arXiv subjects

C. W. Chu

Publications and source records attributed to C. W. Chu.

116 records · Page 7Linked to original sources

arXiv2000

Evidence for the immobile bipolaron formation in the paramagnetic state of the magnetoresistive manganites

Recent research suggests that the charge carriers in the paramagnetic state of the magnetoresistive manganites are small polarons. Here we report studies of the oxygen-isotope effects on the intrinsic resistivity and thermoelectric power in several ferromagnetic manganites. The precise measurements of these isotope effects allow us to make a quantitative data analysis. Our results do not support a simple small-polaron model, but rather provide compelling evidence for the presence of small immobile bipolarons, i.e., pairs of small polarons. Since the bipolarons in the manganites are immobile, the present result alone appears not to give a positive support to the bipolaronic superconductivity theory for the copper-based perovskites.

cond-mat.str-el↗

arXiv2000

Field-Induced Crossover and Colossal Magnetoresistance in La(0.7)Pb(0.3)MnO(3)

A field-induced crossover is observed in the resistivity and magnetization (M) of a La(0.7)Pb(0.3)MnO(3) single crystal. The field-dependence of the resistivity and M suggests that a small spin-canted species with mean-field-like interactions dominates at low fields (H), whereas, individual spins and 3D Ising/Heisenberg models describe the high-H behavior rather well. Around the ferromagnetic transition, an H-induced destruction of the small spin-canted magnetic polarons is accompanied by large magnetoresistance.

cond-mat.str-el↗

arXiv2000

A Possible Crypto-Superconducting Structure in a Superconducting Ferromagnet

We have measured the dc and ac electrical and magnetic properties in various magnetic fields of the recently reported superconducting ferromagnet RuSr2GdCu2O8. Our reversible magnetization measurements demonstrate the absence of a bulk Meissner state in the compound below the superconducting transition temperature. Several scenarios that might account for the absence of a bulk Meissner state, including the possible presence of a sponge-like non-uniform superconducting or a crypto-superconducting structure in the chemically uniform Ru-1212, have been proposed and discussed.

cond-mat.supr-con↗

arXiv1999

Absence of a Bulk Meissner State in RuSr2GdCu2O8

We have systematically investigated the magnetic, electrical, and structural properties of RuSr2GdCu2O8, in which a long-range ferromagnetic order and superconductivity have been previously reported to coexist. Based on the reversible magnetization results, we conclude that the bulk Meissner state does not exist in this compound and that the condensation energy associated with superconductivity is negligible. The absence of a bulk Meissner state and the superconductivity detected are thus attributed to the possible appearance of a sponge-like crypto-superconducting fine structure in RuSr2GdCu2O8 samples that are found to be chemically homogeneous to 1-2 mu m and electrically uniform to \~ 10 mu m across the sample.

cond-mat.supr-con↗

arXiv1999

c-Axis Superfluid Response and Pseudogap in High-Tc Superconductors

To gain insight into the out-of-plane electrodynamics of high-$T_c$ superconduct ors we have measured the absolute values and temperature dependence of the c-a xis magnetic penetration depth $λ_c(T)$ for two typical single layer high -$T_c$ cuprates, La$_{2-x}$Sr$_x$CuO$_4$ and HgBa$_2$CuO$_{4+x}$ as a function o f doping. A distinct change in the behaviour of $λ_c$ is observed near 0. 20 holes/Cu, which is related to the opening of the normal state pseudogap. The variation of $λ_c$ with doping is qualitatively similar to that of the in -plane component, $λ_{ab}$, which suggests that the c-axis superfluid r esponse is mainly governed by the superconducting condensation energy. The stron g doping dependence of $λ_c(0)$ for p<0.20 provides an explanation for the discrepancies in the literature.

cond-mat.supr-con↗

arXiv1999

Universal scaling of Hall resistivity in clean and moderately clean limits for Hg- and Tl-based superconductors

The mixed-state Hall resistivity and the longitudinal resistivity in HgBa_{2}CaCu_{2}O_{6}, HgBa_{2}Ca_{2}Cu_{3}O_{8}, and Tl_{2}Ba_{2}CaCu_{2}O_{8} thin films have been investigated as functions of the magnetic field up to 18 T. We observe the universal scaling behavior between ρ_{xy} and ρ_{xx} in the regions of the clean and the moderately clean limit. The scaling exponent βis 1.9 in the clean limit at high field and low temperature whereas βis 1.0 in the moderately clean limit at low field and high temperature, consistent with a theory based on the midgap states in the vortex cores. This finding implies that the Hall conductivity is also universal in Hg- and Tl-based superconductors.

cond-mat.supr-con↗

arXiv1999

Effects of carrier concentration on the superfluid density of high-T_c cuprates

The absolute values and temperature, T, dependence of the in-plane magnetic penetration depth, $λ_{ab}$, of La_{2-x}Sr_xCuO_4 and HgBa_2CuO_{4+δ} have been measured as a function of carrier concentration. We find that the superfluid density, $ρ_s$, changes substantially and systematically with doping. The values of $ρ_{s}(0)$ are closely linked to the available low energy spectral weight as determined by the electronic entropy just above T_c and the initial slope of $ρ_{s}(T)/ρ_{s}(0)$ increases rapidly with carrier concentration. The results are discussed in the context of a possible relationship between $ρ_s$ and the normal-state (or pseudo) energy gap.

cond-mat.supr-con↗

arXiv1998

Hole Localization in Underdoped Superconducting Cuprates Near 1/8th Doping

Measurements of thermal conductivity versus temperature over a broad range of doping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+δ}$ ($n$=1,2,3) suggest that small domains of localized holes develop for hole concentrations near $p$=1/8. The data imply a mechanism for localization that is intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with oxygen-vacancy clusters.

cond-mat.supr-con↗