2 seminars coming up next week

Thu Jun 16 13:25:31 CEST 2011

 INFORMAL SEMINAR on Chemical Physics

Monday, 20 June   -    11:00 a.m. 

Luigi Stasi Seminar Room,  ICTP Leonardo Building (1st floor)

Yunfeng LIANG    ( Kyoto University )

"Classical and quantum simulations in     earth, energy and environment systems"

Abstract

Recently, we have applied the computational condensed matter physics in liquid-liquid interface, liquid-solid interface and the three phase systems, which we know little in comparison with the bulk condensed matter materials & their surfaces, and are important for earth, energy and environment systems. Of particular, I’ll present three topics, which are relevant to sedimentary Methane Hydrate (MH), CO2 Storage and Sequestration (CCS) and Enhanced Oil Recovery (EOR), respectively.

(1) We have performed molecular dynamics simulations in order to investigate the stability of CH4- and CO2- hydrate. It was found that the cage occupancy of hydrates influences the stability of the hydrates. More importantly, the actual three phase coexistence line of CO2 hydrate may be higher than the value ever measured. Our results show that the “lowered” three phase coexistence line stemmed from the partial occupancy occurred during the lab measurements.

(2) We have investigated the contact angle of CO2/water/silica for two different silica surfaces by using large scale molecular dynamics simulations. It was found that the contact angle increases with pressure for hydrophobic (siloxane) surface while almost constant for the hydrophilic (silanol) surface, which is in excellent agreement with experimental observations. The difference results from the different interactions between the CO2 thin film and silica surfaces.

(3) By using first-principle molecular dynamics, we have studied the interface of new cleaved quartz (0001) surface against liquid water at room temperature. It was found that the quartz surface is not saturated as ideal geminal [Si-(OH)2] surface for all calculations. The rest Si-O* radicals, presumably negatively charged, may help us solve a long-existing mystery in colloid and interface sciences, that the silica surface is negatively charged at PH=7.

In collaboration with Yohei Mikami, Shinya Tsuji, Mia Ledyastuti, Makoto Kunieda and Toshifumi Matsuoka.

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JOINT ICTP/SISSA   COLLOQUIUM   ON CONDENSED MATTER PHYSICS

Wednesday, 22 June -  4:00 p.m.

Luigi Stasi Seminar Room,   ICTP Leonardo Building (1st floor)

Yasutomo J. UEMURA    ( Columbia University, New York )

" Commonality and novelty of unconventional superconductors:  CuO, FeAs, C60, organic and heavy-fermion systems"

Abstract

The year 2011 marks the 100th year anniversary of the discovery of superconductivity by Kammerlingh Onnes. Since then, superconductivity of simple metals, such as Al, Sn, and Hg, was explained by the BCS theory more than 50 years ago. The BCS theory has several key features: (a) pairing mediated by phonons; (b) energy scale of condensing charges, i.e., Fermi energy E_F, being much larger than that of mediating fluctuations hw_B (phonon frequency); (c) the transition temperature Tc scales with hw_B without directly depending on the carrier density.

During recent 30 years, several new superconductors have been discovered: they include high- Tc cuprates, organic BEDT, alkali-doped A3C60, heavy-fermion, and FeAs superconductors which were discovered about three years ago. These systems are much different from BCS superconductors in: (a) pairing seems to be mediated by the spin fluctuations rather than lattice vibrations; (b) effective energy scales of superconducting charges are comparable with energy scales of spin fluctuations, indicating non-retarded interaction; and (c) the transition temperature scales with the superfluid density n_s/m*  which represents the charge energy, suggesting relevance of Bose-Einstein condensation of pre-formed pairs.

In this talk, I will demonstrate striking commonalities of these new unconventional superconductors by showing accumulated experimental results on energy scales and phase diagrams from muon spin relaxation (MuSR), neutron scattering, specific heat, and other measurements, and discuss their implications on pairing and condensation mechanisms. Especially, I will elucidate important roles of collective soft modes relevant to competing states in determining Tc, and advantage of possible resonance of the two key energy scales, E_F, for charges and hw_B for spins, in the process of crossover from Bose Einstein to BCS condensation for the pairing mediated by spin fluctuations.

References:

Y.J. Uemura, Nature Materials 8 (2009) 253-255.

Y.J. Uemura, Physica B404 (2009) 3195-3201.