arXiv subjects

P. Nalbach

Publications and source records attributed to P. Nalbach.

At least 19 recordsLinked to original sources

arXiv2017

Symmetry reduction for tunneling defects due to strong couplings to phonons

Tunneling two-level systems are ubiquitous in amorphous solids, and form a major source of noise in systems such as nano-mechanical oscillators, single electron transistors, and superconducting qubits. Occurance of defect tunneling despite their coupling to phonons is viewed as a hallmark of weak defect-phonon coupling. This is since strong coupling to phonons results in significant phonon dressing and suppresses tunneling in two-level tunneling defects effectively. Here we determine the dynamics of a crystalline tunnelling defect strongly coupled to phonons incorporating the full 3D geometry in our description. We find that inversion symmetric tunnelling is not dressed by phonons whereas other tunnelling pathways are dressed by phonons and, thus, are suppressed by strong defect-phonon coupling. We provide the linear acoustic and dielectric response functions for a crystalline tunnelling defect for strong defect-phonon coupling. This allows direct experimental determination of the defect-phonon coupling. The singling out of inversion-symmetric tunneling states in single tunneling defects is complementary to their dominance of the low energy excitations in strongly disordered solids as a result of inter-defect interactions for large defect concentrations. This suggests that inversion symmetric two-level systems play a unique role in the low energy properties of disordered solids.

cond-mat.mes-hall↗

arXiv2015

Quantum Kibble-Zurek physics in the presence of spatially-correlated dissipation

We study how universal properties of quantum quenches across critical points are modified by a weak coupling to thermal dissipation, focusing on the paradigmatic case of the transverse field Ising model. Beyond the standard quench-induced Kibble-Zurek defect production in the absence of the bath, the bath contributes extra thermal defects. We show that spatial correlations in the noise produced by the bath can play a crucial role: one obtains quantitatively different scaling regimes depending on whether the correlation length of the noise is smaller or larger than the Kibble-Zurek length associated with the quench speed, and the thermal length set by temperature. For the case of spatially-correlated bath noise, additional thermal defect generation is restricted to a window that is both quantum critical and excluded from the non-equilibrium regime surrounding the critical point. We map the dissipative quench problem to a set of effectively independent dissipative Landau-Zener problems. Using this mapping along with both analytic and numerical calculations allows us to find the scaling of the excess defect density produced in the quench, and suggests a generic picture for such dissipative quenches.

cond-mat.quant-gas↗

arXiv2014

Probing chirality fluctuations in molecules by nonlinear optical spectroscopy

Symmetry breaking caused by geometric fluctuations can enable processes that are otherwise forbidden. An example is a perylene bisimide dyad whose dipole moments are perpendicular to each other. Förster-type energy transfer is thus forbidden at the equilibrium geometry since the dipolar coupling vanishes. Yet, fluctuations of the geometric arrangement have been shown to induce finite energy transfer that depends on the dipole variance, rather than the mean. We demonstrate an analogous effect associated with chirality symmetry breaking. In its equilibrium geometry this dimer is non chiral. The linear chiral response which depends on the average geometry thus vanishes. However, we show that certain 2D chiral optical signals are finite due to geometric fluctuations. Furthermore, the correlation time of these fluctuations can be experimentally revealed by the waiting time dependence of the 2D signal.

physics.chem-ph↗

arXiv2014

Cooling a magnetic nanoisland by spin-polarized currents

We investigate cooling of a vibrational mode of a magnetic quantum dot by a spin-polarized tunneling charge current, exploiting the interaction between the magnetization and the vibration. The spin-polarized charge current polarizes the magnetic nanoisland, lowering its energy. Inevitable Ohmic energy losses due to the charge current flow will heat up the vibration. A small but finite coupling between the vibration and the local magnetic moment then permits an energy exchange, resulting in a lower energy, i.e., cooling, of the vibrational mode. We determine parameter regimes for the cooling of the vibration below $50\%$ of its initial value. Lowest final phonon temperature is observed for weak electron-phonon-coupling but similar magnetization-phonon coupling strength. The cooling rate, thereby, increases at first with the magnetization-phonon coupling and then saturates.

cond-mat.mes-hall↗

arXiv2014

Hydration shell effects in the relaxation dynamics of photoexcited Fe-II complexes in water

We study the relaxation dynamics of photoexcited Fe-II complexes dissolved in water and identify the relaxation pathway which the molecular complex follows in presence of a hydration shell of bound water at the interface between the complex and the solvent. Starting from a low-spin state, the photoexcited complex can reach the high-spin state via a cascade of different possible transitions involving electronic as well as vibrational relaxation processes. By numerically exact path integral calculations for the relaxational dynamics of a continuous solvent model, we find that the vibrational life times of the intermittent states are of the order of a few ps. Since the electronic rearrangement in the complex occurs on the time scale of about 100 fs, we find that the complex first rearranges itself in a high-spin and highly excited vibrational state, before it relaxes its energy to the solvent via vibrational relaxation transitions. By this, the relaxation pathway can be clearly identified. We find that the life time of the vibrational states increases with the size of the complex (within a spherical model), but decreases with the thickness of the hydration shell, indicating that the hydration shell acts as an additional source of fluctuations.

physics.atm-clus↗

arXiv2014

Elastic response of [111]-tunneling impurities

We study the dynamic response of a [111] quantum impurity, such as lithium or cyanide in alkali halides, with respect to an external field coupling to the elastic quadrupole moment. Because of the particular level structure of a eight-state system on a cubic site, the elastic response function shows a biexponential relaxation feature and a van Vleck type contribution with a resonance frequency that is twice the tunnel frequency $Δ/\hbar$. This basically differs from the dielectric response that does not show relaxation. Moreover, we show that the elastic response of a [111] impurity cannot be reduced to that of a two-level system. In the experimental part, we report on recent sound velocity and internal friction measurements on KCl doped with cyanide at various concentrations. At low doping (45 ppm) we find the dynamics of a single [111] impurity, whereas at higher concentrations (4700 ppm) the elastic response rather indicates strongly correlated defects. Our theoretical model provides a good description of the temperature dependence of $δv/v$ and $Q^{-1}$ at low doping, in particular the relaxation peaks, the absolute values of the amplitude, and the resonant contributions. From our fits we obtain the value of the elastic deformation potential $γ_t=0.192$ eV.

cond-mat↗

arXiv2013

Vibronic speed-up of the excitation energy transfer in the Fenna-Matthews-Olson complex

We show that the efficient excitation energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions is fueled by underdamped vibrations of the embedding proteins. For this, we present numerically exact results for the quantum dynamics of the excitons in the presence of nonadiabatic vibrational states in the Fenna-Matthews-Olson aggregate employing a environmental fluctuation spectral function derived from experiments. Assuming the prominent 180 cm$^{-1}$ vibrational mode to be underdamped, we observe, on the one hand, besides vibrational coherent oscillations between different excitation levels of the vibration also prolonged electronic coherent oscillations between the initially excited site and its neighbours. On the other hand, however, the underdamped vibrations provide additional channels for the excitation energy transfer and by this increase the transfer speed by up to $30\%$ .

physics.chem-ph↗

arXiv2013

Nonequilibrium quantum fluctuation relations for harmonic systems in nonthermal environments

We formulate exact generalized nonequilibrium fluctuation relations for the quantum mechanical harmonic oscillator coupled to multiple harmonic baths. Each of the different baths is prepared in its own individual (in general nonthermal) state. Starting from the exact solution for the oscillator dynamics we study fluctuations of the oscillator position as well as of the energy current through the oscillator under general nonequilibrium conditions. In particular, we formulate a fluctuation-dissipation relation for the oscillator position autocorrelation function that generalizes the standard result for the case of a single bath at thermal equilibrium. Moreover, we show that the generating function for the position operator fullfills a generalized Gallavotti-Cohen-like relation. For the energy transfer through the oscillator, we determine the average energy current together with the current fluctuations. Finally, we discuss the generalization of the cumulant generating function for the energy transfer to nonthermal bath preparations.

quant-ph↗

arXiv2012

Nonequilibrium Landau-Zener-Stuckelberg spectroscopy in a double quantum dot

We study theoretically nonequilibrium Landau-Zener-Stückelberg (LZS) dynamics in a driven double quantum dot (DQD) including dephasing and, importantly, energy relaxation due to environmental fluctuations. We derive effective nonequilibrium Bloch equations. These allow us to identify clear signatures for LZS oscilations observed but not recognized as such in experiments [Petersson et al., Phys. Rev. Lett. 105, 246804, 2010] and to identify the full environmental fluctuation spectra acting on a DQD given experimental data as in [Petersson et al., Phys. Rev. Lett. 105, 246804, 2010]. Herein we find that super-Ohmic fluctuations, typically due to phonons, are the main relaxation channel for a detuned DQD whereas Ohmic fluctuations dominate at zero detuning.

cond-mat.mes-hall↗

arXiv2012

Competition between relaxation and external driving in the dissipative Landau-Zener problem

We study Landau-Zener transitions in a dissipative environment by means of the quasiadiabatic propagator path-integral scheme. It allows to obtain numerically exact results for the full range of the involved parameters. We discover a nonmonotonic dependence of the Landau-Zener transition probability on the sweep velocity which is explained in terms of a simple physical picture. This feature results from a nontrivial competition between relaxation processes and the external sweep and is not captured by perturbative approaches. In addition to the Landau-Zener transition probability, we study the excitation survival probability and also provide a qualitative understanding of the involved competition of time scales.

cond-mat.stat-mech↗

arXiv2010

Ultraslow quantum dynamics in a sub-Ohmic heat bath

We show that the low-frequency modes of a sub-Ohmic bosonic heat bath generate an effective dynamical asymmetry for an intrinsically symmetric quantum spin -1/2. An initially fully polarized spin first decays towards a quasiequilibrium determined by the dynamical asymmetry, thereby showing coherent damped oscillations on the (fast) time scale of the spin splitting. On top of this, the dynamical asymmetry itself decays on an ultraslow time scale and vanishes asymptotically since the global equilibrium phase is symmetric. We quantitatively study the nature of the initial fast decay to the quasiequilibrium and discuss the features of ultraslow dynamics of the quasiequilibrium itself. The dynamical asymmetry is more pronounced for smaller values of the sub-Ohmic exponent and for lower temperatures, which emphasizes the quantum many-body nature of the effect. The symmetry breaking is related to the dynamic crossover between coherent and overdamped relaxation of the spin polarization and is not connected to the localization quantum phase transition. In addition to this delocalized phase, we identify a novel phase which is characterized by damped coherent oscillations in the localized phase. This allows for a sketch of the zero-temperature phase diagram of the sub-Ohmic spin-boson model with four distinct phases.

cond-mat.stat-mech↗

arXiv2010

Quantum coherent biomolecular energy transfer with spatially correlated fluctuations

We show that the quantum coherent transfer of excitations between biomolecular chromophores is strongly influenced by spatial correlations of the environmental fluctuations. The latter are due either to propagating environmental modes or to local fluctuations with a finite localization length. A simple toy model of a single donor-acceptor pair with spatially separated chromophore sites allows to investigate the influence of these spatial correlations on the quantum coherent excitation transfer. The sound velocity of the solvent determines the wave lengths of the environmental modes, which, in turn, has to be compared to the spatial distance of the chromophore sites. When the wave length exceeds the distance between donor and acceptor site, we find strong suppression of decoherence. In addition, we consider two spatially separated donor-acceptor pairs under the influence of propagating environmental modes. Depending on their wave lengths fixed by the sound velocity of the solvent material, the spatial range of correlations may extend over typical interpair distances, which can lead to an increase of the decohering influence of the solvent. Surprisingly, this effect is counteracted by increasing temperature.

cond-mat.mes-hall↗

arXiv2009

Breakdown of perturbative weak coupling approaches for the biomolecular energy transfer

We show that the biomolecular exciton dynamics under the influence of slow polarization fluctuations in the solvent cannot be described by approaches which are perturbative in the system-bath coupling. For this, we compare results for the decoherence rate of the exciton dynamics of a resumed perturbation theory with numerically exact real-time path-integral results. We find up to one order in magnitude difference in the decoherence rate for realistically slow solvent environments even in the weak coupling regime, while both results coincide for fast environmental noise. This shows explicitely the nonperturbative influence of the bioenvironmental fluctuations and might render current perturbative approaches to biomolecular exciton transport questionable.

cond-mat.soft↗

arXiv2003

Memory effects in Amorphous Solids below 20 mK

At temperatures below 1 K, the capacitance of a glass sample changes due to the application of a DC field in accordance with A. Burins dipole gap theory. However, we now report that below 20 mK, during the first sweep cycle of the DC electric field the capacitance is smaller by about $10^{-5}$ compared to any subsequent sweep. Despite this overall shift the field dependence follows the dipole gap predictions. In a subsequent sweep to higher DC fields the dielectric constant drops by about $10^{-5}$ as soon as the applied field is higher than any field previously applied. A picture involving the dynamics of resonant pairs provides a qualitative description of this behavior.

cond-mat↗

arXiv2002

Memory-effect in glasses at low temperatures

The dielectric constant of amorphous solids at low temperatures is governed by the dynamics of tunneling systems, small groups of atoms which tunnel between quasi equivalent potential minima. Recent experiments showed that at temperatures below 20 mK various glasses exhibit memory for a previously applied electric bias field. A first sweep of an electric bias field may prepare resonant pairs of tunneling systems, which are temporarily formed during the sweep, in metastable states. In subsequent sweeps the same resonant pairs thus significantly contribute to the dielectric constant, leading to a higher dielectric constant. We investigate the dynamics of resonant pairs during a bias field sweep yielding a qualitative explanation of the memory effect.

cond-mat.dis-nn↗

arXiv2002

Dynamics of the Destruction and Rebuilding of a Dipole Gap in Glasses

After a strong electric bias field was applied to a glass sample at temperatures in the millikelvin range its AC-dielectric constant increases and then decays logarithmically with time. For the polyester glass mylar we have observed the relaxation of the dielectric constant back to its initial value for several temperatures and histories of the bias field. Starting from the dipole gap theory we have developed a model suggesting that the change of the dielectric constant after transient application of a bias field is only partly due to relaxational processes. In addition, non-adiabatic driving of tunneling states (TSs) by applied electric fields causes long lasting changes in the dielectric constant. Moreover, our observations indicate that at temperatures below 50 mK the relaxation of TSs is caused primarily by interactions between TSs.

cond-mat.dis-nn↗

arXiv2001

Dephasing of interacting tunneling systems

We investigate the phase coherence time of weakly interacting tunneling systems (TSs). We show that all neighbors of a given TS form together with the TS of interest an entangled cluster as long as the linewidth of the excitation of the neighbor is smaller than its interaction energy with the TS of interest. Thus, the relaxation of all neighbors in the cluster contributes to the dephasing of the TS of interest. This mechanism dominates the transversal decay of the TSs and it explains recent two-pulse echo experiments in which the exponential decay rate could not be explained within spectral diffusion consistent with internal friction data. However, since the proposed mechanism predicts only an exponential decay, the Gaussian like decay at short times remains unexplained.

cond-mat.dis-nn↗

arXiv2001

Dephasing of entangled qubits

We investigate the influence of nearby two-level systems on the dynamics of a qubit. The intrinsic decoherence is given by a coupling of both the qubit and the two-level systems to a heat bath. Assuming weak interactions between the qubit and the two-level systems we show that the dephasing of the qubit follows a multi-exponential decay. For a flat distribution of energy splittings of the two-level systems the multi-exponential behavior simplifies to a good approximation to a single exponential decay. The rate is given by the sum of the dephasing rate of the isolated qubit plus all relaxation rates for the two-level systems out of their present state. This latter contribution sums up to a rate approximately given by the mean coupling between the qubit and the two-level systems times the number of thermally active neighbors.

cond-mat.stat-mech↗