High-temperature superconductivity is closely adjacent to a long-range antiferromagnet, which is called a parent compound. In cuprates, all parent compounds are alike and carrier doping leads to superconductivity, so a unified phase diagram can be drawn. However, the properties of parent compounds for iron-based superconductors show significant diversity and both carrier and isovalent dopings can cause superconductivity, which casts doubt on the idea that there exists a unified phase diagram for them. Here we show that the ordered moments in a variety of iron pnictides are inversely proportional to the effective Curie constants of their nematic susceptibility. This unexpected scaling behavior suggests that the magnetic ground states of iron pnictides can be achieved by tuning the strength of nematic fluctuations. Therefore, a unified phase diagram can be established where superconductivity emerges from a hypothetical parent compound with a large ordered moment but weak nematic fluctuations, which suggests that iron-based superconductors are strongly correlated electron systems.

BiFeO 3 (BFO) films deposited on SrTiO3 (001) substrates and on LaNiO3-coated SrTiO3 (001) substrates with different annealing ambiences of oxygen and nitrogen were studied by using micro-Raman spectroscopy and x-ray diffraction (XRD). XRD showed that the films are in single-phase with rhombohedral structure. According to the analysis of the group theory, 13 Raman-active modes, which can be classified as 4A1 and 9E modes, have been observed in the BiFeO3 films. Raman spectra along the growth direction of the BFO films in the side-view scattering geometry were performed by the Raman mapping technique. The variations of Raman shift and Raman bandwidth in different depths of the films imply the existence of residual strain along the growth direction of the BFO films. These results are very useful for the understanding of the depth dependence of the physical properties including the interface and surface structure of the BFO films.

The origin of nematic order remains one of the major debates in iron-based superconductors. In theories based on spin nematicity, one major prediction is that the spin-spin correlation length at (0,π) should decrease with decreasing temperature below the structural transition temperature T_{s}. Here, we report inelastic neutron scattering studies on the low-energy spin fluctuations in BaFe_{1.935}Ni_{0.065}As_{2} under uniaxial pressure. Both intensity and spin-spin correlation start to show anisotropic behavior at high temperature, while the reduction of the spin-spin correlation length at (0,π) happens just below T_{s}, suggesting the strong effect of nematic order on low-energy spin fluctuations. Our results favor the idea that treats the spin degree of freedom as the driving force of the electronic nematic order.

Recent observations of two nodeless gaps in superconducting CeCu_{2}Si_{2} have raised intensive debates on its exact gap symmetry, while a satisfactory theoretical basis is still lacking. Here we propose a phenomenological approach to calculate the superconducting gap functions, taking into consideration both the realistic Fermi surface topology and the intra- and interband quantum critical scatterings. Our calculations yield a nodeless s^{±}-wave solution in the presence of strong interband pairing interaction, in good agreement with experiments. This provides a possible basis for understanding the superconducting gap symmetry of CeCu_{2}Si_{2} at ambient pressure and indicates the potential importance of multiple Fermi surfaces and interband pairing interaction in understanding heavy fermion superconductivity.

We have reported a Raman scattering investigation of bismuth ferrite (BiFeO(3)) under high pressure up to 50 GPa. Distinct changes in the Raman spectra show evidence for three pressure-induced structural transitions. The abrupt frequency redshifts of the Raman modes near 300 cm(-1) at around 3 GPa are attributed to the modulation of the FeO(6) octahedral tilts. The disappearance of the modes below 250 cm(-1) at 8.6 GPa, together with the enhancement of the two modes in the range of 300-400 cm(-1), indicate the phase transition from the rhombohedral to orthorhombic symmetry. Afterward, the E-3 and E-4 modes disappear at 44.6 GPa, pointing to the occurrence of the orthorhombic-cubic phase transition, which is consistent with the previous postulate that an orthorhombic-cubic transition takes place across the metal-insulator transition at high pressures.

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We use the two fluid model to determine the conditions under which the nuclear spin-lattice lattice relaxation rate, T1, of candidate heavy quantum critical superconductors can exhibit scaling behavior and find that it can occur if and only if their "hidden" quantum critical spin fluctuations give rise to a temperature-independent intrinsic heavy electron spin-lattice relaxation rate. The resulting scaling of T1 with the strength of the heavy electron component and the coherence temperature, T * , provides a simple test for their presence at pressures at which the superconducting transition temperature, Tc, is maximum and is proportional to T * . These findings support the previously noted partial scaling of the spin-lattice relaxation rate with Tc in a number of important heavy electron materials and provide additional evidence that in these materials their optimal superconductivity originates in the quantum critical spin fluctuations associated with a nearby phase transition from partially localized to fully itinerant quasiparticles.PACS numbers: 71.27.+a, 74.70.Tx, 76.60.-k A tantalizing hint that the spin fluctuations seen in the nuclear spin relaxation rate for a number of unconventional superconductors might be the magnetic glue responsible for their superconductivity appears in a scaling relation between that rate and the optimal superconducting transition temperature, T c , that was first noted by Curro et al [1]. In the present communication we focus on understanding this scaling relation for one important member of this family, the heavy electron materials, for which some experimental results are given in Fig. 1 [1, 2]. As may be seen in Fig. 2, finding such a relation appears at first sight highly problematic because the scaling covers a range of temperatures (T c < T < T * ) in the normal state in which both hybridized localized spins and the itinerant heavy electron Kondo liquid contribute to the spin-lattice relaxation rate. However, we find that rigorous Curro T c scaling can become possible if three conditions are met: (1) the maximum in T c occurs at the pressure p L , at which the line marking the boundary between partially localized and fully itinerant behavior for heavy electron quasiparticles, T L , intersects with T c , so that T max c = T L (p L ); (2) at p L the total spin-lattice relaxation rate scales with the coherence temperature, T

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo 2 Ga 8 . Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K 2 at 1 K, suggesting that CeCo 2 Ga 8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature-pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo 2 Ga 8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-twodimensional YbRh 2 Si 2 family. The study of the quasi-one-dimensional CeCo 2 Ga 8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.