Physics - Materials Science Publications (50)

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Physics - Materials Science Publications

In this letter, we report on a family of subwavelength wave manipulation effects experimentally observed in metamaterial plates endowed with heterogeneous populations of tunable resonators. We first document effects in terms of wave attenuation, resulting from the interplay between disorder and heterogeneity, illustrating how random spatial arrangements outperform functionally-graded architectures in terms of achievable total bandgap width. We then proceed to demonstrate the onset of subwavelength localization phenomena---whose manifestation is marked by an unconventional bandgap "push-up" that appears to be germane to the problem of elastic wave propagation. Read More


The magnetic interaction between rare-earth and Fe ions in hexagonal rare-earth ferrites (h-REFeO3), may amplify the weak ferromagnetic moment on Fe, making these materials more appealing as multiferroics. To elucidate the interaction strength between the rare-earth and Fe ions as well as the magnetic moment of the rare-earth ions, element specific magnetic characterization is needed. Using X-ray magnetic circular dichroism, we have studied the ferrimagnetism in h-YbFeO3 by measuring the magnetization of Fe and Yb separately. Read More


We investigate the effect on disorder potential on exciton valley polarization and valley coherence in monolayer WSe2. By analyzing polarization properties of photoluminescence, the valley coherence (VC) and valley polarization (VP) is quantified across the inhomogeneously broadened exciton resonance. We find that disorder plays a critical role in the exciton VC, while minimally affecting VP. Read More


2017Mar
Affiliations: 1School of Physics and Astronomy and CSEC, University of Edinburgh, Edinburgh, UK, 2School of Physics and Astronomy and CSEC, University of Edinburgh, Edinburgh, UK, 3School of Physics and Astronomy and CSEC, University of Edinburgh, Edinburgh, UK, 4ID28, European Synchrotron Radiation Facility, Grenoble, France, 5School of Physics and Astronomy and CSEC, University of Edinburgh, Edinburgh, UK, 6School of Physics and Astronomy and CSEC, University of Edinburgh, Edinburgh, UK

We report that the lowest energy transverse-optic phonon in metallic SnTe softens to near zero energy at the structural transition at $T_C=75 \text{~K}$ and importantly show that the energy of this mode below $T_C$ increases as the temperature decreases. Since the mode is a polar displacement this proves unambiguously that SnTe undergoes a ferroelectric displacement below $T_C$. Concentration gradients and imperfect stoichiometry in large crystals may explain why this was not seen in previous inelastic neutron scattering studies. Read More


We review recent developments in electronic structure calculations that go beyond state-of-the-art methods such as density functional theory (DFT) and dynamical mean field theory (DMFT). Specifically, we discuss the following methods: GW as implemented in the Vienna {\it ab initio} simulation package (VASP) with the self energy on the imaginary frequency axis, GW+DMFT, and ab initio dynamical vertex approximation (D$\Gamma$A). The latter includes the physics of GW, DMFT and non-local correlations beyond, and allows for calculating (quantum) critical exponents. Read More


We investigate the superconducting-gap anisotropy in one of the recently discovered BiS$_2$-based superconductors, NdO$_{0.71}$F$_{0.29}$BiS$_2$ ($T_c$ $\sim$ 5 K), using laser-based angle-resolved photoemission spectroscopy. Read More


We consider the magnetic properties of trinuclear coordinated organometallic complexes in which Coulomb interactions and spin molecular-orbital coupling are relevant. The Mo3S7(dmit)3 crystals considered here consist of two-dimensional layers of decorated honeycomb lattices. Pseudospin-1 moments localized on the organometallic clusters under a trigonal splitting emerge from the interplay of Coulomb repulsion and spin molecular-orbit coupling. Read More


The spontaneous transformations associated with symmetry-breaking phase transitions generate domain structures and defects that may be topological in nature. The formation of these defects can be described according to the Kibble-Zurek mechanism, which provides a generic relation that applies from cosmological to interatomic lengthscales. Its verification is challenging, however, in particular at the cosmological scale where experiments are impractical. Read More


We report the evolution of structural, magnetic and dielectric properties due to partial substitution of Ba by Sr in the high temperature multiferroic YBaCuFeO5. This compound exhibits ferroelectric and antiferromagnetic transitions around 200 K and these two phenomena are presumed to be coupled with each other. Our studies on magnetic and dielectric properties of the YBa1-xSrxCuFeO5 (x = 0. Read More


We report the results of our investigation of magnetic, thermodynamic and dielectric properties of Ca substituted half-doped orthochromite, Dy0.6Ca0.4Fe0. Read More


We have performed a fully relativistic first-principles density functional calculation examining the surface state of bismuth (Bi) (111) multi-layer nanofilm, with up to 20 Bi bilayers, and investigated the Rashba effect and spin texture on the Bi surfaces. We have revealed a giant out-of-plane spin states on the Fermi lines, and the maximum value of the out-of-plane spin component being approximately 40% of the magnitude of the total spin. We have also evaluated the Rashba parameter $\alpha_R \simeq 1. Read More


While current-induced spin-orbit torques (SOTs) have been extensively studied in ferromagnets and antiferromagnets, ferrimagnets have been less studied. Here we report the presence of enhanced spin-orbit torques resulting from negative exchange interaction in ferrimagnets. The effective field and switching efficiency increase substantially as CoGd approaches its compensation point, giving rise to 9 times larger spin-orbit torques compared to that of non-compensated one. Read More


In the present work we analyze some necessary conditions for ignition of solid energetic materials by low velocity impact ignition mechanism. Basing on reported results of {\it ab initio} computations we assume that the energetic activation barriers for the primary endothermic dissociation in some energetic materials may be locally lowered due to the effect of shear strain caused by the impact. We show that the ignition may be initiated in regions with the reduced activation barriers, even at moderately low exothermicity of the subsequent exothermic reactions thus suggesting that the above regions may serve as "hot spots" for the ignition. Read More


We report infrared magneto-spectroscopy studies on thin crystals of an emerging Dirac material ZrTe5 near the intrinsic limit. The observed structure of the Landau level transitions and zero-field infrared absorption indicate a two-dimensional Dirac-like electronic structure, similar to that in graphene but with a small relativistic mass corresponding to a 9.4 meV energy gap. Read More


Quantum Tunneling is ubiquitous across different fields, from quantum chemical reactions, and magnetic materials to quantum simulators and quantum computers. While simulating the real-time quantum dynamics of tunneling is infeasible for high-dimensional systems, quantum tunneling also shows up in quantum Monte Carlo (QMC) simulations that scale polynomially with system size. Here we extend a recent results obtained for quantum spin models {[{Phys. Read More


We demonstrate a topological classification of vortices in three dimensional time-reversal invariant topological superconductors based on superconducting Dirac semimetals with an s-wave superconducting order parameter by means of a pair of numbers $(N_\Phi,N)$, accounting how many units $N_\Phi$ of magnetic fluxes $hc/4e$ and how many $N$ chiral Majorana modes the vortex carries. From these quantities, we introduce a topological invariant which further classifies the properties of such vortices under linking processes. While such processes are known to be related to instanton processes in a field theoretic description, we demonstrate here that they are, in fact, also equivalent to the fractional Josephson effect on junctions based at the edges of quantum spin Hall systems. Read More


We consider the situation when a femtosecond laser pulse creates a hot electron state in half-metallic ferromagnet (e. g. ferromagnetic semiconductor) on a picosecond timescale but do not act directly on localized spin system. Read More


Correlation between nucleation field and internal parameters has been derived analytically for amorphous ferromagnetic microwires. Anisotropy distribution specific for amorphous microwires has been fully taken into account instead of being averaged out. Dependence on some anisotropy distribution parameters of the wire was established numerically. Read More


We report first-principles static and dynamic calculations that clarify the microscopic mechanism of carbon annihilation due to phosphorous treatment upon oxidation of silicon carbide (SiC). We identify the most stable form of the phosphorus (P) in the oxide as the four-fold coordinated with the dangling PO unit and find that the unit attracts carbon ejected from the interface, thus operating as a carbon absorber. This finding provides a microscopic reasoning for the first time for the promotion of the oxidation reaction on one hand and the annihilation of the C-related defects at the interface on the other. Read More


By introducing a superconducting gap in Weyl- or Dirac semi-metals, the superconducting state inherits the non-trivial topology of their electronic structure. As a result, Weyl superconductors are expected to host exotic phenomena such as non-zero-momentum pairing due to their chiral node structure, or zero- energy Majorana modes at the surface. These are of fundamental interest to improve our understanding of correlated topological systems, and moreover practical applications in phase coherent devices and quantum applications have been proposed. Read More


The features of the spin and charge transport of electrons and holes in a metal and a semiconductor were studied using the Boltzmann transport equations. It was shown that the electrons and holes carry the spin in opposite directions in an electrical current. As result, the spin polarization of an electrical current in a metal is substantially smaller than spin polarization of electron gas. Read More


We report a large linear magnetoresistance in Cu$_{2-x}$Te, reaching $\Delta\rho/\rho(0)$ = 250\% at 2 K in a 9 T field. This is observed for samples with $x$ in the range 0.13 to 0. Read More


Recent studies showed that the in-plane and inter-plane thermal conductivities of two-dimensional (2D) MoS2 are low, posing a significant challenge in heat management in MoS2-based electronic devices. To address this challenge, we design the interfaces between MoS2 and graphene by fully utilizing graphene, a 2D material with an ultra-high thermal conduction. We first perform ab initio atomistic simulations to understand the bonding nature and structure stability of the interfaces. Read More


GaN is a key material for lighting and power electronics. Yet, the carrier transport and ultrafast dynamics that are central in GaN devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling of hot carriers. Read More


Arrays of aligned nanorods oriented perpendicular to a support, which are accessible by top-down lithography or by means of shape-defining hard templates, have received increasing interest as sensor components, components for nanophotonics and nanoelectronics, substrates for tissue engineering, sur-faces having specific adhesive or antiadhesive properties and as surfaces with customized wettability. Agglomeration of the nanorods deteriorates the performance of components based on nanorod arrays. A comprehensive body of literature deals with mechanical failure mechanisms of nanorods and design criteria for mechanically stable nanorod arrays. Read More


Intersubband (ISB) polarons result from the interaction of an ISB transition and the longitudinal optical (LO) phonons in a semiconductor quantum well (QW). Their observation requires a very dense two dimensional electron gas (2DEG) in the QW and a polar or highly ionic semiconductor. Here we show that in ZnO/MgZnO QWs the strength of such a coupling can be as high as 1. Read More


We report the preparation of the interface between graphene and the strong Rashba-split BiAg$_2$ surface alloy and investigatigation of its structure as well as the electronic properties by means of scanning tunneling microscopy/spectroscopy and density functional theory calculations. Upon evaluation of the quasiparticle interference patterns the unpertrubated linear dispersion for the $\pi$ band of $n$-doped graphene is observed. Our results also reveal the intact nature of the giant Rashba-split surface states of the BiAg$_2$ alloy, which demonstrate only a moderate downward energy shift upon the presence of graphene. Read More


Water splitting allows the storage of solar energy into chemical bonds (H2+O2) and will help to implement the urgently needed replacement of limited available fossil fuels. Particularly in neutral environment electrochemically initiated water splitting suffers from low efficiency due to high overpotentials caused by the anode. Electro-activation of X20CoCrWMo10-9, a Co-based tool steel resulted in a new composite material (X20CoCrWMo10-9//Co3O4) that catalyzes the anode half-cell reaction of water electrolysis with a so far unequalled effectiveness. Read More


In this article, we explore the anisotropic electron energy loss spectrum (EELS) in monolayer phosphorene based on ab-initio time dependent density functional theory calculations. Similar to black phosphorous, the EELS of undoped monolayer phosphorene is characterized by anisotropic excitonic peaks for energies in vicinity of the bandgap, and by interband plasmon peaks for higher energies. On doping, an additional intraband plasmon peak also appears for energies within the bandgap. Read More


Employing ab initio calculations, we discuss chemical, mechanical, and dynamical stability of MoN-TaN solid solutions together with cubic-like MoN/TaN superlattices, as another materials design concept. Hexagonal-type structures based on low-energy modifications of MoN and TaN are the most stable ones over the whole composition range. Despite being metastable, disordered cubic polymorphs are energetically significantly preferred over their ordered counterparts. Read More


Drift-diffusion model is an indispensable modeling tool to understand the carrier dynamics (transport, recombination, and collection) and simulate practical-efficiency of solar cells (SCs) through taking into account various carrier recombination losses existing in multilayered device structures. Exploring the way to predict and approach the SC efficiency limit by using the drift-diffusion model will enable us to gain more physical insights and design guidelines for emerging photovoltaics, particularly perovskite solar cells. Our work finds out that two procedures are the prerequisites for predicting and approaching the SC efficiency limit. Read More


Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices. A central motivation toward this direction is that antiferromagnetic spin dynamics is expected to be much faster than ferromagnetic counterpart because antiferromagnets have higher resonance frequencies than ferromagnets. Recent theories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs. Read More


An intriguing feature of the magnetic skyrmion in a frustrated magnetic system is its helicity-orbital coupling. When the magnetic dipole-dipole interaction (DDI) is neglected, a skyrmion can show a current-induced rotational motion together with a helicity rotation since the energy is independent of the helicity. Here, we explore the skyrmion dynamics in a frustrated magnetic system based on the $J_{1}$-$J_{2}$-$J_{3}$ classical Heisenberg model explicitly by including the DDI. Read More


Three-dimensional (3D) topological Dirac semimetals (TDSs) are rare but important as a versatile platform for exploring exotic electronic properties and topological phase transitions. A quintessential feature of TDSs is 3D Dirac fermions associated with bulk electronic states near the Fermi level. Using angle-resolved photoemission spectroscopy (ARPES), we have observed such bulk Dirac cones in epitaxially-grown {\alpha}-Sn films on InSb(111), the first such TDS system realized in an elemental form. Read More


Recent studies on the magneto-transport properties of topological insulators (TI) have attracted great attention due to the rich spin-orbit physics and promising applications in spintronic devices. Particularly the strongly spin-moment coupled electronic states have been extensively pursued to realize efficient spin-orbit torque (SOT) switching. However, so far current-induced magnetic switching with TI has only been observed at cryogenic temperatures. Read More


Coherent light-matter interaction can be used to manipulate the energy levels of atoms, molecules and solids. When light with frequency {\omega} is detuned away from a resonance {\omega}o, repulsion between the photon-dressed (Floquet) states can lead to a shift of energy resonance. The dominant effect is the optical Stark shift (1/({\omega}0-{\omega})), but there is an additional contribution from the so-called Bloch-Siegert shift (1/({\omega}o+{\omega})). Read More


Density functional calculations are used to explain the charge transfer doping mechanism by which species physisorptively bonded to graphene can increase its free hole or electron density, without giving rise to defects, and thus maintain a high carrier mobility. Typical dopants studied are FeCl3, AuCl3, SbF5, HNO3, MoO3, Cs2O and O2. These systems do not break the {\pi} bonding of the basal plane are particularly important as these do not degrade the carrier mobility. Read More


Mutiferroic materials where electric polarization is induced by spiral magnetic phase are immensely important for technological applications. Recently, the transition temperature of the spiral phase in YBaCuFeO$_{5}$ is found to rise to the room temperature range through A-site doping. Motivated by this remarkable observation, we investigate the magnetic phase transitions in YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ in the entire doping range $0\le x \le 1$ using first-principles density functional theory calculations (DFT) followed by a Quantum Monte Carlo (QMC) calculation. Read More


We study how the shape of the spinwave resonance lines in rf-voltage induced FMR can be used to extract the spin-wave density of states and the Gilbert damping within the precessing layer in nanoscale magnetic tunnel junctions that possess perpendicular anisotropy. We work with a field applied along the easy axis to preserve the cylindrical symmetry of the uniaxial perpendicularly magnetized systems. We first describe the set-up to study the susceptibility contributions of the spin waves in the field-frequency space. Read More


Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in NbNiTe2, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For NbNiTe2 structure in space group 53 (Pmna) - having an inversion center arising from two glide planes and one mirror plane with a 2-fold rotation and screw axis - a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space group 28 (Pma2), eliminating one glide plane along c, the associated rotation and screw axis, and the inversion center. Read More


Hybrid organic-inorganic perovskites have been established as good candidate materials for emerging photovoltaics, with device efficiencies of over 22 % being reported. There are currently only two organic cations, methylammonium and formamidinium, which produce 3D perovskites with band gaps suitable for photovoltaic devices. Numerous computational studies have identified azetidinium as a potential third cation for synthesizing organic-inorganic perovskites, but to date no experimental reports of azetidinium containing perovskites have been published. Read More


Domains in BaTiO$_3$ induces a regular modulation of uniaxial magnetic anisotropy in CoFeB via an inverse magnetostriction effect. As a result, the domain structures of the CoFeB wedge film and BaTiO$_3$ substrate correlate fully and straight ferroelectric domain boundaries in BaTiO$_3$ pin magnetic domain walls in CoFeB. We use x-ray photoemission electron microscopy and magneto-optical Kerr effect microscopy to characterize the spin structure of the pinned domain walls. Read More


The capability to isolate one to few unit-cell thin layers from the bulk matrix of layered compounds opens fascinating prospects to engineer novel electronic phases. However, a comprehensive study of the thickness dependence and of potential extrinsic effects are paramount to harness the electronic properties of such atomic foils. One striking example is the charge density wave (CDW) transition temperature in layered dichalcogenides whose thickness dependence remains unclear in the ultrathin limit. Read More


This tutorial review presents an overview of the basic theoretical aspects of two-dimensional (2D) crystals. We revise essential aspects of graphene and the new families of semiconducting 2D materials, like transition metal dichalcogenides or black phosphorus. Minimal theoretical models for various materials are presented. Read More


Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to N\'eel's model. Here we report a deviation from this conventional linear 1/d dependence, altered significantly by the microstrain, in Ca and Ti substituted BiFeO3 nanoparticles. Magnetic properties of microstrain-controlled Bi1-xCaxFe1-yTiyO3-delta (y = 0 and x = y) nanoparticles are analyzed as a function of their size ranging from 18 nm to 200 nm. Read More


We study the annealing stability of bottom-pinned perpendicularly magnetized magnetic tunnel junctions based on dual MgO free layers and thin fixed systems comprising a hard [Co/Ni] multilayer antiferromagnetically coupled to thin a Co reference layer and a FeCoB polarizing layer. Using conventional magnetometry and advanced broadband ferromagnetic resonance, we identify the properties of each sub-unit of the magnetic tunnel junction and demonstrate that this material option can ensure a satisfactory resilience to the 400$^\circ$C thermal annealing needed in solid-state magnetic memory applications. The dual MgO free layer possesses an anneal-robust 0. Read More


Leibniz said "Naturam cognosci per analogiam": nature is understood by making analogies. This statement describes a seminal epistemological principle. But one has to be aware of its limitations: quantum mechanics for example at some point had to push Bohr's model of the atom aside to make progress. Read More


We present the structural and dynamical studies of layered vanadium pentaoxide (V2O5). The temperature dependent X-ray diffraction measurements reveal highly anisotropic and anomalous thermal expansion from 12 K to 853 K. The results do not show any evidence of structural phase transition or decomposition of {\alpha}-V2O5, contrary to the previous transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) experiments. Read More


In this paper we employ first-principles calculations to study the mechanical, geometrical, electronic and magnetic properties of Fe atom embedded \textit{s}{}-triazine ($\mathrm{Fe}$-${\mathrm C_6}{\mathrm N_6}$) system under the influence of external environment. Our result show that the binding energy of $\mathrm{Fe}$-${\mathrm C_6}{\mathrm N_6}$ can be modulated by an applied tensile deformation and perpendicular electric field. The non-magnetic semiconducting property of pure \textit{s}{}-triazine sheet (${\mathrm C_6}{\mathrm N_6}$) is found to changed upon embedding of Fe atom in the porous site of the sheet. Read More


Tellurium (Te) films with monolayer and few-layer thickness are obtained by molecular beam epitaxy on a graphene/6H-SiC(0001) substrate and investigated by in situ scanning tunneling microscopy and spectroscopy (STM/STS). We reveal that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1x1) facet of (10-10) of the bulk crystal. The band gap of Te films increases monotonically with decreasing thickness, reaching ~0. Read More