Phases of Mott-Hubbard Bilayers

1
Phases of Mott-Hubbard Bilayers
Jung Hoon Han Sung Kyun Kwan U. ???? Ref
Ribeiro et al, cond-mat/0605284 Han
Jia, cond-mat/0605426
2
Sharp Interface of Band Insulator and Mott
Insulator
SrTi(d0)O3 and LaTi(d1)O3 interfacial layer with
atomic precision was successfully fabricated.
3
Millis-Okamoto Theory
N-layers of Mott insulators are sandwiched
between band insulators Ti t2g orbital states
are studied within Hartree-Fock theory
Okamoto Millis, Nature (2004)
4
Millis-Okamoto Theory
5
Can we do something similar with interface of
two doped Mott insulators?
After all physics of doped Mott insulators is
much richer than that of doped band insulator,
e.g. spin liquid, d-wave superconductivity, and
antiferromagnetism
6
Sharp Interface of two Mott Insulators
Bozovic, Nature (2003)
7
Our Model
Ramesh, Science (2004)

• Layers are oppositely doped with density x of
  holes(doublons)
• Short-range Coulomb coupling across the layers
• Each layer modeled as large-U Hubbard or tJ

8
In this talk we try to answer a naïve
question What are the possible phases of
coupled doped Mott insulators?
9
Phase Diagram of Hubbard Model Recent Efforts
Senechal et al, PRL (2005)
10
Single-layer tJ Model slave boson meanfield
theory
Han, Wang, Lee IJMPB (2001)
11
Lessons from 1D Coupled Chains (A. Seidel)
1D constrained hopping model has (wavefunction)
(charge w.f.) X (spin w.f.) Inter-layer
interaction for the charge sector has effective
Hamiltonian given by 1D attractive Hubbard model
Ribeiro et al, cond-mat/0605284
12
Lessons from 1D Coupled Chains (A. Seidel)
1D analog of paired superfluid phase emerges as
the ground state In the original picture this is
the holon-doublon exciton Existence of exciton
instability is rigorously established in coupled
1D chains
Ribeiro et al, cond-mat/0605284
13
Phases of Mott-Hubbard Bilayers at T0
Han Jia, cond-mat/0605426
14
Spin Liquid Insulator a new phase for bilayers
Exciton pairing acts like a pairing gap for
quasiparticles. Single charge excitation has a
gap due to excitons Insulator

It emerges after magnetic order has melted Spin
Liquid
15
Dichotomy of one-particle vs. two-particle
responses
In the SLI phase, one-particle Greens function
has a charge gap
Two-particle response function such as
conductivity is that of a superfluid due to
transport by excitons
16
Other Phases of Bilayer Mott-Hubbard Model
AFI Antiferromagnetic insulator with (?,?)
ordering Charge gap due both to AFM and exciton
gaps Evolves into SLI when magnetism vanishes
Transport still superfluid-like
m-dSC Magnetic d-wave Superconductor Exciton
gap has vanished d-wave pairing of electrons
Evolves into non-magnetic d-wave superconductor
upon doping
17
Phases of Mott-Hubbard Bilayers from Berkeley
group
Ribeiro et al, cond-mat/0605284
Calculation based on doped carrier
formulation Results are consistent with slave
boson theory
18
New Features of Bilayers

1. Exciton binding is responsible for incoherent
   quasiparticles and charge gap for small doping
2. Exciton binding is responsible for in-plane
   superfluid transport
3. Easy realization of spin liquid without lattice
   frustration