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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Wednesday 14 October · still arriving

Superconductivity

Nodal nematic superconductivity in moiré graphene

Shuwen Sun, Grgur Palle, Kenji Watanabe et al. (+4 more)

Unconventional superconductivity can emerge in various forms in strongly correlated systems. Historically, the most extensively studied examples include nodal and triplet pairing, which were experimentally detected in cuprates and superfluid helium-3, respectively. Beyond these canonical cases, there are also nematic superconductors, in which the rotational symmetry of the lattice is broken in the superconducting state. While there are several examples of nematic superconductors whose pairing emerges inside a preexisting nematic normal state, experimental evidence for superconducting states that intrinsically break rotational symmetry on their own has been exceedingly scarce. Here, we report the experimental realization and identification of an intrinsic multi-component nodal nematic superconducting state in magic-angle twisted trilayer graphene (MATTG). By integrating tunneling spectroscopy with quantum transport measurements, we directly probe the symmetry of low-energy superconducting quasiparticles. Deep inside the superconducting state, we observe a pronounced two-fold anisotropy in the zero-bias tunneling conductance under an in-plane magnetic field, which is strongly suppressed together with superconductivity as the temperature or perpendicular magnetic field is increased. Remarkably, the major axis of anisotropy rotates by up to $90^{\circ}$ across the superconducting dome, indicating an intrinsic, field- and density-tunable symmetry of the low-energy excitations. These behaviors imply a nodal superconducting state in MATTG that spontaneously breaks the six-fold rotational symmetry of the moiré superlattice. Our results establish moiré graphene as a versatile platform for realizing and interrogating nodal nematic superconductivity, opening a new avenue for exploring unconventional symmetry-breaking superconducting states in two-dimensional materials.
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All authors Shuwen Sun, Grgur Palle, Kenji Watanabe, Takashi Taniguchi, Rafael M. Fernandes, Pablo Jarillo-Herrero, Jeong Min Park

Superconductivity

Magnetic Susceptibility of Superconducting Films in the Meissner-London Regime

Ruslan Prozorov

Extracting the London penetration depth, $\lambda(T)$, from the measured magnetic susceptibility, $\chi(T)$, of a superconducting platelet in a perpendicular magnetic field requires knowing the function $\chi(\lambda)$ computed for the actual sample geometry. Thin films are increasingly important in quantum computing and superconducting electronics, most of which operate at ultra-low excitation fields in the vortex-free regime. Understanding their electromagnetic response in the Meissner–London (finite-$\lambda$, no vortices) state is therefore essential for extracting intrinsic properties such as the London penetration depth and superfluid phase stiffness. While numerous prior works have addressed different aspects of this problem, here we focus on providing an analytical semi-empirical representation of $\chi(\lambda)$ based on the 2D sheet-current model and establishing the limits of its applicability using two independent full 3D numerical solutions: finite-element modeling and a volume-current integral equation. As a result, we provide $\chi(\lambda)$ for a disc of radius $a$ and thickness $d$ in closed form, valid for all $\lambda$ and for aspect ratios $\delta\equiv d/(2a)\le0.05$, and accurate to $0.1\%$. The two-dimensional sheet description with Pearl length $\Lambda=2\lambda^2/d$ is adequate for $\delta \lesssim10^{-4}$ and $\lambda \gtrsim 4d$. Thicker films require the full closed form, which includes the three-dimensional corrections. Using the same numerical analysis, we provide an empirical formula that maps rectangular samples—the geometry most commonly used in experiments—onto a disc with an “effective radius” for subsequent calculation of $\chi(\lambda)$, and formulate a complete calibration protocol for whole-sample susceptibility measurements.
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Superconductivity · Cross-listed from cond-mat.quant-gas

Universal enhancement of the pairing gap in a dilute Fermi superfluid

Silas R. Beane, Roland C. Farrell and Cameron P. Shotwell

At zero temperature, a dilute Fermi gas with an attractive contact interaction forms a superfluid with gapped fermionic excitations. We show that the gap at weak coupling can be organized as a trans-series expansion in the interaction strength and the asymptotic Gor'kov-Melik-Barkhudarov gap. Low-energy virtual quasiparticles generate corrections that are linear in the gap with a leading coefficient nearly 25$\times$ larger than the other known coefficients. This substantially increases the gap at intermediate densities, highlighting that systematic corrections are crucial for quantitative predictions.
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Superconductivity

Transport regimes in tunnel ferromagnetic Josephson junctions

R. Satariano, A. A. Golubov, E. Raja et al. (+13 more)

We present a systematic study of the temperature-dependent transport properties of Nb- and Al-based superconductor–insulator–thin superconductor–ferromagnet–superconductor (SIsFS) Josephson junctions. By varying the superconducting material and the thickness of the intermediate superconducting layer, we identify three distinct transport regimes: a tunnel-dominated SIS-like regime, a temperature-driven crossover, and a complex weak-link regime. A unified Usadel-based description connects these regimes through the self-consistent pair potential of the intermediate s-layer. While for Al-based devices an SIS-like transport regime is observed, a self-consistent calculation of the s/F/S proximity electrode, combined with perturbative tunnel coupling to the outer electrode, accounts for Nb devices with different thickness of the superconducting interlayer. The calculation places the 10-nm-interlayer sample in a proximity-supported, strongly suppressed s state, close to the SInFS limit in which the interlayer is effectively driven normal (n). A reduced nonlinear long-junction description captures the same limiting physics. The evolution of I_cR_N(T) thereby provides a direct experimental signature of the three transport regimes. Beyond the fundamental interest, the results establish practical design guidelines: Nb junctions with sufficiently thick s interlayers are well suited for high-speed cryogenic memory elements and tunable superconducting parametric amplifiers, while Al junctions are natural candidates for coherent superconducting quantum circuits
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All authors R. Satariano, A. A. Golubov, E. Raja, R. Ferraiuolo, F. Calloni, H. G. Ahmad, A. Vettoliere, N. Poccia, L. Parlato, G. Ausanio, C. Granata, G. P. Pepe, M. Yu. Kupriyanov, F. Tafuri, D. Montemurro, D. Massarotti

Superconductivity

Bulk superconductivity in quintuple-layer square-planar nickelate La6Ni5O12

Mengzhu Shi, Mei Du, Hongyu Li et al. (+5 more)

Nickelate superconductors, sharing a similar electronic environment with cuprate superconductors, offer a promising platform for unraveling the mechanism of high-temperature superconductivity. However, the realization of superconductivity in these systems has so far been restricted to thin films or bulk samples under high-pressure. This limits the applicability of various experimental probes to study the mechanism of superconductivity, and hinders the potential application of the nickelate superconductors. Exploring bulk superconductivity at ambient pressure is therefore essential to understand its origin and enable broader experimental research. Here, we report the realization of bulk superconductivity at ambient-pressure in quintuple-layer square-planar (SP) nickelate La6Ni5O12. The SP phase is obtained by performing topotactic reaction on the quintuple-layer Ruddlesden-Popper (RP) compound La6Ni5O16. Optimized reduction condition endows La6Ni5O12 with a superconducting state that exhibits onset transition temperature of ~ 18 K, as verified by both zero resistance and diamagnetic shielding effect. Pronounced hysteresis loops revealed by magnetization and torque magnetometry demonstrate the occurrence of vortex pinning. Specific heat measurements on a polycrystalline pellet of La6Ni5O12 reveal a clear superconducting transition at ~18 K and a superconducting volume fraction of ~ 25%, which strongly supports the presence of bulk superconductivity. Our results establish an unprecedented bulk sample platform for investigating superconductivity in nickelates at ambient pressure, which paves the way for further experimental studies into the superconducting mechanism.
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All authors Mengzhu Shi, Mei Du, Hongyu Li, Zhiwei Wang, Zhengjie Wang, Ziji Xiang, Tao Wu, Xianhui Chen

Superconductivity

Andreev-reflection spectroscopy of SrPt3P single crystals

Valeriy A. Stepanov, Nikolai D. Zhigadlo, Erik Piatti et al. (+1 more)

The interplay between inversion symmetry breaking and superconducting pairing mechanisms is a long-standing puzzle of solid-state physics. Ternary platinum phosphide SrPt$_3$P is an excellent case study in this regard, since it shares the distorted anti-perovskite crystal structure of non-centrosymmetric superconductor CePt$_3$Si but preserves the inversion symmetry center, enabling a direct comparison between the two compounds. Despite intense scrutiny, the superconducting pairing strength, gap symmetry, and magnitude of SrPt$_3$P remain an open question. Here, we study the gap structure of high-quality SrPt$_3$P single crystals with $T_c = 8.6$ K by using point-contact Andreev-reflection spectroscopy. The spectra systematically showed symmetric conductance maxima and a zero-bias minimum, as is typical of fully gapped superconductors, and dips at finite voltage $V_\mathrm{dip}$ due to the local transition of the contact region to the resistive state. In contacts with high resistance, where the dips fall sufficiently far from the gap features, the conductance curves can be well fitted with a single isotropic $s$-wave gap. The temperature dependence $\Delta(T)$ of the gap amplitude extracted from the fit is in good agreement with the one predicted by the BCS theory, with a zero-temperature mean value $\Delta(0)=1.88\pm0.08$ meV, and a mean gap ratio $2\Delta/k_\mathrm{B}T_c = 5.2\pm0.2$.
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All authors Valeriy A. Stepanov, Nikolai D. Zhigadlo, Erik Piatti, Dario Daghero

Superconductivity · Cross-listed from physics.flu-dyn

Two-Way Coupled Dynamics of Quantized Vortex Bundle Formation in Spatially Periodic Normal-Fluid Turbulence

Hiromichi Kobayashi, Makoto Tsubota and Rio Yokota

Using fully coupled two-way simulations combining the vortex filament model with the Navier–Stokes equations, we discover that quantized vortices in superfluid $^4$He are systematically rolled up into coherent bundles within normal-fluid vortex tubes. Although co-rotating quantized vortices do not aggregate spontaneously, mutual friction with the normal fluid actively drives their bundling, directly inheriting the spatial hierarchy of classical turbulence. Mutual friction excites high-wavenumber normal-fluid vortex tubes adjacent to quantized vortices, while the superfluid forms a nested structure of thick low-wavenumber outer tubes and fine high-wavenumber inner tubes around the vortex lines. A distinct time lag between superfluid kinetic energy and dissipation fuels the energy cascade, giving rise to Kolmogorov's $k^{-5/3}$ spectrum. Crucially, bundle lifetimes are governed by the ratio of the local coupling time scale $t_\mathrm{cl}$ to the Kolmogorov dissipation time scale $\tau_\eta$ ($t_\mathrm{cl}/\tau_\eta < 20$), leading to enhanced thermal stability at higher temperatures. These findings establish a unified mechanism for two-fluid multiscale vortex organization.
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Superconductivity · Cross-listed from cond-mat.str-el

Multiple Superconducting Phases under Pressure for Different Field Directions in UTe2

Dai Aoki, Atsushi Miyake, Ai Nakamura et al. (+7 more)

The existence of multiple superconducting phases is a striking feature of UTe2, arising from the spin and orbital degrees of freedom and supporting a spin-triplet superconducting state. We present field-temperature phase diagrams under pressure for fields applied along the three principal crystallographic axes, as determined by AC calorimetry measurements. The pressure evolution of the multiple superconducting phases is highly anisotropic with respect to magnetic field direction, revealing rich superconducting phenomena arising from complex fluctuations.
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All authors Dai Aoki, Atsushi Miyake, Ai Nakamura, Yoshiya Homma, Dexin Li, Shota Mizuno, Georg Knebel, Daniel Braithwaite, Jean-Pascal Brison, Jacques Flouquet

Superconductivity

Phase Stability and Anisotropic Superconductivity in Ternary M2B4C2 (M=Ca, Mg) Monolayer

Jakkapat Seeyangnok, Udomsilp Pinsook and Graeme J Ackland

In this work, we investigate the phase stability, electronic structure, and superconducting properties of the two-dimensional ternary borocarbides MB$_4$C$_2$ (M = Ca, Mg) using first-principles calculations and fully anisotropic Migdal-Eliashberg theory. Ab initio molecular dynamics, phonon dispersion, and elastic constant calculations confirm that both monolayers are dynamically, thermally, and mechanically stable. Electronic structure analysis reveals an intrinsic metallic nature with a multiband Fermi surface, where states near the $\Gamma$ point are dominated by in-plane B-$p_x$, $p_y$ orbitals, while zone-boundary states are primarily derived from out-of-plane B-$p_z$ orbitals. The electron-phonon coupling is determined to be $\lambda = 0.64$ for both materials, driven by the rigid covalent B-B and B-C honeycomb networks. By solving the anisotropic Eliashberg equations, we predict that CaB$_4$C$_2$ and MgB$_4$C$_2$ are nodeless two-gap superconductors with critical temperatures ($T_c$) of 33.4 K and 33.8 K, respectively, at a Coulomb pseudopotential of $\mu = 0.10$. Furthermore, the superconducting state remains robust under increased Coulomb repulsion ($\mu = 0.13$), retaining elevated $T_c$ values of 31.2 K and 30.9 K, respectively. These findings establish MB$_4$C$_2$ monolayers as promising candidates for nanoscale superconducting applications and enrich the family of two-dimensional multigap superconductors.
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Tuesday 13 October · still arriving

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

Spectral signatures of spin textures and the emergence of a pseudogap

Matteo Quinzi, Andrea Pintus, Tommaso Chiarotti et al. (+2 more)

Periodic solids with strongly interacting electrons display phenomena – from Mott localization to high-temperature superconductivity – that escape conventional band theory. Efforts to address this unconventional physics have been typically cast in the language of Green's function theories and the associated frequency- and momentum-dependent self-energies capturing strong correlations beyond the reach of mean-field theories. Here, we underscore that frequency- and momentum-dependent self-energies emerge also from mean-field solutions that break translational symmetry, and that these self-energies can display the characteristic fingerprints of strong correlations for certain, physically meaningful, spatial patterns. We showcase this perspective by studying the paradigmatic case of the 2D Hubbard model. We find that already at the mean-field level the doped antiferromagnet displays, in addition to stripes, many quasi-degenerate self-consistent solutions with antiferromagnetic textures organized into antiphase domains separated by hole-rich interfaces, reminiscent of the phase-separation scenario. Remarkably, these solutions map into spectral functions that display very distinctively the phenomenology of the pseudogap, including the nodal-antinodal dichotomy of the self-energy, Fermi arcs, and a complementary Luttinger surface. The result is a conceptually simple description of such an intricate many-body state: the pseudogap emerges as the momentum-space spectral portrait of the real-space interplay between antiferromagnetic domains and the metallic interfaces, which echoes the role of spin-fluctuations identified by nonperturbative methods.
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All authors Matteo Quinzi, Andrea Pintus, Tommaso Chiarotti, Massimo Capone, Nicola Marzari

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

Topological superconductivity in layered d-wave altermagnetic metals

Yijie Mo and Zhongbo Yan

We investigate multilayer d-wave altermagnetic metals with odd-parity helical p-wave pairing. In the minimal bilayer system, interlayer tunneling reconstructs the spin-split Fermi surfaces and generates two layer-hybridized sectors with distinct effective chemical potentials. Without Rashba spin-orbit coupling (RSOC), the emergent mirror symmetry and subsystem chiral symmetry stabilize phases with different numbers of helical Majorana edge-mode pairs and additional counterpropagating pairs, jointly characterized by the mirror Chern number and the winding numbers defined along high-symmetry lines. The combination of RSOC and the altermagnetic exchange field gaps out the helical Majorana pairs while, in a certain parameter regime, leaving another pair of counterpropagating Majorana edge modes gapless, giving rise to second-order topological phases with isolated or twofold Majorana corner modes, as well as a hybrid-order phase in which gapless Majorana edge modes coexist with Majorana corner modes. We identify a layer symmetry that elucidates the physical origin of these phases. For an arbitrary N-layer system, such a symmetry naturally arises from the interlayer-coupling matrix, allowing N-fold Majorana corner modes to emerge when all layer-hybridized sectors enter the nontrivial second-order regime. Our results establish the interplay among the layer degree of freedom, odd-parity pairing, and Fermi-surface configurations as a controllable route to engineering topological superconductivity in layered altermagnetic systems.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Transmission representation of the Josephson free energy beyond the short-junction limit

Anton R. Akhmerov

Beenakker's formula, $U(\varphi)-U(0)=\Delta\sum_n\bigl(1-\sqrt{1-\tau_n\sin^2(\varphi/2)}\bigr)$, expresses the zero-temperature energy-phase relation of a short Josephson junction through its normal-state transmission eigenvalues. We generalize this transmission representation beyond the short-junction limit and express the phase-dependent free energy of a time-reversal-symmetric $s$-wave junction as a Matsubara sum over channels with frequency-dependent transmissions. The representation constrains the small-phase expansion of the free energy. In particular, we prove that the normalized ratio of its quartic and quadratic coefficients satisfies $-1\le\alpha_r\le7/80$ at finite temperature. We also construct a four-site junction with $\alpha_r>0$, which disproves the conjectured upper bound $\alpha_r\le0$. So that these results can be trusted without repeating the derivations, every numbered equation is formally verified in the Lean proof assistant.
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Superconductivity

The role of dependency range in thermally coupled superconducting networks

Nahala Yadid, Bnaya Gross, Shlomo Havlin et al. (+1 more)

Interdependent networks can be highly fragile because dependency links amplify local failures into cascades that trigger global collapse. Much of the theoretical understanding comes from geometric network models, where dependency range is imposed externally and coupling strength is distance independent, and it is unclear how these insights apply to real materials, where interdependence arises from physical transport. Here we study thermally coupled amorphous superconducting networks, in which dependencies emerge from current redistribution and heat-mediated interactions governed by thermal diffusivity. Using an experimentally calibrated model of resistively shunted Josephson junctions, we show the system reproduces the three canonical transition regimes: continuous, nucleation-dominated, and mixed-order. However, because heat-driven dependency simultaneously broadens and weakens with distance, long-range thermal coupling can stabilize the superconducting phase upon heating, in contrast with its destabilizing role in geometric models. Cascades are anisotropic, with one-dimensional front propagation. These results offer an experimentally testable perspective complementing the geometric framework.
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All authors Nahala Yadid, Bnaya Gross, Shlomo Havlin, Nadav M. Shnerb

Superconductivity

Phonon-Mediated Superconductivity near Phase Boundaries in Rhombohedral Multilayer Graphene

Konstantinos Sourounis and Peter M. Oppeneer

The recent observation of superconductivity in untwisted multilayer graphene has sparked intense interest in its microscopic origin. As the displacement field and carrier density are varied, superconductivity emerges in several distinct regions with different temperature and magnetic-field responses. This behavior has been attributed to the presence of distinct electronic phases, raising the possibility that different superconducting regions originate from different microscopic mechanisms. While purely electronic pairing mechanisms have been extensively explored, an in-depth examination of phonon-mediated superconductivity is still missing. Here, we solve the coupled selfconsistent Migdal-Eliashberg equations using input from first-principles calculations and incorporating interaction-driven symmetry breaking of the electronic ground state. Varying the carrier density and displacement field, we reproduce the experimentally observed superconducting regions with comparable superconducting properties such as gap and $T_c$ and predict additional, as-yet undiscovered superconducting phases. Our findings exemplify that several superconductivity regions in rhombohedral graphene can be satisfactorily explained by phonon-mediated pairing.
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Superconductivity

Quantitative Impact of Occupation Smearing Methods on Conventional Superconductivity: from Electronic Structure through Lattice Dynamics to Electron-Phonon Coupling

Zhongyu Wan

In first-principles calculations of metallic systems, the discrete electronic mesh in the Brillouin zone necessitates a smearing function and its width (e.g., the widely used 0.02 Ry Methfessel-Paxton scheme) to enforce continuity of energy and occupation at the Fermi surface. However, the quantitative impact of such empirical parameters on phonon-mediated superconductivity has long been overlooked, leading to their habitual selection by mere convention. Here, using hexahydride CsH6 as a benchmark, we systematically investigate the magnitude and propagation mechanism of smearing-induced errors against the parameter-free optimized tetrahedron method. Long-tailed smearing delocalizes the occupation window, causing substantial Fermi-level shifts, flattening of the density of states, and hardening phonon modes, thereby weakening electron-phonon coupling. Conversely, short-tailed smearing suffers from insufficient sampling, producing density-of-states oscillations and enhanced Fermi-surface nesting; via amplified electronic screening and Peierls instabilities, this softens acoustic modes and generates imaginary frequencies, causing CsH6 to be dynamically unstable and discarded. However, the optimized tetrahedron method reveals that CsH6 is dynamically stable under ambient pressure with a remarkable superconducting transition temperature of 121 K. To prevent subjective empirical parameters from obscuring theoretically promising high-temperature superconductors, we derive a simple scaling relation for the smearing width, applicable to different smearing functions, from the free-electron-gas model, offering an objective, precise, and efficient protocol for parameter selection in high-throughput screening.
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Superconductivity

Effects of the choice of the exchange-correlation functional on high-temperature superconductivity predictions in covalent-cage M(BN)5 family

Zhongyu Wan, Guo-Hua Zhong, Cheng-wei Ju et al. (+2 more)

The recent experimental synthesis of covalent cage superconductors has ignited hopes for ambient-pressure high-temperature superconductivity, while the computational strategy for discovering new materials poses challenges for accurately estimating superconducting properties. Here, we quantified the response of electron-phonon coupling (EPC) properties in M(BN)5 superconductors to different exchange-correlation (XC) effects using high-throughput first-principles calculations. We found that the metallic covalent bonds responsible for superconductivity exhibit anomalously high sensitivity to functional selection uncertainty, distinguishing them from conventional superconductors mediated by metallic bonding. The widely used PBE functional systematically overestimates lattice volumes, inducing phonon softening, enhanced EPC, and consequent Peierls instability. In contrast, PBEsol, which is more accurate through enhanced treatment of short-range interactions, eliminates imaginary frequencies in Rb(BN)5 that appear at the PBE level, yielding an outstanding superconducting critical temperature (Tc) of 113 K at ambient pressure and 138 K at 2 GPa. Enhanced electron localization and short-range correlation in covalent bonds manifest through dual mechanisms: phonon hardening at the lattice scale and band widening at the electronic scale, which collectively suppress Peierls distortion and delocalize antibonding states. Pressure-dependent analysis reveals that compression attenuates phonon discrepancies between XC functionals, whereas electronic properties remain notably sensitive to XC-induced perturbations of fine electronic bands near the Fermi level.
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All authors Zhongyu Wan, Guo-Hua Zhong, Cheng-wei Ju, Ruiqin Zhang, Hai-Qing Lin

Superconductivity

Imaging atomic-scale surface structure and superconducting gap of the misfit layered compound (SnS)1.17NbS2

Dongming Zhao, Shamarita Deb, Lichen Ji et al. (+8 more)

Superlattices of van der Waals-coupled two-dimensional materials offer a platform for engineering electronic band structure and fostering emergent quantum states. Misfit layered compounds provide naturally assembled superlattices, in which crystallographically distinct two-dimensional subsystems coexist within a single bulk crystal. Therein, the presence of both commensurate and incommensurate lattice directions introduces structural modulation, which might alter real-space electronic orders such as superconductivity. Here, we investigate the atomic-scale surface structure and local superconductivity of the misfit layered compound (SnS)1.17NbS2 using structural characterization, electrical transport, and low-temperature scanning tunnelling microscopy (STM). While X-ray diffraction and cross-sectional scanning transmission electron microscopy confirm the bulk misfit structure, STM resolves large SnS- and NbS2-terminated terraces, where the surface atomic registry differs from the bulk misfit structure. Furthermore, tunnelling spectroscopy at 2 K reveals nearly identical superconducting gaps on both terminations, and the spatial variation of the gap shows no visible modulation that corresponds to the misfit lattice. These results establish an atomic-scale picture of the surface structure and superconducting states of this naturally assembled misfit heterostructure.
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All authors Dongming Zhao, Shamarita Deb, Lichen Ji, Souvik Das, Paul Simon, Raúl Cardoso-Gil, Steffen Wirth, Walter Schnelle, Sushmita Chandra, Jian-Feng Ge, Claudia Felser

Superconductivity

Observation of stacked vortex structure with number controllable Majorana zero mode in Bi$_{2}$Te$_{3}$/FeTe heterojunction

Xinru Han, Zhaokun Luo, Quanxin Hu et al. (+16 more)

Creation and control of Majorana zero modes (MZMs) in solid devices is the key challenge in realizing fault tolerant topological quantum computation. Here we report spectroscopic evidence for number controllable MZMs in stoichiometric one quintuple layer Bi$_{2}$Te$_{3}$/FeTe heterostructures, together with the observation of stacked vortices in superconducting islands. Low temperature scanning tunnelling microscopy and spectroscopy reveal a long period moire superlattice arising from lattice mismatch, which induces nanoscale modulation of superconducting gaps. Magnetic vortices exhibit a stacked core structure with two layer derived bound state branches: a dispersive branch evolving into an X-shaped pattern away from the vortex center, and a quantum-limit branch characterized by a spatially unsplit MZMs together with discrete integer quantized Caroli-de Gennes-Matricon states. Notably, MZMs are observed in all measured vortices, and their number can be tuned by magnetic field and the size of Bi$_{2}$Te$_{3}$ islands. These results establish stoichiometric 1QL-Bi$_{2}$Te$_{3}$/FeTe as a patterned superconducting interface that enables isolated and few-vortex Majorana configurations, providing a controlled platform for fuse MZMs.
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All authors Xinru Han, Zhaokun Luo, Quanxin Hu, Xingjian Zhu, Xiaohong Pan, Dan Mu, Xinyu Liang, Zulin Xu, Qiheng Wang, Ryan L. Kong, Mingyuan Li, Zhengliang Zhang, Fazhi Yang, Mojun Pan, Xin Liu, Qiangqiang Gu, Baiqing Lv, Gan Wang, Hong Ding

Monday 12 October

Superconductivity · Cross-listed from hep-th

Type I-II/II-I vortices from dimension-six self-interaction

A. Alonso-Izquierdo, A. González-Parra, K. Slawinska et al. (+2 more)

We show that adding a dimension-six self-interaction to the Abelian-Higgs or Ginzburg Landau model results in a remarkable and rich structure of multi-vortex solutions. Besides the standard type I (attractive) and type II (repulsive) regimes, separated by the Bogomol'nyi-Prasad-Sommerfeld limit (BPS), we identify two new possibilities: type I-II vortices, which attract at short distances and repel at long distances; and type II-I vortices, with the opposite behaviour. Specifically, within the type II-I regime, multi-vortex solutions form chunks of the Abrikosov lattice or clusters. Conversely, in the I-II regime, axially symmetric multi-vortices are stable; nevertheless, depending on the model parameters and topological charge (winding number), they can be either the ground state or not.
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All authors A. Alonso-Izquierdo, A. González-Parra, K. Slawinska, A. Wereszczynski, T. Winyard

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

Pairing from repulsive interactions in twisted WSe2 from functional renormalization

Hannes Braun, Dumitru Călugăru, Pietro M. Bonetti et al. (+9 more)

Motivated by the observation of superconductivity in twisted bilayer WSe$_2$ over an extended range of displacement fields, reaching from the correlated insulator down to zero field, we analyze the correlated electron phases of this system at hole filling $\nu = -1$ and twist angle $3.65^\circ$ using Hartree–Fock, random phase approximation, and functional renormalization group calculations. We derive a continuum model with gate-screened Coulomb interaction from first principles and compare its instabilities to those of a realistic three-orbital Wannier model with a Hubbard interaction and an artificial single-orbital Hubbard model. In both cases, we find a superconducting instability of mixed $d$-wave singlet and $p$-wave triplet character, arising adjacent to inter-valley-coherent spin-density-wave order in the phase diagram spanned by displacement field and interaction strength. Our additional analysis of the pairing mechanism points to inter-valley coherent spin fluctuations as the pairing glue. The extent of the pairing regime and the associated critical temperatures depend on the model input, and we trace differences to the interaction range entering the projected interaction. Within the continuum model, the gate-screened Coulomb interaction yields pairing down to zero displacement field, bringing theory into agreement with experiment.
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All authors Hannes Braun, Dumitru Călugăru, Pietro M. Bonetti, Haoyu Hu, Yan Zhang, Yi Jiang, Quansheng Wu, Kin Fai Mak, Jie Shan, B. Andrei Bernevig, Michael M. Scherer, Laura Classen

Superconductivity

H$_2$-Like Units and Pressure-Tunable High-$T_\mathrm{c}$ Superconductivity in NaH$_6$

Tianran Chen and Taner Yildirim

Hydrogen-rich materials offer a route to high-temperature conventional superconductivity, but often require extreme pressures. Here we report a first-principles study of NaH$_6$, identified in our large-scale hydride search and independently predicted in a subsequent high-throughput study. Several phases compete, with $R\bar{3}m$ and $Pm\bar{3}m$ dominant over broad pressure ranges. The $R\bar{3}m$ phase remains dynamically stable down to 10 GPa, where its calculated $T_\mathrm{c}$ is 34 K, rising to 99 K at 50 GPa. Cubic $Pm\bar{3}m$ becomes thermodynamically competitive at higher pressures and reaches a calculated $T_\mathrm{c}$ of 230 K at 50 GPa, decreasing only weakly upon further compression. Molecular dynamics and phonon-renormalization calculations support structural persistence over the sampled timescale and indicate relatively small anharmonic effects. Despite their high $T_\mathrm{c}$, both phases contain elongated H$_2$-like units, often considered unfavorable for high-$T_\mathrm{c}$ hydrides. Mode-resolved electron-phonon coupling links Raman-active hydrogen vibrations to superconductivity. In $R\bar{3}m$, the linewidths of these modes increase with the pressure-induced tilt of H$_2$ units relative to the threefold axis, alongside the rise in $T_\mathrm{c}$. A minimal symmetry-based model of the vibration-induced second-moment tensor of the hydrogen sublattice shows that these trends are consistent with a significant contribution from response components perpendicular to the threefold axis. NaH$_6$ thus represents an unusual molecular-like hydride in which hydrogen-sublattice reorganization may enable high-$T_\mathrm{c}$ superconductivity at comparatively moderate pressures.
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Superconductivity · Cross-listed from quant-ph

Photon-stimulated decay of $\chi$-matched ancillas causes unheralded bit flips in bosonic codes

Aleksandr Gulkanov

Bosonic codes often read out their syndrome with a transmon ancilla whose computational levels $g$ and $f$ have equal parity and matched dispersive shifts: a decay $f\to e$ is then heralded and leaves the code intact, while the direct $f\to g$ is forbidden by parity. We show that the photons of the code open this decay at first order in the ancilla's decay rate. Dressing by the code mode lets the charge noise that limits the ancilla's $T_1$ drive a photon-stimulated Raman decay $|f,n\rangle\to|g,n+1\rangle$, at $(n+1)(\chi_{ge}/2g_{01})^2$ times the $f\to e$ rate for a smooth noise spectrum. When the matching is set by the detuning, the decay costs $\approx\Gamma_{fe}/\alpha$ per gate and photon, whatever the coupling strength; a deliberate mismatch or matching by a drive lowers this cost, and a qubit ancilla has no such channel. The opened decay is unheralded and flips a cat with probability $\approx 1/2$; being first order, it overtakes the second-order double decay $f\to e\to g$ as the ancilla improves. In a microscopic model of the Ocelot repetition cat code, it makes the bit-flip floor of the gates grow linearly with the cat size, by a factor 1.2-1.7 from four to ten photons, which would limit the noise bias to $\sim 10^3$. The published gate data end at four photons and do not yet test this growth.
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Superconductivity

High-Efficiency Field-Free Superconducting Diodes in Mo$_2$N/FePt Bilayers

Juan S. Cruz, Emilio De Biasi, Juan C. Zapata et al. (+3 more)

Near-ideal, field-free superconducting diode behavior is demonstrated in room-temperature-grown Mo$_2$N/FePt bilayers. The response is consistent with remanent FePt stripe domains producing polarity-dependent thresholds for flux entry and vortex motion. Micromagnetic simulations reveal a periodic stray-field modulation across the stripes and asymmetric field enhancement near their ends at the bridge edges. One current polarity remains nearly dissipationless up to a sizable fraction of the estimated depairing current, whereas the opposite polarity develops dissipation at much lower current, yielding diode efficiencies up to approximately 85% in the thinner bilayer. The nonreciprocity can be continuously tuned by the in-plane magnetic preparation angle and reversibly switched by reverse-field excursions through the hysteretic reversal of the FePt layer. These results support edge-controlled flux entry and vortex motion as a route to high-efficiency superconducting diodes.
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All authors Juan S. Cruz, Emilio De Biasi, Juan C. Zapata, Yeonkyu Lee, Jeehoon Kim, Nestor Haberkorn

Superconductivity

Microscopic Modeling of Interlayer Tunneling in Twisted Bilayer Cuprates

Siddhant Panda, Andreas Kreisel, Laura Fanfarillo et al. (+1 more)

The field of twistronics, recently extended to twisted bilayer cuprate systems, has generated considerable interest due to the prediction of $d+id'$ states close to a $45^\circ$ twist, as suggested by multiple experimental probes. Theoretical studies of these systems using a variety of models and many-body theoretical techniques have revealed multiple competing order parameters. The large variability in experimental results and theoretical predictions demands an accurate microscopic model for these systems, in particular a more realistic description of the interlayer tunneling. In this study, we fit the in-plane hopping and compute the interlayer tunneling amplitudes by calculating the orbital overlap between $d_{x^2-y^2}$ wave functions at each site of the twisted square-lattice model. We observe sign-changing tunneling amplitudes, a strong twist-angle dependence of the direct tunneling term, and long-range tunneling amplitudes that often dominate over direct tunneling. We investigate the band structure, Fermi surfaces, and spectral functions in comparison with those obtained from the phenomenological model and find a substantial redistribution of spectral weight that modifies the competition between different superconducting ground states. We determine the phase diagram as a function of twist angle, filling, and direct tunneling strength, which is controlled by the $c$-axis distance between the monolayers, and observe a reduction in the region of time-reversal-symmetry-breaking (TRSB) $d+id'$ states compared with the phenomenological model. Finally, we investigate the Josephson critical current across the phase diagram and find strongly suppressed critical currents close to a $45^\circ$ twist, consistent with experimental observations.
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All authors Siddhant Panda, Andreas Kreisel, Laura Fanfarillo, P. J. Hirschfeld

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

Topological Chiral Superconductivity from Doping Correlated Charge Transfer Insulator with Orbital Magnetism

Wen Sun, Yuan Yang, Shou-Shu Gong et al. (+1 more)

Topological chiral superconductivity has been extensively studied from the perspective of electronic structures in momentum space, while its emergence from real-space processes in strongly interacting systems remains largely unexplored. Here, we study a doped charge-transfer insulator in a strongly repulsive spinless-fermion model with three sublattices and $2\pi$ flux per unit cell. Combining perturbative analysis, exact diagonalization, and density matrix renormalization group simulations, we establish a topological superconducting ground state with $p+ip$ symmetry. In the large charge-transfer gap limit, chiral charge-transfer excitations generate a correlated-hopping process that provides a microscopic mechanism for electron pairing and its chiral structure. The chiral superconductivity persists into the smaller-gap regime, where chiral excitonic two-particle bound state can emerge. Our results establish a concrete real-space route to topological chiral superconductivity upon doping strongly interacting charge-transfer insulators with orbital magnetism.
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All authors Wen Sun, Yuan Yang, Shou-Shu Gong, Wen Huang

Superconductivity

Tailored VxOy Metal–Insulator-Transition Oxides for Temperature-Triggered Smart Insulation in Superconducting Coils

Mattia Ortino, Davide Uglietti, Andrea Testino et al. (+5 more)

Smart insulation is a turn-to-turn layer that insulates in normal operation and conducts only near a hot spot. It could combine the fast ramping of insulated coils with the passive energy dump of non-insulated coils. Metal-insulator transition (MIT) oxides are natural candidates, but the only one tested in a coil so far, V2O3, switches at 150-170 K, far above the 30-70 K window useful for REBCO magnets. We explore the vanadium-oxygen system to lower the transition. TGA redox processing produced multi-phase powders with an effective transition near 127 K, about 25 K below V2O3. Hydrothermal V3O7, hydrated or annealed, was far too resistive and showed no usable transition. V2O3-epoxy specimens provided a robust reference and an acceptance criterion. Cryogenic measurements revealed a reproducible thermal hysteresis, and the measured specific heat of V2O3 corrects earlier model inputs by close to two orders of magnitude. A first electro-thermal model reproduces current sharing through the smart-insulation layer. Doped V2O3 and V8O15 are identified as the next targets.
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All authors Mattia Ortino, Davide Uglietti, Andrea Testino, Nicola Casati, Alexander Steppke, Wiktoria Wnetkowska, Andrea Masi, Kamil Sedlak

Superconductivity

Superconductivity from altermagnetic fluctuations: the crucial role of sublattice polarization

Mats Barkman, Andreas Kreisel and Brian M. Andersen

Unconventional superconductivity in correlated systems often appears in the vicinity of a magnetically ordered state. This situation is also relevant for altermagnets, characterized by a nonrelativistic spin-split electronic structure in the presence of Néel order. Many proposed altermagnetic candidates, however, never cross the magnetic instability. We study superconductivity arising from altermagnetic fluctuations in such systems and show that the sublattice polarization (SLP) of the normal-state bands plays a pivotal role in determining the superconducting state. SLP, an inherent property of altermagnetic band structures that persists even without magnetic order, prevents Cooper pairs from forming between different sublattices. This suppresses pairing mediated by the nearest-neighbor antiferromagnetic exchange, which favors spin-singlet superconductivity, and leaves longer-range same-sublattice bonds dominant. When the next-nearest-neighbor exchange is ferromagnetic, as expected for frustration-free Néel order, this results in spin-triplet superconductivity. This mechanism is fundamentally different from that in altermagnets with long-range Néel order, where the spin-split Fermi surfaces favor equal-spin pairing. Finally, when longer-range interactions in the disordered phase instead favor singlet pairing, SLP leads to singlet states with characteristic properties distinct from nearest-neighbor pairing states.
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Superconductivity

Molecule-intercalated van der Waals Josephson junctions uncover complex multiband superconductivity

Daniel Margineda, David Caldevilla-Asenjo, Covadonga Álvarez-García et al. (+6 more)

Intercalation of molecular species provides a route to tailor the electronic structure in van der Waals superconductors, yet intercalated compounds have rarely been integrated into phase-sensitive quantum devices. Here, we realize hybrid molecular Josephson junctions based on TaS2/NbSe2 heterostructures. Selective intercalation of the TaS2 electrode induces monolayer-like Ising superconductivity, while the junctions exhibit asymmetric Fraunhofer interference, field-induced and spontaneous Josephson diode effects. The zero-field diode disappears upon reversing the electrode stacking and re-emerges after intercalating the lower NbSe2 electrode. These findings indicate that the hybrid superconducting electrode hosts unconventional superconductivity underlying the nonreciprocal transport, while molecular intercalation acts as a tuning knob for its internal phase structure. This behavior is consistent with a time-reversal-symmetry-breaking multiband superconducting state characterized by a nontrivial relative phase between superconducting condensates. Our results establish hybrid molecular Josephson junctions as a quantum-device platform for phase-sensitive investigation and integration of molecularly engineered superconductors, opening opportunities to uncover and engineer unconventional superconducting phases.
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All authors Daniel Margineda, David Caldevilla-Asenjo, Covadonga Álvarez-García, Daniel Tezze, Raul Arenal, Maxim Ilyn, Celia Rogero, Luis E. Hueso, Marco Gobbi

Superconductivity

Photon echo from Maki-Thompson interference in a fluctuating s-wave superconductor

Hui Zhou and Yuan Wan

We study theoretically the nonlinear optical response of a fluctuating s-wave superconductor driven by two successive terahertz pulses. We show that the response exhibits a photon echo signal that arises from the quantum interference between the time-reversed electron and hole trajectories in the Maki-Thompson process. By contrast, the transport of the fluctuating Cooper pairs responsible for the Aslamazov-Larkin conductivity does not generate an echo. The photon echo signal can be readily measured using terahertz two-dimensional coherent spectroscopy, highlighting its potential to distinguish different fluctuation conductivity mechanisms.
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Superconductivity

Pressure Evolution of the Superconducting Order Parameter of Kagome Superconductor CsV3Sb5

Haiyan Zuo, Wangju Yang, Zhifan Wu et al. (+7 more)

Kagome superconductors AV3Sb5 (A = K, Rb, Cs) offer an outstanding platform for exploring the interplay among charge-density-wave (CDW) order, superconductivity (SC), and nontrivial topological electronic states. For CsV3Sb5, moderate hydrostatic pressure induces a double-dome superconducting phase diagram while monotonically suppressing the CDW phase, which signals an unconventional coupling between SC and CDW. Nevertheless, the microscopic nature of the pressure-modulated superconducting state and its relationship with CDW order remain poorly understood. Here we employ soft directional point-contact spectroscopy (SDPCS), combined with superconducting resistive measurements, to probe the superconducting order parameter of CsV3Sb5 under hydrostatic compression. Our anisotropic PCAR measurements resolve two distinct s-wave superconducting gaps along the c-axis and a strongly anisotropic gap within the ab plane. Two-band Blonder-Tinkham-Klapwijk (BTK) fits to the c-axis spectra yield a large gap satisfying 2Delta1/kBTc > 3.52 and a small gap with 2Deltas/kBTc < 3.52. The spectral-weight-averaged coupling ratio 2Deltaavg/kBTc is enhanced in the pressure window between P1 and P2, suggesting that pressure-induced modifications of the low-energy electronic structure may contribute to the evolution of superconductivity. For ab-plane junctions, the superconducting gap anisotropy is gradually suppressed above P1, accompanied by the emergence of a pronounced peak-dip-hump structure suggestive of enhanced coupling between superconducting quasiparticles and a bosonic excitation. These results shed new light on the intricate entanglement between CDW and SC in kagome superconductivity.
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All authors Haiyan Zuo, Wangju Yang, Zhifan Wu, Yuqing Zhao, Qiuxia Chen, Weiming Lao, Yangpeng Qi, Hechang Lei, Haizi Cong, Cong Ren