Borophene is a monolayer materials made of boron. A perfect planar boropehene called ${\beta }_{12}$ borophene has Dirac cones and they are well reproduced by a tight-binding model according to recent experimental and first-principles calculation results. We explicitly derive a Dirac theory for ${\beta }_{12}$ borophene. Dirac cones are gapless when the inversion symmetry exists, while they are gapped when it is broken. In addition, three-band touching points emerge together with pseudospin triplet fermions when all transfer energy is equal and all onsite energy is equal. The three-band touching is slightly resolved otherwise. We construct effective three-band theories for triplet fermions. We also study the edge states of borophene nanoribbons, which show various behaviors depending on the way of edge terminations.

We report the investigation of the size evolution of lattice dynamics in spin crossover coordination nanoparticles of $\mathrm{[}\mathrm{Fe}\left(\mathrm{pyrazine}\right)\left(\mathrm{Ni}{\left(\mathrm{CN}\right)}_4\right)]$ through nuclear inelastic scattering (NIS) measurements. Vibrational properties in these bistable molecular materials are of paramount importance and NIS permits access to the partial vibrational density of states in both spin states [high spin (HS) and low spin (LS)] from which thermodynamical and mechanical properties can be extracted. We show that the size reduction leads to the presence of inactive metal centers with the coexistence of HS and LS vibrational modes. The confinement effect has only weak impact on the vibrational properties of nanoparticles, especially on the optical modes which remain almost unchanged. On the other hand, the acoustic modes are much more affected which results in the increase of the vibrational entropy and also the Debye sound velocity in the smallest particles $(<10$ nm) in both spin states. This stiffening may be due to the elastic surface stress exerted by the external environment. An evidence of the influence of the host matrix on the vibrational properties of the nanoparticles is also highlighted through the matrix dependence of the sound velocity.

In the first part of this work, an experimental study of the lattice dynamics of spin crossover nanoparticles was performed using the nuclear inelastic scattering (NIS). A size dependence of low energy phonon modes appears under 10 nm, but its origin is not well understood. In this paper, we investigate the phonon confinement effects in the framework of molecular dynamics simulations by modeling three-dimensional nanoparticles considering a cubic lattice with an octahedral pattern. The vibrational density of states is computed and compared to the experiment. The simulations allow one to highlight both the role of the phonon quantification and the role of the size and shape distributions of particles on the extracted parameters leading to a better understanding of the experimental results.

We develop a powerful simulation method that can treat electronic transport in a super-micron-scale open system with atomic vibration at finite temperature. As an application of the developed method to realistic materials, we simulate electronic transport in metallic single-walled carbon nanotubes from nanometer scale to micrometer scale at room temperature. Based on the simulation results, we successfully identify two different crossovers, namely, ballistic to diffusive crossover and coherent to incoherent crossover, simultaneously and with equal footing, from which the mean free path and the phase coherence length can be extracted clearly. Moreover, we clarify the scaling behavior of the electrical resistance and the electronic current in the crossover regime.

Second-harmonic scattering (SHS) experiments provide a unique approach to probe noncentrosymmetric environments in aqueous media, from bulk solutions to interfaces, living cells, and tissue. A central assumption made in analyzing SHS experiments is that each molecule scatters light according to a constant molecular hyperpolarizability tensor ${\mathit{\beta }}^{\left(2\right)}$. Here, we investigate the dependence of the molecular hyperpolarizability of water on its environment and internal geometric distortions, in order to test the hypothesis of constant ${\mathit{\beta }}^{\left(2\right)}$. We use quantum chemistry calculations of the hyperpolarizability of a molecule embedded in point-charge environments obtained from simulations of bulk water. We demonstrate that both the heterogeneity of the solvent configurations and the quantum mechanical fluctuations of the molecular geometry introduce large variations in the nonlinear optical response of water. This finding has the potential to change the way SHS experiments are interpreted: In particular, isotopic differences between ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ could explain recent SHS observations. Finally, we show that a machine-learning framework can predict accurately the fluctuations of the molecular hyperpolarizability. This model accounts for the microscopic inhomogeneity of the solvent and represents a step towards quantitative modeling of SHS experiments.

In the tensor-network framework, the expectation values of two-dimensional quantum states are evaluated by contracting a double-layer tensor network constructed from initial and final tensor-network states. The computational cost of carrying out this contraction is generally very high, which limits the largest bond dimension of tensor-network states that can be accurately studied to a relatively small value. We propose an optimized contraction scheme to solve this problem by mapping the double-layer tensor network onto an intersected single-layer tensor network. This reduces greatly the bond dimensions of local tensors to be contracted and improves dramatically the efficiency and accuracy of the evaluation of expectation values of tensor-network states. It almost doubles the largest bond dimension of tensor-network states whose physical properties can be efficiently and reliably calculated, and it extends significantly the application scope of tensor-network methods.

We present an analytical formulation for the normalization of resonant states at oblique incidence in one- and two-dimensional periodic structures with top and bottom boundaries to homogeneous space, such as photonic crystal slabs and arrays of nanoantennas. The normalization is validated by comparing the resonant state expansion using one and two resonant states with numerically exact results. The predicted changes of resonance frequency and linewidth due to perturbations of refractive index or geometry can be used to study resonantly enhanced refractive index sensing as well as the influence of disorder. In addition, the normalization is essential for the calculation of the Purcell factor.

Strain presents a straightforward tool to tune electronic properties of atomically thin nanomaterials that are highly sensitive to lattice deformations. While the influence of strain on the electronic band structure has been intensively studied, there are only a few works on its impact on optical properties of monolayer transition-metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearest-neighbor tight-binding approximation, we present an analytical view on how uni- and biaxial strain influences the optical fingerprint of TMDs, including their excitonic binding energy, oscillator strength, optical selection rules, and the radiative broadening of excitonic resonances. We show that the impact of strain can be reduced to changes in the lattice structure (geometric effect) and in the orbital functions (overlap effect). In particular, we demonstrate that the valley-selective optical selection rule is softened in the case of uniaxial strain due to the introduced asymmetry in the lattice structure. Furthermore, we reveal a considerable increase of the radiative dephasing due to strain-induced changes in the optical matrix element and the excitonic wave functions.

The physical origin of resonant Wood's anomalies in the reflection spectra of three-dimensional (3D) opaline photonic crystals covered with ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST225) is discussed. For this purpose, the optical reflection spectra are studied for different incident angles of light both experimentally and theoretically. The performed eigenmode analysis reveals that the Wood's anomalies originate from the quasiguided modes which appear in the GST225 capping layer. This conclusion is supported by the simulated electromagnetic near-field distributions of incident light at resonant frequencies. The experimental reflection spectra are in good agreement with theoretical calculations performed by the Fourier modal method in the scattering matrix form.

Subjecting a many-body localized system to a time-periodic drive generically leads to delocalization and a transition to ergodic behavior if the drive is sufficiently strong or of sufficiently low frequency. Here we show that a specific drive can have an opposite effect, taking a static delocalized system into the many-body localized phase. We demonstrate this effect using a one-dimensional system of interacting hard-core bosons subject to an oscillating linear potential. The system is weakly disordered, and is ergodic absent the driving. The time-periodic linear potential leads to a suppression of the effective static hopping amplitude, increasing the relative strengths of disorder and interactions. Using numerical simulations, we find a transition into the many-body localized phase above a critical driving frequency and in a range of driving amplitudes. Our findings highlight the potential of driving schemes exploiting the coherent destruction of tunneling for engineering long-lived Floquet phases.

In addition to its technological relevance, silicon poses a challenge for first principles simulations because it undergoes a semiconductor-to-metal transition upon melting. Moreover, the resulting metallic liquid contains a mixture of metallic and covalent bonding. This coexistence of fundamentally different interactions is difficult to describe within approximate density functional methods, which oftentimes cannot accurately describe these two extremes simultaneously. We report an investigation of the structure, dynamics, and thermodynamics of liquid silicon using ab initio molecular dynamics simulations with three density functional approximations: the local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation, and the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation. We demonstrate that SCAN describes this liquid with better accuracy than the other often-used functionals because it can simultaneously capture covalent and metallic bonding with similar high accuracy.

We propose a model of magnetoelectric effect in doped magnetic ferroelectrics. This magnetoelectric effect does not involve the spin-orbit coupling and is based purely on the Coulomb interaction. We calculate magnetic phase diagram of doped magnetic ferroelectrics. We show that magnetoelectric coupling is pronounced only for ferroelectrics with low dielectric constant. We find that magnetoelectric coupling leads to modification of magnetization temperature dependence in the vicinity of the ferroelectric phase transition. A peak of magnetization appears. We find that magnetization of doped magnetic ferroelectrics strongly depends on the applied electric field.

The recently introduced area of topological magnetism searches for equilibrium structures stabilized by a combination of interactions and specific boundary conditions. Until now, the internal energy of open magnetic chains has been explored. Here, we study the energy landscape of closed magnetic chains with on-site anisotropy coupled with antiferromagnetic exchange and dipolar interactions analytically and numerically. We show that there are many stable stationary states in closed geometries. These states correspond to the noncollinear spin spirals for vanishing anisotropy or to kink solitons for high magnetic anisotropy. Particularly, the noncollinear Möbius magnetic state can be stabilized at finite temperatures in nonfrustrated rings or other closed shapes with an even number of sites without the Dzyaloshinskii-Moriya interaction. We identify the described configurations with the stable stationary states, which appear due to the finite length of a ring.

We report on time-domain stability of the parametric synchronization in a spin-torque nano-oscillator (STNO) based on a magnetic tunnel junction. Time-domain measurements of the instantaneous frequency ($f_i$) of a parametrically synchronized STNO show random short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected RF power, random unlocking of the STNO can be avoided. However, a perfect synchronization characterized by complete suppression of phase noise, so-called phase noise squeezing, can be obtained only at a significantly higher RF power. Our macrospin simulations suggest that a lower temperature and a higher positive ratio of the fieldlike torque to the spin transfer torque reduce the threshold RF power required for phase noise squeezing under parametric synchronization.

We use high-resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of RSb consists of at least two concentric hole pockets at the $\mathrm{\Gamma }$ point and two intersecting electron pockets at the $X$ point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the $\mathrm{\Gamma }$ point. Due to the “lanthanide contraction”, the unit cell of different rare-earth monoantimonides shrinks when changing the rare-earth ion from CeSb to LuSb. This results in the differences in the chemical potentials in these compounds, which are demonstrated by both ARPES measurements and electronic structure calculations. Interestingly, in CeSb, the intersecting electron pockets at the $X$ point seem to be touching the valence bands, forming a fourfold-degenerate Dirac-like feature. On the other hand, the remaining rare-earth monoantimonides show significant gaps between the upper and lower bands at the $X$ point. Furthermore, similar to the previously reported results of LaBi, a Dirac-like structure was observed at the $\mathrm{\Gamma }$ point in YSb, CeSb, and GdSb, compounds showing relatively high magnetoresistance. This Dirac-like structure may contribute to the unusually large magnetoresistance in these compounds.

We develop a theory of the dynamical response of a minimal model of quantum spin ice (QSI) by means of inelastic light scattering. In particular, we are interested in the Raman response of the fractionalized U(1) spin liquid realized in the XXZ QSI. We show that the low-energy Raman intensity is dominated by spinon and gauge fluctuations. We find that the Raman response in the QSI state of that model appears only in the $T_{2g}$ polarization channel. We show that the Raman intensity profile displays a broad continuum from the spinons and coupled spinon and gauge fluctuations, and a low-energy peak arising entirely from gauge fluctuations. Both features originate from the exotic interaction between photon and the fractionalized excitations of QSI. Our theoretical results suggest that inelastic Raman scattering can in principle serve as a promising experimental probe of the nature of a U(1) spin liquid in QSI.

We present results of a detailed theoretical study of the electronic, magnetic, and structural properties of the chalcogenide parent system FeSe using a fully charge-self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we predict a remarkable change of the electronic structure of FeSe which is accompanied by a complete reconstruction of the Fermi surface topology (Lifshitz transition) upon a moderate expansion of the lattice volume. The phase transition results in a change of the in-plane magnetic nesting wave vector from $(\pi ,\pi )$ to $(\pi ,0)$ and is associated with a transition from itinerant to orbital-selective localized magnetic moments. We attribute this behavior to a correlation-induced shift of the Van Hove singularity of the Fe $t_2$ bands at the M point across the Fermi level. Our results reveal a strong orbital-selective renormalization of the effective mass $m^{*}/m$ of the Fe $3d$ electrons upon expansion. The largest effect occurs in the Fe $xy$ orbital, which gives rise to a non-Fermi-liquid-like behavior above the transition. The behavior of the momentum-resolved magnetic susceptibility $\chi \left(\mathbf{q}\right)$ demonstrates that magnetic correlations are also characterized by a pronounced orbital selectivity, suggesting a spin-fluctuation origin of the nematic phase of paramagnetic FeSe. We conjecture that the anomalous behavior of FeSe upon expansion is associated with the proximity of the Fe $t_2$ Van Hove singularity to the Fermi level and the sensitive dependence of its position on external conditions.

Spin-polarized density functional calculations, magnetization, and neutron diffraction (ND) measurements are carried out to investigate the magnetic exchange interactions and strong correlation effects in Yb substituted inverse spinel nickel ferrite. In the pristine form, the compound is found to be a mixed insulator under the Zaanen-Sawatzky-Allen classification scheme as it features both charge transfer and Mott insulator mechanisms. Estimation of magnetic exchange couplings reveals that both octahedral-octahedral and octahedral-tetrahedral spin-spin interactions are antiferromagnetic. This is typical of a spin-frustrated triangular lattice with one of the vertices occupied by tetrahedral spins and the remaining two occupied by octahedral spins. However, since the octahedral-tetrahedral interaction is dominant, it leads to a forced parallel alignment of the spins at the octahedral site which is in agreement with the results of ND measurements. The substituent Yb is found to be settled in +3 charge state, as confirmed from the x-ray photoelectron spectroscopy measurements, to behave like a spin-half-impurity carried by the localized $f_{z(x^2-y^2)}$ orbital. The impurity $f$ spin significantly weakens the antiferromagnetic coupling with the spins at the tetrahedral site, which explains the experimental observation of a decrease in Curie temperature with Yb substitution.

The quantum charge pumping phenomenon connects band topology through the dynamics of a one-dimensional quantum system. In terms of a microscopic model, the Su-Schrieffer-Heeger/Rice-Mele quantum pump continues to serve as a fruitful starting point for many considerations of topological physics. Here we present a generalized Creutz scheme as a distinct two-band quantum pump model. By noting that it undergoes two kinds of topological band transitions accompanying with a Zak-phase difference of $\pi$ and $2\pi$, respectively, various charge pumping schemes are studied by applying an elaborate Peierls phase substitution. Translating into real space, the transportation of quantized charges is a result of cooperative quantum interference effect. In particular, an all-flux quantum pump emerges which operates with time-varying fluxes only and transports two charge units. This makes cold atoms with artificial gauge fields a unique system where this kind of phenomena can be realized.