Quantum spin orbital liquids (QSOLs) are a novel phase of matter, similar to quantum spin liquids, with quantum fluctuations in both spin and orbital degrees of freedom. We use nonequilibrium Green's function theory to study out-of-equilibrium spin transport in an exactly solvable QSOL model put forward by Yao and Lee. We find that the spin transport problem can be mapped to that of a free fermion problem with effective fermionic baths that have a rapidly varying density of states. In the gapless phase, the spin current $I_s\text{−}{V}_s$ relation is thus highly nonlinear, while in the chiral gapped phase, the spin current conductance is quantized to be $1/2\pi$ provided that the contacts are sufficiently wide. The quantized conductance is a signature of the topological nature of the chiral gapped QSOL.

We have investigated the effects of spatial-inversion and time-reversal symmetry breaking on the acoustic phonon branches in chiral MnSi using high-resolution inelastic x-ray scattering and first-principles calculations. We find a momentum-transfer-dependent ( $q$ -dependent) splitting between transverse phonon bands having angular momentum parallel or antiparallel to $q$ . This is understood by a phenomenological theory using a gyrotropic tensor. We observed no significant impact from the time-reversal symmetry breaking induced by a magnetic field (energy shifts < 0.3 meV). This suggests the effect of time-reversal symmetry breaking is small or is restricted to a very-low-energy regime in this material, possibly due to small spin-orbit interaction.

We investigated microscopic lattice states in the donor-acceptor ionic Mott insulator, tetrathiafulvalene-bromanil (TTF-BA), by ${}^{79}\mathrm{Br}\text{−}$nuclear quadrupole resonance (NQR) spectroscopy to explore cross-correlated fluctuations between spin, charge, and the lattice. A ferroelectric transition with lattice dimerization is captured by a NQR line splitting with the critical exponent $\beta$ of $0.40\pm 0.03$, close to the $\beta$ value in the three-dimensional Ising universality class, 0.33, and a peak formation in the spin-lattice relaxation rate $T_1^{-1}$ at the transition temperature $T_{\mathrm{c}}$, of 53 K. Notably, $T_1^{-1}$ does not obey the conventional $T^2$ law expected for the Raman process of phonons even far above $T_{\mathrm{c}}$, indicating the emergence of extraordinary lattice fluctuations. They are very probably associated with polar fluctuations in the paraelectric and paramagnetic phase of TTF-BA and explain the anomalously suppressed paramagnetic spin susceptibility prior to the nonmagnetic ferroelectric order as observed by Sunami et al. [Phys. Rev. Research 2, 043333 (2020)].

As a potential degree of freedom, the chirality of phonons is expected to become a carrier of information. Exploring the phonon mode that can stably maintain chirality along a certain transport path is the basis for its realization. In this work, we find a chiral interface mode that is obviously localized at the line defect in the hexagonal boron nitride intralayer heterojunction, which is topologically protected. The existence of mirror symmetry results in the mode having opposite chiralities on both sides of the interface. For the B-B and N-N interface configurations, the chiral mode at the heavy-atom interface is more localized, which is determined by the local symmetry and atomic vibration intensity. By applying different harmonic forces, the topological chiral phonons at two valleys can be selectively excited and can be directionally transmitted without dissipation. With the decrease of the size of the heterojunction nanoribbon, the chiral interface mode always exists in the structurally stable systems. Our work enriches the research of chiral phonons in quasi-one-dimensional systems and can also provide certain references for the design of chiral phonon devices.

We use density-functional theory calculations to explore the magnetic properties of perovskite rare-earth nickelates $\mathcal{R}\mathrm{Ni}{\mathrm{O}}_3$ by constructing microscopic magnetic models containing all relevant exchange interactions via Wannierization and Green's function techniques. These models elucidate the mechanism behind the formation of antiferromagnetic order with the experimentally observed propagation vector, and explain the reason previous DFT plus Hubbard $U$ calculations favored ferromagnetic order. We perform calculations of magnetic moments and exchange-coupling parameters for different amplitudes of the $R_1^{+}$ breathing-mode distortion, which results in expanded and compressed $\mathrm{Ni}{\mathrm{O}}_6$ octahedra. Our analysis shows that the strong competition between nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic couplings determine the magnetic ordering. The inclusion of spin-orbit coupling demonstrates that the magnetic anisotropy is very small, while the magnetic moments of the short bond nickel atoms tend to zero similar to the collinear case. Finally, we show that nickelates with larger rare-earth ions display overall stronger exchange couplings, resulting in a more stable antiferromagnetic phase. Our results provide a clear picture of the trends of the magnetic order across the nickelate series and give insights into the coupling between magnetic order and structural distortions.

In this work, we study the magnetoelastic behavior of ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x$ ($0.11<x<0.19$) thin films, grown on glass and oxidized Si(100) amorphous substrates, that present an isotropic crystalline texture in the film plane. The magnetoelastic coupling coefficients are obtained through the cantilever deflection technique. We find that the magnetoelastic response is larger for samples grown on glass with respect to those grown on Si(100), and such a response increases for larger Ga concentrations for both substrates used. Furthermore, the increasing substrate temperature during growth does not appear to have a significant effect on magnetoelastic behavior of samples grown on Si(100). From a model that takes into account the elastic grain interaction for isotropic systems, we are able to describe the experimentally observed behavior. We find that the magnetoelastic response of the samples grown on glass are well described by the Voigt model, while the samples on Si(100) present an intermediate response between the Voigt and Reuss models.

Employing a combination of symmetry analysis, low-energy modeling, and ab initio simulations, we predict the presence of magnetic-field-induced Weyl points close to the Fermi level in $\mathrm{Ca}\mathrm{K}{\mathrm{Fe}}_4{\mathrm{As}}_4$. Depending on the relative strengths of the magnetic field and of the spin-orbit coupling, the Weyl fermions can carry a topological charge of $\pm 1$ or $\pm 2$, making $\mathrm{Ca}\mathrm{K}{\mathrm{Fe}}_4{\mathrm{As}}_4$ a rare realization of a double-Weyl semimetal. We further predict experimental manifestations of these Weyl points, both in bulk properties, such as the anomalous Hall effect, and in surface properties, such as the emergence of prominent Fermi arcs. Because $\mathrm{Ca}\mathrm{K}{\mathrm{Fe}}_4{\mathrm{As}}_4$ displays unconventional fully gapped superconductivity below 30 K, our findings open a route to investigate the interplay between superconductivity and Weyl fermions.

We study scattering of itinerant electrons off a magnetic hopfion in a three-dimensional metallic magnet described by a magnetization vector $\mathit{S}\left(\mathit{r}\right)$. A hopfion is a confined topological soliton of $\mathit{S}\left(\mathit{r}\right)$ characterized by an emergent magnetic field $B_{\gamma }\left(\mathit{r}\right)\equiv {\epsilon }_{\alpha \beta \gamma }\mathit{S}·({\nabla }_{\alpha }\mathit{S}\times {\nabla }_{\beta }\mathit{S})/4\ne 0$ with vanishing average value $\langle \mathit{B}\left(\mathit{r}\right)\rangle =0$. We evaluate the scattering amplitude in the opposite limits of large and small hopfion radius $R$ using the eikonal and Born approximations, respectively. In both limits, we find that the scattering cross section contains a skew-scattering component giving rise to the Hall effect within a hopfion plane. That conclusion contests the popular notion that the Hall effect in noncollinear magnetic structures necessarily implies $\langle \mathit{B}\left(\mathit{r}\right)\rangle \ne 0$. In the limit of small hopfion radius $pR\ll 1$, we expand the Born series in powers of momentum $p$ and identify different expansion terms corresponding to the hopfion anisotropy, toroidal moment, and skew scattering.

Recent numerical simulations find a possible chiral spin-liquid state in the intermediate coupling regime of the triangular lattice Hubbard model. Here we provide a simple picture for its origin in terms of a bosonic resonating valence bond (RVB) description. More specifically, we show that such a chiral spin-liquid state can be understood as a quantum disordered tetrahedral spin state stabilized by the four spin ring exchange coupling, which suppresses the order-by-disorder effect toward a stripy spin state. Such a chiral spin-liquid state features a spin Berry phase of $\frac{\pi }{2}$ per triangle. However, we show that the topological property of such a state is totally missed in the Schwinger boson mean-field description as a result of the lack of boson rigidity caused by the no double occupancy constraint.

We study the interplay of strong electron correlations and intratriangle spin exchange at two-thirds filling of the single-orbital Hubbard model on the decorated honeycomb lattice using rotationally invariant slave bosons. We find that the spin exchange tunes between a spin-1 Mott insulator, a metal, and a spin-0 Mott insulator when the exchange is antiferromagnetic. The Mott insulators occur from effective intratriangle multiorbital interactions and are adiabatically connected to the ground state of an isolated triangle. An antiferromagnetic spin exchange, as determined by the Goodenough-Kanamori rules, may occur in coordination polymers from kinetic exchange via the ligands. We characterize the magnetism in the regime where spin triplets dominate. For small $U$ a spin-1 Slater insulator occurs with antiferromagnetic order between triangles. Magnetism in the spin-1 Mott insulator is described by a spin-1 Heisenberg model on a honeycomb lattice, whose ground state is Néel ordered.

Electrically induced formation of valley-polarized free-carrier domains is theoretically investigated in two-dimensional (2D) honeycomb lattice systems exhibiting topological valley transport and the valley Hall effect. The results show that under a strong electric field $E$ applied along a nanostrip, the domains can be formed across the strip when this transverse dimension is comparable to the intervalley diffusion length $L_{iv}$. Further, these domains are found to be characterized by two distinct length scales dependent on $E$ and $L_{iv}$, i.e., the extended diffusion length $L_{\mathrm{ext}}$ ($\propto L_{iv}E$) and the compressed diffusion length $L_{\mathrm{com}}$ ($\propto L_{iv}/E$). The former determines the extension of the domain plateaus, whereas the latter specifies the width of the domain wall. Within each of the domains (excluding a narrow region of the domain wall; $L_{\mathrm{ext}}\gg L_{\mathrm{com}}$), the charge carriers are fully polarized belonging to only one of the valleys, providing a pure bulk valley current inside the considered domain region. The domain properties including the polarization amplitude and domain wall position are analyzed as a function of the applied electric field and intervalley scattering rates at the edges. The position of the domain wall, which determines the relative extension of the plateaus, can be controlled by a transverse current flowing between the Hall-type contacts. The current-voltage characteristic demonstrates a superlinear behavior in the range of electric fields corresponding to the valley-polarization domain formation. This feature is a distinctive signature of the anomalous transport arising from Bloch-band Berry curvature, which enhances the longitudinal conductance and diminishes the channel resistance. By reversing the applied electric field, the valley polarization and localization of the valley-polarized currents can be abruptly switchable. We suggest that the studied scheme of electrically induced domains of valley polarization in the 2D honeycomb lattice systems can be used in novel applications with all-electrical control and manipulation of the valley degree of freedom.

We study spin excitations and the Kondo effect in open-shell nanographenes, motivated by recent scanning tunneling inelastic spectroscopy experiments. Specifically, we consider three systems: the triangulene, the extended triangulene with rocket shape, both with an $S=1$ ground state, and a triangulene dimer with $S=0$ on account of intermolecular exchange. We focus on the consequences of hybridization of the nanographene zero modes with a conducting substrate on the $dI/dV$ line shapes associated with spin excitations. The partially filled nanographene zero modes coupled to the conduction electrons in the substrate constitute multiorbital Anderson impurity models that we solve in the one-crossing approximation, which treats the coupling to the substrate to infinite order. We find that the coupling to the substrate leads to (i) renormalization of the spin flip excitation energies of the bare molecule, (ii) broadening of the spectral features, and (iii) the emergence of zero bias Kondo peaks for the $S=1$ ground states. The calculated substrate induced shift of the spin excitation energies is found to be significantly larger than their broadening, which implies that this effect has to be considered when comparing experimental results and theory.

We use a hybrid $k·p$-theory–tight-binding (HkpTB) model to describe interlayer coupling simultaneously in both Bernal and twisted graphene structures. For Bernal-aligned interfaces, the HkpTB model is parametrized using the full Slonczewski-Weiss-McClure (SWMcC) Hamiltonian of graphite [E. McCann and M. Koshino, Rep. Prog. Phys. 76, 5 (2013)], which is then used to refine the commonly used Bistritzer-MacDonald model for twisted interfaces [J. M. B. Lopes dos Santos et al., Phys. Rev. Lett. 99, 256802 (2007); R. Bistritzer and A. H. MacDonald, Proc. Natl. Acad. Sci. USA 108, 12233 (2011)], by deriving additional terms that reflect all the details of the full SWMcC model of graphite. We find that these terms introduce electron-hole asymmetry in the band structure of twisted bilayers but in twistronic multilayer graphene they produce only a subtle change in moiré miniband spectra, confirming the broad applicability of the BM model for implementing the twisted interface coupling in such systems.

Coulomb blockaded transport of topological superconducting nanowires provides an opportunity to probe the localization of states at both ends of the system in a two-terminal geometry. In addition, it provides a way for checking for subgap states away from the leads. At the same time, Coulomb blockade transport is difficult to analyze because of the interacting nature of the problem arising from the nonperturbative Coulomb interaction inherent in the phenomenon. Here we show that the Coulomb blockade transport can be modeled at the same level of complexity as quantum point contact tunneling that has routinely been used in mesoscopic physics to understand nanowire experiments provided we consider the regime where the tunneling rate is below the equilibration rate of the nanowire. This assumption leads us to a generalized Meir-Wingreen formula for the tunnel conductance which we use to study various features of the nanowire such as Andreev bound states, self-energy, and soft gap. We anticipate that our theory will provide a route to interpret Coulomb blockade transport in hybrid Majorana systems as resulting from features of the nanowire, such as Andreev bound states and soft gaps.

The structure of a 3 × 3 reconstruction formed by Ge deposition onto Al(111) [Al(111)3 × 3-Ge] was investigated by coaxial impact collision ion scattering spectroscopy, which is one of the most surface-sensitive structural analysis techniques. It has been revealed that Al(111)3 × 3-Ge forms a structure with 10 Ge atoms in a unit cell, similar to the recently reported “kagome-like” silicene on Al(111) rather than the previously reported honeycomb lattice. A surface band structure of the Al(111)3 × 3-Ge has also been investigated by angle-resolved photoemission spectroscopy (ARPES). The calculated band structure of our proposed model based on a density functional theory well reproduces ARPES spectra, which also indicates that Al(111)3 × 3-Ge forms a kagome-like structure.

${\mathrm{Ni}}_2{\mathrm{Mn}}_{2-y}{\mathrm{In}}_y$ alloys transform from the martensitic $L{1}_0$ antiferromagnetic ground state near $y=0$ to the austenitic ferromagnetic $L{2}_1$ Heusler phase near $y=1$ due to doping of In impurity for Mn. The off-stoichiometric alloys prepared by rapid quenching are structurally metastable and dissociate into a mixture of $L{2}_1$ (${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{In}$) and $L{1}_0$ (NiMn) phases upon temper annealing. Despite this structural disintegration, the martensitic transformation temperature remains invariant in the temper annealed alloys. Investigations of the local structure of the constituent atoms reveal the presence of strongly coupled ${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{In}$ and NiMn structural units in the temper annealed as well as the rapidly quenched off-stoichiometric ${\mathrm{Ni}}_2{\mathrm{Mn}}_{2-y}{\mathrm{In}}_y$ alloys irrespective of their crystal structure. This random packing of the $L{2}_1$ and $L{1}_0$ structural units seems to be responsible for invariance of martensitic transition temperature in the temper annealed alloys as well as the absence of strain glass transition in rapidly quenched alloys.

Using the Landau-Devonshire approach and available experimental results, we constructed multiwell thermodynamic potential of the layered ferroelectric $\mathrm{Cu}\mathrm{In}{\mathrm{P}}_2{\mathrm{S}}_6$ (CIPS). The analysis of temperature dependences of the dielectric permittivity and lattice constants for different applied pressures unexpectedly reveals the critically important role of a nonlinear electrostriction in this material. With the nonlinear electrostriction included we calculated the temperature and pressure phase diagrams and spontaneous polarization of a bulk CIPS, within the assumed range of applicable temperatures and applied pressures. Using the developed thermodynamic potential, we revealed the strain-induced phase transitions in thin epitaxial СIPS films, as well as the stress-induced phase transitions in СIPS nanoparticles, the shape of which varies from prolate needles to oblate disks. We also revealed the strong influence of a mismatch strain, elastic stress, and shape anisotropy on the phase diagrams and polar properties of a nanoscale CIPS, and derived analytical expressions allowing for elastic control of the nanoscale CIPS polar properties. Hence obtained results can be of particular interest for the strain engineering of nanoscale layered ferroelectrics.

For the technologically relevant spin Hall effect, most theoretical approaches rely on the evaluation of the spin-conductivity tensor. In contrast, for most experimental configurations the generation of spin accumulation at interfaces and surfaces is the relevant quantity. Here, we directly calculate the accumulation of spins due to the spin Hall effect at the surface of a thin metallic layer, making quantitative predictions for different materials. Two distinct limits are considered, both relying on a fully relativistic Korringa-Kohn-Rostoker density functional theory method. In the semiclassical approach, we use the Boltzmann transport formalism and compare it directly with a fully quantum mechanical nonequilibrium Keldysh formalism. Restricting the calculations to the spin-Hall-induced, odd-in-spatial-inversion, contribution in the limit of the relaxation time approximation, we find good agreement between the two methods, where deviations can be attributed to the complexity of Fermi surfaces. Finally, we compare our results with experimental values of the spin accumulation at surfaces as well as the Hall angle and find good agreement for the trend across the considered elements.

Due to thermal and quantum fluctuations the charge density and the electromagnetic field in metallic structures possess randomly changing components, which control the signal to noise ratio in sensors. With the emergence of nanotechnology individual sensors are made with smaller and smaller structures. We calculated and measured the spatial distribution of the mean-square value of the local fluctuation of the charge density and the electromagnetic field in micrometer-size metallic circular and triangular films and found that the spatial distribution of the fluctuation is not uniform; the magnitude of the fluctuation at a fixed frequency can change abruptly by an order of magnitude as the size of the structure is changed. This previously unanticipated result provides for understanding of fundamental physics and help improve the design of devices.