Despite the intense amount of attention and huge potential of two-dimensional (2D) magnets for applications in novel magnetic, magneto-optical, magnetothermal, and magnetoelectronic devices, there is a lack of robust strategy developed to systematically understand magnon-magnon interactions (MMIs) at finite temperature. In this paper, we present a first-principles theoretical method to introduce the finite temperature magnon-magnon interaction into a Heisenberg Hamiltonian through a correction energy. Wick's theorem is used to decouple the four-magnon operators to two-magnon order. We demonstrate the capabilities of this method by studying the strength of MMI in a ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ monolayer. The spin-wave spectrum at finite temperature and the time-dependent spin autocorrelation function are explored. It is found that the magnon relaxation time due to magnon-magnon scattering increases with temperature because of the reduction in magnon energy, while it decreases with wave vector and external magnetic field. Our results provide insight into understanding the magnon damping and energy dissipation in two-dimensional ferromagnetic materials.

In a recent work, new two-dimensional materials, monolayer ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ and ${\mathrm{WSi}}_2{\mathrm{N}}_4$, were successfully synthesized in experiment, and several other monolayer materials with a similar structure, such as ${\mathrm{MoSi}}_2{\mathrm{As}}_4$, were predicted [Hong et al., Science 369, 670 (2020)]. Here, based on first-principles calculations and theoretical analysis, we investigate the electronic and optical properties of monolayer ${\mathrm{MoSi}}_2{\mathrm{N}}_4, {\mathrm{WSi}}_2{\mathrm{N}}_4$, and ${\mathrm{MoSi}}_2{\mathrm{As}}_4$. We show that these materials are semiconductors, with a pair of Dirac-type valleys located at the corners of the hexagonal Brillouin zone. Due to the broken inversion symmetry and the effect of spin-orbit coupling, the valley fermions manifest spin-valley coupling, valley-contrasting Berry curvature, and valley-selective optical circular dichroism. We also construct the low-energy effective model for the valleys, calculate the spin Hall conductivity and the permittivity, and investigate the strain effect on the band structure. Our result reveals interesting valley physics in monolayer ${\mathrm{MoSi}}_2{\mathrm{N}}_4, {\mathrm{WSi}}_2{\mathrm{N}}_4$, and ${\mathrm{MoSi}}_2{\mathrm{As}}_4$, suggesting their great potential for valleytronics and spintronics applications.

Transverse electric (TE) modes cannot propagate through the conducting solids. This is because the continuum of particle-hole excitations of conductors contaminates with the TE mode which damps it out. But in solids hosting tilted Dirac cone that admit a description in terms of a modified Minkowski space-time, the new space-time structure remedies this issue and therefore a tilted Dirac cone material (TDM) supports the propagation of an undamped TE mode which is sustained by charge density fluctuations. The resulting TE mode propagates at fermionic velocities $v_F$, which implies (i) it is strongly confined to the surface of the two-dimensional (2D) TDM and (ii) the TDM acts as a high refractive index $n=c/v_F\sim {10}^2$ medium for TE modes.

We have studied the surface structure of single-crystal, wide-gap semiconductor $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3\left(010\right)$ using x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). The XPS measurements show typical spectra for stoichiometric ${\mathrm{Ga}}_2{\mathrm{O}}_3\left(010\right)$. Annealing in vacuum produced a sharp ($1\times 1$) LEED pattern, characteristic of the monoclinic structure. The XPD angular anisotropies were collected for the Ga $2p_{3/2}$ and O $1s$ core levels. Surface interlayer relaxation up to 8% of the bulk interplanar distance and 0.11–0.14 $\AA$ rumpling are observed at the $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3\left(010\right)$ surface. At the surface, the oxygen atoms shift toward the vacuum with respect to the gallium atoms. The rumpling decreases to zero and and the interplanar distance reaches the bulk value of 1.52 $\AA$ by the sixth atomic layer. The surface structure agrees with that predicted by first-principles density functional theory calculations which, in addition, suggest a significant band gap narrowing of $\approx 1$ eV in the surface layer, due to surface states spatially localized on surface oxygen atoms of ${\mathrm{O}}_{\mathrm{II}}$ type.

We report a two-dimensional artificial lattice for dipolar excitons confined in a GaAs double quantum well. Exploring the regime of large fillings per lattice site, we verify that the lattice depth competes with the magnitude of exciton repulsive dipolar interactions to control the degree of localization in the lattice potential. Moreover, we show that dipolar excitons radiate a narrow-band photoluminescence with a spectral width of a few hundreds of $\mu \mathrm{eV}$ at 340 mK, in both localized and delocalized regimes. This makes our device suitable for explorations of dipolar excitons quasicondensation in a periodic potential.

We present theoretical studies of temperature dependent diamagnetic-paramagnetic transitions in thin quantum rings. Our studies show that the magnetic susceptibility of metal/semiconductor rings can exhibit multiple sign flips at intermediate and high temperatures depending on the number of conduction electrons in the ring ($N$) and whether or not spin effects are included. When the temperature is increased from absolute zero, the susceptibility begins to flip sign above a characteristic temperature that scales inversely with the number of electrons according to $N^{-1}$ or $N^{-1/2}$, depending on the presence of spin effects and the value of $N\mathrm{mod}4$. Analytical results are derived for the susceptibility in the low and high temperature limits, explicitly showing the spin effects on the ring Curie constant.

We develop a method of theoretically investigating the charge, energy, and heat transport in the presence of the charge Kondo correlations. The Coulomb blockade oscillations of heat conductance in the single-electron transistor exhibiting charge Kondo effects are investigated. We explore the Wiedemann-Franz ratio in both charge-single-channel and charge-two-channel Kondo regimes. The close connections of our findings with the recent experiments on multichannel charge Kondo effects are discussed.

We consider a two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI) partially covered by an $s$-wave superconductor, where the uncovered region remains normal but is exposed to a perpendicular Zeeman field. We find analytically and numerically Andreev bound states (ABSs) formed in the normal region and show that, by tuning the SOI to certain values, one can reach a regime where the energy of the lowest ABS becomes pinned close to zero as a function of Zeeman field. In addition, we also consider a configuration with a superconducting vortex and find again ABSs pinned close to zero energy in the topologically trivial phase for a wide range of parameters. Thus, ABSs and Majorana bound states show similar behavior in such structures.

Precise angle-resolved magnetoresistance (ARM) measurements are applied to reveal the origin of symmetry lowering in electron transport and the emergence of a huge number of magnetic phases in the ground state of the antiferromagnetic metal ${\mathrm{HoB}}_{12}$ with fcc crystal structure. By analyzing the polar H-θ-φ magnetic phase diagrams of this compound reconstructed from the experimental ARM data, we argue that nonequilibrium electron density oscillations (dynamic charge stripes) are responsible for the suppression of the indirect Ruderman-Kittel-Kasuya-Yosida exchange along the ⟨110⟩ directions between the nearest neighboring magnetic moments of ${\mathrm{Ho}}^{3+}$ ions in this strongly correlated electron system.

Chemically ordered $L{1}_0$ $\left({\mathrm{Fe}}_{100-x}{\mathrm{Cr}}_x\right)\mathrm{Pt}$ thin films were expitaxially grown on MgO(001) substrates by magnetron sputter-deposition at $770{}^{\circ }\mathrm{C}$. In this sample series, Fe was continuously substituted by Cr over the full composition range. The lattice parameter in the [001] growth direction steadily increases from $L{1}_0$-FePt toward $L{1}_0$-CrPt, confirming the incorporation of Cr in the lattice occupying Fe sites. With the observed high degree of chemical ordering and (001) orientation, strong perpendicular magnetic anisotropy is associated, which persists up to a Cr content of $x=20$ at. %. Similarly, the coercive field in the easy-axis direction is strongly reduced, which is, however, further attributed to a strong alteration of the film morphology with Cr substitution. The latter changes from a well-separated island microstructure to a more continuous film morphology. In the dilute alloy with low Cr content, isolated Cr magnetic moments couple antiferromagnetically to the ferromagnetic Fe matrix. In this case, all Cr moments are aligned parallel, thus forming a ferrimagnetic FeCrPt system. With increasing Cr concentration, nearest-neighbor Cr-Cr pairs start to appear, thereby increasing magnetic frustration and disorder, which lead to canting of neighboring magnetic moments, as revealed by atomistic spin-model simulations with model parameters based on first principles. At higher Cr concentrations, a frustrated ferrimagnetic order is established. With Cr substitution of up to 20 at. %, no pronounced change in Curie temperature, which is in the range of 700 K, was noticed. But with further addition the Curie temperature drops down substantially even down to room temperature at 47 at. % Cr. Furthermore, x-ray magnetic circular dichroism studies on dilute alloys containing up to 20 at. % of Cr revealed similar spin moments for Fe and Cr in the range between 2.1–2.5 ${\mu }_B$ but rather large orbital moments of up to 0.50 $\pm 0.10{\mu }_B$ for Cr. These results were also compared to ab initio calculations.

In this study we investigate the finite-temperature magnetic properties of ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{\mathrm{N}}_x$ $(x=0,3)$ using an effective spin model constructed based on the information obtained by first-principles calculations. We find that assuming the plausible trivalent ${\mathrm{Sm}}^{3+}$ configuration results in a model that can satisfactorily describe the magnetization curves of ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}{\mathrm{N}}_3$. By contrast, the model based on the divalent ${\mathrm{Sm}}^{2+}$ configuration is suitable to reproduce the magnetization curves of ${\mathrm{Sm}}_2{\mathrm{Fe}}_{17}$. These results expand the understanding of how electronic structure affects the magnetic properties of these compounds.

The few-layer transition metal dichalcogenides (TMDs) have been recently suggested as a platform for controlled unconventional superconductivity. We study the manifestations of unconventional triplet pairing in the density of states of a disordered TMD based monolayer. The conventional singlet pairing attraction is assumed to be the dominant pairing interaction. We map the phase diagrams of disordered Ising superconductors in the plane of temperature and the in-plane magnetic field. The latter suppresses singlet and promotes triplet correlations. The triplet order parameters of a trivial (nontrivial) symmetry compete (cooperate) with singlet order parameter which gives rise to a rich phase diagram. We locate the model dependent phase boundaries and compute the order parameters in each of the distinct phases. With this information, we obtain the density of states by solving the Gorkov equation. The triplet components of the order parameters may change an apparent width of the density of states by significantly increasing the critical field. The triplet components of the order parameters lead to the density of states broadening significantly exceeding the broadening induced by magnetic field and disorder in a singlet superconductor.

We investigate the electron-phonon cooling power in disordered electronic systems with a special focus on mesoscopic superconducting proximity structures. Employing the quasiclassical Keldysh Green's function method, we obtain a general expression for the cooling power perturbative in the electron-phonon coupling but valid for arbitrary electronic systems out of equilibrium. We apply our theory to several disordered electronic systems valid for an arbitrary relation between the thermal phonon wavelength and the electronic mean-free path due to impurity scattering. In addition to recovering the known results for bulk normal metals and BCS superconductors, we consider two experimentally relevant geometries of superconductor-normal metal proximity contacts. Both structures feature a significantly suppressed cooling power at low temperatures related to the existence of a minigap in the quasiparticle spectrum. This improved isolation low cooling feature in combination with the high tunability makes such structures highly promising candidates for quantum calorimetry.

We investigate the effects of quenched randomness on topological quantum phase transitions in strongly interacting two-dimensional systems. We focus first on transitions driven by the condensation of a subset of fractionalized quasiparticles (“anyons”) identified with “electric charge” excitations of a phase with intrinsic topological order. All other anyons have nontrivial mutual statistics with the condensed subset and hence become confined at the anyon condensation transition. Using a combination of microscopically exact duality transformations and asymptotically exact real-space renormalization group techniques applied to these two-dimensional disordered gauge theories, we argue that the resulting critical scaling behavior is “superuniversal” across a wide range of such condensation transitions and is controlled by the same infinite-randomness fixed point as that of the 2D random transverse-field Ising model. We validate this claim using large-scale quantum Monte Carlo simulations that allow us to extract zero-temperature critical exponents and correlation functions in $(2+1)\mathrm{D}$ disordered interacting systems. We discuss generalizations of these results to a large class of ground-state and excited-state topological transitions in systems with intrinsic topological order as well as those where topological order is either protected or enriched by global symmetries. When the underlying topological order and the symmetry group are Abelian, our results provide prototypes for topological phase transitions between distinct many-body localized phases.

Recent works on the observation of discrete time-crystalline signatures throw up major puzzles on the necessity of localization for stabilizing such out-of-equilibrium phases. Motivated by these studies, we delve into a clean interacting Floquet system whose quasi-spectrum conforms to the ergodic Wigner-Dyson distribution, yet with an unexpectedly robust, long-lived time-crystalline dynamics in the absence of disorder or fine-tuning. We relate such behavior to a measure zero set of nonthermal Floquet eigenstates with long-range spatial correlations, which coexist with otherwise thermal states at near-infinite temperature and develop a high overlap with a family of translationally invariant, symmetry-broken initial conditions. This resembles the notion of “dynamical scars” that remain robustly localized throughout a thermalizing Floquet spectrum with fractured structure. We dub such a long-lived discrete time crystal formed in partially nonergodic systems, “scarred discrete time crystal” which is distinct by nature from those stabilized by either many-body localization or prethermalization mechanism.

We propose a mathematical model for describing propagating confined modes in domain walls of intermediate angle $\alpha$ $(0<\alpha <\pi /2 \mathrm{radians})$ between domains. The model is obtained from the linearized Bloch equations of motion and under reasonable assumptions that can apply to the scenario of a thick (80 nm) magnetic patch, which simplifies the calculations without a high impact on the model accuracy. The model shows that there is a clear dependence of the local wave number of the confined spin wave on the local angle of domain magnetization with respect to the wall and on the excitation magnetic field frequency. From this model, we can define a local mode index in the wall as a function of such angle and excitation frequency. Therefore, the model can be applied to 1D propagating modes, although it also has physical implications for 2D scenarios where a domain wall merges with a saturated magnetic region. Micromagnetic simulations are in good agreement with the predictions of the model. Our model can also give insight on the effects that curved edge structures may have on the propagating characteristics of spin waves bounded in domain walls.

Antiferromagnetic spintronics is a promising emerging paradigm to develop high-performance computing and communications devices. Antiferromagnetic materials are more abundant than ferromagnets; hence, from a theoretical point of view, it is important to implement simulation tools that can support a data-driven development of materials having specific properties for applications. Here, we present a study focusing on the fundamental properties of antiferromagnetic materials having an easy-plane anisotropy and interfacial Dzyaloshinskii-Moriya interaction (IDMI). An analytical theory is developed and benchmarked against full numerical micromagnetic simulations, describing the main properties of the ground state in antiferromagnets and how it is possible to estimate the IDMI from experimental measurements. The effect of the IDMI on the electrical switching dynamics of the antiferromagnetic element is also analyzed. Our theoretical results have implication in the design of multiterminal heavy-metal/antiferromagnet memory devices.

An easy-plane spin winding in a quantum spin chain can be treated as a transport quantity, which propagates along the chain but has a finite lifetime due to phase slips. In a hydrodynamic formulation for the winding dynamics, the quantum continuity equation acquires a source term due to the transverse vorticity flow. The latter reflects the phase slips and generally compromises the global conservation law. A linear-response formalism for the nonlocal winding transport then reduces to a Kubo response for the winding flow along the spin chain, in conjunction with the parasitic vorticity flow transverse to it. One-dimensional topological hydrodynamics can be recovered when the vorticity flow is asymptotically small. Starting with a microscopic spin-chain formulation, we focus on the asymptotic behavior of the winding transport based on the renormalized sine-Gordon equation, incorporating phase slips as well as Gilbert damping. A generic electrical device is proposed to manifest this physics. We thus suggest winding conductivity as a tangible concept that can characterize low-energy dynamics in a broad class of quantum magnets.