Search Results (15 total)

The temperature dependence of 1∕f noise in individual semiconducting carbon nanotube (CNT) field-effect transistors is used to estimate the distribution of activation energies of the fluctuators D(E) responsible for the noise. D(E) shows a rise at low energy with no characteristic energy scale, and a broad peak at ∼0.4  eV. The peak, responsible for the majority of noise at room temperature, cannot be due to electronic excitations, carrier number fluctuations, or structural fluctuations of the CNT, and likely results from the motion of defects in the dielectric or at the CNT-dielectric interface, or very strongly physisorbed species (binding energy ∼0.4  eV) on the CNT or dielectric surface.

We report measurements of field-effect transistors made of isolated single-walled carbon nanotubes contacted by superconducting electrodes. For large negative gate voltage, we find a dip in the low-bias differential resistance. Remarkably, this dip persists well above the superconducting transition temperature of the electrodes, indicating that it is not caused by superconducting proximity effect from the electrodes. This conclusion is supported by measurements on carbon nanotubes contacted by normal electrodes showing similar features. One possible explanation is superconductivity in the nanotubes, occurring when the gate voltage shifts the Fermi energy into van Hove singularities of the electronic density of states.

We explore the solvation of single conjugated polymer molecules in a smectic liquid crystal using single molecule spectroscopy. We find evidence for two distinct orientations for solvation: the majority of polymers are narrowly orientated parallel to the nematic director, as expected, but, unexpectedly, a second population is aligned perpendicular to the director. These molecules reside in the gaps between the layers. This latter type of solvation has not been previously observed and is not expected when the density variation along the director is sinusoidal.

We present data from one-dimensional unshunted Josephson-junction arrays with ground planes, showing resonant behavior for certain values of the critical current. Due to the hysteresis of the current-voltage characteristics, the number of junction that oscillate on resonance can be controlled. The resonant frequency decreases as more junctions are switched onto the resonance and increases as the array length is increased. We develop a transmission-line model of the arrays that reproduces these experimental observations. We also examine the microscopic origin of this model and compare it to existing models in the literature.

Previous experiments [Barbara et al., Phys. Rev. Lett. 82, 1963 (1999); Vasilić et al., Appl. Phys. Lett. 78, 1137 (2001)] showed that two-dimensional, unshunted Josephson-junction arrays emit radiation coherently when a threshold number of junctions is switched to a radiating state. Here, we report low-power radiation being emitted when the number of radiating junctions is smaller than the threshold number. We show that in this regime the output power is incoherent, i.e., the array behavior is qualitatively different from the coherent regime.

We simulate two-dimensional Josephson-junction arrays, including full mutual-inductance effects, as they are cooled below the transition temperature in a magnetic field. We show numerical simulations of the array magnetization as a function of position, as detected by a scanning superconducting quantum interference device which is placed at a fixed height above the array. The calculated magnetization images show striking agreement with the experimental images obtained by Nielsen et al. [Phys. Rev. B 62, 14 380 (2000)]. The average array magnetization is found to be paramagnetic for many values of the applied field, confirming that paramagnetism can arise from magnetic screening in multiply connected superconductors without the presence of d-wave superconductivity.

We have measured a paramagnetic Meissner effect in Nb–Al₂O₃–Nb Josephson-junction arrays using a scanning superconducting quantum interference device microscope. The arrays exhibit diamagnetism for some cooling fields and paramagnetism for other cooling fields. The measured mean magnetization is always small, 〈Φ_tot-Φ_ext〉<0.3Φ₀ (in terms of flux per unit cell of the array, where Φ₀ is the flux quantum), for the range of cooling fields investigated (-12Φ₀ to 12Φ₀). We demonstrate that a different model of magnetic screening, valid for multiply-connected superconductors, reproduces all of the essential features of paramagnetism that we observe and that no exotic mechanism, such as d-wave superconductivity, is needed for paramagnetism.

We present experimental and theoretical results showing that the distributed electromagnetic environment of a Josephson junction array can cause the junctions in the array to synchronize. Based on our experimental results and our distributed array model, we find that an external load is not necessary for junction synchronization. Also, we show that the high-frequency performance of an array can be significantly better than the performance of a single isolated junction.

The paramagnetic Meissner effect (PME) measured in high-T_C granular superconductors has been attributed to the presence of π junctions between the grains. Here we present measurements of complex ac magnetic susceptibility from two-dimensional arrays of conventional (non-π) Nb/Al/AlOx/Nb Josephson junctions. We measured the susceptibility as a function of the temperature T, the ac amplitude of the excitation field h_ac and the external magnetic field H_dc. The experiments show a strong paramagnetic contribution from the multijunction loops, which manifests itself as a reentrant screening at low temperature, for values of h_ac higher than 50 mOe. A highly simplified model, based on a single loop containing four junctions, accounts for this paramagnetic contribution and the range of parameters in which it appears. This model offers an alternative explanation of PME that does not involve π junctions.

We report measurements of mm-wavelength radiation from two-dimensional arrays of underdamped Josephson junctions. All of our samples emit coherently in a novel synchronized state, which is triggered by a resonance in the array structure. Measurements of the detected power as a function of the number N of active junctions show a threshold, suggesting population inversion. Above threshold, the power scales with N² up to an array size bigger than the free-space radiation wavelength. The highest measured conversion efficiency from dc to ac power is about 17%. Our data are consistent with stimulated emission causing coherence and cannot be explained by existing classical coupling mechanisms.

Femtosecond pump-probe spectroscopy of the equilibrated hydrated electron has been recorded with 35 fs resolution, revealing unprecedented transient features on the 30–80 fs time scales that have been assigned to inertial solvation dynamics of the hydrated electron. These data allow for a rigorous experimental evaluation of the recently published nonadiabatic quantum molecular dynamics simulations of the adiabatic and nonadiabatic dynamics of the hydrated electron.

We have measured the complex ac magnetic susceptibility of unshunted Josephson-junction arrays as a function of temperature T, amplitude of the excitation field h_ac, and external magnetic field H_dc. For small h_ac Meissner screening occurs. For larger h_ac, however, the screening is reentrant in T. This reentrance is not thermodynamic but dynamic and arises from the paramagnetic contribution of multijunction loops. This result gives an alternative explanation of the paramagnetic Meissner effect observed in granular superconductors. Experimental results are in agreement with a simplified model based on a single loop containing four junctions.

We present a detailed analysis of the interaction between two fluxon chains in parallel magnetically coupled long Josephson junctions, one of which is biased (``generator'') while another is unbiased (``detector''). The main effect is that the driven fluxon chain in the generator may drag the chain in the detector. We note that five different regimes of the interaction are possible: both chains may be pinned by the external magnetic field; both may move in a locked state, inducing the same dc voltage in both junctions; in an unlocked state they may move at different velocities; the chain in the detector may remain pinned while the one in the generator is moving; and, finally, in a limited range of parameters the mean detector voltage may be negative, which implies that the detector chain is moving in the direction opposite to that of the chain in the generator. We consider a simplified model based on the assumptions that the fluxon chains are dense and rigid, and that their motion is nonrelativistic. In this model, each chain is represented by a single degree of freedom (its coordinate). Numerical and analytical consideration of the simplified model demonstrates that it is able to reproduce correctly all the dynamical regimes except for the negative-voltage one. To explain the existence of the latter regime, we introduce another model, suggested by the simulations, which is based on the presence of two fluxons and one antifluxon in the generator, and a single fluxon in the detector. The negative voltage is produced by motion of the antifluxon in a bound state with the detector's fluxon. The existence region of this state is limited by its collisions with free fluxons in the generator.

We present results of cw, time-resolved, and spatially resolved spectroscopic studies of emission and absorption in a model conjugated polymer, poly(p-pyridyl vinylene) (PPyV). The redshifted film spectra suggest the formation of aggregated regions. The ∼4× reduction in emission efficiency in films vs solution is attributed to a longer radiative lifetime for aggregate excitons, as is evidenced by time-resolved fluorescence measurements. We present direct optical imaging of aggregates in a conjugated polymer via near-field scanning optical microscopy. The aggregate emission and absorption are found to be localized to partially aligned regions of the film ∼200 nm in size.

The first zero-field step in the current-voltage characteristic of intermediate-length, high-quality, low-loss Nb/Al-AlO_x/Nb Josephson tunnel junctions has been carefully investigated as a function of temperature. When decreasing the temperature, a number of structures develop in the form of regular and slightly hysteretic steps whose voltage position depends on the junction temperature and length. This phenomenon is interesting for the study of nonlinear dynamics and for application of long Josephson tunnel junctions as microwave and millimeter-wavelength oscillators.