Boson–fermion fluctuation dominated model of the superconducting transition in hole-underdoped cuprates

Electron coherent and incoherent pairing instabilities in inhomogeneous bipartite and nonbipartite nanoclusters

“…Our exact calculations of phase diagrams in various bipartite and nonbipartite clusters provide strong evidence for the existence of a narrow, homogeneous (pseudo)gap ∆ s that vanishes near T P s , coexisting with inhomogeneous, weakly temperature dependent broad gap ∆ c , which disappears at higher temperatures T P c > T P s . These phase diagrams display coherent and incoherent pairing (pseudo)gaps and possible superconductivity in agreement with the recent STM measurements in HTSCs [1,2,3,4,5,6,7].…”

“…5 correspond to the pseudogap crossover temperature T * . As temperature is lowered below T * , a spin pseudogap is opened up first, as seen in NMR experiments [22], followed by the gradual disappearance of the spin excitations, consistent with the suppression of low-energy excitations in the HTSCs probed by STM and ARPES [3,4,5,6,7]. In contrast, the local charge gap, ∆ c , evolves smoothly as temperature decreases below T P c .…”

“…In the absence of spin pairing above T P s , the pair fluctuations between the two lowest energy states becomes incoherent. The temperature driven spin-charge separation above T P s resembles an incoherent pairing (IP) phase seen in the HTSCs [2,3,4,5,6]. The charged pairs without spin rigidity above T P s , instead of becoming superconducting, coexist in a nonuniform, charge degenerate IP state similar to a ferroelectric phase [25].…”

“…The vanishing of gaps at quantum critical points, U c = 4.584 and U F = 18.583, indicates energy level crossings and electron instabilities in 4-site clusters for charge and spin, respectively. The charge ∆ c and spin ∆ s gaps versus U in an ensemble of square clusters at N ≈ 3 exhibit at infinitesimal T → 0 the following phases; Phase A: Charge and spin pairing gaps of equal amplitude ∆ s ≡ ∆ P = −∆ c at U ≤ U c describe Bose condensation of electrons similar to BCS-like coherent pairing with a single energy gap; Phase B: Mott-Hubbard like insulator with ∆ c > 0 and gapless S = 1 2 excitations at U c < U < U F describes a spin liquid behavior; Phase C: Parallel (triplet) spin pairing (∆ s < 0) displays S = 3 2 the saturated ferromagnetism at U > U F in Mott-Hubbard insulator for a positive charge gap, ∆ c > 0. Notice, that incoherent opposite spin pairing |∆ s | = ∆ c , different from the charge pairing at U < U c , suggests spin-charge separation for spin and charge degrees at U > U F .…”

“…Strongly correlated electrons in cuprates, manganites and other transition metal oxides exhibit high T c superconductivity, magnetism and ferroelectricity accompanied by spatial inhomogeneities at the nanoscale level [1,2,3,4,5,6,7,8]. Over the past few years there is an increase in interest to electron instabilities in nanoclusters, assembled clusters of correlated materials in various topologies for synthesizing new nanomaterials with unique electronic and magnetic properties [9,10].…”