Z. Phys. B 96, 313-318 (1995) ZEITSCHRIFT FOR PHYSIKB Springer-Verlag 1995

Anisotropic optical properties of single-crystal GdBa2Cu307_ a V. Zelezny 1, D.B. Tanner 1, K. Kamarfis 2, L.P. Kozeeva 3, A.A. Pavlyuk 3 1Department of Physics, University of Florida, Gainesville, FL 32611, USA 2 Research Institute for Solid State Physics of the Hungarian Academyof Sciences, Budapest, Hungary 3 Institute of Inorganic Chemistry, Lavrenteva 3, 630090 Novosihirsk, Russia Received: 20 June 1994

Abstract. The optical spectrum of reduced-Tc does not permit a system to be metallic in two directions GdBa2Cu307_~ has been measured for polarizations par- and insulating (or localized) in a third [2, 3]. allel and perpendicular to the ab plane. The sample was Electrical transport measurements find that the c-axis an oxygen-deficient single crystal with a large face con- has a much smaller conductivity than the ab plane [4]. taining the c axis. The polarized reflectance from this face In a few samples, notably fully oxygenated YBazCu30 7_~, was measured from 20 300 K in the spectral region from the electrical resistance appears to be rather metallic 30-3000cm-1, with 300K data to 30000cm-1. (dpc/dT > 0) along c, suggesting anisotropic three-di- Kramers-Kronig analysis was used to determine the mensional behavior [5, 6]. However other YBa2CU3OT_ ~ spectral dependence of the a b and the c components of samples and other oxide superconductors are more two the dielectric tensor. The optical properties are strongly dimensional, with resistance metallic in the ab plane and anisotropic. The a b-plane response resembles that of oth- insulating along c [7]. Indeed, Forro et al. [8] argue on er reduced-Tc materials whereas the c axis, in contrast, the basis of pressure studies that the c-axis transport is shows only the presence of several phonons. There is a diffusive even in samples for which dpc/dT is positive. complete absence of charge carrier response along c Polarized optical spectroscopy can provide important above and below T~. This observation allows us to set an information about the dimensionality, estimating not only upper limit to the free-carrier spectral weight for trans- the conductivity but also the oscillator strength or spec- port perpendicular to the CuO2 planes. tral weight for transitions along principal directions. These measurements are difficult for the c axis, because PACS: 74.30.Gn; 74.70.Vy; 78.30.Er; 78.65.Ez crystals of this family have the habit to grow in thin plates with minimum dimension along c while thin films tend to grow with the c axis normal to the substrate surfaces. Nevertheless, there have been several c-axis polarized I. Introduction measurements reported. Room temperature measure- ments on Laz_xSrxCuO4 samples which were metallic [9] The amount of anisotropy between the highly-conducting and even superconducting [10] and on metallic LazNiO4 ab planes and the perpendicular c axis in the oxide su- samples [ 11] find essentially insulating behavior along c. perconductors is important because it is related to the In this context, "insulating behavior" means the absence effective dimensionality of these materials. In turn, the of electronic aborption in the far-infrared region, so that dimensionality has implications for the theoretical ap- the spectrum is dominated by features associated with proach that describes the electronic structure. For ex- optical phonons. Tamasaku et al. [10] showed that below ample, if the high T c superconductors are basically an- Tc superconducting samples of Laz_xSrxCuO4 develop a isotropic three-dimensional solids, then one can imagine sharp plasma edge - a measure of the superfluid density. them to have a Fermi surface and may take a Fermi- Recent measurements of Bi2Sr2CaCu208 find highly in- liquid approach towards their properties. In this context, sulating c-axis reflectance, both above and below T~ [12]. we note that band-structure calculations [ 1] give metallic YBa2Cu307_ ~ is an important material for the study behavior in all three directions. If, in contrast, they are of the c axis, on account of the variation in c axis trans- highly two dimensional, then the Fermi-liquid approach port that can be obtained, either by doping or by vari- may not be appropriate. Conventional electronic theory ation in the oxygen content & Several studies have re- ported metallic reflectance for e-axis polarization in * Permanent address: Institute of Physics, CSAV, Prague, YBazCu307_ ~. Bozovic et al. [13] reported a strongly- Czechoslovakia damped plasma edge near 0.1 eV from reflectance mea- 314 surements of YBa2Cu307_ o crystals using an infrared H. Experimental details microscope. Kamba et al. [ 14] reported c-polarized spec- tra for films grown on (110) SrTiO 3 substrates; however, The GdBa2Cu307_ o crystals were grown by spontaneous this experiment was not completely successful on account crystallization from a high-temperature melt consisting of imperfect orientation of the c axis. There are other of 0.7% Gd203, 29% BaO, and 70% CuO. This Cu-rich papers reporting on anisotropy in the near Jr-visible-near and Gd-poor mixture was heated in an alumina crucible uv regions [15-19]. to 1060 C and held there for 5 h. The system was then In the far infrared region, Collins et al. [20] claimed cooled at a rate of 10 degrees/day to 940 C. The remain- to measure the anisotropy of the superconducting gap on ing liquid was decanted while still in the oven, to avoid a mosaic of about 12 crystals with polished surfaces. Per- thermal stress on the crystals. After that, the crystals haps the first reliable single crystal data were presented were cooled to room temperature at a rate of 10-20 de- by Koch et al. [21]. Above Tc an overdamped plasma grees/h. Crystals obtained by this method ranged in size edge seems to appear in the c-axis spectrum while below up to a maximum of 5 • 4 • 1.5 mm 3. Elemental analysis Tc a very sharp plasma-like reflectance edge occurs at showed that the crystals had acquired a considerable 100 cm -1. Homes et al. [22] have measured reduced Tc amount of A1 from the cruicible; the composition was crystals of YBa2Cu307_ ~ (Tc~ 63 K). c-axis polarized found to be GdBal 8CuzsA10.2507_ ~. Further details of electronic absorption is seen in the far infrared for both the growth of these crystals will be presented elsewhere the normal and superconducting states. Below To, there [26]. is a dramatic sharpening of a plasma-like edge at about Several techniques were used to characterize our 60 cm- 1 in the reflectance and evidence for a pseudogap samples. The dc electrical resistance shows two super- at about 200 cm-1 in the optical conductivity. Above Tc conducting drops: one partial at 90 K and the second to the plasma edge is overdamped and the pseudogap ab- zero resistance at 30 K. The magnitude of the resistance sent. is quite high in comparison to high quality samples. The The effect of oxygen variation on the c-axis spectra of optical measurements, discussed below, suggest that the YBa2Cu307_ ~ has been measured by Dfirrler et al. [23] room-temperature resistivity is ,-~ 700 ~-cm, compared and Cooper et al. [24, 25]. The data were taken at room to ,-~200 ~tO-cm for fully oxygenated crystals or films of temperature. Both measurements show a strongly damped YBazCu307 . electronic band for 0 ~ 0 whose strength decreased with The magnetic susceptibility, which we also measured, increasing 0. One thing to note is that at all values of shows a small deviation from a paramagnetic 1/T de- the c-axis spectra clearly show optical phonon features, pendence at 87 K. The oxygen content 0 was estimated so the presence of phonon signatures are not by them- from the Raman spectrum [27, 28]. From this we con- selves evidence for "insulating" behavior; instead, one cluded that the best description of our crystals would be looks for small (or zero) optical conductivity at frequen- by the formula GdBazCu306. 4. This value means that in cies between the phonon features or for near zero reflec- the phase diagram they are located in the vicinity of the tance at the longitudinal-optical phonon minima in the metal/insulator transition but on the metallic side. (The reflectance. (The latter indicates a small value of o- 1(o)) position of this transition is now considered to be at O6~35 at the longitudinal optical frequency.) By these criteria, [-29, 30].) The estimated oxygen content is consistent with only the tetragonal YBa2Cu306 spectra of Dfirrler et al. the lower transition in the resistivity, so we take Tc to be [23] can be said to be highly insulating. 30 K. Observation of the crystal surfaces under a polar- In this paper we report the anisotropy of the far-in- izing microscope revealed ab microtwinning, which pre- frared reflectance parallel to and perpendicular to the ab vented us from distinguishing anisotropy in the ab plane. plane of GdBa2Cu307_ a . Our samples are deficient in This observation, however, demonstrates that the sample oxygen content and inadvertently doped with A1, so that had the orthorhombic crystal structure necessary for su- the transition temperature is --~30 K. This value puts it perconductivity in the RBazCu307 ~ system. at the low end of the range where superconductivity oc- We made reflectance measurements on two faces of curs in the RBa2Cu307_ a system. The crystal studied in the crystal, one which contained the ab plane and one detail was unusually large, 5•215 1.5 mm 3, and had a which contained the c axis, using a Bruker IFSll3v facet with excellent optical quality containing the c axis, interferometric spectrometer over 50-5000 cm- 1 allowing polarized reflectance measurements to be made (0.006-0.6 eV) and using Perkin-Elmer 16U spectrometer without recourse to mosaics, polishing, or microscopes over 1000-30 000 cm-1 (0.12-3.7 eV). For our measure- with their steeply converging beams. ments we used the natural crystal surface unmodified by GdBa2Cu307_ ~ is isomorphic with YBa2Cu307 _ ~; the polishing or any other treatment. The reflectance was only difference is the substitution of Gd for Y. As is well measured relative to an A1 reference mirror and corrected known, this substitution has minimal effect on the phys- for the known reflectance of A1. After measurements, the ical properties, including superconductivity, with the ex- surface of our crystal was coated with A1 and the reflec- ception that the unpaired electrons of the Gd 3+ ions give tance of the coated sample measured, in order to deter- a strong paramagnetic susceptibility and undergo a phase mine accurately the sample area and to estimate the dif- transition at 2.2 K into an antiferromagnetic state. fuse scattering due to any imperfections in the surface. A continuous-flow cryostat was used to cool the sample to a base temperature of 20 K. 315

III. Results 1.0 I I I I I 20 K 50 K The reflectance from the facet containing the c-axis is 100 K 0.8 150 K completely different for the two polarizations, as shown 200 K in Figs. 1-3. Figure 1 shows the 300 K reflectance over ...... 250 K 300 K 100-30 000 cm -1 (0.012-3.7eV) for both the ab-plane 0 0 0.6 and the c-axis directions. The ab-plane reflectance shows .+-O a rapid decrease with increasing frequency. There is a (9 weak structure around 5000cm -t (0.6eV) and an- _.m 0.4 r'v"

Photon Energy (eV) 0.2 0 1 2 3 1.0

0.C I I I I I 200 400 ~O(~600 ) 800 1000 1200

Fig. 3. The reflectance of GdBazCu307_ ~ for radiation polarized 0.8 __ ab plane c axis along the c axis

0.6 0 other around 12 000 cm -1 (1.5 eV). The data, including E o these weak features, resemble closely the Tc=30K YBa2Cu307_ 6 sample reported by Orenstein et al. [31].

N-- The c-axis spectrum is quite flat. The low frequency struc- Q) O2 0.4 ture is associated with optical phonons. In addition, there are broad bands around 8000cm -1 and 16000cm -1 (1 and 2 eV, respectively) and a minimum at our upper frequency limit. The data differ strongly from the c-axis reflectance of the tetragonal phase (0>~0.8), which is 0.2 dominated by a strong feature at 3.8 eV [19]. In partic- ular, Kircher et al. [19] show that there is a rising re- flectance over 2-3.8 eV in the highly oxygen deficient material whereas samples with small values of 5 have a 0.0 decreasing reflectance in this range. These comparisons 0 10000 20000 30000 give us confidence in our characterization of the sample (cm as reduced To, superconducting GdBa2Cu307_ 6. Fig. 1. Polarized reflectance of GdBa2Cu307_ ~ at room temperature Figure 2 shows the ab-plane-polarized reflectance at temperatures between 20 and 300 K. These spectra are similar to those reported in many previous measurements [31-33]. At the lowest temperatures, the reflectance is 1.0 close to unity below 150 cm-1. However, the character- istic shoulder or knee around 430 cm- 1 that is commonly seen in the E [] ab spectrum does not occur in the spectra

o, of Fig. 2. Weak features at 150, 350, and 560cm -1 are due to phonons, visible on account of the reduced carrier concentration in this sample. As temperature is increased, the reflectance drops, with the decrease largest at the lowest frequencies. In contrast to the electronic absorption which domi- CK 50 K --~ 100 K nates the ab-plane optical properties, the reststrahlen 150 K bands of optical phonons are the principal features of the 0.7 200 K ...... 250 K c-axis reflectance, shown in Fig. 3. This reflectance is 300 K characteristic of a semiconductor or insulator, with quite deep minima at the longitudinal optical frequencies. As 0.6 ~ P I I I co--*0, the reflectance decreases and approaches a con- 200 400 ~(~600 800 1000 200 stant value ~--0.4. There is insignificant contribution of free carriers to the spectrum. There is also no clear Fig. 2. The reflectance of GdBa2C%OT_ ~ for radiation polarized evidence for appearance of the midinfrared absorption along the ab plane well-known from ab-plane measurements. The tempera- 316 ture dependence of the spectra is practically negligible; 2000 I there is only a small amount of sharpening of the phonons 20 K 100 K at low temperatures. In particular, the spectra are the 300 K same above and below the temperature where the material 1500 is superconducting. The onset of superconductivity has no effect on the e-axis spectrum in the 30-3000 cm-1 range. 'E o 1000 T IV. Kramers-Kronig analysis b A. ab-plane results see E We used Kramers-Kronig analysis to determine the op- tical conductivity, o"1 (co), and the real part of the di- electric function, e I (co), from our reflectance data. We 0 I I , I ' F ' I r I ' 0 200 400 600 800 1000 1200 used different low-frequency extrapolations for the two ~(Cm -1 ) polarizations: for Ej]_qb we used a Hagen-Rubens Fig. 5. The ab-plane optical conductivity a I (co) of a crystal of formula ~ = 1-A ]/oJ above Tc. On account of the GdBa2Cu3OT_a ,-~ 100% reflectance, below T~, the reflectance was ex- trapolated as ~ = 1 in this temperature range. For Ell c we used a constant reflectance below the lowest measured frequency [34]. At high frequencies a power law con- 200 I ~ I I I tinuation was used. 20 K ...... 300 K The ab-plane dielectric function, shown in Fig. 4, is 150 negative at low frequencies, as expected for a conductor, with a zero-crossing at 3700 cm-a in the near infrared. At low temperatures, el(co) is well described by 100 el (co) = e~ -cop2/co2, as has been shown previously [34].

For our sample, we find cop~5700cm -~. A derivative- ~- 50 like structure from the ab-plane optical phonons can also S be seen superimposed on the electronic background. 0 The ab-plane optical conductivity, shown in Fig. 5, is similar to those published earlier for superconducting samples. Above T~ there is a low-frequency upturn due -50 to the response of the charge carriers, which is Drude- like at low frequencies and which has a non-Drude fall- -100 I I I I , I 200 400 600 800 1000 1200 off in the midinfrared range. With decreasing tempera- ~(Cm -1 ) ture, the zero-frequency conductivity increases and the width of the low frequency upturn narrows until T~ is Fig. 6. The dielectric function el(co ) for ERe of a crystal of GdBazCu307_ reached. Below To, the low-frequency conductivity is

sharply reduced as the charge carriers condense into the 100 r I i I superfluid. Several sharp ab-plane phonon modes can be seen superimposed on the electronic background of Fig. 5.

-1 O0 The frequencies of these phonons are listed in Table 1.

B. c-axis results -300 For Ell c, the real dielectric function, shown in Fig. 6 for our highest and lowest measurement temperatures, is -500 positive everywhere except in the phonon reststrahlen bands, between transverse and longitudinal phonon fre- 20 K 100 K quencies. The low-frequency limiting value is el (0)= 21 -700 ...... 300 K while the midinfrared value is ~'opt ~-7. Most of the os- cillator strength is in the lowest band at 148 cm-1. The values for ~1 (0) and Copt are in good agreement with those -900' , ,,, , r , ~ , T ' , 0 2OO 40~ 60O 8OO 1000 1200 for other perovskites when the contributions of soft modes ~(Om -I) are subtracted. Fig. 4. The ab-plane dielectric function e 1(o~) of a crystal of The phonons are essentially the only things that can GdBa2CU3OT_~ be seen in the c-axis-polarized optical conductivity, which 317

800 i i spectrum of Fig. 2. The difference between these spectra 20 K and other ab-plane spectra may well be explained by the ...... 300 K effect discovered by Reedyk et al. [38]. The spectra differ

600 due to the direction of incidence of the light: along e in most measurements but perpendicular to c in the one reported here. This effect has been studied in some detail I E by Reedyk et al. [38] in PbzSr2(Y/Ca)Cu309_ ~. o 400 7

b B. c-axis spectra

oo Comparing our data to previous measurements of the c axis, we find some similarity with the measurements of Koch et al. [21] and Homes et al. [22]. These two mea- surements find nearly the same positions of the phonon 0 ' 200 400 600 800 1000 1200 bands as occur in our spectra. We find less agreement ~(cm-') with the spectra of Collins et al. [20]. In the latter case, the data look more like the unpolarized spectra of ceramic Fig. 7. The optical conductivity a~(m) for Ell c of a crystal of GdBa2Cu307_ samples, perhaps on account of the mosaic of several small crystals used. The key difference between the spectra presented here and those of other groups [20-22, 24, 25] is the absence Table 1. Frequencies (in cm 1) of Phonon features in of any contribution of free carriers to our e-axis conduc- GdBa2Cu307 tivity. This difference must be due to the reduced Tc of Polarization our crystals. The other samples that have been studied ab plane 150 227 333 420 493 547 680 had T c near 90 K [20, 21] and 63 K [22]. In our case, the e axis 148 176 288 320 560 620 doping level was considerably lower (&~0.6) and To= 30 K. Note." An additional c-axis-polarized band is seen at 800 cm -~, above the maximum of the one-phonon frequencies The absence of any free carrier contribution to our c- polarized spectra has interesting consequences. From a two-component analysis of the ab-plane spectra, we find is shown in Fig. 7. The data are from 20 K and 300 K that most of the free carriers condense into the superfluid, reflectance measurements; there is only small temperature with plasma frequency ~Opsob= 5700 cm-1. This leads to dependence in this polarization. The phonon frequencies an estimate of the ab-plane penetration depth of are listed in Table 1. These phonons are grouped into Jlab = 2800 A. In contrast, the screened plasma frequency three doublets (some are seen as shoulders) and one for e, ~ psc = ~Op~c/~- must be at a frequency lower than singlet. The frequencies are in good agreement with re- 50 cm-1. Otherwise, we would see a plasma minimum ports in the literature for the c-axis phonons [20,21] in similar to what was reported by Tamasaku et al. [10] for YBa2Cu30 7 and definitely different from those for the c- La2_xSrxCuO4. Taking e =21 we get oop~ob=<230cm-t axis phonons [35] in YBazCu306 . This result is further or a penetration depth of 2 r 7 p. confirmation that our sample is representative of super- Assuming a one-electron band scheme and using a conducting material. three-dimensional tight-binding model, O~pc is given by The spectrum also has a clear conductivity peak at

800 cm-1. This frequency is well above the highest pho- 2 __ e2 ~2/7k non frequency in neutron density of states measurements ('DPc h2~O~l~If(~k) [36] and therefore cannot be assigned to a single-phonon absorption. We suggest it is due to a two-phonon process related to the known anomalies [37] in the phonon spec- ge0h2V sin (1) trum around 400 cm-1 (50 meV), although a magnetic absorption cannot be ruled out. where f(e~) is the Fermi-Dirac occupation function, t c is the transfer integral, c is the lattice constant along the V. Discussion c axis, V is the unit cell volume, e0 is the dielectric con- stant of vacuum, and p is the site occupancy. Equation A. ab-plane spectra (1) gives tc < 0.045 meV. This value is extremely low, cor- responding to a temperature of 0.6 K. A three-dimen- The characteristic notch in the conductivity, well known sional model is therefore almost certainly not correct, from ab-plane measurements [31-34], is very difficult to because with this small a value for tc, the Fermi surface see in Fig. 5. One reason is the presence of phonon struc- would be open in the direction of the c axis. With an ture in this spectral region. However, there is a direct open Fermi surface, only a certain portion of the charge relation between the absence of the notch in the conduc- carriers can contribute to intraband optical transitions tivity spectrum and the missing knee in the reflectance polarized E[[ c, so that t c is in fact larger than inferred 318 above. This effect was first pointed out by Kwak [39] in 14. Kamba, S., Petzelt, J., ~elezn3~, V., Pechen, E.V., the context of organic conductors. Using Krasnosvobodtsev, S.I., Gorshunov, B.P. : Solid State Commun. 70, 547 (1989) ogp~ _ ~ Ct c 15. Petrov, M.P., Grachev, A.I., Krasin'kova, M.V., Nechitailov, A.A., Prokofiev, V.V., Poborchy, V.V., Shagin, S.I., Kartenko, o)po~ aE ' (2) N.F.: Solid State Commun. 67, 1197 (1988) 16. Maksimov, E.G., Rashkeev, S.N., Savrasov, S.Y., Uspenskii, where a is the lattice constant along the a axis and E, Y.A.: Plays. Rev. Lett. 63, 1880 (1989) defined by 17. Kircher, J., Alouani, M., Garriga, M., Murugaraj, P., Maier, J., Thompson, C., Cardona, M., Andersen, O.K., Jepsen, O.: 2 _ 2e2a2E Phys. Rev. B44, 217 (1991) ~ h 2 V (3) 18. Geserich, H.P., Koch, B., Diirrler, M., Wolf, Th.: In: Electronic properties of high T~ superconductors and related compounds. is a function of band width and band filling. This analysis Kuzmany, H., Mehring, M., Fink, J. (eds.). Springer Series in gives tc__<0.75 meV, corresponding to a temperature of Solid State Science, Vol. 99, p. 265. Berlin, Heidelberg, New York: Springer 1990 9 K. Thus, down to ,-~ 9 K there is no correlation of elec- 19. Kircher, J., Kelly, M.K., Rashkeev, S., Alouani, M., Fuchs, D., tron motion along the c axis. Note that this temperature Cardona, M.: Phys. Rev. B44, 217 (1991) is definitely below the superconducting transition tem- 20. Collins, R.T., Schlesinger, Z., Holtzberg, F., Feild, C. : Phys. perature. Rev. Lett. 63, 422 (1989) Our results show that the electronic properties of 21. Koch, B., Diirrler, M., Geserich, H.P., Wolf, Th., Roth, G., oxygen deficient GdBazCu307_ a are strongly aniso- Zachmann, G.: In: Electronic properties of high-Tc super- conductors and related compounds. Kuzmany, H., Mehring, tropic. It behaves like a metal, albeit an unusual metal, M., Fink, J. (eds.). Springer Series in Solid State Sciences, in the ab plane while along the c axis it looks like an Vol. 99, p. 290. Berlin, Heidelberg, New York: Springer 1990 insulator. Because our sample is definitely supercon- 22. Homes, C.C., Timusk, T., Liang, R., Bonn, D.A., Hardy, W.N.: ducting, this result has clear implications for the elec- Phys. Rev. Lett. 71, 1645 (1993) tronic structure of the oxide superconductors. Supercon- 23. Dfirrler, M., Zibold, A., Wider, K., Geserich, H.P., Wolf, Th., ductivity can occur in samples which have no coherent Roth, G., Brfiuchle, G., Erb, A., Miiller-Vogt, G., Kircher, J.: In: Electronic properties of high- Tc superconductors. Kuzmany, transport along c. Note, however, that unlike expecta- H., Mehring, M., Fink, J. (eds.). Springer Series in Solid State tions for two-dimensional models [40], - and in contrast Science, Vol. l13, p. 265. Berlin, Heidelberg, NewYork: with the results for Laa_xSrxCuO4 [10] - our sample Springer 1993 shows no development of the plasma edge of the super- 24. Cooper, S.L., Nyhus, P., Reznik, D., Klein, M.V., Lee, W.C., fluid below T~. Thus, even well below T~, there is a strongly Ginsberg, D.M., Veal, B., Paulikas, A.P., Dabrowski, B.: Phys. two-dimensional flavor to its optical properties. Rev. Lett. 70, 1533 (1993) 25. Cooper, S.L., Reznik, D., Kotz, A., Karlow, M.A., Liu, R., We would like to acknowledge enthusiastic discussions with Klein, M.V., Lee, W.C., Giapintzakis, J., Ginsberg, D.M., Veal, P.W. Anderson, P.B. Littlewood, J.R. Schrieffer, and T. Timusk. B., Paulikas, A.P. : Phys. Rev. B47, 8233 (1993) Work at Florida is supported by NSF grants DMR-9101676 and 26. Kozeeva, L.P., Pavlyuk, A.A.: (tmpublished) DMR-9403984. Research in Hungary supported by the Hungarian 27. Burns, G., Dacol, F.H., Field, C., Holtzberg, F.: Solid State National Foundation for Scientific Research under Grant No. 2950 Commun. 77, 367 (1991) and by the Hungarian-American Joint Fund grant 92b-271. 28. Zelezn3~, V., Tanner, D.B., Park, C., Anderson, T.J., Kozeeva, L.P., Pavlyuk, A.A.: Phys. Status Solidi b 177, K37 (1993) 29. 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