as st-DDW, should not exhibit PKE except perhaps for magnetic impurities. Direct determination of st-DDW is also possible through two-magnon Raman scattering, nuclear quadrupolar resonance, or the skyrmions themselves. In a more general context, our work reflects the rich possibilities of emergent behavior in condensed matter systems. 1 arXiv:1308.5357v1 [cond-mat.str-el]