Pressure dependence of the exchange interaction in La2CuO4

We report the results of Raman scattering measurements which determine the pressure dependence of the in-plane exchange coupling constant J in magnetic LarCuO4 for pressures between lbar and 100 kbar. We find that pressure increases the Neel temperature TN much more rapidly than .I. This result together with the theory for a quasi twodimensional magnetic system then suggests that the interplanar exchange coupling decreases with pressure.

Energy (cm-l) sure at which the tetragonal to orthorhombic phase transition passes through room temperature(8).

DISCUSSION
As a result of a residual interlayer exchange interaction JI, an ensemble of two dimensional magnetic sheets having an in-plane correlation length E and staggered magnetization M* may order in three dimensions if the condition TN M JlM2E2 is met (9).Reference (9) suggests that for a choice of parameters consistent with neutron scattering experiments at 1 bar, both t and M increase with the in-plane exchange coupling J. Assuming that the renormalization constants describing the effects of the two-dimensional fluctuations are independent of pressure, the pressure dependence of TN is given by TN 0: .71(P)J3(P) exp (1.4 J(P)/knT).
With this expression in mind, it is instructive to compare the pressure dependence of TN from neutron scattering experimentas , plotted in Fig. 2, with that of J(P), deter- I""I""I""I"" mined by the present Raman scattering experiment.Note that TN has been multiplied by a factor of 3.0 in order to make it equal to J(P) at 1 bar.It is clear that while TN is more sensitive to pressure than J(P), the theoretical expression predicts a much stronger dependence of TN(P) on J(P) than is observed.This discrepancy suggests that pressure rapidly decreases Jl.Alternatively, the renormalization factors describing the effects of the two-dimensional fluctuations may be pressure dependent.More detailed investigation of the two-magnon lineshapes, which may shed light on this point, are underway.

CONCLUSIONS
We have presented here the first direct measurement of the pressure dependence of the in-plane exchange interaction J(P) in magnetic LarCuOd.We find that pressure increases J(P), but apparently either magnetically decouples the CuOs planes or fundamentally affeCt8 the two-dimensional fluctuations of the magnetic excitations of both the oxide superconductors and their insulating parent compounds have played a central role in elucidating the importance of spin fluctuations in high temperature superconductors(1).At the same time, both the superconductivity(2) and the magnetism(3) are remarkably sensitive to pressure, although the fundamental mechanism for this remains unclear.Although Raman scattering has been used to study the pressure dependence of the phonons in several of these materials(4), it has not previously been used to study the effects of pressure on the spin fluctuations.We present here the first report of the pressure dependence of the two-magnon excitation spectrum of magnetic LarCuO4, directly probed by Raman scattering.2. EXPERIMENTAL TECHNIQUES The light scattering experiments were performed on a mechanically polished single crystal of LarCuO4, grown from CuO flux.The as-grown crystal was annealed in nitrogen at 650 C for ten hours, a procedure which reduces the oxygen doping level, yielding an antiferromagnet with a measured Neel temperature of 305 K.The sample was mounted in the (110) orientation in a specially designed high pressure diamond anvil cell, using a stainless steel gasket and krypton as a pressure transmitting medium(5).The pressure was determined by the frequency shift of the luminescence of a small ruby chip placed near the sample.The Raman scattered light was analyzed with a CCD camera and a SPEX triple spectrometerRaman spectra of Br, symmetry for our oriented single crystal sample of LarCu04 at pres_ sum ranging from 1 bar to 100 kbar and laser wavelength 0 of 4579 A are graphed in Fig. 1.The data have been corrected for the wavelength dependent response of the collec-

Figure 1 :
Figure 1: Pressure dependent Raman spectra of Br, symmetry for a LarCuO4 single crystal at room temperature excited by laser light of 4579 A. The spectra are offset vertically for clarity and the separate baselines indicated.

Figure 2 :
Figure 2: Pressure dependences of the Neel temperature TN (0,Reference 3),multiplied by 3.0 for comparison to the exchange interaction J (X).Solid lines are guides for the eye.