Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D12: Tunnelling in Superconductors |
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Sponsoring Units: DCMP Chair: Richard Harris, NIST Room: LACC 402A |
Monday, March 21, 2005 2:30PM - 2:42PM |
D12.00001: Doping dependence of the pseudogap in Pr$_{2-x}$Ce$_x$CuO$_4$ R.L. Greene, M.M. Qazilbash, Yoram Dagan The temperature dependence of the in-plane tunneling conductance was measured for many doping levels of Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ (PCCO) x=0.11-x=0.19 using planar lead/Insulator/PCCO junctions on thin films and single crystals. At low temperatures and zero field the lead features are clearly seen, indicating good tunnel junctions with negligible leakage currents. When superconductivity in the PCCO electrode is quenched a normal state gap is seen at all doping levels studied. We find it to vanish above a certain temperature T*. T* is greater than T$_{c}$ for the underdoped region and it follows T$_{c}$ on the overdoped side. This behavior suggests finite pairing amplitude above T$_{c}$ on the underdoped side. The data and interpretation differ from previously publish ones [1]. [1] L. Alff \textit{et al. }Nature (London) 422, 698 (2003). [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D12.00002: Observation and Modeling of Splitting vs. Non-Splitting of the Zero-bias Conductance Peak in YBCO Planar Tunnel Junctions P.J. Hentges, L.H. Greene, G. Westwood, W.G. Klemperer Planar tunneling spectroscopy is performed on YBCO thin films grown at four different crystallographic orientations. Tunneling insulators are deposited via a patented solution-based zirconia deposition process [Greene et al, J. Mater. Chem. 14, 1 (2004)]. The characteristics of the tunneling conductance and its dependence on magnetic field, temperature and injected current are cataloged with the three different counter- electrode deposition techniques used. Modeling of the data show that by varying the value of the tunneling cone, surface faceting and quasiparticle lifetime, in agreement with the observations, splitting vs. non-splitting of the zero-bias conductance peak can be understood [Hentges, PhD Thesis, UIUC (2004)]. This work is support by the DoE DEFG02-91ER45439, through the FSMRL and the Center for Microanalysis of Materials. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D12.00003: Surface boundary condition and Fermi surface effects in Josephson (001) tilt grain boundary junctions of high temperature superconductors Gerald Arnold, Richard Klemm We calculate the supercurrent across symmetric and asymmetric (001) tilt grain boundary junctions of high temperature superconductors for both specular and random weak tunneling processes. We construct the superconducting half spaces on the left (L) and right (R) sides of the junction, assuming the angle between the (100) axis and the junction normal is $\theta _R$ and $\theta _L$, resp. The Fermi surface is taken to have either the two-dimensional tight-binding hole-doped shape consistent with angle-resolved photoemission experiments, or a nearly circular electron-doped shape, and take the physical surface on each half space to be perfectly flat. Both asymmetric and symmetric junctions with $\theta _R $=0, $\theta _L =\theta$, $\theta _R =\theta _L =\theta$ are constructed, and the superconducting order parameter is take to have $d_{x^2-y^2} $, s-, or extended-s-wave symmetry. For d-wave superconductors the surface boundary conditions with specular tunneling cause the supercurrent $I(\theta )$ to change sign at $\theta =\theta _0 <45^{\circ}$, inconsistent with the tricrystal experiments of Tsuei and Kirtley. For random tunneling, a d-wave interpretation of the tricrystal experiments is possible, but the magnitude of $I(\theta )$, while non zero because of the surface boundary conditions, is greatly reduced from the Ambegaokar-Baratoff value for s-wave superconductors. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D12.00004: Macroscopic quantum tunneling in d-wave high-TC superconductor Kunihiro Inomata, Shigeo Sato, Koji Nakajima, Akihiro Tanaka, Yoshihiko Takano, Huabing Wang, Masanori Nagao, Siro Kawabata, Takeshi Hatano We report the first successful observation of the macroscopic quantum tunneling (MQT) in $d$-wave high-$T_{C}$ superconductor (HTSC). Although, MQT, i.e. the quantum mechanical tunneling of the phase variable of the macroscopic wave function through the potential barrier of a Josephson junction, has been observed at around 300 mK in a conventional $s$-wave low-$T_{C}$ superconductor, it has appeared at approximately 1 K in HTSC because of its higher plasma frequency. On the other hand the $d$-wave pairing symmetry, the other hallmark of HTSC, allows for dissipative quasiparticles excitations within the superconducting energy gap, which may severely interfere with the occurrence of MQT. We also show a feasibility of MQT in spite of an existence of the dissipative quasiparticles. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D12.00005: Impurity-induced Local Density of States in a D-wave Superconductor Carrying a Supercurrent Degang Zhang, C.S. Ting, C.-R. Hu The local density of states (LDOS) and its Fourier component induced by a unitary impurity in a supercurrent-carrying d-wave superconductor are investigated. Both of these quantities possess a reflection symmetry about the line passing through the impurity site and along the supercurrent if it is applied along the antinodal or nodal direction. With increasing supercurrent, both the coherence and resonant peaks in the LDOS are suppressed and slightly broadened. Under a supercurrent along the antinodal direction, the coherence peaks split into double peaks. The modulation wavevectors associated with elastic scatterings of quasiparticles by the defect from one constant-energy piece of the Fermi surface to another are displayed as bright or dark spots in the Fourier space of the LDOS image, and they may be suppressed or enhanced, and shifted depending on the applied current and the bias voltage. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D12.00006: Phenomenological Investigation of the Superconducting Order Modulations in Cuprate Superconductors Lei Jiang, Yan Chen, Zidan Wang Recent high-resolution scanning tunneling spectroscopy experiments on cuprate superconductors such as NCCOC and BSCCO reveal a real-space modulation of local density of states spectrum at low energies in the pseudogap state. Motivated by these experimental findings, we present a phenomenological study of electronic structure by solving the Bogoliubov-de Gennes equations based upon the assumption of the existence of superconducting order modulations. Various one dimensional stripe phase and two dimensional checkerboard pattern with different periodicities are considered in our calculations. We present results for some physical quantities such as local density of states, spectral weights, and the spin susceptibility. Some interesting features are obtained and comparisons with experiments are made as well. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D12.00007: Competing Order and Asymmetric Tunnelling Spectrum Kangjun Seo, Jiangping Hu We argue that the asymmetric tunneling spectrum observed in the Cuprate superconductors stems from the existence of a competing order. The cempetition between the competing order and superconductivity can create an charge depletion region near the surface. The asymmetric response of the depletion region as the function of the external voltage causes the asymmetric tunneling spectrum. The effect is very general in a system which is near the phase boundary of two competing orders favoring different carrier densities. We show that the asymmetry of the point-contact spectroscopy of the heavy fermion superconductor CeCoIn$_5$ is another example of this effect. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D12.00008: Fabrication and characterization of Metal/ native oxide barrier /MgB2 junctions Jihoon Kim, R. Gandikota, R. Singh, N. Newman, J. Rowell Using MBE growth and thermal oxidation in air, we have fabricated cross-bridge junction structures to study the properties of MgB$_{2}$ tunneling. We have measured near-ideal tunneling characteristics from normal metal/native oxide barrier/MgB$_{2}$ structures. Superconducting energy gaps and DOS for MgB$_{2}$ have been inferred from tunneling conductance measurements. We can accurately fit our experimental quasi-particle density of states using a single Dynes function with a value of 2.2meV for the superconducting gap energy and 0.43meV for the thermal broadening energy in MgB$_{2}$ electrode. A potential height and barrier thickness of 0.16 eV and 4.6 nm, respectively, have also been inferred from conductance measurements at high voltage. These results suggest the possibility of using the MgB$_{2}$ native oxide layer as a barrier layer for the fabrication of practical SIS junctions. This research is supported by the ONR under contract number N00014-02-0002. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D12.00009: Observation of a subgap density of states in superconductor-normal-metal bilayers in the Cooper limit Zhenyi Long, M.D. Stewart, Jr., James M. Valles, Jr. We have performed transport and tunneling measurements of ultrathin bilayer films that are composed of a superconductor (Pb) and a normal metal (Ag). With Pb thickness fixed, the bilayer superconducting transition temperature and energy gap decrease exponentially with increasing Ag thickness. Simultaneously, an unexpected density of states (DOS) that increases linearly with energy from the Fermi energy appears within the subgap region. The temperature dependence of the subgap DOS also exhibits deviations from mean field theory expectations. We attribute the subgap states to quasiparticles that become trapped on integrable trajectories in the metal layer. The hybrid superconductor-normal-metal character in the DOS might be a sign of a novel superconductor to metal transition in 2 dimensional electronic systems. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D12.00010: Inelastic Scattering in Point Contacts Between a Normal Ferromagnetic Half-Metal and a Superconductor Charles W. Smith, Paul J. Dolan, Jr. A model for conductance in N/S point contacts, including spin polarization\footnote{G.J. Strijkers, Y. Li, F.Y. Yang, C.L. Chien and J.M. Byers, Phys. Rev. B 63, 104510 (2001).} is modified to incorporate inelastic scattering, i.e., quasiparticle finite-lifetime effects. For the half-metal limit of this model, we present calculations of zero bias conductance as a function of temperature and normalized conductance as a function of voltage, for the entire parameter space of the model. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D12.00011: Tunneling Spectroscopy of Magnetic Excitations in Layered Magnetic Superconductors Marina Hruska, Lev Bulaevskii, Martin Maley, Darryl Smith Modeling magnetic layered superconductors as a stack of Josephson-coupled conduction layers, subgap resonances in the I-V characteristics are predicted, as arising from magnetic excitations by Josephson current. We consider two mechanisms of interaction between the interlayer superconducting phase difference and magnetic moments: the spin-assisted cotunneling and the interaction mediated by the AC magnetic field of the moving Josephson vortex lattice. In the former case, the voltage of the kink in the dc I-V characteristic corresponding to the energy of two-spinwave excitations is found, whereas in the latter we obtain a single peak in voltage corresponding to excitation of a spinwave with momentum determined by the applied magnetic field and interlayer spacing. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D12.00012: Stability of the Finite Bias Tunneling Anomaly in Spin-Paramagnetically Limited Al Films Hank Wu, Philip Adams We will present a detailed study of the finite bias tunneling anomaly in the density of states (DOS) spectrum of spin-paramagnetically limited Al films in high parallel magnetic fields ($\sim6$T). The anomaly arises from a non-perturbative fluctuation mode associated with the formation of virtual Cooper pairs in the paramagnetic phase. We find that the strength and the stability of the mode grows rapidly with increasing film resistivity. Indeed, tilted field measurements in films with sheet resistance $\sim$R$_Q/10$ show that the anomaly persists up to tilt angles corresponding to perpendicular fields in excess of $H_{c2}$! Comparison with the theory I. Aleiner and coworkers will be discussed, along with the implications for nature of DOS spectrum in the high resistance limit. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D12.00013: Superconducting Direct Detectors For Submillimeter Astronomy John Teufel, Minghao Shen, Luigi Frunzio, Christopher Wilson, Daniel Prober, Robert Schoelkopf We are developing superconducting tunnel junctions (STJ) as direct detectors for submillimeter astronomy. Photons with energy greater than the superconducting gap of the aluminum absorber break Cooper pairs and generate excess quasiparticles that are then measured as a tunneling current through the STJ. In order to monitor the response of the detector with large readout bandwidth and maximal sensitivity, we have implemented a novel readout which monitors the impedance of the detector via radio frequency (RF) reflectometry. For calibration of the detector, we have also developed a gold microbridge as an on-chip, submillimeter photon source. When biased, high frequency noise from the microbridge couples via a transmission line to the detector. This allows for a calibrated photon source with near unity coupling, fast chopping, and calculable response. We present recent results in which we measured the ``optical'' properties of the STJ using the microbridge source and RF STJ readout at 300 mK, demonstrating good responsivity, high sensitivity, and fast response times. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D12.00014: Macroscopic resonant tunneling in extremely asymmetric superconducting qubits Philip R. Johnson, William T. Parsons, Frederick W. Strauch, J.R. Anderson, Alex J. Dragt, C.J. Lobb, F.C. Wellstood We predict surprisingly strong macroscopic resonant tunneling (MRT) effects in extremely asymmetric double well superconducting qubits. Observable signatures include a complex pattern of 1-100 MHz splittings in the energy level spectra, arising from the quantum coherent tunnel coupling between states in a shallow upper well and highly excited states in a very deep lower well. Probing quantum tunneling in these devices provides a new test of macroscopic quantum mechanics under extreme asymmetric conditions. Moreover, if our predictions are confirmed, they will have important implications for double well phase qubits. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D12.00015: Electron Tunneling in a High Impedance Environment. M.A. Castellanos-Beltran, K.W. Lehnert Discreteness of charge is a characteristic not usually perceived in regular electric circuits because electrons form a delocalized quantum fluid; any perceived charge is a continuous quantity related to the collective displacement of electrons relative to an ionic background. We attempt to detect the tunneling of individual electrons in a circuit by forcing them to flow continuously through a high-impedance resistor and then hop discretely across a tunnel junction. By measuring the potential of the island, the node of the circuit between the resistor and the tunnel junction, with a fast, sensitive electrometer (a radio-frequency single electron transistor) we will detect individual single electron tunneling events. We describe the preliminary measurements of this circuit, where we are able to resolve the time average potential of the island. We expect to be able to resolve tunneling events at higher frequencies more accurately than has previously been demonstrated [1]. {\\ \\} [1] W. Lu, Z. Ji, L. Pfeiffer, K.W. West, and A.J. Rimberg, {\it Nature} {\bf 423}, 422 (2003). [Preview Abstract] |
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