L. Shan

Fabrication and superconductivity of Na x Ta S 2 crystals

In this paper we report the growth and superconductivity of NaxTaS2 crystals. The structural data deduced from x-ray diffraction pattern shows that the sample has the same structure as 2H-TaS2. A series of crystals with different superconducting transition temperatures (T-c) ranging from 2.5 K to 4.4 K were obtained. It is found that the T-c rises with the increase of Na content determined by energy-dispersive x-ray microanalysis(EDX) of scanning electron microscope (SEM) on these crystals. Compared with the resistivity curve of un-intercalated sample 2H-TaS2 (T-c=0.8 K, T(CDW)approximate to 70 K), no signal of charge density wave (CDW) was observed in samples Na0.1TaS2 and Na0.05TaS2. However, in some samples with lower T-c, the CDW appears again at about 65 K. Comparison between the anisotropic resistivity indicates that the anisotropy becomes smaller in samples with more Na intercalation (albeit a weak semiconducting behavior along c-axis) and thus higher T-c. It is thus concluded that there is a competition between the superconductivity and the CDW. With the increase of sodium content, the rise of T-c in NaxTaS2 is caused mainly by the suppression to the CDW in 2H-TaS2, and the conventional rigid band model for layered dichalcogenide may be inadequate to explain the changes induced by the slight intercalation of sodium in 2H-TaS2.

Influence of carbon concentration on the superconductivity in MgCxNi3

The influence of carbon concentration on the superconductivity (SC) in MgCxNi3 has been investigated by measuring the low temperature specific heat combined with first principles electronic structure calculation. It is found that the specific heat coefficient gamma(n) = C-en/T of the superconducting sample (xapproximate to1) in normal state is twice that of the non-superconducting one (xapproximate to0.85). The comparison of measured gamma(n) and the calculated electronic density of states (DOS) shows that the effective mass renormalization changes remarkably as the carbon concentration changes. The large mass renormalization for the superconducting sample and the low transition temperature T-c(7K) indicate that more than one kind of boson mediated electron-electron interaction exists in MgCxNi3.

Weak-coupling Bardeen-Cooper-Schrieffer superconductivity in the electron-doped cuprate superconductors

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap Delta(T) in electron-doped copper oxides with various T(c)s and Ce-doping concentrations. We show that the temperature dependence of Delta(T) follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where Delta(0)/k(B)T(c)approximate to 1.72 +/- 0.15 and Delta(0) is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak-coupling BCS superconductors.

Magnetic fluctuations in n -type high- T c superconductors reveal breakdown of fermiology: Experiments and Fermi-liquid/RPA calculations

By combining experimental measurements of the quasiparticle and dynamical magnetic properties of optimally electron-doped Pr0.88LaCe0.12CuO4 with theoretical calculations, we demonstrate that the conventional fermiology approach cannot possibly account for the magnetic fluctuations in these materials. In particular, we perform tunneling experiments on the very same sample for which a dynamical magnetic resonance has been reported recently and use photoemission data by others on a similar sample to characterize the fermionic quasiparticle excitations in great detail. We subsequently use this information to calculate the magnetic response within the conventional fermiology framework as applied in a large body of work for the hole-doped superconductors to find a profound disagreement between the theoretical expectations and the measurements: this approach predicts a steplike feature rather than a sharp resonance peak, it underestimates the intensity of the resonance by an order of magnitude, it suggests an unreasonable temperature dependence of the resonance, and most severely, it predicts that most of the spectral weight resides in incommensurate wings which are a key feature of the hole-doped cuprates but have never been observed in the electron-doped counterparts. Our findings strongly suggest that the magnetic fluctuations reflect the quantum-mechanical competition between antiferromagnetic and superconducting orders.

Distinct pairing symmetries in Nd1.85Ce0.15CuO4-y and La1.89Sr0.11CuO4 single crystals: Evidence from comparative tunneling measurements

We used point-contact tunneling spectroscopy to study the superconducting pairing symmetry of electron-doped Nd1.85Ce0.15CuO4-y (NCCO) and hole-doped La1.89Sr0.11CuO4 (LSCO). Nearly identical spectra without zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast, LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave superconductor. Detailed analysis reveals an s-wave component in the pairing symmetry of the NCCO sample with Delta/k(B)T(c)=1.66, a value remarkably close to that of a weakly coupled Bardeen-Cooper-Schriffer (BCS) superconductor. We argue that this s-wave component is formed at the Fermi surface pockets centered at (+/-pi,0) and (0,+/-pi) although a d-wave component may also exist.

Fabrication and superconductivity of NaxTaS2 crystals

In this paper we report the growth and superconductivity of NaxTaS2 crystals. The structural data deduced from x-ray diffraction pattern shows that the sample has the same structure as 2H-TaS2. A series of crystals with different superconducting transition temperatures (T-c) ranging from 2.5 K to 4.4 K were obtained. It is found that the T-c rises with the increase of Na content determined by energy-dispersive x-ray microanalysis(EDX) of scanning electron microscope (SEM) on these crystals. Compared with the resistivity curve of un-intercalated sample 2H-TaS2 (T-c=0.8 K, T(CDW)approximate to 70 K), no signal of charge density wave (CDW) was observed in samples Na0.1TaS2 and Na0.05TaS2. However, in some samples with lower T-c, the CDW appears again at about 65 K. Comparison between the anisotropic resistivity indicates that the anisotropy becomes smaller in samples with more Na intercalation (albeit a weak semiconducting behavior along c-axis) and thus higher T-c. It is thus concluded that there is a competition between the superconductivity and the CDW. With the increase of sodium content, the rise of T-c in NaxTaS2 is caused mainly by the suppression to the CDW in 2H-TaS2, and the conventional rigid band model for layered dichalcogenide may be inadequate to explain the changes induced by the slight intercalation of sodium in 2H-TaS2.

Competition between BCS superconductivity and ferromagnetic spin fluctuations in MgCNi3

The low temperature specific heat of the superconductor MgCNi3 and a nonsuperconductor MgC0.85Ni3 is investigated in detail. An additional contribution is observed from the data of MgCNi3 but absent in MgC0.85Ni3, which is demonstrated to be insensitive to the applied magnetic field even up to 12 T. A detailed discussion on its origin is then presented. By subtracting this additional contribution, the zero field specific heat of MgCNi3 can be well described by the BCS theory with the gap ratio (Delta/k(B)T(c)) determined by the previous tunneling measurements. The conventional s-wave pairing state is further proved by the magnetic field dependence of the specific heat at low temperatures and the behavior of the upper critical field.

Bulk evidence for s-wave pairing symmetry in electron-doped infinite-layer cuprate Sr0.9La0.1CuO2

Low temperature specific heat of the electron-doped (n-type) infinite-layer cuprate

Competition of superconductivity and charge density wave order in NaxTaS2 single crystals

Sr_{0.9}La_{0.1}CuO_{2}

Evidence for s-wave pairing from measurement of the lower critical field in MgCNi3

has been measured. The quasiparticle density of states (DOS) in the mixed state is found to be consistent with the feature of the s-wave pairing symmetry, agreeing very well with the earlier tunnelling measurement, but being contrary to the d-wave symmetry well confirmed for the hole-doped (p-type) cuprates. Our results indicate that the electronic DOS are mainly contributed by the vortex cores in the present sample being contrast to the p-type cuprates in which the vortex cores are abnormal and contribute very limited low energy DOS as evidenced by many means.