J. P. Eisenstein

Formation of a high quality two‐dimensional electron gas on cleaved GaAs

We have succeeded in fabricating a two-dimensional electron gas (2DEG) on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE). A (100) wafer previously prepared by MBE growth is reinstalled in the MBE chamber so that an in situ cleave exposes a fresh (110) GaAs edge for further MBE overgrowth. A sequence of Si-doped AlGaAs layers completes the modulation-doped structure at the cleaved edge. Mobilities as high as 6.1×10^5 cm^2/V s are measured in the 2DEG at the cleaved interface.

Terahertz photoconductivity and plasmon modes in double-quantum-well field-effect transistors

Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors.

Bose-Einstein condensation of excitons in bilayer electron systems.

An exciton is the particle-like entity that forms when an electron is bound to a positively charged ‘hole’. An ordered electronic state in which excitons condense into a single quantum state was proposed as a theoretical possibility many years ago. We review recent studies of semiconductor bilayer systems that provide clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime, where these experiments were performed, is as likely to occur in electron–electron bilayers as in electron–hole bilayers. In current quantum Hall excitonic condensates, disorder induces mobile vortices that flow in response to a supercurrent and limit the extremely large bilayer counterflow conductivity.

Evidence for an anisotropic state of two-dimensional electrons in high Landau levels

We recently reported [PRL 82, 394 (1999)] large transport anisotropies in a two-dimensional electron gas in high Landau levels. These observations were made utilizing both square and Hall bar sample geometries. Simon recently commented [cond-mat/9903086] that a classical calculation of the current flow in the sample shows a magnification of an underlying anisotropy when using a square sample. In this reply we present more recent data obtained with a very high mobility sample, and reiterate that, with or without magnification, an anisotropic state develops in high Landau levels at very low temperatures.

Resonantly Enhanced Tunneling in a Double Layer Quantum Hall Ferromagnet

The tunneling conductance between two parallel 2D electron systems has been measured in a regime of strong interlayer Coulomb correlations. At total Landau level filling

Nematic Fermi Fluids in Condensed Matter Physics

\nu_T=1

High frequency conductivity of the high-mobility two-dimensional electron gas

the tunnel spectrum changes qualitatively when the boundary separating the compressible phase from the ferromagnetic quantized Hall state is crossed. A huge resonant enhancement replaces the strongly suppressed equilibrium tunneling characteristic of weakly coupled layers. The possible relationship of this enhancement to the Goldstone mode of the broken symmetry ground state is discussed.

Vanishing Hall Resistance at High Magnetic Field in a Double-Layer Two-Dimensional Electron System

Correlated electron fluids can exhibit a startling array of complex phases, among which one of the more surprising is the electron nematic, a translationally invariant metallic phase with a spontaneously generated spatial anisotropy. Classicalnematicsgenerally occur in liquids of rod-like molecules; given that electrons are point like, the initial theoretical motivation for contemplating electron nematics came from thinking of the electron fluid as a quantum melted electron crystal, rather than a strongly interacting descendent of a Fermi gas. Dramatic transport experiments in ultra-clean quantum Hall systems in 1999 and in Sr3Ru2O7 in a strong magnetic field in 2007 established that such phases exist in nature. In this article, we briefly review the theoretical considerations governing nematic order, summarize the quantum Hall and Sr3Ru2O7 experiments that unambiguously establish the existence of this phase, and survey some of the current evidence for such a phase in the cuprate and Fe-based high temperature superconductors.

Independently contacted two-dimensional electron systems in double quantum wells

We measure the real and imaginary conductivity sigma(k = 0,omega) of a high-mobility two-dimensional electron gas (2DEG) system at frequencies below and above the momentum scattering rate. The imaginary part of the 2DEG impedance is observed to be inductive, consistent with the Drude model. Using this kinetic inductance, we construct a transmission line by capacitively coupling the 2DEG to an Al Schottky barrier gate separated by 5000 A from the 2DEG. The measured wave velocity and temperature-dependent damping of this transmission line are in good agreement with a simple Drude model. Exciting these modes is equivalent to exciting a 2D plasma mode strongly modified by the interaction between the 2DEG and the gate.

Observation of Quantized Hall Drag in a Strongly Correlated Bilayer Electron System

At total Landau level filling factor nu(tot)=1 a double-layer two-dimensional electron system with small interlayer separation supports a collective state possessing spontaneous interlayer phase coherence. This state exhibits the quantized Hall effect when equal electrical currents flow in parallel through the two layers. In contrast, if the currents in the two layers are equal, but oppositely directed, both the longitudinal and Hall resistances of each layer vanish in the low-temperature limit. This finding supports the prediction that the ground state at nu(tot)=1 is an excitonic superfluid.