The nature of the tetragonal-to-orthorhombic structural transition at Ts ≈ 90 K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization and NMR measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)2As2, suggesting very similar strength of the electronlattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below Ts. This indicates that the structural, or "nematic", phase transition in FeSe is not driven by magnetic fluctuations.PACS numbers: 74.70. Xa, 74.25.Bt, 74.25.Ld, 74.25.nj One of the most intriguing questions in the study of iron-based superconductors concerns the relation between structure, magnetism and superconductivity [1][2][3][4][5][6][7][8][9][10]. Stripe-type antiferromagnetic order often occurs at the same or at a slightly lower temperature than the tetragonal-to-orthorhombic structural distortion and the two types of order are closely related by symmetry. They break the four-fold rotational symmetry of the high-temperature phase, which can be associated with a nematic degree of freedom [4,6]. Superconductivity typically is strongest around the point where the structural transition (T s ) and the antiferromagnetic transition (T N ) are suppressed by pressure or chemical substitution. Whether the magnetic or the structural instability is the primary one, is still under intense debate [10], also because of its relevance to the pairing mechanism [5,6]. Recently, scaling relations between the shear modulus related to the structural distortion, C 66 , and the spinlattice relaxation time T 1 as a measure of the strength of spin fluctuations, have been proposed [7,8] in order to address the above question. They were found to be well satisfied in the Ba(Fe,Co) 2 As 2 system [7,8], where T s and T N are in close proximity to each other, suggesting a magnetically-driven structural transition [7]. Clearly, it is of great interest to see if a relation between shear modulus and spin fluctuations is universally observed in other iron-based materials.FeSe is structurally the simplest iron-based superconductor and has attracted a lot of attention because of a nearly four-fold increase of its T c ≈ 8 K under pressure [11]. Moreover, this system is particularly interesting with respect to the relation of structure and magnetism, since it undergoes a tetragonal-to-orthorhombic structural phase transition at T s ∼ 90 K, similar to that found in the 1111-and 122-type parent compounds [2], but does not order magnetically at ambient pressure [12,13]. Spin fluctuations at low temperatures were, however, observed in nuclear magnetic resonance (NMR) measurements [14]. Surprisingly, the orthorhombic distortion of FeSe is not reduced upon entering the superconducting state [9] in strong contrast to underdoped BaFe 2 As 2 [3,15], indicating different couplings between structure and superconductivity. This strongly motivates fu...

Spin dynamics evolution of BaFe2(As1−xPx)2 was probed as a function of P concentration via 31 P NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility on approaching the AF phase from the superconducting dome. Moreover, magnetically ordered temperature θ deduced from the relaxation rate vanishes at optimal doping. These results provide clear-cut evidence for a quantum-critical point (QCP), suggesting that the AF fluctuations associated with the QCP play a central role in the high-Tc superconductivity.PACS numbers: 74.70. Xa, 74.40.Kb, 74.25.nj Conventional phonon-mediated superconductivity occurs in a normal metal that is well accounted for by Landau's Fermi-liquid (FL) theory. However, the standard FL theory appears to break down above T c in many "exotic" superconductors characterized by unconventional pairing rather than a conventional uniform-sign s-wave pairing function, such as in the heavy fermion materials and cuprates. The origin of the anomalous metallic properties, often referred to as "non-Fermi-liquid" (nFL) behavior, has triggered a growing interest in quantumcritical points (QCPs) that provide a route towards nFL behavior Newly discovered iron-pnictide high-T c superconductivity also appears where antiferromagnetism is suppressed via chemical substitution or pressure [4]. The existence of a QCP in iron pnictides has been suggested [5][6][7][8][9]. Spin-fluctuation-mediated superconductivity associated with the suppression of the antiferromagnetism is one likely scenario [10], but the identification of the mechanism is far from settled [11,12]. The difficulty in examining the superconducting (SC) mechanism could arise from complexity in the materials that can lead to ambiguous interpretations; e.g., non-universal SC gap functions and limited sample quality. It is thus essential to find a suitable model system to examine the mechanism of superconductivity.The isovalent-doped BaFe 2 (As 1−x P x ) 2 can be used as such a model system. It has the highest T c (31 K) among iron-pnictide superconductors known to have line nodes in the SC gap [13][14][15]. Clarifying the mechanism that produces its high-T c nodal gap is thus very important. Since isovalent P-doping is not expected to add carriers [16], BaFe 2 (As 1−x P x ) 2 maintains the compensation condition, i.e. the volume of the hole Fermi surfaces (FSs) is equal to that of the electron FSs. Very clean single-crystals of BaFe 2 (As 1−x P x ) 2 allow de Haasvan Alphen (dHvA) experiments that are the most precise technique to determine FSs, revealing the detailed electronic structure for comparison with band calculations [17,18]. The quasiparticle effective mass increases towards the maximum T c , signaling the enhancement of electron-electron correlation. Such an increase in the quasiparticle mass as well as nFL behavior inferred from resistivity measurements [16,19] can be expected when the system is in proximity to a QCP. However, direct evidence for the...

P and 75 As NMR measurements were performed in superconducting BaFe 2 ͑As 0.67 P 0.33 ͒ 2 with T c =30 K. The nuclear-spin-lattice relaxation rate T 1 −1 and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T 1 −1 in the superconducting ͑SC͒ state decreases without a coherence peak just below T c , as observed in ͑Ba 1−x K x ͒Fe 2 As 2 . In contrast to other iron arsenide superconductors, the T 1 −1 ϰ T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy.Our results suggest that strikingly different SC gaps appear in BaFe 2 ͑As 1−x P x ͒ 2 despite a comparable T c value, an analogous phase diagram, and similar Fermi surfaces to ͑Ba 1−x K x ͒Fe 2 As 2 .

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