Emergence of spin singlets with inhomogeneous gaps in the kagome lattice Heisenberg antiferromagnets Zn-barlowite and herbertsmithite

Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate

“…Sooner or later, the system minimizes its energy by escaping from paramagnetic configurations of localized spins towards AFM or nonmagnetic states. In fact, NMR, NQR and thermal transport experiments lately provided evidence for spin-gapped singlets and the absence of fermionic spin excitations also in the paradigmatic kagome compound Herbertsmithite [23,25,121], which is subject to pronounced magneto-elastic coupling [122,123]-the pairing glue of VBS states.…”

“…Despite a rapidly growing number of candidate materials, no direct evidence for a QSL with itinerant S = 1/2 excitations [21,22], so-called 'spinons', has been reported without any doubt. Particularly for the prime QSL candidates the magnetic ground state remains controversially discussed, e.g., about the presence or absence of a spin gap in Herbertsmithite [23][24][25]. For almost two decades, κ-(BEDT-TTF) 2 Cu 2 (CN) 3 was considered to host a gapless QSL state [21,22,26].…”

Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-barlowite ZnCu$_3$(OD)$_6$FBr

“…dom around 75 K in 19 F NMR [15] and µSR experiments [19]. Therefore, the observed NQR anomalies must be attributed to the EFG, and we conclude that the structural environments at 79 Br sites become somewhat different and more disordered below ∼ 75 K. We note that the spatially averaged crystal structure observed by diffraction techniques maintains the perfect kagome symmetry down to 3 K by neutron powder diffraction and 13 K by synchrotron x-ray diffraction [17].…”

“…Likewise, on the experimental side, each spin liquid candidate material has its own complications, too, often arising from structural disorders. For example, the non-magnetic interlayer Zn 2+ sites of the kagome lattice Heisenberg antiferromagnet (KLHA) herbertsmithite ZnCu 3 (OH) 6 Cl 2 [3][4][5][6][7][8][9][10][11][12][13][14][15] and Zn-barlowite ZnCu 3 (OH) 6 FBr [15][16][17][18][19] are occupied by Cu 2+ defect spins with ∼ 15% [5] and ∼ 5% [18] probability, respectively. These defect spins have been generally believed to account for the enhanced magnetic response observed at low temperatures, and mask the intrinsic behavior of the kagome planes.…”

“…Moreover, these orphaned spins may account for the enhanced magnetic response at low temperatures [20][21][22], even if there are no interlayer Cu 2+ defect spins or spin vacancies [23] within the kagome planes. In fact, our recent 63 Cu nuclear quadrupole resonance (NQR) experiments established that spin singlets gradually emerge with inhomogeneous gaps below ∼ 30 K in both herbertsmithite and Zn-barlowite [15].…”