167 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
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
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
Radiation-induced prodrug activation: extending combined modality therapy for some solid tumours
Combined chemoradiotherapy is the standard of care for locally advanced solid tumours. However, systemic toxicity may limit the delivery of planned chemotherapy. New approaches such as radiation-induced prodrug activation might diminish systemic toxicity, while retaining anticancer benefit. Organic azides have recently been shown to be reduced and activated under hypoxic conditions with clinically relevant doses of radiotherapy, uncaging pazopanib and doxorubicin in preclinical models with similar efficacy as the drug, but lower systemic toxicity. This approach may be relevant to the chemoradiation of glioblastoma and other solid tumours and offers potential for switching on drug delivery from implanted devices. The inclusion of reporters to confirm drug activation, avoidance of off-target effects and synchronisation of irradiation with optimal intratumoral drug concentration will be critical. Further preclinical validation studies of this approach should be encouraged
Tunable nanopatterning of conductive polymers via electrohydrodynamic lithography
[Image: see text] An increasing number of technologies require the fabrication of conductive structures on a broad range of scales and over large areas. Here, we introduce advanced yet simple electrohydrodynamic lithography (EHL) for patterning conductive polymers directly on a substrate with high fidelity. We illustrate the generality of this robust, low-cost method by structuring thin polypyrrole films via electric-field-induced instabilities, yielding well-defined conductive structures with feature sizes ranging from tens of micrometers to hundreds of nanometers. Exploitation of a conductive polymer induces free charge suppression of the field in the polymer film, paving the way for accessing scale sizes in the low submicron range. We show the feasibility of the polypyrrole-based structures for field-effect transistor devices. Controlled EHL pattering of conductive polymer structures at the micro and nano scale demonstrated in this study combined with the possibility of effectively tuning the dimensions of the tailor-made architectures might herald a route toward various submicron device applications in supercapacitors, photovoltaics, sensors, and electronic displays
Determining energy relaxation length scales in two-dimensional electron gases
We present measurements of the energy relaxation length scale in
two-dimensional electron gases (2DEGs). A temperature gradient is established
in the 2DEG by means of a heating current, and then the elevated electron
temperature is estimated by measuring the resultant thermovoltage signal
across a pair of deferentially biased bar-gates. We adapt a model by Rojek and
K\"{o}nig [Phys. Rev. B \textbf{90}, 115403 (2014)] to analyse the
thermovoltage signal and as a result extract , , and the power-law
exponent for inelastic scattering events in the 2DEG. We show that
in high-mobility 2DEGs, can attain macroscopic values of several hundred
microns, but decreases rapidly as the carrier density is decreased. Our
work demonstrates a versatile low-temperature thermometry scheme, and the
results provide important insights into heat transport mechanisms in
low-dimensional systems and nanostructures. These insights will be vital for
practical design considerations of future nanoelectronic circuits.We acknowledge funding from the Leverhulme Trust, UK and the Engineering and Physical Sciences Research Council (EPSRC), UK.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.492633
Demonstration of electron focusing using electronic lenses in low-dimensional system.
We report an all-electric integrable electron focusing lens in n-type GaAs. It is shown that a pronounced focusing peak takes place when the focal point aligns with an on-chip detector. The intensity and full width half maximum (FWHM) of the focusing peak are associated with the collimation of injected electrons. To demonstrate the reported focusing lens can be a useful tool, we investigate the characteristic of an asymmetrically gate biased quantum point contact with the assistance of a focusing lens. A correlation between the occurrence of conductance anomaly in low conductance regime and increase in FWHM of focusing peak is observed. The correlation is likely due to the electron-electron interaction. The reported electron focusing lens is essential for a more advanced electron optics device
Quantisation of Hopping Magnetoresistance Prefactor in Strongly Correlated Two-Dimensional Electron Systems
We report an universal behaviour of hopping transport in strongly interacting
mesoscopic two-dimensional electron systems (2DES). In a certain window of
background disorder, the resistivity at low perpendicular magnetic fields
follows the expected relation . The prefactor decreases exponentially with
increasing electron density but saturates to a finite value at higher
densities. Strikingly, this value is found to be universal when expressed in
terms of absolute resistance and and shows quantisation at and . We suggest a strongly correlated
electronic phase as a possible explanation.Comment: 5 pages, 3 figures, Proceedings of EP2DS 17, Reference adde
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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