Condensed Matter Sack Lunch Seminar

semester, 2009


Tuesday, January 20th 2009
12:20 pm:
There will be no seminar this week.

Tuesday, January 27th 2009
12:20 pm:
Speaker: Prof. Vidya Madhavan, Boston College
Subject: New Developments in Scanning Tunneling Microscopy of High Temperature Superconductors
The speaker is a candidate for the Cond. Matter Experimentalist Asst. Prof. position.

The 'high-Tc problem' remains one of the most important outstanding problems in condensed matter physics today. After two decades of intense research there is no consensus on the basic question: what is the mechanism that responsible for pairing? Even as we explore the possibility that pairing in these unconventional superconductors proceeds without the involvement of a bosonic glue, we have recently made exciting progress in identifying candidates that could potentially mediate pairing. In this talk, I will discuss our recent STM investigations of the electron-doped cuprate superconductor Pr0.88LaCe0.12CuO4-δ (PLCCO) (Tc = 24 K). Our spectra reveal superconducting gaps with coherence peaks that disappear above Tc. In addition, multiple step/peak-like features are observed outside the gap. Such features in STM spectra are suggestive of bosonic excitations that couple strongly to the electrons. Analysis of the data reveals bosonic modes at 10.5±2.5 meV and 3 meV which are much lower in energy than the bosonic mode observed in hole-doped Bi2212. These energy scales match the magnetic resonance modes (spin-excitations) in PLCCO measured by inelastic neutron scattering. I will show that both the local mode energy and the intensity are correlated with the local gap energy scale and discuss the implications of our observations on the pairing issue. I will also briefly discuss recent atomic resolution images and spectroscopy on the parent compound of a pnictide superconductor SrFe2As2. I will compare our data with contradictory LEED on identical samples and explore the possible reasons for this puzzling contradiction.


Thursday, January 29th 2009
1:25 pm:
Speaker: Xian Leng, University of Minnesota
Subject: Interfacial Superconductivity
Note the different day, time and place this week only.

Tuesday, February 3rd 2009
12:20 pm:
There will be no CM Sack Lunch Seminar this week. There will be two CM Seminars.
12:20 pm:
Condensed Matter Seminar in 435 Physics
Speaker: Ania Bleszynski-Jayich, Yale University
Subject: Measurements of persistent currents in normal metal rings using a micromechanical magnetometer
Ania Bleszynski-Jayich is a candidate for the Cond. Matter Experimentalist Asst. Prof. position.

A striking manifestation of quantum mechanics is the existence of a dissipationless current in a non-superconducting metal ring. The persistent current is analogous to electrons orbiting the nucleus in an atom, a more familiar quantum effect. The persistent current is a signature of electronic phase coherence around the ring and offers insight into many issues in mesoscopic physics such as coherence and electron interactions in metals. The prediction that an atom-like persistent current could be observed in a micron size metal ring generated considerable interest. However, the small magnitude of the current and the necessity of measuring it through its associated magnetic moment make persistent current experiments challenging. Few measurements have been reported and inconsistent results have left an unclear picture of the properties of persistent currents. In a novel approach to studying persistent currents we have developed a cantilever based torsional magnetometer capable of detecting a magnetic moment of 1 μB/Hz1/2 (and a force of 1.6 aN/Hz1/2) at a temperature of 300 mK. The rings are integrated directly onto the end of the cantilever. I present measurements of persistent currents in arrays of rings and single rings as a function of temperature, ring size, and magnetic field over a much broader range than previously possible. We compare our results to theory and previous experiments.


Tuesday, February 10th 2009
12:20 pm:
Speaker: Hung-Sheng Chiang, University of Minnesota
Subject: Nonlinear transport in 2D electron systems

Tuesday, February 17th 2009
12:20 pm:
Speaker: Yu Chen, University of Minnesota
Subject: Magnetic field induced superconductivity in Zn nanowires

Tuesday, February 24th 2009
12:20 pm:
Speaker: Eric Garlid, University of Minnesota
Subject: Large Interfacial Spin Accumulation in Lateral Fe/GaAs Devices

Tuesday, March 3rd 2009
12:20 pm:
Speaker: Thomas Dunn, University of Minnesota
Subject: Spin Torque and Magnetic Switching

Tuesday, March 10th 2009
12:20 pm:
Speaker: Matt Parker, Hung-Sheng Chiang, Yen-Hsiang Lin, Te-Yu Chen, University of Minnesota
Subject: Talks for the the APS March Meeting

Tuesday, March 17th 2009
12:20 pm:
There will be no seminar this week.

Thursday, March 26th 2009
12:20 pm:
Speaker: Mike Erickson, University Minnesota
Subject: Temperature dependence of the non-local spin signal in Cu-based lateral spin-valves

Tuesday, March 31st 2009
12:20 pm:
Speaker: Tanner Schulz, University of Minnesota
Subject: Ordered Vortex Arrays in Superconducting Niobium Films

Tuesday, April 7th 2009
12:20 pm:
Speaker: Nin Ji, University of Minnesota
Subject: XMCD study on FeCo based nanocrystals

Tuesday, April 14th 2009
12:20 pm:
Speaker: Lee Wienkes, University of Minnesota
Subject: Optical Absorption Spectra of Mixed Phase Thin Film Semiconductors

Tuesday, April 21st 2009
12:20 pm:
Speaker: Chunyong He
Subject: Doping fluctuation-driven magneto-electronic phase separation in La1-xSrxCoO3 single crystals

Tuesday, April 28th 2009
12:20 pm:
Speaker: Brian Skinner
Subject: Charge inversion and non-monotonic swelling of a large molecule

Tuesday, May 5th 2009
12:20 pm:
Speaker: Elizabeth Smith, University of Minnesota
Subject: Quantifying Protein Expression in Living Cells

Thursday, May 7th 2009
1:25 pm:
Speaker: Matthew Loth, University of Minnesota
Subject: Non-mean-field screening by multivalent counterions
Note different day, time and place: second CM sack Lunch this week

Tuesday, May 12th 2009
12:20 pm:
No seminar this week. Finals week.

Tuesday, September 8th 2009
12:20 pm:
Speaker: Shun Wang, University of Minnesota
Subject: Discovery of a novel smectic liquid crystal phase

Tuesday, September 15th 2009
12:20 pm:
Speaker: Thomas Dunn, University of Minnesota
Subject: Spin Torque and Magnetic Switching

Thursday, September 17th 2009
1:25 pm:
Speaker: Professor Maxim Vavilov, University of Wisconsin - Madison
Subject: Microwave photoconductivity in 2D electron systems with mixed disorder.

I will present a systematic study of the microwave-induced oscillations in the magnetoresistance of a 2D electron gas for mixed disorder including both short-range and long-range components. I will discuss four distinct contributions to the photoconductivity tensor and show that the photoresponse depends crucially on the relative weight of the short-range component of disorder. Depending on the properties of disorder, the theory allows us to identify the temperature range within which the photoresponse is dominated by one of the mechanisms.


Tuesday, September 22nd 2009
12:20 pm:
There will be no seminar this week.

Tuesday, September 29th 2009
12:20 pm:
Speaker: Te-Yu Chen, University of Minnesota
Subject: Nonlinear Magnetic Vortex Dynamics in the Presence of Pinning

Tuesday, October 6th 2009
12:20 pm:
Speaker: LiDong Pan, University of Minnesota
Subject: "Surface Phase Transitions in Liquid Crystal Films"

Tuesday, October 13th 2009
12:20 pm:
Speaker: Matthew Loth, University of Minnesota
Subject: Ionic conductivity on a wetting surface

Tuesday, October 20th 2009
12:20 pm:
Speaker: Matt Parker, University of Minnesota
Subject: Extinction in a Predator-Prey System

Tuesday, October 27th 2009
12:20 pm:
Speaker: Lee Wienkes, University of Minnesota
Subject: Hopping Transport in Amorphous Silicon

During the 1970’s, it was discovered that adding hydrogen to amorphous silicon (a-Si), producing hydrogenated amorphous silicon (a-Si:H), drastically reduced the density of defect states, resulting in an improved material. This opened the door for many discoveries with important technological applications, including the observation of doping, previously thought to be impossible in amorphous semiconductors. Following this revolution in the field, unhydrogenated amorphous silicon (a-Si) was largely left behind despite unsolved questions concerning the transport mechanisms in the material. These questions cover both conductivity and the thermoelectric power. Our group isrevisiting these questions and will present a brief overview of the current knowledge, as well as some new research on unhydrogenated amorphous silicon.


Tuesday, November 3rd 2009
12:20 pm:
Speaker: Brian Skinner, University of Minnesota
Subject: The world's thinnest capacitor

Tuesday, November 10th 2009
12:20 pm:
Speaker: Yen-Hsiang Lin, University of Minnesota
Subject: Indirect magnetic-field-tuned superconductor-insulator transitions

Tuesday, November 17th 2009
12:20 pm:
Speaker: Anthony Hatke, University of Minnesota
Subject: Shubnikov-de Haas Oscillations in Microwave-irradiated 2D Electron Systems

Tuesday, November 24th 2009
12:20 pm:
Speaker: Professor Jorge Vinals, McGill University
Subject: Pattern formation in extended systems.

The evolution of systems driven outside of thermodynamic equilibrium is characterized by strong nonlinearity and the formation of complex spatio-temporal patterns. We illustrate several key concepts by focusing on three prototypical situations: Rayleigh-Benard convection, Faraday waves, and defect motion in soft matter. In the first case, we discuss the appearance of non variational effects in an otherwise overdamped dynamical system, and how they lead to spiral defect chaos, an extended spatio temporally chaotic state. In the second, weak dissipation effects in a near Hamiltonian system account for the formation of quasi crystalline patterns on the surface of a vibrated fluid layer. In the latter case, we discuss topological defect motion in modulated phases, and mechanisms underlying the emergence of long range order.


Tuesday, December 1st 2009
12:20 pm:
Speaker: Feng Guo, University of Minnesota
Subject: The Noise is the signal: Noise Measurements in Magnetic Tunnel Junctions

Tuesday, December 8th 2009
12:20 pm:
Speaker: Stephen Snyder, University of Minnesota
Subject: Magnetic Field Enhanced Superconductivity in Zn Nanowires

Tuesday, December 15th 2009
12:20 pm:
Speaker: Xiang Leng, University of Minnesota
Subject: Electrostatic Tuning of the Superconductor-Insulator Transition

Tuesday, December 22nd 2009
12:20 pm:
Speaker: Hung-Sheng Chiang, University of Minnesota
Subject: Non-linear Magnetoresistance Oscillations in Intensely Irradiated Two-Dimensional Electron Systems Induced by Multi-Photon Processes

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