1,721,005 research outputs found
Optical transitions and sum rules at clean semiconductor surfaces
No full textThe experimental results of surface differential reflectivity and reflectance anisotropy spectroscopy show that in Si(111)-2 x 1, Ge(Ill)-2 x 1 and GaAs(001)-2 x 4 a sum rule for the imaginary part of the (surface) dielectric function is verified both for the isotropic and anisotropic parts of epsilon(s)". It is shown that the sum rule together with the dependence of the spectra upon oxygen contamination are useful in the interpretation of the optical transitions of the above surfaces. In particular, for the case of GaAs(001)-2 x 4 the above analysis has allowed the distinction between optical transitions associated to true surface states and bulk states modified by the surface near the 3 eV critical point
Infrared reflectance anisotropy spectroscopy of Si(111)-2X1: Surface excitons and polarons
No full textThe experimental data regarding the energy distribution of dangling-bond electronic states at the Si(111)-2x1 surface, with special attention to the gap between filled and empty states, are revisited. Clearly some of the results are not compatible. Theorists almost unanimously conclude in favor of a strong (about 0.25 eV) excitonic energy in the optical transitions between surface states. On the experimental side, on the other hand, conflicting results seem to support either large or negligible exciton energy. In order to reconcile all available data, it has been suggested that in highly doped samples a sizeable gap shrinking should occur. We have performed a reflectance anisotropy spectroscopy experiment in order to verify this hypothesis. The optical peak at 0.45 eV associated to dangling-bond transitions does not shift, in highly doped crystals (N-D=4x10(18) cm(-3)), within the experimental accuracy of 20 meV, thereby ruling out the hypothesis of a shrinking of the gap. Under the assumption that all experimental results are correct, an alternative explanation of this puzzling problem, based on the existence of surface polarons together with surface excitons, is suggested. Although very difficult to detect spectroscopically in a direct way, it is argued that polaronic states could play an important role in highly doped samples. A number of possible experiments to clarify the last point are suggested. Accurate theoretical calculations of the polaron energy would also be helpful
Infrared surface absorption in Si(111)2x1 observed with reflectance anisotropy spectroscopy
No full textOptical transitions between surface states associated with dangling bonds in Si(111)2X1 have been measured by means of reflection anisotropy spectroscopy in the near infrared. The results confirm those previously obtained with other optical techniques (namely surface differential reflectivity and photothermal deflection spectroscopies). The method does not require oxidation of the surface and thus opens the way to studying a number of problems in surface physics, including the temperature dependence of surface transitions in Si(111)2x1 and Ge(111)2x1
Spatial Distribution of the International Food Prices: Unexpected Heterogeneity and Randomness
Full text linkGlobalfoodpricesaretypicallyanalysedinatime-seriesframework.Wecomplementthisapproachbyfocusingonthespatialpricedispersionofthecountry-pairbilateraltradeintheinternationalfoodtradenetwork(IFTN),fortenrelevantcommodities.ThemainpurposesaretoverifyiftheLawofOnePrice(LOP)holdsandtoinvestigatetheemergenceofrandomnessintheprice-formationmechanism.Wedistinguishbetweenthe“internal”variance,whichindicatesthemagnitudeofpricediscrimination,andthe“external”variance,thatisameasureofpricedispersion.Wefindthat,forsomecommodities,spatialpricedispersionisremarkableandpersistentovertime(i.e.,failureoftheLOP)andthatthereexistsastrictcorrelationbetweenpricespikesandpeaksinspatialpricevariability.We test whether the price distribution can be replicated through a stochastic process of extraction.Surprisingly, the actual distribution of prices, for several commodities, is well described by a random dis-tribution.Then,theprocessofdataaggregationisnotneutralbecausetheinformationatthemicro-levelscalemightbelostatthemacro-scale,duetothecomplexityoftheIFTN.Finally,wediscusssomepossibleeconomicexplanationsoftheseoutcomesandthemainmethodological,environmental,andpolicyconsequences.1. IntroductionTheendofhungerandtheachievementoffoodsecurityareglobalkey issues explicitly included in the Sustainable Development Goalsagenda (UN, 2015). The interest of the international community isjustifiedbyacknowledgingthecomplexandinterrelatedenvironmentalandsocialdimensionslinkedtofoodmanagement,suchaswaterre-sources(Generoso,2015;DistefanoandKelly,2017;Distefanoetal.,2018b), energy and pollution (Carlsson-Kanyama et al., 2003;D’Odoricoetal.,2018),landuseanddeforestation(OdegardandVanderVoet,2014),andsocialsecurityandhealth(Bellemare,2015;Bush,2010).Thus,abetterunderstandingoffoodmarkets,especiallyinaneraofglobalisation(Duarteetal.,2014;Biewaldetal.,2014;Suweisetal.,2015),iscrucialtoprovidesolidbasesforfoodpoliciesandre-source management (Wang et al., 2016). This interest has been re-inforcedrecently–afterthetwowavesofworldfoodpricecrises(2008and2011)–whereeconomistsanalysedtheaftermathsofprice‘spikes'to assess the short-run effects (Piesse and Thirtle, 2009; Bellemare,2015)andthemaincausesoftemporalfoodpricevolatility(seeDíaz-Bonilla,2016,foradiscussion).Acommonassumptionbehindthesestudies,andusualamongagriculturaleconomists,istheso-calledLawofOnePrice(henceforthLOP):oncepricesareconvertedtoacommoncurrency (including transaction and transport costs), homogeneousgoods should be sold for the same price in different countries(Miljkovic,1999).1TheLOPshouldholdwhengoodsarehighlytraded,at least in spatially separated international markets (Baffes, 1991;GoldbergandVerboven,2005).Onthecontrary,pricedispersion–namely,ahomogeneousproductbeingsoldatdifferentpricesbydifferentexporters–canemerge,forseveralreasons,suchasentrybarriersandgeographicalseparationofmarkets(Krugman,1991;Yangetal.,2017),differentmarginalcosts(Crucini and Yilmazkuday, 2014; Yilmazkuday, 2016), variations inconsumer preferences (Grebitus et al., 2013), and monetary illusion(FehrandTyran,2001).2Followingthisbranchofliterature,weaimattestingthepresenceornotofLOPintheinternationalfoodtradenet-work(IFTN),extendingtheanalysistotheprice-formationmechanism.Theexistentliteratureempiricallyanalysedspatialfood-pricedisper-siononlyattheretaillevel(e.g.,AnaniaandNisticò,2014)but,toourhttps://doi.org/10.1016/j.ecolecon.2019.01.010Received30January2018;Receivedinrevisedform5November2018;Accepted8January2019*Correspondingauthor.E-mailaddress:[email protected](T.Distefano).1Ancillaryhypothesesarethoseconcerningperfectcompetition:agentsareprice-takers,perfectinformation,nofrictionsonfactormobility,individualrationality,andsoon.2Monetaryillusionindicatesthepsychologicaleffectofmakingmistakesduetotheuseofdifferentcurrencies.Ecological Economics 159 (2019) 122–132Available online 28 January 20190921-8009/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Si(111) and Si(100) surfaces observed in air by scanning tunneling microscopy
No full textSi(111) and Si(100) surfaces have been observed by an air-operating scanning tunneling microscope (STM). On the cleaved Si(111) surface STM topographic images show predominantly [211BAR]-oriented monatomic steps. The distribution of the width of the terraces is centered around 4 nm. Si(100) surfaces have been hydrogen-terminated by a treatment with fluoridic acid (HF). The surface appears in the topographic STM images to be quite stable and smooth with presumably no oxide
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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