Publications

Order by: publication date number of citations

• M5-brane prongs, string soliton bound states and wall-crossing

  Varun Gupta, K. Narayan

  SciPost Phys. Core 8, 004 (2025) · published 13 January 2025 | Toggle abstract · pdf

  We study Abelian M5-brane field configurations representing BPS bound states of self-dual string solitons whose locations correspond to the endlines of M2-branes ending on the M5-branes. The BPS equations are obtained from appropriate Bogomolny completion of the effective Abelian low energy functional with two transverse scalars, using two vectors representing the directions along which these endline strings extend. Then we impose boundary conditions on the scalars near the string soliton cores. This leads to a molecule-like equilibrium structure of two non-parallel string solitons at fixed transverse separations, with the M5-brane "prong" deformations comprising two "spikes", each shaped like a ridge. The resulting picture becomes increasingly accurate as one approaches the wall of marginal stability, on which these states decay. There are various parallels with wall-crossing phenomena for string web configurations obtained from D3-brane deformations.

• R-matrices and Miura operators in 5d Chern-Simons theory

  Nafiz Ishtiaque, Saebyeok Jeong, Yehao Zhou

  SciPost Phys. Core 8, 003 (2025) · published 13 January 2025 | Toggle abstract · pdf

  We derive Miura operators for $W$- and $Y$-algebras from first principles as the expectation value of the intersection between a topological line defect and a holomorphic surface defect in 5-dimensional non-commutative $\mathfrak{gl}(1)$ Chern-Simons theory. The expectation value, viewed as the transition amplitude for states in the defect theories forming representations of the affine Yangian of $\mathfrak{gl}(1)$, satisfies the Yang-Baxter equation and is thus interpreted as an R-matrix. To achieve this, we identify the representations associated with the line and surface defects by calculating the operator product expansions (OPEs) of local operators on the defects, as conditions that anomalous Feynman diagrams cancel each other. We then evaluate the expectation value of the defect intersection using Feynman diagrams. When the line and surface defects are specified, we demonstrate that the expectation value precisely matches the Miura operators and their products.

• Entanglement flow in the Kane-Fisher quantum impurity problem

  Chunyu Tan, Yuxiao Hang, Stephan Haas, Hubert Saleur

  SciPost Phys. Core 8, 002 (2025) · published 8 January 2025 | Toggle abstract · pdf

  The problem of a local impurity in a Luttinger liquid, just like the anisotropic Kondo problem (of which it is technically a cousin), describes many different physical systems. As shown by Kane and Fisher, the presence of interactions profoundly modifies the physics familiar from Fermi liquid theory, and leads to non-intuitive features, best described in the Renormalization Group language (RG), such as flows towards healed or split fixed points. While this problem has been studied for many years using more traditional condensed matter approaches, it remains somewhat mysterious from the point of view of entanglement, both for technical and conceptual reasons. We propose and explore in this paper a new way to think of this important aspect. We use the realization of the Kane Fisher universality class provided by an XXZ spin chain with a modified bond strength between two sites, and explore the difference of (Von Neumann) entanglement entropies of a region of length $\ell$ with the rest of the system - to which it is connected with a modified bond - in the cases when $\ell$ is even and odd. Surprisingly, we find out that this difference $\delta S\equiv S^e-S^o$ remains of $O(1)$ in the thermodynamic limit, and gives rise now, depending on the sign of the interactions, to "resonance" curves, interpolating between $-\ln 2$ and $0$, and depending on the product $\ell T_B$, where $1/T_B$ is a characteristic length scale akin to the Kondo length in Kondo problems. $\delta S$ can be interpreted as a measure of the hybridization of the left-over spin in odd length subsystems with the "bath" constituted by the rest of the chain. The problem is studied both numerically using DMRG and analytically near the healed and split fixed points. Interestingly - and in contrast with what happens in other impurity problems - $\delta S$ can, at least to lowest order, be tackled by conformal perturbation theory.

• Spin-1/2 XX chains with modulated gamma interaction

  M. Abbasi, S. Mahdavifar, M. Motamedifar

  SciPost Phys. Core 8, 001 (2025) · published 7 January 2025 | Toggle abstract · pdf

  We study the spin-1/2 XX chain with a modulated Gamma interaction (GI), which results from the superposition of uniform and staggered Gamma terms. We diagonalize the Hamiltonian of the model exactly using the Fermionization technique. We then probe the energy gap and identify the gapped and gapless regions. We also examine the staggered chiral, staggered nematic and dimer order parameters to determine the different phases of the ground state phase diagram with their respective long-range orders. Our findings indicate that the model undergoes first-order, second-order, gapless-gapless, and gapped-gapped phase transitions.

• General relativistic stochastic thermodynamics

  Tao Wang, Yifan Cai, Long Cui, Liu Zhao

  SciPost Phys. Core 7, 082 (2024) · published 11 December 2024 | Toggle abstract · pdf

  Based on the recent work [J. Stat. Phys. 190, 193 (2023), J. Stat. Phys. 190, 181 (2023)], we formulate the first law and the second law of stochastic thermodynamics in the framework of general relativity. These laws are established for a charged Brownian particle moving in a heat reservoir and subjecting to an external electromagnetic field in generic stationary spacetime background, and in order to maintain general covariance, they are presented respectively in terms of the divergences of the energy current and the entropy density current. The stability of the equilibrium state is also analyzed.

  1 citation

• Tunable second harmonic generation in 2D materials: Comparison of different strategies

  Simone Grillo, Elena Cannuccia, Maurizia Palummo, Olivia Pulci, Claudio Attaccalite

  SciPost Phys. Core 7, 081 (2024) · published 4 December 2024 | Toggle abstract · pdf

  Nonlinear optical frequency conversion, where optical fields interact with a nonlinear medium to generate new frequencies, is a key phenomenon in modern photonic systems. However, a major challenge with these techniques lies in the difficulty of tuning the nonlinear electrical susceptibilities that drive such effects in a given material. As a result, dynamic control of optical nonlinearities has remained largely confined to research laboratories, limiting its practical use as a spectroscopic tool. In this work, we aim to advance the development of devices with tunable nonlinear responses by exploring two potential mechanisms for electrically manipulating second-order optical nonlinearity in two-dimensional materials. Specifically, we consider two configurations: in the first, the material does not inherently exhibit second-harmonic generation (SHG), but this response is induced by an external field; in the second, an external field induces doping in a material that already exhibits SHG, altering the intensity of the nonlinear signal. In this work, we have studied these two configurations using a real-time ab-initio approach under an out-of-plane external field and including the effects of doping-induced variations in the screened electron-electron interaction. We then discuss the limitations of current computational methods and compare our results with experimental measurements.

• A $C^\ast$-algebraic view on the interaction of real- and reciprocal space topology in skyrmion crystals

  Pascal Prass, Fabian R. Lux, Emil Prodan, Duco van Straten, Yuriy Mokrousov

  SciPost Phys. Core 7, 080 (2024) · published 4 December 2024 | Toggle abstract · pdf

  Understanding the interaction of real- and reciprocal space topology in skyrmion crystals is an open problem. We approach it from the viewpoint of $C^\ast$-algebras and calculate all admissible Chern numbers of a strongly coupled tight-binding skyrmion system on a triangular lattice as a function of Fermi energy and texture parameters. Our analysis reveals the topological complexity of electronic states coupled to spin textures, and the failure of the adiabatic picture to account for it in terms of emergent electromagnetism. On the contrary, we explain the discontinuous jumps in the real-space winding number in terms of collective evolution in real-, reciprocal, and mixed space Chern numbers. Our work sets the stage for further research on topological dynamics in complex dynamic spin textures coupled to external fields.

• Vector-like quarks: Status and new directions at the LHC

  Avik Banerjee, Elin Bergeaas Kuutmann, Venugopal Ellajosyula, Rikard Enberg, Gabriele Ferretti, Luca Panizzi

  SciPost Phys. Core 7, 079 (2024) · published 4 December 2024 | Toggle abstract · pdf

  Experimental searches for vector-like quarks have until now only considered their decays into Standard Model particles. However, various new physics scenarios predict additional scalars, so that these vector-like quarks can decay to new channels. These new channels reduce the branching ratios into Standard Model final states, significantly affecting current mass bounds. In this article, we quantitatively assess the relevance and observability of single and pair production processes of vector-like quarks, followed by decays into both standard and exotic final states. We highlight the importance of large widths and the relative interaction strengths with Standard Model particles and new scalars. Then, we review the post-Moriond 2024 status of these models in light of available LHC data and discuss potential future strategies to enhance the scope of vector-like quark searches.

  2 citations

• Duality and hidden symmetry breaking in the q-deformed Affleck-Kennedy-Lieb-Tasaki model

  Tyler Franke, Thomas Quella

  SciPost Phys. Core 7, 078 (2024) · published 2 December 2024 | Toggle abstract · pdf

  We revisit the question of string order and hidden symmetry breaking in the $q$-deformed AKLT model, an example of a spin chain that possesses generalized symmetry. We first argue that the non-local Kennedy-Tasaki duality transformation that was previously proposed to relate the string order to a local order parameter leads to a non-local Hamiltonian and thus does not provide a physically adequate description of the symmetry breaking. We then present a modified non-local transformation which is based on a recently developed generalization of Witten's Conjugation to frustration-free lattice models and capable of resolving this issue.

• Functional renormalization group approach for signal detection

  Vincent Lahoche, Dine Ousmane Samary, Mohamed Tamaazousti

  SciPost Phys. Core 7, 077 (2024) · published 2 December 2024 | Toggle abstract · pdf

  This review paper utilizes renormalization group techniques for signal detection in nearly continuous positive spectra. We emphasize the universal aspects of the analogue field-theory approach. The primary objective is to present an extended self-consistent construction of the analogue effective field-theory framework for data, which can be interpreted as a maximum entropy model. In particular, we leverage universality arguments to justify the $\mathbb{Z}_2$ symmetry of the classical action, highlighting the existence of both a large-scale (local) regime and a small-scale (nonlocal) regime. Secondly, in relation to noise models, we observe the universal relationship between phase transitions and symmetry breaking near the detection threshold. Finally, we address the challenge of defining the covariance matrix for tensor-like data. Based on the cutting graph prescription, we note the superiority of definitions that rely on complete graphs of large size for data analysis.