Electron Interaction in Carbon Nanotubes and Reduced-Dimensional Semiconductors
Carbon Nanotubes and Reduced-Dimensional Semiconductors
Physics and applications of low-dimensional nanoscale electronic structures is one of the central areas of modern condensed matter physics. Rapid progress in nanotechnology has enriched physics with novel objects that have unique physical properties governed by quantum effects, and new unique functionalities. Specific areas that are currently being pursued are outlined below.
Electronic Structure and Transport Properties of Carbon Nanotubes
Single-wall carbon nanotubes (SWNTs) are cylindrical, nanometer-diameter, quasi-1D objects made of a wrapped monolayer of carbon, graphene. Due to a wide range of band gap energies, allowed direct optical transitions, and excellent electrical and thermal properties, SWNT-based devices are widely considered as future building blocks for nano-electronics and optoelectronics. SWNTs exhibit a wealth of novel phenomena due to the combined effects of their intrinsic cylindrical shape, 1-dimensionality, the peculiar electronic structure of graphene and electron-electron interactions, including Luttinger liquid behavior, Kondo and Coulomb blockade effects, and formation of strongly bound 1D excitons. One of the most striking examples is the Aharonov-Bohm (AB) effect in CNTs, manifested as modification of the bandstructure under the action of the enclosed magnetic flux, as schematically shown in Fig. 1a, b. In fact, extremely high fields of thousands of Tesla are necessary to resolve a full period of the AB modulation in SWNTs. Only a small fraction of the AB period, or total band gap period, can be explored in semiconducting CNTs with experimentally attainable magnetic fields. Nevertheless, for narrow-gap, (quasi)-metallic nanotubes, such magnetic fields are strong enough to allow continuous tuning of a CNT between a gapped, semiconducting state and a gapless, metallic state (Fig. 1c).
Ongoing activities aim to explore further the effects of strong magnetic fields on electrical and optical properties of SWNTs. These activities emphasizes the use of band gap tunability in an axial magnetic field to probe the electronic structure of narrow-gap and gapless SWNTs, and explore new concepts for optoelectronics.
For more information contact Dmitry Smirnov.
Infrared Spectroscopy of Graphene at High Magnetic Fields
Graphene is the newest member in the family of two-dimensional (2D) carrier systems, which have shown a spectrum of fascinating new physics over the past decades. Graphene, a single atomic sheet of graphite, represents the ultimate 2D material. Moreover, its electronic band structure differs radically from the parabolic bands common to all previous 2D systems. In graphene, the conduction and valence bands meet at two inequivalent, charge-neutral points in momentum space around which the dispersion is linear, leading to the so-called Dirac cones. Much of the interest in graphene stems from an analogy of this dispersion relation to that of relativistic, massless fermions, leading to intriguing new phenomena.
Fig. 1a. Energy band structure of graphene.
In a magnetic field, the linear dispersion relation of graphene leads to an unequally spaced Landau level (LL) spectrum. Most unusually, there exists a LL at E = 0 with a distinctive electron-hole degeneracy. This peculiar behavior of carriers in graphene is further enriched by spin splittings, possible lifting of the Dirac cone valley degeneracy and general many-particle effects, and provides a unique opportunity to probe these exceptional electronic properties of graphene via LL formation.
Infrared measurements reveal both intra- and inter-LL transitions in graphene. The transition energies scale as √B and a simple, non-interacting LL transition interpretation yields a band velocity of c′ ≈ 1.1106m/s. The scaling between the experimental values of these transition energies deviate from a calculated precise 1:(√2 + 1) scaling based upon the single particle picture, and this fact indicates a contribution from many-particle interactions to the transitions.
Project Collaborators:
NHMFL: Z. Jiang, L. C. Tung, and Y.-J. Wang
Columbia University: E. A. Henriksen, P. Kim, and H. L. Stormer
Quantum Cascade Structures Under High Magnetic Fields
Quantum Cascade Lasers (QCLs) are a product of cutting edge band structure engineering. They exploit quantum effects in low-dimensional semiconductor structures lasers to produce wavelength tunability. Multi-quantum wells structures can now be manufactured allowing for the repetition of the optical intersubband transitions of the same electron as it "cascades" down the structure (Fig.1a). There are typically 500-1000 nanometer-thick layers grown into each QCL comprising it's 20 to 100 periods.
Since the energy and size scales of the spatial and magnetic confinements are similar, application of a strong magnetic field has proven to be a sensitive tool to study and control fundamental processes in the physics of intersubband transitions – quantum confinement and intersubband relaxation. An intense magnetic field applied perpendicular to the 2DEG planes of a QCL resonantly modulates electron's lifetime, which causes in turn giant oscillations of laser emission power, laser threshold and quantum efficiency (Fig.1b). High field measurements finally allow retrieving detailed information about the electronic structure, the nature and magnitude of the main intersubband scattering mechanisms – the key parameters for the design of future optoelectronic devices.
References:
- Leuliet, A.; Vasanelli, A.; Wade, A.; Fedorov, G.; Smirnov, D.; Bastard, G. and Sirtori, C., Electron scattering spectroscopy by a high magnetic field in quantum cascade lasers, Phys. Rev. B, 73 (8), 085311 (2006)
- Semtsiv, M.P.; Dressler, S.; Masselink, W.T.; Fedorov, G. and Smirnov, D., Probing the population inversion in intersubband laser by magnetic field spectroscopy, Appl. Phys. Lett., 89 (17), 171105 (2006)
- Faugeras, C.; Wade, A.; Leuliet, A.; Vasanelli, A.; Sirtori, C.; Fedorov, G.; Smirnov, D.; Teissier, R.; Baranov, A. N.; Barate, D. and Devenson, J., Radiative quantum efficiency in an InAs/AlSb intersubband transition, Phys. Rev. B, 74 (11), 113303 (2006)
- Vasanelli, A.; Leuliet, A.; Sirtori, C.; Wade, A.; Fedorov, G.; Smirnov, D.; Bastard, G.; Vinter, B.; Giovannini, M. and Faist, J., Role of elastic scattering mechanisms in GaInAs/AlInAs quantum cascade lasers, Appl. Phys. Lett., 89, 172120 (2006)
For more information contact Dmitry Smirnov.
