Point-contact spectroscopy

Point-contact Spectroscopy (PCS) has served as a powerful tool for investigating the exotic quantum states in strongly correlated electron systems, including superconductivity (SC), heavy electron states and hidden order. For example, in the case of superconductivity, using the theoretical Blonder-Tinkham-Klapwijk (BTK) model taking into account of both the quasiparticle tunneling and Andreev reflections, PCS has been successfully employed to probe the SC gap and order parameter symmetry in several classes of conventional and unconventional superconductors.

Figure1. (a) Enlarged view of the mechanical-PCS setup used in the measurements. (b) The schematic diagram of soft-PCS.(c) The electrical circuit used in the measurements. 

In the typical needle-anvil type point-contact setup, a tip is brought into contact with the sample use a micrometer screw or piezoelectric units. Attocube nanopositioner stacks have been applied in our PCS probe design to continuously move the sample along either x/y/z or any combination of directions with a travel distance of 2.5 (1.5) mm in the vertical (horizontal) direction, where the nanopositioner (ANPZ51/LT) can be monitored by the controller with specific voltage wave functions in the low temperature environment, giving a nm-scale movement for accurate control of the contact. The differential conductance of the point-contact junction is measured using a quasi-four probe configuration and the ac voltage δv as a small modulation to the dc voltage V₀. This is recorded by the lock-in technique as shown in Fig. 1(c).  

Complementary to the needle-anvil type method, the soft point-contact method has also been developed, where a small drop of Ag conductive paint is placed on the sample surface as shown in Fig. 1(b), which has the advantage of good thermal and mechanical stability during the measurements. The soft PCS method can be capsuled into a pressure cell, extending the PCS technique into high pressure environments for simultaneous tuning of the system.