Point Contact Spectroscopy of n-doped Cuprates

Point contact spectroscopy (PCS) is similar to scanning tunneling spectroscopy with the tip in actual physical contact with the sample. PCS is extremely useful especially for gap spectroscopy of superconductors and spin polarization measurements in ferromagnets.

We first fabricated a PCS probe for operation in fields of up to 32 tesla at temperatures down to 0.4 K, at the national high magnetic field laboratory (NHMFL) in Tallahassee, FL (ref. NHMFL reports 13, 14 (2006)) . The motion is controlled by a set of bevel gears. These gears change the direction of motion in order to move the tip perpendicular to the direction of the magnetic field (Fig. 1). A worm gear at the top of the probe allows us to change the contact pressure (and thus vary the barrier strength) from outside the dewar while the sample remains at low temperature. The dI/dV-V data is acquired using an ac-modulation technique. We have designed and built the electronics needed for this technique. All the other required instruments such as voltmeters and lock-in amplifiers are available at the NHMFL. We use the NHMFL data acquisition program to record our PCS data.

Fig. 2 shows PCS data on a single crystal of the n-doped cuprate Pr_2-xCe_xCuO₄ (PCCO) in fields of up to 22 tesla (T = 1.5 K). Our results on PCCO suggest that there are two types of normal state gaps in high temperature superconductors, the “pseudogap” and the correlation gap.

 

Fig. 1. Point contact probe for high magnetic fields.

Fig. 2. Point contact spectra of a Pr2-xCexCuO4 crystal at 1.5 K in different magnetic fields.