We use x-ray spectroscopy at the Ir $L_3/L_2$ absorption edge to study powder samples of the intercalated honeycomb magnet ${\mathrm{Ag}}_3\mathrm{Li}{\mathrm{Ir}}_2{\mathrm{O}}_6$. Based on x-ray absorption and resonant inelastic x-ray scattering measurements, and exact diagonalization calculations including nearest-neighbor Ir-Ir electron hopping integrals, we argue that the intercalation of Ag atoms results in a delocalized electronic structure with enhanced Ir-O hybridization, departing from the local relativistic $j_{\mathrm{eff}}=1/2$ state. We find that the relative orbital contribution to the magnetic moment is increased and the magnetization density is spatially extended and asymmetric in this hybridized state. Our results confirm the importance of metal-ligand hybridization in the magnetism of transition metal oxides and provide empirical guidance for understanding the collective magnetism in intercalated honeycomb iridates.

The effect of the spatially varying spin current on a skyrmion is numerically investigated. It is shown that an inhomogeneous current density induces an elongation of the skyrmion. This elongation can be controlled using current pulses of different strength and duration. Long current pulses lead to a splitting that forms two replicas of the initial skyrmion while for short pulses the elongated skyrmion relaxes back to its initial circular state through rotation in the MHz-GHz frequency range. The frequency is dependent on the strength of the damping coefficient.

We propose the design of a teardrop-shaped magnetic patch as a unidirectional magnetically driven spin wave emitter capable of operating in a wide range of frequencies. We explore its potential through micromagnetic studies in line with vibrational sample magnetometry measurements and ferromagnetic resonance experiments. The proposed system is based on the excitation of a vortex core, acting as a source of spin waves, and a single Bloch domain wall, as a channel for the confinement and propagation of the mode in a sufficiently thick magnetic patch in the single magnetic vortex (SMV) state. The novelty consists in the reconfigurability and simplicity of the system, that is operational without the need of external saturating fields, retaining a single Bloch domain wall and a movable single vortex core. This allows significant suppression of the wave emission by means of an external bias field, which in turn allows a controllable valvelike effect. Following our proposed strategy for cultivating a single vortex core in the shape, and after a thorough micromagnetic study of the most prominent magnetization dynamics in the patch, we show that the SMV state can be obtained in a thick enough (80 nm) teardrop-shaped patch. Micromagnetic results show the potential of this simple structure as a tunable and unidirectional spin wave emitter. Experimental results also suggest that the required magnetic configuration has been experimentally obtained in the structure, in good agreement with micromagnetic simulations.

The disorder and interaction effects on Bogoliubov-Fermi surfaces with preserved inversion symmetry are studied for a low-energy effective model coupled to bosonic degrees of freedom. It is shown that the nonideal Bogoliubov quasiparticles (bogolons) generically induce the odd-frequency pair amplitude which reflects a Cooper pairing at different time. The self-energy of bogolons is mainly contributed by the disorder effects in the low frequency limit as in the usual electron liquid. Depending on the choice of the parameters, there are two kinds of solutions: One is frequency independent (but with sign function of frequency) and the other is proportional to the inverse of the frequency, which exist in both the normal and anomalous parts of the self-energy. These characteristic self-energy structures are clearly reflected in the single-particle spectrum. Since the bogolons are originally composed of electrons, the connection between the two is also sought using the concrete $j=3/2$ fermion model, which reveals that the odd-frequency pairing of bogolons is mainly made of the electrons' odd-frequency pairing.

Surface impedance of type-II superconductors is determined by their local optical conductivity $\sigma \left(\omega \right)$. Standard BCS-like theoretical descriptions of $\sigma \left(\omega \right)$, due to Mattis and Bardeen [Phys. Rev. 111, 412 (1958)] or Zimmermann et al. [Physica C 183, 99 (1991)], do not take pair-breaking processes into account. Therefore they do not provide a quantitative explanation of the microwave response, and in particular, they cannot predict the magnitude of the coherence peak. Here, based on the recently developed concept of Dynes superconductors, which does take also the pair-breaking scattering into account, we provide a simple but complete description of the microwave response of type-II superconductors, concentrating on the unexpected properties of clean superconductors which are often used as cavity materials.

We investigated the high-pressure phases of sulfur using first-principles calculations with high precision. In a previous study, Rudin et al. [Phys. Rev. Lett. 83, 3049 (1999)] predicted that the β-Po type rhombohedral structure is followed by a simple cubic (sc) structure. Strictly limiting the comparison to the work of Rudin et al. and four atoms per unit cell, our high-precision calculations that couple an originally developed structure search method demonstrate that the proposed sc structure is not the ground state at any pressure value, and that compressed S undergoes a first-order phase transition to a body-centered-cubic phase above 500 GPa. The potential energy surface constructed by the pseudopotential employed by Rudin et al. does not conform with that constructed by an all-electron calculation. Our findings extensively contribute to the ongoing effort in forming an accurate picture of the structural phase transitions of compressed S, and also brings to light the importance of careful treatments necessary when calculating high-pressure states using the pseudopotential method.

In this work, we study the topological properties and magnon Hall effect of a three-dimensional ferromagnet in the ABC stacking honeycomb lattice, motivated by the recent inelastic neutron scattering study of $\mathrm{Cr}{\mathrm{I}}_3$. We show that the magnon band structure and Chern numbers of the magnon branches are significantly affected by the interlayer coupling $J_c$, which moreover has a qualitatively different effect in the ABC stacking compared to the AA stacking adopted by other authors. The nontrivial Chern number of the lowest magnon band is stabilized by the next-nearest-neighbor Dzyaloshinskii-Moriya interaction in each honeycomb layer, resulting in the hopping term similar to that in the electronic Haldane model for graphene. However, we also find several gapless Weyl points, separating the nonequivalent Chern insulating phases, tuned by the ratio of the interlayer coupling $J_c$ and the third-neighbor Heisenberg interaction $J_3$. We further show that the topological character of magnon bands results in nonzero thermal Hall conductivity, whose sign and magnitude depend on $J_c$ and the intralayer couplings. Since the interlayer coupling strength $J_c$ can be easily tuned by applying pressure to the quasi-2D material such as $\mathrm{Cr}{\mathrm{I}}_3$, this provides a potential route to tuning the magnon thermal Hall effect in an experiment.

A coupled spin-electron model on a decorated square lattice formed by interconnected trigonal bipyramids is exactly solved by imposing two mobile electrons per each triangular plaquette with the help of a generalized decoration-iteration transformation, which establishes a precise mapping correspondence with the effective Ising model on a square lattice with temperature-dependent interaction. The investigated spin-electron model exhibits two different macroscopically degenerate ground states. The residual entropy of the first ground state, which shows a spontaneous ferromagnetic or ferrimagnetic long-range order depending on character of the exchange coupling, arises from chiral degrees of freedom of the mobile electrons. In contrast, the second ground state is disordered due to a kinetically driven frustration of the localized Ising spins triggered by the hopping term of the mobile electrons. The outstanding reentrant phase transitions connected with temperature induced formation of the spontaneous ferromagnetic or ferrimagnetic order can be found if the spin-electron model is driven sufficiently close to the ground-state phase boundary, but the disordered frustrated phase is the respective ground state. It is verified that the bipartite fermionic entanglement between two mobile electrons within the spontaneously ordered ferromagnetic or ferrimagnetic phase predominantly comes from their charge degrees of freedom, while the one within the disordered frustrated phase comes from both charge and spin degrees of freedom of these particles.

Ir and Pt thickness dependences of the interfacial Dzyaloshinskii-Moriya interaction (iDMI), the perpendicular magnetic anisotropy (PMA), and the damping in Co-based systems grown by sputtering on Si substrates, using Ir or Pt under or overlayer of variable thickness ($0\le t_{\mathrm{Pt}}\le 3.5\mathrm{nm}$ and $0\le t_{\mathrm{Ir}}\le 1.5\mathrm{nm}$), were investigated using Brillouin light scattering. Vibrating sample magnetometry measurements revealed a significant decrease of the areal magnetic moment at saturation as the thickness $t_{\mathrm{Pt}}$ or $t_{\mathrm{Ir}}$ of the overlayer increases, most likely due to the increasing intermixing. The intermixing, found to be negligible for Ir and Pt underlayers, is stronger for Ir than Pt overlayers. Damping measurements for Pt-based systems allowed us to conclude with the strong spin-pumping contribution. The analysis of the $t_{\mathrm{Pt}}$ dependence of damping, within the spin-pumping model, allowed us to determine the spin-diffusion length of Pt, found to be greater when Pt is used as an underlayer. The $t_{\mathrm{Pt}}$ dependence of the effective PMA and iDMI constants, for which similar trends to that of damping were observed, were qualitatively analyzed to separate the contribution of each interface with Co. It was found that these constants are stronger when Co is grown on Pt, most probably due to weaker intermixing at the Pt/Co interface. Linear dependence of damping vs the PMA effective constant was evidenced, confirming their relationship with spin-orbit coupling (SOC). A linear variation of the PMA effective constant vs the iDMI constant was observed in systems using Pt underlayer, whereas quadratic or linear correlations are possible when Pt is used as an overlayer, probably due to change of the interface SOC by the strong intermixing at the Co/Pt interface. The qualitative analysis of the Ir thickness dependence of iDMI and PMA constants revealed a significant (weak) contribution of the Ir/Co interface to PMA (iDMI). The surface iDMI constant of the Co/Ir interface was estimated to be 0.4 ± 0.1 pJ/m, which confirms the similar iDMI sign of Co/Ir and Co/Pt interfaces.

Invigorated by the high-temperature superconductivity in ternary hydrogen-dominated compounds under high pressures, we systematically explored high-pressure phase diagrams, electronic properties, lattice dynamics, and superconductivity of ternary Na-Al-H systems using ab initio methods. We found three stable compounds of ${\mathrm{NaAlH}}_4$, ${\mathrm{NaAlH}}_6$, and ${\mathrm{NaAlH}}_8$, as well as a metastable compound of ${\mathrm{NaAlH}}_7$ under high pressures. Except for ${\mathrm{NaAlH}}_4$, they all containing ${\mathrm{H}}^{-}$ and ${\mathrm{H}}_2$ units. Electronic structure calculations reveal that ${\mathrm{NaAlH}}_7$ and ${\mathrm{NaAlH}}_8$ are metallic, while ${\mathrm{NaAlH}}_4$ and ${\mathrm{NaAlH}}_6$ are semiconductors. In addition, ${\mathrm{NaAlH}}_8$ exhibits a phonon anomaly (dip) in multiple phonon branches along the A-M direction. The phonon anomaly induced by Fermi-surface nesting boosts the electron-phonon coupling strength, and as a result, a superconducting transition temperature $T_c$ of 55 K was produced at 300 GPa.

The nonlinear Hall effect, which is the second-order harmonic charge Hall effect from the Berry curvature dipole in momentum space, has received much attention recently. As the responses to higher harmonics of the driving ac electric field are prominent and measurable, we develop a general nonlinear theory by taking the charge and spin currents as well as the longitudinal and transverse effects into account. We introduce the expansion order of the electric field and Berry curvature multipole moment, where the Berry curvature dipole is a particular one manifesting itself at the second harmonic order and the second expansion order of the electric field. There are four cases with conserving or breaking the time-reversal symmetry (TRS) and inversion symmetry (IS). We find a specific “selection rule” that only longitudinal odd harmonic order charge currents exist for conserving both the TRS and IS, and with breaking both symmetries, all harmonic order charge and spin currents are nonzero. With conserving TRS and breaking IS, the charge Hall current exists at even harmonic order, and the longitudinal charge current occurs at odd harmonic order. Only the longitudinal spin current survives at even harmonic order. With breaking TRS and conserving IS, only odd harmonic order charge and spin currents can appear. Moreover, we observe that every harmonic order current contains a series of infinite-order expansion of the electric field. We further show that the Berry curvature dipole and quadrupole can be determined by measuring the second and fourth harmonic order currents in experiments. This may open a way to explore the higher responses of an ac driving system.

Typical topological systems undergo a topological phase transition in the presence of a strong enough perturbation. We propose an adjustable frustrated toric code with a “topological line” at which no phase transition happens in the system and the topological order is robust against a nonlinear perturbation of arbitrary strength. This important result is a consequence of the interplay between frustration and nonlinearity in our system, which also causes the emergence of other interesting phenomena such as reentrant topological phases and survival of the topological order under local projection operations. Our study opens a window towards more robust topological quantum codes which are the cornerstones of large-scale quantum computing.

Based on the matrix product state (MPS) formalism, we introduce an ansatz for capturing excited states in finite systems with open boundary conditions, providing a very efficient method for computing, e.g., the spectral gap of quantum spin chains. This method can be straightforwardly implemented on top of an existing density-matrix renormalization group or MPS ground-state code. Although this approach is built on open-boundary MPSs, we also apply it to systems with periodic boundary conditions. Despite the explicit breaking of translation symmetry by the MPS representation, we show that momentum emerges as a good quantum number and can be exploited for labeling excitations on top of MPS ground states. We apply our method to the critical Ising chain on a ring and the classical Potts model on a cylinder. Finally, we apply the same idea to compute excitation spectra for 2D quantum systems on infinite cylinders. Again, despite the explicit breaking of translation symmetry in the periodic direction, we recover momentum as a good quantum number for labeling excitations. We apply this method to the 2D transverse-field Ising model and the half-filled Hubbard model; for the latter, we obtain accurate results for, e.g., the hole dispersion for cylinder circumferences up to eight sites.

Recently, the transition metal dichalcogenide ${\mathrm{NbTe}}_2$ was predicted to be a candidate material of topological semimetal. Here we report the magnetotransport data measured in two devices fabricated from ${\mathrm{NbTe}}_2$ nanoflakes. A nonsaturating linear magnetoresistance was observed in both devices at various temperatures. A close analysis shows that the observed linear magnetoresistance is not consistent with the Abrikosov quantum theory; instead, it can be well explained in the framework of the effective-medium theory which describes the classical magnetoresistance of inhomogeneous systems. Our results indicate that the linear magnetoresistance of ${\mathrm{NbTe}}_2$ is most likely a classical magnetoresistance induced by disorders and inhomogeneities. This speculation is supported by the abundant domain structures observed in ${\mathrm{NbTe}}_2$ crystals in transmission electron microscopy measurements.

Structural defects can crucially impact the optical response of monolayer (ML) thick materials as they are considered as efficient trapping sites for excitons. These trapped excitons are located below the free exciton emission. This phenomenon can give rise to a new type of emitters, known as ”single-photon emitters” (SPEs). In this paper we outline the criteria, within our framework, by which single-photon emissions can be enabled in two-dimensional materials and we explore how these criteria can be fulfilled in atomically thin transition-metal dichalcogenides (TMD). In particular, we model the defects effect, in accordance with the most common experimental realisations, on the spatial autocorrelation function of the random disorder potential. Moreover, we provide a way to control the radiative lifetime of these emissions by a hybrid heterostructrue of a ML TMD with a graphene sheet separated by a dielectric material with a controlled thickness, which enables Förster resonance energy transfer process. Our paper predicts that the corresponding SPEs quenched radiative lifetime, which depends strongly on the dielectric environment, will be in the picosecond range. The range of our calculated exciton radiative lifetime in graphene-TMD heterostructure is consistent with that found in recent measurements.

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D’yakonov-Perel’ spin relaxation mechanism in wurtzite GaN.

Elucidating the polymorphism of transition metal dichalcogenide layers and the interplay between structure and properties is a key challenge for the application of these materials. We identify a low-energy metastable phase of monolayer $\mathrm{V}{\mathrm{Se}}_2$ and elucidate its magnetic and electronic properties. This structure is distinct from the previously identified charge density wave (CDW) phase. However, while having rather distinct properties it is very close in energy to the CDW phase and is likely to be realized in experiments. Importantly, local bonding instabilities, as characterized by reconstruction of the electronic structure over a wide energy range, are important for this distortion, which includes both V off-centering in the octahedral coordination cages and a partial disproportionation into two distinct types of V. The phase does not have a ferromagnetic ground state. The results show that the physics of $1T\text{−}\mathrm{V}{\mathrm{Se}}_2$ are richer than previously known with an interplay of Fermi surface instabilities and local bonding effects.

We study the switching behavior of two spin-crossover molecules residing in a nanojunction device consisting of two closely spaced gold electrodes. The spin states are monitored through a real-time measurement of the resistance of the junction. A statistical analysis of the resistance values, the occupation probabilities, and the lifetimes of the respective spin states shows that the two spin-crossover molecules are coupled to each other. We extract the parameters for a minimal model describing the two coupled spin-crossover molecules. Finally, we use the time dependence of factorial cumulants to study the impact of interactions between the two spin-crossover molecules on the switching dynamics.