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Physics Department Current Events

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Physics Department Current Events

Wednesday, May 4, 2016
4:00 p.m. to 5:00 p.m., Marshak Science Building Room 418N
Physics Colloquium
Professor Dorthe Eisele
Chemistry Department, The City College of New York

SOFT SUPRA-MOLECULAR NANOTUBES FOR ROBUST LIGHT HARVESTING?

The most remarkable materials that demonstrate the ability to capture solar energy are natural photosynthetic systems such as found in rather primitive marine algae and bacteria. Their light-harvesting (LH) antennae are crucial components, as they absorb the light and direct the resulting excitation energy efficiently to a reaction center, which then converts these excitations (excitons) into charge-separated states. Nature's highly efficient light-harvesting antennae, like those found in Green Sulfur Bacteria [1], consist of supra-molecular building blocks that self-assemble into a hierarchy of close-packed structures.

In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supra-molecular building block, to close-packed building blocks. Here, I will discuss the impact of hierarchical structure. I will present a model system [1, 2, 3] that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules (as illustrated in the figure). I will show that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained.[4] These results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to Nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft supra-molecular materials.

References:
[1] Orf, G. S. & Blankenship, R. E. “Chlorosome antenna complexes from green photosynthetic bacteria.” Photosynth. Res. (2014).
[2] Eisele, D.M., Knoester, J., Kirstein, S., Rabe, J.P., and Vanden Bout, D.A.: “Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid Substrates.”
Nature Nanotech. 4, (2009) 658-663.
 [3] Eisele, D.M., Cone, C.W., Bloemsma, E.A., Vlaming, C.G.F. van der Kwaak, S.M., Silbey, R.J., Bawendi, M.G., Knoester, J., Rabe, J.P., and Vanden Bout, D.A.: “Utilizing Redox-Chemistry to Elucidate the Nature of Exciton Transitions in Supramolecular Dye Nanotubes.”
Nature Chem. 4, (2012) 655–662.
[4] Eisele, D.M., Arias, D.H., Fu, X., Bloemsma, Steiner, C.P., E., Jensen, R., Rebentrost, P., Eisele, H., Llyod, S., Tokmakoff, A., Knoester, J., Nicastro, D., Nelson, K.A., and Bawendi, M.G.:
“Robust Excitons in Soft Supramolecular Nanotubes.”
PNAS, 111 (2014) E3367-E3375.

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Thursday, May 5, 2016
3:30 p.m. to 4:30 p.m. Marshak Science Building Room 329
Special Condensed Matter Seminar

Chris Hooley
(SUPA, St Andrews)

“A new type of non-Fermi-liquid near the quantum critical point of a
d=2 ferromagnetic metal”

In this seminar I shall consider the low-temperature behaviour of a
two-dimensional metal near a second-order transition to
ferromagnetism.  This is a particular example of the type of problem
referred to by the phrase ‘quantum criticality’.  In general, magnetic
fluctuations near such a quantum critical point are subject to Landau
damping due to their coupling to the conduction electrons.  However,
in the ferromagnetic case this damping must vanish as the wavevector
of the fluctuation tends to zero, since uniform magnetisation (being a
conserved quantity of the model) cannot be damped.

It is usually assumed [1,2], on the basis of a perturbative
calculation, that the Landau-damping rate is proportional to the
modulus of the wavevector, i.e. goes as |q|.  However, in this seminar
I shall describe a calculation (carried out using the functional
renormalisation group) that suggests that this is not the case.
Instead, we find a new type of low-energy fixed-point that corresponds
to a strongly interacting non-Fermi-liquid in which the Landau damping
rate goes as |q|^(3/5), and the specific heat as T^(10/13).  I shall
discuss the physics of this fixed point and where it might be seen in
nature, the relationship of our calculation to other works on nearly
ferromagnetic metals, and possible avenues of future investigation.

This work was done in collaboration with my PhD student Sam Ridgway;
details are available in S.P. Ridgway and CAH, Phys. Rev. Lett. 114,
226404 (2015).

[1] J.A. Hertz, Phys. Rev. B 14, 1165 (1976).
[2] A.J. Millis, Phys. Rev. B 48, 7183 (1993).
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