A numerical approach, based on a local comparative projectional analysis onto symmetry modes of elementary structural units of glasses and their crystalline counterparts, is developed and used for the analysis of atomic vibrations in glasses in terms of phonons for the entire frequency range. The results of such an analysis shed light on the origin of the modes in the controversial low-frequency range, including the boson-peak region. Comparative analyses undertaken for glass, lattice-glass, and polycrystalline models reveal the role of different types of disorder in atomic vibrations.

We calculate the single-particle momentum distribution of a quantum many-particle system in the presence of the Coulomb interaction and a confining potential. The region of intermediate momenta, where the confining potential dominates, marks a crossover from a Gaussian distribution valid at low momenta to a power-law behavior valid at high momenta. We show that for all momenta the momentum distribution can be parametrized by a $q$-Gaussian distribution whose parameters are specified by the confining potential. The real-space pair-correlation function calculated in this way can, in principle, be used to construct improved exchange-correlation functionals to solve electronic structure problems. Furthermore, we find that the functional form of the probability of transitions between the confined ground state and the $n\mathrm{th}$ excited state is invariant under scaling of the ratio $Q^2/{\nu }_n$, where $Q$ is the transferred momentum and ${\nu }_n$ is the corresponding excitation energy. Using the scaling variable $Q^2/{\nu }_n$ the maxima of the transition probabilities can also be expressed in terms of a $q$-Gaussian.

We investigate the ground-state properties of the $K-\mathrm{\Gamma }$ model on a honeycomb lattice using series expansions and numerical exact diagonalizations, where the model includes Kitaev ($K$) and symmetric off-diagonal ($\mathrm{\Gamma }$) interactions. Starting from the weakly interacting dimers on the specific bond, we strengthen the interdimer interactions to the isotropically interacting system. We show that depending on $\mathrm{\Gamma }$ and $K$, the dimer state survives up to the isotropically interacting system, where the phase transition occurs or obeys a phase transition to a magnetically ordered state at an anisotropic interaction. The results are summarized in the phase diagram. We also show that the Kekulé dimerized state is unstable in the isotropic $K-\mathrm{\Gamma }$ model.

The strain-modulated ultrafast spin dynamics of Co embedded exohedral boron fullerene $[\mathrm{Co}\&{\mathrm{B}}_{40}{]}^{-}$ is investigated with high-level ab initio calculations. We find that due to its high ductility and malleability, ${\mathrm{B}}_{40}$ can retain its structure upon both the addition of a magnetic atom and the exertion of mechanical tensile or compressive strain, and that its elasticity modulus is highly anisotropic. The geometric optimization reveals that $[\mathrm{Co}\&{\mathrm{B}}_{40}{]}^{-}$ prefers three different configurations. For the spin-dynamics investigation we focus on one of them, namely the configuration with the Co located near the center of a heptagon (the only geometry in which the boron cage remains intact) as an example to investigate the spin-dynamics features. Several laser-induced spin-flip scenarios are suggested, the details of which strongly depend on the magnitude and the direction of any deformations. Thus, utilizing the coupling between the external mechanical strain and the spin degree of freedom in magnetic exohedral all-boron fullerenes, we demonstrate the realizability of magnetostraintronic devices on molecular systems. The asymmetric modulation of the spin-flip process with respect to the tensile and compressive strain (tension-compression-asymmetric ultrafast spin dynamics) can provide further control of such integrated spin-logic devices.

We study Andreev reflection at the interface of a time-reversal invariant Weyl semimetal without band tilting in the normal or ferromagnetic state with a type-II Weyl semimetal possessing an overtilted anisotropic energy spectrum in the superconducting state. We find that the Andreev reflection at the normal-superconductor interface is tremendously anisotropic, reflecting the anisotropic energy spectrum of the type-II Weyl semimetal. There is a critical junction direction relative to the band tilting direction which, below it, the Andreev reflection acquires distinct properties without any other counterpart. Further, the conductance of this junction is constant and independent of the energy at subgap energies in two extreme limits of the zero and infinite chemical potentials. We can attribute them to the perfect specular and retro Andreev reflections, respectively. In particular, we find a universal value for the zero-energy conductance independent of the chemical potential on each side of the junction. At the interface of the ferromagnet-superconductor, the Andreev reflection can be switched between the specular and retro regimes by varying the exchange energy. This results in different asymptotic values for conductance at high exchange energies depending on if the junction direction is below or above the critical direction. Moreover, we see that tunneling conductance of the normal-ferromagnet-superconductor junction in the thin barrier limit is independent of the potential barrier strength, while it periodically oscillates in terms of the exchange barrier strength. Furthermore, we find a $\pi /4$ phase shift in the periodic oscillation of the conductance of the junctions with directions below and above the critical direction. This highly anisotropic conductance of the junctions studied here can be used as a way, accompanied by the other methods, for detecting the anisotropic spectrum and band-tilting direction of type-II Weyl semimetals.

We study a Josephson junction in a Kitaev chain with particle-hole symmetric nearest-neighbor interactions. When the phase difference across the junction is $\pi$, we show analytically that the full spectrum is fourfold degenerate up to corrections that vanish exponentially in the system size. The Majorana bound states at the ends of the chain are known to survive interactions. Our result proves that the same is true for the zero-energy quasiparticle localized at the junction. We further study finite-size corrections numerically, and show how repulsive interactions lead to stronger end-to-end correlations than in a noninteracting system with the same bulk gap.

We report temperature ($T$)- and pressure ($P$)-dependent resistivity measurements on the isostructural compounds ${\mathrm{U}}_2\mathrm{R}{\mathrm{h}}_{1-x}\mathrm{P}{\mathrm{t}}_x{\mathrm{C}}_2$ ($x=0$, 0.5, and 0.9) from which we construct a $T-P-x$ phase diagram. Compounds with $x=0$ and $x=0.5$ are antiferromagnets below 22.1 and 9.4 K, respectively, and their Néel temperature $\left(T_N\right)$ decreases under applied pressure. For $x=0$, the critical pressure $P_c$ required to suppress $T_N$ to zero temperature is projected to be about 8.8 GPa, but $P_c$ for $x=0.5$ is between 1.6 and 2.1 GPa. At atmospheric pressure, increasing Pt concentration in ${\mathrm{U}}_2\mathrm{R}{\mathrm{h}}_{(1-x)}\mathrm{P}{\mathrm{t}}_x{\mathrm{C}}_2$ tunes magnetic transition temperatures to zero at a critical value of $x_c\approx 0.7$, and, consequently, we surmise the existence of a quantum-phase boundary in the $P-x$ plane at $T=0$ K that extends from ($P=0, x=x_c$) to ($P_c\approx 8.8\mathrm{GPa}, x=0$). For $x=0.9$, superconductivity appears at $T_{\mathrm{c}}=1.09\mathrm{K}$, which decreases at a rate of $\approx -1\mathrm{K}/\mathrm{GPa}$ that is nearly twice that found for ${\mathrm{U}}_2\mathrm{Pt}{\mathrm{C}}_2$ whose $T_{\mathrm{c}}$ is 1.47 K. Together, these results indicate that domes of magnetism and superconductivity formed with $T-P-x$ variations are detached and that the two broken symmetries are independent of each other. Fluctuations in average composition produce rare regions that play an important role in determining physical properties of materials with noninteger $x$.

Synchrotron x-ray photoelectron spectroscopy was used to explore the relationship between the hydroxyl termination and band bending at the $\left(\overline{2}01\right)$ surface of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ bulk single crystals. All as-received $\left(\overline{2}01\right)$ surfaces were terminated with OH groups, with H/OH binding to surface ${\mathrm{O}}_{\mathrm{s}}/\mathrm{G}{\mathrm{a}}_{\mathrm{s}}$ atoms. Removal of this native OH termination produced a large upward shift in band bending of up to 1.0 eV, consistent with strong electron depletion and a semiconductor to insulator-like transition in the near-surface region. Simple surface treatments were used to control the size and stability of the band bending of as-received $\left(\overline{2}01\right)$ surfaces by modifying the nature of the OH termination. NaOH (${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$) treatment consistently produced upward (downward) shifts in band bending and a significant increase (decrease) in the thermal stability of the OH termination that was associated with an increase in the relative density of $\mathrm{G}{\mathrm{a}}_{\mathrm{s}}\text{−}\mathrm{OH}$ (${\mathrm{O}}_{\mathrm{s}}\text{−}\mathrm{H}$) species. Annealing in wet ${\mathrm{O}}_2$ (at 600 °C) produced an extremely stable OH termination and the strongest downward shift in band bending. These effects, combined with the relatively slow dissociation of ${\mathrm{H}}_2\mathrm{O}$ on bare $\left(\overline{2}01\right)$ surfaces, allowed the preparation of surfaces with significant variations in band bending that may prove useful in optimizing the properties of $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ metal-semiconductor contacts and heterojunctions. A comparison of two methods used to determine the absolute band bending at semiconductor surfaces confirmed that bare $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3\left(\overline{2}01\right)$ surfaces are characterized by strong upward band bending (∼0.5 to 1.0 eV) and an electron depletion layer that can be completely removed by the hydroxylation of the surface.

We investigate the effect of long-range Coulomb interaction on the two-electron scattering in the integer quantum Hall regime at bulk filling factor two. We compute the dynamics of the exact two-particle wave function by means of a parallel version of the split-step Fourier method in a 2D potential background reproducing the effect of depleting gates in a realistic heterostructure, with the charge carrier represented by a localized wave packet of edge states. We compare the spatial shift induced by Coulomb repulsion in the final two-electron wave function for two indistinguishable electrons initialized in different configurations according to their Landau index and analyze their bunching probability and the effect of screening. We finally prove the feasibility of this device as a two-qubit conditional phase shifter able to generate controlled entanglement from product states.

Discrete quantum walks are periodically driven systems with discrete time evolution. In contrast to ordinary Floquet systems, no microscopic Hamiltonian exists, and the one-period time evolution is given directly by a series of unitary operators. Regarding each constituent unitary operator as a discrete time step, we formulate discrete space-time symmetry in quantum walks and evaluate the corresponding symmetry protected topological phases. In particular, we study chiral and/or time-glide symmetric topological quantum walks in this formalism. Due to the discrete nature of time evolution, the topological classification is found to be different from that in conventional Floquet systems. As a concrete example, we study a two-dimensional quantum walk having both chiral and time-glide symmetries and identify the anomalous edge states protected by these symmetries.

We report ab initio band diagram and optical absorption spectra of hexagonal boron nitride ($h$-BN), focusing on unravelling how the completeness of the basis set for $GW$ calculations and electron-phonon interactions (EPIs) impact on them. The completeness of the basis set, an issue which was seldom discussed in previous optical spectra calculations of $h$-BN, is found crucial in providing converged quasiparticle band gaps. In the comparison among three different codes, we demonstrate that by including high-energy local orbitals in the all-electron linearized augmented plane waves based $GW$ calculations, the quasiparticle direct and fundamental indirect band gaps are widened by $\sim 0.2$ eV, giving values of 6.81 eV and 6.25 eV, respectively at the $G{W}_0$ level. EPIs, on the other hand, reduce them to 6.62 eV and 6.03 eV respectively at 0 K, and 6.60 eV and 5.98 eV respectively at 300 K. With clamped crystal structure, the first peak of the absorption spectrum is at 6.07 eV, originating from the direct exciton contributed by electron transitions around $K$ in the Brillouin zone. After including the EPIs-renormalized quasiparticles in the Bethe-Salpeter equation, the exciton-phonon coupling shifts the first peak to 5.83 eV at 300 K, lower than the experimental value of $\sim 6.00$ eV. This accuracy is acceptable to an ab initio description of excited states with no fitting parameter.

We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either ${\mathrm{Ir}}^{4+}$ or ${\mathrm{Ir}}^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight-binding calculations including only the Ir $5d$, O $2p$, and O $2s$ orbitals with parameters based on the downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the ${\mathrm{A}}_2{\mathrm{BIrO}}_6$ iridates have essentially zero O $2p$ to Ir $5d$ charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.

A semiconductor quantum dot (QD) is an attractive resource to generate polarization-entangled photon pairs. We study the excitonic spin precession (flip-flop) in a family of QDs with different excitonic fine-structure splitting (FSS) and its impact on the entanglement of photons generated from the excitonic-biexcitonic radiative cascade. Our results reveal that coherent processes leave the time postselected entanglement of QDs with finite FSS unaffected while changing the eigenstates of the system. The flip-flop's precession is observed via quantum tomography through anomalous oscillations of the coincidences in the rectilinear basis. A theoretical model is constructed with the inclusion of an excitonic flip-flop rate and is compared with a two-photon quantum tomography measurement on a QD exhibiting the spin flip-flop mechanism. A generalization of the theoretical model allows estimating the degree of entanglement as a function of the FSS and the spin precession rate. For a finite temporal resolution, the negativity is found to be oscillating with respect to both the FSS and the spin precession rate. This oscillatory behavior disappears for perfect temporal resolution and maximal entanglement is retrieved despite the flip-flop process.

We believe that a necessary first step in understanding the ground-state properties of the spin-$\frac{1}{2}$ kagome Heisenberg antiferromagnet is a better understanding of this model's very large number of low-energy singlet states. A description of the low-energy states that is both accurate and amenable for numerical work may ultimately prove to have greater value than knowing only what these properties are, in particular, when they turn on the delicate balance of many small energies. We demonstrate how this program would be implemented using the basis of spin-singlet dimerized states, though other bases that have been proposed may serve the same purpose. The quality of a basis is evaluated by its participation in all low-energy singlets, not just the ground state. From an experimental perspective, and again in light of the small energy scales involved, methods that can deliver all low-energy states promise more robust predictions than methods that only refine a fraction of these states.

We show how low-energy electronic models derived from the first-principles electronic structure calculations can help to rationalize the magnetic properties of two lacunar spinel compounds $\mathrm{Ga}{M}_4{\mathrm{S}}_8$ with relatively light ($M=\mathrm{V}$) and heavy ($M=\mathrm{Mo}$) transition-metal elements, which are responsible for different spin-orbit interaction strength. In the model, each magnetic lattice point was associated with the ${\left(M_4{\mathrm{S}}_4\right)}^{5+}$ molecule, and the model itself was formulated in the basis of molecular Wannier functions constructed for three magnetic $t_2$ bands. The effects of rhombohedral distortion, spin-orbit interaction, band filling, and the screening of Coulomb interactions in the $t_2$ bands are discussed in details by stressing similarities and differences between ${\mathrm{GaV}}_4{\mathrm{S}}_8$ and ${\mathrm{GaMo}}_4{\mathrm{S}}_8$. The electronic model is further treated in the superexchange approximation, which allows us to derive an effective spin model for the energy and electric polarization ($\mathit{P}$) depending on the relative orientation of spins in the bonds, and study the properties of this model by means of classical Monte Carlo simulations with the emphasis on the possible formation of the skyrmionic phase. While isotropic exchange interactions clearly dominate in ${\mathrm{GaV}}_4{\mathrm{S}}_8$, all types of interactions (isotropic, antisymmetric, and symmetric anisotropic) are comparable in the case of ${\mathrm{GaMo}}_4{\mathrm{S}}_8$. Particularly, large uniaxial exchange anisotropy has a profound effect on the properties of ${\mathrm{GaMo}}_4{\mathrm{S}}_8$. On the one hand, it raises the Curie temperature by opening a gap in the spectrum of magnon excitations. On the other hand, it strongly affects the skyrmionic phase by playing the role of a molecular field, which facilitates the formation of skyrmions, but makes them relatively insensitive to the external magnetic field in the large part of the phase diagram. We predict reversal of the magnetic dependence of $\mathit{P}$ in the case of ${\mathrm{GaMo}}_4{\mathrm{S}}_8$ caused by the reversal of the direction of rhombohedral distortion.

We study a system of $\overline{N}$ interacting triplons (the elementary excitations of a valence-bond solid) described by an effective interacting boson model derived within the bond operator formalism in order to determine the stability and the properties of many-triplon states. In particular, we consider the square lattice spin-$1/2J_1-J_2$ antiferromagnetic Heisenberg model, focus on the intermediate parameter region where a quantum paramagnetic phase sets in, and consider the columnar valence-bond solid as a reference state. Within the bond operator theory, the Heisenberg model is mapped into an effective boson model in terms of triplet operators $t$. The effective boson model is studied at the harmonic approximation and the energy of the triplons and the expansion of the triplon operators $b$ in terms of the triplet operators $t$ are determined. Such an expansion allows us to perform a second mapping and, therefore, determine an effective interacting boson model in terms of the triplon operators $b$. We then consider systems with a fixed number $\overline{N}$ of triplons and study the stability of many-triplon states within a mean-field approximation. We show that many-triplon states are stable, the lowest-energy ones are constituted by a small number of triplons, and the excitation gaps are finite. For $J_2=0.48J_1$ and $0.52J_1$, we also calculate spin-spin and dimer-dimer correlation functions, dimer order parameters, and the bipartite von Neumann entanglement entropy within our mean-field formalism in order to determine the properties of the many-triplon state as a function of the triplon number $\overline{N}$. We find that the spin and the dimer correlations decay exponentially and that the entanglement entropy obeys an area law, regardless of the triplon number $\overline{N}$. Moreover, only for $J_2=0.48J_1$, the spin correlations indicate that the many-triplon states with large triplon number $\overline{N}$ might display a more homogeneous singlet pattern than the columnar valence-bond solid. We also comment on possible relations between the many-triplon states with large triplon number $\overline{N}$ and gapped spin-liquid states.

Tensile strain is a promising tool for the creation and manipulation of magnetic solitonic textures in chiral helimagnets via tunable control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Here, by using in situ resonant small-angle x-ray scattering, we demonstrate that skyrmion and chiral soliton lattices can be achieved as metastable states in FeGe lamella as distinct states under tensile strain and magnetic fields in various orientations with respect to the deformation. The small-angle scattering data can be well accounted for in the framework of the analytical model for a soliton lattice. By using the experimental results and analytical theory, the unwinding of metastable skyrmions in a perpendicular magnetic field as shown by a small-angle scattering experiment was analyzed via micromagnetic simulation.

The spin-$1/2$ $J_1$-$J_2$ Heisenberg model on the square lattice represents one of the simplest examples in which the effects of magnetic interactions may suppress magnetic order, eventually leading to a pure quantum phase with no local order parameters. This model has been extensively studied in the last three decades, with conflicting results. Here, by using Gutzwiller-projected wave functions and recently developed methods to assess the low-energy spectrum, we show the existence of a level crossing between the lowest-energy triplet and singlet excitations for $J_2/J_1\approx 0.54$. This fact supports the existence of a phase transition between a gapless spin liquid (which is stable for $0.48\lesssim J_2/J_1\lesssim 0.54$) and a valence-bond solid (for $0.54\lesssim J_2/J_1\lesssim 0.6$), even though no clear sign of dimer order is visible in the correlations functions. These results, which confirm recent density-matrix renormalization calculations on cylindrical clusters [L. Wang and A. W. Sandvik, Phys. Rev. Lett. 121, 107202 (2018)], reconcile the contradicting results obtained within different approaches over the years.