Heavy-electron Superconductor Research Articles

The scaled-invariant Planckian metal and quantum criticality in Ce1−xNdxCoIn5

The mysterious Planckian metal state, showing perfect T-linear resistivity associated with universal scattering rate, 1/τ = αkBT/ℏ with α ~ 1, has been observed in the normal state of various strongly correlated superconductors close to a quantum critical point. However, its microscopic origin and link to quantum criticality remains an outstanding open problem. Here, we observe quantum-critical T/B-scaling of the Planckian metal state in resistivity and heat capacity of heavy-electron superconductor Ce1−xNdxCoIn5 in magnetic fields near the edge of antiferromagnetism at the critical doping xc ~ 0.03. We present clear experimental evidences of Kondo hybridization being quantum critical at xc. We provide a generic microscopic mechanism to qualitatively account for this quantum critical Planckian state within the quasi-two dimensional Kondo-Heisenberg lattice model near Kondo breakdown transition. We find α is a non-universal constant and depends inversely on the square of Kondo hybridization strength.

Crystalline Electric Field Level Scheme of Non-centrosymmetric CeRhSi3 and CeIrSi3

One of the consequences of antisymmetric spin–orbit interactions due to a lack of inversion symmetry are unusually large upper critical fields observed in heavy-electron superconductors. The upper ...

Scanning tunneling microscopy and spectroscopy studies of the heavy-electron superconductor TlNi2Se2

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Theory perspective: SCES 2016

New discoveries and developments in almost every area of correlated electron physics were presented at SCES 2016. Here, I provide a personal perspective on some of these developments, highlighting some new ideas in computational physics, discussing the ‘hidden order’ challenges of cuprate and heavy electron superconductors, the mysterious bulk excitations of the topological Kondo insulator and new progress in research on quantum spin ice, iron-based superconductors and quantum criticality.

Single-Crystal Neutron Diffraction Study of the Heavy-Electron Superconductor CeNiGe3

A single-crystal neutron diffraction study was performed on anomalous antiferromagnetic ordering in the heavy-electron superconductor CeNiGe3. We observed incommensurate magnetic Bragg reflections characterized by the incommensurate propagation vector \(k_{2} = (0,0.41,1/2)\) below the Neel temperature of 5 K, but no significant magnetic reflection with the commensurate propagation vector \(k_{1} = (1,0,0)\), at which another magnetic reflection was observed in a previous neutron diffraction study with a polycrystalline sample. From the single-crystal study, we suggest that the magnetic phase of CeNiGe3 at ambient pressure is characterized only by the incommensurate propagation vector k2.

Multigap nodeless superconductivity in nickel chalcogenideTlNi2Se2

Low-temperature thermal conductivity measurements were performed on single crystals of $\mathrm{TlNi}{}_{2}\mathrm{Se}{}_{2}$, a nickel chalcogenide heavy-electron superconductor with ${T}_{c}\ensuremath{\simeq}3.7$ K. In zero field, the residual electronic contribution at $T\ensuremath{\rightarrow}0$ K (${\ensuremath{\kappa}}_{0}/T$) was well separated from the total thermal conductivity, which is less than 0.45% of its normal-state value. Such a tiny residual ${\ensuremath{\kappa}}_{0}/T$ is unlikely contributed by the nodal quasiparticles. The nodeless gap structure is supported by the very weak field dependence of ${\ensuremath{\kappa}}_{0}(H)/T$ in low magnetic fields. In the whole field range, ${\ensuremath{\kappa}}_{0}(H)/T$ exhibits an $\mathsf{S}$-shaped curve, as in the case of nickel pnictides $\mathrm{BaNi}{}_{2}\mathrm{As}{}_{2}$ and $\mathrm{SrNi}{}_{2}\mathrm{P}{}_{2}$. This common feature of nickel-based superconductors can be explained by multiple nodeless superconducting gaps.

Finding New Superconductors: The Spin-Fluctuation Gateway to High Tc and Possible Room Temperature Superconductivity

We propose an experiment-based strategy for finding new high transition temperature superconductors that is based on the well-established spin fluctuation magnetic gateway to superconductivity in which the attractive quasiparticle interaction needed for superconductivity comes from their coupling to dynamical spin fluctuations originating in the proximity of the material to an antiferromagnetic state. We show how lessons learned by combining the results of almost three decades of intensive experimental and theoretical study of the cuprates with those found in the decade-long study of a strikingly similar family of unconventional heavy electron superconductors, the 115 materials, can prove helpful in carrying out that search. We conclude that, since Tc in these materials scales approximately with the strength of the interaction, J, between the nearest neighbor local moments in their parent antiferromagnetic state, there may not be a magnetic ceiling that would prevent one from discovering a room temperature superconductor.

Field-induced collective spin-exciton condensation in a quasi-2D dx2–y2-wave heavy electron superconductor

The origin of the spin resonance observed in CeCoIn5 with Inelastic Neutron Scattering is subject to debate. It has been shown recently that in this heavy electron compound at low temperature an instability to a ground state with coexisting dx2-y2-wave superconductivity and Spin Density Wave (SDW) order in a magnetic field is a corollary of the consideration of a collective spin excitation mode in a quasi-2D dx2-y2-wave Pauli-limited superconductor. This provides a natural scenario for the occurence of the puzzling high-field-low-temperature phase highlighted in CeCoIn5. We present perspectives on this ground state transition and propose directions for future experiment.

Muon spin rotation in heavy-electron pauli-limit superconductors

The formalism for analyzing the magnetic field distribution in the vortex lattice of Pauli limited heavy electron superconductors is applied to the evaluation of the vortex lattice static linewidth relevant to Muon Spin Rotation ($\mu$SR) experiment. On the basis of the Ginzburg-Landau expansion for the superconductor free energy we study the evolution with respect to external field of the static linewidth both in the limit of independent vortices (low magnetic field) and in the near $H_\mathrm{c2}^\mathrm{p}(T)$ regime by using an extension of the Abrikosov analysis to Pauli limited superconductors. We conclude that in the Ginzburg-Landau regime with Pauli limit, the electrodynamics of the vortex lattice predicts anomalous variations with applied field of the static linewidth which is a result of the spin response contribution to screening supercurrents that dominates the usual charge response. The model is proposed as a benchmark for comparison with possible other effects including vortex core localized states or interplay with magnetism.

Textured superconductivity in the presence of a coexisting order: Ce115s and other heavy-fermion compounds

Superconductivity in strongly correlated electron systems frequently emerges in proximity to another broken symmetry. In heavy-electron superconductors, the nearby ordered state most commonly is magnetism, and the so-called Ce115 heavy-electron compounds have been particularly instructive for revealing new relationships between magnetism and superconductivity. From measurements of the resistive and bulk transitions to superconductivity in these materials, we find that the resistive transition appears at a temperature considerably higher than the bulk transition when superconductivity and magnetic order coexist, but this temperature difference disappears in the absence of long-range magnetic order. Further, in the pressure–temperature region of coexistence in CeRhIn5, a new anisotropy in the resistive transition develops even though the tetragonal crystal structure apparently remains unchanged, implying a form of textured superconductivity. We suggest that this texture may be a generic response to coexisting order in these and other heavy-fermion superconductors.

Heavy electrons and the symplectic symmetry of spin

The recent discovery of two heavy-fermion materials PuCoGa5 and NpPd5Al2, which transform directly from Curie paramagnets into superconductors, reveals a new class of superconductors where local moments quench directly into the superconducting condensate. Unlike other heavy-electron superconductors, where Cooper pairing is thought to be driven by spin fluctuations, these higher-transition-temperature materials do not seem to be close to a magnetic instability. Large-N expansions have been invaluable in describing heavy-fermion metals, but so far cannot treat superconductivity. Here, we introduce a new class of large-N expansion that uses symplectic symmetry to protect the odd time-reversal parity of spin and sustain Cooper pairs as well-defined singlets. We show that when a lattice of magnetic ions exchange spin with their metallic environment in two distinct symmetry channels, they can simultaneously satisfy both channels by forming a condensate of composite pairs between local moments and electrons. In the tetragonal crystalline environment relevant to PuCoGa5 and NpPd5Al2, the lattice structure selects a natural pair of spin exchange channels, predicting a unique anisotropic paired state with either d- or g-wave symmetry. This pairing mechanism also predicts a large upturn in the NMR relaxation rate above Tc and strong enhancement of Andreev reflection in tunnelling measurements. The Kondo problem—dealing with localized magnetic impurities embedded in a sea of conduction electrons—can be treated on an equal footing with superconductivity for a large system of interacting electrons.

Probing the nodal structures of heavy electron superconductors by means of specific heat measurements in magnetic fields

Measurements of the angle-resolved specific heat C ( H , θ ) have been performed on PrOs 4 Sb 12 and URu 2 Si 2 in order to examine their superconducting gap structures. In the filled skutterudite PrOs 4 Sb 12 , a clear fourfold angular dependence is observed with H rotated in the (1 0 0) planes, whose field variation suggests gap minima along [1 0 0] directions. Interestingly, a reversed angular oscillation of C ( H , θ ) has been observed in the normal state above H c 2 which continues to grow as the high-field antiferroquadrupole phase is approached. In URu 2 Si 2 , C ( H , θ ) with H rotated around the tetragonal c-axis suggests that the nodal structure has rotational symmetry. Comparison between the C ( H ) data for a- and c-axes indicates the existence of point nodes along the [0 0 1] direction.

Dependence of pressure-induced phase transitions on pressure-transmitting media in the heavy-electron superconductor [formula omitted

Elastic neutron-scattering and ac-susceptibility measurements have been performed on a 2 φ single crystal of URu 2 Si 2 under hydrostatic pressure up to ∼ 1.1 GPa using Fluorinert 70 & 77 and Daphne oil 7373 as pressure-transmitting media. Daphne oil produces better hydrostaticity of the applied pressure than Fluorinert, resulting in a slight shift of the phase boundary between hidden order and antiferromagnetic order to a lower pressure. The superconducting phase is also influenced, but never overlaps with the antiferromagnetic phase.

Nodal Structures of Heavy Fermion Superconductors Probed by the Specific-Heat Measurements in Magnetic Fields

Heavy electron superconductors mostly have anisotropic gap function which vanishes (has nodes) for certain directions in the momentum space. Since the nodal structure is closely related to the pairing mechanism, its experimental determination is very important. In anisotropic superconductors, low energy spectrum of the quasiparticles in the vortex state much depends on the nodal structure. In particular, the electronic specific heat has been demonstrated to exhibit the characteristic dependence on the angle between the field and the nodal direction, thereby the nodal structure can be probed experimentally. In this contribution, we present our recent experimental results on the field-orientation dependence of the specific heat on f electron superconductors CeRu 2 , CeCoIn 5 , PrOs 4 Sb 12 , and URu 2 Si 2 , and discuss their gap structures.

No Evidence for “Small-Moment Antiferromagnetism” under Ambient Pressure in URu2Si2: Single-Crystal 29Si NMR Study

The still unidentified hidden order (HO) phase below T 0 = 17.5 K in the heavy-electron superconductor URu 2 Si 2 has been investigated by 29 Si NMR on a single crystal under external magnetic field H ext ∥ c -axis and ambient pressure P . For a previously proposed model of the small-moment antiferromagnetism (SMAF), in which tiny-moment antiferromagnetism (AF) under ambient P is ascribed to spatially homogeneous AF with an ordered moment µ SMAF = 0.02–0.04 µ B , splitting of the line would be expected on the basis of a scaling from the corresponding splitting in the P -induced AF. However, we observed at T = 4.5 K no splitting of the line. This clearly indicates that the SMAF model cannot describe the ambient- P properties. Below T 0 down to ∼14 K, the 29 Si NMR line shows additional broadening over the paramagnetic width. The additional contribution, however, disappears below ∼14 K, which indicates that the HO itself produces almost zero internal field at the 29 Si site along the c -axis at low temperat...

Pressure–temperature phase diagram of the heavy-electron superconductor [formula omitted

The pressure–temperature phase diagram of the heavy-electron superconductor URu 2 Si 2 has been reinvestigated by AC-susceptibility and elastic neutron-scattering (NS) measurements performed on a small single-crystalline rod ( 2 mm in diameter, 6 mm in length) in a Cu–Be clamp-type high-pressure cell ( P < 1.1 GPa ) . At ambient pressure, this sample shows the weakest antiferromagnetic (AF) Bragg reflections reported so far, corresponding to the volume-averaged staggered moment of μ o ∼ 0.011 μ B / U . Under applied pressure, the AF scattering intensity exhibits a sharp increase at P ∼ 0.7 GPa at low temperatures. The saturation value of the AF scattering intensity above 0.7 GPa corresponds to μ o ∼ 0.41 μ B / U , which is in good agreement with that ( ∼ 0.39 μ B / U ) observed above 1.5 GPa in our previous NS measurements. The superconductivity is dramatically suppressed by the evolution of AF phase, indicating that the superconducting state coexists only with the hidden order phase. The presence of parasitic ferro- and/or antiferromagnetic phases with transition temperatures T 1 ⋆ = 120 ( 5 ) , T 2 ⋆ = 36 ( 3 ) and T 3 ⋆ = 16.5 ( 5 ) K and their relationship to the low- T ordered phases are also discussed.

Developments of the theory of spin fluctuations and spin fluctuation-induced superconductivity.

Theory of spin fluctuations as developed in the past 30 years have played important roles in the theory of magnetism in metals, particularly in elucidating the properties around the magnetic instability or quantum critical points. Recently the theory has been extended to deal with the spin fluctuaion-mediated superconductivity with anisotropic order parameters in strongly correlated electron systems. These theoretical developments are briefly reviewed and the high temperature superconductivity of cuprates and organic and heavy electron superconductors are discussed in the light of these theories.

Inhomogeneous Magnetism and Hidden Order in URu2Si2

AbstractFor Abstract see ChemInform Abstract in Full Text.

Quadrupole ordering and multipole interactions in Pr-based compounds

Field–temperature phase diagrams of cubic compounds PrPb3 and PrOs4Sb12with non-magnetic crystalline electric field (CEF) ground state have beenexamined by means of low-temperature magnetization measurements. PrPb3 is atypical Γ3doublet CEF ground state system which undergoes an antiferro-quadrupole(AFQ) transition at TQ = 0.4 K. In a magnetic fieldTQ(H)exhibits re-entrant behaviour depending on the field direction, which can beinterpreted in terms of interactions between induced dipole (and possibly octupole)moments in the AFQ phase. In the heavy-electron superconductor PrOs4Sb12,a field-induced ordered state has been observed above 4 T for allthe principal directions. The results are in favour of a Γ1CEF ground state model in which a level crossing with a Γ5triplet excited state induces a long-range (possibly quadrupolar) ordering in amagnetic field.

Inhomogeneous magnetism and hidden order in URu2Si2

Recent high-pressure studies have been particularly effective in revealing unusual properties of inhomogeneous magnetism in the heavy-electron superconductor URu2Si2. In this paper a narrow selection of the important experimental observations elucidating the pressure-induced two-phase competition is reviewed.