Effects of the environment on nonadiabatic magnetization process in uniaxial molecular magnets at very low temperatures

Saito, Keiji; Miyashita, Seiji; Raedt, de Hans

doi:10.1103/physrevb.60.14553

Cited by 32 publications

(39 citation statements)

References 20 publications

(23 reference statements)

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“…We studied this discrepancy by a quantum master equation and found that even at very low temperatures, the relaxation between levels with the same sign of magnetization easily occurs although it is not allowed in the pure quantum system. 11) As a result of a composite mechanism of nonadiabatic changes of state and a fast relaxation to the ground state, the magnetization process in such molecule magnets shows a step-wise curve which is dierent from the pure quantum case. Due to this eect, amount of the jump at resonant point is dierent from that in the pure quantum mechanical case, which we called the apparent (or deceptive) nonadiabatic process.…”

Section: X1 Introductionmentioning

confidence: 99%

“…We make use of the quantum master equation which we have used in our previous studies of quantum dynamics in dissipative environments. 11,14) The reservoir we consider is simply composed of innite number of phonon and directly connected to the system of interest. Thereby we demonstrate that appearance of the magnetic plateau induced by thermal environment is a robust phenomenon regardless of types of reservoirs.…”

Section: X1 Introductionmentioning

confidence: 99%

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Magnetic Foehn Effect in Adiabatic Transition

Saito

Miyashita

2001

J. Phys. Soc. Jpn.

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The magnetization process as a response to a sweeping magnetic eld in the thermal environment is investigated by using the quantum master equation. In a slow sweeping velocity region where the system shows quasi-adiabatic motion, a magnetic plateau in the magnetization process has been observed in the recent experiment of V 15 [Phys. Rev. Lett. 84 (2000) 3454]. Although the experiment was explained from a view of small capacity of the phonon heat bath, i.e., the phonon bottleneck eect, we found that a magnetic plateau appears almost regardless of the properties of the heat bath in the quasi-adiabatic transition and the appearance of the plateau is quite universal. We call it 'Magnetic Foehn eect'. On the other hand, the magnetic plateau also appears in a fast sweeping velocity region by the Landau{Zener{Stückelberg mechanism which is dierent from the above mechanism in a slow sweeping region, and we study the crossover between them. We also propose an experiment which claries the structure of coupling to the thermal reservoir.KEYWORDS: nonadiabatic transition, magnetic tunneling x1. IntroductionProperties of quantum dynamics have been studied extensively for nanoscale magnets and also in microscopic system with nanostructure. 1{3) Energy levels as a function of the external eld form many avoided level crossing points. When the eld is swept, the probability of each diabatic state is scattered to other states. Such scattering mechanism is well described by Landau, Zener, and Stükelberg (LZS) mechanism 4{6) where the scattering probability is given as a function of the sweeping velocity and energy gap at the avoided level crossing point. The LZS probability was originally derived for a two-level system. Nevertheless it has been successfully applied to multi-level system to analyze phenomena related to the nonadiabatic transition. 1{3,7{9) However, realistic experiments are always exposed to a thermal environment. Hence the study on the nonadiabatic transition with eects of environment is quite crucial for further understanding of time-dependent phenomena in microscopic systems. For example, in the recent experiments of uniaxial molecular magnets such as Mn 12 and Fe 8 which show a step-wise magnetization process, the dissipative eects cannot be neglected even at very low temperatures.1,2,10) In multilevel systems, the nonadiabatic transitions successively occur at the avoided level crossing points. It was found that in these experiments, the shape of magnetization curve cannot be explained only by the modication of the LZS transition probability. We studied this discrepancy by a quantum master equation and found that even at very low temperatures, the relaxation between levels with the same sign of magnetization easily occurs although it is not allowed in the pure quantum system. 11) As a result of a composite mechanism of nonadiabatic changes of state and a fast relaxation to the ground state, the magnetization process in such molecule magnets shows a step-wise curve which is dierent from the pure quantum cas...

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Section: X1 Introductionmentioning

confidence: 99%

Section: X1 Introductionmentioning

confidence: 99%

Magnetic Foehn Effect in Adiabatic Transition

Saito

Miyashita

2001

J. Phys. Soc. Jpn.

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“…For example, in the socalled single molecular magnets, the quantum dynamics of magnetization has been extensively studied. [1][2][3][4][5][6][7][8][9][10] In these systems, the energy levels are discrete, and the quantum mechanical dynamics of a wave function under a sweep of external field is described by the combination of adiabatic and non-adiabatic transitions, where the Landau-Zener formula plays an important role. In addition to the pure quantum mechanical dynamic, effects due to contact with a thermal bath also give important contributions.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of the Density Matrix in Contact with a Thermal Bath and the Quantum Master Equation

Mori

Miyashita

2008

J. Phys. Soc. Jpn.

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We study the structure of the time evolution of the density matrix in contact with a thermal bath in a standard projection operator scheme. In general, the equation of motions of the density matrix with dissipative effects tends to lead any initial state into a steady state. The reduced density matrix of the system in the steady state is obtained by tracing out the degree of freedom of the thermal bath from the density matrix for the equilibrium state of the total system. This reduced matrix is modified by the interaction, and is different from that of the equilibrium of the system alone. In a commonly used equation of motion of the reduced density matrix, we have three terms, i.e., a term of the quantum mechanical evolution of the system density matrix, a term of the non-Markov evolution due to memory effects, and a term depending on the initial total density matrix. We make clear roles of the three terms by explicit calculations of the contributions of the terms to the steady state density matrix. By making use of the role of each term, the properties of the commonly used quantum master equation are examined. For example, if we do not include the imaginary part of the second term, the quantum master equation leads the reduced density matrix into that of the equilibrium of the system without modification. On the other hand, if we include the imaginary part, the steady state density matrix of the system satisfies the master equation up to the second order of the interaction. This property indicates that, in the representation that diagonalizes the system Hamiltonian, the leading order (the second order) terms of the off-diagonal elements of the steady state solution of the master equation agree with those of the density matrix of the equilibrium state.

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“…In order to observe the excited state level crossings discussed in Chapters 3 and 4, a model must allow for non-zero off diagonal matrix elements, α|S z |β , arising from very small anisotropic terms [29]. Such terms would permit a time-dependence where …”

Section: Operating Principles Of Tdrmentioning

confidence: 99%

Tunnel-diode resonator and nuclear magnetic resonance studies of low-dimensional magnetic and superconducting systems

Yeninas

2013

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Effects of the environment on nonadiabatic magnetization process in uniaxial molecular magnets at very low temperatures

Cited by 32 publications

References 20 publications

Magnetic Foehn Effect in Adiabatic Transition

Magnetic Foehn Effect in Adiabatic Transition

Dynamics of the Density Matrix in Contact with a Thermal Bath and the Quantum Master Equation

Tunnel-diode resonator and nuclear magnetic resonance studies of low-dimensional magnetic and superconducting systems

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