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Register a new userTemperature Dependence of the Electrical Transport Properties of Multilayer Graphene
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Multilayer graphene (MLG) thin films are deposited on silicon oxide substrates by mechanical exfoliation (or scotch-tape method) from Kish graphite. The thickness and number of layers are determined from both Atomic Force Microscopy (AFM) and Raman Spectroscopy. Electrical terminals are deposited on MLGs in a four-probe configuration by electron-beam lithography, gold/titanium thermal evaporation, and lift-off. The electrical resistance is measured from room temperature down to 2 K. The electrical resistance of the MLGs shows an increase with decreasing temperature, and then decreases after reaching a maximum value. These results are compared with recent experimental and theoretical data from the literature.
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We apply the semi-classical quantum Boltzmann formalism for the computation of transport properties to multilayer graphene. We compute the electrical conductivity as well as the thermal conductivity and thermopower for Bernal-stacked multilayers with an even number of layers. We show that the window for hydrodynamic transport in multilayer graphene is similar to the case of bilayer graphene. We introduce a simple hydrodynamic model which we dub the multi-fluid model and which can be used to reproduce the results for the electrical conductivity and thermopower from the quantum Boltzmann equation.
Assuming diffusive carrier transport and employing an effective medium theory, we calculate the temperature dependence of bilayer graphene conductivity due to Fermi-surface broadening as a function of carrier density. We find that the temperature dependence of the conductivity depends strongly on the amount of disorder. In the regime relevant to most experiments, the conductivity is a function of T/T*, where T* is the characteristic temperature set by disorder. We demonstrate that experimental data taken from various groups collapse onto a theoretically predicted scaling function.
Resistivity and specific heat measurements were performed in the low carrier unconventional superconductor URu2Si2 on various samples with very different qualities. The superconducting transition temperature (TSC) and the hidden order transition temperature (THO) of these crystals were evaluated as a function of the residual resistivity ratio (RRR). In high quality single crystals the resistivity does not seem to follow a T2 dependence above TSC, indicating that the Fermi liquid regime is restricted to low temperatures. However, an analysis of the isothermal longitudinal magnetoresistivity points out that the T2 dependence may be spoiled by residual inhomogeneous superconducting contribution. We discuss a possible scenario concerning the distribution of TSC related with the fact that the hidden order phase is very sensitive to the pressure inhomogeneity.
The finite-temperature transport properties of FeRh compounds are investigated by first-principles Density Functional Theory-based calculations. The focus is on the behavior of the longitudinal resistivity with rising temperature, which exhibits an abrupt decrease at the metamagnetic transition point, $T = T_m$ between ferro- and antiferromagnetic phases. A detailed electronic structure investigation for $T geq 0$ K explains this feature and demonstrates the important role of (i) the difference of the electronic structure at the Fermi level between the two magnetically ordered states and (ii) the different degree of thermally induced magnetic disorder in the vicinity of $T_m$, giving different contributions to the resistivity. To support these conclusions, we also describe the temperature dependence of the spin-orbit induced anomalous Hall resistivity and Gilbert damping parameter. For the various response quantities considered the impact of thermal lattice vibrations and spin fluctuations on their temperature dependence is investigated in detail. Comparison with corresponding experimental data finds in general a very good agreement.
We report on the fabrication of field-effect transistors based on single and bilayers of the semiconductor WS2 and the investigation of their electronic transport properties. We find that the doping level strongly depends on the device environment and that long in-situ annealing drastically improves the contact transparency allowing four-terminal measurements to be performed and the pristine properties of the material to be recovered. Our devices show n-type behavior with high room-temperature on/off current ratio of ~106. They show clear metallic behavior at high charge carrier densities and mobilities as high as ~140 cm2/Vs at low temperatures (above 300 cm2/Vs in the case of bi-layers). In the insulating regime, the devices exhibit variable-range hopping, with a localization length of about 2 nm that starts to increase as the Fermi level enters the conduction band. The promising electronic properties of WS2, comparable to those of single-layer MoS2 and WSe2, together with its strong spin-orbit coupling, make it interesting for future applications in electronic, optical and valleytronic devices.
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