We have investigated correlation between spin polarization and magnetotransport in a high mobility silicon inversion layer which shows the metal-insulator transition. Increase in the resistivity in a parallel magnetic field reaches saturation at the critical field for the full polarization evaluated from an analysis of low-field Shubnikov-de Haas oscillations. By rotating the sample at various total strength of the magnetic field, we found that the normal component of the magnetic field at minima in the diagonal resistivity increases linearly with the concentration of "spin-up" electrons. 71.30.+h, 73.40.Qv, 73.40.Hm A metal-insulator transition (MIT) observed in Si metal-oxide-semiconductor field-effect transistors [1,2] (Si-MOSFET's) and other systems [3-6] attracts a great deal of attention since it seems to contradict an important result of the scaling theory by Abrahams et al. [7] that the conductance of a disordered two-dimensional (2D) system at zero magnetic field goes to zero for T → 0. In the metallic phase in Si-MOSFET's with high peak electron mobilities of µ peak > ∼ 2 m 2 /V s, the diagonal resistivity ρ xx shows a sharp drop with decreasing temperature from about 2 K [1]. Recent experiments [8,9] show that magnetic fields applied parallel to the 2D plane suppress the low temperature metallic conduction in Si-MOSFET's. Since the parallel magnetic field does not couple the orbital motion of electrons, this fact suggests an important role of the spin of electrons. However, the mechanism of the conduction in the anomalous metallic phase is not clear yet.The 2D systems that show the MIT [1-6] are characterized by strong Coulomb interaction between electrons. The mean Coulomb energy per electron U = (πN s ) 1/2 e 2 /4πε 0 κ is larger than the mean kinetic energy K = πh 2 N s /m * by an order of the magnitude around the critical point for the MIT. Here, N s is the electron concentration, κ is the relative dielectric constant at the interface, and m * is the effective mass of electron. It is estimated that U = 120 K, K = 14 K and the ratio r s = U/K = 8.3 for κ = 7.7 and m * = 0.19m e at N s = 1 × 10 15 m −2 in Si-MOSFET's. The ground state of the insulating phase of high mobility Si-MOSFET's is considered to be a pinned Wigner solid (WS) [10,11]. Magnetic field dependence of the thermal activation energy observed for various angles of the magnetic field was essentially explained by a model based on magnetic interactions in the pinned WS [12,13]. Although the quantum fluctuations change the 2D system into a liquid at higher-N s , electron-electron (e-e) interaction is expected to be still important.In the conduction band of silicon, the spin-orbit interaction is negligible and the spin polarization p = (N ↑ − N ↓ )/N s can always be given in the direction to the magnetic field. Here N ↑ and N ↓ are the concentrations of electrons having an up spin and a down spin, respectively (N s = N ↑ + N ↓ ). In the present work, we investigate the low temperature conduction in a high mobility Si-MOSFET for vario...

Optical Hall conductivity σxy(ω) is measured from the Faraday rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz frequency regime. The Faraday rotation angle (∼ fine structure constant ∼ mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateau-like structure around the Landau-level filling ν = 2. The result, which fits with the behavior expected from the carrier localization effect in the ac regime, indicates that the plateau structure, although not quantized, still exists in the terahertz regime.The quantum Hall effect (QHE), a highlight in the twodimensional electron gas (2DEG) system in strong magnetic fields [1][2][3][4], still harbors, despite its long history, a wealth of important physics. While static properties of the integer QHE have been well understood, we are still some way from a full understanding of dynamical responses in the QHE in the ac or even optical regime. In the static case, the states localized due to disorder with the localization length smaller than the sample size or the inelastic scattering length are crucial in realizing the quantum plateaus for the dc Hall current in a dc electric field [5][6][7][8][9][10][11]. On the other hand, the conventional wisdom for the dynamical response would be that an ac field will delocalize wave functions to make QHE disappear.For relatively low frequencies, the breakdown of QHE in ac fields has a long history of investigation [12]. One issue was whether the delocalization occurs for lowfrequencies (∼ 10 MHz), but the results were not conclusive. Subsequently, experimental study was extended to the microwave regime in the 1980s, where the delocalization as seen in the Hall conductivity σ xy was shown to be absent in the microwave (i.e., gigahertz) regime [13], while the gigahertz responses of the longitudinal conductivity σ xx [9,14,15] were explained with the scaling theory of localization [16]. Thus, a fundamental problem remains as to whether and how QHE is affected in the much higher, terahertz (closer to the optical) frequency regime (ω ∼ 10 12 Hz ∼ 10 −2 eV/h). This is an essential question, since the frequency is exactly the energy scale of interest (i.e., the cyclotron energyhω c ∼ 10 −2 eV for a magnetic field ∼ 10 T, which is the spacing between Landau levels, a prerequisite for QHE).Theoretically, the accurate quantization in QHE is firmly established as a topological (Chern) number [17] in the static case. However, such a picture may not be extended to the ac regime where the topological 'protection' no longer exists. Recently, Morimoto et al. [18] have theoretically examined the ac response of the disordered QHE systems based on the exact diagonalization method, and showed that a plateau-like behavior still exists in σ xy even in the terahertz energy range. This has motivated us to experimentally examine QHE by going beyond the microwave regime, which has so far remained a challenge. An essential experimental ingredient that enables the measurement is a recent development in t...

Two-dimensional (2D) superconductivity was studied by magnetotransport measurements on single-atomic-layer Pb films on a cleaved GaAs(110) surface. The superconducting transition temperature shows only a weak dependence on the parallel magnetic field up to 14 T, which is higher than the Pauli paramagnetic limit. Furthermore, the perpendicular-magnetic-field dependence of the sheet resistance is almost independent of the presence of the parallel field component. These results are explained in terms of an inhomogeneous superconducting state predicted for 2D metals with a large Rashba spin splitting.PACS numbers: 73.20.At,Superconductivity in ultrathin films has been studied for a long time. In Ref.[1], superconductivity was observed even for a few-monolayer thickness in quenchcondensed films of Bi and Pb deposited on a glazed alumina substrate coated with amorphous Ge. Very recently, it has been revealed that superconductivity can occur in single atomic layers of Pb and In grown epitaxially on a Si(111) substrate [2,3]. A single-atomic-layer metal film on an insulating substrate is an interesting system for studies of superconductivity, not only because it is a complete two-dimensional (2D) system but also because of the broken spatial inversion symmetry. The asymmetry of the confining potential in the direction perpendicular to the 2D plane, combined with atomic spin-orbit coupling, is expected to cause the Rashba effect, which lifts the spin degeneracy of the 2D electronic states [4,5]. Actually, angle-resolved photoelectron spectroscopy measurements showed a large Rashba spin splitting of the order of 100 meV on the surfaces of heavy elements, such as Au [6], W [7], and Bi [8], and those of lighter elements, such as Si and Ge, covered with a monolayer of heavy elements, such as Bi [9][10][11].In this Letter, we report magnetotransport measurements on superconducting monolayer Pb films produced by quench condensation onto a cleaved GaAs(110) surface. Here, we focus on the effect of the magnetic field applied parallel to the surface. While the perpendicular component H ⊥ of the magnetic field strongly affects the orbital motion of electrons in the 2D plane, the parallel component H is expected to couple only to the spin degree of freedom. We show that the reduction of the superconducting transition temperature T c is very small even in strong parallel magnetic fields (H ⊥ = 0), which are much larger than the Pauli paramagnetic limit. Furthermore, the H ⊥ dependence of the sheet resistance at low temperature is found to be almost independent of the presence of H . These results are explained by assuming an inhomogeneous superconducting state predicted for Rashba spin-split 2D systems.In order to measure the sheet resistance R sq of ultrathin films, we apply the experimental procedure developed for studies on adsorbate-induced surface inversion layers on InAs [13][14][15] and InSb [16]. In this work, we used a nondoped insulating GaAs single-crystal substrate so as not to create conduction channels in the substrate...

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We have studied the magnetic and transport properties of an ultralow-resistivity two-dimensional electron system in a Si/SiGe quantum well. The spin polarization increases linearly with the in-plane magnetic field and the enhancement of the spin susceptibility is consistent with that in Si-MOS structures. Temperature dependence of resistivity remains metallic even in strong magnetic fields where the spin degree of freedom is frozen out. We also found a magnetoresistance anisotropy with respect to an angle between the current and the in-plane magnetic field.