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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Monday 21 September

Superconductivity

Comment on "Andreev Reflection to Probe Momentum-Dependent Spin Polarization in Altermagnet CrSb"

Igor I. Mazin, Maxim Khodas and Boris Nadgorny

Zhang et al. [Phys. Rev. Lett. 137, 016701 (2026)] report momentum-dependent spin polarization in the altermagnet CrSb from point-contact Andreev reflection along the (0001), (-1-120), and (10-10) directions. We show that the Andreev spin polarization they claim to measure is forbidden by symmetry: it vanishes identically in the diffusive regime for all directions in CrSb, and in the ballistic regime for two of the three orientations measured. It is allowed only along one direction, where the calculated band structure gives ~50%. The large, nearly isotropic values reported follow from an incorrect definition of ballistic Andreev spin polarization that evaluates the spin asymmetry at fixed three-dimensional momentum; the Andreev selection rule is local only in the momentum component parallel to the interface. Moreover, the ballistic regime is not demonstrated, and the fitting procedure is flawed.
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Superconductivity

Weyl Chirality and Kitaev Topology through Bargmann Invariants

Swarup Sangiri and A. Taraphder

We apply a finite-state geometric formulation based on Bargmann invariants to a two-band Weyl Hamiltonian and the one-dimensional Kitaev model of a p-wave superconductor. For the Weyl system, a symmetry-antisymmetrized third-order Bargmann combination selects the chirality-sensitive component of the local pseudospin geometry and extends to a two-band lattice Weyl model. For the Kitaev chain, a normalized ratio involving a third-order Bargmann invariant and a second-order overlap factor removes the dependence on an intermediate state and reproduces the $Z_2$ distinction between its two gapped phases. We further construct a fourth-order loop connecting localized Majorana edge-mode profiles with finite-energy BdG states and an alternating fourth-order loop involving opposite Weyl pseudospin sectors. These constructions illustrate how suitable Bargmann combinations can probe chirality-sensitive, topological, and higher-order geometric information directly through quantum-state overlaps, providing an alternative state-based route without requiring a continuous description of the underlying state-space geometry.
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Superconductivity · Cross-listed from cond-mat.str-el

Kitaev spin liquid in superconducting networks

Guilherme Delfino, Mehmet Dede, Dmitry Green et al. (+3 more)

We propose a realization of the Kitaev honeycomb Hamiltonian – an archetypal spin-liquid model – in a superconducting metamaterial. The architecture consists of Cooper-pair boxes coupled through depleted semiconductor–superconductor heterostructures that do not require spin–orbit coupling. The Cooper-pair boxes encode effective spin degrees of freedom, while normal and anomalous virtual propagation through the heterostructures mediate bond-directional interactions. Two key control parameters are an out-of-plane magnetic flux and the semiconductor Fermi energy. The former controls interference and distinguishes the bond directions, while tuning the latter close to the bottom of the band gives rise to an emergent Nambu-exchange symmetry that enforces the required bond directionality. Through numerical calculations, we identify an operating regime with controlled corrections, with associated energy and length scales within experimental reach. These results establish a route toward equilibrium quantum spin liquids in engineered superconducting networks.
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All authors Guilherme Delfino, Mehmet Dede, Dmitry Green, Michael J. Manfra, Charles M. Marcus, Claudio Chamon

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

Re-entrant p-wave superconductivity in a Chern Dartboard Insulator

Yueyang Wu, Rebecca Chan, Taylor L. Hughes et al. (+1 more)

We formulate an exact theory for a 2-band superconductor with inter-orbital pairing and band Hatsugai-Kohmoto (HK) interactions. We apply this theory to a Chern dartboard insulator and compute the pair susceptibility and pair correlations analytically. We find that in the insulating phase previously predicted by BCS theory (small pairing strength $g$, chemical potential $\mu$ in the gap), the HK interaction can induce superconductivity, and possibly a Chern dartboard superconductor, by lifting the doubly-occupied band and making it cross the chemical potential. From this HK-induced superconductor, we notice that in a significant parameter range (HK interaction strength $U = 1$, $\mu = 0.5$), an insulator-superconductor-insulator re-entrant transition exists. We isolate the re-entrance to a conflation between HK interactions and inter-orbital pairings.
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All authors Yueyang Wu, Rebecca Chan, Taylor L. Hughes, Philip W. Phillips

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

Quantum-interference-driven orbital density wave and high-temperature superconductivity in trilayer nickelates

Youichi Yamakawa and Hiroshi Kontani

Intertwined charge-density-wave (CDW) and spin-density-wave (SDW) orders are a hallmark of high-temperature superconducting multilayer nickelates. In trilayer La4Ni3O_{10}, charge correlations develop at temperatures above the onset of long-range spin order, and the characteristic ordering wavevectors satisfy $Q_{cdw} \approx 2Q_{sdw}$. Here, using a density-wave equation with vertex corrections, we show that quantum interference between short-range SDW fluctuations at $q \approx Q_{sdw}$ on the outer NiO2 layers generates an inter-outer-layer bond order at $Q_{cdw} \approx 2 Q_{sdw}$. This bond order induces a pronounced inner-layer-centered orbital order, with antiphase modulations of the Ni $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ occupations, producing strong orbital polarization but only weak total charge modulation. This intertwined bond-and-orbital order accounts for the layer-selective electronic reconstruction inferred from NMR/NQR and is consistent with Raman spectroscopy and scanning tunnelling microscopy measurements. The same orbital and spin fluctuations also cooperate to stabilize $s_{\pm}$-wave superconductivity through $M_z$ mirror-parity selection rules. Our results provide a unified microscopic framework for intertwined density-wave order and high-Tc superconductivity in multilayer nickelates.
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Superconductivity

Electronic Reconstruction Towards Topological Superconductivity in FeTe

Hongtao Rong, Yang Ge, Zi-Jie Yan et al. (+13 more)

The recent discovery of intrinsic superconductivity in stoichiometric FeTe films has renewed interest in the Te-rich end member of the iron chalcogenides for studies of unconventional and topological superconductivity, yet its intrinsic electronic structure remains unresolved. In this work, we combine molecular beam epitaxy, angle-resolved photoemission spectroscopy (ARPES), electrical transport measurements, density functional theory, and embedded dynamical mean-field theory to track the electronic reconstruction of 20-unit-cell FeTe films as Te annealing progressively removes excess interstitial Fe and drives the system from an antiferromagnetic metal to a superconductor. We find that this evolution is accompanied by recovered quasiparticle coherence, reduced electronic correlations, a Lifshitz transition, and a topological phase transition, yielding dxy-dominated hole and electron pockets that favor inter-pocket scattering. In addition, a shallow dxz/dyz-derived hole band located about 2 meV below the Fermi level may provide an incipient-band pairing channel, while scattering between the two electron pockets at M may offer additional pairing channels. High-resolution polarization-dependent laser ARPES measurements further reveal a topological surface state whose circular dichroism is consistent with the expected orbital-angular-momentum texture of stoichiometric FeTe. These results establish the intrinsic low-energy electronic structure of superconducting FeTe and identify the electronic states most relevant to superconductivity. The coexistence of intrinsic superconductivity and a topological surface state establishes stoichiometric FeTe as a promising platform for exploring topological superconductivity and possible Majorana bound states.
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All authors Hongtao Rong, Yang Ge, Zi-Jie Yan, Haoran Lin, Bing Xia, Xiaoda Liu, Zihao Wang, Pu Xiao, Lok-Kan Lai, Stephen Paplini, Jiatao Song, Jiangang Yang, Peter J. Hirschfeld, Shuolong Yang, Jiabin Yu, Cui-Zu Chang

Superconductivity

Generalized conditions for odd-frequency pairing in superconducting systems

Florian Kayatz, Annica M. Black-Schaffer and Jorge Cayao

Odd-frequency superconducting pairing has been predicted to arise in many systems and is known to lead to phenomena such as paramagnetic Meissner response and long-range superconducting proximity effect. Here, we provide generalized necessary and sufficient conditions for odd-frequency pairing appearing in any superconducting system, from bulk superconductors to superconducting hybrid structures. This generalizes an earlier first-order expression in multiband bulk superconductors to all superconducting systems and to all orders, in both order parameter and frequency. We then apply the derived conditions to several systems, including superconducting-ferromagnet and superconducting Josephson junctions, as well as a transition metal dichalcogenide monolayer proximitized by a conventional superconductor, where the generalized conditions are used to understand the properties of the superconducting state.
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Superconductivity

Interface engineering of spin triplet Cooper pairs for spin-valve implementation

Debashree Nayak, Abhisek Sahoo, Pratap Kumar Sahoo et al. (+1 more)

Spin singlet Cooper pairs can be converted into equal-spin triplet pairs at a superconductor/heavy-metal interface under suitable conditions. Under current carrying conditions, these triplet correlations can give rise to a non equilibrium spin moment in the heavy metal through the predicted supercurrent spin-Hall effect. Here we demonstrate that this current induced triplet spin moment, in conjunction with the magnetic moment of a ferromagnetic layer, can enable a magnetic spin-valve response in Nb/Pt/Ni/Pt/Nb vertical nano-devices. The magnitude of this spin-valve effect depends on the efficiency of singlet-triplet Cooper pair conversion at the Nb/Pt interface. In order to facilitate efficient triplet generation, we deliberately introduced interfacial roughness to introduce finite Rashba spin-orbit coupling at the Nb/Pt interface and an out-of-plane component of magnetic moment at the Ni interface. In contrast, no discernible spin-valve response was observed in devices with smooth interfaces within measurement resolutions. Since the magnetic moment of the Ni layer and the current-induced triplet spin moment in the Pt layer are independently switchable using a magnetic field and bias current, respectively, the spin valve can be controlled via either parameters. These results demonstrates a novel approach to directly utilizing the spin polarized triplet Copper pairs in superconducting spintronic applications.
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All authors Debashree Nayak, Abhisek Sahoo, Pratap Kumar Sahoo, Kartik Senapati

Superconductivity · Cross-listed from quant-ph

Superconducting qubit based on altermagnets

Xue-Feng Pan, Xin-Lei Hei, Franco Nori et al. (+1 more)

Altermagnets, characterized by vanishing net magnetization and momentum-dependent spin splitting, provide a promising platform for next-generation Josephson devices. Here, we exploit the Josephson effect in superconductor-altermagnet-superconductor junctions and show how to engineer prescribed current-phase relations by device design. Based on these programmable Josephson potentials utilizing altermagnetism, we propose a new class of superconducting qubits that combine large anharmonicity with enhanced robustness against decoherence via coherent two-Cooper-pair tunneling. We show that in the $2\phi$-junction regime, this kind of qubit is intrinsically protected against both charge and flux noise due to parity protection. Magnetic flux can be used to precisely control the qubit and, under appropriate bias, this architecture further suppresses charge and flux noise. Our results establish altermagnets as a versatile platform for Josephson-potential engineering and open a new route toward high-performance superconducting qubits combining high coherence, large anharmonicity, and broad tunability.
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All authors Xue-Feng Pan, Xin-Lei Hei, Franco Nori, Peng-Bo Li

Superconductivity

New theoretical model of the dynamic anomalies in HTc superconductors

J. Sosnowski

New theoretical model of the dynamic current-voltage characteristics anomalies in HTc superconductors in slowly varying magnetic field has been proposed. The model is based on an analysis in details of the magnetic flux penetration into HTc superconducting slab in such case. The comparison of model with previous experimental data has been presented. Theoretical analysis resulting from a new solution of diffusion equation is given as well as other one based on phenomenological critical state model.
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Superconductivity · Cross-listed from cond-mat.mes-hall

The Superconducting Talbot Effect in Phased-Array Josephson Junctions

Hechen Ren and Ziying Li

We introduce the superconducting Talbot effect—a macroscopic quantum self-imaging phenomenon occurring when proximitized Cooper pairs propagate through a ballistic two-dimensional electron gas. By configuring periodic superconducting leads into a phased-array Josephson junction with programmable phase differences, we demonstrate active steering of the resulting superconducting Talbot carpet. To overcome transport resolution limits, we design a Vernier-scale collector array that performs sub-wavelength sampling of the fractional Talbot pattern. This approach maps real-space quantum interference with high robustness to disorder, enabling direct extraction of Fermi wavelengths across helical, spin-degenerate, and spin-orbit-split Fermi surfaces. Tight-binding numerical calculations on a square lattice validate the real-space interference patterns. Our results establish a versatile framework for coherent wavefront engineering and quantum materials diagnostics in superconducting optics.
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Superconductivity

Detection of signals in presence of noise through Josephson junction switching currents

O. V. Pountougnigni, R. Yamapi, C. Tchawoua et al. (+2 more)

Josephson junctions can be employed to reveal a sinusoidal signal in presence of Gaussian noise. To mimic realistic setups, the detection is performed linearly ramping the bias current until a switch to the finite voltage occurs; the analysis of the resulting switching currents can be exploited to decide about the presence of the harmonic drive. The signal is applied in two conditions: with an unknown initial phase (incoherent strategy) and with a known initial phase (coherent strategy). In both conditions, the analysis of the efficiency of the detection, performed through the signal-to-noise ratio, as estimated by the Kumar-Carrol index, shows that the dependence upon the Josephson junction ramp rate is beneficial, especially for relatively fast speed. One can conclude that the collection of the switching currents is a robust technique, and thus it is possible to exploit the advantages of a predetermined finite time to collect the data.
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All authors O. V. Pountougnigni, R. Yamapi, C. Tchawoua, V. Pierro, G. Filatrella

Superconductivity · Cross-listed from nucl-th

Spin-One Secondary Pairing in Two-Flavor Color Superconductivity: Spinful Relativistic Superfluidity and Anomaly Matching

Naoki Yamamoto

We study secondary pairing in the two-flavor color-superconducting (2SC) phase, where the residual ungapped quarks form a same-chirality $J^P=1^+$ condensate driven by an attractive instanton-induced interaction. We determine its symmetry realization, quasiparticle structure, anomaly matching, and low-energy effective theory. The pairing gap is necessarily nodal; in particular, the complex axial state has two point nodes and realizes a spinful relativistic superfluid. This state preserves the full chiral symmetry ${\rm SU}(2)_{\rm L}\times{\rm SU}(2)_{\rm R}$ while breaking the modified baryon-number symmetry ${\rm U}(1)_{\tilde{\rm B}}$ and spatial rotations, with rotations about the nodal axis locked to the condensate phase. This locking gives rise to a Berry term, the Mermin-Ho relation, and a type-B orientational Nambu-Goldstone mode in addition to the superfluid phonon. We also show how anomaly matching is reorganized by secondary pairing: the perturbative mixed ${\rm SU}(2)_{\rm L,R}^2{\rm U}(1)_{\tilde{\rm B}}$ anomaly is realized by a Wess-Zumino coupling of the superfluid phonon, whereas the ${\rm SU}(2)_{\rm L}$ and ${\rm SU}(2)_{\rm R}$ Witten anomalies are carried by the point-node Bogoliubov-de Gennes flavor doublets. The resulting theory provides a concrete dense-QCD realization of a spinful relativistic superfluid with nodal fermions required by anomaly matching.
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Superconductivity

Parallel Simulation of Josephson Junctions With Multiplicative Noise

Vincenzo Pierro, Luigi Troiano, Elena Mejuto Villa et al. (+2 more)

Parallel graphic processing units have been employed for fast simulations of the switching dynam- ics of Josephson junctions subject to critical current fluctuations. Such a system is modeled by a nonequilibrium washboard model with multiplicative noise, for which analytical results are lacking. The proposed approach allows us to execute extensive numerical simulation in short time and with relatively inexpensive resources. This allows us to fully characterize the effect of the noise on the junction switching current distributions at realistic bias current sweeprates.
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All authors Vincenzo Pierro, Luigi Troiano, Elena Mejuto Villa, Sergio Pagano, Giovanni Filatrella

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

Direct Charge-4e Superconductivity by Mobilizing Bound Quartets

Jin-Tao Jin, Pengfei Li and Yi Zhou

A direct charge-\(4e\) superconductor exhibits coherent four-electron order without condensation in any charge-\(2e\) pairing channel. We establish three complementary results. First, a particle-number-conserving Hamiltonian with electron hopping and two-body interactions moves bound quartets efficiently: for electron- and pair-hopping amplitudes \(t\) and \(K\) and local gap \(U_0\), virtual pair transfer produces quartet hopping at order \(K^2/U_0\), whereas electron hopping alone contributes only at order \(t^4/U_0^3\). Second, the microscopic theory identifies an exactly solvable parent Hamiltonian at the phase-separation boundary. Its ground state combines quartet off-diagonal long-range order and finite phase stiffness without pair condensation, while rigorous bounds establish quartet order beyond the parent in a family with explicit quartet transfer. Third, a single convergent strong-coupling expansion simultaneously controls quartet kinetics and thermodynamic stability and rules out condensation in every pair channel of the microscopic model. On bipartite hypercubic lattices at half quartet filling, it yields a parametrically controlled homogeneous phase for \(d\geq2\), with quartet order, a linear collective mode, and gapped charge-\(2e\) excitations. Virtual pair transfer thus enables a direct charge-\(4e\) superconducting ground state without pair condensation.
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Superconductivity

Coexistence and Interconversion of Multiple-Order Majorana Modes in Topological Superconductor Films with Varying Thickness

Zhizhong Ding, Chuanbao Zhang, Shunhong Zhang et al. (+4 more)

We theoretically investigate the thickness-dependent evolution of Majorana modes in $C_{2h}$-symmetric topological superconductor films (such as the recently discovered 2M-WS$_2$) proximity coupled with magnetic insulators. For sufficiently thick films, two Majorana bound states coexist as end modes at the surface and interface along a vortex line, with the interfacial mode evolving into a chiral Majorana edge mode upon increasing the proximity-induced exchange field. The intrinsic $C_{2h}$ crystalline symmetry selects two chiral Majorana modes circulating along the hinges on two of the four side surfaces of the film. When the penetration depth of the exchange field is sufficiently shallow, the two circulating modes are localized near the interface, but with qualitatively different subsequent evolutions. One of them further collapses to form two Majorana corner modes, while the other merges with the chiral Majorana mode circulating around the interface. Importantly, the corner modes are well decoupled from the interfacial chiral mode, thereby enabling an unprecedented coexistence of first-, second-, and third-order Majorana modes within a single material platform. We further show that such coexistence persists even in the ultrathin-film limit, where the electric-field-controlled two-dimensional $Z_2$ topology offers an extra advantage to readily interconvert the multiple-order Majorana modes. These findings highlight the pivotal role of the proper crystalline symmetry in enabling emergence, manipulation, and potential braiding of Majorana modes for demonstrating non-Abelian statistics and fault-tolerant quantum computation.
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All authors Zhizhong Ding, Chuanbao Zhang, Shunhong Zhang, Xiaoyu Zhu, Wei Qin, Ping Cui, Zhenyu Zhang

Saturday 19 September

Superconductivity

Bulk Superconductivity in Rocksalt LaN$_{1-x}$

Caeli Benyacko, Lin-Ding Yuan, Josiah A. Turner et al. (+7 more)

We report the synthesis and characterization of lanthanum nitride by direct reaction of lanthanum metal with high pressure nitrogen gas in a laser floating-zone furnace. Combined synchrotron X-ray and neutron diffraction measurement verifies a rocksalt structure with stoichiometry LaN$_{0.94}$, consistent with first principles prediction that LaN is dynamically unstable in the stoichiometric limit. Electrical transport, heat capacity, and magnetization measurements show that LaN$_{0.94}$ is metallic with a superconducting transition at $T_c=5.95$ K. Our data establish LaN as the first bulk superconducting member of the rare earth mononitride family and motivates future exploration of the series using the unique processing space afforded by high-pressure optical reactors.
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All authors Caeli Benyacko, Lin-Ding Yuan, Josiah A. Turner, Laila Reimanis, Siyuan Ji, Cheng Li, Erick A. Lawrence, Hanna Z. Porter, James M. Rondinelli, Stephen D. Wilson

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

Correlation-assisted spin-selective metallicity in strained and bilayer altermagnets

Marnin J. Di Nunzio, Frank Lechermann and Ilya M. Eremin

Altermagnets combine vanishing net magnetization with a momentum-dependent spin-split electronic structure, providing a route to spin-polarized carriers without ferromagnetism. Here, we investigate how electronic correlations, doping, uniaxial strain, and interlayer coupling control altermagnetism within a minimal Hubbard-type model for mono- and bilayer systems. Comparing Hartree–Fock theory with the rotationally invariant slave-boson (RISB) approach, we demonstrate the robustness of altermagnetic order against quasiparticle renormalization and reveal a separation between the onset of magnetic order and the loss of quasiparticle coherence at stronger coupling. In the monolayer, doping produces a pronounced particle–hole asymmetry, while its combination with uniaxial strain generates a fully spin-polarized Fermi surface close to half-filling. In the bilayer, stacking that favors ferroic alignment of the layer altermagnetic order parameters allows their momentum-dependent spin splittings to combine constructively. Weak asymmetric doping additionally induces intra-unit-cell charge order, resulting in fully spin-polarized low-energy carriers. At a bilayer filling of five electrons in four orbitals, we find a continuous paramagnet-to-altermagnet transition followed, at stronger coupling, by an evolution toward a Mott-like regime with strongly suppressed quasiparticle weight. Our results establish correlations, strain, doping, and stacking as complementary means of controlling altermagnetic metals and generating fully spin-polarized Fermi surfaces.
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Superconductivity · Cross-listed from quant-ph

Performance optimization of cascaded traveling wave Josephson parametric amplifiers

Ilari Lilja, Ekaterina Mukhanova, Stanislav Khaldeev et al. (+3 more)

Traveling-wave parametric amplifiers (TWPAs) based on Josephson metamaterials provide broadband gain with near-quantum-limited added noise. Whereas long nonlinear metamaterial devices can deliver high gain, short arrays suffer less from dissipation, pump depletion, and internal standing waves which can degrade noise performance. Here, we demonstrate a cascaded TWPA architecture that combines the advantages of both approaches by employing a short (736-element), low-dissipation Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL)-based TWPA as the first amplification stage, followed by a conventional long (1632-element) TWPA that provides additional gain. The resulting amplifier cascade achieves nearly 30 dB of total gain over a tunable bandwidth of approximately 1 GHz while maintaining added noise close to the quantum limit. Our results establish a cascade of TWPAs as a practical approach for high-gain, broadband, quantum amplification with applications in quantum information processing, multi-mode entanglement, and quantum sensing applications at microwave frequencies.
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All authors Ilari Lilja, Ekaterina Mukhanova, Stanislav Khaldeev, Ilya Golokolenov, Visa Vesterinen, Pertti Hakonen

Superconductivity

Pressure-induced unconventional charge-density-wave states in kagome metal AV3Sb5 (A = K, Rb, Cs)

Zhimian Wu, Linpeng Nie, Ye Yang et al. (+6 more)

Since the discovery of charge density wave (CDW) and superconductivity, kagome metal AV3Sb5 (A = K, Rb, Cs) provides a new platform for exploring novel many-body quantum phenomena. In CsV3Sb5, a stripe-like CDW with commensurate wave vector q = 3/8 was observed under moderate pressures, which leads to a peculiar superconducting double-dome behavior in pressure-dependent phase diagram. Previous density functional theory (DFT) calculations indicate that the pressure-induced stripe-like CDW is beyond conventional phonon softening scenario, suggesting a nontrivial role of electronic correlations. However, an in-depth understanding for the pressure-induced unconventional CDW remains elusive. Here, we performed pressure-dependent 51V nuclear magnetic resonance (NMR) measurements on KV3Sb5 and RbV3Sb5. Although the superconducting double-dome behavior is absent in pressurized KV3Sb5 and RbV3Sb5, a pressure-induced CDW phase, ascribed to a possible incommensurate triple-Q CDW, is identified by NMR spectra in both materials, indicating that the pressure-induced unconventional CDW beyond DFT calculations is a common feature for kagome metal AV3Sb5. In contrast to the stripe-like CDW, the pressure-induced incommensurate triple-Q CDW does not strongly suppress the superconducting temperature (Tc) but coincide with an almost plateau behavior at intermediate pressure regime in the pressure-dependent superconducting phase diagram. Furthermore, by systematically analyzing the Korringa relation between Knight shift and nuclear spin-lattice relaxation rate in AV3Sb5, van Hove singularities (vHSs) driven electronic fluctuations are revealed as an effective knob for the pressure-induced unconventional CDW. Finally, our present findings underscore the pressure-induced unconventional CDW as a novel correlated quantum state in kagome metal AV3Sb5.
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All authors Zhimian Wu, Linpeng Nie, Ye Yang, Kuanglv Sun, Huachen Rao, Dan Zhao, Zhongjun Li, Tao Wu, Xianhui Chen

Superconductivity

Comment on "Topography of Fermi arcs in t-PtBi$_2$ using high-resolution angle-resolved photoemission spectroscopy" arXiv:2503.08841 (cond-mat)

Sergey Borisenko

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy have established surface superconductivity in the Weyl semimetal t-PtBi$_2$, whereas O'Leary et al. concluded from an independent ARPES experiment that superconducting signatures are absent above 3 K. Here we reanalyze the complete deposited data of O'Leary et al. Using only experimentally determined momentum-distribution-curve (MDC) maxima, energy-distribution-curve (EDC) peak positions, and leading edges, we find a strongly anisotropic low-energy gap and dispersion back-bending that closely reproduce the magnitude and angular dependence reported independently by Changdar et al. We further show that the temperature-dependent data of Ref.~\cite{Oleary} originate from the other surface termination and were presented using temperature-dependent energy translations, momentum translations, and momentum rescalings. Registering the original temperature-dependent datasets using the metallic bulk Fermi cutoff reveals finite gaps of approximately 2, 2, 1, and 0.8 meV at the four Fermi crossings. Thus the independently acquired data of O'Leary et al., obtained on different samples and with a different photon energy, confirm rather than contradict the previously reported anisotropic superconducting gap and additionally show that an anisotropic low-energy suppression persists to 19 K.
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Friday 18 September

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

Probing $d$-wave pairing in the $t$-$t'$-$U$ Hubbard model with tensor-backflow wave functions

Xiao Liang

Building on the recently developed tensor-backflow method, which accurately describes energies and stripe states in the Fermi-Hubbard model, we investigate $d$-wave pairing in the two-dimensional $t$-$t'$-$U$ Hubbard model. For the $8\times8$ lattice, we improve the wave function using symmetry projections followed by one Lanczos step, whereas for the $12\times 12$ and $16\times 16$ lattices, we apply one Lanczos step without symmetry projection. We focus primarily on the parameter regime with filling $n=0.875$, on-site repulsion $U=8$ and next-nearest-neighbor hopping $t'=-0.2t$, a regime in which enhanced $d$-wave pairing has been reported. The resulting energies are competitive with those obtained using state-of-the-art neural quantum states. For example, on a $16\times 16$ lattice with periodic boundary conditions, the tensor-backflow wave function after one Lanczos step without explicit symmetry enforcement achieves a relative energy difference of $4.4\times 10^{-3}$ from the symmetry-preserving neural quantum state result. From the real space pair correlations, for $t'=-0.2t$, we observe stronger $d$-wave pair correlations together with the characteristic relative sign between horizontal and vertical bond-pair components. Compared with $t'=0$, finite negative $t'$ increases the spectral weight of the corresponding pair-density matrix, which indicates the enhanced overall $d$-wave pair fluctuations. The leading eigenvalue is enhanced on the $8\times8$ lattice for both with and without symmetry projections. For larger lattices such as $12\times 12$ and $16\times 16$, the leading pair-density matrix spectral weight is distributed among several nearly degenerate eigenmodes rather than being concentrated in a single dominant mode. This indicates enhanced and more broadly distributed $d$-wave pair fluctuations, but no single eigenvalue exhibits the extensive scaling required for ODLRO.
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Superconductivity

Raman scattering in cuprate oxychlorides high-temperature superconductors single-crystals

Chafic Fawaz, Yingzheng Gao, Luca Laveder et al. (+13 more)

We investigate the Raman response of sodium-doped cuprate oxychloride Na$\_x$Ca$\_{2-x}$CuO$\_2$Cl$\_2$ high-temperature superconductors across their entire phase diagram, from the antiferromagnetic to the superconducting region. In addition to the expected Raman-active phonon modes, we detect several additional modes that can be interpreted as being excited via a resonance process enabled by strong electron–phonon coupling. To verify the resonance effect, the Raman response at different incident photon energies was measured. At high energies, there is a well-defined $B\_{1g}$ mode in the antiferromagnetic phase, which can be interpreted as a bimagnon We follow its temperature and doping dependence, providing information on the multimagnon excitation in these cuprates, which can be theoretically linked to the exchange interaction.
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All authors Chafic Fawaz, Yingzheng Gao, Luca Laveder, Lorenzo Menon, Owen Moulding, Rolf Heid, Blair W Lebert, Christophe Bellin, Keevin Beneut, David Santos-Cottin, Ikuya Yamada, Yuichi Okazaki, Hajime Yamamoto, Masaki Azuma, M.-A. Méasson, Matteo d'Astuto

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

Correlation enhanced magnetism at a superconducting cuprate/manganite interface

Vitor A. M. Lima, Marcello Civelli and Walber Hugo de Brito

Motivated by experimental studies on cuprate-manganite heterostructures, we study a correlated bilayer Hubbard model subject to a local interfacial exchange field using cluster dynamical mean-field theory in the underdoped cuprate regime. We find that superconductivity coexists with a manganite-induced Cu spin polarization. Remarkably, throughout this coexistence regime, the superconducting solution carries a larger magnetic moment than the corresponding constrained normal state. Analysis of the magnetic response shows that this reversal is driven by correlations, which enhance magnetism already in the normal state and even more strongly in the superconducting solution. At stronger exchange fields, superconductivity collapses abruptly, displaying hysteresis consistent with a first-order transition, and a strongly spin-polarized pseudogap emerges. These results show that, under strong electronic correlations, superconductivity can enhance rather than suppress interfacial magnetism.
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Superconductivity

Tunneling Spectroscopy Study of Low Baked Bulk Niobium for Superconducting RF Cavity Applications

Ivana Curci, Gregoire Jullien, Luc Maurice et al. (+3 more)

Point-contact tunneling (PCT) spectroscopy was used to probe the local superconducting properties of cavity-grade niobium following surface treatments representative of superconducting radio-frequency (SRF) cavities. Electropolished (EP) samples were compared with samples baked at 120~$^\circ$C for 48 h and subjected to a two-step bake at 75~$^\circ$C for 2 h followed by 120~$^\circ$C for 48 h. Although the superconducting gap $\Delta$ is predominantly distributed around the bulk Nb value (~1.5 meV), regions exhibiting strongly suppressed gaps (minigaps) are observed in 15–29% of junctions on EP samples. Their occurrence decreases to 6% after the 120~$^\circ$C bake and to only 1% after the two-step treatment. Spectra exhibiting minigaps are well described by the Usadel proximity-effect model, consistent with superconducting Nb coupled to normal regions that we attribute to hydride formation. These results provide direct microscopic evidence that low-temperature baking suppresses regions of degraded surface superconductivity in cavity-grade Nb. The pronounced reduction achieved by the two-step bake further supports a link between hydride-related proximity effects and the surface superconducting properties governing SRF cavity performance.
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All authors Ivana Curci, Gregoire Jullien, Luc Maurice, Fabien Eozenou, Patrick Sahuquet, Thomas Proslier

Superconductivity

Strong-coupling multigap superconductivity in the Heusler compound ScAu$_2$Al

Jozef Kačmarčík, Zuzana Pribulová, Yevhen Petrenko et al. (+9 more)

ScAu$_2$Al is the Heusler superconductor with the highest reported transition temperature, Tc, close to 5 K. Here, we combine high-resolution ac calorimetry and electrical-transport measurements with first-principles-based Eliashberg calculations. The large specific-heat jump, ${\Delta}C/{\gamma_nT_c} =$ 2.2, well above the weak-coupling BCS value of 1.43, together with an electron-phonon coupling constant ${\lambda} = $1.1, establishes ScAu2Al as a strong-coupling superconductor. The electronic specific heat is described better by a two-gap {\alpha} model, with $2{\Delta}_S/k_BT_c = $ 4.1 and $2{\Delta}_L/k_BT_c = $ 4.8, than by a single-gap model. The thermodynamic upper critical field follows a conventional WHH-like temperature dependence with $B_{c2}(0) = 0.18$ T, whereas the resistively determined critical field reaches values about three times larger and exhibits a pronounced positive curvature. We attribute the enhanced resistive field scale to superconductivity in disordered grain-boundary and interfacial regions. The experimental results are supported by calculations of the heat capacity and upper critical field within the Eliashberg formalism using Fermi-surface and electron-phonon parameters obtained from first-principles calculations
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All authors Jozef Kačmarčík, Zuzana Pribulová, Yevhen Petrenko, Gabriel Kuderowicz, Paweł Wójcik, Michał J. Winiarski, Szymon M. Królak, Filip Košuth, Pavol Szabó, Tomasz Klimczuk, Bartlomiej Wiendlocha, Peter Samuely

Superconductivity

Failure of Conventional Roughness Metrics in Assessing Field-Limiting Mesoscopic Topography in SRF Nb Films on Cu

Eric M. Lechner, Brandi Redman, Theodore S. Cahall et al. (+1 more)

A characteristic corrugated surface morphology of Nb films on Cu is identified, with mesoscopic features comparable to the London penetration depth and coherence length, and its impact on superconducting radio frequency cavity performance metrics is examined. Magnetic field enhancement factors and superheating field suppression factors are calculated within the London model for a representative corrugated geometry. These results demonstrate that roughness trends derived from buffered chemical polished and electropolished Nb cavities do not capture the impact of nanoscale surface morphology on SRF Nb thin film performance, as even surfaces that possess low average roughness can contain geometric features that generate substantial local field enhancement and significantly suppress the Bean Livingston barrier. The influence of surface roughness on impurity diffusion is also investigated, which highlights the roles of increased surface area and local geometric confinement in modifying near surface impurity distributions. Tracking the effect of impurity diffusion on the evolution of magnetic field enhancement, we show that geometrically confined impurity distributions can mitigate nanoscale magnetic field enhancement substantially.
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All authors Eric M. Lechner, Brandi Redman, Theodore S. Cahall, Anne-Marie Valente-Feliciano

Superconductivity

Topological properties and gap structure of the paired state in twisted TMD bilayers within a two-band effective model

Palash Saha and Michał Zegrodnik

We present a theoretical study motivated by the recent experimental results which demonstrate superconductivity emerging from flat topological bands of twisted transition metal dichalcogenide (TMD) bilayers. To capture the non-trivial band topology of the system, we employ an effective two-band Kane-Mele-like model and substitute it with Coulomb repulsion and inter-site pairing interactions treated at the Hartree-Fock mean-field level. Assuming a real-space pairing scenario, we analyze the resulting superconducting gap symmetry and the stability of the paired state as a function of band filling and applied displacement field. Finally, we highlight the interplay between superconductivity and non-trivial topology, detailing how interaction-induced effects and the evolution of the density of states shape the resulting superconducting phase diagram.
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Superconductivity

Universal Dzyaloshinski-Moriya interaction dictates pairing in unconventional superconductor families

Baishun Yang, Yida Chu, Xuelei Sui et al. (+3 more)

The collinear-antiferromagnetic spin-fluctuation paradigm has long guided unconventional superconductivity research, yet fails to reconcile the noncollinear spin phenomena observed across cuprates, iron-based superconductors, and nickelates. Using extensive first-principles calculations and unbiased large-scale DMRG simulations, we show that Dzyaloshinski-Moriya interaction (DMI)-arising from local inversion-symmetry breaking-is a common ingredient across these families. This DMI unifies hallmark observations in parent compounds-incommensurate orders, spin-wave gaps, and noncollinear textures. Under hole doping, strong DMI drives spin vortices to merge with pi-shifted hole stripes, forming hybrid vortex-hole stripe phases. These phases stabilize charge order while supporting, not suppressing, superconductivity. By contrast, under electron doping, these vortices pin holes and suppress long-range superconductivity. Our results establish DMI as a unifying link between noncollinear magnetism and superconductivity, identifying hole-strip-vortex coupling as a microscopic pairing engine. Given that DMI is common across major superconductor families, these findings challenge the prevailing pairing mechanism and offer an experimentally testable roadmap for materials optimization.
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All authors Baishun Yang, Yida Chu, Xuelei Sui, Haiqing Lin, Shijie Hu, Bing Huang

Superconductivity

Collective excitation-mediated transport in nanoscale Josephson junctions that exhibit quantum confinement

Zhengyuan Liu, Sebastian Scherb, Werner M.J. van Weerdenburg et al. (+3 more)

Quantum confinement can strongly modify transport through Josephson junctions. Here, we study local tunneling transport through nanoscale Josephson junction stacks in the Coulomb blockade regime, where the metallic layers exhibit strong vertical quantum confinement. We find that quasiparticle transport is strongly enhanced by a collective excitation mode intrinsic to the junction and localized in the isolated metallic overlayer. We quantify both the collective-mode energy and the Coulomb gap and show that both exhibit strong layer-dependent modulation, consistent with the modulation of the underlying quantum well states. We further investigate how the collective excitation responds to various perturbations, including mechanical motion and an applied magnetic field. Our results suggest that this collective mode is sensitive to quasiparticles near the Fermi level and may therefore provide an indirect probe of the superconducting state.
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All authors Zhengyuan Liu, Sebastian Scherb, Werner M.J. van Weerdenburg, Daniel Wegner, Nadine Hauptmann, Alexander A. Khajetoorians