Bulletin of the American Physical Society
14th Annual Meeting of the Northwest Section of the APS
Volume 57, Number 7
Thursday–Saturday, October 18–20, 2012; Vancouver, British Columbia, Canada
Session H4: Condensed Matter II |
Hide Abstracts |
Chair: Brad Johnson, Western Washington University Room: SFU Harbour Centre 1520 Barrick Gold Lecture Room |
Saturday, October 20, 2012 1:30PM - 2:06PM |
H4.00001: Conductance Quantization in Graphene Nanostructures Invited Speaker: George Kirczenow Electrical conductances quantized in integer multiples of the fundamental quantum $2e^2/h$ are the hallmark of ballistic quantum transport in nanostructures such as semiconductor quantum point contacts, gold atomic wires, and carbon nanotubes. However, there have been only a few reports of conductance quantization being observed in graphene nanostructures, and the observed behavior presented significant puzzles: Lin {\em et al.} [Phys. Rev. B \textbf{78}, 161409(R) (2008)] and Lian {\em et al.} [Appl. Phys. Lett. \textbf{96}, 103109 (2010)] observed conductance quantization in graphene nanoribbons. However, surprisingly, the conductance steps that they found were orders of magnitude smaller than the ballistic conductance quantum $2e^2/h$. Tombros {\em et al.} [Nature Physics \textbf{7}, 697 (2011)] reported observing conductance quantization in {\em integer} multiples of $2e^2/h$ in a graphene nanoconstriction with curved boundaries. However, the curvature implies the presence of large numbers of atomic-scale steps along the boundaries. In this respect a graphene constriction differs from semiconductor constrictions whose boundaries, being defined electrostatically, are atomically smooth, and this smoothness is widely believed to be crucial for the observation of conductance quantization. In this talk I will review these experiments and the explanations that we have proposed\footnote{S. Ihnatsenka and G. Kirczenow, Phys. Rev. B \textbf{80}, 201407(R) (2009); Phys. Rev. B \textbf{83}, 245442 (2011); Phys. Rev. B \textbf{83}, 245431 (2011); Phys. Rev. B \textbf{85}, 121407(R) (2012); Phys. Rev. B \textbf{86}, 075448 (2012).} of the quantized conductance phenomena that have been observed in graphene nanostructures. [Preview Abstract] |
Saturday, October 20, 2012 2:06PM - 2:18PM |
H4.00002: Proposal for better control of single molecule magnets for quantum information storage Fatemeh Rostamzadeh Renani, George Kirczenow A molecular nano-magnet (MNM) is a single molecule that contains transition metal atoms that endow it with a stable magnetic moment. Transistors based on MNMs are potential candidates for spintronic devices and information storage. Knowledge of the orientation of the molecule's easy axis relative to leads is important for potential spintronic applications of MNMs but it has not been experimentally controllable. Our calculations reveal the possibility of determining the easy axis orientation experimentally by means of current measurements: We find the lowest unoccupied molecular orbital (LUMO) of the Mn$_{12}$-benzoate MNM to be on ligands, unlike the highest occupied molecular orbital which is on the Mn$_{12}$ magnetic core. Therefore, we predict transport via the LUMO not to be subjected to Coulomb blockade. We predict gate controlled switching between Coulomb blockade and coherent resonant tunnelling in transistors based on such MNMs. We propose that this effect can be used to identify specific experimentally realized MNM transistors in which the easy axis is approximately parallel to the direction of the current flow. [Preview Abstract] |
Saturday, October 20, 2012 2:18PM - 2:30PM |
H4.00003: A continued fraction approach to calculating Auger electron spectra Anamitra Mukherjee, Mona Berciu We present a novel real space approach to calculating few body lattice Green's functions for the Anderson impurity model. Using this we compute the two hole impurity Green's function for the impurity coupling to a one dimensional bath. We show that the geometry of the impurity-bath coupling can introduce new features in the impurity spectral function that survive even when the impurity-bath hybridization is weak. We compare these results with the spectral function for the corresponding periodic Anderson model and identify the parameter regime where the impurity spectral function is a faithful representation of the two hole (Auger) spectra of the periodic system. We comment on the generalization of the method to calculating three hole spectral functions and their application to Auger electron spectra in partially filled band. [Preview Abstract] |
Saturday, October 20, 2012 2:30PM - 2:42PM |
H4.00004: Birefringent fermions and their phase transitions on square lattices Nazanin Komeilizadeh, Malcolm Kennett We introduce a tight-binding model for spinless fermions on a square lattice, which when half-filled has low energy excitations with a Dirac-like dispersion. In the vicinity of these Dirac points there are unusual gapless excitations with the feature that there are two different ``speeds of light.'' This is a consequence of a broken chiral symmetry in the model, which occurs in the kinetic energy term, and hence leaves the spectrum gapless in the vicinity of the Dirac points. This chiral symmetry breaking is fundamentally different from spontaneous chiral symmetry breaking that leads to mass generation in field theoretic models. We investigate the effects of interactions in this model and present the associated phase diagrams and phase transitions when there are nearest neighbour and next nearest neighbour interactions. [Preview Abstract] |
Saturday, October 20, 2012 2:42PM - 2:54PM |
H4.00005: Failure of the Holstein model to describe strong electron-phonon coupling Clemens P.J. Adolphs, Mona Berciu The coupling of electrons to phonons and the properties of the resulting quasiparticle, the polaron, are important for understanding many materials, including strongly correlated systems like the cuprates and the manganites. For some materials, the effective electron-phonon (el-ph) coupling $\lambda$ is well known. For others, like the cuprates, estimates range from very small ($\lambda \sim 0.3$) to extremely large ($\lambda \sim 5$). Here, we point out an inconsistency in the widely used theoretical models. Both the Holstein and the Fr\"ohlich model assume that lattice distortions are sufficiently small to allow treating the el-ph coupling as linear. At strong coupling, however, it is well established that a small polaron forms, with potentially considerable lattice distortions, invalidating the original assumption. We use the momentum average approximation to study the effect of higher-order coupling terms in the Holstein model. We show that they have drastic consequences on the properties of the polaron when compared to the linear model and that these effects cannot be captured by a linear model with renormalized parameters. Since linear models fail to describe strongly coupled systems, estimates of $\lambda$ based on those models have to be reevaluated. [Preview Abstract] |
Saturday, October 20, 2012 2:54PM - 3:06PM |
H4.00006: Hop dynamics in glasses Anton Smessaert, J\"org Rottler Although glassy materials are widely used, we still know little about the underlying physics of the glass state. One key challenge is the non-equilibrium nature, which manifests in time-dependent material properties. A link between this ``aging'' and structural changes is still missing. Elementary structural relaxation events, called ``hops,'' have been identified as particles leaving the shell of their immediate neighbors. We present first results of molecular dynamics simulations with a new algorithm that enables us to track these hops throughout the system. Using a standard polymer glass model, we show a complete ``map'' of the hop-dynamics in the bulk. This data allows us to explore the correlations between relaxation events. Our map of hop-events is also useful in the study of dynamical heterogeneities (DH), a concept that arose from the discovery of ``faster'' and ``slower'' moving groups of particles in glasses. Current research aims at connecting their growth to the glass transition, but their observation and study has proven to be challenging. The hop-detection gives us a coarse-grained picture of the dynamics and we can identify DH as spatio-temporal accumulations of hops. This makes a direct study of DH possible and we present preliminary results based on a cluster analysis. [Preview Abstract] |
Saturday, October 20, 2012 3:06PM - 3:18PM |
H4.00007: Electron-phonon coupling in 1D edge-shared cuprates probed by resonant soft x-ray scattering Steven Johnston, W.S. Lee, B. Moritz, Jeroen van den Brink, Z.-X. Shen, T.P. Devereaux Resonant inelastic x-ray scattering (RIXS) is a powerful probe for studying excitations in strongly correlated systems. With continued advancements of the technique, the overall energy resolution has improved to the point of uncovering low-energy bosonic excitations near the elastic line. In this talk we present evidence for coupling to an optical oxygen phonon in the RIXS spectrum of the quasi-1D edge shared cuprate Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$ at the oxygen K-edge. This mode is identified as a compressive mode polarized perpendicular to the chain direction which modulates the Cu-O charge transfer energy which sets the size exchange interaction. By comparing to small cluster calculations we extract a sizable electron-phonon coupling strength and infer an interplay between the electronic, magnetic, and lattice degrees of freedom. [Preview Abstract] |
Saturday, October 20, 2012 3:18PM - 3:38PM |
H4.00008: BREAK |
Saturday, October 20, 2012 3:38PM - 4:14PM |
H4.00009: Ultrafast Dynamics of Polaron Formation Invited Speaker: Susan Dexheimer The formation of localized electronic states reflects the fundamental physics of coupling between electronic and lattice dynamics, as first noted by Landau who in 1933 described the process of polaron formation as ``the electron `digs its own hole' and is trapped there.'' Localization of electronic states plays a critical role in determining the properties of a wide range of materials: polaron formation has a profound impact on charge transport properties of electronic materials, and formation of self-trapped excitons, or exciton-polarons, dramatically changes optical properties and energy transport mechanisms. I will present femtosecond time-resolved studies of the dynamics of the localization process, focusing on the formation and evolution of self-trapped excitons and polarons. The experiments are carried out in quasi-one-dimensional materials in which the strength of the electron-phonon coupling that drives the dynamics can be systematically tuned by varying the material composition. Experiments using femtosecond vibrationally impulsive excitation, in which the system is excited with an optical pulse short compared to the periods of the relevant vibrational modes, allow us to time-resolve the coupled electronic and lattice dynamics as the system evolves from the initially photoexcited delocalized electronic state to form a self-trapped exciton, revealing rapid dynamics involving both optical and acoustic phonon modes. Polaron dynamics are probed using time-resolved terahertz spectroscopy, in which short pulses of far-infrared light are used to monitor the fast photoinduced carrier response, and show localization on the time scale of a single vibrational period of the lattice. [Preview Abstract] |
Saturday, October 20, 2012 4:14PM - 4:26PM |
H4.00010: Specification, design and commissioning of an ultra-low-vibration facility for Scanning probe microscopy experiments Benjamin MacLeod, Yan Pennec, Vincent Wong, Graeme Adamson Scanning probe microscopes are perhaps best known for being able to image individual atoms in real space. A practical complication of this extreme spatial sensitivity is that these instruments are also extremely sensitive to mechanical vibrations; to approach ultimate levels of performance, these microscopes must therefore be operated in an environment with an extremely low level of mechanical vibrations. In this work, the specification, design and commissioning of a new ultra-low-vibration facility recently completed at the University of British Columbia is presented. Based on the pneumatically-suspended inertial slab concept used at NIST's Gaithersburg facility\footnote{Hal Amick, Bea Sennewald, Norman C. Pardue, Clayton Teague, and Brian Scace, Noise Control Engineering Journal 46, 39-47 (1998).} this system will be used as a highly stable platform for a 50mK Scanning Tunneling Microscope system. [Preview Abstract] |
Saturday, October 20, 2012 4:26PM - 4:38PM |
H4.00011: Optical pump wavelength dependence in visible-pump visible-probe spectroscopy of noble metals Derek G. Sahota, Calvin Lobo, Konrad Duch, J. Steven Dodge We have developed a femtosecond visible-pump visible-probe reflectometer with individually tunable pump and probe photon energies. The spectrometer has been used to study optically thick films of the noble metals Au and Cu over a wide variety of pump fluences and photon energies. Through comparison between experimental measurements and two-temperature model (TTM) simulations, we estimate an electron-phonon coupling constant, $g$, of $2.37 \pm 0.11 \times 10^{16}$ Wm$^{-3}$K$^{-1}$ for Au and $1.19 \pm 0.13 \times 10^{17}$ Wm$^{-3}$K$^{-1}$ for Cu, consistent with previous studies. The variation of the optical pump parameters allows a more accurate determination of the electron-phonon coupling constant. The relaxation rate, $\tau$, of the thermally excited electrons is shown to be strongly dependent on the peak electron temperature of the excited sample, and only weakly dependent on the pump photon energy. The static dielectric constant is found to significantly underestimate the dependence of the differential reflectivity on the pump photon energy. [Preview Abstract] |
Saturday, October 20, 2012 4:38PM - 4:50PM |
H4.00012: High Resolution Photoluminescence Spectroscopy of Doped ZnO nanowires Senthil Kumar Eswaran, Faezeh Mohammadbeigi, Deng Zhiwei, Ian Anderson, Simon Watkins ZnO is a material with a very promising optical properties for visible and ultraviolet optoelectronics applications. The control of doping in single crystal epitaxial material is challenging both for n- and p-dopants due to the large n-type background doping typically present. In this paper we discuss recent efforts to dope ZnO nanowires with various n and p-type dopants using the metalorganic chemical vapor deposition growth technique (MOCVD). The nanowire geometry has the advantage of producing near perfect single crystals decoupled from the highly mismatched sapphire substrate. In this way we are able to observed remarkable narrow PL linewidths as low as 0.17 meV in an ensemble of wires. Careful addition of the group III impurities Al and In results in the unambiguous identification of several sharp line bound exciton features. The addition of n-dopants has strong effects on the nanowire morphology, resulting a in a large increase in the lateral growth rate. Antimony is claimed to produce p-type material by several groups, however we show that careful backdoping of single crystals with antimony results in the appearance of a new donor bound exciton transition, providing confirmation of recent channeling measurements, as well as theoretical predictions that Sb prefers to reside on the Zn sublattice. [Preview Abstract] |
Saturday, October 20, 2012 4:50PM - 5:02PM |
H4.00013: THz conductivity measurement of MnSi Laleh Mohtashemi, Amir Farahani, Eric Karhu, Theodore L. Monchesky, J. Steven Dodge We present measurements of the low-frequency optical conductivity of a thin film of MnSi, using time-domain terahertz spectroscopy. At low temperatures and low frequencies, we extract the DC resistivity, scattering life time and plasma frequency from a Drude fit. We obtain a value of $\omega_p\simeq1.0$ eV, which can be used to estimate the renormalization coefficient through comparison with band theory. At higher temperatures, a deviation from Drude behavior is observed, suggesting a loss of quasi-particle coherence. In the region of low temperatures and high frequencies, we see evidence for a crossover to the anomalous power law dependence observed by Mena \textit{et al.}\footnote{F.P. Mena \textit{et al.} Phys. Rev. B. {\bf67}, 241101(R) (2003).} As the temperature increases, the anomalous frequency dependence becomes more pronounced, and the plasma frequency inferred from a Drude fit increases dramatically. Above T$\approx 50$ K, $\sigma_2(\omega)$ develops a negative slope that is inconsistent with both a Drude model and the anomalous power law observed earlier, indicating a sharp pseudogap in the conductivity spectrum. [Preview Abstract] |
Saturday, October 20, 2012 5:02PM - 5:14PM |
H4.00014: Superfluid density in 2D organic superconductors: evidence for d-wave pairing Sonia Milbradt, Andrew Bardin, Colin Truncik, Wendell Huttema, Paul Carriere, Ben Powell, Paul Burn, Shih-Chun Lo, David Broun Organic superconductors are an exciting ``playground'' for low dimensional physics, with a clean, layered structure that exhibits a variety of electronic phases including superconductivity. The interactions responsible for pairing, and the symmetry of the pair wavefunction, continue to be open issues in these materials. To gain further insights, we have carried out microwave spectroscopy of two BEDT-TTF-based superconductors. Penetration depth measurements reveal a strong, linear temperature dependence of superfluid density, indicating line nodes in the order parameter and providing strong evidence for $d$-wave pairing. Measurements of the microwave conductivity allow us to extract the quasiparticle scattering rate, both above \textit{and} below $T_{c}$. In the normal state, the scattering is strong, at several times the thermal energy. Below $T_{c}$ there is a rapid drop in scattering, with a $T^{3}$ temperature dependence characteristic of $d$-wave quasiparticles scattering from antiferromagnetic spin fluctuations. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700