We have performed low-temperature in-plane magnetotransport measurements on twodimensional electron systems induced by deposition of Ag at in situ cleaved surfaces of p-type InSb. The quantum Hall effect was observed even at low magnetic fields around 2 T. The surface electron density and the electron mobility exhibit strong dependence on the Ag-coverage and the annealing temperature in the range of 15-40 K. The annealing effect suggests that the surface morphology strongly affects the properties of the two-dimensional electron systems.A two-dimensional electron system (2DES) at low temperatures and in a strong magnetic field shows the quantum Hall (QH) effect, in which the diagonal resistivity ρ xx vanishes and the Hall resistance is quantized as R H = h/ie 2 for an integer i [1, 2]. Recently, Tsuji et al. performed in-plane magnetotransport measurements on 2DESs induced by deposition of Ag atoms at in situ cleaved surfaces of p-type InAs and observed the QH effect for i = 4 [3,4]. However, a high magnetic field above 10 T was required for the observation of the QH effect. For future research on microscopic current distribution, such as edge channel transport, surface QH systems that can be observed at low magnetic fields are desirable to reduce the difficulty of constructing an experimental system with a scanning microscope. The integer QH effect is expected to appear in the region where the Landau level separationhω c exceeds the level broadeninghτ(ω c τ > 1). Here, ω c is the cyclotron frequency and τ is the scattering time. The dimensionless parameter ω c τ is equivalent to the product of B and the electron mobility µ. It is well-known that InSb has a very small electron effective mass m * = 0.013m e at the Γ point, where m e is the free-electron mass. This value is about a half of that in InAs (m * = 0.026m e ) and thus higher electron mobility (µ = eτ /m * ) is expected. In this letter, we report the observation of the QH effect at cleaved surfaces of InSb. The quantized Hall resistance and the vanishing of ρ xx were clearly observed even at low magnetic fields around 2 T. Besides the dependence on the coverage of adsorbed Ag atoms, annealing effect was studied in the range of 15-40 K. The results suggest that the surface morphology strongly affects the properties of 2DESs.The samples used were cut from a Ge-doped single crystal with an acceptor concentration of 1-2 × 10 21 m −3 . In the case of p-type crystals, the surface electrons are separated from the three-dimensional carriers in the substrate by the depletion layer. Sample preparation and experimental setup are similar to those used in the previous work on InAs surfaces [3,4]. Sample cleaving was done at low temperatures in an ultra-high vacuum chamber with a 4 He cryostat. Magnetotranport data on the (110) cleaved surface were taken in a Hall bar geometry (4 mm × 0.4 mm) using the standard four-probe lock-in technique at 13 Hz with two current electrodes and four voltage electrodes prepared by deposition of gold films onto noncleaved surfaces at r...

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