Heavy-fermion Superconductor Research Articles

Fermi Surface Nesting with Heavy Quasiparticles in the Locally Noncentrosymmetric Superconductor CeRh2As2

Abstract The locally noncentrosymmetric heavy fermion superconductor CeRh2As2 has attracted considerable interests due to its rich superconducting phases, accompanied by possible quadrupole density wave and pronounced antiferromagnetic excitations. To understand the underlying physics, here we report measurements from high-resolution angle-resolved photoemission. Our results reveal fine splittings of the conduction bands related to the locally noncentrosymmetric structure, as well as a quasi-two-dimensional Fermi surface (FS) with strong 4f contributions. The FS shows signs of nesting with an in-plane vector q 1 = (π/a, π/a), which is facilitated by the heavy bands near X ¯ arising from the characteristic conduction-f hybridization. The FS nesting provides a natural explanation for the observed antiferromagnetic spin fluctuations at (π/a, π/a), which might be the driving force for its unconventional superconductivity. Our experimental results can be reasonably explained by density functional theory plus dynamical mean field theory calculations, which can capture the strong correlation effects. Our study not only provides spectroscopic signature of the key factors underlying the field-induced superconducting transition, but also uncovers the critical role of FS nesting and lattice Kondo effect in the underlying magnetic fluctuations.

Heterodimensional Kondo superlattices with strong anisotropy

Localized magnetic moments in non-magnetic materials, by interacting with the itinerary electrons, can profoundly change the metallic properties, developing various correlated phenomena such as the Kondo effect, heavy fermion, and unconventional superconductivity. In most Kondo systems, the localized moments are introduced through magnetic impurities. However, the intrinsic magnetic properties of materials can also be modulated by the dimensionality. Here, we report the observation of Kondo effect in a heterodimensional superlattice VS2-VS, in which arrays of the one-dimensional (1D) VS chains are encapsulated by two-dimensional VS2 layers. In such a heterodimensional Kondo superlattice, we observe the typical Kondo effect but with intriguing anisotropic field dependence. This unique anisotropy is determined to originate from the magnetic anisotropy which has the root in the unique 1D chains in the structure, as corroborated by the first-principles calculation. Our results open up a novel avenue of studying exotic correlated physics in heterodimensional materials.

Coexistence of near- EF Flat Band and Van Hove Singularity in a Two-Phase Superconductor

Quantum many-body systems, particularly, the ones with large near-EF density states, are well known for exhibiting rich phase diagrams as a result of enhanced electron correlations. The recently discovered locally noncentrosymmetric heavy fermion superconductor CeRh2As2 has stimulated extensive attention due to its unusual H−T phase diagram consisting of two-phase superconductivity, antiferromagnetic order, and possible quadrupole-density wave orders. However, the critical near-EF electronic structure remains experimentally elusive. Here, we provide this key information by combining soft-x-ray and vacuum ultraviolet (VUV) angle-resolved-photoemission-spectroscopy measurements and atom-resolved density-functional-theory (DFT)+U calculations. With bulk-sensitive soft x ray, we reveal quasi-2D hole and electron pockets near the EF. On the other hand, under VUV light, the Ce flat bands are resolved with the c−f hybridization persisting up to well above the Kondo temperature. Most importantly, we observe a symmetry-protected fourfold Van Hove singularity (VHS) coexisting with the Ce 4f5/21 flat bands at the X point, which, to the best of our knowledge, has never been reported before. Such a rare coexistence is expected to lead to a large density of states at the zone edge, a large upper critical field of the odd-parity phase, as well as spin and/or charge instabilities with a vector of (1/2, 1/2, 0). Uniquely, it will also result in a new type of f-VHS hybridization that alters the order and fine electronic structure of the VHS and flat bands. Our findings provide not only key insights into the nature of multiple phases in CeRh2As2 but also open up new prospects for exploring the novelties of many-body systems with f-VHS hybridization. Published by the American Physical Society 2024

Phase evolution of Ce-based heavy-fermion superconductors under compression: A combined first-principles and effective-model study

Phase evolution of Ce-based heavy-fermion superconductors under compression: A combined first-principles and effective-model study

Parity games and mystery order in a heavy-fermion superconductor

Parity games and mystery order in a heavy-fermion superconductor

Nodal superconductivity in miassite Rh17S15

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh17S15 (Tc = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δλ(T) and the disorder evolution of the critical superconducting temperature Tc and the upper critical field Hc2(T) in single crystalline Rh17S15. We found a T − linear temperature variation of Δλ(T) below 0.3Tc, which is consistent with the presence of nodal lines in the superconducting gap of Rh17S15. The nodal character of the superconducting state is supported by the observed suppression of Tc and Hc2(T) in samples with a controlled level of non-magnetic disorder introduced by 2.5 MeV electron irradiation. We propose a nodal sign-changing superconducting gap in the A1g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the derived superfluid density. To the best of our knowledge, this establishes miassite as the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form, though it is unlikely that it is present in natural crystals because of unavoidable impurities that quickly destroy nodal superconductivity.

Discovery of Magnetic Phase Transitions in Heavy-Fermion Superconductor CeRh_{2}As_{2}.

We report on the specific heat studies performed on a new generation of CeRh_{2}As_{2} single crystals. Superior quality of the samples and dedicated experimental protocol allowed us to observe an antiferromagneticlike behavior in the normal state and to detect the first-order phase transition of magnetic origin within the superconducting state of the compound. Although in the available literature the physical behavior of CeRh_{2}As_{2} is most often described with the use of quadrupole density wave scenario, we propose an alternative explanation using analogies to antiferromagnetic heavy-fermion superconductors CeRhIn_{5} and Ce_{2}RhIn_{8}.

Field-induced compensation of magnetic exchange as the possible origin of reentrant superconductivity in UTe2

The potential spin-triplet heavy-fermion superconductor UTe2 exhibits signatures of multiple distinct superconducting phases. For field aligned along the b axis, a metamagnetic transition occurs at μ0Hm ≈ 35 T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to Hm. Once the field is tilted away from the b towards the c axis, a reentrant superconducting phase emerges just above Hm. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of μ0Hc2 ≈ 73 T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above Hm in the angular range around 30° caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above Hm.

Spin resonances in heavy-fermion superconductors

Historically heavy-fermion superconductors have been the first discovered unconventional superconductors and can to some extent be regarded as model systems for high-Tc superconductors. A hallmark of the unconventional superconductivity in high-Tc cuprates has been the observation of a spin resonance in the magnetic excitation spectrum probed by inelastic neutron scattering. We review the situation in heavy-fermion superconductors, the possible existence of a spin resonance in the superconducting state and its implications.

Chiral superconductivity in UTe2 probed by anisotropic low-energy excitations

Chiral spin-triplet superconductivity is a topologically nontrivial pairing state with broken time-reversal symmetry, which can host Majorana quasiparticles. The heavy-fermion superconductor UTe2 exhibits peculiar properties of spin-triplet pairing, and the possible chiral state has been actively discussed. However, the symmetry and nodal structure of its order parameter in the bulk, which determine the Majorana surface states, remains controversial. Here we focus on the number and positions of superconducting gap nodes in the ground state of UTe2. Our magnetic penetration depth measurements for three field orientations in three crystals all show the power-law temperature dependence with exponents close to 2, which excludes single-component spin-triplet states. The anisotropy of low-energy quasiparticle excitations indicates multiple point nodes near the ky- and kz-axes in momentum space. These results can be consistently explained by a chiral B3u + iAu non-unitary state, providing fundamentals of the topological properties in UTe2.

Pressure-induced Structural Phase Transition and New Superconducting Phase in UTe2

We report on the crystal structure and electronic properties of the heavy fermion superconductor UTe2 at high pressure up to 11 GPa, as investigated by X-ray diffraction and electrical resistivity experiments. The X-ray diffraction measurements under high pressure using a synchrotron light source reveal anisotropic linear compressibility of the unit cell up to 3.5 GPa, while a pressure-induced structural phase transition is observed above PO–T ∼ 3.5–4 GPa at room temperature, where the body-centered orthorhombic crystal structure with the space group Immm changes into a body-centered tetragonal structure with the space group I4/mmm. The molar volume drops abruptly at PO–T, while the distance between the first-nearest neighbor of U atoms, dU–U, increases, implying a switch from the heavy electronic states to the weakly correlated electronic states. Surprisingly, a new superconducting phase at pressures higher than 7 GPa was detected at Tsc > 2 K with a relatively low upper-critical field, Hc2(0). The resistivity above 3.5 GPa, thus, in the high-pressure tetragonal phase, shows a large drop below 230 K, which may also be related to a considerable change from the heavy electronic states to the weakly correlated electronic states.

Quantum-well states at the surface of a heavy-fermion superconductor

Two-dimensional electronic states at surfaces are often observed in simple wide-band metals such as Cu or Ag (refs. 1–4). Confinement by closed geometries at the nanometre scale, such as surface terraces, leads to quantized energy levels formed from the surface band, in stark contrast to the continuous energy dependence of bulk electron bands2,5–10. Their energy-level separation is typically hundreds of meV (refs. 3,6,11). In a distinct class of materials, strong electronic correlations lead to so-called heavy fermions with a strongly reduced bandwidth and exotic bulk ground states12,13. Quantum-well states in two-dimensional heavy fermions (2DHFs) remain, however, notoriously difficult to observe because of their tiny energy separation. Here we use millikelvin scanning tunnelling microscopy (STM) to study atomically flat terraces on U-terminated surfaces of the heavy-fermion superconductor URu2Si2, which exhibits a mysterious hidden-order (HO) state below 17.5 K (ref. 14). We observe 2DHFs made of 5f electrons with an effective mass 17 times the free electron mass. The 2DHFs form quantized states separated by a fraction of a meV and their level width is set by the interaction with correlated bulk states. Edge states on steps between terraces appear along one of the two in-plane directions, suggesting electronic symmetry breaking at the surface. Our results propose a new route to realize quantum-well states in strongly correlated quantum materials and to explore how these connect to the electronic environment.

Exotic heavy-fermion superconductivity in atomically thin CeCoIn5 films

We report an in situ scanning tunneling microscopy study of atomically thin films of ${\mathrm{CeCoIn}}_{5}$, a $d$-wave heavy-fermion superconductor. Both hybridization and superconducting gaps are observed even in monolayer ${\mathrm{CeCoIn}}_{5}$, providing direct evidence of superconductivity of heavy quasiparticles mediated by purely two-dimensional bosonic excitations. In these atomically thin films, ${T}_{c}$ is suppressed to nearly half of the bulk, but is similar to ${\mathrm{CeCoIn}}_{5}/{\mathrm{YbCoIn}}_{5}$ superlattices containing ${\mathrm{CeCoIn}}_{5}$ layers with the same thickness as the thin films. Remarkably, the out-of-plane upper critical field ${\ensuremath{\mu}}_{0}{H}_{c2\ensuremath{\perp}}$ at zero temperature is largely enhanced from those of bulk and superlattices. The enhanced ${H}_{c2\ensuremath{\perp}}$ well exceeds the Pauli and bulk orbital limits, suggesting the possible emergence of unusual superconductivity with parity mixing caused by the inversion symmetry breaking.

Nested antiferromagnetic spin fluctuations and non-Fermi-liquid behavior in electron-doped CeCo1−xNixIn5

We investigate the electronic state of Ni-substituted ${\mathrm{CeCo}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{In}}_{5}$ by nuclear quadrupole and magnetic resonance (NQR/NMR) techniques. The heavy fermion superconductivity below ${T}_{\mathrm{c}}=2.3$ K for $x=0$ is suppressed by Ni substitutions, and ${T}_{\mathrm{c}}$ reaches zero for $x=0.25$. The $^{115}\mathrm{In}$ NQR spectra for $x=0.125$ and 0.25 can be explained by simulating the electrical field gradient that is calculated for a virtual supercell with density functional theory. The spin-lattice relaxation rate $1/{T}_{1}$ indicates that Ni substitution weakens antiferromagnetic correlations that are not localized near the substituent but instead are uniform in space. The temperature ($T$) dependence of ${({T}_{1}T)}^{\ensuremath{-}1}$ for $x=0.25$ shows a maximum around ${T}_{\mathrm{g}}=2$ K and ${({T}_{1}T)}^{\ensuremath{-}1}$ decreases toward almost zero when temperature is further reduced as if a gap might be opening in the magnetic excitation spectrum; however, the magnetic specific heat and the static magnetic susceptibility evolve smoothly through ${T}_{\mathrm{g}}$ with a $\ensuremath{-}lnT$ dependence. The peculiar $T$ dependence of ${({T}_{1}T)}^{\ensuremath{-}1}$ and non-Fermi-liquid specific heat and susceptibility can be interpreted in a unified way by assuming nested antiferromagnetic spin fluctuations in a quasi-two-dimensional electronic system.

Anomalous electromagnetic response in the spin-triplet superconductor UTe2

The recently discovered heavy-fermion superconductor ${\mathrm{UTe}}_{2}$, as a promising candidate for an odd-parity superconductivity, exhibits anomalous magnetic-field reinforced superconductivity with a huge upper critical field of 35 T. However, the thermodynamic properties of this unconventional superconductivity still remain unclear. Herein we present the results of dc magnetization and heat capacity measurements in ${\mathrm{UTe}}_{2}$ to probe the magnetic response at low temperatures. The obtained isothermal magnetization in the superconducting state of ${\mathrm{UTe}}_{2}$ clearly demonstrates the anomalous magnetic flux pinning effect, which is most likely related to the peculiar electromagnetic response of the spin-triplet pairing state in the hard-magnetization axis (orthorhombic $b$ axis). The ac susceptibility measurements support the anomalous vortex states in ${\mathrm{UTe}}_{2}$. We also found evidence for the Lifshitz transition just above ${H}_{\mathrm{c}2}$ and the anomalous flux pinning anomalies for the easy-magnetization axis ($a$ axis).

Multiple superconducting phases in heavy-fermion metals

Symmetry breaking beyond a global U(1) phase is the key signature of unconventional superconductors. As prototypical strongly correlated materials, heavy-fermion metals provide ideal platforms for realizing unconventional superconductivity. In this article, we review heavy-fermion superconductivity, with a focus on those materials with multiple superconducting phases. In this context, we highlight the role of orbital-selective (matrix) pairing functions, which are defined as matrices in the space of effective orbital degrees of freedom such as electronic orbitals and sublattices as well as equivalent descriptions in terms of intra- and inter-band pairing components in the band basis. The role of quantum criticality and the associated strange-metal physics in the development of unconventional superconductivity is emphasized throughout. We discuss in some detail the recent experimental observations and theoretical perspectives in the illustrative cases of UTe2, CeRh2As2, and CeCu2Si2, where applied magnetic fields or pressure induce a variety of superconducting phases. We close by providing a brief overview of overarching issues and implications for possible future directions.

Muon spin rotation and relaxation in Pr1−xNdxOs4Sb12 : Superconductivity and magnetism in Pr-rich alloys

The Pr-rich end $0\ensuremath{\le}x\ensuremath{\le}0.25$ of the alloy series ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ has been studied using muon spin rotation and relaxation. The end compound ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field associated with broken time-reversal symmetry (TRS) in the superconducting phase. No such field is observed in the Nd-doped alloys for $x\ensuremath{\ge}0.05$, indicating that TRS has been restored. The superfluid density from the vortex-lattice field distribution is insensitive to Nd concentration for $x\ensuremath{\lesssim}0.2$, indicating that TRS restoration does not have a strong effect on the superconducting state. No ${\mathrm{Nd}}^{3+}$ static magnetism, ordered or disordered, is found down to the lowest temperatures of measurement.

Direct observation of the hybridization gap in both the hidden order and large moment antiferromagnetic phases in URu2Si2

Despite extensive research on the heavy fermion superconductor $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ in the past three decades, the nature of the hidden order (HO) phase transition occurring at 17.5 K remains ambiguous. Here we report a comparative scanning tunneling microscopy/spectroscopy (STM/STS) study on different terminations of the parent $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ and Fe-doped samples. A small gap, which was ascribed to the HO parameter by previous STM/STS studies, emerges in both the HO and large moment antiferromagnetic phases on the U terminations, indicating it is not the unique hallmark of the HO parameter. Moreover, a peak-gap-peak structure is observed on the Si terminations. Variations of the two spectral features with Fe concentration and temperature show that they stem from the alteration of $f\text{\ensuremath{-}}c$ hybridization. The higher vanishing temperatures and larger sizes of the gap in the Fe-substituted samples indicate stronger $f\text{\ensuremath{-}}c$ hybridization strength compared to $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$. Our studies demonstrate hybridization is not the driving force for the HO phase transition.

Anisotropic magnetotransport properties of the heavy-fermion superconductor CeRh2As2

We report anisotropic resistivity measurements of the heavy-fermion superconductor CeRh$_2$As$_2$ in magnetic fields up to 16 T and temperatures down to 0.35 K. The measured CeRh$_2$As$_2$ resistivity shows a signature corresponding to the suggested quadrupole density wave order state at $T_0 \sim$ 0.5 K for both measured directions. For a magnetic field applied along the tetragonal $a$ axis, $T_0$ is enhanced with magnetic field reaching $\sim$1.75 K at 16 T. Further, a magnetic field-induced transition occurs at $B_m \sim $ 8.1 T corresponding to a change to a new broken symmetry state. For a magnetic field applied along the $c$ axis, $T_0$ is suppressed below our base temperature $\sim$0.35 K by $B \sim$ 4.5 T, a field close to the previously reported field-induced transition within the superconducting state suggested to be from an even-parity to an odd-parity state. Our results indicate that the multiple superconducting phases in CeRh$_2$As$_2$ are intimately tied to the suppression of the proposed quadrupole-density-wave phase at $T_0$.

Study on superconducting properties of CeIrIn5 thin films grown via pulsed laser deposition

We report the growth of CeIrIn5 thin films with different crystal orientations on a MgF2 (001) substrate using pulsed laser deposition technique. X-ray diffraction analysis showed that the thin films were either mainly a-axis-oriented (TF1) or a combination of a- and c-axis-oriented (TF2). The characteristic features of heavy-fermion superconductors, i.e. Kondo coherence and superconductivity, were clearly observed, where the superconducting transition temperature (T c) and Kondo coherence temperature (T coh) are 0.58 K and 41 K for TF1 and 0.52 K and 37 K for TF2, respectively. The temperature dependencies of the upper critical field (Hc 2) of both thin films and the CeIrIn5 single crystal revealed a scaling behavior, indicating that the nature of unconventional superconductivity has not been changed in the thin film. The successful synthesis of CeIrIn5 thin films is expected to open a new avenue for novel quantum phases that may have been difficult to explore in the bulk crystalline samples.