The normal-state charge transport is studied systematically in high-quality single crystals of BaFe2(As1−xPx)2 (0 ≤ x ≤ 0.71). By substituting isovalent P for As, the spin-density-wave (SDW) state is suppressed and the dome-shaped superconducting phase (Tc 31 K) appears. Near the SDW end point (x ≈ 0.3), we observe striking linear temperature (T ) dependence of resistivity in a wide T -range, and remarkable low-T enhancement of Hall coefficient magnitude from the carrier number estimates. We also find that the magnetoresistance apparently violates the Kohler's rule and is well scaled by the Hall angle ΘH as ∆ρxx/ρxx ∝ tan 2 ΘH . These non-Fermi liquid transport anomalies cannot be attributed to the simple multiband effects. These results capture universal features of correlated electron systems in the presence of strong antiferromagnetic fluctuations.
Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice.1 The electron nematicity has been recently observed in the iron-pnictide 2-6 and cuprate 7-9 hightemperature superconductors. Whether such a tendency of electrons to self-organise unidirectionally has a common feature in these superconductors is, however, a highly controversial issue. In the cuprates, the nematicity has been suggested as a possible source of the pseudogap phase, 7-9 whilst in the iron-pnictides, it has been commonly associated with the tetragonalto-orthorhombic structural phase transition at T s . Here, we provide the first thermodynamic evidence in BaFe 2 (As 1−x P x ) 2 that the nematicity develops well above the structural transition and persists to the nonmagnetic superconducting regime, resulting in a new phase diagram strikingly similar to the pseudogap phase diagram in the cuprates.9,10 Our highly sensitive magnetic anisotropy measurements using microcantilever torque-magnetometry under in-plane field rotation reveal pronounced two-fold oscillations, which break the tetragonal symmetry. Combined with complementary high-resolution synchrotron X-ray and resistivity measurements, our results consistently identify two distinct temperaturesone at T * , signifying a true nematic transition, and the other at T s (< T * ), which we show to be not a true phase transition, but rather what we refer to as a "meta-nematic transition", in analogy to the well-known metamagnetic transition in the theory of magnetism. Our observation of the extended nematic phase above the superconducting dome establishes that the nematicity has primarily an electronic origin, inherent in the normal state of high-temperature superconductors.In the iron pnictides, the antiferromagnetic transition is closely intertwined with the structural phase transition from tetragonal (T) to orthorhombic (O) crystal symmetry. Although recent experiments, including neutron scattering, 2 ARPES, 3,11 STM, 4 and transport measurements, 5,6 have provided evidence for electronic anisotropy, these measurements were carried out either in the low-temperature orthorhombic phase, 2,4,11 where the crystal lattice structure has already broken C 4 symmetry, or in the tetragonal phase under uniaxial strain 3,5,6 that also breaks this symmetry. Therefore, the question remains open whether the electronic anisotropy can exist above the structural transition without an external driving force, including under the superconducting (SC) dome. In the past, the nematic transition in the pnictides has been associated either with the orbital ordering, [12][13][14][15][16][17][18] or with the spontaneous breaking of the Z 2 Ising symmetry between two collinear magnetic ordering wave-vectors Q = (π, 0) and (0, π).19-22 Therefore determining the nature of the nematicity is a key to understanding the microscopic origin of the lattice and magnetic...
In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe(2)As(2) has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe(2)As(2) is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with "octet-line nodes" on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A(1g) superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.
In a superconductor, the ratio of the carrier density, n, to their effective mass, m * , is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two dimensional systems, this ratio n/m * (∼ 1/λ 2 L ) determines the effective Fermi temperature, TF . We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1−xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 K. This structure may arise from quantum fluctuations associated with a quantum critical point (QCP). The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover towards the Bose-Einstein condensate limit driven by quantum criticality.In two families of high temperature superconductors, cuprates and iron-pnictides, superconductivity emerges in close proximity to an antiferromagnetically ordered state, and the critical temperature T c has a dome shaped dependence on doping or pressure [1][2][3]. What happens inside this superconducting dome is still a matter of debate [3][4][5]. In particular, elucidating whether a quantum critical point (QCP) is hidden inside it (Figs. 1A and B) may be key to understanding high-T c superconductivity [4,5]. A QCP marks the position of a quantum phase transition (QPT), a zero temperature phase transition driven by quantum fluctuations [7].The London penetration depth λ L is a property that may be measured at low temperature in the superconducting state to probe the electronic structure of the material, and look for signatures of a QCP. The absolute value of λ L in the zero-temperature limit immediately gives the superfluid density λ −2which is a direct probe of the superconducting state; here m * i and n i are the effective mass and concentration of the superconducting carriers in band i, respectively [8]. Measurements on high-quality crystals are necessary because impurities and inhomogeneity may otherwise wipe out the signatures of the QPT. Another advantage of this approach is that it does not require the application of a strong magnetic field, which may induce a different QCP or shift the zero-field QCP [9].BaFe 2 (As 1−x P x ) 2 is a particularly suitable system for penetration depth measurements as, in contrast to most other Fe-based superconductors, very clean [10] and homogeneous crystals of the whole composition series can be grown [11]. In this system, the isovalent substitution of P for As in the parent compound BaFe 2 As 2 offers an elegant way to suppress magnetism and induce superconductivity [11]. Non-Fermi liquid properties are apparent in the normal state above the superconducting dome ( Fig. 2A) [11,12] and de Haas-van Alphen (dHvA) oscillations [10] have been observed over a wide x range including the superconducting compositions, giving detailed information on the electronic structure. Because P and As are isoelectric, the system remains compensated for all values of x (i.e., volumes of the electron and hole Fermi surfaces...
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