A pseudomagnetic field becomes an experimental reality after the observation of zero-field Landau level-like quantization in strained graphene, but it is not expected that the time-reversal symmetric pseudomagnetic fields will have any effect on the spin degree of freedom of the charge carriers. Here, we demonstrate that spin-orbit coupling (SOC) could act as a bridge between the pseudomagnetic field and spin. In quantum spin Hall (QSH) states, the direction of the spin of edge states is tied to their direction of motion because of the SOC. The pseudomagnetic field affects the clockwise and counterclockwise edge currents of the QSH states and consequently lifts the degenerate edge states of opposite spin orientation. Because of opposite signs of the pseudomagnetic field in two valleys of graphene, the one-dimensional charge carriers at the two opposite edges have different group velocities, and in some special cases, the edge states can only propagate at one edge of the nanoribbon, and the group velocity at the other edge becomes zero.

We show how machine learning techniques based on Bayesian inference can be used to enhance the computer simulation of molecular materials, focusing here on water. We train our machine-learning algorithm using accurate, correlated quantum chemistry, and predict energies and forces in molecular aggregates ranging from clusters to solid and liquid phases. The widely used electronic-structure methods based on density functional theory (DFT) by themselves give poor accuracy for molecular materials like water, and we show how our techniques can be used to generate systematically improvable one- and two-body corrections to DFT with modest extra resources. The resulting corrected DFT scheme is considerably more accurate than uncorrected DFT for the relative energies of small water clusters and different ice structures and significantly improves the description of the structure and dynamics of liquid water. However, our results for ice structures and the liquid indicate that beyond-two-body DFT errors cannot be ignored, and we suggest how our machine-learning methods can be further developed to correct these errors.

For spin-order-driven ferroelectricity, a general theory that includes both purely electronic and ion-displacement contributions simultaneously is lacking. Here we present a realistic method for describing the ion-displacement part of the ferroelectricity and combine it with the pure electronic description proposed earlier to formulate a unified model that can describe any spin order including an incommensurate one. This unified model reproduces first-principles results known for representative multiferroics and provides insight into the origin of ferroelectricity and magnetoelectric coupling. We find that multiferroicity in spiral magnets LiCuVO${}_4$ and TbMnO${}_3$ can be explained by our unified model, but not by previous spin current models. The magnetoelectric effect observed for BiFeO${}_3$ in the cycloidal spiral state is found to originate from the exchange striction.

By direct calculations of the spin gap in the frustrated Heisenberg model on the square lattice, with nearest- ($J_1$) and next-nearest-neighbor ($J_2$) super-exchange couplings, we provide a solid evidence that the spin-liquid phase in the frustrated regime $0.45\lesssim J_2/J_1\lesssim 0.6$ is gapless. Our numerical method is based on a variational wave function that is systematically improved by the application of few Lanczos steps and allows us to obtain reliable extrapolations in the thermodynamic limit. The peculiar nature of the nonmagnetic state is unveiled by the existence of $S=1$ gapless excitations at $k=(\pi ,0)$ and $(0,\pi )$. The magnetic transition can be described and interpreted by a variational state that is built from Abrikosov fermions having a $Z_2$ gauge structure and four Dirac points in the spinon spectrum.

I consider refrigeration and heat engine circuits based on the nonlinear thermoelectric response of point contacts at pinch off, allowing for electrostatic interaction effects. I show that a refrigerator can cool to much lower temperatures than predicted by the thermoelectric figure of merit $ZT$ (which is based on linear-response arguments). The lowest achievable temperature has a discontinuity, called a fold catastrophe in mathematics, at a critical driving current $I=I_{\mathrm{c}}$. For $I>I_{\mathrm{c}}$ one can in principle cool to absolute zero, when for $I<I_{\mathrm{c}}$ the lowest temperature is about half the ambient temperature. Heat backflow due to phonons and photons stops cooling at a temperature above absolute zero, and above a certain threshold turns the discontinuity into a sharp cusp. I also give a heuristic condition for when an arbitrary system's nonlinear response means that its $ZT$ ceases to indicate (even qualitatively) the lowest temperature to which the system can refrigerate.

The thermal conductivity of electron-doped Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals is investigated below 200 K, with an emphasis on the behavior near the magnetic and superconducting ($T_c$) transition temperatures. An enhancement of the in-plane thermal conductivity ${\kappa }_{ab}$ is observed below $T_c$ for all samples, with the greatest enhancement observed near optimal doping. The observed trends are consistent with the scattering of heat carriers by low-energy magnetic excitations. Upon entering the superconducting state, the formation of a spin gap leads to reduced scattering and an enhancement in $\kappa \left(T\right)$. Similarly, an enhancement of $\kappa$ is observed for polycrystalline BaFe${}_2$As${}_2$ below the magnetic transition, and qualitative differences in $\kappa \left(T\right)$ between single crystalline and polycrystalline BaFe${}_2$As${}_2$ are utilized to discuss anisotropic scattering. This study highlights how measuring $\kappa$ near $T_c$ in novel superconductors can be useful as a means to probe the potential role of spin fluctuations.

A series of ultrathin epitaxial films of EuNiO${}_3$ (ENO) were grown on a set of substrates traversing from compressive ($-2.4\%$) to tensile (+2.5%) lattice mismatch. On moving from tensile to compressive strain, transport measurements demonstrate a successively suppressed Mott insulating behavior eventually resulting in a complete suppression of the insulating state at high compressive strain. Corroborating these findings, resonant soft x-ray absorption spectroscopy at the Ni $L_{3,2}$ edge reveals the presence of a strong multiplet splitting in the tensile strained samples that progressively weakens with increasing compressive strain. Combined with cluster calculations, the results show how cumulatively enhanced covalency (i.e., bandwidth) between Ni $d$ and O $p$ orbital derived states leads to the emergent metallic ground state not attainable in the bulk ENO.

We report on the extension and implementation of the Sternheimer-GW method introduced by Giustino et al. [Phys. Rev. B 81, 115105 (2010)] to the case of first-principles pseudopotential calculations based on a plane-waves basis. The Sternheimer-GW method consists of calculating the GW self-energy operator without resorting to the standard expansion over unoccupied Kohn-Sham electronic states. The Green's function is calculated by solving linear systems for frequencies along the real axis. The screened Coulomb interaction is calculated for frequencies along the imaginary axis by using the Sternheimer equation. Analytic continuation to the real axis is performed using Padé approximants. The generalized plasmon-pole approximation is avoided by performing explicit calculations at multiple frequencies using Frommer's multishift solver. We demonstrate our methodology by reporting tests on common insulators and semiconductors, including Si, diamond, LiCl, and SiC. Our calculated quasiparticle energies are in agreement with the results of fully converged calculations based on the sum-over-states approach. As the Sternheimer-GW method yields the complete self-energy $\Sigma (\mathbf{r},{\mathbf{r}}^{\prime },\omega )$ and not only its expectation values on Kohn-Sham states, this work opens the way to nonperturbative GW calculations and to direct calculations of spectral functions for angle-resolved photoemission spectroscopy. As an example of the capabilities of the method we calculate the G${}_0$W${}_0$ spectral functions of silicon and diamond.

We use a self-consistent Hartree-Fock approximation with realistic Coulomb interactions for $\pi$-band electrons to explore the possibility of broken symmetry states in weakly disordered ABC-stacked trilayer graphene. The competition between gapped and gapless broken symmetry states and normal states is studied by comparing total energies. We find that gapped states are favored and that, unlike the bilayer case, gapless nematic broken symmetry states are not metastable. Among the gapped states, the layer antiferromagnetic state is favored over anomalous and spin Hall states.

In this paper we present a paradigmatic tight-binding model for single-layer as well as multilayered semiconducting MoS${}_2$ and similar transition metal dichalcogenides. We show that the electronic properties of multilayer systems can be reproduced in terms of a tight-binding modeling of the single-layer hopping terms by simply adding the proper interlayer hoppings ruled by the chalcogenide atoms. We show that such a tight-binding model makes it possible to understand and control in a natural way the transition between a direct-gap band structure, in single-layer systems, and an indirect gap in multilayer compounds in terms of a momentum/orbital selective interlayer splitting of the relevant valence and conduction bands. The model represents also a suitable playground to investigate in an analytical way strain and finite-size effects.

The rates of a hindered molecular rotation induced by tunneling electrons are evaluated using scattering theory within the sudden approximation. Our approach explains the excitation of copper phthalocyanine molecules (CuPc) on Cu(111) as revealed in a recent measurement of telegraph noise in a scanning tunneling microscopy experiment [Schaffert et al., Nat. Mater. 12, 223 (2013)]. A complete explanation of the experimental data is performed by computing the geometry of the adsorbed system, its electronic structure, and the energy transfer between tunneling electrons and the molecule's rotational degree of freedom. The results unambiguously show that tunneling electrons induce a frustrated rotation of the molecule. In addition, the theory determines the spatial distribution of the frustrated rotation excitation, confirming the striking dominance of two out of four molecular lobes in the observed excitation process. This lobe selectivity is attributed to the different hybridizations with the underlying substrate.

The thermal expansion of α-Fe (ferrite) and γ-Fe (austenite) nanocrystals confined inside multiwalled carbon nanotubes (MWCNTs) is studied in situ, using synchrotron x-ray diffraction, as a function of temperature. While the thermal expansion of ferrite is similar to that of bulk material, a peculiar behavior is evidenced for austenite: The thermal expansion becomes abnormally high above 500 °C. A scenario involving progressive carbon uptake in γ-Fe nanocrystals gives a satisfactory understanding of the phenomenon, and allows one to propose a value of the carbon solubilization rate in γ-Fe particles confined inside MWCNTs.

We study a steady-state nonequilibrium transport between two interacting helical edge states of a two-dimensional topological insulator, described by helical Luttinger liquids, through a quantum dot. For a noninteracting dot, the current is obtained analytically by including the self-energy correction to the dot's Green function. For an interacting dot, we use the equation-of-motion method to study the influence of weak on-site Coulomb interaction on the transport. We find the metal-to-insulator quantum phase transition for attractive $\mathit{or}$ repulsive interactions in the leads when the magnitude of the interaction strength characterized by a charge sector Luttinger parameter $K$ goes beyond a critical value. The critical Luttinger parameter $K_{cr}$ depends on the hopping strengths between the dot and the leads, as well as the energy level of the dot with respect to the Fermi levels of the leads, ranging from the weak-interaction regime for the dot level off-resonance to the strong-interaction regime for the dot in resonance with the equilibrium Fermi level. Near the transition, there are various singular behaviors of current noise, dot density of state, and the decoherence rate (inverse of lifetime) of the dot, which are briefly discussed.

We show that in crystalline insulators, space group symmetry alone gives rise to a topological classification based on the discretization of electric polarization. Using $C_3$ rotational symmetry as an example, we first prove that the polarization is discretized into three distinct classes, i.e., it can only take three inequivalent values. We then prove that these classes are topologically distinct. Therefore, a $Z_3$ topological classification exists, with polarization as a topological class index. A concrete tight-binding model is derived to demonstrate the $Z_3$ topological phase transition. Using first-principles calculations, we identify graphene on a BN substrate as a possible candidate to realize these $Z_3$ topological states. To complete our analysis, we extend the classification of band structures to all 17 two-dimensional space groups. This work will contribute to a complete theory of symmetry-conserved topological phases and also elucidate topological properties of graphenelike systems.

In contrast to a long held belief, it has been shown that $n$-type AlN can be achieved through Si-doping. This is unexplainable from the current theoretical understanding, a situation that hinders further progress in AlN-based ultraviolet (UV) technologies. From first-principles calculations, we find that $n$-type behavior arises under N-rich growth conditions due to high Si solubility and to the formation of $V$${}_{\mathrm{Al}}$-bound Si clusters. We show that metal-rich growth may lead to weak $n$-type behavior due to oxygen impurities binding and deactivating cation vacancies. We provide clues for designing production processes for $n$-type AlN as a base material for potential new UV sources.

We study the carrier injection into a single InGaAs/GaAs quantum dot regulated by a radio frequency surface acoustic wave. We find that the time of laser excitation during the acoustic cycle programs both the emission intensities and time of formation of neutral $\left(X^0\right)$ and negatively charged $\left(X^{-}\right)$ excitons. We identify underlying, characteristic formation pathways of both few-particle states in the time-domain experiments and show that both exciton species can be formed either with the optical pump or at later times by injection of single electrons and holes “surfing” the acoustic wave. All experimental observations are in excellent agreement with calculated electron and hole trajectories in the plane of the two-dimensional wetting layer which is dynamically modulated by the acoustically induced piezoelectric potentials. Taken together, our findings provide insight on both the onset of acoustoelectric transport of electrons and holes and their conversion into the optical domain after regulated injection into a single quantum dot emitter.

On the basis of optical characterization experiments and an eight band $kp$ model, we have studied the effect of Sb incorporation on the electronic structure of InAs quantum dots (QDs). We have found that Sb incorporation in InAs QDs shifts the hole wave function to the center of the QD from the edges of the QD where it is otherwise pinned down by the effects of shear stress. The observed changes in the ground-state energy cannot merely be explained by a composition change upon Sb exposure but can be accounted for when the change in lateral size is taken into consideration. The Sb distribution inside the QDs produces distinctive changes in the density of states, particularly, in the separation between excitation shells. We find a 50$\%$ increase in the thermal escape activation energy compared with reference InAs quantum dots as well as an increment of the fundamental transition decay time with Sb incorporation. Furthermore, we find that Sb incorporation into quantum dots is strongly nonlinear with coverage, saturating at low doses. This suggests the existence of a solubility limit of the Sb incorporation into the quantum dots during growth.

Using a mean-field approximation of the Hubbard model, we investigate the topological phase transition of the graphene system with an intrinsic spin-orbit coupling undergoing a coexistence of a Coulomb interaction and a staggered potential. We find that in an infinite system the antiferromagnetic phase can be classified into four types according to their topological properties: Two of them belong to the normal band-insulator phase, the third type is the quantum spin Hall insulator phase, although the inversion symmetry and the time-reversal symmetry are broken, and the fourth is the spin-polarized quantum Hall insulator phase, a different state of matter possessing Chern number 1 and spin Chern numbers $\frac{1}{2}$, which emerges due to the interplay between the Coulomb repulsion and the breaking of the sublattice symmetry. In addition, the number of edge states in a zigzag graphene ribbon whose parameters belong to one of these types confirms the validity of the analysis of the four phases. All the results obtained from a simplified toy model agree with those acquired from the self-consistent calculation. This indicates that the proposed toy model can exactly and analytically provide the essential information on the phase transition process of such graphene systems.

By employing the T-matrix approach, we investigate a nonmagnetic impurity located on the surface of three-dimensional topological insulators, where the time-reversal symmetry is preserved. It is shown that the images of the local density of states (LDOS) around the single impurity have dip-hump structures with six-fold symmetry at different bias voltages. The peaks are produced by quasiparticle interference, while the local minima at the backscattering wave vectors are due to the absence of backscatterings. With increasing the bias voltage, the peaks and dips move forward to the location of the impurity. These dips at the backscattering wave vectors in the LDOS spectra can be regarded as a signature of the time-reversal symmetry in the topological surface states, which can be observed by scanning tunneling microscopy.

We theoretically analyze correlation effects on the transport properties of a benzene molecule that are mediated by interactions between the motion of the nuclei and the transmitted charge. We focus on the lowest-lying molecular vibrational mode which allows us to derive an analytic expression for the current. The results provide transparent interpretations of various features of the highly nonlinear current-voltage characteristics, which is experimentally accessible through resonant inelastic electron-tunneling spectroscopy.