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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Friday 18 September

Superconductivity · Cross-listed from cond-mat.str-el

Orbital character and photoemission signatures of the A-pocket in infinite-layer nickelates

Leonard M. Verhoff, Michael Seidl, Liang Si et al. (+1 more)

The orbital character of the so-called $A$-pocket in infinite-layer nickelates remains under debate. We investigate bulk NdNiO$_2$ using density-functional theory combined with dynamical mean-field theory and identify the $A$-pocket as a strongly hybridized state with substantial interstitial-$s$, Nd-$d_{xy}$ and Ni-d_${xz/yz}$ contributions. Our angle-resolved photoemission spectroscopy simulations reproduce the main polarization-dependent features measured experimentally. Removing the interstitial-$s$ matrix element leads to a clear disagreement with the measured Fermi surface maps, even though the interstitial-$s$ contribution is not dominant. Suppressing instead the Nd-d$_{xy}$ or Ni-$_{dxz/yz}$ matrix elements leaves the maps almost unchanged, because these are small at the zone corner. More generally, our results demonstrate that assigning the microscopic origin of spectral features in infinite-layer nickelates requires treating orbital hybridizations and photoemission matrix elements together.
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All authors Leonard M. Verhoff, Michael Seidl, Liang Si, Karsten Held

Superconductivity

Digital-analog concept for superconducting perceptron-like neural networks

Andrey E. Schegolev, Vsevolod I. Ruzhickiy, Georgy I. Gubochkin et al. (+7 more)

A promising route to superconducting artificial neural networks is a hybrid digital-Analog architecture that combines digital single-flux-quantum (SFQ) communication with compact Analog nonlinear processing. The study focused on the dynamic conversion of a discrete signal passing through a digital-to-Analog-to-digital (DAD) converter, in which the role of the Analog cell was performed by a $\Sigma$-neuron with a nonlinear transfer function – the basic cell of perceptron-like neural networks. Furthermore, the DAD converter, the elementary functional block of the hybrid architecture, combines a digital-to-analog converter (DAC) and an Analog-to-digital converter (ADC), and re-encodes the Analog $\Sigma$-neuron waveforms as an SFQ pulse sequence. Circuit-level simulations demonstrate how input values encoded by SFQ pulse trains are converted into analog signal levels, transformed by the $\Sigma$-neuron, and mapped back to pulse-based outputs. As a key experimental step, we fabricated and characterised a redesigned $\Sigma$-neuron and measured a sigmoid-like transfer characteristic suitable for activation-function implementation. The extracted response was incorporated into system-level simulations to assess the influence of realistic device parameters on the conversion process. We delineate the operating-range matching requirements for the DAC, neuron, and ADC blocks, supporting the feasibility of the proposed interface as a building block for perceptron-like superconducting neural networks with digital inputs and outputs. Finally, we developed two perceptron networks, one using a mathematical sigmoid activation and the other the measured $\Sigma$-neuron transfer characteristic, which reached classification accuracies of $97.0\%$ and $91.9\%$, respectively, on the MNIST handwritten digit dataset.
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All authors Andrey E. Schegolev, Vsevolod I. Ruzhickiy, Georgy I. Gubochkin, Alexander S. Ionin, Ivan A. Nazhestkin, Mikhail Y. Fominskii, Lyudmila V. Filippenko, Igor I. Soloviev, Maxim V. Tereshonok, Nikolay V. Klenov

Thursday 17 September

Superconductivity

Smectic Superconductivity

Grayson R. Frazier and Erez Berg

We identify a mechanism in which soft nonunitary fluctuations of the pair condensate can suppress phase coherence in a two-dimensional Rashba superconductor proximate to easy-plane ferromagnetic order. This spin polarization couples linearly to the supercurrent through a Lifshitz invariant, acting as an emergent gauge field for the superconducting $\mathrm{U}(1)$ phase. Integrating out soft magnetic fluctuations renormalizes the effective superfluid stiffness and drives the system into a "smectic" superconducting state, characterized by a finite-momentum helical phase texture and a long-wavelength stiffness that vanishes in one direction. Consequently, vortices no longer have a logarithmically divergent energy cost, and the quasi-long-range superconducting order is destroyed at any finite temperature. Weak lattice anisotropy pins the ferromagnetic order, cutting off the smectic-like behaviour at long wavelengths and thereby restoring a finite effective stiffness. Nonetheless, near the onset of magnetic order, the enhanced magnetic susceptibility leads to a vortex-deconfined regime for arbitrarily weak magnetoelectric coupling. The results provide insight into how intertwined superconducting and easy-plane ferromagnetic orders can reshape phase coherence, with relevance to rhombohedral graphene proximitized by a transition-metal dichalcogenide substrate.
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Superconductivity

Metallic Néel order stabilized by coupling between inequivalent Hubbard layers

Yi-Ming Wu, Jessica Yiqi Pan, Hong Yao et al. (+1 more)

Inspired by recent ARPES studies on multilayer ($n\geq3$ layers) cuprate superconductors, we use the unrestricted Hartree-Fock approximation to explore the ground-state phase diagram of two coupled, inequivalently doped square-lattice Hubbard layers. In the decoupled-layer limit, the lightly hole-doped ground state is typically an incommensurate spin-stripe state. However, with sufficiently strong interlayer coupling, stripe order is destabilized relative to a commensurate Néel-ordered metal. The resulting state exhibits a reconstructed Fermi surface with hole pockets centered at $(\pm\pi/2,\pm\pi/2)$ that are similar in character to those seen in experiments. Our results illustrate the qualitatively new physics that can arise from interlayer coupling in multilayer cuprates.
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All authors Yi-Ming Wu, Jessica Yiqi Pan, Hong Yao, Steven A. Kivelson

Superconductivity

Superconductivity in Pb2-xBixPd Single Crystals

S. Srivastava and R. P. Singh

Chemical substitution provides an effective route to tune the structural and superconducting properties and to explore the evolution of superconductivity across related materials. In this work, we investigate the effect of Pb/Bi mixing on superconductivity in possible topological superconductors \ch{Pb2Pd} and $\beta$-\ch{Bi2Pd}, by synthesizing and characterizing single crystals of \ch{Pb_{2-x}Bi_{x}Pd} ($0.2\le x\le1.8$). Increasing Bi content triggers a structural phase transition from non-symmorphic ($I4/mcm$, $x<1$) to layered ($I4/mmm$, $x\ge1$) and induces a monotonic expansion of the lattice parameters. Across the doped range, all superconducting samples behave as weakly-coupled type-II superconductors exhibiting isotropic s-wave superconducting gaps.
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Superconductivity

Type-I superconductivity in a quasi-2D topologically nontrivial YbBi$_2$

Karolina Górnicka, Sudip Malick, Joanna Bławat et al. (+13 more)

Intrinsic superconductivity in stoichiometric materials with nontrivial electronic topology remains uncommon, limiting opportunities to investigate how these two phenomena coexist within a single electronic system. Here, we report bulk type-I superconductivity below $T_c$ $\sim$ 0.9~K in YbBi$_2$, a layered rare-earth compound with a nonsymmorphic crystal structure and a quasi-two-dimensional Fermi surface. Thermodynamic and transport measurements establish the superconducting ground state, while quantum oscillations reveal exceptionally light carriers, with a cyclotron mass as low as 0.07 $m_e$, and a nonzero Berry phase of approximately 0.82 $\pi$. The latter closely matches the calculated value of the corresponding Wilson phase 0.99 $\pi$ for the corresponding orbit near a symmetry-protected band degeneracy. ARPES measurements show good agreement with key features of the calculated electronic structure, providing complementary experimental constraints on the normal-state band structure. The combination of intrinsic type-I superconductivity, light quasi-two-dimensional carriers, and signatures of nontrivial electronic topology identifies YbBi$_2$ as a distinct stoichiometric platform for investigating superconductivity in a topologically nontrivial electronic environment.
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All authors Karolina Górnicka, Sudip Malick, Joanna Bławat, Michał Modrzejewski, Hanna Świątek, Michał J. Winiarski, Federico Mazzola, Ivana Vobornik, Chiara Bigi, Jacob Cook, Brenden R. Ortiz, Andrew F. May, Andrzej P. Kądzielawa, John Singleton, Bartlomiej Wiendlocha, Tomasz Klimczuk

Superconductivity

Generic thermodynamics of condensates with extremely small superfluid density —Application to UTe$_2$ and hidden second phase transition—

Kazushige Machida

Motivated by recent experimental findings on UTe$_2$ via NMR and specific heat experiments, we theoretically explore the thermodynamic signatures of condensates with small superfluid density and characterize their physical properties. We identify the hidden superconducting phase $A_2$ composed of spin-up Cooper pairs at lower temperature ($T$) and field ($H$) under ambient pressure of UTe$_2$ and show that $A_2$ reappears at higher $H$ for all three principal field directions, $a$, $b$, and $c$-axis. The phase transition lines from the $A_1$ phase with spin down pairs to the $A_2$ phase in the $H$-$T$ plane are all horizontal in common, implying a universal physical origin due to the originating from the magnetic-energy gain of the spin-up pairing state with increasing magnetic field. This multiple-phase diagram is extended to these under pressure ($P$), resulting in a simple picture based on non-unitary spin-triplet pairing symmetry $^3B_{3u}$, which enables us to consistently capture the evolution of the superconducting phases throughout the entire $H$-$T$-$P$ phase space.
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Superconductivity

Vortex-mediated spin current injection into two-dimensional superconductor NbSe2

Meng Yang, Xiaolong Yin, Jingjing Liu et al. (+7 more)

Injection of pure spin current into superconductors remains a major challenge in superconducting spintronics. Previous studies have primarily focused on spin-polarized quasiparticles and spin-triplet supercurrents, while vortices, ubiquitous topological defects in type-2 superconductors, have been theoretically proposed as alternative carriers of spin angular momentum, yet direct experimental evidence is still lacking. Here, we report the vortex-mediated spin current injection in NbSe2/LiAl2Fe3O8(LAFO) bilayer, an Ising superconductor/ferrimagnetic insulator heterostructure. Under an out-of-plane temperature gradient and an in-plane magnetic field, the NbSe2/LAFO bilayer shows a pronounced thermoelectric peak near the upper critical magnetic field, which has the opposite sign to the conventional vortex Nernst signal observed in a single-layer NbSe2. The sign reversal suggests that the vortex flow induced by spin current injection is opposite to the flow driven by the temperature gradient, which is consistent with theoretical mechanisms including spin-vorticity transmutation and the inverse vortex spin Hall effect. By mapping the field temperature phase diagram, we reveal that the spin current injection occurs exclusively in the vortex liquid phase of NbSe2. Absence of the thermoelectric signal above the superconducting transition temperature further rules out the quasiparticle contribution. Our results establish vortices as efficient carriers of spin information in superconductors, opening a new route towards vortex-mediated superconducting spintronic devices.
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All authors Meng Yang, Xiaolong Yin, Jingjing Liu, Yifeng Chen, Qinwu Gao, Hongxing Zhu, Danni Huang, Lang Chen, Shuo-Ying Yang, Junxue Li

Superconductivity

Superconductivity at the metal-insulator phase boundary in a bulk nickelate at ambient pressure

Hyo-Bin Ahn, Xinglong Chen, Hong Zheng et al. (+9 more)

The discovery of superconductivity in nickelates has seeded a new field for exploring unconventional superconductivity in transition metal oxides. However, to date superconductivity has only been realized in thin films or under extreme pressure in the bulk. Here we report signatures of superconductivity at ambient pressure in bulk nickelate single crystals of (La1-xPrx)4Ni3O8 and (La1-xYx)4Ni3O8, whose crystal structure comprises interleaving trilayers of square-planar nickel oxide and fluorite-like spacer layers. The parent compound La4Ni3O8 exhibits an insulating ground state, where electrons order into intertwined, insulating charge/spin stripes. Substitution of smaller lanthanide ions disrupts this order, eventually leading to a metallic ground-state. We find that superconductivity emerges in a narrow window proximate to the insulator-metal phase boundary, where both metallic and charge-stripe phases co-exist. The observed low volume-fraction superconductivity is non-percolative, suggesting the prospect of filamentary superconductivity nucleated at the boundary between these phases. However, intergrowth defects that approximate the known infinite layer nickelates are observed in TEM, leaving open the possibility that superconductivity resides here rather than in the trilayer matrix. Remarkably, the electronic phase diagrams of both the Y and Pr series coincide when parameterized by the volume of the fluorite like spacer layers, revealing that this steric parameter profoundly modifies the nickel oxide trilayer electronic structure. Our results identify better understanding of the co-existence region between metallic and charge-ordered phases as a priority for expanding the range of bulk, ambient-pressure nickelate superconductivity and establish spacer-layer engineering as a design tool for exploring this regime of phase competition.
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All authors Hyo-Bin Ahn, Xinglong Chen, Hong Zheng, Yu Zhang, Ramakanta Chapai, Yu Li, Arashdeep S. Thind, Robert F. Klie, Michael R. Norman, Ulrich Welp, J. F. Mitchell, Daniel Phelan

Superconductivity

Crystallographic imperfections and exotic superconductivity of UBe$_{13}$

Andreas Leithe-Jasper, Markus König, Yusei Shimizu et al. (+16 more)

The symmetry of the superconducting gap is related to the symmetry of the crystal structure. In unconventional superconductors, big changes of the critical temperature, critical field or even the gap structure can happen even for small perturbations of the lattice. In this letter, we use microstructuring to study aluminium-free crystals of UBe$_{13}$ which are expected to be closer to "perfect" material than previously studied aluminium-grown single crystals. We compare the effect of minuscule imperfections on the value of the critical temperature and critical field, which, in the case of UBe$_{13}$, has drastic effects, supporting its unconventional nature. We conjecture that this likely arises from the oxidation state of uranium, which is sensitive to its crystal environment. Our findings suggests that uranium-based materials provide not only an excellent reservoir of new unconventional phenomena, but also a way to identify the gap symmetry of unconventional superconductors.
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All authors Andreas Leithe-Jasper, Markus König, Yusei Shimizu, Konstantin Semeniuk, Primož Koželj, Mitja Krnel, Paul Simon, Wilder Carrillo-Cabrera, Nazar Zaremba, Marcel Naumann, Ulrich Burkhardt, Thomas Doert, Yurii Prots, Alfred Amon, Elena Gati, Matthias Vojta, Yuri Grin, Elena Hassinger, Eteri Svanidze

Wednesday 16 September

Superconductivity

Pairing symmetry and superconductivity from long-range Coulomb interactions in the extended t-t'-t”-Jz model for cuprates

U.A. Diaz-Reynoso and F. Mireles

We investigate the pairing symmetry of the ground-state phase diagram of an extended two-dimensional $t$-$t'$-$t''$-$J_z$ model, in which the first-, second-, and third-nearest neighbor electron hopping terms ($t,t'$ and $t''$) and an anisotropic Ising-like antiferromagnetic interaction $J_z$ are treated on the same framework. We find that the dominant pairing symmetry depends sensitively on the sign and magnitude of the hopping parameters $t'$ and $t''$, showing pure $p$-wave, pure $d$-wave, or coexisting pairing channels, highlighting the decisive role of these terms. We further explore the role of the long-range and short-range repulsive interactions in the formation of pairs and analyze the specific case of hole-doped cuprates. Our results indicate that repulsive interactions favors hole pair escaping of the stripe domains. Furthermore, pairing correlation calculations strongly suggest that repulsive Coulomb interactions drive reentrant superconducting behavior at experimentally observed underdoped regime ($\delta\approx0.07$ to $0.15$), as in Neodymium-based cuprates.
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Superconductivity

Surface-condition-mediated supercooling of the A-B transition in confined superfluid helium-3

Daksh Malhotra, Aymar Muhikira, Alexander J. Shook et al. (+1 more)

Chiral $p$-wave superconductors and superfluids are central model systems in the search for topological quantum matter, but in reduced dimensions their properties are inseparable from the boundary conditions imposed by surrounding surfaces. Superfluid $^3$He provides a uniquely clean, well-established spin-triplet $p$-wave condensate in which these boundary conditions can be engineered directly. Here, we use a fourth-sound Helmholtz resonator to study the A–B phase transition of $^3$He confined to a $1.8\,\mu$m cavity after preplating the internal surfaces with $\sim4$ atomic layers of $^4$He. In contrast to our previous fourth-sound measurements, which showed no resolvable hysteresis in the transition temperature, the preplated device exhibits a clear separation between cooling and warming transition temperatures together with stochastic run-to-run variations. The hysteresis decreases toward the highest pressures studied, consistent with previous observations of the pressure dependence of $^4$He-mediated quasiparticle boundary scattering. These results demonstrate that surface preparation can qualitatively modify first-order transition kinetics in confined $^3$He, making boundary conditions an experimentally accessible control parameter for metastability and for future studies of reduced-dimension topological superfluid states.
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All authors Daksh Malhotra, Aymar Muhikira, Alexander J. Shook, John P. Davis

Superconductivity

Multiple Andreev Reflection Spectroscopy of Spin-Resolved Proximity States

A.A. Golubov and M.Yu. Kupriyanov

We predict a coherent multiple Andreev reflection regime in superconducting nanoconstrictions with parallel proximity-induced exchange fields in the electrodes. The subharmonic gap structure is controlled by reconstruction of the spin-resolved electrode spectra, rather than by a rigid displacement of BCS gap edges. This produces split and redistributed anomalies governed by the interval between the outer gap edge and a shifted exchange induced density of states singularity. The effect survives channel transparency averaging and provides a direct transport probe of spin-resolved proximity states.
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Superconductivity

Encapsulation epitaxy of air-stable monolayer superconducting films for quantum circuits and qubits

Xudong Zheng, Sameia Zaman, Kenan Zhang et al. (+22 more)

Two-dimensional (2D) superconductors are an emerging platform for strongly correlated physics and quantum information science. Their reduced dimensionality, atomically flat interfaces, and high crystallinity are attractive for realizing compact lumped-element devices in superconducting circuits. However, synthesizing large-area, monolayer 2D superconductors remains challenging because of their susceptibility to oxidation. Here, we report an "encapsulation epitaxy" mechanism that enables the growth of large-area, air-stable, monolayer superconducting NbSe2 films and explore their use in superconducting quantum circuits. A 2D encapsulation layer, such as graphene or hexagonal boron nitride (hBN), pre-deposited on a 3D substrate (e.g., SiO2 or Si3N4), serves both as a template for epitaxial growth of monolayer NbSe2 (1L-NbSe2) underneath it and as a protective cover. This approach produces uniform, large-area (>1-inch) 1L-NbSe2 with greatly enhanced ambient stability, enabling device fabrication in air. The resulting 1L-graphene/NbSe2 heterostructures exhibit robust superconductivity (Tc ~ 1 K) and enhanced charge density wave order (TCDW ~ 177 K), indicative of high material quality. We further integrate 1L-NbSe2 into superconducting circuits using oxidation-free transfer and superconducting edge-contact techniques. The 1L-NbSe2 exhibits a measured kinetic inductance LK ~ 0.7 nH/square, making it suitable for quantum circuits requiring high-kinetic-inductance elements. Encapsulation epitaxy thus provides a route to air-stable 2D superconductors and van der Waals heterostructures, with potential for wafer-scale, monolithic fabrication of superconducting quantum circuitry.
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All authors Xudong Zheng, Sameia Zaman, Kenan Zhang, Connor A Occhialini, Haowei Xu, Zhien Wang, Fangyuan Liu, Luiz Gustavo Pimenta Martins, Sejoon Lim, Tianyi Zhang, Tilo H. Yang, Jiangtao Wang, Yunyue Zhu, Zachariah Hennighausen, Sein Park, Steven Vitale, Kevin Tibbetts, Stephen Margiotta, Phillip Kim, Cong Su, Ju Li, Riccardo Comin, William D. Oliver, Joel Î-j. Wang, Jing Kong

Superconductivity

Unified Transport and Susceptibility Analysis of a Thin BSCCO Film: From Local Pairing to Global Phase Coherence

Santu Prasad Jana, Bismaya Ranjan Nayak, Sohini Guin et al. (+3 more)

The superconducting transition in thin high-$T_c$ films can be significantly broadened by disorder, spatial inhomogeneity, and phase fluctuations. Here, we study the transition in a 30-nm-thick sputter-grown Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) film through electrical-transport and ac-susceptibility measurements. The resistive transition is examined using a Gaussian distribution of local $T_c$ values and the Ambegaokar–Halperin model, describing spatial variations in superconductivity and thermally activated phase dynamics, respectively. To relate the transport and magnetic responses, we extend the Choy–Stoneham susceptibility model by introducing the superconducting fraction extracted from transport and a temperature-dependent connectivity factor representing the gradual development of Josephson coupling among superconducting regions. The results identify separate temperature regimes corresponding to the onset of local superconductivity, expansion of the superconducting fraction, development of magnetic screening, and establishment of global phase coherence. The transition therefore proceeds progressively, from locally superconducting regions to a connected, phase-coherent state. This analysis provides a common framework for relating spatial inhomogeneity, dissipative phase dynamics, and magnetic screening in inhomogeneous superconducting films.
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All authors Santu Prasad Jana, Bismaya Ranjan Nayak, Sohini Guin, Akshay Naik, Dhavala Suri, Arindam Ghosh

Superconductivity · Cross-listed from cond-mat.str-el

Valley-Helical Superconductivity Driven by Repulsion and Quantum Geometry: Applications to Time-Reversal Symmetric Rhombohedral Graphene

Julian May-Mann, Trithep Devakul and Gal Shavit

Motivated by the recent experimental signatures of chiral superconductivity in valley-polarized graphene- and transition-metal-dichalcogenide-based systems, we investigate the possibility of chiral pairing in the absence of valley polarization or time-reversal symmetry breaking. For systems with opposite Berry curvature in the two valleys, we find that overscreened repulsion (i.e., the Kohn-Luttinger mechanism) can indeed induce pairing of opposite chiralities in each valley, producing a valley-helical state. Moreover, the valley-helical state can outcompete more conventional intervalley paired states at intermediate coupling, despite only the latter having a weak-coupling instability. Using a realistic model of time-reversal-symmetric rhombohedral graphene, we find extended ranges of fillings and displacement fields with dominant valley-helical superconductivity. This occurs for all layer numbers considered here, $4\leq N\leq 8$. We show that the valley-helical states can display topologically projected edge modes, unusual magnetic responses, and intertwined Kekulé-like bond-order; features that can be used to distinguish them from more conventional superconductors.
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Superconductivity · Cross-listed from cond-mat.mtrl-sci

Nitrogen based electride superconductor Nb5Ir3N under pressure: multifunctional physical properties from DFT based first-principles investigation

M. Abdul Hadi Shah, J.H. Abir and S.H. Naqib

Discovery and study of superconducting electrides have opened new avenues in condensed matter physics, driven by their intriguing multifunctional features spanning ambient and high-pressure regimes. This work investigates the ternary nitride superconductor Nb5Ir3N under pressure ranging from 0 to 20 GPa via density functional theory based simulations. Estimated structural parameters agree well with the available data, confirming their reliability and supporting the validity of this analysis. Negative formation enthalpy, together with evaluated elastic constants and phonon spectra, confirm the structural, thermodynamic, mechanical, and dynamical stability of Nb5Ir3N over the entire pressure range. Pressure dependent elastic constants and polycrystalline elastic moduli are investigated. The compound is categorized as ductile in light of estimated mechanical indices. Elastic anisotropy factors indicate that Nb5Ir3N remains anisotropic, with the degree of anisotropy gradually decreasing as pressure increases. Electronic band structure and density of states are calculated with and without spin orbit coupling to investigate its influence on the electronic structure. Calculated optical response reveals substantial intraband contributions in the low energy region, intense ultraviolet absorption, and strong reflectivity. The spectra exhibit optical anisotropy and a broadening at higher pressures. Pressure induced superconducting features are also discussed qualitatively.
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Tuesday 15 September

Superconductivity · Cross-listed from cond-mat.mtrl-sci

Nonlinear electron-phonon interactions from first principles

Zhenbang Dai and Feliciano Giustino

Electron-phonon interactions underpin a variety of phenomena, ranging from transport and superconductivity to polarons and ultrafast carrier dynamics. Despite being one of the most intensely studied subjects in condensed matter physics, research on electron-phonon physics mostly focused on linear, first-order couplings. Second- and higher-order nonlinear couplings are commonly ignored because their calculations are too demanding and we lack computational frameworks that can provide both diagonal and off-diagonal coupling matrix elements. In this work, we report a theory and computational method for computing nonlinear electron-phonon interactions of any order in real materials. Our approach combines the advantages of unit-cell calculations of electron wavefunctions and supercell calculations of phonon perturbations, is systematically improvable, can be used with either semilocal or nonlocal exchange-correlation functionals, and is amenable to Wannier-Fourier interpolation. As a first proof of concept, we illustrate this method by computing second-order electron-phonon coupling matrix elements in diamond, lithium fluoride, and graphite as representative nonpolar semiconductors, polar semiconductors, and metals, respectively. Furthermore, we generalize the ab initio polaron equations to second-order electron-phonon couplings, and we show that second-order couplings are essential to achieve quantitative accuracy in polaron formation energies and hopping barriers. The present methodology will find application in the study of all properties and phenomena that are currently being investigated within the linear electron-phonon coupling approximation, from phonon-mediated superconductivity to excited-states dynamics, both in harmonic and anharmonic systems.
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Superconductivity · Cross-listed from cond-mat.str-el

La substitution studies on the heavy-fermion superconductor CeRh$_2$As$_2$

Sushma Lakshmi Ravi Sankar, Manuel Brando, Jochen Wosnitza et al. (+1 more)

CeRh$_2$As$_2$ has been receiving considerable attention due to its unusual two-phase superconductivity. The superconducting (SC) phase appears at $T_{\mathrm{c}}$ = 0.35 K in an ordered state (phase I) of the Ce-4$f$ moments, which develops below $T_{\mathrm{0}}$ = 0.55 K. The microscopic nature of phase I has not been fully established yet. We report a single-crystal study of the effect of La substitution on these low-temperature phases in Ce$_{1-\textit{x}}$La$_\textit{x}$Rh$_2$As$_2$ up to $x$ = 0.1. The lattice parameters increase monotonically with $x$, corresponding to an effective negative pressure of approximately -0.3 GPa for $x$ = 0.1. While the Ce$^{3+}$ local valence state and the non-Fermi-liquid behavior are preserved, the resistivity coherence maximum $T^{*}_{\mathrm{max}}$ $\approx$ 45 K in the pristine sample shifts to lower temperatures with increasing $x$, indicating a suppression of the Kondo energy scale upon lattice expansion. On the other hand, both $T_{\mathrm{c}}$ and $T_{\mathrm{0}}$ are rapidly suppressed to below 0.1 K for $x$ $>$ 0.05. Notably, such a rapid decrease of $T_{\mathrm{c}}$ under moderate negative pressure is unexpected, but is rather consistent with substitution-induced disorder effect which leads to strong pair breaking in unconventional superconductors. The observed fragility of phase I under both pressure and disorder may imply its itinerant origin.
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All authors Sushma Lakshmi Ravi Sankar, Manuel Brando, Jochen Wosnitza, Seunghyun Khim

Superconductivity

Interplay of dimerization and quasiperiodicity in the superconducting proximity effect of a one-dimensional hybrid ring

Sourav Karmakar and Santanu K. Maiti

Recent studies of the superconducting proximity effect in quasicrystalline and topological systems have opened up a new research direction for exploring how quasiperiodicity and topology influence proximity induced superconductivity. In this work, we investigate spatial variation of the proximity induced pairing amplitude in a hybrid ring composed of a spin singlet superconductor and a normal region described by three different lattice models using the self-consistent Bogoliubov-de Gennes formalism. We first consider the normal region described by the diagonal Aubry-André-Harper (AAH) model. Increasing the quasiperiodic potential enhances spatial fluctuations in the induced order parameter, while progressively suppressing its magnitude in the normal region. Beyond the localization transition, proximity-induced pairing is strongly diminished due to the localized nature of the underlying electronic states. The normal region is then modeled by the Su-Schrieffer-Heeger (SSH) chain to investigate the effect of hopping dimerization. Weak dimerization introduces oscillatory modulations in the induced pairing that extend deep into the normal region, whereas strong dimerization confines these oscillations and significantly reduces the penetration of superconducting correlations. Finally, we study the combined SSH-AAH model to explore the interplay between the dimerized hopping and quasiperiodicity. The results show that the SSH dimerization determines the oscillatory behavior and penetration of the induced pairing, while the AAH potential enhances spatial inhomogeneity and further reduces its magnitude. Together, these two effects provide a versatile means of controlling proximity-induced superconductivity in quasiperiodic hybrid systems.
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Superconductivity

NaFeP: first pairing prediction for an unmade 111 iron phosphide, and the pnictogen-height rule inside one compound

Reinaldo Inácio

Companion to arXiv:2609.10614, which built a physics judge that classifies the gap symmetry of a superconductor candidate from its structure and showed that generation in the wire families re-finds the known canon. Here the judge is applied end to end to the single compound that survived its own screen. From 58 iron-pnictide compositions in the electronic window of BaFe$_2$As$_2$ absent from the Materials Project, a zero-cost literature-and-stability screen leaves one with no bulk synthesis and no band-structure, magnetic or pairing calculation: NaFeP (111-type, P4/nmm, Fe$^{2+}$ d$^6$, arsenic-free). The judge, refined with an explicit Stoner gate and a frame-independent symmetry reader, returns a paramagnetic DFT ground state and an s$\pm$ gap: a clean sign reversal between the hole cylinders at Gamma and the electron sheets at the zone corner, at nesting vector (1/2,1/2,1/2), in every basis and cell tried. Whether the gap is nodeless is where the instrument is tested. The phosphorus height (1.16 Å) lies below the empirical threshold of about 1.33 Åseparating nodal from nodeless pnictides, so the structural rule calls NaFeP nodal. The sealed verdict on a metal-only basis said nodeless; a d+p basis that passes the same gates puts the inner hole cylinder at the node threshold (min|g|/max|g| = 0.07-0.10), and a scan of the phosphorus height reproduces, inside one compound, the ingredients of the height rule: a third hole pocket, a switch of the nesting vector to (1/2,1/2,0), a monotonic approach to the Stoner instability, and near-degeneracy of s$\pm$ and d channels beyond 1.33 Å. The prediction we stand behind is the d+p reading: an s$\pm$ superconductor with a near-nodal inner hole sheet, on the LiFeP side of the rule, with the disagreement between bases reported, not resolved by choice. No Tc is asserted; the discriminating measurement, nodal or nodeless, is stated in advance.
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Monday 14 September

Superconductivity · Cross-listed from cond-mat.mes-hall

Probing proximity-induced superconductivity in bilayer graphene using gate-defined quantum dots

Philipp Schmidt, Mathies Funke, Till Tenger et al. (+8 more)

Van der Waals heterostructures offer a direct way of combining two-dimensional (2D) materials with different electronic properties, such as 2D semiconductors, metals, and superconductors, in a single device. Bilayer graphene (BLG) is particularly attractive in this context, as its electrically tunable band gap enables local control of tunnel barriers and quantum dots. Here, we realize an all-2D hybrid platform based on BLG proximitized by superconducting NbSe$_2$. Using local electrostatic gates, we define tunnel barriers and quantum dots at different distances from the lateral superconductor-semiconductor interface. The quantum dots serve as local spectroscopic probes of the proximitized BLG channel segment, forming tunable superconductor-quantum dot-normal conductor junction devices. Coulomb blockade and finite-bias spectroscopy reveal a proximity-induced superconducting gap of up to $80\,\mathrm{\mu eV}$ and allow to track its evolution with increasing distance from the NbSe$_2$ contact. We find that the local density of states remains suppressed over distances exceeding 1 $\mu$m, consistent with superconducting proximity through a highly ballistic BLG channel. Our results show that BLG-superconductor hybrids offer a controllable platform where quantum dots and quantum point contacts can be well combined with superconductivity.
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All authors Philipp Schmidt, Mathies Funke, Till Tenger, Katarina Stanojević, Hubert Dulisch, Kenji Watanabe, Takashi Taniguchi, Fabian Hassler, Christian Volk, Szabolcs Csonka, Christoph Stampfer

Superconductivity

Current - voltage characteristics in magnetic field for layered superconductor with intrinsic pinning of Josephson vortices

T.B. Charikova, V.N. Neverov, M.R. Popov et al. (+3 more)

The results of measurements of the transverse voltage in the Hall configuration, Uy, under conditions of motion of the charge and vortex subsystems in orhtogonal magnetic and electric fields on epitaxial films of the layered high-temperature superconductor Nd2-xCexCuO4 are presented. the motion of Josephson vortices across CuO2 layers (along the c-axis) is analyzed considering the effects of intrinsic pinning. An exponential dependence of critical depinning current jc(B) for Josephson vortices is established in contrast to the power law dependences for Abrikosov vortices. With increasing external magnetic field after the depinning by a magnetic field at B=Bc, the onset of the vortex creep regime, which evolves into the flux-flow regime at crossover field B=Bf, is observed.
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All authors T.B. Charikova, V.N. Neverov, M.R. Popov, S.D. Popov, N.G. Shelushinina, A.A. Ivanov

Superconductivity

Mode-selective phonon effects on magnetism and superconductivity in trilayer nickelates

C. Alexander Baum, Jun Zhan, Congcong Le et al. (+2 more)

The discovery of high-$T_c$ superconductivity in multilayer nickelates has stimulated intense interest, yet the microscopic origins of their correlated states remain debated. Recently the observed isotope effect of the magnetic state in trilayer nickelates highlights the important role of lattice vibrations in shaping these systems. Here, we investigate the impact of electron-phonon coupling (EPC) on magnetism and superconductivity in trilayer nickelates using ab initio and functional renormalization group calculations. We identify out-of-plane oxygen breathing modes with pronounced EPC and construct symmetry-resolved, layer-dependent couplings. Their relative phases across the three layers generate distinct retarded interactions, producing mode-selective enhancement or suppression of spin-density-wave order and $s_\pm$-wave superconductivity. This selectivity arises from screening-mediated spin-phonon interplay across a hierarchy of energy scales: EPC modifies charge screening at higher energies, reshapes magnetic fluctuations at lower energies, and ultimately influences pairing. Selected modes yield negative isotope coefficients for the magnetic state, qualitatively consistent with experiments. Our results establish how interlayer phonons modify electronically driven order and provide a microscopic framework for understanding isotope effects in trilayer nickelates.
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All authors C. Alexander Baum, Jun Zhan, Congcong Le, Ronny Thomale, Xianxin Wu

Superconductivity

Optical and magnetic signatures of drive-enhanced coherence in phase-disordered superconducting bilayers

Duilio De Santis, Sambuddha Chattopadhyay, Marios H. Michael et al. (+4 more)

Recent experiments demonstrating superconducting-like characteristics in materials above their equilibrium transition temperatures following strong sub-picosecond optical excitation have stimulated theoretical interest in phase-disordered superconductors subject to a periodically modulated superfluid stiffness. Previous analyses of the single-layer optical conductivity have shown that such a drive can enhance the low-frequency $1/\omega$ response, even when the period-averaged stiffness remains unchanged. We extend these studies to bilayer systems, such as the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) cuprate, by considering drives with different symmetries. Specifically, we investigate a layer-symmetric drive, in which the two planes stiffen and soften simultaneously, and a layer-antisymmetric drive, in which one plane stiffens while the other softens. Both drives enhance correlations of the layer-averaged phase and generate comparable superconducting-like optical conductivities. Only the symmetric drive, however, strengthens relative-phase locking and the associated counterflow response to a magnetic field parallel to the planes. Thus, an enhanced $1/\omega$ optical conductivity does not by itself imply enhanced magnetic screening. In view of applications to driven YBCO, where superconducting-like in-plane optics and transient diamagnetism have both been reported, our result provides a symmetry diagnostic: within a phase-synchronization mechanism, the experimentally active low-frequency stiffness modulation must contain a substantial layer-symmetric component.
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All authors Duilio De Santis, Sambuddha Chattopadhyay, Marios H. Michael, Andrea Cavalleri, Gil Refael, Patrick A. Lee, Eugene A. Demler

Superconductivity · Cross-listed from hep-th

Exact group invariant scar towers in two dimensional gauge theories

João Barata, Kiryl Pakrouski and Andrey V. Sadofyev

We construct exact many-body scar towers in two dimensional gauge theories with two flavors of massless fundamental fermions. For oppositely charged fermions, the scar subspace is generated by a gauge-neutral $\eta$-pairing operator, and admits a purely algebraic construction. Exact diagonalization methods reveal anomalously low entanglement and long-range pair correlations in the scar states. Charge conjugation maps the $\eta$-tower to a vector-flavor polarization sector of the equal charge model, yielding an interpretation as a coherent flavor mode with Josephson-like phase dynamics. Finite fermion masses mix the protected pair with an orthogonal channel and destroy the exact tower. The identified scar tower can be algebraically realized in a large family of lattice field theories, revealing a universal character of these states. Our results provide an analytical construction of invariant scar subspaces in a gauge theory with a nontrivial continuum interpretation.
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Superconductivity

Topological superconductors and Majorana fermions based on $X$-wave magnets with $X=p,d,f,g,i $

Motohiko Ezawa

We study superconductors coupled with $X$-wave magnets with $X=p,d,f,g,i$. They are essentially different between altermagnets with $X=d,g,i$ and odd-parity magnets $X=p,f$ due to the compatibility of the spin-singlet formation of the $s$-wave superconductor. The altermagnets and odd-parity magnets show rich superconducting phase diagrams depending on the strength of the magnetic order, the chemical potential and the ratio of the on-site and nearest-neighbor interactions. Especially, we find that the altermagnetic order stabilizes the chiral ($p_{x}+ip_{y}$)-wave superconductor, which is a class D topological superconductor hosting Majorana chiral edge states. It is characterized by the Chern number. Especially, there emerge two chiral edge modes in the $i$-wave altermagnet, which are characterized by the Chern number 2. On the other hand, the $p$ ($f$)-wave odd-parity magnetic order stabilizes the $p$ ($f$)-wave topological superconductor, which is a class DIII topological superconductor hosting Majorana flat bands. It is characterized by the winding number. Majorana flat bands are robust in the presence of the mixing with the $s$-wave superconductivity, which is inevitable for odd-parity magnets although the magnitude of the mixing is tiny.
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Superconductivity

Stripe-Like Superconducting Enhancement and Coexisting Magnetic Texture in an Infinite Layer Nickelate

Ryan Laing, Dung Vu, Jacob Pfund et al. (+7 more)

Despite their promise as structural and electronic analogs to cuprates, nickelate thin films consistently exhibit broadened superconducting transitions in many experiments that remain poorly understood. Local measurements are ideal for revealing the presence of defects or competing phases, which may cause this transition broadening through a phase-separation. Here, we use scanning SQUID microscopy to investigate local superfluid density and magnetic texture of optimally doped $\mathrm{Nd}_{1-x}\mathrm{Eu}_{x}\mathrm{NiO}_{2}$ (NENO) (x=0.25) grown via molecular beam epitaxy. We observed a weakly-magnetic texture coexisting with superconductivity. Spatially resolved susceptibility imaging reveals a highly non-uniform superconducting state, characterized by a robust stripe-like enhancement pattern appearing near the phase transition. By resolving the mesoscopic landscape of electronic and magnetic inhomogeneities, these findings suggest that a competing magnetic phase is a primary contributor to the unusually broad superconducting transitions of these optimally doped NENO samples. Understanding the uncovered superconducting enhancement may provide a path to raising superconducting temperatures in these materials.
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All authors Ryan Laing, Dung Vu, Jacob Pfund, Wenzheng Wei, Frederick J. Walker, Haiyan Tan, Pavel Volkov, Menka Jain, Charles Ahn, Ilya Sochnikov

Superconductivity · Cross-listed from cond-mat.str-el

Charge order before superconductivity in the doped kagome Dirac spin liquid

Yasir Iqbal

Doping a quantum spin liquid is expected to produce a superconductor. We show that for the Dirac spin liquid on the kagome lattice the doped charge orders first. The charge is carried by a chargon doublet whose two gauge components are degenerate on the square and triangular lattices, and the kagome lattice, as the line graph of the honeycomb lattice, splits them. One component disperses and condenses in four valleys. The other has flat lowest bands and stays empty over a window in the bare mass whose width, $(\sqrt6-1)t$, follows from the band structure alone. Within that window, in the mean-field chargon theory constructed from the projective symmetry group of the spin liquid, the doped state forms charge and loop-current crystals with the electromagnetic U(1) unbroken and no superconductivity. Pairing requires a gapped descendant of the spin liquid as the parent and then takes the form of a pair-density wave, with no uniform $d+id$ channel at leading order and none at any order for the crystals at the $M$ point. The predicted charge order is odd under the twofold rotation and has no cubic invariant, which distinguishes it from the charge order of the kagome metals.
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Superconductivity

Wrinkles and Magnetic Flux Trapping in Graphite Nanoflakes: A Possible Source and Manifestation of Room-Temperature Superconductivity

Muhammad Saad, Sergey I. Nikitin, Dmitry A. Tayurskii et al. (+1 more)

The paper reports on a possible manifestation of local high-temperature superconductivity in graphite nanoflakes obtained from the bulk pyrolytic graphite by an extended grinding and subsequent annealing at 670 K in the air. Analysis of transmission electron microscopy (TEM) images shows that such treatment leads to a formation of high-density wrinkle-type defects at the basal-plane surface. The sample reveals the magnetic flux trapping - one of the key signatures of superconductivity - that persists up to 390 K and above. At the same time, both the as-ground (unannealed) and annealed in the vacuum samples manifest negligible flux trapping, and no wrinkles in their TEM images were found. The direct correlation between the appearance of high-density wrinkle arrays and the detection of the trapped magnetic flux above the room temperature strongly suggests that wrinkled regions serve a source of local high-temperature superconductivity in graphite.
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All authors Muhammad Saad, Sergey I. Nikitin, Dmitry A. Tayurskii, Roman V. Yusupov