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arXiv:0806.0166 · Europhysics Letters (EPL)

Controlled preparation and detection of d-wave superfluidity in two-dimensional optical superlattices

Ana María Rey, Rajdeep Sensarma, Simon Fölling, Markus Greiner, Eugene Demler, Mikhail D. Lukin

Published Sep 1, 2009• 45 citations
arXivHTMLPDF

Abstract

d -wave Cooper pairs are believed to be the key for understanding the phenomenon of high-temperature superconductivity in cuprates. These superconductors are an example of the emergence of strong pairing in systems with purely repulsive interactions, similar to superfluid helium 3 and the newly discovered iron oxypnictides. Despite intense studies, there is currently no consensus as to what causes the formation of d -wave Cooper pairs in these materials. Here we propose a novel experimental scheme in which recently demonstrated methods for realizing optical lattices and superlattices are combined to create and to detect, in a controlled way, ultracold-atom d -wave Cooper pairs. Our scheme starts from arrays of isolated plaquettes which incorporate the required d -wave correlations on a short length scale. By tuning the parameters of the potentials, these plaquettes can be coupled to achieve long-range d -wave superfluid correlations, finally arriving at the generic Hubbard model.

DOI
10.1209/0295-5075/87/60001

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d -wave Cooper pairs are believed to be the key for understanding the phenomenon of high-temperature superconductivity in cuprates. These superconductors are an example of the emergence of strong pairing in systems with purely repulsive interactions, similar to superfluid helium 3 and the newly discovered iron oxypnictides. Despite intense studies, there is currently no consensus as to what causes the formation of d -wave Cooper pairs in these materials. Here we propose a novel experimental scheme in which recently demonstrated methods for realizing optical lattices and superlattices are combined to create and to detect, in a controlled way, ultracold-atom d -wave Cooper pairs. Our scheme starts from arrays of isolated plaquettes which incorporate the required d -wave correlations on a short length scale. By tuning the parameters of the potentials, these plaquettes can be coupled to achieve long-range d -wave superfluid correlations, finally arriving at the generic Hubbard model.
This dissertation summarizes my recent work regarding systems of strongly interacting fermionic atoms in optical lattices. This work addresses the combination of two experimental techniques that have been the subject of much recent research in ultracold atom physics. One is the use of optical lattices, which provide a means to realize diverse interaction configurations within a clean, controllable system. The other is the use of magnetically tunable Feshbach resonances to control the strength of the interatomic interaction. Together, these techniques offer the possibility of an experimental realization of many important model Hamiltonians of condensed matter physics, and may also lead to the discovery of new physics. Recent study of this system has shown that strong interactions near Feshbach resonance will lead to the population of multiple lattice bands, and that collisions between atoms on neighboring sites cannot be neglected. These effects lead to a complicated Hamiltonian, but one which can be simplified to an effective single-band model equivalent to the generalized Hubbard model (GHM), which is an extension of the Hubbard model that includes correlated hopping terms. My main results concern the study of this model. The strong correlations between the particles make it difficult to definitively determine the many body physics of the GHM. As a first approach to understanding the GHM in optical lattices, I focus mainly on cases where the problem is greatly simplified by allowing interactions among only small groups of lattice sites. This restriction can be implemented in experiments using an optical superlattice potential. Our results include a proposed scheme (based on double-well superlattices) to empirically verify that the GHM describes this system and to directly measure the various parameters of this model. Other results include exact solutions on four-site square plaquettes, which demonstrate that d-wave excitations can occur in the low-energy states. By using a superlattice to give an array of weakly coupled plaquettes, one can thus produce a d-wave superfluid state. This is of relevance to the study of high-Tc superconductors, although I note certain key differences between the sort of d-wave superfluid described here and that of the superconductors.
Recent experiments on quantum degenerate gases give an opportunity for simulating strongly-correlated electronic systems in optical lattices. It may shed light on some long-standing puzzles in condensed-matter physics, like the nature of high-temperature superconductivity in cuprates that had baffled people over two decades. It is believed that the two-dimensional fermionic Hubbard model, or t-J model, contains the key to this problem; but the difficulty of unveiling the mystery of a strongly-interacting fermionic system is also generally acknowledged. Here, as a substitute, we systematically analyze the property of bosonic t-J model simulated in optical superlattices near unit-filling. In particular, we show the emergence of a strange topological Fermi liquid with Fermi surfaces from a purely bosonic system. We also discuss the possibility of observing these phenomena in ultracold atom experiments. The result may provide some crucial insights into the origin of high-T_{c} superconductivity.
Fermionic atoms confined in a potential created by standing wave light can undergo a phase transition to a superfluid state at a dramatically increased transition temperature. Depending upon carefully controlled parameters, a transition to a superfluid state of Cooper pairs, antiferromagnetic states or d-wave pairing states can be induced and probed under realistic experimental conditions. We describe an atomic physics experiment that can provide critical insight into the origin of high-temperature superconductivity in cuprates.
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