Heavy fermions and superconductivity: “Superconducting spectroscopy” of non-magnetic impurities in CeCu2Si2

Physica 148B (1987) 6-13 North-Holland, Amsterdam

HEAVY F E R M I O N S A N D S U P E R C O N D U C T I V I T Y : " S U P E R C O N D U C T I N G S P E C T R O S C O P Y " OF N O N - M A G N E T I C IMPURITIES IN CeCu2Si 2

F. S T E G L I C H , U. A H L H E I M , U. R A U C H S C H W A L B E and H. SPILLE lnstitut far Festk6rperphysik, Technische Hochschule Darmstadt, D-6000 Darmstadt, Fed. Rep. Germany Received 24 August 1987

We report results on the normal-state (n-state) and the superconducting properties for CeCu2 ~Si2 and the quasi-binary alloys Ce~ xMxCu2 2Si2 with M = La (x ~< 11 at%) and M = Y (x ~< 3 at%). Upon increasing the dopant concentration, the specific heat jump height, AC, is depressed much more strongly than T~. This proves a pair-breaking effect of the La- and Y-induced "Kondo holes" - characterized by temperatures Th.L, and T h.v, that show an anticorrelation to the characteristic temperatures (T~,, T,~) of the underlying Ce 1 xM~Cu 2 2Si2 systems. Compared with La, Y impurities are more efficient in pair breaking and in destroying coherence in the normal state, presumably owing to a less anisotropic scattering potential.

1. Introduction

Electronic quasiparticles with very large effective masses, m* = (200-300)reel, frequently called "heavy fermions", form in certain 4f- and 5f-compounds like CeCu2Si 2 and UBe13 well below a characteristic temperature T* of order 10 K. The origin of these quasiparticles has been the subject of intensive experimental [1] and theoretical [2] research during the past years. This issue will be briefly dealt with in section 2. The other major topic of controversial discussion in this field concerns the nature of the superconducting state in the most exciting of these materials. Ever since the discovery that in CeCu2Si 2 [3] and its U-based counterparts UBe13 [4] and UPt 3 [5] Cooper pairs are formed by such heavy fermions, the possibility of an exotic pairing mechanism has been emphasized. Many empirical facts on heavy-fermion superconductors (HFS) point to an anisotropic Cooper pair state of even parity [6], but it is not yet clear whether the anisotropic superconducting order parameter is a conventional one (i.e. has the symmetry of the lattice [7]), or an unconventional one (with a symmetry lower than that of the lattice [2]). An anisotropic superconducting order parameter, regardless of whether it is a conventional or an unconventional one, should be seriously affected by impurities, in that already ordinary 0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation

potential scattering should lead to pair breaking [8]. In fact, a strong depression of T c upon • alloying in the at% range has been observed for both CeCuzSi 2 [9] and UBe13 [10]. Though substitution of non-f elements (Cu or Be) by doping atoms appears to be particularly efficient in reducing T c of these two compounds, in this paper we will be concerned only with the influence on the superconducting properties of CeCu2Si2, caused by disturbance of the Cesublattice by doping with La and Y. For both Ce 1 xLaxCuzSi 2 and Cel_xYxCuzSi 2 a positively curved Tc(x ) dependence was reported earlier with critical concentrations (as Tc---~0) Xcr = 11at% and 6 a t % , respectively. The observed Tc(x) dependencies for these quasi-binary alloys resemble those of ordinary superconductors containing magnetic or "nearly magnetic" impurities [11]. As was shown in this case, a "superconducting spectroscopy" apt to delineate the magnetic state of the doping atoms can be performed which is based upon the depression of both the specific heat jump, AC, and T~ [11, 12]. Below, we report first experimental results of this same type of superconducting spectroscopy on La and Y impurities replacing Ce in the " K o n d o lattice" CeCu2Si 2. (The term " K o n d o hole" is sometimes used for such a defect structure.) Since earlier investigations [9] on the response of T c on alloying in polycrystalline C e C u z S i 2

F. Steglich et al. / Heavy fermions and superconductivity in CeCu~Si 2

have been hampered severely by a considerable Tc-sCatter for the undoped compounds, here we have chosen to study quasi-binary polycrystalline alloys based on CeCu2.2Si2: Both the normal and superconducting properties of this 10at% Cu excess material have been proven to be reasonably reproducible. For example, Tc's between 0.65 and 0.70K with no exception have been detected for a large number of such CeCu2.2Si 2 samples [9, 13, 14]. Before discussing in section 3 the effect of La and Y impurities on Tc and AC in CeCu2.2Si2, we begin with a brief survey of the normal heavy-fermion state, emphasizing especially the case of our Ce]_xMxCU2.2Si2 alloys.

2. The normal heavy-fermion state with special emphasis o n C e l _ x M x C U 2 . 2 S i 2 (M = La, Y) Many of the observed heavy-fermion phenomena in materials like CeAI 3 [15] and CeCu2Si 2 [3] can be understood qualitatively in terms of independent Kondo ions. This holds true generally for the physical properties above T*, e.g. a Curie-Weiss type susceptibility and an electrical resistivity with negative temperature coefficient. In addition, some of the low-T thermodynamic properties of the afore-mentioned "Kondo lattices" are similar to those of dilute Kondo impurities (well below their characteristic Kondo temperature, TK). For example, the Federived specific heat of CuFe (with T~ = 27 K) depends linearly on temperature below 1 K, CFe = 'YFeT [16]. When scaled to 1 mol-Fe, '~Fe 1 J / K 2 mol-Fe is of the same order as the Sommerfeld coefficient measured for CeAI~ [15] or CeCu2Si 2 [3]. Likewise, a strongly enhanced Pauli paramagnetic susceptibility of the order of 10 -7 m3/mol-Fe (or -Ce) is found for the dilute Kondo alloys as well as for the concentrated Kondo lattices. The whole of these observations leads to the conclusion that heavy fermions form locally at the sites of the Ce (or Fe) ions in the process of a Kondo-type quenching of the f (or d) derived magnetic moments. In contrast to the situation in dilute Kondo alloys, however, heavy fermions in Kondo lat-

7

tices give rise to the formation of a real band structure [17, 18], as verified by recent de Haasvan Alphen experiments on CeCu 6 [19] and UPt 3 [20]. The transition from the high-T regime with dominant single site properties to the low-T regime of coherent heavy fermions is indicated [21] in most of the compounds of interest by a pronounced peak in the electrical resistivity, p(T), near T = T* (cf. fig. 1). This demonstrates that phase coherence for the conduction electrons scattered off the f-ions develops at T < T*. A completely coherent state is, however, not established before the temperature is reduced to Tcoh ~ T*. In several heavy-fermion compounds a maximum has been observed [22] in the temperature dependence of the Sommerfeld coefficient y ( T ) = C(T)/T [22] (cf. fig. 2b). The position of this feature, which may be ascribed [22] to structure in the heavy fermion derived density of states (DOS) at the Fermi level, E v, can be considered an empirical measure of Tcoh [6]. We now discuss how La and Y doping modifies some of the above normal-state (n-state) properties of the C e C u 2 . 2 S i 2 matrix. These results are contained in figs. 1 and 2 and table I. Because La doping may produce a negative chemical pressure on the Ce's, we expect the characteristic temperature T~a of Ce]_xLaxCu2.2Si 2 to be smaller than T* of the C e C u 2 . 2 S i 2 matrix, cf. [23]. On the other hand, a positive chemical pressure caused by substituting Y for Ce should push T* upwards. For low dopant concentrations these differences will presumably be small, but in I

]

0

(Qu.) ~

/

Ceo.97Mo.03Cu2,2Si2 o M= Lo

00

I

100

• M=Y

t

200

T(K

Fig. 1, Resistivity of 3 at% La and 3 at% Y doped CeCuz 2Siz as a function of teniperature in units of the room-temperature value. Arrows mark the positions of the p(T) peaks.

8

F. Steglich et al. I Heavy fermions and superconductivity in CeCu2Si 2

Table I Results characterizing the n-state properties and the superconducting phase transition of Ce~.xMxCu22Si2. %: n-state specific heat coefficient, extrapolated to T = 0. Xm: concentration of magnetic Ce 3~ "impurities" (assumed to be in the fully degenerate J = 5/2 state). T,: position of the resistivity peak. T~, AC: superconducting transition temperature and specific heat jump height, both deduced from the specific heat anomaly (see fig. 2a). M=La

M=Y

x (at%):

0

3

7

9

11

1.5

3

% ( J / K 2 mol-Ce) x m (at%) T~ (K) T c (K) AC ( J / K mol-Ce)

0.60 2.2 20 0.66 0.56

0.74 2.4 25 0.49 0.36

0.68 2.6 23 0.28 0.13

0.66 2.0 23 0.16 0.04

0,71 1.5 b~ 0,08 ~ < 0.01

0.61 b) b) 0.51 0.38

0.62 2.8 21 0,26 0.06

")From ac susceptibility. b)Not measured. a n y case w e e x p e c t to find T~a ~< T,~. This e x p e c t e d b e h a v i o r for T* will b e a p p a r e n t in t h e m e a s u r e m e n t s r e p o r t e d b e l o w for the specific h e a t coefficient, %, a n d t h e Pauli spin susceptibility, X0. S e e m i n g l y c o n t r a s t i n g b e h a v i o r is f o u n d for t h e resistivity, s u p e r c o n d u c t i n g transition t e m p e r a t u r e a n d specific h e a t j u m p height. This l a t t e r t y p e of b e h a v i o r is e x p l a i n e d in section 4. L e t us first a d d r e s s t h e l o w - T dc s u s c e p t i b i l i t y , Xac ( T ) , m e a s u r e d at B = 1 T . B e l o w 5 K o u r d a t a can b e well r e p r e s e n t e d by t h e s u p e r p o s -

I

I

I

C/T

0.s f

B=OT

B=2T ct

0 0

t 0.4

T(K)

b 0

t O./-.. T(K)

Fig. 2. Specific heat of C e C u 2 . 2 S i 2 a s well as of the quasibinary systems Ce 1 xM~Cu225i2 with M = La and Y at B = 0 T (a) and B = 2 T (b). For the sake of clarity, units are per mole of the actual alloy, rather than per mole of Ce. Lines through the data points are intended as guides to the eye; thin solid lines mark idealized specific heat jumps (see text). x =0: x ; x =0.03:0 (La), • (Y); x = 0.09: /~ (La).

ition o f a Pauli s u s c e p t i b i l i t y X0 i d e n t i c a l to t h a t o f t h e CeCu2.2Si2 m a t r i x , X0 = 7.5 x 10 -8 m 3 / m o l C e [24] a n d a C u r i e - W e i s s t e r m - - ( T + 0) 1. T h e l a t t e r i n d i c a t e s t h e p r e s e n c e of s o m e " n o n - t r a n s f o r m e d " Ce 3+ ions with an a v e r a g e c o n c e n t r a t i o n £m = 2 . 3 a t % (as o b t a i n e d f r o m t h e results in t a b l e I) a n d an a v e r a g e W e i s s t e m p e r a t u r e 0 = - 7 K. F o r t h e s e w e a k l y d o p e d s a m p l e s , o u r low-field Xdc(T) d a t a a r e o b v i o u s l y n o t sufficient to r e s o l v e a n y c h a n g e s in X0 ind u c e d by t h e d o p i n g . F o r this p u r p o s e , high-field magnetization measurements are planned. O n t h e o t h e r h a n d , t h e size of t h e S o m m e r f e l d coefficient in t h e n - s t a t e specific h e a t ( e x t r a p o l a t e d to T = 0 a n d given p e r m o l e o f C e in t a b l e I ) , %, is s o m e w h a t l a r g e r for M = L a t h a n for M = Y. Since in a n y single-site m o d e l , y0 ~ - T*, o u r specific h e a t results c o n f i r m the e x p e c t e d b e h a v i o r , i.e. T ~ , ~< T~,. F o r h i g h e r i m p u r i t y c o n c e n t r a t i o n s x in s t o i c h i o m e t r i c C e C u 2 S i 2, considerable differences between La and Y dopings w e r e r e v e a l e d in b o t h X0 a n d %: F o r x = 10 a t % R a u c h s c h w a l b e et al. [24] f o u n d X0 o f t h e Y - d o p e d s y s t e m to b e a b o u t 15% s m a l l e r t h a n for the L a - d o p e d s y s t e m ; a n d B r e d l [25] o b s e r v e d for C e 0 . s Y 0 , 2 C u 2 S i 2 a % v a l u e t h a t was also a b o u t 15% s m a l l e r t h a n for Ce0.sLa0.2CuzSi2. T h e a p p a r e n t l a c k o f a s y s t e m a t i c c h a n g e in % u p o n i n c r e a s i n g t h e c o n c e n t r a t i o n o f a given s p e c i e s o f the d o p a n t (i.e. e i t h e r L a o r Y, cf. t a b l e I ) , as e x p e c t e d in t h e f r a m e w o r k of a n y single-site m o d e l , s h o u l d b e a s c r i b e d to alloying-

F. Steglich et al. / Heavy fermions and superconductivity in CeCu2Si 2

induced changes in the DOS fine-structure at E v which, in the low-concentration range, mask the assumed volume-induced change in the overall magnitude of 3'0. The effect of the doping can, however, be clearly seen in the temperature dependence of 3'(T) (fig. 2b): upon alloying, the pronounced feature near 0 . 4 K in CeCu2.2Si 2 becomes replaced, at lower temperatures, by an only shallow maximum, the position of which depends on the dopant concentration in a rather complex way. It is important to note that the 3'(T) peak shows up at B = 2 T for all La- and Y-doped CeCu2.2Si 2 systems which turn superconducting in the absence of a magnetic field. These findings confirm earlier ones on CeCuxSi 2 samples with varying nominal Cu concentration [26] which suggested that the occurrence of the y(T) peak in the normal state is necessary for the occurrence of heavy-fermion superconductivity in CeCu2Si 2. Finally we turn to the concentration dependence of the electrical resistivity, p(T) of the C e l _ x M x C u 2 . 2 S i 2 systems is very similar to what was observed earlier for both polycrystalline [21] and single crystal [27] samples of CeCu2Si 2. In fig. 1 is shown p(T) for 3 at% Y (filled circle) and for 3 at% La (open circle). (This notation, filled circle for the Y and open symbol for the La data, is used in all the figures.) As seen in fig. 1, the addition of 3 at% Y to CeCu2.2Si2 causes considerably more incoherent scattering than the addition of the same amount of La does: the low-T p(T) peak appears much more pronounced for the Y-doped sample and occurs at a somewhat lower temperature Tp. This indicates that the presence of Y impurities reduces the "coherence regime", when compared with Ce0.97La0.03Cu2.2Si 2. This observation agrees perfectly with published results on stoichiometric CeCu2Si 2 samples doped with 20 at% of either La, Y or Sc and showing that Tp,La > Tp,v > Tp,sc [9]. We wish to emphasize that this trend is opposite to what has to be anticipated for the characteristic "single-ion Kondo temperature" (T~_ a < T~, < Tsc), as discussed above. We, therefore, conclude that in these quasibinary CeCu2SiE-based alloys the position of the low-T resistivity peak should not be used to

9

determine T* as currently assumed [23]. Tp rather serves as an indicator for the effect a given dopant has in disturbing the phase coherence in the low-T Kondo lattice phase. In the following we will see that different dopants also can have quite different effects in modifying the superconducting state in C e C u 2 . 2 S i 2.

3. ]~he superconducting phase transition in Cel_xMxCU2.2Si 2 (M = La, Y)

Our specific heat results taken at zero magnetic field for C e C u 2 . 2 S i 2 and for a few selected La- and Y-doped systems are displayed as C/T vs. T in fig. 2a. The broadened superconducting phase transition anomalies can be replaced by sharp discontinuities so that the total entropy remains unchanged ("equal-areas construction"). For all samples studied, the transition temperatures, To, and the specific heat jump heights, AC, obtained in this way are listed in table I. The results of fig. 2a indicate that, along with a reduction of Tc and AC (cf. figs. 3a and 3b), C/T taken at the same temperature well below T c increases upon increasing the dopant concentration. A substantial linear term in the specific heat in the superconducting state, %T, l

,

,

,

i

[

~

r

i

,

I

AC Cel_xMxCu22Si'2~

~oo

"\\ 1 0.5 0

~ J (3

I

'\1 0

0.5 Tc/ Tco 0 5 x(at%)

Fig. 3. (a) AC/AC~ vs. T~/T~o and (b) T~ vs. x for Cel_,MxCu22Si2 with M = La (O) and Y ( 0 ) . T~0 and A C 0 refer to CeCuz.zSi2. D a s h e d lines: initial slopes; dash-dotted line: extrapolation towards lower temperatures, based on the observation of superconductivity below T~ = 40 m K for a La concentration x = 13 at% [29]. Also shown are the results from the A b r i k o s o v - G o r ' k o v ( A G ) theory and the BCS "law of corresponding states". Solid lines through the data points are guides to the eye.

10

F. Steglich et al. / Heavy fermions and superconductivity in CeCu2SL-

apparently develops which is larger for the Y than for the La doping. In fig. 3b we show the concentration dependence of T c for both the La- and the Y-doped C e e u 2 . 2 S i 2 systems. For comparison, the result of the A b r i k o s o v - G o r ' k o v (AG) theory dealing with stable magnetic moments [28] is included for the case M = La. (Here, the critical concentration XAG ~ 8 . 1 at% is fixed through XAC;= 0.691 T co(d T j dx)- ~[~o). The salient features in fig. 3b are: (i) the initial T~-depression is more than twice as large for Y doping than for La doping - in agreement with earlier findings from lowfield ac susceptibility, g~c(T), measurements

[91; (ii) for Ce~_~La~Cu2.2Si2, T ( x ) is remarkably linear and lies above the A G curve at higher doping concentrations. Similar T~(x) dependencies are well known for ordinary superconductors containing either Kondo impurities with T K >> T~0 [30] (T~0 referring to the undoped material) or "nearly magnetic" impurities, e.g. U in Th [11]. A sensitive way of discriminating between these two cases is to study the depression of the specific heat jump relative to that of T~ [11, 12]. Therefore, in fig. 3a we present the experimental results for AC/ AC 0 vs. To~ T~o. These curves do not follow the BCS "law of corresponding states" (solid straight line) which is expected for "nearly magnetic" impurities that give rise to a "weakening" rather than a "breaking up" of the Cooper pairs [11]. The relative depression of A C is even stronger than predicted for pair breaking by isolated stable local magnetic moments (cf. the universal A G result). The absolute value of the initial slope rn~ = Id(AC/±Co)/d(TJT~0)[ r~'r~,, is larger for the Y doping (1.6) than for the La doping (1.45). By considering the body of work on the so-called " K o n d o superconductors", i.e. superconducting Kondo alloys, we therefore conclude that the non-magnetic La and Y impurities in the superconducting Kondo lattice CeCu22Si2 act like dilute magnetic Kondo impurities in a nonKondo lattice, i.e. ordinary, superconductor [11, 12, 30, 31]. In particular, if we attribute a characteristic temperature T h to these " K o n d o

holes", we expect Th.Y for the Y-induced Kondo hole to be smaller than Th.La, characterizing the La-induced Kondo hole. This latter expectation can be verified by comparison of the results in fig. 3a with theoretical results for m c by Miiller-Hartmann and Zittartz [31] for " K o n d o superconductors":

- m c = 3 - 0.78211

(r - 0.41) 2 - 4.4 ] + ( 7 - 1.90) 2 + 4 +'rr2S(S + 1) '

(~)

where r = ln(Th/To0 ). Physically meaningful T h values are obtained only if the results for Kondo impurities with S = 1/2 are chosen. We find T h , L a ~ 7 0 K (compared to, e.g., ~ t 0 6 K in the case S = 5/2) and Th.v ~-40 K. Using these T h values, we are then able to estimate an effective DOS at E v, Neff(Er), from the initial To-depression [30]:

(drc)

=

dx /x=o

1 8kBNcff(EF)

v2S(S + 1) x (r - 2.56) 2 + 0.5 + "rrZs(s + 1)

(2)

Ncff(EF) is

reduced compared to the 4f-derived resonant DOS at the Ce sites (corresponding to Y~ff = To = 0 - 6 0 J / K 2 mol-Ce, cf. table I) by almost a factor of 2 at the La doping atom (y~ff = 0 . 3 6 J / K 2 mol-Ce) and a factor of about 3 at the Y site (y~ff : 0.21 J / K 2 mol-Ce).

4. Discussion

We have found that the non-magnetic La and Y impurities substituted on periodic Ce sites in the Kondo lattice CeCua 2Si 2 modify the heavyfermion superconducting state in a very similar way as magnetic Kondo impurities (with T K >> Too) do in an ordinary superconductor. Though not carrying a magnetic moment, these " K o n d o holes" can be described by characteristic temperatures Th,La and Th. Y. The most intriguing

F. Steglich et al. / Heavy fermions and superconductivity

result of our analysis is that Th of the Kondo hole shows an anticorrelation to T* of the underlying Kondo lattice: while, as was discussed in section 2, T[, ~< T,~, we find Th.La> Th,y. In particular, the strong depression of A C relative to the depression of Tc proves that the presence of non-magnetic impurities in CeCu2Si 2 gives rise to "pair breaking" rather than "pair weakening". In analogy to what was proposed recently in the case of UBe13 [32] and Ul_xThxBe~3 [33], we wish to ascribe this "pairbreaking" effect (at least) partly to an anisotropic pair state which itself results from a strong Fermi-surface anisotropy inherent to the renorrealized low-T state in a Kondo lattice: The analysis of both upper critical field [26] as well as thermal conductivity 113] results on CeCu2Si 2 suggests that portions of the Fermi surface with (presumably) less heavier electrons and very small superconducting order parameter ("normal regions") coexist with heavy-fermion portions, the latter becoming superconducting below T~0 = 0.6-0.7 K. Ordinary potential scattering of Cooper pairs into those normal portions of the Fermi surface ("normal channels") will break up the pairs [34], Ce impurities in ordinary superconductors like LaAI 2 are "resonant scatterers" that give rise to the formation of "localized excited states" within the energy gap [35]. The latter can fill up the gap efficiently and cause "gapless superconductivity" already at low dopant concentrations [12]. It has been emphasized [36] that "Kondo holes" should act as "resonance scatterers", too. We find this idea to be strongly supported by our observation that the "residual" linear YsT-term in the zerofield specific heat increases with increasing concentration of the respective dopant (either La or Y). The "superconducting spectroscopy" described above reveals that the strength of the "pair breaking" depends critically on the kind of dopant substituted for Ce: Y is more efficient in this respect than La. We infer the scattering of Cooper pairs from La impurities (compared with Y impurities) to be m o r e a n i s o t r o p i c , in that the scattering rate into the "normal channels" is relatively small compared with the scattering rate

in CeCu2Si 2

11

within the "heavy-fermion sheets". Indeed, the effective DOS, averaged over the Fermi surface by the scattering events, is substantially higher (more "heavy-fermion like") for La than it is for Y scattering centers. Our interpretation of these findings starts from the argument [7, 17, 18] that the anisotropies of the Fermi surface in the coherent heavy-fermion state originate in many-body correlations which track the hybridization between local 4f-shells and valence orbits from neighboring sites (ligand states). Whereas the coupling between adjacent Ce planes in CeCu2Si 2 seems to be predominated by the Ce-4f/Cu-3d hybridization [37], the intraplane Ce-Ce coupling will be via 4f-5d hybridization. Thus, it is plausible to assume that the scattering potential of a La impurity (with 5d valence electron) shows an anisotropy that is closer to the anisotropy of the Ce ions forming the Kondo lattice than what holds for an Y impurity (with 4d valence electron). This may also explain why the To-depression by Sc impurities (with 3d valence electron) in CeCu2Si 2 is much more dramatic (xcr < 0.5 at% [9]), presumably because it gives rise to much less anisotropic scattering, than by Y and La impurities. We note that a highly anisotropic scattering rate due to Th impurities in the Ul_xThxBe13 alloy system (x = 2-5 at%) is involved in arguments [32, 33] for the coexistence of two superconducting order parameters on different parts of the Fermi surface below the lower of the two transition temperatures discovered in these alloys by Ott et al. [381. The results of the present study fully confirm the correspondence 19] between the depression of Tc (fig. 3b) on the one hand and the depression of the position of the o(T) peak, Tp (fig. 1), on the other. This underlines, we believe, the importance of a coherent normal state for the occurrence of heavy-fermion superconductivity: For the latter, coherence in the normal heavyfermion state over a distance of, at least, the superconducting coherence length is required. Therefore, the "pair-breaking" effect by nonmagnetic impurities in a superconducting Kondo lattice has a more general meaning than for magnetic impurities in ordinary superconductors,

12

F. Steglich et al. / Heavy fermions and superconductivity in CeCu~Si~

in t h a t it c o n s i s t s o f q u i t e d i f f e r e n t m e c h a n i s m s : Firstly, the afore-mentioned scattering into "norm a l c h a n n e l s " as well as t h e f o r m a t i o n o f " l o c a l ized excited states" within the energy gap break up already existing C o o p e r pairs. Secondly, the scattering from "Kondo holes" apparently leads to a d e s t r u c t i o n o f t h e i t i n e r a c y ( c o h e r e n c y ) o f t h e q u a s i p a r t i c l e s t h e m s e l v e s , i.e. s e v e r e l y h i n d e r s t h e f o r m a t i o n o f C o o p e r pairs. We are presently extending our "supercond u c t i n g s p e c t r o s c o p y " to Sc a n d A g i m p u r i t i e s to c h e c k t h e s e i d e a s as well as to G d a n d M n i m p u r i t i e s in o r d e r to i n v e s t i g a t e to w h a t e x t e n t local m a g n e t i c m o m e n t s a d d to t h e o b s e r v e d " p a i r b r e a k i n g " b y " K o n d o h o l e s " , i.e. n o n m a g n e t i c i m p u r i t i e s o n t h e p e r i o d i c sites of a K o n d o lattice. A m o r e c o m p l e t e p r e s e n t a t i o n o f this w o r k will b e p u b l i s h e d e l s e w h e r e [39].

Acknowledgement W e are g r a t e f u l to C . D , B r e d l , P. F u l d e , N. G r e w e , K. M a k i , E . M f i l l e r - H a r t m a n n , D. R a i n e r , G . R . S t e w a r t , C . M . V a r m a , P. W61fle a n d J. Z i t t a r t z for m a n y s t i m u l a t i n g d i s c u s s i o n s a n d v a l u a b l e s u g g e s t i o n s . T h i s w o r k was s u p p o r t e d b y t h e S o n d e r f o r s c h u n g s b e r e i c h 252 D a r m stadt / Frankfurt / Mainz / Stuttgart.

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