Session Y11: Tunneling in Superconductors: Spectroscopy, Andreev Reflection, and Nanowires

Fingerprints of the superconducting pairing glue via inelastic tunneling spectroscopy

In the past, tunneling spectroscopy has been interpreted as a direct probe of the fermionic density of states in superconductors. However, in this talk we discuss the impact of inelastic tunneling on tunnel spectra and show that depending on the actual system these interpretations have to be revisited. We show how such inelastic tunneling processes can occur in bulk systems and how they affect the analysis of the experimental data. Considering the spin-gap for spin excitations in the high-$T_c$ superconductors we can trace back the peak-dip-hump features observed in many unconventional superconductors to the shift of the spin spectral weight to higher energies below the critical temperature $T_c$ .   

Planar junction tunneling study of single-crystal eutectic phases of Sr$_2$RuO$_4$/Sr$_3$Ru$_2$O$_7$

Despite of intensive study of many years, the precise value of the superconducting energy gap of the odd-parity superconductor Sr$_2$RuO$_4$ has not been fully settled. Many complications exist. The band dependence of superconductivity makes the results from a bulk measurement of the gap difficult to interpret quantitatively. Surface based measurements such as scanning tunneling spectroscopy have to deal with the suppression of the superconducting energy gap on the cleaved $ab$ surface due to surface reconstruction. We performed quasi-particle tunneling measurements of the superconducting energy gap in planar junctions prepared on naturally cleaved edges of a Sr$_2$RuO$_4$ crystal found in the eutectic phase of Sr$_2$RuO$_4$ /Sr$_3$Ru$_2$O$_7$. Cleaving of such eutectic crystals exposes thin Sr$_2$RuO$_4$ “plates” inserted in a Sr$_3$Ru$_2$O$_7$ bulk crystal with the c axis along that of the Sr$_3$Ru$_2$O$_7$ bulk, as shown by X-ray diffraction data. Results obtained on Au-Sr$_2$RuO$_4$/Sr$_3$Ru$_2$O$_7$ tunnel junctions suggest that superconductivity survives on the surface of the “plates”, showing a gap value of ~ 0.2 meV, close to the BCS value for weak-coupling superconductors. Experiments on tunnel junctions made on cleaved crystals of a mesoscopic size are underway.   

Tunneling in Al/Al2O3/Al junctions and its direct link with energy gap and tunneling time across the barrier.

Quantum tunneling has been widely used in order to investigate the density of states of the materials across the barrier and magnetoresistance in magnetic tunnel junctions (MTJs). In spite of the possible applications there is no clear understanding of the barrier parameters as a function of temperature. Measurements of current-voltage (I-V) characteristics of a high quality Al/Al$_2$O$_3$/Al junction at temperatures ranging from 3.5 K to 300 K have been used to extract the barrier properties. Fitting results using Simmons’ model led to a constant value of barrier width $s$$\sim$20.8~$\textrm{\AA}$ and a continuous increase in the barrier height with decreasing temperature. The latter is used to determine the energy band gap temperature dependence and average phonon frequency $\omega$ = 2.05 $\times$ 10$^{13}$ sec$^{-1}$ in Al2O3. Finally from the experimentally extracted barrier height and width parameters we calculate the tunneling time for a solid state tunnel junction. The order of magnitude of this time corresponds to the one obtained in sophisticated experiments.The barrier parameters are used to extract the temperature dependent dwell times in tunneling ($\tau_D$ = 3.6 $\times$ 10$^{-16}$ sec at mid-barrier energies) and locate resonances above the barrier.   

Low temperature tunneling transport in van der Waals contacted superconductor/semiconductor Schottky barriers

We present a comparative study over a large temperature range (2.5-300K) of Schottky barriers formed either by evaporation of normal metals (Au, Al) or by van der Waals contact of mechanically exfoliated under-doped high-Tc Bi-2212 flakes onto moderately doped n-type GaAs and p-type Si semiconductor substrates. Our modified barrier-inhomogeneity model applied to the thermionic emission equation {[}1{]} gives a good description of the temperature evolution of barrier parameters, such as the zero-bias Schottky barrier height $\Phi_{SB}^{0}(T)$, the ideality factor $\eta(T)$ and the flat band barrier height, as the temperature is lowered from high temperatures where thermionic emission dominates to lower temperatures where thermal field emission and field emission (direct tunneling) dominate. At low temperatures for all barriers studied, both $\Phi_{SB}^{0}(T)$ and $\eta^{-1}(T)$ are linear in temperature with zero intercepts. Direct tunneling is verified in the Bi-2212/n-GaAs barriers by the appearance of superconducting density of states curves along with an energy gap $2\Delta$ = 65 meV in good agreement with ARPES and scanning tunneling microscope results by other investigators.\\ {[}1{]} Jurgen H Werner and Herbert H Guttler, Journal of Applied Physics 69, 1522 (1991).   

Current-phase relations in epitaxial Al/InAs nanowire Josephson junctions

Current-phase relations (CPRs) are a fundamental property of Josephson junctions, and in superconductor-normal-superconductor (SNS) junctions they can deviate from sinusoidal behavior at low temperatures. For short junctions, the shape and amplitude of the CPR is directly related to the properties of the Andreev bound states in the junction. Understanding the proximity effect, which is mediated by Andreev reflections, is particularly important in high spin-orbit and topological materials, where proximity-induced topological superconductivity is highly sought after. We used scanning superconducting quantum interference device microscopy to measure the CPR in many epitaxial Al/InAs nanowire junctions. We found CPRs that were very forward-skewed at low temperatures, indicating highly transmitting junctions. We will discuss the temperature, low magnetic field, and gate dependence of the CPRs, and the applicability of the short-junction theory to our junctions.   

Enhanced crossed Andreev reflection in a superconducting ladder

Andreev reflection is a process that happens at the junction of a normal metal (NM) and a superconductor(SC), where the Cooper-pair current in the superconductor gets equal contributions from the electron and hole channels of the normal metal. Crossed Andreev reflection (cAr) is a related process that happens in a system of two NM's independently connected to a SC when the current in the electron channel of the first NM and the current in the hole channel of the second NM together contribute to the Cooper-pair current in the SC. A typical experimental set-up to investigate cAr consists of two ferromagnetic metal leads connected to an s-wave superconductor. We propose an alternative mesoscopic set-up that enhances cAR which contains no ferromagnetic parts. Instead, our set-up consists of two NM's coupled to a superconducting ladder. We calculate the currents in the different channels and demonstrate enhanced cAr.   

Spin polarization measurements of ferromagnetic atomic chains on a superconductor: Part I

Introduction of magnetic defects in superconductors gives rise to spin polarized in-gap Shiba states. Recently chains of magnetic atoms, which give rise to a band of Shiba states, have been proposed as a platform for topological superconductivity. Spectroscopic evidence for in-gap Shiba states and Majorana end mode has been reported in previous studies of self-assembled chains of ferromagnetic Fe atoms on the surface of Pb[1]. In this talk, we introduce the technique of spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM) and discuss how we prepare tips that can show spin contrast at zero magnetic field, without disrupting superconductivity on the Pb surface. We use this technique, combined with the use of a vector magnet to orient the tip magnetization to probe the spin polarization of the Shiba states induced by the Fe atomic chains onto the Pb surface. A key to interpreting such experiments with spin-polarized STM tip is to understand the role of spin-polarization in the setpoint effect, which will be discussed in the next talk. [1] S. Nadj-Perge, I.K. Drozdov, J. Li, H. Chen, S. Jeon, J. Seo, A.H. Macdonald, B.A. Bernevig, A. Yazdani, Science~\textbf{346},602 (2014)   

Spin Polarization Measurements of Ferromagnetic Atomic Chains on a Supercondcutor: Part II

A key property of the Majorana fermions edge mode when realized at the edge of a topological superconductor is their spin. Unlike other low energy excitation in a conventional superconductor, which are made up of time-reverse partners of up and down spin, Majorana is expected to have a definite spin orientation. We utilize the technique of spin-polarized STM as described in the last talk to probe the nature of Majorana excitations in chains of Fe atoms on the surface of Pb. Previous effort on this system has detected signature of Majorana as a zero bias peak at end of such chains [1]. While this previous study shows evidence of ferromagnetism and spin-orbit coupling in such atomic chains on Pb, they did not probe the spin properties of the end mode specifically. We describe energy-resolved spin-polarized STM experiments designed to probe whether the previously reported zero energy end modes are spin-polarized or not. [1] Stevan Nadj-Perge, Ilya K. Drozdov, Jian Li, Hua Chen, Sangjun Jeon, Jungpil Seo, Allan H. MacDonald, B. Andrei Bernevig and Ali Yazdani, Science, 346, 602 (2014)   

Conductance and shot noise in ferromagnet-superconductor epitaxial tunnel junctions

Recently ferromagnet/superconductor hybrids have attracted attention due to the possibility of inducing p-wave superconductivity and of creating novel superconducting-spintronic devices. Most of these devices have a lateral configuration and are non-epitaxial, so they could not provide coherent electron tunneling over the interfaces. Here we investigate the conductance and shot noise in fully epitaxial Fe/MgO/V/MgO/Fe, Fe/MgO/Fe/MgO/V and Fe/MgO/V tunnel junctions with 40 nm thick Vanadium and 2nm thick MgO as a function of the applied bias, temperature (down to 0.3K) and magnetic state. All junctions show presence of finite subgap conductance indicating coherent two-electron transport over the barrier. Moreover, we observe conductance anomalies above the gap suppressed at temperature exceeding critical temperature, which may imply quasiparticle interference effects. High crystalline quality of the MgO barriers is confirmed by the fact that the above gap shot noise is Poissonian (direct tunneling with single barrier) or sub-Poissonian (sequential tunneling over two barriers) and is magnetic state dependent in the last case. The subgap Fano factor shows strong increase supporting multiple Andreev reflections as a possible source of excess subgap conductance.   

Supercurrents and Andreev bound states in a superconducting/strained/superconducting silicene junction

Based on the Dirac-Bogoliubov-de-Gennes equation, we investigated strain effects on the supercurrent and Andreev bound states (ABSs) in a superconducting/strained/superconducting silicene junction. Owing to the novel buckled structure of silicene, the supercurrent and ABS configurations can be effectively controlled by a perpendicular electric field. It was found that the supercurrent strongly depends on the direction of the strain, and the supercurrent exhibits a on/off effect under certain strains. It was demonstrated that the spin-valley symmetry of silicene can induce a spin-valley polarized supercurrent, even though the strength of the supercurrent is strongly modulated by the strain. These findings would potentially provide some intriguing insights into the correlation transport in strained silicene-based superconducting hybrid structures.   

Over-bias Light Emission due to Higher Order Quantum Noise of a Tunnel Junction

Understanding tunneling from an atomically sharp tip to a metallic surface requires to account for interactions on a nanoscopic scale. Inelastic tunneling of electrons generates emission of photons, whose energies intuitively should be limited by the applied bias voltage. However, experiments [Phys. Rev. Lett. 102, 057401 (2009)] indicate that more complex processes involving the interaction of electrons with plasmon polaritons lead to photon emission characterized by over-bias energies. We propose a model of this observation in analogy to the dynamical Coulomb blockade, originally developed for treating the electronic environment in mesoscopic circuits. We explain the experimental finding quantitatively by the correlated tunneling of two electrons interacting with an LRC circuit modeling the local plasmon-polariton mode. To explain the over-bias emission, the non-Gaussian statistics of the tunneling dynamics of the electrons is essential. Reference: F. Xu, C. Holmqvist, and W. Belzig Phys. Rev. Lett. 113, 066801 (2014)   

Enhanced superconductivity at the interface of W/Sr$_{2}$RuO$_{4}$ point contact

Differential resistance measurements are conducted for point contacts (PCs) between the Sr$_{2}$RuO$_{4}$ (SRO) single crystal and the tungsten tip. Since the tungsten tip is hard enough to penetrate through the surface layer, consistent superconducting features are observed. Firstly, with the tip pushed towards the crystal, the zero bias conductance peak (ZBCP) due to Andreev reflection at the normal-superconducting interface increases from 3\% to more than 20\%, much larger than previously reported, and extends to temperature higher than the bulk transition temperature. Reproducible ZBCP within 0.2 mV may also help determine the gap value of SRO, on which no consensus has been reached. Secondly, the logarithmic background can be fitted with the Altshuler-Aronov theory of electron-electron interaction for tunneling into quasi two dimensional electron system. Feasibility of such fitting confirms that spectroscopic information like density of states is probed, and electronic temperature retrieved from such fitting can be important to analyse the PC spectra. Third, at bias much higher than 0.2 mV there are conductance dips due to the critical current effect and these dips persist up to 6.2 K. For more details see Phys. Rev. B 91, 184514 (2015).   

Point contact spectroscopy on materials with high spin-orbit coupling

We will discuss the results of our point-contact spectroscopy experiments on a number of systems with high spin-orbit coupling, including topological insulators and Dirac semimetals. Such materials sometimes give rise to exotic phases of matter like unconventional superconductivity under normal metallic point contacts. On the other hand, the special spin properties of the surface of such materials can be probed using point contacts with superconducting tips. The role of special symmetry-protected properties of the surface states of such systems on Andreev reflection can also be probed. We will also discuss that point contact spectroscopy can be used as an extremely powerful tool for investigating topological systems.   

Fabrication and measurement of multi-terminal mesoscopic Josephson junctions.

We present fabrication and characterization of 3- and 4-terminal mesoscopic Josephson junctions involving InAs quantum well heterostructures [1] and superconducting Al contacts. A cross-shaped nanowire junction region with dimensions of order a few 100 nm is dry-etched in the 2DEG, followed by deposition of superconducting contacts and gating electrodes. These novel 0D devices have been recently predicted to have topological features in their Andreev spectra and finite-bias transport [2]; they may also be useful in efforts towards observation and braiding of Majorana fermions in the solid state [3]. // References: [1] J. Shabani et al. arXiv:1408.1122; [2] R-P Riwar et al. arXiv:1503.06862; B. van Heck et al. Phys. Rev. B 90, 155450 (2014); [3] S. Plissard et al. Nature Nanotechnology 8, 859 (2013)