Publications

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Z2 topological orders in kagome dipolar systems: Feedback from Rydberg quantum simulator

Pengwei Zhao, Gang v. Chen

SciPost Phys. 19, 040 (2025) · published 14 August 2025 | · pdf

The mutual feedback between quantum condensed matter and cold atom physics has been quite fruitful throughout history and continues to inspire ongoing research. Motivated by the recent activities on the quantum simulation of topological orders among the ultracold Rydberg atom arrays, we consider the possibility of searching for topological orders among the dipolar quantum magnets and polar molecules with a kagome lattice geometry. Together with other quantum interactions such as the transverse field, the dipolar interaction endows the kagome system with a similar structure as the Balents-Fisher-Girvin model and thus fosters the emergence of the $\mathbb{Z}_2$ topological orders. We construct a $\mathbb{Z}_2$ lattice gauge theory to access the topological ordered phase and describe the spinon and vison excitations for the $\mathbb{Z}_2$ topological orders. We explain the spectroscopic consequences for various quantum phases as well as the experimental detection. We further discuss the rare-earth kagome magnets, ultracold polar molecules, and cluster Mott insulators for the physical realization.
Hole doping and electronic correlations in Cr substituted BaFe$_{2}$As$_{2}$

Marli dos Reis Cantarino, Kevin R. Pakuszewski, Bjoern Salzmann, Pedro H. A. Moya, Wagner R. da Silva Neto, Gabriel S. Freitas, Pascoal G. Pagliuso, Cris Adriano, Walber H. Brito, Fernando A. Garcia

SciPost Phys. 17, 141 (2024) · published 22 November 2024 | · pdf

For a significant composition range, the suppression of the spin density wave transition temperature ($T_{\text{SDW}}$) in Cr- and Mn-substituted BaFe$_{2}$As$_{2}$ (CrBFA and MnBFA, respectively) coincides as a function of Cr/Mn content, despite the distinct electronic effects of these substitutions. Additionally, for any Cr/Mn content superconductivity (SC) is absent and this topic is particularly less explored in the case of CrBFA. In this work, we employ angle-resolved photoemission spectroscopy (ARPES) and combined density functional theory plus dynamical mean field theory (DFT+DMFT) to address the evolution of the Fermi surface (FS) and electronic correlations in CrBFA. Our findings reveal that incorporating Cr leads to an effective hole doping of the states near the FS, which is well described within the virtual crystal approximation (VCA). Moreover, analysis of the ARPES spectra of the bands with main $d_{yz}$-orbital character reveals a fractional scaling of the imaginary part of self-energy as a function of the binding energy, a signature property of Hund's correlations. Our DFT+DMFT calculations support these experimental findings. We conclude that CrBFA is a correlated electron system for which the changes in the FS as a function of Cr are unrelated to the suppression of $T_{\text{SDW}}$. In addition, we suggest that the absence of SC is primarily due to the competition between Cr local moments and the Fe-derived itinerant spin fluctuations.
Statistical mechanics of exponentially many low lying states

Swapnamay Mondal

SciPost Phys. 18, 103 (2025) · published 19 March 2025 | · pdf

It has recently been argued that for near-extremal black holes, the entropy and the energy above extremality respectively receive a $log T$ and a $T$-linear correction, where $T$ is the temperature. We show that both these features can be derived in a "low but not too low" temperature regime, by assuming the existence of exponentially many low lying states cleanly separated from rest of the spectrum, without using any specific theory. Argument of the logarithm in the expression of entropy is seen to be the ratio of temperature and the bandwidth of the low lying states. We argue that such spectrum might arise in non-supersymmetric extremal brane systems. Our findings strengthen Page's suggestion that there is no true degeneracy for non-supersymmetric extremal black holes.
Adaptive sampling-based optimization of quantics tensor trains for noisy functions: Applications to quantum simulations

Kohtaroh Sakaue, Hiroshi Shinaoka, Rihito Sakurai

SciPost Phys. 19, 038 (2025) · published 14 August 2025 | · pdf

Tensor cross interpolation (TCI) is a powerful technique for learning a tensor train (TT) by adaptively sampling a target tensor based on an interpolation formula. However, when the tensor evaluations contain random noise, optimizing the TT is more advantageous than interpolating the noise. Here, we propose a new method that starts with an initial guess of TT and optimizes it using non-linear least-squares by fitting it to measured points obtained from TCI. We use quantics TCI (QTCI) in this method and demonstrate its effectiveness on sine and two-time correlation functions, with each evaluated with random noise. The resulting QTT exhibits increased robustness against noise compared to the QTCI method. Furthermore, we employ this optimized QTT of the correlation function in quantum simulation based on pseudo-imaginary-time evolution, resulting in ground-state energy with higher accuracy than the QTCI or Monte Carlo methods.
Non-topological edge-localized Yu-Shiba-Rusinov states in CrBr$_3$/NbSe$_2$ heterostructures

Jan P. Cuperus, Daniel Vanmaekelbergh, Ingmar Swart

SciPost Phys. 18, 141 (2025) · published 28 April 2025 | · pdf

Topological superconductivity is predicted to emerge in certain magnet-superconductor hybrid systems. Here, we revisit the heterostructure of insulating monolayer CrBr$_{3}$ and NbSe$_{2}$, for which different conclusions on the presence of topological superconductivity have been reported. Using low-temperature scanning tunneling microscopy and (shot noise) spectroscopy, we find that the superconducting gap well inside the CrBr$_{3}$ islands is not affected by magnetism. At the island edges, we observe Yu-Shiba-Rusinov (YSR) states at a variety of in-gap positions, including zero energy. The absence of topological superconductivity is verified by extensive dI/dV measurements at the CrBr$_{3}$ island edges. Our results ask for a more detailed understanding of the interaction between magnetic insulators and superconductors.
Boundary layers, transport and universal distribution in boundary driven active systems

Pritha Dolai, Arghya Das

SciPost Phys. 19, 088 (2025) · published 8 October 2025 | · pdf

We discuss analytical results for a run-and-tumble particle (RTP) in one dimension in presence of boundary reservoirs. It exhibits "kinetic boundary layers", nonmonotonous distribution, current without density gradient, diffusion facilitated current reversal and optimisation on tuning dynamical parameters, and a new transport effect in the steady state. The spatial and internal degrees of freedom together possess a symmetry, using which we find the eigenspectrum for large systems. The eigenvalues are arranged in two bands which can mix in certain conditions resulting in a crossover in the relaxation. The late time distribution for large systems is obtained analytically; it retains a strong and often dominant "active" contribution in the bulk rendering an effective passive-like description inadequate. A nontrivial "Milne length" also emerges in the dynamics. Finally, a novel universality is proposed in the absorbing boundary problem for dynamics with short-range colored noise. Active processes driven by active reservoirs may thus provide a common physical ground for diverse and new nonequilibrium phenomena.
Chiral adiabatic transmission protected by Fermi surface topology

Isidora Araya Day, Kostas Vilkelis, Antonio L. R. Manesco, Ahmet Mert Bozkurt, Valla Fatemi, Anton R. Akhmerov

SciPost Phys. 18, 098 (2025) · published 18 March 2025 | · pdf

We demonstrate that Andreev modes that propagate along a transparent Josephson junction have a perfect transmission at the point where three junctions meet. The chirality and the number of quantized transmission channels is determined by the topology of the Fermi surface and the vorticity of the superconducting phase differences at the trijunction. We explain this chiral adiabatic transmission (CAT) as a consequence of the adiabatic evolution of the scattering modes both in momentum and real space. The dispersion relation of the junction then separates the scattering trajectories by introducing inaccesible regions of phase space. We expect that CAT is observable in nonlocal conductance and thermal transport measurements. Furthermore, because it does not rely on particle-hole symmetry, CAT is also possible to observe directly in metamaterials.
Mechanical work extraction from an error-prone active dynamic Szilard engine

Luca Cocconi, Paolo Malgaretti, Holger Stark

SciPost Phys. 19, 138 (2025) · published 25 November 2025 | · pdf

Isothermal information engines operate by extracting net work from a single heat bath through measurement and feedback control. In this work, we analyze a {realistic} active Szilard engine operating on a single self-propelled particle by means of steric interaction with an externally controlled mechanical element. In particular, we provide a comprehensive study of how finite measurement accuracy affects the engine's work and power output, as well as the cost of operation. Having established the existence of non-trivial optima for work and power output, we study the dependence of their loci on the measurement error parameters and identify conditions for their positivity under one-shot and cyclic engine operation. We also demonstrate that a suitably defined efficiency of information-to-work conversion, which at equilibrium is bounded above by unity as a consequence of Landauer's principle, may here be made arbitrarily large by increasing the active Péclet number of the particle. Equivalently, for such a nonequilibrium efficiency to remain bounded above by unity, the athermal motion of the bath particles needs to be accounted for explicitly. Notably, the information efficiency for one-shot operation exhibits a discontinuous transition and a non-monotonic dependence on the measurement precision. Finally, we show that cyclic operation improves information efficiency by harvesting residual mutual information between successive measurements.
Sifting composite from elementary models at FCCee and CePC

Giacomo Cacciapaglia, Aldo Deandrea, Abhishek M. Iyer, Andres Pinto

SciPost Phys. 14, 170 (2023) · published 26 June 2023 | · pdf

New Physics models with either an elementary or composite origin are often associated with a similar imprint in a direct search at colliders, case in point being the production of a light pseudoscalar in association with a monochromatic photon from the decay of a Z boson at future $e+e-$ colliders. We exploit the correlation between the discovery of a signal in the Z decays and electroweak precision measurements as a tool to distinguish a composite model from an elementary scalar one. Our results offer an appealing and rich physics case for future colliders and demonstrate how a lepton collider at the Z mass can be a discovery machine for new physics in the Higgs sector.
Interaction and collision of skyrmions in chiral antiferromagnets

George Theodorou, Bruno Barton-Singer, Stavros Komineas

SciPost Phys. 18, 037 (2025) · published 30 January 2025 | · pdf

Skyrmions in an antiferromagnet can travel as solitary waves in stark contrast to the situation in ferromagnets. Traveling skyrmion solutions have been found numerically in chiral antiferromagnets. We study head-on collision events between two skyrmions. We find that the result of the collision depends on the initial velocity of the skyrmions. For small velocities, the skyrmions shrink as they approach, then bounce back and eventually acquire almost their initial speed. For larger velocities, the skyrmions approach each other and shrink until they become singular points at some finite separation and are eventually annihilated. Considering skyrmion energetics, we can determine the regimes of the different dynamical behaviors. Using a collective co-ordinate approach, we reproduce the dynamics of the collisions including the variation of the size of the skyrmions and collapse above a critical velocity.

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