Interaction between Charge-transfer States Studied by Magnetic Field Effects

Organic semiconducting materials, consisting mostly of carbon and hydrogen atoms, provide remarkable promise for electronic applications due to their easy processing, chemical tenability, low costs and environmental-friendly characteristics. For realizing electronic applications such as light emitting diodes and photovoltaic cells, charge-transfer states act as an important intermediate state for recombination and dissociation. Interestingly, magnetic field effects on semiconducting materials have been realized based on the suppression of spin mixing in the charge-transfer states. Although lots of studies have been carried out on investigating the properties of charge-transfer states, little has been done to consider the interaction between them. This thesis aims to reveal the interaction between different kinds of charge-transfer states by using the magnetic field effects. Chapter 1 presents a basic introduction to the organic semiconducting materials and magnetic field effects. Chapter 2 gives a simple description of the experiment procedure, such as device fabrication, magnetic field effects measurement and data analysis. Chapter 3 studies the interaction between intermolecular charge-transfer states. Chapter 4 indicates the interaction between intramolecular charge-transfer states and d electrons. Chapter 5 illustrates the interaction between photo-generated and magnetized charge-transfer states. Chapter 6 introduces the interaction between excitons and free charge carriers in organo-metal halide perovskite materials. Chapter 7 performs the study of magneto-electronic interaction at the interface between Rashba perovskite and ferromagnetic metal. Finally, Chapter 8 gives a short conclusion for the entire study in this dissertation

Sibilant Contrast: Perception, Production, and Sound Change

This study examines sibilant place contrast in the [_i] context in terms of its typology across Chinese dialects and its role in the historical development of Mandarin sibilants. The typology across 170 Chinese dialects reveals that (i) for dialects that have sibilants at three places (dental, palatal, and retroflex), place contrasts in the [_i] context are generally avoided, e.g., */si-ɕi-i/; (ii) for dialects that have sibilants at two places, mostly dental vs. palatal, place contrasts in the [_i] context also tend to be avoided, e.g., */si-ɕi/; (iii) for dialects that do have contrastive dental vs. palatal sibilants in the [_i] context, the place contrast of affricates implies that of fricatives. The first two patterns are mirrored in the sound changes of Mandarin in that contrastive dental and palatal sibilants in the [_i] context that emerged from independent processes have always been enhanced or avoided. In addition, the sound changes also showed an avoidance of contrastive dental vs. palatal in the [_i] context with the shift of palatal sibilants into retroflex sibilants from the 11th to the 14th century. The connection between the synchronic typology and the diachronic changes raises a number of research questions: (i) Does the vowel context affect the perceptual distinctiveness of sibilant place contrasts, e.g., is [si-ɕi] less distinct than [sa-ɕa]? (ii) Do place contrasts differ in perceptual distinctiveness, e.g., is [si-ɕi] less distinct than [si-i]? (iii) Do different manners of articulation differ in perceptual distinctiveness, e.g., is [tsi-tɕi] less distinct than [si-ɕi]? These issues were investigated through a speeded AX-discrimination experiment, which has been shown to be able to evaluate the relative perceptual distinctiveness of sound pairs independent of the listener’s native phonology. Twenty-nine listeners were put under time pressure to judge if a CV pair is the same or not, where the sibilant onsets of the CV pairs contrast in place (e.g., [si-ɕi]) and the vowels were [i] vs. other vowels (e.g., [si-ɕi] vs. [sa-ɕa] vs. [sɹ̩-ɕi]). Assuming that a longer response time indicates less perceptual distinctiveness, the results showed that (i) the [_i] context reduces the perceptual distinctiveness of the place contrasts of dental vs. palatal sibilants; (ii) the introduction of the apical vowel enhances the perceptual distinctiveness between the contrastive sound pairs; (iii) the dental vs. retroflex contrasts are more distinct than the dental vs. palatal contrasts. These findings match the observations in the cross-linguistic typology and the historical development of Mandarin and support the claim that perceptual distinctiveness regulates the phonological system. The reduced distinctiveness of dental vs. palatal sibilants in the [_i] context suggests that contrastive dental vs. palatal sibilants are unstable and are likely to be avoided in sound change. A phonetic study was conducted on the sibilants in Xiangtan, a Chinese dialect reported to have the same sound system as 18th century Mandarin (i.e., [sɹ̩ si i ɻ̩]) with fully contrastive dental vs. palatal sibilants in the [_i] context. It is predicted that in Xiangtan, the pre-[i] dentals in /si tsi tsʰi/ may show signs of being palatalized and thus neutralized with the palatals. Natural productions of /si tsi tsʰi/, /ɕi tɕi tɕʰi/, and /sɹ̩ tsɹ̩ tsʰɹ̩/ syllables with matched tones were recorded from 11 native female speakers of Xiangtan. Center of gravity, energy dispersion, intensity, and duration were extracted for three types of sibilants: Canonical dentals as in /sɹ̩/, canonical palatals as in /ɕi/, and pre-[i] dentals as in /si/. A discriminant analysis was performed by first training a classifier on the canonical dentals and canonical palatals and then using the classifier to predict the place (dental vs. palatal) of the pre-[i] dentals. Native Mandarin listeners were also recruited to identify the isolated first half of the pre-[i] dental sibilants as being dental vs. palatal. The results from both studies showed that (i) some Xiangtan speakers have palatalized the dentals in /si tsi tsʰi/, and (ii) certain speakers variably produce dental and palatal sibilants for the same lexical item. Therefore, the results support the contention that dental and palatal contrasts are perceptually less distinct in the [_i] context and the variation in the realization of the pre-[i] dentals indicates that a merger replicating the development of Mandarin is in process. In general, the perceptual experiment reveals that dental vs. palatal sibilants in the [_i] context form weak contrasts, based on the psychoacoustic similarity of the contrastive elements. The avoidance of weak contrasts is observed in cross-linguistic typology, historical sound change, and speech production. This study thus establishes an empirical connection among the perceptual distinctiveness of sibilant place contrasts, the production of these contrasts, cross-linguistic typology, and historical sound changes

On Black Hole Stability in Critical Gravities

We consider extended cosmological gravities with Ricci tensor and scalar squared terms in diverse dimensions. These theories admit solutions of Einstein metrics, including the Schwarzschild-Tangherlini AdS black holes, whose mass and entropy vanish at the critical point. We perform linearized analysis around the black holes and show that in general the spectrum consists of the usual spin-2 massless and ghost massive modes. We demonstrate that there is no exponentially-growing tachyon mode in the black holes. At the critical point, the massless spin-2 modes have zero energy whilst the massive spin-2 modes are replaced by the log modes. There always exist certain linear combination of massless and log modes that has negative energy. Thus the stability of the black holes requires that the log modes to be truncated out by the boundary condition.Comment: 16 pages, minor corrections, further comments and references adde

A novel circularly polarized filtering antenna with high out‐of‐band radiation rejection level

In this paper, a circularly polarized filtering antenna with high out‐of‐band radiation rejection level is proposed. The entire design consists of a patch and a filtering feeding network, which are connected by metallic pins. First, a high‐selectivity filtering power divider is constructed utilizing two shunted bandpass filter. Next, a 90° phase difference could be realized by adjusting the length of the two outputs of the power divider. The measured transmission coefficient of the feeding network is 6.3 dB at 2 GHz. The measured axial ratio is below 3 dB within the operating band of the proposed antenna, which represents the effectiveness of the proposed feeding network. In addition, the obvious radiation nulls can be found from the antenna measurement because of the transmission zeros introducing by the filtering power divider. The measured gain is 5.73 dB at 2 GHz and decreases dramatically in stopband. Specially, the measured gain is −30 and −41 dB at 1.95 and 2.06 GHz. The proposed antenna has an excellent filtering characteristic compared with those without the filtering feeding network. To sum up, the proposed antenna and filtering power divider are promising in the modern wireless communication systems.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140028/1/mop30929.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/140028/2/mop30929_am.pd