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arXiv:2004.13604 · Physical review. B./Physical review. B

Density dynamics in the mass-imbalanced Hubbard chain

Tjark Heitmann, Jonas Richter, T. Dahm, Robin Steinigeweg

Published Jul 23, 2020• 14 citations
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Abstract

We consider two mutually interacting fermionic particle species on a one-dimensional lattice and study how the mass ratio $\ensuremath{\eta}$ between the two species affects the (equilibration) dynamics of the particles. Focusing on the regime of strong interactions and high temperatures, two well-studied points of reference are given by (i) the case of equal masses $\ensuremath{\eta}=1$, i.e., the standard Fermi-Hubbard chain, where initial nonequilibrium density distributions are known to decay, and (ii) the case of one particle species being infinitely heavy, $\ensuremath{\eta}=0$, leading to a localization of the lighter particles in an effective disorder potential. Given these two opposing cases, the dynamics in the case of intermediate mass ratios $0<\ensuremath{\eta}<1$ is of particular interest. To this end, we study the real-time dynamics of pure states featuring a sharp initial nonequilibrium density profile. Relying on the concept of dynamical quantum typicality, the resulting nonequilibrium dynamics can be related to equilibrium correlation functions. Summarizing our main results, we observe that diffusive transport occurs for moderate values of the mass imbalance and manifests itself in a Gaussian spreading of real-space density profiles and an exponential decay of density modes in momentum space. For stronger imbalances, we provide evidence that transport becomes anomalous on intermediate timescales, and in particular, our results are consistent with the absence of strict localization in the long-time limit for any $\ensuremath{\eta}>0$. Based on our numerical analysis, we provide an estimate for the ``lifetime'' of the effective localization as a function of $\ensuremath{\eta}$.

DOI
10.1103/physrevb.102.045137

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We consider two mutually interacting fermionic particle species on a one-dimensional lattice and study how the mass ratio \ensuremathη\ensuremath{\eta}\ensuremathη between the two species affects the (equilibration) dynamics of the particles. Focusing on the regime of strong interactions and high temperatures, two well-studied points of reference are given by (i) the case of equal masses \ensuremathη=1\ensuremath{\eta}=1\ensuremathη=1, i.e., the standard Fermi-Hubbard chain, where initial nonequilibrium density distributions are known to decay, and (ii) the case of one particle species being infinitely heavy, \ensuremathη=0\ensuremath{\eta}=0\ensuremathη=0, leading to a localization of the lighter particles in an effective disorder potential. Given these two opposing cases, the dynamics in the case of intermediate mass ratios 0<\ensuremathη<10<\ensuremath{\eta}<10<\ensuremathη<1 is of particular interest. To this end, we study the real-time dynamics of pure states featuring a sharp initial nonequilibrium density profile. Relying on the concept of dynamical quantum typicality, the resulting nonequilibrium dynamics can be related to equilibrium correlation functions. Summarizing our main results, we observe that diffusive transport occurs for moderate values of the mass imbalance and manifests itself in a Gaussian spreading of real-space density profiles and an exponential decay of density modes in momentum space. For stronger imbalances, we provide evidence that transport becomes anomalous on intermediate timescales, and in particular, our results are consistent with the absence of strict localization in the long-time limit for any \ensuremathη>0\ensuremath{\eta}>0\ensuremathη>0. Based on our numerical analysis, we provide an estimate for the ``lifetime'' of the effective localization as a function of \ensuremathη\ensuremath{\eta}\ensuremathη.
We study transport in the one-dimensional mass-imbalanced Fermi-Hubbard model in the high-temperature limit, focusing on the case of strong interactions. Prior theoretical and experimental investigations have revealed unconventionally long transport timescales, with complications due to strong finite-size effects. We compute the dynamical current-current correlation function directly in the thermodynamic limit using infinite tensor network techniques. We show that transport in the strong-imbalance limit is dominated by AC resonances, which we compute with an analytic expansion. We study the dephasing of these resonances with mass imbalance, $\ensuremath{\eta}$. In the small-imbalance limit, the model is nearly integrable. We connect these unusual limits by computing the DC conductivity and transport decay time as a function of $\ensuremath{\eta}$ and the interaction strength U/tU/tU/t. We propose an experimental protocol to measure these correlation functions in cold atom experiments.
We study the charge conductivity of the one-dimensional repulsive Hubbard model at finite temperature using the method of dynamical quantum typicality, focusing at half filling. This numerical approach allows us to obtain current autocorrelation functions from systems with as many as 18 sites, way beyond the range of standard exact diagonalization. Our data clearly suggest that the charge Drude weight vanishes with a power law as a function of system size. The low-frequency dependence of the conductivity is consistent with a finite dc value and thus with diffusion, despite large finite-size effects. Furthermore, we consider the mass-imbalanced Hubbard model for which the charge Drude weight decays exponentially with system size, as expected for a nonintegrable model. We analyze the conductivity and diffusion constant as a function of the mass imbalance and we observe that the conductivity of the lighter component decreases exponentially fast with the mass-imbalance ratio. While in the extreme limit of immobile heavy particles, the Falicov-Kimball model, there is an effective Anderson-localization mechanism leading to a vanishing conductivity of the lighter species, we resolve finite conductivities for an inverse mass ratio of $\ensuremath{\eta}\ensuremath{\gtrsim}0.25$.
We study the real-time and real-space dynamics of charge in the one-dimensional Hubbard model in the limit of high temperatures. To this end, we prepare pure initial states with sharply peaked density profiles and calculate the time evolution of these nonequilibrium states, by using numerical forward-propagation approaches to chains as long as 20 sites. For a class of typical states, we find excellent agreement with linear-response theory and unveil the existence of remarkably clean charge diffusion in the regime of strong particle-particle interactions. We additionally demonstrate that, in the half-filling sector, this diffusive behavior does not depend on certain details of our initial conditions, i.e., it occurs for five different realizations with random and nonrandom internal degrees of freedom, single and double occupation of the central site, and displacement of spin-up and spin-down particles.
We study the real-time dynamics of local occupation numbers in a one-dimensional model of spinless fermions with a random on-site potential for a certain class of initial states. The latter are thermal (mixed or pure) states of the model in the presence of an additional static force but become nonequilibrium states after a sudden removal of this static force. For this class and high temperatures, we show that the induced dynamics is given by a single correlation function at equilibrium, independent of the initial expectation values being prepared close to equilibrium (by a weak static force) or far away from equilibrium (by a strong static force). Remarkably, this type of universality holds true in both the ergodic phase and the many-body localized regime. Moreover, it does not depend on the specific choice of a unit cell for the local density. We particularly discuss two important consequences. First, the long-time expectation value of the local density is uniquely determined by the fluctuations of its diagonal matrix elements in the energy eigenbasis. Thus, the validity of the eigenstate thermalization hypothesis is not only a sufficient but also a necessary condition for thermalization. Second, the real-time broadening of density profiles is always given by the current autocorrelation function at equilibrium via a generalized Einstein relation. In the context of transport, we discuss the influence of disorder for large particle-particle interactions, where normal diffusion is known to occur in the disorder-free case. Our results suggest that normal diffusion is stable against weak disorder, while they are consistent with anomalous diffusion for stronger disorder below the localization transition. Particularly, for weak disorder, Gaussian density profiles can be observed for single disorder realizations, which we demonstrate for finite lattices up to 31 sites.
The imbalanced Hubbard model features a transition between dynamic regimes depending on the mass ratio and coupling strength between two different particle species. A slowdown of the lighter particle transport can be attributed to an emergent effective disorder induced by the heavy particles for high mass ratio and strong coupling. This subdiffusive regime has been interpreted as a Griffiths phase, linking the effect to the coexistence of metallic and insulating regions. Here, we investigate the dynamics of the heavy particles, which also reveals subdiffusive behavior, yet cannot be explained within the Griffiths picture. We demonstrate that heavy particles predominantly form small clusters, mainly pairs, during the dynamical process, which reduces their propagation speed and transiently shifts the time-dependent diffusion constant into the subdiffusive regime at late times. The necessary attraction between particles driving this process can be understood within the Born-Oppenheimer approximation. We introduce a classical one-dimensional random walk model that can quantitatively reproduce the subdiffusion dynamics in the strong coupling regime.
Constraints in the dynamics of quantum many-body systems can dramatically alter transport properties and relaxation timescales even in the absence of static disorder.Here, we report on the observation of such constrained dynamics arising from the distinct mobility of two species in the one-dimensional massimbalanced Fermi-Hubbard model, realized with ultracold ytterbium atoms in a state-dependent optical lattice.By displacing the trap potential and monitoring the subsequent dynamical response of the system, we identify suppressed transport and slow relaxation with a strong dependence on the mass imbalance and interspecies interaction strength, consistent with eventual thermalization for long times.Our observations demonstrate the potential for quantum simulators to provide insights into unconventional relaxation dynamics arising from constraints.