Research

The size and dimension of a material has profound effects on its electronic, optical, thermal and mechanical properties. When one or more of the dimensions of a material is reduced to atomic scale, quantum mechanical efforts becomes important. The many-body interactions become more prominent and often dominate over the, giving rise to many correlated phases such as ferromagnetism, antiferromagnetism, superconductivity, charge density waves,  Wigner crystals, etc. Stacking atomically thin layers together provides another route of controlling, manipulating and tailoring the properties of materials via proximity effect and twistronics, leading to designer quantum materials .

Particularly intriguing is the interplay between topological phases and broken symmetries. On the one hand, these novel phases may be topological and protected by various symmetries, such as crystal symmetries and time reversal symmetry. They often manifest in electronic transport studies as quantized conductance that is stable against scattering from disorders. On the other hand, these symmetries may be broken, either spontaneously (e.g. by electronic interactions in the ultra-low density regime) or by external electric and magnetic fields, giving rise to quantum phase transitions that are tunable by external parameters. The rich interplay between topological phases and broken symmetries is at the forefront of condensed matter and materials research, with implications on our understanding of the fundamental processes and phenomena in quantum systems, as well as potential applications such as quantum information science.

 

We are currently studying

Graphene

Consider the following material: it is softer than silk yet stronger than steel; it is transparent but conducts heat and electricity 10-100 times better than copper and silicon; it is produced by school kids every day, yet its discoverers won the Nobel Prize in 2010. This material is graphene, a two-dimensional (2D) a honey-comb lattice of carbon atoms. exhibits unusual energy dispersion relations – the low-lying electrons in single layer graphene behave like massless relativistic Dirac fermions with vanishing density of states at the Dirac point, and a bilayer’s band structure resembles a zero band gap semiconductor. Since recent experimental isolation and measurement of graphene, it has attracted tremendous attention, as the special band structures in single and bi-layer graphenes yield novel aspects to the physics of two-dimensional electron systems. The Dirac spectrum in graphene is predicted to give rise to a number of phenomena, such as quantum spin hall effects, enhanced Coulomb interaction, and suppression of weak localization. Technologically, graphene is an attractive material for nanoscale electronics engineering. As a two-dimensional (2D) relative of carbon nanotubes, it manifests high mobility, high current carrying-capacity and extraordinary thermal conductivity; but in contrast to nanotubes, traditional lithographic techniques can potentially be employed for device synthesis and tailoring transport properties. We are currently investigating the electrical, spin, optical and mechanical transport properties of monolayer, few-layer and twisted few layer graphene.

Black Phosphorus

Phosphorene is single- or few-layers of black phosphorus that is the most stable form of phosphorus. Apart from carbon, it is the only known element with a stable layered allotrope. In single-layer phosphorene, the atoms are arranged in a honeycomb structure, much like graphene, though the atoms are puckered. Like graphene, charge carriers in bulk black phosphorus can have exceedingly high mobility, >50,000 cm2/Vs; but unlike the gapless graphene band structure, bulk black phosphorus hosts a direct band gap. The size of the band gap is thickness dependent, and is predicted to vary from 0.35 eV in bulk to ~2 eV in monolayers. Furthermore, it is also predicted to have unusual properties such as anisotropic transport, large thermoelectric power, and a band gap that is tunable by strain. Recently field effect transistors based on few-layer phosphorene (FLP) have been demonstrated, with mobility ~300–1000 cm2/Vs. Thus, phosphorene is emerging as a new two-dimensional (2D) semiconductor with tremendous promise for electronics, thermal and optoelectronics applications, as well as a model system with interesting physical properties. 

Heterostructures

A wealth of two-dimensional (2D) van der Waals solids exist, which can be metallic, insulator, semiconducting, ferromagnetic or ferroelectric, and that exhibit interesting behavior at low temperatures such as superconductivity or charge density wave formation. Fundamentally new materials not found in nature can be created by stacking individual monolayers of 2D solids. The monolayers are obtained in an intact form and therefore have the capability of forming atomically precise interfaces. The close proximity of the layers perturbs the electronic states in the material leading to new properties. These properties can be tailored both by choice of layers as well as the orientation the layers’ individual crystallographic axes to obtain materials with new properties. The opportunities are almost limitless, and we are only limited by our imagination.