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Among high T_c materials, the YBCO (YBa_2Cu_3O_{7-x}) compounds are special since they have superconducting chains as well as planes. We show that a discontinuity in the density of states as a function of magnetic field may appear at a new energy scale, characteristic of the chain and distinct from that set by the d wave gap. This is observable in experimental studies of the thermodynamical properties of these systems, such as the specific heat.

The transfer operator due to Bogomolny provides a convenient method for obtaining a semiclassical approximation to the energy eigenvalues of a quantum system, no matter what the nature of the analogous classical system. In this paper, the method is applied to integrable systems which are rotationally invariant, in two and three dimensions. In two dimensions, the transfer operator is expanded in a Fourier series in the angle variable, while in three dimensions it is expanded in spherical harmonics. In both cases, when the Fourier coefficients are evaluated using the stationary phase approximation, we arrive at the Einstein-Brillouin-Keller (EBK) quantization conditions. The associated Maslov indices are shown to agree with the results calculated by well-known simple rules. The theory is applied to several rotationally invariant systems, including the hydrogen atom and the isotropic harmonic oscillator in two and three dimensions, the circle billiard, a billiard inside a spherical cavity, and a harmonic potential with a singular magnetic flux line.

Periodic orbit expressions for the density of states lead to spurious results when directly used to calculate quantities of thermodynamic interest. This is because the trace formula is usually valid only for large energies while the calculations make use of it at all energies. We present a prescription for circumventing this problem by isolating contributions which arise from the inaccurate low-energy behavior, and are spurious, from other subdominant contributions which are physical and are not contained in the Weyl series. The method is tested by analyzing the fermionic grand partition function for a disk billiard and may readily be extended to other dynamical systems., Comment: Substantial changes. There are now explicit asymptotic relations as well as a deeper analysis of the analytic structure of the various relations. We dispensed with the harmonic oscillator in order to have a deeper discussion of periodic orbits in billiards

Closed expressions are derived for the resonance widths and Coulomb blockade conductance peak heights in quantum dots for the crossover regime between conserved and broken time-reversal symmetry. The results hold for leads with any number of possibly correlated and inequivalent channels. Our analytic predictions are in good agreement with simulations of both random matrices and a chaotic billiard with a magnetic flux line., Comment: 10 pages, RevTex, 3 Postscript figures

We discuss the symmetry decomposition of the average density of states for the two dimensional potential $V=x^2y^2$ and its three dimensional generalisation $V=x^2y^2+y^2z^2+z^2x^2$. In both problems, the energetically accessible phase space is non-compact due to the existence of infinite channels along the axes. It is known that in two dimensions the phase space volume is infinite in these channels thus yielding non-standard forms for the average density of states. Here we show that the channels also result in the symmetry decomposition having a much stronger effect than in potentials without channels, leading to terms which are essentially leading order. We verify these results numerically and also observe a peculiar numerical effect which we associate with the channels. In three dimensions, the volume of phase space is finite and the symmetry decomposition follows more closely that for generic potentials --- however there are still non-generic effects related to some of the group elements.

Partial dynamical symmetry describes a situation in which some eigenstates have a symmetry which the quantum Hamiltonian does not share. This property is shown to have a classical analogue in which some tori in phase space are associated with a symmetry which the classical Hamiltonian does not share. A local analysis in the vicinity of these special tori reveals a neighbourhood of phase space foliated by tori. This clarifies the suppression of classical chaos associated with partial dynamical symmetry. The results are used to divide the states of a mixed system into ``chaotic'' and ``regular'' classes., Comment: 10 pages, Revtex, 3 figures, Phys. Rev. Lett. in press

We study the effect on quantum spectra of the existence of small circular disks in a billiard system. In the limit where the disk radii vanish there is no effect, however this limit is approached very slowly so that even very small radii have comparatively large effects. We include diffractive orbits which scatter off the small disks in the periodic orbit expansion. This situation is formally similar to edge diffraction except that the disk radii introduce a length scale in the problem such that for wave lengths smaller than the order of the disk radius we recover the usual semi-classical approximation; however, for wave lengths larger than the order of the disk radius there is a qualitatively different behaviour. We test the theory by successfully estimating the positions of scattering resonances in geometries consisting of three and four small disks., Comment: Final published version - some changes in the discussion and the labels on one figure are corrected

We present a semiclassical expansion of the smooth part of the density of states in potentials with some form of symmetry. The density of states of each irreducible representation is separately evaluated using the Wigner transforms of the projection operators. For discrete symmetries the expansion yields a formally exact but asymptotic series in $\hbar$, while for the rotational $SO(n)$ symmetries the expansion requires averaging over angular momentum as well as energy. A numerical example is given in two dimensions, in which we calculate the leading terms of the Weyl expansion as well as the leading periodic orbit contributions to the symmetry reduced level density., Comment: Four of the five figures are appended as a postscript file. The fifth figure is available by snail mail.

Spectrum generating algebras are used in various fields of physics as models to determine quantum structure, including energy levels and transition strengths. The advantage of such models is that their group structure allows an extensive understanding of the system being studied. In addition they possess a simple classical limit, at least for bosonic systems. In this paper we discuss an algebraic model of nuclear structure, the Interacting Boson Model (IBM), and show that in one limit its group structure is particularly simple. For zero angular momentum there is an effective lower dimensional group structure which describes the system both classically and quantum mechanically., Comment: To appear in Nuc. Phys. A. Written in RevTeX. One not-very-important figure available by snail mail

We study the scattering resonances between two confocal hyperbolae and show that the spectrum is dominated by the effect of a single periodic orbit. There are two distinct cases depending on whether the orbit is geometric or diffractive. A generalization of periodic orbit theory allows us to incorporate the second possibility. In both cases we also perform a WKB analysis. Although it is found that the semiclassical approximations work best for resonances with large energies and narrow widths, there is reasonable agreement even for resonances with large widths - unlike the two disk scatterer. We also find agreement with the next order correction to periodic orbit theory., Comment: Written in RevTeX. After \end{document} comes a uuencoded .ps file with two figures