Bulletin of the American Physical Society
11th Annual Meeting of the Northwest Section of APS
Volume 54, Number 6
Thursday–Saturday, May 14–16, 2009; Vancouver, BC, Canada
Session G2: Condensed Matter Physics III |
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Chair: Malcolm Kennett, Simon Fraser University Room: Irving Barber Learning Center 260 |
Saturday, May 16, 2009 1:30PM - 2:06PM |
G2.00001: Optical coupling to spin waves in multiferroic materials Invited Speaker: The coexistence of ferroelectricity and magnetism in multiferroic materials leads to several interesting effects related to the interplay of light with complex electric and magnetic order. One notable example is bismuth ferrite (BiFeO$_3$), a room temperature multiferroic that exhibits a large ferroelectric moment coexisting with a spiral antiferromagnetic phase in the form of a cycloid. I will describe a theory of bulk BiFeO$_3$, which predicts the appearance of several magnon branches related to magnetic fluctuations at integer multiples of the cycloid wavevector. These magnons get admixed with optical phonons at zero wavevector, giving rise to two series of electromagnon resonances in the far infrared spectrum [1], which were recently observed using Raman spectroscopy [2]. I will show that these results are helpful in designing low loss electronic devices based on spin-wave propagation [3]. \\[4pt] [1] R. de Sousa and J.E. Moore, Phys. Rev. B. 77, 012406 (2008). \\[0pt] [2] M. Cazayous, Y. Gallais, A. Sacuto, R. de Sousa, D. Lebeugle, and D. Colson, Phys. Rev. Lett. 101, 037601 (2008). \\[0pt] [3] R. de Sousa and J.E. Moore, Appl. Phys. Lett. 92, 022514 (2008). [Preview Abstract] |
Saturday, May 16, 2009 2:06PM - 2:18PM |
G2.00002: Microwave Spectroscopy of Heavy Fermion Superconductors at MilliKelvin Temperatures Colin Truncik, Wendell Huttema, Paul Carriere, Patrick Turner, David Broun, John Sarrao, Cedomir Petrovic Heavy fermion metals are of immense interest due to the extreme renormalization of quasiparticle mass, the possibility of non-Fermi-liquid physics, and the appearance of superconductivity on the verge of magnetic order, in the vicinity of quantum critical points. We have setup a novel system for high resolution microwave spectroscopy at milliKelvin temperatures, across the frequency range 2 to 40 GHz. We are using this to study the unconventional superconducting states in a number of Ce- and U-based heavy fermion compounds. I will present an overview of the microwave spectroscopy system, and a summary of some of the interesting physics we have observed in these systems. [Preview Abstract] |
Saturday, May 16, 2009 2:18PM - 2:30PM |
G2.00003: Conductivity anisotropy in CaRuO$_3$ Graham Lea, Amir Farahani, Saeid Kamal, Dong-Min Kim, Chang Beom Eom, J. Steven Dodge We present terahertz time-domain spectroscopy measurements of the optical conductivity anisotropy in thin films of CaRuO$_3$. Linearly polarized terahertz-bandwidth pulses are transmitted through the films and extended Drude conductivity parameters in the ab-plane and c-axis are extracted. We observe strong optical mass renormalization at low temperatures in both directions that we attribute to interactions with bosonic modes of the system. For both directions we obtain the plasma frequency from the relation $\omega_{p}^{2} = 1/\rho\tau$, where $\rho$ is the dc resistivity and $\tau$ the scattering lifetime in the low-frequency limit. Contrary to the prediction of band theory,\footnote{I. I. Mazin and D. J. Singh, Phys. Rev. B {\bf 56}, 2556 (1997); {\bf 73}, 189903(E) (2006).} we find that the anisotropy in the plasma frequency is less than ten percent. [Preview Abstract] |
Saturday, May 16, 2009 2:30PM - 2:42PM |
G2.00004: Charge-density wave formation in interacting two-dimensional electronic systems with Landau level mixing Peter Smith, Malcolm Kennett Anisotropic transport in half-filled Landau levels has been explained in terms of charge-density wave (CDW) formation. We use the Hartree-Fock approximation to study the influence of electron-electron interactions and Landau level mixing on the formation of CDWs in two-dimensional electron and hole systems. For the situation of two nearly degenerate eigenstates, we construct a Landau free-energy theory appropriate for competing order parameters that allows for both striped and triangular CDW formation. We find the possibility of coexisting CDW ordering in the two states, along with possible hysteretic behaviour. This physics might be realized using an external parameter such as spin-orbit coupling to tune states into near degeneracy. [Preview Abstract] |
Saturday, May 16, 2009 2:42PM - 2:54PM |
G2.00005: Quantum critical scaling in magnetic field near the Dirac point in graphene Bitan Roy, Igor Herbut Graphene, a monolayer of graphite, exhibits some peculiar electronic properties which are consequences of the pseudo relativistic Dirac like excitations. The anomalous integer quantum Hall effect, i. e. platoues in Hall conductivity $\sigma_{xy}$ at filling factors $f=\pm(4 n+2)$, which can be understood within the framework of non-interacting Dirac like quasiparticles is one of such. On the other hand, the appearance of additional Hall states at filling factors $f=0$ and $f=\pm 1$ at higher magnetic fields calls for electron-electron interactions to be taken into account. Motivated by the recent measurement of the activation energy at the quantum Hall state at the filling factor $f =1$ in graphene, I will discuss the scaling of the interaction-induced gaps in the vicinity of the Dirac point with the magnetic field. The gap at $f =1$ is shown to be bounded from above by $E(1)/C$, where $E(n)={v_F}\sqrt{2nB}$ is the Landau-level energy and $C=5.985+O{1/N}$ is a universal number. The universal scaling functions computed exactly for a large number of Dirac fermions N will also be presented. The sublinear dependence of the gap at the laboratory fields of $10 T [Preview Abstract] |
Saturday, May 16, 2009 2:54PM - 3:14PM |
G2.00006: BREAK
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Saturday, May 16, 2009 3:14PM - 3:50PM |
G2.00007: The metal-insulator transition in vanadium dioxide nanobeams Invited Speaker: Solid materials in which electron-electron correlations are strong can exhibit dramatic phase transitions, at which an abrupt change in the electronic properties occurs with a small accompanying distortion of the lattice. Such transitions could be harnessed to make electronic or optoelectronic devices or sensors embodying different principles from those in present semiconductor technology. A famous example is the metal-insulator transition in vanadium dioxide which occurs at 67 $^{\circ}$C at ambient pressure. VO$_{2}$ is a stable, strong material with a simple structure. Unfortunately though, applications and methodical studies of this and other phase transitions are hindered by broadening, hysteresis and mechanical degradation at the transition, caused by the inevitable domain structure. Nanostructures of the material which are smaller than the characteristic domain size do not show these problems. Using devices made from nanobeams of VO$_{2}$ we have been able to achieve good control of the transition and to determine a number of its properties for the first time. For instance, we find that the metallic phase can be supercooled by more than 50 $^{\circ}$C; that the resistivity of the insulator in coexistence with the metal is independent of temperature; and that the transition occurs via the intermediate M2 phase. We also study nanoelectromechanical effects where reversible buckling of the nanobeam is coupled to the phase transition, and we investigate methods of controlling the phase transition, for example using a gate voltage. [Preview Abstract] |
Saturday, May 16, 2009 3:50PM - 4:02PM |
G2.00008: Gap spectroscopy in underdoped YBa$_2$Cu$_3$O$_{6+x}$ from superfluid spectral weight and intrinsic pair tunneling Wendell Huttema, Patrick Turner, Ruixing Liang, Walter Hardy, Doug Bonn, David Broun Numerous experiments on cuprate high temperature superconductors reveal evidence for two different energy gaps: a normal state pseudogap that grows on underdoping, and a superconducting gap connected to the transition temperature $T_c$ [1]. Here we use two different experiments to probe the superconducting energy gap scale in highly underdoped YBa$_2$Cu$_3$O$_{6+x}$: intrinsic pair tunneling perpendicular to the CuO$_2$ planes; and measurements of in-plane superfluid spectral weight. The measurements have been carried out on special samples in which doping can be tuned continuously using controlled oxygen annealing at room temperature [2]. We present data for $T_c$ in the range of 3~K to 17~K, and show that the superconducting energy gap is indeed linked to $T_c$ (2$\Delta \sim 4 k_B T_c$), vanishing as $T_c \rightarrow 0$. \\[4pt] [1] H\"{u}fner \emph{et al.} Rep. Prog. Phys. 71, 062501 (2008)\\[0pt] [2] Broun \emph{et al.} Phys. Rev. Lett. 99, 237003 (2007) [Preview Abstract] |
Saturday, May 16, 2009 4:02PM - 4:14PM |
G2.00009: Logarithmic flux flow resistivity across the cuprate phase diagram Xiaoqing Zhou, Wendell Huttema, Patrick Turner, David Broun, Ben Morgan, John Waldram, Darren Peets, Ruixing Liang, Walter Hardy, Doug Bonn The dynamic properties of vortices in high quality YBa$_2$Cu$_3$O$_{6+x}$ and Tl$_2$Ba$_2$CuO$_{6+x}$ samples have been studied using high resolution microwave spectroscopy. The flux flow resistivity, a quantity which is intimately related to dissipation from electronic states near the vortex cores when vortices move, has been accurately measured at temperatures far below $T_c$, applied fields far below $B_{c2}$ and dopings that span the entire superconducting region of the cuprate phase diagram. Here we report, for the first time, an universal logarithmic upturn in the temperature dependence of the flux-flow resistivities at sufficiently low temperature. Such upturns have strong resemblance to the ``normal'' resistivities of the highly underdoped cuprates and persist to the highly overdoped side, where the normal state resistivities are metallic. We suggest that this resistivity upturn is an intrinsic property of vortices in cuprates, which has strong implications for the nature of the underdoped cuprate normal state. [Preview Abstract] |
Saturday, May 16, 2009 4:14PM - 4:26PM |
G2.00010: Novel application of the kernel polynomial method to inhomogeneous superconductivity Lucian Covaci, Mona Berciu Inhomogeneities (surface, interfaces, impurities, etc.) in superconductors give rise to interesting phenomena, like broken time-reversal states, bound states near surfaces, etc. Numerical solutions of the self-consistent Bogoliubov-de Gennes mean field equations become computationally intensive for systems whose translational symmetry is broken. We propose a new method of solving the mean-field equations based on the Kernel Polynomial Method by expanding the Green's functions in terms of Chebyshev polynomials and calculating the order parameters self-consistently. The benefits of this method are multiple: usage of large systems, easy implementation of symmetries, multiple bands. Although we apply this method to a specific example (formation of Andreev states in 2D superconductors), it is applicable to any mean-field calculation. [Preview Abstract] |
Saturday, May 16, 2009 4:26PM - 4:38PM |
G2.00011: Time-resolved terahertz photoconductivity of insulating cuprates Amir Farahani, Jesse Petersen, Ruixing Liang, J. Steven Dodge We use a visible pump, terahertz probe technique to study the photoconductivity of the undoped cuprates. We use ultrafast optical pulses ($E_{\mathrm{pump}} = 3.1$~eV) to create photocarriers in high quality single crystals of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$, and time-domain terahertz spectroscopy to probe the resulting photoconductivity. We observe a rapid onset of photoconductivity followed by a non-exponential relaxation on a picosecond timescale. This dynamics is independent of photocarrier concentration over the range of 0.2 to 1.7 percent excitations per copper atom. Assuming a quantum efficiency of unity, we infer a mobility of $\sim$0.2 cm$^2$/Vs, significantly lower than the Hall mobility in chemically doped systems.\footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)} As the fluence is increased, there is a weak decrease in the photoconductivity amplitude. We also measured the frequency dependence of the photoconductivity in the terahertz range, and observe an increase in photoconductivity with frequency up to 600 GHz, suggesting polaronic effects. [Preview Abstract] |
Saturday, May 16, 2009 4:38PM - 4:50PM |
G2.00012: Developments in Scanning Hall Probe Microscopy Taras Chouinard, Ricky Chu, Nigel David, David Broun Low temperature scanning Hall probe microscopy is a sensitive means of imaging magnetic structures with high spatial resolution and magnetic flux sensitivity approaching that of a Superconducting Quantum Interference Device. We have developed a scanning Hall probe microscope with novel features, including highly reliable coarse positioning, in situ optimization of sensor-sample alignment and capacitive transducers for linear, long range positioning measurement. This has been motivated by the need to reposition accurately above fabricated nanostructures such as small superconducting rings. Details of the design and performance will be presented as well as recent progress towards time-resolved measurements with sub nanosecond resolution. [Preview Abstract] |
Saturday, May 16, 2009 4:50PM - 5:02PM |
G2.00013: X Marks the Spot: Scanning for Magnetic Scientific Treasure Using Hall-Effect Sensors Ricky Chu, Nigel David, Taras Chouinard, Adam Schneider, David Broun Scanning Hall probe microscopy is a quantitative magnetic imaging technique that provides high spatial resolution combined with high flux sensitivity, occupying a unique niche in magnetic microscopy [S.J. Bending, Adv. Phys. \textbf{48}, 449 (1999)]. Hall sensors are useful in studying materials with microscopic or nanoscale magnetic structures, like high temperature superconductors and magnetic thin films. Development of conventional semiconductor Hall sensors has stalled due to problems with charge depletion and thermal noise. Sandhu recently produced bismuth Hall probes in an effort to avoid these effects [A. Sandhu \textit{et al}. Jpn. J. Appl. Phys. \textbf{40}, L524 (2001)]. The bismuth probes lack a good model to optimize their performance. I will propose a refinement of the current model with an increased emphasis on material parameters that can be more intuitively manipulated. I will show that the fundamental limit of the Hall probe flux sensitivity is comparable to that of a SQUID, the most sensitive known magnetic sensor. I will also propose a definition for spatial resolution to standardize characterization procedures for Hall sensors. [Preview Abstract] |
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