Zero-bias Conductance Dip Research Articles

Identifying the graphene d-wave superconducting symmetry by an anomalous splitting zero-bias conductance peak

Not until recently, was a gate-tunable, high-temperature superconducting proximity effect in graphene demonstrated experimentally. And usually in d-wave superconductor (SC) hybrid structure, ferromagnetism and spin-triplet states could result in a splitting zero-bias conductance peak (ZBCP). Herein, we theoretically present an anomalous splitting ZBCP in a graphene-based ferromagnet/Rashba spin–orbit coupling (RSOC)/insulator/d-wave SC hybrid structure. With increasing the exchange field from h/EF = 0, the ZBCP starts to turn into a splitting one with a zero-bias conductance dip (ZBCD) sandwiched in between two subpeaks, while from h/EF = 1, the two subpeaks and ZBCD begin to gradually shrink till the ZBCP reappears. The anomalous splitting ZBCP can be modulated by the RSOC strength, magnitude of Fermi wave vector mismatch as well as insulator barrier strength. These peculiar features are ascribed to the novel spin-triplet Andreev reflection in the context of the RSOC, characteristic by the anisotropic d-wave pair symmetry combined with the relativistic nodal fermions, which in turn can be experimentally used to directly identify not only the proximity-induced ferromagnetism and RSOC but d-wave pair symmetry in graphene. These results pave the way to a new class of tunable, high-temperature superconducting spintronic devices based on large-scale graphene.

Superconducting proximity effect in inverted InAs/GaSb quantum well structures with Ta electrodes

We present our recent electronic transport results in top-gated InAs/GaSb quantum well hybrid structures with superconducting Ta electrodes. We show that the transport across the InAs−Ta junction depends largely on the interfacial transparency, exhibiting distinct zero-bias behavior. For a relatively resistive interface, a broad conductance peak is observed at zero bias. When a transparent InAs−Ta interface is achieved, a zero-bias conductance dip appears with two coherent-peak-like features forming at bias voltages corresponding to the superconducting gap of Ta. The conductance spectra of the transparent InAs−Ta junction at different gate voltages can be fit well using the standard Blonder-Tinkham-Klapwijk theory.

Influence of spin–orbit coupling on differential conductance of normal metal/ferromagnetic p wave superconductor junctions

The extended Blonder–Tinkham–Klapwijk model is applied to studying tunneling conductance in normal metal/ferromagnetic p wave superconductor junctions. It is found that the spin–orbit scattering at the interfaces significantly affects the tunneling conductance. The interface spin–orbit coupling (SOC) suppresses the split conductance peaks in the N/ferromagnetic px wave superconductor junction and enhances the zero-bias conductance dip (ZBCD) of py wave superconductor junction. The SOC gives rise to the asymmetry of the conductance spectrum in the ferromagnetic px+py wave superconductor junctions.

Effect of spin fluctuations on charge transport in diffusive normal metal/d-wave superconductor junctions

The effect of spin fluctuations on the transport properties in diffusive normal metal/d-wave superconductor (DN/DS) junctions is studied under various situations by solving the Usadel equation with Nazarov’s generalized boundary condition. Tunneling conductance of the DN/DS junctions is calculated by changing the magnitude of the resistance in DN, Thouless energy in DN, the transparency of the insulating barrier, and the angle between the lobe direction of the d-wave pair potential and the normal to the interface, together with the magnitude of spin fluctuations. A zero-bias conductance dip (ZBCD) and a zero-bias conductance peak (ZBCP) occur in line shapes by way of a coherent Andreev reflection (CAR) around zero energy in the system of DN/DS junctions. We have found that both of the ZBCD and the ZBCP become narrower with the increasing magnitude of spin fluctuations.

Effect of spin fluctuations on tunneling conductance in diffusive normal metal/conventional superconductor junctions

Transport property in diffusive normal metal/conventional superconductor (DN/CS) junctions is studied for the effect of spin fluctuations under various situations by solving the Usadel equation with Nazarov's generalized boundary condition. Tunneling conductance of the DN/CS junctions is calculated by changing the magnitude of the resistance in DN, Thouless energy in DN, and the transparency of the insulating barrier, together with the magnitude of spin fluctuations. A zero-bias conductance dip (ZBCD) and a zero-bias conductance peak (ZBCP) with the width given by Thouless energy occur in line shapes by way of a coherent Andreev reflection (CAR) around zero energy in the system of DN/CS junctions. We have found that both of the ZBCD and the ZBCP sharpen with increasing the magnitude of spin fluctuations in the region of the relatively large resistance in DN.

Ferromagnetic insulator effects in ferromagnetic semiconductor/ferromagnetic insulator/ p-wave superconductor junctions

The effect of the ferromagnetic insulator on tunneling conductance in ferromagnetic semiconductor/ferromagnetic insulator/ p-wave superconductor (FS/FI/P) junctions is studied based on a scattering theory. Three kinds of pairings for the P side are chosen: p x , p y ,p x + ip y waves. It is shown that the spin filtering effect originating from the exchange field in the FI strongly modifies the normalization conductance. Many novel features including the zero-bias conductance dip and splitting are exhibited for fixed spin polarization in the FS. The tunneling spectrum for the heavy holes is much different from that for the light holes due to the different mismatches in the effective mass and Fermi velocity between FS and P.

Charge transport through ferromagnet/two-dimensional electron gas/d-wave superconductor junctions with Rashba spin-orbit coupling

Along the lines of Blonder, Tinkham and Klapwijk, we investigate the charge transport through ferromagnet/two-dimensional electronic gas/d-wave superconductor (F/2DEG/S) junctions in the presence of Rashba spin-orbit (SO) coupling and focus our attention on the interplay between spin polarization and spin precession. At zero spin polarization, the spin-mixing scattering resulted from Rashba SO coupling decreases the zero-bias conductance peak. Under spin polarization, spin precession introduces novel Andreev reflection, which competes with the effect of spin-mixing scattering. If the F layer is a half metal, the later effect is overwhelmed by that of novel Andreev reflection. As a result, the zero-bias conductance dip caused by spin polarization is enhanced, and at strong Rashba SO coupling, a split zero-bias peak is found in the gap. In an intermediate region where the two effects are comparable with each other, the zero-bias conductance shows a reentrant behavior as a function of Rashba SO coupling.

Triplet pairing states with equal spins in ferromagnet/superconductor heterojunctions for noncollinear magnetizations

Four-component Bogoliubov-de Gennes equations are applied to study tunneling conductance spectra of ferromagnet/ferromagnet/d-wave superconductor (F1/F2/d-wave S) tunnel junctions and to find out signs of spin-triplet pairing correlations induced in the proximity structure. The pairing correlations with equal spins arises from the novel Andreev reflection (AR), which requires at least three factors: the usual AR at the F2/S interface, spin flip in the F2 layer, and superconducting coherence kept up in the F2 layer. Effects of angle α between magnetizations of the two F layers, polarizations of the F1 and F2 layers, the thickness of the F2 layer, and the orientation of the d-wave S crystal on the tunneling conductance are investigated. A conversion from a zero-bias conductance dip at α = 0 to a zero-bias conductance peak at a certain value of α can be seen as a sign of generated spin-triplet correlations.

Tunneling conductance anomalies in normal metal/triplet superconductor junction

In the framework of the Blonder-Tinkham-Klapwijk model，we calculate the differential tunneling conductance in normal metal / insulator / normal metal/triplet p-wave superconductor junctions which is studied theoretically as a function of the bias voltage. The results show that there are zero-bias conductance peak, zero-bias conductance dip, double-minimum structures and oscillation components of the differential conductance in the spectra for p-wave superconductor junctions. It can be seen that additional current steps appear in the I-V relationship. The existence of such structures in the conductance spectrum may serve as an evidence that the pairing of Sr2RuO4 has p-wave symmetry.

Tunneling Conductance in Normal Metal/Insulator/TripletSuperconductor Junction

Tunneling conductance in normal metal/insulator/triplet superconductorjunctions is studied theoretically as a function of the bias voltage at zerotemperature and finite temperature. The results show there are zero-biasconductance peak, zero-bias conductance dip and double-minimum structures inthe spectra for p-wave superconductor junctions. The existence of suchstructures in the conductance spectrum can be taken as evidence that thepairing symmetry of Sr2RuO4 is p-wave symmetry.

Andreev reflection inYBa2Cu3O7−δ∕La0.67Ca0.33MnO3: Existence of midgap states in the {110} surface ofYBa2Cu3O7−δ

We use interfacial transport and transmission electron microscopy to study the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}(\mathrm{YBCO})∕{\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}(\mathrm{LCMO})$ junction that form a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductor and a ferromagnet (FM) heterostructure. Zero bias conductance peaks (ZBCPs) caused by Andreev reflection at zero bias were observed in the interfacial differential conductance spectra for the sample with an orderly mixed $c$- and $a$-axis [with (001) and $(100)∕(110)$] orientation of YBCO in the interface. When the YBCO film is oriented with the $c$-axis normal to the interface, a zero bias conductance dip (ZBCD) is observed. These results evidence the existence of surface Andreev bound states (midgap states) in the {110} surface of YBCO, which can lead ZBCP to appear in all directions except the {100} surface of YBCO where ZBCD occurs. Furthermore, the absence of any obvious influence of spin-polarization of LCMO on ZBCP in zero magnetic field implies that the LCMO is not half-metallic in the interface of $\mathrm{YBCO}∕\mathrm{LCMO}$, while strong magnetic field dependence of ZBCP shows that the applied magnetic field enhances the spin polarization of LCMO and suppresses ZBCPs in YBCO.

INTERFACIAL TRANSPORT IN YBa2Cu3O7-δ/La0.67Ca0.33MnO3 HETEROSTRUCTURES

YBa2Cu3O7-δ/La0.67Ca0.33MnO3 heterostructures were fabricated and patterned to study the interfacial transport between a ferromagnet (FM) and a dx2-y2-wave superconductor. Zero bias conductance peaks (ZBCPs) in the differential conductance spectra were observed in the interface where a YBCO layer shows a mixed c- and a-axis orientation. In contrast, a zero bias conductance dip (ZBCD) was observed in the interface with a purely c-axis oriented YBCO layer.

Theory of charge transport in diffusive normal metal/conventional superconductor point contacts

Tunneling conductance in diffusive normal (DN) metal/insulator/$s$-wave superconductor junctions is calculated for various situations by changing the magnitudes of the resistance and Thouless energy in DN and the transparency of the insulating barrier. The generalized boundary condition introduced by Nazarov [Superlattices and Microstructures $25,$ 1221 (1999)] is applied, where the ballistic theory by Blonder, Tinkham, and Klapwijk and the diffusive theory by Volkov, Zaitsev, and Klapwijk based on the boundary condition of Kupriyanov and Lukichev are naturally reproduced. It is shown that the proximity effect can enhance (reduce) the tunneling conductance for junctions with a low (high) transparency. A wide variety of dependencies of tunneling conductance on voltage bias is demonstrated including a U-shaped gap like structure, a zero-bias conductance peak, and a zero-bias conductance dip. The temperature dependence of tunneling conductance is also calculated, and the conditions for the reentrance effect are studied.

Nanometer-scale metallic grains connected with atomic-scale conductors

We describe a technique for connecting a nanometer-scale gold grain to leads by atomic-scale gold point contacts. These devices differ from previous metallic quantum dots in that the conducting channels are relatively well transmitting. We investigate the dependence of the Coulomb blockade on contact resistance. The high-resistance devices display Coulomb blockade and the low-resistance devices display a zero-bias conductance dip, both in quantitative agreement with theory. We find that in the intermediate regime, where the sample resistance is close to h/e2, the I–V curve displays a Coulomb staircase with symmetric contact capacitances.

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.

EFFECTS OF INTERFACE ROUGHNESS AND SPIN-FLIP ON TUNNELSPECTRUM IN FERROMAGNET-d-WAVE SUPERCONDUCTOR JUNCTIONS

Taking into account the spin-polarized effect,rough interface scattering and spin-flip effects,we calculate,within Blonder-Tinkham-Klapwijk Scattering formalism,the spin-polarized tunnel spectrum in the ferromagnet-d-wave superconductor junctions.It is shown that,(1) the conductance exhibits a sharp peak at zero-bias with the increase in spin-flip scattering;(2) the interface roughness can induce a small central peak in the zero-bias conductance dip and decrease the height of the zero-bias conductance peak.The calculated results can account for the tunnel spectrum observed experimentally in La2/3Ba1/3 MnO3/DyBa2Cu3O7 and La2/3 Ba1/3 MnO3/YBa2Cu3O7-δ junctions.

Planar tunneling spectroscopy ofY1−xPrxBa2Cu3O7thin films as a function of crystallographic orientation

We present a systematic study of the charge transport and quasiparticle tunneling properties of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ thin films. Pr doping increases the resistivity along the copper oxide planes and suppresses the superconducting critical temperature ${T}_{c},$ ultimately inducing a superconductor-insulator transition. The tunneling conductance is reproducible and correlated with the crystallographic film orientation. The crystallographic dependence can be divided into two distinct categories: tunneling into (001)-oriented $(c$-axis) films and tunneling into (100)-, (110)-, and (103)-oriented $(ab$-oriented) films. c-axis tunneling data exhibit a conductance dip at zero bias and a broad temperature-dependent peak over $\ensuremath{\sim}15--40$ mV that decreases in magnitude but stays fixed in energy for increasing Pr doping levels. $\mathrm{ab}$-plane tunneling data exhibit a zero-bias conductance peak and a gaplike feature at an energy that scales roughly linearly with ${T}_{c}$ for $x=0.0,$ 0.2, and 0.4. When $x=0.5,$ the resistivity is not linear in temperature and a zero-bias conductance dip is observed. The background conductance that ensures conservation of states in the low-temperature $\mathrm{ab}$-plane data exhibits temperature- and doping-dependent structure over $\ensuremath{\sim}15--40$ mV that is very similar to the peak observed in c-axis tunneling. Finally, analysis of the temperature and magnetic field dependence of the zero bias conductance peak indicates that states are conserved to within $\ensuremath{\sim}20%,$ supporting its interpretation as a feature of a superconducting density of states.

Proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet–d-wave-superconductor junctions

The proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet--$d$-wave-superconductor junctions with {110}-oriented interface are studied by solving self-consistently the Bogoliubov--de Gennes equations within an extended Hubbard model. It is found that the proximity induced order parameter oscillates in the ferromagnetic region. The modulation period is shortened with the increased exchange field while the oscillation amplitude is depressed by the interfacial scattering. With the determined superconducting energy gap, a transfer matrix method is proposed to compute the subgap conductance within a scattering approach. Many interesting features including the zero-bias conductance dip and splitting are exhibited with appropriate values of the exchange field and interfacial scattering strength. The conductance spectrum can be influenced seriously by the spin-flip interfacial scattering. In addition, a sizable local magnetic moment near the {110}-oriented surface of the d-wave superconductor is discussed.

IMPURITIES AND INTERFACE SCATTERING EFFECT ON THE DIFFERENTIAL CONDUCTANCE NORMAL METAL-d-WAVE SUPERCONDUCTOR JUNCTIONS

Taking into account the impurities and rough interface scattering, using Bogoliubov-de Gennes (BdG) equations and Bolonedr-Tinkham-Klapwijk (BTK) model, we calculate the differential conductance of normal metal-d-wave superconductor. It is found that the tunneling spectum depends strongly on both the incident angle electrons and the crystalline axis orientation of superconductor, and exhibits the zero-bias conductance peak under suitable arrangements. Morevover, the zero-bias conductance dip can be splitted into two small conductance dips by the impurities scattering, the tunnel conductance peak at the energy gap and the zero-bias conductance are suppressed and smoothed by the interface roughnes. Our result can explain many experimental measurements on the tunneling spectum of high-Tc superconductors.

Zero-bias conductance dip and phonon features in the superconductor density of states observed on tantalum- and niobium-based tunneling junctions

From a large number of $T\ensuremath{-}T$-oxide---Ag tunneling junctions, with $T=\mathrm{Ta}\mathrm{and}\mathrm{Nb}$, an empirical correlation was observed between the slope $S$ of the normal conductance-versus-voltage characteristics and the strength of the phonon-induced features of the superconductor density of states. This correlation allows for a tentative extrapolation to the bulk tantalum density of states which, when inverted according to the Rowell-McMillan scheme, leads to parameters ($\ensuremath{\lambda}=0.80$, ${\ensuremath{\mu}}^{*}=0.15$) which deviate markedly from values accepted hitherto and stronger high-energy (longitudinal phonon) contributions to ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$. For niobium, high-energy contributions to ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$ stronger than recently assumed are highly probable. The slope $S$ is governed by a zero-bias conductance dip. This anomaly as well as the nonideal features of the tunneling density of states in the superconducting state are attributed to a metal-semiconductor transition layer between the superconductor and its oxide with spatial extent of 3-10 \AA{}, and at least 10 \AA{} in the tantalum and niobium cases, respectively.