LOW TEMPERATURE RESEARCH FACILITY

PROJECT for 2006/2007

Construction Of Gas Handling for a He3-He4 Dilution Refrigerator

Our He3 refrigerator can reach low temperatures down to 0.4 K. However, we have a mini-dilution refrigerator with the potential of reaching 100 mK or even 50 mK. The dilution refrigerator, as its name indicates, uses a mixture of He3-He4 (about 5%-95%) and the circulatuin of He3 through that mixture. The cooling power of such unit goes as T^2, as compared to the exp(-A/T) for an evaporation refrigeration. During the Summer of 2005 Dave Rhodes and Dr. Tahar designed and built a liquid cryogen system around the mini-fridge. One of the crucial components is a helium 1K pot through which the He3 is precooled and condensed before injection into the minifridge. This system when built will be used to reach the said temperatures and study the materials below. With good control on the heat load, we may be able to reach 20 mK. Check out the building parts and as assembled. The gas component include a turbo-molecular pump, a helium sealed rotary pump and a lot of "plumbing", flow control and various vacuum and pressure gauges and valves.

Materials Synthesis

This project will involve one student in the synthesis of ferric chloride intercalated graphite using the two-zone oven method. This entails preparation of the host material (HOPG) samples to the right size (1mmX5mmX10mm), glass ampule cutting, and in-situ synthesis of the chloride salt by burning pure iron wire with chlorine gas, inside the ampule and sealing it. Then, a bake period of usually one to few days follows. One important aspect of any synthesis is the characterization of the sample. That is done using X-ray diffraction, which is very powerful in determining the number of graphite layers sandwitched between the magnetic salt layers. The resulting layered materials constutite two dimentional magnetic and electronic systems, which are further studied using the measurements described below. The student will be involved in the synthesis and characteriztaion of the samples, which very likely will take the first half of the project's time. During the second half, the student will carry out one or two of the measurements. The other measurements will done at a later time during the academic year as an independent study.

Resistivity, Magneto-resistance, and Hall Effect

Electrical transport measurements on these materials are carried out as a function of temperature. We make use of a liquid helium cryostat that houses a 10 Tesla superconducting magnet. The temperature of a sample may be varied from 2 K to 300 K and can be held constant with an accuracy of +/- 0.05K. The collection of data and the regulation of sample temperature is computer controlled. The temperature dependence of the resistance of the sample will sheld light on the conduction mechanism and the electron-phonon scatering, the Hall voltage will measure the type of carriers and their concentration. Due to their layered structure, FeCl3 GIC's are highly anisotropic, and conduction along the ab-plane is orders of magnetude greater and of different mechanism than that along c-axis.

A.C. Magnetic Susceptibility

Materials ac magnetic susceptibility is studied/measured using a Hartshorn Bridge as modified by Maxwell, and an SAR850 Lock-in-Detector in the same temperature range. Attention will be payed to the geometry of the material and the ac susceptibilty along the a-b plane will be measured using the 3He refrigerator for magnetic phase transition investigation, and combined with a-b plane resistivity, the ac susceptibility along c-axis will measured using the superconducting magnet and cryostat.

D.C. Magnetic Susceptibility

The D.C susceptibility is the magnetization or the magnetic moment of the sample. The magnetization is measured using an Oxford Instruments RF SQUID magnetometer with a temperature range extanded down to 0.33K, using a Janis 3He refrigerator. This is done as a further study of the magnetic phase transition.

Specific Heat

Low temperature measurements of sample's heat capacity is done using the pulse and/or the ac method. The ac technique is well suited for heat capacity measurements around phase transition temperatures. The set-up has been tested and run on indium doped tin-lead-tellurides, semiconductor materials with superconducting properties and ferric chloride intercalated graphite. The preliminary results on the GIC's show a phase transition below 2 K, just like previous results on different stage samples.