1,721,032 research outputs found
Nonlinear response and avalanche behavior in metallic glasses
No full textThe response to different stress amplitudes at temperatures below the glass transition temperature is analyzed by mechanical oscillatory excitation of Pd40Ni40P20 metallic glass samples in single cantilever bending geometry. While low amplitude oscillatory excitations are commonly used in mechanical spectroscopy to probe the relaxation spectrum, in this work the response to comparably high amplitudes is investigated. The strain response of the material is well below the critical yield stress even for highest stress amplitudes, implying the expectation of a linear relation between stress and strain according to Hooke's Law. However, a deviation from the linear behavior is evident, which is analyzed in terms of temperature dependence and influence of the applied stress amplitude by two different approaches of evaluation. The nonlinear approach is based on a nonlinear expansion of the stress-strain-relation, assuming an intrinsic nonlinear character of the shear or elastic modulus. The degree of nonlinearity is extracted by a period-by-period Fourier-analysis and connected to nonlinear coefficients, describing the intensity of nonlinearity at the fundamental and higher harmonic frequencies. The characteristic timescale to adapt to a significant change in stress amplitude in terms of a recovery timescale to a steady state value is connected to the structural relaxation time of the material, suggesting a connection between the observed nonlinearity and primary relaxation processes. The second approach of evaluation is termed the incremental analysis and relates the observed response behavior to avalanches, which occur due to the activation and correlation of local microstructural rearrangements. These rearrangements are connected with shear transformation zones and correspond to localized plastic events, which are superimposed on the linear response behavior of the material
Magnetic-Field-Induced Suppression of Jahn-Teller Phonon Bands in (La0.6Pr0.4)0.7Ca0.3MnO3: the Mechanism of Colossal Magnetoresistance shown by Raman Spectroscopy
No full textA long-standing issue in the physics of the colossal magnetoresistance is the role of electron-phonon coupling, which manifests itself as Jahn-Teller polarons. The origin and architecture of polarons makes it possible to study their behavior by Raman spectroscopy, which allows to analyze the polaronic behavior in an applied magnetic field. We performed magnetic-field-dependent Raman spectroscopy on thin films of (La0.6Pr0.4)0.7Ca0.3MnO3 in a range of H = 0-50 kOe and compared the obtained Raman spectra with the magnetic field behavior of the electrical resistivity. In the vicinity of the Curie temperature, TC = 197 K, the intensity of the Jahn-Teller stretching mode at 614 cm-1 and of the bending mode at 443 cm-1 was found to be suppressed and enhanced, respectively. This observed behavior has a remarkable similarity with the field and temperature dependence of the colossal magnetoresistance in (La0.6Pr0.4)0.7Ca0.3MnO3. Our work provides direct evidence that the reduction of the amount of Jahn-Teller polarons at the phase transition is the main mechanism underlying the colossal magnetoresistance
Prigogine-Defay ratio of glassy freezing scales with liquid fragility
No full textA detailed study of published experimental data for a variety of materials on the incremental variation of heat capacity, thermal expansion, and compressibility at glassy freezing reveals a striking dependence of the Prigogine-Defay ratio R on the fragility index m. At high m, R approaches values of ∼1, the Ehrenfest expectation for second-order continuous phase transitions, while R reaches values >20 for low fragilities. We explain this correlation by the degree of separation of the glassy freezing temperature from a hidden phase transition into an ideal low-temperature glass
Crystal melting influenced by particle cooperativity of the liquid
Full text linkRecently, a universal relation between the thermal expansion coefficient of glasses αg, their glass-transition temperature Tg, and the so-called fragility index m of the corresponding supercooled liquid state was found to be valid for more than 200 glass formers, namely αg/m ∝ 1/Tg [Lunkenheimer et al., Nat. Phys. 19, 694 (2023)]. Here we show that this could also have far-reaching consequences for our understanding of crystal melting. Namely, when considering the empirically founded 2/3 rule, stating that the ratio of Tg and the melting temperature Tm is about 2/3 for almost all materials, for crystals a similar relation, αc/m ∝ 1/Tm, should apply. Indeed, we find that the available experimental data are quite consistent with such a relation. This implies that the melting of a crystal into an ordinary (nonsupercooled) liquid is influenced by the fragility, a property quantifying the non-Arrhenius dynamics in the supercooled-liquid state of the material. We argue that this can be explained by a significant enhancement of the “ideal” (noncooperative) melting temperature arising from the cooperativity of the particle motion in the liquid state above Tm. Therefore, a reassessment of the currently widely accepted microscopic understanding of crystal melting, still founded on the general ideas that lead to the time-honored Lindemann melting criterion, may be necessary
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Publisher Correction: Examining the influence of turbulence on viscosity measurements of molten germanium under reduced gravity
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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