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Calorimetric study of the phase transitions in niobium triselenide NbSe3
Solid State Communications, Voi.38, pp. I09-iI2. Pergamon Press Ltd. 1981. Printed in Great Britain.
0038-[098/8t/020109-04502.00/0
CALORIMETRIC STUDY OF THE PHASE TRANSITIONS IN NIOBIUM TRISELENIDE NbSe3 S.Tomid, K . B i l j a k o v i d , D.Djurek and J.R.Cooper I n s t i t u t e of Physics of the U n i v e r s i t y , PB 304, 41001 Zagreb,Yugoslavia P. Monceau Centre de Recherches sur les Tres Basses Temperatures, CNRS, BP 166, 38042 Grenoble - Cedex, France A.Meerschaut Laboratoire de Chimie des Solides, 44072 Nantes- Cedex, France (Received 23 December 1981 by E. F. Bertaut) High resolution s p e c i f i c heat measurements have been performed on the l i n e a r - c h a i n compound NbSe~ between 50K and 160K. Two anomalies were found at 145 K and 58 K, r~spectively. Latent heat was detected at both t r a n s i t i o n s and hysteresis of nearly one degree at the 58K anomaly.
The recently investigated l i n e a r chain conductor niobium t r i s e l e n i d e (NbSe3) forms needle-shaped crystals whose structure is monoclinic and contains Nb chains p a r a l l e l to the b-axes I. Triangular prisms of selenium are stacked on top of each other and the niobium atoms are at the centre of these prisms. Inside "slabs" paral l e l to the b-c plane there e x i s t three types of chains which are characterised by d i f f e r e n t bonding strength between selenium atoms w i t h i n the prism. NbSe3 shows m e t a l - l i k e behaviour in the r e s i s t i v i t y from room temperature down to the helium region. There are two large r e s i s t i ve anomalies at 145K and 59K which are a t t r i b u ted to two unrelated CDW i n s t a b i l i t i e s with wave vectors qi=(0,0.243, O) and q2=(0.5, 0.263, 0.5) respectl'vely2,3. The increase in r e s i s t i v i ty below the t r a n s i t i o n s is a t t r i b u t e d to the decrease in area of the Fermi surface r e s u l t i n g from the opening o f gaps. Wilson proposed that these two CDWs only a f f e c t two typ~s of chains where the Se-Se bonds are stronger ~. The resis t i v e anomalies are very s e n s i t i v e to the appl i e d e l e c t r i c f i e l d and can be suppressed with f i e l d s of the order of 0.I V/cm. The observed n o n lin e a r conductivity below the t r a n s i t i o n temperatures has been i n t e r p r e te d as evidence f o r depinning of CDWs5,6. In view of these i n t r i g u i n g e l e c t r i c a l transport p r o p e r t i e s , t h e measurements o f the thermal properties are also of great i n t e r e s t . In this paper we report s p e c i f i c heat measurements which were made using an improved heat pulse technique previously applied to the cal o r i m e t r i c i n v e s t i g a t i o n of TTF-TCNQ 7. The heat capacity measurements were carried out in several experimental runs. For most runs we used about 6 mg of the s a ~ l e and f o r l a t e n t heat measurements in the 59K t r a n s i t i o n region we used about 14 mg. The small fibers of NbSe3 were sealed together in the s a ~ l e holder with s i l i c o n e high vacuum grease. P a r a s i t i c heat capacities were measured in an independent run and then substracted from the
t o t a l . The absolute temperature of the sink was determined with a platinum thermometer. The resolution in the s p e c i f i c heat was a few parts in 103. We have found two s p e c i f i c heat anomalies at 145 K and 59 K, respectively. The thermodynamic c har ac t e r is t ic s of the phase t r a n s i t i o n s are summarised in Table I.The 145 K t r a n s i tion region (Fig. I.) was only measured while cooling and the cooling rate was about IK/h. I t seems that some p r e t r a n s i t i o n a l effects e x i s t up to about 7 degrees above the c r i t i c a l temperature. From electron d i f f r a c t i o n measurements TsJtsumi et al 8 have reported ID precursor" s c a t t e r i n g , but recent studies by Hodeau et al j indicate that ID d if f u s e sheets are not present. I t is known that NbSe3 is not extremely one dimensional from FS studies9. Furthermore the fact that CDW t r a n s i t i o n s do not remove a l l the FS also supports such a lack of dominant one dimensionality. Hodeau et al att r i b u t e d the d i f f e r e n t results of Tsutsumi et al to the presence of impurities in t h e i r crys t a l s . Fleming et al4 reported 3D diffuse scatt e r i n g at 160 K and 62 K respectively. So i t is probable that the p r e t r a n s i t i o n a l effects we observe above 145 K are associated with this 3D d i f f u s e scattering i . e . short range order, rather than ID precursor e f f e c t s . In contrast to the upper anomaly, .the 58 K anomaly (Fig.2.) is a small and narrow one with a width o f about one degree. The s h i f t of the peaks on heating and cooling is less than one degree. We believe that this is a real s h i f t because the heating and cooling rates were very slow (about I k / h ) . There do not appear to be any p r e t r a n s i t i o n a l effects at this t r a n s i tion. We have estimated the t o t a l change in entropy o f the t r a n s i t i o n s from the areas under the peaks, using the background lines shown in the f i g u r e as dotted lines. We obtained values 0.01 Ro f o r the 145 K anomaly and O.O05Ro f o r the 59K anomaly. In table I these numbers are I09
I lO
THE
PHASE
TRANSITIONS
IN
NIOBIL~I
TRISELENIDE
NbSe 3
Vol.
38,
No.
oO?// oO9-"
18 --
0o
0
0
DO0 /
/
/
© / /
0 0 // / / /
0 / / 0 / 0 / /
o.
{.9
17' --
D p~
/ .d 16 --
22 I
LGI
I
i
I
I
I
140
] 145
I
i
%
I
i
I 150
i
T
I
i
I 155
I
I
K
Fig.1. Specific heat in units of R0 versus f o r the 145K t r a n s i t i o n region as measured while cooling at IK/h. The gas constant Ro=8,31 joules/mole/K.
compared with the t o t a l conduction electron entropy at Tc (}FT~), where the e l e c t r o n i c spec i f i c heat c o e f f i c i e n t F= ~kB2n F, with nF=1 state /eV/ mD atom as calculated by B u l l e t t I0. Thus, negleci~ing any possible changes in the density of states of the uncondensed electrons as a r e s u l t of CDW formation, we estimate that about 25% of the conduction electrons are condensed at T I and a f u r t h e r 30% at Tp. For T I t h i s agrees with the numbers deduces both from transport properties 2 and magnetic susceptibil i t y 1 1 . However f o r T2 i t is lower than the transport value (50-60%) but higher than that from s u s c e p t i b i l i t y (-10%). In the above e s t i mates we neglected the phonon entropy in accordance with the thermodynamic behaviour of a Peierls i n s u l a t o r in the mean f i e l d approximation 12. Namely, as discussed by McMillan 13 electrons near the gap edge cannot respond to l a t t i c e vibrations with wave vectors f u r t h e r than ~o-~ from the nesting vector (where I ° is the coherence length of the l a t t i c e d i s t o r t i o n ) , t h u s the number of phonon modes which p a r t i c i p a t e in the CDW phase t r a n s i t i o n is l i m i t e d . So i f the coherence length ~o is long compared with the l a t t i c e spacing, the phonon entropy is unimportant and the only i ~ o r t a n t c o n t r i b u t i o n comes from single p a r t i c l e (elect r o n i c ) e x c i t a t i o n s across the energy gap. From the X-ray measurements of Fleming et al. 3 ~o is greater than 10 l a t t i c e spacings. How-
ever, although ~o is long, we must say that the shapes of the observed anomalies are rather sharp and are not at a l l BCS l i k e , Instead a BCS l i k e jump in s p e c i f i c heat 1.41FTc as I ~ r ~ i c t e d by conventional mean f i e l d t h e o ~ l J , l ~ is according to the above estimates only 0.014 Ro ( f o r the upper t r a n s i t i o n with 25% condensed electrons) and this would not have been detectable in our experiments. We do not believe that these differences arise from our background subtraction. Therefore we believe that the lower t r a n s i t i o n is indeed sharp and narrow compared with BCS, whereas the upper one is also sharp but is preceded by 3D f l u c t u a t i o n s over about 7 degrees. In order to determine the order of these t r a n s i t i o n s we have also performed several i n dependent experimental runs to measure l a t e n t heat. The l a t e n t heats were only detected in experiments where the sample was cooled extremely slowly from room ten~erature down to the region of the t r a n s i t i o n . We have found a latent heat connected with 145K t r a n s i t i o n both on cooling and heating cycles and estimate i t to be about 0,4 RoTc. For the 5gK t r a n s i t i o n we have found only one i n d i c a t i o n of a l a t e n t heat of about 0,02 RoTc on a heating cycle. I t follows that the thermal prehistory of the sample may be important f o r the l a t e n t heat r e p r o d u c i b i l i t y . The thermal prehistory of the sample could be im]Dortant since s t r a -
2
Vol. 38, No. 2
THE PHASE T ~ N S I T I O N S
IN NIOBIbDI TRISELENIDE NbSe 3
III
f / 8
• Heating
-
~,a
o Cooling J .C~,0"
,~,
~,..o" .•~• qr" s.• S
7 ~CD// J t
y J
I
.w" ~0 /
to"
6 --
0¢~ SOS
0 /
J
I
l
L
I
n
n
L
55
I
I
I
l
60
I
~
[
I
I
I
65
I
U 7O
I
K Fig.2. S p e c i f i c heat versus T f o r the 58K t r a n s i t i o n region as measured w h i l e c o o l i n g and heating at IK/h.
ins induced on c o o l i n g could e f f e c t the nuclea t i o n o f the new phase. I t is g e n e r a l l y b e l i e ved t h a t incommensurate CDW modulations are connected with second order phase t r a n s i t i o n s , whereas commensurate CDWs often lead to f i r s t order transitions (e.g. ref. 15). In NbSe3 the periodicities of the two charge density waves (0,243 and 0,263, respectively) are very close to coT~mensurate values. I t is possible that
domains o f commensurate phase could play a role in the n u c l e a t i o n process which is manifested in the l a t e n t heat. We do not understand why these l a t e n t heats are so large. A l l we can say is t h a t under c e r t a i n e x p e r i mental conditions both t r a n s i t i o n s seem to have sonde f i r s t order character. In summary, we have conducted high resol u t i o n c a l o r i m e t r i c work on NbSe3 and detected
Table I. Parameters obtained f o r NbSe3 near the two phase t r a n s i t i o n s
Tc/K
145 58
•T/K
Cpd T ~S~ 1 ~ /R°
CP/Ro
ACPc /% P
IFTc/R°
Xs/%
0.45
2,5
~ 7
~ 0.01
0.04
25
0.I
1.5
~I
~ 0.005
0.016
30
is the total electronic specific heat coefficient corresponding to the theoretical density of states I state /eV/ Nb atom. x. is the fraction of condensed electrons deduced from the observed transltlon entroples.
If2
THE PHASE T~%NSITIONS
two s p e c i f i c heat anomalies at 145K and 58K, respectively. The 145K anomaly is preceded by a p r e t r a n s i t i o n a l region 7 K wide, while the 58 K anomaly does not show any detectable pret r a n s i t i o n a l effects. The entropy associated with both t r a n s i t i o n s is of the order of 0.01 Ro. Using a density of state at the Fermi level of I state /eV/ Nb atom and neglecting the phonon entropy we f i n d that about 25% and 30% of the conduction electrons are condensed
IN NIOBIL~'M YRISELENIDE
N~bSe3
Voi.
38, No. 2
below Tcl and Tc?, respectively. Under some experimental conditions, l a t e n t heat was observed at both t r a n s i t i o n s , and there was hysteresis at the lower one, so we propose that both of these t r a n s i t i o n s have some f i r s t order character. Acknowledgement - The authors from the I n s t i t u t e of Physics of the U n i v e r s i t y , Zagreb, acknowledge helpful discussions with S.Bari~id and A . B j e l i ~ .
REFERENCES I. Neerschaut A. and Rouxel J . , J . o f Less Comm.Metals 39,197(1975) and Hodeau J . L . , Marezio M., Roucau C., Ayroles R.,~eerschaut A., Rouxel J. and Monceau P., J.Phys.C11,4117(1978) 2. Ong N.P?-~-,and Monceau P., Phys. Rev. B16,3443(1977) 3. Fleming R.M., Moncton D.E. and McWh-an--9.8.,Phys. Rev.B1__88,5560(1978) 4. Wilson J . A . , Phys. Rev. B19,6456(1979) 5. Lee P.#. and Rice T.M.,l~ITys.Rev.B19,3970,(1979) 6. Bardeen J . , Phys. Rev.Lett.42,1498-~F979) 7. Djurek D., Franulovid K., l~-rester M., Tomid S., Giral L. and Fabre J.M., Phys. Rev. Lett.3_88, 715(1977) 8. Tsutsumi K., Tagagaki T . , Yamamoto M., Shiozaki Y., Ido M., Sambongi T., Yamaya K. and Abe Y., Phys. Rev. Lett 39,1675(1977) 9. Monceau P., Sol.State ~"6-mmun.24,331(1977) 10. B u l l e t t D.W., J.Phys. C12,277(~79) 11. Kulick J.D. and Scott ~TT.C., Sol.State Commun.3__22,217(1979) 12. Berlinsky A . J . , Contemp. Phys.17,331(1976) 13. McMillan W.L., Phys. Rev. B16,6-4r'~(1977) 14. Craven R.A., and Mayer S.T'7., Phys. Rev. B16,4583(1977) 15. Friend R.H., Miljak M. and Jerome D., P~-ys. Rev.Lett 4._00,I048(1978)
0038-[098/8t/020109-04502.00/0
CALORIMETRIC STUDY OF THE PHASE TRANSITIONS IN NIOBIUM TRISELENIDE NbSe3 S.Tomid, K . B i l j a k o v i d , D.Djurek and J.R.Cooper I n s t i t u t e of Physics of the U n i v e r s i t y , PB 304, 41001 Zagreb,Yugoslavia P. Monceau Centre de Recherches sur les Tres Basses Temperatures, CNRS, BP 166, 38042 Grenoble - Cedex, France A.Meerschaut Laboratoire de Chimie des Solides, 44072 Nantes- Cedex, France (Received 23 December 1981 by E. F. Bertaut) High resolution s p e c i f i c heat measurements have been performed on the l i n e a r - c h a i n compound NbSe~ between 50K and 160K. Two anomalies were found at 145 K and 58 K, r~spectively. Latent heat was detected at both t r a n s i t i o n s and hysteresis of nearly one degree at the 58K anomaly.
The recently investigated l i n e a r chain conductor niobium t r i s e l e n i d e (NbSe3) forms needle-shaped crystals whose structure is monoclinic and contains Nb chains p a r a l l e l to the b-axes I. Triangular prisms of selenium are stacked on top of each other and the niobium atoms are at the centre of these prisms. Inside "slabs" paral l e l to the b-c plane there e x i s t three types of chains which are characterised by d i f f e r e n t bonding strength between selenium atoms w i t h i n the prism. NbSe3 shows m e t a l - l i k e behaviour in the r e s i s t i v i t y from room temperature down to the helium region. There are two large r e s i s t i ve anomalies at 145K and 59K which are a t t r i b u ted to two unrelated CDW i n s t a b i l i t i e s with wave vectors qi=(0,0.243, O) and q2=(0.5, 0.263, 0.5) respectl'vely2,3. The increase in r e s i s t i v i ty below the t r a n s i t i o n s is a t t r i b u t e d to the decrease in area of the Fermi surface r e s u l t i n g from the opening o f gaps. Wilson proposed that these two CDWs only a f f e c t two typ~s of chains where the Se-Se bonds are stronger ~. The resis t i v e anomalies are very s e n s i t i v e to the appl i e d e l e c t r i c f i e l d and can be suppressed with f i e l d s of the order of 0.I V/cm. The observed n o n lin e a r conductivity below the t r a n s i t i o n temperatures has been i n t e r p r e te d as evidence f o r depinning of CDWs5,6. In view of these i n t r i g u i n g e l e c t r i c a l transport p r o p e r t i e s , t h e measurements o f the thermal properties are also of great i n t e r e s t . In this paper we report s p e c i f i c heat measurements which were made using an improved heat pulse technique previously applied to the cal o r i m e t r i c i n v e s t i g a t i o n of TTF-TCNQ 7. The heat capacity measurements were carried out in several experimental runs. For most runs we used about 6 mg of the s a ~ l e and f o r l a t e n t heat measurements in the 59K t r a n s i t i o n region we used about 14 mg. The small fibers of NbSe3 were sealed together in the s a ~ l e holder with s i l i c o n e high vacuum grease. P a r a s i t i c heat capacities were measured in an independent run and then substracted from the
t o t a l . The absolute temperature of the sink was determined with a platinum thermometer. The resolution in the s p e c i f i c heat was a few parts in 103. We have found two s p e c i f i c heat anomalies at 145 K and 59 K, respectively. The thermodynamic c har ac t e r is t ic s of the phase t r a n s i t i o n s are summarised in Table I.The 145 K t r a n s i tion region (Fig. I.) was only measured while cooling and the cooling rate was about IK/h. I t seems that some p r e t r a n s i t i o n a l effects e x i s t up to about 7 degrees above the c r i t i c a l temperature. From electron d i f f r a c t i o n measurements TsJtsumi et al 8 have reported ID precursor" s c a t t e r i n g , but recent studies by Hodeau et al j indicate that ID d if f u s e sheets are not present. I t is known that NbSe3 is not extremely one dimensional from FS studies9. Furthermore the fact that CDW t r a n s i t i o n s do not remove a l l the FS also supports such a lack of dominant one dimensionality. Hodeau et al att r i b u t e d the d i f f e r e n t results of Tsutsumi et al to the presence of impurities in t h e i r crys t a l s . Fleming et al4 reported 3D diffuse scatt e r i n g at 160 K and 62 K respectively. So i t is probable that the p r e t r a n s i t i o n a l effects we observe above 145 K are associated with this 3D d i f f u s e scattering i . e . short range order, rather than ID precursor e f f e c t s . In contrast to the upper anomaly, .the 58 K anomaly (Fig.2.) is a small and narrow one with a width o f about one degree. The s h i f t of the peaks on heating and cooling is less than one degree. We believe that this is a real s h i f t because the heating and cooling rates were very slow (about I k / h ) . There do not appear to be any p r e t r a n s i t i o n a l effects at this t r a n s i tion. We have estimated the t o t a l change in entropy o f the t r a n s i t i o n s from the areas under the peaks, using the background lines shown in the f i g u r e as dotted lines. We obtained values 0.01 Ro f o r the 145 K anomaly and O.O05Ro f o r the 59K anomaly. In table I these numbers are I09
I lO
THE
PHASE
TRANSITIONS
IN
NIOBIL~I
TRISELENIDE
NbSe 3
Vol.
38,
No.
oO?// oO9-"
18 --
0o
0
0
DO0 /
/
/
© / /
0 0 // / / /
0 / / 0 / 0 / /
o.
{.9
17' --
D p~
/ .d 16 --
22 I
LGI
I
i
I
I
I
140
] 145
I
i
%
I
i
I 150
i
T
I
i
I 155
I
I
K
Fig.1. Specific heat in units of R0 versus f o r the 145K t r a n s i t i o n region as measured while cooling at IK/h. The gas constant Ro=8,31 joules/mole/K.
compared with the t o t a l conduction electron entropy at Tc (}FT~), where the e l e c t r o n i c spec i f i c heat c o e f f i c i e n t F= ~kB2n F, with nF=1 state /eV/ mD atom as calculated by B u l l e t t I0. Thus, negleci~ing any possible changes in the density of states of the uncondensed electrons as a r e s u l t of CDW formation, we estimate that about 25% of the conduction electrons are condensed at T I and a f u r t h e r 30% at Tp. For T I t h i s agrees with the numbers deduces both from transport properties 2 and magnetic susceptibil i t y 1 1 . However f o r T2 i t is lower than the transport value (50-60%) but higher than that from s u s c e p t i b i l i t y (-10%). In the above e s t i mates we neglected the phonon entropy in accordance with the thermodynamic behaviour of a Peierls i n s u l a t o r in the mean f i e l d approximation 12. Namely, as discussed by McMillan 13 electrons near the gap edge cannot respond to l a t t i c e vibrations with wave vectors f u r t h e r than ~o-~ from the nesting vector (where I ° is the coherence length of the l a t t i c e d i s t o r t i o n ) , t h u s the number of phonon modes which p a r t i c i p a t e in the CDW phase t r a n s i t i o n is l i m i t e d . So i f the coherence length ~o is long compared with the l a t t i c e spacing, the phonon entropy is unimportant and the only i ~ o r t a n t c o n t r i b u t i o n comes from single p a r t i c l e (elect r o n i c ) e x c i t a t i o n s across the energy gap. From the X-ray measurements of Fleming et al. 3 ~o is greater than 10 l a t t i c e spacings. How-
ever, although ~o is long, we must say that the shapes of the observed anomalies are rather sharp and are not at a l l BCS l i k e , Instead a BCS l i k e jump in s p e c i f i c heat 1.41FTc as I ~ r ~ i c t e d by conventional mean f i e l d t h e o ~ l J , l ~ is according to the above estimates only 0.014 Ro ( f o r the upper t r a n s i t i o n with 25% condensed electrons) and this would not have been detectable in our experiments. We do not believe that these differences arise from our background subtraction. Therefore we believe that the lower t r a n s i t i o n is indeed sharp and narrow compared with BCS, whereas the upper one is also sharp but is preceded by 3D f l u c t u a t i o n s over about 7 degrees. In order to determine the order of these t r a n s i t i o n s we have also performed several i n dependent experimental runs to measure l a t e n t heat. The l a t e n t heats were only detected in experiments where the sample was cooled extremely slowly from room ten~erature down to the region of the t r a n s i t i o n . We have found a latent heat connected with 145K t r a n s i t i o n both on cooling and heating cycles and estimate i t to be about 0,4 RoTc. For the 5gK t r a n s i t i o n we have found only one i n d i c a t i o n of a l a t e n t heat of about 0,02 RoTc on a heating cycle. I t follows that the thermal prehistory of the sample may be important f o r the l a t e n t heat r e p r o d u c i b i l i t y . The thermal prehistory of the sample could be im]Dortant since s t r a -
2
Vol. 38, No. 2
THE PHASE T ~ N S I T I O N S
IN NIOBIbDI TRISELENIDE NbSe 3
III
f / 8
• Heating
-
~,a
o Cooling J .C~,0"
,~,
~,..o" .•~• qr" s.• S
7 ~CD// J t
y J
I
.w" ~0 /
to"
6 --
0¢~ SOS
0 /
J
I
l
L
I
n
n
L
55
I
I
I
l
60
I
~
[
I
I
I
65
I
U 7O
I
K Fig.2. S p e c i f i c heat versus T f o r the 58K t r a n s i t i o n region as measured w h i l e c o o l i n g and heating at IK/h.
ins induced on c o o l i n g could e f f e c t the nuclea t i o n o f the new phase. I t is g e n e r a l l y b e l i e ved t h a t incommensurate CDW modulations are connected with second order phase t r a n s i t i o n s , whereas commensurate CDWs often lead to f i r s t order transitions (e.g. ref. 15). In NbSe3 the periodicities of the two charge density waves (0,243 and 0,263, respectively) are very close to coT~mensurate values. I t is possible that
domains o f commensurate phase could play a role in the n u c l e a t i o n process which is manifested in the l a t e n t heat. We do not understand why these l a t e n t heats are so large. A l l we can say is t h a t under c e r t a i n e x p e r i mental conditions both t r a n s i t i o n s seem to have sonde f i r s t order character. In summary, we have conducted high resol u t i o n c a l o r i m e t r i c work on NbSe3 and detected
Table I. Parameters obtained f o r NbSe3 near the two phase t r a n s i t i o n s
Tc/K
145 58
•T/K
Cpd T ~S~ 1 ~ /R°
CP/Ro
ACPc /% P
IFTc/R°
Xs/%
0.45
2,5
~ 7
~ 0.01
0.04
25
0.I
1.5
~I
~ 0.005
0.016
30
is the total electronic specific heat coefficient corresponding to the theoretical density of states I state /eV/ Nb atom. x. is the fraction of condensed electrons deduced from the observed transltlon entroples.
If2
THE PHASE T~%NSITIONS
two s p e c i f i c heat anomalies at 145K and 58K, respectively. The 145K anomaly is preceded by a p r e t r a n s i t i o n a l region 7 K wide, while the 58 K anomaly does not show any detectable pret r a n s i t i o n a l effects. The entropy associated with both t r a n s i t i o n s is of the order of 0.01 Ro. Using a density of state at the Fermi level of I state /eV/ Nb atom and neglecting the phonon entropy we f i n d that about 25% and 30% of the conduction electrons are condensed
IN NIOBIL~'M YRISELENIDE
N~bSe3
Voi.
38, No. 2
below Tcl and Tc?, respectively. Under some experimental conditions, l a t e n t heat was observed at both t r a n s i t i o n s , and there was hysteresis at the lower one, so we propose that both of these t r a n s i t i o n s have some f i r s t order character. Acknowledgement - The authors from the I n s t i t u t e of Physics of the U n i v e r s i t y , Zagreb, acknowledge helpful discussions with S.Bari~id and A . B j e l i ~ .
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