We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state and elastoresistance measurements. Our high resolution ARPES allows us to track the Fermi surface deformation from four-fold to two-fold symmetry across the structural transition at ~87 K which is stabilized as a result of the dramatic splitting of bands associated with dxz and dyz character. The low temperature Fermi surface is that a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility as approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice effects and, in the absence of magnetic fluctuations above the structural transition, points clearly towards an electronically driven transition in FeSe stabilized by orbital-charge ordering.Comment: Latex, 8 pages, 4 figure
Cd3As2 is a candidate three-dimensional Dirac semi-metal which has exceedingly high mobility and nonsaturating linear magnetoresistance that may be relevant for future practical applications. We report magnetotransport and tunnel diode oscillation measurements on Cd3As2, in magnetic fields up to 65 T and temperatures between 1.5 K to 300 K. We find the non-saturating linear magnetoresistance persist up to 65 T and it is likely caused by disorder effects as it scales with the high mobility, rather than directly linked to Fermi surface changes even when approaching the quantum limit. From the observed quantum oscillations, we determine the bulk three-dimensional Fermi surface having signatures of Dirac behaviour with non-trivial Berry's phase shift, very light effective quasiparticle masses and clear deviations from the band-structure predictions. In very high fields we also detect signatures of large Zeeman spin-splitting (g ∼ 16).
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
We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (Tc ∼ 6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor ∼2. The quasi-two dimensional Fermi surface consist of nearly-nested electron and hole pockets, suggesting proximity to a spin/charge density wave instability.PACS numbers: 71.18.+y, 74.25.Jb, The recent discovery of superconductivity in ferrooxypnictides [1,2], has generated huge interest as as another possible route towards achieving high T c superconductivity. LaFePO was among the first Fe-based superconductor to be discovered and has a transition temperature of up to T c ≈ 7 K [1]. This compound is isostructural with LaFeAsO, which is non-superconducting and has a spin-density wave (SDW) ground state [4], but becomes a relatively high-T c superconductor (T c ≈ 25 K) when electron doped [3]. By changing the rare-earth ion,Theoretical models suggest that the pairing mechanism in the Fe-based superconductors may be mediated by magnetic fluctuations due to the proximity to a SDW [5,6,7]. Knowing the fine details of the Fermi surface topology, its tendency towards instabilities as well as the strength of the coupling of the quasiparticles to excitations is important for understanding the superconductivity.Quantum oscillations provide a bulk probe of the electronic structure, giving detailed information about the Fermi surface (FS) topology and mass renormalization. To observe quantum oscillations samples must be extremely clean and the upper critical field must be low enough for the normal state to be accessed; LaFePO is a material which fulfils both these requirements. The tetragonal layered structure of LaFePO is made of alternating highly conductive FeP layers and poorly conducting LaO layers stacked along the c axis [1], hence the Fermi surface is expected to be quasi-two dimensional [8]. Here we report extensive measurements of quantum oscillations in torque and transport data. We find that the Fermi surface of LaFePO is composed of quasi twodimensional nearly-nested electron and hole pockets with moderate enhancement of the quasiparticle masses.Single crystals of LaFePO, with dimensions up to 0.2 × 0.2 × 0.04 mm 3 , and residual resistance ratios ρ(300 K)/ρ(10 K) up to 85, were grown from a tin flux [9]. Single crystal x-ray diffraction gives lattice parameters a = 3.941(2)Å, c = 8.507(5)Å, and La/P positions z La = 0.148901(19), z P = 0.63477(10) in agreement with previous results [1]. Torque measurements were performed with piezoresistive microcantilevers [10] down to 0.3 K on different single crystals from the same batch (T c ≈ 6 K); one in Bristol up to 18 T (sample B) and another crystal at the NHMFL, Tallahassee, up to 45 T (sample A). Interplane electrical transport has been measured on a third sample (sample C). Figure 1a shows raw torque measur...
The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations
We report a high-resolution angle-resolved photo-emission spectroscopy study of the evolution of the electronic structure of FeSe1−xSx single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87 K to 58 K for x = 0.15. With increasing S substitution, we find smaller splitting between bands with dyz and dxz orbital character and weaker anisotropic distortions of the low temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in FeSe1−xSx. Finally, we detect the small inner hole pocket for x=0.12, which is pushed below the Fermi level in the orbitally-ordered low temperature Fermi surface of FeSe. arXiv:1508.05016v1 [cond-mat.supr-con]
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.
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