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New Quantum Materials and Transport

 I. Coordinator:
Feng-Chuan Chuang (NSYSU): fchuang@mail.nsysu.edu.tw
Horng-Tay Cheng (NTHU): jeng@phys.nthu.edu.tw
 
II. Core Members:

Chao-Cheng Kaun (Academia Sinica)
Chun-Wei Pao (Academia Sinica),
Chih-Kai Yang (NCCU)
Tsan-Chuen Leung (CCU)
Hung-Chung Hsueh ( TKU)
 
III. Major Directions:

With common interests in the computational research tools, the thematic group members endeavor earnestly to collaborative frontier research projects. We focus on exploring novel quantum mechanical phenomena of new materials and transport properties. Three working groups are formed according to the topical interests:
(A) Quantum transport in mesoscopic/nanoscopic system: (Y.-C. Chen, C.-C. Kaun, and C.-W. Pao)
(B) First-principles calculations on quantum new materials: (C.-K. Yang, F.-C. Chuang, and T-C.)
(C) Excited-States and time-dependent first-principles calculations: (T-C. Leung, H.-C. Hsueh)
 
IV. Activities:

(i) Working groups and working group meetings.
(ii) Semiannual workshop.
(iii) Seminar & visiting international scholars.
 
V. Highlight of Results (2015-2016):
(a)


Prof. Feng-Chuan Chuang reported the existence of quantum spin Hall (QSH) phases III-V elements in the 2D buckled honeycomb structure, including hydrogenation on one or both sides of the films to simulate substrate effects. The band gap is found to be as large as 855 meV for the hydrogenated TlBi film, making this class of III-V materials suitable for room temperature applications. Furthermore, TlBi remains topologically nontrivial with a large band gap at various hydrogen coverages, indicating the robustness of its band topology against bonding effects of substrates. Finally, we demonstrated that hydrogenated GaBi placed on top of Si(111) exhibits the QSH phase.
Reference:
Christian P. Crisostomo, Liang-Zi Yao, Zhi-Quan Huang, Chia-Hsiu Hsu, Feng-Chuan Chuang*, Hsin Lin*, Marvin Albao, and Arun Bansil, Robust Large Gap Two-Dimensional Topological Insulators in Hydrogenated III-V Bilayers, Nano Lett. 15, 6568 (2015).

(b)

Prof. C. C. Kaun’s group studied the electron transport through a magnesium porphine molecule adsorbed on an ultrathin NaCl bilayer by using first-principles calculations based on density functional theory and nonequilibrium Green’s function formalism. The conductance of the consisted tip-vacuum-molecule-NaCl-metal junction depends on the orientation of the molecule on the insulating surface and the tip position above the molecule, which is mediated largely by the molecular pz orbital. The movement of molecule results in a perturbation to the spatial extension of these orbitals, leading to different conductions.
Reference:
K.-P. Dou, J.-S. Tai, and C.-C. Kaun*, "Conductance of a Single Magnesium Porphine Molecule on an Insulating Surface", J. Phys. Chem. C 119, 25129 (2015).

(c)

Figure (a) Schematic of the dibenzenedithiol single-molecule junction; (b) ZT of the thermoelectric nanojunction is crossing over quantum mechanical to the classical phonon transport regime and through competition between electron's and phonon's thermal conductance.
Prof. Yu-Chang Chen and Prof. Chun-Wei Pao collaborate to construct the phase diagram of the figure of merit (ZT) for the dibenzenedithiol (DBDT) single-molecule junction from the theoretical point of view. They investigate its electric conductance (σ), Seebeck coefficient (S), and electronic thermal conductance (κel) in the framework of density functional theory combined with the Lippmann-Schwinger formalism in scattering approach. They observe that the conductance is around 0.0012 G0, and the nanojunction is p-type with the value of the Seebeck coefficient at room temperature around 40 μV/A, in agreement with the results of an experiments. In addition, They also investigate the phonon's thermal conductance (kph) using (i) the weak-link model suitable for ballistic phonon quantum transport mechanism in the low-temperature regime and (ii) the non-equilibrium molecular dynamics (NEMD) simulation in the high-temperature classical regime. They found that ZT reveals the power law behavior that falls into four phases because of the competition between kel and kph and the crossover from the quantum to classical phonon transport mechanism for kph. i.e., ZT ∝ Tx where x=2, 0, 2.26, and 3 in different temperature regimes labeled by I, II, III, and IV, respectively.

 σ S kel   μV/A
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