Saint Louis University

Seminar Announcements
September 11, 5:00pm – 6:00pm : Dr. Scott W. Schmucker (Zyvex Labs).

Graduate program
The Saint Louis University Department of Physics is now accepting applications for the Integrated and Applied Sciences (IAS) Ph.D. program in Nanomaterials and Condensed Matter Physics track. Please contact Dr. Kuljanishvili, Dr. Solenov, or Dr. Wisbey for more information.  

And now for something completely different....

Each and every trade has its favourite tools, some more powerful than others. A plumber would not get by without a good wrench, a carpenter needs a hammer, a mechanic a screwdriver and so on. Theoretical physicists prefer action principles.

This preference is natural given that many of the phenomena we are interested in are associated with deviations from some minimum energy equilibrium state. It is well known that, once you understand a problem from the variational point-of-view, you have a very powerful tool at your hands. However, it is also generally accepted that this approach is restricted to conservative systems.

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PumpkinVisit the Saint Louis University Society of Physics Students (SPS) for the latest activities.  

Computational study of dolphins and whales hydrodynamics

Mouse over images to expand
In a collaboration with Dr. Mark McQuilling from the Dept. of Aerospace and Mechanical Engineering, Dr. Potvin and his students are studying the hydrodynamic drag of cetaceans with the aim of understanding how much energy these animals spend to travel and feed. His team uses computer simulations of the flows about the body of these whales to figure out the forces that resist their motion through the water. The color picture in the top left shows a pressure map on the body (as represented by the so-called Cp - pressure coefficient). Here one sees the pressure to be highest near the head, and lowest over the middle third of the body. Here the fins and flukes have been removed. The effects of those are determined via water tunnel investigations. The simulations are based on Computational Fluid Dynamics (CFD), where the equation of motions of the water particles (F = ma !) are calculated on each one of the tetrahedrons making up the mesh shown in the top middle photo. To save computer time, the mesh is at its highest resolution (ie with the smallest tetrahedrons) near the body where the flows are deflected the most (and where they are more complicated). These calculation are performed here at SLU, either on workstations or on a large computer cluster (top right).

Recent publications 

Flux dynamics, ac losses, and activation energies in (Ba0.6K0.4)Fe2As2 bulk superconductor
M. Nikolo, X. Shi, E.S. Choi, J. Jiang, J.D. Weiss, E.E. Hellstrom, Journal of Low Temperature Physics, 178, 188 (2015). 
Flux pinning and thermally assisted flux flow (TAFF) are studied in a (Ba0.6K0.4)Fe2As2 (Tc=38.3 K) bulk samples in magnetic fields up to 18 T via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1997 Hz in all fields. The frequency (f) shift of the susceptibility curves is modeled by the Anderson-Kim Arrhenius law f = f0 exp(-Ea/kT) to determine flux activation energy Ea/k as a function of ac field Hac and dc magnetic flux density μ0 Hdc. Ea/k ranges from 8822 K (761 meV) at μ0 Hdc = 0 T to 1100 K (95 meV) at 18 T for Hac =80 A/m (1 Oe). The energies drop very quickly in a non-linear manner as μ0 Hdc increases from 0 T to 1 T, and more gradually, in a linear like manner, as μ0 Hdc increases further to 18 T, suggesting some kind of vortex transition. For ac fields of 400 A/m (5 Oe) and higher, the Arrhenius model starts breaking down, at around μ0 Hdc = 2 T. As dc magnetic flux density increases further, this breakdown becomes significant for μ0 Hdc = 15 and 18 T at ac fields of 400 A/m and higher. Extensive mapping of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the ac field, frequency, in addition to the dc magnetic flux density.

Magneto-transport properties and thermally activated flux flow in Ba(Fe0.91Co0.09)2As2 superconductor
M. Nikolo, X. Shi, E.S. Choi, J. Jiang, J.D. Weiss, E.E. Hellstrom, Journal of Superconductivity and Novel Magnetism, 27, 2231 (2014).
Thermally assisted flux flow (TAFF) based on magneto-resistivity and ac susceptibility measurements is studied in a Ba(Fe0.91Co0.09)2As2 (Tc = 25.3 K) sample in magnetic fields up to 18 T. In addition to the upper critical field µ0Hc2 and the coherence length ξ(0), the flux flow activation energy U(T,H) has also been determined. The resistive transition width is proportional to µ0H, in contrast to Tinkham's theoretical prediction. By applying Fisher's model, the glass melting transition temperature Tg, which occurs in the upper TAFF state and not in the zero resistivity vortex solid regime, is calculated. The onset of TAFF temperature and the crossover temperature Tx from TAFF to flux flow are determined. By contrasting the ac susceptibility data with the resistivity data, considerable flux penetration appears even in the zero resistivity state, in addition to ac losses. The H-T phase diagram is drawn and shows weak pinning regime as the field approaches µ0Hc2, and the strength of the weak pinning decreases to zero with increasing magnetic field from 0 T to 18 T.

Mars Shows Signs of Having Flowing Water, Possible Niches for Life, NASA Says  NYT 

Earth like planet Kepler-186fBy KENNETH CHANG September 28, 2015

Scientists have for the first time confirmed liquid water flowing on the surface of present-day Mars, a finding that will add to speculation that life, if it ever arose there, could persist now.

"This is tremendously exciting," James L. Green, the director of NASA's planetary science division, said during a news conference on Monday. "We haven't been able to answer the question, ‘Does life exist beyond Earth?' But following the water is a critical element of that. We now have, I think, great opportunities in the right locations on Mars to thoroughly investigate that."

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