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2002


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Constructing Boosting algorithms from SVMs: an application to one-class classification.

Rätsch, G., Mika, S., Schölkopf, B., Müller, K.

IEEE Transactions on Pattern Analysis and Machine Intelligence, 24(9):1184-1199, September 2002 (article)

Abstract
We show via an equivalence of mathematical programs that a support vector (SV) algorithm can be translated into an equivalent boosting-like algorithm and vice versa. We exemplify this translation procedure for a new algorithm—one-class leveraging—starting from the one-class support vector machine (1-SVM). This is a first step toward unsupervised learning in a boosting framework. Building on so-called barrier methods known from the theory of constrained optimization, it returns a function, written as a convex combination of base hypotheses, that characterizes whether a given test point is likely to have been generated from the distribution underlying the training data. Simulations on one-class classification problems demonstrate the usefulness of our approach.

ei

DOI [BibTex]

2002


DOI [BibTex]


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Phase information in the recognition of natural images

Braun, D., Wichmann, F., Gegenfurtner, K.

Perception, 31(ECVP Abstract Supplement):133, 25th European Conference on Visual Perception, August 2002 (poster)

Abstract
Fourier phase plays an important role in determining global image structure. For example, when the phase spectrum of an image of a flower is swapped with that of a tank, we usually perceive a tank, even though the amplitude spectrum is still that of the flower. Similarly, when the phase spectrum of an image is randomly swapped across frequencies, that is its Fourier energy is randomly distributed over the image, the resulting image becomes impossible to recognise. Our goal was to evaluate the effect of phase manipulations in a quantitative manner. Subjects viewed two images of natural scenes, one of which contained an animal (the target) embedded in the background. The spectra of the images were manipulated by adding random phase noise at each frequency. The phase noise was the independent variable, uniformly distributed between 0° and ±180°. Subjects were remarkably resistant to phase noise. Even with ±120° noise, subjects were still 75% correct. The proportion of correct answers closely followed the correlation between original and noise-distorted images. Thus it appears as if it was not the global phase information per se that determines our percept of natural images, but rather the effect of phase on local image features.

ei

Web [BibTex]

Web [BibTex]


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Forward models in visuomotor control

Mehta, B., Schaal, S.

J Neurophysiol, 88(2):942-53, August 2002, clmc (article)

Abstract
In recent years, an increasing number of research projects investigated whether the central nervous system employs internal models in motor control. While inverse models in the control loop can be identified more readily in both motor behavior and the firing of single neurons, providing direct evidence for the existence of forward models is more complicated. In this paper, we will discuss such an identification of forward models in the context of the visuomotor control of an unstable dynamic system, the balancing of a pole on a finger. Pole balancing imposes stringent constraints on the biological controller, as it needs to cope with the large delays of visual information processing while keeping the pole at an unstable equilibrium. We hypothesize various model-based and non-model-based control schemes of how visuomotor control can be accomplished in this task, including Smith Predictors, predictors with Kalman filters, tapped-delay line control, and delay-uncompensated control. Behavioral experiments with human participants allow exclusion of most of the hypothesized control schemes. In the end, our data support the existence of a forward model in the sensory preprocessing loop of control. As an important part of our research, we will provide a discussion of when and how forward models can be identified and also the possible pitfalls in the search for forward models in control.

am

link (url) [BibTex]

link (url) [BibTex]


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The contributions of color to recognition memory for natural scenes

Wichmann, F., Sharpe, L., Gegenfurtner, K.

Journal of Experimental Psychology: Learning, Memory and Cognition, 28(3):509-520, May 2002 (article)

Abstract
The authors used a recognition memory paradigm to assess the influence of color information on visual memory for images of natural scenes. Subjects performed 5-10% better for colored than for black-and-white images independent of exposure duration. Experiment 2 indicated little influence of contrast once the images were suprathreshold, and Experiment 3 revealed that performance worsened when images were presented in color and tested in black and white, or vice versa, leading to the conclusion that the surface property color is part of the memory representation. Experiments 4 and 5 exclude the possibility that the superior recognition memory for colored images results solely from attentional factors or saliency. Finally, the recognition memory advantage disappears for falsely colored images of natural scenes: The improvement in recognition memory depends on the color congruence of presented images with learned knowledge about the color gamut found within natural scenes. The results can be accounted for within a multiple memory systems framework.

ei

PDF Web DOI [BibTex]

PDF Web DOI [BibTex]


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Detection and discrimination in pink noise

Wichmann, F., Henning, G.

5, pages: 100, 5. T{\"u}binger Wahrnehmungskonferenz (TWK), February 2002 (poster)

Abstract
Much of our information about early spatial vision comes from detection experiments involving low-contrast stimuli, which are not, perhaps, particularly "natural" stimuli. Contrast discrimination experiments provide one way to explore the visual system's response to stimuli of higher contrast whilst keeping the number of unknown parameters comparatively small. We explored both detection and contrast discrimination performance with sinusoidal and "pulse-train" (or line) gratings. Both types of grating had a fundamental spatial frequency of 2.09-c/deg but the pulse-train, ideally, contains, in addition to its fundamental component, all the harmonics of the fundamental. Although the 2.09-c/deg pulse-train produced on our display was measured using a high-performance digital camera (Photometrics) and shown to contain at least 8 harmonics at equal contrast, it was no more detectable than its most detectable component; no benefit from having additional information at the harmonics was measurable. The addition of broadband 1-D "pink" noise made it about a factor of four more detectable than any of its components. However, in contrast-discrimination experiments, with an in-phase pedestal or masking grating of the same form and phase as the signal and 15% contrast, the noise did not improve the discrimination performance of the pulse train relative to that of its sinusoidal components. We discuss the implications of these observations for models of early vision in particular the implications for possible sources of internal noise.

ei

Web [BibTex]

Web [BibTex]


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Training invariant support vector machines

DeCoste, D., Schölkopf, B.

Machine Learning, 46(1-3):161-190, January 2002 (article)

Abstract
Practical experience has shown that in order to obtain the best possible performance, prior knowledge about invariances of a classification problem at hand ought to be incorporated into the training procedure. We describe and review all known methods for doing so in support vector machines, provide experimental results, and discuss their respective merits. One of the significant new results reported in this work is our recent achievement of the lowest reported test error on the well-known MNIST digit recognition benchmark task, with SVM training times that are also significantly faster than previous SVM methods.

ei

PDF DOI [BibTex]

PDF DOI [BibTex]


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Contrast discrimination with sinusoidal gratings of different spatial frequency

Bird, C., Henning, G., Wichmann, F.

Journal of the Optical Society of America A, 19(7), pages: 1267-1273, 2002 (article)

Abstract
The detectability of contrast increments was measured as a function of the contrast of a masking or “pedestal” grating at a number of different spatial frequencies ranging from 2 to 16 cycles per degree of visual angle. The pedestal grating always had the same orientation, spatial frequency and phase as the signal. The shape of the contrast increment threshold versus pedestal contrast (TvC) functions depend of the performance level used to define the “threshold,” but when both axes are normalized by the contrast corresponding to 75% correct detection at each frequency, the (TvC) functions at a given performance level are identical. Confidence intervals on the slope of the rising part of the TvC functions are so wide that it is not possible with our data to reject Weber’s Law.

ei

PDF [BibTex]

PDF [BibTex]


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Support Vector Machines and Kernel Methods: The New Generation of Learning Machines

Cristianini, N., Schölkopf, B.

AI Magazine, 23(3):31-41, 2002 (article)

ei

[BibTex]


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Application of Monte Carlo Methods to Psychometric Function Fitting

Wichmann, F.

Proceedings of the 33rd European Conference on Mathematical Psychology, pages: 44, 2002 (poster)

Abstract
The psychometric function relates an observer's performance to an independent variable, usually some physical quantity of a stimulus in a psychophysical task. Here I describe methods to (1) fitting psychometric functions, (2) assessing goodness-of-fit, and (3) providing confidence intervals for the function's parameters and other estimates derived from them. First I describe a constrained maximum-likelihood method for parameter estimation. Using Monte-Carlo simulations I demonstrate that it is important to have a fitting method that takes stimulus-independent errors (or "lapses") into account. Second, a number of goodness-of-fit tests are introduced. Because psychophysical data sets are usually rather small I advocate the use of Monte Carlo resampling techniques that do not rely on asymptotic theory for goodness-of-fit assessment. Third, a parametric bootstrap is employed to estimate the variability of fitted parameters and derived quantities such as thresholds and slopes. I describe how the bootstrap bridging assumption, on which the validity of the procedure depends, can be tested without incurring too high a cost in computation time. Finally I describe how the methods can be extended to test hypotheses concerning the form and shape of several psychometric functions. Software describing the methods is available (http://www.bootstrap-software.com/psignifit/), as well as articles describing the methods in detail (Wichmann&Hill, Perception&Psychophysics, 2001a,b).

ei

[BibTex]

[BibTex]


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Contrast discrimination with pulse-trains in pink noise

Henning, G., Bird, C., Wichmann, F.

Journal of the Optical Society of America A, 19(7), pages: 1259-1266, 2002 (article)

Abstract
Detection performance was measured with sinusoidal and pulse-train gratings. Although the 2.09-c/deg pulse-train, or line gratings, contained at least 8 harmonics all at equal contrast, they were no more detectable than their most detectable component. The addition of broadband pink noise designed to equalize the detectability of the components of the pulse train made the pulse train about a factor of four more detectable than any of its components. However, in contrast-discrimination experiments, with a pedestal or masking grating of the same form and phase as the signal and 15% contrast, the noise did not affect the discrimination performance of the pulse train relative to that obtained with its sinusoidal components. We discuss the implications of these observations for models of early vision in particular the implications for possible sources of internal noise.

ei

PDF [BibTex]

PDF [BibTex]


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Optimal linear estimation of self-motion - a real-world test of a model of fly tangential neurons

Franz, MO.

SAB 02 Workshop, Robotics as theoretical biology, 7th meeting of the International Society for Simulation of Adaptive Behaviour (SAB), (Editors: Prescott, T.; Webb, B.), 2002 (poster)

Abstract
The tangential neurons in the fly brain are sensitive to the typical optic flow patterns generated during self-motion (see example in Fig.1). We examine whether a simplified linear model of these neurons can be used to estimate self-motion from the optic flow. We present a theory for the construction of an optimal linear estimator incorporating prior knowledge both about the distance distribution of the environment, and about the noise and self-motion statistics of the sensor. The optimal estimator is tested on a gantry carrying an omnidirectional vision sensor that can be moved along three translational and one rotational degree of freedom. The experiments indicate that the proposed approach yields accurate results for rotation estimates, independently of the current translation and scene layout. Translation estimates, however, turned out to be sensitive to simultaneous rotation and to the particular distance distribution of the scene. The gantry experiments confirm that the receptive field organization of the tangential neurons allows them, as an ensemble, to extract self-motion from the optic flow.

ei

PDF [BibTex]

PDF [BibTex]


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Chirality-specific nonlinear spectroscopies in isotropic media

Fischer, P., Albrecht, A.

BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN, 75(5):1119-1124, 2002, 10th International Conference on Time-Resolved Vibrational Spectroscopy (TRVS 2001), OKAZZAKI, JAPAN, MAY 21-25, 2001 (article)

Abstract
Sum or difference frequency generation (SFG or DFG) in isotropic media is in the electric-dipole approximation only symmetry allowed for optically active systems. The hyperpolarizability giving rise to these three-wave mixing processes features only one isotropic component. It factorizes into two terms, an energy (denominator) factor and a triple product of transition moments. These forbid degenerate SFG, i.e., second harmonic generation, as well as the existence of the linear electrooptic effect (Pockels effect) in isotropic media. This second order response also has no static limit, which leads to particularly strong resonance phenomena that are qualitatively different from those usually seen in the ubiquitous even-wave mixing spectroscopies. In particular, the participation of two (not the usual one) excited states is essential to achieve dramatic resonance enhancement, We report our first efforts to see such resonantly enhanced chirality specific SFG.

pf

DOI [BibTex]

DOI [BibTex]


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The chiral specificity of sum-frequency generation in solutions

Fischer, P., Beckwitt, K., Wise, F., Albrecht, A.

CHEMICAL PHYSICS LETTERS, 352(5-6):463-468, 2002 (article)

Abstract
Sum-frequency generation in isotropic media is in the electric-dipole approximation the only symmetry allowed for chiral systems. We demonstrate that the sum-frequency intensity from an optically active liquid depends quadratically on the difference in concentration of the two enantiomers. The dominant contribution to the signal is found to be due to the chirality specific electric-dipolar three-wave mixing nonlinearity. Selecting the polarization of all fields allows the chiral electric-dipolar contributions to the bulk sum-frequency signal to be discerned from any achiral magnetic-dipolar and electric-quadrupolar contributions. (C) 2002 Published by Elsevier Science B.V.

pf

DOI [BibTex]

DOI [BibTex]


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On optical rectification in isotropic media

Fischer, P., Albrecht, A.

LASER PHYSICS, 12(8):1177-1181, 2002 (article)

Abstract
Coherent nonlinear optical processes at second-order are only electric-dipole allowed in isotropic media that are optically active. Sum-frequency generation in chiral liquids has recently been observed, and difference-frequency and optical rectification have been predicted to exist in isotropic chiral media. Both Rayleigh-Schrodinger perturbation theory and the density matrix approach are used to discuss the quantum-chemical basis of optical rectification in optically active liquids. For pinene we compute the corresponding orientationally averaged hyperpolarizability, and estimate the light-induced dc electric polarization and the consequent voltage across a measuring capacitor it may give rise to near resonance.

pf

[BibTex]

[BibTex]


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Learning robot control

Schaal, S.

In The handbook of brain theory and neural networks, 2nd Edition, pages: 983-987, 2, (Editors: Arbib, M. A.), MIT Press, Cambridge, MA, 2002, clmc (inbook)

Abstract
This is a review article on learning control in robots.

am

link (url) [BibTex]

link (url) [BibTex]


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Chemical processes during solid state reaction of carbon with alkali salts prepared for gravimetric hydrogen storage measurements

Skakalova, V., Quintel, A., Choi, Y. M., Roth, S., Becher, M., Hirscher, M.

{Chemical Physics Letters}, 365(3-4):333-337, 2002 (article)

mms

[BibTex]

[BibTex]


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Micromagnetic analysis of nucleation-hardened nanocrystalline PrFeB magnets

Kronmüller, H., Goll, D.

{Scripta Materialia}, 47(8):551-556, 2002 (article)

mms

[BibTex]

[BibTex]


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The Verwey transition in magnetite as studied by means of definite impurity doping

Walz, F., Brabers, J. H. V. J., Brabers, V. A. M.

{Zeitschrift f\"ur Metallkunde}, 93(10):1095-1102, 2002 (article)

mms

[BibTex]

[BibTex]


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Antiparallel ruthenium coupling in doped La1.2Sr1.8Mn2-xRuxO7

Weigand, F., Gold, S., Schmid, A., Geissler, J., Goering, E., Dorr, K., Krabbes, G., Ruck, K.

{Applied Physics Letters}, 81(11):2035-2037, 2002 (article)

mms

[BibTex]

[BibTex]


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Micromagnetic theory of the pinning of domain walls at phase boundaries

Kronmüller, H., Goll, D.

{Physica B}, 319(1-4):122-126, 2002 (article)

mms

[BibTex]

[BibTex]


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Diffusion of oxygen in CdSe-photo sensor arrays

Klement, U., Ernst, F., Baretzky, B., Plitzko, J. M.

{Materials Science and Engineering B-Solid State Materials for Advanced Technology}, 94(2-3):123-130, 2002 (article)

mms

[BibTex]

[BibTex]


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Magnetic imaging with full-field soft X-ray microscopy

Fischer, P., Eimüller, T., Goll, D., Stoll, H., Puzic, A., Schütz, G., Denbeaux, G.

{Zeitschrift f\"ur Metallkunde}, 93(5):377-382, 2002 (article)

mms

[BibTex]

[BibTex]


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Magneto-optical studies of current distributions in high-Tc superconductors

Jooss, C., Albrecht, J., Kuhn, H., Leonhardt, S., Kronmüller, H.

{Reports on Progress in Physics}, 65(5):651-788, 2002 (article)

mms

[BibTex]

[BibTex]


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Dominant thermal defects in B2-FeAl

Bester, G., Meyer, B., Fähnle, M., Fu, C. L.

{Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing}, 323(1-2):487-490, 2002 (article)

mms

[BibTex]

[BibTex]


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Hydrogen desorption kinetics of nanostructured MgH2 composite materials

von Zeppelin, F., Reule, H., Hirscher, M.

{Journal of Alloys and Compounds}, 330, pages: 723-726, 2002 (article)

mms

[BibTex]

[BibTex]


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Hydrogen in mechanically prepared nanostructured h-BN: a critical comparison with that in nanostructured graphite

Wang, P., Orimo, S., Matsushima, T., Fujii, H., Majer, G.

{Applied Physics Letters}, 80(2):318-320, 2002 (article)

mms

[BibTex]

[BibTex]


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Enhancement of the critical current density by silver sheeting of YBCO thin films

Kienzle, M., Albrecht, J., Warthmann, R., Kronmüller, H., Leonhardt, S., Joos, C.

{Physical Review B}, 66, 2002 (article)

mms

[BibTex]

[BibTex]


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Pt magnetization profile in a Pt/Co bilayer studied by resonant magnetic x-ray reflectometry

Geissler, J., Goering, E., Justen, M., Weigand, F., Schütz, G., Langer, J., Schmitz, D., Maletta, H., Mattheis, R.

{Physical Review B}, 65, 2002 (article)

mms

DOI [BibTex]

DOI [BibTex]


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Imaging magnetic microstructures with soft x-ray microscopies

Fischer, P., Denbeaux, G., Nolting, F., Goll, D., Eimüller, T., Quitmann, C., Schütz, G.

{Transactions of the Magnetics Society of Japan}, 2, pages: 234-237, 2002 (article)

mms

[BibTex]

[BibTex]


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Positron age-momentum correlation (AMOC) measurements on organic liquids

Castellaz, P., Siegle, A., Stoll, H.

{Journal of Nuclear and Radiochemical Sciences}, 3, pages: R1-R7, 2002 (article)

mms

[BibTex]

[BibTex]


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Arm and hand movement control

Schaal, S.

In The handbook of brain theory and neural networks, 2nd Edition, pages: 110-113, 2, (Editors: Arbib, M. A.), MIT Press, Cambridge, MA, 2002, clmc (inbook)

Abstract
This is a review article on computational and biological research on arm and hand control.

am

link (url) [BibTex]

link (url) [BibTex]


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Scalable techniques from nonparameteric statistics for real-time robot learning

Schaal, S., Atkeson, C. G., Vijayakumar, S.

Applied Intelligence, 17(1):49-60, 2002, clmc (article)

Abstract
Locally weighted learning (LWL) is a class of techniques from nonparametric statistics that provides useful representations and training algorithms for learning about complex phenomena during autonomous adaptive control of robotic systems. This paper introduces several LWL algorithms that have been tested successfully in real-time learning of complex robot tasks. We discuss two major classes of LWL, memory-based LWL and purely incremental LWL that does not need to remember any data explicitly. In contrast to the traditional belief that LWL methods cannot work well in high-dimensional spaces, we provide new algorithms that have been tested on up to 90 dimensional learning problems. The applicability of our LWL algorithms is demonstrated in various robot learning examples, including the learning of devil-sticking, pole-balancing by a humanoid robot arm, and inverse-dynamics learning for a seven and a 30 degree-of-freedom robot. In all these examples, the application of our statistical neural networks techniques allowed either faster or more accurate acquisition of motor control than classical control engineering.

am

link (url) [BibTex]

link (url) [BibTex]


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Theory of crystal field states for heavy rare-earth impurities in MgB2

Welsch, F., Kremer, R., Fähnle, M.

{Journal of Physics-Condensed Matter}, 14(43):10273-10279, 2002 (article)

mms

[BibTex]

[BibTex]


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Micromagnetism and microstructure - tailoring of high- performance permanent magnets

Goll, D.

{Zeitschrift f\"ur Metallkunde}, 93(10):1009-1018, 2002 (article)

mms

[BibTex]

[BibTex]


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Interplay between chemical and magnetic roughness of Pt in a Pt/Co bilayer investigated with X-ray resonant magnetic reflectometry

Geissler, J., Goering, E., Weigand, F., Schütz, G.

{Zeitschrift f\"ur Metallkunde}, 93(10):946-952, 2002 (article)

mms

[BibTex]


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Direct correlation of Cr 3d orbital polarization and O K-edge X-ray magnetic circular dichroism of epitaxial CrO2 films

Goering, E., Bayer, A., Gold, S., Schütz, G., Rabe, M., Rüdiger, U., Güntherodt, G.

{Europhysics Letters}, 58(6):906-911, 2002 (article)

mms

[BibTex]

[BibTex]


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Surface magnetization reversal of sputtered CrO2

Goering, E., Gold, S., Will, J.

{Zeitschrift f\"ur Metallkunde}, 93(5):372-376, 2002 (article)

mms

[BibTex]

[BibTex]


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Shear-strain-related nonlinear magnetoelastic properties of epitaxial films

Komelj, M., Fähnle, M.

{Physical Review B}, 65(9), 2002 (article)

mms

[BibTex]

[BibTex]


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NMR studies of hydrogen diffusion in the hydrogen-stabilized Laves phase compound C15-HfTi2Hx

Eberle, U., Majer, G., Skripov, A. V., Kozhanov, V. N.

{Journal of Physics-Condensed Matter}, 14(2):153-164, 2002 (article)

mms

[BibTex]

[BibTex]


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The existence of more than one jump process of hydrogen in palladium-silver alloys - an NMR study

Züchner, H., Barlag, H., Majer, G.

{Journal of Alloys and Compounds}, 330-332, pages: 448-453, 2002 (article)

mms

[BibTex]

[BibTex]


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Electric field distribution at low-angle grain boundaries in high-temperature superconductors

Jooss, C., Guth, K., Born, V., Albrecht, J.

{Physical Review B}, 65(1), 2002 (article)

mms

[BibTex]

[BibTex]


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Finite element calculations on the single-domain limit of a ferromagnetic cube - a solution to \microMAG Standard Problem No. 3

Hertel, R., Kronmüller, H.

{Journal of Magnetism and Magnetic Materials}, 238(2-3):185-199, 2002 (article)

mms

[BibTex]

[BibTex]


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Diffusion of oxygen in CdSe-photosensor arrays

Klement, U., Ernst, F., Baretzky, B., Plitzko, J. M.

{Materials Science and Engineering B}, 94, pages: 123-130, 2002 (article)

mms

[BibTex]

[BibTex]


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Influence of the grain boundary phase transitions on the diffusion-related properties

Straumal, B., Baretzky, B.

{Diffusion and Defect Forum}, 216-217, pages: 53-64, 2002 (article)

mms

[BibTex]

[BibTex]


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Magneto-optical studies of flux pinning in high-temperature superconductors

Jooss, C., Albrecht, J.

{Zeitschrift f\"ur Metallkunde}, 93(10):1065-1070, 2002 (article)

mms

[BibTex]

[BibTex]


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From the bulk to monatomic wires: An ab initio study of magnetism in Co systems with various dimensionality

Komelj, M., Ederer, C., Davenport, J. W., Fähnle, M.

{Physical Review B}, 66(14), 2002 (article)

mms

[BibTex]

[BibTex]


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The initial stages of oxidation of γ-TiAl: an X-ray photoelectron study

Schmiedgen, M., Graat, P. C. J., Baretzky, B., Mittemeijer, E. J.

{Thin Solid Films}, 415(1-2):114-122, 2002 (article)

mms

[BibTex]

[BibTex]


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Morphology and thermal stability of metal contacts on crystalline organic thin films

Dürr, A. C., Schreiber, F., Kelsch, M., Carstanjen, H. D., Dosch, H.

{Advanced Materials}, 14(13-14):961-963, 2002 (article)

mms

[BibTex]

[BibTex]


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Magnetic anisotropy of textured CrO2 thin films investigated by X-ray magnetic circular dichroism

Goering, E., Justen, M., Geissler, J., Rüdiger, U., Rabe, M., Güntherodt, G., Schütz, G.

{Applied Physics A-Materials Science \& Processing}, 74(6):747-753, 2002 (article)

mms

[BibTex]

[BibTex]