ELECTRICAL TRANSITIONS IN PURE AND DOPED V2O3

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ELECTRICAL TRANSITIONS IN PURE AND

DOPED V2O3

H. Kuwamoto, H. Keer, J. Keem, S. Shivashankar, L. van Zandt, J. Honig

To cite this version:

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JOURNAL DE PHYSIQUE ColZoque C4, supplkment au no 10, Tome 37, Octobre 1976, page C4-35

ELECTRlCAL TRANSITIONS IN

PURE AND DOPED V203

(*)

H. KUWAMOTO, H. V. KEER, J. E. KEEM,

S. A. SHIVASHANKAR, L. L. VAN ZANDT and J. M. HONIG Department of Physics and Chemistry

Purdue University West Lafayette, Indiana 47907 U. S. A.

ResumB. - Nous avons mesure les proprikt6s Blectriques et thermiques de (CrsV1-~)203, 0 4 X < 0,03 et de (TizV1-z)203, 0 < X < 0,06. Le dopage au chrome (0 < X < 0,018) augmente

la temperature de la transition basse temperature et augmente la resistivit6 de la phase metallique ;

il rend la transition haute temperature mieux d6finie puis diminue l'amplitude de cette transition. Cette dernikre transition dans V203 dope a 1 % de chrome est caractQisQ par des discontinuites en forme de marche de la resistivit6 et par des pics lors des balayages en analyse thermique differentielle. L'Btude de V203 dop6 au titane montre que I'amplitude de meme que la temperature de la transition basse temperature (Tt) diminue enormement lorsque le dopage au Ti augmente. Pour 5,6 % de Ti, Tt decroit de faqon abrupte de 35 K a 0 K. L'energie d'activation de la phase semiconductrice diminue exponentiellement et tend vers zero dans cette gamme de concentration.

En rapprochant ces resultats avec d'autres (resistivite et chaleur specifique), on peut penser que la transition haute tempkrature de ( C r z V ~ - x ) ~ 0 3 n'est pas une transition de Mott mais est peutdtre une phase C. D. W. De plus, (TizV~-x)203 dont la structure n'est pas alt6rke par le dopage au Ti est un materiau interessant pour tester les theories sur la disparition brutale des transitions isolant metal.

Abstract. - Extensive electrical and thermal studies on transitions in ( C r x V ~ - x ) ~ O 3,0 X G0.03,

and in (TixV~-x)203, 0 X 0.06, have been undertaken. An increase in Cr-doping (0 < X s0.018)

raises the temperature of the low-temperature transition and elevates the resistivity of the metallic phase ; it first sharpens and later diminishes the high-temperature transition. The latter (transition) in 1 at. % of Cr-doped V203 is characterized by stepwise discontinuities in resistivity and by sharp spikes in D S c scans. Experimental work on Ti-doped V20 3 shows that the size and tempera-

ture of the low-temperature transition (Tt) diminish drastically with increasing Ti content. At a Ti concentration of ca. 5.6 at. %, Tt drops abruptly from 35 to (presumably) 0 K, The activation energy of the semiconducting phase diminishes nearly exponentially to zero in this composition range. The above findings, in conjunction with other evidence obtained from resistivity and heat capacity measurements, suggest that the high-temperature transition in ( C r x V ~ - z ) ~ 0 3 system is not readily compatible with the Mott transition model but is consistent with the existence of Overhauser charge density waves. Further, the (TizV1-z)~O 3 system, whose structure is essentially unaltered by Ti doping, provides a testing ground for theories on the abrupt disappearance of metal-insulator transitions.

1. Introductory comments.

-

Research on the electrical properties of V 2 0 3 dates back t o 1946, when Marc Foex [ l ] reported a dramatic semiconductor- metal transition in this compound a t 170 K ; changes in resistivity of 4

&

orders of magnitude were obtained on compressed tablets. These observations were confirmed i n studies o n single crystals of V 2 0 3 published by Morin [2] in 1959. More extensive measurements were reported by Feinleib and Paul [3] in 1967, who observed changes i n resistivity of 7 orders of magnitude ; these workers also drew attention to a previously undetected anomalous rise in resistivity with temperature above 350 K. Comparable data obtained by the Purdue Group [4] are shown as Curve (a) in figure 1.

(*) Research supported by National Science Foundation, MRL Program DMR7203018A04.

Room temperature resistivities (p) quoted in ref. [l-41 and by other workers all lie in the range

5 X 10-" < p

<

10-2

IR

cm, which exceed by two to four orders of magnitude the resistivities of bona Jide metals such as copper. This raises the interesting possibility that V 2 0 3 might lie a t the very edge of a crystal configuration that barely sustains itinerant electrons in very narrow bands. If so, one might, by further dilating the lattice, achieve a n insulating state in which electron correlations play a dominant role, as in Mott-Hubbard insulators. The first experiments addressed to this problem were performed in 1969 and 1970 by McWhan and associates [5] a t Bell Telephone Laboratories. Using C r 2 0 3 as a n additive t o dilate the lattice, these workers indeed found a second metal-insulator transition above 350 K. Their findings have since been duplicated elsewhere [4, 61 ; representative data of the Purdue

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C4-36 H. KUWAMOTO, H. V. KEER, J. E. KEEM, S. A. SHIVASHANKAR, L. L. VAN ZANDT AND J. M. HONIG

FIG. l .

-

Electrical resistivity ( p ) versus reciprocal temperature (7') (V1-~Cr~)z03 single crystals.

Group are shown in Curve (b) of figure 1. It is seen that the resistivity of alloys containing 1 at.

%

Cr increases by approximately 2 to 2

+

orders of magnitude from 320 to 350 K and then drops again rapidly with a further rise in temperature. We distinguish this high temperature (H-T) transition from the low temperature (L-T) transition at ca. 175 K, in which there occur changes in lattice symmetry and magnetic order. X ray diffraction experiments by the Bell Group 151 and by others [6] have shown that the H-T transition is accompanied by small shifts in lattice parameters and in atomic positions without alteration of lattice symmetry. The unit cell volume of the semiconducting phase above 400 K, exceeds that of the metallic phase. These various factors led McWhan and coworkers [5, 91 repeatedly to propose that the high temperature transformation in Cr-doped V203 is a manifestation of the Mott transition.

This view has been questioned several times, as des- cribed in several review articles [10]. More recently, Zeiger [l11 attempted to interpret the transition in terms of a shift in the degree of band overlap resulting from changes in the V-V and V-0 distances with temperature, Honig and coworkers [4] tentatively proposed that charge carriers are trapped at and severely scattered by the boundaries separating the two phases that have been identified by X-rays as

coexisting in the transition temperature range. We shall indicate below that it is questionable whether any of the above interpretations can be sustained.

In a group of related publications McWhan and associates [5, 91 as well as Kimizuka et al. [l21 were also the first to document in 1973 that incorporation of Ti into V203 progressively shifts the low temperature transition downward in temperature. The transition disappears altogether when the amount of Ti in the alloy exceeds 5.5 at

%

; the higher temperature anomaly encountered in pure V203 is also eliminated. Presumably, the metallic characteristics of (Ti,V, -,),03 for X

2

0.055 arise from the contraction

of the lattice without change in symmetry. This work was later extended by the Purdue Group [13) and is still in progress.

It is the purpose of the present paper to provide a status report of recent experimental work in these areas and to assess the current understanding of physical phenomena in V203 and its alloys with Cr203 and Ti203.

2. Heats of transition. - That an elementary application of the Mott model to the high temperature transition of (Cr,.,,Vo.gg)203 runs into problems was confirmed by recent heat capacity measurements [14]. These show that the enthalpy of the corundum (semiconducting) phase exceeds the enthalpy of the a corundum (metallic) phase by 200 J/mole Cr-V203, corresponding to an entropy change AS E S , - S , = 0.15 e. U. This value is in

good agreement with the value AS = 0.19 e. U. calculated by McWhan et al. [5, 151 who applied the Clausius-Clapeyron equation to their published phase diagram. However, the electronic entropy of the Mott insulating phase should be lower than that of the metallic phase by approximately this amount ;

i. e., the calculated loss of entropy upon localizing the degenerate electron gas is AS

--

-

0.15 e. U.

Hence, if the Mott model is invoked, an additional mechanism must be sought which compensates for this entropy loss. Such an override mechanism is to be found in the spin disordering effects that inevi- tably accompany the electron localization process. Rudimentary caIculations based on the formula

AS = R In (2 S

+

1) yield a value AS

=

2.2 e. U. ; this now is of the proper sign but exceeds the experimental value by an order of magnitude. To overcome this new problem it was originally argued 1151 that the metallic phase itsdf had a partially ordered spin configuration, but subsequent investigations using NMR techniques [l61 have failed to rovide support- ing evidence for this conjecture.

A second mechanism which could be invoked to

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ELECTRICAL TRANSITIONS I N PURE AND DOPED V203 C4-37 altered, the need to have the lattice participate to

the extent of overriding the anticipated electron localization effects requires considerable modifica- tion of the Mott transition model as propounded by its originator [19]. Thus, far-reaching modifications seem to be required just to explain the thermal data. 3. High temperature resistivity data. - A second

problem arises in conjunction with resistivity data taken at high temperature ; these are displayed in figure 2. It should first be remarked that for pure

Temperature

(K)

FIG. 2.- ~lectrical resistivity of (a) Vz03 and (6) (Vn.ggCr0.01)20 3 single crystals through the high-temperature

anomalous region.

V203 the resistivity p increases anolnalously with temperature between 300 and 500 K and thereafter remains almost steady. These findings corroborate earlier studies .at Purdue University and elsewhere [3,4,20], but arein distinct contrast to the results of the .Bell Group. [21, 221 which shows p declining exponentially with rising T above 500 K. The origin of the discrepancy is not clear, but it has been noted in our laboratory that accidental contamination of V203 by Cr can lead to this type of behaviour.

Of considerable interest in this connection is the resistivity of (Cro~o,Vo~gg)203 which above 600 K is not far above the p values encountered in the 180-300 K range where the alloy is conceded to be metallic. One must be very careful in using resistivities as an indicator concerning the presence or absence of a Matt state. It is .generally taken for granted that a Mott insulator is. characterized by a localized electron configuration. However, it is frequently over- looked that the excited states needed for electrical conduction may very well be itinerant in, character ;

this matter is referred to in several reviews by Mott

et al. [23]. Therefore, experimental evidence invoking Seebeck measurements [4] to argue .against a Mott insulating state in the alloy is inconclusive. On the

other hand, it is difficult to conceive of a Mott state so highly conducting that its electrical properties above 600 K can scarcely be distinguished from the metallic properties of the allop,at 300 K.

4. Hysteresis and domain effects.

-

A third set of

observations which bears on the transition mechanism is shown in figure 3 ; here p is plotted against T in

(V, .,Cr, .,), 0, grown by arc-rnelf~ng lechn~ques

50 300 T e m p e r a t u r e (K) L 0, (c) a -

;

? = , .

* J j J ~ j h

-

4 , t , F , 6 270 280 290 300 31 0 T e m p e r a t u r e (K) J

FIG. 3.

-

High-temperature anomaly in (V0.99Cr0.01)203 single crystal grown by arc-melting technique : (a) temperature hysteresis of electrical resistivity (p), (b) DSc scans taken with

increasing temperature ; process endothermic, and (c) D S c scans taken with decreasing temperature ; process exothermic. part (a) and the D S c scans against T in parts (b) and (c). Both involve new detailed measurements on single crystals over the interval of the high temperature transition in (Cro~o,Vo~gg),03. It is seen :

(1) That a substantial hysteresis loop is encountered in both sets of measurements, confirming earlier observations [4] ; a 50 degree interval separates the heating and cooling branches.

(2) When carried through slowly, the transition involves small, stepwise discontinuities i~ resistivity

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C4-38 H. KUWAMOTO, H. V. KEER, 3. E. KEEM, S. A. SHIVASHANKAR, L. L. VAN ZANDT AND J. M. HONIG (3) Correspondingly, the D S c scans reveal a series

of sharp spikes superposed on a smoothly varying background. Again, their number, size, and disposi- tion varies with every cycle on every specimen but the spikes are always present.

(4) These ancmalies are encountered both on single crystal specimens grown from the melt by arc transfer techniques or grown by chemical vapor transport (CVT) using TeC1, ; numerical values do depend to some extent on the method of sample preparation. These observations fail to confirm earlier speculations [22] that CVT samples grown at lower temperature might be qualitatively different in properties from samples prepared from the melt.

The simplest explanation for the discontinuities in p and the spikes in D S c scans invokes the concept of domains. The anomalies occur over the same region of temperature where the a and

p

corundum phases have been identified as coexisting ; apparently, domains of the emerging phase nucleate and grow a t the expense of the original phase. Since resistivity changes of the type displayed in figure 3 can occur only if the domain size is large (i. e. comparable to sample cross-section), these regions should be visible in optical and electron microscopy studies. Indeed, such domains have been observed by us with these techniques.

The above observations again strongly imply that the lattice is heavily involved in the high temperature transition. In particular, it is difficult to understand how a Mott transition, driven by electron correlation effects that strongly depend on electron densities and which should thus be a unique function of temperature, can exhibit the enormous hysteresis phenomena displayed in figure 3. Again, if the

P

phase were a Mott insulator, then in the transition region localized and itinerant electrons would have to coexist in the domain walls separating the two phases. This peaceful

coexistence runs counter the very essence of the Mott model : once the localized - itinerant transition or its reverse is initiated the process should go to com- pletion catastrophically.

5. Variation of electrical properties with alloy composition.

-

The variation of electrical characteristics with the composition of the alloy system poses additional problems. As first noted by Pettifer and coworkers [G] for ceramic specimens, and as confirmed in our laboratory L241 in experiments with single crystals, an increase of Cr or A1 content in the range 0.006 X 0.018 causes a rise in the resistivity of the metallic phase and a narrowing of the temperature range over which this phase is stable. Towards the upper limiting value of the x range (i. e., just before the metallic range disappears altogether), for which a curve of type (c) depicted in figure 1 is typical,

p lies in the range of 1 Cl cm. Taking n as twice the

V3+ concentration as an upper limit, this corresponds

to a charge carrier mobility of p = 2 X 1OF4 cm2/V S ;

on the basis of the numerical estimates provided in ref. 131, a mean free path 1 = 10-l' cm is calculated. Clearly, these values are inconsistent with any itinerant electron scheme. Furthermore, at 700 K the resistivity of the high temperature phase now is two orders of magnitude below that of the nzetaZZic phase. (See also ref. [22], where a similar anomaly is dis- ~ l a ~ e d . )

There are two possible ways of resolving this dilemma. One is to clarm that alloys in the composition range 0.006 x 0.018 really are not metallic. This, however, is contradicted by the temperature variation of the resistivity itself, as well as by Seebeck coefficient measurements [4] that cannot readily be reconciled with semiconducting characteristics of materials. Furthermore, NMR

51V

shifts obey the Korringa relation [16]. There is, however, some indication that the phase under consideration shows abnormal metallic characteristics : The susceptibi- lity [21,25,26] is very high and varies either according to the Curie or the Curie-Weiss Law, rather than being independent of T as would be expected for metallic Pauli paramagnetism. The second approach is to claim that the effective charge carrier density is very much lower than assumed earlier, in which event the mobility and mean free paths would become much larger and could perhaps be rendered compatible with value appropriate for narrow band metals. It will be argued below that this latter approach merits further investigation.

6. A possible model for the transitions. - In view of the foregoing discussion, a suggestion due to Overhauser (l) appears to be increasingly attractive, namely that the transitions in V,O, and its alloys be linked to the presence of charge and/or spin density waves in these materials. As a variant on the above, the transitions could also be associated with the excitonic insulator model propounded by Kohn 1271. At this early stage it is premature to assess the relative merits of each model ; we therefore discuss only briefly the implications of Overhauser's model [28].

If one or more charge density waves (CDW) existed in the alloys, pairs of gaps might open up a t positions $- Q of reciprocal space which could touch the Fermi surface. If the surface area gapped in this manner is sufficiently extensive then only a fraction of all carriers in the metal would move without impediment. The gap size and extent of gapping of the Fermi surface depend on the amplitude and, more importantly, on the Q vector of the charge density waves, both of which generally should be a function of temperature. Thus, the high temperature MI transition might be an indication either that additional CDW's are formed or else that Q has become commensurate with the lattice parameter. Either

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ELECTRICAL TRANSITIONS IN PURE AND DOPED V2O3 C4-39

case couId lead to a complete gapping of the Fermi surface. With further rise in temperature the gap size shrinks, thus explaining the restoration of temperature-independent conductivity in the alloys above 600 K. The metallic phase between 180 and 350 K could be reconciled with a narrow band metal wherein carriers move with moderate mobility, but in which the number of freely m3ving carriers is increasingly blocked by further gapping as X increaces towards

its upper limit of 0.018.

At the lower temperature range the phase of the spin-up CDW could shift with respect to that of the spin-down component. This would set up a spin density wave (SDW), which, with a wave vector commensurate with the lattice distance along

<

110

>,

could set up antiferromagnetic ordering. The hysteresis in resistivity could arise from the temperature lag in motion of the Cr atoms ; the Cr displacements should occur to minimize the coulomb energies that would arise if the charge density waves were estahlished without readjustment of atomic positions.

The main features of the electrical properties of the alloys are thus seen to accord with properties to be anticipated from Overhauser CDW and SDW. Obviously, before the implications of this model are further explored a search for the existence of these states should be initiated : Neutron, X-ray, and electron diffraction patterns should exhibit satellite peaks that cannot be indexed on any Bragg scheme. Examples of this type of experiment have recently been reported for the transition metal layer dichal- cogenides that also undergo M1 transitions. However, until such time as proof for this type of wave might be forthcoming for Cr-doped V203, the present explanation is quite speculative.

7. Effect of Ti,03 on electrical properties of V,03.

-

Incorporation of Ti203 into V203 has the same effect as compression of the lattice. As a result, the semiconductor-metal transition in V203 at low temperature is gradually reduced in temperature and ultimately eliminated with an increase of X in

(TixV,-x)203. Representative results are shown as a plot of log p vs 103/T in figure 4. The variation of transition temperature (T,), activation energy for resistivity (E,) in the semiconducting range, size of the discontinuity in resistivity (Ap) and enthalpy of transition (AH) with X is shown in figure 5 and

compared with Tt values published by workers at Bell Telephone Laboratories [5, 91. It is seen that the agreement between the sets of

T,

measurements is moderately good.

What is of interest here is that the addition of Ti,O, preserves the lattice symmetry in both the metallic and insulating phases of V203. This makes it possible to study in detail the manner in which the metal-insulator transition terminates as x approaches

FIG. 4.

-

Electrical resistivity (p) versus reciprocal tempera- ture (T) for single crystals of several compositions of

( V I - ~ T ~ ~ ) Z O ~ .

FIG. 5. - Plots of metal-semiconductor transition temperature (Tt), activation energy (&a), and discontinuity in resistivity (Ap) versus composition (X) in (V1-~Ti~)203. Triangles locate enthalpy of transition (AH). Broken line represents Tt values reported

in ref. 191.

the critical composition X, above which the alloy

system remains entirely metallic.

Examination of figures 4 and 5 shows that T, drops very nearly linearly with increasing X from 140

to 35 K ; then as X passes X, 0.055 there is an

abrupt dropoff in Tt from 35 to (presumaljly) 0 K. On the other hand, the activation energy E, diminishes

from 0.15 eV to very small values almost exponen- tially ; no sharp discontinuity is observed. Finally, the size of the discontinuity in resistivity (hp) across the metal-insulator transition drops from approximately 7

3

o. m. (orders of magnitude) at X = 0 to

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(24-40 H. KUWAMOTO, H. V. KEER, J. E. KEEM, S. A. SHIVASHANKAR, L. L. VAN ZANDT AND J. M. HONIG to zero at X = X,. Experience has shown that Ap cannot be interpreted as signifying the occurrence is a reasonably good measure of the enthalpy change of critical phenomenon for which X, locates a critical

AH, as indicated by the few A H measurements which point.

could be carried out prior to the submission of this Clearly, much more work needs to be carefully article. If this parallelism holds for X

2

0.01, figure 5 assembled before firm conclusions can be drawn

would imply that AH does not approach zero asympto- with regard to the properties of the (TixV, -,),O, tically, i. e., that the change in physical characteristics alloy system.

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