Conference on quantum fluids and strongly correlated systems

Paris 2008

Gora’s Lessons

• Theory is not done for theory’s sake. You can be a respectable theorist even if your predictions can be checked in experiments

• When you are senior and very important you should still enter new fields and learn about new areas of physics

• You do not hide behind the backs of younger collaborators. Do your own calculations!

• Forgetting something from Landau and Lifshitz can not be excused under any circmstances

• Entertainment is an important part of physics discussions

Lattice modulation experiments with fermions in optical lattice

Dynamics of Hubbard model

Ehud Altman Weizmann InstituteDavid Pekker Harvard UniversityRajdeep Sensarma Harvard UniversityEugene Demler Harvard University

Thanks to I. Bloch, T. Esslinger, M. Lukin, A.M. Rey

Harvard-MIT

Antiferromagnetic and superconducting Tc of the order of 100 K

Atoms in optical lattice

Antiferromagnetism and pairing at sub-micro Kelvin temperatures

Fermionic Hubbard modelFrom high temperature superconductors to ultracold atoms

Fermions in optical lattice

t

U

t

Hubbard model plus parabolic potential

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, 0

• X-Ray and neutron scattering

• Bragg spectroscopy, TOF noise correlations

• ARPES ???

• Optical conductivity• STM

• Lattice modulation experiments

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• Lattice modulation experiments with d-wave

superfluids

Mott state of fermions

in optical lattice

Signatures of incompressible Mott state

Suppression in the number of double occupancies Esslinger et al. arXiv:0804.4009

Signatures of incompressible Mott stateResponse to external potential I. Bloch, A. Rosch, et al., arXiv:0809.1464

Radius of the cloud as a functionof the confining potential

Next step: observation of antiferromagnetic order

Comparison with DMFT+LDA models suggests that temperature is above the Neel transition

However superexchange interactions have already been observed

Lattice modulation experiments with fermions in optical lattice.

Mott state

Related theory work: Kollath et al., PRA 74:416049R) (2006) Huber, Ruegg, arXiv:0808:2350

Lattice modulation experimentsProbing dynamics of the Hubbard model

Measure number of doubly occupied sites

Main effect of shaking: modulation of tunneling

Modulate lattice potential

Doubly occupied sites created when frequency matches Hubbard U

Lattice modulation experimentsProbing dynamics of the Hubbard model

T. Esslinget et al., arXiv:0804.4009

Mott state

Regime of strong interactions U>>t.

Mott gap for the charge forms at

Antiferromagnetic ordering at

“High” temperature regime

“Low” temperature regime

All spin configurations are equally likely.Can neglect spin dynamics.

Spins are antiferromagnetically ordered or have strong correlations

Schwinger bosons and slave fermions

Fermion hopping

Doublon production due to lattice modulation perturbation

Second order perturbation theory. Number of doublons

Propagation of holes and doublons is coupled to spin excitations.Neglect spontaneous doublon production and relaxation.

“Low” Temperature

d

h 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

Incoherent part:dispersion

Propagation of holes and doublons strongly affected by interaction with spin waves

Propogation of doublons and holes

Spectral function: Oscillations reflect shake-off processes of spin waves

Hopping creates string of altered spins: bound states

Comparison of Born approximation and exact diagonalization: Dagotto et al.

“Low” Temperature

Rate of doublon production

• Low energy peak due to sharp quasiparticles

• Broad continuum due to incoherent part

“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)

Propogation of doublons and holesHopping 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

Doublon decay and relaxation

Energy Released ~ U

Energy carried by

spin excitations ~ J =4t2/U

Relaxation requires creation of ~U2/t2

spin excitations

Relaxation of doublon hole pairs in the Mott state

Relaxation rate

Large U/t : Very slow Relaxation

Alternative mechanism of relaxation

LHB

UHB

• Thermal escape to edges

• Relaxation in compressible edges

Thermal escape time

Relaxation in compressible edges

Lattice modulation experiments with fermions in optical lattice.

Detecting d-wave superfluid state

• consider a mean-field description of the superfluid

• s-wave:

• d-wave:

• anisotropic s-wave:

Setting: BCS superfluid

Can we learn about paired states from lattice modulation experiments? Can we distinguish pairing symmetries?

Modulating hopping via modulation

of the optical lattice intensity

Lattice modulation experiments

where

3 2 1 0 1 2 3

3

2

1

0

1

2

3

• Equal energy contours

Resonantly exciting quasiparticles with

Enhancement close to the bananatips due to coherence factors

Distribution of quasi-particles

after lattice modulation

experiments (1/4 of zone)

Momentum distribution of

fermions after lattice modulation

(1/4 of zone)

Can be observed in TOF experiments

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 (red=up, blue=down)

Lattice modulation as a probe of d-wave superfluids

Scanning tunneling spectroscopy of high Tc cuprates

Conclusions

Experiments with fermions in optical lattice openmany interesting questions about dynamics of the Hubbard model

Thanks to:

Harvard-MIT

Happy birthday Gora!