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2009 METALS, SUPERCONDUCTORS...Evolution of the Fermi surface of BaFe 2 (As 1−x P x ) 2on entering the superconducting domeThe Fermi surface topology of the iron-pnictides is widelythought to play a key role in determining the type of magneticor superconducting order that occurs in these materialsat low temperature. Measurements of the de HaasvanAlphen (dHvA) effect provide a unique way of measuringthe Fermi surface of these materials. Prior to thepresent work, measurement had only been possible in thestoichiometric end members of the various families. However,to gain an understanding of how the bulk Fermi surfaceevolves as the correlation effects responsible for superconductivitybecome strong, it is necessary to measurethe highest T c superconducting samples. With the notableexception of the low T c phosphide material LaFePO, otheriron-pnictides need to tuned, either chemically or with pressurein order for them to become superconducting. Chemicallytuning often adds disorder, for example replacing Feby Co in the series Ba(Fe 1−x Co x ) 2 As 2 . This decreases dramaticallythe dHvA signal, making it unobservable even inhigh pulsed fields. Recently, it was discovered that substitutingAs for P in the series BaFe 2 (As 1−x P x ) 2 producedhigh T c superconductivity without changing the carrier concentration(both As and P are in the 3+ state so this is anisoelectric substitution). Importantly, this substitution doesnot appear to produce significant disorder in the conductingplane, making this an excellent candidate system for adHvA study.Measurements were made in Toulouse on BaFe 2 (As 1−x P x ) 2samples with three different values of x, with T c rangingfrom 12 K to 30 K. These measurements were supplementedwith other data obtained using superconductingmagnets in Osaka (17 T), the hybrid magnet in Tallahassee(45 T), and a resistive magnet in Nijmegen (33 T).Figure 88: Variation of the size of the electron orbits (α andβ) and the effective mass of quasiparticle moving on the β orbitas a function of phosphorous fraction x in BaFe 2 (As 1−x P x ) 2 .The variation of T c with x is also shown. The data show that theelectron Fermi surfaces shrink and the effective mass increasesas x decreases and T c becomes larger. Data for samples withx = 0.64,0.72, and 1.0 were obtained using dc fields in Osaka,Nijmegen and Tallahassee.Band-structure calculations suggest that the Fermi surfaceof these materials consists of quasi-two-dimensional electronand hole sheets. The total volumes of the electron andhole sheets are exactly equal (compensated metal). In ourexperiment only the electron Fermi surfaces were observed.This follows a trend found for other iron-phosphides wherethe mean free path on the electron Fermi surface is muchlonger than on the hole sheets.Figure 87: Measured de Haas-van Alphen data for samplesof BaFe 2 (As 1−x P x ) 2 . The left panels show raw torque data atT = 1.5 K. The right panels show the oscillatory part of the torqueand the corresponding fast Fourier transforms, for samples withx = 0.41 and 0.56. No oscillations were observed for the highestT c = 30 K sample (x = 0.33).The main conclusion is that the data show that the volumeof the electron sheets (and via charge neutrality also thehole sheets) shrink linearly and the effective masses becomestrongly enhanced with decreasing x. Calculationsshow that it is unlikely that these changes are a simpleconsequence of the one-electron bandstructure but insteadthey likely originate from many-body interactions. Thesechanges may be intimately related to the high T c unconventionalsuperconductivity in this system. These results arereported in detail in Shishido et al. arXiv:0910.3634.D. Vignolles, B. Vignolle, C. ProustA. Carrington, A.F. Bangura, A.I. Coldea, P.M.C. Rourke (Bristol University), A. McCollam (HMFL, Nijmegen), H.Shishido, S. Tonegawa, K. Hashimoto, S. Kasahara, H. Ikeda, T. Terashima, Y. Matsuda, T. Shibauchi (University ofKyoto), R. Settai, Y. Ōnuki (University of Osaka)65