Experimental Techniques and Facilities
Low Temperature, High Magnetic Field Facility
The facilities which we have developed are based on three ingredients:
- the development of measurement configurations aimed at exploring the low frequency electrodynamics of highly conducting solids;
- the facility allowing these experiments to be conducted at low temperatures and high magnetic fields; and,
- materials preparation and characterization facility.
Low energy electrodynamics
During the past few years, we have developed various techniques to investigate the frequency dependent conductivity and dielectric constant of (mainly) highly conducting materials over a broad frequency range. The focus of this effort is the low frequency response, at frequencies below what conventional optical methods cover. Conventional lock-in techniques are employed to extract both components of the optical conductivity in the audio-frequency range while at radio frequencies, vector impedance meters and home-designed rf bridge configurations are used. The upper radio-frequency range, up to 2 GHz, is covered by a network analyzer which measures the transmission and reflection coefficient with the sample inserted in the circuit. Various resonant cavities with high quality factors have been designed and built in the microwave and millimeter wave spectral range, operating at various frequencies between 2.5 and 150 GHz, where resonant techniques are necessary7 to investigate the perturbation due to the specimens. Various configurations are employed and these depend on the sample geometry and measurement frequency among other factors. A so-called coherent source spectrometer, based on Backward Wave Oscillator (BWO) sources and on a quasioptical arrangement has recently been constructed. We have acquired a range of compact oscillators in the 2-7 cm-1 range, while external magnetic field driven BWO sources cover the entire submillimeter range up to 25 cm-1. A few years ago, we have also purchased a Bruker IFS 113v Fourier Transform Spectrometer which operates from 20 - 5500 cm-1. We have also developed various novel experimental arrangements [1-4] which will enable us to conduct high precision measurements of the optical conductivity of highly conducting solids, and such technique development forms the essential part of our research effort focus.
Images
We have recently completed the construction of a low temperature, high field optical facility and now we are able to conduct experiments down to 0.35 K (using a continuous flow Oxford He3 system) in the presence of external magnetic fields up to 9T. The spectral ranges which we cover include the radiofrequency, microwave, millimeter and submillimeter wave, and FTR frequencies.
Coaxial cable and waveguide access allows experiments to be conducted up to 150 GHz employing various resonant structure configurations. Optical access allows for quasioptical and optical (reflection and transmission) experiments covering the submillimeter wave and FIR spectral range.
Materials Preparation and Characterization
We have established a materials preparation and characterization facility with the aim of growing large single crystals of highly anisotropic organic and inorganic compounds. We have grown large single crystals of the so-called Bechgaard salts, and their sulphur analogs. The crystals produced are used not only by us but as well as other groups involved in the study of dc transport, ESR, elastic properties and photoemission. X-ray and composition analysis are routinely performed on the specimens which are prepared together with the dc resistivity measurements in the 1.2 - 300 K temperature range. An experienced chemist technician supervises the facility.
References:
1. S. Sridhar, D. Reagor, G. Grüner, "Complex Conductivity Measurements Between 26 and 110 GHz Using Complex Impedance Bridges," Rev. Sci. Instum. 56, 1946. (1985).
2. T.W. Kim, W.P. Beyermann, D. Reagor, G. Grüner, "Complex Conductivity Measurements at Several Frequencies in the Millimeter Wave Spectral Range," Rev. Sci. Instr. 59, 1219 (1988).
3. O. Klein, S. Donovan, M. Dressel, and G. Grüner, "Microwave Cavity Perturbation Technique: Part I: Principles," International Journal of Infrared and Millimeter Waves, 14, 2423-2457 (1993).
S. Donovan, O. Klein, M. Dressel, K. Holczer, and G. Grüner, "Microwave Cavity Perturbation Technique: Part II: Experimental Scheme," International Journal of Infrared and Millimeter Waves, 14, 2459-2487 (1993).
M. Dressel, O. Klein, S. Donovan, and G. Grüner, "Microwave Cavity Perturbation Technique: Part III: Applications," International Journal of Infrared and Millimeter Waves, 14, 2489-2517 (1993).
4. A. Schwartz, M. Dressel, A. Blank, T. Csiba, and G. Grüner, A.A. Volkov, B.P. Gorshunov, and G.V. Kozlov, "Resonant Techniques for Studying the Complex Electrodynamic Response of Conducting Solids in the Millimeter and Submillimeter Wave Spectral Range," Rev. Sci. Instrum., Vol. 66, No. 4, 2943-2953 (1995).
5. G. Grüner, "Waveguide Configuration Optical Spectroscopy," in Millimeter and Submillimeter Wave Spectroscopy of Solids, edited by G. Grüner, Series: Topics in Applied Physics, Vol. No. 74, Ch. 4., 111-166, Springer Verlag, Berlin (1998).