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An emerging device architecture for infrared imaging is the curved focal-plane array which benefits from several optical advantages over the traditional flat design. However, the curving process introduces additional strain in the active region which must be taken into account. Type-II superlattices, a promising alternative to traditional bulk materials for use in infrared photodetectors, is a candidate material for use in these devices, but the transport properties of these highly heterogeneous materials are not straightforward and can be affected by different material conditions, such as superlattice disorder and external strain. We present a comprehensive study of the internal QE calculated for a curved device that incorporates finite element analysis (FEA) modeling, nonequilibirium Green's functions (NEGF) calculations, and Gaussian Process (GP) regression. FEA is used for predicting the strain configuration throughout the active region induced by the curving procedure of the device. NEGF is used to calculate the vertical hole mobility for a select set of strain configurations, from which the internal quantum efficiency of the device is approximated to predict performance under strained conditions. Then this data set is used to train a GP model that maps the quantum efficiency QE predictions onto the spatial coordinates of the curved device, based on the strain configuration predicted using FEA. This analysis is performed for ideal and disordered SLs to understand both the fundamental and practical limitations of the performance of these materials in curved devices.

We analytically calculate the anomalous transverse electric and thermal currents in massive and tilted Dirac systems, using $ \beta $-borophene as a representative material, and report on conditions under which the corresponding Lorenz number $\left(\mathcal{L}_{an}\right)$ deviates from its classically accepted value $\left(\mathcal{L}_{0}\right)$. The deviations in the high-temperature regime are shown to be an outcome of the quantitative difference in the respective kinetic transport expressions for electric $\left(\sigma\right)$ and thermal $\left(\kappa\right)$ conductivity, and are further weighted through a convolution integral with a non-linearly energy-dependent Berry curvature that naturally arises in a Dirac material. In addition, the tilt and anisotropy of the Dirac system that are amenable to change via external stimulus are found to quantitatively influence $ \mathcal{L}_{an} $. The reported deviations from $\mathcal{L}_{0} $ hold practical utility inasmuch as they allow an independent tuning of $\sigma $ and $ \kappa $, useful in optimizing the output of thermoelectric devices., Comment: 4 pages; 5 figures

The GERmanium Detector Array (GERDA) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta ($0\nu\beta\beta$) decay of $^{76}$Ge. The technological challenge of GERDA is to operate in a "background-free" regime in the region of interest (ROI) after analysis cuts for the full 100$\,$kg$\cdot$yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around $Q_{\beta\beta}$ for the $0\nu\beta\beta$ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos ($2\nu\beta\beta$) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for GERDA Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of $16.04^{+0.78}_{-0.85} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched BEGe data set and $14.68^{+0.47}_{-0.52} \cdot 10^{-3}\,$cts/(kg$\cdot$keV$\cdot$yr) for the enriched coaxial data set. These values are similar to the one of Gerda Phase I despite a much larger number of detectors and hence radioactive hardware components.

We analytically establish an anomalous transverse flow of heat in 8-\textit{Pmmn} borophene, one of the several two-dimensional (2D) allotropes of Boron (B). The dispersion of this allotrope contains a pair of anisotropic and tilted Dirac cones which are gapped by placing the 2D \textit{B} sheet under an intense circularly-polarized illumination. A gap in the Dirac dispersion leads to a finite Berry curvature and connected anomalous Hall effects. In the case of thermoelectrics, this manifests as a heat current perpendicular to the temperature gradient - the thermal Hall effect. A quantitative calculation of the attendant thermal Hall conductivity reveals dependence on the intrinsic anisotropy and tilt of the Dirac cone. Further, by estimating the longitudinal thermal conductivity using the Weidemann-Franz law, we also outline steps to compute the thermal Hall angle that gauges the generation efficiency of such transverse heat processes. Finally, we touch upon the idea of thermal rectification wherein the direction of flow of the anomalous heat reverses through a simple switch of the polarization of incident light and is of interest in thermal logic circuits., Comment: 5 pages, 3 figures

This work presents an in-depth investigation of the properties of complexes composed of hydrogen, silicon or oxygen with carbon, which are major unintentional impurities in undoped GaN. This manuscript is a complement to our previous work on carbon--carbon and carbon-vacancy complexes. We have employed a first-principles method using Heyd-Scuseria-Ernzerhof hybrid functionals within the framework of generalized Kohn-Sham density functional theory. Two H--C, four Si--C and five O--C complexes in different charge states have been considered. After full geometry relaxations, formation energies, binding energies and both thermal and optical transition levels were obtained. The calculated energy levels have been systematically compared with the experimentally observed carbon related trap levels. Furthermore, we computed vibrational frequencies for selected defect complexes and defect concentrations were estimated in the low, mid and high carbon doping scenarios considering two different cases where electrically active defects: (a) only carbon and vacancies and (b) not only carbon and vacancies but also hydrogen, silicon and oxygen. We confirmed that $\mathrm{C_N}$ is a dominant acceptor in GaN. In addition to it, substantial amount of $\mathrm{Si_{Ga}-C_N}$ complex exists in a neutral form. This complex is a likely candidate for unknown form of carbon observed in undoped $n$-type GaN., Comment: 19 pages, 23 figures, 8 tables

The heat capacity of conduction electrons obtained from the Sommerfeld expansion is shown to be tunable via the Rashba and Dresselhaus spin-orbit coupling parameters. Using an AlInSb/InSb/AlInSb as a representative heterostructure with alterable well and asymmetric barrier regions, the heat capacity is found to be higher for the spin-down electrons and suffers a reduction for wider wells. A further lowering is obtained through the application of an uniaxial strain. Finally, we suggest a method to determine the spin lifetimes for spins relaxing via the D'yakonov-Perel' mechanism from experimental estimates of thermodynamic potentials such as the Helmholtz free energy and the heat capacity., Comment: 4 pages; 3 figures

We report on a possible optical tuning of the spin Hall conductivity in mono-layer transition metal dichalcogenides. Light beams of frequencies much higher than the energy scale of the system (the \textit{off-resonant} condition) does not excite electrons but rearranges the band structure. The rearrangement is quantitatively established using the Floquet formalism. For such a system of mono-layer transition metal dichalcogenides, the spin Hall conductivity (calculated with the Kubo expression in presence of disorder) exhibits a drop at higher frequencies and lower intensities. Finally, we compare the spin Hall conductivity of the higher spin-orbit coupled WSe$_{2} $ to MoS$_{2} $; the spin Hall conductivity of WSe$_{2}$ was found to be larger., Comment: 4 pages, 3 figures

We study the optical behaviour of an arrangement in which the interface between a linear and non-linear dielectric media is covered by an embedded mono-layer of transition metal dichalcogenides (TMDC). The optical behaviour is qualitatively obtained through transmission and reflection coefficients which are a function of the third order non-linear susceptibility of the Kerr-type dielectric and the inter-band optical conductivity of the TMDC mono-layer. The inter-band optical conductivity of the TMCD mono-layer is calculated using the Kubo formalism from linear response theory. In particular, we theoretically demonstrate that the optical response of this arrangement can be switched between total internal reflection and a normal transmission regime by controlling the intensity of the incident radiation. The reflection and transmission functions, additionally, are shown to be amenable to further control by altering the inter-band optical conductivity of the embedded TMDC mono-layer. The optical conductivity is an outcome of its energy dispersion; we specifically choose two TMDC mono-layers, MoS$_{2}$ and WSe$_{2}$, which have nearly identical dispersion parameters apart from a much stronger spin-orbit coupling in the latter. The stronger spin-orbit coupling in WSe$_{2}$ does not significantly alter the inter-band optical conductivity to manifest in an enhanced reflection spectrum. However, we find that application of an external perturbation such as strain could be effectively used to modulate the overall optical response. We conclude by offering a brief overview of the applicability of our proposed scheme in devices that employ an all-optical switching mechanism through optical bistability which is the hallmark of a non-linear dielectric., Comment: 12 pages, 8 figures

Polarization-sensitive devices rely on meta-materials to exhibit varying degrees of absorption of light of a given handedness. The chiral surface states of a topological insulator(TI) selectively absorb right and left circularly polarized light in the vicinity of the Dirac cone reaching its maximum of unity at the $ \Gamma $ point. In this letter, we demonstrate that a band gap open TI with C$_{2v}$ symmetry which is represented through a combination of Rashba and Dresselhaus Hamiltonians alters the preferential absorption of left and right circularly polarized light allowing a smooth variation of the circular dichroism(CD). This variation in CD, reflected in a range of positive and negative values is shown to be a function of the Rashba and Dresselhaus coupling parameters. Additionally, we draw a parallel between the varying CD and the emerging field of valley-electronics in transition metal dichalcogenides and note the possibility of the chiral states as a basis toward quantum information processing., Comment: 4 pages, 6 figures

Various forms of carbon based complexes in GaN are studied with first-principles calculations employing Heyd-Scuseria-Ernzerhof hybrid functional within the framework of density functional theory. We consider carbon complexes made of the combinations of single impurities, i.e. $\mathrm{C_N-C_{Ga}}$, $\mathrm{C_I-C_N}$ and $\mathrm{C_I-C_{Ga}}$, where $\mathrm{C_N}$, $\mathrm{C_{Ga}}$ and $\mathrm{C_I}$ denote C substituting nitrogen, C substituting gallium and interstitial C, respectively, and of neighboring gallium/nitrogen vacancies ($\mathrm{V_{Ga}}$/$\mathrm{V_N}$), i.e. $\mathrm{C_N-V_{Ga}}$ and $\mathrm{C_{Ga}-V_N}$. Formation energies are computed for all these configurations with different charge states after full geometry optimizations. From our calculated formation energies, thermodynamic transition levels are evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy. Furthermore, the lattice relaxation energies (Franck-Condon shift) are computed to obtain optical activation energies, which are observed in experimental techniques such as deep level optical spectroscopy. We compare our calculated values of activation energies with the energies of experimentally observed C-related trap levels and identify the physical origins of these traps, which are unknown before., Comment: 17 pages, 15 figures, 12 tables. Replacement for published version

The zero gap surface states of a 3D-topological insulator host highly mobile Dirac fermions with spin locked to the momentum. The high mobility attributed to absence of back scattering is reduced in presence of impurities on the surface. In particular, we discuss and compare scattering times for localized impurities on the surface, scattering between states of opposite helicity located on different surfaces coupled through a hybridization potential, and the role of magnetic impurities. Magnetic impurities give rise to an additional spin suppression factor. The role of warped bands and its influence on topological factors that can enhance the overall surface mobility is examined., Comment: 8 pages, 6 figures

The zero gap surface states of a 3D-topological insulator host Dirac fermions with spin locked to the momentum. The gap-less Dirac fermions exhibit electronic behaviour different from those predicted in conventional materials. While calculations based on a simple linear dispersion can account for observed experimental patterns, a more accurate match is obtained by including higher order $ \overrightarrow{k}$ terms in the Hamiltonian. In this work, in presence of a time reversal symmetry breaking external magnetic field and higher order warping term, alteration to the topologically ordained Berry phase of $ (2n + 1)\pi $, momentum relaxation time, and the magneto-conductivity tensors is established., Comment: 6 pages 4 figures

In a recent letter [Phys. Rev. Lett. 110, 177406 (2013)], presenting a spectroscopic study of the electrons emitted from the GaN p-cap of a forward-biased InGaN/GaN light-emitting diode (LED), the authors observed at least two distinct peaks in the electron energy distribution curves (EDCs), separated by about 1.5 eV, and concluded that the only viable explanation for the higher-energy peak was Auger recombination in the LED active region. We present full-band Monte Carlo simulations suggesting that the higher-energy peaks in the measured EDCs are probably uncorrelated with the carrier distribution in the active region. This would not imply that Auger recombination, and possibily Auger-induced leakage, play a negligible role in LED droop, but that an Auger signature cannot be recovered from the experiment performed on the LED structure under study. We discuss, as an alternative explanation for the observed EDCs, carrier heating by the electric field in the band-bending region.

We propose an efficient reduced-order technique for electronic structure calculations of semiconductor nanostructures, suited for inclusion in full-band quantum transport simulators. The model is based on the linear combination of bulk bands obtained by the empirical pseudopotential method, combined with the use of problem-matched basis functions numerically generated from the singular value decomposition. The efficiency and accuracy of the proposed approach are demonstrated in the case of the dispersion relation of hole subbands in an unstrained GaN layer., Comment: Submitted to Computer Physics Communications on September 30, 2012