Abstract
We report the observation of a bulk quantum Hall effect in the quasi-two-dimensional conductor $\ensuremath{\eta}\ensuremath{-}{\mathrm{Mo}}_{4}{\mathrm{O}}_{11}.$ The Hall resistance exhibits well-defined quantized plateaus, coincident with pronounced minima in the diagonal resistance. We propose a model involving (i) tiny quasi-two-dimensional electron and hole pockets, left over from an imperfectly nested charge-density wave, and (ii) the exchange of carriers between these mobile states and the localized charge-density-wave condensate. Together with a magnetic field, these factors result in well-separated bands of mobile states superimposed on a background continuum of localized states---the key ingredients for a quantum Hall effect. We also discuss the implications of these findings in the light of recent predictions concerning chiral metallic surface states in bulk quantum Hall systems.
Highlights
In the realm of high magnetic fields, there is immenseinterest in the physicsof low-dimensional conducting systemswhen the magneticenergy becomescomparableto the electronic (Fermi) energy- i.e. the quantum limit
In a strictly two-dimensional (2D) electron system, it is well known that the quantum Hall effect (QHE) and the fractional QHE are observed under such conditions, so-calledbecausethe Hall conductanceis quantized over extended intervals in magnetic field [I]
The field-induced-spindensity-wave condensate plays the same role as the Anderson localized states in the conventional 2D QHE, by pinning the Fermi energy in these mobility gaps over extended intervals in Imagnetic field
Summary
In the realm of high magnetic fields, there is immenseinterest in the physicsof low-dimensional conducting systemswhen the magneticenergy (orbital and Zeeman) becomescomparableto the electronic (Fermi) energy- i.e. the quantum limit. The possibility of observing a quantum Hall effect was discussed.In particular, the striking similarity between q-M04011 and the semimetallicInAs/GaSb superlatticesystemwith closelymatched electron and hole densitieswas noted [12] Both systemsmay be thought of as arrays of weakly coupled 2D electron systems. As a result, both materials undergo a field-induced transition to a semiconducting state when the quantum limit is reached and the hole and electron bands uncross [l 11.It is these properties of q-Mo401, which motivated the present study, namely, the possibility of observing a QHE in a truly bulk Q2D systemin reasonablelaboratory fields, i.e. Symmetric and asymmetricaveragesof the resistancesnieasured with the magnetic field applied parallel and antiparallel to the at-axis yield the diagonal and transverseHall (R,,) components of the resistance tensor respectively. The majority of the measurementswere conducted in supeiconducting solenoidsand dilution refrigerators, though resisfive magnets and a 3He refrigerator were alsoemployedfor someexperiments
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