arXiv subjects

Ulrich Ebling

Publications and source records attributed to Ulrich Ebling.

9 recordsLinked to original sources

arXiv2022

Triplet character of 2D-fermion dimers arising from $s$-wave attraction via spin-orbit coupling and Zeeman splitting

We theoretically study spin-$1/2$ fermions confined to two spatial dimensions and experiencing isotropic short-range attraction in the presence of both spin-orbit coupling and Zeeman spin splitting - a prototypical system for developing topological superfluidity in the many-body sector. Exact solutions for two-particle bound states are found to have a triplet contribution that dominates over the singlet part in an extended region of parameter space where the combined Zeeman- and center-of-mass-motion-induced spin-splitting energy is large. The triplet character of dimers is purest in the regime of weak $s$-wave interaction strength. Center-of-mass momentum is one of the parameters determining the existence of bound states, which we map out for both two- and one-dimensional types of spin-orbit coupling. Distinctive features emerging in the orbital part of the bound-state wave function, including but not limited to its $p$-wave character, provide observable signatures of unconventional pairing.

cond-mat.quant-gas↗

arXiv2020

Signatures of the BCS-BEC crossover in the yrast spectra of Fermi quantum rings

We study properties of the lowest energy states at non-zero total momentum (yrast states) of the Hubbard model for spin-1/2 fermions in the quantum ring configuration with attractive on-site interaction at low density. In the one-dimensional (1D) case we solve the Hubbard model using the Bethe ansatz, while for the crossover into the 2D regime we use the Full-Configuration-Interaction Quantum Monte-Carlo method (FCIQMC) to obtain the yrast states for the spin-balanced Fermi system. We show how the yrast excitation spectrum changes from the 1D to the 2D regime and how pairing affects the yrast spectra. We also find signatures of fragmented condensation for certain yrast states usually associated with dark solitons.

cond-mat.quant-gas↗

arXiv2020

Eliminating the wave function singularity for ultracold atoms by similarity transformation

A hyperbolic singularity in the wave-function of $s$-wave interacting atoms is the root problem for any accurate numerical simulation. Here we apply the transcorrelated method, whereby the wave-function singularity is explicitly described by a two-body Jastrow factor, and then folded into the Hamiltonian via a similarity transformation. The resulting non-singular eigenfunctions are approximated by stochastic Fock-space diagonalisation with energy errors scaling with $1/M$ in the number $M$ of single-particle basis functions. The performance of the transcorrelated method is demonstrated on the example of strongly correlated fermions with unitary interactions. The current method provides the most accurate ground state energies so far for three and four fermions in a rectangular box with periodic boundary conditions.

cond-mat.quant-gas↗

arXiv2017

Einstein-de Haas Effect in a Dipolar Fermi Gas

We show that an analogue to the classical Einstein-de Haas effect can appear in ultracold dipolar Fermi gases. The anisotropic nature of dipole-dipole interactions can lead to a transfer of magnetization into orbital angular momentum. Remarkably, distinct from a Bose-Einstein condensate, this transfer is accompanied by twisting motion, where individual spin components rotate in opposite directions with larger orbital angular momenta than the full system, possibly leading to easier experimental observation of the effect. This feature is induced by the deformation of the Fermi surface and the direction of the twisting motion can be controlled by an s-wave scattering length or external magnetic field, possibly providing a method of measuring scattering lengths of strongly dipolar atomic species.

cond-mat.quant-gas↗

arXiv2016

Spin relaxation in a one-dimensional large-spin degenerate Fermi gas

In this work, we study the dynamics of an atomic harmonically trapped large-spin Fermi gas in one dimension (1D). We investigate the interplay of different collision processes. Coherent spin oscillations, driven by spin-changing forward scattering are captured by a mean-field description and scale linearly with density regardless of the dimension of the system. Conversely, "incoherent" collision processes which e.g. lead to the damping of spin oscillations, behave differently. In the usual three-dimensional (3D) case, the rate of incoherent processes increases faster with density than mean-field effects, but in 1D it increases slower. This means, that in the 1D case, incoherent collisions become more important at lower densities. We study these effects by deriving and integrating a quantum Boltzmann equation. We demonstrate that the well known fact that in one dimension, interaction-induced correlations become more dominant at low densities can be observed in far-from-equilibrium spin dynamics.

cond-mat.quant-gas↗

arXiv2014

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.

cond-mat.quant-gas↗

arXiv2013

Giant spin oscillations in an ultracold Fermi sea

Collective behavior in many-body systems is the origin of many fascinating phenomena in nature ranging from swarms of birds and modeling of human behavior to fundamental magnetic properties of solids. We report on the first observation of collective spin dynamics in an ultracold Fermi sea with large spin: We observe 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 observe a stronger damping associated with particle-hole excitations. As a striking feature, we find a high-density regime where excited spin configurations are collisionally stabilized.

cond-mat.quant-gas↗

arXiv2013

Engineering spin waves in a high-spin ultracold Fermi gas

We report on the detailed study of multi-component 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 sign of the counterflow spin-currents are effectively controlled. A comparison of our data with numerical and analytical results shows excellent agreement.

cond-mat.quant-gas↗

arXiv2011

Spin segregation via dynamically induced long-range interaction in a system of ultracold fermions

We investigate theoretically the time evolution of a one-dimensional system of spin-1/2 fermions in a harmonic trap after, initially, a spiral spin configuration far-from equilibrium is created. We predict a spin segregation building up in time already for weak interaction under realistic experimental conditions. The effect relies on the interplay between exchange interaction and the harmonic trap, and it is found for a wide range of parameters. It can be understood as a consequence of an effective, dynamically induced long-range interaction that is derived by integrating out the rapid oscillatory dynamics in the trap.

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