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Register a new userMelting of interference in the fractional quantum Hall effect: Appearance of neutral modes
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Added by
Rajarshi Bhattacharyya
Publication date
2018
fields
Physics
and research's language is
English
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Electrons living in a two-dimensional world under a strong magnetic field - the so-called fractional quantum Hall effect (FQHE) - often manifest themselves as fractionally charged quasiparticles (anyons). Moreover, being under special conditions they are expected to be immune to the environment, thus may serve as building blocks for future quantum computers. Interference of such anyons is the very first step towards understanding their anyonic statistics. However, the complex edge-modes structure of the fractional quantum Hall states, combined with upstream neutral modes, have been suspected to prevent an observation of the much sought after interference of anyons. Here, we report of finding a direct correlation between the appearance of neutral modes and the gradual disappearance of interference in a Mach-Zehnder interferometer (MZI), as the bulk filling factor is lowered towards Landau filling { u}_B =1; followed by a complete interference quench at { u}_B=1. Specifically, the interference was found to start diminishing at { u}_B~1.5 with a growing upstream neutral mode, which was detected by a born upstream shot noise in the input quantum point contact (QPC) to the MZI. Moreover, at the same time a { u}_QPC =1/3 conductance plateau, carrying shot-noise, appeared in the transmission of the QPC - persisting until bulk filling { u}_B =1/2. We identified this conductance plateau to result from edge reconstruction, which leads to an upstream neutral mode. Here, we also show that even the particle-like quasiparticles are accompanied by upstream neutral modes, therefore suppressing interference in the FQHE regime.
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Counter propagating (upstream) chiral neutral edge modes, which were predicted to be present in hole-conjugate states, were observed recently in a variety of fractional quantum Hall states (v=2/3,v=3/5,v=8/3 & v=5/2), by measuring charge noise that resulted after partitioning the neutral mode by a constriction (denoted, as NrightarrowC). Particularly noticeable was the observation of such modes in the v=5/2 fractional state - as it sheds light on the non-abelian nature of the states wavefunction. Yet, the nature of these unique, upstream, chargeless modes and the microscopic process in which they generate shot noise, are not understood. Here, we study the ubiquitous v=2/3 state and report of two main observations: First, the nature of the neutral modes was tested by colliding two modes, emanating from two opposing sources, in a narrow constriction. The resultant charge noise was consistent with local heating of the partitioned quasiparticles. Second, partitioning of a downstream charge mode by a constriction gave birth to a dual process, namely, the appearance of an upstream neutral mode (CrightarrowN). In other words, splitting hole conjugated type quasiparticles will lead to an energy loss and decoherence, with energy carried away by neutral modes.
Electronic systems harboring one dimensional helical modes, where the spin and momentum of the electron are locked, have lately become an important field of its own. When coupled to a conventional superconductor, such systems are expected to manifest topological superconductivity, a unique phase that gives rise to exotic Majorana zero modes. Even more interesting are fractional helical states which have not been observed before and which open the route for the realization of the generalized para fermions quasiparticles. Possessing non abelian exchange statistics, these quasiparticles may serve as building blocks in topological quantum computing. Here, we present a new approach to form protected one dimensional helical and fractional helical edge modes in the quantum Hall regime. The novel platform is based on a carefully designed double quantum well structure in a high mobility GaAs based system. In turn, the quantum well hosts two sub bands of 2D electrons, each tuned to the quantum Hall effect regime. By electrostatic gating of different areas of the structure, counter propagating integer, as well as fractional, edge modes, belonging to Landau levels with opposite spins are formed, rendering the modes helical. We demonstrate that due to spin protection, these helical modes remain ballistic, without observed mixing for large distances. In addition to the formation of helical modes, this new platform can be exploited as a rich playground for an artificial induction of compounded fractional edge modes, as well as construction of interferometers based on chiral edge modes.
When electrons are confined in two dimensions and subjected to strong magnetic fields, the Coulomb interactions between them become dominant and can lead to novel states of matter such as fractional quantum Hall liquids. In these liquids electrons linked to magnetic flux quanta form complex composite quasipartices, which are manifested in the quantization of the Hall conductivity as rational fractions of the conductance quantum. The recent experimental discovery of an anomalous integer quantum Hall effect in graphene has opened up a new avenue in the study of correlated 2D electronic systems, in which the interacting electron wavefunctions are those of massless chiral fermions. However, due to the prevailing disorder, graphene has thus far exhibited only weak signatures of correlated electron phenomena, despite concerted experimental efforts and intense theoretical interest. Here, we report the observation of the fractional quantum Hall effect in ultraclean suspended graphene, supporting the existence of strongly correlated electron states in the presence of a magnetic field. In addition, at low carrier density graphene becomes an insulator with an energy gap tunable by magnetic field. These newly discovered quantum states offer the opportunity to study a new state of matter of strongly correlated Dirac fermions in the presence of large magnetic fields.
Interferometry provides direct evidence for anyon statistics. In the fractional quantum Hall effect, interferometers are susceptible to dephasing by neutral modes. The latter support chargeless quasiparticles (neutralons) which propagate upstream along the edge and obey fractional statistics. Here we show that on a suitably engineered bilayer fractional quantum Hall edge, which is an experimentally available platform, the neutral modes can be gapped while leaving the desired charge modes gapless. The gapping mechanism is akin to a four-particle pairing superconductivity. Our considered bilayer structure can be shaped as an anyonic interferometer. We also discuss experimental charge transport signatures of the neutral mode gap.
We demonstrate the emergence of the quantum Hall (QH) hierarchy in a 2D model of coupled quantum wires in a perpendicular magnetic field. At commensurate values of the magnetic field, the system can develop instabilities to appropriate inter-wire electron hopping processes that drive the system into a variety of QH states. Some of the QH states are not included in the Haldane-Halperin hierarchy. In addition, we find operators allowed at any field that lead to novel crystals of Laughlin quasiparticles. We demonstrate that any QH state is the groundstate of a Hamiltonian that we explicitly construct.
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