These are optical photos of the PPMS resistivity pucks. The CMMP group has three of these so that samples can be contacted while one is in the system. Each puck has three stations that include four contacts for transport measurements. These contacts can be accessed either by our break-out box or the PPMS controller's resistivity bridge depending upon the application.
The large gold square is a heat sink to provide thermal conductivity to the PPMS temperature annulus. Ideally the samples should be heat-sunk to this block for ideal temperature control. While this is practical if the samples have insulating substrates, it should be noted that this surface is also electrically grounded. In the case of semiconducting or conducting sample substrates a piece of Kapton tape or other insulator should be placed under the sample. That is what has bee done in the present case.
Contacting samples is an art, and CMMP will not do that for you. Conducting silver or graphite paints can be used, as well as indium, indium solder, or a wire bonder. In many cases forming good contacts becomes a material science issue as contacts need to be annealed, or surface oxides need to be removed. A test station allows the integrity of the contacts to be tested at room temperature using the test electronics to be used in one's experiment.
The proper placement of contacts also depends upon other factors such as the type of measurement one wishes to accomplish. In cases of an irregular sample, the contacts might be best placed on the four corners to allow for a van der Pauw approach. In the case of a Hall measurement of carrier densities, one might wish to even pattern the sample into a Hall bar and attach contacts accordingly.
The availability of three sets of contacts, labelled I+, I-, V+ and V- allows one to mount three samples for traditional 4-point transport measurements. One need not assign the contacts to I+, I-,
etc., unless one is using the PPMS resistivity bridge. In other cases, one is limited only by one's imagination. One can wire channel A for 4-point resistance, channel B for 2-point resistance on two samples, and channel C for a Hall measurement. One can use channel A for one's sample, B for a Hall device to measure applied field exactly, and channel C for a thermometer. In more complex devices one can simultaneously measure longitudinal and transverse voltage drops across samples patterned into bars with multiple pairs of contacts using several channels to assign all of the necessary contacts to be accessed by the breakout box.
Data provided by this technique is complex permittivity, e = e' + ie", as a function of frequency, and complex permeability, u = u' + iu", as a function of frequency. This is often expressed as a function of loss tangent, e"/e' and u"/u'.
