arXiv subjects

J. Herbrych

Publications and source records attributed to J. Herbrych.

At least 37 records · Page 2Linked to original sources

arXiv2021

Ballistic transport in integrable quantum chains with degenerate spectra

We study the ballistic transport in integrable lattice models, i.e., the spin XXZ and Hubbard chains, close to the noninteracting limit. The stiffnesses of spin and charge currents reveal, at high temperatures, a discontinuous reduction (jump) when the interaction is introduced. We show that the jumps are related to the large degeneracy of the parent noninteracting models. These degeneracies are properly captured by the degenerate perturbation calculations which may be performed for large systems. We find that the discontinuities and the quasilocality of the conserved current in this limit can be traced back to the nonlocal character of an effective interaction. From the latter observation we identify a class of observables which show discontinuities in both models. We also argue that the known local conserved quantities are insufficient to explain the stiffnesses in the Hubbard chain in the regime of weak interaction.

cond-mat.str-el↗

arXiv2021

Diffusion in the Anderson model in higher dimensions

We present an extended microcanonical Lanczos method (MCLM) for a direct evaluation of the diffusion constant and its frequency dependence within the disordered Anderson model of noninteracting particles. The method allows to study systems beyond $10^6$ sites of hypercubic lattices in $ d = 3- 7$ dimensions. Below the transition to localization, where we confirm dynamical scaling behavour, of interest is a wide region of incoherent diffusion, similar to percolating phenomena and to interacting many-body localized systems.

cond-mat.str-el↗

arXiv2021

Interaction-induced topological phase transition and Majorana edge states in low-dimensional orbital-selective Mott insulators

Topological phases of matter are among the most intriguing research directions in Condensed Matter Physics. It is known that superconductivity induced on a topological insulator's surface can lead to exotic Majorana modes, the main ingredient of many proposed quantum computation schemes. In this context, the iron-based high critical temperature superconductors are a promising platform to host such an exotic phenomenon in real condensed-matter compounds. The Coulomb interaction is commonly believed to be vital for the magnetic and superconducting properties of these systems. This work bridges these two perspectives and shows that the Coulomb interaction can also drive a canonical superconductor with orbital degrees of freedom into the topological state. Namely, we show that above a critical value of the Hubbard interaction the system simultaneously develops spiral spin order, a highly unusual triplet amplitude in superconductivity, and, remarkably, Majorana fermions at the edges of the system.

cond-mat.str-el↗

arXiv2020

Block orbital-selective Mott insulators: a spin excitation analysis

We present a comprehensive study of the spin excitations - as measured by the dynamical spin structure factor $S(q,ω)$ - of the so-called block-magnetic state of low-dimensional orbital-selective Mott insulators. We realize this state via both a multi-orbital Hubbard model and a generalized Kondo-Heisenberg Hamiltonian. Due to various competing energy scales present in the models, the system develops periodic ferromagnetic islands of various shapes and sizes, which are antiferromagnetically coupled. The 2$\times$2 particular case was already found experimentally in the ladder material BaFe$_2$Se$_3$ that becomes superconducting under pressure. Here we discuss the electronic density as well as Hubbard and Hund coupling dependence of $S(q,ω)$ using density matrix renormalization group method. Several interesting features were identified: (1) An acoustic (dispersive spin-wave) mode develops. (2) The spin-wave bandwidth establishes a new energy scale that is strongly dependent on the size of the magnetic island and becomes abnormally small for large clusters. (3) Optical (dispersionless spin excitation) modes are present for all block states studied here. In addition, a variety of phenomenological spin Hamiltonians have been investigated but none matches entirely our results that were obtained primarily at intermediate Hubbard $U$ strengths. Our comprehensive analysis provides theoretical guidance and motivation to crystal growers to search for appropriate candidate materials to realize the block states, and to neutron scattering experimentalists to confirm the exotic dynamical magnetic properties unveiled here, with a rich mixture of acoustic and optical features.

cond-mat.str-el↗

arXiv2020

Block-spiral magnetism: An exotic type of frustrated order

Competing interactions in Quantum Materials induce novel states of matter such as frustrated magnets, an extensive field of research both from the theoretical and experimental perspectives. Here, we show that competing energy scales present in the low-dimensional orbital-selective Mott phase (OSMP) induce an exotic magnetic order, never reported before. Earlier neutron scattering experiments on iron-based 123 ladder materials, where OSMP is relevant, already confirmed our previous theoretical prediction of block-magnetism (magnetic order of the form $\uparrow\uparrow\downarrow\downarrow$). Now we argue that another novel phase can be stabilized in multi-orbital Hubbard models, the {\it block-spiral state}. In this state, the magnetic islands form a spiral propagating through the chain but with the blocks maintaining their identity, namely rigidly rotating. This new spiral state is stabilized without any apparent frustration, the common avenue to generate spiral arrangements in multiferroics. By examining the behaviour of the electronic degrees of freedom, parity breaking quasiparticles are revealed. Finally, a simple phenomenological model that accurately captures the macroscopic spin spiral arrangement is also introduced, and fingerprints for the neutron scattering experimental detection of our new state are provided.

cond-mat.str-el↗

arXiv2020

Vanishing Wilson ratio as the hallmark of quantum spin-liquid models

We present numerical results for finite-temperature $T>0$ thermodynamic quantities, entropy $s(T)$, uniform susceptibility $χ_0(T)$ and the Wilson ratio $R(T)$, for several isotropic $S=1/2$ extended Heisenberg models which are prototype models for planar quantum spin liquids. We consider in this context the frustrated $J_1$-$J_2$ model on kagome, triangular, and square lattice, as well as the Heisenberg model on triangular lattice with the ring exchange. Our analysis reveals that typically in the spin-liquid parameter regimes the low-temperature $s(T)$ remains considerable, while $χ_0(T)$ is reduced consistent mostly with a triplet gap. This leads to vanishing $R(T \to 0)$, being the indication of macroscopic number of singlets lying below triplet excitations. This is in contrast to $J_1$-$J_2$ Heisenberg chain, where $R(T \to 0)$ either remains finite in the gapless regime, or the singlet and triplet gap are equal in the dimerized regime.

cond-mat.str-el↗

arXiv2020

Resistivity and its fluctuations in disordered many-body systems: from chains to planes

We study a quantum particle coupled to hard-core bosons and propagating on disordered ladders with $R$ legs. The particle dynamics is studied with the help of rate equations for the boson-assisted transitions between the Anderson states. We demonstrate that for finite $R < \infty$ and sufficiently strong disorder the dynamics is subdiffusive, while the two-dimensional planar systems with $R\to \infty$ appear to be diffusive for arbitrarily strong disorder. The transition from diffusive to subdiffusive regimes may be identified via statistical fluctuations of resistivity. The corresponding distribution function in the diffusive regime has fat tails which decrease with the system size $L$ much slower than $1/\sqrt{L}$. Finally, we present evidence that similar non--Gaussian fluctuations arise also in standard models of many-body localization, i.e., in strongly disordered quantum spin chains.

cond-mat.str-el↗

arXiv2019

Novel Magnetic Block States in Low-Dimensional Iron-Based Superconductors

Inelastic neutron scattering recently confirmed the theoretical prediction of a $\uparrow\uparrow\downarrow\downarrow$-magnetic state along the legs of quasi-one-dimensional (quasi-1D) iron-based ladders in the orbital-selective Mott phase (OSMP). We show here that electron-doping of the OSMP induces a whole class of novel block-states with a variety of periodicities beyond the previously reported $π/2$ pattern. We discuss the magnetic phase diagram of the OSMP regime that could be tested by neutrons once appropriate quasi-1D quantum materials with the appropriate dopings are identified.

cond-mat.str-el↗

arXiv2018

Non-Equilibrium Mass Transport in the 1D Fermi-Hubbard Model

We experimentally and numerically investigate the sudden expansion of fermions in a homogeneous one-dimensional optical lattice. For initial states with an appreciable amount of doublons, we observe a dynamical phase separation between rapidly expanding singlons and slow doublons remaining in the trap center, realizing the key aspect of fermionic quantum distillation in the strongly-interacting limit. For initial states without doublons, we find a reduced interaction dependence of the asymptotic expansion speed compared to bosons, which is explained by the interaction energy produced in the quench.

cond-mat.quant-gas↗

arXiv2018

Spin dynamics of the block orbital-selective Mott phase

Iron-based superconductors display a variety of magnetic phases originating in the competition between electronic, orbital, and spin degrees of freedom. Previous theoretical investigations of the multi-orbital Hubbard model in one dimension revealed the existence of an orbital-selective Mott phase (OSMP) with block spin order. Recent inelastic neutron scattering (INS) experiments on the BaFe$_2$Se$_3$ ladder compound confirmed the relevance of the block-OSMP. Moreover, the powder INS spectrum reveled an unexpected structure, containing both low-energy acoustic and high-energy optical modes. Here we present the theoretical prediction for the dynamical spin structure factor within a block-OSMP regime using the density-matrix renormalization group method. In agreement with experiments we find two dominant features: low-energy dispersive and high-energy dispersionless modes. We argue that the former represents the spin-wave-like dynamics of the block ferromagnetic islands, while the latter is attributed to a novel type of local on-site spin excitations controlled by the Hund coupling.

cond-mat.str-el↗

arXiv2017

Efficiency of fermionic quantum distillation

We present a time-dependent density-matrix renormalization group investigation of the quantum distillation process within the Fermi--Hubbard model on a quasi-1D ladder geometry. The term distillation refers to the dynamical, spatial separation of singlons and doublons in the sudden expansion of interacting particles in an optical lattice, i.e., the release of a cloud of atoms from a trapping potential. Remarkably, quantum distillation can lead to a contraction of the doublon cloud, resulting in an increased density of the doublons in the core region compared to the initial state. As a main result, we show that this phenomenon is not limited to chains that were previously studied. Interestingly, there are additional dynamical processes on the two-leg ladder such as density oscillations and selftrapping of defects that lead to a less efficient distillation process. An investigation of the time evolution starting from product states provides an explanation for this behaviour. Initial product states are also considered, since in optical lattice experiments such states are often used as the initial setup. We propose configurations that lead to a fast and efficient quantum distillation.

cond-mat.str-el↗

arXiv2017

Possible Bicollinear Nematic State with Monoclinic Lattice Distortions in Iron Telluride Compounds

Iron telluride FeTe is known to display bicollinear magnetic order at low temperatures together with a monoclinic lattice distortion. Because the bicollinear order can involve two different wavevectors $(π/2,π/2)$ and $(π/2,-π/2)$, symmetry considerations allow for the possible stabilization of a nematic state with short-range bicollinear order coupled to monoclinic lattice distortions at a $T_S$ higher than the temperature $T_N$ where long-range bicollinear order fully develops. As a concrete example, the three-orbitals spin-fermion model for iron telluride is studied with an additional coupling $\tildeλ_{12}$ between the monoclinic lattice strain and an orbital-nematic order parameter with $B_{2g}$ symmetry. Monte Carlo simulations show that with increasing $\tildeλ_{12}$ the first-order transition characteristic of FeTe splits and bicollinear nematicity is stabilized in a (narrow) temperature range. In this new regime the lattice is monoclinically distorted and short-range spin and orbital order breaks rotational invariance. A discussion of possible realizations of this exotic state is provided.

cond-mat.str-el↗

arXiv2017

Self-consistent approach to many-body localization and subdiffusion

An analytical theory, based on the perturbative treatment of the disorder and extended into a self-consistent set of equations for the dynamical density correlations, is developed and applied to the prototype one-dimensional model of many-body localization. Results show a qualitative agreement with numerically obtained dynamical structure factor in the whole range of frequencies and wavevectors, as well as across the transition to the nonergodic behavior. The theory reveals the singular nature of the one-dimensional problem, whereby on the ergodic side the dynamics is subdiffusive with dynamical conductivity $σ(ω) \propto |ω|^α$, i.e., with vanishing d.c. limit $σ_0=0$ and $α<1$ varying with disorder, while we get $α>1$ in the localized phase.

cond-mat.str-el↗

arXiv2017

Density correlations and transport in models of many-body localization

We present a review of recent theoretical results concerning the many-body localization (MBL) phenomenon, with the emphasis on dynamical density correlations and transport quantities. They are shown to be closely related, providing a comprehensive description of the ergodic-to-nonergodic transition, consistent with experimental findings. While the focus is set mostly on the one-dimensional model of interacting spinless fermions, we also present evidence for the absence of full MBL in the one-dimensional Hubbard model and for the density-wave decay induced by the inter-chain coupling.

cond-mat.str-el↗

arXiv2017

Effective realization of random magnetic fields in compounds with large single-ion anisotropy

We show that spin $S=1$ system with large and random single--ion anisotropy can be at low energies mapped to a $S=1/2$ system with random magnetic fields. This is for example realized in \mbox{Ni(Cl$_{1-x}$Br$_{x}$)$_2$-4SC(NH$_2$)$_2$} compound (DTNX) and therefore it represents a long sought realization of random local (on-site) magnetic fields in antiferromagnetic systems. We support the mapping by numerical study of $S=1$ and effective $S=1/2$ anisotropic Heisenberg chains and find excellent agreement for static quantities and also for the spin conductivity. Such systems can therefore be used to study the effects of local random magnetic fields on transport properties.

cond-mat.str-el↗

arXiv2017

Dynamics of locally coupled agents with next nearest neighbor interaction

We consider large but finite systems of identical agents on the line with up to next nearest neighbor asymmetric coupling. Each agent is modelled by a linear second order differential equation, linearly coupled to up to four of its neighbors. The only restriction we impose is that the equations are decentralized. In this generality we give the conditions for stability of these systems. For stable systems, we find the response to a change of course by the leader. This response is at least linear in the size of the flock. Depending on the system parameters, two types of solutions have been found: damped oscillations and reflectionless waves. The latter is a novel result and a feature of systems with at least next nearest neighbor interactions. Analytical predictions are tested in numerical simulations.

math.DS↗

arXiv2016

Universal dynamics of density correlations at the transition to many-body localized state

Within one-dimensional disordered models of interacting fermions we perform a numerical study of several dynamical density correlations, which can serve as hallmarks of the transition to the many-body localized state. Results confirm that density-wave correlations exhibit quite abrupt change with the increasing disorder, with nonvanishing long-time value characteristic for nonergodic phase. In addition, our results reveal in a wide time-window a logarithmic variation of correlations in time, which we can bring in connection with the anomalous behavior of the dynamical conductivity near the transition. Our result support the view that transition to many-body localization can be characterized by universal dynamical exponents.

cond-mat.str-el↗

arXiv2016

Light induced magnetization in a spin S=1 easy-plane antiferromagnetic chain

The time evolution of magnetization induced by circularly polarized light in a $S=1$ Heisenberg chain with large, easy--plane anisotropy is studied numerically and analytically. Results at constant light frequency $Ω=Ω_0$ are interpreted in terms of absorption lines of the electronic spin resonance spectrum. Applying a time dependent light frequency $Ω=Ω(t)$, so called chirping, is shown to be an efficient procedure in order to obtain within a short time a large, controlled value of the magnetization $M^z$. Furthermore, comparison with a $2$ - level model provides a qualitative understanding of the induced magnetization process.

cond-mat.str-el↗