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Warren E. Pickett

Publications and source records attributed to Warren E. Pickett.

At least 19 recordsLinked to original sources
arXiv2026

First-principles evidence for conventional superconductivity in a quasicrystal approximant

Quasicrystals (QCs) host long-range order without translational symmetry, a regime in which the very foundations of BCS theory are not straightforwardly applicable, yet experiments on QCs and their approximant crystals (ACs) point to conventional, $s$-wave, electron-phonon coupled superconductivity. Here we test the predictive power of the electron-phonon framework in a representative decagonal AC from first principles. Using state-of-the-art \textit{ab initio} methods, we compute the superconducting properties of the recently discovered AC Al$_{13}$Os$_4$ and quantitatively reproduce its bulk $T_\text{c}$. This constitutes, to our knowledge, the first \textit{ab initio} determination of $T_\text{c}$ for an AC and establishes that the electron-phonon framework is predictive in these systems as well. Using the generalized quasichemical approximation for alloy modeling in the decagonal Al--Os family, we predict tunable superconductivity in Al$_{13}$Os$_{4-x}$Re$_x$ and Al$_{13}$Os$_{4-x}$Ir$_x$; in particular, Al$_{13}$Re$_4$ is dynamically stable and estimated to have a $T_\text{c}$ about 30% above Al$_{13}$Os$_4$. Finally, we discuss the role of ACs as high-fidelity proxies for their parent QCs. Although long-range quasiperiodicity may introduce subtle electronic features, our findings indicate that the key ingredients for superconductivity are already encoded in the local structural motifs preserved by the AC. This places the Al--Os and Al--Re families among the most promising candidates for the highest-$T_\text{c}$ quasicrystalline superconductivity.

cond-mat.supr-con↗
arXiv2026

Fast Real-Axis Eliashberg Calculations: Full-bandwidth solutions beyond the constant density of states approximation

Experimentally relevant signatures of superconductivity require access to real-frequency quantities, such as the spectral functions, optical response, and transport properties, yet Migdal-Eliashberg calculations are commonly performed on the imaginary axis and then analytically continued, a step that is numerically delicate and can obscure physically relevant spectral features. Here we present a practical route to solving the finite-temperature Migdal-Eliashberg equations directly on the real-frequency axis, while retaining the effects from the full-bandwidth electronic structure. Our formulation accounts for particle-hole asymmetry through an energy-dependent electronic density of states, avoiding the constant density of states approximation often used in real-axis calculations, and includes a static screened Coulomb contribution. We introduce an efficient numerical technique to solve the Migdal-Eliashberg integrals whose computational cost scales linearly with the real-frequency grid, making high-resolution, full-bandwidth real-axis calculations feasible and providing direct access to the interacting Green's function and derived observables without analytic continuation. As an illustration, we apply the method to H$_{3}$S, where a van-Hove singularity near the Fermi level produces strong particle-hole asymmetry. The full-bandwidth solution yields noticeably different spectra than the constant density of states approximation and brings the superconducting gap and lineshapes into closer agreement with experiment, highlighting when band-structure details are essential. Furthermore, the methods presented here open the door to time-dependent, nonequilibrium simulations within Eliashberg theory.

cond-mat.supr-con↗
arXiv2026

Time Reversal Symmetry Breaking and {\it Fragile Magnetic Superconductors}

Roughly twenty reports (as of 2025) of time-reversal-symmetry breaking (TRSB) states in low critical temperature (T$_c$) superconducting (SC), otherwise conventional Fermi liquid, metals have emerged primarily from muon spin relaxation ($μ$SR) data. The detected fields, inferred from the current interpretation of depolarization data, are similar in magnitude and not far above the lower limit of detection, corresponding to magnetizations of no more than 10$^{-3}$ $μ_B$/atom. These materials comprise a new class of {\it fragile magnetic superconductors} modeled as triplet pairing. The measured SC state properties, excepting only the fields detected below T$_c$, are representative of low T$_c$ singlet BCS SCs, not showing unusual coherence lengths or critical fields. While it is recognized that the muon does affect the sample by displacing nearby atoms and impacting magnetic interaction parameters, the measurement process, changing the system from sample $\rightarrow$ sample+$μ^+$ thereby breaking TRS, may deserve further scrutiny. This overview provides a survey of the environment of the muon, from the normal state to the superfluid state, where the induced supercurrent and Yu-Shiba-Rusinov gap states provide coupling of the muon moment to the superfluid. The unusual topological superconductor LaNiGa$_2$, currently modeled as non-unitary triplet, is used as a case study. Supposing that the prevailing $μ$SR inference of a small spontaneous field within the bulk of theSC obtains, the current picture of (possibly non-unitary) triplet pairing is discussed and an attractive alternative for LaNiGa$_2$ is noted.

cond-mat.supr-con↗
arXiv2026

Superfluid Density, Penetration Depth, Condensate Density

Fascination with the concept of superconducting (SC) {\it superfluid density} $ρ_s$ has persisted since the beginning of superconductivity theory, with numerical values of an actual density rarely provided. Over time $ρ_s$, addressed mostly in cuprate and following high temperature superconductors, has become synonymous with the normalized (unitless) inverse square of the magnetic penetration depth $λ_L$ (the London expression, with superfluid density denoted $n_s$), with interest primarily on its temperature $T$ dependence that is expected to reflect the T-dependence of the SC gap amplitude and gap symmetry. In conventional superconductors, generalized expressions from the London penetration depth via Ginzburg-Landau theory, then to BCS theory provide updated pictures of the supercurrent density-vector potential relationship. The BCS value $λ_{band}$ is distinct from any particle density, instead involving particle availability at the Fermi surface and Fermi velocity as the determining factors, thus providing a basis for a more fundamental theory and understanding of what is being probed in penetration depth studies. The number density of superconducting electrons ${\cal N}_s(T$=0) -- the scalar SC {\it condensate density} -- is provided, first from a phenomenological estimate but then supported by BCS theory. A straightforward relation connecting ${\cal N}_s(0)$ to the density of dynamically transporting carriers in the normal state at $T_c$ is obtained. Numerical values of relevant material parameters including $λ_{band}$ and ${\cal N}_s$ are provided for a few conventional SCs.

cond-mat.supr-con↗
arXiv2025

Why Compressed Metal Hydrides are Near-room-temperature Superconductors

This contribution provides a partial response to the titular statement since, it will be claimed,the ``why'' is not yet understood, but there is a pathway for achieving a more complete understanding. The sense of the community has been that, given a prospective metal hydride and pressure, the energy landscape can be surveyed computationally for thermodynamic and dynamic stability, the Eliashberg spectral function with its required input (energy bands, phonon modes, coupling matrix elements) can be calculated, and the critical temperature T$_c$ obtained. Satisfyingly large values of the electron-phonon coupling strength $λ$=2-3 at high mean frequency are obtained, giving very reasonable agreement with existing high T$_c$ hydrides. Typically 80-85\% of $λ$ is attributable to high frequency H vibrations. This much was envisioned by Ashcroft two decades ago, so why should there be any angst? This paper addresses more specifically the question {\it why hydrogen?} Light mass is indeed a factor, but with possibilities not yet explored. This paper provides a concise overview of related formal developments occurring sporadically over several decades that, when implemented, could resolve the question of {\it why hydrogen, why so high T$_c$.} The dearth of success of numerous high throughput searches proposing higher T$_c$ materials, especially hydrides, is touched on briefly. Based on as yet unapplied developments in simplifying effects of atomic displacement, it is proposed that there is a straightforward path toward a deeper understanding of ``metallic hydrogen superconductivity" in conjunction with added computational efficiency, and that some human-learning should assist in focusing the search for higher T$_c$ superconductors.

cond-mat.supr-con↗
arXiv2025

Single layer clathrane: A potential superconducting two-dimensional (2D) hydrogenated metal borocarbide

We propose a new family of two-dimensional (2D) metal-borocarbide clathrane superconductors, derived from three-dimensional (3D) MM$^\prime$B$_6$C$_6$ clathrates. First-principles calculations reveal that hydrogen passivation and surface metal decoration stabilize the M$_2$M$^\prime$B$_8$C$_8$H$_8$ monolayers. These 2D systems exhibit tunable superconductivity governed by hole concentration, structural anisotropy, and electron-phonon coupling. We find that in-plane anisotropy competes with superconductivity, reducing \tc\ despite favorable doping. Biaxial strain mitigates this anisotropy, enhances Fermi surface nesting, and increases \tc\ by an average of 15.5~K. For example, the \tc\ of Sr$_3$B$_8$C$_8$H$_8$ is predicted to increase from 11.3~K to 22.2~K with strain engineering. These findings identify 2D clathranes as promising, strain-tunable superconductors and highlight design principles for optimizing low-dimensional superconducting materials.

cond-mat.supr-con↗
arXiv2023

Symmetry Enforced Fermi Surface Degeneracies Observed in Time-Reversal Symmetry-Breaking Superconductor LaNiGa$_2$

LaNiGa$_2$ is superconductor that breaks time-reversal symmetry in the superconducting state without any known nearby magnetism. Recently, single crystals of LaNiGa$_2$ have been synthesized, revealing a nonsymmorphic Cmcm space group. Here, we report measurements of the electronic structure of LaNiGa$_2$ throughout the three-dimensional Brillouin zone (BZ) using angle-resolved photoemission spectroscopy (ARPES). Our findings show broad consistency with density functional theory (DFT) calculations and provide evidence for degeneracies in the electronic structure that are predicted from the space group. The calculations also predict four Fermi surfaces which cross the purported nodal plane and should therefore form two degenerate pairs. We report evidence for those predicted symmetry enforced degeneracies as well as accidental near degeneracies throughout the BZ. These degeneracies and near-degeneracies may play a role in the pairing mechanism of LaNiGa$_2$. Our results provide insight into the interplay between structure, Fermiology, and superconductivity in unconventional superconductors with nonsymmorphic space group.

cond-mat.supr-con↗
arXiv2023

Comment on "Nontrivial Quantum Geometry and the Strength of Electron-Phonon Coupling", arXiv:2305.02340, J. Yu, C. J. Ciccarino, R. Bianco, I. Errea, P. Narang, B. A. Bernevig

The titular manuscript invites a description of the background of research on MgB$_2$,whose unique electron-phonon coupling strength and record superconducting T$_c$ for such a material at ambient pressure was demonstrated by the combined efforts of several groups two decades ago in terms of conventional but highly developed Migdal-Eliashberg theory. This Comment provides some of the basic features of the theoretical understanding of MgB$_2$ and draws contrasts with the model of Yu {\it et al.}

cond-mat.supr-con↗
arXiv2023

Room Temperature Superconductivity: the Roles of Theory and Materials Design

For half a century after the discovery of superconductivity, materials exploration for better superconductors proceeded without knowledge of the underlying mechanism. The 1957 BCS theory cleared that up: the superconducting state occurs due to pairing of electrons over the Fermi surface. Over the following half century higher critical temperature T$_c$ was achieved only serendipitously as new materials were synthesized. Meanwhile the formal theory of phonon-coupled superconductivity at the material-dependent level became highly developed: given a known compound, its value of T$_c$, the superconducting gap function, and several other properties of the superconducting state became available independent of further experimental input. More recently, density functional theory based computational materials design has progressed to a predictive level -- new materials can be predicted on the basis of various numerical algorithms. Taken together, these capabilities enable theoretical prediction of new superconductors. Here the process that resulted in three new highest temperature superconductors, predicted numerically, confirmed experimentally -- SH$_3$, LaH$_{10}$, and YH$_9$ -- is recounted. These hydrides have T$_c$ in the 200-280K range at megabar pressures, and here the development will be chronicled. Current activities and challenges are discussed, together with Regularities in compressed hydrides that can guide further exploration.

cond-mat.supr-con↗
arXiv2022

Field-induced Bose-Einstein condensation and supersolid in the two-dimensional Kondo necklace

The application of an external magnetic field of sufficient strength to a spin system composed of a localized singlet can overcome the energy gap and trigger bosonic condensation and so provide an alternative method to realize exotic phases of matter in real materials. Previous research has indicated that a spin Hamiltonian with on-site Kondo coupling may be the effective many-body Hamiltonian for $\text{Ba}_2\text{NiO}_2\text{(AgSe)}_2$ (BNOAS) and here we study such a Hamiltonian using a tensor network ansatz in two dimensions. Our results unveil a phase diagram which indicates the underlying phases of BNOAS. We propose, in response to the possible doping-induced superconductivity of BNOAS, a fermionic model for further investigation. We hope that our discovery can bring up further interest in both theoretical and experimental researches for related nickelate compounds.

cond-mat.mtrl-sci↗
arXiv2022

Low Valence Nickelates: Launching the Nickel Age of Superconductivity

The discovery of superconductivity in thin films ($\sim$10 nm) of infinite-layer hole-doped NdNiO$_2$ has invigorated the field of high-temperature superconductivity research, reviving the debate over contrasting views that nickelates that are isostructural with cuprates are either (1) sisters of the high-temperature superconductors, or (2) that differences between nickel and copper at equal band filling should be the focus of attention. Each viewpoint has its merits, and each has its limitations, suggesting that such a simple picture must be superseded by a more holistic comparison of the two classes. Several recent studies have begun this generalization, raising a number of questions without suggesting any consensus. In this paper, we organize the findings of the electronic structures of $n$-layered NiO$_2$ materials ($n$= 1 to $\infty$) to outline (ir)regularities and to make comparisons with cuprates, with the hope that important directions of future research will emerge.

cond-mat.supr-con↗
arXiv2021

Dirac lines and loop at the Fermi level in the Time-Reversal Symmetry Breaking Superconductor LaNiGa$_2$

Unconventional superconductors have Cooper pairs with lower symmetries than in conventional superconductors. In most unconventional superconductors, the additional symmetry breaking occurs in relation to typical ingredients such as strongly correlated Fermi liquid phases, magnetic fluctuations, or strong spin-orbit coupling in noncentrosymmetric structures. In this article, we show that the time-reversal symmetry breaking in the superconductor LaNiGa$_2$ is enabled by its previously unknown topological electronic band structure. Our single crystal diffraction experiments indicate a nonsymmorphic crystal structure, in contrast to the previously reported symmorphic structure. The nonsymmorphic symmetries transform the $k_z=π/c$ plane of the Brillouin zone boundary into a node-surface. Band-structure calculations reveal that distinct Fermi surfaces become degenerate on the node-surface and form Dirac lines and a Dirac loop at the Fermi level. Two symmetry related Dirac points remain degenerate under spin-orbit coupling. ARPES measurements confirm the calculations and provide evidence for the Fermi surface degeneracies on the node-surface. These unique topological features enable an unconventional superconducting gap in which time-reversal symmetry can be broken in the absence of other typical ingredients. LaNiGa$_2$ is therefore a topological crystalline superconductor that breaks time-reversal symmetry without any overlapping magnetic ordering or fluctuations. Our findings will enable future discoveries of additional topological superconductors.

cond-mat.supr-con↗
arXiv2021

MoB2 under Pressure: Superconducting Mo Enhanced by Boron

The discovery of the first high critical temperature (Tc) transition metal diboride superconductor, MgB2 structure alpha-MoB2 under pressure with Tc up to 32 K at 100 GPa, provides new input into some unexplained aspects of electron-phonon coupling in intermetallic compounds. We establish that MoB2 is a phonon-mediated superconductor but has little in common with MgB2 (Tc=40 K at zero pressure). MoB2 is a strongly metallic, three dimensional, multi-Fermi surface material, becoming of additional interest because it displays a frequency separation of Mo and B vibrations that mirrors that of metal superhydrides with Tc approaching room temperature. This separation, which is unusual in intermetallic compounds, allows an analysis separately for Mo and B providing, amongst the other parameters essential for understanding phonon coupling, the matrix elements for scattering by the individual atoms. Strong coupling (lambda(Mo)=1.48) is provided by Mo (total lambda=1.67). A factor of 15 weaker coupling to each B atom is compensated by that coupling being to mean high frequency modes around 85-90 meV (maximum of 140 meV), versus 18-20 meV for Mo. As a result, B enhances Tc by 43% over the Mo-only value, to 33 K, corresponding to the experimental value. These results provide a guideline for designing higher Tc materials from a cooperation of strong coupling from heavy atoms with weakly coupled light atoms. The new high Tc paradigm discovered here highlights the need for studying and engineering larger ionic scattering matrix elements.

cond-mat.supr-con↗
arXiv2021

UTe$_2$: a nearly insulating half-filled $j=\frac{5}{2}$ $5f^3$ heavy fermion metal

Correlated band theory implemented as a combination of density functional theory with exact diagonalization [DFT+U(ED)] of the Anderson impurity term with Coulomb repulsion $U$ in the open 14-orbital $5f$ shell is applied to UTe$_2$. The small gap for $U$=0, evidence of the half-filled $j=\frac{5}{2}$ subshell of $5f^3$ uranium, is converted for $U$=3 eV to a flat band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off-stoichiometry, as observed experimentally. The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by DFT+U(ED) to differ by a factor of seven, indicating a strong site distinction, while the anisotropy factor $η=0.18$ is similar for all three sites. The calculated uranium moment $ ^{1/2}$ of 3.5$μ_B$ is roughly consistent with the published experimental Curie-Weiss values of 2.8$μ_B$ and 3.3$μ_B$ (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hund's rule values for an $f^3$ ion. The $U$=3 eV spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong $5f$ character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of UTe$_2$ is that of a half-filled $j=\frac{5}{2}$ subshell with two half-filled $m_j=\pm\frac{1}{2}$ orbitals forming a narrow gap by hybridization, then driven to a conducting state by configuration mixing (spin-charge fluctuations). UTe$_2$ displays similarities to UPt$_3$ with its $5f$ dominated Fermi surfaces rather than a strongly localized Kondo lattice system.

cond-mat.str-el↗
arXiv2020

Accidental Degeneracy in k-space, Geometrical Phase, and the Perturbation of $π$ by Spin-orbit Interactions

Since closed lines of {\it accidental} electronic degeneracies were demonstrated to be possible, even frequent, by Herring in 1937, no further developments arose for eight decades. The earliest report of such a nodal loop in a real material -- aluminum -- is recounted and elaborated on. Nodal loop semimetals have become a focus of recent activity, with emphasis on other issues. Band degeneracies are, after all, the origin of topological phases in crystalline materials. Spin-orbit interaction lifts accidental band degeneracies, with the resulting spectrum being provided here. The geometric phase $γ(C)=\pmπ$ for circuits $C$ surrounding a line of such degeneracy cannot survive completely unchanged. The change depends on how the spin is fixed during adiabatic evolution. For spin fixed along the internal spin-orbit field, $γ(C)$ decreases to zero as the circuit collapses around the line of lifted degeneracy. For spin fixed along a perpendicular axis, the conical intersection persists and $γ(C)=\pmπ$ is unchanged.

cond-mat.mes-hall↗
arXiv2020

Topological and Thermoelectric Properties of Double Antiperovskite Pnictides

Doubling the perovskite cell (double perovskite) has been found to open new possibilities for engineering functional materials, magnetic materials in particular. This route should be applicable to the antiperovskite (aPV) class. In the pnictide based double aPV (2aPV) class introduced here magnetism is very rare, and we address them as new topological materials, possibly with thermoelectric interest. We have found that the 2aPV supercell provides a systematically larger band gap that can serve to inhibit bulk conductivity, and also large spin-orbit coupling (SOC) for band inversion. We present examples from a broad study of double antiperovskites focusing on the X$_6$AA$'$B$_2$ configuration, where X is the alkaline earth element and A and B are the group 5A pnictogens. We find that an "extended s" state at the valence band minimum, described alternatively as a cation valence state or a modulated interstitial planewave state, plays a crucial role in both topological and thermoelectric properties. Several of these compounds may house topological phases, while transport calculations indicate they may also find themselves useful in thermoelectric applications.

cond-mat.mtrl-sci↗
arXiv2020

Superconducting Symmetry Phases and Dominant bands in (Ca-) Intercalated AA- Bilayer Graphene

Built on a realistic multiband tight-binding model, mirror symmetry is used to map a calcium-intercalated bilayer graphene Hamiltonian into two independent single layer graphene-like Hamiltonians with renormalized hopping. The quasiparticles exhibit two types of chirality. Here a quasi-particle consists of two electrons from opposing layers where possess an additional quantum number called "cone index" which can be regarded as the eigenvalue of mirror symmetry operations. To obtain tight-binding parameters, both effective monolayer Schrodinger equations are solved analytically and fitted to first-principles band structure results. Two quasi-particles (four electrons) can team up to build a Cooper pair with even or odd chirality. Treatment of the pairing Hamiltonian leads to two decoupled gap equations. The pairing of quasi-particles with different cone indexes is forbidden. The decoupled gap equations are solved analytically to obtain all the possible superconducting phases. Two nearly "flat bands" crossing the Fermi energy, each related to the graphene-like structures, are responsible for two distinct superconductivity gaps that emerge. Depending on how much these bands are affected by the intercalant and which is closer to the Fermi energy, distorted s-wave or d-wave superconductivity may become dominant. Numerical calculations reveal that d-wave superconductivity is dominant in both sectors. For these two dominant phases, within the range of 0-6 K which superconductivity has been observed, numerically the transition from single-gap to dual-gap superconductivity is possible. Adopting the two-gap viewpoint of superconductivity in C$_6$CaC$_6$, the dominant $d$-wave states should have the same critical temperature. Around $T_c=2K$ these two relations intersect, otherwise, superconductivity has been realized just in one of these two sectors and disappears in the other one.

cond-mat.supr-con↗