We report observations of coherent phonon oscillations in solution-processed polycrystalline stibnite $({\mathrm{Sb}}_2$${\mathrm{S}}_3)$ photovoltaic thin films using transient absorption spectroscopy. Detailed optical spectroscopy correlated by extensive first-principles lattice dynamics calculations indicates that the coherent $B_{3g}$ longitudinal optical phonon mode with a frequency of 63.74 ± 0.05 ${\mathrm{cm}}^{-1}$ (or 1.911 ± 0.002 THz) at room temperature is generated via the impulsive stimulated-Raman-scattering mechanism. These strong electron-phonon interactions indicate a dominant energy-loss channel in these materials that could impose a fundamental limit on their solar energy conversion efficiency.

In this paper, we propose a practical way to stabilize half-hydrogenated graphene (graphone). We show that the dipole moments induced by a hexagonal-boron nitride (h-BN) substrate on graphene stabilize the hydrogen atoms on one sublattice of the graphene layer and suppress the migration of the adsorbed hydrogen atoms. Based upon first principle spin polarized density of states calculations, we show that the graphone obtained in different graphene/h-BN heterostructures exhibits a half metallic state. We propose to use this exotic material for spin valve systems and other spintronics devices.

We present a distinct mechanism for the formation of bound states in the continuum (BICs). In chiral quantum systems, there appear zero-energy states in which the wave function has finite amplitude only in one of the subsystems defined by the chiral symmetry. When the system is coupled to leads with a continuum energy band, part of these states remain bound. We derive some algebraic rules for the number of these states depending on the dimensionality and rank of the total Hamiltonian. We examine the transport properties of such systems including the appearance of Fano resonances in some limiting cases. Finally, we discuss experimental setups based on microwave dielectric resonators and atoms in optical lattices where these predictions can be tested.

We present the simulation, fabrication, and optical characterization of plasmonic gold bowtie nanoantennas on a semiconducting GaAs substrate as geometrical parameters such as size, feed gap, height, and polarization of the incident light are varied. The surface-plasmon resonance was probed using white light reflectivity on an array of nominally identical, 35-nm-thick gold antennas. To elucidate the influence of the semiconducting, high-refractive-index substrate, all experiments were compared using nominally identical structures on glass. Besides a linear shift of the surface-plasmon resonance from 1.08 to 1.58 eV when decreasing the triangle size from 170 to 100 nm on GaAs, we observed a global redshift by 0.25 $\pm$ 0.05 eV with respect to nominally identical structures on glass. By performing polarization-resolved measurements and comparing results with finite-difference time-domain simulations, we determined the near-field coupling between the two triangles composing the bowtie antenna to be $\sim 8$ times stronger when the antenna is on a glass substrate compared to when it is on a GaAs substrate. The results obtained have strong relevance for the integration of lithographically defined plasmonic nanoantennas on semiconducting substrates and therefore for the development of novel optically active plasmonic-semiconducting nanostructures.

Antiphase domain boundaries (APDBs) in the $(n\times 1)\left(n=4,5\right)$ reconstructions of the ${\mathrm{SrTiO}}_3\left(110\right)$ surface were studied with scanning tunneling microscopy, x-ray photoemission spectroscopy, and density functional theory (DFT) calculations. Two types of APDBs form on each reconstruction; they consist of ${\mathrm{Ti}}_x$${\mathrm{O}}_y$ vacancy clusters with a specific stoichiometry. The presence of these clusters is controlled by the oxygen pressure during annealing. The structural models of the vacancy clusters are resolved with DFT, which also shows that their relative stability depends on the chemical potential of oxygen. The surface band bending can be tuned by controlling the vacancy clusters at the domain boundaries.

We investigate the optical excitation spectra and the photoluminescence depolarization dynamics in bilayer ${\mathrm{WS}}_2$. A different understanding of the optical excitation spectra in the recent photoluminescence experiment by Zhu et al. (arXiv:1403.6224) in bilayer ${\mathrm{WS}}_2$ is proposed. In the experiment, four excitations (1.68, 1.93, 1.99, and 2.37 eV) are observed and identified to be the indirect exciton for the $\Gamma$ valley, trion, A exciton, and B exciton excitations, respectively, with the redshift for the A exciton energy measured to be 30$\sim$50 meV when the sample synthesized from monolayer to bilayer. According to our study, by considering that there exist both the intralayer and charge-transfer excitons in the bilayer ${\mathrm{WS}}_2$, with interlayer hopping of the hole, there exists an excimer state composed by the superposition of the intralayer and charge-transfer exciton states. Accordingly, we show that the four optical excitations in the bilayer ${\mathrm{WS}}_2$ are the A charge-transfer exciton, ${\mathrm{A}}^{\prime }$ excimer, ${\mathrm{B}}^{\prime }$ excimer, and B intralayer exciton states, respectively, with the calculated resonance energies showing good agreement with the experiment. In our picture, the speculated indirect exciton, which involves a high-order phonon absorption/emission process, is not necessary. Furthermore, the binding energy for the excimer state is calculated to be 40 meV, providing reasonable explanation for the experimentally observed energy redshift of the A exciton. Based on the excimer states, we further derive the exchange interaction Hamiltonian. Then the photoluminescence depolarization dynamics due to the electron-hole exchange interaction is studied in the pump-probe setup by the kinetic spin Bloch equations. We find that there is always a residual photoluminescence polarization that is exactly half of the initial one, lasting for an infinitely long time, which is robust against the initial energy broadening and strength of the momentum scattering. This large steady-state photoluminescence polarization indicates that the photoluminescence relaxation time is extremely long in the steady-state photoluminescence experiment, and can be the cause of the anomalously large photoluminescence polarization, nearly 100$\%$, observed in the experiment by Zhu et al. in the bilayer ${\mathrm{WS}}_2$. This steady state is shown to come from the unique form of the exchange interaction Hamiltonian, under which the density matrix evolves into the one which commutes with the exchange interaction Hamiltonian.

The coupling of geometrical and electronic properties is a promising venue to engineer conduction properties in graphene. Confinement added to strain allows for interplay of different transport mechanisms with potential device applications. To investigate strain signatures on transport in confined geometries, we focus on graphene nanoribbons (GNRs) with circularly symmetric deformations. In particular, we study GNRs with an inhomogeneous out-of-plane Gaussian deformation, connected to reservoirs. We observe an enhancement of the density of states in the deformed region, accompanied with a decrease in the conductance, signaling the presence of confined states. The local density of states exhibits a sixfold symmetric structure with an oscillating sublattice occupation asymmetry that persists for a wide range of energy and model parameters.

Transition-metal heterostructures offer the fascinating possibility of controlling orbital degrees of freedom via strain. Here, we investigate theoretically the degree of orbital polarization that can be induced by epitaxial strain in ${\mathrm{LaNiO}}_3$ films. Using combined electronic structure and dynamical mean-field theory methods we take into account both structural distortions and electron correlations and discuss their relative influence. We confirm that Hund's rule coupling tends to decrease the polarization and point out that this applies to both the $d^8\underline{L}$ and $d^7$ local configurations of the Ni ions. Our calculations are in good agreement with recent experiments, which revealed sizable orbital polarization under tensile strain. We discuss why full orbital polarization is hard to achieve in this specific system and emphasize the general limitations that must be overcome to achieve this goal.

Magnetoelectric coupling in pyroelectric ${\mathrm{CaBaCo}}_4$${\mathrm{O}}_7$ is investigated using ab intio calculations and Landau theory. The former shows that exchange striction is strong enough to produce a giant change in electric polarization upon ferrimagnetic ordering, comparable to the experimentally determined value of $\sim 17{\mathrm{mC}/\mathrm{m}}^2$. Furthermore, Landau theory demonstrates that magnetoelastic coupling in ${\mathrm{CaBaCo}}_4$${\mathrm{O}}_7$ is responsible for the strong magnetoelectric coupling appearing close to the magnetic phase transition.

We analyze the dynamics of a Luttinger model following a quench in the electron-electron interaction strength, where the change in the interaction strength occurs over a finite time scale $\tau$. We study the Loschmidt echo (the overlap between the initial and final state) as a function of time, both numerically and within a perturbation scheme, treating the change in the interaction strength as a small parameter, for all $\tau$. We derive the corrections appearing in (a) the Loschmidt echo for a finite quench duration $\tau$, (b) the scaling of the echo following a sudden $\left(\tau \to 0\right)$ quench, and (c) the scaling of the echo after an adiabatic $\left(\tau \to \infty \right)$ quench. We study in detail the limiting cases of the echo in the early-time and infinite-time limits, and provide scaling arguments to understand these in a general context. We also show that our perturbative results are in good agreement with the exact numerical ones.

We study the role of spin-orbit interactions in the coupled magnetoelectric dynamics of a ferromagnetic film coated with an electrical conductor. While the main thrust of this work is phenomenological, several popular simple models are considered microscopically in some detail, including Rashba and Dirac two-dimensional electron gases coupled to a magnetic insulator, as well as a diffusive spin Hall system. We focus on the long-wavelength magnetic dynamics that experiences current-induced torques and produces fictitious electromotive forces. Our phenomenology provides a suitable framework for analyzing experiments on current-induced magnetic dynamics and reciprocal charge pumping, including the effects of magnetoresistance and Gilbert-damping anisotropies, without a need to resort to any microscopic considerations or modeling. Finally, some remarks are made regarding the interplay of spin-orbit interactions and magnetic textures.

Using the small angle neutron scattering (SANS) technique we investigated the vortex lattice (VL) in the mixed state of the stannide superconductor ${\mathrm{Yb}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$. We find a single domain VL of slightly distorted hexagonal geometry for field strengths between 350 and 18 500 G and temperatures between $T=0.05$ and 6.5 K. We observe a clear in-plane rotation of the VL for different magnetic field directions relative to the crystallographic axes. We also find that the hexagonal symmetry of the VL is energetically favorable in ${\mathrm{Yb}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ for external fields oriented along axes of different symmetries: twofold [110], threefold [111], and fourfold [100]. The observed behavior is different from other conventional and unconventional superconductors. The superconducting state is characterized by an isotropically gapped order parameter with an amplitude of $\Delta \left(0\right)=1.57\pm 0.05$ meV. At the lowest temperatures, the field dependence of the magnetic form factor in our material reveals a London penetration depth of ${\lambda }_L=2508\pm 17$ Å and a Ginzburg coherence length of $\xi =100\pm 1.3$ Å, i.e., it is a strongly type-II superconductor, $\kappa ={\lambda }_L/\xi =25$.

Configurationally disordered crystalline boron carbide, with the composition ${\mathrm{B}}_4\mathrm{C}$, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatom's picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: (1) bipolar defects and (2) rotation of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon an increase in temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites; meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground-state configuration and restore the rhombohedral system $(\mathrm{R}3\mathrm{m})$. The restoration of inversion symmetry yielding the full rhombohedral symmetry $\left(R\overline{3}\mathrm{m}\right)$ on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered ${\mathrm{B}}_4\mathrm{C}$ phases reveals a tendency to stabilize the ordered ground-state configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on ${\mathrm{B}}_4\mathrm{C}$. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational disorder in ${\mathrm{B}}_4\mathrm{C}$.

The magnetic phase diagrams of models for quasi-one-dimensional compounds belonging to the iron-based-superconductor family are presented. The five-orbital Hubbard model and the real-space Hartree–Fock approximation are employed, supplemented by density functional theory to obtain the hopping amplitudes. Phase diagrams are constructed by varying the Hubbard $U$ and Hund $J$ couplings at zero temperature. The study is carried out at electronic density (electrons per iron) $n=5.0$, which is of relevance for the already-known material ${\mathrm{TlFeSe}}_2$, and also at $n=6.0$, where representative compounds still need to be synthesized. At $n=5.0$ there is a clear dominance of staggered spin order along the chain direction. At $n=6.0$ and with the realistic Hund coupling $J/U=0.25$, the phase diagram is far richer, including a variety of “block” states involving ferromagnetic clusters that are antiferromagnetically coupled, in qualitative agreement with recent density matrix renormalization group calculations for the three-orbital Hubbard model in a different context. These block states arise from the competition between ferromagnetic order (induced by double exchange and prevailing at large $J/U$) and antiferromagnetic order (dominating at small $J/U$). The density of states and orbital compositions of the many phases are also provided.

The time-dependent numerical renormalization group method (TDNRG) [Anders et al., Phys. Rev. Lett. 95, 196801 (2005)] was recently generalized to multiple quenches and arbitrary finite temperatures [Nghiem et al., Phys. Rev. B 89, 075118 (2014)] by using the full density matrix approach [Weichselbaum et al., Phys. Rev. Lett. 99, 076402 (2007)]. The formalism rests solely on the numerical renormalization group (NRG) approximation. In this paper, we numerically implement this formalism to study the response of a quantum impurity system to a general pulse and to periodic driving, in which a smooth pulse or a periodic train of pulses is approximated by a sufficient number of quenches. We show how the NRG approximation affects the trace of the projected density matrices and the continuity of the time evolution of a local observable. We also investigate the long-time limit of a local observable upon switching from a given initial state to a given final state as a function of both the pulse shape and the switch-on time, finding that this limit is improved for smoother pulse shapes and longer switch-on times. This lends support to our earlier suggestion that the long-time limit of observables, following a quench between a given initial state and a given final state, can be improved by replacing a sudden large and instantaneous quench by a sequence of smaller ones acting over a finite time interval: longer switch-on times and smoother pulses, i.e., increased adiabaticity, favor relaxation of the system to its correct thermodynamic long-time limit. For the case of periodic driving, we compare the TDNRG results to the exact analytic ones for the noninteracting resonant level model, finding better agreement at short to intermediate time scales in the case of smoother driving fields. Finally, we demonstrate the validity of the multiple-quench TDNRG formalism for arbitrary temperatures by studying the time evolution of the occupation number in the interacting Anderson impurity model in response to a periodic switching of the local level from the mixed valence to the Kondo regime at low, intermediate, and high temperatures.

Heterostructures composed of ferromagnetic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}$${\mathrm{MnO}}_3$, ferromagnetic ${\mathrm{SrRuO}}_3$, and superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ were studied experimentally. Structures of composition $\mathrm{Au}/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}$${\mathrm{MnO}}_3/{\mathrm{SrRuO}}_3/{\mathrm{YBa}}_2$${\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ were prepared by pulsed laser deposition, and their high quality was confirmed by x-ray diffraction and reflectometry. A noncollinear magnetic state of the heterostructures was revealed by means of superconducting quantum interference device magnetometry and polarized neutron reflectometry. We have further observed superconducting currents in mesa structures fabricated by deposition of a second superconducting Nb layer on top of the heterostructure, followed by patterning with photolithography and ion-beam etching. Josephson effects observed in these mesa structures can be explained by the penetration of a triplet component of the superconducting order parameter into the magnetic layers.

We report on intraband photocurrent spectroscopy of sixfold stacked Ge/Si quantum dots embedded in a Si matrix and aligned along the growth direction. The dots are formed in a shape of pyramids with the average lateral size of 18 nm. The $n$-type heterostructures show broad spectral response ranging from 5 to 20 $\mu \mathrm{m}$, depending on the polarization of the incoming infrared light. The normal incidence photocurrent peak centered around 12–15 $\mu \mathrm{m}$ is attributed to the transitions from the electron states localized in the Si region adjacent to the dots to continuum states of the Si matrix. The electron confinement is caused by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge/Si quantum dots. Using the Ge content and dot shape determined by Raman and scanning tunneling microscopy analysis as input parameters for three-dimensional band structure simulations, a good agreement between measured and calculated electron binding energy is obtained. Photoluminescence spectroscopy and measurements of temperature dependence of dark conductance are used to correlate photocurrent results.

We consider a sequence of quantized Lorentzian pulses of noninteracting electrons impinging on a quantum point contact and study the waiting time distribution (WTD), for any transmission and any number of pulses. As the degree of overlap between the electronic wave functions is tuned, the WTD reveals how the correlations between particles are modified. In the weak overlap regime, the WTD is made of several equidistant peaks, separated by the same period as the incoming pulses, contained in an almost exponentially decaying envelope. In the other limit, the WTD of a single quantum channel subjected to a constant voltage is recovered. In both cases, the WTD stresses the difference between the fluctuations induced by the scatterer and the ones encoded in the incoming quantum state. A clear crossover between these two situations is studied with numerical and analytical calculations based on scattering theory.

The study, tailoring, and control of the coupling between magnetic layers triggered the development of spintronics and has been up to now focused on systems comprising at least one ferromagnetic layer. Here we present a system where two antiferromagnetic layers separated by an ultrathin tunnel barrier interact with each other. We have studied with neutron and x-ray diffraction two sets of Cr/MgO/Cr trilayers, with either thin (five monolayers or less) or thick (eight monolayers or more) MgO layers, along with isolated Cr layers used as references. While the behavior of the trilayers with thick MgO layers can be explained by the strain state of each layer, that of trilayers with thin barriers cannot be related to the well known behavior of isolated chromium layers. We indeed report in these samples the presence of anomalous magnetic phases (modified propagation vector, direction of spins and propagation vectors inconsistent with the strain state of the sample) that we ascribe to the existence of a tunnel magnetic coupling between the AF layers through the insulating barrier. This unexpected finding opens opportunities for spintronics using solely antiferromagnets.