个人简介
Prof. Chumin Wang
Prof. Chumin Wang
National Autonomous University, Mexico
标题: Thermoelectric transport in segmented metallic nanowires
摘要: 
Direct conversion between thermal and electrical energies by thermoelectric devices is becoming an important alternative for the clean energy generation. Nanowires seem to be promising candidates, whose efficiency is determined by the dimensionless thermoelectric figure-of-merit (ZT) that can be calculated by using the Boltzmann formalism [1]. The inherent correlation between the thermoelectric quantities, such as electrical and thermal conductivities, makes difficult to improve the value of ZT. For example, there is a proportional relationship between these two conductivities in metals, according to the Wiedemann-Franz law, becausevalence electrons carry both electric charge and heat energy. In general, these two conductivities are relatively easy to measure but quite difficult to model at the atomic scale, sincethe multiple scattering out of thermodynamic equilibrium should be included. Moreover, for non-periodic heterostructures, the absence of reciprocal space requires new methods for their study. Nowadays, the electronic states in artificial structures is of great importance in the condensed matter physics and materials science, because they introduce many new physical properties essential for industrial applications of nanoscale devices. In general, the structural disorder of a solid can profoundly modify the localization and transport of its elementary excitations. For example, the single electronic states are all extended in periodic lattices and exponentially localized in randomly disordered systems of one and two dimensions [2].

It is well known that the metals are poor thermoelectric materials. However,segmented nanowires have a band structure by design, which combines with a properly placed chemical potential by applying a gate voltage could archive a good thermoelectric power [3,4]. In this work, we present a comparative study of the thermoelectricity in periodic and quasiperiodically segmented nanowires with macroscopic length by using a novel renormalization plus convolution method [5] developed for the Kubo-Greenwood formula,in which tight-binding and Born models are respectively used for the study of electric and lattice thermal conductivities [6]. This method combines the convolution theorem with the real-space renormalization technique, being able to address multidimensional non-periodic systems with 10^{24} atoms. Analytical solutions of the Kubo-Greenwood formula are found for the ballistic DC and AC conductivities in periodic nanowires, where quantized DC conductance steps are observed, in agreement with experimental data [7]. Finally, our results confirm the existence of a maximum ZT around electronic band edges, whose magnitude grows with the reduction of nanowire cross-section area. Moreover, we observe a clear enhancement of ZT in quasiperiodically segmented nanowires with respect to the periodic ones, mainly due to the reduction of its thermal conductivity at low temperatures, caused by the scattering of lowfrequencyacoustic phononsat the long-range quasiperiodically located interfaces. In fact, it is hard to block the transmission of such phonons since they do not feel local defects neither impurities [8]. 

This work has been supported by CONACyT-252943 and UNAM-DGAPA-IN106317. Computations were performed at Miztli of DGTIC-UNAM.

[1] T.M. Tritt (Ed.), Thermal Conductivity-Theory, Properties and Applications (Kluwer, 2004) p. 3.
[2] E. Abrahams, P.W. Anderson, D.C. Licciardello, and T.V. Ramakrishnan, Phys. Rev. Lett. 42, 673 (1979).
[3] Y. Tian, M.R. Sakr, J.M. Kinder, D. Liang, M.J. MacDonald, R.L.J. Qiu, H.-J. Gao, and X.P.A. Gao, Nano Lett.12, 6492 (2012).
[4] S.C. Andrews, M.A. Fardy, M.C. Moore, S. Aloni, M. Zhang, V. Radmilovicbd and P. Yang, Chem. Sci.2, 706 (2011).
[5] V. Sanchez and C. Wang, Phys. Rev. B70, 144207 (2004).
[6] C. Wang, F. Salazar, and V. Sanchez, Nano Lett. 8, 4205 (2008).
[7] R. de Picciotto, H.L. Stormer, L.N. Pfeiffer, K.W. Baldwin, and K.W. West, Nature411, 51 (2001).
[8] J. E. Gonzalez, V. Sanchez, and C. Wang, J. Electron. Mater. 46, 2724 (2017).
简介: 
Chumin Wang received the B.S., M.S., and Ph.D. degrees in physics from the National Autonomous University of Mexico (UNAM). He was a Postdoctoral Associate at the Department of Physics, University of California, Berkeley, from 1993 to 1994. He is currently a full professor and researcher at the Materials Research Institute, UNAM. His research interests include strongly correlated electron systems and elementary excitations in quasicrystals as well as in porous semiconductors.