We have measured tunneling rates out of an electron layer trapped at a liquid-helium-vacuum interface in the presence of a magnetic field. When the field is transverse to the escape direction we find a striking suppression of the tunneling rates: a field of 3000 G at 40 mK reduces the tunneling current by four orders of magnitude. As the temperature increases the magnetic suppression of tunneling diminishes until it disappears completely above 250 mK. By contrast parallel fields have no effect on the tunneling rates.PACS numbers: 73.40.Gk, 73.20.Dx The transport of electrons through potential barriers in the presence of a magnetic field is a powerful tool for understanding the physics of tunneling in the presence of correlations and in various applications such as the design and control of tunneling devices [1][2][3][4][5]. Experiments on magnetotunneling (MT) have thus far focused on semiconductor heterostructures [1,5] where the magnetic field has only a slight effect on the tunneling rates. The relative importance of the magnetic field may be estimated by comparing the time to complete a cyclotron precession, ~ o^T 1 , to a characteristic time scale of tunneling which is usually taken as [6] T ~ f** dz/sj2 [U(z) -E]/m. Here cv c = eB/m*c is the cyclotron frequency, ra, e, and E are the electronic mass, charge, and total energy, U(z) is its potential energy, B is the magnetic field amplitude, and 21,22 are the turning points of the potential.In this Letter we report on measurements of MT for surface state electrons trapped at a liquid-heliumvacuum interface. In this system the barrier size and the electron densities are readily tuned in situ over a wide range of parameters that are not accessible in heterostructures [7][8][9]. The barriers can be made very wide and shallow so that r ~ 20 psec is two orders of magnitude larger than in heterostructures and the strong magnetic field limit (00 C T > 1) is reached for fields as low as 2500 G. For these barriers we observe a strong suppression of the tunneling rates in the presence of a transverse magnetic field B t (transverse to the direction of tunneling and parallel to the electron layer). As illustrated in Figs. 1-3 the tunneling currents depend exponentially on B*l and exhibit an unexpected temperature and density dependence.Our experimental setup consists of a cylindrical cell of height 2.5=b0.05 mm that is half filled with liquid helium supporting the electron disk. The electrons are prevented from escaping to the sides with a negatively biased guard ring of diameter 18±0.05 mm. A voltage V t applied between the top plate of the cell and the grounded bottom plate creates an external field with which the electrons can be pressed toward or extracted from the surface. The cell is positioned at the center of a double split superconducting magnet that can provide up to 0.5 T of either transverse and/or parallel field. The electron density is determined from the plasmon spectrum of the electron disk which is measured with an rf spectrometer. From the known 2D ...

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We measured the escape rates of surface-state electrons from an electron layer confined at the liquidheliumvacuum interface in the temperature range of 30 -450 mK, for densities (0.02 -2.2) X10' cm We compared the measured escape rates with calculated tunneling rates in a model where the interactions between the escaping electron and the other electrons are described by an effective single-particle potential. Below 200 mK the escape rates were temperature independent. The single-particle rates were enhanced exponentially as the density was increased up to a critical density n, . In this regime the calculated and measured rates were in good agreement. At n, we observe a one order of magnitude steep rise and the time development of density becomes extremely nonlinear. We show that this nonlinearity can be explained by the electron impact excitation of helium atoms on the walls of the cell. As the barrier is raised so that the escape rates decrease below 5.0X 10 sec ', a new mechanism seems to dominate the escape and the rates become very weakly density and external field dependent. Thermally activated escape was observed above 250 rnK and the activation energies were in good agreement with the calculated values.

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