In the last 30 days

• Open/close Spin wave dispersion based on the quasiparticle self-consistent GW method: NiO, MnO and α-MnAs

  Takao Kotani and Mark van Schilfgaarde 2008 J. Phys.: Condens. Matter 20 295214 Tag this article

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  We present calculations of spin wave dispersions for antiferromagnetic MnO, NiO and ferromagnetic α-MnAs based on the recently developed quasiparticle self-consistent GW method (QS GW); we have already shown that QS GW gave a good quasiparticle picture for MnO and NiO in comparison with optical experiments. To obtain the spin wave dispersions, we have developed a method to calculate the transverse dynamical spin susceptibility in the random-phase approximation. This is a general method applicable not only to QS GW, but also to any first-principle method which gives the non-interacting (one-body) Hamiltonian to represent quasiparticles, e.g. the Kohn–Sham Hamiltonian in the density functional theory. In the method, we first calculate the non-interacting spin susceptibility from the supplied non-interacting Hamiltonian; then we obtain the spin susceptibility where the size of the effective interaction is determined so as to satisfy a sum rule. For MnO and NiO, the obtained spin wave dispersions show good agreement with experiments, in contrast to the cases in the local density approximation (LDA) and in the LDA+ U. These results support our claim that the independent-particle picture is powerful enough even for materials like NiO and MnO classified to the Mott insulator, that is, the quasiparticle pictures by QS GW work well to describe their linear responses. For α-MnAs, we find a collinear ferromagnetic ground state in QS GW, while this phase is unstable in the LDA.

• Open/close Pyroelectric properties of Al(In)GaN/GaN hetero- and quantum well structures

  O Ambacher et al 2002 J. Phys.: Condens. Matter 14 3399 Tag this article

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  The macroscopic nonlinear pyroelectric polarization of wurtzite Al _x Ga _{1- x}N, In _x Ga _{1- x}N and Al _x In _{1- x}N ternary compounds (large spontaneous polarization and piezoelectric coupling) dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GaN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. Models of polarization-induced effects in GaN-based devices so far have assumed that polarization in ternary nitride alloys can be calculated by a linear interpolation between the limiting values of the binary compounds. We present theoretical and experimental evidence that the macroscopic polarization in nitride alloys is a nonlinear function of strain and composition. We have applied these results to interpret experimental data obtained in a number of InGaN/GaN quantum wells (QWs) as well as AlInN/GaN and AlGaN/GaN transistor structures. We find that the discrepancies between experiment and ab initio theory present so far are almost completely eliminated for the AlGaN/GaN-based heterostructures when the nonlinearity of polarization is accounted for. The realization of undoped lattice-matched AlInN/GaN heterostructures further allows us to prove the existence of a gradient in spontaneous polarization by the experimental observation of two-dimensional electron gases (2DEGs). The confinement of 2DEGs in InGaN/GaN QWs in combination with the measured Stark shift of excitonic recombination is used to determine the polarization-induced electric fields in nanostructures. To facilitate inclusion of the predicted nonlinear polarization in future simulations, we give an explicit prescription to calculate polarization-induced electric fields and bound interface charges for arbitrary composition in each of the ternary III-N alloys. In addition, the theoretical and experimental results presented here allow a detailed comparison of the predicted electric fields and bound interface charges with the measured Stark shift and the sheet carrier concentration of polarization-induced 2DEGs. This comparison provides an insight into the reliability of the calculated nonlinear piezoelectric and spontaneous polarization of group III nitride ternary alloys.

• Open/close Network-forming phase separation of colloidal suspensions

  Hajime Tanaka et al 2005 J. Phys.: Condens. Matter 17 L143 Tag this article

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  Colloidal suspension can be regarded as an ideal model system of emulsions, protein solutions, foods, and inks. When there are strong attractive interactions between colloidal particles, they aggregate, phase separate, and sometimes form gel. The basic understanding of the resulting formation of superstructures is of crucial importance from both the scientific and industrial viewpoints. Here we provide clear experimental evidence suggesting that phase separation of colloidal suspensions can take the following kinetic pathway accompanying a metastable transient gel state: upon the phase separation, a percolated network is formed by a hierarchical clustering mechanism even at an extremely low colloid volume fraction (<10 ⁻³). Then the network structure coarsens with time under the influence of the connectivity and the resulting self-generated mechanical stress. The similarity of this behaviour to droplet-forming viscoelastic phase separation in a dilute polymer solution suggests that colloid phase separation may be classified as viscoelastic phase separation.

• Open/close Antiferromagnetic LaFeO₃ thin films and their effect on exchange bias

  J W Seo et al 2008 J. Phys.: Condens. Matter 20 264014 Tag this article

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  Antiferromagnetic (AFM) orthoferrites are interesting model systems for exploring the correlation between their crystalline and AFM domains and the resulting exchange bias when coupled to a ferromagnetic layer. In particular, LaFeO ₃ (LFO) has a Néel temperature, T _N = 740 K, which is the highest in the orthoferrite family. The recent developments of synchrotron radiation-based photoelectron emission microscopy (PEEM) have provided the possibility of studying AFM domain structures as well as the magnetic coupling between the AFM and the adjacent ferromagnetic (FM) layer, domain by domain. Thin films of LFO have proved excellent candidates for such studies because their AFM domains are well defined and large enough to be readily imaged by PEEM. This paper reviews the growth, structural and magnetic properties of LFO thin films as well as exchange coupling to a FM layer. The strong correlation between structural and AFM domains in this material allows us to investigate the exchange coupling as a function of the domain configuration, which can be changed by using different substrate material and substrate orientation. A significant increase of the exchange bias field by a factor of about 10 was obtained when LFO was diluted with Ni atoms in the volume part. In this sample, the structural domain boundary became corrugated due to substitutional defects. Our results indicate that the details of the precise domain boundary configuration strongly affect the exchange coupling.

• Open/close Electrowetting: from basics to applications

  Frieder Mugele and Jean-Christophe Baret 2005 J. Phys.: Condens. Matter 17 R705 Tag this article

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  Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects—rather than a unique one—are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices.

• Open/close The structural properties of liquid, solid and amorphous sulphur

  R Winter et al 1990 J. Phys.: Condens. Matter 2 SA215 Tag this article

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  A series of neutron diffraction experiments have been used to investigate the properties of liquid, solid and amorphous sulphur. The liquid was studied from 131 degrees C to 230 degrees C, i.e. at conditions around the λ -transition (T _λ=159 degrees C). The amorphous sample was studied at -196 degrees C and the crystalline solid at room temperature. The data have been transformed to give an accurate pair correlation function g(r) from which the absolute numbers of atoms in given molecular configurations may be determined for distances less than 8 AA. A comparison of the solid and liquid g(r) data indicates which intermolecular bonds are broken on melting. The liquid data seem to be inconsistent with the picture of 'crown'-shaped S ₈ molecules below T _λ , and with the proposed S ₈-to-polymer transition at T _λ. Also it has been found that the diffraction pattern may be analyzed as though the liquid was an assembly of roughly spherical molecular units. In this case each unit would contain about six atoms, and the λ-transition could be related to the percolation limit for these units. The amorphous material was studied at the quenching temperature of -196 degrees C and its structure is intermediate between that of the parent liquid and the crystalline solid. It is concluded that a fundamental diffraction analysis of the states of sulphur has thrown new light on long standing problems.

• 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 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 Mechanical and electronic properties of B₁₂-based ternary crystals of orthorhombic phase

  Xue Jiang et al 2010 J. Phys.: Condens. Matter 22 315503 Tag this article

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  Using first-principles calculations, the structural, mechanical and electronic properties of the experimentally synthesized B ₁₂-based ternary crystals (AlMgB ₁₄, AlNaB ₁₄, AlLiB ₁₄, Mg ₂B ₁₄, MgSi ₂B ₁₂, MgC ₂B ₁₂, Li ₂Si ₂B ₁₂ and Li ₂C ₂B ₁₂) have been investigated. The theoretical equilibrium lattice constants of these crystals agree with the experimental values. The Vickers hardness ( H _v) estimated from the theoretical Young's moduli ranges from 20 to 30 GPa, and the MgC ₂B ₁₂ compound ( H _v = 31.4 GPa) is harder than α-boron. Based on the electron density of states and Mulliken population analysis, the origination of hardness and interaction between the interstitial atoms and the B ₁₂ framework were discussed. Scaled bond order of the B–B bonds was used to interpret the hardness of these B ₁₂-based ternary compounds. The crystal hardness is primarily determined by the B ₁₂ icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron transfer from metal to B atoms. We also calculated the ideal tensile strength of AlMgB ₁₄ and MgC ₂B ₁₂ and found that the 001 and 010 directions are their cleavage directions under tensile strains, respectively.

• 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.