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Penetration depth measurements of organic superconductors

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Title: Penetration depth measurements of organic superconductors
Author(s): Olheiser, Tyson A.
Director of Research: Giannetta, Russell W.
Doctoral Committee Chair(s): Van Harlingen, Dale J.
Doctoral Committee Member(s): Giannetta, Russell W.; Stone, Michael; Liss, Tony
Department / Program: Physics
Discipline: Physics
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Penetration Depth Tunnel Diode Oscillator Cooling Rate Organic Superconductor Antiferromagnetic Insulator Deuterated Superfluid Density D-wave
Abstract: The temperature dependence of the magnetic penetration depth (λ) has been measured in single crystals of fully deuterated κ-(ET)2Cu[N(CN)2]Br. This material separates into superconducting (SC) and antiferromagnetic (AFI) phases. The relative amounts of SC and AFI phases has been shown to depend on the cooling rate of these materials in the vicinity of 80 K. In varying the cooling rate from 0.0025-180 K/min, the Meissner fraction appears to vary logarithmically with the cooling time, suggesting that these organics act like a glassy system near 80 K where SC nucleation grains are formed. In addition, the penetration depth of deuterated κ-(ET)2Cu[N(CN)2]Br consistently displays a power law temperature dependence with an exponent of n = 1.6-1.8 for Meissner fractions ranging from 0.03-0.60. This suggests that these samples are robust dirty d-wave superconductors like their undeuterated counterpart. With abnormally large penetration depths resulting from traditional analysis, an aluminum plating technique was used to measure the absolute penetration depth of deuterated κ-(ET)2Cu[N(CN)2]Br. The large values of λ(0) were confirmed with this technique. Measurements yielded values that increased from 29-320 μm with increasing cooling rate. This finding motivated the development of a SC spherical grain model where the deuterated sample was assumed to be a collection of independent spheres of undeuterated κ-(ET)2Cu[N(CN)2]Br. The model yielded grain sizes on the order of 5-40 μm, which agrees well with literature. Also, high and low temperature data was checked for self consistency. In addition to this work, the temperature dependence of the interplane penetration depth of undeuterated κ-(ET)2Cu[N(CN)2]Br was performed. With higher order corrections to the traditional analysis accounted for, a value of λ⊥ = 130 μm was measured. Also, the temperature dependence of the superfluid density was similar to in-plane behavior, suggesting that interplane transport in these materials is coherent despite being highly anisotropic and definitely 2D in nature.
Issue Date: 2010-08-20
Rights Information: Copyright 2010 Tyson Olheiser
Date Available in IDEALS: 2010-08-20
Date Deposited: 2010-08

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