Publications

Order by: publication date number of citations

• Current-induced re-entrant superconductivity and extreme nonreciprocal superconducting diode effect in valley-polarized systems

  Yu-Chen Zhuang, Qing-Feng Sun

  SciPost Phys. 20, 021 (2026) · published 26 January 2026 | Toggle abstract · pdf

  The superconducting diode effect (SDE) refers to the nonreciprocity of superconducting critical currents. Generally, the SDE has a positive and a negative critical currents $j_{c±}$ corresponding to two opposite directions with unequal amplitudes. It is demonstrated that an extreme nonreciprocity where two critical currents can become both positive (or negative) has been observed in twisted graphene systems. In this work, we theoretically propose a possible mechanism to realize an extreme nonreciprocal SDE. Based on a simple microscopic model, we demonstrate that depairing currents required to dissolve Cooper pairs can be remodulated under the interplay between valley polarizations and applied currents. Near the disappearance of the superconductivity, the remodulation is shown to induce extreme nonreciprocity and also the current-induced re-entrant superconductivity where the system has two different critical current intervals. Our study may provide new horizons for understanding the coexistence of superconductivity and spontaneous valley polarizations, and pave a way for designing SDE with 100% efficiency.

• Spin degrees of freedom incorporated in conformal group: Introduction of an intrinsic momentum operator

  Seiichi Kuwata

  SciPost Phys. Proc. 14, 034 (2023) · published 24 November 2023 | Toggle abstract · pdf

  Considering spin degrees of freedom incorporated in the conformal generators, we introduce an intrinsic momentum operator $\pi_\mu$, which is feasible for the Bhabha wave equation. If a physical state $\psi_{ph}$ for spin s is annihilated by the $\pi_\mu$, the degree of $\psi_{ph}$, deg $\psi_{ph}$, should equal twice the spin degrees of freedom, $2 ( 2 s + 1)$ for a massive particle, where the multiplicity $2$ indicates the chirality. The relation deg $\psi_{ph}$ = 2(2s+1) holds in the representation $R_5$ (s,s), irreducible representation of the Lorentz group in five dimensions.

• Quenching a Fermi superfluid across the BEC-BCS crossover

  Moritz Breyer, Daniel Eberz, Andreas Kell, Michael Köhl

  SciPost Phys. 18, 053 (2025) · published 14 February 2025 | Toggle abstract · pdf

  We study the response of a Fermi superfluid to a rapid change of the interaction strength. For a broad range of quench parameters, the order parameter exhibits damped oscillations, however, we also identify quench regimes in which these oscillations are absent. By comparing our data to a numerical model we find that the damping time constants are explainable by a dephasing in a local density approximation, however, the oscillation frequencies, while being comparable to the superconducting order parameter, display in detail significant differences to the integrable BCS (Bardeen-Cooper-Schrieffer) model.

• A differential-geometry approach to operator mixing in massless QCD-like theories and Poincaré-Dulac theorem

  Matteo Becchetti

  SciPost Phys. Proc. 7, 032 (2022) · published 21 June 2022 | Toggle abstract · pdf

  We review recent progress on operator mixing in the light of the theory of canonical forms for linear systems of differential equations and, in particular, of the Poincar\'e-Dulac theorem. We show that the matrix $A(g) = -\frac{\gamma(g)}{\beta(g)} =\frac{\gamma_0}{\beta_0}\frac{1}{g} + \cdots $ determines which different cases of operator mixing can occur, and we review their classification. We derive a sufficient condition for $A(g)$ to be set in the one-loop exact form $A(g) = \frac{\gamma_0}{\beta_0}\frac{1}{g}$. Finally, we discuss the consequences of the unitarity requirement in massless QCD-like theories, and we demonstrate that $\gamma_0$ is always diagonalizable if the theory is conformal invariant and unitary in its free limit at $g =0$.

• Cons-training tensor networks: Embedding and optimization over discrete linear constraints

  Javier Lopez-Piqueres, Jing Chen

  SciPost Phys. 18, 192 (2025) · published 17 June 2025 | Toggle abstract · pdf

  In this study, we introduce a novel family of tensor networks, termed constrained matrix product states (MPS), designed to incorporate exactly arbitrary discrete linear constraints, including inequalities, into sparse block structures. These tensor networks are particularly tailored for modeling distributions with support strictly over the feasible space, offering benefits such as reducing the search space in optimization problems, alleviating overfitting, improving training efficiency, and decreasing model size. Central to our approach is the concept of a quantum region, an extension of quantum numbers traditionally used in $U(1)$ symmetric tensor networks, adapted to capture any linear constraint, including the unconstrained scenario. We further develop a novel canonical form for these new MPS, which allow for the merging and factorization of tensor blocks according to quantum region fusion rules and permit optimal truncation schemes. Utilizing this canonical form, we apply an unsupervised training strategy to optimize arbitrary objective functions subject to discrete linear constraints. Our method's efficacy is demonstrated by solving the quadratic knapsack problem, achieving superior performance compared to a leading nonlinear integer programming solver. Additionally, we analyze the complexity and scalability of our approach, demonstrating its potential in addressing complex constrained combinatorial optimization problems.

• The thermoelectric conversion efficiency problem: Insights from the electron gas thermodynamics close to a phase transition

  I. Khomchenko, A. Ryzhov, F. Maculewicz, F. Kurth, R. Hühne, A. Golombek, M. Schleberger, C. Goupil, Ph. Lecoeur, A. Böhmer, G. Benenti, G. Schierning, H. Ouerdane

  SciPost Phys. Core 8, 015 (2025) · published 7 February 2025 | Toggle abstract · pdf

  The bottleneck in modern thermoelectric power generation and cooling is the low energy conversion efficiency of thermoelectric materials. The detrimental effects of lattice phonons on performance can be mitigated, but achieving a high thermoelectric power factor remains a major problem because the Seebeck coefficient and electrical conductivity cannot be jointly increased. The conducting electron gas in thermoelectric materials is the actual working fluid that performs the energy conversion, so its properties determine the maximum efficiency that can theoretically be achieved. By relating the thermoelastic properties of the electronic working fluid to its transport properties (considering noninteracting electron systems), we show why the performance of conventional semiconductor materials is doomed to remain low. Analyzing the temperature dependence of the power factor theoretically in 2D systems and experimentally in a thin film, we find that in the fluctuation regimes of an electronic phase transition, the thermoelectric power factor can significantly increase owing to the increased compressibility of the electron gas. We also calculate the ideal thermoelectric conversion efficiency in noninteracting electron systems across a wide temperature range neglecting phonon effects and dissipative coupling to the heat source and sink. Our results show that driving the electronic system to the vicinity of a phase transition can indeed be an innovative route to strong efficiency enhancement, but at the cost of an extremely narrow temperature range for the use of such materials, which in turn precludes potential development for the desired wide range of thermoelectric energy conversion applications.

• DsTau: study of tau neutrino production with nuclear emulsion in the CERN-SPS

  Ali Murat Guler

  SciPost Phys. Proc. 8, 094 (2022) · published 13 July 2022 | Toggle abstract · pdf

  Tau neutrino is one of the least known particle of Standard Model; there are only few measurements with a limited statistics. The DONUT experiment first reported the tau neutrino interaction cross-section but their measurement suffers from large systematic error of more than $50\%$ which is mainly due to uncertainty in the tau neutrino flux prediction. The tau neutrino cross section is an essential ingredient in neutrino experiments and its precise measurement would enable a search for new physics effects such as testing the Lepton Universality in neutrino interactions. The main goal of the DsTau experiment is to measure an inclusive differential cross-section of a $D_s$ production with a consecutive decay to tau lepton in p-A interactions at the CERN-SPS. The measurement of DsTau will reduce the systematic uncertainty in tau neutrino interaction cross-section to $10\%$ level. The results from the pilot run and the prospect for physics runs in 2021-2022 will be discussed.

• Velocity gauge formulation of nonlinear optical response in Floquet quantum systems

  S. Sajad Dabiri, Reza Asgari

  SciPost Phys. Core 8, 049 (2025) · published 21 July 2025 | Toggle abstract · pdf

  Using the velocity gauge formalism, we develop a theoretical framework for computing the nonlinear optical responses of time-periodic quantum systems. This approach complements the length gauge formulation and offers distinct advantages in both numerical and analytical treatments, particularly for atomic and solid-state systems with well-defined momentum-space structures. By applying our framework to the Rabi model, we derive numerical solutions in the velocity gauge and compare them with the length gauge, demonstrating full agreement between the two formulations. Our findings reveal rich optical phenomena, including photon-assisted transitions, frequency mixing effects, and emergent Floquet-induced photocurrents that are absent in static systems. We demonstrate that nonlinear responses in Floquet-driven systems exhibit resonances at integer multiples of the driving frequency, providing insights into ultrafast spectroscopy and Floquet engineering of quantum materials. The present formulation establishes a bridge between theoretical models and experimental observations in driven quantum systems, with potential applications in quantum optics, photonics, and next-generation optoelectronic devices.

• Measurements of sub-jet fragmentation with ALICE

  James Mulligan

  SciPost Phys. Proc. 10, 020 (2022) · published 10 August 2022 | Toggle abstract · pdf

  High-energy jets offer rich opportunities to study quantum chromodynamics, from investigating the limits of perturbative calculability to constraining the emergent properties of the quark-gluon plasma (QGP). In these proceedings, we present new measurements of the fragmentation properties of jets. We report distributions of the sub-jet momentum fraction $z_r$ measured in pp and Pb-Pb collisions with ALICE at the Large Hadron Collider. These measurements serve as input to test the universality of jet fragmentation in the QGP, and offer a path to elucidate jet quenching effects in the large-$z$ region.

• Asymptotics of transverse momentum broadening in dense QCD media

  Paul Caucal

  SciPost Phys. Proc. 10, 018 (2022) · published 10 August 2022 | Toggle abstract · pdf

  We study the asymptotic behaviour of the transverse momentum broadening distribution of an energetic quark or gluon propagating through dense QCD matter, in the large system size $L$ limit, taking into account radiative corrections in the double logarithmic approximation. Thanks to a connection between the evolution of the jet quenching parameter $\hat{q}$ and the formation of traveling wave fronts in nonlinear physics, we obtain a formula for the $L$ dependence of the characteristic transverse momentum scale $Q_s$ of the distribution valid up to terms of order $1/\ln(L)$. We briefly discuss the physical implications of this formula for jet quenching and small-$x$ phenomenology.