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Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2019"








Journal ArticleDOI
TL;DR: In this paper, the shear modulus of open cell polyurethane thermoformed auxetic foams from 3-point and 4-point bending tests was identified.
Abstract: This work describes the identification of the shear modulus of open cell polyurethane thermoformed auxetic foams from 3-point and 4-point bending tests. The foams are incorporated in sandwich beams with carbon fibre/epoxy face skins, and benchmarked against similar sandwich structures made with the conventional counterpart open cell foam. Three types of beams are tested: one with auxetic foams, another type related to a conventional foam core with the same thickness of the auxetic porous materials, and a third type of beam consisting in conventional foam with a thickness corresponding to an iso-weight configuration to the auxetic specimen. The auxetic foam has a shear modulus 7% lower than the one of the bulk conventional specimens, but higher shear stresses at large deformations and a smoother strain stiffening response compared to the beams with the conventional thinner core. The paper also highlights the low shear wave speed of these auxetic foams compared to other porous polymers used in helmet and head protection applications, as well as potential uses of the quasi-zero-stiffness behaviour here observed for the auxetic foam sandwich beam.

31 citations





Journal ArticleDOI
TL;DR: In this paper, the effects of thermo-mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open-cell polyurethane foam are related to changes in chemical bonding and cell structure.
Abstract: The effects of thermo-mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open-cell polyurethane foam are related to changes in chemical bonding and cell structure. Applied volumetric compression, conversion temperature and duration are reported on foam Young's modulus, Poisson's ratio and structural stability. Fourier transform infrared spectral analysis on samples converted at and above 160°C strongly indicates a hydrogen bond interaction increase in urea groups (C=O---H-N) and an increase in hydrogen bonding population. Spectral changes inferred soft segment degradation following extensive heat exposure (200°C, 180 minutes), specifically a shift and intensity change in CH2 and C-O-C polyol bands and a broad baseline increase between 3600 and 2400 cm-1. These changes are linked to: i) resistance to dimensional recovery over time and during re-heating, ii) Poisson's ratio becoming negative, then zero in tension or marginally positive in compression, iii) Young's Modulus reducing then increasing, iv) mass loss, evidenced by thermogravimetric analysis increasing from 150°C. The minimum mean values of Poisson's ratio of ~-0.2 (to 10% compression/tension) are comparable to other studies. All samples that retain their imposed compression over time are isotropic, with near constant Young's moduli and Poisson's ratio (to 10 % compression/tension).



Journal ArticleDOI
TL;DR: In this paper, the authors investigated the properties of Cu2MnSnS4 for photovoltaic applications such as containing only earth abundant and non-toxic elements, and suitable absorp...
Abstract: Cu2MnSnS4 shares several promising properties with the widely investigated Cu2ZnSnS4 for photovoltaic applications such as containing only earth abundant and non-toxic elements, and suitable absorp ...


Journal ArticleDOI
TL;DR: In this article, a Raman-optimized (RO) device substrate and an optimized mapping approach are presented for acquisition of high-resolution Raman spectra, achieving enhancement factors as high as 120 with respect to signals measured on standard SiO2/Si substrates.
Abstract: The on-surface synthesis of graphene nanoribbons (GNRs) allows for the fabrication of atomically precise narrow GNRs. Despite their exceptional properties which can be tuned by ribbon width and edge structure, significant challenges remain for GNR processing and characterization. In this contribution, we use Raman spectroscopy to characterize different types of GNRs on their growth substrate and to track their quality upon substrate transfer. We present a Raman-optimized (RO) device substrate and an optimized mapping approach that allows for acquisition of high-resolution Raman spectra, achieving enhancement factors as high as 120 with respect to signals measured on standard SiO2/Si substrates. We show that this approach is well-suited to routinely monitor the geometry-dependent low-frequency modes of GNRs. In particular, we track the radial breathing-like mode (RBLM) and the shear-like mode (SLM) for 5-, 7- and 9-atom wide armchair GNRs (AGNRs) and compare their frequencies with first-principles calculations.


Journal ArticleDOI
TL;DR: The pairing mechanism in these systems is due to short range attractive interactions between fermions and the size of the Cooper pair is either comparable to the interparticle separation or it can be as big as a nucleus, which is still relatively small in size.
Abstract: In cold atoms and in the crust of neutron stars the pairing gap can reach values comparable with the Fermi energy. While in nuclei the neutron gap is smaller, it is still of the order of a few percent of the Fermi energy. The pairing mechanism in these systems is due to short range attractive interactions between fermions and the size of the Cooper pair is either comparable to the inter-particle separation or it can be as big as a nucleus, which is still relatively small in size. Such a strong pairing gap is the result of the superposition of a very large number of particle-particle configurations, which contribute to the formation of the Copper pairs. These systems have been shown to be the host of a large number of remarkable phenomena, in which the large magnitude of the pairing gap plays an essential role: quantum shock waves, quantum turbulence, Anderson-Higgs mode, vortex rings, domain walls, soliton vortices, vortex pinning in neutron star crust, unexpected dynamics of fragmented condensates and role of pairing correlations in collisions on heavy-ions, Larkin-Ovchinnikov phase as an example of a Fermi supersolid, role pairing correlations control the dynamics of fissioning nuclei, self-bound superfluid fermion droplets of extremely low densities.





Journal ArticleDOI
TL;DR: In order to predict and establish spin Hamiltonians for real magnetic materials, a variety of first principles have been developed, based on density functional theory and wave function methodologies, with a focus on their practical capabilities and limitations.
Abstract: Implicit in the study of magnetic materials is the concept of spin Hamiltonians, which emerge as the low-energy theories of correlation-driven insulators. In order to predict and establish such Hamiltonians for real materials, a variety of first principles $ab$-$initio$ methods have been developed, based on density functional theory and wavefunction methodologies. In this review, we provide a basic introduction to such methods and the essential concepts of low-energy Hamiltonians, with a focus on their practical capabilities and limitations. We further discuss our recent efforts toward understanding a variety of complex magnetic systems that present unique challenges from the perspective of $ab$-$initio$ approaches.


Journal ArticleDOI
TL;DR: On the Compressibility Properties of the Wine-Rack like Carbon Allotropes and Related Poly(phenylacetylene) Systems Edera P. Degabriele, Daphne Attard, James N. Grima-Cornish, Roberto Caruana-Gauci, Ruben Gatt1, Kenneth E. Evans3 and Joseph N Grima* as mentioned in this paper
Abstract: On the Compressibility Properties of the Wine-Rack like Carbon Allotropes and Related Poly(phenylacetylene) Systems Edera P. Degabriele1, Daphne Attard1, James N. Grima-Cornish2, Roberto Caruana-Gauci1, Ruben Gatt1, Kenneth E. Evans3 and Joseph N. Grima*1,2 1Metamaterials Unit, Faculty of Science, University of Malta, Msida MSD 2080, Malta. 2Department of Chemistry, Faculty of Science, University of Malta, Msida MSD 2080, Malta. 3College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK.


Journal ArticleDOI
TL;DR: In this paper, an isolated skyrmion in a square-lattice cluster, interacting with electron spins in a current-carrying quantum wire, is considered, and a quantum formulation of spin-dependent currents via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff-Baym ansatz (GKBA) is employed.
Abstract: Magnetic skyrmions, topologically protected vortex-like configurations in spin textures, are of wide conceptual and practical appeal, notably in relation to the making of so-called race-track memory devices. Skyrmions can be created, steered, and destroyed with magnetic fields and/or (spin) currents. Here the authors focus on the latter mechanism, analyzed via a microscopic treatment of the skyrmion–current interaction. The system considered is an isolated skyrmion in a square-lattice cluster, interacting with electron spins in a current-carrying quantum wire. For the theoretical description, a quantum formulation of spin-dependent currents via nonequilibrium Green's functions (NEGF) within the generalized Kadanoff–Baym ansatz (GKBA) is employed. This is combined with a treatment of skyrmions based on classical localized spins, with the skyrmion motion described via Ehrenfest dynamics. With the mixed quantum–classical scheme, the authors assess how time-dependent currents can affect the skyrmion dynamics, and how this in turn depends on electron–electron and spin–orbit interactions in the wire. This study shows the usefulness of a quantum–classical treatment of skyrmion steering via currents, as a way for example to validate/extract an effective, classical-only, description of skyrmion dynamics from a microscopic quantum modeling of the skyrmion–current interaction. (Less)


Journal ArticleDOI
TL;DR: In this paper, the authors compare two different transition-metal dichalcogenide (TMDC) combinations: MoS2-WSe2 and MoSe2-Se2.
Abstract: In heterobilayers consisting of different transition-metal dichalcogenide (TMDC) monolayers, optically excited electron-hole pairs can be spatially separated into the adjacent layers due to a type-II band alignment. However, they remain Coulomb correlated and form interlayer excitons (ILEs), which recombine radiatively. While these ILEs are observed in several TMDC material combinations, their characters and properties depend on the specific system. Herein, some of these peculiarities are demonstrated by comparing studies performed on two different heterobilayer combinations: MoS2-WSe2 and MoSe2-WSe2.

Journal ArticleDOI
TL;DR: In this paper, the correlated femtosecond electron dynamics in finite graphene clusters, such as nanoribbons, are described by an extended Hubbard model that takes into account the overlap of adjacent orbitals and hopping between up to third-nearest neighbors.
Abstract: A new approach to study the correlated femtosecond electron dynamics in finite graphene clusters, such as nanoribbons, is presented here. The systems are described by an extended Hubbard model that takes into account the overlap of adjacent orbitals and hopping between up to third-nearest neighbors. The model is solved by the nonequilibrium Green functions approach combined with different self-energy approximations, including the second-Born and GW self-energy, to take into account electronic correlations. The description allows us to predict the correlated nonequilibrium dynamics of excited graphene nanostructures of arbitrary geometry containing up to 100 carbon atoms for up to 25 fs.