Ab initio study of anharmonicity in high pressure Cmca-4 metallic hydrogen

Hydrogen is the only atom for which the Schr odinger equation is solvable. Consisting only of a proton and an electron, hydrogen is the lightest element and, nevertheless, is far from being simple. Under ambient conditions, it forms diatomic molecules H2 in gas phase, but di erent temperature and pressures lead to a complex phase diagram, which is not completely known yet. Solid hydrogen was rst documented in 1899 [1] and was found to be isolating. At higher pressures, however, hydrogen can be metallized. In 1935 Wigner and Huntington predicted that the metallization pressure would be 25 GPa [2], where molecules would disociate to form a monoatomic metal, as alkali metals that lie below hydrogen in the periodic table. The prediction of the metallization pressure turned out to be wrong: metallic hydrogen has not been found yet, even under a pressure as high as 320 GPa. Nevertheless, extrapolations based on optical measurements suggest that a metallic phase may be attained at 450 GPa [3]. The interest of material scientist in metallic hydrogen can be attributed, at least to a great extent, to Ashcroft, who in 1968 suggested that such a system could be a hightemperature superconductor [4]. The temperature at which this material would exhibit a transition from a superconducting to a non-superconducting state (Tc) was estimated to be around room temperature. The implications of such a statement are very interesting in the eld of astrophysics: in planets that contain a big quantity of hydrogen and whose temperature is below Tc, superconducting hydrogen may be found, specially at the center, where the gravitational pressure is high. This might be the case of Jupiter, whose proportion of hydrogen is about 90%. There are also speculations suggesting that the high magnetic eld of Jupiter is due to persistent currents related to the superconducting phase [5]. Metallization and superconductivity of hydrogen has puzzled scientists for decades, and the community is trying to answer several questions. For instance, what is the structure of hydrogen at very high pressures? Or a more general one: what is the maximum Tc a phonon-mediated superconductor can have [6]? A great experimental e ort has been carried out pursuing metallic hydrogen and trying to answer the questions above; however, the characterization of solid phases of hydrogen is a hard task. Achieving the high pressures needed to get the sought phases requires advanced technologies. Diamond anvil cells (DAC) are commonly used devices. These devices consist of two diamonds with a tip of small area; for this reason, when a force is applied, the pressure exerted is very big. This pressure is uniaxial, but it can be turned into hydrostatic pressure using transmitting media. Nowadays, this method makes it possible to reach pressures higher than 300 GPa, but even at this pressure hydrogen does not show metallic properties. A recently developed technique that is an improvement of DAC can reach pressures as high as 600 GPa [7], so it is a promising step forward in high pressure physics. Another drawback is that the electronic density of the structures is so low that X-ray di raction patterns have low resolution. For these reasons, ab initio studies are an important source of knowledge in this eld, within their limitations. When treating hydrogen, there are many subtleties in the calculations: as the atoms are so light, the ions forming the crystalline lattice have signi cant displacements even when temperatures are very low, and even at T=0 K, due to Heisenberg's uncertainty principle. Thus, the energy corresponding to this zero-point (ZP) motion is signi cant and has to be included in an accurate determination of the most stable phase. This has been done including ZP vibrational energies within the harmonic approximation for a range of pressures and at T=0 K, giving rise to a series of structures that are stable in their respective pressure ranges [8]. Very recently, a treatment of the phases of hydrogen that includes anharmonicity in ZP energies has suggested that relative stability of the phases may change with respect to the calculations within the harmonic approximation [9]. Many of the proposed structures for solid hydrogen have been investigated. Particularly, the Cmca-4 structure, which was found to be the stable one from 385-490 GPa [8], is metallic. Calculations for this structure, within the harmonic approximation for the ionic motion, predict a Tc up to 242 K at 450 GPa [10]. Nonetheless, due to the big ionic displacements, the harmonic approximation may not su ce to describe correctly the system. The aim of this work is to apply a recently developed method to treat anharmonicity, the stochastic self-consistent harmonic approximation (SSCHA) [11], to Cmca-4 metallic hydrogen. This way, we will be able to study the e ects of anharmonicity in the phonon spectrum and to try to understand the changes it may provoque in the value of Tc. The work is structured as follows. First we present the theoretical basis of the calculations: Density Functional Theory (DFT) for the electronic calculations, phonons in the harmonic approximation and the SSCHA. Then we apply these methods to Cmca-4 hydrogen and we discuss the results obtained. In the last chapter we draw some conclusions and propose possible future work.

As2Te3-ren isolatzaile topologikorako presiopeko trantsizioaren ab-initio azterketa

Materia kondentsatuko sikan erronka nagusietako bat naturako materialen izaera eza- gutu eta ezaugarritzea da. Orain dela urte batzuk arte ezagutzen genituen material guztiak, eroale, erdieroale edo isolatzaileak ziren, materialeko balentzia elektroien izae- raren arabera. Azken urteotan sikako arlo honetan burututako lanek eman dute bere fruitua, materiaren egoera berri bat aurkitu baita naturan [1]: isolatzaile topologikoa. Isolatzaile topologikoak material isolatzaileak dira baina ertza eroalea dute. Egoera eroale hauek dira material berri honen berezkotasuna. Egoerok sistemaren topologia dela eta existitzen dira eta sistemaren simetriaren bidez babestuta daudenez, deusez- taezinak dira. Hall isolatzaile kuantikoa izan zen isolatzaile topologikoen gaia teorikoki garatzen hasteko inspirazio iturria eta esperimentalki beranduago aurkitu ziren [2]. Lan ugari egiten ari da materiaren egoera berri honen teoria osatu eta era honetako material berriak aurkitzeko. Gaur egun isolatzaile topologiko ezagunenetarikoak kalogenuro fami- liakoak dira. Talde honetakoa da 2008.urtean estrainekoz aurkitu zen hiru dimentsiotako isolatzaile topologikoa: Bi1

Spin-orbita elkarrekintza eta trukatze-korrelazio eremu eskalar eta bektorialaren ab-initio kalkuluak LAPW metodoaren bidez: Bismutoaren adibidea

Gradu amaierako lan honetan, LAPW metodoa aztertu da solidoen propietate elektronikoak era teorikoan ikertzeko eta efektu erlatibistek hauengan duten eragina zenbatesteko tresna teoriko bezala. Konkretuki spin-orbita elkarrekintzan zentratu gara, eta hau konputazionalki inplementatzeko bigarren bariazionalaren metodoa aztertu da. Bestalde, Spin-DFT teoriaren barruan spin-orbita kodifikatzen duen trukatze-korrelazio eremu bektorialaren azterketa labur bat egin da, ekarpen erlatibista beste ikuspuntu batetik aztertu eta informazio osagarria lortzeko asmoz.

La leyenda de la violación de Lucrecia y la articulación del relato del reinado de Tarquinio el Soberbio en Tito Livio ("Ab Urbe Condita" 1.49-60)

[ES] El propósito de este trabajo es el de mostrar cómo Livio elabora el episodio de la violación de Lucrecia de tal manera que se conecta con los restantes episodios del reinado de Tarquinio el Soberbio, formando una unidad tan estrecha y bien dispuesta que éste se convierte en el punto culminante de un proceso que comienza con el inicio del reinado del último monarca romano. Livio no se limita a seleccionar y disponer en una simple secuencia temporal los hechos que la tradición atribuía al segundo Tarquinio, sino que los elabora y ordena con vistas a conseguir un relato efectivo desde la perspectiva literaria y narrativa y, como consecuencia de ello, convincente desde el punto de vista moral, una de sus principales preocupaciones.

A avaliação da atenção básica e o trabalho dos agentes comunitários de saúde: implantação do PMAQ-AB na região do Médio Paraíba Fluminense

Este estudo tem por objeto a implantação do Programa Nacional de Melhoria do Acesso e da Qualidade da Atenção Básica (PMAQ-AB) na região do Médio Paraíba fluminense sob o olhar dos Agentes Comunitários de Saúde e dos gestores da Atenção Básica. A indagação norteadora desta pesquisa foi: em que medida o Programa Nacional de Melhoria do Acesso e da Qualidade da Atenção Básica (PMAQ-AB) capta a importância do trabalho dos Agentes Comunitários de Saúde? Partindo do pressuposto que o desenvolvimento do PMAQ não se dá, no concreto, de forma linear nem livre de tensões, e que o trabalho do ACS, por seu caráter de mediação, é um campo privilegiado para análise sobre essas tensões. O objetivo geral do estudo é analisar a implantação do Programa Nacional de Melhoria do Acesso e da Qualidade da Atenção Básica (PMAQ-AB) nos municípios da região do Médio Paraíba do Estado do Rio de Janeiro, tomando como eixo de análise o processo de trabalho dos agentes comunitários de saúde. O estudo apresenta uma abordagem quantitativa e qualitativa, através de questionários, entrevistas e grupos focais e o método de análise dos dados qualitativos foi de base interpretativa com apoio do referencial da Hermenêutica-Dialética. O campo da pesquisa foi a região do Médio Paraíba fluminense. Os resultados evidenciam que na opinião dos gestores, o PMAQ aparece como uma estratégia norteadora do processo de trabalho das equipes, que possibilita o resgate de alguns princípios da ESF, já os ACS apontam que o seu primeiro contato com o PMAQ foi fundamentado na pactuação do cumprimento de metas que resultariam no pagamento de um incentivo financeiro. Percebemos que não existe uma crítica quanto a implantação do PMAQ, nem por parte da gestão, muito menos por parte das equipes. Não encontramos nos relatos dos gestores nenhuma menção ao trabalho do ACS, sua participação e importância em todas as fases do PMAQ. Os gestores consideram que o PMAQ promoveu uma mudança positiva na forma de organização do trabalho dos ACS. Os ACS não perceberam mudanças em sua rotina a partir da implantação do PMAQ, exceto pela questão da sobrecarga de trabalho. Conclui-se que o PMAQ-AB não capta a potencialidade e o trabalho dos ACS. A participação do ACS nas fases do PMAQ é identificada somente no aspecto instrumental, ou seja, dele ser um produtor, um registrador de informações.Para os gestores o PMAQ surge como algo que abre um horizonte positivo, de maior envolvimento e retomada de alguns processos pelas EAB, mas não é assim, de fato, que a questão chega até os ACS e, por consequência, nem para as equipes.

Revised ab initio natural band offsets of all group IV, II-VI, and III-V semiconductors

Using an all-electron band structure approach, we have systematically calculated the natural band offsets between all group IV, III-V, and II-VI semiconductor compounds, taking into account the deformation potential of the core states. This revised approach removes assumptions regarding the reference level volume deformation and offers a more reliable prediction of the "natural" unstrained offsets. Comparison is made to experimental work, where a noticeable improvement is found compared to previous methodologies.

AB，ABA型乙丙嵌段共聚物的合成、结构及作为乙丙橡胶改性聚丙烯增容剂的研究

本文试图用Solvay型的Ziegler-Natta催化剂合成PP－EPR，PP－EPR－PP嵌段共聚物，作为乙丙橡胶（EPT）和聚丙烯（PP）的增容剂，来提高聚丙烯的抗冲击强度，并从力学性能、动态力学谱和形态等方面研究增容剂的增容效果。Ziegler-Natta催化剂能否合成乙丙嵌段共聚物是一个有争议的问题。为此，我们利用改进型的Solvay δ-TiCl_3-Et_2AlCl在已烷中加压淤浆聚合丙烯，发现在120分钟之前，聚丙烯的分子量随时间迅速增加，超过120分钟则变缓慢，并趋向于平衡。这说明催化剂活性中心上的活性链最小有120分钟的时间，若在该时间内用聚合过程中换反应单体的办法，有可能合成嵌段共聚物。乙丙嵌段共聚物形成的直接证明是利用(PP－PE)_(50)。通过气相色谱检测发现，抽真空3分钟可以保证换反应气体的纯度，因而用气相聚合可以得到各段纯净的(PP－PE)_(50)多嵌段共聚物，该嵌段的~(13)c-NMR研究表明，在35.68ppm处存在以化学键相连的长乙烯和长丙烯链的特征共振峰。PGC的研究发现，其在热裂解中产生的G碎片比PP／P混物的多，GC－MS的研究表明G碎片由含7个碳的烃组成的混合物，组分之一具有嵌段共聚物的裂解特征。对PP－EPR系列产物和PP－EPR－PP（5－60－20）用扭摆法进和动态力学分析表明，这些嵌段共聚物只在－30℃左右有一个玻璃化转变。而相应的共混物则分别在－50℃和5℃出现两个玻璃化转变，且各T_g不随组成比和共混方法而变化。这是由于嵌段共聚物中各段间化学键的作用，使各段的T_g内移，从而使较靠近的两个T_g合二为一，在动态力学谱上只表现出一个T_g。粘弹谱仪测定的结果基本上同扭摆法的结果。尽管我们尚未准确地测定出乙丙嵌段共聚物中EPR段的分子量，但我们弄清了PP段的立体构型、等规度、分子量、结晶度和EPR段的乙丙比、无规乙丙共聚物的含量、含有长序列乙烯的结晶度等结构因素。用不同段长的PP－EPR作PP／EPT共混物的增容剂，发现降低PP－EPR中PP段的分子量，三元共混物的力学性能明显升高，而增加EPR段的分子量即聚合时间，其无缺口冲击强度先增加而后又降低，说明有一个EPR段最佳长度范围。根据该现象我们提出模型并进行了解释。结晶度的规律与冲击强度的规律相同，对冲击强度提高较大的增容剂，共混物中PP的结晶度降低，但抗张性能却升高，说明增容剂在两相界面起到主价的连接作用。扭摆法和粘弹谱仪测定的动态力学谱表明，增容剂的加入减小了聚丙烯结晶无序化转变，使PP的T_g突出出来。形态的研究说明，PP－EPR还起了“乳化剂”的作用，使EPT在PP连续相中均匀分散，且其微区大小适中。在PP／EPT（85／15）中加入4％的PP－EPR（5－30）嵌段共聚物，室温（20℃）的“冲击屈服强度”与PP／EPT的相同，－20℃的冲击强度为112kg·cm/cm~2，是PP／EPT的1.5倍，－40℃为72kg·cm/cm~2，是PP／EPT的1.9倍，在应力－应变实验中，三元共混物PP／EPT／PP－EPR（5－30）的σ_b*ε_b为2.97*10~5。比相应的PP／EPT（2.27*10~5）有所提高。说明PP－EPR（5－30）对PP／EPT有良好的增容效果，比文献中使用的PP－EPR（15－55）效果好。用PP－EPR－PP三嵌段共聚物作PP／EPT的增容剂，实验证明比PP－EPR二嵌段共聚物有更好的增容效果。例如，在PP／EPT（85／15）中加入4％的PP－EPR－PP（5－60－20），试样不但在20℃，而且在－20℃均未被冲断；在－20℃的“冲击屈服强度”是PP／EPT冲击强度的1.4倍，PP的9.0倍，－40℃的冲击强度是PP／EPT的2.2倍，PP的8.4倍；且其σ_b*ε_b（2.62*10~5）比PP／EPT（2.60*10~5）的有一定的提高，比PP的（2.45*10~5）也高。对PP／EPT／PP－EPR－PP（5－60－20）三元共混物的试样在－40℃冲断面的形态进行研究表明，加入增容剂的共混物断面凹凸不平，是韧性断裂的特征，且在断裂过程中EPT微区被牵拉出的EPT较多，说明PP和EPT的相界面的作用力较大，增容剂起到了主价键的连接作用。动态力学谱表明，增容剂的加入降低了PP结晶无序化转变。而液氮冷冻的脆断面的形态说明，PP－EPR－PP起到了“乳化剂”的作用，使EPT在PP连续相中大小均匀地分散开。