Charge pair hopping and Bose-Einstein condensation in underdoped Mott insulators

Abstract

Recently, a renormalized Hamiltonian has been derived by continuing spin states from the Mott limit of the t-J model to the underdoped region. We show that it naturally leads to a pairing mechanism in which the pair has a dual character. Its spin part is a spinon singlet which condenses at T * . The charge part is a real-space holon pair formed at Tp < T * , which undergoes Bose-Einstein condensation at Tc < Tp. While neither is observable separately, the combination is. The mechanism is consistent with the small superfluid density, the decline of Tc at small doping, and the existence of pairs above Tc in cuprates, indicated by the observation of diamagnetism and Nernst effect.

Spin-charge split pairing in underdoped cuprate superconductors: support from low-$T$ specific heat

“…It behaves like a mobile hole pair of charge +2e with concentration x/2, which lives in a d-wave band; its spin part is an RVB singlet (concentration 1 − x), already ordered at T * . This is seen in the calculated electron pair Green's function [10]…”

“…Recently a different, logically consistent, pairing mechanism has been found in the spin-charge separated state of a renormalized t-J model in which the spin and the charge of the electrons are paired separately [9,10]. As an electron hops on the lattice with an amplitude t avoiding double occupancy, its spin behaves like a localized moment, coupled with neighbors with an antiferromagnetic interaction J, as in the Mott insulator (x = 0).…”

“…By requiring that the spin phases are the same (by symmetry) as those in the insulator (x = 0), one obtains a qualitatively correct phase diagram for underdoped cuprates, as detailed in [9,10]. Briefly, the phase above T * -the strange metal -has no quasiparticles of any kind.…”

“…The smallness of x causes holons to form real-space pairs below T p < T * via the strong coupling term [10]. As it propagates, the phase of the bound (holon) pair is locked to that of the spin RVB ordered state, creating a physical (gauge invariant) pair with a split character.…”

Coexistence of $s$- and $d$-wave gaps due to pair-hopping and exchange interactions

“…The extended s-wave superconducting gap composed of ∆ dd (k, ε) and ∆ αβ (k, ε), which emerges due to the pair-hopping interaction via the SK mechanism, corresponds to the kinetic-energy-driven superconductivity of the single-band t-J model [72][73][74][75][76][77][78][79][80]. In the kinetic-energy-driven superconductivity, the charge carriers form the superconducting pairs to gain kinetic energy.…”