Synthesis, structural and transport properties of the hole-doped Superconductor Pr_{1-x}Sr_xFeAsO

1 MB·English

by  Gang Mu

Synthesis, structural and transport properties of the hole-doped Superconductor
Pr Sr FeAsO
1−x x
∗
Gang Mu, Bin Zeng, Xiyu Zhu, Fei Han, Peng Cheng, Bing Shen, and Hai-Hu Wen
National Laboratory forSuperconductivity, Institute of PhysicsandBeijingNationalLaboratory forCondensed Matter Physics,
Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, People’s Republic of China
Superconductivity was achieved in PrFeAsO by partially substituting Pr3+ with Sr2+. The elec-
trical transport properties and structure of this new superconductor Pr1−xSrxFeAsO at different
9 doping levels (x = 0.05∼ 0.25) were investigated systematically. It was found that the lattice con-
0 stants(a-axisandc-axis)increasemonotonouslywithSrorholeconcentration. Thesuperconducting
0 transition temperature at about 16.3 K (95% ρn) was observed around the doping level of 0.20∼
2 0.25. A detailed investigation was carried out in the sample with doping level of x = 0.25. The

n domination of hole-like charge carriers in this material was confirmed by Hall effect measurements.
a The magnetoresistance (MR) behavior can be well described by a simple two-band model. The
J uppercritical fieldofthesamplewithTc =16.3K(x=0.25) wasestimated tobebeyond45Tesla.

3 Ourresultssuggest thatthehole-dopedsamplesmayhavehigheruppercritical fieldscomparingto
1 theelectron-doped ones, dueto thehigher quasi-particle density of states at theFermi level.

] PACSnumbers: 74.10.+v,74.25.Fy,74.62.Dh,74.25.Dw
n
o
c I. INTRODUCTION crease monotonously with doped concentration of Sr or
-
r holenumbers. Thephysicalpropertiesofaselectedsam-
p
ple with x = 0.25 were investigated in depth. The con-
u The discovery of superconductivity at 26 K in FeAs-
ducting chargecarriersinthis sample werecharacterized
.s basedlayeredquaternarycompound LaFeAsO1−xFx has to be hole type by the Hall effect measurements. And it
t attracted great interests in the fields of condensed mat-
a isfoundthatthe MRdatashowagoodtwo-bandbehav-
m ter physics and material sciences.1,2 The family of the ior. Wealsoestimatedtheuppercriticalfieldofthesame
FeAs-based superconductors has been extended rapidly
- sample based on the Ginzburg-Landau theory as well as
d and it can be divided into three categories. The first theWerthamer-Helfand-Hohenberg(WHH)formula.18It
n category has the general formula of REFeAsO where
is suggested that the upper critical fields in the hole-
o RE stands for the rare earth elements and is abbrevi-
doped samples may be higher than that in the electron-
c ated as the FeAs-1111 phase.3 The second class is for-
[ doped ones.
mulated as (Ba, Sr)1−xKxFe2As2 which is denoted as
2 FeAs-122 for simplicity.4,5 The third type Li FeAs has
x
v an infinite layered structure (denoted as FeAs- 111).6,7,8
II. EXPERIMENTAL DETAILS
6
As for the FeAs-1111 phase, most of the discovered su-
8
perconductors are characterized as electron doped ones
9 and the superconducting transition temperature has The Pr1−xSrxFeAsO samples were prepared using a
0 two-step solid state reaction method. In the first step,
0. been quickly raised to Tc = 55∼ 56 K via replacing PrAs and SrAs were prepared by reacting Pr flakes (pu-
lanthanum with other rare earth elements.9,10,11,12,13,14
1 rity99.99%),Srflakes(purity99.9%)andAsgrains(pu-
About the hole-doped side, however, since the first
8 rity 99.99%) at 500 oC for 8 hours and then 700 oC
0 hole-doped superconductor La1−xSrxFeAsO with Tc ≈ for 16 hours. They were sealed in an evacuated quartz
: 25 K was discovered,15,16 only the Nd-based system
v tube when reacting. Then the resultant precursors were
i Nd1−xSrxFeAsOwithTc =13.5Kwasreported.17 Obvi- thoroughly grounded together with Fe powder (purity
X ously, there is an extensive space to explore more super-
99.95%)and Fe O powder (purity 99.5%) in stoichiom-
r conductorsinthehole-dopedsidebasedontheFeAs-1111 2 3
a phaseandtofurtherextendthefamilyoftheFeAs-based etry as given by the formula Pr1−xSrxFeAsO. All the
weighing and mixing procedures were performed in a
superconductors. And it is also significant to investigate
glovebox with a protective argonatmosphere. Then the
the basic physical properties of the hole-doped system
mixtures werepressedinto pellets andsealedin a quartz
based on the FeAs-1111 phase.
tube withanatmosphereof20%Ar. The materialswere
Inthispaperwereportanewroutetoeasilysynthesize heatedupto1150oCwitharateof120oC/hrandmain-
the hole-doped superconductors based on the FeAs-1111 tained for 60 hours. Then a cooling procedure was fol-
phase,SrdopedPr1−xSrxFeAsO,withthe maximumsu- lowed. ItisimportanttonotethattheuseofSrAsasthe
perconducting transitiontemperatureof16.3K.We car- startingmaterial, insteadofusing SrCO or SrO,is very
3
riedoutasystematicstudyontheevolutionofthesuper- essential to synthesize the high quality samples, and to
conductivityandthelatticeconstantswiththecontentof muchsuppress the secondaryimpurity phases. This new
SrorholeconcentrationinthesystemofPr1−xSrxFeAsO. route makes our hole-doped samples easily reproduced.
We found that the a-axis and c-axis lattice constants in- The X-ray diffraction (XRD) measurements of our
2

3
0.5 (a) Pr1-xSrxFeAsO
m) 0.4 x = 0.25
c
0.3 m) 2

m c
0T
( 0.2 ~ 150 K

9T m
0.1 ( 1
Experimental data at 0 T
0.0 0+ATn fit
0 10 20 30 40 50
0
u/mol) -20 (FHbC )= 10 Oe 2 m/mol) 02 ’’ 0 50 100 1T50 (K)200 250 300
m A -2
(e -4 -4 10 -4 Hac = 0.1 Oe FIG. 2: (Color online) The resistivity curve at 0 T in the
M ( ’ f = 333 Hz temperature region up to 300 K for the sample with x =
-6 -6
0.25. A flattening of resistivity in high temperature region
ZFC 0 5 10 15 20 25
is obvious, which seems to be a common feature of the hole-
-8 T (K) doped FeAs-based superconductors4,15,16. The red solid line
0 5 10 15 20 25 30 shows the fit below about 150 K using the formula ρ=ρ0+
ATn.
T (K)
FIG. 1: (Color online) (a) Temperature dependence of resis- indicating the quite high quality of our sample, and the
tivity for the Pr0.75Sr0.25FeAsO sample under two different onsettransitiontemperatureisabout16.3Ktakingacri-
magnetic fields 0 T and 9 T near the superconducting tran- terion of 95% ρ . A magnetic field of 9 T only depresses
n
sition. A rather sharp transition can be seen at zero field. the onset transition temperature about 2.5 K but makes
(b)Temperaturedependenceofdcmagnetizationforthezero the superconducting transition broader. The former be-
field cooling (ZFC) and field cooling (FC) process at H = 10
havior may indicate a rather high critical field in our
Oe. Theinset shows theacsusceptibility of thesame sample
sample, while the latter reflected the weak link between
measured with Hac = 0.1 Oe, f = 333 Hz. thegrains.19Fig.1(b)showsthezerofieldcooledandalso
the field cooled dc magnetization of the same sample at
10 Oe. And the diamagnetic transition measured with
sampleswerecarriedoutbyaMac-ScienceMXP18A-HF
ac susceptibility technique is shown in the inset of Fig.1
equipment with θ-2θ scan. The dc magnetization mea-
(b). Aroughestimatefromthediamagneticsignalshows
surements were done with a superconducting quantum
that the superconducting volume fraction of the present
interference device (Quantum Design, SQUID, MPMS7)
sampleisbeyond50%,confirmingthebulksuperconduc-
and the ac susceptibility of the samples were measured
tivity in our samples. The onset critical temperature by
on the Maglab-12T (Oxford) with an ac field of 0.1 Oe
magnetic measurements is roughly corresponding to the
andafrequencyof333Hz. TheresistanceandHalleffect
zero resistivity temperature.
measurements were done using a six-probe technique on
Shown in Fig. 2 is the temperature dependence of re-
the Quantum Design instrument physical property mea-
sistivityunderzerofieldupto300Kforthesamesample
surementsystem(PPMS)withmagneticfieldsupto9T.
as shown in Fig. 1. The resistivity data in the normal
The temperature stabilization was better than 0.1% and
state were fitted using the formula
the resolution of the voltmeter was better than 10 nV.
ρ=ρ +ATn, (1)
0
III. EXPERIMENTAL DATA AND DISCUSSION as we had done in the F-doped LaFeAsO system.19
As represented by the red solid line in Fig. 2, the
A. Resistive and diamagnetic transition data below about 150 K can be roughly fitted with
the fitting parameters ρ = 0.306 mΩ cm and n =
0
InFig.1(a)wepresentatypicalsetofresistivedatafor 2.000. The fine quadratic dependent behavior of the re-
thesamplePr1−xSrxFeAsOwithx=0.25under0Tand sistivity, which is consistent with the prediction of the
9 T near the superconducting transition. One can see Fermi-liquidtheory,maysuggestaratherstrongscatter-
thattheresistivitytransitionatzerofieldisrathersharp ing between electrons in the present system in the low
3

2100 4.00
2
10 x=0.05
s) 1800 x=0.10 (a)
nt x=0.15 3.99
y (cou 11250000 003 103 200 211 114 212 xx==00..2205 o Aa ()3.98
ntensit 690000 101 110 111 FeAs 112 004 104 105 213 204 006 220 214 106 3.97
I * * *
300

8.66
0 (b)
20 25 30 35 40 45 50 55 60 65 70 8.64
)
A
2 (degree) o (8.62
c
8.60
FIG. 3: (Color online) X-ray diffraction patterns for
8.58
Pr1−xSrxFeAsO samples with different doping levels: x =
0.05, 0.10, 0.15, 0.20 and 0.25. Onecan see thatall themain 8.56
peakscan beindexedtothetetragonal ZrCuSiAs-typestruc-
ture. Small amount of impurity phases were denoted by red -0.1 0.0 0.1 0.2
asterisks.
Sr content x
temperature region. In high temperature region above FIG.4: (Coloronline)Dopingdependenceof(a)a-axislattice
constant; (b) c-axis lattice constant. The black filled squares
about 150 K, however,a flattening of resistivity was ob-
represent data from our measurements. For electron-doped
served clearly. The similar behavior has been observed
side, the variable ”x” represents the fluorine concentration.
in other hole-doped FeAs-1111 systems La1−xSrxFeAsO ThedataoftheundopedandF-dopedcases(redfilledcircles)
and Nd1−xSrxFeAsO,15,16,17 and also in the FeAs-122 are takenfrom thework of Ren et al.11 Onecan seethat the
system(Ba,Sr)1−xKxFe2As2.4 Wehavepointedoutthat lattice constant roughly show a monotonous variation versus
thisbehaviormaybeacommonfeatureofthehole-doped dopingfrom electron doped region to hole-doped one.
FeAs-based superconductors16.
that both the a-axis and c-axis lattice constants expand
B. Doping dependence of lattice constants and monotonously from 11% F-doped PrFeAsO to the Sr-
superconducting properties doped samples. This indicates that the strontium atoms
gointothecrystallatticeofthePrFeAsOsystembecause
The XRD patterns for the samples with the nominal theradiiofSr2+(1.12˚A)islargerthanthatofPr3+(1.01
doping levels of 0.05∼ 0.25 are shown in Fig. 3. Some ˚A). It is worth noting that the extent of the lattice ex-
small peaks from small amount of FeAs impurity phase pandingisappreciablysmallerthanthatintheSr-doped
whichweredenotedbyredasteriskcanstillbe seen, and LaFeAsO system, where the maximum onset transition
the sample with x = 0.05 has a bit more impurity phase temperature can be as high as 26 K.15,16 In some cases,
FeAs than other samples. However, it is clear that all weseeaslightdropofresistivityattemperaturesashigh
the main peaks can be indexed to the FeAs-1111 phase as 28 K. So we believe that a further increase in Tc is
with the tetragonal ZrCuSiAs-type structure. By hav- possible if more strontium can be chemically doped into
ing a closer scrutiny one can find that the diffraction this system, like in the case of Nd1−xSrxFeAsO.17
peaksshiftslightlytothelow-anglesidewhenmorestron- In Fig. 5 we show the resistivity data of our sam-
tium are doped into the samples, suggesting an expand- ples made at variousnominaldoping levels ofSr ranging
ing effect of the lattice constants. The similar behav- from x = 0.05 to 0.25. A clear but rounded resistivity
ior has been observed in the Sr-doped LaFeAsO system anomaly can be seen around 155∼175 K when the dop-
previously.16 By using the software of Powder-X20, we ing level is 5%. And a tiny resistivity drop at about 6 K
took a general fit to the XRD data of each sample and whichmaybe inducedbythe antiferromagneticordering
determinedthelatticeconstants. Thedopingdependence of Pr3+ ions or superconductivity can be observed. At
of the lattice constants calculated from the XRD data, this time it is difficult to discriminate between the two
along with the data of the undoped and 11% F-doped scenarios since the magnitude of the resistivity drop is
PrFeAsO samples11, are presented in Fig. 4. It is clear quite small. At the doping levels of 0.10∼ 0.25, resis-
4

9 Pr1-xSrxFeAsO 9 20K 30K
8 8 40K 50K (a)
7 x=0.05 cm) 7 7153K0K 110405KK
m) 6 xx==00..1105 0H = 0 T 56 126000KK 128205KK
c 5 xx==00..2205 (y 4 225705KK
x
m 4 3 300K
( 3 2
1
2
0
1
-1
0 0 1 2 3 4 5 6 7 8 9
0 50 100 150 200 250 300 0H (T)
1.0
T (K)
(b)
C)0.8
FIG.5: (Coloronline)Temperaturedependenceofresistivity 3 m/0.6 Pr1-xSrxFeAsO
0o.f2s5a.mOpnleescPanr1−sexeSrtxhFateAthsOe rwesiitshtivxit=y a0n.0o5m,a0ly.10is, s0u.1p5p,re0s.s2e0d, -8 0 0.4 x = 0.25
1
gradually by doping more Sr into the system. The maxi- (H0.2
mumonsettransition temperatureappearsaroundthenomi- R
nal doping level of 0.20∼ 0.25.
0.0
-0.2
0 50 100 150 200 250 300
tivity anomaly in high temperature regime is suppressed
gradually and it eventually evolves into a flattening be- T (K)
haviorathighdopinglevels. Alsothemagnitudeofresis-
tivity reduces obviously compared with the sample with FIG.6: (Color online)Halleffectmeasurementsforonesam-
5% doping, suggesting that more and more conducting plePr0.75Sr0.25FeAsO.(a)Hallresistivityρxy versusthemag-
chargecarrierswere introduced into the samples. At the netic field µ0H at different temperatures. (b) Temperature
same time, the superconductivity emerges with doping dependenceoftheHallcoefficient RH. Ahugehumpappears
and becomes optimal with an onset transition tempera- in the intermediate temperature region, which seems to be a
commonfeatureforthehole-dopedFeAs-basedsuperconduc-
tureof16.3Kwhenthedopinglevelisx=0.20∼0.25. It
tors.
is worth noting that the resistivity at high temperatures
at a high doping levelof holes behaves in a different way
comparedwiththatintheelectron-dopedsamples11,21,22
where the resistivity anomaly is suppressed completely get more information about the conducting carriers, we
andthe resistivityalwayspresentsa metallic behaviorin measuredtheHalleffectofthesamplewithx=0.25(the
that regime. While in the hole-doped samples, the re- same one as shown in Fig. 1). Fig. 6(a) shows the mag-
sistivity anomaly is smeared much slower.15,16,17 In the neticfielddependenceofHallresistivity(ρ )atdifferent
xy
Nd1−xSrxFeAsO system,17 for example, it is found that temperatures. Inthe experimentρxy wastakenasρxy =
the structural transition from tetragonal to orthorhom- [ρ(+H)- ρ(-H)]/2 ateachpoint to eliminate the effectof
bic occurs even in the sample with superconductivity. the misaligned Hall electrodes. It is clear that all curves
Doping more holes may lead to stronger suppression to inFig. 6(a)havegoodlinearityversusthemagneticfield.
the structural transition as well as the SDW order, this Moreover,ρ ispositiveatalltemperaturesbelow250K
xy
mayleaveapotentialspacetoincreasethesuperconduct- givinga positive Hall coefficientR =ρ /H, whichac-
H xy
ing transition temperature in the hole-doped FeAs-1111 tually indicates that hole type charge carriers dominate
phase. the conduction in the present sample.
The temperature dependence of R is shown in Fig.
H
6(b). Very similar to that observed in La1−xSrxFeAsO
C. Hall effect samples,15,16 theHallcoefficientR revealsahugehump
H
in the intermediate temperature regime and the value of
It is known that for a conventional metal with Fermi R decreases down to zero at about 250 K, then it be-
H
liquid feature, the Hall coefficient is almost independent comesslightlynegativeabovethattemperature. Herewe
of temperature. However, this situation is changed for employasimpletwo-bandscenariowithdifferenttypesof
a multiband material23 or a sample with non-Fermi liq- carriers to interpret this behavior. We have known that
uid behavior, such as the cuprate superconductors.24 To for a two-bandsystem in the low-fieldlimit, the Hall co-
5
efficient R can be written as
H
RH = σ12(σR11++σσ222)R22, (2) 4 34 / (%)0 Pr0.75Sr0.25FeAsO 22340500KKKK
%) 2 50K
where σi (i = 1,2) is the conductance for different types ( 3 1 7150K0K
orefpcrheaserngtesctahrerieHrsalilncdoeiffffiecreiennttbfaonrdesa,cahntdypRei o=f c−a1rr/ineires /0 2 00 50H/100 (T/m15 c2m0) 25 111346050KKK
separately with n the concentration of the charge carri- 180K
ers for the differenit bands. We attribute the strongtem- 1 220205KK
perature dependence of RH in the present system to the (a) 225705KK
complicatedvariationoftheconductanceσ withtemper- 0
i
ature, which reflects mainly the temperature dependent -2 -1 0 1 2 3 4 5 6 7 8 9 10
behaviorofthescatteringrelaxationtime. Andthesign- 0H (T)
cdhiffaenrgeinntgteyffpeecstooffchRaHrgemcaayrriinedrsic(aetleectthreonparensdenhcoeleotfytpweo) ) 4 %) 1.5 345000KKK FFFiiittt 345000KKK
in the present system. The conductance σi of electron- (% 3 (/0 1.0 110405KK FFiitt 110405KK
like and hole-like carriersmay vary differently with tem- 0
0.5
perature,andtheelectron-likecarriersbecomedominant / 2
when the temperature is higher than about 250 K. 0.0
Thissimpletwo-bandmodelisconsistentwiththeMR 1 0 2 4 6 8
data (will be addressed in the next section). However, if 0H (T)
(b)
there are two types of carriers in two bands, we may ex- 0
pect ρ to be quadratic in magnetic field. The linear
xy 0 50 100 150 200 250 300
behavior in the ρ ∼H curve shown in Fig. 6(a) seems
xy
to be not agree with this scenario. So further measure- T (K)
ments with higher magnetic fields are required.
FIG. 7: (Color online) (a) Field dependence of MR ∆ρ/ρ0
at different temperatures for the sample with x = 0.25. The
inset shows Kohler plot of the same sample. It is clear that
D. Magnetoresistance
Kohlers rule is not obeyed. (b) Temperature dependence of
MR for the same sample determined at 9 T. Shown in the
Magnetoresistance is a very powerful tool to investi- inset is the theoretical fit to the field dependent MR data
gate the electronic scattering process and the informa- using a two-band model (see text).
tion about the Fermi surface.23,25 Field dependence of
MR, for the sample with x = 0.25 at different tempera-
tures is shown in the main frame of Fig. 7(a). Here MR with α and β the fitting parameters which were related
was expressed as ∆ρ/ρ = [ρ(H)−ρ ]/ρ , where ρ(H) totheconductancesandmobilitiesforthechargecarriers
0 0 0
andρ representthelongitudinalresistivityatamagnetic intwobands. Thefittingresultswereshownbythe solid
0
field H and that at zero field, respectively. One can see lines in the inset of Fig. 7(b). It is clear that Eq. 3
thatthecurveobtainedat20Krevealsaratherdifferent can describe our data quite well. This argument can be
feature compared with the data at temperatures above further confirmed by seeing about the situation of the
25K.Alsothe magnitude ofMR(obtainedat9T) at20 so-calledKohlerplot. Thesemiclassicaltransporttheory
K reached2 times ofthat at 25K giving a sharpdropin haspredicted thatthe Kohlerrule,whichcanbe written
the ∆ρ/ρ ∼ T curve when increasing the temperature as
0
asrevealedinthemainframeofFig. 7. Weattributethis ∆ρ µ H
0
behavior to the presence of fluctuant superconductivity =F( ), (4)
ρ ρ
0 0
in the low temperature region (∼16.3 K-20 K). At tem-
peratures higher than 150 K, the value of MR vanishes willbeheldifonlyoneisotropicrelaxationtimeispresent
to zero gradually. inasingle-bandsolid-statesystem.26 Eq. (4)meansthat
As for the case in the moderate temperature region, the ∆ρ/ρ vs µ H/ρ curves for different temperatures
0 0 0
the ∆ρ/ρ ∼H curve reveals a clear nonlinear behavior. should be scaled to a universal curve if the Kohler rule
0
In the inset of Fig. 7(b) we present the data at five is obeyed. The scaling based on the Kohler plot for the
typicaltemperaturesinthisregion. Thesedatawerethen present sample is revealed in the inset of Fig. 7(a). An
fitted based on a simple two-band model which gave the obvious violation of the Kohler rule can be seen on this
following formula plot. We attribute this behavior to the presence of a
multi-band effect in the present system.
∆ρ (µ0H)2 Actually, theoretical researches and angle-resolved
= , (3)
ρ α+β×(µ H)2 photoemission spectroscopic (ARPES) studies have
0 0
6
using Eq. 5. One can see a quite good fit in the inset
of Fig. 8 as revealed by the red solid line. The zero
0.40 temperature upper critical field was determined to be
0.35 Pr1-xSrxFeAsO Hc2(0) ≈ 52 T from the fitting process. Actually one
) 0.30 x = 0.25 can also determine the slope of Hc2(T) near Tc, which is
m 60 found to be about -4.0 T/K in the present sample. By
c 0.25 01TT 50 GOLn sTeht eoof rtyh eFit using the WHH formula18 the value of zero temperature
m 0.20 3T T) 40 resistivity upper critical field Hc2(0) can be estimated through:
(xx 00..1105 579TTT H (0c22300 H (0)=−0.693T (dHc2) . (6)
10 c2 c dT T=Tc
0.05 0
0 2 4 6 8 1012141618
0.00 T (K) Taking Tc= 16.3 K, we get Hc2(0) ≈ 45.1 T. Regard-
0 5 10 15 20 25 30 ing the relatively low value of Tc=16.3 K in the present
sample, this value of upper critical field H (0) is actu-
c2
T (K)
ally quite high. The slope of dH (T)/dT| in the hole-
c2 Tc
doped sample is clearly larger than that in the electron-
FIG. 8: (Color online) Temperature dependence of resistiv- doped samples. In the hole-doped La1−xSrxFeAsO
ityforthePr0.75Sr0.25FeAsOsampleunderdifferentmagnetic superconducting samples, we also found much larger
fields. The onset transition temperature defined by 95%ρn dH (T)/dT| when comparing it with the F-doped
shifts with the magnetic field slowly. The inset shows the c2 Tc
LaFeAsOsample.19,27 This maybe understoodasdueto
phase diagram derived from the resistive transition curves.
the higherquasiparticledensity ofstates(DOS)nearthe
The onset transition point is presented by the blue open cir-
cles. The red solid line shows the theoretical curve based on Fermi levelin the hole-doped samples. In a dirty type-II
theGL theory (Eq. 5). superconductor, it was predicted29 that −dHc2/dT|Tc ∝
γ withγ thenormalstatespecificheatcoefficientwhich
n n
is proportional to the DOS at E . It is thus reason-
F
shown a rather complicated Fermi surface and energy- able to ascribe the higher value of dHc2/dT|Tc to higher
band structure in the FeAs-based superconductors. Our DOSinthehole-dopedsamples. Theoreticalcalculations
data from measuring the Hall effect and MR are consis- do show that the DOS in the hole-doped side is larger
tent with these conclusions. than that in the electron-doped samples.30,31 The mea-
surements onlowercriticalfields32 andspecific heat33 in
Ba K Fe As reveal that the superfluid density and
0.6 0.4 2 2
E. Upper critical field the normalstate DOS is about 5 to 10 times largerthan
that in the F-doped REFeAsO system. If the normal
Finally, we attempted to estimate the upper critical state DOS is really larger in the hole-doped systems,
field of the sample with x = 0.25 from the resistivity higher upper critical fields may be achieved in the hole-
data. Temperature dependence of resistivity under dif- doped FeAs-1111 samples provided that the supercon-
ferent magnetic fields is shown in the main frame of Fig. ducting transition temperature can be improved to the
8. Similar to that found in the F-doped LaFeAsO poly- same scale.
crystallinesamples,19,27 theonsettransitionpoint,which
reflects mainly the upper critical field in the configura-
tion of Hkab-plane, shifts more slowly than the zero re- IV. CONCLUDING REMARKS
sistivity pointtolow temperaturesunder fields. We take
a criterion of 95%ρ to determine the onset transition
n In summary, bulk superconductivity was achieved by
points under different fields, which are presented by the
substituting Pr3+ with Sr2+ in PrFeAsO system. A sys-
blue open circles in the inset of Fig. 6. From these data
tematic evolution of superconductivity and the lattice
we can roughly estimate the upper critical field of this
samplebasedontheGinzburg-Landau(GL)theory. The constantswithdopinginholedopedPr1−xSrxFeAsOwas
discovered. By doping more Sr into the parent phase
followingequationhavebeenextractfromtheGLtheory
PrFeAsO, the anomaly of resistivity at about 165 K is
and used successfully on other samples:19,27,28
suppressed gradually and the superconductivity eventu-
ally sets in. The a-axis and c-axis lattice constants in-
1−t2 crease monotonously with Sr concentration. The maxi-
Hc2(T)=Hc2(0)1+t2, (5) mum superconducting transition temperature Tc = 16.3
K is found to appear around the nominal doping level x
where t = T/T is the reduced temperature and H (0) =0.20∼0.25. The positiveHallcoefficientR inawide
c c2 H
is the upper critical field at zero temperature. Taking temperaturerangesuggeststhattheholetypechargecar-
T = 16.3 K and H (0) as the adjustable parameter,the riersdominatetheconductioninthissystem. Thestrong
c c2
measured data in the inset of Fig. 8 were then fitted temperature dependence and sign-changing effect of R
H
7
were attributed to a multi-band effect and have been in- Acknowledgments
terpreted based on a simple two-band model with differ-
ent types of charge carriers. This argument was further
confirmed by the nonlinear field dependence of MR and
the violation of the Kohler rule. Interestingly, the slope
of the upper critical magnetic field vs. temperature near Weacknowledgethe helpofXRDexperimentsfromL.
T seems to be much higher than that of the electron- H.YangandH.Chen. ThisworkissupportedbytheNat-
c
doped samples. This is attributed to the higher DOS in uralScienceFoundationofChina,theMinistryofScience
the hole-doped samples than in the electron-doped ones. and Technology of China (973 project: 2006CB01000,
Thismayprovideanewwaytoenhancetheuppercritical 2006CB921802), the Knowledge Innovation Project of
field in the hole-doped FeAs-1111 superconductors. Chinese Academy of Sciences (ITSNEM).
∗
Electronic address: [email protected] (2008).
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