Showing papers by "Solid State Physics Laboratory published in 2016"

Single-spin CCD

Abstract: Spin-based electronics or spintronics relies on the ability to store, transport and manipulate electron spin polarization with great precision. In its ultimate limit, information is stored in the spin state of a single electron, at which point quantum information processing also becomes a possibility. Here, we demonstrate the manipulation, transport and readout of individual electron spins in a linear array of three semiconductor quantum dots. First, we demonstrate single-shot readout of three spins with fidelities of 97% on average, using an approach analogous to the operation of a charge-coupled device (CCD). Next, we perform site-selective control of the three spins, thereby writing the content of each pixel of this 'single-spin charge-coupled device'. Finally, we show that shuttling an electron back and forth in the array hundreds of times, covering a cumulative distance of 80 μm, has negligible influence on its spin projection. Extrapolating these results to the case of much larger arrays points at a diverse range of potential applications, from quantum information to imaging and sensing.

Correction: One step in situ synthesis of CeO2 nanoparticles grown on reduced graphene oxide as an excellent fluorescent and photocatalyst material under sunlight irradiation.

Abstract: CeO2 nanoparticles (NPs) with average particle size of ∼17 nm were grown on graphene sheets by simply mixing cerium chloride as the Ce precursor with graphene oxide (GO) in distilled water and the simultaneous reduction of GO to reduced graphene oxide (rGO), followed by a one-step hydrothermal treatment at 150 °C. A unique blue to green tuneable luminescence was observed as a function of the excitation wavelength. With this method, significant applications of rGO-CeO2 nanocomposites in many optical devices could be realized. The photocatalytic activity of the as-synthesized CeO2 and rGO-CeO2 nanocomposite was investigated by monitoring the degradation of methylene blue (MB) dye under direct sunlight irradiation. The rGO-CeO2 nanocomposite exhibited excellent photocatalytic activity compared to CeO2 NPs by degrading 90% of the MB dye in 10 min irradiation under sunlight. This property of rGO-CeO2 nanocomposites was ascribed to the significant suppression of the recombination rate of photo-generated electron-hole pairs due to charge transfer between rGO sheets and CeO2 NPs and the smaller optical band-gap in the rGO-CeO2 nanocomposite.

Computer-automated tuning of semiconductor double quantum dots into the single-electron regime

Abstract: We report the computer-automated tuning of gate-defined semiconductor double quantum dots in GaAs heterostructures. We benchmark the algorithm by creating three double quantum dots inside a linear array of four quantum dots. The algorithm sets the correct gate voltages for all the gates to tune the double quantum dots into the single-electron regime. The algorithm only requires (1) prior knowledge of the gate design and (2) the pinch-off value of the single gate T that is shared by all the quantum dots. This work significantly alleviates the user effort required to tune multiple quantum dot devices.

Cerium oxide nanoparticles promote neurogenesis and abrogate hypoxia-induced memory impairment through AMPK-PKC-CBP signaling cascade.

Abstract: Structural and functional integrity of the brain is adversely affected by reduced oxygen saturation, especially during chronic hypoxia exposure and often encountered by altitude travelers or dwellers. Hypoxia-induced generation of reactive nitrogen and oxygen species reportedly affects the cortex and hippocampus regions of the brain, promoting memory impairment and cognitive dysfunction. Cerium oxide nanoparticles (CNPs), also known as nanoceria, switch between +3 and +4 oxidation states and reportedly scavenge superoxide anions, hydrogen peroxide, and peroxynitrite in vivo. In the present study, we evaluated the neuroprotective as well as the cognition-enhancing activities of nanoceria during hypobaric hypoxia. Using polyethylene glycol-coated 3 nm nanoceria (PEG-CNPs), we have demonstrated efficient localization of PEG-CNPs in rodent brain. This resulted in significant reduction of oxidative stress and associated damage during hypoxia exposure. Morris water maze-based memory function tests revealed that PEG-CNPs ameliorated hypoxia-induced memory impairment. Using microscopic, flow cytometric, and histological studies, we also provide evidences that PEG-CNPs augmented hippocampus neuronal survival and promoted neurogenesis. Molecular studies revealed that PEG-CNPs promoted neurogenesis through the 5′-adenine monophosphate-activated protein kinase–protein kinase C–cyclic adenosine monophosphate response element-binding protein binding (AMPK-PKC-CBP) protein pathway. Our present study results suggest that nanoceria can be translated as promising therapeutic molecules for neurodegenerative diseases.

Nano-iron pyrite seed dressing: a sustainable intervention to reduce fertilizer consumption in vegetable (beetroot, carrot), spice (fenugreek), fodder (alfalfa), and oilseed (mustard, sesamum) crops

Abstract: Continuous agricultural innovations are required to feed the exploding human population through natural or artificial resources. Though light is ample on earth, two-third of unavailable ocean and one-third of available soil are major limiting factors to free growth. Excessive fertilizer usage is irreversibly altering the chemical ecology of soil, further reducing the available area. Seed metabolism might be a potential answer to this resource crunch. Without genetic modification and thus maintaining the existing biodiversity, manipulation of seed metabolism at the very onset of germination is a sustainable alternative. The current work presents seed priming with iron pyrite (FeS2) prior to sowing as one such sustainable and innovative intervention to reduce fertilizer consumption in vegetable (beetroot, carrot), spice (fenugreek), fodder (alfalfa), and oilseed (mustard, sesamum) crops. A 12-h seed pretreatment in an aqueous suspension of nano-iron disulfide/pyrite (FeS2) resulted in significant yield increase in the above crops. While agriculturists aim to restore the natural genomic diversity of different domesticated crops, environmental engineers require technologies to reduce fertilizer consumption without compromising agricultural yields, thereby making the planet more sustainable. This nanoscale seed pretreatment approach using FeS2, otherwise a benign earth abundant mineral, suggests the sustainable opportunity to translate this technology to other crops thereby enhancing the global agricultural production.

Hierarchical 3D NiCo 2 O 4 nanoflowers as electrode materials for high performance supercapacitors

Abstract: Three-dimensional NiCo2O4 nanoflowers have been directly grown on a stainless steel substrate surface, using a facile chemical bath deposition (CBD) technique. The deposited thin films were characterized for their structural, morphological and electrochemical properties by using XRD, SEM, cyclic voltammetry and charge discharge methods. The 3D NiCo2O4 nanoflowers were used as working electrode to measure the supercapacitor performance. The 3D NiCo2O4 nanoflowers exhibit high specific capacitance of 543 Fg−1 at current density 1 Ag−1. The capacitance loss was 9.4 % after 1000 cycles at a current density of 3 Ag−1. This shows a good cycle stability and high rate capability of 3D NiCo2O4 nanoflowers. From this investigation it can be concluded that the low cost and environmental friendly CBD technique could be used to deposit efficient 3D NiCo2O4 nanoflowers for supercapacitor application.

Enhanced CO gas sensing properties of Cu doped SnO2 nanostructures prepared by a facile wet chemical method.

Abstract: We report the synthesis of Cu doped SnO2 nanostructures with enhanced CO gas sensing properties by a facile wet chemical method. The effects of Cu doping on the structural and optical properties of SnO2 nanostructures were investigated using X-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) with energy dispersive X-ray spectroscopy, Raman spectroscopy and photoluminescence spectroscopy. FESEM studies revealed the presence of nanosheets and nanodisc-like structures in Cu doped SnO2 samples. Gas sensing studies showed that the sensor prepared using 1% Cu doped SnO2 nanostructures exhibits highly enhanced CO gas sensing properties as compared to pure SnO2 nanostructures and shows excellent selectivity for CO with negligible interference from CH4, CO2 and NO2. The possible mechanism for the enhanced CO gas sensing properties of Cu doped SnO2 nanostructures is proposed.

Measuring the Degeneracy of Discrete Energy Levels Using a GaAs/AlGaAs Quantum Dot.

Abstract: Two new transport measurements in GaAs quantum dots--one addressing spin-related transport, another quantum state degeneracy--use methods that could be adopted in other systems.

Computer-automated tuning of semiconductor double quantum dots into the single-electron regime

Abstract: We report the computer-automated tuning of gate-defined semiconductor double quantum dots in GaAs heterostructures. We benchmark the algorithm by creating three double quantum dots inside a linear array of four quantum dots. The algorithm sets the correct gate voltages for all the gates to tune the double quantum dots into the single-electron regime. The algorithm only requires (1) prior knowledge of the gate design and (2) the pinch-off value of the single gate $T$ that is shared by all the quantum dots. This work significantly alleviates the user effort required to tune multiple quantum dot devices.

Design and fabrication of an innovative three-axis Hall sensor

Abstract: The measurement of all three components of a magnetic field, simultaneously to high precision with Hall sensors, remains a challenge. Given the high precision of state-of-the-art conventional uniaxial Hall sensors, this is disappointing. Currently, three-axis Hall sensors suffer from either, or a combination, of the following: large spatial distribution between active areas; high signal noise; cross-sensitivity between measurement axes due to angular errors or the planar Hall effect; the inability to measure at a single point in space and time. A new type of three-axis Hall sensor is proposed, consisting of three sets of uniaxial Hall sensors in a small active volume. The feasibility of the proposed sensor has been proven in a prototype with an active volume as small as 200 μm × 200 μm × 200 μm. Due to its unique configuration, the new sensor addresses current three-axis Hall sensor limitations: it provides a high spatial resolution of 30 μm × 30 μm × 1 μm for each field component; full field vector measurements practically at a single point in space and time; and a reduction of the planar Hall effect by a factor of 35. Angular errors between the individual Hall sensors in the prototype lie between 0.1° and 0.5°, above the tolerable error for non-corrected measurements. However, once understood they can be taken into account. With proper calibration, this type of three-axis Hall sensor has great potential for high-accuracy three-axis magnetic field measurements and is particularly suitable for field mapping of magnets.

Current-Controlled Spin Precession of Quasistationary Electrons in a Cubic Spin-Orbit Field.

Abstract: Space- and time-resolved measurements of spin drift and diffusion are performed on a GaAs-hosted two-dimensional electron gas. For spins where forward drift is compensated by backward diffusion, we find a precession frequency in the absence of an external magnetic field. The frequency depends linearly on the drift velocity and is explained by the cubic Dresselhaus spin-orbit interaction, for which drift leads to a spin precession angle twice that of spins that diffuse the same distance.

Experimental signatures of the inverted phase in InAs/GaSb coupled quantum wells

Abstract: Transport measurements are performed on InAs/GaSb double quantum wells at zero and finite magnetic fields applied parallel and perpendicular to the quantum wells. We investigate a sample in the inverted regime where electrons and holes coexist, and compare it with another sample in the noninverted semiconducting regime. The activated behavior in conjunction with a strong suppression of the resistance peak at the charge neutrality point in a parallel magnetic field attest to the topological hybridization gap between electron and hole bands in the inverted sample. We observe an unconventional Landau level spectrum with energy gaps modulated by the magnetic field applied perpendicular to the quantum wells. This is caused by a strong spin-orbit interaction provided jointly by the InAs and the GaSb quantum wells.

Influence of laser repetition rate on the structural and optical properties of GaN layers grown on sapphire (0001) by laser molecular beam epitaxy

Abstract: High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices.

Equilibrium free energy measurement of a confined electron driven out of equilibrium

Abstract: The Jarzynski equality relates nonequilibrium thermodynamics and equilibrium states. We realize a coupled quantum dot-electron reservoir system in which the time resolved observation of the tunneling dynamics is used to explicitly measure the exerted work and dissipated heat per single charge. We determine accurate values of the equilibrium free energy change over a large range of final state energies by driving the system far from equilibrium.

Spin-Orbit Coupling at the Level of a Single Electron.

Abstract: We utilize electron counting techniques to distinguish a spin-conserving fast tunneling process and a slower process involving spin flips in AlGaAs/GaAs-based double quantum dots. By studying the dependence of the rates on the interdot tunnel coupling of the two dots, we find that as many as 4% of the tunneling events occur with a spin flip related to spin-orbit coupling in GaAs. Our measurement has a fidelity of 99% in terms of resolving whether a tunneling event occurred with a spin flip or not.

Nano iron pyrite (FeS2) exhibits bi-functional electrode character

Abstract: Sustainable charge storage devices require materials that are environmentally benign, readily moldable, easily synthesizable, and profitable for applications in the electronics industry. Nano iron pyrite (FeS2) is one such material, which is applicable in diverse areas like photovoltaic devices to seed dressing in agriculture. In this work, we propose an innovative application of nano FeS2viz., as a symmetric charge storage device that is flexible, portable, and lightweight; along with its fabrication details. The device consists of a (H3PO4)/polyvinyl alcohol (PVA) electrolyte gel sandwiched between two similar electrodes made up of FeS2/poly-aniline (PA), upon which graphite sheets are used as current collectors. Electrodes were characterized by XRD, FTIR and SEM. The device was calibrated by cyclic voltammetry and charge–discharge cycle. In its present laboratory prototype form, it powers solid-state electronic devices and electric motors. Further refinements of this device will open up new avenues in the field of sustainable charge storage devices and low power electronics.

Piezo, ferro and dielectric properties of ceramic-polymer composites of 0-3 connectivity

Abstract: Ferroelectric PZT-PVDF composites (0–3 connectivity) of specific dimensions were made utilizing a hot press technique. Linear variation in the experimental density of the pellets made from 0.1 to 0.8 PZT volume fraction was observed. The surface morphology of the composite has been micro-graphed using SEM. The variation of the dielectric properties of the composites on PZT volume fractions is reported and analyzed. It is observed that there was an increment in dielectric constant (ϵr) with an increase in temperature and volume fractions. The piezoelectric strain coefficient (d33) increases with an increase in the volume fraction and voltage coefficient (g33) were calculated. Comparisons were made with the Furukawa model to ϵr and d33. The remnant polarization and coercive field were analyzed from the hysteresis plots.

Band gap and broken chirality in single-layer and bilayer graphene

Abstract: Chirality is one of the key features governing the electronic properties of single- and bilayer graphene: the basics of this concept and its consequences on transport are presented in this review. By breaking the inversion symmetry, a band gap can be opened in the band structures of both systems at the K-point. This leads to interesting consequences for the pseu-dospin and, therefore, for the chirality. These consequences can be accessed by investigating the evolution of the Berry phase in such systems. Experimental observations of Fabry–Perot interference in a dual-gated bilayer graphene device are finally presented and are used to illustrate the role played by the band gap on the evolution of the pseudospin. The presented results can be attributed to the breaking of the chirality in the energy range close to the gap.

Nanosecond-timescale spin transfer using individual electrons in a quadruple-quantum-dot device

Abstract: The ability to coherently transport electron-spin states between different sites of gate-defined semiconductor quantum dots is an essential ingredient for a quantum-dot-based quantum computer. Previous shuttles using electrostatic gating were too slow to move an electron within the spin dephasing time across an array. Here, we report a nanosecond-timescale spin transfer of individual electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation rates at a so-called hot spot, we can upper bound the shuttle time to at most 150 ns. While actual shuttle times are likely shorter, 150 ns is already fast enough to preserve spin coherence in, e.g., silicon based quantum dots. This work therefore realizes an important prerequisite for coherent spin transfer in quantum dot arrays.

Dislocation density investigation on MOCVD-grown GaN epitaxial layers using wet and dry defect selective etching

Abstract: Results on the investigations of the dislocation etch pits in the GaN layers grown on sapphire substrate by metal organic chemical vapor deposition are revealed by wet chemical etching, and dry etching techniques are reported. The wet etching was carried out in molten KOH, and inductively coupled plasma (ICP) was used for dry etching. We show that ICP using dry etching and wet chemical etching using KOH solution under optimal conditions give values of dislocation density comparable to the one obtained from the high-resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy investigations. Investigated threading dislocation density is in the order of ~109/cm2 using different techniques.

The seed stimulant effect of nano iron pyrite is compromised by nano cerium oxide: regulation by the trace ionic species generated in the aqueous suspension of iron pyrite

Abstract: A brief seed pretreatment of 12 hours, in an aqueous suspension of synthesized nano iron pyrite (FeS2), significantly increases the yield of spinach and other crops. The effector mechanism is not clear. An aqueous suspension of FeS2, produces very trace amounts of H2O2, Fe2O3, FeS, FeSO4, Fe(SO4)3, SO2, S and H+ ionic species. Thus for 12 hours, seeds are exposed to this complex aqueous suspension. Among these trace species, H2O2 and Fe2O3 are known seed stimulants and plant growth promoters respectively. In this work, an attempt has been made to quench one of these trace compounds generated in the aqueous suspension of FeS2, viz., H2O2; and the long-term effect on the mature plant was monitored. To test this, along with FeS2, an agriculturally relevant inorganic peroxide scavenger viz., nano cerium oxide (CeO2) was introduced into this system. Four seed pretreatment regimens were followed for the spinach viz., (i) control (water), (ii) FeS2 + water, (iii) CeO2 + water, (iv) FeS2 + CeO2 + water; and growth was monitored for the next 80 days. It was found that, at maturity, CeO2 and FeS2 + CeO2, resulted in significantly smaller leaves, as compared to the control and FeS2; furthermore, FeS2 resulted in leaves with increased chlorophyll and carbohydrate. Thus, the data indicates that by quenching the H2O2, the seed-stimulant effect of FeS2 is compromised. So, while the FeS2 + water suspension functions as a seed vigor enhancer, CeO2 + water on the contrary, functions as a ‘seed vigor reducer’. It is noteworthy, that CeO2 is used by Chinese farmers, as a micro-nutrient to increase crop production. Current data indicates that, it delays germination of seeds, whereas FeS2 hastens germination. Thus such an approach could be used for hastening or delaying germination, manipulating weed population, seed storage in critical conditions, timing the life-cycle of a plant and developing more energy-efficient plants, especially in regions, where there is limited sunlight during significant parts of the year.

Nuclear magnetic resonance and nuclear spin relaxation in AlAs quantum well probed by ESR

Abstract: The study of nuclear magnetic resonance and nuclear spin-lattice relaxation was conducted in an asymmetrically doped to $n\ensuremath{\sim}1.8\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}16$ nm AlAs quantum well grown in the $[001]$ direction. The dynamic polarization of nuclear spins due to a hyperfine interaction resulted in the so-called Overhauser shift of two-dimensional conduction electron spin resonance. The maximum shifts achieved in the experiments are several orders of magnitude smaller than in GaAs-based heterostructures, indicating that the hyperfine interaction is weak. The nuclear spin-lattice relaxation time extracted from the decay of the Overhauser shift over time turned out to depend on the filling factor of the two-dimensional electron system. This observation indicates that nuclear spin-lattice relaxation is mostly due to the interaction between electron and nuclear spins. The Overhauser shift diminishes resonantly when the rf radiation of certain frequencies was applied to the sample. This effect served as an indirect, yet powerful, method for nuclear magnetic resonance detection: NMR quadrupole splitting of $^{75}\mathrm{As}$ nuclei was clearly resolved. Theoretical calculations performed describe well these experimental findings.

Nanosecond-timescale spin transfer using individual electrons in a quadruple-quantum-dot device

Abstract: The ability to coherently transport electron-spin states between different sites of gate-defined semiconductor quantum dots is an essential ingredient for a quantum-dot-based quantum computer. Previous shuttles using electrostatic gating were too slow to move an electron within the spin dephasing time across an array. Here we report a nanosecond-timescale spin transfer of individual electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation rates at a so-called `hot spot', we can upper bound the shuttle time to at most 150 ns. While actual shuttle times are likely shorter, 150 ns is already fast enough to preserve spin coherence in e.g. silicon based quantum dots. This work therefore realizes an important prerequisite for coherent spin transfer in quantum dot arrays.

Structured back gates for high-mobility two-dimensional electron systems using oxygen ion implantation

Abstract: We present a reliable method to obtain patterned back gates compatible with high mobility molecular beam epitaxy via local oxygen ion implantation that suppresses the conductivity of an 80 nm thick silicon doped GaAs epilayer. Our technique was optimized to circumvent several constraints of other gating and implantation methods. The ion-implanted surface remains atomically flat which allows unperturbed epitaxial overgrowth. We demonstrate the practical application of this gating technique by using magneto-transport spectroscopy on a two-dimensional electron system (2DES) with a mobility exceeding 20 × 106 cm2/V s. The back gate was spatially separated from the Ohmic contacts of the 2DES, thus minimizing the probability for electrical shorts or leakage and permitting simple contacting schemes.