14 research outputs found
Imaging magnetic polarons in the doped Fermi-Hubbard model
Polarons are among the most fundamental quasiparticles emerging in
interacting many-body systems, forming already at the level of a single mobile
dopant. In the context of the two-dimensional Fermi-Hubbard model, such
polarons are predicted to form around charged dopants in an antiferromagnetic
background in the low doping regime close to the Mott insulating state.
Macroscopic transport and spectroscopy measurements related to high
materials have yielded strong evidence for the existence of such quasiparticles
in these systems. Here we report the first microscopic observation of magnetic
polarons in a doped Fermi-Hubbard system, harnessing the full single-site spin
and density resolution of our ultracold-atom quantum simulator. We reveal the
dressing of mobile doublons by a local reduction and even sign reversal of
magnetic correlations, originating from the competition between kinetic and
magnetic energy in the system. The experimentally observed polaron signatures
are found to be consistent with an effective string model at finite
temperature. We demonstrate that delocalization of the doublon is a necessary
condition for polaron formation by contrasting this mobile setting to a
scenario where the doublon is pinned to a lattice site. Our work paves the way
towards probing interactions between polarons, which may lead to stripe
formation, as well as microscopically exploring the fate of polarons in the
pseudogap and bad metal phase
Can Three-Body Recombination Purify a Quantum Gas?
Three-body recombination in quantum gases is traditionally associated with heating, but it was recently found that it can also cool the gas. We show that in a partially condensed three-dimensional homogeneous Bose gas three-body loss could even purify the sample, that is, reduce the entropy per particle and increase the condensed fraction η. We predict that the evolution of η under continuous three-body loss can, depending on small changes in the initial conditions, exhibit two qualitatively different behaviors-if it is initially above a certain critical value, η increases further, whereas clouds with lower initial η evolve towards a thermal gas. These dynamical effects should be observable under realistic experimental conditions
Ultracold field-linked tetratomic molecules
Ultracold polyatomic molecules offer intriguing new opportunities in cold
chemistry, precision measurements, and quantum information processing, thanks
to their rich internal structure. However, their increased complexity compared
to diatomic molecules presents a formidable challenge to employ conventional
cooling techniques. Here, we demonstrate a new approach to create ultracold
polyatomic molecules by electroassociation in a degenerate Fermi gas of
microwave-dressed polar molecules through a field-linked resonance. Starting
from ground state NaK molecules, we create around tetratomic
(NaK) molecules, with a phase space density of at a temperature
of , more than times colder than previously realized
tetratomic molecules. We observe a maximum tetramer lifetime of
in free space without a notable change in the presence of an
optical dipole trap, indicating these tetramers are collisionally stable. The
measured binding energy and lifetime agree well with parameter-free
calculations, which outlines pathways to further increase the lifetime of the
tetramers. Moreover, we directly image the dissociated tetramers through
microwave-field modulation to probe the anisotropy of their wave function in
momentum space. Our result demonstrates a universal tool for assembling
ultracold polyatomic molecules from smaller polar molecules, which is a crucial
step towards Bose--Einstein condensation (BEC) of polyatomic molecules and
towards a new crossover from a dipolar Bardeen-Cooper-Schrieffer (BCS)
superfluid to a BEC of tetramers. Additionally, the long-lived FL state
provides an ideal starting point for deterministic optical transfer to deeply
bound tetramer states
Many-Body Decay of the Gapped Lowest Excitation of a Bose-Einstein Condensate.
We study the decay mechanism of the gapped lowest-lying axial excitation of a quasipure atomic Bose-Einstein condensate confined in a cylindrical box trap. Owing to the absence of accessible lower-energy modes, or direct coupling to an external bath, this excitation is protected against one-body (linear) decay, and the damping mechanism is exclusively nonlinear. We develop a universal theoretical model that explains this fundamentally nonlinear damping as a process whereby two quanta of the gapped lowest excitation mode couple to a higher-energy mode, which subsequently decays into a continuum. We find quantitative agreement between our experiments and the predictions of this model. Finally, by strongly driving the system below its (lowest) resonant frequency, we observe third-harmonic generation, a hallmark of nonlinear behavior
Direct observation of incommensurate magnetism in Hubbard chains
The interplay between magnetism and doping is at the origin of exotic
strongly correlated electronic phases and can lead to novel forms of magnetic
ordering. One example is the emergence of incommensurate spin-density waves
with a wave vector that does not match the reciprocal lattice. In one dimension
this effect is a hallmark of Luttinger liquid theory, which also describes the
low energy physics of the Hubbard model. Here we use a quantum simulator based
on ultracold fermions in an optical lattice to directly observe such
incommensurate spin correlations in doped and spin-imbalanced Hubbard chains
using fully spin and density resolved quantum gas microscopy. Doping is found
to induce a linear change of the spin-density wave vector in excellent
agreement with Luttinger theory predictions. For non-zero polarization we
observe a decrease of the wave vector with magnetization as expected from the
Heisenberg model in a magnetic field. We trace the microscopic origin of these
incommensurate correlations to holes, doublons and excess spins which act as
delocalized domain walls for the antiferromagnetic order. Finally, when
inducing interchain coupling we observe fundamentally different spin
correlations around doublons indicating the formation of a magnetic polaron
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Can Three-Body Recombination Purify a Quantum Gas?
Three-body recombination in quantum gases is traditionally associated with
heating, but it was recently found that it can also cool the gas. We show that
in a partially condensed three-dimensional homogeneous Bose gas three-body loss
could even purify the sample, that is, reduce the entropy per particle and
increase the condensed fraction . We predict that the evolution of
under continuous three-body loss can, depending on small changes in the initial
conditions, exhibit two qualitatively different behaviours - if it is initially
above a certain critical value, increases further, whereas clouds with
lower initial evolve towards a thermal gas. These dynamical effects
should be observable under realistic experimental conditions
Quantifying hole-motion-induced frustration in doped antiferromagnets by Hamiltonian reconstruction
Abstract Unveiling the microscopic origins of quantum phases dominated by the interplay of spin and motional degrees of freedom constitutes one of the central challenges in strongly correlated many-body physics. When holes move through an antiferromagnetic spin background, they displace the positions of spins, which induces effective frustration in the magnetic environment. However, a concrete characterization of this effect in a quantum many-body system is still an unsolved problem. Here we present a Hamiltonian reconstruction scheme that allows for a precise quantification of hole-motion-induced frustration. We access non-local correlation functions through projective measurements of the many-body state, from which effective spin-Hamiltonians can be recovered after detaching the magnetic background from dominant charge fluctuations. The scheme is applied to systems of mixed dimensionality, where holes are restricted to move in one dimension, but SU(2) superexchange is two-dimensional. We demonstrate that hole motion drives the spin background into a highly frustrated regime, which can quantitatively be described by an effective J 1–J 2-type spin model. We exemplify the applicability of the reconstruction scheme to ultracold atom experiments by recovering effective spin-Hamiltonians of experimentally obtained 1D Fermi-Hubbard snapshots. Our method can be generalized to fully 2D systems, enabling promising microscopic perspectives on the doped Hubbard model
Quantifying hole-motion-induced frustration in doped antiferromagnets by Hamiltonian reconstruction
Unveiling the microscopic origins of quantum phases dominated by the interplay of spin and motional degrees of freedom constitutes one of the central challenges in strongly correlated many-body physics. When holes move through an antiferromagnetic spin background, they displace the positions of spins, which induces effective frustration in the magnetic environment. However, a concrete characterization of this effect in a quantum many-body system is still an unsolved problem. Here we present a Hamiltonian reconstruction scheme that allows for a precise quantification of hole-motion-induced frustration. We access non-local correlation functions through projective measurements of the many-body state, from which effective spin-Hamiltonians can be recovered after detaching the magnetic background from dominant charge fluctuations. The scheme is applied to systems of mixed dimensionality, where holes are restricted to move in one dimension, but SU(2) superexchange is two-dimensional. We demonstrate that hole motion drives the spin background into a highly frustrated regime, which can quantitatively be described by an effective J1–J2-type spin model. We exemplify the applicability of the reconstruction scheme to ultracold atom experiments by recovering effective spin-Hamiltonians of experimentally obtained 1D Fermi-Hubbard snapshots. Our method can be generalized to fully 2D systems, enabling promising microscopic perspectives on the doped Hubbard model
First and second sound in a compressible 3D Bose fluid
The two-fluid model is fundamental for the description of superfluidity. In
the nearly-incompressible-liquid regime, it successfully describes first and
second sound, corresponding, respectively, to density and entropy waves, in
both liquid helium and unitary Fermi gases. Here, we study the two sounds in
the opposite regime of a highly compressible fluid, using an ultracold K
Bose gas in a three-dimensional box trap. We excite the longest-wavelength mode
of our homogeneous gas, and observe two distinct resonant oscillations below
the critical temperature, of which only one persists above it. In a microscopic
mode-structure analysis, we find agreement with the hydrodynamic theory, where
first and second sound involve density oscillations dominated by, respectively,
thermal and condensed atoms. Varying the interaction strength, we explore the
crossover from hydrodynamic to collisionless behavior in a normal gas.Comment: Main text (5 pages, 4 figures), Supplemental Material (14 pages, 8
figures
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Research data supporting "First and Second Sound in a Compressible 3D Bose Fluid"
The supporting data found in the zip folder contains the experimental data points and theory curves for the paper, while the read-me file provides associated details