We present a Green's function formalism to investigate the topological properties of weakly interacting one-dimensional topological insulators, including the bulk-edge correspondence and the quantum criticality near topological phase transitions, and using the interacting Su-Schrieffer-Heeger model as an example. From the many-body spectral function, we find that closing of the bulk gap remains a defining feature even if the topological phase transition is driven by interactions. The existence of edge state in the presence of interactions can be captured by means of a $T$-matrix formalism combined with Dyson's equation, and the bulk-edge correspondence is shown to be satisfied even in the presence of interactions. The critical exponent of the edge state decay length is shown to be affiliated with the same universality class as the noninteracting limit.

The computational cost of calculating the self-energy matrices used in first-principles transport-property calculations is proportional to the cube of the lateral length of electrodes. Therefore, the clarification of transport properties is difficult because the system size increases when the transition region structure becomes complicated owing to lattice defects such as adatoms, substitutional doping, vacancies, and lattice distortions. In this study we propose an improved procedure to calculate the self-energy matrices in the electrodes to reduce computational costs of electron-transport calculations without degrading the accuracy. This procedure accurately reproduces the self-energy matrices of the supercell-structured electrodes from the generalized Bloch states of the primitive unit cell. Furthermore, we carry out electron-transport calculations on fluorine-adsorbed graphene sheets connected to semi-infinite graphene electrodes and find the dependence of the electron transmission on the symmetry of the arrangement of adatoms perpendicular to the transport direction.

To understand recent works on classical and quantum spin equations and their topological classification, we develop a unified mathematical framework for bosonic Bogoliubov–de Gennes (BdG) systems and associated classical wave equations; it applies not just to equations that describe quantized spin excitations in magnonic crystals but more broadly to other systems that are described by a BdG Hamiltonian. Because here the generator of dynamics, the analog of the Hamiltonian, is para-Hermitian (also known as pseudo- or Krein-Hermitian) but not Hermitian, the theory of Krein spaces plays a crucial role. For systems which are thermodynamically stable, the classical equations can be expressed as a “Schrödinger equation” with a Hermitian Hamiltonian. We then apply the Cartan-Altland-Zirnbauer classification scheme: To properly understand what topological class these equations belong to, we need to conceptually distinguish between symmetries and constraints. Complex conjugation enters as a particle-hole constraint (as opposed to a symmetry), since classical waves are necessarily real-valued. Because of this distinction, only commuting symmetries enter in the topological classification. Our arguments show that the equations for spin waves in magnonic crystals are a system of class A, the same topological class as quantum Hamiltonians describing the integer quantum Hall effect. Consequently, the magnonic edge modes first predicted by Shindou et al. [Phys. Rev. B 87, 174427 (2013)] are indeed analogs of the quantum Hall effect, and their net number is topologically protected.

Quantum interference between time-reversed electron paths in two dimensions (2D) leads to the well-known weak localization correction to resistance. If spin-orbit coupling is present, the resistance correction is negative, termed weak antilocalization (WAL). Here, we report the observation of WAL coexisting with exchange coupling between itinerant electrons and localized magnetic moments. We use low-temperature magnetotransport measurements to investigate the quasi-two-dimensional, high-electron-density interface formed between ${\mathrm{SrTiO}}_3$ and the antiferromagnetic Mott insulator ${\mathrm{NdTiO}}_3$. As the magnetic field angle is gradually tilted away from the sample normal, the data reveal the interplay between strong $k$-cubic Rashba-type spin-orbit coupling and a substantial magnetic exchange interaction from local magnetic regions. The resulting quantum corrections to the conduction are in excellent agreement with existing models and allow sensitive determination of the small magnetic moments ($22{\mu }_B$ on average), their magnetic anisotropy, and mutual coupling strength. This effect is expected to arise in other 2D magnetic materials systems.

We show that a gap parameter can fully describe the merging of Dirac cones in semi-Dirac materials from the $K$ and $K^{\prime }$ points into the common $M$ point in the Brillouin zone. We predict that the gap parameter manifests itself by enhancing the thermoelectric figure of merit $zT$ as the chemical potential crosses the gap, followed by a sign change in the Seebeck coefficient around the same point. Subsequently, whenever the chemical potential crosses the gap potential parameter, there is a well-balanced maximum of the power factor and the efficiency of the thermoelectrics. An optimal operating point where comaximization of the power efficiency occurs is consequently singled out for the best thermoelectric performance. Our work paves the way for the use of two-dimensional semi-Dirac materials for thermoelectric applications.

Spin-Seebeck effect (SSE) is an effective route to realize pure spin current by using spin-polarized electrons or spin-wave transport in magnetic systems. Here we propose a route, i.e., by using spin-orbit coupling (SOC), to achieve robust SSE characterized by pure thermal spin current. The material examples are constructed on achiral nanotubes, and the theoretical results reveal that (i) as temperature gradient is applied along the nanotubes, thermal spin-up and spin-down currents with opposite flow directions are produced without any accompanying charge current, (ii) the SSE is robust against decoherence and nonuniform interchain SOC, (iii) the thermal spin currents display a multioscillation feature with increasing device temperatures, supporting their potential device applications in thermal spin-current multiswitcher, and (vi) strain engineering in the radical direction of nanotubes is an effective way to improve SSE and to control pure thermal spin current. These inspiring spin transport behaviors in achiral molecular systems put forward a mechanism to realize the robust SSE characterized by pure spin current and develop the research field of spin-orbito-caloritronics, which focuses on the interplay of electrons' spin and orbital degrees of freedom in the presence of temperature gradient.

We show theoretically that in the superconducting nanostructures the gapped states of different topology are not always protected by separating gapless states. Depending on the structure design parameters, they can be either protected or not, with a protection–unprotection transition separating these two distinct situations. We build up a general theoretical description of the transition vicinity in the spirit of Landau theory. We speculate that similar protection–unprotection transitions may also occur for other realizations of topological protection in condensed matter systems.

The so called phase-coherence length $l_{\phi }$ and its relation to the geometrical dimensions of a sample determine the electronic transport regime. Different approaches are established for extracting $l_{\phi }$ from magnetotransport data of mesoscopic systems and need to be cross-checked by using experimental data on the same model system in order to ensure an overall consistent theoretical description. Suitable model systems for testing this consistency are single GaN:Ge nanowires. Their magnetoconductance at low temperatures exhibits universal conductance fluctuations as well as weak localization effects. We find that the values of $l_{\phi }$ obtained from the established analysis of the magnitude of the conductance fluctuations rms($\mathrm{\Delta }G$) decrease with increasing measurement current, whereas the corresponding values of $l_{\phi }$ determined by the analyses of the correlation field $B_{\text{C}}$ and the weak localization effect yield the same value for $l_{\phi }$ independent of the measurement current used. We apply and modify the existing theoretical framework for bias-dependent differential conductance fluctuations, in order to explain the decrease of the conductance fluctuations rms($\mathrm{\Delta }G$) with increasing current density in our dc measurements. This leads to the same values of $l_{\phi }$ independent of the analysis approach applied to the same set of data.

Melting experiments require rapid data acquisition due to instabilities of the molten sample and optical drifting due to the high required laser power. In this work, the melting curve of zirconium has been determined up to 80 GPa and 4000 K using in situ fast x-ray diffraction (XRD) in a laser-heated diamond anvil cell. The main method used for melt detection was the direct observance of liquid diffuse scattering in the XRD patterns and it has been proven to be a reliable melting diagnostic. The effectiveness of other melting criteria such as the appearance of temperature plateaus with increasing laser power is also discussed.

We examine the interaction between precipitates and grain boundaries, which undergo shear-coupled motion. The elastic problem, emerging from grain boundary perturbations and an elastic mismatch strain induced by the precipitates, is analyzed. The resulting free elastic energy contains interaction terms, which are derived numerically via the integration of the elastic energy density. The interaction of the shear-coupled grain boundary and the coherent precipitates leads to potential elastic energy reductions. Such a decrease of the elastic energy has implications for the grain boundary shape and also for the solubility limit near the grain boundary. By energy minimization we are able to derive the grain boundary shape change analytically. We apply the results to the Fe-C system to predict the solubility limit change of cementite near an $\alpha$-iron grain boundary.

Systems that break reciprocity offer new possibilities for controlling wave propagation. Here we study the scattering properties of coupled resonator systems that are under dynamic modulation. Strong linear nonreciprocal transmission is manifested in the acoustic regime by introducing an initial spatial phase bias to the space-time modulated coupled resonators. A theoretical model is developed to characterize the system and the results are in good agreement with experimental observations. Our work opens up new opportunities for designing compact nonreciprocal devices and developing acoustic topological insulators.

We investigate the $S=2$ antiferromagnetic quantum spin chain with the exchange and single-ion anisotropies in a magnetic field, using the numerical exact diagonalization of finite-size clusters and the level spectroscopy analysis. It is found that a magnetization plateau possibly appears at half of the saturation magnetization for some suitable anisotropy parameters. The level spectroscopy analysis indicates that the 1/2 magnetization plateau is formed by two different mechanisms, depending on the anisotropy parameters. The phase diagram of the 1/2 plateau states and some typical magnetization curves are also presented. In addition, the biquadratic interaction is revealed to enhance the plateau induced by the Haldane mechanism.

Strong dependence of the Josephson energy on the magnetization orientation in Josephson junctions with ferromagnetic interlayers and spin-orbit coupling opens a way to control magnetization by Josephson current or Josephson phase. Here we investigate the perspectives of magnetization control in superconductor/ferromagnet/superconductor (S/F/S) Josephson junctions on the surface of a 3D topological insulator hosting Dirac quasiparticles. Due to the spin-momentum locking of these Dirac quasiparticles a strong dependence of the Josephson current-phase relation on the magnetization orientation is realized. It is demonstrated that this can lead to splitting of the ferromagnet's easy axis in the voltage driven regime. We show that such a splitting can lead to stabilization of an unconventional fourfold degenerate ferromagnetic state.

Superhard bulk materials with large bulk modulus like diamond and cubic BN (c-BN) show high thermal conductivity (κ) reaching 2000–3000 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. The large modulus means large group velocity, which contributes to high lattice thermal conductivity. However, whether the hardening of phonon bands would increase or decrease the phonon scattering rate through the phonon-phonon scattering phase space is not evident and should depend on the material type. In this work, we target cubic silicon nitride ($\mathrm{c}\text{−}\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$), which was recently developed and found to be the third superhardest material next to the diamond and c-BN. The κ of polycrystalline $\mathrm{c}\text{−}\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$ was measured with the time-domain thermoreflectance method, and thermal conductivity of 23 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ was obtained at room temperature. The first-principles thermal conductivity calculations identified that this translates to a single-crystal value of about 88 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, which is more than an order of magnitude smaller than those of diamond and c-BN. The calculation further identified that difference arises from the large scattering phase space and anharmonic amplitude of $\mathrm{c}\text{−}\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$, which are two to three times larger than the values of diamond. It is found that the scattering phase space of $\mathrm{c}\text{−}\mathrm{S}{\mathrm{i}}_3{\mathrm{N}}_4$ optical phonons with small dispersion anomalously increases with the frequency.

The magneto-optic Kerr effect (MOKE) is a well known and handy tool to characterize ferro-, ferri-, and antiferromagnetic materials. Many of the MOKE techniques employ effects solely linear in magnetization $\mathit{M}$. Nevertheless, a higher-order term being proportional to ${\mathit{M}}^2$ and called quadratic MOKE (QMOKE) can additionally contribute to the experimental data. Here, we present detailed QMOKE spectroscopy measurements in the range of 0.8–5.5 eV based on a modified eight-directional method applied on ferromagnetic bcc Fe thin films grown on MgO substrates. From the measured QMOKE spectra, two further complex spectra of the QMOKE parameters $G_s$ and $2G_{44}$ are yielded. The difference between those two parameters, known as $\mathrm{\Delta }G$, denotes the strength of the QMOKE anisotropy. Those QMOKE parameters give rise to the QMOKE tensor $\mathit{G}$, fully describing the perturbation of the permittivity tensor in the second order in $\mathit{M}$ for cubic crystal structures. We further present experimental measurements of ellipsometry and linear MOKE spectra, wherefrom permittivity in the zeroth and the first order in $\mathit{M}$ are obtained, respectively. Finally, all those spectra are described by ab initio calculations.

We examine the question of whether a $\pi$ junction can spontaneously form in a Josephson junction between two topological superconductors. We study a junction between two time-reversal-invariant topological superconductors and show this system goes through a series of multiple transitions between a 0-junction phase, where the free energy has its minimum for a superconducting phase difference of zero, and a $\pi$-junction phase, where the free energy has its minimum for a superconducting phase difference of $\pi$. These transitions occur in the absence of a Coulomb blockade or magnetic impurities. Rather, they are driven by the spin-orbit coupling in the junction, and can be probed, for example, by measuring the tunneling density of states or the critical current as a function of the junction's length or its Fermi velocity.

We use the constrained random phase approximation to derive from first principles the Ru-$t_{2g}$ Wannier-function-based model for the Kitaev spin-liquid candidate material $\alpha \text{−}{\mathrm{RuCl}}_3$. We find the nonlocal Coulomb repulsion to be sizable compared to the local one. In addition we obtain the contribution to the Hamiltonian from the spin-orbit coupling and find it to also contain non-negligible nonlocal terms. We invoke strong-coupling perturbation theory to investigate the influence of these nonlocal elements of the Coulomb repulsion and the spin-orbit coupling on the magnetic interactions. We find that the nonlocal Coulomb repulsions cause a strong enhancement of the magnetic interactions, which deviate from experimental fits reported in the literature. Our results contribute to the understanding and design of quantum spin-liquid materials via first-principles calculations.

Unveiling the physics that governs the intertwining between the nanoscale self-organization and the dynamics of insulator-to-metal transitions (IMTs) is key for controlling on demand the ultrafast switching in strongly correlated materials and nanodevices. A paradigmatic case is the IMT in ${\mathrm{V}}_2{\mathrm{O}}_3$, for which the mechanism that leads to the nucleation and growth of metallic nanodroplets out of the supposedly homogeneous Mott insulating phase is still a mystery. Here, we combine x-ray photoemission electron microscopy and ultrafast nonequilibrium optical spectroscopy to investigate the early-stage dynamics of isolated metallic nanodroplets across the IMT in ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films. Our experiments show that the low-temperature monoclinic antiferromagnetic insulating phase is characterized by the spontaneous formation of striped polydomains, with different lattice distortions. The insulating domain boundaries accommodate the birth of metallic nanodroplets, whose nonequilibrium expansion can be triggered by the photoinduced change of the $3d$-orbital occupation. We address the relation between the spontaneous nanotexture of the Mott insulating phase in ${\mathrm{V}}_2{\mathrm{O}}_3$ and the timescale of the metallic seeds growth. We speculate that the photoinduced metallic growth can proceed along a nonthermal pathway in which the monoclinic lattice symmetry of the insulating phase is partially retained.

We study the generic band structures of the five-dimensional (5D) Weyl semimetal, in which the band degeneracies are 2D Weyl surfaces in the momentum space, and may have nontrivial linkings with each other if they carry nonzero second Chern numbers. We prove a number of theorems constraining the topological linking configurations of the Weyl surfaces, which can be viewed as a 5D generalization of the celebrated doubling theorem for 3D Weyl semimetal. As a direct physical consequence of these constraints, the 5D Weyl semimetal hosts a rich structure of topological boundary states. We show that on the 4D boundary of the 5D Weyl semimetal, there are 3D chiral Fermi hypersurfaces protected by bulk Weyl surfaces. On top of that, for bulk Weyl surfaces that are linked and carry nonzero second Chern numbers, the associated boundary 3D Fermi hypersurfaces will shrink to singularities at certain energies, which trace out a protected 1D Weyl nodal arc, in analogy to the Fermi arc on the 3D Weyl semimetal surface.

The rich structure of solid state physics provides us with Dirac materials the effective theory of which enjoys the Lorentz symmetry. In nonsymmorphic lattices, the Lorentz symmetry can be deformed in a way that the null energy-momentum vectors will correspond to the on-shell condition for tilted Dirac cone dispersion. In this sense, tilted Dirac/Weyl materials can be viewed as solid state systems where the effective spacetime is non-Minkowski. In this work, we show that the polarization tensor for tilted Dirac cone systems acquires a covariant form only when the spacetime is considered to be an appropriate deformation of the Minkowski spacetime that is compatible with the dispersion. As a unique consequence of the deformation of the geometry of the spacetime felt by the electrons in tilted Dirac cone materials, the Coulomb density-density interactions will generate corrections in both longitudinal and transverse channels. Therefore the transverse photons also participate in mediating the Coulomb forces, implying emergent Amperean forces associated with the tilt of the spacetime.