Full-counting statistics of energy transport of molecular junctions in the polaronic regime

We investigate the full-counting statistics (FCS) of energy transport carried by electrons in molecular junctions for the Anderson-Holstein model in the polaronic regime. Using two-time quantum measurement scheme, generating function (GF) for the energy transport is derived and expressed as a Fredholm determinant in terms of Keldysh nonequilibrium Green's function in the time domain. Dressed tunneling approximation is used in decoupling the phonon cloud operator in the polaronic regime. This formalism enables us to analyze the time evolution of energy transport dynamics after a sudden switch-on of the coupling between the dot and the leads towards the stationary state. The steady state energy current cumulant GF in the long time limit is obtained in the energy domain as well. Universal relations for steady state energy current FCS are derived under finite temperature gradient with zero bias and this enables us to express the equilibrium energy current cumulant by a linear combination of lower order cumulants. Behaviors of energy current cumulants in steady state under temperature gradient and external bias are numerically studied and explained. Transient dynamics of energy current cumulants is numerically calculated and analyzed. The universal scaling of normalized transient energy cumulants is found under both temperature gradient and external bias

Thermodynamics of energy, charge and spin currents in thermoelectric quantum-dot spin valve

We provide a thermodynamically consistent description of energy, charge and spin transfers in a thermoelectric quantum-dot spin valve in the collinear configuration based on nonequilibrium Green's function and full counting statistics. We use the fluctuation theorem symmetry and the concept of entropy production to characterize the efficiency with which thermal gradients can transduce charges or spins against their chemical potentials, arbitrary far from equilibrium. Close to equilibrium, we recover the Onsager reciprocal relations and the connection to linear response notions of performance such as the figure of merit. We also identify regimes where work extraction is more efficient far then close from equilibrium.Comment: 13 pages, 4 figures; accepted in Phys. Rev.

The Role of Defects in the Metal-Nonmetal Transition in Metallic Oxide Films

Metal-nonmetal transition, or more specifically, metal-insulator transition (MIT) has been one of the most intriguing topics in condensed matter physics. Two theories describing fundamental driving mechanisms of MIT has been well-established over time: Mott-Hubbard theory and Anderson localization theory. The former mainly deals with contribution of electron interactions/correlations to the MIT, and the latter focuses on the role of disorder. However, it is an open topic how a system behaves when both effects exist in a system. This study mostly takes interest in a type of MIT induced by dimensionality-crossover in the transition metal oxides (TMOs) systems. TMO system is a perfect playground for studying the underlying mechanisms behind MIT due to wide presence of strong electron interactions in the d-band, and the existence of oxygen vacancies as an unavoidable form of disorder. The focus of this study is mainly on the role played by disorder. A metallic TMO system SrVO3 (SVO) was chosen to perform the investigation due to its simple structure and lack of magnetic ordering. Well-ordered SVO thin films have been fabricated in a layer-by-layer fashion on crystalline SrTiO3 (001) substrates. Surface structural characterization and morphology images suggest that the SVO films are of high quality with correct symmetry and atomically flat surfaces. The structural and chemical composition characterization indicates the existence of a significant amount of oxygen vacancies in the first three layers of the SVO films, coinciding with the critical thickness for the MIT, which has been confirmed by spectroscopic analysis which reveals zero density of states at the Fermi level for films with thickness below 3 unit cell (u.c.). Transport measurements reveal weakly localized lnT behavior for metallic SVO films close to the critical thickness, agreeing with the picture of a 2D disordered correlated system. Negative magnetoresistance observed in the weakly localized films is consistent with the prediction that disorder dominates over correlation effects. Moreover, by deliberately introducing more disorder into metallic SVO films, MIT can also be induced. Through our research, we conclude that the disorder effect is the major driving mechanism for MIT

Short time dynamics of molecular junctions after projective measurement

In this work, we study the short time dynamics of a molecular junction described by Anderson-Holstein model using full-counting statistics after projective measurement. The coupling between the central quantum dot (QD) and two leads was turned on at remote past and the system is evolved to steady state at time $t=0$, when we perform the projective measurement in one of the lead. Generating function for the charge transfer is expressed as a Fredholm determinant in terms of Keldysh nonequilibrium Green's function in the time domain. It is found that the current is not constant at short times indicating that the measurement does perturb the system. We numerically compare the current behaviors after the projective measurement with those in the transient regime where the subsystems are connected at $t=0$. The universal scaling for high-order cumulants is observed for the case with zero QD occupation due to the unidirectional transport at short times. The influences of electron-phonon interaction on short time dynamics of electric current, shot noise and differential conductance are analyzed

Three-terminal normal-superconductor junction as thermal transistor

We propose a thermal transistor based on a three-terminal normal-superconductor (NS) junction with superconductor terminal acting as the base. The emergence of heat amplification is due to the negative differential thermal conductance (NDTC) effect for the NS diode in which the normal side maintains a higher temperature. The temperature dependent superconducting energy gap is responsible for the NDTC. By controlling quantum dot levels and their coupling strengths to the terminals, a huge heat amplification factor can be achieved. The setup offers an alternative tuning scheme of heat amplification factor and may find use in cryogenic applications.Comment: 6 pages, 3 figure

Modulating near-field thermal transfer through temporal drivings: a quantum many-body theory

The traditional approach to studying near-field thermal transfer is based on fluctuational electrodynamics. However, this approach may not be suitable for nonequilibrium states due to dynamic drivings. In our work, we introduce a theoretical framework to describe the phenomenon of near-field heat transfer between two objects when subjected to periodic time modulations. We utilize the machinery of nonequilibrium Green's function to derive general expressions for the DC energy current in Floquet space. Furthermore, we also obtain the energy current under the condition of small driving amplitude. The external drivings create a nonequilibrium state, which gives rise to various effects such as heat-transfer enhancement, heat-transfer suppression, and cooling. To illustrate these phenomena, we conduct numerical calculations on a system of Coulomb-coupled quantum dots, and specifically investigate the scenario of periodically driving electronic reservoir. In our calculations, we employ the $G_0W_0$ approximation, which does not require self-consistent iteration and is suitable for weak Coulomb interaction. Our theoretical formalism can be applied to study near-field energy transfer between two metallic plates under periodic time modulations.Comment: 12 pages, 2 figure