Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B20: Phase Transitions and Excitations at Surfaces |
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Sponsoring Units: DCMP Chair: Roland Stumpf, Sandia Natl Lab Livermore Room: LACC 407 |
Monday, March 21, 2005 11:15AM - 11:27AM |
B20.00001: Multi-state tunneling conductance of Si (100) dimers at 4 K Yan Pennec, David Fortin, Mark Freeman, Michael Horn von Hoegen We present an STM measurement of the Si(100) surface at 4 K. It shows various phase reconstructions depending on the tunneling conditions, and some flickering dimers. We recorded the telegraph-noise-like spectrum on top of a single dimer. Instead of the thermally activated bi-stable system measured previously at 80 K (Hata et al PRL 86, 3084 (2001)), we found an 8-state system. We correlate the 8 states with the $2^3$ possible conformations between the measured dimer row and its first neighbors along [-110]. The discrete conductance levels found at a fixed position are found to split, crossover and branch in a spatially-resolved scan across a single dimer row. The excitation of the dimer flickering appears driven by the interaction of the hot electrons and the dimers. This new data exposes a significant electronic transport between the rows, a point which has been ignored in most modeling of the surface. Work supported by iCORE and CIAR. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B20.00002: Real-Time Microscopy of Second Order Phase Transitions on Pb/Ge(111) S. Chiang, Y. Sato Using the Low Energy Electron Microscope (LEEM), we have studied phase transitions on the Pb/Ge(111) surface. The critical temperature of the phase transition from $\beta $ to (1x1) depends strongly on the coverage, occurring at 180C for coverage below 1ML and at 270C for coverages above 1.33ML. LEEM data clearly show this difference in the critical temperature derives from the different mechanisms of the transformation. For coverages between 0.33ML and 1ML, where the $\alpha -(\surd $3x$\surd $3)R30\r{ } phase coexists with the $\beta $-($\surd $3x$\surd $3)R30\r{ } phase below the critical temperature and with the (1x1) phase above it, the reversible phase transformation from $\beta $ to (1x1) shows typical second order phase transition behavior, i.e., the intensity of the $\beta $ phase gradually changes with temperature. For coverages above 1 ML, where the surface is either the low temperature $\beta $ phase or the higher temperature (1x1) phase, the system shows an unusual reversible second order phase transition, with fluctuating domains. The frequency analysis of the critical fluctuations will be presented, and the different mechanisms for both of the second order phase transitions mentioned above will be discussed. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B20.00003: Defect-induced period-doubling perturbation on Si(111)4x1-In Geunseop Lee, Sang-Yong Yu, Hanchul Kim, Ja-Yong Koo We investigated using STM and LEED the influence of defects at room temperature on the quasi-one dimensional Si(111)4$\times $1-In surface which changes into a 4$\times $2 (or 8$\times $2) phase below 120 K. Various types of defects (vacancy, step edge, and phase shift boundary) and adatoms (H$_{2}$, O$_{2}$, and In) were found to induce local period-doubling ($\times $2) modulations at room temperature. The $\times $2 modulated region shows metallic $I-V$ characteristics, having little change from that of the defect-free 4$\times $1 region despite the difference in topology in the image. Therefore, the defect-induced $\times $2 modulation is discriminated from the low-temperature phase that was reported to be insulating. Using the first-principles calculations, the $\times $2 modulation is found to originate from a different 4$\times $2 structure of the clean surface that is stabilized by the presence of defects. The nature of the phase transition of this In/Si(111) system and the influence of the defects will be discussed. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B20.00004: Observation of Finite-Size Effects on a Structural Phase Transition of 2D Nano-Islands Ing-Shouh Hwang, Shih-Hsin Chang, Chung-Kai Fang, Tien T. Tsong The monolayer Pb-covered Si(111) surface exhibits a 1$\times $1-Pb phase at room temperature. The structure undergoes a reversible transformation into a $\sqrt 7 \times \sqrt 3 $-Pb reconstruction at low temperatures. With a variable-temperature scanning tunneling microscope (STM), we study the phase transition of Pb nano-islands on Si(111). Our observations indicate that no coverage change occurs across the phase transition. On the 1$\times $1 structure, Pb adatoms are located at the T$_{1}$ site and the coverage is exactly 1 ML. The $\sqrt 7 \times \sqrt 3 $ phase is basically a distorted 1$\times $1 structure with some Pb adatoms slightly displaced from their T$_{1}$ site to form trimer rows. This phase transition may involve only small displacements of Pb atoms without breaking the Pb-Si bonds. Our STM observations also show that the transition temperature decreases with decreasing domain (island) size [1]. The boundaries of the nano-islands also influence the transition. At temperatures around the transition temperature, temporal fluctuations in structures can be seen. Careful examination of the change in the surface structure near the transition temperature reveals the fast dynamics associated with the thermal fluctuations. [1] I.-S. Hwang et al., Phys. Rev. Lett. \textbf{93}, 106101 (2004). [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B20.00005: Cross-over to Mean Field Behavior in Small-World Nanomaterials Xingjun Zhang, M.A. Novotny, T. Dubreus Recently the suggestion was made to include small-world network effects into nanoscale materials [1,2]. Small-world connections lead to (slightly modified) mean-field behavior of model systems [3]. Consequently there is the possibility for a new class of nanomaterials governed by the mean-field fixed point. Here we describe finite-scale scaling investigations of ferromagnetic Ising models adding small-world connections to either $d=1$ or $d=2$ lattices with $V$ sites. In particular, we demonstrate how finite- size scaling works by scaling with a 'length' given by the logarithm of $V$ rather than a linear dimension of the system. Results are compared to studies of a related model with dilute long-range interactions [4]. \newline [1] M.A. Novotny and S.M. Wheeler, Brazilian J. Phys, vol. 34, p. 395 (2004). \newline [2] M.A. Novotny, et al, J. Appl. Phys, in press (preprint cond-mat/0410589). \newline [3] M.B. Hastings, Phys. Rev. Lett., vol. 91, 098701 (2003). \newline [4] R.T. Scalettar, Physica A, vol. 170, p. 282 (1991). [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B20.00006: Direct Evidence of the Charge Ordered Phase Transition of Indium Nanowires on Si(111) Se Jun Park, Han-Woong Yeom, Suk-Hwa Min, Do-Hyang Park, In-Whan Lyo The self-organized indium nanowires on Si(111) surface is an ideal model system exhibiting an 1D metallic character on a surface. As temperature is lowered, this system undergoes a reversible phase transition from a (4 x 1) phase at room temperature to a (8 x 2) phase at $\sim $125 K via periodicity doubling along the nanowires. By means of low-temperature scanning tunneling microscopy and spectroscopy, we investigated the temperature dependent electronic structures of the system. We found that the phase transition is truly a metal-insulator transition with an energy gap opening and the low-temperature phase is indeed \textit{charge ordered }along the wires, with the out-of-phase distribution of the occupied and unoccupied states$^{1}$. The analysis of charge orderings near defects above $T_{c}$ reveals that a lattice distortion can be distinguished from a charge ordering. A fluctuating charge-ordered state is also found in In wires terminated by two out-of-phase defects. Our observations provide the crucial evidence for the CDW ground state of the system. $^{1}$S. J. Park, H. W. Yeom, S. H. Min, D. H. Park, and I. W. Lyo, Phys. Rev. Lett. 93, 106402 (2004) [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B20.00007: Charge Order in Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ Films C.S. Nelson, M. Rajeswari, M. Overby, V. Smolyaninova, R. Kennedy X-ray scattering experiments were carried out on epitaxial Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ films grown on LaAlO$_{3}$, SrTiO$_{3}$, and NdCaAlO$_{4}$ substrates. Incommensurate peaks, which are believed to be indicative of the presence of charge order, with a purely in-plane wavevector were observed in all films at low temperatures. Additional characteristics of the charge order peaks were measured using resonant x-ray scattering techniques, and the magnitude of the wavevector, the ordering temperature, the direction of the lattice modulation, and the correlation length were found to vary as a function of substrate. The effects of film thickness were also studied for films grown on SrTiO$_{3}$ substrates, and only minor differences were observed for films of similar quality that differed in thickness by a factor of 2. Finally, of all the films investigated, the highest ordering temperature was observed in a $\sim $200 nm thick film grown on a SrTiO$_{3}$ substrate, in which the charge order peaks persisted to temperatures above 300 K. The work at Brookhaven was supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B20.00008: Strain Phase Diagram of SrTiO$_3$ Thin Films Feizhou He, B.O. Wells, S.P. Alpay, S.M. Shapiro SrTiO$_3$ thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of oxide films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained, epitaxial films of SrTiO$_3$ were grown on different substrates. The structural phase transition temperature $T_c$ increases from 105 K in bulk STO to 167 K for films under tensile strain and 330 K for films with compressive strain. The measured temperature-strain phase diagram is qualitatively consistent with theory [1], however the increase in $T_c$ is much larger than predicted in all cases. The symmetry of the phases involved in the transition is different from the corresponding bulk structures largely because of epitaxial constraint, the clamping effect. Thus the shape of the STO unit cell is tetragonal at all temperatures. The possibility exists of a very unique low temperature phase with orthorhombic symmetry ($Cmcm$) but tetragonal unit cell shape. More generally, we have characterized at least three different manifestations of the clamping effect, showing it is much more subtle than usually recognized. This work is supported through NSF DMR-0239667, DMR-0132918, by the Research Corp, and at BNL by the US DOE DE-AC02-98CH10886. [1] N. A. Pertsev, A. K. Tagantsev and N. Setter, Phys. Rev. B61, R825 (2000). [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B20.00009: Phase Transitions in Hexane Physisorbed onto Graphite Carlos Wexler, Michael Roth, Cary Pint We report the results of molecular dynamics (MD) simulations of hexane physisorbed onto the basal plane of graphite. At low temperatures the system forms a herringbone solid. For monolayer coverages a solid to nematic liquid crystal transition takes place at $T_1 = 138 \pm 2$K followed by another transition at $T_2 = 176 \pm 3$K into an isotropic fluid. We characterize the different phases by calculating various order parameters, coordinate distributions, energetics, spreading pressure and correlation functions, most of which are in reasonable agreement with available experimental evidence. In addition, we perform simulations where the Lennard-Jones interaction strength, corrugation potential strength and dihedral rigidity are varied in order to better characterize the nature of the two transitions through. We find that both phase transitions are facilitated by a ``footprint reduction'' of the molecules via tilting, and to a lesser degree via creation of gauche defects in the molecules. Furthermore, submonolayer coverages see a diminished role for the nematic phase, whereas supramonolayer coverages present a more robust nematic. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B20.00010: Vibrational recognition of adsorption sites for CO adsorbed on transition-metal surfaces Ismaila Dabo, Cody Friesen, Andrzej Wieckowski, Nicola Marzari We have studied the vibrational properties of carbon monoxide adsorbed on transition-metal surfaces, using density-functional perturbation theory in the GGA-PBE approximation. The full dispersions for CO adsorbed on high-symmetry surface sites have been calculated, obtaining excellent agreement with spectroscopic measurements. This agreement in predicting absolute frequencies and frequency shifts as a function of the adsorption site is at variance with the poor performance of most exchange- correlation functionals in describing the energetics of adsorption. We provide a simple physical picture justifying this accuracy. These results open the possibility to directly connecting experimental vibrational spectra with the microscopic details of adsorption geometries. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B20.00011: The role of localization in interpreting laser-grating experiments on H diffusion on Pt(111) Stefan Badescu, Wone-Keun Han, See-Chen Ying Recently, H diffusion on Pt(111) surfaces has been studied with laser-grating methods [1]. The puzzling fact is that the new values obtained for the diffusion barrier E$_{b}$ of $\sim $160 meV at low coverage and of $\sim $105 meV at high coverage are much higher than the corresponding barrier measured through quasielastic helium atom scattering [2] ($\sim $ 68 meV). We interpret the diffusive motion observed in these experiments as evidence of quantum tunneling of the H atoms after thermally activation to higher vibrational bands [3] in the presence of surface disorder. The first excited band has an Anderson localization length small compared with the grating separation, while still larger than the characteristic length scale in the helium scattering experiment. This accounts for the apparent discrepancy of the two experiments. Numerical simulations are under way to substantiate this argument. [1] C. Z. Zheng \textit{et al}., Phys. Rev. B \textbf{77}, 205402 (2004) [2] A. P. Graham \textit{et al.}, J. Chem. Phys. \textbf{111}, 1676 (1999) [3] S. C. Badescu \textit{et al.}, Phys. Rev. Lett. \textbf{88}, 136101 (2002) [Preview Abstract] |
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B20.00012: Atomic-scale Observation of Condensation and Fluctuation of Charge Density Wave in quasi-1D Metal-Insulator Transition Se Jun Park, Joung Real Ahn, Han-Woong Yeom, Eun-Young Choi, Woo-Jin Jung, In-Whan Lyo Metal-insulator transition of two-dimensional In nanowire arrays on Si(111) surface is studied near the transition temperature ($T_{c})$. While low energy electron diffraction (LEED) analysis of the system shows a continuous order behavior, however, it is found that both high temperature metallic phase and low temperature insulating phase of charge density waves (CDW) coexist as nanometer-scale domains below $T_{c}$. More strikingly, below $T_{c}$, nano-scale metallic domains exhibits strong one-dimensional (1D) fluctuation associated with CDW condensates in forms of short, fluctuating 1D segments. Similar events are also found at the domain boundaries between metallic and insulating domains, however, with stronger transverse coupling of CDW fluctuations. These are the first observation of CDW condensation and fluctuation in real-space. In these phenomena, defects appear to play crucial roles in maintaining metallic domains below $T_{c}$. [Preview Abstract] |
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B20.00013: Field-Induced Reversible Phase Manipulation in Metal-Insulator Transition using Scanning Tunneling Microscopy Se Jun Park, Chang-Sub Byun, Do-Hyang Park, In-Whan Lyo Reversible electronic switching between insulating and metallic phases is a novel idea that may allow new types of field effect devices feasible.$^{1}$ Here we demonstrate the reversible manipulation between metallic and insulating phases in two-dimensional In nanowire arrays on Si(111) surface near the metal-insulator transition temperature ($T_{c})$. The electronic switching of phases was induced by local electric field applied by the probe tip of a scanning tunneling microscope. The field-dependent hysteresis behavior was also observed in tip height measurements as a function of the sample bias, under the constraint of constant tunneling current. A model including the intrinsic bi-stability of the nanometer-scale domains of In nanowire arrays will be discussed. $^{1}$C. Ahn, J. Triscone, J. Mannhart, \textit{Nature }6952, 1015 (2003) [Preview Abstract] |
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B20.00014: Scattering times of image-state electrons on flat and stepped bimetallic Ag/Pt surfaces Serban Smadici, Richard Osgood Jr. Scattering dynamics of electrons on flat Ag/Pt(111) and stepped Ag/Pt(997) surfaces was probed by femtosecond angle-resolved 2-photon-photoemission. An asymmetric splitting of the n=1 image-state dispersion for 1 ML-Ag covered stepped Pt(997) surface was clearly observed and is tentatively attributed to diffraction of the final state electron. Our femtosecond pump-probe setup enabled measurement of the 2-photon cross-correlation traces, which access the scattering dynamics of image-state electrons. In contrast to the Ag(111) surface, for 2 ML-Ag covered Pt(111) surface, the lifetime of the n=2 image state was observed to be larger than that of the n=1 image state. This behavior is attributed to the overlap of image-state wave functions with Pt substrate bulk electrons and is in agreement with the alignment of the image-state energies with respect to projected bulk bands. The momentum dependence of the image-state scattering time for the stepped 2 ML Ag/Pt(997) surface and flat 2 ML Ag/Pt(111) surface will also be discussed. This work was supported by DOE under contract number DE-FG02-90ER14104. [Preview Abstract] |
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