A microscopic approach that is based on the multisubband semiconductor Bloch equations formulated in the basis of a 14-band $\mathbf{k}·\mathbf{p}$ model is employed to compute the temporal dynamics of photocurrents in GaAs quantum wells following excitation with femtosecond laser pulses. This approach provides a transparent description of the interband, intersubband, and intraband excitations, fully includes all resonant as well as off-resonant excitations, and treats the light-matter interaction nonperturbatively. For linearly polarized excitations, the photocurrents contain contributions from shift and rectification currents. We numerically compute and analyze these currents generated by excitation with femtosecond laser pulses for [110]- and [111]-oriented GaAs quantum wells. It is shown that the often employed perturbative ${\chi }^{\left(2\right)}$ approach breaks down for peak fields larger than about 10 kV/cm, and that nonperturbative effects lead to a reduction of the peak values of the shift and rectification currents and to temporal oscillations that originate from Rabi flopping. In particular, we find a complex oscillatory photon energy dependence of the magnitudes of the shift and rectification currents. Our simulations demonstrate that this dependence is the result of mixing between the heavy- and light-hole valence bands. This is a surprising finding since the band mixing has an even larger influence on the strength of the photocurrents than the absorption coefficient. For [110]-oriented GaAs quantum wells, the calculated photon energy dependence is compared to experimental results, and good agreement is obtained. This validates our theoretical approach.

We predict a robust quantum interference phenomenon in a semiconductor P-N junction: with a local pump on one side of the junction, the response of a local probe on the other side behaves as if the disturbance emanates not from the pump but instead from its mirror image about the junction. This phenomenon follows from the matching of Fermi surfaces of the constituent materials, thus it is robust against the details of the junction (e.g., width, potential profile, and even disorder), in contrast to the widely studied anomalous focusing caused by negative refraction. The recently fabricated P-N junctions in 2D semiconductors provide ideal platforms to explore this phenomenon and its applications to dramatically enhance charge and spin transport as well as carrier-mediated long-range correlation.

We investigate the thermodynamic stability of carbon-rich icosahedral boron carbide at different compositions, ranging from ${\mathrm{B}}_4\mathrm{C}$ to ${\mathrm{B}}_2\mathrm{C}$, using first-principles calculations. Apart from ${\mathrm{B}}_4\mathrm{C}$, generally addressed in the literature, ${\mathrm{B}}_{2.5}\mathrm{C}$, represented by ${\mathrm{B}}_{10}{\mathrm{C}}_2^p$(C-C), where ${\mathrm{C}}^p$ and (C-C) denote a carbon atom occupying the polar site of the icosahedral cluster and a diatomic carbon chain, respectively, is predicted to be thermodynamically stable under high pressures with respect to ${\mathrm{B}}_4\mathrm{C}$ as well as pure boron and carbon phases. The thermodynamic stability of ${\mathrm{B}}_{2.5}\mathrm{C}$ is determined by the Gibbs free energy G as a function of pressure $p$ and temperature $T$, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. The stability range of ${\mathrm{B}}_{2.5}\mathrm{C}$ is then illustrated through the $p\text{−}T$ phase diagrams. Depending on the temperatures, the stability range of ${\mathrm{B}}_{2.5}\mathrm{C}$ is predicted to be within the range between 40 and 67 GPa. At $T$ $\gtrsim$ 500 K, the icosahedral ${\mathrm{C}}^p$ atoms in ${\mathrm{B}}_{2.5}\mathrm{C}$ configurationally disorder at the polar sites. By investigating the properties of ${\mathrm{B}}_{2.5}\mathrm{C}$, e.g., elastic constants and phonon and electronic density of states, we demonstrate that ${\mathrm{B}}_{2.5}\mathrm{C}$ is both mechanically and dynamically stable at zero pressure, and is an electrical semiconductor. Furthermore, based on the sketched phase diagrams, a possible route for experimental synthesis of ${\mathrm{B}}_{2.5}\mathrm{C}$ as well as a fingerprint for its characterization from the simulations of x-ray powder diffraction pattern are suggested.

We develop a phase-field model that describes the elastic distortion of a ferroelastic material with cubic anisotropy due to an arbitrary dislocation network and a uniform external load. The dislocation network is characterized using the Nye tensor and enters the formulation via a set of incompatibility constraints for the internal strain field. The long-range elastic response of the material is obtained by minimization of the free energy that accounts for higher-order terms of the order parameters and symmetry-adapted strain gradients. The influence of dislocations on the microstructure is studied using a static equilibrium analysis of a material without dislocations and with a random array of parallel edge dislocations. A minimal continuum dislocation dynamics is then used to investigate the simultaneous evolution of the network of geometrically necessary dislocations and the internal strain field. The model developed here is directly applicable to single-phase cubic crystals with an arbitrary degree of anisotropy as well as to ferroelastic materials undergoing temperature-driven cubic-to-tetragonal phase transitions.

We report on the investigation of perpendicular exchange bias in $\mathrm{FePt}\left(001\right)/\mathrm{NiO}\left(\overline{1}\overline{1}1\right)$ orthogonal exchange couple with FePt partially $\mathrm{L}{1}_0$ ordered. From initial magnetization curve measurement and magnetic domain imaging, we find that, for the as-grown bilayer structure, the FePt layer experiences a small-angle magnetization rotation when it is magnetized near to saturation in film normal direction. After field cooling, the bilayer structure shows a significant enhancement of perpendicular magnetic anisotropy, indicating the field mediated coupling between the spins across the FePt/NiO interface. According to Koon's theoretical calculation on the basis of lowest energy ferromagnetic/antiferromagnetic coupling configuration for compensated spins at antiferromagnetic side, we consider slightly slanted Ni spins at the interface off the $\left(\overline{1}\overline{1}1\right)$ easy plane can stabilize the spin coupling between FePt and NiO and result in the observed exchange bias in this paper. This consideration was further confirmed by stripe domain width calculation.

Muon spin rotation ($\mu \mathrm{SR}$) and ${}^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1-y}{\mathrm{Y}}_y{\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2\%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20\%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20\%$. Low-energy spin fluctuations give rise to a peak in ${}^{19}\mathrm{F}$ NMR $1/T_1$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by ${}^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

Our recent success in triggering dendritic flux instabilities in $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films by applying magnetic fields at ultrahigh rates is followed here by a detailed study of the effect as a function of the field ramp rate, ${\dot{B}}_a$, and temperature, $T$. We trace the borderline in the ${\dot{B}}_a-T$ plane separating regions of smooth, gradual flux penetration and dendritic flux avalanches. In addition, we describe the changes in the dendritic morphology in the instability region as a result of changes in either ${\dot{B}}_a$ or $T$. Our experimental results, showing a monotonic increase of the avalanche threshold field ramp rate with temperature, are discussed in the framework of existing theories. On the basis of these theories we also explain the high stability of YBCO to dendritic avalanches as compared to, e.g., $\mathrm{Mg}{\mathrm{B}}_2$, identifying the flux flow resistivity, rather than any of the thermal parameters, as the main parameter governing the film stability.

In this paper, we develop a multiscale approach to model intrinsic dissipation under high frequency of vibrations in solids. For vibrations with a timescale comparable to the phonon relaxation time, the local phonon distribution deviates from the equilibrium distribution. We extend the quasiharmonic (QHM) method to describe the dynamics under such a condition. The local deviation from the equilibrium state is characterized using a nonequilibrium stress tensor. A constitutive relation for the time evolution of the stress component is obtained. We then parametrize the evolution equation using the QHM method and a stochastic sampling approach. The stress relaxation dynamics is obtained using mode Langevin dynamics. Methods to obtain the input variables for the Langevin dynamics are discussed. The proposed methodology is used to obtain the dissipation rate $E_{\mathrm{dissip}}$ for different cases. Frequency and size effect on $E_{\mathrm{dissip}}$ are studied. The results are compared with those obtained using nonequilibrium molecular dynamics (MD).

The pressure-temperature $\left(P\text{−}T\right)$ phase diagram of ${\mathrm{N}}_2\text{−}{\mathrm{O}}_2$ mixture with a composition of ${\mathrm{N}}_2$-48 mol % ${\mathrm{O}}_2$ has been investigated using x-ray diffraction and the phase stability of a hexagonal phase (space group: $P6$/mmm), with the kagome lattice examined under high-pressure and low-temperature conditions. While the phase appears as a low-temperature phase of the cubic phase $\left(Pm3n\right)$ with the structure of $\gamma \text{−}{\mathrm{O}}_2$ or $\delta \text{−}{\mathrm{N}}_2$ and is stable in a wide range of pressures and temperatures, it transforms to lower symmetry monoclinic or orthorhombic phases at lower temperature, accompanied with a distortion of the kagome lattice. Based on Rietveld refinements, the monoclinic and orthorhombic phases are found to be in the $P{2}_1/a$ and Cmmm space groups, respectively. In magnetization measurements, a magnetic transition is observed with a relatively large drop of magnetization, corresponding to the cubic-to-hexagonal phase transition. This suggests that the hexagonal phase has a certain magnetic ordered state that arises from the molecular magnetic moment of ${\mathrm{O}}_2$.

Using a time-dependent electrical investigation of the all-optical switching in ferrimagnetic and ferromagnetic Hall crosses via the anomalous Hall effect, intriguing insights into the rich physics underlying the all-optical switching are provided. We demonstrate that two different all-optical magnetization switching mechanisms can be distinguished; a “single pulse” switching for ferrimagnetic GdFeCo alloys, and a “two regimes” switching process for both ferrimagnetic TbCo alloys and ferromagnetic Pt/Co multilayers. We show that the latter takes place at two different time scales, and consists of a steplike helicity-independent multiple-domain formation within the first 1 ms followed by a helicity-dependent remagnetization on several tens of milliseconds.

We present a robust tight-binding description, based on the Slater-Koster formalism, of the band structure of ${\mathrm{H}}_3\mathrm{S}$ in the Im$\overline{3}m$ structure, stable in the range of pressure $P=180–220$ GPa. We show that the interatomic hopping between the $3s$ and $3p$ orbitals (and partially between the $3p$ orbitals themselves) of sulfur is fundamental to capturing the relevant physics associated with the Van Hove singularities close to the Fermi level. Comparing the model so defined with density functional theory calculations we obtain a very good agreement not only of the overall band structure but also of the low-energy states and the Fermi surface properties. The description in terms of Slater-Koster parameters permits us also to evaluate at a microscopic level a hopping-resolved linear electron-lattice coupling which can be employed for further tight-binding analyses also at a local scale.

${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is a prototypical multiband superconductor with three bands crossing the Fermi level. These bands exhibit distinct dimensional characteristics, with one quasi-2D $\gamma$ band and two quasi-1D $\alpha$ and $\beta$ bands. This leads to the expectation that the superconductivity on the $\gamma$ band may be only weakly Josephson-coupled to that on the other two bands. Based on an explicit microscopic weak coupling calculation appropriate for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we study the collective Leggett modes associated with the relative phase oscillations between the bands and show that a relatively soft Leggett mode exists due to the comparatively weaker interband Josephson coupling. These calculations also provide insight into why the superconducting gap magnitudes may be comparable on all three bands, despite the noticeable differences between the $\gamma$ and $\alpha /\beta$ bands. The analyses can be readily applied to other multiband superconductors.

We experimentally demonstrate the coupling at zero magnetic field of an isotopically pure erbium-doped yttrium orthosilicate crystal (${}^{167}\mathrm{Er}:\mathrm{YSO}$) to a three-dimensional superconducting cavity with a $Q$ factor of ${10}^5$. A tunable loop-gap resonator is used and its resonance frequency is tuned to observe the hyperfine transitions of the erbium sample. The observed spectrum differs from what is predicted by the published spin Hamiltonian parameters. The narrow cavity linewidth also enables the observation of asymmetric line shapes for these hyperfine transitions. Such a broadly tunable superconducting cavity (from 1.6 to 4.0 GHz in the current design) is a promising device for building hybrid quantum systems.

We present a reformulation of the hairy-probe method for introducing electronic open boundaries that is appropriate for steady-state calculations involving nonorthogonal atomic basis sets. As a check on the correctness of the method we investigate a perfect atomic wire of Cu atoms and a perfect nonorthogonal chain of H atoms. For both atom chains we find that the conductance has a value of exactly one quantum unit and that this is rather insensitive to the strength of coupling of the probes to the system, provided values of the coupling are of the same order as the mean interlevel spacing of the system without probes. For the Cu atom chain we find in addition that away from the regions with probes attached, the potential in the wire is uniform, while within them it follows a predicted exponential variation with position. We then apply the method to an initial investigation of the suitability of graphene as a contact material for molecular electronics. We perform calculations on a carbon nanoribbon to determine the correct coupling strength of the probes to the graphene and obtain a conductance of about two quantum units corresponding to two bands crossing the Fermi surface. We then compute the current through a benzene molecule attached to two graphene contacts and find only a very weak current because of the disruption of the $\pi$ conjugation by the covalent bond between the benzene and the graphene. In all cases we find that very strong or weak probe couplings suppress the current.

The recent discovery of superconductivity under high pressure in the two-leg ladder compound ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ [H. Takahashi et al., Nat. Mater. 14, 1008 (2015)] opens a broad avenue of research, because it represents the first report of pairing tendencies in a quasi-one-dimensional iron-based high-critical-temperature superconductor. Similarly, as in the case of the cuprates, ladders and chains can be far more accurately studied using many-body techniques and model Hamiltonians than their layered counterparts, particularly if several orbitals are active. In this publication, we derive a two-orbital Hubbard model from first principles that describes individual ladders of ${\mathrm{BaFe}}_2{\mathrm{S}}_3$. The model is studied with the density matrix renormalization group. These first reported results are exciting for two reasons: (i) at half-filling, ferromagnetic order emerges as the dominant magnetic pattern along the rungs of the ladder, and antiferromagnetic order along the legs, in excellent agreement with neutron experiments; and (ii) with hole doping, pairs form in the strong coupling regime, as found by studying the binding energy of two holes doped on the half-filled system. In addition, orbital selective Mott phase characteristics develop with doping, with only one Wannier orbital receiving the hole carriers while the other remains half-filled. These results suggest that the analysis of models for iron-based two-leg ladders could clarify the origin of pairing tendencies and other exotic properties of iron-based high-critical-temperature superconductors in general.

The mechanism of the room temperature (RT) ferromagnetism in C-doped ZnO and the trend that the higher the C concentration, the smaller the saturation magnetic moment $\left(M_{\mathrm{s}}\right)$, have remained controversial and puzzling for a long time. Using density-functional calculations with hybrid functional, we qualitatively understand these experimental phenomena. The substitutional ${\mathrm{C}}_{\mathrm{O}}$ defects dominate in Zn-rich conditions and $n$-type materials. They carry 1 or $2{\mu }_{\mathrm{B}}/\mathrm{C}$, depending on the position of the Fermi level. In low C concentrations and at RT, the ${\mathrm{C}}_{\mathrm{O}}$ defects are kept isolated and prefer to couple ferromagnetically, thus the $M_{\mathrm{s}}$ is larger. As the C concentration increases, nonmagnetic ${\mathrm{C}}_2$ dimers form through binding of two ${\mathrm{C}}_{\mathrm{O}}$ defects or binding of one ${\mathrm{C}}_{\mathrm{O}}$ defect with one interstitial C, thus decreasing the $M_{\mathrm{s}}$. Our results also suggest that the ferromagnetism from ${\mathrm{C}}_{\mathrm{O}}$ defects can be quenched by higher annealing temperatures. Furthermore, the $p$-type conductivity from carbon dopant is not supported in the present study.

The ternary semiconductors NiZrSn and CoZrBi with $C{1}_b$ crystal structure are introduced by calculating their basic structural, electronic, and phononic properties using density functional theory. Both the gradient-corrected PBE functional and the hybrid functional HSE06 are employed. While NiZrSn is found to be a small-band-gap semiconductor ($E_g=0.46$ eV in PBE and 0.60 eV in HSE06), CoZrBi has a band gap of 1.01 eV in PBE (1.34 eV in HSE06). Moreover, effective masses and deformation potentials are reported. In both materials $ABC$, the intrinsic point defects introduced by species $A$ (Ni or Co) are calculated. The Co-induced defects in CoZrBi are found to have a higher formation energy compared to Ni-induced defects in NiZrSn. The interstitial Ni atom (${\mathrm{Ni}}_i$) as well as the $V_{\mathrm{Ni}}{\mathrm{Ni}}_i$ complex introduce defect states in the band gap, whereas the Ni vacancy ($V_{\mathrm{Ni}}$) only reduces the size of the band gap. While ${\mathrm{Ni}}_i$ is electrically active and may act as a donor, the other two types of defects may compensate extrinsic doping. In CoZrBi, only the $V_{\mathrm{Co}}{\mathrm{Co}}_i$ complex introduces a defect state in the band gap. Motivated by the reported use of NiZrSn for thermoelectric applications, the Seebeck coefficient of both materials, both in the $p$-type and the $n$-type regimes, is calculated. We find that CoZrBi displays a rather large thermopower of up to $500\mu \mathrm{V}/\mathrm{K}$ when $p$ doped, whereas NiZrSn possesses its maximum thermopower in the $n$-type regime. The reported difficulties in achieving $p$-type doping in NiZrSn could be rationalized by the unintended formation of ${\mathrm{Ni}}_i^{2+}$ in conjunction with extrinsic acceptors, resulting in their compensation. Moreover, it is found that all types of defects considered, when present in concentrations as large as 3%, tend to reduce the thermopower compared to ideal bulk crystals at $T=600$ K. For NiZrSn, the calculated thermodynamic data suggest that additional Ni impurities could be removed by annealing, leading to precipitation of a metallic ${\mathrm{Ni}}_2\mathrm{ZrSn}$ phase.

We present a magnetically biased graphene-ferrite structure discriminating the transverse electric (TE) and transverse magnetic (TM) plasmonic modes of graphene. In this structure, the graphene TM plasmons interact reciprocally with the structure. In contrast, the graphene TE plasmons exhibit nonreciprocity. This nonreciprocity is manifested in unidirectional TE propagation in a frequency band close to the interband threshold frequency. The proposed structure provides a unique platform for the experimental demonstration of the unusual existence of the TE plasmonic mode in graphene.