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, 1999, 41, . 5 From Roentgen to Ioffe, from Giessen to Saint Petersburg Relations Between Russian and German Physics A. Scharmann Physics Institute, JustusLiebigUniversity Giessen, D 35392 Giessen, Germany E-mail: Arthur.Scharmann@exp1.physik.uni-giessen.de Two former professors of physics at Giessen university contributed significantly to the development of the Ioffe Institute: Wilhelm Conrad Roentgen as a teacher of Abram Ioffe, and Wilhelm Hanle, whose effect found various applications, e. g., in the spectroscopy of hot electrons in low dimensional structures. A few examples will illustrate how topics of their scientific work found a continuation in the research activities at Giessen, but also in the collaboration between Giessen and Saint Petersburg. They range from sodium chloride, the old Roentgen / Ioffe material where we couldprove the existence of anunusual isotope effect in nickel-doped crystals, over level-crossing experiments in gases, to GaAs/A1As superlattices, where level-anticrossing spectroscopy of excitons reveals detailed information about recombination processes and interface quality. A short summary of the efforts to keep the traditionally close and good relations between Russian and German physics vivid completes the report.

A talk given by a physicist from Giessen, Germany, at Gyulai would have been the inventors of the F center since a conference with the title Physics at the turn of the during their extensive experiments on the influence of X-rays 21 century naturally starts with the beginning of the 20 and light upon the electrical conduction in crystals [4] they century and Wilhelm Conrad Roentgen, winner of the first certainly dealt with such defects.

Nobel prize in 1901. He became well known in the One year after Roentgens death in 1923 another physicist scientific community already before detecting the X-rays by who became important for Giessen University, namely an experiment which he did during his time in Giessen Wilhelm Hanle, owner of the physics chair at Giessen where he owned the physics chair from 1879 to 1888. from 1941 to 1969, reported [5] on an effect which later Rowland proved in 1876 that a moved electrostatic charge, was named after him. This Hanle effect, depolarization a so called convection current, had the same magnetic effect of (resonance) fluorescence by an external magnetic field due to destruction of the coherence existing at zero-field like a normal conduction current. Roentgen repeated first in level-crossing, is still one of the most accurate methods for 1885 these experiments in a much improved form and then measuring atomic lifetimes. Its main advantage are the low extended them to the first experimental proof of Maxwells densities at which the experiments can be done in order to dielectric displacement current [1]. For this purpose he avoid disturbing effects like collision broadening etc.

used an apparatus in which a disk made from a dielectric For that reason it has been widely used in the Ioffe material was rotating between two ring electrodes. The Institute [6] as well as in Giessen. Two examples shall upper one was grounded, the lower one consisted of two be given here: The group of Boris Zakharchenya studied halfs with opposite electric potentials. This caused two the photoluminescence of hot electrons created in a times per turn a change of sign of the polarization inside GaAs/AlGaAs multiple quantum well structure by excitation the dielectric, and Roentgen succeeded in an unambiguous with a krypton laser [7]. In the emission, a pronounced proof of the magnetic field connected to this displacement peak shows up at the high energy edge of the spectrum current. In this field we find our first example for relations stemming from the recombination of electrons from the between Russian and German physics, since the Russian point of photocreation, that means prior to any energy physicist A. Eichenwald later continued these experiments relaxation. The degree of polarization in this peak decreases at the engineering school in Moscow and was the first to with increasing magnetic field in the form of a typical Hanle demonstrate the quantitative agreement of this magnetic field 2 with that of a normal conduction current [2].

curve P(B)/P(0) = 1+4c 0 -1 and allows to determine More important for the anniversary we celebrate this the lifetime 0 of the hot electrons which, in that case, week was of course the long-lasting connection between corresponds to the emission time of a LO phonon. It is Roentgen and Ioffe. At the end of a century in which the obvious that such results can be obtained only in experiments existence of quarks and Z bosons has been proved, it is which avoid complicating factors like, for instance, phonon very illustrative to read about the years from 1902 to 1905 heating leading to a large spread in the times observed.

when the founder of our Physical-Technical Institute spent The second example is a result from my institute [7].

as Roentgens student and assistant at Munich University. In In this case, the level-crossing technique is used in a his book Vstretchi s fizikami [3] he gives a very detailed rather different context. Light emitted from the helium description of Roentgen as an excellent experimentator, but 41D level excited by atomic collisions with He+ or Ne+ very conservative physicist, who did not allow to use the projectiles with a polarization parallel to the ion beam word electron in his institute. Without this restriction, has been registrated in dependence of external electric and perhaps, Roentgen and Ioffe instead of Pohl, Gudden and magnetic fields. For zero electric field only the Hanle From Roentgen to Ioffe, from Giessen to Saint Petersburg Relations Between Russian and German... effect influences the intensity I(B) of this (polarized) light diagonalization of the appropriate spin hamiltonian with help which therefore has Lorentzian lineshape with a half-width of the programme V-epr [10] it is possible to understand depending on the lifetime of the excited state. At non- and fit completely the angular dependence observed for zero, but constant electric field the quadratic Stark effect an [001] rotation. What was interesting and puzzling with causes several additional level crossings, but only those these results is the structure observed on the resonance. A with m = 2 can be observed with the experimental deconvolution of the integrated spectrum yields 5 Gaussians geometry used. So a splitting of the Hanle curve into with an intensity ratio 1 : 6 : 13 : 6 : 1.

three Lorentzians results whose intensities directly reflect The problem that this cannot be explained by a superhythe sublevel populations. perfine interaction with surrounding chlorine nuclei already From these examples you may see the common interests arose in the case of silver chloride doped with Ni2+, where which existed in our institutes a long time, and not only in a similar structure was observed [11]. The authors explained this field. But all of you are aware of the problems ham- this by a different distribution of the two chlorine isotopes on pering the relations between Russian and (West) German the four neighbouring lattice sites in the plane perpendicular physics. Already Roentgen and Ioffe shared a fate which to the centre axis. Taking into account the natural abundance 35 many of us experienced for a long time, too, namely to live of Cl being three times larger than that of Cl one really in countries being for some time very unfriendly to each calculates probabilities of 1 : 6 : 13 : 6 : 1 for the different other. This becomes obvious from Roentgens statement configurations. The idea behind is a distortion of the Of course I could not publish during the war against square due to different vibrational amplitudes as a result Russia a scientific article together with a Russian physicist of the different isotope masses. This distortion leads to (in [3]), explaining the long delay (from 1913 to 1921) an orthorombic component of the crystal field and by that between the two parts of their common paper [4] about to a deviation of the resonance position proportional to Electrical conductivity in some crystals and the influence x2 - y2. So the model can explain the existence of 5 lines of irradiation on it. I still remember my first visit in this with the intensity ratio observed, but until now no direct beautiful town on occasion of the International Conference determination of the real distribution of the isotopes was on Luminescence in 1972 and the strong barriers between done. Therefore we performed an electron-nuclear double East and West which existed at that time. Fortunately the resonance (ENDOR) experiment on this system. The result situation became better and better in the beginning of the is a large number of ENDOR lines, which again with help Eighties. Especially in my field, interaction of radiation with of the V-epr programme can be completely analyzed.

matter, the scientific community in Russia and Germany is Analyzing the ENDOR intensities for the Cl respectively very indebted to Albrecht Winnacker, who participates in Cl nuclei at the 100 position over the five lines, one really this conference, and Kurt Schwarz, at that time in Riga, finds a decrease for chlorine 37, an increase for chlorine 35, for organizing a series of Soviet- (now Russian-) German and surprisingly on the central line a ratio 1:5 instead of the seminars on Point defects in insulators and deep level natural isotope ratio (1 : 3). So our ENDOR experiments centers in semiconductors, and to the Russian Academy of completely proved the model.

Sciences and Deutsche Forschungsgemeinschaft for funding. After the drastic changes in 1989 further programmes From these seminars, starting with the first meeting were started to support the cooperation between Russian at Heidelberg 1983, emanated a vivid scientific exchange and German physicists. Here I want to mention especially between Giessen and Leningrad / Saint Petersburg. During the Volkswagen Foundation which financed from 1990 to one of these research stays we came back to Steinsalz 1998 eighteen projects with scientific groups from Saint (NaC1), the old material of Roentgen and Ioffe [4]. Here, Petersburg, among them eleven with participation of the however, in a special form, namely doped with divalent Ioffe institute. Altogether 17 million DM were given to nickel. This incorporation needs a charge compensation Russia (10 percent of them to Saint Petersburg), a sum by sodium vacancies. Andrey Badalyan detected in EPR which amounts to nearly one half of the total funding two different centre configurations, one with the vacancy for Central and Eastern Europe. An important role in along an 100, the other along an 110 direction [9]. this context played Gottfried Landwehr from Wrzburg We concentrated one the first species. We have three of University as a head of the selection committee.

them, besides the one shown with a nickel-vacancy axis I am very glad that one of the eleven projects mentioned along [001], two others along [100] and [010] respectively. above was carried out by Boris Zakharchenyas and Pavel What do we expect from this 3d8 system Its level scheme in Baranovs groups from Ioffe institute and my group at a cubic environment with tetragonal distortion can be found Giessen. Its topic was photoluminescence of hot electrons in many textbooks. For our EPR experiment only the orbital and magnetic resonance in quantum well structures and susinglet ground state with triplet spin configuration (the two d perlattices. GaAs/AlAs superlattices are grown by molecular electrons couple to S = l) is relevant. The axial crystal field beam epitaxy and consist of a sequence of alternating GaAs causes a fine structure splitting between the three mS states and AlAs layers. This periodical arrangement shows up competing with the Zeeman effect which makes the situation nicely in high resolution transmission electron microscopy.

a little bit more complicated when the external magnetic In addition, Roentgens discovery also plays an important field is not directed along the centre axis. But via exact role for the characterization of such nanostructures, X-ray 5 , 1999, 41, . 820 A. Scharmann diffraction supplying complementary information about layer of the radiative levels. The first level anticrossing (with dimensions. Since these are of the order of a few mono- increasing magnetic field) for the normal interface leads to layers, drastic quantum confinement effects appear. We a population increase of the level, from which light linearly studied a series of samples with nominally 5.5 monolayers polarized along [110] is emitted. The second anticrossing (1.73 nm) GaAs and 8.5 monolayers (2.65 nm) A1As. For accordingly increases the intensity of the [110] light. For quantum wells with such dimensions the energy of the the inverted interface the sequence is reversed. In that way lowest electron state in A1As is lower than that in GaAs, so one can even distinguish at which interface the recombining excitons are formed from Xz electrons in the A1As layer and exciton is localized [14]. Since the whole luminescence from -holes in the GaAs layer. This situation characterizes is a dynamic process and all levels are coupled by rate a type II superlattice and leads to a localization of the equations a population increase of one radiative level may excitons at the interfaces. Due to the very low temperatures cause a decrease in the other. Another consequence is of our experiments (1.5 K) only the energetically lowest the different dependence of ODMR of different excitons on excitons (more exactly: the heavy hole excitons) play a the microwave chopping frequency we use to modulate the role. The radius of such an exciton, which in the bulk effect [15]. This provides an additional possibility to study material amounts to more than 10 nm, is strongly reduced in exciton dynamics and to unravel different contributions.

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