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Register a new userDiffusion Thermopower of Quantum Hall States Measured in Corbino Geometry
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We have measured the diffusion thermopower of a quantum Hall system in a Corbino setup. A concentric electron-temperature gradient is introduced by irradiating microwaves, via a coplanar waveguide, near the outer rim of a circular mesa of a two-dimensional electron gas. The resulting radial thermovoltages exhibit sawtooth-like oscillations with the magnetic field, taking large positive (negative) values just below (above) integer fillings with sign reversal at the center of the quantum Hall plateaus. The behavior is in agreement with a recent theory [Y. Barlas and K. Yang: Phys. Rev. B 85 (2012) 195107], which treats disorder within the self-consistent Born approximation.
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Using the Onsager relation between electric and heat transport coefficients, and considering the very different roles played by the quantum Hall condensate and quasiparticles in transport, we argue that near the center of a quantum Hall plateau thermopower in a Corbino geometry measures {it entropy per quasiparticle per quasiparticle charge}. This relation indicates that thermopower measurement in a Corbino setup is a more direct measure of quasiparticle entropy than in a Hall bar. Treating disorder within the self-consistent Born approximation, we show through an explicit microscopic calculation that this relation holds on an integer quantum Hall plateau at low temperatures. Applying this to non-Abelian quantum Hall states, we argue that Corbino thermopower at sufficiently low temperature becomes temperature-independent, and measures the quantum dimension of non-Abelian quasiparticles that determines the topological entropy they carry.
For certain measurements, the Corbino geometry has a distinct advantage over the Hall and van der Pauw geometries, in that it provides a direct probe of the bulk 2DEG without complications due to edge effects. This may be important in enabling detection of the non-Abelian entropy of the 5/2 fractional quantum Hall state via bulk thermodynamic measurements. We report the successful fabrication and measurement of a Corbino-geometry sample in an ultra-high mobility GaAs heterostructure, with a focus on transport in the second and higher Landau levels. In particular, we report activation energy gaps of fractional quantum Hall states, with all edge effects ruled out, and extrapolate the conductivity prefactor from the Arrhenius fits. Our results show that activated transport in the second Landau level remains poorly understood. The development of this Corbino device opens the possibility to study the bulk of the 5/2 state using techniques not possible in other geometries.
Competition between liquid and solid states in two-dimensional electron system is an intriguing problem in condensed matter physics. We have investigated competing Wigner crystal and fractional quantum Hall ( FQH ) liquid phases in atomically thin suspended graphene devices in Corbino geometry. Low temperature magnetoconductance and transconductance measurements along with $IV$ characteristics all indicate strong charge density dependent modulation of electron transport. Our results show unconventional FQH phases which do not fit the standard Jains series for conventional FQH states, instead they appear to originate from residual interactions of composite fermions in partially filled higher Landau levels. And at very low charge density with filling factors $ u lesssim$ 1/5, electrons crystallize into an ordered Wigner solid which eventually transforms into an incompressible Hall liquid at filling factors around $ u leq$ 1/7. Building on the unique sample structure, our experiments pave the way for enhanced understanding of the ordered phases of interacting electrons.
This paper has been superseded by a new preprint: Kun Yang and Bertrand I. Halperin, arXiv:0901.1429.
We propose a Corbino-disk geometry of a graphene membrane under out-of-plane strain deformations as a convenient path to detect pseudo-magnetic and electric fields via electronic transport. The three-fold symmetric pseudo-magnetic field changes sign six times as function of angle, leading to snake states connecting the inner and outer contacts and to nearly quantized transport. For dynamical strain obtained upon AC gating, the system supports an AC pseudo-electric field which, in the presence of the pseudo-magnetic field, produces a net electronic charge current in the absence of an external voltage, via a pseudo-Hall effect.
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