• 커뮤니티
  • 세미나/콜로퀴움
세미나/콜로퀴움

APCTP-POSTECH Distinguished Lecture[09.07.06]

본문

"
“Carbon Nanotube Superconductivity: An Update”
  

ØSpeaker :Prof. Ping Sheng (HKUST, Clear Water Bay, Kowloon, Hong Kong)
ØPlace : #512, Hogil Kim Memorial Bldg., POSTECH
ØDate & Time : July 6 (Mon), 15:00 - 16:00

  
ØAbstract
Superconductivity in carbon nanotubes is a topic of intriguing interest.  While the small-diameter nanotubes
Are  predicted  to  have  enhanced  electron-phonon  coupling—a  key  element responsible  for  nanotube
superconductivitythe associated increase in fluctuation effects is unfavorable to the  manifestation  of  a
superconducting transition.  The possibility of a Peierls transition in thin nanotubes is a further  deterrent  to
superconductivity.  It  follows that the existence of coupling  between  the  nanotubes  is  important  to  the
realization of its superconducting behavior, since the transverse coherence can suppress  fluctuations  and
lower the Peierls transition temperature, thereby making the  appearance  of  a  superconducting  transition
possible.   Owing to  its  ordered  and  closely-spaced  pore  structure (with  a  13.6  Å  center-to-center
separation between the 0.7 nm diameter pores), the AFI zeolite (composition: Al12P12O48) with embedded
4-Å carbon nanotubes constitutes an ideal material  for  the  observation  of  nanotube  superconductivity.
  
In this talk  I  will  describe  the  eight-year effort  that  resulted  in  the  two  new  developments  in  2007
(on improved sample fabrication) and  2008 (on making electrical contacts),  leading  to  reasonably  easy
observation of nanotube  superconductivity with high reliability  and  repeatability.  I n particular,  we  have
measured the specific heat signal for the superconducting transition as well as the resistive superconducting
transition.  The coupled carbon nanotube arrays can yield both nearly 1D superconducting behavior as well
as the Josephson array behavior, with their attendant magnetic characteristics.  The overall physical picture
that emerges is that of a coupled Josephson array consisting of aligned nanotubes crossing over from  an
individually fluctuating 1D system to a coherent 3D superconductor, mediated by a Kosterlitz-Thouless (KT)
transition which establishes quasi long range order in the lateral plane perpendicular to the  c-axis  of  the
nanotubes.  The attainment of global coherence is seen at 5K and below, accompanied by the appearance
of a well-defined supercurrent gap at 2K.  While the existence of the superconducting transition in nanotube
arrays is now beyond reasonable doubt, there are still many aspects  of  the  data  which  have  yet  to  be
theoretically understood.  I will present some important lessons learned from the eight-year effort and what
they tell us about the mesoscopic electronic states in most carbon nanotubes.  In particular, these lessons
may shed light on why nanotube superconductivity was not observed and/or confirmed earlier.
  
                   Contact Person : Prof. Wokyung Sung(054-279-2061, wsung@postech.ac.kr)"
top_btn