First-principles calculations are conducted to investigate the magnetic properties of the ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ compounds. We reveal that the ${\mathrm{Na}}_2{\mathrm{IrO}}_3$'s local structural distortions are essential for an accurate description of the magnetism of such systems. They provide a feasible explanation for the experimentally observed antiferromagnetic zigzag magnetic ground state. We demonstrate that the underlying competition between the spin-orbit coupling and the crystal-field splitting rules the crystal structure and profoundly influences the strength of the magnetic exchange interactions. We unambiguously identify that the $D_{4h}$-type distortions, due to the Jahn-Teller effect, are disclosed in an elongation along the ${\mathrm{IrO}}_6$ polyhedra's apical axis in the $ac$ plane. On the other hand, the Ir atoms off-centering in the basal plane (perpendicular to the local $D_{4h}$ elongation axis) arises owing to an inhomogeneous cationic charge distribution of ${\mathrm{Ir}}^{4+}/{\mathrm{Na}}^{+}$ in the transition-metal layer resulting in both effects being decisive in controlling the magnetism of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ together with the spin-orbit interactions.

The electric quadrupolar state in the frustrated quantum magnet ${\mathrm{Tb}}_{2+\delta }{\mathrm{Ti}}_{2-\delta }{\mathrm{O}}_7$ has been studied by means of ultrasonic and magnetostriction measurements. A single crystal showed an elastic anomaly around 0.4 K, manifesting a long-range quadrupole ordering. By investigating the anisotropy of the magnetoelastic responses, we found a crossover temperature for the strongly correlated quadrupole state, below which the experimental data of the elastic constant and magnetostriction become qualitatively different from their calculations based on a single-ion model. We suppose that relatively high onset temperature of the quadrupole correlation compared with the transition temperature is ascribed to the geometrical frustration effect, and this correlated state seems to be responsible for the unusual properties in the paramagnetic state of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

Ferromagnetic resonance modes in a single rectangular ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ microstrip were directly imaged using time-resolved scanning transmission x-ray microscopy combined with a phase-locked ferromagnetic resonance excitation scheme, and the findings were corroborated by micromagnetic simulations. Although under uniform excitation in a single confined microstructure typically standing spin waves are expected, all imaged spin waves showed a nonstationary character both at and off resonance, the latter being additionally detected with microantenna-based ferromagnetic resonance. The effect of the edge quality on the spin waves was observed in micromagnetic simulations.

Tunable magnon propagation is essential for constructing magnonic devices. The magnonic spin-valve effect was recently realized in a magnetic bilayer heterostructure. Similar to a standard electronic spin valve, switching from parallel to antiparallel magnetic orientations in the bilayer achieves comparatively high and low polarized magnon transmission. Here, we propose a trilayer magnonic spin-valve structure to improve controllability over polarized magnon propagation. In addition to high and low magnon transmission, we find a medium magnon transmission behavior, where magnons from the bottom layer jump across the middle magnetic layer and land in the top layer. Coupled magnon modes in the trilayer structure give rise to the magnon transmission feature. We investigate the effects of the external magnetic field and spin-orbit torque, and we can drive a $150\%$ change in the medium transmission without magnetization reversal. This effect avoids the slow reversal process and increases the trilayer valve's flexibility. These findings render the possibility of dynamically manipulable magnonic information devices with multiple output states.

Crystallographic analysis and thermoelectric studies of solid solutions ${\mathrm{Rh}}_{1\text{−}x}{\mathrm{Ir}}_x{\mathrm{Te}}_2$ $(0\le x\le 1)$ are reported. All compositions show layered structures belonging to the $P\overline{3}m1$ space group at room temperature. ${\mathrm{IrTe}}_2$ presents a first-order phase transition from the hexagonal to the triclinic lattice ($P\overline{1}$ space group), which is monitored by synchrotron radiation x-ray powder diffraction. In the cooling-down process the transition appears at 240 K while in the warming-up process it begins at 280 K, showing a remarkable hysteresis. All compositions show a strong metallic behavior with enhanced Pauli paramagnetism and two regimes in the electrical resistivity. These regimes are associated with electron-electron scattering (at low temperature $\rho \sim T^2$) and electron-phonon coupling (higher temperatures $\rho \sim T$). The Seebeck coefficient shows hole-type carriers for all the compounds.

The design of spin-transport-based devices such as magnon transistors or spin valves will require multilayer systems composed of different magnetic materials with different physical properties. Such layered structures can show various interface effects, one class of which is proximity effects, where a certain physical phenomenon that occurs in one layer leaks into another one. In this work a magnetic proximity effect is studied in trilayers of different ferro- and antiferromagnetic materials within an atomistic spin model. We find the magnetic order in the central layer—with lower critical temperature—is enhanced, even for the case of an antiferromagnet surrounded by ferromagnets. We further characterize this proximity effect via the magnon spectra which are specifically altered, especially for the case of the antiferromagnet in the central layer.

Owing to charge-free property, magnon is promising to achieve dissipationless transport without Joule heating. Valley degree of freedom has also been fully expressed in electronic structures, enabling information coding and manipulation via valley-based qubits. Considering the conceptual importance and application potential, it is highly desirable to couple valley to magnon excitation and realize their modulations. We demonstrate valley magnons and valley modulations in a kagome ferromagnetic lattice, with Dzyaloshinskii-Moriya and staggered exchange interactions. The staggered exchange interaction breaks spatial inversion symmetry, leading to magnon valley Hall effect. When the Dzyaloshinskii-Moriya interaction is further included, the valley degeneracy is lifted, exhibiting net anomalous Hall effect and a series of topological phase transitions. Single-edge heat transport of magnons can occur, with propagating currents along the edge and local circulating currents. These findings give full play to spin and valley degrees of freedom and add another dimension to magnonic device paradigms.

We develop a minimal theory for the recently observed metal-insulator transition (MIT) in two-dimensional (2D) moiré multilayer transition metal dichalcogenides (mTMDs) using Coulomb disorder in the environment as the underlying mechanism. In particular, carrier scattering by random charged impurities leads to an effective 2D MIT approximately controlled by the Ioffe-Regel criterion, which is qualitatively consistent with the experiments. We find the necessary disorder to be around $5–10\times {10}^{10}{\mathrm{cm}}^{-2}$ random charged impurities in order to quantitatively explain much, but not all, of the observed MIT phenomenology as reported by two different experimental groups. Our estimate is consistent with the known disorder content in TMDs.

Correlated phenomena such as magnetism and the Mott phase are a very controversial issues in two-dimensional transition metal dichalcogenides (TMDCs). Intending to find the value of the correlation strength and understanding the origin of ferromagnetic order in TMDCs, we first identify relevant, low-energy degrees of freedom on both octahedral $1T$ and trigonal prismatic $2H$ structures in $3d\text{−}M{X}_2$ ($M=\mathrm{Ti},\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni};X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) and then determine the strength of the effective Coulomb interactions between localized $d$ electrons from the first principles using the constrained random-phase approximation. The on-site Coulomb interaction values lie in the range $1.4–3.7\mathrm{eV} (1.1–3.6\mathrm{eV})$ for the $1T$ structure ($2H$ structure) and depend on the ground-state electronic structure, $d$-electron number, and correlated subspace. For most of the $3d$-TMDCs, we obtain $1<U/W_b<2$ (the bandwidth $W_b$), which turn out to be larger than the corresponding values in elementary transition metals. Based on the calculated $U$ and exchange $J$ interaction, we check the condition to be fulfilled for the formation of the ferromagnetic order by the Stoner criterion. The results indicate that experimentally observed $\mathrm{Mn}{X}_2$ ($X=\text{S}$, Se) and $\mathrm{V}{X}_2$ ($X=\text{S}$, Se) have an intrinsic ferromagnetic behavior in pristine form, although V-based materials are close in vicinity to the critical point separating the ferromagnetic from the paramagnetic phase.

This paper reports the presence of extended-range ordering in the atomic pair-correlation function of amorphous silicon $\left(a\text{−}\mathrm{Si}\right)$ using ultralarge atomistic models obtained from Monte Carlo and molecular-dynamics simulations. The extended-range order manifests itself in the form of radial oscillations on the length scale of 20–40 Å, which is examined by directly analyzing the radial distribution of atoms in distant coordination shells and comparing the same with those from a class of partially ordered networks of Si atoms and disordered configurations of crystalline silicon from an information-theoretic point of view. The effect of the oscillations on the first sharp diffraction peak (FSDP) in the structure factor is addressed by obtaining a semianalytical expression for the static structure factor of $a\text{−}\mathrm{Si}$, and calculating an estimate of the error of the intensity of the FSDP associated with the truncation of radial information from distant shells. The results indicate that the extended-range oscillations do not have any noticeable effects on the position and intensity of the FSDP, which is primarily determined by the medium-range atomic correlations of up to a length of 20 Å in amorphous silicon.

The electronic and topological properties of Bi(110) ultrathin films epitaxially grown on a Cu(111) substrate are investigated using scanning tunneling microscopy/spectroscopy, combined with density functional theory calculations. Bilayer-by-bilayer growth of the ultrathin Bi(110) films is observed, which manifests a structure of black-phosphorus-like Bi bilayers (BLs), with thickness up to 4 BLs. The surface atomic buckling heights in the 1-BL and 2-BL films are clearly identified to depend on the stacking modes with respect to the well-ordered Bi atoms in the adlayer covered on the Cu(111) surface. Our results demonstrate that while the electronic and topological properties of 1-BL films greatly depend on the stacking modes between the Bi(110) bilayer and the adlayer, the 2-BL films show well-decoupled electronic properties from the Cu substrate and nontrivial topologies robust against surface atomic buckling height benefitting from the interbilayer coupling. Our calculations further show that besides the parameter of buckling heights, the topological nontrivial-to-trivial transition can also be induced by changing the interbilayer distance and the vertical intralayer bond length.

Two-dimensional electron gases in ${\mathrm{SrTiO}}_3$-based heterostructures provide a platform to study the real-time evolution of the macroscopic state with a variation of the carrier density and the impact of structural properties on the emergence of the superconducting state. We have explored the isothermal evolution of the electron gas in ${\mathrm{AlO}}_x/{\mathrm{SrTiO}}_3$ by measuring the variation of resistance with continuous gate-voltage-controlled tuning of its carrier density. It is seen that condensation of the ordered phase leads to nonmonotonic isotherms within the superconducting dome. The timescale for dynamic change following changes in gate voltage is measured across the phase transition. It is found to be tens of seconds near the onset of superconductivity, significantly larger compared to the normal state. Such a large timescale governing the kinetics of the phase transition presumably arises from the strong impact of structural defects and distortions of the substrate on the development of superconducting islands.

InAs/GaSb hosts a broken-gap band alignment that has been shown to generate helical topological edge states. Upon the introduction of Mn into the structure, it has been predicted to host a quantized anomalous Hall effect. Here, we show that dilute Mn doping on InAs in InAs/GaSb allows a tuning of the Fermi level, the introduction of paramagnetism, but also has a nontrivial impact on the band alignment of the system. The measurement of Shubnikov–de Haas oscillations, cyclotron resonance, and a nonlinear Hall effect in Mn-doped samples indicate the coexistence of a high mobility two-dimensional electron gas and a hole gas. Conversely, in undoped InAs/GaSb, pure-$n$-type transport is observed. We hypothesize that Mn acceptor levels can pin the Fermi energy near the valence band edge of InAs, far from the interface, which introduces a strong band bending to preserve the band offset at the InAs/GaSb interface. The realization of the quantized anomalous Hall effect in this structure will thus require careful control of the band alignment to preserve topological insulating character.

We study hydrodynamic electron transport in Corbino graphene devices. Due to the irrotational character of the flow, the forces exerted on the electron liquid are expelled from the bulk. We show that in the absence of Galilean invariance, force expulsion produces qualitatively new features in thermoelectric transport: (i) it results in drops of both voltage and temperature at the system boundaries and (ii) in conductance measurements in pristine systems, the electric field is not expelled from the bulk. We obtain thermoelectric coefficients of the system in the entire crossover region between charge neutrality and high electron density regime. The thermal conductance exhibits a sensitive Lorentzian dependence on the electron density. The width of the Lorentzian is determined by the fluid viscosity. This enables determination of the viscosity of electron liquid near charge neutrality from purely thermal transport measurements. In general, the thermoelectric response is anomalous: It violates the Matthiessen's rule, the Wiedemann-Franz law, and the Mott relation.

Using a diagrammatic scheme, we study the acoustoelectric effects in two-dimensional (2D) hexagonal Dirac materials due to the sound-induced pseudogauge field. We analyze both uniform and spatially dispersive currents in response to copropagating and counterpropagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic transverse charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components $j_T\propto A_L{A}_T^{*}$. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component $j_T\sim sin\left(6\theta \right)$ is achieved that stems from the inherited threefold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic $\sim {sin}^2\left(3\theta \right)$ or ${cos}^2\left(3\theta \right)$. We quantitatively estimate the pseudogauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D hexagonal Dirac materials.

Permanent localization of electrons inside a graphene quantum dot (GQD) is known to be forbidden, as a manifestation of Klein tunneling. However, an electron which scatters on a GQD may be transiently trapped inside and one known practice is the usage of magnetic field. These electronic states discussed here, called quasibound states, are scattering resonances typically characterized by a finite lifetime (trapping time). In this paper, we present a theoretical perspective concerning the opportunity to enhance the lifetime of quasibound states excited in a GQD placed in a uniform magnetic field, using circularly polarized light. Generally speaking, electron trapping inside GQDs is achievable for certain well-defined conditions, for instance, magnetic field intensity. We report here that the trapping time of an electron inside a GQD may be successfully enhanced by adjusting the light intensity while keeping the magnetic field constant.

We report the results of magnetic susceptibility $\chi$ and ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR) measurements on a three-dimensional hyperhoneycomb lattice compound $\left[\right({\mathrm{C}}_2{\mathrm{H}}_5{)}_3\mathrm{NH}{]}_2{\mathrm{Cu}}_2{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3$ (CCCO). The average value of the antiferromagnetic (AFM) exchange coupling between the ${\mathrm{Cu}}^{2+}$ $(S=1/2)$ spins was determined to be $J\sim 50$ K from the $\chi$ measurements. No long-range magnetic ordering has been observed down to $T=50$ mK, although NMR lines become slightly broader at low temperatures below 1 K. The broadening of the NMR spectrum observed below 1 K reveals that the Cu spin moments remain at this temperature, suggesting a non-spin-singlet ground state. The temperature and magnetic field dependence of $1/T_1$ at temperatures above 20 K is well explained by paramagnetic thermal spin fluctuations where the fluctuation frequency of ${\mathrm{Cu}}^{2+}$ spins is higher than the NMR frequency of the order of megahertz. However, a clear signature of the slowing down of the ${\mathrm{Cu}}^{2+}$ spin fluctuations was observed at low temperatures where $1/T_1$ shows a thermally activated behavior. The magnetic field dependence of the magnitude of the spin excitation gap suggests that the magnetic behaviors of CCCO are characterized as an AFM chain at low temperatures.

We report on the structural, magnetic, and superconducting properties of single and double atomic layers of Mn on a clean and unreconstructed Nb(110) substrate. Low-temperature scanning tunneling spectroscopy measurements reveal a proximity-induced superconducting state and in-gap Yu-Shiba-Rusinov bands in the Mn thin films, which are found to grow pseudomorphically on the Nb surface. Spin-polarized scanning tunneling microscopy measurements reveal a $c(2\times 2)$ antiferromagnetic (AFM) order in the Mn layers, with an out-of-plane spin orientation. First-principles density functional theory calculations confirm the experimentally observed magnetic state, which is understood as the consequence of a strong intralayer and interlayer nearest-neighbor AFM exchange coupling. These results are expected to be of importance for the design of superconducting AFM spintronic systems and quantum information technologies.

In this Letter we investigate the properties of a quantum impurity model in the presence of additional many-body interactions between mobile carriers. The fundamental question which is addressed here is how the interactions in the charge and spin sectors of an itinerant system affect the quantum impurity physics in the vicinity of the intermediate coupling fixed point. To illustrate the general answer to this question we discuss a two-channel charge Kondo circuit model. We show that the electron-electron interactions resulting in the formation of a massive spin mode in an itinerant electron subset drive the system away from the unstable non-Fermi-liquid (NFL) fixed point to the stable Fermi-liquid (FL) regime. We discuss the thermoelectric response as a benchmark for the NFL-FL crossover.