Spin Liquids and Antiferromagnetic Order in the Shastry-Sutherland-Lattice CompoundYb2Pt2Pb

Abstract

We present measurements of the magnetic susceptibility χ and the magnetization M of single crystals of metallic Yb2Pt2Pb, where localized Yb moments lie on the geometrically frustrated Shastry-Sutherland Lattice (SSL). Strong magnetic frustration is found in this quasi-two dimensional system, which orders antiferromagnetically (AF) at TN =2.02 K from a paramagnetic liquid of Ybdimers, having a gap ∆=4.6 K between the singlet ground state and the triplet excited states. Magnetic fields suppress the AF order, which vanishes at a 1.25 T quantum critical point. The spin gap ∆ persists to 1.5 T, indicating that the AF degenerates into a liquid of dimer triplets at T=0. Quantized steps are observed in M(B) within the AF state, a signature of SSL systems. Our results show that Yb2Pt2Pb is unique, both as a metallic SSL system that is close to an AF quantum critical point, and as a heavy fermion compound where geometrical frustration plays a decisive role.PACS numbers: 75.10. Kt,75.20.Hr,75.30.Kz Much interest has focused on systems with geometrical frustration, where conventional antiferromagnetic (AF) order is suppressed in favor of more exotic ground states. The Shastry Sutherland Lattice (SSL) is one of the simplest frustrated systems [1], consisting of planes of orthogonal dimers of moments with interdimer coupling J ′ and the intradimer coupling J. The T=0 phase diagram has two limiting behaviors, depending on J ′ /J. Nonordering dimers are found for small J ′ /J, distinguished by an energy gap ∆ between the singlet and triplet states of the dimer. Insulating SrCu 2 (BO 3 ) 2 exemplifies this disordered 'spin liquid'(SL) regime [2][3][4]. Conversely, AF order with gapless magnetic excitations is favored for large J ′ /J, and the RB 4 (R= Gd,Tb,Dy,Ho,Er) compounds may represent this limit [5][6][7][8]. A T=0 transition between the SL and AF phases has been predicted for J ′ /J ≃0.6 -0.7 [3, 9-11], although symmetry-based arguments [12]

Magnetic structure ofYb2Pt2Pb: Ising moments on the Shastry-Sutherland lattice

“…From a wholly experimental perspective, the presence of two antiferromagnetic domains would likely also lead to significant hysteresis in the temperature dependencies of the low field magnetic susceptibility when the sample is cooled into the antiferromagnetic state in either zero field [zero field cooled (ZFC)] or in a field that is larger than 3 T [field cooled (FC)], the saturation field. Our measurements find no evidence for hysteresis in the ZFC or FC susceptibilities χ (T ), or in the field dependence of the magnetization M(B), other than near the quantized magnetization steps [18][19][20].…”

“…The formation of the antiferromagnetically ordered state in these sublattices destroys the Shastry-Sutherland lattice that was suggested on the basis of the crystal structure in the ab plane [17][18][19][20]33]. Figures 3(c) and 3(d) show that each sublattice is a square lattice of dimers with parallel moments that are all Yb1 or all Yb2 type within a given plane, and that these dimers are stacked antiferromagnetically along the c axis into ladders with alternating Yb1-Yb2 rungs.…”

“…In metallic Yb 2 Pt 2 Pb, Yb moments form the orthogonal dimers of the SSL planes, and AF order appears below the Néel temperature T N = 2.07 K [17]. The exchange interactions J 5 K and J /J 1 deduced from the magnetic susceptibility χ (T ) [18] are much smaller in Yb 2 Pt 2 Pb than in the RB 4 , and a field of only 1.23 T, applied along the (110) or (−110) dimer bond directions, partially suppresses the B = 0 AF order, accompanied by quantized M(B) steps [18][19][20] that culminate in the saturation of M(B) at 3 T. The magnetic structures of SSL systems reflect the frustration of short-ranged magnetic interactions, and in Yb 2 Pt 2 Pb, where the Yb moments are large and classical with strong Ising anisotropy that confines * maronson@bnl.gov them to the SSL plane, we can expect AF states that are analogous to but distinct from those found in SrCu 2 (BO 3 ) 2 at high fields, where Heisenberg quantum spins S = 1/2 are aligned perpendicular to the SSL layers.…”

Single Crystal Growth and Anisotropic Magnetic Properties of Peanut-Shaped Cage Compound Tm6Cr4Al43

“…One of the remarkable features of this family is that the cage structure of these compounds, expressed as R 2 Tr 2 Al 24 , has a peanut shape and it includes two rare-earth ions inside. The distance between the two rare-earth ions is about 3.4 Å, which is comparable with 3.51 Å of Yb 2 Pt 2 Pd [5], indicating possible magnetic dimer formation in the cage structure. In addition, guest-ion modes of the two rare-earth ions, if exists, may have unconventional features.…”

SCES2013 Summary: Experiment

“…1, that there are other routes to a quantum-critical state. Though not easily tunable, spatial dimensionality [34], geometrical frustration of magnetic ions [41,42,43,44] and, interestingly, the lack of periodicity in quasi-crystals [45,46] lead to physical properties that are consistent with expectations of quantum criticality. Whether it will be possible to understand properties of these materials within the context of the conventional model of criticality remains to be seen, but it seems unlikely.…”