High Temperature Superconductivity in the Past Twenty Years Part 1 ...

228second derivative of tunnel current d 2 I/dV 2 corresponds toeach phonon mode [65] . For HTS, however, no bosonmediating electron pairing has been identified. Oneexplanation could be that electron pair formation [66] andrelated electron-boson interactions are heterogeneous at theatomic scale, and therefore affect its characterization.In August 2001, a team of physicists used angleresolvedphotoemission spectroscopy to probe electrondynamics, i.e., velocity and scattering rate for threedifferent families of copper oxide superconductors [52] . Theysaw in all of these materials an abrupt change of electronvelocity from 50 meV to 80 meV, which could not beexplained by any known process other than to invokecoupling with the phonons associated with the movement ofthe oxygen atoms. This suggests that electron-phononcoupling strongly influences the electron dynamics in theHTSs and must therefore be included in any microscopictheory of superconductivity.With the advances in d 2 I/dV 2 spectroscopy usingscanning tunneling microscopy, it has become possible tostudy bosonic modes directly at the atomic scale [67] . In 2006,Lee and Davis et al. [69] measured the superconductingenergy gap Δ and the phonon energy Ω of Bi 2 Sr 2 CaCu 2 O 8+δat the atomic scale for the first time, and gave the d 2 I/dV 2imaging studies of the Bi 2 Sr 2 CaCu 2 O 8+δ . During experimentalresearch, they found intense disorder of electronbosoninteraction energies at the nanometer scale, alongwith the expected modulations in d 2 I/dV 2[69],[70] . Changingthe density of holes has minimal effects on both the averagemode energies and the modulations, indicating that thebosonic modes are unrelated to electronic or magneticstructure. Instead, comparing Δ(r) and Ω(r) images findsthat the color bar is reversed to show directly how higher Ωis correlated to lower Δ, and the spatial correlations of Δ(r)and Ω(r) both change together with doping. Moreover, themodes appear to be local lattice vibrations, as substitutionof 18 O for 16 O throughout the material reduces the averagemode energy by approximately 6 percent, the expectedeffect of this isotope substitution on lattice vibrationfrequencies [71] . Significantly, the mode energies are alwaysspatially anti-correlated with the superconducting pairinggapenergies, suggesting the interplay between these latticevibration modes and the superconductivity.3.3 Circulating Currents TheoryIn 1989, Varma from Bell Laboratories stated theradical idea that HT-superconductivity and relatedphenol-mena occur in certain materials becausequantum-mechanical fluctuations in these materialsincrease as temperature decreases [43] . In 1996, Varmafurther explained the nature of the fluctuations in his theory[49] : in copper oxide materials, also known as cuprates,superconductivity is associated with the formation of a newstate of matter in which electric current loops formspontaneously, going from copper to oxygen atoms andback to copper. His theory concluded that thequantum-mechanical fluctuations are the fluctuations ofJOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 2, JUNE 2008these current loops. As proposed by Varma [49],[72] , there aretwo possible circulating currents phases preservingtranslation symmetry of the lattice as they correspond to 4or 2 current loops per unit cell referred as Θ I and Θ II phases,respectively. In 2002, an angle resolved photoemissionspectroscopy measurement observed a dichroic signal in theBi 2 Sr 2 CaCu 2 O 8+δ system consistent with the phase Θ II [73] .In 2006, a French-German team of experimentalscientists [74] performed polarized elastic neutron scatteringexperiments to test the magnetic moments of this secondcirculating current state which actually had never beenattempted before. They successfully reported the firstsignature of a novel magnetic order in the pseudo gap stateof YBa 2 Cu 3 O 6+δ . In these experiments a beam of neutronschanges its direction as well as the direction of its magnetizationin a manner closely related to the geometricalarrangement of the current loops inside the material inwhich the beam is made to pass. The pattern of theobserved magnetic scattering corresponds to the oneexpected in the circulating current theory of the pseudo gapstate with two current loops per CuO 2 unit cell, phaseΘ II [49],[72] . The fact that two experiments came to the sameconclusion with different techniques and in differentcuprate compounds means the confidence in the results. Amicroscopic theory of HT-superconductivity could alsosuggest ways of fabricating room temperature superconductors,possibly with materials more amenable toindustrial fabrication than the cuprates.3.4 PseudogapOne of many exotic features beyond the BCS theory isthe observation of a pseudogap in the electron spectralfunction near the antinodal points (π, 0) and (0, π) at atemperature, i.e. pseudogap temperature, T * >> T c . In aconventional BCS superconductor, this gapping processoccurs simultaneously with the superconductivity at T c . Ithas been debated about the relationship between thepseudogap and the superconductivity in cuprates. Onescenario assumes that the pseudogap Δ p marks only acompeting or coexisting order with the superconductivityand it has nothing to do with the pairing origin. However,another picture, namely the Anderson’s RVBmodel [38],[75],[76] predicted that the spin-singlet pairing in theRVB state, which causes the formation of the pseudogap,may lend its pairing strength to the mobile electrons andcause them to naturally pair and then to condense at T c .According to this picture there should be a closerelationship between the pseudogap and thesuperconductivity.The pseudogap is seen in all HTSs and seems to berelated to the superconducting gap in the sense that itevolves smoothly into the superconducting gap and has thesame d-wave symmetry. Unlike a conventional superconductinggap, the pseudogap magnitude is temperature independent.Tunneling, angle-resolved photoemission spectroscopyand optical data indicate that the magnitude of thegap does not change significantly on going through T c .