Nature of strong hole pairing in doped Mott antiferromagnets

Abstract

Cooper pairing instability in a Fermi liquid is well understood by the BCS theory, but pairing mechanism for doped Mott insulators still remains elusive. Previously it has been shown by density matrix renormalization group (DMRG) method that a single doped hole is always self-localized due to the quantum destructive interference of the phase string signs hidden in the t-J ladders. Here we report a DMRG investigation of hole binding in the same model, where a novel pairing-glue scheme beyond the BCS realm is discovered. Specifically, we show that, in addition to spin pairing due to superexchange interaction, the strong frustration of the phase string signs on the kinetic energy gets effectively removed by pairing the charges, which results in strong binding of two holes. By contrast, if the phase string signs are “switched off” artificially, the pairing strength diminishes significantly even if the superexchange coupling remains the same. In the latter, unpaired holes behave like coherent quasiparticles with pairing drastically weakened, whose sole origin may be attributed to the resonating-valence-bond (RVB) pairing of spins. Such non-BCS pairing mechanism is therefore beyond the RVB picture and may shed important light on the high-Tc cuprate superconductors.

Charge modulation as fingerprints of phase-string triggered interference

“…One immediate "complication" is by now well established: two holes in a t − J ladder will immediately bind in a "Cooper pair", where yet again the phase strings play a crucial role in the binding mechanism [27]. Turning to the truly finite density case on many-rung ladders (or the 2D square lattice) the DMRG computations already showed a long time ago the 214-like stripes [9].…”

Quasiparticle collapsing in an anisotropict−Jladder

“…In particular, the breakdown of the quasiparticle in a form of spin-charge separation has been conjectured in the study of doped Mott insulators, notably the high-T c cuprates 1-8 . However, no consensus has been reached yet on how a quasiparticle precisely falls into parts in such strongly correlated electron systems.A t-J square ladder as a quasi one-dimensional (1D) doped Mott insulator system has been intensively investigated [9][10][11][12][13][14][15][16][17][18][19][20][21] . Such systems are beyond a purely 1D system due to the presence of closed loops of various sizes, and can be accurately studied by the DMRG numerical method 22 .…”

Localization in a t−J -Type Model with Translational Symmetry

“…1 and 2, respectively. In the following, we show that such two doped holes are actually paired up to become a mobile object freely moving on a gapped spin background just like the case at µ a = µ b = 0 [26]. Define the binding energy of two injected holes by [26]…”

“…The phase string effect is caused by the nearest-neighbor hopping of the hole, but will get "scrambled" by introducing a sufficiently large next-nearest-neighbor hopping term, [25]. In the following DMRG calculation, we fix t/J = 3 as the same in [18,20,26] and focus on the µ a = t and µ b = 0 to study the ground states of the one hole and two hole doped cases. At µ a = µ b = 0, we recover the isotropic 2-leg t-J ladder results [18,20,26].…”