Tunneling Conductance Spectra Research Articles

Tunneling conductance in normal metal–triplet superconductor junction

We calculate the tunneling conductance spectra of a normal metal/insulator/triplet superconductor from the reflection amplitudes using the Blonder–Tinkham–Klapwijk formula. For the triplet superconductor we assume one special p-wave order parameter having line nodes and two two-dimensional f-wave order parameters with line nodes breaking the time-reversal symmetry. Also we examine nodeless pairing potentials. The tunneling peaks are due to the formation of bound states for each surface orientation at discrete quasiparticles trajectory angles. The tunneling spectra can be used to distinguish the possible candidate pairing states of the superconductor Sr 2RuO 4.

Charge current in a ferromagnet-superconductor junction with the pairing state of broken time-reversal symmetry

We calculate the tunneling conductance spectra of a ferromagnetic metal/insulator/superconductor using the Blonder-Tinkham-Klapwijk (BTK) formulation. Two possible states for the superconductor are considered with the time reversal symmetry ($\cal{T}$) broken, i.e., $d_{x^2-y^2}+is$, or $d_{x^2-y^2}+id_{xy}$. In both cases the tunneling conductance within the gap is suppressed with the increase of the exchange interaction due to the suppression of the Andreev reflection. In the $(d_{x^2-y^2}+is)$-wave case the peaks that exist when the ferromagnet is a normal metal in the amplitude of the s-wave component due to the bound state formation are reduced symmetrically, with the increase of the exchange field, while in the $(d_{x^2-y^2}+id_{xy})$-wave case the residual density of states within the gap develops a dip around E=0 with the increase of the exchange field. These results would be useful to discriminate between $\cal{T}$-broken pairing states near the surface in high-$T_c$ superconductors

Tunneling conductance of normal metal/dx2−y2-wave superconductor junctions in the presence of broken time-reversal symmetry states near interfaces

In order to clarify the influence of (the presence of) the broken time-reversal symmetry state (BTRSS) induced near the interface, tunneling conductance spectra in normal metal/${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductor junctions are calculated on the basis of the quasiclassical Green's function method. The spatial dependence of the pair potential in the superconductor side is determined self-consistently. We discuss two types of the symmetry on the BTRSS: (i) ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+is$-wave state and (ii) ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}{+id}_{\mathrm{xy}}$-wave state. It is shown that the amplitude of the subdominant component $(is$ wave or ${\mathrm{id}}_{\mathrm{xy}}$ wave) is quite sensitive to the transmission coefficient of the junction. As the results, the splitting of the zero-bias conductance peak due to the BTRSS inducement is detectable only at junctions with small transmission coefficients for both cases. When the transmission coefficients are relatively large, the explicit peak splitting does not occur and the difference in the two cases appears in the height of the zero-bias peaks.

Two-gap state density in MgB(2): a true bulk property or a proximity effect?

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound MgB(2) directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of MgB(2) is used as a tip in the STM experiment. The "sample" is chosen to be a 2H- NbSe(2) single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages V(S) approximately +/-3.8 mV and V(L) approximately +/-7.8 mV. With rising temperature both peaks disappear at the T(C) of the bulk MgB(2), a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.

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.

Low-temperature STM/STS of high- Tc superconductors

We have carried out low-temperature scanning tunneling microscopy and spectroscopy studies on high- T c superconductors, Bi 2Sr 2CaCu 2O 8+ δ and YBa 2Cu 3O 7− δ single crystals. We present an experimental method and STM/STS results of these materials. Samples were cleaved below 20 K in order to obtain clean surfaces. For YBa 2Cu 3O 7− δ , we observe an atomic image of BaO and charge density waves image on the CuO chain layer. Differential tunneling conductance spectrum of YBa 2Cu 3O 7− δ has a clear superconducting energy gap along 100 nm path.

Parity-Sensitive Measurements Based on Ferromagnet/Superconductor Tunneling Junctions

A method for identifying the parity of unconventional superconductors based on tunneling spectroscopy is proposed. For a model of calculation, we adopt a ferromagnet/superconductor (F/S) junction, the tunneling current of which is spin polarized. The tunneling conductance spectra are shown to be quite sensitive to the direction of the magnetization axis in the ferromagnet only when the superconductor has odd parity. Therefore, it is possible to distinguish the parity of the superconductor by performing tunneling spectroscopy in F/S junctions.

Influence of impurity scattering on tunneling conductance in normal-metal–d-wave superconductor junctions

Tunneling conductance spectra between a normal metal / d-wave superconductor junction under the presence of bulk impurities in the superconductor are studied. The quasiclassical theory has been applied to calculate the spatial variation of the pair potential and the effect of impurity scattering has been introduced by t-matrix approximation. The magnitude of a subdominant s-wave component at the interface is shown to robust against the impurity scattering while that for a subdominant $d_{xy}$-wave component is largely suppressed with the increase of the impurity scattering rate. The zero-bias conductance peak due to the zero-energy Andreev bound states is significantly broadened for the case of Born limit impurity compared with that of unitary limit impurity.

Charge current in ferromagnet-triplet- superconductorjunctions

We calculate the tunnelling conductance spectra of a ferromagneticmetal/insulator/triplet-superconductor junction from thereflection amplitudes using the Blonder-Tinkham-Klapwijk formula.For the triplet superconductor, we assume one special p-wave orderparameter, having line nodes, and two two-dimensional f-wave orderparameters with line nodes, breaking the time-reversal symmetry.Also we examine nodeless pairing potentials. The evolution of thespectra with the exchange potential depends solely on the topologyof the gap. The weak Andreev reflection within the ferromagnetresults in the suppression of the tunnelling conductance andeliminates the resonances due to the anisotropy of the pairingpotential. The tunnelling spectrum splits asymmetrically withrespect to E = 0 under the influence of an external magnetic field.The results can be used to distinguish between the possiblecandidate pairing states of the superconductor Sr2RuO4.

Temperature dependence of tunneling spectra in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ single crystals

Abstract During past years, scanning tunneling spectroscopy (STS) investigations of high temperature superconductors have revealed unusual characteristics of the superconducting gap. We present here the temperature dependence of the tunneling conductance spectra on YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ single crystals. We discuss similarities and differences between these two compounds. In particular we note that while in Bi2Sr2CaCu2O8+δ a pseudogap is observed above the critical temperature, even in overdoped samples, we find no indication of a pseudogap in optimally doped YBa2Cu3O7−δ above Tc. This is consistent with other striking differences between the two systems and supports the interpretation of the pseudogap in terms of strong superconducting correlations.

D-wave superconductivity of the hole-doped(SrCa)10Cu17O29ladder compound

We performed tunneling experiments on single crystals of hole-doped two-leg ladders $(\mathrm{SrCa}{)}_{10}{\mathrm{Cu}}_{17}{\mathrm{O}}_{29}$ with ${T}_{c}\ensuremath{\cong}75$ K and, for reference purposes, on textured bulk samples of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Bi2212). Point-contact and break-junction (Bi2212 only) junction configurations were used. The point-contact junctions, both superconductor--insulator--normal-metal and superconductor--normal-metal, were using Ag counterelectrodes. In all experiments the tunneling direction was along ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ planes (for the ladders) and ${\mathrm{CuO}}_{2}$ planes (for Bi2212). The recorded tunneling conductance spectra were reproducible with sharp, well-defined energy-gap structures. All junctions exhibited a pronounced zero-bias conductance peak disappearing at the critical temperature. Experimental results are consistent with the formation of zero-energy Andreev bound states at the surface of the superconductors and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave symmetry of the order parameter $\ensuremath{\Delta}$ in both compounds.

Tunneling Conductance and Spatial Dependences of Pair Potentials in Normal Metal-Triplet Superconductor Junctions

Tunneling conductance spectra in normal metal / insulator / triplet superconductor junctions are theoretically investigated. For the triplet paring states, we select several kinds of the symmetries...

Effect of Superconducting Fluctuations in Tunneling Conductance Spectra

The influence of superconducting fluctuations on the conductance spectra of superconductor-insulator-normal metal (S-I-N) tunnel junctions is considered. We discuss how the tunneling conductance is affected as the strength of fluctuations is changed close to the superconducting critical transition.

TUNNELING CONDUCTANCE OF A NORMAL METAL/INSULATOR/Sr 2RuO 4 JUNCTION

A theory of tunneling conductance spectra for a normal metal/insulator/Sr 2RuO 4 junction is presented. We assume several types of air potentials with triplet symmetries that are promising candidates for Sr 2RuO 4. The calculated conductance spectra show either zero bias peaks of gap-like structures depending on the orientations of the junctions. The existence of residual components in the gap reflects a non-unitary property of the superconducting states. Based on the calculation of a temperature dependence, we verify that a measurement temperature below 100 mK is required to determine the pairing symmetry experimentally.

Theory of Tunneling Conductance for Normal Metal/Insulator/ Triplet Superconductor Junction

Tunneling conductance spectra of normal metal/insulator/triplet superconductor junctions are investigated theoretically. As triplet paring states we select several types of symmetries that are promising candidates for the superconducting states in UPt 3 and in Sr 2 RuO 4 . The calculated conductance spectra are sensitive to the orientation of the junction which reflects the anisotropy of the pairing states. They show either zero-bias conductance peaks or gap-like structures depending on the orientation of the junctions. The existence of a residual density of states, peculiar to nonunitary states, is shown to have a significant influence on the properties of the conductance spectra. Present results serve as a guide for the experimental determination of the symmetry of the pair potentials in UPt 3 and Sr 2 RuO 4 .

Electron Resonances in Sharp Tips and Their Role in Tunneling Spectroscopy

The influence of the shape and electronic structure of the tip in scanning tunneling spectroscopy is studied by a combined theoretical and experimental approach. Tunneling conductance spectra of a Cu(111) surface are used to detect and characterize the electronic states of W tips. The characteristic tunneling conductance of the Cu(111) surface state is the only feature present in spectra recorded with a blunt tip, while spectra obtained with sharp tips display also tip resonances. By means of a self-consistent tight-binding model of realistic tip structures we show that a sharp W tip exhibits, in contrast with blunt tips, narrow resonant states around the Fermi level.

Theory of tunneling and photoemission spectroscopy for high-temperature superconductors.

A comprehensive analysis is presented of the tunneling conductance and angle-resolved photoemission spectra in high-temperature superconductors. It is shown that unexplained features of the tunneling and photoemission spectra such as broad backgrounds, dips, and asymmetry of the tunneling conductance can arise in a model of spin-fluctuation mediated inelastic tunneling. Effects of directionality in tunneling play an important role in determining the behavior of the tunneling conductance. \textcopyright{} 1996 The American Physical Society.

Theory for tunneling spectroscopy of anisotropic superconductors.

A theory for the tunneling spectroscopy of a normal-metal--insulator--anisotropic-superconductor (N-I-S) junction is presented. In anisotropic superconductors, the effective pair potential felt by the quasiparticles depends on their wave vectors in contrast to the case of isotropic s-wave superconductors. By introducing the effect into the Blonder-Tinkham-Klapwidjk formula, a conductance formula for N-I-S junctions is obtained. It is shown that the conductance spectra are a function not only of the amplitudes of pair potentials but also of their phases. Tunneling conductance spectra calculated for various symmetries strongly depend on the relation between the tunneling direction and crystalline axes. In some crystalline angle regions of d-wave superconductors, a zero-energy peak in the conductance spectra is calculated. This conductance peak reflects the existence of anomalous bound states around the insulator--anisotropic-superconductor interface. The relation between the tunneling conductance spectra and the local density of states of superconductors is discussed. \textcopyright{} 1996 The American Physical Society.

Tunneling observation of phonon contribution to the pairing interaction in oxide superconductors

Tunneling conductance spectra were observed by point contact junction of SIN on single crystals of BSCCO (2212). Outside of the definite gap structure edge, significant fine structures were observed almost reproducibly ranging to 0.2 eV. The conductance derivative peaks both in positive and negative bias range were found in well one by one correspondence with the phonon spectrum peaks reported by Renker et al. [1]. Most of the peak modes were attributed to the optical phonons due to the oxygen oscillations in and around the [CuO 2] n net-plane. Numerical computation in terms of the Eliashberg gap equation was carried out, and the electron-phonon coupling intensity function α 2 F( ω) was elucidated and it turns out that the breathing mode relating to the polaronic valence fluctuations are predominant as compared with the B 1 g mode regarding the spin correlations.

Theory of Tunneling Spectroscopy of d-Wave Superconductors.

A tunneling theory for a normal metal--insulator-- $d$-wave superconductor junction is presented. In contrast to the $s$-wave superconductor, the tunneling conductance spectra strongly depend on the tunneling direction relative to the crystalline axes, and do not always represent the bulk density of states. Zero-bias conductance peaks are expected in $\mathrm{ab}$-plane tunneling. The present theory systematically explains various experimental results in the tunneling spectroscopy of high- ${T}_{c}$ superconductors.