D. M. Kennes

Transient superconductivity from electronic squeezing of optically pumped phonons

Recent developments in advanced light sources have made it possible to transiently alter the electronic properties of materials by exciting specific atomic vibrations in solids. This study provides a theoretical framework for these experiments.

Entanglement scaling of excited states in large one-dimensional many-body localized systems

We study the properties of excited states in one-dimensional many-body localized (MBL) systems using a matrix product state algorithm. First, the method is tested for a large disordered non-interacting system, where for comparison we compute a quasi-exact reference solution via a Monte Carlo sampling of the single-particle levels. Thereafter, we present extensive data obtained for large interacting systems of L~100 sites and large bond dimensions chi~1700, which allows us to quantitatively analyze the scaling behavior of the entanglement S in the system. The MBL phase is characterized by a logarithmic growth (L)~log(L) over a large scale separating the regimes where volume and area laws hold. We check the validity of the eigenstate thermalization hypothesis. Our results are consistent with the existence of a mobility edge.

Renormalization group approach to time-dependent transport through correlated quantum dots

We introduce a real time version of the functional renormalization group which allows to study correlation effects on nonequilibrium transport through quantum dots. Our method is equally capable to address (i) the relaxation out of a nonequilibrium initial state into a (potentially) steady state driven by a bias voltage and (ii) the dynamics governed by an explicitly time-dependent Hamiltonian. All time regimes from transient to asymptotic can be tackled; the only approximation is the consistent truncation of the flow equations at a given order. As an application we investigate the relaxation dynamics of the interacting resonant level model which describes a fermionic quantum dot dominated by charge fluctuations. Moreover, we study decoherence and relaxation phenomena within the ohmic spin-boson model by mapping the latter to the interacting resonant level model.

Oscillatory dynamics and non-Markovian memory in dissipative quantum systems.

The nonequilibrium dynamics of a small quantum system coupled to a dissipative environment is studied. We show that (i) the oscillatory dynamics close to a coherent-to-incoherent transition is significantly different from the one of the classical damped harmonic oscillator and that (ii) non-Markovian memory plays a prominent role in the time evolution after a quantum quench.

Extending the range of real time density matrix renormalization group simulations

We discuss a few simple modifications to time-dependent density matrix renormalization group (DMRG) algorithms which allow to access larger time scales. We specifically aim at beginners and present practical aspects of how to implement these modifications within any standard matrix product state (MPS) based formulation of the method. Most importantly, we show how to ‘combine’ the Schrodinger and Heisenberg time evolutions of arbitrary pure states |ψ〉 and operators A in the evaluation of 〈A〉ψ(t)=〈ψ|A(t)|ψ〉. This includes quantum quenches. The generalization to (non-)thermal mixed state dynamics 〈A〉ρ(t)=Tr[ρA(t)] induced by an initial density matrix ρ is straightforward. In the context of linear response (ground state or finite temperature T>0) correlation functions, one can extend the simulation time by a factor of two by ‘exploiting time translation invariance’, which is efficiently implementable within MPS DMRG. We present a simple analytic argument for why a recently-introduced disentangler succeeds in reducing the effort of time-dependent simulations at T>0. Finally, we advocate the python programming language as an elegant option for beginners to set up a DMRG code.

Universal quench dynamics of interacting quantum impurity systems

The equilibrium physics of quantum impurities frequently involves a universal crossover from weak to strong reservoir-impurity coupling, characterized by single-parameter scaling and an energy scale

Transport in quasiperiodic interacting systems: From superdiffusion to subdiffusion

{T}_{K}

Spin and thermal conductivity of quantum spin chains and ladders

(Kondo temperature) that breaks scale invariance. For the noninteracting resonant level model, the nonequilibrium time evolution of the Loschmidt echo after a local quantum quench was recently computed explicitly [R. Vasseur, K. Trinh, S. Haas, and H. Saleur, Phys. Rev. Lett. 110, 240601 (2013)]. It shows single-parameter scaling with variable

Efficiency and power of a thermoelectric quantum dot device

{T}_{K}t

Transport properties of the one-dimensional Hubbard model at finite temperature

. Here, we scrutinize whether similar universal dynamics can be observed in various interacting quantum impurity systems. Using density matrix and functional renormalization group approaches, we analyze the time evolution resulting from abruptly coupling two noninteracting Fermi or interacting Luttinger liquid leads via a quantum dot or a direct link. We also consider the case of a single Luttinger liquid lead suddenly coupled to a quantum dot. We investigate whether the field-theory predictions for the universal scaling as well as for the large-time behavior successfully describe the time evolution of the Loschmidt echo and the entanglement entropy of microscopic models. Our study shows that for the considered local quench protocols the above quantum impurity models fall into a class of problems for which the nonequilibrium dynamics can largely be understood based on the knowledge of the corresponding equilibrium physics.