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1D, 2D … and 3D spin liquids are everywhere !

At the atomic scale, magnetism is of purely quantum origin. Most magnetic properties of materials are nonetheless described in a classical framework : the effect of quantum fluctuations usually vanishes at the macroscopic scale. Our motivation in quantum magnetism is to unravel and study exotic state of matters that defeat this paradigm and can be described only by quantum mechanics. As such, the spin liquid state appears as a macroscopic entanglement of 2 spin states, realizing a quantum alternative to the classical Neel state commonly observed in antiferromagnetic materials. On theoretical grounds the spin liquid state is favored in weakly connected spin networks, when each spin interacts with the fewest possible neighbors, a situation best realized in quasi-one dimensional materials. In two (2D) and even more in three (3D) dimensions the stability of the spin liquid state is still highly debated. Through a combination of macroscopic measurements (SQUID) and local probe techniques (μSR and NMR), we have shown that the novel PbCuTe2O6 oxide which possesses a 3D frustrated hyperkagome magnetic structure, features the main signatures of a spin liquid state. It appears to be the first spin liquid candidate on a 3D magnetic lattice with pure S=1/2 spins.

a) Magnetization of the PbCuTe2O6 compound measured in a 3He cryostat to reach low temperatures (T>0.45 K) for various applied fields. The vertical arrow points to a magnetic anomaly at 0.86K which is also detected in the heat capacity data (top right inset). Hyperkagome network of Cu ions constituting a 3D magnetic lattice (bottom left inset). b) Relaxation rate of the muon spins depicting a strong slowing down of the spin dynamics below 1K and slow fluctuations persisting at very low temperatures. c) NMR shift (symbols) proportional to the local susceptibility compared to the macroscopic susceptibility (dash) and to the theoretical expectation for the hyperkagome model.

The novel insulating quantum magnet PbCuTe2O6 is synthesized by our colleagues at National Taiwan University. The magnetic Cu2+ ions in this material interact antiferromagnetically with an interaction strength of 22 K. A first hint of the low temperature magnetic properties, close to the ground state, is provided by SQUID measurements of the thermal magnetization using a 3He cryostat. We observed no standard magnetic transition as a result of the strong frustration at play in the compound. Frustration, which helps melting any kind of long range order, arises here from the specific magnetic lattice so-called “hyperkagome”, made of corner-sharing triangles building up a 3D structure. A small anomaly around 0.86 K is nonetheless clearly visible, although it is difficult to infer whether it arises from a marginal fraction of the polycrystalline sample or if it is the subtle signature of a bulk transition which would then disqualify the possibility of a spin liquid ground state. This issue is unambiguously addressed by complementary local probe µSR (performed at the Paul Scherrer Institute,) and NMR measurements : the spin dynamics is found to slow down drastically around 1 K but no on-site frozen magnetism could be detected in the bulk of the sample, providing the first fingerprint of a spin liquid ground state. Additionally, the finite T->0 susceptibility demonstrates the absence of an energy gap in the excitation spectrum and suggests long range spin-spin correlation, thus a type of spin liquid known as critical.

Reference :

Spin Liquid State in the 3D Frustrated Antiferromagnet PbCuTe2O6 : NMR and Muon Spin Relaxation Studies
P. Khuntia, F. Bert, P. Mendels, B. Koteswararao, A. V. Mahajan, M. Baenitz, F. C. Chou, C. Baines, A. Amato, Y. Furukawa
Phys. Rev. Lett. 116, 107203 (2016).

Contact :

Fabrice Bert