1,614 research outputs found
Instabilities, nonhermiticity and exceptional points in the cranking model
A cranking harmonic oscillator model, widely used for the physics of fast
rotating nuclei and Bose-Einstein condensates, is re-investigated in the
context of PT-symmetry. The instability points of the model are identified as
exceptional points. It is argued that - even though the Hamiltonian appears
hermitian at first glance - it actually is not hermitian within the region of
instability.Comment: 4 pages, 1 figur
Chirality of wave functions for three coalescing levels
The coalescence of three levels has particular attractive features. Even
though it may be difficult to realise such event in the laboratory (three
additional real parameters must be adjusted), to take up the challenge seems
worthwhile. In the same way as the chiral behaviour of a usual EP can give a
direction on a line, the state vectors in the vicinity of an EP3 provide an
orientation in the plane. The distinction between left and right handedness
depends on the distribution of the widths of the three levels in the vicinity
of the point of coalescence.Comment: Manuscript has been discussed in June 2007 with the experimental
group under Professor Achim Richter at the TU Darmstadt. It has been
presented at the 6th International Workshop on Pseudo Hermitian Hamiltonians,
London, 16-18 July 2007. An expanded version is being prepared for
publication. 3 Figures, 11 page
Shell Structures and Chaos in Deformed Nuclei and Large Metallic Clusters
A reflection-asymmetric deformed oscillator potential is analysed from the
classical and quantum mechanical point of view. The connection between
occurrence of shell structures and classical periodic orbits is studied using
the ''removal of resonances method'' in a classical analysis. In this
approximation, the effective single particle potential becomes separable and
the frequencies of the classical trajectories are easily determined. It turns
out that the winding numbers calculated in this way are in good agreement with
the ones found from the corresponding quantum mechanical spectrum using the
particle number dependence of the fluctuating part of the total energy. When
the octupole term is switched on it is found that prolate shapes are stable
against chaos whereas spherical and oblate cases become chaotic. An attempt is
made to explain this difference in the quantum mechanical context by looking at
the distribution of exceptional points which results from the matrix structure
of the respective Hamiltonians. In a similar way we analyse the modified
Nilsson model and discuss its consequences for nuclei and metallic clusters.Comment: to appear in Physica Scripta., CNLS-94-02, a talk given at the Nobel
sponsored conference SELMA 94 "New Nuclear Phenomena in the Vicinity of
Closed Shell" (Stockholm and Uppsala, 29 Aug.- 3 Sept. 1994
A Simple Shell Model for Quantum Dots in a Tilted Magnetic Field
A model for quantum dots is proposed, in which the motion of a few electrons
in a three-dimensional harmonic oscillator potential under the influence of a
homogeneous magnetic field of arbitrary direction is studied. The spectrum and
the wave functions are obtained by solving the classical problem. The ground
state of the Fermi-system is obtained by minimizing the total energy with
regard to the confining frequencies. From this a dependence of the equilibrium
shape of the quantum dot on the electron number, the magnetic field parameters
and the slab thickness is found.Comment: 15 pages (Latex), 3 epsi figures, to appear in PhysRev B, 55 Nr. 20
(1997
Coupling of whispering-gallery modes in size-mismatched microdisk photonic molecules
Mechanisms of whispering-gallery (WG) modes coupling in microdisk photonic
molecules (PMs) with slight and significant size mismatch are numerically
investigated. The results reveal two different scenarios of modes interaction
depending on the degree of this mismatch and offer new insight into how PM
parameters can be tuned to control and modify WG-modes wavelengths and
Q-factors. From a practical point of view, these findings offer a way to
fabricate PM microlaser structures that exhibit low thresholds and directional
emission, and at the same time are more tolerant to fabrication errors than
previously explored coupled-cavity structures composed of identical
microresonators.Comment: 3 pages with 5 figures (to appear in Opt. Lett. 2007
Orbital Magnetism in Small Quantum Dots with Closed Shells
It is found that various kind of shell structure which occurs at specific
values of the magnetic field leads to the disappearance of the orbital
magnetization for particular magic numbers of small quantum dots with an
electron number .Comment: 4 pages, latex file, four figures as postscript files, to appear at
JETP Letters, December 199
Statistical Fluctuations of Electromagnetic Transition Intensities in pf-Shell Nuclei
We study the fluctuation properties of E2 and M1 transition intensities among
T=0,1 states of A = 60 nuclei in the framework of the interacting shell model,
using a realistic effective interaction for pf-shell nuclei with a Ni56 as a
core. It is found that the B(E2) distributions are well described by the
Gaussian orthogonal ensemble of random matrices (Porter-Thomas distribution)
independently of the isobaric quantum number T_z. However, the statistics of
the B(M1) transitions is sensitive to T_z: T_z=1 nuclei exhibit a Porter-Thomas
distribution, while a significant deviation from the GOE statistics is observed
for self-conjugate nuclei (T_z=0).Comment: 8 pages, latex, 3 figures (ps format
Phase transitions in open quantum systems
We consider the behaviour of open quantum systems in dependence on the
coupling to one decay channel by introducing the coupling parameter
being proportional to the average degree of overlapping. Under critical
conditions, a reorganization of the spectrum takes place which creates a
bifurcation of the time scales with respect to the lifetimes of the resonance
states. We derive analytically the conditions under which the reorganization
process can be understood as a second-order phase transition and illustrate our
results by numerical investigations. The conditions are fulfilled e.g. for a
picket fence with equal coupling of the states to the continuum. Energy
dependencies within the system are included. We consider also the generic case
of an unfolded Gaussian Orthogonal Ensemble. In all these cases, the
reorganization of the spectrum occurs at the critical value of
the control parameter globally over the whole energy range of the spectrum. All
states act cooperatively.Comment: 28 pages, 22 Postscript figure
Observation of extremely slow hole spin relaxation in self-assembled quantum dots
We report the measurement of extremely slow hole spin relaxation dynamics in
small ensembles of self-assembled InGaAs quantum dots. Individual spin
orientated holes are optically created in the lowest orbital state of each dot
and read out after a defined storage time using spin memory devices. The
resulting luminescence signal exhibits a pronounced polarization memory effect
that vanishes for long storage times. The hole spin relaxation dynamics are
measured as a function of external magnetic field and lattice temperature. We
show that hole spin relaxation can occur over remarkably long timescales in
strongly confined quantum dots (up to ~270 us), as predicted by recent theory.
Our findings are supported by calculations that reproduce both the observed
magnetic field and temperature dependencies. The results suggest that hole spin
relaxation in strongly confined quantum dots is due to spin orbit mediated
phonon scattering between Zeeman levels, in marked contrast to higher
dimensional nanostructures where it is limited by valence band mixing.Comment: Published by Physical Review
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