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Hysteresis in Optical Megagauss Spectroscopy
Berlin, 06. September 2006
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Pulsed magnetic fields in excess of 100 T, that is one Megagauss, can only be generated at the expense of very short pulse duration, typically microseconds. The pulse structure of the so-called transient field exposes the investigated materials to the same field magnitude in very short sequence, leading to different observations at the two times as the sample did not return to thermodynamical equilibrium.
Whereas sample-intrinsic dynamical phenomena of the spins or charges have been observed, analysed and understood, a large number of observations in the optical spectroscopy of semiconductors in Megagauss fields could not be readily explained prior to this work.
A complex analysis is performed on the impact of the magnetic field generation onto the observations with emphasis on both, microscopic responses of the sample material as well as macroscopic effects of a reasonably well conducting system. The latter, well known to manifest themselves as eddy currents, can be demonstrated to affect the macroscopic observations that are made in any such experiment in a way as to modify or obscure microscopic information. Moreover, it can be shown that a completely new class of observations occurs directly connected with eddy currents.
As a striking example, in cyclotron resonance measurements of bulk Indium Antimonide, InSb, a very standard material and technique, completely unexpected transmission drops occur additionally to the observation of the cyclotron resonance itself. The most intriguing property of which is its asymmetric occurrence only whenever the field is rising to a maximum of either polarity and not on the return to zero field. It can be shown that this hysteresis phenomenon depends on the field derivative rather than the time structure of the pulse, indicating an eddy current related phenomenon.
This can be understood by the existence of additional charge carriers which are generated by the Hall like electric field that builds up to balance an azimuthal eddy current. In a very rough application of the Drude model, a quantitatively successful calculation and simulation is performed. En route to this groundbreaking result a new technique is applied to measure the transient conductivity of bulk semi-conducting specimen contactlessly in a transient magnetic field using existent magnetization measurement equipment.
Finally, with demonstrations of the dependence of cyclotron resonances on both sample radius (size) and sweep rate / derivative of magnetic field for various mercury based semiconductors it was clearly shown, that eddy current related phenomena must be analysed whenever a conductor is exposed to a transient magnetic field, even in other than optical experiments.
Keywords: Hysteresis, bulk semiconductors, Indium Antimonide, Mercury based semiconductors, transient magnetic fields, Megagauss, eddy currents, field derivative, cyclotron resonance, optical spectroscopy, semiconductor dynamics, transient conductivity, Drude model
Rezension von Prof. Junichiro Kono, PhD
Rice University, Houston, Texas, USA
Kontakt über email@example.com (in Englisch)
“With currently available high magnetic field technology, megagauss magnetic fields (i.e. fields greater than 100 T) can be produced only in pulsed form. There are several different methods for producing such high magnetic fields, but the most convenient and powerful one is the so-called Single-Turn Coil Method, which was employed in this thesis work …
This method has produced a great variety of new experimental results over the past decades that provide insight into the charge and spin states of electrons and holes in solid state materials placed in ultrahigh magnetic fields.
However, traditionally, in the analyses of experimental data taken in such high pulsed magnetic fields, dynamical aspects have been totally neglected. Namely, the applied high magnetic fields have been treated as quasi-stationary, under the assumption that the time scale of the pulsed magnetic field (i.e., microseconds) can be considered to be infinitely long compared to all other dynamical phenomena occurring inside the sample. This is a crude approximation that breaks down in some situations, and this is exactly what this thesis has investigated.
It presents great details of the electrodynamics in analyzing phenomena induced by time-varying magnetic fields, starting from Maxwell’s equations. This is one of the most detailed and accurate analyses of this physical problem I have ever encountered in the literature. In Particular, it takes into account all possible effects associated with eddy currents.
The Highlight of this thesis work is … (a study of) Indium Antimonide, where the peculiar characters of what is referred to as “Hysteresis of the Third Type” are fully described and … quantitatively demonstrated through a model with realistic parameters. I consider this to be a great achievement.
… this thesis work provides significant new insight into the behaviour of conductors in pulsed magnetic fields. This is the first systematic study of this sort, to my knowledge, and shed light on previously neglected aspects of high-field solid state spectroscopy.”
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