Bulletin of the American Physical Society
15th Annual Meeting of the Northwest Section of the APS
Volume 59, Number 6
Thursday–Saturday, May 1–3, 2014; Seattle, Washington
Session C2: Condensed Matter I |
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Chair: Matt McCluskey, Washington State University Room: Alder Commons 107 |
Friday, May 2, 2014 1:30PM - 2:00PM |
C2.00001: The Dynamic Hubbard Model: what might be missing in current descriptions of strongly correlated electrons in solids Invited Speaker: Frank Marsiglio Electrons move around and they interact with one another via the Coulomb interaction. When electrons form extended (i.e. Bloch) states in metals they do the same thing. Yet very often they form a collective exotic state like superconductivity. Is this the consequence of pairing via an attractive interaction, or is something more subtle at work? This talk will attempt to do several things: (i) present a ``state of the union'' summary of our understanding of superconductivity, (ii) explain the physics of the Dynamic Hubbard model (DHM) in terms that undergraduates can understand, and (iii) present some results achieved for the DHM. [Preview Abstract] |
Friday, May 2, 2014 2:00PM - 2:12PM |
C2.00002: \textit{d}-Wave Superconductivity and Quasiparticle Dynamics in $\rm KFe_2As_2$ Megan A. Boothby, A.J. Koenig, Wendell A. Huttema, Colin J.S. Truncik, Natalie C. Murphy, David Deepwell, Xianhui Chen, David M. Broun Recent work in superconductivity focuses largely on unconventional superconductors that have a layered structure, such as the pnictide, $\rm KFe_2As_2$. Among other potential benefits, these tend to display a much higher critical temperature than conventional superconductors. It is always interesting to investigate the mechanism for forming Cooper pairs. In our experiment, we probe the nodal structure of the superconducting energy gap in $\rm KFe_2As_2$ to determine pairing symmetry by using milliKelvin microwave spectroscopy. We find that the superfluid density has a linear temperature dependence, which provides compelling evidence for line nodes and \textit{d}-wave pairing. We also investigate the relaxation dynamics of thermally excited quasiparticles, wherein we discover a rapid collapse in scattering below $\rm T_c$, much like the high-$\rm T_c$ cuprate superconductors. I will present surface impedance data taken at temperatures down to 0.1K, from which we obtain complex microwave conductivity and superfluid density. [Preview Abstract] |
Friday, May 2, 2014 2:12PM - 2:24PM |
C2.00003: Vibrating vortices as a probe of superconductivity Natalie Murphy, Xiaoqing Zhou, Eric Thewalt, Wendell Huttema, Colin Truncik, Kevin Morse, John Sarrao, David Broun A characteristic property of ordered phases of matter is the spectrum of quasiparticle excitations they support at low energies. Examples include phonons in crystals, rotons in superfluid $^{\mathrm{4}}$He, and Bogoliubov quasiparticles in BCS-type superfluids such as $^{\mathrm{3}}$He and metallic superconductors. ~In the case of superfluid $^{\mathrm{3}}$He, a set of particularly beautiful experiments by the Lancaster group used vibrating wire resonators to probe the quasiparticle spectrum. In short, the dynamics of the vibrating wire are modified by their interaction with the surrounding quasiparticle gas, and are detected electrically. We have developed an analogous technique for studying quasiparticles in superconductors, with the vibrating wire replaced by superconducting vortices oscillating at microwave frequencies. The key discovery that allows the technique to work is that, in certain unconventional superconductors, the vortex core contains very few states - the dynamics of the moving vortex instead are dominated by its interactions with the gas of extended quasiparticle states in which it is embedded. I will establish the validity of technique using data on high temperature superconductors, and then show how it can be used to obtain new physics in the heavy fermion superconductor CeCoIn$_{\mathrm{5}}$. [Preview Abstract] |
Friday, May 2, 2014 2:24PM - 2:36PM |
C2.00004: Optics with microwaves in heavy fermions David Broun In so-called ``heavy-fermion'' metals, the hybridization of the conduction band with electrons localized in partially filled $f$ orbitals leads to the formation of heavy quasiparticles, for which the effective mass can be renormalized by a factor of 100 or more. However, the itinerant nature of these quasiparticles competes with a tendency to form more conventional, magnetically ordered states. These materials are therefore situated near a quantum critical point --- a zero-temperature phase transition driven by the competition between kinetic energy and potential energy --- a conflict between itinerancy and localization that lies at the heart of all correlated electron materials. Along with mass enhancement, the scattering dynamics in heavy fermion compounds also undergo a strong renormalization. This critical slowing-down brings important electronic timescales, such as electronic scattering rates, down into the GHz range, where optical-type measurements and analyses can be carried out with microwaves. We have developed a dilution-refrigerator-based system for carrying out these measurements, and have used it to study a range of heavy fermion materials such as CeCoIn$_5$, UBe$_{13}$ and URu$_2$Si$_2$. An overview of our most striking results will be presented. [Preview Abstract] |
Friday, May 2, 2014 2:36PM - 2:48PM |
C2.00005: Quantum well state induced oscillation of pure spin currents in Fe/Au/Pd(001) systems Eric Montoya, Bret Heinrich, Erol Girt In normal metals, such as Au, Cu, and Ag, the transport of pure spin current is well described by spin diffusion theory. In nonmagnetic materials having a large Stoner enhancement, such as Pd and Pt, strong spin-spin correlation effects lead to local fluctuating magnetic moments known as paramagnons. Interaction with these paramagnons leads to decoherence of spin currents on much shorter length scales than in normal metals. Since spin transport through Au and Pd is governed by different mechanisms, it is interesting to investigate spin transport in Au/Pd heterostructures. GaAs/ 2.3 nm Fe/ $d$ nm Au/ 9.7 nm Pd/ 4.1 nm Au samples were studied, where $d$, the Au spacer thickness, has been varied. The ferromagnetic resonance spin pumping mechanism was used to generate spin current at the Fe/Au interface. The net spin pumping rapidly decreased with increase in $d$. The rate was too great to be caused by spin diffusion in Au, indicating reflection of the spin current at the Au/Pd interface. Furthermore, the spin pumping exhibited an oscillatory dependence on $d$. This represents, for the first time, the formation of quantum well states that affect the transport of spin currents involving contributions of electrons across the whole Fermi surface. [Preview Abstract] |
Friday, May 2, 2014 2:48PM - 3:15PM |
C2.00006: Break |
Friday, May 2, 2014 3:15PM - 3:45PM |
C2.00007: Chalcogenide semiconductors for energy applications Invited Speaker: Janet Tate Materials discovery and research into the fundamental mechanisms at work in new materials development drives new technology, and vice versa. Chalcogenide semiconductors could be important components of several next-generation energy-related devices: as transparent conductors or channel layer in thin-film transistors or transparent transistors, as $p$-type membranes and absorbers in solar cells, and as light emitters in LEDs. I will discuss the wider challenges in some of these applications, why this particular materials set is relevant, the broad skills and collaborative effort necessary for success in this type of research, and some new results from the OSU collaboration. This work is partially supported by the National Science Foundation under DMR1035513. [Preview Abstract] |
Friday, May 2, 2014 3:45PM - 3:57PM |
C2.00008: Spin flip times of donor bound electrons in GaAs and InP as a function of magnetic field Todd Karin, Russell Barbour, Patrick Wihelm, Kai-Mei Fu Donor bound electrons in III-V semiconductors could provide a qubit with high optical homogeneity and strong optical transitions. The fundamental limit to their performance in quantum information devices is the electron spin flip time \(T_1\). However, the specific mechanisms responsible for spin flips of donor bound electrons are not well understood. We have measured the spin flip time \(T_1\) for electron spins bound to donors in GaAs and InP as a function of magnetic field and donor density in order to help elucidate the mechanisms responsible for spin flips. Measuring \(T_1\) in this way probes the fundamental limits for using bound electrons in semiconductors for quantum information applications. [Preview Abstract] |
Friday, May 2, 2014 3:57PM - 4:09PM |
C2.00009: Room-temperature persistent photoconductivity in strontium titanate Marianne Tarun, Farida Selim, Matthew McCluskey Strontium titanate (SrTiO$_{3})$ is an oxide material with unique properties and is often used as a substrate for oxide thin films such as high-temperature superconductors. Persistent photoconductivity (PPC) is investigated in SrTiO$_{3}$ single crystals at room temperature. Defects and impurities can have a strong effect on the electrical properties of SrTiO$_{3}$. Our prior work showed that hydrogen impurities form a defect complex in SrTiO$_{3}$ which we tentatively ascribed to a strontium vacancy passivated by two hydrogen atoms. The defect, alternatively, could be a partially passivated titanium vacancy. When a thermally treated sample is exposed to sub-bandgap light at room temperature, the free-electron concentration increases by over two orders of magnitude. After the light is terminated, the enhanced conductivity persists for several days, with negligible decay. We tentatively attribute the PPC to the excitation of an electron from a titanium vacancy defect into the conduction band, with a high barrier for recapture. The presence of titanium vacancies was investigated through positron lifetime measurements. Wavelength-dependence measurements showed an optical excitation threshold for photoconductivity of 2.9 eV. [Preview Abstract] |
Friday, May 2, 2014 4:09PM - 4:21PM |
C2.00010: Strain-dependence of the structure and ferroic properties of epitaxial NiTiO3 thin films grown on different substrates T. Varga, T.C. Droubay, M.E. Bowden, S.A. Stephens, S. Manandhar, V. Shutthanandan, R.J. Colby, B.C. Kabius, E. Apra, S.A. Chambers Polarization-induced weak ferromagnetism has been predicted a few years back in perovskite MTiO$_{3}$ (M$=$Fe,Mn,Ni). We set out to stabilize this metastable perovskite structure by growing NiTiO$_{3}$ epitaxially on different substrates, and to control the polar and magnetic properties via strain. Epitaxial Ni$_{1-x}$Ti$_{1-y}$O$_{3}$ films of different Ni/Ti ratios and thicknesses were deposited on Al$_{2}$O$_{3}$, Fe$_{2}$O$_{3}$/Al$_{2}$O$_{3}$, and LiNbO$_{3}$ substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of substrate choice, film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy shows that substrate-exerted strain has a marked effect on the structure and crystalline quality of the films. Physical property measurements reveal a dependence of the N\'{e}el transition, the weak ferromagnetism, and lattice polarization on strain, and suggest that the choice of substrate and film stoichiometry can be used to control the ferroic properties in NiTiO$_{3}$ thin films. Our results are also consistent with the theory prediction that the ferromagnetism in acentric NiTiO$_{3}$ is polarization-induced. [Preview Abstract] |
Friday, May 2, 2014 4:21PM - 4:33PM |
C2.00011: Observation of internal excitation transfer in luminescent lanthanide materials by time-resolved X-ray absorption spectroscopy Joseph Pacold, David Tatum, Gerald Seidler, Kenneth Raymond Luminescent lanthanide materials have a broad range of established and emerging applications, including fluorescent lighting and displays, laser materials, dyes for biomedical assays, and wavelength-converting coatings for solar cells. Most of these applications make use of parity-forbidden transitions with the partially filled 4f shells of the trivalent lanthanide ions, which lead to emission lines at wavelengths spanning the visible spectrum. The desired excited state of the lanthanide ion is typically populated by energy transfer (ET) from a strongly absorbing ``sensitizer,'' rather than by direct excitation. We have observed a transient X-ray absorption spectroscopy signal associated with the ET in several luminescent dyes and a suspension of an inorganic Eu phosphor. This opens up a novel approach to studies of internal ET in luminescent materials, complementary to previous measurements at optical wavelengths. In addition, the transient signal shows an unexpected change in 5d electronic structure accompanying the 4f-4f transition, which suggests that the intrashell excitation is associated with a change in the degree of mixing of 4f-5d states. [Preview Abstract] |
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