We show that quasi-one-dimensional quantum wires can be written onto the surface of magnetic topological insulator (MTI) thin films by gate arrays. When the MTI is in a quantum anomalous Hall state, MTI/superconductor quantum wires have especially broad stability regions for both topological and nontopological states, facilitating creation and manipulation of Majorana particles on the MTI surface.

We introduce the concept of plasmonic parametric resonance (PPR) as a novel way to amplify high angular momentum plasmonic modes of nanoparticles by means of a simple uniform optical pump. In analogy with parametric resonance in dynamical systems, PPR originates from the temporal modulation of one of the parameters governing the evolution of the state of the system. As opposed to conventional localized surface plasmon resonances (LSPR), we show that in principle any plasmonic mode of arbitrarily high order is accessible by PPR with a spatially uniform optical pump. Moreover, in contradistinction with other mechanisms of plasmonic amplification, the coherent nature of PPR lends itself to a more straightforward experimental detection approach. The threshold conditions for PPR are analytically derived. Schemes of experimental realization and detection are also discussed.

We propose a model for the Co charge disproportionation in $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{C}{\mathrm{a}}_x\mathrm{Co}{\mathrm{O}}_{4\pm \delta }$ ($0.4\le x\le 0.7$) and $\mathrm{L}{\mathrm{a}}_{1.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Co}{\mathrm{O}}_4$ layered cobaltites based on the photon energy, azimuthal angle, and polarization dependence of the scattered intensity of selected reflections in a resonant x-ray scattering experiment. Tetragonal superlattice $(h/2,h/2,l)$-type reflections were detected at room temperature for all the samples independently from the Ca (or Sr) doping rate in agreement with a checkerboard ordering of two different Co sites. The corresponding average charge disproportion is accounted for by a semiempirical model from which the imaginary part of the resonant atomic scattering factors of each of the two Co atoms is obtained resulting in about $0.5\pm 0.1$ electron. Moreover, no forbidden $(h/4,h/4,l)$-type reflections were observed at room temperature indicating the lack of any local anisotropy ordering in contrast with the behavior found in the related layered manganites. Finally, we found the pattern of small distortions $(\sim 0.05\AA )$ around the two Co sites compatible with the resonant x-ray scattering results. The symmetry of this displacement pattern is consistent with the $A2mm$ (or Ammm) orthorhombic structure of the ordered phase and the structural transition is accounted for by the condensation of two soft modes—${\mathrm{X}}_1{}^{+}\left({\mathrm{B}}_{2\mathrm{u}}\right)$ and ${\mathrm{X}}_1{}^{+}\left({\mathrm{A}}_1\right)$—acting on the oxygen atoms.

The model of dimers adsorption on hexagonal lattice with different orientations to surface and hard-spheres lateral interactions has been studied at nonzero temperature. The transfer-matrix method was used as the main one and the Monte Carlo method was used for checking of some extreme cases. Adsorption isotherms, dependencies of the entropy from the density of the adsorption layer and of the energy from the system temperature at certain points of the phase space, were computed. It was found that at least the first ten phases of the ground state still persist at nonzero temperatures.

The Gibbs free energy is the fundamental thermodynamic potential underlying the relative stability of different states of matter under constant-pressure conditions. However, computing this quantity from atomic-scale simulations is far from trivial, so the potential energy of a system is often used as a proxy. In this paper, we use a combination of thermodynamic integration methods to accurately evaluate the Gibbs free energies associated with defects in crystals, including the vacancy formation energy in bcc iron, and the stacking fault energy in fcc nickel, iron, and cobalt. We quantify the importance of entropic and anharmonic effects in determining the free energies of defects at high temperatures, and show that the potential energy approximation as well as the harmonic approximation may produce inaccurate or even qualitatively wrong results. Our calculations manifest the necessity to employ accurate free energy methods such as thermodynamic integration to estimate the stability of crystallographic defects at high temperatures.

Magnetic properties of a trigonal ferromagnet $\mathrm{N}{\mathrm{d}}_2\mathrm{F}{\mathrm{e}}_{17}$ have been studied on single crystals in steady (14 T) and pulsed (32 T) magnetic fields. The easy-magnetization direction lies close to the [120] axis, deviating from the basal plane by $2.9^{\circ }$ (at $T=5\mathrm{K}$). Of particular interest is the low-temperature magnetization process along the high-symmetry axis [001], which is the hard direction. This process is discontinuous and involves two first-order phase transitions (FOMPs). One of them (at 20 T) is a symmetry FOMP similar to that observed in $\mathrm{S}{\mathrm{m}}_2\mathrm{F}{\mathrm{e}}_{17}$. The second transition (at 10.4 T) is unusual: as the magnetization turns abruptly toward the applied field, it also changes its azimuthal orientation (the angle $\phi$) by ${60}^{\circ }$. Both transitions can be reasonably accounted for by the presence of a significant sixth-order trigonal anisotropy term.

We report the structural, magnetic, and transport properties of the polycrystalline CrVTiAl alloy along with first-principles calculations. The alloy crystallizes in a LiMgPdSn-type structure with a lattice parameter of 6.14 Å at room temperature. The absence of the (111) peak along with the presence of a weak (200) peak indicates the antisite disorder of Al with Cr and V atoms, which is different from the pure ${\mathrm{DO}}_3$ type. Magnetization measurements reveal a magnetic transition near 710 K, a coercive field of $\sim 100$ Oe at 3 K, and a moment of $\sim {10}^{-3}{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ These observations are indicative of fully compensated ferrimagnetism in the alloy, which is confirmed by theoretical modeling. The temperature coefficient of resistivity is found to be negative, signaling the semiconducting nature. However, the absence of exponential dependence indicates the semiconducting nature with gapless/spin-gapless behavior. Electronic and magnetic properties of CrVTiAl for all three possible crystallographic configurations are studied theoretically. All the configurations are found to be different forms of semiconductors. The ground-state configuration is a fully compensated ferrimagnet with band gaps of 0.58 and 0.30 eV for the spin-up and -down bands, respectively. The next-higher-energy configuration is also fully compensated ferrimagnetic but has a spin-gapless semiconducting nature. The highest-energy configuration corresponds to a nonmagnetic, gapless semiconductor. The energy differences among these configurations are quite small ($<1\mathrm{mRy}/\mathrm{atom}$), which hints that, at finite temperatures, the alloy exists in a disordered phase, which is a mixture of the three configurations. By taking into account the theoretical and experimental findings, we conclude that CrVTiAl is a fully compensated ferrimagnet with a predominantly spin-gapless semiconducting nature.

Boundary surfaces of nodal gap superconductors can host Andreev bound states (ABS), which develop a paramagnetic response under external RF field in contrast to the bulk diamagnetic response of the bulk superconductor. At low temperature, this surface paramagnetic response dominates and enhances the nonlinear RF response of the sample. With a recently developed photoresponse imaging technique, the anisotropy of this “paramagnetic” nonlinear Meissner response, and its current direction (angular) and RF power dependence has been systematically studied. A theoretical model describing the current flow in the surface paramagnetic Andreev bound state, the bulk diamagnetic Meissner state, and their response to optical illumination is proposed and it shows good agreement with the experimental results.

We have recently demonstrated that the fast momentum relaxation due to electron-phonon scattering of hot electrons excited into the conduction band of GaAs leads to the formation of hot-electron ensembles spread over the Brillouin zone. In the present work, we study the energy relaxation of hot-electron ensembles in GaAs, by means of ab initio calculations and time-, energy-, and momentum-resolved spectroscopy. We theoretically show that when the temperature decreases, the energy relaxation time ascribed to electron-phonon interaction becomes faster than at ambient temperature and prove that this is indeed the case in the experimental results.

We investigate the critical properties of the phase transition towards complex tensor order that has been proposed to occur in spin-orbit-coupled superconductors. For this purpose, we formulate the bosonic field theory for fluctuations of the complex irreducible second-rank tensor order parameter close to the transition. We then determine the scale dependence of the couplings of the theory by means of the perturbative renormalization group (RG). For the isotropic system, we generically detect a fluctuation-induced first-order phase transition. The initial values for the running couplings are determined by the underlying microscopic model for the tensorial order. As an example, we study three-dimensional Luttinger semimetals with electrons at a quadratic band-touching point. Whereas the strong-coupling transition of the model receives substantial fluctuation corrections, the weak-coupling transition at low temperatures is rendered only weakly first order due to the presence of a fixed point in the vicinity of the RG trajectory. If the number of fluctuating complex components of the order parameter is reduced by cubic anisotropy, the theory maps onto the field theory for frustrated magnetism.

Half-Heusler alloys provide a new platform for deriving a host of topologically exotic compounds through the inherent flexibility of tuning their hybridization strength (via lattice parameters), spin-orbit strength, substitution/doping, etc. Using the first-principles calculation within the density functional theory, we explore the possibility of realizing a topological insulating phase in a new half-Heusler material ZrIrBi. We discovered three routes through which ZrIrBi can be transformed to exhibit a topological nontrivial phase. They are (i) a hydrostatic expansion by 1% causing a band inversion with zero gap, (ii) a uniaxial strain along (001) direction which opens a band gap while preserving the inverted band order, and (iii) substitution of 50% Bi by As and 50% Zr by Hf forming the compounds $\mathrm{ZrIr}({\mathrm{As}}_{0.5}{\mathrm{Bi}}_{0.5}$) and $({\mathrm{Zr}}_{0.5}{\mathrm{Hf}}_{0.5}$)IrBi again showing a topologically nontrivial band inversion. A definitive proof of the surface conduction in all three cases are done by simulating surface band structures. We report the formation energies and the phonon dispersion for the three cases to confirm the chemical and mechanical stability of the compounds.

Despite intense interest in realizing topological phases across a variety of electronic, photonic, and mechanical platforms, the detailed microscopic origin of topological behavior often remains elusive. To bridge this conceptual gap, we show how hallmarks of topological modes—boundary localization and chirality—emerge from Newton's laws in mechanical topological systems. We first construct a gyroscopic lattice with analytically solvable edge modes, and show how the Lorentz and spring restoring forces conspire to support very robust “dangling bond” boundary modes. The chirality and locality of these modes intuitively emerges from microscopic balancing of restoring forces and cyclotron tendencies. Next, we introduce the highlight of this work, an experimentally realistic mechanical nonequilibrium (Floquet) Chern lattice driven by ac electromagnets. Through appropriate synchronization of the ac driving protocol, the Floquet lattice is “pushed around” by a rotating potential analogous to an object washed ashore by water waves. Besides hosting “dangling bond” chiral modes analogous to the gyroscopic boundary modes, our Floquet Chern lattice also supports peculiar half-period chiral modes with no static analog, i.e., analogs of anomalous Floquet Chern insulators edge modes. With key parameters controlled electronically, our setup has the advantage of being dynamically tunable for applications involving arbitrary Floquet modulations. The physical intuition gleaned from our two prototypical topological systems is applicable not just to arbitrarily complicated mechanical systems, but also photonic and electrical topological setups.

Graphene is a two-dimensional material that has been highly regarded with its unique features to excite surface plasmonic waves. In this paper, we present the Anderson localization of surface plasmons in monolayer graphene. Here we proposed an active plasmonic device that consists of a monolayer graphene on silicon random grating to trap the surface plasmons in local cavities that are created by random multiple scattering. The quality factor of localized graphene surface plasmons (GSPs) is greater than the corresponding factor for uniformly distributed GSPs in periodic silicon substrate (reported before) up to three times. The field intensity of spatially localized GSPs in monolayer graphene is increased by a factor of 15 compared to GSPs in periodic grating. Our simulation results also show that the bandwidth of excitation spectrum of GSPs is extended as a result of introducing randomness in period in order to realize the desired random grating structure. Although the tuning of resonance frequencies of Anderson localized GSPs is a challenging task due to its random nature, here we tune the resonance frequencies of localized surface plasmons by using an external gate voltage for adjusting the Fermi level of monolayer graphene.

The interlayer gallery between two adjacent sheets of van der Waals materials is expected to modify properties of atoms and molecules confined at the atomic interfaces. Here, we directly image individual hydrogen atom intercalated between two graphene sheets and investigate its dynamics by scanning tunnelling microscopy (STM). The intercalated hydrogen atom is found to be remarkably different from atomic hydrogen chemisorbed on the external surface of graphene. Our STM measurements, complemented by first-principles calculations, show that the hydrogen atom intercalated between two graphene sheets has dramatically reduced potential barriers for elementary migration steps. Especially, the confined atomic hydrogen dissociation energy from one of the graphene sheet is reduced to 0.34 eV, which is only about a third of a hydrogen atom chemisorbed on the external surface of graphene. This offers a unique platform for direct imaging of the atomic dynamics of confined atoms. Our results suggest that the atomic interfaces of van der Waals materials provide a confined environment to tune the dynamics process of confined atoms or molecules.

We present an accurate interatomic potential for graphene, constructed using the Gaussian approximation potential (GAP) machine learning methodology. This GAP model obtains a faithful representation of a density functional theory (DFT) potential energy surface, facilitating highly accurate (approaching the accuracy of ab initio methods) molecular dynamics simulations. This is achieved at a computational cost which is orders of magnitude lower than that of comparable calculations which directly invoke electronic structure methods. We evaluate the accuracy of our machine learning model alongside that of a number of popular empirical and bond-order potentials, using both experimental and ab initio data as references. We find that whilst significant discrepancies exist between the empirical interatomic potentials and the reference data—and amongst the empirical potentials themselves—the machine learning model introduced here provides exemplary performance in all of the tested areas. The calculated properties include: graphene phonon dispersion curves at 0 K (which we predict with sub-meV accuracy), phonon spectra at finite temperature, in-plane thermal expansion up to 2500 K as compared to NPT ab initio molecular dynamics simulations and a comparison of the thermally induced dispersion of graphene Raman bands to experimental observations. We have made our potential freely available online at [http://www.libatoms.org].

We analyze the coupling of elastic lattice deformations to the magnon degrees of freedom of magnon Dirac materials. For a honeycomb ferromagnet we find that, as happens in the case of graphene, elastic gauge fields appear coupled to the magnon pseudospinors. For deformations that induce constant pseudomagnetic fields, the spectrum around the Dirac nodes splits into pseudo-Landau levels. We show that when a Dzyaloshinskii-Moriya interaction is considered, a topological gap opens in the system and a Chern-Simons effective action for the elastic degrees of freedom is generated. Such a term encodes a phonon Hall viscosity response, entirely generated by quantum fluctuations of magnons living in the vicinity of the Dirac points. The magnon Hall viscosity vanishes at zero temperature, and grows as temperature is raised and the states around the Dirac points are increasingly populated.

Magnetic proximity effect of a topological insulator in contact with a ferromagnet is reported in thin-film bilayers of 15-nm-thick ${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_3$ on either 15- or 40-nm-thick ${\mathrm{SrRuO}}_3$ on (100) ${\mathrm{SrTiO}}_3$ wafers. ${\mathrm{SrRuO}}_3$ is an itinerant ferromagnet which has long been considered weak, thus any observation of a significant magnetic proximity effect in the present system should help elucidate the mechanism of this magnetism and might be utilized in device applications. Magnetotransport results of the bilayers were compared with those of reference films of 15-nm ${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_3$ and 15- or 40-nm ${\mathrm{SrRuO}}_3$. Comparison of the temperature coefficient of resistance $\left[\right(1/R)\times dR/dT$, which is qualitatively proportional to the magnetization] of the bilayer and reference ferromagnetic film normalized above $T_c$ shows a clear suppression in the bilayer by about 50% just below $T_c$, indicating a weaker proximity magnetization in the bilayer. Resistance hysteresis loops versus field at $1.85\pm 0.05$ K in the bilayer and reference films show a clear magnetic proximity effect, where the peak resistance of the bilayer at the coercive field shifts to lower fields by $\sim 30\%$ compared to a hypothetical bilayer of two resistors connected in parallel with no interaction between the layers. Narrowing of the coercive peaks of the bilayers as compared to those of the reference ferromagnetic films by 25–35% was also observed, which represents another signature of the magnetic proximity effect.

We report transverse-field (TF) muon spin rotation experiments on single crystals of the topological superconductor ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ with nominal concentrations $x=0.15$ and 0.18 ($T_c\sim 3$ K). The TF spectra ($B=10$ mT), measured after cooling to below $T_c$ in field, did not show any additional damping of the muon precession signal due to the flux line lattice within the experimental uncertainty. This puts a lower bound on the magnetic penetration depth $\lambda \ge 2.3\mu \mathrm{m}$. However, when we induce disorder in the vortex lattice by changing the magnetic field below $T_c$, a sizable damping rate is obtained for $T\to 0$. The data provide microscopic evidence for a superconducting volume fraction of $\sim 70\%$ in the $x=0.18$ crystal and thus bulk superconductivity.