Publications

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THz higher-order topological photonics in Ge-on-Si heterostructures

Ian Colombo, Pietro Minazzi, Emiliano Bonera, Fabio Pezzoli, Jacopo Pedrini

SciPost Phys. Core 7, 039 (2024) · published 4 July 2024 | · pdf

We design germanium-based higher-order topological cavities for terahertz applications by breaking the symmetry of a two-dimensional photonic crystal following the Su-Schrieffer-Heeger model. Calculations demonstrate the parity inversion of the electric field in differently deformed unit cells. The interface between domains of opposite topology presents edge and corner modes. The former are chiral, locking light propagation to its helicity. The latter prove that Ge-based structures can be used as high-order topological photonic crystals. These findings can accelerate the development of Si-photonic components working in a spectral range of high technological interest.
Ground state of the staggered Heisenberg-Γ honeycomb model in a magnetic field

Mojtaba Ahmadi-Yazdi, Mohammad-Hossein Zare, Hamid Mosadeq, Farhad Fazileh

SciPost Phys. Core 7, 030 (2024) · published 15 May 2024 | · pdf

We study the ground state properties of the S=1/2 staggered Heisenberg-$\Gamma$ honeycomb model under a magnetic field based on analytical and numerical methods. Our calculations show that the conventional zigzag and stripy phases are favored because of the staggered Heisenberg interaction away from the pure $\Gamma$ limit. In our classical analysis, we find that the field induces a series of competing magnetic phases with relatively large unit cells in the region sandwiched between the two magnetic phases with long-range ordering. In the quantum treatment, these large magnetic unit cells are destabilized by strong quantum fluctuations that result in the stabilization of a gapless quantum spin liquid behavior. In a honeycomb $\Gamma$ magnet, we disclose an intermediate-field gapless quantum spin liquid phase driven by a tilted field away from the out-of-plane direction only for a narrow region between the low-field zigzag and high-field fully polarized phases.
Muonium reaction in MgO: A showcase for the final steps of ion implantation

Rui C. Vilão, Ali Roonkiani, Apostolos G. Marinopoulos, Helena V. Alberto, João M. Gil, Ricardo B. L. Vieira, Robert Scheuermann, Alois Weidinger

SciPost Phys. Core 8, 056 (2025) · published 26 August 2025 | · pdf

We present an in-depth investigation of the implantation of positive muons in magnesium oxide (MgO). Muonium, the positive muon plus an electron is an analogue of the hydrogen atom. This study describes the final stage of the implantation process, from muon diffusion over the potential barrier and the stopping by an inelastic reaction to the final embedding of the muon into the lattice structure. A special aspect is a relatively long-lived intermediate configuration which lasts for several hundred nanoseconds or more and is accessible to muon spin spectroscopy. The model presented here provides a framework for the analysis of the general case of ion implantation.
Physics-informed neural networks viewpoint for solving the Dyson-Schwinger equations of quantum electrodynamics

Rodrigo Carmo Terin

SciPost Phys. Core 8, 054 (2025) · published 20 August 2025 | · pdf

Physics-informed neural networks (PINNs) are employed to solve the Dyson-Schwinger equations of quantum electrodynamics (QED) in Euclidean space, with a focus on the non-perturbative generation of the fermion's dynamical mass function in the Landau gauge. By inserting the integral equation directly into the loss function, our PINN framework enables a single neural network to learn a continuous and differentiable representation of the mass function over a spectrum of momenta. Also, we benchmark our approach against a traditional numerical algorithm showing the main differences among them. Our novel strategy, which is expected to be extended to other quantum field theories, is the first step towards forefront applications of machine learning in high-level theoretical physics.
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 | · 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.
Electromagnetic multipole expansions and the logarithmic soft photon theorem

Geoffrey Compère, Dima Fontaine, Kevin Nguyen

SciPost Phys. Core 8, 066 (2025) · published 10 October 2025 | · pdf

We study the general structure of the electromagnetic field in the vicinity of spatial infinity. Starting from the general solution of the sourced Maxwell equations written in terms of multipole moments as obtained by Iyer and Damour, we derive the expansion of the electromagnetic field perturbatively in the electromagnetic coupling. At leading order, where the effect of long-range Coulombic interactions between charged particles is neglected, we discover infinite sets of antipodal matching relations satisfied by the electromagnetic field, which extend and sometimes correct previously known relations. At next-to-leading order, electromagnetic tails resulting from these Coulombic interactions appear, which affect the antipodal matching relations beyond those equivalent to the leading soft photon theorem. Moreover, new antipodal matching relations arise, which we use to re-derive the classical logarithmic soft photon theorem of Sahoo and Sen. Our analysis largely builds upon that of Campiglia and Laddha, although it invalidates the antipodal matching relation which they originally used in their derivation. The antipodal matching relations and the proof of the classical logarithmic soft photon theorem agree with an earlier analysis of Bhatkar, which we generalize using other methods.
Energy-saving fast-forward scaling

Takuya Hatomura

SciPost Phys. Core 8, 019 (2025) · published 11 February 2025 | · pdf

We propose energy-saving fast-forward scaling. Fast-forward scaling is a method which enables us to speed up (or slow down) given dynamics in a certain measurement basis. We introduce energy costs of fast-forward scaling, and find possibility of energy-saving speedup for time-independent measurement bases. As concrete examples, we show such energy-saving fast-forward scaling in a two-level system and quantum annealing of a general Ising spin glass. We also discuss the influence of a time-dependent measurement basis, and give a remedy for unwanted energy costs. The present results pave the way for realization of energy-efficient quantum technologies.
Chiral edge states emerging on anyon-net

Atsushi Ueda, Kansei Inamura, Kantaro Ohmori

SciPost Phys. Core 8, 062 (2025) · published 30 September 2025 | · pdf

We propose a symmetry-based approach to constructing lattice models for chiral topological phases, focusing on non-Abelian anyons. Using a 2+1D version of anyon chains and modular tensor categories (MTCs), we ensure exact MTC symmetry at the microscopic level. Numerical simulations using tensor networks demonstrate chiral edge modes for topological phases with Ising and Fibonacci anyons. Our method contrasts with conventional solvability approaches, providing a new theoretical avenue to explore strongly coupled 2+1D systems, revealing chiral edge states in non-Abelian anyonic systems.
Thermodynamics of adiabatic quantum pumping in quantum dots

Daniele Nello, Alessandro Silva

SciPost Phys. Core 7, 067 (2024) · published 4 October 2024 | · pdf

We consider adiabatic quantum pumping through a resonant level model, a single-level quantum dot connected to two fermionic leads. Using the tools of adiabatic expansion, we develop a self-contained thermodynamic description of this model accounting for the variation of the energy level of the dot and the tunnelling rates with the thermal baths. This enables us to study various examples of pumping cycles computing the relevant thermodynamic quantities, such as the entropy produced and the dissipated power. These quantities are compared with the transport properties of the system, i.e. the pumped charge and the charge noise. Among other results, we find that the entropy production rate vanishes in the charge quantization limit while the dissipated power is quantized in the same limit.
$2+1$ dimensional Floquet systems and lattice fermions: Exact bulk spectral equivalence

Thomas Iadecola, Srimoyee Sen, Lars Sivertsen

SciPost Phys. Core 8, 035 (2025) · published 3 April 2025 | · pdf

A connection has recently been proposed between periodically driven systems known as Floquet insulators in continuous time and static fermion theories in discrete time. This connection has been established in a $(1+1)$-dimensional free theory, where an explicit mapping between the spectra of a Floquet insulator and a discrete-time Dirac fermion theory has been formulated. Here we investigate the potential of static discrete-time theories to capture Floquet physics in higher dimensions, where so-called anomalous Floquet topological insulators can emerge that feature chiral edge states despite having bulk bands with zero Chern number. Starting from a particular model of an anomalous Floquet system, we provide an example of a static discrete-time theory whose bulk spectrum is an exact analytic match for the Floquet spectrum. The spectra with open boundary conditions in a particular strip geometry also match up to finite-size corrections. However, the models differ in several important respects. The discrete-time theory is spatially anisotropic, so that the spectra do not agree for all lattice terminations, e.g. other strip geometries or on half spaces. This difference can be attributed to the fact that the static discrete-time model is quasi-one-dimensional in nature and therefore has a different bulk-boundary correspondence than the Floquet model.

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