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Abstract:

LF-µSR studies have previously been used to study the diffusive 1D motion of solitons and polarons in conducting polymers. This type of study was also applied to investigating the diffusive motion of spinons in spin-1/2 antiferromagnetic chains. Recently the method has been extended to examples of 2D layered triangular spin lattices which can support quantum spin liquid states, such as 1T-TaS₂ and YbZnGaO₄. These systems are found to show spin dynamics that matches well to 2D spin diffusion, such a model being found to provide a much better fit to the data than previously proposed models for spin correlations in such systems. In YbZnGaO₄ the diffusion rate shows a clear crossover between classical and quantum regimes as T falls below the exchange coupling J. That the spin diffusion approach works well in the high T classical region might be expected, but it is found that it also works equally well in the low T quantum region where quantum entanglement controls the spin dynamics. Measurement of the diffusion rate allows a T dependent length scale to be derived from the data that can be assigned to a quantum entanglement length ξ_E. Another entanglement measure, the Quantum Fisher Information F_Q can also be obtained from the data and its T dependence is compared to that of ξ_E.

Abstract:

We study magnetism in the κ-(ET)2X family of charge-transfer salts using implanted muon spectroscopy in conjunction with detailed ab initio electronic structure calculations using density functional theory (DFT). ET stands for the electron donor molecule bis(ethylendithio)tetrathiafulvalene and X is an anion. The DFT calculations are used to establish molecular spin distributions, muon stopping sites, and dipolar field parameters, that allow us to make a quantitative interpretation of the experimental results. Materials in the κ-(ET)2X family with X = Ag2(CN)3 and X = Cu2(CN)3 have attracted particular interest, as they have the attributes of quantum spin liquids, showing no magnetic ordering down to 30 mK in zero field μSR and in NMR, despite having exchange couplings of order 200-250 K. In contrast, the material with X = Cu[N(CN)2]Cl has an antiferromagnetic (AF) ordering transition with TN in the region of 23-30 K. In order to better understand the muon spectroscopy signature of magnetism in this whole family of compounds at both low and high magnetic fields, we look in detail at the case X = Cu[N(CN)2]Cl. As the first step in our study, the spin density distribution for the ET dimer is calculated using DFT and used to simulate the 3.7 T H1-NMR spectrum of this salt, with the spectrum showing good agreement with that measured previously [K. Miyagawa, A. Kawamoto, Y. Nakazawa, and K. Kanoda, Phys. Rev. Lett. 75, 1174 (1995)0031-900710.1103/PhysRevLett.75.1174]. Best match to the data is found for antiferromagnetic interlayer ordering and an ordered moment per dimer of 0.25μB. DFT is also used to explore muon stopping sites for this salt, finding one set of sites resulting from muonium addition to C=C double bonds in the ET layer, with muons stopping in the anion layer forming another group of sites. The dipolar fields associated with each of the stopping sites is computed and these are compared with the precession frequencies observed in the ZF-μSR spectrum [M. Ito, T. Uehara, H. Taniguchi, K. Satoh, Y. Ishii, and I. Watanabe, J. Phys. Soc. Jpn. 84, 053703 (2015)0031-901510.7566/JPSJ.84.053703]. Best match to the ZF-μSR spectrum is obtained with the mode of interlayer ordering having FM character and an ordered moment per dimer of 0.31μB for muon sites in the anion layer and 0.36μB for muonium sites in the ET layer. New measurements of TF-μSR spectra for fields up to 8 T are reported and analyzed to obtain the best estimate of the magnetic order parameter under different measurement conditions, allowing us to observe the variation of TN with applied field and the field-induced transverse canting of the moments.

Abstract:

Although muons are primarily regarded as a local spin probe, they can also access the charge state of an atom or molecule via quadrupolar level crossing resonance (QLCR) spectroscopy. We use Li+TCNQ– (TCNQ = 7,7,8,8-tetracyanoquinodimethane), a simple charge transfer salt, to test the potential of this technique in molecular systems by studying the interaction of a positive muon with the TCNQ nitrogen atoms. We show that both a positive muon and muonium are able to add to the nitrogen, leading to a singlet spin state for the addition molecule. This produces a characteristic three line QLCR spectrum, with the observed line positions and intensities determined by the principal values and orientation of the electric field gradient tensor at the nitrogen. Ab initio calculation of this field gradient and the resulting QLCR spectrum give good agreement with the experiment. A nonresonant background contribution to the relaxation rate also provides evidence for spin excitations rapidly diffusing along the TCNQ chains. These reflect mobile unpaired electrons introduced by muonium addition. It is thus shown that a single set of muon measurements can be sensitive to both spin and charge degrees of freedom in the same molecular material.