Interfacial growth from vapor has been extensively studied. However, a straightforward picture of the growth mode under different growth conditions is still lacking. In this paper, we develop a comprehensive interfacial growth theory based on the stochastic approach. Using a critical interisland separation, we construct a general phase diagram of the growth modes. It has been revealed that if the Ehrlich-Schwoebel barrier $E_{\mathrm{ES}}$ is smaller than a critical value, the interfacial growth proceeds in a layer-by-layer (LBL) mode at any deposition rate. However, if $E_{\mathrm{ES}}$ is larger than the critical value, LBL growth occurs only at very small or very large deposition rates relative to the intralayer hopping rate, and multilayer (ML) growth occurs at a moderate deposition rate. Experiments with zinc oxide growth by chemical vapor deposition have been designed to qualitatively demonstrate the theoretical model. By changing the flux of the carrier gas (nitrogen gas) in chemical vapor deposition, we realize LBL, ML, and then reentrance of LBL homoepitaxial growth of ZnO successively. Moreover, we find that surface kinetics of ZnO is suppressed by decreasing oxygen partial pressure by comparing the experimental observations and theoretical models, which is supported by our recent first-principles calculations. Since the influence of the substrate and the growth species on growth can approximately be represented by binding energy and surface kinetics, we suggest that the phase diagram is essential for interfacial growth of different materials by vapor deposition.

Oxygen partial pressure during vapor phase growth plays a critical role in determining the microstructure and other properties of oxides. However, it remains unclear how it affects the growth mechanism on the atomic scale. In this article, we take ZnO(0001) surface as a model case and demonstrate the influence of oxygen partial pressure on surface adsorption and diffusion of intrinsic adatoms by first-principles calculations. Two typical reconstructions of ZnO(0001) surface, denoted as $(2\times 2)$-O and $n_3$, are utilized to model the oxygen-rich condition and oxygen-poor condition, respectively. The $(2\times 2)$-O refers to the surface with an oxygen adatom in a $(2\times 2)$ supercell, while the $n_3$ stands for the surface with triangular pits of edge length $n=3$. We find that under the oxygen-rich condition in which $(2\times 2)$-O forms, adsorption of the O adatom is always more energetically favorable than the Zn adatom. Under oxygen-poor condition in which $n_3$ forms, however, the preferential adsorbate changes from O adatom to Zn adatom as the oxygen partial pressure decreases. The O adatom is less diffusive than the Zn adatom on both reconstructed surfaces. The diffusion barriers of both Zn and O on $n_3$ are higher than their counterparts on $(2\times 2)$-O. Insufficient surface diffusion leads to a high nucleation rate; therefore, a second-layer nucleus may form before the completion of the first-layer on $n_3$. It suggests that ZnO growth under oxygen-poor condition, in comparison with the oxygen-rich condition, is more likely to proceed with the three-dimensional island growth mode and result in a rougher surface.

We compute various current-correlation functions of electrons flowing from a topological nanowire to the tip of a superconducting scanning tunnel microscope and identify fingerprints of a Majorana bound state. In particular, the spin resolved cross correlations are shown to display a clear distinction between the presence of a such an exotic state (negative correlations) and an Andreev bound state (positive correlations). Similarity and differences with measurements with a normal tunnel microscope are also discussed, like the robustness to finite temperature, for instance.

We study a spin-orbital model for $4d^1$ or $5d^1$ Mott insulators in ordered double perovskites with strong spin-orbit coupling. This model is conveniently written in terms of pseudospin and pseudo-orbital operators representing multipoles of the effective $j=\frac{3}{2}$ angular momentum. Similarities between this model and the effective theories of Kitaev materials motivate the proposal of a chiral spin-orbital liquid with Majorana fermion excitations. The thermodynamic and spectroscopic properties of this quantum spin liquid are characterized using parton mean-field theory. The heat capacity, spin-lattice relaxation rate, and dynamic structure factor for inelastic neutron scattering are calculated and compared with the experimental data for the spin liquid candidate ${\mathrm{Ba}}_2{\mathrm{YMoO}}_6$. Moreover, based on a symmetry analysis, we discuss the operators involved in resonant inelastic x-ray scattering (RIXS) amplitudes for double-perovskite compounds. In general, the RIXS cross sections allow one to selectively probe pseudospin and pseudo-orbital degrees of freedom. For the chiral spin-orbital liquid in particular, these cross sections provide information about the spectrum for different flavors of Majorana fermions.

There is increasing recognition that the multiorbital nature of the $3d$ electrons is important to the proper description of the electronic states in the normal state of the iron-based superconductors. Earlier studies of the pertinent multiorbital Hubbard models identified an orbital-selective Mott phase, which anchors the orbital-selective behavior seen in the overall phase diagram. An important characteristics of the models is that the orbitals are kinetically coupled, i.e., hybridized, to each other, which makes the orbital-selective Mott phase especially nontrivial. A $U\left(1\right)$ slave-spin method was used to analyze the model with nonzero orbital-level splittings. Here we develop a Landau free-energy functional to shed further light on this issue. We put the microscopic analysis from the $U\left(1\right)$ slave-spin approach in this perspective, and show that the intersite spin correlations are crucial to the renormalization of the bare hybridization amplitude towards zero and the concomitant realization of the orbital-selective Mott transition. Based on this insight, we discuss additional ways to study the orbital-selective Mott physics from a dynamical competition between the interorbital hybridization and collective spin correlations. Our results demonstrate the robustness of the orbital-selective Mott phase in the multiorbital models appropriate for the iron-based superconductors.

We use the slave-spin mean-field approach to study particle-hole symmetric one- and two-band Hubbard models in the presence of Hund's coupling interaction. By analytical analysis of the Hamiltonian, we show that the locking of the two orbitals vs orbital selective Mott transition can be formulated within a Landau-Ginzburg framework. By applying the slave-spin mean field to impurity problems, we are able to make a correspondence between impurity and lattice. We also consider the stability of the orbital selective Mott phase to the hybridization between the orbitals and study the limitations of the slave-spin method for treating interorbital tunnelings in the case of multiorbital Bethe lattices with particle-hole symmetry.

We report investigations on the magnetotransport in LaSb, which exhibits extremely large magnetoresistance (XMR). Foremost, we demonstrate that the resistivity plateau can be explained without invoking topological protection. We then determine the Fermi surface from Shubnikov–de Haas (SdH) quantum oscillation measurements and find good agreement with the bulk Fermi pockets derived from first-principles calculations. Using a semiclassical theory and the experimentally determined Fermi pocket anisotropies, we quantitatively describe the orbital magnetoresistance, including its angle dependence. We show that the origin of XMR in LaSb lies in its high mobility with diminishing Hall effect, where the high mobility leads to a strong magnetic-field dependence of the longitudinal magnetoconductance. Unlike a one-band material, when a system has two or more bands (Fermi pockets) with electron and hole carriers, the added conductance arising from the Hall effect is reduced, hence revealing the latent XMR enabled by the longitudinal magnetoconductance. With diminishing Hall effect, the magnetoresistivity is simply the inverse of the longitudinal magnetoconductivity, enabling the differentiation of the electron and hole contributions to the XMR, which varies with the strength and orientation of the magnetic field. This work demonstrates a convenient way to separate the dynamics of the charge carriers and to uncover the origin of XMR in multiband materials with anisotropic Fermi surfaces. Our approach can be readily applied to other XMR materials.

Layered transition-metal dichalcogenide (TMD) materials, i.e., $1T\text{−}{\mathrm{TiSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$, harbor a second-order charge density wave (CDW) transition where phonons play a key role for the periodic modulations of conduction electron densities and associated lattice distortions. We systematically study the transport and capacitance characteristics over a wide temperature range of Schottky barriers formed by intimately contacting freshly exfoliated flakes of $1T\text{−}{\mathrm{TiSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$ to $n$-type GaAs semiconductor substrates. The extracted temperature-dependent parameters (zero-bias barrier height, ideality, and built-in potential) reflect changes at the TMD/GaAs interface induced by CDW formation for both TMD materials. The measured built-in potential reveals chemical-potential shifts relating to CDW formation. With decreasing temperature a peak in the chemical-potential shifts during CDW evolution indicates a competition between electron energy redistributions and a combination of lattice strain energies and Coulomb interactions. These modulations of chemical potential in CDW systems, such as $1T\text{−}{\mathrm{TiSe}}_2$ and $2H\text{−}{\mathrm{NbSe}}_2$ harboring second-order phase transitions, reflect a corresponding conversion from short-range to long-range order.

Motivated by the recent experiments [Scientific Reports 6, 23051 (2016); Phys. Rev. Lett. 114, 096602 (2015)], we theoretically investigate Cooper pair splitting current in a graphene-based Cooper pair beam splitter geometry. By considering the graphene-based superconductor as an entangler device, instead of normal [two-dimensional (2D)] BCS superconductor, we show that the Cooper pair splitting current mediated by the crossed Andreev process is amplified compared to its normal superconductor counterpart. This amplification is attributed to the strong suppression of the local normal Andreev reflection process (arising from the Cooper pair splitting) from the graphene-based superconductor to lead via the same quantum dot, in comparison to the usual 2D superconductor. Due to the vanishing density of states at the Dirac point of undoped graphene, a doped graphene-based superconductor is considered here and it is observed that Cooper pair splitting current is very insensitive to the doping level in comparison to the usual 2D superconductor. The transport process of nonlocal spin-entangled electrons also depends on the type of pairing, i.e., whether the electron-hole pairing is onsite, intersublattice or the combination of both. The intersublattice pairing of graphene causes the maximum nonlocal Cooper pair splitting current, whereas the presence of both pairings reduces the Cooper pair splitting current.

Coherent thermal radiation emitters based on diffraction gratings inscribed on surface of a polar material, such as silicon carbide, always possess high angular dispersion resulting in wideband-dispersive or monochromatic-directive emission. In this paper, we identify roots of the high angular dispersion as the rapid surface phonon polariton (SPhP) resonance of the material surface and the misalignment of the dispersion curve of the diffraction orders of the grating with respect to light line. We minimize the rapid variation of SPhP resonance by compensating the material dispersion using bundled graphene sheets and mitigate the misalignment by a proper choice of the grating design. Utilizing a modified form of rigorous coupled wave analysis to simultaneously incorporate atomic-scale graphene sheets and bulk diffraction gratings, we accurately compute the emissivity profiles of the composite structure and demonstrate reduction in the angular dispersion of thermal emission from as high as ${30}^{\circ }$ to as low as $4^{\circ }$ in the SPhP dominant wavelength range of 11–12 $\mu \text{m}$. In addition, we demonstrate that the graphene sheets via their tunable optical properties allow a fringe benefit of dynamical variation of the angular dispersion to a wide range.

We present a numerical investigation of the exciton energy and oscillator strength in type-II nanowire quantum dots. For a single quantum dot, the poor overlap of the electron part and the weakly confined hole part of the excitonic wave function leads to a low oscillator strength compared to type-I systems. To increase the oscillator strength, we propose a double quantum dot structure featuring a strongly localized exciton wave function and a corresponding fourfold relative enhancement of the oscillator strength, paving the way towards efficient optically controlled quantum gate applications in the type-II nanowire system. The simulations are performed using a computationally efficient configuration-interaction method suitable for handling the relatively large nanowire structures.

Manipulation of acoustic wave fronts by thin and planar devices, known as metasurfaces, has been extensively studied, in view of many important applications. Reflective and refractive metasurfaces are designed using the generalized reflection and Snell's laws, which tell that local phase shifts at the metasurface supply extra momentum to the wave, presumably allowing arbitrary control of reflected or transmitted waves. However, as has been recently shown for the electromagnetic counterpart, conventional metasurfaces based on the generalized laws of reflection and refraction have important drawbacks in terms of power efficiency. This work presents a new synthesis method of acoustic metasurfaces for anomalous reflection and transmission that overcomes the fundamental limitations of conventional designs, allowing full control of acoustic energy flow. The results show that different mechanisms are necessary in the reflection and transmission scenarios for ensuring perfect performance. Metasurfaces for anomalous reflection require nonlocal response, which allows energy channeling along the metasurface. On the other hand, for perfect manipulation of anomalously transmitted waves, local and nonsymmetric response is required. These conclusions are interpreted through appropriate surface impedance models which are used to find possible physical implementations of perfect metasurfaces in each scenario. We hope that this advance in the design of acoustic metasurfaces opens new avenues not only for perfect anomalous reflection and transmission but also for realizing more complex functionalities, such as focusing, self-bending, or vortex generation.

We report the atomic-scale imaging of the buried Cd-Si(111)-$7\times 7$ interface with a low temperature scanning tunneling microscopy (STM). The Cd(0001) films grown on Si(111)-$7\times 7$ reveal the electronic growth mode, and manifest a series of quantum-well states. In the low-bias STM images, not only the $7\times 7$ reconstruction but also individual Si adatoms buried by thick Cd islands are clearly visible. The two successive layers of Cd islands exhibit the distinct contrasts due to the quantum size effect. Moreover, we found a small gap appeared at Fermi level owing to the Anderson localization induced by interface scattering. The perfect transparency of Cd films can be attributed to the anisotropic electron motion, i.e., lateral electron localization and transverse motion like free-electron.

Shear banding is a nucleation-controlled process in metallic glasses (MGs) involving multiple temporal-spatial scales, which hinders a concrete understanding of its structural origin down to the atomic scale. Here, inspired by the morphology of composite materials, we propose a different perspective of MGs as a hard particle-reinforced material based on atomic-scale structural heterogeneity. The local stable structures indicated by a high level of local fivefold symmetry (L5FS) act as hard “particles” which are embedded in the relatively soft matrix. We demonstrate this concept by performing atomistic simulations of shear banding in CuZr MG. A shear band is prone to form in a sample with a high degree of L5FS which is slowly quenched from the liquid. An atomic-scale analysis on strain and the structural evolution reveals that it is the strain gradient effect that has originated from structural heterogeneity that facilitates shear transformation zones (STZs) to mature shear bands. An artificial composite model with a high degree of strain gradient, generated by inserting hard MG strips into a soft MG matrix, demonstrates a great propensity for shear banding. It therefore confirms the critical role strain gradient plays in shear banding. The strain gradient effect on shear banding is further quantified with a continuum model and a mechanical instability analysis. These physical insights might highlight the strain gradient as the hidden driving force in transforming STZs into shear bands in MGs.

We report high-pressure Raman experiments of black phosphorus up to 24 GPa. The linewidths of first-order Raman modes $A_g^1, B_{2g}$, and $A_g^2$ of the orthorhombic phase show a minimum at 1.1 GPa. Our first-principles density functional analysis reveals that this is associated with the anomalies in electron-phonon coupling at the semiconductor to topological insulator transition through inversion of valence and conduction bands marking a change from trivial to nontrivial electronic topology. The frequencies of $B_{2g}$ and $A_g^2$ modes become anomalous in the rhombohedral phase at 7.4 GPa, and new modes appearing in the rhombohedral phase show anomalous softening with pressure. This is shown to originate from unusual structural evolution of black phosphorous with pressure, based on first-principles theoretical analysis.

We present a technique to compute the microcanonical thermodynamical properties of a many-body quantum system using tensor networks. The density of States, and more general spectral properties, are evaluated by means of a Hubbard-Stratonovich transformation performed on top of a real-time evolution, which is carried out via numerical methods based on tensor networks. As a consequence, the free energy and thermal averages can be also calculated. We test this approach on the one-dimensional Ising and Fermi-Hubbard models. Using matrix product states, we show that the thermodynamical quantities as a function of temperature are in very good agreement with the exact results.

Electronic, magnetic, and transport properties of the antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure, promising for the AFM spintronics, are studied from first principles using the Vienna ab initio simulation package. We investigate the site occupation of sublattices and the lattice parameters of three competing phases. We analyze the factors that determine which of the three conceivable structures will prevail. We then estimate formation energies of possible defects for the experimentally prepared lattice structure. ${\mathrm{Mn}}_{\mathrm{Cu}}$ and ${\mathrm{Cu}}_{\mathrm{Mn}}$ antisites as well as $\mathrm{Mn}\leftrightarrow \mathrm{Cu}$ swaps and vacancies on Mn or Cu sublattices were identified as possible candidates for defects in CuMnAs. We find that the interactions of the growing thin film with the substrate and with vacuum as well as the electron correlations are important for the phase stability while the effect of defects is weak. In the next step, using the tight-binding linear muffin-tin orbital method for the experimental structure, we estimate transport properties for systems containing defects with low formation energies. Finally, we determine the exchange interactions and estimate the Néel temperature of the AFM-CuMnAs alloy using the Monte Carlo approach. A good agreement of the calculated resistivity and Néel temperature with experimental data makes it possible to draw conclusions concerning the competing phases.

We study the low-temperature magnetic phase transitions and phase diagram of one member of the lanthanide family of frustrated compounds, ${\text{SrGd}}_2{\text{O}}_4$, using Monte Carlo simulation techniques. A detailed experimental study of two members of the ${\text{SrLn}}_2{\text{O}}_4$ family of compounds, ${\text{SrHo}}_2{\text{O}}_4$ and ${\text{SrGd}}_2{\text{O}}_4$, has recently been carried out by Young et al. [O. Young, Ph.D. thesis, University of Warwick, 2013; O. Young, G. Balakrishnan, M. R. Lees, and O. A. Petrenko, Phys. Rev. B 90, 094421 (2014)]. A phase diagram for ${\text{SrGd}}_2{\text{O}}_4$ was mapped out in the field-temperature ($H-T$) plane from magnetization, susceptibility, and specific-heat measurements and several ordered phases were identified. However, the detailed nature of these phases was not determined. We have used a model of classical Heisenberg spins to investigate the low-temperature behavior. Our simulation results qualitatively agree with the experimental findings. By collecting data from all measured thermodynamic quantities, a $H-T$ phase diagram is constructed. It reveals four separate regions of phases with unique magnetic ordering. We have identified the nature of the order in each of these phases.

We have combined elastic and inelastic neutron scattering techniques, magnetic susceptibility, and resistivity measurements to study single-crystal samples of ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$, which contain the superconducting phase that has a transition temperature of $\sim 31$ K. In the inelastic neutron scattering measurements, we observe both the spin-wave excitations resulting from the block antiferromagnetic ordered phase and the resonance that is associated with the superconductivity in the superconducting phase, demonstrating the coexistence of these two orders. From the temperature dependence of the intensity of the magnetic Bragg peaks, we find that well before entering the superconducting state, the development of the magnetic order is interrupted, at $\sim 42$ K. We consider this result to be evidence for the physical separation of the antiferromagnetic and superconducting phases; the suppression is possibly due to the proximity effect of the superconducting fluctuations on the antiferromagnetic order.

Density functional theory in principle predicts correct ground-state properties for all materials. However, the rocksalt structure of MnO has been obtained only by the high-level random phase approximation and diffusion Monte Carlo (DMC). Here, we propose and test for MnO, FeO, CoO, and NiO that a semilocal density functional can solve this problem by properly including both self-interaction and van der Waals corrections. The importance of the latter was previously unanticipated. The MnO structural energy difference from SCAN+$r\text{VV10+}U$ agrees well with that from DMC. Here SCAN is a recent semilocal exchange-correlation functional, $r\text{VV10}$ is the revised Vydrov-Van Voorhis long-range van der Waals correction, and the onsite Hubbard $U$ is taken from linear response.