434 research outputs found
Decoherence of a Superposition of Macroscopic Current States in a SQUID
We show that fundamental conservation laws mandate parameter-free mechanisms
of decoherence of quantum oscillations of the superconducting current between
opposite directions in a SQUID -- emission of phonons and photons at the
oscillation frequency. The corresponding rates are computed and compared with
experimental findings. The decohering effects of external mechanical and
magnetic noise are investigated
Tunneling with dissipation and decoherence for a large spin
We present rigorous solution of problems of tunneling with dissipation and
decoherence for a spin of an atom or a molecule in an isotropic solid matrix.
Our approach is based upon switching to a rotating coordinate system coupled to
the local crystal field. We show that the spin of a molecule can be used in a
qubit only if the molecule is strongly coupled with its atomic environment.
This condition is a consequence of the conservation of the total angular
momentum (spin + matrix), that has been largely ignored in previous studies of
spin tunneling.Comment: 4 page
Quantum dynamics of crystals of molecular nanomagnets inside a resonant cavity
It is shown that crystals of molecular nanomagnets exhibit enhanced magnetic
relaxation when placed inside a resonant cavity. Strong dependence of the
magnetization curve on the geometry of the cavity has been observed, providing
evidence of the coherent microwave radiation by the crystals. A similar
dependence has been found for a crystal placed between Fabry-Perot
superconducting mirrors. These observations open the possibility of building a
nanomagnetic microwave laser pumped by the magnetic field
Dislocation-induced spin tunneling in Mn-12 acetate
Comprehensive theory of quantum spin relaxation in Mn-12 acetate crystals is
developed, that takes into account imperfections of the crystal structure and
is based upon the generalization of the Landau-Zener effect for incoherent
tunneling from excited energy levels. It is shown that linear dislocations at
plausible concentrations provide the transverse anisotropy which is the main
source of tunneling in Mn-12. Local rotations of the easy axis due to
dislocations result in a transverse magnetic field generated by the field
applied along the c-axis of the crystal, which explains the presence of odd
tunneling resonances. Long-range deformations due to dislocations produce a
broad distribution of tunnel splittings. The theory predicts that at subkelvin
temperatures the relaxation curves for different tunneling resonances can be
scaled onto a single master curve. The magnetic relaxation in the thermally
activated regime follows the stretched-exponential law with the exponent
depending on the field, temperature, and concentration of defects.Comment: 17 pages, 14 figures, 1 table, submitted to PR
Oscillation of the tunnel splitting in nanospin systems within the particle mapping formalism
The oscillation of tunnel splitting in the biaxial spin system within
magnetic field along the anisotropy axis is analyzed within the particle
mapping approach, rather than in the (\theta-\phi) spin coherent-state
representation. In our mapping procedure, the spin system is transformed into a
particle moving in the restricted geometry whose wave function subjects
to the boundary condition involving additional phase shift. We obtain the new
topological phase that plays the same role as the Wess-Zumino action in spin
coherent-state representation. Considering the interference of two possible
trajectories, instanton and anti-instanton, we get the identical condition for
the field at which tunneling is quenched, with the previous result within spin
coherent-state representation.Comment: 11 pages, 1 figure; Some typographical errors have been correcte
Quantum nucleation in ferromagnets with tetragonal and hexagonal symmetries
The phenomenon of quantum nucleation is studied in a ferromagnet in the
presence of a magnetic field at an arbitrary angle. We consider the
magnetocrystalline anisotropy with tetragonal symmetry and that with hexagonal
symmetry, respectively. By applying the instanton method in the
spin-coherent-state path-integral representation, we calculate the dependence
of the rate of quantum nucleation and the crossover temperature on the
orientation and strength of the field for a thin film and for a bulk solid. Our
results show that the rate of quantum nucleation and the crossover temperature
depend on the orientation of the external magnetic field distinctly, which
provides a possible experimental test for quantum nucleation in nanometer-scale
ferromagnets.Comment: 19 pages and 3 figures, Final version and accepted by Phys. Rev. B
(Feb. B1 2001
Superradiation from Crystals of High-Spin Molecular Nanomagnets
Phenomenological theory of superradiation from crystals of high-spin
molecules is suggested. We show that radiation friction can cause a
superradiation pulse and investigate the role of magnetic anisotropy, external
magnetic field and dipole-dipole interactions. Depending on the contribution of
all these factors at low temperature, several regimes of magnetization of
crystal sample are described. Very fast switch of magnetization's direction for
some sets of parameters is predicted.Comment: 10 pages, 3 figure
A Unified Account of the Moral Standing to Blame
Recently, philosophers have turned their attention to the question, not when a given agent is blameworthy for what she does, but when a further agent has the moral standing to blame her for what she does. Philosophers have proposed at least four conditions on having “moral standing”:
1. One’s blame would not be “hypocritical”.
2. One is not oneself “involved in” the target agent’s wrongdoing.
3. One must be warranted in believing that the target is indeed blameworthy for the wrongdoing.
4. The target’s wrongdoing must some of “one’s business”.
These conditions are often proposed as both conditions on one and the same thing, and as marking fundamentally different ways of “losing standing.” Here I call these claims into question. First, I claim that conditions (3) and (4) are simply conditions on different things than are conditions (1) and (2). Second, I argue that condition (2) reduces to condition (1): when “involvement” removes someone’s standing to blame, it does so only by indicating something further about that agent, viz., that he or she lacks commitment to the values that condemn the wrongdoer’s action. The result: after we clarify the nature of the non-hypocrisy condition, we will have a unified account of moral standing to blame. Issues also discussed: whether standing can ever be regained, the relationship between standing and our "moral fragility", the difference between mere inconsistency and hypocrisy, and whether a condition of standing might be derived from deeper facts about the "equality of persons"
Spin-parity dependent tunneling of magnetization in single-molecule magnets
Single-molecule magnets facilitate the study of quantum tunneling of
magnetization at the mesoscopic level. The spin-parity effect is among the
fundamental predictions that have yet to be clearly observed. It is predicted
that quantum tunneling is suppressed at zero transverse field if the total spin
of the magnetic system is half-integer (Kramers degeneracy) but is allowed in
integer spin systems. The Landau-Zener method is used to measure the tunnel
splitting as a function of transverse field. Spin-parity dependent tunneling is
established by comparing the transverse field dependence of the tunnel
splitting of integer and half-integer spin systems.Comment: 4 pages, 6 figure
Properties of low-lying states in some high-nuclearity Mn, Fe and V clusters: Exact studies of Heisenberg models
Using an efficient numerical scheme that exploits spatial symmetries and spin
parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for the high nuclearity spin clusters, Mn_{12}, Fe_8 and V_{15}.
The largest calculation involves the Mn_{12} cluster which spans a Fock space
of a hundred million. Our results show that the earlier estimates of the
exchange constants need to be revised for the Mn_{12} cluster to explain the
level ordering of low-lying eigenstates. In the case of the Fe_8 cluster,
correct level ordering can be obtained which is consistent with the exchange
constants for the already known clusters with butterfly structure. In the
V_{15} cluster, we obtain an effective Hamiltonian that reproduces exactly, the
eight low-lying eigenvalues of the full Hamiltonian.Comment: Revtex, 12 pages, 16 eps figures; this is the final published versio
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