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Physics Condensed Matter Superconductivity

cond-mat.supr-con Watch Superconductivity

Wednesday 12 August

Superconductivity

Floquet Quasiparticle Poisoning of Frozonium

Haoyu Guo and Debanjan Chowdhury

Periodic driving can suppress the Josephson nonlinearity of a fluxonium superconducting circuit, producing a nearly harmonic Floquet spectrum at isolated freezing points [K. Lewellen et al., Newton 2, 100434 (2026)]. Here we show that this dynamically frozen behavior does not generically suppress quasiparticle-induced dissipation in the resulting frozonium circuit. We formulate quasiparticle processes in the frozonium using a Floquet framework and analyze both drive-assisted Cooper-pair breaking and tunneling of pre-existing quasiparticles. Pair generation is controlled by gap-breaking thresholds at high drive frequencies, while multiphoton resonances produce pronounced rate enhancements at lower frequencies. Quasiparticle tunneling exhibits connected resonance structures organized by the harmonic Floquet-Magnus spectrum near the freezing point, with resonant hybridization generating characteristic avoided crossings. Our results show that suitable operating regimes must balance dynamical freezing against quasiparticle loss and provide a framework for identifying experimental drive parameters away from harmful resonances.
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Superconductivity

Spin-polarized supercurrents and Josephson diode effect in altermagnets

Janus F. Niebuhr, Matthias Eschrig and Danilo Nikolić

We present a systematic theoretical study of the Josephson effect in junctions consisting of a d-wave altermagnet (AM) placed between two BCS superconductors (SC). In general, the SC/AM interfaces are spin-active and modeled by spin-dependent $\delta$ potentials, allowing for an arbitrary direction of the local exchange field vector. The model is formulated within the fully quantum (Gor'kov) and quasiclassical (Eilenberger) Green's function technique, applied to two distinct cases of (i) a weakly spin-polarized AM (exchange field much smaller compared to the Fermi energy) and (ii) a strongly spin-polarized AM (exchange field comparable to the Fermi energy). We apply our model to the SC/AM/SC geometry, accounting for the Josephson current-phase relation (CPR). In the weakly spin-polarized regime, the CPR displays the normal Josephson effect. Irrespective of the orientation of the altermagnet, the junction undergoes the $0-\pi$ transition. Depending on the orientation, the system displays the features similar to those of a ferromagnetic or an antiferromagnetic junction. To investigate the spin-polarized currents and nonreciprocal transport as the central results of the present work, we put the main focus on the strongly spin-polarized regime. Within this regime, we distinguish two cases. A coplanar exchange field profile across the junctions displays the normal Josephson effect; however, with a pure and stable long-range second harmonic in the CPR. In contrast, a noncoplanar exchange field profile gives rise to the so-called quantum geometric phases across the junction, leading to the absence of the phase-inversion center in the Josephson CPR. As a result, a Josephson diode effect emerges with a significant charge diode efficiency larger than 30% and a perfect spin diode efficiency of 100%.
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Superconductivity

Thickness-Driven Superconductor-Insulator Transition in (Cu,C)-1234 and Proximity-Induced Superconductivity Recovery in (Cu,C)-1234/YBCO Heterostructure

Zhihang Xu, Detian Yang, Ping Zhu et al. (+4 more)

Superconducting proximity effect and related thickness-driven property evolution remain an important issue in understanding high temperature superconductors. Among proximity systems, superconductor-superconductor (S-S') is special for the existence of intrinsic superconductivity in both materials. Such platform allows the different superconducting orders to compete, couple and reconstruct at the interface. In this paper, (Cu,C)-1234/YBCO heterostructure grown on LAO (001) with fixed thickness of bottom YBCO layer as 150 nm and varied thickness of top (Cu,C)-1234 layer as 188nm, 87 nm, 18nm and estimated 1.2 nm were fabricated and component films were preserved. Electrical transport characterization indicated that as the thickness decrease the (Cu,C)-1234 film degrades and underwent the superconductor-insulator transition (SIT) from thicker to less than 18 nm. In contrast, superconductivity is re-established in transport measurements when the insulating (Cu,C)-1234 layer is coupled to superconducting YBCO As the (Cu,C)-1234 thickness is further reduced to approximately 1.2 nm, the recovered superconductivity is strongly suppressed. The observed thickness dependence is consistent with a scenario in which interfacial coupling restores superconductivity over a finite thickness range before increasing disorder and dimensional confinement dominate in the two-dimensional limit. This work establishes a promising platform for investigating interfacial coupling between cuprate superconductors and provides new insight into the superconducting proximity effect in high-temperature superconducting heterostructures.
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All authors Zhihang Xu, Detian Yang, Ping Zhu, Ruoxian Sun, Xiaoyang Cai, Yanqun Guo, Chuanbing Cai

Superconductivity

Acoustic Plasmon Resonance: Breaking the Anderson Stiffness Paradigm in Quasi-Two-Dimensional Superconducting Films

V. M. Kovalev and A. V. Chaplik

Recent experiments on superconducting films have revealed an acoustic plasmon mode that depends critically on the superconducting transition, directly challenging the long-standing Anderson-Higgs paradigm regarding the stiffness of the plasma spectrum in superconductors. In this Letter, we provide a microscopic theoretical framework that explains this behavior and establishes the physical conditions under which classical Anderson-Higgs constraints are bypassed. We demonstrate that in films of finite thickness, the transverse redistribution of normal and superfluid charge densities enables a unique coupling mechanism to electromagnetic radiation - a feature fundamentally absent in the conventional Carlson-Goldman scenario. Our theory predicts an acoustic mode whose dispersion, temperature scaling, and dependence on film thickness are in remarkable agreement with recent experimental observations. By delineating the regime of this acoustic response, we reconcile the observed electromagnetic activity of collective excitations with the fundamental principles of superconductivity.
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Tuesday 11 August

Superconductivity

Yttrium Superhydrides Revisited: Advanced Experimental and Theoretical Studies of YH$_6$, YH$_9$ and YH$_{10}$

Dmitrii V. Semenok, Pedro N. Ferreira, Di Zhou et al. (+10 more)

Yttrium polyhydrides are benchmark materials in high-pressure superconductivity, yet several key properties of the Y-H system remain insufficiently characterized. Here we combine contact transport, contactless radio-frequency measurements, pulsed-field experiments, and first-principles calculations to reinvestigate YH$_6$, YH$_9$, and YH$_{10}$ in the pressure range 140-213 GPa. Yttrium hydrides YH$_6$ ($\textit{$T_c$}$ = 218-221 K) and YH$_9$ ($\textit{$T_c$}$ = 235-237 K) demonstrate narrow superconducting transitions ($\textit{${\Delta}$T$_c$}$ = 2-5 K), approaching the limit imposed by thermal fluctuations. Pulsed-field measurements on YH$_6$ up to 60 T establish an extended superconducting phase diagram with a linear slope $\textit{dB$_{c2}$/dT}$ = -0.52 T/K, pronounced transition broadening above 30 T, and negligible normal-state magnetoresistance. We report the radio-frequency AC susceptibility study of YH$_6$, providing evidence for superconductivity via high-frequency field screening in a contactless geometry. Experiments involving Pd incorporation, Pd thin-film sputtering, and Al alloying show strong suppression of high-temperature superconductivity, with no transitions detected above 78-120 K. Finally, using density-functional theory with the stochastic self-consistent harmonic approximation, superconducting density-functional theory, and full-bandwidth Migdal-Eliashberg calculations, we show that anharmonic effects substantially reduce the predicted $\textit{$T_c$}$ of cubic YH$_{10}$ to approximately 260-270 K. These results strongly disfavor room-temperature superconductivity in binary yttrium superhydrides.
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All authors Dmitrii V. Semenok, Pedro N. Ferreira, Di Zhou, Fabian Jőbstl, Andrey V. Sadakov, Kirill S. Pervakov, Burkhan I. Massalimov, Toni Helm, Ryosuke Akashi, Vladimir M. Pudalov, Viktor V. Struzhkin, Christoph Heil, Ivan A. Troyan

Superconductivity

Light induced superconducting diode effect in patterned films

Evan M. Wilson, Hou-Tong Chen and Alexander V. Balatsky

Structured light offers a route to control superconducting transport without permanently modifying the material or applying a static bias. Here we show that structured optical driving can generate a superconducting diode response in patterned superconducting films with asymmetric holes. Using generalized time-dependent Ginzburg Landau simulations, we find that optical driving produces rectified dc photovoltages and zero bias directional supercurrent imbalance in a junction free geometry, with continuous-drive diode efficiencies of order $10^{-3}$ and pulsed efficiencies reaching $10^{-2}$. The response is controlled by both the hole array and the optical mode. Increasing the number of asymmetric holes enhances rectification, reversing circular helicity reverses the diode polarity, and the optical spatial mode strongly modifies the magnitude and polarity of the directional response. Pulsed excitation enhances the zero bias line cut current imbalance to the percent level. For linearly polarized illumination, the asymmetric metacrystal converts the drive into local chiral supercurrent motion, inducing an inverse Faraday effect like mechanism for dynamical time reversal symmetry breaking. These results establish patterned superconducting films as a viable platform for light-tunable superconducting diode behavior.
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Superconductivity

Discriminating superconducting fluctuations from the pseudogap in Bi$_2$Sr$_2$Ca$_{n-1}$Cu$_n$O$_{2n+4+\delta} (n = 2,3)$: A magnetotransport study

Shunpei Yamaguchi, Nae Sasaki, Shintaro Adachi et al. (+13 more)

Understanding the normal state is essential for uncovering the mechanism of high-$T_c$ superconductivity. We investigate magnetotransport in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ single crystals over a wide doping range. While the in-plane resistivity and Hall coefficient show strong pseudogap-induced temperature dependence, the $T^2$ Hall-angle behavior and the modified Kohler's rule remain robust across all dopings. The onset temperatures of the pseudogap are clearly distinct from superconducting fluctuations, although they scale with the pseudogap magnitudes with a factor consistent with a $d$-wave superconductor. These results demonstrate that the pseudogap does not arise from superconducting fluctuations and instead suggest that it may originate from preformed Cooper pairing in the BCS-BEC crossover regime.
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All authors Shunpei Yamaguchi, Nae Sasaki, Shintaro Adachi, Keiichi Harada, Yuki Teramoto, Shintaro Matsuda, Tomohiro Usui, Takenori Fujii, Takashi Noji, Itsuhiro Kakeya, Haruka Taniguchi, Michiaki Matsukawa, Atsushi Miyake, Hajime Ishikawa, Koichi Kindo, Takao Watanabe

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

High-pressure electride superconductor Li5N for multifunctional applications: A theoretical insight into the physical properties

M. Abdul Hadi Shah and S.H. Naqib

This study aims to unveil the physical properties of multifunctional Li5N electride under high pressure in the range of 150-350 GPa through first principles analysis within the density functional theory.

Superconductivity

Quadrupolar phase transition in superconducting lanthanum hydride

Abhishek Raghav, Kousuke Nakano, Marco Cherubini et al. (+2 more)

Lanthanum hydride (LaH$_{10}$) has been widely studied for its high superconducting critical temperature of 250 K at about 170 GPa pressure. Although the structural ${R\bar{3}m}$-to-${Fm\bar{3}m}$ transition under pressure connected to the emergence of the superconducting phase in this material is broadly understood, the detailed characterization of its nature and its order parameter are still missing. By applying the cluster multipole moment analysis to the hydrogen sublattice, we reveal that this transition is triggered by a quadrupolar $T_{2g}$ order parameter, and we provide evidence for its weak first-order nature. By performing path integral molecular dynamics coupled to a message-passing atomic cluster expansion (MACE) neural network potential, trained on Perdew-Burke-Ernzerhof (PBE) density functional theory configurations, we show that the collapse of the order parameter at the transition is simultaneously associated with the discontinuous softening of the optical $T_{2g}$ phonons. Their symmetry lets them carry a non-negligible electron-phonon coupling in LaH$_{10}$, while the weak first-order nature of the transition makes them soft. The presence of structural instabilities with low-frequency quadrupolar distortions can be a key ingredient to enhance superconductivity in superhydrides and provides guidance for the discovery of new high-$T_c$ superconductors in hydrogen-rich compounds.
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All authors Abhishek Raghav, Kousuke Nakano, Marco Cherubini, Ryotaro Arita, Michele Casula

Monday 10 August

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

Some features of high-temperature superconductivity on flat bands

V.R. Shaginyan, A.Z. Msezane and S.A. Artamonov

In this letter, we examine how the presence of flat band leads to the formation of a high-temperature superconductor even in the case of repulsive pairing interactions. We also show that in the case of flat bands, the high-temperature superconducting state deforms the flat band, tilting it and making the effective mass finite. As a result, neither the superfluid weight nor the supercurrent disappear. Our results are in good agreement with experimental data.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Topological phase rectification via Aharonov-Bohm interference in a Majorana–quantum-dot interferometer

Jia Liu, Hao-Yuan Yang, Yuan Hong et al. (+5 more)

We propose and theoretically investigate a topological superconducting rectifier based on a quantum-dot–Majorana interferometer. The Aharonov-Bohm phase, controlled by a magnetic flux threading the interferometer loop, tunes the quantum interference between a trivial $2\pi$-periodic quantum-dot channel and a topological $4\pi$-periodic Majorana channel. At non-integer flux, this interference generates a persistent current background $I_{\rm off}$ that shifts the current-phase relation into a unipolar regime, in which the supercurrent flows strictly in one direction. We introduce a signed unipolarity factor $\eta_u$, with $|\eta_u|>0.5$ defining the unipolar regime, and establish its quantitative relationship to the conventional diode efficiency $\eta$. The unipolarity proves robust against variations of the quantum-dot level, spin polarization, and Majorana hybridization, is enhanced by stronger Majorana coupling and Rashba spin-orbit interaction, and persists at realistic temperatures and under quasiparticle poisoning. We further propose a topological diode figure of merit $\mathcal{Z}_{\rm TD}$, defined from the Fourier spectrum of $\eta_u$, whose nonzero value provides a model-independent signature of the $4\pi$-periodic Majorana channel and distinguishes topological from trivial rectification mechanisms. Our findings establish the quantum-dot–Majorana interferometer as a promising route toward high-performance topological superconducting diodes with clear experimental signatures accessible via standard dc transport measurements.
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All authors Jia Liu, Hao-Yuan Yang, Yuan Hong, Li Ma, Feng Chi, Zi-Chuan Yi, Li-Ming Liu, Zhen-Guo Fu

Superconductivity

Superconductivity, pseudogap and marginal Fermi liquid in a relative-momentum-local theory

Jian-Jian Miao

Cuprate superconductors host \(d\)-wave superconductivity (SC), a pseudogap (PG) with Fermi arcs and a strange metal (SM) with incoherent excitations and temperature-linear resistivity. These regimes emerge from the same doped Mott system but are usually described separately. Here a relative-momentum-local (RML) theory is constructed, with the exact spin-singlet Cooper representation of antiferromagnetic superexchange in the \(t\)-\(J\) model serving as a common thread. Pair correlations organize the low-energy behaviour of this Cooper channel into phase-coherent static SC, phase-incoherent quasi-static PG and phase-incoherent dynamic SM. A static RML prescription yields an exactly solvable PG Hamiltonian with nodal electron poles, antinodal gaps, resolution-broadened Fermi arcs and gapless charge-\(2e\) Cooper surfaces. A Cooper-surface Lifshitz transition generates logarithmic enhancements in the specific-heat coefficient and charge response without a spin singularity. The particle-particle continuum at finite centre-of-mass momentum produces Ohmic damping, while a dynamic RML prescription yields a one-loop self-energy with marginal-Fermi-liquid scaling. Spectral-weight transfer in the physical pair spectrum diagnoses the PG–SM crossover, whereas nonzero phase stiffness and global phase coherence identify SC. The RML theory relates single-particle, pair-sensitive and thermodynamic probes to three infrared organizations of one Cooper channel. A decisive experimental signature would be zero-energy Cooper-surface ridges in two-electron angle-resolved photoemission spectroscopy (2e-ARPES).
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Superconductivity · Cross-listed from cond-mat.mes-hall

Disorder-robust trivial Majorana-like states from smooth confinement in chiral superconducting nanowires

Eslam Ahmed, Jorge Cayao and Yukio Tanaka

Near-zero-energy states in Majorana nanowires can arise from topologically trivial mechanisms such as smooth spatial inhomogeneity and disorder, making zero-energy pinning alone insufficient evidence of bulk topology. Here we identify a real-space mechanism governing their robustness to symmetry-preserving disorder. For a chiral-symmetric Bogoliubov-de Gennes Hamiltonian, we decompose a low-energy state into two normalized components of opposite chirality and show that disorder-induced splitting is bounded by their spatial overlap. We demonstrate this result in a finite Rashba nanowire with smooth chemical potential and pairing profiles. Below the bulk topological transition, smooth confinement produces partially separated chiral components with exponentially small overlap, yielding globally trivial Majorana-like Andreev bound states that remain near zero energy even under strong scalar, nonmagnetic disorder. The chiral overlap therefore provides a direct diagnostic of the protection of low-energy states against local perturbations, independent of the bulk topological invariant.
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Superconductivity · Cross-listed from cond-mat.mes-hall

Probing crystal-field modulations with magnetic adatoms on the incipient charge-density-wave superconductor $2H$-NbS$_2$

Werner M.J. van Weerdenburg, Margarete Huisinga, Constantin Flommersfeld et al. (+2 more)

The interplay between multiple quantum phases in layered materials may lead to incipient quantum behavior, where the material's ground state is close to a phase transition and sensitive to local disorder. The transition metal dichalcogenide material $2H$-NbS$_2$ exhibits incipient charge-density-wave behavior along with a well-developed superconducting state, creating a scenario where the local lattice instabilities play a crucial role. Here we present how an individual magnetic atom on $2H$-NbS$_2$ can be applied as a local sensor to reveal hidden crystal-field modulations. By manipulating the adatom across the surface with the tip of a scanning tunneling microscope, we measure variations in the Yu-Shiba-Rusinov (YSR) excitation spectra and map the local environment around an intrinsic point defect. We find that while the superconducting state is spatially uniform, the YSR excitation energy strongly depends on the position of the atom. We determine that the main contribution to this effect originates from variations in the local crystal-field environment. These results establish a new approach to investigate crystal-field modulations at the atomic scale and reveal how defects and lattice instabilities shape the atomic landscape of an incipient charge-density-wave material.
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All authors Werner M.J. van Weerdenburg, Margarete Huisinga, Constantin Flommersfeld, Lisa M. Rütten, Katharina J. Franke

Superconductivity

Revealing Hidden Unconventional Pairing through Nonreciprocal Transport

Wen-Bo Dai, Ming Gong, Xianxin Wu et al. (+2 more)

Identifying the pairing symmetry of Cooper pairs is a fundamental step toward understanding the microscopic mechanisms of unconventional superconductors. However, experimental identification remains a formidable challenge, particularly when unconventional pairing is obscured by a dominant $s$-wave component that masks its spectroscopic signatures. Here, we develop a symmetry-resolved framework to identify superconducting pairing symmetry through nonreciprocal conductance upon exchanging source and detector terminals in multiterminal devices. We show that nonreciprocal transport arises from symmetry-breaking components of the superconducting order parameter and exhibits a characteristic angular dependence that encodes the momentum-space structure of the pairing gap. In particular, time-reversal-breaking singlet pairing induces nonreciprocal charge transport, while spin-triplet pairing generates nonreciprocal spin responses, providing distinct transport fingerprints of the underlying order. We demonstrate this mechanism using representative models of iron-based and noncentrosymmetric superconductors and outline experimental protocols for multiterminal measurements. Our results advance the theoretical understanding of nonreciprocal transport in superconductors, and establish it as a symmetry-selective probe for identifying hidden unconventional pairing in a wide range of superconducting materials.
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All authors Wen-Bo Dai, Ming Gong, Xianxin Wu, Chui-Zhen Chen, X. C. Xie

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

Geometrical approach and topological electron density in the $p_x + ip_y$ superconductor

Karyn Le Hur, Kabir Narayanan and Óscar Mañas Chambert

We present an analysis on the geometrical and physical nature of the $p_x + ip_y$ superconductor on the square lattice, with an emphasis on the topological phase transition at half-filling. We develop a local topological marker from specific Dirac points within the Brillouin zone, which is introduced via the addition of two one-dimensional (1D) $\mathbb{Z}$ $(\mathbb{Z}_2$) invariants defined on the Bloch sphere. We relate this topological marker to the electron spectral function integrated on frequency through the local momentum-resolved electron density, which may be measured via Angle Resolved Photoemission Spectroscopy (ARPES), and show that it remains well-protected including temperature effects. Integrating on a small area around a specific point in momentum space associated to the measure uncertainty, this also reveals the Van Hove logarithmic profile of the density of states in the derivative of the local marker while preserving the topological information. Topological transitions correspond to a protected semi-metal. We analyse the real space representation of this topological marker from correlation functions. We present physical responses such as the topological superfluid density.
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Superconductivity

Exchange-induced suppression of superconductivity in a nano-skyrmion lattice - superconductor hybrid

Dongfei Wang, Wenbin Li, Eric Mascot et al. (+1 more)

Engineered magnet-superconductor hybrids have recently been identified as promising platforms for the investigation of topological superconductivity. Beyond ferro- and antiferromagnetic structures, coupling non-collinear spin textures, such as skyrmion lattices, to superconductors offers an exciting route for creating and manipulating unconventional superconducting states. In this work, by preparing monolayer Fe on Ir(111) thin films grown epitaxially on a Nb(110) surface, we realize a hybrid system of a nano-skyrmion lattice proximitized to a superconducting substrate. Scanning tunneling spectroscopy shows that superconductivity becomes suppressed by the Fe nano-skyrmion lattice, with both the superconducting gap and coherence peaks disappearing. Tight-binding calculations reveal that with increasing exchange coupling, the gap is progressively filled up and eventually superconductivity gets quenched. These results reveal microscopic constraints for designing topologically non-trivial states based on magnet-superconducting heterostructures.
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All authors Dongfei Wang, Wenbin Li, Eric Mascot, Roland Wiesendanger

Superconductivity

Prediction of BaBiO$_3$-like superconducting perovskites in K-doped SrAsO$_3$

Zhihong Yuan, Rui Liu, Pengyu Zheng et al. (+1 more)

Using first-principles calculations, we predict a new perovskite compound SrAsO$_3$ . The undoped cubic phase has pronounced soft-phonon instabilities, which are gradually suppressed upon K doping the Sr site. The cubic phase becomes dynamically stable for K-doping levels above approximately 60%, and the stabilized K-doped phases are metallic with predicted conventional phonon mediated superconductivity. Moreover, the inclusion of nonlocal exchange interactions broadens the electronic bandwidth, enhances the electron-phonon coupling (EPC) strength, and increases the superconducting transition temperature ($T_c$) of these doped compounds. In particular, the HSE06 hybrid exchange-correlation functional corrected EPC constant $\lambda$ reaches 1.41 for Sr$_{0.4}$K$_{0.6}$AsO$_3$, corresponding to a predicted $T_c$ of 44.3 K. These results suggest that SrAsO$_3$ is a BaBiO$_3$-like superconducting perovskite driven by strong electron-phonon coupling.
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All authors Zhihong Yuan, Rui Liu, Pengyu Zheng, Zhiping Yin

Sunday 9 August

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

Layer-Locked Chiral Topological Superconductivity

Yijie Mo and Zhongbo Yan

We uncover a universal mechanism for realizing layer-locked topological phases. Guided by it, we investigate the realization of layer-locked chiral topological superconductivity-the superconducting analogue of the quantum anomalous layer Hall effect-in a nonsymmorphic bilayer antiferromagnetic system with s-wave pairing. We identify three distinct gate-tunable topological phases and establish a direct correspondence between the nearly quantized layer-resolved Chern numbers and the layer-locking behavior of chiral Majorana edge states, vortex-core Majorana zero modes, and nearly quantized thermal Hall responses.
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Superconductivity

Excitron-Induced Pair Fluctuations Reveal Superconductivity in the Electron Gas

Yasutami Takada

Understanding how superconductivity can emerge in dilute electronic systems remains a central challenge in condensed matter physics. By performing first-principles calculations of the electron self-energy \Sigma(k,iw_n) in the low-density three-dimensional electron gas, we identify a sharp divergence at r_s ~ 8 and T ~ 10^{-4}E_ F, signaling a second-order phase transition. This critical behavior originates from one-dimensional superconducting fluctuations mediated by virtual excitations of an excitron—a quasi-1D electronic composite formed by an electron and longitudinal electron-hole pairs. Although the superconducting mechanism itself is plasmon-mediated, the excitron channel provides a unique window into its fluctuation dynamics. Near the transition, we observe a pseudogap and a linear-in-T inverse electron lifetime, reminiscent of phenomena in high-T_c materials. These results reveal an unexpected route by which plasmon-driven superconductivity manifests in the dilute 3D electron gas through quasi-1D excitron dynamics.
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Saturday 8 August

Superconductivity

Quantum Metric Induced Critical Current Anomaly in Flat Band Josephson Junctions

Zhong C.F. Li, Yuxuan Deng, Dmitri K. Efetov et al. (+1 more)

In well-established theories of Josephson junctions, the superconducting critical current \( I_\mathrm{c} \) increases as the normal state conductance \( \mathcal{G} \) increases. However, in a recent experiment in twisted bilayer graphene (TBG) based Josephson junctions, unexpectedly, it was observed that the increase of the critical current is accompanied by a decrease of the normal state conductance. We call this phenomenon the critical current anomaly. In this work, we point out that in the TBG-based Josephson junction, due to the suppression of the conventional Josephson current by the flatness of the band and the quantum metric enabled Josephson current (QMJC), the critical current anomaly can occur. The QMJC appears if the quantum metric length is comparable or longer than the junction length. We show that both \( \mathcal{G} \) and \( I_\mathrm{c} \) have the conventional and the quantum metric contributions, and there are parameter regimes in which \( I_\mathrm{c} \) increases even when \( \mathcal{G} \) decreases. We first demonstrate the critical current anomaly by a simple modified Lieb-lattice model both analytically and numerically. The incredible consistency with the experimental results is demonstrated using a realistic six-band model of twisted bilayer graphene. Therefore, we suggest that the critical current anomaly observed in the experiment provide strong evidence of QMJC which were ignored in well-established theories of Josephson junctions.
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All authors Zhong C.F. Li, Yuxuan Deng, Dmitri K. Efetov, K. T. Law

Superconductivity

Effects of high-pressure synthesis on phase formation and superconducting properties of PrFeAsO1-xFx

Priya Singh, Konrad Kwatek, Tatiana Zajarniuk et al. (+3 more)

Motivated by recent reports of enhanced superconducting performance in several families of iron-based superconductors (IBS) processed by high-pressure (HP) synthesis, we investigate the influence of high gas pressure and high-temperature synthesis (HP-HTS) process on the structural, microstructural, electrical transport, and magnetic properties of Pr-based oxypnictide PrFeAsO1-xFx (Pr1111) using the processing conditions of 0.5 GPa for 1 h previously optimized for other IBS families. Representative underdoped (x = 0.2), optimal doped (x = 0.3), and overdoped (x = 0.5) compositions from the ambient-pressure electronic phase diagram of Pr1111 are selected to evaluate its composition-dependent effects of HP-HTS. The results demonstrate that HP-HTS enhances fluorine incorporation, improves phase formation, and produces a denser microstructure with improved grain connectivity in the underdoped and optimal doped compositions. Magnetic measurements reveal increased in the superconducting transition temperature (Tc) of ~1 K for x = 0.2 and ~6 K for x = 0.3, whereas only a marginal improvement in the critical current density is observed. Electrical resistivity measurements of the underdoped composition show a slight increase in Tc accompanied by a broader resistive transition, indicating residual structural inhomogeneity. In contrast, the overdoped composition exhibits increased impurity phase segregation, accompanied by suppression of superconductivity. These results demonstrate that the effectiveness of HP-HTS in Pr1111 is strongly composition dependent and governed by the interplay among fluorine incorporation, phase stability, and microstructural evolution, highlighting the need for further optimization of the HP-HTS processing conditions.
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All authors Priya Singh, Konrad Kwatek, Tatiana Zajarniuk, Tomasz Cetner, Jan Mizeracki, Shiv J. Singh

Friday 7 August

Superconductivity

Onset of the transitional flux-avalanche regime in bulk NbTi controlled by the thermal boundary conductance

Irina Abaloszewa, Victor V. Chabanenko and Aleksander Abaloszew

Thermomagnetic avalanches in type-II superconductors occur in two qualitatively different regimes, electromagnetically controlled in thin films and thermally limited in bulk samples, distinguished by the sign of the temperature derivative of the threshold field $H_\text{th}(T)$. The two regimes are separated by a critical thermal boundary conductance $h_c$, and a non-monotonic $H_\text{th}(T)$ has been predicted in the transitional region where the interface conductance $h$ approaches $h_c$. We approach this region in a bulk NbTi disk by raising the interface coupling above the pure-nonadecane baseline with a silver-filled interface layer. Whereas the pure interface gives a monotonically decreasing $H_\text{th}(T)$, the silver-filled interface produces a non-monotonic dependence not previously realized in a bulk superconductor: a temperature interval of positive slope, $dH_\text{th}/dT > 0$, terminating in a maximum at $T^* \approx 6.1$–$6.4$~K. The effect is reproduced for two independent silver-filled compositions; in a third, with the highest loading, the low-temperature decrease is absent altogether and $H_\text{th}(T)$ is flat up to the same $T^*$, the evolution expected for stronger coupling. The position of the maximum is set by the intrinsic properties of NbTi, independent of the silver content. The onset of the positive-slope interval coincides in temperature with a change of the avalanche morphology from narrow channeled fingers to broad fronts. The reversal of the sign of $dH_\text{th}/dT$ is a direct experimental signature of the onset of the transitional regime, in which heat removal during the instability becomes dynamically relevant.
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Superconductivity

Emergent magnetic pseudogap from phase fluctuations and hierarchy of scales in two-dimensional superconductors

Xu-Cheng Wang and Yang Qi

Preformed pairs and phase fluctuations are believed to play a vital role in predicting the charge pseudogap in the normal state of two-dimensional superconductors. In this work, we extend this idea and further identify the emergent magnetic pseudogap from pure phase fluctuations without invoking any competing order. We examine the NMR relaxation rate $1/T_1T$ by evaluating the bubble contribution and leading-order vertex correction within perturbation theory. It is found that the magnetic pseudogap, manifesting as a smooth suppression of $1/T_1T$ in the normal state, is characterized by a temperature scale $T_\text{mPG}$ distinct from the superconducting gap $\Delta_\text{SC}$ and transition temperature $T_c$. The onset scales of both charge and magnetic pseudogap are dominated by the competition of BKT correlation length $\xi(T)$ and BCS coherence length $\xi_\text{BCS}$. Moreover, the vertex correction is shown to be irrelevant for $d$-wave pairing, while it becomes prominent in $s$-wave systems and drives a coherent enhancement of $1/T_1T$ at lower temperatures just above $T_c$. We attribute this normal-state enhancement of $1/T_1T$ to the diverging coherence peak at the $s$-wave superconducting gap edge, which shares the same spirit as the celebrated Hebel-Slichter peak in the BCS theory. Analogous to the coherent Hebel-Slichter peak, regularization by Fermi-liquid-like scatterings is important and is characterized by a scattering length $\ell$. The normal-state coherent enhancement of $1/T_1T$ is hence described by the competition of $\xi(T)$ and $\ell$, through which the coherence scale $T_\text{coh}$ is determined. As a result, the complete evolution of $1/T_1T$ is understood quantitatively in a unified picture as the interplay among hierarchy of scales $\xi(T)$, $\xi_\text{BCS}$ and $\ell$.
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Superconductivity · Cross-listed from cond-mat.stat-mech

Statistical stability of random potentials to thermal and quantum activation

Luis Filsinger and Roland Willa

In numerous physical, chemical, and biological systems the dynamics can be reduced to the motion of state variables in a complex potential landscape. In case the manifold is known, the motion and response of the embedded object can be described deterministically up to stochastic effects usually associated with a noise. In contrast, if the manifold is unknown, the static and dynamic response of the state variable may be used as a spectroscopic tool to characterize the potential landscape. Inspired by a seminal work of L.\ Embon and co-workers, [Sci.\ Rep.\ 5, 7598 (2015)] we investigate the statistical properties of potential minima, in particular, their stability to thermal and quantum activation. For Gaussian random manifolds, we derive an algebraic expression to evaluate the statistical probability of the potential character (value, slope, curvature, ...). With this tool, we compute the expectation value for the rate of thermal and quantum activation and link these findings to the principal characteristics of the Gaussian potential, i.e., its Green's function. This link provides the opportunity to access information on the potential's Green's function by studying the activation behavior of an object in this manifold.
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Superconductivity

Origin of the superconductor-insulator transition in disordered two-dimensional films

Alexander Weitzel, Lea Pfaffinger, Animesh Panda et al. (+8 more)

Theory predicts the superconductor-to-insulator transition (SIT) to emerge from the competition between Anderson localization, which tends to localize single-particle wavefunctions, and superconductivity, which establishes long-range correlations in the superconducting order parameter. In two-dimensional (2D) superconducting films, the transition temperature $T_\text{c}$ at which resistance vanishes, $R_\Box(T_\text{BKT}){=}0$, is set by the Berezinskii-Kosterlitz-Thouless (BKT) mechanism and satisfies $T_\text{BKT}< T_{c0}$, where $T_{c0}$ is the mean-field transition temperature. In weakly disordered samples $T_\text{BKT}\lesssim T_{c0}$, whereas increasing disorder drives $T_\text{BKT}\ll T_{c0}$ near the SIT. Whether the finite-temperature transition retains its BKT character throughout this crossover remains an open question. Here, we investigate the evolution of both sheet resistance $R_\Box(T)$ and superfluid stiffness $J_s(T)$ over a wide range of disorder strength $W$. We establish that even near the SIT, the finite-temperature transition from the superconducting to the resistive state remains of BKT type. However, as disorder approaches the critical value, the zero temperature superfluid phase stiffness, $J_s(0)$, is found to vanish rapidly while $T_{c0}$ remains finite, which we attribute to quantum phase fluctuations as the drive for the zero-temperature transition. Three decades after its experimental discovery by Haviland, Liu, and Goldman, our measurements clarify the origin of the SIT in 2D films.
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All authors Alexander Weitzel, Lea Pfaffinger, Animesh Panda, Arpan Das, Ilaria Maccari, Simon Reinhardt, Sven Linzen, Evgenii Il'ichev, Nicola Paradiso, Ferdinand Evers, Christoph Strunk

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

Universal Magnetoresistance Scaling in Layered Pd-based Multiband Metals Beyond Compensated Semimetal Regime

Kenjiro Okawa and Takao Sasagawa

We systematically investigated the magnetoresistance ratio (MR) of single-crystalline, nonmagnetic layered Pd-based metals, including centrosymmetric PdTe2, PdPb2, and beta-PdBi2 and noncentrosymmetric alpha-PdBi. Our study identifies a distinct class of large MR in multiband, high-carrier-density systems. Unlike well-studied extremely large MR materials, such as Dirac and Weyl semimetals described by simple compensated-carrier models, these compounds possess complex Fermi surfaces, as validated by our first-principles calculations. Nevertheless, they exhibit a remarkably simple MR scaling governed by carrier mobility, manifested in systematic dependencies on magnetic field, temperature, and the residual resistivity ratio (RRR). The validity of Kohler's rule in high-RRR crystals indicates that MR is governed by a single effective scattering time, even in these multiband systems. The field and RRR dependences of MR follow an intermediate power-law behavior between linear and quadratic, attributable to imperfect carrier compensation and a distribution of carrier mobilities. Among the studied compounds, alpha-PdBi exhibits the largest MR, reaching 1500% (2 K, 7 T), owing to its exceptionally high RRR (approximately 660). However, when compared on an equal-RRR basis, its MR is smaller than that of its centrosymmetric counterparts. This trend suggests that additional scattering channels arising from spin-orbit-induced band splitting in noncentrosymmetric systems reduce the effective carrier mobility. Our results establish a new class of large MR in clean multiband metals where complex electronic structures give rise to emergent single-parameter scaling, highlighting the interplay between disorder, mobility, and symmetry.
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Thursday 6 August

Superconductivity

Determination of the properties of a superconducting single crystal FeSe using an EPR spectroscopy

S.I. Bondarenko, A.A. Prokhorov, N.N. Galtsov et al. (+3 more)

Using an EPR spectrometer, the properties of single-crystal FeSe were studied in a magnetic field up to 6000 Oe at temperatures from 3.5 K to 8 K in the superconducting state and at temperatures above 8K and up to 25K in the normal state. It was shown that by measuring non-resonant spectrometer signals in zero and weak magnetic fields up to 30 Oe, it is possible to determine critical temperature Tc, the superconducting transition width, and the value of the first critical magnetic field Hc1 in FeSe single-crystal. Resonant EPR signals are observed in fields at 1400 Oe and 3400 Oe, which corresponds to the paramagnetism of doubly ionized iron atoms (Fe2+) in the FeSe crystal. Non-resonant EPR signals in a magnetic field up to 6000 Oe at temperatures from 3.5K to 8K are a nonlinear function of the field, and correspond to the superconducting mixed state of the FeSe, but at temperatures above 8K (up to 25K) they are field independent.
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All authors S.I. Bondarenko, A.A. Prokhorov, N.N. Galtsov, V.P. Timofeev, V.P. Koverya, A.V. Krevsun

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

Temperature-tunable spin-wave refraction using superconducting control elements

Pim H. Vree, Merel A. Bouma, Michael Borst et al. (+3 more)

Spin waves are promising signal carriers for microwave control at the micrometer scale. However, realizing low-damping, tunable control of spin-wave propagation remains a central challenge. Here we use magnetic shielding by superconducting control elements to tune the local spin-wave dispersion and realize temperature-controlled refraction of spin waves in a thin-film magnetic insulator. Using magnetic imaging based on spins in diamond, we characterize the refractive index and demonstrate both positive and negative refraction as well as wavefront shaping by the superconductors. The observed refraction is explained by a geometrical analysis of the superconductivity-induced modification of the hyperbolic spin-wave dispersion. Our results demonstrate that superconductors enable tunable spin-wave optical elements, opening new opportunities for microwave control in classical or quantum information devices.
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All authors Pim H. Vree, Merel A. Bouma, Michael Borst, Tomas T. Osterholt, Rembert A. Duine, Toeno van der Sar

Superconductivity

Emergence of Bogoliubov Fermi Surfaces in hybrid Al/InAs heterostructures

S. Feyrer, V. Dimic, I. Lobato et al. (+15 more)

We investigate the microwave electrodynamics of a proximitized two-dimensional electron gas in hybrid superconductor/semiconductor heterostructures. Using lumped-element resonators with inductor wires oriented relative to an in-plane magnetic field, we directly probe the superfluid stiffness via the kinetic inductance. As the field increases, the resonance frequency exhibits a non-monotonic and strongly anisotropic evolution that cannot be explained by orbital pair breaking alone. We show that this behavior is consistent with the emergence of Bogoliubov Fermi surfaces, which selectively suppress the supercurrent response depending on the direction of the magnetic field. Microscopic calculations of the stiffness tensor capture the observed anisotropy driven by the interplay of Zeeman and orbital Fulde-Ferrell effects. Our results establish microwave stiffness measurements as a sensitive probe of anisotropic gapless superconductivity in hybrid systems.
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All authors S. Feyrer, V. Dimic, I. Lobato, A. Kirchner, P. Drexler, L. Rupp, D. Bougeard, T. Lindemann, S. Gronin, G. Gardner, M. J. Manfra, G. F. R. Ruiz, C. A. Balseiro, L. Arrachea, M. Aprili, N. Paradiso, C. Strunk, L. Tosi