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81.
Marco Bernardi Derek Vigil-Fowler Chin Shen Ong Jeffrey B. Neaton Steven G. Louie 《Proceedings of the National Academy of Sciences of the United States of America》2015,112(17):5291-5296
Hot carrier dynamics critically impacts the performance of electronic, optoelectronic, photovoltaic, and plasmonic devices. Hot carriers lose energy over nanometer lengths and picosecond timescales and thus are challenging to study experimentally, whereas calculations of hot carrier dynamics are cumbersome and dominated by empirical approaches. In this work, we present ab initio calculations of hot electrons in gallium arsenide (GaAs) using density functional theory and many-body perturbation theory. Our computed electron–phonon relaxation times at the onset of the Γ, L, and X valleys are in excellent agreement with ultrafast optical experiments and show that the ultrafast (tens of femtoseconds) hot electron decay times observed experimentally arise from electron–phonon scattering. This result is an important advance to resolve a controversy on hot electron cooling in GaAs. We further find that, contrary to common notions, all optical and acoustic modes contribute substantially to electron–phonon scattering, with a dominant contribution from transverse acoustic modes. This work provides definitive microscopic insight into hot electrons in GaAs and enables accurate ab initio computation of hot carriers in advanced materials.Hot carriers (HCs) generated by the absorption of light or injection at a contact are commonly found in many advanced technologies (1–9). In electronics, the operation of high-speed devices is controlled by HC dynamics, and HC injection is a key degradation mechanism in transistors (10, 11). In solar cells and plasmonics, recent work has focused on extracting the kinetic energy of HCs before cooling (7, 9), a process defined here as the energy loss of HCs, ultimately leading to thermal equilibrium with phonons. HC dynamics is also crucial to interpret time-resolved spectroscopy experiments used to study excited states in condensed matter (12). This situation has sparked a renewed interest in HCs in a broad range of materials of technological relevance.Experimental characterization of HCs is challenging because of the subpicosecond timescale associated with the electron–phonon (e-ph) and electron–electron (e-e) scattering processes regulating HC dynamics. For example, HCs can be studied using ultrafast spectroscopy, but microscopic interpretation of time-resolved spectra requires accurate theoretical models. However, modeling of HCs thus far has been dominated by empirical approaches, which do not provide atomistic details and use ad hoc parameters to fit experiments (13, 14). Notwithstanding the pioneering role of these early studies, the availability of accurate ab initio computational methods based on density functional theory (DFT) (15) and many-body perturbation theory (16) enables studies of HCs with superior accuracy, broad applicability, and no need for fitting parameters.Hot electrons in gallium arsenide (GaAs) are of particular interest because of the high electron mobility and multivalley character of the conduction band. Electrons excited at energies greater than ∼0.5 eV above the conduction band minimum (CBM) can transfer from the Γ to the L and X valleys, with energy minima at ∼0.25 and ∼0.45 eV above the CBM, respectively (17). Such intervalley scattering processes play a crucial role in hot electron cooling and transport at high electric fields.Ample experimental data exist on hot electron transport and cooling in GaAs (12, 18–21). The interpretation of these experiments relies on Monte Carlo simulations using multiple parameters fit to experimental results. For example, Fischetti and Laux (13) used two empirical deformation potentials to model electron scattering induced by optical and acoustic phonons. Additionally, Fischetti and Laux (13) used simplified band structure and phonon dispersions. We note that, because multiple parameter sets can fit experimental results, the HC scattering rates due to different physical processes obtained empirically are not uniquely determined (13, 14).Although heuristic approaches can provide some insight into HC dynamics of well-characterized materials (e.g., GaAs), there is a lack of generally applicable, predictive, and parameter-free approaches to study HCs.Here, we carry out ab initio calculations of hot electrons in GaAs with energies up to 5 eV above the CBM. Our ability to use extremely fine grids in the Brillouin zone (BZ) allows us to resolve hot electron scattering in the conduction band with unprecedented accuracy. We focus here on three main findings. First, our overall computed e-ph scattering rates are in excellent agreement with those in previous semiempirical calculations in ref. 13 that combine multiple empirical parameters. The advantage of our approach is the ability to compute the electronic band and momentum dependence of the e-ph scattering rates without fitting parameters. Second, we show that both optical and acoustic modes contribute substantially to e-ph scattering, with a dominant scattering from transverse acoustic (TA) modes. This result challenges the tenet that HCs lose energy mainly through longitudinal optical (LO) phonon emission. Third, our calculations provide valuable means for quantitative interpretation of experiments of hot electron cooling in GaAs. In particular, the ultrafast (∼50 fs) e-ph relaxation times that we compute at the onset of the X valley are in excellent agreement with the fastest decay time observed in ultrafast optical experiments (18, 19, 21). This signal was attributed by some (18) to e-e scattering and by others (21) to e-ph scattering. The excellent agreement with time decay signals in time-resolved experiments shows the dominant role of e-ph scattering for hot electron cooling at low carrier density.Our approach combines electronic band structures computed ab initio using the GW (where G is the Green function, W is the screened Coulomb potential, and GW is the diagram employed for the electron exchange-correlation interactions) method (16) with phonon dispersions from density functional perturbation theory (DFPT) (22), and it is entirely free of empirical parameters. We compute the e-ph matrix elements using a Wannier function formalism (23) on very fine BZ grids and are able to resolve e-ph scattering for the different conduction band valleys. The e-e rates for hot electrons—also known as impact ionization (II) rates—are computed using the GW method (16, 24), and thus include dynamical screening effects. Additional details of our calculations are discussed in Methods. 相似文献
82.
Peter Chin Wan Fung 《Chinese medicine》2009,4(1):10-6
This article hypothesizes that the Chinese medicine meridian system is a special channel network comprising of skin with abundant nerves and nociceptive receptors of various types, and deeper connective tissues inside the body with the flowing interstitial fluid system. These meridian channels provide efficient migratory tracks mainly due to durotaxis (also including chemotaxis) for mast cells, fibroblasts and other cells to migrate and carry out a number of physiological functions. Acupuncture acting on meridian channel causes cytoskeletal remodeling through mechanotransduction, leading to regulation of gene expression and the subsequent production of related proteins. Also, stimulation on cell surface can trigger Ca2+ activities, resulting in a cascade of intra- and inter-cellular signaling. Moreover, nerve endings in the meridian channels interact with mast cells and induce the degranulation of these cells, leading to the release of many specific biomolecules needed for homeostasis, immune surveillance, wound healing and tissue repair. Acupoint along a meridian channel is a functional site to trigger the above functions with specificity and high efficiency. 相似文献
83.
Prenatal diagnosis of fetal cystic hygroma is very important in clinical medicine. In this series, we report our work of detecting cystic hygroma using three-dimensional (3-D) ultrasound (US). We reviewed our computer database of prenatal diagnosis on cystic hygroma in National Cheng Kung University Hospital from May 1995 to June 2000. All the fetuses were initially scanned by a high-resolution, real-time, 2-D US scanner and subsequently by a 3-D US scanner. In total, 23 cases of fetal cystic hygroma were diagnosed in utero. The range of gestational age at prenatal diagnosis by US was between 11 and 24 weeks, and 91% were diagnosed before 21 weeks. Among them, 8 cases (35%) were Turner syndrome (45,X), and 10 cases (43%) were complicated with hydrops fetalis. Although the diagnostic rates by 2-D US and 3-D US were both 100% (23 of 23), notably, 3-D US can provide additional vivid illustrations in 3-D after various modes of reconstruction, but 2-D US cannot. In conclusion, 3-D US may add novel visual depiction of the lesion in 3-D after reconstruction and, thus, assists substantially in prenatal consultation. 相似文献
84.
Kim D Hur DY Kim YS Lee K Lee Y Cho D Kang JS Kim YI Hahm E Yang Y Yoon S Kim S Lee WB Park HY Kim YB Hwang YI Chang KY Lee WJ 《Human immunology》2002,63(7):576-587
Burkitt lymphoma (BL) is a tumor with the characteristics of germinal center B cells. We previously reported that the CM1 (centrocyte/-blast marker 1) molecule is expressed only in germinal center B cells, specifically, in a subpopulation of centroblasts and centrocytes. In the present study, we investigated the apoptosis induced by anti-CM1 in the Ramos and Raji human BL cell lines. The Ramos is protected from apoptosis by the crosslinking of sIgM and the calcium ionophore by the ligation of CD40 with anti-CD40 monoclonal antibodies (mAb) or soluble CD40 ligand (sCD40L). In this investigation on the effect of CM1 on apoptosis in BL cell lines, we found that cellular signaling by CM1 induces apoptosis and decreases cell viability, in BL cell lines cultured for 24 hours with protein-G agarose beads conjugated anti-CM1 mAb. Stimulation by CD40 ligated with sCD40L protected Raji cells from CM1-induced apoptosis, but did not protect Ramos cells. Furthermore, after anti-CM1 mAb stimulation, CD95 expression was upregulated and CD40 expression was unaltered or slightly decreased in Ramos cells, whereas CD95 was downregulated and CD40 was slightly upregulated in Raji cells. The engagement of CD40 by sCD40L enhanced CD95 expression, but the level of CM1 expression was unchanged in Ramos. However, sCD40L downregulated both CD95 and CM1 expression in Raji. In addition, the caspase-8 specific inhibitor blocked CM1-induced apoptosis in Ramos cells, but not in Raji cells. Increased mitochondrial membrane permeabilization was observed only in Raji cells. Moreover, the effector caspase inhibitor, z-DEVD, blocked CM1-mediated apoptosis in both cell lines. We found that CM1-induced apoptosis is achieved via different initiation pathways, which are cell-type dependent. 相似文献
85.
Background
Regulatory T cells (Tregs) play a pivotal role in the persistence of hepatitis C virus infection. The aim of this study was to evaluate the frequency and function of Tregs in patients with chronic hepatitis C (CHC).Methods
We enrolled 44 CHC patients with elevated alanine aminotransferase (ALT) levels (CH group), 13 CHC patients with persistent normal ALT levels (PNALT group), and 14 age-matched healthy subjects (HS group; controls). Tregs were identified as CD4+, CD25+, and forkhead box P3 (Foxp3)+ T lymphocytes, using three-color fluorescence-activated cell sorting (FACS). The frequency of Tregs was determined by calculating the percentage of CD4+CD25high T cells among CD4 T cells. CD127 and CD45RA were also analyzed for subsets of Tregs. The levels of serum transforming growth factor (TGF)-β and interleukin (IL)-10 in immunosuppressive assays were detected by enzyme-linked immunosorbent assay (ELISA). The immunosuppressive abilities of Tregs were evaluated by measuring their ability to inhibit the proliferation of effector cells.Results
Higher proportions of Tregs were found in the CH and PNALT groups compared with the HS group. The populations of CD127 low/negative and CD45RA negative cells were higher in the CH group than in the PNALT group. The expressions of IL-10 and TGF-β in the CH and PNALT groups were significantly higher than those in the HS group. In addition, the immunosuppressive ability of Tregs from the CH group was increased relative to that in the PNALT and the HS group.Conclusions
CHC patients, irrespective of liver function, had higher frequencies of Tregs than healthy subjects; however, only CHC patients with inflammation showed enhanced immunosuppressive function of Tregs. 相似文献86.
Mai Xu Tse Chiu Wong Shui Yee Wong Kwai Sang Chin Kwok Leung Tsui Renee Y. Hsia 《The Journal of emergency medicine》2013
Background
In Hong Kong Emergency Departments (EDs), the timeliness of providing high-quality services has been compromised by the increasing attendance of non-emergent patients in addition to the unpredictable arrival of emergency patients.Objectives
We sought to quantify the impact of the presence of emergent patients and other related factors on the delay in service for non-emergent patients.Methods
We conducted a retrospective study in patients who visited the ED of a large hospital in Hong Kong from July 1, 2009 to June 30, 2010. We estimated waiting and length of stay (LOS) for individual non-emergent patients registered during day and evening shifts. Using multiple linear regression, we estimated waiting time and LOS as a function of the presence of emergent patients and other related factors such as patient demographics and clinical factors. In particular, we evaluated the influence of the arrival or presence of emergent patients on the odds of violating the 120-min waiting time target for semi-urgent patients.Results
The arrival of a new emergent patient prolonged the waiting time and LOS of a non-emergent patient by 14.9% (95% confidence interval [CI] 14.2–15.5) and 10.8% (95% CI 10.6–11.0), respectively. An additional patient-hour needed for an emergent patient increased the probability of violating the waiting time target for non-emergent patients (odds ratio 2.3, 95% CI 2.2–2.4).Conclusions
The arrival of an emergent patient significantly prolonged the waiting time and LOS for non-emergent patients. Discouraging non-urgent ED utilization and building a real-time decision-support system are critical methods needed to relieve staff pressure and guide contingent resource reallocation when emergent patients arrive. 相似文献87.
Efficacy and safety of combination simvastatin and colesevelam in patients with primary hypercholesterolemia 总被引:4,自引:0,他引:4
Knapp HH Schrott H Ma P Knopp R Chin B Gaziano JM Donovan JM Burke SK Davidson MH 《The American journal of medicine》2001,110(5):352-360
PURPOSE: To examine the efficacy and safety of colesevelam hydrochloride, a novel, nonsystemic, lipid-lowering agent, when coadministered with starting doses of simvastatin in a multicenter, randomized, double-blind, placebo-controlled trial. PATIENTS AND METHODS: Subjects with hypercholesterolemia (plasma low density lipoprotein [LDL] cholesterol level > 160 mg/dL and triglyceride level < or = 300 mg/dL) were randomly assigned to receive daily doses of placebo (n = 33), colesevelam 3.8 g (recommended dose, n = 37), simvastatin 10 mg (n = 35), colesevelam 3.8 g with simvastatin 10 mg (n = 34), colesevelam 2.3 g (low dose, n = 36), simvastatin 20 mg (n = 39), or colesevelam 2.3 g with simvastatin 20 mg (n = 37), for 6 weeks. RESULTS: Mean LDL cholesterol levels decreased relative to baseline in the placebo group (P < 0.05) and in all active treatment groups (P < 0.0001). For groups treated with combination therapy, the mean reduction in LDL cholesterol level was 42% (-80 mg/dL; P < 0.0001 compared with baseline), which exceeded the reductions for simvastatin 10 mg (-26%, -48 mg/dL) or 20 mg (-34%, -61 mg/dL) alone, or for colesevelam 2.3 g (-8%, -17 mg/dL) or 3.8 g (-16%, -31 mg/dL) alone (P < 0.001). The effects of combination therapy on serum HDL cholesterol and triglyceride levels were similar to those for simvastatin alone. Side effects were similar among treatment groups, and there were no clinically important changes in laboratory parameters. CONCLUSION: Coadministration of colesevelam and simvastatin was effective and well tolerated, providing additive reductions in LDL cholesterol levels compared with either agent alone. 相似文献
88.
89.
Cheng Len Sy Hung Chin Tsai Shue Ren Wann Susan Shin-Jung Lee Yung Ching Liu Yao Shen Chen 《Journal of microbiology, immunology, and infection》2008,41(4):301-306
BACKGROUND AND PURPOSE: Hepatocyte growth factor (HGF) is a multifunctional cytokine that has been found to be elevated in tuberculous and bacterial meningitis, but no evaluation has been undertaken of its usefulness in identifying various forms of aseptic meningitis. METHODS: In a retrospective study, the levels of HGF in the cerebrospinal fluid of 65 patients were measured prior to treatment. The association of HGF with non-infectious diseases and clinically or microbiologically proven bacterial, tuberculous, viral, fungal and parasitic meningitis was observed, along with its relation to other parameters of the cerebrospinal fluid. RESULTS: Forty six of the 65 patients (71%) were diagnosed as having meningitis. Cerebospinal fluid HGF level was significantly elevated in patients with meningitis compared with patients with non-infectious diseases (1501 vs 578 pg/mL; Mann-Whitney U test, p=0.001). The highest HGF level was found in bacterial meningitis (2699 pg/mL), followed by tuberculous meningitis (1540 pg/mL), viral meningitis (1431 pg/mL), fungal meningitis (714 pg/mL) and parasitic meningitis (174 pg/mL). There was no association between HGF level and other parameters of the cerebrospinal fluid (Pearson's correlation test). CONCLUSION: Cerebrospinal fluid HGF may offer additional information in the classification of meningitis. This may assist in patient management when no pathogen is cultured from the cerebrospinal fluid and when other parameters of the cerebrospinal fluid demonstrate equivocal results. 相似文献
90.
Hyungoo Kang Sol An Woo Jung Lee Ga Ryang Kang Sangwon Kim Su-Mi Hur Keewook Paeng Myungwoong Kim 《RSC advances》2018,8(43):24166
We demonstrate the synthesis of poly(methyl methacrylate) (PMMA) and poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) brushes on crosslinked random copolymer thin films, compositionally varied poly(styrene-r-glycidyl methacrylate) (P(S-r-GMA)), which can be further functionalized with a molecule featuring an initiator group upon crosslinking to form highly stable thin films. With careful optimizations, PMMA brushes were successfully grown from the surfaces of initiator functionalized P(S-r-GMA) via surface-initiated atom transfer radical polymerization. The grafting densities of the PMMA and P(S-b-MMA) brushes were effectively controlled to be in different density regimes by controlling the composition of P(S-r-GMA) and post-crosslinking functionalization methods. Synthesized BCP brushes were stable upon repetitive washing and thermal annealing processes even at high grafting density, highlighting that the outstanding stability of crosslinked P(S-r-GMA) thin films enables close examination of the morphology of thermally annealed P(S-b-MMA) brushes in different grafting density regimes.Crosslinkable epoxy copolymers enable achieving highly stable P(S-b-MMA) brushes with controlled grafting density for close examination of phase separation behaviors. 相似文献