Theoretical scanning tunneling microscopy (STM) images for all group-III and -V dopants on the GaAs (110) surface are calculated using density functional theory (DFT). In addition, a geometrical model based on the covalent radii of the dopants and substrate atoms is used to interpret the images. We find that the covalent radius of the dopant determines the geometry of the surface, which in turn determines the contrast seen in the STM images. Our model allows bond lengths to be predicted with an error of less than 4.2% and positions to be predicted with an average deviation of only 0.19 Å compared to positions from fully relaxed DFT. For nitrogen we demonstrate good qualitative agreement between simulated and experimental STM images for dopants located in the first three surface layers. We are able to explain differences in both the contrast and positions of bright and dark features in the STM image based on our geometrical model. We then provide a detailed quantitative analysis of the positions of the bright features for nitrogen dopants in the second layer. The agreement of the DFT calculation with experiment is excellent, with the positions of the peaks in simulated and experimental STM scans differing by less than 2% of the lattice constant. For dopants other than nitrogen, we compare the calculated STM image contrast with the available experimental data and again find good agreement.

Depositing Au on a graphene derivative, which involves substituting four C atoms with three N atoms in a $3\times 3$ cell graphene, we realized a topological insulator of the Kane-Mele model with a gap of 50 meV surrounding the Dirac point of graphene. In this material, we observed an anomalous band inversion (BI) protected by the symmetry with character $e$ of group ${\mathrm{C}}_{3\mathrm{V}}$. The symmetry constrains two $e$ bands with mirror-symmetry combination and mirror-antisymmetry combination (MAC) of Au and N orbitals degenerate at $\mathrm{\Gamma }$, whereas the interaction of ${\pi }^{*}$ of graphene on the $e$-MAC band tends to lift this degenerate, resulting in that the ${\pi }^{*}$ and $e$-MAC band exchange their orbital components near $\mathrm{\Gamma }$, causing thus a discontinued BI.

A recent study predicted that a 2D single layer of antimony in buckled honeycomb as well as asymmetric washboard structures, named antimonene, is stable at high temperature and displays semiconducting properties. Based on first-principles, spin-polarized density functional calculations, we investigated chemisorption of selected adatoms and physisorption of molecules on two antimonene phases. Since adspecies-adspecies interaction is minimized by using large supercells, our results mimic the effects of isolated, single adatoms or molecules. We found that molecules such as ${\mathrm{H}}_2,{\mathrm{O}}_2$, and ${\mathrm{H}}_2\mathrm{O}$ neither form strong chemical bonds nor dissociate; they are physisorbed with a weak binding energy without affecting the properties of antimonene. The adatoms, such as H, Li, B, C, N, O, Al, In, Si, P, Cl, Ti, As, and Sb, are chemisorbed with significant binding energy, whereby the atomic and electronic structures are modified locally. Boron and carbon adatoms are implemented into buckled antimonene crystal leading to a local reconstruction of the crystal. Nitrogen gives rise to Stone-Wales type defects. The localized states originating from adatoms give rise to diversity of electronic structure. The lowest conduction and highest valence bands of antimonene in asymmetric washboard structures have very high curvature. Once combined with adatom states, these bands offer a variety of features. Specific adatoms lead to spin polarization, attain magnetic moments, and can attribute a half-metallic character to antimonene.

Spin excitations in atomic-scale nanostructures have been investigated with inelastic scanning tunneling spectroscopy, sometimes with conflicting results. In this work, we present a theoretical viewpoint on a recent experimental controversy regarding the spin excitations of Co adatoms on Pt(111). While one group [Balashov et al., Phys. Rev. Lett. 102, 257203 (2009)] claims to have detected them, another group reported their observation only after the hydrogenation of the Co adatom [Dubout et al., Phys. Rev. Lett. 114, 106807 (2015)]. Utilizing time-dependent density functional theory in combination with many-body perturbation theory, we demonstrate that, although inelastic spin excitations are possible for Cr, Mn, Fe, and Co adatoms, their efficiency differs. While the excitation signature is less pronounced for Mn and Co adatoms, it is larger for Cr and Fe adatoms. We find that the tunneling matrix elements or the tunneling cross-section related to the nature and symmetry of the relevant electronic states are more favorable for triggering the spin excitations in Fe than in Co. An enhancement of the tunneling and of the inelastic spectra is possible by attaching hydrogen to the adatom at the appropriate position.

The graphene twist bilayer represents the prototypical system for investigating the stacking degree of freedom in few-layer graphenes. The electronic structure of this system changes qualitatively as a function of angle, from a large-angle limit in which the two layers are essentially decoupled—with the exception of a 28-atom commensuration unit cell for which the layers are coupled on an energy scale of $\approx 8\mathrm{meV}$—to a small-angle strong-coupling limit. Despite sustained investigation, a fully satisfactory theory of the twist bilayer remains elusive. The outstanding problems are (i) to find a theoretically unified description of the large- and small-angle limits, and (ii) to demonstrate agreement between the low-energy effective Hamiltonian and, for instance, ab initio or tight-binding calculations. In this article, we develop a low-energy theory that in the large-angle limit reproduces the symmetry-derived Hamiltonians of Mele [Phys. Rev. B 81, 161405 (2010)], and in the small-angle limit shows almost perfect agreement with tight-binding calculations. The small-angle effective Hamiltonian is that of Bistritzer and MacDonald [Proc. Natl. Acad. Sci. (U.S.A.) 108, 12233 (2011)], but with the momentum scale $\mathrm{\Delta }K$, the difference of the momenta of the unrotated and rotated special points, replaced by a coupling momentum scale $g^{\left(c\right)}=\frac{8\pi }{\sqrt{3}a}sin\frac{\theta }{2}$. Using this small-angle Hamiltonian, we are able to determine the complete behavior as a function of angle, finding a complex small-angle clustering of van Hove singularities in the density of states (DOS) that after a “zero-mode” peak regime between $0.{90}^{\circ }<\theta <0.{15}^{\circ }$ limits $\theta <0.{05}^{\circ }$ to a DOS that is essentially that of a superposition DOS of all bilayer stacking possibilities. In this regime, the Dirac spectrum is entirely destroyed by hybridization for $-0.25<E<0.25\text{eV}$ with an average band velocity $\approx 0.3v_F^{\left(\text{SLG}\right)}$ (where SLG denotes single-layer graphene). We study the fermiology of the twist bilayer in this limit, finding remarkably structured constant energy surfaces with multiple Lifshitz transitions between $K$- and $\mathrm{\Gamma }$-centered Fermi sheets and a rich pseudospin texture.

Motivated by the recent isolation of the trivial insulator germanane, a fully hydrogenated germanene, we show that partially substituting hydrogen atoms with iodine on only one side of the material creates a two-dimensional topological insulator with a large band gap of 0.49 eV. This functionalization opens up different routes for the observation of the quantum spin Hall effect in fully two-dimensional materials. We also show that by creating nanoroads with the pattern functionalization of germanane by iodine in an ordered or disordered way, topologically protected interfaces states arise at the boundary of germanane/iodinated germanane.

We study the Rényi entanglement entropy and the Shannon mutual information for a class of spin-1/2 quantum critical XXZ chains with random coupling constants which are partially correlated. In the XX case, distinctly from the usual uncorrelated random case where the system is governed by an infinite-disorder fixed point, the correlated-disorder chain is governed by finite-disorder fixed points. Surprisingly, we verify that, although the system is not conformally invariant, the leading behavior of the Rényi entanglement entropies are similar to those of the clean (no randomness) conformally invariant system. In addition, we compute the Shannon mutual information among subsystems of our correlated-disorder quantum chain and verify the same leading behavior as the $n=2$ Rényi entanglement entropy. This result extends a recent conjecture concerning the same universal behavior of these quantities for conformally invariant quantum chains. For the generic spin-1/2 quantum critical XXZ case, the true asymptotic regime is identical to that in the uncorrelated disorder case. However, these finite-disorder fixed points govern the low-energy physics up to a very long crossover length scale, and the same results as in the XX case apply. Our results are based on exact numerical calculations and on a numerical strong-disorder renormalization group.

We demonstrate electrical control of the neutral exciton fine-structure splitting in a single InAs/GaAs self-assembled quantum dot by significantly reducing the splitting to near zero through the application of a vertical electric field in the fast electron tunneling regime. This is verified by performing high-resolution photocurrent spectroscopy of the two fine-structure split exciton eigenstates as a function of reverse bias voltage. Using the qubit initialization scheme for a quantum-dot hole spin based on rapid electric-field ionization of a spin-polarized exciton, our results suggest a practical approach towards achieving qubit initialization with near-unity fidelity in the absence of magnetic fields.

This work studies the limits of far- and near-field electromagnetic responses of subwavelength scatterers, like the unitary limit of lossless scatterers and the ideal absorption limit of lossy particles. These limit behaviors are described in terms of analytic formulas that approximate finite-size effects while rigorously including radiative corrections. This analysis predicts the electric and/or magnetic limit responses of both metallic and dielectric nanoparticles while quantitatively describing near-field enhancements.

We consider a system of spinless fermions on the honeycomb lattice with substrate-induced modulated electrostatic potentials tripling the unit cell. The resulting non-Abelian $\mathcal{S}U\left(2\right)$ gauge fields act cooperatively to realize a quadratic band crossing point (QBCP). Using a combination of mean-field theory and renormalization group techniques, we show that in the QBCP regime, arbitrarily weak repulsive electronic interactions drive the system into the quantum anomalous Hall state. This proves that substrate-induced local voltages are an effective knob to induce the spontaneous formation of a topological quantum phase.

Topological crystalline insulators define a new class of topological insulator phases with gapless surface states protected by crystalline symmetries. In this work, we present a general theory to classify topological crystalline insulator phases based on the representation theory of space groups. Our approach is to directly identify possible nontrivial surface states in a semi-infinite system with a specific surface, of which the symmetry property can be described by 17 two-dimensional space groups. We reproduce the existing results of topological crystalline insulators, such as mirror Chern insulators in the $pm$ or $pmm$ groups, $C_{nv}$ topological insulators in the $p4m,p31m$, and $p6m$ groups, and topological nonsymmorphic crystalline insulators in the $pg$ and $pmg$ groups. Aside from these existing results, we also obtain the following results: (1) there are two integer mirror Chern numbers (${\mathbb{Z}}^2$) in the $pm$ group but only one ($\mathbb{Z}$) in the $cm$ or $p3m1$ group for both the spinless and spinful cases; (2) for the $pmm$ ($cmm$) groups, there is no topological classification in the spinless case but ${\mathbb{Z}}^4$ (${\mathbb{Z}}^2$) classifications in the spinful case; (3) we show how topological crystalline insulator phase in the $pg$ group is related to that in the $pm$ group; (4) we identify topological classification of the $p4m,p31m$, and $p6m$ for the spinful case; (5) we find topological nonsymmorphic crystalline insulators also existing in $pgg$ and $p4g$ groups, which exhibit new features compared to those in $pg$ and $pmg$ groups. We emphasize the importance of the irreducible representations for the states at some specific high-symmetry momenta in the classification of topological crystalline phases. Our theory can serve as a guide for the search of topological crystalline insulator phases in realistic materials.

In epitaxial thin films of magnetite a large change in magnetization across the Verwey transition has been observed. In the Verwey state, the sample magnetization appeared to be strongly reduced, in some samples even close to zero. Using superconducting quantum interference magnetometry, x-ray absorption near edge spectroscopy, and polarized neutron reflectometry, a simple rotation of the magnetization vector due to a change in magnetocrystalline anisotropy was excluded. The experimental data rather suggest an intrinsic loss of magnetic moment due to a possible transition into a low or intermediate spin state of ${\mathrm{Fe}}^{2+}$. This observation discloses a different aspect of the Verwey transition.

The remarkable intrinsic magnetic properties of the $\mathrm{LaF}{\mathrm{e}}_{12}{\mathrm{B}}_6$ compound have been studied by neutron powder diffraction (NPD) and magnetization measurements. The NPD measurement reveals that $\mathrm{LaF}{\mathrm{e}}_{12}{\mathrm{B}}_6$ exhibits an antiferromagnetic (AFM) structure that can be described with a magnetic propagation vector of (¼, ¼, ¼) below $T_{\mathrm{N}}$. In the amplitude-modulated model used for the refinement, the Fe magnetic moments are confined to the $ab$ plane with a maximum value of $0.43 {\mu }_{\mathrm{B}}$ at 1.5 K. It is shown that the AFM state can be transformed to a ferromagnetic (FM) state via a field-induced first-order transition accompanied with a huge magnetic hysteresis. The $\mathrm{LaF}{\mathrm{e}}_{12}{\mathrm{B}}_6$ compound is not only the unique stable $R\mathrm{F}{\mathrm{e}}_{12}{\mathrm{B}}_6$ phase, along the rare-earth $R$ series but also presents unique magnetic behavior for a purely $3d$ itinerant electron system, including particularly low ordering temperature $T_{\mathrm{N}}=36\mathrm{K}$, remarkably small Fe moment, unusual amplitude-modulated magnetic arrangement, and a multicritical point in the magnetic phase diagram. In addition, we reveal that at 2 K, the AFM-FM transition is abrupt, leading to a large increase of the Fe magnetic moment up to $1.55{\mu }_{\mathrm{B}}$; the magnetization curve presents ultrasharp steps, giving rise to an unusual staircaselike behavior.

A consistent theory of ferromagnetic resonance in superparamagnetic nanoparticles with uniaxial anisotropy of arbitrary strength is developed. Being free from restrictions on the material parameters of the particles and on the spectrometer characteristics, the approach is well suitable for the interpretation of experimental data as well as for cross-checking approximate models. Ferromagnetic resonance absorption lines of single particles and noninteracting random ensembles of those are presented. It is shown that the multicomponent spectra inherent to highly anisotropic particles in the athermic limit persist for the superparamagnetic ones until temperature attains a certain level defined by the interplay between the intensity of thermal fluctuations and the anisotropy magnitude. The inference that orientational spread of the particle easy axes is a precondition for the occurrence of multipeak absorption lines in superparamagnetic systems is refuted: We demonstrate that ensembles of coherently oriented nanoparticles are able to render well-defined two-component spectra provided their anisotropy is considerable and the tilt of the easy axes to the magnetizing field is close to ${90}^{\circ }$. However, the orientational spread of the easy axes, if it exists, sharpens the spectrum contour. Concerning the role of polydispersity, it is found that a fraction of small particles always smoothes the spectrum as those particles produce wide, nearly symmetrical absorption lines with a single maximum.

In mechanically driven superfluid turbulence, the mean velocities of the normal- and superfluid components are known to coincide: ${\mathit{U}}_{\text{n}}={\mathit{U}}_{\text{s}}$. Numerous laboratory, numerical, and analytical studies showed that under these conditions, the mutual friction between the normal- and superfluid velocity components also couples their fluctuations: ${\mathit{u}}_{\text{n}}^{\prime }(\mathit{r},t)\approx {\mathit{u}}_{\text{s}}^{\prime }(\mathit{r},t)$, almost at all scales. We show that this is not the case in thermally driven superfluid turbulence; here the counterflow velocity ${\mathit{U}}_{\text{ns}}\equiv {\mathit{U}}_{\text{n}}-{\mathit{U}}_{\text{s}}\ne 0$. We suggest a simple analytic model for the cross-correlation function $〈{\mathit{u}}_{\text{n}}^{\prime }(\mathit{r},t)·{\mathit{u}}_{\text{s}}^{\prime }({\mathit{r}}^{\prime },t)〉$ and its dependence on $U_{\text{ns}}$. We demonstrate that ${\mathit{u}}_{\text{n}}^{\prime }(\mathit{r},t)$ and ${\mathit{u}}_{\text{s}}^{\prime }(\mathit{r},t)$ are decoupled almost in the entire range of separations $|\mathit{r}-{\mathit{r}}^{\prime }|$ between the energy-containing scale and intervortex distance.

Recent developments in the search for topological superconductivity have brought lattices of magnetic adatoms on a superconductor into intense focus. In this work we will study ferromagnetic chains of adatoms on superconducting surfaces with Rashba spin-orbit coupling. Generalizing the deep-impurity approach employed extensively in previous works to arbitrary subgap energies, we formulate the theory of the subgap spectrum as a nonlinear matrix eigenvalue problem. We obtain an essentially analytical description of the subgap spectrum, allowing an efficient study of the topological properties. Employing a flat-band Hamiltonian sharing the topological properties of the chain, we evaluate the $\mathbb{Z}$-valued winding number and discover five distinct topological phases. Our results also confirm that the topological band formation does not require the decoupled Shiba energies to be fine-tuned to the gap center. We also study the properties of Majorana bound states in the system.

Alkali-metal (potassium) adsorption on FeSe thin films with thickness from 2 unit cells (UC) to 4 UC on $\mathrm{SrTi}{\mathrm{O}}_3$ grown by molecular beam epitaxy is investigated with a low-temperature scanning tunneling microscope. At appropriate potassium coverage (0.20–0.25 monolayer), the tunneling spectra of the films all exhibit a superconductinglike gap which is overall larger than 11 meV (five times the gap value of bulk FeSe) and decreases with increasing thickness, and two distinct features of characteristic phonon modes at ∼11 and ∼21 meV. The results reveal the critical role of the interface-enhanced electron-phonon coupling for possible high-temperature superconductivity in ultrathin FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3$ and is consistent with recent theories. Our study provides compelling evidence for the conventional pairing mechanism for this type of heterostructure superconducting system.

We investigate the magnetic and electronic properties of the $\mathrm{Gd}{T}_2{\mathrm{Zn}}_{20}$ ($T=\mathrm{Fe}$ and Co) compounds using x-ray resonant magnetic scattering (XRMS), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD). The XRMS measurements reveal that ${\mathrm{GdCo}}_2{\mathrm{Zn}}_{20}$ has a commensurate antiferromagnetic spin structure with a magnetic propagation vector $\vec{\tau }=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ below the Néel temperature ($T_N\sim 5.7$ K). Only the Gd ions carry a magnetic moment forming an antiferromagnetic structure with magnetic representation ${\mathrm{\Gamma }}_6$. For the ferromagnetic ${\mathrm{GdFe}}_2{\mathrm{Zn}}_{20}$ compound, an extensive investigation was performed at low temperature and under magnetic field using XANES and XMCD. A strong XMCD signal of about $12.5\%$ and $9.7\%$ is observed below the Curie temperature ($T_C\sim 85\mathrm{K}$) at the Gd $L_2$ and $L_3$ edges, respectively. In addition, a small magnetic signal of about $0.06\%$ of the jump is recorded at the Zn $K$ edge, suggesting that the Zn $4p$ states are spin polarized by the Gd $5d$ extended orbitals.

We report a reinvestigation of superconducting ${\text{Sn}}_{1-x}{\text{In}}_x\text{Te}$ at both low and high In doping levels. Considering the system over a broad composition range in a single study allows us to characterize a significant change in the properties as a function of $x$: The system evolves from a weakly coupled $p$-type superconductor to a strongly coupled $n$-type superconductor with increasing indium content. Hall effect measurements show that the carrier density does not vary monotonically with indium content; a change from $p$-type to $n$-type is observed near 10% In doping. This is contrary to expectations dictating that In should be a $p$-type dopant in semiconducting SnTe, because it has one less valance electron than Sn. A crystallographic search for point defects at high $x$ indicates that the material remains ideal NaCl-type over a wide composition range. Density functional theory calculations for In-doped SnTe support a picture where In does not act as a trivial hole dopant but instead forms a distinct, partly filled In $5s-\mathrm{Te}5p$ hybridized state centered around ${\text{E}}_F$, which is very different from what is seen for other nominal hole dopants such as Na, Ag, and vacant Sn sites.