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Superconductivity in niobium triselenide
S o l i d S t a t e C o ~ n m i c a t l o n s , Voi.36, pp.983-988. Pergamon P r e s s L t d . 1980. P r i n t e d i n G r e a t B r i t a l n .
SUPERCONDUCTIVITY
IN NIOBIUM TRISELENIDE
C.M. Bastuscheck and R.A. Buhrman S c h o o l o f A p p l i e d and E n g i n e e r i n g P h y s i c s , I t h a c a , NY 14853
Cornell University,
J . D . Kulick and J . C . S c o t t L a b o r a t o r y o f Atomic and S o l i d S t a t e P h y s i c s , Ithaca, NY 14853
Cornel! University,
(Received 12 3une 1980 by A.G. Chynoweth) Sensitive magnetic susceptibility measurements on NbSe 3 reveal a large temperature dependent diamagnetism and flux trapping below about 2K. Flux exclusinn is ~,nh;mced by etching and compacting the polycrystalllne samples, as expected for a weakly coupled assembly o'f superconducting crystallites. Single crystal resistivity measurements reveal an anomaly at 2K, which is depressed in temperature by applied meg.eric fields. However, the temperature and field dependence of the res|stivlty, especially the fact that it does not vanish, are .oL explained.
Niobium t r i s e l e n i d e i s an a n i s o t r o p i c metallic compound which has been the o b j e c t o f considerable recent attention, primarily because o f the n o n - l i n e a r c o n d u c t i v i t y [1,2,3,] associated with the onset of two distinct charge-denslty wave distortions [4,5,6]. In addition, unusual r e s i s t i v i t y b e h a v i o r a t low temp e r a t u r e has been r e p o r t e d [ 7 , 8 ] . While M e i s s h e r e f f e c t measurements have been p r e s e n t e d [ 9 ] as e v i d e n c e f o r a s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e which i n c r e a s e s r a p i d l y w i t h p r e s s u r e , t h i s Communication r e p o r t s the f i r s t c o n clusive e v i d e n c e t h a t the m a t e r i a l is superconducting at zero pressurez namely f l u x e x c l u s i o n and f l u x trapping. The f i e l d and ten~p e r a t u r e dependence of t h e s u p e r c o n d u c t i n g d i a magnetism i s q u i t e s i m i l a r t o t h a t o f t h e a n a l ogous compound TaSe~ [ l O ] w h i c h , l i k e NbSe~, grows as s m a l l f i b r o u s c r y s t a l l i t e s . We sh~w t h a t the f i b r o u s n a t u r e of NbSe 3 i s an i m p o r t a n t f a c t o r i n d e t e r m i n i n g the m a g n e t i c s u p e r c o n d u c t i n g b e h a v i o r of a macroscopic sample. The compound was p r e p a r e d by the u s u a l method [ 1 1 ] o f h e a t i n g s c o i c h i o m e t r i c amounts o f e l e m e n t a l n i o b i u m and s e l e n i u m f o r s e v e r a l weeks i n an e v a c u a t e d q u a r t z t u b e . S i n g l e c r y s tals approximately 0.01 x O . l x lO mm were obtained for resistivity measurements by p r o loneed heating of the reaction mixture at 700~C w i t h a t e m p e r a t u r e g r a d i e n t of §O°C a l o n g the 15 cm t u b e . P o l y c r y s t a l l i n e samples f o r susceptibility measurements were o b t a i n e d by r e a c t i o n a t 800°C f o r two weeks w l t h r e l a t i v e l y rapid cooling (6 d a y s ) t o room t e m p e r a t u r e . This material formed in mats of s m a l l f i b e r s e n d r i b b o n s , w h i c h bad l e n g t h s from 0 . 0 5 t o I mm and c r o s s d i m e n s i o n s f r o m 2 t o IO, m. DebyeScherrer x-ray diffraction was u s e d t o c o n f i r m t h e c o m p o s i t i o n of t h e p o l y c r y s t a l l t n e sample. T h i r t e e n l i n e s were o b s e r v e d and i n d e x e d w i t h t h e p u b l i s h e d [ 5 ] NbSe~ structure. A careful s e a r c h was made f o r d i f f r a c t i o n which c o u l d be
a s s o c i a t e d w i t h any o f t h e d i s e l e n l d e p o l y t y p e s and none was found ( a t the l e v e l o f one p a r t i n a t h o u s a n d ) . Scanning e l e c t r o n m i c r o s c o p y w i t h a r e s o l u t i o n of IOOX f a i l e d t o r e v e a l any v i s i b l e s u r f a c e i n h o m o g e n e i t y o t h e r than s t r i a t i o n s and c r a c k s r u n n i n g p a r a l l e l t o the b a x i s . Scanning Auger m i c r o a n a l y s i s , w i t h depth profiling, (2, horizontal resolution and l O 0 ~ d e p t h r e s o l u t i o n ) showed t h a t the s u r f a c e s o f t y p i c a l c r y s t a l s are s e l e n i u m r i c h r e l a t i v e t o the b u l k , but no o t h e r c o m p o s i t i o n a l lnhomog e n e i t y c o u l d be d e t e c t e d . R e s i s t i v i t y measurements were made as a f u n c t i o n o f t e m p e r a t u r e , m a g n e t i c f i e l d and c u r rent density, using conventional four-probe a . c . t e c h n i q u e s . To d a t e , 16 samples have been examined, 8 being aligned w i t h the m a g n e t i c field of a superconducting solenoid parallel to t h e b - a x i s ( t h e f i b e r d i r e c t i o n and the a x i s o f highest conductivity) 4 w i t h t h e f i e l d a l o n g a* (i.e. p e r p e n d i c u l a r t o t h e r i b b o n ) and 4 w i t h the field along the c-axis. Room t e m p e r a t u r e r e s £ s t i v i ~ i e s f e l l i n r.he range 0 . 3 - 2 . 0 x 10 - 3 Rcm, and r e s i d u a l r e s i s t a n c e r a t i o s liar = p ( 3 O O K ) / p ( 4 . 2 K ) ] v a r i e d between 20 and 250. A l l samples e x h i b i t e d t h e p r e v i o u s l y r e ported [1,2,3] resistive a n o m a l i e s a t 140K and 59K. I n a l l samples a d r o p i n r e s i s t i v i t y was o b s e r v e d t o o c c u r between 2 and 2.§K (see f i g u r e l a ) . As w i t h t h e d a t a p u b l i s h e d by Haen e t al. [ 7 , 8 ] t h e d e t a i l s o f t h i s low t e m p e r a t u r e anomaly were f o u n d t o be sample (and h i s t o r y ) dependent, the drop in resistance amounting to between 5% and 70% o f i t s v a l u e a t 4K. There i s no o b v i o u s c o r r e l a t i o n o f t h e m a g n i t u d e o f the resistive anomaly w i t h RRR, r e s i s t a n c e v a l u e a t 4K, sample d i m e n s i o n o r t h e b a t c h from which t h e c r y s t a l s were s e l e c t e d . The m a g n e t i c f i e l d dependence o f t h e r e sistivity can be seen i n f i g u r e s I and 2 f o r
983
SUPERCONDUCTIVITY IN NIOBIUM TRISELENIDE
,t.L~rL~j
H=27kG
E u
~
S
~
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I
=L
f i g u r e 3 ] . T h i s c u r r e n t c o r r e s p o n d s to a power d i s s i p a t i o n of l e s s than 1 pW, hence we r u l e ouc h e a t i n g as a source o f the n o n - l i n e a r i c y . A l l the measurements o f 0(T,H) discussed in the p r e c e d i n g p a r a g r a p h s were made i n the l i n ear r e g i o n o f t h e I - Y c h a r a c [ e r i s t i c s . M a g n e t i z a t i o n measurements were made on t h r e e samples p r e p a r e d Co i n v e s t i g a t e the e f f e c t o f changes tn I n t e r - f i b e r c o u p l i n g . A11 measurements were made on m a t e r i a l grown i n a single
[
[
20 kbars
[
o
I I I
I I I I t t t I t t
20 (.9 t,,,* 15 -!I0
%
%
\
I #
tube. Sample A consisted of 94 mg
°"""--~-%~'-~ 2
T, Fig. 1 a) Resistivity several values of applied
I
.3
4
K vs. field
temperature for p a r a l l e l co the
current.
b) Ef£ecctve critical field, extracted from curves o f t h e type shown i n f i g u r e a, a s e x p l a i n e d in t h e t e x t . f i e l d s p a r a l l e l t o the b and a* axes. The data f o r the c d i r e c t i o n Is t n t e ~ e d t a c e between the b e h a v t o u r shown. The data o f f i g u r e l a a l l o w us t o d e f i n e an e f f e c t i v e upper c r i t i c a l field, H*c~T), as the f i e l d r e q u i r e d t o depress t o T the t e m p e r a t u r e o f the r e s i s t i v e anomaly. ( F o r convenience t h i s t e m p e r a t u r e was a r b i t r a r i l y sel e c t e d as t h a t a t which the r e s i s t a n c e drops by I0% o f i t s 4K v a l u e . ) As shown i n f i g . l b , f o r f i e l d s a p p l i e d a l o n g the b - a x i s ~c2 i s on the s c a l e o f lO kgauss, whereas f o r Ella the v a l u e i s s e v e r a l hundred gauss. H : 2 ( l l c ) i s I n the k i l o g a u s s r a n g e , though noc so l a r g e es harc2(llb). F u r t h e r work i s b e i n g done t o c l a r i f y this anisocropy. As r e p o r t e d by Haed ec a l . [ 7 , 8 ] the I - V characteristic is highly non-linear. We have made measurements i n the range o f c u r r e n t dens l a y 0.0005< J < 0.50 A/cm2 and f i n d t h a t the non-Ohmic b e h a v t o u r onsets a t a p p r o x i m a t e l y lO'2A/cm 2 (i.e. O.I~A i n our s a m p l e s ) . [See
to form a cylinder 3 mm long by 2.5 n~
d i a m e t e r . Sample C c o n s i s t e d of 135 mg o f s i m i l a r l y compacted m a t e r l a l , buc the f i b r o u s NbSe 3 was etched tn dilute H2S04 (in which selenium iS s o l u b l e ) , r i n s e d and a i r d r i e d b e f o r e compaction. The compacted samples were removed f r o g the d i e press and t r a n s f e r r e d co the c r y o s t a t , where measurements were p e r f o r m e d a t zero p r e s sure The m a g n e t i z a t i o n measurements were made w i t h SQUID magnetometers £n two s e p a r a t e c r y o s c a r s , one c o o l e d w i t h a pumped 3He poc, the o c h e r by a d i l u t i o n refrigerator. The s t a t i c m a g n e t i z a t i o n , H. , was measured by sweeping ac t e m p e r a t u r e tn c o n s t a n t m a g n e t i c f i e l d H, i n fields from less than O.1 mOe to g r e a t e r than lO00e. The t e m p e r a t u r e dependent ac s u s c e p t i bility was d e t e r m i n e d by measuring the m a g n e t i z a t i o n response to a s m a l l ac f i e l d ( N 0 . 5 mOe ac 0.08 Hz) a p p l i e d p a r a l l e l co the static field. F i g . 4 shows Xd ~ Hdc/H vs T f o r the three samples m e a s u r e ~ i n c o n s t a n t applied field of 80 mOe. All t h r e e samples show a strongly t e m p e r a t u r e dependent d i a m a g n e t i s m a t low t e m p e r a t u r e s , but even ac the l o w e s t t e m p e r acute o n l y a f r a c t i o n o f t o t a l f l u x e x p u l s i o n iS a c h i e v e d i n t h i s f i e l d . Sample A ( a s - g r o w n ) had an o b s e r v a b l e d i a m a g n e t i c s i g n a l to above 1.2 K~ where lC became s m a l l e r than the r e s o | u l i o n o f the i n s t r u m e n t . At the s c a l e o f F i g . 4 the s i g n a l cannot be seen above 0 . 6 K. Sample B ( c o m p a c t e d ) shows n o t o n l y more f l u x e x p u l s ion at all t e m p e r a t u r e s , but a l s o a s l i g h t l y d i f f e r e n ~ shape f o r X vs T. Sample C ( e t c h e d and compacted) shows s t i l l stronger flux expulsi o n and a t e m p e r a t u r e dependence i n which ~ t e n d s t o l e v e l o f f a t low t e m p e r a t u r e s . shape o f t h e Hdc(T) curve f o r each sample changes b u t s l i g h t l y with applied field for H a t l e a s t as h i g h as 20 Oe, o n l y the magnitude does noc s c a l e w i t h a p p l i e d f i e l d (see below~ and F i g . 6 ) . ~dc(T) showed no t h e r m a l h y s t e r e s i s ; warming and t r a c e s are i d e n t i c a l . F i g u r e § shows the t e m p e r a t u r e dependent d i a m a g n e t i s m and f l u x t r a p p i n g f o r sample B ( c o m p a c t e d ) near the t e m p e r a t u r e where the low t e m p e r a t u r e r e s i s t i v e anomaly has been observed ( i n the a s - g r o ~ n m a t e r i a l ) . Between 0.5 K and 1.1 K the t e m p e r a t u r e dependent s u s c e p t i b i l ities of samples A and B are v e r y s i m i l a r in shape, w i t h X(B) a p p r o x i m a t e l y f i f t y rimes x(A). We c o n s i d e r t h i s s i m i l a r i t y to be e v i dence t h a t the c o m p a c t i o n has caused no i r r e •
b%
O
quartz
of the f i b r o u s m a t e r i a l l o o s e l y assembled i n a volume 3 x 4 x 10 nrn. Sample B c o n s i s t e d of 90 mg o f t h e m a t e r i a l compacted u s i n g a h y d r a u l i c p r e s s w i t h a p p l i e d p r e s s u r e s of a p p r o x i m a t e l y
H=OkG
2,5
Vol. 36, No. I I
Vol.
36, No. 11
SUPEKCONDUCTIVITY IN NIOBIUN TRISELENIDE
9~
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8
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at
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986
SUPERCONDUCTIVITY IN NIOBItrH TRISELENIDE
NbSe 3
V o l . 36, No. 11
Xcx: vs T H = 8 0 mOe
Samples:
'o_
o A As grown
X
A B o
Compacted
C Etched and compacted
O
I
~
0
I
~
0.4
0,8
T,
K
Fig." 4 X vs T for three samples (see text). Total Melssner effect is M = - H / 4 w ) . Uncertalnty in absolute magnitude is 50% but the curves are Intercomparable wlchin 20%. The magnetic field was applied above 3 K, the sample was cooled to 30 r~, and the change in magnetization was recorded while the sample was warmed slowly. The solid lines guide the eye.
versible pressure change in the NbSe 3. Above 1.2 K the expected change in the susceptibility of the as-grown sample (assuming this similarIcy) is smaller than the resolution of the magnetometer. Flux trapping was observed in all three samples, and was observed in applied fields as small as 3 mOe. The dependence of .M~c(~f.~ on the applied field H is shown in Fig ~ r the etched and compacted material. Mdc i s p r o p o r t i o n a l to H o n l y in t h e very low field limit (H 0.5 mOe), while above O.l Oe M is nearly constant. This behavior was observed in al~ three samples, and is similar to that observed i n TaSe 3 [lO] and (SN) x [12,13]. As i s the case for any hystereric magnetic system, this plot of Mdc(T) vs H is not equivalent to a direct measurement of M vs H made by sweeping the field. We have made such measurements and find that [H(H)/H]T.~ons~ decreases much more slowly than~ as H is increase d • [M(T)/H]L~e t e m p e r a t u r e dependence o f the low f r e quency ac s u s c e p t i b i l i t y was n e a r l y i d e n t i c a l t o M (T) f o r each sample, and i n ~he v e r y low field limit Xac(T)~Xdc(T). In higher fields Xac(T) had the same temperature dependence as
Xd c buc, w h i l e Xd C f e l l q u i c k l y as H was i n c r e a s e d o v e r 0 . 5 mOe, Xac d e c r e a s e d o n l y v e r y slowly, falling Co h a l f i~s z e r o f i e l d v a l u e ac H ~5OO Oe. The diamagnetism and flux trapping clearly demonstrate that NbSe 3 contains superconducting elements with zero applied pressure below 2.1 ± 0.2 K. The onset of the magnetic transltion correlates well with the drop in the reslstlvtty observed a t 2.1 ± 0.2K. The magnetic and resistive behavior is not thac of a bulk superconductor, nor is the diamagnetic susceptlb i l i t y that expected for a collection of isolated superconducting fibers, being larger, non-llnear i n H, a n d s t r o n g l y v a r y i n g ac low temperatures. The o b s e r v e d m a g n e t i c b e h a v i o r i s consistent with the behavior o f an ensemble o f small superconductoring elements with weak inter-element coupling. The t e m p e r a t u r e and field dependence of the diamagnetism is determ i n e d by t h e strength of the inter-element coupling and by the condition o f fluxold quantization about multiply connected current p a t h s . The behavior o [ the as-grown NbSe- sample indicates chat here the Inter-elemen~ coupling is
s u f f i c i e n t l y weak that only at quite low temper-
Vol. 36, No. I I
SUPERCONDUCTIVITY IN NIOBIUM TRISELENIDE
987
1.5
Sample B
NbSe 3
H=I.20e
'0 - -
X
o
Me)c
I.O
L~ Trapped flux
<3 x
/ IOO
1 V x'°°
0.5
I
T,
K
Fig. 5 Flux expulsion while cooling and flux release upon warming after the field was removed at T = 0.48 K, The vertical scale is the same for bach curves (bach are normalized to H - 1.2 Oe).
a t u r e s and low f i e l d s can s i g n i f i c a n t diamagneti c s u p e r c u r r e n t s be s u s t a i n e d between the s u p e r conducting filaments. E v i d e n t l y the e f f e c t of c o m p a c t i o n and e t c h i n g is co i n c r e a s e the tnter-tlement coupling. The d i s c r e p a n c y between o u r d a t a and t h a t of Monceau e t a l [9], who observed d i a m a g n e t i s m o n l y under p r e s s u r e , can b e u n d e r s t o o d in terms o f the g r e a t e r s e n s i t i v i t y of our SQUID magnetometer, and the v e r y s m a l l magnitude o f Xdc f o r f i b r o u s NbSe~ i n f i e l d s o f I Oe and l a r g e r . I n t h e p r e s e n t w~rk i t i s f o u n d tha~ Xdc a p p r o a c h e s s e v e r a l t e n t h s of c o r a l f l u x e x c l u s i o n ac tow t e m p e r a t u r e in the v e r y low f i e l d ( 0.5 mOe) l i m i t , f a l l s c o - - t O - 2 o f t o t a l f l u x e x c l u s i o n by O . 1 0 e and by a n o t h e r f a c t o r o f tO a t 1.00e. These o b s e r v a t i o n s are s u g g e s t i v e of a transition i n which s e p a r a t e s m a l l r e g i o n s , w i t h dimensions much t e s s than the p e n e t r a t i o n depth, are individually superconducting below a b o u t 2 . l K and become p r o g r e s s i v e l y coupled to g i v e weak phase coherence o v e r much l a r g e r r e g i o n s a t tower t e m p e r a t u r e . T h i s g r a d u a l paracoherent~:oherent transition [ 1 4 ] would a r i s e from a distribution o f weak J o s e p h s o n c o u p l i n g s between the s u p e r c o n d u c t i n g r e g i o n s . Such a p i c t u r e c o u l d a l s o account f o r t h e e l e c t r i c a l dissipation and non-Ohmic b e h a v l o u r observed b e l o w 2K a t v e r y low c u r r e n t d e n s i t y levels i n t h a t f l u x - f l o w would o c c u r r e a d i l y a t the J u n c -
cions,
each one of which i s c h a r a c t e r i z e d by a However, t h i s model appears i n c o n s i s t e n t w i t h the e v i d e n c e tha£ the s i n g l e c r y s t a l s used f o r the r e s i s t i v i t y measurements are physically and s t o t c h i o m e t r i c a l l y homogeneous on a s c a l e o f lO0~. We cannot yec r u l e o u t the p o s s i b i l i t y t h a t the e x p l a n a t i o n l i e s w i t h l n h o m o g e n e i t y on a microscopic scale. Since the v a l u e s o f iH*sc2Yield a coherence l e n g t h ~ l ~ 20~, which comparable t o the l a t t i c e dimensions of NbSe~, [ 5 ] I t i s c o n c e i v a b l e t h a t t h e s u p e r c o n d u c t i n g e l e m e n t s are some of t h e s i x i n d i v i d u a l c h a i n s i n the u n i t c e l t . The p r o p e r t i e s would then be e x t r e m e l y s e n s i t i v e to strains and
critical current.
impurities. Whatever t h e e x p l a n a t i o n , NbSe 3 i s an unu s u a l s u p e r c o n d u c t o r . F u r t h e r work i s i n p r o gress to clarify its electromagnetic properties, and to l o o k more c l o s e l y f o r evidence of inhomogeneity. This research was supported by t h e National Science Foundation under grants DHR77-O9879 and DMR79-O9177 and by HSF through the Cornett University Materials Science C e n t e r , g r a n t number DH376-81083. We wish to thank Prof. T. N. Rhodin and Dr. L. Rathbun for performing the Auger Microprobe analysis.
988
SUPERCONDUCTIVITY IN NIOBIUM TRISELEI~ZDE
VoZ. 36, No. I1
m
SampleC
NbSe3
~
0.48K
/ [
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/
K
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2 0.79 K
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Ft S. 6 H vs H. H(T) m e a s u r e m e n t s i n many { l e l d s a r e r e p l o c c e d co g i v e H(H) {or s e l e c t e d temperatures. The dashed LLne r e p r e s e n t s c o r a l
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REFERENCES
L. 2. 3. 4.
P. Honceau, N.P. Ong, A.H. P o r t i s , A. Heecschauc and J. Rouxel, Phys. Rev. L e f t . 37, 602 (L976). P. Haen, G. Waysand, G. Both, A. Wa£ncaL, J. Phys. (Parts)37,CA-179 (1976). N.P. On8 and P; .~"onceau, Phys. Rev. BL.~6, 3443 (L977). K. Tsucsumi, T. T a k a g a k i , H. Yamamoto, Y. S h i o z a k i , H. 1do, T. Sambonsi, K.
g.
(L977). I0.
C.H. B a s t u s c h e c k , R.A. Buhrman and J .
11.
C. S c o t t , B u l l . Am. Phys. Soc. 24, 387 (L979). A. Heerschauc and J. RouxeL, J. Less Common HecaLs 39, L97 (1975). R.H. Dee, D.H. DoZLard, B.G. TurreLL
L2.
¥amaya and Y. Abe, P h y s . Rev. L a c e . 39,
5. 6. 7. 8.
1675 (L977). J . L . Kodesu, H. Harezto, C. Roucau, R. Ayrotes, A. Heerschauc, J. RouxeL and P. Honceau, 3. Phys. C 1L, 41L7 (L978). R.H. FLemins, D.E. Honcton and D.R. HcWhan, Phys. Rev. B 18, 5560 (1978). P. Haen, P. L a p i e r r e , P. Honceau, H. Nunez ReLuetro and J. RLchard, SoLLd State Commun. 26, 725 (1978). P. Heart, J.H. Hignoc, P. Honceau and H. Nunez R e g u e i r o , J . 39, C6-703 (1978).
Phys.
(Paris)
P. Honceau, J . P e y r a r d , J . R i c h a r d and P. M o t t n i e , Phys. Rev. L e e r . 39, L6L
L3. l&.
and J . F . CaroLan, S o l i d S c a r e Commun. 2a, 469 (L977). C.H. B a s t u s c h e c k , R.A. Buhrman, K. Temkin and J . C . S c o t t . B u l l . Am.
Phys. 5oc. 23, 305 (L978). G. Daucscher, Y. Zmry and L. Gunther,
Phys. Ray. SlO, 4598 ( L 9 7 4 ) , and s e v e r a l
papers Ln Proc. Conf. lnhomogeneous Superconductors, Berkeley Sprin 8, W.V. 1979, D.V. Gubser, T.L. FrancovtLlo, $.A. Wolf and J.R. L e i b o v t t z Eds., A . I . P . Conf. Pro¢. Vol. 58, 1980.
SUPERCONDUCTIVITY
IN NIOBIUM TRISELENIDE
C.M. Bastuscheck and R.A. Buhrman S c h o o l o f A p p l i e d and E n g i n e e r i n g P h y s i c s , I t h a c a , NY 14853
Cornell University,
J . D . Kulick and J . C . S c o t t L a b o r a t o r y o f Atomic and S o l i d S t a t e P h y s i c s , Ithaca, NY 14853
Cornel! University,
(Received 12 3une 1980 by A.G. Chynoweth) Sensitive magnetic susceptibility measurements on NbSe 3 reveal a large temperature dependent diamagnetism and flux trapping below about 2K. Flux exclusinn is ~,nh;mced by etching and compacting the polycrystalllne samples, as expected for a weakly coupled assembly o'f superconducting crystallites. Single crystal resistivity measurements reveal an anomaly at 2K, which is depressed in temperature by applied meg.eric fields. However, the temperature and field dependence of the res|stivlty, especially the fact that it does not vanish, are .oL explained.
Niobium t r i s e l e n i d e i s an a n i s o t r o p i c metallic compound which has been the o b j e c t o f considerable recent attention, primarily because o f the n o n - l i n e a r c o n d u c t i v i t y [1,2,3,] associated with the onset of two distinct charge-denslty wave distortions [4,5,6]. In addition, unusual r e s i s t i v i t y b e h a v i o r a t low temp e r a t u r e has been r e p o r t e d [ 7 , 8 ] . While M e i s s h e r e f f e c t measurements have been p r e s e n t e d [ 9 ] as e v i d e n c e f o r a s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e which i n c r e a s e s r a p i d l y w i t h p r e s s u r e , t h i s Communication r e p o r t s the f i r s t c o n clusive e v i d e n c e t h a t the m a t e r i a l is superconducting at zero pressurez namely f l u x e x c l u s i o n and f l u x trapping. The f i e l d and ten~p e r a t u r e dependence of t h e s u p e r c o n d u c t i n g d i a magnetism i s q u i t e s i m i l a r t o t h a t o f t h e a n a l ogous compound TaSe~ [ l O ] w h i c h , l i k e NbSe~, grows as s m a l l f i b r o u s c r y s t a l l i t e s . We sh~w t h a t the f i b r o u s n a t u r e of NbSe 3 i s an i m p o r t a n t f a c t o r i n d e t e r m i n i n g the m a g n e t i c s u p e r c o n d u c t i n g b e h a v i o r of a macroscopic sample. The compound was p r e p a r e d by the u s u a l method [ 1 1 ] o f h e a t i n g s c o i c h i o m e t r i c amounts o f e l e m e n t a l n i o b i u m and s e l e n i u m f o r s e v e r a l weeks i n an e v a c u a t e d q u a r t z t u b e . S i n g l e c r y s tals approximately 0.01 x O . l x lO mm were obtained for resistivity measurements by p r o loneed heating of the reaction mixture at 700~C w i t h a t e m p e r a t u r e g r a d i e n t of §O°C a l o n g the 15 cm t u b e . P o l y c r y s t a l l i n e samples f o r susceptibility measurements were o b t a i n e d by r e a c t i o n a t 800°C f o r two weeks w l t h r e l a t i v e l y rapid cooling (6 d a y s ) t o room t e m p e r a t u r e . This material formed in mats of s m a l l f i b e r s e n d r i b b o n s , w h i c h bad l e n g t h s from 0 . 0 5 t o I mm and c r o s s d i m e n s i o n s f r o m 2 t o IO, m. DebyeScherrer x-ray diffraction was u s e d t o c o n f i r m t h e c o m p o s i t i o n of t h e p o l y c r y s t a l l t n e sample. T h i r t e e n l i n e s were o b s e r v e d and i n d e x e d w i t h t h e p u b l i s h e d [ 5 ] NbSe~ structure. A careful s e a r c h was made f o r d i f f r a c t i o n which c o u l d be
a s s o c i a t e d w i t h any o f t h e d i s e l e n l d e p o l y t y p e s and none was found ( a t the l e v e l o f one p a r t i n a t h o u s a n d ) . Scanning e l e c t r o n m i c r o s c o p y w i t h a r e s o l u t i o n of IOOX f a i l e d t o r e v e a l any v i s i b l e s u r f a c e i n h o m o g e n e i t y o t h e r than s t r i a t i o n s and c r a c k s r u n n i n g p a r a l l e l t o the b a x i s . Scanning Auger m i c r o a n a l y s i s , w i t h depth profiling, (2, horizontal resolution and l O 0 ~ d e p t h r e s o l u t i o n ) showed t h a t the s u r f a c e s o f t y p i c a l c r y s t a l s are s e l e n i u m r i c h r e l a t i v e t o the b u l k , but no o t h e r c o m p o s i t i o n a l lnhomog e n e i t y c o u l d be d e t e c t e d . R e s i s t i v i t y measurements were made as a f u n c t i o n o f t e m p e r a t u r e , m a g n e t i c f i e l d and c u r rent density, using conventional four-probe a . c . t e c h n i q u e s . To d a t e , 16 samples have been examined, 8 being aligned w i t h the m a g n e t i c field of a superconducting solenoid parallel to t h e b - a x i s ( t h e f i b e r d i r e c t i o n and the a x i s o f highest conductivity) 4 w i t h t h e f i e l d a l o n g a* (i.e. p e r p e n d i c u l a r t o t h e r i b b o n ) and 4 w i t h the field along the c-axis. Room t e m p e r a t u r e r e s £ s t i v i ~ i e s f e l l i n r.he range 0 . 3 - 2 . 0 x 10 - 3 Rcm, and r e s i d u a l r e s i s t a n c e r a t i o s liar = p ( 3 O O K ) / p ( 4 . 2 K ) ] v a r i e d between 20 and 250. A l l samples e x h i b i t e d t h e p r e v i o u s l y r e ported [1,2,3] resistive a n o m a l i e s a t 140K and 59K. I n a l l samples a d r o p i n r e s i s t i v i t y was o b s e r v e d t o o c c u r between 2 and 2.§K (see f i g u r e l a ) . As w i t h t h e d a t a p u b l i s h e d by Haen e t al. [ 7 , 8 ] t h e d e t a i l s o f t h i s low t e m p e r a t u r e anomaly were f o u n d t o be sample (and h i s t o r y ) dependent, the drop in resistance amounting to between 5% and 70% o f i t s v a l u e a t 4K. There i s no o b v i o u s c o r r e l a t i o n o f t h e m a g n i t u d e o f the resistive anomaly w i t h RRR, r e s i s t a n c e v a l u e a t 4K, sample d i m e n s i o n o r t h e b a t c h from which t h e c r y s t a l s were s e l e c t e d . The m a g n e t i c f i e l d dependence o f t h e r e sistivity can be seen i n f i g u r e s I and 2 f o r
983
SUPERCONDUCTIVITY IN NIOBIUM TRISELENIDE
,t.L~rL~j
H=27kG
E u
~
S
~
,e
I
=L
f i g u r e 3 ] . T h i s c u r r e n t c o r r e s p o n d s to a power d i s s i p a t i o n of l e s s than 1 pW, hence we r u l e ouc h e a t i n g as a source o f the n o n - l i n e a r i c y . A l l the measurements o f 0(T,H) discussed in the p r e c e d i n g p a r a g r a p h s were made i n the l i n ear r e g i o n o f t h e I - Y c h a r a c [ e r i s t i c s . M a g n e t i z a t i o n measurements were made on t h r e e samples p r e p a r e d Co i n v e s t i g a t e the e f f e c t o f changes tn I n t e r - f i b e r c o u p l i n g . A11 measurements were made on m a t e r i a l grown i n a single
[
[
20 kbars
[
o
I I I
I I I I t t t I t t
20 (.9 t,,,* 15 -!I0
%
%
\
I #
tube. Sample A consisted of 94 mg
°"""--~-%~'-~ 2
T, Fig. 1 a) Resistivity several values of applied
I
.3
4
K vs. field
temperature for p a r a l l e l co the
current.
b) Ef£ecctve critical field, extracted from curves o f t h e type shown i n f i g u r e a, a s e x p l a i n e d in t h e t e x t . f i e l d s p a r a l l e l t o the b and a* axes. The data f o r the c d i r e c t i o n Is t n t e ~ e d t a c e between the b e h a v t o u r shown. The data o f f i g u r e l a a l l o w us t o d e f i n e an e f f e c t i v e upper c r i t i c a l field, H*c~T), as the f i e l d r e q u i r e d t o depress t o T the t e m p e r a t u r e o f the r e s i s t i v e anomaly. ( F o r convenience t h i s t e m p e r a t u r e was a r b i t r a r i l y sel e c t e d as t h a t a t which the r e s i s t a n c e drops by I0% o f i t s 4K v a l u e . ) As shown i n f i g . l b , f o r f i e l d s a p p l i e d a l o n g the b - a x i s ~c2 i s on the s c a l e o f lO kgauss, whereas f o r Ella the v a l u e i s s e v e r a l hundred gauss. H : 2 ( l l c ) i s I n the k i l o g a u s s r a n g e , though noc so l a r g e es harc2(llb). F u r t h e r work i s b e i n g done t o c l a r i f y this anisocropy. As r e p o r t e d by Haed ec a l . [ 7 , 8 ] the I - V characteristic is highly non-linear. We have made measurements i n the range o f c u r r e n t dens l a y 0.0005< J < 0.50 A/cm2 and f i n d t h a t the non-Ohmic b e h a v t o u r onsets a t a p p r o x i m a t e l y lO'2A/cm 2 (i.e. O.I~A i n our s a m p l e s ) . [See
to form a cylinder 3 mm long by 2.5 n~
d i a m e t e r . Sample C c o n s i s t e d of 135 mg o f s i m i l a r l y compacted m a t e r l a l , buc the f i b r o u s NbSe 3 was etched tn dilute H2S04 (in which selenium iS s o l u b l e ) , r i n s e d and a i r d r i e d b e f o r e compaction. The compacted samples were removed f r o g the d i e press and t r a n s f e r r e d co the c r y o s t a t , where measurements were p e r f o r m e d a t zero p r e s sure The m a g n e t i z a t i o n measurements were made w i t h SQUID magnetometers £n two s e p a r a t e c r y o s c a r s , one c o o l e d w i t h a pumped 3He poc, the o c h e r by a d i l u t i o n refrigerator. The s t a t i c m a g n e t i z a t i o n , H. , was measured by sweeping ac t e m p e r a t u r e tn c o n s t a n t m a g n e t i c f i e l d H, i n fields from less than O.1 mOe to g r e a t e r than lO00e. The t e m p e r a t u r e dependent ac s u s c e p t i bility was d e t e r m i n e d by measuring the m a g n e t i z a t i o n response to a s m a l l ac f i e l d ( N 0 . 5 mOe ac 0.08 Hz) a p p l i e d p a r a l l e l co the static field. F i g . 4 shows Xd ~ Hdc/H vs T f o r the three samples m e a s u r e ~ i n c o n s t a n t applied field of 80 mOe. All t h r e e samples show a strongly t e m p e r a t u r e dependent d i a m a g n e t i s m a t low t e m p e r a t u r e s , but even ac the l o w e s t t e m p e r acute o n l y a f r a c t i o n o f t o t a l f l u x e x p u l s i o n iS a c h i e v e d i n t h i s f i e l d . Sample A ( a s - g r o w n ) had an o b s e r v a b l e d i a m a g n e t i c s i g n a l to above 1.2 K~ where lC became s m a l l e r than the r e s o | u l i o n o f the i n s t r u m e n t . At the s c a l e o f F i g . 4 the s i g n a l cannot be seen above 0 . 6 K. Sample B ( c o m p a c t e d ) shows n o t o n l y more f l u x e x p u l s ion at all t e m p e r a t u r e s , but a l s o a s l i g h t l y d i f f e r e n ~ shape f o r X vs T. Sample C ( e t c h e d and compacted) shows s t i l l stronger flux expulsi o n and a t e m p e r a t u r e dependence i n which ~ t e n d s t o l e v e l o f f a t low t e m p e r a t u r e s . shape o f t h e Hdc(T) curve f o r each sample changes b u t s l i g h t l y with applied field for H a t l e a s t as h i g h as 20 Oe, o n l y the magnitude does noc s c a l e w i t h a p p l i e d f i e l d (see below~ and F i g . 6 ) . ~dc(T) showed no t h e r m a l h y s t e r e s i s ; warming and t r a c e s are i d e n t i c a l . F i g u r e § shows the t e m p e r a t u r e dependent d i a m a g n e t i s m and f l u x t r a p p i n g f o r sample B ( c o m p a c t e d ) near the t e m p e r a t u r e where the low t e m p e r a t u r e r e s i s t i v e anomaly has been observed ( i n the a s - g r o ~ n m a t e r i a l ) . Between 0.5 K and 1.1 K the t e m p e r a t u r e dependent s u s c e p t i b i l ities of samples A and B are v e r y s i m i l a r in shape, w i t h X(B) a p p r o x i m a t e l y f i f t y rimes x(A). We c o n s i d e r t h i s s i m i l a r i t y to be e v i dence t h a t the c o m p a c t i o n has caused no i r r e •
b%
O
quartz
of the f i b r o u s m a t e r i a l l o o s e l y assembled i n a volume 3 x 4 x 10 nrn. Sample B c o n s i s t e d of 90 mg o f t h e m a t e r i a l compacted u s i n g a h y d r a u l i c p r e s s w i t h a p p l i e d p r e s s u r e s of a p p r o x i m a t e l y
H=OkG
2,5
Vol. 36, No. I I
Vol.
36, No. 11
SUPEKCONDUCTIVITY IN NIOBIUN TRISELENIDE
9~
I0
8
T,42K
E 6
4
o,~ I
NbSe 3
I
HII ~*
I
~',o.'o'°~' T= 1.4 K
I 0.2
I 04
I 0.6
I 0.8
I 1.0 H,
m 1.2
I 1.4
I 1.6
I 1.8
2.0
kG
FLg. 2 F t e l d dependence o f t h e c e s t s c t v t ~ y ~empera~ures a b o v e and b e l o w t h e ~ r a n s l ~ l o n .
at
NbSe 3
42K
E
e
I
~L T
3 --
2 --
e
T ,L
"="
1.4 K t It,Ill
O0001
i 0.001
!
, I I ,111 0.01
j, Fig. 3 restsctvtcy ¢=ansic£on.
I
e s Itiesl O.I
A/cm z
Current denstcy d e p e n d e n c e oE t h e ac cempe=acures a b o v e and b e L o v t h e
*
I
Iol 1.0
986
SUPERCONDUCTIVITY IN NIOBItrH TRISELENIDE
NbSe 3
V o l . 36, No. 11
Xcx: vs T H = 8 0 mOe
Samples:
'o_
o A As grown
X
A B o
Compacted
C Etched and compacted
O
I
~
0
I
~
0.4
0,8
T,
K
Fig." 4 X vs T for three samples (see text). Total Melssner effect is M = - H / 4 w ) . Uncertalnty in absolute magnitude is 50% but the curves are Intercomparable wlchin 20%. The magnetic field was applied above 3 K, the sample was cooled to 30 r~, and the change in magnetization was recorded while the sample was warmed slowly. The solid lines guide the eye.
versible pressure change in the NbSe 3. Above 1.2 K the expected change in the susceptibility of the as-grown sample (assuming this similarIcy) is smaller than the resolution of the magnetometer. Flux trapping was observed in all three samples, and was observed in applied fields as small as 3 mOe. The dependence of .M~c(~f.~ on the applied field H is shown in Fig ~ r the etched and compacted material. Mdc i s p r o p o r t i o n a l to H o n l y in t h e very low field limit (H 0.5 mOe), while above O.l Oe M is nearly constant. This behavior was observed in al~ three samples, and is similar to that observed i n TaSe 3 [lO] and (SN) x [12,13]. As i s the case for any hystereric magnetic system, this plot of Mdc(T) vs H is not equivalent to a direct measurement of M vs H made by sweeping the field. We have made such measurements and find that [H(H)/H]T.~ons~ decreases much more slowly than~ as H is increase d • [M(T)/H]L~e t e m p e r a t u r e dependence o f the low f r e quency ac s u s c e p t i b i l i t y was n e a r l y i d e n t i c a l t o M (T) f o r each sample, and i n ~he v e r y low field limit Xac(T)~Xdc(T). In higher fields Xac(T) had the same temperature dependence as
Xd c buc, w h i l e Xd C f e l l q u i c k l y as H was i n c r e a s e d o v e r 0 . 5 mOe, Xac d e c r e a s e d o n l y v e r y slowly, falling Co h a l f i~s z e r o f i e l d v a l u e ac H ~5OO Oe. The diamagnetism and flux trapping clearly demonstrate that NbSe 3 contains superconducting elements with zero applied pressure below 2.1 ± 0.2 K. The onset of the magnetic transltion correlates well with the drop in the reslstlvtty observed a t 2.1 ± 0.2K. The magnetic and resistive behavior is not thac of a bulk superconductor, nor is the diamagnetic susceptlb i l i t y that expected for a collection of isolated superconducting fibers, being larger, non-llnear i n H, a n d s t r o n g l y v a r y i n g ac low temperatures. The o b s e r v e d m a g n e t i c b e h a v i o r i s consistent with the behavior o f an ensemble o f small superconductoring elements with weak inter-element coupling. The t e m p e r a t u r e and field dependence of the diamagnetism is determ i n e d by t h e strength of the inter-element coupling and by the condition o f fluxold quantization about multiply connected current p a t h s . The behavior o [ the as-grown NbSe- sample indicates chat here the Inter-elemen~ coupling is
s u f f i c i e n t l y weak that only at quite low temper-
Vol. 36, No. I I
SUPERCONDUCTIVITY IN NIOBIUM TRISELENIDE
987
1.5
Sample B
NbSe 3
H=I.20e
'0 - -
X
o
Me)c
I.O
L~ Trapped flux
<3 x
/ IOO
1 V x'°°
0.5
I
T,
K
Fig. 5 Flux expulsion while cooling and flux release upon warming after the field was removed at T = 0.48 K, The vertical scale is the same for bach curves (bach are normalized to H - 1.2 Oe).
a t u r e s and low f i e l d s can s i g n i f i c a n t diamagneti c s u p e r c u r r e n t s be s u s t a i n e d between the s u p e r conducting filaments. E v i d e n t l y the e f f e c t of c o m p a c t i o n and e t c h i n g is co i n c r e a s e the tnter-tlement coupling. The d i s c r e p a n c y between o u r d a t a and t h a t of Monceau e t a l [9], who observed d i a m a g n e t i s m o n l y under p r e s s u r e , can b e u n d e r s t o o d in terms o f the g r e a t e r s e n s i t i v i t y of our SQUID magnetometer, and the v e r y s m a l l magnitude o f Xdc f o r f i b r o u s NbSe~ i n f i e l d s o f I Oe and l a r g e r . I n t h e p r e s e n t w~rk i t i s f o u n d tha~ Xdc a p p r o a c h e s s e v e r a l t e n t h s of c o r a l f l u x e x c l u s i o n ac tow t e m p e r a t u r e in the v e r y low f i e l d ( 0.5 mOe) l i m i t , f a l l s c o - - t O - 2 o f t o t a l f l u x e x c l u s i o n by O . 1 0 e and by a n o t h e r f a c t o r o f tO a t 1.00e. These o b s e r v a t i o n s are s u g g e s t i v e of a transition i n which s e p a r a t e s m a l l r e g i o n s , w i t h dimensions much t e s s than the p e n e t r a t i o n depth, are individually superconducting below a b o u t 2 . l K and become p r o g r e s s i v e l y coupled to g i v e weak phase coherence o v e r much l a r g e r r e g i o n s a t tower t e m p e r a t u r e . T h i s g r a d u a l paracoherent~:oherent transition [ 1 4 ] would a r i s e from a distribution o f weak J o s e p h s o n c o u p l i n g s between the s u p e r c o n d u c t i n g r e g i o n s . Such a p i c t u r e c o u l d a l s o account f o r t h e e l e c t r i c a l dissipation and non-Ohmic b e h a v l o u r observed b e l o w 2K a t v e r y low c u r r e n t d e n s i t y levels i n t h a t f l u x - f l o w would o c c u r r e a d i l y a t the J u n c -
cions,
each one of which i s c h a r a c t e r i z e d by a However, t h i s model appears i n c o n s i s t e n t w i t h the e v i d e n c e tha£ the s i n g l e c r y s t a l s used f o r the r e s i s t i v i t y measurements are physically and s t o t c h i o m e t r i c a l l y homogeneous on a s c a l e o f lO0~. We cannot yec r u l e o u t the p o s s i b i l i t y t h a t the e x p l a n a t i o n l i e s w i t h l n h o m o g e n e i t y on a microscopic scale. Since the v a l u e s o f iH*sc2Yield a coherence l e n g t h ~ l ~ 20~, which comparable t o the l a t t i c e dimensions of NbSe~, [ 5 ] I t i s c o n c e i v a b l e t h a t t h e s u p e r c o n d u c t i n g e l e m e n t s are some of t h e s i x i n d i v i d u a l c h a i n s i n the u n i t c e l t . The p r o p e r t i e s would then be e x t r e m e l y s e n s i t i v e to strains and
critical current.
impurities. Whatever t h e e x p l a n a t i o n , NbSe 3 i s an unu s u a l s u p e r c o n d u c t o r . F u r t h e r work i s i n p r o gress to clarify its electromagnetic properties, and to l o o k more c l o s e l y f o r evidence of inhomogeneity. This research was supported by t h e National Science Foundation under grants DHR77-O9879 and DMR79-O9177 and by HSF through the Cornett University Materials Science C e n t e r , g r a n t number DH376-81083. We wish to thank Prof. T. N. Rhodin and Dr. L. Rathbun for performing the Auger Microprobe analysis.
988
SUPERCONDUCTIVITY IN NIOBIUM TRISELEI~ZDE
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