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Register a new userPossible hydrogen doping and enhancement of Tc (= 35 K) in a LaFeAsO-based superconductor
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We report that the incorporation of hydroxide ions (OH)- significantly enhances the superconducting transition temperature (Tc) in the LnFeAsO-based superconductors (Ln1111: Ln = La, Ce and Pr). For La1111, Tc of the (OH)- incorporated sample synthesized using high-pressure technique becomes 35 K, which is higher by 7 K than the typical optimally-doped La1111 superconductors. Similar enhancement in Tc is also observed for Ce1111 and Pr1111. 1H-NMR measurement have confirmed the existence of hydrogen atoms in the samples. Accompanying the (OH)- incorporation, the lattice parameters are largely contracted, down to the values which have never been attained by any other dopings/substitutions.
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We investigated the crystal structure and superconducting properties of As-grown and high-pressure-annealed PrO0.5F0.5BiS2. We found that the high-pressure annealing generates uniaxial lattice contraction along the c axis. Both As-grown and high-pressure-annealed PrO0.5F0.5BiS2 show bulk superconductivity. The Tc of PrO0.5F0.5BiS2 is clearly enhanced from Tczero = 3.6 K to Tczero = 5.5 K by high-pressure annealing. Unexpectedly, the semiconducting characteristics is relatively enhanced by high-pressure annealing. Namely, we assume that the enhancement of Tc can not be understood by an increase of electron carriers. Having considered these facts, we conclude that the enhancement of Tc correlates with uniaxial lattice contraction along the c axis in PrO0.5F0.5BiS2.
The Nernst effect in metals is highly sensitive to two kinds of phase transition: superconductivity and density-wave order. The large positive Nernst signal observed in hole-doped high-Tc superconductors above their transition temperature Tc has so far been attributed to fluctuating superconductivity. Here we show that in some of these materials the large Nernst signal is in fact caused by stripe order, a form of spin / charge modulation which causes a reconstruction of the Fermi surface. In LSCO doped with Nd or Eu, the onset of stripe order causes the Nernst signal to go from small and negative to large and positive, as revealed either by lowering the hole concentration across the quantum critical point in Nd-LSCO, or lowering the temperature across the ordering temperature in Eu-LSCO. In the latter case, two separate peaks are resolved, respectively associated with the onset of stripe order at high temperature and superconductivity near Tc. This sensitivity to Fermi-surface reconstruction makes the Nernst effect a promising probe of broken symmetry in high-Tc superconductors.
Pnictides provide an opportunity to study the effects of quantum criticality in a multi-band high temperature superconductor. Quasiparticle mass divergence near optimal doping, observed in two major classes of high-temperature superconductors, pnictides and cuprates, is a direct experimental indicator of enhanced electronic interactions that accompany quantum criticality. Whether quasiparticles on all Fermi surface pockets in BaFe2(As1-xPx)2 are affected by quantum criticality is an open question, which specific heat measurements at high magnetic fields can directly address. Here we report specific heat measurements up to 35T in BaFe2(As1-xPx)2 over a broad doping range, 0.44 <= x <= 0.6. We observe saturation of C/T in the normal state at all dopings where superconductivity is fully suppressed. Our measurements demonstrate that quasiparticle mass increases towards optimal doping in multiple pockets, some of which exhibit even stronger mass enhancement than previously reported from quantum oscillations of a single pocket.
Measurements of the critical current density (Jc) by magnetization and the upper critical field (Hc2) by magnetoresistance have been performed for hafnium-doped MgB2. There has been a remarkable enhancement of Jc as compared to that by ion irradiation without any appreciable decrease in Tc, which is beneficial from the point of view of applications. The irreversibility line extracted from Jc shows an upward shift. In addition, there has been an increase in the upper critical field which indicates that Hf partially substitutes for Mg. Hyperfine interaction parameters obtained from time differential perturbed angular correlation (TDPAC) measurements revealed the formation of HfB and HfB2 phases along with the substitution of Hf. A possible explanation is given for the role of these species in the enhancement of Jc in MgB2 superconductor.
We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.
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