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Pressure-induced changes in the electronic structure of solids

Description: A variety of high-pressure metalization and metal-semimetal transitions, crystallographic phase transitions, and equation of state and lattice vibrational anomalies are reviewed in terms of the concepts of electronic transition and pressure-induced loss of covalency. 46 refs., 10 figs.
Date: July 1, 1985
Creator: McMahan, A.K.
Partner: UNT Libraries Government Documents Department

Metallization of some simple systems

Description: We discuss the metallization of Xe, Ar, He, I/sub 2/, H/sub 2/, and N/sub 2/ in terms of some recent theoretical work and shock-wave experiments. New shock-wave data on liquid hydrogen and deuterium leads to a predicted pressure above 3 Mbar for the appearance of a monatomic metal phase. We expect CsI to become metallic near 0.8 Mbar.
Date: January 1, 1981
Creator: Ross, M. & McMahan, A.K.
Partner: UNT Libraries Government Documents Department

Shell structure effects in compressed aluminum

Description: Self-consistent augmented-plane-wave (APW) calculations of the T = 0 aluminum pressure-volume isotherm are reported from normal density to essentially the free-electron limit, and are compared to predictions based on statistical models. Two distinct density regimes are observed in which the APW and statistical model results differ fundamentally. In the first the L shell electrons are pressure ionized and in the second the K shell electrons are pressure ionized. The first of these occurs in the range 5-75 TPa, and the second in the range 2.3 x 10/sup 3/ - 1.1 x 10/sup 5/ TPa.
Date: June 16, 1977
Creator: McMahan, A. K. & Ross, M.
Partner: UNT Libraries Government Documents Department

Positron annihilation and pressure-induced electronic s-d transition

Description: The polycrystalline, partial annihilation rates for positrons in compressed cesium have been calculated using the linear muffin-tin orbitals method. These results suggest that the pressure-induced electronic s-d transition in Cs should be directly observable by momentum sensitive positron annihilation experiments.
Date: June 1, 1985
Creator: McMahan, A.K. & Skriver, H.L.
Partner: UNT Libraries Government Documents Department

Combined local-density and dynamical mean field theory calculations for the compressed lanthanides Ce, Pr, and Nd

Description: This paper reports calculations for compressed Ce (4f{sup 1}), Pr (4f{sup 2}), and Nd (4f{sup 3}) using a combination of the local-density approximation (LDA) and dynamical mean field theory (DMFT), or LDA+DMFT. The 4f moment, spectra, and the total energy among other properties are examined as functions of volume and atomic number for an assumed face-centered cubic (fcc) structure. These materials are seen to be strongly localized at ambient pressure and for compressions up through the experimentally observed fcc phases ({gamma} phase for Ce), in the sense of having fully formed Hund's rules moments and little 4f spectral weight at the Fermi level. Subsequent compression for all three lanthanides brings about significant deviation of the moments from their Hund's rules values, a growing Kondo resonance at the fermi level, an associated softening in the total energy, and quenching of the spin orbit since the Kondo resonance is of mixed spin-orbit character while the lower Hubbard band is predominantly j = 5/2. while the most dramatic changes for Ce occur within the two-phase region of the {gamma}-{alpha} volume collapse transition, as found in earlier work, those for Pr and Nd occur within the volume range of the experimentally observed distorted fcc (dfcc) phase, which is therefore seen here as transitional and not part of the localized trivalent lanthanide sequence. The experimentally observed collapse to the {alpha}-U structure in Pr occurs only on further compression, and no such collapse is found in Nd. These lanthanides start closer to the localized limit for increasing atomic number, and so the theoretical signatures noted above are also offset to smaller volume as well, which is possibly related to the measured systematics of the size of the volume collapse being 15%, 9%, and none for Ce, Pr, and Nd, respectively.
Date: March 30, 2005
Creator: McMahan, A K
Partner: UNT Libraries Government Documents Department

Ab initio calculation of tight-binding parameters

Description: We calculate ab initio values of tight-binding parameters for the f- electron metal Ce and various phases of Si, from local-density functional one-electron Hamiltonian and overlap matrix elements. Our approach allows us to unambiguously test the validity of the common minimal basis and two-center approximations as well as to determine the degree of transferability of both nonorthogonal and orthogonal hopping parameters in the cases considered.
Date: December 1, 1997
Creator: McMahan, A. K. & Klepeis, J. E.
Partner: UNT Libraries Government Documents Department

Shock anomaly and s-d transition in high-pressure lanthanum

Description: Linear-muffin-tin orbital calculations of the band structure and pressure-volume isotherms for fcc La, both at zero and finite temperatures. The calculated bulk modulus shows a rapid stiffening in the range from 40 to 50% compression, due to termination of the 6s to 5d electronic transition. When combined with a simple Slater model analysis, these results yield a temperature dependent peak in the lattice Grueneisen parameter. Experimental confirmation of this peak is found in an anomalous stiffening seen in the shock compression data for La, and it may also have some bearing on the observed saturation of the superconducting transition temperature in La around 200 kbar.
Date: July 23, 1981
Creator: McMahan, A.K.; Skriver, H.L. & Johansson, B.
Partner: UNT Libraries Government Documents Department

Magnetic and thermodynamic properties of the 3-D periodic anderson lattice hamiltonian

Description: Tight-binding models capture many of the qualitative features of interaction-induced effects in solids. For example, the simplest such model, the single-band Hubbard Hamiltonian, describes the Mott insulating phase which occurs in correlated systems, despite the fact that the one electron band is nominally only half-filled, as well as the tendency towards magnetic order. Both phenomena occur in the transition metal oxides. The Periodic Anderson Model (PAM) is a step towards incorporating more complex orbital structure. It contains a pair of orbitals on each site--a delocalized conduction band and a set of highly correlated, localized states. The PAM successfully describes conditions for transitions between antiferromagnetic order of the local moments and phases in which these moments are quenched into singlets paired with conduction electrons. These phenomena are central to heavy fermion systems. The pressure-induced volume collapse in Ce has also been attributed to Kondo-like quenching of the local f moments in this metal, as has been discussed in the context of the impurity Anderson Model. The authors describe Quantum Monte Carlo (QMC) calculations of the magnetic and thermodynamic properties of the PAM in three dimensions. Previous QMC studies have been reported in one and two dimensions. A focus of our attention will be on the density of states and the specific heat. The organization of this paper is as follows. They first introduce the PAM and outline some of its properties. Next, a brief presentation of the Quantum Monte Carlo, Maximum Entropy, and Hartree-Fock methods is given. They then show the equilibrium magnetic properties of the PAM, including the spin correlations between conduction and localized orbitals, and antiferromagnetic correlations in the localized band, before turning to the thermodynamics and the density of states. A concluding section describes connections of this work to the problem of the rare earth volume collapse transitions.
Date: September 10, 1998
Creator: Huscrot, C.; McMahan, A. K.; Pollock, E. I & Scalettar, R. T.
Partner: UNT Libraries Government Documents Department

Novel high energy density materials: Synthesis by megabar hot pressing. LDRD final report

Description: The goal of this work was to demonstrate proof-of-principle existence of a new class of high energy density materials (HEDMs). These proposed novel solids are derived from first and second row elements arranged in a uniform, three-dimensional network. Thus, every bond in these systems is energetic, in contrast to conventional energetic materials that store energy only within individual molecules. Recent predictions have suggested that a number of possible compounds including a polymeric form of nitrogen can be synthesized at high pressures and recovered metastably at ambient conditions. Specifically, polymeric nitrogen is predicted to have an energy density about three times that of a typical explosive. Such extended solid HEDMs offer dramatic new opportunities as explosives, monopropellants, or as environmentally clean fuels. The authors utilized the laser heated diamond anvil cell as the synthesis route for establishing proof-of-principle existence. They conducted high pressure studies of pure molecular nitrogen samples and completely revised the previously published equation-of-state. They also pursued studies of carbon monoxide, a compound that is isoelectronic with nitrogen and exhibits very similar high pressure phase transformations. Carbon monoxide polymerizes under pressure into a solid that can be recovered and may be energetic.
Date: April 1, 1996
Creator: Lorenzana, H.E.; Yoo, C.S.; Lipp, M.; Barbee, T. III; McMahan, A.K. & Mailhiot, C.
Partner: UNT Libraries Government Documents Department

Progress towards the synthesis of polymeric nitrogen

Description: Current conventional energetic compounds rely on strong covalent bonds within individual molecules for energy storage. A new class of energetic compounds has been recently proposed that entirely replaces weak van der Waals interactions with strong covalent bonds arranged in a continuous, uniform network, thus tremendously enhancing the energy per volume. In particular, recent theoretical calculations have suggested that a phosphorus-like or polymeric form of nitrogen may exist metastably at atmospheric pressure as a hard, insulating solid with an enhanced energy per unit volume. It is predicted that the polymeric phase of nitrogen should be stable at high pressure. And therefore the megabar diamond anvil cell might provide the ideal vehicle for carrying out proof-of-existence experiments. currently, the authors are bringing to bear technologies for achieving multimegabar pressures and temperatures of several thousand K. These conditions are necessary to rearrange the bonds of strongly covalent systems into highly energetic configurations. There is no doubt that the transformations will show strong hysteresis making the initial synthesis difficult, but for these very same reasons, these new compounds potentially will be metastable at ambient conditions in their energetic state. They discuss their results and progress to date, indicating that they are well on their way to understanding the high pressure equation-of-state of sold N{sub 2}.
Date: June 1, 1994
Creator: Lorenzana, H. E.; McMahan, A. K.; Yoo, C. S. & Barbee, T. W. III
Partner: UNT Libraries Government Documents Department

LDA+DMFT Approach to Materials with Strong Electronic Correlations

Description: LDA+DMFT is a novel computational technique for ab initio investigations of real materials with strongly correlated electrons, such as transition metals and their oxides. It combines the strength of conventional band structure theory in the local density approximation (LDA) with a modern many-body approach, the dynamical mean-field theory (DMFT). In the last few years LDA+DMFT has proved to be a powerful tool for the realistic modeling of strongly correlated electronic systems. In this paper the basic ideas and the set-up of the LDA+DMFT(X) approach, where X is the method used to solve the DMFT equations, are discussed. Results obtained with X=QMC (quantum Monte Carlo) and X=NCA (non-crossing approximation) are presented and compared. By means of the model system La{sub 1-x}Sr{sub x}TiO{sub 3} we show that the method X matters qualitatively and quantitatively. Furthermore, they discuss recent results on the Mott-Hubbard metal-insulator transition in the transition metal oxide V{sub 2}O{sub 3} and the {alpha}-{gamma} transition in the 4f-electron system Ce.
Date: December 2, 2001
Creator: Held, K; Nekrasov, I A; Keller, G; Eyert, V; Blumer, N; McMahan, A K et al.
Partner: UNT Libraries Government Documents Department

Kondo-like 4f delocalization in Gd at high pressure

Description: We present resonant inelastic x-ray scattering (RIXS) and x-ray emission spectroscopy (XES) results which suggest Kondo-like aspects in the delocalization of 4f electrons in Gd metal to 113 GPa. Analysis of the RIXS data reveal a prolonged and continuous process throughout the entire pressure range, so that the volume collapse transition at 59 GPa is only part of the delocalization phenomenon. Moreover, the L{sub {gamma}1} XES spectra indicate no apparent change in the bare 4f moment across the collapse, suggesting that Kondo screening is responsible for the expected Pauli-like behavior in magnetic susceptibility.
Date: November 28, 2005
Creator: Maddox, B R; Lazicki, A; Yoo, C S; Iota, V; Chen, M; McMahan, A K et al.
Partner: UNT Libraries Government Documents Department

New cubic phase of lithium nitride to 200 GPa

Description: We present a new cubic ({gamma}) Li{sub 3}N phase discovered above 40({+-}5) GPa. Structure and electronic bands are examined at high pressure with synchrotron x-ray diffraction and inelastic x-ray scattering in a diamond anvil cell, and also with first-principles calculations. We observe a dramatic band-gap widening and volume collapse at the phase transition. {gamma}-Li{sub 3}N remains extremely stable and ionic to 200 GPa, with predicted metallization near 8 TPa. The high structural stability, wide band-gap and simple electronic structure of {gamma}-Li{sub 3}N are analogous to that of such lower valence closed-shell solids as NaCl, MgO and Ne, meriting its use as a low-Z internal pressure standard.
Date: July 19, 2005
Creator: Lazicki, A; Maddox, B; Evans, W; Yoo, C S; McMahan, A K; Pickett, W E et al.
Partner: UNT Libraries Government Documents Department


Description: The solid-state properties of most elements are now well understood on the basis of quantum physics - with few exceptions, notably the element number 94, plutonium. Plutonium has six crystalline phases at ambient pressure, some of which are separated by unusual phase transitions with large discontinuities in volume, exhibit negative thermal expansion coefficients, or form exotic low-symmetry structures. The main challenge to explain these anomalous properties is that the characteristic ingredient of actinides, the 5f electronic states, are in the cross-over regime between the localized and delocalized (itinerant) behaviour in Pu. The early part of the actinide series with the 5f states being itinerant, i.e. part of the metallic bond, culminates with Pu; starting with Am (Z = 95), the 5f states are localized, nonbonding, and resemble the 4f states in lanthanides. Both itinerant and localized regimes are well covered by existing theories, but they cannot be simply interpolated due to the importance of dynamical electron-electron correlations. Here we present accurate quantum Monte Carlo calculations achieving previously inaccessible resolution. Obtained results demonstrate that interplay of the full Coulomb interaction vertex with spin-orbital coupling is crucial for understanding the experimentally observed spectral properties of plutonium near the Fermi level.
Date: April 1, 2010
Creator: Gorelov, E; Kolorenc, J; Wehling, T; Hafermann, H; Lichtenstein, A I; Shick, A B et al.
Partner: UNT Libraries Government Documents Department

Utilizing Nano-focussed Bremstrahlung Isochromat Spectroscopy (nBIS) to Determine the Unoccupied Electronic Structure of Pu

Description: Understanding the behavior of 5f electrons remains an unrealized ambition of condensed matter physics [1,2]. Recently, there has been a large amount of interest in the actinides, particularly plutonium, driven by the complex and intriguing behavior of Pu and several of its compounds [3-5]. This has prompted both theoretical and experimental investigations of 5f metals and compounds. Of the different allotropes of Pu, the d-phase is of particular interest because of the high symmetry crystal structure and the stability of the phase to low temperatures when alloyed with small amounts of trivalent elements. Consequently much of the recent experimental and theoretical work has focused on this allotrope. From an experimental point of view, the reactivity and radioactivity of Pu, and the complexity of the phase diagram, make it exceedingly complicated to collect high-quality data. Investigations of these complex behaviors all point back to being caused by the intriguing interplay of the various electron states and in particular the behavior of the 5f states. While there are a number of ongoing experimental efforts directed at determining the occupied electronic structure of Pu, there is essentially no experimental data on the unoccupied electronic structure of Pu. We aim to determine the conduction band (unoccupied) electronic structure of Pu and other actinides in a phase specific fashion and emphasizing bulk contributions by using Nano-focussed Bremstrahlung Isochromat Spectroscopy (nBIS). Bremstrahlung Isochromat Spectroscopy (BIS) is the high-energy variant of inverse photoelectron spectroscopy (IPES: electron in, photon out), which is essentially the time reversal of photoelectron spectroscopy (photon in, electron out). IPES can be used to follow the dispersion of electronic states in ordered samples. Owing to its low energies, IPES is usually very surface sensitive. However, by working at higher energies (>200 eV), we will sample preferentially for bulk properties, downgrading the impact of surface ...
Date: November 1, 2005
Creator: Butterfield, M T; Tobin, J G; Teslich, N E; Bliss, R A; Wall, M A; McMahan, A K et al.
Partner: UNT Libraries Government Documents Department