Polydopamine-Coated Poly-Lactic Acid Aerogels as Scaffolds for Tissue Engineering Applications

Poly-L-lactic acid (PLLA) aerogel-based scaffolds were obtained from physical PLLA gels containing cyclopentanone (CPO) or methyl benzoate (BzOMe) molecules. An innovative single step method of solvent extraction, using supercritical CO2, was used to achieve cylindrical monolithic aerogels. The pore distribution and size, analyzed by SEM microscopy, were found to be related to the crystalline forms present in the physical nodes that hold the gels together, the stable alpha'-form and the metastable co-crystalline epsilon-form, detected in the PLLA/BzOMe and PLLA/CPO aerogels, respectively. A higher mechanical compressive strength was found for the PLLA/CPO aerogels, which exhibit a more homogenous porosity. In vitro biocompatibility tests also indicated that monolithic PLLA/CPO aerogels exhibited greater cell viability than PLLA/BzOMe aerogels. An improved biocompatibility of PLLA/CPO monolithic aerogels was finally observed by coating the surface of the aerogels with polydopamine (PDA) obtained by the in situ polymerization of dopamine (DA). The synergistic effect of biodegradable polyester (PLLA) and the biomimetic interface (PDA) makes this new 3D porous scaffold, with porosity and mechanical properties that are tunable based on the solvent used in the preparation process, attractive for tissue engineering applications

Direct femtosecond laser ablation of copper with an optical vortex beam

Laser surface structuring of copper is induced by laser ablation with a femtosecond optical vortex beam generated via spin-to-orbital conversion of the angular momentum of light by using a q-plate. The variation of the produced surface structures is studied as a function of the number of pulses, N, and laser fluence, F. After the first laser pulse (N = 1), the irradiated surface presents an annular region characterized by a corrugated morphology made by a rather complex network of nanometer-scale ridges, wrinkles, pores, and cavities. Increasing the number of pulses (2 < N < 100), the surface texture progressively evolves towards larger structures, while the central, non-ablated area is gradually decorated by nanoparticles produced during laser ablation. At large number of pulses (200 < N < 1000), a micro-tip with a nanostructured surface forms in the center of the irradiated area, which eventually disappears at still larger number of pulses (N > 1000) and a deep crater is formed. The nanostructure variation with the laser fluence, F, also evidences an interesting dependence, with a coarsening of the structure morphology as F increases. Our experimental findings demonstrate that direct femtosecond laser ablation with optical vortex beams produces interesting patterns not achievable by the more standard beams with a Gaussian intensity profile. They also suggest that appropriate tuning of the experimental conditions (F, N) can allow generating micro- and/or nano-structured surface for any specific application

Vector vortex beams generated by q-plates as a versatile route to direct fs laser surface structuring

We report an experimental investigation on direct laser surface structuring with femtosecond vector vortex beams generated by means of q-plates with topological charges q = 1, 3/2, 2, 5/2. Structured light beams with spatially variant state of polarization and intensity are generated and applied to multi-pulse irradiation of a solid crystalline silicon target. The creation of a variety of surface structures, like laser induced periodic surface structures, multi-spot arrays and shaped ablation craters, is demonstrated by direct laser surface structuring with vector vortex beams at different values of q. The features of the surface structures are compared with the vector vortex beam characteristics at the focal plane, evidencing their relationship with the polarization and intensity profile of the laser beams. Our experimental findings show that vector vortex beams produced by q-plates can offer a valuable and versatile route to imprint unconventional surface structures on a solid target through a mask-free ablative process and step scan processing

Surface structures with unconventional patterns and shapes generated by femtosecond structured light fields

We present an investigation on ultrashort laser surface structuring with structured light fields generated by various q-plates. In particular, q-plates with topological charges q = 1, 3/2, 2, 5/2 are used to generate femtosecond (fs) vector vortex beams, and form complex periodic surface structures through multi-pulse ablation of a solid crystalline silicon target. We show how optical retardation tuning of the q-plate offers a feasible way to vary the fluence transverse distribution of the beam, thus allowing the production of structures with peculiar shapes, which depend on the value of q. The features of the generated surface structures are compared with the vector vortex beam characteristics at the focal plane, by rationalizing their relationship with the local state of the laser light. Our experimental findings demonstrate how irradiation with fs complex light beams can offer a valuable route to design unconventional surface structures

Surface Structuring with Polarization-Singular Femtosecond Laser Beams Generated by a q-plate

In the last few years femtosecond optical vortex beams with di erent spatial distributions of the state of polarization (e.g. azimuthal, radial, spiral, etc.) have been used to generate complex, regular surface patterns on di erent materials. Here we present an experimental investigation on direct femtosecond laser surface structuring based on a larger class of vector beams generated by means of a q-plate with topological charge q = +1/2. In fact, voltage tuning of q-plate optical retardation allows generating a family of ultrashort laser beams with a continuous spatial evolution of polarization and uence distribution in the focal plane. These beams can be thought of as a controlled coherent superposition of a Gaussian beam with uniform polarization and a vortex beam with a radial or azimuthal state of polarization. The use of this family of ultrashort laser beams in surface structuring leads to a further extension of the achievable surface patterns. The comparison of theoretical predictions of the vector beam characteristics at the focal plane and the generated surface patterns is used to rationalize the dependence of the surface structures on the local state of the laser beam, thus o ering an e ective way to either design unconventional surface structures or diagnose complex ultrashort laser beams

Symmetry, spin and orbital character of a van-Hove singularity in proximity to a Lifshitz transition in Sr4_4Ru3_3O10_{10}

The physics of strongly correlated electron materials is often governed by Van Hove singularities (VHss) in the vicinity of the Fermi energy. The divergence of the density of states generated by the VHss can promote electron-electron interactions and the emergence of new phases such as superconductivity, ferromagnetism, metamagnetism, nematicity and density wave orders. The shape and intensity of this divergence depends sensitively on the order and symmetry of the VHs, and hence a detailed understanding of the low-energy electronic structure is essential to understand the role of VHss in emergent phases. A family of materials with a large diversity of emergent phases that can be related to VHss close to the Fermi energy is the Ruddlesden-Popper series of the strontium ruthenates. Here we study the low-energy electronic structure at the surface of ferromagnetic Sr$_4$Ru$_3$O$_{10}$ by scanning tunneling microscopy and spectroscopy at millikelvin temperatures. We identify multiple VHss close to the Fermi energy and establish their spin character. Using quasiparticle interference we extract the orbital character and symmetry of the VHs closest to the Fermi energy, enabling us to identify a new mechanism for a field-induced Lifshitz transition facilitated by spin-orbit coupling as the origin of the metamagnetic behaviour in Sr$_4$Ru$_3$O$_{10}$.Comment: 25 pages, 5 figures and supplementary materia

Multiferroic quantum material Ba2Cu1−xMnxGe2O7 (0 ≤ x ≤ 1) as a potential candidate for frustrated Heisenberg antiferromagnet

Multiferroic Ba2CuGe2O7 was anticipated as a potential member of the exciting group of materials hosting a skyrmion or vortex lattice because of its profound Dzyaloshinskii–Moriya interaction (DMI) and the absence of single ion anisotropy (SIA). This phase, however, could not be evidenced and instead, it exhibits a complex incommensurate antiferromagnetic (AFM) cycloidal structure. Its sister compound Ba2MnGe2O7, in contrast, is characterized by a relatively strong in-plane exchange interaction that competes with a non-vanishing SIA and the weak DMI, resulting in a quasi-two-dimensional commensurate AFM structure. Considering this versatility in the magnetic interactions, a mixed solid solution of Cu and Mn in Ba2Cu1−xMnxGe2O7 can hold an interesting playground for its interactive DMI and SIA depending on the mixed spin states of the transition metal ions towards the skyrmion physics. Here, we present a detailed study of the micro- and macroscopic spin structure of the Ba2Cu1−xMnxGe2O7 solid solution series using high-resolution neutron powder diffraction techniques. We have developed a remarkably rich magnetic phase diagram as a function of the applied magnetic field and x, which consists of two end-line phases separated by a potentially quantum-critical phase at x = 0.57. An AFM conical structure at zero magnetic field is demonstrated to persist up to x = 0.50. Our results provide crucial information on the spin structure and magnetic properties, which are necessary for the general understanding and theoretical developments on multiferroicity in the frame of skyrmion type or frustrated AFM lattice where DMI and SIA play an important role

Interplay of ferromagnetism and spin-orbit coupling in Sr4Ru3O10

Funding: IB acknowledges funding through the International Max Planck Research School for Chemistry and Physics of Quantum Materials, MN, CT and PW through EP/R031924/1 and EP/T031441/1 and LCR from the Royal Commission of the Exhibition of 1851. TEM measurements were supported through grants EP/R023751/1, EP/L017008/1 and EP/T019298/1. YN acknowledges support through the ERC grant ERC-714193-QUESTDO held by Phil King.The ground state of metamagnetic materials can be controlled by magnetic field, promising new functionalities for spintronics applications. Yet, a microscopic understanding of the interplay of the electronic structure with the susceptibility to emergent orders is often missing, but would greatly facilitate optimization of the properties of metamagnetic materials. Here, we use low temperature scanning tunneling microscopy (STM) and spectroscopy to study the metamagnetism in the trilayer ruthenate Sr4Ru3O10, combining STM-based magnetostriction measurements with quasiparticle-interference imaging (QPI) to elucidate the role of the microscopic electronic structure in the macroscopic metamagnetic properties. Our results highlight the importance of the orthorhombicity of the material for its metamagnetic properties, confirmed by magnetization measurements. Our QPI results show clear signatures of the minority spin bands crossing the Fermi energy, and provide a link between the ferromagnetic properties, spin-orbit coupling and the orthorhombicity of the crystal structure.PostprintPeer reviewe

Magnetic-field tunable intertwined checkerboard charge order and nematicity in the surface layer of Sr2RuO4

C.A.M. acknowledges funding from EPSRC through EP/L015110/1, LCR from the Royal Commission for the Exhibition of 1851, A.W.R. from EPSRC through EP/P024564/1, P.W. from EPSRC through EP/R031924/1, and C.M.Y. and P.W. through EP/S005005/1. V.G., R.F., R.B., A.G., A.V. and P.W. acknowledge support from the Bilateral Project "Atomic-scale imaging of the superconducting condensate in the putative triplet superconductor Sr2RuO4: a platform for topological quantum computations?" in a joint Royal Society of Edinburgh and CNR Bilateral Scheme CUP B56C18003920005.In strongly correlated electron materials, the electronic, spin, and charge degrees of freedom are closely intertwined. This often leads to the stabilization of emergent orders that are highly sensitive to external physical stimuli promising opportunities for technological applications. In perovskite ruthenates, this sensitivity manifests in dramatic changes of the physical properties with subtle structural details of the RuO6 octahedra, stabilizing enigmatic correlated ground states, from a hotly debated superconducting state via electronic nematicity and metamagnetic quantum criticality to ferromagnetism. Here, it is demonstrated that the rotation of the RuO6 octahedra in the surface layer of Sr2RuO4 generates new emergent orders not observed in the bulk material. Through atomic-scale spectroscopic characterization of the low-energy electronic states, four van Hove singularities are identified in the vicinity of the Fermi energy. The singularities can be directly linked to intertwined nematic and checkerboard charge order. Tuning of one of these van Hove singularities by magnetic field is demonstrated, suggesting that the surface layer undergoes a Lifshitz transition at a magnetic field of ≈32T. The results establish the surface layer of Sr2RuO4 as an exciting 2D correlated electron system and highlight the opportunities for engineering the low-energy electronic states in these systems.Publisher PDFPeer reviewe