Intrinsic loss has long been thought to be a deteriorative factor in wave propagations. However, for metamaterials in which the real part of the permittivity or permeability vanishes, the role of loss is highlighted and can even lead to a topological transition of dispersion from a closed elliptic curve to an open hyperbolic curve. Based on two-dimensional transmission lines with lumped elements, anisotropic $\mu$-near-zero metamaterials with engineered losses are realized. Loss-induced abnormal propagations including collimation and field enhancement governed by the topological transition are experimentally observed in transmission-line-based metamaterials.

Coherent phonons in single-wall carbon nanotubes (SWNTs) are observed as oscillations of the differential absorption coefficient as a function of time by means of pump-probe spectroscopy. For the radial breathing mode (RBM) of a SWNT, the coherent phonon signal is understood to be a result of the modulated diameter-dependent energy gaps due to the coherent RBM phonon oscillations. However, this mechanism might not be the dominant contribution to other phonon modes in the SWNT. In particular, for the $G$-band phonons, which correspond to bond-stretching motions, we find that the modulation of the interatomic optical dipole (electron-photon) matrix element gives rise to a strong coherent $G$-band phonon intensity comparable to the coherent RBM phonon intensity. We also further discuss the dependence of coherent $G$-band and RBM phonon amplitudes on the laser excitation pulse width.

The spin Hall and its inverse effects, driven by the spin orbit interaction, provide an interconversion mechanism between spin and charge currents. Since the spin Hall effect generates and manipulates spin current electrically, to achieve a large effect is becoming an important topic in both academia and industries. So far, materials with heavy elements carrying a strong spin orbit interaction, provide the only option. We propose here a new mechanism, using the surface roughness in ultrathin films, to enhance the spin Hall effect without heavy elements. Our analysis based on Cu and Al thin films suggests that surface roughness is capable of driving a spin Hall angle that is comparable to that in bulk Au. We also demonstrate that the spin Hall effect induced by surface roughness subscribes only to the side-jump contribution but not the skew scattering. The paradigm proposed in this paper provides the second, not if only, alternative to generate a sizable spin Hall effect.

Epitaxial growth on metal substrates has been shown to be the most powerful approach in producing large-scale high-quality monolayer graphene, yet it remains a major challenge to realize uniform bilayer graphene growth. Here we carry out a comparative study of the atomistic mechanisms for bilayer graphene growth on the (111) surfaces of Cu and Ni, using multiscale approaches combining first-principles calculations and rate-equation analysis. We first show that the relatively weak graphene-Cu interaction enhances the lateral diffusion and effective nucleation of C atoms underneath the graphene island, thereby making it more feasible to grow bilayer graphene on Cu. In contrast, the stronger graphene-Ni interaction suppresses the lateral mobility and dimerization of C atoms underneath the graphene, making it unlikely to achieve controlled growth of bilayer graphene on Ni. We then determine the critical graphene size beyond which nucleation of the second layer will take place. Intriguingly, the critical size exhibits an effective inverse “Ehrlich-Schwoebel barrier” effect, becoming smaller for faster C migration from the Cu surface to the graphene-Cu interface sites across the graphene edge. These findings allow us to propose a novel alternating growth scheme to realize mass production of bilayer graphene.

The Zitterbewegung of a wave packet in a zigzag graphene nanoribbon is theoretically investigated. The coupling between edge states and bulk states results in intriguing properties. Apart from the oscillation in position perpendicular to the direction of motion, we also observe an oscillation along the direction of propagation which is not present in semiconductor nanowires or infinite graphene sheets. We also observe a resonance of its amplitude with respect to the central momentum of the wave packet. We show here that this longitudinal Zitterbewegung is caused by the interplay between bulk and edge states, which is a unique property of a zigzag nanoribbon.

We present a comprehensive study of thermoelectric transport properties of a quantum nanoelectromechanical system (NEMS) described by a single-electron transistor (SET) coupled to a quantum nanomechanical resonator (NR). The effects of a quantum NR on the electronic current are investigated with special emphasis on how the SET-NR coupling strength plays a role in such a NEMS. We find that the SET-NR coupling is not only able to suppress or enhance the thermoelectric current but can also switch its direction. The effect of the NR on the thermoelectric coefficients of the SET is studied and we find that even a small SET-NR coupling could dramatically suppress the figure of merit $ZT$. Lastly, we investigate the backaction of electronic current on the NR and possible routes of heating or cooling the NR are discussed. We find that by appropriately tuning the gate voltage the backaction can be eliminated, which could find possible applications to enhance the sensitivity of detection devices.

The knowledge of the structures that can exist in compounds containing helium is of interest for understanding the conditions where and if this inert element can form structures where closed shell electrons of helium can participate in bonding that is not describable exclusively by van der Waals interactions alone. In this study we examine stable mixtures of He and ${\mathrm{H}}_2\mathrm{O}$ at high pressures using a first-principles structure searching method. We find a thermodynamically stable structure that can be characterized by interactions comparable in strength to that of conventional hydrogen bonds. An orthorhombic structure with space group Ibam is identified that has progressively lower enthalpy with increasing pressure above 296 GPa than a mixture of He and ${\mathrm{H}}_2\mathrm{O}$. This mechanically and dynamically stable structure is found at pressures that are now becoming accessible to high-pressure techniques.

We have studied the transport properties of transmission electron microscope (TEM) lamellae obtained from a pyrolytic graphite sample of grade B (SPI-II) with electrical contacts at the edges of the graphene layers. The temperature, magnetic field, input current dependence of the resistance as well as the current-voltage characteristic curves are compatible with the existence of granular superconductivity below 5 K. TEM pictures of the studied lamellae reveal clear differences of the embedded interfaces to those existing in more ordered pyrolytic samples, which appear to be the origin for the relatively lower temperatures at which the granular superconductivity is observed.

To extract a pure atomic-resolution hologram around hydrogen from a two-dimensional (2D) neutron intensity distribution around a single crystal, enhancement of the holographic signal and suppression of the diffuse scattering are important; sample cooling can achieve both. To estimate the performance of sample cooling in achieving these requirements, the angular anisotropy of the neutron intensities around a ${\mathrm{PdH}}_{0.78}$ single crystal was measured at 300 K and 100 K. In addition to the reduction in the diffuse scattering intensity, the pattern itself changed markedly upon sample cooling, thus indicating a significant modification of the hydrogen lattice dynamics. The reconstruction from a hologram, which was obtained by subtracting the diffuse scattering pattern, shows images of H and Pd nuclei as negative and positive peaks, respectively. In particular, such reconstruction at 100 K exhibits distinct H images at distant positions owing to the suppression of hydrogen motion. These features are important for the determination of hydrogen sites in various materials.

We present a first-principles computational study of solid ${}^4\mathrm{He}$ at $T=0$ K and pressures up to $\sim 160$ GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrödinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid ${}^4\mathrm{He}$ does not increase appreciably with compression for $P\ge 85$ GPa. Also, we show that the Lindemann ratio in dense solid ${}^4\mathrm{He}$ amounts to $0.10$ almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed ${}^4\mathrm{He}$. In particular, we disclose huge discrepancies of at least $\sim 50\%$ in the calculated ${}^4\mathrm{He}$ kinetic energies using both the QH DFT and present DFT-DMC methods.

We determine the magnetic and magnetotransport properties of 33 nm thick ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films epitaxially deposited by rf-magnetron sputtering on unpoled (011) ${\left[{\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3\right]}_{0.68}-{\left[{\mathrm{PbTiO}}_3\right]}_{0.32}$ (PMN-PT) substrates. The magnetoresistance (MR), as well as the magnetization reversal, strongly depend on the in-plane crystallographic direction of the epitaxial (011) ${\mathrm{Fe}}_3{\mathrm{O}}_4$ film and strain. When the magnetic field is applied along [100], the magnetization loops are slanted and the sign of the longitudinal MR changes from positive to negative around the Verwey transition at 125 K on cooling. Along the $\left[01\overline{1}\right]$ direction, the loops are square shaped and the MR is negative above the switching field across the whole temperature range, just increasing in absolute value when cooling from 300 K to 150 K. The value of the MR is found to be strongly affected by poling the PMN-PT substrate, decreasing in the [100] direction and slightly increasing in the $\left[01\overline{1}\right]$ direction upon poling, which results in a strained film.

We present a detailed study of the ferromagnetic/ferroelectric heterostructure ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3/{\left[\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3\right]}_{0.68}-{\left[{\mathrm{PbTiO}}_3\right]}_{0.32}$ (011), where reversible electrical switching induces a 10 K shift of the magnetic Curie temperature ${\mathrm{T}}_C$. Two distinct polarization configurations can be set in the ferroelectric substrate and are stable at remanence; reciprocal space maps highlight the accompanying lattice parameter changes which impose a biaxial strain on the manganite thin film. The magnetic response to the strain changes is probed by temperature dependent Mn $L_{3,2}$ x-ray magnetic circular dichroism and resistance measurements. X-ray natural linear dichroism spectra for both strain states probe the valence charge anisotropy: The existing population imbalance between out-of-plane and in-plane oriented orbitals increases further with tensile strain, favoring orbital occupation in the surface plane. Multiplet and density functional theory calculations support the emerging picture that an increase in tensile in-plane strain leads to an increased energy difference between the two $\mathrm{e}{}_g$ orbitals and a larger Mn-O-bond length. Increasing the electron-lattice coupling and reducing the $\mathrm{e}{}_g$ electron itinerancy that leads to ferromagnetism due to double exchange coupling, results ultimately in lower ${\mathrm{T}}_C$ values.

Protected by the chiral symmetry, three-dimensional chiral topological insulators are characterized by an integer-valued topological invariant. How this invariant could emerge in physical observables is an important question. Here, we show that the magnetoelectric polarization can identify the integer-valued invariant if we gap the system without coating a quantum Hall layer on the surface. The quantized response is demonstrated to be robust against weak perturbations. We also study the topological properties by adiabatically coupling two nontrivial phases, and find that gapless states appear and are localized at the boundary region. Finally, an experimental scheme is proposed to realize the Hamiltonian and measure the quantized response with ultracold atoms in optical lattices.

Dynamical Jahn-Teller effect has escaped so far direct observation in metallic systems. It is particularly believed to be quenched also in correlated conductors with orbitally degenerate sites such as cubic fullerides. Here, the Gutzwiller approach is extended to treat electron correlation over metals with Jahn-Teller active sites and applied to the investigation of the ground state of ${\mathrm{K}}_3{\mathrm{C}}_{60}$. It is shown that dynamical Jahn-Teller instability fully develops in this material when the interelectron repulsion $U$ on ${\mathrm{C}}_{60}$ sites exceeds some critical value. The latter is found to be lower than the current estimates of $U$, meaning that dynamical Jahn-Teller effect takes place in all cubic fullerides. This leads to strong splitting of LUMO orbitals on ${\mathrm{C}}_{60}$ sites and calls for reconsideration of the role of orbital degeneracy in the Mott-Hubbard transition in fullerides.

Density-functional-theory calculations are combined with many-body perturbation theory in order to elucidate the geometry, electronic, and optical properties of the $wz-\mathrm{GaN}(1\overline{1}00$) surface, i.e., the so-called $m$-plane. The optical absorption and reflection anisotropy related to electronic transitions between surface states are identified by comparison with measured data covering transition energies from 2.4 up to 5.4 eV. Our results show a surface relaxation mechanism consistent with the electron counting rule that causes a moderate buckling of the GaN surface dimers and gives rise to two distinct surface states: The doubly occupied N dangling bonds form a surface band that is resonant with the GaN valence-band edge at the center of the Brillouin zone, whereas the empty Ga dangling bonds occur within the GaN band gap closely following the dispersion of the conduction-band edge. These two states contribute strongly to the formation of surface excitons that redshift the optical absorption with respect to the bulk optical response. The surface optical absorption i.e., the excitonic onset below the bulk band gap followed by a broad absorption band at higher energies related to the dispersion of the surface band structure, is calculated in agreement with the experimental data.

The broken inversion symmetry at the surface of a metallic film (or, more generally, at the interface between a metallic film and a different metallic or insulating material) greatly amplifies the influence of the spin-orbit interaction on the surface properties. The best known manifestation of this effect is the momentum-dependent splitting of the surface-state energies (Rashba effect). Here we show that the same interaction also generates a spin polarization of the bulk states when an electric current is driven through the bulk of the film. For a semi-infinite jellium model, which is representative of metals with a closed Fermi surface, we prove as a theorem that regardless of the shape of the confinement potential, the induced surface spin density at each surface is given by $\mathbf{S}=-\gamma \hslash \widehat{\mathbf{z}}\times \mathbf{j}$, where $\mathbf{j}$ is the particle current density in the bulk, $\widehat{\mathbf{z}}$ the unit vector normal to the surface, and $\gamma =\frac{\hslash }{4m{c}^2}$ contains only fundamental constants. For a general metallic solid, $\gamma$ becomes a material-specific parameter that controls the strength of the interfacial spin-orbit coupling. Our theorem, combined with an ab initio calculation of the spin polarization of the current-carrying film, enables a determination of $\gamma$, which should be useful in modeling the spin-dependent scattering of quasiparticles at the interface.

We report on the possibility of valley number fractionalization in graphene with a topological defect that is accounted for in the Dirac equation by a pseudomagnetic field. The valley number fractionalization is attributable to an imbalance in the number of one-particle states in one of the two Dirac points with respect to the other. The difference in the number of one-particle states is manifest and can be exactly evaluated thanks to an external uniform magnetic field. Although the external magnetic field is precluded, the net valley number results in being dependent only on the flux of the pseudomagnetic field. We also discuss the analogous effect that the topological defect might lead to the induced spin polarization of the charge carriers in graphene.

Low-energy states of quantum spin liquids are thought to involve partons living in a gauge-field background. We study the spectrum of Majorana fermions of the Kitaev honeycomb model on spherical clusters. The gauge field endows the partons with half-integer orbital angular momenta. As a consequence, the multiplicities do not reflect the point-group symmetries of the cluster, but rather its projective symmetries, operations combining physical and gauge transformations. The projective symmetry group of the ground state is the double cover of the point group.

Recent experiments point to a variety of intermetallic systems which exhibit exotic quadrupolar orders driven by the Kondo coupling between conduction electrons and localized quadrupolar degrees of freedom. Using a Luttinger $k·p$ Hamiltonian for the conduction electrons, we study the impact of such quadrupolar order on their energies and wave functions. We discover that such quadrupolar orders can induce a nontrivial Berry curvature for the conduction electron bands, leading to a nonvanishing optical gyrotropic effect. We estimate the magnitude of the gyrotropic response in a candidate quadrupolar material, ${\mathrm{PrPb}}_3$, and discuss the resulting Faraday rotation in thin films.