September 15, 2008

Superconductors Under a Pile of Regular Metal Could Have Critical Temperatures of 200K instead of 50K

Theorists propose that for certain types of superconductors, contact with a metal layer could greatly increase the transition temperatures of these materials—in some cases by as much as an order of magnitude.

This relates to recent research which suggests that superconductors do not achieve their best performance because of quantum traffic jams with electrons The piling on of regular metal can help unblock the electron traffic jam.

Designing ways to raise the superconducting transition temperature (Tc) has always been an important goal of condensed matter research. In the past twenty years, two families of superconducting materials with transition temperature above 50 K have been discovered: the cuprates and more recently, the iron-pnictides. Many believe that some cuprate compounds should be very high temperature superconductors (that is, with a Tc~200 K) were it not for the fact that the superconducting carriers, the Cooper pairs, have a low mobility. Writing in Physical Review B, Erez Berg and Steve Kivelson of Stanford University and Dror Orgad of The Hebrew University in Jerusalem turn this logic around and suggest that making contact between a nominally low-mobility superconductor and a high-mobility metal will increase the mobility of Cooper pairs in the superconductor and raise Tc.

Berg et al. consider a two-dimensional lattice where Δ0 is the attraction between two electrons on the same site. To mimic the poor Cooper pair mobility, they either assume that the probability that electron pairs “hop” from site to site is zero, or the electron pairs can only hop in one direction. As constructed, this model exhibits finite Δ0 but zero ρs and cannot be superconducting—at least not in all directions—even at zero temperature. Since Tc is set by ρs, the trick is to find a way to increase ρs by modifying the system so the electrons can move around more easily. Berg et al. therefore propose to put the two-dimensional lattice in contact with a normal metallic layer (Fig. 1). Electrons can now hop to the metal and ρs increases. They demonstrate that for an appropriate choice of the electron transfer parameter between the two layers, the Tc of the composite system can be raised to a substantial fraction of Δ0/kB. Plugging in the numbers appropriate for the relevant cuprate compounds, this amounts to increasing Tc from around 10 K to over 100 K, assuming the interfaces are “ideal” as in the model.

The fact that coupling a normal metal to a low ρs superconductor can raise the Tc actually has a close analogy to what is found in electronic devices called Josephson junction arrays (Fig. 1). A Josephson junction array consists of many superconducting islands connected by insulating materials. In such a system, the quantum tunneling of the Cooper pairs between neighboring islands eventually triggers superconductivity through the entire array. Once the superconducting island is smaller than a certain size, however, even a single extra Cooper pair would significantly raise the electrostatic energy (the so-called charging energy) of the island. When this extra charging energy overwhelms the kinetic-energy gain of allowing the Cooper pairs to spread out, the system becomes insulating. Under this condition the system has a finite Δ0 (proportional to the bulk Tc of the superconducting island) but zero ρs—just as in the model from Berg et al.

There has been other advances this year to create superconducting compounds that can reach 195K or the temperature of dry ice.

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