arXiv subjects

Leander Reascos

Publications and source records attributed to Leander Reascos.

3 recordsLinked to original sources

arXiv2024

SymPT: a comprehensive tool for automating effective Hamiltonian derivations

The Schrieffer-Wolff transformation (SWT) is a foundational perturbative method for deriving effective Hamiltonians in quantum systems by systematically eliminating couplings between pairs of energy distant subspaces. Despite recent advancements, the implementation of SWTs for sufficiently complex systems remains computationally challenging and often requires extensive calculations that are prone to errors. In this work, we introduce an analytical software tool, SymPT (Symbolic Perturbation Theory), designed to automate the SWT and its extensions. Building on a universal framework developed in recent research, SymPT provides a systematic and generalizable solution for deriving the generator of the transformation, enabling accurate computation of effective Hamiltonians for arbitrary perturbative systems. The tool supports both time-independent and time-periodic Hamiltonians, extending beyond standard SWT to incorporate arbitrary coupling elimination, block-diagonalization and full-diagonalization routines, thus enabling precise handling of systems with intricate energy structures.

quant-ph↗

arXiv2024

Closed-Form Generators for Perturbative Transformations in Static and Periodically Driven Quantum Systems

We introduce a unified framework for the practical implementation of perturbative transformations that overcomes the limitations of existing construction methods. The central result is a closed-form operator expression for the generator of a broad class of perturbative transformations in systems composed of finite-dimensional and bosonic subspaces. We further generalize the construction to time-dependent systems with periodic perturbations, obtaining a solution that remains valid across low-, intermediate-, and high-frequency regimes, provided that the transition channels eliminated by the transformation remain sufficiently detuned. The derived framework remains independent of the particular perturbative scheme and therefore applies to a large class of perturbative approaches for both time independent and time dependent transformations. We demonstrate the scope and accuracy of the method by deriving effective dispersive interactions in exemplary anharmonic and periodically driven light--matter systems.

quant-ph↗

arXiv2020

Berry: A code for the differentiation of Bloch wavefunctions from DFT calculations

Density functional calculations of electronic structures of materials is one of the most used techniques in theoretical solid state physics. These calculations retrieve single electron wavefunctions and their eigenenergies. The berry suite of programs amplifies the usefulness of DFT by ordering the eigenstates in analytic bands, allowing the differentiation of the wavefunctions in reciprocal space. It can then calculate Berry connections and curvatures and the second harmonic generation conductivity. The berry software is implemented for two dimensional materials and was tested in hBN and InSe. In the near future, more properties and functionalities are expected to be added.

cond-mat.mes-hall↗