Low-frequency 1/f γ noise
is ubiquitous, even in high-end electronic devices. Recently, it was
found that adsorbed O2 molecules provide the dominant contribution
to flux noise in superconducting quantum interference devices. To
clarify the basic principles of such adsorbate noise, we have investigated
low-frequency noise, while the mobility of surface adsorbates is varied
by temperature. We measured low-frequency current noise in suspended
monolayer graphene Corbino samples under the influence of adsorbed
Ne atoms. Owing to the extremely small intrinsic noise of suspended
graphene, we could resolve a combination of 1/f γ and Lorentzian noise induced by the presence of Ne.
We find that the 1/f γ noise
is caused by surface diffusion of Ne atoms and by temporary formation
of few-Ne-atom clusters. Our results support the idea that clustering
dynamics of defects is relevant for understanding of 1/f noise in metallic systems.
We have measured magnetoresistance of suspended graphene in the Corbino geometry at magnetic fields up to B = 0.15 T, i.e., in a regime uninfluenced by Shubnikov-de Haas oscillations. The low-temperature relative magnetoresistance [R(B) − R(0)]/R(0) is strong, approaching 100% at the highest magnetic field studied, with a quite weak temperature dependence below 30 K. A decrease in the relative magnetoresistance by a factor of two is found when charge carrier density is increased to |n| 3 × 10 10 cm −2 . Furthermore, we find a shift in the position of the charge neutrality point with increasing magnetic field, which suggests that magnetic field changes the screening of Coulomb impurities around the Dirac point. The gate dependence of the magnetoresistance allows us to characterize the role of scattering on long-range (Coulomb impurities, ripples) and short-range disorder (adatoms, atomic defects), as well as to separate the bulk resistance from the contact one. Based on the analysis of the magnetoresistance, we propose a more reliable method to extract the bulk mobility, which does not require prior knowledge of the contact resistance. It is thus demonstrated that studying magnetoresistance in the Corbino geometry is an extremely valuable tool to characterize high-mobility graphene samples, in particular, in the vicinity of the Dirac point.
Low dimensional fermionic quantum systems are exceptionally interesting because they reveal distinctive physical phenomena, including among others, topologically protected excitations, edge states, frustration, and fractionalization. Our aim was to confine 3He on a suspended carbon nanotube to form 2-dimensional Fermi-system. Here we report our measurements of the mechanical resonance of the nanotube with adsorbed sub-monolayer down to 10 mK. At intermediate coverages we have observed the famous 1/3 commensurate solid. However, at larger monolayer densities we have observed a quantum phase transition from 1/3 solid to an unknown, soft, and mobile solid phase. We interpret this mobile solid phase as a bosonic commensurate crystal consisting of helium dimers with topologically-induced zero-point vacancies which are delocalized at low temperatures. We thus demonstrate that 3He on a nanotube merges both fermionic and bosonic phenomena, with a quantum phase transition between fermionic solid 1/3 phase and the observed bosonic dimer solid.
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