J'P' Eisenstein, Caltech, DMR0242946

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Nature of Excitonic Bose Condensation in the
Quantum Hall Regime
J.P. Eisenstein, Caltech, DMR-0242946
When two layers of electrons are brought close
together in the presence of an intense magnetic
field a new state of matter emerges. In this new
state electrons in one layer become bound, in
effect, to the voids between electrons in the
other layer. These electronhole complexes,
known as excitons, are similar to Cooper pairs in
a superconductor, only they possess no net
charge. Like Cooper pairs, excitons are bosons
and may undergo Bose-Einstein condensation. The
nature of this exciton condensate has been the
focus of our research in recent years. In spite
of the similarity to superconductivity, the
nature of the transition to the excitonic state
remains poorly understood. Much is known about
the system when the layers are far apart and no
excitons are present, and when the layers are
close together and the exciton condensate is
well-developed. It is the intermediate, or
critical, region which remains mysterious. We
have made a number of significant discoveries
concerning the transition region. Most recently,
we find that the spin configuration of the system
changes abruptly upon crossing the critical
point. This is an unexpected result, since it is
usually assumed that the spins of all electrons
in the system are aligned by the magnetic field.
Our results, which involve the application of
nuclear magnetic resonance methods, reveal that
the degree of spin alignment increases when the
excitons form. This is an important finding and
suggests that the underlying phase transition is
first-order.
B
Schematic illustration of exciton condensation in
a double layer 2D electron system in the presence
of a large perpendicular magnetic field, B. In
the upper image the layers are far apart and the
electrons in one layer ignore those in the other.
In the lower image the layers are much closer
together. Electrons in each layer bind onto
holes in the opposite layer. These complexes are
excitons and, being bosons, undergo Bose-Einstein
condensation. The nature of the transition
between the two regimes is poorly understood has
been the focus of our recent work.
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Nature of Excitonic Bose Condensation in the
Quantum Hall Regime
J.P. Eisenstein, Caltech, DMR-0242946
Graduate Students Lisa Tracy, Melinda J.
Kellogg and Ian B. Spielman, Xerxes
Lopez-Yglesias Collaborators Loren Pfeiffer and
Ken West, Bell Labs
Related Publications Spin Transition in a
Strongly Correlated Bilayer Two-Dimensional
Electron System, Physical Review Letters 94,
076803 (2005). Bose Einstein Condensation of
Excitons in Bilayer Electron Systems, Nature
432, 691 (2004). Half Full or Half Empty?
Science, 305, 950 (2004). Onset of Interlayer
Phase Coherence in a Bilayer Two-Dimensional
Electron System Effect of Layer Density
Imbalance, Phys. Rev. B, 70, 081303(R) (2004).
Tunneling conductance vs. interlayer voltage in
double layer 2D electron system before and after
destruction of nuclear polarization with NMR
pulse. Sharp peak near V0 signals presence of
exciton condensate. These data prove that spin
polarization of excitonic phase is larger than
that of the weakly-coupled phase.
Selected Invited Presentations Spin Dependent
Onset of Exciton Condensation in Bilayer Quantum
Hall Systems March 2005 APS meeting, Los
Angeles, (Tracy). Observations of Nascent
Superfluidity in a Bilayer 2D Electron System at
? 1, 16th Intl. Conference on Electronic
Properties of Two Dimensional Systems,
Albuquerque, July 2005 (Kellogg). Bose
Condensation of Excitons and the Quantum Hall
Effect, EuroConference on Ultra-Cold Gases and
their Applications Bose Einstein Condensation,
San Feliu de Guixols, Sept. 2005 (Eisenstein).