Superconductivity in 4-Angstrom carbon nanotubes—a short review

Abstract

We give an up-to-date review of the superconducting phenomena in 4-Angstrom carbon nanotubes embedded in aligned linear pores of the AlPO 4 -5 (AFI) zeolite, first discovered in 2001 as a fluctuation Meissner effect. With the introduction of a new approach to sample synthesis around 2007, new data confirming the superconductivity have been obtained. These comprise electrical, specific heat, and magnetic measurements which together yield a consistent yet complex physical picture of the superconducting state, largely owing to the one-dimensional (1D) nature of the 4-Angstrom carbon nanotubes. For the electrical transport characteristics, two types of superconducting resistive behaviors were reproducibly observed in different samples. The first type is the quasi 1D fluctuation superconductivity that exhibits a smooth resistance drop with decreasing temperature, initiating at 15 K. At low temperatures the differential resistance also shows a smooth increase with increasing bias current (voltage). Both are unaffected by an applied magnetic field up to 11 Tesla. These manifestations are shown to be consistent with those of a quasi 1D superconductor with thermally activated phase slips as predicted by the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. The second type is the quasi 1D to 3D superconducting crossover transition, which was observed to initiate at 15 K with a slow resistance decrease switching to a sharp order of magnitude drop at $7.5 K. The latter exhibits anisotropic magnetic field dependence and is attributed to a Berezinskii-Kosterlitz-Thouless (BKT)-like transition that establishes quasi-long-range order in the plane transverse to the c-axis of the aligned nanotubes, thereby mediating a 1D to 3D crossover. The electrical data are complemented by magnetic and thermal specific heat bulk measurements. By using both the SQUID VSM and the magnetic torque technique, the onset of diamagnetism was observed to occur at $15 K, with a rapid increase of the diamagnetic moment below $7 K. The zero-field-cooled and field-cooled branches deviated from each other below 7 K, indicating the establishment of a 3D Meissner state with macroscopic phase coherence. The superconductivity is further supported by the specific heat measurements, which show an anomaly with onset at 15 K and a peak at 11-12 K. In the 3D superconducting state, the nanotube arrays constitute a type-II anisotropic superconductor with H c1 z 60 to 150 Oe, coherence length x z 5 to 15 nm, London penetration length l z 1.5 mm, and Ginzburg-Landau k z 100. We give a physical interpretation to the observed phenomena and note the challenges and prospects ahead.

Superconductivity in Bundles of Double-Wall Carbon Nanotubes

“…Since the coherence length of the wire is given by , where is the quantum flux, d = 15 nm and Tesla, we get its value to be 24.3 nm. This is noted to be on the same order of magnitude as the superconducting coherence length in 4-Angstrom SWNTs (6–15 nm)1123 and B-doped SWNTs (17 nm)24. From this value of the coherence length we may also estimate the inverse proximity effect, since for our sample normal metal electrodes were used, and the critical temperature T c of a superconducting wire would consequently be suppressed in accordance with the formula 25, with T c0 being the critical temperature in the absence of the inverse proximity effect.…”

A disorder-enhanced quasi-one-dimensional superconductor

“…On cooling crystals A – C in a dilution refrigerator, we uncover a two-step superconducting transition characteristic of strongly anisotropic q1D superconductors35363738 (Fig. 4a–c).…”

“…4g,h,j), but is rapidly suppressed by a magnetic field. Low transverse phase stiffness is common in q1D superconductors: for example, bulk phase coherence in carbon nanotube arrays is quenched by 2–3 T, yet pairing persists up to 28 T36. The superconducting volume fraction corresponding to the magnitude of this Meissner effect is also unusually low: <0.1%.…”

Highly fluorescent C-dots obtained by pyrolysis of quaternary ammonium ions trapped in all-silica ITQ-29 zeolite

“…There are precedents in the literature in which crystalline aluminophosphate AlPO-5 with monodirectional structure constituted by channels containing tripropylammonium ion has been used for the preparation of C-dots, 27 but this zeotype AlPO-5 has been found better suited for the formation of single wall carbon nanotubes with very narrow diameter. [28][29][30][31][32][33][34][35] Results and Discussion.…”