3,916 research outputs found
Observation of a two-dimensional spin-lattice in non-magnetic semiconductor heterostructures
Tunable magnetic interactions in high-mobility nonmagnetic semiconductor
heterostructures are centrally important to spin-based quantum technologies.
Conventionally, this requires incorporation of "magnetic impurities" within the
two-dimensional (2D) electron layer of the heterostructures, which is achieved
either by doping with ferromagnetic atoms, or by electrostatically printing
artificial atoms or quantum dots. Here we report experimental evidence of a
third, and intrinsic, source of localized spins in high-mobility GaAs/AlGaAs
heterostructures, which are clearly observed in the limit of large setback
distance (=80 nm) in modulation doping. Local nonequilibrium transport
spectroscopy in these systems reveals existence of multiple spins, which are
located in a quasi-regular manner in the 2D Fermi sea, and mutually interact at
temperatures below 100 milliKelvin via the Ruderman-Kittel-Kasuya-Yosida (RKKY)
indirect exchange. The presence of such a spin-array, whose microscopic origin
appears to be disorder-bound, simulates a 2D lattice-Kondo system with
gate-tunable energy scales.Comment: 7 pages + 4 figs. To appear in Nature Physics. This is the original
submitted version. Final version will be posted six months after publication.
The Supplementary Information can be downloaded from:
http://www.physics.iisc.ernet.in/~arindam/Supplementary_Information_NPHYS-2006-08-0
0812B.pd
Y coupled terahertz quantum cascade lasers
Here we demonstrate a Y coupled terahertz (THz) quantum cascade laser (QCL)
system. The two THz QCLs working around 2.85 THz are driven by independent
electrical pulsers. Total peak THz output power of the Y system, with both arms
being driven synchronously, is found to be more than the linear sum of the peak
powers from the individual arms; 10.4 mW compared with 9.6 mW (4.7 mW + 4.9
mW). Furthermore, we demonstrate that the emission spectra of this coupled
system are significantly different to that of either arm alone, or to the
linear combination of their individual spectra.Comment: 9 pages, 3 figure
Reversible Mode Switching in Y coupled Terahertz Lasers
Electrically independent terahertz (THz) quantum cascade lasers (QCLs) are
optically coupled in a Y configuration. Dual frequency, electronically
switchable emission is achieved in one QCL using an aperiodic grating, designed
using computer-generated hologram techniques, incorporated directly into the
QCL waveguide by focussed ion beam milling. Multi-moded emission around 2.9 THz
is inhibited, lasing instead occurring at switchable grating-selected
frequencies of 2.88 and 2.92 THz. This photonic control and switching behaviour
is selectively and reversibly transferred to the second, unmodified QCL via
evanescent mode coupling, without the transfer of the inherent grating losses
Transport Through an Electrostatically Defined Quantum Dot Lattice in a Two-Dimensional Electron Gas
Quantum dot lattices (QDLs) have the potential to allow for the tailoring of
optical, magnetic and electronic properties of a user-defined artificial solid.
We use a dual gated device structure to controllably tune the potential
landscape in a GaAs/AlGaAs two-dimensional electron gas, thereby enabling the
formation of a periodic QDL. The current-voltage characteristics, I(V), follow
a power law, as expected for a QDL. In addition, a systematic study of the
scaling behavior of I(V) allows us to probe the effects of background disorder
on transport through the QDL. Our results are particularly important for
semiconductor-based QDL architectures which aim to probe collective phenomena.Comment: 6 pages, 4 figure
Thermoelectric Properties of Electrostatically Tunable Antidot Lattices
We report on the fabrication and characterization of a device which allows
the formation of an antidot lattice (ADL) using only electrostatic gating. The
antidot potential and Fermi energy of the system can be tuned independently.
Well defined commensurability features in magnetoresistance as well as
magnetothermopower are obsereved. We show that the thermopower can be used to
efficiently map out the potential landscape of the ADL.Comment: 4 pages, 3 figures; to appear in Appl. Phys. Let
Evolution of entanglement within classical light states
We investigate the evolution of quantum correlations over the lifetime of a
multi-photon state. Measurements reveal time-dependent oscillations of the
entanglement fidelity for photon pairs created by a single semiconductor
quantum dot. The oscillations are attributed to the phase acquired in the
intermediate, non-degenerate, exciton-photon state and are consistent with
simulations. We conclude that emission of photon pairs by a typical quantum dot
with finite polarisation splitting is in fact entangled in a time-evolving
state, and not classically correlated as previously regarded
Giant Stark effect in the emission of single semiconductor quantum dots
We study the quantum-confined Stark effect in single InAs/GaAs quantum dots
embedded within a AlGaAs/GaAs/AlGaAs quantum well. By significantly increasing
the barrier height we can observe emission from a dot at electric fields of
-500 kV/cm, leading to Stark shifts of up to 25 meV. Our results suggest this
technique may enable future applications that require self-assembled dots with
transitions at the same energy
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