Thermoelectric Measurements of Ni Nanojunctions
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Thermoelectric Measurements of Ni Nanojunctions See Kei Lee1, Ryo Yamada1 and Hirokazu Tada1 ABSTRACT We investigated the thermoelectric voltage (TEV) of atomic contacts of nickel (Ni) by using a scanning tunneling microscope. The TEV of nanoscale junctions show fluctuation in stepwise manner. Histogram analysis of TEV observed in the Ni point contact with the conductance of 1.2 G0 (G0 = 2e2/h is the quantum of charge conductance) revealed multiple voltage peaks at larger and smaller values observed at conductance of 2.5 G0, which showed a single sharp voltage peak. Fluctuation observed in our results suggest that there is transition of the transport channel distribution caused by the thermal motion of Ni atoms. INTRODUCTION Studies on thermoelectric effects (TE) in atomic and molecular junctions have attracted much attention because these junctions can realize a narrow distribution of the energy of the electrons participating in the transport process which can contribute to maximum thermoelectric efficiency [1-2]. The efficiency of thermoelectric devices is represented as: ZT =
σ S 2T (κ e + κ l )
where σ is the electrical conductivity, S is the Seebeck coefficient, and κe and κl are the electronic and lattice part of the thermal conductivity, respectively. A greater ZT would indicates a greater efficiency of the device. Nanoscale materials have shown much promise as good thermoelectric material because of two reasons [3-6]: i) Low-dimensionality and quantum size effects could improve the Seebeck coefficient and ii) Small feature sizes enhance phonon scattering on nanoscale interfaces and reduce thermal conductivity. Application of nanoscale thermoelectric devices can be beneficial as it can be integrated into chip sets and converting the accumulated waste heat into usable electric energy [7]. Electronic transport phenomena in low-dimensional systems can be roughly divided intotwo categories: ballistic transport and diffusive transport. This two transport are realized when the wire length is shorter and longer, respectively, than the mean free path of electron. It had been discussed that a higher thermoelectric effiency value can be expected for a system showing ballistic transport [6]. Seebeck coefficient of the quantized transport system is predicted to change oscillatory as a function of the conductance [7], showing the maximum Seebeck coefficient at (n+1/2)G0. This behavior is experimentally observed in 2D electron gas system [8]. The thermoelectic characteristics of atomic junction is interesting because the junction shows ballistic transport even at room temperature. The thermorelectric behaviors of gold, silver and Cu atomic contacts were investigated by a mechanically controlled break junction [9]. The Seebeck coefficient randomly scattered around 0 at G = nG0 and small positive peaks were observed around G = (n+1/2)G0 (n; integer), which resembled with what expected by the theory.
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In this work, we observed the thermo
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