We investigate charge transport of $pn$ and $npn$ junctions made from silicene, a Si analogue of graphene. The conductance shows the distinct gate-voltage dependencies peculiar to the topological and nontopological phases, where the topological phase transition is caused by external electric field. Namely, the conductance is (not) suppressed in the $np$ ($nn$) regime when both sides are topological, and in the $nn$ ($np$) regime when one side is topological and the other side is nontopological. Furthermore, we find that the conductance is almost quantized to be 0, 1, and 2. Our findings will open a new way to nanoelectronics based on silicene.

We investigate theoretically the switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes using self-interaction corrected density-functional theory combined with the nonequilibrium Green's-function method for quantum transport. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond and reduces by about one order of magnitude the transmission coefficient at the Fermi level, and thus the linear response conductance. Furthermore, protonation downshifts in energy the position of the highest occupied molecular orbital, so that the current of the protonated species is lower than that of the unprotonated one along the entire bias range investigated, from $-1.5$ to 1.5 V. A second protonation at the opposite thiol group has only minor effects and no further drastic reduction in transmission takes place. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation.

We calculate the single-particle spectral function for doped bilayer graphene in the low energy limit, described by two parabolic bands with zero band gap and long range Coulomb interaction. Calculations are done using thermal Green's functions in both the random phase approximation (RPA) and the fully self-consistent GW approximation. Consistent with previous studies RPA yields a spectral function which, apart from the Landau quasiparticle peaks, shows additional coherent features interpreted as plasmarons, i.e., composite electron-plasmon excitations. In the GW approximation the plasmaron becomes incoherent and peaks are replaced by much broader features. The deviation of the quasiparticle weight and mass renormalization from their noninteracting values is small which indicates that bilayer graphene is a weakly interacting system. The electron energy loss function, $\mathrm{Im}[-{\epsilon }_q^{-1}\left(\omega \right)]$ shows a sharp plasmon mode in RPA which in the GW approximation becomes less coherent and thus consistent with the weaker plasmaron features in the corresponding single-particle spectral function.

We present the ab initio calculation of the electronic structure and magnetic properties of Bi${}_2$Fe${}_4$O${}_9$. This compound crystallizes in the orthorhombic crystal structure with the Fe${}^{3+}$ ions forming the Cairo pentagonal lattice implying strong geometric frustration. The neutron diffraction measurements reveal nearly orthogonal magnetic configuration, which at first sight is rather unexpected since it does not minimize the total energy of the pair of magnetic ions coupled by the Heisenberg exchange interaction. Here, we calculate the electronic structure and exchange integrals of Bi${}_2$Fe${}_4$O${}_9$ within the $\mathrm{LSDA}+U$ method. We obtain three different in-plane ($J_3=36$ K, $J_4=73$ K, $J_5=23$ K) and two interplane ($J_1=10$ K, $J_2=12$ K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi${}_2$Fe${}_4$O${}_9$ needs a more complicated model than the ideal Cairo pentagonal lattice with only two exchange parameters in the plane. However, if one takes into account only the two largest exchange integrals, then according to the ratio $x\equiv J_3/J_4=0.49<\sqrt{2}$ [a critical parameter for the ideal Cairo pentagonal lattice, see Rousochatzakis, Läuchli, and Moessner, Phys. Rev. B 85, 104415 (2012)] the ground state should be the orthogonal magnetic configuration in agreement with experiment. The microscopic origin of different exchange interactions is also discussed.

An analytical perturbative approach is presented to calculate the out-of-plane precessional motion of the magnetization of an in-plane free layer submitted to the spin torque of a perpendicular polarizer in the macrospin model. We consider the effects of the uniaxial anisotropy field, of an in-plane applied field, and of the perturbation due to the in-plane reference layer on the otherwise circular and monochromatic oscillations of the free layer. We calculate the frequency change due to these perturbations, the amplitude of the second harmonics, and the critical current for the existence of oscillations. This approach is rather general in the treatment of harmonics in spin torque oscillators.

We study the Rényi mutual information of classical systems characterized by a transfer matrix. We first establish a general relationship between the Rényi mutual information of such classical mixtures of configuration states and the Rényi entropy of a corresponding Rokhsar-Kivelson–type quantum superposition. We then focus on chiral and nonchiral kagome-ice systems, classical spin liquids on the kagome lattice, which respectively have critical and short-range correlations. Through a mapping of the chiral kagome ice to the quantum Liftshitz critical field theory, we predict a universal subleading term in the Rényi mutual information of this classical spin liquid, which can be realized in the pyrochlore spin ice in a magnetic field. We verify our prediction with direct numerical transfer-matrix computations, and further demonstrate that the nonchiral kagome ice (and the corresponding quantum Rokhsar-Kivelson superposition) is a topologically trivial phase. Finally, we argue that the universal term in the mutual information of the chiral kagome ice is fragile against the presence of defects.

Dislocation motion governs the strength and ductility of metals, and the Peierls stress (${\sigma }_p$) quantifies dislocation mobility. ${\sigma }_p$ measurements carry substantial uncertainty in face-centered cubic (fcc) metals, and ${\sigma }_p$ values can differ by up to two orders of magnitude. We perform first-principles simulations based on orbital-free density functional theory (OFDFT) to calculate the most accurate currently possible ${\sigma }_p$ for the motion of $\frac{1}{2}\langle 110\rangle \left\{111\right\}$ dislocations in fcc Al. We predict the ${\sigma }_p$s of screw and edge dislocations (dissociated in their equilibrium state) to be $1.9\times {10}^{-4}G$ and $4.9\times {10}^{-5}G$, respectively ($G$ is the shear modulus). These values fall within the range of measurements from mechanical deformation tests (10${}^{-4}$–10${}^{-5}G$). OFDFT also finds a new metastable structure for a screw dislocation not seen in earlier simulations, in which a dislocation core on the glide plane does not dissociate into partials. The corresponding ${\sigma }_p$ for this undissociated dislocation is predicted to be $1.1\times {10}^{-2}G$, which agrees with typical Bordoni peak measurements (10${}^{-2}$–10${}^{-3}G$). The calculated ${\sigma }_p$s for dissociated and undissociated screw dislocations differ by two orders of magnitude. The presence of undissociated, as well as dissociated, screw dislocations may resolve the decades-long mystery in fcc metals regarding the two orders of magnitude discrepancy in ${\sigma }_p$ measurements.

The pressure effect on exchange bias (EB) and coercive fields, $H_{\mathrm{E}}$ and $H_{\mathrm{C}}$, in phase-separated ferromagnetic/antiferromagnetic (FM/AFM) CaMn${}_{1-x}$Ru${}_x$O${}_3$ manganites with 0.06 ≤ $x$ ≤ 0.15 was studied by magnetization measurements performed in the temperature range 10–200 K and under hydrostatic pressure up to 11 kbar. Both $H_{\mathrm{E}}$ and $H_{\mathrm{C}}$ exhibit intriguing dependence on Ru doping and on applied pressure. It was found that $H_{\mathrm{E}}$ is apparent only at $x$ < 0.1 and decreases progressively with increasing doping, while $H_{\mathrm{C}}$ depends nonmonotonically on Ru content, reaching a maximum at around $x$ ∼ 0.09. The $H_{\mathrm{E}}$ was found to increase strongly under applied pressure within doping range 0.06 ≤ $x$ ≤ 0.1, while $H_{\mathrm{C}}$ exhibits irregular behavior, namely, increases with increasing pressure for $x$ $=$ 0.15, changes nonmonotonically for $x$ $=$ 0.1, decreases for $x$ $=$ 0.08, and is almost invariable for $x$ $=$ 0.06. Complex pressure and Ru-doping effects on $H_{\mathrm{E}}$ and $H_{\mathrm{C}}$ are explained within a model involving size-variable nanoscale FM regions (droplets) embedded in an AFM matrix. The enhancement of EB with increasing pressure is attributed to the reduction in the FM droplet size, evidenced by both pressure dependence of spontaneous FM moment and $H_{\mathrm{E}}$ dependence upon cooling field $H_{\mathrm{cool}}$. The impact of FM droplet size on the EB was further evidenced by the magnetic field effect, which, in contrast to the pressure effect, leads to a growth of the FM droplets. The intricate $H_{\mathrm{C}}$ dependencies on both pressure and Ru content are understandable in a view of the transition from the multidomain state to the single-domain one, induced by droplet size decrease with increasing pressure or with doping lowering. Concisely, owing to the unique mechanism of valence modification, the external pressure appears to be an effective tool for controlling the EB and the coercivity in Ru-doped CaMnO${}_3$ perovskites.

The experimentally established phase diagram of the half-filled Hubbard model features the existence of three distinct finite-temperature regimes, separated by extended crossover regions. A number of crossover lines can be defined to span those regions, which we explore in quantitative detail within the framework of dynamical mean-field theory. Most significantly, the high-temperature crossover between the bad metal and Mott-insulator regimes displays a number of phenomena marking the gradual development of the Mott insulating state. We discuss the quantum critical scaling behavior found in this regime, and propose methods to facilitate its possible experimental observation. We also introduce the concept of quantum Widom lines and present a detailed discussion that highlights its physical meaning when used in the context of quantum-phase transitions.

The electronic states induced by misfit dislocations at zincblende CdTe/CdS, CdS/ZnS, and InP/GaP heterointerfaces and their impacts on the valence band offsets are investigated using first-principles calculations. The (100) and (110) heterointerfaces including perfect edge dislocations with a Burgers vector of $\frac{a}{2}\left[1\overline{1}0\right]$ and a line vector of [001] are considered for each system. Two types of dislocation core structures are found: The “closed” core has four-membered rings only and contains threefold and fivefold coordinated cations, whereas the “open” core has ten-membered rings that involve threefold coordinated atoms. The closed core forms at the (110) interfaces and is energetically more favorable than the open core. The characteristics of dislocation-induced electronic states are heavily dependent on the system and core structure, but all have localized states in the valence and conduction bands. The localized states in the valence band mostly have anion-orbital characteristics, whereas those in the conduction band mainly have cation-orbital characteristics. Some, but not all, dislocation cores also induce electronic states in the gap between the higher of the valence band maxima and the lower of the conduction band minima in the two phases that constitute the interface. The explicit treatment of misfit dislocations changes the valence band offset between the regions at a distance of ∼1 nm or more from the heterointerface by typically ∼0.1 eV.

The layered dichalcogenide MoS${}_2$ has many unique physical properties in low dimensions. Recent experimental Raman spectroscopies report an anomalous blue shift of the in-plane $E_{2g}^1$ mode with decreasing thickness, a trend that is not understood. Here, we combine experimental Raman scattering and theoretical studies to clarify and explain this trend. Special attention is given to understanding the surface effect on Raman frequencies by using a force constants model based on first-principles calculations. Surface effects refer to the larger Mo-S force constants at the surface of thin film MoS${}_2$, which results from a loss of neighbours in adjacent MoS${}_2$ layers. Without surface effects, the frequencies of both out-of-plane $A_{1g}$ and in-plane $E_{2g}^1$ modes decrease with decreasing thickness. However, the $E_{2g}^1$ mode blue shifts while the $A_{1g}$ mode red shifts once the surface effect is included, in agreement with the experiment. Our results show that competition between the thickness effect and the surface effect determines the mechanical properties of two-dimensional MoS${}_2$, which we believe applies to other layered materials.

A key step towards dissipationless transport devices is the quantum anomalous Hall effect, which is characterized by an integer quantized Hall conductance in a ferromagnetic insulator with strong spin-orbit coupling. In this work, the anomalous Hall effect due to the impurity scattering, namely the extrinsic anomalous Hall effect, is studied when the Fermi energy crosses with the topologically nontrivial conduction band of a quantum anomalous Hall system. Two major extrinsic contributions, the side-jump and skew-scattering Hall conductivities, are calculated using the diagrammatic techniques in which both nonmagnetic and magnetic scattering are taken into account simultaneously. The side-jump Hall conductivity changes its sign at a critical sheet carrier density for the nontrivial phase, while it remains sign unchanged for the trivial phase. The critical sheet carrier densities estimated with realistic parameters lie in an experimentally accessible range. The results imply that a quantum anomalous Hall system could be identified in the good-metal regime.

Thin films of topological insulator Bi${}_2$Se${}_3$ were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature $T_C$, resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above $T_C$. Such negative magnetoresistance was only observed for Bi${}_2$Se${}_3$ layers thinner than $t\sim 4$ nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.

We investigated the electronic structure of Na${}_2$IrO${}_3$ using optical spectroscopy, first-principles calculation, and x-ray absorption spectroscopy. We found that the electronic structure of Na${}_2$IrO${}_3$ is mainly determined by anisotropic hopping interactions and spin-orbit coupling. Due to the hopping interaction, the orbital character of the bands near the Fermi level deviates from the spin-orbit coupling-induced $J_{\mathrm{eff}}$ $=$ 1/2 states. Polarization-dependent O 1$s$ x-ray absorption spectroscopy showed that the $J_{\mathrm{eff}}$ $=$ 1/2 state of an Ir atom can be mixed with the $J_{\mathrm{eff}}$ $=$ 3/2 state of the neighboring Ir atom. This result implies that mixing between the $J_{\mathrm{eff}}$ $=$ 1/2 and 3/2 states in the valence state should be carefully considered in proposed exotic states of Na${}_2$IrO${}_3$, such as topological insulator and quantum spin liquid states.

Optical pumping of excited exciton states in semiconductor quantum wells is a tool for the realization of ultracompact terahertz (THz) lasers based on stimulated optical transition between excited ($2p$) and ground ($1s$) exciton states. We show that the probability of two-photon absorption by a $2p$ exciton is strongly dependent on the polarization of both photons. Variation of the threshold power for THz lasing by a factor of 5 is predicted by switching from linear to circular pumping. We calculate the polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency.

We have revised the transmission line model with the purpose of simplifying the electromagnetic characterization of metasurfaces with both electric and magnetic responses. The revised model is an efficient tool for the case when a metasurface is located on top of a finite-thickness substrate or inside a finite-thickness slab. We have derived analytical expressions for the grid series impedance and shunt admittance of a substrated metasurface. The presented theory is illustrated by a numerical example of an array consisting of Mie-resonant silicon nanospheres. We have retrieved individual polarizabilities of a single sphere in a uniform host medium and compared them with the predictions of classical Mie theory. Further, we have studied a more practical case when the array of Si spheres is located on top of a finite-thickness glass substrate.

Electronic, transport, and spin properties of grain boundaries (GBs) are investigated in electrostatically doped graphene at finite electron densities within the Hartree and Hubbard approximations. We demonstrate that depending on the character of the GBs, the states residing on them can have a metallic character with a zero group velocity or can be fully populated losing the ability to carry a current. These states show qualitatively different features in charge accumulation and spin polarization. We also demonstrate that the semiclassical Thomas-Fermi approach provides a satisfactory approximation to the calculated self-consistent potential. The conductance of GBs is reduced due to enhanced backscattering from this potential.

We demonstrate experimentally that magnetism in ZnO, TiO${}_2$, CeO${}_2$, and SnO${}_2$ nanocrystals (NCs) has a fluctuant nature that varies with capping surfactant type and concentration. By developing a forced hydrolysis approach with additional postprocessing for the synthesis and surfactant capping of these NCs, we effectively avoid the influence of size, shape, and magnetic impurities on the magnetic behavior of NCs, thus revealing the systematic influence of the capping surfactants on the NC magnetism. The x-ray photoelectron spectroscopy results and theoretical calculations clearly show that the magnetism fluctuation with surfactant concentration can be attributed to the periodic variation of spins, which arises from the concentration-dependent electron transfer from surfactants to NCs. Our results not only explain the previously reported seemingly irregular magnetism induced by capping surfactants but also provide an effective approach to tune or optimize the NC magnetism.

Different types of self-similar states are found in quasiperiodic systems characterized by topological invariants—the Chern numbers. We show that the topology introduces a competing length in the self-similar band edge states transforming peaks into doublets of size equal to the Chern number. This length intertwines with quasiperiodicity and introduces an intrinsic scale, producing Chern beats and nested regions where the fractal structure becomes smooth. Chern numbers also influence the zero-energy mode that, for quasiperiodic systems, which exhibit exponential localization, is related to the ghost of the Majorana: the remnant of the edge localized topological state that delocalizes at the onset to a topological transition. In superconducting wires, the exponentially decaying profile of the edge localized Majorana modes also encode fingerprints of the Chern states that reside in close proximity to zero energy.

Irradiation with 1.4 GeV ${}^{208}$Pb ions was used to induce artificial disorder in single crystals of iron-arsenide superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ and to study its effects on the temperature-dependent London penetration depth and transport properties. A study was undertaken on overdoped single crystals with $x=0.108$ and $x=0.127$ characterized by notable modulation of the superconducting gap. Irradiation corresponding to the matching fields of $B_{\varphi }=6$ T and 6.5 T with doses $2.22\times {10}^{11}$ $\mathit{\text{d}}$/cm${}^2$ and $2.4\times {10}^{11}$ $\mathit{\text{d}}$/cm${}^2$, respectively, suppresses the superconducting $T_c$ by approximately 0.3 to 1 K. The variation of the low-temperature penetration depth in both pristine and irradiated samples is well described by the power law $\Delta \lambda \left(T\right)=A{T}^n$. Irradiation increases the magnitude of the prefactor $A$ and decreases the exponent $n$, similar to the effect of irradiation in optimally-doped samples. This finding supports universal $s_{\pm }$ pairing in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ compounds for the entire Co doping range.