Magnetotransmission spectroscopy was employed to study the valley Zeeman effect in large area monolayer ${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$. The extracted values of the valley $g$ factors for both A and B excitons were found to be similar with $g_v\simeq -4.5$. The samples are expected to be strained due to the CVD growth on sapphire at high temperature ($700{}^{\circ }\mathrm{C}$). However, the estimated strain, which is maximum at low temperature, is only $\simeq 0.2\%$. Theoretical considerations suggest that the strain is too small to significantly influence the electronic properties. This is confirmed by the measured value of the valley $g$ factor, and the measured temperature dependence of the band gap, which are almost identical for CVD and mechanically exfoliated ${\mathrm{MoS}}_2$.

Nickel orthoborate $\mathrm{N}{\mathrm{i}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$ having a complex orthorhombic structure Pnnm (No. 58, $Z=2$) of the kotoite type is known for quite a long time as an antiferromagnetic material below $T_{\mathrm{N}}=46\mathrm{K}$, but up to now its physical properties including the lattice dynamics have not been explored. Six $\left[\mathrm{Ni}{\mathrm{O}}_6\right]$ units of $2a$ and $4f$ types are linked via rigid $\left[\mathrm{B}{\mathrm{O}}_3\right]$ groups and these structural particularities impose restrictions on the lattice dynamics and spin-phonon interactions. We performed the symmetry analysis of the phonon modes at the center of the Brillouin zone. The structural parameters and phonon modes were calculated using the dmol3 program. We report and analyze results of infrared and Raman studies of phonon spectra measured in all required polarizations. Most of the even and odd phonons predicted on the basis of the symmetry analysis and theoretical calculations were reliably identified in the measured spectra. Clear evidence of the spin-phonon interaction was found for some particular phonons below $T_{\mathrm{N}}$. An unexpected emergence of several very narrow and weak phonon lines was observed in the infrared absorption spectra exactly at the magnetic ordering temperature $T_{\mathrm{N}}$. Moreover, anomalous behavior was found for some Raman phonons. The emergence of new phonon modes in the infrared and Raman spectra exactly at $T_{\mathrm{N}}$ proves the existence of a magnetostructural phase transition of a new type in $\mathrm{N}{\mathrm{i}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$. A possible nature of this transition is discussed.

Metallic glasses are commonly found to favor denser packing structures and icosahedral order in experiments, simulations, and theoretical models. Here we present a molecular dynamics simulation study of Cu-Zr metallic glasses, prepared through a pressure-mediated pathway. The resulting glasses exhibit anomalous structure-property relationships; these glasses are less energetically stable, concomitant with a denser atomic packing and a significant increase in icosahedral short-range order. The enhanced icosahedral order is shown to be accompanied by a pressure-mediated change in chemical short-range order. The results demonstrate that in amorphous alloys (nonmonatomic), theoretical frameworks of the two-order-parameter model must be generalized to account for chemical degrees of freedom.

Noting the structural relationships between phases of carbon and boron carbide with phases of boron nitride and boron subnitride, we investigate their mutual solubilities using a combination of first-principles total energies supplemented with statistical mechanics to address finite temperatures. Thus we predict the solid-state phase diagram of boron-carbon-nitrogen (B-C-N). Owing to the large energy costs of substitution, we find that the mutual solubilities of the ultrahard materials diamond and cubic boron nitride are negligible, and the same for the quasi-two-dimensional materials graphite and hexagonal boron nitride. In contrast, we find a continuous range of solubility connecting boron carbide to boron subnitride at elevated temperatures. An electron-precise ternary compound ${\mathrm{B}}_{13}\mathrm{CN}$ consisting of ${\mathrm{B}}_{12}$ icosahedra with NBC chains is found to be stable at all temperatures up to melting. It exhibits an order-disorder transition in the orientation of NBC chains at approximately $T=500$ K. We also propose that the recently discovered binary ${\mathrm{B}}_{13}{\mathrm{N}}_2$ actually has composition ${\mathrm{B}}_{12.67}{\mathrm{N}}_2$.

We investigate the dynamics of a phonon-mediated superconductor driven out of equilibrium. The electronic hopping amplitude is ramped down in time, resulting in an increased electronic density of states. The dynamics of the coupled electron-phonon model is investigated by solving Migdal-Eliashberg equations for the double-time Keldysh Green's functions. The increase of the density of states near the Fermi level leads to an enhancement of superconductivity when the system thermalizes to the new state at the same temperature. We provide a time- and momentum-resolved view on this thermalization process and show that it involves fast processes associated with single-particle scattering and much slower dynamics associated with the superconducting order parameter. The importance of electron-phonon coupling for the rapid enhancement and the efficient thermalization of superconductivity is demonstrated, and the results are compared to a BCS time-dependent mean-field approximation.

Bulk-related conduction electron spin resonance and conduction hole spin resonance were investigated in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, a three-dimensional topological insulator. Electrons in the conduction band and holes in the valence band both have spin ½. The effective $g$-factors for conduction electrons are equal to 27.3 ± 0.15 for magnetic field parallel to the c axis and 19.48 ± 0.07 for magnetic field perpendicular to the c axis, whereas for conduction holes 29.90 ± 0.09 for magnetic field parallel and 18.96 ± 0.04 for magnetic field perpendicular to the c axis, respectively. Nonparabolicity effects were not observed in the investigated low carrier concentration range, below $8\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$. Large $g$-factors, higher by an order of magnitude than the free electron value, are due to strong spin-orbit interactions in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. The striking similarity of the spin resonances due to conduction electrons and holes confirms the peculiar symmetry between the conduction and valence bands of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, both having similar effective masses and spin character.

We unveil novel spin textures in an itinerant fermion model on a frustrated triangular lattice in the limit of low electronic density. Using hybrid Monte Carlo simulations on finite clusters we identify two types of nanoscale spin textures in the background of ${120}^{\circ }$ order: (i) a planar ferromagnetic cluster, and (ii) and a noncoplanar cluster with spins oriented perpendicular to the ${120}^{\circ }$ plane. Both these textures lead to localization of the electronic wave functions and are in turn stabilized by the concomitant charge modulations. The noncoplanar spin texture is accompanied by an unusual scalar chirality pattern. A well defined electric charge and magnetic moment associated with these textures allow for their easy manipulation by external electric and magnetic fields—a desirable feature for data storage. We identify a localization-delocalization behavior for electronic wave functions which is unique to frustrated magnets and propose a general framework for stabilizing similar spin textures in spin-charge coupled systems.

In transition metal oxide systems, there exists a serious discrepancy between the theoretical quasiparticle energies and the experimental photoemission energies. To improve the accuracy of electronic structure calculations for these systems, we use the all-electron mixed basis $\mathit{GW}$ method, in which single-particle wave functions are accurately described by the linear combinations of plane waves and atomic orbitals. We adopt the full $\omega$ integration to evaluate the correlation part of the self-energy and compare the results with those obtained by plasmon pole models. We present the quasiparticle energies and band gap of titanium dioxide (${\mathrm{TiO}}_2$) and zinc oxide (ZnO) within the one-shot $\mathit{GW}$ approximation. The results are in reasonable agreement with experimental data in the case of ${\mathrm{TiO}}_2$ but underestimated by about 0.6–1.4 eV from experimental data in the case of ZnO, although our results are comparable to previous one-shot $\mathit{GW}$ calculations. We also explain a new approach to perform $\omega$ integration very efficiently and accurately.

We report on a systematic experimental study on the chlorination of monatomic silicene layer on Ag(111) by scanning tunneling microscopy. Monolayer silicene on Ag(111) can form 4×4, (√13×√13)R ± 13.9°, and (2√3×2√3)R30° reconstructions due to their different buckling configurations. We found that at low dosage, Cl atoms attach to the upper buckled Si atoms without changing the buckling configuration of the silicene monolayer. However, at high coverage, the global buckling configuration will be significantly changed, resulting in new ordered structures. Interestingly, all monolayer silicene structures, regardless of their initial reconstructions, tend to form a local silicene 1×1 structure at the saturation coverage. The mechanism for chlorination of monolayer silicene is explained.

We explore a long Josephson contact transporting Cooper pairs between one-dimensional (1D) charge-neutral chiral Majorana modes in the leads via charged Dirac chiral modes in the normal region. We investigate the regimes of (i) transparent contacts and (ii) tunnel junctions implemented in 3D topological insulator/superconductor/magnet hybrid structures. The setup acts as a SQUID controlled by the magnetic flux enclosed by the chiral loop of the normal region. This chirality leads to the fractional $h/e$-periodic pattern of critical current. The current-phase relation can have sawtoothlike shape with spikes at unusual even phases of $2\pi n$.

The surface of W(110) exhibits a spin-orbit-induced Dirac-cone-like surface state, which is of mainly $d_{z^2}$ orbital character near $\overline{\mathrm{\Gamma }}$, although it is strongly influenced by the twofold ${\mathrm{C}}_{2v}$ surface symmetry. Its distinctive k-dependent spin polarization along $\overline{\mathrm{\Gamma }}\overline{\mathrm{H}}$ is revealed by spin- and angle-resolved photoemission excited with $p$- and $s$-polarized light. The spin texture of the surface state is found to change sign upon switching from $p$- to $s$-polarized light. Based on electronic-structure calculations, this behavior is explained by the orbital composition of the Dirac-cone-like state. The dominant part of the state has even mirror symmetry and is excited by $p$-polarized light. A minor part with odd symmetry is excited by $s$-polarized light and exhibits a reversed spin polarization. Our study demonstrates in which way spin-orbit interaction combines the spin degree of freedom with the orbital degree of freedom and opens a way to manipulate the spin information gathered from the Dirac-cone-like surface state by light. Our results prove that “spin control” is not restricted to topological surface states with $p$-type orbital symmetry in topological insulators.

Simulations are carried out based on the dynamical mean-field theory (DMFT) in order to investigate the properties of correlated thin films for various values of the chemical potential, temperature, interaction strength, and applied transverse electric field. Application of a sufficiently strong field to a thin film at half filling leads to the appearance of conducting regions near the surfaces of the film, whereas in doped slabs the application of a field leads to a conductivity enhancement on one side of the film and a gradual transition to the insulating state on the opposite side. In addition to the inhomogeneous DMFT, a local density approximation (LDA) is considered in which the particle density $n$, quasiparticle residue $Z$, and spectral weight at the Fermi level $A(\omega =0)$ of each layer are approximated by a homogeneous bulk environment. A systematic comparison between the two approaches reveals that the less expensive LDA results are in good agreement with the DMFT approach, except close to the metal-to-insulator transition points and in the layers immediately at the film surfaces. LDA values for $n$ are overall more reliable than those for $Z$ and $A(\omega =0)$. The hysteretic behavior (memory effect) characteristic of the bulk doping driven Mott transition persists in the slab.

We study the fate of the surface states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably, these two disorder types have a dramatically different effect on the surface states. Our soft x-ray angle-resolved photoemission spectroscopy (ARPES) measurements combined with numerical simulations show that unitary surface disorder pushes the Dirac state to inward quintuplet layers, burying it below an insulating surface layer. As a consequence, the surface spectral function becomes weaker but retains its quasiparticle peak. This is in contrast to Gaussian disorder, which smears out the quasiparticle peak completely. At the surface of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, neon sputtering adds additional unitary scatterers to the Gaussian disorder of the adsorbates. Since the introduced unitary disorder pushes the surface state to inward layers, the effects of Gaussian disorder are reduced. As a result the ARPES signal becomes sharper upon sputtering.

Often real samples of graphene consist of islands of both monolayer and bilayer graphene. Bound states in such hybrid quantum dots are investigated for (i) a circular single-layer graphene quantum dot surrounded by an infinite bilayer graphene sheet and (ii) a circular bilayer graphene quantum dot surrounded by an infinite single-layer graphene. Using the continuum model and applying zigzag boundary conditions at the single-layer–bilayer graphene interface, we obtain analytical results for the energy levels and the corresponding wave spinors. Their dependence on perpendicular magnetic and electric fields are studied for both types of quantum dots. The energy levels exhibit characteristics of interface states, and we find anticrossings and closing of the energy gap in the presence of a bias potential.

A discrete version of deposition-diffusion equations appropriate for description of step flow on a vicinal surface is analyzed for a two-dimensional grid of adsorption sites representing the stepped surface and explicitly incorporating kinks along the step edges. Model energetics and kinetics appropriately account for binding of adatoms at steps and kinks, distinct terrace and edge diffusion rates, and possible additional barriers for attachment to steps. Analysis of adatom attachment fluxes as well as limiting values of adatom densities at step edges for nonuniform deposition scenarios allows determination of both permeability and kinetic coefficients. Behavior of these quantities is assessed as a function of key system parameters including kink density, step attachment barriers, and the step edge diffusion rate.

We report a giant linear magnetoelectric coupling in strained ${\mathrm{BiMnO}}_3$ thin films in which the disorder associated with an islanded morphology gives rise to extrinsic relaxor ferroelectricity that is not present in bulk centrosymmetric ferromagnetic crystalline ${\mathrm{BiMnO}}_3$. Strain associated with the disorder is treated as a local variable, which couples to the two ferroic order parameters, magnetization $\vec{M}$ and polarization $\vec{P}$. A straightforward “gas under a piston” thermodynamic treatment explains the observed correlated temperature dependencies of the product of susceptibilities and the magnetoelectric coefficient together with the enhancement of the coupling by the proximity of the ferroic transition temperatures close to the relaxor freezing temperature. Our interpretation is based on a trilinear coupling term in the free energy of the form $\vec{L}·(\vec{P}\times \vec{M})$, where $\vec{L}$ is a hidden antiferromagnetic order parameter, previously postulated by theory for ${\mathrm{BiMnO}}_3$. This phenomenological invariant not only preserves inversion and time-reversal symmetry of the strain-induced interactions but also explains the pronounced linear magnetoelectric coupling without using the more conventional higher order biquadratic interaction proportional to ${(\vec{P}·\vec{M})}^2$.

The electrical dc transport properties of hexagonal magnetic equiatomic alloys of nominal composition ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{NiGe}$ $(x=0.2\text{and}0.25)$ have been investigated experimentally as well as theoretically using first-principles electronic structure calculations. Thermal hysteresis in the magnetization data indicates that the alloys undergo a first-order martensitic transition. Both the alloys show unusual nonmetallic resistivity behavior and a noticeable amount of training effect in resistivity when thermally cycled through the first-order martensitic transition. We observe moderate negative magnetoresistance ($\sim -11.5$% for 150 kOe) at 5 K (well below the martensitic transition temperature) associated with clear virgin line effect for both the alloys. We have adapted different flavors of density functional theory approach to understand the experimentally observed nonmetallic transport behavior.

Anomalously large fluctuation diamagnetism around the superconducting critical temperature has been recently observed in iron selenide (FeSe) [Kasahara et al. (unpublished)]. This indicates that superconducting fluctuations (SCFs) play a more significant role in FeSe, which supposedly has a two-band structure, than in the familiar single-band superconductors. Motivated by the data on FeSe, SCF-induced diamagnetism is examined in a two-band system, on the basis of a phenomenological approach with a Ginzburg-Landau functional. The obtained results indicate that the SCF-induced diamagnetism may be more enhanced than that in a single-band system due to the existence of two distinct fluctuation modes. Such enhancement of diamagnetism unique to a two-band system seems consistent with the large diamagnetism observed in FeSe, though still far from a quantitative agreement.

The electronic and magnetic properties of the collapsed-tetragonal ${\mathrm{CaPd}}_2{\mathrm{As}}_2$ superconductor (SC) with a transition temperature of 1.27 K have been investigated by ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The temperature ($T$) dependence of the nuclear spin lattice relaxation rates ($1/T_1$) and the Knight shifts indicate the absence of magnetic correlations in the normal state. In the SC state, $1/T_1$ measured by ${}^{75}\mathrm{As}$ NQR shows a clear Hebel-Slichter (HS) peak just below $T_{\mathrm{c}}$ and decreases exponentially at lower $T$, confirming a conventional $s$-wave SC. In addition, the Volovik effect, also known as the Doppler shift effect, has been clearly evidenced by the observation of the suppression of the HS peak with applied magnetic field.