High-pressure x-ray diffraction measurements have been carried out on the intermetallic semiconductor ${\mathrm{FeGa}}_3$ and the equation of state for ${\mathrm{FeGa}}_3$ has been determined. First-principles based DFT calculations within the GGA approximation indicate that although the unit cell volume matches well with the experimentally obtained value at ambient pressure, it is significantly underestimated at high pressures and the difference between them increases as pressure increases. $\mathrm{GGA}+U$ calculations with increasing values of $U_{\mathrm{Fe}\left(3d\right)}$ (on-site Coulomb repulsion between the Fe $3d$ electrons) at high pressures, correct this discrepancy. Further, the $\mathrm{GGA}+U$ calculations also show that along with $U_{\mathrm{Fe}\left(3d\right)}$, the Fe $3d$ bandwidth also increases with pressure and around a pressure of 4 GPa, a small density of states appear at the Fermi level. High-pressure resistance measurements carried out on ${\mathrm{FeGa}}_3$ also clearly show a signature of an electronic transition. Beyond the pressure of 19.7 GPa, the diffraction peaks reduce in intensity and are not observable beyond $\sim 26\mathrm{GPa}$, leading to an amorphous state.

We have studied the spin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet FeGe in a temperature range from 225 K up to $T_C\approx 278.7\mathrm{K}$ by small-angle neutron scattering. The method we have used is based on [Grigoriev et al., Phys. Rev. B 92, 220415(R) (2015)] and was extended here for the application in polycrystalline samples. We confirm the validity of the anisotropic spin-wave dispersion for FeGe caused by the Dzyaloshinskii-Moriya interaction. We have shown that the spin-wave stiffness $A$ for the FeGe helimagnet decreases with a temperature as $A\left(T\right)=194[1-0.7{(T/T_C)}^{4.2}]{\mathrm{meV}\AA }^2$. The finite value of the spin-wave stiffness $A=58{\mathrm{meV}\AA }^2$ at $T_C$ classifies the order-disorder phase transition in FeGe as being the first-order one.

Although chiral magnetic materials have emerged as a potential ingredient in future spintronic memory devices, there are few comprehensive studies of magnetic properties in scalably grown thin films. We present growth, systematic physical and magnetic characterization, and microwave absorption spectroscopy of B20 FeGe thin films. We also perform micromagnetic simulations and apply analytical theory to understand the dynamical magnetic behavior of this material. We find magnetic resonance features in both the helical and field-polarized magnetic states, which are well explained by micromagnetic simulations and analytical calculations. In particular, we show the resonant enhancement of spin waves along the FeGe film thickness that has a wave vector matching the helical vector. Using our analytic model, we also describe the resonance frequency of a helical magnetic state in bulk materials and thin films, which depends strongly on its untwisting field. Our results pave the way for understanding and manipulating high frequency spin waves in thin-film chiral magnet FeGe near room temperature.

We report measurements of the NMR line shape of 4.685% abundant ${}^{29}\mathrm{Si}$ in single-crystal silicon for four different crystallographic orientations relative to the applied magnetic field. To avoid significant inhomogeneous broadening, the sample crystals are immersed in a susceptibility-matched liquid and the proton NMR line shape in the liquid is used to measure the residual susceptibility mismatch and to shim the applied field. The observed line shapes are in good agreement with disorder-averaged spin dynamics simulations performed on a $4\times 4\times 4$ unit-cell lattice with periodic boundary conditions. The splitting of resolved doublet features is used to measure the asymmetry $\mathrm{\Delta }J$ of the nearest-neighbor indirect dipole or $J$-coupling tensor, with the result $\mathrm{\Delta }J=90\pm 20$ Hz. All resolved doublets can be identified with specific nearest- and next-nearest-neighbor isolated spin pairs that occur in the disorder ensemble.

We theoretically investigate the effect of transverse magnetic anisotropy on spin-flip assisted tunneling through atomic-spin chains. Using a phenomenological approach and first-order perturbation theory, we analytically calculate the inelastic tunneling current, differential conductance, and atomic-spin transition rates. We predict the appearance of additional steps in the differential conductance and a pronounced increase in the spin-flip transition rate which at low voltages scales quadratically with the ratio of the transverse anisotropy energy and the sum of the longitudinal anisotropy energy and the exchange energy. Our results provide intuitive qualitative insight into the role played by transverse anisotropy in inelastic tunneling spectroscopy of atomic chains and can be observed under realistic experimental conditions.

We study two-dimensional noncentrosymmetric nodal superconductors under Zeeman field and clarify the field-angle dependence of topological properties. It has been shown that the nodal excitation acquires an excitation gap due to the Zeeman field perpendicular to antisymmetric spin-orbit coupling, and then gapful topological superconductivity is realized [A. Daido and Y. Yanase, Phys. Rev. B 94, 054519 (2016)]. We show that the system undergoes a gapful-gapless transition against tilting of the field. The gapless phase remains to show a finite band gap and unusual Majorana edge states in between the bulk bands. The Majorana edge states naturally propagate in a same direction between oppositely oriented edges. We elucidate relations of such unidirectional Majorana edge states with chiral edge states in the gapful topological superconducting phase and previously studied Majorana flat bands at zero Zeeman field. A compact formula of topological invariants characterizing the edge states is given. The gapful-gapless topological phase transition and associated evolution of Majorana states are demonstrated in a model for $D+p$-wave superconductivity. Experimental realization in recently fabricated cuprate heterostructures and heavy-fermion thin films is discussed.

We propose a possible way to realize topological superconductivity with application of laser light to superconducting cuprate thin films. Applying Floquet theory to a model of $d$-wave superconductors with Rashba spin-orbit coupling, we derive an effective model and discuss its topological nature. Interplay of the Rashba spin-orbit coupling and the laser light effect induces the synthetic magnetic fields, thus making the system gapped. Then the system acquires the topologically nontrivial nature which is characterized by Chern numbers. The effective magnetic fields do not create the vortices in superconductors, and thus the proposed scheme provides a promising way to dynamically realize a topological superconductor in cuprates. We also discuss an experimental way to detect the signature.

Shear transformation (ST) plays a decisive role in determining the mechanical behavior of metallic glasses, which is believed to be a stress-assisted thermally activated process. Understanding the dependence in its activation energy on the stress imposed on the material is of central importance to model the deformation process of metallic glasses and other amorphous solids. Here a theoretical model is proposed to predict the variation of the minimum energy path (MEP) associated with a particular ST event upon further deformation. Verification based on atomistic simulations and calculations are also conducted. The proposed model reproduces the MEP and activation energy of an ST event under different imposed macroscopic strains based on a known MEP at a reference strain. Moreover, an analytical approach is proposed based on the atomistic calculations, which works well when the stress varies linearity along the MEP. These findings provide necessary background for understanding the activation processes and, in turn, the mechanical behavior of metallic glasses.

We present results of an experimental study of structural, magnetotransport, and magnetic properties of a disordered system which consists of the strained crystalline CoFe nanogranules with the size of 2–5 nm embedded into the B-Al-O oxide matrix with a large number of dispersed Fe or Co atoms. They act in the matrix as magnetic ions and contribute essentially to the magnetization at $T$ ⩽ 25 K. The conductivity of the system follows the $\text{ln}T$ law on the metallic side of the metal-insulator transition in the wide range of metal content variation $x=49-56\mathrm{at}.\%$ that formally corresponds to the conductivity of the array of granules with strong tunnel coupling between them. We found that scaling power laws in the dependence of anomalous Hall effect (AHE) resistivity ${\rho }_{\mathrm{AHE}}$ vs longitudinal resistivity ρ strongly differ if temperature $T$ or metal content $x$ are variable parameters. The obtained results are interpreted in terms of the model of two sources of AHE emf arising from metallic nanogranules and insulating tunneling regions, respectively. We suggest that the tunneling AHE component can be caused by the recently predicted scattering assisted mechanism [A. V. Vedyayev et al., Phys. Rev. Lett. 110, 247204 (2013)] and is strongly shunted due to generation of local circular Hall current.

Our research presents theoretical and numerical investigations of the dynamics and thermal transport properties in various locally resonant nanophononic metamaterials. Using finite element analysis, we show that the hybridization between the local resonances of the branched nanopillars and the bulk phonon modes of the host nanostructure can alter the phonon dispersion spectrum and greatly reduce the group velocities, leading to significant thermal conductivity reduction. According to the configuration of the periodic nanostructure, we propose a cantilever-in-mass model to theoretically analyze and control the resonance hybridization band. The influence of nanopillar number and size on the resonance hybridization frequency is systematically explored by both theoretical analysis and numerical simulation. Excellent agreement between theoretical results and numerical simulations reveals that the locally resonant frequencies can be accurately predicted by the proposed analytical model. Remarkably, the thermal conductivity of the resonant branched nanostructure can be tailored close to zero at the vicinity of local resonances with flat dispersion curves.

The random magnetic field produced by nuclear spins has long been viewed as the dominating source of decoherence in the quantum-dot based spins. Here we obtain in both exact and analytical manner the dynamics of spin qubits coupled to nuclear spin environments via the hyperfine interaction, going beyond the weak system-bath interaction and Markovian approximation. We predict that the detailed-balance breaking produced by chemical potential gradient in nuclear baths leads to the rapid oscillations of populations, quantum coherence and entanglement, which are absent in the conventional case (i.e., Overhauser noise). This is attributed to the nonequilibrium feature of the system as shown in the relation between the oscillation period and the chemical potential imbalance. Our results reveal the essentiality of nonequilibriumness with detailed-balance breaking for enhancing the dynamical coherence and entanglement of spin qubits. Moreover, our exact solution explicitly demonstrates that the non-Markovian bath comprised by nuclear spins can preserve the collective quantum state, due to the recovery of coherence. Finally, we propose an experiment using ultracold trapped ions to observe these nonequilibrium and memory effects.

Synchronization of spin torque nano-oscillators (STNOs) has been a subject of extensive research as various groups try to harness the collective power of STNOs to produce a strong enough microwave signal at the nanoscale. Achieving synchronization has proven to be, however, rather difficult for even small arrays while in larger ones the task of synchronization has eluded theorists and experimentalists altogether. In this work we solve the synchronization problem, analytically and computationally, for networks of STNOs connected in series. The procedure is valid for networks of arbitrary size and it is readily extendable to other network topologies. These results should help guide future experiments and, eventually, lead to the design and fabrication of a nanoscale microwave signal generator.

We construct a two-dimensional (2D) lattice model that is argued to realize a gapped chiral spin liquid with (Ising) non-Abelian topological order. The building blocks are spin-$\frac{1}{2}$ two-leg ladders with $\text{SU}\left(2\right)$-symmetric spin-spin interactions. The two-leg ladders are then arranged on rows and coupled through $\text{SU}\left(2\right)$-symmetric interactions between consecutive ladders. The intraladder interactions are tuned so as to realize $c=1/2$ Ising conformal field theory, a fact that we establish numerically via density matrix renormalization group studies. Time-reversal breaking interladder interactions are tuned so as to open a bulk gap in the 2D lattice system. This 2D system supports gapless chiral edge modes with Ising non-Abelian excitations but no charge excitations, in contrast to the Pfaffian non-Abelian fractional quantum Hall state.

We report a density-matrix renormalization group study of the $t\text{−}J$ model with nearest $(t_1$ and $J_1)$ and next-nearest $(t_2$ and $J_2)$ interactions on a four-leg cylinder with concentration $\delta =1/8$ of doped holes. We observe an astonishingly complex interplay between uniform $d$-wave superconductivity (SC) and strong spin and charge-density wave ordering tendencies (SDW and CDW). Depending on parameters, the CDWs can be commensurate with period 4 or 8. By comparing the charge ordering vectors with $2k_F$, we rule out Fermi surface nesting-induced density wave order in our model. Magnetic frustration (i.e., $J_2/J_1\sim 1/2)$ significantly quenches SDW correlations with little effect on the CDW. Typically, the SC order is strongly modulated at the CDW ordering vector and exhibits $d$-wave symmetry around the cylinder. There is no evidence of a near-degenerate tendency to pair-density wave (PDW) ordering, charge $4e$ SC, or orbital current order.

The composition dependence of the natural band alignment at nonpolar ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/{\mathrm{Al}}_y{\mathrm{Ga}}_{1-y}\mathrm{N}$ heterojunctions is investigated via hybrid functional based density functional theory. Accurate band-gap data are provided using Heyd-Scuseria-Ernzerhof (HSE) type hybrid functionals with a composition dependent exact-exchange contribution. The unstrained band alignment between zincblende (zb) ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ semiconductor alloys is studied within the entire ternary composition range utilizing the Branch-point technique to align the energy levels related to the bulklike direct ${\mathrm{\Gamma }}_{\mathrm{v}}\to {\mathrm{\Gamma }}_{\mathrm{c}}$ and indirect, pseudodirect, respectively, ${\mathrm{\Gamma }}_{\mathrm{v}}\to X_{\mathrm{c}}$ type transitions in zb-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$. While the zb-GaN/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ band edges consistently show a type-I alignment, the relative position of fundamental band edges changes to a type-II alignment in the Al-rich composition ranges of zb-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{AlN}$ and zb-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/{\mathrm{Al}}_y{\mathrm{Ga}}_{1-y}\mathrm{N}$ systems. The presence of a direct-indirect band-gap transition at $x_{\mathrm{c}}=0.63$ in zb-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ semiconductor alloys gives rise to a notably different composition dependence of band discontinuities in the direct and indirect energy-gap ranges. Below the critical direct-indirect Al/Ga-crossover concentration, the band offsets show a close to linear dependence on the alloy composition. In contrast, notable bowing characteristics of all band discontinuities are observed above the critical crossover composition.

We perform transport measurements on double quantum dots defined in Ge/Si core/shell nanowires and focus on Pauli spin blockade in the regime where tens of holes occupy each dot. We identify spin blockade through the magnetic field dependence of the leakage current. We find both a dip and a peak in the leakage current at zero field. We analyze this behavior in terms of quantum dot parameters such as coupling to the leads, interdot tunnel coupling, as well as spin-orbit interaction. We estimate a lower bound on the spin-orbit parameter corresponding to an upper bound of $l_{\mathrm{so}}=500$ nm for the Rashba spin-orbit length. We also extract effective Landé $g$ factors up to 8.0 from field-dependent spin blockade measurements.

Long-range interactions in triple quantum dots in the Kondo regime are investigated by accurately solving the three-impurity Anderson model. For the occupation configuration of $(N_1,N_2,N_3)=(1,0,1)$, a long-range antiferromagnetic exchange interaction ($J_{\mathrm{AF}}$) is demonstrated and induces a crossover from the separated Kondo singlet to the long-range spin singlet state between edge dots. In the long-range spin singlet phase, a long-range overlapping or entanglement of Kondo clouds is discovered, which induces a transition peak in the spectral function of the middle dot under equilibrium conditions. Under nonequilibrium conditions, the long-range entanglement of the Kondo clouds is characterized by the conductance peak at zero bias, which can be observed in experiments.

We theoretically investigate the nonlinear optical pulse responses of excitons in a thin film where the excitonic center-of-mass motion is confined. A large interaction volume between excitons and radiation yields particular coupled states with radiative decay times reaching several femtoseconds. By considering two polarization directions of light, we reveal that these fast-decay modes dominantly survive in an optical Kerr spectra even under a massive nonradiative damping $\mathrm{\Gamma }=30$ meV. The results clearly show that there is an optimal combination of the incident pulse width and the film thickness for maximizing the integrated intensity of nonlinear signals.

We study the manifestation of the nonperturbative effects of interaction in sequential tunneling between a quasi-one-dimensional system of chiral quantum Hall edge channels and a multilevel quantum dot (QD). We use the formal scattering theory approach to the bosonization technique to present an alternative derivation of the Fermi-edge singularity effect and demonstrate the origin of its universality. This approach allows us to address, within the same framework, plasmon-assisted sequential tunneling to relatively large dots and investigate the interaction-induced level broadening. The results are generalized by taking into account the dispersion in the spectrum of plasmons in the QD. We then discuss their modification in the presence of neutral modes, which can be realized either in a QD with two chiral strongly interacting edge channels or in a three-dimensional QD in the Coulomb blockade regime. In the former case a universal behavior of the tunneling rate is discovered.