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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Thursday 1 October

Superconductivity

Selective suppression of electronic orders via interlayer coupling in superconducting bilayer nickelate thin films

Ziao Han, Lifen Xiang, Tianren Wang et al. (+11 more)

The discovery of spin-density-wave (SDW) order in bilayer nickelates has intensified interest in its interplay with superconductivity. Unlike cuprates, where doping rapidly suppresses the Néel temperature, the SDW transition temperature ($T_{\mathrm{SDW}}$) in bilayer nickelates is robust against oxygen annealing and even increases under pressure. Here, we combine oxygen annealing with isovalent rare-earth ($A$-site) substitution to effectively apply $c$-axis uniaxial pressure, realizing superconducting bilayer nickelate films with $T_{\mathrm{SDW}}$ suppressed from 150 K to 70 K. Notably, while SDW order is weakened but remains, a second charge-like anisotropy order is completely eliminated in the superconducting state. Polarization-resolved O $K$-edge X-ray absorption and electronic structure calculations show that strengthened interlayer coupling reconstructs the Fermi surface and weakens the SDW. These findings, consistent with a spin-spinless stripe ground state, provide new insight into the mechanism of density wave formation and their interplay with superconductivity.
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All authors Ziao Han, Lifen Xiang, Tianren Wang, Congcong Le, Jun Zhan, Siyi Lei, Sonia Francoual, Qisi Wang, Jiangping Hu, Tao Xiang, Ronny Sutarto, Xianxin Wu, X. J. Zhou, Zhihai Zhu

Superconductivity

Highly anisotropic collective modes of altermagnetic superconductors with Bogoliubov Fermi surfaces

Huaisong Zhao, Peng Zou, Xia-Ji Liu et al. (+1 more)

We investigate collective modes in charge-neutral altermagnetic superconductors with $d$-wave spin-split bands, focusing on the impact of emergent Bogoliubov Fermi surfaces. In the absence of a magnetic field, the BCS state supports collective modes that inherit the underlying $d$-wave symmetry, resulting in pronounced momentum-space anisotropy. The broken time-reversal symmetry further couples these modes to spin-density fluctuations, allowing their signatures to emerge in the spin dynamical structure factor. Upon applying a magnetic field, Bogoliubov Fermi surfaces emerge and qualitatively reshape the collective-mode dynamics through gapless particle-hole excitations and strongly anisotropic Landau damping. The damping is strongest along the momentum-space diagonal, where it can suppress the low-energy phonon mode at small momenta. Remarkably, the suppressed phonon is accompanied by the emergence of an in-gap structure in the Higgs mode and an associated pronounced enhancement of the spin response. These results establish Bogoliubov Fermi surfaces as a mechanism for generating highly anisotropic collective-mode dynamics and identify spin response as a sensitive probe of in-gap Higgs excitations in altermagnetic superconductors.
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All authors Huaisong Zhao, Peng Zou, Xia-Ji Liu, Hui Hu

Wednesday 30 September

Superconductivity

Electronic structure and two-orbital model of the quadlayer La$_5$Ni$_4$O$_{13}$

Jian-Xiang Sun, Haokan Xiao, Cui-Qun Chen et al. (+1 more)

The discovery of pressure-induced superconductivity in Ruddlesden–Popper (RP) nickelates has stimulated extensive interest in high-T$_c$ superconductors. Here, we systematically study the electronic properties of the quadlayer RP nickelate La$_5$Ni$_4$O$_{13}$ under ambient pressure, 5% isotropic compressive strain, and 4% $c$-axis uniaxial strain using density functional theory (DFT) and random phase approximation (RPA) calculations. DFT calculations show that isotropic strain broadens the Ni-$e_g$ bands and induces charge transfer from O-$p$ to Ni-$d$ orbitals, whereas $c$-axis uniaxial strain selectively shifts the $d_{z^2}$-derived bonding1 band upward while leaving the $d_{x^2-y^2}$ dispersion nearly unchanged. From Wannier downfolding, we construct a quadlayer two-orbital model that reproduces the low-energy Ni-$e_g$ bands. Our model reveals that under ambient pressure and 5% isotropic strain, the Fermi surface consists of two electron pockets ($\alpha$ and $\delta$) and three hole pockets ($\beta$, $\beta^{\prime}$, and $\beta^{\prime \prime}$), while under uniaxial strain, a $\gamma$ hole pocket with $d_{z^2}$ orbital character emerges. RPA calculations reveal that the leading spin response shifts from $\mathbf{q}\approx(2\pi/3,2\pi/3)$ at ambient pressure to $\mathbf{q}\approx(\pi,\pi)$ under both strain conditions and is enhanced under $c$-axis compression. These results suggest that $c$-axis compression may provide a favorable route to superconductivity in the quadlayer nickelate analogous to that in bilayer and trilayer nickelates.
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All authors Jian-Xiang Sun, Haokan Xiao, Cui-Qun Chen, Dao-Xin Yao

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

Resonances control when multiterminal Josephson currents reduce to two-terminal couplings

A. Bar{\i}ş Özgüler

Multiterminal Josephson junctions, in which one weak link couples three or more superconductors, are studied as hosts of topological Andreev bands, multi-pair supercurrents and tunable circuit elements. Their currents are modeled both as pairwise networks of two-terminal couplings and through genuinely multiterminal processes such as quartets. We ask how accurate the pairwise description is and what controls its error. In a microscopic scattering model of planar junctions, we compare exact currents with the best pairwise approximation, which allows arbitrary nonsinusoidal couplings. In ensembles of disordered three- and four-terminal junctions, the pairwise terms capture nearly all of the energy variation, yet the median current error ranges from 8.0 to 21.2 percent, about twice the energy error, because currents weigh the multiterminal harmonics more strongly. Designed devices extend the comparison to sixteen terminals. We identify a mechanism that controls this error. A normal-region mode near the Fermi level that couples to three or more terminals produces large nonpairwise currents, and detuning it with a gate suppresses them. Shifting only this mode, selected from normal-state properties, predicts the gate dependence of the error in eight three- and four-terminal devices without fitting currents. Finite-gap calculations in two clean devices confirm a drop from tens of percent near resonance to below one percent. An analytic single-level model gives a sufficient detuning for pairwise accuracy at any terminal count. Planar junctions thus inherit the resonant and cotunneling regimes known from quantum dots, and normal-state properties identify the mode that selects between them.
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Superconductivity

Electronic Reconstruction across the Tilt-Free Transition in La$_3$Ni$_2$O$_7$

Mengjie Kong, Gergely Németh, Yingpeng Yu et al. (+5 more)

The emergence of high-$T_c$ superconductivity in pressurized La$_3$Ni$_2$O$_7$ is intimately linked to a structural transition that suppresses the tilts of the NiO$_6$ octahedra, yet its impact on the electronic structure remains poorly understood. Here, we probe the electronic response across this tilt-free transition at $T_{st}\simeq 544$ K using broadband infrared-to-visible reflectivity at ambient pressure, covering photon energies from 15 meV to 3.2 eV. We observe a pronounced redistribution of spectral weight over an exceptionally broad energy range, with spectral weight transferred from excitations between 1 and 3 eV toward low-energy excitations below 1 eV. Most strikingly, two low-energy interband excitations progressively converge and merge upon entering the tilt-free phase, revealing a substantial reconstruction of the finite-energy electronic structure. These changes point to a reconstruction of the bilayer Ni $3d_{z^2}$-derived electronic states, whose interlayer coupling is central to proposed mechanisms of superconductivity in La$_3$Ni$_2$O$_7$. Our results establish the tilt-free transition as a direct route to reorganizing the electronic degrees of freedom implicated in high-$T_c$ superconductivity and provide an ambient-pressure reference for the electronic structure of the superconducting state.
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All authors Mengjie Kong, Gergely Németh, Yingpeng Yu, Bosen Wang, Jianping Sun, Jinguang Cheng, Ferenc Borondics, Bastien Michon

Superconductivity

A Different Perspective on Superconductivity in Crystalline Graphene: Exploiting Energetics

Ke Wang, Shicong Song and K. Levin

A central mystery of crystalline graphene is why superconductivity is so widespread yet often confined to strange slivers near boundaries between distinct isospin-ordered metals. In this paper, we apply a “two-parent” energetic framework which we show can explain this unusual form of superconductivity without specifying the details of the necessarily present pairing attraction. First-order transitions are crucial here: when two normal isospin-ordered states are degenerate in free energy, even a small net superconducting energy gain may stabilize an equilibrium superconductor. We demonstrate how this is possible even though the small energy gain from pairing is reduced by the expense of reconstructing the normal metal, which is needed to achieve superconducting compatibility. The first order degeneracy also gives superconductivity a choice between two normal state parents, favoring the state with the largest net free energy gain.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Fingerprints of unconventional pairing in superconductor-hole-gas heterostructures

Serafim S. Babkin, Karsten Flensberg, Jeroen Danon et al. (+1 more)

Superconductor and two-dimensional hole gas heterostructures are promising platforms for quantum devices, but their intrinsic properties, e.g., unconventional pairings, are hard to access experimentally. Here, we show that the in-plane-field dependence of the superfluid stiffness and spin susceptibility exhibits nonanalyticities encoding the induced gap structure in the hole gas. Additionally, Fourier harmonics of the stiffness tensor's field-angle dependence, accessible via kinetic-inductance measurements, provide fingerprints of unconventional pairing and cubic Rashba coupling.
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All authors Serafim S. Babkin, Karsten Flensberg, Jeroen Danon, Maksym Serbyn

Superconductivity

Spectroscopic Evidence for Nontrivial Band Topology in Superconducting FeTe/MnTe Heterostructure

Shiwu Su, Yu Liang, Tongrui Li et al. (+14 more)

FeTe has long been regarded as a nonsuperconducting antiferromagnetic metal with trivial band topology, but recent advances in stoichiometry control have begun to challenge this picture. Here we use higher-order epitaxy on zinc-blende MnTe to stabilize near-stoichiometric FeTe with strongly suppressed interstitial Fe and a superconducting transition onset near 13 K. Angle-resolved photoemission spectroscopy reveals markedly enhanced quasiparticle coherence, well-defined Fe-derived hole bands, and a nearly two-dimensional Dirac-cone-like state near the Fermi level. First-principles calculations identify an inversion between odd- and even-parity bands, yielding nontrivial $Z_2$ topology and a Dirac surface state consistent with experiment. These results elucidate the intrinsic electronic structure of stoichiometric superconducting FeTe and provide evidence for nontrivial band topology, positioning FeTe/MnTe as a promising platform for exploring topological superconductivity.
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All authors Shiwu Su, Yu Liang, Tongrui Li, Zhen Wang, Yuzhe Wang, Xianglin Li, Sen Liao, Pengxu Ran, Jiexiong Sun, Shengtao Cui, Zhe Sun, Zhengtai Liu, Jishan Liu, Mao Ye, Jing Tao, Donglai Feng, Juan Jiang

Tuesday 29 September

Superconductivity

Reply to "Comment on 'Topography of Fermi arcs in t-PtBi2 using high-resolution angle-resolved photoemission spectroscopy'"

Adam Kaminski, Paul C. Canfield and Benjamin Schrunk

In a recent work [1] we demonstrated that the Fermi arcs in t-PtBi2 are indeed disconnected segments of the Fermi surface. We showed that the surface state band crosses EF between Weyl points and is has finite band gap at all other momenta until it merges with bulk bands. We did not find evidence of a superconducting gap in the spectra down to 3K. In recent Comment, Sergey Borisenko reanalyzed data from repository posted with our paper and concluded that a superconducting gap with i-wave symmetry is present in our data up to temperatures of 19K. We argue that this claim arises from incorrect identification of the location of Fermi arc and confusion between band gap and superconducting gap. This error was compounded by the fact that Author of the Comment analyzed data from interpolated images used for mapping the Fermi surface that are not suitable for extraction of energy gap on meV scale. This fact was previously communicated to the Author of the Comment during e-mail exchange. We maintain that our conclusion of absence of a superconducting gap above 3 K are valid and consistent with recent STM measurements performed on the same samples, which found surface superconductivity with Tc = 3K, below the temperature at which the data for our ARPES paper was measured.
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Superconductivity

Imaging phase winding in topological superconductors with a fork-tip Josephson STM

Vladislav Poliakov and Archisman Panigrahi

The complex phase winding in a topological superconductor remains challenging to access by conventional real-space probes. We propose a scheme to probe this winding with a fork-tip Josephson STM near an impurity, where two superconducting tips form an interferometer sensitive to phase differences between two positions. The signal remains accessible even if the relative phase between the fork-tip and sample fluctuates, establishing a route towards a local phase-sensitive STM probe of topological superconductivity. We numerically compute the interference pattern to demonstrate that the magnitude and phase of the order parameter can be reconstructed as a function of angle, providing a route towards resolving the longstanding open experimental problem of directly detecting phase winding in topological superconductors.
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Superconductivity · Cross-listed from cond-mat.str-el

Neural-quantum-state based downfolding of the three-band Emery model for cuprates and nickelates

Hannah Lange, Julius F. A. Tirpitz and Annabelle Bohrdt

Understanding the physics underlying high-temperature superconductivity in cuprates and, more recently, infinite-layer nickelates has remained a central challenge in condensed-matter physics. We establish neural quantum states (NQS), specifically Hidden Fermion Determinant States (HFDS), as a scalable variational approach to the three-band Emery model of the copper- and nickel-oxide layers in these materials. After benchmarking HFDS against matrix product states on width-two geometries, we study ground states of fully two-dimensional (2D) systems of up to $10\times10$ unit cells ($300$ sites). We characterize the momentum-space distribution of dopants and find a pronounced electron-hole dichotomy similar to cuprate experiments. We further downfold the three-band model to effective single-band descriptions by constructing interacting Wannier functions. We consider a wide range of parameters – from the charge-transfer regime relevant to cuprates to the Hubbard-Mott regime of nickelates, as well as systematic scans of the charge-transfer gap that has been demonstrated to impact the critical superconducting temperatures. Across all regimes, the effective model significantly deviates from the usual Fermi-Hubbard model: The typical ratio $U/t$ is enhanced, some parameters experience a significant doping dependence, and sizable terms beyond the conventional Hubbard model are present, most notably a density-assisted hopping $t_n$. Notably, in all effective models, $t_n$ has the largest contribution to particle-hole asymmetry, rather than next-nearest-neighbor hopping contributions. The effective parameters sensitively depend on the charge-transfer energy, doping, and interaction ratios. Our results establish HFDS as an efficient tool for studying the 2D Emery model and demonstrate that single-band descriptions can require interaction terms generated by the underlying multi-band models.
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Superconductivity

Anisotropic interface-confined superconductivity in FeTe-based heterostructures

Bicky Singh Moirangthem, Kamal R. Joshi, Zi-Jie Yan et al. (+4 more)

Interface-confined superconductivity emerges from the interaction of electronic states across chemically distinct boundaries, providing a route to engineer superconducting phases where magnetism and topology coexist. Determining the intrinsic nature of such superconductivity, however, is challenging because the superconducting layer is only a few nanometers thick and buried beneath several normal layers. Here, we measure the whole-sample Meissner response of six $\mathrm{FeTe}$-based heterostructures in a uniform magnetic field using a frequency-domain tunnel-diode resonator. In the ultrathin limit, the conventional normalization of the measured susceptibility, $\chi(T\to0)=-1$, fails by tens of percent. We establish the appropriate calibration and invert $\chi(T)$ to determine the London penetration depth $\lambda(T)$. Two key results emerge. First, the broad transitions observed in $\chi(T)$ arise naturally from the extreme geometry and large $\lambda$, without requiring chemical or structural inhomogeneity; the extracted $\lambda(T)$ closely tracks the resistive transition. Second, $\lambda(T)$ and the corresponding superfluid density are inconsistent with a fully gapped isotropic $s$-wave state and instead indicate a strongly anisotropic order parameter possibly with line nodes or deep gap minima. The inferred $\lambda(0)$ is of order $1\,\mu$m, consistent with an independent analysis of the 2D phase stiffness. Despite the distinct chemical, magnetic, and topological character of the three overlayers, all six FeTe heterostructures exhibit similar low-temperature power-law behavior, with no systematic dependence of the superconducting response on overlayer identity. These results point to the interfacial $\mathrm{FeTe}$ layer as the common origin of superconductivity.
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All authors Bicky Singh Moirangthem, Kamal R. Joshi, Zi-Jie Yan, Pu Xiao, Lok-Kan Lai, Cui-Zu Chang, Ruslan Prozorov

Superconductivity

Reversible field-free superconducting diode effect controlled by an antiferromagnet

Filip Krizek, Kamil Olejník, Tobias Edwinson et al. (+19 more)

The semiconductor diode, which allows current to flow preferentially in one direction, is a fundamental building block of numerous modern electronic circuits. Its superconducting analogue—the superconducting diode effect—enables directional dissipationless current flow and may provide similar functionality in future superconducting quantum circuits. Realization of such a nonreciprocal supercurrent requires broken time-reversal symmetry. At zero applied field, this has been typically associated either with intrinsic unconventional superconductivity or extrinsic spin-split electronic states induced by magnetic proximity. Here we demonstrate a field-free superconducting diode effect in conventional superconducting Al proximitized by collinear antiferromagnetic CuMnAs, whose electronic structure breaks time-reversal symmetry without generating spin splitting. By tuning the proximity effect through an insulating AlAs interlayer and correlating the reversal of the diode polarity with the reversal of the remanent Néel state, we establish that the antiferromagnet controls the superconducting diode effect. Our results show that neither spin-split bands nor net magnetization is required for a magnetically controlled field-free superconducting diode effect, extending superconducting nonreciprocity to a broader class of collinear compensated magnets.
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All authors Filip Krizek, Kamil Olejník, Tobias Edwinson, August Jacobi, Athira Suresh, Andrej Farkaš, Jan Kraus, Vít Novák, Christoph Müller, Vojtěch Pařízek, Niclas Heinsdorf, Peter Wadley, Oliver Amin, Kevin Edmonds, Tomas Jungwirth, Libor Šmejkal, Anna Birk Hellenes, Sumit Ghosh, Michal Mazur, Dominik Kriegner, Lucas Casparis, Saulius Vaitiekėnas

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

Correlation-assisted topological and metamagnetic transitions in Rashba-coupled superconductors: $t$-$J$-$U$ model study

Tushar Dey, Maciej Fidrysiak and Józef Spałek

Unconventional superconductivity commonly emerges in systems characterized by strong electronic correlations, with layered copper-oxides serving as a canonical example. Observation of spin-momentum locking within Bi-family of the cuprates, compatible with the presence of non-negligible Rashba-type spin-orbit coupling (RSOC), calls for an investigation of the joint effects of electronic correlations and RSOC on pairing in copper-oxide and related superconductors. Employing statistically-consistent variational approximation (SGA), we carry out such an analysis by constructing the phase diagram for the case of square-lattice t-J-U model incorporating RSOC. We also investigate the effects of time-reversal-symmetry breaking by Zeeman field, as well as characterize emergent topological superconducting (TSC) states. Chern number $C = \pm 4$ TSC is found in a broad regime of on-site Coulomb repulsion close to half-filling. The latter is not governed by correlations and emerges also within the weak-coupling Bogoliubov-de Gennes (BdG) scheme. Yet, we identify a distinct $C = \pm 2$ TSC state that is driven specifically by electronic correlations via a topological transition occurring with no bulk quasiparticle gap closure, and is accompanied by discontinuous metamagnetic and Lifshitz transitions. Moreover, a qualitatively distinct doping evolution of the $d$- and $p$-wave components of the underlying mixed-parity SC order parameter above the metal-to-insulator transition is demonstrated. Our work points toward the relevance of joint correlation and RSOC effects beyond BdG scheme to phase diagrams of RSOC-coupled superconductors.
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Superconductivity · Cross-listed from cond-mat.str-el

Uniform-Charge-Density $d$-Wave Superconductivity in the Pure $t$-$J$ Model at $1/8$ Doping on an Infinite Cylinder

Guangyu Yu and Zheng Zhu

The ground state of the square-lattice $t$-$J$ model at $1/8$ hole doping has been the subject of a long-standing controversy, with conflicting numerical reports of stripe order versus superconductivity. Here, we address this debate using the variational uniform matrix product state (VUMPS) method, which optimizes the wave function directly in the thermodynamic limit along the cylinder axis and eliminates the boundary pinning effects that can arise in finite-cylinder calculations. On an infinite cylinder of circumference $L_y = 8$, we find a uniform-charge-density $d$-wave superconducting ground state with quasi-long-range pairing correlations. The dominant pairing occurs at zero center-of-mass momentum, while no sizable finite-momentum pairing component is detected. Twisted-boundary-condition calculations reveal a finite superconducting phase stiffness, with the order-parameter phase winding smoothly to follow the applied flux, providing an independent probe of the robustness of the superconductivity. Moreover, both charge and spin correlations decay rapidly, with no signatures of long-range stripe or magnetic order, in contrast to an $L_y=6$ cylinder at $1/6$ doping where the same method readily identifies stripe order. Our results provide methodologically distinct evidence in the long-standing stripe–superconductivity debate in the pure $t$-$J$ model and reveal uniform $d$-wave superconductivity in a minimal model of doped Mott insulators.
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Superconductivity · Cross-listed from cond-mat.quant-gas

Finite-momentum trimers and Cooper quartets in a one-dimensional Fermi gas with coexistent $s$- and $p$-wave interactions

Yixin Guo

We study finite-momentum trimers and Cooper quartets in a one-dimensional two-component Fermi gas with coexistent $s$- and $p$-wave interactions, regularized in the relative momentum of each pair so that they remain Galilean invariant. Pauli blocking alone then makes the trimer dispersion deviate from free center-of-mass motion, so the trimer energy can be lowest at finite total momentum. Allowing this motion expands the $abb$ region among pairs and trimers and removes the region where an odd-wave pair is lowest. The $aabb$ quartet lies below all these branches at every sampled coupling, and in a strip starting on the crossing of the two dimer-pair channels its lowest breakup channel is a moving $abb$ trimer and an atom. At weak interspecies attraction, a variational bound shows that two odd-wave Cooper pairs bind into a quartet, whose binding exceeds that of the coexisting trimer by orders of magnitude as the attraction weakens. At stronger interspecies attraction, the quartet binding relative to two unlike-species dimers disappears when the odd-wave interaction is switched off, although the even-wave interaction dominates the interaction energy. Relative to the in-vacuum case, the Fermi sea enhances the weak-coupling quartet binding, and at fixed scattering lengths the quartet binding grows with density while the coexisting trimer becomes much shallower. These results show that neither the most strongly bound pair nor the dominant interaction energy alone determines whether the quartet binds.
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Superconductivity · Cross-listed from cond-mat.mtrl-sci

Tuning the secondary electron yield and electronic conductivity of MgZnO multilayers synthesized by Atomic layer deposition

Mathieu Lafarie, Sarah Dadouch, Fréderic Miserque et al. (+4 more)

Secondary electron emission (SEE), arising from the interaction of energetic electrons with material surfaces, can induce deleterious effects in vacuum radio-frequency (RF) systems, notably multipacting, thereby degrading performance and threatening device integrity. Despite extensive efforts, effective and widely adopted mitigation strategies remain limited. Here, we report an approach to simultaneously tailor SEE and electrical conductivity using nanometric multi-element multilayers deposited by atomic layer deposition (ALD). The design and fabrication of these heterostructures are presented, and a range of films is systematically characterized to establish correlations between emission properties, electrical conductivity, chemical and structural features. We demonstrate that such heterostructures enable combinations of total electron emission yield (TEEY) and conductivity unattainable in single-phase materials, including regimes combining high TEEY with relatively high conductivity and low TEEY with low conductivities. These results open new avenues for the rational design of functional surfaces for vacuum electronic applications.
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All authors Mathieu Lafarie, Sarah Dadouch, Fréderic Miserque, Yunlin Zheng, Jocelyne Leroy, Mohamed Belhaj, Thomas Proslier

Superconductivity

Evanescent-wave Johnson Noise from Superconductors

Hruday Mallubhotla, Gustav Romare, Ilya Esterlis et al. (+3 more)

We compute the evanescent-wave Johnson noise (EWJN) in the vacuum half-space above a superconductor, and the resulting relaxation time ($T_1$) of spin and charge qubits placed at nanometer distances from the surface. The electromagnetic response is described by a single microscopic transverse current-response kernel $Q(q, \omega)$ for a BCS superconductor. This is computed for varying densities of both non-magnetic impurities and magnetic impurities, for arbitrary frequency and temperature and for wave vectors $q \ll k_F$ (the Fermi wavevector). When combined with the fluctuation-dissipation theorem and the nonlocal surface impedances of the half-space, this yields the magnetic and electric field noise at any distance $z \gg k_F^{-1}$ from the surface, from which we obtain $T_1$. Just below $T_c$ the magnetic noise is enhanced relative to the normal state by the coherence (Hebel-Slichter-type) peak of the dissipative conductivity and drops exponentially at lower temperatures; the electric noise shows no coherence peak. The theory predicts that there is a zero-temperature noise floor induced by magnetic impurities. In the gapless regime produced by pair breaking, the finite subgap density of states $\nu(0)$ yields a temperature-independent noise spectral density and a relaxation rate bounded by $T_1^{-1}(T)\le[\nu(0)/\nu_F]^{2}\,T_{1,N}^{-1}(T)$ for $T\ll T_c$, with equality in the extreme nonlocal regime. Here $\nu(0)$ and $\nu_F$ are the superconducting and normal-state densities of states at the Fermi energy, and $T_{1,N}(T)$ is the relaxation time the same electrode would produce in its normal state at the same temperature.
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All authors Hruday Mallubhotla, Gustav Romare, Ilya Esterlis, Maxim Vavilov, Robert Joynt, Alex Levchenko

Monday 28 September

Superconductivity

Enhanced Critical Currents and Irreversibility Fields in YBa$_2$Cu$_4$O$_8$ Films through Ca-Substitution

Jiangteng Liu, Shuhei Funaki, Yuki Ogimoto et al. (+6 more)

The intrinsic carrier concentration of superconducting materials may not be optimal, and different superconducting properties may have distinct optimal doping levels. Here, we show that Ca substitution increases the hole concentration in intrinsically underdoped epitaxial (Y$_{1-x}$Ca$_x$)Ba$_2$Cu$_4$O$_8$ (YCa124) films with $x \leq 0.10$, driving YCa124 toward optimal doping, increasing $T_c$ and $J_c$. At 40 K, 10% Ca doping increases $J_c$ by factors of 2.4 and 10 at 0.03 and 6 T, respectively, compared with the undoped film. The field dependence of $J_c$ follows $J_c \propto B^{-0.5}$, independent of Ca content, indicating that Ca substitution does not alter the dominant pinning mechanism. This field dependence is consistent with pinning by planar Y-125 stacking-fault intergrowths observed by electron microscopy. In addition, Ca substitution slows thermally activated vortex motion (creep) at $T \gtrsim 15$ K, while at lower temperatures the creep rate $S$ shows an unusual decrease with increasing field. Finally, we consider how doping tunes the upper bound on $J_c$, set by the depairing current density $J_d$, and the lower bound on creep, set by the Ginzburg parameter $G_i$. We find positive correlations between $J_c$ and $J_d$, and between $S$ and $G_i$, and compare them with trends across a broad range of superconductors.
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All authors Jiangteng Liu, Shuhei Funaki, Yuki Ogimoto, Bryan Zhang, Ryoya Nagaura, Ryuji Yoshida, Takeharu Kato, Masashi Miura, Serena Eley

Superconductivity

Periodic Trends and a Physics-Based Multistage Search for High-$T_c$ Hydride Superconductors

Tianran Chen and Taner Yildirim

Hydrogen-rich materials under pressure are candidates for conventional phonon-mediated superconductivity, but first-principles electron–phonon-coupling (EPC) calculations are computationally demanding. We develop an interpretable multistage workflow for prioritizing binary hydride superconductors before full EPC calculations. Using controlled cubic $X_m$H$_n$ prototypes at 200~GPa, we introduce the spectral moment $A_1$ as an intermediate measure of electron–phonon perturbation strength. In its linewidth-based construction, $A_1$ remains finite as harmonic phonons soften, allowing related structures with imaginary harmonic modes to be screened; it does not establish superconductivity or physical realizability for unstable structures. For dynamically stable cubic systems, $A_1$ correlates strongly with the H-$1s$ density-of-states fraction at the Fermi level, $\alpha$, a projected Fermi-surface descriptor, $\beta$, and atomic number density, $\rho$. These trends are summarized heuristically by $A_1\approx h\alpha\beta\rho$, where $h$ is framework- and pressure-dependent; the descriptor is used for ranking, not as a universal $T_c$ predictor. We incorporate $\alpha\beta\rho$ and enthalpic competitiveness into a three-stage evolutionary search comprising multiobjective structural screening, harmonic-phonon stability filtering, and coarse density-functional perturbation-theory (DFPT) EPC calculations. Application to more than 100,000 binary hydride structures at 50 and 200~GPa identifies candidates and prioritizes NaH$_6$ structural families. Higher-accuracy calculations show that $Pm\bar{3}m$ NaH$_6$ is dynamically stable down to 50~GPa and has a harmonic Allen–Dynes estimate of $T_c\approx230$~K. The framework combines electronic descriptors with progressively more expensive stability and EPC calculations for scalable hydride exploration.
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Superconductivity · Cross-listed from cond-mat.str-el

t-J model at 50

Józef Spałek

I briefly overview the original formulation of the t-J model starting from Hubbard model, as well as stress its unique features, i.e., advantages and shortcomings when discussing its physical properties. Particular emphasis is put on a brief characterization of connection between magnetism, unconventional superconductivity, and the Mott-Hubbard localization of the correlated carriers in a narrow band. In the second part I summarize the model generalization to the three–orbital situation. The main purpose is to list some basic features of the physics associated with t–J model.
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Superconductivity

Improving the critical current density of the $\mathrm{V}_{0.59}\mathrm{Ti}_{0.40}\mathrm{Ce}_{0.01}$ alloy superconductor through successive cold-working and annealing at different temperatures

Asi Khandelwal, SK. Ramjan, Basudev Padhi et al. (+5 more)

The critical current density ($J_c$) of V-Ti alloy superconductors is strongly influenced by the size and distribution of microstructural defects that pin magnetic flux lines. In this work, the effect of successive cold working and annealing (SCA) at 550{\deg}C on the microstructure, superconducting properties, and flux-pinning behaviour of the V$_{0.59}$Ti$_{0.40}$Ce$_{0.01}$ alloy is investigated and compared with the previously reported SCA at 450 and 650{\deg}C. During SCA at 550{\deg}C, the superconducting transition temperature increases gradually with successive processing steps. The first annealing after cold rolling to 50% thickness produces a significant enhancement in $J_c$ over the measured field range, whereas subsequent SCA cycles result in only marginal changes. The $J_c$ remains relatively weakly dependent on magnetic field over wide magnetic-field range, and the final cold-worked sample exhibits a finite $J_c$ up to 8.5 T. Pinning force density analysis shows that grain boundaries dominate flux pinning in low magnetic fields, whereas dislocations and $\beta$-$\alpha'$ interfaces become the dominant pinning centres in higher fields. Comparison of the different SCA temperatures shows that 650{\deg}C provides the highest low-field $J_c$, whereas 450{\deg}C gives the best high-field performance. In contrast, SCA at 550{\deg}C provides most balanced field dependence and the largest enhancement in $J_c$ relative to the corresponding as-cast alloy. Although the highest absolute $J_c$ in the high-field regime is achieved after the third SCA cycle at 450{\deg}C, a comparable value is obtained after only the first annealing at 550{\deg}C. These results demonstrate that the intermediate annealing temperature of 550{\deg}C provides an effective balance between defect generation, phase evolution, and recovery, resulting in enhanced flux pinning over a wide magnetic-field range.
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All authors Asi Khandelwal, SK. Ramjan, Basudev Padhi, L. S. Sharath Chandra, Archna Sagdeo, Kranti Kumar, Sudip Pal, M. K. Chattopadhyay

Superconductivity

Low-Temperature Stabilization of $\delta$-NbN Superconducting Thin Films through Energy-Selective Ion Beam Sputtering

Yifan Liu, Junwoo Lee, Mingyu Xu et al. (+3 more)

Achieving high-quality superconducting thin films at low temperatures is a central challenge for scalable quantum device integration and Complementary Metal-Oxide-Semiconductor (CMOS)-compatible fabrication. Here, we demonstrate a room-temperature magnetron sputtering approach for synthesizing niobium nitride (NbN) thin films using a novel energy-selective ion source that enables independent control of ion energy and ion flux. This capability provides a powerful route to precisely tailor surface kinetics and crystallization pathways, overcoming longstanding limitations in stabilizing high-quality NbN at reduced temperatures. The resulting films were characterized by superconducting transport measurements, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Compared with conventional reactive sputtering, ion beam-assisted growth yields a substantial narrowing of the transition width at the superconducting temperature, indicating improved phase purity and electronic homogeneity. Furthermore, this work introduces energy-selective ion control as a general, non-equilibrium route to stabilize metastable superconducting phases at low temperatures, offering a new design paradigm for thin-film quantum materials.
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All authors Yifan Liu, Junwoo Lee, Mingyu Xu, Weiwei Xie, Robert J. Cava, Qi Hua Fan

Superconductivity

Unconventional superconductivity in locally non-centrosymmetric CeNi$_2$As$_2$

Félix Morineau, Jan Knapp, Javier Landaeta et al. (+12 more)

Recent years have seen intense research on heavy fermion superconductivity, inspired by the discovery of multiple superconducting phases in UTe$_\text{2}$ and CeRh$_\text{2}$As$_\text{2}$. In the latter material, two superconducting phases observed under applied magnetic field oriented along the crystallographic $\textit{c}$ direction are associated with local inversion symmetry breaking. To advance understanding of these phenomena, it is highly desirable to find further examples of heavy fermion superconductivity in materials with locally non-centrosymmetric structure. We have succeeded in this quest, by growing single crystals of the CaBe$_\text{2}$Ge$_\text{2}$ isomorph of CeNi$_\text{2}$As$_\text{2}$. The resulting superconductivity brings more than we had foreseen. In addition to providing the opportunity to compare and contrast with that of CeRh$_\text{2}$As$_\text{2}$, the condensation occurs from an incoherent normal state quantitatively similar to that of UBe$_\text{13}$. Our findings therefore raise profound questions not just about superconductivity in the locally non-centrosymmetric structures, but about unconventional superconductivity itself.
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All authors Félix Morineau, Jan Knapp, Javier Landaeta, Thomas Lühmann, Lea Richter, Petra Knappova, Soumen Ash, Sushma Lakshmi Ravi Sankar, Konstantin Semeniuk, Elena Hassinger, Christoph Geibel, Manuel Brando, Daniel F. Agterberg, Andrew P. Mackenzie, Seunghyun Khim

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

Bogoliubov coupled-cluster theory for su(2) Hamiltonians

Swarnamoy Ghosh, Thomas M. Henderson and Gustavo E. Scuseria

Coupled-cluster theory is the method of choice for weakly correlated systems, but in strongly correlated systems where the mean-field reference is qualitatively poor, it can break down badly. These failures can be ameliorated by using symmetry-broken mean-field references. For systems that spontaneously break spin symmetry, we might favor an unrestricted reference. In systems that instead break number symmetry, we turn to Bogoliubov coupled cluster theory. In this work, we apply Bogoliubov coupled-cluster to Hamiltonians with an underlying su(2) algebra to study the pairing Hamiltonian and the spin XXZ and J1-J2 models by exploiting the correspondence between fermionic-pair operators and spin-1/2 operators. For the pairing problem, the reference is the usual Bardeen–Cooper–Schrieffer (BCS) quasiparticle vacuum, while for spin systems we use the analogous spin-BCS reference, which breaks Sz symmetry and provides a flexible starting point for strongly correlated regimes. Correlation is incorporated through a hierarchy of coupled-cluster approximations. Beyond energies, we compute the response density matrices for the evaluation of spin–spin correlation functions for the XXZ and J1-J2 Heisenberg models, and the pairing parameter and particle-number variance for the pairing Hamiltonian. We also derive the corresponding relaxed density matrices, including the effects of orbital relaxation. Benchmark calculations for the half-filled pairing Hamiltonian and spin models demonstrate that Bogoliubov coupled cluster provides a systematically improvable wave-function-based approach for su(2) Hamiltonians, including geometrically frustrated spin systems requiring complex and noncollinear BCS references.
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Sunday 27 September

Superconductivity

Superconducting Pairing Symmetry on Geometric-Algebra Foundations via the Scalar-Projection Method

Youping Dai

We formulate the low-energy theory of superconducting pairing in the geometric algebra $Cl_{3,0}$. Conventional BCS mean-field theory and its extensions ($s$-, $p$-, $d$-wave, etc.) have algebraic counterparts in an eight-dimensional real multivector space. The construction is a two-level projection: matrix blocks are represented as Clifford multivectors, and their grade-0 projected geometric products extract the scalar blocks from which Hamiltonians are built. The result is an algebraic reconstruction, physically equivalent to the standard complex-matrix formulation. In this reconstruction, the dichotomy of pairing channels acquires an algebraic root: it follows from the fermionic identity $B^\dagger(V)=-B^\dagger(V^{\mathsf T})$, and the Fierz rearrangement is realized by the map $\Phi:G\mapsto Ge_{31}$ (with $e_{31}^2=-1$). This yields a kernel/matrix dictionary: at the effective-kernel level grades $0\oplus3$ correspond to spin singlets and grades $1\oplus2$ to triplets; at the pairing-matrix level the singlet plane is $\mathrm{span}\{e_2,e_{31}\}$. The BCS limit has a double grade identity: kernel grade 0 (scalar glue) and vertex grade 2, a bivector ($i\sigma_2\leftrightarrow e_{31}$). We state three algebraic results: (i) a decomposition theorem for BdG quantum geometry with a null-texture criterion, applied to the chiral-state controversy in CsV$_3$Sb$_5$; (ii) a Fierz sign duality in the four-dimensional Minkowski algebra, with a chirality-reversal extension to two-node Weyl-semimetal models; (iii) an error bound and condition-number structure for tomographic reconstruction under a linear-response probe model. Illustrations include a glue-generation mechanism, a one-loop algebraic renormalization flow whose feeding constant coincides in magnitude with the $\mathfrak{su}(2)$ Casimir, closed-form $\mu=0$ inter-node spectra, and a UTe$_2$ two-phase analysis.
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Superconductivity · Cross-listed from cs.ET

JFS-CryoMem: A Cryogenic Memory with Voltage-Controlled Superconducting Devices and Femtojoule-Scale Write/Read Energies

Md Rahatul Islam Udoy, Md Mazharul Islam, Juan P. Mendez et al. (+7 more)

Scalable cryogenic systems require memory that combines nonvolatile storage, selective access, low thermal disturbance, and compatibility with superconducting electronics. We present a cryogenic memory architecture that integrates a voltage-controlled Josephson junction field-effect transistor (JJFET) selector with a ferroelectric superconducting quantum interference device (FeSQUID) storage element, hereafter termed JFS-CryoMem. The JJFET provides gate-controlled cell selection, whereas the FeSQUID stores information in stable remanent-polarization states. JFS-CryoMem features separate read and write path mechanisms that support nondestructive readout and independent optimization of programming and sensing conditions. The architecture is evaluated using experimentally calibrated compact models that reproduce the measured electrical characteristics of both constituent devices. We demonstrate selective programming using a half-bias scheme, nonvolatile state retention, and distinguishable readout in a $4 \times 4$ array while accounting for the selected cell and all unselected parallel branches. We then extend the analysis to arrays up to $16 \times 16$ and examine how array scaling alters current distribution, column-equivalent resistance, readout separation, required bitline current, and read energy. The results reveal the principal sensing and energy tradeoffs associated with larger arrays and identify the operating conditions required to preserve read distinguishability as the array grows. JFS-CryoMem provides a device-to-array framework for cryogenic memory in quantum, high-performance, and space-oriented computing systems.
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All authors Md Rahatul Islam Udoy, Md Mazharul Islam, Juan P. Mendez, Denis Mamaluy, Aaron J. Muhowski, Samuel Hawkins, William M. Martinez, Courtney Sovinec, Wei Pan, Ahmedullah Aziz

Saturday 26 September

Superconductivity

Spin pumping reveals vortex-mediated angular-momentum dissipation at the superconducting transition

Maximilian Mangold, Alex Burg, Franz Weidenhiller et al. (+4 more)

Spin pumping into superconductors is generally expected to become less efficient below the critical temperature $T_\textrm{c}$ because quasiparticle transport freezes out. Here we report a pronounced enhancement of the spin pumping-induced magnetic damping in Py/Nb heterostructures, confined to a narrow temperature interval immediately below the superconducting transition. The damping exceeds its normal-state value and therefore cannot be explained within the conventional quasiparticle picture of spin transport. The enhancement coincides with the finite-resistance vortex-creep regime of the Nb layers and disappears upon cooling into the pinned vortex state, indicating that mobile Abrikosov vortices provide an additional channel for spin-angular-momentum dissipation. These results establish ferromagnetic resonance as a sensitive probe of dissipative vortex dynamics in superconductors.
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All authors Maximilian Mangold, Alex Burg, Franz Weidenhiller, Thomas N.G. Meier, Hiroto Adachi, Lin Chen, Christian Back

Friday 25 September

Superconductivity

Quantum Geometric Origin of Yu-Shiba-Rusinov States

Rui-Xing Zhang

In this work, we revisit the classic Yu-Shiba-Rusinov (YSR) problem of magnetic impurities in $s$-wave superconductors and uncover a quantum geometric origin of the YSR bound-state structure. The key geometric quantity is the momentum-space average of Bloch band projector over low-energy electrons, whose matrix rank, positive eigenvalues, and corresponding eigenvectors directly control the number, energies, and symmetry representations of the YSR states, respectively. As a result, a superconductor with nontrivial normal-state quantum geometry can host more bound states than its trivial counterpart, even if they share the same electronic dispersion. We apply our theory to monolayer $1H$-NbSe$_2$ and find that the geometric texture of its single superconducting band enforces three symmetry-distinct YSR channels for a magnetic point impurity at Nb or Se sites. Our work opens a route to probing normal-state wavefunction geometry through local YSR spectroscopy.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Plasmonic Modes in hybrid Josephson Junction Arrays

Alex Kirchner, Simon Feyrer, Vjeko Dimić et al. (+9 more)

We present microwave characterization measurements of Josephson junction arrays (JJAs) based on epitaxial Al-InAs quantum well heterostructure. The Josephson inductance of the constituent planar junctions can be derived by probing the low-energy plasmon modes of these devices. Applying an out-of-plane magnetic field gives rise to a Fraunhofer-like diffraction pattern, from which the current-phase relation and the effective transparency can be extracted. JJAs can be used to achieve high inductances, suitable for the implementation of quantum circuits. They also provide an excellent test-bed for studying the microscopic excitations of hybrid superconductor-semiconductor devices associated with the presence of Andreev states. Here, we demonstrate their tunability over a broad range of out-of-plane magnetic fields.
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All authors Alex Kirchner, Simon Feyrer, Vjeko Dimić, Johanna Berger, Davide Curcio, Giorgio Biasiol, Michael Prager, Matthias Kronseder, Dominique Bougeard, Nicola Paradiso, Christoph Strunk, Leandro Tosi