Collinear magnets in honeycomb lattices under the action of time-dependent strains are investigated. Given the limits of high-frequency periodically varying deformations, we derive an effective Floquet theory for spin systems that results in the emergence of a spin chirality. We find that the coupling between magnons and spin chirality depends on the details of the strain such as the spatial dependence and applied direction. Magnonic fluctuations about the ferromagnetic state are determined, and it is found that spatially homogeneous strains drive the magnon system into topologically protected phases. In particular, we show that certain uniform strain fields play the role of an out-of-plane nearest-neighbor Dzyaloshinskii-Moriya interaction. Furthermore, we explore the application of nonuniform strains, which lead to a confinement of magnon states that for uniaxial strains propagates along the direction that preserves translational symmetry. Our work demonstrates a direct way in which to manipulate the magnon spectrum based on time-dependent strain engineering that is relevant for exploring topological transitions in quantum magnonics.

In this paper we present a theoretical study of the magnetic properties of the ${\mathrm{CoCu}}_2{\mathrm{O}}_3$ compound. The magnetic effective exchange interactions and zeroth-field splitting were computed using ab initio methods, then the magnetic order and transition temperature were determined using classical Monte Carlo simulations. We showed that, unlike other members of the $A{\mathrm{Cu}}_2{\mathrm{O}}_3$ family, the presence of an additional magnetic atom, associated with a large folding of the puckered layers in the ($\vec{a}, \vec{b}$) directions, induces a magnetic pattern based on coupled three-leg ladders, quite different from the two-leg structural ladders. The propagation vector has been found to be $\vec{q}=(0,\frac{1}{2},\frac{1}{2})$. It is associated with a doubly degenerated ground state suggesting a doubling of the unit cell. The large ${\mathrm{Co}}^{2+}$-ion anisotropy was shown to be of crucial importance in the high transition temperature observed in this compound.

The transport properties, resistance, Hall effect, and low $T$ magnetoresistance for very oxygen overdoped nonsuperconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ (Bi2201) thin films are reported. From 20 to 300 K, the temperature dependence of the resistance is well described by a law of the form $a+b{T}^{4/3}$, theoretically predicted to occur in the presence of ferromagnetic fluctuations. In addition, this prediction is reinforced by the analysis of the transverse and the longitudinal low $T$ magnetoresistance. Interestingly, the presence of a weak disorder causing low $T$ electronic localization allows us to evidence very short dephasing lengths, as observed in other systems with ferromagnetic fluctuations.

The quadratic band crossing points (QBCPs) at the Fermi level in two dimensions have been proposed to be unstable under electron-electron interactions. The possible interaction-driven states include the quantum anomalous Hall (QAH) state and various nematic ordered states. In this paper, motivated by the discovery of ferromagnetic van der Waals layered metal-organic framework ${\mathrm{CrCl}}_2{\left(\mathrm{pyrazine}\right)}_2$, we theoretically propose that a single layer of ${\mathrm{CrCl}}_2{\left(\mathrm{pyrazine}\right)}_2$ might realize one or some of these interaction-driven states based on the QBCP protected by $C_4$ symmetry. By introducing short-range density-density type repulsion interactions into this system, we have found the phase diagram depending on different interaction ranges and strengths. The exotic phases include the staggered chiral flux state manifesting the QAH effect, the site-nematic insulator, and the site-nematic Dirac semimetal state. The QAH state is robust against perturbations breaking the QBCP but it is weakened by increasing temperature. The metal-organic framework is tunable by changing the transition-metal elements, which might improve the gap size and stability of this interaction-induced QAH state.

The observation is made that a linear dispersion in any dimension under acoustic-phonon- deformation-potential scattering theoretically prescribes a constant charge transport distribution, required for the boxcar profile known to maximize the thermoelectric figure of merit. A linear dispersion squeezed by two transport gaps for optimized bandwidth under scattering by phonon deformation then theoretically constitutes a globally optimal qualitative band structure that may arise in real systems.

Simulations of extended quantum systems are typically performed by extrapolating results of a sequence of finite-system-size simulations to the thermodynamic limit. In the quantum Monte Carlo community, twist-averaging was pioneered as an efficient strategy to eliminate one-body finite size effects. In the dynamical mean field community, cluster generalizations of the dynamical mean field theory were formulated to study systems with nonlocal correlations. In this work, we put the twist-averaging and the dynamical cluster approximation variant of the dynamical mean field theory onto equal footing, discuss commonalities and differences, and compare results from both techniques to the standard periodic boundary technique. At the example of Hubbard-type models with local, short-range and Yukawa-like longer range interactions we show that all methods converge to the same limit, but that the convergence speed differs in practice. We show that embedding theories are an effective tool for managing both one-body and two-body finite size effects, in particular if interactions are averaged over twist angles.

Binary transition metal nitride with a graphene-like structure and strong magnetic properties is rare. Based on the first-principles calculations, we design two kinds of $M{\mathrm{N}}_4$ ($M$ = transition metal) monolayers, which are transition metal nitrides with a planar structure, made up of $M{\mathrm{N}}_4$ units aligned in the rhombic and square patterns. The two structural lattices have robust stability and good compatibility with different metal atoms, and the underlying mechanism is the combination of $s{p}^2$ hybridization, coordinate bond, and $\pi$ conjugation. With the metal atom changing from V, Cr, Mn, Fe to Co, the total charge of $M{\mathrm{N}}_4$ system increases by one electron in turn, which results in continuous adjustability of the electronic and magnetic properties. The planar ligand field is another feature of the two $M{\mathrm{N}}_4$ lattices, which brings about the special splitting of five suborbitals of $3d$ metal atom and gives rise to strong magnetism. Moreover, room-temperature ferromagnetism in square-${\mathrm{CoN}}_4$ monolayer with the Curie temperatures of 321 K is determined by solving the Heisenberg model combined with Monte Carlo method.

For a relativistic particle confined to a Möbius strip, the effective Dirac equation is first given in the thin-layer quantization formalism. We find that an effective gauge potential results from the rotation transformation of the adapted frame moving on the Möbius strip, and that an effective mass results from the rescaling transformation determined by the metric tensor of the Möbius strip. It is intriguing that the geometry-induced gauge potential can provide an effective monopole magnetic field for the particle with spin, and can induce quantum spin Hall effects. As potential applications, an effective monopole magnetic field and spin Hall effects can be generated and manipulated by designing the geometry of a two-dimensional nanodevice.

We have studied a near-surface two-dimensional electron gas based on an InAs quantum well on a GaAs substrate. In devices without a dielectric layer we estimated large electron mobilities on the order of ${10}^5{\mathrm{cm}}^2/\mathrm{Vs}$. We have observed quantized conductance in a quantum point contact, and determined the $g$ factor. Using samples with an epitaxial Al layer, we defined multiple Josephson junctions and found the critical current to be gate tunable. Based on multiple Andreev reflections the semiconductor-superconductor interface is transparent, with an induced gap of $125\mathrm{\mu }\mathrm{eV}$. Our results suggest that this InAs system is a viable platform for use in hybrid topological superconductor devices.

In this work, we theoretically investigate the light-induced topological phases and finite-size crossovers in a paradigmatic quantum spin Hall (QSH) system with high-frequency pumping optics. Taking the HgTe quantum well for an example, our numerical results show that circularly polarized light can break time-reversal symmetry and induce the quantum anomalous Hall (QAH) phase. In particular, the coupling between the edge states is spin dependent and is related not only to the size of the system, but also to the strength of the polarized pumping optics. By tuning the two parameters (system width and optical pumping strength), we obtain four transport regimes, namely, QSH, QAH, edge conducting, and normal insulator. These four different transport regimes have contrasting edge conducting properties, which will feature prominently in transport experiments on various topological materials.

We present the results of ${}^{69,71}\mathrm{Ga}$-NMR measurements on an organic antiferromagnet $\lambda$-(BEDSe-${\mathrm{TTF})}_2{\mathrm{GaCl}}_4$ [BEDSe-TTF = bis(ethylenediseleno)tetrathiafulvalene], with comparison with reports on $\lambda$-(${\mathrm{BETS})}_2{\mathrm{GaCl}}_4$ [BETS = bis(ethylenedithio)tetraselenafulvalene] [T. Kobayashi et al., Phys. Rev. B 102, 235131 (2020)]. We found that the dynamics of two crystallographically independent ethylene groups induce two types of quadrupolar relaxation in the high-temperature region. As the ethylene motion freezes, hyperfine (HF) interactions develop between $\pi$ spin and Ga nuclear spin below 100 K, and thereby, magnetic fluctuations of the $\pi$-spin system are detected even from the Ga site. The HF interaction in $\lambda$-(${\mathrm{BETS})}_2{\mathrm{GaCl}}_4$ was more than twice as large as in $\lambda$-(BEDSe-${\mathrm{TTF})}_2{\mathrm{GaCl}}_4$, implying that the short contacts between Cl atoms and the chalcogens of the fulvalene part are essential for the transferred HF interaction. We propose that NMR using nuclei in anion layers is useful for studying interlayer interactions in organic conductors, which have not been studied experimentally. In addition, because the mechanism of the transferred HF interaction is considered the same as the $\pi \text{−}d$ interaction in isostructural Fe-containing $\lambda$-type salts, our findings aid in the understanding of their physical properties.

The anomalous spin diffusion of the integrable easy-axis Heisenberg chain originates in the ballistic transport of symmetry sectors with nonzero magnetization. Ballistic transport is replaced by normal dissipative transport in all magnetization sectors upon introducing the integrability-breaking perturbations, including external driving. Such behavior implies that the diffusion constant obtained for the integrable model is relevant for the spread of spin excitations but not for the spin conductivity. We present numerical results for closed systems and driven open systems, indicating that the diffusion constant shows a discontinuous variation as the function of perturbation strength.

Higher-order topological phases (HOTPs) hold gapped bulk bands and topological boundary states localized in boundaries with codimension higher than one. In this paper, we provide a unified construction and topological characterization of HOTPs for the full Altland-Zirnbauer tenfold symmetry classes, based on a method known as nested band inversion surfaces (BISs). Specifically, HOTPs built on this method are decomposed into a series of subsystems, and higher-order topological boundary states emerges from the interplay of their first-order topology. Our analysis begins with a general discussion of HOTPs in continuous Hamiltonians for each symmetry class, then moves on to several lattice examples illustrating the topological characterization based on the nested-BIS method. Despite the example minimal models possessing several spatial symmetries, our method does not rely on any spatial symmetry and can be easily extended into arbitrary orders of topology in dimensions. Furthermore, we extend our discussion to systems with asymmetric boundary states induced by two different mechanisms, namely, crossed BISs that break a ${\mathcal{C}}_4$ rotation symmetry and non-Clifford operators that break certain chiral-mirror symmetries of the minimal models.

Mechanical deformations in $\alpha -{\mathcal{T}}_3$ lattices induce local pseudomagnetic fields of opposite directionality for different valleys. When this strain is equipped with a dynamical drive, it generates a complementary valley-asymmetric pseudoelectric field, which is expected to accelerate electrons. We propose that by combining these effects by a time-dependent nonuniform strain, tunable valley filtering devices can be engineered that extend beyond the static capabilities. We demonstrate this by implementing an oscillating Gaussian bump centered in a four-terminal Hall bar $\alpha -{\mathcal{T}}_3$ setup and calculating the induced pseudoelectromagnetic fields analytically. Within a recursive Floquet Green-function scheme, we determine the time-averaged transmission and valley polarization, as well as the spatial distributions of the local density of states and current density. As a result of the periodic drive, we detect novel energy regimes with a highly valley-polarized transmission, depending on $\alpha$. Analyzing the spatial profiles of the time-averaged local density of states and current density we can relate these regimes to the pseudoelectromagnetic fields in the setup. By means of the driving frequency, we can manipulate the valley-polarized states, which might be advantageous for future device applications.