TcSUH
Bi-Weekly Seminar
Magnetoelectric Effects, Spin Frustration, and Ferroelectricity in Multiferroic Manganites
by: Bernd Lorenz
Date: Friday November 18, 2005
Time: 1:00 pm – 2:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The interaction between electric and magnetic fields in matter and/or between dielectric and magnetic orders is one of the fundamental problems in condensed matter physics. The magnetoelectric effect that allows the control of magnetic (dielectric) properties by electric (magnetic) fields is of principal physical interest and it bears the potential for the development of a new type of magnetoelectric memory. The topic has attracted renewed interest quite recently with the discovery of the coexistence of ferroelectricity and magnetic orders in multiferroic rare earth manganites. We discuss the complex physical properties of multiferroic RMnO3 and RMn2O5 (R=rare earth, Y) and show that magnetic frustration as well as strong spin-lattice coupling are the origin of a wealth of interesting phenomena such as incommensurate magnetic orders, frustration-induced ferroelectricity, magnetic field control of ferroelectric polarization, etc. The interactions between the Mn spins, the rare earth magnetic moments, and the ferroelectric polarization in these compounds give rise to an unprecedented phase complexity, e.g. as observed in hexagonal HoMnO3. In orthorhombic RMn2O5, our high-resolution thermal expansion measurements provide unambiguous proof that the ferroelectric transitions are accompanied by strong structural anomalies resulting in anisotropic lattice strain along the principal crystallographic directions.
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Bi-Weekly Seminar
Magnetic-Ordering-Related Phonon and Crystal Field Anomalies in Rare Earth Manganites
Date: Friday October 28, 2005
Time: 1:00 pm – 2:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The complex relationships among the lattice distortions, magnetism, and dielectric and transport properties of rare earth manganites RMnO3 (R = rare earth, Y, Sc) with both orthorhombic and hexagonal structure are attracting increasing interest. The role of structural distortions is widely recognized, but there are only a few studies on their variation with R and how this affects the spin-phonon and electron-phonon coupling. The results of recent experiments on the variations with R of the Raman spectra of orthorhombic RMnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) will be reported. In this series, with decreasing radius rR of R (R=La to Eu), the magnetic transition temperature TN to A-type antiferromagnetic (A-AFM) ordering of Mn3+ decreases from ~140 K to ~40 K. With further decrease of rR (R=Gd to Ho), however, the magnetic structure below TN changes from A-AFM to an incommensurate antiferromagnetic one (IC-AFM) with sine-wave ordering of the Mn3+ moments. It will be shown that the change of magnetic structure correlates with strong mixing of phonon modes involving in-plane Mn-O stretchings and bendings of MnO6 octahedra. The strong spin-phonon coupling is evidenced by phonon softening at T<TN in A-AFN, but not in IC-AFM manganites.Another promising experimental approach&nash;temperature-dependent crystal field IR spectroscopy–will be discussed and the first results on crystal field anomalies near TN in hexagonal RMnO3 (R = Ho, Er, Tm, Yb) will be reported.
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Bi-Weekly Seminar
First Experimental Test of the Incorrect Assumption that Continuous Columnar Pinning Centers Produce the Highest Jc in Superconductors
by: Dr. Alberto Gandini
Date: Friday September 30, 2005
Time: 1:00 pm – 2:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The improvement of critical current in HTS by optimization of pinning center morphology has been a crucial area for research since the HTS's discovery. In the past decade it has been stated in numerous papers that the optimum pinning centers are provided by continuous columnar defects. This conventional wisdom has never been questioned, nor experimentally tested. This has led several researchers to believe that the highest Jc could only be achieved by means of continuous columnar defects. Columnar defects have been assumed to provide the highest Jc because theoretically they have been shown to maximize the pinning potential. However, pinning theory completely neglects that, as the pinning center density increases, the current percolation is reduced, and hence Jc decreases. Recently we argued that percolation has a larger effect on Jc than previously expected. We proposed that discontinuous pinning centers, which reduce the loss of current percolation, would result in a higher Jc. An experiment was performed to directly compare continuous and discontinuous pinning, using high-energy ions. We now present the surprising experimental result that, in clear contrast with the conventional belief, Jc for discontinuous pinning is much higher than for continuous. This experiment indicates that the superior percolation achieved by discontinuous pinning outweighs the decrease in pinning potential. Record Jc ~ 300 KA/cm2 at 77 K was achieved in melt-textured YBCO for pinning which was 67% discontinuous. This work stands as the first experimental test of the postulate that continuous columnar pinning centers produce the highest Jc, and shows that the postulate is incorrect.
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Bi-Weekly Seminar
Condensed Matter Physics Phenomena in Biological Systems
Date: Thursday April 21, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
This talk will give a brief overview of condensed matter physics phenomena in biological systems, including diamagnetism, quantum tunneling in electron transfer reactions, excitons, charge density waves, and (albeit speculative) proposals of biological superconductivity. I will then, time permitting, discuss some of our own research, such as the production of nonlinear harmonics by enzyme complexes and motor proteins in the plasma membrane and the inner membranes of mitochondria. At low frequencies, we use a high-[Tc] SQUID to directly probe the current response, which greatly reduces electrode polarization effects. We have been studying, in vivo, budding yeast (Saccharomyces cerevisie) and, in vitro, cytochrome c, a mitochondrial membrane protein in the respiratory chain. Also of interest are the electric and magnetic properties of tubulin, which self-assembles to form microtubules in the cytoskeleton.
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Bi-Weekly Seminar
Theory of Inhomogeneous High-Temperature Superconductivty
by: Prof. W. P. Su
Date: Thursday January 27, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Inhomogeneity is a hallmark of the high-temperature superconductors as evidenced by many experiments. A natural interpretation of that can be found in a phenomenological model of d-wave superconductivity, which is an extended Hubbard model with onsite repulsion and nearest-neighbor attractive interaction. This model gives rise to a phase diagram which is strikingly similar to the observed one in the cuprates. A central result of the model is that below a critical doping concentration, the system is unstable with respect to phase separation between the antiferromagetic state and the d-wave superconducting state. Such a state has a vanishing compressibility, therefore it is easily rendered inhomogeneous by the random dopant potentials.
As a microscopic origin of the intersite attractive force, a tight-binding version of the Little’s exciton model has been examined. Quantum Monte Carlo calculations indicate that the purely repulsive interaction between conduction electrons and exciton electrons (electronic polarization) can indeed induce phase separation and superconductivity, where are manifestations of the intersite attractive force.
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