Laboratory of Theoretical and Experimental Superconductive Tunnelling

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Fundamental study of unconventional superconductors	Experimental techniques Point-contact spectrosopy is a simple but powerful experimental technique that allows directly measuring the energy gap in a superconductor, and to determine its symmetry in the reciprocal space, thanks to quantum phenomena that occur at the interface between the superconductor and a normal metal. A point contact (i.e. a contact whose radius is smaller than the electronic mean free path and of the coherence length in the superconductor) is made between a normal metal (N) and the superconductor under study (S). This is achieved for example by pressing a very sharp metallic tip against the sample surface, as in the standard "needle-anvil" tecnique.  We actually developed a pressure-less, "soft" technique in which the contact is made by putting a small dro of Ag conductive paint on the sample surface. This has some advantages in very small (or thin) samples, and ensures greater mechanical and thermal stability. If the potential barrier at the S/N interface is small, the conduction is dominated by Andreev reflection: when the voltage across the contact is smaller than the energy gap (divided by the electronic charge e) incident electrons are reflected back as holes and a Cooper pair is transmitted into S. The spectrum  (i.e. the differential conductance of the contact as a function of the voltage across the contact itself) then shows a characteristic shape from which, thanks to suitable models, it is possible to extract the desired information about the number, the amplitude and the symmetry of the superconducting gap (that means, of the superconducting order parameter). In case of strong-coupling superconductors, further information about the characteristic energy (or even the spectrum) of the bosonic excitations that act as the pairing glue can also be obtained from a suitable analysis of the spectra. If the potential barrier at the S/N interface is large (i.e. there is an insulating surface layer between N and S) the conduction is dominated by the tunnelling of quasiparticles. In this case, the low-temperature spectrum allows directly obtaining the density of states, and provides information on the spectrum of the mediating bosonic modes.  <back	Contact configuration in the "soft" PCS technique Sketch of  the trajectories of quasiparticles involved in Andreev reflection at the N/S interface Calculated point-contact spectra for the case of a single-gap, s-wave superconductor, from the pure Andreev regime (SN junction, Z=0) to the pure tunnel regime (SIN junction, Z=10)
	The Josephson effect is due to the tunneling of Cooper pairs between two superconductors. SIS or SIS' junctions whose conductance is dominated by quasiparticle tunnelling and Cooper pair tunnellingcan be obtained by using the point-contact technique (if the tip is made of a superconductor) or the break-junction technique. In this case, a sample of the superconductor under study is broken at low temperature in two parts that are then are then readjusted until a weak link is formed between them. Under irradiation with microwaves, these junctions can show the Shapiro steps typical of the inverse ac Josephson effect. <back	I-V characteristics of a Josephson break junction in MgB2
	Transport measurements in low-resistance materials are carried out by using the four-probe configuration, eliminating thermoelectric effects and applying suitable corrections for anisotropic or layered materials. Depending on the shape of the sample under study and on the purpose of the measure, different contact configurations are used: standard (for regularly shaped bulk materials or patterned films), collinear (for films or thin samples), van der Pauw (for films or thin samples of irregular shape), Montgomery (for large single crystals). <back	Resistance of a Ba(Fe,Co)2As2 thin film (measured with the van der Pauw method)

LaTEST - Department of Applied Science and Technology, Politecnico di Torino
corso Duca degli Abruzzi 24, 10129 Torino (TO) - Italy