O. Portugall

Excitons in carbon nanotubes with broken time-reversal symmetry.

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields (>55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or dark).

Magnetic Brightening of Carbon Nanotube Photoluminescence through Symmetry Breaking

We report that symmetry breaking by a magnetic field can drastically increase the photoluminescence quantum yield of single-walled carbon nanotubes, by as much as a factor of 6, at low temperatures. To explain this we have developed a theoretical model based on field-dependent exciton band structure and the interplay of Coulomb interactions and the Aharonov-Bohm effect. This conclusively explains our data as the first experimental observation of dark excitons 5-10 meV below the bright excitons.

Alignment Dynamics of Single-Walled Carbon Nanotubes in Pulsed Ultrahigh Magnetic Fields

We have measured the dynamic alignment properties of single-walled carbon nanotube (SWNT) suspensions in pulsed high magnetic fields through linear dichroism spectroscopy. Millisecond-duration pulsed high magnetic fields up to 56 T as well as microsecond-duration pulsed ultrahigh magnetic fields up to 166 T were used. Because of their anisotropic magnetic properties, SWNTs align in an applied magnetic field, and because of their anisotropic optical properties, aligned SWNTs show linear dichroism. The characteristics of their overall alignment depend on several factors, including the viscosity and temperature of the suspending solvent, the degree of anisotropy of nanotube magnetic susceptibilities, the nanotube length distribution, the degree of nanotube bundling, and the strength and duration of the applied magnetic field. To explain our data, we have developed a theoretical model based on the Smoluchowski equation for rigid rods that accurately reproduces the salient features of the experimental data.

ARMS: A Successful European Program for an 80 T User Magnet

ARMS is the acronym for Advanced Research Magnet Systems, a project under the European Unions 5th Framework Research Infrastructures program (HPRI-CT-1999-50007). Eight partners throughout Europe cooperated in ARMS, which was coordinated by the University of Oxford, UK. The objective of the project was to build an 80 T user magnet to be installed at LNCMP, Toulouse, France. The approach chosen was the coil-ex/coil-in method, whereby a large outer coil was energized by LNCMPs unique, 14 MJ capacitor bank. The inner coil was energized by a fast, 100 kJ bank at the peak of the outer coils field. In this paper the evolution of the coils, the conductors and other materials is outlined as is the testing and ultimate, successful use in physics experiments up to 76 T. A view of the future direction of high pulsed fields in Europe, post-ARMS, will also be given

The Megagauss facility of the HUMBOLDT high magnetic field center in Berlin

Abstract We report on concept. realisation, and first experimental results obtained with the preliminary megagauss equipment of the HUMBOLDT High Magnetic Field Center. The present megagauss set-up is a handy, transportable single-turn coil equipment using only 20% of the capacity of the final installation, i.e. 50 kJ at 60 kV producing about 100 and 200 T in the single-turn coils of 10 and 5 mm diameter, respectively. Test reports on magnetooptical measurements at low temperature are presented.

MAGNETO SPECTROSCOPY OF SINGLE-WALLED CARBON NANOTUBES

Carbon nano-tubes (CNTs) are attracting tremendous interest as the object of fundamental studies in condensed matter and molecular physics as well as possible functional units for future nano-devices. Some of the most striking features of CNTs are related to the symmetry breaking effect of high magnetic fields threading the tube axis. In the present paper we review the results of recent magneto-optical studies of single-walled CNTs in pulsed magnetic fields up to 71 T and at temperatures between 4.2 and 300 K. We present clear evidence for (a) the effect of the Aharonov-Bohm phase on the bandstructure of CNTs, (b) the existence of dark excitons in CNTs due to Coulomb mixing and (c) the effect of magnetic brightening, i.e. the increase of the quantum yield of CNTs under the influence of an external field. Experimental data was derived from large ensembles of individualized tubes in either aligned films or in liquid suspension. A brief discussion of the effect of dynamic magnetic alignment of suspended tube...

OPTICAL PROCESSES IN SINGLE-WALLED CARBON NANOTUBES THREADED BY A MAGNETIC FLUX

J. KONO,1,† S. ZARIC, J. SHAVER, X. WEI, S. A. CROOKER, O. PORTUGALL, G. L. J. A. RIKKEN, R. H. HAUGE, and R. E. SMALLEY Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, U.S.A. †E-mail: kono@rice.edu National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, U.S.A. National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A. Laboratoire National des Champs Magnetiques Pulses, 31432 Toulouse Cedex 04, France Department of Chemistry, Rice University, Houston, Texas 77005, U.S.A.

HIGH SENSITIVITY MEGAGAUSS SPECTROSCOPY

The present paper describes unique experimental techniques developed for minimizing disturbances due to transient electromagnetic fields during a capacitor discharge into a small single turn coil, generating magnetic fields above the megagauss limit. Results of cyclotron resonance measurements are presented as example of the capability of the experimental set-up to resolve transmission changes being smaller than one percent.

The 3D Analogue of the Quantum Hall Effect in HgSe:Fe and Its Temperature Dependence

We present temperature dependent measurements of oscillations in the Hall resistance in Hg1−x Fe x Se referred as “the 3D analogue of the quantum Hall effect” [1] in magnetic fields up to 18 Tesla and temperatures between 4.2 and 50 K. Highly doped material (x ≈ 0.1) exhibits a strong modulation of the Hall resistance which cannot be explained by fluctuations of the carrier concentration in a system with a pinned Fermi energy. The measurements are compared with simultaneously recorded Shubnikov-de Haas-data.

The Temperature Dependence of the Three-Dimensional Analogue of the Quantum Hall Effect in Semimagnetic Hg_{1-x}Fe_{x}Se

Ηg1x Fex Se with Fe concentrations above 5 x 1018cm-3 is a mixed valence system with a Fermi energy pinned to the localised donor state [1]. Oscillations of the density of states caused by an applied external magnetic field, thus lead to a bidirectional charge transfer between the impurities and the quasi-free electron gas. These fluctuations of the free carrier concentration could be observed as modulations of the Hall voltage [2] with an amplitude of about 0.8 % at T = 4.2 K We measured the temperature dependence of this effect in fields up to 17.5 T and at temperatures up to 30 K in an unoriented sample with an iron concentration of 5 x 1019cm-3 and compared it with simultaneously recorded Shubnikov-de Haas oscillations. We found no phase shifting between Hall and Shubnikov-de Haas oscillations, thus scattering effects as a cause for the Hall voltage modulations described by Mani et al. [3] can be excluded. We observed an independence of the average free carrier concentration from temperature in the considered range. Mobility measurements up to 30 K showed only a decrease of about 10 % so that a temperature independent Landau-level halfwidth can be used to calculate the concentration of the quasi-free carriers f f (E) D(E)dE (Fig. 1). The thermal broadening of the Hall oscillations is therefore essentially caused by the smoothing of the Fermi distribution f (E).