A team of researchers from the Istituto Nanoscienze-CNR in Pisa, the University of Salamanca, RWTH Aachen University, HFML-FELIX and the Institute for Molecules and Materials (IMM) have now discovered how the electronic transport in the quantum Hall regime of ultra-clean graphene is governed predominantly by electron-phonon scattering, i.e. scattering of electrons with thermally excited lattice vibrations. It enables extending the well-accepted notion of phonon-limited resistivity in ultra-clean graphene from zero magnetic field to the quantum Hall regime and works towards an understanding of the mysterious room-temperature quantum Hall effect in graphene. The results have recently been published in Nature Communications.
Ultra-clean graphene
There are still many unknowns on the room temperature quantum Hall effect discovered in Nijmegen in 2007 and ultra-clean graphene devices are required to resolve this mystery.
The interdisciplinary team of researchers demonstrated that graphene encapsulated in hexagonal boron nitride (hBN) realizes a novel transport regime, where dissipation in the QH phase is governed predominantly by electron-phonon scattering, i.e. scattering of electrons with thermally excited lattice vibrations (and not by impurity scattering as in “dirty” systems). For this, the team used ultra-clean graphene and very high magnetic fields. “Our colleagues in Salamanca and Aachen have made ultra-clean graphene devices (graphene on BN) and have come to Nijmegen to measure their resistance as a function of temperature (4 .. 300 K) and magnetic field (0 .. 35 T)”, professor Zeitler says.
Quantum Hall effect
The Quantum Hall effect (QHE), one of the most fundamental effects in 2D systems with three Nobel Prizes associated to it, is a well-accepted concept in physics describing the behaviour of 2D electrons in a magnetic. Apart from its significance for fundamental physics, it also establishes a standard on how to precisely define electrical resistance.
The current studies represent a combined experimental and theoretical approach to understand the quantum Hall effect in graphene at elevated temperatures. “Experiments have shown that for very strong magnetic fields applied to 2D systems, the Hall resistance becomes quantized, RH = h/ne2 and only depends on the charge of the electron and Planck’s constant, two fundamental constants of nature. However, normally one requires rather low temperatures, a few degrees above absolute zero, to observe it”, Zeitler explains. “With our present work we can now pinpoint the behaviour of the QHE in graphene as a function of temperature and its robustness at elevated temperatures to electron-phonon scattering as we are used to explaining electronic transport without a magnetic field”.
