Quantum spin liquids (QSLs) lack the broken symmetries that are associated with conventional order, due to strong quantum fluctuations that originate with geometrical frustration or low dimensionality. The low energy properties of QSLs derive from the long-ranged entanglement of electronic states and the overall topology of the resulting many-body wave functions. The signature properties of a QSL are the presence of fractionalized excitations, fundamentally different from single electron states, and as well excitations that are nonlocal- often with topological character.

At present, much of what is known about QSLs comes from experimental and theoretical results on frustrated insulators. Very little is yet known about their metallic counterparts, although it is expected that their phase behaviors could be much richer. The discovery of intrinsically metallic QSLs where frustrated moments are coupled to itinerant conduction electrons has the potential to realize and to test theoretical predictions of exotic states and phases like unconventional superconductivity, ferromagnetism, and Dirac metals where excitations have photon-like dispersions.

This project seeks to identify and to study the phase stability and fundamental excitations of metallic QSL systems, focusing on metallic analogs of low dimensional and geometrically frustrated insulators.

Topics of particular interest to this program include the following:

  • Novel T=0 Phase Transitions The formation of a QSL in a metal arises at T=0 from the collapse of a magnetically ordered state, possibly involving an electronic instability where moment- bearing electrons are incorporated into the Fermi surface, or even as a fundamental change in the system’s topology. How does this quantum critical point differ from other types of T=0 phase transitions, and how do we characterize the critical modes and the quantum fluctuations associated with formation of magnetic moments in metals with strong correlations?


  • New routes to unconventional superconductivity P. W. Anderson’s RVB model played a formative role for metallic QSLs, where mobile charge forms the Cooper pairs in unconventional superconductors. Previously, the approach to obtaining metallic QSLs involved doping known QSL systems, virtually all of which are Mott insulators. The cuprates are rare examples where this approach is successful in correlated insulators. QSLs that are intrinsically metallic could potentially be a better starting point, and it is significant that there are now several candidate systems where superconductivity could feasibly coincide with the collapse of magnetic order.


  • Effects of dimensionality in QSLs Coupling frustrated magnetic subsystems with mobile charges whose dimensionality is different or the same, such as a spin chain embedded in a metal that is quasi-one dimensional or alternatively fully three dimensional. How do the fractionalized excitations and Fermi surfaces of the spin chain subsystems change as they are increasingly coupled to the metal? Are there new types of moment formation and compensation mechanisms at play in low dimensional systems?

This program combines exploratory synthesis (Aronson), with the characterization of the fundamental excitations using a combination of electron spectroscopy (Tjeng), inelastic neutron scattering (Aronson), STM (Burke), and NMR (H. Takagi and K. Kitagawa).

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