Lithium Lattice Laboratory
A new experiment on degenerate Fermi gases in honeycomb optical lattices.
In recent years, ultracold quantum gases have demonstrated impressive results in the quantum simulation of condensed matter phenomena, e.g. the Superfluid to Mott insulator transition, or Anderson localization of matter waves in a random potential. As a result, ultracold quantum gases are considered ideal in order to investigate systems that cannot easily be simulated numerically. A particularly interesting system in which many curious phenomena are predicted to occur is fermions in a honeycomb lattice, which is the underlying geometry of graphene (Nobel prize 2010).
First of all, at half filling, the band structure of a honeycomb lattice features a linear dispersion relation, as for massless particles described by the Dirac equation. As a consequence, relativistic effects such as Klein tunneling (lossless transmission through a barrier) or Zitterbewegung (a rapid motion that arises from the interference of positive and negative energy states) are expected to occur. While these effects were predicted over 80 years ago, they have yet to be directly and unambiguously observed.
Furthermore, due to the two-dimensional geometry with low coordination number, quantum fluctuations in such a system are quite strong. There have therefore been predictions of a variety of novel quantum phases, including a gapless spin liquid phase. Although the atoms in such a phase are strongly correlated, there is no magnetic order even for T=0. Such spin liquid phases are thought to play a major role in high-TC superconductivity. Indeed, it has been estimated that in a doped honeycomb lattice, superconductivity might be possible at room temperature!
Planned experimental sequence:
- Generate atomic beam of 6Li in an oven
- Decelerate atoms in a Zeeman slower
- Capture atoms in magneto-optical trap
- Transfer atoms to strong optical trap
- Evaporative cooling of a spin mixture in optical trap (near Feshbach resonance) to molecular BEC
- Crossover to degenerate Fermi gas
- Load into 2D potential (generated by a blue-detuned laser in a TEM01 mode)
- Load into honeycomb lattice generated by holographic projection of potential
- Perform site-resolved fluorescence imaging of single atoms
On the afternoon of January 28, 2014, we successfully produced the first molecular BEC of Li2 atoms with our apparatus. The images show the momentum distribution at a homogeneous magnetic field of 760 G. From right to left, the depth of the dipole trap is reduced, leading to the distinctive bimodal distribution. For small trap depths (far right), a quasi-pure molecular BEC remains.
In January 2013, we saw the first MOT in our vacuum chamber. After optimizing and characterizing the MOT, we will cool the atoms further by evaporation in an optical trap
As of December 2011, we have locked our first laser (see error signal). We are currently finalizing the plan of the vacuum chamber and hope to soon have our first Lithium MOT.
From left to right:
- Dr. Wolfgang Limmer (scientific staff)
- Dr. Wladimir Schoch (scientific staff)
- Thomas Paintner (Ph.D. student)
- Daniel Hoffmann (Ph.D. student)
- Prof. Dr. Johannes Hecker Denschlag (project leader)
- ...and you?!?