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Special Seminar
What is Going on in the Iron Pnictides?
by: Dr. Qimiao Si
Date: Thursday April 09, 2009
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Since the surprising discovery of high Tc superconductivity in the iron pnictides in early 2008, a large body of physical properties have already been accumulated. In this talk, I will attempt to draw a coherent picture about the microscopic physics of these systems.
Based on the fact that they are "bad metals," I will make the case that these materials lie at the boundary between Mott localization and itinerancy [1,2,3]. The incoherent electronic excitations are modeled in terms of localized magnetic moments, with J1-J2 superexchange interactions on the iron square lattice. Such a magnetic frustration leads to a (pi,0) collinear antiferromagnetic ordering and a reduced ordered moment [1], and naturally yields a magnetism-induced structural phase transition; both are observed experimentally. The coupling of the local moments to the coherent electronic excitations tunes the strength of antiferromagnetic order, leading to a magnetic quantum critical point [2,3]; emerging evidence for quantum criticality will be summarized. The implications of these considerations for superconductivity will be discussed.
[1] Q. Si and E. Abrahams, PRL101, 076401 (2008)[2] J. Dai et al, PNAS 106, 4118 (2009)[3] Q. Si et al, NJP (2009) -- arXiv.org:0901.4112
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Special Seminar
Magnetotransport in Heavy Fermion Metals CeMIn5 (M = Co, Ir): The Influence of Antiferromagnetic Fluctuations
by: Steffen Wirth
Date: Thursday November 06, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Heavy fermion metals are often characterized by a variety of relevant energy scales and competing interactions which may result in such fascinating phenomena as quantum criticality and unconventional superconductivity. Therefore, these materials have advanced to suitable model systems by means of which electronic interactions can be studied in detail. This will be discussed for two classes of heavy fermion metals: First, results of detailed magneto-transport investigations on YbRh2Si2 will be described. In the second part, we present an overview of magnetotransport measurements on CeCoIn5 and CeIrIn5. Pressure-dependent Hall measurements on CeCoIn5 exhibit a well developed feature that can unambiguously be related to spin fluctuations associated with the departure from Landau Fermi liquid behavior. Magnetotransport measurements on CeIrIn5 indicate a precursor state to superconductivity. The relation of this precursor state to the so-called pseudogap in high-TC cuprate superconductors will be discussed.
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Special Seminar
Brief Overview of Recent Developments on Iron-based Superconductors
by: Prof. Yan Chen
Date: Friday October 03, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Superconductivity has always attracted considerable attention from people in various fields due to its striking quantum phenomena. The recent discovery of high-temperature superconductivity in the iron-based pnictide materials has made a significant impact in the field of condensed matter physics. The superconducting transition temperature has been quickly raised to about 55K and this novel superconductor does not belong to the same categories as those of known superconductors such as copper oxide high-Tc superconductors and conventional BCS-type superconductors. In this talk, I will give a very brief introduction to the recent research progress as well as a perspective on these new iron-based superconductors, in particular, the theoretical and experimental efforts toward cracking the mechanism.
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Special Seminar
Cluster-based Model, a New Route Towards Description of Complex Atomic Structures
by: Prof. Chuang Dong
Date: Friday September 26, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Atomic structures are generally described by crystallography, which is entirely based on planar periodicity. However such a method largely fails in describing complex atomic structures such as complex metallic alloys and disordered systems. Alternative to planar periodicity is spherical periodicity arising from resonance between electron wave and static atomic strictures, which has been proven widely present in disordered systems. One unique advantage is that only local 1st-neighbor cluster structure information is needed to depict the global structure. It implies that a structure, no matter how complex it can be from crystallographic viewpoint, consists simply of a cluster plus a few glue atoms, or in cluster formula (cluster)1(glue atom)x. Such a cluster-plus-glue-atom model is applied to give the composition formulas of a variety of complex structures such as crystalline intermetallic compounds, bulk metallic glasses, quasicrystals, and even multi-element solid solution limit alloys.
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Special Seminar
Current Carrying State and Its Implication in a D-Wave Superconductor
by: Dr. Jian-Xin Zhu
Date: Tuesday July 08, 2008
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
A fundamental property of all known superconductors is the formation of Cooper pairs in the superconducting state. A far-reaching implication of this fact is the quantization of magnetic flux in multiply connected superconducting geometries. In this talk, I will discuss the magnetic flux dependence of order parameter and supercurrent in a hollow $d$-wave superconducting cylinder. It is shown that the existence of line nodal quasiparticles in a pure $d_{x^2-y^2}$ pairing state gives rise to an $hc/e$ periodicity in the order parameter and a first-order quantum phase transition for a large system size. We demonstrate that the flux periodicity in the supercurrent is sensitive to the detailed electronic band structure and electron filling factor. In particular, we find that, in cooperation with the increase of the cylinder circumference, the $hc/2e$ periodicity can be restored significantly in the supercurrent by avoiding the particle-hole symmetry point. A similar study of a $d_{x^2-y^2}+id_{xy}$ pairing state verifies the peculiarity of unconventional superconductors with nodal structure. I will also discuss the possibility of an impurity quantum phase transition as driven by the supercurrent.
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