In the first part of the present paper, the two-photon absorption spectrum of liquid water is calculated using an ab initio real-time many-body approach. Correlation effects, such as single-particle and excitonic effects, are included, with the latter showing large impact on the shape and peak positions of the two-photon spectrum. A broad peak at 10.0 eV is calculated, in excellent agreement with the experimental reference. In the second part, we show the impact of the on- and off-diagonal elements of the self-energy operator upon $GW$ self-consistency (depending on the number of quasiparticle-corrected unoccupied bands) on the electronic band gap of liquid water at the $\mathrm{\Gamma }$ point. The off-diagonal self-energy elements increase the electronic gap, correcting previous results reported by W. Chen et al. [Phys. Rev. Lett. 117, 186401 (2016)] by 0.52 eV, and putting at the same time the non-self-consistent bootstrap approximation for the vertex function into question. Furthermore, depending on the calculated band gap using different $GW$ flavors, the absolute position of the absorption band of the nonlinear spectrum is affected.

We consider a helical system of fermions with a generic spin- (or pseudo-spin-) orbit coupling. Using the equation of motion approach for the single-particle distribution functions and a mean-field decoupling of the higher-order distribution functions, we find a closed form for the charge- and spin-density fluctuations in terms of the charge- and spin-density linear-response functions. Approximating the nonlocal exchange term with a Hubbard-like local-field factor, we obtain coupled spin- and charge-density response matrices beyond the random phase approximation, whose poles give the dispersion of four collective spin-charge modes. We apply our generic technique to the well-explored two-dimensional system with Rashba spin-orbit coupling and illustrate how it gives results for the collective modes, Drude weight, and spin-Hall conductivity, which are in very good agreement with the results obtained from other more sophisticated approaches.

The coherent charge transport through an illuminated graphene ribbon is studied as a function of electronic doping and characteristics of the electromagnetic driving, for monochromatic circularly polarized light. We focus on the DC current carried by 2D bulk carriers which is dominant (over edge transport) for short and wide enough samples. We investigate how the ballistic conductance suppression, due to photon resonances between the valence and conduction bands, evolves when the experimentally tunable parameters are varied. The residual conductance can be associated with evanescent states and related to dynamical gaps in the Floquet quasienergy spectrum.

We show that nonuniform superconductors of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type conduct electric current in a way which is very different from the usual case. We discuss both equilibrium and nonequilibrium properties using a modified Ginzburg-Landau formalism. Among the novel features are the existence of two different critical currents and two distinct stable states able to carry a given current, the possibility of superconducting domain walls, and also a spontaneous supercurrent in a ring geometry.

Quantum spin clusters provide a platform for the experimental study of many-body entanglement. Here we address a simple model of a single-molecule nanomagnet featuring $N$ interacting spins in a transverse field. The field can control an entanglement transition (ET). We calculate the magnetization, low-energy gap, and neutron-scattering cross section and find that the ET has distinct signatures, detectable at temperatures as high as 5% of the interaction strength. The signatures are stronger for smaller clusters.

Chiral superconductors support chiral edge modes and potentially spontaneous edge currents at their boundaries. Motivated by the putative multiband chiral $p$-wave superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we study the influence of the interference between different bands at the edges, which may appear in the presence of moderate edge disorder or in edge tunneling measurements. We show that interband interference can strongly modify the measurable quantities at the edges when the order parameter exhibits phase difference between the bands. This is illustrated by investigating the edge dispersion and the edge current distribution in the presence of interband mixing, as well as the conductance at a tunneling junction. The results are discussed in connection with the putative chiral $p$-wave superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. In passing, we also discuss similar interference effects in multiband models with other pairing symmetries.

We study the effects of spin-orbit coupling on the magnetoconductivity in diffusive cylindrical semiconductor nanowires. Following up on our former study on tubular semiconductor nanowires, we focus in this paper on nanowire systems where no surface accumulation layer is formed but instead the electron wave function extends over the entire cross section. We take into account the Dresselhaus spin-orbit coupling resulting from a zinc-blende lattice and the Rashba spin-orbit coupling, which is controlled by a lateral gate electrode. The spin relaxation rate due to Dresselhaus spin-orbit coupling is found to depend neither on the spin density component nor on the wire growth direction and is unaffected by the radial boundary. In contrast, the Rashba spin relaxation rate is strongly reduced for a wire radius that is smaller than the spin precession length. The derived model is fitted to the data of magnetoconductance measurements of a heavily doped back-gated InAs nanowire and transport parameters are extracted. At last, we compare our results to previous theoretical and experimental studies and discuss the occurring discrepancies.

The spectral approach to infinite disordered crystals is applied to an Anderson-type Hamiltonian to demonstrate the existence of extended states for nonzero disorder in 2D lattices of different geometries. The numerical simulations shown prove that extended states exist for disordered honeycomb, triangular, and square crystals. This observation stands in contrast to the predictions of scaling theory, and aligns with experiments in photonic lattices and electron systems. The method used is the only theoretical approach aimed at showing delocalization. A comparison of the results for the three geometries indicates that the triangular and honeycomb lattices experience transition in the transport behavior for similar levels of disorder, which is to be expected from the planar duality of the lattices. This provides justification for the use of artificially prepared triangular lattices as analogues for honeycomb materials, such as graphene. The analysis also shows that the transition in the honeycomb case happens more abruptly compared to the other two geometries, which can be attributed to the number of nearest neighbors. We outline the advantages of the spectral approach as a viable alternative to scaling theory and discuss its applicability to transport problems in both quantum and classical 2D systems.

Spin-polarized electronic structures are the cornerstone of spintronics, and have thus attracted a significant amount of interest; in particular, researchers are looking into how to modulate the electronic structure to enable multifunctional spintronics applications, especially in half-metallic systems. However, the control of the spin polarization has only been predicted in limited two-dimensional systems with spin-polarized Dirac structures and is difficult to achieve experimentally. Here, we report the modulation of the electronic structure in the light-induced excited state in a typical half-metal, $\mathrm{L}{\mathrm{a}}_{1/2}\mathrm{S}{\mathrm{r}}_{1/2}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$. According to the spin-transport measurements, there appears a light-induced increase in magnetoresistance due to the enhanced spin scattering, which is closely associated with the excited spin polarization. Strikingly, the light-induced variation can be enhanced via alcohol processing and reduced by oxygen annealing. X-ray photoelectron spectroscopy measurements show that in the chemical process, a redox reaction occurs with a change in the valence of Mn. Furthermore, first-principles calculations reveal that the change in the valence of Mn alters the electronic structure and consequently modulates the spin polarization in the excited state. Our findings thus report a chemically tunable electronic structure, demonstrating interesting physics and the potential for multifunctional applications and ultrafast spintronics.

Analytic expressions for chemical potentials without any approximations are derived for all types of extrinsic (doped) gapped Dirac-cone materials including gapped graphene, silicene, germanene, and single-layer transition-metal dichalcogenides. In setting up our derivations, a reliable piecewise-linear model has been established for calculating the density of states in molybdenum disulfide, showing good agreement with previously obtained numerical results. For spin- and valley-resolved band structures, a decrease of chemical potential with increasing temperature is found as a result of enhanced thermal populations of an upper subband. Due to the broken symmetry with respect to electron and hole states in ${\mathrm{MoS}}_2$, the chemical potential is shown to cross a zero-energy point at sufficiently high temperatures. It is important to mention that the chemical potential at a fixed temperature can still be tuned by varying the doping density and band structure of a system with an external electric or strain field. Since a thermal-convolution path (or a chemical-potential-dependent response function for the thermal convolution of fermions) starting from zero temperature must be selected in advance before obtaining finite-temperature properties of any collective quantities, e.g., polarizability, plasmon modes, and damping, a control of their thermal dependence within a certain temperature range is expected for field-tunable extrinsic gapped Dirac-cone materials.

Exotic spin-multipolar ordering in spin transition metal insulators has so far eluded unambiguous experimental observation. A less studied, but perhaps more feasible fingerprint of multipole character emerges in the excitation spectrum in the form of quadrupolar transitions. Such multipolar excitations are desirable as they can be manipulated with the use of light or electric field and can be captured by means of conventional experimental techniques. Here we study single crystals of multiferroic ${\mathrm{Sr}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ and observe a two-magnon spin excitation appearing above the saturation magnetic field in electron spin resonance (ESR) spectra. Our analysis of the selection rules reveals that this spin excitation mode does not couple to the magnetic component of the light, but it is excited by the electric field only, in full agreement with the theoretical calculations. Due to the nearly isotropic nature of ${\mathrm{Sr}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$, we identify this excitation as a purely spin-quadrupolar two-magnon mode.

The magnetism of ordered and disordered ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ is explained using a model involving double exchange and superexchange. An important feature of this model is the majority spin hybridization in the large coupling limit, which results in ferromagnetism rather than ferrimagnetism as in ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$. The ferromagnetic insulating ground state in the ordered phase is explained. The essential role played by the Ni-Mn superexchange between the Ni $e_g$ electron spins and the Mn $t_{2g}$ core electron spins in realizing this ground state is outlined. In the presence of antisite disorder, the model system is found to exhibit a tendency of becoming a spin glass at low temperatures, while it continues to retain a ferromagnetic transition at higher temperatures, similar to recent experimental observations [D. Choudhury et al., Phys. Rev. Lett. 108, 127201 (2012)]. This reentrant spin glass or reentrant ferromagnetic behavior is explained in terms of the competition of the ferromagnetic double exchange between the Ni $e_g$ and the Mn $e_g$ electrons, and the ferromagnetic Ni-Mn superexchange, with the antiferromagnetic antisite Mn-Mn superexchange.

A computationally efficient, yet general, free-energy modeling scheme is developed based on first-principles calculations. Finite-temperature disorder associated with the fast (electronic and magnetic) degrees of freedom is directly included in the electronic structure calculations, whereas the vibrational free energy is evaluated by a proposed model that uses elastic constants to calculate average sound velocity of the quasiharmonic Debye model. The proposed scheme is tested by calculating the lattice parameter, heat capacity, and single-crystal elastic constants of $\alpha$-, $\gamma$-, and $\delta$-iron as functions of temperature in the range 1000–1800 K. The calculations accurately reproduce the well-established experimental data on thermal expansion and heat capacity of $\gamma$- and $\delta$-iron. Electronic and magnetic excitations are shown to account for about 20% of the heat capacity for the two phases. Nonphonon contributions to thermal expansion are 12% and 10% for $\alpha$- and $\delta$-Fe and about 30% for $\gamma$-Fe. The elastic properties predicted by the model are in good agreement with those obtained in previous theoretical treatments of paramagnetic phases of iron, as well as with the bulk moduli derived from isothermal compressibility measurements [N. Tsujino et al., Earth Planet. Sci. Lett. 375, 244 (2013)]. Less agreement is found between theoretically calculated and experimentally derived single-crystal elastic constants of $\gamma$- and $\delta$-iron.

We derive the linearized ferromagnetic high-frequency susceptibility tensor in explicit form, which can be evaluated for arbitrary micromagnetic energy functionals and without imposing a priori symmetry constraints. For energy functionals defined spatially, our method can be used to describe the magnon dispersion. Our approach is also applicable to magnetic systems in a nonsaturated state and can thus be combined with micromagnetic simulations. To find the equilibrium states of the magnetization, we suggest a trajectory-dependent solver, which allows for fast fitting of experimental ferromagnetic resonance spectra.

Recently, Prakash et al. have discovered bulk superconductivity in single crystals of bismuth, which is a semimetal with extremely low carrier density. At such low density, we argue that conventional electron-phonon coupling is too weak to be responsible for the binding of electrons into Cooper pairs. We study a dynamically screened Coulomb interaction with effective attraction generated on the scale of the collective plasma modes. We model the electronic states in bismuth to include three Dirac pockets with high velocity and one hole pocket with a significantly smaller velocity. We find a weak-coupling instability, which is greatly enhanced by the presence of the hole pocket. Therefore we argue that bismuth is the first material to exhibit superconductivity driven by retardation effects of Coulomb repulsion alone. By using realistic parameters for bismuth we find that the acoustic plasma mode does not play the central role in pairing. We also discuss a matrix element effect, resulting from the Dirac nature of the conduction band, which may affect $T_c$ in the $s$-wave channel without breaking time-reversal symmetry.

Usually $Z_2$ topological insulators are protected by time-reversal symmetry. Here we present a new type of $Z_2$ topological insulators in a cubic lattice which is protected by a novel hidden symmetry, while time-reversal symmetry is broken. The hidden symmetry has a composite antiunitary operator consisting of fractional translation, complex conjugation, sublattice exchange, and local gauge transformation. Based on the hidden symmetry, we define the hidden symmetry polarization and $Z_2$ topological invariant to characterize the topological insulators. The surface states have band structures with an odd number of Dirac cones, where pseudospin-momentum locking occurs. When the hidden symmetry-breaking perturbations are added on the boundaries, a gap opens in the surface band structure, which confirms that the topological insulator and the surface states are protected by the hidden symmetry. We also discuss the realization and detection of this new kind of $Z_2$ topological insulator in optical lattices with ultracold atom techniques.

Polymorph of $8-Pmmn$ borophene exhibits anisotropic tilted Dirac cones. In this work, we explore the consequences of the tilted Dirac cones in magnetotransport properties of a periodically modulated borophene. We evaluate modulation-induced diffusive conductivity by using linear response theory in low temperature regime. The application of weak spatial modulation (electric, magnetic or both) gives rise to the magnetic-field-dependent nonzero oscillatory drift velocity which causes Weiss oscillation in the longitudinal conductivity at low magnetic field. The Weiss oscillation is studied in the presence of a weak spatial electric, magnetic, and both modulations individually. The tilting of the Dirac cones gives rise to an additional contribution to the Weiss oscillation in longitudinal conductivity. Moreover, it also enhances the frequency of the Weiss oscillation and modifies its amplitude too. Most remarkably, it is found that the presence of both out-of-phase electric and magnetic modulations can cause a sizable valley polarization in diffusive conductivity. The origin of valley polarization lies in the opposite tilting of the two Dirac cones at two valleys.