Volume 80, Number 4, November 2007
Article Number 47002
Number of page(s) 5
Section Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties
Published online 23 October 2007
EPL, 80 (2007) 47002
DOI: 10.1209/0295-5075/80/47002

Transport properties of massless Dirac fermions in an organic conductor $\alpha$-(BEDT-TTF)2I3 under pressure

N. Tajima1, S. Sugawara2, M. Tamura1, R. Kato1, Y. Nishio2 and K. Kajita2

1  RIKEN - Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan
2  Department of Physics, Toho University - Miyama 2-2-1, Funabashi-shi, Chiba 274-8510, Japan

received 12 May 2007; accepted in final form 21 September 2007; published November 2007
published online 23 October 2007

A zero-gap state with the Dirac cone-type energy dispersion was found in an organic conductor $\alpha$-(BEDT-TTF)2I3 under high hydrostatic pressures. This is the first two-dimensional zero-gap state discovered in bulk crystals with layered structures. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermions with anisotropic Fermi velocity. From the galvano-magnetic measurements, the density and mobilities of electrons and holes were determined in the temperature region between 77 K and 2 K. In this region, the carrier density (n) depends on temperature (T) as $n\propto T^{2}$ and decreases by about four orders of magnitude. On the other hand, the sheet resistance per BEDT-TTF layer $(R_{{\rm S}})$ stays almost constant in the region. The value is written as $R_{{\rm S}}$ = gh/e2 in terms of the quantum resistance h/e2 = 25.8 k$\Omega $, where g is a parameter that depends weakly on temperature.

71.20.Rv - Polymers and organic compounds.
72.15.Gd - Galvanomagnetic and other magnetotransport effects.
75.47.-m - Magnetotransport phenomena; materials for magnetotransport.

© Europhysics Letters Association 2007