Reemerging superconductivity at 48 K across quantum criticality in iron chalcogenides

Pressure plays an essential role in the induction1 and control2,3 of superconductivity in iron-based superconductors. Substitution of a smaller rare-earth ion for the bigger one to simulate the pressure effects has surprisingly raised the superconducting transition temperature Tc to the record high 55 K in these materials4,5. However, Tc always goes down after passing through a maximum at some pressure and the superconductivity eventually tends to disappear at sufficiently high pressures1-3. Here we show that the superconductivity can reemerge with a much higher Tc after its destruction upon compression from the ambient-condition value of around 31 K in newly discovered iron chalcogenide superconductors. We find that in the second superconducting phase the maximum Tc is as high as 48.7 K for K0.8Fe1.70Se2 and 48 K for (Tl0.6Rb0.4)Fe1.67Se2, setting the new Tc record in chalcogenide superconductors. The presence of the second superconducting phase is proposed to be related to pressure-induced quantum criticality. Our findings point to the potential route to the further achievement of high-Tc superconductivity in iron-based and other superconductors.Comment: 20 pages and 7 figure

Magnetic properties of the dense Kondo system CeB6 in high magnetic fields

This work reports on the phase diagram study of the dense Kondo system CeB6 by means of magnetization (M) and transverse magnetoresistance (MR) measurements in magnetic fields (B) to 1ST, in the temperature (T) range from 1.8 to 40 K. The zero field resistivity displays Kondo behavior in the paramagnetic P-phase, while two discontinuities at low T were observed at TQ ≈3.5 K and TN ≈ 2.3 K, the boundaries of the antiferro-quadrupolar (AFQ), and antiferromagnetic (AF) phases, respectively. The isothermal transverse MR is large and negative at low T, and it shows a distinct feature when the P-AFQ line of the B-T phase diagram is crossed. Similarly, the M vs. B curves at low temperatures show upward discontinuities at the phase boundary. Both the features in MR and M at the P-AFQ line correlate well in T, revealing the gradual increase of the T for the onset of AFQ ordering with B. No evidence for reentrant behavior could be observed in fields to 1ST. © 2001 Published by Elsevier Science B.V

ELECTRON-SPIN-RESONANCE OF ND3+ AND YB3+ IN CEFE4P12

5020148221482

ENHANCEMENT OF THE INTERGRANULAR SUPERCONDUCTING PROPERTIES IN BI2SR2CACU2O8 WITH LI ADDITIONS

The superconducting properties of Li-doped Bi2Sr2CaCu2O8 compounds, with nominal composition (Bi2Sr2CaCu2O8)1-xLix(0 less-than-or-equal-to x less-than-or-equal-to 0.55), have been investigated by means of measurements of AC and DC magnetic susceptibility (chi(AC), chi(DC)), electrical resistivity, X-ray diffraction, and metallographic analysis. These measurements revealed that the superconducting transition temperature T(c) increased gradually with the Li concentration x, and that the changes in chi(AC) and chi(DC) due to the onset of superconductivity increased as well. The real and imaginary components of chi(AC) versus T data revealed that a second transition appeared below the onset of superconductivity at T(c), possibly due to the weak superconducting links that make up the intergranular regions. This second transition is strongly dependent on the applied magnetic field H, and it shifts towards higher temperatures with x. The maximum T(c)'s of the intra- and intergrain regions, maximum chi(AC) and chi(DC) signals and maximum intergranular Bean Model critical current density occur for the compound with x=0.3. Li addition enhances the superconducting properties of the weak intergranular links, making these regions more homogeneous and less sensitive to magnetic fields.2114167111312

CRYSTAL-FIELD EFFECTS IN THE ESR-SPECTRA OF CE1-XGDXFE4P12, CE1-XDYXFE4P12, CE1-XERXFE4P12

74101047105

MAGNETOELASTIC CONTRIBUTION TO THE ELASTIC-CONSTANTS OF TERBIUM, DYSPROSIUM AND ERBIUM

405303

Far-infrared absorptivity of UPt3.

The absorptivity of the heavy-fermion compound UPt3 is measured from 2 to 1000 cm-1 (0.25"124 meV) at temperatures between 1.2 K and room temperature. Above 50 cm-1 (6.2 meV) the absorptivity is relatively temperature independent, while below that frequency the absorptivity is very temperature dependent, in accord with the dc resistivity. By performing a Kramers-Kronig transformation of the data, augmented with recently published results at higher frequencies, the complex conductivity is obtained. The low-temperature conductivity may be characterized by free carriers which undergo frequency-dependent scattering and have a concomitant frequency-dependent mass enhancement, ≫(), with ≫(0)=65. The data indicate a bare free-carrier plasma frequency of 2.1×104 cm-1 (2.6 eV). Combining these results with the measured specific heat for UPt3 gives for the low-frequency effective mass m*=240m, and for the optical band mass mb=3.7m. The carrier density is close to one electron per formula unit. The far-infrared absorptivity measurement indicates that the scattering rate begins to rise with an 2 dependence, while the measured dc resistivity has a T2 dependence at temperatures below 2 K. To account for the far-infrared data, the carrier scattering rate "(,T) can be written as "(,T)1/42+(pT)2, with an experimental upper limit of p=1. This is not consistent with electron-electron scattering, for which p=2. © 1988 The American Physical Society

Far-infrared absorptivity of UPt3.

The absorptivity of the heavy-fermion compound UPt3 is measured from 2 to 1000 cm-1 (0.25"124 meV) at temperatures between 1.2 K and room temperature. Above 50 cm-1 (6.2 meV) the absorptivity is relatively temperature independent, while below that frequency the absorptivity is very temperature dependent, in accord with the dc resistivity. By performing a Kramers-Kronig transformation of the data, augmented with recently published results at higher frequencies, the complex conductivity is obtained. The low-temperature conductivity may be characterized by free carriers which undergo frequency-dependent scattering and have a concomitant frequency-dependent mass enhancement, ≫(), with ≫(0)=65. The data indicate a bare free-carrier plasma frequency of 2.1×104 cm-1 (2.6 eV). Combining these results with the measured specific heat for UPt3 gives for the low-frequency effective mass m*=240m, and for the optical band mass mb=3.7m. The carrier density is close to one electron per formula unit. The far-infrared absorptivity measurement indicates that the scattering rate begins to rise with an 2 dependence, while the measured dc resistivity has a T2 dependence at temperatures below 2 K. To account for the far-infrared data, the carrier scattering rate "(,T) can be written as "(,T)1/42+(pT)2, with an experimental upper limit of p=1. This is not consistent with electron-electron scattering, for which p=2. © 1988 The American Physical Society

ESR determination of the crystal field parameters of Nd, Dy, Er, and Yb doped skutterudite CeFe4P12

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Despite extensive research on the skutterudites for the last decade, their electric crystalline field ground state is still a matter of controversy. By using electron spin resonance measurements we show that an appropriate description of the ground state can be given for the T-h cubic symmetry of the Ce1-xRxFe4P12 (R = Dy, Er, Yb, x less than or similar to 0.003) skutterudite compounds. From the analysis of the ESR data the three crystal field parameters, B-4(c), B-6(c) and B-6(t) were determined for each of these rare-earths at the Ce3+ site. (c) 2009 Elsevier B.V. All rights reserved.4041932813284Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)US Department of Energy [DE-FG02-98ER45707]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)US Department of Energy [DE-FG02-98ER45707