Session Y12: Thermodynamic and Transport Properties

Anisotropic Electronic Transport Properties of ReSe2 Field-Effect Transistors

Few-layer van der Waals (vdW) semiconductors have been at the forefront of optimizing electronic transport due to their weak inter-layer forces, tunable band gaps, and ability to create unique layered structures.  ReSe₂ has shown to have anisotropic electronic and optoelectronic properties induced by interlayer coupling, in addition to having a native oxide, ReO₂, which makes it an interesting material for electronic transistor applications.  In this study, we have experimentally characterized the electronic transport of n-type and p-type ReSe₂ field effect transistors.  This project also probes the temperature dependence of the anisotropic properties of these devices.   

The magnetotransport in the quantum wires comprised of vertically stacked quantum dots

We investigate a periodic system of vertically stacked InAs/GaAs quantum dots (VSQD) subjected to a two-dimensional confining harmonic potential and a magnetic field in the symmetric gauge. Given the tiny length scales, adequate lateral confinement, and strong vertical coupling involved in the experiments, the VSQD system has become known for mimicking the standard semiconducting quantum wires. An exact analytical diagnosis of the problem allows us to show the system's direct relevance to the physics of musical sounds, magnetization, magnetotransport, and designing of the optical amplifiers. The results suggest making the most of the system for applications in single-electron devices and

quantum state transfer in the quantum computation.

EPL Europhys. Lett. {\bf 127}, 37004 (2019).

Mod. Phys. Lett. B {\bf 34}, 2150173 (2020).

Nature Physics (submitted).   

Probing the Dynamics of Electric Double-Layer Formation Over Wide Time Scales (10-9 – 10+5 s) in the Ionic Liquid DEME-TFSI

Ionic liquids (ILs) have found wide use as the gate dielectric in a variety of novel transistors, allowing large carrier concentrations to be induced. These concentrations arise from the formation of an electric double layer (EDL) at the interface between the liquid and the channel, a process that yields an extremely high effective areal capacitance. ILs have been used to gate a variety of different materials, including polymers, oxides and low-dimensional materials, and have led to the discovery of exciting fundamental phenomena. In this work, we investigate the transient response of DEME-TFSI [N, N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)-imide] based ionic-liquid (IL) planar capacitors, studying this response over time scales as short as a few nanoseconds, to as long as several days. Our measurements point to the existence of three distinct mechanisms for charging/discharging of the IL. The fastest of these is associated with the development of a standard polarization charge in the bulk of the liquid dielectric, which dominates at times shorter than ~10^-6 s. The second process is attributed to electric double layer formation, which is initiated after ~10^-6 s but which takes as long as ~10^-2 s to reach completion. Finally, we also identify the presence of a pseudocapacitance that arises from electrochemical reactions; this process is only activated at voltages above ~2.5 V and is relatively slow. Indeed, we find evidence that full discharging of this pseudocapacitance can take as long as 10⁵ s (i.e., days). Overall, our findings provide useful insight into the mechanisms for slow ion dynamics in ILs, and highlight the constraints that these dynamics place on the potential operational speed of IL-based transistors.   

Ballistic and diffusive phonon heat transport studied by pulse propagation in one dimension

Several kinds of heat pulses are inserted into a one-dimensional lattice.   Their propagation is studied in classical mechanics, which gives insight into ballistic versus diffusive behavior.  The results are modeled using quasiparticle pictures, especially the phonon Boltzmann equation.  Ballistic propagation is easily modeled, has interesting dependence on the shape of the initial insertion, and gives insight into new aspects of Boltzmann theories of systems inhomogeneous in space and time.  Phonon scattering provides an approach to diffusive propagation.  Scattering by impurities with altered mass give new features to the propagating pulse, as does anharmonic phonon-phonon coupling.  The Boltzmann-predicted pulse shapes using relaxation-time approximation show a crossover toward diffusion, but do not agree closely with the simulations.   

Electrical transport of graphene-contacted thin layer pentagonal transition metal dichalcogenide (TMDC)

2D materials with puckered pentagonal structures have been predicted to show exotic new properties due to in-plane anisotropy. Yet there have been limited experimental transport studies on such materials due to the scarcity of this type of materials and difficulties in achieving ohmic contact, especially for thin layers that have larger band gaps. Most of the current studies focus on Palladium diselenide PdSe2, which is air stable and can be mechanically exfoliated into thin layers. In this work, we use graphene to contact atomically thin PdSe2, which significantly reduces the contact resistance, therefore facilitated magnetotransport studies at low temperatures and enabled observation of new features that have never been discussed in previous studies.   

A simple method to calculate specific heat at constant pressure from first principles

Calculating specific heat at constant pressure (Cp) entirely from first-principles has historically proven difficult, and what few methods which do exist are complex and require significant effort to implement.  In this work, we describe a different method for calculating Cp which is entirely based on first-principles, density functional theory.  It is conceptually simple, computationally efficient and easy to implement because it avoids the need for finite temperature density functional theory.  In order to demonstrate the accuracy, precision and validity of our method, we present and discuss data and results for well-studied, benchmark semiconducting materials Si and Ge, where our results agree well with the prevailing literature.  We also go further and calculate Cp for wurtzite GaN, a material for which there is no prevailing consensus.  We use our calculated values and a review of the literature to determine a set of recommended values for GaN.   

Decoupling between propagating acoustic waves and two-level systems in hydrogenated amorphous silicon

Tunneling two-level systems are the dominant energy loss mechanisms for amorphous solids at low temperatures. Hydrogenated amorphous silicon is one of the few materials that shows mechanical loss  values below the “glassy range,” which indicates a low density of tunneling two-level systems  at low temperature. We have measured the specific heat  of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition, which shows a large density of two-level systems. Annealing at 200 °C, well below the growth temperature, does not significantly affect the already-low mechanical loss, but irreversibly reduces the specific heat by an order of magnitude at 2 K, indicating a large reduction in the density of two-level systems. We compare the specific heat with the internal friction results, which suggest that the two-level systems are uncharacteristically decoupled from acoustic waves, both before and after annealing. Our analysis yields an anomalously low value of the coupling constant , which increases upon annealing but remains anomalously low. The results suggest that the coupling constant value is lowered by the presence of hydrogen.   

Refinement of the Two-Band Model Using Nonsaturating Magnetothermoelectric Coefficients in the Highly Compensated Semimetal NbSb2

With growing interest in the magnetothermoelectric properties of semimetals, it becomes pertinent to develop and analyze methods for classifying and modeling their thermoelectric responses. We study the temperature and magnetic field dependence of the Seebeck and Nernst coefficients in the compensated semimetal NbSb₂.  At low temperatures and high fields, we find that the Seebeck coefficient increases quadratically and the Nernst coefficient increases linearly as a function of field without signs of saturation up to 14T.  We present a new analysis of magnetothermoelectricity in highly compensated semimetals, where the linear and quadratic field dependences of the Seebeck and Nernst signals impose stringent limitations on 2 band model fitting of the electrical conductivity, tightly constraining the carrier densities and mobilities [1,2].  We will discuss the implication of our results on the nonsaturating nature of the magnetoresistance and magnetothermoelectric effects.

    

1D Coulomb drag in the ultra-low temperature limit

We report temperature dependence measurements of 1D Coulomb drag in GaAs/AlGaAs quantum wires.  Utilizing enhanced filtering and a nuclear magnetization stage, 1D electron temperatures below 10 mK have been achieved.  The drag resistance R_D exhibits the standard modulation as the wires’ subband occupancy is varied.  The temperature dependence of R_D as a function of both subband occupancy and interwire separation has been studied.  The qualitative and quantitative functional shape of this dependence is utilized to determine the parametric evolution of the dominant drag inducing scattering mechanisms in the ultra-low temperature limit.   

Competing drag inducing mechanism in quasi-1D quantum wires

We report 1D Coulomb drag measurements in laterally-coupled single layer GaAs/AlGaAs quantum wires. The drag resistance RD exhibits the standard modulation as the wires’ subband occupancy is varied. In contrast to previous Coulomb drag studies in one-dimensional GaAs systems, both symmetric and anti-symmetric components to the drag signals are observed upon current reversal, highlighting the presence of competing drag-inducing mechanisms. The dependence of each contribution is studied as a function of both current amplitude and temperature. The anti-symmetric results are compared with theories for momentum transfer-induced drag while the symmetric contribution is interpreted in terms of a rectification of charge fluctuations.

This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) OoS. Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government   

Thermal runaway of Si-based metasurfaces under high-power illumination

Si-based dielectric metasurfaces can be used to make compact optical components such as lenses, polarizers, gratings, and highly reflective surfaces.  Si is a particularly attractive material for use in high-power-density applications such as non-linear microscopy, telecommunications, and laser-propelled light sails because of its low absorptive loss at infrared frequencies.  In this work, we assessed the thermal stability of radiatively cooled Si/SiO₂ metasurfaces subjected to high optical fluxes (>1 GW m^-2) of 1550 nm light using a broad-spectrum and temperature-dependent absorption model for Si that we assembled from multiple literature sources.  We found that nonlinear absorption effects at low temperatures in conjunction with a shrinking bandgap and rising free-carrier absorption at high temperatures can result in a thermal runaway effect that ultimately destroys the metasurface.  Furthermore, we find that small hotspots caused by defects or dust can push an otherwise thermally stable metasurface into thermal runaway.  We will discuss in detail the physical phenomena driving this thermal runaway in Si, evaluate the absorption and emission contributions of the SiO₂ layer, and set limits on the maximum optical intensity a Si-based metasurface can survive.   

Universal features of dynamics of localized electronic states

In a number of systems, electronic transport can be viewed as dynamics of localized electronic states. However, there is no universal theory that describes time evolution of localized states. In this work, we present universal features of such motion. They originate from the concept of adiabatic change of character near an avoided crossing. It is shown that localized electronic states move by interchanging positions with adjacent localized states that form an avoided crossing. Avoided crossings formed by two adjacent electronic states would be adiabatic, whereas avoided crossing formed by two distant electronic states would be diabatic. To illustrate this idea, we develop a numerical model based on Anderson localized states. Based on our observations, we discuss universal features of relaxation time. The presented perspective could give new insights on Metal-Insulator transitions and electron transport in nanostructures, superlattices, and disordered semiconductors.     

Wannier interpolation of electron-phonon coupling in doped semiconductors

The accurate theoretical calculation of the electron-phonon coupling interaction (EPCI) on very fine reciprocal space grids is of crucial importance in order to predict and interpret transport experiments. Such demanding evaluation of the EPCI has become possible for undoped semiconductors with the use of the Wannier interpolation technique, in conjunction with the development of fast and highly scalable ab-initio computational infrastructures. On the contrary, despite the plethora of recent advances, the realistic description of the EPCI for doped 3d and 2d semiconductors on fine reciprocal space grids has been hindered by the lack of a well-defined and fast method to interpolate the interaction. In this talk we overcome this limitation and propose a precise, stable and fast technique based on first-principles calculations and Wannier interpolation to obtain the correct EPCI for the case of doped 3d and 2d semiconductors. For polar materials, this is of particular importance since state-of-the-art transport calculations incorrectly employ the EPCI evaluated in the undoped setup, leading to a large underestimation of the dielectric screening and a proportional overestimation of the interaction, and thus to an incorrect evaluation of the transport coefficients.   

A comparative study of phonon anharmonicity and lattice thermal transport with perturbation theory and molecular dynamics

Phonon properties are crucial to determine the intrinsic lattice thermal transport of strongly anharmonic materials. Using the state-of-the-art perturbation theory (PT) including up to the fourth-order anharmonicity and molecular dynamics (MD) with accurate first-principles-based machine learning potential, we calculated the phonon lifetimes and lattice thermal conductivities of strongly anharmonic materials Tl₃VSe₄ and BaAg₂Te₂. We find that PT underestimates the phonon scatterings in the entire Brillouin zone for Tl₃VSe₄ and BaAg₂Te₂ at room temperature, and the calculated phonon lifetimes based on MD simulations show a better agreement with experimental results. Using unified theory on top of accurate phonon properties, we reveal a significant two-channel lattice thermal transport in these strongly anharmonic materials at high temperatures. Our results pave the avenue for future studies of phonon properties and ultralow lattice thermal conductivities of strongly anharmonic crystals beyond the conventional PT realm.   

Thermoelectric transport through a quantum dot embedded between Majorana bound states

We study a system composed of leads connected to a quantum dot (QD) embedded between two 1D topological superconductors hosting Majorana bound states (MBSs) at their ends. These topological nanowires are treated by means of the Kitaev effective model, which physical realizations have been achieved using hybrid structures semiconductors/superconductors (such as InAs-Al), in presence of the magnetic field. The MBSs in such structures interact between them through a coupling that exponentially decays with the wire length. The interplay between discrete energy levels and continuum energy produces interference phenomena, such as the Fano effect. This leads to a violation of the Wiedemann-Franz law and to an increment in the thermoelectric quantities of the system consequently. Working components mentioned above are present in our system, thus our goal is to provide thermoelectric features of the QD influenced by the connection of MBSs belonging to both topological wires. Our results are obtained in the linear response regime using a temperature difference $\Delta T$. Our findings show that the thermoelectric quantities can be tuned by controlling the tunneling couplings in the system, as well as the phase difference between topological superconductors, leading to its suppression.