Browsing by Author "Ho, PL"

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BCS-like superconductivity in MgCNi3
(2003-02-01)
Lin, JY
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Ho, PL
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Huang, HL
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Lin, PH
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Zhang, YL
The low-temperature specific heat C(T,H) of the superconductor MgCNi3 has been measured in detail. DeltaC/gamma(n)T(c)=1.97 is estimated from the anomaly at T-c. At low temperatures, the electronic contribution in the superconducting state follows C-es/gamma(n)T(c)approximate to7.96 exp(-1.46T(c)/T). The magnetic-field dependence of gamma(H) is found to be linear with respect to H. T-c estimated from the McMillan formula agrees well with the observed value. All the specific-heat data appear to be consistent with each other within the moderate-coupling BCS context. It is amazing that such a superconductor unstable to ferromagnetism behaves so conventionally. The Debye temperature Theta(D)=287 K and the normal state gamma(n)=33.6 mJ/mol K-2 are determined for the present sample.
Electronic and magnetic instability in Pr0.65Ca0.25Sr0.1MnO3
(2003-09-01)
Mollah, S
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Huang, HL
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Liu, SJ
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Ho, PL
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Huang, WL
Electronic and magnetic instability in polycrystalline Pr0.65Ca0.25Sr0.1MnO3 manganite has been established from a systematic study of resistivity (p), magnetization (M) and specific heat (C). The sample shows a charge ordering transition at T-CO similar to 200 K, an antiferromagnetic (AFM) ordering transition at T-N similar to 175 K, a metal-insulator (MI) transition at T-MI similar to 80 K and an unusual magnetic ordering transition at T-M(*) similar to 45 K in the absence of magnetic fields. Though the C data do not show any anomaly at T-MI for MI transition, these illustrate a much smaller anomaly than expected one at T-M* and is suppressed by magnetic fields. This indicates that the unusual magnetic ordering in this sample is of canted, fluctuated or phase separation of AFM and ferromagnetic in nature which is established from the T-H phase diagram, as well as the thermal and magnetic hysteresis in p, M and C. (C) 2003 Elsevier Science B.V. All rights reserved.
H-c2 of MgCNi3 determined from the specific heat and resistivity measurements
(2002-10-01)
Lin, JY
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Lin, PH
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Ho, PL
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Huang, HL
;
Zhang, YL
H-c2 of MgCNi3 has been determined from the specific heat C and resistivity rho measurements in the same sample. The results from rho are nearly identical with those determined from the anomaly in C. Furthermore, utilizing the relation gamma(H) proportional to H and the value of dgamma/dH, the obtained value of H-c2 is the same as that by the WHH model, if the spin paramagnetic effect and the spin-orbit interaction are taken into account. The results of this comparison have strong implications on the order parameter of MgCNi3.
Magnetic field dependence of low-temperature specific heat of the spinel oxide superconductor LiTi2O4
(2004-08-01)
Sun, CP
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Lin, JY
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Mollah, S
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Ho, PL
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Yang, HD
Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi2O4 has been elaborately investigated. In the normal state, the obtained electronic coefficient of specific heat gamma(n)=19.15 mJ/mol K-2, the Debye temperature Theta(D)=657 K and some other parameters are compared with those reported earlier. The superconducting transition at T(c)similar to11.4 K is very sharp (DeltaT(c)similar to0.3 K) and the estimated deltaC/gamma(n)T(c) is similar to1.78. In the superconducting state, the best fit of data leads to the electronic specific heat C-es/gamma(n)T(c)=9.87 exp(-1.58 T-c/T) without field and gamma(H)proportional toH(0.95) with fields. In addition, H-c2(0)similar to11.7 T, H-c(0)similar to0.32 T, xi(GL)(0)similar to55 A, lambda(GL)(0)similar to1600 Angstrom, and H-c1(0)similar to26 mT are estimated from the Werthamer-Helfand-Hohenberg (WHH) theory or other relevant relations. All results from the present study indicate that LiTi2O4 can be well described by a typical type-II, BCS-like, moderate coupling, and fully gapped superconductor in the dirty limit. It is further suggested that LiTi2O4 is a moderately electron-electron correlated system.
Pressure effects on the transition temperature of superconducting MgCxNi3
(2003-09-01)
Yang, HD
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Mollah, S
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Huang, WL
;
Ho, PL
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Huang, HL
The effect of hydrostatic pressure (P) up to 17 kbar on the superconducting transition temperature (T-C) of the newly discovered intermetallic nonoxide perovskite superconductor MgCxNi3 has been reported. T-C is found to increase with increasing P at a rate of dT(C)/dPsimilar to0.0134 to 0.0155 K/kbar depending on the value of carbon content x. The absolute value of dT(C)/dP for MgCxNi3 is about the same as that of intermetallic RNi2B2C (R denotes rare earth) and metallic superconductors but about one order of magnitude smaller than that of the most recently and intensively studied superconductor MgB2. However, the d ln T-C/dPsimilar to0.001 81 to 0.002 24 kbar(-1) and the rate of change of T-C with unit cell volume (V), d ln T-C/d ln Vsimilar to-3.18 to -2.58 of MgCxNi3 are having the comparable magnitude to that of MgB2 with opposite sign. The increase of T-C with P in MgCxNi3 can be explained in the framework of density of states (DOS) effect.
X-ray absorption and optical spectroscopy studies of (Mg1-xAlx)B-2
(2003-09-01)
Yang, HD
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Liu, HL
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Lin, JY
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Kuo, MX
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Ho, PL
X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg1-xAlx) B-2 for x=0.0, 0.1, 0.2, 0.3, and 0.4. The important role of B 2psigma hole states to superconductivity has been identified, and the decrease in the hole carrier number is quantitatively determined. The rate of the decrease in the hole concentration agrees well with the theoretical calculations. On the other hand, while the evolution of the electronic structure is gradual through the doping range, T-c suppression is most significant at x=0.4. These results suggest that the superstructure in (Mg1-xAlx)B-2, in addition to the sigma holes, can affect the lattice dynamics and contributes to the T-c suppression effect. Other possible explanations like the topological change of the sigma band Fermi surface are also discussed.