Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe2(As1-xPx){2} as a function of isoelectric substitution (As/P) for 0.41

The physics of quantum critical phase transitions connects to some of the most difficult problems in condensed matter physics, including metal-insulator transitions, frustrated magnetism and high-temperature superconductivity. Near a quantum critical point, a new kind of metal emerges, the thermodynamic and transport properties of which do not fit into the unified phenomenology for conventional metals-the Landau Fermi-liquid theory-characterized by a low-temperature limiting T-linear specific heat and a T 2 resistivity 1 . Studying the evolution of the temperature dependence of these observables as a function of a control parameter leads to the identification of both the presence and the nature of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe 2 (As 1−x P x ) 2 below the critical temperature T c by suppressing superconductivity with high magnetic fields. At sufficiently low temperatures, the resistivity of all compositions (x 0.31) crosses over from a linear to a quadratic temperature dependence, consistent with a low-temperature Fermi-liquid ground state. As compositions with optimal T c are approached from the overdoped side, this crossover becomes steeper, consistent with models of quantum criticality where the effective Fermi temperature T F goes to zero.The iron-based superconductors are part of a family of unconventional superconductors that exhibit several competing orders. The parent material BaFe 2 As 2 is a tetragonal paramagnet at high temperature and becomes an orthorhombic metallic antiferromagnet at ∼140 K (ref. 2). As the material is electron doped, hole doped or isovalently substituted this transition is rapidly suppressed, giving rise to superconductivity. In this work, we attempt to understand the nature of the low-T metallic state of the Fe-based superconductor BaFe 2 (As 1−x P x ) 2 by suppressing the superconductivity in a high magnetic field. Even though BaFe 2 (As 1−x P x ) 2 is isovalently substituted, we will describe the chemical composition-temperature (x-T ) phase diagram using language commonly applied to electron/hole-doped compounds, namely 'underdoped' refers to materials that exhibit a structural/magnetic instability, and 'overdoped' for paramagnetic compounds that do not. For this material, the maximum T c (optimal doping) occurs at x = 0.30.BaFe 2 (As 1−x P x ) 2 is a multi-band compound with both electronand hole-like carriers and the magnetoresistance is therefore a sum of contributions from all Fermi surfaces. In Fig. 1 we illustrate the magnetoresistance as a function of temperature and field for a range of compositions from x = 0.31 to x = 0.73 and T c spanning 29.5 K to 0 K. For all temperatures measured, a quadratic magnetoresistance fit captures most of the data and the intercept ρ 0,T is extrapolated (shown by black lines in Fig. 1). At low fields, this fit deviates from the quadratic dependence in the near-optimally doped samples, even at temperatures T > T c , although the deviation ostensibly disappears a...

Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of YbRh2Si2. We have carried out de Haas-van Alphen measurements in the magnetic-field range 8 T < or = H < or = 16 T, and directly observe field dependence of the extremal Fermi surface areas. Our data support the theory that a low-field "large" Fermi surface, including the Yb 4f quasihole, is increasingly spin split until a majority-spin branch undergoes a Lifshitz transition and disappears at H0 approximately 10 T, without requiring 4f localization at H0.

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A new algorithm for extracting de Haas-van Alphen frequencies and effective masses from calculated band energies is presented. The algorithm creates an interpolated kspace "super cell," which is broken into slices perpendicular to the desired magnetic field direction. Fermi surface orbits are located within each slice, and de Haas-van Alphen frequencies and effective masses are calculated. Orbits are then matched across slices, and extremal orbits determined. This technique has been successful in locating extremal orbits not previously noticed in the complicated topology of existing UPt 3 band-structure data; these new orbits agree with experimental de Haas-van Alphen measurements on this material, and solidify the case for a fullyitinerant model of UPt 3 .