SciCombinator

Discover the most talked about and latest scientific content & concepts.

Journal: Journal of physics. Condensed matter : an Institute of Physics journal

28

We study by molecular dynamics the structural properties of single layer hexagonal boron nitride (h-BN) in comparison to graphene. We show that the Tersoff bond order potential developed for BN by Albe et al (1997 Radiat. Eff. Defects Solids 141 85-97) gives a thermally stable hexagonal single layer with a bending constant κ = 0.54 eV at T = 0. We find that the non-monotonic behaviour of the lattice parameter, the expansion of the interatomic distance and the growth of the bending rigidity with temperature are qualitatively similar to those of graphene. Conversely, the energetics of point defects is extremely different: instead of Stone-Wales defects, the two lowest energy defects in h-BN involve either a broken bond or an out-of-plane displacement of a N atom to form a tetrahedron with three B atoms in the plane. We provide the formation energies and an estimate of the energy barriers.

Concepts: Energy, Molecule, Atom, Chemical bond, Thermodynamics, Force, Potential energy, Boron nitride

28

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility χ in the normal state with temperature and under hydrostatic pressure. The temperature dependences of χ and its anisotropy Δχ = χ(‖) - χ(⊥) are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy Δχ is very large and comparable to the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, the significant pressure effect on χ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure-dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability, with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height Z of chalcogen species from the Fe plane. The change of Z under pressure determines a large positive pressure effect on χ, which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level.

Concepts: Magnetic field, Fundamental physics concepts, Condensed matter physics, Magnetism, Thermodynamics, Permeability, Magnetic susceptibility, Ideal gas

28

A theoretical investigation of the possible existence of chiral polaron formation in graphene is reported. We present an analytical method to calculate the ground-state of the electron-phonon system within the framework of the Lee-Low-Pines theory. On the basis of our model, the influence of electron-optical phonon interaction on the graphene electronic spectrum is investigated. We considered only the doubly degenerate optical phonon modes of E(2g) symmetry near the zone center Γ. It is analytically shown that the energy dispersions of both valence and conduction bands of the pristine graphene differ significantly from those obtained through the standard electron self-energy calculations arising from the electron-optical phonon interactions. In this paper, we also show for the first time that the degenerate band structure of the graphene promotes the chiral polaron formation. Furthermore, due to the k-dependent nature of the polaronic self-energy, in analogy with quantum chromodynamics, we also propose a running electron-phonon coupling constant as a function of energy.

Concepts: Photon, Physics, Particle physics, Quantum field theory, Quantum electrodynamics, Chirality, Phonon, Standard Model

28

LiFeAs is one of the new class of iron superconductors with a bulk [Formula: see text] in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared by self-flux growth techniques with significantly improved properties, including a much decreased residual gamma, γ® (≡C/T as T → 0), in the superconducting state. Thus, in contrast to previous explanations of a finite γ® in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual γ in LiFeAs is instead a function of sample quality. Further, since LiFeAs has been characterized as nodeless with multiple superconducting gaps, we report here on its specific heat properties in zero and applied magnetic fields, to compare to similar results on nodal iron superconductors. For comparison, we also investigate LiFe(0.98)Cu(0.02)As, which has the reduced T© of ≈9 K and an H(c2) of 15 T. Interestingly, although presumably both LiFeAs and LiFe(0.98)Cu(0.02)As are nodeless, they clearly show a non-linear dependence of the electronic density of states (∝ specific heat γ) at the Fermi energy in the mixed state with the applied field, similar to the Volovik effect for nodal superconductors. However, rather than indicating nodal behavior, the satisfactory comparison with a recent theory for γ(H) for a superconductor with two isotropic gaps in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in a magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor.

Concepts: Electron, Magnetic field, Fundamental physics concepts, Condensed matter physics, Diamagnetism, Unconventional superconductor, Absolute zero, High-temperature superconductivity

28

We present a technique for analyzing the full three-dimensional density profiles of planar crystal-fluid interfaces in terms of density modes. These density modes can also be related to crystallinity order parameter profiles which are used in coarse-grained, phase field type models of the statics and dynamics of crystal-fluid interfaces and are an alternative to crystallinity order parameters extracted from simulations using local crystallinity criteria. We illustrate our results for the hard sphere system using finely resolved, three-dimensional density profiles from a density functional theory of fundamental measure type.

Concepts: Scientific method, Condensed matter physics, Density functional theory, Phase transition, C, Currying, Archimedes, Windows PowerShell

28

The A-site ordered perovskite oxide with the formula CaCu(3)Sn(4)O(12) has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group [Formula: see text] (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO(6) network is extremely tilted, giving rise to a square planar coordination for Cu(2+) cations. The non-magnetic character of Sn(4+) offers an excellent opportunity to probe the magnetism of Cu(2+) at the A sublattice in CaCu(3)Sn(4)O(12). Magnetic susceptibility shows that this compound is ferromagnetic below T© = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu(2+) spins with a magnetic moment of 0.5 μ(B)/Cu atom.

Concepts: Magnetic field, Crystallography, Fundamental physics concepts, Magnet, Paramagnetism, Magnetism, Neutron diffraction, Magnetic susceptibility

28

A communication protocol is proposed in which vibron-mediated quantum state transfer takes place in a molecular lattice. We consider two distant molecular groups grafted on each side of the lattice. These groups form two quantum computers where vibrational qubits are implemented and received. The lattice defines the communication channel along which a vibron delocalizes and interacts with a phonon bath. Using quasi-degenerate perturbation theory, vibron-phonon entanglement is taken into account through the effective Hamiltonian concept. A vibron is thus dressed by a virtual phonon cloud whereas a phonon is clothed by virtual vibronic transitions. It is shown that three quasi-degenerate dressed states define the relevant paths followed by a vibron to tunnel between the computers. When the coupling between the computers and the lattice is judiciously chosen, constructive interference takes place between these paths. Phonon-induced decoherence is minimized and a high-fidelity quantum state transfer occurs over a broad temperature range.

Concepts: Photon, Energy, Quantum mechanics, Schrödinger equation, Quantum field theory, Qubit, Quantum entanglement, Quantum decoherence

28

Fluorescence yield (FY) detected x-ray absorption spectra (XAS) of 3d transition metal ions are calculated from the integrated 2p3d resonant x-ray emission spectra. The resulting FY-XAS spectra are compared with the normal XAS spectra corresponding to the absorption cross section and significant deviations between the two spectra are found. This implies that the assumption that the FY-XAS spectrum identifies with the XAS spectrum is disproved. Especially for the early transition metal systems the differences between the FY-XAS and XAS are large, due to the opening of inelastic decay channels from selected x-ray absorption final states. The theoretical calculations show that the difference between FY detection and XAS is largest for the detection in depolarized geometry. The calculations are compared with experimental spectra for oxides and coordination compounds for Fe(2+), Co(2+) and Ni(2+) systems. The implications for the sum rules in XAS and magnetic circular dichroism experiments are discussed.

Concepts: Spectroscopy, Electromagnetic radiation, Zinc, Transition metal, X-ray absorption spectroscopy, Absorption spectroscopy, Emission spectrum, X-ray absorption fine structure

28

Electrical transport properties of (La(1-x)X(x))MnO(3) (X = Ca, Sr), which exhibits the so-called colossal magnetoresistance (CMR) effect, are investigated in the framework of linear response theory combined with first-principles calculation. The calculation is performed by applying the coherent potential approximation and Kubo-Greenwood formula to the KKR Green’s function method. In order to reveal the mechanism that underlies the CMR effects, the dc conductivities of the system in A (layered)-, C (stripe)-, and G (111)-type antiferromagnetic states as well as the local moment disordered state are calculated as a function of x. It is found that the conductivity of the system strongly depends on the magnetic structure and is highly anisotropic in the cases of A- and C-type antiferromagnetic structures. Also, it is concluded that a drastic change in the conductivity is expected when the system undergoes a magnetic transition, and that this might be closely related to the observed CMR effect. It is pointed out that, for such a drastic change in the conductivity to occur associated with the magnetic transition, the half-metallic nature of the Mn local electronic structure must be essential.

Concepts: Scientific method, Effect, Magnetism, Trigraph, Colossal magnetoresistance, Magnetoresistance, Electric and magnetic fields in matter, Giant magnetoresistance

28

Bismuth sodium titanate (Bi(0.5)Na(0.5))TiO(3) (BNT) is considered to be one of the most promising lead-free alternatives to piezoelectric lead zirconate titanate (PZT). However, the effect of dopants on the material has so far received little attention from an atomic point of view. In this study we investigated the effects of cobalt-doping on the formation of additional phases and determined the preferred lattice site of cobalt in BNT. The latter was achieved by comparing the measured x-ray absorption near-edge structure (XANES) spectra to numerically calculated spectra of cobalt on various lattice sites in BNT. (Bi(0.5)Na(0.5))TiO(3) + x mol% Co (x = 0.0, 0.5, 1.0, 2.6) was synthesized via solid state reaction. As revealed by SEM backscattering images, a secondary phase formed in all doped specimens. Using both XRD and SEM-EDX, it was identified as Co(2)TiO(4) for dopant levels >0.5 mol%. In addition, a certain amount of cobalt was incorporated into BNT, as shown by electron probe microanalysis. This amount increased with increasing dopant levels, suggesting that an equilibrium forms together with the secondary phase. The XANES experiments revealed that cobalt occupies the octahedral B-site in the BNT perovskite lattice, substituting Ti and promoting the formation of oxygen vacancies in the material.

Concepts: Condensed matter physics, Lead, Revealed preference, Piezoelectricity, Boron, Dopant, Ferroelectricity, Lead zirconate titanate