Nuclear relaxation and Overhauser effect in a nearly-one-dimensional Heisenberg system

Abstract

The spin dynamics in a nearlywne&imensional Heisenberg systemthe solid free radical Tanol (2,2,6,6-tetramethyl+piperidinol-1&xyl)is studied through dynamical-nuclear-polarization {DNP) experiments (Overhauser efFect) and proton spin-lattice relaxation-time (T,) measurements. As the couplings between the electronic spins and the protons have been determined in Tanol, absolute deteraunations of the values of the electronic-spin-frequency correlation functions at the electronic Larmor frequency (co,) and at the nuclear I.armor frequency {eo")can be achieved by performing both DNP and T, measurements. The value obtained at eo, is in agreement with theoretical calculations in one-dimensional Heisenberg systems using the method of Tahir-Kheli and McFadden. The value obtained at co" together with the frequency dependence of T, are explained by introducing a cutofF efFect in the spin-difFusion process in one dimension. A derivation is given which expresses the magnitude of the cutofF efFect in terms of the interchain couplings. An evaluation of the interchain couplings, using T& measurements, is presented. The values of the interchain couplings, which are consistent with both the T, measurements and the'Neel temperature are J~J2 10 'J, where J is the intrachain exchange (J/k =4.1 K), and Ji and J2 are the interchain couplings along two axis perpendicular to the chain. Ho~ever, it cannot be excluded that, in Tanol, the disturbance of the spin-difFusion process is due to finite-length efFects.

Theoretical and experimental studies on one-dimensional magnetic systems

“…One of the most spectacular effects of the diffusive behavio~r in one dimension shows up in the spin-lattice relaxation time of nuclear spins, since insertion of (5.29) into (3.4) shows that the spin-lattice relaxation time T 1 vanishes with the square root of the Larmor frequency oJ L (Boucher et al 1974, AhmedBakheit et al 1974). This is a typical one-dimensional effect, and actually T 1 does not vanish because of weak three-dimensionM diffusion and also because of dipole interactions, which do not commute with the total spin (Boucher et al 1976).…”

“…(5) Smith and Fricdberg (1968), Skalyo et al (1970 a, b), Butterworth and Woollam (1969), Bottermann et al (1969), Nagata and Tazuki (1970), Spence et al (1969), Nagata et al (1972), Nagata and Tazuki (1972), Ferrieu (1974), Borsa and Mali (1974).…”

“…Finite temperature results were given by Griffiths (1964 c). The application of other methods to the ground state of a Heisenberg chain is discussed in the following papers : Katsura and Inawashiro (1964), Silverstein and Soos (1970), Davis (1960), Bulaevskii (1963), Yamada (1969), Todani and Kawasaki (1973). The reader is also referred to the review of Thompson…”

“…A special case of disorder is dilution by diamagnetic ions , Thorpe 1975, Hone and Petzinger 1972, Matsubara 1974 Steiner et al and the influence of such impurities on the three-dimensional transition temperature has recently been studied by Hone et al (1975). Another example is the experimental search for the theoretically predicted dimerization in certain types of chains ; the first successful experiments have been done recently, on VI_xCrxO 2 (Pouget et al 1974) and on TTFCuSaC4(CF3) 4 (Bray et al 1975). And as a last example, studies of the behaviour of one-dimensional magnets in an external field, especially the dynamics, may be mentioned.…”

Electron-proton couplings in TCNQ compounds and dimensionality of triplet excitons in (ϕ3AsCH3)(TCNQ)2at room temperature

“…Special attention has been paid to one-dimensional (l-D) systems because in these systems [(co) exhibits a divergence as co-+0. After the method had been tested in 1-D insulating magnetic systems (Heisenberg chains) [1][2][3][4][5][6][7], it was used in tetracyanoquinondimethane (TCNQ) salts [8][9][10][11][12][13][14][15][16][17], which are 1-D itinerant spin systems. The aim was to obtain information about the motion of the triplet excitons in semiconducting compounds, or about the motion of the conduction electrons in metal-like compounds.…”