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Lattice%20modulation%20experiments%20with%20fermions%20in%20optical%20lattices%20and%20more

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Lattice modulation experiments with fermions in optical lattices and more Nonequilibrium dynamics of Hubbard model Ehud Altman Weizmann Institute – PowerPoint PPT presentation

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Title: Lattice%20modulation%20experiments%20with%20fermions%20in%20optical%20lattices%20and%20more


1
Lattice modulation experiments with fermions in
optical lattices and more
  • Nonequilibrium dynamics of Hubbard model

Ehud Altman Weizmann
Institute David Pekker
Harvard University Rajdeep Sensarma
Harvard University Eugene Demler
Harvard University
2
Fermionic Hubbard model
From high temperature superconductors to
ultracold atoms
Atoms in optical lattice
Antiferromagnetism and pairing at sub-micro
Kelvin temperatures
3
Fermions in optical lattice
U
Hubbard model plus parabolic potential
t
t
Probing many-body states
Electrons in solids
Fermions in optical lattice
  • Thermodynamic probes
  • i.e. specific heat
  • System size, number of doublons
  • as a function of entropy, U/t, w0
  • X-Ray and neutron
  • scattering
  • Bragg spectroscopy,
  • TOF noise correlations
  • ARPES

???
  • Lattice
  • modulation
  • experiments
  • Optical conductivity
  • STM

4
Outline
  • Introduction. Recent experiments with fermions in
    optical lattice. Signatures of Mott state
  • Lattice modulation experiments in the Mott state.
    Linear response theory
  • Comparison to experiments
  • Lifetime of repulsively bound pairs
  • Lattice modulation experiments with d-wave
    superfluids

5
  • Mott state of fermions
  • in optical lattice

6
Signatures of incompressible Mott state
Suppression in the number of double occupancies
Esslinger et al. arXiv0804.4009
7
Signatures of incompressible Mott state
Response to external potential I. Bloch, A.
Rosch, et al., arXiv0809.1464
Radius of the cloud as a function of the
confining potential
Comparison with DMFTLDA models suggests that
temperature is above the Neel transition
Next step observation of antiferromagnetic order
However superexchange interactions have already
been observed
8
Radius of the cloud high temperature expansion
Starting point zero tunneling. Expand in
t/T. Interaction can be arbitrary
Minimal cloud size for attractive interactions
Observed
experimentally
by the Mainz group
Competition of interaction energy and entropy
Theory first two terms in t/T
expansion
9
Lattice modulation experiments with fermions in
optical lattice.Mott state
Related theory work Kollath et al., PRA
74416049R) (2006)
Huber, Ruegg, arXiv08082350
10
Lattice modulation experiments Probing
dynamics of the Hubbard model
Measure number of doubly occupied sites
Main effect of shaking modulation of tunneling
Doubly occupied sites created when frequency w
matches Hubbard U
11
Lattice modulation experiments Probing
dynamics of the Hubbard model
R. Joerdens et al., arXiv0804.4009
12
Mott state
Regime of strong interactions Ugtgtt.
High temperature regime
All spin configurations are equally likely. Can
neglect spin dynamics.
Spins are antiferromagnetically ordered or have
strong correlations
13
Schwinger bosons and Slave Fermions
14
Schwinger bosons and slave fermions
Fermion hopping
Propagation of holes and doublons is coupled to
spin excitations. Neglect spontaneous doublon
production and relaxation.
Doublon production due to lattice modulation
perturbation
Second order perturbation theory. Number of
doublons
15
Propagation of holes and doublons strongly
affected by interaction with spin waves
Assume independent propagation of hole and
doublon (neglect vertex corrections)
Self-consistent Born approximation Schmitt-Rink
et al (1988), Kane et al. (1989)
Spectral function for hole or doublon
Sharp coherent part dispersion set by J, weight
by J/t
16
Propogation of doublons and holes
Spectral function Oscillations reflect
shake-off processes of spin waves
Comparison of Born approximation and exact
diagonalization Dagotto et al.
Hopping creates string of altered spins bound
states
17
Rate of doublon production
  • Low energy peak due to sharp quasiparticles
  • Broad continuum due to incoherent part

18
High Temperature
Atomic limit. Neglect spin dynamics. All spin
configurations are equally likely.
Aij (t) replaced by probability of having a
singlet
Assume independent propagation of doublons and
holes. Rate of doublon production
Ad(h) is the spectral function of a single
doublon (holon)
19
Propogation of doublons and holes
Hopping creates string of altered spins
Retraceable Path Approximation Brinkmann Rice,
1970
Consider the paths with no closed loops
Spectral Fn. of single hole
Doublon Production Rate
Experiments
20
Lattice modulation experiments. Sum rule
Ad(h) is the spectral function of a single
doublon (holon)
Sum Rule
Experiments
Possible origin of sum rule violation
  • Nonlinearity
  • Doublon decay

The total weight does not scale quadratically
with t
21
Lattice modulation experiments Probing
dynamics of the Hubbard model
R. Joerdens et al., arXiv0804.4009
22
Doublon decay rateinspired by experiments in ETH
23
Relaxation of doublon hole pairs in the Mott state
Energy Released U
  • Relaxation requires
  • creation of U2/t2
  • spin excitations
  • Energy carried by
  • spin excitations
  • J 4t2/U

Relaxation rate
Very slow Relaxation
Large U/t
24
Alternative mechanism of relaxation
UHB
  • Thermal escape to edges

LHB
  • Relaxation in compressible edges

m
Thermal escape time
Relaxation in compressible edges
25
Doublon decay in a compressible state
How to get rid of the excess energy U?
Compressible state Fermi liquid description
Doublon can decay into a pair of quasiparticles
with many particle-hole pairs
26
Doublon decay in a compressible state
Decay amplitude
27
Doublon decay in a compressible state
Fermi liquid description
Single particle states
Doublons
Interaction
Decay
Scattering
28
Doublon decay in a compressible state
Decay rate contained in self-energy
Self-consistent equations for doublon
29
Doublon decay in a compressible state
30
Lattice modulation experiments with fermions in
optical lattice.Detecting d-wave superfluid state
31
Setting BCS superfluid
  • consider a mean-field description of the
    superfluid
  • s-wave
  • d-wave
  • anisotropic s-wave

Can we learn about paired states from lattice
modulation experiments? Can we distinguish
pairing symmetries?
32
Lattice modulation experiments
Modulating hopping via modulation of the optical
lattice intensity
where
  • Equal energy
  • contours

Resonantly exciting quasiparticles with
Enhancement close to the banana tips due to
coherence factors
33
Lattice modulation as a probe of d-wave
superfluids
Momentum distribution of fermions after lattice
modulation (1/4 of zone)
Distribution of quasi-particles after lattice
modulation experiments (1/4 of zone)
Can be observed in TOF experiments
34
Lattice modulation as a probe of d-wave
superfluids
number of quasi-particles
density-density correlations
  • Peaks at wave-vectors connecting tips of bananas
  • Similar to point contact spectroscopy
  • Sign of peak and order-parameter (redup,
    bluedown)

35
Scanning tunneling spectroscopy of high Tc
cuprates
36
Conclusions
Experiments with fermions in optical lattice
open many interesting questions about dynamics of
the Hubbard model
Thanks to
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