J. Prakash et al. / Physica C 469 (2009) 300–304
303
of
of
q
q
in the linear regime is 4.6 ꢃ 10ꢀ2
m
X
cm Kꢀ1. The T2 behavior
amer–Helfand–Hohenberg (WHH) formula [23], the zero field
below 150 K signifies a strong electron–electron correlation.
upper critical field Hc2(0) can be calculated (Hc2(0) = ꢀ0.693Tc
The temperature dependence of electrical resistivity (low tem-
(dHc2=dTÞ
). The slope of dHc2/dT} is estimated from H–T phase
T¼TC
perature data) for La0.8Na0.2O0.8F0.2FeAs is shown in Fig. 3(iii). For
this sample too, the resistivity anomaly (at ꢁ150 K) disappears
and the resistivity drops abruptly to zero below 23 K, indicating
diagram and is found to be ꢀ2.97 and ꢀ1.18 T/K for the ‘x’ = 0.15
and ‘x’ = 0.2 phase. Using the value of transition temperature,
Tc = 30.9 K and 23 K, we find Hc2 = 63.5 T and 19 T for La0.85
-
a
superconducting transition. The residual resistivity value
Na0.15O0.85F0.15FeAs and La0.8Na0.2O0.8F0.2FeAs, respectively. These
values are smaller than the reported Hc2 value of La(O/F)FeAs and
(La/K)(O/F)FeAs [5]. So the incorporation of sodium ion at La site
enhances the Tc but on the other hand suppresses the Hc2 value
as compared to KF doped LaOFeAs [5]. This indicates the role of so-
dium ion on the superconducting properties of La(O/F)FeAs. Using
the value of Hc2 (0) we can also calculate the mean field Ginzburg–
(RRR = q300 31) is found to be 13.79 for La0.8Na0.2O0.8F0.2FeAs. This
/q
value of RRR suggests good metallic conductivity and better inter-
grain connectivity in comparison to La0.85Na0.15O0.85F0.15FeAs
(RRR = 7.27). Further, the temperature dependence of Seebeck coef-
ficient (S) for the ‘x’ = 0.15 phase is shown in inset of Fig. 3(iii). In
the measured temperature range, S has a negative value. This is
similar to that found in F doped LaOFeAs and provides evidence
of electron doping [21]. The Seebeck coefficient varies from
ꢀ34
creases in magnitude as the temperature is lowered further. We
note that increases approximately linearly with for
Landau coherence length by the formula (nGL = (U0/2pYc2)
1/2). By
taking U0 = 2.07 ꢃ 10ꢀ7 G cm2 and the calculated Hc2 values, we
estimate a coherence length of ꢁ23 Å and 42 Å for samples with
‘x’ = 0.15 and ‘x’ = 0.2, respectively. These values are higher than
that reported for La(O/F)FeAs [5].
l
V/K at 300 K to a value of ꢁ ꢀ135
l
V/K at ꢁ80 K then de-
S
T
150 K ꢄ T ꢄ 255 K and a deviation occurs at high temperature
above 255 K. This behavior is somewhat similar to the T depen-
dence of resistivity in this temperature range (as shown in inset
of Fig. 3(ii)). Below 150 K, S decreases with temperature faster than
a linear rate and passes through a minimum at Tmin and exhibits a
maximum at 80 K. Using the Mott expression S = p2kBT (2eTF)ꢀ1
In conclusion, we have successfully synthesized a new series of
oxypnictides superconductors with nominal compositions of La1ꢀx
-
NaxO1ꢀxFxFeAs with ‘x’ = 0.15 and 0.20 using sodium fluoride as a
fluorinating agent at relatively low temperature (1080 °C) by
sealed tube method. By substituting NaF in LaOFeAs, a systematic
change of superconducting transition temperature is observed
and the maximum Tc is found to be 30.9 K. This is the highest tran-
sition temperature in Lanthanum based oxypnictides containing
fluoride ions and synthesized at ambient pressure. With increasing
temperature (T), the resistivity above Tc crosses over from a T2
dependence due to electron–electron interaction to a linear T
dependence. The thermoelectric power measurement indicates
that the dominant carriers are electrons. From magnetoresistance
studies the value of the upper critical field (Hc2) is estimated to
be 64 T corresponding to a coherence length (n GL) of ꢁ23 Å for
‘x’ = 0.15 composition. The increase of Tc with sodium ion while a
decrease with potassium ions [5] clearly indicate the role of dopant
size (lattice) in controlling on the superconducting transition
temperature.
and taking the value at 40 K (S = ꢀ103
lV/K), we calculated the
Fermi energy ꢁ 0.014 eV. This value is smaller than that reported
for LaO0.89F0.11FeAs [21] and clearly out of the range because of
strong electronic correlation. We believe that detailed Hall coeffi-
cient measurements are required to explain these subtle changes.
The upper critical field is one of the important parameters to
characterize superconductivity. To get more information about
Hc2, we have studied the temperature dependence of the resistivity
under different magnetic fields which is shown in Fig. 4 for La0.85
-
Na0.15O0.85F0.15FeAs. Similar measurements were also carried out
for La0.8Na0.2O0.8F0.2FeAs. It is clear that the onset temperature
shifts with magnetic field weakly, but the zero resistivity temper-
ature shifts more rapidly to lower values. This allows one to deter-
mine the upper critical field of these materials. Taking the very
onset of the transition as the upper critical field point Tc (Hc2) im-
plies that almost all cooper pairs are broken at this temperature
and magnetic field. By taking a criterion of 90% and 10% of normal
Acknowledgements
A.K.G. and S.P. thank DST, Government of India for financial
support. J.P. and S.J.S. thank CSIR and UGC, Government of India,
respectively, for fellowships.
state resistivity (qn), we calculated the upper critical field Hc2 and
the irreversibility field H ꢂ (T), respectively. The H–T phase diagram
for each sample is shown in inset of Fig. 3. By using the Werth-
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4
3
2
1
0
dHc2/dT|T=T
c
= - 2.97 T/K
1.5
1.2
0.9
0.6
0.3
0.0
22 24 26
28 30 32
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H = 0 T
H = 1 T
H = 2 T
H = 3 T
H = 4 T
12
16
20
24
28
32
Temperature (K)
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Fig. 4. Temperature dependence of the electrical resistivity of La0.85Na0.15O0.85F0.15-
FeAs under different magnetic fields. Inset shows the temperature dependence of
estimated upper critical field (j) and irreversibility field ( ).