We propose a fully ab initio theory to compute the electron density response under the perturbation in the local field. This method is based on our recently developed local dielectric response theory [Phys. Rev. B 92, 241107(R) (2015)], which provides a rigorous theoretical framework to treat local electronic excitations in both finite and extended systems beyond the commonly employed dipole approximation. We have applied this method to study the electronic part of the molecular polarizability of water in ice $\mathrm{I}h$ and liquid water. Our results reveal that the crystal field of the hydrogen-bond network has strong anisotropic effects, which significantly enhance the out-of-plane component and suppress the in-plane component perpendicular to the bisector direction. The contribution from the charge transfer is equally important, which increases the isotropic molecular polarizability by $5–6$%. Our study provides insights into the dielectric properties of water, which form the basis to understand electronic excitations in water and to develop accurate polarizable force fields of water.

Recently, the existence of local magnetic moment in a hydrogen adatom on graphene was confirmed experimentally [González-Herrero et al., Science 352, 437 (2016)]. Inspired by this breakthrough, we theoretically investigate the top-site adatom on trilayer graphene (TLG) by solving the Anderson impurity model via self-consistent mean field method. The influence of the stacking order, the adsorption site, and external electric field are carefully considered. We find that, due to its unique electronic structure, the situation of TLG is drastically different from that of the monolayer graphene. First, the adatom on rhombohedral stacked TLG (r-TLG) can have a Fano-shaped impurity spectral density, instead of the normal Lorentzian-like one, when the impurity level is around the Fermi level. Second, the impurity level of the adatom on r-TLG can be tuned into an in-gap state by an external electric field, which strongly depends on the direction of the applied electric field and can significantly affect the local magnetic moment formation. Finally, we systematically calculate the impurity magnetic phase diagrams, considering various stacking orders, adsorption sites, doping, and electric field. We show that, because of the in-gap state, the impurity magnetic phase of r-TLG will obviously depend on the direction of the applied electric field as well. All our theoretical results can be readily tested in experiment, and may give a comprehensive understanding about the local magnetic moment of an adatom on TLG.

We calculate the optical conductivity of an anisotropic two-dimensional system with Rashba spin-flip excitation within the Kubo formalism. We show that the anisotropic Rashba effect caused by an external field significantly changes the magnitude of the spin splitting. Furthermore, we obtain an analytical expression for the longitudinal optical conductivity associated with interband transitions as a function of the frequency for arbitrary polarization angle. We find that the diagonal components of the optical conductivity tensor are direction dependent and the optical absorption spectrum exhibits a strongly anisotropic absorption window. The height and width of this absorption window are very sensitive to the anisotropy of the system. While the height of absorption peak increases with increasing effective mass anisotropy ratio, the peak intensity is larger when the light polarization is along the armchair direction. Moreover, the absorption peak width becomes broader as the density-of-states mass or Rashba interaction is enhanced. These features in the optical absorption spectrum can be used to determine parameters relevant for spintronics.

We investigate charge fractionalizations in artificial Tomonaga-Luttinger liquids (TLLs) composed of two capacitively coupled quantum Hall edge channels (ECs) in graphene. The interaction strength of the artificial TLLs can be controlled through distance $W$ between the ECs. We show that the fractionalization ratio $r$ and the TLL mode velocity $v$ vary with $W$. The experimentally obtained relation between $v$ and $r$ follows a unique function predicted by the TLL theory. We also show that charged wave packets are reflected back and forth multiple times at both ends of the TLL region.

An interacting bilayer electron system provides an extended platform to study electron-electron interaction beyond single layers. We report here experiments demonstrating that the layer densities of an asymmetric bilayer electron system oscillate as a function of perpendicular magnetic field that quantizes the energy levels. At intermediate fields, this interlayer charge transfer can be well explained by the alignment of the Landau levels in the two layers. At the highest fields where both layers reach the extreme quantum limit, however, there is an anomalous, enhanced charge transfer to the majority layer. Surprisingly, when the minority layer becomes extremely dilute, this charge transfer slows down as the electrons in the minority layer condense into a Wigner crystal. Furthermore, by examining the quantum capacitance of the dilute layer at high fields, the screening induced by the composite fermions in an adjacent layer is unveiled. The results highlight the influence of strong interaction in interlayer charge transfer in the regime of very high fields and low Landau level filling factors.

The characteristics of the cooperative Mott-Peierls metal-insulator transition (MIT) of ${\mathrm{VO}}_2$ can be altered by employing epitaxial strain. While the most commonly used substrate for this purpose is isostructural rutile ${\mathrm{TiO}}_2$, thin films often suffer from interdiffusion of Ti ions near the interface. Exploiting this phenomena, we investigate the nature of interfacial ${\mathrm{V}}^{4+}/{\mathrm{Ti}}^{4+}$ cation intermixing and its effects on the MIT using scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS), soft x-ray absorption spectroscopy (XAS), and hard x-ray photoelectron spectroscopy (HAXPES), along with supporting density functional theory (DFT) calculations. We find that the reduced orbital occupancy in highly Ti incorporated ${\mathrm{VO}}_2$ is responsible for suppressing the MIT. Interdiffused films are found to be metallic at all measured temperatures, despite a resolute dimerization inferred from x-ray absorption data at lower temperatures. Our results demonstrate that the Mott physics can be suppressed in doped ${\mathrm{VO}}_2$, while a lattice dimerization remains thermodynamically favorable.

Electromagnetically induced transparency in metamaterials allows to engineer structures which transmit narrow spectral ranges of radiation while exhibiting a large group index. Implementation of this phenomenon frequently calls for strong near-field coupling of bright (dipolar) resonances to dark (multipolar) resonances in the metamolecules comprising the metamaterials. The sharpness and contrast of the resulting transparency windows thus depends strongly on how closely these metamolecules can be placed to one another, placing constraints on fabrication capabilities. In this manuscript, we demonstrate that the reliance on near-field interaction strength can be relaxed, and the magnitude of the electromagnetic-induced transparency enhanced, by exploiting the long-range coupling between metamolecules in periodic lattices. By placing dolmen structures resonant at THz frequencies in a periodic lattice, we show a significant increase of the transparency window when the in-plane diffraction is tuned to the resonant frequency of the metamolecules, as confirmed by direct mapping of the THz near-field amplitude across a lattice of dolmens. Through the direct interrogation of the dark resonance in the near field, we show the interplay of near- and far-field couplings in optimizing the response of planar dolmen arrays via diffraction-enhanced transparency.

We present a systematic study of spin and lattice dynamics in the quasi-one-dimensional spiral magnet ${\mathrm{CuBr}}_2$, using Raman scattering in conjunction with infrared and neutron spectroscopy. Along with the development of spin correlations upon cooling, we observe a rich set of broad Raman bands at energies that correspond to phonon-dispersion energies near the one-dimensional magnetic wave vector. The low-energy bands further exhibit a distinct intensity maximum at the spiral magnetic ordering temperature. We attribute these unusual observations to two possible underlying mechanisms: (1) formation of hybrid spin-lattice excitations and/or (2) “quadrumerization” of the lattice caused by spin-singlet entanglement in competition with the spiral magnetism.

We compute the chiral magnetic effect (CME) in multi-Weyl semimetals (multi-WSMs) based on the chiral kinetic theory. Multi-WSMs are WSMs with multiple monopole charges that have nonlinear and anisotropic dispersion relations near the Weyl points, and we need to extend the conventional computation of the CME in WSMs with linear dispersion relations. The topological properties of the CME in multi-WSMs are investigated in detail for not only static magnetic fields but also time-dependent (dynamic) ones. We propose an experimental setup to measure the multiple monopole charge via the topological nature hidden in the dynamic CME.

We theoretically investigate how each orbital and valley play a role in high thermoelectric performance of SnSe. In the hole-doped regime, two kinds of valence band valleys contribute to its transport properties: one is the valley near the U-Z line, mainly consisting of the Se-$p_z$ orbitals, and the other is the one along the $\mathrm{\Gamma }$-Y line, mainly consisting of the Se-$p_y$ orbitals. Whereas the former valley plays a major role in determining the transport properties at room temperature, the latter one also offers comparable contribution and so the band structure exhibits multivalley character by increasing the temperature. In the electron-doped regime, the conduction band valley around the $\mathrm{\Gamma }$ point solely contributes to the thermoelectric performance, where the quasi-one-dimensional electronic structure along the $a$ axis is crucial. This study provides important knowledge for the thermoelectric properties of SnSe, and will be useful for future searches for high-performance thermoelectric materials.

We calculate the ac optical response to circularly polarized light of a Weyl semimetal (WSM) with varying amounts of tilt of the Dirac cones. Both type-I and -II (overtilted) WSMs are considered in a continuum model with broken time-reversal symmetry. The Weyl nodes appear in pairs of equal energies but of opposite momentum and chirality. For type I, the response of a particular node to right-hand polarized (RHP) and left-hand polarized (LHP) light is distinct only in a limited range of photon energy $\mathrm{\Omega }, \frac{2}{1+C_2/v}<\frac{\mathrm{\Omega }}{\mu }<\frac{2}{1-C_2/v}$ with $\mu$ the chemical potential and $C_2$ the tilt associated with the positive chirality node assuming the two nodes are oppositely tilted. For the overtilted case (type II), the same lower bound applies but there is no upper bound. If the tilt is reversed, the RHP and LHP responses are also reversed. We present corresponding results for the Hall angle.

The atomic and electronic structure of ${\mathrm{SrTiO}}_3$(001) surfaces is investigated by low-energy electron diffraction, Auger electron spectroscopy, atomic force microscopy, ultraviolet photoemission spectroscopy (UPS), and theoretical calculations. While the surface is ${\mathrm{TiO}}_2$-terminated after annealing at $1225{}^{\circ }\mathrm{C}$ for 72 h, an SrO-terminated surface is realized along with a $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ reconstruction after annealing at $1275{}^{\circ }\mathrm{C}$ for 72 h. The stability of the surface structure revealing a $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ is evaluated by first-principles calculations, which shows that the most stable surface is a periodically SrO-deficient surface. Furthermore, UPS measured on the SrO-terminated $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ surface shows an upward band bending as compared to the ${\mathrm{TiO}}_2$-terminated surface. Neither in-gap nor metallic states are formed by doping electrons to the $\sqrt{2}\times \sqrt{2}R{45}^{\circ }$ surface in contrast to the ${\mathrm{TiO}}_2$-terminated surface.

The nonclassical modification of plasmon-assisted fluorescence enhancement is theoretically explored by placing two-level dipole emitters at the narrow gaps encountered in canonical plasmonic architectures, namely, dimers and trimers of different metallic nanoparticles. Through detailed simulations, in comparison with appropriate analytical modeling, it is shown that within classical electrodynamics and for the reduced separations explored here, fluorescence enhancement factors of the order of ${10}^5$ can be achieved, with a divergent behavior as the particle touching regime is approached. This remarkable prediction is mainly governed by the dramatic increase in excitation rate triggered by the corresponding field enhancement inside the gaps. Nevertheless, once nonclassical corrections are included, the amplification factors decrease by up to two orders of magnitude, and a saturation regime for narrower gaps is reached. These nonclassical limitations are demonstrated by simulations based on the generalized nonlocal optical response theory, which accounts in an efficient way not only for nonlocal screening but also for the enhanced Landau damping near the metal surface. A simple strategy to introduce nonlocal corrections to the analytic solutions is also proposed. It is therefore shown that the nonlocal optical response of the metal imposes more realistic, finite upper bounds to the enhancement feasible with ultrasmall plasmonic cavities, thus providing a theoretical description closer to state-of-the-art experiments.

The magnetocrystalline anisotropy (MAE) of ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}(0\le x\le 0.25)$ alloys are investigated using the singular point detection technique and density functional theory. A slight reduction in MAE as compared to that of ${\mathrm{Ni}}_2\mathrm{MnGa}$ is observed due to Pt substitution. The calculated MAE varies almost linearly with the orbital moment anisotropy. A competition between the elastic and the chemical contributions explains the observed trend of the MAE with increasing Pt content. The large MAE in combination with the previously reported increase of the martensitic transition temperature makes these alloys promising candidates for ferromagnetic shape memory applications near room temperature.

We analyze the emergence of a process zone of improper ferroelastic class. We derive expressions for the zone emergence temperature and the size of the incipient zone in terms of experimentally accessible physical parameters. Our numerical estimates suggest that the region of existence of the process zone typically occupies a considerable part of the phase diagram.

We investigate the substitution of early transition metals (Zr, Hf, and Nb) in Ni-based binary glass-forming metallic melts and the impact on structural and dynamical properties by using a combination of neutron scattering, electrostatic levitation (ESL), and isotopic substitution. The self-diffusion coefficients measured by quasielastic neutron scattering (QENS) identify a sluggish diffusion as well as an increased activation energy by almost a factor of 2 for ${\mathrm{Hf}}_{35}{\mathrm{Ni}}_{65}$ compared to ${\mathrm{Zr}}_{36}{\mathrm{Ni}}_{64}$. This finding can be explained by the locally higher packing density of Hf atoms in ${\mathrm{Hf}}_{35}{\mathrm{Ni}}_{65}$ compared to Zr atoms in ${\mathrm{Zr}}_{36}{\mathrm{Ni}}_{64}$, which has been derived from interatomic distances by analyzing the measured partial structure factors. Furthermore, QENS measurements of liquid ${\mathrm{Hf}}_{35}{\mathrm{Ni}}_{65}$ prepared with ${}^{60}\mathrm{Ni}$, which has a vanishing incoherent scattering cross section, have demonstrated that self-diffusion of Hf is slowed down compared to the concentration weighted self-diffusion of Hf and Ni. This implies a dynamical decoupling between larger Hf and smaller Ni atoms, which can be related to a saturation effect of unequal atomic nearest-neighbor pairs, that was observed recently for Ni-rich compositions in Zr-Ni metallic melts. In order to establish a structure-dynamics relation, measured partial structure factors have been used as an input for mode-coupling theory (MCT) of the glass transition to calculate self-diffusion coefficients for the different atomic components. Remarkably, MCT can reproduce the increased activation energy for ${\mathrm{Hf}}_{35}{\mathrm{Ni}}_{65}$ as well as the dynamical decoupling between Hf and Ni atoms.

We introduce and explore a family of self-dual models of single-particle motion in quasiperiodic potentials, with hopping amplitudes that fall off as a power law with exponent $p$. These models are generalizations of the familiar Aubry-André model. For large enough $p$, their static properties are similar to those of the Aubry-André model, although the low-frequency conductivity in the localized phase is sensitive to $p$. For $p\lesssim 2.1$ the Aubry-André localization transition splits into three transitions; two distinct intermediate regimes with both localized and delocalized states appear near the self-dual point of the Aubry-André model. In the intermediate regimes, the density of states is singular continuous in much of the spectrum, and is approximately self-similar: states form narrow energy bands, which are divided into yet narrower subbands; we find no clear sign of a mobility edge. When $p<1$, localized states are not stable in random potentials; in the present model, however, tightly localized states are present for relatively large systems. We discuss the frequency-dependence and strong sample-to-sample fluctuations of the low-frequency optical conductivity, although a suitably generalized version of Mott's law is recovered when the power law is slowly decaying. We present evidence that many of these features persist in models that are away from self-duality.

The effects of a magnetic field on spin glass dynamics are explored for a ${\mathrm{Cu}}_{0.887}{\mathrm{Mn}}_{0.113}$ thin film of thickness $\mathcal{L}=20$ nm in a multilayer configuration. An experimental protocol removes uncertainties associated with the time dependence of the field-cooled magnetization $M_{\text{FC}}(t,T)$. Activated dynamics is exhibited after the spin glass correlation length $\xi (t,T)$ has reached $\mathcal{L}$, creating a quasiequilibrium state. The activation energy depends upon the strength of the magnetic field $H$. The magnitude of the activation energy diminishes as $H^2$, the coefficient of which is proportional to the number of correlated spins. A quantitative fit requires a “pancakelike” correlated region, associated with the $T=0$ phase transition for a spin glass in $D=2$ dimensions.

Angular-momentum conservation has served as a guiding principle in the interplay between spin dynamics and mechanical rotations. However, in an antiferromagnet with vanishing magnetization, new fundamental rules are required to properly describe spin-mechanical phenomena. Here we show that the Néel order dynamics affects the mechanical motion of a rigid body by modifying its inertia tensor in the presence of strong magnetocrystalline anisotropy. This effect depends on temperature when magnon excitations are considered. Such a spin-mechanical inertia can produce measurable consequences at small scales.

Recent experiments reveal that the honeycomb ruthenium trichloride $\alpha -\mathrm{RuC}{\mathrm{l}}_3$ is a prime candidate of the Kitaev quantum spin liquid (QSL). However, there is no theoretical model which can properly describe its experimental dynamical response due to the lack of a full understanding of its magnetic interactions. Here, we propose a general scheme to calculate the magnetic interactions in systems (e.g., $\alpha -\mathrm{RuC}{\mathrm{l}}_3$) with nonnegligible orbital moments by constraining the directions of orbital moments. With this scheme, we put forward a minimal $J_1-K_1-{\mathrm{\Gamma }}_1-J_3-K_3$ model for $\alpha -\mathrm{RuC}{\mathrm{l}}_3$ and find that: (I) The third nearest neighbor (NN) antiferromagnetic Heisenberg interaction $J_3$ stabilizes the zigzag antiferromagnetic order; (II) The NN symmetric off-diagonal exchange ${\mathrm{\Gamma }}_1$ plays a pivotal role in determining the preferred direction of magnetic moments and generating the spin wave gap. An exact diagonalization study on this model shows that the Kitaev QSL can be realized by suppressing the NN symmetric off-diagonal exchange ${\mathrm{\Gamma }}_1$ and the third NN Heisenberg interaction $J_3$. Thus, we not only propose a powerful general scheme for investigating the intriguing magnetism of $J_{\mathrm{eff}}=1/2$ magnets, but also point out future directions for realizing the Kitaev QSL in the honeycomb ruthenium trichloride $\alpha -\mathrm{RuC}{\mathrm{l}}_3$.