Kevin Mathias Mertes

Small-angle shubnikov-de haas measurements in a 2D electron system: the effect of a strong In-plane magnetic field

Measurements in magnetic fields applied at small angles relative to the electron plane in silicon MOSFETs indicate a factor of 2 increase of the frequency of Shubnikov-de Haas oscillations at H>H(sat). This signals the onset of full spin polarization above H(sat), the parallel field above which the resistivity saturates to a constant value. For H<H(sat), the phase of the second harmonic of the oscillations relative to the first is consistent with scattering events that depend on the overlap instead of the sum of the spin-up and spin-down densities of states. This unusual behavior may reflect the importance of many-body interactions.

Distribution of Tunnel Splittings in Mn12 Acetate

In magnetic fields applied parallel to the anisotropy axis, the relaxation of the magnetization of Mn(12)-acetate measured for different sweep rates collapses onto a single scaled curve. The form of the scaling implies that the dominant symmetry-breaking process responsible for tunneling is a locally varying second-order transverse anisotropy, forbidden by tetragonal symmetry in the perfect crystal, which gives rise to a broad distribution of tunnel splittings in a real crystal of Mn(12) acetate. Different forms applied to even- and odd-numbered steps provide a clear distinction between even resonances (associated with crystal anisotropy) and odd resonances (which require a transverse magnetic field).

Mn12-acetate: a prototypical single molecule magnet

Abstract Single molecule magnets display fascinating quantum mechanical behavior, and may have important technological applications for information storage and quantum computation. A brief review is given of the physical properties of Mn12-acetate, one of the two prototypical molecular nanomagnets that have been most intensively investigated. Descriptions and discussions are given of the Mn12 magnetic cluster and the fundamental process of quantum tunneling of a nanoscopic spin magnetization; the distinction between thermally-assisted tunneling and pure quantum tunneling, and a study of the crossover between the two regimes; and a review of earlier investigations that suggest that the tunneling in this system is due to locally varying second-order crystal anisotropy which gives rise to a distribution of tunnel splittings. In the second part of the paper, we report results obtained by a new experimental method that confirm our earlier conclusion that the tunnel splittings in Mn12 are distributed rather than single-valued, as had been generally assumed.

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

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. {copyright} {ital 1999} {ital The American Physical Society}

Temperature dependence of the resistivity of a dilute two-dimensional electron system in high parallel magnetic field

We report measurements of the resistance of silicon metal-oxide-semiconductor field-effect transistors as a function of temperature in high parallel magnetic fields where the two-dimensional system of electrons has been shown to be fully spin polarized. In a field of 10.8 T, insulating behavior is found for densities up to n{sub s}{approx}1.35x10{sup 11}cm{sup -2}{approx}1.5n{sub c}; above this density the resistance is a very weak function of temperature, varying less than 10% between 0.25 and 1.90 K. At low densities {rho}{yields}{infinity} more rapidly as the temperature is reduced than in zero field and the magnetoresistance {Delta}{rho}/{rho} diverges as T{yields}0.

Abrupt crossover between thermally activated relaxation and quantum tunneling in a molecular magnet

We report Hall sensor measurements of the magnetic relaxation of Mn12 acetate as a function of magnetic field applied along the easy axis of magnetization. Detailed data taken at a series of closely spaced temperatures between 0.24 K and 1.4 K indicate an abrupt shift between thermally activated and ground-state tunneling over a narrow range of temperature.

Ground-state tunneling in Mn 12 -acetate

We report Hall sensor measurements of the magnetic relaxation of

Hall coefficient of a dilute two-dimensional electron system in a parallel magnetic field

{\mathrm{Mn}}_{12}

Classical versus quantum effects in the B=0 conducting phase in two dimensions

-acetate as a function of magnetic field applied along the easy axis of magnetization for a series of closely spaced temperatures between 0.24 K and 1.9 K. The occasional absence (or suppression) of ground-state tunneling under conditions where one would expect it to be readily observable is attributed to the presence of a broad distribution of tunnel splittings.

Local measurements of magnetization in Mn12 crystals

Measurements in magnetic fields applied at a small angle with respect to the two-dimensional plane of the electrons of low-density silicon metal-oxide--semiconductor field-effect transistors indicate that the Hall coefficient is independent of parallel field from