| Date & time | Speaker & affiliation | Talk title & abstract |
| Jan 25 4:00pm | Lijun Zhu (UCR) | Helicity
order: hidden order parameter in
URu2Si2 Abstract: We propose that the ``hidden order parameter" in URu2Si2 is a helicity order which must arise, if the Pomeranchuk criteria for the spin-antisymmetric Landau parameters with respect to the stability of a Fermi liquid state are violated. In a simple model, we calculate the specific heat, linear and nonlinear magnetic susceptibilities and the change of transition temperature in a magnetic field with such an order parameter, and obtain quantitative agreement with experiments in terms of two parameters extracted from the data. Signatures in other experiments are also discussed. |
| Jan 26 1:30pm (note the unusual day and time) | Philippe Bourges (Laboratoire Léon Brillouin, CEA Saclay, France) | Magnetic order in the pseudogap phase of high-Tc
superconductors
Abstract:
One of the leading issues in high-Tc
superconductors is the origin of the pseudogap phase in
underdoped cuprates. Using polarized elastic neutron
diffraction, we identify a novel magnetic order in the
YBa2Cu3O6+x system. The observed magnetic order preserves
translational symmetry as proposed for orbital moments in the
circulating current theory of the pseudogap state. To date, it
is the first direct evidence of a hidden order parameter
characterizing the pseudogap phase in
high-Tc cuprates. (cond-mat/0509210) |
| Feb 1 4:00pm | Vivek Aji (UCR) | Effect of spin orbit interaction on time
reversal violating states in YBCO
Abstract:
Recent neutron
scattering experiments [1] have verified the existence of ordered
magnetic moments in underdoped YBCO superconductors. Such moments were
predicted to exist based on the violation of time reversal symmetry in
the underdoped regime of the Cuprates [2]. We study the effects of
spin orbit interaction in the observed symmetry breaking. Our primary
motivation is to understand why the observed moment has a substantial
in-plain component, while a theory based on orbital currents
would predict no such component should exist. We find that
spin orbit interaction does indeed induce in plane
ferromagnetism in YBCO.[1] Fauque et al., cond-mat/0509210. [2] Simon and Varma, PRL 89, 247003 (2002). |
| Feb 8 4:00pm | open | no seminar this week |
| Feb 15 4:00pm | Ajay Gopinathan (UCSB) | The Physics of Crawling Cells and Ballistic Bacteria
Abstract:
One recurring theme in biological systems is the translation of mechanical
forces exerted by growing polymers into motion. In this talk, I shall discuss
the physics behind this process in the context of our recent work on two such
systems. In crawling cells, polymerization of the protein actin at the leading
edge produces the protrusive force that drives the motion. The bacterial
pathogen, Listeria monocytogenes, harnesses the same mechanism. When a
bacterium infects a cell, it creates a dense network of the biopolymer actin
(the actin comet tail) whose polymerization propels the bacterium through the
cell and into neighboring cells. I shall describe how we model the underlying
physics in both these systems. Using finite element simulations to describe the
elastic actin comet tail, we quantitatively reproduce many distinctive features
of actin propulsion that have been observed experimentally, including stepped
motion, tail shape and the propulsion of flat surfaces. In addition, I will
discuss analytical work to describe the dynamics of the leading edge of a
crawling cell. We show that distinct dynamical structures that form at the
leading edge during motion simply correspond to different modes of the cell
membrane's dynamics. In particular, we find that membrane motion can be
wave-like, corresponding to membrane ruffling, or unstable, corresponding to
the tendency to form finger-like extensions called filopodia.
|
| Feb 21 4:00pm (note the unusual day and time) | Christopher Mudry (Paul Scherrer Institute, Switzerland) | Universal Scaling Relations in Strongly
Anisotropic Materials Abstract: We
consider the critical temperature in strongly anisotropic
antiferromagnetic materials, with weak coupling between stacked
planes, in order to determine the interplane coupling constant from
experimentally measured susceptibilities. We present theoretical
arguments for a universal relation between interplane coupling and
susceptibility shown numerically by Yasuda et. al.,
Phys. Rev. Lett. 94, 217201 (2005). We predict a more general
scaling function if the system is close to a quantum critical point, a
similar relation for other susceptibilities than considered in Yasuda
et. al., and the validity of these relations for more general phase
transitions. |
| Feb 22 4:00pm | Ashwin Vishwanath (Berkeley) | TBA |
Seminars for Fall 2005
Seminars for Winter 2006