Tunneling α2F(ω) on high Tc A15 and B1 compounds

Physica 135B (1985) 198-202 North-Holland, Amsterdam

T U N N E L I N G a2F(~o) ON HIGH T c A15 A N D B1 C O M P O U N D S K.E. KIHLSTROM Dept. of Physics, Westmont College, 955 La Paz Road, Santa Barbara, CA 93108, USA

R.W. S I M O N TRW, I Space Park, Redondo Beach, CA 90278, USA

S.A. W O L F Code 6634, Naval Research Laboratory, Washington, DC 20375, USA

Recent advances in artificial tunneling barriers have resulted in excellent tunnel junctions on several materials whose native oxide are of poor quality. Among the high T c A15 compounds tunneling a2F(o~) data are now available for Nb~Sn, Nb3A1 and Nb3Ge. Among B1 compounds there have been no published a 2F(o)) results. Since this is another class of high Tc materials, the contrast would be interesting. This paper will give the first tunneling a 2F(co)spectrum for the B 1 compound NbN. The spectrum show two large phonon peaks at energies of 13 meV and 47 meV. The A value of 1.46 +-0A0 along with the 2 A / k B T c of 4.25 show this to be a strong coupled material. In addition, we will present a 2F(~o)as a function of composition for A15 V-Si, comparing it with previously measured A15 compounds. Here in V3Si unlike the other A15 compounds measured, there is no evidence of mode softening which had been presented as an explanation for the high Tc's in the A15 compounds. Also unusual for the A15 compounds previously measured by tunneling, is that V3Si remains weak coupled as silicon concentration increases. Both the A value (remaining about 1.0) and the 2 A / k B T ~ of about 3.5 show this.

1. Introduction The B1 and A15 c o m p o u n d s have represented high T¢ superconductivity for m a n y years. Both systems have held promise for appllications and also for increasing our understanding of superconductivity. It is this latter issue that this p a p e r addresses. Tunneling is the most direct p r o b e of superconductivity but has b e e n a difficult m e a s u r e m e n t to make. This is for two basic reasons. First, tunneling only probes about a coherence length into the material ( < 5 0 / ~ in m a n y A15 and B1 compounds). Thus the samples must be well made up to the surface. Second, there must be a good quality insulating barrier to tunnel through. Unfortunately, m a n y of these c o m p o u n d s do not produce native oxide barriers of suffficient quality for good tunneling measurements. In the A15 c o m p o u n d s progress was m a d e primarily through the use of insulating silicon

barriers developed by R u d m a n and Beasley [1]. G o o d quality tunnel junctions produced o~2F(o)), the e l e c t r o n - p h o n o n spectral function, for Nb3AI (Kwo and Geballe [2]), N b 3 G e (Kihlstrom et al. [3, 4]), and Nb3Sn ( R u d m a n and Beasley [5]). In each case as the " B " element concentration (A1, G e , Sn) approached stoichiometry there was phonon m o d e softening ( m o v e m e n t of the lowest energy p h o n o n p e a k to lower energies). See fig. 1 for Nb3Ge. This results in increased values for A and Tc. M o d e softening as an explanation for high Tc's in the A15's conflicts with the traditional view that a high value for N(0), the density of states at the Fermi surface, is responsible. This was based on early heat capacity data on V3Si (by Kunzler et al. [6]) and Nb3Sn (by Vieland and Wicklund [7] which gave high values for N(0). Subsequent heat capacity results on Nb3Sn (Stewart et al. [8]), NB3A1 (Cort et al. [9]), Nb3Ge (Stewart et al. [10], Kihlstrom et al. [4]), giving lower values for N(0) in these A15's

0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

K.E. Kihlstrom et al. / Tunneling aZF(to) on A15 and B1 compounds ....

I ....

I .... A=I.03

0.6 n : ~ ~ C ~

I .... X=0.81 *+

A-'2.62

X=1.05

A 3.51

X=1.46

"0 oo

A 3.82

X =1.70

aa

0.4

@

t, oJ

0.2

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20

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ENERGY (meV) Fig. 1. The e l e c t r o n - p h o n o n spectral function a2F(to) for four N b - G e samples with Tc's of 7.0, 16.8, 20.1 and 21.2 K, respectively. Note the strengthening and movement to lower energies of the lowest energy p h o n o n peak.

raised questions as to the validity of attributing high Tc to high N(0) in the A15's. But V3Si certainly does have a high value for N(0) (Junod and Muller [11]). Thus it would not need mode softening to explain its high T c. Whether there is mode softening in V3Si was a prime goal of this work. In the B1 compounds, while tunnel junctions with good quality current-voltage (I-V) characteristics have been made (for example Van Dover and Bacon [12]), obtaining a2F(to) has been much more difficult. We reported a preliminary a 2F(to) spectra for NbN.9C.1 [13] and Gurvitch et al. presented partial spectra (up to 32meV) for NbN [14] but neither of these spectra were able to give quantitative data for the microscopic parameters such as A,/z* and (o~). In this work we will give the full a2F(to) spectrum for NbN along with the microscopic parameters.

199

con (as in ref. 1). A Pb counterelectrode was then deposited. The current-voltage (I-V) curves for the three junctions are shown in fig. 2. The NbN samples were made by U H V rf sputtering (see Wolf et al. [18]) using a niobium target and an argon (75mTorr)/nitrogen (10mTorr) gas mixture. There was no intentionally added carbon but similarly prepared samples [19] had typically a 2% level of carbon inclusion. The tunneling barrier was formed using chemical annodization [20] where the sample is placed in a saturated solution of boric acid and water. An electric current is then passed through the solution to the sample causing oxide ions from the solution to form an insulating barrier on the NbN. This process has the advantages of plugging up pinholes in the barrier (more current is drawn to them) and allowing higher resistance junctions to form than possible with the ordinary native oxide. Fig. 3 shows the I-V curve for the NbN/oxide/Pb tunnel junction. There was no evidence of a proximity knee structure.

V - Si/SiOx/Pb T=1.5 K SAMPLE 1 : T c = 10.1 K Av - Si = 1.39 mV SAMPLE 2: T c = 1 3 . 8 K A v - s i = 2 . 1 0 m V SAMPLE 3: Tc=15.4K Av-si=2.41 mV

D I--

2. Experimental details

Film and junction preparation are described elsewhere in detail for the V3Si [15] and NbN [16]. The V3Si samples were made by electron beam coevaporation (see Hammond [17] with thicknesses of 700 A and 2500 A (both gave excellent tunneling results). The barrier was 25-30/~ sili-

0

1

2

3 4 VOLTAGE (mV)

5

6

7

Fig. 2. The current-voltage characteristics for the three V-Si samples measured in this study.

K.E. Kihlstrom et al. / Tunneling aZF(to) on A 1 5 and B1 compounds

200

5.0[_,

, ' ' I ....

I''

' ' I ....

I ' ' r '

L

NbN/OXIDE/Pb T = 1.5°K T c = 14.0 ,ANbN = 2 , 5 6 m V

J' Nb-Ge

4,5r

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E + N-s°

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I

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....

15

17.5

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20 ATOMIC PERCENT

1 22.5

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(Ge,AJ~,Sn)

Fig. 4. 2 A I k B T ~ versus c o m p o s i t i o n (atomic per cent B elem e n t ) for Nb3Sn, Nb3AI, and Nb3Ge. I 4

I 5

I 6

I 7

VOLTAGE (mV)

Fig. 3. T h e c u r r e n t - v o l t a g e characteristics for N b N / o x i d e / P b . T h e r e was no e v i d e n c e of a " k n e e - s t r u c t u r e " characteristic of a proximity layer.

3. Experimental results The ratio 2 A I I k B T c, a measure of the coupling strength, is shown in fig. 4 for NbAI, NbSn, NbGe. In each case the material is weak coupled away from stoichiometry but becomes strong coupled as stoichiometry is approached (and as Tc increases). For V-Si however, the material remains weak coupled throughout. Fig. 5 shows t h e a 2 F ( c o ) spectra for the three V-Si samples. Once again the V-Si behaves very differently than Nb-AI, N b - G e or Nb-Sn. There is no evidence of mode softening with increased Si concentration. Correspondingly, there is little change in A, the electron-phonon coupling constant which goes from 0.76 to 0.91. Table I shows parameters for low Tc (off-stoichometry) and high Tc (near stoichiometry) samples of NbAI, NbGe, NbSn and VSi (as well as for NbN which will be discussed shortly). In the three niobium-based A15's, the A value changed more strongly than in V-Si going from 1.0 up to 1.7. 2 A I k T c for the NbN samples was about 4.25,

well above the BCS limit of 3.51 which indicates this to be a strong coupled superconductor. (In agreement with both our earlier results [13] and those of Gurvitch et al. [14].) Fig. 6 shows the deviation from the BCS density of states for NbN where the experimentally measured values are compared with those calculated from the generated aeF(00). The agree-

I

0.7

I

I

T

V-Si ........ A = ....

0.6

1.4mV

A=2.1 mV A=2.4mV

Tc=

10.1 K

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0.5

0.4 ~

0.3

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50

ENERGY (meV)

Fig. 5. T h e e l e c t r o n - p h o n o n t h r e e V - S i samples.

spectral function a2F(w) for

201

K.E. Kihlstrom et al. / Tunneling otZF(to) on A15 and B1 compounds

Table I Parameters for low- and high-T¢ samples Sample

(To)

Tc

zl

A

~*

(to)

(tO,og)

(oJz)

2.15 3.15

2A kBTo 3.56 4.45

NbAI NbAI

(low) (high)

14.0 16.4

1.2 1.7

0.13 0.15

13.3 11.4

11.2 9.5

226 181

NbSn NbSn

(low) (high)

12.0 17.8

1.85 3.30

3.58 4.27

1.13 1.63

0.15 0.17

14.2 13.5

12.4 11.4

234 231

NbGe NbGe

(low) (high)

7.0 21.2

1.03 3.82

3.4 4.2

0.81 1.70

0.13 0.11

14.3 12.4

12.7 10.4

241 196

W3Si V3Si

(low) (high)

10.1 15.4

1.39 2.41

3.2 3.6

0.76+-0.15 0.89+-0.06

0.14---0.07 0.11-+0.04

21.1 21.5

19.2 19.2

512 520

14.0

2.56

4.25

1.46-+0.10

0.33-+0.16

20.8

15.5

673

NbN

ment is by no means perfect but is similar to that seen in the A15's. Fig. 7 shows the a ZF(0)) trace for one of the NbN samples. The other sample was also measured to obtain a 2F(0)) and did produce the same general features but the program did not converge completely to experimental results and was not used in the quantitative analysis. In both samples there was no evidence of inelastic scattering in the tunneling barrier. The spectrum shows two main peaks of comparable size, one at 13 meV and the other at 47 meV (with a shoulder at about 40 meV). These features were c o m m o n to both spectra and observable both from first and second derivative data. Our earlier sample [13] (NbN.9C.1) also showed the two main peaks. The lower energy peak (13meV) agrees with the ot2F(0)) from

I

1

i

I

\

8

I

~ .....

NbN

I

I

x%

-2

s¢

I

I

10

20

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I

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30 40 50 ENERGY (meV)

i

i

NbN T c = 14.0 K A = 2.56 mV ~ = 1.46

0.6 0.5

"3

0.4

%

0.3 0.2 0.1 0 0

10

20

30

40

50

60

70

ENERGY(meV)

Fig. 7. Electron-phonon spectral function of ot2F(to) for NbN with Tc= 14 K.

I_

EXPERIMENTAL CALCULATED_

0

-'

i

0.7

I

I

60

70

Fig. 6. Experimental versus calculated deviation from the BCS density of states for the NbN sample.

Gurvitch et al. The high energy peak is presumably due to the light nitrogen atoms in the comp o u n d contributing to a high frequency mode. This added strength may help give NbN its relatively high T c. The e l e c t r o n - p h o n o n coupling constant A = 1.46 _+ 0.10 is characteristic of a strong coupled superconductor (consistent with the high value obtained for 2 A / k B T c ) . As seen in table I, the high energy peak contributes to high values of {O)), {O)2) and (%og)-

202

K.E. Kihlstrom et al. / Tunneling a2F(to) on A15 and B1 compounds

4. Conclusions

References

F r o m the A 1 5 ' s the main result is that unlike in N b - A 1 , N b - G e and N b - S n , V - S i shows no evidence of m o d e softening. In o n e sense, this is not surprising in that since V3Si is a high density of states material its high T c can be explained by that alone. This is in contrast with N b 3 G e which has a significantly lower N ( 0 ) but higher T c. P r e s u m a b l y with m o d e softening V3Si w o u l d have a significantly higher Tc than N b 3 G e . In a n o t h e r sense this is surprising in that it leads to the conclusion that there are two separate m e c h a n i s m s at w o r k in the A15 c o m p o u n d s to p r o d u c e high Tc'S. This m a y be a distinction b e t w e e n the v a n a d i u m based and the n i o b i u m based A15's. Otherwise, w h e t h e r the A15 is an equilibrium c o m p o u n d (i.e. V3Si ) versus metastable (i.e. N b 3 G e ) near stoichiometry m a y be a factor. M o r e i n f o r m a t i o n will be n e e d e d to decide. N b N is s h o w n to be strong c o u p l e d as seen b o t h by its large 2 A / k B T c (4.25) and A (1.46 +-0.10) values. T h e e l e c t r o n - p h o n o n spectra function a 2 F ( w ) shows two strong peaks at 13 and 47 m e V (with a s h o u l d e r at 40 meV). T h e high e n e r g y p e a k leads to high values of ( w ) , (o92) and

[1] D.A. Rudman and M.R. Beasley, Appl. Phys. Lett. 36 (1980) 1010. [2] J. Kwo and T.H. Geballe, Phys. Rev. B23 (1981) 3230. [3] K.E. Kihlstrom and T.H. Geballe, Phys. Rev. B24 (1981) 4101. [4] K.E. Kihlstrom, D. Mael and T.H. GebaUe, Phys. Rev. B29 (1984) 150. [5] D.A. Rudman and M.R. Beasley, Phys. Rev. B30 (1984) 2590. [6] J.E. Kunzler, J.P. Maita, H.J. Levinstein and E.J. Ryder, Phys. Rev. 143 (1966) 390. [7] L.J. Vieland and A.W. Wicklund, Phys. Rev. 166 (1968) 424. [8] G.R Stewart, B. Cort and G.W. Webb, Phys. Rev. B24 (1981) 3841. [9] B. Cort, G.R. Stewart, C.L. Snead, Jr., A.R. Sweedler and S. Moehlecke, Phys. Rev. B24 (1981) 3794. [10] G.R. Stewart, L.R. Newkirk and F.A. Valencia, Solid State Commun. 26 (1978) 417. [11] A. Junod, J.L. Jorda, M. Pelizzone and J. Muller, Phys. Rev. B29 (1984) 1189. [12] R.B. van Dover and D.D. Bacon, IEEE Trans. Magn. MAG-19 (1983) 951. [13] K.E. Kihlstrom, R.W. Simon and S.A. Wolf, Bull Am. Phys. Soc, 29 (1984) 483. [14] M. Gurvitch, J.P. Remeika, J.M. Rowell, J. Geerk and W.P. Lowe, to be published in IEEE Trans. Magn. [15] K.E. Kihlstrom, Phys. Rev. B (in press). [16] K.E. Kihlstrom, R.W. Simon and S.A. Wolf, Phys. Rev. B32 (1985) 1843. [17] For details on the e-beam evaporator, see R.H. Hammond, IEEE Trans. Magn. MAG-11 (1975) 201; J. Vac. Technol. 15 (1978) 382. [18] S.A. Wolf, James J. Kennedy and Martin Nisenoff, J. Vac. Soc. Tech. 13 (1976) 145. [19] S.A. Wolf, I.L. Singer, E.J. Cukauskas, T.L. Francavilla and E.F. Skelton, J. Vac. Soc. Tech. 17 (1980) 411. S.A. Wolf, D.U. Gubser, T.L. Francavilla and E.F. Skelton, J. Vac. Soc. Tech. 18 (1981) 253. E.F. Skelton, S.A. Wolf and T.L. Francavilla, J. Vac. Soc. Tech. 18 (1981) 259. [20] R.W. Simon, P.M. Chaikin and S.A. Wolf, IEEE Trans. Magn. MAG-19 (1983) 957.

(,0log). Acknowledgements W e w o u l d like to t h a n k G e r a l d B. A r n o l d , S i m o n B e n d i n g , Sergio Celasti, T . L . FrancaviUa, W.W. Fuller, T . H . Geballe, D . U . G u b s e r , R . H . H a m m o n d , D a v i d J o n e s , Walter L o w e and D. Van Vechten for valuable discussions and experim e n t a l assistance.