Two-Dimensional Phononic Crystals: Disorder Matters

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201

Revista Praticando Capoeira: materialidade e representações

O presente estudo analisa a revista Praticando Capoeira, entendendo-a como vestígios de uma rede de práticas, capaz de dar a ver a luta de sentidos em que foi produzida e as relações de força que determinaram sua estruturação. O impresso em questão foi produzido pela editora D+T a partir de 1999, vendido principalmente em bancas de revistas até 2009, período em que foram publicadas 44 edições. Por intermédio dos dispositivos editoriais do impresso e pelas representações elaboradas e postas a circular, buscou-se compreender a organização do mundo da capoeira, considerado como um campo social. Uma revista, por sua agilidade de produção e circulação, consegue captar aspectos do cotidiano ao mesmo tempo em que as informações, os embates e as mudanças acontecem. Para trabalharmos com esse objeto, que se converte em fonte de pesquisa, são necessários considerar três elementos distintos, mas articulados, isto é, considerá-lo como suporte de textos, além de atentar para os próprios textos e para o discurso produzido pelo encadeamento de sentidos no ato da leitura. Ancorado na História Cultural e nas reflexões sobre as lutas de representação, a pesquisa entende a coleção de revistas como objetos culturais que guardam as marcas de sua fabricação. Com base na materialidade da revista, trabalha-se com o periódico nos termos de uma arqueologia dos objetos, em busca de pistas, para localizar os sinais das práticas editoriais, capazes de revelar os vestígios das estratégias, utilizadas pelos editores para a organização e regramento da leitura do impresso. A revista Praticando Capoeira é um documento e um monumento sobre o mundo da capoeira, uma testemunha do presente para o futuro, que proporciona a recordação do nosso tempo. As representações estão nos textos e imagens, materializadas no objeto em função de sua fórmula editorial. Identificamos que a revista apresenta a organização do campo com base nos grupos de capoeira e considera a prática da capoeiragem como um dos elementos da identidade brasileira, com potencial esportivo e pedagógico

Acoustic phonon propagation in ultra-thin Si membranes under biaxial stress field

We report on stress induced changes in the dispersion relations of acoustic phonons propagating in 27 nm thick single crystalline Si membranes. The static tensile stress (up to 0.3 GPa) acting on the Si membranes was achieved using an additional strain compensating silicon nitride frame. Dispersion relations of thermally activated hypersonic phonons were measured by means of Brillouin light scattering spectroscopy. The theory of Lamb wave propagation is developed for anisotropic materials subjected to an external static stress field. The dispersion relations were calculated using the elastic continuum approximation and taking into account the acousto-elastic effect. We find an excellent agreement between the theoretical and the experimental dispersion relations

Thermal conductivity of MoS2 polycrystalline nanomembranes

Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. Afavorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS₂ nanosheets in a nanoscale grain-size limit.Wereport thermal conductivity measurements on 5 nm thick polycrystalline MoS₂ by means of 2-laser Raman thermometry. The free-standing, drum-like MoS₂ nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS₂ and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 W m⁻¹ K⁻¹. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed

Phonon Transport in the Gigahertz to Terahertz Range: Confinement, Topology and Second Sound

Perspectives on studying phonon transport in the gigahertz to terahertz range with Brillouin light scattering and frequency-domain thermoreflectance.Transport of heat and hypersound with gigahertz (GHz) to terahertz (THz) phonons is crucial for heat management in electronics, mediating signal processing with microwave radiation, thermoelectrics, and various types of sensors based on nanomechanical resonators. Efficient control of heat and sound transport requires new materials, novel experimental techniques, and a detailed knowledge of the interaction of phonons with other elementary excitations. Wave-like heat transport, also known as second sound, has recently attracted renewed attention since it provides several opportunities for overcoming some of the limitations imposed by diffusive transport (Fourier’s regime). The frequency-domain detection of GHz-to-THz phonons can be carried out in a remote, non-destructive, and all-optical manner. The ongoing development of nanodevices and metamaterials made of low-dimensional nanostructures will require spatially resolved, time-resolved, and anisotropic measurements of phonon-related properties. These tasks can be accomplished with Brillouin light scattering (BLS) and various newly developed variants of this method, such as pumped-BLS. In the near future, pumped-BLS is expected to become useful for characterizing GHz topological nanophononics. Finally, second-sound phenomena can be observed with all-optical methods like frequency-domain thermoreflectance.This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme (No. 101003436) and the Polish National Science Centre (No. UMO-2018/31/D/ST3/03882). J.S.R. acknowledges financial support from the Spanish Ministerio de Economıa, Industria y Competitividad for its support through Grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program and Grant No. PID2020-119777GB-I0016 (THERM2MAIN)

Imperfect phononic crystals work too : The effect of translational and mid-plane symmetry breaking on hypersound propagation

Translationally symmetric nanostructures, termed phononic crystals (PnCs), offer control over the propagation of acoustic phonons in the gigahertz (GHz) range for signal-processing applications and thermal management at sub-Kelvin temperatures. In this work, we utilize Brillouin light scattering to investigate the impact of symmetry breaking on GHz phonon propagation in PnCs made of holey silicon nanomembranes. We show that the lattice of thimble-like holes leads to broken mid-plane symmetry and, hence, to anticrossing acoustic band gaps. With the rising level of uncorrelated translational disorder, the phononic effects are gradually suppressed, starting at higher frequencies. Strikingly, the low-frequency partial Bragg bandgap remains robust up to the highest level of disorder

Progress and Perspectives on Phononic Crystals

Review and Perspectives article on recent developments and future prospects of phononic crystals.Phononic crystals (PnCs) control the transport of sound and heat similar to the control of electric currents by semiconductors and metals or light by photonic crystals. Basic and applied research on PnCs spans the entire phononic spectrum, from seismic waves and audible sound to gigahertz phononics for telecommunications and thermal transport in the terahertz range. Here, we review the progress and applications of PnCs across their spectrum, and we offer some perspectives in view of the growing demand for vibrational isolation, fast signal processing, and miniaturization of devices. Current research on macroscopic low-frequency PnCs offers complete solutions from design and optimization to construction and characterization, e.g., sound insulators, seismic shields, and ultrasonic imaging devices. Hypersonic PnCs made of novel low-dimensional nanomaterials can be used to develop smaller microelectromechanical systems and faster wireless networks. The operational frequency, compactness, and efficiency of wireless communications can also increase using principles of optomechanics. In the terahertz range, PnCs can be used for efficient heat removal from electronic devices and for novel thermoelectrics. Finally, the introduction of topology in condensed matter physics has provided revolutionary designs of macroscopic sub-gigahertz PnCs, which can now be transferred to the gigahertz range with advanced nanofabrication techniques and momentum-resolved spectroscopy of acoustic phonons.This project has received funding from the European Union’s Horizon 2020 research and innovation programme (No. 101003436), the Foundation for Polish Science (No. POIR.04.04.00-00-5D1B/18), and the Polish National Science Centre (No. UMO-2018/31/D/ST3/03882)

Naturen i Oslo er verdt milliarder. Verdsetting av urbane økosystemtjenester fra grønnstruktur

mosaikk av grønne parker, frodige bakgårder, kolonihager, urbane skoger, våtmarker, bekker, lver, innsjøer, gamle trær, kystlinje, fjord og øyer, som bidrar til innbyggernes livskvalitet ved å i mulighet for rekreasjon, læring og andre opplevelser. I tillegg til dette utgjør urbane økosyste- mene viktige leveområder for det rike biologiske mangfoldet vi finner i Oslo og bidrar til å regulere aturlige prosesser i byen. Tap av grønnstruktur og reduksjon av økosystemtjenestene de leve- er vil kunne medføre betydelige kostnader, for eksempel som følge av redusert evne til flom- emping eller i form av redusert livskvalitet og dårligere helse for innbyggerne. I tillegg reduseres yens attraktivitet for næringsliv og turisme. Økonomisk verdsetting av urbane økosystemtjenester. Urbane økosystemtjenester kommer n stor befolkning til gode og flere bystudier viser at de kan ha økonomiske verdier i milliardklas- en. For å kunne gjøre et raskt anslag av den økonomiske betydningen av grønnstruktur må man velge økosystemtjenester som relativt lett kan kvantifiseres, samtidig som de representerer verdier for mange på tvers av byen. Økonomiske verdsettingsstudier viser at regulerende øko- ystemtjenester som flomdemping, og støttende økosystemtjenester som leveområde for dyr krever inngående studier. Fritidsbruk kan være lettere å anslå samtidig som studier viser at re- kreasjon ofte er den dominerende verdien for bybefolkningen; gjennom daglige opplevelsesver- ier og mer langsiktig allmenn psykisk og fysisk helse. Denne rapporten er i hovedsak en verdioverføringsstudie. Verdioverføring er nødvendig om øko- omisk verdsetting av økosystemtjenester skal gjennomføres når man ikke har tid eller ressurser il ny datainnsamling. Med eksemplene for Oslo i denne rapporten har vi ønsket å se hvor langt vi kunne komme ved å bruke forholdsvis enkel metoder som ikke krever nye undersøkelser, for å vise at verdsetting kan gjøres med relativt enkle grep og gi verdifull informasjon til arbeid med g debatt om byutvikling. Verdien av erstatningsansvar for bytrær. Det finnes 0,7-1,2 millioner bytrær med høyde over 5 meter i Oslos byggesone. Hver Osloborger deler altså byrommet med 1-2 store trær. Oslo Kommune krever at skade på bytrær eid av det offentlige erstattes etter en bestemt takstmodell om tar høyde for treets tilstand og stedsspesifikke kvaliteter, deriblant økosystemtjenester. akstmodellen viser at Oslo kommune har satt en gjennomsnittsverdi på ca. 40 000 kroner på ytrærne på offentlig grunn. Vi har anvendt takstmodellen på alle bytrær (både på privat og of- entlig grunn) i byggesonen for å anslå den samlede verdien av disse bytrærne. Samlet erstat- ingsansvar for alle store bytrær i byggesonen blir da på mellom 28 og 42 milliarder kroner (av- engig av antall og kvaliteten på bytrærne). Dette tallet utgjør en kapitalverdi (altså ikke per år). Norge, Oslo, urbane økosystemtjenester, grønnstruktur, Økonomisk verdsetting, Urban ecosystem services, Blue-green infrastructure, economic valuation, Norway, Oslo© Norsk institutt for naturforskning. Publikasjonen kan siteres fritt med kildeangivelse

Hypersonic phonon propagation in one-dimensional surface phononic crystal

Hypersonic, thermally activated surface acoustic waves propagating in the surface of crystalline silicon patterned with periodic stripes were studied by Brillouin light scattering. Two characteristic directions (normal and parallel to the stripes) of surface acoustic waves propagation were examined exhibiting a distinctive propagation behavior. The measured phononic band structure exhibits diverse features, such as zone folding, band gap opening, and hybridization to local resonance for waves propagating normal to the stripes, and a variety of dispersive modes propagating along the stripes. Experimental results were supported by theoretical calculations performed using finite element method