Crossover between magnetic and electric edges in quantum Hall systems

Description: 

Quantum Hall systems ( Nobel 1985, 1998), had made a paradigm shift  in our understanding of phase transitions in condensed matter systems through the introduction of topological phases (Nobel 2016). Such topological phases in Quantum Hall systems are realized in two dimensional gases of electrons formed in a sandwitch piece between two semiconductors called heterostructure in the presence of a very high uniform transverse magnetic field and at extremely low temperature. The main and must identifier of such topological phases is an perfectly insulating bulk surrounded by a current carrying edge states at the systems' boundary (edge) with an uni-directional flow called chirality. The drift velocity of the current carrying edge electrons in such chiral states is proportional to the product of the Electric and Magnetic field that are perpendicular to each other. Now the researchers in UK ( Group of Alain Nogaret in Bath and his collaborators in Cambridge) and in Indian Institute of Technolgy, Delhi ( Puja Mondal Ankip Kumar and Sankalpa Ghosh), have demonstrated that such chiral edge states can be created in the bulk of the system purely by modulating the uniform magnetic field of conventional Quantum Hall systems. The drift velocity in this case is proportional to the product of the magnetic field and the magnetic field gradient ( variation). The experiment not only created such chiral magnetic edge states, but also been able to change the separation between such magnetic edge states in the bulk with the conventional electrostatic edge states at the edge. The experiment recorded a transition from the magnetic edge state dominated transport to conventional electrostatic edge dominated transport in a highly controlled way by measuring the resistivity of the sample as a function of the applied magnetic field and the bias voltage. The peaks of such oscillating resistance as a function of increasing magnetic field shows a clear direction reversal as a function of biasing electrostatic voltage ( not present if the magnetic field is uniform) that fully matches with a detailed theoretical calculation of the band structure of the system and and the quantum mechanical calculation of the conductivity using it. The results provides a novel twist to the conventional understanding of the Quantum Hall systems and might be interesting in general to the explore the rich bulk-edge correspondence in various topological phases in condensed matter . The work is funded by a UGC-UKIERI grant. The results were recently published in https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.081302

Contact details: 

Dr. Sankalpa Ghosh

Department of Physics

sankalpa[at]physics.iitd.ac.in