892 research outputs found
Spin tunneling and topological selection rules for integer spins
We present topological interference effects for the tunneling of a single
large spin, which are caused by the symmetry of a general class of magnetic
anisotropies. The interference originates from spin Berry phases associated
with different tunneling paths exposed to the same dynamics. Introducing a
generalized path integral for coherent spin states, we evaluate transition
amplitudes between ground as well as low-lying excited states. We show that
these interference effects lead to topological selection rules and spin-parity
effects for integer spins that agree with quantum selection rules and which
thus provide a generalization of the Kramers degeneracy to integer spins. Our
results apply to the molecular magnets Mn12 and Fe8.Comment: 4 pages, 3 EPS figures, REVTe
Berry-phase blockade in single-molecule magnets
We formulate the problem of electron transport through a single-molecule
magnet (SMM) in the Coulomb blockade regime taking into account topological
interference effects for the tunneling of the large spin of a SMM. The
interference originates from spin Berry phases associated with different
tunneling paths. We show that in the case of incoherent spin states it is
essential to place the SMM between oppositely spin-polarized source and drain
leads in order to detect the spin tunneling in the stationary current, which
exhibits topological zeros as a function of the transverse magnetic field.Comment: 4 pages, Revtex 4, 4 EPS figure
Quantum Computing in Molecular Magnets
Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.Comment: 13 pages, 2 figures, PDF, version published in Nature, typos
correcte
Crystal Field -AS_z^2 Does Not Produce One-Phonon Transitions With Delta S_z=+-2 [Comment on EPL 46, 692 (1999) by Leuenberger and Loss]
Recently Leuenbeger and Loss suggested a theory of phonon-assisted relaxation
in a molecular nanomagnet Mn-12 that "contrary to previous results is in
reasonably good agreement ... with all experimental parameter values known so
far". The purpose of this Comment is to show that the model of Leuenberger and
Loss and its comparison with experiment are premised upon their incorrect use
of the linear formula for the strain tensor. The spin-phonon coupling
introduced by Leuenberger and Loss disappears if the nonlinear term in the
strain tensor is taken into account.Comment: 2 pages, no figures, submitted to EP
Spin relaxation in Mn12-acetate
We present a comprehensive derivation of the magnetization relaxation in a
Mn12-acetate crystal based on thermally assisted spin tunneling induced by
quartic anisotropy and weak transverse magnetic fields. The overall relaxation
rate as function of the magnetic field is calculated and shown to agree well
with data including all resonance peaks. The Lorentzian shape of the resonances
is also in good agreement with recent data. A generalized master equation
including resonances is derived and solved exactly. It is shown that many
transition paths with comparable weight exist that contribute to the relaxation
process. Previously unknown spin-phonon coupling constants are calculated
explicitly.Comment: 4 pages,4 EPS figures,LaTeX(europhys.sty);final version accepted for
EP
Multi-photon Rabi oscillations in high spin paramagnetic impurity
We report on multiple photon monochromatic quantum oscillations (Rabi
oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of
Mn (S=5/2) impurities in MgO. We find that when the microwave magnetic
field is similar or large than the anisotropy splitting, the Rabi oscillations
have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model.
We show that these new frequencies come from multiple photon coherent
manipulation of the multi-level spin impurity. We develop a model based on the
crystal field theory and the rotating frame approximation, describing the
observed phenomenon with a very good agreement.Comment: International Conference: Resonance in Condensed Matter Altshuler 10
Time-dependent density-matrix functional theory for biexcitonic phenomena
We formulate a time-dependent density-matrix functional theory (TDDMFT)
approach for higher-order correlation effects like biexcitons in optical
processes in solids based on the reduced two-particle density-matrix formalism
within the normal orbital representation. A TDDMFT version of the Schr\"odinger
equation for biexcitons in terms of one- and two-body reduced density matrices
is derived, which leads to finite biexcitonic binding energies already with an
adiabatic approximation. Biexcitonic binding energies for several bulk
semiconductors are calculated using a contact biexciton model
Mode-selective coupling of coherent phonons to the Bi2212 electronic band structure
Cuprate superconductors host a multitude of low-energy optical phonons. Using
time- and angle-resolved photoemission spectroscopy, we study coherent phonons
in BiSrCaYCuO. Sub-meV
modulations of the electronic band structure are observed at frequencies of
and THz. For the dominant mode at 3.94 THz, the
amplitude of the band energy oscillation weakly increases as a function of
momentum away from the node. Theoretical calculations allow identifying the
observed modes as CuO-derived phonons. The Bi- and Sr-derived
modes which dominate Raman spectra in the relevant frequency range are
absent in our measurements. This highlights the mode-selectivity for phonons
coupled to the near-Fermi-level electrons, which originate from CuO
planes and dictate thermodynamic properties.Comment: 7 pages, 3 figure
Analytical approach to semiconductor Bloch equations
Although semiconductor Bloch equations have been widely used for decades to
address ultrafast optical phenomena in semiconductors, they have a few
important drawbacks: (i) Coulomb terms between free electron-hole pairs require
Hartree-Fock treatment which, in its usual form, preserves excitonic poles but
loses biexcitonic resonances. (ii) Solving the resulting coupled differential
equations imposes heavy numerics which completely hide the physics. This can be
completely avoided if, instead of free electron-hole pairs, we use correlated
pairs, i.e., excitons. Their interactions are easy to handle through the
recently constructed composite-exciton many-body theory, which allows us to
\emph{analytically} obtain the time evolution of the polarization induced by a
laser pulse. This polarization comes from Coulomb interactions between virtual
excitons, but also from Coulomb-free fermion exchanges, which are dominant at
large detuning
Magnetic quantum coherence effect in Ni4 molecular transistors
We consider the electron transport in single molecule magnet transistors in
the presence of Zeeman spin splitting and magnetic quantum coherence (MQC). The
Zeeman interaction is extended along the leads, thereby producing gaps in the
energy spectrum which allow electron transport with spin polarized along a
certain direction. The MQC induces an effective coupling between localized spin
states and continuum spin states in the single molecule magnet and leads,
respectively. We investigate the conductance at zero temperature as a function
of the applied bias and magnetic field, and show that the MQC is responsible
for the appearence of resonances. Accordingly, we name them MQC resonances.Comment: 5 pages, Revtex
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