32 research outputs found
Spectra and magnetic properties of large spins in external fields
Journals published by the American Physical Society can be found at http://publish.aps.org/Spectra and magnetic properties of large spins J (e.g., spins possessed by ions or molecules), placed into a crystal electric field (CEF) of an arbitrary symmetry point group, are shown to change drastically when J changes by 1/2 or 1. At a fixed field symmetry and configuration of its N extrema situated at the p-fold symmetry axis, physical characteristics of the spin depend periodically on J with the period equal to p. The problem of the spectrum and eigenstates of the large spin J is equivalent to an analogous problem for a scalar charged particle confined to a sphere S-2 and placed into the magnetic field of the monopole with the charge J. This analogy, as well as the strong difference between close values of J, stems from the Berry phase occurring in the problem. For energies close to the extrema of the.CEF, the problem can be formulated as Harper's equation on the sphere. The (2J + 1)-dimensional space of states is split into smaller multiplets of classically degenerated states. These multiplets in turn are split into submultiplets of states transforming according to specific irreducible representations of the symmetry group determined by J and p. We classify possible configurations and corresponding spectra. Experimental realizations of large spins in a symmetric environment are proposed and physical effects observable in these systems are analyzed. [S1050-2947(99)00709-X]
Berry's phase for large spins in external fields
It is shown that even for large spins the fundamental difference between
integer and half-integer spins persists. In a quasi-classical description this
difference enters via Berry's connection. This general phenomenon is derived
and illustrated for large spins confined to a plane by crystalline electric
fields. Physical realizations are rare-earth Nickel Borocarbides. Magnetic
moments for half-integer spin
(Dy, ) and magnetic susceptibilities for integer spin
(Ho, ) are calculated. Experiments are proposed to furnish evidence
for the predicted fundamental difference.Comment: 4 pages RevTe
Theoretical model for the superconducting and magnetically ordered borocarbides
We present a theory of superconductivity in presence of a general magnetic
structure in a form suitable for the description of complex magnetic phases
encountered in borocarbides. The theory, complemented with some details of the
band structure and with the magnetic phase diagram, may explain the nearly
reentrant behaviour and the anisotropy of the upper critical field of HoNi2B2C.
The onset of the helical magnetic order depresses superconductivity via the
reduction of the interaction between phonons and electrons caused by the
formation of magnetic Bloch states. At mean field level, no additional
suppression of superconductivity is introduced by the incommensurability of the
helical phase.Comment: 8 pages, 2 figures. Published version, one important reference adde
Phenomenological Theory of Superconductivity and Magnetism in HoDyNiBC
The coexistence of the superconductivity and magnetism in the
HoDyNiBC is studied by using Ginzburg-Landau theory. This
alloy shows the coexistence and complex interplay of superconducting and
magnetic order. We propose a phenomenological model which includes two magnetic
and two superconducting order parameters accounting for the multi-band
structure of this material. We describe phenomenologically the magnetic
fluctuations and order and demonstrate that they lead to anomalous behavior of
the upper critical field. The doping dependence of in
HoDyNiBC showing a reentrance behavior are analyzed
yielding a very good agreement with experimental data.Comment: 4 pages, 3 figures, REVTeX, submitted to PR
Interference effects in isolated Josephson junction arrays with geometric symmetries
As the size of a Josephson junction is reduced, charging effects become
important and the superconducting phase across the link turns into a periodic
quantum variable. Isolated Josephson junction arrays are described in terms of
such periodic quantum variables and thus exhibit pronounced quantum
interference effects arising from paths with different winding numbers
(Aharonov-Casher effects). These interference effects have strong implications
for the excitation spectrum of the array which are relevant in applications of
superconducting junction arrays for quantum computing. The interference effects
are most pronounced in arrays composed of identical junctions and possessing
geometric symmetries; they may be controlled by either external gate potentials
or by adding/removing charge to/from the array. Here we consider a loop of N
identical junctions encircling one half superconducting quantum of magnetic
flux. In this system, the ground state is found to be non-degenerate if the
total number of Cooper pairs on the array is divisible by N, and doubly
degenerate otherwise (after the stray charges are compensated by the gate
voltages).Comment: 9 pages, 6 figure
Continuum field description of crack propagation
We develop continuum field model for crack propagation in brittle amorphous
solids. The model is represented by equations for elastic displacements
combined with the order parameter equation which accounts for the dynamics of
defects. This model captures all important phenomenology of crack propagation:
crack initiation, propagation, dynamic fracture instability, sound emission,
crack branching and fragmentation.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett. Additional
information can be obtained from http://gershwin.msd.anl.gov/theor
Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers
In antiferromagnetically coupled superlattices grown on (001) faces of cubic
substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or
Fe/Cr, the magnetic states evolve under competing influence of bilinear and
biquadratic exchange interactions, surface-enhanced four-fold in-plane
anisotropy, and specific finite-size effects. Using phenomenological
(micromagnetic) theory, a comprehensive survey of the magnetic states and
reorientation transitions has been carried out for multilayer systems with even
number of ferromagnetic sub-layers and magnetizations in the plane. In
two-layer systems (N=2) the phase diagrams in dependence on components of the
applied field in the plane include ``swallow-tail'' type regions of
(metastable) multistate co-existence and a number of continuous and
discontinuous reorientation transitions induced by radial and transversal
components of the applied field. In multilayers (N \ge 4) noncollinear states
are spatially inhomogeneous with magnetization varying across the multilayer
stack. For weak four-fold anisotropy the magnetic states under influence of an
applied field evolve by a complex continuous reorientation into the saturated
state. At higher anisotropy they transform into various inhomogeneous and
asymmetric structures. The discontinuous transitions between the magnetic
states in these two-layers and multilayers are characterized by broad ranges of
multi-phase coexistence of the (metastable) states and give rise to specific
transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio