Coupled Maxwell and time-dependent orbital-free calculations are implemented and tested to describe the interaction of electromagnetic waves and matter. The currents and induced fields predicted by the orbital-free calculations are compared to time-dependent density functional calculations and very good agreement is found for various systems including jellium sheets, jellium spheres, atomistic sheets, and icosahedron clusters.

We introduce a simple scheme in order to account for the coupling with longitudinal phonons within the first-principles Bethe-Salpeter approach based on many-body perturbation theory. This allows one to evaluate the reduction of exciton binding energies observed in polar semiconductors. The electron-phonon coupling is modeled from the macroscopic dielectric response in the infrared which, in turn, is calculated through density functional perturbation theory. In this way, the additional computational cost determined by our method is negligible. We first illustrate our approach in the case of bulk ZnS. Then, we discuss the limit of the Bethe-Salpeter equation in the case of strongly delocalized excitons and we show how the excitonic series of bulk ${\mathrm{Cu}}_2\mathrm{O}$ is well reproduced.

We employ transformation optics to optimize the generic nonlinear wave interaction of second-harmonic generation (SHG) from a pair of touching metallic wires. We demonstrate a six orders of magnitude increase in efficiency with respect to SHG from a single wire, and a ten orders of magnitude increase in the second-harmonic scattered power by increasing the background permittivity. These results have clear implications for the design of nanostructured metallic frequency-conversion devices. Finally, we exploit our analytic solution of a nontrivial nanophotonic geometry as a platform for performing a critical comparison of the strengths, weaknesses, and validity of other prevailing theoretical approaches previously employed for nonlinear wave interactions at the nanoscale.

The quantum spin Hall (QSH) insulator, ensured by the time-reversal ($\mathcal{T}$) symmetry, has been a conceptual milestone of complex topological phases and well known to realize the quantum anomalous Hall effect when $\mathcal{T}$ symmetry is broken by introducing ferromagnetism. Here, based on first-principles calculations and a tight-binding model, we show that, unlike previous $\mathcal{T}$-symmetry breaking, the QSH phase can survive under the antiferromagnetic long-range order in a RbCuSe/CsMnP heterobilayer consisting of a nonmagnetic QSH insulator RbCuSe and an antiferromagnetic insulator CsMnP. The calculated spin Chern number, ${\mathcal{Z}}_2$ invariant, and gapless edge state confirm the topological nontrivial phase clearly. Moreover, the role of effective and Rashba spin-orbit coupling and the magnitude of antiferromagnetism are discussed to reveal the underlying physical mechanism. Our results may lead to further scientific and technological advances in topological magnetism and antiferromagnetic spintronics.

While defects such as oxygen vacancies in correlated materials can modify their electronic properties dramatically, understanding the microscopic origin of electronic correlations in materials with defects has been elusive. Lanthanum nickelate with oxygen vacancies, ${\mathrm{LaNiO}}_{3-x}$, exhibits the metal-to-insulator transition as the oxygen vacancy level $x$ increases from the stoichiometric ${\mathrm{LaNiO}}_3$. In particular, ${\mathrm{LaNiO}}_{2.5}$ exhibits a paramagnetic insulating phase, also stabilizing an antiferromagnetic state below $T_N\simeq 152\mathrm{K}$. Here, we study the electronic structure and energetics of ${\mathrm{LaNiO}}_{3-x}$ using first principles. We find that ${\mathrm{LaNiO}}_{2.5}$ exhibits a “site-selective” paramagnetic Mott insulating state at $T\simeq 290\mathrm{K}$ as obtained using density functional theory plus dynamical mean field theory ($\mathrm{DFT}+\mathrm{DMFT}$). The Ni octahedron site develops a Mott insulating state with strong correlations as the Ni $e_g$ orbital is half-filled while the Ni square-planar site with apical oxygen vacancies becomes a band insulator. Our oxygen vacancy results cannot be explained by the pure change of the Ni oxidation state alone within the rigid band-shift approximation. Our $\mathrm{DFT}+\mathrm{DMFT}$ density of states explains that the peak splitting of unoccupied states in ${\mathrm{LaNiO}}_{3-x}$ measured by the experimental x-ray absorption spectra originates from two nonequivalent Ni ions in the vacancy-ordered structure.

We present a study of the propagation of three-dimensional (3D) bipolar electromagnetic ultrashort pulses in an inhomogeneous array of semiconductor carbon nanotubes (CNTs) in the presence of a control high-frequency (HF) electric field. The inhomogeneity is present in the form of a layer with an increased concentration of conduction electrons, which acts as a barrier for the propagation of ultrashort electromagnetic pulses through the CNT array. The dynamics of the pulse is described by a nonlinear equation for the vector potential of the electromagnetic field (it takes the form of a 3D generalization of the sine-Gordon equation), derived from the Maxwell's equations and averaged over the period of the HF control field. By means of systematic simulations, we demonstrate that, depending on the amplitude and frequency of the HF control, the ultrashort pulse approaching the barrier layer either passes it or bounces back. The layer's transmissivity for the incident pulse is significantly affected by the amplitude and frequency of the HF control field, with the reflection coefficient nearly vanishing in intervals that make up a discrete set of transparency windows, which resembles the effect of the electromagnetically induced transparency. Having passed the barrier, the ultrashort pulse continues to propagate, keeping its spatiotemporal integrity. The results may be used for the design of soliton valves, with the transmissivity of the soliton stream accurately controlled by the HF field.

For a driven-dissipative quantum many-body system prepared in a spontaneous broken-symmetry steady state, in addition to the Goldstone mode, the soft fluctuation modes provide important insight into the system's dynamics. Using a microscopic polariton laser theory, we find a rich transformation behavior of discrete and continuum soft modes in a two parameter (pump density and cavity dissipation rate) space. Our theory yields a unified picture of a variety of seemingly disconnected physical concepts including Mott transition, Mollow spectra or relaxation oscillations, polaritonic Bardeen-Cooper-Schrieffer gaps, and Goldstone companion modes. In particular, transformation paths connect the Goldstone companion modes with the Mollow analog modes across a line of exceptional points.

Topological interface states in one-dimensional electronic and photonic systems are currently intensively investigated. We demonstrate the coupling of topologically confined states: By concatenation of three substructures, where the outer embedding structures have opposite signs of reflection phases to the embedded structure, we realize a system of coupled interface states showing mode splitting. We theoretically and experimentally show that a topological transition occurs for this coupled system. The demonstration of these systems is put on a solid foundation by first realizing the substructures relevant for interface states between topologically distinct one-dimensional photonic crystals. We experimentally demonstrate band closing and band inversion for a redistribution of the optical path to the constitutive materials of the structures. The band inversion is demonstrated by the emergence of interface states at metal-dielectric interfaces and the findings are supported by ellipsometric measurements.

We theoretically investigate the Floquet generation of second-order topological superconducting (SOTSC) phase in the high-temperature platform both in two dimension (2D) and three dimension (3D). Starting from a $d$-wave superconducting pairing gap, we periodically kick the mass term to engineer the dynamical SOTSC phase within a specific range of the strength of the drive. Under such dynamical breaking of time-reversal symmetry (TRS), we show the emergence of the weak SOTSC phase, harboring eight corner modes, i.e., two zero-energy Majorana per corner, with vanishing Floquet quadrupole moment. On the other hand, our study interestingly indicates that upon the introduction of an explicit TRS breaking Zeeman field, the weak SOTSC phase can be transformed into strong SOTSC phase, hosting one zero-energy Majorana mode per corner, with quantized quadrupole moment. We also compute the Floquet Wannier spectra that further establishes the weak and strong nature of these phases. We numerically verify our protocol computing the exact Floquet operator in open boundary condition and then analytically validate our findings with the low energy effective theory (in the high-frequency limit). The above protocol is applicable for 3D as well as where we find one dimensional (1D) hinge mode in the SOTSC phase. We then show that these corner modes are robust against moderate disorder and the topological invariants continue to exhibit quantized nature until disorder becomes substantially strong. The existence of zero-energy Majorana modes in these higher-order phases is guaranteed by the antiunitary spectral symmetry.

Pressure dependence of the thermodynamic critical field $B_c$ in elemental aluminum was studied by means of the muon-spin rotation-relaxation technique. Pressure enhances the deviation of $B_c\left(T\right)$ from parabolic behavior, expected for a typical type-I superconductor, thus suggesting the weakening of the gap ratio $\langle \alpha \rangle =\langle \mathrm{\Delta }\rangle /k_B{T}_c$ ($\langle \mathrm{\Delta }\rangle$ is the average value of the superconducting energy gap, $T_c$ is the transition temperature, and $k_B$ is the Boltzmann constant). With the pressure increase from 0.0 to $\simeq 1.6$ GPa, $\langle \alpha \rangle$ decreases almost linearly from 1.73(1) to 1.67(1). Our results imply, therefore, that in elemental aluminum, the gap ratio $\langle \alpha \rangle$ is smaller than the weak-coupled BCS prediction ${\alpha }_{\mathrm{BCS}}\simeq 1.764$, and it is even further reduced under pressure.

We show that the chiral multifold fermions present a dual Haldane sphere problem in momentum space. Owing to the Berry monopole at the degenerate point, a dual Landau level emerges in the trace of quantum metric, with which a quantized geometric invariant is defined through a surface integration. We further demonstrate potential manifestations in the measurable, physical observables. With a lower bound derived for the finite spread of Wannier functions, anomalous phase coherence is identified accordingly for the flat band superconductivity. We briefly comment on the stability of these results under perturbations. Potential experimental probes of the quantum metric are also discussed.

High-resolution spectroscopic studies of $\mathrm{LaAl}{\mathrm{O}}_3$ single crystal doped with holmium ions are reported. Polarized and unpolarized absorption and luminescence spectra were measured in the broad spectral range from 2000 to $23000\mathrm{c}{\mathrm{m}}^{–1}$ at temperatures of 4.5–5 K. Additional measurements were fulfilled using site-selective laser spectroscopy. Energies and symmetry properties of the corresponding wave functions of crystal-field levels of ${\mathrm{Ho}}^{3+}$ ions which substitute for ${\mathrm{La}}^{3+}$ ions in $\mathrm{LaAl}{\mathrm{O}}_3$ at sites with the $D_3$ symmetry were determined with high accuracy, and, on this basis, crystal-field calculations were performed. A thorough analysis of spectral line profiles with the fine doublet structure corresponding to singlet-doublet transitions in the trigonal crystal field was made, which showed the existence of the random deformation splitting phenomenon. The value of deformation splitting lies in the region $0.3–0.8\mathrm{c}{\mathrm{m}}^{–1}$ that substantially exceeds the widths of the doublet hyperfine splitting. The observed line shapes were successfully modeled, assuming the interaction of the ${\mathrm{Ho}}^{3+}$ ions with random deformations of the crystal lattice induced by point defects and ferroelastic domain boundaries.

Motivated by the search for type-II multiferroics, we present a comprehensive optical study of a complex oxide family of type-II multiferroic candidates: $\mathrm{RbFe}{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_2, \mathrm{RbFe}{\left(\mathrm{Se}{\mathrm{O}}_4\right)}_2$, and $\mathrm{RbFe}{\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$. We employ rotational-anisotropy second harmonic generation spectroscopy (RA SHG), a technique sensitive to point symmetries, to address discrepancies in literature-assigned point/space groups and to identify the correct crystal structures. At room temperature we find that our RA SHG patterns rotate away from the crystal axes in $\mathrm{RbFe}{\left(A{\mathrm{O}}_4\right)}_2$ ($A=\mathrm{Se},\mathrm{S}$), which identifies the lack of mirror symmetry and in-plane two-fold rotational symmetry. Also, the SHG efficiency of $\mathrm{RbFe}{\left(\mathrm{Se}{\mathrm{O}}_4\right)}_2$ is two orders of magnitude stronger than $\mathrm{RbFe}{\left(A{\mathrm{O}}_4\right)}_2$ ($A=\mathrm{Mo},\mathrm{S}$), which suggests broken inversion symmetry. Additionally, we present temperature-dependent linear optical characterizations near the band edge of this family of materials using ultraviolet-visible absorption spectroscopy. Included is experimental evidence of the band gap energy and band gap transition type for this family. Subband gap absorption is also presented, which reveals prominent optical transitions, some with an unusual central energy temperature dependence. Furthermore, we find that by substituting the $A$ site in $\mathrm{RbFe}{\left(A{\mathrm{O}}_4\right)}_2$ ($A=\mathrm{Mo},\mathrm{Se},\mathrm{S}$), the aforementioned transitions are spectrally tunable. Finally, we discuss the potential origin and impact of these tunable transitions.

Experimental and numerical investigation of phonon optical modes of MgO as a function of temperature (from 300 to 1400 K) and pressure (from 0 to 21 GPa) are here presented. Infrared reflectivity measurements were performed to probe energies and widths of the optical phonons, as well as of the multiphonon processes affecting the spectral shape, over a variation of the unit-cell volume exceeding 20%. Calculations within quasiharmonic approximation (QHA) account well for the volume dependence of the optical phonon energies observed in high-pressure experiments, while they fail at larger volumes, corresponding to the highest investigated temperatures. Moreover, QHA calculations more closely predict energies of transverse optical (TO) modes than those of longitudinal optical (LO) ones. This can be ascribed to known limitations in the modeling of the effective charges ($Z^{*}$) and dielectric constant (${\varepsilon }_{\infty }$) that lead to an underestimation of the LO-TO splitting. Based on the comparison of our experimental and theoretical results, we propose an empirical analytical expression for ${Z^{*}}^2/{\varepsilon }_{\infty }$ as a function of the atomic cell volume. Density-functional perturbation theory including phonon-phonon scattering up to the third order of the lattice potential expansion is used to calculate phonon widths. These calculations reproduce and explain remarkably well the nontrivial volume dependence of both TO and LO phonons linewidths determined by the experiments.

We use a magnetic field to manipulate the pinned spins in an Fe/FeO bilayer at room temperature, observing the behavior of the pinned magnetic moment via magnetometry. Our experiment reveals a linear relationship between the exchange bias field and the pinned magnetic moment. Based on the pinned uncompensated spin density at the interface as a function of the pinned moment, we provide a quantitative description of the exchange bias effect via exchange interaction of ferromagnetic and antiferromagnetic spins.

We study the effects of anisotropic order parameters on the temperature dependence of London penetration depth anisotropy ${\gamma }_{\lambda }\left(T\right)$. After ${\mathrm{MgB}}_2$, this dependence is commonly attributed to distinct gaps on multiband Fermi surfaces in superconductors. We have found, however, that the anisotropy parameter may depend on temperature also in one-band materials with anisotropic order parameters $\mathrm{\Delta }(T,{\mathit{k}}_F)$; a few such examples are given. We have also found that for different order parameters, the temperature dependence of $\mathrm{\Delta }\left(T\right)/\mathrm{\Delta }\left(0\right)$ can be represented with good accuracy by the interpolation suggested by Einzel [J. Low Temp. Phys 131, 1 (2003)], which simplifies considerably the evaluation of ${\gamma }_{\lambda }\left(T\right)$. Of particular interest are mixed order parameters of two symmetries for which ${\gamma }_{\lambda }\left(T\right)$ may go through a maximum for a certain relative weight of two phases. Also, for this case we find that the ratio ${\mathrm{\Delta }}_{max}\left(0\right)/T_c$ may exceed substantially the weak-coupling limit of 1.76. It, however, does not imply strong coupling; rather, it is due to significantly anisotropic angular variation of $\mathrm{\Delta }$.

Cycloidal spin orders are common in multiferroics. One of the prototypical examples is ${\mathrm{BiFeO}}_3$ (BFO) which shows a large polarization and a cycloidal antiferromagnetic (AFM) order at room temperature. Here, we employ Landau theory and phase-field simulations to analyze the coupled switching dynamics of polarization and cycloidal AFM orders in BFO. We identify 14 types of transitional spin structures between two cycloids and nine electric-field-induced spin switching paths. We demonstrate the electric-field-induced rotation of wave vectors of the cycloidal spins and discover two types of cycloidal spin switching dynamics: fast local spin flips and slow rotation of wave vectors. Also, we construct roadmaps to achieve the switching between any two spin cycloids through multistep applications of electric fields. This paper provides a theoretical framework for the phenomenological description of spin cycloids and a fundamental understanding of the switching mechanisms to achieve electrical control of magnetic orders.

Graphene with honeycomb structure, critically important in understanding physics of matter, exhibits exceptionally unusual half-integer quantum Hall effect and unconventional electronic spectrum with quantum relativistic phenomena. Particularly, graphenelike structure can be used for realizing topological insulator which inspires an intrinsic topological protection mechanism with strong immunity for maintaining coherence of quantum information. These various peculiar physics arise from the unique properties of Dirac cones which show high hole degeneracy, massless charge carriers, and linear intersection of bands. Experimental observation of Dirac cones conventionally focuses on the energy-momentum space with bulk measurement. Here, we demonstrate a direct observation of the movement of Dirac cones from single-photon dynamics in photonic graphene under different biaxial strains. Sharing the same spirit of wave-particle nature in quantum mechanics, we identify the movement of Dirac cones by dynamically detecting the edge modes and extracting the diffusing distance of the packets with accumulation and statistics on individual single-particle registrations. Our results of observing movement of Dirac cones from single-photon dynamics, together with the method of direct observation in real space by mapping the band structure defined in momentum space, paves a novel way to understand and explore a variety of artificial structures.