Michael von Ortenberg

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.

Magnetoresistance and upper critical fields of oriented TlBaCaCuO thin films in pulsed high magnetic fields up to 40 T

The magnetoresistance was measured on oriented thin film TlBaCaCuO samples in pulsed high magnetic fields up to 40 T, by a PSD method at frequencies of 200 kHz, for the two fundamental orientations of the film in the magnetic field. T c was defined at the midpoint of the transition curve. We obtained -d B c2 // /d T =20 T/K and -d B c2 ⊥ /d T =0.6 T/K, for the magnetic field parallel and perpendicular to the film plane, respectively, which give a large anisotropy of the upper critical field B c2 ( T ) of 33. The estimated coherence lengths are ξ ⊥ (0)=0.8 A and ξ // (0)=28 A.

The application of high magnetic fields in solid state physics

Abstract Frontiers in High Magnetic Fields may induce Frontiers in Materials Engineering and vice versa especially for materials combining semiconductor and magnetic properties in semimagnetic semiconductors. ZnMn2As2 is a new material of the class (Zn1−xMnx)3As2. For x= 2 3 , however, the usual statistical substitutionof the Zn-ion by Mn2+ condenses into an ordered layer structure. The different magnetic phases are determined by neutron scattering, compared with low temperature magnetization data and detected directly in the DC-magnetoresistance of the low mobility holes, whose conductivity properties are essentially determined by the internal spin polarization of the material.

FIR and transport properties of II–VI narrow gap semiconductors in high magnetic fields

Abstract In HgSe:Fe the Fe++-state acts as a donor degenerate with the conduction band. For a threshold concentration of nFe=5×1018cm−3 the Fermi level gets pinned to the donor energy and spatial correlation effects of the ionized donors become important. Different experimental methods are presented to probe the density of states near the Fermi surface in the mixed valence regime. No final model exists so far to explain the imaging of the density of states at the Fermi energy in all relevant experimental data. For further studies epitaxial layers of Q2D are desirable. As preliminary result for this issue epitaxial layers of pure HgSe have been successfully grown and checked by magnetospectroscopy.

Pure and Combined Spin-Flip of Delocalized Electrons

A review of high-magnetic field electron-spin spectroscopy of delocalized electrons in solids is given. Due to spin-orbit coupling there is always a strong influence of the delocalized motion of the electrons on the spin properties. A series a experimental results on combined spin-flip and related resonances in connection with different magnetic field generators up to 1,000 T is presented and discussed.

Optical radiation: counterpart of high magnetic fields

Abstract High magnetic fields provide an excellent tool to tune the energy levels of charge carriers. Any probe energy with a fixed value can be used to investigate the level system in question. Along with internal energies of the system, electromagnetic radiation proves to be an ideal probe. In this way magneto-optics has dominated due to its efficiency in the study of energy-band structures in solids. Selected examples for the investigation of semimagnetic semiconductors and magnetic materials covering a wide range of radiation energies and magnetic fields demonstrate the operation of magneto-optics in relation to other magneto-spectroscopic methods.

Transient Megagauss Fields on Indium Antimonide and Mercury Cadmium Telluride

We present a careful analysis of previously reported data obtained in cyclotron resonance measurements in InSb using CO2 radiation and transient magnetic fields in the range of ≈ 50T. With the consideration of eddy currents incorporating Drude model like conductivity tensor we developed a quantitative determination of conductivity in transient fields. Using these values an explanation is found for data previously not understood. According to our considerations many other hysteretic phenomena can be qualitatively represented.

CYCLOTRON RESONANCE IN MEGAGAUSS FIELDS

A review of magneto-optical experiments on semiconductor structures in megagauss fields is presented, featuring both the single-turn coil-generator and the explosive flux compression. Special emphasis is laid on the transient character of the fields resulting in unexpected and sophisticated response of the materials investigated.

HgSe and HgSe:Fe in extreme magnetic fields

We present a tight binding approach to model energy levels of HgSe in high magnetic field. In this model we are able to explain all given experimental findings in low field and high field as well as in strained samples of lower dimension.

MAGNETOOPTICAL EXPERIMENTS USING THE EXPLOSIVE DRIVEN FLUX COMPRESSION UP TO 1000T

We discuss three recent magnetooptical experiments using the explosive driven flux-compression technique that have been performed in cooperation of the Berlinian with the Russian group of VNIIEF, Sarov. We present an overview on the experimental techniques touching the frontiers of physics as well as a detailed discussion of the results obtained on the semiconducting materials GaN, GaAs and HgSe. Special emphasis will be laid on the interpretation in context with theoretical predictions and analysis that go beyond the ordinary k•p-formalism but are also valid in the limit of the HOFSTADTER butterfly.