Showing papers by "Solid State Physics Laboratory published in 2010"
TL;DR: In this paper, the chemical modification of single-layer graphene sheets with aromatic diazonium ions (carbon attachment) is analyzed by confocal Raman spectroscopy, which allows the controlled transformation of sp2 to sp3 carbon proceeds in two separate steps.
Abstract: Graphene modifications with oxygen or hydrogen are well known in contrast to carbon attachment to the graphene lattice. The chemical modification of graphene sheets with aromatic diazonium ions (carbon attachment) is analyzed by confocal Raman spectroscopy. The temporal and spatial evolution of surface-adsorbed species allows accurate tracking of the chemical reaction and identification of intermediates. The controlled transformation of sp2 to sp3 carbon proceeds in two separate steps. The presented derivatization is faster for single-layer graphene and allows controlled transformation of adsorbed diazonium reagents into covalently bound surface derivatives with enhanced reactivity at the edge of single-layer graphene. On bilayer graphene the derivatization proceeds to an adsorbed intermediate, which reacts slower to a covalently attached species on the carbon surface.
160 citations
TL;DR: Graphene, a single layer of carbon atoms forming a perfectly stable and clean two-dimensional crystal with very few defects, has been proclaimed to be a new revolutionary material for electronics as mentioned in this paper.
141 citations
TL;DR: In this paper, a resonant LC-circuit with high sensitivity to small capacitance changes is employed to measure the quantum capacitance in graphene as a function of charge carrier density.
Abstract: We report on measurements of the quantum capacitance in graphene as a function of charge carrier density. A resonant LC-circuit giving high sensitivity to small capacitance changes is employed. The density of states, which is directly proportional to the quantum capacitance, is found to be significantly larger than zero at and around the charge neutrality point. This finding is interpreted to be a result of potential fluctuations with amplitudes of the order of 100 meV in good agreement with scanning single-electron transistor measurements on bulk graphene and transport studies on nanoribbons.
125 citations
TL;DR: In this article, ground and excited state transport through small (d≈70 nm) graphene quantum dots is investigated, and successive spin filling of orbital states is detected by measuring the difference between ground-state energies as a function of a magnetic field.
Abstract: We investigate ground and excited state transport through small (d≈70 nm) graphene quantum dots. The successive spin filling of orbital states is detected by measuring the difference between ground-state energies as a function of a magnetic field. For a magnetic field in-plane of the quantum dot the Zeeman splitting of spin states is measured. The results are compatible with a g factor of 2, and we detect a spin-filling sequence for a series of states which is reasonable given the strength of exchange interaction effects expected by comparing Coulomb interaction energy and kinetic energy of charge carriers in graphene.
125 citations
TL;DR: In this article, a ZnO coated one-port surface acoustic wave (SAW) resonator was used as a sensor for the selective detection of ammonia in the presence of water vapors.
Abstract: ZnO coated one-port surface acoustic wave (SAW) resonators are used as ammonia sensor in the present study. The cross-sensitivity of the sensor with commonly available volatile organic compounds (VOCs)/gases has been analyzed. An interesting feature through which the presence of ammonia can be differentiated from rest of the vapors/gases has been observed. The differential frequency shift (Δf) of the sensor is positive for liquor ammonia and negative for the rest. Further Δf for ammonia in the presence of water vapors is found to be positive. The sensor has good sensitivity, selectivity, reversibility and repeatability. The results are analyzed by considering possible sensing mechanisms such as electrical loading, elastic loading and mass loading. The results suggest the possibility of utilizing the ZnO coated sensor for the efficient selective detection of ammonia.
92 citations
TL;DR: In this paper, a single-layer graphene quantum dot coupled to source and drain via two constrictions is shown to have conductance resonances in the Coulomb blockade regime as well as localized states in real space.
Abstract: We present scanning-gate images of a single-layer graphene quantum dot which is coupled to source and drain via two constrictions. We image and locate conductance resonances of the quantum dot in the Coulomb-blockade regime as well as resonances of localized states in the constrictions in real space, which are interpreted in light of previous transport experiments. In addition our technique allows us to estimate the extent of these localized states, namely, radii of about 10--13 nm.
92 citations
TL;DR: Experiments qualitatively validate the predictions of the fluctuation theorem as the first evidence of this theorem in the nonequilibrium quantum regime by using a quantum coherent conductor.
Abstract: We experimentally demonstrate the validity of nonequilibrium fluctuation relations by using a quantum coherent conductor. In equilibrium the fluctuation-dissipation relation leads to the correlation between current and current noise at the conductor, namely, the Johnson-Nyquist relation. When the conductor is voltage biased so that the nonlinear regime is entered, the fluctuation theorem has predicted similar nonequilibrium fluctuation relations, which hold true even when the Onsager-Casmir relations are broken in magnetic fields. Our experiments qualitatively validate the predictions as the first evidence of this theorem in the nonequilibrium quantum regime.
89 citations
TL;DR: In this paper, the magnetoresistance of a side-gated ring structure etched out of single-layer graphene was investigated and the relative phases of the wave functions in the interfering paths were induced by changing the voltage applied to the side gate or the back gate.
Abstract: We investigate the magnetoresistance of a side-gated ring structure etched out of single-layer graphene. We observe Aharonov?Bohm oscillations with about 5% visibility. We are able to change the relative phases of the wave functions in the interfering paths and induce phase jumps of ? in the Aharonov?Bohm oscillations by changing the voltage applied to the side gate or the back gate. The observed data can be interpreted within existing models for 'dirty metals'.
85 citations
TL;DR: In this article, the authors investigated the crossover between weak localization and weak antilocalization in InAs of different diameters (75 nm-140 nm-217 nm) for a magnetic field applied perpendicularly to the nanowire axis, and extracted the spin orbit and coherence lengths using a quasi-one-dimensional model of the conductance.
Abstract: We investigate the crossover between weak localization and weak antilocalization in InAs nanowires of different diameters (75 nm-140 nm-217 nm). For a magnetic field applied perpendicularly to the nanowire axis, we extract the spin orbit and coherence lengths using a quasi-one-dimensional model of the conductance. We find a spin-orbit length inversely proportional to the width of the nanowire. When a parallel magnetic field is applied, we observe that the weak-antilocalization contribution is less affected by the magnetic field than in the perpendicular case.
81 citations
TL;DR: In this article, the conductance behavior of atomic-size contacts made of ferromagnetic metals (Co) or noble metals (Au) with magnetoresistance (MR) was studied.
Abstract: We present a comprehensive study of the conductance behavior of atomic-size contacts made of ferromagnetic metals (Co) or noble metals (Au) with ferromagnetic electrodes (Co). In order to separate the influence of the large electrodes from the influence of the contacts themselves, we used different sample geometries. These include combinations of nonmagnetic electrodes connected to magnetic bridges and vice versa as well as different orientations of the magnetic field. The magnetoresistance (MR) curves show very rich behavior with strong MR ratios (MRR). In all geometries the MRR values are of comparable size, reaching up to a few thousand percent in the tunneling regime. We study the possible influence of the micromagnetic order of the domains in the vicinity of the contact as well as ballistic MR, giant MR, tunnel MR, atomically enhanced anisotropic MR (AAMR), and magnetostriction. We conclude that AAMR is the most important origin for the MR at high magnetic fields $(|B|g2\text{ }\text{T})$, while magnetostriction, tunnel MR, and giant MR govern the low-field regime $(|B|l2\text{ }\text{T})$.
55 citations
TL;DR: In this article, the authors studied a double quantum dot in different coupling regimes and observed additional structures inside the finite-bias triangles, which can be attributed to electronic excited dot states, while others are probably due to modulations of the transmission of the tunnel barriers connecting the system to source and drain leads.
Abstract: We study a graphene double quantum dot in different coupling regimes. Despite the strong capacitive coupling between the dots, the tunnel coupling is below the experimental resolution. We observe additional structures inside the finite-bias triangles, part of which can be attributed to electronic excited dot states, while others are probably due to modulations of the transmission of the tunnel barriers connecting the system to source and drain leads.
TL;DR: In this paper, the effects of Li substitution on the structural, dielectric, and ferroelectric properties of the lead-free ceramic system (K 05− x Li x Na 05 )(Nb 09 Ta 01 )O 3 (KNN) with x ǫ= 0, 0015, 0045, and 0060 have been synthesized by conventional solid state reaction followed by high-energy ball milling The average particle size of all the milled powders as determined from the TEM analysis was about 35nm These powders were sintered at 1050
TL;DR: In this paper, the Dresselhaus spin-orbit interaction (SOI) of a series of two-dimensional electron gases hosted in GaAs/AlGaAs and InGaAs/GaAs (001) quantum wells (QWs) is measured by monitoring the precession frequency of the spins as a function of an in-plane electric field.
Abstract: The Dresselhaus spin-orbit interaction (SOI) of a series of two-dimensional electron gases hosted in GaAs/AlGaAs and InGaAs/GaAs (001) quantum wells (QWs) is measured by monitoring the precession frequency of the spins as a function of an in-plane electric field. The measured spin-orbit-induced spin splitting is linear in the drift velocity, even in the regime where the cubic Dresselhaus SOI is important. We relate the measured splitting to the Dresselhaus coupling parameter $\ensuremath{\gamma}$, the QW confinement, the Fermi wave number ${k}_{\text{F}}$, and strain effects. From this, $\ensuremath{\gamma}$ is determined quantitatively, including its sign.
TL;DR: In this article, the authors reported a breakthrough in the spin coherence times of hole ensembles, confined in the so-called natural QDs, in narrow GaAs/AlGaAs quantum wells at temperatures below 500 mK.
Abstract: For the realization of scalable solid-state quantum-bit systems, spins in semiconductor quantum dots (QDs) are promising candidates. A key requirement for quantum logic operations is a sufficiently long coherence time of the spin system. Recently, hole spins in III–V-based QDs were discussed as alternatives to electron spins, since the hole spin, in contrast to the electron spin, is not affected by contact hyperfine interaction with the nuclear spins. Here, we report a breakthrough in the spin coherence times of hole ensembles, confined in the so-called natural QDs, in narrow GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently, time-resolved Faraday rotation and resonant spin amplification techniques deliver hole-spin coherence times, which approach in the low magnetic field limit values above 70 ns. The optical initialization of the hole spin polarization, as well as the interconnected electron and hole spin dynamics in our samples, are well reproduced using a rate equation model.
TL;DR: In this paper, the electronic eigenstates of graphene quantum dots of realistic size (up to 80 nm diameter) were investigated in the presence of a perpendicular magnetic field and Coulomb-blockade measurements were performed near the Dirac point.
Abstract: We investigate the electronic eigenstates of graphene quantum dots of realistic size (up to 80 nm diameter) in the presence of a perpendicular magnetic field $B$. Numerical tight-binding calculations and Coulomb-blockade measurements performed near the Dirac point exhibit the transition from the linear density of states at $B=0$ to the Landau-level regime at high fields. Details of this transition sensitively depend on the underlying graphene lattice structure, bulk defects, and localization effects at the edges. Key to the understanding of the parametric evolution of the levels is the strength of the valley-symmetry-breaking $K\text{\ensuremath{-}}{K}^{\ensuremath{'}}$ scattering. We show that the parametric variation in the level variance provides a quantitative measure for this scattering mechanism. We perform measurements of the parametric motion of Coulomb-blockade peaks as a function of magnetic field and find good agreement. We demonstrate that the magnetic-field dependence of graphene energy levels may serve as a sensitive indicator for the properties of graphene quantum dots and, in further consequence, for the validity of the Dirac picture.
TL;DR: The results suggest that, compared with scraping cells or using citrate buffer, the most suitable detachment method for V‐79 cells is detachment by trypsin and keeping cells in the stirred cell suspension until measurement, which provides the highest cell viability, less visible damage on SEM micrographs and leaves the metabolic rate of cells unchanged.
TL;DR: In this article, the mean field theory and the probability law of Zn 1− x Mn x Cr 2 O 4 nanoparticles were used to calculate the magnetic exchange energies and the magnetic susceptibility.
TL;DR: In this paper, the crystal structure and microstructure of Ni09Zn01O were refined at room temperature in both the Fm{\overline 3}m and R{overline 2}m space groups, and it was shown that below the Neel point (458 K), where magnetic ordering triggers the presence of a trigonal strain, the common usage of a higher-symmetry non-admissible space group for crystal structure analysis via the Rietveld method may result in both an incorrect structure description and incorrect micro-structure parameters
Abstract: The crystal structure and microstructure of as-prepared and annealed Ni09Zn01O were refined at room temperature in both the Fm{\overline 3}m and R{\overline 3}m space groups It is shown that below the Neel point (458 K), where magnetic ordering triggers the presence of a trigonal strain, the common usage of a higher-symmetry non-admissible space group for crystal structure and microstructure analysis via the Rietveld method may result in both an incorrect structure description and incorrect microstructure parameters (size and strain) More realistic microstructure data can be obtained by whole powder pattern modelling of the powder diffraction data Increasing the annealing temperature causes a reduction of the trigonal distortion as well as an increase in domain size Simultaneously, the Raman spectra become less resolved, a clear indication of domain growth and structural evolution of the structure towards cubic symmetry (R{\overline 3}m → Fm{\overline 3}m)
TL;DR: In this article, a buffer layer of titanium (Ti) is deposited prior to the catalyst deposition and the growth was carried out using chemical vapor deposition (CVD) technique and a significant increase in emission current density was achieved by the use of titanium buffer layer due to much less dense growth of CNTs of smaller diameter.
TL;DR: In this article, the evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and is consistent with the explanation of the self-consistent charge localization.
Abstract: Quantum point contacts (QPCs) implemented in p-type GaAs/AlGaAs heterostructures are investigated by low-temperature electrical conductance spectroscopy. Besides one-dimensional conductance quantization, a pronounced extra plateau is found at about 0.7(2e2/h) which possesses the characteristic properties of the so-called "0.7 anomaly" known from experiments with n-type samples. The evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and is consistent with the explanation of the 0.7 anomaly based on self-consistent charge localization. These observations are robust when lateral electrical fields are applied which shift the relative position of the electron wave function in the QPC, testifying to the intrinsic nature of the underlying physics.
TL;DR: In this paper, a decrease in the value of room temperature dielectric constant is observed with the increase in Ni concentration, which has been explained in terms of space charge polarization and Koop's two layer model.
Abstract: Li 0.4−0.5 x Zn 0.2 Ni x Fe 2.4−0.5 x O 4 ferrites with x varying from 0.02 to 0.1 in steps of 0.02 have been synthesized by the citrate precursor method and investigated for their dielectric properties. A decrease in the value of room temperature dielectric constant is observed with the increase in Ni concentration. It has been explained in terms of space charge polarization and Koop's two layer model. The variation in dielectric constant with frequency shows dispersion while a resonance peak could be seen in the variation of dielectric loss with frequency. With increase in temperature the dielectric constant and dielectric loss increased, which have been discussed in terms of polarization and the Debye-type of dispersion. Possible mechanism contributing to the above process is discussed.
TL;DR: LiTiMg-ferrite radome is used as superstrate or radome, which controls the radiation, reception, and scattering from a printed antenna or array by applying a dc magnetic bias field in the plane of the ferrite, orthogonal to the RF magnetic field as discussed by the authors.
TL;DR: In this article, high-mobility two-dimensional electron gases in AlxGa1-xAs heterostructures were investigated by employing Schottkygate-dependent measurements of the samples' electron density and mobility.
Abstract: In this work, we investigate high-mobility two-dimensional electron gases in AlxGa1-xAs heterostructures by employing Schottky-gate-dependent measurements of the samples' electron density and mobility. Surprisingly, we found that two different sample configurations can be set in situ with mobilities differing by a factor of more than two in a wide range of densities. This observation is discussed in the context of charge redistributions between the doping layers and is relevant for the design of future gateable high-mobility electron gases.
TL;DR: In this article, a correlated study involving structure analysis, temperature dependence of dielectric functions and the complex impedance spectroscopy analysis was carried out in lanthanum modified bismuth titanate (BT, Bi4−xLaxTi3O12; x = 0, 0.75) ceramics.
Abstract: A correlated study involving structure analysis, temperature dependence of dielectric functions and the complex impedance spectroscopy analysis was carried out in lanthanum modified bismuth titanate (BT, Bi4−xLaxTi3O12; x = 0, 0.75) ceramics. A series addition of three RQ circuits has been shown to remarkably fit the complex impedance data for both x = 0 and 0.75 samples, which were attributed to crystalline layer, plate boundary and grain boundary microelements. Temperature dependence DC-conductance of these microelements has revealed that lanthanum-ion substitution is mainly taking place in the perovskite blocks, which also led to decrease in orthorhombic splitting. This site preferred substitution was also consistent with shift in the oxygen ion-jump relaxation peak in the temperature dependence of the dissipation factor study.
TL;DR: In this article, the effects of the La doping on the dielectric and piezoelectric properties of the ceramics were systematically examined and the results of X-ray diffraction suggest that all the Ceramics crystallize into a single-phase pervoskite structure.
Abstract: Lead zinc niobate–lead zirconium titanate, 0.3Pb(Zn 1/3 Nb 2/3 )O 3 –0.7Pb(Zr 0.51 Ti 0.49 )O 3 (PZN–PZT), ceramics doped with different amounts of lanthanum (La) were prepared by mechano-chemical alloying (MCA) method accompanied by sintering. The results of X-ray diffraction suggest that all the ceramics crystallize into a single-phase pervoskite structure. The SEM analysis of the ceramic samples reveals a dense microstructure with homogeneous distribution of grains. The bulk density of the ceramics increases with the increase of La content. The effects of the La doping on the dielectric and piezoelectric properties of the ceramics were systematically examined. An increase in La concentration causes the Curie temperature ( T c ) to decrease and the phase transition to become more diffused. The piezoelectric charge co-efficient ( d 33 ) and electromechanical coupling constant ( k p ) show improvement with the increase of La doping level in the ceramic system. The d 33 , k p and percentage strain of the 3 mol%-La doped PZN–PZT ceramic were observed to be 575 pC/N, 64 and 22%, respectively.
TL;DR: In this article, the Coulomb interaction was used to scatter electrons far from equilibrium in a cold two-dimensional electron system, where the interaction between excited electrons and the degenerate Fermi liquid induces a positive charge in a nanoscale region which would be negatively charged for diffusive transport at local thermal equilibrium.
Abstract: Scattering of otherwise ballistic electrons far from equilibrium is investigated in a cold two-dimensional electron system. The interaction between excited electrons and the degenerate Fermi liquid induces a positive charge in a nanoscale region which would be negatively charged for diffusive transport at local thermal equilibrium. In a three-terminal device we observe avalanche amplification of electrical current, resulting in a situation comparable to the Venturi effect in hydrodynamics. Numerical calculations using a random-phase approximation are in agreement with our data and suggest Coulomb interaction as the dominant scattering mechanism.
TL;DR: In this article, a study of Mn-doped InAs quantum wells reveals unexpected metastable behaviour of magnetotransport phenomena at sub-kelvin temperatures, in structures that show at the same time the quantum Hall effect in high magnetic fields.
Abstract: A study of Mn-doped InAs quantum wells reveals unexpected metastable behaviour of magnetotransport phenomena at sub-kelvin temperatures, in structures that show at the same time the quantum Hall effect in high magnetic fields. These findings bridge the physics of two-dimensional carrier systems with phenomena specific to magnetically doped semiconductors.
TL;DR: In this article, high-mobility two-dimensional electron gases in AlGaAs heterostructures were investigated by employing Schottkygate-dependent measurements of the samples' electron density and mobility.
Abstract: We investigate high-mobility two-dimensional electron gases in AlGaAs heterostructures by employing Schottky-gate-dependent measurements of the samples' electron density and mobility. Surprisingly, we find that two different sample configurations can be set in situ with mobilities diering by a factor of more than two in a wide range of densities. This observation is discussed in view of charge redistributions between the doping layers and is relevant for the design of future gateable high-mobility electron gases.
TL;DR: The observation of Aharonov–Bohm oscillations in nanoribbons of Bi2Se3 opens the way for electronic transport experiments in nanoscale three-dimensional topological insulators.
Abstract: The observation of Aharonov–Bohm oscillations in nanoribbons of Bi2Se3 opens the way for electronic transport experiments in nanoscale three-dimensional topological insulators.
TL;DR: The surface morphology and structural characteristics of the quantum dots analyzed by atomic force microscope revealed that the density of the InAs quantum dots first increased and then decreased with the amount of InAs coverage; whereas density decreased with increase in growth temperature.
Abstract: Self-assembled InAs quantum dots (QDs) were grown on germanium substrates by metal organic chemical vapor deposition technique. Effects of growth temperature and InAs coverage on the size, density, and height of quantum dots were investigated. Growth temperature was varied from 400 to 450 °C and InAs coverage was varied between 1.40 and 2.35 monolayers (MLs). The surface morphology and structural characteristics of the quantum dots analyzed by atomic force microscope revealed that the density of the InAs quantum dots first increased and then decreased with the amount of InAs coverage; whereas density decreased with increase in growth temperature. It was observed that the size and height of InAs quantum dots increased with increase in both temperature and InAs coverage. The density of QDs was effectively controlled by growth temperature and InAs coverage on GaAs buffer layer.