234 research outputs found
Coherent spin rotation in the presence of a phonon-bottleneck effect
A characteristic of spin reversal in the presence of phonon-bottleneck is the
deviation of the magnetization cycle from a reversible function into an opened
hysterezis cycle. In recent experiments on molecular magnets (e.g. V and
Ru), the zero-field level repulsion was sufficiently large to ensure an
otherwise adiabatic passage through zero-field and the magnetization curves can
be described by using only a phonon-bottleneck model. Here, we generalize the
phonon-bottleneck model into a model able to blend the non-adiabatic dynamics
of spins with the presence of a non-equilibrium phonon bath. In this simple
phenomenological model, Bloch equations are written in the eigenbasis of the
effective spin Hamiltonian, considered to be a two-level system at low
temperatures. The relaxation term is given by the phonon-bottleneck mechanism.
To the expense of calculus time, the method can be generalized to multi-level
systems, where the notion of Bloch sphere does not apply but the density matrix
formalism is still applicable.Comment: as published in EuroPhys Letter
Butterfly hysteresis loop and dissipative spin reversal in the S=1/2, V15 molecular complex
Time resolved magnetization measurements have been performed on a spin 1/2
molecular complex, so called V. Despite the absence of a barrier,
magnetic hysteresis is observed over a timescale of several seconds. A detailed
analysis in terms of a dissipative two level model is given, in which
fluctuations and splittings are of same energy. Spin-phonon coupling leads to
long relaxation times and to a particular "butterfly" hysteresis loop.Comment: LaTeX/RevTeX, 3 figures.Approved for publication in PR
Magnetic strong coupling in a spin-photon system and transition to classical regime
We study the energy level structure of the Tavis-Cumming model applied to an
ensemble of independent magnetic spins coupled to a variable number of
photons. Rabi splittings are calculated and their distribution is analyzed as a
functin of photon number and spin system size . A sharp
transition in the distribution of the Rabi frequency is found at . The width of the Rabi frequency spectrum diverges as
at this point. For increased number of photons , the Rabi
frequencies converge to a value proportional to . This
behavior is interpreted as analogous to the classical spin resonance mechanism
where the photon is treated as a classical field and one resonance peak is
expected. We also present experimental data demonstrating cooperative, magnetic
strong coupling between a spin system and photons, measured at room
temperature. This points towards quantum computing implementation with magnetic
spins, using cavity quantum-electrodynamics techniques.Comment: Received 8 April 2010; revised manuscript received 17 June 2010;
published 14 July 201
Spin-Orbit Coupling Fluctuations as a Mechanism of Spin Decoherence
We discuss a general framework to address spin decoherence resulting from
fluctuations in a spin Hamiltonian. We performed a systematic study on spin
decoherence in the compound K[VAsO(DO)]
8DO, using high-field Electron Spin Resonance (ESR). By analyzing the
anisotropy of resonance linewidths as a function of orientation, temperature
and field, we find that the spin-orbit term is a major decoherence source. The
demonstrated mechanism can alter the lifetime of any spin qubit and we discuss
how to mitigate it by sample design and field orientation.Comment: submitte
Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies
On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the
ability to precisely position nanosized samples in antinodes of the
electro-magnetic field for maximal magnetic interaction. A method is developed
to entrap micro-crystals containing spins in a well defined location on a
substrate's surface. Traditional cavity ESR measurements are then performed on
a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr spins were
entrapped as well and measured while approaching the lower limit of the ESR
sensitivity. This method suggests the feasibility of on-chip ESR measurements
at dilution refrigerator temperatures by enabling the positioning of samples
atop an on-chip superconducting cavity.Comment: to appear in Journal of Applied Physic
Tunable multi-photon Rabi oscillations in an electronic spin system
We report on multi-photon Rabi oscillations and controlled tuning of a
multi-level system at room temperature (S=5/2 for Mn2+:MgO) in and out of a
quasi-harmonic level configuration. The anisotropy is much smaller than the
Zeeman splittings, such as the six level scheme shows only a small deviation
from an equidistant diagram. This allows us to tune the spin dynamics by either
compensating the cubic anisotropy with a precise static field orientation, or
by microwave field intensity. Using the rotating frame approximation, the
experiments are very well explained by both an analytical model and a
generalized numerical model. The calculated multi-photon Rabi frequencies are
in excellent agreement with the experimental data
Coherent Quantum Dynamics of a Superconducting Flux Qubit
We have observed coherent time evolution between two quantum states of a
superconducting flux qubit comprising three Josephson junctions in a loop. The
superposition of the two states carrying opposite macroscopic persistent
currents is manipulated by resonant microwave pulses. Readout by means of
switching-event measurement with an attached superconducting quantum
interference device revealed quantum-state oscillations with high fidelity.
Under strong microwave driving it was possible to induce hundreds of coherent
oscillations. Pulsed operations on this first sample yielded a relaxation time
of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These
results are promising for future solid-state quantum computing.Comment: submitted 2 December 2002; accepted 4 February 200
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