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Magnetic Excitation of CuGeO3 under Applied Pressure

M. Niahi, IC. Kakurai, Y. Fujii

Neutron Scattering Z&oratory, Institute for Solid State Physics# Thr Universily of Tokyo ri E E 1 v E SEP 1 7 1997 O S T I

106-1 Shirnkata, Tohi, I b a d , 319-11, Japan

M. Yethiraj, D. A. Ternant, S. E. Nagler, 1. k Femandcz-Baca Solid State Division, Oak Ridgr NatioMl Labomtoty, Oak Ridge, ZV 37831 -6033, USA

0. F’ujita, and J. Akimitsu

Department Physics, Aoyama-Gakuin Um*verd@ Chitosedal, Setagaya-ky Tokyo 157, Japan *

Abstract 7 I-- Magnetic excitations of the sph-Pcierh compound CuC3eO3 under applied pressure of 2 GPa

have been studied. The dispersion dong the chain direction up to zone boundary has been

obtained. The spin-Peierls gap energy iuncnases to 4.2 meV and the zone baundary energy

decreases to 14.1 meV. The pres~un dependence of dispersion relation can be interpreted by

0

e3 0

e3 0 a0 tho incram of ~ - ~ ~ ~ - n ~ i ~ ~ intra-chain interaction under =lid pms~re causing

b- ofboth the spin-Peierh gap energy and transition kmperoturr.

m 7

keywords, spin-Peierls, CuGeO3, high p m m , magnetic excitation

Corresponding Author

Madcam Nidi Scattering Labaratory, BSP, Tbc U d v d V Of Tokyo

105-1, Shhhta, Tokai, Ib~raki, 319-11, Jqm Tek +81-3-3479-4892 ~f +81-29-282-5782 Fa: +81-3-3402-9449 or +81-29-282-8709

email: nishi@Prtd.issp.u-tokyo.ac.jp - OF THIS r.xxmmT Is UB6lMrnD “The submitted manuscript has been authored by a contractor of the US. Government under contract No. DE-AC05-960R22464. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish 01 reproduce the published form of the contribution, or allow others to do so, for U.S. Government purposes.”

ORNL is managed by Lockheed ? Martin Energy Research Corp. under Contract No. DE-ACO5-96OR22464 for the U.S. Department of Energy.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabli- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily collstitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof.

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During the past four years many experimental studies have been performed on the spin- Poierls (SP) compound, CuGe03. Provious neutron scattaring experiment under high

pressure, 1.8 GPa in Ref. 1 revealed that the c-axis slightly elongates, while the b-axis

remarkably shortens by a factor of 4 more than the lattice contraction due to the temperature

change from RT to 5 K. It is natura to expect that the interchain exchange interactfon Jb

inCnases and the intra-chain interaction JC remains unchanged upon applying 1.8 GPa. Inagaki

and Fukuyama [2J have calciated the phase diagram of antifenomagnOtic (AF) and SP states

by treating Jb end the spin-lattice coupling 7 in the mean field approximation. According to

this approach, SP phase becomes unstable With increasing Jbat constant . However,

experimental results by Refs.1 and 3 show the increase of SP transition temperature, T . and SP gap, & fram 14 K to 23 K and &om 2 meV to 4 meV, mpedvely with increasing

presswe. The mean field results therefore seem to bc in conflict with these pressure

oxperimms.

Thc temperahue depenawcc of the magnetic susceptibility in cuQco3 above Tqreported by

Hasa et a2. [4] cannot be well described by &e spin S=1/2 onedhendonal, nearest neighbor

Heisenberg AF model by Bonner and Fisher [SI. Riera and Dobry (61 reasonably described

the experimental CUNC of the magnetic susceptibility with nearest-neighbor (M) and next- nearest-neighbor interadtion (=), JC and J ~ c , respectively with W2d% d.36 and J-160 K

Castilla et al. [73 deduced W.24, J r l 5 0 K and dimerization parameter 64.03. The object

of this paper is to determine J2c from tiic pressure dependence in order to estimate the

importance oftbc competing intcractiom in C U M 3 SYSkm.

A CuGc03 sin& crystal with the size 5 mmg x 9 mm was grown by the traveling-solvent

floating-me method. The inelastic ncutmn-scatteIing cxpcriment was @ o d on the H8-3

triple-axis specmme!ter installed at HFIR of Oak Ridge National Laboratory. The final neutron

energy was fixed to be 13.6 meV from the (002) reflection of a pyrolytic graphite (PG) analyzer. The single crystal was mounted with the (0, k, 2) scattering plane in an aluminum

micro CCII using Fluorinca 75 BS thc ~IZSSWC-M~~~~ fluid and clamped type hi& pres sur^

cell [8] set in CT14 cryostat. The value of applied pressure was estimated consistently by the

lattice constants, b and c, of CuGe03 as compared with the data of Ref, I.

The magnetic excitation profiles at 5 K arc shown in Fig. 1. Both experimental conditions

are used witb horizontal collimation, 48'-40'-40'-120' and vertical bent PO analyzer as shown

in Fig. 1 (a) and with another one, 48-40'-100'-120' and large flat PG analyzer as shown in

Fig. 1 (b). The consistency of both conditions are checked at Q= (0,1,0.68). In Fig. 2, the

dispersion curvcs along c*-axh both under high pressure, 2 GPa and ambient pressure (Ae) arc shown together. SP gap energy at 2 GPa becomes about twice of that at AP, but the zone boundary (233) energy decreases fiom 16 mcV to 14 meV at 5 K. Just adjusting the parameters

of the dimerized chain with only the nn exchange, as calculated numerically by Bonner and

Bltrte [9] would yield the change in the dimerization parameter kO.06 to 0.17 and in the

exchange constant tiom Jfe10.5 meV (121 IC) to 9.8 m V (I 14 K) upon the pressure increase

from AP to 2 GPa.. On the other hand the structural study under high pressure [3]

unambiguously revealed that the displaccmcnt dz of the Cu atoms, believed to govern the dimerization parsmeter, does decrease upon applying the pressure. To resolve this

inconsistency we tried to estimate nnn interaction io the frame of the spin wave theory [IO] to

describe the pressure dependent dispersion including the SP gap h p phenomenologically. Jb

is estimated from the energy.dif'€ercnce at Q=(O, 0,0.5) and (0, 1,0.5). The fit &ults arc indicated by the solid lines in Fig. 2 and the ratio a d z d J ~ increased from 0.1662 f 0.0059 to

0.1802 f 0.0075 at 2 GPa. Although the values arc smaller than the critical vslue for a

finite energy gap npo&d theoretically PI, the increase of it may indicate the enhancement of

the competing interaction between JC and Jzrupon applying the pressure. A morc sophisticated

analysis of the pressure dependent dispersion including the nnn intra-chain exchange

interaction, dimerization and interchain exchange interaction is desirable. The mnfirmation of double gap and thc contribution of Jb will be reported c l s e w k [ 1 I].

This work was done at the Oak Ridge National Laboratory under US - Japan Cooperation

Program in Ncutron Scattering and supported by US DOE under contract No. DE-ACOS-

840R21400 with Martin Marietta Energy Systems, Inc.

.- -

References

(1 J M. Nishi et al.. Phys. Rev. B 52 (1995) R6959.

[2] S. hag& and H. Fukuyama, J. Phys. Soc. Jpn. 52 (1983) 3620.

131 S. Katano et al., Phys. Rev, B 52 (1995) 15364.

[SI M. Hast et at., Phys. Rev. Lea. 70 (1993) 3651.

[5] J.C. Bonncr and M.E. Fisher, Phys. Rev. 135 (1964) A640.

[6] J. R i m and A. Dob~y, Phys. Rev. B 51 (1995) 16098.

G. Castilla et al., Phys. Rev. Lett. 75 (1995) 1823.

[8] A. Onodara et al., Jpn. J. Appl. Php. 26 (1994) 152.

191 J.C. Bonncr and H.W.J. Blate, Phys. Rev. B 25 (1982) 6959. 8'

[IO] W, Marshall and S.W. Lvvesoy, Theory of Thermal Neutron Scattering, (ed. Oxford at the Clamdon R s s ) (1971) 305.

[I 1 J M. Nishi et Sl., in pieparation.

Figure Captions

Fig. 1 Neutron inelastic scattering spectra for different Q at 2 OPo and 5 R Enorgy 6caw

were done under the conditions of collimations 48'4-40'-120' and 48'-40'-100'-120' at (a) and (b), respectively. Solid lines are the results of Gaussian fitting.

Fig. 2 Ths dispersion curves along c * d s at 5 K under 2 GPa and ambient pressure. Solid

lines am the fitting curyts by spin wave formula add4 SP gap.

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Fig. 1 (a) M. Nishi et db

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Fig. 2 M. Nishi et al.

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