Optical properties of the quasi-two-dimensional single-component molecular Mott insulator [Au(Et-thiazdt)${}_2]$ (Et-thiazdt = N-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate) have been investigated under pressure at room temperature. At 1.5 GPa, [Au(Et-thiazdt)${}_2]$ undergoes an insulator to metal transition (IMT). Optical conductivity spectra exhibit a clear Drude peak at high pressure. In addition, we observed a clear anisotropy of pressure-induced modifications of the electronic structure. With increasing pressure, along the molecule stacks, a strong increase of the spectral weight below 1 eV is observed, while in the transverse direction, it remains barely constant with a redistribution from midinfrared to low energy. Besides the increase of the singly occupied molecular orbital (SOMO) bandwidth, calculations show that the SOMO-1 bands cross the Fermi level at the transition. Moreover, we have calculated the optical conductivity as a function of pressure to provide a picture of the compound physics under 1 eV. Our results indicate that the pressure-induced IMT is simultaneously due to a bandwidth and a band-filling phenomenon that imply both Mott physics and uncorrelated charge carriers.

We identify direct-transition photoemission peaks from the bulk valence bands of Si in energy- and momentum-resolved photoemission from Si(111)-($7\times 7$) using polarized 6-eV laser light. Time-resolved study of spectral line shapes of the peaks under interband excitation by 2-eV femtosecond-laser pulses shows the ultrafast transient spectral-width broadening and its recovery associated with a low-energy peak shift. The changes reveal the dynamics of screening effects by electron-hole plasma, hot-hole relaxation, and band renormalization in photoexcited Si, showing strong many-body effects in relaxation at excitation density less than ${10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$.

The real-space atomic surface structure of mercury cadmium telluride was successfully achieved on the $\left(111\right)B$ surface of $\mathrm{H}{\mathrm{g}}_{0.78}\mathrm{C}{\mathrm{d}}_{0.22}\mathrm{Te}$ by ultrahigh-vacuum scanning tunneling microscopy (STM). The work casts light on the reconstructions of the $\left(111\right)B$ surface unraveling a $(2\times 2)$ surface reconstruction induced by adatom adsorption of Cd. The other $(2\times 2)$ surface reconstruction is clarified to be induced by the single Te vacancy, which is more stable than the reconstruction of multivacancies in contrast to the prevailing view. The simulated STM images are in good agreement with the experiments. We also observed an in situ morphology transition from the $(1\times 1)$ structure to those $(2\times 2)$ reconstructions, implying the stability of the reconstructions.

We present a comparative theoretical study of magnetic resonance within the polaron pair recombination (PPR) and the triplet exciton-polaron quenching (TPQ) models. Both models have been invoked to interpret the photoluminescence detected magnetic resonance (PLDMR) results in $\pi$-conjugated materials and devices. We show that resonance line shapes calculated within the two models differ dramatically in several regards. First, in the PPR model, the line shape exhibits unusual behavior upon increasing the microwave power: it evolves from fully positive at weak power to fully negative at strong power. In contrast, in the TPQ model, the PLDMR is completely positive, showing a monotonic saturation. Second, the two models predict different dependencies of the resonance signal on the photoexcitation power, $P_L$. At low $P_L$, the resonance amplitude $\mathrm{\Delta }I/I$ is $\propto P_L$ within the PPR model, while it is $\propto P_L^2$ crossing over to $P_L^3$ within the TPQ model. On the physical level, the differences stem from different underlying spin dynamics. Most prominently, a negative resonance within the PPR model has its origin in the microwave-induced spin-Dicke effect, leading to the resonant quenching of photoluminescence. The spin-Dicke effect results from the spin-selective recombination, leading to a highly correlated precession of the on-resonance pair partners under the strong microwave power. This effect is not relevant for TPQ mechanism, where the strong zero-field splitting renders the majority of triplets off resonance. On the technical level, the analytical evaluation of the line shapes for the two models is enabled by the fact that these shapes can be expressed via the eigenvalues of a complex Hamiltonian. This bypasses the necessity of solving the much larger complex linear system of the stochastic Liouville equations. Our findings pave the way towards a reliable discrimination between the two mechanisms via cw PLDMR.

We study the linear response of doped three-dimensional Dirac and Weyl semimetals to vector potentials, by calculating the wave-vector- and frequency-dependent current-current response function analytically. The longitudinal part of the dynamic current-current response function is then used to study the plasmon dispersion and the optical conductivity. The transverse response in the static limit yields the orbital magnetic susceptibility. In a Weyl semimetal, along with the current-current response function, all these quantities are significantly impacted by the presence of parallel electric and magnetic fields (a finite $\mathbf{E}·\mathbf{B}$ term) and can be used to experimentally explore the chiral anomaly.

We present a first-principles investigation of the electronic properties of monolayer ${\mathrm{WS}}_2$ coated with an overlayer of Fe. Our ab initio calculations reveal that the system is a half-metallic ferromagnet with a gap of $\sim 1$ eV for the majority spin channel. Furthermore, the combined effect of time-reversal symmetry breaking due to the magnetic Fe overlayer and the large spin-orbit coupling induced by W gives rise to nondegenerate K and ${\mathrm{K}}^{\prime }$ valleys. This has a tremendous impact on the excited-state properties induced by externally applied circularly polarized light. Our analysis demonstrates that the latter induces a singular hot-spot structure of the transition probability around the K and ${\mathrm{K}}^{\prime }$ valleys for right and left circular polarization, respectively. We trace back the emergence of this remarkable effect to the strong momentum dependent spin-noncollinearity of the valence band involved. As a main consequence, a strong valley-selective magnetic circular dichroism is obtained, making this system a prime candidate for spintronics and photonics applications.

The effect of applying an external electric field on doping-induced spin-orbit splitting of the lowest conduction-band states in a bismuth-doped zinc oxide nanowire is studied by performing electronic structure calculations within the framework of density functional theory. It is demonstrated that spin splitting in Bi-doped ZnO nanowires could be tuned and enhanced electrically via control of the strength and direction of the applied electric field, thanks to the nonuniform and anisotropic response of the ZnO:Bi nanowire to external electric fields. The results reported here indicate that a single ZnO nanowire doped with a low concentration of Bi could function as a spintronic device, the operation of which is controlled by applied lateral electric fields.

Noncentrosymmetric metals are anticipated to exhibit a dc photocurrent in the nonlinear optical response caused by the Berry curvature dipole in momentum space. Weyl semimetals (WSMs) are expected to be excellent candidates for observing these nonlinear effects because they carry a large Berry curvature concentrated in small regions, i.e., near the Weyl points. We have implemented the semiclassical Berry curvature dipole formalism into an ab initio scheme and investigated the second-order nonlinear response for two representative groups of materials: the TaAs-family type-I WSMs and the ${\mathrm{MoTe}}_2$-family type-II WSMs. Both types of WSMs exhibited a Berry curvature dipole in which type-II Weyl points are usually superior to the type-I WSM because of the strong tilt. Corresponding nonlinear susceptibilities in several materials promise a nonlinear Hall effect in the dc field limit, which is within the experimentally detectable range.

Resonant inelastic x-ray scattering (RIXS) experiments performed at the oxygen $K$ edge on the iridate perovskites ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ reveal a sequence of well-defined dispersive modes over the energy range up to $\sim 0.8\mathrm{eV}$. The momentum dependence of these modes and their variation with the experimental geometry allows us to assign each of them to specific collective magnetic and/or electronic excitation processes, including single and bimagnons, and spin-orbit and electron-hole excitons. We thus demonstrate that dispersive magnetic and electronic excitations are observable at the O $K$ edge in the presence of the strong spin-orbit coupling in the $5d$ shell of iridium and strong hybridization between Ir $5d$ and O $2p$ orbitals, which confirm and expand theoretical expectations. More generally, our results establish the utility of O $K$-edge RIXS for studying the collective excitations in a range of $5d$ materials that are attracting increasing attention due to their novel magnetic and electronic properties. Especially, the strong RIXS response at O $K$ edge opens up the opportunity for investigating collective excitations in thin films and heterostructures fabricated from these materials.

Silicene, the two-dimensional allotrope of silicon, is predicted to exist in a low-buckled honeycomb lattice, characterized by semimetallic electronic bands with graphenelike energy-momentum dispersions around the Fermi level (represented by touching Dirac cones). Single layers of silicene are mostly synthesized by depositing silicon on top of silver, where, however, the different phases observed to date are so strongly hybridized with the substrate that not only the Dirac cones, but also the whole valence and conduction states of ideal silicene appear to be lost. Here, we provide evidence that at least part of this semimetallic behavior is preserved by the coexistence of more silicene phases, epitaxially grown on Ag(111). In particular, we combine electron energy loss spectroscopy and time-dependent density functional theory to characterize the low-energy plasmon of a multiphase-silicene/Ag(111) sample, prepared at controlled silicon coverage and growth temperature. We find that this mode survives the interaction with the substrate, being perfectly matched with the $\pi$-like plasmon of ideal silicene. We therefore suggest that the weakened interaction of multiphase silicene with the substrate may provide a unique platform with the potential to develop different applications based on two-dimensional silicon systems.

Describing correlated electron systems near phase transitions has been a major challenge in computational condensed-matter physics. In this paper, we apply highly accurate fixed-node quantum Monte Carlo techniques, which directly work with many-body wave functions and simulate electron correlations, to investigate the metal-to-insulator transition of a correlated hydrogen lattice. By calculating spin and charge properties, and analyzing the low-energy Hilbert space, we identify the transition point and identify order parameters that can be used to detect the transition. Our results provide a benchmark for density functional theories seeking to treat correlated electron systems.

Re-based double perovskites (DPs) have garnered substantial attention due to their high Curie temperatures ($T_C$) and display of complex interplay of structural and metal-insulator transitions (MIT). Here we systematically study the ground-state electronic and structural properties for a family of Re-based DPs $A_{2}B{\mathrm{ReO}}_6$ ($A$=Sr, Ca and $B$=Cr, Fe), which are related by a common low-energy Hamiltonian, using density functional theory +$U$ calculations. We show that the on-site interaction $U$ of Re induces orbital ordering (denoted C-OO), with each Re site having an occupied $d_{xy}$ orbital and a C-type alternation among $d_{xz}/d_{yz}$, resulting in an insulating state consistent with experimentally determined insulators ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$, ${\mathrm{Ca}}_2{\mathrm{CrReO}}_6$, and ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$. The threshold value of $U_{Re}$ for orbital ordering is reduced by inducing $E_g$ octahedral distortions of the same C-type wavelength (denoted C-OD), which serves as a structural signature of the orbital ordering; octahedral tilting also reduces the threshold. The C-OO and the concomitant C-OD are a spontaneously broken symmetry for the Sr-based materials (i.e., $a^0{a}^0{c}^{-}$ tilt pattern), while not for the Ca-based systems (i.e., $a^{-}{a}^{-}{b}^{+}$ tilt pattern). Spin-orbit coupling does not qualitatively change the physics of the C-OO/C-OD, but can induce relevant quantitative changes. We prove that a single set of $U_{Cr},U_{Fe},U_{Re}$ capture the experimentally observed metallic state in ${\mathrm{Sr}}_2{\mathrm{FeReO}}_6$ and insulating states in other three systems. We predict that the C-OO is the origin of the insulating state in ${\mathrm{Sr}}_2{\mathrm{CrReO}}_6$, and that the concomitant C-OD may be experimentally observed at sufficiently low temperatures (i.e., space group $P{4}_2/m$) in pure samples. Additionally, given our prescribed values of $U$, we show that the C-OO induced insulating state in ${\mathrm{Ca}}_2{\mathrm{CrReO}}_6$ will survive even if the C-OD amplitude is suppressed (e.g., due to thermal fluctuations). The role of the C-OO/C-OD in the discontinuous, temperature driven MIT in ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$ is discussed.

We investigate excitation dynamics in the system of a quantum dipole emitter (QDE) coupled to a located nearby metal nanoparticle (MNP), which exhibits a dipolar localized surface plasmon (LSP) resonance at the frequency of the QDE radiative transition, in the presence of a strong external resonant electromagnetic field. Considering the QDE-field interactions in the regime of strong QDE-field coupling, we show that the feedback provided by the MNP on the QDE (due to the LSP excitation with the field generated by the dipole moment of the QDE transition) influences significantly the coherent process of Rabi oscillations, resulting in the occurrence of additional satellite frequencies in the radiation spectrum scattered by the QDE-MNP configuration. The relative ratio of high harmonics depends strongly on the QDE-MNP separation, an important characteristic feature that can be used for observing this effect and can be exploited, for example, for controlling distances at the nanoscale.

We present a framework to calculate the anisotropic and nonlinear photoconductivity for two band systems with application to graphene. In contrast to the usual perturbative (second order in the optical field strength) techniques, we calculate photoconductivity to all orders in the optical field strength. In particular, for graphene, we find the photoresponse to be giant (at large optical field strengths) and anisotropic. The anisotropic photoresponse in graphene is correlated with polarization of the incident field, with the response being similar to that of a half-wave plate. We predict that the anisotropy in the simultaneous measurement of longitudinal (${\sigma }_{xx}$) and transverse $\left({\sigma }_{yx}\right)$ photoconductivity, with four probes, offers a unique experimental signature of the photovoltaic response, distinguishing it from the thermal-Seebeck and bolometric effects in photoresponse.

Epitaxial Bi(111) films were subject to many and partly even controversial studies on the semimetal-semiconductor transition triggered by a robust quantum confinement. The residual conductance was ascribed to conducting surface channels. We investigated ultrathin crystalline Bi films on Si(111) as a function of film thickness $d$ between 20 and 100 bilayers by means of electric transport measurements. Varying temperature and magnetic field, we disentangled two transport channels. One remains indeed metallic at all thicknesses investigated and exhibits a slightly increasing conductance as a function of $d$, whereas the second is activated with a $d^{-1}$ dependence of the activation energy, indicating a quasiharmonic confining potential. Both channels reflect the electronic properties of the entire film and do not allow us to strictly separate surface and bulk states. While there is clearly no bulk conductivity, the activated channel is consistently described as electronic excitation into the partly occupied quantum well states, which are also responsible for the metallic conductance and preferentially located close to both interfaces of the film.

We study theoretically the detection of the topological phase transition occurring in Rashba nanowires with proximity-induced superconductivity using a quantum dot. The bulk states lowest in energy of such a nanowire have a spin polarization parallel or antiparallel to the applied magnetic field in the topological or trivial phase, respectively. We show that this property can be probed by the quantum dot created at the end of the nanowire by external gates. By tuning one of the two spin-split levels of the quantum dot to be in resonance with nanowire bulk states, one can detect the spin polarization of the lowest band via transport measurement. This allows one to determine the topological phase of the Rashba nanowire independently of the presence of Majorana bound states.

We study Landau-Zener-Stückelberg-Majorana (LZSM) interferometry under the influence of projective readout using a charge qubit tunnel-coupled to a fermionic sea. This allows us to characterize the coherent charge-qubit dynamics in the strong-driving regime. The device is realized within a silicon complementary metal-oxide-semiconductor (CMOS) transistor. We first read out the charge state of the system in a continuous nondemolition manner by measuring the dispersive response of a high-frequency electrical resonator coupled to the quantum system via the gate. By performing multiple fast passages around the qubit avoided crossing, we observe a multipassage LZSM interferometry pattern. At larger driving amplitudes, a projective measurement to an even-parity charge state is realized, showing a strong enhancement of the dispersive readout signal. At even larger driving amplitudes, two projective measurements are realized within the coherent evolution resulting in the disappearance of the interference pattern. Our results demonstrate a way to increase the state readout signal of coherent quantum systems and replicate single-electron analogs of optical interferometry within a CMOS transistor.