Recent experimental studies of Ge nanoislands on silicon-on-insulator (SOI) substrates have provided a defect-free strain relaxation mechanism through the bending of the substrate. Here, using atomistic Monte Carlo simulations and analytical modeling, we couple this relaxation mechanism with interdiffusion and alloying and observe composition profiles that are completely different from those observed in flat nanoislands. Moreover, for comparable SOI and island thicknesses, intermixing can be greatly reduced and Ge content in the islands is highly preserved.

We disentangle complex Si(111) hole subbands dispersion obtained by angle-resolved photoelectron spectroscopy and classify the subbands into three valence components. Through this procedure, quantum numbers and in-plane effective masses of the subbands are determined. The in-plane effective masses are close to the effective masses of a Si bulk valence band obtained by the first-principles calculations. Out-of-plane effective masses are obtained within the framework of triangular potential approximation. The results are compared to the reported theoretical results examining the validity of the theory. Energy separations of the subbands, order of the subbands, and out-of-plane effective masses are found to be basically in good agreement with the theoretical results.

We revisit the problem of quantum localization of many-body states in a quantum dot and the associated problem of relaxation of an excited state in a finite correlated electron system. We determine the localization threshold for the eigenstates in Fock space. We argue that the localization-delocalization transition (which manifests itself, e.g., in the statistics of many-body energy levels) becomes sharp in the limit of a large dimensionless conductance (or, equivalently, in the limit of weak interaction). We also analyze the temporal relaxation of quantum states of various types (a “hot-electron state,” a “typical” many-body state, and a single-electron excitation added to a “thermal state”) with energies below, at, and above the transition.

We show how the elusive toroidal dipole moment appears as a radiative excitation eigenmode in a metamolecule resonator that is formed by pairs of plasmonic nanorods. We analyze one such nanorod configuration—a toroidal metamolecule. We find that the radiative interactions in the toroidal metamolecule can be qualitatively represented by a theoretical model based on an electric point dipole arrangement. Both a finite-size rod model and the point dipole approximation demonstrate how the toroidal dipole moment is subradiant and difficult to excite by incident light. By means of breaking the geometric symmetry of the metamolecule, the toroidal mode can be excited by linearly polarized light and appears as a Fano resonance dip in the forward scattered light. We provide simple optimization protocols for maximizing the toroidal dipole mode excitation. This opens up possibilities for simplified control and driving of metamaterial arrays consisting of toroidal dipole unit-cell resonators.

Half metal magnets are of great interest in the field of spintronics because of their potential full spin polarization at the Fermi level $\left(E_{\mathrm{F}}\right)$ and low magnetization damping. The high Curie temperature and the predicted 0.7 eV minority spin gap make the $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ Heusler compound very promising for applications. We investigated the half-metallic magnetic character of this compound using spin-resolved photoemission, ab initio calculation, and ferromagnetic resonance. At the surface of $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$, a gap in the minority spin channel is observed, leading to $100\%$ spin polarization. However, this gap is 0.3 eV below $E_{\mathrm{F}}$, and a minority spin state is observed at $E_{\mathrm{F}}$. We show that a minority spin gap at $E_{\mathrm{F}}$ can nevertheless be recovered either by changing the chemical composition of the compound or by covering the surface by Mn, MnSi, or MgO. This spin-gap recovery results in extremely small damping coefficients, reaching values as low as $7\times {10}^{-4}$.

The domain wall motion along a helix-shaped nanowire is studied for the case of spin-current driving via the Zhang-Li mechanism. The analysis is based on the collective variable approach. Two effects are ascertained: (i) the curvature results in the appearance of the Walker limit for a uniaxial wire, and (ii) the torsion results in effective shift of the nonadiabatic spin torque parameter $\beta$. The latter effect changes considerably the domain wall velocity and can result in negative domain wall mobility. This effect can be also used for an experimental determination of the nonadiabatic parameter $\beta$ and damping coefficient $\alpha$.

Partial nesting between two connected or disconnected regions of the Fermi surface leads to fractional powers of the Coulomb scattering lifetime as a function of temperature. This result is first demonstrated for a toy band structure where partial nesting occurs within a single band and between different regions of the Brillouin zone. A comparison is then made to a multiband scenario by studying the scattering rate of an effective two-orbital model that was proposed in the context of multiorbital superconductors. In the process, various model independent features affecting the temperature exponent $n$ are identified. The logarithmically divergent contributions of the lowest order vertex correction to the multiorbital susceptibility, and the role played by nesting in suppressing these divergences, is analyzed. The relevance of these results is discussed keeping the recently observed anomalous resistivity in the Co doped iron superconductor LiFeAs as a backdrop.

We have studied the pressure-induced structural and electronic transitions of GeS to 49 GPa, using diamond anvil cells, microconfocal Raman spectroscopy, synchrotron angle-resolved x-ray diffraction, and electrical resistance measurements. The results indicate that layered GeS phase I (Pnma) undergoes two structural phase transitions: first to phase II $(P{2}_1/m)$ at ∼9 GPa and then to phase III (Cmcm) at ∼32 GPa. These transitions occur sluggishly over large pressure ranges and accompany an increase of the nearest coordination number of Ge and S atoms from three in phase I and II to five in phase III, resulting in an extended network of densely packed layer structures in phase III. We suggest anisotropic compression enhanced shears to be responsible for the transitions. Phase II also undergoes an electronic insulator-metal transition and becomes metallic above 20 GPa, The temperature-dependent resistance change of phase II further indicates a gradual band-gap closing of phase II $(P{2}_1/m)$.

In itinerant ferromagnets, the quenched disorder is predicted to dramatically affect the ferromagnetic to paramagnetic quantum phase transition driven by external control parameters at zero temperature. Here we report a study on Fe-doped ${\mathrm{Cr}}_2\mathrm{B}$, which, starting from the paramagnetic parent, orders ferromagnetically for Fe-doping concentrations $x$ larger than $x_{\mathrm{c}}=2.5\%$. In parent ${\mathrm{Cr}}_2\mathrm{B}, {}^{11}\mathrm{B}$ nuclear magnetic resonance data reveal the presence of both ferromagnetic and antiferromagnetic fluctuations. The latter are suppressed with Fe doping, before the ferromagnetic ones finally prevail for $x>x_{\mathrm{c}}$. Indications for non-Fermi-liquid behavior, usually associated with the proximity of a quantum critical point, were found for all samples, including undoped ${\mathrm{Cr}}_2\mathrm{B}$. The sharpness of the ferromagneticlike transition changes on moving away from $x_{\mathrm{c}}$, indicating significant changes in the nature of the magnetic transitions in the vicinity of the quantum critical point. Our data provide some constraints for understanding quantum phase transitions in itinerant ferromagnets in the limit of weak quenched disorder.

We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of 72%. We measure energy relaxation times at the symmetry point of 5 and $10\mu \mathrm{s}$, corresponding to 7 and $20\mu \mathrm{s}$ when relaxation through the resonator due to Purcell effect is subtracted out, and levels of flux noise of 2.6 and $2.7\mu {\mathrm{\Phi }}_0/\sqrt{\text{Hz}}$ at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. The strong coupling between the qubits and the cavity leads to a large, cavity-mediated, qubit-qubit coupling. This coupling, which is characterized spectroscopically, reaches 38 MHz. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.

By combining the Baeriswyl wave function with equilibrium and time-dependent variational principles, we develop a nonequilibrium formalism to study quantum quenches for two-dimensional spinless fermions with nearest-neighbor hopping and repulsion. The variational ground-state energy, the charge-density wave (CDW) order parameter, and the short-time dynamics agree convincingly with the results of numerically exact simulations. We find that, depending on the initial and final interaction strength, the quenched system either exhibits oscillatory behavior or relaxes to a time-independent steady state. The time-averaged expectation value of the CDW order parameter rises sharply when crossing from the steady-state regime to the oscillating regime, indicating that the system, being nonintegrable, shows signs of thermalization with an effective temperature above or below the equilibrium critical temperature, respectively.

Applying a symmetric bulk bipartition to the one-dimensional Affleck-Kennedy-Lieb-Tasaki valence-bond solid (VBS) states for the integer spin-$S$ Haldane gapped phase, we can create an array of fractionalized spin-$S$/2 edge states with the super unit cell $l$ in the reduced bulk system, and the topological properties encoded in the VBS wave functions can be revealed. The entanglement Hamiltonian (EH) with even $l$ corresponds to the quantum antiferromagnetic Heisenberg spin-$S$/2 model. For the even integer spins, the EH still describes the Haldane gapped phase. For the odd integer spins, however, the EH just corresponds to the quantum antiferromagnetic Heisenberg half-odd integer-spin model with spinon excitations, characterizing the critical point separating the topological Haldane phase from the trivial gapped phase. Our results thus demonstrate that the topological bulk property not only determines its fractionalized edge states but also the quantum criticality associated with the topological phase, where the elementary excitations are precisely those fractionalized edge degrees of freedom confined in the bulk of the topological phase.

Within the framework of density functional theory and Hedin's $GW$ approximation for single-particle excitations, we present quasiparticle band structures and densities of states for two gallium oxide polymorphs: rhombohedral $\alpha -{\mathrm{Ga}}_2{\mathrm{O}}_3$ and monoclinic $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$. The gap problem is attacked. In addition, their electron effective mass tensors are given. Solving the Bethe-Salpeter equation we also calculate excitonic optical spectra of the two polymorphs. The treatment of excitonic effects allows for a trustable prediction of optical properties from the band gap to the ultraviolet region. In addition, for few other polymorphs we also discuss the frequency-dependent dielectric tensor within the independent-particle approximation (random phase approximation) and densities of states on density functional level. We demonstrate that apart from subtle details, the overall densities of states and optical spectra, in particular the isotropically averaged spectra, are rather similar for all polymorphs, while the electronic dielectric constants vary with the structure. For all polymorphs, complete sets of elastic constants are given.

We investigate the role of electronic-vibrational coupling in resonant electron transport through single-molecule junctions, taking into account that the corresponding coupling strengths may depend on the charge and excitation state of the molecular bridge. Within an effective-model Hamiltonian approach for a molecule with multiple electronic states, this requires to extend the commonly used model and include vibrationally dependent electron-electron interaction. We use Born-Markov master equation methods and consider selected models to exemplify the effect of the additional interaction on the transport characteristics of a single-molecule junction. In particular, we show that it has a significant influence on local cooling and heating mechanisms, it may result in negative differential resistance, and it may cause pronounced asymmetries in the conductance map of a single-molecule junction.

Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in-plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blueshift of the emission with increasing temperature is well described using a two-oscillator model for the temperature dependence of the band gap.

Using hybrid density functional calculations, we address the structural properties, formation energies, and charge transition levels of a variety of oxygen defects in GaAs. The set of considered defects comprises the bridging O atom in a As-O-Ga configuration, interstitial O atoms in tetrahedral sites, and O atoms substitutional to either Ga $\left({\mathrm{O}}_{\mathrm{Ga}}\right)$ or As atoms $\left({\mathrm{O}}_{\mathrm{As}}\right)$. In addition, we consider an As vacancy containing two O atoms, for which the most stable configurations are found through the use of molecular dynamics simulations, and defect complexes involving a ${\mathrm{O}}_{\mathrm{As}}$ defect bound to either one or two ${\mathrm{As}}_{\mathrm{Ga}}$ antisites, denoted ${\mathrm{As}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{As}}$ and $\left({\mathrm{As}}_{\mathrm{Ga}}\right){}_2\text{−}{\mathrm{O}}_{\mathrm{As}}$, respectively. We find that the bridging O defect and the ${\mathrm{As}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{As}}$ and $\left({\mathrm{As}}_{\mathrm{Ga}}\right){}_2\text{−}{\mathrm{O}}_{\mathrm{As}}$ complexes are the most stable oxygen defects in GaAs. The actual occurrence of these defects is examined against two criteria. The first criterion concerns the stability against O dissociation and is evaluated via the calculation of dissociation energies. The second criterion involves the defect formation at thermodynamic equilibrium and is inferred from the comparison between the formation energy of the oxygen defect and that of its O-related dissociation product (bridging O defect). Both the ${\mathrm{As}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{As}}$ and $\left({\mathrm{As}}_{\mathrm{Ga}}\right){}_2\text{−}{\mathrm{O}}_{\mathrm{As}}$ complexes satisfy these criteria and are stable against O dissociation. Further analysis in cooled-down conditions leads us to dismiss the ${\mathrm{As}}_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{As}}$ defect due to the more favorable bonding of two rather than one ${\mathrm{As}}_{\mathrm{Ga}}$ antisites. The conclusion that only the bridging O defect and the $\left({\mathrm{As}}_{\mathrm{Ga}}\right){}_2\text{−}{\mathrm{O}}_{\mathrm{As}}$ complex are expected to occur is in accord with experimental observations.

At ambient pressure the element cerium shows a metastable ($t_{1/2}\sim 40$ years) double-hexagonal close-packed $\beta$ phase that is positioned between two cubic phases, $\gamma$ and $\alpha$. With modest pressure the $\beta$ phase can be suppressed, and a volume contraction (17%) occurs between the $\gamma$ and the $\alpha$ phases as the temperature is varied. This phenomenon has been linked to subtle alterations in the $4f$ band. In order to rationalize the presence of the metastable $\beta$ phase, and its position in the phase diagram, we have computed the stacking fault formation energies of the cubic phases of cerium using an axial interaction model. This model links the total energy differences between hexagonal closed-packed stacking sequences and stacking fault energetics. Total energies are calculated by density functional theory and by dynamical mean-field theory merged with density functional theory. It is found that there is a large difference in the stacking fault energies between the $\alpha$ and the $\gamma$ phase. The $\beta \text{-phase}$ energy is nearly degenerate with the $\gamma$ phase, consistent with previous third-law calorimetry results, and dislocation dynamics explain the pressure and temperature hysteretic effects.

The phase-transition sequence of $0.67\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3})-0.37\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-0.37PT) single crystals driven by the electric ($E$) field and temperature is comprehensively studied. Based on the strain-$E$ field loop, polarization-$E$ field loop, and the evolution of domain configurations, the $E$ field along the ${\left[011\right]}_C$ induced phase transitions have been confirmed to be as follows: tetragonal ($T$) → monoclinic $\left(M_C\right)\to$ single domain orthorhombic ($O$) phase. As the $E$ field decreases, the induced $O$ phase cannot be maintained and transformed to the $M_C$ phase, then to the coexistence state of $M_C$ and $T$ phases. In addition, the complete sets of dielectric, piezoelectric, and elastic constants for the ${\left[011\right]}_C$-poled domain-engineered PMN-0.37PT single crystal were measured at room temperature, which show high longitudinal dielectric, piezoelectric, and electromechanical properties $({\varepsilon }_{33}^T=10661,d_{33}=1052\mathrm{pC}/\mathrm{N}$, and $k_{33}=0.766)$. Our results revealed that the $M_C$ phase plays an important role in the high electromechanical properties of this domain-engineered single crystal. The temperature dependence of the domain configuration revealed that the volume fraction of the $M_C$ phase decreases with temperature accompanied by the reduction of ${\varepsilon }_{33}^T,d_{31}$, and $k_{31}$ due to the substantially smaller intrinsic properties of the $T$ phase.

Organic-inorganic hybrid halide perovskites, in which the A cations of an ${\mathrm{ABX}}_3$ perovskite are replaced by organic cations, may be used for photovoltaic and solar thermoelectric applications. In this contribution, we systematically study three lead-free hybrid perovskites, i.e., methylammonium tin iodide ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{SnI}}_3$, ammonium tin iodide ${\mathrm{NH}}_4{\mathrm{SnI}}_3$, and formamidnium tin iodide $\mathrm{HC}{\left({\mathrm{NH}}_2\right)}_2{\mathrm{SnI}}_3$ by first-principles calculations. We find that in addition to the commonly known motif in which the corner-shared ${\mathrm{SnI}}_6$ octahedra form a three-dimensional network, these materials may also favor a two-dimensional (layered) motif formed by alternating layers of the ${\mathrm{SnI}}_6$ octahedra and the organic cations. These two motifs are nearly equal in free energy and are separated by low barriers. These layered structures features many flat electronic bands near the band edges, making their electronic structures significantly different from those of the structural phases composed of three-dimension networks of ${\mathrm{SnI}}_6$ octahedra. Furthermore, because the electronic structures of $\mathrm{HC}{\left({\mathrm{NH}}_2\right)}_2{\mathrm{SnI}}_3$ are found to be rather similar to those of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{SnI}}_3$, formamidnium tin iodide may also be promising for the applications of methylammonium tin iodide.