Here we report the optical and x-ray absorption spectra of the wide-band-gap oxide MgO using density functional theory and many-body perturbation theory (MBPT). Our comprehensive study of the electronic structure shows that while the band gap is underestimated with the exchange-correlation functional PBEsol (4.58 eV) and the hybrid functional HSE06 (6.58 eV) compared to the experimental value (7.7 eV), it is significantly improved (7.52 eV) and even overcompensated (8.53 eV) when quasiparticle corrections are considered. Inclusion of excitonic effects by solving the Bethe-Salpeter equation (BSE) yields the optical spectrum in excellent agreement with experiment. Excellent agreement is observed also for the O and Mg K-edge absorption spectra, demonstrating the importance of the electron-hole interaction within MBPT. Projection of the electron-hole coupling coefficients from the BSE eigenvectors on the band structure allows us to determine the origin of prominent peaks and identify the orbital character of the relevant contributions. The real-space projection of the lowest energy exciton wave function of the optical spectrum indicates a Wannier-Mott type, whereas the first exciton in the O K edge is more localized.

Although charge-spin separation has important consequences for the properties of one-dimensional (1D) Mott-Hubbard insulators, matrix elements in some of its dynamical correlation functions involve the coupling of spin and charge degrees of freedom. The corresponding interplay of the 1D Mott-Hubbard insulator's charge and spin degrees of freedom is an issue of both fundamental and technological interest, for instance, concerning their dynamics at subpicosecond timescales. On the other hand, the up- and down-spin one-particle spectral functions are the simplest dynamical correlation functions that involve excitation of both the charge and spin degrees of freedom. They are thus suitable to extract basic useful physical information on the above interplay at finite magnetic fields. Here the line shape of such functions at and in the $(k,\omega )$ plane's vicinity of their cusp singularities is studied for the Mott-Hubbard insulator described by the 1D Hubbard model with one fermion per site at zero and finite magnetic fields. At zero field, they can be accessed in terms of electrons by photoemission experiments. At finite field, such functions and corresponding interplay of correlations and magnetic-field effects refer to an involved nonperturbative many-particle problem that is poorly understood. The Mott-Hubbard gap that separates the addition and removal spectral functions is calculated for all spin densities and interaction values. The qualitative differences in the one-particle properties of the Mott-Hubbard insulator and corresponding doped insulator are also investigated. The relation of our theoretical results and predictions to both condensed-matter and ultracold spin-$1/2$ atom systems is discussed.

The superconducting phase of the ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ (Hg-1201) and ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$ (Hg-1223) cuprates has been investigated by Raman spectroscopy under hydrostatic pressure. Our analysis reveals that the increase of $T_{\mathrm{c}}$with pressure is slower in the Hg-1223 cuprate compared to Hg-1201 due to a charge carrier concentration imbalance accentuated by pressure in Hg-1223. We find that the energy variation under pressure of the apical oxygen mode from which the charge carriers are transferred to the ${\mathrm{CuO}}_2$ layer, is the same for both the Hg-1223 and Hg-1223 cuprates and it is controlled by the interlayer compressibility. At last, we show that the binding energy of the Cooper pairs related to the maximum amplitude of the $d$-wave superconducting gap at the antinodes does not follow $T_{\mathrm{c}}$with pressure. It decreases while $T_{\mathrm{c}}$increases. In the particular case of Hg-1201, the binding energy collapses from 10 to 2 $K_B{T}_{\mathrm{c}}$ as the pressure increases up to 10 GPa. These direct spectroscopic observations joined to the fact that the binding energy of the Cooper pairs at the antinodes does not follow $T_{\mathrm{c}}$either with doping, raises the question of its link with the pseudogap energy scale which follows the same trend with doping.

Coherent spin manipulations of spin-$\frac{3}{2}$ color center ensembles in 6H-SiC crystal have been studied in high magnetic fields using methods of pulsed electron paramagnetic resonance, Rabi oscillations, and pulsed electron-electron double resonance under optical alignment conditions of the spin level populations. Rabi oscillation experiments show room temperature coherent control of these spin-$\frac{3}{2}$ color center ensembles in strong magnetic fields. A sharp decrease of the spin-lattice relaxation time $T_1$, ∼40 times, was observed in 6H-SiC at magnetic field of ∼3.5 T with increasing temperature from 100 to 300 K, while the spin-spin relaxation time $T_2$ is only shortened by ∼1.3 times. With an increase in the magnetic field, the times $T_1$ and $T_2$ were shown to decrease. The relaxation time $T_1$ in the case of magnetic field directed along the axis of the spin-$\frac{3}{2}$ center is ∼2 times longer than $T_1$ in magnetic field perpendicular to this axis. Relaxation times of the spin center in crystal grown with a reduced concentration of an isotope ${}^{29}\mathrm{Si}$ are significantly longer than crystal, with the natural content of isotopes. With a decrease in the ${}^{29}\mathrm{Si}$ content in our experiments by a factor of ∼5, the effective nuclear spin bath in SiC is reduced by a factor of ∼2. In a zero magnetic field resonance, transitions are allowed as magnetic dipole transitions with frequency ${\omega }_0$ which correspond to the zero-field splitting. In zero magnetic field and in fixed magnetic fields, the Rabi frequency was shown, using so-called “Feynman-Vernon-Hellwarth transformation,” to be ${\omega }_R=\left|\gamma \right|B_1$. In pulsed electron-electron double resonance experiments, a change in the intensity of the electron spin echo signal corresponding to one of the spin-allowed fine structure transitions is recorded depending on the sweep of the second frequency. The experiments show the possibility to coherently detect the optical spin alignment between $M_S=\pm \frac{3}{2}$ via optically pumped silent $M_S=\pm \frac{1}{2}$ sublevels of the spin-$\frac{3}{2}$ color centers, including a detection of Rabi oscillations.

We study edge modes in topological semimetals, which have an energy band structure of ordinary semimetals but can be characterized by a Chern number. More specifically, we focus on a Qi-Wu-Zhang-type square-lattice model and a Haldane-type honeycomb model, both of which exhibit antichiral edge modes whose wave packets propagate in the same direction at both parallel edges of the strip. To obtain these analytical solutions of the edge modes, we apply the transfer matrix method, which was developed in the previous work [Phys. Rev. B 101, 014442 (2020)]. As a result, we show that the bulk-edge correspondence is broken down for a certain range of the model parameters. More precisely, when increasing the strength of a hopping amplitude of the Qi-Wu-Zhang-type model, the edge modes abruptly disappear, although the nontrivial Chern number does not change. In the Haldane-type model, for varying the model parameters, the edge modes do not necessarily disappear, and the nontrivial Chern number does not change. However, the energy spectral flows of the edge modes from the valence band to the conduction band are abruptly broken at a certain set of the model parameters.

Computational physics and chemistry are called to play a very important role in the development of new technologies based on two-dimensional (2D) materials, reducing drastically the number of trial and error experiments needed to obtain meaningful advances in the field. Here, we present a thorough theoretical study of the structural and electronic properties of the single-layer, double-layer, and bulk transition metal dichalcogenides ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, ${\mathrm{MoTe}}_2$, ${\mathrm{WS}}_2$, ${\mathrm{WSe}}_2$, and ${\mathrm{WTe}}_2$ in the $2H$ phase, for which only partial experimental information is available. We show that the properties of these systems depend strongly on the density functional theory approach used in the calculations and that inclusion of weak dispersion forces is mandatory for a correct reproduction of the existing experimental data. By using the most accurate functionals, we predict interlayer separations, direct and indirect band gaps, and spin-orbit splittings in those systems for which there is no experimental information available. We also discuss the variation of these properties with the specific chalcogen and transition metal atom.

We present two methods for constructing a flat-band (FB) system having a flat energy dispersion over the entire Brillouin zone within the tight-binding model, where the resulting Hamiltonian may not be easily obtained by existing methods based on a bipartite graph and line graph techniques. In the first method, we derive a set of conditions equivalent to the appearance of FBs for a given graph structure. This method allows parameters to be tuned so that systems with a small number of sites per unit cell have a FB. In the second method, we show that FB systems can be obtained by removing or adding sites to an existing FB system under specific rules. In particular, the site addition method enables us to construct multiple FB systems stemming from a single FB system. The FB system obtained by the second method has the characteristic that the component ratios in the FB eigenstate are partially common to the original system. We illustrate how lattices having a FB can be constructed by applying the latter method starting from an existing lattice such as a kagome lattice, demonstrating that a wide variety of lattices can possess a FB in the band structure.

We present an investigation into the optical properties of ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ at different temperatures from 300 down to 5 K over a broad frequency range. The optical conductivity at 5 K is decomposed into infrared-active phonons, a far-infrared band $\alpha$, a mid-infrared band $\beta$, and a high-energy absorption edge. By comparing the measured optical conductivity to first-principles calculations and the optical response of other nickelates, we find that ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ features evident charge-stripe fluctuations. The $\beta$ band is attributed to electronic transitions between the gapped Ni-$d_{x^2-y^2}$ bands due to fluctuating charge stripes, while the high-frequency absorption edge corresponds to the onset of transitions involving other high-energy bands. Furthermore, an analysis of the temperature-dependent optical spectral weight reveals a $T^2$ law, which is likely to originate from strong correlation effects.

Employing exact diagonalization, we systematically study the anisotropic Heisenberg model which is related to rare-earth triangular-lattice materials. We probe its full 3D phase diagram afresh and identify a large region of quantum spin liquid (QSL) phase which can extend to the QSL region of the $J_1\text{–}{J}_2$ triangular Heisenberg model. Furthermore, we explore the magnetization curves of different phases and reproduce the 1/3-magnetization plateau in the quantum spin liquid phase region. More importantly, to study the possible chemical disorders in real materials, we consider the randomness of exchange interactions and find no spin glass order. And there is a large region of random-singlet phase which contains strongly random spin networks, dominated by two-spin singlets, four-spin singlets and other singlet domains. Our comprehensive ED study can give detailed insightful understanding of the microscopic Hamiltonian related to the ${\mathrm{YbMgGaO}}_4$ and some other related rare-earth triangular-lattice materials.

In this paper, we derive a general formula for the quantized fractional corner charge in two-dimensional $C_n$-symmetric higher-order topological insulators. We assume that the electronic states can be described by the Wannier functions and that the edges are charge neutral, but we do not assume vanishing bulk electric polarization. We expand the scope of the corner charge formula obtained in previous works by considering more general surface conditions, such as surfaces with higher Miller index and surfaces with surface reconstruction. Our theory is applicable even when the electronic states are largely modulated near system boundaries. It also applies to insulators with nonvanishing bulk polarization, and we find that in such cases the value of the corner charge depends on the surface termination even for the same bulk crystal with $C_3$ or $C_4$ symmetry, via a difference in the Wyckoff position of the center of the $C_n$-symmetric crystal.

Light can interact with magnetism in materials. Motivated by the Edelstein effect, whereby a static electric field can generate magnetization in metals, in this work we theoretically and computationally demonstrate that static magnetization can also be generated through light in semiconductors. Such an effect is essentially a second-order nonlinear response and can be considered as a generalization of the Edelstein effect. This nonlinear Edelstein effect (NLEE) applies to semiconductors under both linearly and circularly polarized light, and there are no constraints from either spatial inversion or time-reversal symmetry. With ab initio calculations, we reveal several prominent features of NLEE. We find that the light-induced orbital magnetizations can be significantly greater than the spin magnetizations, in contrast to standard intrinsic magnetism where the orbital magnetic moment is strongly quenched under crystal field. We show that in multilayer (multisublattice) materials, different ferromagnetic and ferrimagnetic structures can be realized under photon pumping, depending on the interlayer (intersublattice) symmetry. It is also possible to switch the magnetic ordering in antiferromagnetic materials. The relationship between NLEE and other magneto-optic effects, including the inverse Faraday effect and inverse Cotton-Mouton effect, is also discussed.

Chiral Majorana hinge modes are characteristic of a second-order topological superconductor in three dimensions. Here we systematically study pairing symmetry and the leading pairing channels in Dirac materials with the point group $D_{2h}$, and find that the $s+id$-wave pairing superconductivity may exist as a consequence of competition between $s$- and $d$-wave pairing. The superconducting state is topologically nontrivial and possesses Majorana hinge and surface modes. The chiral Majorana hinge modes can be characterized by a winding number of the quadrupole moment or quantized quadruple moment at the symmetrically invariant point. Possible relevance to superconductivity in ${\mathrm{ZrTe}}_5$ is discussed. Our findings suggest the strong spin-orbital coupling, crystalline symmetries, and electron-electron interaction in the Dirac materials may provide a platform to realize chiral Majorana hinge modes with no proximity effect or external fields.

The time evolution of local operators in quantum compass models is characterized by simplicity as it can be represented as expanding and contracting strings of operators. Here we present an analytical solution to the problem of growth of a local energy operator in a quantum compass model on a Bethe lattice. We find a linear increase in time of the average operator length and a diffusive spreading of the operator length distribution. Using a moment method, we evaluate the local energy autocorrelation function that shows a Lorentzian shape at low frequencies. Furthermore, using a stochastic method, we visualize the expansion of the string cloud.

Topological materials host fascinating low dimensional gapless states at the boundary. As a prominent example, helical topological edge states (TESs) of two-dimensional topological insulators and their stacked three-dimensional equivalent, weak topological insulators (WTIs), have sparked research enthusiasm due to their potential application in the next generation of electronics/spintronics with low dissipation. Here, we propose the layered superconducting material CaSn as a WTI with nontrivial $Z_2$ as well as nodal-line semimetal protected by crystalline nonsymmorphic symmetry. Our systematic angle-resolved photoemission spectroscopy (ARPES) investigation on the electronic structure exhibits excellent agreement with the calculation. Furthermore, scanning tunneling microscopy/spectroscopy (STM/STS) at the surface step edge shows signatures of the expected TES. These integrated evidences from ARPES, STM/STS measurement, and corresponding ab initio calculation strongly support the existence of TES in the nonsymmorphic nodal-line semimetal CaSn, which may become a versatile material platform to realize multiple exotic electronic states as well as topological superconductivity.

The long coherence time of a single nitrogen vacancy (NV) center spin in diamond is a crucial advantage for implementing quantum information processing. However, the realization of strong coupling between single NV spins is challenging. Here, we propose a method to greatly enhance the interaction between two single NV spins in diamond which are only weakly coupled to an electromechanical cavity. Owing to the presence of a critical point for the linearized electromechanical subsystem, the coupling between a single NV spin and the high-frequency polariton (formed by the mechanical and cavity modes) can be fully decoupled, but the coupling between the single NV spin and the low-frequency polariton is, however, greatly enhanced. Thus, AC Stark shift of the single NV spin can be measured. With the low-frequency polariton as a quantum bus, a strong coupling between two single NV centers is achievable. This effective strong coupling can ensure coherent quantum-information exchange between two spin qubits in the weakly coupled spin-cavity electromechanical system.

We present the structural, electronic, vibrational, and photoluminescence properties of polycrystalline ${\mathrm{LaV}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_4$ $(x=0–0.2)$ samples at room temperature. The substitution of Nb at the V site shows the fascinating structural and optical behavior due to their isoelectronic character and larger ionic radii of ${\mathrm{Nb}}^{5+}$ as compared to the ${\mathrm{V}}^{5+}$. The Rietveld refinement of x-ray diffraction patterns demonstrate that the $x=0$ sample exists in a monoclinic $(P{2}_1/n)$ phase, whereas for the $x>0$, both monoclinic and scheelite-tetragonal $(I{4}_1/a)$ phases coexist in a certain proportion. Interestingly, a monotonous enhancement in the Raman spectral intensity with Nb substitution is correlated with the substitution induced increase in the scheelite-tetragonal phase. The x-ray absorption measurements reveal that the La ions exist in a trivalent oxidation state, while V and Nb cations possess $5+$ oxidation state in tetrahedral coordination. Moreover, the Fourier-transform infrared (FTIR) spectra indicate that the Nb substitution give origin to some additional IR modes owing to the deformation of the ${\mathrm{VO}}_4^{3-}$ tetrahedra and mixing of monoclinic and tetragonal phases. The photoluminescence measurements on these samples exhibit broadband spectra and their deconvolution designate the availability of more than one electron-hole pairs recombination center.

Systematic change of structural transition and high-spin (HS) to low-spin (LS) transition of ${\mathrm{Fe}}^{2+}$ in synthetic $\left({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4}\right)\mathrm{O}$-ferropericlase for pressures up to that of the lower mantle region were investigated using synchrotron x-ray diffraction (XRD) and synchrotron Mössbauer spectroscopic methods. The XRD patterns and the Mössbauer spectra were measured up to 160 GPa at room temperature. The results of the synchrotron XRD analysis indicate that the cubic structure of $\left({\mathrm{Mg}}_{0.6}{\mathrm{Fe}}_{0.4}\right)\mathrm{O}$-ferropericlase is maintained up to 160 GPa. The Mössbauer spectra at 19.8 and 24.0 GPa consist of three doublets assigned to HS ${\mathrm{Fe}}^{2+}$ at the octahedral site. At pressures from 61 to 136 GPa, a singlet assigned to LS ${\mathrm{Fe}}^{2+}$ is added to the three HS ${\mathrm{Fe}}^{2+}$ doublets, and its area ratio with respect to the HS ${\mathrm{Fe}}^{2+}$ doublets gradually increase with increasing pressure. At pressures above 136 GPa, the Mössbauer spectra consist of only an LS ${\mathrm{Fe}}^{2+}$ singlet, implying that all Fe at these pressures is in a LS state. The resulting spin crossover pressure interval from 61 to 136 GPa indicates the coexistence of both HS and LS ${\mathrm{Fe}}^{2+}$ at pressure conditions from the upper part to the bottom of the lower mantle.