Browsing by Author "Zhang, Rui"
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Item Benchmark measurements and simulations of dose perturbations due to metallic spheres in proton beams(Elsevier, 2013) Newhauser, Wayne D.; Rechner, Laura; Mirkovic, Dragan; Yepes, Pablo; Koch, Nicholas C.; Titt, Uwe; Fontenot, Jonas D.; Zhang, RuiMonte Carlo simulations are increasingly used for dose calculations in proton therapy due to its inherent accuracy. However, dosimetric deviations have been found using Monte Carlo code when high density materials are present in the proton beamline. The purpose of this work was to quantify the magnitude of dose perturbation caused by metal objects. We did this by comparing measurements and Monte Carlo predictions of dose perturbations caused by the presence of small metal spheres in several clinical proton therapy beams as functions of proton beam range and drift space. Monte Carlo codes MCNPX, GEANT4 and Fast Dose Calculator (FDC) were used. Generally good agreement was found between measurements and Monte Carlo predictions, with the average difference within 5% and maximum difference within 17%. The modification of multiple Coulomb scattering model in MCNPX code yielded improvement in accuracy and provided the best overall agreement with measurements. Our results confirmed that Monte Carlo codes are well suited for predicting multiple Coulomb scattering in proton therapy beams when short drift spaces are involved.Item Commit Phase Variations in Timestamp-based Software Transactional Memory(2008-02-11) Zhang, Rui; Budimli?, Zoran; Scherer, William N., IIITimestamp-based Software Transactional Memory (STM) validation techniques use a global shared counter and timestamping of objects being written to reason about sequencing of transactions and their linearization points, while reducing the number of unnecessary validations that have to be performed, thus improving overall system performance. During the commit phase of a timestamp-based validation scheme, several actions have to be performed: locking of the objects being written to the memory, atomically incrementing a shared timestamp counter, updating timestamps for objects being committed to memory, performing a final validation of the transaction’s consistency, and atomically effecting the transaction’s changes to the outside world. The order and manner in which these actions are performed can affect both the correctness of the STM implementation and the overall system performance. We identify several commit sequence designs, prove their correctness, and analyze their performance. We identify cases where timestamps do not have to be unique for different transactions committing concurrently, and cases where unnecessary updates of the global shared counter — which can trigger extra vaidations in other transactions, hurting performance — can be avoided. We evaluate these commit sequence designs on a set of benchmarks on a 16 processor SunFire SMP machine. We show that a carefully chosen commit sequence can improve overall system performance by up to 14% over the current state of the art single counter timestamp-based validation techniques, and we show that it is possible to obtain high performance without incurring space overhead proportional to the number of objects in the system.Item Composability for Application-Specific Transactional Optimizations(2010-01-21) Zhang, Rui; Budimlić, Zoran; Scherer, William N., IIISoftware Transactional Memory (STM) has made great advances towards acceptance into mainstream programming by promising a programming model that greatly reduces the complexity of writing concurrent programs. Unfortunately, the mechanisms in current STM implementations that enforce the fundamental properties of transactions — atomicity, consistency, and isolation — also introduce considerable performance overhead. This performance impact can be so significant that in practice, programmers are tempted to leverage their knowledge of a specific application to carefully bypass STM calls and instead access shared memory directly. While this technique can be very effective in improving performance, it breaks the consistency and isolation properties of transactions, which have to be handled manually by the programmer for the specific application. It also tends to break another desirable property of transactions: composability. In this paper, we identify the composability problem and propose two STM system extensions to provide transaction composability in the presence of direct shared memory reads by transactions. Our proposed extensions give the programmer a similar level of flexibility and performance when optimizing the STM application as the existing practices, while preserving composability. We evaluate our extensions on several benchmarks on a 16-way SMP. The results show that our extensions provide performance competitive with hand-optimized non-composable techniques, while still maintaining transactional composability.Item Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe2−xNixAs2(American Physical Society, 2018) Man, Haoran; Zhang, Rui; Park, J.T.; Lu, Xingye; Kulda, J.; Ivanov, A.; Dai, PengchengWe use transport and inelastic neutron-scattering measurements to investigate single crystals of iron pnictide BaFe2−xNixAs2(x=0,0.03), which exhibit a tetragonal-to-orthorhombic structural transition at Ts and stripe antiferromagnetic order at TN(Ts≥TN). Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave-vector S1(1,0) and its 90∘ rotated wave-vector S2(0,1) under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain the single domain orthorhombic antiferromagnetic phase of BaFe2−xNixAs2 induces resistivity and spin excitation anisotropy at temperatures above zero pressure Ts. In the uniaxial pressure-free detwinned sample, spin excitation anisotropy between S1(1,0) and S2(0,1) first appears in the paramagnetic orthorhombic phase below Ts. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above Ts in iron pnictides.Item Doping effects of Cr on the physical properties of BaFe1.9−xNi0.1CrxAs2(American Physical Society, 2018) Gong, Dongliang; Xie, Tao; Zhang, Rui; Birk, Jonas; Niedermayer, Christof; Han, Fei; Lapidus, S.H.; Dai, Pengcheng; Li, Shiliang; Luo, HuiqianWe present a systematic study on the heavily Cr doped iron pnictides BaFe1.9−xNi0.1CrxAs2 by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from x=0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Néel temperature TN coincides with the tetragonal-to-orthorhombic structural transition temperature Ts, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing x until x=0.5, and then decreases with further increasing x. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the FeAs4 tetrahedron along the c axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around x=0.5. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.Item Doping evolution of antiferromagnetism and transport properties in nonsuperconducting BaFe2−2xNixCrxAs2(American Physical Society, 2015) Zhang, Rui; Gong, Dongliang; Lu, Xingye; Li, Shiliang; Laver, Mark; Niedermayer, Christof; Danilkin, Sergey; Deng, Guochu; Dai, Pengcheng; Luo, HuiqianWe report elastic neutron scattering and transport measurements on the Ni and Cr equivalently doped iron pnictide BaFe2−2xNixCrxAs2. Compared with the electron-doped BaFe2−xNixAs2, the long-range antiferromagnetic (AF) order in BaFe2−2xNixCrxAs2 is gradually suppressed with vanishing ordered moment and Néel temperature near x=0.20 without the appearance of superconductivity. A detailed analysis on the transport properties of BaFe2−xNixAs and BaFe2−2xNixCrxAs2 suggests that the non-Fermi-liquid behavior associated with the linear resistivity as a function of temperature may not correspond to the disappearance of the static AF order. From the temperature dependence of the resistivity in overdoped compounds without static AF order, we find that the transport properties are actually affected by Cr impurity scattering, which may induce a metal-to-insulator crossover in highly doped BaFe1.7−yNi0.3CryAs2.Item Doping evolution of antiferromagnetism and transport properties in nonsuperconducting BaFe2−2xNixCrxAs2(American Physical Society, 2015) Zhang, Rui; Gong, Dongliang; Lu, Xingye; Li, Shiliang; Laver, Mark; Niedermayer, Christof; Danilkin, Sergey; Deng, Guochu; Dai, Pengcheng; Luo, HuiqianWe report elastic neutron scattering and transport measurements on the Ni and Cr equivalently doped iron pnictide BaFe2−2xNixCrxAs2. Compared with the electron-doped BaFe2−xNixAs2, the long-range antiferromagnetic (AF) order in BaFe2−2xNixCrxAs2 is gradually suppressed with vanishing ordered moment and Néel temperature near x=0.20 without the appearance of superconductivity. A detailed analysis on the transport properties of BaFe2−xNixAs and BaFe2−2xNixCrxAs2 suggests that the non-Fermi-liquid behavior associated with the linear resistivity as a function of temperature may not correspond to the disappearance of the static AF order. From the temperature dependence of the resistivity in overdoped compounds without static AF order, we find that the transport properties are actually affected by Cr impurity scattering, which may induce a metal-to-insulator crossover in highly doped BaFe1.7−yNi0.3CryAs2.Item Electronic nematic correlations in the stress-free tetragonal state of BaFe2−xNixAs2(American Physical Society, 2015) Man, Haoran; Lu, Xingye; Chen, Justin S.; Zhang, Rui; Zhang, Wenliang; Luo, Huiqian; Kulda, J.; Ivanov, A.; Keller, T.; Morosan, Emilia; Si, Qimiao; Dai, PengchengWe use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped BaFe2−xNixAs2 iron pnictides in uniaxial-strained and external-stress-free detwinned states. Using a specially designed in situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial-strained tetragonal state of BaFe2−xNixAs2 below a temperature T∗, previously identified as a signature of the electronic nematic phase, is also present in the stress-free tetragonal phase below T**(Item Electronic specific heat in BaFe2−xNixAs2(American Physical Society, 2016) Gong, Dongliang; Xie, Tao; Lu, Xingye; Ren, Cong; Shan, Lei; Zhang, Rui; Dai, Pengcheng; Yang, Yi-feng; Luo, Huiqian; Li, ShiliangWe have systematically studied the low-temperature specific heat of the BaFe2−xNixAs2 single crystals covering the whole superconducting dome. Using the nonsuperconducting heavily overdoped x=0.3 sample as a reference for the phonon contribution to the specific heat, we find that the normal-state electronic specific heats in the superconducting samples may have a nonlinear temperature dependence, which challenges previous results in the electron-doped Ba-122 iron-based superconductors. A model based on the presence of ferromagnetic spin fluctuations may explain the data between x=0.1 and x=0.15, suggesting the important role of Fermi-surface topology in understanding the normal-state electronic states.Item Enhanced representation of soil NO emissions in the Community Multiscale Air Quality (CMAQ) model versionﾠ5.0.2(Copernicus Publications, 2016) Rasool, Quazi Z.; Zhang, Rui; Lash, Benjamin; Cohan, Daniel S.; Cooter, Ellen J.; Bash, Jesse O.; Lamsal, Lok N.Modeling of soil nitric oxide (NO) emissions is highly uncertain and may misrepresent its spatial and temporal distribution. This study builds upon a recently introduced parameterization to improve the timing and spatial distribution of soil NO emission estimates in the Community Multiscale Air Quality (CMAQ) model. The parameterization considers soil parameters, meteorology, land use, and mineral nitrogen (N) availability to estimate NO emissions. We incorporate daily year-specific fertilizer data from the Environmental Policy Integrated Climate (EPIC) agricultural model to replace the annual generic data of the initial parameterization, and use a 12 km resolution soil biome map over the continental USA. CMAQ modeling for July 2011 shows slight differences in model performance in simulating fine particulate matter and ozone from Interagency Monitoring of Protected Visual Environments (IMPROVE) and Clean Air Status and Trends Network (CASTNET) sites and NO2 columns from Ozone Monitoring Instrument (OMI) satellite retrievals. We also simulate how the change in soil NO emissions scheme affects the expected O3 response to projected emissions reductions.Item Incorporating GOES Satellite Photosynthetically Active Radiation (PAR) Retrievals to Improve Biogenic Emission Estimates in Texas(Wiley, 2018) Zhang, Rui; White, Andrew T.; Biazar, Arastoo Pour; McNider, Richard T.; Cohan, Daniel S.This study examines the influence of insolation and cloud retrieval products from the Geostationary Operational Environmental Satellite (GOES) system on biogenic emission estimates and ozone simulations in Texas. Compared to surface pyranometer observations, satellite‐retrieved insolation and photosynthetically active radiation (PAR) values tend to systematically correct the overestimation of downwelling shortwave radiation in the Weather Research and Forecasting (WRF) model. The correlation coefficient increases from 0.93 to 0.97, and the normalized mean error decreases from 36% to 21%. The isoprene and monoterpene emissions estimated by the Model of Emissions of Gases and Aerosols from Nature are on average 20% and 5% less, respectively, when PAR from the direct satellite retrieval is used rather than the control WRF run. The reduction in biogenic emission rates using satellite PAR reduced the predicted maximum daily 8 h ozone concentration by up to 5.3 ppbV over the Dallas‐Fort Worth (DFW) region on some days. However, episode average ozone response is less sensitive, with a 0.6 ppbV decrease near DFW and 0.3 ppbV increase over East Texas. The systematic overestimation of isoprene concentrations in a WRF control case is partially corrected by using satellite PAR, which observes more clouds than are simulated by WRF. Further, assimilation of GOES‐derived cloud fields in WRF improved CAMx model performance for ground‐level ozone over Texas. Additionally, it was found that using satellite PAR improved the model's ability to replicate the spatial pattern of satellite‐derived formaldehyde columns and aircraft‐observed vertical profiles of isoprene.Item Inferring Atmospheric Particulate Matter Concentrations from Chinese Social Media Data(Public Library of Science, 2016) Tao, Zhu; Kokas, Aynne; Zhang, Rui; Cohan, Daniel S.; Wallach, DanAlthough studies have increasingly linked air pollution to specific health outcomes, less well understood is how public perceptions of air quality respond to changing pollutant levels. The growing availability of air pollution measurements and the proliferation of social media provide an opportunity to gauge public discussion of air quality conditions. In this paper, we consider particulate matter (PM) measurements from four Chinese megacities (Beijing, Shanghai, Guangzhou, and Chengdu) together with 112 million posts on Weibo (a popular Chinese microblogging system) from corresponding days in 2011–2013 to identify terms whose frequency was most correlated with PM levels. These correlations are used to construct an Air Discussion Index (ADI) for estimating daily PM based on the content of Weibo posts. In Beijing, the Chinese city with the most PM as measured by U.S. Embassy monitor stations, we found a strong correlation (R = 0.88) between the ADI and measured PM. In other Chinese cities with lower pollution levels, the correlation was weaker. Nonetheless, our results show that social media may be a useful proxy measurement for pollution, particularly when traditional measurement stations are unavailable, censored or misreported.Item Inside Time-based Software Transactional Memory(2007-07-06) Zhang, Rui; Budimlić, Zoran; Scherer, William N., IIIWe present a comprehensive analysis and experimental evaluation of time-based validation techniques for Software Transactional Memory (STM). Time-based validation techniques emerge recently as an effective way to reduce the validation overhead for STM systems. In a time-based strategy, information based on global time enables the system to avoid a full validation pass in many cases where it can quickly prove that no consistency violation is possible given the time information for the current transaction and the object it is attempting to open. We show that none of the current timebased strategies offers the best performance across various applications and thread counts. We also show an adaptive technique which has the potential to achieve an overall best performance based on time information and show some preliminary results we have.Item Local breaking of fourfold rotational symmetry by short-range magnetic order in heavily overdoped Ba(Fe1−xCux)2As2(American Physical Society, 2017) Wang, Weiyi; Song, Yu; Hu, Ding; Li, Yu; Zhang, Rui; Harriger, L.W.; Tian, Wei; Cao, Huibo; Dai, PengchengWe investigate Cu-doped Ba(Fe1−xCux)2As2 with transport, magnetic susceptibility, and elastic neutron scattering measurements. In the heavily Cu-doped regime where long-range stripe-type antiferromagnetic order in BaFe2As2 is suppressed, Ba(Fe1−xCux)2As2 (0.145≤x≤0.553) samples exhibit spin-glass-like behavior in magnetic susceptibility and insulating-like temperature dependence in electrical transport. Using elastic neutron scattering, we find stripe-type short-range magnetic order in the spin-glass region identified by susceptibility measurements. The persistence of short-range magnetic order over a large doping range in Ba(Fe1−xCux)2As2 likely arises from local arrangements of Fe and Cu that favor magnetic order, with Cu acting as vacancies relieving magnetic frustration and degeneracy. These results indicate locally broken fourfold rotational symmetry, suggesting that stripe-type magnetism is ubiquitous in iron pnictides.Item Longitudinal Spin Excitations and Magnetic Anisotropy in Antiferromagnetically Ordered BaFe2As2(American Physical Society, 2013) Wang, Chong; Zhang, Rui; Wang, Fa; Luo, Huiqian; Regnault, L.P.; Dai, Pengcheng; Li, YuanWe report on a spin-polarized inelastic neutron-scattering study of spin waves in the antiferromagnetically ordered state of BaFe2As2. Three distinct excitation components are identified, with spins fluctuating along the c axis, perpendicular to the ordering direction in the ab plane and parallel to the ordering direction. While the first two “transverse” components can be described by a linear spin-wave theory with magnetic anisotropy and interlayer coupling, the third “longitudinal” component is generically incompatible with the local-moment picture. It points toward a contribution of itinerant electrons to the magnetism that is already in the parent compound of this family of Fe-based superconductors.Item Modulation of mechanics and electromechanical force in prestin transfected membrane tethers(2007) Zhang, Rui; Anvari, BahmanThe voltage-dependent movement, or electromotility, of cochlear outer hair cells (OHCs) contributes to cochlear amplification in mammalian hearing. Prestin, a transmembrane protein expressed in the lateral wall of the OHSs, is essential for electromotility, but molecular details of its function are unknown. Using a combined optical tweezers and whole-cell voltage clamping system, we have investigated the contribution of prestin to the mechanics and electromechanical force (EMF) of membrane tethers, and related these parameters to prestin-associated non-linear capacitance (NLC). Our study included three test groups consisting of (i) untransfected; (ii) wild type (WT) prestin-transfected; and (iii) mutant (A100W) prestin-transfected human embryonic kidney (HEK) cells. While there was no difference in membrane effective viscosity among the three cell types, tethers from WT prestin-transfected HEK cells demonstrated altered membrane mechanical parameters and increased EMF values than control untransfected HEK cells. We found that A100W mutation in prestin, which eliminates NLC also diminishes EMF, but does not affect membrane mechanical parameters. These results suggest that prestin-associated charge transfer is associated with generation of EMF in the membrane, but independent of the effect of prestin on membrane mechanics. Based on our results, we propose synergistic effects of prestin and the membrane in the generation of NLC and electromotility. The information these results provide is important for understanding protein/membrane interactions, prestin properties and the origin of electromotility.Item Nematic Crossover in BaFe2As2 under Uniaxial Stress(American Physical Society, 2015) Ren, Xiao; Duan, Lian; Hu, Yuwen; Li, Jiarui; Zhang, Rui; Luo, Huiqian; Dai, Pengcheng; Li, YuanRaman scattering can detect spontaneous point-group symmetry breaking without resorting to single-domain samples. Here, we use this technique to study BaFe2As2, the parent compound of the “122” Fe-based superconductors. We show that an applied compression along the Fe-Fe direction, which is commonly used to produce untwinned orthorhombic samples, changes the structural phase transition at temperature Ts into a crossover that spans a considerable temperature range above Ts. Even in crystals that are not subject to any applied force, a distribution of substantial residual stress remains, which may explain phenomena that are seemingly indicative of symmetry breaking above Ts. Our results are consistent with an onset of spontaneous nematicity only below Ts.Item Nematic Fluctuations in the Non-Superconducting Iron Pnictide BaFe1.9−xNi0.1CrxAs2(Frontiers Media S.A., 2022) Gong, Dongliang; Yi, Ming; Wang, Meng; Xie, Tao; Zhang, Wenliang; Danilkin, Sergey; Deng, Guochu; Liu, Xinzhi; Park, Jitae T.; Ikeuchi, Kazuhiko; Kamazawa, Kazuya; Mo, Sung-Kwan; Hashimoto, Makoto; Lu, Donghui; Zhang, Rui; Dai, Pengcheng; Birgeneau, Robert J.; Li, Shiliang; Luo, Huiqian; Rice Center for Quantum MaterialsThe main driven force of the electronic nematic phase in iron-based superconductors is still under debate. Here, we report a comprehensive study on the nematic fluctuations in a non-superconducting iron pnictide system BaFe1.9−xNi0.1CrxAs2 by electronic transport, angle-resolved photoemission spectroscopy (ARPES), and inelastic neutron scattering (INS) measurements. Previous neutron diffraction and transport measurements suggested that the collinear antiferromagnetism persists to x = 0.8, with similar Néel temperature TN and structural transition temperature Ts around 32 K, but the charge carriers change from electron type to hole type around x = 0.5. In this study, we have found that the in-plane resistivity anisotropy also highly depends on the Cr dopings and the type of charge carriers. While ARPES measurements suggest possibly weak orbital anisotropy onset near Ts for both x = 0.05 and x = 0.5 compounds, INS experiments reveal clearly different onset temperatures of low-energy spin excitation anisotropy, which is likely related to the energy scale of spin nematicity. These results suggest that the interplay between the local spins on Fe atoms and the itinerant electrons on Fermi surfaces is crucial to the nematic fluctuations of iron pnictides, where the orbital degree of freedom may behave differently from the spin degree of freedom, and the transport properties are intimately related to the spin dynamics.Item Nematic magnetoelastic effect contrasted between Ba(Fe1−xCox)2As2 and FeSe(American Physical Society, 2016) Hu, Yuwen; Ren, Xiao; Zhang, Rui; Luo, Huiqian; Kasahara, Shigeru; Watashige, Tatsuya; Shibauchi, Takasada; Dai, Pengcheng; Zhang, Yan; Matsuda, Yuji; Li, YuanTo elucidate the origin of nematic order in Fe-based superconductors, we report a Raman scattering study of lattice dynamics, which quantify the extent of C4-symmetry breaking, in BaFe2As2 and FeSe. FeSe possesses a nematic ordering temperature Ts and orbital-related band-energy split below Ts that are similar to those in BaFe2As2, but unlike BaFe2As2 it has no long-range magnetic order. We find that the Eg phonon-energy split in FeSe becomes substantial only well below Ts, and its saturated value is much smaller than that in BaFe2As2. Together with reported results for the Ba(Fe1−xCox)2As2 family, the data suggest that magnetism exerts a major influence on the lattice.Item Nematic Quantum Critical Fluctuations in BaFe2−xNixAs2(American Physical Society, 2016) Liu, Zhaoyu; Gu, Yanhong; Zhang, Wei; Gong, Dongliang; Zhang, Wenliang; Xie, Tao; Lu, Xingye; Ma, Xiaoyan; Zhang, Xiaotian; Zhang, Rui; Zhu, Jun; Ren, Cong; Shan, Lei; Qiu, Xianggang; Dai, Pengcheng; Yang, Yi-feng; Luo, Huiqian; Li, ShiliangWe have systematically studied the nematic fluctuations in the electron-doped iron-based superconductor BaFe2−xNixAs2 by measuring the in-plane resistance change under uniaxial pressure. While the nematic quantum critical point can be identified through the measurements along the (110) direction, as studied previously, quantum and thermal critical fluctuations cannot be distinguished due to similar Curie-Weiss-like behaviors. Here we find that a sizable pressure-dependent resistivity along the (100) direction is present in all doping levels, which is against the simple picture of an Ising-type nematic model. The signal along the (100) direction becomes maximum at optimal doping, suggesting that it is associated with nematic quantum critical fluctuations. Our results indicate that thermal fluctuations from striped antiferromagnetic order dominate the underdoped regime along the (110) direction. We argue that either there is a strong coupling between the quantum critical fluctuations and the fermions, or more exotically, a higher symmetry may be present around optimal doping.