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Abstract Motivated by the recent observation of superconductivity with T c ~ 80 K in pressurized La3Ni2O7 1, we explore the structural and electronic properties of A 3Ni2O7 bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La3Ni2O7 at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations between T c and the in-plane Ni-O-Ni bond angles for La3Ni2O7. Moreover, two structural phases with significant c + octahedral rotations and in-plane bond disproportionations are uncovered for A = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Ni e g manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb3Ni2O7 as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.

Considerable evidence shows that the heavy fermion material UTe2 is a spin-triplet superconductor, possibly manifesting time-reversal symmetry breaking, as measured by Kerr effect below the critical temperature, in some samples. Such signals can arise due to a chiral orbital state or possible nonunitary pairing. Although experiments at low temperatures appear to be consistent with point nodes in the spectral gap, the detailed form of the order parameter and even the nodal positions are not yet determined. Thermal conductivity measurements can extend to quite low temperatures, and varying the heat current direction should be able to provide information on the order parameter structure. Here, we derive a general expression for the thermal conductivity of a spin-triplet superconductor and use it to compare the low-temperature behavior of various states proposed for UTe2.

A growing number of superconducting materials display evidence for spontaneous time-reversal symmetry breaking (TRSB) below their critical transition temperatures. Precisely what this implies for the nature of the superconducting ground state of such materials, however, is often not straightforward to infer. We review the experimental status and survey different theoretical mechanisms for the generation of TRSB in superconductors. In cases where a TRSB complex combination of two superconducting order parameter components is realized, defects, dislocations and sample edges may generate superflow patterns that can be picked up by magnetic probes. However, even single-component condensates that do not break time-reversal symmetry in their pure bulk phases can also support signatures of magnetism inside the superconducting state. This includes, for example, the generation of localized orbital current patterns or spin-polarization near atomic-scale impurities, twin boundaries and other defects. Signals of TRSB may also arise from a superconductivity-enhanced Ruderman-Kittel-Kasuya-Yosida exchange coupling between magnetic impurity moments present in the normal state. We discuss the relevance of these different mechanisms for TRSB in light of recent experiments on superconducting materials of current interest.

Abstract In weakly coupled BCS superconductors, only electrons within a tiny energy window around the Fermi energy, E F , form Cooper pairs. This may not be the case in strong coupling superconductors such as cuprates, FeSe, SrTiO3 or cold atom condensates where the pairing scale, E B , becomes comparable or even larger than E F . In cuprates, for example, a plausible candidate for the pseudogap state at low doping is a fluctuating pair density wave, but no microscopic model has yet been found which supports such a state. In this work, we write an analytically solvable model to examine pairing phases in the strongly coupled regime and in the presence of anisotropic interactions. Already for moderate coupling we find an unusual finite temperature phase, below an instability temperature T i , where local pair correlations have non-zero center-of-mass momentum but lack long-range order. At low temperature, this fluctuating pair density wave can condense either to a uniform d-wave superconductor or the widely postulated pair-density wave phase depending on the interaction strength. Our minimal model offers a unified framework to understand the emergence of both fluctuating and long range pair density waves in realistic systems.

In 2001 superconductivity with a high critical temperature of 39 K was discovered in MgB2, but efforts since then to identify other diboride-family superconductors have been mostly unsuccessful. Here, the authors report the discovery of superconductivity in pressurized WB2, originating from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2.

Abstract The Eliashberg theory of superconductivity accounts for the fundamental physics of conventional superconductors, including the retardation of the interaction and the Coulomb pseudopotential, to predict the critical temperature T c. McMillan, Allen, and Dynes derived approximate closed-form expressions for the critical temperature within this theory, which depends on the electron–phonon spectral function α 2 F(ω). Here we show that modern machine-learning techniques can substantially improve these formulae, accounting for more general shapes of the α 2 F function. Using symbolic regression and the SISSO framework, together with a database of artificially generated α 2 F functions and numerical solutions of the Eliashberg equations, we derive a formula for T c that performs as well as Allen–Dynes for low-T c superconductors and substantially better for higher-T c ones. This corrects the systematic underestimation of T c while reproducing the physical constraints originally outlined by Allen and Dynes. This equation should replace the Allen–Dynes formula for the prediction of higher-temperature superconductors.

Abstract The single-layered ruthenate Sr2RuO4 is one of the most enigmatic unconventional superconductors. While for many years it was thought to be the best candidate for a chiral p-wave superconducting ground state, desirable for topological quantum computations, recent experiments suggest a singlet state, ruling out the original p-wave scenario. The superconductivity as well as the properties of the multi-layered compounds of the ruthenate perovskites are strongly influenced by a van Hove singularity in proximity of the Fermi energy. Tiny structural distortions move the van Hove singularity across the Fermi energy with dramatic consequences for the physical properties. Here, we determine the electronic structure of the van Hove singularity in the surface layer of Sr2RuO4 by quasi-particle interference imaging. We trace its dispersion and demonstrate from a model calculation accounting for the full vacuum overlap of the wave functions that its detection is facilitated through the octahedral rotations in the surface layer.

Abstract Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap $${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$ Δ k α , for all momenta k on the Fermi surface of every band α. While there are a variety of techniques for determining $$|{\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha |$$ ∣ Δ k α ∣ , no general method existed to measure the signed values of $${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$ Δ k α . Recently, however, a technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where $${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$ Δ k α has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured, is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the $${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$ Δ k α it generates to the $${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$ Δ k α determined from single-atom scattering in FeSe where s± energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for $${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$ Δ k α of opposite sign.

Recent resonant inelastic x-ray scattering (RIXS) experiments have detected a significant high-energy spin-excitation anisotropy in the nematic phase of the enigmatic iron-based superconductor FeSe, whose origin remains controversial. We apply an itinerant model previously used to describe the spin-excitation anisotropy as measured by neutron scattering measurements, with magnetic fluctuations included within the RPA approximation. The calculated RIXS cross section exhibits overall agreement with the RIXS data, including the high energy spin-excitation anisotropy.

Experiments indicate an abrupt change in the pairing gap near the nematic transition in the FeSe1−x S x iron-based superconductor. Here, Setty et al. propose to explain them via a novel spin-1/2 paired state with topologically protected zero-energy excitations over a finite area nodal surface.

The unconventional superconductor Sr_{2}RuO_{4} has been the subject of enormous interest over more than two decades, but until now the form of its order parameter has not been explicitly determined. Since groundbreaking NMR experiments revealed recently that the pairs are of dominant spin-singlet character, attention has focused on time-reversal symmetry breaking linear combinations of s-, d-, and g-wave one-dimensional (1D) irreducible representations. However, a state of the form d_{xz}+id_{yz} corresponding to the two-dimensional representation E_{g} has also been proposed based on some experiments. We present a systematic study of the stability of various superconducting candidate states, assuming that pairing is driven by the fluctuation exchange mechanism, including a realistic three-dimensional Fermi surface, full treatment of both local and nonlocal spin-orbit couplings, and a wide range of Hubbard-Kanamori interaction parameters U,J,U^{′},J^{′}. The leading superconducting instabilities are found to exhibit nodal even-parity A_{1g}(s^{′}) or B_{1g}(d_{x^{2}−y^{2}}) symmetries, similar to the findings in two-dimensional models without longer-range Coulomb interaction which tends to favor d_{xy} over d_{x^{2}−y^{2}}. Within the so-called Hund's coupling mean-field pairing scenario, the E_{g}(d_{xz}/d_{yz}) solution can be stabilized for large J and specific forms of the spin-orbit coupling, but for all cases studied here the eigenvalues of other superconducting solutions are significantly larger when the full fluctuation exchange vertex is included in the pairing kernel. Additionally, we compute the spin susceptibility in relevant superconducting candidate phases and compare to recent neutron scattering and nuclear magnetic resonance (NMR) Knight shift measurements. It is found that d_{xz}+id_{yz} order supports a neutron resonance in its superconducting phase, in contrast to a recent experiment [K. Jenni et al., Phys. Rev. B 103, 104511 (2021)2469-995010.1103/PhysRevB.103.104511], whereas s^{′}+id_{x^{2}−y^{2}} does not. Furthermore, comparison of the Knight shift reveals that s^{′}+id_{x^{2}−y^{2}} exhibits a larger low-temperature shift than d_{xz}+id_{yz}.

The interplay between superconductivity and charge density wave (CDW) in 2H-NbSe2 is still not fully understood. Here, Cho et al. use controlled disorder to probe the interplay between these two phases in 2H-NbSe2 and find that superconductivity initially competes with CDW but eventually long-range CDW order assists superconductivity.

We use dirty d-wave BCS theory to calculate absolute superfluid density, residual specific heat, Volovik effect, and thermal conductivity and compare to experiments on the cuprate superconductor La_{2−x}Sr_{x}CuO_{4}, showing that the theory provides a surprisingly good account of the data across the overdoped region. The starting point is an empirical angle-resolved photoemission spectroscopy–based parametrization of the electronic structure, including substantial Fermi-liquid renormalizations. Furthermore, a proper treatment of the less-explored weak out-of-plane dopant disorder limit is found to be essential. We then show that the same approach captures the low energy physics of another important overdoped cuprate, Tl-2201, thought to be much “cleaner” since it exhibits quantum oscillations, low residual resistivities, and small superconducting state Sommerfeld coefficients. We conclude that the low energy properties of cuprates are remarkably well described in the overdoped regime by dirty d-wave theory, without the need to introduce physics beyond the Landau-BCS paradigm.

The Hubbard model is the simplest model that is believed to exhibit superconductivity arising from purely repulsive interactions and has been extensively applied to explore a variety of unconventional superconducting systems. Here we study the evolution of the leading superconducting instabilities of the single-orbital Hubbard model on a two-dimensional square lattice as a function of onsite Coulomb repulsion U and band filling by calculating the irreducible particle-particle scattering vertex obtained from dynamical cluster approximation (DCA) calculations, and compare the results to both perturbative Kohn-Luttinger (KL) theory as well as the widely used random phase approximation (RPA) spin-fluctuation pairing scheme. Near half-filling, we find remarkable agreement of the hierarchy of the leading pairing states among these three methods, implying adiabatic continuity between weak- and strong-coupling pairing solutions of the Hubbard model. The d_{x^{2}−y^{2}}-wave instability is robust to increasing U near half-filling as expected. Away from half-filling, the predictions of KL and RPA at small U for transitions to other pair states agree with DCA at intermediate U as well as recent diagrammatic Monte Carlo calculations. RPA results fail only in the very dilute limit, where it yields a d_{xy} ground state instead of a p-wave state established by diagrammatic Monte Carlo and low-order perturbative methods, as well as our DCA calculations. We discuss the origins of this discrepancy, highlighting the crucial role of the vertex corrections neglected in the RPA approach. Overall, a comparison of the various methods over the entire phase diagram strongly suggests a smooth crossover of the superconducting interaction generated by local Hubbard interactions between weak and strong coupling.

A single crystal of isovalently substituted Ba(Fe_{1−x}Ru_{x})_{2}As_{2} (x=0.24) is sequentially irradiated with 2.5 MeV electrons up to a maximum dose of 2.1×10^{19} e^{−}/cm^{2}. The electrical resistivity is measured in situ at T=22 K during the irradiation and ex situ as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature T_{c} decreases and the residual resistivity ρ_{0} increases. We find that electron irradiation leads to the fastest suppression of T_{c} compared to other types of artificially introduced disorder, probably due to the strong short-range potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed T_{c} versus ρ_{0} is fully compatible with s_{±} pairing, in contrast to earlier claims that this model leads to a too rapid suppression of T_{c} with scattering.

Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity.

We study the incommensurate charge ordered states in the model using the Gutzwiller mean field theory on large systems. In particular, we explore the properties of incommensurate charge modulated states referred to as nodal pair density waves (nPDW) in the literature. nPDW states intertwine site and bond charge order with modulated d-wave pair order, and are characterized by a nonzero amplitude of uniform pairing; they also manifest a dominant intra-unit cell d-density wave form factor. To compare with a recent scanning tunneling microscopy (STM) study (Hamidian et al 2015 Nat. Phys.12 150) of the cuprate superconductor BSCCO-2212, we compute the continuum local density of states (LDOS) at a typical STM tip height using the Wannier function based approach. By Fourier transforming Cu and O sub-lattice LDOS we also obtain bias-dependent intra-unit cell form factors and spatial phase difference. We find that in the nPDW state the behavior of form factors and spatial phase difference as a function of energy agrees remarkably well with the experiment.This is in contrast to commensurate charge modulated states, which we show do not agree with experiment. We propose that the nPDW states are good candidates for the charge density wave phase observed in the superconducting state of underdoped cuprates. [ABSTRACT FROM AUTHOR]

Crystalline FeSe is known to display strong nematic order below a weak tetragonal-orthorhombic structural transition around K, and a superconducting transition at . Recently, it was shown that electron irradiation, which creates pointlike potential scattering defects, has the surprising effect of enhancing Tc while suppressing Ts. Here we discuss a possible scenario for such an effect, which postulates a competition between superconductivity and nematic order. The transition to the nematic state is modeled by a mean field theory of a d-wave Pomeranchuk instability, together with a Cooper pairing interaction in both one- and multiband models. The effect of nonmagnetic impurities on both orders is treated on equal footing within the Born approximation. We find evidence that disorder can indeed enhance Tc while suppressing the competing nematic order, but only in a multiband situation. We discuss our results in the context of experimental data on FeSe crystals. [ABSTRACT FROM AUTHOR]

Abstract Visualizing atomic-orbital degrees of freedom is a frontier challenge in scanned microscopy. Some types of orbital order are virtually imperceptible to normal scattering techniques because they do not reduce the overall crystal lattice symmetry. A good example is d xz /d yz (π,π) orbital order in tetragonal lattices. For enhanced detectability, here we consider the quasiparticle scattering interference (QPI) signature of such (π,π) orbital order in both normal and superconducting phases. The theory reveals that sublattice-specific QPI signatures generated by the orbital order should emerge strongly in the superconducting phase. Sublattice-resolved QPI visualization in superconducting CeCoIn5 then reveals two orthogonal QPI patterns at lattice-substitutional impurity atoms. We analyze the energy dependence of these two orthogonal QPI patterns and find the intensity peaked near E = 0, as predicted when such (π,π) orbital order is intertwined with d-wave superconductivity. Sublattice-resolved superconductive QPI techniques thus represent a new approach for study of hidden orbital order.

We generalize the theory of Cooper-pairing by spin excitations in the metallic antiferromagnetic state to include situations with electron and/or hole pockets. We show that Cooper-pairing arises from transverse spin waves and from gapped longitudinal spin fluctuations of comparable strength. However, each of these interactions, projected on a particular symmetry of the superconducting gap, acts primarily within one type of pocket. We find a nodeless -wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations support nodal -wave symmetry on the hole pockets. Our results may be relevant to the asymmetry of the AF/SC coexistence state in the cuprate phase diagram. [ABSTRACT FROM AUTHOR]

The pseudogap phase in cuprate superconductors is predicted to be a pair density wave state (PDW) but experimental evidence has been lacking. Here, the authors detect the temperature evolution of energy gap modulations and scattering interference signature suggesting the Bi2Sr2CaDyCu2O8 pseudogap phase contains a PDW.

Motivated by recent experimental reports of significant spin–orbit coupling (SOC) and a sign-changing order-parameter in the Li _1− _x Fe _x (OHFe) _1− _y Zn _y Se superconductor with only electron pockets present, we study the possible Cooper-pairing symmetries and their quasiparticle interference (QPI) signatures. We find that each of the resulting states— s -wave, d -wave and helical p -wave—can have a fully gapped density of states consistent with angle-resolved photoemission spectroscopy experiments and, due to SOC, are a mixture of spin singlet and triplet components leading to intra- and inter-band features in the QPI signal. Analyzing predicted QPI patterns we find that only the spin-triplet dominated even parity A _1 _g ( s -wave) and B _2 _g ( d -wave) pairing states are consistent with the experimental data. Additionally, we show that these states can indeed be realized in a microscopic model with atomic-like interactions and study their possible signatures in spin-resolved STM experiments.

Visão panorâmica da saúde no mundo e a inserção do home care. Descreve a Organização Mundial da Saúde (OMS) em seu aspecto histórico, enfatizando a participação brasileira em sua fundação e alguns do fatos e eventos mais relevantes que antecederam essa criação. Também descreve a historia da Organização Panamericana de Saúde (OPS), fundada em dezembro de 1902 e a celebração do seu primeiro centenário em 2002. Uma visão geral da saúde no mundo é oferecida, incluindo-se os aspectos da globalização, como fato irreversível, mudanças demográficas e epidemiológicas, dependência e o trabalho de prestação de cuidados de home care, serviços de home care e politicas de home care.

We generalize the theory of Cooper-pairing by spin excitations in the metallic antiferromagnetic state to include situations with electron and/or hole pockets. We show that Cooper-pairing arises from transverse spin waves and from gapped longitudinal spin fluctuations of comparable strength. However, each of these interactions, projected on a particular symmetry of the superconducting gap, acts primarily within one type of pocket. We find a nodeless ${{{\rm d}}_{{{x}^{2}}-{{y}^{2}}}}$ -wave state is supported primarily by the longitudinal fluctuations on the electron pockets, and both transverse and longitudinal fluctuations support nodal ${{{\rm d}}_{{{x}^{2}}-{{y}^{2}}}}$ -wave symmetry on the hole pockets. Our results may be relevant to the asymmetry of the AF/SC coexistence state in the cuprate phase diagram.