Space momentum

It has long been known from statistical mechanical theory that a Bose-Einstein ideal gas, which at low temperatures would show condensation of molecules into die ground translational state (a condensation in momentum space rather than in position space), should show a third-order phase transition at the temperature at which this condensation starts. Nonnal helium ( He) is a Bose-Einstein substance, but is far from ideal at low temperatures, and the very real forces between molecules make the >L-transition to He II very different from that predicted for a Bose-Einstein gas. 

Gutzwiller M C 1967 Phase-integral approximation in momentum space and the bound states of an atom J Math. Phys. 8 1979... 

An eminent researcher at the boundaries between physics and chemistry, Howard Reiss, some years ago explained the difference between a solid-state chemist and a solid-state physicist. The first thinks in configuration space, the second in momentum space so, one is the Fourier transform of the other. 

In particular, we see that each mode in momentum space contributes the quantity... 

The nondiagonal elements are needed to preserve the complementary character of wave mechanics, e.g., to permit the description of a certain physical situation in ordinary space as well as in momentum space. 

Ordinarily, the term phase space refers to the conjunction of configuration and momentum space. We use it here for configuration-velocity space. 

Let < j(k) be the Klein-Gordon amplitude corresponding to a spin zero particle localized at the origin at time t = 0. Since in momentum space the space displacement operator is multiplication by exp (— tk a), the state localized at y at time t = 0 is given by exp (—ik-y) (k). This displaced state by condition (b) above must be orthogonal to (k), i.e. 

We shall call an amplitude ( ), which satisfies Eq. (9-516), a transversal amplitude.20 We can summarize the above statements as follows in momentum space, a one-photon amplitude u(k) is defined on the forward light cone, i.e., for Jc2 = 0, k0 > 0, and satisfies the subsidiary... 

Inasmuch as the inscribed sphere corresponds to only 226 electrons per unit cube, it seems likely that the density of energy levels in momentum space has become small at 250.88, possibly small enough to provide a satisfactory explanation of the filled-zone properties. However, there exists the possibility that the Brillouin polyhedron is in fact completely filled by valence electrons. If there are 255.6 valence electrons per 52 atoms at the composition Cu6Zn8, and if the valence of copper is one greater than the valence of zinc, then it is possible to determine values of the metallic valences of these elements from the assumption that the Brillouin polyhedron is filled. These values are found to be 5.53 for copper and 4.53 for zinc. The accuracy of the determination of the metallic valences... 

The theory of superconductivity based on the interaction of electrons and phonons was developed about thirty years ago. I 4 In this theory the electron-phonon interaction causes a clustering of electrons in momentum space such that the electrons move in phase with a phonon when the energy of this interaction is greater than the phonon energy hm. The theory is satisfactory in most respects. 

One Important aspect of the supercomputer revolution that must be emphasized Is the hope that not only will It allow bigger calculations by existing methods, but also that It will actually stimulate the development of new approaches. A recent example of work along these lines Involves the solution of the Hartree-Fock equations by numerical Integration In momentum space rather than by expansion In a basis set In coordinate space (2.). Such calculations require too many fioatlng point operations and too much memory to be performed In a reasonable way on minicomputers, but once they are begun on supercomputers they open up several new lines of thinking. 

The transformations connecting the coordinate-space wave function, v[/(R), to the momentum-space wave function, k), are... 

The discretized momentum-space wave function corresponding to a momentum of ki% is denoted by 44. As with the discretized spatial wave function [Eq. (37)], the discretized momentum wave functions are also normalized so that 4/ p = 1 (i.e., = i/ ki) V ). 

The wave function in momentum space is given by the Fourier transform of the coordinate-space wave function... 

The discretized momentum space wave function, is therefore given by... 

Mermin s "generalised crystallography" works primarily with reciprocal space notions centered around the density and its Fourier transform. Behind the density there is however a wave function which can be represented in position or momentum space. The wave functions needed for quasicrystals of different kinds have symmetry properties - so far to a large extent unknown. Mermin s reformulation of crystallography makes it attractive to attempt to characterise the symmetry of wave functions for such systems primarily in momentum space. 

In the next setion we review some key concepts in Mermin s approach. After that we summarise in section III some aspects of the theory of (ordinary) crystals, which would seem to lead on to corresponding results for quasicrystals. A very preliminary sketch of a study of the symmetry properties of momentum space wave functions for quasicrystals is then presented in section IV. 

We notice that neither the momentum distribution nor the reciprocal form factor seems to carry any information about the translational part of the space group. The non diagonal elements of the number density matrix in momentum space, on the other hand, transform under the elements of the space group in a way which brings in the translational parts explicitly. 

Combining the inverses of (III. 14) and (III. 16) we get the natural expansion for a general element of the number density matrix in momentum space ... 

Using (III. 16) we can also write this Fourier component in terms of the momentum space orbitals as... 

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