Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P2: Quantum Oscillations, Superconductivity, and Pseudogaps in Nanoscaled Metal Films and Islands |
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Sponsoring Units: DCMP Chair: Mei-Yin Chou, Georgia Institute of Technology Room: Oregon Ballroom 202 |
Wednesday, March 17, 2010 8:00AM - 8:36AM |
P2.00001: One-Dimensional Shell Effects in Thin Metal Films Invited Speaker: During epitaxial growth and thermal processing of metal thin films on semiconductors, the system often self-organizes into domains or islands of preferred heights. This extra stability for specific film thicknesses has an electronic origin; confinement of the electrons in the film causes the formation of quantum-well states, which cross the Fermi level periodically as the film thickness increases. These periodic Fermi level crossings give rise to one-dimensional shell effects, akin to the shell effects associated with the periodic table. As a result, large variations in film properties are expected, including the surface energy that determines the stability and preferred heights of the film, the work function and other surface properties, and the superconducting gap and transition temperature. This talk will present highlights in recent advances on this subject, including angle-resolved photoemission measurements of the electronic structure and experimental and theoretical determinations of the amplitude, damping, period, and phase relationships of the property variations. The roles played by the substrate and the effects of phonon scattering will also be discussed. See T. Miller, M. Y. Chou, and T.-Chiang, ``Phase relations associated with one-dimensional shell effects in thin metal films,'' Phys. Rev. Lett. \textbf{102}, 236803 (2009), and references therein. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 9:12AM |
P2.00002: Superconductivity at the Two-dimensional Limit Invited Speaker: Superconductivity in the extreme two-dimensional limit is studied on ultra-thin lead (Pb) films down to two atomic layers, where only a single channel of quantum well states exist. Scanning tunneling spectroscopy reveals that local superconducting order remains robust until two atomic layers, where the transition temperature abruptly plunges to lower values depending sensitively on the exact atomic structure of the film. Our result shows that Cooper pairs can still form in the last two dimensional channel of electron states, although their binding are strongly affected by the substrate [1]. In this presentation, I will also discuss this new result in comparison with several recent experimental results on ultra-thin metallic films performed using local STS probes and macroscopic transport measurements.\\[4pt] [1] ``Superconductivity at the Two-dimensional Limit,'' Shengyong Qin, Jungdae Kim, Qian Niu and Chih-Kang Shih, Science 324, 1314 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:48AM |
P2.00003: Quantum-size effects in ultrathin Pb-islands on Si(111): From quantum well states to the reduction of the superconducting gap Invited Speaker: Using low-temperature scanning tunneling spectroscopy at 5 and 50 K, we studied the linewidth of unoccupied quantum-well states in ultrathin Pb islands, grown on Si(111) on two different Pb/Si interfaces. A quantitative analysis of the differential conductance spectra allowed us to determine the electron- electron (e-e), electron-phonon (e-ph), and the interface contributions to the lifetime. Layer-dependent ab initio calculations of the e-ph linewidth contribution are in excellent agreement with the data. Importantly, the sum of the calculated e-e and e-ph lifetime broadening follows the experimentally observed quadratic energy dependence [1]. The energy gap of these superconducting Pb islands, in a thickness range between 60 and 5 monolayer, was found to decrease from its bulk value as a function of inverse island thickness. Corresponding values of the critical temperature Tc, estimated using the bulk gap-to-Tc ratio, are in quantitative agreement with ex-situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer-dependent ab initio density functional calculations for freestanding Pb films show that the e-ph coupling constant, determining Tc, decreases with diminishing film thickness [2].\\[4pt] [1] I.-P. Hong, C. Brun, F. Patthey, I. Yu. Sklyadneva, X. Zubizarreta, R. Heid, V. M. Silkin, P. M. Echenique, K. P. Bohnen, E. V. Chulkov, and W.-D. Schneider, Phys. Rev. B (RC) 80, 081409 (2009).\\[0pt] [2] C. Brun, I.-P. Hong, F. Patthey, I. Yu. Sklyadneva, R. Heid, P. M. Echenique, K. P. Bohnen, E. V. Chulkov, and W.-D. Schneider, Phys. Rev. Lett. 102, 207002 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:24AM |
P2.00004: Pseudogap Mediated by Quantum-Size Effects in Pb Islands Invited Speaker: Pseudogap is an important state of high temperature superconducting (HTS) materials. The origin of it, however, is still a debated issue in the HTS field. Bulk conventional superconductor is known not to have any pseudogap state. However, in our recent experiment, we discovered that nanostructured Pb island grown on Si(111)-(7x7) surface possesses a pseudogap. In many aspects, the behavior of this pseudogap is similar to what has been observed for HTS with scanning tunneling spectroscopy (STS). In this talk, I will present our results of the novel pseudogap, or in special cases a pseudopeak, around the Fermi level for Pb islands on Si(111) using STS measurements at high energy resolution. These gap or peak features persist to temperatures as high as $\sim $80 K and are uniquely related to the quantum well nanostructure of the Pb islands. Through a systematic analysis of the STS spectra of various Pb island thicknesses, we found that electron-phonon scattering and quantum confinement conspire to give rise to the pseudogap features in this system. This novel manifestation of quantum size effects has not been investigated before and may shine light in understanding the pseudogap state in HTS. Our findings also add a new dimension for the quantum confinement effects in metallic thin films/islands in addition to the quantum well states and thickness dependent superconductivity. \\[4pt] References: [1] Kedong Wang, Xieqiu Zhang, M. M. T. Loy, T.-C. Chiang, Xudong Xiao, ``Pseudogap Mediated by Quantum-Size Effects in Lead Islands'', Phys. Rev. Lett. 102, 076801 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 11:00AM |
P2.00005: Tuning the Quantum Stability and Superconductivity of Ultrathin Metal Alloys Invited Speaker: Superconductivity is inevitably suppressed in reduced dimensionality. In this talk, I will show how quantum confinement of itinerant electrons in a soft metal, Pb, can be exploited to stabilize atomically-smooth crystalline superconductors with lateral dimensions of the order of a few millimeters and vertical dimensions of only a few atomic layers. The thin film growth experiments were done in ultrahigh vacuum and the superconductive properties of the films were probed ex-situ with (contactless) magnetic susceptibility measurements and dc transport measurements. These extremely thin superconductors show no indication of defect-or fluctuation-driven suppression of superconductivity and sustain enormous supercurrents of up to ten percent of the theoretical depairing current density. Their magnetic hardness implies a superconducting critical state with strong vortex pinning that is attributed to quantum trapping of vortices. Our study paints a conceptually appealing picture of a model nanoscale superconductor with calculable critical state properties and surprisingly strong phase coherence. These results are contrasted with in-situ measurements of the order parameter using local probes such as scanning tunneling spectroscopy. I will furthermore discuss how the superconductive properties of these films can be tailored by alloying or `Fermi surface engineering'. This includes the possibility of kinetically stabilizing metallic alloys of thermodynamically immiscible elements through size quantization. Superconductivity data from these alloys films narrow down the possible mechanisms for the general Tc suppression in Pb-based alloy films and furthermore reveal that critical supercurrents are already established in two-monolayer thin films. [Preview Abstract] |
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