Carrier Mean Free Path Research Articles

Scaling behavior of resistivity tails at the edge of superconducting transition in Bi 2Sr 2CaCu 2O 8+δ epitaxial films

The behavior of the resistivity just below the mean-field transition temperature has been investigated in Bi 2Sr 2CaCu 2O 8+δ epitaxial films with various oxygen contents. Analysis of the temperature dependence of the resistivity was performed in terms of the Kosterlitz-Thouless charge-unbiding theory and the Ginzburg-Landau Coulomb-gas model for vortex fluctuations. A crossover from two- to three-dimensional vortex configuration was observed as the temperature approaches the Kosterlitz-Thouless transition temperature from above. The carrier mean free path and the effective interplane coupling as well as their dependences on the oxygen content were evaluated.

Spin dynamics study in polyaniline: macroscopic and microscopic transport property relationship

Proton spin lattice relaxation times T 1 have been measured in the frequency range 7–95 MHz on a set of unstretched and stretched polyaniline films, together with d.c. conductivity. A highly anisotropic quasi 1D spin diffusion is evidenced in all the samples. It is shown that while stretching increases parallel d.c. conductivity, it favours interchain hoppings at the microscopic scale. These results are interpreted in a coherent picture that links the macroscopic transport properties to the microscopic ones. In particular, d.c. conductivity anisotropy is shown to be of geometrical nature when the microscopic transport anisotropy is very high. It is also concluded that the carrier mean free path is not consistent with metallic conductivity.

Analysis of an electro-optical switching device due to intrinsic photoconductivity

We analyse an electro-optical device consisting of a stack of undoped alternating layers of narrow- and wide-gap materials (e.g. GaAs/AlGaAs) together with a series resistor under constant voltage bias. The device depends crucially on the photoconductivity of the narrow-gap layers with a width of the order of a carrier mean free path. The Franz-Keldysh effect, the charge separation of photoexcited carriers in a static field, and the ballistic part of the current are responsible for the resulting very large electro-optical nonlinearity. In a Fabry-Perot cavity a room temperature electro-optical bistability is obtained at light intensities smaller than 10 mW cm-2.

Direct measurement of spin dependent mean free paths in metals (abstract)

We describe results obtained using a new spin valve structure that is specifically designed to measure the spin up and spin down mean free paths in ferromagnetic metals. We report how these mean free paths can be measured more directly and with greater accuracy than previous methods, which were based mostly on indirect evidence from magnetoresistance in ternary alloys. The technique is based on the ‘‘backed’’ spin valve layered structure: substrate/seed/80 Å FeMn/50 Å NiFe/23 Å Cu/20 Å NiFe/t Å b/50 Å Ta where the back layer of material b with thickness t is probed by the rest of the structure, which forms a spin polarized conduction electron source. As t is varied the majority carrier mean free path λ+b in the layer b is obtained directly from the form taken by the change in film conductance between parallel and antiparallel magnetization states, ΔG, whose solution of the Boltzmann transport equation shows is well approximated by the form ΔG=ΔG0+ΔGb{1−exp([t−tx]/βλ+b)}, where ΔG0 arises from the 20 Å NiFe layer, tx is the high resistivity region of intermixing at the b layer interfaces, and β≂1 from observations with b=Cu. The minority carrier mean free path in a ferromagnetic layer b, λb, is obtained by comparing ΔG and λ+b with ΔG′, and λb′+, of a nonmagnetic b′ layer of similar resistivity to b; it is a less direct measurement than that of λ+ since it relies on the connection between conductivity and mean free path for the minority subband. We have obtained room temperature results for Ni80Fe20 (λ+=46±3 Å, 0<λ<6 Å), Fe (λ+=15±2 Å, λ=21±5 Å), and Co (λ+=55±4 Å, 0<λ<8 Å). The connection of spin dependent conductivity and mean free paths in ferromagnetic metals is crucial in exploring the mechanism of giant magnetoresistance and, more broadly, is central in all theories of transport in magnetic metals; this new technique should prove a powerful tool in measuring these fundamental quantities.

Size and temperature dependence of electrical transport properties of vacuum evaporated Bi2Se3 films

The Bi2Se3 films of thicknesses in the range 50 to 546 nm are vacuum deposited on glass substrates at room temperature. After annealing for 1 h at 423 K, the electrical resistivity, Hall coefficient, and the thermoelectric power are measured. It is observed that the resistivity is strongly affected by both the film thickness and temperature, and the activation energy (ΔE) is found to vary from 0.116 to 0.279 eV with the decrease in film thickness. The Hall coefficient (RH) as well as the mobility increase with increasing film thickness. Also, the thermoelectric power varies in a similar manner. It is found that the deposits were all of n-type and the carrier concentrations (n) measured by different methods have the order of 1020 cm−3. The analysis combining the data from thermoelectric power and electrical conductivity measurements, by using the effective mean free path theory of size effect in thin films developed by Pichard et al. leads to the determination of mean free path, carrier concentration, Fermi energy, and the parameter Ug = (d ln 1g/d ln E), where Ug is a parameter determining the energy dependence of the mean free path of carriers in infinitely thick films.

Relationship between thermal diffusivity and mean free path

The thermal diffusivity is a parameter that determines the speed and attenuation of a thermal wave propagating through a material. In this paper we show, using the kinetic model for thermal transport, that the thermal diffusivity is also proportional to the mean free path of the heat carriers. Thus, the thermal diffusivity has the added feature of being directly related to an important transport property of the excitations that carry the heat. Thermal diffusivity and mean free path data are presented for an amorphous insulating solid, Pyrex 7740 glass, in the temperature range from 25 to 300 K.

Evidence for mean free path fluctuation induced pinning in YBa2Cu3O7 and YBa2Cu4O8 films.

The critical current ${\mathit{j}}_{\mathit{c}}$ and the pinning energy ${\mathit{U}}_{\mathit{c}}$ have been determined for three types of yttrium-based superconducting films from current ${\mathit{j}}_{\mathit{s}}$ and dynamic relaxation Q=d ln${\mathit{j}}_{\mathit{s}}$/d ln(dB/dt) data by means of the generalized inversion scheme. For B2 T and T80 K the temperature dependence of ${\mathit{j}}_{\mathit{c}}$ and ${\mathit{U}}_{\mathit{c}}$ for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of ${\mathit{T}}_{\mathit{c}}$ is not observed.

Analysis of the dc conductivity of the quasi-one-dimensional charge-density-wave conductor (fluoranthene)2X.

It has been shown recently that the dc conductivity (along the highly conducting axis) of fluoranthene radical cation salts exhibits the behavior of a quasi-one-dimensional conductor with a Peierls transition at about 180 K to a charge-density-wave ground state. In the high-temperature range fluctuations of the order parameter lead to a pseudogap in the electronic density of states, whereas below the phase transition the energy gap is BCS-like. To analyze the conductivity data, a simple band-structure model is used and related to optical data. In spite of the occurrence of the Peierls transition, it is assumed that three-dimensional effects and fluctuations are strong enough for the conduction to be essentially due to electron-hole transport in bands and not polaronlike. Therefore the conductivity is determined simply by the Boltzmann equation with deformation-potential scattering. (a) This model yields a good fit of experimental data using the BCS-like gap below the transition and the pseudogap as determined from paramagnetic spin susceptibility above the transition. (b) It is possible to obtain the temperature dependence of the concentration, the mobility, the relaxation time, and the mean free path of carriers. (c) One can check a fundamental criterion for the applicability of the model. (d) We obtain from the fit parameters reasonable estimates of the quantities determining the deformation-potential scattering. Moreover, it is demonstrated that the theory can be used to determine the full temperature dependence of the gap/pseudogap directly from conductivity data.

Temperature dependence of the anomalous leakage current in polysilicon-on-insulator MOSFET's

A new analytical model is presented for the temperature and bias dependence of the anomalous leakage current based on thermionic field emission via grain boundary traps in the gate-drain overlap region in polysilicon-on-insulator MOSFET's. The existing model based on pure field emission (tunneling) via grain boundary traps does not include a temperature dependence and therefore cannot explain the observed strong temperature dependence of leakage at low gate voltages, as well as the weaker temperature dependence at high gate voltages, which the new analytical model presented in this paper can. Below 150 K, we believe that impact ionization due to the increasing carrier mean free path leads to the observed increase in the leakage current with decreasing temperature. Since the analytical model does not include impact ionization, it cannot model the leakage current at low temperatures. >

Hall effect near the mobility edge

Perturbative renormalization-group procedure has been employed to discuss the behavior of weak-field Hall conductivity near the mobility edge in amorphous semiconductors. It is found that the anomalous sign can occur for carrier mean free paths less than a certain critical magnitude. Under limited, but presumably pratical conditions, the conductivity and Hall mobilities are linked by a simple relation, which may lead to reassessment of the value of the Hall measurements.

Study of band-edge parameters in a-Si:H alloys by polarized electroabsorption effects

Polarized electroabsorption method has been used to evaluate the carrier mean free path as well as mobility near the conduction and valence band edges in hydrogenated amorphous silicon alloys. The results show that the electron mobility reaches about 13 cm2/Vs for device quality undoped a-Si:H, while the hole mobility is about 15% of that for electron. Alloying with Ge or C leads to a continuous reduction of mobilities with the largest drop for (Ge,C) concentrations above 10%.

Transport and boundary scattering in confined geometries: Analytical results.

We utilize a geometric argument to determine the effects of boundary scattering on the carrier mean-free path in samples of various cross sections. Analytic expressions for samples with rectangular and circular cross sections are obtained. We also outline a method for incorporating these results into calculations of the thermal conductivity.

Effective carrier mean-free path in confined geometries

The concept of exchange length is used to determine the effects of boundary scattering on transport in samples of circular and rectangular cross section. Analytical expressions are presented for an effective mean free path for transport in the axial direction. The relationship to the phonon thermal conductivity is discussed.

80–111 GHz quasi-optical measurement of the complex conductivities of YBa2Cu3O7 superconducting thin films

Complex conductivities of YBa2Cu3O7 high-temperature superconducting thin films were measured from 80 to 111 GHz using quasi-optical techniques. From transmission measurements, a peak was obtained in σ1 versus temperature. Measurements of the frequency dependence of this peak position found it shifted to higher temperatures with increasing frequency. The data for both σ1 and σ2, as a function of frequency, were fitted using a BCS/effective medium model. This fit indicates that the observed peak in σ1 is a result of the temperature dependence of the carrier mean free path combined with effects resulting from the granularity of the films.

110 GHz Si MSM photodetectors

Summary form only given. The authors report an Si MSMPD (metal-semiconductor-metal photodetector) with a 3.7-ps response time and 110-GHz bandwidth. They also present an experimental and theoretical study of the factors that are important to the speed of Si MSMPDs, such as the photon absorption length, carrier diffusion length, carrier mean free path, finger spacing, and finger width. They also discuss new possibilities for even faster ( approximately 400 GHz) Si-based high-speed MSMPDs. >

Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors

We present a detailed and thorough study of a wide variety of quantum well infrared photodetectors (QWIPs), which were chosen to have large differences in their optical and transport properties. Both n- and p-doped QWIPs, as well as intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasicontinuum, and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot carrier mean free path, net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity. These results allow a better understanding of the optical and transport physics and thus a better optimization of the QWIP performance.

Diffusion in a short base

The usual diffusion-equation description of transport in the base of a junction transistor breaks down if the base length is comparable to the minority carrier mean free path. We present a rigorous analytic treatment of this problem, based on an exact solution of the Boltzmann transport equation (BTE). A key ingredient of our approach is formulation of the boundary conditions for the distribution function f( r, k, t) at the base-emitter interface. Numerical solution of the BTe shows that there are significant corrections (of order 100%) to the diffusion-equation estimates of both the static current gain and the frequency cut-off in a short-base bipolar junction transistor (BJT). In a model where the electron scattering mean free path and the recombination length are both assumed independent of energy, the steady-state BTE reduces to an integral equation for the electron concentration. We present an approximate analytical solution of this equation that gives an excellent agreement with the exact numerical solution. The analytic solution is asymptotically exact in the limit of ultra-short base lengths, where the minority-carrier transport can be regarded as thermionic, as well as in the long-base limit (where the diffusion equation is rigorously valid). On the basis of our analytic solution, we propose and test a new expression for the effective (concentration-dependent) diffusivity, that interpolates between the diffusive and thermionic limits.

Magnetic penetration depth of kappa -(BEDT-TTF)2Cu

We present results of the dc magnetization of single-crystalline \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu[N(CN${)}_{2}$]Br. From the slope of the linear M(lnH) dependence of the reversible magnetization, the magnetic penetration depth \ensuremath{\lambda}(T) is estimated. At low temperatures, \ensuremath{\lambda}(T) shows only a weak T dependence, consistent with conventional pairing. For the in-plane London penetration depth we find ${\ensuremath{\lambda}}_{\mathit{L}}$(0)=(650\ifmmode\pm\else\textpm\fi{}20) nm. Small deviations from a pure BCS behavior indicate some tendency towards intermediate or strong-coupling and/or a somewhat reduced carrier mean free path.

Far-infrared hopping conductivity in the CuO chains of a single-domain YBa2Cu3O7- delta crystal.

We report on the observation of far-infrared hopping conductivity in the CuO chains of a single-domain YBa${}_{2}$Cu${}_{3}$O${}_{7}$${}_{\mathrm{\ensuremath{-}}}$${}_{\mathrm{\ensuremath{\delta}}}$ crystal. Our results suggest that at low temperatures the mean free path of carriers in the chains is mainly determined by scattering at oxygen vacancies, while at higher temperatures scattering at phonons occurs additionally.

On the characteristic lengths of the highTcoxide superconductors

Abstract We discuss the fundamental characteristic lengths, namely ζ0 the zero temperature coherence length, ΛL the London penetration depth, and l the carrier mean free path, for the oxide high Tc superconductors in relation to the latest experimental measurements. We examine theoretically why ζ0 is so small and argue that its smallness is independent of the nature of the pairing mechanism of superconductivity. We also compare various measurements from the point of view of Pippard's theory for both the normal and the superconducting states respectively. Several questions are posed by these interpretations of the experimental data.