Thermodynamics of volume collapse transitions in cerium and related compounds

We present a non-linear elastic model of a coherent transition with discontinuous volume change in an isotropic solid. The model reproduces the anomalous thermodynamics typical of coherent equilibrium including intrinsic hysteresis (for a pressure driven experiment) and a negative bulk modulus. The novelty of the model is that the statistical mechanics solution can be easily worked out. We find that coherency leads to an infinite-range density--density interaction, which drives classical critical behavior. The pressure width of the hysteresis loop shrinks with increasing temperature, ending at a critical point at a temperature related to the shear modulus. The bulk modulus softens with a 1/2 exponent at the transition even far from the critical point. Many well known features of the phase diagram of Ce and related systems are explained by the model.Comment: Acta Materialia, in pres

Alterung der Gesellschaft: Neue Herausforderungen für die Gründungsförderung in Deutschland? Ergebnisse einer empirischen Untersuchung bei Wirtschaftskammern und kommunalen Einrichtungen 2005 bis 2006

Die Alterung der Gesellschaft und deren Auswirkungen stellten in den vergangenen fünf Jahren ein sehr stark und mitunter kontrovers diskutiertes Thema dar. Die Konsequenzen der Bevölkerungsalterung sind vielschichtig und betreffen bei weitem nicht nur die Sicherheit der sozialen Sicherungssysteme (Börsch-Supan 2004; Schmidt 2004, 2005). Dabei wurde bis dato nur in sehr begrenztem Umfang die Verbindung zwischen demografischem Wandel, Gründungsgeschehen und Gründungsförderung betrachtet. Im Rahmen einer Studie für das BMWi hat sich das RWI Essen umfassend mit diesem Konnex auseinandergesetzt und hierzu jüngst den Endbericht vorgelegt (Engel et al. 2007). Gegenstand des vorliegenden Beitrags ist eine Aufbereitung der wesentlichen Erkenntnisse aus der Befragung bei Wirtschaftskammern und kommunalen Einrichtungen in den Jahren 2005 und 2006.Darüber hinaus werden Ergebnisse vertiefender Analysen dargestellt, die sich mit der Positionierung der Kammern in der Förderlandschaft bzw. denKooperationsstrategien detailliert auseinandersetzen. Hintergrund hierfür ist einerseits die Diskussion bezüglich der übersichtlichen Gestaltung des deutschen Förder- bzw. Beratungssystems (Klemmer et al. 1996; Kailer 2001: 113). Handlungsalternativen bieten sich vor allem in der engeren Abstimmung bzw. Bündelung von Angeboten verschiedener Einrichtungen. Andererseits hat die Nachfrage gerade älterer gründungsinteressierter Personen1 erheblich zugenommen. Die in den letzten Jahren zu beobachtende Veränderung im Kreis der Gründungsinteressierten gibt einen Vorgeschmack auf das Kommende.Da die Ausstattung mit Personal mit dieser Entwicklung scheinbar nicht Schritt hielt, erhöht sich der Druck auf die Gründungsberatung zu mehr Effizienz. Ausgehend davon spricht vieles dafür, sich mit den Anforderungen und Potenzialen älterer Gründungsinteressierter sowie mit den Einschätzungen der Kammern zu möglichen Anpassungserfordernissen in der Gründungsberatung auseinanderzusetzen. Das Ziel dieser Studie besteht darin, hierzu einen ersten, bundesweiten Überblick aus dem Blickwinkel der Gründungsintermediäre zu bieten

Viscoelasticity and metastability limit in supercooled liquids

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter \lambda, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where \lambda is larger than a critical value \lambda_c the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if \lambda < \lambda_c a kinetic spinodal is present and the time needed to experimentally observe it scales as exp[\omega/(\lambda_c-\lambda)^2], where \omega is roughly the ratio between surface tension and enthalpy of fusion

Domain enhanced interlayer coupling in ferroelectric/paraelectric superlattices

We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.Comment: Latest version as was published in PR

Droplet Fluctuations in the Morphology and Kinetics of Martensites

We derive a coarse grained, free-energy functional which describes droplet configurations arising on nucleation of a product crystal within a parent. This involves a new `slow' vacancy mode that lives at the parent-product interface. A mode-coupling theory suggests that a {\it slow} quench from the parent phase produces an equilibrium product, while a {\it fast} quench produces a metastable martensite. In two dimensions, the martensite nuclei grow as `lens-shaped' strips having alternating twin domains, with well-defined front velocities. Several empirically known structural and kinetic relations drop out naturally from our theory.Comment: 4 pages, REVTEX, and 3 .eps figures, compressed and uuencoded, Submitted to Phys. Rev. Let

Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures

A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is presented, which enables the thermodynamic description of dense laminar polydomain states in epitaxial ferroelectric thin films. The theory explicitly takes into account the mechanical substrate effect on the polarizations and lattice strains in dissimilar elastic domains (twins). Numerical calculations are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic substrates. The "misfit strain-temperature" phase diagrams are developed for these films, showing stability ranges of various possible polydomain and single-domain states. Three types of polarization instabilities are revealed for polydomain epitaxial ferroelectric films, which may lead to the formation of new polydomain states forbidden in bulk crystals. The total dielectric and piezoelectric small-signal responses of polydomain films are calculated, resulting from both the volume and domain-wall contributions. For BaTiO3 films, strong dielectric anomalies are predicted at room temperature near special values of the misfit strain.Comment: 19 pages, 8 figure

Multi-clonal SARS-CoV-2 neutralization by antibodies isolated from severe COVID-19 convalescent donors.

The interactions between antibodies, SARS-CoV-2 and immune cells contribute to the pathogenesis of COVID-19 and protective immunity. To understand the differences between antibody responses in mild versus severe cases of COVID-19, we analyzed the B cell responses in patients 1.5 months post SARS-CoV-2 infection. Severe, and not mild, infection correlated with high titers of IgG against Spike receptor binding domain (RBD) that were capable of ACE2:RBD inhibition. B cell receptor (BCR) sequencing revealed that VH3-53 was enriched during severe infection. Of the 22 antibodies cloned from two severe donors, six exhibited potent neutralization against authentic SARS-CoV-2, and inhibited syncytia formation. Using peptide libraries, competition ELISA and mutagenesis of RBD, we mapped the epitopes of the neutralizing antibodies (nAbs) to three different sites on the Spike. Finally, we used combinations of nAbs targeting different immune-sites to efficiently block SARS-CoV-2 infection. Analysis of 49 healthy BCR repertoires revealed that the nAbs germline VHJH precursors comprise up to 2.7% of all VHJHs. We demonstrate that severe COVID-19 is associated with unique BCR signatures and multi-clonal neutralizing responses that are relatively frequent in the population. Moreover, our data support the use of combination antibody therapy to prevent and treat COVID-19

Martensitic growth in ZrO2--An in situ, small particle, TEM study of a single-interface transformation

An in situ TEM experiment on martensitic growth was performed using submicron ZrO2 particles of a square-platelet shape. The transformation was between the orthorhombic and the monoclinic phases and involved a simple shear plus a dilatation in the shear plane. The o/m interface propagated at a speed of 2 nm/s, while maintaining a sharp habit plane which was stepped on the unit cell scale. The average inclination of this stepped interface obeyed the invariant plane strain condition. While no long-range stresses were present, dislocation-like line contrast was revealed at the steps. These results are analyzed in terms of the coherency dislocation concept. Fundamental properties, such as the interfacial energy, Peierls stress and nucleus size, have been deduced.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28555/1/0000357.pd