1,720,997 research outputs found
Exploring Many Body Interactions with Raman Spectroscopy
Full text linkMany-body interactions are cornerstones of contemporary solid state physics research.
Especially, phonon related interactions such as phonon-phonon coupling, spin-phonon
coupling and electron-phonon coupling constantly present new challenges. To study
phonon related many-body interactions, temperature dependent Raman spectroscopy is
employed. Firstly, a new design and construction of a Raman microscope aimed at high
collection eciency, positional and thermal stability is discussed. The application of
the home-built Raman microscope is shown in the context of two types of novel materials; Cr2Ge2Te6 (spin-phonon coupling) and Bi2Te3-xSex (phonon-phonon coupling). Cr2Ge2Te6 is one of the rare class of ferromagnetic semiconductors and recent thermal transport studies suggest the spin and lattice are strongly coupled in its cousin compound Cr2Si2Te6. In this work, the spin-phonon coupling in Cr2Ge2Te6 has been revealed in multiple ways: we observed a split of two phonon modes due to the breaking of time reversal symmetry; the anomalous hardening of an additional three modes; and a dramatic enhancement of the phonon lifetimes. It is well-known that the phonon-phonon interaction plays a signicant role in determining the thermal transport properties of thermoelectrics. A comprehensive study of the phonon dynamics of Bi2Te3-xSex has been performed. We found that the unusual temperature dependence of dierent phonon modes originates from both cubic and quartic anharmonicity. These results are consistent with the resonance bonding mechanism, suggesting that the resonance bonding may be a common feature for conventional thermoelectrics. In the Raman spectra of Bi2Te2Se, the origin of the extra Raman feature has been debated for decades. Through a temperature dependent Raman study, we were able to prove the feature is generated by a Te-Se antisite induced local mode. The anomalous linewidth of the local mode as well as the anharmonic behavior were explained through a statistical analysis. Then, a wavelet transform and data clustering based spike-removal algorithm is proposed to resolve the erroneous spikes caused by cosmic rays in Raman spectra. Data sets from real measurements were analyzed to demonstrate the capability of the algorithm. Finally, the results of a minor project are presented. Thin lms of the high dielectric material SrTiO3 grown via a new method on silicon(100) are studied by ellipsometry. The results suggest that two layers of SrTiO3 are formed during the growth.Ph.D
Proximity Effect and Tunneling Spectroscopy of High-temperature Superconductor-semiconductor Hybrid Structures
Full text linkThis thesis presents Andreev scattering studies of a variety of high-temperature superconductor Bi2Sr2CaCu2O8+x-normal material (SN) junctions. The SN devices were fabricated with our newly-developed technique, called the mechanical-bonding method. High-temperature superconductivity was observed in topological insulators Bi2Se3 and Bi2Te3 (as well as graphite) by proximity to the high-temperature superconductor Bi2Sr2CaCu2O8+x. This proximity effect was further confirmed by the application of a magnetic field and observation of a shifting in the energy of the Andreev-reflected quasiparticles as a result of diamagnetic screening supercurrents. However, the ability to probe the topological surface states in Bi2Se3 and Bi2Te3 was limited, due to the Fermi energy being deep in the bulk conduction band. Thus we turned to Bi2Te2Se, which is quite insulating in the bulk. The differential conductance measurements on Bi2Sr2CaCu2O8+x/Bi2Te2Se devices demonstrated a zero-bias conductance peak, suggesting evidences of a new excitation emerging at the interface between the two materials. Furthermore, the mechanical bonding method was employed as a technique to probe the density of states of Bi2Sr2CaCu2O8+x at various doping levels, observing the superconducting gap and the pseudogap of Bi2Sr2CaCu2O8+x at various temperatures.Ph.D
Optical Spectroscopy of Two-dimensional Electronic Materials
No full textThis thesis presents optical studies of several low-dimensional electronic materials. Reflectance, transmittance, ellipsometry and Raman scattering were employed to measure the fundamental excitations of the materials under study. The electrodynamics of BSCCO two-dimensional atomic crystals were investigated as a function of thickness. The thickness dependence of the optical response indicates a tendency of the BSCCO surface to become insulating. Furthermore, the characteristic energy scale of magnetic fluctuations was observed to increase in the atomic crystals. A series of superconducting CuPh.D.2016-05-18 00:00:0
Exploring Many Body Interactions with Raman Spectroscopy
No full textMany-body interactions are cornerstones of contemporary solid state physics research.
Especially, phonon related interactions such as phonon-phonon coupling, spin-phonon
coupling and electron-phonon coupling constantly present new challenges. To study
phonon related many-body interactions, temperature dependent Raman spectroscopy is
employed. Firstly, a new design and construction of a Raman microscope aimed at high
collection eciency, positional and thermal stability is discussed. The application of
the home-built Raman microscope is shown in the context of two types of novel materials; Cr2Ge2Te6 (spin-phonon coupling) and Bi2Te3-xSex (phonon-phonon coupling). Cr2Ge2Te6 is one of the rare class of ferromagnetic semiconductors and recent thermal transport studies suggest the spin and lattice are strongly coupled in its cousin compound Cr2Si2Te6. In this work, the spin-phonon coupling in Cr2Ge2Te6 has been revealed in multiple ways: we observed a split of two phonon modes due to the breaking of time reversal symmetry; the anomalous hardening of an additional three modes; and a dramatic enhancement of the phonon lifetimes. It is well-known that the phonon-phonon interaction plays a signicant role in determining the thermal transport properties of thermoelectrics. A comprehensive study of the phonon dynamics of Bi2Te3-xSex has been performed. We found that the unusual temperature dependence of dierent phonon modes originates from both cubic and quartic anharmonicity. These results are consistent with the resonance bonding mechanism, suggesting that the resonance bonding may be a common feature for conventional thermoelectrics. In the Raman spectra of Bi2Te2Se, the origin of the extra Raman feature has been debated for decades. Through a temperature dependent Raman study, we were able to prove the feature is generated by a Te-Se antisite induced local mode. The anomalous linewidth of the local mode as well as the anharmonic behavior were explained through a statistical analysis. Then, a wavelet transform and data clustering based spike-removal algorithm is proposed to resolve the erroneous spikes caused by cosmic rays in Raman spectra. Data sets from real measurements were analyzed to demonstrate the capability of the algorithm. Finally, the results of a minor project are presented. Thin lms of the high dielectric material SrTiO3 grown via a new method on silicon(100) are studied by ellipsometry. The results suggest that two layers of SrTiO3 are formed during the growth.Ph.D
Optical Spectroscopy of Two-dimensional Electronic Materials
Full text linkThis thesis presents optical studies of several low-dimensional electronic materials. Reflectance, transmittance, ellipsometry and Raman scattering were employed to measure the fundamental excitations of the materials under study. The electrodynamics of BSCCO two-dimensional atomic crystals were investigated as a function of thickness. The thickness dependence of the optical response indicates a tendency of the BSCCO surface to become insulating. Furthermore, the characteristic energy scale of magnetic fluctuations was observed to increase in the atomic crystals. A series of superconducting CuPh.D.2016-05-18 00:00:0
Proximity Effect and Tunneling Spectroscopy of High-temperature Superconductor-semiconductor Hybrid Structures
No full textThis thesis presents Andreev scattering studies of a variety of high-temperature superconductor Bi2Sr2CaCu2O8+x-normal material (SN) junctions. The SN devices were fabricated with our newly-developed technique, called the mechanical-bonding method. High-temperature superconductivity was observed in topological insulators Bi2Se3 and Bi2Te3 (as well as graphite) by proximity to the high-temperature superconductor Bi2Sr2CaCu2O8+x. This proximity effect was further confirmed by the application of a magnetic field and observation of a shifting in the energy of the Andreev-reflected quasiparticles as a result of diamagnetic screening supercurrents. However, the ability to probe the topological surface states in Bi2Se3 and Bi2Te3 was limited, due to the Fermi energy being deep in the bulk conduction band. Thus we turned to Bi2Te2Se, which is quite insulating in the bulk. The differential conductance measurements on Bi2Sr2CaCu2O8+x/Bi2Te2Se devices demonstrated a zero-bias conductance peak, suggesting evidences of a new excitation emerging at the interface between the two materials. Furthermore, the mechanical bonding method was employed as a technique to probe the density of states of Bi2Sr2CaCu2O8+x at various doping levels, observing the superconducting gap and the pseudogap of Bi2Sr2CaCu2O8+x at various temperatures.Ph.D
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
Variations 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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