Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe$_{2}$As$_{2}$

1.7 MB·

by  Krzysztof Gofryk

Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe As
2 2
Krzysztof Gofryk,1 Minghu Pan,1 Claudia Cantoni,1
Bayrammurad Saparov,1 Jonathan E. Mitchell,1 and Athena S. Sefat1
1Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
(Dated: January 8, 2014)
We use multi-scale techniques to determine the extent of local inhomogeneity and superconduc-
tivity in Ca Pr Fe As single crystal. The inhomogeneity is manifested as a spatial varia-
0.86 0.14 2 2
tion of praseodymium concentration, local density of states, and superconducting order parameter.
We show that the high-T superconductivity emerges from clover-like defects associated with Pr
c
dopants. ThehighestT isobservedinboththetetragonalandcollapsedtetragonalphases,andits
c
4
filamentarynatureisaconsequenceofnon-uniformPrdistributionthatdevelopslocalized,isolated
1
superconducting regions within the crystals.
0
2

n Introduction. –Thediscoveryofunconventionalsuper- [STM]) measurements. EELS results indicate that the
a conductivityinFe-basedsuperconductors[1,2]openeda Prdistributionisnotuniform,whileSTMshowsanelec-
J
new era in superconductivity research [3]. These new tronic inhomogeneity, which is evidenced as a spatial
7
superconductors share several characteristics with their variation of both local density of states and the super-

high-T relatives based on copper (cuprates) [4]. First, conductingorderparameter. Theresultssuggestthatthe
] c
n both are d-electron materials having layered structures highest T , associated with clover-like defect, resides in
c
o
with characteristic CuO and FeAs/FeSe planes where the close vicinity of the Pr atoms and forms isolated su-
2
c
the superconductivity is believed to originate. Second, perconducting regions. STM also confirms that a signifi-
-
r their parent compounds are antiferromagnetic and su- cant part of the sample (up to 30 %) remains in the nor-
p
perconductivityarisesbyapplicationofpressureorwhen mal state when cooled below T , in agreement with bulk
u c
s they are doped by electrons or holes. Aside from intro- studies. Furthermore, we find that the high-Tc super-
t. ducing the charge carriers, the role of doping in iron- conductivity in Ca0.86Pr0.14Fe2As2 is observed in both
a
based superconductors, as well as in cuprates and heavy tetragonal and collapsed tetragonal phases. We discuss
m
fermions, is still unclear, nor it is well understood how the implications of this study on the role of disorder and
-
d the dopants are distributed in the material. In addition, itsrelationshipwithsuperconductivity,andgiveperspec-
n to promote superconductivity, the dopants are potential tives with extension to other exotic superconductors.
o sources of inhomogeneity such as phase separation and
c crystalline or electronic disorder. In many models of su- Methods. – Single crystals of Ca0.86Pr0.14Fe2As2 were
[
perconductivity,itisassumedthatthedopantatomsare grown out of FeAs flux with the typical size of about
1 distributeduniformlyinthematerial. However,thereare 2 1.5 0.2mm3[10]. Theenergy-dispersivespectroscopy
× ×
v many indications for nanoscale inhomogeneity in these (EDS) analysis points to inhomogeneous distribution of
7
materials [5, 6]. Pr dopant on the micron size. Several spots on differ-
2
ent crystals from the same batch have been used for the
4 Theissuesofdopingandinhomogeneityarewellexem-
analysis giving a heterogenous Pr concentration ranging
1
plified in Ca Pr Fe As superconductor [7, 8]. This
1. electron dope1d−xsystxem2can2show T 45 K, the highest from 11.75 to 15.33 %. The average concentration has
0 c ≈ beenestablishedas14%,whichisusedinthepaper. The
among the pnictides with ThCr Si crystal structure [9–
4 2 2 magnetic susceptibility were measured using a Quantum
11]. Such a high T in a simple structure stimulated a
1 c Design MPMS-7 device. The electrical resistivity and
huge scientific interest in this material from both, fun-
: heatcapacityweremeasuredusingafourwireandrelax-
v
damental and applied point of view. Despite extensive
i ation methods, respectively, implemented in a Quantum
X studies, the origin of the high-T superconducting state
c DesignPPMS-14setup. Thelowtemperaturediffraction
r inPr-dopedCaFe2As2 anditsfilamentarynaturearestill experimentswerecarriedoutusingaPANalyticalX’Pert
a under debate (see Refs. 12–16).
PRO MPD x-ray diffractometer with Cu-K radiation.
α1
Here we address these problems by investigating STEM and EELS were performed using a Nion Ultra-
the local electronic inhomogeneous state in Pr-doped STEM 200 microscope. The samples were imaged us-
CaFe As . We perform extensive electronic and struc- ingthehighangleannulardarkfielddetector(HAADF),
2 2
tural studies of single crystals of Ca Pr Fe As which selects diffracted electrons that have undergone
0.86 0.14 2 2
superconductor (onset T = 45 K) by use of macro- elasticscatteringincloseproximitytothenucleiyielding
c
(magnetic susceptibility, electrical resistivity, specific an intensity nearly proportional to Z2 and consequent
heat) and micro-scale (scanning transmission electron chemical information. The STM and scanning tunnel-
microscopy [STEM] coupled with electron energy loss ing spectroscopy (STS) experiments were carried out in
spectroscopy [EELS] and scanning tunneling microscopy a home-built low temperature scanning tunneling micro-
2
scope system. The samples were cleaved in ultra-high (a) (b)
vacuum and then loaded into the STM head for investi-
gations at low temperatures.
Bulk properties. – It has been reported that
Ca Pr Fe As superconductor exhibits a transforma-
1 x x 2 2
−
tion from tetragonal (T) to non-magnetic collapsed
tetragonal (cT) phase at 50-70 K [7, 8, 14]. We per-
∼
form x-ray diffraction measurements and use the (008)
Bragg peak, collected off the Ca Pr Fe As single
0.86 0.14 2 2
crystalsathigh-angleregions,asaprobeofthec-axislat-
ticeparameter. Surprisingly,themajority( 80%)ofour FIG. 2. (Color online) (a) The normalized magnetic suscep-
∼
Ca Pr Fe As samples do not show the transition tibility measured in zero-field-cooled (ZFC) and field cooled
0.86 0.14 2 2
(see Fig.1), and only a small fraction of our single crys- (FC) regimes and the temperature dependence of the elec-
trical resistivity of Ca Pr Fe As (b) low temperature
tals ( 20 %) exhibits T cT transformation at the low 0.86 0.14 2 2
tempe∼ratures. Notwithst−anding the differences, the high specific heat of Ca0.86Pr0.14Fe2As2 presented as C/T vs. T2
(see text).
T superconducting state in Ca Pr Fe As shows
c 0.86 0.14 2 2
similar characteristics for the samples with or without
collapsed tetragonal phase transition (see Supplemental
Materialinmoredetail[17]). Belowwefocusonthecrys- is unclear [18, 19]. In addition, the Sommerfeld coeffi-
tals that do not show T cT transition, as they best cient which is a measure of the density of states at the
represent the crystal prod→uct of the reaction batch. Fermi level, is as large as 50 mJ/mol K2. In general,
Typical low-temperature DC magnetic susceptibility theoriginoftheresidualγr observedinsuperconducting
and electrical resistivity of Ca Pr Fe As single materials could be caused by pair breaking effects in an
0.86 0.14 2 2
crystals are shown in Fig.2a. A diamagnetic behavior, unconventional superconductor, crystallographic defects
characteristic of superconducting state, is observed in and disorder, and/or spin glass behavior. However, as
thesusceptibilitybelow45Ktogetherwithanadditional weshowbelow, arelativelylargevalueofthelinearterm
field repulsion below 20 K. However, similarly to pre- in the specific heat of Ca0.86Pr0.14Fe2As2 is most proba-
vious reports, the mag∼nitude of the susceptibility is very bly associated with regions of the sample which are not
small and a maximum superconducting shielding corre- superconducting.
sponds to 5% at 2 K [7, 8]. The temperature vari- Nano-scale properties. – All the bulk results strongly
∼
ation of the electrical resistivity of Ca0.86Pr0.14Fe2As2 suggest that nanoscle inhomogeneity may be an impor-
crystals is presented in Fig.2a. As seen, the resistivity tant factor in Ca Pr Fe As . To explore this hy-
0.86 0.14 2 2
shows a rapid drop at Tc = 45 K (onset Tc), character- pothesis and the origin of the inhomogeneous super-
istic of superconducting state. However, the supercon- conducting state in more detail, we have performed
ducting transition is broad and zero resistance may not microscopic studies using STEM and STM measure-
evenbeobserved(seeSupplementalMaterialinmorede- ments. Aberration-corrected STEM and EELS provide
tail [17]). Figure 2b shows the low temperature depen- an adequate tool for probing the distribution of ele-
dence the specific heat presented as C/T vs. T2. An ments within the bulk with angstrom spatial resolution.
upturn seen at low temperature is similar to that ob- The best geometry for analyzing the Pr distribution in
served in other Fe-based superconductors and its origin Ca Pr Fe As is a cross sectional one, in which the
0.86 0.14 2 2
electron beam is parallel to the [100] axis of the crystal.
Inthisprojection, eachatomiccolumncontainsonlyone
(a) (008) (b) (008)
type of atoms: Fe, As, or Ca/Pr. Ca and Pr have sig-
Ca0.86Pr0.14Fe2As2 T Ca0.86Pr0.14Fe2As2
nificantly different atomic numbers, producing different
HAADF intensity. Therefore, the patchy contrast in the
HAADFimageofFig.3aoriginatestoalargeextentfrom
avariationinthenumberofthePratomspresentineach
Ca/Pr column, and provides a first indication for a non-
T cT
uniformPrdistribution,whichcausesdark(Pr-poor)and
bright (Pr-rich) patches. Figures 3b-f validate this hy-
pothesis,anddemonstratetheatomicresolutionofEELS
FIG. 1. (Color online) The temperature dependence of the in this particular case. Fig 3b is another HAADF image
(008)diffractionpeakofCa Pr Fe As forsamplesthat in which the beam is now slowly scanned over a coarse
0.86 0.14 2 2
(a) do not show and (b) show the collapsed tetragonal phase grid, allowingfortheacquisitionofanEELspectrumfor
transition. Thehorizontaldashedlinemarksthetransforma- each image pixel (spectrum image). Fig.3f shows the Ca
tion region (see text) (see text).
map obtained by plotting the Ca-L integrated intensity.
3
We notice that brighter spots in the Ca map correspond a b c d e
tiPfimolre,adsgaimoen,fdmaFvenirigdc.se3tpehvoeetarrsnesidfano.rfte,ThthteaoisskicemionsluubalmtetatnnttesheroewuspihstolohyswihatniciogqinhnuiidtrnheeddefiicclHiiaenAtneeAcdpyDrbooFy-f HAADF Ca-L 4^01 x 4 stnuoc( ytisnet0n1×)i 111117778878901000000 10 HA2A0DF
the arrows in Fig.3b and c. The small Ca peak in Fig.3f f distance (pixels)
ctcaroasarnstrnhieooosntpwonibnnebdtyahttoettharbeirbCitugahthicectodkensnttceoessnspvtomartarasitapiootfiinonFn∼Fisg6i.gi03n.%c3.dsoa,TfmahtchpeqselueeCirvtaehadircciobakntyncieoeusnnsss--, stnuoC )stnuoc( ytisnetni 116802000000000distan1c0e (pixe2lC0s)a -L
ing the log-ratio method for the corresponding low-loss (g) (h) (i)
45 % 10 %
EELspectrumimage[20]. Thethicknessofthesampleis
in fact uniform along each atomic plane in Fig.3b, with
an average thickness of 0.3 electronic mean free paths.
Onceestablishedatomic-scalespatialsensitivityofEELS
tovariationsinPrconcentration,morefocusedspectrum
images were acquired as shown in Fig.3g. In this case, 35 % 0
the spectrum image covers a 13 u.c. 13 u.c. region
× FIG. 3. (Color online) Sensitivity of STEM/EELS to the Pr
within the basal plane, the same geometry used in STM
distribution: (a) image of Ca Pr Fe As along the [100]
0.86 0.14 2 2
experiments. The relative Pr concentration is shown in
zone axis. Due to the difference in Z, the atomic columns in
Fig.3i, which clearly displays a clustering of Pr atoms the Ca(Pr) plane show different brightness depending on the
within regions of a few u.c. in size, in contrast with the number of Pr atoms they contain. (b) simultaneous HAADF
quite uniform distribution for the relative Fe concentra- signal obtained while a spectrum image is acquired by scan-
tion (see Fig.3h). We note that the Pr and Fe signals ningtheelectronbeamoverasmallregionona27 100grid.
×
(c)Caelementalmapobtainedbyplottingtheintegratedin-
in the plane view maps do not show atomic resolution.
tensity under the Ca-L edge. (d) thickness map from the
This is due in part to the coarser grid used and beam 2,3
zero-loss spectrum for the same region in (b) and (c). (e, f)
broadening effects, but also to the larger disorder asso-
line scans of (b) and (c) at the position indicated by the ar-
ciated with the (001) surface, which is observed also in rows. Praseodymium distribution probed by STEM/EELS:
STM and explains the small ( 3%) variation in the Fe (g) simultaneous HAADF image produced while the electron
∼
relative concentration. Despite the signal delocalization, beam scans a region of 13 u.c. 13 u.c. within the ab plane
×
a simple statistic of Fig.3i reveals that, if we assume the using a 70 70 grid. Maps of the Fe (h) and Pr (i) rela-
×
tiveconcentrationsestimatedusingtheFe-M ,Ca-L ,and
Pr atoms are located only in the regions with intensity 2,3 2,3
Pr-N edges (see text).
abovethemeanvalue(coloredredtowhiteinFig.3i),and 4,5
ignore contributions from low-intensity pixels below the
25th percentile, the biggest cluster in Fig.3i would con-
tain 10 Pr atoms. Considering that the electron beam consistoffouratomicfeaturesattachedwiththreebright
∼
probes a thickness of about 8 unit cells along c, a mono- tails. Under different bias, the defects turn into bright
layer slice of the sample in the ab plane would show very spots (see Supplemental Material in more detail [17]),
small Pr nuclei containing only a few atoms and spaced similar to the observations of Zeljkovic et al. [15], where
only a few unit cells apart. such defects are identified to be Pr-dopants. Interest-
ingly,asimilarclover-likestructurehasbeenrecentlyob-
Figure 4a displays a typical topographic image taken
served in doped Bi Se topological insulator [26, 27]. In
at4.2Kfortheinsitu-cleavedCa Pr Fe As single 2 3
0.86 0.14 2 2
the surrounding area of these defects, the surface lattice
crystal. To visualize the defects, we flatten the STM
becomes highly disordered in comparison to the unper-
image by compressing the overall roughness of these
turbed 2 1 stripe structure. Such ”disordered” region
bright spots in order to enhance the atomic contrast
×
merges smoothly into the 2 1 structure, and is likely a
of these dopants. As shown in this large scale image
×
reconstruction of the Ca layer [15].
(42 nm 42 nm), the majority of the sample surface is
×
coveredbytheso-called2 1stripes. Asthein-planelat- Pointtunnelingspectratakenat4.2Konordered2 1
× ×
tice constant is 3.98 ˚A, a high resolution image of two regions (see lower star in Fig.4b) demonstrates a clear
stripes, inset of Fig.4b, reveals a 2 1 (7.8 ˚A 4.1 ˚A) superconducting gap (green curve in Fig.4c). The STS
× ×
structure. Such 2 1 striped network has been reported curves can be fit to the Dynes function [28] using a gap
×
frequently on the surface of various 122 Fe-based super- value of ∆ 12 meV, yielding the ratio 2∆/k T 6.3.
B c
∼ ∼
conductors,resultingfromahalflayerofBa/Sr/Caafter This is consistent with reported values for other 122 Fe-
cleaving (see Refs.21–25). Furthermore, clover-like de- based superconductors [29, 30] and it is also expected
fects, around 1-2 nm in size, are found disrupting the for a moderate-coupling BCS superconductor. However,
2 1stripedstructure. Figure4bshowsthateachdefects suchagapstructurevariesspatially;theSTScurvesmea-
×
4
with praseodymium dopants create regions of strong lo-
calized superconductivity in this material. Gap inhomo-
geneity exists in most doped superconductors, including
Fe-based materials, however, the majority of the mate-
rials are bulk superconductors, showing only nanoscale
variation of the gap (see Refs. 15, 33–38). The appear-
ance of a non-superconductive phase and related phase
separation explains the filamentary nature of the super-
conductivity in this material and is in agreement with
the bulk measurements. The STM and STEM/EELS
data,togetherwithpreviousstudiesonthismaterial[15]
indicate that the clover-like defects observed in the to-
pographic image can be identified with Pr dopants, and
mark the site of localized regions of strong high-T su-
c
perconductivity. Furthermore, the size of the large-gap
red regions in Fig.4d is very similar to the size of the
red/yellow clusters in Fig.3i, indicating that the Pr clus-
(c)
tersareindeedverysmall,composedof3or4atoms,and
separated from other clusters by a few unit cells.
FIG.4. (Coloronline)Surfacemorphologiesofinsitu-cleaved Summary and outlook. – In summary, we have inves-
Ca0.86Pr0.14Fe2As2 single crystal and corresponding super- tigated the effects of electronic inhomogeneity and fil-
conducting gaps: (a) 42 nm 42 nm STM topographic im-
× amentary superconductivity in Ca0.86Pr0.14Fe2As2. We
age of the cleaved surface, taken with bias V = -0.3 V and
use an extensive macro- and micro-probe measurements
I = 50 pA. The STM image shown here is flattened to re-
t
and show that the inhomogeneity is manifested as a spa-
move the overall roughness and enhance the atomic contrast
of dopants. (b) Higher resolution STM image showing local cial variation of both, local density of states, and the
2 1 structure. (c) dI/dV curves measured on clover-like de- superconducting order parameter. Our analysis shows
×
fect (red) and on ordered 2 1 region (green). The locations that the inhomogeneous and strongly localized high-T
× c
are marked by yellow stars in panel b. (d) Superconducting superconducting state emerges from clover-like defects,
gap map derived from dI/dV spectra grid over a larger area
and is a consequence of a Pr distribution which is non
of84nm 84nmsize. (e)ThreerepresentativedI/dV spectra
× uniform although on a very small scale. In addition, a
for non-superconducting (blue) and superconducting (green,
significant part of the sample remains in normal state
red) regions. The dI/dV spectra grid was taken with V =
-100 mV, I = 10 pA, and modulation 3 mV (see text). at 4.2 K, in agreement with the bulk studies. Interest-
t
ingly, the presence of the high-T superconductivity in
c
Ca Pr Fe As is observed in both, tetragonal and
0.86 0.14 2 2
sured on the clover-like defect (see upper star in Fig.4b) collapsed tetragonal phases. It has been shown recently
show a gap size as large as 30 meV with no prominent thattheelectronicstructureisstronglyreconstructedbe-
coherencepeaks(redcurveinFig.4c). Tofurtherinvesti- low the cT transition in CaFe As , leading to disappear-
2 2
gatethegapdistributionwetookadifferentialtunneling ance of the hole pocket and magnetism at the center of
conductance spectra (dI/dV versus V) survey on a large theBrillouinzone[39,40]. Ifsimilarsituationisobserved
surface area, using the previously explored method [31– inPr-dopedCaFe As , thiswouldwarrantaquestionon
2 2
33] (see Fig.4d). Fig.4e shows three representative tun- thenatureofsuperconductivityandtheroleofinterband
nelingspectraforsuperconducting(redandgreencurves) spin fluctuation pairing mechanism in this material, and
and normal state (blue curve) regions. The gap map in other unconventional superconductors.
demonstrates a distinct nanoscale phase separation be- ThisworkwassupportedbytheDepartmentofEnergy,
tweensuperconductingandnon-superconductingregions. Basic Energy Sciences, Materials Sciences and Engineer-
Over 2/3 area (green color) has a gap around 15 meV, ing Division.
while 1/3 of the material is in normal state at 4.2 K.
∼
Thesuperconductingregionsareconnectedtoeachother,
therefore the presence of the percolating superconduct-
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Supplemental Material for ”Local inhomogeneity and filamentary superconductivity
in Pr-doped CaFe As ”
2 2
K. Gofryk, M. Pan, C. Cantoni, B. Saparov, J. E. Mitchell, and A. S. Sefat
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
(Dated: January 8, 2014)
PACSnumbers: 74.70.Xa,74.55.+v,74.62.Dh,68.37.Ma
In this Supplement we provide additional most prominent anomalies (see arrows in Figs.S1g and
4 information regarding: (i) transport properties h) and plot H vs. T in Figs.S1i and j. As seen, for both
1 of Ca Pr Fe As samples with and without samples T (defined at a maximum in dρ/dT) decreases
0.86 0.14 2 2 c
0 collapsed tetragonal phase transition and (ii) upon applied magnetic field with the slope of dHc2 = -
2 dT
STM measurements of Ca Pr Fe As . 6 T K 1. From the slope of the critical field, at high
0.86 0.14 2 2 −
n magneticfields,thevalueofH maybeevaluatedbythe
c2
a
Werthammer-Helfand-Hohenberg(WHH)model.[1,2]In
J
thisapproach(takingintoaccounttheorbitaleffectonly;
7
Makiparameterα=0)thecriticalfieldmaybeexpressed
TRANSPORT PROPERTIES
n] as Hco2rb = 0.69× Tc ddHTc2 Tc and for Ca0.86Pr0.14Fe2As2
this gives 180 T. Interestingly, for sample showing the
o As stated in the manuscript, the majority ( 80 %) (cid:2) (cid:3)
c of our Ca Pr Fe As samples did not show∼the col- collapsed tetragonal transition another anomaly having
r- lapsedtet0r.a8g6ona0.l1p4ha2set2ransition,andonlyasmallfrac- ddHTc2 = -2 T K−1 and Hc2 = 59 T can be identified and
p tracked.
tion ( 20 %) exhibits T cT transformation at low
u ∼ − Alltheresultsobtainedindicatethepresenceofthefil-
temperatures. Figs.S1a and b show the temperature
s amentary and inhomogeneous superconducting state in
. dependence of the (008) Bragg peak, collected off an
t Ca Pr Fe As with significant spacial distribution
a Ca Pr Fe As single crystals at high-angle regions, 0.86 0.14 2 2
m as 0a.8p6rob0e.14of c2-axi2s lattice parameter. For samples with of Tc. Furthermore, the transport data might suggest
larger amount of the high temperature superconducting
- thecT transition(Fig.S1a),the(008)peakincreaseswith
d phase in in Ca Pr Fe As is present in the tetrago-
lowering temperature, characteristic of formation of the 0.86 0.14 2 2
n nalcrystalstructure,ascomparedtothecollapsedtetrag-
o collapsed tetragonal phase at about 50 K. The absence
onal one. At this point we don’t know the origin of this
c of this transformation in other Ca Pr Fe As sin-
[ 0.86 0.14 2 2 behaviour. Thiscouldbesimplyassociatedwithadiffer-
gle crystal can be seen in Fig.S1b. The Figs.S1c and ent degree of inhomogeneity in these two samples, how-
1 d show the temperature dependence of the electrical re-
ever it could also have a more fundamental origin, re-
v sistivity measured for these two particular samples. In
lated to the nature of the superconductivity in Pr-doped
7
both pieces of crystal, a drop of the resistivity is ob-
2 served at 45 K (onset T ) characteristic of supercon- CaFe2As2 (see the main text).
4 ∼ c
ducting state. As can be seen from the figures an ad-
1
. ditional transition is observed at 20 K and for some
1 ∼ STM MEASUREMENTS
samples a zero resistance value has been observed be-
0
4 low 20 K, as exemplified in Fig.S1d. The temperature
Figure S2 shows a large-scale topographic STM im-
1 dependenceoftheelectricalresistivity,measuredforsev-
age acquired at 4 K for the in situ-cleaved Pr-doped
: eral values of the magnetic field applied parallel to the
v CaFe As single crystal. The size is 84 nm 84 nm,
Xi c-axis of the crystal of Ca0.86Pr0.14Fe2As2 is shown in taken2wi2th bias = -0.5 V. In addition to lin×ear stripe
figures S1e and f. The superconducting anomaly is grad-
structure existing all over the surface, bright spots with
r ually depressed and the transition is broadened with the
a similar size and uniform distribution were also observed,
field applied. As seen from the figures, the magnetic
similar to the observations of Zeljkovic et al. [3], where
field has a slightly different effect on the superconduct-
such defects are believed as Pr-dopant. As shown in the
ing state to the samples with and without the collapsed
paper, the the bright spots actually have a clover-like
tetragonal phase transition. Figs.S1g and h show the
structure in fllatten STM image.
temperature dependence of derivative of the electrical
resistivity of those two samples at different magnetic
fields. The dρ/dT curves show broad anomalies consis-
tent with the presence of the inhomogeneous supercon-
ducting state and significant spacial distribution of Tc in [S1] E. Helfand and N. R. Werthamer, Phys. Rev. 147, 288
Ca0.86Pr0.14Fe2As2 (see the main text). We track the (1966).
2
[S2] N. R. Werthamer, E. Helfand, and P. C. Hohenberg, Chu, and J. E. Hoffman, Phys. Rev. B 87, 201108(R)
Phys. Rev. 147, 295 (1966). (2013).
[S3] I. Zeljkovic, D. Huang, Can-Li Song, B. Lv, Ching-Wu
3
(a) T (b) T
cT
(c) (d)
(e) (f)
(g) (h)
T T
d d
/ /
r r
d d
(i) (j)
FigureS1: (Coloronline)ThecrystallographicandtransportdatameasuredonthesametwopiecesofCa Pr Fe As single
0.86 0.14 2 2
crystals. Thetemperaturedependenceofthe(008)diffractionpeakofCa Pr Fe As illustratesfor(a)transformationfrom
0.86 0.14 2 2
hightemperaturetetragonalphase(T)tolowtemperaturecollapsedtetragonalphase(cT)andfor(b)lackofthistransformation
forthissample. Thehorizontaldashedlinesmarkthetransformationregion. (c,d)Thetemperaturedependenceoftheelectrical
resistivityforthesetwosamples. (e,f)ρ(T)measuredatseveraldifferentmagneticfields. (g,h)Thetemperatureandmagnetic
field dependencies of the derivative of the electrical resistivity. Arrows mark the anomalies at dρ/dT. (i, j) H vs. T (see
c2
text).
4
FigureS2: (Coloronline)Alargescale, topographicSTMimage(raw)takenat4Kfortheinsitu-cleavedCa Pr Fe As
0.86 0.14 2 2
single crystal sample. The size is 84 nm 84 nm, taken with bias = -0.5 V (see text).
×