3,179 research outputs found
Two-hole ground state wavefunction: Non-BCS pairing in a - two-leg ladder system
Superconductivity is usually described in the framework of the
Bardeen-Cooper-Schrieffer (BCS) wavefunction, which even includes the
resonating-valence-bond (RVB) wavefunction proposed for the high-temperature
superconductivity in the cuprate. A natural question is \emph{if} any
fundamental physics could be possibly missed by applying such a scheme to
strongly correlated systems. Here we study the pairing wavefunction of two
holes injected into a Mott insulator/antiferromagnet in a two-leg ladder using
variational Monte Carlo (VMC) approach. By comparing with density matrix
renormalization group (DMRG) calculation, we show that a conventional BCS or
RVB pairing of the doped holes makes qualitatively wrong predictions and is
incompatible with the fundamental pairing force in the - model, which is
kinetic-energy-driven by nature. By contrast, a non-BCS-like wavefunction
incorporating such novel effect will result in a substantially enhanced pairing
strength and improved ground state energy as compared to the DMRG results. We
argue that the non-BCS form of such a new ground state wavefunction is
essential to describe a doped Mott antiferromagnet at finite doping.Comment: 11 pages, 5 figure
Upper Pseudogap Phase: Magnetic Characterizations
It is proposed that the upper pseudogap phase (UPP) observed in the high-Tc
cuprates correspond to the formation of spin singlet pairing under the bosonic
resonating-valence-bond (RVB) description. We present a series of evidence in
support of such a scenario based on the calculated magnetic properties
including uniform spin susceptibility, spin-lattice and spin-echo relaxation
rates, which consistently show that strong spin correlations start to develop
upon entering the UPP, being enhanced around the momentum (\pi, \pi) while
suppressed around (0, 0). The phase diagram in the parameter space of doping
concentration, temperature, and external magnetic field, is obtained based on
the the bosonic RVB theory. In particular, the competition between the Zeeman
splitting and singlet pairing determines a simple relation between the
"critical" magnetic field, H_{PG}, and characteristic temperature scale, T0, of
the UPP. We also discuss the magnetic behavior in the lower pseudogap phase at
a temperature Tv lower than T0, which is characterized by the formation of
Cooper pair amplitude where the low-lying spin fluctuations get suppressed at
both (0, 0) and (\pi, \pi). Properties of the UPP involving charge channels
will be also briefly discussed.Comment: 11 pages, 5 figures, final version to appear in PR
Spin and charge modulations in a single hole doped Hubbard ladder -- verification with optical lattice experiments
We show that pronounced modulations in spin and charge densities can be
induced by the insertion of a single hole in an otherwise half-filled 2-leg
Hubbard ladder. Accompanied with these modulations is a loosely bound structure
of the doped charge with a spin-1/2, in contrast to the tightly bound case
where such modulations are absent. These behaviors are caused by the
interference of the Berry phases associated a string of flipped spins (or
"phase strings") left behind as a hole travels through a spin bath with a
short-range anti-ferromagnetic order. The key role of the phase strings is also
reflected in how the system respond to increasing spin polarization, increasing
the on-site repulsion, addition of a second hole, and increasing asymmetry
between intra- and inter-chain hopping. Remarkably, all these properties
persist down to ladders as short as sites. They can therefore be
studied in cold atom experiments using the recently developed fermion
microscope.Comment: 5 pages, 4 figure
Pairing versus phase coherence of doped holes in distinct quantum spin backgrounds
We examine the pairing structure of holes injected into two \emph{distinct}
spin backgrounds: a short-range antiferromagnetic phase versus a symmetry
protected topological phase. Based on density matrix renormalization group
(DMRG) simulation, we find that although there is a strong binding between two
holes in both phases, \emph{phase fluctuations} can significantly influence the
pair-pair correlation depending on the spin-spin correlation in the background.
Here the phase fluctuation is identified as an intrinsic string operator
nonlocally controlled by the spins. We show that while the pairing amplitude is
generally large, the coherent Cooper pairing can be substantially weakened by
the phase fluctuation in the symmetry-protected topological phase, in contrast
to the short-range antiferromagnetic phase. It provides an example of a non-BCS
mechanism for pairing, in which the paring phase coherence is determined by the
underlying spin state self-consistently, bearing an interesting resemblance to
the pseudogap physics in the cuprate.Comment: 9 pages, 6 figure
- …