Selected Publications

Here is a list, by no means compete, of selected research publications/exhibited artistic works/educational publications.

Wu, M. K. Wu, P. M. Chen, Y. Y. Chang, C. C. Wei, P. C. Lan, T. W. Wu, J. R. “Apparatus and method for enhancing figure of merit in composite thermoelectric materials with aerogel.” Patent 18151892.9-1211. December 3, 2018.

Wu, M. K. Wu, P. M. Chen, Y. Y. Chang, C. C. Wei, P. C. Lan, T. W. Wu, J. R. “Apparatus and method for enhancing figure of merit in composite thermoelectric materials with aerogel.” U.S.A. Patent 2018-0204992-A1 July 19, 2018.

Wu, M.K. Wu,P.M. Chen, Y.Y. Chang, C.C. Wei, P.C. Lan, T.W. Wu, J.R.“藉由凝膠增強 優值係數之復合熱電材料之裝置和方法.” R.O.C. Patent I628816 July 1, 2018.

E. Y. Ma, Y. T. Cui, K. Ueda, S. Tang, K. Chen, N. Tamura, P. M. Wu, J. Fujioka, Y. Tokura, Z. X. Shen. “Mobile metallic domain walls in an all-in-all-out magnetic insulator.” Science, 350, 538-541 (2015). Citations: 14

Among a vast array of magnetic insulators, which are distinct from ferroelectric insulators, there has been theoretical expectation but no direct experimental evidence for conduction at magnetic domain wall boundaries. Understanding this effect can provide additional insight into metal-insulator transitions, for example in antiferromagnetic materials. This paper combines bulk and local conductivity measurements to directly resolve domain walls in real space, and prove the boundary is conducting, in the all-in all-out pyrochlore iridate Nd2Ir2O7. A key feature of this work is the utilization of a scanning microwave impedance microscope (sMIM) to image the conductive domain walls. I helped measure resistive transport and Seebeck coefficient versus temperature for bulk material and interpret bulk and sMIM data.

C. H. Wang, T. K. Chen, C. C. Chang, C. H. Hsu, Y. C. Lee, M. J. Wang, P. M. Wu, M. K. Wu. “Disordered Fe vacancies and superconductivity in potassium-intercalated iron selenide (K2-xFe4+ySe5).” Europhys. Lett., 111 27004 (2015). Citations: 8

Understanding the parent phase of high Tc superconductors can help to unlock the mystery behind why Tc’s are so high in these materials. In the high Tc potassium-intercalated FeSe compounds, we developed a novel synthesis approach to grow crystals with homogenous chemical compound and crystal phase, and thus, enabled the identification of the correct parent phase in these materials. We then show that it is the Fe-vacancy order-disorder transition that is responsible for emergence of superconductivity from the parent phase. As Tc’s for the Fe-chalcogenides are now rivaling those of cuprates, we believe that this study may help direct studies to shed further light on why the cuprates are also such high temperature superconductors. I helped design experiment, interpret data, and write the paper.

P. M. Wu, S. Ishii, K. Tanabe, K. Munakata, R. H. Hammond, K. Tokiwa, T. H. Geballe, M. R. Beasley. “Synthesis and ionic liquid gating of hexagonal WO3 thin films.” Appl. Phys. Lett. 106, 042602 (2015). Citations: 6

We demonstrated growth of the hexagonal crystal phase of WO3 via thin film epitaxy techniques for the first time. WO3 is an electro-chromic material with wide ranging applications such as smart windows and hydrogen sensing. The hexagonal phase is unstable in the bulk, but may be found as small grains embedded in the bulk. This phase has previously been linked to high Tc (exceeding 90 K) superconductivity in surface doped tungsten bronzes. After stabilizing the phase in thin film form, we utilized ionic liquid gating, a novel and powerful electrostatic gating technique, to introduce charge into the film, and observed a metal-insulator transition at sufficiently high doping densities. While superconductivity is not observed, our results do not yet rule it out due to large phase space of electrostatic gating voltages, warranting further study. I helped design experiment, fabricated the thin film samples with ionic liquid gate, measured transport properties, analyzed data and wrote the paper.

H. H. Chang, C. C. Chang, Y. Y. Chiang, J. Y. Luo, P. M. Wu, C. M. Tseng, Y. C. Lee, Y. R. Wu, Y. T. Hsieh, M. H. Wen, M. J. Wang, M. K. Wu. “Growth and characterization of superconducting β-FeSe type iron chalcogenide nanowires.” Supercond. Sci. Technol. 27 (2014) 025015. Citations: 4

This study presented the successful growth of FeSe (and related family of materials such as FeSeTe and FeSeS) nanowires via a laser ablation and anneal technique. Nano-devices based on these chalcogenide nanowires were measured, and interestingly, only the original FeSe nanowire, which is superconducting in the bulk form, was found to be not superconducting, while the doped materials retained superconductivity. This study seeded additional works revealing the importance of vacancies (or defects) to the emergence of superconductivity in the chalcogenide superconductors. I helped design experiment, fabricate nanowire devices, analyze data and write the paper.

S. F. Svensson, E. A. Hoffmann, N. Nakpathomkun, P. M. Wu, H. Q. Xu, H. A. Nilsson, D. Sanchez, V. Kashcheyevs, H. Linke. “Nonlinear thermovoltage and thermocurrent in quantum dots.” New J. Phys. 15 (2013) 105011. Citations: 48

This work provides a rigorous fundamental study of nonlinear thermoelectric effects in quantum dots embedded in nanowires. The quantum dots can be formed either with in-line heterojunctions in the nanowire or via Schottky contacts to the nanowire. In the latter system of nanowires, which at sufficiently low temperature can be described by a single impurity Anderson model, evidence for thermally induced spin-charge separation is observed. This paper shows that thermal currents are a powerful way to controllably probe transport effects in nanowire quantum dot systems. I helped with CBE material synthesis, thermovoltage measurements and interpreting data.

P. M. Wu+, J. Gooth+, X. Zianni, S. F. Svensson, J. G. Gluschke, K. A. Dick, C. Thelander, K. Nielsch, H. Linke. “Large Thermoelectric Power Factor Enhancement Observed in InAs Nanowires.” Nano Letters. (2013). Citations: 46

This work provides strong experimental evidence for theoretically predicted enhanced power factor in nanowires due to quantum confinement effects. Additionally, we show experimentally for the first time how the thermovoltage (or Seebeck coefficient) increases to ~ mV/K range as the nanowire Fermi level drops below the first conduction subband. These effects and underlying root causes suggest routes forward to enhance thermoelectric performance in similar nanostructures and bulk materials. I helped design the experiment, take the thermoelectric measurements, analyze data and write the paper.

P. M. Wu, N. Anttu, H. Q. Xu, L. Samuelson, M.-E. Pistol. “Colorful InAs Nanowire Arrays: From strong to weak absorption with geometrical tuning.” Nano Letters. (2012). Citations: 64

This paper demonstrates both experimentally and theoretically how the optically visible color of III-V nanowires can be tuned by simply modifying nanowire and/or array geometrical parameters. The ability to tune absorption in these sparse arrays bodes well for concentrated III-V nanowire photovoltaics and photodetection. I co-designed experiment, fabricated nanowire array, helped measure reflectance and co-wrote the paper.

P. M. Wu, P. Li, H. Zhang, A.M. Chang. “Evidence for the Formation of Quasi-Bound-State in an Asymmetrical Quantum Point Contact.” Phys. Rev. B. 85, 085305 (2012). Citations: 18

We show that careful control of the confinement potential of electrons in two-dimensional AlGaAs-GaAs high mobility electron systems enables a study of rich electron-electron interaction physics. The strongly asymmetric design of our devices leads to momentum mismatched entrance and exits for the 1D electron channel and helps to stabilize a quasi-bound state. Our transport results hint at unusual spin and correlated electron states, with one tantalizing possibility a zigzag 1D Wigner crystal state, in the point contacts. I designed experiment, fabricated the quantum point contact samples, performed measurements, analyzed data and wrote the paper.

P. Li, P. M. Wu, Y. Bomze, I. V. Borzenets, G. Finkelstein, A. M. Chang. “Switching Currents Limited by Single Phase Slips in One-Dimensional Superconducting Al Nanowires.” Phys. Rev. Lett. 107, 137004 (2011). Citations: 38

In this work we detail how extremely thin Al (aluminium) nanowires are switched from the superconducting state to the normal state by current induced transitions and fluctuations. Due to the very narrow width of the Al superconductor, a single phase slip, which is a topological event where the superconducting order parameter shifts by 2π, triggers the switch. At low temperatures, our results provide the strongest experimental evidence for a previously predicted effect, that the phase slips by macroscopic quantum tunnelling. The extremely thin width of the Al nanowire was a technical feat achieved by depositing Al on the edge of an atomically sharp InP (Indium Phosphide) ridge. I helped take resistive transport measurements, interpret analysis results and write the paper.

F. C. Hsu, J. Y. Luo, K. W. Yeh, T. K. Chen, T. W. Huang, P. M. Wu, Y. C. Lee, Y. L. Huang, Y. Y. Chu, D. C. Yan, and M. K. Wu. “Superconductivity in the PbO-type structure α-FeSe.” Proc. Natl. Acad. Sci. 105, 14262-14264 (2008). Citations: 1830

This is the first report of superconductivity at Tc~8-10 K in the tetragonal FeSe structure. Along with a subsequent report on Te substitution by our team (Yeh et al. EPL 84, 37002 (2008), Citations: 484), this work initiated studies into the Fe-chalcogenide based superconductors. I performed resistive transport measurements, primarily focusing on the Hall effect in bulk crystals, analyzed data and co-wrote the paper.