Growth of bismuth and antimony-based chalcohalide single crystals by physical vapor transport method

AVBVICVII ternary compounds with special structure show strong anisotropy, which can be attributed to the double chains along b axis. The crystals have attracted widespread interests because of their unique...

Extreme Clusters of Grains in Random Microstructures of Polycrystals

It was experimentally observed that in polycrystalline materials under low macro loading of the specimen the first sites of failure initiation take place in the specific clusters of few grains. In some grains of these extreme clusters, the local (meso-) strains and stresses are high enough to cause first damages or plastic slips. In the stochastic microstructure of polycrystals, the formation of an extreme cluster is random and rare. Nevertheless, they govern the failure process initiation and can severely affect the reliability of polycrystalline machine parts. It is time and resource consuming to search and investigate extreme clusters on the real specimens of polycrystalline materials experimentally. A theoretical tool is desirable. Here we present the powerful computational method to look for extreme clusters, to investigate their possible patterns, and to evaluate the absolute maximums of local strains/stresses that can be achieved in these clusters. The experimentally observed clusters consist of few (3-4) preferably oriented neighboring grains or even of one big supergrain. The strain and stress bursts arise due to an interaction of the grains. One can expect that in bigger clusters, larger local bursts of fields can be generated. We found the typical forms of the extreme clusters (small and big) in four different polycrystals with grains of a weak and strong anisotropy for the case of uniaxial tension. In all regarded cases, the extreme clusters have the forms of the symmetrical patterns. In big clusters of highly anisotropic grains, the maximum of mesostrain exceeds the macrostrain by several times. In clusters of weakly anisotropic grains, the local strain concentration is rather moderate (tens of percents).

The Investigation on Initiation and Propagation of Hydraulic Fractures in Shale Reservoir

Hydraulic fracturing is a necessary technique for shale gas exploitation. In order to have efficient stimulation treatment, a complex fracture network has to be developed, whereas with rich bedding planes and natural fractures, the mechanism of forming a fracture network is not fully understood and it is so tricky to predict propagation and initiation of hydraulic fracture. Therefore, in this paper, considering the strong anisotropy of shale reservoir, numerical simulation has been conducted to analyze fracture propagation and initiation on the basis of finite element and damage mechanics. Simulation results indicate that hydraulic fracture is not merely controlled by in situ stress due to strong anisotropy in shale. With plenty of bedding planes, hydraulic fracture tends to have initiation and propagation along the bedding plane. In particular, this influence becomes stronger with low strength and high development density of bedding planes. Additionally, in combination with natural fracture and bedding plane, the initiation point is usually on a natural fracture plane, causing relatively small breakdown pressure. In the process of fracture propagation, hydraulic fracture connects with natural fractures and bedding planes, forming dendritic bifurcation and more complicated paths. Numerical simulation proves that bedding plane and natural fracture are vital factors of hydraulic fracture. Compared to natural fracture, the bedding plane has a stronger impact on hydraulic fracture propagation. For the initiation of hydraulic fracture, natural fracture is the major effecting factor. The outcome of this study is able to offer theoretical guidance for hydraulic fracturing in shale.

Optical conductivities in triple fermions with different monopole charges

We investigate the linear optical conductivities of the newly-discovered triple-component semimetals. Due to the exactly flat band, the optical conductivity relates to the transition between the zero band and the conduction band directly reflecting the band structure of the conduction electrons in contrast to the other materials. For the low-energy models with various monopole charges, the diagonal conductivities show strong anisotropy. The ω-dependence of interband conductivities for a general low-energy model is deduced. The real part of the interband σ_xx always linearly depends on the optical frequency, while the one of σ_zz is proportional to ω^{2/n-1}. This can be a unique fingerprint of the monopole charge. For the lattice models, there also exists the optical anomalous Hall conductivity, where a sign change may appear. The characteristic frequencies of the kink structures are calculated, strictly. Our work will help us to establish the basic picture of linear optical response in topological triple-component semimetals and identify them from other materials.

Discriminating Between Spatial and Temporal Variations in Seismic Anisotropy at Active Volcanoes

<p>This thesis addresses the measurement and interpretation of seismic anisotropy around active volcanoes via shear wave splitting analysis. An overpressured magma reservoir will exert a stress on the surrounding country rock that may or may not be manifest as observable strain. Shear wave splitting analysis can be a useful indicator of stress in the crust and hence, the pressure induced by magma movement. Changes in shear wave splitting have already been observed at Mt. Ruapehu following eruptions in 1995/1996 and are inferred to be caused by changes in local stress in response to magma pressure. One of the main problems with the interpretation of temporal changes in shear wave splitting is the possibility of spatial variations being sampled along differing raypaths and being interpreted as temporal changes. Using a dense observational network and an automated shear wave splitting analysis, we examine local earthquakes occurring in 2008 within 100 km of Mt. Ruapehu. We note a strong azimuthal dependence of the fast direction of anisotropy (phi) and so introduce a spatial averaging technique and a two-dimensional tomography of recorded delay times (dt), to observe the spatial variation in more detail. Using this new method of mapping shear wave splitting parameters, we have created a benchmark of spatial variations in shear wave anisotropy around Mt. Ruapehu, against which future temporal changes may be measured. The observed anisotropy is used to define regions in which phi agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which phi aligns with structural features such as fault strikes, suggesting structural anisotropy. Data from past deployments of three-component seismometers have been analysed in the same way as those recorded during the 2008 experiment and the results compared. We identify a stable region of strong anisotropy, interpreted to be caused by schistose mineral alignment, and a transient region of strong anisotropy centred on the volcano during the major magmatic eruption of 1995. We also introduce a method of analysing temporal variations in seismic anisotropy at active volcanoes by using tight clusters of earthquakes and highly correlated multiplets. At Mt. Ruapehu, changes in shear wave splitting parameters associated with the 2006 and 2007 phreatic eruptions are detected using a cluster of earthquakes to the west of the volcano. Similar analyses using another cluster and multiplets from the stable region of strong anisotropy do not reveal temporal changes, although examination of the waveform codas of the repeating earthquakes reveals systematic changes that we interpret as being caused by seismic scatterers associated with the 2006 and 2007 eruptions. These scatterers appear to contaminate the shear wave coda and so inhibit the detection of any subtle changes in shear wave splitting parameters. Finally, we apply some of these methods to data from the 2008 eruption of Okmok volcano, Alaska. Shear wave splitting analysis at Okmok reveals a change in anisotropy associated with the 2008 eruption. This change however, is attributed to a change in dominant hypocentre location. Multiplet analysis at Okmok volcano reveals a similar scatterer contamination of the shear wave arrival. This spurious phase is interpreted to be an S to P conversion from interaction with the magma reservoir.</p>

Discriminating Between Spatial and Temporal Variations in Seismic Anisotropy at Active Volcanoes

<p>This thesis addresses the measurement and interpretation of seismic anisotropy around active volcanoes via shear wave splitting analysis. An overpressured magma reservoir will exert a stress on the surrounding country rock that may or may not be manifest as observable strain. Shear wave splitting analysis can be a useful indicator of stress in the crust and hence, the pressure induced by magma movement. Changes in shear wave splitting have already been observed at Mt. Ruapehu following eruptions in 1995/1996 and are inferred to be caused by changes in local stress in response to magma pressure. One of the main problems with the interpretation of temporal changes in shear wave splitting is the possibility of spatial variations being sampled along differing raypaths and being interpreted as temporal changes. Using a dense observational network and an automated shear wave splitting analysis, we examine local earthquakes occurring in 2008 within 100 km of Mt. Ruapehu. We note a strong azimuthal dependence of the fast direction of anisotropy (phi) and so introduce a spatial averaging technique and a two-dimensional tomography of recorded delay times (dt), to observe the spatial variation in more detail. Using this new method of mapping shear wave splitting parameters, we have created a benchmark of spatial variations in shear wave anisotropy around Mt. Ruapehu, against which future temporal changes may be measured. The observed anisotropy is used to define regions in which phi agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which phi aligns with structural features such as fault strikes, suggesting structural anisotropy. Data from past deployments of three-component seismometers have been analysed in the same way as those recorded during the 2008 experiment and the results compared. We identify a stable region of strong anisotropy, interpreted to be caused by schistose mineral alignment, and a transient region of strong anisotropy centred on the volcano during the major magmatic eruption of 1995. We also introduce a method of analysing temporal variations in seismic anisotropy at active volcanoes by using tight clusters of earthquakes and highly correlated multiplets. At Mt. Ruapehu, changes in shear wave splitting parameters associated with the 2006 and 2007 phreatic eruptions are detected using a cluster of earthquakes to the west of the volcano. Similar analyses using another cluster and multiplets from the stable region of strong anisotropy do not reveal temporal changes, although examination of the waveform codas of the repeating earthquakes reveals systematic changes that we interpret as being caused by seismic scatterers associated with the 2006 and 2007 eruptions. These scatterers appear to contaminate the shear wave coda and so inhibit the detection of any subtle changes in shear wave splitting parameters. Finally, we apply some of these methods to data from the 2008 eruption of Okmok volcano, Alaska. Shear wave splitting analysis at Okmok reveals a change in anisotropy associated with the 2008 eruption. This change however, is attributed to a change in dominant hypocentre location. Multiplet analysis at Okmok volcano reveals a similar scatterer contamination of the shear wave arrival. This spurious phase is interpreted to be an S to P conversion from interaction with the magma reservoir.</p>

Anisotropic Differences in the Thermal Conductivity of Rocks: A Summary from Core Measurement Data in East China

The study of thermal conductivity anisotropy is of great importance for more accurate heat flow calculations, geodynamic studies, development and utilization of hot dry rock, and simulation of heat transfer in geological reservoirs of nuclear waste, and so on. To study the thermal conductivity anisotropy of rocks, 1158 cores from 60 boreholes in East China were tested for thermal conductivity, including thermal conductivity values parallel to (λ∥) and perpendicular to (λ⊥) structural planes of basalt, mudstones, gneisses, sandstones, carbonates, evaporites, and metamorphic rocks. The thermal conductivity anisotropy is not obvious for sand, clay, and evaporate, and the average anisotropic factors of 1.19 ± 0.22, 1.18 ± 0.17, and 1.18 ± 0.17 for tuff/breccia, granitoid and contact metamorphic rocks, respectively, indicate that these three rocks have strong anisotropy characteristics. Finally, the effect of thermal conductivity anisotropy on heat flow is studied and discussed in detail, showing that the results of thermal conductivity tests have a significant effect on the calculation of heat flow and thermal structure, and the data show that a deviation of about 10% in thermal conductivity causes a deviation of about 11% in heat flow, which may lead to a misperception of deep thermal structure studies. The regular and anisotropic characteristics of thermal conductivity of various rocks in Eastern China obtained in this paper can provide parameter support for projects such as heat flow calculations, thermal structure studies, and geothermal resource development and utilization.