Showing papers by "Solid State Physics Laboratory published in 2006"

Counting statistics of single electron transport in a quantum dot.

Abstract: We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.

Structural, dielectric and magnetic properties of NiCuZn ferrite grown by citrate precursor method

Abstract: Fine powders of Ni0.6 − xCuxZn0.4Fe2O4, where 0 ≤ x ≤ 0.4 was prepared by the citrate precursor method. The powders were calcined at 600 °C for a duration of 1 h and sintered at 900 °C for 3 h. The effect of compositional variation on structural, dielectric and magnetic properties of the Cu substituted NiZn ferrite was investigated. X-ray diffraction measurements confirmed the formation of single-phase cubic spinel structure at such low temperature of calcinations and sintering. A maximum density of 4.7 g/cm3 was obtained for the Cu substituted ferrite. The grain sizes were estimated from the SEM micrographs. The addition of copper promoted grain growth, resulting in increase in grain size. Curie temperature, however, was understandably lowered with the increase in Cu content. A saturation magnetization value of 92 emu/g was obtained for the composition x = 0.2. This value is higher than that reported for ferrites prepared by the conventional ceramic method. Ferrite with Cu concentration of x = 0.4, showed the highest value of initial permeability.

Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene

Abstract: We present Raman spectroscopy measurements on single- and few-layer graphene flakes. Using a scanning confocal approach we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab-initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry.

Terahertz Plasma Waves in Gated Graphene Heterostructures

Abstract: Plasma waves in graphene-based heterostructures with massless (neutrino-like) two-dimensional electron gas and with a highly conducting substrate (n+-Si) serving as a gate and an isolating gate layer (SiO2) are studied. Using the developed model, we show that the sufficiently long plasma waves exhibit a linear (sound-like) dispersion with the wave velocity determined by the gate layer thickness and the gate voltage. The plasma wave velocity in graphene heterostructures can significantly exceed the plasma wave velocity in the commonly employed semiconductor gated heterostructures. The gated graphene heterostructures can be used in different novel voltage tunable THz devices which utilize the plasma waves.

Spatially resolved Raman spectroscopy of single- and few-layer graphene

Abstract: We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry.

Counting statistics and super-Poissonian noise in a quantum dot: Time-resolved measurements of electron transport

Abstract: We present time-resolved measurements of electron transport through a quantum dot. The measurements were performed using a nearby quantum point contact as a charge detector. The rates for tunneling through the two barriers connecting the dot to source and drain contacts could be determined individually. In the high bias regime, the method was used to probe excited states of the dot. Furthermore, we have detected bunching of electrons, leading to super-Poissonian noise. We have used the framework of full counting statistics to model the experimental data. The existence of super-Poissonian noise suggests a long relaxation time for the involved excited state, which could be related to the spin relaxation time.

Magnetic field symmetry and phase rigidity of the nonlinear conductance in a ring.

Abstract: We have performed nonlinear transport measurements as a function of a perpendicular magnetic field in a semiconductor Aharonov-Bohm ring connected to two leads. While the voltage-symmetric part of the conductance is symmetric in the magnetic field, the voltage-antisymmetric part of the conductance is not symmetric. These symmetry relations are compatible with the scattering theory for nonlinear mesoscopic transport. The observed asymmetry can be tuned continuously by changing the gate voltages near the arms of the ring, showing that the phase of the nonlinear conductance in a two-terminal interferometer is not rigid, in contrast with the case for the linear conductance.

Structural properties and impedance spectroscopy of excimer laser ablated Zr substituted BaTiO3 thin films

Abstract: BaZr0.15Ti0.85O3 thin films were deposited on Pt-coated Si substrates using the pulsed excimer laser ablation technique. X-ray diffraction and atomic force microscope techniques were used to study the structural characteristics of the films. Films with good crystalline quality, with an average grain size of 0.5 µm were obtained, under various oxygen background pressures. Ferroelectric hysteresis loops recorded on the films deposited at 26.66 Pa oxygen pressure showed the best properties. To gain a further understanding of the electrical properties of these films, impedance spectroscopy was used and data acquired at several different temperatures. AC conductivity plots showed the presence of space charge conduction at low frequencies; however, at high frequencies, all the curves merged and expectedly showed an almost dc conduction behaviour. The activation energy obtained from ac conductivity data may be attributed to oxygen vacancy motion.

Anisotropic optical conductivity and two colors of MgB 2

Abstract: We present the anisotropic optical conductivity of ${\mathrm{MgB}}_{2}$ between 0.1 and $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at room temperature obtained on single crystals of different purity by spectroscopic ellipsometry and reflectance measurements. The bare (unscreened) plasma frequency ${\ensuremath{\omega}}_{p}$ is almost isotropic and equal to $6.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which contrasts some earlier reports of a very small value of ${\ensuremath{\omega}}_{p}$. The data suggests that the $\ensuremath{\sigma}$ bands are characterized by a stronger electron-phonon coupling ${\ensuremath{\lambda}}_{tr}$ but smaller impurity scattering ${\ensuremath{\gamma}}_{\mathrm{imp}}$, compared to the $\ensuremath{\pi}$ bands. The optical response along the boron planes is marked by an intense interband transition at $2.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, due to which the reflectivity plasma edges along the $a$ and $c$ axes are shifted with respect to each other. As a result, the sample spectacularly changes color from a blueish-silver to the yellow as the polarization is rotated from the in-plane direction toward the $c$ axis. The optical spectra are in good agreement with the published ab initio calculations. The remaining discrepancies can be explained by the relative shift of $\ensuremath{\sigma}$ bands and $\ensuremath{\pi}$ bands by about $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ compared to the theoretical band structure, in agreement with the de Haas-van Alphen experiments. The widths of the Drude and the interband peaks are both very sensitive to the sample purity.

Fano effect in a quantum-ring-quantum-dot system with tunable coupling

Abstract: Transport measurements are presented on a quantum ring that is tunnel-coupled to a quantum dot. When the dot is in the Coulomb blockade regime, but strongly coupled to the open ring, Fano line shapes are observed in the current through the ring, when the electron number in the dot changes by 1. The symmetry of the Fano resonances is found to depend on the magnetic flux penetrating the area of the ring and on the strength of the ring-dot coupling. At temperatures above T=0.65 K the Fano effect disappears, while the Aharonov-Bohm interference in the ring persists up to T=4.2 K. Good agreement is found between these experimental observations and a single-channel scattering matrix model including decoherence in the dot.

A comparative analysis of one-port Colpitt and two-port Pierce SAW oscillators for DMMP vapor sensing

Abstract: This paper reports an experimental study on chemical sensitivity and noise behavior of Colpitt and Pierce SAW oscillators based on 433.92 MHz one-port and two-port SAW resonator devices, respectively. The oscillators are characterized for phase noise as well as Allan variance. The sensitivity for DMMP (di-methyl methyl phosphonate) vapor is imparted by providing a diallyl-bisphenol polymer precursor as a selective interface coated on to both the SAW devices in an identical manner. Comparative vapor sensing experiments are carried out under simultaneous exposure to the same DMMP vapor stream in different ranges of vapor concentration and duration. A new data acquisition method is employed which is wireless and automated. It uses an antenna pickup at the input of a spectrum analyzer with PC control interface. The results show that the two-port Pierce oscillator is more stable but less sensitive than the one-port Colpitt oscillator. It is found that under the conditions of analyte exposure durations longer than 30 s the one-port Colpitt oscillator yields higher signal to noise ratios. The results are interpreted in terms of phase response characteristics and different sensitivities of the one-port and two-port SAW devices towards mass loading and viscoelastic effects.

Phase coherence in the inelastic cotunneling regime.

Abstract: Two quantum dots with tunable mutual tunnel coupling have been embedded in a two-terminal Aharonov-Bohm geometry. Aharonov-Bohm oscillations investigated in the cotunneling regime demonstrate coherent tunneling through nonresonant states. Visibilities of more than 0.8 are measured indicating that phase-coherent processes are involved in the elastic and inelastic cotunneling. An oscillation-phase change of pi is detected as a function of bias voltage at the inelastic cotunneling onset.

Solvation, transduction and independent component analysis for pattern recognition in SAW electronic nose

Abstract: This paper revisits the fundamental basis for signal generation in polymer coated SAW vapor sensors and applies the independent component analysis (ICA) for feature extraction from the SAW sensor array data to explore whether the independent components could represent analyte-specific solvation parameters and whether they could form the feature vector for reliable pattern classification. Thermodynamic partitioning of analytes between vapor and polymer phases is treated as independent contributions from different solvation mechanisms, each associated with characteristic ‘environment swap’ energy. The overall equilibrium partition coefficient of an analyte is modeled as product of partial partition coefficients associated with different solvation mechanisms. The polymer films on SAW devices are treated to be acoustically thin. The theory of signal generation accounts for effects from both the mass as well as the viscoelastic loadings. It explains the signal amplification factor due to viscoelastic effects, and models the sensor signal to be proportional to the equilibrium partition coefficient. Thus, the logarithmic signal becomes a linear combination of the partial free energies associated with various solvation mechanisms. A linear-solvation-energy relationship (LSER) like factorization is assumed for the partial free energies where the latter are expressed as product of analyte and complimentary polymer associated solvation parameters. The problem of sensor array signal analysis is then treated as a blind source separation problem with the analyte solvation parameters being the independent sources, the polymer solvation parameters being the mixing weights and the log(signals) being the measured variables. The FastICA algorithm with Gram-Schmidt orthogonalization is applied to determine independent components. The principal component analysis (PCA) is done as pre-processing step for ICA. An experimental SAW sensor array data available in the literature [Rose-Pehrsson et al., Anal. Chem. 60 (1988) 2801–2811] is used to seek validation for our approach, and to examine the role of ICA in SAW sensor array signal processing. In brief, the paper establishes a direct relationship between the independent components and the analyte solvation parameters, and presents ICA as an effective method for feature extraction for pattern recognition in SAW electronic noses.

Study on the degradation of low-density polyethylene in the presence of cobalt stearate and benzil

Abstract: Degradable polymers are in great demand for a variety of applications such as packaging, agriculture, and medicine. Polyolefins blended with photodegradants/biodegradants are potential candidates for replacing the nondegradable thermoplastics in areas where litter abatement poses problems. In the present article, the effect of metallic photoinitiators like cobalt stearate and a combination of metallic/nonmetallic photoinitiators, i.e., a mixture of cobalt stearate and benzil, on the photooxidative degradation of low-density polyethylene (LDPE) films have been investigated. Attempts have been made to correlate the results as a function of mixed additives. Films of LDPE containing varying amounts of cobalt stearate and a combination of benzil and cobalt stearate were prepared. The photodegradation of these films has been monitored by various techniques like FTIR spectroscopy, differential thermal analysis, and density and viscosity measurements. Cobalt stearate was highly effective in accelerating the photodegradation of LDPE films at low concentrations. The addition of benzil to cobalt stearate decreased the rate of photodegradation compared to cobalt stearate alone. A retarding effect was observed when benzil alone was added to LDPE. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 236–243, 2006

Zn,Ni ferrite/NiO nanocomposite powder obtained from acetylacetonato complexes

Abstract: The results on the synthesis, microstructure, structure and DC magnetization studies of nanocomposite Zn,Ni ferrite/NiO powder obtained by thermal decomposition of acetylacetonato complexes are reported in this paper. According to the results obtained by inductively coupled plasma optical emission spectroscopy (ICP-OES) element analysis and multiphase Rietveld refinement, the three samples made are composed of spinel-ferrite (86.7%–96.7%) and NiO (3.3%–13.3%) phases. The compositions of the spinel-ferrite (SP) phase in the investigated samples, S1–S3, are Zn0.72Ni0.24Fe1.98O4, Zn0.56Ni0.29Fe2.07O4 and Zn0.40Ni0.40Fe2.10O4, respectively. Due to the cation deficiency in spinels, created vacancies induce a partial change in the cation valence, . The vacancy distribution is found to be random at 8a and 16d cation sites, except in sample S3, where all vacancies are over octahedral sites. The x-ray line broadening due to crystallite size effect is found to be isotropic for all spinels, while the x-ray line broadening due to the strain effect is anisotropic. A correlation between the Zn2+ occupancy of the tetrahedral site and the 650 cm−1 Raman peak intensities is shown. The observed coercivity decrease and shift in hysteresis loop in the samples are caused by the interaction between spinel and NiO phase. The results of M(H) measurements point to the properties of an ensemble of interacting nanoparticles. High saturation magnetization values and superparamagnetic behaviour at room temperature point to the technological significance of the title compounds.

Classical hall effect in scanning gate experiments

Abstract: Scanning gate experiments on a two-dimensional electron gas in the regime of the classical Hall effect are presented. The Hall resistance is recorded while tuning the local potential by applying a voltage to the metallic tip of a scanning force microscope. In diffusive samples and at zero magnetic field an intriguing Hall resistance pattern arises that is attributed to tip-induced inhomogeneous current flow. Measurements at small, i.e., nonquantizing, magnetic fields reveal an additional Hall resistance pattern due to the tip-induced inhomogeneous electron density in the Hall cross. Deviations of the measurements on higher-mobility samples from expectations based on symmetry arguments are used to distinguish the diffusive from the mesoscopic transport regime. Finite-element-method modeling for the diffusive regime and trajectory calculations for ballistic electrons allow a concise interpretation of the measurements.

Two-subband quantum Hall effect in parabolic quantum wells

Abstract: The low-temperature magnetoresistance of parabolic quantum wells displays pronounced minima between integer filling factors. Concomitantly the Hall effect exhibits overshoots and plateaulike features next to well-defined ordinary quantum Hall plateaus. These effects set in with the occupation of the second subband. We discuss our observations in the context of single-particle Landau fan charts of a two-subband system empirically extended by a density dependent subband separation and an enhanced spin splitting ${g}^{*}$.

Dielectric, ferroelectric and pyroelectric properties of PMNT ceramics

Abstract: In the present study, different PMNT samples in the MPB range were prepared in single perovskite phase using Columbite technique. After poling, dielectric constant (er) of PMNT 70/30 samples decreases and of PMNT 68/32 and PMNT 66/34 samples increases. Polarization vs. electric field (P–E) hysteresis loops of PMNT samples show their ferroelectric nature. Transition temperature (Tc) obtained from pyroelectric coefficient ( p i ) variation with temperature increases with the increase in PT content in PMNT samples near MPB and agrees with Tc obtained from the dielectric study.

Anisotropic reversible mixed-state properties of superconducting carbon-doped Mg ( B 1 − x C x ) 2 single crystals

Abstract: We report on magnetic and torque measurements on carbon-doped $\mathrm{Mg}{({\mathrm{B}}_{1\ensuremath{-}x}{\mathrm{C}}_{x})}_{2}$ single crystals with $x=0.038$, 0.066, and 0.095. One-band Ginzburg Landau (GL) and London theories are applied to derive the reversible parameters: i.e., the upper critical fields ${H}_{c2}$, the coherence lengths $\ensuremath{\xi}$, the penetration depths $\ensuremath{\lambda}$, the anisotropy $\ensuremath{\gamma}$, the lower critical fields ${H}_{c1}$, and the GL parameter $\ensuremath{\kappa}$ for fields parallel and perpendicular to the crystallographic $ab$ plane. Due to the persistence of two-band superconductivity in the investigated doping range, these parameters turn out to depend not only on temperature, as in the case of one-band superconductors, but also on field. Therefore, the evaluation was carried out separately for two field regions: first for fields near ${H}_{c1}$ and second close to ${H}_{c2}$. In general, the upper critical field increases and the anisotropy decreases upon carbon doping. All further reversible parameters are significantly modified as well. These modifications are compared for single crystals with different carbon concentration.

Dielectric behaviour of microwave sintered rare-earth doped BaTiO3 ceramics

Abstract: Ferroelectric Ba 1− x Re x TiO 3 (Re = Ce, Nd and Pr) with x = 0.003 were prepared through microwave sintering route. The dielectric studies were made on these samples as a function of frequency and temperature and high values of dielectric constant has been obtained in most of the samples. Large values of room temperature dielectric constant obtained in the present study is attributed to microwave sintering. Impedance measurements were also carried out from room temperature to 600 °C and in a wide frequency range. The complex impedance plots (imaginary versus real values of Z , M and Y ) have brought out interesting features of the samples. These plots enable the separation of grain, grain boundary and electrode effects.

Dielectric spectroscopy of a high-polarization ferroelectric liquid crystal

Abstract: Dielectric spectroscopy investigations in the frequency range 50 Hz to 1 MHz have been carried out on a new ferroelectric liquid-crystalline material (S-(-)-4-(2-n-hexylpropionyloxy)biphenyl-4′-(3-methyl-4-decyloxy)benzoate) possessing a relatively large spontaneous polarization (P s ∼ 240 nC cm−2) and containing a lateral methyl group on the aromatic ring of the alkoxybenzoate unit. The effect of temperature on the dielectric relaxation modes has been investigated in the SmC* and N* phases. From dielectric dispersion data, relaxation frequency and dielectric strength of all detected relaxation modes have been evaluated and discussed. A new surface-like mode of relaxation frequency ∼11 kHz and dielectric strength 3.8, is seen to appear in the SmC* phase.

Correlation between oxygen isotope effects on transition temperature and magnetic penetration depth in high-temperature superconductors close to optimal doping

Abstract: The oxygen-isotope $(^{16}\mathrm{O}\text{\ensuremath{-}}^{18}\mathrm{O})$ effect (OIE) on the in-plane magnetic penetration depth ${\ensuremath{\lambda}}_{ab}(0)$ in optimally doped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$, and in slightly underdoped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ and ${\mathrm{Y}}_{0.8}{\mathrm{Pr}}_{0.2}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ was studied by means of muon-spin rotation. A substantial OIE on ${\ensuremath{\lambda}}_{ab}(0)$ with an OIE exponent ${\ensuremath{\beta}}_{\mathrm{O}}=\ensuremath{-}d\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}{\ensuremath{\lambda}}_{ab}(0)∕d\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}{M}_{\mathrm{O}}\ensuremath{\approx}\ensuremath{-}0.2$ (${M}_{\mathrm{O}}$ is the mass of the oxygen isotope), and a small OIE on the transition temperature ${T}_{c}$ with an OIE exponent ${\ensuremath{\alpha}}_{\mathrm{O}}=\ensuremath{-}d\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}{T}_{c}∕d\phantom{\rule{0.2em}{0ex}}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}{M}_{\mathrm{O}}\ensuremath{\simeq}0.02--0.1$ were observed. The observation of a substantial isotope effect on ${\ensuremath{\lambda}}_{ab}(0)$, even in cuprates where the OIE on ${T}_{c}$ is small, indicates that lattice effects play an important role in cuprate high-temperature superconductors.

Plasma effects in lateral Schottky junction tunneling transit-time terahertz oscillator

Abstract: We study the plasma oscillations in a two-dimensional electron channel with a reverse-biased Schottky junction. Using the developed model we show that the negative dynamic conductivity of the Schottky junction associated with the tunneling injection and electrontransit- time effect can result in the self-excitation of plasma oscillations (plasma instability) in the quasineutral portion of the channel serving as a resonant cavity. The spectrum of plasma oscillations and the conditions of their self-excitations are expressed via the structure parameters. The instability can be used in a novel diode device - lateral Schottky junction tunneling transit-time terahertz oscillator.

Model calculation of the scanned field enhancement factor of CNTs

Abstract: The field enhancement factor of a carbon nanotube (CNT) placed in a cluster of CNTs is smaller than an isolated CNT because the electric field on one tube is screened by neighbouring tubes. This screening depends on the length of the CNTs and the spacing between them. We have derived an expression to compute the field enhancement factor of CNTs under any positional distribution of CNTs using a model of a floating sphere between parallel anode and cathode plates. Using this expression we can compute the field enhancement factor of a CNT in a cluster (non-uniformly distributed CNTs). This expression is used to compute the field enhancement factor of a CNT in an array (uniformly distributed CNTs). Comparison has been shown with experimental results and existing models.

Effect of field-dependent core size on reversible magnetization of high-κ superconductors

Abstract: The field dependence of the vortex core size {zeta}(B) is incorporated in the London model, in order to describe reversible magnetization M(B,T) for a number of materials with large Ginzburg-Landau parameter {kappa}. The dependence {zeta}(B) is directly related to deviations in M(ln B) from linear behavior prescribed by the standard London model. A simple method to extract {zeta}(B) from the magnetization data is proposed. For most materials examined, {zeta}(B) so obtained decreases with increasing field and is in qualitative agreement both with behavior extracted from {micro}SR and small-angle neutron-scattering data and with that predicted theoretically.

Lead-strontium titanate glass ceramics: II-dielectric behaviour

Abstract: Dielectric properties of glass ceramic samples prepared in the system [(Pb1-xSrx)O.TiO2)]- [2SiO2.B2O3]-[K2O]-[BaO] have been studied. Dielectric constant vs temperature plots of lead rich glass ceramics samples show a peak similar to Curie peak in case of ceramic lead strontium titanate. The Curie temperature of tetragonal crystalline phase of various glass ceramics has been found to decrease with Pb/Sr ratio in the initial glass. This complements X-ray diffraction (XRD) studies carried out previously to establish that the crystallites in various glass ceramic samples are (Pb1−xSrx)TiO3 solid solutions.

Probing the Kondo density of states in a three-terminal quantum ring

Abstract: By measuring the Kondo effect in a quantum ring connected to three terminals, we show that it is possible to determine which lead is strongly coupled to the ring and gives rise to a Kondo resonance, and which lead is weakly coupled. In the configuration where the ring is strongly coupled to only two leads, we use the third weakly coupled lead to probe the Kondo density of states (DOS). By applying a bias voltage between the two strongly coupled leads, we show directly the splitting of the out-of-equilibrium Kondo DOS.