36 research outputs found
Thermal transport properties of disordered spin-1/2 systems
This work studies heat transport of bond-disordered spin-1/2 chains. As an
example, the XX case is analyzed, which corresponds to a model of
noninteracting spinless fermions. Within the fermion representation, the
single-particle eigenenergies are determined numerically, which allow one to
compute transport coefficients. Since the ballistic transport properties of a
clean chain are destroyed by disorder, the focus is on the frequency dependence
of the thermal conductivity and on a qualitative comparison with the spin
conductivity, both at finite temperatures.Comment: 2 pages, 1 figure. To be published in the Proceedings of SCES '05,
Vienna, in Physica
Domain-wall melting as a probe of many-body localization
Motivated by a recent optical-lattice experiment by Choi et al.[Science 352,
1547 (2016)], we discuss how domain-wall melting can be used to investigate
many-body localization. First, by considering noninteracting fermion models, we
demonstrate that experimentally accessible measures are sensitive to
localization and can thus be used to detect the delocalization-localization
transition, including divergences of characteristic length scales. Second,
using extensive time-dependent density matrix renormalization group
simulations, we study fermions with repulsive interactions on a chain and a
two-leg ladder. The extracted critical disorder strengths agree well with the
ones found in existing literature.Comment: 4+2 pages, 4+2 figure
Thermal and optical conductivity in the Holstein model at half-filling and at finite temperatures in the Luttinger-liquid and charge-density-wave regime
Electron-phonon interactions play a key role in many branches of solid-state
physics. Here, our focus is on the transport properties of one-dimensional
systems, and we apply efficient real-time matrix-product state methods to
compute the optical and thermal conductivities of Holstein chains at finite
temperatures and filling. We validate our approach by comparison with
analytical results applicable to single polarons valid in the small polaron
limit. Our work provides a systematic study of contributions to the thermal
conductivity at finite frequencies and elucidates differences in the spectrum
compared to the optical conductivity, covering both the Luttinger-liquid and
charge-density-wave regimes. Finally, we demonstrate that our approach is
capable of extracting the DC conductivities as well. Beyond this first
application, several future extensions seem feasible, such as, the inclusion of
dispersive phonons, different types of local electron-phonon coupling, and a
systematic study of drag effects in this electron-phonon coupled system.Comment: Main text: 7 pages, 4 figures. Supplementary Material: 4 pages, 6
figure
Transport properties of low-dimensional quantum spin systems
This thesis studies transport properties of low-dimensional quantum spin systems with a particular focus on the transport of heat and spin. While the integrable anisotropic spin-1/2 chain exhibits ballistic transport (finite Drude weights), the numerical results of this work show that heat and spin transport of non-integrable models (spin ladder, frustrated and dimerized chain) is not ballistic, characterized by vanishing Drude weights in the thermodynamic limit. The results for the thermal conductivity of spin ladders are compared to experimental data. For the case of the spin-1/2 Heisenberg chain, this work contains detailed results for the temperature and magnetic field dependence of the Drude weights. Finally, bond-disordered spin systems are considered, namely the spin-1/2 chain and the spin ladder. In the first case, the transport coefficients are computed by means of exact diagonalization and discussed as a function of frequency, temperature, and magnetic field. In the second case, spectral properties of the elementary excitation of disordered spin ladders are analyzed using diagrammatic perturbation theory and numerical techniques.Diese Arbeit beschäftigt sich mit den Transporteigenschaften niedrigdimensionaler Quanten-Spinsysteme, wobei das Hauptinteresse dem Wärme- und Spintransport gilt. Während die integrable anisotrope Spin-1/2 Kette ballistische Transporteigenschaften (endliche Drude-Gewichte) aufweist, belegen die numerischen Ergebnisse dieser Arbeit, dass der Wärme- und Spintransport von nicht-integrablen Systemen (Spinleiter, frustrierte und dimerisierte Kette), nicht ballistisch ist, gekennzeichnet durch das Verschwinden der Drude-Gewichte im thermodynamischen Limes. Darüber hinaus werden Ergebnisse für die Wärmeleitfähigkeit von Spinleitern mit experimentellen Daten verglichen. Für den Fall der Spin-1/2 Heisenberg-Kette werden detaillierte Ergebnisse für die Temperatur- und Magnetfeldabhängigkeit der Drude-Gewichte präsentiert. Schließlich werden ungeordnete Spinsysteme betrachtet, und zwar Spin-1/2 Ketten und Leitern. Im ersten Fall werden die Transportkoeffizienten mittels exakter Diagonalisierung berechnet und als Funktion von Frequenz, Temperatur und Unordnungsstärke diskutiert. Im zweiten Fall werden spektrale Eigenschaften der elementaren Anregungen von ungeordneten Spinleitern analysiert, wobei diagrammatische Störungstheorie und numerische Techniken verwendet werden
Many-body localization characterized from a one-particle perspective
We show that the one-particle density matrix can be used to
characterize the interaction-driven many-body localization transition in closed
fermionic systems. The natural orbitals (the eigenstates of ) are
localized in the many-body localized phase and spread out when one enters the
delocalized phase, while the occupation spectrum (the set of eigenvalues of
) reveals the distinctive Fock-space structure of the many-body
eigenstates, exhibiting a step-like discontinuity in the localized phase. The
associated one-particle occupation entropy is small in the localized phase and
large in the delocalized phase, with diverging fluctuations at the transition.
We analyze the inverse participation ratio of the natural orbitals and find
that it is independent of system size in the localized phase.Comment: 5 pages, 3 figures; v2: added two appendices and a new figure panel
in main text; v3: updated figur