Influence on superconductivity in the parity mixing superconductor Li2T3B(T:Pt,Pd) by non-magnetic impurity and defect doping (original) (raw)
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Superconductor Science and Technology, 2010
We report on magnetisation data obtained as a function of temperature and magnetic field in Li 2 (P d 0.8 P t 0.2) 3 B and Li 2 P d 3 B. Reversible magnetisation curves were plotted as √ M vs. T. This allow study of the asymptotic behavior of the averaged order parameter amplitude (gap) near the superconducting transition. Results of the analysis show, as expected, a mean field superconducting transition for Li 2 P d 3 B. On contrary, a large deviation from the mean field behavior is revealed for Li 2 (P d 0.8 P t 0.2) 3 B. This is interpreted as due to the strength of the non s-wave spin-triplet pairing in this Pt-containing compound which produces nodes in the order parameter and consequently, phase fluctuations. The diamagnetic signal above T c (H) in Li 2 P d 3 B is well explained by superconducting Gaussian fluctuations, which agrees with the observed mean field transition. For Li 2 (P d 0.8 P t 0.2) 3 B the diamagnetic signal above T c (H) is much higher than the expected Gaussian values and appears to be well explained by three dimensional critical fluctuations of the lowes-Landau-level type, which somewhat agrees with the scenariou of a phase mediated transition.
NMR study of noncentrosymmetric superconductor Li2Pt3B
Physica C: Superconductivity and its Applications, 2007
Measurements of Knight shift and nuclear spin-lattice relaxation time for 7 Li , 11 B and 195 Pt has been performed in powdered sample of newly discovered noncentrosymmetric superconductor, Li 2 Pt 3 B. We discuss the local density of states at each atomic site. The result shows the platinum 5d electrons plays a dominant role for the electrical properties in this compound.
Physica C: Superconductivity, 2005
Electronic structure of recently discovered isotypic ternary borides Li 2 Pd 3 B and Li 2 Pt 3 B, with noncentrosymmetric crystal structures, is studied with a view to understanding their superconducting properties. Estimates of the Fermi-surface averaged electron-phonon matrix element and Hopfield parameter are obtained in the rigid ion approximation of Gaspari and Gyorffy [Phys. Rev. Lett. 28 (1972) 801]. The contribution of the lithium atoms to the electron-phonon coupling is found to be negligible, while both boron and palladium atoms contribute equally strongly to the Hopfield parameter. There is a significant transfer of charge from lithium, almost the entire valence charge, to the B-Pd(Pt) complex. The electronic structure and superconducting properties of Li 2 Pd 3 B, thus, can be understood from the viewpoint of the compound being composed of a connected array of B-Pd tetrahedra decoupled from the backbone of Li atoms, which are connected by relatively short bonds. Our results suggest that conventional s-wave electron-phonon interaction without explicit consideration of SO coupling can explain qualitatively the observed T c in Li 2 Pd 3 B. However, such an approach is likely to fail to describe superconductivity in Li 2 Pt 3 B.
Experimental and first-principles studies on superconductivity in noncentrosymmetric La3Se4
Physical Review B
We report the synthesis and detailed characterization of superconducting La 3 Se 4 , with T c ∼ 8.5 ± 0.1 K, using x-ray diffraction, electrical transport, magnetization, and heat capacity measurements. La 3 Se 4 crystallizes in the noncentrosymmetric cubic Th 3 P 4-type structure with space group I43d. Characteristic superconducting parameters such as the lower critical field, upper critical field, thermodynamic critical field, coherence length, penetration depth, and Ginzburg-Landau parameter have been determined. The specific heat jump at T c , C/γ T c = 2.04 ± 0.05, exceeds the value for a weakly coupled BCS superconductor, and the electronphonon coupling constant is found to be λ ep = 0.87 ± 0.02, suggesting superconductivity in La 3 Se 4 is in the strong-coupling regime. The estimated upper critical field is well below the calculated Pauli limit, and the Maki parameter value (α < 1) indicates that the superconducting upper critical field is dominated by orbital pair breaking. From density functional theory based first-principles simulations we observe the number of states at the Fermi energy is dominated mainly by d and f electrons of La. Furthermore, we observe band crossings along the high-symmetry k lines in the vicinity of the Fermi energy. These bands are observed to split due to the removal of spin degeneracy associated with spin-orbit coupling, with the splitting energy E ASOC ≈ 65 meV.
Impurity effects in superconducting UPt3
Superconducting UPt, is characterized by a novel and complex magnetic field-temperature phase diagram, with two superconducting transitions at Tel and Tc2 in zero field. We have studied the effects of Pd and Y impurities on the zero field superconducting properties of UPt,. Resistance measurements show that both dopants increase the residual resistivity and decrease the spin fluctuation temperature in the normal state. T,, is depressed by both dopants, but more effectively by Pd. 1 TCl -TC21 is essentially unaffected by Y doping, but increases dramatically with Pd doping.
Magnetic properties of the layered superconductor Li0.48(THF)0.3HfNCl with Tc∼26K
Journal of Magnetism and Magnetic Materials, 2001
We report on DC-magnetization measurements on an oriented polycrystalline sample of the layered superconductor Li (THF) HfNCl having ¹ &26 K. For the applied "eld perpendicular to the basal-plane (ab-plane), we found that diamagnetic moments appear at temperature as high as ¹&2;¹ , with increasing strength of the applied "eld. This indicates the existence of strong two-dimensional superconducting #uctuations. The anisotropy of superconducting parameters such as the upper critical "eld and Ginzburg}Landau coherence length demonstrates the highly anisotropic character for the present superconductor. It is suggested that Li (THF) HfNCl is a quasi-two-dimensional superconductor.
Synthesis and search for superconductivity in LiBC
Solid State Communications, 2002
Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al , we have attempted to synthesise Li deficient Li x BC ( x = 1, 0.8, 0.6 and 0.4) and look for superconductivity in this system. Our synthesis procedure, following the recipe for MgB 2 , involves reaction of elemental components in a Ta crucible at 900 o C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P6 3 /mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, was observed in the temperature range of 300 to 4 K. This is possibly related to the presence of disorder in the B-C stacking; evidence for which is suggested from a study of the vibrational modes of Li x BC through infrared spectroscopy.
Coexistence of 3d-Ferromagnetism and Superconductivity in [(Li 1− x Fe x )OH](Fe 1− y Li y )Se
Angewandte Chemie International Edition, 2014
Superconducting [(Li 1−x Fe x)OH](Fe 1−y Li y)Se (x ≈ 0.2, y ≈ 0.08) was synthesized by hydrothermal methods and structurally characterized by single crystal X-ray diffraction. The crystal structure contains anti-PbO type (Fe 1−y Li y)Se layers separated by layers of (Li 1−x Fe x)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism emerges from the iron atoms in the (Li 1−x Fe x)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic with superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor, which gives evidence for a spontaneous vortex phase where both orders coexist. 57 Fe-Mössbauer spectra, 7 Li-NMR spectra, and µSR experiments consistently support this rare situation, especially in a bulk material where magnetism emerges from a 3d-element.
Evidence for quasi-two-dimensional superconductivity in electron-doped Li 0.48 ( THF ) y HfNCl
Physical Review B, 2000
Dc-magnetization and NMR measurements were carried out on a layered superconductor Li 0.48 (THF) y HfNCl having T c ϳ26 K. For the magnetic field applied perpendicular to the basal plane (ab plane͒ above 10 kOe, we found a pronounced broadening of the superconducting transition in temperature dependence of magnetization and the substantial diamagnetic signals were observed as high as 2T c , indicating the existence of superconducting fluctuations. Analysis based on the anisotropic Ginzburg-Landau model reveals that the present system is a highly anisotropic superconductor. 7 Li-NMR signals were observed around zero Knight shift, indicating that the local Fermi-level density of states, N(E F ), at Li site is practically nothing and the superconductivity is derived from the HfNCl layer. We have shown the unambiguous evidence for the quasi-two-dimensional superconducting character in this system.