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Register a new userGeometric Resonance of Composite Fermions near Bilayer Quantum Hall States
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Via the application of parallel magnetic field, we induce a single-layer to bilayer transition in two-dimensional electron systems confined to wide GaAs quantum wells, and study the geometric resonance of composite fermions (CFs) with a periodic density modulation in our samples. The measurements reveal that CFs exist close to bilayer quantum Hall states, formed at Landau level filling factors $ u=1$ and 1/2. Near $ u=1$, the geometric resonance features are consistent with half the total electron density in the bilayer system, implying that CFs prefer to stay in separate layers and exhibit a two-component behavior. In contrast, close to $ u=1/2$, CFs appear single-layer-like (single-component) as their resonance features correspond to the total density.
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We observe geometric resonance features of composite fermions on the flanks of the even denominator { u} = 1/2 fractional quantum Hall state in high-mobility two-dimensional electron and hole systems confined to wide GaAs quantum wells and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The features provide a measure of how close to { u} = 1/2 the system stays single-component and supports a composite fermion Fermi sea before transitioning into a { u} = 1/2 fractional quantum Hall state, presumably the two-component {Psi}331 state.
Two-dimensional interacting electrons exposed to strong perpendicular magnetic fields generate emergent, exotic quasiparticles phenomenologically distinct from electrons. Specifically, electrons bind with an even number of flux quanta, and transform into composite fermions (CFs). Besides providing an intuitive explanation for the fractional quantum Hall states, CFs also possess Fermi-liquid-like properties, including a well-defined Fermi sea, at and near even-denominator Landau level filling factors such as $ u=1/2$ or $1/4$. Here, we directly probe the Fermi sea of the rarely studied four-flux CFs near $ u=1/4$ via geometric resonance experiments. The data reveal some unique characteristics. Unlike in the case of two-flux CFs, the magnetic field positions of the geometric resonance resistance minima for $ u<1/4$ and $ u>1/4$ are symmetric with respect to the position of $ u=1/4$. However, when an in-plane magnetic field is applied, the minima positions become asymmetric, implying a mysterious asymmetry in the CF Fermi sea anisotropy for $ u<1/4$ and $ u>1/4$. This asymmetry, which is in stark contrast to the two-flux CFs, suggests that the four-flux CFs on the two sides of $ u=1/4$ have very different effective masses, possibly because of the proximity of the Wigner crystal formation at small $ u$.
We study the role of anisotropy on the transport properties of composite fermions near Landau level filling factor $ u=1/2$ in two-dimensional holes confined to a GaAs quantum well. By applying a parallel magnetic field, we tune the composite fermion Fermi sea anisotropy and monitor the relative change of the transport scattering time at $ u=1/2$ along the principal directions. Interpreted in a simple Drude model, our results suggest that the scattering time is longer along the longitudinal direction of the composite fermion Fermi sea. Furthermore, the measured energy gap for the fractional quantum Hall state at $ u=2/3$ decreases when anisotropy becomes significant. The decrease, however, might partly stem from the charge distribution becoming bilayer-like at very large parallel magnetic fields.
Composite fermions in fractional quantum Hall (FQH) systems are believed to form a Fermi sea of weakly interacting particles at half filling $ u=1/2$. Recently, it was proposed (D. T. Son, Phys. Rev. X 5, 031027 (2015)) that these composite fermions are Dirac particles. In our work, we demonstrate experimentally that composite fermions found in monolayer graphene are Dirac particles at half filling. Our experiments have addressed FQH states in high-mobility, suspended graphene Corbino disks in the vicinity of $ u=1/2$. We find strong temperature dependence of conductivity $sigma$ away from half filling, which is consistent with the expected electron-electron interaction induced gaps in the FQH state. At half filling, however, the temperature dependence of conductivity $sigma(T)$ becomes quite weak as expected for a Fermi sea of composite fermions and we find only logarithmic dependence of $sigma$ on $T$. The sign of this quantum correction coincides with weak antilocalization of composite fermions, which reveals the relativistic Dirac nature of composite fermions in graphene.
Composite fermion metal states emerge in quantum Hall bilayers at total Landau level filling factor $ u_T$=1 when the tunneling gap collapses by application of in-plane components of the external magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index.
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