Optimization of trial wave functions for use in quantum Monte Carlo with application to LiH

Methods for both partial and full optimization of wavefunction parameters are explored, and these are applied to the LiH molecule. A partial optimization can be easily performed with little difficulty. But to perform a full optimization we must avoid a wrong minimum, and deal with linear-dependency, time step-dependency and ensemble-dependency problems. Five basis sets are examined. The optimized wavefunction with a 3-function set gives a variational energy of -7.998 + 0.005 a.u., which is comparable to that (-7.990 + 0.003) 1 of Reynold's unoptimized \fin ( a double-~ set of eight functions). The optimized wavefunction with a double~ plus 3dz2 set gives ari energy of -8.052 + 0.003 a.u., which is comparable with the fixed-node energy (-8.059 + 0.004)1 of the \fin. The optimized double-~ function itself gives an energy of -8.049 + 0.002 a.u. Each number above was obtained on a Bourrghs 7900 mainframe computer with 14 -15 hrs CPU time.

Spin polarized neutron matter and magnetic susceptibility within the Brueckner-Hartree-Fock approximation

The Brueckner-Hartree-Fock formalism is applied to study spin polarized neutron matter properties. Results of the total energy per particle as a function of the spin polarization and density are presented for two modern realistic nucleon-nucleon interactions, Nijmegen II and Reid93. We find that the dependence of the energy on the spin polarization is practically parabolic in the full range of polarizations. The magnetic susceptibility of the system is computed. Our results show no indication of a ferromagnetic transition which becomes even more difficult as the density increases.

Spin-polarized Hartree-Fock-Slater calculations in atoms and diatomic molecules with the finite element method

We present spin-polarized Hartree-Fock-Slater calculations performed with the highly accurate numerical finite element method for the atoms N and 0 and the diatomic radical OH as examples.

Wave functions for the methane molecule

If the potential field due to the nuclei in the methane molecule is expanded in terms of a set of spherical harmonics about the carbon nucleus, only the terms involving s, f, and higher harmonic functions differ from zero in the equilibrium configuration. Wave functions have been calculated for the equilibrium configuration, first including only the spherically symmetric s term in the potential, and secondly including both the s and the f terms. In the first calculation the complete Hartree-Fock S.C.F. wave functions were determined; in the second calculation a variation method was used to determine the best form of the wave function involving f harmonics. The resulting wave functions and electron density functions are presented and discussed

Spin-polarized electronic-structure of MnBiAl

The self-consistent spin-polarized band-structure calculation of ferromagnetic compound MnBiAl in its low-temperature phase has been performed. In this paper the calculation results are given. Comparison with the results of MnBi is performed in order to find the effect on electronic structure by doping with Al.

Symmetry controlled spin polarized conductance in Au nanowires

The fact that the resistance of propagating electrons in solids depends on their spin orientation has led to a new field called spintronics. With the parallel advances in nanoscience, it is now possible to talk about nanospintronics. Many works have focused on the study of charge transport along nanosystems, such as carbon nanotubes, graphene nanoribbons, or metallic nanowires, and spin dependent transport properties at this scale may lead to new behaviors due to the manipulation of a small number of spins. Metal nanowires have been studied as electric contacts where atomic and molecular insertions can be constructed. Here we describe what might be considered the ultimate spin device, namely, a Au thin nanowire with one Co atom bridging its two sides. We show that this system has strong spin dependent transport properties and that its local symmetry can dramatically change them, leading to a significant spin polarized conductance.

Non-Zeeman splitting for a spin-resolved STM with a Kondo adatom in a spin-polarized two-dimensional electron gas

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effect of spin-polarized subbands in the inhomogeneous hole gas providing the indirect exchange in GaMnAs bilayers

The magnetic order resulting from the indirect exchange in the metallic phase of a (Ga,Mn)As/GaAs double layer structure is studied via Monte Carlo simulation. The polarization of the hole gas is taken into account, establishing a self-consistency between the magnetic order and the electronic structure. The Curie-Weiss temperatures calculated for these low-dimensional systems are in the range of 50-80 K, and the dependence of the transition temperature with the GaAs separation layer is established. (C) 2003 Published by Elsevier B.V.

Comment on Wave functions for a Duffin-Kemmer-Petiau particle in a time-dependent potential [J. Math. Phys. 48, 073515 (2007)]

It is shown that the paper Wave functions for a Duffin-Kemmer-Petiau particle in a time-dependent potential by Merad and Bensaid [J. Math. Phys. 48, 073515 (2007)] is not correct in using inadvertently a non-Hermitian Hamiltonian in a formalism that does require Hermitian Hamiltonians.

Classes of exact wave functions for general time-dependent Dirac Hamiltonians in 1+1 dimensions

The construction of two classes of exact solutions for the most general time-dependent Dirac Hamiltonian in 1+1 dimensions was discussed. The extension of solutions by introduction of a time-dependent mass was elaborated. The possibility of existence of a generalized Lewis-Riesenfeld invariant connected with such solutions was also analyzed.

Spin-polarized transport in II-VI diluted magnetic semiconductors superlattices

We studied the spin-polarized charge densities in II-VI-based diluted magnetic superlattices formed of p-doped ZnTe:Mg/ZnTe:TM/ZnTe:Mg non-magnetic/magnetic/non-magnetic layers, with TM standing for transition metal. The calculations were performed within a self-consistent k.p method, in which are also taken into account the exchange correlation effects in the local density approximation. Our results show a limit for the width of the non-magnetic layer for which the difference between the opposite spin charge densities is maximized, indicating the best conditions to obtain full polarization by varying the TM content. We also discuss these effects in the calculated photoluminescence spectra. Our findings point to the possibility of engineering the spin-polarized charge distribution by varying the widths of the magnetic and non-magnetic layers and/or varying the TM concentration in the magnetic layers, thus providing a guide for future experiments. (c) 2012 Elsevier B.V. All rights reserved.

Spin polarized transport in manganese oxide double-perovskite thin films

Gegenstand dieser Arbeit war die Untersuchung von metallischen gemischtvalenten Manganaten und magnetischen Doppelperowskiten. Aufgrund ihres großen negativen Magnetowiderstandes (MW) sind diese halbmetallischen Oxide interessant für mögliche technische Anwendungen, z.B. als Leseköpfe in Festplatten. Es wurden die kristallographischen, elektronischen und magnetischen Eigenschaften von epitaktischen Dünnschichten und polykristallinen Pulverproben bestimmt.Epitaktische Dünnschichten der Verbindungen La_0.67Ca_0.33MnO₃ und La_0.67Sr_0.33MnO₃ wurdenmit Kaltkathodenzerstäubung und Laserablation auf einkristallinen Substraten wie SrTiO₃abgeschieden. Mit Hall-Effekt Messungen wurde ein Zusammenbruch der Ladungsträgerdichte bei der Curie-Temperatur T_C beobachtet.Mit dem Wechsel des Dotierungsatoms A von Ca (T_C=232 K) zu Sr (T_C=345 K)in La_0.67A_0.33MnO₃ konnte die Feldsensitivität des Widerstandes bei Raumtemperatur gesteigert werden. Um die Sensitivität weiter zu erhöhen wurde die hohe Spinpolarisation von nahezu 100% in Tunnelexperimenten ausgenutzt. Dazu wurden biepitaktische La_0.67Ca_0.33MnO₃ Schichten auf SrTiO₃ Bikristallsubstraten hergestellt. Die Abhängigkeit des Tunnelmagnetowiderstandes (TMW) vom magnetischen Feld, Temperatur und Strum war ein Schwerpunkt der Untersuchung. Mittels spinpolarisierten Tunnelns durch die künstliche Korngrenze konnte ein hysteretischer TMW von 70% bei 4 K in kleinen Magnetfeldern von 120 Oe gemessen werden. Eine weitere magnetische Oxidverbindung, der Doppelperowskit Sr₂FeMoO₆ miteine Curie-Temperatur oberhalb 400 K und einem großen MW wurde mittels Laserablation hergestellt. Die Proben zeigten erstmals das Sättigunsmoment, welches von einer idealen ferrimagnetischen Anordnung der Fe und Mo Ionen erwartet wird. Mit Hilfe von Magnetotransportmessungen und Röntgendiffraktometrie konnte eine Abhängigkeit zwischen Kristallstruktur (Ordnung oder Unordnung im Fe, Mo Untergitter) und elektronischem Transport (metallisch oder halbleitend) aufgedeckt werden.Eine zweiter Doppelperowskit Ca₂FeReO₆ wurde im Detail als Pulverprobe untersucht. Diese Verbindung besitzt die höchste Curie-Temperatur von 540 K, die bis jetzt in magnetischen Perowskiten gefunden wurde. Mit Neutronenstreuung wurde eine verzerrte monoklinische Struktur und eine Phasenseparation aufgedeckt.

Compact Singlet S Helium Wave functions(corrected)

This is a corrected version of the Phys. Rev. A 74,14501 (2006) article. The result is improved slightly from that in the original paper.

Ignition conditions for inertial confinement fusion targets with a nuclear spin-polarized DT fuel

The nuclear fusion cross-section is modified when the spins of the interacting nuclei are polarized. In the case of deuterium?tritium it has been theoretically predicted that the nuclear fusion cross-section could be increased by a factor d = 1.5 if all the nuclei were polarized. In inertial confinement fusion this would result in a modification of the required ignition conditions. Using numerical simulations it is found that the required hot-spot temperature and areal density can both be reduced by about 15% for a fully polarized nuclear fuel. Moreover, numerical simulations of a directly driven capsule show that the required laser power and energy to achieve a high gain scale as d-0.6 and d-0.4 respectively, while the maximum achievable energy gain scales as d0.9.