Signature of a randomness-driven spin-liquid state in a frustrated magnet

J. Khatua1 , M. Gomilšek 2,3, J. C. Orain4, A. M. Strydom 5,6, Z. Jagličić 7,8, C. V. Colin 9, S. Petit 10, A. Ozarowski 11, L. Mangin-Thro 12, K. Sethupathi 1,13, M. S. Ramachandra Rao 13,14, A. Zorko 2,3 and P. Khuntia 1,13,15

1 Department of Physics, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India.

2 Jožef Stefan Institute, Jamova c. 39, 1000 Ljubljana, Slovenia.

3 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska u. 19, 1000 Ljubljana, Slovenia.

4 Paul Scherrer Institute, Bulk MUSR group, LMU 5232 Villigen PSI, Switzerland.

5 Highly Correlated Matter Research Group, Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa.

6 Max Planck Institute for Chemical Physics of Solids, 40 Nöthnitzerstr., Dresden D-01187, Germany.

7 Faculty of Civil and Geodetic Engineering, University of Ljubljana, 1000 Ljubljana, Slovenia.

8 Institute of Mathematics, Physics and Mechanics, 1000 Ljubljana, Slovenia.

9 Institut Néel, Université Grenoble Alpes, CNRS, Grenoble 38042, France.

10 LLB, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France.

11 National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA.

12 Institut Laue-Langevin, 38042 Grenoble, France.

13 Quantum Centre for Diamond and Emergent Materials, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India.

14 Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India.

15 Functional Oxide Research Group, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India.



Geometrically frustrated magnets are defined as a subclass of magnetic materials where all pair-wise interactions cannot be satisfied simultaneously. They usually lack classical long-range order and host strong quantum fluctuations. Some of them, dubbed Quantum Spin Liquids (QSL), even host fractionalized excitations, in contrast with spin-wave excitations found in conventional magnets [1]. Despite tremendous experimental efforts, however, proven experimental realizations remain rare. Indeed, this physics is extremely sensitive to perturbations such as next nearest-neighbor interactions, or magnetic anisotropy. In this paper, we pursue an alternative route, where disorder plays a prominent role. Indeed, disorder is unavoidable in real materials. However, provided quantum fluctuations are intrinsically strong, it can act as a new prism in revealing many interesting quantum phenomena [2-4]. Recently, it has been suggested that frustrated magnets, with quenched disorder in the form of material defects or a broad distribution of exchange interaction strengths, can exhibit a randomness-induced spin-liquid state [5]. In this work, we focus on the antiferromagnet Li4CuTeO6 (henceforth LCTO). Using neutron diffraction, magnetic measurements, specific heat, muon spectroscopy, density functional theory calculations and exact diagonalization, we establish that LCTO is a realization of this enigmatic novel ground state. In particular, neutron diffraction has proven to be of primary importance in understanding and unravelling the disorder issue in this material.

Rietveld refinement of data collected at room temperature on D1B (CRG@ILL) evidences a large anti-site disorder between Li+ and Cu2+ in LCTO. Our analysis shows that the majority of Cu2+ ions (84%) at the 2d crystallographic site, form random-length spin chains, while a minority of Cu2+ ions (~7%) at defect 4g sites strongly couple to these spin chains, leading to significant frustration. This structure results in a model of randomly depleted 1D spin chains with exchange J, to which randomly occupied sites couple via an exchange J’ (see Figure c for the definition of exchange coupling constants), hence introducing strong frustration. The large and negative value of the Curie–Weiss temperature reflects strong antiferromagnetic J and J’ interactions. The data show that this compound neither undergoes a phase transition to long-range magnetic order nor spin-freezing down to at least 45 mK. Furthermore, specific heat and magnetization results reveal a data collapse behavior, which suggests the presence of a random-singlet state. Muon spin relaxation measurements corroborate a dynamic ground state, which is attributed to the presence of subdominant interchain interactions that couple the chains in a disordered network. Our results thus establish that LCTO hosts a randomness-induced spin-liquid-like state in a frustrated magnet

This work was carried out under the supervision of P Khuntia. Claire Colin, Lucile Mangin-Thro and Sylvain Petit performed the D1B neutron scattering experiments and their analysis.

Fig.1: Crystal structure and spin model of LCTO. (a)Rietveld refinement of neutron-diffraction data at room temperature for the incident wavelength 1.28 . The solid circles represent the observed intensity (Obs.), whereas the black solid line is the calculated intensity (Cal.). (b) Visualization of one unit cell of LCTO where edge-sharing TeO6 and CuO6 octahedra connect Cu2+ ions on 2d (Cu2) sites with an exchange J through a Cu–OO–Cu super-superexchange (SSE) bridge around Te6+. Additionally, corner-sharing CuO6 octahedra connect Cu2+ ions on 2d (Cu2) and 4g (Cu3) sites with an exchange J’ through a nearly-linear Cu–O–Cu superexchange (SE) bridge. (c) Resulting spin model of randomly depleted 1D spin chains of Cu2 sites with antiferromagnetic exchange J, to which randomly occupied Cu3 sites couple via an antiferromagnetic exchange J’.  


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