A computational study within the framework of density functional theory in the local density approximation (LDA) is presented for native defects and doping in ${\mathrm{ZnGeN}}_2$. Gap corrections are taken into account using an LDA+$U$ approach and finite size corrections for charged defects are evaluated in terms of an effective charge model, introduced in this paper. The donor or acceptor characteristics of each of the cation and N vacancies and the two cation antisite defects are determined as well as their energies of formation under different chemical potential conditions. These are then used to determine defect concentrations and Fermi level pinning self-consistently. The cation antisite defects are found to have significantly lower formation energy than the cation vacancies. At a typical growth temperature of 1200 K, the charge neutrality condition pins the Fermi level close to the crossing of the formation energies of the ${\mathrm{Zn}}_{\mathrm{Ge}}^{-1}$ acceptor with the ${\mathrm{Ge}}_{\mathrm{Zn}}^{2+}$ shallow donor. Since this point lies closer to the valence-band maximum (VBM), intrinsic $p$-type doping would result at the growth temperature and will persist at room temperature if the defect concentrations are frozen in. It is the highest and of order ${10}^{16}{\mathrm{cm}}^{-3}$ for the most Ge-poor condition. On the other hand, for the most Ge-poor condition, it drops to ${10}^{13}{\mathrm{cm}}^{-3}$ at 1200 K and to almost zero at 300 K because then the Fermi level is too close to the middle of the gap. Oxygen impurities are found to strongly prefer the ${\mathrm{O}}_{\mathrm{N}}$ substitutional site and are found to be shallow donors with a very low energy of formation. It can only be suppressed by strongly reducing the oxygen partial pressure relative to that of nitrogen. At high temperatures, however, introduction of oxygen will be accompanied by compensating ${\mathrm{Zn}}_{\mathrm{Ge}}^{-2}$ acceptors and would lead to negligible net doping. The prospects for Ga base $p$-type doping are evaluated. While good solubility is expected, site competition between Zn and Ge sites is found to lead to a compensation problem similar to that of the two antisites and leads to $p$-type doping of the same level of ${10}^{16}{\mathrm{cm}}^{-3}$.

Predictive modeling of the thermal conductivity of nanostructures remains a large challenge. Ab initio studies rely on parametrized models of scattering rates of the nanostructure elements. Simplified interatomic potentials, on the other hand, often fail in the quantitative prediction of thermal conductivity. We have developed a simple and short-ranged orthogonal tight-binding model for the thermal properties of Si. We systematically introduce separate handles to tune the value and slope of the potential energy with respect to interatomic distance. Furthermore, we add an embedding potential in our model to capture the correct structural stability trend in Si. The model shows excellent transferability to the thermal properties of diamond Si along with good reproduction of the relative stabilities of different structures. We demonstrate the improvement over earlier tight-binding models and contrast the transferability of our model with simple interatomic potentials.

We demonstrate the dynamics and optical control of a large quantum mechanical solid state spin system consisting of a donor electron spin strongly coupled to the 9/2 nuclear spin of ${}^{115}\mathrm{In}$ in the semiconductor ZnO. Comparison of electron spin dynamics observed by time-resolved pump-probe spectroscopy with density matrix theory reveals nuclear spin pumping via optically oriented electron spins, coherent spin-spin interaction, and quantization effects of the ten nuclear spin levels. Modulation of the optical electron spin orientation at frequencies above 1 MHz gives evidence for fast optical manipulation of the nuclear spin state.

Energy spectra both of the conduction and valence bands of the HgTe quantum wells with a width close to the Dirac point were studied experimentally. Simultaneous analysis of the Shubnikov–de Haas oscillations and the Hall effect over a wide range of electron and hole densities yields surprising results: the top of the valence band is strongly split by spin-orbit interaction while the splitting of the conduction band is absent, within experimental accuracy. This holds true for the structures with normal and inverted band ordering. The results obtained are inconsistent with the results of $\mathit{kP}$ calculations, in which the smooth electric field across the quantum well is only reckoned in. It is shown that taking into account the asymmetry of the quantum-well interfaces within a tight-binding method gives reasonable agreement with the experimental data.

We investigate van der Waals (vdW) heterostructures made of germanene, stanene, or silicene with hexagonal boron nitride (h-BN). The intriguing topological properties of these buckled honeycomb materials can be maintained and further engineered in the heterostructures, where the competition between the substrate effect and external electric fields can be used to control the tunable topological phase transitions. Using such heterostructures as building blocks, various vdW topological domain walls (DW) are designed, along which there exist valley polarized quantum spin Hall edge states or valley-contrasting edge states which are protected by valley(spin)- resolved topological charges and can be tailored by the patterning of the heterojunctions and by external fields.

We show that the electron mobility in ideal, free-standing two-dimensional “buckled” crystals with broken horizontal mirror (${\sigma }_{\mathrm{h}}$) symmetry and Dirac-like dispersion (such as silicene and germanene) is dramatically affected by scattering with the acoustic flexural modes (ZA phonons). This is caused both by the broken ${\sigma }_{\mathrm{h}}$ symmetry and by the diverging number of long-wavelength ZA phonons, consistent with the Mermin-Wagner theorem. Non-${\sigma }_{\mathrm{h}}$-symmetric, “gapped” 2D crystals (such as semiconducting transition-metal dichalcogenides with a tetragonal crystal structure) are affected less severely by the broken ${\sigma }_{\mathrm{h}}$ symmetry, but equally seriously by the large population of the acoustic flexural modes. We speculate that reasonable long-wavelength cutoffs needed to stabilize the structure (finite sample size, grain size, wrinkles, defects) or the anharmonic coupling between flexural and in-plane acoustic modes (shown to be effective in mirror-symmetric crystals, like free-standing graphene) may not be sufficient to raise the electron mobility to satisfactory values. Additional effects (such as clamping and phonon stiffening by the substrate and/or gate insulator) may be required.

We present a generalized model to describe the lattice thermal conductivity of low-dimensional (low-D) and disordered systems. The model is a straightforward generalization of the Debye-Peierls and Allen-Feldman schemes to arbitrary dimensions, accounting for low-D effects such as differences in dispersion, density of states, and scattering. Similar in spirit to the Allen-Feldman approach, heat carriers are categorized according to their transporting capacity as propagons, diffusons, and locons. The results of the generalized model are compared to experimental results when available, and equilibrium molecular dynamics simulations otherwise. The results are in very good agreement with our analysis of phonon localization in disordered low-D systems, such as amorphous graphene and glassy diamond nanothreads. Several unique aspects of thermal transport in low-D and disordered systems, such as milder suppression of thermal conductivity and negligible diffuson contributions, are captured by the approach.

Despite fermion doubling, a two-dimensional quasi-relativistic spin-1/2 system can still lead to true fractionalization of electrical charge, when a massive ordered phase supports a “half-vortex”. Such topological defect is possible when the order parameter in form of Dirac mass is described by two $U\left(1\right)$ angles, and each of them winds by an angle $\pi$ around a point. We demonstrate that such a mass configuration in an eight-dimensional Dirac Hamiltonian exhibits only a single bound zero mode, and therefore binds the charge of $e/2$. In graphene, for example, such an ordered phase is provided by the easy-plane spin-triplet Kekule valence bond solid. We argue that an application of an in-plane magnetic field can cause an excitonic instability toward such ordered phase, even for weak repulsion, when the on-site, nearest-neighbor, and second neighbor components of it are of comparable strengths.

Epitaxial growth of Ag on Fe(100) and postdeposition relaxation have been studied in several experiments. We provide a first-principles density functional theory analysis of key adatom interaction energies and diffusion barriers controlling growth and relaxation kinetics for the submonolayer regime, as these have not been assessed previously. A cluster expansion approach is used to obtain an extensive set of conventional lateral interactions between adatoms on fourfold hollow adsorption sites. We find robust oscillatory decay of pair interactions with increasing separation, and of trio interactions with increasing perimeter length. First- and second-nearest-neighbor pair interactions, as well as compact linear and bent trio interactions, dominate. The adatom terrace diffusion barrier is estimated to be $E_{\mathrm{d}}\approx 0.39$ eV. We also provide a limited analysis of unconventional interactions for which one adatom is at the bridge-site transition state for hopping and one or more others are at fourfold hollow sites. Energy barriers for diffusion along island edges can be determined with the aid of both conventional and unconventional interactions.

A general formula for the orbital magnetic moment of interacting electrons in solids is derived using the many-electron Green's function method. The formula factorizes into two parts, a part that contains the information about the one-particle band structure of the system and a part that contains the effects of exchange and correlations carried by the Green's function. The derived formula provides a convenient yet rigorous means of including the effects of exchange and correlations beyond the commonly used local density approximation of density functional theory.

Strong topological insulators may have nonzero weak indices. The nonzero weak indices allow for the existence of topologically protected helical states along line defects of the lattice. If the lattice admits line defects that connect opposite surfaces of a slab of such a “weak-and-strong” topological insulator, these states effectively connect the surface states at opposite surfaces. Depending on the phases accumulated along the dislocation lines, this connection results in a suppression of in-plane transport and the opening of a spectral gap or in an enhanced density of states and an increased conductivity.

We predict that a temperature gradient can induce a magnon-mediated intrinsic torque in systems with a nontrivial magnon Berry curvature. With the help of a microscopic linear response theory of nonequilibrium magnon-mediated torques and spin currents we identify the interband and intraband components that manifest in ferromagnets with Dzyaloshinskii-Moriya interactions and magnetic textures. To illustrate and assess the importance of such effects, we apply the linear response theory to the magnon-mediated spin Nernst and torque responses in a kagome lattice ferromagnet.

We study the Ruderman-Kittel-Kasuya-Yosida effective exchange interaction between magnetic impurities embedded on the edges of transition metal dichalcogenide flakes, using a three-orbital tight-binding model. Electronic states lying midgap of the bulk structure have a strong one-dimensional (1D) character, localized on the edges of the crystallite. This results in exchange interactions with $1/r$ (or slower) decay with distance $r$, similar to other 1D systems. Most interestingly, however, the strong spin-orbit interaction in these materials results in sizable noncollinear Dzyaloshinskii-Moriya interactions between impurities, comparable in size to the usual Ising and in-plane components. Varying the relevant Fermi energy by doping or gating may allow one to modulate the effective interactions, controlling the possible helical ground state configurations of multiple impurities.

We report a strong angular asymmetry of optical second-harmonic generation (SHG) in plasmonic crystals formed by an Au grating on top of an anisotropic bismuth-thulium iron garnet (BTIG) film. We found that a weak anisotropy-driven angular SHG asymmetry of the crystalline BTIG is resonantly enhanced and reaches 95% as the surface plasmon polariton (SPP) is excited at the Au/BTIG interface. The asymmetry introduced by an interference of the odd and even (with respect to the angle of incidence) SHG fields is attributed to the anisotropy of the BTIG film. The angular SHG spectra are reproduced using a simple model with a resonant SPP-induced SHG contribution from the anisotropic Au/BTIG interface. The observed asymmetry of the SHG provides valuable insight into the mechanisms of nonlinear plasmonics and can noticeably expand its abilities regarding active light conversion and manipulation.

Motivated by recent numerical discovery of a gapped spin liquid phase in spin-$1/2$ triangular-lattice $J_1-J_2$ Heisenberg model, we classify symmetric $Z_2$ spin liquids on triangular lattice in the Abrikosov-fermion representation. We find 20 phases with distinct spinon symmetry quantum numbers, eight of which have their counterparts in the Schwinger-boson representation. Among them we identify two promising candidates (#1 and #20), which can realize a gapped $Z_2$ spin liquid with up to next nearest neighbor mean-field amplitudes. We analyze their neighboring magnetic orders and valence bond solid patterns, and find one state (#20) that is connected to 120-degree Neel order by a continuous quantum phase transition. We also identify gapped nematic $Z_2$ spin liquids in the neighborhood of the symmetric states and find three promising candidates (#1, #6, and #20).

We propose a theory of metals at the spin-density-wave quantum-critical point in spatial dimension $d=2$. We provide a first estimate of the full set of critical exponents (dynamical exponent $z=2.13$, correlation length $\nu =1.02$, spin susceptibility $\gamma =0.96$, electronic non-Fermi liquid ${\eta }_{\tau }^f=0.53$, spin-wave Landau damping ${\eta }_{\tau }^b=1.06$), which determine the universal power laws in thermodynamics and response functions in the quantum-critical regime relevant for experiments in heavy-fermion systems and iron pnictides. We present approximate numerical and analytical solutions of Polchinski-Wetterich–type flow equations with soft frequency regulators for an effective action of electrons coupled to spin-wave bosons. Performing the renormalization group in frequency instead of momentum space allows to track changes of the Fermi-surface shape and to capture Landau damping during the flow. The technique is easily generalizable from models retaining only patches of the Fermi surface to full, compact Fermi surfaces.

We present a systematic assessment of the structural properties, the electronic density of states, the charge densities, and the phase stabilities of ${\mathrm{AgInSe}}_2$ and ${\mathrm{AuInSe}}_2$ using screened-exchange hybrid density functional theory, and compare their properties to those of ${\mathrm{CuInSe}}_2$. For ${\mathrm{AgInSe}}_2$, hybrid density functional theory properly captures several experimentally measured properties, including the increase in the band gap and the change in the direction of the lattice distortion parameter $u$ in comparison to ${\mathrm{CuInSe}}_2$. While the electronic properties of ${\mathrm{AuInSe}}_2$ have not yet been experimentally characterized, we predict it to be a small gap ($\approx 0.15$ eV) semiconductor. We also present the phase stability of ${\mathrm{AgInSe}}_2$ and ${\mathrm{AuInSe}}_2$ according to screened-exchange density functional theory, and compare the results to predictions from conventional density functional theory, results tabulated from several online materials data repositories, and experiment (when available). In comparison to conventional density functional theory, the hybrid functional predicts phase stabilities of ${\mathrm{AgInSe}}_2$ in better agreement with experiment: discrepancies in the calculated formation enthalpies are reduced by approximately a factor of 3, from $\approx 0.20$ eV/atom to $\approx 0.07$ eV/atom, similar to the improvement observed for ${\mathrm{CuInSe}}_2$. We further predict that ${\mathrm{AuInSe}}_2$ is not a stable phase, and can only be present under nonequilibrium conditions.

We provide an atomistic tight-binding description of a few carriers confined in ambipolar ($n-p$) double quantum dots defined in a semiconducting carbon nanotube. We focus our attention on the charge configuration in which Pauli blockade of the current flow is observed [F. Pei et al., Nat. Nanotechnol. 7, 630 (2012); E. A. Laird et al., Nat. Nanotechnol. 8, 565 (2013)] with a single excess electron in the $n$ dot and a single hole in the $p$ dot. We use the configuration interaction approach to determine the spin-valley structure of the states near the neutrality point and discuss its consequences for the interdot exchange interaction, the degeneracy of the energy spectrum, and the symmetry of the confined states. We calculate the transition energies lifting the Pauli blockade and analyze their dependence on the magnetic field vector. Furthermore, we introduce bending of the nanotube and demonstrate its influence on the transition energy spectra. The best qualitative agreement with the experimental data is observed for nanotubes deflected in the gated areas in which the carrier confinement is induced.

We report nonlinear resonance fluorescence of an individual semiconductor quantum dot. By driving a single semiconductor quantum dot via a two-photon transition, we probe the linewidth of two-photon excitation processes and show that, similar to their single-photon counterparts, they are close to being Fourier limited at low temperatures. The evolution of the population of excitonic states with increasing Rabi energy exhibits a clear S-shaped behavior, indicative of the nonlinear response via the two-photon excitation process. Numerical calculations of the nonlinear response using a four-level atomic system representing the manifold of excitonic and biexcitonic states in the quantum dot are in excellent agreement with our experiments and reveal the effect of interactions with LA phonons in the solid-state environment. Finally, we demonstrate the formation of dressed states emerging from a nonlinear two-photon interaction between the quantum dot and the optical excitation field. The nonlinear optical dressing induces a mixing of all four excitonic states that allows direct optical tuning of the polarization selection rules and energies of the dressed states in the artificial atom. We expect our results to play a pivotal role for the generation of nonclassical photon pairs desired for applications in quantum communication and fundamental experiments on quantum optical properties of photons.