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arXiv:1708.03810 · Physical Review Letters

Localized Magnetic Moments with Tunable Spin Exchange in a Gas of Ultracold Fermions

Luis Riegger, Nelson Darkwah Oppong, M. Höfer, Diogo Fernandes, Immanuel Bloch, Simon Fölling

Published Apr 4, 2018• 139 citations
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Abstract

We report on the experimental realization of a state-dependent lattice for a two-orbital fermionic quantum gas with strong interorbital spin exchange. In our state-dependent lattice, the ground and metastable excited electronic states of ^{173}Yb take the roles of itinerant and localized magnetic moments, respectively. Repulsive on-site interactions in conjunction with the tunnel mobility lead to spin exchange between mobile and localized particles, modeling the coupling term in the well-known Kondo Hamiltonian. In addition, we find that this exchange process can be tuned resonantly by varying the on-site confinement. We attribute this to a resonant coupling to center-of-mass excited bound states of one interorbital scattering channel.

DOI
10.1103/physrevlett.120.143601

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We report on the experimental realization of a state-dependent lattice for a two-orbital fermionic quantum gas with strong interorbital spin exchange. In our state-dependent lattice, the ground and metastable excited electronic states of ^{173}Yb take the roles of itinerant and localized magnetic moments, respectively. Repulsive on-site interactions in conjunction with the tunnel mobility lead to spin exchange between mobile and localized particles, modeling the coupling term in the well-known Kondo Hamiltonian. In addition, we find that this exchange process can be tuned resonantly by varying the on-site confinement. We attribute this to a resonant coupling to center-of-mass excited bound states of one interorbital scattering channel.
We report on the experimental observation of a novel interorbital Feshbach resonance in ultracold (173)Yb atoms. This opens up the possibility of tuning the interactions between the (1)S(0) and (3)P(0) metastable state, both possessing zero total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via interorbital cross thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy. Our measurements are well described by a generalized two-channel model of the orbital-exchange interactions.
Motivated by the impressive recent advance in manipulating cold ytterbium atoms, we explore and substantiate the feasibility of realizing the Coqblin-Schrieffer model in a gas of cold fermionic 173Yb^{173}\mathrm{Yb}173Yb atoms. Making use of different AC polarizabillity of the electronic ground state (electronic configuration 1S0^{1}\mathrm{S}_{0}1S0​) and the long lived metastable state (electronic configuration 3P0^{3}\mathrm{P}_{0}3P0​), it is substantiated that the latter can be localized and serve as a magnetic impurity while the former remains itinerant. The exchange mechanism between the itinerant 1S0^{1}\mathrm{S}_{0}1S0​ and the localized 3P0^{3}\mathrm{P}_{0}3P0​ atoms is analyzed and shown to be antiferromagnetic. The ensuing SU(6) symmetric Coqblin-Schrieffer Hamiltonian is constructed, and, using the calculated exchange constant JJJ, perturbative renormalization group (RG) analysis yields the Kondo temperature TK{T}_{K}TK​ that is experimentally accessible. A number of thermodynamic measurable observables are calculated in the weak-coupling regime T>TKT>{T}_{K}T>TK​ (using perturbative RG analysis) and in the strong-coupling regime T<TKT<{T}_{K}T<TK​ (employing known Bethe ansatz techniques).
We report on the experimental observation of a strongly interacting gas of ultracold two-electron fermions with an orbital degree of freedom and magnetically tunable interactions. This realization has been enabled by the demonstration of a novel kind of Feshbach resonance occurring in the scattering of two (173)Yb atoms in different nuclear and electronic states. The strongly interacting regime at resonance is evidenced by the observation of anisotropic hydrodynamic expansion of the two-orbital Fermi gas. These results pave the way towards the realization of new quantum states of matter with strongly correlated fermions with an orbital degree of freedom.
We report on the investigation of the scattering properties between the ground state 1S0^{1}S_{0}1S0​ and the metastable state 3P0^{3}P_{0}3P0​ of the fermionic isotope of 171Yb^{171}\mathrm{Yb}171Yb. We measure the sss-wave scattering lengths in the two-orbital collision channels as aeg+=225(13)a0{a}_{eg}^{+}=225(13){a}_{0}aeg+​=225(13)a0​ and ${a}_{eg}^{\ensuremath{-}}=355(6){a}_{0}$, using clock transition spectroscopy in a three-dimensional optical lattice. The results show that the interorbital spin-exchange interaction is antiferromagnetic, indicating that 171Yb^{171}\mathrm{Yb}171Yb is a promising isotope for the quantum simulation of the Kondo effect with the two-orbital system.
We report on the experimental observation of a strongly interacting gas of ultracold two-electron fermions with an orbital degree of freedom and magnetically tunable interactions.This realization has been enabled by the demonstration of a novel kind of Feshbach resonance occurring in the scattering of two 173 Yb atoms in different nuclear and electronic states.The strongly interacting regime at resonance is evidenced by the observation of anisotropic hydrodynamic expansion of the two-orbital Fermi gas.These results pave the way towards the realization of new quantum states of matter with strongly correlated fermions with an orbital degree of freedom.
Strongly correlated materials feature multiple electronic orbitals, which are crucial to accurately understanding their many-body properties. In such multiband models, quantum interference can lead to flat energy bands with large degeneracy that gives rise to itinerant magnetic phases. We report on signatures of a ferrimagnetic state realized in a Lieb lattice with ultracold fermions, characterized by antialigned magnetic moments with antiferromagnetic correlations, and concomitant with a finite spin polarization. The signatures remain robust when increasing repulsive interactions from the weakly interacting to the Heisenberg regime and emerge when continuously tuning the lattice unit cell from a square to a Lieb geometry. Our flexible approach paves the way toward exploring exotic phases, such as quantum spin liquids in kagome lattices and heavy fermion behavior in Kondo models.