Vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ undergoes a phase transition between an insulating monoclinic phase and a conducting rutile phase. Like other correlated electron systems, the properties of $\mathrm{V}{\mathrm{O}}_2$ can be extremely sensitive to small changes in external parameters such as strain. In this paper, we investigate a compressively strained $\mathrm{V}{\mathrm{O}}_2$ film grown on (001) quartz substrate in which the phase transition temperature $\left(T_{\mathrm{c}}\right)$ has been depressed to 325 K from the bulk value of 340 K. Infrared and optical spectroscopy reveals that the lattice dynamics of this strained film are similar to unstrained $\mathrm{V}{\mathrm{O}}_2$. However, some of the electronic interband transitions of the strained $\mathrm{V}{\mathrm{O}}_2$ film are significantly shifted in energy from those in unstrained $\mathrm{V}{\mathrm{O}}_2$. The lattice dynamics remain largely unchanged while the $T_{\mathrm{c}}$ and some of the electronic interband transitions differ substantially from the bulk values, which highlights the role of electronic correlations in driving this metal-insulator phase transition.

The sliding Luttinger liquid approach is applied to study fractional topological insulators (FTIs). We show that a FTI is the low energy fixed point of the theory for realistic spin-orbit and electron-electron interactions. We find that the topological phase pertains in the presence of an interaction that breaks the spin invariance, and its boundaries are even extended by those terms. Finally we show that the one-dimensional chiral anomaly in the Luttinger liquid leads to the emergence of topological Chern-Simons terms in the effective gauge theory of the FTI state.

We address the radiative emission of individual germanium extrinsic centers in ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ epilayers grown on germanium substrates. Microphotoluminescence experiments demonstrate the capability of high temperature emission (70 K) and complex exciton configurations (neutral exciton $X$ and biexciton $XX$, positive $X^{+}$ and negative $X^{-}$ charged excitons) of these quantum emitters. Finally, we investigate the renormalization of each energy level showing a large and systematic change of the binding energy of $XX$ and $X^{+}$ from positive to negative values (from $\sim +5$ meV up to $\sim -7$ meV covering $\sim 70$ meV of the emission energy) with increasing quantum confinement. These light emitters, grown on a silicon substrate, may exhibit energy-degenerate $X$ and $XX$ energy levels. Furthermore, they emit at the highest detection efficiency window of Si-based single-photon detectors. These features render them a promising device platform for the generation of entangled photons in the time-reordering scheme.

We report magnetotransport studies of ultrathin Sb films. Sb has been identified as a topologically nontrivial element; as a semimetal, however, the interior states dominate the transport in bulk samples. In the ultrathin films studied here, quantum confinement suppresses the interior transport such that surface transport accounts for about $15\%$ of the conduction in 10-bilayer-thick Sb structures. For thicknesses between 5 and 16 bilayers, the conduction increases linearly with film thickness, extrapolating to a finite remnant conductivity at zero film thickness. Weak antilocalization (WAL) is observed at low magnetic fields with thickness independent values of the phase breaking length and prefactor ($\alpha$) implying surface transport coupled to residual interior conduction. At high fields we see an evolution of the magnetoresistance (MR) field dependence from parabolic to sublinear as a function of film thickness. The data are reproduced by a simple model combining parallel parabolic MR from the interior and WAL from the surface.

Plasmon excitations in metallic armchair graphene nanoribbons are investigated using the random phase approximation. An exact analytical expression for the polarization function of Dirac fermions is obtained, valid for arbitrary temperature and doping. We find that at finite temperatures, due to the phase space redistribution among inter-band and intra-band electronic transitions in the conduction and valence bands, the full polarization function becomes independent of temperature and position of the chemical potential. It is shown that for a given width of nanoribbon there exists a single plasmon mode whose energy dispersion is determined by the graphene's fine structure constant. In the case of two Coulomb-coupled nanoribbons, this plasmon splits into in-phase and out-of-phase plasmon modes with splitting energy determined by the inter-ribbon spacing.

We study the photoresponse of two-dimensional semi-Dirac semimetals and three-dimensional Dirac semimetals to off-resonant circularly polarized light. For two-dimensional semi-Dirac semimetals we find that incident light does not open a gap in the spectrum, in contrast to the case of purely linear dispersion. For the three-dimensional case we find that applying a circularly polarized light, one can tune from a trivial insulator to three-dimensional Dirac semimetal with an inverted gap. We propose and show that application of light leads to an intriguing platform for generation, motion, and merging of three-dimensional Dirac points.

We performed tunable resonance Raman spectroscopy on samples highly enriched in the (5,5), (6,6), (7,7), and (8,8) armchair structures of metallic single-wall carbon nanotubes. We present Raman excitation profiles (REPs) for both the radial breathing mode and $G$-band phonons of these species. $G$-band excitation profiles are shown to resolve the expected incoming and outgoing resonances of the scattering process. Notably, the profiles are highly asymmetric, with the higher-energy outgoing resonance weaker than the incoming resonance. These results are comparable to the asymmetric excitation profiles observed previously in semiconducting nanotubes, introduce a different electronic type, and broaden the structural range over which the asymmetry is found to exist. Modeling of the behavior with a third-order quantum model that accounts for the $k$ dependence in energies and matrix elements, without including excitonic effects, is found to be insufficient for reproducing the observed asymmetry. We introduce an alternative fifth-order model in which the REP asymmetry arises from quantum interference introduced by phonon-mediated state mixing between the $E_{11}^M$ and $K$-momentum excitons. Such state mixing effectively introduces a nuclear coordinate dependence in the transition dipole moment and thus may be viewed as a non-Condon effect from a molecular perspective. This result unifies a molecularlike picture of nanotube transitions (introduced by their excitonic nature) with a condensed matter approach for describing their behavior.

Enabling efficient nonreciprocal acoustic devices is challenging, yet very desirable for a variety of applications, including acoustic imaging, underwater communications, energy concentration and harvesting, signal processing, and noise control. We discuss the theory and design of a fully linear compact acoustic circulator based on spatiotemporal modulation of the effective acoustic index, providing a compact and practical way to realize large sound circulation at any desired frequency. Our proposal enables tunable isolation levels of over 40 dB, with insertion losses as low as 0.3 dB, in a noise-free, integrable, frequency scalable device whose total size does not exceed $\lambda /6$.

The spin motive force driven by the dynamics of the skyrmion structure formed in a nanomagnetic disk is numerically investigated. Due to the existence of the magnetic structure along the disk edge, the collective mode of the magnetization is modified from that of the bulk skyrmion lattice obtained by using the periodic boundary condition. For a single-skyrmion disk, the dynamics of the skyrmion core and the edge magnetization induce the spin motive force, and a measurable AC voltage is obtained by two probes on the disk. For a multi-skyrmions disk, the phase-locked collective mode of skyrmions is found in the lowest resonant frequency where the amplitude of the AC voltage is enhanced by the cascade effect of the spin motive force. We also investigate the effect of the Rashba spin-orbit coupling on the spin motive force.

Analytical expressions for the exchange interaction between $J$ multiplets of interacting metallic centers are derived on the basis of a complete electronic model which includes the intrasite relativistic effects. A common belief that this interaction can be approximated by an isotropic form $\propto {\mathbf{J}}_1·{\mathbf{J}}_2$ (or $\propto {\mathbf{J}}_1·{\mathbf{S}}_2$ in the case of interaction with an isotropic spin) is found to be ungrounded. It is also shown that the often used “$1/U$ approximation” for the description of the kinetic contribution of the exchange interaction is not valid in the case of $J$ multiplets. The developed theory can be used for microscopic description of exchange interaction in materials containing lanthanides, actinides, and some transition-metal ions.

Employing first-principles electronic-structure calculations in conjunction with the frozen-magnon method, we calculate exchange interactions, spin-wave dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin gapless semiconductor (SGS) compounds ${\text{Mn}}_2\text{CoAl}, {\text{Ti}}_2\text{MnAl}, {\text{Cr}}_2\text{ZnSi}, {\text{Ti}}_2\text{CoSi}$, and ${\text{Ti}}_2\text{VAs}$. We find that their magnetic behavior is similar to the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice exchange interactions play an essential role in the formation of the magnetic ground state and in determining the Curie temperature $T_{\mathrm{c}}$. All compounds, except ${\text{Ti}}_2\text{CoSi}$, possess a ferrimagnetic ground state. Due to the finite energy gap in one spin channel, the exchange interactions decay sharply with the distance, and hence magnetism of these SGSs can be described considering only nearest- and next-nearest-neighbor exchange interactions. The calculated spin-wave dispersion curves are typical for ferrimagnets and ferromagnets. The spin-wave stiffness constants turn out to be larger than those of the elementary $3d$ ferromagnets. Calculated exchange parameters are used as input to determine the temperature dependence of the magnetization and $T_{\mathrm{c}}$ of the SGSs. We find that the $T_{\mathrm{c}}$ of all compounds is much above the room temperature. The calculated magnetization curve for ${\text{Mn}}_2\text{CoAl}$ as well as the Curie temperature are in very good agreement with available experimental data. This study is expected to pave the way for a deeper understanding of the magnetic properties of the inverse-Heusler-based SGSs and enhance the interest in these materials for application in spintronic and magnetoelectronic devices.

The center-of-mass structure factor, $S\left(Q\right)$, of liquid hydrogen and liquid deuterium has been measured up to 5 GPa, mostly along the melting line from 50 to 296 K. Good quality data were achieved thanks to a novel synchrotron x-ray technique that can isolate the very weak x-ray scattering signal of the micrometric volume of hydrogen compressed in the diamond anvil cell. $S\left(Q\right)$ is dominated by a broad peak and hence, its wave-vector position, $Q_m$, is used to appreciate the structural changes in the system. An isotope effect in the position of $Q_m$ is observed that can be explained by a density effect. The shift of $Q_m$ towards higher $Q$ as density increases is followed over a threefold compression. Two simple liquid type evolutions are disclosed with a crossover between them around 37 ${\mathrm{nm}}^{-3}$/molecule. An interpretation is proposed based on the change in the zero-point motional renormalization of the interaction from anharmonic to harmonic.

${\mathrm{BiMnO}}_3$ is the only transition-metal perovskite oxide that is insulating and shows strong ferromagnetism in bulk. This distinctive behavior would make it a promising candidate as a magnetoelectric multiferroic if it was also a polar material, but experiments have shown that bulk ${\mathrm{BiMnO}}_3$ has either a very small polarization (below $0.1\mu \mathrm{C}/{\mathrm{cm}}^2$) or, most likely, that it is a paraelectric. There is also experimental evidence that the polarization in ${\mathrm{BiMnO}}_3$ films grown on ${\mathrm{SrTiO}}_3$ can be as high as $20\mu \mathrm{C}/{\mathrm{cm}}^2$. Despite the interest in these behaviors, the diagram of ${\mathrm{BiMnO}}_3$ as a function of epitaxial strain has remained largely unexplored. Here, we use first-principles to predict that, both under enough compressive and tensile epitaxial strain, ${\mathrm{BiMnO}}_3$ films are ferroelectric with a giant polarization around $100\mu \mathrm{C}/{\mathrm{cm}}^2$. The phases displayed by the films are similar to those experimentally found for ${\mathrm{BiFeO}}_3$ in similar conditions—at compressive strains, the film is supertetragonal with a large component of the polarization pointing out of plane, while at tensile strains the polarization points mostly in plane. As in ${\mathrm{BiFeO}}_3$ films, these phases are antiferromagnetic—the orbital ordering responsible for ferromagnetism in ${\mathrm{BiMnO}}_3$ is absent in the polar phases. Our calculations also show that the band gap of some of these ${\mathrm{BiMnO}}_3$ films is substantially smaller than gaps typically found in ferroelectric oxides, suggesting it may be a suitable material for photovoltaic applications.

We develop, from first principles, a general and compact formalism for predicting the electromagnetic response of a metamaterial with nonmagnetic inclusions in the long-wavelength limit, including spatial dispersion up to the second order. Specifically, by resorting to a suitable multiscale technique, we show that the effective medium permittivity tensor and the first- and second-order tensors describing spatial dispersion can be evaluated by averaging suitable spatially rapidly varying fields, each satisfying electrostatic-like equations within the metamaterial unit cell. For metamaterials with negligible second-order spatial dispersion, we exploit the equivalence of first-order spatial dispersion and reciprocal bianisotropic electromagnetic response to deduce a simple expression for the metamaterial chirality tensor. Such an expression allows us to systematically analyze the effect of the composite spatial symmetry properties on electromagnetic chirality. We find that even if a metamaterial is geometrically achiral, i.e., it is indistinguishable from its mirror image, it shows pseudo-chiral-omega electromagnetic chirality if the rotation needed to restore the dielectric profile after the reflection is either a $0^{\circ }$ or ${90}^{\circ }$ rotation around an axis orthogonal to the reflection plane. These two symmetric situations encompass two-dimensional and one-dimensional metamaterials with chiral response. As an example admitting full analytical description, we discuss one-dimensional metamaterials whose single chirality parameter is shown to be directly related to the metamaterial dielectric profile by quadratures.

We investigated the iron-based ladder compounds (Ba,$\mathrm{Cs}){\mathrm{Fe}}_2{\mathrm{Se}}_3$. Their parent compounds ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ and ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ have different space groups, formal valences of Fe, and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, x-ray diffraction, and powder neutron diffraction measurements were conducted to obtain a temperature and composition phase diagram of this system. Block magnetism observed in ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ is not so fragile against Ba doping. A new type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$, but interladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless a finite $T$-linear contribution in specific heat was obtained at low temperatures.

We present a comparative calculation of the electronic structure of the high critical temperature superconductor Co-doped ${\text{BaFe}}_2{\text{As}}_2$ and its parent compound at the electron correlation level by the embedded cluster method; the electron correlation is calculated through the second-order Møller-Plesset perturbation theory. The superconducting doped material is represented by the ${\text{Ba}}_4{\text{CoFe}}_4{\text{As}}_8$ cluster. The analysis of the orbital populations in this cluster reveals the formation of an antiferromagnetic order in the Fe plane with a spin-density increase on the central Co atom with respect to the spin density of the central Fe atom of the undoped case. This increase is associated with an increase of the $d_{z^2}$ orbital population of the central atom. However, the formation mechanism of the local magnetic moment implies also a spin transfer from the nearest-neighbor Fe atoms and from the next-nearest-neighbor As atoms to the central Co atom, and it corresponds to a $J_1-J_2$ antiferromagnetic Heisenberg model. Some particular features of $d_{yz}$ and $d_{x^2-y^2}$ orbitals in the triplet and in the singlet cluster states are interpreted to correspond to a spinless fermion. This result, as well as the result relative to the formation mechanism of the magnetic moments, can be connected with a model of resonating-valence-bond (RVB) superconductors suggested recently by Poilblanc et al. [Phys. Rev. B 89, 241106 (2014)] and based on the Anderson RVB theory.

The perovskite antiferromagnetic $(T_{\mathrm{N}}\sim 220\mathrm{K})$ insulator ${\text{EuNiO}}_3$ undergoes at ambient pressure a metal-to-insulator transition at $T_{\mathrm{MI}}=460\mathrm{K}$ which is associated with a simultaneous orthorhombic-to-monoclinic distortion, leading to charge disproportionation. We have investigated the change of the structural and magnetic properties of ${\text{EuNiO}}_3$ with pressure (up to $\sim 20\mathrm{GPa}$) across its quantum critical point (QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and ${}^{151}\mathrm{Eu}$ nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure, we find that after a small increase of $T_{\mathrm{N}}$ ($p\le 2\mathrm{GPa}$) and the induced magnetic hyperfine field $B_{\mathrm{hf}}$ at the ${}^{151}\mathrm{Eu}$ nucleus ($p\le 9.7\mathrm{GPa}$), both $T_{\mathrm{N}}$ and $B_{\mathrm{hf}}$ are strongly reduced and finally disappear at $p_{\mathrm{c}}\cong 10.5\mathrm{GPa}$, indicating a magnetic QCP at $p_{\mathrm{c}}$. The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressure-induced insulator-to-metal transition occurs at $p_{\mathrm{IM}}\cong 6\mathrm{GPa}$, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high-pressure electrical resistance data on ${\text{EuNiO}}_3$ at low temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as $T^n$, with $n=1.6$, whereas it becomes a normal Fermi liquid, $n=2$, for pressures above $\sim 15\mathrm{GPa}$. On the basis of the obtained data, a magnetic phase diagram in the ($p, T$) space is suggested.

Electron transport in graphene is along the sheet but junction devices are often made by stacking different sheets together in a “side-contact” geometry which causes the current to flow perpendicular to the sheets within the device. Such geometry presents a challenge to first-principles transport methods. We solve this problem by implementing a plane-wave-based multiple-scattering theory for electron transport. This implementation improves the computational efficiency over the existing plane-wave transport code, scales better for parallelization over large number of nodes, and does not require the current direction to be along a lattice axis. As a first application, we calculate the tunneling current through a side-contact graphene junction formed by two separate graphene sheets with the edges overlapping each other. We find that transport properties of this junction depend strongly on the AA or AB stacking within the overlapping region as well as the vacuum gap between two graphene sheets. Such transport behaviors are explained in terms of carbon orbital orientation, hybridization, and delocalization as the geometry is varied.

Silicon nanocrystals prepared by thermal decomposition of silicon-rich 2–5-nm-thick ${\mathrm{SiO}}_x$ layers ($0.64\le x\le 1$) are investigated using time-resolved terahertz spectroscopy. The samples consist of a superlattice of isolated monolayers composed of Si nanocrystals with controlled variable size and filling fraction. Experiments with variable optical pump fluence over almost two orders of magnitude allow us to determine the depolarization fields in the structure. Careful consideration of the local fields along with Monte Carlo calculations of the microscopic conductivity of Si nanocrystals supported by structural characterization of the samples provide detailed information about the electrical connectivity of nanocrystals and about the charge transport among them. Well below the percolation threshold, nanocrystals grow mostly isolated from each other. In thicker or in more Si-enriched layers, nanocrystals merge during their growth and form tens-of-nanometer-sized photoconducting Si structures with a good electrical connection. In addition, in thick ${\mathrm{SiO}}_x$ layers, imperfectly connected clusters of Si nanocrystals are observed which develop probably at the end of the growth process and allow only limited charge transport due to energy barriers.

Single Co atoms adsorbed to missing-row reconstructed Au(110) exhibit a zero-bias peak in spectra of the differential conductance acquired with a low-temperature scanning tunneling microscope. This peak is assigned to the Abrikosov-Suhl resonance, which arises due to the Kondo effect. The surface reconstruction gives rise to Co adsorption sites with a local environment of anisotropic substrate heights. To adequately extract the spatial evolution of the resonance amplitude from spectra of the differential conductance changes in the tip-surface separation have to be taken into account.