J. W. Xiong

Multiple bosonic mode coupling in the electron self-energy of (La2-xSrx)CuO4

High resolution angle-resolved photoemission spectroscopy data along the (0, 0)-(pi,pi) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La2-xSrx)CuO4 samples in the normal state. Fine structure at energies of (40-46) meV and (58-63) meV, and possible fine structure at energies of ( 23 - 29) meV and ( 75 - 85) meV, have been identified. These observations indicate that, in (La2-xSrx) CuO4, more than one bosonic modes are involved in the coupling with electrons.

Measurement of an anisotropic energy gap in single plane Bi2Sr2-xLaxCuO6+delta

We report angle-resolved photoemission spectra both above and below T-c in the single-plane cuprate superconductor Bi2Sr2-xLaxCuO6+8. The superconducting state measurements show a highly anisotropic excitation gap with a maximum magnitude smaller than that of the bilayer compound Bi2Sr2CaCu2O8 by a factor of 3. For a range of doping, the gap persists well above T-c, behavior previously associated with underdoped bilayer cuprates. The anisotropy and magnitude of the normal-state gap are very similar to the superconducting state gap, indicating that the two gaps may have a common origin in a pairing interaction.We report angle-resolved photoemission spectra both above and below T{sub c} in the single-plane cuprate superconductor Bi{sub 2}Sr{sub 2{minus}x}La{sub x}CuO{sub 6+{delta}}. The superconducting state measurements show a highly anisotropic excitation gap with a maximum magnitude smaller than that of the bilayer compound Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} by a factor of 3. For a range of doping, the gap persists well above T{sub c}, behavior previously associated with underdoped bilayer cuprates. The anisotropy and magnitude of the normal-state gap are very similar to the superconducting state gap, indicating that the two gaps may have a common origin in a pairing interaction. {copyright} {ital 1997} {ital The American Physical Society}

Crystal growth and superconductivity of heavily La-doped Bi-2201 single crystals

Abstract A number of La-doped Bi-2201 single crystals were prepared by self-flux method. The qualities of the crystals have been checked by X-ray diffraction patterns (XRD) and electron diffraction patterns from transmission electron microscopy (TEM). The correlation between the actual composition of La determined by Energy-Dispersive X-ray (EDX) analysis and the c -axis parameter determined from XRD shows a linear behavior, being consistent with previous reports. A new superconducting phase with T c as high as 36 K has been discovered for the first time in heavily La-doped Bi 2 Sr 2− x La x CuO 6+ y single crystals ( x ≥0.8) in which one normally could not observe the superconductivity. Since our samples have been post-annealed in flowing oxygen and heavily La-doped samples are normally in the region with an antiferromagnetic background, it is natural to attribute the appearance of the high T c superconductivity to the electronic phase separation induced by the cooperative interactions between the antiferromagnetic background and the mobile holes (given by excess oxygen).

Pseudogap, superconducting energy scale, and Fermi arcs of underdoped cuprate superconductors

Through the measurements of magnetic field dependence of specific heat in La2-xSrxCuO4 in zero temperature limit, we determined the nodal slope v(Delta) of the quasiparticle gap. It is found that v(Delta) has a very similar doping dependence of the pseudogap temperature T-* or value Delta(p). Meanwhile the virtual maximum gap at (pi,0) derived from v(Delta) is found to follow the simple relation Delta(q)=0.46k(B)T(*) upon changing the doping concentration. This strongly suggests a close relationship between the pseudogap and superconductivity. It is further found that the superconducting transition temperature is determined by both the residual density of states of the pseudogap phase and the nodal gap slope in the zero temperature limit, namely, T(c)approximate to beta v(Delta)gamma(n)(0), where gamma(n)(0) is the extracted zero temperature value of the normal state specific heat coefficient which is proportional to the size of the residual Fermi arc k(arc). This shows that the superconductivity may be formed by forming a new gap on the Fermi arcs near nodes below T-c. These observations mimic the key predictions of the SU(2) slave boson theory based on the general resonating-valence-bond picture.

Electronic specific heat and low-energy quasiparticle excitations in the superconducting state of La2˛xSrxCuO4 single crystals

Low-temperature specific heat has been measured and extensively analyzed on a series of La2-xSrxCuO4 single crystals from underdoped to overdoped regime. From these data the quasiparticle density of state in the mixed state is derived and compared to the predicted scaling law C-vol/TrootH=f(T/rootH) of d-wave superconductivity. It is found that the scaling law can be nicely followed by the optimally doped sample (x = 0.15) in quite a wide region of (T/rootH less than or equal to 8K/rootT). However, the region for this scaling becomes smaller and smaller toward more underdoped region: a clear trend can be seen for samples from x=0.15 to 0.069. Therefore, generally speaking, the scaling quality becomes worse on the underdoped samples in terms of scalable region of T/rootH. This feature in the underdoped region is explained as due to the low-energy excitations from a second order ( for example, antiferromagnetic correlation, d-density wave, spin-density wave, or charge-density wave order) that may coexist or compete with superconductivity. Surprisingly, deviations from the d-wave scaling law have also been found for the overdoped sample (x=0.22), while the scaling law is reconciled for the overdoped sample, when the core size effect is taken into account. An important discovery of present work is that the zero-temperature data follow the Voloviks relation Deltagamma(T=0)=ArootH quite well for all samples investigated here; although the applicability of the d-wave scaling law to the data at finite temperatures varies with doped-hole concentration. We also present the doping dependence of some parameters, such as the residual linear term gamma(0), the alpha value, etc. It is suggested that the residual linear term (gamma(0)T) of the electronic specific heat observed in all cuprate superconductors is probably due to the inhomogeneity, either chemical or electronic in origin. The field-induced reduction of the specific heat in the mixed state is also reported. Finally, implications on the electronic phase diagram are suggested.

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.

Evidence of coherent c-axis charge transport in underdoped La2-xSrxCuO4 superconductors

The electronic structures of 2 X 2 polar (003) surfaces of LiCoO2 have been studied using the first-principles. projector augmented-wave (PAW) method within the generalized gradient approximation (GGA), based on the density functional theory (DFT). Several geometry structure models were considered with different terminations in order to obtain the most stable structure. From cleavage energies of separation it reveals that, for LiCoO2 (003), the configuration with 0 termination on one side and Li termination on the other side was the most stable one. Because the exposed Li-O groups easily react with the electrolytes so that the structure is destroyed, this conclusion can indirectly explain the phenomenon that LiCoO2 coated by MgO possesses the more stable structure [J. Electrochem. Soc. 149, A466 (2002)]. It is also found that structure relaxation is considerably large, and the electronic density of states (DOS) shows large difference between (003) surfaces and bulk, with, for example, a decrease of the band gap between the valance band and the empty band in (003) surfaces region compared to the bulk. It is possible to reconstruct for the polar (003) surfaces of LiCoO2 crystallite.

Effects of the Nd4Ba2Cu2Oy phase on texturing and properties of melt-textured NdBa2Cu3Oy

A series of melt-textured NdBa2Cu3Oy bulks with controlled amount of Nd4Ba2Cu2Oy additions has been fabricated by the newly developed technology. The optimal formation temperature increases with increasing the amount of Nd4Ba2Cu2Oy addition. Also a remarkable enhancement of the magnetic levitation force is induced by Nd4Ba2Cu2Oy addition. The optimal amount of Nd4Ba2Cu2Oy can be expressed as Nd4Ba2Cu3Oy + 0.4Nd(4)Ba(2)Cu(2)O(y). SEM studies show that microcracks in the bulks can be effectively reduced by Nd4Ba2Cu2Oy particles. The enhancement of the magnetic levitation force is attributed to the suppression of weak links in grains

Structural, transport and magnetic properties of PrBa2−xSrxCu3O7-δ

Abstract Structural, electrical transport and magnetic properties of polycrystalline samples of PrBa 2−x Sr x Cu 3 O 7-δ were investigated. The results were discussed based on the variation of hybridization between Pr ions and O2p of the CuO 2 planes induced by Sr doping.

Manipulating vortex motion by thermal and lorentz force in high-temperature superconductors

By using thermal and Lorentz force, the vortex motion is successfully manipulated in the mixed state of underdoped La