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Register a new userExperimental Determination of the Lorenz Number in Cu0.01Bi2Te2.7Se0.3 and Bi0.88Sb0.12
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Nanostructuring has been shown to be an effective approach to reduce the lattice thermal conductivity and improve the thermoelectric figure of merit. Because the experimentally measured thermal conductivity includes contributions from both carriers and phonons, separating out the phonon contribution has been difficult and is mostly based on estimating the electronic contributions using the Wiedemann-Franz law. In this paper, an experimental method to directly measure electronic contributions to the thermal conductivity is presented and applied to Cu0.01Bi2Te2.7Se0.3, [Cu0.01Bi2Te2.7Se0.3]0.98Ni0.02, and Bi0.88Sb0.12. By measuring the thermal conductivity under magnetic field, electronic contributions to thermal conductivity can be extracted, leading to knowledge of the Lorenz number in thermoelectric materials.
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We present a double-resonant Raman mode in few-layer graphene, which is able to probe the number of graphene layers reliably. This so-called N mode on the low-frequency side of the G mode results from a double-resonant Stokes/anti-Stokes process combining a LO and a ZO phonon. Simulations of the double-resonant Raman spectra in bilayer graphene show very good agreement with the experiments. The investigation of the out-of-plane ZO phonon for layer number determination is expected to be transferable to other layered materials like boron nitride.
Mn$_2$Au is an important antiferromagnetic (AF) material for spintronics applications. Due to its very high Neel temperature of about 1500 K, some of the basic properties are difficult to explore, such as the AF susceptibility and the exchange constants. Experimental determination of these properties is further complicated in thin films by unavoidable presence of uncompensated and quasiloose spins on antisites and at interfaces. Using x-ray magnetic circular dichroism (XMCD), we have measured the spin and orbital contribution to the susceptibility in the direction perpendicular to the in-plane magnetic moments of a Mn$_2$Au(001) film and in fields up to 8 T. By performing these measurements at a low temperature of 7 K and at room temperature, we were able to separate the loose spin contribution from the susceptibility of AF coupled spins. The value of the AF exchange constant obtained with this method for a 10 nm thick Mn$_2$Au(001) film equals to (24 $pm$ 5) meV.
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