1/f Dielectric Fluctuations at the Glass Transition
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Mat. Res. Soc. Symp. Proc. Vol. 455 ©1997 Materials Research Society
B was not measurably different from 1.00 over at least a decade in temperature below the spin glass transition. This implies, in an activated kinetics picture, 1/f noise over an extremely broad range of frequency, e.g. many more decades than can be measured. The question is whether this is a generic signature of a thermodynamic glass phase and whether it occurs in structural glasses. In this paper detailed measurements of dielectric polarization noise as a function of temperature and frequency in glycerol and PVC are described. Thermal equilibrium noise in the electric polarization of a lossy dielectric material is expected based on the fluctuation-dissipation theorem. Johnson noise of the conduction electrons in a resistor is a well known analog. Thermal noise was observed in the magnetization of spin-glasses using very sensitive SQUID detectors[9]. A systematic investigation of thermal dielectric polarization noise and FDT in glycerol was recently reported[4,5]. The FDT voltage noise produced by dielectric polarization fluctuations within a sample-filled capacitor is given by: Sv = 4 kbT c"CO /IC12 co - 4 kbT (
I"/c')/IClo
(1)
where kb is the Boltzmann constant, T is temperature, co is angular frequency, C is the capacitance of the sample-filled cell, Co is the empty cell geometrical capacitance, c', c" are the real and imaginary components of the dielectric susceptibility. Experimental Methods The capacitance sample cell consisted of copper plates separated by teflon or polypropylene spacer rings (0.1 - 0.4mm thickness, 0.5-4.0 cm 2 area). The geometrical capacitances, CO, ranged from 5 pF to 45pF (± 10%). The sample cell was attached to a copper cold finger in vacuum within a liquid-nitrogen cooled cryostat which sits on a vibration isolation table within an RF shielded room. The operational temperature range was 80K-400K. A custom built preamplifier utilizing an Analog Devices 549L ultra-low current noise JFET op-amp, and a 1011 ohm (Victoreen) input resistor, was mounted inside the cryostat adjacent to the sample [5] (See fig. I inset). The stray capacitance in this arrangement was limited to < 1 pF. The preamplifier could be used for low temperature measurements by maintaining it at a much higher temperature than the sample using its self heating and deliberately poor thermal grounding. The preamp output was further amplified with a Stanford Research pre-amp and then digitized using a personal computer-based data acquisition card and spectrum analyzer. 10"-10 397 K
PVC
Z10-11 402 K
C 10-12 -U)i 1
10 Q
Figure 1. Voltage noise spectra for PVC at two temperatures in the vicinity of its transition. Inset: the measurement circuit located within the cryostat
k
ikglass AD
10
10"-1.1
1
10
100
1000
f (Hz)
254
One must be careful in dealing with the amplifier background. Amplifier current noise produces voltage noise inversely proportional to the sample capacitance at low frequencies with a 1/f 2 dependence. The relative proport
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