Metamaterials (MTMs) in a metal/insulator/metal (MIM) configuration have drawn much attention recently, but the resonances in such systems are still not fully understood. Here, we employ a rigorous mode expansion method to analytically study the resonance properties of a model MIM MTM where the top metallic layer consists of an array of metallic stripes. Our analyses, supported by full-wave simulations and microwave experiments, provide a unified platform to understand the resonances in such systems, in which two previously established models are found valid only at certain extreme conditions. In particular, the resonance in such a system undergoes a transition from a vertical Fabry-Pérot type to a transverse type as the spacer thickness shrinks, and the resonance frequency saturates at a particular value in the thin-spacer limit. Finally, we derive a set of analytical formulas to describe how the essential properties (i.e., resonance frequency and quality factor) of the resonance depend on the structural details of the system and verify these analytical relationships by full-wave simulations in MIM systems with complex microstructures.

We present a study of metastability regions in the in-plane magnetic field versus temperature phase diagram of graphene and intercalated graphite superconductors. Due to the vanishing density of states, undoped graphene requires a finite BCS interaction $V_c$ to become superconducting (any finite doping drives this critical value to zero). Above $V_c$, superconducting graphene under in-plane magnetic field displays the conventional low temperature first-order transition (FOT) to the normal phase, but the width of the associated metastability region (normalized to the zero-temperature critical field) vanishes when doping goes to zero and the interaction approaches $V_c$. In the case of intercalated graphite superconductors, modeled as two-dimensional two-band superconductors (a graphene-like band and a metallic interlayer band), a critical graphene intraband interaction is required for the appearance of a second metastability region in the superconducting region of the phase diagram. The width of this metastability region also goes to zero as the graphene intraband interaction approaches, from above, its critical value and the metastability region vanishes at the zero-temperature supercooling field associated with the metallic interlayer band. Slightly above this critical value, the low-temperature FOT line bifurcates at an intermediate temperature into a FOT line and a second-order transition line.

In the framework of the hydrodynamic model describing the response of electrons in a metal, we show that arrays of very narrow and shallow metallic slits have an optical response that is influenced by the spatial dispersion in metals arising from the repulsive interaction between electrons. As a simple Fabry-Perot model is not accurate enough to describe the structure's behavior, we propose considering the slits as generalized cavities with two modes, one being propagative and the other evanescent. This very general model allows us to conclude that the impact of spatial dispersion on the propagative mode is the key factor explaining why the whole structure is sensitive to spatial dispersion. As the fabrication of such structures with relatively large gaps compared to previous experiments is within our reach, this work paves the way for future much needed experiments on nonlocality.

We investigate the valley depolarization dynamics and valley Hall effect of exciton due to the electron-hole exchange interaction in mono- and bilayer ${\mathrm{MoS}}_2$ by solving the kinetic spin Bloch equations. The effect of the exciton energy spectra by the electron-hole exchange interaction is explicitly considered. For the valley depolarization dynamics, in the monolayer ${\mathrm{MoS}}_2$, it is found that in the strong scattering regime, the conventional motional narrowing picture in the conventional strong scattering regime is no longer valid, and a novel valley depolarization channel is opened. For the valley Hall effect of exciton, in both the mono- and bilayer ${\mathrm{MoS}}_2$, with the exciton equally pumped in the K and ${\mathrm{K}}^{\prime }$ valleys, the system can evolve into the equilibrium state where the valley polarization is parallel to the effective magnetic field due to the exchange interaction. With the drift of this equilibrium state by applied uniaxial strain, the exchange interaction can induce the momentum-dependent valley/photoluminesence polarization, which leads to the valley/photoluminesence Hall current. Specifically, the disorder strength dependence of the valley Hall conductivity is revealed. In the strong scattering regime, the valley Hall conductivity decreases with the increase of the disorder strength; whereas in the weak scattering regime, it saturates to a constant, which can be much larger than the one in Fermi system due to the absence of the Pauli blocking.

We investigate the effects of long-range potential fluctuations and electron-electron interactions on the electronic and magnetic properties of graphene nanoribbons with zigzag edges using an extended mean-field Hubbard model. We show that electron-electron interactions make the edge states robust against potential fluctuations. When the disorder is strong enough, the presence of electron-hole puddles induces a magnetic phase transition from antiferromagnetically coupled edge states to ferromagnetic coupling, in agreement with recent experimental results.

Unconventional thermal effects across the first-order antiferromagnetic (AFM)-ferromagnetic (FM) transition in an intermetallic alloy are reported. They show instances of warming when heat is extracted from the sample, and cooling when heat is provided to the sample across the AFM-FM transition in Ta-doped ${\text{HfFe}}_2$, thus providing indisputable evidence of metastable supercooled AFM and superheated FM states, respectively. Such thermal effects are observed in a magnetic solid prepared from commercially available materials. The transition proceeds in multiple steps which is interpreted in the framework of quenched disorder broadening of the AFM-FM transition and classical nucleation theory. Measurements in the presence of a magnetic field conform to the above framework.

We consider orthoferrite and orthochromite perovskites with the formula $\mathit{RM}{\mathrm{O}}_3$, where $R$ is a lanthanide and $M$ is Fe or Cr. We identify an atomistic interaction that couples the magnetic moments of the $R$ and $M$ cations with the oxygen octahedral rotations characterizing these crystals. We show that this interaction results in an effective magnetic field acting on the $R$ atoms, which in turn explains several intriguing features of these compounds, such as the existence (or absence) of a net magnetization associated with the $R$ sublattice and its crystallographic direction. Further, considered together with the couplings described by L. Bellaiche et al. [J. Phys.: Condens. Matter 24, 312201 (2012)], this interaction explains why some $\mathit{RM}{\mathrm{O}}_3$ compounds display a compensation temperature at which the magnetization cancels and changes sign, while others do not.

In high-superconducting transition temperature ($T_{\mathrm{c}}$) iron-based superconductors, interband sign reversal ($s_{\pm }$) and sign preserving ($s_{++}$) $s$-wave superconducting states have been primarily discussed as the plausible superconducting mechanism. We study Co impurity scattering effects on the superconductivity in order to achieve an important clue on the pairing mechanism using single-crystal ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Se}$ and depict a phase diagram of a FeSe system. Both superconductivity and structural transition/orbital order are suppressed by the Co replacement on the Fe sites and disappear above $x$ = 0.036. These correlated suppressions represent a common background physics behind these physical phenomena in the multiband Fermi surfaces of FeSe. By comparing experimental data and theories so far proposed, the suppression of $T_{\mathrm{c}}$ against the residual resistivity is shown to be much weaker than that predicted in the case of general sign reversal and full gap $s_{\pm }$ models. The origin of the superconducting paring in FeSe is discussed in terms of its multiband electronic structure.

This paper addresses the electronic properties of $\mathrm{N}{\mathrm{d}}_3\mathrm{R}{\mathrm{u}}_4\mathrm{A}{\mathrm{l}}_{12}$ (hexagonal crystal structure) with focus on its magnetic anisotropy that allows a comparison between single-ion and two-ion mechanisms when comparing to ${\mathrm{U}}_3\mathrm{R}{\mathrm{u}}_4\mathrm{A}{\mathrm{l}}_{12}$. We performed magnetization measurements on a single crystal in static and pulsed magnetic fields as well as neutron-diffraction experiments. $\mathrm{N}{\mathrm{d}}_3\mathrm{R}{\mathrm{u}}_4\mathrm{A}{\mathrm{l}}_{12}$ is a strongly anisotropic uniaxial ferromagnet with a Curie temperature of 39 K. The magnetic moments are aligned collinearly along the [001] axis. The magnetic structure of $\mathrm{N}{\mathrm{d}}_3\mathrm{R}{\mathrm{u}}_4\mathrm{A}{\mathrm{l}}_{12}$ has orthorhombic symmetry for which the crystallographic Nd site is split into two magnetically inequivalent positions, Nd1 and Nd2. The Nd1 and Nd2 atoms exhibit reduced magnetic moments, 0.95 and $2.66{\mu }_{\mathrm{B}}$, as compared to the free $\mathrm{N}{\mathrm{d}}^{3+}$-ion value ($3.28{\mu }_{\mathrm{B}}$). We discuss this finding in terms of crystal-field effects and competing exchange and anisotropy interactions. Since the single-ion mechanism in $\mathrm{N}{\mathrm{d}}_3\mathrm{R}{\mathrm{u}}_4\mathrm{A}{\mathrm{l}}_{12}$ leads to uniaxial anisotropy and the two-ion mechanism of the actinide analog ${\mathrm{U}}_3\mathrm{R}{\mathrm{u}}_4\mathrm{A}{\mathrm{l}}_{12}$ is known to lead to planar anisotropy, this paper demonstrates the decisive influence of these different mechanisms on the magnetic anisotropy.

We consider a superconducting nanowire proximity coupled to a superconductor/ferromagnet/superconductor (S/F/S) junction, where the magnetization penetrates into a superconducting segment in a nanowire decaying as $\sim exp[-\frac{\mid n\mid }{\xi }]$, where $n$ is the site index and the $\xi$ is the decay length. We tune chemical potential and spin-orbit coupling so that the topological superconducting regime hosting the Majorana fermion is realized for long $\xi$. We find that when $\xi$ becomes shorter, zero energy state at the interface between a superconductor and a ferromagnet splits into two states at nonzero energy. Accordingly, the behavior of the Josephson current is drastically changed due to this “zero mode-nonzero mode crossover.” By tuning the model parameters, we find an almost second-harmonic current-phase relation $sin2\phi$, where $\phi$ is the phase difference of the junction. Based on the analysis of Andreev bound state (ABS), we clarify that the current-phase relation is determined by coupling of the states within the energy gap. We find that the emergence of crossing points of ABS is a key ingredient to generate $sin2\phi$ dependence in the current-phase relation. We further study both the energy and $\phi$ dependence of pair amplitudes in the ferromagnetic region. For large $\xi$, an odd-frequency spin-triplet $s\text{-wave}$ component is dominant. The magnitude of the odd-frequency pair amplitude is enhanced at the energy level of ABS.

The binary chalcogenides ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ are the most widely studied topological insulators. Although the quantum anomalous Hall effect has recently been observed in magnetically doped ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, this compound has been studied to a much lesser extent. Here, by using energy-resolved quasiparticle interference mapping, we investigate the scattering events of pristine ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ surfaces. We find that, in addition to the Dirac fermions, another strongly spin-polarized surface resonance emerges at higher energies in the unoccupied electronic states. Although the two surface states are of different origin, i.e., topologically protected and trivial, respectively, both show strongly directional scattering properties and absence of backscattering. A comparison with ab initio calculations demonstrates that this is a direct consequence of their spin-momentum–locked spin texture which is found to exhibit an opposite rotational sense for the trivial state and the Dirac state.

Manganites are known to often show colossal electroresistance (CER) in addition to colossal magnetoresistance. The ${\left(\mathrm{L}{\mathrm{a}}_{1-y}\mathrm{P}{\mathrm{r}}_y\right)}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ (LPCMO) system has a peculiar CER behavior in that little change of magnetization occurs. We use a magnetic force microscope to uncover the CER mechanism in the LPCMO system. In contrast to the previous belief that current reshapes the ferromagnetic metallic (FMM) domains, we show that the shape of the FMM domains remain virtually unchanged after passing electric current. Instead, it is the appearance of a tiny fraction of FMM “bridges” that is responsible for the CER behavior.

In this paper, we perform a detailed analysis of the exact holographic mapping first introduced in arXiv:1309.6282, which was proposed as an explicit example of holographic duality between quantum many-body systems and gravitational theories. We obtain analytic results for free fermion systems that not only confirm previous numerical results, but also elucidate the exact relationships between the various physical properties of the bulk and boundary systems. These analytic results allow us to study the asymptotic properties that are difficult to probe numerically, such as the near-horizon regime of the black-hole geometry. We shall also explore a few interesting but hitherto unexplored bulk geometries, such as that corresponding to a boundary critical fermion with a nontrivial dynamical critical exponent. Our analytic framework also allows us to study the holographic mapping of some of these boundary theories in dimensions 2+1 or higher.

The quantum well states of a film can be used to sample the electronic structure of the parent bulk material and determine its band parameters. We highlight the benefits of two-dimensional film band mapping, with respect to complex bulk analysis, in an angle-resolved photoemission spectroscopy study of the $5d$ states of Au(111). Discrete $5d$-derived quantum well states of various orbital characters form in Au(111) films and span the width of the corresponding bulk bands. For sufficiently thick films, the dispersion of these states samples the bulk band edges, as confirmed by first-principles calculations, thus providing the positions of the critical points of bulk Au in agreement with previously determined values. In turn, this analysis identifies several $d$-like surface states and resonances with large spin splittings that originate from the strong spin-orbit coupling of the Au $5d$ atomic levels.

We study coherent transport in a system of a periodic linear chain of quantum dots placed between two parallel quantum wires. We show that resonant-tunneling conductance between the wires exhibits a Rabi splitting of the resonance peak as a function of Fermi energy in the wires, indicating the emergence of strong coupling between the system constituents. The underlying mechanism of the strong coupling regime is conservation of the quasimomentum in a periodic system that leads to transition resonances between electron states in a quantum dot chain and quantum wires. A perpendicular magnetic field, by breaking the system's left-right symmetry, gives rise to a fine structure of the conductance lineshape.

The surface spin states for bismuth thin films are investigated using an $s{p}^3$ tight-binding model. The model explains most experimental observations using angle-resolved photoemission spectroscopy, including the Fermi surface, the band structure with Rashba spin splitting, and the quantum confinement in the energy band gap of the surface states. A large out-of-plane spin component also appears. The surface states penetrate inside the film to within approximately a few bilayers near the Brillouin-zone center, whereas they reach the center of the film near the Brillouin-zone boundary.

A relativistic spinor with spin 3/2 is historically called a Rarita-Schwinger spinor. The right- and left-handed chiral degrees of freedom for the massless Rarita-Schwinger spinor are independent and are thought of as the left and right Weyl fermions with helicity $\pm 3/2$. We study three orbital spin-1/2 Weyl semimetals in the strong spin-orbital coupling limit with time reversal symmetry breaking. We find that in this limit the systems can be a $J_{\mathrm{eff}}=1/2$ Weyl semimetal or a $J_{\mathrm{eff}}=3/2$ semimetal, depending on the Fermi level position. The latter near Weyl points includes degrees of freedom of both Rarita-Schwinger-Weyl and Weyl. A nonlocal potential separates the Weyl and Rarita-Schwinger-Weyl degrees of freedom, and a relativistic Rarita-Schwinger-Weyl semimetal emerges. This recipe can be generalized to a mulit-Weyl semimetal and Weyl fermions with pairing interaction to obtain high monopole charges. Similarly, a spatial-inversion-breaking Raita-Schwinger-Weyl semimetal may also emerge.

Using many-body configuration interaction techniques, we show that Wigner crystallization occurs at the zigzag edges of graphene at surprisingly high electronic densities up to $0.8{\text{nm}}^{-1}$. In contrast with one-dimensional electron gas, the flatband structure of the edge states makes the system interaction dominated, facilitating electronic localization. The resulting Wigner crystal manifests itself in pair-correlation functions, and evolves smoothly as the edge electron density is lowered. We also show that the crystallization affects the magnetization of the edges. While the edges are fully polarized when the system is charge neutral (i.e., high density), above the critical density, the spin-spin correlations between neighboring electrons go through a smooth transition from antiferromagnetic to magnetic coupling as the electronic density is lowered.

We report on a theoretical study of the hole states in II-IV quantum dots of spherical and ellipsoidal shapes, described by smooth potential confinement profiles that can be modeled by Gaussian functions in all three dimensions. The universal dependencies of the hole energy, $g$ factor, and localization length on the quantum dot barrier height, as well as the ratio of effective masses of the light and heavy holes are presented for the spherical quantum dots. The splitting of the fourfold degenerate ground state into two doublets is derived for anisotropic (oblate or prolate) quantum dots. Variational calculations are combined with numerical ones in the framework of the Luttinger Hamiltonian. Constructed trial functions are optimized by comparison with the numerical results. The effective hole $g$ factor is found to be independent of the quantum dot size and barrier height and is approximated by a simple universal expression depending only on the effective mass parameters. The results can be used for interpreting and analyzing experimental spectra measured in various structures with quantum dots of different semiconductor materials.

The impact of lattice type, period, porosity, and thickness of two-dimensional silicon phononic crystals on the reduction of thermal conductance by coherent modification of phonon dispersion is investigated using the theory of elasticity and the finite element method. Increases in the period and porosity of the phononic crystal affect the group velocity and phonon density of states and, as a consequence, reduce the in-plane thermal conductance of the structure as compared to the unpatterned membrane. This reduction does not depend significantly on the lattice type and thickness of phononic crystals. Moreover, the reduction is strongly temperature dependent and strengthens as the temperature is increased.