Antiferromagnetic and ferromagnetic configurations of hematite α-Fe${}_2$O${}_3$ structures have been investigated by first-principles methods, which have been used to theoretically analyze the local structural and magnetic effects due to the presence of interstitial atoms such as hydrogen and oxygen. This study is based on the projector- augmented wave method within the local spin density approximation (LSDA) in addition to an on-site Coulomb correction of the Fe $d$ orbitals (i.e., the LSDA+$U^{\mathrm{SIC}}$ method). The results demonstrate that this correction potential is important in achieving an accurate description of both the structural and the magnetic properties of α-Fe${}_2$O${}_3$. The ground state of α-Fe${}_2$O${}_3$ is the antiferromagnetic phase. The presence of oxygen vacancies, interstitial oxygen, and an interstitial hydrogen decreases the local Fe magnetic moment $|M_s\left(\mathrm{Fe}\right)|$ in both the ferro- and antiferromagnetic phases, although the hydrogen has a rather modest effect. The density of states calculations demonstrate that the presence of interstitial atoms and defects yields a small reduction in the material's electronic gap.

We present $GW$ calculations of the topological insulators Bi${}_2$Se${}_3$, Bi${}_2$Te${}_3$, and Sb${}_2$Te${}_3$ within the all-electron full-potential linearized augmented-plane-wave formalism. Quasiparticle effects produce significant qualitative changes in the band structures of these materials when compared to density functional theory (DFT), especially at the $\Gamma$ point, where band inversion takes place. There, the widely used perturbative one-shot $GW$ approach can produce unphysical band dispersions, as the quasiparticle wave functions are forced to be identical to the noninteracting single-particle states. We show that a treatment beyond the perturbative approach, which incorporates the off-diagonal $GW$ matrix elements and thus enables many-body hybridization to be effective in the quasiparticle wave functions, is crucial in these cases to describe the characteristics of the band inversion around the $\Gamma$ point in an appropriate way. In addition, this beyond one-shot $GW$ approach allows us to calculate the values of the $Z_2$ topological invariants and compare them with those previously obtained within DFT.

In this experimental and theoretical study, it was found that the emission pattern of the doubly positively charged exciton complex $X^{2+}$ strongly depends on the nature of the involved excited hole states as well as the quantum dot symmetry. The two-hole system in the final state of the $X^{2+}$ recombination for the investigated high-symmetry pyramidal InGaAs quantum dots does not exhibit the singlet-tripletlike arrangement previously observed for the two-electron counterpart. Instead, the final states exhibit two true doublets, in accordance with group-theoretical predictions. Asymmetry is manifested in the photoluminescence spectra of $X^{2+}$ by a significant splitting of one doublet, which is a spectral feature exhibited to some degree by all of the measured quantum dots. The analysis demonstrates that an external magnetic field elevates the symmetry of the quantum dots. This work highlights the exciton complex $X^{2+}$ as a very sensitive probe of the quantum dot shape as well as the nature of the involved quantum states. Thus, its spectral features are very suitable for an efficient uninvasive postgrowth symmetry characterization of quantum dots.

We use ab initio calculations to investigate the geometry, band structures, and optical properties of hydrogen functionalized graphene, where hydrogen atoms are attached periodically to $A$-site carbon atoms to form $\sqrt{3}\times \sqrt{3}$, $2\times 2$, $3\times 3$, $4\times 4$, and $5\times 5$ supercells. The adsorbed hydrogen atoms distort the carbon atoms vertically, with almost no modification of the in-plane structures. The ground states show ferromagnetic order, with a total energy of a few tens meV lower than the nonmagnetic ground state. The hydrogen adsorption opens a gap that depends on the supercell size. Optical conductivities are calculated and show fine structures at the band edge. Taking into account the spin-orbit coupling, the nonzero off-diagonal components of the conductivity predict Kerr and Faraday effects without an external magnetic field.

Rates of exciton transitions between heavily P-doped silicon crystallites are calculated, and their dependencies on the phosphorus concentration are found. It is shown that doping is capable of considerable acceleration of resonant exciton transfer that becomes an even more efficient process in dense ensembles of silicon crystallites than the radiative interband recombination.

Self-healing mechanisms of vacancy defects in graphene and silicene are studied using first-principles calculations. We investigated host adatom adsorption, diffusion, vacancy formation, and revealed atomistic mechanisms in the healing of single, double, and triple vacancies of single-layer graphene and silicene. Silicon adatom, which is adsorbed to silicene at the top site forms a dumbbell-like structure by pushing one Si atom underneath. The asymmetric reconstruction of the single vacancy in graphene is induced by the magnetization through the rebonding of two dangling bonds and acquiring a significant magnetic moment through the remaining unsaturated dangling bond. In silicene, three twofold coordinated atoms surrounding the single vacancy become fourfold coordinated and nonmagnetic through rebonding. The energy gained through new bond formation becomes the driving force for the reconstruction. Under the external supply of host atoms, while the vacancy defects of graphene heal perfectly, the Stone-Wales defect can form in the course of healing of silicene vacancy. The electronic and magnetic properties of suspended, single-layer graphene and silicene are modified by reconstructed vacancy defects.

We investigate, assess, and suggest possibilities for a measurement of the local spin susceptibility of a conducting low-dimensional electron system. The basic setup of the experiment we envisage is a source-probe one. Locally induced spin density (e.g., by a magnetized atomic force microscope tip) extends in the medium according to its spin susceptibility. The induced magnetization can be detected as a dipolar magnetic field, for instance, by an ultrasensitive nitrogen-vacancy center based detector, from which the spatial structure of the spin susceptibility can be deduced. We find that one-dimensional systems, such as semiconducting nanowires or carbon nanotubes, are expected to yield a measurable signal. The signal in a two-dimensional electron gas is weaker, though materials with a high enough $g$-factor (such as InGaAs) seem promising for successful measurements.

We study the Higgs amplitude mode in the $s$-wave superfluid state on the honeycomb lattice inspired by recent cold atom experiments. We consider the attractive Hubbard model and focus on the vicinity of a quantum phase transition between semimetal and superfluid phases. On either side of the transition, we find collective mode excitations that are stable against decay into quasiparticle pairs. In the semimetal phase, the collective modes have “Cooperon” and exciton character. These modes smoothly evolve across the quantum phase transition, and become the Anderson-Bogoliubov mode and the Higgs mode of the superfluid phase. The collective modes are accommodated within a window in the quasiparticle-pair continuum, which arises as a consequence of the linear Dirac dispersion on the honeycomb lattice, and allows for sharp collective excitations. Bragg scattering can be used to measure these excitations in cold atom experiments, providing a rare example wherein collective modes can be tracked across a quantum phase transition.

Contrary to previous studies that identified the ground state crystal structure of the entire $R_3$Co series ($R$ is a rare earth) as orthorhombic $Pnma$, we show that Y${}_3$Co undergoes a structural phase transition at $T_t\simeq 160$ K. Single crystal neutron diffraction data reveal that at $T_t$ the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the $Pnma$ structure of Y${}_3$Co. In particular, a transversal acoustic phonon mode along the $\left(00\xi \right)$ direction has imaginary frequencies at $\xi <1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $\left(00\xi \right)$ phonon mode below a critical temperature $T_t$. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y${}_3$Co as a system with the charge density wave instability.

Under a magnetic field along its axis, domain-wall motion in a uniaxial nanowire is much slower than in the fully anisotropic case, typically by several orders of magnitude (the square of the dimensionless Gilbert damping parameter). However, with the addition of a magnetic field transverse to the wire, this behavior is dramatically reversed; up to a critical field strength, analogous to the Walker breakdown field, domain walls in a uniaxial wire propagate faster than in a fully anisotropic wire (without a transverse field). Beyond this critical field strength, precessional motion sets in, and the mean velocity decreases. Our results are based on leading-order analytic calculations of the velocity and critical field as well as numerical solutions of the Landau-Lifshitz-Gilbert equation.

The magnetic properties and electronic structure of cobalt phthalocyanine (CoPc, spin-$\frac{1}{2}$) molecular chains have been studied using density functional theory with a hybrid exchange functional, over a wide range of chain geometries. Our theoretical results for the exchange interactions in the known phases $\alpha$-CoPc ($J/k_B\sim 85$ K) and $\beta$-CoPc ($J/k_B\sim 2$ K) are in quantitative agreement with recent magnetic measurements; we also find the computed exchange interaction agrees qualitatively with recent measurements by inelastic tunneling spectroscopy on thin films. The computed exchange interactions are much larger than those in copper phthalocyanine (CuPc), and are predicted to rise to a maximum of $J/k_B\sim 400$ K when the molecules are face-on. The dominant exchange mechanism is expected to be superexchange arising from the direct hopping between $d_{z^2}$ orbitals.

We have determined effective mass parameters for ultrapure diamond by time-resolved cyclotron resonance of optically excited carriers. The transverse and longitudinal electron masses are found to be $m_t/m_0=0.280\left(2\right)$ and $m_l/m_0=1.56\left(2\right)$, where $m_0$ is the free-electron mass. The Luttinger parameters are extracted as ${\gamma }_1=2.67\left(2\right),{\gamma }_2=-0.40\left(2\right)$, and ${\gamma }_3=0.68\left(2\right)$, which give the light- and heavy-hole bands with considerable anisotropy and the split-off hole mass. The resonance widths indicate scattering times longer than 100 ps and carrier mobilities as high as ${10}^6{{\mathrm{cm}}^2{\mathrm{V}}^{-1}\mathrm{s}}^{-1}$ at 10 K.

We investigate how decoherence affects the entanglement established between two quantum dots in microcavities, and propose a tomographic scheme able to measure the entangled state. The scheme we consider establishes the entanglement via the exchange and measurement of a photon. Making the realistic assumption of noise dominated by pure-dephasing processes, we find that the photon must be exchanged and measured on time scales shorter than the quantum dots’ characteristic dephasing time for appreciable levels of entanglement to be achieved. The tomographic scheme is able to reconstruct the full density matrix of the quantum dots, and requires only single spin rotations and the injection of an additional photon. Remarkably, we find that the additional photon need not be exchanged and measured on a time scale shorter than the dephasing time for accurate tomography, and also allows many to be used in order to increase the measurement signal.

The determining factor of the bulk properties of doped Si is the column rather than the row in the periodic table from which the dopants are drawn. It is unknown whether the basic properties of dopants at surfaces and interfaces, steadily growing in importance as microelectronic devices shrink, are also solely governed by their column of origin. The common light impurity P replaces individual Si atoms and maintains the integrity of the dimer superstructure of the Si(001) surface, but loses its valence electrons to surface states. Here we report that isolated heavy dopants are entirely different: Bi atoms form pairs with Si vacancies, retain their electrons, and have highly localized, half-filled orbitals.

We demonstrate that entanglement of two macroscopic nanoelectromechanical resonators—coupled to each other via a common detector, a tunnel junction—can be generated by running a current through the device. We introduce a setup that overcomes generic limitations of proposals suggesting to entangle systems via a shared bath. At the heart of the proposal is an Andreev entangler setup, representing an experimentally feasible way of entangling two nanomechanical oscillators. Instead of relying on the coherence of a (fermionic) bath, in the Andreev entangler setup, a split Cooper pair that coherently tunnels to each oscillator mediates their coupling and thereby induces entanglement between them. Since entanglement is in each instance generated by Markovian and non-Markovian noisy open system dynamics in an out-of-equilibrium situation, we argue that the present scheme also opens up perspectives to observe dissipation-driven entanglement in a condensed-matter system.

We study the interaction of electromagnetic waves with two nested parallel metallic wire arrays, such that the unit cell of the structure has two inequivalent parallel wires. We develop a formalism to solve scattering problems using effective medium methods and prove that the electromagnetic coupling of the two sets of wires may result in the emergence of sharp Fano resonances.

We consider states of bosons in two dimensions that do not support anyons in the bulk, but nevertheless have stable chiral edge modes that are protected even without any symmetry. Such states must have edge modes with central charge $c=8k$ for integer $k$. While there is a single such state with $c=8$, there are, naively, two such states with $c=16$, corresponding to the two distinct even unimodular lattices in 16 dimensions. However, we show that these two phases are the same in the bulk, which is a consequence of the uniqueness of signature $(8k+n,n)$ even unimodular lattices. The bulk phases are stably equivalent, in a sense that we make precise. However, there are two different phases of the edge corresponding to these two lattices, thereby realizing a novel form of the bulk-edge correspondence. Two distinct fully chiral edge phases are associated with the same bulk phase, which is consistent with the uniqueness of the bulk since the transition between them, which is generically first order, can occur purely at the edge. Our construction is closely related to $T$ duality of toroidally compactified heterotic strings. We discuss generalizations of these results.

We discuss advantages and limitations of the spin noise spectroscopy for characterization of interacting quantum dot systems on specific examples of individual singly and doubly charged quantum dot molecules (QDMs). It is shown that all the relevant parameters of the QDMs, including tunneling amplitudes with spin-conserving and spin-nonconserving interactions, decoherence rates, Coulomb repulsions, anisotropic $g$ factors and the distance between the dots, can be determined by measuring properties of the spin noise power spectrum.

We investigate the link between superconductivity and atomic vibrations in Nb${}_3$Sn films with a thickness ranging from 10 to 50 nm. The challenge of measuring the phonon density of states (PDOS) of these films has been tackled by employing the technique of nuclear inelastic scattering by ${}^{119}$Sn isotopes to reveal the Sn-partial phonon density of states. With the support of ab initio calculations, we evaluate the effect of reduced film thickness on the PDOS. This approach allows us to estimate the changes in superconducting critical temperature $T_c$ induced by phonon confinement, which turned out to be limited to a few tenths of K. The presented method is successful for the Nb${}_3$Sn system and paves the way for more systematic studies of the role of phonon confinement in Sn-containing superconductors.

Recently, it has been found that, when operated at large input power, the linewidth $\Delta f$ of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime, a hot spot coexists with regions which are still superconducting. Surprisingly, $\Delta f$ was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature $T_b$. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of $T_b$. In the presence of a spatial gradient of the junction parameters' critical current and resistance, $\Delta f$ deceases with increasing $T_b$, similar to the experimental observation.