Thermopower oscillations of a quantum-point contact

Power, Efficiency and Fluctuations in a Quantum Point Contact as Steady-State Thermoelectric Heat Engine

The trade-off between large power output, high efficiency and small fluctuations in the operation of heat engines has recently received interest in the context of thermodynamic uncertainty relations (TURs). Here we provide a concrete illustration of this trade-off by theoretically investigating the operation of a quantum point contact (QPC) with an energy-dependent transmission function as a steady-state thermoelectric heat engine. As a starting point, we review and extend previous analysis of the power production and efficiency. Thereafter the power fluctuations and the bound jointly imposed on the power, efficiency, and fluctuations by the TURs are analyzed as additional performance quantifiers. We allow for arbitrary smoothness of the transmission probability of the QPC, which exhibits a close to step-like dependence in energy, and consider both the linear and the non-linear regime of operation. It is found that for a broad range of parameters, the power production reaches nearly its theoretical maximum value, with efficiencies more than half of the Carnot efficiency and at the same time with rather small fluctuations. Moreover, we show that by demanding a non-zero power production, in the linear regime a stronger TUR can be formulated in terms of the thermoelectric figure of merit. Interestingly, this bound holds also in a wide parameter regime beyond linear response for our QPC device.

Generating pure spin current with spin-dependent Seebeck effect in ferromagnetic zigzag graphene nanoribbons

Pure spin current is of great importance in spintronics and may be achieved by spin dependent Seebeck effect (SDSE) in magnetic systems. Zigzag-edged graphene nanoribbons (ZGNRs) are very well-known 2D magnetic nanostructures. However, perfect and pristine ZGNRs either in the anti-ferromagnetic ground state or in the ferromagnetic (FM) state are not capable of producing pure spin current by SDSE at low temperature. In this work, by density functional theory calculations, we propose a scheme for generating pure spin current using SDSE in FM-ZGNRs by introducing antidots. Specifically, by creating a hexagonal antidot with either armchair edges or zigzag edges in the scattering region, we can get finite Seebeck thermopower for both spin channels with opposite signs, leading to the opposite flow directions of the two spin channels. The mechanism is well explained by the cooperation of the varying localization features of states around the Fermi level and the antidot induced scattering potential. By slightly tuning the chemical potential, pure spin current can be achieved. The size and edge shape effects have also been systematically studied. The findings indicate a novel scheme for thermally generating pure spin current in zigzag graphene nanoribbons and may find important application in graphene based spintronics.

Extracting the Energy Sensitivity of Charge Carrier Transport and Scattering

It is a challenge to extract the energy sensitivity of charge carriers' transport and scattering from experimental data, although a theoretical estimation in which the existing scattering mechanism(s) are preliminarily assumed can be easily done. To tackle this problem, we have developed a method to experimentally determine the energy sensitivities, which can then serve as an important statistical measurement to further understand the collective behaviors of multi-carrier transport systems. This method is validated using a graphene system at different temperatures. Further, we demonstrate the application of this method to other two-dimensional (2D) materials as a guide for future experimental work on the optimization of materials performance for electronic components, Peltier coolers, thermoelectricity generators, thermocouples, thermopiles, electrical converters and other conductivity and/or Seebeck-effect-related sensors.

Enhanced thermoelectric figure of merit in thin GaAs nanowires

Combining density functional theory and the nonequilibrium Green's function method, we investigate the thermoelectric properties of thin GaAs nanowires (NWs). After identifying the most stable structures for GaAs NWs, either in wurtzite (wz) or zinc blende (zb) stacking, we present a systematic analysis on the thermoelectric properties of these NWs and their dependence on stacking type (wz or zb), size of NWs, and temperature. Although bulk GaAs is a well-known poor thermoelectric material, the thermoelectric figure of merit, ZT, is significantly enhanced in thin GaAs NWs. Typically, the room temperature ZT of a 1.1 nm-diameter GaAs NW reaches as high as 1.34, exhibiting more than 100-fold improvement over the bulk counterpart, which is attributed to both the reduced thermal conduction and enhanced power factor in thin NWs. Adopting their unique electronic characteristics, further enhancement is possible through surface engineering, for example, the introduction of surface roughness or dopants.

Scattering theory of nonlinear thermoelectricity in quantum coherent conductors

We construct a scattering theory of weakly nonlinear thermoelectric transport through sub-micron scale conductors. The theory incorporates the leading nonlinear contributions in temperature and voltage biases to the charge and heat currents. Because of the finite capacitances of sub-micron scale conducting circuits, fundamental conservation laws such as gauge invariance and current conservation require special care to be preserved. We do this by extending the approach of Christen and Büttiker (1996 Europhys. Lett. 35 523) to coupled charge and heat transport. In this way we write relations connecting nonlinear transport coefficients in a manner similar to Mott's relation between the linear thermopower and the linear conductance. We derive sum rules that nonlinear transport coefficients must satisfy to preserve gauge invariance and current conservation. We illustrate our theory by calculating the efficiency of heat engines and the coefficient of performance of thermoelectric refrigerators based on quantum point contacts and resonant tunneling barriers. We identify, in particular, rectification effects that increase device performance.

Thermoelectric transport properties in atomic scale conductors

The thermoelectric transport properties in atomic scale conductors consisting of a Si atom connected by two electrodes are investigated. It is found that both the electrical current and the heat current have two contributions, one from the voltage and the other from the temperature gradient. The quantities such as the Seebeck thermopower and the thermal conductance that characterize the thermoelectric transport properties of the tunnel atomic junction are studied quantitatively with a first-principles technique within the framework of Landauer-Buttiker formalism in the linear response regime. A finite thermopower only exists in a very narrow range where the energy derivative of the transmission function is nonzero. The thermopower anomaly is observed in the tunneling regime in this device but this does not violate the thermodynamic law with respect to the heat current.