Valley Polarization in Si(100) at Zero Magnetic Field

The fractional quantum Hall (FQH) regime of the Si two-dimensional electron system (2DES) in enhancement-mode field-effect transistors of Si/SiGe heterostructures was probed via electrical transport measurements. At n ∼ 2.6 × 10 11 /cm 2 with µ = 1.6 × 10 6 cm 2 /V s, signatures of FQH states at filling factors ν = 4/5, 6/5, and 10/7 were observed, in addition to the FQH states reported in previous studies. The temperature dependence of the FQH states is investigated and comparison is made with previous work done on modulation-doped samples. Results indicate that robustness of the FQH states is dependent on the nature of disorder in the 2DES. 1The discoveries of the integer quantum Hall (IQH) effect[1] and the fractional quantum Hall (FQH) effect [2] have spurred much research on the ground states of two-dimensional (2D) electrons in the presence of strong magnetic fields over the past 30 years. Thanks to to the constantly refined III-V epitaxial technology, the quality of GaAs/AlGaAs heterostructures has been continually improving, leading to the observation of a few tens of FQH states in GaAs [3]. While large in number, most of the FQH states can be understood within the theoretical framework of composite fermions (CFs) [4]. In the CF model, the FQH effect, originally arising from electron-electron interaction, is mapped to the IQH effect arising from Landau quantization of the orbital motion of CFs.Soon after the discoveries of the IQH and FQH effects, it was realized that additional degrees of freedom of 2DESs such as spins, layers, subbands, and valleys, may produce new correlated states which do not exist in a one-component 2DES [5]. Celebrated examples include Skyrmions in systems with small Zeeman splitting [6,7], even-denominator FQH states [8,9], and the excitonic superfluid in bilayer systems [10]. While much effort towards the understanding of the roles of spins, layers, and subbands in the FQH regime has been made using high-quality GaAs quantum wells, experimental studies on the effects of valleys have been relatively sparse, only a few reports on 2D electrons in Si [11,12] and AlAs [13][14][15][16] available, partly due to the less impressive material quality of multi-valley semiconductors.Indeed, the electron mobilities of the devices used in these studies were smaller than that of GaAs by almost two orders of magnitude [3,17]. Nevertheless, looking back at the history of discoveries of new states in GaAs, we may expect that more interesting physical phenomena in multi-valley systems will emerge, once materials with higher mobility are available.Recently, we reported a fabrication process for making undoped (100)-oriented Si/SiGe FETs and observed an electron mobility of 1.6 × 10 6 cm 2 /V s [18], approximately eight times higher than that of typical modulation-doped Si/SiGe quantum wells [19]. Such improvement in material quality naturally prompted us to perform magneto-transport experiments and search for new states in Si 2DESs. In this paper, we focus on the FQH regime ν < 2, where the...

Section: Fig 1 (A)supporting

confidence: 63%

Fractional quantum Hall effect of two-dimensional electrons in high-mobility Si/SiGe field-effect transistors

Pan

Tsui

et al. 2012

“…Experimentally measured E VS ranges from vanishingly small, to several hundreds of µeV, 9,30,31 to possibly a few meV. 32 Furthermore, to achieve controllability, spin qubits are generally located near or at the interface between the host and the barrier materials. Dangling bonds, charge traps, and other defects are inevitably present at the many interfaces of a semiconductor heterostructure, and the coherence properties of a spin qubit in a nanostructure are not as clearly understood and measured as in bulk Si.…”

Section: Introductionmentioning

confidence: 99%

Spin relaxation in a Si quantum dot due to spin-valley mixing

Huang

2014

103

We study the relaxation of an electron spin qubit in a Si quantum dot due to electrical noise. In particular, we clarify how the presence of conduction-band valleys influences spin relaxation. In single-valley semiconductor quantum dots, spin relaxation is through the mixing of spin and envelope orbital states via spin-orbit interaction. In Si, the relaxation could also be through the mixing of spin and valley states. We find that the additional spin relaxation channel, via spinvalley mixing and electrical noise, is indeed important for an electron spin in a Si quantum dot. By considering both spin-valley and intra-valley spin-orbit mixings and Johnson noise in a Si device, we find that the spin relaxation rate peaks at the hot spot, where the Zeeman splitting matches the valley splitting. Furthermore, because of a weaker field dependence, the spin relaxation rate due to Johnson noise could dominate over phonon noise at low magnetic fields, which fits well with recent experiments.

“…The multiplicity of the Hilbert space brought about by the existence of equivalent valleys has been shown to hamper spin QC. [71][72][73][74][75] At the same time, the interface potential gives rise to a valley-orbit coupling, which has been studied extensively in recent years, both experimentally [76][77][78][79][80][81] and theoretically. [82][83][84][85][86][87][88][89][90] Addressing specific valley eigenstates is a profound, challenging and unresolved problem.…”

Section: Introductionmentioning

confidence: 99%

Coherent electrical rotations of valley states in Si quantum dots using the phase of the valley-orbit coupling

Culcer

2012

A gate electric field has a small but non-negligible effect on the phase of the valley-orbit coupling in Si quantum dots. Finite interdot tunneling between valley eigenstates in a double quantum dot is enabled by a small difference in the phase of the valley-orbit coupling between the two dots, and it in turn allows controllable rotations of two-dot valley eigenstates at a level anticrossing. We present a comprehensive analytical discussion of this process, with estimates for realistic structures.