Two-dimensional Weak Localization Effect Research Articles

Weak localization effect in the strongly ferromagnetic Kondo compoundUCo0.5Sb2

Experimental results of the magnetization $(M)$, specific heat $({C}_{p})$, electrical resistivity $(\ensuremath{\rho})$, magnetoresistance (MR), and thermoelectric power $(S)$ are presented for single crystals of the tetragonal $\mathrm{U}{\mathrm{Co}}_{0.5}{\mathrm{Sb}}_{2}$ compound. Anomalies in the $M\mathit{(}T\mathit{)}$, $[{C}_{p}\mathit{(}T\mathit{)}]$, $\ensuremath{\rho}\mathit{(}T\mathit{)}$, $S\mathit{(}T\mathit{)}$ dependencies have allowed us to establish that $\mathrm{U}{\mathrm{Co}}_{0.5}{\mathrm{Sb}}_{2}$ undergoes a long-range ferromagnetic ordering at ${T}_{C}=64.5(2)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of the magnetization measured fairly far below ${T}_{C}$ might be explained by considering both types of magnetic excitations: i.e., the spin-wave- and Stoner-type excitations. The $M\mathit{(}T\mathit{)}$ dependence is well represented by the relation $1\ensuremath{-}M(T)∕{M}_{s}=B{T}^{3∕2}+{B}_{1}{T}^{3∕2}\mathrm{exp}(\ensuremath{-}\mathrm{\ensuremath{\Delta}}∕T)$ with the determined spin-wave stiffness constant $D\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.2em}{0ex}}{\mathrm{\AA{}}}^{2}$ and a Stoner gap $\mathrm{\ensuremath{\Delta}}=69(2)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The behavior of the electrical resistivity below about ${T}_{C}∕2$ is discussed in terms of the electron scattering on magnons with a dispersion relation ${E}_{q}=\mathrm{\ensuremath{\Delta}}+D{q}^{2}$ and the two-dimensional weak localization effect described by a \ensuremath{-}ln $T$ dependence. Negative magnetoresistance is observed over a wide range of temperatures below 150 K. The relative large field and temperature changes in the transverse MR around ${T}_{C}$ are thought to be due to the ferromagnetic order or/and damped critical fluctuations by the applied fields. Below 40 K the magnetoresistance behavior may be understood on the basis of the predominant elastic scattering predicted for two-dimensional systems with the weak localization effect. The thermoelectric power data indicate $\mathrm{U}{\mathrm{Co}}_{0.5}{\mathrm{Sb}}_{2}$ to be a $p$-type material with a moderate $S$ value of $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{V}∕\mathrm{K}$ at room temperature. The ${T}^{3∕2}$ dependence observed in the $S\mathit{(}T\mathit{)}$ curves for $T\ensuremath{\leqslant}30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is consistent with the electron-magnon scattering process. The high anisotropy and temperature dependence of the $S\mathit{(}T\mathit{)}$ curves may be explained by adopting a phenomenological model for a Kondo lattice.

Electronic Transport Properties of IncipientGraphitic Domains Formation in PAN Derived Carbon Nanofibers

The carbon nanofibers used in this work were derived from a polyacrylonitrile (PAN)/N, N-dimethyl formamide (DMF) precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity, /spl sigma/, was measured at temperatures between 1.9 and 300 K and transverse magnetic field between -9 and 9 T. Zero magnetic field conductivity /spl sigma/(0,T) was found to increase monotonically with the temperature with a convex /spl sigma/(0,T) versus T curve. Conductivity increases with the external transverse magnetic field, revealing a negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of -75 % at 1.9 K and 9 T. The magnetic field dependence of the conductivity and the temperature dependence of the zero-field conductivity are best described using the two-dimensional weak localization effect.

Magnetic-field-induced superconductor–metal-insulator transitions in bismuth metal graphite

Bismuth-metal graphite (MG) has a unique layered structure where Bi nanoparticles are encapsulated between adjacent sheets of nanographites. The superconductivity below $T_{c}$ (= 2.48 K) is due to Bi nanoparticles. The Curie-like susceptibility below 30 K is due to conduction electrons localized near zigzag edges of nanographites. A magnetic-field induced transition from metallic to semiconductor-like phase is observed in the in-plane resistivity $\rho_{a}$ around $H_{c}$ ($\approx$ 25 kOe) for both $H$$\perp$$c$ and $H$$\parallel$$c$ ($c$: c axis). A negative magnetoresistance in $\rho_{a}$ for $H$$\perp$$c$ (0$<H\leq$3.5 kOe) and a logarithmic divergence in $\rho_{a}$ with decreasing temperature for $H$$\parallel$$c$ ($H$ $>$ 40 kOe) suggest the occurrence of two-dimensional weak localization effect.

Two-Dimensional Weak Localization Effect in Stage-4 MoCl5 Graphite Intercalation Compound

The c-axis resistivity ρc(H, T) of stage-4 MoCl5 GIC between 1.9 and 50 K has been measured with and without an external magnetic field along the c axis (0≤H≤44 kOe). The interior G layers form a bottleneck to the c-axis conduction. The T and H dependence of ρc is mainly determined from that of the bottleneck resistivity proportional to the in-plane resistivity of interior graphite layers. A logarithmic behavior of ρc in the form of In(T) and In(H) indicates that the two-dimensional weak localization occurs in the interior graphite layers. The T and H dependence of the in-plane conductivity derived from ρc is discussed in terms of scaling relation predicted from the theory of two-dimensional weak localization.

Two-dimensional weak-localization effect in the stage-4MoCl5graphite intercalation compound

The c-axis resistivity ${\ensuremath{\rho}}_{c}(H,T)$ of a stage-4 ${\mathrm{MoCl}}_{5}$ graphite intercalation compound between 1.9 and 50 K has been measured with and without an external magnetic field along the c axis $(0&lt;~H&lt;~44\mathrm{kOe}).$ The interior graphite (G) layers form a bottleneck to the c-axis conduction. The T and H dependences of ${\ensuremath{\rho}}_{c}$ are mainly determined from that of the bottleneck resistivity which is proportional to the in-plane resistivity of interior graphite layers. An observed logarithmic behavior of ${\ensuremath{\rho}}_{c}$ in the form of $\mathrm{ln}(T)$ and $\mathrm{ln}(H)$ indicates that the two-dimensional weak localization occurs in the interior G layers. The T and H dependences of the in-plane conductivity derived from ${\ensuremath{\rho}}_{c}$ are well described by a scaling function of $H{\ensuremath{\tau}}_{\ensuremath{\varepsilon}}$ at low T and low H, where ${\ensuremath{\tau}}_{\ensuremath{\varepsilon}}$ is the characteristic relaxation time for inelastic scattering and is dependent on $T({\ensuremath{\tau}}_{\ensuremath{\varepsilon}}\ensuremath{\approx}{T}^{\ensuremath{-}0.2}).$

Two-dimensional weak localization effects in high temperature superconductor Nd2−x CexCuO4−δ

A systematic study of the resistivity and Hall effect in single crystal Nd{2-x}Ce{x}CuO{4-d} films (0.12 < x < 0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x = 0.12) and in situ superconducting optimally doped (x = 0.15) or overdoped (x = 0.18) samples in a high magnetic field B > B{c2}. The phase coherence time and the effective thickness of a CuO{2} conducting layer {d} (~ 1.5 A) have been estimated by fitting 2D weak localization theory expressions to the magnetoresistivity data for magnetic fields perpendicular to the {ab} plane and in plane. Estimates of the parameter {d} ensure the condition of strong carrier confinement and justify a model consisting of almost decoupled 2D metallic sheets for the Nd{2-x}Ce{x}CuO{4-d} single crystal.

C-axis resistivity of graphite intercalation compounds

The c-axis resistivity of stage-2, 3, 4, 5, 6, 7, and 9 graphite intercalation compounds (GICs) has been measured in the temperature range between 4.2 and 300 K with and without an external magnetic field along the c-axis. In these compounds the c-axis conduction is dominated by the in-plane conduction because of the highly anisotropic resistivity. The temperature dependence of strongly depends on the stage number. The stage-2 and 3 GICs show a metallic behaviour: increasing with increasing temperature. The logarithmic behaviour in is observed in a limited temperature range for stages 5 and 6, and negative longitudinal magnetoresistance is observed for stages 3 to 7, indicating that the two-dimensional weak localization effect may occur mainly in the interior graphite layers with a small charge transfer. The c-axis resistivity of stage-3 to 9 GICs shows a unusual thermal hysteresis in the temperature range between 180 and 240 K. It has two local minima at critical temperatures - 210 K) and (= 223 - 231 K) depending on the stage number. The carriers in the bounding graphite layers with a large charge transfer are scattered by enhanced fluctuations due to electric dipole moments of molecules. There may occur a two-dimensional- (2D-) like dipole ordered phase below and a 3D-like dipole ordered phase below .

C-axis resistivity of MoCl5 graphite intercalation compounds.

The c-axis resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ of stage-2, -3, -4, -5, and -6 ${\mathrm{MoCl}}_{5}$ graphite intercalation compounds (GIC's) has been measured in the temperature range between 4.2 and 300 K with and without an external magnetic field along the c axis. The resistivity for stage-3, -4, and -5 ${\mathrm{MoCl}}_{5}$ GIC's shows a logarithmic increase (lnT) with decreasing temperature at low temperatures. The stage-4, -5, and -6 ${\mathrm{MoCl}}_{5}$ GIC's show a negative longitudinal magnetoresistance (MR), while the stage-2 and -3 ${\mathrm{MoCl}}_{5}$ GIC's show a positive longitudinal MR. Because of high anisotropy in resistivity, the c-axis conduction is dominated by the in-plane conduction. The logarithmic behavior and negative MR in ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ are discussed in terms of the two-dimensional weak localization effect occurring in the graphite basal plane. The possibility of the Kondo effect is not ruled out because of possible exchange interactions between conduction electrons and ${\mathrm{Mo}}^{5+}$ spins having an effective magnetic moment of 1.83--1.95${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. The temperature dependence of ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ varies with the stage number: metallic behavior for stage-2 and semiconductorlike behavior for stage-5 ${\mathrm{MoCl}}_{5}$ GIC. The resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ of stage-3 ${\mathrm{MoCl}}_{5}$ GIC has a local maximum around 267 K. These behaviors of ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ are also discussed in terms of conduction mechanisms including conduction paths, phonon- and impurity-assisted hopping, and small polaron effect. \textcopyright{} 1996 The American Physical Society.

Resistance of ceramic samples: 2D localization and time dependence

In this paper, we report results of giant magnetoresistance obtained in a ceramic sample. The results show that between 40 mK and 30 K, the resistance increases as log(T) when temperature decreasing, which could be associated with the two-dimensional weak localization effect. The time dependence of the resistance after changing the magnetic field was also measured at different temperatures. At temperatures higher than 2 K, the decay of resistance is associated with the decay of the magnetization. Bellow this temperature, however, the observed large variation of the resistance with time after changing the magnetic field may be associated with the reconfiguration of magnetic structure.

Two-Dimensional Weak Localization Effect in Amorphous Cu–Mg Alloys

Both electronic structure and electron transport properties in the amorphous Cu–Mg alloys have been studied, using flash-evaporated thin films and melt-spun ribbons. The Hall effect measurements revealed the validity of the free electron model in this system. The temperature dependence of the electrical resistivity of the amorphous bulk sample can be well explained in the context of the generalized Faber-Ziman theory. However, a substantial deviation from this prediction occurs at low temperatures in thin film samples, when the film thickness is reduced below 500 A. The deviation is attributed to the two-dimensional weak localization effect. The critical temperature T D , at which the upturn deviation becomes noticeable, is found to increase sharply with increasing the sheet resistance. Its value reached about 50 K for the 50 A amorphous Cu 0.4 Mg 0.6 alloy.