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We have studied the structural and superconducting properties of β-FeSe under pressures up to 26 GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3) GPa, much smaller than the rest of Fe based superconductors. At 12 GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high-pressure orthorhombic phase has a higher Tc reaching 34 K at 22 GPa.

We report on specific heat ͑C p ͒, transport, Hall probe, and penetration depth measurements performed on Fe͑Se 0.5 Te 0.5 ͒ single crystals ͑T c ϳ 14 K͒. The thermodynamic upper critical field H c2 lines has been deduced from C p measurements up to 28 T for both H ʈ c and H ʈ ab, and compared to the lines deduced from transport measurements ͑up to 55 T in pulsed magnetic fields͒. We show that this thermodynamic H c2 line presents a very strong downward curvature for T → T c which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that H c2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T / T c ϳ 0.99 ͑for H ʈ ab͒ which is the consequence of a very small value of the coherence length c ͑0͒ϳ4 Å ͓and ab ͑0͒ϳ15 Å͔, confirming the strong renormalization of the effective mass ͑as compared to DMFT calculations͒ previously observed in ARPES measurements ͓A. Phys. Rev. Lett. 104, 097002 ͑2010͔͒. H c1 measurements lead to ab ͑0͒ = 430Ϯ 50 nm and c ͑0͒ = 1600Ϯ 200 nm and the corresponding anisotropy is approximatively temperature independent ͑ϳ4͒, being close to the anisotropy of H c2 for T → T c . The temperature dependence of both ͑ϰT 2 ͒ and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.

The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is one of the most intriguing properties of the iron-based superconductors (Fe SC), and has relevance for the cuprates as well. Establishing the critical electronic modes behind nematicity remains a challenge, however, because their associated susceptibilities are not easily accessible by conventional probes. Here, using FeSe as a model system, and symmetry-resolved electronic Raman scattering as a probe, we unravel the presence of critical charge nematic fluctuations near the structural/nematic transition temperature, T S ∼ 90 K. The diverging behavior of the associated nematic susceptibility foretells the presence of a Pomeranchuk instability of the Fermi surface with d-wave symmetry. The excellent scaling between the observed nematic susceptibility and elastic modulus data demonstrates that the structural distortion is driven by this d-wave Pomeranchuk transition. Our results make a strong case for chargeinduced nematicity in FeSe.nematicity | superconductivity | Raman scattering

International audienceWe performed high pressure resonant Raman experiments on well characterized purified single-wall carbon nanotubes up to 40 GPa using argon as pressure transmitting medium. We used two different excitating wavelengths, at 632.8 nm and 514.5 nm. In contrast with other studies no clear sign of phase transformation is observed up to the highest studied pressure of 40 GPa. Our results suggest that the progressive disappearance of the radial breathing modes observed while increasing pressure should not be interpreted as the sign of a structural phase transition. Moreover, a progressive change of profile of the tangential modes is observed. For pressures higher than 20 GPa the profile of those modes is the same for both laser excitations. We conclude that a progressive loss of resonance of single-wall carbon nanotubes under pressure might occur. In addition, after high pressure cycle we observed a decrease of intensity of the radial breathing and tangential modes and a strong increase of the D band

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We studied the effect of disorder on the superconducting properties of polycrystalline MgB2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2Δ0/kBTc = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses Tc and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dHc2/dT at Tc can be obtained without sacrificing more than a few degrees in Tc. The upper critical field of the sample after irradiation exceeds 28 T at T → 0.