Publications

Order by: publication date number of citations

• Random matrices and the free energy of Ising-like models with disorder

  Nils Gluth, Thomas Guhr, Alfred Hucht

  SciPost Phys. 17, 122 (2024) · published 30 October 2024 | Toggle abstract · pdf

  We consider an Ising model with quenched surface disorder, the disorder average of the free energy is the main object of interest. Explicit expressions for the free energy distribution are difficult to obtain if the quenched surface spins take values of $± 1$. Thus, we choose a different approach and model the surface disorder by Gaussian random matrices. The distribution of the free energy is calculated. We chose skew-circulant random matrices and analytically compute the characteristic function of the free energy distribution. From the characteristic function we numerically calculate the distribution and show that it becomes log-normal for sufficiently large dimensions of the disorder matrices, and in the limit of infinitely large matrices tends to a Gaussian. Furthermore, we establish a connection to the central limit theorem.

• National Borders among Families: Removal and ‘Bare Life’ in India

  Salah Punathil

  Mig. Pol. 3, 005 (2024) · published 30 October 2024 | Toggle abstract · pdf

  This paper is about the state-driven process of ‘migrant ‘illegality’ (Genova 2002) and its impact on the life of Bengali speaking residents in the Assam state of India. While the movement of people across borders between India and present-day Bangladesh has been historical and complex, this ethnographic work explores how the state-driven process of migrant illegality and the production of ‘bare life’ have disrupted intimate relations and family life among this population in Assam. While the recent NRC (National Register of Citizens) update in Assam identified 1.9 million people as illegal migrants, there has been bureaucratic enactment of ‘migrant illegality’ by Assam Border Police for the last several years. The institutional procedures, court documents and narratives of the select cases of ‘detected’ as well as ‘detained’ migrants from ethnographic fieldwork reveal how the absence of formal papers and errors in the family records, kinship relations and property inheritance among the poor migrant families transforms actual citizens to ‘illegal migrants’ in the bureaucratic manoeuvring and reduces them to their ‘bare life’. The paper also shows how prejudice, arbitrariness, and contradictions feed into the bureaucratic process and lead to intense crises among family units, as several families have both Indians and alleged ‘Bangladeshis’ in their homes today. The paper argues that the major consequence of this state-driven ‘migrant illegality’ in the last two decades has been the creation of national borders among families, unsettling intimate relations and shared spaces.

  1 citation

• Massive scalar clouds and black hole spacetimes in Gauss-Bonnet gravity

  Iris van Gemeren, Tanja Hinderer, Stefan Vandoren

  SciPost Phys. Core 7, 069 (2024) · published 29 October 2024 | Toggle abstract · pdf

  We study static black holes in scalar-Gauss-Bonnet (sGB) gravity with a massive scalar field as an example of higher curvature gravity. The scalar mass introduces an additional scale and leads to a strong suppression of the scalar field beyond its Compton wavelength. We numerically compute sGB black hole spacetimes and scalar configurations and also compare with perturbative results for small couplings, where we focus on a dilatonic coupling function. We analyze the constraints on the parameters from requiring the curvature singularity to be located inside the black hole horizon $r_h$ and the relation to the regularity condition for the scalar field. For scalar field masses $m r_h \gtrsim 10^{-1}$, this leads to a new and currently most stringent bound on sGB coupling constant $\alpha$ of $\alpha/r_h^2\sim 10^{-1}$ in the context of stellar mass black holes. Lastly, we look at several properties of the black hole configurations relevant for further work on observational consequences, including the scalar monopole charge, Arnowitt–Deser–Misner mass, curvature invariants and the frequencies of the innermost stable circular orbit and light ring.

  2 citations

• Six-derivative Yang-Mills couplings in heterotic string theory

  Mohammad R. Garousi

  SciPost Phys. 17, 121 (2024) · published 28 October 2024 | Toggle abstract · pdf

  In this work, we present a comprehensive analysis of the structure of six-derivative bosonic couplings in heterotic string theory. First, we determine the maximal covariant and Yang-Mills gauge invariant basis, which consists of 801 independent coupling constants. By imposing T-duality constraints on the circular reduction of these terms, we obtain 468 relations between the coupling constants at the six-derivative order and the known couplings at lower derivative orders. Through the use of field redefinitions, we are able to eliminate the remaining 333 coupling constants. Remarkably, we find that the Yang-Mills field strength only appears through the trace of two field strengths or their derivatives. Finally, we perform further field redefinition to rewrite the remaining couplings in a canonical form characterized by 85 independent couplings.

  2 citations

• Josephson current through the SYK model

  Luca Dell'Anna

  SciPost Phys. 17, 120 (2024) · published 24 October 2024 | Toggle abstract · pdf

  We calculate the equilibrium Josephson current through a disordered interacting quantum dot described by a Sachdev-Ye-Kitaev model fully contacted by two BCS superconductors, such that all modes of the dot contribute to the coupling, which encodes hopping and spin-flip processes. We show that, at zero temperature and at the conformal limit, i.e. in the strong interacting limit, the Josephson current is suppressed by $U$, the strength of the interaction, as ln(U)/U and becomes universal, namely it gets independent on the superconducting gap. At finite temperature, instead, it depends on the ratio between the gap and the temperature. A proximity effect exists but the self-energy corrections induced by the coupling with the superconducting leads seem subleading as compared to the interaction self-energy and the tunneling matrix for large number of particles. Finally we compare the results of the original four-fermion model with those obtained considering zero interaction, two-fermions and a generalized $q$-fermion model.

• Supersymmetric polynomials and algebro-combinatorial duality

  Dmitry Galakhov, Alexei Morozov, Nikita Tselousov

  SciPost Phys. 17, 119 (2024) · published 24 October 2024 | Toggle abstract · pdf

  In this note we develop a systematic combinatorial definition for constructed earlier supersymmetric polynomial families. These polynomial families generalize canonical Schur, Jack and Macdonald families so that the new polynomials depend on odd Grassmann variables as well. Members of these families are labeled by respective modifications of Young diagrams. We show that the super-Macdonald polynomials form a representation of a super-algebra analog $\mathsf{T}(\widehat{\mathfrak{gl}}_{1|1})$ of Ding-Iohara-Miki (quantum toroidal) algebra, emerging as a BPS algebra of D-branes on a conifold. A supersymmetric modification for Young tableaux and Kostka numbers are also discussed.

  3 citations

• Carrollian $\mathscr Lw_{1+\infty}$ representation from twistor space

  Laura Donnay, Laurent Freidel, Yannick Herfray

  SciPost Phys. 17, 118 (2024) · published 22 October 2024 | Toggle abstract · pdf

  We construct an explicit realization of the action of the $\mathscr Lw_{1+∞}$ loop algebra on fields at null infinity. This action is directly derived by Penrose transform of the geometrical action of $\mathscr Lw_{1+∞}$ symmetries in twistor space, ensuring that it forms a representation of the algebra. Finally, we show that this action coincides with the canonical action of $\mathscr Lw_{1+∞}$ Noether charges on the asymptotic phase space.

  4 citations

• Conventional single-gap $s-$wave superconductivity and hidden peak effect in single crystals of Mo$_8$Ga$_{41}$ superconductor

  Sunil Ghimire, Kyuil Cho, Kamal R. Joshi, Makariy A. Tanatar, Zhixiang Hu, Cedomir Petrovic, Ruslan Prozorov

  SciPost Phys. 17, 117 (2024) · published 21 October 2024 | Toggle abstract · pdf

  London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo$_8$Ga$_41$. The full temperature range superfluid density, $\rho_s(T)$, is consistent with the clean isotropic $s-$wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating an anharmonic vortex pinning potential. The field dependence of the effective critical current density, $j_{c}(H)$, reveals an unusual result. While in the ZFC protocol, $j_{c}(H)$ is monotonically suppressed by the magnetic field, it exhibits a profound "hidden" peak effect in the FC protocol, that is, without a vortex density gradient. We suggest a possible novel mechanism for such a peak effect, which involves both static and dynamic aspects of vortex pinning.

  1 citation

• Single-gap isotropic $s-$wave superconductivity in single crystals AuSn$_4$

  Sunil Ghimire, Kamal R. Joshi, Elizabeth H. Krenkel, Makariy A. Tanatar, Marcin Kończykowski, Romain Grasset, Paul C. Canfield, Ruslan Prozorov

  SciPost Phys. 17, 116 (2024) · published 21 October 2024 | Toggle abstract · pdf

  London, $\lambda_L (T)$, and Campbell, $\lambda_{C} (T)$, penetration depths were measured in single crystals of a topological superconductor candidate AuSn$_4$. At low temperatures, $\lambda_L (T)$ is exponentially attenuated and, if fitted with the power law, $\lambda(T) \sim T^n$, gives exponents $n>4$, indistinguishable from the isotropic single $s-$wave gap Bardeen-Cooper-Schrieffer (BCS) asymptotic. The superfluid density fits perfectly in the entire temperature range to the BCS theory. The superconducting transition temperature, $T_c = 2.40 ± 0.05$ K, does not change after 2.5 MeV electron irradiation, indicating the validity of the Anderson theorem for isotropic $s-$wave superconductors. Campbell penetration depth before and after electron irradiation shows no hysteresis between the zero-field cooling (ZFC) and field cooling (FC) protocols, consistent with the parabolic pinning potential. Interestingly, the critical current density estimated from the original Campbell theory decreases after irradiation, implying that a more sophisticated theory involving collective effects is needed to describe vortex pinning in this system. In general, our thermodynamic measurements strongly suggest that the bulk response of the AuSn$_4$ crystals is fully consistent with the isotropic $s-$wave weak-coupling BCS superconductivity.

• Codebase release 1.3 for QArray

  Barnaby van Straaten, Joseph Hickie, Lucas Schorling, Jonas Schuff, Federico Fedele, Natalia Ares

  SciPost Phys. Codebases 35-r1.3 (2024) · published 21 October 2024 | Toggle abstract · src

  Semiconductor quantum dot arrays are a leading architecture for the development of quantum technologies. Over the years, the constant capacitance model has served as a fundamental framework for simulating, understanding, and navigating the charge stability diagrams of small quantum dot arrays. However, while the size of the arrays keeps growing, solving the constant capacitance model becomes computationally prohibitive. This paper presents an open-source software package able to compute a $100 × 100$ pixels charge stability diagram of a 16-dot array in less than a second. Smaller arrays can be simulated in milliseconds - faster than they could be measured experimentally, enabling the creation of diverse datasets for training machine learning models and the creation of digital twins that can interface with quantum dot devices in real-time. Our software package implements its core functionalities in the systems programming language Rust and the high-performance numerical computing library JAX. The Rust implementation benefits from advanced optimisations and parallelisation, enabling the users to take full advantage of multi-core processors. The JAX implementation allows for GPU acceleration.

  3 citations