Session S51: Thermodynamic & Transport Properties of Semiconductors

Magneto-Inter-Subband Oscillations in GaAs quantum wells with three populated subbands placed in tilted magnetic fields.

The effect of tilted magnetic fields on magnetotransport is studied in GaAs quantum wells with three populated subbands. In perpendicular fields magneto-intersubband oscillations (MISO) are observed. These oscillations obey the relation $\Delta _{\mathbf{ij}} \quad =$\textbf{(E}$_{\mathbf{i}}$\textbf{-E}$_{\mathbf{j}}$\textbf{)}$=$\textbf{\textit{k}}\textbf{ž}$\omega _{\mathbf{c}}$, where \textbf{E}$_{\mathbf{i\, }}_{\, }$is the energy of the bottom of $i$-th subband and \textbf{\textit{k}} is an integer. MISO are periodic in the inverse magnetic field and show three frequencies \textbf{f}$_{\mathbf{ij\, }}$\textbf{\textasciitilde }$\Delta _{\mathbf{ij}}$\textbf{.} \quad Due to \textbf{E}$_{\mathbf{1,2}}$\textbf{\textless \textless E}$_{\mathbf{3}}$ two MISO oscillate at high frequencies (HF) demonstrating a beat pattern with the beat frequency \textbf{f}$_{\mathbf{b}} \quad =$\textbf{(f}$_{\mathbf{13}}$\textbf{-f}$_{\mathbf{23}}$\textbf{)/2\textasciitilde }$\Delta_{\mathbf{12}}$. With increasing tilt angle at small magnetic fields, \textbf{ž}$\omega_{\mathbf{c}}$\textbf{\textless }$\Delta _{\mathbf{12\, ,\, }}$the periodicity of HF-MISO changes indicating a change in the subband gap $\Delta_{\mathbf{12}}$\textbf{.} The dependence of $\Delta_{\mathbf{12\, }}$on the parallel magnetic field is found to be in a good agreement with existing theory. At larger parallel magnetic fields and \textbf{ž}$\omega_{\mathbf{c}}$\textbf{ \textgreater }$\Delta_{\mathbf{12\, ,\, }}$the high frequency beating disappears leaving only HF-MISO with single frequency \textbf{f}$=$\textbf{(f}$_{\mathbf{13}} \quad +$\textbf{f}$_{\mathbf{23}}$\textbf{)/2}.\textbf{ }It indicates a magnetic breakdown between the lower two subbands. Investigations of the 2D electron system in the regime of the magnetic breakdown are presented.   

Blockade in a silicon double quantum dot via the valley degree of freedom

Measuring electrical transport through double quantum dots (DQDs) is a useful way of illuminating several aspects of the states of the carriers. We show transport measurements through a silicon DQD formed in a mesa etched nanowire. Comparing the data at positive and negative bias voltage we observe a size asymmetry in the region of allowed current typically associated with Pauli spin blockade (PSB). However, the qualitative features of the asymmetry in our data, including i) lack of odd/even filling, ii) same polarity of asymmetry across many bias triangles, iii) lack of systematic dependence on magnetic field, and iv) a dependence on gate voltages, are all in disagreement with the predictions of PSB. In contrast, we have developed a model based on the selective filling of valley states in the DQD and the conservation of the valley degree of freedom during tunneling that predicts all of the qualitative features in our data.   

Negative Differential Conductance from Space Charge Limited Currents in Semiconductors

Applying the theory of space charge limited currents (SCLC), we show that negative differential conductance can arise from doubly occupied traps that are nearly degenerate with the bottom of the conduction band. Using degenerate state perturbation theory, the Coulomb energy of the doubly occupied traps is shown to depend on the hybridization with the conduction band states. Initially, when carriers are injected into the solid, traps begin to fill while the conduction band states stay relatively empty and thus accessible to trapped electrons via hopping. Trap and conduction states continue to be filled as current is increased, and the energy of trapped electrons begins to rise. A critical current is reached whereupon a further increase in current leads to a reduction of filled traps (i.e. a reduction of space charge in the solid), and thus a corresponding decrease in voltage. This trend in the current-voltage characteristic curves persists until the bottom of the conduction band has been filled, then voltage rises with current.   

Vertical electronic transport in van de waals heterostructures

In this work, we will introduce the theoretical investigation of the vertical electronic transport in various heterostructrues by using both tight-binding method and first-principles calculations. Counterintuitively, we find that the maximum electronic transport is achieved at very limited scattering regions but not at large overlapped catering regions. Based on this finding, we design a special setup to measure the tunneling effect in rotated bilayer systems.   

First principles lattice thermal conductivity of Li$_{\mathrm{2}}$Se, Li$_{\mathrm{2}}$Te and alloys: phase space guidelines for thermal transport

The lattice thermal conductivities ($k)$ of Li$_{\mathrm{2}}$Se, Li$_{\mathrm{2}}$Te and alloys are examined using a first-principles Peierls-Boltzmann transport methodology. The dominant resistance to heat-carrying acoustic phonons in Li$_{\mathrm{2}}$Se and Li$_{\mathrm{2}}$Te comes from the interactions of these modes with two optic phonons, aoo scattering. In typical cubic and hexagonal materials ($e.g.$, Si, GaAs, AlN) aoo scattering does not play a considerable role in determining $k$, as it requires significant bandwidth and dispersion of the optic phonon branches, both present in Li$_{\mathrm{2}}$Se and Li$_{\mathrm{2}}$Te. We discuss how these properties and other features of the phonon dispersion ($e.g.$, bunching of the acoustic branches and an acoustic-optic frequency gap) combine to determine the overall conductivity of a material. Thus, microscopic scattering phase space arguments are generalized to give a more comprehensive view of intrinsic thermal transport in crystalline solids. We note that these general considerations are important for the discovery and design of new `high$ k$' and `low $k$' materials for thermal management applications.   

Thermal Conductivity Accumulation Function of Silicon-Germanium Alloy from Thermoreflectance and First-Principles

Phonons are the dominant heat carriers in semiconductors. Alloying changes their properties by introducing mass disorder and altering the bonding environment. In this study, we determine the thermal conductivity accumulation functions of silicon-germanium alloys using broadband frequency-domain thermoreflectance experiments. The accumulation function describes the cumulative mean free path-dependent contributions to thermal conductivity and provides a measure for determining how alloying alters thermal conductivity compared to pure semiconductors. The experimental results are compared to calculations based in density functional theory, lattice dynamics, the virtual crystal approximation, and the Boltzmann transport equation. In both thermoreflectance and lattice dynamics we find alloying increases the proportional accumulation of long MFP phonons.   

Study of Thermal properties of VO2 and multilayer VO2 thin films for application in Thermal Switches.

Ultrafast nature of the phase transition near room temperature in VO$_{\mathrm{2}}$ makes it attractive material for applications in electronics and optical devices however utilization of corresponding drastic change in thermo-physical properties are rarely reported. In this study we investigate thermal and electronic properties of VO$_{\mathrm{2}}$ thin films on various substrates across the transition temperature to seek possibility of utilizing VO$_{\mathrm{2}}$ based thermal switches for applications in thermal devices. In addition, the interfacial heat transfer in VO$_{\mathrm{2}}$/metal multilayer thin film is mediated by phonons at low temperature, and when temperature is elevated beyond phase transition temperature, the interface thermal conductance is mediated mainly by both phons and electrons. VO$_{\mathrm{2}}$-multilayers approach is studied to utilize the switching interface thermal conductance in order to obtain higher thermal conductivity switch ratio than what can be achieved in intrinsic VO$_{\mathrm{2}}$. Thermal conductivities and interface thermal conductance of VO$_{\mathrm{2}}$ and VO$_{\mathrm{2}}$ multilayer thin films are measured using the time-domain thermoreflectance (TDTR) method. We will discuss interplay of phononic and electronic component to thermal conductivity in the light of Wiedemann--Franz law across the metal to insulator state of VO$_{\mathrm{2}}$ films.   

Efros-Shklovskii variable range hopping conductivity without Coulomb gap

In doped semiconductors and Coulomb glasses, in the limit of weak coupling, the electron conductivity primarily proceeds by phonon-assisted tunneling or hopping between different sites through the insulating gaps that separate them. Electron conduction can occur both through nearest-neighbor hopping and through cotunneling of electrons between distant sites via a chain of intermediate virtual states. In the presence of some disorder, the latter mechanism dominates at low temperatures, where the length of the hops grows to optimize the conductivity. This transport mechanism was introduced by Mott, and is called variable range hopping. When the Coulomb interaction between localized electrons is taken into account, it can be shown that at a sufficiently low temperature, variable range hopping conductivity obeys the Efros-Shklovskii (ES) law, which has been observed in a number of amorphous semiconductors and granular metal systems at low temperatures. ES conductivity has been long understood as the result of a soft, Coulomb gap at the Fermi level. However, such a theory overlooks the presence of spatial correlations between site energies and their possible effects on electrical conductivity. In this talk, we show both analytically and numerically that in systems where spatial correlations must be taken into account, ES conductivity may persist far outside the Coulomb gap, in contrast to conventional transport theory for doped semiconductors and Coulomb glasses where ES conductivity only occurs within the Coulomb gap.   

Maximum non-saturating magnetoresistance in MoTe2

The search for exotic materials with a linear magnetoresistance (MR) is one of the most challenging tasks of the condensed matter community and materials science. Here, we investigated the magnetoresistance behavior of high-quality single crystals MoTe2. A large linear non-staturated MR in a magnetic field of 60 T, was observed with a maximum at a temperature of $T =$ 45 K. The detailed field and temperature dependencies will be presented. Our results not only provide a general scaling approach for the anisotropic MR but also are crucial for correctly understanding the mechanism of the linear MR, including the origin of the remarkable ``turn-on'' behavior in the resistance versus temperature curve.   

Negative thermal expansion above a quantum phase transition

Strong, thermally persistent, isotropic negative thermal expansion (NTE) is unusual and has been observed in only a handful of materials. Scandium trifluoride (ScF$_{\mathrm{3}})$ features large isotropic thermal expansion persistent over a 1000K range of temperature. More interestingly, no structural phase transition has been reported above 0.4K and it retains the simple cubic structure up to its high melting point of 1800K, which is unusual compared with other transition metal trifluorides. Here, we present a combined inelastic x-ray scattering (IXS) and x-ray diffraction study of ScF$_{\mathrm{3}}$, which reveals some exciting features of this material. The low-energy (\textasciitilde 1 meV) vibrational modes corresponding to M and R points of simple cubic Brillouin zone could explain NTE in ScF3, and we find that the low temperature IXS data show a central peak which is especially strong at these points. In addition, the whole M-R branch undergoes unusual softening at low temperature. We determine that this mode softens nearly to zero energy as the temperature approaches to 0K. These signature portend an approach to a quantum phase transition of this insulating, nonmagnetic simple cubic perovskite material ScF3. The central peak, soft mode and thermal expansion could all be consequences of this incipient transition. The connections we have established in the phenomenology of ScF3 may be present in other perovskites as well as other materials that display strong NTE   

First Principles Study of structural characteristics and phase change mechanism of Ge-Sb-Te based materials

Using \textit{ab initio} density functional theory, we investigate the structural properties and their phase transition mechanism of the crystalline and amorphous phases of Ge-Sb-Te (GST) based phase change materials, which would be utilized for phase change random access memory. Among various stochiometries of GST, we focus on compositions along the (GeTe)$_n$(Sb$_2$Te$_3$)$_m$ pseudo-binary line, denoted simply by $(n,m)$ with integer $n$ and $m$. We explore various GST materials corresponding $(n,m)$ sets including (1,0), (0,1), (1,1), (2,1) and (1,2) by modeling their both phases. Especially, their amorphous phases can be constructed based on experimental data available or molecular dynamics (MD) simulations performing melt-quench processes. To understand the phase transition mechanism, we evaluate their coordination numbers, radial distribution functions, and angle distribution functions, which enables us to identify the characteristic local geometry representing each phase. We further investigate the thermal properties of various phases by evaluating their phonon densities of states obtained by Fourier-transforming the velocity autocorrelation functions calculated directly from our MD simulation.   

Nonlinear THz absorption and cyclotron resonance in InSb.

The emergence of coherent high-field terahertz (THz) sources in the past decade has allowed the exploration of nonlinear light-matter interaction at THz frequencies. Nonlinear THz response of free electrons in semiconductors has received a great deal of attention. Such nonlinear phenomena as saturable absorption and self-phase modulation have been reported. InSb is a narrow-gap (bandgap 0.17 eV) semiconductor with a very low electron effective mass and high electron mobility. Previous high-field THz work on InSb reported the observation of ultrafast electron cascades via impact ionization. We study the transmission of an intense THz electric field pulse by an InSb wafer at different incident THz amplitudes and 10 K temperature. Contrary to previous reports, we observe an increased transmission at higher THz field. Our observation appears similar to the saturable THz absorption reported in other semiconductors. Along with the increased absorption, we observe a strong modulation of the THz phase at high incident fields, most likely due to the self-phase modulation of the THz pulse. We also study the dependence of the cyclotron resonance on the incident THz field amplitude. The cyclotron resonance exhibits a lower strength and frequency at the higher incident THz field.   

Bound excitons at nitrogen and bismuth isoelectronic impurities

When nitrogen and bismuth dopants are simultaneously incorporated into a host lattice such as gallium arsenide (GaAs) or gallium phosphide (GaP), each dopant species contributes to the evolution of the electronic structure. Bound excitons in these systems luminescence from localized states whose distinctive radiative signatures provide invaluable clues into the nature of impurity clustering and inter-impurity interactions within the host lattice. Spectroscopic studies of these states will be presented for a series of samples grown by molecular beam epitaxy. Research was supported by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC36-08GO28308 and by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100.   

Electron-Phonon Renormalization of Electronic Band Structures of C Allotropes and BN Polymorphs

The effect of lattice vibration on electronic band structures has been mostly neglected in first-principles calculations because the electron-phonon (e-ph) renormalization of quasi-particle energies is often small ($< 100$ meV). However, in certain materials, such as diamond, the electron-phonon coupling reduces the band gap by nearly 0.5 eV, which is comparable to the many-body corrections of the electronic band structures calculated using the density functional theory (DFT). In this work, we compared two implementations of the Allen-Heine-Cardona theory in the EPW code and the ABINIT package respectively. Our computations of Si and diamond demonstrate that the ABINIT implementation converges much faster. Using this method, the e-ph renormalizations of electronic structures of three C allotropes (diamond, graphite, graphene) and four BN polymorphs (zincblend, wurtzite, mono-layer, and layered-hexagonal) were calculated. Our results suggest that (1) all of the zero-point renormalizations of band gaps in these materials, except for graphene, are larger than 100 meV, and (2) there are large variations in e-ph renormalization of band gaps due to differences in crystal structure.   

Non-adiabatic effects on the optical response of driven systems

Periodically driven systems have received renewed interest due to their capacity to engineer non-trivial effective Hamiltonians. A characteristic of such systems is how they respond to weak periodicity-breaking drive, as for example when a laser is pulsed instead of continuous wave. We develop semi-classical equations of motion of a wave packet in the presence of electric and magnetic fields which are turned on non-adiabatically. We then show the emergence of significant corrections to electronic collective excitations and optical responses of topological insulator surface states, Weyl metals and semiconductor mono-chalcogenides.