Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N11: Structural and Magnetic Phase Transitions |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Craig Fennie, Argonne National Laboratory Room: Colorado Convention Center Korbel 1F |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N11.00001: Ti-V alloy phase diagram: DFT-based cluster approach Michael R. Fellinger, John W. Wilkins, Dallas R. Trinkle V is an important alloying element for Ti since it stabilizes the high-temperature bcc phase of Ti at lower temperatures, while minimizing the formation of intermetallic compounds [1]. Theoretical determination of the Ti-V phase diagram must take into account structures based on both bcc and hcp lattices over the full V concentration range. With the cluster expansion formalism for alloys [2], DFT calculations determine the energies of all structures necessary for constructing the cluster expansion for the energetics of the alloy. Preliminary results indicate the DFT-based cluster approach determines solubility limits and transformation temperatures that are in good agreement with the experimental phase diagram [3]. \newline [1] M. J. Donachie, Jr., \textit{Titanium: A Technical Guide}. ASM International: Metals Park, OH (1988). [2] D. de Fontaine, Solid State Phys. \textbf{47}, 33 (1994). [3] T. B. Massalski, et al., \textit{Binary Alloy Phase Diagrams}, Vol. 2. ASM International: Metals Park, OH (1986). [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N11.00002: Phase Transition to Modulated Cubic Phase in AuZn alloy S. M. Shapiro, J. C. Lashley, W. Ratcliff, D. J. Thoma, J. L. Smith AuZn is believed to undergo a martensitic transition from a high-temperature cubic phase to a reported low-temperature trigonal phase around 60K. Specific heat studies revealed that the transition is second-order for the 50-50 alloy, and first-order for off-stoichiometric compositions. Elastic neutron diffraction studies on a single crystal of the 50-50 alloy showed the surprising result that the martensitic transformation is suppressed and new Bragg peaks appear at q$_{o }$= (1/3,1/3,0), in a continuous manner. The low-temperature structure can be viewed as cubic with a 3-fold increase in the cell along the [110] direction, similar to what has been observed in premartensitic phases. Attempts to measure the phonon dispersion curves using inelastic x-ray scattering will also be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N11.00003: Structural Studies on First-Order Phase Transitions inRPd$_{2}$Ga$_{2}$ Robin Macaluso, L. Chapon, E. Goremychkin, R. Osborn, J. Mitchell, B. Rainford We have investigated the structure of RPd$_{2}$Ga$_{2}$ (R = La, Ce) compounds by neutron powder diffraction. For the first time, a first-order structural transition is observed at T$_{t}$ = 70 K and T$_{t}$ = 125 K for CePd$_{2}$Ga$_{2}$ and LaPd$_{2}$Ga$_{2}$, respectively. The high-temperature structure (T $>$ T$_{t})$ for both compounds is the tetragonal CaBe$_{2}$Ge$_{2}$ type with lattice parameters of a = 4.4791(4) {\AA} and c = 9.83732(17) {\AA} and a = 4.83185(19) {\AA} and c = 10.7548(5) {\AA} for LaPd2Ga2 and CePd2Ga2 at 305 K, respectively. Below T$_{t }$ the symmetry of both structures descends to an orthorhombic space group, \textit{Pmmn}. Lattice parameters at 2 K are $a $= 6.07032(3) {\AA}; $b $= 12.90921(6) {\AA}; $c $= 9.87899(5) {\AA} and $a$ = 6.3996(4), $b$ = 11.9508(8), $c$ = 9.9291(7) for LaPd$_{2}$Ga$_{2}$ and CePd$_{2}$Ga$_{2}$. In this talk, evidence for the order of the structural transition will be presented and the low and high temperature structures will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N11.00004: High pressure phases of alkali ternary borohydrides Ravhi Kumar, Andrew Cornelius Insitu synchrotron x-ray diffraction experiments were carried out on MBH$_{4}$ (M = K and Rb) borohydrides at high pressures up to 26 GPa using diamond anvil cells. KBH$_{4}$ undergoes a structural transition at 4 GPa to a tetragonal phase from cubic and then to an orthorhombic phase around 7 GPa which are very similar to NaBH$_{4}$ investigated earlier [1]. However, RbBH$_{4}$ shows, a direct transition from the ambient cubic to an orthorhombic phase at 2.9 GPa, followed by a monoclinic phase at 8 GPa. Complementary high pressure Raman experiments, support the transitions observed in the diffraction experiments. The results will be presented in detail. [1]. Ravhi S. Kumar and Andrew L. Cornelius, Appl.Phys.Lett., 87,261916 (2005) This work is supported in part by the U.S. Department of Energy (DOE) under Award Number DE-FG36 05GO85028. HPCAT is supported by DOE-BES, DOE-NNSA,NSF, and the W.M. Keck Foundation. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N11.00005: First-principles study of the Jahn-Teller distortion in transition metal dihydrides. Ramiro Quijano, Romeo de Coss The transition metal dihydrides TiH$_{2}$ and ZrH$_{2}$ present the fluorite structure (CaF$_{2})$ at high temperature but undergoes a tetragonal distortion with c/a$<$1 at low temperature. Early electronic band structure calculations have shown that TiH$_{2}$ and ZrH$_{2}$ in the cubic phase display a very flat band at the Fermi level. Thus the low temperature tetragonal distortion has been associated to a Jahn-Teller effect. Previous total energy calculations of the tetragonal distortion for TiH$_{2}$ within the Density Functional Theory (DFT), find that the ground state correspond to a tetragonal structure with c/a$>$1, in contradiction with the experimental observation (c/a$<$1). In the present work, we have performed full-potential LAPW calculations using the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA) for the exchange correlation functional energy. Special attention was paid to the convergence of the total-energy calculations, since in TiH$_{2}$ the energy differences for a tetragonal distortion at constant volume are only fractions of 1 mRy. We find that the ground state of TiH$_{2}$ and ZrH$_{2}$ corresponds to a tetragonal distorted fluorite structure with c/a$<$1, in agreement with the experimental observations. The same behavior is predicted for HfH$_{2}$. The electronic band structure of the three systems is analyzed in the context of the Jahn-Teller effect. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N11.00006: Phonon-based Mechanisms for Structural Transformations in Pu Turab Lookman, Avadh Saxena, Robert Albers Plutonium undergoes a number of transformations that involve changes in unit cell shape and numbers of atoms. We focus on the fcc to monoclinic $\delta \rightarrow \alpha^{\prime}$ transformation and suggest displacive mechanisms that can describe the transformation. We also explore signatures of the mechanisms in phonon dispersion curves and present a phase diagram in paramter space. The implications for the $\beta \rightarrow \alpha$ transformation, assuming a displacive mechanism, will also be explored. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N11.00007: Classical and quantum formalism of mode-based atomic-scale descriptions of lattice distortions K.H. Ahn, Jichan Moon We present classical and quantum mechanical atomic scale theory of lattice distortions using atomic scale modes and their constraint equations. The formalism is demonstrated for a 1-dimensional chain and a 2-dimensional square lattice. We comment on the application of the mode-based atomic-scale approach for systems with strong anharmonicity and structural phase transitions. \newline \newline [1] K. H. Ahn, T. Lookman, A. Saxena, and A. R. Bishop, Phys. Rev. B 68, 092101 (2003); \newline [2] K. H. Ahn, T. Lookman, and A. R. Bishop, Nature 428, 401 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N11.00008: Homogeneous nucleation of Lennard-Jones liquids Hui Wang, Harvey Gould, W. Klein We investigate the homogeneous nucleation of a Lennard-Jones liquid as a function of the degree of supercooling. The umbrella sampling method, with the size of the largest cluster as the order parameter, is used to compute the free energy cost of the nucleating droplets. Their saddle point nature is verified using an intervention technique. For moderate supercooling the nucleating droplets are found to be compact and spherical, with the core having fcc symmetry and the surface bcc. As the system is quenched near the spinodal, which corresponds to the vanishing of the free energy barrier, the nucleating droplets become more diffuse and anisotropic with no well defined core or surface. The environment of these nucleating droplets form randomly stacked planes, which is consistent with the spinodal nucleation picture. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N11.00009: Time-domain thermoreflectance of FeRh across magnetic/structural phase transition D.A. Walko, J. Wang, D.G. Cahill, J.-U. Thiele, E.E. Fullerton As FeRh is heated above $\sim100^{\circ}$ C, an antiferromagnetic to ferromagnetic phase transition is accompanied by an abrupt expansion in its lattice parameter of $\sim$0.5\%. This first-order phase transition has a large temperature hysteresis (often $>20^{\circ}$). Ultrafast laser pulses have been shown to heat thin FeRh films through the phase transitions on a picosecond time scale. We have used time-domain thermoreflectance (TDTR) in the temperature range 35 to 160$^{\circ}$ C to study FeRh films grown on MgO substrates. The TDTR measurements were performed with a mode-locked Ti:sapphire laser; the sample was slightly heated with the near-infrared pump beam, and small changes in the sample's reflectivity were observed with the delayed probe beam using lock-in detection. We used TDTR to measure the thermal conductance of the film/substrate interface. Additionally, the small temperature excursions produced in TDTR allowed us to observe the transient behavior of FeRh at various temperatures across the phase transition. The reflectivity is affected by fast changes in the film's optical properties and in its thickness, and we discuss the effects of hysteresis on the measurement. Work supported by U.S. Department of Energy. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N11.00010: Thermodynamic and transport anisotropic properties of RVSb3 crystals Athena S. Sefat, Sergey L. Bud'ko, Paul C. Canfield The RVSb3 series (R= La, Nd, Sm, Gd - Dy) offers the possibility of studying the magnetic ordering in materials with a single, crystallographically unique, rare-earth site. The anisotropic magnetization M(H, T), resistivity, and heat capacity C(T) results, on flux-grown crystals, will be presented. All of the compounds are metallic, and all, with the exceptions of non-magnetic LaVSb3 and ferromagnetic CeVSb3, show features typical of antiferromagnetic order below 10 K. For CeVSb3, the easy axis of magnetization is parallel to c in the ordered state, whereas for the antiferromagnetic RVSb3 members of Pr, Nd, Tb, and Dy, the crystalline fields confine the spins close to the a-axis. Given that CeVSb3 is a rare example of a Ce based ferromagnet, we measured the pressure dependence of Tc up to 10 kbar and found it to increase at a rate of 0.14 K/kbar. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N11.00011: X-ray resonant magnetic scattering study of the spin-flop transition in Gd$_5$Ge$_4$ Lizhi Tan, Shibabrata Nandi, Andreas Kreyssig, Shuang Jia, Alan Goldman, Robert McQueeney, Paul Canfield, Jonathan Lang, Zahirul Islam, Thomas Lograsso, Deborah Schlagel, Vitalij Pecharsky, Karl Gschneidner Gd$_5$Ge$_4$ crystallizes in the orthorhombic space group Pnma. Below 127~K the Gd moments order antiferromagnetically in a layered structure with a magnetic unit cell same as the chemical unit cell. The ferromagnetic Gd-rich slabs are stacked antiferromagnetically along b-axis The magnetic moments are primarily aligned along the c-axis. X-ray resonant magnetic scattering was used to study a fully reversible spin-flop transition in a single crystal Gd$_5$Ge$_4$ and to elucidate details of the magnetic structure in the spin-flop phase. The Gd moments at the three Wyckoff sites flop from c-axis antiferromagnetically aligned to a-axis antiferromagnetically aligned in a critical field $H_{sf} = 9$ kOe applied along c-axis at $T = 10$ K. The magnetic space group changes from $Pnm'a$ to $Pn'm'a'$ at all three sites. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N11.00012: Pressure-Dependent Magnetization Studies of Two Rare Earth-Based Intermetallic Systems R.P. Guertin, E.S. Choi, B. Andraka, C.R. Rotundu, W. McCallum, Y. Janssen Pressure dependent magnetization studies have been performed on two rare earth-based ternary intermetallic systems, R$_{2}$CoIn$_{8}$, where R=Gd, Dy and Pr, and Pr$_{6}$Ni$_{2}$Si$_{3}$, the n=2 member of the Pr$_{(n+1)(n+2)}$Ni$_{n(n+1)+2}$Si$_{n(n+1)}$ family. The pressure dependence of the magnetization was measured for 2$<$T$<$300 K, 0$<$H$<$ 9 T and hydrostatic pressures 0$\le $P$\le $8 kbar using a vibrating sample magnetometer. For the R$_{2}$CoIn$_{8}$ system, R=Dy and Gd order antiferromagnetically at T$_{N}$=17.0 and 34.5 K, respectively and dT$_{N}$/dP=+0.1 K/kbar for R=Dy and +0.4 K/kbar for R=Gd. Pr$_{2}$CoIn$_{8}$ is a van Vleck paramagnet, indicating a crystalline electric field (CEF) singlet ground state. For Pr$_{6}$Ni$_{2}$Si$_{3}$, the Curie temperature (T$_{C}$= 35.0 K) and the saturation magnetization (1.35 $\mu _{B}$/Pr) decrease non-linearly with increasing pressure, consistent with a pressure-induced increase in the CEF splitting. The Pr$^{3+}$ ground state is presumably a singlet, as the local Pr symmetry is very low. Preliminary high field VSM data suggest that a CEF level crossing occurs at 10.5 T where magnetization increases sharply to above 3.0 $\mu _{B}$/Pr. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N11.00013: The equivalency between hydrostatic pressure and Si doping in the giant magnetocaloric compound Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ studied by X-Ray Magnetic Circular Dichroism. Y.C. Tseng, D. Haskel, J. Lang, S. Sinogeikin, Ya. Mudryk, V.K. Pecharsky, K. Gschneidner Jr. The effect of pressure (P$\le $150 kbar) upon the magnetic properties of giant magnetocaloric material Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ (x=0.125, 0.5) was investigated by x-ray magnetic circular dichroism measurements in a diamond anvil cell. The ferromagnetic Curie temperature, Tc, increases linearly with pressure albeit with different slopes dTc/dP for x=0.125 and 0.5. This pressure dependence of Tc, together with a discontinuity in Tc (P) at $\sim $274K, are also observed in the x-T phase diagram. The equivalency of pressure and Si demonstrates that the magnetic properties in this class of materials are controlled by volume, and not by preferential substitution of Si/Ge at certain lattice sites. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N11.00014: Fluctuation induced first-order phase transitions in a dipolar Ising ferromagnetic slab Rafael M. Fernandes, Harry Westfahl Jr. We investigate the size effects on the magnetic phase diagram of an Ising ferromagnetic slab with finite width and finite thickness in which two interactions compete: the short-range strong exchange interaction and the long-range weak dipolar one. We show that the homogeneous ordered state is unstable towards the formation of a modulated phase and that thermal fluctuations induce a first-order Brazovskii transition. By considering striped and bubble modulated configurations, we show that the first has a lower energy and a higher spinodal limit and that in the most stable ordered phase the order parameter is modulated along the limited direction but uniform along the unlimited one. This effect is shown to be a consequence of finite lengths and of Dirichlet boundary conditions in a system with competing interactions. Applications of this model to the domain structure of thin films are discussed, specially for the case of MnAs:GaAs thin films, for which qualitative behaviors of the number of domains and of the mean value of modulation as functions of the temperature are outlined. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N11.00015: Hidden zero-temperature bicritical point in the 2D anisotropic Heisenberg model: Monte Carlo simulations and novel finite-size scaling Chenggang Zhou, David Landau, Thomas Schulthess The bicritical point in the phase diagram of the 2D anisotropic Heisenberg antiferromagnet in a field has not been fully resolved by Monte Carlo simulations. A recent study [Phys. Rev. B \textbf{72}, 064443 (2005)] showed an upper bound for the bicritical temperature. By performing quite detailed Monte Carlo simulations near the apparent spin-flop line, we found this system was governed by a single-spin Hamiltonian, which terminates the renormalization group flow of a finite-size 2D nonlinear $\sigma $ model. Using a novel finite-size scaling analysis, we confirm that the bicritical point in two dimensions is Heisenberg-like and occurs at T=0. Thus, the uncertainty in the phase diagram is completely removed [Phys. Rev. B \textbf{74}, 064407 (2006)]. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700