Localized normal excitations and resonance anomalies above the gap in tunneling current of high Tc superconductors

Physica C 162-164 (1989) 1049-1050 North-HoUand

LOCALIZED NORMAL EXCITATIONS AND RESONANCE ANOMALIES ABOVE THE GAP IN TUNNELING CURRENT OF HIGH T SUPERCONDUCTORS c

S. Tak~cs, ~. Beha~ka, A. P l e c e n t k and V.M. S v i s t u n o v +

Institute of Electrical Engineering of Electro-Physical Research Centre, Slovak Academy of Sciences, 842 39 Bratislava, Czechoslovakia +Physico-Technical Institute of Acad. Sci. Ukrainian SSR, Oonetsk, USSR From the temperature and magnetic-field dependences of the tunneling structures on YBaCuO/Pb thin films structures, interesting results concerning the energy gaps of Pb, YBaCuO and possibly proximity-effect influenced other structures are obtained. The additional resonances above the gap can be connected with the existence of localized baglike excitations in the 2-dimensional fermion system of Cu-O planes. The calculated energy changes of the bags by changing the number of the included quasiparticles are in the range of the experimental results for the resonance steps. Many essential results can be obtained from

75 - 82 K, the transition width about i0 K and

tunneling measurements on superconductors. This is valid for high Tc superconductors, too (e. g.

the critical current density 104 A/cm 2. The sur-

(energy gap, spectral function of the electron -

phology and disorded orientation of individual

phonon interaction, etc.). Many studies were

granulae. The Pb counterelectrode was deposited

face of the films was s ~ o t h with granular mor-

performed on polycrystals, single crystals and

directly on the surface of 0.7 um thick YBaCuO

thin films. Some controversial tunneling results

film without additional artificial barrier.

are not surprising due to the complexity of the

The tunnel junctions were realized as quasi-

structure with unusual properties (anisotropy,

particle tunneling type connections without ex-

small coherence length, inhomogeneities, proxi-

perimental evidence of pair tunneling. Only jun-

mity effect, different tunneling mechanisms,

ctions were further investigated where Pb gap

etc.). 1 Even more complex resonance-like struc-

was observed in tunneling structures. Similar

tures were obtained in tunneling current well I above the gap, for which different mechanisms

results were received with Pb point contacts. Figure i illustrates the temperature depen-

can be responsible 1'2 (localization of electrons in normal regions near the counterelectrode, 3

baglike excitations in the superconductor,

sur-

face states, interaction with impurities and phonons, magnons, etc.). These structures are observed in the majority of prepared samples and

N"

their temperature dependence resembles the BCS one. We give here some results of tunneling measurements on YBaCuO/Pb thin film structures with main emphasis on resonances above the gap.

m + o

U(4.2 K) = 142.5 mV 100.25 mV 13.69 mV

The tunneling structures were prepared on the base of YBaCuO superconducting thin films synthe-

T,K

sized by our low-temperature process of vacuum coevaporation.41he zero resistance Tc was about 0921-4534/89/$03.50 © Elsevier Science Publishers B.V.

(North-Holland)

FIGURE 1

60

1050

S. Takdcs et al. / Localized normal excitations and resonance anomalies

dence of YBaCuo gap and of two lowest resonances, which are nearly p e r i o d i c . 1 The resonances above the gap can be caused by the existence of m o r e - p a r t i c l e bags. For the 3dimensional (3D) sytem, the gap should be of the order of the Fermi energy to get bags with lower e x c i t a t i o n energy than the BCS e x c i t a t i o n s . 2'3 Analogous c a l c u l a t i o n s can be p e r f o r med for 2D and 1D systems. We give here only the e s s e n t i a l r e s u l t s . The baglike e x c i t a t i o n s are in the 2D case e n e r g e t i c a l l y favourable with respect to BCS e x c i t a t i o n s , i f the l o c a l i z a t i o n p o t e n t i a l i s of the order of the gap. The tD m o r e - p a r t i c l e bags are always below the BCS e x c i t a t i o n s . For our purposes, it is important how

the energy of the bags changes by changing the number of included quasiparticles. This quantity is in the 2D case approximately given by E = ~ if

-60

-40

-20

(2.3 - 4)Z~¢~ 37 - 64 meV Z~ ~ 16 meV

0

20

40

60

FIGURE 2

the l o c a l i z a t i o n p o t e n t i a l equals EF and ~ ,

r e s p e c t i v e l y . The temperature dependence of the resonances should be given by those of the gap,

t h a t these structures are connected with the

which i s confirmed by the experiments.

p r o x i m i t y e f f e c t induced superconductivity in Pb

Typical tunneling s t r u c t u r e s at 4.2 K and f o r d i f f e r e n t magnetic f i e l d s ( o r i e n t e d perpendicu-

or some Pb a l l o y s . The resonances in the samples given here are

l a r to the surface of f i l m s ) are shown in Figure

of r e l a t i v e l y simple shape, in c o n t r a d i c t i o n to

2. The s t r u c t u r e s disappearing at temperatures

some r e s u l t s where more complicated peak s t r u c -

higher than T

tures were also present (see 1 and the references

C

of Pb are ascribed to Pb e l e c t r o -

de. The presence of some Pb a l l o y (which should

t h e r e i n ) . This makes the low-number p a r t i c l e

be a type I I superconductor) is i n d i c a t e d by the

bags 5 u n l i k e l y to be the cause of the resonances

existence of P b - l i k e gap at f i e l d s above 1.4 T.

The m o r e - p a r t i c l e bags should lead to simple

Only the lowest resonance at approximately

resonance s t r u c t u r e s . Nevertheless, one cannot

40 mV i s given in Figure 2. I t s m a g n e t i c - f i e l d

exclude the mechanisms mentioned above, as they

dependence seems to be complicated. I t i s very

may lead to resonance e f f e c t s , too.

i n t e r e s t i n g t h a t t h i s resonance is present up to

REFERENCES 1. ~. Be~a~)xa et a l . , IEEE Trans. on Magn. MAG-25 (1989) 2583.

very high magnetic f i e l d without changing i t s shape s u b s t a n t i a l l y (and without any sign of

2. S. Tak~cs, Mod. Phys. Lett. B 3 (1989) 421.

magnetic s p l i t t i n g . ) . In a d d i t i o n to t h i s resonance, there are some other peaks j u s t above the gap which are very s e n s i t i v e to the applied magnetic f i e l d (they are vanishing already at B > 0 . 6

T). We suppose

3. M. geinstein, Mod. Phys. Lett. B 1 (1987) 327 4. ~. Chromik et al., IEEE Trans. on Magn. MAG-25 (1989) 2484.