CARNEGIE MELLON UNIVERSITY -
UNIVERSITY OF PITTSBURGH

Weekly Physics Seminar Schedule
October 13, 2008 - October 20, 2008

Monday, October 13, 2008, Joint Physics Colloquium, Andrei Frolov, Simon Frazer University, "Simulating the Big Bang," 4:30 PM, 102 Thaw Hall, PITT

Abstract:  The idea of inflation (a period of rapid quasi-exponential expansion of the Universe) neatly solves several issues in cosmology. While the Universe is inflating, its contents are cold. Eventually, inflation ends and the field driving the inflation must decay, depositing energy into high-energy particles. This process, known as reheating, starts the hot big bang as we know it, and could allow a glimpse of physics at energies we know very little about. I will discuss a simple scalar field model of reheating, which for all its simplicity has rich physics involving parametric resonance, non-linear evolution, and turbulence. I will illustrate this dynamical behavior with simulation using a new numerical solver I developed.

Wednesday, October 15, 2008, High Energy Physics Seminar, Pasha Bolokhov, University of Pittsburgh, "Non-abelian Vortices in N=1 SQCD", 4:30 PM, Wean 7316, CMU

Thursday, October 16, 2008, Condensed Matter Biological Physics Seminar, Jian-Jun Pan, Dept. of Physics, Carnegie Mellon Univ., Annual Review, "The Superstructure of an Antimicrobial Peptide, Alamethicin, in Lipid Membranes", 4:30 PM, Wean 7316*, CMU (*Note the special location)

Abstract:  Alamethicin (Alm) is one of the most extensively studied antimicrobial peptides that affect the plasma membrane, and several models that involve a channel structure formed by monomer aggregation have been proposed. The Alm channel structure has been observed experimentally by x-ray and neutron in-plane scattering, and channel size has been estimated. The barrel stave model suggests that there are 6-8 Alm monomers per channel.

In this work we investigate the effect of membrane hydration and hydrophobic mismatch on the Alm channel superstructure in an oriented multilayer sample by x-ray scattering. Wide angle x-ray (WAXS) scattering near 1.4 inverse Angstroms indicates that the lipid chain region is not much perturbed by the incorporation of up to 10 mole percent Alm. Low angle x-ray scattering (LAXS) indicates that when the sample is very dry, which promotes interactions between neighboring bilayers, a body centered tetragonal crystal packing of Alm channels is formed. As the hydration level increases closer to biological conditions, the separation between bilayers increases, the interbilayer interactions weaken, and the crystalline order disappears while considerable diffuse scattering remains. The effect of hydrophobic mismatch is examined for two mono-unsaturated lipids, diC18:1PC and diC22:1PC, that differ in bilayer thickness by 7.3 Angstroms. There is also additional in-plane scattering at a medium q of 0.7 inverse Angstroms that our analysis suggests may not be from the Alm channel structure.

Thursday, October 16, 2008, Quantum Information Seminar, Vlad Gheorghiu, Carnegie Mellon Univ., "Secret Sharing with Graph States (cont.)", 4:30 PM, Wean 7423*, Coffee and doughnuts at 4:00 PM in Wean 7423, CMU (*Note the special location)

Abstract: I will continue the discussion about secret sharing with graph states, focusing on quantum secret sharing.  I will present some personal research ideas which may apply to secret sharing protocols.  Main reference is arXiv:0808.1532 [quant-phy] by Damian Markham and Barry C. Sanders.

Thursday*, October 16, 2008, High Energy Physics Seminar, Ahmed Idilbi, Duke University, "Factorization and Resummation for Color Octet Production in Effective Theory", 4:30 PM, Doherty Hall Room A310*, CMU (*Note the special date, and location)

Friday, October 17, 2008, Research Talk to Graduate Students, Dr. Michael Wood-Vasey, 3:00 pm, 103 Allen Hall, PITT

Friday, October 17, 2008, Condensed Matter Seminar, Prof. Martin Weinelt, Max Born Institute, Berlin, "Hot Spots and Spin Waves," 4:30 PM, 319 Allen Hall, PITT

Abstract:  Recent experiments demonstrate that significant demagnetization of thin ferromagnetic films can be achieved within a picosecond upon optical excitation. Within this timescale, the excited electronic system and the underlying lattice are not in equilibrium and it seems that the transient hot electron population is responsible for the change of the magnetization. It remains controversial to date, which microscopic processes are fast enough to provoke femtomagnetism. To approach these problems we combined time-, angle- and energy-resolved two-photon photoemission with spin-resolved electron detection and investigated ultrathin iron and cobalt films on Cu(100).

In purely non-relativistic electron-electron or electron-phonon scattering a spin-flip is not possible because the Coulomb operator does not act on the spin part of the electron wave-function. Electrons only undergo a spin flip in the presence of spin-orbit coupling significantly enhanced at hybridization points in the band structure. We have identified these so-called spin hot-spots by linear magnetic dichroism. Initial bulk and surface states with minority spin-character are the source for the dichroic intensities and the apparent dichroic lifetimes of the image-potential states. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band-structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level.

Experimental access to the spin-dependent relaxation processes of low-energy electrons in d-band ferromagnets proves a challenge, as the dynamics occur on the femtosecond timescale and it is difficult to distinguish between refilling and the various scattering processes. For these purposes the well-defined dispersing image-potential-state electron can be employed as test charge or primary electron. While at the minimum of the image- potential band the decay rate is determined by the strongly spin-dependent density of d-states, the increase of the decay rate with energy above the band bottom is governed by spin-independent states only. We observe twice the increase in decay rate for minority-spin electrons than for majority-spin electrons on thin iron films. The factor of two is explained if electron magnon scattering is included. Then minority-spin electrons may also scatter into the majority-spin bands via magnon emission and thus gain twice the phase space of their majority-spin counterparts. These magnon-enhanced electron scattering processes allow for transfer of angular momentum of hot electrons on a femtosecond timescale.