Phase separation in a lattice model of a superconductor with pair hopping

We have studied the extended Hubbard model with pair hopping in the atomic limit for arbitrary electron density and chemical potential. The Hamiltonian considered consists of (i) the effective on-site interaction U and (ii) the intersite charge exchange interactions I, determining the hopping of electron pairs between nearest-neighbour sites. The model can be treated as a simple effective model of a superconductor with very short coherence length in which electrons are localized and only electron pairs have a possibility of transferring. The phase diagrams and thermodynamic properties of this model have been determined within the variational approach, which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. We have also obtained rigorous results for a linear chain (d = 1) in the ground state. Moreover, at T = 0 some results derived within the random phase approximation (and the spin-wave approximation) for d = 2 and 3 lattices and within the low-density expansions for d = 3 lattices are presented. Our investigation of the general case (as a function of the electron concentration n and as a function of the chemical potential μ) shows that, depending on the values of interaction parameters, the system can exhibit not only the homogeneous phases, superconducting (SS) and nonordered (NO), but also the phase separated states (PS: SS-NO). The system considered exhibits interesting multicritical behaviour including tricritical points.     

Pair tunneling in a two-dimensional superconductor

Pair tunneling to a two-dimensional superconductor is reported for the first time. It is found that flux quantization disappears due to the nature of the two-dimensional superconductor, while the magnitude of the pair tunneling current is unaffected. Also due to the specific behavior of the system under consideration, a crossover to conventional three-dimensional behavior is observed at low temperatures.     

Spin-filter tunneling in superconducting mesa structures with a ferromagnetic manganite interlayer

We have studied the current transport and magnetism in epitaxial hybrid superconducting mesa structures consisting of a cuprate superconductor and superconducting niobium with a manganite LaMnO₃ (LMO) interlayer. We have shown experimentally using magnetic resonance that the magnetization, magnetic anisotropy parameters, and transition temperature to the ferromagnetic state of the interlayer of the structures are analogous to those of an autonomous LMO film grown on a neodymium gallate substrate. The estimate of the barrier height obtained from the dependence of the characteristic resistance of mesa structures on the interlayer thickness has shown the barrier height variation with the thickness in the range of 5–30 mV. The temperature dependences of the conductivity of the mesa structure in the range between superconducting transition temperatures of the superconductors can be described in the theory taking into account the d-wave nature of the superconductivity for one of the electrodes and the spin-filtering of carriers passing through the tunnel interlayer. Spin-filtering is confirmed by the tunnel magnetoresistance and the high sensitivity of mesa structures to a weak external magnetic field in a voltage interval smaller than the gap of niobium.     

Phase and amplitude collective modes of an incommensurate spin density wave

We examine the collective modes of an incommensurate quasi-one-dimensional spin density wave associated with oscillations in the phase and amplitude of its complex order parameter. Using a linear response formalism that ensures gauge- and translational-invariance the effects of the Coulomb repulsion in the particle-particle channel are shown to simply renormalise the velocity of the massless phase mode to a value higher than the Fermi velocity. Analytic results for the frequency and damping of the massive amplitude mode are presented. These two longitudinal collective modes remain decoupled for arbitrary wavevector q.     

Interband resonant tunneling in superconductor heterostructures in a quantizing magnetic field

The resonant tunneling of electrons through quasistationary levels in the valence band of a quantum well in double-barrier structures based on III–V materials with type-II heterojunctions is considered in a quantizing magnetic field directed perpendicularly to the interfaces. The transmission coefficients of the tunnel structure for transitions from states corresponding to different Landau levels are calculated using the Kane model. It is shown that transitions with a unit change in the Landau level index n as a result of mixing of the wave functions of states with opposite spin orientations are possible on the interfaces due to spin-orbit coupling. The probability of such transitions can be comparable to the probability of transitions without a change in the Landau level index for InAs/AlGaSb/GaSb resonant-tunneling structures. Fiz. Tverd. Tela (St. Petersburg) 40, 2121–2126 (November 1998)     

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the scanning tunneling microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) d(x(2)-y(2)) orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the fourfold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and "direct" tunneling into Cu-O planes are also discussed.     

Impurity pair modes in a diatomic linear chain: nearest neighbour pair

The behaviour of the impurity modes due to a pair of substitutional impurities characterized by both mass as well as force-constant changes and occupying nearest neighbour positions in a diatomic linear chain, is studied. The results are compared with those for the case of impurity pairs occupying next nearest neighbour sites discussed earlier as well as the existing three dimensional calculations of Elliott and Pfeuty. The nearest neighbour impurity pair gap and local modes can be interpreted in terms of two single impurities substituted in the two different sublattices unlike the next nearest neighbour pair modes. The inband resonant modes are totally new features characteristic of the pair. Finally, the predictions of the theory are compared with the available experimental data for Si-impurity-pair-complexes and qualitative agreement is shown.     

Zeeman effects on d-wave superconductor and tunneling spectrum in normal-metal/d-wave superconductor tunnel junction

We study the Zeeman effect on the d-wave superconductor and tunneling spectrum in normal-metal(N)/d-wave superconductor(S) junction by applying a Zeeman magnetic field to the S. It is shown that: (1) the Zeeman magnetic field can lead to the S gap decreasing, and with the increase in Zeeman energy, the superconducting state is changed to the normal state, exhibiting a first-order phase transition; (2) the Zeeman energy difference between the two splitting peaks in the conductance spectrum is equal to2h0 (h0 is the Zeeman energy); (3) both the barrier strength of interface scattering and the temperature can lower the magnitudes of splitting peaks, of which the barrier strength can lead to the splitting peaks becoming sharp and the temperature can smear out the peaks,however, neither of them can influence the Zeeman effect.     

ac Josephson effect and resonant Cooper pair tunneling emission of a single Cooper pair transistor

We measure the high-frequency emission of a single Cooper pair transistor (SCPT) in the regime where transport is only due to tunneling of Cooper pairs. This is achieved by coupling on chip the SCPT to a superconductor-insulator-superconductor junction and by measuring the photon assisted tunneling current of quasiparticles across the junction. This technique allows a direct detection of the ac Josephson effect of the SCPT and provides evidence of Landau-Zener transitions for proper gate voltage. The emission in the regime of resonant Cooper pair tunneling is also investigated. It is interpreted in terms of transitions between charge states coupled by the Josephson effect.     

Theory of interlayer tunneling in bilayer quantum Hall ferromagnets

Spielman et al. [Phys. Rev. Lett. 84, 5808 (2000] recently observed a large and sharp Josephson-like zero-bias peak in the tunnel conductance of a bilayer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size and destroy the predicted Josephson effect. We predict that the peak would be split and shifted by an in-plane magnetic field in a way that maps the dispersion relation of the ferromagnet's Goldstone mode. We also predict resonant structures in the dc I-V characteristic under bias by an ac electric field.     

Retardation-effect-induced plasmon modes in a silica-core gold-shell nanocylinder pair

The retardation-effect-induced plasmon modes in a silica-core gold-shell nanocylinder pair is investigated by the two-dimension finite difference time domain method. We show that for light polarized perpendicular to the axis connecting the pair, the spectrum depends sensitively on the size of nanocylinder pair. As the size increases, several retardation-induced non-dipolar plasmon modes including multipolar modes appear in the spectrum and the resonance wavelength and strength of its plasmon modes can be tuned by changing separation width between the nanocylinder pair. Both extinction spectra as a function of size of the core-shell nanocylinder and near-field intensity associated with the coupling resonance modes are reported.     

Vibrational modes induced by a pair of impurities

Presence of a polarizable impurity atom in an ionic crystal induces a localized vibrational mode of the lattice. The frequency of the mode is distinct from the propagating modes of the pure crystal. It is shown in this paper that the frequency of the mode depends on the polarizability of the impurity atom and on the distance of separation between two adjacent impurities.     

Electron interaction with vibrational modes in tunneling through a single molecular electronic level

Within the adiabatic approximation, it is shown that the effective electron-phonon interaction Hamiltonian for tunneling through a single molecular electronic level contains two different contributions. The interference of the two interaction channels can lead to either the enhancement or suppression of phonon generation. Conditions determining the intensity of excitation of vibrational modes of the molecule are found.     

Low-frequency interlayer vibration modes in two-dimensional layered materials

Two-dimensional (2D) layered materials have been attracted tremendous research interest because of their novel photoelectric properties. If a single atomic layer instead of individual atoms is taken as a rigid motion object, two unique interlayer vibrations, i.e. compression/breathing and shear motions, at ultra-low frequencies can be expected and actually have been observed in many layered materials. The vibrations stem from the interlayer van der Waals interaction and can be well described by a conventional linear-chain model in most cases. The vibration frequencies strongly depend on layer thickness, which enables an accurate determination of layer numbers. A quick and nondestructive determination of flake thickness is particularly important for the materials, since the physical properties can be dramatically changed in the cases of several atomic layers. As a measure of interlayer coupling, the low-frequency modes are also sensitive to the stacking methods of atomic layers and the overlapping of different kinds of 2D materials. This allows the modes to play a key role in the applications like van der Waals heterojunctions. In this paper, we will give a brief review on the experimental observations and theoretical understanding of the interlayer modes in several typical 2D systems, as well as their actual and potential applications.