Tilman Enss | Ruprecht-Karls-Universität Heidelberg
Winter term 2016/2017: Theory of Ultracold Atoms
Summer term 2016: Condensed Matter Theory
Winter term 2015/2016: Many-body physics with ultracold atoms
Summer term 2015: Condensed Matter Theory
Winter term 2014/2015: Theory of Ultracold Atoms
Summer term 2014: Condensed Matter Theory
Winter term 2013/2014: Special topics in quantum mechanics
Summer term 2013: Quantum mechanics
Winter term 2012/2013: Many-body methods in solid state physics
more on teaching
Nicolò Defenu (Postdoc)
Former students and team members
Moritz Drescher (PhD student)
Martin Braß (Master student)
Sergej Trenkenschu (Master student)
Dominik Lorenz (Bachelor student, Heidelberg 2016: Quench dynamics in the
Daniel Issing (Bachelor student, Heidelberg 2015: Bethe ansatz and
quench dynamics for 1D integrable quantum systems)
Bruno Faigle-Cedzich (Bachelor student, Heidelberg 2015: Shear
viscosity of two-component Fermi gas)
My research focuses on many-body theory, mainly dynamics and
transport in strongly correlated quantum systems, ranging from
ultracold atomic gases to superconductors and quantum wires.
I work on the development of several modern and advanced
theoretical tools including the functional renormalization group
(fRG), Hamiltonian flows, Luttinger-Ward calculations, transfer-matrix DMRG and the newly developed
articles on Google Scholar
- Quench dynamics in 1d chains and Many-body localization (MBL), using
the Lightcone renormalization group
- Many-body dynamics and transport in interacting ultracold atomic gases
- Quantum critical behavior in disordered correlated electron systems
- Nonequilibrium dynamics and transport in quantum dots and Luttinger liquids
- Raman and optical spectroscopy in cuprates
- Functional renormalization group method (fRG)
- Aging behavior in stochastic systems without detailed balance
Isolated quantum systems and universality under extreme conditions,
Observation of quantum-limited spin transport in strongly interacting two-dimensional Fermi gases
We measure the transport and thermodynamic properties of two-dimensional ultracold Fermi gases through their demagnetization in a magnetic field gradient. Using a phase-coherent spin-echo sequence, we are able to separate bare spin diffusion from the Leggett-Rice effect, in which demagnetization is slowed by the precession of spin current around the local magnetization. We find that the bare transverse spin diffusivity reaches a minimum of 1.7(6)ℏ/m, where m is the bare particle mass, when the two-dimensional scattering length is comparable to the inverse Fermi wave vector...
C. Luciuk, S. Smale, F. Böttcher, H. Sharum, B. A. Olsen, S. Trotzky, T. Enss, and J. H. Thywissen, arXiv:1612.00815.
Evidence for ferromagnetic instability in a repulsive Fermi gas of ultracold atoms
Ferromagnetism is among the most spectacular manifestations of interactions within many-body fermion systems. In contrast to weak-coupling phenomena, it requires strong repulsion to develop, making a quantitative description of ferromagnetic materials notoriously difficult. This is especially true for itinerant ferromagnets, where magnetic moments are not localized into a crystal lattice. In particular, it is still debated whether the simplest case envisioned by Stoner of a homogeneous Fermi gas with short-range repulsive interactions can exhibit ferromagnetism at all. In this work, we positively answer this question by studying a clean model system consisting of a binary spin-mixture of ultracold Li6 atoms, whose repulsive interaction is tuned via a Feshbach resonance...
G. Valtolina, F. Scazza, A. Amico, A. Burchianti, A. Recati, T. Enss, M. Inguscio, M. Zaccanti, and G. Roati,
We report the experimental measurement of the equation of state of a two-dimensional Fermi gas with attractive s-wave interactions throughout the crossover from a weakly coupled Fermi gas to a Bose gas of tightly bound dimers as the interaction strength is varied. We demonstrate that interactions lead to a renormalization of the density of the Fermi gas by several orders of magnitude. We compare our data near the ground state and at finite temperature with predictions for both fermions and bosons from quantum Monte Carlo simulations and Luttinger-Ward theory. Our results serve as input for investigations of close-to-equilibrium dynamics and transport in the two-dimensional system.
L. Bayha, D. Kedar, P. A. Murthy, M. Neidig, M. G. Ries, A. N. Wenz,
G. Zürn, S. Jochim, and T. Enss,
Phys. Rev. Lett. 116,
Viewpoint in Physics:
Journey from Classical to Quantum in Two Dimensions, by Meera
Atome in zwei Dimensionen (popular article in German, Phys. Unserer Zeit 47, 113 (2016)).
slides of talk in Budapest, 2015.
Nonlinear spin diffusion and spin rotation in a trapped Fermi gas
Transverse spin diffusion in a polarized, interacting Fermi gas leads to the Leggett-Rice effect, where the spin current precesses around the local magnetization. With a spin-echo sequence both the transverse diffusivity and the spin-rotation parameter γ are obtained; the sign of γ reveals the repulsive or attractive character of the effective interaction. In a trapped Fermi gas the spin diffusion equations become nonlinear, and their numerical solution exhibits an inhomogeneous spin state even at the spin echo time. While the microscopic diffusivity and γ increase at weak coupling, their apparent values inferred from the trap-averaged magnetization saturate in agreement with a recent experiment for a dilute ultracold Fermi gas.
Tilman Enss, Phys. Rev. A 91, 023614 (2015).
Observation of the Leggett-Rice effect in a unitary Fermi
PRL Editors' suggestion
We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of 40K precesses about the local magnetization. As predicted by Leggett and Rice, precession is observed both in the Ramsey phase of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At unitarity, we measure a Leggett-Rice parameter γ=1.08(9) and a bare transverse spin diffusivity D0⊥
=2.3(4)ℏ/m for a normal-state gas initialized with full polarization and at one fifth of the Fermi temperature, where m is the atomic mass. For a unitary gas, γ→0 as temperature is increased to the Fermi temperature. Tuning the scattering length a, we find that a sign change in γ occurs in the range 0<(kFa)-1
≤1.3, where kF is the Fermi momentum. We discuss how γ reveals the effective interaction strength of the gas, such that a change in γ indicates a switching of branch, between a repulsive and an attractive Fermi gas.
S. Trotzky, S. Beattie, C. Luciuk, S. Smale, A. B. Bardon, T. Enss,
E. Taylor, S. Zhang, and J. H. Thywissen,
Phys. Rev. Lett. 114, 015301 (2015).
Purification and many-body localization in cold atomic gases
We propose to observe many-body localization in cold atomic gases by realizing a Bose-Hubbard chain with binary disorder and studying its non-equilibrium dynamics. In particular, we show that measuring the difference in occupation between even and odd sites, starting from a prepared density-wave state, provides clear signatures of localization. As hallmarks of the many-body localized phase we confirm, furthermore, a logarithmic increase of the entanglement entropy in time and Poissonian level statistics. Our numerical density-matrix renormalization group calculations for infinite system size are based on a purification approach which allows to perform the disorder average exactly, thus producing data without any statistical noise, and with maximal simulation times of up to a factor 10 longer than in the clean case.
Felix Andraschko, Tilman Enss, and Jesko Sirker, Phys. Rev. Lett. 113, 217201 (2014).
Universal equation of state and pseudogap in the two-dimensional Fermi gas
We determine the thermodynamic properties and the spectral function for a homogeneous two- dimensional Fermi gas in the normal state using the Luttinger-Ward, or self-consistent T-matrix, approach. The density equation of state deviates strongly from that of the ideal Fermi gas even for moderate interactions, and our calculations suggest that temperature has a pronounced effect on the pressure in the crossover from weak to strong coupling, consistent with recent experiments. We also compute the superfluid transition temperature for a finite system in the crossover region. There is a pronounced pseudogap regime above the transition temperature: the spectral function shows a Bogoliubov-like dispersion with backbending, and the density of states is significantly suppressed near the chemical potential. The contact density at low temperatures increases with interaction and compares well with both experiment and zero-temperature Monte Carlo results.
Marianne Bauer, Meera M. Parish, and Tilman Enss, Phys. Rev. Lett. 112, 135302 (2014).
Damping of the quadrupole mode in a two-dimensional Fermi gas
In a recent experiment [E. Vogt et al., Phys. Rev. Lett. 108, 070404 (2012)], quadrupole and breathing modes of a two-dimensional Fermi gas were studied. We model these collective modes by solving the Boltzmann equation via the method of phase-space moments up to fourth order, including in-medium effects on the scattering cross section. In our analysis, we use a realistic Gaussian potential deformed by the presence of gravity and magnetic field gradients. We conclude that the origin of the experimentally observed damping of the quadrupole mode, especially in the weakly interacting (or even non-interacting) case, cannot be explained by these mechanisms.
Silvia Chiacchiera, Dany Davesne, Tilman Enss, and Michael Urban, Phys. Rev. A 88, 053616 (2013).
M. Urban, S. Chiacchiera, D. Davesne,
T. Enss, and P.-A. Pantel, J. Phys.: Conf. Ser. 497, 012028 (2014).
Quantum critical transport in the unitary Fermi gas
The thermodynamic and transport properties of the unitary Fermi gas
at finite temperature T are governed by a quantum critical point at
T=0 and zero density. We compute the universal shear viscosity to
entropy ratio η/s in the high-temperature quantum critical regime
T≫|μ| and find that this strongly coupled quantum fluid comes
close to perfect fluidity η/s=ℏ/4πkB
. Using a
controlled large-N expansion we show that already at the first
non-trivial order the equation of state and the Tan contact density
C agree well with the most recent experimental measurements and
theoretical Luttinger-Ward and Bold Diagrammatic Monte Carlo
Enss, Phys. Rev. A 86, 013616 (2012).
talk at ERG 2012 conference
Enss, Büro für Denkmalforschung und Denkmalpflege
Excitation spectra and rf-response near the polaron-to-molecule
transition from the functional renormalization group
A light impurity in a Fermi sea undergoes a transition from a
polaron to a molecule for increasing interaction. We develop a
new method to compute the spectral functions of the polaron
and molecule in a unified framework based on the functional
renormalization group with full self-energy feedback. We
discuss the energy spectra and decay widths of the attractive
and repulsive polaron branches as well as the molecular bound
state and confirm the scaling of the excited state decay rate
near the transition. The quasi-particle weight of the polaron
shifts from the attractive to the repulsive branch across the
transition, while the molecular bound state has a very small
residue characteristic for a composite particle. We propose an
experimental procedure to measure the repulsive branch in a
Li6 Fermi gas using rf-spectroscopy and calculate the
Richard Schmidt and Tilman Enss, Phys. Rev. A 83, 063620 (2011)
slides of talk
at ERG 2014 conference