We use the methods of topological quantum chemistry to explore the topology of phonons on time-reversal symmetric crystals with the structure of the planar honeycomb (layer group $p6/mmm$). This approach is not tied to a particular model of atomic vibrations, but is applied to the most general dynamical matrix constrained only by the symmetries of the system. We show that four distinct fragile topological phonon phases are generically possible. Truncating the dynamical matrix to third-nearest neighbors yields a model that realizes the different phonon topologies, characterized by the existence of phononic edge and corner modes and by Wilson loops with winding numbers one and two. Fitting the dynamical matrix to the ab initio phonon bands shows that graphene is not very far from a topologically nontrivial phonon phase.

Owing to the interplay between charge, spin, and orbits of electrons, spin-orbit torque (SOT) has attracted considerable interest during the past decade. Despite substantial progress, the existing quantification methods of SOT still have their respective restrictions on magnetic anisotropy, entanglement between SOT effective fields, and lack of corrections for the thermal gradient and planar Hall effect. Thus, it is still essential to accurately characterize SOT across diverse samples. In this study, with the aim of removing the above-mentioned restrictions and enabling universal SOT quantification, we report the characterization of sign and amplitude of SOT by angular measurement, during which magnetization coherently rotates as enforced by an external magnetic field. First, we validate the applicability of our angular measurement method in a perpendicularly magnetized [Co-Ni]/Pt heterostructure by showing excellent agreement with the results of conventional quantification methods. Remarkably, the thermoelectric effect, i.e., the anomalous Nernst effect (ANE) arising from the temperature gradient, can be self-consistently disentangled and quantified by field dependence. The superiority of this angular measurement method has been further demonstrated in a Cu/CoTb/Cu sample with large ANE but negligible SOT, and in a [Co-Ni]/Pt sample with weak perpendicular magnetic anisotropy (PMA), for which the conventional quantification methods face difficulty and even yield a fatal error. By providing a comprehensive and versatile way to characterize SOT and thermoelectric effects in diverse heterostructures, our results provide an important foundation for the spin-orbit study and interdisciplinary research of thermal spintronics.

A formulation for thermal noise in the stochastic form of the Landau-Lifshitz-Bloch equation used for modeling the magnetization dynamics at elevated temperatures is presented. The diffusion coefficients for thermal fluctuations are obtained via the Fokker-Plank equation using the mean-field approximation of the field in defining the free energy. The presented model leads to a mean magnetization consistent with the equilibrium magnetization for small and large particles. The distribution of the magnetization magnitude is of the Maxwell-Boltzmann type. The presented model was tested by studying the equilibrium magnetization in macrospin particles at high temperatures. The model is appealing for multiscale modeling, such as modeling heat assisted magnetic recording systems and all-optical magnetization reversal.

Despite experimental evidence for the two-dimensional topological Berezinskii-Kosterlitz-Thouless (BKT) transition in superconducting films and ${}^4\mathrm{He}$ superfluid films, the observation of a BKT transition in condensed matter systems has proven to be difficult. Potential signatures of BKT transitions were reported in two-dimensional magnets, however, the observation of a classical BKT transition in nominally three-dimensional systems has naturally not been investigated as the transition requires an underlying $XY$ model with spins confined to a plane. Here we report that the temperature dependence of the electron paramagnetic resonance (EPR) linewidth observed in ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Mn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ $(x=0.1 \mathrm{and} 0.04)$ as well as in certain other three-dimensional (3D) manganites undergoing antiferromagnetic transitions is described satisfactorily by the BKT model. We explain this unexpected observation of signatures of a two-dimensional topological phase transition in 3D systems in terms of an effective two-dimensional $XY$ easy plane anisotropy induced by the magnetic field applied in the EPR experiment that allows for the mediation of long-range vortex-like correlations between spin clusters formed due to phase segregation. This conclusion is supported by a re-analysis of EPR results reported earlier in La-doped ${\mathrm{CaMnO}}_3$ and a nonmanganite compound ${\mathrm{BaNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. We infer that field-induced BKT correlations in a 3D system provide a step towards the observation of a BKT transition in a suitably chosen condensed matter system by the application of an appropriate magnetic field.

We propose a finite-temperature phase diagram for the two-dimensional spin-$1/2$ $J_1-J_2$ XXZ antiferromagnet on a triangular lattice. Our analysis, based on a composite fermion representation, yields several phases. This includes a zero-temperature helical spin liquid with $N=6$ anisotropic Dirac cones, and with nonzero vector chirality implying a broken ${\mathbb{Z}}_2$ symmetry. It is terminated at $T=0$ by a continuous quantum phase transition to a ${120}^{\circ }$ ordered state around $J_2/J_1\approx 0.089$ in the XX limit; these phases share a double degeneracy, which persists to finite $T$ above the helical spin liquid. By contrast, at $J_2/J_1\simeq 0.116$, the transition into a stripe phase appears as first order. We further discuss experimental and numerical consequences of the helical order and the anisotropic nature of the Dirac dispersion.

Mn-based antiperovskite compounds in the form ${\mathrm{Mn}}_{3}AX$, where $A$ is a main group element and $X$ is C or N, undergo magnetostructural transitions with which these materials acquire magnetocaloric, giant magnetoresistance, and spin-transport properties, which can be modified or tailored by manipulating the compositions of numerous compounds. This enables closer investigations and better understandings of the underlying principles governing these properties. ${\mathrm{Mn}}_{3-x}{\mathrm{Ni}}_x\mathrm{GaC}$, which is a derivative of the prototype ${\mathrm{Mn}}_3\mathrm{GaC}$ antiperovskite, would normally be expected to form a cubic structure with a homogeneous composition. Contrary to this, we find that the addition of Ni leads to a heterogenous compound consisting of an antiperovskite part and a ${\mathrm{Ni}}_2\mathrm{MnGa}$ Heusler insertions. The system shows kinetic arrest features, which we study as a function of Ni composition using the techniques of x-ray diffraction, magnetization, and neutron diffraction under a magnetic field.

We show that the magnetic response of atomically thin materials with a Dirac spectrum and spin-orbit interactions can exhibit strong dependence on electron-electron interactions. While graphene itself has a very small spin-orbit coupling, various two-dimensional (2D) compounds “beyond graphene” are good candidates to exhibit the strong interplay between spin-orbit and Coulomb interactions. Materials in this class include dichalcogenides (such as ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$), silicene, germanene, and 2D topological insulators described by the Kane-Mele model. We present a unified theory for their in-plane magnetic field response leading to “anomalous,” i.e., electron interaction dependent, transition moments. Our predictions can be potentially used to construct unique magnetic probes with high sensitivity to electron correlations.

Defect pyrochlore oxides have attracted great interest as a promising luminescent material due to their flexible composition and high electron/hole mobility. In this paper, we investigate the structural and electronic properties of lanthanide-doped (Ln) defect pyrochlore oxides ${\mathrm{KNbWO}}_6:0.125{\mathrm{Ln}}^{3+}$ by using first-principles calculations. We perform structural optimizations of various defect pyrochlore models and calculate their electronic structures, revealing that hydration has a significant influence on both the local symmetry around the ${\mathrm{Eu}}^{3+}$ ion and the band structure with an alteration of their luminescent behavior. In the hydrated compounds, the electric-dipole ${}^{5}D_0\text{−}{}^{7}F_2$ transition is found to be partially suppressed by a raised local symmetry, and the water molecules in the compounds can mediate the nonradiative energy transfer between the activator ${\mathrm{Eu}}^{3+}$ ions and the host. It turns out that the oxygen vacancies are detrimental to luminescence as they reduce the ${\mathrm{Eu}}^{3+}$ ion in its vicinity to the ${\mathrm{Eu}}^{2+}$ ion and also serve as traps for conduction electrons excited by incident light. Our calculations for ${\mathrm{KNbWO}}_6:0.125{\mathrm{Ln}}^{3+}$ (Ln=Ce, Pr, Nd, Pm, Sm) support that the defect pyrochlore oxide ${\mathrm{KNbWO}}_6$ can also be used as a luminescence host for ${\mathrm{Ln}}^{3+}$ ion doping, giving valuable insight into a variation trend in luminescent properties of these materials at the atomic level.

The Weyl semimetal NbP exhibits a very small Fermi surface consisting of two electron and two hole pockets, whose fourfold degeneracy in $k$ space is tied to the rotational symmetry of the underlying tetragonal crystal lattice. By applying uniaxial stress, the crystal symmetry can be reduced, which successively leads to a degeneracy lifting of the Fermi-surface pockets. This is reflected by a splitting of the Shubnikov–de Haas frequencies when the magnetic field is aligned along the $c$ axis of the tetragonal lattice. In this study, we present the measurement of Shubnikov–de Haas oscillations of single-crystalline NbP samples under uniaxial tension, combined with state-of-the-art calculations of the electronic band structure. Our results show qualitative agreement between calculated and experimentally determined Shubnikov–de Haas frequencies, demonstrating the robustness of the band-structure calculations upon introducing strain. Furthermore, we predict a significant shift of the Weyl points with increasing uniaxial tension, allowing for an effective tuning to the Fermi level at only 0.8% of strain along the $a$ axis.

In this paper, we explore the decoherence dynamics of a probing spin coupled to a spin bath, where the spin bath is given by a controllable 1D transverse-field Ising chain. The 1D transverse-field Ising chain with free-end boundary condition is equivalent to a modified Kitaev model with nonlocal Majorana bound states in its topological phase. We find that the non-Markovian decoherence dynamics of the probing spin can manifest the topological structure of the spin chain. By controlling the external magnetic field on the Ising chain, we find a close relationship between the topological phase transition and the non-Markovian dynamics in the real-time domain. We also investigate the corresponding quantum entanglement dynamics in this topological system.

Singlet fission is commonly defined to involve a process by which an overall singlet state with local triplet structure spin decoheres into two triplet states, thereby completing the fission process. This process, often defined in loose terms involving the multiplicity of the overall state, is investigated here using a uniform Heisenberg spin chain subject to a dephasing environmental interaction. We introduce results from quantum information theory which enables the quantification of coherence and entanglement in a bi- and multipartite system. The calculated measures of these quantum effects can be linked to observables, such as magnetization and total spin, with simulations of the model and using theoretical methods. We demonstrate that these observables can act as a proxy for the coherence and entanglement measures. The decay of both of these between the two local triplets can be monitored, enabling a clear definition of the spin-decoherence process in singlet fission.

We have computed static and dynamic properties of an electron coupled to hard-core-boson (HCB) degrees of freedom in one spatial dimension. The polaron, an electron dressed with HCB excitations, remains light even in the strong-coupling limit as its effective mass remains of the order of the free-electron mass. This result is in a sharp contrast to the Holstein model where the electron effective mass increases exponentially with the electron-phonon coupling. HCB degrees of freedom mediate the attractive potential between two electrons that form a bound singlet bipolaron state at any nonzero coupling strength. In the low-frequency regime of the electron spectral function we observe a quasiparticle (QP) band that is separated from the continuum of states only in the central part of the Brillouin zone. The quasiparticle weight approaches zero as the QP band enters the continuum where it obtains a finite lifetime. At finite temperature an electron can annihilate thermally excited HCBs. Such thermally activated processes lead to a buildup of the spectral weight below the QP band. While the investigated model bears a resemblance with the Holstein model, we point out many important differences that originate from the binary HCB excitation spectrum, which in turn mimics spin-$\frac{1}{2}$ degrees of freedom.

Motivated by the recently observed insulating states in twisted bilayer graphene, we study the nature of the correlated insulating phases of the twisted bilayer graphene at commensurate filling fractions. We use the continuum model and project the Coulomb interaction onto the flat bands to study the ground states by using a Hartree-Fock approximation. In the absence of the hexagonal boron nitride substrate, the ground states are the intervalley coherence states at charge neutrality (filling $\nu =0$, or four electrons per moiré cell) and at $\nu =-1/4$ and $-1/2$ (three and two electrons per cell, respectively) and the $C_2\mathcal{T}$ symmetry-broken state at $\nu =-3/4$ (one electron per cell). The hexagonal boron nitride substrate drives the ground states at all $\nu$ into $C_2\mathcal{T}$ symmetry broken-states. Our results provide good reference points for further study of the rich correlated physics in the twisted bilayer graphene.

Quantum many-body edge and extended magnon excitations from the 1/3 plateau of the anisotropic Heisenberg model on an open ${AB}_2$ chain in a magnetic field $h$ are unveiled using the density matrix renormalization group and exact diagonalization. By tuning both the anisotropy and $h$ in the rich phase diagram, the edge states penetrate in the bulk, whose gap closes in a symmetry-protected topological Kosterlitz-Thouless transition. Also, we witness the squeezed chain effect, the breaking of the edge state degeneracy, and a topological change of the excitations from gapped magnons with quadratic long-wavelength dispersion to a linear spinon dispersion in the Luttinger liquid gapless phase as the anisotropy $\lambda$ approaches the critical point from the $\lambda >0$ side of the phase diagram.

We apply a full-polaron master equation and a weak-coupling non-Markovian master equation to describe the steady-state time-averaged properties of a driven two-level system, an electron coherently tunneling between double quantum dots (DQDs), interacting with a bosonic phonon bath. Comparing the results obtained using these two master equations with those from a recent DQD experiment and its corresponding weak-coupling theoretical method, we find that the original parameter set used in the experiment and theoretical method is not in the weak-coupling parameter regime. By using the full-polaron master equation with a slight adjustment on only the value of the interdot separation in the original experimental parameter set, we find that a reasonable fit to the experimentally measured time-averaged steady-state population data can be achieved. The adjusted interdot separation is within the possible values allowed by the geometry of the surface gates that define the DQD in the experiment. Our full-polaron equation approach does not require the special renormalization scheme employed in their weak-coupling theoretical method, and can still describe the experimental results of driving-induced phonon-enhanced steplike shoulder behaviors in the experiment. This demonstrates that the full-polaron master equation approach is a correct and efficient tool to describe the steady-state properties of a driven spin-boson model in the case of strong system-environment coupling.

Motivated by the recent discovery of the Mott insulating phase and unconventional superconductivity due to the flat bands in twisted bilayer graphene, we propose more generic ways of getting two-dimensional (2D) emergent flat-band lattices using either 2D Dirac materials or ordinary electron gas (2DEG) subject to moderate periodic orbital magnetic fields with zero spatial average. We find stark contrast between Schrödinger and Dirac electrons, i.e., the former show recurring “magic” values of the magnetic field when the lowest band becomes flat, whereas, for the latter, the zero-energy bands are asymptotically flat without magicness. By examining the Wannier functions localized by the smooth periodic magnetic fields, we are able to explain these nontrivial behaviors using minimal tight-binding models on a square lattice. In particular the magicness of the 2DEG can be understood in terms of destructive quantum interference similar to classic flat-band lattice models. The two cases can be interpolated by varying the $g$ factor or effective mass of a 2DEG and by taking into account the Zeeman coupling, which also leads to flat bands with nonzero Chern numbers for each spin. Our paper provides flexible platforms for exploring interaction-driven phases in 2D systems with on-demand superlattice symmetries.

Recently experiments have shown very significant spin activity in biological molecules such as DNA, proteins, oligopeptides, and aminoacids. Such molecules have in common their chiral structure, time reversal symmetry and the absence of magnetic exchange interactions. The spin activity is then assumed to be due to either the intrinsic spin-orbit (SO) interaction or SO coupled to the presence of strong local sources of electric fields. Here we derive an analytical tight-binding Hamiltonian model for oligopeptides that contemplates both intrinsic SO and Rashba interaction induced by hydrogen bonding. We use a lowest order perturbation theory band-folding scheme and derive the reciprocal space intrinsic and Rashba type Hamiltonian terms to evaluate the spin activity of the oligopeptide and its dependence on molecule uniaxial deformations. SO strengths in the tens of meV are found and explicit spin active deformation potentials. We find a rich interplay between responses to deformations both to enhance and diminish SO strength that allow for experimental testing of the orbital model. Qualitative consistency with recent experiments shows the role of hydrogen bonding in spin activity. Hydrogen bonding as the source of spin activity further enhances, coupled to chirality, the ubiquity of spin effects that may be pervasive and functional in biological molecular structures.

Recent experiments on semiconductor quantum dots have demonstrated the ability to utilize a large quantum dot to mediate superexchange interactions and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. Taking advantage of this opportunity requires a deeper understanding of how to control superexchange interactions in these systems. Here, we consider a triple-dot system arranged in linear and triangular geometries. We use configuration interaction calculations to investigate the interplay of superexchange and nearest-neighbor exchange interactions as the location, detuning, and electron number of the mediating dot are varied. We show that superexchange processes strongly enhance and increase the range of the net spin-spin exchange as the dots approach a linear configuration. Furthermore, we show that the strength of the exchange interaction depends sensitively on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, two-dimensional quantum dot arrays.

We generate translationally invariant systems exhibiting many-body localization from all-bands-flat single-particle lattice Hamiltonians dressed with suitable short-range many-body interactions. This phenomenon, dubbed many-body flatband localization, is based on symmetries of both single-particle and interaction terms in the Hamiltonian, and it holds for any interaction strength. We propose a generator of corresponding Hamiltonians which covers both interacting bosons and fermions for arbitrary lattice dimensions, and we provide explicit examples of such models in one and two lattice dimensions. We also explicitly construct an extensive set of local integrals of motion for this set of models. Our results can be further generalized to long-range interactions as well as to systems lacking translational invariance.