arXiv subjects

Hauke Biss

Publications and source records attributed to Hauke Biss.

5 recordsLinked to original sources

arXiv2025

Hybridization of the amplitude mode in a confined fermionic superfluid

In phase transitions, spontaneous symmetry breaking results in a non-zero order parameter and two collective excitations: the Goldstone and the amplitude mode. These modes, which define key properties of superconductors and fermionic superfluids, are well-understood in homogeneous systems. However, their behavior under strong confinement remains largely unexplored, particularly when their excitation energy becomes comparable to the imposed discrete level spacing. In this scenario, hybridization between different collective modes is expected to take place. Here, we show how the amplitude mode hybridizes with a spatial mode in a confined fermionic superfluid. Using lattice modulation spectroscopy, we observe the evolution of the mode throughout the entire crossover from the Bardeen-Cooper-Schrieffer (BCS) state to Bose-Einstein condensation (BEC) of molecules. In the BCS regime, the excitation energy is located at twice the pairing gap, then gradually becomes an in-gap excitation in the strongly correlated regime. Further to the BEC limit, the excitation energy approaches twice the level spacing. The spectral weight of this mode vanishes when approaching the superfluid critical temperature. Our experimental results are in excellent agreement with an effective field theory, providing strong evidence that amplitude oscillations hybridize with and eventually transform into breathing oscillations of the order parameter. The strong modification of the excitation spectrum reveals how confinement and finite-size effects impact fundamental modes of symmetry-broken states.

cond-mat.quant-gas↗

arXiv2021

Comparing fermionic superfluids in two and three dimensions

Understanding the origins of unconventional superconductivity has been a major focus of condensed matter physics for many decades. While many questions remain unanswered, experiments have found that the systems with the highest critical temperatures tend to be layered materials where superconductivity occurs in two-dimensional (2D) structures. However, to what extent the remarkable stability of these strongly correlated 2D superfluids is related to their reduced dimensionality is still an open question. In this work, we use dilute gases of ultracold fermionic atoms as a model system to directly observe the influence of dimensionality on strongly interacting fermionic superfluids. We achieve this by measuring the superfluid gap of a strongly correlated quasi-2D Fermi gas over a wide range of interaction strengths and comparing the results to recent measurements in 3D Fermi gases. We find that the superfluid gap follows the same universal function of the interaction strength in both systems, which suggests that there is no inherent difference in the stability of fermionic superfluidity between two- and three-dimensional quantum gases. Finally, we compare our data to results from solid state systems and find a similar relation between the interaction strength and the gap for a wide range of two- and three-dimensional superconductors.

cond-mat.quant-gas↗

arXiv2021

Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas

Ultracold atomic gases are a powerful tool to experimentally study strongly correlated quantum many-body systems. In particular, ultracold Fermi gases with tunable interactions have allowed to realize the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. However, large parts of the excitation spectrum of fermionic superfluids in the BEC-BCS crossover are still unexplored. In this work, we use Bragg spectroscopy to measure the full momentum-resolved low-energy excitation spectrum of strongly interacting ultracold Fermi gases. This enables us to directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. We also use our spectra to determine the evolution of the superfluid gap and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, towards the BCS regime a calculation that includes the effects of particle-hole correlations shows better agreement with our data.

cond-mat.quant-gas↗

arXiv2020

Observation of superfluidity in a strongly correlated two-dimensional Fermi gas

Understanding how strongly correlated two-dimensional (2D) systems can give rise to unconventional superconductivity with high critical temperatures is one of the major unsolved problems in condensed matter physics. Ultracold 2D Fermi gases have emerged as clean and controllable model systems to study the interplay of strong correlations and reduced dimensionality, but direct evidence of superfluidity in these systems has been missing. Here, we demonstrate superfluidity in an ultracold 2D Fermi gas by moving a periodic potential through the system and observing no dissipation below a critical velocity v$_{\rm c}$. We measure v$_{\rm c}$ as a function of interaction strength and find a maximum in the crossover regime between bosonic and fermionic superfluidity. Our measurement establishes ultracold Fermi gases as a powerful tool for studying the influence of reduced dimensionality on strongly correlated superfluids.

cond-mat.quant-gas↗

arXiv2020

Sound propagation and quantum limited damping in a two-dimensional Fermi gas

Strongly interacting two-dimensional Fermi systems are one of the great remaining challenges in many-body physics due to the interplay of strong local correlations and enhanced long-range fluctuations. Here, we probe the thermodynamic and transport properties of a 2D Fermi gas across the BEC-BCS crossover by studying the propagation and damping of sound modes. We excite particle currents by imprinting a phase step onto homogeneous Fermi gases trapped in a box potential and extract the speed of sound from the frequency of the resulting density oscillations. We measure the speed of sound across the BEC-BCS crossover and compare the resulting dynamic measurement of the equation of state both to a static measurement based on recording density profiles and to Quantum Monte Carlo calculations and find reasonable agreement between all three. We also measure the damping of the sound mode, which is determined by the shear and bulk viscosities as well as the thermal conductivity of the gas. We find that the damping is minimal in the strongly interacting regime and the diffusivity approaches the universal quantum bound $\hbar/m$ of a perfect fluid.

cond-mat.quant-gas↗