Polarization, denoting the precession direction with respect to the background magnetization, is an intrinsic degree of freedom of a spin wave. Using magnetic textures to control the spin-wave polarization is fundamental and indispensable toward reprogrammable polarization-based magnonics. Here, we show that due to the intrinsic cubic anisotropy, a ${90}^{\circ }$ antiferromagnetic domain wall naturally acts as a spin-wave retarder (waveplate). Moreover, for a ${90}^{\circ }$ domain wall pair developed by introducing a second domain in a homogenous antiferromagnetic wire, the sign of the retarding effect can be flipped by simply switching the magnetization direction of the intermediate domain.

We theoretically investigate the magnetoelastic coupling (MEC) and its effect on magnon transport in two-dimensional antiferromagnets with a honeycomb lattice. MEC coefficients along with magnetic exchange parameters and spring constants are computed for monolayers of transition-metal trichalcogenides with Néel magnetic order (${\text{MnPS}}_3$ and ${\text{VPS}}_3$) and zigzag order (${\text{CrSiTe}}_3$, ${\text{NiPS}}_3$, and ${\text{NiPSe}}_3$) by ab initio calculations. Using these parameters, we predict that the spin-Nernst coefficient is significantly enhanced due to magnetoelastic coupling. Our study shows that although Dzyaloshinskii-Moriya interaction can produce spin-Nernst effect in these materials, other mechanisms such as magnon-phonon coupling should be taken into account. We also demonstrate that the magnetic anisotropy is an important factor for control of magnon-phonon hybridization and enhancement of the Berry curvature and thus the spin-Nernst coefficient. Our results pave the way toward gate tunable spin current generation in two-dimensional magnets by spin-Nernst effect via electric field modulation of MEC and anisotropy.

Stable compounds in the V-N binary system are systematically investigated and four new phases are found: $Pbam\text{−}{\mathrm{V}}_5{\mathrm{N}}_2, Pnma\text{−}{\mathrm{V}}_2\mathrm{N}, P\overline{3}m1\text{−}{\mathrm{V}}_2{\mathrm{N}}_3$, and $I4/mcm\text{−}{\mathrm{VN}}_2$. All the predicted high-pressure vanadium nitrides are dynamically stable at ambient pressure. Moreover, the thermodynamic stability of vanadium nitrides in the temperature range of 0–1500 K at different pressures (0, 20, 40, 60, and 120 GPa) was also evaluated within the harmonic approximation. The sequence of phases of ${\mathrm{V}}_2\mathrm{N}$ under pressure is $\varepsilon {\text{-Fe}}_2\text{N}\text{type}\to \zeta {\text{-Fe}}_2\text{N}\text{type}\to {\text{Fe}}_2\text{C}\text{type}\to Pnma{\text{-V}}_2\text{N}$. In addition, relative stability and lattice dynamics properties of several vanadium mononitrides are systematically calculated and discussed. Structural features, mechanical properties, electronic structures, and chemical bonding of all the V-N compounds are analyzed at 0 GPa. Among these vanadium nitrides, WC-type VN has the highest Vickers hardness ($\sim 37\mathrm{GPa}$) and superior fracture toughness (4.3–6.1 MPa ${\mathrm{m}}^{1/2}$), which mainly originate from its strong V-N bonding as well as its strong three-dimensional V-N covalent bond network. The configuration of the strong and short N-N covalent bonds enables the new phase $I4/mcm\text{−}{\mathrm{VN}}_2$ to exhibit good mechanical properties. Our results also reveal that the formation of a strong covalent-bond network topology in a crystal is a fundamental principle for designing a hard or superhard structure.

Melting of solids usually starts with a thin liquid layer forming on the surface at temperatures below their bulk melting point. While the liquid phase formation in the so-called premelting is understood to be governed by the interface energies among the coexisting solid, liquid, and vapor phases, the influence of the underlying crystal structures on its kinetics, and atomistic mechanisms are unknown, and consequently, ignored in the theory of melting. Here we report the first observation of a strong influence of the underlying crystal structure on melting kinetics with the resulting localization and delocalization behavior of the surface disordering and the liquid layers. With increasing temperature, the surface disordering remains in the localized state with a constant thickness in fcc crystals, whereas it becomes delocalized by growing steadily into the bulk in bcc crystals. In both cases, the surface melting is found to occur with highly correlated atomic motion in the form of extended atomic chains and loops that emit from the disordered surface and transmit to the bulk. This newly discovered mechanism in surface melting is behind the surface melting kinetics: The close packed fcc crystal can effectively impede the correlated atomic motion and limit the surface disordering to only the localized region on surface, while the more openly packed bcc crystal allows for its proliferation by tunneling from the surface into the bulk of the crystals. These findings provide valuable insights for future development of new theories of crystal melting.

The strongly correlated system $\mathrm{Ho}{}^{11}\mathrm{B}_{12}$ with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry, and heat capacity measurements at temperatures in the range of 3–300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field (CF) at holmium sites $B_4=-0.333$ meV and $B_6=-2.003$ meV (in Stevens notations), with an unconventional large ratio $B_6/B_4$ pointing to the dominant role of conduction electrons in the formation of a CF potential. The molecular field in the antiferromagnetic (AFM) state $B_{\mathrm{loc}}=\left(1.75\pm 0.1\right)$ T has been directly determined from the INS spectra together with short-range order effects detected in the paramagnetic state. A comparison of measured magnetization in diluted ${\mathrm{Lu}}_{0.99}{\mathrm{Ho}}_{0.01}{\mathrm{B}}_{12}$ and concentrated ${\mathrm{HoB}}_{12}$ single crystals showed a strong suppression of Ho magnetic moments by AFM exchange interactions in holmium dodecaboride. To account explicitly for the short-range AFM correlations, a self-consistent holmium dimer model was developed that allowed us to reproduce successfully field and temperature variations of the magnetization and heat capacity in the cage-glass phase of ${\mathrm{HoB}}_{12}$ in external magnetic fields.

The hybridization of magnons (spin waves) with phonons, if sufficiently strong and comprising of long wavelength excitations, may offer a new playground when manipulating the magnetically ordered systems with light. Applying a magnetic field to a quasi-two-dimensional antiferromagnet, $\mathrm{Fe}{\mathrm{PS}}_3$, we tune the magnon-gap excitation to coincide with the initially lower-in-energy phonon modes. Hybrid magnon-phonon modes, the magnon polarons are unveiled with the demonstration of a pronounced avoided crossing between the otherwise bare magnon and phonon excitations. The magnon polarons in $\mathrm{Fe}{\mathrm{PS}}_3$ are traced with Raman scattering experiments. However, as we show, they also couple directly to terahertz photons, evoking their further explorations in the domain of antiferromagnetic optospintronics. The magnon-phonon coupling is also discussed as a possible reason of the magnon mode splitting observed in the absence of a magnetic field.

We show here theoretically and experimentally that a Rashba-split electron state inside a ferromagnet can efficiently convert a dynamical spin accumulation into an electrical voltage. The effect is understood to stem from the Rashba splitting but with a symmetry linked to the magnetization direction. It is experimentally measured by spin pumping in a $\mathrm{CoFeB}|\mathrm{MgO}$ structure where it is found to be as efficient as the inverse spin Hall effect at play when Pt replaces MgO, with the extra advantage of not affecting the magnetic damping in the ferromagnet.

We consider a multiparticle fermionic state to study the magnetization dynamics in an XX spin chain model. The advantage this state provides is the freedom in choosing the initial magnetization profile at each spin site. We characterize the $t=0$ properties of this state, in particular, the probability for the state to have $n$ fermions, fidelity, the two-point and the four-point correlators, and spin-spin correlations. The dynamics of the domain-wall state formed by considering the ground states with magnetizations $m_0$ and $-m_0$ in the left and right half of the chain, respectively, results in the growth of a flat plateau $(m=0)$ away from the origin. In contrast, the magnetization dynamics of the domain wall state initialized via the multiparticle state reveals the absence of such a region. Our studies of the particle-number fluctuation for the multiparticle state with all sites having identical magnetization reveals linear dependence with respect to the system size, as opposed to the $lnl$ dependence which is obtained for the ground state with the same magnetization.

Ising models with frustrated next-nearest-neighbor interactions present a rich array of modulated phases. These phases, however, assemble and relax slowly, which hinders their computational study. In two dimensions, strong fluctuations further hamper determining their equilibrium phase behavior from theoretical approximations. The exact numerical transfer matrix (TM) method, which bypasses both difficulties, can serve as a benchmark method once its own numerical challenges are surmounted. Building on our recent study [Hu and Charbonneau, Phys. Rev. B 103, 094441 (2021)], in which we evaluated the two-dimensional axial next-nearest-neighbor Ising model with transfer matrices, we here extend the effective usage of the TM method to Ising models with biaxial, diagonal, and third-nearest-neighbor frustration models. The high-accuracy TM numerics help resolve various physical ambiguities about these reference models, thus providing a clearer overview of modulated phase formation in two dimensions.

We present a microscopic derivation of the Cooper-pair insulator (CPI), a topologically ordered counterpart of the $s$-wave superconductor. For this, we study a generalized model of electrons with attractive interactions with the unitary renormalization group method. The effective Hamiltonian for the CPI corresponds to a gapped, insulating state of quantum matter arising solely from interparticle interactions (and without any need for disorder or coupling to an external magnetic field). The CPI ground-state manifold displays signatures of topological order, including a fourfold degeneracy. The CPI effective Hamiltonian can be written in terms of Wilson loops and contains a topological $\theta$-term known to be equivalent to the Chern-Simons term in two spatial dimensions. We show that the long-ranged many-particle entanglement content of the CPI ground state is driven by interhelicity two-particle scattering processes. The state with $\theta =0$ possesses the largest bipartite entanglement entropy (EE) and scales logarithmically with subsystem size ($L$). Passage from the CPI to the $s$-wave BCS superconducting ground state at $T=0$ under RG is demonstrated through the replacement of the long-ranged entanglement of the former by the short-ranged entanglement of the latter. A study of the renormalization of the entanglement in $k$ space shows that the CPI state possesses a hierarchy of scales of entanglement, and that this hierarchy collapses in the BCS state. Our work offers clear evidence for the microscopic origins of topological order in a prototypical system and lays the foundation for similar investigations in other systems of correlated electrons.

We consider how electron-phonon interaction influences the insulator-metal transitions driven by doping in a strongly correlated system. Using the polaronic version of the generalized tight-binding method, we investigate a multiband two-dimensional model taking into account both charge density displacement and transitive types of electron-lattice contributions. For adiabatic ratio $t\gg {\omega }_0$, we analyze a wide electron-phonon parameter range and demonstrate the relationship between transition features and such properties of the system as polaron and bipolaron crossovers, the redistribution of the spectral weight due to the multiparticle effects of Coulomb and electron-phonon interactions, orbital selectivity, flat-band formation, and pseudogap behavior of various origins.

Using a tight-biding model, we elaborate that the previously discovered out-of-plane polarized helical edge spin current caused by Rashba spin-orbit coupling can be attributed to the fact that in a strip geometry, a positive momentum eigenstate does not always have the same spin polarization at the edge as the corresponding negative momentum eigenstate. In addition, in the presence of a magnetization pointing perpendicular to the edge, an edge charge current is produced, which can be chiral or nonchiral depending on whether the magnetization lies in-plane or out-of-plane. The spin polarization near the edge develops a transverse component orthogonal to the magnetization, which is antisymmetric between the two edges and tends to cause a noncollinear magnetic order between the two edges. If the magnetization only occupies a region near one edge, or in an irregular shaped quantum dot, this transverse component has a nonzero average, rendering a gate-voltage-induced magnetoelectric torque without the need of a bias voltage. We also argue that other types of spin-orbit coupling that can be obtained from the Rashba type through a unitary transformation, such as the Dresselhaus spin-orbit coupling, will have similar effects too.

The half-valley-metal (HVM) states where the gap of one valley is closed while the other valley remains semiconducting are quite crucial for achieving 100% valley polarization and valley-Hall-effect. However, the symmetry of materials makes the HVM states scarce. In this work, using Floquet theory, we demonstrate the laser-dressed HVM states in two-dimensional (2D) gapped-Dirac materials. We show that as a circularly polarized laser is applied to a 2D gapped-Dirac material, the gaps of the two valleys (K and K’) can be regulated by varying the photon energy, amplitude and chirality of the laser. At specific photon energies and laser amplitudes, the linear energy-momentum dispersion of Dirac materials is restituted in one valley while the gap in the other valley is preserved. On the basis of first-principles calculations, we also propose a promising candidate material, boron antimonide (BSb) monolayer to achieve the laser-dressed HVM states. More interestingly, the Berry curvature in the two valleys can be tuned by changing the laser parameters.

We present a comprehensive theory of the Dyakonov-Shur (DS) plasma instability in current-biased graphene transistors. Using the hydrodynamic approach, we derive equations describing the DS instability in the two-dimensional electron fluid in graphene at arbitrary values of electron drift velocity. These nonlinear equations together with Maxwell's equations are used for numerical analysis of the spatial and temporal evolution of the graphene electron system after the DS instability is triggered by random current fluctuations. We analyze conditions necessary for the onset of the DS instability and the properties of the final stationary state of the graphene electron system. We demonstrate that the instability results in the coherent anharmonic oscillatory state of the electron fluid and calculate both the spatial distribution and the power of the electromagnetic radiation generated by the graphene transistor in the DS instability regime.

Different channels over which electrons scatter between parts of the Fermi surface are the key to various electronic quantum matters, such as superconductivity and density waves. We consider an effective model in higher dimensions where each of the two pockets in the original model maps to (the Landau levels of) two Dirac fermions. We discover an emergent channel when two Dirac fermions from different pairs annihilate, where the presence of a strong density wave is essential. We support our analysis with numerical calculations on model examples in the vicinity of ferromagnetic and antiferromagnetic orders. We also discuss interesting consequences on electron interaction channels that beyond-mean-field fluctuations may induce.

Topological nodal-line semimetals offer an interesting research platform to explore exotic phenomena associated with its torus-shaped Fermi surface. Here, we study magnetotransport in the massive nodal-line semimetal with spin-orbit coupling and finite Berry curvature distribution which exists in many candidates. The magnetic field leads to a deformation of the Fermi torus through its coupling to the orbital magnetic moment, which turns out to be the main scenario of the magnetoresistivity (MR) induced by the Berry curvature effect. We show that a small deformation of the Fermi surface yields a positive MR $\propto B^2$, different from the negative MR by pure Berry curvature effect in other topological systems. As the magnetic field increases to a critical value, a topological Lifshitz transition of the Fermi surface can be induced, and the MR inverts its sign at the same time. The temperature dependence of the MR is investigated, which shows a totally different behavior before and after the Lifshitz transition. Our work uncovers a unique scenario of the MR induced solely by the deformation of the Fermi surface and establishes a relation between the Fermi surface topology and the sign of the MR.

The spin-orbit entangled quantum states in $4d/5d$ compounds, e.g., the $J_{\mathrm{eff}}=\frac{1}{2}$ and $J_{\mathrm{eff}}=\frac{3}{2}$ states, have attracted great interests for their unique physical roles in unconventional superconductivity and topological states. Here, the key role of tetragonal distortion is clarified, which determines the ground states of $4d^1/5d^1$ systems to be the $J_{\mathrm{eff}}=\frac{3}{2}$ one (e.g., ${\mathrm{K}}_2{\mathrm{NbCl}}_6$) or the $S=\frac{1}{2}$ one (e.g., ${\mathrm{Rb}}_2{\mathrm{NbCl}}_6$). By tuning the tetragonal distortion via epitaxial strain, the occupation weights of $d_{xy}/d_{yz}/d_{xz}$ orbitals can be subtly modulated, competing with the spin-orbit coupling. Consequently, quantum phase transitions between the $S=\frac{1}{2}$ state and the $J_{\mathrm{eff}}=\frac{3}{2}$ state, as well as between different $J_{\mathrm{eff}}=\frac{3}{2}$ states, can be achieved, resulting in significant changes of local magnetic moments. Our prediction points out a reliable route to engineer new functionality of $J_{\mathrm{eff}}$ states in these quantum materials.

Additional orbital degrees of freedom in multiband quantum systems may offer more intriguing possibilities for spin-orbit (SO) control. Here we explore the Rashba and Dresselhaus SO couplings in realistic AlInAs/GaInAs triple wells subjected to top $\left(V_{\mathrm{T}}\right)$ and back $\left(V_{\mathrm{B}}\right)$ gate potentials, allowing for flexible triple-occupancy control for electrons. By performing a self-consistent Poisson-Schrödinger calculation, we determine all the relevant Rashba (Dresselhaus) SO terms of both intraband ${\alpha }_{\nu }$ $\left({\beta }_{\nu }\right)$ and interband ${\eta }_{\mu \nu }$ $\left({\mathrm{\Gamma }}_{\mu \nu }\right)$ types. For a structurally symmetric triple well, we achieve a crossing of energy levels between the first and second subbands and sequentially an anticrossing between the second and third subbands, accompanied with a double band swapping, simply via adjusting $V_{\mathrm{B}}$. As a consequence, when $V_{\mathrm{B}}$ varies, an emerging interchange of the first-subband ${\alpha }_1$ $\left({\beta }_1\right)$ and second-subband ${\alpha }_2$ $\left({\beta }_2\right)$ SO terms, and of ${\alpha }_2$ $\left({\beta }_2\right)$ and ${\alpha }_3$ $\left({\beta }_3\right)$ respectively for the second and third subbands, i.e., double SO interchange, occurs, greatly fascinating for selective SO control among distinct subbands in spintronic devices. Further, near the two-band swapping points, across which electron transfer among the three local wells (triple well) takes place, detailed interchanging features of nonlinear SO control are also contrasting. Remarkably, in addition to Rashba terms, we also realize a wide-range control of Dresselhaus couplings, which are usually immune to electrical manipulation. Regarding the interband Rashba and Dresselhaus SO contributions, ${\eta }_{\mu \nu }$ and ${\mathrm{\Gamma }}_{\mu \nu }$ exhibit either a resonant behavior or a steplike jump across band-swapping points, depending on the parity and spatial distribution of electron wave functions. By varying the barrier height of the two inner barriers in the triple-well configuration, dependence of relevant SO terms on engineered structures, which are either structurally symmetric or asymmetric, is also discussed. Interestingly, in the latter asymmetric case, we realize a seemingly symmetric configuration by adjusting $V_{\mathrm{B}}$, in which ${\alpha }_1$ and ${\alpha }_2$ essentially vanish while ${\alpha }_3$ is nonzero, providing a handle for suppressing electron spin relaxation of chosen subbands. Our results will stimulate more experiments probing unusual SO features in multiband and multiwell quantum systems and act as a guide for more proposals designed for spintronic devices.

The transition between ergodic and many-body localized (MBL) phases is expected to occur via an avalanche mechanism, in which ergodic bubbles that arise due to local fluctuations in system properties thermalize their surroundings leading to delocalization of the system, unless the disorder is sufficiently strong to stop this process. We propose an algorithm based on neural networks that allows us to detect the ergodic bubbles using experimentally measurable two-site correlation functions. Investigating the time evolution of the system, we observe a logarithmic in time growth of the ergodic bubbles in the MBL regime. The distribution of the size of ergodic bubbles converges during time evolution to an exponentially decaying distribution in the MBL regime, and a power-law distribution with a thermal peak in the critical regime, supporting thus the scenario of delocalization through the avalanche mechanism. Our algorithm permits us to pinpoint the quantitative differences in the time evolution of systems with random and quasiperiodic potentials, as well as to identify rare (Griffiths) events. Our results open different pathways in studies of the mechanisms of thermalization of disordered many-body systems and beyond.

$\mathrm{Ce}{\mathrm{Rh}}_6{\mathrm{Ge}}_4$ is an unusual example of a stoichiometric heavy fermion ferromagnet, which can be cleanly tuned by hydrostatic pressure to a quantum critical point. To understand the origin of this anomalous behavior, we have characterized the magnetic ordering and crystalline electric field (CEF) scheme of this system. While magnetic Bragg peaks are not resolved in neutron powder diffraction, coherent oscillations are observed in zero-field $\mu \mathrm{SR}$ below $T_{\mathrm{C}}$, which are consistent with in-plane ferromagnetic ordering consisting of reduced Ce moments. From analyzing the magnetic susceptibility and inelastic neutron scattering, we propose a CEF-level scheme which accounts for the easy-plane magnetocrystalline anisotropy, where the low lying first excited CEF exhibits significantly stronger hybridization than the ground state. These results suggest that the orbital anisotropy of the ground state and low-lying excited state doublets are important for realizing anisotropic electronic coupling between the $f$ and conduction electrons, which gives rise to the highly anisotropic hybridization observed in photoemission experiments.