The Kitaev quantum wire (KQW) model with open boundary possesses two Majorana edge modes. When the local chemical potential on a defect site is much higher than that on other sites and than the hopping energy, the electron hopping is blocked at this site. We show that the existence of such a defect on a closed KQW also gives rise to two low-energy modes, which can simulate the edge modes. The energies of the defect modes vanish to zero as the local chemical potential of the defect increase to infinity. We develop a quantum Langevin equation to study the transport of KQW for both open and closed cases. We find that when the lead is contacted with the site beside the defect, we can observe two splitted peaks around the zero-bias voltage in the differential conductance spectrum, whereas if the lead is contacted with the bulk of the quantum wire far from the the defect or the open edges, we cannot observe any zero-bias peak.

The interdependence between training and magnetization reversal in granular Co-CoO exchange bias (EB) systems prepared by O ion implantation in Co thin films is demonstrated by polarized neutron reflectometry. While high-fluence O-implanted thin films show reduced relative training values and no asymmetry in magnetization reversal (all reversals take place by domain wall nucleation and motion), low-fluence O ion implantation results in an increased relative training and a magnetization reversal asymmetry between the first descending and the first ascending branches. Whereas the untrained decreasing field reversal occurs mainly by domain wall nucleation and motion, traces of a domain rotation contribution are evidenced in the increasing field reversal. This is explained by the evolution of the CoO structure and the contribution of the out-of-plane magnetization with ion implantation. The amount of incorporated O, which determines the threshold between both behaviors, is around 20 at.%. This reveals that the interdependence between training and magnetization reversal is insensitive to the morphology of the constituents (i.e., granular or layered), indicating that this is an intrinsic EB effect, which can be conveniently tailored by the interplay between the intrinsic properties of the investigated materials and ion implantation.

The reversal of spins in a magnetic material as they relax toward equilibrium is accompanied by the release of Zeeman energy, which can lead to accelerated spin relaxation and the formation of a well-defined self-sustained propagating spin-reversal front known as magnetic deflagration. To date, studies of ${\mathrm{Mn}}_{12}$-acetate single crystals have focused mainly on deflagration in large longitudinal magnetic fields, and they found a fully spin-reversed final state. We report a systematic study of the effect of a transverse magnetic field on magnetic deflagration, and we demonstrate that in small longitudinal fields the final state consists of only partially reversed spins. Further, we measured the front speed as a function of applied magnetic field. The theory of magnetic deflagration, together with a modification that takes into account partial spin reversal, fits the transverse field dependence of the front speed but not its dependence on the longitudinal field. The most significant result of this study is the finding of a partially spin-reversed final state, which is evidence that the spins at the deflagration front are also only partially reversed.

We present a combined theoretical and experimental study of the infrared (IR) and electronic absorption spectra in a molecular beam experiment as well as an analysis of spin dynamics in the clusters ${\mathrm{Co}}_3^{+}\text{(}\mathrm{EtOH}\text{)}$ and ${\mathrm{Co}}_3^{+}\text{(}\mathrm{EtOH},{\mathrm{H}}_2\mathrm{O}\text{)}$. The calculated IR and ground-state absorption spectra show very good agreement with experiment. By using high-level quantum chemistry methods, laser-induced ultrafast spin-flip scenarios in these structures are predicted. For the spin flip in ${\mathrm{Co}}_3^{+}\text{(}\mathrm{EtOH}\text{)}$, our investigation indicates a 5 meV tolerance with respect to the laser detuning and a 6.5 meV tolerance with respect to the pulse spectral broadening, which are quite acceptable for the experimental implementation. In addition, we find that with the increase of the laser detuning the fitness of the processes gradually decays to zero on both sides but with different origins. This joint study of the homotrinuclear clusters provides insight into the experimentally observed spectra and optical properties, and steps towards the optical control of molecular magnetism for future spintronics application.

Several first-principles calculations of positron-annihilation characteristics in solids have added gradient corrections to the local-density approximation within the theory by Arponen and Pajanne [Ann. Phys. (NY) 121, 343 (1979)] since this theory systematically overestimates the annihilation rates. As a further remedy, we propose to use gradient corrections for other local-density approximation schemes based on perturbed hypernetted chain and on quantum Monte Carlo results. Our calculations for various metals and semiconductors show that the proposed schemes generally improve the positron lifetimes when they are confronted with experiment. We also compare the resulting positron affinities in solids with data from slow-positron measurements.

Chemical functionalization of graphene holds promise for various applications ranging from nanoelectronics to catalysis, drug delivery, and nanoassembly. In many of these applications, it is critical to assess the rates of electromigration-directed motion of adsorbates along the surface of current-carrying graphene due to the electron wind force. In this paper, we develop an accurate analytical theory of electromigration of bivalent functional groups (epoxide, amine) on graphene. Specifically, we carefully analyze various factors contributing to the electron wind force, such as lattice effects and strong scattering beyond Born approximation, and derive a simple analytical expression for this force. Further, we perform accurate electronic-structure-theory calculations to parametrize the obtained analytical expression. The obtained results can be generalized to different functional groups and adsorbates, e.g., alkali atoms on graphene.

A narrow-band transmission filter is demonstrated near normal incidence that operates through relaxation of supported-mode selection rules and is explained in the context of group theory. We calculated the transverse magnetic and transverse electric dispersion relations of a dielectric grating in the subwavelength and near-wavelength region using finite element modal analysis and determine the modes' corresponding irreducible representations. Coupling to select transverse magnetic modes at normal incidence is optimized to yield broadband high reflectance that acts as the background for the transmission filter. While some modes couple at normal incidence, others are shown to remain inaccessible due to symmetry mismatch. Away from normal incidence, the reduced symmetry relaxes the selection rules, enabling weak coupling between the incident field and these symmetry-protected modes. This weak coupling produces narrow transmission bands within the opaque background. Furthermore, by choosing the plane of incidence to include or exclude the grating periodicity, we show that orthogonal mode sets can independently be selected to couple to the incident light, yielding separate transmission bands. This spectral filtering is experimentally demonstrated with a suspended silicon grating in the infrared spectrum ($7–14\mu \mathrm{m}$), which agrees well with simulated transmittance spectra and modal analysis.

The transition from an indirect to a direct gap semiconductor in unstrained as well as compressively and tensile strained ${\mathrm{Sn}}_x{\mathrm{Ge}}_{1-x}$ alloys is investigated as a function of the Sn content 0 ≤ $x$ ≤ 1 by means of both a very accurate supercell approach and the more approximate virtual crystal approximation (VCA). In the local density approximation we calculate the bowing parameter of the lattice constant of unstrained ${\mathrm{Sn}}_x{\mathrm{Ge}}_{1-x}$ alloys. Provided that pseudopotentials suitable for the VCA are used, the random supercell and VCA approaches yield consistent bowing parameters for the lattice constant of −0.21 and −0.28 Å, respectively, in the entire Sn concentration range. The band structures and energy gaps are calculated using the modified Becke-Johnson potential, which, for Ge, yields a one-electron band gap in very good agreement with experimental data. The crossover from an indirect to a direct gap semiconducting alloy is determined at about 4.5% Sn in unstrained ${\mathrm{Sn}}_x{\mathrm{Ge}}_{1-x}$. When ${\mathrm{Sn}}_x{\mathrm{Ge}}_{1-x}$ is grown commensurately and thus strained on Ge(100), a transition to a direct gap is also observed but at Sn concentrations of about 10%. We finally predict the direct and indirect band gaps as a function of the in-plane lattice constant and Sn concentration for ${\mathrm{Sn}}_x{\mathrm{Ge}}_{1-x}$ alloys grown on (100) substrates.

We study the electric and thermoelectric transport properties of correlated quantum dots coupled to two ferromagnetic leads and one superconducting electrode. Transport through such hybrid devices depends on the interplay of ferromagnetic-contact-induced exchange field, superconducting proximity effect, and correlations leading to the Kondo effect. We consider the limit of large superconducting gap. The system can be then modeled by an effective Hamiltonian with a particle-nonconserving term describing the creation and annihilation of Cooper pairs. By means of the full density-matrix numerical renormalization group method, we analyze the behavior of electrical and thermal conductances, as well as the Seebeck coefficient as a function of temperature, dot level position, and strength of the coupling to the superconductor. We show that the exchange field may be considerably affected by the superconducting proximity effect and is generally a function of Andreev bound-state energies. Increasing the coupling to the superconductor may raise the Kondo temperature and partially restore the exchange-field-split Kondo resonance. The competition between ferromagnetic and superconducting proximity effects is reflected in the corresponding temperature and dot level dependence of both the linear conductance and the (spin) thermopower.

Neutral one-hole, one-electron excitations converging to the $\mathrm{He}1s$ ionization limit of He bilayers on a metal surface are studied with linearly polarized synchrotron radiation and time-of-flight techniques. Comparing signals of emitted electrons, emitted He atoms in metastable excited states, and fluorescence photons as a function of photon energy and polarization with theoretical results, a detailed picture of the photoexcitation is obtained. We show that the width of the photoabsorption peaks is governed in an anisotropic way by the large zero point motion of the He atoms; broadening directly correlates with the shape of the electronic clouds of the absorbing states.

The electrically driven nanowire systems are of great importance to nanoscience and engineering. Due to strong nonlinearity, chaos can arise, but in many applications it is desirable to suppress chaos. The intrinsically high-dimensional nature of the system prevents application of the conventional method of controlling chaos. Remarkably, we find that the phenomenon of coherence resonance, which has been well documented but for low-dimensional chaotic systems, can occur in the nanowire system that mathematically is described by two coupled nonlinear partial differential equations, subject to periodic driving and noise. Especially, we find that, when the nanowire is in either the weakly chaotic or the extensively chaotic regime, an optimal level of noise can significantly enhance the regularity of the oscillations. This result is robust because it holds regardless of whether noise is white or colored, and of whether the stochastic drivings in the two independent directions transverse to the nanowire are correlated or independent of each other. Noise can thus regularize chaotic oscillations through the mechanism of coherence resonance in the nanowire system. More generally, we posit that noise can provide a practical way to harness chaos in nanoscale systems.

We report the metal (0–80 GPa)–semiconductor ($oC$40, 80–120 GPa)–metal ($oC$24, >120 GPa) transitions of lithium under compression, as observed through simultaneous electrical resistance and x-ray diffraction measurements. The energy gap in the $oC$40 semiconductor quickly decreases with applied pressure, and Li reverts back to a metallic state accompanied by a structural transition to $oC$24. The large $\rho$ value for $oC$24 indicates that this re-entrant metallic phase is a so-called “poor metal.” Our results reveal a characteristic phase diagram that demonstrates the existence of a semiconductor phase between two distinct metallic phases.

Recent interest in novel functionalities arising at domain walls of ferroic materials naturally calls for a microscopic understanding. To this end, first-principles calculations have been performed in order to provide solid evidence of polar distortions in the twin walls of nonpolar CaTiO${}_3$ and magnetic CaMnO${}_3$. We show that such polar displacements arise from rotation and/or tilting octahedral distortions—cooperatively acting at the twin wall in both considered systems—rather than from a proper secondary ferroelectric instability, as often believed. Our results are in excellent agreement with experimental observations of domain walls in CaTiO${}_3$. In addition, we show that magnetic properties at the twin wall in CaMnO${}_3$ are also modified, thus suggesting an unexplored route to achieve and detect multiferroic ordering in a single-phase material.

Measurements of the specific heat jump at the onset of superconducting transition in the iron-pnictide compounds revealed strong variation of its magnitude as a function of doping that is peaked near the optimal doping. We show that this behavior is a direct manifestation of the coexistence between spin-density wave and superconducting orders and the peak originates from thermal fluctuations of the spin-density waves near the end point of the coexistence phase—a tetracritical point. Thermal fluctuations result in a power-law dependence of the specific heat jump that is stronger than the contribution of mass renormalization due to quantum fluctuations of spin-density waves in the vicinity of the putative critical point beneath the superconducting dome.

The electronic structure of ${\text{BaTi}}_2$${\text{As}}_{2}O$, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by angle-resolved photoemission spectroscopy and first-principles calculation. The experimental electronic structure shows a multiorbital nature and three-dimensional character, which is consistent with the calculated results. An anomalous temperature-dependent spectral weight redistribution and broad line shape indicate the incoherent nature of the spectral function. At the density-wave-like transition temperature around 200 K, a partial gap opens at the Fermi patches. These findings suggest that ${\text{BaTi}}_2$${\text{As}}_{2}O$ is likely a charge density wave material in the strong-interaction regime.

We study the origin of the cubic to tetragonal and tetragonal to monoclinic structural transitions in ${\mathrm{KCrF}}_3$, and the associated change in orbital order, paying particular attention to the relevance of super-exchange in both phases. We show that super-exchange is not the main mechanism driving these transitions. Specifically, it is not strong enough to be responsible for the high-temperature cubic to tetragonal transition and does not yield the type of orbital order observed in the monoclinic phase. The energy difference between the tetragonal and the monoclinic structure is tiny, and most likely results from the interplay between volume, covalency, and localization effects. The transition is rather driven by Slater exchange than super-exchange. Nevertheless, once the monoclinic distortions are present, super-exchange helps in stabilizing the low-symmetry structure. The orbital order we obtain for this monoclinic phase is consistent with the magnetic transition at 80 K.

We demonstrate the significance of the Fock exchange in accurately describing dopant geometry on the superexchange interaction between the magnetic ions in doped semiconductors. The result emphasizes the important role of the Coulombic and electronic contributions to the Hellmann-Feynman forces, which have been overlooked in methods using generalized gradient approximation and local density approximation functionals. Using hydrogen-doped ZnO:Co as an example, we show that the hybrid functional can result in a new geometry of hydrogenic complex, which mediates ferromagnetic interaction through a combination of double exchange and Jahn-Teller effect.

We study the tunneling of conduction electrons through a (110)-oriented single-barrier heterostructure grown from III-V semiconductor compounds. It is shown that, due to low spatial symmetry of such a barrier, the tunneling current through the barrier leads to an electron spin polarization. The inverse effect, generation of a direct tunneling current by spin polarized electrons, is also predicted. We develop the microscopic theory of the effects and show that the spin polarization emerges due to the combined action of the Dresselhaus spin-orbit coupling within the barrier and the Rashba spin-orbit coupling at the barrier interfaces.

We report the results of a combined microwave polarization-dependence and power-dependence study of the microwave-radiation-induced magnetoresistance oscillations in high mobility GaAs/AlGaAs heterostructure devices at liquid helium temperatures. The diagonal resistance was measured with the magnetic field fixed at the extrema of the radiation-induced magnetoresistance oscillations, as the microwave power was varied at a number of microwave polarization angles. The results indicate a nonlinear relation between the oscillatory peak or valley magnetoresistance and the microwave power, as well as a cosine square relation between the oscillatory peak or valley magnetoresistance and the microwave polarization angle. The main features are briefly compared with the predictions of existing models.