We found atypical pressure dependence in the transport measurements of the metal to insulator transition (MIT) in epitaxial thin films of vanadium sesquioxide (${\mathrm{V}}_2{\mathrm{O}}_3$). Three different crystallographic orientations and four thicknesses, ranging from 40 to 500 nm, were examined under hydrostatic pressures $\left({\mathrm{P}}_h\right)$ of up to 1.5 GPa. All of the films at transition exhibited a four order of magnitude resistance change, with transition temperatures ranging from 140 to 165 K, depending on the orientation. This allowed us to build pressure-temperature phase diagrams of several orientations and film thicknesses. Interestingly, for pressures below 500 MPa, all samples deviate from bulk behavior and show a weak transition temperature $\left(T_c\right)$ pressure dependence $(d{T}_c/d{P}_h=1.2\times {10}^{-2}\pm 0.3\times {10}^{-2}\mathrm{K}/\mathrm{MPa}),$ which recovers to bulklike behavior $(3.9\times {10}^{-2}\pm 0.3\times {10}^{-2}\mathrm{K}/\mathrm{MPa})$ at higher pressures. Furthermore, we found that pressurization leads to morphological but not structural changes in the films. This indicates that the difference in the thin film and bulk pressure-temperature phase diagrams is most probably due to pressure-induced grain boundary relaxation, as well as both plastic and elastic deformations in the film microstructure. These results highlight the difference between bulk and thin films behaviors.

We compare the optical absorption of extended systems using the density-density and current-current linear response functions calculated within many-body perturbation theory. The two approaches are formally equivalent for a finite momentum $\mathbf{q}$ of the external perturbation. At $\mathbf{q}=\mathbf{0}$, however, the equivalence is maintained only if a small $q$ expansion of the density-density response function is used. Moreover, in practical calculations, this equivalence can be lost if one naively extends the strategies usually employed in the density-based approach to the current-based approach. Specifically, we discuss the use of a smearing parameter or of the quasiparticle lifetimes to describe the finite width of the spectral peaks and the inclusion of electron-hole interaction. In those instances, we show that the incorrect definition of the velocity operator and the violation of the conductivity sum rule introduce unphysical features in the optical absorption spectra of three paradigmatic systems: silicon (semiconductor), copper (metal) and lithium fluoride (insulator). We then demonstrate how to correctly introduce lifetime effects and electron-hole interactions within the current-based approach.

We investigate the transition from regular behavior towards chaos for Rydberg excitons in the presence of an external magnetic field. To this end, we develop a measure for chaos that is robust with respect to fluctuations in the density of states. We find that irrespective of whether or not the magnetic field is oriented along a high-symmetry direction of the crystal, all antiunitary symmetries of the system are broken for large field strengths. This result emphasizes the influence of phonons on the symmetry of Rydberg excitons. In addition, we find that the appearance of Landau levels above the band gap results in modified level spacing statistics that resemble systems without broken symmetry.

We show that two-dimensional (2D) honeycomb gold (HG) could be thermodynamic and lattice dynamic stable owing in part to the relativistic effect and electronic configuration. HG exhibits a covalent characteristic in its bonding and is a semiconductor with an energy gap of 0.1 eV at the Brillouin zone $K$ point caused by strong spin-orbit coupling. The gap can be further widened to about 0.3 eV if HG is tailored into nanoribbons with the armchair type of edges. In contrast, 2D close-packed gold (CPG) is metallic with a small effective mass. Both HG and CPG are more transparent to visible light than graphene. They are expected to outperform graphene as a semiconducting material in an electronic logic device and as a transparent conducting material in fabricating a display device, respectively.

The transport properties of a quantum dot coupled to ferromagnetic contacts and attached to a topological superconductor wire hosting Majorana zero-energy modes at its ends are studied theoretically in the Kondo regime. By using the numerical renormalization group method, the temperature and dot level dependence of the spectral function, the conductance, and its spin polarization are studied for different coupling strengths to a topological superconductor. It is shown that the transport characteristics are generally determined by the interplay of three competing energy scales, resulting from Kondo correlations, a ferromagnetic-lead-induced exchange field, and topological-wire-induced splitting. These two splittings are found to have opposite signs. Moreover, they can compensate for each other in an appropriate parameter regime.

We have conducted resonant angle-resolved photoemission spectroscopy of well-defined ${\mathrm{CeO}}_2$(111) and $c–{\mathrm{Ce}}_2{\mathrm{O}}_3$(111) model surfaces, revealing distinct $f$ contributions in the valence band of the two compounds. In conjunction with density functional theory calculations, we show that the $f$ contribution in ${\mathrm{CeO}}_2$ is of a covalent nature, arising from hybridization with the $\mathrm{O}2p$ bands. In contrast, $c–{\mathrm{Ce}}_2{\mathrm{O}}_3$ exhibits an almost nondispersive $f$ state at 1.3 eV, which is indicative of almost negligible c-f hybridization.

We study the origin of layer dependence in band structures of two-dimensional (2D) materials. We find that the layer dependence, at the density functional theory (DFT) level, is a result of quantum confinement and the nonlinearity of the exchange-correlation functional. We use this to develop an efficient scheme for performing DFT and $GW$ calculations of multilayer systems. We show that the DFT and quasiparticle band structures of a multilayer system can be derived from a single calculation on a monolayer of the material. We test this scheme on multilayers of ${\mathrm{MoS}}_2$, graphene, and phosphorene. This new scheme yields results in excellent agreement with the standard methods at a fraction of the computation cost. This helps overcome the challenge of performing fully converged $GW$ calculations on multilayers of 2D materials, particularly in the case of transition-metal dichalcogenides, which involve very stringent convergence parameters.

We demonstrate that spectrally diverse multiple magnetic dipole resonances can be excited in all-dielectric structures lacking rotational symmetry, in contrast to conventionally used spheres, disks, or spheroids. Such multiple magnetic resonances arise from hybrid Mie-Fabry-Perot modes, and can constructively interfere with induced electric dipole moments, thereby leading to novel multifrequency unidirectional scattering. Here we focus on elongated dielectric nanobars, whose magnetic resonances can be spectrally tuned by their aspect ratios. Based on our theoretical results, we suggest all-dielectric multimode metasurfaces and verify them in proof-of-principle microwave experiments. We also believe that the demonstrated property of multimode directionality is largely responsible for the best efficiency of all-dielectric metasurfaces that were recently shown to operate across multiple telecom bands.

We devote this work to the study of the mean-field phase diagram of the $\nu =0$ quantum Hall state in bilayer graphene and the computation of the corresponding neutral collective modes, extending the results of recent works in the literature. Specifically, we provide a detailed classification of the complete orbital-valley-spin structure of the collective modes and show that phase transitions are characterized by singlet modes in orbital pseudospin, which are independent of the Coulomb strength and suffer strong many-body corrections from short-range interactions at low momentum. We describe the symmetry breaking mechanism for phase transitions in terms of the valley-spin structure of the Goldstone modes. For the remaining phase boundaries, we prove that the associated exact $\text{SO}\left(5\right)$ symmetry existing at zero Zeeman energy and interlayer voltage survives as a weaker mean-field symmetry of the Hartree-Fock equations. We extend the previous results for bilayer graphene to the monolayer scenario. Finally, we show that taking into account Landau level mixing through screening does not modify the physical picture explained above.

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

We study influence of image forces on conductance of ferroelectric tunnel junctions. We show that the influence of image forces is twofold: (i) they enhance the electroresistance effect due to polarization hysteresis in symmetric tunnel junctions at nonzero bias, and (ii) they produce the electroresistance effect due to hysteresis of dielectric permittivity of ferroelectric barrier. We study dependence of ferroelectric tunnel junction conductance on temperature and show that image forces lead to strong conductance variation with temperature.

Based on the models of series and parallel connections of the two phases in a composite, analytic approximations are derived for the elastic constants (Young's modulus, shear modulus, and Poisson's ratio) of elastically isotropic two-phase composites containing second phases of various volume fractions, shapes, and regular distributions. Comparison with a plentitude of finite element simulations and numerous previous experimental investigations shows a large consistency between the results and the analytic expressions derived, confirming the adequacy of the present approach. Compared with previous classical models, the present model has several advantages, including its simplicity, accuracy of prediction, and universal applicability.

We theoretically study the current-induced spin torque in antiferromagnetic tunnel junctions, composed of two semi-infinite antiferromagnetic layers separated by a tunnel barrier, in both clean and disordered regimes. We find that the torque enabling electrical manipulation of the Néel antiferromagnetic order parameter is out of plane, $\sim \mathbf{n}\times \mathbf{p}$, while the torque competing with the antiferromagnetic exchange is in plane, $\sim \mathbf{n}\times (\mathbf{p}\times \mathbf{n})$. Here, $\mathbf{p}$ and $\mathbf{n}$ are the Néel order parameter direction of the reference and free layers, respectively. Their bias dependence shows behavior similar to that in ferromagnetic tunnel junctions, the in-plane torque being mostly linear in bias, while the out-of-plane torque is quadratic. Most importantly, we find that the spin transfer torque in antiferromagnetic tunnel junctions is much more robust against disorder than that in antiferromagnetic metallic spin valves due to the tunneling nature of spin transport.

We present a microscopic theory of Raman scattering in a two-dimensional Fermi liquid (FL) with Rashba and Dresselhaus types of spin-orbit coupling and subject to an in-plane magnetic field $\left(\vec{B}\right)$. In the long-wavelength limit, the Raman spectrum probes the collective modes of such a FL: the chiral spin waves. The characteristic features of these modes are a linear-in-$q$ term in the dispersion and the dependence of the mode frequency on the directions of both $\vec{q}$ and $\vec{B}$. All of these features have been observed in recent Raman experiments on ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ quantum wells.

We report the magnetization dynamics excited by picosecond acoustic pulses confined in the acoustic cavity of freestanding nickel films. By detecting both sides of the sample we show that acoustic pulses moving back and forth inside the cavity efficiently control the magnetization precession. The multiple round trips of acoustic pulses, which survive for 12 bounces, enhance the amplitude of the magnetization precession by a factor of 4.5. Even though the conditions of total reflection are met in the freestanding film, the acoustic pulses have a large attenuation as compared to similar Ni films on $\mathrm{Si}{\mathrm{O}}_2$ substrate, for which we speculate that it is related to a stress induced by the substrate. Our results constitute a guideline for simple designs of acoustic cavities and for achieving a very large amplification of the magnetization precession angle. It is potentially useful for high-frequency magneto-optical modulators.

The electric manipulation of antiferromagnets has become an area of great interest recently for zero-stray-field spintronic devices, and for their rich spin dynamics. Generally, the application of antiferromagnetic media for information memories and storage requires a heterostructure with a ferromagnetic layer for readout through the exchange-bias field. In magnetoelectric and multiferroic antiferromagnets, the exchange coupling exerts an additional impediment (energy barrier) to magnetization reversal by the applied magnetoelectric energy. We proposed and verified a method to overcome this barrier. We controlled the energy required for switching the magnetic domains in magnetoelectric ${\mathrm{Cr}}_2{\mathrm{O}}_3$ films by compensating the exchange-coupling energy from the ferromagnetic layer with the Zeeman energy of a small volumetric spontaneous magnetization found for the sputtered ${\mathrm{Cr}}_2{\mathrm{O}}_3$ films. Based on a simplified phenomenological model of the field-cooling process, the magnetic and electric fields required for switching could be tuned. As an example, the switching of antiferromagnetic domains around a zero-threshold electric field was demonstrated at a magnetic field of 2.6 kOe.

We investigated the effects of excited many-electron states in the optical control of the magnetic state in undoped Mott-Hubbard insulator. To derive the spin Hamiltonian in material under optical pumping one has used a many-electron approach based on the X-operator representation. Extending the projection operators approach on arbitrary energy spectra of the Mott-Hubbard insulator, we obtained the Hamiltonian of superexchange interaction in analytical form. The Hamiltonian includes the spin-exciton variables which are usually missing in discussion on the magnetic response to optical pumping. The superexchange is also not additive over contributions from the ground and optical excited states, and nonzero contributions to the Dzyaloshinskii-Moriya interaction are induced in insulators with different spins at the ground and excited cell states. As a test, a microscopic background for the optical induced superexchange was analyzed in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ (further La214) and ${\mathrm{FeBO}}_3$ with spins 1/2 and 5/2, respectively.

We constructed an effective tight-binding model with five Cr $3d$ orbitals for LaOCrAs according to first-principles calculations. Based on this model, we investigated possible superconductivity induced by interactions in doped LaOCrAs using the functional renormalization group (FRG). We find that there are two domes of superconductivity in electron-doped LaOCrAs. With increasing electron doping, the ground state of the system evolves from G-type antiferromagnetism in the parent compound to an incipient $s_{\pm }\text{−}\mathrm{wave}$ superconducting phase dominated by electron bands derived from the $d_{3z^2-r^2}$ orbital as the filling is above 4.2 electrons per site on the $d$ orbitals of Cr. The gap function has strong octet anisotropy on the Fermi pocket around the zone center and diminishes on the other pockets. In electron over-doped LaOCrAs, the system develops a $d_{x^2-y^2}$-wave superconducting phase and the active band derives from the $d_{xy}$ orbital. In between the two superconducting domes, a time-reversal symmetry breaking $s+id$ SC phase is likely to occur. The typical transition temperature for SC is estimated to lie between that of iron pnictides and cuprates. We also find the $s_{\pm }\text{−}\mathrm{wave}$ superconducting phase in the hole-doped case.