Browsing by Author "Lee, T. K."

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Experimental evidence for a two-gap structure of superconducting NbSe(2): A specific-heat study in external magnetic fields
(2007-12-01)
Huang, C. L.
;
Lin, J. -Y.
;
Chang, Y. T.
;
Sun, C. P.
;
Shen, H. Y.
To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe(2), comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H perpendicular to c) to and parallel (H parallel to c) to the c axis of the crystal and using the two-gap model, we conclude that (1) more than one energy scale of the order parameter is required for superconducting NbSe(2) due to the thermodynamic consistency; (2) Delta(L)=1.26 meV and Delta(S)=0.73 meV are obtained; (3) N(S)(0)/N(0)=11%-20%; (4) the observation of the kink in gamma(H) curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe(2); and (5) Delta(S) is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.
Experimental evidence for a two-gap structure of superconducting NbSe2: A specific-heat study in external magnetic fields
(2007-12-01)
Huang, C. L.
;
Lin, J. -Y.
;
Chang, Y. T.
;
Sun, C. P.
;
Shen, H. Y.
To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H perpendicular to c) to and parallel (H parallel to c) to the c axis of the crystal and using the two-gap model, we conclude that (1) more than one energy scale of the order parameter is required for superconducting NbSe2 due to the thermodynamic consistency; (2) Delta(L)=1.26 meV and Delta(S)=0.73 meV are obtained; (3) N-S(0)/N(0)=11%-20%; (4) the observation of the kink in gamma(H) curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe2; and (5) Delta(S) is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.
Kondo interactions and magnetic correlations in CePt2 nanocrystals
(2007-04-13)
Chen, Y. Y.
;
Huang, P. H.
;
Ou, M. N.
;
Wang, C. R.
;
Yao, Y. D.
The evolution of the Kondo effect and antiferromagnetic (AF) correlations with size reduction in CePt2 nanoparticles (3.1-26 nm) is studied by analysis of the temperature-dependent specific heat and magnetic susceptibility. The AF correlations diminish with size reduction. The Kondo effect predominates at small particle size with trivalent, small Kondo temperature (T-K) magnetic regions coexisting with strongly mixed-valent, large T-K nonmagnetic regions. We discuss the role of structural disorder, background density of states and the electronic quantum size effect on the results.
Strange superconductivity near an antiferromagnetic heavy-fermion quantum critical point
(2019-03-15)
Chang, Y. Y.
;
Hsu, F.
;
Kirchner, S.
;
Mou, C. Y.
;
Lee, T. K.
The heavy fermion systems CeMIn5 with M = Co, Rh, Ir, the "115" family, provide a prototypical example of strange superconductivity with unconventional d-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductivity and its link to antiferromagnetic quantum criticality of the strange metal state are still long-standing open issues. We propose a microscopic mechanism for strange superconductors, based on the coexistence and competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond spin-liquid near the antiferromagnetic quantum critical point via a large-N Kondo-Heisenberg model and renormalization group analysis beyond the mean-field level. The interplay of these two effects provides a qualitative understanding on how superconductivity emerges from the strange metal state and the observed superconducting phase diagrams for CeMIn5.
Variational phase diagram of the electron-doped two-dimensional extended t-J model
(2007-04-01)
Shih, C. T.
;
Wu, J. J.
;
Chou, C. P.
;
Chen, Y. C.
;
Lee, T. K.
In this paper, we will present the zero-temperature phase diagram of the two-dimensional extended t - J model studied by the variational Monte Carlo (VMC) method. The introduction of the next and third nearest neighbor hopping terms t ' and t '' breaks the symmetry of the phase diagram. For the electron doped systems (t ' positive), the phase boundary of antiferromagnetism (AFM) is pushed to larger doping density, and the superconducting (SC) region is suppressed, which are consistent with the experimental phase diagram.