Silicon carbide and other group-IV binary materials with 1:1 stoichiometry are trivial semiconductors. Using first-principles calculations combined with a tight-binding model, we demonstrate two-dimensional (2D) silicon carbide materials (siligraphenes) with 1:3 and 3:1 stoichiometries are topological insulators (TIs) superior to graphene. Dirac cones and topologically nontrivial electronic structures are predicted in these binary honeycomb lattices. The band gaps opened at the Dirac points due to the spin-orbital coupling (SOC) are several orders of magnitude larger than the graphene value. Such interesting properties also exist in their analogs of other group-IV elements, such as g-$\mathrm{Ge}{\mathrm{C}}_3$, g-${\mathrm{Ge}}_3\mathrm{C}$, g-$\mathrm{Ge}{\mathrm{Si}}_3$, and g-${\mathrm{Ge}}_3\mathrm{Si}$. The TI states predicted in these unique lattices broaden the application field of group-IV binary materials and open the door to searching for 2D TIs with enhanced SOC.

Using one- and two-dimensional random arrays of coupled metallic nanowires as a generic example of disordered plasmonic systems, we demonstrate that the structural disorder induces localization of light in these nanostructures at a deep-subwavelength scale. The ab initio analysis is based on solving the complete set of three-dimensional Maxwell equations. We find that random variations of the radius of coupled plasmonic nanowires are sufficient to induce the Anderson localization (AL) of surface plasmon polaritons (SPPs), the size of these trapped modes being significantly smaller than the optical wavelength. Remarkably, the optical-gain coefficient, needed to compensate for losses in the plasmonic components of the system, is much smaller than the loss coefficient of the metal, which is obviously beneficial for the realization of the AL in plasmonic nanostructures. The dynamics of excitation and propagation of the Anderson-localized SPPs are addressed too.

We report on hybrid Monte Carlo simulations of the tight-binding model with long-range Coulomb interactions for the electronic properties of graphene. We investigate the spontaneous breaking of sublattice symmetry corresponding to a transition from the semimetal to an antiferromagnetic insulating phase. Our short-range interactions thereby include the partial screening due to electrons in higher energy states from ab initio calculations based on the constrained random phase approximation [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)]. In contrast to a similar previous Monte Carlo study [M. V. Ulybyshev et al., Phys. Rev. Lett. 111, 056801 (2013)], we also include a phenomenological model that describes the transition to the unscreened bare Coulomb interactions of graphene at half-filling in the long-wavelength limit. Our results show, however, that the critical coupling for the antiferromagnetic Mott transition is largely insensitive to the strength of these long-range Coulomb tails. They hence confirm the prediction that suspended graphene remains in the semimetal phase when a realistic static screening of the Coulomb interactions is included.

By means of first-principles $GW$ calculations, we have studied the electronic structure properties of $M{X}_2$ ($M=\text{Mo}$, W; $X=\text{S}$, Se, Te) bilayers, including hybrid structures of $M{X}_2$ building blocks. The effect of spin-orbit coupling on the electronic structure and the effect of van der Waals interaction on the geometry were taken into account. All the homogeneous bilayers are identified as indirect band-gap materials, with an increase of the band gap when Mo is changed to W, and a decrease of the band gap when the atomic number of $X$ is increased. The same behavior is also observed for hybrid bilayers with common chalcogen atoms, while bilayers with common metal atoms have a direct band gap. Finally, it is shown that due to their particular band alignment, some heterobilayers enable electron-hole separation, which is of interest for solar cell applications.

We develop an effective model to describe the statistical properties of exciton fine structure splitting (FSS) and polarization angle in quantum dot ensembles (QDEs) using only a few symmetry-related parameters. The connection between the effective model and the random matrix theory is established. Such effective model is verified both theoretically and experimentally using several rather different types of QDEs, each of which contains hundreds to thousands of QDs. The model naturally addresses three fundamental issues regarding the FSS and polarization angels of QDEs, which are frequently encountered in both theories and experiments. The answers to these fundamental questions yield an approach to characterize the optical properties of QDEs. Potential applications of the effective model are also discussed.

Organic photovoltaic cells have improved in efficiency from 1% two decades ago to over 10% today. Continued improvement necessitates a theoretical understanding of the factors determining efficiency. Organic photovoltaic efficiency can be parameterized in terms of open-circuit voltage, short-circuit current, and fill factor. Here we present a theory that explains the dependencies of open-circuit voltage on semiconductor energy levels, light intensity, solar cell and light-source temperatures, charge-carrier recombination, and external fluorescence efficiency. The present theory also explains why recombination at the donor-acceptor heterointerface is a dominant process in heterojunction-based cells. Furthermore, the Carnot efficiency appears, highlighting the connection to basic thermodynamics. The theory presented here is consistent with and builds on the experimental and theoretical observations already in the literature. Crucially, the present theory can be straightforwardly derived in a line-by-line fashion using standard tools from statistical physics.

We investigate quasi-one-dimensional atomic chains on Si(553)-Au with a scanning tunneling microscope (STM). The observed periodicity at the Si step edge can be altered by the STM and depends on the magnitude of the tunneling current. In a recent report this reversible structural transition was attributed to transient doping with a characteristic time scale of a few milliseconds [S. Polei et al., Phys. Rev. Lett. 111, 156801 (2013)]. Here we explore the evolution of the STM topography as a function of the magnitude of the tunneling current for a wide temperature range. Based on a decomposition of topographic line profiles and a detailed Fourier analysis we conclude that all observed current-dependent STM topographies can be explained by a time-averaged linear combination of two fluctuating step-edge structures. These data also reveal the precise relative alignment of the characteristic STM features for both phases along the step edges. A simple diagram is developed, presenting the relative contribution of these phases to the STM topography as a function of tunneling current and temperature. Time- and current-dependent measurements of fluctuations in the tunneling current reveal two different transition regimes that are related to two specific current injection locations within the surface unit cell. A method based on spatially resolved $I$($z$) curves is presented that enables a quantitative analysis of contributing phases.

Exploiting the two-point measurement statistics, we propose a quantum measurement scheme of current with limited resolution of electron counting. Our scheme is equivalent to the full counting statistics in the long-time measurement with the ideal resolution, but is theoretically extended to take into account the resolution limit of actual measurement devices. Applying our scheme to a resonant level model, we show that the limited resolution of current measurement gives rise to a positive excess noise, which leads to a deviation from the Johnson-Nyquist relation. The deviation exhibits universal single-parameter scaling with the scaling variable $Q\equiv S_{\mathrm{M}}/S_0$, which represents the degree of the insufficiency of the resolution. Here, $S_0$ is the intrinsic noise, and $S_{\mathrm{M}}$ is the positive quantity that has the same dimension as $S_0$ and is defined solely by the measurement scheme. For the lack of the ideal resolution, the deviation emerges for $Q<1$ as $2exp[-{\left(2\pi \right)}^2/Q]$ having an essential singularity at $Q=0$, which followed by the square root dependence $\sqrt{Q/4\pi }$ for $Q\gg 1$. Our findings offer an explanation for the anomalous enhancement of noise temperature observed in Johnson noise thermometry.

Glass surfaces were patterned by milling periodic trench structures with wavelengths from 150 to 750 nm in a focused ion-beam (FIB) system. Upon exposure to 30 keV Ga${}^{+}$ ion irradiation under an incidence angle of 52° with respect to the surface normal, those patterns were found to transform into “ripple”-like nanostructures. Their evolution was monitored in situ for ion fluence from 5 × 10${}^{16}$ to 2.4 × 10${}^{18}$ Ga${}^{+}$ ions/cm${}^2$ using the scanning electron microscope incorporated in the FIB. With increasing fluence, the wavelengths of the ripples remain constant (and equal to their original feature size) while they propagate across the surface, in a direction which coincides with the projection of the ion beam's incident direction onto the surface. The propagation velocities $v$, being in the range (60−100) nm/(10${}^{17}$ ions cm${}^{-2}$), were determined to be inversely proportional to the wavelengths λ, with $v$ = $A$ + $B$/${\lambda }^2$ (the constants $A$ and $B$ derive from various ion beam and target material parameters). Such a dependence is in accordance with the predictions of a theoretical two-field continuum model of ripple formation which treats the surface as being composed of a thin layer of mobile species on the substrate. A quantitative agreement of the experimental findings with this model is obtained if the presence of a viscous flow is effective in that thin surface layer. Ex situ atomic force microscopy was employed to derive the final crest-to-valley amplitudes of the ripple structures; their values were ∼50−100 nm and thus similar to the depth of the original trenches. On the pristine surface areas (which have not been patterned), ripples were also formed by ion bombardment. Their wavelength increases with ion fluence Φ from ∼250 to ∼420 nm, following a dependence λ ∝ ${\Phi }^{0.19}$.

We have studied the nucleation and growth of Au clusters at submonolayer and greater coverages on the $h$-BN nanomesh grown on Rh(111) by means of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). STM reveals that submonolayer Au deposited at 115 K nucleates within the nanomesh pores and remains confined to the pores even after warming to room temperature. Whereas there is a propensity of monoatomic high islands at low temperature, upon annealing, bi- and multilayer Au clusters emerge. Deposition of higher coverages of Au similarly results in Au clusters primarily confined to the nanomesh pores at room temperature. XPS analysis of core-level electronic states in the deposited Au shows strong final-state effects induced by restricted particle size dominating for low Au coverage, with indications that larger Au clusters are negatively charged by interaction through the $h$-BN monolayer. DFT calculations suggest that the structure of the Au clusters transitions from monolayer to bilayer at a size between 30 and 37 atoms per cluster, in line with our experiment. Bader charge analysis supports the negative charge state of deposited Au.

We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics with the temperature-dependent effective potential method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free energy for the paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium nitride. The influence of lattice dynamics on the transition between the two phases is demonstrated by constructing the temperature-pressure phase diagram.

We compute damping of mechanical oscillations of a cantilever that contains flipping paramagnetic spins. This kind of damping is mandated by the dynamics of the total angular momentum, $\text{spin}+\text{mechanical}$, and it does not require an external magnetic field. Rigorous expression for the damping rate is derived in terms of the imaginary part of the magnetic susceptibility. The effect of spins on the quality factor of the cantilever can be significant in cantilevers of small length that have large concentration of paramagnetic spins of atomic and/or nuclear origin.

We fabricated a perpendicularly magnetized bit pattern media using a hexagonally close-packed auto-assembled anodic alumina template with 100 nm and 50 nm periods by depositing a Co/Pt multilayer to form an ordered array of ferromagnetic nanodots, so-called nanobumps. We used Hall resistance measurements and magnetic force microscopy to characterize the dot-by-dot magnetization reversal mechanism under applied field. The role of interdot exchange coupling and dipolar coupling are investigated. Then we focus on separating the various origins of switching field distribution (SFD) in this system, namely dipolar interactions, intrinsic anisotropy distribution, and template packing faults. Finally we discuss the influence of triangular dipolar frustrations on the energy stability of demagnetized and half-switched states based on an Ising model, including local exchange coupling. The impact of SFD and lattice defects lines between misoriented ordered domains on the magnetic configurations is studied in detail.

We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system ${\text{SeCuO}}_3$ using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetometry agrees reasonably well with the rotation of the $\mathbf{g}$ tensor above $T\gtrsim 50$ K. Below 50 K, the $\mathbf{g}$ tensor is temperature independent, while macroscopic magnetic axes continue to rotate. Additionally, the susceptibility anisotropy has a temperature dependence which can not be reconciled with the isotropic Heisenberg model of interactions between spins. ESR linewidth analysis shows that anisotropic exchange interaction must be present in ${\text{SeCuO}}_3$. These findings strongly support the presence of anisotropic exchange interactions in the Hamiltonian of the studied system. Below $T_N=8$ K, the system enters a long-range antiferromagnetically ordered state with easy axis along the ${\langle \overline{1}01\rangle }^{*}$ direction. Small but significant rotation of magnetic axes is also observed in the antiferromagnetically ordered state suggesting strong spin-lattice coupling in this system.

We report a study of the lattice dynamics in superconducting NaFeAs ($T_c$ = 8 K) and doped ${\text{NaFe}}_{0.97}$${\text{Co}}_{0.03}$As ($T_c$ = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on isostructural and isoelectronic LiFeAs, anomalous broadening of $E_g$(As) and $A_{1g}$(Na) modes upon cooling is observed in both samples. In addition, in the Co-doped sample, a superconductivity-induced renormalization of the frequency and linewidth of the $B_{1g}$(Fe) vibration is observed. This renormalization cannot be understood within a single band and simple multiband approaches. A theoretical model that includes the effects of spin-density wave correlations along with sign-changing $s$-wave pairing state and interband scattering has been developed to explain the observed behavior of the $B_{1g}$(Fe) mode.

Symmetry protected topological (SPT) phases are a minimal generalization of the concept of topological insulators to interacting systems. In this paper, we describe the classification and properties of such phases for three-dimensional (3D) electronic systems with a number of different symmetries. For symmetries representative of all classes in the famous 10-fold way of free-fermion topological insulators/superconductors, we determine the stability to interactions. By combining with results on bosonic SPT phases, we obtain a classification of electronic 3D SPT phases for these symmetries. In cases with a normal $U\left(1\right)$ subgroup we show that this classification is complete. We describe the nontrivial surface and bulk properties of these states. In particular, we discuss interesting correlated surface states that are not captured in a free-fermion description. We show that in many, but not all, cases, the surface can be gapped while preserving symmetry if it develops intrinsic topological order.

The block entanglement entropy and fluctuations are investigated in one dimension in finite-size correlated electron systems using the Gutzwiller wave function as a prototype correlated electron state. Entanglement entropy shows logarithmic divergence for all values of the correlation projection parameter $g$, as predicted by conformal field theories for critical systems, but the central charge requires finite-size corrections. There is an infinite correlation length corresponding to correlation between the same kinds of spins, for all values of $g$. A scaling form for the block entropy, as a function of $g$ and the system size $N$, is proposed which predicts a metal-insulator crossover at $N^{1/3}g\approx 0.24$ for $N<{10}^5$. Bipartite fluctuations in the number of particles in a block, and the spin fluctuations also obey an approximate scaling. A relation is found between the block entropy and the bipartite spin fluctuations. Our results show some correspondence with an experiment on Ni nanochains.

Electron-holes, phonons, and plasmons come in close proximity to each other in the low-energy range of the excitation spectrum of two-dimensional (2D) crystals, breaking the validity of the weakly interacting-quasiparticles picture. By including the lattice oscillations into the scheme of time-dependent density-functional theory, we open a pathway to the ab initio treatment of the coupled low-energy excitations in 2D crystals. With the use of graphene as an important test system, we find the strong coupling of the elementary excitations, giving rise to new hybrid collective modes. The total (including both the electronic and ionic response) dielectric function ${\varepsilon }_{\text{tot}}\left(\omega \right)$ is constructed and the picture of the low-energy excitation spectrum of graphene is redrawn.