The electronic properties of the Mn:GaSe interface, produced by evaporating Mn at room temperature on a $\epsilon$-GaSe(0001) single-crystal surface, have been studied by soft x-ray spectroscopies, and the experimental results are discussed at the light of ab initio DFT+U calculations of a model ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se} \left(x=0.055\right)$ surface alloy. Consistently with these calculations that also predict a high magnetic moment for the Mn ions $\left(4.73\text{–}4.83 {\mu }_B\right)$, XAS measurements at the Mn $L$ edge indicate that Mn diffuses into the lattice as a ${\mathrm{Mn}}^{2+}$ cation with negligible crystal-field effects. Ab initio calculations also show that the most energetically favorable lattice sites for Mn diffusion are those where Mn substitutes Ga cations in the Ga layers of the topmost Se-Ga-Ga-Se sandwich. Mn $s$ and $p$ states are found to strongly hybridize with Se and Ga $p$ states, while weaker hybridization is predicted for Mn $d$ states with Se $s$ and $p$ orbitals. Furthermore, unlike other Mn-doped semiconductors, there is strong interaction between the $\mathrm{Ga}\text{−}s$ and $\mathrm{Mn}\text{−}{d}_{z^2}$ states. The effects of hybridization of Mn $3d$ electrons with neighboring atoms are still clearly detectable from the characteristic charge-transfer satellites observed in the photoemission spectra. The Mn $3d$ spectral weight in the valence band is probed by resonant photoemission spectroscopy at the Mn $L$ edge, which also allowed an estimation of the charge transfer ($\mathrm{\Delta }=2.95$ eV) and Mott-Hubbard ($U=6.4$ eV) energies on the basis of impurity-cluster configuration-interaction model of the photoemission process. The Mott-Hubbard correlation energy $U$ is consistent with the $U_{\mathrm{eff}}$ on-site Coulomb repulsion parameter (5.84 eV) determined for the ab initio calculations.

We consider an implementation of the Elitzur-Vaidman bomb experiment in a dc-biased electronic Mach-Zehnder interferometer with a leakage port on one of its arms playing the role of a “lousy bomb.” Many-body correlations tend to screen out manifestations of interaction-free measurement. Analyzing the correlations between the current at the interferometer's drains and at the leakage port, we identify the limit where the originally proposed single-particle effect is recovered. Specifically, we find that in the regime of sufficiently diluted injected electron beam and short measurement times, effects of quantum-mechanical wave-particle duality emerge in the cross-current correlations.

We present results and analyses from first-principles calculations aimed at exploring the size- and edge-dependent properties of a wide range of MXene nanoribbons cut from two-dimensional (2D) semiconducting MXenes. The nanoribbons are classified by their edge types (armchair versus zigzag), the composition and sequencing of the terminating atomic lines, and the lowest-energy structural models of their 2D counterparts. The semiconducting versus metallic nature of the nanoribbons is well explained using an electron counting rule for the edge dangling bonds. For semiconducting nanoribbons, the band-gap evolution as a function of ribbon size is shown to be dependent on the lowest-energy structural model, and determined by a combination of factors such as quantum confinement, the energetic location of the edge states, and the strength of the $d\text{−}d$ hybridization. Nanoribbons cut from 2D MXenes with asymmetric surfaces are found to have bent ground-state structures with curvatures increasing as the size of the ribbon decreases.

The inherent asymmetry of the electric transport in graphene is attributed to Klein tunneling across barriers defined by $pn$ interfaces between positively and negatively charged regions. By combining conductance and shot noise experiments, we determine the main characteristics of the tunneling barrier (height and slope) in a high-quality suspended sample with Au/Cr/Au contacts. We observe an asymmetric resistance $R_{\mathrm{odd}}=100-70\mathrm{\Omega }$ across the Dirac point of the suspended graphene at carrier density $|n_{\mathrm{G}}|=(0.3-4)\times {10}^{11}{\mathrm{cm}}^{-2}$, while the Fano factor displays a nonmonotonic asymmetry in the range $F_{\mathrm{odd}}\sim 0.03$–0.1. Our findings agree with analytical calculations based on the Dirac equation with a trapezoidal barrier. Comparison between the model and the data yields the barrier height for tunneling, an estimate of the thickness of the $pn$ interface $d<20$ nm, and the contact region doping corresponding to a Fermi level offset of $\sim -18$ meV. The strength of pinning of the Fermi level under the metallic contact is characterized in terms of the contact capacitance $C_c=19\times {10}^{-6} \text{F}/{\mathrm{cm}}^2$. Additionally, we show that the gate voltage corresponding to the Dirac point is given by the difference in work functions between the backgate material and graphene.

Topological Dirac semimetal is a newly discovered class of materials which has attracted intense attention. This material can be viewed as a three-dimensional (3D) analog of graphene and has linear energy dispersion in bulk, leading to a range of exotic transport properties. Here we report direct quantum transport evidence of the 3D Dirac semimetal phase of layered material $\mathrm{ZrT}{\mathrm{e}}_5$ by angular-dependent magnetoresistance measurements under high magnetic fields up to 31 T. We observed very clear negative longitudinal magnetoresistance induced by chiral anomaly under the condition of the magnetic field aligned only along the current direction. Pronounced Shubnikov–de Hass (SdH) quantum oscillations in both longitudinal magnetoresistance and transverse Hall resistance were observed, revealing anisotropic light cyclotron masses and high mobility of the system. In particular, a nontrivial π-Berry phase in the SdH oscillations gives clear evidence for the 3D Dirac semimetal phase. Furthermore, we observed clear Landau level splitting under high magnetic field, suggesting possible splitting of the Dirac point into Weyl points due to broken time-reversal symmetry. Our results indicate that $\mathrm{ZrT}{\mathrm{e}}_5$ is an ideal platform to study 3D massless Dirac and Weyl fermions in a layered compound.

The embedding potential defined on the boundary surface of a semi-infinite crystal relates the value and normal derivative of generalized Bloch states propagating or decaying toward the interior of the crystal. It becomes Hermitian when the electron energy $\epsilon$ is located in a projected bulk band gap at a given wave vector $\mathbf{k}$ in the surface Brillouin zone (SBZ). If one plots the real eigenvalues of the embedding potential for a time-reversal invariant insulator in the projected bulk band gap along a path $\epsilon ={\epsilon }_0\left(\mathbf{k}\right)$ passing between two time-reversal invariant momentum (TRIM) points in the SBZ, then, they form Kramers doublets at both end points. We will demonstrate that the $Z_2$ topological invariant, $\nu$, which is either 0 or 1, depending on the product of time-reversal polarizations at the two TRIM points, can be determined from the two different ways these eigenvalues are connected between the two TRIM points. Furthermore, we will reveal a relation, $\nu =P$ mod 2, where $P$ denotes the number of poles that the embedding potential exhibits along the path. We also discuss why gapless surface states crossing the bulk band gap inevitably occur on the surface of topological band insulators from the view point of the embedding theory.

The possibility of obtaining optical gain in an ensemble of semiconductor nanocrystals without involvement of multiexcitons is very attractive for low-threshold laser applications. Here, we reexamine theoretically the conditions required to reach this single-exciton gain regime in nanocrystals. We show that the electron-phonon interaction can play a very positive role, in addition to the exciton-exciton interaction. In presence of both interactions, the optical gain regime can be reached even when the population of nanocrystals containing single excitons is below 10%. For these reasons, we suggest that ultrasmall nanocrystals, or nanocrystals with deep defects at their surface, could be promising materials for light amplification.

In a common paradigm, the electronic structure of condensed matter is divided into weakly and strongly correlated compounds. While conventional band theory usually works well for the former class, many-body effects are essential for the latter. Materials such as the familiar ${\mathrm{SrTiO}}_3$ (STO) compound that bridge or even abandon this characterization scheme are highly interesting. Here, it is shown, by means of combining density functional theory with dynamical mean-field theory, that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly correlated $t_{2g}$ low-energy quasiparticles and localized “in-gap” states of dominant $e_g$ character with a subtle correlation signature. We furthermore touch base with recent experimental work and study the surface instability towards magnetic order.

We show that the effective electron-electron interaction $v_{\text{eff}}\left[n\right](\mathbf{r},{\mathbf{r}}^{\prime })=\mathrm{erf}\left[|\mathbf{r}-{\mathbf{r}}^{\prime }|b\left(n\right)\right]/|\mathbf{r}-{\mathbf{r}}^{\prime }|$, with $n$ being the electronic density and $b\left(n\right)$ fixed from the compressibility sum rule, is remarkably accurate in both three and two dimensions, competing with the actual state-of-the-art exchange-correlation static kernels. Using this result, we propose a simple isotropic approximation for the quasi-two-dimensional regime, which gives a realistic wave vector analysis of the correlation energy, in the context of the linear response time-dependent density functional theory.

It is inferred from the chemical shift of muon spin rotation $\left(\mu \mathrm{SR}\right)$ spectra that muons implanted in pristine (fully oxidized) mayenite, ${\left[{\mathrm{Ca}}_{12}{\mathrm{Al}}_{14}{\mathrm{O}}_{32}\right]}^{2+}\left[{\square }_5{\mathrm{O}}^{2-}\right]$ (C12A7, with $\square$ referring to the vacant cage), are bound to ${\mathrm{O}}^{2-}$ at the cage center to form ${\mathrm{OMu}}^{-}$ (where Mu represents muonium, a muonic analog of the H atom). However, an isolated negatively charged state $({\mathrm{Mu}}^{-}$, an analog of ${\mathrm{H}}^{-})$ becomes dominant when the compound approaches the state of electride ${\left[{\mathrm{Ca}}_{12}{\mathrm{Al}}_{14}{\mathrm{O}}_{32}\right]}^{2+}\left[{\square }_{4}2{\mathrm{e}}^{-}\right]$ as a result of the reduction process. Moreover, the ${\mathrm{OMu}}^{-}$ state in the pristine specimen exhibits depolarization of paramagnetic origin at low temperatures (below $\sim 30$ K), indicating that ${\mathrm{OMu}}^{-}$ accompanies a loosely bound electron in the cage that can be thermally activated. This suggests that interstitial muons (and hence H) forming a “cage electron-hydroxyl” complex can serve as electron donors in C12A7.

We demonstrate that recent advances in nanoscale thermal transport and temperature manipulation can be brought to bear on the problem of tailoring thermal radiation from wavelength-scale composite bodies. We show that such objects—complicated arrangements of phase-change chalcogenide (${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$) glasses and metals or semiconductors—can be designed to exhibit strong resonances and large temperature gradients, which in turn lead to large and highly directional emission at midinfrared wavelengths. We find that partial directivity depends sensitively on a complicated interplay between shape, material dispersion, and temperature localization within the objects, requiring simultaneous design of the electromagnetic scattering and thermal properties of these structures. Our calculations exploit a recently developed fluctuating-volume current formulation of electromagnetic fluctuations that rigorously captures radiation phenomena in structures with strong temperature and dielectric inhomogeneities, such as those studied here.

Recent scanning tunneling microscopy experiments on electron transport through iron(II) phthalocyanine (FePc) molecules adsorbed on the Au(111) surface have revealed that the measured Kondo conductance signature depends strongly on the specific adsorption site. To understand the physical origin of experimental observations, particularly the variation of Kondo features with the molecular adsorption site, we employ a combined density functional theory (DFT) and hierarchical equations of motion (HEOM) approach to investigate the electronic structure and Kondo correlation in FePc/Au(111) composite systems. The calculation results indicate that, for the on-top adsorption configuration, the two degenerate spin-unpaired $d_{\pi }$ orbitals on the Fe center are coupled indirectly through substrate band states, leading to the Fano-like antiresonance line shape in the $dI/dV$ spectra, while for the bridge adsorption configuration, the environment-induced couplings are largely suppressed because of the two different spin-unpaired $d$ orbitals. Therefore, our work suggests that the environment-induced coupling as an essential physical factor could greatly influence the Fano-Kondo features in magnetic molecule/metal composites, and the crucial role of local orbital degeneracy and symmetry is discovered. These findings provide important insights into the electron correlation effects in complex solid-state systems. The usefulness and practicality of the combined DFT+HEOM method is also highlighted.

We develop coarse-graining tensor renormalization group algorithms to compute physical properties of two-dimensional lattice models on finite periodic lattices. Two different coarse-graining strategies, one based on the tensor renormalization group and the other based on the higher-order tensor renormalization group, are introduced. In order to optimize the tensor network model globally, a sweeping scheme is proposed to account for the renormalization effect from the environment tensors under the framework of second renormalization group. We demonstrate the algorithms by the classical Ising model on the square lattice and the Kitaev model on the honeycomb lattice, and show that the finite-size algorithms achieve substantially more accurate results than the corresponding infinite-size ones.

To explore the origin of the suppression of the charge density wave (CDW) in ${\mathrm{Bi}}^{2-}$ square sheet of $R_2{\mathrm{O}}_2\mathrm{Bi}$ ($R$: rare-earth elements), we have investigated the band structures and phonon dispersions of ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Bi}$ and ${\mathrm{Er}}_2{\mathrm{O}}_2\mathrm{Bi}$. We have found that the large spin-orbit coupling of Bi atoms together with the chemical pressure effect reduces the Fermi surface nesting, which results in the suppression of the CDW instability in $R_2{\mathrm{O}}_2\mathrm{Bi}$. The pressure effect has been checked for a similar compound, ${\mathrm{Er}}_2{\mathrm{O}}_2\mathrm{Sb}$, which shows a series of structural distortions in the Sb layer under volume contraction, from herringbone type, to 1D-ladder type, and to square sheet. Furthermore, we show that the observed local disorder in the Bi square sheet of ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Bi}$ can be explained by the phonon softening anomaly, which is also expected to describe the anomalous resistivity upturn observed in ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Bi}$ upon cooling.

Ge-rich $\mathrm{G}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{i}}_x$ alloys have been investigated using spectroscopic ellipsometry and photoluminescence at room temperature. Special emphasis was placed on the compositional dependence of the lowest-energy interband transitions. For $x\le 0.05$, a compositional range of particular interest for modern applications, we find $E_0=0.799\left(1\right)+3.214\left(45\right)x+0.080\left(44\right)x^2$ (in eV) for the lowest direct gap. The compositional dependence of the indirect gap is obtained from photoluminescence as $E_{\mathrm{ind}}=0.659\left(4\right)+1.18\left(17\right)x$ (in eV). We find no significant discrepancies between these results and the extrapolations from measurements at higher Si concentrations. Such discrepancies had been suggested by recent work on $\mathrm{G}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{i}}_x$ films on Si. Accurate knowledge of the interband transition energies is an important requirement for the design of devices incorporating Ge-rich $\mathrm{G}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{i}}_x$ alloys and for the understanding of more complex systems, such as ternary $\mathrm{G}{\mathrm{e}}_{1-x-y}\mathrm{S}{\mathrm{i}}_x\mathrm{S}{\mathrm{n}}_y$ alloys, in terms of its binary constituents.

A detailed study of the $g$-factor anisotropy of electrons and holes in InAs/${\mathrm{In}}_{0.53}{\mathrm{Al}}_{0.24}{\mathrm{Ga}}_{0.23}\mathrm{As}$ self-assembled quantum dots emitting in the telecom spectral range of $1.5–1.6\mu \mathrm{m}$ (around 0.8 eV photon energy) is performed by time-resolved pump-probe ellipticity technique using a superconducting vector magnet. All components of the $g$-factor tensors are measured, including their spread in the quantum dot (QD) ensemble. Surprisingly, the electron $g$ factor shows a large anisotropy changing from $g_{\mathrm{e},x}=-1.63$ to $g_{\mathrm{e},z}=-2.52$ between directions perpendicular and parallel to the dot growth axis, respectively, at an energy of 0.82 eV. The hole $g$-factor anisotropy at this energy is even stronger: $|g_{\text{h},x}|=0.64$ and $|g_{\text{h},z}|=2.29$. On the other hand, the in-plane anisotropies of electron and hole $g$ factors are small. The pronounced out-of-plane anisotropy is also observed for the spread of the $g$ factors, determined from the spin dephasing time. The hole longitudinal $g$ factors are described with a theoretical model that allows us to estimate the QD parameters. We find that the QD height-to-diameter ratio increases while the indium composition decreases with increasing QD emission energy.

When a contaminant diffuses on the surface of a nanomechanical resonator, the motions of the two become correlated. Despite being a high-order effect in the resonator-particle coupling, such correlations affect the system dynamics by inducing dissipation of the resonator energy. Here, we consider this diffusion-induced dissipation in the cases of multiple particles adsorbed on carbon nanotube and graphene resonators. By solving the stochastic equations of motion, we simulate the ringdown of the resonator, in order to determine the resonator energy decay rate. We find two different scalings with the number of adsorbed particles $K$ and particle mass $m$. In the regime where the adsorbates are inertially trapped at an antinode of vibration, the dissipation rate $\mathrm{\Gamma }$ scales with the total adsorbed mass $\mathrm{\Gamma }\propto Km$. In contrast, in the regime where particles diffuse freely over the resonator, the dissipation rate scales as the product of the total adsorbed mass and the individual particle mass: $\mathrm{\Gamma }\propto K{m}^2$.

We study amplified spontaneous emission (ASE) from wavelength-scale composite bodies—complicated arrangements of active and passive media—demonstrating highly directional and tunable radiation patterns, depending strongly on pump conditions, materials, and object shapes. For instance, we show that under large enough gain, $\mathcal{PT}$ symmetric dielectric spheres radiate mostly along either active or passive regions, depending on the gain distribution. Our predictions are based on a recently proposed fluctuating-volume-current formulation of electromagnetic radiation that can handle inhomogeneities in the dielectric and fluctuation statistics of active media, e.g., arising from the presence of nonuniform pump or material properties, which we exploit to demonstrate an approach to modeling ASE in regimes where Purcell effect (PE) has a significant impact on the gain, leading to spatial dispersion and/or changes in power requirements. The nonlinear feedback of PE on the active medium, captured by the Maxwell-Bloch equations but often ignored in linear formulations of ASE, is introduced into our linear framework by a self-consistent renormalization of the (dressed) gain parameters, requiring the solution of a large system of nonlinear equations involving many linear scattering calculations.

We study magnetic domains in ${\text{Nd}}_2{\text{Fe}}_{14}\text{B}$ single crystals using high resolution magnetic force microscopy (MFM). Previous MFM studies and small angle neutron scattering experiments suggested the presence of nanoscale domains in addition to optically detected micrometer-scale ones. We find, in addition to the elongated, wavy nanodomains reported by a previous MFM study, that the micrometer-sized, star-shaped fractal pattern is constructed of an elongated network of nanodomains $\sim 20$ nm in width, with resolution-limited domain walls thinner than 2 nm. While the microscopic domains exhibit significant resilience to an external magnetic field, some of the nanodomains are sensitive to the magnetic field of the MFM tip.