On the local aspect of valleytronics

Valley magnetic moments play a crucial role in valleytronics in 2D hexagonal materials. Traditionally, based on studies of quantum states in homogeneous bulks, it is widely believed that only materials with broken structural inversion symmetry can exhibit nonvanishing valley magnetic moments. Such constraint excludes from relevant applications those with inversion symmetry, as specifically exemplified by gapless monolayer graphene despite its technological advantage in routine growth and production. This work revisits valley-derived magnetic moments in a broad context covering inhomogeneous structures as well. It generalizes the notion of valley magnetic moment for a state from an integrated total quantity to the local field called "local valley magnetic moment" with space-varying distribution. In suitable inversion-symmetric structures with inhomogeneity, e.g., zigzag nanoribbons of gapless monolayer graphene, it is shown that the local moment of a state can be nonvanishing with sizable magnitude, while the corresponding total moment is subject to the broken symmetry constraint. Moreover, it is demonstrated that such local moment can interact with space-dependent electric and magnetic fields manifesting pronounced field effects and making possible a local valley control with external fields. Overall, a path to "local valleytronics" is illustrated which exploits local valley magnetic moments for device applications, relaxes the broken symmetry constraint on materials, and expands flexibility in the implementation of valleytronics

Abelian and Non-Abelian Quantum Geometric Tensor

We propose a generalized quantum geometric tenor to understand topological quantum phase transitions, which can be defined on the parameter space with the adiabatic evolution of a quantum many-body system. The generalized quantum geometric tenor contains two different local measurements, the non-Abelian Riemannian metric and the non-Abelian Berry curvature, which are recognized as two natural geometric characterizations for the change of the ground-state properties when the parameter of the Hamiltonian varies. Our results show the symmetry-breaking and topological quantum phase transitions can be understood as the singular behavior of the local and topological properties of the quantum geometric tenor in the thermodynamic limit.Comment: 5 pages, 2 figure

The Euler Number of Bloch States Manifold and the Quantum Phases in Gapped Fermionic Systems

We propose a topological Euler number to characterize nontrivial topological phases of gapped fermionic systems, which originates from the Gauss-Bonnet theorem on the Riemannian structure of Bloch states established by the real part of the quantum geometric tensor in momentum space. Meanwhile, the imaginary part of the geometric tensor corresponds to the Berry curvature which leads to the Chern number characterization. We discuss the topological numbers induced by the geometric tensor analytically in a general two-band model. As an example, we show that the zero-temperature phase diagram of a transverse field XY spin chain can be distinguished by the Euler characteristic number of the Bloch states manifold in a (1+1)-dimensional Bloch momentum space

Analyzing Flow Patterns in Unsaturated Fractured Rock of Yucca Mountain Using an Integrated Modeling Approach

Abstract This paper presents a series of modeling investigations to characterize percolation patterns in the unsaturated zone of Yucca Mountain, Nevada, a proposed underground repository site for storing high-level radioactive waste. The investigations are conducted using a modeling approach that integrates a wide variety of moisture, pneumatic, thermal, and isotopic geochemical field data into a comprehensive three-dimensional numerical model through model calibration. This integrated modeling approach, based on a dualcontinuum formulation, takes into account the coupled processes of fluid and heat flow and chemical isotopic transport in Yucca Mountain's highly heterogeneous, unsaturated fractured tuffs. In particular, the model results are examined against different types of field-measured data and used to evaluate different hydrogeological conceptual models and their effects on flow patterns in the unsaturated zone. The objective of this work to provide understanding of percolation patterns and flow behavior through the unsaturated zone, which is a crucial issue in assessing repository performance. Introduction Since the 1980s, the unsaturated zone (UZ) of the highly heterogeneous, fractured tuff at Yucca Mountain, Nevada, has been investigated by the U.S. Department of Energy as a possible repository site for storing high-level radioactive waste. Characterization of flow and transport processes in fractured rock of the Yucca Mountain UZ has received significant attention and generated tremendous interest in scientific communities over the last two decades. During this long and extensive study, many types of data have been collected from the Yucca Mountain UZ, and these data have helped to develop a conceptual understanding of various physical processes within the UZ system. The complexity of geological conditions and physical processes within the Yucca Mountain has posed a tremendous challenge for site-characterization effort, while quantitative evaluation of fluid flow, chemical transport, and heat transfer has proven to be essential. The need for quantitative investigations of flow and transport at the Yucca Mountain site has motivated a continual effort in developing and applying large, mountain-scale flow and transport models [e.g., Wu et al., 1999a and The site characterization studies of the unsaturated tuff at the Nevada Test Site and at Yucca Mountain began in the late 1970s and early 1980s. Those early hydrological, geological, and geophysical investigations of Yucca Mountain and the surrounding region were conducted to assess the feasibility of the site as a geological repository for storing high-level radioactive waste and to provide conceptual understanding of UZ flow processes In the early 1990s, more progress was made in UZ model development. Wittwer et al. [1992, 1995] developed a three-dimensional (3-D) site-scale model that incorporated several geological and hydrological complexities, such as geological layering, degree of welding, fault offsets, and different matrix and fracture properties. The 3-D model handled fracture-matrix flow using an effective continuum method (ECM) and was applied to evaluate various assumptions concerning faults and infiltration patterns. Using the ECM concept, Ahlers et al. [1995a, 1995b] continued development of the UZ site-scale model with increased numerical and spatial resolution. Their studies considered more processes, such as gas and heat flow analyses, and introduced an inverse modeling approach for estimating model-input properties. However, more comprehensive UZ models were not developed until a couple of years later, when the UZ models were developed for total system performance assessment-viability assessment (TSPA-VA) [e.g., Wu et al., 1999a and The next generation of UZ models included those primarily developed for the TSPA-site recommendation (SR) calculations [e.g., Wu et al., 2002a; Moridis et al., 2003; Robinson et al., 2003]. These TSPA-SR models were enhanced from the TSPA-VA model. More 4 importantly, the newer models took into account the coupled processes of flow and transport in highly heterogeneous, unsaturated fractured porous rock, and were applied to analyzing the effect of current and future climates on radionuclide transport through the UZ system. The site-scale UZ flow and transport models developed during the site characterization of Yucca Mountain have built upon the past research as well as the above-referenced work and many other studies [e.g., McLaren et al., 2000; Robinson et al., 1996 and 1997; This paper presents the results of our continuing effort to develop a realistic and representative UZ flow model to characterize the Yucca Mountain UZ system. More specifically, we focus on analyzing unsaturated flow patterns in the Yucca Mountain UZ under various climates and different hydrogeological conceptual models using an integrated modeling approach. This effort integrates different field-observed data, such as water potential, liquid saturation, perched water, gas pressure, chloride, and temperature logs into one single 3-D UZ flow and transport model. Using the dual-permeability modeling approach, the integrated modeling effort provides consistent model predictions for different, but inter-related hydrological, pneumatical, geochemical, and geothermal processes in the UZ. More importantly, such an integrated modeling exercise will improve the model's capability and credibility in describing and predicting current and future conditions and processes of the UZ system. At the same time, the combined model calibration will present a consistent check on modeled percolation fluxes and reveal better insight into the UZ flow patterns. The modeling study of this work consists of (1) UZ model description; (2) model development and calibration using liquid saturation, water potential, perched water, and pneumatic data; (3) assessing percolation patterns and flow behavior using thermal and geochemical data; and (4) simulated percolation pattern analysis. Conceptual Model of UZ Flow Over the past two decades, extensive scientific investigations have been conducted for the site characterization of Yucca Mountain, including data collection from surface mapping, sampling from many deep and shallow boreholes, constructing underground tunnels, and field testing [e.g., Rousseau, 1998]. of faults and perched water on the UZ system. As illustrated in In fact, the PTn's capability of attenuating episodic percolation fluxes has been observed in field tests of water release into the PTn matrix In addition, the possibility for capillary barriers exists at both upper and lower PTn contacts, as well as within the PTn layers PTn layers. However, the extent of lateral flow diversion within the PTn remains a topic of debate. For example, a recent study using a conceptual model with transitional changes in rock properties argues that lateral diversion may be small Perched Water Perched water has been encountered in a number of boreholes at Yucca Mountain, including UZ-14, SD-7, SD-9, SD-12, NRG-7a, G-2, and WT-24 ( Perched water may occur where percolation flux exceeds the capacity of the geological media to transmit vertical flux in the UZ [Rousseau et al., 1998]. Several conceptual models have been investigated for explaining the genesis of perched water at Yucca Mountain [e.g., Wu et al., 1999b and Faults In addition to possible capillary and permeability barriers, major faults in the UZ are also expected to play an important role in controlling percolation flux. Permeability within faults is much higher than that in the surrounding tuff Pneumatic permeability measurements taken along portions of faults revealed low airentry pressures, indicating that large fracture apertures are present in the fault zones. Highly brecciated fault zones may act as vertical capillary barriers to lateral flow. Once water is diverted into a fault zone, however, its high permeability could facilitate rapid vertical flow through the unsaturated system Heterogeneity A considerable amount of field data, obtained from tens of boreholes, two underground tunnels, and hundreds of outcrop samples at the site, constrains the distribution of rock properties within different units. In general, field data indicate that the Yucca Mountain formation is more heterogeneous vertically than horizontally, such that layer-wise representations are found to provide reasonable approximations of the complex geological system. This is because many model calibration results using this approximation are able to match different types of observation data obtained from different locations and depths [e.g., Bandurraga and Bodvarsson, 1999; 8 In summary, as shown in Ambient water flow in the UZ system is at a quasi-steady state condition, subject to spatially varying net infiltration on the ground surface. Hydrogeological units/layers are internally homogeneous, unless interrupted by faults or altered. There may exist capillary barriers in the PTn unit, causing lateral flow. Perched water results from permeability barrier effects. Major faults serve as fast vertical flow pathways for laterally diverted flow. Numerical Modeling Approach and Model Description Because of the complexity of the UZ geological system and the highly nonlinear nature of governing equations for UZ flow and transport, numerical modeling approaches were used in this study. Numerical simulations were carried out using the TOUGH2 and T2R3D codes [Pruess, 1991; Wu et al., 1996]. Most UZ flow simulations in this study were performed using an unsaturated flow module of the TOUGH2 code, which solves Richards' equation. Two active phases (liquid and gas) and heat flow was simulated using a two-phase fluid and heat flow module. Tracer and geochemical transport runs were carried out with the T2R3D code. Numerical Model Grids There are two 3-D numerical model grids used in this study, as shown in plan view in consists of 980 mesh columns of fracture and matrix continua, 86,440 gridblocks, and 350,000 connections in a dual-permeability grid. Vertically, the thermal grid has an average of 45 computational grid layers. Modeling Fracture-Matrix Interaction Modeling fracture and matrix flow and interaction under multiphase, multicomponent, isothermal or nonisothermal conditions has been a key issue for simulating fluid and heat flow in the Yucca Mountain UZ. Different modeling approaches have been tested for handling fracture-matrix interaction at Yucca Mountain The dual-permeability methodology considers global flow and transport occurring not only between fractures but also between matrix gridblocks. In this approach, the rockvolume domain is represented by two overlapping (yet interacting) fracture and matrix 10 continua, and local fracture-matrix flow and transport is approximated as a quasi-steady state. When applied in this work, however, the traditional dual-permeability concept is first modified by using an active fracture model Model Input Parameters Since the Richards' and two-active-phases flow equations are used in modeling unsaturated flow of water and air through fracture and matrix, relative permeability and capillary pressure curves are needed for the two media. In addition, other intrinsic fracture and matrix properties are also needed, such as porosity, permeability, density, and fracture geometric parameters, as well as rock thermal properties. In our modeling study, the van Genuchten models of relative permeability and capillary pressure functions The model input parameters of fractured and matrix rock were determined by two steps: (1) using field and laboratory measurements Model Boundary Conditions The ground surface of the mountain (or the tuff-alluvium contact in the area of significant alluvial cover) is taken as the top model boundary, while the water table is treated as the bottom model boundary. For flow simulations, net infiltration is applied to fractures along the top boundary using a source term. The bottom boundary at the water table is treated as a Dirichlet-type boundary. All the lateral boundaries, as shown in Net infiltration of water, resulting from precipitation that penetrates the top-soil layer of the mountain, is the most important factor affecting the overall hydrological, geochemical and thermal-hydrological behavior of the UZ. Net infiltration is the ultimate source of groundwater recharge and deep-zone percolation through the UZ, and provides a vehicle for transporting radionuclides from the repository to the water table. To cover the various possible scenarios and uncertainties of current and future climates at Yucca Mountain, we have incorporated a total of nine net infiltration maps, provided by US Geological Survey (USGS) scientists [Hevesi and Flint, 2000; As shown in Model Calibration The complexities of the heterogeneous geological formation at the Yucca Mountain UZ, A total of 18 flow simulation scenarios are studied in this work, as listed in In these calibrations, the gas flow model uses the UZ thermal model grid Comparison of model simulation results and field-measured pneumatic data for boreholes UZ-7a is shown in Flow Patterns and Analyses The primary objective of modeling UZ flow at Yucca Mountain is to estimate percolation flux through the UZ system. This is because percolation is the most critical factor that affects overall repository performance under current and future climates. However, in situ Past studies [e.g., Wu et al., 2002a] have shown that it is very difficult even to quantify the range of percolation fluxed by using hydrological data alone. Percolation patterns inside the UZ strongly depend on infiltration rates and their spatial distribution, among other factors. Therefore, over the past two decades, significant research effort has been devoted to estimating the infiltration rates [e.g., Flint et al., 1996; Hevesi and Flint, 2000; 17 Simulated Percolation Fluxes Percolation Patterns at Repository: Percolation fluxes at the repository horizon, as predicted using 18 3-D UZ flow simulation results of Flow Pattern Analyses Simulated In this study, heat flow simulations use the 3-D thermal model grid Temperature distributions at the bottom boundary of the thermal model are taken from deep-borehole-measured temperature profiles All Cl transport simulations were run using the T2R3D code for 100,000 years to approximate the current, steady-state condition under the infiltration scenarios considered. Chloride is treated as a conservative component transported through the UZ, subject to advection, diffusion, and first-order delay. The mechanical dispersion effect through the fracture-matrix system was ignored. A constant molecular diffusion coefficient of 2.032 × 10 -9 m 2 /s is used for matrix diffusion for Cl and the half-life for radioactive decay is 301,000 years. Concluding Remarks This paper presents a large-scale modeling study to characterize percolation patterns in the unsaturated zone of Yucca Mountain, Nevada, a proposed underground repository site for storing high-level radioactive waste. The modeling studies are conducted using an integrated modeling approach, which incorporates a wide variety of field data into a This study summarizes our current research effort to characterize UZ flow patterns at Yucca Mountain. Even with the significant progress made in quantitative evaluation of UZ flow and transport processes at the site using numerical models over the last two decades, there still exist a number of limitations and shortcomings with these models and their results. In general, accuracy and reliability of UZ site-scale models and simulation results are critically dependent on the accuracy of estimated model-related properties and other types of input parameters as well as hydrogeological conceptual models. The main limitations and uncertainties with the current UZ site-scale models are (1) the lack of indepth knowledge of the mountain system (including the geological and conceptual models and the availability of field and laboratory data), and (2) the approximations of a large volume-averaged modeling approach. As a result, continual research effort is still 26 needed toward a better understanding of the Yucca Mountain UZ system

A Note on Temperature and Energy of 4-dimensional Black Holes from Entropic Force

We investigate the temperature and energy on holographic screens for 4-dimensional black holes with the entropic force idea proposed by Verlinde. We find that the "Unruh-Verlinde temperature" is equal to the Hawking temperature on the horizon and can be considered as a generalized Hawking temperature on the holographic screen outside the horizons. The energy on the holographic screen is not the black hole mass $M$ but the reduced mass $M_0$, which is related to the black hole parameters. With the replacement of the black hole mass $M$ by the reduced mass $M_0$, the entropic force can be written as $F=\frac{GmM_0}{r^2}$, which could be tested by experiments.Comment: V4: 13 pages, 4 figures, title changed, discussions for experiments added, accepted by CQ

Soy intake and breast cancer risk in Singapore Chinese Health Study

We investigated the effects of soy isoflavone intake on breast cancer in a prospective study of 35 303 Singapore Chinese women enrolled during April 1993 to December 1998 in the Singapore Chinese Health Study. At recruitment, each subject was personally administered a validated semiquantitative food frequency questionnaire covering 165 food and beverage items. As of December 31 2005, 629 had developed breast cancer following an accumulation of 338 242 person-years. Using Cox regression and adjusting for age at interview, year of interview, dialect group, education, family history of breast cancer, age when periods became regular, parity, menopausal status, body mass index (BMI), n-3 fatty acid, and other covariates, we found breast cancer risk was reduced significantly in association with high soy intake. Relative to women with lower (below median) soy intake (<10.6 mg isoflavone per 1000 Kcal), women with higher (above median) intake showed a significant 18% risk reduction (relative risk (RR)=0.82, 95% confidence interval (CI)=0.70–0.97). This inverse association was apparent mainly in postmenopausal women (RR=0.74, 95% CI=0.61–0.90), and was not observed in premenopausal women (RR=1.04, 95% CI=0.77–1. 40). Among postmenopausal women, the soy–breast cancer association was stronger in those above median BMI (RR=0.67, 95% CI=0.51–0.88) than in leaner women (RR=0.83, 95% CI=0.62–1.11). Duration of follow-up modified the soy–breast cancer association, the effect being twice as large among women with 10+ vs fewer years of follow-up. Neither oestrogen nor progesterone receptor status of the tumours materially influenced the association. These prospective findings suggest that approximately 10 mg of isoflavones per day, obtained in a standard serving of tofu, may have lasting beneficial effects against breast cancer development

In situ epitaxial engineering of graphene and h-BN lateral heterostructure with a tunable morphology comprising h-BN domains

Graphene and hexagonal boron nitride (h-BN), as typical two-dimensional (2D) materials, have long attracted substantial attention due to their unique properties and promise in a wide range of applications. Although they have a rather large difference in their intrinsic bandgaps, they share a very similar atomic lattice; thus, there is great potential in constructing heterostructures by lateral stitching. Herein, we present the in situ growth of graphene and h-BN lateral heterostructures with tunable morphologies that range from a regular hexagon to highly symmetrical star-like structure on the surface of liquid Cu. The chemical vapor deposition (CVD) method is used, where the growth of the h-BN is demonstrated to be highly templated by the graphene. Furthermore, large-area production of lateral G-h-BN heterostructures at the centimeter scale with uniform orientation is realized by precisely tuning the CVD conditions. We found that the growth of h-BN is determined by the initial graphene and symmetrical features are produced that demonstrate heteroepitaxy. Simulations based on the phase field and density functional theories are carried out to elucidate the growth processes of G-h-BN flakes with various morphologies, and they have a striking consistency with experimental observations. The growth of a lateral G-h-BN heterostructure and an understanding of the growth mechanism can accelerate the construction of various heterostructures based on 2D materials

Synthesis of Mesoporous Silica@Co–Al Layered Double Hydroxide Spheres: Layer-by-Layer Method and Their Effects on the Flame Retardancy of Epoxy Resins

Hierarchical mesoporous silica@Co–Al layered double hydroxide (m-SiO2@Co–Al LDH) spheres were prepared through a layer-by-layer assembly process, in order to integrate their excellent physical and chemical functionalities. TEM results depicted that, due to the electrostatic potential difference between m-SiO2 and Co–Al LDH, the synthetic m-SiO2@Co–Al LDH hybrids exhibited that m-SiO2 spheres were packaged by the Co–Al LDH nanosheets. Subsequently, the m-SiO2@Co–Al LDH spheres were incorporated into epoxy resin (EP) to prepare specimens for investigation of their flame-retardant performance. Cone results indicated that m-SiO2@Co–Al LDH incorporated obviously improved fire retardant of EP. A plausible mechanism of fire retardant was hypothesized based on the analyses of thermal conductivity, char residues, and pyrolysis fragments. Labyrinth effect of m-SiO2 and formation of graphitized carbon char catalyzed by Co–Al LDH play pivotal roles in the flame retardance enhancement