Showing papers by "Solid State Physics Laboratory published in 2014"

Seed treatment with iron pyrite (FeS2) nanoparticles increases the production of spinach

Abstract: Certain nano-materials are known to have plant growth promoting effects, which could find applications in agriculture. We drew inspiration from the nano-factories of deep-sea hydrothermal vents; where iron pyrite nanoparticles serve as fertilizer to sustain chemoautotrophic life forms. We synthesized such iron pyrite nanoparticles in a controlled environment and used them as seed treatment agent (Pro-fertilizer). For us, the term ‘pro-fertilizers’ represents those materials that cause enhanced plant growth with minimum interference to the soil ecosystem when used for seed treatment. We conducted multi-location field trials on spinach crops, since it is a globally popular crop, consumed as both fresh (salads) and processed food. The spinach seeds were treated for 14 hours in an aqueous suspension of iron pyrite nanoparticle (FeS2 + H2O) and thereafter directly sown in the field setup for the experiment. The control seeds were only treated in water for the same duration and sown directly in the field. After 50 days, the crop yields from iron-pyrite nanoparticle treated seeds and control seeds were evaluated. The plants developed from iron pyrite nanoparticle treated seeds exhibited significantly broader leaf morphology, larger leaf numbers, increased biomass; along with higher concentration of calcium, manganese and zinc in the leaves when compared to the plants developed from control seeds. We further investigated the possible mechanism resulting in the biomass enhancement following seed-treatment. Our results indicate that there is an enhanced breakdown of stored starch in the iron pyrite treated seeds resulting in significantly better growth. This raises the possibility of developing iron pyrite nanoparticles as a commercial seed-treatment agent (pro-fertilizer) for spinach crops.

Fabry-Pérot interference in gapped bilayer graphene with broken anti-Klein tunneling.

Abstract: We report the experimental observation of Fabry-Perot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device’s electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1-μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Perot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.

Cavity quantum electrodynamics with many-body states of a two-dimensional electron gas

Abstract: Light-matter interaction has played a central role in understanding as well as engineering new states of matter. Reversible coupling of excitons and photons enabled groundbreaking results in condensation and superfluidity of nonequilibrium quasiparticles with a photonic component. We investigated such cavity-polaritons in the presence of a high-mobility two-dimensional electron gas, exhibiting strongly correlated phases. When the cavity was on resonance with the Fermi level, we observed previously unknown many-body physics associated with a dynamical hole-scattering potential. In finite magnetic fields, polaritons show distinct signatures of integer and fractional quantum Hall ground states. Our results lay the groundwork for probing nonequilibrium dynamics of quantum Hall states and exploiting the electron density dependence of polariton splitting so as to obtain ultrastrong optical nonlinearities.

Characterization of Qubit Dephasing by Landau-Zener-Stückelberg-Majorana Interferometry

Abstract: Controlling coherent interaction at avoided crossings and the dynamics there is at the heart of quantum information processing. A particularly intriguing dynamics is observed in the Landau-Zener regime, where periodic passages through the avoided crossing result in an interference pattern carrying information about qubit properties. In this Letter, we demonstrate a straightforward method, based on steady-state experiments, to obtain all relevant information about a qubit, including complex environmental influences. We use a two-electron charge qubit defined in a lateral double quantum dot as test system and demonstrate a long coherence time of ${T}_{2}\ensuremath{\simeq}200\text{ }\text{ }\mathrm{ns}$, which is limited by electron-phonon interaction.

Anomalous sequence of quantum Hall liquids revealing a tunable Lifshitz transition in bilayer graphene.

Abstract: Bilayer graphene is a unique system where both the Fermi energy and the low-energy electron dispersion can be tuned. This is brought about by an interplay between trigonal warping and the band gap opened by a transverse electric field. Here, we drive the Lifshitz transition in bilayer graphene to experimentally controllable carrier densities by applying a large transverse electric field to a h-BN-encapsulated bilayer graphene structure. We perform magnetotransport measurements and investigate the different degeneracies in the Landau level spectrum. At low magnetic fields, the observation of filling factors $\ensuremath{-}3$ and $\ensuremath{-}6$ quantum Hall states reflects the existence of three maxima at the top of the valence-band dispersion. At high magnetic fields, all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample. This enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum. The observed evolution of the degeneracies, therefore, reveals the presence of a Lifshitz transition in our system.

Plasmon hybridization in pyramidal metamaterials: a route towards ultra-broadband absorption

Abstract: Pyramidal metamaterials are currently developed for ultra-broadband absorbers. They consist of periodic arrays of alternating metal/dielectric layers forming truncated square-based pyramids. The metallic layers of increasing lengths play the role of vertically and, to a less extent, laterally coupled plasmonic resonators. Based on detailed numerical simulations, we demonstrate that plasmon hybridization between such resonators helps in achieving ultra-broadband absorption. The dipolar modes of individual resonators are shown to be prominent in the electromagnetic coupling mechanism. Lateral coupling between adjacent pyramids and vertical coupling between alternating layers are proven to be key parameters for tuning of plasmon hybridization. Following optimization, the operational bandwidth of Au/Ge pyramids, i.e. the bandwidth within which absorption is higher than 90%, extends over a 0.2-5.8 µm wavelength range, i.e. from UV-visible to mid-infrared, and total absorption (integrated over the operational bandwidth) amounts to 98.0%. The omni-directional and polarization-independent high-absorption properties of the device are verified. Moreover, we show that the choice of the dielectric layer material (Si versus Ge) is not critical for achieving ultra-broadband characteristics, which confers versatility for both design and fabrication. Realistic fabrication scenarios are briefly discussed. This plasmon hybridization route could be useful in developing photothermal devices, thermal emitters or shielding devices that dissimulate objects from near infrared detectors.

Experimental probe of topological orders and edge excitations in the second Landau level

Abstract: This paper reports careful experiments designed to shed light on the natures of the fractional quantized Hall states observed at filling fractions 5/2, 7/3, and 8/3 in very high quality GaAs quantum wells.

Insulating state and giant nonlocal response in an InAs/GaSb quantum well in the quantum Hall regime.

Abstract: We present transport measurements performed in InAs=GaSb double quantum wells. At the electronhole crossover tuned by a gate voltage, a strong increase in the longitudinal resistivity is observed with increasing perpendicular magnetic field. Concomitantly with a local resistance exceeding the resistance quantum by an order of magnitude, we find a pronounced nonlocal resistance signal of almost similar magnitude. The coexistence of these two effects is reconciled in a model of counterpropagating and dissipative quantum Hall edge channels providing backscattering, shorted by a residual bulk conductivity.

Spin-Relaxation Anisotropy in a GaAs Quantum Dot

Abstract: We report that the electron spin-relaxation time T1 in a GaAs quantum dot with a spin-1/2 ground state has a 180° periodicity in the orientation of the in-plane magnetic field. This periodicity has been predicted for circular dots as being due to the interplay of Rashba and Dresselhaus spin orbit contributions. Different from this prediction, we find that the extrema in the T1 do not occur when the magnetic field is along the [110] and [11¯0] crystallographic directions. This deviation is attributed to an elliptical dot confining potential. The T1 varies by more than 1 order of magnitude when rotating a 3 T field, reaching about 80 ms for the optimal angle. We infer from the data that in our device the signs of the Rashba and Dresselhaus constants are opposite.

Nanoceria based electrochemical sensor for hydrogen peroxide detection

Abstract: Oxidative stress is a condition when the concentration of free radicals and reactive molecular species rise above certain level in living systems. This condition not only perturbs the normal physiology of the system but also has been implicated in many diseases in humans and other animals. Hydrogen peroxide (H2O2) is known to be involved in induction of oxidative stress and has also been linked to a variety of ailments such as inflammation, rheumatoid arthritis, diabetes, and cancer in humans. It is one of the more stable reactive molecular species present in living systems. Because of its stability and links with various diseases, sensing the level of H2O2 can be of great help in diagnosing these diseases, thereby easing disease management and amelioration. Nanoceria is a potent candidate in free radical scavenging as well as sensing because of its unique redox properties. These properties have been exploited, in the reported work, to sense and quantify peroxide levels. Nanoceria has been synthesized using different capping agents: Hexamethylene-tetra-amine (HMTA) and fructose. CeO2-HMTA show rhombohedral and cubic 6.4 nm particles whereas CeO2-fructose are found to be spherical with average particle diameter size 5.8 nm. CeO2-HMTA, due to the better exposure of the active (200) and (220) planes relative to (111) plane, exhibits superior electrocatalytic activity toward H2O2 reduction. Amperometric responses were measured by increasing H2O2 concentration. The authors observed a sensitivity of 21.13 and 9.6 μA cm−2 mM−1 for CeO2-HMTA and CeO2-fructose, respectively. The response time of 4.8 and 6.5 s was observed for CeO2-HMTA and CeO2-fructose, respectively. The limit of detection is as low as 0.6 and 2.0 μM at S/N ratio 3 for CeO2-HMTA and CeO2-fructose, respectively. Ceria-HMTA was further tested for its antioxidant activity in an animal cell line in vitro and the results confirmed its activity.

Characterizing wave functions in graphene nanodevices: Electronic transport through ultrashort graphene constrictions on a boron nitride substrate

Abstract: The nature of localized edge states in edge-disordered graphene nanoconstrictions connected to external leads is studied. Contrary to the general belief, the results show that the localization length is significantly longer than the size of the nanoconstriction, which results into charge spill over into the bulk as well.

Metal oxide SAW E-nose employing PCA and ANN for the identification of binary mixture of DMMP and methanol

Abstract: An Electronic nose (E-nose) made up of an array of metal oxide thin film based SAW sensors is proposed as a detection system for the binary mixture of DMMP (simulant of nerve agent) and methanol at room temperature The SAW devices are one port surface acoustic wave (SAW) resonators having center frequency of 4339 MHz The E-nose sensor array has SAW sensors coated with four different chemical sensitive coatings of ZnO, TeO2, SnO2 and TiO2 in order to recognize the individual components in a binary mixture of DMMP (in ppb levels) and methanol (in sub ppm levels) Principal Component Analysis (PCA) as a data pre-processing technique and Artificial Neural Network (ANN) as a pattern classification technique have been applied for obtaining a clear discrimination and correct classification

Cerium oxide nanoparticles prevent apoptosis in primary cortical culture by stabilizing mitochondrial membrane potential.

Abstract: Cerium oxide nanoparticles (CNPs) of spherical shape have unique antioxidant capacity primarily due to alternating + 3 and + 4 oxidation states and crystal defects. Several studies revealed the protective efficacies of CNPs in cells and tissues against the oxidative damage. However, its effect on mitochondrial functioning, downstream effectors of radical burst and apoptosis remains unknown. In this study, we investigated whether CNPs treatment could protect the primary cortical cells from loss of mitochondrial membrane potential (Δψm) and Δψm-dependent cell death. CNPs with spherical morphology and size range 7-10 nm were synthesized and utilized at a concentration of 25 nM on primary neuronal culture challenged with 50 μM of hydrogen peroxide (H2O2). We showed that optimal dose of CNPs minimized ROS content of the cells and also curbed related surge in cellular calcium flux. Importantly, CNPs treatment prevented apoptotic loss of cell viability. Reduction in the apoptosis could be successfully attributed to the maintenance of Δψm and restoration of major redox equivalents NADH/NAD(+) ratio and cellular ATP. These findings, therefore, suggest possible route of CNPs protective efficacies in primary cortical culture.

Suppressed decay of a laterally confined persistent spin helix

Abstract: We experimentally investigate the dynamics of a persistent spin helix in etched GaAs wire structures of $2--80\ensuremath{\mu}\text{m}$ width. Using magneto-optical Kerr rotation with high spatial resolution, we determine the lifetime of the spin helix. A few nanoseconds after locally injecting spin polarization into the wire, the polarization is strongly enhanced as compared to the two-dimensional case. This is mostly attributed to a transition to one-dimensional diffusion, strongly suppressing diffusive dilution of spin polarization. The intrinsic lifetime of the helical mode is only weakly increased, which indicates that the channel confinement can only partially suppress the cubic Dresselhaus spin-orbit interaction.

Imaging the Conductance of Integer and Fractional Quantum Hall Edge States

Abstract: We measure the conductance of a quantum point contact while the biased tip of a scanning probe microscope induces a depleted region in the electron gas underneath. At a finite magnetic field, we find plateaus in the real-spacemaps of the conductance as afunction of tip position atinteger(ν ¼ 1,2,3,4,6,8) and fractional (ν ¼ 1=3, 2=3, 5=3, 4=5) values of transmission. They resemble theoretically predicted compressible and incompressible stripes of quantum Hall edge states. The scanning tip allows us to shift the constriction limiting the conductance in real space over distances of many microns. The resulting stripes of integer and fractional filling factors are rugged on scales of a few hundred nanometers, i.e., on a scale much smaller than the zero-field elastic mean free path of the electrons. Our experiments demonstrate that microscopic inhomogeneities are relevant even in high-quality samples and lead to locally strongly fluctuating widths of incompressible regions even down to their complete suppression for certain tip positions.ThemacroscopicquantizationoftheHallresistancemeasuredexperimentallyinanonlocalcontact configuration survives in the presence of these inhomogeneities, and the relevant local energy scale for the ν ¼ 2 state turns out to be independent of tip position.

Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition

Abstract: Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO2/Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications.

Evidence of Fowler–Nordheim Tunneling in Gate Leakage Current of AlGaN/GaN HEMTs at Room Temperature

Abstract: Dependence of gate leakage current on Al mole fraction of AlGaN/GaN high-electron mobility transistors (HEMTs) is studied using temperature-dependent current-voltage and capacitance-voltage characteristics. The reverse leakage current is mostly dominated by Poole-Frenkel (PF) emission in the structures used in this brief. However, it is observed that at higher mole fractions, due to higher electric field across the barrier, Fowler-Nordheim (FN) tunneling also contributes to the gate leakage current even at room temperature and above. An expression for critical temperature below which FN tunneling component becomes comparable with or more than PF emission component is presented. It is concluded that the dominant gate leakage mechanisms in III-N HEMTs are dependent on mole fraction of the barrier material and the temperature. However, the relative strengths of PF emission and FN tunneling are also influenced by various process-dependent parameters.

Dynamics of a localized spin excitation close to the spin-helix regime

Abstract: The time evolution of a local spin excitation in a (001)-confined two-dimensional electron gas subjected to Rashba and Dresselhaus spin-orbit interactions of similar strength is investigated theoretically and compared with experimental data. Specifically, the consequences of the finite spatial extension of the initial spin polarization is studied for non-balanced Rashba and Dresselhaus terms and for finite cubic Dresselhaus spin-orbit interaction. We show that the initial out-of-plane spin polarization evolves into a helical spin pattern with a wave number that gradually approaches the value $q_0$ of the persistent spin helix mode. In addition to an exponential decay of the spin polarization that is proportional to both the spin-orbit imbalance and the cubic Dresselhaus term, the finite width $w$ of the spin excitation reduces the spin polarization by a factor that approaches $\exp(-q_0^2 w^2/2)$ at longer times.

Unidirectional spin-orbit interaction and spin-helix state in a (110)-oriented GaAs/(Al,Ga)As quantum well

Abstract: The Dresselhaus spin-orbit interaction is quantitatively investigated in a (110)-oriented GaAs quantum well by means of time- and spatially resolved Kerr rotation. The experimental results directly demonstrate a unidirectional out-of-plane spin-orbit interaction that linearly depends on the electron momentum along the $[1\overline{1}0]$ direction and vanishes for the electron momentum along the [001] direction. Spatially resolved measurements of the diffusion-driven spin precession dynamics provide evidence of the formation of a persistent spin-helix state in this system.

Spin-orbit splitting and effective masses in p-type GaAs two-dimensional hole gases

Abstract: We present magnetotransport measurements performed on two-dimensional hole gases embedded in carbon doped $p$-type GaAs/AlGaAs heterostructures grown on [001] oriented substrates. A pronounced beating pattern in the Shubnikov--de Haas oscillations proves the presence of strong spin-orbit interaction in the device under study. We estimate the effective masses of spin-orbit-split subbands by measuring the temperature dependence of the Shubnikov--de Haas oscillations at different hole densities. While the lighter heavy-hole effective mass is not energy dependent, the heavier heavy-hole effective mass has a prominent energy dependence, indicating a strong spin-orbit induced nonparabolicity of the valence band. The measured effective masses show qualitative agreement with self-consistent numerical calculations.

Increasing the ν = 5/2 gap energy: an analysis of MBE growth parameters

Abstract: The fractional quantized Hall state at the filling factor = 5/2 is of special interest due to its possible application for quantum computing. Here we report on the optimization of growth parameters that allowed us to produce twodimensional electron gases (2DEGs) with a 5/2 gap energy up to 135mK. We concentrated on optimizing the molecular beam epitaxy (MBE) growth to provide high 5/2 gap energies in ‘as-grown’ samples, without the need to enhance the 2DEGs properties by illumination or gating techniques. Our findings allow us to analyse the impact of doping in narrow quantum wells with respect to conventional DX-doping in AlxGa1 xAs. The impact of the setback distance between doping layer and 2DEG was investigated as well. Additionally, we found a considerable increase in gap energy by reducing the amount of background impurities. To this end growth techniques like temperature reductions for substrate and effusion cells and the reduction of the Al mole fraction in the 2DEG region were applied.

Micro-structural evaluation of Ti/Al/Ni/Au ohmic contacts with different Ti/Al thicknesses in AlGaN/GaN HEMTs

Abstract: In this work, ohmic contacts were formed by varying the Ti/Al thickness ratio in the metal stack of Ti/Al/Ni/Au on Al .28 Ga .72 N/GaN HEMT epistructure followed by annealing in the temperature range 740–860 °C by rapid thermal processor (RTP). The contacts were electrically characterized for contact resistance ( R c ) and the sheet resistance ( R s ) of AlGaN/GaN epistructure. The ohmic contacts formed by Ti/Al metal thickness ratio of 1/5 exhibited lowest R c values and better surface morphology compared to the contacts formed by other Ti/Al metal thickness ratios. The difference observed in the electrical characterization of these contacts was correlated with their X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) analyses. The surface morphology of the ohmic metal post annealing showed two distinct regions in scanning electron microscope (SEM) images. The energy dispersive X-ray analysis (EDAX) identified these regions as Ni–Al and Au–Al rich. Ni–Al rich region is believed to be responsible for rough morphology. Further, the contact formed with Ti/Al metal thickness ratio 1/5 showed less number of elemental Al and Ti atoms and therefore was correlated with lower oxidation probability of the contact compared to ohmic contact formed by other metal thickness ratios.

Solidification Modeling: Evolution, Benchmarks, Trends in Handling Turbulence, and Future Directions

Abstract: A systematic evolution of the solidification modeling is presented in this article. An approach starting from the basic governing equations to the intricate modeling of the alloy solidification using different approaches has been reviewed. Important advantages and issues related to different formulations and the use of fixed/moving grids for the modeling of solidification have been discussed. This article outlines the important solidification modeling approaches used in the literature. The mathematical description of the most frequently employed methods for modeling of solidification has been presented providing adequate references for other solidification models. This article highlights an important subdomain of solidification modeling, namely, the modeling of solidification processes having significant turbulence (such as welding, casting, and Czochralski crystal growth). A review of the use of different turbulence models along with the state-of-the-art techniques in these areas is presented. The paper also describes the important benchmarking studies (both experimental and numerical modeling results) used for the validation of solidification of both pure metals and alloys. Finally, the physical and numerical complexities associated with the solidification modeling phenomena along with the important challenges and future directions are presented.

Functionalization Of Carbon Nanotubes With Metal Phthalocyanine For SELECTIVE Gas Sensing Application

Abstract: Composites of hydroxylated single-walled carbon nanotubes and copper phthalocyanine (CuPc) has been obtained. Phthalocyanines get noncovalently adsorbed onto CNTs surface by π–π stacking. Conversion of β-form of phthalocyanine to α-form gives clear evidence of composite formation. Only carboxyl and hydroxyl CNTs react with phthalocyanine while no reaction occurs for amided CNTs. High selectivity in the thin film resistors of SWNT–OH + CuPc composites toward NOx gases is observed. The response for NOx is 10 times and 100 times more then that of ammonia and Sox, respectively. This makes the SWNT–OH + CuPc composite a candidate for detection of NOx—a common pollutant.