Jasper Simon Krauser

Probing superfluids in optical lattices by momentum-resolved Bragg spectroscopy

Strongly correlated many-body systems show various exciting phenomena in condensed matter physics such as high-temperature superconductivity and colossal magnetoresistance. Recently, strongly correlated phases could also be studied in ultracold quantum gases possessing analogies to solid-state physics, but moreover exhibiting new systems such as Fermi-Bose mixtures and magnetic quantum phases with high spin values. Particularly interesting systems here are quantum gases in optical lattices with fully tunable lattice and atomic interaction parameters. While in this context several concepts and ideas have already been studied theoretically and experimentally, there is still great demand for new detection techniques to explore these complex phases in detail.

Multiband spectroscopy of ultracold fermions: observation of reduced tunneling in attractive Bose-Fermi mixtures.

We perform a detailed experimental study of the band excitations and tunneling properties of ultracold fermions in optical lattices. Employing a novel multiband spectroscopy for fermionic atoms, we can measure the full band structure and tunneling energy with high accuracy. In an attractive Bose-Fermi mixture we observe a significant reduction of the fermionic tunneling energy, which depends on the relative atom numbers. We attribute this to an interaction-induced increase of the lattice depth due to the self-trapping of the atoms.

Coherent multi-flavour spin dynamics in a fermionic quantum gas

Quantum gases are useful toy models for the study of quantum magnetism. Exquisite control of a spinor gas of fermionic atoms in an optical lattice has now been demonstrated, opening up the exploration of quantum magnetism with high spins.

Giant Spin Oscillations in an Ultracold Fermi Sea

Collective Coherent Spin Dynamics Ultracold gases have shown considerable promise for the quantum simulation of more complicated systems, such as correlated electrons in solids. Usually, researchers use two hyperfine states of the atoms to correspond to the spin up and down states of the electrons; however, these gases typically have a much richer internal state structure. Krauser et al. (p. 157) observed the coherent behavior of a gas of potassium-40 atoms that had 10 accessible internal spin states and that evolved through collisions. The spin state of the system oscillated as a whole, a surprising finding given that the atoms are fermions. Collective behavior in many-body systems is the origin of many fascinating phenomena in nature, ranging from the formation of clouds to magnetic properties of solids. We report on the observation of collective spin dynamics in an ultracold Fermi sea with large spin. As a key result, we observed long-lived and large-amplitude coherent spin oscillations driven by local spin interactions. At ultralow temperatures, Pauli blocking stabilizes the collective behavior, and the Fermi sea behaves as a single entity in spin space. With increasing temperature, we observed a stronger damping associated with particle-hole excitations. Unexpectedly, we found a high-density regime where excited spin configurations are collisionally stabilized. Our results reveal the intriguing interplay between microscopic processes either stimulating or suppressing collective effects in a fermionic many-body system. Long-lived oscillations of the internal state of a trapped fermionic potassium-40 gas occur within a single spatial mode.

Intrinsic photoconductivity of ultracold fermions in optical lattices.

We report on the experimental observation of an analog to a persistent alternating photocurrent in an ultracold gas of fermionic atoms in an optical lattice. The dynamics is induced and sustained by an external harmonic confinement. While particles in the excited band exhibit long-lived oscillations with a momentum-dependent frequency, a strikingly different behavior is observed for holes in the lowest band. An initial fast collapse is followed by subsequent periodic revivals. Both observations are fully explained by mapping the system onto a nonlinear pendulum.

Engineering spin waves in a high-spin ultracold Fermi gas.

We report on the detailed study of multicomponent spin waves in an s=3/2 Fermi gas where the high spin leads to novel tensorial degrees of freedom compared to s=1/2 systems. The excitations of a spin-nematic state are investigated from the linear to the nonlinear regime, where the tensorial character is particularly pronounced. By tuning the initial state we engineer the tensorial spin-wave character, such that the magnitude and the sign of the counterflow spin currents are effectively controlled. A comparison of our data with numerical and analytical results shows good agreement.

Relaxation Dynamics of an Isolated Large-Spin Fermi Gas Far from Equilibrium

A fundamental question in many-body physics is how closed quantum systems reach equilibrium. We address this question experimentally and theoretically in an ultracold large-spin Fermi gas where we find a complex interplay between internal and motional degrees of freedom. The fermions are initially prepared far from equilibrium with only a few spin states occupied. The subsequent dynamics leading to redistribution among all spin states is observed experimentally and simulated theoretically using a kinetic Boltzmann equation with full spin coherence. The latter is derived microscopically and provides good agreement with experimental data without any free parameters. We identify several collisional processes, which occur on different time scales. By varying density and magnetic field, we control the relaxation dynamics and are able to continuously tune the character of a subset of spin states from an open to a closed system.

Bragg spectroscopy in optical lattices

Strongly correlated many-body systems are at the heart of many recent problems in condensed matter physics ranging from high-temperature superconductivity to colossal magnetoresistance. A wealth of analytic tools was developed to tackle the mechanics of these system. Excitations define many important characteristics in many-body systems which can be measured by e.g. angle-resolved photoemission. But due to their complex nature, inherent defects as well as experimental difficulties many open questions remain.