| 41. |
- Odell, Anders, et al.
(författare)
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Investigation of the Conducting Properties of a Photoswitching Dithienylethene Molecule
- 2010
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Ingår i: ACS NANO. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 4:5, s. 2635-2642
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Tidskriftsartikel (refereegranskat)abstract
- Photoswitching molecules are attractive candidates as organic materials for optoelectronics applications because light impulses can switch them between states with different conducting characteristics. Here, we report a fully self-consistent density functional theory calculation of the electron transport properties of photoswitching dithienylethene attached to Au leads in both the open and closed conformations. The molecule is found to be a good conductor in both conformations, with the low-bias current for the closed one being about 20 times larger than that of the open. Importantly, the current voltage characteristics away from the linear response are largely determined by molecular orbital rehybridization in an electric field, in close analogy to what happens for Mn-12 molecules. However, in the case of dithienylethene attached to Au, such a mechanism is effective also in conditions of strong electronic coupling to the electrodes. This makes the dithienylethene family an intriguing materials platform for constructing highly conducting organic optoelectronics switches.
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| 42. |
- Punkkinen, Marko Patrick John, et al.
(författare)
-
Compressive Surface Stress in Magnetic Transition Metals
- 2011
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 106:5, s. 057202-
-
Tidskriftsartikel (refereegranskat)abstract
- Because of the increased electron density within the surface layer, metal surfaces are generally expected to have tensile surface stress. Here, using first-principles density functional calculations, we demonstrate that in magnetic 3d metals surface magnetism can alter this commonly accepted picture. We find that the thermodynamically stable surfaces of chromium and manganese possess compressive surface stress. The revealed negative surface stress is shown to be ascribed to the enhanced magnetic moments within the surface layer relative to the bulk values.
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| 43. |
- Punkkinen, M.P.J., et al.
(författare)
-
Surface properties of 3d transition metals
- 2011
-
Ingår i: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 91:27, s. 3627-3640
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Tidskriftsartikel (refereegranskat)abstract
- Using the projector augmented wave method within density functional theory, we present a systematic study of the layer relaxation, surface energy and surface stress of 3d transition metals. Comparing the calculated trends for the surface energy and stress with those obtained for 4d and 5d metals we find that magnetism has a significant effect on the surface properties. Enhanced surface magnetic moments decrease the size of the surface relaxation, lower the surface energy and surface stress, leading to compressive stress in Cr and Mn.
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| 44. |
- Ropo, M., et al.
(författare)
-
First-principles atomistic study of surfaces of Fe-rich Fe-Cr
- 2011
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23:26, s. 265004-
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Tidskriftsartikel (refereegranskat)abstract
- The surface properties of Fe-rich ferromagnetic Fe-Cr alloys are investigated using a first-principles quantum-mechanical method. In dilute alloys, the surfaces are dominated by Fe, whereas the Cr-containing surfaces become favorable when the bulk Cr concentration exceeds the limit of similar to 10 atomic per cent. The abrupt change in the surface behavior is the consequence of complex competing magneto-chemical interactions between the alloying atoms. Considering the quantities of various features: equilibrium surface profiles, chemical potentials, segregation energies, surface energies, magnetic moments, mixing energies and pair interactions, within a wider range of bulk and surface concentrations enables us to build a comprehensive picture of the physics of Fe-Cr surfaces. Using the present achievements many previously controversial results can now be merged into a consistent model of Fe-rich Fe-Cr alloys.
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| 45. |
- Stishov, Sergei M., et al.
(författare)
-
Lost Heat Capacity and Entropy in the Helical Magnet MnSi
- 2010
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 105:23, s. 236403-
-
Tidskriftsartikel (refereegranskat)abstract
- The heat capacity of MnSi at B = 0 and B = 4 T was measured in the temperature range 2.5-100 K. To analyze the data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. The calculated phonon frequencies were confirmed by means of inelastic neutron scattering. The analysis of the data suggests the existence of negative contributions to the heat capacity and entropy of MnSi at T > T-c that may imply a specific ordering in the spin subsystem in the paramagnetic phase of MnSi.
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| 46. |
- Zhang, Hualei, et al.
(författare)
-
Alloying effects on the elastic parameters of ferromagnetic and paramagnetic Fe from first-principles theory
- 2011
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 110:7, s. 073707-
-
Tidskriftsartikel (refereegranskat)abstract
- The elastic properties of paramagnetic face-centered-cubic (fcc) Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys are investigated using the exact muffin-tin orbitals density functional method in combination with the coherent-potential approximation. We find that the theoretical lattice parameter of fcc Fe is strongly enlarged by Al, V, and Rh and slightly reduced by Si, Cr, and Co, while it remains nearly constant with Mn and Ni. Both positive and negative alloying effects appear for the elastic constants C(ij)(x) of fcc Fe. These findings are in contrast to those obtained for ferromagnetic body-centered-cubic (bcc) Fe alloys, where all alloying elements considered here are predicted to enlarge the lattice parameter and decrease the C(11)(x) and C(12)(x) elastic constants of bcc Fe. With some exceptions, alloying has much larger effects on ferromagnetic bcc alloys than on paramagnetic fcc ones. Based on the theoretical elastic parameters of the paramagnetic fcc and ferromagnetic bcc phases, simple parameterizations in terms of chemical composition of the equilibrium lattice constants, single-crystal elastic constants, and polycrystalline elastic moduli of Fe-based alloys are presented.
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| 47. |
- Zhang, Hualei, et al.
(författare)
-
Density-functional study of paramagnetic iron
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:14, s. 140411-
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Tidskriftsartikel (refereegranskat)abstract
- By using density-functional theory in combination with the coherent-potential approximation and the disordered local magnetic moment picture, we demonstrate that the competing high-temperature cubic phases of paramagnetic Fe correspond to two distinct total energy minima in the tetragonal (Bain) configurational space. Both the face-centered-cubic (fcc) and the body-centered-cubic (bcc) lattices are dynamically stable, and at static conditions the fcc structure is found to be the thermodynamically stable phase. The theoretical bcc and fcc bulk parameters are in agreement with the experimental data. Due to the shallow energy minimum around the bcc structure, increasing temperature is predicted to stabilize the bcc (δ) phase against the fcc (γ) one.
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| 48. |
- Zhang, Hualei, et al.
(författare)
-
Single-crystal elastic constants of ferromagnetic bcc Fe-based random alloys from first-principles theory
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:18, s. 184105-
-
Tidskriftsartikel (refereegranskat)abstract
- The elastic properties of ferromagnetic Fe1-xMx (M=Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys in the body-centered-cubic (bcc) crystallographic phase have been studied using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The theoretical lattice parameters and the single-crystal elastic constants agree well with the available experimental data. The most significant alloying effects are found for Al, Si, and Ni additions. All elements enlarge the lattice parameter and decrease the C-11, C-12, and C' elastic constants and the bulk modulus of bcc Fe. At the same time, C-44 is found to increase with Al, Si, V, Cr, or Mn and remain nearly constant with Co, Ni, and Rh. Accordingly, the elastic anisotropy of bcc Fe increases with all alloying elements considered here. The calculated alloying effects on the single-crystal elastic constants are shown to originate from volume effects in combination with the peculiar electronic structure of bcc Fe.
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| 49. |
- Zhang, Hualei, et al.
(författare)
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Static equation of state of bcc iron
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:13, s. 132409-
-
Tidskriftsartikel (refereegranskat)abstract
- Body-centered-cubic (bcc) iron is one of the most investigated solid-state systems. Using four different density-functional methods, we show that there is a magnetic transition close to the ground-state volume of bcc Fe, which originates from the particular magnetic band structure. The common equation of state functions, used to determine the basic ground-state physical quantities from the calculated total energies, cannot capture the physics of this magnetic transition leading to serious underestimation of the Fe bulk modulus. Ignorance of the magnetic transition found here is reflected by large scatter of the published theoretical bulk moduli of ferromagnetic bcc Fe. Due to the low performance of the exchange-correlation functionals, most of the erroneous results are accidentally in good agreement with the experimental values. The present finding is of fundamental importance, especially taking into account that bcc Fe is frequently used as a test system in assessing the performance of exchange-correlation approximations or total-energy methods.
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| 50. |
- Zhang, Hualei, et al.
(författare)
-
Theoretical elastic moduli of ferromagnetic bcc Fe alloys
- 2010
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 22:27, s. 275402-
-
Tidskriftsartikel (refereegranskat)abstract
- The polycrystalline elastic parameters of ferromagnetic Fe1-xMx (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 <= x <= 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poisson's ratio (nu) of bcc Fe. The complex composition dependence of the C-44 single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Young's modulus (E), and Debye temperature (Theta). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects.
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