Estudo Geralhttps://estudogeral.sib.uc.ptThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 25 Aug 2019 04:40:41 GMT2019-08-25T04:40:41Z50181Expansão em bosões térmicos no modelo de Heisenberghttp://hdl.handle.net/10316/2175Title: Expansão em bosões térmicos no modelo de Heisenberg
Authors: Brajczewska, Marta
Abstract: Neste trabalho, o método de expansão em bosões térmicos (TBE) é estudado e aplicado no domínio do magnetismo. Recorreu-se ao modelo de Heisenberg para um conjunto de spins localizados numa rede cristalina.
Os graus de liberdade intrínsecos do sistema são descritos na aproximação do campo médio, enquanto os graus de liberdade colectivos são descritos por meio de bosões. A TBE possibilita a definição de ondas de spin a temperatura finita, alargando assim a aplicação da teoria dos magnões até à vizinhança da temperatura crítica.
Este trabalho discute o problema cinemático da teoria das ondas de spin. Mostra-se aqui que um ingrediente essencial da resolução do problema cinemático reside na redução do espaço de fase acessível aos magnões, em particular para S >= 1.
É efectuada uma comparação com as outras teorias de renormalização dos magnões, nomeadamente com a renormalização cinemática da aproximação de fases aleatórias (RPA) e a renormalização dinâmica da aproximação dos magnões renormalizados (MRA). Comparam-se também os resultados obtidos com alguns dados experimen-tais para os compostos EuO, EuS e MnF2.
Wed, 01 Jan 1992 00:00:00 GMThttp://hdl.handle.net/10316/21751992-01-01T00:00:00ZSelf-expansion and compression of charged clusters of stabilized jelliumhttp://hdl.handle.net/10316/12366Title: Self-expansion and compression of charged clusters of stabilized jellium
Authors: Vieira, Armando; Fiolhais, Carlos; Brajczewska, Marta; Perdew, John P.
Abstract: In a positively charged metallic cluster, surface tension tends to enhance the ionic density with respect to its bulk value, while surface-charge repulsion tends to reduce it. Using the stabilized jellium model, we examine the self-expansion and compression of positively charged clusters of simply metals. Quantal results from the Kohn-Sham equations using the local density approximation are compared with continuous results from the liquid drop model. The positive background is constrained to a spherical shape. Numerical results for the equilibrium radius and the elastic stiffness are presented for singly and doubly positively charged aluminum, sodium, and cesium clusters of 1-20 atoms. Self-expansion occurs for small charged clusters of sodium and cesium, but not of aluminum. The effect of the expansion or compression on the ionization energies is analyzed. For Al6, we also consider net charges greater than 2+. The results of the stabilized jellium model for self-compression are compared with those of other models, including the SAPS (spherical averaged pseudopotential model)
Mon, 01 Jan 1996 00:00:00 GMThttp://hdl.handle.net/10316/123661996-01-01T00:00:00ZSelf-compression of metallic clusters under surface tensionhttp://hdl.handle.net/10316/41590Title: Self-compression of metallic clusters under surface tension
Authors: Perdew, John P.; Brajczewska, Marta; Fiolhais, Carlos
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/10316/415901993-01-01T00:00:00ZDominant density parameters and local pseudopotentials for simple metalshttp://hdl.handle.net/10316/12330Title: Dominant density parameters and local pseudopotentials for simple metals
Authors: Fiolhais, Carlos; Perdew, John P.; Armster, Sean Q.; MacLaren, James M.; Brajczewska, Marta
Abstract: The properties of the simple metals are controlled largely by three density parameters: the equilibrium average valence electron density 3/4πrs3, the valence z, and the density on the surface of the Wigner-Seitz cell, represented here by the equilibrium number Nint of valence electrons in the interstitial region. To demonstrate this fact, and as a refinement of the ‘‘stabilized jellium’’ or ‘‘structureless pseudopotential’’ model, we propose a structured local electron-ion pseudopotential w(r) which depends upon either rs and z (‘‘universal’’ choice for Nint), or rs, z, and Nint for each metal (‘‘individual’’ potential). Calculated binding energies, bulk moduli, and pressure derivatives of bulk moduli, evaluated in second-order perturbation theory, are in good agreement with experiment for 16 simple metals, and the bulk moduli are somewhat better than those calculated from first-principles nonlocal norm-conserving pseudopotentials. Structural energy differences agree with those from a nonlocal pseudopotential calculation for Na, Mg, and Al, but not for Ca and Sr. Our local pseudopotential w(r) is analytic for all r, and displays an exponential decay of the core repulsion as r→∞. The decay length agrees with that of the highest atomic core orbital of s or p symmetry, corroborating the physical picture behind this ‘‘evanescent core’’ form. The Fourier transform or form factor w(Q) is also analytic, and decays rapidly as Q→∞; its first and only zero Q0 is close to conventional or empirical values. In comparison with nonlocal pseudopotentials, local ones have the advantages of computational simplicity, physical transparency, and suitability for tests of density functional approximations against more-exact many-body methods
Sun, 01 Jan 1995 00:00:00 GMThttp://hdl.handle.net/10316/123301995-01-01T00:00:00ZIonization energy and electron affinity of a metal cluster in the stabilized jellium model: Size effect and charging limithttp://hdl.handle.net/10316/12365Title: Ionization energy and electron affinity of a metal cluster in the stabilized jellium model: Size effect and charging limit
Authors: Seidl, Michael F; Perdew, John P.; Brajczewska, Marta; Fiolhais, Carlos
Abstract: We report the first reliable theoretical calculation of the quantum size correction c which yields the asymptotic ionization energy I(R) = W + ((1/2) + c)/R + O(R–2) of a simple-metal cluster of radius R. Restricted-variational electronic density profiles are used to evaluate two sets of expressions for the bulk work function W and quantum size correction c: the Koopmans expressions, and the more accurate and profile-insensitive Delta SCF expressions. We find c [approximate] –0.08 for stabilized (as for ordinary) jellium, and thus for real simple metals. We present parameters from which the density profiles may be reconstructed for a wide range of cluster sizes, including the planar surface. We also discuss how many excess electrons can be bound by a neutral cluster of given size. Within a continuum picture, the criterion for total-energy stability of a negatively charged cluster is less stringent than that for existence of a self-consistent solution
Fri, 15 May 1998 00:00:00 GMThttp://hdl.handle.net/10316/123651998-05-15T00:00:00ZMetal-cluster ionization energy: A profile-insensitive exact expression for the size effecthttp://hdl.handle.net/10316/12352Title: Metal-cluster ionization energy: A profile-insensitive exact expression for the size effect
Authors: Seidl, Michael F; Perdew, John P.; Brajczewska, Marta; Fiolhais, Carlos
Abstract: The ionization energy of a large spherical metal cluster of radius R is I(R)=W+(+c)/R, where W is the bulk work function and c≈-0.1 is a material-dependent quantum correction to the electrostatic size effect. We present 'Koopmans' and 'displaced-profile change-in-self-consistent-field' expressions for W and c within the ordinary and stabilized-jellium models. These expressions are shown to be exact and equivalent when the exact density profile of a large neutral cluster is employed; these equivalences generalize the Budd-Vannimenus theorem. With an approximate profile obtained from a restricted variational calculation, the 'displaced-profile' expressions are the more accurate ones. This profile insensitivity is important, because it is not practical to extract c from solutions of the Kohn-Sham equations for small metal clusters
Thu, 15 May 1997 00:00:00 GMThttp://hdl.handle.net/10316/123521997-05-15T00:00:00ZCompression of metallic clusters in the stabilized jellium modelhttp://hdl.handle.net/10316/41572Title: Compression of metallic clusters in the stabilized jellium model
Authors: Brajczewska, Marta; Fiolhais, Carlos; Vieira, Armando; Perdew, John P.
Sat, 01 Jan 1994 00:00:00 GMThttp://hdl.handle.net/10316/415721994-01-01T00:00:00ZA new pseudopotential for simple metalshttp://hdl.handle.net/10316/41557Title: A new pseudopotential for simple metals
Authors: Fiolhais, Carlos; Brajcewska, Marta; Fiolhais, Manuel; Perdew, John P.; Armster, S. Q.
Sat, 01 Jan 1994 00:00:00 GMThttp://hdl.handle.net/10316/415571994-01-01T00:00:00ZCharged metal clusters : atomistic versus continuous background continuous descriptionshttp://hdl.handle.net/10316/41681Title: Charged metal clusters : atomistic versus continuous background continuous descriptions
Authors: Nogueira, Fernando; Vieira, Armando; Brajczewska, Marta; Fiolhais, Carlos
Thu, 01 Jan 1998 00:00:00 GMThttp://hdl.handle.net/10316/416811998-01-01T00:00:00ZThermal boson expansion for the Heisenberg ferromagnethttp://hdl.handle.net/10316/41958Title: Thermal boson expansion for the Heisenberg ferromagnet
Authors: Brajczewska, Marta; Fiolhais, Carlos; Providência, João da
Wed, 01 Jan 1986 00:00:00 GMThttp://hdl.handle.net/10316/419581986-01-01T00:00:00ZApplication of the thermal boson expansion to the Heisenberg ferromagnets EuO and EuShttp://hdl.handle.net/10316/41957Title: Application of the thermal boson expansion to the Heisenberg ferromagnets EuO and EuS
Authors: Fiolhais, Carlos; Brajczewska, Marta; Providência, João da
Thu, 01 Jan 1987 00:00:00 GMThttp://hdl.handle.net/10316/419571987-01-01T00:00:00ZEnergetics of small clusters of stabilized jellium : continuum and shell-structure effectshttp://hdl.handle.net/10316/41772Title: Energetics of small clusters of stabilized jellium : continuum and shell-structure effects
Authors: Brajczewska, Marta; Fiolhais, Carlos; Perdew, John P.
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/10316/417721993-01-01T00:00:00ZVolume shift and charge instability of simple-metal clustershttp://hdl.handle.net/10316/12326Title: Volume shift and charge instability of simple-metal clusters
Authors: Brajczewska, Marta; Vieira, Armando; Fiolhais, Carlos
Abstract: Experiment indicates that small clusters show changes (mostly contractions) of the bond lengths with respect to bulk values. We use the stabilized jellium model to study the self-expansion and self-compression of spherical clusters (neutral or ionized) of simple metals. Results from Kohn — Sham density functional theory are presented for small clusters of Al and Na, including negatively-charged ones. We also examine the stability of clusters with respect to charging
Mon, 01 Jan 1996 00:00:00 GMThttp://hdl.handle.net/10316/123261996-01-01T00:00:00ZDecay of charged stabilized jellium clustershttp://hdl.handle.net/10316/41775Title: Decay of charged stabilized jellium clusters
Authors: Vieira, Armando; Brajczewska, Marta; Fiolhais, Carlos
Sun, 01 Jan 1995 00:00:00 GMThttp://hdl.handle.net/10316/417751995-01-01T00:00:00ZDependence of metal surface properties on the valence-electron density in the stabilized jellium modelhttp://hdl.handle.net/10316/4511Title: Dependence of metal surface properties on the valence-electron density in the stabilized jellium model
Authors: Brajczewska, Marta; Henriques, C.; Fiolhais, Carlos
Abstract: The stabilized jellium model is the simplest model which yields realistic results for the physical properties of simple metals. For the surface properties, its single input is the valence-electron density, which is described by the density parameter rs. We remark that the surface energy and the work function as a function of rs, within that model, are reasonably approximated by power laws and compare that behaviour with similar descriptions found in the literature and with experiment. We also present a simple relationship between the surface energy and the bulk modulus, which is well fitted by the power of the density parameter (when the effective valence is taken to be z*=1). Another simple relationship between the work function and the bulk modulus is shown.
Mon, 01 Jan 2001 00:00:00 GMThttp://hdl.handle.net/10316/45112001-01-01T00:00:00ZApplication of the thermal boson expansion to the Heisenberg antiferromagnet MnF2http://hdl.handle.net/10316/42217Title: Application of the thermal boson expansion to the Heisenberg antiferromagnet MnF2
Authors: Brajczewska, Marta; Fiolhais, Carlos; Providência, João da
Mon, 01 Jan 1990 00:00:00 GMThttp://hdl.handle.net/10316/422171990-01-01T00:00:00ZDiscontinuous phase transition obtained with the thermal boson expansionhttp://hdl.handle.net/10316/41575Title: Discontinuous phase transition obtained with the thermal boson expansion
Authors: Fiolhais, Carlos; Brajczewska, Marta; Providência, João da
Fri, 01 Jan 1993 00:00:00 GMThttp://hdl.handle.net/10316/415751993-01-01T00:00:00ZCollective and intrinsic degrees of freedom in the Heisenberg ferromagnet.http://hdl.handle.net/10316/40857Title: Collective and intrinsic degrees of freedom in the Heisenberg ferromagnet.
Authors: Providência, João da; Brajczewska, Marta; Fiolhais, Carlos
Abstract: A separation between collective and intrinsic degrees of freedom in the Heisenberg model of a ferromagnet at a finite temperature is achieved by reformulating the thermal boson expansion previously obtained by the present authors. In the new approach, all the collective modes (spin waves) have wavenumbers lower than a certain value. The intrinsic energy turns out to be the usual mean-field energy minus a correlation energy due to the excitations of spin waves. The influence of the cutoff in momentum space on the critical temperature is studied and the temperature dependence of the spin-wave renormalisation factor and the magnetisation are calculated.
Sun, 01 Jan 1989 00:00:00 GMThttp://hdl.handle.net/10316/408571989-01-01T00:00:00Z