In the last 30 days

• Open/close Magnetically induced hyperthermia: size-dependent heating power of γ-Fe₂O₃ nanoparticles

  Michael Lévy et al 2008 J. Phys.: Condens. Matter 20 204133 Tag this article

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  By combining magnetic properties with nanosized biocompatible materials, superparamagnetic nanoparticles may serve as colloidal heating mediators for cancer therapy. This unique potential has attracted attention for designing new magnetic nanoparticles with high efficiency heating properties. Their heating power under high frequency magnetic field is governed by the mechanisms of magnetic energy dissipation for single-domain particles due both to internal Néel fluctuations of the particle magnetic moment and to the external Brownian fluctuations. These mechanisms are highly sensitive to the crystal size, the particle material, and the solvent properties. Here we explore the heating properties of maghemite particles with large particle sizes, in the range 15–50 nm, synthesized through a new procedure which includes a hydrothermal process. Particle shape and size distribution, hydrodynamic volume, and magnetic anisotropy are characterized, respectively, by transmission electron microscopy, dynamic magnetically induced birefringence, and ferromagnetic resonance. Together with our previous data on low diameter particles (Fortin J P et al 2007 J. Am. Chem. Soc 129 2628–35), this study provides the whole size dependence of heating efficiency in the range 5–50 nm and assesses the balance between Néel and Brownian contributions to thermal losses. In agreement with theoretical predictions, the heating efficiency shows a maximum for an optimal size of about 15 nm.

• Open/close Electronic properties of graphene nanostructures

  F Molitor et al 2011 J. Phys.: Condens. Matter 23 243201 Tag this article

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  In this review, recent developments in the fabrication and understanding of the electronic properties of graphene nanostructures are discussed. After a brief overview of the structure of graphene and the two-dimensional transport properties, the focus is put on graphene constrictions, quantum dots and double quantum dots. For constrictions with a width below 100 nm, the current through the constriction is strongly suppressed for a certain back gate voltage range, related to the so-called transport gap. This transport gap is due to the formation of localized puddles in the constriction, and its size depends strongly on the constriction width. Such constrictions can be used to confine charge carriers in quantum dots, leading to Coulomb blockade effects.

• Open/close Cation migration and magnetic ordering in spinel CoFe₂O₄ powder: micro-Raman scattering study

  T Yu et al 2002 J. Phys.: Condens. Matter 14 L613 Tag this article

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  Micro-Raman scattering was used to characterize spinel CoFe ₂O ₄ at high temperature up to 870 K and under an external magnetic field up to 6.0 kOe. It was found that the rapid increase in the linewidth of the Raman modes was related to the inter-site cation migration starting at ~390 K. A red-shift of the Raman peaks induced by the magnetic ordering was observed upon applying a magnetic field at room temperature. Phase analysis of CoFe ₂O ₄ powder was also carried out by means of x-ray diffraction and micro-Raman spectroscopy in this work.

• Open/close QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

  Paolo Giannozzi et al 2009 J. Phys.: Condens. Matter 21 395502 Tag this article

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  QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

• Open/close Spin structure transition in La_{1.6 − x}Nd_0.4Sr_xCuO₄ superconductors

  J F Ding et al 2010 J. Phys.: Condens. Matter 22 275701 Tag this article

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  The field and temperature dependencies of the spin structures in the normal states of La _{1.6 − x}Nd _0.4Sr _x CuO ₄ ( x = 0.10, 0.12, and 0.15) single crystals have been studied by measuring the magnetoresistance and susceptibility. A negative magnetoresistance appears just below the spin-ordering temperature for the magnetic fields parallel to the CuO ₂ plane, which can be attributed to the spin-flop transition of the special spin structure in the normal state of the system. The anisotropic variations of susceptibilities with temperature for all the three specimens can be described in the framework of the crystal-field theory. The well fitted broad peaks of the in-plane susceptibilities χ _ab for the specimens suggest that the susceptibilities are dominated by Nd ^{3 +} , and thus the spin reorientation of Cu ^{2 +} in the CuO ₂ plane can not be observed from the study of the susceptibility.

• Open/close Zinc oxide nanostructures: growth, properties and applications

  Zhong Lin Wang 2004 J. Phys.: Condens. Matter 16 R829 Tag this article

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  Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.

• Open/close Correlation-crystal-field analysis of Nd³⁺(4f³) energy-level structures in various crystal hosts

  E Rukmini et al 1994 J. Phys.: Condens. Matter 6 5919 Tag this article

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  We have performed an in-depth correlation-crystal-field (CCF) analysis of the energy-level structures in 10 Nd ³⁺(4f ³) crystal systems: NdF ₃, Nd ₂Te ₄O ₁₁, NdVO ₄, NdPO ₄, Nd ³⁺:LiYF ₄, Nd ³⁺:LaVO ₄, Nd ³⁺:LaCl ₃, Nd ³⁺:BaY ₂F ₈, Nd ³⁺:YAlO ₃ and Nd ³⁺:LuA1O ₃. A model Hamiltonian employing 20 free-ion parameters, appropriate one-electron crystal-field interaction parameters and also selected two-particle CCF interaction parameters was diagonalized within the complete 364 SLJM _J basis set of the 4f ³ electronic configuration. Inclusion of the fourth-rank g ₂ ⁽⁴⁾, g _10A ⁽⁴) and g _10B ⁽4 ⁾ CCF operators in the phenomenological energy-level fits yields an overall improved agreement between calculated and empirical energy levels besides eliminating major discrepancies between calculated and observed crystal-field splittings within the anomalous ²H(2) ^11/2 multiplet of Nd ³⁺ ion. The fits are also in qualitative agreement with the ab initio calculations of CCF effects for lanthanide ions.

• Open/close First principles studies of multiferroic materials

  Silvia Picozzi and Claude Ederer 2009 J. Phys.: Condens. Matter 21 303201 Tag this article

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  Multiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: (a) multiferroics where ferroelectricity is driven by hybridization or purely structural effects, with BiFeO ₃ as the prototype material, and (b) multiferroics where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin-, charge-, or orbital-ordering, with rare-earth manganites as prototypes. As for the first class of multiferroics, first principles calculations are shown to provide an accurate qualitative and quantitative description of the physics in BiFeO ₃, ranging from the prediction of large ferroelectric polarization and weak ferromagnetism, over the effect of epitaxial strain, to the identification of possible scenarios for coupling between ferroelectric and magnetic order. For the second class of multiferroics, ab initio calculations have shown that, in those cases where spin-ordering breaks inversion symmetry (e.g. in antiferromagnetic E-type HoMnO ₃), the magnetically induced ferroelectric polarization can be as large as a few µC cm ⁻². The examples presented point the way to several possible avenues for future research: on the technological side, first principles simulations can contribute to a rational materials design, aimed at identifying spintronic materials that exhibit ferromagnetism and ferroelectricity at or above room temperature. On the fundamental side, ab initio approaches can be used to explore new mechanisms for ferroelectricity by exploiting electronic correlations that are at play in transition metal oxides, and by suggesting ways to maximize the strength of these effects as well as the corresponding ordering temperatures.

• Open/close The SIESTA method for ab initio order-N materials simulation

  José M Soler et al 2002 J. Phys.: Condens. Matter 14 2745 Tag this article

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  We have developed and implemented a selfconsistent density functional method using standard norm-conserving pseudopotentials and a flexible, numerical linear combination of atomic orbitals basis set, which includes multiple-zeta and polarization orbitals. Exchange and correlation are treated with the local spin density or generalized gradient approximations. The basis functions and the electron density are projected on a real-space grid, in order to calculate the Hartree and exchange-correlation potentials and matrix elements, with a number of operations that scales linearly with the size of the system. We use a modified energy functional, whose minimization produces orthogonal wavefunctions and the same energy and density as the Kohn-Sham energy functional, without the need for an explicit orthogonalization. Additionally, using localized Wannier-like electron wavefunctions allows the computation time and memory required to minimize the energy to also scale linearly with the size of the system. Forces and stresses are also calculated efficiently and accurately, thus allowing structural relaxation and molecular dynamics simulations.

• Open/close Fddd structure in AB-type diblock copolymers

  Kohtaro Yamada et al 2006 J. Phys.: Condens. Matter 18 L421 Tag this article

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  It is well known that lamellar structure, hexagonal structure of cylinder domains, gyroid and body-centred cubic structure of spherical domains all exist as equilibrium structures in AB-type diblock copolymers. We shall show in the present letter that, in addition to the above four phases, there is another equilibrium phase, the so-called Fddd structure which is an interconnected but uniaxial structure. We confirm this conclusion by two different methods. One is the mode-expansion including a substantial number of modes. The other is direct simulations of the time-evolution equation in three dimensions.