We provide topological classification of possible phases with the symmetry of the planar phase of superfluid ${}^3\mathrm{He}$. Compared to the B phase [class DIII in classification of A. Altland and M. R. Zirnbauer, Phys. Rev. B 55, 1142 (1997)], it has an additional symmetry, which modifies the topology. We analyze the topology in terms of explicit mappings from the momentum space and also discuss explicitly topological invariants for the B phase. We further show how the bulk-boundary correspondence for the three-dimensional (3D) B phase can be inferred from that for the 2D planar phase. A general condition is derived for the existence of topologically stable zero modes at the surfaces of 3D superconductors with class-DIII symmetries.

An advanced three-phase phase field approach (PFA) is suggested for a nonequilibrium phase interface that contains an intermediate phase, in particular, a solid-solid interface with a nanometer-sized intermediate melt (IM). A thermodynamic potential in the polar order parameters is developed that satisfies all thermodynamic equilibrium and stability conditions. The special form of the gradient energy allowed us to include the interaction of two solid-melt interfaces via an intermediate melt and obtain a well-posed problem and mesh-independent solutions. It is proved that for stationary 1D solutions to two Ginzburg-Landau equations for three phases, the local energy at each point is equal to the gradient energy. Simulations are performed for $\beta \leftrightarrow \delta$ phase transformations (PTs) via IM in an HMX energetic material. The obtained energy IM width dependence is described by generalized force-balance models for short- and long-range interaction forces between interfaces but not far from the melting temperature. A force-balance model is developed that describes phase field results even 100 K below the melting temperature. The effects of the ratios of width and energies of solid-solid and solid-melt interfaces, temperature, and the parameter characterizing interaction of two solid-melt interfaces, on the structure, width, energy of the IM and interface velocity are determined by finite element method. Depending on parameters, the IM may appear by continuous or discontinuous barrierless disordering or via critical nucleus due to thermal fluctuations. The IM may appear during heating and persist during cooling at temperatures well below than it follows from sharp-interface approach. On the other hand, for some parameters when IM is expected, it does not form, producing an IM-free gap. The developed PFA represents a quite general three-phase model and can be extended to other physical phenomena, such as martensitic PTs, surface-induced premelting and PTs, premelting/disordering at grain boundaries, and developing corresponding interfacial phase diagrams.

We present experimental results demonstrating the phenomenon of acoustic transparency with a significant slowdown of sound propagation realized with a series of paired detuned acoustic resonators (DAR) side-attached to a waveguide. The phenomenon mimics the electromagnetically induced transparency in atomic physics. By arranging four identical DAR pairs along the waveguide with an equal subwavelength separation between adjacent pairs, we show that this arrangement features unique properties of narrow-band transmission and strong dispersion. In particular, we demonstrate side-lobe suppression of more than 20 dB on both sides of the transparency window, and we quantify directly (using a pulse propagation) the acoustic slowdown effect, resulting in the sound group velocity of ∼9.8 m/s (i.e. in the group refractive index of 35). We find very similar values of the group refractive index by using measurements of the phase of the transmitted wave. It is also shown that a direct coupling exists between the DAR in each pair, which cannot be explained by the interference of waves radiated from those resonators. This detrimental coupling becomes noticeable for small values of detuning and also if the cross-sectional area of the neck of the resonators is increased.

An investigation of the vibrational density of states (VDOS) in silver nanocrystals is performed using Raman scattering. A specific sample architecture, setup configuration, and original elaboration process are used in order to take simultaneously advantage of spectrally and spatially localized surface plasmon resonance, optical amplification, and dark-field spectroscopy. Disentangling the contributions of atom vibrations and electron-hole excitations (i.e., the so-called “background” in surface-enhanced Raman scattering) is performed. The extracted VDOS is successfully compared with theoretical ones obtained by atomic scale simulations. The effects of size, strain, and disorder on the VDOS are analyzed; in particular, the strain effect is investigated experimentally using the geometrical phase analysis coupled with high-resolution transmission electron microscopy. This work offers an opportunity to examine thermodynamic properties, like specific heat, at the nanoscale.

We show that germanene nanoroads embedded in completely hydrogenated germanene (germanane) exhibits a quantum spin Hall effect (QSHE). These nanoroads can be obtained experimentally by local hydrogen dissociation from germanane. Using first-principle calculations we predict that germanene nanoroads with zigzag interfaces show dissipationless conducting channels with in-plane and out-of-plane spin textures.

Anisotropic Dirac cones can appear in a number of correlated electron systems, such as cuprate superconductors and deformed graphene. We study the influence of long-range Coulomb interaction on the physical properties of an anisotropic graphene by using the renormalization group method and $1/N$ expansion, where $N$ is the flavor of Dirac fermions. Our explicit calculations reveal that the anisotropic fermion velocities flow monotonously to an isotropic fixed point in the lowest energy limit in clean graphene. We then incorporate three sorts of disorders, including random chemical potential, random gauge potential, and random mass, and show that the interplay of Coulomb interaction and disorders can lead to rich and unusual behaviors. In the presence of strong Coulomb interaction and a random chemical potential, the fermion velocities are driven to vanish at low energies and the system turns out to be an exotic anisotropic insulator. In the presence of Coulomb interaction and other two types of disorders, the system flows to an isotropic low-energy fixed point more rapidly than the clean case, and exhibits non-Fermi liquid behaviors. We also investigate the nonperturbative effects of Coulomb interaction, focusing on how the dynamical gap is affected by the velocity anisotropy. It is found that the dynamical gap is enhanced (suppressed) as the fermion velocities decrease (increase), but is suppressed as the velocity anisotropy increases.

We determine the wavelength dependence of the two-photon absorption cross section in rubrene single crystals both by direct measurement of nonlinear transmission and from the two-photon excitation spectrum of the photoluminescence. The peak two-photon absorption coefficient for $b$-polarized light was found to be $(4.6\pm 1)\times {10}^{-11}$ m/W at a wavelength of $850\pm 10$ nm. It is 2.3 times larger for $c$-polarized light. The lowest energy two-photon excitation peak corresponds to an excited state energy of $2.92\pm 0.04$ eV and it is followed by a vibronic progression of higher energy peaks separated by $\sim$$0.14$ eV.

The recently discovered iron-based superconductors have chainlike As layers. These layers generate an additional three-dimensional hole pocket and conelike electron pockets. The former is attributed to the Ca $d$ and As1 $p_z$ orbitals and the latter are attributed to the anisotropic Dirac cone, contributed by As1 $p_x$ and $p_y$ orbitals. We find that large gaps on these pockets open in the collinear antiferromagnetic ground state of ${\text{CaFeAs}}_2$, suggesting that the chainlike As layers are strongly coupled to FeAs layers. Moreover, due to the low symmetry crystal induced by the As layers, the bands attributed to FeAs layers in $k_y=\pi$ plane are twofold degenerate but in $k_x=\pi$ plane are lifted. This degeneracy is protected by a hidden symmetry $\widehat{{\rm Y}}=\widehat{T}{\widehat{R}}_y$. Ignoring the electron cones, the materials can be well described by a six-band model, including five Fe $d$ and As1 $p_z$ orbitals. We suggest that these features may help us to identify the sign change and pairing symmetry in iron-based superconductors.

${\text{MoS}}_2$, a member of transition metal dichalcogenides (TMDs), has recently emerged as an interesting two-dimensional material due to its unique mechanical, thermal, electronic and optical properties. Unlike graphene which possesses massless Dirac fermions with ultrahigh electron mobility, monolayer ${\text{MoS}}_2$ is a direct band gap semiconductor. An interesting question arises: Can monolayer ${\text{MoS}}_2$ also possess massless Dirac fermions with ultrahigh electron mobility? Here, using first-principles calculations, we show that a monolayer ${\text{MoS}}_2$ allotrope, which consists of repeated square-octagon rings (abbreviated as so-${\text{MoS}}_2$ to distinguish it from the normal hexagonal lattice, h-${\text{MoS}}_2$) possesses both massless Dirac fermions and heavy fermions. Distinct from the $p$-orbital Dirac fermions of graphene, the Dirac fermions of so-${\text{MoS}}_2$ are $d$ electrons and possess a Fermi velocity comparable to that of graphene. The Dirac cone structure in so-${\text{MoS}}_2$ demonstrated here greatly enriches our understanding on the physical properties of TMDs and opens up possibilities for developing high-performance electronic or spintronic devices.

The influence of a gate voltage on magnetic anisotropy is investigated in a thin Fe film epitaxially grown on a Ag(1,1,10) substrate and covered by MgO. Oscillations in step-induced magnetic anisotropy due to quantum well states (QWS) confined in the Fe film are observed and shown to persist up to room temperature at low Fe thicknesses. By systematically examining the voltage and thickness dependence of the magnetic hysteresis loop characteristics, we identify two distinct effects by which an applied voltage modifies the magnetic anisotropy. The first effect is due to voltage-induced changes to interfacial perpendicular magnetic anisotropy which, due to the vicinal geometry, leads to changes in the effective in-plane uniaxial magnetic anisotropy. A second effect is observed at lower film thicknesses and shows nonmonotonic voltage-induced effects on magnetic anisotropy. This nonmonotonic behavior coincides with the onset of significant QWS-induced effects on magnetic anisotropy and suggests a link between QWS- and voltage-induced anisotropy changes.

The spin-1/2 alternating Heisenberg chain system ${\mathrm{Na}}_3{\mathrm{Cu}}_2{\mathrm{SbO}}_6$ features two relevant exchange couplings: $J_{1a}$ within the structural Cu${}_2$O${}_6$ dimers and $J_{1b}$ between the dimers. Motivated by the controversially discussed nature of $J_{1a}$, we perform extensive density-functional-theory (DFT) calculations, including DFT + $U$ and hybrid functionals. Fits to the experimental magnetic susceptibility using high-temperature series expansions and quantum Monte Carlo simulations yield the optimal parameters $J_{1a}=-217$ K and $J_{1b}=174$ K with the alternation ratio $\alpha =J_{1a}/J_{1b}\simeq -1.25$. For the closely related system Na${}_2$Cu${}_2$TeO${}_6$, DFT yields substantially enhanced $J_{1b}$, but weaker $J_{1a}$. The comparative analysis renders the buckling of the chains as the key parameter altering the magnetic coupling regime. Numerical simulation of the dispersion relations of the alternating-chain model clarify why both antiferromagnetic and ferromagnetic $J_{1a}$ can reproduce the experimental magnetic susceptibility data.

We present a comprehensive study of magnetic properties and high wave-vector magnon excitations in epitaxial Fe(001) films grown on Ir(001) substrate. The magnetic properties are investigated by magneto-optical Kerr effect for various thicknesses of Fe film from 4 up to 27 monolayers. The magnon dispersion relation is obtained by means of spin-polarized electron energy loss spectroscopy. By comparing the experimental and theoretical results, we comment on the effect of the strain induced tetragonal distortion in the Fe film and the electronic hybridizations between the film and the substrate on the exchange interaction and magnon dispersion relation. The thickness-dependent measurements show that the magnon dispersion relation does not change significantly while changing the film thickness in the regime, where the film is uniformly strained. In addition, the measurements are also performed on thicker Fe films, where the films are relaxed into a bulklike structure. The magnon energies close to the zone boundary differ from the ones measured on thick Fe(110) films on W(110). However, the magnon dispersion relation measured for both systems can be well described within the Heisenberg model by using the same effective exchange parameters.

We describe a simple method to determine, from ab initio calculations, the complete orientation dependence of interfacial free energies in solid-state crystalline systems. We illustrate the method with an application to precipitates in the Al-Ti alloy system. The method combines the cluster expansion formalism in its most general form (to model the system's energetics) with the inversion of the well known Wulff construction (to recover interfacial energies from equilibrium precipitate shapes). Although the inverse Wulff construction only provides the relative magnitude of the various interfacial free energies, absolute free energies can be recovered from a calculation of a single, conveniently chosen, planar interface. The method is able to account for essentially all sources of entropy (arising from phonons, bulk point defects, as well as interface roughness) and is thus able to transparently handle both atomically smooth and rough interfaces. The approach expresses the resulting orientation dependence of the interfacial properties using symmetry adapted bases for general orientation dependent quantities. As a byproduct, this paper thus provides a simple and general method to generate such basis functions, which prove useful in a variety of other applications, for instance to represent the anisotropy of the so-called constituent strain elastic energy.

Via a two-step deposition and post-annealing procedure, K-doped WO${}_3$ thin films with reproducible transport properties are obtained. We observe a larger critical field H${}_{c2}$ along the $c$ axis, consistent with the picture of the Fermi surface containing one-dimensional bands along this direction. Reducing the film thickness results in a superconductor to insulator transition. Scanning electron microscopy (SEM) images show that KWO${}_3$ crystallites become less connected as the deposition time is reduced, providing a microscopic explanation for the transport behavior. In the superconducting films, a resistive anomaly is observed similar to bulk crystals, with a characteristic temperature that shifts lower with decreasing film thickness. The competing electronic effects manifest as a suppression of the density of states at the Fermi level, observed using point contact tunneling spectroscopy, demonstrating that disorder-induced increased Coulomb interactions are present. Using the theory of Belitz for the reduction of $T_c$ due to disorder, we can infer that the film with the highest observed $T_c$ has a relatively large disorder dependent electron-phonon interaction parameter $\sim$1.2. Understanding microscopically why certain films display higher $T_c$ will aid in the search for the trace high-$T_c$ superconducting anomalies observed in lightly surface doped bronzes.

Recently, the terahertz (THz) photon-assisted tunneling (PAT) through a two-level InAs quantum dot (QD) has been successfully realized in experiment [Phys. Rev. Lett. 109, 077401 (2012)]. The Coulomb interaction in this device is comparable with the energy difference between the two energy levels. We theoretically show that the main peaks of the experiment can be well derived by our model analysis. Furthermore, we find additional peaks, which were not addressed in the InAs QD experiment and may be further identified in experiments. In particular, we show that, to observe the interesting photon-induced excited state resonance in InAs QD, the Coulomb interaction should be larger than the THz photon frequency.

The study of the interaction potential between static charges within Monte Carlo simulation of graphene is carried out. The numerical simulations are performed in the effective lattice field theory with noncompact ($3+1$)-dimensional Abelian lattice gauge fields and ($2+1$)-dimensional staggered lattice fermions. It is shown that for all considered temperatures the interaction can be well described by the Debye screened potential created by two-dimensional electron-hole excitations. At low temperatures Debye mass $m_D$ plays a role of order parameter of the insulator-semimetal phase transition. In the semimetal phase at high temperature graphene effective field theory reveals the properties of weakly interacting two-dimensional plasma of fermion excitations.

We present a combined experimental and theoretical study on the local magnetism of the Co ions in the spin-chain compound ${\text{CoV}}_2$${\text{O}}_6$, which crystallizes in two different allotropic phases, $\alpha$- and $\gamma$-${\text{CoV}}_2$${\text{O}}_6$. Using x-ray magnetic circular dichroism, we have found an exceptionally high and a moderate orbital contribution to the magnetism in $\alpha$-${\text{CoV}}_2$${\text{O}}_6$ and $\gamma$-${\text{CoV}}_2$${\text{O}}_6$, respectively. Full-multiplet calculations indicate that the differences in the magnetic behavior of $\alpha$- and $\gamma$-${\text{CoV}}_2$${\text{O}}_6$ phases originate from different local distortions of the ${\text{CoO}}_6$ octahedra. In particular, the strong compression of the ${\text{CoO}}_6$ octahedra together with the unusually small octahedral crystal field splitting in $\alpha$-${\text{CoV}}_2$${\text{O}}_6$ lead to a strong mixture of $t_{2g}$ and $e_g$ orbitals which, via the local atomic Coulomb and exchange interactions, results in an exceptionally large orbital moment.

We have carried out detailed bulk and local probe studies on the hexagonal oxides Ba${}_3$M${\mathrm{Ir}}_2$$O_9$ (M=Sc,Y), where Ir is expected to have a fractional oxidation state of $+4.5$. In the structure, Ir-Ir dimers are arranged in an edge-shared triangular network parallel to the $ab$ plane. Whereas only weak anomalies are evident in the susceptibility data, clearer anomalies are present in the heat capacity data—around 10 K for Ba${}_3$Sc${\mathrm{Ir}}_2$$O_9$ and at 4 K for Ba${}_3$Y${\mathrm{Ir}}_2$$O_9$. Our ${}^{45}$Sc nuclear magnetic resonance (NMR) line shape (first-order quadrupole split) is symmetric at room temperature but becomes progressively asymmetric with decreasing temperatures indicating the presence of developing inequivalent Sc environments. This is suggestive of distortions in the structure which could arise from progressive tilt/rotation of the IrO${}_6$ octahedra with a decrease in temperature $T$. The ${}^{45}$Sc NMR spectral weight shifts near the reference frequency with decreasing $T$ indicating the development of magnetic singlet regions. Around 10 K, a significant change in the spectrum takes place with a large intensity appearing near the reference frequency but with the spectrum remaining multipeak. It appears from our ${}^{45}$Sc NMR data that in Ba${}_3$ScIr${}_2$$O_9$ significant disorder is still present below 10 K. In the case of Ba${}_3$YIr${}_2$$O_9$, the ${}^{89}$Y NMR spectral lines are asymmetric at high temperatures but become nearly symmetric (single magnetic environment) below $T\sim 70$ K. Our ${}^{89}$Y spectra and $T_1$ measurements confirm the onset of long-range ordering from a bulk of the sample at 4 K in this compound. Our results suggest that Ba${}_3$YIr${}_2$$O_9$ might be structurally distorted at room temperature (via, for example, tilt/rotations of the IrO${}_6$ octahedra) but becomes progressively a regular triangular lattice with decreasing $T$. The effective magnetic moments and magnetic entropy changes are strongly reduced in Ba${}_3$YIr${}_2$$O_9$ as compared to those expected for a $S=1/2$ system. Similar effects have been found in other iridates which naturally have strong spin-orbit coupling.