Coulomb effects in a nanoscale SINIS junction

We study a system of two superconductors connected by a small normal grain. We consider the modification of the Josephson effect by the Coulomb interaction on the grain. Coherent charge transport through the junction is suppressed by Coulomb repulsion. An optional gate electrode may relax the charge blocking and enhance the current leading to the single Cooper pair transistor effect. Temperature dependences of critical current and of the minigap induced in the normal grain by the proximity to superconductor are studied. Both temperature and Coulomb interaction suppress critical current and minigap, but their interplay may lead to the nonmonotonic and even reentrant temperature dependence.     

Evidence for a minigap in YBCO grain boundary Josephson junctions

Self-assembled YBaCuO diffusive grain boundary submicron Josephson junctions offer a realization of a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current-voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Their persistence at large voltages is evidence of the long lifetime of the antinodal (high energy) quasiparticles.     

Theory of tunneling spectroscopy in the Larkin-Ovchinnikov state

We present a theory of tunneling spectroscopy for normal metal/Larkin-Ovchinnikov state junctions in which the spatial periodic modulation in the pair potential amplitude is taken into account. The tunneling spectra show the characteristic line shapes reflecting the minigap structures under the periodic pair potentials depending on the boundary condition of the pair potentials at the junction interface. These features are qualitatively different from the tunneling spectra of the Fulde-Ferrell state. We propose an experimental setup which identifies the superconducting state of CeCoIn5.     

Evidence of Hofstadter's Fractal Energy Spectrum in the Quantized Hall Conductance

The energy spectrum of a two-dimensional electron system in a perpendicular homogeneous magnetic field and a weak lateral superlattice potential with square symmetry is composed of Landau bands with recursive internal subband structure. The Hall conductance in the minigaps is anticipated to be quantized in integer multiples of e(2)/h that vary erratically from minigap to minigap in accordance with a Diophantine equation. Hall measurements on samples with the requisite properties uncover this long searched for evidence of Hofstadter's butterflylike energy spectrum.     

One-dimensional minigaps in inversion layer subbands on high index surfaces of InSb

In metal-oxide-semiconductor structures prepared on high index surfaces ofp-InSb onedimensional minigaps are observed in the quasi-two-dimensional subband structure of the inversion layer electrons at low temperatures. From anisotropic structure in the inversion channel conductance energetical position and size of the minigap are deduced and studied in dependence on surface orientation. The measured energetical position of the minigap is well described by two models, the valley projection model by Sham et al. and the staircase interface model by Volkov et al. In the magnetoconductance the Shubnikov-de Haas oscillations are found to contain several periods, reflecting the possible electron orbits of a two-dimensional electron gas with a one-dimensional minigap in a magnetic field applied normal to the two-dimensional plane. The size of the minigap determined from magnetic breakdown is found to be comparable to the one deduced from the anisotropic structure in the zero-field conductance.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

超导邻近效应在正常金属层中引起的反常小能隙现象

将散射矩阵方法推广到超导双结系统，对正常金属/正常金属/超导结的隧道电导进行了研究.中间正常金属层中的电子和其Andreev反射空穴之间的相位相干性导致了隧道电导出现反常小能隙现象.综合考虑相干和顺序遂穿，计算结果理想的解释了最近Gupta等人在实验中观测到的反常小能隙现象.     

Effect of reverse substrate bias on the minigap in Si inversion layers

The influence of a reverse substrate bias on the minigap of Si inversion layers on (10, 1, 1) and (8, 1, 1) surfaces has been investigated. Substrate bias shifts the position of the gap to higher gate voltages but the center of the gap remains at a fixed mobile charge density. Reverse bias increases the magnitude of the minigap by as much as 30%.     

Phase controlled superconducting proximity effect probed by tunneling spectroscopy

Using a dual-mode STM-AFM microscope operating below 50 mK we measured the local density of states along small normal wires connected at both ends to superconductors with different phases. We observe that a uniform minigap can develop in the whole normal wire and in the superconductors near the interfaces. The minigap depends periodically on the phase difference. The quasiclassical theory of superconductivity applied to a simplified 1D model geometry accounts well for the data.     

dc Josephson effect in metallic single-walled carbon nanotubes

The dc Josephson effect is investigated in a single-walled metallic carbon nanotube connected to two superconducting leads. In particular, by using the Luttinger liquid theory, we analyze the effects of the electron-electron interaction on the supercurrent. We find that in the long junction limit the strong electronic correlations of the nanotube, together with its peculiar band structure, induce oscillations in the critical current as a function of the junction length and/or the nanotube electron filling. These oscillations represent a signature of the Luttinger liquid physics of the nanotube, for they are absent if the interaction is vanishing. We show that this effect can be exploited to reverse the sign of the supercurrent, realizing a tunable π-junction.     

Coulomb blockade of proximity effect at large conductance

We consider the proximity effect in a normal dot coupled to a bulk superconducting reservoir by the tunnel contact with large normal conductance. Coulomb interaction in the dot suppresses the proximity minigap induced in the normal part of the system. We find exact expressions for the thermodynamic and tunneling minigaps as functions of the junction's capacitance. The tunneling minigap interpolates between its proximity-induced value in the regime of weak Coulomb interaction to the Coulomb gap in the regime of strong interaction. In the intermediate case a nonuniversal two-step structure of the tunneling density of states is predicted. The charge quantization in the dot is also studied.     

Plasmons at the interface between a corrugated semiconductor film and a binary metallic superlattice

We investigate the minigaps of the dispersion relation of plasmon polaritons that propagate at the interface between a corrugated semiconductor film and a binary metallic superlattice. Minima of the reflectivity for p-polarized light are calculated and their frequencies are used to obtain the ‘optical’ dispersion relations which are compared with the corresponding electromagnetic dispersion curves, in the effective medium theory. Results show that the minigap enlarges with the corrugation strength.     

Magnetic levels in superlattices

A magnetic field parallel to the layers of a GaAs---GaAlAs superlattice leads to a quantisation of the subband dispersion relation. The discrete energy levels are calculated with a semiclassical quantisation scheme and it is shown that within the energy width of the subbands, closed orbits, and in the superlattice minigap open orbits, are formed. Experimentally this behaviour is observed as sharp peaks in the interband Landau level absorption for energies within the subband width (closed orbits) and the disappearance of these peaks at higher energies (where no closed orbits exist).     

Band hybridization effect in InAs/GaSb based quantum wells

We develop a simple way to investigate the band hybridization effect in the present of the many-body interactions in an InAs/GaSb based quantum wells with different widths. The exchange self-energy and energy gap are obtained analytically at the long wave limit. An electron-like and a hole-like dispersion relations were obtained and a minigap about several meV is observed at the intercross of the electron and hole dispersion relations. Our theoretical results show that the widths of the quantum well have crucial role on the band hybridization in such a system.     

Phase coherence of the electron and hole in a ferromagnetic film in proximity with a superconductor

The scattering matrix approach between the clean and dirty limits is developed for the study of tunneling spectra in a ferromagnetic film in proximity to a superconductor. The minigap and the damped oscillation from ``0" to ``π" state in tunneling conductance are attributed to the phase coherence of the electrons and the corresponding Andreev-reflected holes in the ferromagnetic film. The calculated results provide a reasonable explanation for the behavior observed in recent experiments.     

"Wormhole" geometry for entrapping topologically protected qubits in non-abelian quantum hall states and probing them with voltage and noise measurements

We study a tunneling geometry defined by a single point-contact constriction that brings to close vicinity two points sitting at the same edge of a quantum Hall liquid, shortening the trip between the otherwise spatially separated points along the normal chiral edge path. This wormhole-like geometry allows for entrapping bulk quasiparticles between the edge path and the tunnel junction, possibly realizing a topologically protected qubit if the quasiparticles have non-Abelian statistics. We show how either noise or simpler voltage measurements along the edge can probe the non-Abelian nature of the trapped quasiparticles.     

Plasmons in a planar graphene superlattice

Plasmon collective excitations are studied in a planar graphene superlattice formed by periodically alternating regions of gapless graphene and of its gapped modification. The plasmon dispersion law is determined both for the quasi-one-dimensional case (the Fermi level is located within the minigap) and for the quasi-two-dimensional case (the Fermi level is located within the miniband). The problem concerning the absorption of modulated electromagnetic radiation at the excitation of plasmons is also considered.     

Anharmonic Bloch Oscillation of Electrons in Biased Superlattices

The oscillatory motion of electrons in a periodic potential under a constant applied electric field, known as Bloch oscillations (BO), is one of the most striking and intriguing quantum effects and was predicted more than eighty years ago. Oscillating electrons emit electromagnetic radiation and here we consider this BO effect for emission in the THz region. To date, it has been assumed that the Bloch oscillation of an electron is anharmonic oscillation, therefore with radiation emitted at the single Bloch frequency. We analyze scenarios when Bloch oscillations can be accompanied by the emission of radiation not only at the Bloch frequency but also with double and triple Bloch frequencies. The first scenario means that electrons could jump over neighboring Stark states. The second scenario of anharmonic emission is coupled to an opening of the minigap in the miniband.     

Nonmonotonic behavior of resistance in a superconductor-Luttinger liquid junction

Transport through a superconductor-Luttinger liquid junction is considered. When the interaction in the Luttinger liquid is repulsive, the resistance of the junction with a sufficiently clean interface shows nonmonotonic temperature or voltage dependence due to the competition between the superconductivity and the repulsive interaction. The result is discussed in connection with recent experiments on single-wall carbon nanotubes in contact with superconducting leads.     

The Wiedemannben Franz law in a normal metalben superconductor junction

In this paper the influence of superconducting correlations on the thermal and charge conductances in a normal metal-superconductor (NS) junction in the clean limit is studied theoretically. First we solve the quasiclassical Eilenberger equations, and using the obtained density of states we can acquire the thermal and electrical conductances for the NS junction. Then we compare the conductance in a normal region of an NS junction with that in a single layer of normal metal (N). Moreover, we study the Wiedemann-Franz (WF) law for these two cases (N and NS). From our calculations we conclude that the behaviour of the NS junction does not conform to the WF law for all temperatures. The effect of the thickness of normal metal on the thermal conductivity is also theoretically investigated in the paper.     

Density of states anomalies in hybrid superconductor-ferromagnet-normal metal structures

The results of calculations of the spatially-resolved density of states (DOS) in an S(F/N) bilayer are presented (S is a superconductor, F is a metallic ferromagnet, N is a normal metal) within quasiclassical theory in the dirty limit. Analytical solutions are obtained in the case of thin F, N layers which demonstrate the peculiar features of DOS in this system. The dependences of the minigap and the DOS peak positions on the exchange energy and parameters of the layers are studied numerically.