We use aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, atomic force microscopy, and the density functional theory to study the structural and electronic characteristics of graphene grown on the C face of SiC. We show that for high growth temperatures the graphene/SiC($000\overline{1}$) interface is dominated by a thin amorphous film which strongly suppresses the epitaxy of graphene on the SiC substrate. This film maintains an almost fixed thickness regardless of the number of the overlying graphene layers, while its chemical signature shows the presence of C, Si, and O. Structurally, the amorphous area is inhomogeneous, as its Si concentration gradually decreases while approaching the first graphene layer, which is purely $s{p}^2$ hybridized. Ab initio calculations show that the evaporation process and the creation of Si vacancies on the C face of SiC strongly enhance the surface disorder and designate defect areas as preferential sublimation sites. Based on these features, we discuss differences and similarities between the C-only buffer layer that forms on the Si face of SiC and the thicker C-Si-O amorphous film of the C face.

The Berezinskii-Kosterlitz-Thouless (BKT) phase transition drives the unbinding of topological defects in many two-dimensional systems. In the two-dimensional Coulomb gas, it corresponds to an insulator-conductor transition driven by charge deconfinement. We investigate the global topological properties of this transition, both analytically and by numerical simulation, using a lattice-field description of the two-dimensional Coulomb gas on a torus. The BKT transition is shown to be an ergodicity breaking between the topological sectors of the electric field, which implies a definition of topological order in terms of broken ergodicity. The breakdown of local topological order at the BKT transition leads to the excitation of global topological defects in the electric field, corresponding to different topological sectors. The quantized nature of these classical excitations, and their strict suppression by ergodicity breaking in the low-temperature phase, afford striking global signatures of topological-sector fluctuations at the BKT transition. We discuss how these signatures could be detected in experiments on, for example, magnetic films and cold-atom systems.

We present results for the optical conductivity of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ within density functional theory by including spin-orbit and correlation effects as implemented in the generalized gradient approximation. We identify the various interband transitions and show that the underlying quasimolecular-orbital nature of the electronic structure in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ translates into distinct features in the optical conductivity. Most importantly, the parity of the quasimolecular orbitals appears to be the main factor in determining strong and weak optical transitions. We also present optical conductivity calculations for $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ and discuss the similarities and differences with ${\mathrm{Na}}_2{\mathrm{IrO}}_3$.

We present evidence for “image biexcitons” within organic-inorganic perovskite-coated silver gratings. These composite quasiparticles are formed by the interaction between an exciton and its image in the metal mirror below, with binding energy 100 meV at room temperature. By changing the polar and azimuthal angles of the incident light, we observe strong coupling between excitons and surface plasmon polaritons on the grating, with Rabi splittings of 150 and 125 meV for the exciton and biexciton, respectively. A detailed analysis of the field polarizations and dipole orientations shows how these Rabi couplings arise from the strongly compressed field volume.

We report an order-$N$ approach to compute the Kubo Hall conductivity for disorderd two-dimensional systems reaching tens of millions of orbitals, and realistic values of the applied external magnetic fields (as low as a few Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity ${\sigma }_{xy}$ using a wave-packet propagation method and a continued fraction expansion for the computation of diagonal and off-diagonal matrix elements of the Green functions. The validity of the method is demonstrated by comparison of results with brute-force diagonalization of the Kubo formula, using (disordered) graphene as the system of study. This approach to mesoscopic system sizes is opening an unprecedented perspective for so-called reverse engineering in which the available experimental transport data are used to get a deeper understanding of the microscopic structure of the samples. Besides, this will not only allow addressing subtle issues in terms of resistance standardization of large-scale materials (such as wafer scale polycrystalline graphene), but will also enable the discovery of new quantum transport phenomena in complex two-dimensional materials, out of reach with classical methods.

We present a comprehensive study of the band bending and alignment at the interface of $\gamma \text{−}{\text{Al}}_2{\text{O}}_3/{\text{SrTiO}}_3$ heterostructures by hard x-ray photoelectron spectroscopy. Our measurements find no signs for a potential gradient within the polar $\gamma \text{−}{\text{Al}}_2{\text{O}}_3$ film as predicted by the basic electronic reconstruction scenario. We present evidence for a band bending on the ${\text{SrTiO}}_3$ side of the interface, yielding a roughly 600 meV deep potential trough, which reaches below the chemical potential and has a spatial expansion of 3–5 unit cells. The band offset between the bulk valence bands is determined to be also approximately 600 meV, corresponding to aligned bands at the interface. Finally, the spatial confinement of the interfacial two-dimensional electron system is derived from the chemically shifted ${\text{Ti}}^{3+}$ photoemission signal in the Ti $2p$ core level spectra, measured at various photoelectron detection angles. It is found to be in excellent agreement with the spatial depth of the potential trough.

The decoration of graphene samples with adatoms or nanoparticles leads to the enhancement of spin-orbit interactions as well as to the introduction of symmetry-breaking effects that could have drastic effects on spin and electronic transport phenomena. We present an analysis based on symmetry considerations and examine the impact on the scattering matrix for graphene systems containing defects that enhance spin-orbit interactions, while conserving the electronic total angular momentum. We show that the appearance and dominance of skew scattering, and the related observation of valley and/or spin Hall effect in decorated graphene samples, suggests the set of symmetries that adatom perturbations should satisfy. We further show that detailed measurements of the transport and elastic times as a function of carrier concentration make it possible to not only extract the strength of the spin-orbit interaction, as suggested before, but also obtain the amplitude of the symmetry-breaking terms introduced. To examine how different terms would affect measurements, we also present calculations for typical random distributions of impurities with different perturbations, illustrating the detailed energy dependence of different observables.

A theoretical description of the measured differential conductance through magnetic atoms on a $\mathrm{C}{\mathrm{u}}_2\mathrm{N}/\mathrm{Cu}(100)$ substrate is presented [Otte et al., Phys. Rev. Lett. 103, 107203 (2009)]. In particular, we analyze the case of a weakly coupled Co/ Fe dimer. The starting point of our model is an ionic Hamiltonian which describes the inelastic electron tunneling excitations and the Kondo resonances as due to atomic spin fluctuations associated with electron cotunneling processes in the tip-atom-surface system. The interaction terms of this Hamiltonian are written in the basis set of the eigenstates of the atomic part, which in the present case includes the crystalline and Zeeman fields and also, a Heisenberg exchange coupling between the two spins. The appropriate Green's functions that define the differential conductance spectra are calculated by means of the equation-of-motion method. We obtain, in this form, a very satisfactory description of the overall experimental findings related to the behavior of the spin state excitations and Kondo resonance structures in the presence of an applied magnetic field.

Co-based Heusler compounds are ferromagnetic with a high Curie temperature and a large magnetization density, and thus are promising for spintronic applications. In this paper, we perform a systematic ab initio study of two principal spin-related phenomena, namely, anomalous Hall effect and current spin polarization, in ${\mathrm{Co}}_2$-based Heusler compounds ${\mathrm{Co}}_2\mathrm{Fe}X$ ($X=\mathrm{Al}, \mathrm{Ga}, \mathrm{In}, \mathrm{Si}, \mathrm{Ge}, \mathrm{Sn}$) in the cubic ${\mathrm{L}2}_1$ structure within the density functional theory with the generalized gradient approximation (GGA). The accurate all-electron full-potential linearized augmented plane-wave method is used. First, we find that the spin polarization of the longitudinal current ($P^L$) in ${\mathrm{Co}}_2\mathrm{Fe}X$ ($X=\mathrm{Al}, \mathrm{Ga}, \mathrm{In}, {\mathrm{Al}}_{0.5}{\mathrm{Si}}_{0.5}, \text{and} \mathrm{Sn}$) is $\sim 100$% even though that of the electronic states at the Fermi level ($P^D$) is not. Further, the other compounds also have a high current spin polarization with $P^L>85$%. This indicates that all the ${\mathrm{Co}}_2\mathrm{Fe}X$ compounds considered are promising for spin-transport devices. Interestingly, $P^D$ is negative in ${\mathrm{Co}}_2\mathrm{Fe}X$ ($X=\mathrm{Si}, \mathrm{Ge}, \text{and} \mathrm{Sn}$), differing in sign from the $P^L$ as well as that from the transport experiments. Second, the calculated anomalous Hall conductivities (AHCs) are moderate, being within 200 S/cm, and agree well with the available experiments on a highly ${\mathrm{L}2}_1$ ordered ${\mathrm{Co}}_2\mathrm{FeSi}$ specimen although they differ significantly from the reported experiments on other compounds where the B2 antisite disorders were present. Surprisingly, the AHC in ${\mathrm{Co}}_2\mathrm{FeSi}$ decreases and then changes sign when Si is replaced by Ge and finally by Sn. Third, the calculated total magnetic moments agree well with the corresponding experimental ones in all the studied compounds except ${\mathrm{Co}}_2\mathrm{FeSi}$ where a difference of 0.3 ${\mu }_B/\mathrm{f}.\mathrm{u}.$ exists. We also perform the GGA plus on-site Coulomb interaction $U$ calculations in the $\mathrm{GGA}+U$ scheme. We find that including the $U$ affects the calculated total magnetic moment, spin polarization and AHC significantly, and in most cases, unfortunately, results in a disagreement with the available experimental results. All these interesting findings are discussed in terms of the underlying band structures.

Here we study orthorhombic $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3\left(\mathrm{LCMO}\right)$ where $x=0.2,0.25$, and $0.3$ measuring the magnetization, the magneto-Seebeck, and magnetovolume contraction as a function of temperature and magnetic field to examine the influence of lattice, spin, and electronic degrees of freedom. Making the assumption that these contributions are independent of each other allows us to isolate the purely magnetic contribution to the total entropy change. This is compared directly with the value predicted from a mean-field plus Bean-Rodbell model. The model allows the first order character of the transition to be determined, as well as assigning numeric values to the number of magnetic ions that make up preformed magnetic clusters, and to the magnitude of the spread in $T_C$. We quantify how the clusters, the $T_C$ spread values, and the electronic contribution, impact deleteriously on the available entropy change in fixed magnetic field, as the first order nature of the transition strengthens.

It is crucially important to investigate the effects of temperature on magnetic properties such as critical phenomena, nucleation, pinning, domain wall motion, and coercivity. The Landau-Lifshitz-Gilbert (LLG) equation has been applied extensively to study dynamics of magnetic properties. Approaches of Langevin noises have been developed to introduce the temperature effect into the LLG equation. To have the thermal equilibrium state (canonical distribution) as the steady state, the system parameters must satisfy some condition known as the fluctuation-dissipation relation. In inhomogeneous magnetic systems in which spin magnitudes are different at sites, the condition requires that the ratio between the amplitude of the random noise and the damping parameter depend on the magnitude of the magnetic moment at each site. Focused on inhomogeneous magnetic systems, we systematically showed agreement between the stationary state of the stochastic LLG equation and the corresponding equilibrium state obtained by Monte Carlo simulations in various magnetic systems including dipole-dipole interactions. We demonstrated how violations of the condition result in deviations from the true equilibrium state. We also studied the characteristic features of the dynamics depending on the choice of the parameter set. All the parameter sets satisfying the condition realize the same stationary state (equilibrium state). In contrast, different choices of parameter set cause seriously different relaxation processes. We show two relaxation types, i.e., magnetization reversals with uniform rotation and with nucleation.

Bose-Einstein condensates of exciton-polaritons are described by a Schrödinger system of two equations. Nonlinearity due to exciton interactions gives rise to a frequency band of dark soliton solutions, which are found analytically for the lossless zero-velocity case. The soliton's far-field value varies from zero to infinity as the operating frequency varies across the band. For positive detuning (photon frequency higher than exciton frequency), the exciton wave function becomes discontinuous when the operating frequency exceeds the exciton frequency. This phenomenon lies outside the parameter regime of validity of the Gross-Pitaevskii (GP) model. Within its regime of validity, we give a derivation of a single-mode GP model from the initial Schrödinger system and compare the continuous polariton solitons and GP solitons using the healing length notion.

We uncover four spin-charge ordered ground states in the strong coupling limit of the Kondo lattice model on triangular geometry. The results are obtained using Monte Carlo simulations, with a classical treatment of localized moments. Two of the states at one-third electronic filling ($n=1/3$) consist of decorated ferromagnetic chains coupled antiferromagnetically with the neighboring chains. The third magnetic ground state is noncollinear, consisting of antiferromagnetic chains separated by a pair of canted ferromagnetic chains. An even more unusual magnetic ground state, a variant of the ${120}^{\circ }$ Yafet-Kittel phase, is discovered at $n=2/3$. These magnetic orders are stabilized by opening a gap in the electronic spectrum: a “band effect.” All the phases support modulations in the electronic charge density due to the presence of magnetically inequivalent sites. In particular, the charge ordering pattern found at $n=2/3$ is observed in various triangular lattice systems, such as $2H-{\mathrm{AgNiO}}_2$, $3R-{\mathrm{AgNiO}}_2$, and ${\mathrm{Na}}_x{\mathrm{CoO}}_2$.

Density functional theory calculations of the vibrational modes of ${\mathrm{Bi}}_{12}{\mathrm{SiO}}_{20}$ are presented, with a detailed assignment and geometry investigation of the vibrational modes in the sillenite structure. We show that the symmetry and geometry of the strong Raman peaks allow direct probing of the Bi-O(1) and Bi-O(2) bonds. The physical significance of the vibrational modes is considered to discuss experimental data on the trivalent sillenites and show evidence of O(1) and/or O(2) vacancies in the Bi-O framework. The infrared modes are also discussed to motivate future systematic studies of sillenites. The Born effective charges of sillenites are studied and the results show the existence of large and anisotropic charges. The magnitude of the LO-TO splits is also calculated and compared with experiment.

Orbital-density-dependent functionals, such as the Perdew-Zunger or the Koopmans-compliant functionals, are used to remove unphysical self-interaction energies and to restore missing piece-wise linearity in approximate formulations of density-functional theory (DFT). At variance with functionals of the total density, orbital-density-dependent functionals are typically not invariant with respect to unitary transformations of the occupied states. Such additional degrees of freedom require an extension of established approaches for direct minimization that preserve their numerical robustness and efficiency, and make it possible to apply these advanced electronic-structure functionals to large or complex systems. In this work we adapt the ensemble-DFT algorithm [N. Marzari, D. Vanderbilt, and M. C. Payne, Phys. Rev. Lett. 79, 1337 (1997)] to the case of orbital-density-dependent functionals, partitioning the variational problem into a nested loop of (i) minimizations with respect to unitary transformations at a fixed orbital manifold, that lead to a projected, unitary-covariant functional of the orbitals only that enforces the Pederson condition and (ii) variational optimization of the orbital manifold for this projected functional. We discuss in detail both general and functional-dependent trends and suggest a procedure to efficiently exploit the combination of the different minimization strategies. The overall formulation allows for a stable, robust, and efficient algorithm, yielding great improvements over conventional techniques.

In this study, the magnetic ground state of iridium perovskites $({\text{Sr}}_{n+1}{\text{Ir}}_n{\text{O}}_{3n+1}$, where $n=1$, 2, and $\infty )$ was considered using muon spin spectroscopy $\left(\mu \text{SR}\right)$. When probed by muons in ${\text{Sr}}_2{\text{IrO}}_4$ and ${\text{Sr}}_3{\text{Ir}}_2{\text{O}}_7$ $(n=1,2)$, the internal field $\left(B_{\mathrm{loc}}\right)$ showed clear signs of a magnetic order in two stages at transition temperatures $T_{\mathrm{N}}\simeq 230\text{K}$ and $T_{\mathrm{m}}\simeq 90\text{K}$ in ${\text{Sr}}_2{\text{IrO}}_4$ and $T_{\mathrm{N}}\simeq 280\text{K}$ and $T_{\mathrm{m}}\simeq 70\text{K}$ in ${\text{Sr}}_3{\text{Ir}}_2{\text{O}}_7$. In contrast, no long-range magnetic order was observed in orthorhombic ${\text{SrIrO}}_3$ $(n=\infty )$. Based on the known magnetic structure in ${\text{Sr}}_2{\text{IrO}}_4$ and ${\text{Sr}}_3{\text{Ir}}_2{\text{O}}_7$, we successfully identified muon sites in these compounds from the magnitude of $B_{\mathrm{loc}}$ in the first stage $(T_{\mathrm{m}}\le T\le T_{\mathrm{N}})$. Below $T_{\mathrm{m}},B_{\mathrm{loc}}$ probed by a fraction of muons occupying sites near the apical oxygen of ${\text{IrO}}_6$ octahedra exhibited a further increase but remained mostly unchanged for sites close to the in-plane oxygen. While such behavior cannot be explained by the alteration of the Ir spin structure, it is consistent with the selective appearance of ordered magnetic moments on the apical oxygen. The oxygen polarization was also in line with the reported magnetization anomalies in these compounds below $\sim T_{\mathrm{m}}$. A possible link between the oxygen polarization and ferroelectric (multiferroic) behavior in ${\text{Sr}}_2{\text{IrO}}_4$ was considered.

In the presence of random hyperfine fields, the noise spectrum, $\langle \delta s_{\omega }^2\rangle$, of a spin ensemble represents a narrow peak centered at $\omega =0$ and a broad wing reflecting the distribution of the hyperfine fields. In the presence of an ac drive, the dynamics of a single spin acquires additional harmonics at frequencies determined by both the drive frequency and the local field. These harmonics are reflected as additional peaks in the noise spectrum. We study how the ensemble-averaged $\langle \delta s_{\omega }^2\rangle$ evolves with the drive amplitude, ${\omega }_{dr}$ (in the frequency units). Our main finding is that additional peaks in the spectrum, caused by the drive, remain sharp even when ${\omega }_{dr}$ is much smaller than the typical hyperfine field. The reason is that the drive affects only the spins for which the local Larmour frequency is close to the drive frequency. The shape of the low-frequency Rabi-peak in $\langle \delta s_{\omega }^2\rangle$ is universal with both the position and the width, being of the order of ${\omega }_{dr}$. When the drive amplitude exceeds the width of the hyperfine field distribution, the noise spectrum transforms into a set of sharp peaks centered at harmonics of the drive frequency.

It is well known that carbon nanotubes (CNTs) possess ultrahigh thermal conductivity that is comparable to bulk diamond. However, no research has studied the possible low thermal conductivity of different CNTs so far. By performing nonequilibrium molecular dynamic simulations, we reveal that the perfect graphyne nanotube (GNT) exhibits an unprecedentedly low thermal conductivity (below 10 W/mK at room temperature), which is generally two orders of magnitude lower than that of ordinary CNTs and even lower than the values reported for defected, doped, and chemically functionalized CNTs. By performing phonon polarization and spectral energy density analysis, we observe that the ultralow thermal conductivity stems from the unique atomic structure of the GNT, consisting of the weak acetylenic linkage ($sp$ C-C bonds) and the strong hexagonal ring ($s{p}^2$ C-C bonds), which results in a large vibrational mismatch between these two components, and thus induces significantly inefficient heat transfer. Moreover, the thermal transport in GNT with a large number of acetylenic linkages is dominated by the low frequency longitudinal modes in the linkage. Such strong confinement of the low frequency thermal energy results in the extremely low thermal conductivity due to the flattened phonon dispersion curves (low phonon group velocities). The exploration of the abnormal thermal transport of GNTs paves the way for design and application of the relevant devices that could benefit from the ultralow thermal conductivity, such as thermoelectrics for energy conversion.