Yttrium doped La1-xYxO0.9F0.1FeAs superconductors: Hall and thermopower studies

Article abstract of DOI:10.1016/j.physc.2010.04.013

The effect of yttrium substitution at the lanthanum site on the superconducting properties of La_1-xY_xO_0.9F_0.1FeAs ('x' = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffractio

Full text of DOI:10.1016/j.physc.2010.04.013

Physica C 470 (2010) 511–515
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Yttrium doped La1ꢀxYxO0.9F0.1FeAs superconductors: Hall and thermopower studies
S.J. Singh a, J. Prakash b, S. Patnaik a, A.K. Ganguli b,
*
a School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067, India
b Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of yttrium substitution at the lanthanum site on the superconducting properties of
La1ꢀxYxO0.9F0.1FeAs (‘x’ = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray
diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y2O3 is observed.
The temperature dependence of resistivity measurements confirm the superconducting transition tem-
perature (Tc) of 34.8 ( 0.05) K and corresponding Meissner transition at 34.3 K in the ‘x’ = 0.3 composition
which is higher than that reported for the parent phase (LaO0.9F0.1FeAs (Tc = 28 K)). Further increase in the
concentration of yttrium leads to broadening and suppression of the superconducting transition. The
value of Hc2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient (S) shows a neg-
ative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient
(RH) also remains negative throughout the temperature range supporting the thermopower results. The
lattice parameters (a and c) decreases and the charge-carrier density increases with yttrium doping.
Ó 2010 Elsevier B.V. All rights reserved.
Received 27 March 2010
Received in revised form 14 April 2010
Accepted 22 April 2010
Available online 17 May 2010
Keywords:
Oxypnictides
Superconductivity
Hall effect
1. Introduction
pounds to study the effect of Y doping on the charge carrier con-
centration and its effect on the superconducting transition
The mechanism of superconductivity in high temperature
superconducting materials is still a challenge. In the absence of a
proper theory, it is important to explore new materials in order
to obtain superconductors with high superconducting transition
temperature (Tc), high critical field (Hc) and high critical current
(Jc). In the last 20 years researchers have found new superconduc-
tors like the cuprates [1] and MgB2 [2]. In 2008, Kamihara et al. [3]
discovered new iron-based superconducting materials La(O/F)FeAs
with transition temperature of 26 K. Superconductivity in this class
of FeAs based materials [4,5] is surprising since most of Fe based
compounds exhibit strong magnetic behavior. Following the initial
work of Kamihara et al., AFe2As2, AFeAs [6,7] and iron chalcogenide
[8] superconductors were discovered. The parent compound LnOF-
eAs shows a structural transition followed by antiferromagnetic
transition on cooling [9]. Like the cuprates, superconductivity oc-
curs through doping (here electron doping with F) or applied pres-
sure by suppression of the structural transition and the
antiferromagnetic state. The highest transition temperature (Tc)
achieved in this class of materials is ꢁ55 K [10]. The superconduc-
ting transition in Ln(O/F)FeAs compounds increases by substitution
of smaller rare earth metal ions like Ce, Pr, Nd, Sm and Gd. Yttrium
doping at rare earth site in LaO0.6FeAs [11] and Ce(O/F)FeAs [12]
leads to enhancement in the transition temperature. In the present
work, we have synthesized Y-doped La1ꢀxYxO0.9F0.1FeAs com-
temperature by Hall and thermopower studies. We have also
investigated the effect of Y doping on the upper critical field of
La1ꢀxYxO0.9F0.1FeAs for the first time.
2. Experimental
Polycrystalline
samples
with
nominal
compositions
La1ꢀxYxO0.9F0.1FeAs (x = 0.0, 0.1, 0.2, 0.3, 0.5 and 0.6) were synthe-
sized by the solid state method. La, La2O3, Y2O3, LaF3 and FeAs with
high purity were used as starting materials. All chemical manipu-
lations were performed in a nitrogen filled glove box. FeAs was ob-
tained by reacting Fe chips and As powder at 800 °C for 24 h. The
raw materials were taken according to stoichiometric ratios and
sealed in evacuated silica ampoules (10ꢀ4 Torr) and heated at
900 °C for 30 h. The resulting powder was pelletized, wrapped in
Ta foil and sealed in silica tube. These were heated at 1180 °C for
30 h. The phase identification of the samples was carried out using
X-ray diffractometry with Co K
tion patterns of the finely ground powders were recorded with
Co K radiation in the 2h range of 10°–70°. The lattice parameters
a radiation. Powder X-ray diffrac-
a
were obtained from a least squares fit to the observed d values.
Resistivity, Hall effect and rf susceptibility measurements were
carried out using a cryogenic 8 T cryogen-free magnet in conjunc-
tion with a variable temperature insert (VTI). The samples were
cooled in helium vapor and the temperature was measured with
an accuracy of 0.05 K using a calibrated Cernox sensor wired to a
* Corresponding author. Tel.: +91 11 26591511; fax: +91 11 26854715.
E-mail address: ashok@chemistry.iitd.ernet.in (A.K. Ganguli).
0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2010.04.013

512
S.J. Singh et al. / Physica C 470 (2010) 511–515
Lakeshore 340 temperature controller. Standard four probe tech-
nique was used for transport measurements. The external mag-
netic field (0–5 T) was applied perpendicular to the probe current
direction and the data were recorded during the warming cycle
with heating rate of 1 K/min. The inductive part of the magnetic
susceptibility was measured using a tunnel diode based RF pene-
tration depth technique [13]. The sample was kept inside an induc-
tor that formed a part of an LC circuit of an ultrastable oscillator
(ꢁ2.3 MHz). A change in the magnetic state of the sample results
in a change in the inductance of the coil and is reflected as a shift
in oscillator frequency which was measured by an Agilent 53131A
counter. The thermoelectric power measurement of each sample
was carried out by bridge geometry where a temperature gradient
was created across the sample, in the temperature range of 13–
300 K using a homemade setup attached to the cold finger head
of the close cycle refrigerator. The samples were directly attached
to two Au/Fe + Chromel thermocouple using silver paint. The volt-
refined lattice parameters are smaller than the parent LaOFeAs
compound [3,14]. Both the lattice parameters (a and c) decrease
with the increase in yttrium content (Fig. 1(ii)) up to x = 0.4 due
to smaller size of Y3+ ion as compared to La3+. For the x P 0.5 com-
position the lattice parameters seems to be constant with yttrium
substitution indicating the solubility limit of yttrium ion at the lan-
thanum site. Also for the x = 0.6 composition appreciable amount
of Y2O3 (ꢁ40%) as secondary phase was observed. From the above
studies we can consider the solid solution of La1ꢀxYxO0.9F0.1FeAs to
be valid in the range of 0 6 x 6 0.4. Chong et al. [15] reported the
synthesis of YO0.9F0.1FeAs compound which shows appreciable
amount of Y2O3, YAs and FeAs as secondary phases. This suggests
that the synthesis of pure Y(O/F)FeAs phase is difficult under ambi-
ent condition. Tropeano et al. [16] also observed the presence of
(La, Y)As and (La, Y)OF as secondary phases in La1ꢀxYxO0.85F0.15FeAs
compounds.
The zero field resistivity as a function of temperature for the
nominal compositions La1ꢀxYxO0.9F0.1FeAs (x = 0, 0.1, 0.2, 0.3, 0.5
and 0.6) are shown in Fig. 2(i). The resistivity decreases monoto-
nously with decreasing temperature and the onset of superconduc-
ting transition occurs at 7 and 32 K for ‘x’ = 0.1 and ‘x’ = 0.2
compositions respectively whereas the parent compound
(LaO0.9F0.1FeAs) shows a transition temperature of ꢁ28 K. Thus
the superconducting transition temperature initially decreases
for the Y-doped (‘x’ = 0.1) composition as compared to parent
LaO0.9F0.1FeAs compound. Similar behavior was earlier observed
age difference,
DV, between the hot and cold end of the sample was
measured by a Keithley 2182 Nanovoltmeter using copper wires.
3. Results and discussion
Fig. 1(i) shows the powder X-ray diffraction patterns for
La1ꢀxYxO0.9F0.1FeAs (‘x’ = 0, 0.1, 0.2, 0.3, 0.5 and 0.6). Majority of
the observed reflections could be satisfactorily indexed on the ba-
sis of the tetragonal ZrCuSiAs type structure. Minor amount of Y2O3
was observed as a secondary phase for x > 0.1 compositions along
with LaOF phase in some compositions. The variation of the refined
lattice parameters with yttrium content is shown in Fig. 1(ii). The
6
LaO
La
La
La
La
F FeAs
(i)
Y
Y
Y
Y
Y
O
F
F
F
F
F
FeAs
5
4
3
2
1
0
O
O
O
O
FeAs
FeAs
FeAs
FeAs
(i)
La1-xYxO0.9F0.1FeAs
La
O
x = 0
x = 0.1
*
*
*
*
x = 0.2
x = 0.3
x = 0.5
0
50
100
150
200
250
x = 0.6
*
Temperature (K)
10
20
30
40
50
60
70
2θ (in degree)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Tc = 34.8 K
4.035
8.720
La0.7Y0.3O0.9F0.1FeAs
(ii)
4.030
4.025
4.020
4.015
4.010
4.005
8.715
8.710
8.705
8.700
8.695
8.690
8.685
8.680
0.0
T = 33.5 K
c
-0.5
-1.0
0
25
50
(ii)
T (K)
0
5
10 15 20 25 30 35 40 45
Temperature (K)
0.0
0.1
0.2
0.3
0.4
0.5
Concentration of Y3+ ion (x)
Fig. 2. (i) The temperature dependence of resistivity
(q) as a function of
temperature for La1ꢀxYxO0.9F0.1FeAs. (ii) The variation of resistivity with respect to
temperature for La0.7Y0.3O0.9F0.1FeAs in the low temperature region. The inset shows
the magnetic susceptibility as a function of temperature up to 50 K for the x = 0.3
composition.
Fig. 1. (i) Powder X-ray diffraction patterns of La1ꢀxYxO0.9F0.1FeAs sintered at
1180 °C. The impurity phases are Y2O3(ꢂ) and LaOF (O). (ii) The variation of lattice
parameters with yttrium content (x) for La1ꢀxYxO0.9F0.1FeAs.

S.J. Singh et al. / Physica C 470 (2010) 511–515
513
in Y-doped CeO0.9F0.1FeAs [12] where too we observed the mini-
0.6
0.5
0.4
0.3
0.2
0.1
0.0
4
3
2
1
0
dH /dT|
= - 2.51 T/K
mum Tc at a composition with 0.1 mol of Y per formula unit. How-
ever, the x = 0.3 composition showed the highest Tc of ꢁ34.8 K. On
further increase in Y-content, the transition temperature shifts to
lower temperatures. The x = 0.5 and 0.6 compositions showed tran-
sition temperature 31.4 and 28 K respectively. Note that beyond
x = 0.4 does not lead to decrease in lattice parameters and hence
we may assume that the Y-content does not increase further. The
transition is rather broad for composition with x > 0.3. The criteria
used for the determination of Tc are the same as that used earlier
[14,17] and schematically shown in Fig. 2(ii). The residual resistiv-
H
H
27
30
33
36
T (K)
H = 0 T
H = 1 T
H = 2 T
H = 3 T
H = 4 T
ity value (RRR = q250/q35) of 4.5, 1.5, 7.1, 17.2 for sample x = 0, 0.1,
0.2 and 0.3 respectively, suggests good metallic conductivity and
reasonable normal-state connectivity. The inductive part of the rf
magnetic susceptibility attesting the onset of bulk diamagnetism
is shown in inset of Fig. 2(ii). The increase in Tc for the x = 0.3 com-
position (34.8 K) as compared to x = 0 composition (28 K) indicates
that the smaller Y3+ ions as compared to La3+ enhances the chem-
ical pressure which leads to increase in the superconducting tran-
sition temperature. However Y(O/F)FeAs compound showed Tc at
ꢁ10 K [15] which is quite lower as compared to La0.7Y0.3O0.9F0.1-
FeAs (Tc = 34.8 K). Earlier studies on the oxygen deficient
La1ꢀxYxO0.6FeAs compounds (synthesized by high pressure tech-
nique) shows similar enhancement in Tc with yttrium doping with
maximum Tc at 43 K for x = 0.5 composition [11] which is higher
than our yttrium doped compounds synthesized at ambient pres-
sure. It has been observed that the oxypnicide compounds synthe-
sized by high pressure method have higher Tc as compared to
compounds synthesized at ambient pressure, like, La(O/F)FeAs
shows Tc at 26 K [3] and 43 K [18] when synthesized at ambient
pressure and at high pressure respectively. Tropeano et al. [16]
have reported the superconductivity in 15% F-doped compositions
of La1ꢀxYxO0.85F0.15FeAs with maximum Tc of ꢁ40 K for x = 0.5.
However for our La1ꢀxYxO0.9F0.1FeAs compounds maximum Tc
was observed at ꢁ35 K for x = 0.3 (Fig. 3). This difference in maxi-
mum Tc (Fig. 3) could be due to the difference in the fluoride con-
tent of our sample (10%) and Tropeano’s sample (15%). Thus there
is an interplay of the geometrical parameter and the superconduc-
ting temperature probably through the increase in overlap of
orbitals.
20 22 24 26 28 30 32 34 36
Temperature (K)
Fig. 4. Temperature dependence of the electrical resistivity of La0.7Y0.3O0.9F0.1FeAs
under varying magnetic fields. Inset shows the temperature dependence of upper
critical field (j) and irreversibility field ( ) as a function of temperature.
weakly with magnetic field, but the zero resistivity temperature
shifts much more rapidly to lower temperatures, leading to a
broadening and increase in band width of the superconducting
transition, characteristic of the type-II superconductivity. Using a
criterion of 90% and 10% of normal-state resistivity (qn), we have
calculated the upper critical field Hc2 and the irreversibility field
Hꢂ(T) respectively. The H–T phase diagram for each sample is
shown in inset of Fig. 4. By using the Werthamer–Helfand–Hohen-
berg (WHH) formula [19], the zero field upper critical field Hc2(0)
can be calculated:
Hc2ð0Þ ¼ ꢀ0:693 TcðdHc2=dTÞ
T¼Tc
The slope of dHc2/dT estimated from the phase diagram (shown
in inset) is ꢀ2.51 for La0.7Y0.3O0.9F0.1FeAs. Using the value of tran-
sition temperature (Tc) of 34.8 K, we find Hc2 = 60.5 T. This value
is smaller than the reported Hc2 value of La(O/F)FeAs [14] but high-
er than that reported for Yb doped LaO0.8F0.2FeAs (ꢁ46 T) [20] and
approximately similar to Sb doped La(O/F)FeAs1ꢀxSbx(ꢁ72 T) [21].
The temperature dependence of thermoelectric power (S)
(Fig. 5) shows a negative value for all the Y-doped samples. This
is similar to that found in F-doped LaOFeAs and provides evidence
of electron doping. Upon Y or F doping, the upturn of thermoelec-
tric power seen in the parent compound, LaOFeAs [22] disappears,
and the room temperature value of thermopower increases. The
We have compared the temperature dependence of the electri-
cal resistivity under variable magnetic fields (up to 4 T) for
La0.7Y0.3O0.9F0.1FeAs, which exhibits the highest-Tc in the
La1ꢀxYxO0.9F0.1FeAs series (Fig. 4). It is clear that the Tc (onset) shifts
45
40
35
30
0
-30
-60
Max T for 10% F
doped La Y (O/F)FeAs
25
Max T for 15% F
doped La Y (O/F)FeAs and
40% oxygen deficient
La Y O FeAs
20
-90
15
LaO0.9F0.1FeAs
La
Y
O
FeAs [Ref. 11]
1-x
x
0.6
-120
-150
La0.8Y0.2O0.9F0.1FeAs
10
5
10 % F doped La
15 % F-doped La
Y
O FeAs [prsent work]
F
1-x
x 0.9 0.1
0.85 0.15
La0.7 0.3 0.9 0.1
Y
O
F
FeAs
FeAs
Y
O
F
FeAs [Ref. 16]
1-x
x
La0.5 0.5 0.9 0.1
Y
O F
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0
50
100
150
200
250
300
Yttrium Content (x)
Temperature (K)
Fig. 3. Variation of Tc with yttrium content (x) for La1ꢀxYxO0.6FeAs (j), 10% F-doped
Fig. 5. Temperature variation of Seebeck coefficient from 14 K to room temperature
La1ꢀxYxO0.9F0.1FeAs ( ) and 15% F-doped La1ꢀxYxO0.85F0.15FeAs ( ).
for La1ꢀxYxO0.9F0.1FeAs compared with the parent LaO0.9F0.1FeAs.

514
S.J. Singh et al. / Physica C 470 (2010) 511–515
negative sign of thermoelectric power suggest that electrons are
the major charge carriers. From Fig. 5, it is clear that all Y-doped
compounds have approximately the same value of thermopower
LaO0.9F0.1FeAs at 40 K and Y-doped sample La0.5Y0.5O0.9F0.1FeAs at
50 K (Fig. 6(i). It is clear that qxy is linear with magnetic field and
is negative at all temperatures above the critical temperature, indi-
cating that the normal-state conduction of these samples is domi-
nated by electron-like charge carriers. From this set of data, the
(ꢁꢀ30
lV/K) at room temperature. Like the parent compound
[22], in the higher temperature region the value of S increases lin-
early as a function of temperature and indicates a metallic behav-
ior. On lowering the temperature the magnitude of the
thermoelectric power increases to a maximum and then starts
decreasing as the temperature is lowered further before the super-
conducting transition. As the concentration of Y is increased, the
maximum value of Seebeck coefficient decreases. Band structure
calculations have shown that LnO/FFeAs compounds are multiband
materials and that several electron and hole pockets contribute to
the electrical transport [23]. We note that thermoelectric power (S)
Hall coefficient RH = qxy/H was determined and shown in
Fig. 6(ii). It is known that the Hall coefficient is nearly a constant
with varying temperature for a normal metal with Fermi-liquid
feature. However, this situation changes for a multiband material
or in non-Fermi-liquid behavior, such as cuprate superconductors.
The temperature dependence of Hall coefficient suggests either a
multiband effect or some unusual scattering process. Using the sin-
gle band equation n = 1/RHe to evaluate the charge-carriers density,
we obtain n = 0.71 ꢃ 1021 cmꢀ3 and 1.1 ꢃ 1021 cmꢀ3 at 50 K for
La0.7Y0.3O0.9F0.1FeAs and La0.5Y0.5O0.9F0.1FeAs respectively whereas
LaO0.9F0.1FeAs shows a value n = 0.63 ꢃ 1021 cmꢀ3 at 40 K. It is
clear that with increasing the concentration of Y, the charge-carrier
density increases. The Hall coefficient (RH) is negative in the entire
temperature range, suggesting that the superconductivity in all
these samples is dominated by electron type charge carriers and
supports the thermopower studies. It should be noted that both
the families of oxypnictides, (La/Y)(O/F)FeAs and La(O/F)FeAs have
a low charge-carrier density. This would give support to a theoret-
ical proposal that the iron-based superconductors have very low
superfluid density [23].
increases approximately linearly with
T
in the range
150 K 6 T 6 255 K and a deviation occurs at high temperature
(above 255 K). Below 150 K, thermoelectric power S showed a
downward deviation from linearity. This behavior must be due to
the different temperature (T) dependencies of the multiband con-
tributions to the transport properties; at low T (below 150 K),
charge transport is governed mainly by electron carriers whereas
at higher T, the contribution of hole carriers must be considered
like in the other multiband superconductors [2].
In order to get more information about the charge carriers, we
have studied the Hall resistivity ‘
q
xy’ for the parent F-doped sample
4. Conclusions
We have successfully synthesized the Y-doped La oxypnictide
superconductors (La1ꢀxYxO0.9F0.1FeAs) by sealed tube method.
The highest transition temperature of 34.8 K is obtained for the
x = 0.3 composition with an upper critical field of 60.5 T. Note that
the maximum Tc depends not only with Y-content but also on F-
content. The Hall and the thermopower studies indicate that the
majority of charge carriers are electrons.
0
-1
-2
-3
(i)
Acknowledgements
AKG and SP thank DST, Govt. of India for financial support. JP
and SJS thank CSIR, Govt. of India, for fellowships.
LaO0.9F0.1FeAs
La0.5Y0.5O0.9F0.1FeAs
-4
-5
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