Characteristics of the wavelength of ripples on icicles

It is known that the wavelength of the ripples on icicles in nature is of centimeter-scale. Such study on morphological instability of ice-water interface during ice growth from flowing supercooled water film with one side being a free surface has recently been made [K. Ueno, Phys. Rev. E 68, 021603 (2003)]. This is a first theoretical study taking into account the influence of the shape of the water-air surface on the growth condition of infinitesimal disturbances of the ice-water interface. A simpler formula to determine the wavelength of the ripples than that in the previous paper is derived. It seems that the wavelength of ripples is insensitive to the water supply rates, diameters of the icicles and surrounding air temperatures. The details of dependence of the wavelengh of ripples on these parameters are investigated.Comment: 15 pages, 6 figure

Morphological instability of the solid-liquid interface in crystal growth under supercooled liquid film flow and natural convection airflow

Ring-like ripples on the surface of icicles are an example of morphological instability of the ice-water interface during ice growth under supercooled water film flow. The surface of icicles is typically covered with ripples of about 1 cm in wavelength, and the wavelength appears to be almost independent of external temperature, icicle radius, and volumetric water flow rate. One side of the water layer consists of the water-air surface and growing ice is the other. This is one of the more complicated moving phase boundary problems with two interfaces. A recent theoretical work [K. Ueno, Phys. Rev. E 68, (2003) 021603] to address the underlying instability that produces ripples is based on the assumption of the absence of airflow around icicles. In this paper, we extend the previous theoretical framework to include a natural convection airflow ahead of the water-air surface and consider whether the effect of natural convection airflow on the wavelength of ripples produced on an ice surface is essential or not.Comment: 19 pages, 5 figure

Compreendendo os sinais: São relevantes os modelos de medidas múltiplas da eficácia do professor para fornecer informações consistentes para professores e diretores?

There is increasing policy interest in the use of standards-based multiple measure teacher evaluation systems that include both observational and value-added measures of teacher effectiveness. The growing literature that assesses the relationships between these measures does so mainly in academic settings using a validity lens. While valuable in their own right, this evidence from research-based settings provides little evidence about how teachers and principals receive the different signals from multiple measures of effectiveness when implemented in district contexts. Using pairwise correlations and a series of ordinary least squares regressions, this study assesses the relationships between value-added measures of teacher effectiveness and an observational measure of teacher practice as implemented in a district's pilot of a new standards-based multiple-measure teacher evaluation system. We find moderate correlations between value-added and observation-based measures, indicating that teachers will receive similar but not entirely consistent signals from each performance measure. We conclude by highlighting considerations for districts working to develop and implement standards-based multiple-measure teacher evaluation systems. Cada vez hay más interés en el uso de los sistemas de medidas múltiples de evaluación docente basados en estándares con medidas de observación y de valor añadido de la eficacia docente. La creciente literatura que evalúa las relaciones entre estas medidas lo hace principalmente en el ámbito académico utilizando la lente de la validez. Si bien valiosa, la evidencia basada en la investigación proporciona poca información sobre cómo los profesores y directores reciben las diferentes señales de los múltiples medidas de efectividad cuando se implementa en contextos distritales. Utilizando correlaciones por pares y una serie de regresiones de mínimos cuadrados ordinarios este estudio evalúa la relación entre las medidas de valor agregado de la eficacia docente y una medida observacional de la práctica docente como se aplica en una prueba piloto en un distrito con un sistema de múltiples medidas. Encontramos correlaciones moderadas entre el valor agregado y las medidas basadas en la observación, lo que indica que los docentes recibían señales enteramente coherentes similares pero no de cada medida de desempeño. Concluimos resaltando consideraciones para los distritos que trabajan para desarrollar e implementar sistemas de evaluación docente de medidas múltiples basadas en estándares. Há um crescente interesse em utilizar sistemas de medidas múltiplas de avaliação de professores baseada em padrões, que incluem tanto medidas observacionais e de valor agregado sobre a eficácia do professor. A crescente literatura acadêmica avaliando a relação entre essas medidas usa principalmente o lente de validade. Embora valiosa, as pesquisas baseadas em evidências fornece pouca informação sobre como professores e diretores recebem os diferentes sinais de medidas múltiplas de eficácia quando implementado em contextos distritais. Usando correlações em pares e uma série de regressões este estudo avalia a relação entre as medidas de valor agregado de eficácia do professor e uma medida de observação da prática docente aplicada em um teste piloto em um distrito escolar com um sistema de medidas múltiplas. Foram encontradas correlações moderadas entre as medidas de valor acrescentado baseados na observação, indicando que os professores receberam sinais inteiramente coerentes semelhantes, embora não de cada medida de desempenho. Concluímos, destacando considerações para os distritos que trabalham para desenvolver e implementar sistemas de avaliação de professores com base em padrões de medidas múltiplas.

Numerical and experimental verification of a theoretical model of ripple formation in ice growth under supercooled water film flow

Little is known about morphological instability of a solidification front during the crystal growth of a thin film of flowing supercooled liquid with a free surface: for example, the ring-like ripples on the surface of icicles. The length scale of the ripples is nearly 1 cm. Two theoretical models for the ripple formation mechanism have been proposed. However, these models lead to quite different results because of differences in the boundary conditions at the solid-liquid interface and liquid-air surface. The validity of the assumption used in the two models is numerically investigated and some of the theoretical predictions are compared with experiments.Comment: 30 pages, 9 figure

Selkäreppusalauksista p-adisissa approksimaatiohiloissa

Tiivistelmä. Tällä hetkellä käytössä olevat salausmenetelmät perustuvat pääosin vaikeisiin matemaattisiin ongelmiin, kuten kokonaisluvun tekijöihinjakoon ja diskreetin logaritmin ongelmaan. Kvanttitietokoneiden kehittyessä nämä salausmenetelmät saadaan kuitenkin helposti murrettua, joten uusien salausten kehittäminen on tärkeää. Erityisesti hiloihin perustuvia salausmenetelmiä pidetään lupaavina kvanttiturvallisuuden kannalta. Selkäreppuongelmalla tarkoitetaan tilannetta, jossa on annettuna jono positiivisia kokonaislukuja ja jokin kokonaisluku S. Lukua S voidaan ajatella selkärepun tilavuutena ja jonon alkioita eri esineiden tilavuuksina. Tarkoituksena on valita jonosta sopivat esineet, joilla annettu selkäreppu saadaan tasan täyteen. Selkäreppuongelma on helppo ratkaista, jos annettuja esineitä on vähän, mutta se vaikeutuu huomattavasti, kun esineiden määrää kasvatetaan. Selkäreppuongelmaan perustuvia julkisen avaimen salausmenetelmiä kutsutaan selkäreppusalauksiksi. Merkle ja Hellman kehittivät ensimmäisen selkäreppusalauksen vuonna 1978. Sen toiminta perustui havaintoon, että selkäreppuongelma ratkeaa helposti, jos annettu jono on superkasvava. Pian kuitenkin huomattiin, että tämä salausmenetelmä on helppo murtaa esimerkiksi Shamirin hyökkäyksellä. Merklen ja Hellmanin selkäreppusalausta voidaan kehittää p-adisia lukuja ja niiden approksimaatiohiloja käyttämällä. Näin rakennetut salausmenetelmät eroavat Merklen ja Hellmanin selkäreppusalauksesta siinä, että niissä salaisena avaimena käytettävä jono ei ole superkasvava, vaan vähenevä jono p-adisia lukuja. Salaovena näissä salauksissa käytetään p-adisella itseisarvolla toteutuvaa vahvaa kolmioepäyhtälöä. Tämän eroavaisuuden vuoksi p-adiset selkäreppusalaukset kestävät edellä mainitun Shamirin hyökkäyksen. Näille salauksille voidaan kuitenkin muodostaa muita hyökkäyksiä esimerkiksi LLL-algoritmia hyödyntämällä. LLL-hyökkäyksessä lasketaan hilalle redusoidut kantavektorit ja yritetään löytää niiden joukosta selkokielinen viesti. Yksinkertaisimmassa p-adisessa selkäreppusalauksessa LLL-hyökkäyksen rakentamiseen riittää salatun viestin paljastuminen hyökkääjälle. Salauksen turvallisuutta voidaan kasvattaa lisäämällä viestin lähettäjälle yksi salainen avain enemmän. Tässä versiossa salattuun viestiin summataan yksi komponentti lisää, mikä vaikeuttaa LLL-hyökkäyksiä. Tällöin hyökkäyksen muodostamiseen ei riitä pelkkä salatun viestin paljastuminen, vaan lisäksi vaaditaan tietoa viestin lähettäjän salaisesta avaimesta tai selkokielisestä viestistä. Lisätyn avaimen avulla vastaanottaja pystyy myös varmistamaan viestin lähettäjän, mikä on merkittävä etu edelliseen selkäreppusalaukseen verrattuna. Salausten turvallisuutta voidaan lisätä kasvattamalla hilan dimensiota, koska tällöin LLL-hyökkäykset vaikeutuvat. Turvallisuuden rajana voidaan pitää hilan dimensiota 60

Digitaaliset allekirjoitukset

Tiivistelmä. Kryptografialla tarkoitetaan viestin tai tiedoston salaamista sellaiseen muotoon, että ulkopuoliset henkilöt eivät saa siitä selvää. Salattuun viestiin voidaan myös liittää digitaalinen allekirjoitus, jonka avulla viestin vastaanottaja voi varmistaa, keneltä viesti on tullut. Tutkielman alkupuolella esitellään tärkeitä lukuteoriaan liittyviä lauseita ja määritelmiä, joihin digitaalisten allekirjoitussysteemien toiminta perustuu. Tämän jälkeen käsitellään yksityiskohtaisesti ja esimerkkien avulla RSA-, ElGamal- ja DSA-allekirjoitussysteemien toimintaperiaatteet

High frequency mechanical excitation of a silicon nanostring with piezoelectric aluminum nitride layers

A strong trend for quantum based technologies and applications follows the avenue of combining different platforms to exploit their complementary technological and functional advantages. Micro and nano-mechanical devices are particularly suitable for hybrid integration due to the easiness of fabrication at multi-scales and their pervasive coupling with electrons and photons. Here, we report on a nanomechanical technological platform where a silicon chip is combined with an aluminum nitride layer. Exploiting the AlN piezoelectricity, Surface Acoustic Waves are injected in the Si layer where the material has been localy patterned and etched to form a suspended nanostring. Characterizing the nanostring vertical displacement induced by the SAW, we found an external excitation peak efficiency in excess of 500 pm/V at 1 GHz mechanical frequency. Exploiting the long term expertise in silicon photonic and electronic devices as well as the SAW robustness and versatility, our technological platform represents a strong candidate for hybrid quantum systems

Scaling in the correlation energies of two-dimensional artificial atoms

We find an unexpected scaling in the correlation energy of artificial atoms, i.e., harmonically confined two-dimensional quantum dots. The scaling relation is found through extensive numerical examinations including Hartree-Fock, variational quantum Monte Carlo, density-functional, and full configuration-interaction calculations. We show that the correlation energy, i.e., the true ground-state total energy subtracted by the Hartree-Fock total energy, follows a simple function of the Coulomb energy, confimenent strength and, the number of electrons. We find an analytic expression for this function, as well as for the correlation energy per particle and for the ratio between the correlation and total energies. Our tests for independent diffusion Monte Carlo and coupled-cluster results for quantum dots -- including open-shell data -- confirm the generality of the obtained scaling. As the scaling is also well applicable to $\gtrsim$ 100 electrons, our results give interesting prospects for the development of correlation functionals within density-functional theory.Comment: Accepted to Journal of Physics: Condensed Matte

Phonological development of Finnish speaking children at 3;6 and associations to previous and simultaneous lexical ability

Previous studies of Finnish children's phonological development focus mainly on children under 2;0. Earlier findings have suggested that phonological and lexical development are strongly associated at an early age. However, less is known about the longitudinal association. This study describes the phonological skills of Finnish-speaking children at 3;6 and compares them with early lexicon size at 2;0 and lexical ability at 3;6 (N = 67). The children's phonological development was measured using The Finnish Phonology Test. Lexical development was evaluated using the Finnish, long-form version of the Communicative Development Inventory at 2;0 and the Boston naming test at 3;6 At 3;6, all children mastered the vowels and diphthongs fully, and most of the children also mastered the consonants, with the exception of the phonemes/d/and/r/. Phonotactic skills had also been acquired well at this group level, although the word-medial and, especially, word-initial consonant clusters were still challenging. The percentage of phonemes correct was 95. Both paradigmatic and phonotactic skills at 3;6 were significantly associated with lexicon size at 2;0. In addition phonotactic skills correlated with naming ability at 3;6. Lexical development at 2;0 explained 21% of the variance in the phonological development at 3;6, whereas, the explaining value of simultaneous lexical skill was limited (9%). Present findings propose that associations between lexical and phonological skills weaken as phonological skills become more honed.Peer reviewe

Vacancy-Impurity Complexes in Highly Sb-Doped Si Grown by Molecular Beam Epitaxy

Positron annihilation measurements, supported by first-principles electron-structure calculations, identify vacancies and vacancy clusters decorated by 1–2 dopant impurities in highly Sb-doped Si. The concentration of vacancy defects increases with Sb doping and contributes significantly to the electrical compensation. Annealings at low temperatures of 400–500 K convert the defects to larger complexes where the open volume is neighbored by 2–3 Sb atoms. This behavior is attributed to the migration of vacancy-Sb pairs and demonstrates at atomic level the metastability of the material grown by epitaxy at low temperature.Peer reviewe