Fabrice BERT1, Jean-Christophe Orain1,2, Panchanan Khuntia1,3, Lucy Clark4, Bernard Bernu5, Philippe Mendels1
1 Spectroscopies des Matériaux Quantiques, Laboratoire de Physique des Solides, bat. 510, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay, France.
2 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
3Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India
4Departments of Chemistry and Physics, School of Physical Sciences, University of Liverpool, Liverpool L69 7ZD, United Kingdom
5 LPTMC, UMR 7600 of CNRS, UPMC, Paris-Sorbonne, F-75252 Paris Cedex 05, France
Beyond the now archetypal spin liquid material herbertsmithite, we have explored different frustrated structures which open new tracks to tackle the still difficult problem of the Heisenberg antiferromagnetic model on the kagome lattice and/or avoid the also difficult problem of defects.
PbCuTe2O6 emerged recently as a unique example of a Cu2+ (S=1/2) based quantum spin liquid (QSL) with a three dimensional lattice and dominant interactions building up a 3D network of corner sharing triangles, coined “hyperkagome”. We used sub-kelvin magnetization, NMR and mSR to establish the absence of magnetic freezing down to 20mK and the persistence of slow fluctuations . The finite susceptibility at low T suggests a QSL state with a spinon Fermi Surface which evolves below 1K, as witnessed by a different relaxation regime, though still keeping the QSL character. A recent inelastic neutron scattering experiment revealed the fractional nature of the magnetic excitations forming a continuum of spinons, thus providing the last important evidence for the spin liquid nature of the ground state . Interestingly, in the materials context cumulated over 20 years, no signature of defect contribution was detected which allows further investigations of the spin textures induced by controlled spin-less substitutions.
In [NH4]2[C7H14N][V7O6F18], the V4+ ions (d1), with less Jahn-Teller effect compared to Cu2+ (3d9), form a unique S = ½ breathing kagome lattice which consists of alternating equilateral triangles, preserving the full frustration of the isotropic model and the spin liquid ground state[3,4]. Combining NMR measurement of the local susceptibility and state-of-the-art series expansion analysis, we could evaluate the ratio J’/J=0.55 for the interactions of the two sets of triangles. In line with recent theoretical results from variational methods and DMRG, we found that the spinon excitations are gapless and lead to a metallic-like low T heat capacity in this strong insulator. This experimental study should trigger novel theory approaches of the kagome problem through a J’-> J limit.
 P. Khuntia, F. Bert, P. Mendels, B. Koteswararao, A. V. Mahajan, M. Baenitz, F. C. Chou, C. Baines, A. Amato, and Y. Furukawa, Phys. Rev. Lett. 116, 107203 (2016)
 S. Chillal et al, preprint arXiv:1712.07942
 L. Clark, J.-C. Orain, F. Bert, M.A. De Vries, F.H. Aidoudi, R.E. Morris, P. Lightfoot, J.S. Lord, M.T.F. Telling, P. Bonville, J.P. Attfield, P. Mendels and A. Harrison, Phys. Rev. Lett. 110, 207208 (2013)
 J.-C. Orain, B. Bernu, P. Mendels, L. Clark, F. H. Aidoudi, P. Lightfoot, R. E. Morris and F. Bert, Phys. Rev. Lett. 118, 237203 (2017)
 C. Repellin, Y-C He, F. Pollmann, Phys. Rev. B 96, 205124 (2017); Y. Iqbal, D. Poilblanc, R. Thomale, F. Becca, Phys. Rev. B 97, 115127 (2018)