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Register a new userThe low-density spin susceptibility and effective mass of mobile electrons in Si inversion layers
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We studied the Shubnikov-de Haas (SdH) oscillations in high-mobility Si-MOS samples over a wide range of carrier densities $nsimeq (1-50) times 10^{11}$cm$^{-2}$, which includes the vicinity of the apparent metal-insulator transition in two dimensions (2D MIT). Using a novel technique of measuring the SdH oscillations in superimposed and independently controlled parallel and perpendicular magnetic fields, we determined the spin susceptibility $chi^*$, the effective mass $m^*$, and the $g^*$-factor for mobile electrons. These quantities increase gradually with decreasing density; near the 2D MIT, we observed enhancement of $chi^*$ by a factor of $sim 4.7$.
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We report measurements of the spin susceptibility in dilute (rs up to 10) AlAs two-dimensional (2D) electrons occupying a single conduction-band valley with an anisotropic in-plane Fermi contour, characterized by longitudinal and transverse effective masses, ml and mt. As the density is decreased, the spin susceptibility is significantly enhanced over its band value, reflecting the role of interaction. Yet the enhancement is suppressed compared to the results of quantum Monte Carlo based calculations that take the finite thickness of the electron layer into account but assume an isotropic effective mass equal to sqrt(ml.mt). Proper treatment of an interacting 2D system with an anisotropic effective mass therefore remains a theoretical challenge.
We report measurements of the spin susceptibility and the electron effective mass for two-dimensional electrons confined at the interfaces of MgxZn1-xO/ZnO single heterostructures (x = 0.05, 0.08, and 0.11), grown by molecular-beam epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through control of the barrier composition, the electron density was systematically varied in the range of 5.6 x 10^11 to 1.6 x 10^12 cm^-2, corresponding to a range of 3.1 < rs < 5.2, where rs is the average electron spacing measured in units of the effective Bohr radius. We used the coincidence technique, where crossings of the spin-split Landau levels occur at critical tilt angles of magnetic field, to evaluate the spin susceptibility. In addition, we determined the effective mass from the temperature dependence of the Shubnikov-de Haas oscillations measured at the coincidence conditions. The susceptibility and the effective mass both gradually increase with decreasing electron density, reflecting the role of electron-electron interaction.
We report effective hole mass ($m^{*}$) measurements through analyzing the temperature dependence of Shubnikov-de Haas oscillations in dilute (density $p sim 7 times 10^{10}$ cm$^{-2}$, $r_{s} sim 6$) two-dimensional (2D) hole systems confined to a 20 nm-wide, (311)A GaAs quantum well. The holes in this system occupy two nearly-degenerate spin subbands whose $m^{*}$ we measure to be $sim $ 0.2 (in units of the free electron mass). Despite the relatively large $r_{s}$ in our 2D system, the measured $m^{*}$ is in good agreement with the results of our energy band calculations which do not take interactions into account. We hen apply a sufficiently strong parallel magnetic field to fully depopulate one of the spin subbands, and measure $m^{*}$ for the populated subband. We find that this latter $m^{*}$ is surprisingly close to the $m^{*}$ we measure in the absence of the parallel field. We also deduce the spin susceptibility of the 2D hole system from the depopulation field, and conclude that the susceptibility is enhanced by about 50% relative to the value expected from the band calculations.
We report measurements of the spin susceptibility in dilute two-dimensional electrons confined to a 45$AA$ wide AlAs quantum well. The electrons in this well occupy an out-of-plane conduction-band valley, rendering a system similar to two-dimensional electrons in Si-MOSFETs but with only one valley occupied. We observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased, following closely the prediction of quantum Monte Carlo calculations and continuing at finite values through the metal-insulator transition.
In materials without spatial inversion symmetry the spin degeneracy of the conduction electrons can be lifted by an antisymmetric spin-orbit coupling. We discuss the influence of this spin-orbit coupling on the spin susceptibility of such superconductors, with a particular emphasis on the recently discovered heavy Fermion superconductor CePt3Si. We find that, for this compound (with tetragonal crystal symmetry,) irrespective of the pairing symmetry, the stable superconducting phases would give a very weak change of the spin susceptibility for fields along the c-axis and an intermediate reduction for fields in the basal plane. We also comment on the consequences for the paramagnetic limiting in this material.
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