GaN is of great importance for optoelectronic and power electronic devices, and carbon is one of its most common impurities, used also actively for preparing semi-insulating buffer layers. Experimental investigations reveal at least five carbon-related centers, however, theory could provide definite assignment so far for only two of those: in terms of simple carbon substitutionals on nitrogen $\left({\mathrm{C}}_{\mathrm{N}}\right)$ and gallium $\left({\mathrm{C}}_{\mathrm{Ga}}\right)$ sites. We apply here an optimized hybrid functional (which reproduces the band gap and is Koopmans’ compliant) to investigate small carbon complexes. We find that an increasing carbon concentration, beyond the compensation of the $n$-type doping by the acceptor ${\mathrm{C}}_{\mathrm{N}}$, makes its complexes with ${\mathrm{C}}_{\mathrm{Ga}}$ and ${\left(\mathrm{C}-\mathrm{N}\right)}_{\mathrm{N}}$ split interstitials competitive in formation energy. Except for $p$-doped samples, these complexes are electrically passive, so the Fermi level is pinned a little below midgap, ensuring the semi-isolating behavior even for high carbon concentrations. The charge transition levels, calculated by the optimized functional, allow the interpretation of all carbon-related deep-level spectra as well as of their relation to the carbon-to-donor ratio.

Since the charged mode is much faster than the neutral modes on quantum Hall edges at large filling factors, the edge may remain out of equilibrium in thermal conductance experiments. This sheds light on the observed imperfect quantization of the thermal Hall conductance at $\nu =8/3$ and can increase the observed thermal conductance by two quanta at $\nu =8/5$. Under certain unlikely but not impossible assumptions, this might also reconcile the observed thermal conductance at $\nu =5/2$ with not only the PH-Pfaffian order but also the anti-Pfaffian order.

We study adiabatic charge pumping through a Majorana bound state tunnel coupled to multiple normal leads. We show that, for most of the parameters, such a pump does not lead to any net pumped charge between the various leads unless a multiply connected geometry is implemented. We introduce an Aharonov–Bohm ring geometry at the junction to implement such a multiply connected geometry. We further show that the Fourier transform of the pumped charge with respect to the flux inserted through the ring shows a clear distinction between the case of an Andreev bound state and the Majorana bound state. Hence such a Fourier analysis can serve as a diagnostic for the detection of Majorana bound states in the proposed geometry.

One of the typical features of a Majorana zero mode (MZM) at the end of a topological superconductor is a zero-bias peak in the tunneling spectroscopy of the normal-lead-superconductor junction. In this paper we study a model with one phonon mode coupling to the superconductor lead of the normal-lead-superconductor junction, which can be viewed as an electron-lead/phonon-coupled-MZM/hole-lead structure. The phonon-coupled MZM acts as a series of channels in which an electron can turn into a hole by absorbing and emitting phonons. These channels present in the local density of states (LDOS) as a series of stripes, generating the corresponding peaks in the tunneling spectroscopy. In LDOS, the electron-phonon interaction narrows and redistributes the weight among stripes. In the tunneling spectroscopy, the heights of peaks present a feature of the multiphonon process. With these investigations, our work illuminates the mechanism of phonon-assisted Andreev reflection at a Majorana zero mode.

The contribution of bulk inversion asymmetry to the total spin-orbit coupling is commonly neglected for group III-V nanowires grown in the generic [111] direction. We have solved the complete Hamiltonian of the circular nanowire accounting for bulk inversion asymmetry via exact numerical diagonalization. Three different symmetry classes of angular momentum states exist, which reflects the threefold rotation symmetry of the crystal lattice about the [111] axis. A particular group of angular momentum states contains degenerate modes which are strongly coupled via the Dresselhaus Hamiltonian, which results in a significant energy splitting with increasing momentum. Hence, under certain conditions Dresselhaus spin-orbit coupling is relevant for [111] InAs and [111] InSb nanowires. We demonstrate momentum-dependent energy splittings and the impact of Dresselhaus spin-orbit coupling on the dispersion relation. In view of possible spintronics applications relying on bulk inversion asymmetry we calculate the spin expectation values and the spin texture as a function of the Fermi energy. Finally, we investigate the effect of an axial magnetic field on the energy spectrum and on the corresponding spin polarization.

We report on results of our analysis of the $c$-axis infrared conductivity, ${\sigma }_c\left(\omega \right)$, of bilayer ${\mathrm{LnBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (Ln=La, Nd, Y) and trilayer ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ high-$T_{\mathrm{c}}$ superconductors. The analysis employs the multilayer model involving the conductivity of the bilayer or trilayer unit, ${\sigma }_{\mathrm{bl}}\left(\omega \right)$, and that of the spacing layers separating the latter units, ${\sigma }_{\mathrm{int}}\left(\omega \right)$. For the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ sample with concentration of holes $p=0.09$, our fitting of the data strongly suggests that at low temperatures, the conductivity ${\sigma }_{\mathrm{bl}}\left(\omega \right)$ possesses a pronounced and narrow Drude peak. For samples with $p\ge 0.115$ however, the fitting indicates that ${\sigma }_{\mathrm{bl}}\left(\omega \right)$ is, at low temperatures, dominated by a mode at a finite energy in the range from 30 to 60 meV. The properties of this resonance are in accord with those of a collective mode that appears in the spectra of ${\sigma }_{\mathrm{bl}}\left(\omega \right)$ calculated using a microscopic gauge-invariant theory of ${\sigma }_c\left(\omega \right)$ by J. Chaloupka and coworkers [Phys. Rev. B 79, 184513 (2009)]. The frequency and spectral weight of the latter mode are determined by the magnitude of the splitting between the bonding and the antibonding band of the bilayer or trilayer unit. Our results, in conjunction with the microscopic theory, thus demonstrate that in moderately underdoped bilayer and trilayer high-$T_{\mathrm{c}}$ cuprates the bilayer (or trilayer) splitting is already developed. The observed doping dependence is consistent with results from angular resolved photoemission spectroscopy.

The compound $\mathrm{BaIrS}{\mathrm{i}}_2$ was successfully synthesized under high pressure at 6 GPa. $\mathrm{BaIrS}{\mathrm{i}}_2$ crystalizes in an orthorhombic symmetry with the noncentrosymmetric space group $C{222}_1$ and the lattice parameters $a=15.0492\left(1\right)\AA , b=8.0311\left(1\right)\AA$, and $c=8.0314\left(1\right)\AA$. $\mathrm{BaIrS}{\mathrm{i}}_2$ shows superconductivity below a critical temperature $T_c\sim 6\mathrm{K}$ and an upper critical field ${\mu }_0{H}_{c2}\left(0\right)\sim 6\mathrm{T}$ with a Ginzburg-Landau parameter ${\kappa }_{\mathrm{GL}}\sim 87$. Specific-heat measurements suggest that $\mathrm{BaIrS}{\mathrm{i}}_2$ is an intermediate-coupling superconductor with a moderate electron-phonon coupling constant ${\lambda }_{ep}\sim 0.8$. The Sommerfeld constant is ${\gamma }_n\sim 5.5\mathrm{mJ}/\mathrm{mole}{\mathrm{K}}^2$. Phonon mediation seems to be dominant for Cooper pairing in $\mathrm{BaIrS}{\mathrm{i}}_2$. The ab initio calculations revealed that $\mathrm{BaIrS}{\mathrm{i}}_2$ has four Fermi surfaces split by spin-orbit coupling. The Cooper pairing seems to occur between electrons without spin degeneracy.

Magnetization, specific heat, and electron spin resonance (ESR) measurements are carried out to clarify the low-energy excitations for the $S=1/2$ polyhedral clusters $\left\{\mathrm{M}{\mathrm{o}}_{72}{\mathrm{V}}_{30}\right\}$ and $\left\{{\mathrm{W}}_{72}{\mathrm{V}}_{30}\right\}$. The clusters provide unique model systems of a kagomé network on a quasisphere. The linear field variation of magnetization at low temperatures indicates that the ground state is singlet for both clusters. The temperature and the magnetic field dependence of specific heat shows a distinct difference between the two clusters with differing structural symmetries. In $\left\{{\mathrm{W}}_{72}{\mathrm{V}}_{30}\right\}$ with $I_{\mathrm{h}}$ symmetry, the existence of several tens of low-energy singlet excited states below the lowest triplet excited state is revealed. The specific heat of the slightly distorted $\left\{\mathrm{M}{\mathrm{o}}_{72}{\mathrm{V}}_{30}\right\}$ with $D_{5\mathrm{h}}$ symmetry leads to a drastic decrease of the singlet contribution, which is consistent with the partial lifting of the frustration and the decrease of degenerated low-energy states. The singlet excitation existence is confirmed further by the temperature dependence of the ESR spectra. Comprehensive experimental studies have demonstrated unique low-energy excitations of the spherical kagomé networks and their sensitivity to the cluster symmetry.

Based on micromagnetic simulation and analysis of Thiele's equation, in this work we demonstrate that ratchet motion of a skyrmion can be induced by a biharmonic in-plane magnetic field $h_x\left(t\right)=h_1\sin \left(m\omega t\right)+h_2\sin (n\omega t+\phi )$, provided that integers $m$ and $n$ are coprime and that $m+n$ is odd. Remarkably, the speed and direction of the ratchet motion can be readily adjusted by the field amplitude, frequency, and phase, with the maximum speed being over 5 m/s and the direction rotatable over 360°. The origin of the skyrmion ratchet motion is analyzed by tracing the excitation spectra of the dissipation parameter $\mathcal{D}$ and the skyrmion position R, and it shows that the dissipative force plays a key role in the appearance of ratchet motion. Such a ratchet motion of a skyrmion is distinguished from those caused by single-frequency ac drives reported in the literature, and from that driven by pulsed magnetic fields as also predicted in this work. Our results show that skyrmion ratchet effect under biharmonic forces shares some common features with those found in many soliton systems, and the facile controllability of both the skyrmion speed and direction should be useful in practice.

Ferromagnetic Weyl semimetals exhibit an anomalous Hall effect, a consequence of their topological properties. In the noninteracting case, the derivative of the orbital magnetization with respect to chemical potential is proportional to this anomalous Hall effect, the Středa formula. Motivated by compounds such as ${\text{Mn}}_3\text{Sn}$, here we investigate how interactions modeled by a Hubbard $U$ impact on both quantities when the Fermi energy is either aligned with the Weyl nodes or away from them. Using dynamical mean-field theory, we first find interaction-induced Mott- and band-insulating transitions. In the Weyl semimetal regime, away from insulators, interactions lead to an increase in the imbalance between the densities of spin species induced by a Zeeman term $h$. This increased imbalance leads to an increase of the anomalous Hall effect that can also be understood from the displacement of the Weyl nodes and topological arguments. This interaction-induced spin imbalance also compensates the reduction in orbital magnetization of each spin species that comes from smaller quasiparticle weight. In the small interaction regime, the combined effects lead to an orbital magnetization that depends weakly on interaction and still changes linearly with chemical potential at small doping. In the intermediate and strong correlation regimes, the localization due to interaction affects strongly the orbital magnetization, which becomes small. A quasiparticle picture explains the anomalous Hall effect but does not suffice for the orbital magnetization. We propose a modified Středa formula to relate anomalous Hall effect and orbital magnetization in the weak correlation limit. We also identify mirror and particle-hole symmetries of the lattice model that explain, respectively, the vanishing of the anomalous Hall effect at $h=0$ for all $U$ and $\mu$ of the orbital magnetization at half-filling $\mu =0$ for all $U$ and $h$.

Self-assembled InAs quantum dots (QDs), which have long hole-spin coherence times and are amenable to optical control schemes, have long been explored as building blocks for qubit architectures. One such design consists of vertically stacking two QDs to create a QD molecule (QDM) and using the spin-mixing properties of “moleculelike” coupled hole states for all-optical qubit manipulation. In this paper, the first of two papers, we introduce the incorporation of dilute ${\mathrm{GaBi}}_x{\mathrm{As}}_{1-x}$ alloys in the barrier region between the two dots. ${\mathrm{GaBi}}_x{\mathrm{As}}_{1-x}$ is expected to increase the spin mixing of the molecular states needed for qubit operations by raising the barrier valence-band edge and spin-orbit splitting. Using an atomistic tight-binding model, we compute the properties of ${\mathrm{GaBi}}_x{\mathrm{As}}_{1-x}$ and the modification of hole states that arise when the alloy is used in the barrier of an InAs QDM. An atomistic treatment is necessary to correctly capture nontraditional alloy effects such as the band-anticrossing valence band. It also allows for the study of configurational variances and clustering effects of the alloy. We find that in InAs QDMs with a GaBiAs interdot barrier, electron states are not strongly affected by the inclusion of Bi. However, hole states are much more sensitive to the presence and configuration of Bi in the barriers. By independently studying the alloy-induced strain and electronic scattering off Bi and As orbitals, we conclude that an initial increase in QDM hole-state energy at low Bi concentration is caused by the alloy-induced strain. We further find that the decrease in QDM hole energy at higher Bi concentrations can only be explained when both alloy strain and orbital effects are considered. In our second paper, we use the understanding developed here to discuss how the alloyed barriers contribute to enhancement in hole spin-mixing and the implications for QDM qubit architectures.

We report investigation of the geometry and electronic structure of iridium atoms adsorbed onto graphene through a combined experimental and theoretical study. Ir atoms were deposited onto a flake of graphene on a Pt(111) surface and found to form clusters even at low temperatures. The areal density of the observed clusters on the graphene flake suggests the clusters are most likely pairs of Ir atoms. Theoretical ab initio density functional (DFT) calculations indicate that these Ir dimers are oriented horizontally, near neighboring “bridge” sites of the graphene lattice, as this configuration has the strongest adsorption energy of all high-symmetry configurations for the Ir dimer. A large peak in the local density of states (LDOS) at the Dirac point energy was measured via scanning tunneling spectroscopy, and this result is reproduced by a DFT calculation of the LDOS. The peak at the Dirac point energy is found to be from the Ir $s$ and $p$ states. The LDOS in the monomer case was also calculated, and is found to significantly differ from the experimentally determined data, further supporting the hypothesis of low-temperature clustering.

We consider a quantum quench from the strongly correlated ground state of the Kondo model to a Fermi sea. We calculate the overlap between the ground states before and after the quench, as well as the Loschmidt echo, i.e., the transition amplitude between the initial state and the evolved state at a time $t$ after the quench. The overlap is known to determine the dynamics of the echo at large times. We show, in addition, that the overlap depends algebraically on the emergent Kondo length, with a power-law exponent that is the difference of long and short-time contributions that appear in the echo. Our result suggests that, in general, there may be more information contained in the overlap than previously recognized.

A theoretical study of the thermoelectric current and energy harvesting in an interacting double-quantum-dot system, connected to reservoirs held at different chemical potentials and temperatures, is presented. Using a rate-equation approach, the current is evaluated for different energetic configurations of the double quantum dot. We discuss in detail the current-temperature gradient relations (the thermoelectric analog to current-voltage relations) and demonstrate that, due to interactions, the current is nonmonotonically dependent on thermal bias. This interaction-induced nonmonotonicity influences the possibility of harvesting thermal energy from the double quantum dot, and it is shown that in some configurations, energy cannot be harvested at all. We analyze the conditions under which energy can, indeed, be harvested and converted to useful electrical power and the optimal conditions for thermoelectric energy conversion.

The high-$T_c$ superconducting cuprates are recognized as doped Mott insulators. Several studies indicate that as a function of doping and temperature, there is a crossover from this regime into a phase characterized as a marginal Fermi liquid. Several calculations of the doped Mott insulating phase indicate that the Fermi surface defines small pockets which at the higher doping levels switch to a full closed Fermi surface, characteristic of a more metallic system. Here we use femtosecond laser-based pump-probe techniques to investigate the structure of the Fermi surface in the underdoped region of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\delta }$ and compare it with that associated with the optimally doped material. We confirm the concept of a small pocket in the underdoped system consistent with theoretical predictions in this strongly correlated state.

We report thermal expansion and magnetostriction of the cubic non-Kramers system ${\mathrm{PrIr}}_2{\mathrm{Zn}}_{20}$ with a nonmagnetic ${\mathrm{\Gamma }}_3$ ground-state doublet. In previous experiments, antiferroquadrupolar order at $T_{\mathrm{Q}}=0.11$ K and a Fermi liquid state at around $B_{\mathrm{c}}\approx 5\mathrm{T}$ for $\mathit{B}\parallel \left[001\right]$, indicative of possible ferrohastatic order, were discovered. For magnetic fields $\mathit{B}\parallel \left[001\right]$, the low-temperature longitudinal and transverse thermal expansion and magnetostriction are highly anisotropic. The resulting volume strain is very small, indicating that the Pr valence remains nearly constant as a function of magnetic field. We conclude that the Fermi liquid state around $B_{\mathrm{c}}$ forms through a very little change in $c\text{−}f$ hybridization. This result is in sharp contrast to Ce- and Yb-based Kramers Kondo lattices, which show significantly larger volume strains due to the high sensitivity of the Kondo temperature to hydrostatic pressure.

The sine-Gordon—equivalently, the massive Thirring—Hamiltonian is ubiquitous in low-dimensional physics, with applications that range from cold atom and strongly correlated systems to quantum impurities. We study here its nonequilibrium dynamics using the quantum quench protocol—following the system as it evolves under the sine-Gordon Hamiltonian from initial Mott-type states with large potential barriers. By means of the Bethe ansatz, we calculate exactly the Loschmidt amplitude, the fidelity, and work distribution characterizing these quenches for different values of the interaction strength. Some universal features are noted as well as an interesting duality relating quenches in different parameter regimes of the model.

We theoretically investigate the orbital effects of an in-plane magnetic field on the spectrum of a quantum dot embedded in a two-dimensional electron gas (2DEG). We derive an effective two-dimensional Hamiltonian where these effects enter in proportion to the flux penetrating the 2DEG. We quantify the latter in detail for harmonic, triangular, and square potential of the heterostructure. We show how the orbital effects allow one to extract a wealth of information, for example, on the heterostructure interface, the quantum dot size and orientation, and the spin-orbit fields. We illustrate the formalism by extracting this information from recent measured data [L. C. Camenzind et al., arXiv:1804.00162; Nat. Commun. 9, 3454 (2018)].

Flexoelectricity is a form of electromechanical coupling that has recently emerged because, unlike piezoelectricity, it is theoretically possible in any dielectric material. Two-dimensional (2D) materials have also garnered significant interest because of their unusual electromechanical properties and high flexibility, but the intrinsic flexoelectric properties of these materials remain unresolved. In this work, using atomistic modeling accounting for charge-dipole interactions, we report the intrinsic flexoelectric constants for a range of two-dimensional materials, including graphene allotropes, nitrides, graphene analogs of group-IV elements, and the transition metal dichalcogenides (TMDCs). We accomplish this through a proposed mechanical bending scheme that eliminates the piezoelectric contribution to the total polarization, which enables us to directly measure the flexoelectric constants. While flat 2D materials like graphene have low flexoelectric constants due to weak $\pi -\sigma$ interactions, buckling is found to increase the flexoelectric constants in monolayer group-IV elements. Finally, due to significantly enhanced charge transfer coupled with structural asymmetry due to bending, the TMDCs are found to have the largest flexoelectric constants, including ${\mathrm{MoS}}_2$ having a flexoelectric constant ten times larger than graphene.