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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Wednesday 9 September

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

Orbital-selective electronic modes induced by doping and originating from distinct spin excitations: an essential feature of orbital-selective Mott transition

Masanori Kohno

The nature of the orbital-selective Mott transition (OSMT) remains elusive. This study shows that, by doping an orbitally degenerate spin-1 antiferromagnetic insulator in the Kanamori-Hubbard model, electronic modes emerge in the band gap in an orbital-selective manner, exhibiting momentum-shifted spin-mode dispersion relations. Electronic modes originating from conventional spin excitations, including the low-energy one, appear in the doped orbital, while those from inter-orbital spin excitations appear in the undoped orbital. These emergent modes constitute an essential feature of the OSMT.
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Superconductivity

Record-Breaking Elemental Superconductivity in Tetralayer Kagome Borophene

Yingnan Liu, Yan Liu, Renyu Duan et al. (+4 more)

Superconductivity above the liquid-nitrogen temperature remains rare in two-dimensional elemental crystals, where strong covalent bonding often yields high phonon frequencies but insufficient electron-phonon coupling. Here, using first-principles calculations and fully anisotropic Migdal-Eliashberg theory, we predict tetralayer kagome borophene (TKB) stabilized by ABAB covalent stacking, as a liquid-nitrogen-temperature elemental superconductor. With a predicted critical temperature of 102 K, TKB sets a record-high value among previously reported elemental superconductors. Unlike known high-Tc boron-based superconductors dominated by in-plane sigma-bonding states and high-frequency in-plane B-B stretching modes, TKB realizes an out-of-plane s-pz-bonding-mediated pairing mechanism, in which interlayer s-pz bonding states at the Fermi level are strongly coupled to low-frequency out-of-plane vibrations of boron atoms. These results reveal a distinct out-of-plane pairing channel in multilayer borophene and establish covalent stacking engineering as a potential route for high-Tc superconductivity in two-dimensional materials.
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All authors Yingnan Liu, Yan Liu, Renyu Duan, Menghui Wang, Meiling Xu, Hanyu Liu, Shoutao Zhang

Superconductivity

Eight-unit-cell electronic modulations in cuprates originating from local molecular orbitals

Zhiheng Yao, Sixuan Chen, Jianfa Zhao et al. (+10 more)

The pair density wave (PDW) state with eight-unit-cell (8a0) periodicity has been widely regarded as the primary order in cuprates, yet its existence and origin remain subjects of intense debate. Using spectroscopic imaging scanning tunneling microscopy, we observe spatial modulations of the electronic states with approximately 8a0 periodicity in both the superconducting and insulating regimes of hole-doped Ca2CuO2Cl2 cuprate. We find that the 8a0 spatial patterns are generated by the formation of molecular orbitals by doped holes, which organize into 4a0*4a0 plaquettes as the basic unit. Our results identify the 4a0 molecular orbital as the fundamental electronic building block in cuprates, while the 8a0 PDW represents a spatial subharmonic that emerges at sufficiently high doping.
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All authors Zhiheng Yao, Sixuan Chen, Jianfa Zhao, Shusen Ye, Weixiang Qu, Ning Xia, Yuling Dai, Luchuan Shi, Hongrui Zhang, Zhenqi Hao, Changqing Jin, Shuo Yang, Yayu Wang

Tuesday 8 September

Superconductivity

Generating is not discovering: a pre-registered physics judge for AI-proposed superconductors, calibrated on six known superconductors and one negative control

Reinaldo Inácio

Generative models now propose millions of "stable" crystal structures, and their own referees have shown that stability is not discovery; no public benchmark asks whether a candidate has the physics to do a job. For superconductors the job is twofold: to pair, with a definite gap symmetry, and to become wire. We present a physics judge that answers both from a structure alone (DFT+U, a gated Wannier model, the full rank-4 RPA susceptibility on a $24^3$ mesh, the linearized gap equation, an irreducible-representation classifier, a manufacturability funnel) and measure what it says about what generators propose. With answer keys pre-registered before computing, the judge was calibrated on six known superconductors (Nb, Nb$_3$Sn, MgB$_2$, BaFe$_2$As$_2$, La$_{2-x}$Sr$_x$CuO$_4$, YBa$_2$Cu$_3$O$_7$), recovering the expected class in all six after two dated corrections. On the ruthenium analogue of the iron pnictide, with two runs and their criterion pre-registered the day before, it returns a null verdict at the same physical interaction where iron gives s$\pm$ ($\lambda_1 \le 0.0007$ versus 0.096; critical U 4.17 versus 0.97 eV); the pre-registered equal-alpha run fails the letter of the criterion, and we declare the resolution in favour of absolute U as post-hoc. The funnel reproduces the industrial map with thresholds fixed before running. Three generation audits follow: a descriptor sweep over 150 Materials Project metals re-finds the canon; 1,248 MatterGen structures yield 0 candidates that are new, stable and carry a pairing motif; mechanism filters over 47,893 compounds, validated by a blind hold-out, re-derive the community's analogies: geometry plus d-count is necessary but not sufficient. Generation re-finds what is known; judgment is the bottleneck. We propose a public benchmark over the eleven lists of AI-proposed superconductor candidates, none of which classifies gap symmetry.
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Superconductivity

Origin of Flat Bands and Role of Electron Correlation in Lutetium Hydrides

Anmol Lamichhane, Adam Denchfield, Hyeondeok Shin et al. (+3 more)

Lutetium hydrides (LuH$_x$, $1.75 \leq x \leq 3$) form a diverse series of phases, several of which superconduct under pressure. Characterizing their electronic properties has remained challenging owing to a high propensity for hydrogen defect formation, and recent angle-resolved photoemission (ARPES) measurements reveal puzzling flat-band regions that position these materials as candidates where superconductivity and flat-band physics may intersect. Here, by combining density functional theory, dynamical mean-field theory, and the constrained random-phase approximation, we uncover the microscopic origin and correlation nature of these flat bands. Across all compositions, the screened on-site Coulomb interaction is larger for H-s states than for Lu-$d$ states due to compact hydrogen orbitals. Nevertheless, these systems remain weakly correlated metals: the nearly filled H-$s$ shell admits little charge fluctuation, so its large interaction acts as a static level shift rather than a source of correlation. Although hydrogen primarily occupies tetrahedral sites at $x=2$, we discover that anti-site defects–where hydrogens occupy slightly unfavorable octahedral sites–generate both the ARPES flat-band features and the low-energy optical absorption peak, attesting to the usual defective nature of such materials in experimental samples. We further find that correlation strength is governed primarily by hydrogen orbital filling at these sites rather than the interaction magnitude itself. Consequently, we identify hydrogen orbital filling as the fundamental organizing principle dictating correlation and low-energy flat-band physics in lutetium hydrides.
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All authors Anmol Lamichhane, Adam Denchfield, Hyeondeok Shin, Panchapakesan Ganesh, Russell J. Hemley, Hyowon Park

Superconductivity · Cross-listed from cs.CE

Fixed-mesh based approach for modeling of superconducting magnetic bearings

Elias Paakkunainen, Bárbara Maria Oliveira Santos, Gabriel dos Santos et al. (+3 more)

Numerical simulations of superconducting magnetic bearings (SMBs) are complicated by the relative motion between high temperature superconductors and the magnetic guideway, which commonly requires repeated updates of the geometry and mesh. We present a fixed-mesh based approach for modeling the motion induced effects in which a coordinate transformation is applied only to the air region between the moving regions. The transformation can be included in a time-dependent anisotropic material tensor, leaving the mesh during the simulation and the rest of the formulation unchanged. To provide numerical examples, the approach is implemented in 2D using the finite element method with the $j$-$a$ formulation and is combined with homogenization and circuit coupling for double-crossed loops of coated conductors. The open-source model implementations are made available. The proposed approach is verified and validated against existing modeling approaches and experimental results from the literature for SMBs. Two simulation examples are presented, and in both of them the levitation force predicted by the fixed-mesh model agrees with the experimental results, yielding $R^2$ values above 0.99. For a verification problem, it is shown that the fixed-mesh model converges towards the remeshed reference solution under mesh refinement as expected. For an SMB undergoing vertical and horizontal translations, the homogenized fixed-mesh model reproduces the lateral-force response of a fully resolved moving-geometry reference model. Additionally, the homogenized fixed-mesh model is computationally more efficient than the reference model, indicating a significant reduction in the number of degrees of freedom and the computation time. The results demonstrate accurate and computationally efficient simulation of translational motion with the fixed-mesh based approach.
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All authors Elias Paakkunainen, Bárbara Maria Oliveira Santos, Gabriel dos Santos, Timo Tarhasaari, Paavo Rasilo, Sebastian Schöps

Superconductivity

Junction-free polarity-tunable superconducting diode effect enables XNOR logic operation

Alapan Bera and Soumik Mukhopadhyay

The realization of the superconducting diode effect (SDE), characterized by non-reciprocal supercurrent, is a major step towards dissipationless logic circuits. A polarity-tunable superconducting diode with high diode efficiency primarily consists of Josephson-junction architectures that couple two superconducting regions via a weak link. Realizing an SDE without a weak-link junction would reduce fabrication complexity, lower electrical noise, and minimize dependence on junction quality. Here, we report an all-vdW, highly efficient, junction-free SDE in NbSe2, enabled by proximity-induced symmetry breaking from a strong uniaxial ferromagnet Fe3GeTe2 (F3GT). Superconducting diode efficiency of up to 30% is achieved at a small magnetic field of 50 mT. A field-free diode rectification effect is realized with diode polarity that is tunable by the F3GT spin polarization. Furthermore, by employing the active-field dependence of the SDE, a two-input exclusive-NOR (XNOR) logic gate operation is demonstrated. The findings highlight the potential utilization of magnetic proximity effect towards realizing junction-free superconducting diodes for high-performance, energy-efficient logic circuits.
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Superconductivity

Fabrication And Characterization Of High-Quality Nb/Al-AlO$_x$/Nb Cross-Type Josephson Tunnel Junctions Utilising CMP-based Planarisation Techniques

Alexander Stoll, Andreas Reifenberger, Daniel Hengstler et al. (+2 more)

Josephson tunnel junctions (JJs) are the fundamental building blocks of today's most advanced superconducting electronic components, such as qubits and superconducting quantum interference devices (SQUIDs). Given the ongoing demand for scalability of these devices on the wafer-scale, it is crucial to control the fabrication process as precisely as possible to ensure uniform quality and reproducibility. Today, window-type JJs are often used in the production of DC SQUIDs and while they are reliable and reproducible, they suffer from alignment inaccuracies caused by photolithography and unavoidable parasitic capacitances, thus limiting the energy sensitivity of DC SQUIDs. These problems can be circumvented by producing JJs with cross-type geometry, which allows for smaller junction areas and eliminates parasitic capacitances. Here we discuss the fabrication of Nb/Al-AlOx/Nb-based cross-type JJs, where the trilayer is embedded in sputter-deposited SiO$_2$ to allow for planarisation of the structures and to ensure the reliable insulation of the sidewalls of the bottom electrode. Instead of lift-off processes that require a lot of time and potentially leave unwanted wings along the edges of microstructures behind that might compromise subsequent layers, we use chemical-mechanical polishing (CMP) for the removal of excess SiO$_2$. This is not only much faster, but produces smooth and uniform surfaces, which in turn effectively improves the reliability of our JJ production process resulting in a high yield of over $90\,\%$ on wafer-scale. In addition, we discuss the influence of fabrication details on the quality and electrical properties of our JJs with different junction areas down to $1\,\mathrm{\mu m} \times 1\,\mathrm{\mu m}$ distributed across the wafer and extract the junction specific quality parameters from their IV-characteristics and their Fraunhofer patterns.
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All authors Alexander Stoll, Andreas Reifenberger, Daniel Hengstler, Andreas Fleischmann, Christian Enss

Superconductivity

Thomas-Fermi screening of electrostatic fields in a type-I superconductor

Nikhil Seeja Sivakumar, Tyler James, Frederick Carlisle et al. (+2 more)

The empirical London equations make distinct predictions for a superconductor's longitudinal and transverse electrodynamic response. Conventionally, this framework attributes the Meissner effect to the transverse component, with the longitudinal response predicted to remain unchanged on entering the superconducting state, i.e. a static electric field is screened over the Thomas-Fermi length ($\lambda_{\mathrm{TF}} \sim 1$~Å), just as in the nonsupercondcuting metal. J.~E.~Hirsch~[Phys.~ Rev.~B~69,~214515~(2004)] has developed an alternative formalism, which predicts that the longitudinal screening length should instead be governed by the London penetration depth as the system is cooled below the superconducting transition temperature ($T_{\mathrm{c}}$). Here we use a combination of qPlus atomic force microscopy and scanning tunneling microscopy/spectroscopy of single-crystal Pb(111) at $\sim$~340~mK~ \mbox{($T/T_{\mathrm{c}}\sim0.05$)} in an attempt to detect any modifications in electrostatic screening of the longitudinal tip field upon entering the superconducting phase. We quench superconductivity using a magnetic field of 200 mT normal to the Pb(111) surface. By measuring force-distance curves and local field-emission resonance spectra at the same sample position in the presence and absence of the magnetic field, we constrain the change in screening between the normal and superconducting state to less than $\sim 1 \%$. This is between two to three orders of magnitude smaller than that predicted by Hirsch's theory, and entirely consistent with unmodified Thomas-Fermi screening across the superconducting transition.
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All authors Nikhil Seeja Sivakumar, Tyler James, Frederick Carlisle, Philip Moriarty, Brian Kiraly

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

High-resolution angle-resolved photoemission spectroscopy with tunable magnetic field

Jairo Obando-Guevara, Huu-Thong Le, Emiliano Corcino Aracena et al. (+7 more)

The control and perturbation of quantum phenomena with magnetic fields is an indispensable tool in materials research. Recently, in-situ field tuning was implemented into angle-resolved photoemission spectroscopy (magneto-ARPES), which provides direct momentum and energy-resolved information of the field-dependent electronic structure. However, aberrations of the electron trajectories were shown to be substantial, leading to significant spectral distortions and broadening even in small fields. Here we show that the electronic structure can be recovered from magneto-ARPES spectra with high accuracy while maintaining high momentum resolution even when strong trajectory aberrations are present. We studied Bi$_2$Se$_3$ in a dipole field of a coil using a laser-based ARPES system. The electronic structure was reconstructed in post-processing using detailed electron trajectory simulations. We identify two-dimensional (2D) momentum mapping, a micron beam spot size, and precise numerical field simulations as critical technical requirements for high-resolution magneto-ARPES. We show how circular dichroism can provide additional information about the coupling of the magnetic field to the spin. The experimental achievements and the scaling laws from our simulations provide a road map towards magneto-ARPES in larger fields.
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All authors Jairo Obando-Guevara, Huu-Thong Le, Emiliano Corcino Aracena, Sayan Singha, Kyungchan Lee, Yingdong Guan, Zhiqiang Mao, Dezhe Jin, Chaoxing Liu, Heike Pfau

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

Theory of Kondo hybridization wave in Kondo lattice

Yin Zhong

Recent scanning tunneling microscopy experiments have discovered emergent spatially modulated Kondo hybridization wave (KHW) order in the heavy fermion superconductor UTe$_2$ and the artificial Kondo lattice system 1T/1H-TaS$_2$, challenging the conventional paradigm of spatially uniform Kondo hybridization in heavy fermion physics. Here, we develop a microscopic theory for KHW order based on the canonical square-lattice Kondo lattice model within the large-$N$ fermionic mean-field approximation. We systematically identify stable modulated KHW phases and establish their ground-state phase diagram. The prominent $\boldsymbol{Q}=(0,\pi)$ KHW phase yields uniaxial stripe modulation of the Kondo hybridization gap, which faithfully reproduces the spatial modulation pattern observed in UTe$_2$. Moreover, its inherent unit-cell-doubling modulation precisely accounts for the spectroscopic features measured in 1T/1H-TaS$_2$. We further predict a characteristic in-plane conductivity anisotropy that serves as a definitive transport fingerprint to discriminate KHW states with distinct ordering wavevectors. Our work provides a microscopic foundation for the newly observed KHW order and establishes a unified theoretical framework for understanding emergent modulated hybridization phenomena in heavy fermion materials.
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Superconductivity

Geometry-induced current decrowding in superconducting 3D constrictions

Igor Bogush and Oleksandr Dobrovolskiy

Current crowding is a ubiquitous limitation in nanoscale devices, where confinement and sharp features distort current flow, generating localized current-density hotspots and premature failure. In superconductors, this effect suppresses the order parameter and promotes vortex nucleation, reducing the critical current below the intrinsic depairing limit. Here, we demonstrate that 3D shaping of superconducting nanoarchitectures overcomes geometric current crowding through spatial current redistribution, establishing a geometry-induced decrowding effect. Using finite-element modeling based on the time-dependent Ginzburg–Landau equation, we reveal that 3D constrictions exhibit a pronounced and tunable response to moderate in-plane magnetic fields—a functionality absent in planar geometries. This field-controlled geometry enables critical-current modulation and unveils new vortex-dynamics regimes, including a non-reciprocal critical current. Furthermore, curvature and finite thickness fundamentally alter vortex nucleation by enabling apex-mediated entry of single vortex lines, followed by their 3D splitting into two vortex filaments—a mechanism that does not occur in 2D manifolds. Our findings demonstrate 3D geometric engineering as a design paradigm for superconducting nanoarchitectures, offering control over current distribution and vortex dynamics in devices approaching the depairing limit.
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Superconductivity · Cross-listed from quant-ph

Instantaneous-Frame Theory of Strongly Driven Parametric Gates

Kentaro Kubo, Shinichi Inoue, Jushin Tei et al. (+3 more)

Parametric two-qubit gates using tunable couplers are a promising approach to scalable superconducting quantum processors. However, conventional theories formulated in the idle eigenbasis break down in the large-amplitude regime required for fast gate operations. Here, we show that parametric gates are more naturally and accurately described as coherent rotations between instantaneous eigenstates rather than idle eigenstates. This approach may appear counterintuitive, since instantaneous eigenbases are usually associated with adiabatic dynamics. However, they also form a natural moving frame for driven dynamics, where the non-Abelian Berry connection governs transitions and provides a compact description of the exchange and effective $ZZ$ couplings. We show that this theory achieves quantitative agreement with exact numerical simulations across representative tunable-coupler architectures, including double-transmon, capacitively shunted double-transmon, and single-transmon couplers. The description remains accurate even in regimes where idle-frame descriptions fail, while substantially reducing the reliance on computationally demanding exact simulations. These results establish an instantaneous-frame theory of parametric gates beyond conventional idle-frame models and provide a new perspective on driven quantum dynamics.
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All authors Kentaro Kubo, Shinichi Inoue, Jushin Tei, Yinghao Ho, Yasunobu Nakamura, Hayato Goto

Superconductivity

$1/3$-Flux Bound States in Multicomponent Superconductor Exhibiting Non-Abelian Statistics of $\mathbb {Z}_3$ Parafermions

Xing-Yu Wu, Y. H. Wang, X. C. Xie et al. (+1 more)

Multicomponent superconductors (MSCs) are predicted to host magnetic flux quanta carrying arbitrary fractions of the superconducting flux quantum. Recent advances have raised the expectation that $1/3$ flux quanta may emerge in MSCs with $C_3$ rotational symmetry. Electrons bound to such fractional flux are long regarded to form anyons, yet their explicit braiding statistics remain unexplored. We propose that these $1/3$-flux bound states ($1/3$-FBSs) exhibit the intriguing non-Abelian statistics of $\mathbb{Z}_3$ parafermions. Under no-double-occupancy constraint, two successive braiding operations of $1/3$-FBSs are equivalent to a single $\mathbb Z_3$ parafermion braiding operation. The parafermion parity encoding the braiding outcome can be read out via the fermionic occupation number of the $1/3$-FBSs. Combined with a crossed-Andreev-reflection-induced Hadamard gate, one can realize the complete set of $\mathbb{Z}_3$ parafermion braiding operations.
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All authors Xing-Yu Wu, Y. H. Wang, X. C. Xie, Yijia Wu

Superconductivity

Néel-Vector Control of the Josephson Diode Effect in $\mathcal{PT}$-symmetric Antiferromagnets

Xian-Tang Xu, Xun-Jiang Luo, Mingliang Tian et al. (+1 more)

The interplay of superconductivity and magnetism gives rise to rich phenomena in Josephson junctions. In this Letter, we study Josephson junctions formed by conventional $s$-wave superconductors and a $\PT$-symmetric collinear antiferromagnet modeled on CuMnAs. Using microscopic modeling and symmetry analysis, we show that these junctions exhibit both the Josephson diode effect and $\varphi_{0}$-junction states. Remarkably, both effects are controlled by the Néel vector: rotating it by $90^{\circ}$ switches off both, while reversing it switches the diode polarity. To reveal the microscopic mechanism, we develop a channel-resolved scattering theory that accurately captures the anomalous phases and establishes the exact condition for the diode effect. The interplay of the channel current-phase relations yields a sizable diode efficiency, tunable by both the magnitude and direction of the exchange field. Furthermore, a Green-function reduction identifies a single renormalized $\PT$-degenerate band as the transport carrier and precisely reproduces the full current amplitudes. Our work establishes $\PT$-symmetric antiferromagnets as versatile platforms for field-free, highly tunable Josephson diodes and $\varphi_{0}$ junctions.
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All authors Xian-Tang Xu, Xun-Jiang Luo, Mingliang Tian, Ning Hao

Superconductivity

Magnetochiral anisotropy of thermally activated phase slips in noncentrosymmetric superconductors

Alex Levchenko

Resistive tails of narrow superconducting wires below the critical temperature are governed by phase slips, thermally activated near the transition and quantum at lower temperatures. We develop the theory of thermally activated phase slips in wires lacking inversion symmetry, modeled microscopically as a diffusive wire formed from a two-dimensional electron gas with Rashba spin-orbit coupling in an in-plane magnetic field. The phase-slip resistance acquires magnetochiral anisotropy: at fixed current it changes upon reversal of the field, and at fixed field upon reversal of the current, while the linear-response resistance remains field symmetric as required by Onsager reciprocity. The nonreciprocity originates from an asymmetry of the phase-slip activation barrier that is odd in both current and field. It is generated by the Lifshitz invariants of the free energy, the cubic gradient term and the momentum-odd quartic vertex, together with even gradient terms promoted to odd ones by the helical ground state, precisely the combination that determines the superconducting diode effect. The barrier asymmetry is therefore fixed by the diode efficiency of the same wire through a universal numerical ratio, with no adjustable parameters. The kinetic Lifshitz invariant, which makes relaxation and noise of the order parameter nonreciprocal, affects only the fluctuation prefactor at a parametrically subleading level. Because the asymmetry resides in the activation exponent, the magnetochiral signal is amplified relative to the diode asymmetry by the ratio of barrier height to temperature, and appears as a field-antisymmetric splitting of the exponential slopes of differential-resistance traces routinely used to identify thermally activated phase slips.
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Superconductivity · Cross-listed from cond-mat.quant-gas

Large-Chern-number flat bands, anomalous Dirac cones, and unconventional superfluidity in square-lattice systems with SU(N) non-Abelian gauge fields

Wentao Zhong and Zhongbo Yan

We study topological band structures and superfluid phases in two-dimensional square-lattice systems with homogeneous SU($N$) non-Abelian gauge fields. Starting from an SU(4) gauge-field model related to the Hofstadter model with flux $\alpha=1/4$, we show that the lowest and highest bands are isolated Chern bands that carry Chern numbers $C=-4$ and give rise to four chiral edge modes in a strip geometry. Remarkably, although the two middle bands touch and form $16$ gapless Dirac cones, their combined Chern number is $C=8$. We then generalize the construction to SU($N$) systems and reveal an even–odd structure of the band topology: when $N$ is odd, all bands are isolated and carry nonzero Chern numbers; when $N$ is even, the two middle bands touch at $N^{2}$ Dirac points, while all other bands remain isolated and topologically nontrivial. We find that the uppermost and lowermost bands become increasingly flat and their Berry curvature becomes more uniform as $N$ increases, providing a promising platform for realizing fractional Chern insulating phases. We further examine the spin-$3/2$ SU(4) model with on-site attractive Hubbard interactions, exploring its superfluid phases at partial filling. We find that the non-Abelian gauge field breaks the hidden SO(5) degeneracy of the quintet pairing and selects distinct nematic superfluid states. For fillings in the middle-band regime, the resulting spectrum can host topologically protected Bogoliubov Fermi surfaces. Our results provide a starting point for exploring both topological band physics and unconventional superfluidity in synthetic SU($N$) cold-atom systems.
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Superconductivity

Signatures of nodal superconductivity in stoichiometric FeTe

Cequn Li, Zi-Jie Yan, Yang Ge et al. (+13 more)

Superconductivity in stoichiometric FeTe opens access to the FeTe endpoint of the Fe(Se,Te) phase diagram, yet the nature of its superconducting pairing state remains unresolved. In this work, we combine scanning superconducting quantum interference device (SQUID) microscopy, electrical transport, scanning tunneling microscopy and spectroscopy (STM/S), and mean-field calculations to investigate the local superfluid response and pairing state of FeTe thin films with tunable stoichiometry. Even in stoichiometric FeTe, we observe micrometer-scale spatial variations in both superfluid stiffness and superconducting transition temperature $T_c$, while the London penetration depth remains non-saturating down to 0.02$T_c$ and follows a power-law temperature dependence with an exponent of approximately 1-1.5. Together with a V-shaped low-energy density of states and two-gap modeling, these results indicate a superconducting state with gap nodes or deep minima, consistent with either a $d$-wave or nodal $s$-wave superconducting state. Our findings establish stoichiometric FeTe as a distinct superconducting regime that departs from the trend toward more isotropic gaps at intermediate Se/Te compositions, providing a new benchmark for modern microscopic theories of iron-chalcogenide superconductivity. Our work also reveals a crossover from weak to rapid suppression of $T_c$ as superfluid stiffness decreases, connecting FeTe to the broader phenomenology observed in unconventional superconductors.
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All authors Cequn Li, Zi-Jie Yan, Yang Ge, Zihao Wang, Bing Xia, Stephen Paolini, Pu Xiao, Lok-Kan Lai, Jiatao Song, Austin R. Kaczmarek, Lujin Min, Kenji Yasuda, Peter J. Hirschfeld, Jiabin Yu, Cui-Zu Chang, Katja C. Nowack

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

Selective coupling of high-order phonons in La2-xSrxCuO4

Ke-Jun Xu, Paulina Majchrzak, Nathan Giles-Donovan et al. (+14 more)

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.
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All authors Ke-Jun Xu, Paulina Majchrzak, Nathan Giles-Donovan, Sangheon Kim, Jaewon Choi, Jun Okamoto, Ganesha Channagowdra, Hsiao-Yu Huang, Di-Jing Huang, Seiki Komiya, Takao Sasagawa, Shuichi Wakimoto, Wei-Sheng Lee, Zhi-Xun Shen, Thomas P. Devereaux, Dung-Hai Lee, Robert J. Birgeneau

Monday 7 September

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

Hamiltonian fingerprints of a collective mode across the BCS-BEC crossover

Yogeshwar Prasad

Collective excitations can survive profound changes in the microscopic degrees of freedom that generate them. We track a layer-odd collective response from an exact weak-coupling hybridisation anchor into the paired regime of an attractive Hubbard bilayer and find that its Hamiltonian sensitivities evolve in a manner consistent with a crossover from single-particle interlayer hybridisation towards collective pair superexchange. At weak coupling, an exact identity for identical tunnel-coupled layers fixes a relative-phase Gaussian kernel zero at $2t_h$, independent of the intralayer hopping. Sign-free determinant quantum Monte Carlo reveals an interaction-driven softening of the many-body spectral scale without analytic continuation, while Gaussian response theory supplies the continuous pole interpretation and shows a shift from layer-density to relative-pair-phase character. In the paired regime, the characteristic response scale acquires a growing fractional sensitivity to the intralayer hopping and an interaction dependence partway towards the $1/U$ law. These results show how a collective excitation can be tracked across a fermion-to-composite-boson crossover while constraining changes in its microscopic drive.
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Superconductivity · Cross-listed from hep-ph

Quantum-Geometric Meissner Effect in Magnetized Color Superconductors

Kazuya Mameda and Noriyuki Sogabe

We find a quantum-geometric Meissner response in magnetized two-flavor color-superconducting (2SC) quark matter. Landau quantization quenches the transverse quasiparticle dispersion, suppressing the conventional Fermi-surface contribution and giving rise to a Meissner response governed by the quantum geometry of the Landau levels. In the strong-field regime, the response becomes dominated by the quantum metric of the lowest Landau level (LLL), and the leading scaling of the transverse Meissner mass is consequently set by the pairing gap, in contrast to the chemical potential scaling of conventional color superconductors. This unconventional scaling has a topological origin, as the LLL quantum metric is constrained by its Chern number. The reduced transverse Meissner mass provides potential implications for kHz quasi-periodic oscillations in magnetars.
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Superconductivity

Interference-controlled magnetic bound states and subgap transport in a Square–octagon superconductor

Ravi Kiran

The square–octagon lattice provides a useful setting for studying magnetic adatoms because its multisublattice structure allows different adsorption geometries to probe distinct combinations of the host electronic states. We study magnetic adatoms in an $s$-wave superconducting square–octagon lattice, focusing on the interplay between adsorption geometry, narrow and flat-band states, and subgap transport. We find that the large spectral weight associated with a flat band does not necessarily imply strong impurity–host coupling. For symmetric square-hollow adsorption, destructive interference strongly suppresses the overlap with the flat-band manifold, whereas top and octagon-hollow adsorption retain substantial overlap. This selectivity is reflected in the magnetic bound states, with the square-hollow channel supporting a predominantly atomic state, while the top and octagon-hollow states are strongly hybridized with the superconducting host. The effect develops continuously as the relevant band becomes flatter and remains pronounced in a material-motivated parameter regime. In a normal–superconductor–normal geometry, these differences are also reflected in the nonlocal subgap conductance. At a representative exchange splitting, square-hollow adsorption suppresses the low-bias conductance relative to the clean device, whereas octagon-hollow adsorption enhances it, with the square-hollow response showing a much sharper dependence on exchange splitting. Our results show that the magnetic-adatom response in narrow and flat-band superconductors is governed by the available spectral weight and by the symmetry and wave-function structure of the impurity–host hybridization.
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Superconductivity

Doping-controlled topological superconducting transition in misfit layer compounds

Hugo Le Du, Robin Salvatore, Justine Cordiez et al. (+12 more)

Achieving topological superconductivity is a key goal in quantum physics, offering a path to fault-tolerant quantum computers. A central challenge in this field is to continuously drive a material through a topological quantum phase transition to directly observe the evolution from trivial to topological superconductivity. However, finding a robust platform that allows such extreme and precise tuning remains a challenge. Here, we demonstrate a doping-controlled phase transition from a conventional to a topological superconducting state in the bulk misfit layer compound (LaxPb1-xSe)1.14(NbSe2)2. We reveal a non-monotonic phase diagram characterized by two distinct superconducting regimes separated by a non-superconducting phase at a precise doping. In the highly doped regime, the superconducting phase becomes remarkably sensitive to non-magnetic disorder, and orientation-selective in-gap modes emerge at atomic step edges. Supported by Bogoliubov-de Gennes calculations, these emergent spatial signatures are consistent with a time-reversal-symmetric crystalline-topological order parameter. Our results establish misfit compounds as a platform to engineering topological superconductivity.
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All authors Hugo Le Du, Robin Salvatore, Justine Cordiez, Ludovica Zullo, Arindam Mukherjee, Daniel Schmieg, Dominik Volavka, Francois Debontridder, Marie Herve, Tomas Samuely, Shunsuke Sasaki, Florent Pawula, Etienne Janod, Laurent Cario, Tristan Cren

Superconductivity

Nearly Isotropic Vortex Solid in $\mathbf{(La,Pr)_{3}Ni_{2}O_{7}}$ Thin Films

Yaolong Bian, Yaqi Chen, Heng Wang et al. (+14 more)

The discovery of superconductivity in bulk bilayer nickelates has established a new platform for exploring high-$T_c$ superconductivity beyond the cuprates. The role of the Ni $3d_{z^2}$-derived $\gamma$ band in the superconductivity of bilayer nickelates remains unresolved. By performing simultaneous resistance and diamagnetism measurements on (La,Pr)$_3$Ni$_2$O$_7$ thin films, we map the vortex melting phase diagram for both in-plane and out-of-plane magnetic fields. For $H\parallel c$, the geometric confinement effect gives rise to pancake vortices. Remarkably, the anisotropy parameter of the vortex melting field $\gamma_{H_m} \equiv H_m^{ab}/H_m^c$ decreases monotonically with decreasing temperature and approaches unity at low temperatures. Within the anisotropic Ginzburg–Landau scaling, $H_m^{ab}/H_m^c = \sqrt{\rho_s^{ab}/\rho_s^c}$ tracks the superfluid-density anisotropy. Such a vortex solid implies a nearly isotropic superfluid density, which is irreconcilable with the strictly two-dimensional $3d_{x^2-y^2}$-derived bands, but naturally explained by a substantial interlayer superfluid contribution from the $3d_{z^2}$-derived $\gamma$ band. Our results provide thermodynamic evidence for a substantial contribution of the $\gamma$ band to superconductivity in bilayer nickelate thin films.
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All authors Yaolong Bian, Yaqi Chen, Heng Wang, Guangdi Zhou, Fei Peng, Zichen Lv, Jiaqiang Cai, Yifan Chen, Wenjie Meng, Ze Wang, Haoliang Huang, Daohua Zhang, Mingliang Tian, Jinfeng Jia, Qi-kun Xue, Zhuoyu Chen, Jinglei Zhang

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

Coupled structural and electronic evolution under pressure in CuIr2Se4, CuRh2S4, and CuRh2Se4

M. Emi, M. Shiomi, K. Kojima et al. (+15 more)

Spinel chalcogenides provide a platform for investigating the interplay among metallic, superconducting, and pressure-induced insulating states. Here, we combine synchrotron powder X-ray diffraction and electrical-resistivity measurements to investigate the pressure evolution of CuIr2Se4, CuRh2S4, and CuRh2Se4 over pressure ranges extending beyond those previously explored. High-pressure diffraction reveals closely related monoclinic supercells in all three compounds. For CuIr2Se4 and CuRh2S4, constrained profile fits based on structural models relaxed using density functional theory are compatible with Phase-IV-type bond-disproportionated structures, whereas the data for CuRh2Se4 establish a compatible monoclinic unit cell without resolving its atomic-scale ordering pattern. Insulating-like transport develops abruptly over a narrow pressure range in CuIr2Se4 but more gradually over broader pressure ranges in the Rh-based compounds, in close correspondence with their respective structural transformations. We also establish previously unreported bulk superconductivity in CuIr2Se4 at ambient pressure: zero resistance is attained at 0.29 K, and the accompanying ac diamagnetic response is consistent with nearly complete superconducting shielding. These results establish a close relationship between the formation of the high-pressure monoclinic phases and the evolution toward insulating transport, and demonstrate that transition-metal and chalcogen substitutions tune the characteristic pressure scales and the competition with superconductivity within a closely related structural framework.
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All authors M. Emi, M. Shiomi, K. Kojima, K. Sugimoto, T. Karasawa, H. Suzuki, M. Takahashi, K. Oka, H. Kadobayashi, S. Kawaguchi-Imada, N. Hirao, T. Ohashi, D. Ito, T. Kubo, M. Matsushita, M. Nohara, K. Matsubayashi, N. Katayama

Sunday 6 September

Superconductivity

Symmetry-selective field-induced triplet superconductivity in Ising-superconductor monolayer NbSe$_2$

Sudipta Biswas, Sudhansu S. Mandal and A. Taraphder

We investigate superconductivity in monolayer NbSe$_2$, an Ising superconductor, under an in-plane Zeeman field $h_x$, focusing on the emergence of symmetry selected equal-spin triplet pairing. Using a self-consistent Bogoliubov–de Gennes approach with realistic hopping parameters for monolayer NbSe$_2$ on a triangular lattice, we determine the energetically favored singlet pairing states in a range of chemical potentials $\mu$ for onsite, nearest-neighbor, and next-nearest-neighbor pairing channels, and map the resulting phase diagram in $(h_x,\mu)$ plane. A momentum-resolved analysis reveals distinct dominant contributions in order parameters arise from the surroundings of $\Gamma$, $K$, and $K'$ points of the Brillouin zone. We find that an in-plane magnetic field helps to induce a triplet pairing whose symmetry is determined by the parent singlet state, upon opening the triplet interaction channel. For the nonlocal pairing channels, chiral-$d$ pairing is energetically favored and it induces chiral-$p$ equal-spin triplet component with opposite chirality. We further find that Rashba spin-orbit coupling, relevant to substrate coupling and electrostatic gating, suppresses the superconducting orders and reduces the critical field. Our results establish a direct link between the symmetry of the parent singlet condensate and the emergent triplet superconductivity, highlighting monolayer NbSe$_2$ as a promising platform for field-tunable mixed-parity superconducting states.
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Superconductivity

Superconductivity in a Bulk Moire Superlattice Material

Subham Naik, Paul Monson, Susanta Manna et al. (+7 more)

Moire materials provide a versatile platform realizing emergent electronic states arising from enhanced correlation due to flat bands. A variety of phenomena including superconductivity, low dimensional ferromagnetism, Mott insulating phase, and topological phenomena have been reported in such systems. To date, moire phenomena have been predominantly explored in artificially assembled low dimensional van der Waals structures, where relative twist and lattice alignment are controlled during device fabrication. Recently, intrinsically grown bulk moire crystals have emerged as a complementary materials platform, in which lattice mismatch between constituent layers generates a coherent moire superlattice throughout the bulk crystal. Here, we report the evidence of bulk superconductivity in single crystals of a recently reported bulk Moire material [Sr6TaS8]1+x[TaS2]8 at ambient pressure. The material exhibits a bulk superconducting transition at Tc = 2.5 K, evidenced consistently by electrical transport, magnetic susceptibility, and heat capacity measurements on single crystals. Structural characterization by X-ray diffraction and atomic resolution electron microscopy establishes the composite, incommensurate crystal structure and scanning tunnelling microscopy reveals a long-wavelength surface modulation due to the incommensurate structure. The observation of bulk superconductivity establishes superconductivity as an emergent phase in this intrinsically synthesized moire material and highlights bulk moire crystals as a promising platform for investigating correlated quantum phenomena beyond artificially assembled two-dimensional structures.
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All authors Subham Naik, Paul Monson, Susanta Manna, Prabuddhakant Mishra, Soumyojit Chatterjee, Sandip Kuila, Partha Pratim Jana, M. B. Sreedhara, Rahul Sharma, Gohil S. Thakur

Superconductivity

$^{139}$La nuclear quadrupole resonance studies of pressurized La$_4$Ni$_3$O$_{10}$

Meng Zhang, Zhuo Wang, Yantao Cao et al. (+5 more)

Density-wave (DW) orders are considered as competing orders to unconventional superconductivity and are commonly seen in a variety of superconductors including but not limited to the recently discovered Ruddlesden-Popper-phase nickelates. By utilizing $^{139}$La nuclear quadrupole resonance, we systematically investigate into the nature of DW orders and their evolution under pressure in La$_4$Ni$_3$O$_{10}$. Spin and charge DW orders are found to be intertwined in this material, which is in stark contrast to those in La$_3$Ni$_2$O$_7$. Short-range DW orders are observed near 150 K, well above the development of long-range DW orders at around 139 K. Upon applying a hydrostatic pressure of 2.3 GPa, the transition temperatures of the short-range and long-range orders decrease at rates of 1 K/GPa and 10 K/GPa, respectively. Our results thus affirm that both spin density wave and charge density wave as competing orders with the superconducting state in La$_4$Ni$_3$O$_{10}$, and provide new insights into the interplay between DW orders and unconventional superconductivity.
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All authors Meng Zhang, Zhuo Wang, Yantao Cao, Yang Yuan, Kangjian Luo, Shanxiang Gao, Hanjie Guo, Yongkang Luo

Saturday 5 September

Superconductivity

Effect of non-homogeneous non-magnetic Impurities in Superconductors: A comparison between Microscopic and Macroscopic theories

Carlos Redondo Herrero, Tejas Guruswamy, Orlando Quaranta et al. (+1 more)

Non-homogeneous, non-magnetic impurities in superconductivity have recently gained traction as a promising approach to reduce surface resistance and increase the superheating field. We aim to deepen the understanding of impurities' role from a theoretical perspective by utilizing Eilenberger's equation, and to investigate the validity of a more phenomenological approach based on London's equation for determining the magnetic field profile within the superconductor. We show that the microscopic and macroscopic theories produce identical electromagnetic field distributions for any spatial distributions of impurities inside the superconductors. However, the two approaches provide different magnetic field-related quantities. The macroscopic model only gives the Bean-Livingston barrier, while the microscopic theory provides the experimentally-relevant superheating field. Based on the microscopic formalism, we determine a family of impurity profiles that make the superheating field $H_{\rm{sh}}$ equal to the critical field $H_c$, which is the maximum achievable.
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All authors Carlos Redondo Herrero, Tejas Guruswamy, Orlando Quaranta, Akira Miyazaki

Superconductivity

Microscopic calculation of coherence lengths and magnetic penetration depth in multiband superconductors

Tristan Ryoma Fuchs, Takuya Nomoto, Hikaru Watanabe et al. (+1 more)

We present an extended Ginzburg-Landau (GL) method for calculating the superconducting coherence length and magnetic penetration depth at temperatures well below the transition temperature $T_{\mathrm c}$. In contrast to conventional GL theory, which expands the free energy in both the order parameters and their gradients, our method applies a perturbative expansion only to the covariant-gradient terms, while retaining the full dependence on the superconducting order parameters. The coefficients of these terms are determined from finite differences of microscopic free energies evaluated at small imposed pair momenta. The method applies to both single-band and multiband superconductors and therefore provides a framework for incorporating more realistic electronic structures. For the models examined here, the extended GL method agrees well with real-space Bogoliubov-de Gennes (BdG) calculations over a wide temperature range, while requiring substantially less computational effort.
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All authors Tristan Ryoma Fuchs, Takuya Nomoto, Hikaru Watanabe, Ryotaro Arita