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arXiv:1308.0603 · Physical Review Letters

Realizing a Kondo-Correlated State with Ultracold Atoms

Johannes Bauer, Christophe Salomon, Eugene Demler

Published Nov 20, 2013• 84 citations
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Abstract

We propose a novel realization of Kondo physics with ultracold atomic gases. It is based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential. We show that different situations displaying Kondo physics can be realized when Feshbach resonances between the species are tuned by a magnetic field and the trapping frequency is varied. We illustrate that a mixture of 40K and 23Na atoms can be used to generate a Kondo-correlated state and that momentum resolved radio frequency spectroscopy can provide unambiguous signatures of the formation of Kondo resonances at the Fermi energy. We discuss how tools of atomic physics can be used to investigate open questions for Kondo physics, such as the extension of the Kondo screening cloud.

DOI
10.1103/physrevlett.111.215304

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•Jan 1, 2013•0 citations
We propose a novel realization of Kondo physics with ultracold atomic gases. It is based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential. We show that different situations displaying Kondo physics can be realized when Feshbach resonances between the species are tuned by a magnetic field and the trapping frequency is varied. We illustrate that a mixture of 40K and 23Na atoms can be used to generate a Kondo-correlated state and that momentum resolved radio frequency spectroscopy can provide unambiguous signatures of the formation of Kondo resonances at the Fermi energy. We discuss how tools of atomic physics can be used to investigate open questions for Kondo physics, such as the extension of the Kondo screening cloud.
We propose a simple but novel scheme to realize the Kondo effect with ultracold atoms. Our system consists of a Fermi sea of spinless fermions interacting with an impurity atom of different species which is confined by an isotropic potential. The interspecies attraction can be tuned with an s-wave Feshbach resonance so that the impurity atom and a spinless fermion form a bound dimer that occupies a threefold-degenerate p orbital of the confinement potential. Many-body scatterings of this dimer and surrounding spinless fermions occur with exchanging their angular momenta and thus exhibit the SU(3) orbital Kondo effect. The associated Kondo temperature has a universal leading exponent given by TK∝exp[-π/(3apkF3)] that depends only on an effective p-wave scattering volume ap and a Fermi wave vector kF. We also elucidate a Kondo singlet formation at zero temperature and an anisotropic interdimer interaction mediated by surrounding spinless fermions. The Kondo effect thus realized in ultracold atom experiments may be observed as an increasing atom loss by lowering the temperature or with radio-frequency spectroscopy. Our scheme and its extension to a dense Kondo lattice will be useful to develop new insights into yet unresolved aspects of Kondo physics.
We recently proposed a novel realization of Kondo physics with ultracold atomic gases and illustrated that a mixture of 40 K and 23 Na atoms has suitable properties for the generation of a Kondo-correlated state with experimentally accessible scales. This system fortuitously satisfies rather special conditions. Here we discuss an alternative realization based on confinement induced resonances which could also be applicable for other mixtures. We first explain the general principle of how to engineer the Kondo correlated state like this. Then we present results for local spectral functions from numerical renormalization group (NRG) calculations for the appropriate effective Anderson impurity model and also predict the experimentally measurable radio frequency response.
We show that a composite spin-\textonehalf{} Kondo system can be formed by coupling a superconducting quantum interference device (SQUID) to the internal hyperfine states of a trapped ultracold atomic Fermi gas. Here, the SQUID, or flux qubit, acts as an effective magnetic impurity that induces spin-flip scattering near the Fermi energies of the trapped gas. Although the ultracold gas and SQUID are at vastly different temperatures, the formation of a strongly correlated Kondo state between the two systems is found when the gas is cooled below the Kondo temperature. We find that the Kondo temperature of this hybrid system is within current experimental limits. Furthermore, the momentum distribution of the trapped fermions is calculated, which clearly shows an experimental signature of the Kondo screening length. In addition to probing Kondo physics, the controllability of this system can be used to systematically explore the relaxation and equilibration of a strongly correlated system that has been initially prepared in a selected nonequilibrium state.
The Kondo effect in condensed-matter systems manifests itself most sharply in their transport measurements. Here we propose an analogous transport signature of the orbital Kondo effect realized with ultracold atoms. Our system consists of imbalanced Fermi seas of two components of fermions and an impurity atom of different species which is confined by an isotropic potential. We first apply a $\ensuremath{\pi}/2$ pulse to transform two components of fermions into two superposition states. Their interactions with the impurity atom then cause a ``transport'' of fermions from majority to minority superposition states, whose numbers can be measured after applying another $3\ensuremath{\pi}/2$ pulse. In particular, when the interaction of one component of fermions with the impurity atom is tuned close to a confinement-induced ppp-wave or higher partial-wave resonance, the resulting conductance is shown to exhibit the Kondo signature, i.e., universal logarithmic growth by lowering the temperature. The proposed transport measurement will thus provide a clear evidence of the orbital Kondo effect accessible in ultracold atom experiments and pave the way for developing new insights into Kondo physics.
We create weakly bound Feshbach molecules in ultracold Fermi gas 40 K by sweeping a magnetic field across a broad Feshbach resonance point 202.2 G with a rate of 20 ms/G and perform the dissociation process using radio-frequency (RF) technology. From RF spectroscopy, we obtain the binding energy of the weakly bound molecules in the vicinity of Feshbach resonance. Our measurement also shows that the number of atoms generated from the dissociation process is different at various magnetic fields with the same RF amplitude, which gives us a deeper understanding of weakly bound Feshbach molecules.
Trapped cold atoms at very low temperatures, now readily accessible experimentally, can be controlled and serve as a testing ground for many theoretical physical properties which are paralleled in condensed matter systems. Ultracold trapped atom experiments show promise to recreate analogous condensed matter systems in a controlled environment and to create novel many-body quantum states of matter. The tunability of trapping laser intensities and magnetically controlled Feshbach interactions are the main tools for varying lattice hopping strengths and inter-particle interactions. In this thesis, we search for the phase of a many-body system of cold fermion atoms that can convert to boson molecules and vice versa. We then look at the microscopic details of two atoms that interact to form a quasibound molecule. Our two-atom model explains the mechanism behind the magnetically tunable interparticle interaction known as the Feshbach resonance. We find a dependence of the bound pair formation on the two-particle energy. Our results show that if a magnetic field can be fine tuned to sufficient precision, of order 10 mG, a narrow resonance can be achieved where longer lived bound states can be created. This narrow resonance reconciliates the two-body physics with the effective Hamiltonian that describes the many-body system.