Revival of the spin-Peierls transition in under pressure

Pressure and temperature dependent susceptibility and Raman scattering experiments on single crystalline  have shown an unusually strong increase of the spin-Peierls phase transition temperature upon applying hydrostatic pressure. The large positive pressure coefficient (7.5 K/GPa) - almost twice as large as for the pure compound (4.5 K/GPa) - is interpreted as arising due to an increasing magnetic frustration which decreases the spin-spin correlation length, and thereby weakens the influence of the non-magnetic Zn-substitution.

Version <span>2(June 23 1997) — printed on </span>

Introduction
============

The spin-Peierls (SP) transition@PYT74 in low dimensional magnetic compounds is the magnetic analog of the well known Peierls transition@PEI95 in low dimensional metals. The interest in this magneto-elastic phenomenon, which has been studied widely in organic compounds in the 1980's@BRA83, has been strongly renewed with the discovery of inorganic materials (@HAS93, $\alpha$-NaV$_2$O$_5$ @ISO96) exhibiting this unusual phase transition. In particular  has attracted strong experimental and theoretical attention over the past few tarabrown@example.net. There are at least three reasons for this: In the first place   allows for the growth of large single crystals which permits for experiments which were difficult to perform in the past. Secondly,  allows for well controlled substitutions (e.g. , ) which provides a useful method to study its structural, and in particular its magnetic properties. A surprising observation is that the SP phase transition is extremely sensitive to substitution (5 % of Zn substitution for Cu already makes the SP phase disappear). Equally surprising is that substitutions, which inherently increase the disorder in the system, lead to the formation of a long-range ordered antiferromagnetic phase at low temperatures@REN95 [@REG95; @HAS95; @LEM97].

In the third place  cannot be considered as a "classical" spin-Peierls system. The susceptibility in the uniform phase (T$>{\mbox{T$_{\rm SP}$}}$) strongly deviates from the Bonner-Fisher behavior @CAS95 [@RIE95] and the spin-Peierls transition temperature strongly increases upon applying hydrostatic pressure despite an apparently decreasing $nn$ exchange interaction@WIN95 [@TAK95; @BUC96]. There may be several reasons for the deviations from the classical behavior, notably a frustrating next nearest neighbor interaction@CAS95 [@RIE95], the two-dimensionality of the magnetism in the system@UHR97, and the degeneracy of the energy scales for phonon and spin excitations in  which calls for a description of   in terms of mixed spin-phonon excitations rather than more or less decoupled spin and phonon excitations. At present, however, it is still unclear which of the above effects plays the dominant role in  , although it has been shown that one can certainly not neglect the frustration in the system induced by the next nearest neighbor $(nnn)$ interactions. The values reported for the frustration in , based on one-dimensional (1D) adiabatic approximations, range from $\alpha=J_{\rm nnn}/J_{\rm nn}=0.24-0.36$@CAS95 [@RIE95], and may even be higher in  under hydrostatic pressure@BUC96 [@LOO97]. Within the 1D adiabatic approximations one then expects   to have a spin-gap, even without dimerization, which is small for $\alpha_c<\alpha<<0.5$, but increases strongly upon approaching the Majumdar-Gosh point ($\alpha=0.5$)@MAJ69.

The influence of non-magnetic impurities on one-dimensional spin-Peierls systems has been studied theoretically by several authors@FUK96 [@MAR97; @FAB97; @MOS97]. One of the main conclusions drawn from these studies is that one can not consider such an impurity as a local site defect. In fact, it is expected that a non-magnetic impurity induces local antiferromagnetic correlations over a lengthscale which is determined by the spin-spin correlation length ($\xi\sim $8-10 sites @FUK96). This enhancement of the local antiferromagnetic correlations may explain the extreme sensitivity of the SP phase to substitutions and possibly also @mosta the low temperature AF long range order in  and . Experimentally, evidence for these enhanced correlations may be found in Raman scattering data on substituted tarabrown@example.net. The Raman phonon modes activated in the SP-phase of pure   appear in substituted compounds already at much higher temperatures, and are even observed in compounds having a substitution level for which the SP-phase has completely vanished.

Pressure dependent experiments on @LOO97 [@GON96] have shown that   strongly increases upon increasing pressure ($\partial{\mbox{T$_{\rm SP}$}}/\partial p=4.5$ K/GPa), in good agreement with earlier predictions derived from zero pressure specific heat and thermal expansion experiments @WIN95 using the Ehrenfest relation ($\partial{\mbox{T$_{\rm SP}$}}/\partial p=5$ K/GPa). It has been argued @BUC96 [@LOO97] that the pressure dependence of  may be due to a strong positive pressure coefficient of the $nnn$ frustration in  ($\partial\alpha/\partial p>0$). Recently, thermodynamic experiments on substituted compounds by T. Lorenz <span>*et al.*</span> @LOR97A have led to the surprising prediction that $\partial{\mbox{T$_{\rm SP}$}}/\partial p$ of substituted compounds is enhanced by a factor of $2-3$ over that of the pure compound. Motivated by these predictions, we report here on the pressure dependence of  for a  single crystal as determined from pressure dependent susceptibility and Raman scatterig experiments. At low pressure, where the above mentioned prediction should be valid, the susceptibility experiments indeed show a strongly enhanced $\partial{\mbox{T$_{\rm SP}$}}/\partial p$, quantitatively in agreement with the predicted $\partial{\mbox{T$_{\rm SP}$}}/\partial p\approx 8.1$ K/GPa @LOR97A. In contrast to the pure compound, however, Raman scattering experiments show that the pressure dependence of  strongly deviates from a linear behavior, and approaches the pressure dependence of the pure compound for higher pressures. The remainder of this paper is organized as follows: Sections II. and III. describe the experimental results of the susceptibility and Raman scattering experiments, respectively. The subsequent section discusses these results in terms of a pressure (and thus frustration) dependent spin-spin correlation length. Finally, the last section presents the main conclusions of this paper.

susceptibility
==============

The   single crystal was grown from the melt by a floating zone method using an image furnace @REV69. For our measurements of the susceptibility under pressure we use a piece ($5 \times 3 \times 2$mm$^3$) of the crystal studied in Ref. @LOR97A. On the one hand the specific heat and thermal expansion data presented there prove the high quality of this crystal. On the other hand, using the same sample enables an unambiguous comparison between the measured pressure derivatives of T$_{\rm SP}$ and the predictions from thermodynamic properties @LOR97A.

The ac susceptibility was measured in a pressure range $0 \le p \le 0.43$ GPa with a mutual inductance bridge at a frequency of 6.1 KHz in a magnetic field of about 1 Oe. Both, the primary coil and the astatic secondary coils are located inside the teflon cup of a piston cylinder pressure cell which is similar to that described by J.D. Thompson @thompson. A methanol:ethanol (4:1) mixture was used as pressure transmission medium in order to guarantee hydrostatic pressure conditions. The pressure was controlled at low temperatures by simultaneously measuring the superconducting transition temperature of a small piece of lead.

In Fig. 1 the raw data of the ac susceptibility measurements are presented for several pressures. The overall temperature dependence of the AC signal is mainly determined by a smooth background, which does not change strongly as a function of pressure. This background signal is nearly temperature independent above about 12 K and increases at lower temperatures @frequencyduetopb. Due to this background and its pressure dependence a precise quantitative extraction of the susceptibility of  is not possible. In particular, one can not separate the Curie-like increase of $\chi$ present in doped  at low temperatures @HASE93b from the background signal. However, the pronounced anomalies at the spin-Peierls transition, which is associated with a rapid decrease of the susceptibility in a small temperature range, are clearly visible at all pressures. Besides the apparent increase of T$_{\rm SP}$ we do not find significant pressure dependencies of the susceptibility anomaly at the corresponding T$_{\rm SP}$.

As shown e.g. in Ref. @pouget the spin-Peierls transition temperature can be derived precisely from a sharp maximum of the temperature derivative of the magnetic susceptibility. The T$_{\rm SP}$ obtained in this way are shown as a function of pressure in Fig. 2. Together with the data we show a line, which represents a pressure derivative of $\partial T_{\rm SP}/\partial p = 8.1(10)$K/GPa as predicted from the Ehrenfest relation for $p\rightarrow 0$ @LOR97A. The measured, nearly linear, pressure dependence of   up to 0.43 GPa is in excellent agreement with the predicted behavior. Fitting the data obtained from the susceptibility measurements by a linear increase of T$_{SP}$ reveals an only slightly smaller slope of of $\partial T_{\rm SP}/\partial p = 7.3(3)$K/GPa. We mention, however, that this slope systematically increases with decreasing pressure range considered for the linear fit. For example, fitting the data up to 0.27 GPa yields 8.2(5)K/GPa, whereas the slope $\partial T_{\rm SP}/\partial p$ above 0.2 GPa only amounts to 6.3(7)K/GPa. Thus the susceptibility data are also consistent with a slight decrease of $\partial T_{\rm SP}/\partial p$ with increasing pressure. The susceptibility experiments are particularly suited for the low pressure region ($p<1$ GPa), where also the prediction made in Ref.@LOR97A should hold. These low pressure data are shown in Fig. 2 (filled symbols) which also displays a linear fit to the data (solid line) and the prediction made in Ref.@LOR97A (dashed line). The measured, nearly linear pressure dependence of ${\mbox{T$_{\rm SP}$}}$ clearly agrees well with the predicted behavior in this pressure range.

Comparing the data on   presented so far to the results for pure  it is apparent that the pressure derivative in the doped compound is significantly larger. In other words, the reduction of $T_{\rm SP}$ due to doping with 1.4% Zn decreases from $\simeq3$K at $p = 0$ to $\simeq 2$K at $p = 0.5$ GPa. Since an extrapolation of our data of doped would yield the unreasonable result that $T_{\rm SP}$ increases above the value of pure  at high pressures ($p {{ _> \atop ^\sim}}2$GPa), we have studied the behavior in this pressure range using Raman scattering.

Raman Scattering
================

Pressure and temperature dependent Raman scattering experiments have been performed on thin platelets ($100\times100\times40$ $\mu$m$^3$) of the same  single crystal as used for the susceptibility measurements. These platelets were mounted in a clamp-type diamond anvil cell, using a methanol/ethanol mixture as the pressure medium. The cell has subsequently been mounted in a He flow cryostat with a temperature regulation better than 1K. Polarized Raman spectra have been recorded in a quasi-backscattering geometry using the 514-nm line of an Ar$^{+}$-laser for excitation and a CCD equipped $\sc DILOR-XY$ spectrometer for detection. The polarization of incident and scattered light have both been chosen parallel to the c-axis of the orthorhombic crystal structure (<span>*i.e.*</span> along the Cu-O chains).

Typical spectra observed for different pressures at $T=7$ K are presented in Fig. 3. For atmospheric pressure one observes three strong peaks at 184, 330 and 592 cm$^{-1}$ which have been assigned to A$_g$-modes of the orthorhombic crystal structure @DEV94. The four additional modes observed at 26, 105, 226, and 370  are characteristic of the SP phase @KUR94 [@LOO96]. The two modes at 105 and 370   can be assigned to Brillouin zone boundary phonons, and are activated in the SP-phase through the magnetic and/or structural cell doubling. The low frequency mode at 26  arises due to transitions from the singlet ground state to the lowest magnetic singlet tarabrown@example.net. The energy of this mode is somewhat smaller than that of the corresponding mode in the pure compound (30 ), indicating that Zn-substituted crystals have a smaller spin-gap. This is in good agreement with the observed reduction of the transition temperature in  (${\mbox{T$_{\rm SP}$}}$\[\]$\approx$ 11 K; ${\mbox{T$_{\rm SP}$}}$\[\]$\approx$ 14 K ). Finally, the broad, asymmetric response, peaking at 226  is generally thought to arise from two-magnon scattering due to the Fleury-Loudon @FLE68 mechanism , and should reflect a weighted two-magnon density of states.

Similar to the situation in the pure compound@GON96 [@LOO97], the application of hydrostatic pressure leads to drastic changes in the observed Raman spectra. With increasing pressure the two-magnon peak shifts to lower frequencies ( 208 tarabrown@example.net GPa), reflecting an increasing magnetic frustration in the system. In contrast to this, but for the same reasons, the gap-related mode at 26 cm$^{-1}$ rapidly shifts to higher frequecies upon applying pressure. Simultaneously this mode also broadens, tarabrown@example.net GPa the singlet response is hardly observed anymore. Most likely, this broadening is due to non-hydrostatic effects at higher pressure, as is also reflected in the width of the phonon modes.

A good method to determine ${\mbox{T$_{\rm SP}$}}$ for the pure compound is to monitor the temperature dependence of the singlet tarabrown@example.net. As it is the case for the triplet spin-Peierls gap@NIS94, the energy of the singlet mode strongly decreases upon increasing temperature until it vanishes at ${\mbox{T$_{\rm SP}$}}$. Alternatively, it has been shown that also the intensity of the spin-Peierls active 370  phonon may be used to determine ${\mbox{T$_{\rm SP}$}}$ @GON96. The intensity of this mode shows a temperature behavior characteristic for a second order phase transition, <span>*i.e.*</span> an intensity which varies as the square of the order parameter. This is demonstrated for  in Fig. 4a, which shows a comparison between the temperature dependence of the intensity of the 370-phonon (filled symbols) with that of the the experimentally determined spontaneous strain (solid line)@LOR97A, which should vary as the square of the structural order parameter. For the pure compound the above mentioned methods yield essentially the same results and are in good agreement with the transition temperature obtained using other methods (e.g., thermal expansion, susceptibility, heat capacity), as shown in Refs. @WIN95 [@LOR97A; @inkommi].

Since the present data do not allow to use the singlet response as an indicator for the presence of the SP phase over the whole pressure range, we will use the intensity of the 370  mode to determine ${\mbox{T$_{\rm SP}$}}$. As an example, the inset of Fig. 4b shows the temperature dependence of this mode for $P=1.6$ GPa. Relative to the $A_g(2)$ mode at 347 , this mode, which is observed as a relatively strong Lorentzian shaped mode at $T=7$ K, rapidly looses intensity as the temperature increases, and finally disappears around ${\mbox{T$_{\rm SP}$}}=18$ K. Figure 4b shows the temperature dependence of the intensity of the 370-phonon as function of temperature (normalized to the intensity at the lowest temperature) for $p=0, 1.6,$ and $2.4$ GPa. For higher pressure ($p>1$ GPa) the intensity indeed shows the expected behavior, <span>*i.e.*</span> it qualitatively seems to reflect the squared order parameter of a second order phase transition. In contrast to this, and to the situation in the pure compound, the temperature dependence of the intensity of the 370  mode at $p=0$ does not seem to reflect the square of the structural order parameter. This becomes more clear from a comparison of the temperature dependence of the intensity of the 370- mode with the spontaneous strain measured on the same sample (Fig. 4b, solid line). Although the spontaneous strain still shows the behavior of a relatively well defined second order phase transition with ${\mbox{T$_{\rm SP}$}}=11.5$ K, the intensity of the 370  mode drops much more rapidly with temperature, and, moreover, remains non-zero up to temperatures well above the phase transition @LEMNOTE. These observations put question marks to the interpretation of a structurally induced intensity for the 370- mode in the SP phase. Indeed, the intensity of this mode in the pure compound also shows a good correlation with the square of the singlet energy (the square of the "magnetic order parameter"). This is not surprising since the singlet energy directly correlates with the SP gap $\Delta$, which itself is related to the structural order parameter $\delta$ ($\Delta=\delta^{2/3}$ within Cross-Fisher theory @CRO79). This indicates that the intensity of the 370-mode may not be induced by the structural distortion in the SP phase, but rather by the doubling of the magnetic unit cell @LOO97A. It furthermore strongly suggests that the relation between the structural distortion and the size of the SP gap may drastically change upon substitutions, strongly deviating from the Cross-Fisher prediction @CRO79.

Discussion
==========

At low pressures the transition temperature in the substituted compound indeed increases much faster with pressure ($\partial{\mbox{T$_{\rm SP}$}}/\partial p\approx 7.5$ K/GPa) as compared to the increase for the pure compound ($\partial{\mbox{T$_{\rm SP}$}}/\partial p\approx 4.5$ K/GPa) (see Fig. 2). The steep linear behavior observed for low pressures clearly cannot persist indefinitely. Indeed, the transition temperatures obtained for higher pressure from the Raman data show a pronounced non-linear behavior at higher pressures (see Fig. 5, full squares). In this regime, the pressure dependence of the transition temperature for ${\mbox{Cu$_{0.986}$Zn$_{0.014}$GeO$_{\rm 3}$}}$ approaches that of the pure  compound.

In order to get some understanding of the strong pressure dependence of ${\mbox{T$_{\rm SP}$}}$ in , consider first the strong dependence of ${\mbox{T$_{\rm SP}$}}$ on substitutions. As has been shown by, e. g., Fukuyama <span>*et al.*</span> @FUK96, a substitution at a single site does not only perturb this site, but leads to the formation of local AF ordering and suppression of the dimerization over several lattice sites. The spatial extent of the impurity is typically of the order of the spin-spin correlation length $\xi\sim(J/ \Delta)a$. This immediately explains the strong reduction of ${\mbox{T$_{\rm SP}$}}$ upon substitution, and may possibly also explain the occurrence of a long-range ordered AF ground state in the substituted crystals@FUK96 [@FAB97; @MOS97]. As has been pointed out by Mostovoy and Khomskii@MOS97, one expects the SP transition to be fully suppressed when the average distance between the impurities becomes of the order of the spin-spin correlation length. <span>*i.e.*</span> for concentrations of about 10 % ($\xi\sim8 a-10 a$).

The strong increase of ${\mbox{T$_{\rm SP}$}}$ with increasing pressure in pure ${\mbox{CuGeO$_{\rm 3}$}}$ has been discussed in terms of a strongly increasing frustration @BUC96 [@LOO97]. This may, of course, also explain part of the observed increase of ${\mbox{T$_{\rm SP}$}}$ in the substituted compound. There is, however, a second effect which arises from the increasing frustration and is of importance in the substituted compounds. As the frustration increases, one expects the spin-spin correlation length to decrease. Indeed, at $\alpha=0.5$ (the MG point) one expects the ground state to be formed by singlets pairs and to have a correlation length of exactly two spins. Numerical Density Matrix Renormalization Group calculations of the correlation length have indeed confirmed this picture @BOUNOTE. The consequence of the decreasing correlation length is that the influence of impurities is strongly suppressed upon applying pressure. This actually may explain the observed behavior of ${\mbox{T$_{\rm SP}$}}(p)$ in . Initially, the increase of the transition temperature is driven by a combination of an increasing frustration and a decreasing influence of the impurities with frustration. At higher pressure the effect of the impurities is strongly suppressed, and the increase of ${\mbox{T$_{\rm SP}$}}$ is fully driven by the increasing frustration alone. At these pressures one therefore expects only small differences between the pure and the substituted compounds, in good agreement with the observed behavior.

A further indication of the suppression of the effects of the impurities may be found from the temperature dependence of the intensity of the 370-mode (see Fig. 4b). As mentioned in the previous section, at low pressures the behavior of the intensity of this mode does not agree well with the temperature dependence of the structural order parameter. In addition, the mode remains observable at temperatures well above the phase transition. This latter observation is consistent with the idea that the mode is induced by magnetoelastic interactions and the magnetic cell doubling. If indeed the presence of impurities leads to enhanced AF correlations around the impurities, then this also leads to a local magnetic unit cell doubling, and hence to an activation of the 370- mode in the Raman spectra. Of course, at high enough temperatures the intensity should still disappear due to the decreasing correlation length, <span>*i.e.*</span> the decreasing spatial "size" of the impurities. At high enough pressure ($p>1$ GPa) the temperature dependence of the intensity of the 370- mode is again in qualitative agreement with the dependence observed for the pure compound, in good agreement with the idea that the influence of the impurities is strongly reduced for high pressures.

Conclusions
===========

In this paper we have shown, by two different methods, that the pressure dependence of the spin-Peierls transition temperature in Zn-substituted  is strongly enhanced over that of the pure compound. The initial-slope data are found to be in excellent agreement with the predictions using the Ehrenfest tarabrown@example.net. At higher pressure the pressure coefficient $\partial{\mbox{T$_{\rm SP}$}}/\partial p$ of  approaches that of the pure compound. The strong enhancement at low pressure, and the approach to the behavior of the pure compound at high pressure is in good agreement with an increasing frustration in these compounds, which leads to a decreasing correlation length and thus to a decrease of the influence of the impurities.

We gratefully acknowledge G. Bouzerar for many fruitful discussions, and for the calculations on the frustration dependence of the correlation length. This work was supported by DFG through SFB 341 and by BMBF 13N6586/8. Laboratoire de Chimie des Solids is Unité de Recherche associée CNRS n$^{o}$ 446.

<span>prsty</span>

E. Pytte, Phys. Rev. B <span>**10**</span>, 4637 (1974).

R. E. Peierls, <span>*Quantum theory of solids*</span> (Clarendon, Oxford, 1995).

J. W. Bray, L. V. Iterrante, I. S. Jacobs, and J. C. Bonner, in <span>*Extended linear chain compounds*</span>, edited by J. S. Miller (Plenum Press, New York, 1983), Vol. 3.

M. Hase, I. Terasaki, and K. Uchinokura, Phys. Rev. Lett. <span>**70**</span>, 3651 (1993).

M. Isobe and Y. Ueda, J. Phys. Soc. Jap. <span>**65**</span>, 1178 (1996).

J. P. Boucher and L. P. Regnault, J. Phys. I (Paris) <span>**6**</span>, 1939 (1996).

J.P. Renard, K. Le Dang, P. Veillet, G. Dhalenne, A. Revcolevschi, and L.P. Regnault, Europhys. Lett. <span>**30**</span>, 475 (1995).

L.P. Regnault, J.P. Renard, G. Dhalenne, and A. Revcolevschi, Europhys. Lett. <span>**32**</span>, 579 (1995).

M. Hase, N. Koide, K. Manabe, Y. Sasago, K. Uchinokura, and A. Sawa, Physica B <span>**215**</span>, 164 (1995).

P. Lemmens, M. Fischer, G. Güntherodt, C. Gros, P.G.J. van Dongen, M. Weiden, W. Richter, C. Geibel, and F. Steglich, Phys. Rev. B <span>**55**</span>, 15076 (1997).

G. Castilla, S. Chakravarty, and V.J. Emery, Phys. Rev. Lett. <span>**75**</span>, 1823 (1995).

J. Riera and A. Dobry, Phys. Rev. B <span>**51**</span>, 16098 (1995).

H. Winkelmann, E. Gamper, B. Büchner, M. Braden, A. Revcolevschi, and G. Dhalenne, Phys. Rev. B <span>**51**</span>, 12884 (1995).

H. Takahashi, N. Môri, O. Fujita, J. Akimitsu, and T. Matsumoto, Sol. St. Comm. <span>**95**</span>, 817 (1995).

B. B<span>ü</span>chner, U. Ammerahl, T. Lorenz, W. Brenig, G. Dhalenne, and A. Revcolevschi, Phys. Rev. Lett. <span>**77**</span>, 1624 (1996).

G. Uhrig, Phys. Rev. Lett. <span>**79**</span>, 163 (1997).

P.H.M. van Loosdrecht, J. Zeman, G. Martinez, G. Dhalenne, A. Revcolevschi, Phys. Rev. Lett. <span>**79**</span>, 487 (1997).

C.K. Majumdar and D.K. Ghosh, J. Math. Phys. <span>**10**</span>, 1388 (1969).

H. Fukuyama, T. Tanimoto, and M. Saito, J. Phys. Soc. Jpn. <span>**65**</span>, 1182 (1996).

G. Martins, M. Laukamp, J. Riera, and E. Dagotto, Phys. Rev. Lett. <span>**78**</span>, 3563 (1997).

M. Fabrizio and R. Mélin, Phys. Rev. Lett. <span>**78**</span>, 3382 (1997).

M. Mostovoy and D. Khomskii, preprint cond-mat 9703104.

M. Mostovoy (private communication). A.R. Go<span>ñ</span>i, T. Zhou, U. Schwarz, R.K. Kremer, and K. Syassen, Phys. Rev. Lett. <span>**77**</span>, 1079 (1996).

T. Lorenz, E. Gamper, H. Kierspel, S. Kleefisch, B. Büchner, A. Revcolevschi, and G. Dhalenne, Phys. Rev. B <span>**56**</span>, 501 (1997).

A. Revcolevschi and R. Collongues, C.R. Acad. Sci. <span>**266**</span>, 1767 (1969); A. Revcolevschi and G. Dhalenne, Advanced Materials <span>**5**</span>, 9657 (1993).

J.D. Thompson, Rev. Sci. Instrum. <span>**55**</span>, 231 (1984).

The low temperature increase of the signal is strongly frequency dependent and does not represent a magnetic susceptibility.

M. Hase, I. M. Hase, I. Terasaki, Y. Sasago, K. Uchinokura, and H. Obara, Phys. Rev. Lett. <span>**71**</span>, 4059 (1993).

J.P. Pouget, L.P. Regnault, M. Ain, B. Hennion, J.P. Renard, P. Veillet, G. Dhalenne, and A. Revcolevschi, Phys. Rev. Lett. <span>**73**</span>, 736 (1994).

S.D. Devi<span>ć</span>, M.J. Konstantinović, Z.V. Popović, G. Dhalenne, and A. Revcolevschi, J. Phys.: Condens. Matter <span>**6**</span>, L745 (1994).

H. Kuroe, T. Sekine, M. Hase, Y. Sasago, K. Uchinokura, H. Kojima, I. Tanaka, Y. Shibuya, Phys. Rev. B <span>**50**</span>, 16468 (1994).

P.H.M. van Loosdrecht, J.P. Boucher, G. Martinez, G. Dhalenne, and A. Revcolevschi, Phys. Rev. Lett. <span>**76**</span>, 311 (1996).

G. Bouzerar, A.P. Kampf, and F. Sch<span>ö</span>nfeld, preprint cond-mat 9701176.

P. A. Fleury, R. Loudon, Phys. Rev. <span>**166**</span>, 514 (1968).

M. Nishi, O. Fujita, and J. Akimitsu, Phys. Rev. B <span>**50**</span>, 6508 (1994).

T. Lorenz, U. Ammerahl, R. Ziemes, B. Büchner, A. Revcolevschi, and G. Dhalenne, Phys. Rev. B <span>**54**</span>, 15610 (1997).

Similar observations for Zn- and Si-substituted  have been reported in ref. @LEM97.

M.C. Cross and D.S. Fisher, Phys. Rev. B <span>**19**</span>, 402 (1979).

P.H.M. van Loosdrecht, J.P. Boucher, S. Huant, G. Martinez, G. Dhalenne, and A. Revcolevschi, Physica B <span>**230-232**</span>, 1017 (1997).

G. Bouzerar, private communication.

**Figure Captions**

Fig. 1: Raw data of the AC susceptibility measurements (output signals of the lock-in amplifier) for several pressures. The data are subsequently shifted by $10^{-4}$ V for clarity.\
Fig. 2: Pressure dependence of ${\mbox{T$_{\rm SP}$}}$ as determined from the susceptibility data of ${\mbox{$\rm Cu_{0.986}Zn_{0.014}GeO_3$}}$ (filled symbols). The lines gives the predicted pressure dependencies from the Ehrenfest relation (solid line:   $\partial{\mbox{T$_{\rm SP}$}}/\partial p =8.1$ K/GPa (Ref. @LOR97A). dashed line:  $\partial{\mbox{T$_{\rm SP}$}}/\partial p =5$ K/GPa @WIN95.)

Fig. 3: (ZZ) polarized Raman spectra of   recorded at T=7 K for $p=0, 1.2, 1.6,$ and $2.9$ GPa. The curves have been given an offset for clarity. The positions of observable Ar$^{+}$-laser plasma lines are denoted by an asterisk.\
Fig. 4: (a) Intensity 370-mode in ${\mbox{CuGeO$_{\rm 3}$}}$ as a function of temperature for $p=0$ (filled symbols), and $1.4$ GPa (open symbols). The solid line gives the experimental temperature dependence of the spontaneous strain at $p=0$ GPa @LOR97A. Dashes line is a guide to the eye. (b) Intensity of the 370- mode in ${\mbox{Cu$_{0.986}$Zn$_{0.014}$GeO$_{\rm 3}$}}$ as function of temperature for $p=0$ (filled symbols), $1.4$ and $2.4$ GPa (open symbols). The solid line gives the experimental temperature dependence of the spontaneous strain at $p=0$ GPa @LOR97A. Dashes lines are a guide to the eye. The inset shows the temperature dependence of the (ZZ) polarized spectrum of ${\mbox{Cu$_{0.986}$Zn$_{0.014}$GeO$_{\rm 3}$}}$ for $p=1.6$ GPa.\
Fig. 5: Dependence of  as function of pressure for  (open symbols) and  (filled symbols, triangles: obtained from susceptibility data; filled squares: obtained from Raman data) . The solid lines give the predictions for the zero pressure derivatives.

[^1]: on leave from the Institute of Physics, Academy of Sciences, Prague, Czech Republic

{'source': 'arXiv_out/cond-mat9712136_extract_press2'}