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Register a new userElectron spin resonance in S=1/2 antiferromagnets at high temperature
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We study the electron spin resonance (ESR) of low-dimensional spin systems at high temperature, and test the Kubo-Tomita theory of exchange narrowing. In finite-size systems (molecular magnets), we found a double-peak resonance which strongly differs from the usual Lorentzian. For infinite systems, we have predictions for the linewidth and lineshape as a function of the anisotropy strength. For this, we have used an interpolation between a non-perturbative calculation of the memory function at short times (exact diagonalization) and the hydrodynamic spin-diffusion at long times. We show that the Dzyaloshinskii-Moriya anisotropies generally induce a much larger linewidth than the exchange anisotropies in two dimensions, contrary to the one-dimensional case.
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Cu(C$_8$H$_6$N$_2$)Cl$_2$, a strong-rung spin-1/2 Heisenberg ladder compound, is probed by means of electron spin resonance (ESR) spectroscopy in the field-induced gapless phase above $H_{c1}$. The temperature dependence of the ESR linewidth is analyzed in the quantum field theory framework, suggesting that the anisotropy of magnetic interactions plays a crucial role, determining the peculiar low-temperature ESR linewidth behavior. In particular, it is argued that the uniform Dzyaloshinskii-Moriya interaction (which is allowed on the bonds along the ladder legs) can be the source of this behavior in Cu(C$_8$H$_6$N$_2$)Cl$_2$.
The electron spin resonance spectrum of a quasi 1D S=1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie--Weiss and Ne`{e}l temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S=1/2 antiferromagnetic spin chain with a uniform Dzyaloshinskii--Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.
Linked cluster series expansions about the Ising limit are used to study ground state preperties, viz. ground state energy, magnetization and excitation spectra, for mixed spin S=(1/2,1) quantum ferrimagnets on simple bipartite lattices in 1, 2, and 3-dimensions. Results are compared to second-order spin wave theory and, in general, excellent agreement is obtained.
Motivated by various spin-1/2 compounds like Cs$_2$CuCl$_4$ or $kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, we derive a Raman-scattering operator {it `a la} Shastry and Shraiman for various geometries. For T=0, the exact spectra is computed by Lanczos algorithm for finite-size clusters. We perform a systematic investigation as a function of $J_2/J_1$, the exchange constant ratio: ranging from $J_2=0$, the well known square-lattice case, to $J_2/J_1=1$ the isotropic triangular lattice. We discuss the polarization dependence of the spectra and show how it can be used to detect precursors of the instabilities of the ground state against quantum fluctuations.
We present a multi-frequency Electron Spin Resonance (ESR) study in the range of 4 GHz to 420 GHz of the quasi-one-dimensional, non-dimerized, quarter-filled Mott insulators, delta-(EDT-TTF-CONMe_2)_2X (X=AsF_6, Br). In the high temperature orthorhombic phase above T~190 K, the magnitude and the temperature dependence of the high temperature spin susceptibility are described by a S = 1/2 Heisenberg antiferromagnetic chain with J_AsF6=298 K and J_Br=474 K coupling constants for X=AsF_6 and Br respectively. We estimate from the temperature dependence of the line width an exchange anisotropy, J/J of ~2 * 10^{-3}. The frequency dependence of the line width and the g-shift have an unusual quadratic dependence in all crystallographic orientations that we attribute to an antisymmetric exchange (Dzyaloshinskii--Moriya) interaction.
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