We report a combined study of the specific heat and de Haas-van Alphen effect in the iron-pnictide superconductor BaFe2(As(1-x)P(x))2. Our data when combined with results for the magnetic penetration depth give compelling evidence for the existence of a quantum critical point close to x=0.30 which affects the majority of the Fermi surface by enhancing the quasiparticle mass. The results show that the sharp peak in the inverse superfluid density seen in this system results from a strong increase in the quasiparticle mass at the quantum critical point.

Efforts to understand the microscopic origin of superconductivity in the cuprates are dependent on knowledge of the normal state. The Hall number in the low-temperature, high-field limit n H (0) has a particular importance because, within conventional transport theory, it is simply related to the number of charge carriers, so its evolution with doping gives crucial information about the nature of the charge transport. Here we report a study of the high-field Hall coefficient of the single-layer cuprates Tl 2 Ba 2 CuO 6+δ (Tl2201) and (Pb/La)-doped Bi 2 Sr 2 CuO 6+δ (Bi2201), which shows how n H (0) evolves in the overdoped-so-called strange metal-regime of cuprates. We find that n H (0) increases smoothly from p to 1 + p, where p is the number of holes doped into the parent insulating state, over a wide range of doping. The evolution of n H correlates with the emergence of the anomalous linear-in-temperature term in the low-temperature in-plane resistivity. The results could suggest that quasiparticle decoherence extends to dopings well beyond the pseudogap regime.

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We report a de Haas-van Alphen (dHvA) oscillation study of the 111 iron pnictide superconductors LiFeAs with Tc ≈ 18 K and LiFeP with Tc ≈ 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and shows quasi-nested electron and hole bands. The effective masses generally show significant enhancement, up to ∼ 3 for LiFeP and ∼ 5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs respectively.Identification of the particular structural and electronic characteristics that drive superconductivity in the iron-based materials continues to be a central experimental and theoretical question in the field. A successful theory needs to explain trends, such as the variation of T c and also the structure of the superconducting energy gap. In most of the iron arsenides the parent materials have a non-superconducting, antiferromagnetically ordered ground state. Disruption of this magnetic order leads to superconductivity and then eventually a nonsuperconducting paramagnetic ground state. A good example is the BaFe 2 (As 1−x P x ) 2 series which has a maximum T c =30 K when x 0.33 [1, 2]. Here BaFe 2 As 2 has a magnetic ground state whereas BaFe 2 P 2 is a paramagnet and neither superconduct.The 111-family of iron-pnictides LiFeAs 1−x P x , is unique because both LiFeAs and its counterpart LiFeP superconduct and are non-magnetic with T c ∼ 18 K [3, 4] and ∼ 5 K [5], respectively. Also, penetration depth measurements have shown that LiFeAs is fully gapped [6,7], whereas LiFeP has gap nodes [7]. Establishing whether this switch of pairing structure is linked to changes in the topology and orbital character of the Fermi surface (FS) provides an stringent test of candidate theories for the superconducting pairing in these materials.Magneto-quantum oscillation effects such as the de Haasvan Alphen (dHvA) effect are a powerful probe of the threedimensional bulk Fermi surface and have been successfully used to study a variety of iron-based superconductors [8,9]. In this Letter, we present a study of the dHvA oscillations in both LiFeP and LiFeAs which establishes that the bulk Fermi surface topology of these compounds is in good agreement with DFT calculations. Furthermore, by comparing the values of the extracted effective masses of the quasiparticles to the calculated band masses, we find significant orbit dependence to the mass enhancement factors which is likely linked The top panels show the raw pulsed field torque data in units of the change in the cantilever resistance at T = 1.5 K. The arrow indicates the position of the irreversible field. The middle panels show the oscillatory part of the torque after subtraction of a smooth background. The bottom panels show FFTs of the torque. For the pea...

Heavy electrons with extremely strong Coulomb repulsions can condense into a fully gapped s-wave superconducting state.