We investigate the anisotropy of the optical properties of thin Fe films on GaAs(001) from first-principles calculations. Both intrinsic and magnetization-induced anisotropy are covered by studying the system in the presence of spin-orbit coupling and external magnetic fields. We use the linearized augmented plane wave method, as implemented in the wien2k density functional theory code, to show that the $C_{2v}$ symmetric anisotropy of the spin-orbit coupling fields at the Fe/GaAs(001) interface manifests itself in the corresponding anisotropy of the optical conductivity and the polar magneto-optical Kerr effect. While their magnetization-induced anisotropy is negligible, the intrinsic anisotropy of the optical properties is significant and reflects the underlying $C_{2v}$ symmetry of the Fe/GaAs(001) interface. This suggests that the effects of anisotropic spin-orbit coupling fields in experimentally relevant Fe/GaAs(001) slabs can be studied by purely optical means.

In this paper we present a comprehensive and detailed analysis of carrier/exciton wave function extension in large low-strain ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}$As quantum dots (QDs). They exhibit rather shallow confinement potential with electron/hole localization energy below 30 meV and confinement strength substantially weakened in comparison to typical epitaxial quasi-zero-dimensional semiconductor nanostructures. The aim of this study is to investigate the influence of different factors on the wave function (probability density distribution) for carriers or excitons in this regime, i.e., object shape anisotropy as well as strain, piezoelectricity, and Coulomb interactions, and to identify the physical mechanisms determining the properties of optical emission. To probe the wave function symmetry, polarization-resolved photoluminescence has been performed, and the spatial extensions of the corresponding probability densities have been verified in magneto-optical measurements. The observed diamagnetic coefficients in the range of (15–31) $\mu \mathrm{eV}/{\mathrm{T}}^2$ reflect large in-plane QD size. These studies also enable us to investigate the importance of light hole states admixture to the valence band ground state in such nanostructures, which can be addressed via the degree of linear polarization of emission as well as the exciton $g_X$ factor. The linear-polarization-resolved measurements revealed an exceptionally low exciton fine structure splitting of $5\hspace{0.5em}\mu \mathrm{eV}$ on average as well as a low emission polarization degree of −0.05, with the polarization perpendicular to the QD elongation direction dominating. The increased light hole contribution to the lowest energy hole level is reflected in the decreased exciton $g_X$ factor (in the range of 0–1) and is consistent with the results of the eight-band k·p modelling. Based on the temperature dependence of the diamagnetic coefficient, the problem of individual QD uniformity has additionally been discussed. To evaluate the impact of the confinment potential and the structure geometry on the optical properties of the QDs, a comparison between the investigated dots and InAs/InGaAlAs/InP quantum dashes exhibiting a much deeper confining potential is presented.

We calculate the room-temperature complex conductivity $\sigma \left(\omega \right)$ of suspended and supported graphene at terahertz frequencies $\left(100\mathrm{GHz}–10\mathrm{THz}\right)$ by employing a self-consistent coupled simulation of carrier transport and electrodynamics. We consider a wide range of electron $\left(n={10}^{12}–{10}^{13}{\mathrm{cm}}^{-2}\right)$ and impurity $\left(N_{\mathrm{i}}=8\times {10}^{10}–2\times {10}^{12}{\mathrm{cm}}^{-2}\right)$ densities. For graphene supported on ${\mathrm{SiO}}_2$, there is excellent agreement between the calculation with clustered impurities and the experimentally measured $\sigma \left(\omega \right)$. The choice of substrate $({\mathrm{SiO}}_2$ or $h$-BN) is important at frequencies below 4 THz. We show that carrier scattering with substrate phonons governs transport in supported graphene for $N_{\mathrm{i}}/n<0.1$. Electron-impurity interactions dominate for $N_{\mathrm{i}}/n>0.1$, and transport enters the electron-hole puddle regime for $N_{\mathrm{i}}/n>0.5$. The simple Drude model, with an effective scattering rate $\Gamma$ and Drude weight $D$ as parameters, fits the calculated $\sigma \left(\omega \right)$ for supported graphene very well, owing to electron-impurity scattering. $\Gamma$ decreases with increasing $n$ faster than $n^{-1/2}$ and is insensitive to electron-electron interaction. Both electron-electron and electron-impurity interactions reduce the Drude weight $D$, and its dependence on $n$ is sublinear.

A density functional theory (DFT) study of the discrete breathers (DBs) in graphane (fully hydrogenated graphene) was performed. To the best of our knowledge, this is the first demonstration of the existence of DBs in a crystalline body from the first-principle simulations. It is found that the DB is a robust, highly localized vibrational mode with one hydrogen atom oscillating with a large amplitude along the direction normal to the graphane plane with all neighboring atoms having much smaller vibration amplitudes. DB frequency decreases with increase in its amplitude, and it can take any value within the phonon gap and can even enter the low-frequency phonon band. The concept of DB is then used to propose an explanation to the recent experimental results on the nontrivial kinetics of graphane dehydrogenation at elevated temperatures.

Utilizing open Fe nanocorrals built by atom manipulation, we demonstrate self-regulated Gd atom trapping in open quantum corrals. The number of Gd atoms trapped is exactly determined by the diameter of the corral. The quantization can be understood as a self-regulating process, arising from the long-range interaction between Gd atoms and the open corral. We illustrate with arrays of open corrals that such atom trapping can suppress unwanted statistical fluctuations. Our approach opens a potential pathway for nanomaterial design and fabrication with atomic-level precision.

We study the dynamics of a two-dimensional ensemble of randomly distributed classical Heisenberg spins with isotropic RKKY and weaker anisotropic dipole-dipole couplings. Such ensembles may give rise to the flux noise observed in SQUIDs with a $1/f^{\alpha }$ power spectrum with $\alpha \lesssim 1$. We solve numerically the Landau-Lifshitz-Gilbert equations of motion in the dissipationless limit. We find that Ising-type fluctuators, which arise from spin clustering close to a spin-glass critical behavior with $T_c=0$, give rise to $1/f^{\alpha }$ noise. Even weak anisotropic interactions lead to a crossover from the Heisenberg-type criticality to the much stronger Ising-type criticality. The temperature-dependent exponent $\alpha \left(T\right)\lesssim 1$ increases and approaches unity when the temperature is lowered. This mechanism acts in parallel to the spin diffusion mechanism. Whereas the latter is sensitive to the device geometry, the spin-clustering mechanism is largely geometry independent.

In the framework of disordered local moment approach by using magnetic sampling method, we suggested a model that takes into account the magnetic disorder in paramagnetic Fe with point defects. We calculate solution enthalpies of substitutional (Nb, V) and interstitial (C, N) impurities in paramagnetic face-centered cubic Fe and obtain results that are in agreement with available experimental data. It is found that both interstitial and substitutional atoms may favor the local magnetic polarization of the Fe host around the impurities by decreasing the potential energy of the system. The possibility of a formation of predominantly ferromagnetic Fe clusters around carbon in the temperature range of overcooled austenite is discussed.

The nucleation kinetics of the formation of ordered ferroelectric phases from the glassy relaxor states of ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$ (PMN) and of solid solutions of PMN and ${\mathrm{PbTiO}}_3$ is investigated. A delay time, dependent on field and temperature, is found to precede a rather rapid establishment of macroscopic polarization. The qualitative form of the temperature dependence follows prior theory for the lag preceding homogeneous nucleation of a crystal from a glass. However, the increase of the lag time as the “melting” line is approached from below is stronger than expected.

The crystallographic, magnetic, and thermal properties of polycrystalline $\mathrm{Bi}{\mathrm{Mn}}_2{\mathrm{PO}}_6$ and its nonmagnetic analog $\mathrm{Bi}{\mathrm{Zn}}_2{\mathrm{PO}}_6$ are investigated by x-ray diffraction, magnetization $M$, magnetic susceptibility $\chi$, heat capacity $C_p$, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements versus applied magnetic field $H$ and temperature $T$ as well as by density-functional band theory and molecular-field calculations. Both compounds show a strong monotonic lattice softening on cooling, where the Debye temperature decreases by a factor of two from ${\Theta }_{\mathrm{D}}\sim 650$ K at $T=300$ K to ${\Theta }_{\mathrm{D}}\sim 300$ K at $T=2$ K. The $\chi \left(T\right)$ data for $\mathrm{Bi}{\mathrm{Mn}}_2{\mathrm{PO}}_6$ above 150 K follow a Curie-Weiss law with a Curie constant consistent with a Mn${}^{+2}$ spin $S=5/2$ with $g$ factor $g=2$ and an antiferromagnetic (AFM) Weiss temperature ${\theta }_{\mathrm{CW}}\simeq -78$ K. The $\chi$ data indicate long-range AFM ordering below $T_{\mathrm{N}}\simeq 30$ K, confirmed by a sharp $\lambda$-shaped peak in $C_{\mathrm{p}}\left(T\right)$ at 28.8 K. The magnetic entropy at 100 K extracted from the $C_{\mathrm{p}}\left(T\right)$ data is consistent with spin $S=5/2$ for the Mn${}^{+2}$ cations. The band-theory calculations indicate that $\mathrm{Bi}{\mathrm{Mn}}_2{\mathrm{PO}}_6$ is an AFM compound with dominant interactions $J_1/k_{\mathrm{B}}\simeq 6.7$ K and $J_3/k_{\mathrm{B}}\simeq 5.6$ K along the legs and rungs of a Mn two-leg spin-ladder, respectively. However, sizable and partially frustrating interladder couplings lead to an anisotropic three-dimensional magnetic behavior with long-range AFM ordering at $T_{\mathrm{N}}\simeq 30$ K observed in the $\chi$, $C_{\mathrm{p}}$, and NMR measurements. A second magnetic transition at $\approx$10 K is observed from the $\chi$ and NMR measurements but is not evident in the $C_{\mathrm{p}}$ data. The $C_{\mathrm{p}}$ data at low $T$ suggest a significant contribution from AFM spin waves moving in three dimensions and the absence of a spin-wave gap. A detailed analysis of the NMR spectra indicates commensurate magnetic order between 10 and 30 K, while below 10 K additional features appear that may arise from an incommensurate modulation and/or spin canting. The commensurate order is consistent with microscopic density functional calculations that yield a collinear Néel-type AFM spin arrangement both within and between the ladders, despite the presence of multiple weak interactions frustrating this magnetic structure of the Mn spins. Frustration for AFM ordering and the one-dimensional spatial anisotropy of the three-dimensional spin interactions are manifested in the frustration ratio $f=|{\theta }_{\mathrm{CW}}|/T_{\mathrm{N}}\simeq 2.6$, indicating a suppression of $T_{\mathrm{N}}$ from 68 K in the absence of these effects to the observed value of about 30 K in $\mathrm{Bi}{\mathrm{Mn}}_2{\mathrm{PO}}_6$.

The superconducting and magnetic properties of phase-separated $A_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ compounds are known to depend on postgrowth heat treatments and cooling profiles. This paper focuses on the evolution of microstructure on annealing and how this influences the superconducting properties of ${\mathrm{Rb}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ single crystals. We find that the minority phase in the as-grown crystal has increased unit cell anisotropy ($c/a$ ratio), reduced Rb content, and increased Fe content compared to the main phase. The microstructure is rather complex, with two-phase mesoscopic plate-shaped features aligned along $\left\{113\right\}$ habit planes. The minority phases are strongly faceted on the $\left\{113\right\}$ planes, which we have shown to be driven by minimizing the volume strain energy introduced as a result of the phase transformation. Annealing at 488 K results in coarsening of the mesoscopic plate-shaped features and the formation of a third distinct phase. The subtle differences in structure and chemistry of the minority phase(s) in the crystals are thought to be responsible for changes in the superconducting transition temperature. In addition, scanning photoemission microscopy has clearly shown that the electronic structure of the minority phase has a higher occupied density of states of the low binding energy Fe3d orbitals, which is characteristic of crystals that exhibit superconductivity. This demonstrates a clear correlation between the Fe-vacancy-free phase with high $c/a$ ratio and the electronic structure characteristics of the superconducting phase.

The electronic properties of the two-dimensional material silicene are strongly influenced by the application of a perpendicular electric field $E_z$ and of an exchange field $M$ due to adatoms positioned on the surface or a ferromagnetic substrate. Within the random phase approximation, we investigate how electron-electron interactions are affected by these fields and present analytical and numerical results for the dispersion of plasmons, their lifetime, and their oscillator strength. We find that the combination of the fields $E_z$ and $M$ brings a spin and valley texture to the particle-hole excitation spectrum and allows the formation of spin- and valley-polarized plasmons. When the Fermi level lies in the gap of one spin in one valley, the intraband region of the corresponding spectrum disappears. For zero $E_z$ and finite $M$ the spin symmetry is broken and spin polarization is possible. The lifetime and oscillator strength of the plasmons are shown to depend strongly on the number of spin and valley type electrons that form the electron-hole pairs.

We provide a general theoretical framework to describe the electromagnetic properties of viscous charged fluids, consisting, for example, of electrons in certain solids or plasmas. We confirm that finite viscosity leads to multiple modes of evanescent electromagnetic waves at a given frequency, one of which is characterized by a negative index of refraction, as previously discussed in a simplified model by one of the authors. In particular, we explain how optical spectroscopy can be used to probe the viscosity. We concentrate on the impact of this on the coefficients of refraction and reflection at the sample-vacuum interface. Analytical expressions are obtained relating the viscosity parameter to the reflection and transmission coefficients of light. We demonstrate that finite viscosity has the effect to decrease the reflectivity of a metallic surface, while the electromagnetic field penetrates more deeply. While on a phenomenological level there are similarities to the anomalous skin effect, the model presented here requires no particular assumptions regarding the corpuscular nature of the charge liquid. A striking consequence of the branching phenomenon into two degenerate modes is the occurrence in a half-infinite sample of oscillations of the electromagnetic field intensity as a function of distance from the interface.

An algorithm for imaginary time evolution of a fermionic projected entangled pair state with ancillas from infinite temperature down to a finite temperature state is presented. As a benchmark application, it is applied to spinless fermions hopping on a square lattice subject to $p$-wave pairing interactions. With a tiny bias it allows one to evolve the system across a high-temperature continuous symmetry-breaking phase transition.

We present a controlled microscopic study of mobile holes in the spatially anisotropic (Abelian) gapped phase of the Kitaev honeycomb model. We address the properties of (i) a single hole (its internal degrees of freedom as well as its hopping properties); (ii) a pair of holes [their (relative) particle statistics and interactions]; and (iii) the collective state for a finite density of holes. We find that each hole in the doped model has an eight-dimensional internal space, characterized by three internal quantum numbers: the first two “fractional” quantum numbers describe the binding to the hole of the fractional excitations (fluxes and fermions) of the undoped model, while the third “spin” quantum number determines the local magnetization around the hole. The fractional quantum numbers also encode fundamentally distinct particle properties, topologically robust against small local perturbations: some holes are free to hop in two dimensions, while others are confined to hop in one dimension only; distinct hole types have different particle statistics, and in particular, some of them exhibit nontrivial (anyonic) relative statistics. These particle properties in turn determine the physical properties of the multihole ground state at finite doping, and we identify two distinct ground states with different hole types that are stable for different model parameters. The respective hopping dimensionalities manifest themselves in an electrical conductivity approximately isotropic in one ground state and extremely anisotropic in the other one. We also compare our microscopic study with related mean-field treatments, and discuss the main discrepancies between the two approaches, which in particular involve the possibility of binding fractional excitations as well as the particle statistics of the holes. On a technical level, we describe the hopping of mobile holes via a quasistationary approach, where effective hopping matrix elements are calculated between ground states with stationary holes at different positions. This approach relies on the fact that the model remains exactly solvable in the presence of stationary holes, and that the motion of sufficiently slow holes does not generate bulk excitations in a gapped phase. When the bare hopping amplitude is much smaller than the energy gap, many of our results, in particular those on the hopping properties and the particle statistics, are exact.

Hybrid functionals serve as a powerful practical tool in different fields of computational physics and quantum chemistry. On the other hand, their applicability for the case of correlated $d$ and $f$ orbitals is still questionable and needs more considerations. In this article we formulate the on-site occupation dependent exchange correlation energy and effective potential of hybrid functionals for localized states and connect them to the on-site correction term of the $\text{DFT}\hspace{0.17em}+\hspace{0.17em}U$ method. The resultant formula indicates that the screening of the onsite electron repulsion is governed by the ratio of the exact exchange in hybrid functionals. Our derivation provides a theoretical justification for adding a $\text{DFT}\hspace{0.17em}+\hspace{0.17em}U$-like on-site potential in hybrid-DFT calculations to resolve issues caused by overscreening of localized states. The resulting scheme, hybrid $\text{DFT}\hspace{0.17em}+\hspace{0.17em}{\mathrm{V}}_{\mathrm{w}}$, is tested for chromium impurity in wurtzite AlN and vanadium impurity in 4H-SiC, which are paradigm examples of systems with different degrees of localization between host and impurity orbitals.

We study a variety of questions related to entanglement in symmetry-protected phases. These phases are analogous to topological insulators in that they are short-range entangled states with symmetry-protected edge or surface states. We show that the now standard bulk-edge correspondence relating the entanglement spectrum to the gapless edge spectrum holds for these phases as well. We further consider the question of coupling these models to gauge fields which we show gives an alternative way to construct different kinds of long-range entangled states without symmetry. We also argue that this procedure yields models with perturbatively stable edge or surface states at a variety of interface types. The non-onsite nature of the edge symmetry plays an important role in our considerations.

Time-reversal symmetry breaking superconductors are exotic phases of matter with fascinating properties, which are, however, encountered rather sparsely. Here we identify the possibility of realizing such a superconducting ground state that exhibits an $f+is$ pairing symmetry in strained graphene. Although the underlying attractive interactions need to be sufficiently strong and comparable in pristine graphene to support such pairing state, we argue that strain can be conducive for its formation even for weak interactions. We show that quantum-critical behavior near the transition is controlled by a multicritical point, characterized by various critical exponents computed here in the framework of an $\epsilon$ expansion near four space-time dimensions. Furthermore, a vortex in this mixed superconducting state hosts a pair of Majorana fermions supporting a quartet of insulating and superconducting orders, among which is the topologically nontrivial quantum spin Hall insulator. These findings suggest that strained graphene could provide a platform for the realization of exotic superconducting states of Dirac fermions.

Topologically ordered systems in the presence of symmetries can exhibit new structures which are referred to as symmetry-enriched topological (SET) phases. We introduce simple methods to detect certain SET orders directly from a complete set of topologically degenerate ground-state wave functions. In particular, we first show how to directly determine the characteristic symmetry fractionalization of the quasiparticles from the reduced density matrix of the minimally entangled states. Second, we show how a simple generalization of a nonlocal order parameter can be measured to detect SET phases. The usefulness of the proposed approach is demonstrated by examining two concrete model states which exhibit SET phases: (i) a spin-1 model on the honeycomb lattice and (ii) the resonating valence bond (RVB) state on a kagome lattice. We conclude that the spin-1 model and the RVB state are in the same SET phases.

We use the dynamical mean-field approximation to study singularities in the self-energy and a two-particle irreducible vertex induced by the metal-insulator transition of the disordered Falicov-Kimball model. We set general conditions for the existence of a critical metal-insulator transition caused by a divergence of the imaginary part of the self-energy. We calculate explicitly the critical behavior of the self-energy for the symmetric and asymmetric disorder distributions. We demonstrate that the metal-insulator transition is preceded by a pole in a two-particle irreducible vertex. We show that unlike the singularity in the self-energy the divergence in the irreducible vertex does not lead to nonanalyticities in measurable physical quantities. We reveal universal features of the critical metal-insulator transition that are transferable also to the Mott-Hubbard transition in the models of the local Fermi liquid.