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Kamdem Thaddee Thesis Defense 11/5/12

Thesis Defense

Kamdem Thaddee

Monday, November 5, 2012
3:30pm (334 JFB)

Title: No Title

Abstract:

We performed a systematic study of bipolar and unipolar diodes based on MEH-PPV, using electronic and magneto-transport measurements with magnetic field in the range 0–180 mT and admittance spectroscopy in the frequencies varying from 1Hz to 10MHz. The admittance spectra of bipolar devices reveal two relaxation processes with distinct time scales that are influenced by the magnetic field. The slower process, which dominates the device capacitance at frequencies less than 10 Hz, is attributed to the trap-assisted monomolecular recombination. The second faster process is attributed to the electron-hole bimolecular recombination kinetics. decreases by approximately 30% under magnetic field of 30 mT. We observed that at low frequencies, the differential capacitance of bipolar devices is positive at low biases voltages, turns negative at intermediate biases, and becomes positive again at stronger biases. By carefully selecting bias voltage, we were able to tune some bipolar diodes from the state with the negative capacitance to the state with the positive capacitance just by applying magnetic field. The magneto-conductance has a characteristic cutoff frequency that shifts to higher frequencies with increasing bias voltages. In particular, the magneto-conductance at 10 MHz in a bipolar device was measured to be 4.5 % in the magnetic field of magnitude 30 mT. For bipolar devices, the frequency-dependent response of the device admittance to the small magnetic field is identical to the response of the admittance to the small increase in the bias voltage in zero magnetic field. We found that the response of the admittance on the magnetic field is consistent with the polaron-polaron model of OMAR. The admittance of unipolar diodes did not reveal any magnetic field.

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This Week's Colloquium: James Pinfold, Nov. 1, 2012

James Pinfold
University of Alberta

Thursday, Nov. 1, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:00pm (102 JFB)

Title: The MoEDAL Experiment at the LHC - Searching for the Highly Ionizing Particle Avatars of New Physics

Abstract:

MoEDAL (the Mo nopole and Exotics Detector at the Lhc) was approved by CERN research board in March 2012. It is the 7th and newest LHC experiment designed to search for highly ionizing particle avatars of New Physics. I will give a brief historical introduction to the search for highly ionizing particles, concentrating on the magnetic monopole, and then describe the experiment and its revolutionary physics potential.

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Thursday Colloquium: Carsten Rott, Oct. 25, 2012

Carsten Rott
(CCAPP) at The Ohio State University

Thursday, Oct. 25, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:00pm (102 JFB)

Title: Latest Results from the IceCube Neutrino Observatory

Abstract:

The world's largest neutrino observatory IceCube is comprised of more than 5000 photomultiplier tubes on 86 strings installed in the Antarctic ice cap at depths from 1450 to 2450 m. An air shower array, called IceTop, is located at the surface. Construction of this multi-purpose detector was completed in December 2010. The detector has been taking data since the deployment of the first string in January 2005. Data has been analyzed for high energy neutrino events of astro-physical origin, cosmic rays, transient sources, dark matter annihilation signals, and various others. After a review of the latest results, the talk will conclude by giving an outlook for potential upgrades to the IceCube detector.

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Special Colloquium: Alan Drew, Oct 24, 2012.

Alan Drew
Queen Mary, University of London

Thursday, Oct. 24, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:15pm (102 JFB)

Title: Local Probe Investigation of Spin & Charge Dynamics in Organic Semiconductors

Abstract:

Organic semiconductors fall into a class of materials that shows significant potential for future applications and as a result, the field is becoming extremely topical. This is due to their ease of processing, low cost, highly tuneable electronic properties, favorable mechanical properties and long spin coherence times. The latter point makes them extremely promising for future spintronic applications. However, there is a lack of suitable techniques that can yield information on intrinsic spin and charge carrier dynamics in organic materials. For example, many of the experimental techniques available that probe the spin polarization of charge carriers in inorganic spintronic devices/materials are not always applicable to organic materials. Muon spectroscopy is a technique that has rarely been applied to study spintronic problems in inorganic systems, yet is ideally suited to studying them in organic semiconductors.

Low energy muon spin rotation can directly measure the depth resolved spin polarization of charge carriers in organic spin injection devices [1]. After giving a brief introduction to muon spin rotation in the context of these results, I will go on to demonstrate that it is possible to separate out the various contributions to spin decoherence in organic spin valves, differentiating between interface and bulk spacer layer effects [2].

A more exotic application of the muon technique, known as avoided level crossing (ALC) spectroscopy, can be used to probe the spin dynamics in organic semiconductors on a molecular lengthscale [3,4]. After briefly introducing this application of muons, I will go on to present measurements of temperature dependent electron spin relaxation rates, on a series of organic molecules of different morphology and molecular structure. These measurements, when combined with some of the latest results on the mass-dependence of electron spin relaxation rates, offer clues as to the underlying relaxation mechanism in organic semiconductors [4,5].

Finally, taking advantage of the intrinsic spatial sensitive of ALC spectroscopy, I will show how laser excited ALC spectroscopy (a technique I am currently developing) can offer unique insight into electron transfer in organic molecules. Not only is this an important process in organic electronics/spintronics, it also is fundamental to many biological processes, including photosynthesis, DNA repair and cell respiration.

[1] A. J. Drew et al., Nature Materials 8, 109 (2009)
[2] L. Schulz et al., Nature Materials 10, 39 (2011)
[3] A. J. Drew et al., Phys. Rev. Lett. 100, 116601 (2008)
[4] L. Schulz et al., Phys. Rev. B 84, 085209 (2011)
[5] L. Nuccio et al., submitted.

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