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Register a new userRelaxation Dynamics of Meso-Reservoirs
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We study the phenomenology of maximum-entropy meso-reservoirs, where we assume that their local thermal equilibrium state changes consistently with the heat transferred between the meso-reservoirs. Depending on heat and matter carrying capacities, the chemical potentials and temperatures are allowed to vary in time, and using global conservation relations we solve their evolution equations. We compare two-terminal transport between bosonic and fermionic meso-reservoirs via systems that tightly couple energy and matter currents and systems that do not. For bosonic reservoirs we observe the temporary formation of a Bose-Einstein condensate in one of the meso-reservoirs from an initial nonequilibrium setup.
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The efficient conversion of thermal energy to mechanical work by a heat engine is an ongoing technological challenge. Since the pioneering work of Carnot, it is known that the efficiency of heat engines is bounded by a fundamental upper limit, the Carnot limit. Theoretical studies suggest that heat engines may be operated beyond the Carnot limit by exploiting stationary, non-equilibrium reservoirs that are characterized by a temperature as well as further parameters. In a proof-of-principle experiment, we demonstrate that the efficiency of a nano-beam heat engine coupled to squeezed thermal noise is not bounded by the standard Carnot limit. Remarkably, we also show that it is possible to design a cyclic process that allows for extraction of mechanical work from a single squeezed thermal reservoir. Our results demonstrate a qualitatively new regime of non-equilibrium thermodynamics at small scales and provide a new perspective on the design of efficient, highly miniaturized engines.
We study how a system of one-dimensional spin-1/2 fermions at temperatures well below the Fermi energy approaches thermal equilibrium. The interactions between fermions are assumed to be weak and are accounted for within the perturbation theory. In the absence of an external magnetic field, spin degeneracy strongly affects relaxation of the Fermi gas. For sufficiently short-range interactions, the rate of relaxation scales linearly with temperature. Focusing on the case of the system near equilibrium, we linearize the collision integral and find exact solution of the resulting relaxation problem. We discuss the application of our results to the evaluation of the transport coefficients of the one-dimensional Fermi gas.
Non-Hermitian skin effect, namely that the eigenvalues and eigenstates of a non-Hermitian tight-binding Hamiltonian have significant differences under open or periodic boundary conditions, is a remarkable phenomenon of non-Hermitian systems. Inspired by the presence of the non-Hermitian skin effect, we study the evolution of wave-packets in non-Hermitian systems, which can be determined using the single-particle Greens function. Surprisingly, we find that in the thermodynamical limit, the Greens function does not depend on boundary conditions, despite the presence of skin effect. We proffer a general proof for this statement in arbitrary dimension with finite hopping range, with an explicit illustration in the non-Hermitian Su-Schrieffer-Heeger model. We also explore its applications in non-interacting open quantum systems described by the master equation, where we demonstrate that the evolution of the density matrix is independent of the boundary condition.
We study the spin dynamics in a high-mobility two-dimensional electron gas confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of the spin relaxation is found: as the magnetic field becomes stronger, the spin relaxation time first increases quadratically but then changes to a linear dependence, before it eventually becomes oscillatory, whereby the longitudinal and transverse times reach maximal values at even and odd filling Landau level factors, respectively. We show that the suppression of spin relaxation is due to the effect of electron gyration on the spin-orbit field, while the oscillations correspond to oscillations of the density of states appearing at low temperatures and high magnetic fields. The transition from quadratic to linear dependence can be related to a transition from classical to Bohm diffusion and reflects an anomalous behavior of the two-dimensional electron gas analogous to that observed in magnetized plasmas.
Transition metal dichalcogenides represent an ideal testbed to study excitonic effects, spin-related phenomena and fundamental light-matter coupling in nanoscopic condensed matter systems. In particular, the valley degree of freedom, which is unique to such direct band gap monolayers with broken inversion symmetry, adds fundamental interest in these materials. Here, we implement a Tamm-plasmon structure with an embedded MoSe2 monolayer and study the formation of polaritonic quasi-particles. Strong coupling conditions between the Tamm-mode and the trion resonance of MoSe2 are established, yielding bright luminescence from the polaritonic ground state under non-resonant optical excitation. We demonstrate, that tailoring the electrodynamic environment of the monolayer results in a significantly increased valley polarization. This enhancement can be related to change in recombination dynamics shown in time-resolved photoluminescence measurements. We furthermore observe strong upconversion luminescence from resonantly excited polariton states in the lower polariton branch. This upconverted polariton luminescence is shown to preserve the valley polarization of the trion-polariton, which paves the way towards combining spin-valley physics and exciton scattering experiments.
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