Superconductivity without phonons

Abstract

The idea of superconductivity without the mediating role of lattice vibrations (phonons) has a long history. It was realized soon after the publication of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity 50 years ago that a full treatment of both the charge and spin degrees of freedom of the electron predicts the existence of attractive components of the effective interaction between electrons even in the absence of lattice vibrations--a particular example is the effective interaction that depends on the relative spins of the electrons. Such attraction without phonons can lead to electronic pairing and to unconventional forms of superconductivity that can be much more sensitive than traditional (BCS) superconductivity to the precise details of the crystal structure and to the electronic and magnetic properties of a material.

Heavy fermion superconductor CeCu2Si2under high pressure: Multiprobing the valence crossover

“…This means that the midpoint should actually be situated at a marginally lower p VCO . Regardless of this detail, and compared to the rather qualitative lines in previous p-T phase diagrams, [23][24][25][26] the present one is the first signature of the 4f electron delocalization based on experiment. Finally, extrapolating this line, p CeCu 2 Si 2 cr (= p V ) ∼ = 4.5 ± 0.2 GPa can be deduced, so that both coordinates of the CEP are determined within the experimental errors.…”

Comprehensive scenario for single-crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba1−xKx)FeLiu1, Tanatar2, Straszheim3 et al. 2014Phys. Rev. B6215722View full textAdd to dashboardCite

“…A characteristic feature of the scenario, suggested for magnetically mediated superconductivity [13][14][15], is systematic doping evolution of all electronic properties, in particular, of electrical resistivity. Superconducting T c has maximum at a point where the line of the second-order magnetic transition goes to T = 0 (quantum critical point, QCP).…”

NMR studies of pseudogap and electronic inhomogeneity in Bi2Sr2CaCu2OCrocker1, Dioguardi2, apRoberts-Warren3 et al. 2011Phys. Rev. B

“…7 yields a slope of 103(11) (s K) −1/2 , which is remarkably close to the theoretical value 187 (s K) −1/2 , suggesting that quasiparticle scattering rather than antiferromagnetic fluctuations is the dominant mechanism for the oxygen spin lattice relaxation. This result suggests that either (1) the oxygen is completely insensitive to the spin fluctuations, (2) there is a second electronic degree of freedom that couples to the oxygen, or (3) there or no antiferromagnetic spin fluctuations in the overdoped regime. Since ξ ∼ a at T = T m , it is likely that short range spin fluctuations continue to exist down to T c .…”

“…The parent state of these materials is a Mott insulator, but for sufficient hole doping the conductivity becomes metallic and high temperature superconductivity emerges with T c ∼ 100 K and d-wave symmetry. 1,2 Other strongly correlated electron systems such as the heavy fermions, the ironbased superconductors and the organic superconductors also exhibit superconductivity in close proximity to an antiferromagnetism, [3][4][5] but the cuprates are unique in that they also exhibit a pseudogap in the normal state over a broad range of dopings. 6 It remains unclear whether the partial suppression of the low energy density of states in the pseudogap is the result of a new thermodynamic phase [7][8][9] or a crossover in magnetic behavior driven by the proximate Mott insulating state.…”