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First order quantum breakdown of superconductivity
An international team of scientists published an article entitled “First-order quantum breakdown of superconductivity in an amorphous superconductor” in the journal Nature Physics (Impact Factor: 18.1). The article explores the fundamental nature of transformation from superconducting to insulating ground state of amorphous Indium Oxide in increase of disorder. Researcher Dr. Mikhail Feigel’man from Jozef Stefan Institute, in collaboration with researchers from the Neel Institute (CNRS Grenoble) and Karlsruhe Institute of Technology, provided a groundbreaking framework for understanding of unexpected experimental demonstration of sharp disappearance of superconductivity of thin films at ultra-low temperatures, upon increase of their normal-state resistance, that is in sharp contrast with standard paradigm of continuous (second-order) superconducting-insulator transitions existing for more than three decade.
First-order nature of this transition is understood in terms of energy competition between two low-temperature states of matter: the superconducting condensate of electron pairs on one hand, and the insulator made out of bound electron pairs with Coulomb repulsion between them, on another hand. Strongly disordered superconducting films became popular due to the search for a “super inductor” - a new element that is badly needed for practical use in superconducting quantum circuits, qubits and detectors. The observed and explained first-order transition demonstrates existence of an upper bound on kinetic inductance that can be reached in a superconducting film.
Andreev conductance via an absolute ferromagnet
International team including Mikhail Feigel’man, researcher at CENN Nanocenter and Jozef Stefan Institute, and his colleagues Pavel Ostrovsky and Mistafa Ismagambetov from MPI Stuttgart,
published an article entitled Andreev Conductance in Disordered SF Junctions with Spin-Orbit Scattering in the special issue of the Journal of Low Temperature Physics dedicated to the memory of famous theoretical physicist Alexander Andreev who first explains how charge transport between superconductor and normal metal takes place, in spite of an energy gap in the former. The authors explore possibility of low-voltage (sub-gap) electron transfer between usual superconductor with a singlet Cooper pairing and a “half-metal” - a very strong ferro-magnet where electrons with a single spin polarization only are present at the Fermi surface. On a first sight such a process is impossible since electrons in a superconductor are bound to pairs with opposite spin projections.
The authors demonstrated, surprisingly, that this is not the case if superconductor is strongly disordered, so directions of electrons momenta change very fast in time due scattering by disorder. The trick is that scattering of electrons always contains a so-called “spin-orbital” component which couples directions of momenta and of spin of electrons. In result, spin of electron pairs is not a good quantum number anymore, it contains both singlet and triplet component in a coherent superposition. Triplet component appears due to random spin-orbit impurity scattering, thus it vanishes upon averaging over disorder. However, it does not affect coherent transport of two electrons in the triplet spin configuration across a border between Half-Metal and Superconductor. It was demonstrated by exact analytical calculation that conductance dI/dV is determined by the comparison between the rate of spin-orbit scattering 1/τso and superconducting energy gap Δ.
The studied phenomenon can be employed for the experimental method to measure τso in strongly disordered superconductors.
Unexpected low-energy excitations in strongly disordered superconductors
Researcher Dr. Mikhail Feigel’man from Jozef Stefan Institute and CENN Nanocenter, in collaboration with colleagues Anton Khvalyuk, Thibault Charpentier, Nicolas Rock and Benjamin Sacepe from the Neel Institute (CNRS Grenoble) provided a novel framework for understanding of unexpected experimental demonstration of a strong temperature evolution of superfluid stiffness Θ (aka inverse kinetic inductance) at very low temperatures T much below critical temperature Tc. Usual superconductors normally exhibit exponential dependence like δΘ ~ exp(-Δ/T) due to the gap Δ in the quasi-particle’s excitation spectrum.
In the joint experimental-theoretical paper entitled “Near power-law temperature dependence of the superfluid stiffness in strongly disordered superconductors” the authors first demonstrate that in strongly disordered amorphous Indium Oxide a quite different dependence δΘ ~ Ta (with a ≈ 2) takes place. Secondly, it was shown that such a dependence results from strong spatial fluctuations of superconducting gap Δ(r) which come due to very strong disorder inherent for such a superconductor. It was demonstrated both by semi-quantitative analytical theory and by related numerical simulations that the above “near-power-law” behavior originates from localized low-energy modes which normally do not exist in more standard superconductors.
Local density of states correlations in the Lévy-Rosenzweig-Porter random matrix ensemble
A recent study by Dr. Aleksey Lunkin (Nanocenter CENN) in collaboration with Konstantin Tikhonov (CFM) demonstrates how seemingly abstract random matrices can provide deep insights into the dynamics of complex quantum systems.
Random matrix theory has long been a cornerstone for understanding quantum chaos and disorder. In this work, the authors focus on a special class of random matrices with heavy-tailed elements, which effectively capture the impact of disorder and rare, strong interactions. Despite their mathematical simplicity, these matrices reproduce rich and universal dynamical behavior observed in realistic quantum settings.
The researchers show that the evolution of local quantum states is governed by a single emergent energy scale. Their analytical framework, in perfect agreement with extensive numerical simulations, reveals universal patterns once the results are properly rescaled.
A key highlight of the study is the discovery of a stretched-exponential decay of the return probability—a hallmark of systems with widely distributed relaxation times, characteristic of non-ergodic quantum dynamics.
Localization reshapes the Josephson effect in disordered wires
Localization reshapes the Josephson effect in disordered wires
A new theoretical study by Mustafa E. Ismagambetov (MPI Stuttgart), Aleksey V. Lunkin (Nanocenter CENN), and Pavel M. Ostrovsky (MPI Stuttgart) reveals that strong disorder fundamentally alters how supercurrent flows in long, metallic Josephson junctions.
In clean systems, the critical current between two superconductors is linked to the normal-state conductance via the classical Ambegaokar–Baratoff relation. But when disorder becomes strong enough to localize electronic states, this relation breaks down. Using a nonlinear sigma model approach, the authors analyze how localization affects superconducting transport across the entire parameter space.
Their analysis identifies three distinct regimes governed by the interplay between disorder and the superconducting gap. In the fully localized regime, quantum interference suppresses the supercurrent more strongly than the normal-state resistance would suggest, signaling a breakdown of conventional expectations.
The findings open a new perspective on superconductivity in mesoscopic and disordered systems, with direct implications for designing coherent devices in noisy, real-world environments.
Density-of-States and its correlations in Rosenzveig-Porter matrix models with heavy tails via Super-symmetry approach.
Ljubljana team composed of Elizaveta Safonova, Aleksey Lunkin and Mikhail Feigel’man, together with Vladimir Kravtsov (ICTP, Trieste) managed to extend very efficient super-symmetric field theory approach to various quantum systems for matrix models of the Rosenzveig-Porter type with strong diagonal disorder to the situation when off-diagonal matrix elements are drafted from Levy-type distributions with a “heavy tail” - so that second moment of the distribution diverges. Previously this type of models was analyzed by more elementary means only, so that the properties of spectrum at low energy scales were out of reach. Levy-Rosenzveig-Porter model is known to demonstrate a broad variety of different quantum phases: ergodic, non-ergodic extended (NEE), as well as localized one – depending on the model parameters.
Super-symmetry approach was made efficient for the heavy-tail problems due to the use of the functional Hubbard-Stratonovich transformation introduced few decades ago by Yan Fyodorov and A. Mirlin. In result, the authors were able to calculate exactly the average Density-of-States (DoS) in the whole range of model parameters, as well as the correlations of DoS at various energy scales: from level spacing δ to Thouless energy Eth >> δ where level repulsion takes place, and then up to much larger scale of minigap width Γ>> Eth where the obtained results match exactly the ones obtained by other means in a recent study by A. Lunkin and K. Tikhonov. The obtained results extend applicability of the powerful supersymmetric method to a wide class of systems, with potential applications to a new problems related to the domain of Many-Body Localization.
Stirring the false vacuum via interacting quantized bubbles on a 5,564-qubit quantum annealer
In a study published in Nature Physics, researchers from the Jožef Stefan Institute, Forschungszentrum Jülich, University of Leeds and the Institute of Science and Technology Austria (ISTA) employed a 5,564-qubit D-Wave quantum annealer to simulate the dynamics of false vacuum decay, a quantum tunneling process from a metastable quantum state to a true vacuum state, central to quantum field theory and non-equilibrium phenomena such as phase transitions and dynamical metastability. False vacuum decay, characterized by the nucleation and expansion of true vacuum bubbles, presents substantial theoretical and experimental challenges due to its inherently non-equilibrium and non-perturbative nature.
The authors implemented an effective quantum spin model that captures both the initial nucleation and subsequent coherent interactions of vacuum bubbles within a dissipative environment. Quantized bubble formation and evolution were observed in real time, revealing coherent scaling behavior in the driven many-body dynamics for more than 1,000 intrinsic qubit time units. These observations establish quantum annealers as a viable platform for probing real-time false vacuum decay and other non-equilibrium quantum phase transition phenomena in large-scale quantum systems.
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