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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Thursday 27 August

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

Occupation-Driven Josephson Diode in a Symmetric Junction

Jianxiong Zhai, Zelei Zhang and Jiawei Yan

We propose a Josephson diode mechanism in which nonreciprocity arises not from a conventional asymmetric Andreev spectrum but from nonequilibrium occupation of the current-carrying states engineered by attached reservoirs. We realize this mechanism in a double-quantum-dot junction, where a phase-textured nonlocal reservoir acts as a quantum Zeno selector: rapid dissipation freezes out the bright state directly coupled to the jump operator $L$, while preserving an orthogonal dark Andreev channel whose supercurrent remains comparable to that of the lossless junction. In the infinite-gap limit, the steady-state current factorizes as $I_{\rm ss}=I_A P_\gamma$, so that even when the Andreev current $I_A$ is strictly reciprocal, the phase asymmetry of $P_\gamma$ alone can produce a Josephson diode effect through reservoir engineering. We further show that local Coulomb repulsion can drive the system toward a nearly ideal diode regime via a dark-pair resonance. Using Keldysh-Lindblad calculations, we demonstrate that our results remain robust for realistic junctions with a finite superconducting gap and dissipation.
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Superconductivity · Cross-listed from physics.app-ph

Diamond quantum-sensing platform with integrated boron-doped diamond microwave antenna and thermometer

Masahiro Ohkuma, Ryo Matsumoto, Shintaro Adachi et al. (+5 more)

Wide-field nitrogen-vacancy (NV) magnetic imaging at cryogenic temperatures requires microwave excitation and reliable knowledge of the temperature near the sensing region. Here, we report an integrated diamond quantum-sensing platform combining an ensemble of NV centers with a boron-doped diamond (BDD) microwave antenna and thermometer formed on the same diamond substrate. The BDD antenna provides microwave excitation for optically detected magnetic resonance measurements, and the BDD thermometer monitors the thermal environment near the NV sensing region. The BDD thermometer detected laser-induced local heating that was not clearly resolved by a stage-mounted thermometer. Using this platform, we imaged the temperature-dependent Meissner response of multiple cuprate superconductors while recording the temperature. These results demonstrate that the integrated BDD–NV platform provides a practical approach for cryogenic wide-field magnetic imaging with integrated microwave delivery and local thermometry.
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All authors Masahiro Ohkuma, Ryo Matsumoto, Shintaro Adachi, Shinobu Onoda, Kensei Terashima, Takao Watanabe, Yoshihiko Takano, Keigo Arai

Superconductivity · Cross-listed from quant-ph

A Superconducting Phase Transition Single-Electron Transistor

Gorka Aizpurua-Iraola, Thomas H. Swift, Felix-Ekkehard von Horstig et al. (+6 more)

Quantum computers require fast and accurate methods for qubit state detection. Phase-transition sensors exploit the abrupt change between two physical states of a material to achieve enhanced sensitivity and have enabled advanced detectors for quantum technologies, such as superconducting nanowire single-photon detectors. However, this sensing principle has not yet been applied to semiconductor spin qubits. Here, we demonstrate a superconducting phase-transition radio-frequency single-electron transistor (PTSET), a charge sensor for semiconductor spin qubits whose response is enhanced by a superconducting-to-normal phase transition. The transition is engineered by linking the sensor current to a low-critical-current, high-kinetic-inductance inductor integrated into the radio-frequency matching network. We demonstrate improvements in sensitivity of one and two orders of magnitude over conventional rfSETs in the large- and small-signal regimes, respectively. Our results establish phase-transition sensing as a route towards ultrasensitive, integrated charge sensors for semiconductor quantum computing and point to broader applications, including cryogenic photon detection for radio astronomy.
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All authors Gorka Aizpurua-Iraola, Thomas H. Swift, Felix-Ekkehard von Horstig, Domenic Prete, James Kirkman, Grayson M. Noah, Fabio Olivieri, Alberto Gomez-Saiz, M. Fernando Gonzalez-Zalba

Superconductivity · Cross-listed from cond-mat.quant-gas

Crossing the Rotational Sound Barrier in a Quantum Solvent

Baptiste Coquinot, Giacomo Bighin, Mikhail Lemeshko et al. (+1 more)

Molecules embedded in superfluids provide an experimentally controllable platform for investigating impurity physics. Here, we investigate a driven molecule rotating in a superfluid environment, including helium and Bose–Einstein condensates, at rotation frequencies similar to the bath dynamics. Within the experimentally relevant platform of an optical centrifuge, we show that the rotor remains localized up to a characteristic harmonic frequency that can realistically exceed the excitation energies of the bath, enabling access to ultrafast rotating impurities. In the co-rotating frame, the bath excitations experience a rotational Doppler shift, generating angular-momentum-resolved resonances absent in equilibrium angulon theory. We identify a dissipative rotational sound barrier at which the molecule resonantly emits bath excitations and undergoes strong angular momentum exchange with the surrounding medium. Overall, we establish the dynamical phase diagram of the driven rotor in a quantum solvent and introduce a generic platform for investigating fast driven rotating impurities in quantum many-body systems.
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All authors Baptiste Coquinot, Giacomo Bighin, Mikhail Lemeshko, Ragheed Alhyder

Superconductivity

Stabilization of Interband Phase Solitons in Two-Band Noncentrosymmetric Superconducting Rings

Yuriy Yerin, Boris Malomed, Stefan-Ludwig Drechsler et al. (+1 more)

Two-band superconductors maintain a relative interband phase which can carry winding soliton excitations in a superconducting ring, supported by independent winding numbers in the two bands. In rings of superconductors obeying the inversion symmetry the interband phase solitons are metastable states, separated from the uniform ground state by the energy of screening currents. In this work we find that, breaking the inversion symmetry strongly enough, one can make the soliton a true ground state. In that case, a magneto-electric coupling, absent in centrosymmetric materials, contributes critically above a certain threshold, a relevant, free-energy term, odd with respect to the winding number, which biases the energy balance in favor of a particular winding sign. Once the bias outweighs the energy cost that originally made the soliton metastable, a phase soliton with a finite winding number becomes the ground state, with chirality set by the applied field. In current–flux measurements performed in equilibrium states, the effect is demonstrated by field-odd soliton branches, that replace the metastable ones existing in mesoscopic rings, built by two-component superconductors, realizing a magneto-electric analog of the Little–Parks fluxoid-branch physics in the interband relative-phase sector.
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All authors Yuriy Yerin, Boris Malomed, Stefan-Ludwig Drechsler, F. Sebastian Bergeret

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

Topologically protected perfect crossed Andreev reflection in flux-engineered quantum wire junctions

Abhiram Soori

Generating non-locally entangled electron pairs via Cooper-pair splitting is vital for solid-state quantum information processing. However, isolating the underlying crossed Andreev reflection (CAR) is challenging due to competing transport processes like electron tunneling (ET) and local Andreev reflection (AR). Here, we propose a flux-tunable four-terminal normal metal-superconductor junction that achieves deterministic, 100% efficient CAR. We demonstrate that at exactly half a magnetic flux quantum ($\phi=\pi$), exact destructive Aharonov-Bohm and Peierls interferences structurally forbid ET and AR respectively. By tuning the central junction hopping, electron reflection is also suppressed to zero. Using the Cauchy argument principle, we prove that this suppression manifests as a quantized topological winding number, guaranteeing a topologically protected unit CAR probability. We establish that this regime is characterized by a strictly positive cross-correlation shot noise, providing an unambiguous experimental signature of Cooper-pair splitting. Furthermore, this perfect CAR is nearly broadband within the superconducting gap and remarkably robust against structural disorder, offering a highly resilient architecture for deterministic nonlocal entanglement generation.
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Superconductivity

Strain-driven orbital-selective reconstruction and bicollinear-to-stripe evolution in FeTe

Zhenfeng Ouyang, Yin Chen, Yi-Heng Tian et al. (+4 more)

FeTe, as a representative parent material among iron-based superconductors, provides an ideal platform for exploring the interplay among orbital-selective correlations, magnetism, and unconventional superconductivity. However, a unified picture of the correlated electronic structure and magnetism of FeTe under strain remains to be fully clarified. Here, combining density functional theory plus dynamical mean-field theory and Heisenberg model analysis, we uncover an orbital-selective reconstruction of the correlated electronic structure and reveal a strain-driven trajectory from bicollinear to stripe antiferromagnetism (AFM) via an intermediate competing staggered $n$-mer AFM regime in FeTe. Moderate strain gives rise to a regime where more coherent quasiparticles coexist with suppressed local moments. Further strain drives FeTe into an incoherent correlated regime with robust local moments and Fe-$3d_{z^2}$-dominated low-energy states. These results establish a strain-driven trajectory across distinct magnetic and correlated electronic states in FeTe.
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All authors Zhenfeng Ouyang, Yin Chen, Yi-Heng Tian, Jia-Ming Wang, Rong-Qiang He, Kai Liu, Zhong-Yi Lu

Wednesday 26 August

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

How quantum is quantum geometry?

Ady Stern and Felix von Oppen

The quantum geometric tensor - the Berry curvature together with the quantum metric - now underlies a long list of observables, from the anomalous Hall effect to the superfluid weight of a flat band. We ask which of these observables actually require quantum mechanics. To answer this question, we study a purely classical system: a point particle carrying a classical magnetic moment $\boldsymbol{\ell}$ that precesses in a momentum-dependent magnetic field $\mathbf{B}(\mathbf{p})$. Within Hamiltonian classical mechanics, the component of $\boldsymbol{\ell}$ along the field reproduces the Berry-curvature phenomena, while its precessing transverse component reproduces the quantum-metric phenomena. The particle acquires a position spread whose second moment is the metric, an orbital magnetic moment, and - most strikingly - an inertial mass generated by a position-dependent force, and with it a nonzero Drude weight in a system that is nominally dispersionless.
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Superconductivity

Supercurrent detection and manipulation of topological phase transitions in Shiba-Majorana hybrid systems

Debika Debnath, Ioannis Ioannidis, Paramita Dutta et al. (+2 more)

The non-Abelian statistics of Majorana zero modes has inspired numerous proposals for their detection and manipulation in topological superconductors. Implementations based on magnetic adatoms deposited on superconductors draw particular attention due to their capabilities for precise atomic manipulation and the control over disorder. Here, we propose a scheme for detecting changes in the ground state parity of a topologically non-trivial adatom system by passing supercurrent through their low-energy modes. We unravel characteristic discontinuities in the critical current driven by zero-energy level crossings. We apply these findings to a setup where the Majorana coupling is mediated by a single control magnetic adatom hosting a Yu-Shiba-Rusinov state, and test the robustness of our results against finite temperatures and different tunneling regimes. Our findings introduce a non-invasive approach for reading out and controlling the ground state parity of Majorana states in Shiba-Majorana hybrid systems.
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All authors Debika Debnath, Ioannis Ioannidis, Paramita Dutta, Mircea Trif, Thore Posske

Superconductivity · Cross-listed from hep-th

Why Cooper pairs live in AdS2: a spectral analysis of the Yukawa-SYK model

Veronika Carolin Stangier and Jörg Schmalian

We establish the spectral foundation of the geometric formulation of Cooper pairing in the Yukawa–Sachdev–Ye–Kitaev model. Starting from the large-$N$ bilocal effective action, we derive the Gaussian fluctuation kernel in the even-frequency spin-singlet Cooper channel. Following the construction of Maldacena and Stanford, we determine the kernel eigenvalue $k\left(h\right)$ analytically for arbitrary conformal weight $h$ and resolve the pairing fluctuations into the continuous and discrete sectors of the associated de-Sitter space Laplacian. We show that the superconducting instability and the universal low-energy fluctuations near it reside entirely in the continuous scattering sector, while the discrete modes remain non-critical. Expanding the kernel about the lowest continuum mode yields a Klein–Gordon action on ${\rm dS}_2$, restricted to the continuous spectral subspace. This is precisely the subspace on which the inverse Radon transform to ${\rm AdS}_2$ is well defined. The projected bilocal Cooper-pair field can therefore be mapped onto a scalar matter field propagating in ${\rm AdS}_2$, without requiring any further spectral restriction on the bulk theory. Our results provide the missing microscopic justification for the projection implicit in earlier holographic formulations and clarify how a local bulk field emerges from the bilocal pairing theory.
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Superconductivity

Exact pair density wave in topological moire flat bands and universal superfluid stiffness

Zhengzhi Wu, Ming-rui Li and Hong Yao

Pair-density waves (PDWs) are unconventional superconducting states beyond the BCS paradigm, but identifying them unambiguously in microscopic lattice models is generally challenging due to various competing orders. In particular, an exact realization of PDW in interacting models with topological bands remains elusive. Here we construct an interacting model on a twisted bilayer checkerboard lattice (TBCB) featuring topological flat bands, and rigorously show that an exact PDW ground state can be induced by quantum geometric nesting (QGN), owing to its momentum-space nonsymmorphic symmetry. We further prove that the superfluid stiffness of any nondegenerate QGN superconductor (including the PDW studied here and uniform superconductor) obeys the universal relation $D_s=2N_{\rm flat}\nu(1-\nu)m_{\rm pair}^{-1}$, where $N_{\rm flat}$, $\nu$, and $m_{\rm pair}$ are the number of flat bands, flat band filling, and the two-particle effective mass, respectively. Our results reveal a rich interplay among PDW order, quantum geometry, and band topology.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Multicomponent Magnetic Domain Walls in Rhombohedral Graphene

Mainak Das, Nemin Wei and Chunli Huang

Spatial textures of magnetic order, such as domain walls and skyrmions, are fundamental objects in magnetism. In rhombohedral multilayer graphene, magnetic order involves spin and valley degrees of freedom, opening the possibility of qualitatively new spatial textures. Here, we explore this possibility through a microscopic study of a one-dimensional domain wall in the valley-imbalanced quarter-metal phase of rhombohedral graphene. We uncover two different classes of domain walls. One resembles a conventional magnetic domain wall, locally rotating between the two bulk states, whereas the other is intrinsically multicomponent and explores states that are not occupied in either bulk domain. Which texture is realized is controlled by the competition between intervalley Hund's coupling and spin-orbit coupling, and we identify experimental signatures to distinguish them. For a superconducting junction formed across the wall, we further show that the superconducting phase difference couples directly to the intervalley-coherent phase of the texture, allowing precession of this internal phase to generate a voltage. This mechanism provides an explanation for the resistive domain walls recently observed in chiral superconductors [arXiv2605.13303] and predicts accompanying magnetization oscillations that could be detected using GHz-frequency AC probes. Our theory establish a microscopic connection between multicomponent magnetic textures, their internal dynamics, and superconducting transport.
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Superconductivity

Electrical manipulation of oxygen stoichiometry in multiterminal YBa$_2$Cu$_3$O$_{7-\delta}$ junctions

Daniel Stoffels, Caio C. Quaglio-Gomes, Nicolas Lejeune et al. (+7 more)

Local manipulation of oxygen stoichiometry offers a route to control the electronic properties of complex oxides, yet the selective modification of individual current-carrying branches through oxygen redistribution remains unexplored in multiterminal high-temperature superconducting junctions. In a YBa$_2$Cu$_3$O$_{7-\delta}$ Y-shaped three-terminal device, we demonstrate the possibility to electrically control oxygen vacancy migration on a hand-picked terminal while largely preserving the other two. Oxygen-depleted propagating fronts are directly visualized by the resulting change in optical reflectivity and they are linked to the evolution of the electrical response. The process is highly directional and determined by the polarity of the applied current, allowing for the creation of either a converging or a diverging propagating front from the central node of the Y-shaped device. The associated changes in resistance exhibit relaxation on a timescale of minutes, driven by the vacancy concentration gradient. Effects of oxygen migration are also mapped by Kelvin Probe Force Microscopy and Scanning Laser Microscopy, which probe work-function changes and spatially resolved variations in the superconducting transition, respectively. Notably, the Tc contrast revealed by the latter provides a quantitative handle on the underlying oxygen content, enabling direct visualization of oxygen redistribution. Finite-element modeling and nanoprobe X-ray diffraction qualitatively reproduce the observed vacancy redistribution. These results establish a post-fabrication route to locally tune properties of superconducting multiterminal devices such as nanocryotrons, yTron, and tunable weak links.
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All authors Daniel Stoffels, Caio C. Quaglio-Gomes, Nicolas Lejeune, Emile Fourneau, Pedro Schio, Huidong Li, Lourdes Fabrega, Anna Palau, Maycon Motta, Alejandro V. Silhanek

Superconductivity

Symmetry Origins of the Field-Free Superconducting Diode Effect in the Kagome Superconductor CsV$_3$Sb$_5$

Xin-Jie Liu, Shengbiao Sun, Ke-Fan Song et al. (+14 more)

Field-free superconducting diode effects require both inversion-symmetry breaking and an internal time-reversal-symmetry (TRS) breaking field, making them sensitive probes of hidden order in superconductors. In centrosymmetric kagome AV$_3$Sb$_5$, the inversion symmetry generally should generally preclude the observation of the superconducting diode effect. Furthermore, though TRS breaking has been reported in the superconducting regime of CsV$_3$Sb$_5$, whether it is generated by superconductivity or inherited from charge-density-wave (CDW) order remains unresolved. Here we show that pristine CsV$_3$Sb$_5$ devices exhibit no intrinsic field-free superconducting diode effect, whereas surface oxidation or asymmetric etching activates a large nonreciprocal supercurrent. Moreover, the response is stochastic, with sweep-dependent polarity and magnitude, indicating metastable TRS-breaking domain configurations. Small out-of-plane magnetic fields stabilize the superconducting diode response, consistent with field selection of such domains. Finally, when long-range CDW order is suppressed by Ti doping, the SDE disappears. Our results establish the symmetry requirements for the field-free SDE in CsV$_3$Sb$_5$, reveal its stochastic domain-controlled character, and link superconducting-state TRS breaking to CDW-related order.
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All authors Xin-Jie Liu, Shengbiao Sun, Ke-Fan Song, Jia-Peng Peng, Xilin Feng, Lang Xiao, Tong Liu, Qilin Han, Ya-Qing Bie, Ning Kang, Xiaosong Wu, Yanfei Wu, Shouguo Wang, Kam Tuen Law, Shuo Wang, Dapeng Yu, Ben-Chuan Lin

Superconductivity

Fluxtube Bouquets and Type-1.5 Clustering in Superfluid Neutron Star Cores

Adarsh Karekkat, Gabriele Montefusco and Marco Antonelli

We study mesoscopic configurations of a neutron superfluid coupled to a proton superconductor in the outer core of a neutron star. The condensates are described by a two-component Ginzburg-Landau free energy with local couplings, neglecting genuine phase-gradient entrainment. In two spatial dimensions, we minimize the free energy using quasi-periodic boundary conditions and constrained phase-imprinting calculations to study vortex-fluxtube and fluxtube-fluxtube interactions. We find that, for locally attractive couplings in the free energy, vortex-fluxtube overlap is energetically favoured and several pre-existing proton fluxtubes can bind around a neutron vortex, forming finite vortex-centred aggregates that we call fluxtube bouquets. These bouquet configurations may become so dense that a vortex can effectively accommodate several quanta of magnetic flux. We also confirm the possible presence of a type-1.5-like regime and find that it survives in the zero-entrainment regime considered here. In this type-1.5 regime, the fluxtube-fluxtube interaction is repulsive at short distances and attractive at intermediate distances, leading to self-assembled clusters while the individual fluxtubes remain topologically distinct. Possible implications for dissipative coupling and transport in neutron stars are discussed.
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Superconductivity · Cross-listed from quant-ph

Phase-sensitive cascade quantum amplifier with nearly noiseless operation

Ilari Lilja, Ekaterina Mukhanova, Michael Perelshtein et al. (+5 more)

Phase-sensitive parametric devices enable quadrature-selective amplification with the potential for sub-quantum-limited noise performance. In this work, we investigate the operation of a SQUID-based Josephson Parametric Amplifier (JPA), comparing its performance in the phase-preserving and phase-sensitive regimes. The device, fabricated using VTT SWAPS technology, is driven in a three-wave mixing configuration and characterized in a reflection-based measurement setup at millikelvin temperatures. To directly probe the noise performance at low JPA gains, we employ a cascaded amplification scheme in which a Traveling-Wave Parametric Amplifier (TWPA) provides low-noise pre-amplification of the JPA output. In a phase-preserving operation, the JPA exhibits near-quantum-limited performance with a system noise temperature of $351\pm53$ mK at 6 GHz. In contrast, phase-sensitive operation yields a minimum system noise temperature of $94\pm12$ mK, well below the standard quantum limit of 288 mK. Our results demonstrate that a JPA-TWPA amplifier cascade opens the door to direct, high-fidelity probing of quantum devices without the need for background noise subtraction.
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All authors Ilari Lilja, Ekaterina Mukhanova, Michael Perelshtein, Kirill Petrovnin, Stanislav Khaldeev, Visa Vesterinen, Gheorghe Sorin Paraoanu, Pertti Hakonen

Superconductivity

Nearly Isotropic Quantum-Critical Transport in Single-Crystal CeNiC2

Hanming Ma, Jun Gouchi, Dilip Bhoi et al. (+4 more)

Pressure-induced superconductivity and $T$-linear resistivity have been reported in polycrystalline CeNiC$_2$, but orientational averaging has left the directional character of the critical scattering unresolved. We report pressure-dependent resistivity of high-quality single crystals for current along each crystallographic axis. These crystals have substantially lower residual resistivity and a slightly higher maximum onset $T_c$ than the polycrystalline sample, placing superconductivity in a cleaner transport regime. Near $P_c \approx 9.5-10$ GPa, the normal-state resistivity becomes nearly $T$-linear along every axis, the fitted residual resistivity is strongly enhanced, and superconductivity forms a narrow dome. For $I \parallel b$, the $T$-linear normal state remains nearly unchanged in magnetic fields up to 9 T applied along $a$ and $c$; the upper critical field is large and only moderately anisotropic. The common evolution along all three axes establishes a nearly isotropic quantum-critical transport regime, inconsistent with a simple low-dimensional spin-fluctuation picture and implicates valence fluctuations as the leading source of critical scattering associated with the superconducting dome.
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All authors Hanming Ma, Jun Gouchi, Dilip Bhoi, Toru Shigeoka, Bosen Wang, J.-G. Cheng, Yoshiya Uwatoko

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

Realization of Air-Stable Two-Dimensional Superconductor Nb2Pd3Te5 With Quasi-One-Dimensional Pair Density Modulation

Jiayi Wang, Hui Guo, Hao Zhang et al. (+12 more)

Two-dimensional (2D) superconductors provide a fertile platform for exploring reduced-dimensional superconductivity and emergent quantum phenomena. Incorporating quasi-one-dimensional (quasi-1D) structural motifs into 2D superconductors offers a powerful route to engineer strong electronic anisotropy, enabling unconventional superconducting states and anisotropic superconducting transport functionalities. However, such systems remain rarely realized. Here we report the realization of a 2D superconductor Nb2Pd3Te5, exhibiting an intrinsic quasi-1D pair density modulation. Monolayer and bilayer Nb2Pd3Te5 is synthesized via van-der-Waals epitaxy. Using ultralow-temperature scanning tunneling microscopy/spectroscopy, we observe the quasi-1D crystal structure and superconductivity below ~0.6 K with a pronounced quasi-1D pair density modulation. Remarkably, both monolayer and bilayer Nb2Pd3Te5 show strong air stability. Our findings establish atomically 2D Nb2Pd3Te5 as a robust and promising platform for exploring novel low-dimensional quantum phenomena and anisotropy-enabled superconducting devices.
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All authors Jiayi Wang, Hui Guo, Hao Zhang, Haowei Chen, Peixuan Li, Xianghe Han, Ziang Wang, Siyu Xu, Qian Fang, Haohao Xu, Shixuan Du, Chengmin Shen, Hui Chen, Wang Yao, Hong-Jun Gao

Superconductivity

Distinct Surface and Bulk Superconductivity in the Kagome Superconductor SrSn$_3$

Qun Zhu, Yong-Wei Wang, Ji-Hai Zhang et al. (+6 more)

Surface and bulk superconductivity may possess fundamentally different superconducting properties in quantum materials with nontrivial electronic structures, yet their superimposed spectroscopic signatures often prevent direct experimental access to each superconducting channel. Here we reveal, in epitaxial films of the kagome superconductor SrSn$_3$, distinct surface and bulk superconducting channels with markedly different superconducting gaps, upper critical fields, and vortex-core electronic states by tuning the tunneling junction resistance in scanning tunneling spectroscopy. The surface superconductivity is characterized by a thickness-independent superconducting gap and an enhanced upper critical field, whereas the bulk superconducting channel exhibits a larger superconducting gap that decreases with reducing film thickness and a much lower upper critical field. Within magnetic vortex cores, robust non-split zero-bias conductance peaks are observed exclusively in the surface superconducting channel, while pronounced zero-bias suppression is consistently associated with the bulk superconducting channel. These findings demonstrate that the vortex-core electronic structure depends sensitively on the underlying superconducting channel, providing new insight into vortex-bound states in topological quantum materials.
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All authors Qun Zhu, Yong-Wei Wang, Ji-Hai Zhang, Qiang-Jun Cheng, Chen-Yu Hu, Jun-Zhong Wang, Qi-Kun Xue, Xu-Cun Ma, Can-Li Song

Superconductivity

Grain Boundary Engineering Effect on Vortex Matter in Superconducting Films

Qun Wang, Ting Chen, Ya-Xun He et al. (+6 more)

Grain boundaries (GBs) in polycrystalline superconducting films act as a double-edged sword: they can pin vortices or degrade superconductivity through Josephson-like weak-link coupling. Here, we demonstrate that sputtering pressure tunes GB coupling in NbTiN films and visualize its consequences for vortex matter. The 5 mTorr film exhibits dispersed grain orientations and a two-step resistive transition under field, signaling intergranular weak-link behavior. In contrast, the 7 mTorr film develops a (111) texture, a single-step transition, higher critical current density, a second magnetization peak, and a {\delta}l-type pinning response consistent with improved GB coupling. Cryogenic magnetic force microscopy reveals a spatially heterogeneous, cluster-like vortex configuration in the 5 mTorr film, whereas the 7 mTorr film hosts a more uniform distribution with enhanced local order. These results establish a connection between deposition-controlled GB connectivity, macroscopic weak-link transport, and microscopic vortex organization, providing a practical route to tailor vortex pinning in polycrystalline superconducting films.
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All authors Qun Wang, Ting Chen, Ya-Xun He, Xing-Jian Liu, Jian-Wen Sun, Kang-Hong Yin, Fang-Ting Lin, Shi-Xun Cao, Jun-Yi Ge

Tuesday 25 August

Superconductivity

Probing bulk superconductivity in centrosymmetric Te-doped PtBi$_2$ single crystals

Kilian Srowik, Pablo Pedrazzini, Soumen Ash et al. (+17 more)

The Weyl semimetal $\gamma$-PtBi$_2$ has been shown to be one of the most promising novel materials, recently proposed as a topological i-wave superconductor. A crucial requirement for observing this physics is the absence of inversion symmetry in its trigonal $P31m$ crystal structure. Centrosymmetry has been reported to be readily restored in the $P\overline{3}m1$ structure upon electron doping, partially substituting Bi with as little as 2% Te. In this work, we synthesized Te-doped PtBi$_{2-x}$Te$_{x}$ samples and thoroughly investigated the bulk superconductivity of selected single crystals with nominal composition PtBi$_{1.96}$Te$_{0.04}$, exhibiting the highest superconducting volume fraction of $\sim100$%. Single crystal XRD measurements confirm the centrosymmetric $P\overline{3}m1$ structure in our samples, whereas bulk superconductivity with a critical temperature of $T_\mathrm{c} \approx 2.4\,$K was observed in magnetization and specific heat measurements. The upper critical fields for two different orientations were determined as $H^{\parallel}_{\mathrm{c2}}(0)\approx 6.3\,$kOe for in-plane and $H^{\perp}_{\mathrm{c2}}(0)\approx 4.7\,$kOe for out-of-plane magnetic fields. Robust superconductivity was also found in resistance and point contact measurements, where the latter showed a slight enhancement of the critical temperature up to $T_\mathrm{c} \sim 3\,$K. Finally, ARPES measurements corroborate the centrosymmetric structure of our samples, showing a single surface termination and the absence of Fermi arcs.
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All authors Kilian Srowik, Pablo Pedrazzini, Soumen Ash, Oksana Kvitnitskaya, Andrii Kuibarov, Susmita Changdar, Oleksandr Suvorov, Volodymyr Bezguba, Alexander Kordyuk, Rafał Kurleto, Dawid Wutke, Reza Firouzmandi, Robert Kluge, Swarnamayee Mishra, Alexander Mistonov, Saicharan Aswartham, Jochen Geck, Sergey Borisenko, Laura T. Corredor, Bernd Büchner

Superconductivity

Enhanced Superconductivity in Multilayer FeSe Films by Simplified Molecular Beam Epitaxy

Maria Hilse, Hemian Yi, Zhe Chen et al. (+7 more)

Multi-unit-cell (UC) e̱ta}-FeSe films grown on SrTiO3(100) continue to attract attention because of the significant enhancement in the superconducting transition temperature (Tc) compared to that in bulk FeSe. In prior reports of molecular beam epitaxy (MBE)-grown e̱ta}-FeSe/SrTiO3(100), elaborate growth protocols have been used to achieve enhanced Tc, leading to a general belief that careful pre-treatment of the SrTiO3 substrate and post-growth annealing in ultrahigh vacuum (UHV) are essential. Here, we report a greatly simplified protocol for the MBE growth of superconducting multi-UC e̱ta}-FeSe films on SrTiO3(100), eliminating the need for careful substrate pre-treatment and post-growth UHV annealing while still achieving an enhanced Tc. With appropriate capping, epitaxial films with 14 UC thickness exhibit a zero-resistance transition temperature Tc ~ 20 K in ex situ electrical transport measurements. The MBE optimization process is guided by the growth-parameter dependencies of film morphology and structural properties, as characterized by reflection high-energy electron diffraction, X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy.
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All authors Maria Hilse, Hemian Yi, Zhe Chen, Jessica L. Thompson, Kalana D. Halanayake, Danielle Reifsnyder Hickey, Seong H. Kim, Cui-Zu Chang, Nitin Samarth, Roman Engel-Herbert

Superconductivity

Disorder-tuned crossing of monopole and conventional pairing instabilities in multi-Weyl semimetals

Enrique Muñoz and Rodrigo Soto-Garrido

We study how non-magnetic impurity scattering affects the emergence and possible coexistence of pairing instabilities in a two-node multi-Weyl semimetal. Within an explicitly specified projected impurity kernel—valley diagonal and momentum independent across each Fermi pocket, the leading behaviour of non-magnetic disorder in the small-pocket window $q^{\max}_{\rm intra}\xi_{\rm dis}\ll1\ll|2\mathbf Q|\xi_{\rm dis}$, treated at leading order in Born and Abrikosov–Gor'kov theory—quenched disorder tunes the leading pairing instability from a topologically nontrivial monopole channel to a conventional ($s$-wave) one, extending our earlier clean-system analysis into the disordered regime. A Born self-energy calculation in the chiral band basis then gives: (i) a band-isotropic conventional channel that is Anderson protected against intra-node scalar disorder ($\eta_s=1$), introduced phenomenologically at the projected-band level; solving the competition for arbitrary $\eta_s$ yields the crossing criterion $(1-\eta_s)/(1-\eta_m)<T_{c0}^{(s)}/T_{c0}^{(m)}$, so the mechanism tolerates substantial loss of conventional-channel protection; and (ii) a rank-one monopole sector fixing $f_m$ as the exact eigenfunction with $\eta_m(J)=1/(J+2)$, exact given that kernel. The crossing location in $\Gamma_N/T_{c0}^{(m)}$ is set by $\eta_m(J)$, $\eta_s$, and $r=T_{c0}^{(s)}/T_{c0}^{(m)}$. From the clean projected BdG Hamiltonian the pure monopole nodes carry Berry charge $\pm J$, distinct from the gapped conventional solution; the clean nodal thermodynamics is charge-dependent, $N_{\rm SC}(E)\propto E^{2/J}$ and $C\propto T^{1+2/J}$, and the residual density of states shows a threshold only for $J=1$. The crossing lies in the moderately metallic regime, $\mu/\Gamma_N\simeq11$–$14$ for the illustrative $T_{c0}^{(m)}/\mu=0.133$ used in the figures.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Electronic cooling of a TiW alloy normal-metal island using Nb-based superconducting tunnel junctions

Joel Hätinen, Renan Pires Loreto, Arvind Kumar et al. (+2 more)

TiW thin films remain non-superconducting down to millikelvin temperatures and are compatible with large-scale CMOS manufacturing, making them attractive for cryogenic and quantum tunnel-junction devices, such as normal-metal-insulator-superconductor (NIS) thermal sensors and electron refrigerators. We demonstrate significant electronic cooling of a TiW-based normal-metal island and electron thermometry by using TiW-Al-AlOx-Nb NIS tunnel-junction stacks. The NIS thermometer enables local electron temperature measurements from 0.5 to 8.32 K. NIS cooling is observed between 0.6 and 3.5 K, with maximum absolute and relative temperature reductions of 217 mK and 27%, respectively. Calculations using tunnel-junction parameters obtained independently from current-voltage fits reproduce the measured optimum-voltage scale, which lies substantially below the ideal low-temperature prediction.
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All authors Joel Hätinen, Renan Pires Loreto, Arvind Kumar, Jani Taskinen, Mika Prunnila

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

Influence of the inter-orbital interaction and kinetic terms on superconductivity: a simple two-orbital Hubbard model

Vito Marino, Diego Florez-Ablan, Luca F. Tocchio et al. (+2 more)

We investigate a minimal two-orbital Hubbard model with intra- and inter-orbital nearest-neighbor hopping $t$ and $\tilde{t}$, as well as intra- and inter-orbital density-density interactions $U$ and $U'$ by means of variational ansätze based on Jastrow-Slater wave functions within quantum Monte Carlo techniques. To focus on the electronic mechanisms of superconductivity, we restrict the variational ansätze to uniform nonmagnetic states with an explicit pairing amplitude and compute the pairing correlations as a function of filling and model parameters. At $U/t=10$, superconducting correlations are highly enhanced by the presence of inter-orbital terms, $U'$ and $\tilde{t}$. For $\tilde{t}=0$, a finite value of $U'$ effectively screens the intra-orbital repulsion $U$, producing a shift in the superconducting dome. Consequently, inter-orbital repulsion yields a sizable increase in electron pairing compared to the single-orbital baseline. Furthermore, introducing a finite $\tilde{t}$ provides an additional boost to superconducting correlations, an effect driven by the simultaneous presence of flat and broad bands in the electronic structure.
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All authors Vito Marino, Diego Florez-Ablan, Luca F. Tocchio, Massimo Capone, Federico Becca

Superconductivity

Josephson effect in bipolar magnetic semiconductors

Polireddi Naveen and Abhiram Soori

We theoretically investigate equilibrium currents in a one-dimensional Josephson junction incorporating a bipolar magnetic semiconductor (BMS). We show that the intrinsic exchange splitting of the spin-resolved bands enables purely electrical control of the $0$–$\pi$ transition through gate-tunable modulation of the BMS chemical potential, eliminating the need for an external magnetic field. This provides a viable route toward electrically tunable $\pi$-junction behavior and highlights the potential of BMS-based Josephson devices for phase-controllable superconducting electronics. Furthermore, in the presence of Rashba spin–orbit coupling, we find an anomalous Josephson effect characterized by a finite equilibrium supercurrent at zero phase difference. This behavior originates from the intrinsic breaking of time-reversal symmetry associated with the spin-polarized electronic structure of the BMS. Interestingly, despite the simultaneous breaking of time-reversal and inversion symmetries—conditions often associated with nonreciprocal superconducting transport—we do not observe a Josephson diode effect. Our results therefore highlight an important distinction between anomalous Josephson transport and superconducting nonreciprocity: the former does not necessarily imply a finite critical-current asymmetry between opposite current directions.
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Superconductivity

Phase-controlled perfect nonlocal spin and charge diode effects in a four-terminal Josephson junction with $p$-wave magnets

Lovy Sharma, Bimal Ghimire and Manisha Thakurathi

We theoretically investigate charge and spin transport in a four-terminal Josephson junction with a normal-metal barrier. The top and bottom superconducting leads are equal-spin triplet $p_y$-wave superconductors, while the left and right leads are $p$-wave magnets with proximity-induced conventional $s$-wave superconductivity. When the transverse macroscopic phase difference between the top and bottom leads is set to zero, a longitudinal phase bias generates a pure transverse spin current with perfect 100% nonreciprocity. Remarkably, a finite transverse phase difference preserves the perfect spin-diode effect while simultaneously inducing a perfect charge-diode effect, enabling fully nonreciprocal spin and charge transport. Moreover, the spin-diode efficiency exhibits sharp, step-like switching as a function of both the gate voltage applied to the barrier and the crystallographic orientation of the $p$-wave magnet, providing independent and experimentally accessible knobs for controlling the diode polarity. The diode response remains robust against asymmetric interface couplings, nonmagnetic disorder, variations in the relative singlet and triplet pairing strengths, temperature, and junction dimensions, demonstrating that the effect is not a consequence of fine-tuned parameters. These findings establish the proposed four-terminal junction as a highly tunable and structurally robust platform for dissipationless, phase-controlled spin and charge rectification, with potential applications in superconducting spintronics.
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Monday 24 August

Superconductivity

Nonequilibrium pulse dynamics and metastable latching in nonlinear kinetic inductance detectors

M. Rouble, C. Albert, P. Day et al. (+3 more)

Microwave kinetic inductance detectors are typically operated at high readout power to raise the detector signal above system noise. At sufficiently large readout power, the current-dependent kinetic inductance couples the detector response to its readout bias. Using a nonlinear resonator framework and time-domain circuit calculations, we show that the amplitude, shape, and relaxation time of the driven detector's response depend on both the absorbed energy and on the readout bias. Strongly driven bias points produce amplified, extended, and non-exponential pulse responses. Qualitative agreement between calculated and measured pulse responses indicates that these effects are dominated by the driven nonlinear resonator dynamics rather than by altered quasiparticle dynamics. Beyond resonance bifurcation, sufficiently large pulse events drive the resonator between stable branches, resulting in a metastable latched state which persists after the quasiparticle transient has decayed. The pulse energy required for branch switching is set by the readout bias, suggesting a mode of triggered detection with an in-situ tunable threshold. Although nonlinear operation requires calibration of the bias- and energy-dependent response, the enhanced pulse amplitude and duration, together with tunable latching and the ability to select these parameters via the readout operating state, are likely to be of interest for single-photon and rare-event experiments, especially those limited by amplifier or system noise.
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All authors M. Rouble, C. Albert, P. Day, M. Dobbs, H. G. Leduc, J. Montgomery

Superconductivity

Intertwined spin-charge stripe order and polar lattice distortion in La$_{3}$Ni$_{2}$O$_{7}$

Xiaoying Li, Wenqian Tu, Run Lv et al. (+4 more)

The low-temperature density-wave state of La$_3$Ni$_2$O$_7$ hosts pronounced spin-density-wave (SDW) order, while recent experiments further reveal charge redistribution and a concomitant lattice-symmetry lowering. However, the microscopic relationship among spin, charge, and lattice remains unclear. Using first-principles calculations, we investigate the pressure evolution of the electronic structure and static spin susceptibility of La$_3$Ni$_2$O$_7$, together with the energetics and lattice response of representative magnetic configurations. We trace the SDW instability to strong Fermi-surface nesting and find that the high-pressure spin response closely tracks $T_{\mathrm C}$, suggesting spin-fluctuation-mediated pairing. Among the candidate magnetic states considered, the spin-charge-stripe states emerge as energetically favored and dynamically stable, developing pronounced disproportionation of both the local Ni moments and the Ni–O bond lengths. Remarkably, the lowest-energy a-stripe state spontaneously relaxes into the experimentally proposed polar Am2m structure through a polar distortion along the b axis. These results establish a unified picture in which spin, charge, and lattice responses are strongly intertwined in the low-pressure density-wave state, while spin fluctuations remain a plausible ingredient of superconductivity under pressure.
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All authors Xiaoying Li, Wenqian Tu, Run Lv, Li'e Liu, Dingfu Shao, Yuping Sun, Wenjian Lu

Superconductivity

Competing Extended-$s$- and $d$-Wave Pairing from Distinct Spin-Fluctuation Channels in Stoichiometric $\mathrm{FeTe}$

Yichen Hua, Wen-lin Yang, Jian-Jian Miao et al. (+2 more)

The recent observation of superconductivity in stoichiometric $\mathrm{FeTe}$ raises the question of how pairing develops in this tetragonal 11-type chalcogenide once interstitial Fe is removed. We construct an experimentally constrained five-orbital tight-binding model from first-principles calculations and treat electronic correlations and pairing within the fluctuation-exchange approximation. The linearized Eliashberg equation yields competing extended-$s$- and $d_{x^2-y^2}$-wave spin-singlet pairing instabilities. Near stoichiometric filling, spin fluctuations near $(\pi,0)$ and $(0,\pi)$ in the unfolded one-Fe Brillouin zone connect the $\Gamma/M$ hole pockets with the $X/Y$ electron pockets and favor an extended-$s$ gap that changes sign between the hole and electron sheets. Upon electron doping, depletion of the hole pockets shifts the dominant scattering toward the $X$–$Y$ channel near $(\pi,\pi)$, making the $d_{x^2-y^2}$-wave state the leading instability, with nodal lines that avoid most of the Fermi surface. The relative strengths of the two pairing channels vary with filling and interaction strength as the dominant spin-fluctuation channel changes. Under matched interaction strength, temperature, and filling, the extended-$s$ eigenvalue is larger in $\mathrm{FeTe}$ than in $\mathrm{FeSe}$ throughout the range considered, while the $d_{x^2-y^2}$-wave eigenvalue is also generally larger, particularly under electron doping. These results give concrete gap structures against which spectroscopic measurements of stoichiometric $\mathrm{FeTe}$ can be compared.
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All authors Yichen Hua, Wen-lin Yang, Jian-Jian Miao, Hu Xu, Changming Yue