arXiv subjects

J. E. Thomas

Publications and source records attributed to J. E. Thomas.

At least 19 recordsLinked to original sources

arXiv2026

Revealing the Spin Hydrodynamics of a Spin-Imbalanced Unitary Fermi Gas

Hydrodynamics governs diverse collective phenomena in nature, from the expansion of a quarkgluon plasma to viscous electron flow in quantum materials. In strongly interacting hydrodynamic fluids, the transport of spin remains poorly understood. Here, we investigate spin transport in the hydrodynamic regime of a spin-imbalanced unitary Fermi gas confined in a uniform optical box. By quenching a spatially periodic optical potential that modulates both the density and spin polarization, we observe the relaxation of the many-body system, which determines both the spin diffusivity and the spin Seebeck/Peltier coefficients in a homogeneous quantum gas. Our measurements provide parameter-free benchmarks for microscopic theories and establish an ultracold atom platform for studying spin caloritronics in strongly correlated matter.

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arXiv2026

Chirality-Induced Spin Currents in a Fermi Gas

We observe and model spin currents arising from chirality and effective spin-exchange interactions in a weakly interacting $^6$Li Fermi gas. Chirality is introduced by a static displacement between the center of the trapped atoms and the center of an applied magnetic bowl, which produces left- or right-handed spatially varying spin rotation. Spin current is directly observed via oscillations in the centers of mass of the spin-up and spin-down components, which appear to bounce off of or pass through one another, depending on the degree of handedness and s-wave scattering length. We show that this behavior obeys a driven oscillator equation with an effective spin-dependent driving force. Our measurements demonstrate chirality-induced spin selectivity via the direction of the current flow, extending CISS phenomena to Fermi gases.

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arXiv2024

Universal Density Shift Coefficients for the Thermal Conductivity and Shear Viscosity of a Unitary Fermi Gas

We measure universal temperature-independent density shifts for the thermal conductivity $κ_T$ and shear viscosity $η$, relative to the high temperature limits, for a normal phase unitary Fermi gas confined in a box potential. We show that a time-dependent kinetic theory model enables extraction of the hydrodynamic transport times $τ_η$ and $τ_κ$ from the time-dependent free-decay of a spatially periodic density perturbation, yielding the static transport properties and density shifts, corrected for finite relaxation times.

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arXiv2024

Collective dynamical Fermi suppression of optically-induced inelastic scattering

We observe strong dynamical suppression of optically induced loss in a weakly interacting Fermi gas as the $s$-wave scattering length is increased. The single, cigar-shaped cloud behaves as a large spin lattice in energy space with a tunable Heisenberg Hamiltonian. The loss suppression occurs as the lattice transitions into a magnetized state, where the fermionic nature of the atoms inhibits interactions. The data are quantitatively explained by incorporating spin-dependent loss into a quasi-classical collective spin vector model, the success of which enables the application of optical control of effective long-range interactions to this system.

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arXiv2024

Energy-resolved spin correlation measurements: Decoding transverse spin dynamics in weakly interacting Fermi gases

We study transverse spin dynamics on a microscopic level by measuring energy-resolved spin correlations in weakly interacting Fermi gases (WIFGs). The trapped cloud behaves as a many-body spin-lattice in energy space with effective long-range interactions, simulating a collective Heisenberg model. We observe the flow of correlations in energy space in this quasi-continuous system, revealing the connection between the evolution of the magnetization and the localization or spread of correlations. This work highlights energy-space correlation as a new observable in quantum phase transition studies of WIFGs, decoding system features that are hidden in macroscopic measurements.

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arXiv2023

Verifying a quasi-classical spin model of perturbed quantum rewinding in a Fermi gas

We systematically test a quasi-classical spin model of a large spin-lattice in energy space, with a tunable, reversible Hamiltonian and effective long-range interactions. The system is simulated by a weakly interacting Fermi gas undergoing perturbed quantum rewinding using radio-frequency(RF) pulses. The model reported here is found to be in a quantitative agreement with measurements of the ensemble-averaged energy-resolved spin density. This work elucidates the effects of RF detunings on the system and measurements, pointing the way to new correlation measurement methods.

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arXiv2022

Hydrodynamic Relaxation in a Strongly Interacting Fermi Gas

We measure the free decay of a spatially periodic density profile in a normal fluid strongly interacting Fermi gas, which is confined in a box potential. This spatial profile is initially created in thermal equilibrium by a perturbing potential. After the perturbation is abruptly extinguished, the dominant spatial Fourier component exhibits an exponentially decaying (thermally diffusive) mode and a decaying oscillatory (first sound) mode, enabling independent measurement of the thermal conductivity and the shear viscosity directly from the time-dependent evolution.

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arXiv2021

Energy-Resolved Information Scrambling in Energy-Space Lattices

Weakly interacting Fermi gases simulate spin-lattices in energy-space, offering a rich platform for investigating information spreading and spin coherence in a large many-body quantum system. We show that the collective spin vector can be determined as a function of energy from the measured spin density, enabling general energy-space resolved protocols. We measure an out-of-time-order correlation function in this system and observe the energy dependence of the many-body coherence.

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arXiv2021

Spin-Energy Correlation in Degenerate Weakly-Interacting Fermi Gases

Weakly interacting Fermi gases exhibit rich collective dynamics in spin-dependent potentials, arising from correlations between spin degrees of freedom and conserved single atom energies, offering broad prospects for simulating many-body quantum systems by engineering energy-space "lattices," with controlled energy landscapes and site to site interactions. Using quantum degenerate clouds of $^6$Li, confined in a spin-dependent harmonic potential, we measure complex, time-dependent spin-density profiles, varying on length scales much smaller than the cloud size. We show that a one-dimensional mean field model, without additional simplifying approximations, quantitatively predicts the observed fine structure. We measure the magnetic fields where the scattering lengths vanish for three different hyperfine state mixtures to provide new constraints on the collisional (Feshbach) resonance parameters.

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arXiv2019

Measuring the Hydrodynamic Linear Response of a Unitary Fermi Gas

We directly observe the hydrodynamic linear response of a unitary Fermi gas confined in a box potential and subject to a spatially periodic optical potential that is translated into the cloud at speeds ranging from subsonic to supersonic. We show that the time-dependent change of the density profile is sensitive to the thermal conductivity, which controls the relaxation rate of the temperature gradients and hence the responses arising from adiabatic and isothermal compression.

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arXiv2018

Probing Energy-Dependent Feshbach Resonances by Optical Control

Optical control enables new high resolution probes of narrow collisional (Feshbach) resonances, which are strongly dependent on the relative momentum of colliding atom pairs, and important for simulating neutron matter with ultracold atomic gases. We demonstrate a two-field optical vernier, which expands kHz (mG) magnetic field detunings near a narrow resonance into MHz optical field detunings, enabling precise control and characterization of the momentum-dependent scattering amplitude. Two-photon loss spectra are measured for the narrow resonance in $^6$Li, revealing rich structure in very good agreement with our theoretical model. However, anomalous frequency shifts between the measured and predicted two-photon spectra are not yet explained.

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arXiv2018

Designer Spatial Control of Interactions in Ultracold Gases

Designer optical control of interactions in ultracold atomic gases has wide application, from creating new quantum phases to modeling the physics of black holes. We demonstrate spatial control of interactions in a two-component cloud of $^6$Li fermions, using electromagnetically induced transparency (EIT) to create a "sandwich" of resonantly and weakly interacting regions. Interaction designs are imprinted on the trapped cloud by two laser beams and manipulated with just MHz changes in the frequency of one beam. We employ radio-frequency spectroscopy to measure the imprinted 1D spatial profiles of the local mean-field interactions and to demonstrate that the tuning range of the scattering length is the same for both optical and magnetic control. All of the data are in excellent agreement with our continuum-dressed state theoretical model of optical control, which includes both the spatial and momentum dependence of the interactions.

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arXiv2018

Atom Pairing in Optical Superlattices

We study the pairing of fermions in a one-dimensional lattice of tunable double-well potentials using radio-frequency spectroscopy. The spectra reveal the coexistence of two types of atom pairs with different symmetries. Our measurements are in excellent quantitative agreement with a theoretical model, obtained by extending the Green's function method of Orso et al., [Phys. Rev. Lett. 95, 060402 (2005)], to a bichromatic 1D lattice with finite harmonic radial confinement. The predicted spectra comprise hundreds of discrete transitions, with symmetry-dependent initial state populations and transition strengths. Our work provides an understanding of the elementary pairing states in a superlattice, paving the way for new studies of strongly interacting many-body systems.

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arXiv2016

Fermi Gases in the Two-Dimensional to Quasi-Two-Dimensional Crossover

We tune the dimensionality of pancake-shaped strongly-interacting $^6$Li Fermi gas clouds from two-dimensional (2D) to quasi-2D, by controlling the ratio of the radial Fermi energy $E_F$ to the harmonic oscillator energy $hν_z$ in the tightly confined direction. In the 2D regime, where $E_F<<hν_z$, the measured radio frequency resonance spectra are in agreement with 2D-BCS theory. In the quasi-2D regime, where $E_F\simeq hν_z$, the measured spectra deviate significantly from 2D-BCS theory. For both regimes, the measured cloud radii disagree with 2D-BCS mean field theory, but agree approximately with predictions using a free energy derived from the Bethe-Goldstone equation.

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arXiv2015

Two-field optical methods to control magnetic Feshbach resonances

Using an optically-trapped mixture of the two lowest hyperfine states of a $^6$Li Fermi gas, we observe two-field optical tuning of the narrow Feshbach resonance by up to 3 G and an increase in spontaneous lifetime near the broad resonance from $0.5$ ms to $0.4$ s. We present a new model of light-induced loss spectra, employing continuum-dressed basis states, that agrees in shape and magnitude with measurements for both broad and narrow resonances.

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arXiv2014

Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases

We measure the density profiles for a Fermi gas of $^6$Li containing $N_1$ spin-up atoms and $N_2$ spin-down atoms, confined in a quasi-two-dimensional geometry. The spatial profiles are measured as a function of spin-imbalance $N_2/N_1$ and interaction strength, which is controlled by means of a collisional (Feshbach) resonance. The measured cloud radii and central densities are in disagreement with mean-field Bardeen-Cooper-Schrieffer theory for a true two-dimensional system. We find that the data for normal-fluid mixtures are reasonably well fit by a simple two-dimensional polaron model of the free energy. Not predicted by the model is a phase transition to a spin-balanced central core, which is observed above a critical value of $N_2/N_1$. Our observations provide important benchmarks for predictions of the phase structure of quasi-two-dimensional Fermi gases.

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arXiv2014

Shear viscosity of a universal Fermi gas near the superfluid phase transition

We precisely measure the shear viscosity for a resonantly interacting Fermi gas as a function of temperature, from nearly the ground state through the superfluid phase transition at a critical temperature $T_c$. Using an iterative method to invert the data, we extract the {\it local} shear viscosity coefficient $α_S(θ)$ versus reduced temperature $θ$, revealing previously hidden features. We find that $α_S$ begins to decrease rapidly with decreasing $θ$ well above $T_c$, suggesting that preformed pairs play an important role. Further, we observe that the derivative $α_S'(θ)$ has a maximum at $T_c$. We compare the local data to several microscopic theories. Finally, we determine the local ratio of the shear viscosity to the entropy density.

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arXiv2014

Anomalous minimum in the shear viscosity of a Fermi gas

We measure the static shear viscosity $η$ in a two-component Fermi gas near a broad collisional (Feshbach) resonance, as a function of interaction strength and energy. We find that $η$ has both a quadratic and a linear dependence on the interaction strength $1/({k_{FI}a})$, where $a$ is the s-wave scattering length and $k_{FI}$ is the Fermi wave vector for an ideal gas at the trap center. For energies above the superfluid transition, the minimum in $η$ as a function of interaction strength is significantly shifted toward the BEC side of resonance, to $1/(k_{FI}a)\simeq 0.25$.

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