Bulletin of the American Physical Society
2013 Annual Fall Meeting of the APS Prairie Section
Volume 58, Number 15
Thursday–Saturday, November 7–9, 2013; Columbia, Missouri
Session C2: Condensed Matter Physics II |
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Chair: Helmut Kaiser, University of Missouri Research Reactor Room: Memorial Union Stotler I&II |
Friday, November 8, 2013 10:30AM - 11:06AM |
C2.00001: Site-inversion versus frustration in the CoAl$_2$O$_4$ spinel: A neutron diffraction study at MURR Invited Speaker: David Vaknin Neutron diffraction studies of a well defined CoAl$_2$O$_4$ polycrystalline sample reveal the role that site-inversion plays in determining the magnetic properties of this system. The A-B site-inversion in the spinel system is a long standing issue. In this regard CoAl$_2$O$_4$ is notorious as Co-Al site-inversion can exceed 20\% and, so far, could not be totally eliminated. In this talk, I will review previous reports including more recent ones that emphasize Co-Co next-nearest-neighbor coupling in the diamond-like Co sublattice that is claimed to lead to exotic spin-liquid structure or to be on the boundary of a recently predicted phase diagram between a collinear antiferromagnetic and a spiral magnetic structure. I will present the various views on the phase diagram of CoAl$_2$O$_4$ as a function of Co-Al site inversion parameter and contrast them with our neutron diffraction studies. [Preview Abstract] |
Friday, November 8, 2013 11:06AM - 11:18AM |
C2.00002: Adsorption-Induced Conformational Changes in Porous Materials Matthew Connolly, Carlos Wexler In physisorption it is normally assumed that the adsorbent is structurally inert, its porous conformation unchanged by the adsorption; this assumption underlies most conceptual/theoretical framework used for characterization of porous solids. Recently, the mechanical response of the material has come under scrutiny: when a gas enters pores with sizes comparable with the range of the van der Walls forces an excess pressure/tension is created. Here we present a theoretical, computational and experimental demonstration of conformational expansion (breathing) of graphene-like adsorbents upon adsorption: Molecular dynamics simulations show the potential for supercritical hydrogen to open new pores in carbons. Grand Canonical Monte Carlo perturbative calculations demonstrate a reduction of the free energy of strip-shaped pores with gas loading upon a conformational change that increases the net size of micropores. Experimentally, reversible pore expansion during adsorption was measured by x-ray scattering for graphene oxide frameworks. These breathing modes have significant consequences for medium- to high-pressure adsorption, with modified adsorption isotherms that may require re-interpretation of standard models. Supported by DOE DE-FG02-07ER46411, ACS-PRF 52696-ND5, and NSF 1069091. [Preview Abstract] |
Friday, November 8, 2013 11:18AM - 11:30AM |
C2.00003: Diffusion of Squalene in n-alkanes and squalane Bruce Kowert Squalene, an intermediate in the biosynthesis of cholesterol, has a 24-carbon backbone with six methyl groups and six isolated double bonds. Capillary flow techniques have been used to determine the translational diffusion constant, $D$, of squalene at room temperature in several nonpolar solvents; they include squalane, $n$-hexadecane, and three $n$-octane-squalane mixtures. Values of $r$, squalene's hydrodynamic radius, are calculated from the Stokes-Einstein relation and decrease as the viscosity increases. These solvent-dependent $r$ values are a consequence of the relative sizes of squalene and the solvents. The Stokes-Einstein limit assumes the solute is much larger than the solvent; this is not the case for our solutions. A number of $n$-alkane solutes diffusing in $n$-alkane solvents also have $r$ values that decrease as the viscosity increases. The $r$ values for squalene and these $n$-alkanes have a common dependence on the ratio of the solvent's van der Waals volume to that of the solute probe. The translational motion of squalene appears to be similar to that of $n$-alkane solutes with similar chain lengths diffusing in $n$-alkane solvents; $n$-tetracosane is an example. [Preview Abstract] |
Friday, November 8, 2013 11:30AM - 12:06PM |
C2.00004: Emergence of coherence in the charge density wave state of intercalated 2H-NbSe2. Invited Speaker: S. Rosenkranz Charge density waves (CDWs) are ubiquitous in condensed matter and associated with many exotic phenomena of great interest. This symmetry breaking state is described by a complex order parameter with an amplitude and a phase. In the conventional view of CDW formation, based on clean, weak-coupling systems, finite amplitude and long-range phase coherence set in simultaneously at a single temperature, the CDW transition temperature, but the situation may be dramatically different at strong coupling or in the presence of disorder. We have studied in detail the CDW formation in pristine and intercalated samples of 2H-NbSe2, a CDW material with strong electron-phonon coupling, combining structural (X-ray), spectroscopic (photoemission and scanning tunneling microscopy) and transport probes. In contrast to the conventional view, we find two separate characteristic temperature scales that can be widely separated. The higher crossover scale marks the onset of short-range CDW correlations, with non-zero amplitude and associated gap in the electronic spectra, but with an incoherent phase of the order parameter. The lower scale is a phase transition that marks the onset of global phase coherence and long-range CDW order. The observation of a persistent gap in the absence of long-range order and the absence of coherent excitations are strikingly similar to the characteristics of the pseudogap state observed in cuprate superconductors and other systems. Our observations thus emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide new insights into the significance of phase incoherence in the realization of pseudogap phases. [Preview Abstract] |
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