We show that the intensity of pumped states near Weyl point is different when pumped with left- and right-handed circular-polarized light, which leads to a special circular dichroism (CD) in time-dependent angle-resolved photoemission spectra (ARPES). We derive the expression for the CD of time-dependent ARPES, which is directly related to the chirality of Weyl fermions. Based on the above derivation, we further propose a method to determine the chirality for a given Weyl point from the CD of time-dependent ARPES. The corresponding CD spectra for TaAs has then been calculated from first principles, which can be compared with future experiments.

In a recent publication [S. Groth et al., Phys. Rev. B 93, 085102 (2016)], we have shown that the combination of two complementary quantum Monte Carlo approaches, namely configuration path integral Monte Carlo [T. Schoof et al., Phys. Rev. Lett. 115, 130402 (2015)] and permutation blocking path integral Monte Carlo [T. Dornheim et al., New J. Phys. 17, 073017 (2015)], allows for the accurate computation of thermodynamic properties of the spin-polarized uniform electron gas over a wide range of temperatures and densities without the fixed-node approximation. In the present work, we extend this concept to the unpolarized case, which requires nontrivial enhancements that we describe in detail. We compare our simulation results with recent restricted path integral Monte Carlo data [E. W. Brown et al., Phys. Rev. Lett. 110, 146405 (2013)] for different energy contributions and pair distribution functions and find, for the exchange correlation energy, overall better agreement than for the spin-polarized case, while the separate kinetic and potential contributions substantially deviate.

We use a combination of the coherent potential approximation and dynamical mean field theory to study magnetic properties of the ${\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x$ alloy from first principles. Calculated uniform magnetic susceptibilities have a Curie-Weiss-like behavior, and extracted effective temperatures are in agreement with the experimental results. The individual squared magnetic moments obtained as a function of nickel concentration follow the same trends as experimental data. An analysis of the ionic and spin weights shows a possibility of high-spin to intermediate- and low-spin state transitions at high temperatures.

In this work, we present resonant tail-state regime absorption enhanced organic photovoltaics. We combine periodically structured ${\mathrm{TiO}}_2$ bottom electrodes with P3HT-PCBM bulk-heterojunction solar cells in an inverted device configuration. The wavelength-scale patterns are transferred to the electron-selective bottom electrodes via direct laser interference patterning, a fast method compatible with roll-to-roll processing. Spectroscopic and optoelectronic device measurements suggest polarization-dependent absorption enhancement along with photocurrent generation unambiguously originating from the population of tail states. We discuss the effects underlying these absorption patterns with the help of electromagnetic simulations using the discontinuous Galerkin time domain method. For this, we focus on the total absorption spectra along with spatially resolved power loss densities. Our simulations stress the tunability of the absorption resonances towards arbitrary wavelength regions.

Zeeman splitting of quantum-confined states of excitons in InGaAs quantum wells (QWs) is experimentally found to depend strongly on quantization energy. Moreover, it changes sign when the quantization energy increases with a decrease in the QW width. In the 87-nm QW, the sign change is observed for the excited quantum-confined states, which are above the ground state only by a few meV. A two-step approach for the numerical solution of the two-particle Schrödinger equation, taking into account the Coulomb interaction and valence-band coupling, is used for a theoretical justification of the observed phenomenon. The calculated variation of the $g$-factor convincingly follows the dependencies obtained in the experiments.

The theoretical study of grain boundaries (GBs) in polycrystalline semiconductors is currently stalemated by their complicated nature, which is difficult to extract from any direct experimental characterization. Usually, coincidence-site-lattice models are constructed simply by aligning two symmetric planes ignoring various possible reconstructions. Here, we propose a general self-passivation rule to determine the low-energy GB reconstruction and find new configurations for the CdTe Σ3 (112) GBs. First-principles calculations show that it has lower formation energies than the prototype GBs adopted widely in previous studies. Surprisingly, the reconstructed GBs show self-passivated electronic properties without deep-level states in the band gap. Based on the reconstructed configurations, we revisited the influence of $\mathrm{CdC}{\mathrm{l}}_2$ post-treatment on the CdTe GBs and found that the addition of both Cd and Cl atoms in the GB improves the photovoltaic properties by promoting self-passivation and inducing $n$-type levels, respectively. The present study provides a new route for further studies of GBs in covalent polycrystalline semiconductors and highlights that previous studies on the GBs of multinary semiconductors, which are based on the unreconstructed prototype GB models, should be revisited.

Quantum spin Hall (QSH) insulators with gapless edge states have potential applications in designing low-dissipation devices. In spite of many predictions, to verify the QSH phase in graphene layered materials experimentally is still difficult due to the obstacle in achieving spin-orbit coupling strong enough. We propose a Rashba system of graphene bilayer gapped by dielectric layers and show it can host a valley-polarized QSH phase even when the Rashba interaction approached zero. Such a system exhibits asymmetric topological quantum phase transitions under opposite interlayer biases, due to the dielectric-potential induced inversion asymmetry in the absence of interlayer bias. Specifically, the quantum valley Hall phase exists in zigzag-edged nanoribbons under the bias in one direction but is absent under the reverse bias. These topological phenomena can be well understood by the competition among the dielectric-induced potential, Rashba interaction, and interlayer bias in modulating the bulk band gap. Moreover, the phase diagram is given and the corresponding phase boundaries are derived analytically. Our findings provide a possible way to detect the QSH-related asymmetric topological quantum phenomena in graphene bilayer based on the current experimental technology.

We present a study of the surface structure of the Ag-In-RE (RE: rare-earth elements Gd, Tb, and Yb) complex intermetallics using scanning tunneling microscopy and low-energy electron diffraction. The surface of the Ag-In-Yb approximant prepared by sputter-annealing methods under ultrahigh-vacuum conditions produces a flat (100) surface with no facets. However, the Ag-In-Gd and Ag-In-Tb 1/1 approximants, which have a surface miscut of about ${12}^{\circ }$ relative to the (100) plane, develop surface facets along various crystallographic directions. The structure of each facet can be explained as a truncation of the rhombic triacontahedral clusters, i.e., the main building blocks of these systems. Despite their differences in atomic structure, symmetry, and density, the facets show common features. The facet planes are In rich. The analysis of the nearest-neighbor atom distances suggests that In atoms form bonds with the RE atoms, which we suggest is a key factor that stabilizes even low-density facet planes.

We report a density-functional study on the evolution of the electronic and lattice structure in ${\mathrm{BiFeO}}_3$ with injected electrons and holes. First, the self-trapping of electrons and holes was investigated. We found that the injected electrons tend to be localized on Fe sites due to the local lattice expansion, the on-site Coulomb interaction of $\mathrm{Fe}3d$ electrons, and the antiferromagnetic order in ${\mathrm{BiFeO}}_3$. The injected holes tend to be delocalized if the on-site Coulomb interaction of $\mathrm{O}2p$ is weak (in other words, $U_{\mathrm{O}}$ is small). Single-center polarons and multicenter polarons are formed with large and intermediate $U_{\mathrm{O}}$, respectively. With intermediate $U_{\mathrm{O}}$, multicenter polarons can be formed. We also studied the lattice distortion with the injection of carriers by assuming the delocalization of these carriers. We found that the ferroelectric off-centering of ${\mathrm{BiFeO}}_3$ increases with the concentration of the electrons injected and decreases with that of the holes injected. It was also found that a structural phase transition from $R3c$ to the nonferroelectric $Pbnm$ occurs, with the hole concentration over $8.7\times {10}^{19}{\text{cm}}^{-3}$. The change of the off-centering is mainly due to the change of the lattice volume. The understanding of the carrier localization mechanism can help to optimize the functionality of ferroelectric diodes and the ferroelectric photovoltage devices, while the understanding of the evolution of the lattice with carriers can help tune the ferroelectric properties by the carriers in ${\mathrm{BiFeO}}_3$.

We present first-principles calculations of metallic atomic hydrogen in the 400–600 GPa pressure range in a tetragonal structure with space group $I{4}_1/amd$, which is predicted to be its first atomic phase. Our calculations show a band structure close to the free-electron-like limit due to the high electronic kinetic energy induced by pressure. Bands are properly described even in the independent electron approximation fully neglecting the electron-electron interaction. Linear-response harmonic calculations show a dynamically stable phonon spectrum with marked Kohn anomalies. Even if the electron-electron interaction has a minor role in the electronic bands, the inclusion of electronic exchange and correlation in the density response is essential to obtain a dynamically stable structure. Anharmonic effects, which are calculated within the stochastic self-consistent harmonic approximation, harden high-energy optical modes and soften transverse acoustic modes up to a 20% in energy. Despite a large impact of anharmonicity has been predicted in several high-pressure hydrides, here the superconducting critical temperature is barely affected by anharmonicity, as it is lowered from its harmonic 318 K value only to 300 K at 500 GPa. We attribute the small impact of anharmonicity on superconductivity to the absence of softened optical modes and the fairly uniform distribution of the electron-phonon coupling among the vibrational modes.

A breakdown of the spin dependence of the electron reflection due to organic molecules or amorphous carbon deposited onto a metallic film has been observed in the past. The goal of the present work is to further elucidate the physics of this phenomenon by studying it in ways not yet studied in the past. The most intriguing observation of the present study is that the breakdown phenomenon appears in a relatively well defined electron energy range between 2 and about 200 eV kinetic energy. Outside this energy range the breakdown phenomenon is not observed. However, an explanation of the breakdown phenomenon is still missing.

${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ has been synthesized and magnetic measurements have been carried out, revealing that $M_{\text{sat}}=0.92$ MA/m at $T=300$ K. The $M$ versus $T$ curve shows a broad peak around $T=160$ K. The anisotropy constant $K_1$, estimated at $T=300$ K, is 0.25 MJ/${\mathrm{m}}^3$. Theoretical analysis of ${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ system has been carried out and extended to the full range of ${\mathrm{Fe}}_5{\mathrm{Si}}_{1-x}{\mathrm{P}}_x{\mathrm{B}}_2,{\mathrm{Fe}}_5{\mathrm{P}}_{1-x}{\mathrm{S}}_x{\mathrm{B}}_2$, and $\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_5{\mathrm{SiB}}_2$ compositions. The electronic band structures have been calculated using the full-potential local-orbital minimum-basis scheme (FPLO-14). The calculated total magnetic moments are 9.20, 9.15, 9.59, and $2.42{\mu }_B$ per formula units of ${\mathrm{Fe}}_5{\mathrm{SiB}}_2,{\mathrm{Fe}}_5{\mathrm{PB}}_2,{\mathrm{Fe}}_5{\mathrm{SB}}_2$, and ${\mathrm{Co}}_5{\mathrm{SiB}}_2$, respectively. In agreement with experiment, magnetocrystalline anisotropy energies (MAE's) calculated for $T=0$ K change from a negative (easy-plane) anisotropy $-0.28$ MJ/${\mathrm{m}}^3$ for ${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ to the positive (easy-axis) anisotropy 0.35 MJ/${\mathrm{m}}^3$ for ${\mathrm{Fe}}_5{\mathrm{PB}}_2$. Further increase of the number of $p$ electrons in ${\mathrm{Fe}}_5{\mathrm{P}}_{1-x}{\mathrm{S}}_x{\mathrm{B}}_2$ leads to an increase of MAE up to $0.77$ MJ/${\mathrm{m}}^3$ for the hypothetical ${\mathrm{Fe}}_5{\mathrm{P}}_{0.4}{\mathrm{S}}_{0.6}{\mathrm{B}}_2$ composition. Volume variation and fixed spin moment calculations (FSM) performed for ${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ show an inverse relation between MAE and magnetic moment in the region down to about 15% reduction of the spin moment. The alloying of ${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ with Co is proposed as a practical realization of magnetic moment reduction, which ought to increase MAE. MAE calculated in virtual crystal approximation (VCA) for a full range of $\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_5{\mathrm{SiB}}_2$ compositions reaches the maximum value of 1.16 MJ/${\mathrm{m}}^3$ at Co concentration $x=0.3$, with the magnetic moment $7.75{\mu }_B$ per formula unit. Thus, $\left({\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}\right){}_5{\mathrm{SiB}}_2$ is suggested as a candidate for a rare-earth free permanent magnet. For the stoichiometric ${\mathrm{Co}}_5{\mathrm{SiB}}_2$ there is an easy-plane magnetization, with the value of $\text{MAE}=-0.15$ MJ/${\mathrm{m}}^3$.

We study the $I-V$ characteristics of ${\mathrm{S}}_{\text{T}}\text{/n/N}$ contacts, where ${\mathrm{S}}_{\text{T}}$ is a BCS superconductor S with a built-in exchange field $h$, n represents a normal metal wire, and N a normal metal reservoir. The superconductor ${\mathrm{S}}_{\text{T}}$ is separated from the n wire by a spin filter which allows the passage of electrons with a certain spin direction so that only fully polarized triplet Cooper pairs penetrate into the n wire. We show that both the subgap conductance ${\sigma }_{\text{sg}}$ and the excess current $I_{\text{exc}}$, which occur in conventional S/n/N contacts due to Andreev reflection (AR), exist also in the considered system. In our case, they are caused by unconventional AR that is not accompanied by spin flip. The excess current $I_{\text{exc}}$ exists only if $h$ exceeds a certain magnitude $h_{\text{c}}$. At $h<h_{\text{c}}$, the excess current is converted into a deficit current $I_{\text{def}}$. The dependencies of the differential conductance and the current $I_{\text{exc}}$ are presented as a function of voltage and $h$.

Two recent theoretical works studied the role of Kitaev interactions in the newly observed incommensurate magnetic order in the hyper-honeycomb ($\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$) and stripy-honeycomb ($\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$) iridates. Each of these works analyzed a different model ($JK\mathrm{\Gamma }$ versus coupled zigzag chain model) using a contrasting method (classical versus soft-spin analysis). The lack of commonality between these works precludes meaningful comparisons and a proper understanding of these unusual orderings. In this study, we complete the unfinished picture initiated by these two works by solving both models with both approaches for both three-dimensional (3D) honeycomb iridates. Through comparisons between all combinations of models, techniques, and materials, we find that the bond-isotropic $JK\mathrm{\Gamma }$ model consistently predicts the experimental phase of $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ regardless of the method used, while the experimental phase of $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ can be generated by the soft-spin approach with eigenmode mixing irrespective of the model used. To gain further insights, we solve a one-dimensional (1D) quantum spin-chain model related to both 3D models using the density matrix renormalization group method to form a benchmark. We discover that in the 1D model, incommensurate correlations in the classical and soft-spin analysis survive in the quantum limit only in the presence of the symmetric-off-diagonal exchange $\mathrm{\Gamma }$ found in the $JK\mathrm{\Gamma }$ model. The relevance of these results to the real materials is also discussed.

The photoinduced dynamics of a potassium-tetracyanoquinodimethane (K-TCNQ) single crystal in the generalized Peierls phase are evaluated via time-resolved vibrational spectroscopy. The transient reflectivity spectrum of the photoinduced state in the mid-IR range shows a decrease in the height and width of the reflectivity band because of the electron-molecular-vibration-coupled CN stretching mode at approximately 2180 ${\mathrm{cm}}^{-1}$. This spectral change suggests that the photoexcitation of the charge transfer in TCNQ molecules induces melting of the dimerization of the molecules. From detailed analysis of the spectral evolution, the relaxation time constant from the photoinduced state to the dimerized state is estimated to be approximately 0.6 ps. Even after the recovery of the dimerization, a fluctuation is still observed, probably because of a domain-wall soliton. The fluctuation gradually dissipates with a time constant of approximately 2.3 ps. Direct observation of the dimerization process reveals the true dynamics of the photoinduced cooperative phenomenon within this system.

Universal exact conditions guided the construction of most ground-state density functional approximations in use today. We derive the relation between the entropy and Mermin free energy density functionals for thermal density functional theory. Both the entropy and sum of kinetic and electron-electron repulsion functionals are shown to be monotonically increasing with temperature, while the Mermin functional is concave downwards. Analogous relations are found for both exchange and correlation. The importance of these conditions is illustrated in two extremes: the Hubbard dimer and the uniform gas.

Long-range orbital order has been investigated in ${\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{V}}_2{\mathrm{O}}_4$ as a function of doping ($x$) using first-principles density functional theory calculations including the effects of Coulomb correlation and spin-orbit interaction within GGA+$U$ and GGA+$U$+SO approximations. Through a detailed analysis of corresponding Wannier orbital projections of the vanadium $d$ bands, we have clearly established that for $x\le 0.6$, the orbital order at V sites consists of a linear superposition of $d_{xz}$ and $d_{yz}$ orbitals of the type $d_{xz}\pm d_{yz}$. Within each $ab$ plane a ferro-orbital ordering of either $d_{xz}+d_{yz}$ or $d_{xz}-d_{yz}$ is observed which alternates in successive $ab$ planes along the $c$ direction. On the contrary, for $x>0.6$, it is the $d_{xz}$ or $d_{yz}$ orbital that orders at V sites in successive $ab$ planes along $c$ direction (so-called $A$-type ordering). At Fe sites, we observe an orbital ordering of $d_{x^2-y^2}$ orbitals for $x\le 0.6$ and $d_{z^2}$ orbitals for $x>0.6$. The effect of spin-orbit interaction on orbital ordering is found to be not significant in the entire range of doping studied.

We present a polarization-resolved photoluminescence excitation study of the absorption spectrum of a $p$-shell neutral exciton in a single CdTe/ZnTe quantum dot. We find that the fine structure of the $p$-shell exciton is completely analogous to the fine structure of the $s$-shell exciton, including the selection rules and the effects of a magnetic field applied in Faraday and Voigt configurations. The energy spectrum of the $p$-shell exciton is found to be well described by introducing respective isotropic and anisotropic constants of the exchange interaction between a $p$-shell electron and a $p$-shell hole. The typical values of these exchange constants averaged over several randomly selected quantum dots yield ${\delta }_0^{pp}=(0.92\pm 0.16)$ meV and ${\delta }_1^{pp}=(0.58\pm 0.25)$ meV. Additionally, we demonstrate that the nonresonant relaxation of the $p$-shell exciton conserves the exciton spin to a very high degree for both bright and dark exciton configurations.

We report electrical characterization of quantum dots formed by introducing pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires. Regular Coulomb oscillations are observed over a wide gate voltage span, indicating that WZ segments create significant barriers for electron transport. We find a direct correlation of transport properties with quantum dot length and corresponding growth time of the enclosed ZB segment. The correlation is made possible by using a method to extract lengths of nanowire crystal phase segments directly from scanning electron microscopy images, and with support from transmission electron microscope images of typical nanowires. From experiments on controlled filling of nearly empty dots with electrons, up to the point where Coulomb oscillations can no longer be resolved, we estimate a lower bound for the ZB-WZ conduction-band offset of 95 meV.