Geometrical Frustration  and Quantum Criticality

in the Shastry-Sutherland Lattice System  Yb2Pt2Pb

Meigan Aronson

Stony Brook University and Brookhaven National  Laboratory


    It is increasingly believed that quantum critical points are a ubiquitous feature of many different families of correlated electron compounds, separating magnetically ordered phases from  heavy Fermi liquids. The properties of metals near QCPs are manifestly not Fermi-liquid like, and their instabilities include unconventional superconductivity and partial ‘nematic’ order. In some cases, electronic delocalization is found at or near the QCP, and it has long been considered a central feature of the physics of heavy fermions. Recently, a different sort of QCP has been proposed for heavy fermions, where strong frustration, possibly due to geometrical frustration,  prohibits magnetic order.

    We report here an experimental investigation of high quality single crystals of Yb2Pt2Pb, which forms on the geometrically frustrated Shastry-Sutherland lattice (SSL). Yb2Pt2Pb is a highly anisotropic and strongly frustrated system which orders antiferromagnetically at TN=2.07 K, with no suggestion of Kondo physics.  The B=0 ground state is a nonmagnetic singlet, due to dimer formation, resulting in a ~ 4-5 K gap between the singlet and triplet states of these dimers.  Since TN is comparable to the dimer gap, antiferromagnetic order emerges from a paramagnetic state of uncoordinated dimers, i.e.  a valence bond solid. Magnetic fields suppress TN to zero at 1.2 T,  while the dimer gap closes at 1.5 T, indicating that the antiferromagnetic phase is fully surrounded by the valence bond solid. A complex field and temperature phase diagram is found, including a mass enhanced Fermi liquid phase at high fields. Yb2Pt2Pb seems to combine many of the features that we expect from unfrustrated heavy fermions with those found in  the insulating SSL system SrCu2(BO3)2.


This research was carried in collaboration with Dr. Moosung Kim, and was supported by the National Science Foundation.

 

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