Publications

Order by: publication date number of citations

• DFT2kp: Effective kp models from ab-initio data

  João Victor V. Cassiano, Augusto de Lelis Araújo, Paulo E. Faria Junior, Gerson J. Ferreira

  SciPost Phys. Codebases 25 (2024) · published 5 February 2024 | Toggle abstract · pdf

  The $k\cdot p$ method, combined with group theory, is an efficient approach to obtain the low energy effective Hamiltonians of crystalline materials. Although the Hamiltonian coefficients are written as matrix elements of the generalized momentum operator $\pi= p+ p_{SOC}$ (including spin-orbit coupling corrections), their numerical values must be determined from outside sources, such as experiments or ab initio methods. Here, we develop a code to explicitly calculate the Kane (linear in crystal momentum) and Luttinger (quadratic in crystal momentum) parameters of $k\cdot p$ effective Hamiltonians directly from ab initio wavefunctions provided by Quantum ESPRESSO. Additionally, the code analyzes the symmetry transformations of the wavefunctions to optimize the final Hamiltonian. This is an optional step in the code, where it numerically finds the unitary transformation $U$ that rotates the basis towards an optimal symmetry-adapted representation informed by the user. Throughout the paper, we present the methodology in detail and illustrate the capabilities of the code applying it to a selection of relevant materials. Particularly, we show a "hands-on" example of how to run the code for graphene (with and without spin-orbit coupling). The code is open source and available at https://gitlab.com/dft2kp/dft2kp.

  6 citations

• Interference dynamics of matter-waves of SU($N$) fermions

  Wayne Jordan Chetcuti, Andreas Osterloh, Luigi Amico, Juan Polo

  SciPost Phys. 15, 181 (2023) · published 30 October 2023 | Toggle abstract · pdf

  Interacting N-component fermions spatially confined in ring-shaped potentials display specific coherence properties. The orbital angular momentum per particle of such systems can be quantized to fractional values specifically depending on the particle-particle interaction. Here we demonstrate how to monitor the state of the system through homodyne (momentum distribution) and self-heterodyne system's expansion. For homodyne protocols, the momentum distribution is affected by the particle statistics in two distinctive ways. The first effect is a purely statistical one: at zero interactions, the characteristic hole in the momentum distribution around the momentum k=0 opens up once half of the SU(N) Fermi sphere is displaced. The second effect originates from the interaction: The fractionalization in the interacting system manifests itself by an additional "delay" in the flux for the occurrence of the hole, that now becomes a characteristic minimum at k=0. We demonstrate that the angular momentum fractional quantization is reflected in the self-heterodyne interference as specific dislocations in interferograms. Our analysis demonstrates how the study of the interference fringes grants us access to both number of particles and number of components of SU(N) fermions.

  6 citations

• Spin conductivity of the XXZ chain in the antiferromagnetic massive regime

  Frank Göhmann, Karol K. Kozlowski, Jesko Sirker, Junji Suzuki

  SciPost Phys. 12, 158 (2022) · published 12 May 2022 | Toggle abstract · pdf

  We present a series representation for the dynamical two-point function of the local spin current for the XXZ chain in the antiferromagnetic massive regime at zero temperature. From this series we can compute the correlation function with very high accuracy up to very long times and large distances. Each term in the series corresponds to the contribution of all scattering states of an even number of excitations. These excitations can be interpreted in terms of an equal number of particles and holes. The lowest term in the series comprises all scattering states of one hole and one particle. This term determines the long-time large-distance asymptotic behaviour which can be obtained explicitly from a saddle-point analysis. The space-time Fourier transform of the two-point function of currents at zero momentum gives the optical spin conductivity of the model. We obtain highly accurate numerical estimates for this quantity by numerically Fourier transforming our data. For the one-particle, one-hole contribution, equivalently interpreted as a two-spinon contribution, we obtain an exact and explicit expression in terms of known special functions. For large enough anisotropy, the two-spinon contribution carries most of the spectral weight, as can be seen by calculating the f-sum rule.

  6 citations

• Slightly broken higher-spin current in bosonic and fermionic QED in the large-$N$ limit

  Zheng Zhou, Yin-Chen He

  SciPost Phys. 15, 072 (2023) · published 29 August 2023 | Toggle abstract · pdf

  We study the slightly broken higher-spin currents in various CFTs with U(1) gauge field, including the tricritical QED, scalar QED, fermionic QED and QED-Gross-Neveu-Yukawa theory. We calculate their anomalous dimension by making use of the classical non-conservation equation and the equations of motion. We find a logarithmic asymptotic behaviour ($\gamma_s\sim 16/(N\pi^2)$ log s ) of the anomalous dimension at large spin $s$, which is different from other interacting CFTs without gauge fields and may indicate certain unique features of gauge theories. We also study slightly broken higher-spin currents of the SU(N)$_1$ WZW model at $d=2+\epsilon$ dimensions by formulating them as the QED theory, and we again find its anomalous dimension has a logarithmic asymptotic behaviour with respect to spin. This result resolves the mystery regarding the mechanism of breaking higher spin currents of Virasoro symmetry at $d=2+\epsilon$ dimensions, and may be applicable to other interesting problems such as the $2+\epsilon$ expansion of Ising CFT.

  6 citations

• Finite temperature spin diffusion in the Hubbard model in the strong coupling limit

  Oleksandr Gamayun, Arthur Hutsalyuk, Balázs Pozsgay, Mikhail B. Zvonarev

  SciPost Phys. 15, 073 (2023) · published 30 August 2023 | Toggle abstract · pdf

  We investigate finite temperature spin transport in one spatial dimension by considering the spin-spin correlation function of the Hubbard model in the limiting case of infinitely strong repulsion. We find that in the absence of a magnetic field the transport is diffusive, and derive the spin diffusion constant. Our approach is based on asymptotic analysis of a Fredholm determinant representation. The obtained results are in agreement with Generalized Hydrodynamics approach.

  6 citations

• Stability and asymptotic interactions of chiral magnetic skyrmions in a tilted magnetic field

  Bruno Barton-Singer, Bernd J. Schroers

  SciPost Phys. 15, 011 (2023) · published 17 July 2023 | Toggle abstract · pdf

  Using a general framework, interaction potentials between chiral magnetic solitons in a planar system with a tilted external magnetic field are calculated analytically in the limit of large separation. The results are compared to previous numerical results for solitons with topological charge $\pm 1$. A key feature of the calculation is the interpretation of Dzyaloshinskii-Moriya interaction (DMI) as a background $SO(3)$ gauge field. In a tilted field, this leads to a $U(1)$-gauged version of the usual equation for spin excitations, leading to a distinctive oscillating interaction profile. We also obtain predictions for skyrmion stability in a tilted field which closely match numerical observations.

  6 citations

• Hidden-orders of uranium compounds

  Hisatomo Harima

  SciPost Phys. Proc. 11, 006 (2023) · published 5 June 2023 | Toggle abstract · pdf

  Hidden order is often discussed in strongly correlated electron systems. In many cases, it shows no magnetic responses, so that the order parameter is electric, but accompanies few lattice distortion. In the Jahn-Teller cases, the ordered state gains its energy by changing inter-site mixing strengths, leading a larger structural distortion with lowering symmetry. On the contrary, in strongly correlated electron systems, the strong intra-site Coulomb energy causes the the symmetry lowering. It is not necessary to bring the lattice distortion for lowering local site symmetry. In fact, lower symmetry of electronic state than the symmetry of the crystal structure can be found for UTe$_2$, UGe$_2$ similarly for URu$_2$Si$_2$.

  6 citations

• Functional renormalization group for non-Hermitian and $\mathcal{PT}$-symmetric systems

  Lukas Grunwald, Volker Meden, Dante M. Kennes

  SciPost Phys. 12, 179 (2022) · published 30 May 2022 | Toggle abstract · pdf

  We generalize the vertex expansion approach of the functional renormalization group to non-Hermitian systems. As certain anomalous expectation values might not vanish, additional terms as compared to the Hermitian case can appear in the flow equations. We investigate the merits and shortcomings of the vertex expansion for non-Hermitian systems by considering an exactly solvable $\mathcal{PT}$-symmetric non-linear toy-model and reveal, that in this model, the fidelity of the vertex expansion in a perturbatively motivated truncation schema is comparable with that of the Hermitian case. The vertex expansion appears to be a viable method for studying correlation effects in non-Hermitian systems.

  6 citations

• Topological defects in Floquet circuits

  Mao Tian Tan, Yifan Wang, Aditi Mitra

  SciPost Phys. 16, 075 (2024) · published 13 March 2024 | Toggle abstract · pdf

  We introduce a Floquet circuit describing the driven Ising chain with topological defects. The corresponding gates include a defect that flips spins as well as the duality defect that explicitly implements the Kramers-Wannier duality transformation. The Floquet unitary evolution operator commutes with such defects, but the duality defect is not unitary, as it projects out half the states. We give two applications of these defects. One is to analyze the return amplitudes in the presence of "space-like" defects stretching around the system. We verify explicitly that the return amplitudes are in agreement with the fusion rules of the defects. The second application is to study unitary evolution in the presence of "time-like" defects that implement anti-periodic and duality-twisted boundary conditions. We show that a single unpaired localized Majorana zero mode appears in the latter case. We explicitly construct this operator, which acts as a symmetry of this Floquet circuit. We also present analytic expressions for the entanglement entropy after a single time step for a system of a few sites, for all of the above defect configurations.

  6 citations

• Impurity Knight shift in quantum dot Josephson junctions

  Luka Pavešić, Marta Pita Vidal, Arno Bargerbos, Rok Žitko

  SciPost Phys. 15, 070 (2023) · published 24 August 2023 | Toggle abstract · pdf

  Spectroscopy of a Josephson junction device with an embedded quantum dot reveals the presence of a contribution to level splitting in external magnetic field that is proportional to $\cos\phi$, where $\phi$ is the gauge-invariant phase difference across the junction. To elucidate the origin of this unanticipated effect, we systematically study the Zeeman splitting of spinful subgap states in the superconducting Anderson impurity model. The magnitude of the splitting is renormalized by the exchange interaction between the local moment and the continuum of Bogoliubov quasiparticles in a variant of the Knight shift phenomenon. The leading term in the shift is linear in the hybridisation strength $\Gamma$ (quadratic in electron hopping), while the subleading term is quadratic in $\Gamma$ (quartic in electron hopping) and depends on $\phi$ due to spin-polarization-dependent corrections to the Josephson energy of the device. The amplitude of the $\phi$-dependent part is largest for experimentally relevant parameters beyond the perturbative regime where it is investigated using numerical renormalization group calculations. Such magnetic-field-tunable coupling between the quantum dot spin and the Josephson current could find wide use in superconducting spintronics.

  6 citations