Splitting Of Zero-bias Conductance Peak Research Articles

Disorder-induced suppression of the zero-bias conductance peak splitting in topological superconducting nanowires.

We investigate the effect of three types of intrinsic disorder, including that in pairing energy, chemical potential, and hopping amplitude, on the transport properties through the superconducting nanowires with Majorana bound states (MBSs). The conductance and the noise Fano factor are calculated based on a tight-binding model by adopting a non-equilibrium Green’s function method. It is found that the disorder can effectively lead to a reduction in the conductance peak spacings and significantly suppress the peak height. Remarkably, for a longer nanowire, the zero-bias peak could be reproduced by weak disorder for a finite Majorana energy splitting. It is interesting that the shot noise provides a signature to discriminate whether the zero-bias peak is induced by Majorana zero mode or disorder. For Majorana zero mode, the noise Fano factor approaches zero in the low bias voltage limit due to the resonant Andreev tunneling. However, the Fano factor is finite in the case of a disorder-induced zero-bias peak.

Multiband coupling effect on splitting of zero-bias conductance peak of (1 1 0) surface states in [formula omitted

The electronic density of states (DOS) of (110) surface states in an YBCO high temperature superconductor were theoretically studied within a two-band approach for a semi-infinite-thickness film. As the conducting chain and plane layers in the material exhibit the superconductivity, the dispersions of those layers were taken into account, and modeled by the tight-binding bands under consideration to the second next nearest neighbor state. The impact of the chain-plane coupling strength was investigated in the calculation. The model parameters were extracted from the photoemission experiments. For comparison, the superconducting order parameter was modeled either dx2-y2 or dx2-y2+Δs, where Δs is the s-wave component. It was found that the zero-bias conductance peak of the plane layer in the case of dx2-y2+Δs order parameter can immediately split even if there is no coupling strength between the bands. However, in another case, this peak splits into two peaks with the presence of the coupling strength. Particularly, the splitting of the peak strongly depends on the size of the coupling strength.

Interplay between antiferromagnetic order and spin polarization in ferromagnetic metal/electron-doped cuprate superconductor junctions

Recently we proposed a theory of point-contact spectroscopy and argued that the splitting of zero-bias conductance peak in electron-doped cuprate superconductor point-contact spectroscopy is due to the coexistence of antiferromagnetic (AF) and $d$-wave superconducting orders [Phys. Rev. B 76, 220504(R) (2007)]. Here we extend the theory to study the tunneling in the ferromagnetic metal/electron-doped cuprate superconductor (FM/EDSC) junctions. In addition to the AF order, the effects of spin polarization, Fermi-wave vector mismatch between the FM and EDSC regions, and effective barrier are investigated. It is shown that there exists midgap surface-state contribution to the conductance to which Andreev reflections are largely modified due to the interplay between the exchange field of ferromagnetic metal and the AF order in EDSC. Low-energy anomalous conductance enhancement can occur which could further test the existence of AF order in EDSC. Finally, we propose a more accurate formula in determining the spin-polarization value in combination with the point-contact conductance data.

Impurity scattering effect on charge transport in high-Tc cuprate junctions

It is known that the zero-bias conductance peak (ZBCP) is expected in tunneling spectra of normal-metal/high-Tc cuprate junctions because of the formation of the midgap Andreev resonant states (MARS) at junction interfaces. In the present review, we report the recent theoretical study of impurity scattering effects on the tunneling spectroscopy. In the former part of the present paper, we discuss impurity effects in the normal metal. We calculate tunneling conductance for diffusive normal metal (DN)/high-Tc cuprate junctions based on the Keldysh Green function technique. Besides the ZBCP due to the MARS, we can expect ZBCP of a different origin, i.e., caused by coherent Andreev reflection (CAR) assisted by the proximity effect in DN. Their relative importance depends on the angle α between the interface normal and the crystal axis of high-Tc superconductors. At α=0 we find the ZBCP by the CAR for low-transparency junctions with small Thouless energies in DN; this is similar to the case of diffusive normal metal/insulator/s-wave superconductor junctions. Under increase of α from zero to π/4, the contribution of MARS to ZBCP becomes more prominent and the effect of the CAR is gradually suppressed. Such complex spectral features would be observable in conductance spectra of high-Tc junctions at very low temperatures. In the latter part of our paper, we study impurity effects in superconductors. We consider impurities near the junction interface on the superconductor side. The conductance is calculated from the Andreev and the normal reflection coefficients, which are estimated by using the single-site approximation in an analytic calculation and by the recursive Green function method in a numerical simulation. We find splitting of the ZBCP in the presence of the time reversal symmetry. Thus the zero-field splitting of ZBCP in the experiment does not perfectly prove the existence of a broken time reversal symmetry state.

Study of the tunneling effect on quasi-2D organic superconductors κ-(ET) 2X

We study the tunneling spectroscopy via Andreev bound states, which can be used to determine the pairing symmetry if one can prepare well-treated surfaces in the superconducting plane. In the present study, we investigate the tunneling spectrum in systems having multiple Fermi surfaces, where we focus on organic superconductors κ-(ET) 2 X. We show that the multiplicity of the Fermi surfaces can lead to a splitting of the zero-bias conductance peak (ZBCP). We propose that the presence/absence of the ZBCP splitting is used as a probe to distinguish the pairing symmetry in κ-(ET) 2 X.

Zero-bias conductance peak splitting due to multiband effect in tunneling spectroscopy of organic superconductors

We study how the multiplicity of the Fermi surface affects the zero-bias peak in conductance spectra of tunneling spectroscopy. As case studies, we consider models for organic superconductors $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ and (TMTSF)$_2$ClO$_4$. We find that multiplicity of the Fermi surfaces can lead to a splitting of the zero-bias conductance peak (ZBCP). We propose that the presence/absence of the ZBCP splitting is used as a probe to distinguish the pairing symmetry in $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$.

Generalized method of image and the tunneling spectroscopy in high-Tcsuperconductors

A generalized method of image is developed to investigate the tunneling spectrum from the metal into a class of states, with the tight-binding dispersion fully included. The broken reflection symmetry is shown to be the necessary condition for the appearance of the zero-bias conductance peak (ZBCP). Applying this method to the d-wave superconductor yields results in agreement with experiments regarding the splitting of ZBCP's in magnetic fields. Furthermore, a ZBCP is predicted for tunneling into the (110) direction of the d-density-wave state, providing a signature to look for in experiments.

Doping and magnetic field dependence of in-plane tunneling into YBa(2)Cu(3)O(7-x): possible evidence for the existence of a quantum critical point.

We present tunneling measurements into (1,1,0) YBa(2)Cu(3)O(7-x) films at various doping levels around the optimum. We find that, above a certain doping level near optimum doping, a spontaneous zero bias conductance peak splitting, delta, appears. It increases with doping. It also increases with magnetic field applied along the c axis, for both underdoped and overdoped films. The low field susceptibility chi = d delta /dH/(H-->0) is maximum, possibly diverging when the spontaneous value of delta goes to zero. These results suggest a transition from a pure d(x(2)-y(2)) to a d+id(xy) or d+is order parameter.

Interface effect and tunneling conductance of a normal metal/d-wave superconductor junction

The interface effect on the broken time reversal symmetry state formed at the interface of normal metal/d-wave superconductor junctions is studied on the basis of the quasiclassical Green's function method. The magnitude of the induced subdominant s-wave component of the pair potential which breaks time reversal symmetry is reduced with the increase of the magnitude of the transparency of the junctions. We also present tunneling conductance spectra in the junction based on the self-consistently determined pair potential where the zero-bias conductance peak (ZBCP) is split due to the s-wave component. Whether the splitting of ZBCP is observed or not in actual experiments strongly depends on the transparency of the junctions.

Anomaly of zero-bias conductance peaks in ferromagnet/d-wave superconductor junctions

The Blonder-Tinkham-Klapwijk approach is applied to studying spin-polarized quasiparticle transport in ferromagnet $(\mathrm{FM})/d$-wave superconductor (SC) junctions by taking into account the roughness of the interfacial barrier, broken time-reversal symmetry (BTRS) states near the surface of the SC, and exchange interactions in the FM. It is shown that (1) the exchange splitting in the FM decreases the height of the zero-bias conductance peak (ZBCP) and may induce a zero-bias conductance dip (ZBCD), (2) the presence of the BTRS states in the SC may make the ZBCP split into two peaks, and (3) the interface roughness obstructs the ZBCP splitting and decreases the height of the ZBCP. The calculated results can account for the ZBCD observed experimentally in ${\mathrm{La}}_{2/3}{\mathrm{Ba}}_{1/3}{\mathrm{MnO}}_{3}/{\mathrm{DyBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ and ${\mathrm{La}}_{2/3}{\mathrm{Ba}}_{1/3}{\mathrm{MnO}}_{3}/{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ junctions.

Zero-bias anomaly of the tunneling spectroscopy on different crystal orientation of YBa 2Cu 3O 7−δ epitaxial thin films

We report the tunneling spectroscopy on (100), (110) and (001) surfaces of YBa 2Cu 3O 7−δ epitaxial thin films using planar junctions made of Ag/Natural barrier/YBCO. Tunneling I–V curves are measured at various temperatures and in different magnetic fields. Both the experimental spectra of the (100) and (110) films show zero-bias conductance peak (ZBCP). Surprisingly, we observed a narrower peak on top of the broader ZBCP for tunneling through the (100) films. The splitting of zero-bias conductance peak predicted by the theory was not observed for either parallel or perpendicularly applied magnetic field up to 6 teslas. Both the ZBCP of the (100) and (110) YBa 2Cu 3O 7−δ films can be suppressed by the applied magnetic fields and disappear at the temperatures above T c.