Well-defined Shells Research Articles

Formation of vortex clusters and giant vortices in mesoscopic superconducting disks with strong disorder

Merged, or giant, multi-quanta vortices (GVs) appear in very small superconductors near the superconducting transition due to strong confinement of magnetic flux. Here we present evidence for a new, pinning-related, mechanism for vortex merger. Using Bitter decoration to visualise vortices in small Nb disks, we show that confinement in combination with strong disorder causes individual vortices to merge into clusters/GVs well below Tc and Hc2, in contrast to well-defined shells of individual vortices found in the absence of pinning.

Pinning-Induced Formation of Vortex Clusters and Giant Vortices in Mesoscopic Superconducting Disks

Merged, or giant, multiquanta vortices (GVs) are known to appear in very small superconductors near the superconducting transition due to strong confinement of magnetic flux. Here we present evidence for a new, pinning-related, mechanism for vortex merger. Using Bitter decoration to visualize vortices in small Nb disks with varying degrees of disorder, we show that confinement in combination with strong disorder causes individual vortices to merge into clusters or even GVs well below Tc and Hc2, in contrast to well-defined shells of individual vortices found in the absence of pinning.

Structure and Self-Diffusion of Water Molecules in Chabazite: A Molecular Dynamics Study

Using classical molecular dynamics (MD) simulations, we have studied some structural and diffusive properties of water molecules adsorbed in chabazite. In particular, we have investigated the variation of the self-diffusion coefficient of the water molecules as a function of their concentration and the nature of the hydration shells of the extraframework Ca++ ions with varying concentrations of water. Our study indicates that the well-defined and stable hydration shells of this ion play an important role in the diffusion process. The diffusion anisotropy is computed at T = 600 K. It is compared with theoretical results based on jump models and qualitatively compared with pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments of a single chabazite crystal at 293 K and with tracer diffusion studies.

From cylindrical-channel mesoporous silica to vesicle-like silica with well-defined multilamella shells and large inter-shell mesopores

Highly regular vesicle-like silica consisting of concentric shells of silica and voids in alternation have been prepared in a multilamellar vesicles (MLVs) template created with a P123 surfactant and a hydrophobic additive 1,3,5-triisopropylbenzene (TIPB) by hydrolysis and condensation of tetraethoxysilane (TEOS). It is shown that the morphology of the mesoporous silica is mainly controlled by the molar ratio of TIPB : P123. Cylindrical-channel (rod-like) mesoporous silica with hexagonal structure was obtained at low molar ratio of TIPB : P123, while a vesicle-like mesoporous silica was prepared at high molar ratio of TIPB : P123. TIPB at low concentration was merely dissolved into the hydrophobic core of the P123 cylindrical micelle, but it was forced to spread into the palisade layer at high concentration and ultimately transformed the cylindrical micelle phase into the MLV phase. The key role of TIPB has been demonstrated in increasing the average packing parameter (P) of P123, and this opens the way to synthesize vesicle-like silica simply by controlling the molar ratio of TIPB : P123.

Molecular Modeling of Interactions between l-Lysine and Functionalized Quartz Surfaces

Molecular modeling techniques have been used to investigate the interaction of L-lysine in aqueous medium with silanol and methyl sites onto quartz substrates. The substrate effect has been studied for partially hydrophilic surfaces formed by silanol and methyl groups with a ratio of 1:5 and hydrophobic fully methylated surfaces. Molecular dynamics and static calculations indicate that L-lysine does not show any significant interaction with fully methylated surfaces, while its interaction with hydroxylated/methylated surfaces is dominated by electrostatic and H-bond terms. Accordingly, on fully methylated surfaces there is no preferential orientation of L-lysine with respect to the surface, while for hydroxylated/methylated surfaces the L-lysine-surface interaction mainly depends on the molecular orientation, with a preferred geometry involving the ammonium group pointing toward the silanol site. The structure of water shells around L-lysine molecules was shown to be strongly affected by the relative hydrophilic/hydrophobic character of the surfaces. In particular, the order is almost completely lost for partially hydrophilic surfaces, while well-defined hydration shells around L-lysine are obtained for hydrophobic surfaces.

The fabrication of hollow spherical copper sulfide nanoparticle assemblies with 2-hydroxypropyl-β-cyclodextrin as a template under sonication

2-Hydroxypropyl-β-cyclodextrin was used as a template to fabricate hollow spherical copper sulfide nanoparticle assemblies in the presence of sonication. The as-prepared spheres were uniform in shape and have well-defined shells composed of one-layered CuS nanoparticles. The interaction between the Cu ions and HP-β-CD was confirmed by the FTIR. The effects of the sonication were studied and a possible self-assembly mechanism was discussed.

On rotational dynamics of an NH4+ ion in water

The structural, thermodynamic, and dynamical properties of a single ammonium ion, NH4+, in liquid water were investigated using molecular dynamic simulation techniques. Many-body polarizable model potentials were employed to describe the molecular interactions. The water molecules were found to form well-defined hydration shells with a preferred orientation around the NH4+ ion as expected. We also found that the average water dipole moment is enhanced in the first hydration shell. The translational and rotational motion of NH4+ was examined via the velocity, angular-momentum, and reorientation-autocorrelation functions. The computed rotational diffusion coefficients of the NH4+ ion in water, which were determined from the angular momentum autocorrelation function and the angular mean-square displacement, are 0.093 and 0.067×1012 rad2/s, respectively. These results are in good agreement with the experimental nuclear magnetic resonance value of 0.075×1012 rad2/s.

Growth simulations of silver shells on copper and palladium nanoclusters

The growth of Ag-Pd and Ag-Cu core-shell nanoclusters is studied by molecular-dynamics simulations on realistic time scales. The metals are modeled by many-body potentials derived in the framework of the second-moment approximation to the tight-binding model. First, the energy barriers of the most relevant diffusion processes of Ag adatoms on Pd and Cu cores are calculated, and then growth simulations are performed for several values of the deposition flux and of the temperature. We find that well-defined Ag shells on both Pd and Cu cores are grown. In particular, single-layer shells with a few defects can be obtained in a wide range of temperatures. The main difference between the Ag-Pd and the Ag-Cu systems is that in the former better structures are obtained at low temperatures, while in the latter, the best shells are grown at high temperatures. These behaviors are explained in terms of the ability (inability) to incorporate Ag adatoms into the Pd or Cu cores.

An Anomalous X-ray Diffraction Study of Yttrium(III) Hydration

Anomalous X-ray Diffraction (AXD) methods were used to determine the hydration structure of Y3+ in a 3.5 m solution of yttrium chloride in water. Data were gathered at two wavelengths below the absorption edge of Y3+. The data were corrected for attenuation, inelastic scattering and background and container scattering. They were then renormalized and put on an absolute scale of electron units. Fourier transformation of the results provided information on the Y3+ hydration. It is found that the short range order around Y3+ is well established with two well-defined hydration shells. The first, with Y3+···H2O centered at around 2.30 Å contains 8 water molecules, and the second with Y3+···O and Y3+···Cl- is centered at 4.77 Å. The results are in excellent agreement with a recent EXAFS+XANES study. Moreover, it can be shown that the contributions to the first hydration shell from O and H can be resolved in to two peaks, with maxima at 2.28 and 2.95 Å respectively. However, it is not possible to accurately determine coordination numbers for these Y3+···O and Y3+···H correlations independently due to the approximate nature of the calculations.

Evolution of the Local Order in 1,3,5-Trifluorobenzene from the Liquid State up to Supercritical Conditions

The local order in 1,3,5-trifluorobenzene has been investigated under isothermal (573 K) and isobaric (16 MPa) conditions from the liquid to the supercritical domain (critical point: Tc = 530 K and pc = 4 MPa) using neutron diffraction and molecular dynamics simulation. A progressive evolution of the local ordering is observed when the density decreases from liquid (300 K, 0.1 MPa) to gas values (573 K, 16 MPa) in the supercritical domain. The translational ordering, which is characterized by two well-defined shells of neighboring molecules at ambient conditions, is gradually weakened and extend only to the first shell as the density decreases in the supercritical domain. In contrast, although there is also a loss of the long-range orientational ordering (at about 7 A) and a tendency to reach a more random distribution, the very short orientational ordering extending on a spatial range of about 4 A is still present. Therefore, the signature of stacked pairs of molecules existing in the liquid state at am...

The structure of liquid and supercritical benzene as studied by neutron diffraction and molecular dynamics

The local ordering in benzene has been investigated under isothermal and isobaric conditions from the liquid to the supercritical domain using neutron diffraction. The experimental results have been analyzed at the light of molecular dynamics simulation. A progressive evolution of local ordering is observed when the density decreases when going from liquid to gaslike values in the supercritical domain. The translational ordering, which is characterized by three well-defined shells of neighboring molecules at ambient conditions, is gradually weakened, and extends only to the first shell as the density decreases in the supercritical domain (under isothermal compression). In the same way, the orientational ordering of benzene, which exists under ordinary conditions where parallel and perpendicular configurations of neighboring molecules are slightly favored, is lost in the supercritical state and is found to be almost completely isotropic.

Quantum dot structures and devices with sharp adjustable electronic shells

Quantum-dot (QD) heterostructures with up to five well-defined electronic shells have been fabricated using self-assembled QDs grown by molecular beam epitaxy (MBE). Shape-engineered stacks of self-aligned QDs with improved uniformity have been used to increase the gain in the active region of laser diodes. Lasing is observed in the upper QD shells for small gain media, and progresses towards the QD ground states for longer cavity lengths. At 77 K thresholds of J th =15 A/cm 2 were obtained for a 2 mm cavity lasing in the first excited state (p-shell), and J th =125 A/cm 2 for a 1 mm cavity lasing in n=3 (d-shell). At 300 K for a 1 mm cavity, J th is 490 A/cm 2 with lasing in n=4 (f-shell). For an increased QD density, J th is smaller than 100 A/cm 2 at room temperature. State-filling spectroscopy is also used to study the effects of alloy intermixing in QD ensembles having well-defined electronic shells and with various densities. Rapid thermal annealing (RTA) is performed on QD samples grown with different intersublevel energy spacings. For InAs/GaAs QDs, the intersublevel is tuned between ∼90 and 25 meV. The intense and sharp shell structures observed in photoluminescence (PL) indicate unambiguously that the QDs retained their zero-dimensional (0D) density of states after the diffusion of the potential, which also causes strong blueshifts (over ∼200 meV) and a pronounced narrowing of the inhomogeneously broadened emission (down to ∼12 meV).

Near-surface InAs/GaAs quantum dots with sharp electronic shells

The interaction between zero-dimensional states and surface states is studied using near-surface quantum dot (QD) ensembles with well-defined electronic shells. The inhomogeneous broadening of self-assembled InAs/GaAs QDs increases from ∼30 to more than ∼46 meV as the distance of the QDs from the surface is changed from 100 to 5.0 nm. Simultaneously, a decrease of the radiative recombination intensity by ∼3 orders of magnitude, and a red-shift of ∼65 meV are observed. For QDs capped with less than ∼10 nm, remarkable charge transfers between the QD and surface states lead to optical memory effects lasting over time-scales of several minutes.

Experimental Evidence for a Second Coordination Sphere Water Molecule in the Hydration Structure of YbDTPA – Insights for a Re-Assessment of the Relaxivity Data of GdDTPA

The low temperature (–100 °C) X-ray structure of the complex K2[Yb(DTPA)(H2O)] has been determined. The metal ion is at the center of a tricapped trigonal prism and is nine-coordinate, binding to the three nitrogens and five oxygens of the ligand and one water molecule. From the structure obtained, three well-defined hydration shells can be observed consisting of: i) one coordinated water molecule; ii) several water molecules in the outer coordination sphere of the YbIII ion and iii) one water molecule surprisingly close to the metal center and hydrogen-bonded to proximate carboxylate groups. A variable-field and temperature NMRD study of the corresponding Gd complex has been performed and the data interpreted by taking into account the presence of second-sphere water molecule(s). The results of the analysis are in excellent agreement with the X-ray structural data.

Coupled InAs/GaAs quantum dots with well-defined electronic shells

Artificial molecules are studied using coupled quantum-dot (QD) ensembles with well-defined electronic shells. The coupling strength between the zero-dimensional states is varied by changing the distance between two layers of stacked self-assembled InAs/GaAs QDs. For strongly coupled QDs grown with a 4 nm spacer, state-filling spectroscopy reveals a shift of the QD symmetric state to lower energies by ∼23 meV. The wetting layer states are also strongly coupled because of the shallow confinement, resulting in a redshift of its symmetric state by ∼26 meV.

Widely tunable self-assembled quantum dot lasers

Quantum dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular beam epitaxy. At 77 K, we tune the lasers from the first to the fourth excited state of the QDs by varying the cavity length, this covers a wavelength range from 869 to 963 nm. At room temperature, we obtain lasing from the second to the fourth excited state covering the 938 to 984 nm wavelength range. For high injection currents, a large part of the QD ensemble contributes at once to the stimulated emission yielding a lasing emission linewidth having a full width at half maximum of 25 nm. By increasing the energy spacing between the QD energy level contributing to the lasing and the wetting layer energy levels, improved thresholds at higher temperatures are observed, leading to lasing below 100 A/cm2 at room temperature.

Intermixing in quantum-dot ensembles with sharp adjustable shells

State-filling spectroscopy is used to study the effects of alloy intermixing in quantum-dot (QD) ensembles having well-defined electronic shells. Rapid thermal annealing is performed on samples of self-assembled QDs grown with different intersublevel energy spacings. For InAs/GaAs QDs, the intersublevel is tuned between ∼90 and 25 meV. The intense and sharp shell structures observed in photoluminescence indicate unambiguously that the QDs retained their zero-dimensional density of states after the diffusion of the potential, which also causes strong blueshifts (up to ∼200 meV) and a pronounced narrowing of the inhomogeneously broadened emission (down to ∼12 meV).

Lasing in quantum-dot ensembles with sharp adjustable electronic shells

Quantum-dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular-beam epitaxy. Shape-engineered stacks of self-aligned QDs with improved uniformity are used to increase the gain in the active region. Lasing is observed in the upper QD shells for small-gain media, and progresses towards the QD ground states for longer cavity lengths. We obtained at 77 K thresholds of Jth=15 A/cm2 for a 2 mm cavity lasing in the first excited state (p shell), and Jth=125 A/cm2 for a 1 mm cavity lasing in n=3 (d shell). At 300 K for a 1 mm cavity, Jth is 490 A/cm2 with lasing in n=4 (f shell).

Modeling of Jets from Comet Hale–Bopp (C/1995 O1): Observations from the Vainu Bappu Observatory

Jet and shell structures from Comet Hale–Bopp during the period October 1996 to October 1997 using data from the Vainu Bappu Observatory are modeled. Evolution of jet and shell structures during the period of observations could be attributed to activity from sources near +65°, +35°, +5°, −5°, −35°, −65° in latitude. Although no deliberate attempt was made to place the sources at symmetric latitudes, the best fit shows a remarkable symmetry. Due to changing solar illumination geometry, while only the southern sources were apparently active during 1996, most of the sources appeared active in February 1997 and October 1997. In the 10 April image, under a near pole-on solar illumination, curiously, only the sources at +65°, +5°, and −5° appear to be active and not the source at +35°. Pole positions which gave a reasonably good fit to the observed sets of shells varied from 260° to 290° in right ascension and −50° to −65° in declination. Around 10 April, while the gas production from the equatorial sources is expected to be low due to near grazing incidence of the Sun light, the fractional area of the high latitude source is limited for a spherically symmetric nucleus. To explain the well-defined shells due to rotation of the jets, we assume that the source at +65° occupies 10% of the longitude belt and 10° in latitude. Assuming a radius of 35 km for the comet corresponding to the upper limit of H. A. Weaver and P. L. Lamy (1999, Earth Moon Planets, in press) and Z. Sekanina (1999b, Earth Moon Planets, in press), the area of 56 km2 for this source could account for 29% of the observed total production rate of 4.0×1030 molecules s−1 by D. G. Schleicher et al. (1997, Bull. Am. Astron. Soc.29, 1033). The intricate shell patterns during February–May 1997 could not be exactly replicated using the model of a single nucleus. Complexity of the shell structures can be better explained using the binary model of Comet Hale–Bopp suggested by Sekanina (1999a, Earth Moon Planets, in press; 1998b, Astrophys. J.509, L133–L136) with sizes of 70 and 30 km across. Approximate estimates of the longitude of the sources active between February and May 1997 are presented. The lower limits on the dust to gas mass production ratio are estimated to be, 4.8±1.1, 3.4±1.0, and 6.2±1.3 on 18 February, 10 April, and 2 May, 1997, respectively.

Manipulating the energy levels of semiconductor quantum dots

Artificial atoms with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QD's) grown by molecular-beam epitaxy. State-filling spectroscopy of the zero-dimensional transitions between confined electrons and holes demonstrates that the energy levels are readily tunable. One to five confined levels, with an interlevel energy spacing between 25 and 90 meV, are obtained by adjusting the growth temperature or with post growth annealings. The uniformity and reproducibility of InAs/GaAs QD's are optimized by adjusting growth parameters affecting the evolution and the equilibrium shape of the QD's: the amount of strained material deposited, and the annealing time following the InAs deposition. Well-defined excited states are also obtained with stacked layers of vertically self-assembled QD's.