Dzyaloshinsky-Moriya interaction in vesignieite: A route to freezing in a quantum kagome antiferromagnet

We report an electron spin resonance investigation of the geometrically frustrated spin-1/2 kagome antiferromagnet vesignieite, BaCu$_3$V$_2$O$_8$(OH)$_2$. Analysis of the line widths and line shifts indicates the dominance of in-plane Dzyaloshinsky-Moriya anisotropy that is proposed to suppress strongly quantum spin fluctuations and thus to promote long-range ordering rather than a spin-liquid state. We also evidence an enhanced spin-phonon contribution that might originate from a lattice instability and discuss the origin of a low-temperature mismatch between intrinsic and bulk susceptibility in terms of local inhomogeneity

Dynamic Structural Flexibility of Fe-MOF-5 Evidenced by ⁵⁷Fe Mössbauer Spectroscopy

Temperature-dependent ⁵⁷Fe Mössbauer spectra were collected on Fe[subscript x]Zn[subscript 4−x](1,4-benzenedicarboxylate)₃ (Fe-MOF-5). When measured under an Ar atmosphere, the data at higher temperatures reveal thermal population of the lowest-lying electronic excited state, as expected for low symmetry tetrahedral ferrous ions. In the presence of N₂, however, the temperature dependence becomes exaggerated and the spectra cannot be fitted to a single species. A fluctuating electric field gradient at the Fe nuclei best explains these data and suggests dynamic structural distortions induced by weak interactions with N₂. This direct evidence of dynamic behaviour at MOF open metal sites is relevant for the use of MOF SBUs in catalysis, gas separation, and other applications that invoke similar phenomena

Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite

We report $^{35}$Cl NMR, ESR, $\mu$SR and specific heat measurements on the $S=1/2$ frustrated kagom\'e magnet kapellasite, $\alpha-$Cu$_3$Zn(OH)$_6$Cl$_2$, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction $J_1$ and give an energy scale for the competing interactions $|J| \sim 10$ K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term $D$, with $|D/J|\sim 3$%. These findings validate a posteriori the use of the $J_1 - J_2 - J_d$ Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagom\'e lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by $^{35}$Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and $\mu$SR in the low-$T$, correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes.Comment: 15 pages, 11 figures. To appear in Phys. Rev.

Spin dynamics and disorder effects in the S = 1/2 kagome Heisenberg spin liquid phase of kapellasite

International audienceWe report 35 Cl NMR, ESR, µSR and specific heat measurements on the S = 1/2 frustrated kagomé magnet kapellasite, α−Cu3Zn(OH)6Cl2, where a gapless spin liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbour exchange interaction J1 and give an energy scale for the competing interactions |J| ∼ 10 K. The study of the temperature-dependent ESR lineshift reveals a moderate symmetric exchange anisotropy term D, with |D/J| ∼ 3 %. These findings validate a posteriori the use of the J1 − J2 − J d Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagomé lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by 35 Cl NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and µSR in the low-T , correlated, spin liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low energy modes

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

Collective behaviour of electrons, frustration induced quantum fluctuations and entanglement in quantum materials underlie some of the emergent quantum phenomena with exotic quasi-particle excitations that are highly relevant for technological applications. Herein, we present our thermodynamic and muon spin relaxation measurements, complemented by ab initio density functional theory and exact diagonalization results, on the recently synthesized frustrated antiferromagnet Li4CuTeO6, in which Cu2+ ions (S = 1/2) constitute disordered spin chains and ladders along the crystallographic [101] direction with weak random inter-chain couplings. Our thermodynamic experiments detect neither long-range magnetic ordering nor spin freezing down to 45 mK despite the presence of strong antiferromagnetic interaction between Cu2+ moments leading to a large effective Curie-Weiss temperature of -154 K. Muon spin relaxation results are consistent with thermodynamic results. The temperature and magnetic field scaling of magnetization and specific heat reveal a data collapse pointing towards the presence of random-singlets within a disorder-driven correlated and dynamic ground-state in this frustrated antiferromagnet

Antisymmetric Magnetic Interactions in Oxo-Bridged Copper(II) Bimetallic Systems

The antisymmetric magnetic interaction is studied using correlated wave-function-based calculations in oxo-bridged copper bimetallic complexes. All of the anisotropic multispin Hamiltonian parameters are extracted using spin-orbit state interaction and effective Hamiltonian theory. It is shown that the methodology is accurate enough to calculate the antisymmetric terms, while the small symmetric anisotropic interactions require more sophisticated calculations. The origin of the antisymmetric anisotropy is analyzed, and the effect of geometrical deformations is addressed.