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The CESR and spin wave studies of A-15 films in normal and superconducting states
~
Solid State Communications, Voi.36, pp. I17-122. Pergamon Press Ltd. 1980. Printed in Great Britain.
THE CESR AND SPIN WAVE STUDIES OF A-IS FILMS IN NOP/~AL AND SUPERCONDUCTING STATES S.~.Ekbote, S.K.Gupta and A.V.Narllkar National Physical Laboratory, Hillside Road, New Delhi-llOOl2, India
(Received 18 J u l y 1980 by S. Amelinckx) The electron spin resonance studies have been reported for A-15 superconductors, namely Nb3Ge, Nb3Si and V3SI possessIng different T c values.and CESR, Platzmar~-Wolff type spin waves, and spin waves of antiferromagnetic type are observed in all the samples. It is found that Tc o f Nb3Ge depends upon the presence and separation of spin wave absorptions from the CESR, and Tc is found to increase when the separation is reduCed. It is concluded that the exchange interactions In.the conduction band, as manifested by the behaviour of spin waves, are of antiferromagnetic type and they are responsible tot superconductivity in A-15 materials studied°
1. Introduction
Our earlier work I-3, on ~b3Ge ot TC~22o65K, in the normal state, showed the presence of CESR line toQether with two other absorptions which were identltied as spin waves having antiterromaQnetic character. All these lines had however vanished when the material was in the superconducting state.The disapPearanCe o t lines at 4.2K strongly suggested that the pair formation in superconducting state was due to exchange interactions, contrary to the BCS mechanism. In this paPer we report studies of Nb3Ge films with To varying from 8K to 22.65K. For the sake of comparison, the results obtained on two other A-15 materials namely Nb3Si and V3Si are also reported.
In the previous papersl-4,we had reported our CESR and spin wave resonance studies of high Tc(Tc~22.65K ) sb3Oe films of A-15 structure in normal and superconducting states. A detailed discussion or difficulties involved in the observation of CESR in the normal and superconducting states was given in the above publications. It was established that under favourable conditions CESR and spin waves resonanCe was possible in thin A-15 films° In 9eneral, there are two main COlldltions which are to be satisfied for the observation of CESR. Firstly, the thickness of the films must be of the appropriate order so that the microwave and magnetic fields can penltrate the material in the normal and superconducting states. Secondly, the upper critical field Hc2, of the superconducting films should be large enough so that the applied field for the CESR does not destroy the superconducting state. These conditions are well discussed by Kaplan5 and Aol and Swlhart6. Kaplan has shown that the observation of CESR in a thin film of superconductor is possible if the thickness of the film is nearly equal to ~V'2, where A is the penltration depth in the superconducting state. According to Pippard7, the skin depth ~ is generally of the same order a s A a n d for a sample of thickness less than ~ t h e constraints due to the Meilener effect and skin depth are circumvented. The calculations of Kaplan were extended by Aoi and Swihart for CESR in thin films of superconductors, taking into aCcount the changes in the diffusion and spin relaxation of the quasi particles in the superconducting state. Besides, the A-15 films are especially favourable for CESR studies as the degeneracy of their d-levels at the Fermi surface is considerably removed 8, making their spln-lattiCe rel~xatlon times to be of the order of IO-8-sec. In view of all these conslderations the CESR studies oz A-15 films was undertaken °
2. Experimental Thin films of A-15 ,,~/perc(mductors supperted deposited on alumina and having thickness 2OOOA O, were systematiCally investigated using the Varian V-4502 x-band ESR spectrOmeter and IOOKC/S magnetic field modulation. The spectra at liquid air temperature and above upto 45OK were obtained using Varlan V-4547 variable temperature accessory° The Varian V-4545A cavity with proper accessory was used for recording the spectra at the liquid helium temperature. The sample was pasted on the narrow wall ot the liquid helium cavity as pointed out by Aoi and Swlhar~ 6 that such a sample mounting arrangement is most favourable for observing CESR° The g-values were measured with respect to D P R ~ standard and the line width measurements were accurate upto + Ol(~m, 3. Results Fig.l, a, b, c, respectively depict the CESR and spin wa'~ spectra of three dizzerent A-15 superconductors, Nb3Ge(Tc~22 °65K)• v3Si(Tc~I7K) and Nb3Si(To~I5K) . Also shown in the figure(d), is the spectra obtained for a non-superconductlng saJnple Nb-Si. It is evident that a group of three lines is 117
I18
A-15 FILMS IN NORMAL ~ D
SUPERCONDUCTING STATES
Vol. 36, No. 2
WAVES o.
C[SR ~ ta )
~
~
v
5000 O, 2700 Oe
3700 Oe
~,_
NI~3 S(
Tc
=iSK ~
~
]
(©]
2700
Fig.1
:
CESR and spin wave spectra in Nb3Ge, V3Si, Nb3Si and Nb-Si.
of the same order aS calculated frOm the Fermi velocity VF and electron mean free II path of A-15 compounds. These facts confirm that the absorption near g=2 is indee~ due to the conduction electrons, and spin waves of anti ferromagnetic type should not be contused with Platzmann-Wolff spin waves of Fic. 2. Fir.3 shows the room temperature CESR and soin wave studies of Nb3Ge films of different Tc. The Tc of these samples varies from 8 to 22.65K. In all these samples, the line a t t r i b u t e d as CESR is always at g~2 and it is accompanied by spin waves in the couplet form. T h u s while the g values of CESR do not vary much from sample to sample, the position of spin waves varies considerably and an interesting observation is that for low T c samples these lines are quite away ~rom the CESR while for hlch T c samples t h e y are closer to the CESR (Table I). The positions of spin waves are orientation aependent ant these curves are for such orientations.of the applied magnetic field tot which the spin waves tot each sample are located farthest from the CESR llne. The orlentatlen studies ot the sample with Tc=22o65K have been depicted in Fig.4. It is observeu that on changing the orientation ot the magnetic field the two side absorptions merge into each other and then uJ - cOi D*K ~'..o ... .., I together they move towards the CESR llne, where ~ = 2~sH°/~Is the resonance frequency and at a certain angle all the three lines for CESR while~)5-_2~8Hs/~ is the resonm e r q e . Beyond this, the c o m b i n e d llne of ance f r e q u e n c y for spin wave modes. D* is side bands tends to move towards the high defined as anisOtroplc dlfruslon coefficifield side ot the CESR line. On further ent and K is the wa~,~ vector. In a slab rotation ot the magnetic field the twO lines ot thickness t, K takes on values multiples return to their earlier positions passing through the CESR line and get separated. ot T/t. The orientation behavlour is also This behaviour was observed in all samples similar to paramagnetlc spin waves behavldown to iiqui~ nitrogen t e m p e r a t u r e . This our. Moreover, the ~ R can be calculated clearly shows that the origin and properfrOm CESR by the Elllott IO relation. ties of all the three lines are interrelated. The behaviour ot two side absorptions i with the uirectlon of the magnetic field where ~Ris the collision time of conduction suggests strongly that they are spin wave electrons, ~ . I s the spin lattice relaxation absorptions in the conduction band . It is time calculated from ~ H , the line width discussed earller 2-4 that the spin waves and /~g,the difference between ge.l and gobAo___ are manifestations of exchange interactions The calculated value of ~ from ~ a t l o n of antiferromagnetlc type in the conductloll 2 is of the order of IO-14 sac. and it is
always present when the material is in the A-15 t o m , while only on~ line is to be seen when the A-15 phase is absent. The g values of all the lines lie near 2. It can also be seen that the llne which was previously established I as CESR is distinct from the two si~e absorptions ill a couplet ~orm O~ated on the low £iela si~e. The curve d)Is tot the non superconducting Nb-Si sample which depicts only one absorption near g=2. The x-ray diffraction analysis ot this sample failed to reveal the presence ot &-15 phase. The A-15 structure is characterlseG by orthogonal chains of densely packed transition metal ions. This strongly suggests that the lines in the couplet form can be most likely to be due to the ClOSe proximity of transltlcn metal ions 4. The detailed studies of Nb3Si, v3Si and Nb-Si samples is however a subject matter ot a separate communication to be published elsewhere. It may be pointed out that the two side absorptions, in the couplet form, which are identifled as spin waves of antlferromagnetic type, ~re distinct from the Platzmann.Wolff ~ lines which could also be resolved in some of the samples o~ Nb3Ge (Fig. 2). It is found that these lines obey the dispersion law, given by Platzmann-Wolf~,
~
V o l . 36, No. 2
A-15 FILM5 IN NORMAL AND SUPERCONDUCTING STATES
~
Fig.2
l
~
- i
IIUII
CESR
Tc(K) ' Line
I. 2.
' '
H
a) 2 2 . 6
~,xtmue~ clistance 'of
Siqnal
Width
g-value
n pp(Oe)
'
|
TVM
CESR, Platzmann-Wolzt type spin waves and a n t i f e r r o m a g n e t i c type ot spin w a v e s in a Nb3Ge sample.
TABLE
S.NO.'
119
43.5
spin w a v e s f r o m C E S R s i g n a l (Oe)
2.026
I14.8
21.3
47.5
2 .O43
623.2
c) 1 6 . 8
49.0
2 .O18
738 .O
52.O
2.079
836.4
b)
I
3. 4•
'
d)
8.0
|
t
¢(SR
,f~
S~N Y ~ S
3000 OI
wm~s
4
2
8
0
~
~ !
Fig.3
z
CESR and spin waves in Nb3Ce samples with v a r y i n g Tc(a) 22.65K (h) 21.3M (c) 16.8K and (d) 8.OK.
120
A-I 5 FILMS IN NORMAL AND SUPERCONDUCTING
Oo ClESR
C[SR
~"
~
- :3000 Oo
3 5 0 0 Oe e . 6 d '
~Oee.
~
V-
Orientation behavlour of CESR and spin waves in Nb3Ce sample of Tc~22.65K ( ~ i s the orientation angle of the tield).
NO00e
Vol. 36, No. 2
band, discu§sed by Vonsovskli add I g y m ~ 12, Overhauser I~ and Chan and Heine 14. The temperature variation of representative samples of Nb3Ge with T c of 21°3oK and 22.65K is shown in Figs. 5 and 6. It can be seen that the intensity of spin waves of antiferromagnetic type increases and their position is altered negliglbly. The spectra at 4.2K 2.e. when the samples are in the superconducting state, show neither the CESR line nor the spin waves absorptions. The vanishing of these lines gives a strong credence to the formation of sOme type of antiferrOmagnetic state in the conduction bandi-4.
4. D!SOU~S~OD8
3 ~ 0 o, e -,~o"
Fiq.4
STATES
The ~pectra of flgol suggest beyona any reaso~able aoubt tnat the presence of two side absorptions is relateu iutlmetely wltn tf,e A-15 str~cture. In the [l~, supercoxi~uctiA,~ Nb-~i sample, where the A-15 structure is not formed, the siae absorptioi,a are uot observe~. AS polntea out earlier2,3,4 the aenbe p a c k ~ 9 of transition ioL,s in ~-~5 challis Can leau to a~tirerromagnetic type of exchange interactions° In the A-15 superconductors the Fermi level is lOCated at the band edge of the sharp d level and when the A~I5 order is absent the d band is flattened and is lOCated se.oe the Fe .i eve14. The band at the Fermi sUrfaCe
T • 426 K
r"
2eo0 0q ~000,
3 6 0 0 Oe r
T * 300 K
5600 Oe
TeffOK
F
34i00 Oe !
OK Oe
Fig.5
TeItOK
T "4.2K
z
0
Temperature variation of CESR end spin waves for Nb3Ge with Tc=2"I.65K.
A-15 FILMS IN NORMAL AND SUPERCONDUCTING
Vol. 36, No. 2 ~
3
,
WAVES
,
~
~
I~ ~
/--,,,
=°* K.., 3 0 0 0
Ot
I
STATES
12 !
show clearly that the intensity o£ the spin waves is increasing as the temperature is lowered and, as discussed previously 3, it seems that some ordered m a ~ e t l c state is being stabilized. The observations at 4.2K, as will be seen, support this strongly. At 4.2K, when the samples are in superconducting state, all absorptions are vanished (Figs.5 & 6). The disappearance of CESR, which is contrary to theoretical calCUlations of Kapian 5 a~d Aol and Swihart6 who have argued that CESR should be observed in the superconducting state it it is found also in the normal state, suggests strongly 2-4 that the electrons responsible for superconductivity are bound magnetically. The disappearance of spin waves in the superconducting state further confirms this, as it is the manifestation of stabilized antlferromagnetlc state which can be excited only with the help ot high microwave frequencies and high magnetic tlel~s which wOUld necessarily revert the sample normal. 5o Conclusions
I0 KOe
Fig.6
!
T=4.2K
0 !
Temperature variation of CESR and spin waves tot N'o3Ge with Tc-21.3K.
reduces the possibly of exchange interaction in the conduction band and hence the two side absorptions are absent. This shows that the presence of spin waves is relevant to superconductivity. Further, the importance of these spin waves can be realised more when we analyse the result oZ fig.3. It can be seen that when Tc is low the separation between the CESR and the spin waves is large, while when Tc is high they are very much closer. This confirms that in high TC Nb3Ge samples d level lies at the Fermi surface and there are pronotu~ced s-d interactions, while in low Tc samples the Itinerary of d electrons at Fermi surface is very much diminished and s-d interactions are considerably reduced. It is generally known from the band structure of A-15 superconductor that when Tc is high a sharp a level is present at the Fermi surface which is expected to enhance s-d interactions. However, when Tc is low, a flat d band at the Fermi surface results which is expected to reduce the s-d interactions. According to Overhauser 13 and Chart and Helne 14 the presence of a sharp d band at the Fermi surface overlapped by s electrons gives rise to collective electron antlferrCma~netlsm in the conduction band known as the SDW state. Stearns has suggested that the increase in itlneraCy of d electr.oDs leads to the antlferromagnetlc c o u p l l n ~ 5 FrOm the above discussion, it is clear that when s-d exchange interactions o~ antlferromaqnetic type move towsrds conduction electrons Tc is enhanced, Furthermore, the temperature variation of these samples
I. CESR and spin waves have been observed in the normal state or A-15 superconductors studied, nanely Nb3Ge, Nb3Si ana V33i. 2. Absence or spin waves in the non-superconducting Nb-si sample, z:ot possessing the A-15 structurep suggests strongly that the occurrence ot s u ~ r c o n d u c t l v l t y is related to the presence or spin waves, which are manifestations of exchange interactions in conduction band and whose origin lles in the densely packed chains of transition ions ot a-15 lattice. 3. Studies of different ~b3Ge samples with difterent Tc values show that T c enhances when the exchange interactions move towards the CESR llne. The orientation response observed strongly suggests that the concerned interactions are of ant Ire rrOmagnet ic type. 4. Disappearance of CESR and span waves at 4.2K in the superconducting state conforms that the electrons responsible are bound through exchange izlteractlons of antiferromagnetic type and the resulting state can be a spi~ density wave state.
Acknowledgements - The authors would llke to thank Dr.J.E.Evetts and Dr.R.Scmekh of the University of Cambridge for providing high T c ~I/ms Ot t~b3~e. They are grateful to Pro£essor Jean Butter or the Swiss Federal Institute ot Technology in Lausanne, Dr.G.K.Shenoy of Argonne National Laboratory and Dr.V.U.S.RaO ot Pittsburgh Energy Technology Centre for helpful discussions, and to Professor K.P. Sinha and Professor R. Srinivasan ot the Indian Institute of Science, Bangalore for encouragement. The authors thank the Director ~atlOnal Physical Laboratory[ tot provision of facilities. This work was rundee by the Department of Science and Technoloqy.
122
A-15 FILMS IN NORMAL AND SUFERCONDUCTING STATES
Vol. 36. No. 2
References I.
S.s.Ekbote, ~.K.~upta and A.V. ~arllkar, Indian J.Phys. (~ov .1979) In Press.
2.
S.~,.Ekbote, S.K.Gupta and A.V. sarlikar, ~ndlan J.Phys.(Jan.1980) In Press.
3.
S.~.Ekbote, S.K.Gupta and A.V. ~arlikar, Current Science 49,144
8.
D.Dew-Hughes, Cryogenics 15 no.S, 435 (1975).
9.
P.MoPlatzmar~n a~d P.A.Wolff, Phys. Rev. Letters 18,280, (1967).
IO°
R.J.Elliott, Phys.Rev096,266, (1954).
11.
A.J.Arko, D.H.Lowndes, F.AoMuller, L.W. Roeland, JoWOlfrat, F.M oMUeller and G..W.Webb, Journal de Physique 39, C6,412(1978). S.V.VonsovsRii and %~u,A.Izyumov, Soy° Phys •Uspekhl, 5, 723 (19 63 ) ° A.W.Overhauser, Phys.Rev.128,1437 (1962) o S.K.Chan and V.Heine, J.Phys.F,3, 975 (1973). M.B.Stearns, Phys. Rev.B 4,4069 an~
(19~o). 4.
S.~.Zkbote and A.V.~arlikar, Material Research Bulletim (June 1980) In Press.
5o
J.I.Kaplan, Phys.Letters 19,266,
12. 13.
(1965). 6. 7.
K.Aol and J.C.Swihart, Phys. Rev. 8.2, 2555, (1970). A.D. Plppard, Proc. Roy.Soc.A 191, 385 (1947),
14. 15.
4o6l ( t 9 7 . t ) .
Solid State Communications, Voi.36, pp. I17-122. Pergamon Press Ltd. 1980. Printed in Great Britain.
THE CESR AND SPIN WAVE STUDIES OF A-IS FILMS IN NOP/~AL AND SUPERCONDUCTING STATES S.~.Ekbote, S.K.Gupta and A.V.Narllkar National Physical Laboratory, Hillside Road, New Delhi-llOOl2, India
(Received 18 J u l y 1980 by S. Amelinckx) The electron spin resonance studies have been reported for A-15 superconductors, namely Nb3Ge, Nb3Si and V3SI possessIng different T c values.and CESR, Platzmar~-Wolff type spin waves, and spin waves of antiferromagnetic type are observed in all the samples. It is found that Tc o f Nb3Ge depends upon the presence and separation of spin wave absorptions from the CESR, and Tc is found to increase when the separation is reduCed. It is concluded that the exchange interactions In.the conduction band, as manifested by the behaviour of spin waves, are of antiferromagnetic type and they are responsible tot superconductivity in A-15 materials studied°
1. Introduction
Our earlier work I-3, on ~b3Ge ot TC~22o65K, in the normal state, showed the presence of CESR line toQether with two other absorptions which were identltied as spin waves having antiterromaQnetic character. All these lines had however vanished when the material was in the superconducting state.The disapPearanCe o t lines at 4.2K strongly suggested that the pair formation in superconducting state was due to exchange interactions, contrary to the BCS mechanism. In this paPer we report studies of Nb3Ge films with To varying from 8K to 22.65K. For the sake of comparison, the results obtained on two other A-15 materials namely Nb3Si and V3Si are also reported.
In the previous papersl-4,we had reported our CESR and spin wave resonance studies of high Tc(Tc~22.65K ) sb3Oe films of A-15 structure in normal and superconducting states. A detailed discussion or difficulties involved in the observation of CESR in the normal and superconducting states was given in the above publications. It was established that under favourable conditions CESR and spin waves resonanCe was possible in thin A-15 films° In 9eneral, there are two main COlldltions which are to be satisfied for the observation of CESR. Firstly, the thickness of the films must be of the appropriate order so that the microwave and magnetic fields can penltrate the material in the normal and superconducting states. Secondly, the upper critical field Hc2, of the superconducting films should be large enough so that the applied field for the CESR does not destroy the superconducting state. These conditions are well discussed by Kaplan5 and Aol and Swlhart6. Kaplan has shown that the observation of CESR in a thin film of superconductor is possible if the thickness of the film is nearly equal to ~V'2, where A is the penltration depth in the superconducting state. According to Pippard7, the skin depth ~ is generally of the same order a s A a n d for a sample of thickness less than ~ t h e constraints due to the Meilener effect and skin depth are circumvented. The calculations of Kaplan were extended by Aoi and Swihart for CESR in thin films of superconductors, taking into aCcount the changes in the diffusion and spin relaxation of the quasi particles in the superconducting state. Besides, the A-15 films are especially favourable for CESR studies as the degeneracy of their d-levels at the Fermi surface is considerably removed 8, making their spln-lattiCe rel~xatlon times to be of the order of IO-8-sec. In view of all these conslderations the CESR studies oz A-15 films was undertaken °
2. Experimental Thin films of A-15 ,,~/perc(mductors supperted deposited on alumina and having thickness 2OOOA O, were systematiCally investigated using the Varian V-4502 x-band ESR spectrOmeter and IOOKC/S magnetic field modulation. The spectra at liquid air temperature and above upto 45OK were obtained using Varlan V-4547 variable temperature accessory° The Varian V-4545A cavity with proper accessory was used for recording the spectra at the liquid helium temperature. The sample was pasted on the narrow wall ot the liquid helium cavity as pointed out by Aoi and Swlhar~ 6 that such a sample mounting arrangement is most favourable for observing CESR° The g-values were measured with respect to D P R ~ standard and the line width measurements were accurate upto + Ol(~m, 3. Results Fig.l, a, b, c, respectively depict the CESR and spin wa'~ spectra of three dizzerent A-15 superconductors, Nb3Ge(Tc~22 °65K)• v3Si(Tc~I7K) and Nb3Si(To~I5K) . Also shown in the figure(d), is the spectra obtained for a non-superconductlng saJnple Nb-Si. It is evident that a group of three lines is 117
I18
A-15 FILMS IN NORMAL ~ D
SUPERCONDUCTING STATES
Vol. 36, No. 2
WAVES o.
C[SR ~ ta )
~
~
v
5000 O, 2700 Oe
3700 Oe
~,_
NI~3 S(
Tc
=iSK ~
~
]
(©]
2700
Fig.1
:
CESR and spin wave spectra in Nb3Ge, V3Si, Nb3Si and Nb-Si.
of the same order aS calculated frOm the Fermi velocity VF and electron mean free II path of A-15 compounds. These facts confirm that the absorption near g=2 is indee~ due to the conduction electrons, and spin waves of anti ferromagnetic type should not be contused with Platzmann-Wolff spin waves of Fic. 2. Fir.3 shows the room temperature CESR and soin wave studies of Nb3Ge films of different Tc. The Tc of these samples varies from 8 to 22.65K. In all these samples, the line a t t r i b u t e d as CESR is always at g~2 and it is accompanied by spin waves in the couplet form. T h u s while the g values of CESR do not vary much from sample to sample, the position of spin waves varies considerably and an interesting observation is that for low T c samples these lines are quite away ~rom the CESR while for hlch T c samples t h e y are closer to the CESR (Table I). The positions of spin waves are orientation aependent ant these curves are for such orientations.of the applied magnetic field tot which the spin waves tot each sample are located farthest from the CESR llne. The orlentatlen studies ot the sample with Tc=22o65K have been depicted in Fig.4. It is observeu that on changing the orientation ot the magnetic field the two side absorptions merge into each other and then uJ - cOi D*K ~'..o ... .., I together they move towards the CESR llne, where ~ = 2~sH°/~Is the resonance frequency and at a certain angle all the three lines for CESR while~)5-_2~8Hs/~ is the resonm e r q e . Beyond this, the c o m b i n e d llne of ance f r e q u e n c y for spin wave modes. D* is side bands tends to move towards the high defined as anisOtroplc dlfruslon coefficifield side ot the CESR line. On further ent and K is the wa~,~ vector. In a slab rotation ot the magnetic field the twO lines ot thickness t, K takes on values multiples return to their earlier positions passing through the CESR line and get separated. ot T/t. The orientation behavlour is also This behaviour was observed in all samples similar to paramagnetlc spin waves behavldown to iiqui~ nitrogen t e m p e r a t u r e . This our. Moreover, the ~ R can be calculated clearly shows that the origin and properfrOm CESR by the Elllott IO relation. ties of all the three lines are interrelated. The behaviour ot two side absorptions i with the uirectlon of the magnetic field where ~Ris the collision time of conduction suggests strongly that they are spin wave electrons, ~ . I s the spin lattice relaxation absorptions in the conduction band . It is time calculated from ~ H , the line width discussed earller 2-4 that the spin waves and /~g,the difference between ge.l and gobAo___ are manifestations of exchange interactions The calculated value of ~ from ~ a t l o n of antiferromagnetlc type in the conductloll 2 is of the order of IO-14 sac. and it is
always present when the material is in the A-15 t o m , while only on~ line is to be seen when the A-15 phase is absent. The g values of all the lines lie near 2. It can also be seen that the llne which was previously established I as CESR is distinct from the two si~e absorptions ill a couplet ~orm O~ated on the low £iela si~e. The curve d)Is tot the non superconducting Nb-Si sample which depicts only one absorption near g=2. The x-ray diffraction analysis ot this sample failed to reveal the presence ot &-15 phase. The A-15 structure is characterlseG by orthogonal chains of densely packed transition metal ions. This strongly suggests that the lines in the couplet form can be most likely to be due to the ClOSe proximity of transltlcn metal ions 4. The detailed studies of Nb3Si, v3Si and Nb-Si samples is however a subject matter ot a separate communication to be published elsewhere. It may be pointed out that the two side absorptions, in the couplet form, which are identifled as spin waves of antlferromagnetic type, ~re distinct from the Platzmann.Wolff ~ lines which could also be resolved in some of the samples o~ Nb3Ge (Fig. 2). It is found that these lines obey the dispersion law, given by Platzmann-Wolf~,
~
V o l . 36, No. 2
A-15 FILM5 IN NORMAL AND SUPERCONDUCTING STATES
~
Fig.2
l
~
- i
IIUII
CESR
Tc(K) ' Line
I. 2.
' '
H
a) 2 2 . 6
~,xtmue~ clistance 'of
Siqnal
Width
g-value
n pp(Oe)
'
|
TVM
CESR, Platzmann-Wolzt type spin waves and a n t i f e r r o m a g n e t i c type ot spin w a v e s in a Nb3Ge sample.
TABLE
S.NO.'
119
43.5
spin w a v e s f r o m C E S R s i g n a l (Oe)
2.026
I14.8
21.3
47.5
2 .O43
623.2
c) 1 6 . 8
49.0
2 .O18
738 .O
52.O
2.079
836.4
b)
I
3. 4•
'
d)
8.0
|
t
¢(SR
,f~
S~N Y ~ S
3000 OI
wm~s
4
2
8
0
~
~ !
Fig.3
z
CESR and spin waves in Nb3Ce samples with v a r y i n g Tc(a) 22.65K (h) 21.3M (c) 16.8K and (d) 8.OK.
120
A-I 5 FILMS IN NORMAL AND SUPERCONDUCTING
Oo ClESR
C[SR
~"
~
- :3000 Oo
3 5 0 0 Oe e . 6 d '
~Oee.
~
V-
Orientation behavlour of CESR and spin waves in Nb3Ce sample of Tc~22.65K ( ~ i s the orientation angle of the tield).
NO00e
Vol. 36, No. 2
band, discu§sed by Vonsovskli add I g y m ~ 12, Overhauser I~ and Chan and Heine 14. The temperature variation of representative samples of Nb3Ge with T c of 21°3oK and 22.65K is shown in Figs. 5 and 6. It can be seen that the intensity of spin waves of antiferromagnetic type increases and their position is altered negliglbly. The spectra at 4.2K 2.e. when the samples are in the superconducting state, show neither the CESR line nor the spin waves absorptions. The vanishing of these lines gives a strong credence to the formation of sOme type of antiferrOmagnetic state in the conduction bandi-4.
4. D!SOU~S~OD8
3 ~ 0 o, e -,~o"
Fiq.4
STATES
The ~pectra of flgol suggest beyona any reaso~able aoubt tnat the presence of two side absorptions is relateu iutlmetely wltn tf,e A-15 str~cture. In the [l~, supercoxi~uctiA,~ Nb-~i sample, where the A-15 structure is not formed, the siae absorptioi,a are uot observe~. AS polntea out earlier2,3,4 the aenbe p a c k ~ 9 of transition ioL,s in ~-~5 challis Can leau to a~tirerromagnetic type of exchange interactions° In the A-15 superconductors the Fermi level is lOCated at the band edge of the sharp d level and when the A~I5 order is absent the d band is flattened and is lOCated se.oe the Fe .i eve14. The band at the Fermi sUrfaCe
T • 426 K
r"
2eo0 0q ~000,
3 6 0 0 Oe r
T * 300 K
5600 Oe
TeffOK
F
34i00 Oe !
OK Oe
Fig.5
TeItOK
T "4.2K
z
0
Temperature variation of CESR end spin waves for Nb3Ge with Tc=2"I.65K.
A-15 FILMS IN NORMAL AND SUPERCONDUCTING
Vol. 36, No. 2 ~
3
,
WAVES
,
~
~
I~ ~
/--,,,
=°* K.., 3 0 0 0
Ot
I
STATES
12 !
show clearly that the intensity o£ the spin waves is increasing as the temperature is lowered and, as discussed previously 3, it seems that some ordered m a ~ e t l c state is being stabilized. The observations at 4.2K, as will be seen, support this strongly. At 4.2K, when the samples are in superconducting state, all absorptions are vanished (Figs.5 & 6). The disappearance of CESR, which is contrary to theoretical calCUlations of Kapian 5 a~d Aol and Swihart6 who have argued that CESR should be observed in the superconducting state it it is found also in the normal state, suggests strongly 2-4 that the electrons responsible for superconductivity are bound magnetically. The disappearance of spin waves in the superconducting state further confirms this, as it is the manifestation of stabilized antlferromagnetlc state which can be excited only with the help ot high microwave frequencies and high magnetic tlel~s which wOUld necessarily revert the sample normal. 5o Conclusions
I0 KOe
Fig.6
!
T=4.2K
0 !
Temperature variation of CESR and spin waves tot N'o3Ge with Tc-21.3K.
reduces the possibly of exchange interaction in the conduction band and hence the two side absorptions are absent. This shows that the presence of spin waves is relevant to superconductivity. Further, the importance of these spin waves can be realised more when we analyse the result oZ fig.3. It can be seen that when Tc is low the separation between the CESR and the spin waves is large, while when Tc is high they are very much closer. This confirms that in high TC Nb3Ge samples d level lies at the Fermi surface and there are pronotu~ced s-d interactions, while in low Tc samples the Itinerary of d electrons at Fermi surface is very much diminished and s-d interactions are considerably reduced. It is generally known from the band structure of A-15 superconductor that when Tc is high a sharp a level is present at the Fermi surface which is expected to enhance s-d interactions. However, when Tc is low, a flat d band at the Fermi surface results which is expected to reduce the s-d interactions. According to Overhauser 13 and Chart and Helne 14 the presence of a sharp d band at the Fermi surface overlapped by s electrons gives rise to collective electron antlferrCma~netlsm in the conduction band known as the SDW state. Stearns has suggested that the increase in itlneraCy of d electr.oDs leads to the antlferromagnetlc c o u p l l n ~ 5 FrOm the above discussion, it is clear that when s-d exchange interactions o~ antlferromaqnetic type move towsrds conduction electrons Tc is enhanced, Furthermore, the temperature variation of these samples
I. CESR and spin waves have been observed in the normal state or A-15 superconductors studied, nanely Nb3Ge, Nb3Si ana V33i. 2. Absence or spin waves in the non-superconducting Nb-si sample, z:ot possessing the A-15 structurep suggests strongly that the occurrence ot s u ~ r c o n d u c t l v l t y is related to the presence or spin waves, which are manifestations of exchange interactions in conduction band and whose origin lles in the densely packed chains of transition ions ot a-15 lattice. 3. Studies of different ~b3Ge samples with difterent Tc values show that T c enhances when the exchange interactions move towards the CESR llne. The orientation response observed strongly suggests that the concerned interactions are of ant Ire rrOmagnet ic type. 4. Disappearance of CESR and span waves at 4.2K in the superconducting state conforms that the electrons responsible are bound through exchange izlteractlons of antiferromagnetic type and the resulting state can be a spi~ density wave state.
Acknowledgements - The authors would llke to thank Dr.J.E.Evetts and Dr.R.Scmekh of the University of Cambridge for providing high T c ~I/ms Ot t~b3~e. They are grateful to Pro£essor Jean Butter or the Swiss Federal Institute ot Technology in Lausanne, Dr.G.K.Shenoy of Argonne National Laboratory and Dr.V.U.S.RaO ot Pittsburgh Energy Technology Centre for helpful discussions, and to Professor K.P. Sinha and Professor R. Srinivasan ot the Indian Institute of Science, Bangalore for encouragement. The authors thank the Director ~atlOnal Physical Laboratory[ tot provision of facilities. This work was rundee by the Department of Science and Technoloqy.
122
A-15 FILMS IN NORMAL AND SUFERCONDUCTING STATES
Vol. 36. No. 2
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