Using the continuum model, we investigate the confined states and the corresponding wave functions of ABC-stacked trilayer graphene (TLG) quantum dots (QDs). First, a general infinite-mass boundary condition is derived and applied to calculate the electron and hole energy levels of a circular QD in both the absence and presence of a perpendicular magnetic field. Our analytical results for the energy spectra agree with those obtained by using the tight-binding model, where a TLG QD is surrounded by a staggered potential. Our findings show that ($i$) the energy spectrum exhibits intervalley symmetry $E_K^e\left(m\right)=-E_{K^{\prime }}^h\left(m\right)$ for the electron (e) and hole (h) states, where $m$ is the angular momentum quantum number, ($ii$) the zero-energy Landau level (LL) is formed by the magnetic states with $m⩽0$ for both Dirac valleys, that is different from monolayer and bilayer graphene QD with infinite-mass potential in which only one of the cones contributes, and ($iii$) groups of three quantum Hall edge states in the tight-binding magnetic spectrum approach the zero LL, which results from the layer symmetry in TLG QDs.

Scanning tunneling microscopy (STM) measurements of the (110) surface of magnetite showed the coexistence of two reconstructions: the known $(1\times 3)$ row reconstruction and a surprising atomic structure of high complexity which occupies a small fraction of the surface. Oxygen vacancies on the ${\mathrm{Fe}}_3{\mathrm{O}}_4$(110) B-terminated surface have previously been determined to be the most energetically favorable surface termination of those considered [Li et al., Surf. Sci. 601, 876 (2007)]. However, this study only investigated oxygen vacancies which were threefold coordinated. Here, first principles calculations indicate that twofold coordinated oxygen represents the most energetically stable oxygen vacancy on the B-terminated (110) surface of magnetite. STM simulations reveal that the structure that occupies a small fraction of the surface corresponds to this energetically favorable B-terminated ${\mathrm{Fe}}_3{\mathrm{O}}_4$(110) surface. The oxygen vacancies form an ordered array: Along the $\left[\overline{1}10\right]$ direction, every second twofold coordinated oxygen atom is vacant, and vacancies are separated by 6 Å. In adjacent twofold coordinated oxygen rows, the vacancies are shifted in the $\left[\overline{1}10\right]$ direction by 3 Å. Density functional theory calculations of the spin density distributions indicate that surface and subsurface octahedrally coordinated iron atoms are charge ordered. The charge ordering and existence of oxygen vacancies act to reduce the surface charge. However, other polarity compensation mechanisms may be at play to stabilize the surface.

We consider an interacting quantum dot working as a coherent source of single electrons. The dot is tunnel coupled to a reservoir and capacitively coupled to a gate terminal with an applied ac potential. At low frequencies, this is the quantum analog of the $RC$ circuit with a purely dynamical response. We investigate the quantized dynamics as a consequence of ac pulses with large amplitude. Within a Keldysh-Green function formalism we derive the time-dependent current in the Coulomb blockade regime. Our theory thus extends previous models that considered either noninteracting electrons in nonlinear response or interacting electrons in the linear regime. We prove that the electron emission and absorption resonances undergo a splitting when the charging energy is larger than the tunnel broadening. For very large charging energies, the additional peaks collapse and the original resonances are recovered, though with a reduced amplitude. Quantization of the charge emitted by the capacitor is reduced due to Coulomb repulsion and additional plateaus arise. Additionally, we discuss the differential capacitance and resistance as a function of time. We find that to leading order in driving frequency the current can be expressed as a weighted sum of noninteracting currents shifted by the charging energy.

For multicomponent electron scattering states, we derive a general relation between the Wigner group delay and the Bohmian dwell time. It is found that the definition of group delay should account for the phase of the spinor wave functions of propagating modes. The difference between the group delay and dwell time comes from both the interference delay and the decaying modes. For barrier tunneling of helical electrons on a surface of topological insulators, our calculations including the trigonal-warping term show that the decaying modes can contribute greatly to the group delay. The derived relation between the group delay and the dwell time is helpful to unify the two definitions of tunneling time in a quite general situation.

We investigate phase separation including elastic coherency effects in the bulk and at surfaces and find a reduction of the solubility limit in the presence of free surfaces. This mechanism favors phase separation near free surfaces even in the absence of external stresses. We apply the theory to hydride formation in nickel, iron, and niobium and obtain a reduction of the solubility limit by up to two orders of magnitude at room temperature in the presence of free surfaces. We develop in particular a scale bridging description of the solubility limit in the low-temperature regime, where the long-ranged elastic effects are expressed through a geometrical solubility modification factor, which expresses the difference to bulk systems. This expression allows to include elastic coherency effects near surfaces, e.g., in ab initio simulations.

We investigate the spontaneous emission rate of a two-level quantum emitter next to a composite medium made of randomly distributed metallic inclusions embedded in a dielectric host matrix. In the near field, the Purcell factor can be enhanced by two orders of magnitude relative to the case of a homogeneous metallic medium and reaches its maximum precisely at the insulator-metal transition. By unveiling the role of the decay pathways in the emitter's lifetime, we demonstrate that, close to the percolation threshold, the radiation emission process is dictated by electromagnetic absorption in the heterogeneous medium. We show that our findings are robust against change in material properties and shape of inclusions and apply for different effective-medium theories as well as for a wide range of transition frequencies.

Understanding the effect of vibrations in optically active nanosystems is crucial for successfully implementing applications in molecular-based electro-optical devices, quantum information communications, single photon sources, and fluorescent markers for biological measurements. Here, we present a first-principles microscopic description of the role of phonons on the isotopic shift presented in the optical emission spectrum associated to the negatively charged silicon-vacancy color center in diamond. We use the spin-boson model and estimate the electron-phonon interactions using a symmetrized molecular description of the electronic states and a force-constant model to describe molecular vibrations. Group theoretical arguments and dynamical symmetry breaking are presented in order to explain the optical properties of the zero-phonon line and the isotopic shift of the phonon sideband.

In this paper, the magnetization reversal of exchange-biased Py/CoO vortex structures was investigated. The vortex structures were prepared by film deposition onto closely packed arrays of silica particles. By zero field cooling below the Néel temperature of the antiferromagnetic CoO, a so called circular exchange bias can be imprinted. By completing the same cooling process in an applied magnetic field, either a displaced vortex or even a $c$ state can be stabilized, which depends strongly on the CoO layer thickness. A strong athermal training effect mainly on the descending branch after the second loop is observed, which is accompanied by a strong reduction in loop shift, indicating a rearrangement of the CoO spin configuration during reversal. By analyzing the nucleation and annihilation field of the exchange-biased loops, a strong asymmetry is observed, which hints to a magnetization reversal via formation of a distorted viscous vortex. This asymmetry gets more pronounced with increasing CoO layer thickness. Furthermore, we investigated the blocking temperature distribution of the Py/CoO sample series. Below the mean blocking temperature, an increased coercive field is observed when the vortex is cooled in a field larger than the annihilation field compared to zero field cooling, revealing the competition between the vortex state formation and the alignment of spins along the cooling-field direction.

The high efficiency of perovskite solar cells benefits from the high density of photoinduced free carriers. We studied how exciton and free carriers, as the two major photoproducts, coexist inside the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ perovskite. A new density-resolved spectroscopic method was developed for this purpose. The density-dependent coexistence of excitons and free carriers over a wide density range was experimentally observed. The quantitative analysis on the density-resolved spectra revealed a moderate exciton binding energy of 24 ± 2 meV. The results effectively proved that the strong ionic polarization inside the perovskite has a negligible contribution to exciton formation. The spectra also efficiently uncovered the effective mass of electron-hole pairs. Our spectroscopic method and the results profoundly enrich the understanding of the photophysics in perovskite materials for photovoltaic applications.

We study the magnetic and electronic phases of a one-dimensional (1D) magnetic adatom chain on a 2D superconductor. In particular, we confirm the existence of a “self-organized” 1D topologically nontrivial superconducting phase within the set of subgap Yu-Shiba-Rusinov states formed along the magnetic chain. This phase is stabilized by incommensurate spiral correlations within the magnetic chain that arise from the competition between short-range ferromagnetic and long-range antiferromagnetic electron-induced exchange interactions, similar to a recent study for a 3D superconductor [M. Schecter et al., Phys. Rev. B 93, 140503(R) (2016)]. The exchange interactions along diagonal directions are also considered and found to display behavior similar to a 1D substrate when close to half filling. We show that the topological phase diagram is robust against local superconducting order parameter suppression and weak substrate spin-orbit coupling. Lastly, we study the effect of a direct ferromagnetic exchange coupling between the adatoms, and find the region of spiral order in the phase diagram to be significantly enlarged in a wide range of the direct exchange coupling.

The atomic and electronic band structures of GaP/Si(001) heterointerfaces were investigated by ab initio density functional theory calculations. Relative total energies of abrupt interfaces and mixed interfaces with Si substitutional sites within a few GaP layers were derived. It was found that Si diffusion into GaP layers above the first interface layer is energetically unfavorable. An interface with Si/Ga substitution sites in the first layer above the Si substrate is energetically the most stable one in thermodynamic equilibrium. The electronic band structure of the epitaxial GaP/Si(001) heterostructure terminated by the $(2\times 2)$ surface reconstruction consists of surface and interface electronic states in the common band gap of two semiconductors. The dispersion of the states is anisotropic and differs for the abrupt Si-Ga, Si-P, and mixed interfaces. Ga $2p$, P $2p$, and Si $2p$ core-level binding-energy shifts were computed for the abrupt and the lowest-energy heterointerface structures. Negative and positive core-level shifts due to heterovalent bonds at the interface are predicted for the abrupt Si-Ga and Si-P interfaces, respectively. The distinct features in the heterointerface electronic structure and in the core-level shifts open new perspectives in the experimental characterization of buried polar-on-nonpolar semiconductor heterointerfaces.

Silver opens up interesting perspectives in the fabrication of complex systems based on heteroepitaxial layers after the growth of a silicene layer on its (111) face has been proposed. In this work we explore different synthesis methods of hexagonal boron nitride ($h$-BN) and graphene sheets on silver. The resulting layers have been examined by high-resolution scanning tunneling microscopy. A comparison of the interfacial electronic band structure upon growth of the distinct two-dimensional (2D) layers has been performed by scanning tunneling spectroscopy and complementary first-principle calculations. We demonstrate that the adsorption of the 2D layers has an effect on the binding energy of the Shockley state and the surface potential by lowering the local work function. These effects are larger in the case of graphene where the surface state of Ag(111) is depopulated due to charge transfer to the graphene. Furthermore, we show that the electronic properties of the $h$-BN/silver system can be tuned by employing different thicknesses of silver ranging from a few monolayers on Cu(111) to the single crystal Ag substrate.

We investigate the effect of Rashba spin-orbit coupling (SOC) on the optoelectronic properties of $n$- and $p$-type monolayer ${\mathrm{MoS}}_2$. The optical conductivity is calculated within the Kubo formalism. We find that the spin-flip transitions enabled by the Rashba SOC result in a wide absorption window in the optical spectrum. Furthermore, we evaluate the effects of the polarization direction of the radiation, temperature, carrier density, and the strength of the Rashba spin-orbit parameter on the optical conductivity. We find that the position, width, and shape of the absorption peak or absorption window can be tuned by varying these parameters. This study shows that monolayer ${\mathrm{MoS}}_2$ can be a promising tunable optical and optoelectronic material that is active in the infrared to terahertz spectral range.

By means of first-principle calculations, we report a stoichiometric crystal structure of BiSb with broken space-inversion symmetry. This structure is insulating in bulk and has nontrivial band topology. We observe a pressure driven Weyl semimetallic electronic phase transition in this BiSb system without a crystal phase change. The obtained Weyl semimetallic phase exists in the 4.0–6.0 GPa pressure range. We find that a total of 12 pairs of Weyl points, 12 monopoles and 12 antimonopoles, exist in the Brillouin zone. Additionally, the spin texture of the bulk BiSb compound appears to be electrically controllable when the interlink between pressure and an electric field is exploited. This produces novel manipulable topological transport properties in this system.

We consider nonlocal correlations in iron in the vicinity of the $\alpha -\gamma$ phase transition within the spin rotationally invariant dynamical mean-field theory (DMFT) approach, combined with the recently proposed spin-fermion model of iron. The obtained nonlocal corrections to DMFT yield a decrease of the Curie temperature of the $\alpha$ phase, leading to an agreement with its experimental value. We show that the corresponding nonlocal corrections to the energy of the $\alpha$ phase are crucially important to obtain the proximity of energies of $\alpha$ and $\gamma$ phases in the vicinity of the iron $\alpha -\gamma$ transformation.

Adsorption of Au on Si(553) results in the self-assembly of highly ordered step arrays of one-dimensional (1D) Au atomic wires along the step direction. Charge transfer from the terrace to the step edge causes every third Si atom at the step edge to exhibit a partially filled dangling bond hosting a single fully spin-polarized electron which forms in an ordered 1D spin chain along the step. The interstep correlation of this threefold periodicity in neighboring Si step edges and the geometry of the unit cell has been determined by means of high-resolution spot profile analysis low-energy electron diffraction, scanning tunneling microscopy, and density functional theory. While the twofold periodicity of the Au wires exhibits a weak interwire interaction, leading to streaks in the diffraction pattern, the correlation of the Si step edge atoms is by far a stronger interaction, resulting in clear spots. The corresponding unit cell spanned by threefold ordered step edge atoms can be described as a centered structure which is magnetically frustrated and may stabilize a (two-dimensional) quantum spin liquid.

To probe the charge-scattering mechanism in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ single crystal, we analyzed the temperature and magnetic-field dependence of the Seebeck coefficient ($S$). The large saturation value of $S$ at high field clearly demonstrates the linear energy dispersion of a three-dimensional Dirac fermion. A wide tunability of the charge-scattering mechanism has been realized by varying the strength of the magnetic field and the carrier density via In doping. With the increase in magnetic field, the scattering time crosses over from being nearly energy independent to a regime of linear dependence. On the other hand, the scattering time enters into the inverse energy-dependent regime and the Fermi surface strongly modifies with 2% In doping at Cd sites. With further increase in In content from 2% to 4%, we did not observe any Shubnikov–de Haas oscillation up to a 9 T field, but the magnetoresistance is found to be quite large as in the case of the undoped sample.

Self-rotation occurs in an exciton-polariton condensate in a two-dimensional semiconductor microcavity pumped by a nonresonant Gaussian laser beam. A wave packet of the condensate spontaneously rotates around the center of the pumped region at a constant frequency breaking the rotation symmetry of the system. When two self-rotating condensates are created with an appropriate distance, synchronization occurs between the dynamics of the self-rotating condensates.

A general model for treating the effects of three-dimensional (3D) interface roughness (IFR) in layered semiconductor structures has been developed and experimentally verified. The configurational average of the IFR potential produces an effective grading potential in the out-of-plane direction, which significantly alters the energy spectrum of the structure. The scattering self-energy of the 3D IFR is also derived. Under strong IFR, this scattering effect is shown to be dominant over phonon interaction and impurity scattering. When applied to intersubband transitions, these theoretical predictions explain the experimentally observed anomalous energy shift and unusual broadening in the intersubband transitions in III-nitride superlattices.

The deformation of graphene due to the chemisorption of hydrogen atoms on its surface and the long-range elastic interaction between hydrogen atoms induced by these deformations are investigated using a multiscale approach based on first principles, empirical interactions, and continuum modeling. Focus is given to the intrinsic low-temperature structure and interactions. Therefore, all calculations are performed at $T=0$, neglecting possible temperature or thermal fluctuation effects. Results from different methods agree well and consistently describe the local deformation of graphene on multiple length scales reaching 500 $\AA$. The results indicate that the elastic interaction mediated by this deformation is significant and depends on the deformation of the graphene sheet both in and out of plane. Surprisingly, despite the isotropic elasticity of graphene, within the linear elastic regime, atoms elastically attract or repel each other depending on (i) the specific site they are chemisorbed; (ii) the relative position of the sites; (iii) and if they are on the same or on opposite surface sides. The interaction energy sign and power-law decay calculated from molecular statics agree well with theoretical predictions from linear elasticity theory, considering in-plane or out-of-plane deformations as a superposition or in a coupled nonlinear approach. Deviations on the exact power law between molecular statics and the linear elastic analysis are evidence of the importance of nonlinear effects on the elasticity of monolayer graphene. These results have implications for the understanding of the generation of clusters and regular formations of hydrogen and other chemisorbed atoms on graphene.