Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out Density Functional Theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements.Comment: 13 pages, 5 figures accepted for publication on European Journal of Physics

Crossover between magnetism and superconductivity in low H-doped LaFeAsO

By a systematic study of the hydrogen-doped LaFeAsO system by means of dc resistivity, dc magnetometry, and muon-spin spectroscopy we addressed the question of universality of the phase diagram of rare-earth-1111 pnictides. In many respects, the behaviour of LaFeAsO_(1-x)H_(x) resembles that of its widely studied F-doped counterpart, with H^- realizing a similar (or better) electron-doping in the LaO planes. In a x = 0.01 sample we found a long-range SDW order with T_n = 119 K, while at x = 0.05 the SDW establishes only at 38 K and, below T_c = 10 K, it coexists at a nanoscopic scale with bulk superconductivity. Unlike the abrupt M-SC transition found in the parent La-1111 compound, the presence a crossover region makes the H-doped system qualitatively similar to other Sm-, Ce-, or Nd-1111 families.Comment: to appear in Journal of Physics: Condensed Matte

Strain derivative of thermoelectric properties as a sensitive probe for nematicity

The nematic instability is an undebatable ingredient of the physics of iron-based superconductors. Yet, its origin remains enigmatic as it involves a fermiology with an intricate interplay of lattice-, orbital-, and spin degrees of freedom. It is well known that thermoelectric transport is an excellent probe for revealing even subtle signatures of instabilities and pertinent fluctuations. In this paper, we report a strong response of the thermoelectric transport properties of two underdoped 1111 iron-based superconductors to a vanishingly small strain. By introducing the strain derivative of the Seebeck and the Nernst coefficients, we provide a description of the nematic order parameter, proving the existence of an anisotropic Peltier-tensor beside an anisotropic conductivity tensor. Our measurements reveal that the transport nematic phenomenology is the result of the combined effect of both an anisotropic scattering time and Fermi surface distortions, pointing out that in a realistic description, abreast of the spin fluctuations also the orbital character is a fundamental ingredient. In addition, we show that nematic fluctuations universally relax in a Curie–Weiss fashion above TS in all the elasto-transport measurements and we provide evidences that nematicity must be band selective

Role of magnetic dopants in the phase diagram of Sm1111 pnictides: The Mn case

The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally-doped SmFeAsO0.88-F0.12 iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide range of manganese contents (x) by means of muon-spin spectroscopy (muSR), both in zero- and in transverse fields, respectively, to probe the magnetic and the superconducting order. The pure superconducting phase (at x < 0.03) is replaced by a crossover region at intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x = 0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of Tc and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors [M. Gastiasoro et al., ArXiv 1606.09495], which includes correlation-enhanced RKKY-couplings between the impurity moments.Comment: 10 pages, 13 figures. Accepted on Physical Review

Tauroursodeoxycholic acid administration as adjuvant therapy in cirrhotic patients on transplantation waiting lists.

BACKGROUND/AIMS: The lack of organ availability and an increased number of end-stage cirrhotic patients has led to the lengthening of liver transplantation waiting lists. The progressive worsening of clinical and functional performance in patients awaiting the graft is one of the factors implicated in the increased mortality during the wait and in poor transplantation outcome. In this work our aim was to evaluate the effects of tauroursodeoxycholic acid administration on biochemical, clinical and functional parameters in a group of cirrhotic patients consecutively placed onto our liver transplantation waiting list. METHODOLOGY: Ten cirrhotic patients underwent biochemical, clinical and functional evaluation at the time of entering on our liver transplantation waiting list, then tauroursodeoxycholic acid was administered until liver transplantation. Complete evaluation was repeated every 2 months. The results were compared to those of a comparable historical control group that had undergone liver transplantation the year before the study. RESULTS: All patients were transplanted within 6 months from insertion on the waiting list. Longitudinal analysis of the treated group showed that cholestasis and cytolisis parameters constantly decreased and that gamma-glutamyl transpeptidase was significantly lower compared to baseline values at the 4th month of therapy. Clinical and functional parameters remained stable during follow-up. Comparison with the control group showed that gamma-glutamyl transpeptidase, alkaline phosphatase and both aminotransferases were reduced at the 4th month of therapy. Fewer days of hospital stay and less intensive care were required in the treated group. CONCLUSIONS: Treatment of end-stage cirrhotic patients awaiting liver transplantation with tauroursodeoxycholic acid improves biochemical parameters of cytolisis and cholestasis, and furthermore helps to maintain clinical and functional stability during the wait. Improved biochemical conditions and steady clinical-functional performance may promote better short-term transplant outcome

Role of Dirac cones in magnetotransport properties of REFeAsO (RE = rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE = rare earth). On one hand we carry out density functional theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-Gamma and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements