Hard x-ray photoelectron spectroscopy (HAXPES) is used to investigate the intrinsic electronic properties of single crystal epitaxial CdO(100) thin films grown by metal organic vapor phase epitaxy (MOVPE). The reduced surface sensitivity of the HAXPES technique relaxes stringent surface preparation requirements, thereby allowing the measurement of as-grown samples with intrinsically higher carrier concentration ($n=2.4\times {10}^{20}{\text{cm}}^{-3}$). High-resolution HAXPES spectra of the valence band and core levels measured at photon energy of 6054 eV are presented. The effects of conduction band filling and band gap renormalization are discussed to explain the observed binding energy shifts. The measured bandwidth of the partially occupied conduction band feature indicates that a plasmon contribution may be present at higher carrier concentrations. The Cd 3$d$${}_{5/2}$ and O 1$s$ core-level line shapes are found to exhibit an increased asymmetry with increased carrier concentration, interpreted as evidence for final state screening effects from the carriers in the conduction band. Alternatively the core-level line shape is interpreted as arising from strong conduction electron plasmon satellites. The nature of these two competing models to describe core-level line shapes in metallic oxides is explored.

Hexagonal warping provides an anisotropy to the dispersion curves of the helical Dirac fermions that exist at the surface of a topological insulator. A subdominant quadratic in the momentum term leads to an asymmetry between conduction and valence band. A gap can also be opened through magnetic doping. We show how these various modifications to the Dirac spectrum change the polarization function of the surface states and employ our results to discuss their effect on the plasmons. In the long wavelength limit, the plasmon dispersion retains its square-root dependence on its momentum, $\mathbf{q}$, but its slope is modified and it can acquire a weak dependence on the direction of $\mathbf{q}$. Further, we find the existence of several plasmon branches, one of which is damped for all values of $\mathbf{q}$, and extract the plasmon scattering rate for a representative case.

Metasurfaces and metamaterials promise arbitrary rerouting of light using two-dimensional (2D) planar arrangements of electric and magnetic scatterers, respectively, 3D stacks built out of such 2D planes. An important problem is how to self-consistently model the response of these systems in a manner that retains dipole intuition yet does full justice to the self-consistent multiple scattering via near-field and far-field retarded interactions. We set up such a general model for metamaterial lattices of complex 2D unit cells of poly-atomic basis as well as allowing for stacking in a third dimension. In particular, each scatterer is quantified by a magnetoelectric polarizability tensor and Ewald lattice summation deals with all near-field and long-range retarded electric, magnetic, and magnetoelectric couplings self-consistently. We show in theory and experiment that grating diffraction orders of dilute split ring lattices with complex unit cells show a background-free signature of magnetic dipole response. For denser lattices experiment and theory show that complex unit cells can reduce the apparent effect of bianisotropy, i.e., the strong oblique-incidence handed response that was reported for simple split ring lattices. Finally, the method is applied to calculate transmission of finite stacks of lattices. Thereby our simple methodology allows us to trace the emergence of effective material constants when building a 3D metamaterial layer by layer, as well as facilitating the design of metasurfaces.

We report on the optical response of different magnetic oxides (manganites, ruthenates, and magnetite) and show that for photons with energies ≈1.5−3.5 eV the intensity of light reflected by these epitaxial oxide thin films is strongly modulated by magnetic fields. We argue that this behavior is related to the magnetopolaron physics, and the magnitude of the magnetoreflectance is a fingerprint and a measure of the electron-lattice coupling strength. Our research provides the basis for a fresh experimental approach to the physics of strongly correlated electronic systems.

We derive and discuss general physical bounds on the electromagnetic scattering and absorption of passive structures. Our theory, based on passivity and power conservation, quantifies the minimum and maximum allowed scattering for an object that absorbs a given level of power. We show that there is a fundamental tradeoff between absorption and overall scattering suppression for each scattering harmonic, providing a tool to quantify the performance of furtive sensors, regardless of the applied principle for scattering suppression. We illustrate these fundamental limitations with examples of light scattering from absorbing plasmonic nanoparticles and loaded dipole antennas, envisioning applications to the design of cloaked sensors and absorbers with maximized absorption efficiency.

We study the local density of states (LDOS) in a finite photonic crystal, in particular in the frequency range of the band gap. We propose an original point of view on the band gap, which we consider to be the result of vacuum fluctuations in free space that tunnel in the forbidden range in the crystal. As a result, we arrive at a model for the LDOS that is in two major items modified compared to the well-known expression for infinite crystals. First, we modify the Dirac $\delta$ functions to become Lorentzian with a width set by the crystal size. Second, building on characterization of the fields versus frequency and position we calculated the fields in the band gap. We start from the fields at the band edges, interpolated in space and position, and incorporating the exponential damping in the band gap. We compare our proposed model to exact calculations in one dimension using the transfer matrix method and find very good agreement. Notably, we find that in finite crystals, the LDOS inside the band gap depends on frequency, on position, and on crystal size, in contrast to the well-known results for infinite crystals where the LDOS is zero, independent of frequency and position.

Andreev reflection in a monolayer molybdenum disulfide superconducting-normal (S/N) hybrid junction is investigated. We find, by using a modified Dirac Hamiltonian and the scattering formalism, that the perfect Andreev reflection happens at normal incidence with $p$-doped S and N regions. The probability of the Andreev reflection and the resulting Andreev conductance, in this system, are demonstrated to be large in comparison with the corresponding gapped graphene structure. We further investigate the effect of a topological term ($\beta )$ in the Hamiltonian and show that it results in an enhancement of the Andreev conductance with $p$-doped S and N regions, while in the corresponding structure with an $n$-doped S region it is strongly reducible in comparison. This effect can be explained in terms of the dependence of the Andreev reflection probability on the sign of $\beta$ and the chemical potential in the superconducting region.

We discuss the temperature $\left(T\right)$ dependence of the generalized density of states $G(\omega ,T)$ and of the phonon intensities of the filled skutterudite compound YbFe${}_4$Sb${}_{12}$ investigated by inelastic neutron scattering in an extensive range of energy-momentum phase space. The inelastic response has been measured at $T=$ 1.7, 50, 100, 200, and 290 K at a thermal time-of-flight spectrometer with an incident wavelength of 1.3 Å. The experimentally determined signal is compared with $T$-dependent ab initio powder averaged lattice dynamics calculations. An additional set of $T$-dependent spectra is calculated with thermal displacement parameters extracted from neutron diffraction data within the incoherent approximation. $G(\omega ,T)$ shows a nonuniform intensity variation with $T$ due to the specific thermal displacement parameters of Yb, Fe, and Sb. We demonstrate that this temperature variation can be well reproduced by the two simulation approaches. In particular, the ab initio powder averaged lattice dynamics calculation is capable of reproducing the momentum and energy characteristics of the phonon form factors of YbFe${}_4$Sb${}_{12}$.

We provide a quantum theoretical description of the magnetic polarization induced by intense circularly polarized light in a material. Such effect—commonly referred to as the inverse Faraday effect—is treated using beyond-linear response theory, considering the applied electromagnetic field as external perturbation. An analytical time-dependent solution of the Liouville–von Neumann equation to second order is obtained for the density matrix and used to derive expressions for the optomagnetic polarization. Two distinct cases are treated, the long-time adiabatic limit of polarization imparted by continuous wave irradiation, and the full temporal shape of the transient magnetic polarization induced by a short laser pulse. We further derive expressions for the Verdet constants for the inverse, optomagnetic Faraday effect and for the conventional, magneto-optical Faraday effect and show that they are in general different. Additionally, we derive expressions for the Faraday and inverse Faraday effects within the Drude-Lorentz theory and demonstrate that their equality does not hold in general, but only for dissipationless media. As an example, we perform initial quantum mechanical calculations of the two Verdet constants for a hydrogenlike atom and we extract the trends. We observe that one reason for a large inverse Faraday effect in heavy atoms is the spatial extension of the wave functions rather than the spin-orbit interaction, which nonetheless contributes positively.

We investigate the properties of a hybrid single-electron transistor, involving a small superconducting island sandwiched between normal metal leads, which is driven by dc plus ac voltages. In order to describe its properties we derive from the microscopic theory a set of coupled equations. They consist of a master equation for the probability to find excess charges on the island, with rates depending on the distribution of nonequilibrium quasiparticles. Their dynamics follows from a kinetic equation which accounts for the excitation by single-electron tunneling as well as the relaxation and eventual recombination due to the interaction with phonons. Our low-temperature results compare well with recent experimental findings obtained for ac-driven hybrid single-electron turnstiles.

We calculate the differential conductance at a probe inserted in the weak link of a topological Josephson junction, consisting of a semiconducting nanowire deposited on top of two separated superconductors. Our aim is to understand how the peculiar features in the spectrum of Andreev bound states, arising due to the presence of Majorana bound states at the ends of the two topological superconducting wires defining the junction, can be determined through a measurement of the differential conductance. We find that when the probe allows a single propagating mode, the differential conductance presents a dip at zero voltage of zero conductance close to the position where the spectrum exhibits the topologically protected crossing. This can be viewed as a signature of the presence of Majorana states, which does not require fermion parity conservation and is robust against parameters' changes, as well as disorder. On the contrary, when the probe allows two or more propagating modes the differential conductance resembles the spectrum of Andreev bound states. This has been established making use of both numerical and analytical methods.

The layered, noncentrosymmetric heavy element ${\text{PbTaSe}}_2$ is found to be superconducting. We report its electronic properties accompanied by electronic-structure calculations. Specific heat, electrical resistivity, and magnetic-susceptibility measurements indicate that PbTaSe${}_2$ is a moderately coupled, type-II BCS superconductor ($T_c=3.72$ K, Ginzburg–Landau parameter $\kappa =17$) with an electron-phonon coupling constant of ${\lambda }_{ep}=0.74$. Electronic-structure calculations reveal a single bulk three-dimensional Dirac cone at the $K$ point of the Brillouin zone derived exclusively from its hexagonal Pb layer; it is similar to the feature found in graphene except there is a 0.8 eV gap opened by spin-orbit coupling. The combination of large spin-orbit coupling and lack of inversion symmetry also results in large Rashba splitting on the order of tenths of an eV.

A comprehensive investigation of rotatable anisotropy in a Fe${}_{0.8}$Ga${}_{0.2}$ thin film with a stripe domain structure has been performed comparing static and dynamic measurements. The stripes' domain formation and their rotation under a transverse magnetic field have been imaged by magnetic force microscopy. The rotatable anisotropy field $H_{\mathrm{rot}}$ was determined by fitting the frequency evolution of the dipole-dominated magnetostatic spin-wave mode versus the in-plane orientation of the stripe domains, measured by Brillouin light scattering in the absence of any dc or ac magnetic field. We obtained $H_{\mathrm{rot}}\approx 1.35$ kOe, which is nearly ten times larger than the crystallographic in-plane anisotropy field. By applying a dc magnetic field along the stripes' axis, $H_{\mathrm{rot}}$ decreases, and eventually vanishes for saturated in-plane magnetization. At remanence, we established a quantitative relationship between static and dynamic properties, that is, the stripes' rotation angle and the in-plane angle dependence of spin-wave frequency.

I present the results of first principles calculations of the electronic structure and magnetic interactions for the recently discovered superconductor YFe${}_2$Ge${}_2$ and use them to identify the possible nature of superconductivity and quantum criticality in this compound. I find that the Fe $3d$ derived states near the Fermi level show a rich structure with the presence of both linearly dispersive and heavy bands. The Fermi surface exhibits nesting between hole and electron sheets that manifests as a peak in the susceptibility at $(1/2,1/2)$. The antiferromagnetic spin fluctuations associated with this peak may be responsible for mediating the superconductivity in this compound resulting in a $s_{\pm }$ state similar to that of the previously discovered iron-based superconductors. I also find that various magnetic orderings are almost degenerate in energy, which indicates that the proximity to quantum criticality is due to competing magnetic interactions.

We report calculations of the electronic structure and magnetic properties of YFe${}_2$Ge${}_2$ and discuss the results in terms of the observed superconductivity near magnetism. We find that YFe${}_2$Ge${}_2$ is a material near a magnetic quantum critical point based on comparison of standard density functional results that predict magnetism with experiment. The band structure and Fermi surfaces are very three dimensional and higher conductivity is predicted in the $c$-axis direction. The magnetism is of Stoner type and is predominately from an in-plane ferromagnetic tendency. The interlayer coupling is weak giving a perhaps two dimensional character to the magnetism, which is in contrast to the conductivity and may be important for suppressing the ordering tendency. This is compatible with a triplet superconducting state mediated by spin fluctuations.

We report the transport results of underdoped Bi${}_{2+x}$Sr${}_{2-x}$CuO${}_{6+\delta }$ ($0.05\le x\le 0.40$) single crystals. A close relationship between the upturn of in-plane resistivity [${\rho }_{ab}\left(T\right)$] and the sign of magnetoresistance in the normal state upon Bi substitutions is found. Combining the results of the field and angular dependence of magnetoresistance, the model fitting of ${\rho }_{ab}\left(T\right)$, as well as the Hall coefficient results, we suggest a crossover from weak to strong localization of charge carriers in the underdoped Bi${}_{2+x}$Sr${}_{2-x}$CuO${}_{6+\delta }$, which may be responsible for the rather narrow superconducting dome in this system.

We use Ehrenfest dynamics and time-dependent density functional theory to calculate electronic stopping power $S_e$ of energetic ions in graphitic targets from first principles. By treating core electrons as valence electrons within the projected augmented wave framework, we demonstrate that this approach provides an accurate description of $S_e$ for a wide range of ions and ion energies, even when not only valence, but also core electron excitations are essential. Our impact-parameter-dependent approach capable of describing the stopping of both low- and high-energy ions is a significant step forward in $S_e$ calculations, as it makes it possible to monitor projectile charge state during impacts, estimate contributions of core and valence electron excitations to $S_e$, and it gives a quantitative description of electronic stopping in the cross-over region for bulk solids and nanostructures from first principles.

We investigate the properties of the vortex singularities in two-component exciton-polariton condensates in semiconductor microcavities in the presence of transverse-electric–transverse-magnetic (TE–TM) splitting of the lower polariton branch. This splitting does not change qualitatively the basic (lemon and star) geometry of half-quantum vortices (HQVs), but results in warping of both the polarization field and the supercurrent streamlines around these entities. The TE–TM splitting has a pronounced effect on the HQV energies and interactions, as well as on the properties of integer vortices, especially on the energy of the hedgehog polarization vortex. The energy of this vortex can become smaller than the energies of HQVs. This leads to modification of the Berezinskii-Kosterlitz-Thouless transition from the proliferation of half-vortices to the proliferation of hedgehog-based vortex molecules.

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices is now of intense research focus, and it becomes particularly important to evaluate the role played by crystalline defects on their properties. In this paper, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First, the link between the presence of structural defects and the recombination intensity of trapped excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replicas of exciton recombinations. Second, the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations, indicating extremely compact excitons in h-BN.

When a constriction is realized in a 2D quantum spin Hall system, electron tunneling between helical edge states occurs via two types of channels allowed by time-reversal symmetry, namely spin-preserving (p) and spin-flipping (f) tunneling processes. Determining and controlling the effects of these two channels is crucial to the application of helical edge states in spintronics. We show that, despite that the Hamiltonian terms describing these two processes do not commute, the scattering matrix entries of the related 4-terminal setup always factorize into products of p-term and f-term contributions. Such factorization provides an operative way to determine the transmission coefficients $T_p$ and $T_f$ related to each of the two processes, via transconductance measurements. Furthermore, these transmission coefficients are also found to be controlled independently by a suitable combination of two gate voltages applied across the junction. This result holds for an arbitrary profile of the tunneling amplitudes, including disorder in the tunnel region, enabling us to discuss the effect of the finite length of the tunnel junction, and the space modulation of both magnitude and phase of the tunneling amplitudes.