Pseudomodes and the corresponding transformation of the temperature-dependent bath correlation function

In open system approaches with non-Markovian environments, the process of inserting an individual mode (denoted as "pseudomode") into the bath or extracting it from the bath is widely employed. This procedure, however, is typically performed on basis of the spectral density (SD) and does not incorporate temperature. Here, we show how the - temperature-dependent - bath correlation function (BCF) transforms in such a process. We present analytic formulae for the transformed BCF and numerically study the differences between factorizing initial state and global thermal (correlated) initial state of mode and bath, respectively. We find that in the regime of strong coupling of the mode to both system and bath, the differences in the BCFs give rise to pronounced differences in the dynamics of the system.Comment: 12 pages, 4 figure

Excitonic Wave Function Reconstruction from Near-Field Spectra Using Machine Learning Techniques

A general problem in quantum mechanics is the reconstruction of eigenstate wave functions from measured data. In the case of molecular aggregates, information about excitonic eigenstates is vitally important to understand their optical and transport properties. Here we show that from spatially resolved near field spectra it is possible to reconstruct the underlying delocalized aggregate eigenfunctions. Although this high-dimensional nonlinear problem defies standard numerical or analytical approaches, we have found that it can be solved using a convolutional neural network. For both one-dimensional and two-dimensional aggregates we find that the reconstruction is robust to various types of disorder and noise

Probing weak dipole-dipole interaction using phase-modulated non-linear spectroscopy

Phase-modulated non-linear spectroscopy with higher harmonic demodulation has recently been suggested to provide information on many-body excitations. In the present work we theoretically investigate the application of this method to infer the interaction strength between two particles that interact via weak dipole-dipole interaction. To this end we use full numerical solution of the Schr\"odinger equation with time-dependent pulses. For interpretation purpose we also derive analytical expressions in perturbation theory. We find one can detect dipole-dipole interaction via peak intensities (in contrast to line-shifts which typically are used in conventional spectroscopy). We provide a detailed study on the dependence of these intensities on the parameters of the laser pulse and the dipole-dipole interaction strength. Interestingly, we find that there is a phase between the first and second harmonic demodulated signal, whose value depends on the sign of the dipole-dipole interaction.Comment: 12 pages, 8 figures, Supporting information provided with the source file

Analytic Representations of Bath Correlation Functions for Ohmic and Superohmic Spectral Densities Using Simple Poles

We present a scheme to express a bath correlation function (BCF) corresponding to a given spectral density (SD) as a sum of damped harmonic oscillations. Such a representation is needed, for example, in many open quantum system approaches. To this end we introduce a class of fit functions that enables us to model ohmic as well as superohmic behavior. We show that these functions allow for an analytic calculation of the BCF using pole expansions of the temperature dependent hyperbolic cotangent. We demonstrate how to use these functions to fit spectral densities exemplarily for cases encountered in the description of photosynthetic light harvesting complexes. Finally, we compare absorption spectra obtained for different fits with exact spectra and show that it is crucial to take properly into account the behavior at small frequencies when fitting a given SD.Comment: 16 pages, 9 figure

Vibronic Lineshapes of PTCDA Oligomers in Helium Nanodroplets

Oligomers of the organic semiconductor PTCDA are studied by means of helium nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasi continuous spectra, resembling the broad oligomer spectra

Break-up of Rydberg superatoms via dipole-dipole interactions

We investigate resonant dipole-dipole interactions between two "superatoms" of different angular momentum, consisting of two Rydberg-blockaded atom clouds where each of them carries initially a coherently shared single excitation. We demonstrate that the dipole-dipole interaction breaks up the superatoms by removing the excitations from the clouds. The dynamics is akin to an ensemble average over systems where only one atom per cloud participates in entangled motion and excitation transfer. Our findings should thus facilitate the experimental realization of adiabatic exciton transport in Rydberg systems by replacing single sites with atom clouds.Comment: 10 pages, 5 figure

Non-Markovian Quantum State Diffusion for Temperature-Dependent Linear Spectra of Light Harvesting Aggregates

Non-Markovian Quantum State Diffusion (NMQSD) has turned out to be an efficient method to calculate excitonic properties of aggregates composed of organic chromophores, taking into account the coupling of electronic transitions to vibrational modes of the chromophores. NMQSD is an open quantum system approach that incorporates environmental degrees of freedom (the vibrations in our case) in a stochastic way. We show in this paper that for linear optical spectra (absorption, circular dichroism) no stochastics is needed, even for finite temperatures. Thus, the spectra can be obtained by propagating a single trajectory. To this end we map a finite temperature environment to the zero temperature case using the so-called thermofield method. The resulting equations can then be solved efficiently by standard integrators.Comment: 14 pages, 4 figure

Spectral properties of molecular oligomers. A non-Markovian quantum state diffusion approach

Absorption spectra of small molecular aggregates (oligomers) are considered. The dipole-dipole interaction between the monomers leads to shifts of the oligomer spectra with respect to the monomer absorption. The line-shapes of monomer as well as oligomer absorption depend strongly on the coupling to vibrational modes. Using a recently developed approach [Roden et. al, PRL 103, 058301] we investigate the length dependence of spectra of one-dimensional aggregates for various values of the interaction strength between the monomers. It is demonstrated, that the present approach is well suited to describe the occurrence of the J- and H-bands

Nanoelectromechanical rotary current rectifier

Nanoelectromechanical systems (NEMS) are devices integrating electrical and mechanical functionality on the nanoscale. Because of individual electron tunneling, such systems can show rich self-induced, highly non-linear dynamics. We show theoretically that rotor shuttles, fundamental NEMS without intrinsic frequencies, are able to rectify an oscillatory bias voltage over a wide range of external parameters in a highly controlled manner, even if subject to the stochastic nature of electron tunneling and thermal noise. Supplemented by a simple analytic model, we identify different operational modes of charge rectification. Intriguingly, the direction of the current depends sensitively on the external parameters