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Ralf Schützhold

Publications and source records attributed to Ralf Schützhold.

At least 55 records · Page 3Linked to original sources
arXiv2019

Hierarchy of double-time correlations

The hierarchy of correlations is an analytical approximation method which allows us to study non-equilibrium phenomena in strongly interacting quantum many-body systems on lattices in higher dimensions. So far, this method was restricted to equal-time correlators $\langle\hat A_μ(t)\hat B_ν(t)\rangle$. In this work, we generalize this method to double-time correlators $\langle\hat A_μ(t)\hat B_ν(t')\rangle$, which allows us to study effective light cones and Green functions and to incorporate finite initial temperatures.

cond-mat.stat-mech↗
arXiv2019

Doublon bottleneck in the ultrafast relaxation dynamics of hot electrons in 1T-TaS_2

Employing time-resolved photoelectron spectroscopy we analyze the relaxation dynamics of hot electrons in the charge density wave / Mott material 1T-TaS_2. At 1.2 eV above the Fermi level we observe a hot electron lifetime of 12 +- 5 fs in the metallic state and of 60 +- 10 fs in the broken symmetry ground state - a direct consequence of the reduced phase space for electron-electron scattering determined by the Mott gap. Boltzmann equation calculations which account for the interaction of hot electrons in a Bloch band with a doublon-holon excitation in the Mott state provide insight into the unoccupied electronic structure in the correlated state.

cond-mat.str-el↗
arXiv2019

Boltzmann relaxation dynamics in the strongly interacting Fermi-Hubbard model

Via the hierarchy of correlations, we study the Mott insulator phase of the Fermi-Hubbard model in the limit of strong interactions and derive a quantum Boltzmann equation describing its relaxation dynamics. In stark contrast to the weakly interacting case, we find that the scattering cross sections strongly depend on the momenta of the colliding quasi-particles and holes. Therefore, the relaxation towards equilibrium crucially depends on the spectrum of excitations. For example, for particle-hole excitations directly at the minimum of the (direct) Mott gap, the scattering cross sections vanish such that these excitations can have a very long life-time.

quant-ph↗
arXiv2019

WKB approach to pair creation in spacetime-dependent fields

Besides tunneling in static potential landscapes, for example, the Wentzel-Kramers-Brillouin (WKB) approach is a powerful nonperturbative approximation tool to study particle creation due to time-dependent background fields, such as cosmological particle production or the Sauter-Schwinger effect, i.e., electron-positron pair creation in a strong electric field. However, our understanding of particle creation processes in background fields depending on both space and time is rather incomplete. In order to venture into this direction, we propose a generalization of the WKB method to truly spacetime-dependent fields and apply it to the case of a spacetime-dependent mass.

hep-th↗
arXiv2019

Analog of cosmological particle creation in electromagnetic waveguides

We consider an electromagnetic waveguide with a time-dependent propagation speed $v(t)$ as an analog for cosmological particle creation. In contrast to most previous studies which focus on the number of particles produced, we calculate the corresponding two-point correlation function. For a small step-like variation $δv(t)$, this correlator displays characteristic signatures of particle pair creation. As another potential advantage, this observable is of first order in the perturbation $δv(t)$, whereas the particle number is second order in $δv(t)$ and thus stronger suppressed for small $δv(t)$.

quant-ph↗
arXiv2019

Particle pair creation by inflation of quantum vacuum fluctuations in an ion trap

The creation of matter and structure in our universe is currently described by an intricate interplay of quantum field theory and general relativity. Signatures of this process during an early inflationary period can be observed, while direct tests remain out of reach. Here, we study an experimental analog of the process based on trapped atomic ions. We create pairs of phonons by tearing apart quantum vacuum fluctuations. Thereby, we prepare ions in an entangled state of motion. Controlling timescales and the coupling to environments should permit optimizing efficiencies while keeping the effect robust via established tools in quantum information processing (QIP). This might also permit to cross-fertilize between concepts in cosmology and applications of QIP, such as, quantum metrology, experimental quantum simulations and quantum computing.

quant-ph↗
arXiv2019

Dynamically assisted nuclear fusion

We consider the prototypical deuterium-tritium fusion reaction. At intermediate initial kinetic energies (in the keV regime), a major bottle-neck of this reaction is the Coulomb barrier between the nuclei, which is overcome by tunneling. Here, we study whether the tunneling probability can be enhanced by an additional electromagnetic field, such as an x-ray free electron laser (XFEL). We find that this dynamical assistance should be feasible with present-day or near future technology.

nucl-th↗
arXiv2019

Quantum simulation of spontaneous pair creation in 2D optical lattices

One of the fundamental predictions of Quantum Electrodynamics (QED) is the spontaneous creation of particle--antiparticle pairs from vacuum in presence of a very strong electric field. Under these extreme conditions a strongly bound state can fetch an otherwise unobservable electron from the Dirac sea, leaving behind a hole representing a positron. Although generally known for many decades, the effect has not yet been demonstrated experimentally. We propose an analogue model of the quantum Dirac field, realized by ultra--cold fermionic atoms in an optical lattice, aiming at an experimental simulation of this intriguing non--perturbative phenomenon. Numerical simulations demonstrate the effect of spontaneous pair creation in the optical analogue system, in qualitative agreement with QED: in the adiabatic regime the vacuum can be destabilized only by supercritical fields exceeding a critical threshold.

quant-ph↗
arXiv2019

Time reversal and quantum Loschmidt echo in optical lattices

A quantum Loschmidt echo (also referred to as quantum time mirror) corresponds to an effective time inversion after which the quantum wave function reverses its previous time evolution and eventually reaches its initial distribution again. We propose a comparably simple protocol for such an effective time reversal for ultra-cold atoms in optical lattices which should be easier to realize experimentally than previous proposals.

quant-ph↗
arXiv2018

Interaction of a Bose-Einstein condensate with a gravitational wave

Partly motivated by recent proposals for the detection of gravitational waves, we study their interaction with Bose-Einstein condensates. For homogeneous condensates at rest, the gravitational wave does not directly create phonons (to lowest order), but merely affects existing phonons or indirectly creates phonon pairs via quantum squeezing -- an effect which has already been considered in the literature. For inhomogeneous condensate flows such as a vortex lattice, however, the impact of the gravitational wave can directly create phonons. This more direct interaction can be more efficient and could perhaps help bringing such a detection mechanism for gravitational waves a step closer towards experimental realizability -- even though it is still a long way to go. Finally, we argue that super-fluid Helium might offer some advantages in this respect.

quant-ph↗
arXiv2018

Environment induced pre-thermalization in the Mott-Hubbard model

Via the hierarchy of correlations, we study the strongly interacting Fermi-Hubbard model in the Mott insulator state and couple it to a Markovian environment which constantly monitors the particle numbers $\hat n_μ^\uparrow$ and $\hat n_μ^\downarrow$ for each lattice site $μ$. As expected, the environment induces an imaginary part $γ$ (i.e., decay rate) of the quasi-particle frequencies $ω_{\mathbf{k}}\toω_{\mathbf{k}}-iγ$ and tends to diminish the correlations between lattice sites. Surprisingly, the environment does also steer the state of the system on intermediate time scales $\mathcal{O}(1/γ)$ to a pre-thermalized state very similar to a quantum quench (i.e., suddenly switching on the hopping rate $J$). Full thermalization occurs via local on-site heating and takes much longer.

cond-mat.str-el↗
arXiv2018

Discrete worldline instantons

The semiclassical approximation of the worldline path integral is a powerful tool to study nonperturbative electron-positron pair creation in spacetime-dependent background fields. Finding solutions of the classical equations of motion, i.e. worldline instantons, is possible analytically only in special cases, and a numerical treatment is nontrivial as well. We introduce a completely general numerical approach based on an approximate evaluation of the discretized path integral that easily and robustly gives the full semiclassical pair production rate in nontrivial multi-dimensional fields, and apply it to some example cases.

hep-th↗
arXiv2018

Analog Sauter-Schwinger effect in semiconductors for spacetime-dependent fields

The Sauter-Schwinger effect predicts the creation of electron-positron pairs out of the quantum vacuum via tunneling induced by a strong electric field. Unfortunately, as the required field strength is extremely large, this fundamental prediction of quantum field theory has not been verified experimentally yet. Here, we study under which conditions and approximations the interband tunneling in suitable semiconductors could be effectively governed by the same (Dirac) Hamiltonian, especially for electric fields which depend on space and time. This quantitative analogy would allow us to test some of the predictions (such as the dynamically assisted Sauter-Schwinger effect) in this area by means of these laboratory analogs.

cond-mat.mes-hall↗
arXiv2017

Sauter-Schwinger pair creation dynamically assisted by a plane wave

We study electron-positron pair creation by a strong and constant electric field superimposed with a weaker transversal plane wave which is incident perpendicularly (or under some angle). Comparing the fully non-perturbative approach based on the world-line instanton method with a perturbative expansion into powers of the strength of the weaker plane wave, we find good agreement - provided that the latter is carried out to sufficiently high orders. As usual for the dynamically assisted Sauter-Schwinger effect, the additional plane wave induces an exponential enhancement of the pair-creation probability if the combined Keldysh parameter exceeds a certain threshold.

hep-ph↗
arXiv2017

Dynamically assisted Sauter-Schwinger effect - non-perturbative versus perturbative aspects

The Sauter-Schwinger effect predicts the creation of electron-positron pairs out of the quantum vacuum by a strong and slowly varying electric field. This effect can be dynamically assisted by an additional weaker time-dependent field, which may drastically enhance the pair-creation probability. In previous studies, it has been found that the enhancement may crucially depend on the temporal shape of this weaker pulse, e.g., a Gaussian profile $\exp\{-(ωt)^2\}$ or a Sauter pulse $1/\cosh^2(ωt)$ behave quite differently. In order to understand this difference, we make a perturbative expansion in terms of the weaker field while treating the strong electric field non-perturbatively. For a large class of profiles including the Sauter pulse, already the sum of the zeroth-order and the first-order amplitudes of this perturbative expansion yields good agreement. For other cases, such as a Gaussian or sinusoidal profile, this is not true in general and higher orders can yield the dominant contribution - where the dominant order depends on the chosen parameters. Our findings are confirmed by numerical simulations and help us to sort previous results into a bigger picture.

hep-th↗
arXiv2016

Prefactor in the dynamically assisted Sauter-Schwinger effect

The probability of creating an electron-positron pair out of the quantum vacuum by a strong electric field can be enhanced tremendously via an additional weaker time-dependent field. This dynamically assisted Sauter-Schwinger effect has already been studied in several works. It has been found that the enhancement mechanism depends on the shape of the weaker field. For example, a Sauter pulse $1/\cosh(ωt)^2$ and a Gaussian profile $\exp(-ω^2 t^2)$ exhibit significant, qualitative differences. However, so far most of the analytical studies were focused on the exponent entering the pair-creation probability. Here, we study the subleading prefactor in front of the exponential using the worldline instanton method. We find that the main features of the dynamically assisted Sauter-Schwinger effect, including the dependence on the shape of the weaker field, are basically unaffected by the prefactor. To test the validity of the instanton approximation, we compare the number of produced pairs to a numerical integration of the full Riccati equation.

hep-th↗
arXiv2016

Doubly assisted Sauter-Schwinger effect

We study electron-positron pair creation by a strong and slowly varying electric field, assisted by a weaker and more rapidly changing field (e.g., in the keV regime) plus an additional high-energy (say MeV) photon. It turns out that this combination can yield a pair creation probability which is exponentially larger than in the case where one (or more) of the three ingredients is missing. Apart from a deeper understanding of these non-perturbative phenomena, this double enhancement may pave the way for an experimental verification of this fundamental prediction.

hep-th↗