arXiv subjects

J. A. Fagan

Publications and source records attributed to J. A. Fagan.

3 recordsLinked to original sources

arXiv2010

Large Magnetic Susceptibility Anisotropy of Metallic Carbon Nanotubes

Through magnetic linear dichroism spectroscopy, the magnetic susceptibility anisotropy of metallic single-walled carbon nanotubes has been extracted and found to be 2-4 times greater than values for semiconducting single-walled carbon nanotubes. This large anisotropy is consistent with our calculations and can be understood in terms of large orbital paramagnetism of electrons in metallic nanotubes arising from the Aharonov-Bohm-phase-induced gap opening in a parallel field. We also compare our values with previous work for semiconducting nanotubes, which confirm a break from the prediction that the magnetic susceptibility anisotropy increases linearly with the diameter.

cond-mat.mes-hall↗

arXiv2008

Length-dependent Raman spectroscopy of single-walled carbon nanotubes: the effect of dispersant on defects

We compare Raman spectra from aqueous suspensions of length-separated single-walled carbon nanotubes (SWCNTs) dispersed using either polymer adsorption of single-stranded DNA or miscelle encapsulation with sodium deoxycholate surfactant. The Raman spectral features, other than the D-band, increase monotonically with nanotube length in both dispersion schemes. The intensity ratio of the disorder-induced D to G' Raman bands decays as a function of SWCNT length, proportional to 1/L, as expected for endcap defects. While the UV-vis absorption and fluorescence also increase with length for both dispersants, the fluorescence intensity is dramatically lower for DNA-wrapped SWCNTs of equal length. The similarities in the length-dependent D/G' ratios exclude defects as an explanation for the fluorescence decrease in DNA versus deoxycholate dispersions.

cond-mat.mtrl-sci↗

arXiv2008

Magneto-spectroscopy of Highly-Aligned Carbon Nanotubes: Identifying the Role of Threading Magnetic Flux

We have investigated excitons in highly-aligned single-walled carbon nanotubes (SWCNTs) through optical spectroscopy at low temperature (1.5 K) and high magnetic fields ($\textbf{\textit{B}}$) up to 55 T. SWCNT/polyacrylic acid films were stretched, giving SWCNTs that are highly aligned along the direction of stretch ($\hat{n}$). Utilizing two well-defined measurement geometries, $\hat{n}\parallel\textbf{\textit{B}}$ and $\hat{n}\perp\textbf{\textit{B}}$, we provide unambiguous evidence that the photoluminescence energy and intensity are only sensitive to the $\textbf{\textit{B}}$-component parallel to the tube axis. A theoretical model of one-dimensional magneto-excitons, based on exchange-split `bright' and `dark' exciton bands with Aharonov-Bohm-phase-dependent energies, masses, and oscillator strengths, successfully reproduces our observations and allows determination of the splitting between the two bands as $\sim4.8$ meV for (6,5) SWCNTs.

cond-mat.mes-hall↗