Superconductivity at 32K and anisotropy in Tl0.58Rb0.42Fe1.72Se2 crystals

0.52 MB·English

by  Hangdong Wang

Superconductivity at 32K and anisotropy in Tl Rb Fe Se crystals
0.58 0.42 1.72 2
Hangdong Wang,1,2 Chihen Dong,1 Zujuan Li,1 Shasha Zhu,1
Qianhui Mao,1 Chunmu Feng,1 H. Q. Yuan,1 and Minghu Fang1,∗
1Department of Physics, Zhejiang University, Hangzhou 310027, China
2Department of Physics, Hangzhou Normal University, Hangzhou 310036, China
(Dated: January 4, 2011)
Single crystals of Tl0.58Rb0.42Fe1.72Se2 are successfully grown with the superconducting transition
1 temperaturesTconset=32K andTczero=31.4K.TheHallcoefficient exhibitsamulti-bandbehavior,
1 whichisverysimilartothatofallotherFe-basedsuperconductors. Wefoundthatthesusceptibility
0 atthenormalstatedecreaseswithdecreasingthetemperature,indicatingastrongantiferromagnetic
2 (AFM)spinfluctuationatthenormalstate, whichmight berelated tothesuperconductivity(SC).
We also determined the upper critical fields in ab-plane and along c-axis. The anisotropy of the
n superconductivity determined by the ratio of Hc2ab and Hc2c is estimated to 5.0, which is larger
a
J thanthatin(Ba,K)Fe2As2 andBaFe2−xCoxAs2,butsmallerthanthatincupratesuperconductors.

3
PACSnumbers: 74.70.Ad;71.35.Ji;74.25.-q;74.25.Op

]
n
ab
o Sincesuperconductivityat26KinLaO1−xFxFeAswas ofthesuperconductivitydeterminedbytheratioofHc2
c
c discovered by Kamihara et al.[1], the iron-based super- and Hc2 is estimated to be 5.0.
r- conductors have receivedworldwide attention in the last Single crystals were grown by the Bridgeman method.
p three years. There have been four major types of iron- First, Rb Se, Tl Se, Fe and Se powders with high pu-
2 2
u
based compounds reported to exhibit SC after doping rity (99.99%) were mixed in an appropriate stoichiom-
s
. or under high pressures, i.e. 1111-type ReFeAsO (Re = etry and were put into alumina crucibles and sealed in
t
a rare earth)[1–6], 122-type BFe2As2 (B=Ba, Sr or Ca)[7– evacuated silica tube. The mixture was heated up to
m 9], 111-type AFeAs (A = alkali metal)[10], and 11-type 950oC and held for 6 hours. Then the melting mixture
- tetragonal FeTe1−xSe(S)x[11, 12]. All these iron-based was cooled down to 700oC in the cooling rate of 3oC/h
d
superconductorsshareacommonlayeredstructurebased and finally the furnace was cooled to room temperature
n
o on a square planar Fe2+ layer,tetrahedrallycoordinated with the power shut off. The obtained single crystals
c pnictogen (P, As) or chalcogen (S, Se, Te) anions. The show the flat shiny surface and are easy to cleave. The
[ emergenceofsuperconductivityupondestructionoflong- compositionofcrystals,Tl0.58Rb0.42Fe1.72Se2,wasdeter-
1 range antiferromagnetic (AFM) order is qualitatively minedbyusinganEnergyDispersiveX-raySpectrometer
v similar to observations in the layered cuprate high tem- (EDXS). The structure of single crystals was character-
2 perature superconductors. However,in the cuprates,the ized by X-ray diffraction (XRD). Magnetic susceptibil-
6 repulsiveinteractionamongtheelectronsissostrongthat ity measurements were carried out using the Quantum
4
the parent compounds are Mott insulators. By contrast, Design MPMS-SQUID.The measurementsof resistivity,
0
. all iron-based parents above mentioned are metallic. Hall effect and magneto-resistance were done using the
1
Very recently,superconductivity at about 30Kwas re- Quantum Design Physical Properties Measurement Sys-
0
1 ported in KxFe2Se2 [13] and Cs0.8Fe2Se1.96 [14]. Our tem PPMS-9.
1 group[15]haveconfirmedthatsuperconductivityat31K Figure1showstheXRD(Fig.1a)patternofpowderob-
v: occurringinthenewly(Tl,K)FexSe2 compound(denoted tained by groundingthe crystalsandsingle crystalXRD
i as 122-type iron-chalcogenide) is also in proximity of an (Fig.1b) pattern for Tl0.58Rb0.42Fe1.72Se2. Most peaks
X
AFMinsulator. Wefoundthat(Tl,K)Fe1.5Se2 withaFe- in the powder XRD pattern can be well indexed with a
r
vacancy super-lattice is an AFM insulator with the Neel ThCr Si -type structure (space group: I4/mmm). The
a 2 2
temperature, TN=250 K, which can be regarded as the latticeparameters,a =3.896˚A,andc =14.303˚Awasob-
parent of this system. With increasing the Fe-content, tained by the fitting XRD data. One small peak marked
theAFMorderisreduced. Whenthemagnetismiselim- by a triangle belongs to FeSe compound due to small
inated, superconductivity at31Kemerges. In this letter, amountexistingbetweencrystals. Only(00l)peakswere
wereportthesuccessfulgrowingofthesinglecrystalsfor observed in the single crystal XRD pattern, indicating
another new compound Tl0.58Rb0.42Fe1.72Se2. The on- that the crystallographic c axis is perpendicular to the
set and zero-resistivity transition temperature were esti- plane of the single crystal. The interesting is that there
matedtobe32Kand31.4K,respectively. Wefoundthat isanotherseriesof(00l)peaks(markedbythe asterisks)
the Hall coefficient exhibits a multi-band behavior. The with c = 14.806˚A, which is larger than that of the main
susceptibility result indicates a strong AFM spin fluctu- phase, implying that there may be a modulation struc-
ationatthe normalstate. We alsodeterminedtheupper turealongc axisduetothe existenceofFe-vacancy. But
criticalfieldsinab-planeandalongc-axis. Theanisotropy there is only a smallpeak appearinginthe powderXRD
2

2.0
002 008
3 0 a.u.) 011...505 (002a) 004 110 112 006 105 114 200 107/ 116/ 213 206/ 215/ 118
1 *
( 0.0
ty 008
si 6 (b) 6
n 0
te 4 004 0
n
I 2 002 010
0
*
* *
0
10 20 30 40 50 60 70
2 (Degree)
FIG.1: (Coloronline)(a)Powderand(b)SinglecrystalX-ray
diffraction pattern of Tl0.58Rb0.42Fe1.72Se2
pattern. This modulation structure was also observedin
CsxFe2Se2 crystals[14].
Figure 2(a) shows the resistivity in ab-plane, ρab(T),
and along c axis, ρc(T), as a function of temperature
for Tl0.58Rb0.42Fe1.72Se2 crystal. At higher tempera-
tures,bothρab(T)andρc(T)exhibitasemiconductor-like
behavior, and displays a maximum resistivity at about
150 K, and shows a metallic behavior below 150 K. A
metal-insulator transition occurs at about 150K. Simi-
lar behavior of ρab(T) was also observed in KxFe2Se2
[13], CsxFe2Se2 [14], (Tl,K)FexSe2[15] and RbxFe2Se2
[17, 18] crystals. We identified that the temperature of
the maximum resistivity in ρab(T) depends on the ac-
tual Fe-content in the crystals for the similar system
(Tl,K)FexSe2 [15]. It should be noted that the ab-
solute value of resistivity at the normal state is quite
large, for example, ρab is of 10 mΩ cm at 300 K, which
is much larger than that in other typical iron-based
superconductors. This may be attributed to the this
compound residing in the under-doping region in the
phase diagram, as discussed in (Tl,K)FexSe2 system by
us.[15]Another,fromthe ratiovalue ofρc/ρab=30-45es-
timatedfromtheresistivitydataatthenormalstate,the
anisotropy is smaller than that in (Tl,K)FexSe2 system,
where ρc/ρab=70-80 [15].
Inordertoexploretheelectronicstructureinthiscom-
pound,wemeasuredtheHallcoefficientRH. Figure2(b)
shows the temperature dependence of Hall coefficient,
RH(T), forTl0.58Rb0.42Fe1.72Se2 crystal. The interesting
isthatthechangeinthesignalofRH(T)at214Koccurs
from positive to negative, which is the typical behavior
of the multi-band of the electronic band and a common FIG. 2: (Color online)(a) Temperature dependence of ab-
feature of Fe-based superconductors. Above 214K, the plane and c-axis resistivity. (b)Temperature dependence of
Hall coefficient. (c) Temperature dependenceof dc magnetic
positivevalue ofRH implies thatthe carrierinthis com-
susceptibilityforbothzerofieldcooling(ZFC)andfieldcool-
pound is dominated by holes. The negative value of RH
ing processes (FC) at a magnetic field of H= 5 Oe applied
from 214K to Tc indicates that the carrier in the com- alongcaxisand(d)inab-planeforTl0.58Rb0.42Fe1.72Se2crys-
poundisdominatedbyelectrons. Thisresultmeansthat tal.
3
4
l) H = 1T
o
m 3 H c
/
u
m
e 2

3
- 0
1 H // c
( 1
Tl Rb Fe Se
0.58 0.42 1.72 2
0
0 100 200 300 400
T (K)
FIG. 3: (Color online) The magnetic susceptibility measured
at1TeslaforTl0.58Rb0.42Fe1.72Se2 crystalwiththemagnetic
field along and perpendicularto c-axis.
there are both hole pocket and electron pocket near the
Fermi energy, which is very similar to that of all other
Fe-based superconductors although there are Fe vacan-
ciesontheFe-planarinTl0.58Rb0.42Fe1.72Se2 compound.
Inthe (Tl,K)FexSe2 system[15],wefoundthatnosignal
change of RH occurs, indicating that the carrier is dom-
inated by electrons from Tc to 350 K. The difference of
the electron structure between both may be due to the
difference between K and Rb radii.
From both ρab(T) and ρc(T), we can see that a
onset
sharp superconducting transition occurs at Tc =32
mid zero
K, Tc =31.5 K and Tc =31.4 K , confirmed by
the existence of diamagnetism below Tc (see Fig. 2c
and 2d). As the magnetic field is applied in c axis,
the diamagnetic signal at the superconducting state is
very large and over negative unity due to larger demag-
netization factor. When the magnetic field is applied
in ab plane, the demagnetization effect can being ig-
nored,diamagneticsignalreachesto 98%atlowtemper-
atures. All of these demonstrate a bulk SC emerging in
the Tl0.58Rb0.42Fe1.72Se2 crystal, which is different than FIG. 4: (Color online) The temperature dependence of the
that observedin (Tl,K)FexSe2 by us [15], where no bulk resistivity withmagnetic field(a) parallel and(b)perpendic-
SC was observed in the range of 1.70 ≤ x < 1.78. ulartotheab-plane,respectively. (c)Thetemperaturedepen-
Figure 3 shows the magnetic susceptibility at the nor- dence of upper critical field Hc2(T) for Tl0.58Rb0.42Fe1.72Se2
crystal
malstateasafunctionoftemperature,χc(T)andχab(T),
fortheTl0.58Rb0.42Fe1.72Se2crystalwithamagneticfield
of 1 T applied parallel and perpendicular to the c-axis.
A drop in susceptibility at about 30 K corresponds to χc dropwithdecreasingthetemperatureimpliesastrong
thesuperconductingtransition. Whenthemagneticfield AFM spin fluctuation at the normal state, which might
was applied along c-axis, the susceptibility, χc(T) value be related to the SC.
above 150 K decreases linearly with decreasing the tem- The temperature dependence of resistivity from 20K
perature, then shows a little increase until the super- to 34K with different magnetic fields applied along c-
conducting transition occurs. When the magnetic field axis and in ab-plane is shown in Fig. 4(a) and 4(b).
was appliedin ab-plane, the χab value decreaseswelllin- We defined the Tc as the temperature where the re-
earlywithdecreasingthetemperatureinthetemperature sistivity was 90% drop at the superconducting transi-
rangeof30K≤T≤400K.Thebehaviorofbothχab and tion to determine the upper critical fields, Hc2. The
4
Hc2(T) determined in this way is shown in Fig. 4(c). and 2009CB929104,and PCSIRT of the Ministry of Ed-
The Hc2(T) exhibits a rather linear temperature depen- ucation of China (Grant No. IRT0754).
dence for both directions. Thus we can easily get the
ab
value of the slope for both directions: -dHc2 /dT|Tc
c
=10.0 T/K , -dHc2 /dT|Tc =2.0 T/K.The Hc2(0) value
can be estimated by the Werthamer-Helfand-Hohenberg
∗ Electronic address: [email protected]
(WHH)equation[16]Hc2(T)=0.693[-(dHc2/dT)]Tc with
[1] Y. Kamiharaet al., J. Am.Chem. Soc. 130, 3296(2008)
Tc=32 K, to be 221 T and 44.2 T with the mag-
[2] X.H. Chen et al., Nature(London) 453, 761(2008)
netic field applied in ab-plane and along c-axis, respec-
[3] G.F. Chen et al., Phy.Rev.Lett. 100, 247002(2008)
ab c
tively. The anisotropy Hc2 (0)/ Hc2 (0) is about 5.0, [4] Z.A. Ren et al., Europhys.Lett. 82, 57002 (2008)
which is larger than that in Ba0.6K0.4Fe2As2[20] and [5] H.H. Wen et al., Europhys. Lett. 82, 17009 (2008)
Ba(Fe0.92Co0.08)2As2 [21]. [6] C. Wang, et al., Europhys. Lett.83, 67006 (2008)
In summary, we successfully grew the single crystals [7] M. Rotteret al., Phys.Rev. Lett.101, 107006 (2008)
[8] A.S. Sefat et al., Phys.Rev.Lett. 101. 117004 (2008)
ofTl0.58Rb0.42Fe1.72Se2withthesuperconductingtransi-
onset zero [9] L. J. Li et al., New J. Phys. 11. 025008 (2009)
tiontemperature Tc =32K andTc =31.4K.The
[10] X. C. Wang et al., Solid State Commun. 148, 538-540
Hall coefficient exhibits a multi-band behavior, which is
(2008)
very similar to that of all other Fe-based superconduc- [11] K.-C.Hsuetal.,Proc.Natl.Acad.Sci.U.S.A.105,14262
tors although there are Fe vacancies on the Fe-planar (2008)
in Tl0.58Rb0.42Fe1.72Se2 compound. We also determined [12] M. H.Fang et al., Phys.Rev.B 78, 224503 (2008)
theuppercriticalfieldsinab-planeandalongc-axis. The [13] J. G. Guo et al., Phys.Rev.B 82, 180520(R) (2010)
[14] A. Krzton-Maziopa et al., arXiv:Condmat/1012.3637
anisotropy of the superconductivity determined by the
ab c [15] M. H.Fang et al., arXiv:Condmat/1012.5236
ratioofHc2 andHc2 isestimatedto5.0,whichislarger [16] N. R.Werthamer et al., Phys. Rev.147, 295 (1966)
than that in Ba0.6K0.4Fe2As2 and BaFe1.92Co0.08As2, [17] J. J. Ying et al., arXiv:Condmat/1012.5552
but smaller than that in cuprate superconductors. [18] A. F. Wang et al., arXiv:Condmat/1012.5525
This work is supported by the Nature Science [19] C. H.Li et al., arXiv:Condmat/1012.5637
Foundation of China (Grant No. 10974175 and [20] Z.S. Wang et al., Phys.Rev. B 78, 140501(R) (2008)
10934005), the National Basic Research Program of [21] N. Niet al., Phy.Rev.B 78, 214515 (2008)
China (973 Program) under grant No. 2011CBA00103