Hydrogen atom adsorption on high-quality graphene on Ir(111) [gr/Ir(111)] is investigated using high-resolution electron energy loss spectroscopy. The evolution of the vibrational spectrum, up to 400 meV, of gr/Ir(111) upon increasing hydrogen atom exposures is measured. The two dominant binding configurations of atomic hydrogen are identified as (1) graphanelike hydrogen clusters on the parts of the graphene more strongly interacting with the Ir(111) surface and (2) dimers bound more weakly to the freestanding parts of the graphene. The graphanelike surface structures lead to increased corrugation of the graphene sheet, yielding graphane-related phonon components. Additionally, a recent theoretical prediction of the existence of a bending character for a LO/TO graphane chair phonon mode is experimentally verified. No clear evidence was found for hydrogen bound on both sides of a high-quality graphene sheet and phonon features strongly suggest interactions between graphanelike hydrogen clusters and Ir atoms in the substrate.

An efficient phonovoltaic (pV) material requires a highly energetic optical phonon $\left(E_{p,\mathrm{O}}\gg k_{\mathrm{B}}T\right)$ with linewidth dominated by the electron-phonon (e-p) coupling and resonant with its electronic band gap $\left(\mathrm{\Delta }{E}_{e,g}\right)$, as discussed in Paper I [C. Melnick and M. Kaviany, Phys. Rev. B 93, 094302 (2016)]. No current material combines these properties. While graphite (graphene) has the former two, it lacks a band gap. Opening and tuning the band gap in graphite is challenging due to the stability of the Dirac point, e.g., under a uniaxial strain $<0.25$. We tune its band gap through partial hydrogenation using extensive ab initio calculations and find a stable graphame structure with $\mathrm{\Delta }{E}_{e,g}\simeq E_{p,\mathrm{O}}\simeq 200$ meV, ${\mathrm{C}}_{128}{\mathrm{H}}_{1\times 24}$. We calculate the e-p coupling in tuned ${\mathrm{C}}_{128}{\mathrm{H}}_{1\times 24}$ and graphene and show that the transition from $\pi \text{−}{\pi }^{*}$ (graphene) to $\sigma \text{−}{\sigma }^{*}$ (graphane) bands suppresses the electron-phonon coupling, such that optical phonons in ${\mathrm{C}}_{128}{\mathrm{H}}_{1\times 24}$ primarily downconvert, and it does not achieve a high figure of merit $\left(Z_{\mathrm{pV}}<0.1\right)$. Ab initio phonon-phonon couplings are calculated for graphane and graphene to support this result. Overall, we develop a material with $E_{p,\mathrm{O}}\simeq \mathrm{\Delta }{E}_{e,g}\gg k_{\mathrm{B}}T$ and a method for tuning and evaluating pV materials.

We study electron transport at the edge of a generic disordered two-dimensional topological insulator, where some channels are topologically protected from backscattering. Assuming the total number of channels is large, we consider the edge as a quasi-one-dimensional quantum wire and describe it in terms of a nonlinear sigma model with a topological term. Neglecting localization effects, we calculate the average distribution function of transmission probabilities as a function of the sample length. We mainly focus on the two experimentally relevant cases: a junction between two quantum Hall (QH) states with different filling factors (unitary class) and a relatively thick quantum well exhibiting quantum spin Hall (QSH) effect (symplectic class). In a QH sample, the presence of topologically protected modes leads to a strong suppression of diffusion in the other channels already at scales much shorter than the localization length. On the semiclassical level, this is accompanied by the formation of a gap in the spectrum of transmission probabilities close to unit transmission, thereby suppressing shot noise and conductance fluctuations. In the case of a QSH system, there is at most one topologically protected edge channel leading to weaker transport effects. In order to describe ‘topological’ suppression of nearly perfect transparencies, we develop an exact mapping of the semiclassical limit of the one-dimensional sigma model onto a zero-dimensional sigma model of a different symmetry class, allowing us to identify the distribution of transmission probabilities with the average spectral density of a certain random-matrix ensemble. We extend our results to other symmetry classes with topologically protected edges in two dimensions.

Based on density functional calculations with different exchange-correlation functionals we propose three new atomic structures for gold-induced nanowires on Ge(001) surfaces for Au coverages higher than one monolayer. We start the investigations from the original and a modified giant missing row (GMR) structure, the Au-trimer stabilized Ge ridge model. We replace successively Ge atoms at the wire ridge or facets by Au ones. Comparing the relative formation energies the new models are shown to be energetically more favorable than the GMR model proposed previously for lower coverages. Indeed, additional Au atoms at the wire surface stabilize novel geometries. The new models are able to explain several features of the nanowire structure observed by scanning tunneling microscopy and the electronic states found experimentally by angle-resolved photoemission spectroscopy.

Interaction between localized plasmons in isolated proximal nanostructures is a well-studied phenomenon. Here we explore plasmon-plasmon interactions in connected extended systems. Such systems can now be easily produced experimentally using graphene. However, the mechanisms of plasmonic interactions in extended systems are not well understood. We employ finite-element methods to study these interactions in graphene superlattice nanoribbon arrays with a periodically modulated electrochemical potential or number of layers. We find a rich variation in the resulting plasmonic resonances depending on the dimensions, the electrochemical potentials (doping), and the separation of the nanoribbon segments, and we demonstrate the involvement of both transverse and longitudinal plasmon-plasmon interactions. For example, unlike predictions based on the well-known “orbital hybridization model,” the energies of the resulting hybrid plasmonic resonances in the extended system can lie between the energies of the plasmons in the individual components. Our results demonstrate that the plasmonic spectra of graphene superlattice structures can be easily adjusted, continuously tuned, and used to enhance optical fields in the infrared and terahertz regions of the electromagnetic spectrum.

Fieldlike spin-orbit torque in FeMn/Pt bilayers with ultrathin polycrystalline FeMn has been characterized through planar Hall effect measurements. A large effective field of $2.05\times {10}^{-5}$ to $2.44\times {10}^{-5}\mathrm{Oe}\left({\mathrm{A}}^{-1}{\mathrm{cm}}^2\right)$ is obtained for FeMn in the thickness range of 2–5 nm. The experimental observations can be reasonably accounted for by using a macrospin model under the assumption that the FeMn layer is composed of two spin sublattices with unequal magnetizations. The large effective field corroborates the spin Hall origin of the effective field, considering the much smaller uncompensated net moments in FeMn as compared to NiFe. The effective absorption of spin current by FeMn is further confirmed by the fact that spin current generated by Pt in NiFe/FeMn/Pt trilayers can only travel through the FeMn layer with a thickness of 1–4 nm. By quantifying the fieldlike effective field induced in NiFe, a spin diffusion length of 2 nm is estimated in FeMn, consistent with values reported in the literature by ferromagnetic resonance and spin-pumping experiments.

In the pseudogap regime of the cuprates, a period-2 charge order breaks a ${\mathbb{Z}}_2$ symmetry, reflecting a broken translational symmetry. Therefore, the interaction of charge order and quenched disorder due to potential scattering, can, in principle, be treated as a random field Ising model. A numerical analysis of the ground state of such a random field Ising model reveals local, glassy dynamics in both two and three dimensions. The dynamics are treated in the glassy limit as a heat bath which couples to the itinerant electrons, leading to an unusual electronic non-Fermi-liquid. If the dynamics are strong enough, the electron spectral function has no quasiparticle peak and the effective mass diverges at the Fermi surface, precluding quantum oscillations. In contrast to charge density, $d$-density wave order (reflecting staggered circulating currents) does not directly couple to potential disorder, allowing it to support quantum oscillations. At fourth order in Landau theory, there is a term consisting of the square of the $d$-density wave order parameter, and the square of the charge order. This coupling could induce parasitic charge order, which may be weak enough for the Fermi liquid behavior to remain uncorrupted. Here, we argue that this distinction must be made clear, as one interprets quantum oscillations in cuprates.

We report Shubnikov–de Haas (SdH) oscillation measurements on FeSe under high pressure up to $P=16.1$ kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at $P\sim 8$ kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative $d{T}_c/dP$ observed in a range between $P\sim 8$ and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: $k_B{T}_c/E_F\sim 0.1$ even at $P=16.1$ kbar.

The universal bimodal distribution of transmission eigenvalues in lossless diffusive systems underpins such celebrated phenomena as universal conductance fluctuations, quantum shot noise in condensed matter physics, and enhanced transmission in optics and acoustics. Here, we show that in the presence of absorption, the density of the transmission eigenvalues depends on the confinement geometry of the scattering media. Furthermore, in an asymmetric waveguide, the densities of the reflection and absorption eigenvalues also depend on the side from which the waves are incident. With increasing absorption, the density of absorption eigenvalues transforms from a single-peak to a double-peak function. Our findings open an additional avenue for coherent control of wave transmission, reflection, and absorption in random media.

A method is proposed for the inclusion of electron correlation in the calculation of the temperature dependence of band structures arising from electron-phonon coupling. It relies on an efficient exploration of the vibrational phase space along the recently introduced thermal lines. Using the $G_0{W}_0$ approximation, the temperature dependence of the direct gaps of diamond, silicon, lithium fluoride, magnesium oxide, and titanium dioxide is calculated. Within the proposed formalism, a single calculation at each temperature of interest is sufficient to obtain results of the same accuracy as in alternative, more expensive methods. It is shown that many-body contributions beyond semilocal density functional theory modify the electron-phonon coupling strength by almost $50\%$ in diamond, silicon, and titanium dioxide, but by less than $5\%$ in lithium flouride and magnesium oxide. The results reveal a complex picture regarding the validity of semilocal functionals for the description of electron-phonon coupling.

Epitaxial $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\left(\mathrm{LSMO}\right)\text{/}\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Fe}{\mathrm{O}}_3$ (LSFO) superlattices on (111)-oriented $\mathrm{SrTi}{\mathrm{O}}_3$ substrates with sublayer thicknesses ranging from 3 to 60 unit cells (u.c.) were synthesized and characterized. Detailed analysis of their structural, electronic, and magnetic properties were performed to explore the effect of sublayer thickness on the magnetic structure and exchange coupling at (111)-oriented perovskite oxide interfaces. In the ultrathin limit (3–6 u.c.), we find that the antiferromagnetic (AF) properties of the LSFO sublayers are preserved with an out-of-plane canting of the AF spin axis, while the ferromagnetic (FM) properties of the LSMO sublayers are significantly depressed. For thicker LSFO layers ($>9$ u.c.), the out-of-plane canting of the AF spin axis is only present in superlattices with thick LSMO sublayers. As a result, exchange coupling in the form of spin-flop coupling exists only in superlattices which display both robust ferromagnetism and out-of-plane canting of the AF spin axis.

In this work we introduce a symmetry classification for electronic density waves which break translational symmetry due to commensurate wave-vector modulations. The symmetry classification builds on the concept of extended point groups: symmetry groups which contain, in addition to the lattice point group, translations that do not map the enlarged unit cell of the density wave to itself, and become “nonsymmorphic”-like elements. Multidimensional representations of the extended point group are associated with degenerate wave vectors. Electronic properties such as (nodal) band degeneracies and topological character can be straightforwardly addressed, and often follow directly. To further flesh out the idea of symmetry, the classification is constructed so as to manifestly distinguish time-reversal invariant charge (i.e., site and bond) order, and time-reversal breaking flux order. For the purpose of this work, we particularize to spin-rotation invariant density waves. As a first example of the application of the classification we consider the density waves of a simple single- and two-orbital square lattice model. The main objective, however, is to apply the classification to two-dimensional ($\text{2D}$) hexagonal lattices, specifically the triangular and the honeycomb lattices. The multicomponent density waves corresponding to the commensurate $M$-point ordering vectors are worked out in detail. To show that our results generally apply to $2\text{D}$ hexagonal lattices, we develop a general low-energy $\text{SU}\left(3\right)$ theory of (spinless) saddle-point electrons.

We study hexagonal spin-channel (“triplet”) density waves with commensurate $M$-point propagation vectors. We first show that the three $Q=M$ components of the singlet charge density and charge-current density waves can be mapped to multicomponent $Q=0$ nonzero angular momentum order in three dimensions ($\text{3D}$) with cubic crystal symmetry. This one-to-one correspondence is exploited to define a symmetry classification for triplet $M$-point density waves using the standard classification of spin-orbit coupled electronic liquid crystal phases of a cubic crystal. Through this classification we naturally identify a set of noncoplanar spin density and spin-current density waves: the chiral spin density wave and its time-reversal invariant analog. These can be thought of as $3\text{D}L=2$ and 4 spin-orbit coupled isotropic $\beta$-phase orders. In contrast, uniaxial spin density waves are shown to correspond to $\alpha$ phases. The noncoplanar triple-$M$ spin-current density wave realizes a novel $2\text{D}$ semimetal state with three flavors of four-component spin-momentum locked Dirac cones, protected by a crystal symmetry akin to nonsymmorphic symmetry, and sits at the boundary between a trivial and topological insulator. In addition, we point out that a special class of classical spin states, defined as classical spin states respecting all lattice symmetries up to global spin rotation, are naturally obtained from the symmetry classification of electronic triplet density waves. These symmetric classical spin states are the classical long-range ordered limits of chiral spin liquids.

We investigate the orbital effects of a strong external magnetic field on the ground-state properties of a two-dimensional (2D) Holstein polaron, employing variational approaches based on exact diagonalization. From the ground-state energy and the wave function, we calculate the electron-phonon correlation function, the average phonon number, and the Drude weight and investigate the evolution of a 2D Holstein polaron as a function of the magnetic flux. Although the external magnetic field affects the polaron throughout the parameter regime, we show that the magnetic field has a stronger effect on a loosely bound (spatially extended) polaron. We also find that the magnetic field can be used as a tuning parameter, particularly for a weakly coupled polaron, to reduce the spatial extent of a large polaron.

We consider the optical conductivity ${\sigma }_1\left(\omega \right)$ in the metallic phase of the one-dimensional Hubbard model. Our results focus on the vicinity of half filling and the frequency regime around the optical gap in the Mott insulating phase. By means of a density-matrix renormalization group implementation of the correction-vector approach, ${\sigma }_1\left(\omega \right)$ is computed for a range of interaction strengths and dopings. We identify an energy scale $E_{\mathrm{opt}}$ above which the optical conductivity shows a rapid increase. We then use a mobile impurity model in combination with exact results to determine the behavior of ${\sigma }_1\left(\omega \right)$ for frequencies just above $E_{\mathrm{opt}}$ which is in agreement with our numerical data. As a main result, we find that this onset behavior is not described by a power law.

Despite considerable interest in layered transition metal dichalcogenides (TMDs), such as $M{X}_2$ with $M=(\mathrm{Mo},\mathrm{W})$ and $X=(\mathrm{S},\mathrm{Se},\mathrm{Te})$, the physical origin of their topological nature is still poorly understood. In the conventional view of topological phase transition (TPT), the nontrivial topology of electron bands in TMDs is caused by the band inversion between metal $d$- and chalcogen $p$-orbital bands where the former is pulled down below the latter. Here, we show that, in TMDs, the TPT is entirely different from the conventional speculation. In particular, $M{\mathrm{S}}_2$ and $M\mathrm{S}{\mathrm{e}}_2$ exhibits the opposite behavior of TPT such that the chalcogen $p$-orbital band moves down below the metal $d$-orbital band. More interestingly, in $M\mathrm{T}{\mathrm{e}}_2$, the band inversion occurs between the metal $d$-orbital bands. Our findings cast doubts on the common view of TPT and provide clear guidelines for understanding the topological nature in new topological materials to be discovered.

We report the density-functional calculations that provide a firm theoretical framework to identify the multivacancies and unravel the underlying physics in the most stable silicon carbide polytype 4H-SiC. The calculations with the generalized gradient approximation (GGA) for the Si and C monovacancy, $V_{\mathrm{Si}}$ and $V_{\mathrm{C}}$, have clarified the significantly lower formation energy of the C vacancy accompanied by the efficient pairing relaxation of the surrounding Si dangling bonds. Our GGA calculation also predicts a stable next-neighbor $V_{\mathrm{C}}{V}_{\mathrm{C}}$ divacancy which is lower in the formation energy than the usual nearest-neighbor divacancy $V_{\mathrm{C}}{V}_{\mathrm{Si}}$ discussed in the past. We also perform the calculations with the hybrid functional and confirm the stability of the next-neighbor $V_{\mathrm{C}}{V}_{\mathrm{C}}$ divacancy. Our calculations indeed clarify that it is possible to detect the $V_{\mathrm{C}}{V}_{\mathrm{C}}$ with its peculiar hyperfine coupling constants by the electron paramagnetic resonance (EPR) measurements. Based on the structural characteristics and the energetics for the monovacancy, we further propose an extended dangling-bond-counting (EDBC) model to choose the energetically favorable topological network of the vacant sites for the multivacancy. The GGA calculations combined with the EDBC model reveal that $V_3$ and $V_6$ are energetically favorable. The stable $V_3$ is a nearest-neighbor complex of $V_{\mathrm{C}}\text{−}{V}_{\mathrm{Si}}\text{−}{V}_{\mathrm{C}}$, whereas the $V_6$ is the high-symmetry $V_5$ (the central $V_{\mathrm{Si}}$ surrounded by four $V_{\mathrm{C}})$ plus a symmetry-breaking next-neighbor $V_{\mathrm{C}}$. We perform the GGA calculations for the electronic structure of such $V_3$ and $V_6$ and discuss the possibility of detecting these multivacancies. In particular, the EPR-detected ANN1 center is provisionally identified as the doubly positive trivacancy $V_{\mathrm{C}}\text{−}{V}_{\mathrm{Si}}\text{−}{V}_{\mathrm{C}}$.