Sharp Zero-bias Conductance Peak Research Articles

Intrinsic topological superconductivity with exactly flat surface bands in the quasi-one-dimensional A2Cr3As3 (A=Na, K, Rb, Cs) superconductors

A spin-U(1)-symmetry protected momentum-dependent integer-$Z$-valued topological invariant is proposed to time-reversal-invariant (TRI) superconductivity (SC) whose nonzero value will lead to exactly flat surface band(s). The theory is applied to the weakly spin-orbit coupled quasi-1D A$_2$Cr$_3$As$_3$ (A=Na, K, Rb, Cs) superconductors family with highest $T_c$ up to 8.6 K with $p_z$-wave pairing in the $S_z=0$ channel. It's found that up to the leading atomic spin-orbit-coupling (SOC), the whole (001) surface Brillouin zone is covered with exactly-flat surface bands, with some regime hosting three flat bands and the remaining part hosting two. Such exactly-flat surface bands will lead to very sharp zero-bias conductance peak in the scanning tunneling microscopic spectrum. When a tiny subleading spin-flipping SOC is considered, the surface bands will only be slightly split. The application of this theory can be generalized to other unconventional superconductors with weak SOC, particularly to those with mirror-reflection symmetry.

Tunneling spectrum in triplet p-wave superconductor-silicene junctions

The tunneling spectrum in normal/triplet p- wave superconductor (N/p S)-undoped and doped silicene junction is studied based on Blonder-Tinkham-Klapwijk (BTK) theory. Three kinds of pairings for p S are chosen: p x , p y , p x + ip y waves. It is found that the normalized conductance (NC) strongly depends on the kind of pairings, the insulating gap which varies in magnitude with an applied electric field (Ez ) and the chemical potential in the normal silicene region. For N/p S-undoped silicene junctions, the Andreev reflection (AR) and the NC can be switched on or off by the applied electric field while the transport can occur for all the bias voltage in doped silicene junctions. A sharp zero-bias conductance peak (ZBCP) is obtained in N/p S-doped silicene junction that is independent of the values of the insulating gap of silicene and distinctly different from the normal/d- wave superconductor-silicene junctions. In addition, for N/p x S-silicene junctions the NC exhibits a double-minimum structure when the bias voltage eV = 0.8Δ0 with Δ0 the zero-temperature energy gap of p S while a little peak appears for N/p y S case. In N/p x +ip y S-silicene junctions a sharp peak and a conductance dip appear at eV = 0.8Δ0 and eV = Δ0 , respectively. It is hoped that the results obtained may guide the design of silicene devices.

Spin-polarized proximity effect in superconducting junctions

We study spin dependent phenomena in superconducting junctions in both ballistic and diffusive regimes. For ballistic junctions we study both ferromagnet/ s- and d-wave superconductor junctions and two-dimensional electron gas/ s-wave superconductor junctions with Rashba spin-orbit coupling. It is shown that the exchange field always suppresses the conductance while the Rashba spin-orbit coupling can enhance it. In the latter part of the article we study the diffusive ferromagnet/insulator/ s- and d-wave superconductor (DF/I/S) junctions, where the proximity effect can be enhanced by the exchange field in contrast to common belief. This resonant proximity effect in these junctions is studied for various situations: conductance of the junction and density of states of the DF are calculated by changing the heights of the insulating barriers at the interfaces, the magnitudes of the resistance in DF, the exchange field in DF, the transparencies of the insulating barriers and the angle between the normal to the interface and the crystal axis of d-wave superconductors α. It is shown that the resonant proximity effect originating from the exchange field in DF strongly influences the tunneling conductance and density of states. We clarify the followings: for s-wave junctions, a sharp zero bias conductance peak (ZBCP) appears due to the resonant proximity effect. The magnitude of this ZBCP can exceed its value in normal states in contrast to that observed in diffusive normal metal/superconductor junctions. We find similar structures to the conductance in the density of states. For d-wave junctions at α = 0 , we also find a result similar to that in s-wave junctions. The magnitude of the resonant ZBCP at α = 0 can exceed that at α / π = 0.25 due to the formation of the mid gap Andreev resonant states. To cite this article: T. Yokoyama, Y. Tanaka, C. R. Physique 7 (2006).

Scanning tunneling microscopy/spectroscopy of ferromagnetic Ni clusters on graphite and BSCCO high-Tc superconductor

Abstract A brief description of the basic principles of the theory of scanning tunneling microscopy and spectroscopy is given. This paper presents results of our experimental investigations of Ni clusters deposited on ab-surface of Bi2Sr2CaCu2O8+y (BSCCO) single crystals or highly oriented pyrolytic graphite (HOPG) substrates. The microclusters were produced by the plasma gas condensation or evaporator with integral flux monitor. The scanning tunneling spectroscopy was performed at 24 K and at room temperature. The spectra measured above the Ni clusters and directly above the substrate reveal remarkable differences. Point contact tunneling (PCT) measurements on the overdoped regime Bi2Sr2CaCu2O8+y single crystals show sharp zero bias conductance peak (ZBCP) at zero bias voltage. The ZBCP observation gives strong evidence that the order parameter is of d-wave symmetry for overdoped BSCCO single crystals.

Magnetic-field dependence of tunneling conductance spectra of La 0.67Sr 0.33MnO 3/YBa 2Cu 3O 7− δ junctions in ultra-low temperatures

We precisely measured conductance spectra near the zero-bias conductance peak (ZBCP) in La 0.67Sr 0.33MnO 3/YBa 2Cu 3O 7− δ junctions, which reflects the formation of the surface bound states in the d-wave superconductors. When the conductance spectra of the junctions were measured at ultra-low temperatures (<150 mK), very sharp ZBCPs were observed. We found the notable features of the magnetic field response of the ZBCP, i.e., the coexistence of spin-dependent and spin-independent components in the ZBCP and the apparent absence of the Zeeman splitting.

Observation of Andreev surface bound states in the 3-K phase region of Sr2RuO4.

The tunneling spectrum of the superconducting phase with Tc approximately 3.0 K has been measured in the Ru-embedded region of Sr2RuO4 using cleaved junctions. A sharp zero-bias conductance peak (ZBCP) has been observed below 3 K. All characteristics of this ZBCP suggest that it originates from Andreev surface bound states, indicating that the pairing in the 3-K phase is also non- s-wave. Below the bulk Tc of Sr2RuO4 (approximately 1.5 K), a crossover from sharp to bell-shaped ZBCP was found. This supports the theory that there is a phase transition in the 3-K phase region near the bulk Tc.

Point-contact tunneling on Bi 2Sr 1.94La 0.06CuO 6+δ single crystals

We have performed point-contact tunneling measurements on overdoped Bi 2Sr 1..94La 0.06CuO 6+δ (Bi2201) single crystals with gold tips. Sharp zero bias conductance peak (ZBCP) was observed. The data were analyzed with extended BTK theory. We found that the dx 2-y 2-wave symmetry gap parameter can fit the data nicely.

Normal electron tunnelling in ramp-type YBCO-junctions prepared by laser ablation

Ramp-type YBa 2Cu 3O 7/insulator/YBa 2Cu 3O 7 junctions have been prepared by laser ablation. The dependence of the differential conductance on the voltage is measured at different temperatures and magnetic fields. The junctions are thermally stable and can be cooled down to liquid helium temperature and warmed up again to room temperature several times without quality loss. At temperatures below 35K they show clear indications of an energy gap and in the same temperature range a very sharp zero-bias conductance peak.