We use epitaxial strain to spatially tune the bottom of the surface-state band $E_{\mathrm{SS}}$ of Ag(111) islands on Nb(110). Bulk and surface-state contributions to the Ag(111) local density of states (LDOS) can be separated with scanning tunneling spectroscopy. For thick islands ($\approx 20$ nm), the Ag surface states are decoupled from the Ag bulk states and the superconductive gap induced by proximity to Nb is due to bulk states only. However, for thin islands (3–4 nm), surface-state electrons develop superconducting correlations as identified by a complete energy gap in the LDOS when $E_{\mathrm{SS}}$ is smaller than but close to the Fermi level. The induced superconductivity in this case is of a two-band nature and appears to occur when the surface-state wave function reaches down to the Ag/Nb interface.

We use a spin-polarized tight-binding model Hamiltonian and the Landauer transport formalism to investigate the electronic transport properties of carbon nanotubes where different types of holes have been drilled through their sidewalls. We focus on zigzag edged defects with different atomic configurations since these systems enable the emergence of magnetic properties. We show that a number of hole geometries, magnetic states, and electronic spins yield attractive transport properties, such as ON/OFF switching for the electronic current, and nontrivial dependence of transmission with hole size.

Bulk superfluid helium supports two sound modes: first sound is an ordinary pressure wave, while second sound is a temperature wave, unique to superfluid systems. These sound modes do not usually exist independently, but rather variations in pressure are accompanied by variations in temperature, and vice versa. We studied the coupling between first and second sound in dilute ${}^3\mathrm{He}$–superfluid ${}^4\mathrm{He}$ mixtures, between 1.6 and 2.2 K, at ${}^3\mathrm{He}$ concentrations ranging from 0% to 11%, under saturated vapor pressure, using a quartz tuning fork oscillator. Second sound coupled to first sound can create anomalies in the resonance response of the fork, which disappear only at very specific temperatures and concentrations, where two terms governing the coupling cancel each other, and second sound and first sound become decoupled.

Doped strontium titanate becomes superconducting at a density as low as $n=5\times {10}^{17}{\mathrm{cm}}^{-3}$, where the Fermi energy is orders of magnitude smaller than the longitudinal-optical-phonon frequencies. In this limit, the only optical mode with a frequency which is smaller than the Fermi energy is the plasmon. In contrast to metals, the interaction strength is weak due to screening by the crystal, which allows the construction of a controllable theory of plasmon superconductivity. We show that plasma mediated pairing alone can account for the observed transition temperatures only if the dielectric screening by the crystal is reduced in the slightly doped samples compared with the insulating ones. We also discuss unique features of the plasmon mechanism, which appear in the tunneling density of states above the gap.

Bilayer graphene under a magnetic field has an octet of quasidegenerate levels due to spin, valley, and orbital degeneracies. This zero-energy Landau level is resolved into several incompressible states whose nature is still elusive. We use a Hartree-Fock treatment of a realistic tight-binding four-band model to understand the quantum ferromagnetism phenomena expected for integer fillings of the octet levels. We include the exchange interaction with filled Landau levels below the octet states. This Lamb-shift-like effect contributes to the orbital splitting of the octet. We give phase diagrams as a function of applied bias and magnetic field. Some of our findings are in agreement with experiments. We discuss the possible appearance of phases with orbital coherence.

A solid-state-based charge qubit is presented. The system consists of a one-dimensional wire with a pair of qubits embedded at its center. It is shown that the system supports collective states localized in the left and right sides of the wire and therefore, as a whole, performs as a single qubit. The couplings between the ground and excited states of the two central qubits are inversely proportional making them fully asynchronized and allowing for coherent manipulation and gate operations. Initialization and measurement devices, such as leads and charge detectors, connected to the edges of the wire are modeled by a continuum of energy states. The coupling to the continuum is discussed using the effective non-Hermitian Hamiltonian. At weak continuum coupling, all internal states uniformly acquire small decay widths. This changes dramatically as the coupling strength increases: the width distribution undergoes a sharp restructuring and is no longer uniformly divided among the eigenstates. Two broad resonances localized at the ends of the wire are formed. These superradiant states (analogous to Dicke states in quantum optics) effectively protect the remaining internal states from decaying into the continuum and hence increase the lifetime of the qubit. Environmental noise is introduced by considering random Gaussian fluctuations of electronic energies. The interplay between decoherence and superradiance is studied by solving the stochastic Liouville equation. In addition to increasing the lifetime, the emergence of the superradiant states increases the qubit coherence.

We apply a method called “force-enhanced atomic refinement” (FEAR) to create a computer model of amorphous silicon $\left(a\text{-Si}\right)$ based upon the highly precise x-ray diffraction experiments of Laaziri et al. [Phys. Rev. Lett. 82, 3460 (1999)]. The logic underlying our calculation is to estimate the structure of a real sample $a\text{-Si}$ using experimental data and chemical information included in a nonbiased way, starting from random coordinates. The model is in close agreement with experiment and also sits at a suitable energy minimum according to density-functional calculations. In agreement with experiments, we find a small concentration of coordination defects that we discuss, including their electronic consequences. The gap states in the FEAR model are delocalized compared to a continuous random network model. The method is more efficient and accurate, in the sense of fitting the diffraction data, than conventional melt-quench methods. We compute the vibrational density of states and the specific heat, and we find that both compare favorably to experiments.

The heteroepitaxial interface formed by copper deposited onto the tantalum (001) surface is studied by surface x-ray diffraction and ab initio calculations. The analysis of the crystal truncation rods reveals the presence of a wetting layer of copper made of two atomic planes pseudomorphic to the tantalum substrate, with the upper most atomic planes significantly deformed. These findings are in total agreement with the results of density-functional-theory calculations. The presence of the wetting layer confirms a Stranski-Krastanov growth mode and is thought to explain the extremely fast atomic diffusion of copper during the dewetting process in the solid state at high temperature.

The electronic band structure of crystals is generally influenced by the periodic arrangement of their constituent atoms. Specifically, the emerging two-dimensional (2D) layered structures have shown different band structures with respect to their stacking configurations. Here, based on first-principles density-functional theory calculations, we demonstrate that the band structure of the recently synthesized 2D ${\mathrm{Ca}}_2\mathrm{N}$ electride changes little for the stacking sequence as well as the lateral interlayer shift. This intriguing invariance of band structure with respect to geometrical variations can be attributed to a complete screening of ${\left[{\mathrm{Ca}}_2\mathrm{N}\right]}^{+}$ cationic layers by anionic excess electrons delocalized between the cationic layers. The resulting weak interactions between 2D dressed cationic layers give rise to not only a shallow potential barrier for bilayer sliding but also an electron-doping-facilitated shear exfoliation. Our findings open a route for exploration of the peculiar geometry-insensitive electronic properties in 2D electride materials, which will be useful for future thermally stable electronic applications.

We analyze shot noise in a magnetic tunnel junction with a two-level quantum dot attached to the magnetic electrodes. The considerations are limited to the case when some transport channels are suppressed at low temperatures. Coupling of the two dot's levels to the electrodes are assumed to be generally different and also spin dependent. To calculate the shot noise we apply the approach based on the full counting statistics. The approach is used to account for experimental data obtained in magnetic tunnel junctions with organic barriers. The experimentally observed Fano factors correspond to the super-Poissonian statistics, and also depend on relative orientation of the electrodes' magnetic moments. We have also calculated the corresponding spin shot noise, which is associated with fluctuations of spin current.

The recent discovery of hidden spin polarization emerging in bulk electronic states of specific nonmagnetic crystals is a fascinating phenomenon, though hardly explored yet. Here, we study from a theoretical perspective nonmagnetic ${\mathrm{BaNiS}}_2$, recently suggested to exhibit a giant Rashba-like spin-orbit splitting of the bulk bands, despite the absence of heavy elements. We employ density functional theory and Green's functions calculations to reveal the exact spin textures of both bulk and surface. We predict unambiguous signatures of spin-polarized electronic states at the surface, which reflect the bulk Rashba splitting and which could be experimentally measured with sufficient resolution: this would constitute a clear report of a bulk-Rashba-induced spin splitting at the surface of centrosymmetric crystals.

Tungsten (W) films were grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy demonstrated that these as-grown films are highly epitaxial and single crystalline with the $\left[00l\right]$ orientation. A special lattice stacking for the W/STO interface is observed to significantly reduce the lattice mismatching, which can be explained by the coincidence lattice model. The Hall effect has been investigated over the temperature range of 4–330 K. An anomalous transition of the major charge carriers from holes to electrons was observed in these W films upon cooling. The threshold temperature, in which the sign of the Hall coefficient $R_{\mathrm{H}}$ was reversed, was found to increase with the film thinning. With the sample's thickness reduced to several unit cells, its major carriers remained electrons even at room temperature. Calculations using the density functional perturbation theory revealed that such a transition from $p$ type to $n$ type could be attributed to the appearance of an electron pocket along the M-Γ direction induced by the lattice mismatching between the W film and $\mathrm{SrTi}{\mathrm{O}}_3$ substrate.

We perform first-principles total-energy calculations to investigate stabilities and electronic properties of hexagonal boron nitride ($h$-BN) bilayers under biaxial tensile strains. The possible stacking patterns of $h$-BN bilayers are investigated in detail. We show that the interlayer distances between two layers in $h$-BN bilayers can be changed under applied strains, and furthermore, they can decrease and increase depending on the stacking patterns of $h$-BN bilayers. We find that the band gaps are tunable by applying strains. We also find that tensile strains can give rise to a transformation from an indirect- to a direct-gap semiconductor in the case of the most stable stacking bilayer. These results indicate the high importance of $h$-BN bilayers as future electronic and optoelectronic device materials.

The polarization response of molecules or meta-atoms to external electric and magnetic fields, which defines the electromagnetic properties of materials, can either be direct (electric field induces electric moment and magnetic field induces magnetic moment) or indirect (magnetoelectric coupling in bianisotropic scatterers). Earlier studies suggest that there is a fundamental bound on the indirect response of all passive scatterers: It is believed to be always weaker than the direct one. In this paper, we prove that there exist scatterers which overcome this bound substantially. Moreover, we show that the amplitudes of electric and magnetic polarizabilities can be negligibly small as compared to the magnetoelectric coupling coefficients. However, we prove that if at least one of the direct-excitation coefficients vanishes, magnetoelectric coupling effects in passive scatterers cannot exist. Our findings open a way to a new class of electromagnetic scatterers and composite materials.

We study the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities embedded in $p$-doped transition metal dichalcogenide triangular flakes. The role of underlying symmetries is exposed by analyzing the interaction as a function of impurity separation along zigzag and armchair trajectories, in specific parts of the sample. The large spin-orbit coupling in these materials produces strongly anisotropic interactions, including a Dzyaloshinskii-Moriya component that can be sizable and tunable. We consider impurities hybridized to different orbitals of the host transition metal and identify specific characteristics for onsite and hollow site adsorption. In the onsite case, the different components of the interaction have similar magnitude, while for the hollow site, the Ising component dominates. We also study the dependence of the interaction with the level of hole doping, which supplies a further degree of tunability. Our results could provide ways of controlling helical long range spin order in magnetic impurity arrays embedded in these materials.

We show that a particle emitting in close proximity to a magneto-optical metamaterial substrate can experience a repulsive force if the magnetization is found along the surface plane. An analytical condition for the existence of such a repulsive force is obtained within the near-field approximation. Significantly, the repulsive force can be tuned by varying the filling fraction in a stack of two alternating layers of a metallic magneto-optical material and a dielectric. Potential applications can be envisaged for nanomechanical devices, particularly since similar metamaterial architectures have already been developed experimentally.

In the present paper, we propose that the chiral magnetic effect, the direct consequence of the presence of Weyl points in the band structure, can be detected by its coupling to certain phonon modes, which behave like pseudoscalars under point group transformations. Such coupling can generate resonance between intrinsic plasmon oscillation and the corresponding phonon modes, leading to dramatic modification of the optical response by the external magnetic field, which provides a way to study chiral magnetic effect by optical measurements.

We perform a study of domain walls in Co/${\mathrm{Ir}}_n$/Pt(111) ($n=0,\cdots ,6$) films by a combined approach of first-principles calculations and spin-dynamics simulations. We determine the tensorial exchange interactions and the magnetic anisotropies for the Co overlayer in both fcc and hcp geometries, depending on the number of Ir buffer layers. We find strong ferromagnetic nearest-neighbor isotropic exchange interactions between the Co atoms and an out-of-plane magnetic anisotropy for the films in fcc geometry. Our simulations show that the magnetic domain walls are of Néel type, and their rotational sense (chirality) is changed upon the insertion of an Ir buffer layer compared to the pristine Co/Pt(111) system. Our spin-dynamics simulations indicate a twisting of the spins with respect to the planar domain-wall profile on the triangular lattice. We discuss this domain-wall twisting using symmetry arguments and in terms of an appropriate micromagnetic continuum model considering extra energy terms compared to the available literature.

The electronic structure and magnetism of the quaternary Heusler alloy NiFeMnSn are studied using the full-potential linearized augmented plane-wave (FPLAPW) method. The calculation for the perfectly LiMgPdSn-type ordered crystal structure (type I) of NiFeMnSn shows a high spin polarization ($\sim 76$%) with a ferromagnetic ground state. The total spin magnetic moment is in good agreement with the Slater-Pauling rule. The structural investigations using neutron diffraction at 500 K, and Mössbauer spectroscopy at 300 K on the NiFeMnSn alloy, prepared using an arc melting, show the presence of atomic site disorder. The electronic structure calculation for the disordered structure shows that the site disorder destroys the nearly half-metallic nature of this alloy. The magnetization measurements indicate that the Curie temperature is well above room temperature ($\sim 405$ K) as desired for the spintronics application.