Weyl semimetals are characterized by an even number of bulk Weyl points and surface Fermi arcs connecting their surface projections. It has been shown that when a magnetic field is applied perpendicular to the surface, an unusual kind of quantum oscillations appear. Semiclassically, these oscillations are associated with electron motion in momentum space on the surface Fermi arcs as well as tunneling between the top and bottom surfaces through the bulk chiral Landau level. In this work, we develop an effective surface theory for Weyl semimetals by integrating out the bulk, leaving only the top and bottom infinite surfaces. We use our model to analyze the Landau level spectrum and compare the results with a full three-dimensional calculation as well as the semiclassical theory. We find that the surface theory is a good approximation for the three-dimensional model and agrees with the semiclassical analysis whenever the Weyl points are far from each other. We analytically analyze the case of overlapping Weyl points with short Fermi arcs and find an energy gap which appears as a phase shift in the quantum oscillations, in agreement with our numerics. We also study the case of a type-II Weyl semimetal and show that the absence of the bulk chiral Landau level prevents the formation of surface Landau levels.

We calculate momentum-dependent self-energies due to self-consistent spin and charge-density fluctuations within a single-band Hubbard model for hole-doped cuprates. We find that the dynamical density instability (mainly paramagnonlike spin fluctuations) arising from the van Hove singularity at the antinodal point produces strongly anisotropic mass renormalization, increasing from the nodal direction to the antinodal one. This gives rise to a coherent Fermi arc pattern in which although the self-energy dressed Green's function has poles at all Fermi momenta, the corresponding spectral weight decreases from the nodal to the antinodal direction. We study the feedback effects of the anisotropic self-energy and Fermi arc on the charge-density wave (CDW) instability by direct computations of charge susceptibility and the quasiparticle interference pattern. We find that that due to the loss of spectral weight at the antinodal points, the CDW nesting weakens, and the corresponding hot spot shifts from the antinodal region to the tip of the Fermi arc below the magnetic Brillouin zone. The results are in good agreement with experiments, reproducing the observed slow doping dependence of the CDW wave vector. Our investigation therefore provides a mechanism for the Fermi arc in cuprates, with various testable predictions such as a sharp, first-order-like phase transition of the CDW order on the higher-doping side.

Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion. In this work, we show that a ZnO film consisting of well-packed micrometer-sized ZnO crystals exhibits purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission, even under optical excitation more than 25 times above the excitonic lasing threshold. The lasing mechanism is shifted from the exciton-exciton scattering to the exciton-electron scattering with increasing temperature from 3 to 150 K. The exciton-electron scattering process continues to exist with further increasing temperature from 150 to 300 K. Thus, we present distinct experimental evidence that the room-temperature excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. We also argue that the long carrier diffusion length and the low optical loss nature of the micrometer-sized ZnO crystals, as compared to those of ZnO nanostructures, plays a key role in showing room-temperature excitonic lasing.

The radiative properties of most structures are intimately connected to the way in which their constituents are ordered on the nanoscale. We have proposed a new representation for radiative heat transfer formalism in many-body systems. In this representation, we explain why collective effects depend on the morphology of structures, and how the arrangement of nanoparticles and their material affects the thermal properties in many-body systems. We investigated the radiative heat transfer problem in fractal (i.e., scale invariant) structures. In order to show the effect of the structure morphology on the collective properties, the radiative heat transfer and radiative cooling are studied and the results are compared for fractal and nonfractal structures. It is shown that fractal arranged nanoparticles display complex radiative behavior related to their scaling properties. We showed that, in contrast to nonfractal structures, heat flux in fractals is not of large-range character. By using the fractal dimension as a means to describe the structure morphology, we present a universal scaling behavior that quantitatively links the structure radiative cooling to the structure gyration radius.

The origin of the electronic nematicity and its remarkable material dependence are famous longstanding unsolved issues in Fe-based superconductors. To attack these issues, we focus on the in-plane anisotropy of the resistivity: In the nematic state in FeSe, the relation ${\rho }_x>{\rho }_y$ holds, where ${\rho }_{x\left(y\right)}$ is the resistivity along the longer (shorter) Fe-Fe axis. In contrast, the opposite anisotropy ${\rho }_x<{\rho }_y$ is realized in other undoped Fe-based superconductors. Such nontrivial material dependence is naturally explained in terms of the strongly orbital-dependent inelastic quasiparticle scattering realized in the orbital-ordered state. The opposite anisotropy between FeSe $\left({\rho }_x>{\rho }_y\right)$ and other undoped compounds $\left({\rho }_x<{\rho }_y\right)$ reflects the difference in the number of hole pockets. We also explain the large in-plane anisotropy of the thermoelectric power in the nematic state.

The existence or absence of nonanalytic cusps in the Loschmidt-echo return rate is traditionally employed to distinguish between a regular dynamical phase (regular cusps) and a trivial phase (no cusps) in quantum spin chains after a global quench. However, numerical evidence in a recent study (J. C. Halimeh and V. Zauner-Stauber, arXiv:1610.02019) suggests that instead of the trivial phase, a distinct anomalous dynamical phase characterized by a novel type of nonanalytic cusps occurs in the one-dimensional transverse-field Ising model when interactions are sufficiently long range. Using an analytic semiclassical approach and exact diagonalization, we show that this anomalous phase also arises in the fully connected case of infinite-range interactions, and we discuss its defining signature. Our results show that the transition from the regular to the anomalous dynamical phase coincides with ${\mathbb{Z}}_2$-symmetry breaking in the infinite-time limit, thereby showing a connection between two different concepts of dynamical criticality. Our work further expands the dynamical phase diagram of long-range interacting quantum spin chains, and can be tested experimentally in ion-trap setups and ultracold atoms in optical cavities, where interactions are inherently long range.

One of the most important signatures of Weyl materials is the chiral anomaly. So far, there has been little study of chiral anomaly of Weyl magnons, featured by nontrivial band crossings at paired Weyl nodes of opposite chirality, while the anomaly for its electronic counterpart can routinely be observed in experiments. Here a road map for realizing and detecting the chiral anomaly of Weyl magnons in stacked honeycomb ferromagnets that are shown to be a Weyl magnetic material is provided. Using the Aharonov-Casher effect that is about the interaction between magnetic moments and electric fields, the magnon motion in honeycomb ferromagnetic layers can be quantized into magnonic Landau levels by a proper inhomogeneous electric field. The zeroth magnonic Landau level is chiral so that, under a magnetic field gradient, Weyl magnons propagate unidirectionally from one Weyl node to the other and reverse their chiralities, resulting in the magnonic chiral anomaly. A net magnon current carrying spin and heat through the zeroth magnonic Landau level depends linearly on the magnetic field gradient and the electric field gradient in the linear transport region. The linear spin and heat conductance can serve as the signatures of magnonic chiral anomaly. Furthermore, an experimental realization is proposed.

We use a perturbative approach to study the effects of interfacial spin-orbit coupling in magnetic multilayers by treating the two-dimensional Rashba model in a fully three-dimensional description of electron transport near an interface. This formalism provides a compact analytic expression for current-induced spin-orbit torques in terms of unperturbed scattering coefficients, allowing computation of spin-orbit torques for various contexts, by simply substituting scattering coefficients into the formulas. It applies to calculations of spin-orbit torques for magnetic bilayers with bulk magnetism, those with interface magnetism, a normal-metal/ferromagnetic insulator junction, and a topological insulator/ferromagnet junction. It predicts a dampinglike component of spin-orbit torque that is distinct from any intrinsic contribution or those that arise from particular spin relaxation mechanisms. We discuss the effects of proximity-induced magnetism and insertion of an additional layer and provide formulas for in-plane current, which is induced by a perpendicular bias, anisotropic magnetoresistance, and spin memory loss in the same formalism.

Realistic representation of finite temperature dielectric functions of noble metals is crucial in describing the optical properties of advancing applications in plasmonics and optical metamaterials. However, the atomistic origins of the temperature dependence of noble metals’ dielectric functions still lack full explanation. In this paper, we implement electronic structure calculations as well as ellipsometry experiments to study the finite temperature dielectric functions of noble metals Au, Ag, and Cu. Theoretically, the intraband dielectric function is described by the Drude model, of which the important quantity electron lifetime is obtained by considering the electron-phonon, electron-electron, and electron-surface scattering mechanism. The electron-phonon coupling is key to determining the temperature dependence of electron lifetime and intraband dielectric function. For the interband dielectric function, it arises from the electronic interband transition. Due to the limitation of incorporating electron-phonon coupling into the interband transition scheme, the temperature dependence of the interband dielectric function is mainly determined by the thermal expansion effect. Experimentally, variable angle spectroscopic ellipsometry measures the dielectric functions of Au and Ag over the temperature range of 300–700 K and spectral range of 2–20 µm. Those experimental measurements are consistent with theoretical results and thus verify the theoretical models for the finite temperature dielectric function.

We perform projective quantum Monte Carlo simulations of zigzag graphene nanoribbons within a realistic model with long-range Coulomb interactions. Increasing the relative strength of nonlocal interactions with respect to the on-site repulsion does not generate a phase transition but has a number of nontrivial effects. At the single-particle level we observe a marked enhancement of the Fermi velocity at the Dirac points. At the two-particle level, spin- and charge-density-wave fluctuations compete. As a consequence, the edge magnetic moment is reduced but the edge dispersion relation increases in the sense that the single-particle gap at momentum $q=\pi /|a_1|$ grows. We attribute this to nonlocal charge fluctuations which assist the spin fluctuations to generate the aforementioned gap. In contrast, the net result of the interaction-induced renormalization of different energy scales is a constant spin-wave velocity of the edge modes. However, since the particle-hole continuum is shifted to higher energies—due to the renormalization of the Fermi velocity—Landau damping is reduced. As a result, a roughly linear spin-wave-like mode at the edge spreads out through a larger part of the Brillouin zone.

The pyrochlore metal ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$ has been recently investigated by second-harmonic generation (SHG) reflectivity. In this paper, we develop a general formalism that allows for the identification of the relevant tensor components of the SHG from azimuthal scans. We demonstrate that the secondary order parameter identified by SHG at the structural phase transition is the $x^2-y^2$ component of the axial toroidal quadrupole. This differs from the $3z^2-r^2$ symmetry of the atomic displacements associated with the $I\overline{4}m2$ crystal structure that was previously thought to be its origin. Within the same formalism, we suggest that the primary order parameter detected in the SHG experiment is the $3z^2-r^2$ component of the magnetic quadrupole. We discuss the general mechanism driving the phase transition in our proposed framework, and suggest experiments, particularly resonant x-ray scattering ones, that could clarify this issue.

The sensing capability of a single optically bright electronic spin in diamond can be enhanced by making use of proximal dark nuclei as ancillary spins. Such systems, so far realized only in bulk diamond, can provide orders of magnitude higher sensitivity and spectral resolution in the case of magnetic sensing, as well as improved readout fidelity and state storage time in quantum information schemes. Nanodiamonds offer opportunities for scanning and embedded nanoscale probes, yet electronic-nuclear spin complexes have so far remained inaccessible. Here, we demonstrate coherent control of a ${}^{13}\mathrm{C}$ nuclear spin located 4 Å from a nitrogen-vacancy center in a nanodiamond and show coherent exchange between the two components of this hybrid spin system. We extract a free precession time $T_2^{*}$ of $26 \mu \mathrm{s}$ for the nuclear spin, which exceeds the bare-electron free-precession time in nanodiamond by two orders of magnitude.

The inversion symmetry breaking at the interface between different materials generates a strong interfacial spin-orbit coupling (ISOC) that may influence the spin and charge transport in hybrid structures. Here we use a simple analytically solvable model to study in the ballistic approximation various spin transport phenomena induced by ISOC in a bilayer metallic system. In this model a nonequilibrium steady state carrying a spin current is created by applying a spin-dependent bias across the metallic junction. Physical observables are then calculated using the scattering matrix approach. In particular we calculate the absorption of the spin current at the interface (the interface spin loss) and study the interface spin-to-charge conversion. The latter consists of an in-plane interface charge current generated by the spin-dependent bias applied to the junction, which can be viewed as a spin-galvanic effect mediated by ISOC. Finally, we demonstrate that ISOC leads to an interfacial spin-current swapping; that is, the “primary” spin current flowing through the spin-orbit active interface is necessarily accompanied by a “secondary” swapped spin current flowing along the interface and polarized in the direction perpendicular to that of the primary current. Using the exact spin continuity equation, we relate the swapping effect to the interfacial spin loss and argue that this effect is generic and independent of the ballistic approximation used for specific calculations.

To model polaronic behavior in strongly correlated transition-metal oxides with ab initio methods, one typically requires a level of theory beyond that of local density or general gradient density functional theory (DFT) approximations to account for the strongly correlated $d$-shell interactions of transition-metal oxides. In the present work, we utilize density functional theory with additional on-site Hubbard corrections $\left(\mathrm{DFT}+U\right)$ to calculate polaronic properties in two lithium ion battery cathode materials, ${\mathrm{Li}}_x{\mathrm{FePO}}_4$ and ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$, and two photocatalytic materials, ${\mathrm{TiO}}_2$ and ${\mathrm{Fe}}_2{\mathrm{O}}_3$. We investigate the effects of the $+U$ on-site projection on polaronic properties. Through systematic comparison with hybrid functional calculations, it is shown that $+U$ projection in these model materials can impact upon the band gap, polaronic hopping barrier, and polaronic eigenstate offset from the band edges in a nontrivial manner. These properties are shown to have varying degrees of coupling and dependence on the $+U$ projection in each example material studied, which has important implications for arriving at systematic material predictions of polaronic properties in transition-metal oxides.

We consider core-shell nanowires with conductive shell and insulating core and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e., along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e., thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell.

Solid ${\mathrm{NH}}_4\mathrm{F}$  displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, x-ray diffraction, and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made.

We report the development of a hybrid numerical/analytical model capable of mapping the spatially varying distributions of the local ferromagnetic resonance (FMR) frequency and dynamic magnetic susceptibility in a wide class of patterned and compositionally modulated magnetic structures. Starting from the numerically simulated static micromagnetic state, the magnetization is deliberately deflected orthogonally to its equilibrium orientation, and the magnetic fields generated in response to this deflection are evaluated using micromagnetic software. This allows us to calculate the elements of the effective demagnetizing tensor, which are then used within a linear analytical formalism to map the local FMR frequency and dynamic magnetic susceptibility. To illustrate the typical results that one can obtain using this model, we analyze three micromagnetic systems boasting nonuniformity in either one or two dimensions, and successfully explain the spin-wave emission observed in each case, demonstrating the ubiquitous nature of the Schlömann excitation mechanism underpinning the observations. Finally, the developed model of local FMR frequency can be used to explain how spin waves could be confined and steered using magnetic nonuniformities of various origins, rendering it a powerful tool for the mapping of the graded magnonic index in magnonics.

We report the effect of magnetic field induced quantum tunneling and relaxation transitions between orthogonal configurations in polyatomic systems where no tunneling is expected. Typical situations of this kind occur in molecular systems and local centers in crystals in ground and excited electronic $T$ states, subject to the $T\otimes e$ problem of the Jahn-Teller effect, where the wave functions of the three tetragonally distorted configurations are orthogonal. A detailed microscopic theory of this effect shows how the magnetic field violates the orthogonality between the latter allowing for tunneling and relaxations, which decrease in strong fields due to the induced decoherence. The novel effect is demonstrated experimentally as a big, sharp peak in ultrasound attenuation by ${\mathrm{Cr}}^{2+}$ centers in ZnSe:${\mathrm{Cr}}^{2+}$ in the magnetic field $B=0.15$ T at the temperature below 8 K. It may influence a variety of magnetic, electronic, and photonic properties of any system in an electronic $T$ state.