arXiv subjects

C. H. Wang

Publications and source records attributed to C. H. Wang.

50 records · Page 3Linked to original sources

arXiv2004

Anomalous Magnetoresistance in Pb-doped Bi$_2$Sr$_2$Co$_2$O$_y$ Single Crystals

Magnetoresistance (MR) of the Bi$_{2-x}$Pb$_x$Sr$_2$Co$_2$O$_y$ ($x$=0, 0.3, 0.4) single crystals is investigated systematically. A nonmonotonic variation of the isothermal in-plane and out-of-plane MR with the field is observed. The out-of-plane MR is positive in high temperatures and increases with decreasing $T$, and exhibits a pronounced hump, and changes the sign from positive to negative at a centain temperature. These results strongly suggest that the observed MR consists of two contributions: one \emph{negative} and one \emph{positive} component. The isothermal MR in high magnetic fields follows a $H^2$ law. While the negative contribution comes from spin scattering of carriers by localized-magnetic-moments based on the Khosla-Fischer model.

cond-mat.str-el↗

arXiv2004

Hysteresis and Anisotropic Magnetoresistance in Antiferromagnetic $Nd_{2-x}Ce_xCuO_{4}$

The out-of-plane resistivity ($ρ_c$) and magnetoresistivity (MR) are studied in antiferromangetic (AF) $Nd_{2-x}Ce_xCuO_{4}$ single crystals, which have three types of noncollinear antiferromangetic spin structures. The apparent signatures are observed in $ρ_c(T)$ measured at the zero-field and 14 T at the spin structure transitions, giving a definite evidence for the itinerant electrons directly coupled to the localized spins. One of striking feature is an anisotropy of the MR with a fourfold symmetry upon rotating the external field (B) within ab plane in the different phases, while twofold symmetry at spin reorientation transition temperatures. The intriguing thermal hysteresis in $ρ_c(T,B)$ and magnetic hysteresis in MR are observed at spin reorientation transition temperatures.

cond-mat.str-el↗

arXiv2004

Effect of oxygen content on the transport properties and magnetoresistance in [Ca$_{2}$CoO$_{3-δ}$]$_{0.62}$[CoO$_{2}$] single crystals

Transport property is investigated in [Ca$_{2}$CoO$_{3-δ}$]$_{0.62}$[CoO$_{2}$] single crystals obtained by varying annealing conditions. The $ρ_{ab}(T)$ exhibits a resistivity minimum, and the temperature corresponding to this minimum increases with the loss of oxygen content, indicative of the enhancement of spin density wave (SDW). Large negative magnetoresistance (MR) was observed in all single crystals [Ca$_{2}$CoO$_{3-δ}$]$_{0.62}$[CoO$_{2}$], while a magnetic-field-driven insulator-to-metal (IM) transition in oxygen annealed samples. These results suggest a ferromagnetic correlation in system enhanced by oxygen content. In addition, a low temperature thermal activation resistivity induced by fields was observed in single crystals annealed in oxygen atmosphere.

cond-mat.str-el↗

arXiv2004

Dimensional crossover and anomalous magnetoresistivity in single crystals $Na_xCoO_2$

The in-plane ($ρ_{ab}$) and c-axis ($ρ_c$) resistivities, and the magnetoresistivity of single crystals $Na_xCoO_2$ with x = 0.7, 0.5 and 0.3 were studied systematically. $ρ_{ab}(T)$ shows similar temperature dependence between $Na_{0.3}CoO_2$ and $Na_{0.7}CoO_2$, while $ρ_c(T)$ is quite different. A dimensional crossover from two to three occurs with decreasing Na concentration from 0.7 to 0.3. The angular dependence of in-plane magnetoresistivity for 0.5 sample shows a \emph{"d-wave-like"} symmetry at 2K, while the \emph{"p-wave-like"} symmetry at 20 K. These results give an evidence for existence of a \emph{spin ordering orientation} below 20 K turned by external field, like the stripes in cuprates.

cond-mat.str-el↗

arXiv2004

Magnetic and Transport Properties in $CoSr_2Y_{1-x}Ca_xCu_2O_7$ ($x$=0$\sim$0.4)

Magnetic and transport properties of $Co Sr_2 Y_{1-x} Ca_x Cu_2 O_7$ ($x=0 \sim 0.4$) system have been investigated. A broad maximum in M(T) curve, indicative of low-dimensional antiferromagnetic ordering originated from $CoO_{1+δ}$ layers, is observed in Ca-free sample. With increasing Ca doping level up to 0.2, the M(T) curve remains almost unchanged, while resistivity is reduced by three orders. Higher Ca doping level leads to a drastic change of magnetic properties. In comparison with the samples with $x=0.0 \sim 0.2$, the temperature corresponding to the maximum of M(T) is much lowered for the sample $x$=0.3. The sample $x$=0.4 shows a small kink instead of a broad maximum and a weak ferromagnetic feature. The electrical transport behavior is found to be closely related to magnetic properties for the sample $x$=0.2, 0.25, 0.3, 0.4. It suggests that $CoO_{1+δ}$ layers are involved in charge transport in addition to conducting $CuO_2$ planes to interpret the correlation between magnetism and charge transport. X-ray photoelectron spectroscopy studies give an additional evidence of the the transfer of the holes into the $CoO_{1+δ}$ charge reservoir.

cond-mat.str-el↗

arXiv2004

Giant electron-electron scattering in the Fermi-liquid state of Na_0.7CoO_2

The in-plane resistivity, rho, and thermal conductivity, kappa, of a single crystal of Na_0.7CoO_2 were measured down to 40 mK. Verification of the Wiedemann-Franz law, kappa/T = L_0/rho as T -> 0, and observation of a T^2 dependence of rho at low temperature, rho = rho_0 + AT^2, establish the existence of a well-defined Fermi-liquid state. The measured value of coefficient A reveals enormous electron-electron scattering, characterized by the largest Kadowaki-Woods ratio, A/gamma^2, encountered in any material. The rapid suppression of A with magnetic field suggests a possible proximity to a magnetic quantum critical point. We also speculate on the possible role of magnetic frustration and proximity to a Mott insulator.

cond-mat.str-el↗

arXiv2004

Infrared spectroscopy of the charge ordering transition in Na$_{0.5}$CoO$_2$

We report infrared spectra of a Na$_{0.5}$CoO$_2$ single crystal which exhibits a sharp metal-insulator transition near 50 K due to the formation of charge ordering. In comparison with x=0.7 and 0.85 compounds, we found that the spectral weight associated with the conducting carriers at high temperature increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm$^{-1}$. A hump near 800 cm$^{-1}$ develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong anti-resonance feature of phonon spectra. At lower temperature $T_{co}$, an optical gap develops at the magnitude of 2$Δ\approx3.5k_BT_{co}$, evidencing an insulating charge density wave ground state. Our experimental results and analysis unequivocally point towards the importance of charge ordering instability and strong electron-phonon interaction in Na$_x$CoO$_2$ system.

cond-mat.str-el↗

arXiv2004

Measurement of the Branching Fractions for $B \to ωK$ and $B \to ωπ$

We report improved measurements of branching fractions for charmless hadronic two-body {\it B} meson decays containing an $ω$ meson in the final state. The results are based on a data sample of 78 fb$^{-1}$ collected on the $Υ(4S)$ resonance by the Belle detector. We measure the branching fractions ${\mathcal B}(B^+ \to ωK^+) = (6.5^{+1.3}_{-1.2}\pm 0.6)\times 10^{-6}$ and ${\mathcal B}(B^+ \to ωπ^+) = (5.7^{+1.4}_{-1.3}\pm 0.6)\times 10^{-6}.$ We give 90% confidence upper limits for ${\mathcal B}(B^0 \to ωK^0) < 7.6\times 10^{-6}$ and ${\mathcal B}(B^0 \to ωπ^0) < 1.9\times 10^{-6}.$ We also obtain the partial rate asymmetries ${\mathcal A}_{CP}=0.06^{+0.21}_{-0.18}\pm 0.01$ for $B^\pm \to ωK^\pm$ and ${\mathcal A}_{CP}=0.50^{+0.23}_{-0.20}\pm 0.02$ for $B^\pm \to ωπ^\pm.$

hep-ex↗

arXiv2001

A Prototype of Upgrading Bepc Control System

We will use EPICS toolkit [1] to build a prototype for upgrading BEPC control system. The purposes are for the following three aspects: (1) Setup a network based distributed control system with EPICS. (2) Study some front-end control technology. (3) Merge the existing control systems into EPICS. The hardware architecture consists of three physical layers. The front-end layer is VME crate, PowerPC, VME I/O modules and interface boards (VME-CAMAC and Fieldbus). The back-end layer is SUN workstation, which is used as application development. The two layers are connected via the middle layer which is 100M Ethernet. In order to preserve our investment in the hardware, the existing CAMAC hardware will remain. The CAMAC will be connected to the VME via CAMAC serial highway [2]. The operator interface will be developed with EPICS DM2K. High-level accelerator control applications will be studied as well with EPICS channel access and archive access.

physics.acc-ph↗

arXiv2001

Thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$

The thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$ ($T_c \approx$ 8 K) have been studied. The thermopower is negative from room temperature to 10 K. Combining with the negative Hall coefficient reported previously, the negative thermopower definetly indicates that the carrier in $MgCNi_3$ is electron-type. The nonlinear temperature dependence of thermopower below 150 K is explained by the electron-phonon interaction renormalization effects. The thermal conductivity is of the order for intermetallics, larger than that of borocarbides and smaller than $MgB_2$. In the normal state, the electronic contribution to the total thermal conductivity is slightly larger than the lattice contribution. The transverse magnetoresistance of $MgCNi_3$ is also measured. It is found that the classical Kohler's rule is valid above 50 K. An electronic crossover occures at $T^* \sim 50 K$, resulting in the abnormal behavior of resistivity, thermopower, and magnetoresistance below 50 K.

cond-mat.supr-con↗

arXiv2001

Control System of the Bepcii

Recently the Chinese Academy of Sciences has chosen BEPCII as the future development of the BEPC, i.e. upgrade of both the machine and detector. The luminosity of the machine is expected to increase to 1.0x1033cm-2s-1. The project will be started at the beginning of 2002 and finished with in 3-4 years. The BEPC control system was built in 1987 and upgraded in 1994. According to the design of the BEPCII, a double ring schema will be adopted and a number of new devices will be added in the system. The existing control system has to be upgraded. The BEPCII will be distributed architecture and developed by EPICS. We are going to apply the standard hardware interfaces and mature technologies in the system. A number of VME IOCs will be added in the system and the feildbus, PLCs will be used as device control for some kind of equipment. We will keep the existing system in use, such as CAMAC modules and PC front-ends, and merge it into EPICS system. Recently the development of the prototype is in progress. This paper will describe the system design and development issues.

hep-ph↗

arXiv2001

Normal state resistivity, upper critical field and Hall effect in superconducting perovskite $MgCNi_3$

The normal state resistivtity, upper critical field $H_{c2}$ and Hall coefficient $R_H$ in superconducting perovskite $MgCNi_3$ ($T_c \approx 8 K$) have been studied. Above 70 K, $ρ(T)$ fits well curve predicted by Bloch-Grüneisen theory consistently with electron-phonon scattering. $H_{c2}(0)$ was estimated to be about 15.0 Tesla within the weak-coupling BCS theory, and the superconducting coherence length $ξ(0)$ is approximately 47 Å. $R_H$ of $MgCNi_3$ is negative for the whole temperature range which definitely indicates that the carrier in $MgCNi_3$ is electron-type. $R_H$ is temperature independent between $T_c$ and $\sim$ 140 K. Above $\sim$ 140 K, the magnitude of $R_H$ decreases as temperature rises. At T = 100 K, the carrier density is $1.0 \times 10^{22}/cm^3$, which is comparable with that in perovskite $(Ba,K)BiO_3$, and less than that of the metallic binary $MgB_2$.

cond-mat.supr-con↗

arXiv2001

A Study of B meson Decays to Charm at Belle

With ~10fb$^{-1}$ data from Belle, the results of b to c and u decays have been represented here. Several decay modes have been studied, which include charmonium, semileptonic decay and Cabibbo-suppressed decays mode. The preliminary results and the measurements of KM matrix elements will also be presented here.

hep-ex↗

arXiv1997

Detection of Minimum-Ionizing Particles and Nuclear Counter Effect with Pure BGO and BSO Crystals with Photodiode Read-out

Long BGO (Bismuth Germanate) and BSO (Bismuth Silicate) crystals coupled with silicon photodiodes have been used to detect minimum-ionizing particles(MIP). With a low noise amplifier customized for this purpose, the crystals can detect MIPs with an excellent signal-to-noise ratio. The NCE(Nuclear Counter Effect} is also clearly observed and measured. Effect of full and partial wrapping of a reflector around the crystal on light collection is also studied.

physics.ins-det↗