J Supercond Nov Magn (2014) 27:15991602DOI 10.1007/s10948-014-2559-3

LETTER

Superconductivity at 25 K under Hydrostatic Pressurefor FeTe0.5Se0.5 SuperconductorRajveer Jha Rayees A. Zargar A. K. Hafiz H. Kishan V. P. S. Awana

Received: 24 March 2014 / Accepted: 8 April 2014 / Published online: 22 May 2014 Springer Science+Business Media New York 2014

Abstract We report the impact of hydrostatic pressureon the superconductivity and normal-state resistivity ofFeTe0.5Se0.5 superconductor. At the ambient pressure, theFeTe0.5Se0.5 compound shows the superconducting transi-tion temperature T onsetc at above 13 K and T

=0c at 11.5 K.

We measure pressure-dependent resistivity from 250 to 5 K,which shows that the normal-state resistivity increases ini-tially for the applied pressures of up to 0.55 GPa, andthen the same is decreased monotonically with increasingpressure of up to 1.97 GPa. On the other hand, the supercon-ducting transition temperatures (T onsetc and T =0c ) increasemonotonically with increasing pressure. Namely the T onsetcincreases from 13 to 25 K and T =0c from 11.5 to 20 Kfor the pressure range of 01.97 GPa. Our results suggestthat superconductivity in this class of Fe-based compoundsis very sensitive to pressure as the estimated pressure coef-ficient dTc(onset)/dP is 5.8 K/GPa. It may be suggestedthat the FeTe0.5Se0.5 superconductor is a strong electron-correlated system. The enhancement of Tc with applyingpressure is mainly attributed to an increase of charge carriersat the Fermi surface.

Keywords Fe-chalcogenide Hydrostatic pressure High-pressure resistivity Strong electron correlationeffects

R. Jha H. Kishan V. P. S. Awana ()CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg,New Delhi 110012, Indiae-mail: awana@mail.nplindia.ernet.inURL: www.freewebs.com/vpsawana

R. A. Zargar A. K. HafizDepartment of Physics, Jamia Millia Islamia,New Delhi 110025, India

1 Introduction

The discovery of Fe-based superconductor LaFeAsO1xFxwith superconducting transition temperature (Tc) above26 K has attracted much attention in the condensed mat-ter scientific community [1]. In this direction, other ironpnictides and Fe-chalcogenide superconductors have beendeveloped such as LnFeAsO1xFx (Ln = Sm, Nd, Pr,and Ce 1111-family) [25] with Tc of up to 55 K,Ba1xKxFe2As2 with Tc = 38 K [6], LiFeAs with Tc =18 K [7], and Fe1+xSe with Tc = 8.0 K [8]. Interestingly,all the Fe-based superconductors have a layered structuresuch as the FeAs layer in pnictides and FeSe layer in chalco-genide, as similar to that of CuO2 layer in high-temperaturecuprate superconductors [9]. The parent compounds ofall Fe-based superconductors show the spindensitywave(SDW) transition accompanied with a structural changefrom tetragonal to monoclinic at low temperatures [16].The superconductivity can be induced with elemental dop-ing in the parent compounds [111]. In FeSe system, thesuperconductivity can be induced up to 8 K with eitheran excess of iron or the deficiency of selenium [12, 13].Under hydrostatic pressure, the superconductivity of FeSesuperconductor has been increased up to 27 K [14]. TheFeTe1xSex chalcogenide system has the simplest crystalstructure among the other iron-based superconductors thatgives the positive discrimination to its experimental and the-oretical studies related to onsite chemical substitution andthe impact of high pressures [15, 16]. The impact of highpressure on FeSe1xTex compounds showed that the crys-tal structure changes from tetragonal (P4/nmm) phase tothe orthorhombic (Pbnm) at high pressures and low tem-peratures [1719]. Also, the pressure-dependent supercon-ducting properties have been studied for the FeTe0.5Se0.5

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system, which showed the improvement in Tc from 14 to22 K for the pressure of 25.3 kbar [20].

The pressure effect is one of the powerful tools tostudy the superconducting properties by effectively chang-ing the primary parameters such as the lattice constantsthrough bond lengths and angles without doping, whichmay improve the electronic density of states at the Fermisurface. Here, we report the pressure-dependent resistivityfrom 250 K down to 5 K for the various applied pressuresfrom 0 to 1.97 GPa for the FeTe0.5Se0.5 superconductor. TheFeTe0.5Se0.5 compound has been synthesized by solid-statereaction route via vacuum encapsulation, which is crys-tallized in tetragonal structure with space group P4/nmm.The normal-state resistivity decreases with increasing pres-sure, and the superconducting transition temperature Tc isenhanced up to 25 K at 1.97 GPa pressure. Our results arequalitatively in confirmation with ref. [20].

2 Experimental

The bulk polycrystalline FeTe0.5Se0.5 sample was syn-thesized through a standard solid-state reaction route viavacuum encapsulation. The high-purity chemicals Fe, Se,and Te are weighed in the stoichiometric ratio and groundthoroughly in a glove box having pure argon atmosphere.The mixed powder was subsequently pelletized and thenencapsulated in an evacuated (103 Torr) quartz tube. Theencapsulated tube is then heated at 750 C for 12 h andslowly cooled to room temperature. The X-ray diffraction(XRD) was taken at room temperature in the scatteringangular (2) range of 10 80 in equal 2step of 0.02using a Rigaku diffractometer with Cu K ( = 1.54 A).A Rietveld analysis was performed using the standard Full-Prof program.

The pressure-dependent resistivity measurements wereperformed by using HPC-33 Piston type pressure cell withQuantum Design DC resistivity option on Physical Prop-erty Measurements System (PPMS-14T, Quantum Design).Hydrostatic pressures were generated on the sample bya BeCu/NiCrAl clamped piston-cylinder cell, which wasimmersed in a fluid pressure transmitting a medium of Flu-orinert (FC70:FC77 = 1:1) in a Teflon cell. Four proberesistivity methods are used to measure the resistivity of thesample.

3 Results and Discussion

The room temperature observed in and a Rietveld-refinedXRD pattern of the FeTe0.5Se0.5 compound are shown inFig. 1. The compound is fitted in a tetragonal structure withspace group P4/nmm. Within the XRD detection limit, no

impurity phase is seen in the main phase of the compound.A Rietveld-refined lattice parameter is found to be a =3.79(1) A and c = 6.02 (2) A. The inset of the Fig. 1 showsthe unit cell of the FeTe0.5Se0.5 compound, which has beendrawn from the refined parameters. It can be seen that theFe is located at coordinate positions 0.75, 0.25, and 0 andSe/Te at 0.25, 0.25, and 0.272. Our results are quite similarto the earlier reported results [15, 16].

Figure 2 represents the temperature-dependent resistiv-ity (T ) of the FeTe0.5Se0.5 compound from 250 K downto 5 K at the various applied pressures from 0 to 1.97 GPa.It can be seen in Fig. 2 that the normal-state resistivity(250 K) decreases slightly with increasing pressure of upto 0.55 GPa. The normal conduction (T ) is of slightlymetallic nature, and the same improves with pressure. Thedecrease in normal-state resistivity with improved metallicbehavior may take place due to the alignment of distortedstructure of this system resulting in increase of number ofcharge carriers at the Fermi level [17]. Figure 3 shows theenlarged part of the (T ) under pressure from 30 to 10 Kto see the variation of the superconducting transition tem-perature with applied pressure. At zero pressure, T =0c isat 11.5 K, which increases to 12.5 K at 0.35 GPa pressure.Interestingly, with a small increase in pressure from 0.35 to0.55 GPa, the T =0c is increased from 12.5 to 16 K. For fur-ther higher pressures, the T =0c increases gradually to 20 Kwith increasing pressure to 1.97 GPa. It is also seen that withincreasing applied pressure, the superconducting transitionwidth (T onsetc T =0c ) increases and is maximum (5 K)for the highest possible applied pressure of 1.97 GPa.

Figure 4 shows the temperature derivative of resistiv-ity (d/dT ) of the FeTe0.5Se0.5 compound at the variousapplied pressures from 0 to 1.97 GPa. The midpoint of thesuperconducting transition Tc can be seen clearly from thed/dT peak. The height of the transition peak is highestfor the applied pressure of 0.55 GPa, which decreases for

Fig. 1 Rietveld-fitted XRD pattern at room temperature, observed(open circles) and calculated (solid lines), of the FeTe0.5Se0.5compound

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Fig. 2 Temperature-dependent resistivity ( Vs T ) at various appliedpressures in the temperature range of 2505 K of the FeTe0.5Se0.5compound

the further higher applied pressures. It can be clearly seenthat with increasing pressure, the width of the d/dT peakincreases. This shows that broadening of the superconduct-ing transition takes place with pressure. Also, secondarypeaks are seen for higher pressures of 1.68 and 1.97 GPa.The sufficient increase in the broadening of the transitionwith the secondary peaks at higher pressures is indicativeof phase separation. Figure 5 shows the summarized resultsof the three different regions of the superconducting transi-tion temperature T =0c , T midc , and T onsetc versus pressure ofthe FeTe0.5Se0.5 compound. It can be seen in Fig. 5 that thesuperconducting transition temperature (T =0c ) increasesgradually from 11.5 to 20 K for the applied pressure of 01.97 GPa. The obtained highest T onsetc is 25 K for the appliedpressure of 1.97 GPa. The pressure coefficient d T onsetc /dPof FeTe0.5Se0.5 is estimated to be 5.8 K/GPa. This value isclose to earlier reported values of pressure coefficient for theiron-based chalcogenide superconductors [14, 20]. Increasein the Tc with applying pressure of the FeTe0.5Se0.5 system

Fig. 3 Resistivity versus temperature ( Vs T ) plots of theFeTe0.5Se0.5 compound at various applied pressures in the temperaturerange of 3010 K

Fig. 4 Temperature derivative of resistivity of the FeTe0.5Se0.5 com-pound at various applied pressures in the temperature range of 3010 K

is due to the increase in the number of density of state at theFermi surface and enhanced electronphonon coupling con-stant outweighing the lattice stiffening effect. It would bepossible to attain a higher Tc by applying still higher pres-sures. In our study, at present, we are limited to a maximumof 2 GPa of pressure.

In earlier studies on iron-based compounds, it has beensuggested that the superconductivity is correlated with thesuppression of SDW ordering, which can be achieved bydoping such as in PrFeAsO1xFx , NdFeAsO1xFx , andBa1xKxFe2As2 or by applying external pressure as well[2, 3, 6]. In the binary FeSe superconductor, the largest posi-tive pressure coefficient of Tc has been discussed in terms ofspin instabilities [14]. On the other hand, the large pressurecoefficient of the FeTe0.5Se0.5 compound is related to thelarge atomic size of Te, which seems sensible to the crystallattice vibrations [20]. With applying pressure, the decreasein normal-state resistivity suggests that the structure getsaligned and the grain boundary contributions are reduced. It

Fig. 5 T =0c , T midc , and T onsetc versus pressure plots of theFeTe0.5Se0.5 compound

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may also be suggested that the FeTe0.5Se0.5 superconductoris a strong electron-correlated system invoking increasedhybridization of Fe and Se under pressure. In our case,the normal-state resistivity is suppressed with the appliedpressure accompanied with improved metallic behavior.

4 Conclusion

In conclusion, we studied the impact of hydrostatic pres-sure on superconductivity of the FeTe0.5Se0.5 supercon-ductor. The observed pressure coefficient dT onsetc /dP is5.8 K/GPa, which is comparable to earlier reported val-ues for the similar superconducting compounds. The muchhigher pressure coefficient may happen due to the increaseof the density of states at the Fermi level with appliedpressure.

Acknowledgments The authors would like to thank the Directorof NPL, India, for his keen interest in the present work. This workis financially supported by DAE-SRC outstanding investigator awardscheme on search for new superconductors. Rajveer Jha acknowledgesthe CSIR for the senior research fellowship. H. Kishan thanks CSIRfor providing Emeritus Scientist Fellowship.

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Superconductivity at 25 K under Hydrostatic Pressure for FeTe0.5Se0.5 SuperconductorAbstractIntroductionExperimentalResults and DiscussionConclusionAcknowledgmentsReferences