369 research outputs found
Renormalization group study of intervalley scattering and valley splitting in a two-valley system
Renormalization group equations are derived for the case when both valley
splitting and intervalley scattering are present in a two-valley system. A
third scaling parameter is shown to be relevant when the two bands are split
but otherwise distinct. The existence of this parameter changes the
quantitative behavior at finite temperatures, but the qualitative conclusions
of the two-parameter theory are shown to be unaffected for realistic choice of
parameters
Flow diagram of the metal-insulator transition in two dimensions
The discovery of the metal-insulator transition (MIT) in two-dimensional (2D)
electron systems challenged the veracity of one of the most influential
conjectures in the physics of disordered electrons, which states that `in two
dimensions, there is no true metallic behaviour'; no matter how weak the
disorder, electrons would be trapped and unable to conduct a current. However,
that theory did not account for interactions between the electrons. Here we
investigate the interplay between the electron-electron interactions and
disorder near the MIT using simultaneous measurements of electrical resistivity
and magnetoconductance. We show that both the resistance and interaction
amplitude exhibit a fan-like spread as the MIT is crossed. From these data we
construct a resistance-interaction flow diagram of the MIT that clearly reveals
a quantum critical point, as predicted by the two-parameter scaling theory
(Punnoose and Finkel'stein, Science 310, 289 (2005)). The metallic side of this
diagram is accurately described by the renormalization group theory without any
fitting parameters. In particular, the metallic temperature dependence of the
resistance sets in when the interaction amplitude reaches gamma_2 = 0.45 - a
value in remarkable agreement with the one predicted by the theory.Comment: as publishe
Renormalization group study of a two-valley system with spin-splitting
Renormalization group equations in a two-valley system with valley-splitting
and intervalley scattering are derived in the presence of spin-splitting
induced by a parallel magnetic field. The relevant amplitudes in different
regimes set by the relative strengths of the spin and valley splittings and the
intervalley scattering rate are identified. The range of applicability of the
standard formula for the magnetoconductance is discussed
Test of scaling theory in two dimensions in the presence of valley splitting and intervalley scattering in Si-MOSFETs
We show that once the effects of valley splitting and intervalley scattering
are incorporated, renormalization group theory consistently describes the
metallic phase in silicon metal-oxide-semiconductor field-effect transistors
down to the lowest accessible temperatures
Transition from N-Type to P-Type Destroys Ferromagnetism in Semiconducting Sn\u3csub\u3e1-X\u3c/sub\u3eCo\u3csub\u3ex\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e and Sn\u3csub\u3e1-X\u3c/sub\u3eCr\u3csub\u3ex\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e Nanoparticles
This work reports strong correlations between the structural, magnetic and electronic properties of room temperature ferromagnets (RTFM) Sn1-xCoxO2 and Sn1-xCrxO2 for x = 0 to 0.1. The samples prepared by the sol-gel chemical method show RTFM for x \u3c xL with the limiting concentration xL = 0.01 for Co doping and xL = 0.025 for Cr doping. As doping level x is increased from x = 0, the magnetic moment per ion, μ, increases and the lattice volume VL decreases up to x = xL. For x \u3e xL, μ dramatically decreases toward zero and VL increases, the latter suggesting interstitial doping. Thermoelectric measurements showed that the samples are n-type for x \u3c xL and p-type for x \u3e xL, suggesting that the RTFM is intrinsic and it is electron mediated
Mobility-Dependence of the Critical Density in Two-Dimensional Systems: An Empirical Relation
For five different electron and hole systems in two dimensions (Si MOSFET's,
p-GaAs, p-SiGe, n-GaAs and n-AlAs), the critical density, that marks the
onset of strong localization is shown to be a single power-law function of the
scattering rate deduced from the maximum mobility. The resulting curve
defines the boundary separating a localized phase from a phase that exhibits
metallic behavior. The critical density in the limit of infinite
mobility.Comment: 2 pages, 1 figur
Size, Surface Structure, and Doping Effects on Ferromagnetism in SnO\u3csub\u3e2\u3c/sub\u3e
The effects of crystallite size, surface structure, and dopants on the magnetic properties of semiconducting oxides are highly controversial. In this work, Fe:SnO2 nanoparticles were prepared by four wet-chemical methods, with Fe concentration varying from 0% to 20%. Analysis confirmed pure single-phase cassiterite with a crystallite size of 2.6 ± 0.1 nm that decreased with increasing. Fe% doped substitutionally as Fe3+. Pure SnO2 showed highly reproducible weak magnetization that varied significantly with synthesis method. Interestingly, doping SnO2 with Fe \u3c 2.5% produced enhanced magnetic moments in all syntheses; the maximum of 1.6 × 10−4 µB/Fe ion at 0.1% Fe doping was much larger than the 2.6 × 10−6 µB/Fe ion of pure Fe oxide nanoparticles synthesized under similar conditions. At Fe ≥ 2.5%, the magnetic moment was significantly reduced. This work shows that (1) pure SnO2 can produce an intrinsic ferromagnetic behavior that varies with differences in surface structure, (2) very low Fe doping results in high magnetic moments, (3) higher Fe doping reduces magnetic moment and destroys ferromagnetism, and (4) there is an interesting correlation between changes in magnetic moment, bandgap, and lattice parameters. These results support the possibility that the observed ferromagnetism in SnO2 might be influenced by modification of the electronic structure by dopant, size, and surface structure
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