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Modern Physics Letters B

DYNAMICS OF MOLECULAR MAGNET Fe 8 INTERACTING WITH AN INJECTING SPIN-POLARIZED ELECTRON

Guofeng Zhang, Wen Yin, Yingfang Gao, Liang Jiu-Qing, Qiwei Yan

Published May 10, 2004• 0 citations
Publisher

Abstract

Quantum dynamics time evolution of a molecular magnet Fe 8 interacting with an electron nuclear spin is studied by solving the time-dependent Schrödinger equations. It is found that the variation of Fe 8 magnetization and the nuclear spin crucially depends on the interaction strength. The time evolution of the entanglement between the injecting electron and Fe 8 is evaluated. It is observed that the entanglement oscillates in time and is tightly related to the spin variation of the injecting electron. From these characteristics, the technique for the reversing and read-out of Fe 8 spin states is suggested.

DOI
10.1142/s0217984904007086

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Quantum dynamics time evolution of a molecular magnet Fe 8 interacting with an electron nuclear spin is studied by solving the time-dependent Schrödinger equations. It is found that the variation of Fe 8 magnetization and the nuclear spin crucially depends on the interaction strength. The time evolution of the entanglement between the injecting electron and Fe 8 is evaluated. It is observed that the entanglement oscillates in time and is tightly related to the spin variation of the injecting electron. From these characteristics, the technique for the reversing and read-out of Fe 8 spin states is suggested.
We investigate the time evolution of the local spin in a molecular magnet interacting with injected electrons. By solving the time-dependent Schrödinger equations, we find that the variation in the magnetization of the molecular magnet and the electron spin crucially depends on the strength of the exchange interaction. We calculate the time evolution of the entanglement between the injected electron and the molecular magnet. It is found that the entanglement oscillates in time and the oscillations are closely related to the changes in the spins. The study provides an estimation of the feasibility of the encoding and read-out by using the polarization of the molecular magnets and the injected electrons.
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We report observation of coherent quantum oscillations in spin-10 Fe 8 molecular clusters. The powder of magnetically oriented Fe 8 crystallites was placed inside a resonator, in a dc magnetic field perpendicular to the magnetization axis. The field dependence of the ac-susceptibility was measured up to 5 T, at 680 MHz, down to 25 mK. Two peaks in the imaginary part of the susceptibility have been detected, whose positions coincide, without any fitting parameters, with the predicted two peaks corresponding to the quantum splitting of the ground state in the magnetic field parallel and perpendicular to the hard magnetization axis.
We study the time evolution of entanglement of two spins in an anisotropically coupled quantum dot interacting with the unpolarised nuclear spins environment. We assume that the exchange coupling strength in the z direction J z is different from the lateral one J l . We observe that the entanglement decays as a result of the coupling to the nuclear environment and reaches a saturation value, which depends on the value of the exchange interaction difference J = ‖J l − J z ‖ between the two spins and the strength of the applied external magnetic field. We find that the entanglement exhibits a critical behaviour controlled by the competition between the exchange interaction J and the external magnetic field. The entanglement shows a quasi-symmetric behaviour above and below a critical value of the exchange interaction. It becomes more symmetric as the external magnetic field increases. The entanglement reaches a large saturation value, close to unity, when the exchange interaction is far above or below its critical value and a small one as it closely approaches the critical value. Furthermore, we find that the decay rate profile of entanglement is linear when the exchange interaction is much higher or lower than the critical value but converts to a power law and finally to a Gaussian as the critical value is approached from both directions. The dynamics of entanglement is found to be independent of the exchange interaction for an isotropically coupled quantum dot.
We investigate the dynamics of spin and charge in an interacting system consisting of impurity and conducting electrons. The time evolution of spin and charge in the impurity is given by solving the time-dependent Schrödinger equations for the many-body states of the interacting system. By switching on the interaction between impurity and conducting electrons, the spin and charge of the impurity begin oscillations with frequencies that reflect the elementary excitations of the interacting system. The dynamics reflects the basic picture of the Kondo effect.
Molecular magnets are ideal systems to probe the realm that borders quantum and classical physics, as well as to study decoherence phenomena in nanoscale systems. The control of the quantum behavior of these materials and their structural characteristics requires synthesis of new complexes with desirable properties which will allow probing of the fundamental aspects of nanoscale physics and quantum information processing. Of particular interest among the magnetic molecular materials are single-molecule magnets (SMMs) and antiferromagnetic (AFM) molecular wheels in which the spin state of the molecule is known to behave quantum mechanically at low temperatures. In previous experiments the dynamics of the magnetic moment of the molecules is governed by incoherent quantum tunneling. Short decoherence times are mainly due to interactions between molecular magnets within the crystal and interactions of the electronic spin with the nuclear spin of neighboring ions within the molecule. This decoherence problem has imposed a limit to the understanding of the molecular spin dynamics and the sources of decoherence in condensed matter systems. Particularly, intermolecular dipolar interactions within the crystal, which shorten the coherence times in concentrated samples, have stymied progress in this direction. Several recent works have reported a direct measurement of the decoherence time in molecular magnets. This has been done by eliminating the dephasing created by dipolar interactions between neighboring molecules. This has been achieved by a) a dilution of the molecules in a liquid solution to decrease the dipolar interaction by separating the molecules, and b) by polarizing the spin bath by applying a high magnetic field at low temperatures. Unfortunately, both approaches restrict the experimental studies of quantum dynamics. For example, the dilution of molecular magnets in liquid solution causes a dispersion of the molecular spin orientation and anisotropy axes, while the large fields required to polarize the spin bath overcome the anisotropy of the molecular spin. In this thesis I have explored two methods to overcome dipolar interactions in molecular magnets: a) studying the dynamics of molecular magnets in single crystals where the separation between magnetic molecules is obtained by chemical doping or where the high crystalline quality allows observations intrinsic to the quantum mechanical nature of the tunneling of the spin, and b) studying the electronic transport through an individual magnetic molecule which has been carefully placed in a single-electron transistor device. I have used EPR microstrip resonators to measure Fe17Ga molecular wheels within single crystals of Fe18 AFM wheels, as well as demonstrating, for the first time in a Single Molecule Magnet, the complete suppression of a Quantum Tunneling of the Magnetization transition forbidden by molecular symmetry.