Ultracold atoms in optical lattices have recently been all the rage for their fundamental and technological promise. The ability to tune interactions, dimensionality, spin, statistics, and a host of other variables in a completely disorder-free environment opens the possibility for these systems to be used as emulators of model Hamiltonians that describe complex materials. However, measuring their temperature remains a practical challenge as few promising schemes are known. A suitable measurable property is needed such that its magnitude is in a one-to-one correspondence with the temperature of the system.
Writing in Physical Review A, Jean-Sébastien Bernier and collaborators from France and Argentina have demonstrated how to make use of a rather unusual thermometric property—the Raman transition rate for a fermionic gas. For a wide range of well-specified conditions, the Raman signal depends on temperature only via the Fermi distribution function. To implement the method, one simply measures the Raman spectrum of the gas.
Bernier et al. go beyond the thermometric aspect and predict distinct features in the respective Raman spectra, fingerprints as it were, of various regimes of correlation (weak correlations, strongly correlated Fermi liquid, and a Mott insulating phase). The predicted signatures, such as the emergence of broad Hubbard bands in the incompressible Mott regime, would survive at ultralow temperatures and provide for diagnostics of the degree of correlation in and fermionic gases in optical lattices. Naturally, this would also serve to map out the phase diagram of ultracold correlated systems. – Yonko Millev