Condensed Matter Seminars
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Condensed Matter Seminars are in Room 2205
NPB on Mondays @ 4:00pm
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| August 31 | ||
| Speaker | Jacob Jones, Materials Science and Engineering, UF | |
| Title | In situ crystallographic studies of ferroelectrics: from synthesis through device performance | |
| Abstract |
Diffraction is a powerful tool in the characterization of materials. Although it has historically been limited to describing the static and room temperature structure of materials, we have recently been utilizing diffraction techniques to describe the kinetics of various time-dependence microstructural processes in ferroelectric materials. Using neutron, synchrotron X-ray, and laboratory X-ray sources, we describe structural changes including the phase evolution during solid state calcination and thin film crystallization processes, the ferroelectric phase transitions during cooling from high-temperature processing, the domain switching behavior during static and cyclic electric field loading, and the structural changes present at crack tips during mechanical loading. The technical portion of this talk will be preceded by an introduction to the people and capabilities of Jones’ research group and the associated collaborative opportunities. |
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| Host | Amlan Biswas | |
| September 7 (No seminar--Labor Day) | ||
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| September 14 | ||
| Speaker | A.R. Asthagiri, Chemical Engineering, UF |
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| Title | Oxidation and Reactivity of Transition Metal Surfaces |
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| Abstract | (MSWord)
Transition metals (TM) serve as catalysts under oxygen-rich conditions in applications such as natural gas combustion, exhaust gas remediation in lean-burn engines, and the selective oxidation of organic compounds. Under oxygen-rich conditions the metal surface can undergo several structural changes as it begins to oxidize, which in turn can dramatically modify the reactivity of the catalyst. Despite advances in our understanding of the oxidation of several catalytically important TM surfaces there is still disagreement in the exact surface phase that is associated with enhanced reactivity in systems such as CO oxidation on Pt. Therefore there is a need to better understand (1) the oxidation process and the structure of the oxygen phases that develop under various conditions (temperature, partial pressures) and (2) the resulting modifications in reactivity of the catalyst. In this talk, I will first discuss work in our group examining the initial atomic-level steps in the oxidation of Pt and Pd(111) surfaces using Density Functional Theory (DFT), an accurate first-principles method. We have found a novel mechanism for the initiation of oxidation on Pt(111) that results in strongly buckled 1-D oxide chains on the Pt(111) surface. On Pd(111) this mechanism does not occur but instead subsurface oxygen becomes stable at lower oxygen concentration. I will discuss the differences in Pt and Pd that lead to these differences in oxidation mechanisms. I will also present some preliminary results in understanding the reactivity of CO and NO on the 1-D oxide chains on Pt(111). In the second part of my talk I will present examples from our DFT study of several small molecules (H2O, H2, CO, and CH4) on the major oxide surfaces that form on Pd(111). We have found dramatic differences in reactivity between the 2D oxide phase that initially forms on Pd(111) and the bulk oxide that develops at higher oxygen concentrations. These differences can be attributed to changes in both the geometric and electronic structure of the different oxide surfaces. Our work provides new insight into the kinetics of oxidation of TM surfaces and demonstrates the sensitive link between atomic-level structure of the oxide and the reactivity of the oxide phase. |
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| Host | Y. Lee (FG) | |
| September 21 | ||
| Speaker | Nathan Bachman, Schlumberger | |
| Title | Diffusion-relaxation correlations from magnetic resonance for fluid identification during earth exploration | |
| Abstract | One never-ending quest in the oilfield industry is to identify fluids present in the rocks quickly and accurately after the drilling process. Nuclear magnetic resonance (NMR) is an established tool in this quest, with new applications constantly in development. Recent advances at Schlumberger allow for encoding information about the molecular diffusion of the fluids while also measuring the NMR relaxation of hydrogen. The correlations between these two fundamental physics quantities allows for fluid identification. The talk is divided into three sections: 1) a brief introduction to formation evaluation, 2) description of diffusion-relaxation correlations and how they are used to differentiate oil, water and gas, and 3) some "real-world" physics problems that pose a risk to commercial viability. | |
| Host | Y. Lee (FG) | |
| September 28 | ||
| Speaker | Alex Gurevich, NHMFL/FSU | |
| Title | Making cuprates and other new superconductors suitable for applications. | |
| Abstract | The ongoing quest for higher-Tc (HTS) or even room-temperature superconductors (RTS) has been mostly biased toward the paradigm of high-temperature superconductivity based on strong crystalline anisotropy, low carrier density, and the Cooper pairing mediated by exchange of magnetic excitations. The non-phonon pairing tends to favor anisotropic order parameters with nodal lines or sign changes between different parts of the Fermi surface, short coherence lengths, and coexistence of superconductivity with other competing ordered states. Based on the lessons learned from high-Tc cuprates, MgB2, and the recently discovered ferropnictides, I will discuss constrains imposed by unconventional pairing for applications at 77-300K and high fields. In this case the mean field critical temperature Tc and the upper critical field Hc2 no longer become the key figures of merits as the electromagnetic transport is limited by strong vortex fluctuations, current-blocking grain boundaries, and nodes in the order parameter. As a result, new physics and materials science challenges of how to reduce fluctuations and make superconductors usable at high fields and temperatures emerge. For dc applications, this translates into achieving acceptable irreversibility fields H* and critical current densities Jc at 100-300K in a RTS with the Ginzburg number Gi 1. Recent progress in the development of biaxial coated conductor technology and designer nanoparticle correlated pinning in YBCO has shown that these problems can be overcome, resulting in high Jc and H* at 77K. However, these problems are not only specific to YBCO, but would likely occur in HTS with unconventional pairing, high anisotropy, low carrier density, short coherence lengths, long Thomas Fermi screening length and coexistence of superconductivity with other competing ordered states. These features are characteristic of both cuprates and pnictides, which (unlike MgB2) do exhibit strong vortex fluctuations and current blocking by grain boundaries. Since pinning defect nanostructure cannot fully suppress strong vortex fluctuations, the key questions to be addressed are: 1. what kind of pinning nanoscructure can provide maximum irreversibility field H* for a given set of intrinsic superconducting parameters? 2. Given that high Tc, low superfluid density ns and high effective mass anisotropy = mc/ma can greatly enhance thermal fluctuations of vortices and reduce H* well below Hc2, what would be the materials constraints for the minimum ns and so that a superconductor could still carry a finite critical current at 77 or 300K? I will discuss possibilities of tuning materials parameters to ameliorate intrinsic limitations of anisotropic superconductors, and the challenges of making a putative RTS work at 300K. | |
| Host | P. Hirschfeld | |
| October 5 | ||
| Speaker | Janice Musfeldt, Dept. of Chemistry, U. Tennessee | |
| Title | Magnetoelastic interactions in complex materials | |
| Abstract |
Full MSWord text with graphic
The interplay between structure and magnetism in correlated materials is an important factor leading to their complex properties and unique functionalities. The microscopic aspects of these interactions and the effect of magnetic field on local structure are, however, not well established. As candidates for molecular and frustrated solids that can exhibit substantial magnetoelastic coupling, we considered two different physical systems and employed vibrational spectroscopy to probe local lattice distortions. The first focuses on the magnetic field-induced transition to the fully polarized state in the molecular coordination polymer Cu(HF2)(pyz)2BF4. We find that, in reaction to the change of state, the system lowers its magnetic exchange energy. The beauty of the present experiment is that exchange is mediated by a molecular ligand, pyrazine, the value of the coupling being sensitively dependent on the Cu-pyz-Cu path. We follow these distortions by magneto-infrared spectroscopy and demonstrate that they track the magnetization. This finding is relevant to field-driven magnetic ordering transitions in other low-dimensional quantum Heisenberg antiferromagnets like copper halides and complex materials with higher energy scales such as the copper oxides. The second example focuses on spin-lattice interactions in the frustrated Kagome staircase oxide Co3V2O8. Here again, an applied magnetic field is used to drive through a series of magnetic transitions. In this system, magnetoelastic interactions are dominated by a Co center displacement mode that when analyzed in combination with calculated displacement patterns reveals the field-induced local lattice distortions. This result is important for other magnetically frustrated oxides where magnetic ordering-induced lattice distortions are also likely to occur. |
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| Host | P. Hirschfeld | |
| October 12 | ||
| Speaker | Wei Ku, Brookhaven National Laboratory | |
| Title | Ferro-orbital order and strong magnetic anisotropy in the
parent
compounds of Fe-pnictides superconductors
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| Abstract | The puzzling nature of magnetic and lattice phase transitions of iron pnictides is investigated via a first-principles Wannier function analysis of representative parent compound LaOFeAs. A rare ferro-orbital ordering is found to give rise to the recently observed highly anisotropic magnetic coupling, and drive the phase transitions---without resorting to widely employed frustration or nesting picture. The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates. In particular, the strong coupling to the magnons advocates active roles of light orbitons in spin dynamics and electron pairing in iron pnictides. | |
| Host | H.-P. Cheng | |
| October 19 | ||
| Speaker | Sarah J. Hurst, Center for Nanoscale Materials, Argonne National Laboratory | |
| Title | "Three-Dimensional Hybridization" with Polyvalent DNA-Gold Nanoparticle Conjugates | |
| Abstract | .pdf here | |
| Host | S. Hershfield (FG) | |
| October 26 | ||
| Speaker | Emmanuel Rashba , Harvard | |
| Title | Spin-Orbit Effects in Graphene | |
| Abstract | I will start with a brief review of the basic properties of graphene concentrating on three "spins" involved: pseudospin, isospin, and real spin. Next, I will talk about spin-orbit effects in crystals (in the bulk and at the surface) concentrating on some recent experimental data. Finally, I will report on recent work on spin-orbit effects in graphene making emphasize on its SARPES spectra, discussing recent experimental data and theoretical predictions. | |
| Host | D. Maslov | |
| November 2 | ||
| Speaker | Michael Hoch, NHMFL | |
| Title | Nanoscale phase separation and spin dynamics in colossal response transition metal oxides | |
| Abstract | Transition metal oxides exhibit very interesting magnetic and transport properties, many of which have their origin in nanoscale phase separation. The bilayer manganite La1.2 Sr1.8 Mn2O7 and the cobaltite La0.85Sr0.15CoO3 , which show colossal and large magnetoresistance respectively, are examples of systems whose properties can be altered by the application of a magnetic field. Information on electronic structure and phase separation in these two hole-doped systems has previously been obtained from a variety of experiments that include magnetoresistance, ARPES and neutron scattering. Pulsed NMR experiments that we have carried out as a function of temperature provide information on spin dynamics and the evolution of the electronic structure at the local level. The results are discussed in terms of available models for these systems. | |
| Host | N. Sullivan | |
| November 9 | ||
| Speaker | Alexander Punnoose , CUNY | |
| Title | Quantum phase transitions in a dirty-Fermi-liquid
and the
metal-insulator transition in two dimensions.
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| Abstract | Electrons in a disordered system propagate diffusively at sufficiently low temperatures. It is well know that the effects of quantum diffusion in two dimensions are profound, even weak disorder leads to the complete destruction of the fermi-liquid at low temperatures. In this talk, I will discuss recent theoretical progress in our understanding of a dirty-fermi-liquid in the context of scaling theory, the most promising analytical tool available to understand the physics of disordered systems. I will argue that the observed metal-insulator transition in two dimensions corresponds to a quantum critical point with non-fermi-liquid properties. The results of various experiments designed to test the predictions of the scaling theory in the diffusive regime will be detailed. | |
| Host | A. Biswas (FG) | |
| November 16 | ||
| Speaker | Srikanth Hariharan, U. South Florida | |
| Title | Exchange Bias and Magnetocaloric Effect in Nanostructures | |
| Abstract |
Magnetic nanostructures are considered basic building blocks in spintronics and high-density
data storage applications. Surface and configurational effects in oxide nanoparticle assemblies
have been increasingly found to play significant roles in controlling the magnetic anisotropy.
Modification of the surface spin structure in magnetic oxide nanoparticles can be achieved by
methods such as controlling the particle shapes, use of mechanical milling or surfactant
chemistry to alter the coordination of surface atoms and forming interfaces with non-magnetic
metals. We discuss how these effects often lead to novel magnetic properties, useful for
applications, such as tunable exchange bias and enhanced magnetocaloric effect (MCE). We will
present recent experimental results on high-aspect ratio nanostructures and polymer-based
nanocomposites for RF and microwave applications. Lastly, we will also demonstrate MCE as a
powerful tool to study first and second order phase transitions in complex oxides and discuss
the influence of nanostructuring in mixed-phase manganites.
Research supported by DoE, ARO and NSF |
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| Host | A. Hebard | |
| November 23 | ||
| Speaker | Suk Bum Chung , Stanford U. | |
| Title | Detecting the Majorana fermion surface state of 3He-B through spin relaxation | |
| Abstract | The concept of the Majorana fermion has been postulated more than eighty years ago; however, this elusive particle has never been observed in nature. The non-local character of the Majorana fermion can be useful for topological quantum computation. Recently, it has been shown that the 3He-B phase is a time-reversal invariant topological superfluid, with a single component of gapless Majorana fermion state localized on the surface. Such a Majorana surface state contains half the degrees of freedom of the single Dirac surface state recently observed in topological insulators. We show here that the Majorana surface state can be detected through an electron spin relaxation experiment. The Majorana nature of the surface state can be revealed though the striking angular dependence of the relaxation time on the magnetic field direction, 1/T_1 \propto sin2 \theta where \theta is the angle between the magnetic field and the surface normal. The temperature dependence of the spin relaxation rate can reveal the gapless linear dispersion of the Majorana surface state. We propose a spin relaxation experiment setup where we inject an electron inside a nano-sized bubble below the helium liquid surface. | |
| Host | Y. Lee (FG) | |
| November 30 | ||
| Speaker | S-W. Cheong, Department of Physics and Astronomy and Center for Emergent Materials, Rutgers University | |
| Title | Switchable Photo-Diode effect in Ferroelectric BiFeO3 | |
| Abstract | Uni-directional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal-semiconductor interfaces with Schottky barriers. We have recently discovered a diode effect associated with the direction of bulk electric polarization in BiFeO3, which is a ferroelectric with a relatively small optical gap edge of ~2.2 eV. We found that bulk electric conduction in ferroelectric monodomain BiFeO3 crystals is highly non-linear and uni-directional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. Associated with the diode effect, large directional photocurrent at zero bias can be induced by visible light in ferroelectric monodomain BiFeO3; i.e., a significant photovoltaic effect is observed. Our discovery [1] of substantial diode-like and photovoltaic effects in BiFeO3 is of significant importance for fundamental understanding of charge dynamics in leaky multiferroics, and may lead to new design approaches for multifunctional devices combining magnetic, electronic and optical functionalities. [1] T. Choi et al., Science 324, 63 (2009). | |
| Host | Amlan Biswas (FG) | |
| December 7 | ||
| Speaker | Stephen Teitsworth, Duke Unviersity |
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| Title | Electric field domains in semiconductor superlattices | |
| Abstract | Electronic transport systems that possess negative differential resistance (NDR) often exhibit spatially non-uniform electric field distributions, such as moving or pinned electric field domains. In this talk, I will discuss recent theoretical and experimental results concerning NDR and electric field domains in semiconductor superlattices and related systems. After a brief review of the fundamental physical mechanisms that cause NDR, I describe recent theoretical predictions and ongoing experimental measurements concerning the sensitivity of electric field domains to noise. If time permits, I will also describe a proposal to suppress electric field domain formation in a superlattice by using an appropriate shunt layer. | |
| Host | Ho Bun Chan | |
Condensed Matter Seminars
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Condensed Matter Seminars are in Room 2205
NPB on Mondays @ 4:00pm
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| January 11 | ||
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| January 18 (No seminar MLK day) | ||
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| January 25 | ||
| Speaker | Jon Dobson, Keele University, UK | |
| Title | Applications of Magnetic Micro- and Nanoparticles in
Biology and
Medicine
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| Abstract | The use of magnetic micro- and nanoparticles for biomedical applications was first proposed in the 1920s as a way to measure the rehological properties of the cytoplasm. Since that time, magnetic micro- and nanoparticles have been used in a variety of biomedical techniques such as targeted drug delivery, MRI contrast enhancment, immnoassay and cell sorting. More recently, magnetic micro- and nanoparticles have been used to investigate and manipulate cellular processes both in vitro and in vivo. This talk will focus on some of the research our group is doing on (i) magnetic nanoparticle-mediated activation of cellular processes for tissue engineering, stem cell and drug screening applications, (ii) novel methods of magnetic nanoparticle-based gene transfection and hyperthermia, and (iii) imaging and characterization of magnetic iron compounds associated with neurodegenerative diseases. | |
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| February 1 | ||
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| February 8 | ||
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| February 15 | ||
| Speaker | Oskar Vafek, Florida State U. | |
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| Host | P. Hirschfeld | |
| February 22 | ||
| Speaker | Joe Aumentado , NIST | |
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| Host | Y. Lee | |
| March 1 | ||
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| March 8 (No seminar Spring break) | ||
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| March 15 (No seminar APS meeting) | ||
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| March 22 | ||
| Speaker | Ryuji Nomura, Tokyo Institute of Technology |
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| Host | Y. Lee | |
| March 29 | ||
| Speaker | Hedi Matoussi, Florida State U., Dept. of Chemistry | |
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| Host | S. Hershfield | |
| April 5 | ||
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| April 12 | ||
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| April 19 | ||
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| April 26 | ||
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