The optical response of phosphorene can be gradually changed by application of moderate uniaxial compression, as the material undergoes the transition into an indirect gap semiconductor and eventually into a semimetal. Strain tunes not only the gap between the valence band and conduction band local extrema but also the effective masses, and in consequence, the exciton anisotropy and binding strength. In this article, we consider from a theoretical point of view how the exciton stability and the resulting luminescence energy evolves under uniaxial strain. We find that the exciton binding energy can be as large as 0.87 eV in vacuum for 5% transverse strain, placing it amongst the highest for two-dimensional materials. Further, the large shift of the luminescence peak and its linear dependence on strain suggest that it can be used to probe directly the strain state of single layers.

We investigate a Mach-Zehnder interferometer fed by two time-dependently driven single-particle sources, one of them placed in front of the interferometer, the other in the center of one of the arms. As long as the two sources are operated independently, the signal at the output of the interferometer shows an interference pattern, which we analyze in the spectral current, in the charge and energy currents, as well as in the charge current noise. The synchronization of the two sources in this specifically designed setup allows for collisions and absorptions of particles at different points of the interferometer, which have a strong impact on the detected signals. It introduces further relevant time scales and can even lead to a full suppression of the interference in some of the discussed quantities. The complementary interpretations of this phenomenon in terms of spectral properties and tunable two-particle effects (absorptions and quantum exchange effects) are put forward in this paper.

The lattice thermal conductivities (κ) of binary compound materials are examined as a function of hydrostatic pressure $P$ using a first-principles approach. Compounds with relatively small mass ratios, such as MgO, show an increase in κ with $P$, consistent with measurements. Conversely, compounds with large mass ratios that create significant frequency gaps between acoustic and optic phonons (e.g., BSb, BAs, BeTe, BeSe) exhibit decreasing κ with increasing $P$, a behavior that cannot be understood using simple theories of κ. This anomalous $P$ dependence of κ arises from the fundamentally different nature of the intrinsic scattering processes for heat-carrying acoustic phonons in large mass ratio compounds compared to those with small mass ratios. This work demonstrates the power of first-principles methods for thermal properties and advances a broad paradigm for understanding thermal transport in nonmetals.

A graphene antidot lattice, created by a regular perforation of a graphene sheet, can exhibit a considerable band gap required by many electronics devices. However, deviations from perfect periodicity are always present in real experimental setups and can destroy the band gap. Our numerical simulations, using an efficient linear-scaling quantum transport simulation method implemented on graphics processing units, show that disorder that destroys the band gap can give rise to a transport gap caused by Anderson localization. The size of the defect-induced transport gap is found to be proportional to the radius of the antidots and inversely proportional to the square of the lattice periodicity. Furthermore, randomness in the positions of the antidots is found to be more detrimental than randomness in the antidot radius. The charge carrier mobilities are found to be very small compared to values found in pristine graphene, in accordance with recent experiments.

The growth, morphology, and magnetic structure of ultrathin Cr films grown on a Ag(001) substrate are studied using low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), and ab initio density functional theory (DFT) calculations. The presence and temperature dependence of $c(2\times 2)$ half-order spots in the LEED pattern, for low electron energies, along with the presence of characteristic Cr $3d$ bands in the ARPES spectra, confirm the existence of antiferromagnetic ordering for the Cr monolayer case. Our experiments are also consistent with the presence of a $p(1\times 1)$ Ag overlayer on top of the Cr layer, suggesting the existence of a Ag/Cr/Ag(001) sandwich structure at the surface. Our DFT calculations confirm that this is the most favored geometric and magnetic structure of the system. The Cr layer is found to retain a “two-dimensional” character with enhanced Cr $3d$ magnetic moments, despite being buried below a Ag monolayer, due to the absence of significant hybridization between Cr $3d$ and Ag $4d$ electronic states. The coverage dependence of the magnetic ordering indicates a maximum ordering above the expected monolayer coverage, possibly due to intermixing between Ag and Cr atoms in the overlayer.

We demonstrate theoretically the possibility of optimal control of light propagation and exciton transfer in arrays constructed of subnanometer sized noble-metal clusters by using phase-shaped laser pulses and analyze the mechanism underlying this process. The theoretical approach for simulation of light propagation in the arrays is based on the numerical solution of the coupled time-dependent Schrödinger equation and the classical electric field propagation in an iterative self-consistent manner. The electronic eigenstates of individual clusters and the dipole couplings are obtained from ab initio TDDFT calculations. The total electric field is propagated along the array by coupling an external excitation electric field with the electric fields produced by all clusters. A genetic algorithm is used to determine optimal pulse shapes which drive the excitation in a desired direction. The described theoretical approach is applied to control the light propagation and exciton transfer dynamics into a T-shaped structure built of seven ${\mathrm{Ag}}_8$ clusters. We demonstrate that a selective switching of light localization is possible in $\sim 5\mathrm{nm}$ sized cluster arrays which might serve as a building block for plasmonic devices with an ultrafast operation regime.

Relaxor-based ferroelectrics have been known for decades to possess a relatively thick surface layer (“skin”) that is distinct from its interior. Yet while there is consensus about its existence, there are controversies about its symmetry, phase stability, and origin. In an attempt to clarify these issues, we have examined the surface layer of PZN-12%PT. While the bulk transitions from a ferroelastically twinned tetragonal ferroelectric state with in-plane polarization to a cubic paraphase at $T_{\mathrm{c}}=200{}^{\circ }\mathrm{C}$, the skin layer shows a robust labyrinthine nanodomain structure with out-of-plane polarization that persists hundreds of degrees above the bulk Curie temperature. Cross-sectional transmission electron microscopy analysis shows that the resilience of the skin's polarization is correlated with a compositional imbalance: lead vacancies at the surface are charge-compensated by niobium enrichment; the excess of $\mathrm{N}{\mathrm{b}}^{5+}$—a small ion with $d^0$ orbital occupancy—stabilizes the ferroelectricity of the skin layer.

We report the control of spin reorientation (SR) transition in perovskite $\mathrm{D}{\mathrm{y}}_{1-x}\mathrm{S}{\mathrm{m}}_x\mathrm{Fe}{\mathrm{O}}_3$, a whole family of single crystals grown by an optical floating zone method from $x=0$ to 1 with an interval of 0.1. Powder x-ray diffractions and Rietveld refinements indicate that lattice parameters $a$ and $c$ increase linearly with Sm doping concentration, whereas $b$ keeps a constant. Temperature dependence of the magnetizations under zero-field-cooling (ZFC) and field-cooling (FC) processes are studied in detail. We have found a remarkable linear change of SR transition temperature in Sm-rich samples for $x>0.2$, which covers an extremely wide temperature range including room temperature. The $a$-axis magnetization curves under the FC during cooling (FCC) process bifurcate from and then jump back to that of the ZFC and FC warming process in single crystals when $x=0.5-0.9$, suggesting complicated $4f-3d$ electron interactions among $\mathrm{D}{\mathrm{y}}^{3+}\text{−}\mathrm{S}{\mathrm{m}}^{3+},\mathrm{D}{\mathrm{y}}^{3+}\text{−}\mathrm{F}{\mathrm{e}}^{3+}$, and $\mathrm{S}{\mathrm{m}}^{3+}\text{−}\mathrm{F}{\mathrm{e}}^{3+}$ sublattices of diverse magnetic configurations. The magnetic properties from the doping effect on SR transition temperature in these single crystals might be useful in the material physics and device design applications.

We examine the dynamics of a skyrmion moving over a two-dimensional periodic substrate utilizing simulations of a particle-based skyrmion model. We specifically examine the role of the nondissipative Magnus term on the driven motion and the resulting skyrmion velocity-force curves. In the overdamped limit, there is a depinning transition into a sliding state in which the skyrmion moves in the same direction as the external drive. When there is a finite Magnus component in the equation of motion, a skyrmion in the absence of a substrate moves at an angle with respect to the direction of the external driving force. When a periodic substrate is added, the direction of motion or Hall angle of the skyrmion is dependent on the amplitude of the external drive, only approaching the substrate-free limit for higher drives. Due to the underlying symmetry of the substrate the direction of skyrmion motion does not change continuously as a function of drive, but rather forms a series of discrete steps corresponding to integer or rational ratios of the velocity components perpendicular $\left(\langle V_{\perp }\rangle \right)$ and parallel $\left(\langle V_{\left|\right|}\rangle \right)$ to the external drive direction: $\langle V_{\perp }\rangle /\langle V_{\left|\right|}\rangle =n/m$, where $n$ and $m$ are integers. The skyrmion passes through a series of directional locking phases in which the motion is locked to certain symmetry directions of the substrate for fixed intervals of the drive amplitude. Within a given directionally locked phase, the Hall angle remains constant and the skyrmion moves in an orderly fashion through the sample. Signatures of the transitions into and out of these locked phases take the form of pronounced cusps in the skyrmion velocity versus force curves, as well as regions of negative differential mobility in which the net skyrmion velocity decreases with increasing external driving force. The number of steps in the transport curve increases when the relative strength of the Magnus term is increased. We also observe an overshoot phenomena in the directional locking, where the skyrmion motion can lock to a Hall angle greater than the clean limit value and then jump back to the lower value at higher drives. The skyrmion-substrate interactions can also produce a skyrmion acceleration effect in which, due to the nondissipative dynamics, the skyrmion velocity exceeds the value expected to be produced by the external drive. We find that these effects are robust for different types of periodic substrates. Using a simple model for a skyrmion interacting with a single pinning site, we can capture the behavior of the change in the Hall angle with increasing external drive. When the skyrmion moves through the pinning site, its trajectory exhibits a side step phenomenon since the Magnus term induces a curvature in the skyrmion orbit. As the drive increases, this curvature is reduced and the side step effect is also reduced. Increasing the strength of the Magnus term reduces the range of impact parameters over which the skyrmion can be captured by a pinning site, which is one of the reasons that strong Magnus force effects reduce the pinning in skyrmion systems.

We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compounds ${\mathrm{LaPd}}_{x}P{n}_2$ $(Pn=\mathrm{Sb},\mathrm{Bi})$ grown on (100) MgO substrates by molecular beam epitaxy. X-ray diffraction, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy confirm the ${\mathrm{HfCuSi}}_2$ structure of the material with a peculiar pnictogen square net layer. The superconducting transition temperature $T_{\text{c}}$ varies little with Pd concentration. ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ has a higher $T_{\text{c}}$ (3.2 K) by about 20% compared with ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ (2.7 K). Fe substitution of Pd leads to a rapid decay of superconductivity, suggesting that these superconductors are conventional type II.

It is generally assumed that two-dimensional electron gas (2DEG) recently observed at the (001) ${\text{SrTiO}}_3$ surface can be solely derived by oxygen vacancies introduced during ultrahigh vacuum annealing or through ultraviolet irradiation exposure. However, 2DEG entirely due to defect formation may be at odds with the characteristics of high mobility and easy field-effect manipulation required for applications; to that aim, an intrinsic formation mechanism should be preferred. Using advanced ab initio simulations we give evidence that 2DEG at the (001) ${\text{SrTiO}}_3$ surface may even result from purely intrinsic properties of the pristine surface, provided that the surface is SrO terminated. The key concept is that the SrO termination is electron-attractive as a consequence of both the surface-induced polarity and the specific electronic reconstruction, whereas the ${\text{TiO}}_2$ termination is electron-repulsive. It follows that in vacuum-cleaved samples where both terminations are present, 2DEG can result from the structurally ordered superposition of the two kinds of domain, even in the absence of any extrinsic source. On the other hand, in etching-prepared single-terminated ${\text{TiO}}_2$ samples 2DEG should be assumed as entirely derived by extrinsic factors.

Harnessing spin-orbit coupling for the manipulation of spins and magnetization via electric charge currents is the key objective of spin-orbitronics. Towards this end ferromagnetic materials are combined with nonmagnetic materials with strong spin-orbit coupling (typically involving heavy elements). However, many of the nominally nonmagnetic materials are highly susceptible to magnetic proximity effects, and the role of induced moments for spin transport has been controversial. Here we demonstrate that for Pt and Pd increased induced magnetic moments are correlated with strongly reduced spin-Hall conductivities. This observation finds an intuitive explanation in the development of a spin splitting of the chemical potential and the energy dependence of the intrinsic spin-Hall effect determined by first-principles calculations. This work provides simple guidance towards the optimization of spin current efficiencies for devices based on spin-orbit coupling phenomena.

Gapless surface states of time reversal invariant topological insulators are protected by the antiunitary nature of the time-reversal operation. Very recently, this idea was generalized to magnetic structures, in which time-reversal symmetry is explicitly broken, but there is still an antiunitary symmetry operation combining time-reversal symmetry and crystalline symmetry. These topological phases in magnetic structures are dubbed “topological magnetic crystalline insulators.” In this work, we present a general theory of topological magnetic crystalline insulators in different types of magnetic crystals based on the corepresentation theory of magnetic crystalline symmetry groups. We construct two concrete tight-binding models of topological magnetic crystalline insulators, the ${\widehat{C}}_4\widehat{\Theta }$ model and the $\widehat{\tau }\widehat{\Theta }$ model, in which topological surface states and topological invariants are calculated explicitly. Moreover, we check different types of antiunitary operators in magnetic systems and find that the systems with ${\widehat{C}}_4\widehat{\Theta },{\widehat{C}}_6\widehat{\Theta }$, and $\widehat{\tau }\widehat{\Theta }$ symmetry are able to protect gapless surface states. Our work will pave the way to search for topological magnetic crystalline insulators in realistic magnetic materials.

The mean attenuation length, $〈L〉$, of electrons emitted from ion bombarded solids was derived from measured angular-dependent electron yields γ(θ) in combination with Monte Carlo simulations of inelastic (electronic) energy deposition. The transport controlled contributions of excited electrons to the measured electron yields were derived as the integral $g_L$ over $S_e\exp (\text{–}z/L)$, where $S_e(\theta ,z)$ is the electronic energy deposition and $z$ the depth from the surface. The unknown attenuation length $L\equiv 〈L〉$ reflects the average over the energy spectrum and the angular distribution of those internally excited electrons that can reach the solid-vacuum interface and overcome the surface barrier. To determine $L$, the ratios $g_L\left(\theta \right)/g_L\left(0\right)$, calculated for $0⩽L⩽10\mathrm{nm}$, were compared with measured yield ratios γ(θ)/γ(0) for a wide variety of projectile-target combinations and impact energies between 1 and 50 keV (velocity-proportional electronic stopping). The procedure works well at angles at which $S_e$(θ,$z)$ decreases smoothly in the depth region between 1 and 3 nm. The result is $〈L〉=1.5\pm 0.3$ nm, a number basically in accordance with expectation based on estimated data for the inelastic mean free path of low-energy electrons (<25 eV) but a factor of 10 lower than the numbers recently advocated (10–15 nm) to rationalize “internal” electron yields observed with metal-insulator-metal sandwich structures.

We have recently shown [Phys. Rev. B 89, 165314 (2014)] that a noninteracting quantum dot coupled to a one-dimensional topological superconductor and to normal leads can sustain a Majorana mode even when the dot is expected to be empty, i.e., when the dot energy level is far above the Fermi level of the leads. This is due to the Majorana bound state of the wire leaking into the quantum dot. Here we extend this previous work by investigating the low-temperature quantum transport through an interacting quantum dot connected to source and drain leads and side coupled to a topological wire. We explore the signatures of a Majorana zero mode leaking into the quantum dot for a wide range of dot parameters, using a recursive Green's function approach. We then study the Kondo regime using numerical renormalization group calculations. We observe the interplay between the Majorana mode and the Kondo effect for different dot-wire coupling strengths, gate voltages, and Zeeman fields. Our results show that a “0.5” conductance signature appears in the dot despite the interplay between the leaked Majorana mode and the Kondo effect. This robust feature persists for a wide range of dot parameters, even when the Kondo correlations are suppressed by Zeeman fields and/or gate voltages. The Kondo effect, on the other hand, is suppressed by both Zeeman fields and gate voltages. We show that the zero-bias conductance as a function of the magnetic field follows a well-known universality curve. This can be measured experimentally, and we propose that the universal conductance drop followed by a persistent conductance of $0.5e^2/h$ is evidence of the presence of Majorana-Kondo physics. These results confirm that this “0.5” Majorana signature in the dot remains even in the presence of the Kondo effect.

We report density functional theory calculations for ${\mathrm{Mo}}_3{\mathrm{S}}_7{\left(\mathrm{dmit}\right)}_3$. We derive an ab initio tight-binding model from overlaps of Wannier orbitals; finding a layered model with interlayer hopping terms $\sim 3/4$ the size of the in-plane terms. The in-plane Hamiltonian interpolates the kagomé and honeycomb lattices. It supports states localized to dodecahedral rings within the plane, which populate one-dimensional (1D) bands and lead to a quasi-1D spin-one model on a layered honeycomb lattice once interactions are included. Two lines of Dirac cones also cross the Fermi energy.

We show that the dynamical screening of the Coulomb interaction among Cu-$d$ electrons in high-$T_c$ cuprates is strong and that a proper treatment of this effect is essential for a consistent description of the electronic structure. In particular, we find that ab initio calculations for undoped ${\mathrm{La}}_2{\mathrm{CuO}}_4$ in the paramagnetic phase yield an insulator only if the frequency dependence of the Coulomb interaction and the interatomic interaction between $p$ and $d$ electrons are taken into account. We also identify a collective excitation in the screened interaction at 9 eV, which is rather localized on the copper site, and which is responsible for a satellite structure at energy $-13$ eV, located below the $p$ bands.

We apply the nonequilibrium Green's function formalism to the problem of a multiterminal nanojunction subject to an arbitrary time-dependent bias. In particular, we show that taking a generic one-particle system Hamiltonian within the wide-band-limit approximation, it is possible to obtain a closed analytical expression for the current in each lead. Our formula reduces to the well-known result of Jauho et al. [Phys. Rev. B 50, 5528 (1994)] in the limit where the switch-on time is taken to the remote past, and to the result of Tuovinen et al. [Phys. Rev. B 89, 085131 (2014)] when the bias is maintained at a constant value after the switch-on. As we use a partition-free approach, our formula contains both the long-time current and transient effects due to the sudden switch-on of the bias. Numerical calculations performed for the simple case of a single-level quantum dot coupled to two leads are performed for a sinusoidally varying bias. At certain frequencies of the driving bias, we observe “ringing” oscillations of the current, whose dependence on the dot level, level width, oscillation amplitude, and temperature is also investigated.