Session D23: Metals Theory Oxides Interfaces

Transferable tight-binding description of the Fe-C interaction

A coherent transferable tight-binding (TB) parameterization including magnetism has yet to be developed for the Fe-C interaction. Although interatomic potentials have been obtained for this system, recent findings show that the results from these potentials are inconsistent with DFT calculations and do not give an accurate portrayal of chemical bonding in the system. Using dual DFT grid and LCAO calculations within GPAW, we obtain one electron wave functions expanded in a multiple-$\zeta$ LCAO basis. This is then down-folded onto an optimal minimal basis, giving a continuous and transferable description of Fe-C bonding. By constructing a TB energy functional using these bond integrals and a parameterized interatomic repulsion, we show how an accurate description of the energy hierarchy of interstitial carbon in Fe-structures can be achieved. Furthermore, we use the model to calculate elastic properties and energies of a variety of Fe-carbides, defects, and carbon diffusion paths. This simple model based on physical insights may be used to study systems containing thousands of atoms.   

Empirical potentials from a combined density functional theory / genetic algorithm approach: Illustration for Xe in UO2

We have developed a new empirical potential for xenon in uranium dioxide with existing UO2 potentials that achieves accurate xenon defect energetics. This potential was fit to several snapshots of DFT+U molecular dynamics of a single defect configuration using the genetic algorithm code Iterative Potential Refinement (IPR). In IPR, the forces, stresses, and energies from DFT calculations are used to parameterize empirical potentials. Several random sets of parameters are uses to compare against DFT and the genetic algorithm minimizes the error of the parameters with respect to the DFT results. We compare this potential and other xenon potentials to DFT+U using a large set of defect calculations of xenon incorporated into sites with high, intermediate, low strain. Despite only being fit to a single configuration, our new empirical potential gives the very good agreement with DFT+U across a range of xenon incorporation sites and vastly outperforms existing xenon potentials.   

The dielectric response of single-crystal UO$_{2}$ probed by terahertz time-domain spectroscopy

We measured the complex dielectric function, \textit{$\varepsilon =\varepsilon $}$_{r}$\textit{+i$\varepsilon $}$_{i}$, of single-crystal UO$_{2}$ in the temperature (T) range of 5-500 K by terahertz (THz) time-domain spectroscopy to study its dielectric response. A critical temperature point of 60 K is found for the measured temperature dependence of \textit{$\varepsilon $}$_{r}(T)$. The dispersion of \textit{$\varepsilon (\omega )$} in the THz range follows the trend of the damped resonant mode of the transverse optical phonon at \textit{$\omega $}$_{TO}$=8.4 THz. Examining the Lyddane-Sachs-Teller relationship reveals mode-softening of \textit{$\omega $}$_{TO}$ in the temperature range of 60-120 K. We then performed optical-pump THz-probe measurements to study the dynamic dielectric response of UO$_{2}$ following photoexcitation. We observe a small pump-induced change in \textit{$\varepsilon $} that lasts for more than one millisecond, indicating heavy electron behavior of photoexcited 5f electrons resulting from strong electron-lattice interaction. The slow relaxation is attributed to the diffusion of optical phonons generated by photoexcited 5f electrons in the UO$_{2}$ crystal.   

Magnetocaloric Effect of NiFeCoCrPdx High Entropy Alloys

FeCoCrNi is one of many ``high entropy alloys'' (HEAs), which are multicomponent alloys with high entropy of mixing. These materials often have high hardness, and resistance to wear and corrosion, making them attractive for applications. Here, we report on the magnetic entropy change and magnetocaloric effect of the FeCoCrNiPdx system. The addition of Pd to FeCoCrNi has been shown to enable the critical temperature to be tuned from 130 K for x=0 to 500 K for x=2. Isothermal magnetization measurements were made on samples with x ranging from 0 to 0.50 as functions of temperature. The magnetic entropy change ($\Delta $S) was calculated using the thermodynamic Maxwell Relation. We find that Pd additions tune the peak $\Delta $S temperature from 130 to 300K, while modestly increasing the peak $\Delta $S magnitude. Interestingly, the addition of Pd leads to an almost doubling of the relative cooling power (RCP), as well as a notable change in the critical behavior of the material. The RCP of $\sim $35 J/kg for a 1T field change puts the Pd-containing HEAs in competition with other magnetocaloric materials in the 100-200 K operating range. This alloy system's combination of durability and a tunable Curie temperature without appreciable change in cooling power may make this system interesting for magnetocaloric applications.   

Magnetic and crystallographic properties of Cr1-xFexGe

According to previously published bulk measurements, Cr1-xFexGe exhibits a quantum critical point at x=0.75, where it turns from a paramagnet (for x<0.75) into a ferromagnet (for x>0.75). Cr1-xFexGe is a simple cubic B20 (FeSi) crystal. The endpoints of the alloy are binary compounds that have been studied to some degree. FeGe, the better known of the two, is a spiral ferromagnet similar to MnSi. However, less is known for CrGe, which is thought to be a weakly ferromagnetic paramagnet with bulk properties that may be explained by the paramagnon theory. We report new neutron scattering results on Cr1-xFexGe for x=0.6, 0.7, 0.75, 0.8.   

An {\it ab-initio} multiscale method to investigate chemical embrittlement of metals

We present a multiscale method, coupling a small region treated by first-principles quantum mechanics, to a large classical atomistics region. Our method is based on total energy arguments that are applicable to metals. We employ Kohn-Sham (KS) Density Functional Theory (DFT) in the region of interest and couple this to the classical Embedded Atom Method (EAM). Results for the chemical embrittlement of metals due to segregated impurities at grain boundaries (GB) are presented. We study the average interplanar strain surrounding the GB and use this as a measure of the atomic relaxation. We apply our method to study the chemical embrittlement of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities, which are known to segregate to the GB but not embrittle Cu. We find that Bi and Pb weaken and hence embrittle the Cu GB. In contrast Ag impurities at the GB plane increase cohesion.   

Molecular dynamics study of the thermopower of Ag, Au, and Pt nanocontacts

Using molecular dynamics simulations of many junction stretching processes combined with tight-binding-based electronic structure and transport calculations, we analyze the thermopower of silver (Ag), gold (Au), and platinum (Pt) atomic contacts. In all cases we observe that the thermopower vanishes on average within the standard deviation and that its fluctuations increase for a decreasing minimum cross section of the junctions. However, we find a suppression of the fluctuations of the thermopower for the s-valent metals Ag and Au, when the conductance originates from a single, perfectly transmitting channel. Essential features of the experimental results for Au, Ag, and copper (Cu) of Ludoph and van Ruitenbeek [Phys. Rev. B 59, 12290 (1999)], as yet unaddressed by atomistic studies, can hence be explained by considering the atomic and electronic structure at the disordered narrowest constriction of the contacts. For the multivalent metal Pt our calculations predict the fluctuations of the thermopower to be larger by one order of magnitude as compared to Ag and Au, and suppressions of the fluctuations as a function of the conductance are absent. Main features of our results are explained in terms of an extended single-level model.   

First-Principles Study of Hydrogen Permeation in Palladium-Gold Alloys

Density functional theory and lattice model calculations are combined to study the permeability of hydrogen in Pd lightly alloyed with Au. This study shows that small amounts of Au substitutions in Pd lead to, respectively, an increase and decrease of the diffusivity and solubility of hydrogen in the alloy. The competition between these two phenomena depends on temperature and can yield dilute PdAu membranes with a hydrogen permeability higher than pure Pd.   

Hydrogen Embrittlement in Zirconium: a Quasi-Continuum Density Functional Theory Study

The hydrogen embrittlement in Zirconium becomes a very important and emergent issue for academia, industry and policy makers as a result of the Japan nuclear accident. The hydride formation, diffusion and embrittlement in zircolay will impact dramatically on the development of advanced nuclear energy systems, the life time extension of the current nuclear fleet and dry storage of spent nuclear fuel. Quasi-Continuum Density Functional Theory (QCDFT) is a powerful concurrent multiscale method based entirely on density functional theory (DFT) and allows quantum simulations of materials properties of a large system with billions of atoms. Using QCDFT modeling, we found that the presents of hydrogen at the cracktip of zirconium, both on crack surface and in-bulk, will form zirconium hydrides and embrittle the system. The concentration of hydrogen and orientation of crack plays important roles in such embrittlement. The mechanism of hydrogen embrittlement under various loading conditions will be discussed.   

Effects of disorder on the T-dependent bandstructure of purple bronze Li$_2$Mo$_{12}$O$_{34}$

The band structures of ordered and disordered Li$_2$Mo$_{12}$O$_{34}$ are calculated by use of ab-initio DFT-LMTO method. The unusual band dispersion in the z-direction obtained in previous band calculations is confirmed for the ordered structure, and the overall band structure agree reasonably with existing photoemission data. The T-dependent band broadening is calculated from configurations with thermal disorder of the atomic positions within the unit cell. The band structure shows important band broadening of the two bands at the Fermi energy. The bands are particularly sensitive to in-plane movements of Mo sites. Already disorder due to zero-point motion makes a band broadening of the order of 20 meV and creates a sizable band overlap. The effect of Li vacancies on the two bands is relatively small.   

Metamagnetic transition self-propelled by the spin injection

We study metamagnetic phase transition of itinerant electrons controlled by the spin injection mechanism. The current flow between a ferromagnetic metal and a metamagnetic metal produces the non-equilibrium shift of chemical potential for spin-up and spin-down electrons. This shift acts as an effective magnetic field driving the metamagnetic transition between low and high magnetization states of the metamagnet in the vicinity to the contact with the ferromagnet. We show that high magnetization state of the metamagnet self propels into the bulk of the metamagnet and the length of this state has threshold dependence on the electrical current.   

Generalized Statistical Thermodyanmics Applied to Small Material Systems

When characterizing the behavior of small material systems, surface effects can strongly influence the thermodynamic behavior and need to be taken into account in a complete thermal physics analysis. Although there have been a variety of approached proposed to incorporate surface effects, they are often restricted to certain types of systems (e.g., those involving incompressible phases) and often invoke thermodynamics parameters that are often not well-defined for the surface. It is proposed that a generalized statistical mechanics based on the concept of thermodynamic availability (exergy) can be formulated from which the surface properties and their influence on system behavior can be naturally and rigorously obtained. This availability-based statistical thermodynamics will be presented and its use illustrated in a treatment of nucleation during crystallization.   

$d + i d$ superconductivity on the honeycomb bilayer

We demonstrate that for interlayer attractive interactions on bilayer honeycomb lattice with large interlayer hopping, a time reversal symmetry breaking $d$-wave topological superconductor is a dominant phase. We find that small momentum order parameter expansion has $d_{x^2-y^2} + i d_{xy}$ symmetry around both Dirac points and discuss a possible relevance of this state for experiments on graphene bilayer.