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Register a new userHole weak anti-localization in a strained-Ge surface quantum well
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We report a magneto-transport study of a two-dimensional hole gas confined to a strained Ge quantum well grown on a relaxed Si0.2Ge0.8 virtual substrate. The conductivity of the hole gas measured as a function of a perpendicular magnetic field exhibits a zero-field peak resulting from weak anti-localization. The peak develops and becomes stronger upon increasing the hole density by means of a top gate electrode. This behavior is consistent with a Rashba-type spin-orbit coupling whose strength is proportional to the perpendicular electric field, and hence to the carrier density. By fitting the weak anti-localization peak to a model including a dominant cubic spin-orbit coupling, we extract the characteristic transport time scales and a spin splitting energy of ~1 meV. Finally, we observe a weak anti-localization peak also for magnetic fields parallel to the quantum well and attribute this finding to a combined effect of surface roughness, Zeeman splitting, and virtual occupation of higher-energy hole subbands.
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We report on an observation of a fractional quantum Hall effect in an ultra-high quality two-dimensional hole gas hosted in a strained Ge quantum well. The Hall resistance reveals precisely quantized plateaus and vanishing longitudinal resistance at filling factors $ u = 2/3, 4/3$ and $5/3$. From the temperature dependence around $ u = 3/2$ we obtain the composite fermion mass of $m^star approx 0.4,m_e$, where $m_e$ is the mass of a free electron. Owing to large Zeeman energy, all observed states are spin-polarized and can be described in terms of spinless composite fermions.
Low-field magnetoresistance is ubiquitous in low-dimensional metallic systems with high resistivity and well understood as arising due to quantum interference on self-intersecting diffusive trajectories. We have found that in graphene this weak-localization magnetoresistance is strongly suppressed and, in some cases, completely absent. This unexpected observation is attributed to mesoscopic corrugations of graphene sheets which cause a dephasing effect similar to that of a random magnetic field.
The quantum correction to electrical conductivity is studied on the basis of two-dimensional Wolff Hamiltonian, which is an effective model for a spin-orbit coupled (SOC) lattice system. It is shown that weak anti-localization (WAL) arises in SOC lattices, although its mechanism and properties are different from the conventional WAL in normal metals with SOC impurities. The interband SOC effect induces the contribution from the interband singlet Cooperon, which plays a crucial role for WAL in the SOC lattice. It is also shown that there is a crossover from WAL to weak localization in SOC lattices when the Fermi energy or band gap changes. The implications of the present results to Bi-Sb alloys and PbTe under pressure are discussed.
The results of experimental study of interference induced magnetoconductivity in narrow quantum well HgTe with the normal energy spectrum are presented. Analysis is performed with taking into account the conductivity anisotropy. It is shown that the fitting parameter tau_phi corresponding to the phase relaxation time increases in magnitude with the increasing conductivity (sigma) and decreasing temperature following the 1/T law. Such a behavior is analogous to that observed in usual two-dimensional systems with simple energy spectrum and corresponds to the inelasticity of electron-electron interaction as the main mechanism of the phase relaxation. However, it drastically differs from that observed in the wide HgTe quantum wells with the inverted spectrum, in which tau_phi being obtained by the same way is practically independent of sigma. It is presumed that a different structure of the electron multicomponent wave function for the inverted and normal quantum wells could be reason for such a discrepancy.
We show how the weak field magneto-conductance can be used as a tool to characterize epitaxial graphene samples grown from the C or the Si face of Silicon Carbide, with mobilities ranging from 120 to 12000 cm^2/(V.s). Depending on the growth conditions, we observe anti-localization and/or localization which can be understood in term of weak-localization related to quantum interferences. The inferred characteristic diffusion lengths are in agreement with the scanning tunneling microscopy and the theoretical model which describe the pure mono-layer and bilayer of graphene [MacCann et al,. Phys. Rev. Lett. 97, 146805 (2006)].
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