Thermoelectric study of dissipative quantum dot heat engines

This paper examines the thermoelectric response of a dissipative quantum dot heat engine based on the Anderson-Holstein model in two relevant operating limits: (i) when the dot phonon modes are out of equilibrium, and (ii) when the dot phonon modes are strongly coupled to a heat bath. In the first case, a detailed analysis of the physics related to the interplay between the quantum dot level quantization, the on-site Coulomb interaction and the electron-phonon coupling on the thermoelectric performance reveals that an n-type heat engine performs better than a p-type heat engine. In the second case, with the aid of the dot temperature estimated by incorporating a {\it{thermometer bath}}, it is shown that the dot temperature deviates from the bath temperature as electron-phonon interaction becomes stronger. Consequently, it is demonstrated that the dot temperature controls the direction of phonon heat currents, thereby influencing the thermoelectric performance. Finally, the conditions on the maximum efficiency with varying phonon couplings between the dot and all the other macroscopic bodies are analyzed in order to reveal the nature of the optimum junction.Comment: 10 pages, 9 figures, To be published in Phys Rev.

Quantum thermoelectrics based on 2-D Semi-Dirac materials

We show that a gap parameter can fully describe the merging of Dirac cones in semi-Dirac materials from $K$- and $K^\prime$-points into the common $M$-point in the Brillouin zone. We predict that the gap parameter manifests itself by enhancing the thermoelectric figure of merit $zT$ as the chemical potential crosses the gap followed by a sign change in the Seebeck coefficient around the same point. Subsequently, we show that there is also a trade-off feature between the maximum power delivered and the efficiency when the chemical potential crosses the gap parameter. An optimal operating point that minimizes the power-efficiency trade-off is consequently singled out for the best thermoelectric performance. Our work paves the way for the use of 2D semi-Dirac materials for thermoelectric applications.Comment: 5 pages, 5 figure

Role of dual nuclear baths on spin blockade leakage current bistabilities

Spin-blockaded electronic transport across a double quantum dot (DQD) system represents an important advancement in the area of spin-based quantum information. The basic mechanism underlying the blockade is the formation of a blocking triplet state. The bistability of the leakage current as a function of the applied magnetic field in this regime is believed to arise from the effect of nuclear Overhauser fields on spin-flip transitions between the blocking triplet and the conducting singlet states. The objective of this paper is to present the nuances of considering a two bath model on the experimentally observed current bistability by employing a self consistent simulation of the nuclear spin dynamics coupled with the electronic transport of the DQD set up. In doing so, we first discuss the important subtleties involved in the microscopic derivation of the hyperfine mediated spin flip rates. We then give insights as to how the differences between the two nuclear baths and the resulting difference Overhauser field affect the two-electron states of the DQD, and their connection with the experimentally observed current hysteresis curve.Comment: 9 pages, 5 figure

Optimized Fabry-P\'erot cavity engineered nanoscale thermoelectric generators

In this work, we aim to design a heterostructure based nanoscale thermoelectric generator that can maximize the waste-heat conversion efficiency at a given output power. The primary objective to be achieved for this is to realize a boxcar-shaped (bandpass) electronic transmission function (R. S. Whitney, Phys. Rev. Lett. 112, 130601 (2014)). In order to achieve that, we propose the use of an electronic analog of optical Fabry-P\'erot cavity over a central resonant tunneling structure. We further explore the optimum design possibilities by varying the geometry of the cavity wall to ensure a nearly perfect bandpass energy filtering of electrons. Based on our findings, we propose a general design guideline to realize such transmission and demonstrate that such devices can be excellent thermoelectric generators compared to the existing proposals in terms of boosting the output power without a cost in efficiency. It is theoretically demonstrated using the non-equilibrium Green's function technique coupled with self-consistent charging effects that an enhancement in the maximum output power up to $116\%$ can be achieved through this scheme at a $10\%$ higher efficiency as compared to resonant tunneling based devices. Furthermore, an elaborate comparative study of the linear response parameters is also presented and explained in terms of the physical transport properties. This study suggests an optimal device design strategy for an improved thermoelectric generator and sets the stage for a new class of thermoelectric generators facilitated via transmission lineshape engineering

Optimal Single Quantum Dot Heat-to-pure-spin-current Converters

We delve into the conditions under which a quantum dot thermoelectric setup may be tuned to realize an optimal heat-to-pure-spin-current converter. It is well known that a heat-to-pure-spin-current converter may be realized using a non-interacting quantum dot with a spin-split energy spectrum under particle hole symmetry conditions. However, with the inclusion of Coulomb interaction $U$, ubiquitous in typical quantum dot systems, the relevant transport physics is expected to be altered. In this work, we provide a detailed picture of thermoelectric pure spin currents at various Coulomb interaction parameters $U$ and describe the conditions necessary for an exact cancellation of charge transport between energy levels $\epsilon$ and their Coulomb-charged partner levels $\epsilon+U$, so as to yield the largest terminal pure spin currents. A non-trivial aspect pointed out here is that at sufficiently large values of $U$ ($\ge U_0$), pure spin currents tend to optimize at points other than where the particle-hole symmetry occurs. It is also ascertained that a global maximum of pure spin current is generated at a typical value of the interaction parameter $U$. These optimum conditions may be easily realized using a typical gated quantum dot thermoelectric transport setupComment: 8 pages, 5 figures in Physica B (2015