The electronic structure of Au-Sn intermetallic layers of different compositions grown on Au(111) to the thickness of several nanometers has been studied in this paper. The layer, the interface, and the substrate related components in the Au $4f$ and Sn $4d$ core-level spectra obtained using x-ray photoelectron spectroscopy (XPS) vary with deposition parameters to reveal the details of the Au-Sn formation. While AuSn is grown by deposition at room temperature, Au rich compounds form as a result of heat treatment through interdiffusion of Au and Sn. Deposition at high temperature forms more Au rich compositions compared to postannealing at the same temperature due to the kinetic energy of the impinging Sn atoms in the former case. Postannealing, on the other hand, stabilizes the bulk phases such as AuSn and ${\mathrm{Au}}_5\mathrm{Sn}$ and exhibits an activated behavior for transition from the former to the latter with increasing temperature. The XPS valence-band spectra of AuSn and ${\mathrm{Au}}_5\mathrm{Sn}$ layers show good agreement with the density functional theory calculation, indicating that these have the bulk structure reported in literature. However, the influence of antisite defects is observed in ${\mathrm{Au}}_5\mathrm{Sn}$. Low-energy electron-diffraction study reveals that although the AuSn layer is ordered its top surface is disordered at room temperature. Surface order is obtained by annealing or deposition at elevated temperatures and dispersing bands are observed by angle-resolved photoemission spectroscopy. Both electronlike and holelike bands are evident for the ($\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ phase, while a nearly free electronlike parabolic surface state is observed for the $p(3\times 3)R{15}^{\circ }$ phase.

We study the electron and phonon spectral functions as well as superconductivity for a two-dimensional electron gas coupled to dipolar phonons via small-momentum-transfer scattering. The results reported here are obtained through sign-problem-free quantum Monte Carlo simulation. Hence, aside from modeling there are no further approximations. Our results can be relevant to the high-temperature superconductor at the interface between a unit-cell-thick FeSe and ${\mathrm{SrTiO}}_3$. Our results give support to the interpretation that the observed replica band in unit-cell-thick FeSe on ${\mathrm{SrTiO}}_3$ is due to the small-momentum transfer interaction between the FeSe electrons and a branch of polar STO phonons. Moreover, we conclude that while the small-momentum electron-phonon interaction can not induce strong Cooper pairing by itself, it can enhance the Cooper pairing triggered by other pairing interactions.

In this paper, we study the evaporation dynamics of the Sachdev-Ye-Kitaev model, with an initial temperature $T_{\chi }$, by coupling it to a thermal bath with lower temperature $T_{\psi }<T_{\chi }$ modeled by a larger SYK model. The coupling between the small system and the bath is turned on at time $t=0$. Then the system begins to evolve and finally becomes thermalized. Using the Keldysh approach, we analyze the relaxation process of the system for different temperatures and couplings. For marginal or irrelevant coupling, after a short-time energy absorption, we find a smooth thermalization of the small system where the energy relaxes before the system become thermalized. The relaxation rate of effective temperature is found to be bounded by $T$, while the energy thermalization rate increases without saturation when increasing the coupling strength. On the contrary, for the relevant coupling case, both energy and effective temperature show oscillations. We find this oscillations frequency to be coincident with the typical excitation energy of the small SYK system.

Colloidal quantum dots (QDs) of group III-V are considered as promising candidates for next-generation environmentally friendly light emitting devices, yet there appears to be only limited understanding of the underlying electronic and excitonic properties. Using large-scale density functional theory with the hybrid B3LYP functional solving the single-particle states and time-dependent density functional theory accounting for the many-body excitonic effects, we have identified the structural, electronic, and excitonic optical properties of InP, GaP, and GaInP QDs containing up to a thousand atoms or more. The calculated optical gap of InP QD appears in excellent agreement with available experiments, and it scales nearly linearly with the inverse diameter. The radiative exciton decay lifetime is found to increase surprisingly linearly with increasing the dot size. For GaP QDs we predict an unusual electronic state crossover at a diameter of around 1.5 nm, whereby the nature of the lowest unoccupied molecular orbital (LUMO) state switches its symmetry from ${\mathrm{\Gamma }}_5$-like at a larger diameter to ${\mathrm{\Gamma }}_1$-like at a smaller diameter. After the crossover, the absorption intensity of the band-edge exciton states is significantly enhanced. Finally, we find that Vegard's law holds very well for GaInP random alloyed quantum dots down to ultrasmall sizes with less than a hundred atoms. The obtained energy gap bowing parameter of this common-cation compound in QD regime appears positive, size-dependent, and much smaller than its bulk parentage. The volume deformation, dominating over the charge exchange and structure relaxation effects, is mainly responsible for the QD energy gap bowing. The impact of excitonic effects on the optical bowing is found to be marginal. The present work provides a road map for a variety of electronic and optical properties of colloidal QDs in group III-V that can guide spectroscopic studies.

We use density-functional band-structure calculations to explore the origin of the up-up-down-down (UUDD) magnetic order in ${\mathrm{Cu}}_2{\mathrm{GeO}}_4$ with the frustrated $J_1\text{−}{J}_2$ spin chains coupled into layers within the spinel-like crystal structure. In contrast to earlier studies, we find that the nearest-neighbor coupling $J_1$ should be negligibly small, due to a nearly perfect compensation of the ferromagnetic direct exchange and antiferromagnetic superexchange. Under this condition, weak symmetric anisotropy of the exchange couplings gives rise to the UUDD order observed experimentally and also elucidates the nontrivial ordering pattern between the layers, whereas a small Dzyaloshinskii-Moriya interaction causes a spin canting that may generate local electric polarization. We argue that the buckling of the copper chains plays a crucial role in the suppression of $J_1$ in ${\mathrm{Cu}}_2{\mathrm{GeO}}_4$ and sets this compound apart from other $J_1\text{−}{J}_2$ chain magnets.

We use density functional theory to quantitatively compute the effect of light absorption in ferromagnetic materials. We show that, in the presence of spin-orbit coupling, optically induced transitions do not conserve the magnetization and that a systematic induced demagnetization, whose magnitude depends on both the helicity of the light and the direction of the magnetization, is observed. Very differently from the inverse Faraday effect, this mechanism is due to the absorption of light and depends on the magnetic state of each atom, and therefore cannot be described by an effective optomagnetic field. Then, based on these results, we derive a set of parameters which can be used in micromagnetic simulations in order to account for light transition effects on the magnetization dynamics.

Recent experiments on switching antiferromagnetic domains by electric current pulses have attracted a lot of attention to spin-orbit torques in antiferromagnets. In this work, we employ the tight-binding model solver, kwant, to compute spin-orbit torques in a two-dimensional antiferromagnet on a honeycomb lattice with strong spin-orbit interaction of Rashba type. Our model combines spin-orbit interaction, local $s-d$-like exchange, and scattering of conduction electrons on on-site disorder potential to provide a microscopic mechanism for angular-momentum relaxation. We consider two versions of the model: One with preserved and one with broken sublattice symmetry. A nonequilibrium staggered polarization that is responsible for the so-called Néel spin-orbit torque is shown to vanish identically in the symmetric model but may become finite if sublattice symmetry is broken. Similarly, antidamping spin-orbit torques vanish in the symmetric model but become finite and anisotropic in a model with broken sublattice symmetry. As expected, antidamping torques also reveal a sizable dependence on impurity concentration. Our numerical analysis also confirms symmetry classification of spin-orbit torques and strong torque anisotropy due to in-plane confinement of electron momenta.

We study field-driven domain wall (DW) velocities in asymmetric multilayer stacks with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI), both experimentally and by micromagnetic simulations. Using magneto-optical Kerr microscopy under intense and nanoseconds-long magnetic field pulses, we show that DWs in these films propagate at velocities up to hundreds of meters per second and that, instead of the expected decrease in velocity after the Walker field, a long plateau with constant velocity is observed, before breakdown. Both the maximum speed and the field extent of the velocity plateau strongly depend on the values of the spontaneous magnetization and the DMI strength, as well as on the magnetic anisotropy. Micromagnetic simulations reproduce these features in sufficiently wide strips, even for perfect samples. A physical model explaining the microscopic origin of the velocity plateau is proposed.

We present the preparation and characterization of an $\mathrm{E}{\mathrm{r}}_2\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_7$ single crystal. The material was studied by means of x-ray and neutron diffraction, magnetization, AC susceptibility, and specific heat measurements. The defect-fluorite type of cubic structure, previously reported as the crystal structure of $\mathrm{E}{\mathrm{r}}_2\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_7$, is consistent with our x-ray powder diffraction data. The measurements of physical properties were performed in magnetic field applied along principal crystallographic directions. A negligible directional dependence of physical properties is observed at temperatures down to 3 K, and only minor differences can be traced at lower temperatures. A low-temperature anomaly in magnetization and specific heat data and its evolution with applied magnetic field is followed. A frequency-dependent shift of freezing temperature was observed in AC susceptibility. Several origins of the observed anomaly are discussed, leaving a spin-glass state as the most probable explanation.

The alkali metal potassium exhibits complex structures under pressure, including both commensurate and incommensurate phases, however, the transformation kinetics and microscopic mechanisms between these are yet to be elucidated. Here, we investigate the phase transformation behavior between close-packed fcc and incommensurate host-guest structures (KIII). We use multiscale molecular dynamics, with a machine-learned potential which fully reproduces the phase diagram and known phase transitions of potassium. We find that no straightforward, low-energy path exists: The previously proposed displacive transformation mechanisms have impossibly high kinetic barriers. The fcc-KIII transition occurs in a complex and diffusive manner, and involves an intermediate amorphization process during the nucleation of the product phase. Our findings may provide further insight into phase transition theory.

Skyrmions are chiral magnetic textures that have immense potential for applications in spintronic devices. However, their formation is quite challenging and necessitates a subtle balance of the magnetic interactions at play. Here, we study Ta/FeCoB/TaOx trilayer using crossed double wedges, i.e., thickness gradients of FeCoB and of top Ta, which is subsequently oxidized, leading to an oxidation gradient. This enabled us to observe micron-sized skyrmions in the vicinity of different transition regions of the sample: from perpendicular magnetic anisotropy to paramagnetic phase and also from perpendicular to in-plane magnetic anisotropy. These observations can be explained by the isolated skyrmion model taking into account the different energy contributions at play, namely, anisotropy, exchange, Dzyaloshinskii-Moriya, dipolar, and Zeeman. We also qualitatively compare the current-induced motion of skyrmions obtained in different transition regions. Our study not only provides an effective means to form skyrmions by tuning the interfacial magnetic properties but also highlights the differences pertaining to the skyrmions observed in different transition zones, which is extremely crucial for any envisaged application.

We present a study of the digitized quantum annealing protocol proposed by R. Barends et al. [Nature (London) 534, 222 (2016)]. Our analysis, performed on the benchmark case of a transverse Ising chain problem, shows that the algorithm has a well-defined optimal working point for the annealing time ${\tau }_{\mathrm{P}}^{\mathrm{opt}}$, scaling as ${\tau }_{\mathrm{P}}^{\mathrm{opt}}\sim \mathrm{P}$, where $\mathrm{P}$ is the number of digital Trotter steps, beyond which the residual energy error shoots up toward the value characteristic of the maximally disordered state. We present an analytical analysis for the translationally invariant transverse Ising chain case, but our numerical evidence suggests that this scenario is more general, surviving, for instance, the presence of disorder.

Determining phase diagrams and phase transitions semiautomatically using machine learning has received a lot of attention recently, with results in good agreement with more conventional approaches in most cases. When it comes to more quantitative predictions, such as the identification of universality class or precise determination of critical points, the task is more challenging. As an exacting testbed, we study the Heisenberg spin-1/2 chain in a random external field that is known to display a transition from a many-body localized to a thermalizing regime, which nature is not entirely characterized. We introduce different neural network structures and dataset setups to achieve a finite-size scaling analysis with the least possible physical bias (no assumed knowledge on the phase transition and directly inputting wave-function coefficients), using state-of-the-art input data simulating chains of sizes up to $L=24$. In particular, we use domain adversarial techniques to ensure that the network learns scale-invariant features. We find a variability of the output results with respect to network and training parameters, resulting in relatively large uncertainties on final estimates of critical point and correlation length exponent which tend to be larger than the values obtained from conventional approaches. We put the emphasis on interpretability throughout the paper and discuss what the network appears to learn for the various used architectures. Our findings show that a quantitative analysis of phase transitions of unknown nature remains a difficult task with neural networks when using the minimally engineered physical input.

The magnetic structure of Nd in NdFeAsO compound has been investigated by x-ray resonant magnetic scattering at the Nd $L_2$ edge ($E=6.725$ keV) at $1.7\le T\le 15$ K. At $T=1.7$ K we find that the Nd moments are aligned along the crystallographic $c$ direction with the (1, 0, 0) propagation vector, and are arranged antiferromagnetically along the $a$ direction and ferromagnetically along the $b$ and $c$ directions. At $1.7<T\le 15$ K, we observe an unusual temperature dependence of the magnetic Bragg peaks that appear differently at $\mathbf{Q}=(h,0,l)$ and $(0,k,l)$. From the changes of magnetic intensities and our representation analysis, we find that the Nd moments are reoriented toward the $a$ direction with a gradual decrease of the moment component in the $c$ direction with increasing temperature. We conclude that the reorientation of the Nd moment is due to a strong interplay with the Fe moment which is fully ordered in this temperature range.

We report experimental measurements of shot noise in MgO-based magnetic tunnel junctions (MTJs) upon angular magnetization rotation in the free layer. The normalized shot noise (the Fano factor) is found to be sub-Poissonian and exhibits a sinusoidal-like variation over the relative magnetic orientation between the free and the pinned layer inside the MTJs. The observed sub-Poissonian statistics provides direct evidence of electron sequential tunneling mediated by localized states inside the tunnel barrier. The variation of the Fano factor can be interpreted in terms of a semi-classical model that we propose. Based on this model, the variation of the Fano factor can be used to reveal microscopic details of the tunneling barrier.

We present a comprehensive study of the temperature ($T$) dependence of the longitudinal spin Seebeck effect (LSSE) in $\mathrm{Pt}/{\mathrm{EuO}}_{1-x}$ and $\mathrm{Pt}/{\mathrm{La}}_2{\mathrm{NiMnO}}_6$ (LNMO) hybrid structures across their Curie temperatures ($T_c$). Both systems host ferromagnetic interaction below $T_c$, and hence present optimal conditions for testing magnon spin current based theories against ferrimagnetic yttrium iron garnet. Notably, we observe an anomalous Nernst effect generated voltage in bare ${\mathrm{EuO}}_{1-x}$, however, we find LSSE predominates the thermal signals in the bilayers with Pt. The $T$ dependence of the LSSE in small $T$ range near $T_c$ could be fitted to a power law of the form ${(T_c-T)}^P$. The derived critical exponent $P$ was verified for different methods of LSSE representation and sample crystallinity. The results are explained based on the magnon-driven thermal spin pumping mechanism that relates the $T$ dependence of LSSE to the spin mixing conductance ($G_{\text{mix}}$) at the heavy metal/ferromagnet interface, which in turn is known to vary in accordance with the square of the spontaneous magnetization ($M_s$). Additionally, the $T$ dependence of the real part of $G_{\text{mix}}$ derived from spin Hall magnetoresistance measurements at different temperatures for the Pt/LNMO structure further establishes the interdependence.

Motivated by the recent theoretical study by Okubo et al. [Phys. Rev. Lett. 108, 017206 (2012)] on the possible realization of the frustration-induced symmetric skyrmion-lattice state in the $J_1-J_2$ (or $J_1-J_3)$ triangular-lattice Heisenberg model without the Dzyaloshinskii-Moriya interaction, we investigate the ordering of the classical $J_1-J_2$ honeycomb-lattice Heisenberg antiferromagnet under a magnetic field by means of a Monte Carlo simulation, a mean-field analysis and a low-temperature expansion. The model has been known to have an infinite ringlike degeneracy in the wave-vector space in its ground state for $1/6<J_2/J_1<0.5$, in distinction with the triangular-lattice model. As reported by Okumura et al. [J. Phys. Soc. Jpn. 79, 114705 (2010)], such a ringlike degeneracy gives rise to exotic spin liquid states in zero field, e.g., the “ring-liquid” state and the “pancake-liquid” state. In this paper, we study the in-field ordering properties of the model paying attention to the possible appearance of exotic multiple-$q$ states. Main focus is made on the $J_2/J_1=0.3$ case, where we observe a rich variety of multiple-$q$ states including the single-$q$, double-$q$, and triple-$q$ states. While the skyrmion-lattice triple-$q$ state observed in the triangular-lattice model is not realized, we instead observe an exotic double-$q$ state consisting of meron/antimeron-like lattice textures.

The Dirac semimetal ${\mathrm{PdTe}}_2$ was recently reported to be a type-I superconductor with $T_c=1.64$ K and a critical field ${\mu }_0{H}_c=13.6$ mT. Since type-I superconductivity is unexpected for binary compounds, we have conducted muon spin rotation ($\mu \mathrm{SR}$) experiments to probe the superconducting phase on the microscopic scale via its intermediate state. For crystals with a finite demagnetization factor $N$, the intermediate state forms in applied fields $(1-N)H_c<H_a<H_c$. We have carried out transverse field muon spin rotation measurements on a thin disklike crystal with the field perpendicular to ($N_{\perp }=0.87$) and in the plane ($N_{\parallel }=0.08$) of the disk. By analyzing the $\mu \mathrm{SR}$ signal we find that the volume fraction of the normal domains grows quasilinearly with applied field at the expense of the Meissner domain fraction. This then provides solid evidence for the intermediate state and type-I superconductivity in the bulk of ${\mathrm{PdTe}}_2$.