arXiv subjects

Michael Galperin

Publications and source records attributed to Michael Galperin.

At least 19 recordsLinked to original sources

arXiv2026

Mixed quantum-classical evolution in open molecular systems

We consider a mixed quantum-classical formulation for open nonequilibrium molecular systems. We employ the pseudoparticle nonequilibrium Green's function (PP-NEGF) method as a fully quantum description and a starting point for introducing classical nuclear dynamics for an open system. We use this formulation to derive a density-matrix equation of motion describing the quantum-classical evolution. We compare our results with previous formulations and highlight important differences related to the nonequilibrium and open character of the molecular system. We also analyze the approximations needed to reduce full quantum-classical dynamics to the fewest-switches surface hopping (FSSH) method and show that these approximations are unreasonable when adiabatic surfaces come close to each other.

physics.chem-ph↗

arXiv2024

Control of open quantum systems: Manipulation of a qubit coupled to a thermal bath by an external driving field

Fast and reliable manipulation with qubits is fundamental for any quantum technology. The implementation of these manipulations in physical systems is the focus of studies involving optimal control theory. Realistic physical devices are open quantum systems. So far, studies in optimal control theory have primarily utilized the Redfield/Lindblad quantum master equation to simulate the dynamics of such systems. However, this Markov description is not always sufficient. Here, we present a study of qubit control utilizing the nonequilibrium Green's function method. We compare the traditional master equation with more general Green's function results and demonstrate that even in the parameter regime suitable for the application of the Redfield/Lindblad approach, the two methods yield drastically different results when addressing evolution involving mixed states. In particular, we find that, in addition to predicting different optimal driving profiles, a more accurate description of system evolution enables the system to reach the desired final state much more quickly. We argue that the primary reason for this is the significance of the non-Markov description of driven system dynamics due to the effect of time-dependent driving on dissipation.

quant-ph↗

arXiv2024

Two-dimensional spectroscopy of open quantum systems

Two-dimensional spectroscopy is discussed for open quantum systems with multiple simultaneously measurable fluxes. In particular, we discuss a junction where optical measurements of photon flux are complemented with simultaneous transport measurements of electron currents. Theory of two-dimensional spectroscopy in both fluxes is developed employing non-self-consistent nonequilibrium Green's function formulation. Theoretical derivations are illustrated with numerical simulations within generic junction model.

cond-mat.mes-hall↗

arXiv2024

Nonlinear optical spectroscopy of open quantum systems

Development of experimental techniques at nanoscale resulted in ability to perform spectroscopic measurements on single-molecule current carrying junctions. These experiments are natural meeting point for research fields of optical spectroscopy and molecular electronics. We present a pedagogical comparison between perturbation theory expansion of standard nonlinear optical spectroscopy and (non-self-consistent) perturbative diagrammatic formulation of the nonequilibrium Green's functions method (NEGF is widely used in molecular electronics) indicating their similarities and differences. Comparing the two approaches we argue that optical spectroscopy of open quantum systems has to be analyzed within the more general Green's function formulation.

cond-mat.mes-hall↗

arXiv2024

Liouvillian exceptional points of an open driven two-level system

We study the applicability of the Liouvillian exceptional points (LEPs) approach to nanoscale open quantum systems. A generic model of the driven two-level system in a thermal environment is analyzed within the nonequilibrium Green's function (NEGF) and Bloch quantum master equation (QME) formulations. We derive the latter starting from the exact NEGF Dyson equations and highlight the qualitative limitations of the LEP treatment by examining the approximations employed in its derivation. We find that non-Markov character of evolution in open quantum systems does not allow for the introduction of the concept of exceptional points for a description of their dynamics. Theoretical analysis is illustrated with numerical simulations.

quant-ph↗

arXiv2023

Quantum Thermodynamics: Inside-Outside Perspective

We introduce an energy-resolved variant of quantum thermodynamics for open systems strongly coupled to their baths. The approach generalizes the Landauer-Buttiker inside-outside duality method [Phys. Rev. Lett. 120, 107701 (2018)] to interacting systems subjected to arbitrary external driving. It is consistent with the underlying dynamical quantum transport description and is capable of overcoming limitations of the only other consistent approach [New J. Phys. 12, 013013 (2010)]. We illustrate viability of the generalized inside-outside method with numerical simulations for generic junction models.

cond-mat.stat-mech↗

arXiv2023

Exceptional points treatment of cavity spectroscopies

The infrared response of a system of two vibrational modes in a cavity is calculated by an effective non-Hermitian Hamiltonian derived by employing the nonequilibrium Green's functions (NEGF) formalism. Degeneracies of the Hamiltonian (exceptional points, EP) widely employed in theoretical analysis of optical cavity spectroscopies are used in an approximate treatment and compared with the full NEGF. Qualitative limitations of the EP treatment are explained by examining the approximations employed in the calculation.

cond-mat.mes-hall↗

arXiv2020

Entropy and information flow in quantum systems strongly coupled to baths

Considering von Neumann expression for reduced density matrix as thermodynamic entropy of a system strongly coupled to baths, we use nonequilibrium Green's function (NEGF) techniques to derive bath and energy resolved expressions for entropy, entropy production, and information flows. The consideration is consistent with dynamic (quantum transport) description and expressions reduce to expected forms in limiting cases of weak coupling or steady-state. Formulation of the flows in terms of only system degrees freedom is convenient for simulation of thermodynamic characteristics of open nonequilibrium quantum systems. We utilize standard NEGF for derivations in noninteracting systems, Hubbard NEGF is used for interacting systems. Theoretical derivations are illustrated with numerical simulations within generic junction models.

cond-mat.mes-hall↗

arXiv2020

Nonequilibrium dual-boson approach

We develop nonequilibrium auxiliary quantum master equation dual boson method (aux-DB), and argue that it presents a convenient way to describe steady states of correlated impurity models (such as single molecule optoelectronic devices) where electron and energy transport should be taken into account. The aux-DB is shown to provide high accuracy with relatively low numerical cost. Theoretical analysis is followed by illustrative simulations within generic two-level junction model, where the new scheme is benchmarked against numerically exact results.

cond-mat.mes-hall↗

arXiv2020

Green's functions perspective on nonequilibrium thermodynamics of open quantum systems strongly coupled to baths

We give nonequilibrium Green's function (NEGF) perspective on thermodynamics formulations for open quantum systems strongly coupled to baths. Scattering approach implying thermodynamic consideration of a super-system (system plus baths) weakly coupled to external super-baths is compared with consideration of thermodynamics of a system strongly coupled to its baths. We analyze both approaches from the NEGF perspective and argue that the latter yields a possibility of thermodynamic formulation consistent with dynamical (quantum transport) description.

cond-mat.mes-hall↗

arXiv2020

Green's function methods for single molecule junctions

We present a brief pedagogical review of theoretical Green's function methods applicable to open quantum systems out of equilibrium in general, and single molecule junctions in particular. We briefly describe experimental advances in molecular electronics, then discuss different theoretical approaches. We then focus on Green's function methods. Two characteristic energy scales governing the physics are many-body interactions within the junctions, and molecule-contact coupling. We therefore discuss weak interactions and weak coupling, as two limits that can be conveniently treated within, respectively, the standard nonequilibrium Green's function (NEGF) method and its many-body flavors (pseudoparticle and Hubbard NEGF). We argue that the intermediate regime, where the two energy scales are comparable, can in many cases be efficiently treated within the recently introduced superperturbation dual fermion approach. Finally, we review approaches for going beyond these analytically accessible limits, as embodied by recent developments in numerically exact methods based on Green's functions.

cond-mat.mes-hall↗

arXiv2020

Optical properties of periodically-driven open nonequilibrium quantum systems

Characterization and control of matter by optical means is at the forefront of research both due to fundamental insights and technological promise. Theoretical modeling of periodically driven systems is a prerequisite to understanding and engineering nanoscale quantum devices for quantum technologies. Here, we develop a theory for transport and optical response of molecular junctions, open nonequilibrium quantum systems, under external periodic driving. Periodic driving is described using the Floquet theory combined with nonequilibrium Green's function description of the system. Light-matter interaction is modeled employing the self-consistent Born approximation. Generic three-level model is utilized to illustrate effect of the driving on optical and transport properties of junctions.

cond-mat.mes-hall↗

arXiv2019

Markovian Treatment of non-Markovian Dynamics of Open Fermionic Systems

We show that an open fermionic system coupled to continuous environment with unitary system-environment evolution can be exactly mapped onto an auxiliary system consisting of the physical fermion system and a set of discrete fermionic modes subject to non-unitary Lindblad-type system-modes evolution in such a way that reduced dynamics of the fermionic system in the two cases are the same. Conditions for equivalence of reduced dynamics in the two systems are identified and a proof is presented. The study is extension of recent work on Bose systems [D. Tamascelli, A. Smirne, S. F. Huelga, and M. B. Plenio, Phys. Rev. Lett. 120, 030402 (2018)] to open quantum Fermi systems and to multi-time correlation functions. Numerical simulations within generic junction model are presented for illustration.

cond-mat.mes-hall↗

arXiv2019

Flux-conserving diagrammatic formulation of optical spectroscopy of open quantum systems

We present a theoretical approach to optical spectroscopy of open nonequilibrium systems, which generalizes traditional nonlinear optical spectroscopy tools by imposing charge and energy conservation at all levels of approximation. Both molecular and radiation field degrees of freedom are treated quantum mechanically. The formulation is based on the nonequilibrium Green's function (NEGF) approach and a double sided Feynman diagrammatic representation of the photon flux is developed. Numerical simulations are presented for a model system. Our study bridges the theoretical approaches of quantum transport and optical spectroscopy and establishes a firm basis for applying traditional tools of nonlinear optical spectroscopy in molecular optoelectronics.

cond-mat.mes-hall↗

arXiv2019

Numerically exact counting statistics of energy current in the Kondo regime

We use the inchworm Quantum Monte Carlo method to investigate the full counting statistics of particle and energy currents in a strongly correlated quantum dot. Our method is used to extract the heat fluctuations and entropy production of a quantum thermoelectric device, as well as cumulants of the particle and energy currents. The energy--particle current cross correlations reveal information on the preparation of the system and the interplay of thermal and electric currents. We furthermore demonstrate the signature of a crossover from Coulomb blockade to Kondo physics in the energy current fluctuations, and show how the conventional master equation approach to full counting statistics systematically fails to capture this crossover.

cond-mat.mes-hall↗

arXiv2019

Electron Transfer Methods in Open Systems

Utilization of electron transfer methods for description of quantum transport is popular due to simplicity of the formulation and its ability to account for basic physics of electron exchange between system and baths. At the same time, necessity to go beyond simple golden rule-type expressions for rates was indicated in the literature and ad hoc formulations were proposed. Similarly, kinetic schemes for quantum transport beyond usual second order Lindblad/Redfield considerations were discussed. Here we utilize recently introduced by us nonequilibrium Hubbard Green's functions diagrammatic technique to analyze construction of rates in open systems. We show that previous considerations for rates of second and fourth order can be obtained as a particular case of zero and second order Green's function diagrammatic series with bare diagrams. We discuss limitations of previous considerations, stress advantages of the Hubbard Green's function approach in constructing the rates and indicate that standard dressing of the diagrams is a natural way to account for additional baths/degrees of freedom when formulating generalized expressions for the rates.

cond-mat.mes-hall↗

arXiv2019

Electronic friction in interacting systems

We consider effects of strong light-matter interaction on electronic friction in molecular junctions within generic model of single molecule nano cavity junction. Results of the Hubbard NEGF simulations are compared with mean-field NEGF and generalized Head-Gordon and Tully approaches. Mean-field NEGF is shown to fail qualitatively at strong intra-system interactions, while accuracy of the generalized Head-Gordon and Tully results is restricted to situations of well separated intra-molecular excitations, when bath induced coherences are negligible. Numerical results show effects of bias and cavity mode pumping on electronic friction. We demonstrate non-monotonic behavior of the friction on the bias and intensity of the pumping field and indicate possibility of engineering friction control in single molecule junctions.

cond-mat.mes-hall↗

arXiv2019

Hubbard NEGF Analysis of Photocurrent in Nitroazobenzene Molecular Junction

We present combined experimental and theoretical study of photo-induced current in molecular junctions consisting of monolayers of nitroazobenzene oligomers chemisorbed on carbon surfaces and illuminated by UV-Vis light through a transparent electrode. Experimentally observed dependence of the photocurrent on light frequency, temperature and monolayer thickness is analyzed within first principles simulations employing the Hubbard NEGF diagrammatic technique. We reproduce qualitatively correct behavior and discuss mechanisms leading to characteristic behavior of dark and photo-induced currents in response to changes in bias, frequency of radiation, temperature and thickness of molecular layer.

cond-mat.mes-hall↗