Magnetotransport in 2DES's formed in Si-MOSFET's and Si/SiGe quantum wells at low temperatures is reported. Metallic temperature dependence of resistivity is observed for the n-Si/SiGe sample even in a parallel magnetic field of 9 T, where the spins of electrons are expected to be polarized completely. Correlation between the spin polarization and minima in the diagonal resistivity observed by rotating the samples for various total strength of the magnetic field is also investigated. 71.30.+h, 73.40.Qv, 73.20.Dx Metallic temperature dependence of resistivity at a zero magnetic field has been observed in two-dimensional electron systems (2DES's) in Si-MOSFET's [1,2] and other 2D systems [3][4][5][6][7][8] characterized by strong Coulomb interaction between electrons (or holes) [7]. In experiments on Si-MOSFET's, it was found that a magnetic field applied parallel to the 2D plane can suppress the metallic behavior [9][10][11]. This result indicates that the spins of electrons play an important role in the metallic region as well as in the insulating region [12,13].The parallel magnetic field B does not couple the orbital motion of electrons within the 2D plane, but it changes the spin polarization of the 2DES. The spin polarization can be defined as p = (N ↑ − N ↓ )/N s , where N ↑ and N ↓ are the concentrations of electrons having an up spin and a down spin, respectively, and N s is the total electron concentration (N s = N ↑ + N ↓ ). p is expected to increase linearly with the total strength B tot (= B ) of the magnetic field. We have p = B tot /B c for p < 1 and B c = 2πh 2 N s /µ B g v g FL m FL if the system can be considered as Fermi liquid. Here, µ B (=he/2m e ) is the Bohr magneton and the valley degeneracy g v is 2 on the (001) surface of silicon. The strong e-e interaction is expected to change the effective g-factor g FL and the effective mass m FL from g * = 2.0 and m * = 0.19m e in the non-interacting 2DES in a (001) silicon surface.In the present work, we use two n-type silicon samples. A Si-MOSFET sample denoted Si-M has a peak electron mobility of µ peak = 2.4 m 2 /V s at N s = 4 × 10 15 m −2 and T = 0.3 K. The estimated SiO 2 layer thickness is 98 nm. A Si/SiGe sample denoted Si-G was grown by combining gas-source MBE and solid-source MBE. Details of the growth and characterization have been reported elsewhere [14,15] and T = 0.36 K. The samples were mounted on a rotatory thermal stage in a pumped 3 He refrigerator or in a 3 He-4 He dilution refrigerator together with a GaAs Hall generator and resistance thermometers calibrated in magnetic fields.In Ref.[11], some of the present authors have determined the product of g FL and m FL in Si-M from the low-temperature Shubnikov-de Haas oscillations in tilted magnetic field based on work of Fang and Styles [16]. The obtained enhancement factor α = g FL m FL /0.38m e is shown in Fig. 1(a) as a function of r s = π 1/2 (e/h) 2 (m * /κε 0 )N s −1/2 . r s is a dimensionless 1
