| dc.description.abstract | In the quantum Hall (QH) regime, the electrical transport happens along the downstream chiral edge modes (dictated by the external magnetic field) while the bulk remains a vanilla insulator. However, it has been theoretically predicted that a certain class of fractional QH phases may also contain upstream modes together with the downstream ones (counter-propagating modes). Further, these upstream modes may not carry any electrical current but can carry energy. Detecting the charge-neutral upstream modes is challenging and remains critical for the emergence of renormalized modes with exotic quantum statistics for quantum computing. In this context, QH of graphene is an ideal platform with more degrees of freedom like spin, valley, and orbital together with a unique half-filled zeroth Landau level (ν=0 state), where the bulk is not vanilla type but rather can host canted antiferromagnetic (CAF) phase with charge-neutral Goldstone modes or isospin-ferromagnetic (FM) phase (ν≠0) with spin-wave excitations like magnon, etc. This thesis attempts to detect these charge-neutral upstream modes and spin-wave excitations in graphene QH using “Noise Thermometry”, based on heat transport.
First, we present our study on detection of charge neutral upstream modes at hole-conjugate ν=2/3 and 3/5 fractional QH states of bilayer graphene. We observed excess noise along the edge in the upstream direction at ν=2/3 and 3/5 states, providing smoking gun evidence of upstream modes at these fillings, while no noise is detected at integer and particle-like FQH states. The channel length and temperature dependence of the noise in upstream direction at ν=2/3, together with remarkable agreement of theoretically calculated noise, suggest ballistic nature of upstream modes, quite distinct from the diffusive nature reported in GaAs/AlGaAs based system. Next, using similar technique to detect heat flow in the upstream direction, we detected charge neutral spin-wave excitations at the ν=2 QH ferromagnet in bilayer graphene. We generate spin excitation by creating an imbalance in the chemical potential (> Zeeman energy gap) between the edge states of opposite spin, and detect it in the upstream direction due to heat transport by spin-wave. The observed threshold of bias energy (where spin excitation occurs) at different magnetic fields agrees with the expected Zeeman gap. In a slightly different measurement scheme, we tried to detect heat transport signature of charge neutral Goldstone modes present at ν=0 CAF state. Our findings shed light on the competition between the heat transport via goldstone modes and phonon.
Next, we study the effect of periodic magnetic field on Hall conctivity of graphene. The periodic magnetic field over graphene was created by Abrikosov vortices of a type-II superconductor (NbSe2 in this work). We found a density-dependent reduction of the Hall conductivity of graphene as the temperature is lowered from above the superconducting critical temperature of NbSe2, where the magnetic field is uniform, to below, where the magnetic field bunches into an Abrikosov flux lattice. | en_US |
