Thereza Paiva

Observation of antiferromagnetic correlations in the Hubbard model with ultracold atoms

Ultracold atoms in optical lattices have great potential to contribute to a better understanding of some of the most important issues in many-body physics, such as high-temperature superconductivity. The Hubbard model—a simplified representation of fermions moving on a periodic lattice—is thought to describe the essential details of copper oxide superconductivity. This model describes many of the features shared by the copper oxides, including an interaction-driven Mott insulating state and an antiferromagnetic (AFM) state. Optical lattices filled with a two-spin-component Fermi gas of ultracold atoms can faithfully realize the Hubbard model with readily tunable parameters, and thus provide a platform for the systematic exploration of its phase diagram. Realization of strongly correlated phases, however, has been hindered by the need to cool the atoms to temperatures as low as the magnetic exchange energy, and also by the lack of reliable thermometry. Here we demonstrate spin-sensitive Bragg scattering of light to measure AFM spin correlations in a realization of the three-dimensional Hubbard model at temperatures down to 1.4 times that of the AFM phase transition. This temperature regime is beyond the range of validity of a simple high-temperature series expansion, which brings our experiment close to the limit of the capabilities of current numerical techniques, particularly at metallic densities. We reach these low temperatures using a compensated optical lattice technique, in which the confinement of each lattice beam is compensated by a blue-detuned laser beam. The temperature of the atoms in the lattice is deduced by comparing the light scattering to determinant quantum Monte Carlo simulations and numerical linked-cluster expansion calculations. Further refinement of the compensated lattice may produce even lower temperatures which, along with light scattering thermometry, would open avenues for producing and characterizing other novel quantum states of matter, such as the pseudogap regime and correlated metallic states of the two-dimensional Hubbard model.

Observation of spatial charge and spin correlations in the 2D Fermi-Hubbard model

Strong electron correlations lie at the origin of high-temperature superconductivity. Its essence is believed to be captured by the Fermi-Hubbard model of repulsively interacting fermions on a lattice. Here we report on the site-resolved observation of charge and spin correlations in the two-dimensional (2D) Fermi-Hubbard model realized with ultracold atoms. Antiferromagnetic spin correlations are maximal at half-filling and weaken monotonically upon doping. At large doping, nearest-neighbor correlations between singly charged sites are negative, revealing the formation of a correlation hole, the suppressed probability of finding two fermions near each other. As the doping is reduced, the correlations become positive, signaling strong bunching of doublons and holes, in agreement with numerical calculations. The dynamics of the doublon-hole correlations should play an important role for transport in the Fermi-Hubbard model.

Kondo–attractive-Hubbard model for the ordering of local magnetic moments in superconductors

We consider local magnetic moments coupled to conduction electrons with on-site attraction in order to discuss the interplay between pairing and magnetic order. We probe the ground-state properties of this model on a one-dimensional lattice through pair binding energies and several correlation functions calculated by means of density-matrix renormalization group. A phase diagram is obtained (for fixed electron density 1/3) from which we infer that coexistence between magnetic order and superconductivity is robust at the expense of a continuous distortion of the magnetic arrangement of the local moments as evidenced by a strong dependence of the characteristic wave vector

Fermions in 2D Optical Lattices: Temperature and Entropy Scales for Observing Antiferromagnetism and Superfluidity

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Ground-state and finite-temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice

on the coupling constants. This allows us to understand some trends of the coexistence, such as the influence of the rare earth on

Fermions in 3D optical lattices: cooling protocol to obtain antiferromagnetism.

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Compressibility of a Fermionic Mott Insulator of Ultracold Atoms

, as observed experimentally in the borocarbides.

Modulation of charge-density waves by superlattice structures

One of the major challenges in realizing antiferromagnetic and superfluid phases in optical lattices is the ability to cool fermions. We determine constraints on the entropy for observing these phases in two-dimensional Hubbard models using determinantal quantum Monte Carlo simulations. We find that an entropy per particle approximately = ln2 is sufficient to observe the insulating gap in the repulsive Hubbard model at half-filling, or the pairing pseudogap in the attractive case. Observing antiferromagnetic correlations or superfluidity in 2D systems requires a further reduction in entropy by a factor of 3 or more. In contrast with higher dimensions, we find that adiabatic cooling is not useful to achieve the required low temperatures. We also show that double-occupancy measurements are useful for thermometry for temperatures greater than the nearest-neighbor hopping energy.

Signatures of spin and charge energy scales in the local moment and specific heat of the half-filled two-dimensional Hubbard model

We investigate ground state and finite temperature properties of the half-filled Hubbard model on a honeycomb lattice using quantum Monte Carlo and series expansion techniques. Unlike the square lattice, for which magnetic order exists at

Effect of Inhomogeneity on s-wave Superconductivity in the Attractive Hubbard Model

T=0