Eunseong Kim

Supersolid Helium at High Pressure

We have measured the pressure dependence of the supersolid fraction by a torsional oscillator technique. Superflow is found from 25.6 bar up to 136.9 bar. The supersolid fraction in the low temperature limit increases from 0.6% at 25.6 bar near the melting boundary up to a maximum of 1.5% near 55 bar before showing a monotonic decrease with pressure extrapolating to zero near 170 bar.

Effect of 3He impurities on the nonclassical response to oscillation of solid 4He.

We have investigated the influence of impurities on the possible supersolid transition in 4He by systematically enriching isotopically pure samples with 3He. The addition of 3He broadens the onset of nonclassical rotational inertia and shifts it to higher temperature, suggesting that the phenomenon is correlated with the condensation of 3He atoms onto the dislocation network in solid 4He.

Evidence of Supersolidity in Rotating Solid Helium

Supersolidity in a Spin Observing superfluid flow in a solid is a counterintuitive finding that has been accomplished by freezing 4He inside a torsional oscillator and monitoring the oscillating period as the temperature is lowered: A reduction in the oscillating period will be observed at the supersolid transition when the mass of the superfluid decouples from the oscillator and the remaining normal component of the solid. However, extraneous classical effects can also cause this reduction, and so, to confirm supersolid formation, Choi et al. (p. 1512, published online 18 November) performed a slightly different measurement. Rotation was superimposed onto the oscillating motion, and the period and the shear modulus of the system were measured simultaneously. These two quantities exhibited very different responses to the rotation speed, suggesting that supersolidity (rather than classical effects that would also affect the shear modulus) is indeed at the root of the previously observed change in the oscillating period. Measurements on rotating frozen helium support the formation of a quantum, or supersolid, phase. Supersolidity, the appearance of zero-viscosity flow in solids, was first indicated in helium-4 torsional oscillator (TO) experiments. In this apparatus, the irrotationality of the superfluid component causes it to decouple from the underlying normal solid, leading to a reduction in the resonant period of the TO. However, the resonant period may be altered for reasons other than supersolidity, such as the temperature dependence of the elastic modulus of solid helium. Superimposing rotation onto oscillatory measurements may distinguish between supersolidity and classical effects. We performed such simultaneous measurements of the TO and the shear modulus, and observed substantial change in the resonant period with rotational speed where the modulus remained unchanged. This contrasting behavior suggests that the decrease in the TO period is a result of supersolidity.

Thin film transistors using preferentially grown semiconducting single-walled carbon nanotube networks by water-assisted plasma-enhanced chemical vapor deposition

Nearly perfect semiconducting single-walled carbon nanotube random network thin film transistors were fabricated and their reproducible transport properties were investigated. The networked single-walled carbon nanotubes were directly grown by water-assisted plasma-enhanced chemical vapor deposition. Optical analysis confirmed that the nanotubes were mostly semiconductors without clear metallic resonances in both the Raman and the UV-vis-IR spectroscopy. The transistors made by the nanotube networks whose density was much larger than the percolation threshold also showed no metallic paths. Estimation based on the conductance change of semiconducting nanotubes in the SWNT network due to applied gate voltage difference (conductance difference for on and off state) indicated a preferential growth of semiconducting nanotubes with an advantage of water-assisted PECVD. The nanotube transistors showed 10(-5) of on/off ratio and approximately 8 cm2 V(-1) s(-1) of field effect mobility.

Multi-barrier field-emission behavior in PBTTT thin films at low temperatures

We investigated the low-temperature transport mechanism for poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT). The temperature-dependent transport behavior was studied by varying the drain–source electric field and gate bias. The results suggest that low-temperature charge transport is dominated by direct tunneling at low electric fields, while field emission is prevailing for high electric fields with high carrier densities. However, the obtained barrier heights are remarkably greater than expected in a conventional field emission. We propose a simplified model of field emission through quasi-one-dimensional path with multiple barriers which shows good agreement with the results more clearly. Field emission across the domain boundaries may assist in overcoming the transport barriers induced by the interchain disorder, which results in the weak temperature dependence of conductivities and nonlinear current–voltage relation at low temperatures.

Amorphous dithenylcyclopentadienone-carbazole copolymer for organic thin-film transistors

We developed a new high performance amorphous donor–acceptor conjugated copolymer consisting of a dithienylcylclopentadienone subunit as an electron acceptor and carbazole derivative as an electron donor. X-Ray diffraction analysis shows no scattering patterns, indicating that a disordered amorphous solid is formed. Atomic force microscopy (AFM) images show an amorphous surface mophology regardless of the annealing temperature. A high on/off current ratio of approximately 106 and high field effect mobility of 2.2 × 10−2 cm2 V−1 s−1 were obtained with stable output characteristics. The high performance of the amorphous copolymer is ascribed to the relatively low activation energy and low characteristic temperature obtained from a low temperature transport analysis, reflecting that localization of the charge carrier is not substantial in the film.

Scaling analysis of field-tuned superconductor–insulator transition in two-dimensional tantalum thin films

The superconductor–insulator (SI) transition in two-dimensional Ta thin films is investigated by controlling both film thickness and magnetic field. An intriguing metallic phase appears between a superconducting and an insulating phase within a range of film thickness and magnetic field. The temperature and electric field scaling analyses are performed to investigate the nature of the SI transition in the thickness-tuned metallic and superconducting samples. The critical exponents product of νz obtained from the temperature scaling analysis is found to be approximately 0.67 in the entire range of film thickness. On the other hand, an apparent discrepancy is measured in the product of ν(z + 1) by the electric filed analysis. The product values are found to be about 1.37 for the superconducting films and about 1.86 for the metallic films respectively. We find that the discrepancy is the direct consequence of electron heating that introduces additional dissipation channels in the metallic Ta films.

Stress- and temperature-dependent hysteresis of the shear modulus of solid helium

The shear modulus of solid 4He below 200 mK exhibits an unusual increase, the characteristics of which show remarkable similarities to those of the period reduction in torsional oscillator experiments. We systematically studied the drive strain and temperature dependence of the shear modulus at low temperatures. The hysteretic behavior depends strongly on the drive and cooling history, which can be associated with the thermally assisted Granato-Lucke dislocation theory. The phase diagram of the shear modulus is constructed on the basis of the emerging hysteretic behavior.

Simultaneous investigation of shear modulus and torsional resonance of solid He 4

We investigate the origin of a resonant period drop of a torsional oscillator (TO) containing solid

Modified Granato–Lucke Theory with Pinning Length Distribution in Solid 4He

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