[8] In each of these materials, the anisotropic nature of the pairing was implicated by the power-law dependence of the nuclear magnetic resonance (NMR) relaxation rate and specific heat capacity on temperature.
[10] Experimental works by Paul Chaikin's and Michael Naughton's groups as well as theoretical analysis of their data by Andrei Lebed have firmly confirmed unconventional nature of superconducting pairing in (TMTSF)2X (X=PF6, ClO4, etc.)
Müller in 1986, who also discovered that the lanthanum-based cuprate perovskite material LaBaCuO4 develops superconductivity at a critical temperature (Tc) of approximately 35 K (-238 degrees Celsius).
Soon after, in January 1987, yttrium barium copper oxide (YBCO) was discovered to have a Tc of 90 K, the first material to achieve superconductivity above the boiling point of liquid nitrogen (77 K).
(For example, the origin of the attractive force leading to the formation of Cooper pairs may be different from the one postulated in BCS theory.)
[22] Publications in March 2018 provided evidence for unconventional superconducting properties of a graphene bilayer where one layer was offset by a "magic angle" of 1.1° relative to the other.
After more than twenty years of intense research, the origin of high-temperature superconductivity is still not clear, being one of the major unsolved problems of theoretical condensed matter physics.
It appears that unlike conventional superconductivity driven by electron-phonon attraction, genuine electronic mechanisms (such as antiferromagnetic correlations) are at play.
One reason for this is that the materials in question are generally very complex, multi-layered crystals (for example, BSCCO), making theoretical modeling difficult.
Secondly, there is the interlayer coupling model, according to which a layered structure consisting of BCS-type (s symmetry) superconductor can enhance the superconductivity by itself.
[citation needed] In order to solve this unsettled problem, there have been numerous experiments such as photoelectron spectroscopy, NMR, specific heat measurement, etc.
Promising experimental results from various researchers in September 2022, including Weijiong Chen, J.C. Séamus Davis and H. Eisiaki revealed that superexchange of electrons is possibly the most probable reason for high-temperature superconductivity.
NMR measurements of the resonance frequency on YBCO indicated that electrons in the copper oxide superconductors are paired in spin-singlet states.
As these metals go superconducting, electrons with oppositely directed spins couple to form singlet states.
Because the technique is sensitive to the angle of the emitted electrons one can determine the spectrum for different wave vectors on the Fermi surface.
Furthermore, the possibility that junction interfaces can be in the clean limit (no defects) or with maximum zig-zag disorder was taken into account in this tricrystal experiment.
[31] A proposal of studying vortices with half magnetic flux quanta in heavy-fermion superconductors in three polycrystalline configurations was reported in 1987 by V. B. Geshkenbein, A. Larkin and A. Barone in 1987.