Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W23: Metals: Alloys and Compounds |
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Sponsoring Units: DCMP Chair: Stefano Curtarolo, Duke University Room: Colorado Convention Center 110 |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W23.00001: Anomalous electronic correlations in ground state momentum density of Al$_{97}$Li$_3$ B. Barbiellini, J. Kwiatkowska, S. Kaprzyk, A. Bansil, H. Kawata, N. Shiotani We report high resolution Compton scattering measurements on an Al$_{97}$Li$_3$ disordered alloy single crystal for momentum transfer along the [100], [110] and [111] symmetry directions [1]. The results are interpreted via corresponding KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation) first principles computations. By comparing spectra for Al$_{97}$Li$_3 $ and Al, we show that the momentum density in the alloy differs significantly from the predictions of the conventional Fermi liquid picture and that the ground state of Al is modified anomalously by the addition of Li. Work supported in part by the USDOE.\\ \mbox{[1]} J. Kwiatkowska, B. Barbiellini, S. Kaprzyk, A. Bansil, H. Kawata, and N. Shiotani, Phys. Rev. Lett. 96 (2006) 186403. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W23.00002: Pseudo-gap Observed at Martensite Transition in Ni$_{2}$MnGa Single Crystal C.P. Opeil, J.C. Lashley, R.K. Schulze, B. Mihaila, W.L. Hults, J.L. Smith, P.S. Riseborough, L. Manosa, A. Planes Specific heat and coefficient of linear thermal expansion coefficient measurements show that the ferromagnetic shape-memory alloy Ni$_{2}$MnGa single crystal exhibits a pre-martensitic transition (PMT) at T = 214 K and a MT at 196 K. Lee \textit{et al.}\footnote{Y. Lee, J. Y. Rhee, B. N. Harmon, \textit{Physical Review B} \textbf{66}, 054424 (2002).} predicts that magnetically-tuned Fermi-surface nesting at \textbf{q} = (2$\pi $/3a) (1,1,0) is responsible for phonon softening at the PMT. On the basis of temperature dependant angle resolved photoemission (ARPES) measurements, we show that a pseudo-gap opens at 0.3 eV below the Fermi energy at the martensitic transition (MT) thus providing further evidence that the Fermi surface is nested at the MT and is only partially nested at the PMT. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W23.00003: First-principles electronic structure of shape-memory alloy Ni$_2$MnGa Bogdan Mihaila, Alice Acatrinei, Christopher D. Taylor, Cyril P. Opeil, Llu\'is Ma\~nosa The Ni$_2$MnGa memory-shape alloy undergoes both a feromagnetic phase trasition (T$_C$~$\sim$~380~K) and a martensitic transformation (MT, T$_M$~$\sim$~175~K) upon cooling. In addition, the MT is preceeded by a premartensitic (pre-MT) phase transition corresponding to a micromodulated structure accompanied by phonon softening presumably related to Fermi-surface nesting and strong electron-phonon coupling. Here, we report results of a comparison study of first-principles electronic structure calculations and recent angle-resolved photoemission measurements, at temperatures T=219~K, in close proximity with the pre-MT, and T=173~K, in the martensite phase. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W23.00004: Electronic Structure Theory of the Structural Transformation of Shape Memory NiTi Nicholas Hatcher, Oleg Kontsevoi, Arthur Freeman The unique property of displacive phase transformations has created much interest in the study of martensitic materials. Among them, NiTi finds a wide range of applications due to its shape memory behavior; however, the detailed mechanism of its structural evolution during the martensitic transformation is not fully understood. We have investigated the transition paths between the B2, R, B19, and B19' phases using the highly-precise FLAPW method\footnote{Wimmer,\,Krakauer,\,Weinert, \,and\,Freeman,\,Phys.\,Rev.\,B,\,{\bf 24},\,864\,(1981)} to identify the governing processes of the transformation through calculations of the total energy, electronic structure, elastic moduli, and shear energetics. For example, we show the role of topological shifts of the Fermi surface and band structure evolution with the changing monoclinic angle of the B19' and R phases. In addition, the magnetism and magnetic susceptibility of the phases are investigated using both fixed spin moment and Stoner enhancement calculations. To date, we find the B2, B19, and B19' phases to be Stoner enhanced paramagnets. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W23.00005: Possible metastable rhombohedral states of the bcc transition metals Michael Mehl, Daniel Finkenstadt The energy $E(c/a)$ for a bcc element stretched along its [001] axis (the Bain path) has a minimum at c/a = 1, a maximum at c/a = $\sqrt2$, and an elastically unstable\footnote{M. J. Mehl, A. Aguayo, L. L. Boyer, and R. De Coss, {\em Phys. Rev. B} {\bf 70}, 014105 (2004).} local minimum at c/a $>~\sqrt2$. A rhombohedral strain is an alternative method of connecting the bcc and fcc structures. The primitive lattice keeps $R\overline3m$ symmetry, with the angle $\alpha$ changing from 109.4$^\circ$ (bcc), to 90$^\circ$ (simple cubic), to 60$^\circ$ (fcc). We studied this path for the non-magnetic bcc transition metals (V, Nb, Mo, Ta, and W) using both a full-potential LAPW and PAW VASP. Except for Ta, the energy $E(\alpha)$ has a local {\em maximum} at $\alpha=60^\circ$, with local minima near 55$^\circ$ and 70$^\circ$, the later having lower energy. We studied the elastic stability of the 70$^\circ$ minimum structure. Only W is elastically stable in this structure, with the smallest eigenvalue of the elastic tensor at 4~GPa, while the other three elements are unstable. We discuss the possibility that Tungsten is actually metastable in this structure. We also consider the possible epitaxial growth of this structure. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W23.00006: The pressure-induced bcc-hcp structural transformation in iron Babak Sadigh, Micheal Surh We study within the framework of the spin density-functional theory the microscopic mechanism of the bcc-to-hcp martensitic transformation in iron from first principles. We investigate the correlation of the pressure-induced structural instability of the bcc phase with the disappearance of ferromagnetism in this system. We show that our calculations can shed new light on recent novel shock experiments in single-crystalline iron. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W23.00007: Prediction of a metastable cubic phase for the transition metals with hcp ground state. Romeo de Coss, Aaron Aguayo, Gabriel Murrieta The discovery of a metastable phase for a given material is interesting because corresponds to a new bonding and new properties are expected. The calculation of the total-energy along the Bain path is frequently used as a method to find tetragonal metastable states. However, a local minimum in the tetragonal distortion is not a definitive proof of a metastable state, and the elastic stability needs to be evaluated. In a previous work, using the elastic stability criteria for a cubic structure, we have shown that the transition metals with hcp ground state; Ti, Zr, and Hf have a fcc metastable phase [Aguayo, G. Murrieta, and R. de Coss, Phys. Rev. B \textbf{65}, 092106 (2002)]. That result is interesting since the fcc crystal structure does not appear in the current pressure-temperature phase diagram of these metals, and support the experimental observations of fcc Ti and Zr in thin films. In the present work, we extend the elastic stability study of the fcc structure to the non-magnetic transition metals with hcp ground state; Sc, Ti, Y, Zr, Tc, Ru, Hf, Re, and Os. We find that all the metals involved in this study have a metastable fcc structure. From these results, substrates on which the fcc structure of these metals could be growth epitaxially are predicted. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W23.00008: Anharmonic Phonons in Vanadium Alloys and Compounds. Olivier Delaire, Max Kresch, Matthew Lucas, Rebecca Stevens, Jorge Munoz, Brent Fultz Using inelastic neutron scattering, we investigated the temperature-dependence of the phonon density of states (DOS) of pure BCC vanadium and vanadium alloys V-6.25{\%}X, with X a transition metal solute, as well as the A15 compounds V3Si and V3Ge. Phonons in pure vanadium exhibit an anomalous stiffening with increasing temperature up to 1000C. The addition of 6.25{\%} impurities in solid solution changes this behavior. Solutes to the right of vanadium in the periodic table induce a reversal to the expected quasiharmonic softening with thermal expansion, solutes to the left increase the stiffening. V3Si and V3Ge also exhibit strong phonon stiffenings up to 500C. Anharmonic couplings arising from phonon-phonon or electron-phonon interactions are used to explain the departure from the quasiharmonic behavior. Using differential scanning calorimetry, we measured the heat capacity of vanadium and its alloys up to 1400C, and related it to the temperature-dependent phonon DOS. We compare our findings to theoretical studies of the effect of phonon-phonon and electron-phonon couplings on the heat capacity. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W23.00009: Ab-initio calculations of phonon properties for the Nb-Mo alloy using the virtual crystal approximation O. de la Pe\~{n}a-Seaman, R. de Coss, R. Heid, K.-P. Bohnen We have studied the structural, electronic, and lattice dynamic properties of the Nb$_{1-x}$Mo$_x$ alloy within the framework of the density functional perturbation theory, using the mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. The calculations were performed for both LDA and GGA $xc$-functional approximations. The structural parameters were optimized via the total-energy method for the whole range of Mo-concentration (0$\leq$x$\leq$1). The electronic properties were analyzed in terms of the electronic topological transitions (ETT) in the Fermi surface. The calculated phonon dispersion curves for the different Mo-concentrations are in very good agreement with experimental data reported in the literature. We find that LDA results are in general higher in frequency than GGA. The evolution of the Kohn anomaly in the $\Gamma$–H direction observed experimentally in the Nb-Mo alloy is well reproduced by the VCA calculations. Thus, we have shown that the virtual crystal approximation as implemented in the present work is useful for the study of random intermetallic alloys where the alloying elements are adjacent in the periodic table. This research was supported by CONACYT, Mexico under Grant No. 43830-F. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W23.00010: Electronic Gr\"{u}neisen Parameter in the Non-equilibrium Regime Jincheng Wang, Chunlei Guo The Gr\"{u}neisen parameter is a fundamental parameter characterizing the relationship between thermal expansion and specific heat of a solid. Conventionally, electronic Gr\"{u}neisen parameter\textit{ $\gamma $}$_{e}$ is measured in a solid by minimizing the lattice contribution to thermal expansion at low temperatures, and the value of \textit{$\gamma $}$_{e}$ is believed to be a constant. In this report, we perform pump-probe experiments using surface plasmon as the probe technique to resolve the dynamics of acoustic phonons that are impulsively excited in Ag film using fs laser pulses. Our study shows that the conventional value of electronic Gruneisen parameter is not necessarily valid in thermal non-equilibrium distribution immediately following ultrafast pulse excitation. A revised \textit{$\gamma $}$_{e}$ is proposed here to precisely take into account the role of electron pressure in driving acoustic phonons in solids. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W23.00011: A new intermetallic prototype? Verifying new structure predictions in CdPt and PtPd Gus Hart A recent data-mining approach to predicting the structure of intermetallic compounds has suggested that the CdPt and PtPd systems harbor a new crystal structure. Unlike other such predictions, this one is unique. The structure, never seen in any other fcc- based intermetallic, contains only four atoms per unit cell. Furthermore, this structure is the only one of this small size, except L1$_2$, that cannot be characterized as a sequential stacking of planes containing only A or B atoms. The stability of such a structure is tested using an exhaustive search of a cluster-expansion Hamiltonian. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W23.00012: Novel ground states in mixed bcc/fcc, high-/low-spin Fe-(Ni,Pd,Pt) from first-principles Sergey V. Barabash, Roman V. Chepulskii$^3$, Volker Blum$^4$, Alex Zunger Among the observed ordered phases of Fe-X (X=Ni,Pd,Pt), some structures are curiously absent: L1$_2$ Fe$_3$Pt exists, but Fe$_3$Pd and Fe$_3$Ni do not; L1$_0$ FePt and FePd exist, but the stability of L1$_0$ FeNi is debated. Furthermore, the recently measured short range order (SRO) in Fe-rich Fe-Ni is at odds with L1$_2$-type SRO. Theory has been hindered by the appearance of geometric (bcc/fcc) and magnetic (high-spin [HS]/low-spin [LS]) bi-stabilities. We address such bi-stabilities by performing first-principles mixed-basis cluster expansion with added geometric and magnetic ``filters,'' separating structures according to HS/LS and ``degree'' of fcc-ness and bcc-ness. We performed separate fcc HS and (for Fe-Pd) bcc HS cluster expansions. We predict that {\em at low temperatures}: (i) New ordered structures exist, including fcc Pt$_8$Ti-type FeX$_8$ and the (100) superlattices Fe$_2$Pd and Fe$_2$Pt; (ii) {\em All } fcc Fe$_3$X compounds are unstable w.r.t. bcc Fe + L1$_0$ FeX (iii) Fcc FePd is unstable w.r.t. bcc Fe + (100) Fe$_2$Pd superlattice, while L1$_0$ FeNi is, in fact, stable w.r.t. bcc Fe + L1$_2$ FeNi$_3$. At high T, (iv) The SRO in Fe-rich Fe-Ni is governed by (100) superlattice ordering. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W23.00013: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W23.00014: The Role of Displacement Short Range Order in the Determination of Higher Order Correlation Yevgeniy Puzyrev, Don Nicholson, Malcolm Stocks, Gene Ice Scattering experiments are routinely used to measure pair correlation in solid solutions. Only in special cases can model structures that reproduce the pair correlation be used to determine higher order correlations. However, the requirement that measured displacement short range order also be reproduced by the model further constrains the possible higher order correlation. The determination of further restrictions on higher order correlation relies on the use of a model for atomic displacement in terms of local environment. We illustrate this procedure for a 1-d lattice-liquid1using a simple displacement model and for NiFe using ab initio calculations to relate atomic displacements to local environment. \newline (1) Frank H. Stillenger and Salvatore Torquanto, J. Phys. Chem. B 108 19589 (2004). [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W23.00015: Short-range atomic ordering in magnetostrictive Fe-Ga alloys Yingzhou Du, Robert McQueeney, Mianliang Huang, Thomas Lograsso, Deborah Schlagel, Sung Chang, Douglas Robinson Fe$_{1-x}$Ga$_{x}$ alloys are known for having large magnetostriction (MS) while maintaining good ductility. Further improvement of the MS properties appears to be limited by Ga ordering. As Ga composition is increased, the MS coefficient is observed to peak, and then drop rapidly, when the system crosses over from a solid solution to an ordered alloy. X-ray diffuse scattering was used to study the Ga short-range order (SRO) as a function of composition and heat treatment. The data indicate the development of DO$_{3}$-type SRO with increasing Ga composition in the BCC solid solution. For the slow-cooled samples, the correlation length and area of the SRO scattering both increase dramatically beyond the peak in the MS at x = 0.18, indicating that the development of Ga clusters beyond a certain size limits the MS . The trends for the quenched samples are similar, but not as clear as the slow-cooled ones. The results indicate that nanoscale sized Ga clusters act to enhance the magnetoelastic coupling. [Preview Abstract] |
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