We derive first- and second-order piezoelectric coefficients for the zinc-blende III-V semiconductors $\left\{\mathrm{Al},\mathrm{Ga},\mathrm{In}\right\}\text{−}\left\{\mathrm{N},\mathrm{P},\mathrm{As},\mathrm{Sb}\right\}$. The results are obtained within the Heyd-Scuseria-Ernzerhof hybrid-functional approach in the framework of density functional theory and the Berry-phase theory of electric polarization. To achieve a meaningful interpretation of the results, we build an intuitive phenomenological model based on the description of internal strain and the dynamics of the electronic charge centers. We discuss in detail first- and second-order internal strain effects, together with strain-induced changes in ionicity. This analysis reveals that the relatively large importance in the III-Vs of nonlinear piezoelectric effects compared to the linear ones arises because of a delicate balance between the ionic polarization contribution due to internal strain relaxation effects, and the contribution due to the electronic charge redistribution induced by macroscopic and internal strain.

Using first-principles calculations and symmetry analysis, we show that as cation atoms in a zinc blende–based semiconductor are replaced through atomic mutation (e.g., evolve from ZnSe to $\mathrm{CuGaS}{\mathrm{e}}_2$ to $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnGeS}{\mathrm{e}}_4$), the band gaps of the semiconductors will become more and more indirect because of the band splitting at the zone boundary, and in some cases will even form the segregating states. For example, although ZnSe is a direct band gap semiconductor, quaternary compounds $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnGeS}{\mathrm{e}}_4$ and $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ can be indirect band gap semiconductors if they form the primitive mixed CuAu ordered structures. We also find that the stability and the electronic structure of the quaternary polytypes with different atomic ordering are almost negative-linearly correlated. We suggest that these intrinsic properties of the multication semiconductors can have a large influence on the design and device performance of these materials.

In the frame of $\mathbf{k}·\mathbf{p}$ method and variational approach for the effective energy functional of a contact between a three-dimensional topological insulator (TI) and normal insulator (NI), we analytically describe the formation of interfacial bound electron states of two types (ordinary and topological) having different spatial distributions and energy spectra. We show that these states appear as a result of the interplay of two factors: hybridization and band bending of the TI and NI electron states near the TI/NI boundary. These results are corroborated by the density functional theory calculations for the exemplar ${\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{Zn}\mathrm{Se}$ system.

Many-body effects on quantum capacitance, compressibility, renormalized Fermi velocity, and kinetic and interaction energies of massless Dirac electrons in graphene, induced by Coulomb interactions, are analyzed theoretically in the first-order, Hartree-Fock, and random-phase approximations. Recent experimental data on quantum capacitance and renormalized Fermi velocity are analyzed and compared with the theory. The bare Fermi velocity and the effective dielectric constants are obtained from the experimental data. A combined effect of Coulomb interactions and Gaussian fluctuations of the disorder potential is considered.

We demonstrate that a nonperturbative framework for the treatment of the excitations of single-walled carbon nanotubes based upon a field theoretic reduction is able to accurately describe experiment observations of the absolute values of excitonic energies. This theoretical framework yields a simple scaling function from which the excitonic energies can be read off. This scaling function is primarily determined by a single parameter, the charge Luttinger parameter of the tube, which is in turn a function of the tube chirality, dielectric environment, and the tube's dimensions, thus expressing disparate influences on the excitonic energies in a unified fashion. We test this theory explicitly on the data reported by Dukovic et al. [Nano Lett. 5, 2314 (2005)] and Sfeir et al. [Phys. Rev. B 82, 195424 (2010)] and so demonstrate the method works over a wide range of reported excitonic spectra.

We consider a spin-orbit-coupled two-dimensional electron system under the influence of a thermal gradient externally applied to two attached reservoirs. We discuss the generated voltage bias (charge Seebeck effect), spin bias (spin Seebeck effect), and magnetization-dependent thermopower (magneto-Seebeck effect) in the ballistic regime of transport at linear response. We find that the charge thermopower is an oscillating function of both the spin-orbit strength and the quantum well width. We also observe that it is always negative for normal leads. We carefully compare the exact results for the linear response coefficients and a Sommerfeld approximation. When the contacts are ferromagnetic, we calculate the spin-resolved Seebeck coefficient for parallel and antiparallel magnetization configuration. Remarkably, the thermopower can change its sign by tuning the Fermi energy. This effect disappears when the Rashba coupling is absent. Additionally, we determine the magneto-Seebeck ratio, which shows dramatic changes in the presence of a the Rashba potential.

Nuclear magnetic resonance (NMR) and transport measurements have been performed at high magnetic fields and low temperatures in a series of $n$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals. In low-density samples, a complete spin polarization of the electronic system is achieved, as observed from the saturation of the isotropic component of the ${}^{209}\mathrm{Bi}$ NMR shift above a certain magnetic field. The corresponding spin splitting, defined in the phenomenological approach of a 3D electron gas with a large (spin-orbit-induced) effective $g$ factor, scales as expected with the Fermi energy independently determined by simultaneous transport measurements. Both the effective electronic $g$ factor and the “contact” hyperfine coupling constant are precisely determined. The magnitude of this latter reveals a nonnegligible $s$ character of the electronic wave function at the bottom of the conduction band. Our results show that the bulk electronic spin polarization can be directly probed via NMR and pave the way for future NMR investigations of the electronic states in Bi-based topological insulators.

Weyl semimetals are a three-dimensional gapless topological phase in which bands intersect at arbitrary points—the Weyl nodes—in the Brillouin zone. These points carry a topological quantum number known as the chirality and always appear in pairs of opposite chiralities. The notion of chirality leads to anomalous nonconservation of chiral charge, known as the chiral anomaly, according to which charge can be pumped between Weyl nodes of opposite chiralities by an electromagnetic field with nonzero $\mathit{E}·\mathit{B}$. Here, we propose probing the chiral anomaly by measuring the optical activity of Weyl semimetals via circular dichroism. In particular, we observe that applying such an electromagnetic field on this state gives it a nonzero gyrotropic coefficient or a Hall-like conductivity, which may be detectable by routine circular dichroism experiments. This method also serves as a diagnostic tool to discriminate between Weyl and Dirac semimetals; the latter will give a null result. More generally, any experiment that probes a bulk correlation function that has the same symmetries as the gyrotropic coefficient can detect the chiral anomaly as well as differentiate between Dirac and Weyl semimetals.

In this Rapid Communication, an efficient computational material design approach (cluster expansion) is employed for the ferroelectric ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ system. Via exploring a configuration space including over $3\times {10}^6$ candidates, two special cation ordered configurations—either perfect or mixed 1/1 (011) superlattice—are identified with the mostly enhanced ferroelectric polarization by up to about $95\%$ in comparison with the (001) superlattice. We find that the exotic couplings between the antiferrodistortive (AFD) and ferroelectric (FE) modes (e.g., ${\mathrm{AFD}}_x\text{−}{\mathrm{FE}}_z$ and ${\mathrm{AFD}}_{xy}\text{−}{\mathrm{FE}}_z$), which is absent from the PTO and STO, as the origin for the best polarization property of the two superlattices. This understanding should provide fresh ideas to design multifunctional perovskite heterostructures.

Magnetic and electronic states of an Ising-spin Kondo lattice model on a kagome lattice are investigated by a Monte Carlo simulation. In addition to the conventional ferromagnetic and ferrimagnetic orders, we show that this model exhibits several thermally induced phases, such as partially disordered, Kosterlitz-Thouless-like, and loop-liquid states. In the partially disordered state, we show that the magnetic transition is associated with the charge-gap formation. We find that the density of states shows characteristic peaks reflecting the underlying spin texture. On the other hand, in the loop-liquid state, the formation of closed loops of the same spin sites manifests itself in the peaks in the density of states and the optical conductivity. Our results elucidate the peculiar cooperation between thermal fluctuations and the spin-charge interplay in this frustrated itinerant electron system.

Alkali metals display unexpected properties at high pressure, including emergence of low-symmetry crystal structures, which appear to occur due to enhanced electronic correlations among the otherwise nearly free conduction electrons. We investigate the high-pressure electronic and structural ground state of K, Rb, and Cs using x-ray absorption spectroscopy and x-ray diffraction measurements together with $abinitio$ theoretical calculations. The sequence of phase transitions under pressure observed at low temperature is similar in all three heavy alkalis except for the absence of the $oC84$ phase in Cs. Both the experimental and theoretical results point to pressure-enhanced localization of the valence electrons characterized by pseudogap formation near the Fermi level and strong $spd$ hybridization. Although the crystal structures predicted to host magnetic order in K are not observed, the localization process appears to drive these alkalis closer to a strongly correlated electron state.

The doping mechanism of 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ) doped poly(3-hexylthiophene) (P3HT) thin films and solutions is studied by UV-visible and IR absorption spectral analysis. We show that integer charge transfer between the two molecules is observed in this system with 63% of the dopants producing a fully transferred charge in thin film. The primary P3HT doped species in solution and thin film are shown to be bipolarons and polarons, respectively. The absorption signatures of polarons and bipolarons are identified and their cross sections are measured, which can be used to evaluate the influence of processing conditions on the density of impurity dopant-induced polarons in nominally “neat” P3HT films.

The thermal properties of anisotropic crystals are of both fundamental and practical interest, but transport phenomena in anisotropic materials such as graphite remain poorly understood because solutions of the Boltzmann equation often assume isotropy. Here, we extend an analytic solution of the transient, frequency-dependent Boltzmann equation to highly anisotropic solids and examine its predictions for graphite. We show that this simple model predicts key results, such as long $c$-axis phonon mean free paths and a negative correlation of cross-plane thermal conductivity with in-plane group velocity, that were previously observed with computationally expensive molecular-dynamics simulations. Further, using our analytic solution, we demonstrate a method to reconstruct the anisotropic mean free path spectrum of crystals with arbitrary dispersion relations without any prior knowledge of their harmonic or anharmonic properties using observations of quasiballistic heat conduction. These results provide a useful analytic framework to understand thermal transport in anisotropic crystals.

The three-dimensional confinement characterizing a nanocrystal (NC) leads to the formation of discrete electronic states. The energy gap between these states in colloidal NCs can be up to an order of magnitude larger than the vibrational energy of the host material. This large energetic mismatch (not given in self-assembled quantum dots) leads to the expectation that an electron occupying an excited state would be unable to release its energy to vibrations and a “phonon bottleneck” should finally be observed. Using large-scale ab initio calculations and a time-dependent formalism, we show that on the contrary, a phonon bottleneck can be observed only in a narrow window of diameters for CdSe and InAs NCs and should not occur at all in Si NCs. Two relaxation pathways enable fast carrier relaxation. For smaller structures (below 20-Å radius), the coupling strength and energy detuning are such that quantum mechanics allows us to effectively bridge electronic gaps much larger than the vibronic energy. For larger structures, the coupling to passivant modes, although very weak, leads to an efficient picosecond carrier relaxation. This work provides insight into the nature of carrier relaxation in colloidal nanostructures and highlights that defects, of any kind, are not necessary to explain the observed fast carrier relaxation.

The chalcogenides of tin and lead (SnS, SnSe, SnTe, PbS, PbSe, and PbTe) have applications ranging from solar cells to thermoelectrics. Taking rocksalt structured PbTe(001) as the substrate, we explore the coherent (001)-oriented heteroepitaxy of the other five IV-VI semiconductors through first-principles electronic structure calculations. We investigate the effects of lattice strain and its relationship to the interface energy, as well as the electron redistribution, as a function of the epilayer thickness [from 1 to 5 monolayers (ML)] below the dislocation critical point. Analysis of the chemical bonding explains trends including work function shifts and surface rumpling. Among the five combinations studied SnTe/PbTe(001) has the smallest lattice mismatch, resulting in the most stable coherent interface and unique charge transfer behavior. Here, we perform orbital-resolved band structure calculations (with spin-orbit coupling effects) for the SnTe/PbTe(001) system, highlighting its potential use in topological spintronic thin-film devices.

We study a three-terminal spin-valley topological insulator/superconductor/insulator (TI/S/TI) device to realize perfect Cooper-pair splitting in real space. The key point is to construct a single topologically-protected interface state in each TI electrode that is valley helical and spin polarized. It is shown that only the crossed Andreev reflection of electrons is allowed when the opposite topological interface states are engineered in the two TI electrodes, but the normal reflection, local Andreev reflection, as well as the electron cotunneling are entirely prohibited. The quantized and noiseless crossed Andreev reflection is protected by the bulk topological invariants of the TI electrodes and persists in a very long junction $(L>{\xi }_s)$ as well as in moderate disorders.

The size dependence of plastic flow is studied by discrete dislocation dynamical simulations of systems with various amounts of interacting dislocations while the stress is slowly increased. The regions between avalanches in the individual stress curves as functions of the plastic strain were found to be nearly linear and reversible where the plastic deformation obeys an effective equation of motion with a nearly linear force. For small plastic deformation, the mean values of the stress-strain curves obey a power law over two decades. Here and for somewhat larger plastic deformations, the mean stress-strain curves converge for larger sizes, while their variances shrink, both indicating the existence of a thermodynamical limit. The converging averages decrease with increasing size, in accordance with size effects from experiments. For large plastic deformations, where steady flow sets in, the thermodynamical limit was not realized in this model system.

We report on the transport and magnetic properties of iron-deficient $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4\left(\mathrm{F}{\mathrm{e}}_{3-\delta }{\mathrm{O}}_4\right)$ thin films grown with pulsed-laser deposition, where the stoichiometry and amount of cation vacancies are precisely controlled through changes in the oxygen partial pressure during growth. As the stoichiometry evolves from $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ to $\gamma \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$, three distinct structural and magnetic regimes emerge: a $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$-like regime, a $\gamma \text{−}\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$-like regime, and a transition regime. While reflection high-energy electron diffraction measurements reveal that films in all three regimes grow epitaxially cube-on-cube on MgO substrates, the transition-regime films are characterized by an absence of long-range, out-of-plane ordering in the film. Selected area electron diffraction measurements reveal the transition-regime films are well ordered on a local level, but not throughout the entire film. The structural disorder of the transition-regime films does not manifest itself in the transport properties, where a systematic change in resistivity, due primarily to variations in the $\mathrm{F}{\mathrm{e}}^{2+}:\mathrm{F}{\mathrm{e}}^{3+}$ cation ratio, occurs continuously throughout all three regimes. Large differences are observed, however, in the magnetic properties of the transition-regime films, which are reminiscent of magnetically disordered systems. We attribute this unique magnetically disordered state to magnetic frustration arising at the boundaries between the different locally ordered regions.