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Events on Tuesday, March 6th, 2012

R. G. Herb Condensed Matter Seminar
Magnetic interactions and anisotropies in iron superconductors
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Leni Bascones, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC)
Abstract: A major breakthrough took place in 2008 with the discovery of high temperature superconductivity in iron pnictides. The basic structure of these materials are FeAs layers. In these materials superconductivity emerges when doping or applying pressure to an antiferromagnetic compound. Most iron pnictides order with (pi,0) momentum, antiferromagnetic in the x direction and ferromagnetic in the y direction. The magnetic state of iron pnictides is metallic and presents unusual magnetic moment and resistivity anisotropy [1].

The origin of magnetism in iron superconductors is still unsettled. At the heart of the debate are the itinerant (weak coupling) versus localized (strong coupling) nature of magnetism, the role played by the orbital degree of freedom and Hund's coupling and a possible nematic phase. The itinerant picture relies on the metallicity of the magnetic state and an approximate (pi,0) nesting of the Fermi surface. In the strong coupling picture localized moments interact with AF exchange constants J1 and J2 with their first and second nearest neighbors respectively. (pi,0) state can be stabilized with J1< 2 J2, but little is known on the actual value of these constants. Orbital ordering has been proposed as a leading instability and to explain the anisotropic properties.

In this talk I will present our recent calculations which try to make connection between the itinerant and localized picture. We have analyzed the magnetic interactions of iron superconductors on the basis of a five orbital model treated both within Hartree-Fock and Heisenberg approximations [2]. We show that the exchange constants depend on charge and orbital filling and on Hund's coupling with unexpected consequences on the phase diagram [2]. We have also studied the dc and optical conductivities and separated the effect of magnetism and of orbital ordering on the anisotropic properties [3].

[1] For a review see, J. Paglione and R.L. Greene, Nature Phys. 6, 645 (2010).
[2] M.J. Calderon, G. Leon, B. Valenzuela and E. Bascones, arXiv:1107.2279. E. Bascones, M.J. Calderon, B. Valenzuela, PRL 104, 227201 (2010)
[3] B. Valenzuela, E. Bascones, M.J. Calderon, PRL 105, 207202 (2010)
Host: Perkins
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Chaos & Complex Systems Seminar
Perturbation in the system: Creative network practice
Time: 12:05 pm
Place: 4274 Chamberlin
Speaker: Michelle Riel, Artist, Designer, Theorist
Abstract: In art practice, use of computers, electronics, and telecommunications began in the 1960s. What then differs in today's creative network practice? In particular, chance operations of seminal 20th century artists and exploration of systems theory by mid-century artists have become the dynamical systems of today's creatives who employ coding, physical computing, and emerging networked technologies and methods. I relate historical precursors to current work and discuss how network practices employ non-linear dynamic processes.
Host: Sprott
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