Session L04: Applied, Condensed Matter, and Chemical Physics

Analysis of Thermoradiative Thermal Energy Conversion

The thermoradiative cell is a new method for converting heat energy to electrical power, first detailed by Strandberg in 2015. The cell is a p-n junction semiconductor device, similar to a photovoltaic cell but thermodynamically operating in the reverse direction, converting the thermal dark current into electrical power while radiating waste heat to space.

The power and efficiency can be calculated as a function of bandgap in the Shockley-Queisser detailed-balance limit, in which the thermal emissivity of the cell is due to the recombination of electron-hole pairs, and all other recombination losses are ignored. The current produced is directly proportional to the recombination radiation. The fundamental loss mechanism for the thermoradiative cell is the energy carried by the infrared radiation into space from band-to-band recombination of carriers injected across the junction. In an ideal cell, to maximize the efficiency, the emission energy of these photons would precisely equal the bandgap. This can be achieved, for example, using dielectric filters or meta-material filters to recycle emission at other wavelengths back into the cell.

The voltage is proportional to the external bias. These two constraints allow optimization of the optimum bias point for maximum power. Unlike photovoltaic cells, the maximum power operating point is not the same as the maximum efficiency point, and higher efficiency can be achieved at a higher (negative) bias in the ideal case. Incorporating non-ideal losses, however, shifts the maximum efficiency point toward lower bias. Unlike in photovoltaic cells, non-radiative recombination (e.g., Auger losses) will reduce the output current, but will not reduce the conversion efficiency, since the recombination energy is retained in the cell in the form of heat.

Since a thermoradiative cell operates by radiating directly to space, the current produced by themoradiative cells will increase as Stefan-Boltzmann radiation; roughly the fourth power of the temperature. Thus, the power produced is highest at high operating temperatures, and, unlike conventional thermal conversion, increasing radiator temperature increases, the efficiency. Thus, the choice of technology will be toward semiconductors resistant to degradation at high temperature.   

Reentrant fractional quantum Hall state: A charge ordered and broken symmetry phase of composite fermion

Non-interacting composite fermions can be used to account for the majority of fractional quantum Hall states. However, the residual interactions between composite fermions can lead to the formations of more exotic states with complex correlations. We recently observed a bubble phase of composite fermions in a GaAs two-dimensional electron gas sample near Landau level filling factor ν = 5/3. This phase with very interesting correlations was identified via a reentrant behavior of the fractional quantum Hall effect in transport measurements. Our finding reveals a new class of strongly correlated topological phases, driven by the clustering and charge ordering of emergent quasiparticles.   

Single particle and collective localization around ν=1 integer quantum hall plateau in a high mobility two-dimensional electron system

In a two-dimensional electron system, different topologically protected bulk orders can occur depending upon the competition between electron-electron interaction and the disorder potential. In the middle of ν=1 plateau the bulk corresponds to single particle localization, Anderson Insulator, whereas the flanks correspond to a periodic arrangement. This type of localization corresponds to the appearance of Wigner crystal known as Integer Quantum Hall Wigner Solid (IQHWS). The phase boundaries of these states and other properties will be discussed as obtained from transport measurements in modern GaAs samples.   

2D semiconductor FETs: the role of flake vs dielectric thickness"

Layered semiconductors are being actively studied for their possible application in high-performance field effect transistors (FETs). While transistors made of 2D semiconductors have been realized, the effect of their thickness on device performance is not fully understood. Moreover, the validity of conventional metal-oxide-semiconductor (MOS) equations on these 2D device structures is a topic of great interest. In this paper, we examine the role of 2D semiconductor flake and oxide dielectric thickness in the determination of the subthreshold swing of WSe2 FETs. By extracting the subthreshold swing of various devices with different flake and oxide thicknesses, and comparing it to the subthreshold swing expression derived from MOSFET theory when the semiconductor thickness is used as the depletion width, we are able to validate the use of such equation for the determination of the subthreshold swing in 2D semiconductor FETs. Additionally, we explore how the oxide and semiconductor thicknesses affect the determination of the threshold voltage.   

Topological transitions induced by strain in the kagome lattice

We study the effects of a uniform strain on the electronic and topological properties of the 2D kagome lattice using a tight-binding formalism that includes intrinsic and Rashba spin-orbit coupling (SOC). The degeneracy at the Γ point, where a flat-band-parabolic-band touching occurs, evolves into a pair of (tilted) type-I Dirac cones owing to a uniform strain, as shown by effective Hamiltonians, where the anisotropy and tilting of the bands depend on the magnitude and direction of the strain field. Interestingly, we find that the Dirac cones become type-III (including flat dispersions) when the strain is applied along the bonds of the lattice. As expected, the inclusion of intrinsic SOC opens a gap at the emergent Dirac points, making the strained flat band topological, as characterized by a nontrivial Z₂ index. We show that the strain drives the systems into a trivial or topological phase for strains of a few percent, allowing topological transitions via uniform deformations. Additionally, when the Rashba interaction is included, semimetallic phases appear in the topological phase diagrams. These findings suggest an alternative way of engineering anisotropic tilted Dirac bands with tunable topological properties in strained kagome lattices.