Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U12: Point Contact Tunnelling and Proximity Effect |
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Sponsoring Units: DCMP Chair: Milind Kunchur, University of South Carolina Room: LACC 402A |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U12.00001: Andreev Reflections at the Superconductor-Semiconductor Interface Soren Flexner, Paul Welander, Kevin Inderhees, Michael Vissers, James N. Eckstein We present results of experiments involving coupling of a BCS superconductor (niobium) with a heavily doped gallium arsenide based semiconductor system. Silicon doped GaAs is grown by molecular beam epitaxy and capped by InGaAs, with an indium fraction of 30 percent. Silicon delta doping layers increase electron densities into the semi-metallic regime. Intimate contact between superconductor and semiconductor is obtained by \textit{in situ} evaporation of niobium. Evidence of strong Andreev reflections at this interface is observed and analysis of this behavior in accordance with the BTK formalism will be presented. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U12.00002: Investigation of the Superconductor-Ferromagnet Proximity Region Using the Usadel Equation Serge Reymond, Paul SanGiorgio, Tesu Kim, Jinho Kim, Kookrin Char, Malcolm Beasley Due to its simplicity, the Usadel formulation is widely used to study the superconducting properties of heterostructures composed of superconductors (S), ferromagnets (F), and normal metals (N) in the dirty limit. In particular, with very simple boundary conditions and parameters, it quantitatively explains the non-monotonic relation between Tc and the ferromagnet thickness observed in SF bilayers. However, we find that the same simple analysis fails to explain the density of states measured in the F layer by tunneling spectroscopy. Based on experimental data taken in SF and SN bilayers, we discuss the validity of this approach and which of the ingredients -- boundary resistance, spin-orbit scattering, tunneling barrier height, etc. -- play a role in the real problem. Work supported by DoE BES. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U12.00003: Quantum conduction through two narrow half-metallic leads with opposite spin coupled to a superconductor Qian Wang, Chia-Ren Hu, Chin-Sen Ting We study quantum conduction through a two-dimensional junction consisting of two narrow half-metallic leads with opposite spin coupled to a wide superconductor at zero temperature. When a half-metallic lead is coupled to a superconductor, conductance vanishes at zero bias. Adding another half-metallic lead with opposite spin, conductance becomes non-zero due to Andreev reflection at the interface when the distance between the centers of the two leads is smaller than the coherence length of the superconductor. With the use of the Bogoliubov equations, we compute the conductance as a function of the distance between the centers of the two half-metallic leads. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U12.00004: Point-Contact Spectroscopy Study of the Ferromagnetic Superconductor ZrZn$_2$ M.A. Tanatar*, C.S. Turel, J.Y.T. Wei, R.W. Hill, S.M. Hayden Superconductivity and ferromagnetism are competing orders, but in ZrZn$_2$ evidence for superconductivity has been observed in the ferromagnetic state [1]. We have performed point-contact spectroscopy measurements on single-crystal samples of ZrZn$_2$ using Au and Pt-Ir tips. We present differential conductance spectra measured down to 100 mK, along with their magnetic-field evolution. Implications of our data on possible pairing symmetry [2] and pairing mechanism [3] will be discussed. *Permanent address: Inst. Surface Chemistry, N.A.S. Ukraine, Kyiv, Ukraine. [1] C. Pfleiderer et al. Nature \textbf{412,} 58 (2001). [2] M.B. Walker and K.V. Samokhin, Phys. Rev. Lett. \textbf{88,} 207001 (2002). [3] D.J. Singh and I.I. Mazin Phys. Rev. Lett. \textbf{88,} 187004 (2002). [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U12.00005: Tunneling Spectroscopy of Superconductor/Ferromagnet Proximity Effect Bilayers Paul SanGiorgio, Serge Reymond, Tesu Kim, Jinho Kim, Kookrin Char, Malcolm Beasley Previous studies of the superconducting proximity effect in ferromagnetic thin films have primarily focused on the non-monotonicity of the transition temperature as a function of film thickness. This behavior is due to the non-zero center-of-mass momentum acquired by a Cooper pair in this presence of an exchange field, which results in position-space oscillations of the pair density in the ferromagnet. In order to provide a complementary view of this phenomenon, we have made a systematic study of the tunneling density of states on the ferromagnetic side as a function of film thickness. We make our samples in a well-calibrated UHV sputtering chamber, allowing us to probe thicknesses both smaller and larger than the coherence length of the ferromagnet, which is on the order of 1 nm. By careful sample preparation, measurement, and data analysis, we are able to observe minute variations in the density of state, characteristic of samples with thicknesses greater than the coherence length. Work supported by DOE BES [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U12.00006: Thouless energy in Josephson junctions with the barrier composed of strongly correlated materials near the Mott transition A.N. Tahvildar-Zadeh, J.K. Freericks, B.K. Nikoli\'c The Thouless energy was originally introduced as the inverse of the electron dwell time (mod $\hbar$) in a finite-sized piece of diffusive conductor, where the dimensionless conductivity is determined by the ratio of the Thouless energy to the quantum energy level spacing of the wire. The Thouless energy is also useful in characterizing the proximity-effect coupling in Josephson junctions, consisting of two superconductors separated by a barrier layer of diffusive normal metal; the characteristic voltage across the junction $I_c R_N$ is described by a universal function of the Thouless energy. In this talk, we present a generalized form of the Thouless energy for Josephson junctions made of a strongly correlated metal (insulator) as the barrier layer and use it to illustrate how the quasiclassical picture of transport breaks down as the strongly correlated barrier passes through the Mott metal-insulator transition. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U12.00007: LOFF superconductivity in the proximity region of d-wave superconductor/ferromagnet tunneling junctions Kwok-Wai Ng, Mario Freamat In the proximity region of superconductor-ferromagnet tunneling junctions, a signature of the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state is the decaying oscillation of the LOFF order parameter with the distance from the junction barrier. Using N/I/F/Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ (BSCCO) heterostructures, we investigated the dependence of this oscillatory behavior on the d-wave symmetry of the BSCCO superconductive order parameter. Two junctions were prepared on the same BSCCO crystal, in order to probe two different injection angles $\beta $ in the ab-plane: one close to the nodal line direction ($\beta $=45\r{ }) and the other one close to the maximum gap direction ($\beta $=0). The tunneling spectra obtained on N/BSCCO junctions (i.e., no ferromagnetic layer) with the same orientation presented the usual features: high amplitude zero bias conductance peaks (ZBCP) for $\beta $=45\r{ } and V-shaped gaps for $\beta $=0. However, with a 70 {\AA} thick antiferromagnetic interlayer (Fe), the N/I/F/BSCCO junctions show ZBCP for $\beta $=0 and gaps for $\beta $=45\r{ }. We attribute this opposite behavior to the presence of a spatially modulated LOFF order parameter in the Fe layer leading to flipped spectral characteristics at distances dependent on the ferromagnetic exchange energy. The diffusiveness of the structures is rather low, so that the oscillations for the nodal and antinodal injections are almost in antiphase. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U12.00008: Point contact tunneling spectroscopy of a Tl2Ba2CaCu2O8 thin film Daniel Mazur, J. F. Zasadzinski, H. Claus, K. E. Gray, Y. C. Ma, S. L. Yan, N. L. Wang Point contact tunneling spectroscopy was used to investigate the superconducting gap of Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8}$ (Tl- 2212 cuprate) c-axis oriented thin film of near-optimal doping, with T$_{C} $~=~106~K. The quasiparticle peaks were observed at various bias voltages in the range 30-55 mV, and the 2{\_}/k$_{B}$T$_{c}$ ratio coming from our measurement is $\sim $ 6.6-12. This is the first observation of large energy gap in near-optimally doped Tl-2212 by electron tunneling. The largest values match those found in the~infrared response and Raman scattering on Tl- 2212 (single crystals and oriented thin films) published by other groups. We discuss this in the context of experiments done by relevant techniques on related materials, the Tl-2201, Tl-2223 and Bi-2212 cuprates. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U12.00009: Point contact spectrscopy of electron-doped cuprates in a magnetic field of 32 tesla Yun Sung-Hee, Neesha Anderson, Tara Dhakal, Amlan Biswas Electron-doped cuprates have low values of the upper critical field ($H_{c2} \sim 10$ T at 1.5 K) and hence it is possible to study their normal state at low temperatures. Such studies have been done before and showed evidence of a ``pseudogap". However, to understand the origin of this pseudogap and which model of high-$T_c$ superconductivity it supports, it is necessary to study the effect of high magnetic fields on this pseudogap. We have performed point contact spectroscopy experiments using junctions between a normal metal (Pt-Rh) and electron-doped Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO) films for $0.13 < x < 0.17$. To probe the normal state at low temperatures ($\sim$ 0.4 K), we suppressed the superconductivity by applying high magnetic fields (up to 32T). We will show the effects of such high fields on the pseudogap and discuss our results in the context of present theories {\em viz.} preformed pairs and the presence of a quantum critical point. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U12.00010: Evidence for nodeless gap in superconducting Nd1.85Ce0.15CuO4-y: Lei Shan, Yan Huang, Shiliang Li, Pengcheng Dai, Fang Zhou, Jiwu Xiong, Wenxin Ti, Hai-Hu Wen The pairing symmetry in a single crystal of Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ is studied by measuring the point-contact spectroscopy along nodal and anti-nodal directions. For comparison the same measurements on a hole-doped cuprate single crystal of La$_{1.89}$Sr$_{0.11}$CuO$_{4}$ is also presented. A nearly identical spectrum is obtained in Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ for both directions along Cu-O bond and Cu-Cu bond and no any zero bias conductance peak is observed. This is in contrast to the results of La$_{1.89}$Sr$_{0.11}$CuO$_{4}$, in which an angular dependent spectrum is observed with a remarkable zero bias conductance peak in the nodal direction. Our results support an s-wave like symmetry in optimally electron-doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ other than the d-wave dominant symmetry as demonstrated in hole-doped cuprates. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U12.00011: Proximity effect in normal metal high-Tc superconductor contacts Tomas Lofwander We study the proximity effect in good contacts between normal metals and high-Tc ($d_{x^2-y^2}$-wave) superconductors. We present theoretical results for the spatially dependent order parameter and local density of states, including effects of impurity scattering in the two sides, s-wave pairing interaction in the normal metal side (attractive or repulsive) and subdominant s-wave paring in the superconductor side. For the $[100]$ orientation, a real combination $d+s$ of the order parameters is always found. The spectral signatures of the proximity effect in the normal metal include a suppression of the low-energy density of states and a finite-energy peak structure. These features are mainly due to the impurity self-energies, which dominate over the effects of induced pair potentials. For the $[110]$ orientation, for moderate transparencies, induction of a $d+is$ order parameter on the superconductor side leads to a proximity induced $is$-order parameter also in the normal metal. The spectral signatures of this type of proximity effect are potentially useful for probing time-reversal symmetry breaking at a $[110]$ interface. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U12.00012: Observation of the current-phase relation of a characterized superconducting atomic point contact Youiti Ootuka, Hisao Miyazaki, Akinobu Kanda, Yamaguchi Takahide The current-phase relation $I(\varphi )$ in a superconducting atomic point contact (APC) is different from that in a tunnel junction. Beenakker et al. treated this problem based on the idea of Andreev reflection, and obtained a formula for $I(\varphi )$ as a function of transmission coefficient $\tau $ of the contact. Measuring magnetic responses of a superconducting loop with an APC, Koops at al. determined $I(\varphi )$ to find a clear non-sinusoidal behavior. However, no characterization of the point contact was possible in their experiment. In this paper, we propose a new experiment in which we can determine both the transmission coefficients {\{}$\tau _{i}${\}} and $I(\varphi )$ relation: A device we should make is a dc-SQUID which consists of an APC and a tunnel junction. The coefficients {\{}$\tau _{i}${\}} can be determined by analyzing the sub-gap $I-V$ characteristics. On the other hand, $I(\varphi )$ of the point contact is derived from a dependence of the critical current on the magnetic flux, $I_{C}(\Phi )$. Using the EB-lithography and break junction technique, we performed the experiment at 90 mK. The $I-V$ characteristics can be fitted to the theory of multiple Andreev reflection quite well. The critical current is not symmetric to the magnetic field direction nor to the current direction, which means the $I(\varphi )$ is not sinusoidal. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U12.00013: Calculation of superconductor-normal metal point contact conductance with finite gap decay length Vladimir Lukic A numerical method is developed for the calculation of differential conductance in superconductor-normal metal contact junctions using the Bogoliubov-De Gennes equations beyond the Blonder-Tinkham-Klapwijk (BTK) approximation. Effects of finite lengthscales of gap onset and contact potential are calculated self-consistently, and exact quasiparticle momenta are retained. For a physical choice of the parameters, the combination of these effects produces a significant departure from the BTK conductance, most notably a suppression of the excess current above the gap. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U12.00014: Proximity Effect in Nb/Cu/CoFe Trilayers Kyungmoon Kim, Jinho Kim, Jun Hyung Kwon, Kookrin Char, Hyeonjin Doh, Han-Yong Choi We have fabricated the Nb/Cu , Nb/CoFe bilayer and Nb/Cu/CoFe trilayer samples by varying the CoFe or Cu layer thickness using DC magnetron sputtering system and measured their superconducting transition temperature Tc. In Nb/Cu(d$_{Cu})$ and Nb/CoFe(d$_{CoFe})$ bilayers , we observed Tc behavior consistent with conventional SN and SF theory. In Nb/Cu/CoFe trilayer , as we increase d$_{Cu}$ with fixed values of d$_{Nb}$ and d$_{CoFe }$, $_{ }$Tc of Nb/Cu(d$_{Cu})$/CoFe trilayer increased rapidly for d$_{Cu} \quad <$ 5 nm and slowly saturated to a limiting value. We analyzed these data using the method based on Usadel formalism and obtained $\gamma _b ^{N}$=R$_{b}$A/$\rho _{n}\xi _{n}$[Nb/Cu]=0.41 , $\gamma _b ^{F}$= R$_{b}$A/$\rho _{f}\xi _{f}$ [Nb/CoFe] = 0.33, and $\gamma _b ^{F}$[Cu/CoFe]=0.35. In our Nb/Cu/CoFe trilayers, as we increased d$_{CoFe}$ with fixed values of d$_{Nb}$ and d$_{Cu }$, we observed the same dip structure as Nb/CoFe(d$_{CoFe})$. We will explain our data with interface resistance and its implications. [Preview Abstract] |
Thursday, March 24, 2005 10:48AM - 11:00AM |
U12.00015: Andreev reflection at the normal-metal / heavy-fermion superconductor (HFS) interface: Point-contact spectroscopy (PCS) studies of CeCoIn5 L.H. Greene, W.K. Park, J.L. Sarrao, J.D. Thompson Andreev reflection between a normal metal and superconductor with highly disparate fermi surface parameters is investigated with PCS using a gold tip and the HFS, single crystal CeCoIn5. Data are taken from 60 K down to 400 mK [W. K. Park et al, cond- mat/0409090] and applied fields up to 9 T. The contact is shown to be in the Sharvin limit with the enhanced sub-gap conductance arising from Andreev refection. The temperature dependence of the zero-bias conductance data are best fit using the extended Blonder-Tinkham-Klapwijk model with a d-wave order parameter [S. Kashiwaya et al. PRB 53, 2667 (1996)]. The highly- suppressed Andreev signal, a signature of normal-metal/HFS junctions, is quantified and theoretical models to account for this are presented. We acknowledge A.J. Leggett, D. Pines, V. Lukic, J. Elenewski, B.F. Wilkin, A.N. Thaler, P.J. Hentges, K. Parkinson, W.L. Feldmann and support by the DoE DEFG02- 91ER45439, through the FSMRL and the Center for Microanalysis of Materials. [Preview Abstract] |
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U12.00016: Keldysh study of point-contact tunneling between superconductors C.J. Bolech, T. Giamarchi We revisit the problem of point-contact tunnel junctions involving one-dimensional superconductors and present a simple scheme for computing the full current-voltage characteristics within the framework of the non-equilibrium Keldysh Green function formalism. The effects of different spin-pairing symmetries, combined with magnetic fields and finite temperatures, at arbitrary bias voltages are addressed. We propose ways of measuring the effects found when the two sides of the junction have dissimilar superconducting gaps and when the symmetry of the superconducting states does not correspond to spin-singlet pairing. [Preview Abstract] |
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