Crystal structure, physical properties and superconductivity in A Fe Se single x 2 2 crystals X. G. Luo, X. F. Wang, J. J. Ying, Y. J. Yan, Z. Y. Li, M. Zhang, A. ∗ F. Wang, P. Cheng, Z. J. Xiang, G. J. Ye, R. H. Liu and X. H. Chen Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China (Dated: February 1, 2011) 1 Westudiedthecorrelationamongstructureandtransportpropertiesandsuperconductivityinthe 1 differentAxFe2Se2 singlecrystals(A=K,Rb,andCs). Twosetsof(00l)reflectionsareobservedin 0 theX-raysinglecrystaldiffractionpatterns,andarisefromtheintrinsicinhomogeneousdistribution 2 oftheintercalatedalkaliatoms. Theoccurrenceofsuperconductivityisclosely relatedtothec-axis n latticeconstant,andtheAcontentiscrucialtosuperconductivity. Thehumpobservedinresistivity a seemstobeirrelevanttosuperconductivity. Thereexist manydeficiencieswithintheFeSelayersin J AxFe2Se2,whiletheirTc doesnotchangesomuch. Inthissense,superconductivityisrobusttothe 9 Feand Sevacancies. Very high resistivity in thenormal state should arise from such defects in the 2 conductingFeSelayers. AxFe2Se2 (A=K,Rb,andCs)singlecrystalsshowthesamesusceptibility behaviorinthenormalstate,andnoanomalyisobservedinsusceptibilityatthehumptemperature ] inresistivity. TheclearjumpinspecificheatforRbxFe2Se2 andKxFe2Se2 singlecrystalsshowsthe n good bulk superconductivity in these crystals. o c PACSnumbers: 74.70.Xa,74.25.F-,74.62.Bf - r p u Thenewlydiscoverediron-basedsuperconductorshave and it is much more complicated than that in the Fe-Se s attractedworldwideattentioninthe pastthree years[1– systems. The intercalated alkali atoms could be crucial . t 5] because of their high superconducting transition tem- to the superconductivity. The normal-state resistivity a m perature(Tc ashighas55K)andthefactthatsupercon- should be influenced by these vacancies seriously. But ductivity emerges proximity to the magnetically ordered how these vacancies affect the physical properties still - d state. [6, 7], which were considered to be taken as a remains unresolved. n comparison with the superconducting cuprates for find- In this article, we systematically studied the effect of o ing out the mechanism of the high-T superconductivity. c thestartingmaterialsandtheheatingprocessonthesin- c The highest T of the FeAs-basedpnictides reaches55 K [ c glecrystalgrowthforAxFe2Se2(A=K,Rb,andCs),and atambientpressure,whiletheanti-PbOtypeFeSex,own- measured the physical properties of these single crystals 1 ingthe extremelysimple structurewiththe edge-sharing and determined their crystal structures. It is found that v FeSe tetrahedra formed FeSe layers stacking along the 4 two sets of (00l) reflections exist in all the crystals, and 0 c-axis,displaysalowerT of8Katambientpressure[8]. c superconductivity is closely related to the c-axis lattice 7 After T was enhanced to as high as 37 K by applying 6 c constant, indicating that the A content is crucial to the highpressure[9],theeffortsofintroducingstructuresbe- 5 superconductivity. The hump in resistivity arises from . tweentheFeSelayers,justliketheFeAs-analoguesbeing, the defects within the conducting FeSe layers and is ir- 1 successfully induced 30 K superconductivity by interca- relevant to superconductivity. No anomaly is observed 0 lating the alkali (K, Rb, Cs) and Tl atoms into between 1 in magnetic susceptibility at the temperature of hump the FeSe layers[10–15]. The intercalatedFeSe supercon- 1 in resistivity. The clear jump in specific heat for super- v: ductors show some distinct physical properties from the conductingKxFe2Se2andRbxFe2Se2singlecrystalsindi- FeAs-based superconductors, such as, superconductivity i cates the good bulk superconductivity in these crystals. X with very high normal-state resistivity, a broad hump in resistivity. The Fe content is important in control- Single crystals AxFe2Se2 (A = K, Rb, and Cs) were r a ling the magnetic and superconducting properties in the grown by Bridgeman method as described elsewhere[12, 13]. The starting materials and the heating process are iron chalcogenides, additional iron would greatly affect veryimportanttogetsuperconductingsinglecrystal,and its structural and magnetic properties [16], and super- even is slightly changed to dramatically affect its physi- conductivity can be enhanced by the de-intercalation of the interstitial iron [17, 18]. However, vacancies have cal properties. The three different batches of RbxFe2Se2 been found to be able to exist at either A site, or within and three batches of KxFe2Se2 single crystals were got- ten by the slightly change of the heating temperatures theconductingFeSelayersinchemicalformulaAxFe2Se2, and starting materials. The single crystals were charac- terized by X-ray single crystal diffraction, magnetic sus- ceptibility,andelectricaltransportmeasurements. X-ray ∗E-mail: [email protected] single crystal diffraction was performed on a TTRAX3 theta/thetarotatinganodeX-rayDiffractometer(Japan) withCuKαradiationandafixedgraphitemonochroma- 2 150 400 0 8 20 6 4 -25 ) 10 3 100 2 300 m m) 0 C/ c 30 32 34 RbxFe2Se2-1 -9 0 0 -50 1 m RbxFe2Se2-2 ( ( H -75 R 50 RbxFe2Se2-3 200 -10 RbxFe2Se2-4 RbxFe2Se2-2 RbxFe2Se2-3 -100 -20 RbxFe2Se2-4 0 100 0 50 100 150 200 250 300 0 50 100 150 200 250 300 T (K) T (K) FIG. 2: (Color online) Hall coefficient as a function of tem- FIG. 1: (Color online) Temperature dependence of resis- perature for single crystals: RbxFe2Se2-2, RbxFe2Se2-3 and tivity for the three batches of RbxFe2Se2 single crystals: RbxFe2Se2-4. RbxFe2Se2-1 (blueline), RbxFe2Se2-2 (red line), RbxFe2Se2- 3(darkcyanline) andRbxFe2Se2-4(blackline). Theinset is zoomed plot of (b) around Tc. resistivitybehavior andtheir values. The onsetandzero resistance temperature for RbxFe2Se2-2 and RbxFe2Se2- tor. Magnetic susceptibility measurements were carried 3 are 32.0, 32.4 and 31.5, 31.6 K, respectively. Thus one outusingtheQuantumDesignMPMS-SQUID.Themea- can see that the humps in resistivity seems to be irrel- surement of resistivity and magnetoresistance were done evant to superconductivity. The large magnitude of the on the Quantum Design PPMS-9. normal-stateresistivitycomparedtoFeAs-basepnictides [19–21] and FeSe [22] (usually with ρ(300 K) much less The typical temperature dependence of resistivity is than 1 mΩ cm) reflects the existence of many deficien- observedforthreebatchesofRbxFe2Se2singlecrystalsas shown in Fig.1. Among these crystals, RbxFe2Se2-1 was cieswithintheconductingFeSelayersintheseRbxFe2Se2 singlecrystals. Thehumpinresistivityshouldarisefrom obtainedwith nominalcompositionas Rb0.8Fe2Se1.96 by ◦ suchlarge amount of defects within the conducting FeSe being melt at 1080 C and turning off furnace at 950 ◦ layers. Inthissense,superconductivityisquiterobustto C; RbxFe2Se2-2 and RbxFe2Se2-3 came from the same batch with the nominal composition as Rb0.8Fe2Se2 by the vacancies within the FeSe layers. For RbxFe2Se2-4, ◦ althoughtheresistivitystillshowsahumpataround170 being melt at 1030 C and turning off furnace at 700 ◦ K, no superconductivity can be observed, and a strong C; RbxFe2Se2-4 was grown with nominal composition ◦ semiconducting/insulator-likebehaviorisobservedbelow as Rb0.8Fe2Se1.96 by being melt at 1030 C and switch- ◦ 70 K. ingofffurnaceat700 C. TheresistivityofRbxFe2Se2-1 showsaverysmallhumparound290Kandthenbecomes For comparison, we measured the temperature depen- metallic below this temperature with the residual resis- denceoftheHallcoefficientontheexactlysamepiecesof tivity ratio RRR = R(300 K)/R(35 K) ≈ 37.2. Super- RbxFe2Se2-2,RbxFe2Se2-3andRbxFe2Se2-4asshownin conductivity appearsbelow 32.4K andzeroresistance is Fig.2. The Hall coefficient of the superconducting crys- reachedat31.9K.Thesuperconductingtransitionwidth tals RbxFe2Se2-2 and RbxFe2Se2-3 is positive at high of RbxFe2Se2-1 is as narrowas 0.5 K although the resis- temperature, and gradually decreases with decreasing tivityatroomtemperatureisaslargeas70mΩcmat300 temperature and then becomes negative at low temper- K.Thehumptemperatureofresistivity(T )shifts to ature. The sign change of the Hall coefficient is also hump 265 K and 225 K for RbxFe2Se2-2 and RbxFe2Se2-3, re- observed in superconducting Tl0.58Rb0.42Fe1.72Se2 sam- spectively. The RRR decreases to 26.1 and 17.3 for the ples previously [23]. Actually from ARPES results in twocrystals,respectively. Theseresultsindicatethatthe AxFe2Se2 (A = K and Cs) [24, 25], the AxFe2Se2 are metallicity of RbxFe2Se2-2 and RbxFe2Se2-3 is weaker electronover-dopedandonlyelectronpocketscanbeob- compared to RbxFe2Se2-1. However, as we can see from served. However, hole pockets could exist in the super- theinsetofFig.1andTableI,thesuperconductingtransi- conductingsamplesdue tosignchangeofHallcoefficient tiontemperatureseemsnottovarywiththechangeinthe basedonthe Halleffectmeasurements,suggestingapos- 3 ) Units) 01..92 RRRbbbxxxFFFeee222SSSeee222---432 006-1 006-2 008-1 008-2 -10010 0010-2 Arb. Units 11..05 KKxxFFee22SSee22--12 (a) b. RbxFe2Se2-1 y ( KxFe2Se2-3 sity (Ar 0.6 004-1 004-2 ntensit 0.5 Inten 0.3 02-2 I 0.010 20 320 (de40gree5)0 60 70 0 150 0.0 10 /10 (b) 10 20 30 40 50 60 70 T (K) 5 600 FIG. 3: (Color online) The X-ray single crystal diffraction patterns for thedifferent batches RbxFe2Se2 single crystals. m)100 0 28 30 32 c 400 m siblymulti-bandnatureofthesuperconductivity. Forthe sample without superconductivity, the Hall coefficient is ( 50 negative in the whole temperature range. It indicates 200 thatdominantcarrieriselectronfornon-superconducting crystal. Itseemsthatthe holepocketmightbe quiteim- portantfor the superconductivity. Therefore,it needs to be furtherinvestigated. What’smore,noanomalyinthe 0 0 0 50 100 150 200 250 300 HallcoefficientisobservedatT . Itsuggeststhatthe hump humpsinresistivityforAxFe2Se2 crystalsarenotrelated T (K) to a structural or magnetic transition, being contrasting FIG.4: (Coloronline)(a): TheX-raysinglecrystaldiffraction tothefactsthattheanomalyofresistivityinunderdoped FeAs-basedsuperconductorisalwaysrelevanttoastruc- patterns for the different batches KxFe2Se2 single crystals: KxFe2Se2-1(blueline), KxFe2Se2-2(redline) andKxFe2Se2- tural/magnetic transition. 3(black line); (b): The temperaturedependenceof theresis- AsshowninFig. 1,onecanobservealmostthesameTc tivity of these three different KxFe2Se2 single crystals. The forthesinglecrystalswiththedifferentThump. TheX-ray inset is zoomed plot of (b) around Tc. singlecrystaldiffractionwascarriedoutforthesamefour pieces RbxFe2Se2 single crystals shown in Fig. 1 to find out the relationship of superconductivity and structure. inFig. 4. ItshowsthattheKxFe2Se2crystalsexhibitthe The X-ray diffraction (XRD) patterns are shown in Fig. obviously different resistivity behavior. KxFe2Se2-1 was 3. Surprisingly, two sets of (00l) reflections are observed grownusing K0.8(FeSe)2 as starting materials and being inallthefoursamples. Thetwoc-axislatticeparameters melt at 1030 ◦C for 3 hours. KxFe2Se2-2 and KxFe2Se2- c1andc2areobtained(listedinTable I).The c-axislat- 3 were grown by using K0.8(FeSe)2 as starting materials tice parameters c1 and c2 correspond to the two sets of andbeing melt at 1030◦C for 2 hoursand 950◦C for 20 reflectionswithweakandstrongintensities,respectively. hours,respectively. InFig.4a,twosetsof(00l)reflections Thesetwodistinctsetsofreflectionscouldarisefromthe canbeobserved,andthisbehavioristhesamewiththose inhomogeneousdistributionoftheintercalatedRbatoms. intheRbxFe2Se2samples,suggestingthattheinhomoge- Considering the fact that the superconducting crystals neous distribution of the intercalated alkaliatoms in the show nearly fully shielding fraction, the reflections with crystalsiscommonfeature. Forthenon-superconducting c2shouldberesponsibleforthesuperconductivity. From sample, the c-axis lattice constant c1 is smaller by more the superconducting to non-superconducting crystal, c1 than 0.64%, while c2 is larger by 0.15% than those in is reduced by 0.55% while c2 is enhanced by more than the superconducting samples. These results are consis- 0.14%. It is found that the insulator-like behavior is en- tentwiththeresultsobservedintheRbxFe2Se2 samples, hanced with losing superconductivity. It indicates that indicating that the content of alkali atom plays a crucial superconductivitymayexistwithinalimitedrangeofthe roleforthe occurrenceofsuperconductivity. KxFe2Se2-1 c-axis lattice parameter. In other word, the Rb content showsabroadresistivityhumpatabout220Kandsuper- is crucialto the occurrenceof superconductivity because conductivityat31.7K.Forthe KxFe2Se2-2, Thump shifts the c-axis lattice parameter strongly depends on the Rb to 120 K and superconductivity shows up at 30.3 K. Al- content. though the superconductivity disappears in KxFe2Se2-3, We then carefully measured resistivity and the XRD the Thump for KxFe2Se2-3 is higher than that for the su- patterns for three pieces of KxFe2Se2 crystals, as shown perconducting KxFe2Se2-2, strongly demonstrating that 4 TABLEI:Thec-axislatticeparametersc1andc2correspondingtothetwosetsofreflectionswithweakandstrongintensities, respectively. Tczero,Tconset,thehumptemperatureinresistivity(Thump)andresidualresistivityratio(RRR=R(300K)/R(35 K)) for all thecrystals AxFe2Se2 (A=K,Rb,Cs). sample name c1 (˚A) c2 (˚A) Tczero (K) Tconset (K) Thump (K) RRR RbxFe2Se2-1 14.873 14.569 31.9 32.4 290 37.2 RbxFe2Se2-2 14.873 14.582 31.5 32.0 265 26.1 RbxFe2Se2-3 14.874 14.574 31.6 32.4 225 17.3 RbxFe2Se2-4 14.792 14.604 170 KxFe2Se2-1 14.292 14.086 31.2 31.7 220 21.2 KxFe2Se2-2 14.282 14.062 29.2 30.8 120 0.65 KxFe2Se2-3 14.201 14.107 160 CsxFe2Se2 15.556 15.285 28.3 30.3 nohump <300 K 16.3 ) nit 0.6 4.7 1.5 b. U (a) CsxFe2Se2 (a) H = 5 T RbxFe2Se2-1 r 0.4 1.2 A 4.6 y ( H // ab t si 0.2 0.9 n e 4.5 t n I 0.0 H // c 0.6 10 20 30 40 50 60 70 2 (degree) 4.4 1.5 mol) (b) RbxFe2Se2-2 1.0 mol) (b) u / 2.0 u / m m H // ab )1.0 -3 0e 0.5 -3 0e cm (1 1.9 H // c (1 ( 0.09 0.0 1.5 0.5 0.06 2.7 (c) RbxFe2Se2-4 0.03 1.2 0.00 2.4 26 28 30 32 34 0.0 H // c 0.9 0 100 200 300 2.1 T (K) H // ab 0.6 FIG. 5: (Color online) (a): The X-ray single crystal diffrac- 1.8 0 100 200 300 tion pattern for the CsxFe2Se2 single crystal; (b): The tem- peraturedependenceoftheresistivityoftheCsxFe2Se2 single T (K) crystal. The inset is zoomed plot of (b) around Tc. FIG. 6: (Color online) The temperature dependence of the susceptibility with the magnetic field of 5T applied parallel and perpendicular to the c-axis for (a): RbxFe2Se2-1; (b): thesuperconductivityisnotcorrelatedtothehumpinre- RbxFe2Se2-2; (c): RbxFe2Se2-4. sistivity. The position of the hump in resistivity reflects the vacancy level within the conducting FeSe layers. It suggests that the vacancies within the FeSe layers have served with Tonset = 30.3 K and Tzero = 28.3 K. In c c much weaker correlation to superconductivity than the the XRD patterns of Fig.5a,very smallreflections corre- content of intercalated alkali atoms does. sponding to c1 can still be found except for the main re- Figure5showstheX-raysinglecrystaldiffractionpat- flectionswithc2. Itsuggeststheinhomogeneousdistribu- tern and the temperature dependence of resistivity for tionofalkaliatomsiscommonforalltheAxFe2Se2 single CsxFe2Se2 single crystal. Totally metallic resistivity can crystals. It is worthy to note that the superconductivity be observed below 300 K. Superconductivity was ob- always shows up around 30 K for the crystals AxFe2Se2 5 withchangingtheintercalatedalkaliatomAfromK,Rb anomaly can be found at T , suggesting that hump hump to Cs. The T seems not to depend on the ionic radii inresistivitycannotbeascribedtoamagnetictransition. c of the intercalated alkali atoms although the supercon- Fig.7 shows the susceptibility with the magnetic field of ductivity strongly depends on the A content. As shown 5 T applied parallel and perpendicular to the c-axis for in Fig.1 and Fig.4b,the hump in resistivity changes pro- KxFe2Se2-2andCsxFe2Se2,respectively. Thesimilarbe- nouncedly for the same alkali atom case, while the Tc is haviorwiththoseofRbxFe2Se2 crystalsshowninFig.6is nearlythesame(about30K).Verylargenormal-statere- observed. These results indicate that although the elec- sistivityisobservedinalltheaboveAxFe2Se2singlecrys- tronicpropertieschangedramaticallyfromsystemtosys- tals, suggesting that large amount of deficiencies within tem and from crystal to crystal, the magnetic property theconductingFeSelayersforallofthesecrystals. Based does not change a lot. on these observations, T seems to be robust to the va- c cancies within the FeSe layers. 900 (a) Tc ) 2 0T 4.3 K (a) KxFe2Se2-2 3.0 ol 800 9T m H = 5 T / 4.2 J m Rb Fe Se x 2 2 ( 700 2.5 H // ab T )4.1 1 ) C / Tc -1 l - ol 600 o H // c m K Fe Se m 2.0 x 2 2 u u4.0 m m 24 27 3 0 33 36 e e (b) Cs Fe Se 3 -3 0 x 2 2 1.5 - 10 (b) (13.0 ( 2 K) 12 RbxFe2Se2 ol KxFe2Se2 1.0 m H // ab 8 J/ m 2.9 ( H // c 0.5 T 4 / C 0 50 100 150 200 250 300 T (K) 0 FIG. 7: (Color online) The temperature dependence of the 21 24 27 30 33 36 susceptibility with the magnetic field of 5 T applied parallel andperpendiculartothec-axisfor(a): KxFe2Se2-2;and(b): T (K) CsxFe2Se2. FIG.8: (Coloronline)(a): Theheatcapacityasafunctionof temperatureforRbxFe2Se2-2andKxFe2Se2-1atthemagnetic MagneticsusceptibilitywasmeasuredontheAxFe2Se2 fieldof0Tand9Tappliedalongc-axis. (b): Theheatcapacity single crystals to investigate the correlation between the differencebetween0Tand9TforRbxFe2Se2-2andKxFe2Se2- normal-state resistivity and magnetism. Figure 6 shows 1. A clear heat peak was observed, indicating the good bulk theanisotropicmagneticsusceptibilitywiththemagnetic superconductivity. field of 5 T applied within the ab-plane and along the c-axis for RbxFe2Se2-1, RbxFe2Se2-2, RbxFe2Se2-4, re- Fig.8a shows the temperature dependence of the spe- spectively. Although the samples show very different re- cific heat (C/T) around T at the magnetic field of 0 T c sistivity behavior,such as the different Thumps, the mag- and 9 T for RbxFe2Se2-2 and KxFe2Se2-1, respectively. nitude ofresistivityandTc,the normal-statesusceptibil- At 0 T, one can see the clear specific heat anomaly at ity shows the quite similar behavior to each other. As T . Although 9 T is far lower than the upper critical c the field is applied within the ab-plane, the magnitude field, which is estimated higher than 100 T,[12, 13] the of susceptibility varies within 20% in the normal state anomaly in specific heat is completely suppressed. The andthe susceptibility itself showsa broadminimum. No specific heat jump (C(0T)-C(9T))/T against T is plot- 6 ted in Fig.8b for the two crystals. The heat capacity suggests that the humps in resistivity in AxFe2Se2 are jumps for RbxFe2Se2-2 and KxFe2Se2-1 crystals shows not related to a structural or magnetic transition, being almostthe samebehavior. The clearheatcapacity jump contrasting to the facts that the anomaly of resistivity in the superconducting samples definitely indicates the in the underdoped FeAs-based superconductor is always good bulk superconductivity in these crystals. relevant to a structural/magnetic transition. The X-ray single crystal diffraction patterns reveals In conclusion, we systematically studied the struc- two sets of (00l) reflections existing in all the crystals of turebythe X-raysinglecrystaldiffractionandmeasured AthxeFset2aSret2in.gSucochmtpwoosisteiotsnoafnrdeflheecatitoinngssptrroocnegslsy.dAepltehnodugohn the transport properties in the AxFe2Se2 single crys- tals. All the samplesshow two sets of (00l)reflections in the superconducting phase is dominant, the trace of sec- X-ray single crystal diffraction patterns, indicating the ond phase is still observed as shown in Fig.5a. These intrinsically inhomogeneous distribution of the interca- results indicate the existence of inhomogeneousdistribu- lated A atoms. The occurrence of Superconductivity is tion of the A atoms in all the crystals. It is found that closely relatedto the c-axis lattice parameter,indicating superconductivity is closely related to the c-axis lattice that the A content is crucial to the superconductivity constant, indicating that the A content is crucial to the superconductivity because the c-axis lattice parameter in AxFe2Se2. The very large magnitude of the normal- state resistivity reflects the large amount of deficiencies stronglydependsontheAcontent. TheAcontentinsin- withintheconductingFeSelayers. Thehumpinresistiv- gle crystals is quite sensitive to the nominalcomposition ityshouldoriginatefromthesedefectsandisfoundtobe andconditionofcrystalgrowth. Therefore,itisnoteasy irrelevant to the superconductivity. In this sense, super- to grow the single crystal with superconductivity. The conductivity is robust to the vacancies within the FeSe very large normal-state resistivity relative to other iron layers. No anomaly in susceptibility is observed to be pnictide superconductors suggests the large amount of associated with the hump in resistivity. The clear jump deficiencies within the conducting FeSe layers for all the AxFe2Se2 single crystals. The humpinresistivity should idnucstpiencgifiscinhgleeatcrfyosrtaRlsbixnFdei2cSaete2satnhde gKoxoFde2bSuel2k ssuuppeerrccoonn-- arise from such defects, and seems to be irrelevant to ductivity in these crystals. superconductivity. Despite of the existence of many de- ficiencieswithintheconductingFeSe layersinAxFe2Se2, ACKNOWLEDGEMENT This work is supported Tc does not change a lot with varying A from K to Rb by the Natural Science Foundation of China and by the and Cs. Therefore, superconductivity seems robust to Ministry of Science and Technology of China, and by the such vacancies. No anomaly is observed in magnetic Chinese Academy of Sciences. susceptibility at the hump temperature in resistivity. 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