We theoretically propose that the single-crystal formed TaS is a new type of topological metal, hosting ring-shaped gapless nodal lines and triply degenerate points (TDPs) in the absence of spin-orbit coupling (SOC). In the presence of SOC, each TDP splits into four TDPs along the high-symmetric line in momentum space, and one of the nodal rings remains closed due to the protection of the mirror reflection symmetry, while another nodal ring is fully gapped and transforms into six pairs of Weyl points (WPs) carrying opposite chirality. The electronic structures of the projected surfaces are also discussed, the unique Fermi arcs are observed, and the chirality remains or vanishes depending on the projection directions. On the (010) projected surface, one may observe a Lifshitz transition. The new type of topological metal TaS is stable and experimentally achievable, and the coexistence of topological nodal lines, WPs, and TDPs in TaS makes it a potential candidate to study the interplay between different types of topological fermions.

We report the experimental observation of anomalies appearing in the reflection of intense $p$-polarized 100-femtosecond (fs) laser pulses at a nonmetallic material surface with a grating structure. The reflectivity was measured in air as a function of the angle of incidence at a Si grating. The results have exhibited an abrupt decrease to create a sharp dip at a specific incident angle of $\sim {24}^{\circ }$, where the grating surface was deeply ablated along the edge of the grooves. Similar to the so-called Wood's anomalies, the observed angle-dependent reflectivity provides direct evidence that surface plasmon polaritons (SPPs) can resonantly be excited at the interface between air and the nonmetallic material surface, as the intense fs laser pulse produces a high density of free electrons to form a metal-like layer on the Si grating surface. Calculation for a model target reproduces well the experimental results to confirm the excitation of SPPs on the Si grating, demonstrating the generation of enhanced near fields for the periodic ablation of a target surface.

Sensitivity of entanglement Hamiltonian spectrum to boundary conditions is considered as a phase detection parameter for delocalized-localized phase transition. By employing one-dimensional models that undergo delocalized-localized phase transition, we study the shift in the entanglement energies and the shift in the entanglement entropy when we change boundary conditions from periodic to antiperiodic. Specifically, we show that both these quantities show a change of several orders of magnitude at the transition point in the models considered. Therefore, this shift can be used to indicate the phase-transition points in the models. We also show that both these quantities can be used to determine mobility edges separating localized and delocalized states.

We explore the transport properties of a periodically modulated $\alpha -{\mathcal{T}}_3$ lattice in the presence of a perpendicular magnetic field. The effect of the Berry phase on electrical conductivity oscillation, so-called Weiss oscillation, caused by the modulation-induced nonzero drift velocity of charge carriers is investigated. Employing linear response theory within the low-temperature regime, we analyze Weiss oscillation as a function of the external magnetic field for both electrically and magnetically modulated $\alpha -{\mathcal{T}}_3$ lattices numerically as well as analytically. The Berry phase makes this hexagonal lattice structure behave differently than other two-dimensional fermionic systems. It causes a significant valley polarization in magnetoconductivity. Most interestingly, the combined effect of both modulations breaks the particle-hole symmetry and causes a smooth transition from even (odd) to odd (even) filling fraction corresponding to the density of states peaks by means of the Berry phase.

We use Ca doping during growth of one- and two-monolayer-thick MgO films on Ag(100) to identify the adsorption sites of individual adatoms with scanning tunneling microscopy. For this we combine atomic resolution images of the bare MgO layer with images of the adsorbates and the substitutional Ca atoms taken at larger tip-sample distance. For Ho atoms, the adsorption sites depend on MgO thickness. On the monolayer, they are distributed on the O and bridge sites according to the abundance of those sites, $1/3$ and $2/3$, respectively. On the MgO bilayer, Ho atoms populate almost exclusively the O site. A third species adsorbed on Mg is predicted by density functional theory and can be created by atomic manipulation. Au atoms adsorb on the bridge sites for both MgO thicknesses, while Co and Fe atoms prefer the O sites, again for both thicknesses.

We investigate the distribution of waiting times between electrons emitted from a periodically driven single-electron turnstile. To this end, we develop a scheme for analytic calculations of the waiting time distributions for arbitrary periodic driving protocols. We illustrate the general framework by considering a driven tunnel junction before moving on to the more involved single-electron turnstile. The waiting time distributions are evaluated at low temperatures for square-wave and harmonic driving protocols. In the adiabatic regime, the dynamics of the turnstile is synchronized with the external drive. As the nonadiabatic regime is approached, the waiting time distribution becomes dominated by cycle-missing events in which the turnstile fails to emit within one or several periods. We also discuss the influence of finite electronic temperatures. The waiting time distributions provide a useful characterization of the driven single-electron turnstile with complementary information compared to what can be learned from conventional current measurements.

Resonant microwave absorption of two-dimensional electron systems in AlGaAs/GaAs heterostructures with a single Corbino disk geometry has been studied. Axisymmetric dark plasmon modes have been excited using a near-field excitation technique, and their magnetodispersion have been determined in the presence of a perpendicular magnetic field. Plasma excitations observed have been compared to bright plasmon modes in a single disk of identical geometry. Dark plasmon modes have been found to have considerably longer lifetimes compared to bright plasmon modes due to the inhibition of superradiant losses.

The periodically driven quantum Ising chain has recently attracted a large attention in the context of Floquet engineering. In addition to the common paramagnet and ferromagnet, this driven model can give rise to new topological phases. In this work, we systematically explore its quantum phase diagram by examining the properties of its Floquet ground state. We specifically focus on driving protocols with time-reversal invariant points, and demonstrate the existence of an infinite number of distinct phases. These phases are separated by second-order quantum phase transitions, accompanied by continuous changes of local and string order parameters, as well as sudden changes of a topological winding number and of the number of protected edge states. When one of these phase transitions is adiabatically crossed, the correlator associated to the order parameter is nonvanishing over a length scale which shows a Kibble-Zurek scaling. In some phases, the Floquet ground state spontaneously breaks the discrete time-translation symmetry of the Hamiltonian. Our findings provide a better understanding of topological phases in periodically driven clean integrable models.

Bragg projection ptychography (BPP) is a coherent x-ray diffraction imaging technique which combines the strengths of scanning microscopy with the phase contrast of x-ray ptychography. Here we apply it for high resolution imaging of the phase-shifted crystalline domains associated with epitaxial growth. The advantages of BPP are that the spatial extent of the sample is arbitrary, it is nondestructive, and it gives potentially diffraction limited spatial resolution. Here we demonstrate the application of BPP for revealing the domain structure caused by epitaxial misfit in a nanostructured metallic thin film. Experimental coherent diffraction data were collected from a niobium thin film, epitaxially grown on a sapphire substrate as the beam was scanned across the sample. The data were analyzed by BPP using a carefully selected combination of refinement procedures. The resulting image shows a close packed array of epitaxial domains, shifted with respect to each other due to misfit between the film and its substrate.

The Haldane spin-chain compound ${\mathrm{Tb}}_2{\mathrm{BaNiO}}_5$ has been known to order antiferromagnetically below $\left(T_N=\right)63\mathrm{K}$. The present magnetic studies on the polycrystals bring out that there is another magnetic transition at a lower temperature $(T_2=)25\mathrm{K}$ with pronounced magnetic-field-induced metamagnetic and metaelectric behaviors. Multiferroic features are found below $T_2$ only and not at $T_N$. The most intriguing observation is that the observed change in dielectric constant (Δε′) is intrinsic and largest (e.g., ∼18% at 15 K) within this Haldane spin-chain family $R_2{\mathrm{BaNiO}}_5$. Taking into account the fact that this trend (that is, the largest value of Δε′ for the Tb case within this family) correlates well with a similar trend in $T_N$ (with the values of $T_N$ being ∼55, 58, 53, and 32 K for Gd, Dy, Ho, and Er cases), we believe that the explanation usually offered for this $T_N$ behavior in rare-earth systems is applicable for this Δε′ behavior as well. That is, single-ion anisotropy following crystal-field splitting is responsible for the extraordinary magnetodielectric effect in this Tb case. This work provides a pathway in the field of multiferroics to promote magnetoelectric coupling.

We investigated the pressure effect on the magnetization of the soft material ${\mathrm{CsCuCl}}_3$. We also measured the lattice distortion under the longitudinal magnetic fields at the ambient pressure. While the $ab$ plane shrinks in all the quantum phases below $T_{\mathrm{N}}$ at the ambient pressure, its magnitude is much larger in the intermediate 2-1-coplanar or the IC3 phase with a large quantum spin fluctuation than in the low field phase. We found the pressure induced quantum phases; the uud phase for $H\parallel c$ and the IC5 phase for $H\parallel b^{*}$. We also found the large reduction of the magnetization both below and above $T_{\mathrm{N}}$ and the enhancement of $T_{\mathrm{N}}$ by pressure. $d{T}_{\mathrm{N}}/dP$ is much larger in the intermediate field phase than in the low field phase. From these results, we could draw the rough magnetic phase diagram under pressure in a high field region. While all the quantum phases below $T_{\mathrm{N}}$ are stabilized by pressure, the degree of the stability by pressure is much larger in the quantum phase with a large quantum spin fluctuation than in the low field phase. In the soft material such as ${\mathrm{CsCuCl}}_3$, we propose that the $ab$ plane shrinks spontaneously so as to enhance $T_{\mathrm{N}}$ and the quantum spin fluctuation in high field quantum phases under pressure.

The variational tensor network renormalization approach to two-dimensional (2D) quantum systems at finite temperature is applied to a model suffering the notorious quantum Monte Carlo sign problem—the orbital $e_g$ model with spatially highly anisotropic orbital interactions. Coarse graining of the tensor network along the inverse temperature $\beta$ yields a numerically tractable 2D tensor network representing the Gibbs state. Its bond dimension $D$—limiting the amount of entanglement—is a natural refinement parameter. Increasing $D$ we obtain a converged order parameter and its linear susceptibility close to the critical point. They confirm the existence of finite order parameter below the critical temperature $T_c$, provide a numerically exact estimate of $T_c$, and give the critical exponents within $1\%$ of the 2D Ising universality class.

While high-throughput density functional theory (DFT) has become a prevalent tool for materials discovery, it is limited by the relatively large computational cost. In this paper, we explore using DFT data from high-throughput calculations to create faster, surrogate models with machine learning (ML) that can be used to guide new searches. Our method works by using decision tree models to map DFT-calculated formation enthalpies to a set of attributes consisting of two distinct types: (i) composition-dependent attributes of elemental properties (as have been used in previous ML models of DFT formation energies), combined with (ii) attributes derived from the Voronoi tessellation of the compound's crystal structure. The ML models created using this method have half the cross-validation error and similar training and evaluation speeds to models created with the Coulomb matrix and partial radial distribution function methods. For a dataset of 435 000 formation energies taken from the Open Quantum Materials Database (OQMD), our model achieves a mean absolute error of 80 meV/atom in cross validation, which is lower than the approximate error between DFT-computed and experimentally measured formation enthalpies and below 15% of the mean absolute deviation of the training set. We also demonstrate that our method can accurately estimate the formation energy of materials outside of the training set and be used to identify materials with especially large formation enthalpies. We propose that our models can be used to accelerate the discovery of new materials by identifying the most promising materials to study with DFT at little additional computational cost.

Utilizing the magneto-optic Kerr effect and Kerr microscopy measurements, we investigated the antiferromagnetic (AFM) domain switching process at different magnetic fields in a single-crystalline Fe/CoO bilayer grown on MgO(001) substrate. In spite of the zero-net magnetic moment in the CoO layer, we find that the activation energy barrier of CoO AFM domain switching decreased at larger magnetic field. To separate the different behaviors of domain nucleation and domain wall motion during the CoO spin switching process, a new analytical method was developed. Using this method, we found that the CoO domain nucleation energy barrier exhibited a jump at a critical magnetic field while the CoO domain wall motion experienced only a tiny energy barrier variation. The field-dependent behaviors of the energy barriers were attributed to the formation of a spiral domain wall in the Fe layer during its magnetization reversal and this was supported by micromagnetic simulations.

By combining Brillouin light scattering and micromagnetic simulations, we studied the spin-wave (SW) dynamics of a Co/Pd thin film multilayer, which features a stripe domain structure at remanence. The periodic up and down domains are separated by corkscrew type domain walls. The existence of these domains causes a scattering of the otherwise bulk and surface SW modes, which form mode families, similar to a one-dimensional magnonic crystal. The dispersion relation and mode profiles of SWs are measured for the transferred wave vector parallel and perpendicular to the domain axis.

$\mathrm{C}{\mathrm{o}}_3\mathrm{Si}$ was recently reported to exhibit remarkable magnetic properties in the nanoparticle form [B. Balasubramanian et al., Appl. Phys. Lett. 108, 152406 (2016)], yet better understanding of this material should be promoted. Here we report a study on the crystal structures of $\mathrm{C}{\mathrm{o}}_3\mathrm{Si}$ using an adaptive genetic algorithm and discuss its electronic and magnetic properties from first-principles calculations. Several competing phases of $\mathrm{C}{\mathrm{o}}_3\mathrm{Si}$ have been revealed from our calculations. We show that the hexagonal $\mathrm{C}{\mathrm{o}}_3\mathrm{Si}$ structure reported in experiments has lower energy in the nonmagnetic state than in the ferromagnetic state at zero temperature. The ferromagnetic state of the hexagonal structure is dynamically unstable with imaginary phonon modes and transforms into a new orthorhombic structure, which is confirmed by our structure searches to have the lowest energy for both $\mathrm{C}{\mathrm{o}}_3\mathrm{Si}$ and $\mathrm{C}{\mathrm{o}}_3\mathrm{Ge}$. Magnetic properties of the experimental hexagonal structure and the lowest-energy structures obtained from our structure searches are investigated in detail.

We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the first-order transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this “healing-length” distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Bragg-glass phase. This suggests that the healing length may be a key quantity to model confinement effects in the first-order transition of extremely layered vortex nanocrystals.

We have carried out an analysis of magnetic data in 69 uranium, 7 neptunium, and 4 plutonium ferromagnets with the spin fluctuation theory developed by Takahashi [Y. Takahashi, J. Phys. Soc. Jpn. 55, 3553 (1986)]. The basic and spin fluctuation parameters of the actinide ferromagnets are determined and the applicability of the spin fluctuation theory to actinide $5f$ system has been discussed. Itinerant ferromagnets of the $3d$ transition metals and their intermetallics follow a generalized Rhodes-Wohlfarth relation between $p_{\mathrm{eff}}/p_{\mathrm{s}}$ and $T_{\mathrm{C}}/T_0$, viz., $p_{\mathrm{eff}}/p_{\mathrm{s}}\propto {(T_{\mathrm{C}}/T_0)}^{-3/2}$. Here, $p_{\mathrm{s}}$, $p_{\mathrm{eff}}$, $T_{\mathrm{C}}$, and $T_0$ are the spontaneous and effective magnetic moments, the Curie temperature, and the width of spin fluctuation spectrum in energy space, respectively. The same relation is satisfied for $T_{\mathrm{C}}/T_0<1.0$ in the actinide ferromagnets. However, the relation is not satisfied in a few ferromagnets with $T_{\mathrm{C}}/T_0\sim 1.0$ that corresponds to local moment system in the spin fluctuation theory. The deviation from the theoretical relation may be due to several other effects not included in the spin fluctuation theory such as the crystalline electric field effect on the $5f$ electrons from ligand atoms. The value of the spontaneous magnetic moment $p_{\mathrm{s}}$ increases linearly as a function of $T_{\mathrm{C}}/T_0$ in the uranium and neptunium ferromagnets below ${(T_{\mathrm{C}}/T_0)}_{\mathrm{kink}}=0.32\pm 0.02$, where a kink structure appears in relation between the two quantities. $p_{\mathrm{s}}$ increases more weakly above ${(T_{\mathrm{C}}/T_0)}_{\mathrm{kink}}$. A possible interpretation with the $T_{\mathrm{C}}/T_0$ dependence of $p_{\mathrm{s}}$ is given.