Response to parallel magnetic field of a dilute two-dimensional electron system across the metal-insulator transition

Abstract

The response to a parallel magnetic field of the very dilute insulating two-dimensional system of electrons in silicon metal-oxide-semiconductor field-effect transistors is dramatic and similar to that found on the conducting side of the metal-insulator transition: there is a large initial increase in resistivity with increasing field, followed by saturation to a value that is approximately constant above a characteristic magnetic field of about 1 T. This is unexpected behavior in an insulator that exhibits Efros-Shklovskii variable-range hopping in zero field, and appears to be a general feature of very dilute electron systems. ͓S0163-1829͑99͒50932-6͔Until quite recently, it was believed that all twodimensional systems of electrons ͑or holes͒ are necessarily localized in the absence of a magnetic field in the limit of zero temperature. This conclusion was based on the scaling theory for noninteracting electrons of Abrahams et al., 1 was further confirmed theoretically for weakly interacting electrons, 2,3 and received experimental confirmation in a number of materials, including thin films 4 and ͑high-density͒ silicon metal-oxide-semiconductor field-effect transistors ͑MOSFET's͒. 5,6 In the last several years, however, measurements in very dilute two-dimensional systems have provided evidence of a transition from insulating to conducting behavior with increasing electron ͑hole͒ density above some low critical value on the order of 10 9 -10 11 cm Ϫ2 . 7-13 At these very low densities the energy of electron-electron interactions exceeds the Fermi energy by an order of magnitude or more, and correlations thus provide the dominant energy in the problem. Dilute, strongly interacting two-dimensional systems are currently the focus of intense theoretical interest, and have elicited a spate of theoretical attempts to account for the presence and nature of the unexpected conducting phase.One of the most interesting characteristics of the conducting phase is its dramatic response to a magnetic field applied parallel to the plane of the two-dimensional system. For example, the resistivity of very high-mobility silicon MOS-FET's increases by almost three orders of magnitude with increasing field, saturating to a new value in fields above ϳ2 -3 T. 14,15 A similar effect was observed in p-GaAs/Al x Ga 1Ϫx As heterostructures 11 confirming that this giant positive magnetoresistance is a general property of dilute conducting two-dimensional ͑2D͒ systems. 16 In Ref. 17, it was reported that the metal-insulator transition in Si MOSFET's shifts toward higher electron densities in a parallel magnetic field of the order of a few T, while at higher magnetic fields, the effect saturates. We note that a parallel magnetic field couples only to the spins of the electrons and not to their orbital motion. Spins are thus known to play a crucial role, and it has been suggested that full alignment of the electrons results in the complete suppression of the anomalous conducting phase.In this paper we report that the response of the very dilut...

Ambipolar Gate Effect and Low Temperature Magnetoresistance of UltrathinLa0.8Ca0.2MEblen‐Zayas1, Bhattacharya2, Staley3 et al. 2005Phys. Rev. Lett.

“…In the past decade, there has been a resurgence of interest in the possible ground states of highly correlated two-dimensional electron gases (2DEGs) [1], especially regarding the existence of a metallic state at low temperatures. Both high mobility 2DEGs and highly disordered amorphous ultrathin metal films exhibit large positive magneto-resistance for in-plane magnetic fields at low temperatures [2]. This is consistent with theoretical arguments that attribute the existence of a metallic ground state to strong antiferromagnetic or singlet correlations between electrons [3] in the appropriate range of densities.…”

Magnetoresistance of a two-dimensional electron gas in a parallel magnetic field

“…For the temperature dependence, using our analytical results for charged impurity scattering [16], we predict for B = 0 and B ≥ B c (6) Note that, for g s = 1, the Fermi energy increases by a factor 2 compared to g s = 2. Correspondingly, the temperature dependence is weaker for a spin-polarized EG compared to an unpolarized one.…”

“…In recent experiments [1][2][3][4][5][6][7][8][9], the transport properties of the two-dimensional electron gas (2D EG) in Silicon inversion layers and GaAs heterostructures have been studied by applying a parallel magnetic field. The motivation to study transport properties came from a renewed interest into the metal-insulator transition in a 2D EG [10][11][12].…”

Small-Angle Shubnikov–de Haas Measurements in a 2D Electron System: The Effect of a Strong In-Plane Magnetic Field

“…The resistance, R xx , is shown in Fig. 1 [7,8,10,12,13], the in-plane (f 0) magnetoresistance rises dramatically with increasing field and saturates above a density-dependent field H sat ͑n s ͒ [14]. The sample was then rotated to make a small angle f with respect to the field, so that the in-plane component was almost equal to the total field H k ഠ H, while the projection in the perpendicular direction, H Ќ ഠ fH, remained relatively small even in high fields.…”