Session Q11: Quantum Gases in Optical Lattices I

Magnetically mediated hole pairing in fermionic ladders of ultracold atoms

Doped antiferromagnets are prime examples of strongly correlated systems and hold the potential to shed light on the emergence of high-T_C superconductivity in cuprate materials via pairing of mobile dopants. Here we report on the microscopic observation of hole pairing due to the magnetic correlations in a Fermi-Hubbard system by using quantum gas microscopy. We prepare two-leg ladders where we disable tunnelling between the rungs in order to suppress Pauli repulsion and thus drastically increase binding energy of the holes. Observing pairs of holes predominantly occupying the same rung of the ladder allows us to extract a binding energy slightly below the superexchange energy. For higher doping levels we find indications of crystallization of hole pairs.   

Preparation of the spin-Mott state: a spinful Mott insulator of repulsively bound pairs

We observe and study a special ground state of bosons with two spin states in an optical lattice:the spin-Mott insulator, a state that consists of repulsively bound pairs which is insulating for bothspin and charge transport. Because of the pairing gap created by the interaction anisotropy, it canbe prepared with low entropy and can serve as a starting point for adiabatic state preparation.We find that the stability of the spin-Mott state depends on the pairing energy, and observe twoqualitatively different decay regimes, one of which exhibits protection by the gap.   

Mott transition and magnetism of SU(3) fermions in a square lattice

The SU(N) Fermi-Hubbard model is attracting strong ongoing interest for its connections to multi-orbital solid-state systems and its rich predicted phase diagrams. In this work we study the SU(3) Fermi-Hubbard model in the two-dimensional square lattice using determinant Quantum Monte Carlo (DQMC), as a function of the interaction strength U/t and temperature. Previous results in the strongly interacting limit, U/t >> 1, and one particle per site propose the existence of 2-sublattice ordering at high-T and 3-sublattice ordering at low-T. Here we explore if that also occurs at weak and moderate values of the interaction strength, and study the evolution of the compressibility and structure factors to understand at which interaction strengths the system opens a charge gap and when it develops magnetic ordering. These results provide guidance for ongoing optical lattice experiments with ¹⁷³Yb or ⁸⁷Sr quantum gas microscopes.   

Experimental measurement and manipulation of unitary fermionic p-wave interactions in a 3D optical lattice

Resonantly enhanced and controllable p-wave interactions in ultracold atomic systems are a promising test bed for realizing exotic many-body states.  However, strong three-body loss has traditionally limited experiments to systems of low dimensionality.  In this work, we report on experimental spectroscopic measurements of Feshbach-enhanced p-wave interactions of 40K, where three-body loss is suppressed by loading spin polarized atoms in a deep 3D optical lattice.  The resultant interaction energy shifts are in good agreement with analytic predictions of the two-body problem across the Feshbach resonance. The observed lifetimes of strongly interacting pairs (doublons) are consistent with predictions of two-body loss from dipolar relaxation and exceed tens of milliseconds. Measured Rabi oscillations between interacting and non-interacting doublons demonstrate the possibility of coherent manipulation of p-wave Feshbach molecules. These pioneering experiments test theoretical predictions of two-body p-wave interactions and opens doors towards realizing novel many-body states with them.

    

Particle fluctuations and the failure of simple effective models for many-body localized phases

This talk contributes to the ongoing debate whether a many-body localized phase (MBL) of a spinless fermion model with potential disorder and nearest-neighbor interactions (t-V model) exists or not. In particular, recent evidence suggests that there is a continuing subdiffusive particle transport even deep in the putative MBL phase [1, 2, 3]. Therefore, we investigate and compare the particle number fluctuations in the many-body localized phase with those in the non-interacting case (Anderson localization) and in effective models where only interaction terms diagonal in the Anderson basis are kept. We demonstrate that these types of simple effective models cannot account for the particle number fluctuations observed in the MBL phase of the microscopic model. As a consequence, it appears questionable if the microscopic model possesses an exponential number of exactly conserved local charges. If such a set of conserved local charges does not exist, then particles are expected to ultimately delocalize for any finite disorder strength.

[1] Phys. Rev. Lett. 124, 243601 (2020), [2] Phys. Rev. B 103, 024203 (2021), [3] Annals of Physics 435, 168481 (2021)   

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We study the ground state phases of interacting bosons in the presence of a 2D Aubry-André potential. By using a a mean-field percolation analysis, we focus on several superlattice and quasicrystalline regimes of the 2D Aubry-André model, including generalisations that account for a tilting or skewing of the potential. We show that barriers to the onset of macroscopic phases naturally arise from weakly modulated domains in the 2D Aubry-André model. This leads to the formation of mixed phases, in which the macroscopic properties are dominated by a minority of the system. The phase diagrams then exhibit substantially different features when compared against crystalline systems, including a lobe-like or wave-like appearance of the Bose glass, sharp extrusions and extended domains with weak percolation. By studying the 2D Aubry-André model across multiple regimes, we have shown that the unique properties of mixed phases are not distinct to a small set of parameters.   

Coherent control of localization in modulated quasiperiodic lattices

We report experiments demonstrating reversible coherent control of a localization phase transition by phasonic modulation. As background, we recently reported phasonic spectroscopy of a quantum gas in an artificial quasicrystal realized by a bichromatic optical lattice [1]. In that work, we studied how rapid phase modulation of the secondary optical lattice (phason modulation) causes atoms to absorb energy at high harmonics of the drive frequency. In this talk, we discuss phase modulation in a lower frequency regime where excitation into higher bands is not significant but the modulation frequency is high enough to avoid intraband excitations [2]. In particular, we investigated the phase modulation amplitude dependence of the localization properties of the quasicrystal. In this regime, the effect of the phase modulation is to renormalize the effective lattice depth of the phase modulated secondary lattice [1]. By monitoring transport in the modulated bichromatic lattice, we observed that the effective variation of the secondary lattice depth causes a number of localization-delocalization transitions as the phason drive amplitude is increased. These results open a new path to dynamical coherent control of the transport properties of quantum matter.

[1] S. Rajagopal et al., Phys. Rev. Lett. 123, 223201 (2019)

[2] G.Sun and A. Eckardt, Phys. Rev. Res. 2, 013241 (2020)   

Quantum criticality and universality in the $p$-wave paired Aubry-Andr\'{e}-Harper model

We investigate the quantum criticality and universality in Aubry-Andr\'{e}-Harper (AAH) model with $p$-wave superconducting pairing $\Delta$ in terms of the generalized fidelity susceptibility (GFS). We show that the higher-order GFS is more efficient in spotlighting the critical points than lower-order ones, and thus the enhanced sensitivity is propitious for extracting the associated universal information from the finite-size scaling in quasiperiodic systems. The GFS obeys power-law scaling for localization transitions and thus scaling properties of the GFS provide compelling values of critical exponents. Specifically, we demonstrate that the fixed modulation phase $\phi=\pi$ alleviates the odd-even effect of scaling functions across the Aubry-Andr\'{e} transition with $\Delta=0$, while the scaling functions

for odd and even numbers of system sizes with a finite $\Delta$ cannot coincide irrespective of the value of $\phi$.

A thorough numerical analysis with odd number of system sizes reveals the correlation-length exponent $\nu $$\simeq$ 1.000 and the dynamical exponent $z$ $\simeq$ 1.388 for transitions from the critical phase to the localized phase, suggesting the unusual universality class of localization transitions in the AAH model with a finite $p$-wave superconducting pairing lies in a different universality class from the Aubry-Andr\'{e} transition. The results may be testified in near term state-of-the-art experimental settings.   

Bose Glass Transition in a Two-Dimensional Optical Quasicrystal

A quasicrystal is a non-periodic yet long-range ordered structure, which provides an fascinating testing ground for a variety of phenomena, particularly those of disordered many-body systems. One such intriguing phenomenon is localization, which can typically be induced by either the presence of sufficient disorder, or strong interactions, leading to Anderson or Mott insulators respectively. Interestingly, the interplay of interactions and disorder can produce a new state, the so-called Bose glass phase.

In this talk I will report on the realisation of a two-dimensional Bose glass phase, and the observation of the localization transition driven by disorder, which was detected with both condensate fraction and coherence measurements. Remarkably, interactions can revive the coherence of the localized state, leading to a shift in the phase transition. Our findings pave the way for further studying novel phenomena of disordered systems, such as many-body localization.

    

Contour-time approach to the disordered Bose-Hubbard model in the strong coupling regime

There has been considerable recent interest in the disordered Bose Hubbard model (BHM) in recent years, particularly in the context of thermalization and many-body localization.  We develop a two-particle irreducible (2PI) strong-coupling approach to the disordered BHM that allows us to treat both equilibrium and out-of-equilibrium situations.  We obtain equations of motion for spatio-temporal correlations and explore their equilibrium solutions.  We study the equilibrium phase diagram as a function of disorder strength and discuss applications of the formalism to out-of-equilibrium situations.