From (03C0, 0) magnetic order to superconductivity with (03C0, 03C0) magnetic resonance in Fe1.02Te12212xSex

From (03C0, 0) magnetic order to superconductivity with (03C0, 03C0) magnetic...

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NATUREMATERIALSDOI:10.1038/NMAT2800 LETTERS K (rlu)

H (rlu) K (rlu)

T = 2 K

Counts (per ~80 s) 2 meV

4 meV

6 meV

8 meV x = 0.19 x = 0.38 x = 0.05

(H, 1¬H)

H (rlu)

Fe1.02(Te1¬x Sex)

Figure 4 | Difference of microscopic magnetic properties between samples with and without bulk superconductivity. a, Neutron scattering intensity map on the (HK0) plane for the x=0.19 sample without bulk superconductivity (hω=0meV). b, Neutron scattering intensity map on the (HK0) plane for the x=0.38 sample with bulk superconductivity (hω =0meV). Both samples exhibit quasi-elastic scattering near (1/2, 0), but the scattering in the x=0.19 sample is much stronger than that in the x=0.38 sample. The data shown in a and b were measured using the multiaxis crystal spectrometer at NIST. c, Typical inelastic neutron scattering transverse scans through (1/2,1/2) (or (pi,pi)) at fixed energy measured from a single crystal of

Fe1.02Te0.95Se0.05 at 2K. The solid lines through the data are Gaussian fits to the magnetic excitations. For clarity, data and fits are shifted along the y axis by an arbitrary amount. The position of the background for each scan is indicated by a horizontal line. The black horizontal bar represents the expected

Q-resolution.

and has a superconducting volume fraction of ∼90%. As shown in Fig. 4a,b, both samples exhibit quasi-elastic scattering centred near (pi,0). With the data normalized to phonon intensity, (pi,0) magnetism in the x = 0.19 sample is a factor of 2.3(2) stronger than in the x = 0.38 sample with the same correlation length of 4.4(4) Å. This short-range AFM order at (pi,0) coexists with (pi,pi) spin fluctuations for which a spin-gap and a magnetic resonance formsforthex =0.38sampleinthebulksuperconductingstate.For a sample with x = 0.27, well into region I, spin excitations at the same wave vector have been reported29, suggesting that magnetic scatteringat(pi,pi)and(pi,0)coexistoverawidecompositionrange.

To explore this coexistence, we carried out inelastic neutron scattering measurements on an x =0.05 single crystal. This sample exhibits long-range (pi,0) magnetic order with an ordered moment of 1.68(6) µB/Fe. In Fig. 4c we illustrate typical transverse inelastic neutron scattering scans centred at (pi,pi) that were measured up to 8meV, and show the presence of well-defined magnetic excitations similar to those we reported earlier for an x = 0.4 sample30. For the lower-energy excitation at 2meV, we find a peak with a width broader than the Q-resolution of the instrument. The flat-top structure suggests that it consists of two components that separate through dispersion at higher energies. As previously found for an x = 0.4 sample, the corresponding dispersion relation extrapolates to incommensurate points (1/2+ε, 1/2−ε) with ε = 0.10(1), however, with a softer dispersion velocity of 62(5)meVÅ as compared with 345(2)meVÅ for the x =0.4 bulk superconducting sample30.

As this is an alloy where composition and local properties must vary throughout, it is difficult to determine the length scale over which these different magnetic properties coexist in our samples. Possible situations include homogeneous, intrinsic coexistence and a two-phase description where (pi,0) and (pi,pi) magnetic correlations exist in distinct volume fractions. It is also possible that (pi,0) magnetism exists only in rare regions of the alloy as an impurity or Griffiths phase. Irrespectively, we find that bulk superconductivity occurs only when (pi,0) magnetic correlations are strongly suppressed, suggesting that they are not favourable to superconducting pairing. The gradual increase of the bulk superconducting transition temperature (Tcχ in Fig. 1a) and the superconductingvolumefraction(−4piχ inFig. 1b)withincreasing

Se content for x > 0.29 suggests that the detrimental effects of (pi,0) magnetic correlations still exist in region I, progressively weakening with increasing x. A negative role for (pi,0) magnetic correlations is further corroborated by our neutron scattering measurements carried out on Fe1.1(Te0.62Se0.38), with excess Fe.

Although the 38% Se content for Fe1.02(Te0.62Se0.38) would place the sample in region I, excess Fe drives the sample into region I. Both susceptibility and specific heat measurements indicate that bulk

LETTERS NATUREMATERIALSDOI:10.1038/NMAT2800 superconductivity is suppressed; the resistivity follows a logarithmic temperature dependence indicating weakly localized charge carriers (see Supplementary Information), consistent with our previous report25.Ourneutronscatteringmeasurements(seeSupplementary Information) show an absence of low-energy magnetic scattering at (pi,pi) but clearly defined magnetic short-range ordering at (pi,0). This result indicates that magnetic correlations of the (pi,0) variety are destructive to superconductivity and contribute to weak charge carrier localization. For bulk superconducting samples it is at the (pi,pi) magnetic wave vector that a spin gap and a magnetic resonance are formed, a result consistent with s± pairing symmetry17–19 and indicating a similar mechanism behind iron chalcogenide and iron pnictide superconductivity. Insummary,wehaveexploredthedichotomybetween(pi,0)and

(pi,pi) magnetism in the Fe1.02(Te1−xSex) system. For low Se content, long-range magnetic order is formed with a magnetic wave vector

(pi,0). Dynamic magnetic correlations with a (pi,pi) wave vector however,docoexistinawiderangeofthephasediagram.Increasing Se doping tunes the relative strength of these distinct correlations. Magnetic correlations near (pi,0) are antagonistic to superconductivity and associated with weak charge carrier localization in an intermediate region between long-range (pi,0) AFM order and superconductivity. Bulk superconductivity occurs only in a composition range where (pi,0) magnetic correlations are sufficiently suppressed and (pi,pi) spin fluctuations associated with the nearly nesting Fermi surface dominate. This indicates that iron chalcogenide and iron pnictide superconductors, despite a competing magnetic instability intheformer,haveasimilarmechanismforsuperconductivity.

Methods

Fe (Te Se ) single crystals used in this study were synthesized using a flux method and were shown to be tetragonal phase with the space group P4/nmm at room temperature by X-ray and neutron diffraction measurements . The compositions of crystals were determined using an energy-dispersive X-ray spectrometer. As early studies revealed that the superconductivity of this system is sensitive to Fe non-stoichiometry , we have chosen samples with y ∼ 0.02 to explore the evolution from (pi,0) magnetic order to superconductivity with (pi,pi) magnetic resonance. We measured resistivity with a four-probe method, the Hall effect with a five-probe method and specific heat with an adiabatic relaxation technique using a commercial physical property measurement system. d.c. magnetic susceptibility was measured using a commercial superconducting quantum interference device. Neutron diffraction experiments were carried out on the four-circle diffractometer E5 and the two-axis diffractometer E4 at the BER I reactor at the Helmholtz–Zentrum, and neutron scattering measurements probing magnetic short-range order and excitations were carried out using the multiaxis crystal spectrometer and disc chopper spectrometer instruments at NIST. Inelastic neutron scattering measurements were carried out on the x =0.05 sample using the triple-axis spectrometer IN8 operated by the Institut Laue-Langevin, with the sample mounted with the c axis vertical, giving us access to spin excitations within the basal ab plane.

Received 6 April 2010; accepted 8 June 2010; published online 18 July 2010

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Acknowledgements

The work at Tulane is supported by the NSF under grant DMR-0645305 for materials and equipment, and the DOE under DE-FG02-07ER46358 for personnel. Work at AMRI was supported by DARPA through grant HR 01-09-1-0047. Work at NIST is in part supported by the NSF under grant DMR-0454672. Work at the Johns Hopkins University Institute for Quantum Matter is supported by the DOE under grant DE-FG02-08ER46544. D.N.A. and K.P. acknowledge the Deutsche Forschungsgemeinschaft for support under the priority program SPP 1458 and contract AR 613/1-2.

Author contributions

T.J.L, J.H., B.Q., D.F. and Z.Q.M. carried out sample growth, transport property and specific heat measurements (T.J.L and J.H. contributed equally). Neutron scattering measurements were carried out by W.B., M.R., S.A.J.K., K.P., S.M., D.N.A., A.H., Y.Q., V.T., A.T.S., J.A.R. and C.B. Magnetic susceptibility was measured by A.R., H.P. and L.S.

Additional information

The authors declare no competing financial interests. Supplementary information accompanies this paper on w.nature.com/naturematerials. Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions. CorrespondenceandrequestsformaterialsshouldbeaddressedtoZ.Q.M.orW.B.

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