Showing papers by "Solid State Physics Laboratory published in 2011"
TL;DR: In this paper, independent single-shot readout of two electron spins in a double quantum dot has been demonstrated, and the results provide a possible route to the realization and efficient characterization of multiqubit quantum circuits based on single quantum dot spins.
Abstract: Measurement of coupled quantum systems plays a central role in quantum information processing. We have realized independent single-shot read-out of two electron spins in a double quantum dot. The read-out method is all-electrical, cross-talk between the two measurements is negligible, and read-out fidelities are ~86% on average. This allows us to directly probe the anticorrelations between two spins prepared in a singlet state and to demonstrate the operation of the two-qubit exchange gate on a complete set of basis states. The results provide a possible route to the realization and efficient characterization of multiqubit quantum circuits based on single quantum dot spins.
195 citations
TL;DR: In this article, the authors compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene and independently find an interlayer screening length in the order of four to five layers.
Abstract: substrates. We compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene. The ab initio calculations show that the work function of single- and bilayer graphene is mainly given by a variation of the Fermi energy with respect to the Dirac point energy as a function of doping, and that electrostatic interlayer screening only becomes relevant for thicker multilayer graphene. From the Raman G-line shift and the comparison of the Kelvin probe data with the ab initio calculations, we independently find an interlayer screening length in the order of four to five layers. Furthermore, we describe in-plane variations of the work function, which can be attributed to partial screening of charge impurities in the substrate, and result in a nonuniform charge density in single-layer graphene.
190 citations
TL;DR: In this review, recent developments in the fabrication and understanding of the electronic properties of graphene nanostructures are discussed and the focus is put on graphene constrictions, quantum dots and double quantum dots.
Abstract: In this review, recent developments in the fabrication and understanding of the electronic properties of graphene nanostructures are discussed. After a brief overview of the structure of graphene and the two-dimensional transport properties, the focus is put on graphene constrictions, quantum dots and double quantum dots. For constrictions with a width below 100 nm, the current through the constriction is strongly suppressed for a certain back gate voltage range, related to the so-called transport gap. This transport gap is due to the formation of localized puddles in the constriction, and its size depends strongly on the constriction width. Such constrictions can be used to confine charge carriers in quantum dots, leading to Coulomb blockade effects.
128 citations
TL;DR: In this article, the authors used the Aqueous Chemical Growth (ACG) technique to synthesize monodispersed spherical particles of chromium (III) oxide, α-Cr2O3, from a diluted solution of KCr(SO4)2·12H2O using the ACG technique.
86 citations
TL;DR: In this article, a low-temperature experimental test of the fluctuation theorem for electron transport through a double quantum dot is presented, where the rare entropy-consuming system trajectories are detected in the form of single charges flowing against the source-drain bias by using time-resolved charge detection with a quantum point contact.
Abstract: We present a low-temperature experimental test of the fluctuation theorem for electron transport through a double quantum dot The rare entropy-consuming system trajectories are detected in the form of single charges flowing against the source-drain bias by using time-resolved charge detection with a quantum point contact We find that these trajectories appear with a frequency that agrees with the theoretical predictions even under strong nonequilibrium conditions, when the finite bandwidth of the charge detection is taken into account
63 citations
TL;DR: In this paper, a real-time detection of electron tunneling in a graphene quantum dot is presented, and the role of localized states in the tunneling process is investigated. But the authors focus on the single-electron charging events on the dot.
Abstract: We present real-time detection measurements of electron tunneling in a graphene quantum dot. By counting single-electron charging events on the dot, the tunneling process in a graphene constriction and the role of localized states are studied in detail. In the regime of low charge detector bias we see only a single time-dependent process in the tunneling rate which can be modeled using a Fermi-broadened energy distribution of the carriers in the lead. We find a nonmonotonic gate dependence of the tunneling coupling attributed to the formation of localized states in the constriction. Increasing the detector bias above ${V}_{b}=2$ mV results in an increase of the dot-lead transition rate related to back action of the charge detector current on the dot.
58 citations
01 Oct 2011-Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems
TL;DR: In this paper, the anisotropic etching properties of KOH, tetra-methyl ammonium hydroxide (TMAH), and ethylene di-amine pyro-catechol (EDP) solutions were compared.
Abstract: Bulk micromachining in Si (110) wafer is an essential process for fabricating vertical microstructures by wet chemical etching. We compared the anisotropic etching properties of potassium hydroxide (KOH), tetra-methyl ammonium hydroxide (TMAH) and ethylene di-amine pyro-catechol (EDP) solutions. A series of etching experiments have been carried out using different etchant concentration and temperatures. Etching at elevated temperatures was found to improve the surface quality as well as shorten the etching time in all the etchants. At 120°C, we get a smooth surface (Ra = 21.2 nm) with an etching rate 12.2 μm/min in 40wt% KOH solution. At 125°C, EDP solution (88wt%) was found to produce smoothest surface (Ra = 9.4 nm) with an etch rate of 1.8 μm/min. In TMAH solution (25wt%), the best surface roughness was found to be 35.6 nm (Ra) at 90°C with an etch rate of 1.18 μm/min. The activation energy and pre-exponential factor in Arrhenius relation are also estimated from the corresponding etch rate data.
54 citations
TL;DR: In this article, the shape of the confinement potential in the longitudinal and transverse directions of a point contact was extracted from the transport data and used to predict which interaction-induced states can best form in quantum point contacts.
Abstract: Quantum point contacts are fundamental building blocks for mesoscopic transport experiments and play an important role in recent interference and fractional quantum Hall experiments. However, it is unclear how electron?electron interactions and the random disorder potential influence the confinement potential and give rise to phenomena such as the mysterious 0.7 anomaly. Novel growth techniques of AlXGa1?XAs heterostructures for high-mobility two-dimensional electron gases enable us to investigate quantum point contacts with a strongly suppressed disorder potential. These clean quantum point contacts indeed show transport features that are obscured by disorder in standard samples. From these transport data, we are able to extract those parameters of the confinement potential that describe its shape in the longitudinal and transverse directions. Knowing the shape (and hence the slope) of the confinement potential might be crucial for predicting which interaction-induced states can best form in quantum point contacts.
52 citations
TL;DR: A comprehensive review of fundamental issues, device architectures, technology development and applications of HgCdTe-based avalanche photodiodes (APAs) is presented in this article, where high gain, above 5×103, a low excess noise factor close to unity, and fast response in the range of pico-seconds has been achieved by electron-initiated avalanche multiplication for SWIR, MWIR, and LWIR detector applications involving low optical signals.
Abstract: This paper presents a comprehensive review of fundamental issues, device architectures, technology development and applications of HgCdTe based avalanche photodiodes (APD). High gain, above 5×103, a low excess noise factor close to unity, THz gain-bandwidth product, and fast response in the range of pico-seconds has been achieved by electron-initiated avalanche multiplication for SWIR, MWIR, and LWIR detector applications involving low optical signals. Detector arrays with good element-to-element uniformity have been fabricated paving the way for fabrication of HgCdTe-APD FPAs.
52 citations
TL;DR: The effect of electron-phonon coupling in a graphene and an InAs nanowire double quantum dot is studied to reveal oscillations of the DQD current periodic in energy detuning between the two levels.
Abstract: Graphene and InAs nanowires are both promising materials for coherent spin manipulation, but coupling between a quantum system and its environment leads to decoherence. Here, the contribution of electron–phonon coupling to decoherence in graphene and InAs nanowire is studied.
51 citations
TL;DR: In this article, a polycrystalline sample of Zr-doped barium titanate (BaTiO3) was prepared by conventional solid state reaction method by XRD and SEM, and electrical properties (dielectric, ferroelectric and impedance spectroscopy) were measured in wide range of frequency and temperature.
Abstract: A polycrystalline sample of Zr-doped barium titanate (BaTiO3) was prepared by conventional solid state reaction method. The effect of Zr (0·15) on the structural and microstructural properties of BaTiO3 was investigated by XRD and SEM. The electrical properties (dielectric, ferroelectric and impedance spectroscopy) were measured in wide range of frequency and temperature. With substitutions of Zr, the structure of BaTiO3 changes from tetragonal to rhombohedral. Lattice parameters were found to increase with substitution. The room temperature dielectric constant increases from ∼ 1675 to ∼ 10586 and peak dielectric constant value increases from ∼ 13626 to ∼ 21023 with diffuse phase transition. Impedance spectroscopy reveals the formation of grain and grain boundary in the material and found to decrease with increase in temperature.
TL;DR: In this paper, the electron exchange process via a quantum coherent conductor between two reservoirs is described, which yields the fluctuation theorem (FT) in mesoscopic transport, which is semiquantitatively validated in the current and noise measurement in an Aharonov-Bohm ring.
Abstract: Mesoscopic systems provide us a unique experimental stage to address nonequilibrium quantum statistical physics. By using a simple tunneling model, we describe the electron exchange process via a quantum coherent conductor between two reservoirs, which yields the fluctuation theorem (FT) in mesoscopic transport. We experimentally show that such a treatment is semiquantitatively validated in the current and noise measurement in an Aharonov-Bohm ring. The experimental proof of the microreversibility assumed in the derivation of FT is presented.
TL;DR: It is reported that microwaves can also excite tunnelling transitions between states with different spin, and it is shown that the dominant mechanism responsible for violation of spin conservation is the spin–orbit interaction.
Abstract: Tunnelling transitions triggered by microwave irradiation between coupled quantum dots have generally been assumed to be spin-conserving This study shows that this condition is violated in the presence of spin–orbit coupling, thus opening new possibilities for manipulating a two–spin qubit system by microwave irradiation
TL;DR: In this paper, the shape of the confinement potential of the GaAs/AlGaAs heterostructures of two-dimensional (2D) point contacts with a strongly suppressed disorder potential was analyzed.
Abstract: Quantum point contacts are fundamental building blocks for mesoscopic transport experiments and play an important role in recent interference- and fractional quantum Hall experiments. However, it is not clear how electron-electron interactions and the random disorder potential influence the confinement potential and give rise to phenomena like the mysterious 0.7 anomaly. Novel growth techniques of GaAs/AlGaAs heterostructures for high-mobility two-dimensional electron gases enable us to investigate quantum point contacts with a strongly suppressed disorder potential. These clean quantum point contacts indeed show transport features that are obscured by disorder in standard samples. From this transport data, we are able to extract the parameters of the confinement potential which describe its shape in longitudinal and transverse direction. Knowing the shape (and hence the slope) of the confinement potential might be crucial to predict which interaction-induced states can form in quantum point contacts.
TL;DR: In this article, the density and temperature-dependent conductance of graphene nanoribbons with varying aspect ratio was investigated, and individual resonances showed signatures of multilevel transport in some regimes, and stochastic Coulomb blockade in others.
Abstract: We investigate the density- and temperature-dependent conductance of graphene nanoribbons with varying aspect ratio. Transport is dominated by a chain of quantum dots forming spontaneously due to disorder. Depending on ribbon length, electron density, and temperature, single or multiple quantum dots dominate the conductance. Between conductance resonances, cotunneling transport at the lowest temperatures turns into activated transport at higher temperatures. The density-dependent activation energy resembles the Coulomb gap in a quantitative manner. Individual resonances show signatures of multilevel transport in some regimes, and stochastic Coulomb blockade in others.
TL;DR: In this paper, the energy relaxation channels of hot electrons far from thermal equilibrium in a degenerate two-dimensional electron system are investigated in transport experiments in a mesoscopic three-terminal device.
Abstract: The energy relaxation channels of hot electrons far from thermal equilibrium in a degenerate two-dimensional electron system are investigated in transport experiments in a mesoscopic three-terminal device. We observe a transition from two dimensions at zero magnetic field to quasi-one-dimensional scattering of the hot electrons in a strong magnetic field. In the two-dimensional case, electron-electron scattering is the dominant relaxation mechanism, while the emission of optical phonons becomes more and more important as the magnetic field is increased. The observation of up to 11 optical phonons emitted per hot electron allows us to determine the onset energy of longitudinal-optical phonons in GaAs at cryogenic temperatures with a high precision Eph = 36.0 ± 0.1 meV. Numerical calculations of electron-electron scattering and the emission of optical phonons underline our interpretation in terms of a transition to one-dimensional dynamics.
TL;DR: In this article, the effects of increasing Sn content on the dielectric, polarization, strain and piezoelectric behavior of the ceramics were studied, and it was shown that an increase in Sn concentration causes the Curie temperature (T c ) to decrease from 115°C to 45°C and the phase transition to become more diffuse.
01 Dec 2011-Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems
TL;DR: In this paper, the effect of residual stress on the electromechanical properties of a shunt switch has been analyzed in respect of resonant frequency, pull down voltage and switching speed.
Abstract: Studies have been carried out on a RF MEMS shunt switch to analyze the effect of residual stress on its electromechanical characteristics. This paper presents the simulated results as well as theoretically calculated results of a shunt switch due to the presence of residual stress gradient in respect of resonant frequency, pull down voltage and switching characteristics. The effect of introduction of holes in the beam is also studied. The calculated results, corresponding to the switch (without holes) at zero residual stress, of resonant frequency, pull-down voltage and switch on and off time are 28.14 kHz, 28.2 V, 16.35 μsec and 8.6 μsec respectively. Modal analysis of the both the structures (with and without holes) are carried out for different values of residual stress gradients. Modal analysis predicted that higher values of tensile stress gradient are not favorable for switching action. The pull-down voltages and switch on and off times are simulated at different stress gradients. With the increase in compressive stress gradient, the pull-down voltage is found to increase, whereas, switch on and off times is decreased. Corresponding to −20 MPa/μm residual stress gradient, the resonant frequency, pull-down voltage and switch on and off times are found to be 74.5 kHz, 63.5 V, 7.5 μsec and 3.36 μsec respectively. Introduction holes in the structure modified these values to 63.77 kHz, 53.1 V, 8.7 μsec, 3.92 μsec respectively.
TL;DR: In this article, the effects of excessive heating of a Titanium microbolometer were investigated and it was shown that even though the power supplied in pulse mode cannot damage the element physically, it may be sufficient for significant performance degradations.
TL;DR: In this article, the authors studied spin dephasing and spin diffusion in a highmobility two-dimensional electron system, embedded in a GaAs/AlGaAs quantum well grown in the [110] direction, by a two-beam Hanle experiment.
Abstract: We have studied spin dephasing and spin diffusion in a high-mobility two-dimensional electron system, embedded in a GaAs/AlGaAs quantum well grown in the [110] direction, by a two-beam Hanle experiment. For very low excitation density, we observe spin lifetimes of more than 16 ns, which rapidly decrease as the pump intensity is increased. Two mechanisms contribute to this decrease: The optical excitation produces holes, which lead to a decay of electron spin via the Bir-Aronov-Pikus mechanism and recombination with spin-polarized electrons. By scanning the distance between the pump and probe beams, we observe the diffusion of spin-polarized electrons over more than 20 $\ensuremath{\mu}$m. For high pump intensity, the spin polarization in a distance of several micrometers from the pump beam is larger than at the pump spot, due to the reduced influence of photogenerated holes.
TL;DR: In this article, the authors performed scanning-gate microscopy on a quantum point contact and observed that the conducting channel is shifted in real space when asymmetric gate voltages are applied.
Abstract: We perform scanning-gate microscopy on a quantum-point contact. It is defined in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs heterostructure, giving rise to a weak disorder potential. The lever arm of the scanning tip is significantly smaller than that of the split gates defining the conducting channel of the quantum-point contact. We are able to observe that the conducting channel is shifted in real space when asymmetric gate voltages are applied. The observed shifts are consistent with transport data and numerical estimations.
TL;DR: In this article, the spin dynamics of resident holes in a p-modulation-doped GaAs/Al 0.3Ga0.7As single quantum well were investigated.
Abstract: We investigate spin dynamics of resident holes in a p-modulation-doped GaAs/Al0.3Ga0.7As single quantum well. Time-resolved Faraday and Kerr rotation, as well as resonant spin amplification, are utilized in our study. We observe that nonresonant or high-power optical pumping leads to a resident hole spin polarization with opposite sign with respect to the optically oriented carriers, while low-power resonant optical pumping only leads to a resident hole spin polarization if a sufficient in-plane magnetic field is applied. The competition between two different processes of spin orientation strongly modifies the shape of resonant spin amplification traces. Calculations of the spin dynamics in the electron-hole system are in good agreement with the experimental Kerr rotation and resonant spin amplification traces and allow us to determine the hole spin polarization within the sample after optical orientation, as well as to extract quantitative information about spin dephasing processes at various stages of the evolution.
TL;DR: In this article, the structural and optical properties of yttria stabilized zirconia (YSZ) thin films grown by pulsed laser deposition (PLD) technique and in situ crystallized at different substrate temperatures (Ts = 400°C, 500°C and 600°C).
TL;DR: In this article, high quality microwave dielectrics based on Mn 4+ doped ZST have been prepared with ZST nanopowder without using any sintering aid and the room temperature dielectric constant remains almost unchanged in the low and microwave frequency regions (1-MHz to 9.3 GHz).
TL;DR: In this article, a study of Cl2/BCl3-based inductively coupled plasma (ICP) was conducted using thick photoresist mask for anisotropic etching of 50 μm diameter holes in a GaAs wafer at a relatively high average etching rate for etching depths of more than 150 μm.
Abstract: A study of Cl2/BCl3-based inductively coupled plasma (ICP) was conducted using thick photoresist mask for anisotropic etching of 50 μm diameter holes in a GaAs wafer at a relatively high average etching rate for etching depths of more than 150 μm. Plasma etch characteristics with ICP process pressure and the percentage of BCl3 were studied in greater detail at a constant ICP coil/bias power. The measured peak-to-peak voltage as a function of pressure was used to estimate the minimum energy of the ions bombarding the substrate. The process pressure was found to have a substantial influence on the energy of heavy ions. Various ion species in plasma showed minimum energy variation from 1.85 eV to 7.5 eV in the pressure range of 20 mTorr to 50 mTorr. The effect of pressure and the percentage of BCl3 on the etching rate and surface smoothness of the bottom surface of the etched hole were studied for a fixed total flow rate. The etching rate was found to decrease with the percentage of BCl3, whereas the addition of BCl3 resulted in anisotropic holes with a smooth veil free bottom surface at a pressure of 30 mTorr and 42% BCl3. In addition, variation of the etching yield with pressure and etching depth were also investigated.
TL;DR: In this paper, the metallic tip of a scanning force microscope is used to induce a potential in a fully controllable double quantum dot defined via local anodic oxidation in a GaAs/AlGaAs heterostructure.
Abstract: The metallic tip of a scanning force microscope operated at $300\text{}\mathrm{m}\mathrm{K}$ is used to locally induce a potential in a fully controllable double quantum dot defined via local anodic oxidation in a GaAs/AlGaAs heterostructure. Using scanning gate techniques we record spatial images of the current through the sample for different numbers of electrons on the quantum dots (i.e., for different quantum states). Owing to the spatial resolution of current maps, we are able to determine the spatial position of the individual quantum dots, and investigate their apparent relative shifts due to the voltage applied to a single gate.
TL;DR: Measurements of Coulomb resonances, including constriction resonances and Coulomb diamonds prove the functionality of the graphene quantum dot with a charging energy of approximately 4.5 meV and demonstrate that the complex pattern superimposing the quantum dot energy spectra is due to the formation of additional localized states with increasing magnetic field.
Abstract: We present transport measurements on a strongly coupled graphene quantum dot in a perpendicular magnetic field. The device consists of an etched single-layer graphene flake with two narrow constrictions separating a 140 nm diameter island from source and drain graphene contacts. Lateral graphene gates are used to electrostatically tune the device. Measurements of Coulomb resonances, including constriction resonances and Coulomb diamonds prove the functionality of the graphene quantum dot with a charging energy of approximately 4.5 meV. We show the evolution of Coulomb resonances as a function of perpendicular magnetic field, which provides indications of the formation of the graphene specific 0th Landau level. Finally, we demonstrate that the complex pattern superimposing the quantum dot energy spectra is due to the formation of additional localized states with increasing magnetic field.
TL;DR: In this article, the interpretation of such images is complex and not very intuitive under certain circumstances: scanning a graphene quantum dot (QD) in the Coulomb-blockaded regime, they observe an apparent shift of features in scanning-gate images as a function of gate voltages, which cannot be a real shift of the physical system.
Abstract: Scanning-probe techniques have been developed to extract local information from a given physical system In particular, conductance maps obtained by means of scanning-gate microscopy (SGM), where a conducting tip of an atomic-force microscope is used as a local and movable gate, seem to present an intuitive picture of the underlying physical processes Here, we argue that the interpretation of such images is complex and not very intuitive under certain circumstances: scanning a graphene quantum dot (QD) in the Coulomb-blockaded regime, we observe an apparent shift of features in scanning-gate images as a function of gate voltages, which cannot be a real shift of the physical system Furthermore, we demonstrate the appearance of more than one set of Coulomb rings arising from the graphene QD We attribute these effects to screening between the metallic tip and the gates Our results are relevant for SGM on any kind of nanostructure, but are of particular importance for nanostructures that are not covered with a dielectric, eg graphene or carbon nanotube structures
TL;DR: Results clearly revealed that the SWCNT-PPy nanocomposite facilitated the electron transfer from CuNP to Pt electrode and provided an electrochemical approach for the determination of NO(x) and exhibited good reproducibility and retained stability over a period of one month.
TL;DR: In this paper, a generic circuit model of microbolometer Infrared detector that can be used to simulate the electrical and thermal performance of micro-bolometers using SPICE like circuit simulator is presented.
Abstract: In this paper, we present a generic circuit model of microbolometer Infrared detector that can be used to simulate the electrical and thermal performance of microbolometers using SPICE like circuit simulator. Using this model, we have studied the effects of various parameters on the microbolometer performance by simulations in PSPICE for its verification. We have validated the model with the performance of our titanium microbolometers being developed at our laboratory. We have tuned the model parameters for these microbolometers and have shown that the simulated performance agrees with the measured performance reasonably well with the variety of measurement condition. The validated model has been used to fine tune the design of our titanium microbolometers as it allows us to monitor some internal parameters also that are not easy to measure in practical devices, like instantaneous temperature of microbolometer for a varying IR intensity falling on it. The proposed model is generic and therefore, using similar procedure it may be used for other types of microbolometers also like amorphous-Si based, vanadium oxide based etc., once it is validated for that.