States In Two-dimensional Electron Systems Research Articles

Unconventional superconductivity in two-dimensional electron systems with long-range correlations

Properties of superfluid states of two-dimensional electron systems with critical antiferromagnetic fluctuations are investigated. These correlations are found to result in the emergence of rapidly varying in the momentum space terms in all components of the mass operator, including the gap function $\Delta({\bf p})$. It is shown that a domain, where these terms reside, shrinks with the temperature, leading to a significant difference between the temperature $T_c$, at which superconductivity is terminated, and the temperature $T^*$, where the gap in the single-particle spectrum vanishes.

Anisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field

We report the observation of an acute sensitivity of the anisotropic longitudinal resistivity of two-dimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic field. In the third and higher Landau levels, at filling fractions $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9/2$, 11/2, etc., the in-plane field can lead to a striking interchange of the ``hard'' and ``easy'' transport directions. In the second Landau level the normally isotropic resistivity and the weak $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2$ quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic.

Possible triplet superconductivity in MOSFETs

A theory that predicts a spin-triplet, even-parity superconducting ground state in two-dimensional electron systems is re-analyzed in the light of recent experiments showing a possible insulator-to-conductor transition in such systems. It is shown that the observations are consistent with such an exotic superconductivity mechanism, and predictions are made for experiments that would further corroborate or refute this proposal.

Incompressible, metallic and insulating states of two-dimensional systems: Recent experiments

This paper provides a brief summary of some recent experimental results regarding the states of two-dimensional electron systems at high magnetic fields. The states include the incompressible, quantum Hall liquid at integer and fractional Landau-level fillings, the metallic-like state of the flux-particle composite Fermions at half-filling and the insulating phase at low fillings. Some unifying features of these states and their interrelation are also discussed.

Enhanced finite-wave-vector conductivity at multiple even-denominator filling factors in two-dimensional electron systems.

High-frequency surface acoustic waves (g2 GHz, \ensuremath{\lambda}2 \ensuremath{\mu}m) have been propagated on high mobility ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs heterostructures, allowing measurement of the finite-wave-vector conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{x}\mathit{x}}$(q,\ensuremath{\omega}). We find enhanced ${\mathrm{\ensuremath{\sigma}}}_{\mathit{x}\mathit{x}}$(q,\ensuremath{\omega}) at multiple even-denominator filling factors \ensuremath{\nu}=1/2, 3/2, 1/4, and 3/4 with ${\mathrm{\ensuremath{\sigma}}}_{\mathit{x}\mathit{x}}$(q,\ensuremath{\omega}) increasing linearly in q. The enhanced ${\mathrm{\ensuremath{\sigma}}}_{\mathit{x}\mathit{x}}$(q,\ensuremath{\omega}) at \ensuremath{\nu}=1/2 persists to high temperatures (g4 K), above the temperatures at which the fractional quantum Hall effect is observed. Empirically, the effects represent a series of robust states in two-dimensional electron systems that support gapless excitations. These data are strikingly consistent with a recent theory describing Fermi-surface formation at even denominator \ensuremath{\nu}.

Landau level quantization and superconductivity

A microscopic calculation of vortex-lattice states in two-dimensional electron systems at strong magnetic fields is made taking fully the Landau level quantization into account within the mean field scheme. Results of the order parameter and the local density of states are presented both in the limit of pairing in a single Landau level and in the semiclassical regime of weaker fields, and differences from the Abrikosov vortex state are discussed.

Density of States of Two-Dimensional Electron Systems at High Magnetic Fields

The density of states of the two-dimensional electron gas at high magnetic fields has been measured via magnetization. These experiments, along with recent specific heat, magneto-capacitance and activated transport measurements of the density of states, will be reviewed and compared. The common result, that a significant DOS exists in the Landau gap, will be discussed in the framework of current theoretical work.

Density of states and de Haas-van Alphen effect in two-dimensional electron systems.

The density of states of two-dimensional electron systems in GaAs/AlGaAs single-layer and multilayer heterostructures has been determined through measurements of the high-field magnetization. Our results reveal a substantial density of states between Landau levels, even in high-mobility single quantum wells. There is no existing theoretical explanation for this anomaly.