Investigations of the Impact of Nuclear Quadrupole Moments on the Low-Frequency Elastic and Dielectric Susceptibility of a Polymer Glass at Low Temperatures

Table of Contents

1 Introduction and Motivation

2 Theoretical Background

2.1 Low-Temperature Dynamics in Glasses

2.1.1 Glass Formation and Thermal Properties

2.1.2 Structure of Amorphous Materials

2.1.3 Energy States in a Double Well Potential

2.1.4 The Standard Tunnelling Model

2.1.5 Frequency Response of Amorphous Samples

2.2 Nuclear Quadrupole Moments

2.2.1 Nuclear Quadrupole Resonance

2.2.2 Resonant Susceptibility

2.2.3 Relaxation Mechanisms

3 Experimental Methods

3.1 Working at Low Temperatures

3.1.1 Cooling Methods

3.1.2 Thermometry

3.2 Polymer Glass FR-122P

3.2.1 Chemical Structure

3.2.2 Sample Production

3.2.3 Material Choice

3.3 Acoustic Measurements

3.3.1 Double Paddle Oscillator

3.3.2 Measurement Strategy

3.3.3 Probing the Nuclear Bath

3.4 Dielectric Measurements

4 Results and Analysis

4.1 Preparatory Measurements

4.1.1 High Temperatures

4.1.2 Background

4.1.3 Voltage Scaling

4.1.4 Thermalisation

4.2 Complex Acoustic and Dielectric Susceptibilities

4.2.1 Temperature Overview

4.2.2 Acoustic Measurements

4.2.3 Dielectric Measurements

4.3 Numerical Calculations

4.3.1 Standard Tunnelling Model

4.3.2 Resonant Susceptibility

4.3.3 Additional Relaxation Rate

4.4 Impact of RF Bias

5 Discussion

5.1 Validity of Data

5.2 Conclusion

5.2.1 Interpretation of Results

5.2.2 Comparison to Related Work

5.3 Outlook

6 Summary

Research Objectives and Core Themes

This thesis investigates the impact of nuclear quadrupole moments (NQM) on the low-frequency elastic and dielectric properties of the polymer glass FR-122P at temperatures ranging from 7 mK to 600 mK. The research aims to determine whether observed deviations from the Standard Tunnelling Model (STM) arise primarily from an additional relaxation mechanism between NQM and atomic tunnelling systems or from modifications to the tunnelling system distribution function.

• Characterization of low-temperature dynamics in glasses and the role of atomic tunnelling systems.
• Theoretical and experimental study of nuclear quadrupole moments and their interaction with tunnelling degrees of freedom.
• Development and validation of a novel experimental module to selectively drive nuclear quadrupole transitions via RF bias.
• Comparative analysis of acoustic and dielectric susceptibility measurements against numerical simulations based on the STM and its extensions.

Excerpt from the Book

Summary of Chapters

1 Introduction and Motivation: Summarizes the history of glass research and identifies the research gap regarding the impact of nuclear quadrupole moments on the low-temperature physics of amorphous systems.

2 Theoretical Background: Provides the theoretical foundation, detailing the Standard Tunnelling Model and the theory of nuclear quadrupole moments and their interactions.

3 Experimental Methods: Describes the cooling techniques, the fabrication of the FR-122P samples, the acoustic measurements using a double paddle oscillator, and the design of the novel RF bias module.

4 Results and Analysis: Presents the experimental findings for acoustic and dielectric measurements and compares them to numerical simulations of the STM and its proposed extensions.

5 Discussion: Evaluates the validity of the obtained data, interprets the results within the context of previous research, and offers an outlook on future investigations.

6 Summary: Concludes the thesis by summarizing the experimental approach and the core findings regarding the role of NQM in the relaxation processes of polymer glasses.

Keywords

Polymer glass, FR-122P, low-temperature physics, nuclear quadrupole moments, NQM, Standard Tunnelling Model, STM, elastic susceptibility, dielectric susceptibility, atomic tunnelling systems, relaxation mechanisms, double paddle oscillator, acoustic measurements, cryogenics.

Frequently Asked Questions

What is the primary focus of this master's thesis?

The thesis focuses on examining how nuclear quadrupole moments (NQM) affect the low-frequency elastic and dielectric properties of the polymer glass FR-122P at temperatures below 1 Kelvin.

What are the central themes discussed in this work?

The work covers the structural properties of glasses, the Standard Tunnelling Model, the theoretical physics of nuclear quadrupole moments, and advanced low-temperature measurement techniques.

What is the main research question?

The research investigates whether the observed anomalies in the dielectric and acoustic properties of NQM-containing glasses are caused by an additional relaxation pathway between NQM and tunnelling systems or by structural changes (a pseudogap) in the tunnelling system distribution.

Which experimental methods were employed?

The research used low-frequency acoustic measurements via a custom-fabricated double paddle oscillator (DPO) and dielectric susceptibility measurements in a capacitance bridge, both conducted in a 3He/4He dilution cryostat.

What does the main part of the thesis cover?

The main part encompasses the theoretical derivation of tunnelling interactions, the experimental characterization of the FR-122P polymer, the fabrication of the DPO, and the development of an RF bias module for nuclear quadrupole transition driving.

Which keywords best characterize the work?

Key terms include Polymer glass, FR-122P, nuclear quadrupole moments (NQM), Standard Tunnelling Model (STM), and low-temperature acoustics.

What makes the material FR-122P particularly suitable for this study?

FR-122P contains bromine atoms with a large, well-defined nuclear quadrupole moment. Furthermore, the bromine atoms are covalently bonded to carbon, providing a uniform chemical and electrical environment, which simplifies the interpretation of the quadrupole splitting.

Did the RF bias module succeed in isolating NQM effects?

While the module allowed for RF excitation, it was determined that the observed changes in susceptibility were predominantly due to thermal heating caused by eddy currents in the gold-coated surface of the DPO, setting an upper limit on the method's sensitivity for detecting NQM transitions.