Influences of impurities on the electron density and transmission of a finite HgTe/CdTe quantum well sample are studied numerically in the framework of the Landauer-Büttiker formula. In a geometry of a slab with finite width, electrons in helical edge states protected by the time-reversal symmetry can tunnel through nonmagnetic impurities at two resonant energy levels. Electrons of one side can tunnel to the other side and the quantized conductance can be broken down. For a small sample with impurity, the transmission coefficient can even drop to zero for the crosswalk between the helical edge states at two sample sides. The distance between helical edge states is critical for the suppression of the transmission. The $s$ orbital and $p$ orbital of pseudospins affect the quantized spin Hall current in a different way for their different energies.

To overcome deficiencies of conventional density functional theory (DFT) utilizing the standard approximation for the exchange-correlation, the revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) has been used for calculating the formation and migration energies of the vacancy in Si. It is demonstrated that the hybrid approach gives a much more accurate electronic description of the bulk and the vacancy. The correct description of the band gap and the donor transition levels obtained with the HSE06 functional builds confidence in predictions of the vacancy acceptor states. The calculated migration energies of the vacancy with different charge states agree well with low-temperature annealing measurements and, together with formation energies, provide an excellent estimate of the activation energy of vacancy-mediated self-diffusion in silicon.

We present a microscopic theory for transport of the spin polarized charge density wave with both electrons and holes in the $\left(111\right)$ GaAs quantum wells. We analytically show that, contradicting to the commonly accepted belief, the spin and charge motions are bound together only in the fully polarized system but can be separated in the case of low spin polarization or short spin lifetime even when the spatial profiles of spin density wave and charge density wave overlap with each other. We further show that, the Coulomb drag between electrons and holes can markedly enhance the hole spin diffusion if the hole spin motion can be separated from the charge motion. In the high-spin polarized system, the Coulomb drag can boost the hole spin diffusion coefficient by more than one order of magnitude.

Temperature-dependent self-organized formation of nanoripples on TiO${}_2$(110) surfaces irradiated by low-energy Ar${}^{+}$ beams has been investigated by scanning tunnelling microscopy in UHV conditions. Under the experimental conditions employed (2 keV Ar${}^{+}$ and oblique incidence, 75${}^{\circ }$ off-normal, $T$ = 120, 300, 620, and 720 K) on the irradiated surface the ripple structure of periodicity ∼10 nm has developed. Interestingly, the orientation of the nanopatterns switches reversibly by 90${}^{\circ }$ with the systematic change of the substrate temperature during irradiation. We have demonstrated that formation of the surface nanomorphology is determined by the interplay between the erosion of the monatomic step edges at grazing incidence, anisotropic surface diffusion along the favoured crystallographic orientation and, at elevated temperatures, the diffusion into the bulk of the excess Ti ions. As indicated by density functional theory (DFT) calculations used for modelling the diffusion processes on the ion irradiated TiO${}_2$(110), the significant surface mass transport required to form the nanoripples is dominated by the highly mobile Ti atoms diffusing in assistance of O adatoms.

A theoretical and experimental description of the threshold, amplitude, and stability of a self-oscillating nanowire in a field emission configuration is presented. Two thresholds for the onset of self-oscillation are identified, one induced by fluctuations of the electromagnetic environment and a second revealed by these fluctuations by measuring the probability density function of the current. The ac and dc components of the current and the phase stability are quantified. An ac to dc ratio above 100% and an Allan deviation of $1.3\times {10}^{-5}$ at room temperature can be attained. Finally, it is shown that a simple nonlinear model cannot describe the equilibrium effective potential in the self-oscillating regime due to the high amplitude of oscillations.

A clear gate-voltage-tunable weak antilocalization and a giant magnetoresistance of ∼400$\%$ are observed at 1.9 K in single-layer graphene with an out-of-plane field. A large magnetoresistance value of 275$\%$ is obtained even at room temperature, implying potential applications of graphene in magnetic sensors. Both the weak antilocalization and giant magnetoresistance persists far away from the charge neutrality point, in contrast to previous reports, and both effects originate from charged impurities. Interestingly, the signatures of Shubnikov–de Haas oscillations and the quantum Hall effect are also observed for the same sample.

We investigate spin-dependent conductance through a quantum Aharonov-Bohm ring containing localized electrons which interact with the propagating flow of electrons via exchange interaction of the ferromagnetic or antiferromagnetic type. We analyze the conductance oscillations as a function of both the chemical potential (particle concentration) and external magnetic field. It is demonstrated that the amplitude of the conductance oscillations in the ballistic regime is determined by the value of the noncompensated spin localized in the ring. The results are in agreement with the concept of fractional quantization of the ballistic conductance, proposed by us earlier [Phys. Rev. B 71, 113311 (2005)].

Several mean-field computations have revealed the existence of an out-of-equilibrium dynamical transition induced by quantum-quenching an isolated system starting from its symmetry-broken phase. In this work we focus on the quantum ${\varphi }^4$ $N$-component field theory. By taking into account dynamical fluctuations at the Hartree-Fock level, corresponding to the leading order of the $1/N$ expansion, we derive the critical properties of the dynamical transition beyond mean-field theory (including at finite temperature). We find diverging time and length scales, dynamic scaling, and aging. Finally, we unveil a relationship with critical coarsening, an off-equilibrium regime that can be induced by quenching from the symmetric toward the symmetry-broken phase.

We have undertaken a detailed study of the Cr surface and Cr/MgO interface electronic properties by symmetry- and angle-resolved photoemission spectroscopy on a Cr(001) surface covered or not with 1.2 monolayer MgO. For the bare Cr(001) film, the experimental bulk dispersions are in good agreement with the theoretical band structure, and characteristic antiferromagnetic features like the antiferromagnetic gap at the $X$ point are observed. However, by covering the Cr film by 1.2 monolayer MgO, the observed experimental dispersions are strongly perturbed as a result of the existence of metal-induced gap states in the MgO film. By carrying out normal emission measurements, we detect two 2D localized states for the Cr(001) surface that persist at the Cr/MgO interface: a ${\Delta }_1$ state at very low binding energy, which is usually associated with Cr surface magnetism, and an additional resonant ${\Delta }_5$ state, which shall play a crucial role for transport and coupling in Cr/MgO epitaxial systems.

We study magnetization transport at infinite temperature in several spin ladder systems as well as in next-nearest-neighbor coupled spin chains. In the integrable ladder considered we analytically show that the transport is ballistic in sectors with nonzero average magnetization, while numerical simulations of a nonequilibrium stationary setting indicate an anomalous transport in the zero-magnetization sector. For other systems, isotropic Heisenberg ladder and spin chains, showing eigenlevel repulsion typical of quantum chaotic systems, numerical simulations indicate diffusive transport.

In the $s=1/2$ antiferromagnetic spin chain material TiPO${}_4$, the formation of a spin gap takes place in a two-step process with two characteristic temperatures, $T^{*}=111$ K and $T_{\mathrm{SP}}=74$ K. We observe an unusual lattice dynamics over a large temperature regime as well as evidence for an orbital instability preceding the spin-Peierls transition. We relate different intrachain exchange interactions of the high temperature compared to the spin-Peierls phase to a modification of the orbital ordering pattern. In particular, our observation of a high-energy excitation of mixed electronic and lattice origin suggests an exotic dimerization process different from other spin-Peierls materials.

We study three-dimensional $Z_2$ topological phases enriched by $Z_2^T$ time-reversal symmetry with bosonic bulk excitations. Some of these phases can be constructed by simply coupling the three-dimensional symmetry protected topological phases with $Z_2\times Z_2^T$ symmetry to a deconfined $Z_2$ gauge field. Besides these simple phases, we also construct two special root phases, whose boundary can have an extra $Z_2$ topological order in addition to their bulk topological order, and the boundary anyon excitations can have fractional time-reversal transformation $\mathcal{T}$ with ${\mathcal{T}}^4=-1$. In particular, the boundary $e$ and $m$ anyons of one of the two root phases are interchanged under time-reversal transformation, and they must be degenerate and orthogonal with each other. Eventually we obtain ${\left({\mathbb{Z}}_2\right)}^2\oplus {\left({\mathbb{Z}}_2\right)}^2$ classification (8 different phases) for $Z_2^T$-enriched $Z_2$ topological phases with bosonic bulk excitations in three dimensions.

We investigate theoretically electromagnetic plane-wave scattering by self-complementary metasurfaces. By using Babinet's principle extended to metasurfaces with resistive elements, we show that the frequency-independent transmission and reflection are realized for normal incidence of a circularly polarized plane wave onto a self-complementary metasurface, even if there is diffraction. Next, we consider two special classes of self-complementary metasurfaces. We show that self-complementary metasurfaces with rotational symmetry can act as coherent perfect absorbers, and those with translational symmetry compatible with their self-complementarity can split the incident power equally, even for oblique incidences.

We report on the optical properties of the CuCl microcavities that exhibit fluctuations in their refractive index profiles along the cavity structures. The angle-resolved reflectance spectra of the CuCl microcavities with a cavity length of half-integer multiples for the $Z_3$ exciton wavelength clearly show three cavity-polariton modes and minor modes. The Rabi splitting energies observed in the cavity-polariton modes are widely controlled by changing the cavity length of the microcavity. To analyze the spectral profile and the Rabi splitting energies qualitatively, the refractive index profiles of the constituent layers in the CuCl microcavities are evaluated using spectroscopic ellipsometry, and the reflectance spectra of the CuCl microcavities with the fluctuations in the refractive index profiles of the microcavity structures are calculated using the nonlocal response theory. The calculated reflectance spectra reproduce very well the cavity-polariton modes and the minor modes, including their energies and spectral profiles, in the observed reflectance spectra, which indicates an essential role of the fluctuation effect of the refractive indices of constituent layers in controlling the Rabi splitting energies and the quality factor of the microcavity.

We consider the effects of dephasing on field-induced coherent charge and energy transport in molecular junctions. Within generic models we show that dephasing controls the relative intensities of energy and charge fluxes, and that the dependence of the energy flux on the dephasing rate is nonmonotonic. We further demonstrate the possibility for laser-controlled charge-energy separation in multiterminal molecular junctions, a prerequisite for engineering low-heating stable nanoscale devices.

Helium atom scattering (HAS) was used to study the antimony Sb(111) surface beyond the hard-wall model. HAS angular distributions and drift spectra show a number of selective adsorption resonance features, which correspond to five bound-state energies for He atoms trapped in the surface-averaged He-Sb(111) potential. As their best representation, a 9-3 potential with a depth of $4.4\pm 0.1$ meV was determined. Furthermore, the charge density corrugation of the surface was analyzed using close-coupling calculations. By using a hybrid potential, consisting of a corrugated Morse potential (short range) and a 9-3 potential (long range), a peak-to-peak corrugation of 17% was obtained. The kinematic focusing effects that occurred were in good agreement with surface phonon dispersion curves from already published density functional perturbation theory calculations.

A quartet of hydrogens can be dynamically bound to group-14 atoms, the resulting complexes having even valence. In macroscopic assemblies and at high pressures these can give way by dynamically assisted (quantum) disproportionation to complexes with odd valence and hence, in principle, to metallic tendencies. A new extended metallic composition of, for example, GeH${}_3$ is investigated by first-principles methods within density functional theory. Its stoichiometry and its very existence is a direct consequence of inclusion of nuclear quantum dynamical contributions to the free energy. From enthalpic comparisons GeH${}_3$ augmented with hydrogen appears preferred beyond 175 GPa, where three candidate structures are competitive, these being A15, $P{4}_2/mmc$, and $Cccm$. The pressure at which GeH${}_3$ makes its appearance is significantly influenced by zero-point energy, and quantum effects play an important role in a notable trend towards disproportionation. The ensuing systems are all metallic and the superconducting transition temperature of GeH${}_3$ has been estimated as about 100 K or higher (a common range for the three prospective structures). The general proposition of quantum disproportionation at elevated pressures appears extendable to other high hydrides.

We present an investigation of Mn $\delta$-doped layers in Si(001) grown by molecular beam epitaxy. We discovered that a Pb surfactant has significant effect on the structural and magnetic properties of the submonolayer of Mn, which depends on the Si capping layer growth temperature $T_{\mathrm{Si}}$, and the Mn coverage ${\theta }_{\mathrm{Mn}}$. The results presented in this paper identify three regions in the growth-phase diagram characterized by distinct magnetic behaviors and crystal structures. In one region, x-ray absorption fine structure and transmission electron microscopy experiments indicate that MnSi nanocrystallites form with B2-like crystal type structures. At the optimal growth conditions, $T_{\mathrm{Si}}$ $=$ 200${}^{\circ }$C and ${\theta }_{\mathrm{Mn}}=0.26$ monolayer, a ferromagnetic phase develops with a Curie temperature $T_C>400$ K and a saturation moment $m_{\mathrm{sat}}=1.56$ ${\mu }_B$/Mn, whereas $T_C$ drops to zero for a control sample prepared without Pb. For $T_{\mathrm{Si}}>200{}^{\circ }$C MnSi-B20 type precipitates form with a $T_C\approx 170$ K. Rutherford backscattering spectrometry shows that the increase in the remanent magnetization in these two phases is possibly correlated with an increase in the Mn substitutional fraction, which suggests that a Si${}_{1-x}$Mn${}_x$ dilute magnetic semiconductor may be forming in the matrix between the precipitates. Density functional calculations show that Pb changes the pathway by which the Mn atoms access the Si substitutional sites, Mn${}_{\mathrm{Si}}$. While the Pb increases the formation energy of Mn${}_{\mathrm{Si}}$ at the Si surface, it enables substitutional incorporation by lowering the formation energy of Si vacancies by 0.92 eV.

We describe an experimental technique for associating the satellite lines in a rare-earth optical spectrum caused by a defect with the rare-earth ions in crystal sites around that defect. This method involves measuring the hyperfine splitting caused by a magnetic dipole-dipole interaction between host ions and a magnetic defect. The method was applied to Ce${}^{3+}$:EuCl${}_3·$6H${}_2$O to assign 13 of the outermost 22 satellite lines to sites. The assignments show that the optical shift of a satellite line is loosely dependent on the distance to the dopant. The interaction between host and dopant ions is purely dipole-dipole at distances greater than 7 Å, with an additional contribution, likely superexchange, at distances less than 7 Å.

We report the complex dielectric function of high-quality $A{\text{Fe}}_2{\text{As}}_2$, ($A=\text{Ca}$, Sr, Ba) single crystals with $T_{\mathrm{N}}\approx 150\text{K}$, $200\text{K}$, and $138\text{K}$, respectively, determined by broadband spectroscopic ellipsometry at temperatures $10\le T\le 300\text{K}$ and wave numbers from $100{\text{cm}}^{-1}$ to $52000{\text{cm}}^{-1}$. In ${\text{CaFe}}_2{\text{As}}_2$ we identify the optical spin-density-wave gap $2{\Delta }_{\mathrm{SDW}}\approx 1250{\text{cm}}^{-1}$. The $2{\Delta }_{\mathrm{SDW}}/\left(k_{\mathrm{B}}{T}_{\mathrm{N}}\right)$ ratio, characterizing the strength of the electron-electron coupling in the spin-density-wave state, amounts to $\approx 12$ in ${\text{CaFe}}_2{\text{As}}_2$, significantly larger than the corresponding values for the ${\text{SrFe}}_2{\text{As}}_2$ and ${\text{BaFe}}_2{\text{As}}_2$ compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both ${\text{SrFe}}_2{\text{As}}_2$ and ${\text{CaFe}}_2{\text{As}}_2$ and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in ${\text{CaFe}}_2{\text{As}}_2$ at a temperature $T^{*}\approx 80\text{K}$, well below the Néel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. The temperature dependence of the infrared phonons reveals clear anomalies at the corresponding Néel temperatures of the investigated compounds. In ${\text{CaFe}}_2{\text{As}}_2$, the phonons exhibit signatures of SDW fluctuations above $T_N$ and some evidence for anomalies at $T^{*}$. Investigation of all three materials in the visible spectral range reveals a spin-density-wave-induced suppression of two absorption bands systematically enhanced with decreasing atomic number of the intercalant. A dispersion analysis of the data in the entire spectral range clearly shows that ${\text{CaFe}}_2{\text{As}}_2$ is significantly more metallic than the other two compounds. Our results single out ${\text{CaFe}}_2{\text{As}}_2$ in the class of ${\text{ThCr}}_2{\text{Si}}_2$-type iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems.