Andreev Reflection Spectroscopy
Point Contact Anreev Reflection (PCAR) technique require careful measurements of I-V of the point contact as well as conductance G=dI/dV in the voltage range from 0 to approximately 10Δ. Altrernatively, one can apply sufficient field to suppress superconductivity. It is very important to know the transport regime in which the measurements are done.
Depending on the type of experiment, spin polarization can be defined in a number of ways. The PCAR technique measures the transport spin polarization. The correct interpretation of the spin polarization measurements requires a clear identification of the transport regime (diffusive vs ballistic) and our probes are designed to provide this information. The measurements in the ballistic (Sharvin) regime (mean free path L is larger than the size of the contact d) is generally easier to interpret as the number of quantum channels and their transparencies are well defined.
Experimental setup:
The PCAR technique requires low temperature measurements, so that the tip is in a superconducting state. This is one of the limitations of the technique, as it is impossible to measure some of the insulating magnetic materials, such as Fe3O4, for instance, which has a Verwey transition at about 120K. All measurements are done in a four-probe geometry in liquid He4 cryostat, with the differential conductance dI/dV obtained by standard ac lock-in detection. The second derivative d2I/dV2 can also be recorded to obtain the point contact phonon spectra. In general, it is advantageous to reduce the temperature even lower, e.g. to 0.3K.
Fig.1 - An experimental setup: He cryostat, adjustment mechanism with the superconducting point (tip), and standard electronics/data acquisition system.
We have two different low temperature probes for spin polarization measurements; one is based on a mechanical adjustment mechanism, and the other on a more sensitive piezo-controlled activator. In a mechanical adjustment, a shaft connected to a differential type screw is used to change the position of the tip by about 10 µm per revolution, allowing fairly good control over the contact resistance of the junction. After the contact is established, the shaft is disconnected from the sample stage to eliminate the temperature gradient and increase the stability of the contact (see Fig.2).
Fig.2 - Schematics of the probe with the mechanical adjustment mechanism.
This design allowed us to measure the resistance of the contact in a broad temperature range (see Fig.3), allowing independent determination of the mean free path and the contact size, which are defined by the following formulas respectively:
Fig.3 - Sn-MnAs point contact resistance of and in-plane resistivity data for the same sample of type A MnAs in the temperature range (~200-240K) where both dependencies are approximately linear. Inset: Resistivity of MnAs between 4K and 300K.
For instance, for MnAs epitaxial film and Sn tip with the typical contact resistance Rc~10Ω the linear regime corresponds to the temperature range of approximately 200-245K with dRc/dT=0.22Ω/K and /dT=0.35·10-6Ω.cm/K, resulting in d=15nm and L=330nm. These estimates show that L>>d, indicating that the measurements are done in the ballistic regime and consequently the ballistic formulas can be safely used for the data analysis.
A number of superconducting tips, such as Sn, Pb, Nb, and others can be used. These tips can be either mechanically polished, or electrochemically etched (see Fig. 4).
Fig.4 - Electrochemical etching set-up for Nb tips
The Scanning Electron Micrograph of a typical Nb tip fabricated by electrochemical etching is shown below:

Copyright ©2006 Boris Edward Nadgorny