An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach

The Landauer approach provides a conceptually simple way to calculate the intrinsic thermoelectric (TE) parameters of materials from the ballistic to the diffusive transport regime. This method relies on the calculation of the number of propagating modes and the scattering rate for each mode. The modes are calculated from the energy dispersion (E(k)) of the materials which require heavy computation and often supply energy relation on sparse momentum (k) grids. Here an efficient method to calculate the distribution of modes (DOM) from a given E(k) relationship is presented. The main features of this algorithm are, (i) its ability to work on sparse dispersion data, and (ii) creation of an energy grid for the DOM that is almost independent of the dispersion data therefore allowing for efficient and fast calculation of TE parameters. The inclusion of scattering effects is also straight forward. The effect of k-grid sparsity on the compute time for DOM and on the sensitivity of the calculated TE results are provided. The algorithm calculates the TE parameters within 5% accuracy when the K-grid sparsity is increased up to 60% for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation is strongly influenced by the transverse K density (K perpendicular to transport direction) but is almost independent of the transport K density (along the transport direction). The DOM and TE results from the algorithm are bench-marked with, (i) analytical calculations for parabolic bands, and (ii) realistic electronic and phonon results for $Bi_{2}Te_{3}$.Comment: 16 Figures, 3 Tables, submitted to Journal of Computational electronic

Equilibrium domain structure in a ferromagnetic film coated by a superconducting film

International audienceno abstrac

Equilibrium domain structure in a ferromagnetic film coated by a superconducting film

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Equilibrium domain structure in a ferromagnetic film coated by a superconducting film

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Capturing the Cumulative Effect in the Pump Probe Transient Thermoreflectance Technique using Network Identification by Deconvolution Method

International audienceNetwork Identification by Deconvolution (NID) method is used to capture the heat cumulative effect in the homodyne configuration of the Pump-Probe Transient Thermoreflectance (PPTTR) experiment. This cumulative effect is very important in the interpretation of the PPTTR which is becoming widely used for the extraction of thin film thermal conductivity. We show that this intrinsic behavior can be introduced as a cumulative effect weight function in the time constant spectrum of the structure under study. We show how the main features of this weight function change when we change the laser repetition rate and/or the laser pump beam modulation frequency, and how these changes may affect the extraction of the thermal properties of the sample under study, particularly the thermal conductivity and the interface thermal resistance. Numerical simulations of the PPTTR experiment are used to validate the application of NID method. Limitations of the method will also be discussed

Thermal and thermomechanical study of micro-refrigerators on a chip based on semiconductor heterostructures

International audienceWe present results from optical characterization of active solid-state SiGe/Si thermionic micro coolers with sizes ranging from 40/spl times/40 up to 100/spl times/100 micron square. These devices have achieved 7K cooling at 100/spl deg/C ambient temperature. These micro refrigerators can be used to remove hot spots in IC chips and achieve localized temperature control. Transient thermoreflectance measurements have shown that the cooling speed of these thin film coolers is on the order of 20-30 microseconds, 10/sup 4/ times faster than the commercial Bi/sub 2/Te/sub 3/ thermoelectric coolers. We characterized several micro-refrigerators devices by various optical non-contact methods such as interferometry or thermoreflectance. Maximum surface temperature and displacement was measured for a variety of devices sizes. The contribution of Peltier/thermoionic effect at interfaces and Joule heating inside the structure were separated by studying their different current dependence. Cooling is proportional to the current while Joule heating is proportional to the square of the current. We found that these two terms have different device size area dependence. This was explained by the fact that cooling occurs on top of the device and thus the cooling temperature is proportional to the sum of the device and substrate thermal resistances while the temperature rise due to Joule heating is only proportional to the substrate thermal resistance. This shows that the dominant source of heat is in the buffer layer below the device or in the substrate itself