| 31. |
- Al-Zoubi, Noura, et al.
(författare)
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Tetragonality of carbon-doped ferromagnetic iron alloys : A first-principles study
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:1, s. 014112-
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Tidskriftsartikel (refereegranskat)abstract
- Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.
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| 32. |
- Alam, Aftab, et al.
(författare)
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Effect of disorder on the electronic properties of graphene : A theoretical approach
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:8, s. 085454-
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Tidskriftsartikel (refereegranskat)abstract
- In order to manipulate the properties of graphene, it is very important to understand the electronic structure in the presence of disorder. We investigate, within a tight-binding description, the effects of disorder in the on-site (diagonal disorder) term in the Hamiltonian as well as in the hopping integral (off-diagonal disorder) on the electronic dispersion and density of states by the augmented space recursion method. Extrinsic off-diagonal disorder is shown to have dramatic effects on the two-dimensional (2D) Dirac cone, including asymmetries in the band structures as well as the presence of discontinuous bands (because of resonances) in certain limits. Disorder-induced broadening, related to the scattering length (or lifetime) of Bloch electrons, is modified significantly with increasing strength of disorder. We propose that our methodology is suitable for the study of the effects of disorder in other 2D materials, such as a boron nitride monolayer.
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| 33. |
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| 34. |
- Allaf Navirian, Hengameh, et al.
(författare)
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Acoustically driven ferroelastic domain switching observed by time-resolved x-ray diffraction
- 2010
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 81:2
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Tidskriftsartikel (refereegranskat)abstract
- Domain polarization switching in potassium dihydrogen phosphate (KH2PO4, KDP) induced by a propagating strain wave has been observed with time-resolved x-ray diffraction. A pulsed electric field with amplitude of 6 kV/cm and duration of 1 mu s was applied along the crystallographic c axis. The field-induced strain waves emanating from the sample surfaces are the result of the converse piezoelectric effect. In the center of the probed surface two waves interfered constructively inducing ferroelastic domain switching, in the absence of an external electric field, at a delay of 3 mu s, corresponding to acoustic propagation at a velocity found to be 1500 m/s.
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| 35. |
- Alling, Björn, 1980-, et al.
(författare)
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A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:10, s. 104203-
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Tidskriftsartikel (refereegranskat)abstract
- We use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.
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| 36. |
- Alling, Björn, et al.
(författare)
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Configurational disorder effects on adatom mobilities on Ti1-xAlxN(001) surfaces from first principles
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : american physical society. - 1098-0121 .- 1550-235X. ; 85:24, s. 245422-
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Tidskriftsartikel (refereegranskat)abstract
- We use metastable NaCl-structure Ti0.5Al0.5N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution during film growth. First-principles calculations were employed to obtain energy potential maps of Ti and Al adsorption on an ordered TiN(001) reference surface and a disordered Ti0.5Al0.5N(001) solid-solution surface. The energetics of adatom migration on these surfaces are determined and compared to isolate effects of configurational disorder. The results show that alloy surface disorder dramatically reduces Ti adatom mobilities. Al adatoms, in sharp contrast, experience only small disorder-induced differences in migration dynamics.
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| 37. |
- Alling, Björn, et al.
(författare)
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Dynamic and structural stability of cubic vanadium nitride
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 91:5, s. 054101-
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Tidskriftsartikel (refereegranskat)abstract
- Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.
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| 38. |
- Alling, Björn, 1980-, et al.
(författare)
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Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American institute of physics. - 1098-0121 .- 1550-235X. ; 82, s. 184430-
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Tidskriftsartikel (refereegranskat)abstract
- Two different methods for the modeling of a magnetically disordered CrN stateusing a supercell approach are investigated. They are found to give equivalentresults of the total energy, being also similar to results obtained with an effectivemedium approach. Furthermore, CrN is shown to be better described using aLDA+U framework for the treatment of electron-electron correlations as comparedto GGA or LDA calculations. Modeling the cubic paramagnetic phase with ourmodels for magnetic disorder and considering the strong electron correlations, thetemperature and pressure induced phase transitions in CrN can be explained.
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| 39. |
- Alling, Björn, et al.
(författare)
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Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:13
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1-xCuxMnSb alloys as a case study. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase T-C in Cu-rich samples.
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| 40. |
- Alling, Björn
(författare)
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Metal to semiconductor transition and phase stability of Ti1-xMgxNy alloys investigated by first-principles calculations
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 89:8, s. 085112-
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Tidskriftsartikel (refereegranskat)abstract
- Titanium nitride based materials are applied in several technological applications owing to their stability at high temperatures, mechanical and optical properties, as well as good electrical conductivity. Here, I use theoretical first-principles methods to investigate the possibilities for a semiconducting state as well as the phase stability of Ti1-x MgxNy ternary alloys and compounds. I demonstrate that B1 (NaCl) solid solutions of Ti1-xMgxN with x less than= 0.5 are thermodynamically stable with respect to all previously reported phases in the Ti-Mg-N system. For the composition x = 0.5, an ordered TiMgN2 phase with an L1(1)-type order of Ti and Mg on the metal sublattice is predicted to be the configurational ground state at low temperatures. Other ordered phases present in neighboring materials systems are considered but are found to be unstable. The electronic origin of the stability of the B1 structure solid solutions is identified. A metal to semiconductor transition is observed as the Mg content is increased to x = 0.5. TiMgN2 as well as disordered Ti0.5Mg0.5N solid solution are investigated with hybrid functional calculations and predicted to be semiconductors with band gaps of 1.1 eV and around 1.3 eV, respectively, the latter depending on the details of the Ti and Mg configuration.
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