arXiv subjects

Aifei Zhang

Publications and source records attributed to Aifei Zhang.

5 recordsLinked to original sources

arXiv2026

Non-universal localization transition in the quantum Hall effect probed through broken-symmetry states of graphene

The quantum Hall effect hosts quantum phase transitions in which the localization length, that is the size of disorder-induced bulk localized states, is governed by universal scaling from percolation theory. However, this universal character is not systematically observed in experiments, including very recent ones in extremely clean devices. Here we explore this non-universality by systematically measuring the localization length in broken-symmetry quantum Hall states of graphene. Depending on the nature and gap size of these states, we observe differences of up to a tenfold in the minimum localization length, accompanied by clear deviations from universal scaling. Our results, as well as the previously observed non-universality, are fully captured by a simple picture based on the co-existence of localized states from two successive sub-Landau levels.

cond-mat.mes-hall↗

arXiv2026

Ballistic-to-diffusive transition in engineered counter-propagating quantum Hall channels

Exotic quantum Hall systems hosting counter-propagating edge states can show seemingly non-universal transport regimes, usually depending on the size of the sample. We experimentally probe transport in a quantum Hall sample engineered to host a tunable number of counter-propagating edge states. The latter are coupled by Landauer reservoirs, which force charge equilibration over a tunable effective length. We show that charge transport is determined by the balance of up- and downstream channels, with a ballistic regime emerging for unequal numbers of channels. For equal numbers, we observe a transition to a critical diffusive regime, characterized by a diverging equilibration length. Our approach allows simulating the equilibration of hole-conjugate states and other exotic quantum Hall effects with fully controlled parameters using well-understood quantum Hall states.

cond-mat.mes-hall↗

arXiv2026

Quantized heat flow in the Hofstadter butterfly

When subjected to a strong magnetic field, electrons on a two-dimensional lattice acquire a fractal energy spectrum called Hofstadter's butterfly. In addition to its unique recursive structure, the Hofstadter butterfly is intimately linked to non-trivial topological orders, hosting a cascade of ground states characterized by non-zero topological invariants. These states, called Chern insulators, are usually understood as replicas of the ground states of the quantum Hall effect, with electrical and thermal conductances that should be quantized, reflecting their topological order. The Hofstadter butterfly is now commonly observed in van-der-Waals heterostructures-based moiré superlattices. However, its thermal properties, particularly the quantized heat flow expected in the Chern insulators, have not been investigated, potentially questioning their similarity with standard quantum Hall states. Here we probe the heat transport properties of the Hofstadter butterfly, obtained in a graphene~/~hexagonal boron nitride moiré superlattice. We observe a quantized heat flow, uniquely set by the topological invariant, for all investigated states of the Hofstadter butterfly: quantum Hall states, Chern insulators, and even symmetry-broken Chern insulators emerging from strong electronic interactions. Our work firmly establishes the universality of the quantization of heat transport and its intimate link with topology.

cond-mat.mes-hall↗

arXiv2025

Non-ohmic to ohmic crossover in the breakdown of the quantum Hall states in graphene under broadband excitations

Graphene, through the coexistence of large cyclotron gaps and small spin and valley gaps, offers the possibility to study the breakdown of the quantum Hall effect across a wide range of energy scales. In this work, we investigate the breakdown of the QHE in high-mobility graphene Corbino devices under broadband excitation ranging from DC up to 10 GHz. We find that the conductance is consistently described by variable range hopping (VRH) and extract the hopping energies from both temperature and field-driven measurements. Using VRH thermometry, we are able to distinguish between a cold and hot electron regime, which are dominated by non-ohmic VRH and Joule heating, respectively. Our results demonstrate that breakdown in the quantum Hall regime of graphene is governed by a crossover from non-ohmic, field-driven VRH to ohmic, Joule-heating-dominated transport.

cond-mat.mes-hall↗

arXiv2024

Vanishing bulk heat flow in the nu=0 quantum Hall ferromagnet in monolayer graphene

Under high perpendicular magnetic field and at low temperatures, graphene develops an insulating state at the charge neutrality point. This state, dubbed $ν=0$, is due to the interplay between electronic interactions and the four-fold spin and valley degeneracies in the flat band formed by the $n=0$ Landau level. Determining the ground state of $ν=0$, including its spin and valley polarization, has been a theoretical and experimental undertaking for almost two decades. Here, we present experiments probing the bulk thermal transport properties of monolayer graphene at $ν=0$, which directly probe its ground state and collective excitations. We observe a vanishing bulk thermal transport, in contradiction with the expected ground state, predicted to have a finite thermal conductance even at very low temperature. Our result highlight the need for further investigations on the nature of $ν=0$.

cond-mat.mes-hall↗