philwu

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Research


III/V Nanowire Growth and Electrical, Thermal and Optical Properties

My research at Lund and the nmC@LU consists of epitaxial growth of III/V nanowires and characterizing the nanowire electrical, thermal and optical properties.

Here is an example of some neat nanowire growth. A vision and one of the next challenges in nanowire electronics is the 3D integration of nanowire building blocks. Here are images showing that capillary forces associated with a liquid-air meniscus between two nanowires provides a simple, controllable technique to bend vertical nanowires into designed, interconnected assemblies.

I have also investigated the optical response of arrays of nanowires. The nanowires are arranged in a square lattice, with different arrangements of nanowire to nanowire spacing from 300 nm to 900 nm. This size coincides with the wavelengths of the human visible light spectrum. Understanding how light interferes and interacts with these arrays will give insights to building better photodetectors and photovoltaic solar cells. The colors shown are the actual visible color taken from an optical microscope!

Colorful InAs Nanowire Arrays


Links: Lund Physics; nmC@LU 

Low Dimensional Transport in Quantum Dots and Wires
My research at Duke focused on several interesting questions, with an overarching concern being, “How does charge flow at the nanoscale?” Modern lithography techniques allow the fabrication of test systems with dimensions on the order of a hundred nanometers, which becomes comparable in size to the Fermi wavelength, or the spread of the electron’s “waviness.” For systems this small, non-classical quantum effects could dominate the electronic transport behavior. This is clearly seen in the Coulomb blockade of semiconducting quantum dots and coherent electron focusing in quantum point contacts. Low dimensional effects were also observed in one-dimensional superconducting nanowires. 

Quantum point contacts are constrictions (1D wires) that can be tuned to allow electrical transport of a single or multiple quantized spin degenerate conductance modes. The transport manifests as plateaus at specific values in the conductance as the size of the constriction is tuned and can largely be described by a model of non-interacting electrons. However, below the last plateau or in the single mode limit, unusual conductance features are observed and cannot be explained within a non-interacting electron framework. In particular, in asymmetric point contact geometries, I observed unusual resonances below the single mode limit and in certain configurations, a complete destruction of the last plateau. We ascribe such features to quasibound state formation from momentum mismatch in the asymmetrical devices and have found additional evidence for strong interaction effects in this regime. Although not fully understood, these observations may be related to incipient lattice formation (Wigner crystallization) in the channel.

An issue at the heart of 1D superconductivity is the behavior of the current induced superconducting to normal state transition. Superconducting nanowires are ideally better suited to reach the depairing limit than bulk counterparts, but in practice, phase slippage or switching events prevent reaching the maximum possible critical current. A key finding in our work is that single phase slips can destroy the superconducting state, and in the thinnest superconducting nanowires, the low temperature phase slip arises from macroscopic quantum tunneling.

The image on the left shows a double quantum dot, with the dark areas the substrate surface and the light features the top metallic gates. A 2D electron gas resides beneath the surface. The dots are formed in the underlying 2DEG by applying voltages to the gates to field effect deplete the 2DEG regions beneath the metallic gates.


Iron Based Superconductivity

In addition to the research on low dimensional transport, I am participating in the study of the new Fe based superconductors. This work is in collaboration with the Superconductivity Lab under M.K. Wu at the Institute of Physics, Academia Sinica, Taiwan. The layered FeAs based superconductors have Tc as high as 55 K, making them the only non-cuprate high temperature superconductors known to date. Our collaboration discovered the superconducting β-FeSe compound. Although Tc is ~ 8 K, the crystal lattice structure of β-FeSe is by far the simplest of the Fe superconductors, as it consists of only Fe and Se arranged in a layered tetrahedral conformation (shown on the right). Se also has much less stringent safety requirements in the laboratory compared to the highly toxic As. Subsequent studies have found Te doping and high pressure can significantly enhance Tc. This strongly suggests that the structural conformation of the system plays an important role in the emergent superconductivity. Understanding the structural phase transition in β-FeSe may elucidate how superconductivity arises in the Fe-based materials, and give key insights in the search for new high Tc superconductors.