arXiv subjects

C. L. Zhang

Publications and source records attributed to C. L. Zhang.

43 records · Page 3Linked to original sources

arXiv2006

Relating supercooling and glass-like arrest of kinetics for phase separated systems: studies on doped CeFe$_2$ and (La,Pr,Ca)MnO$_3$

Coexisting ferromagnetic and antiferromagnetic phases over a range of temperature as well as magnetic field have been reported in many materials of current interest, showing disorder-broadened 1st order transitions. Anomalous history effects observed in magnetization and resistivity are being explained invoking the concepts of kinetic arrest akin to glass transitions. From magnetization measurements traversing novel paths in field-temperature space, we obtain the intriguing result that the regions of the sample which can be supercooled to lower temperatures undergo kinetic-arrest at higher temperatures, and vice versa. Our results are for two diverse systems viz. the inter-metallic doped CeFe$_2$ which has an antiferromagnetic ground state, and the oxide La-Pr-Ca-Mn-O which has a ferromagnetic ground state, indicating the possible universality of this effect of disorder on the widely encountered phenomenon of glass-like arrest of kinetics.

cond-mat.str-el↗

arXiv2005

Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3

TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T_N < 41K is accompanied by ferroelectric order for T < 28K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally-modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.

cond-mat.str-el↗

arXiv2003

Variable-range-hopping conductivity of half-doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$

We report measurements of in-plane $ρ_{ab}$ and out-of-plane $ρ_{c}$ resistivities on a single crystal of the half-doped bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$. In the temperature $T$ range 220 to 300 K, the resistive anisotropy $ρ_{c}/ρ_{ab}=A+B/T$ ($A$ and $B$ constants), which provides evidence for the variable-range-hopping conduction in the presence of a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic field $H$ and $\sin^{2}ϕ$ dependences of the negative magnetoresistivity $\ln [ρ_{i}(T,H,ϕ)/ρ_{i}(T,H=0)]$ ($i=ab,c$), where $ϕ$ is the in-plane angle between the magnetic field and the current.

cond-mat.str-el↗

arXiv2003

Spin-wave scattering at low temperatures in manganite films

The temperature $T$ and magnetic field $H$ dependence of the resistivity $ρ$ has been measured for La$_{0.8-y}$Sr$_{0.2}$MnO$_{3}$ (y=0 and 0.128) films grown on (100) SrTiO$_{3}$ substrates. The low-temperature $ρ$ in the ferromagnetic metallic region follows well $ρ(H,T)=ρ_{0}(H)+A(H)ω_{s}/\sinh (\hbar ω_{s}/2k_{B}T)+B(H)T^{7/2}$ with $ρ_{0}$ being the residual resistivity. We attribute the second and third term to small-polaron and spin-wave scattering, respectively. Our analysis based on these scattering mechanisms also gives the observed difference between the metal-insulator transition temperatures of the films studied. Transport measurements in applied magnetic field further indicate that spin-wave scattering is a key transport mechanism at low temperatures.

cond-mat.str-el↗

arXiv2003

Small-polaron hopping conductivity in bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

We report anisotropic resistivity measurements on a La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal over a temperature $T$ range from 2 to 400 K and in magnetic fields $H$ up to 14 T. For $T\geq 218$ K, the temperature dependence of the zero-field in-plane $ρ_{ab}(T)$ resistivity obeys the adiabatic small polaron hopping mechanism, while the out-of-plane $ρ_{c}(T)$ resistivity can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between the resistivity and magnetization, we developed a model which allows the determination of $ρ_{ab,c}(H,T)$. The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.

cond-mat.str-el↗

arXiv2002

Low-temperature electrical transport in bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

The temperature $T$ and magnetic field $H$ dependence of anisotropic in-plane $ρ_{ab}$ and out-of-plane $ρ_{c}$ resistivities have been investigated in single crystals of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. Below the Curie transition temperature $T_c=$ 125 K, $ρ_{ab}$ and $ρ_{c}$ display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K $\leq T \leq$ 110 K), both $ρ_{ab}(T)$ and $ρ_{c}(T)$ follow a $T^{9/2}$ dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient $B_{ab}^{fit}$ and the ratio of the exchange interaction $J_{ab}/J_c$ obtained by fitting the data are in excellent agreement with the calculated $B_{ab}$ based on the two-magnon model and $J_{ab}/J_c$ deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower $T$, in the non-metallic regime ($T<$ 50 K), {\it both} the in-plane $σ_{ab}$ and out-of-plane $σ_{c}$ conductivities obey a $T^{1/2}$ dependence, consistent with weak localization effects. Hence, this demonstrates the three-dimensional metallic nature of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ at $T<T_c$.

cond-mat.mtrl-sci↗

arXiv1999

Relationship Between Conductivity and Phase Coherence Length in Cuprates

The large ($10^2 - 10^5$) and strongly temperature dependent resistive anisotropy $η= (σ_{ab}/σ_c)^{1/2}$ of cuprates perhaps holds the key to understanding their normal state in-plane $σ_{ab}$ and out-of-plane $σ_{c}$ conductivities. It can be shown that $η$ is determined by the ratio of the phase coherence lengths $\ell_i$ in the respective directions: $σ_{ab}/σ_c = \ell_{ab}^2/\ell_{c}^2$. In layered crystals in which the out-of-plane transport is incoherent, $\ell_{c}$ is fixed, equal to the interlayer spacing. As a result, the T-dependence of $η$ is determined by that of $\ell_{ab}$, and vice versa, the in-plane phase coherence length can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped $YBa_2Cu_3O_y$ ($6.3 < y < 6.9$) and $Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}$ ($0 < x \leq 0.55$) and show that $σ_{ab}$ of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped $Nd_{2-x}Ce_{x}CuO_{4-y}$, the dependence $σ_{ab}(η)$ indicates a crossover from incoherent to coherent transport in the c-direction.

cond-mat.supr-con↗