Average Hopping Length Research Articles

Conduction through Localized States in a Single Crystal of the TlGa[sub 0.5]Fe[sub 0.5]Se[sub 2] Alloy

Layered single crystals of the TlGa0.5Fe0.5Se2 alloy in a dc electric field at temperatures ranging from 128 to 178 K are found to possess variable-range-hopping conduction along natural crystal layers through states localized in the vicinity of the Fermi level. The parameters characterizing the electrical conduction in the TlGa0.5Fe0.5Se2 crystals are estimated as follows: the density of states near the Fermi level NF = 2.8 × 1017 eV−1 cm−3, the spread in energy of these states ΔE = 0.13 eV, the average hopping length Rav = 233 A, and the concentration of deep-lying traps N t = 3.6 × 1016 cm−3.

Microscopic analysis of low-frequency flux noise in YBa2Cu3O7 direct current superconducting quantum interference devices

We use low-temperature scanning electron microscopy combined with superconducting quantum interference device (SQUID) detection of magnetic flux to image vortices and to investigate low-frequency flux noise in YBa2Cu3O7 thin film SQUIDs. The low-frequency flux noise shows a nonlinear increase with magnetic cooling field up to 60μT. This effect is explained by the surface potential barrier at the SQUID hole. By correlating flux noise data with the spatial distribution of vortices, we obtain information on spatial fluctuations of vortices on a microscopic scale, e.g., an average vortex hopping length of ∼10nm.

Single-photon detection mechanism in a quantum dot transistor

We study the transport mechanisms in a quantum dot MODFET by tuning the localization induced by charge stored on the quantum dots with light. The temperature dependence of the resistivity of a macroscopic sample reveals a hopping transport when the dots contain an excess of electrons. The resistance of a mesoscopic sample however, which is capable of detecting single photons, exhibits a much weaker dependence upon temperature. This points towards source-drain tunnelling as a transport mechanism and is confirmed by a statistical analysis of the single-photon-induced conductance steps. The complexity of the conducting paths increases as the average hopping length reduces.

Charge transfer in TlFeS2 and TlFeSe2

The temperature dependences of the conductivity and the thermoelectric coefficient in TlFeS2 and TlFeSe2 samples have been investigated in the temperature range 85–400 K. The variable-range hopping conduction has been established. It is found that the density of localized states NF near the Fermi level is 1.7×1018 and 3.3×1018 eV−1 cm−3, and the average hopping length R is 109 and 104 A for TlFeS2 and TlFeSe2, respectively. The non-Arrhenius (activationless) behavior of the hopping conductivity is established in the temperature region T<200 K for TlFeS2 and T<250 K for TlFeSe2.

Electrical resistance in sheets composed of multiwalled carbon nanotubes

We report growth of sheets (7–92 μm in thickness) composed of multiwalled carbon nanotubes (MCNs) and an experimental study of d.c. electrical resistance R( T) in the temperature range 7< T<300 K. The exp[( T 0/ T) −1/4] temperature dependence below ∼50 K, which is Mott's law of three-dimensional variable range hopping (3D VRH) conduction, is found. The result is supported by the estimated average hopping length, δ=0.16 μm, much smaller than our sample thickness. The temperature dependence of electrical resistance below 100 K normalized to their 100 K value, R( T)/ R(100 K), is in good agreement with that for an individual MCN. This agreement suggests that the transport mechanism for our sheets and for the individual MCN is the same.

AC Conductivity and dielectric anomaly in annealed EuSe

AbstractThe complex permittivity of annealed n‐type EuSe is measured in the temperature range T = 4.2 to 295 K for frequencies 103 to 2.4 × 1010 Hz. Using further the results of the crystallographic characterization and the dc conductivity σdc, Hall effect, and optical transmittance measurements one finds that shallow donors associated with Se vacancies and a decrease in their concentration due to annealing can explain the T and annealing dependences of σdc and σac. At T = 295 K where σdc is high, no frequency dispersion of σac is found even at microwave frequencies. When σdc decreases drastically at low temperatures a strong frequency dependence of σac is seen. It follows the relation for ac impurity hopping conduction proposed by Pollak: σac ⋐ ωs with s &lt; 1 at low frequencies and 1 &lt; s ≦ 2 at high frequencies. A maximum hopping frequency τ−1 = 4.0 × 109 s−1 and an average hopping length R = 9 Å at low temperatures is estimated. The annealing dependence of the dielectric constant at low temperatures can be explained by the polarizability αd of neutral Se vacancies. From the measured αd = 3.2 × 106 Å3 the carrier effective mass m* = 0.28 is estimated.

High Frequency Conductivity of EuSe and EuO

The a.c. conductivity of EuSe and EuO has been measured in the frequency range 103-2.3 × 1010 Hz for the temperatures 4.2-293 K. In both materials the a.c. conductivity is proportional to ωn at temperatures where the d.c. conductivity is low with n = 0.5-0.8 at low frequencies, f ⩽ 106 Hz and n = 1.85-2 at higher frequencies. The results indicate that at these temperatures the frequency dependence of the a.c. conductivity may saturate at microwave frequencies. At temperatures where the d.c.-conductivity is high the frequency dependence of the conductivity is small or may even disappear. This and the relatively high values of carrier concentration and mobility in EuSe at room temperature indicate that the band conduction dominates at high temperatures. Consideration of different models for the frequency dependent conductivity leads to thermally activated hopping as the most likely process at low and high frequencies. Interpreting the high frequency data with a hopping model we have estimated the maximum hopping frequency and the average hopping length in EuSe. Both in EuSe and EuO the strong positive frequency dispersion at temperatures where the d.c. conductivity is low supports the idea of carrier localization.