Below is a selection of completed projects
Enter novel quantum state via patterned strain engineering
The harnessing of interlayer coupling is crucial to creating and hosting many distinctively different electronic properties in a 2D system. In the meantime, while researchers worldwide tried to discover new possibility in twistronics, we tried to think differently: can we develop a new approach to engineer the electronic band structure of 2D materials without the requirement to carefully misalign the two lattice structures? In other words, can we artificially and easily create a superlattice or structure in which the lattice has been distorted and/or misorientated to achieve a similar goal, or even something new?
Driven by this motivation, we came up with an idea and a device design, to artificially create the parterned strain (lattice deformation) in 2D materials via nanofabrication. The research team develops new techniques to etch the surface of hexagonal boron nitride (hBN) substrates, then enabling the graphene or other 2D materials placed upon it to conform to the surface topography and be lattice deformed accordingly. With these techniques, the substrate topography can be arbitrarily defined via nanolithography with the potential to approach 2.5D and 3D patterning, thereby opening up more possibilities. The collaborated research team demonstrates the existence of two novel Hall effects at zero magnetic fields (or more precisely, without breaking time-reversal symmetry). In addition to opening a new avenue for fundamental research into quantum geometrical and topological phenomena, their approach to band engineering will also be of great help to the future applications in 2D materials and quantum electronics.
Hall effects in artificially corrugated bilayer graphene without breaking time-reversal symmetry
Sheng-Chin Ho, Ching-Hao Chang, et al., Nature Electronics 4, 116–125 (2021).
See also: Behind the paper by Sheng-Chin in Nature Portfolio Communities.
Unveiling the Exotic Zigzag Wigner Spin Chain
The elusive and exotic zigzag Wigner crystallization in one-dimensional (1D) quantum wires has finally been observed. Wigner crystal – a crystalline phase where electrons are strongly correlated and self-organize themselves in a lattice array – has proven to be challenging to observe in experiments since its prediction more than 80 years ago by Eugene Wigner. To date, it has only been observed in 2D systems; in 1D systems there have only been hints of its existence. Many non-trivial Wigner-like phases with interesting derived properties, such as the so-called zigzag Wigner crystal, have also been suspected but remained largely elusive.
Ho and colleagues design a clever and innovative setup which combines real-space charge and spin detection together with conductance measurements to demonstrate this largely elusive, exotic Wigner-like phase and its spin properties in 1D quantum wires. Such a quantum spin chain with a variety of electrically controllable spin phases in solid-state systems is not merely of fundamental interest, but also has potential technological applications, e.g. serving as a quantum mediator to couple qubits that are physically apart.
Imaging the zigzag Wigner crystal in confinement-tunable quantum wires
Sheng-Chin Ho et al., Phys. Rev. Lett. 121, 106801 (2018).
Selected as "Featured in Physics" and "Editors’ Suggestion"
See also: Synopsis by Michael Schirber in Physics
All-Electric Spin Transistor
The spin field-effect transistor envisioned by Datta and Das opens a gateway to spin information processing. Although the coherent manipulation of electron spins in semiconductors is now possible, the realization of a functional spin field-effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch, spin relaxation and the spread of spin precession angles. In this project, we present an all-electric and all-semiconductor spin field-effect transistor in which these obstacles are overcome by using two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin–orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins (spin injection, manipulation and detection) in a purely electrical manner. Such a device is compatible with large-scale integration and holds promise for future spintronic devices for information processing.
All-electric all-semiconductor spin field-effect transistor
Pojen Chuang, Sheng-Chin Ho et al., Nature Nanotechnology 10, 35 (2015).
See also: News & Views by Marc Cahay in Nature Nanotechnology 10, 21-22 (2015); Research Highlights by David Ciudad in Nature Materials 14, 134 (2015).
Electrical Control of Coherent Spin Dynamics
As the collapse of Moore's Law seems to be foreseeable, there is considerable interest in being able to control the spin dynamics because this could lead to the development of a range of spintronic devices that in principle are much faster and use less energy than their electronic counterparts. In order to implement successfully such concepts into, for example, quantum computing, it is necessary to controllably generate, manipulate, and detect spin currents by electrical means and so minimize, or eliminate, the use of ferromagnetic contacts or external magnetic fields. We are among the first to demonstrate on-chip spin injector, detector, and coherent spin control by using quantum point contacts, the simplest geometries for integrated quantum devices, through spin-orbit and/or electron-electron interactions. This on-chip spintronic technique can avoid several serious problems associated with conventional methods of spin injection – such as the impedance mismatch, which drastically limits the spin pumping efficiency of the injected current. Most importantly, it allows the devices to be used for rapid control of spin dynamics and their spatial separation and correlated phases, sailing toward the ultrahigh speed quantum information processing.
Controlled spatial separation of spins and coherent dynamics in spin-orbit-coupled nanostructures
Shun-Tsung Lo et al., Nature Communications 8, 15997 (2017).
All-electrical injection and detection of a spin-polarized current using 1D conductors
T.-M. Chen et al., Phys. Rev. Lett. 109, 177202 (2012).
Bias-controlled spin polarization in quantum wires
T.-M. Chen et al., Appl. Phys. Lett. 93, 032101 (2008).