Critical properties of superconducting Ba1-xKxFe2As2

Article abstract of DOI:10.1016/j.physc.2009.09.004

Magnetisation and magneto-resistance measurements have been carried out on superconducting Ba_1-xK_xFe₂As₂ samples with x = 0.40 and 0.50. From high field magnetization hysteresis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The J_C determined from the high field data is >10⁴ A/cm² at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 T. From the variation of the T_C onset with applied field, dH_C2/dT at T_C was obtained to be -7.708 T/K and -5.57 T/K in the samples with x = 0.40 and 0.50.

Full text of DOI:10.1016/j.physc.2009.09.004

Physica C 470 (2010) 8–11
Contents lists available at ScienceDirect
Physica C
journal homepage: www.elsevier.com/locate/physc
Critical properties of superconducting Ba1 K Fe As
Àx x
2
2
1
*
A. Bharathi , Shilpam Sharma, S. Paulraj, A.T. Satya, Y. Hariharan , C.S. Sundar
Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 February 2009
Received in revised form 28 August 2009
Accepted 2 September 2009
Available online 17 September 2009
Magnetisation and magneto-resistance measurements have been carried out on superconducting
Ba1ÀxK
x
Fe
2 2
As samples with x = 0.40 and 0.50. From high field magnetization hysteresis measurements
carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using
4
2
the Bean critical state model. The J
T. The superconducting transitions were also measured resistively in increasing applied magnetic fields
up to 12 T. From the variation of the T onset with applied field, dHC2/dT at T
was obtained to be À7.708
T/K and À5.57 T/K in the samples with x = 0.40 and 0.50.
Ó 2009 Elsevier B.V. All rights reserved.
C
determined from the high field data is >10 A/cm at 4.2 K and
5
C
C
Keywords:
Resistivity
H
C2
J
C
x 2 2
Ba1ÀxK Fe As
1
. Introduction
perature with increase in external pressure, if optimally doped
9]. However, in contrast to cuprates, it is seen that the parent
compound can be self doped with the application of moderate
pressure: for example, in the CaFe As system, application of
[
The recent discovery of superconductivity in Fe based arsenides
[
1] has elicited enormous excitement amongst condensed matter
2
2
physicists, since many of the properties in these compounds bear
a strong resemblance to the much investigated high temperature
superconducting oxides. Several classes of compounds have been
discovered systematically in the last few months [1–4]. The two
major families of FeAs superconductors that have been investi-
gated are REOFeAs systems, broadly termed 1111 compounds [1],
1.5 GPa results in the disappearance of the SDW state and the
occurrence of superconductivity at 19 K [10]. Similar pressure in-
duced superconductivity is also observed in the BaFe
2 2
As and
SrFe As systems, which however is controversial [11]. Much of
2
2
the early literature in this compound was devoted to understand-
ing the SDW transition and the associated structural changes in the
pristine compound and its disappearance with chemical substitu-
tion [12].
2 2
where RE is an early rare-earth and the two layered MFe As sys-
tems, termed as the 122 compounds [2], where M stands for an
alkaline earth, Ca, Sr, Ba or the divalent rare earth Eu. Hole and
electron doping in the REO layers of the 1111 compounds induce
the superconducting state at the expense of the spin density wave
In a short span of time, there have been several studies on the
characterization of the superconducting state of the arsenide
superconductors. These include the measurement of critical fields,
superconducting gaps, penetration depth, coherence lengths, etc.
[13–24]. Many of these experiments have been carried out on sin-
gle crystalline samples, to investigate the anisotropic properties
[14–17]. While there are discrepancies in the reported values of
coherence lengths, penetration depths, etc., the consensus arrived
at indicates that the arsenide system is a type-II superconductor
with a Ginzburg–Landau parameter, K to be ꢀ80 [17]. A variety
of experiments have pointed to the existence of two superconduc-
ting gaps [18–21], as is also supported by electronic structure cal-
culations [22]. The anisotropy in the physical properties of 122
arsenide appears to be much lesser than in the cuprate supercon-
ductor, suggesting that it is a 3D superconductor [14–17,24]. The
proximity to SDW transition, existence of two superconducting
gaps, and 3D nature of superconductivity have to form the crucial
(
SDW) ground state present in the pristine sample [5]. Likewise, K
substitution at the M site [2] and Co substitution at the Fe [6] site
in the MFe As compounds, introduce holes/electrons respectively
2
2
in the system giving rise to superconductivity, destroying the SDW
state [7]. Of the two classes of compounds, the 122 compounds
have been more thoroughly investigated, since single crystals were
available form a very early stage [8]. Like the cuprates these com-
pounds show an increase in the superconducting transition tem-
*
Corresponding author. Tel./fax: +91 44 27480081.
E-mail address: bharathi@igcar.gov.in (A. Bharathi).
Retired from service.
1
0
921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2009.09.004

A. Bharathi et al. / Physica C 470 (2010) 8–11
9
underpinnings of the theoretical models of superconductivity in
this system.
2
.5
.0
Ba1-xK Fe As x=0.40
x 2 2
In this paper we present the critical properties, HC2 and J
Fe As superconductor (x = 0.40 and x = 0.50) in fields up to
6 T. We employ magneto-resistance measurements up to 12 T to
C
of the
(a)
2
Ba1ÀxK
x
2
2
1
obtain an estimate of the upper critical field HC2(0). The high field
magnetization measurements up to 16 T were used to obtain the
critical current density at 4.2 K and 20 K. The experiments have
been carried out in two samples, that have been prepared in two
different ways (see below) resulting in K concentrations, x = 0.40
and x = 0.50.
1.5
1.0
0.5
0
.0
0.4
-0.8
-
(
b)
0
50
100 150
250 300
0.0
T(K)
0
50
100
150
200
2
. Experimental details
T (K)
Towards a controlled synthesis of these arsenide samples we
employ a stainless steel chamber that can be pressure locked with
0 bar of pure Ar gas, to prevent vitalization of As. This procedure
x 2 2
Fig. 1. The variation of resistivity with temperature in Ba1ÀxK Fe As for x = 0.40.
Inset (a) shows the structure of the unit cell [7] and (b) shows the diamagnetic
5
signal in the same sample.
dispenses off with the use of sealing in quartz tubes and this we
believe is a safer way to synthesise these arsenides. We have fol-
lowed two recipes for synthesis: In the first method (sample 1)
we first synthesise FeAs by heat treating an intimate mixture Fe
powder (99.999%) and As powder (99.9999%) at 600 °C for 6 h un-
der pressure. Stoichiometric quantities of Ba, K and FeAs of compo-
second sample that has a K fraction of x = 0.50 has a room temper-
ature resistivity of 1.2 m
tivity was 37.39 K with
susceptibility was 35.7 K. The normal state resistivity shown in
Fig. 1, does not exhibit the linear temperature dependence seen
X
D
cm and the T
C
onset determined resis-
onset by ac
T
C
of ꢀ2 K and the T
C
sition Ba1Àx
x
K Fe
2
As
2
with nominal x = 0.45 are then weighed and
assembled together into Ta crucibles in a He/Ar containing dry
box. This is assembled into the stainless steel tube and filled with
C
in high T cuprates or the Bloch–Gruneisen behaviour seen in
5
0 bar of ultra pure Ar and ramped at 50 °C/h to 800 °C followed by
MgB [27]. Instead the temperature dependence of resistivity in
2
2
soaking at that temperature for 10 h. The product is then ground
and pelletised in the dry box and subsequently sintered at 950 °C
for 5 h. In the second method (sample 2) we skipped the FeAs step
and started with a mixture of Fe and As powders and Ba and K
chunks. This was heat treated at 800 °C for 4 h followed by pelletis-
ing and sintering at 950 °C for 5 h.
the normal state varies as T up to 70 K, beyond which it becomes
sublinear in T, very similar to that seen in A15 compounds [28].
The residual resistance ratio (RRR) is large at ꢀ8, similar to that ob-
tained by Rotter et al. [26].
The resistivity measurements carried out in the 5–45 K temper-
ature for various external magnetic fields up to 12 T are shown in
Fig. 2 for the x = 0.40 sample. It is clear from the figure that the
application of magnetic field does not broaden the superconduc-
The X-ray characterisation of the samples were done in a STOE
diffractometer operating in the Bragg–Brentano geometry, indi-
cated the formation of the I4/mmm, ThCr
amount of unreacted FeAs was seen in sample 1, whereas in sam-
ple 2 in addition to FeAs some Ba As14 was also present [25]. Using
2
Si
2
structure. A small
x 2 2
ting transitions in the Ba1ÀxK Fe As samples, unlike that seen in
the oxyarsenides [13], implying that there is not much granularity
or anisotropy in the double FeAs system. The lack of significant
anisotropy has been amply demonstrated on single crystals of
3
the STOE program the lattice parameters were obtained to be
a = 0.39159 nm and c = 1.3318 nm in sample 1 and a = 0.3899 nm
and c = 1.3411 nm in sample 2 respectively. Using these lattice
constants and the experimental variation of a,c lattice parameters
with K concentration [26] the K contents were estimated to be
x ꢀ 0.40 and x ꢀ 0.50 in sample 1 and sample 2.
the Ba122 compounds [14–16]. The plots of the T
C
onsets (defined
0.4
Ba1-xK Fe As
The resistivity versus temperature was measured in the four
probe geometry, and ac susceptibility measurements were carried
out using a mutual inductance set-up, in a dipper cryostat. The
magneto-resistance measurements were carried out in the four
probe geometry in an apparatus with field capability of up to
x
2
2
x=0.40
0.3
TC
0
6
1
T
T
2 T
1
2 T operable in the 6–300 K temperature range. The magnetiza-
0.2
tion measurements were carried out in a CRYOGENIC make, liquid
helium based, vibrating sample magnetometer operating at
2
0.4 Hz capable of a magnetic field range of up to 16 T and temper-
ature variation from 2 K to 300 K.
0.1
3
. Results and discussion
0.0
Fig. 1 shows the resistivity measured in the sample with
3
0
35
40
45
x = 0.40 in the 4–300 K temperature range. The room temperature
resistivity is 2.2 m cm, and the sharp superconducting transition
T(K)
X
is clearly seen. The inset (b) of Fig. 1 shows the diamagnetic signal
in the same sample. The resistivity onset occurs at 38.87 K and the
Fig. 2. The resistance measured as function of temperature for sample 1 is shown
for three magnetic fields, 0 T, 6 T and 12 T, measurements at all fields are not shown
for clarity. The dotted line shows how the T onset was determined for the 12 T
C
data.
D
T
C
was ꢀ2 K (obtained from the temperature difference between
onset and downset). The diamagnetic onset occurs at 37.5 K. The

1
0
A. Bharathi et al. / Physica C 470 (2010) 8–11
1
1
1
4
2
0
8
6
4
2
0
Ba1-xK Fe As x=0.40
Linear Fit
14
12
10
8
Ba1-x
Linear Fit
K
x
Fe
As x=0.50
2 2
x
2
2
4
2
0
2
4
Ba1-xK Fe As
x
2
2
(
a)
(b)
x=0.40
T=4.2K
6
-
-
4
2
0
-16
-12
-8
-4
0
4
8
12
16
37
38
T (K)
39
34
35
36
37
B (T)
T (K)
Fig. 4. Variation of magnetization as a function magnetic field (0–16 T to À16 T and
back 0 T) showing the magnetic hysteresis in Ba1Àx Fe As , x = 0.40, at 4.2 K.
Fig. 3. The variation of the T
application of magnetic field for Ba1Àx
line in each of the panels is the linear fit used to obtain dHC2/dT at T
C
onset obtained from R(T) measurements with the
K
x
2
2
K
x
Fe As (a) x = 0.40 and (b) x = 0.50, the solid
2
2
C
.
4
.2 K
as shown in Fig. 2), as a function of applied external field are dis-
played in Fig. 3a and b for x = 0.40 and x = 0.50 respectively. The
5
Ba K Fe As
1-x x 2 2
10
20 K
x=0.40
linear fit of the HC2 versus T curve gives a value of dHC2/dT at T
C
in x = 0.40 sample to be À7.708 T/K and in the sample with
x = 0.50 to be À5.57 T/K. Based on WHH formula [29] in the dirty
limit, this results in an evaluation of HC2(0) in the two samples
to be 202 T and 147 T respectively which is much larger than the
Chandrasekhar–Clogston paramagnetic limit [30] of ꢀ70 T for this
compound.
4
1
0
3
10
(a)
In the dirty limit (dHC2/dT)Tc is proportional to the normal state
resistivity and linear coefficient of specific heat [29]. The lower val-
ues of the normal state resistivity and T
C
in the x = 0.50 sample may
0
4
8
12
16
be responsible for the lower (dHC2/dT)Tc observed in this sample as
compared to that with x = 0.40. These values of (dHC2/dT)Tc seen in
Fig. 3 are similar to that observed perpendicular to the c axis in sin-
Ba K Fe As x=0.50
5
1-x
x
2
2
gle crystals of Ba1Àx
dHC2/dT)Tc values are generic to the arsenide compounds and are
particularly large when compared to that observed in high T
superconductors and C substituted MgB [31]. Recent high field
x
K Fe
As
2 2
[9]. It should be noted that these large
10
(
C
2
4
measurements seem to suggest that HC2(0) in these systems is in
the paramagnetic limited regime [32].
The magnetization hysteresis curves were measured in the two
samples at 4.2 K and 20 K. For this measurement the magnetic
field was ramped from 0 T to 16 T in the forward direction and
then ramped to 16 T in the reverse direction and then back to
10
4.2 K
(b)
20 K
3
1
0
0
T. The measured magnetization in the entire forward and reverse
0
4
8
12
16
cycling of magnetic field at 4.2 K is shown in Fig. 4 for the x = 0.40
sample. It is clear from the figure that the hysteretic behaviour
persists up to a large field of 16 T at 4.2 K. It is also clear from
Fig. 4 that the hysteretic curve is superposed on a ferromagnetic
background, possibly arising from a small amount of unreacted
FeAs present in the sample [33]. A much larger ferromagnetic
background was seen in polycrystalline samples in the oxyarse-
B (T)
Fig. 5. The variation the critical current J
loops at 4.2 K and 20 K in Ba1Àx Fe As (a) x = 0.40 and (b) x = 0.50.
C
obtained from magnetization hysteresis
K
x
2
2
larger than that observed in specifically processed C doped MgB
31].
2
[
nides [23]. Using the hysteretic magnetization data, J
using the Bean critical state model, J = 2(M )/d, where M
À M
and M are the measured magnetization in the forward and
reverse cycle at a given magnetic field and d is the diameter of
the grains. With an average grain size of 10 m obtained from
scanning electron microscopy, J is evaluated at 4.2 K and 20 K
for x = 0.40 and x = 0.50, which are shown in Fig. 5a and b respec-
tively. It is clear from the figure that at 4.2 K, J is larger than
0 A/cm even at 12 T. Such large values have also been observed
in the oxyarsenides at 5 K [23]. These J values are however much
C
is evaluated
C
+
À
+
À
4. Summary
l
In conclusion we have measured dHC2/dT by field dependent
resistivity versus temperature measurements and J by field
dependent magnetization measurements of superconducting
Fe As . The dHC2/dT and the critical current density ob-
C
C
C
Ba1Àx
K
x
2
2
4
2
1
tained from magnetization measurements in this compound are
C
notably high.

A. Bharathi et al. / Physica C 470 (2010) 8–11
11
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