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arXiv:cond-mat/0302107 · Physical review. B, Condensed matter

Landau-Zener-Stueckelberg effect in a model of interacting tunneling systems

Dmitry A. Garanin

Published Jul 11, 2003• 24 citations
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Abstract

The Landau-Zener-Stueckelberg (LZS) effect in a model system of interacting tunneling particles is studied numerically and analytically. Each of N tunneling particles interacts with each of the others with the same coupling J. This problem maps onto that of the LZS effect for a large spin $S=N/2.$ The mean-field limit $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}$ corresponds to the classical limit $\stackrel{\ensuremath{\rightarrow}}{S}\ensuremath{\infty}$ for the effective spin. It is shown that the ferromagnetic coupling $J>0$ tends to suppress the LZS transitions. For $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}$ there is a critical value of J above which the staying probability P does not go to zero in the slow sweep limit, unlike the standard LZS effect. For $J<0$ LZS transitions are boosted and $P=0$ for a set of finite values of the sweep rate for $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}.$ Various limiting cases such as strong and weak interaction, slow and fast sweep are considered analytically. It is shown that the mean-field approach works well for arbitrary N if the interaction J is weak.

DOI
10.1103/physrevb.68.014414

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The Landau-Zener-Stueckelberg (LZS) effect in a model system of interacting tunneling particles is studied numerically and analytically. Each of N tunneling particles interacts with each of the others with the same coupling J. This problem maps onto that of the LZS effect for a large spin S=N/2.S=N/2.S=N/2. The mean-field limit N\ensuremath→\ensuremath∞\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}N\ensuremath→​\ensuremath∞ corresponds to the classical limit S\ensuremath→\ensuremath∞\stackrel{\ensuremath{\rightarrow}}{S}\ensuremath{\infty}S\ensuremath→​\ensuremath∞ for the effective spin. It is shown that the ferromagnetic coupling J>0J>0J>0 tends to suppress the LZS transitions. For N\ensuremath→\ensuremath∞\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}N\ensuremath→​\ensuremath∞ there is a critical value of J above which the staying probability P does not go to zero in the slow sweep limit, unlike the standard LZS effect. For J<0J<0J<0 LZS transitions are boosted and P=0P=0P=0 for a set of finite values of the sweep rate for N\ensuremath→\ensuremath∞.\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty}.N\ensuremath→​\ensuremath∞. Various limiting cases such as strong and weak interaction, slow and fast sweep are considered analytically. It is shown that the mean-field approach works well for arbitrary N if the interaction J is weak.
The asymptotic staying probability PPP in the Landau-Zener effect with interaction is analytically investigated at fast sweep $\ensuremath{\epsilon}=\ensuremath{\pi}{\ensuremath{\Delta}}^{2}∕(2\ensuremath{\hbar}v)⪡1$. We have rigorously calculated the value of I0{I}_{0}I0​ in the expansion $P\ensuremath{\cong}1\ensuremath{-}\ensuremath{\epsilon}+{\ensuremath{\epsilon}}^{2}∕2+{\ensuremath{\epsilon}}^{2}{I}_{0}$ for arbitrary couplings and relative resonance shifts of individual tunneling particles. The results essentially differ from those of the mean-field approximation. It is shown that strong long-range interactions such as dipole-dipole interaction generate huge values of I0{I}_{0}I0​ because a flip of one particle strongly influences many others. However, in the presence of strong static disorder, making the resonances for individual particles shifted with respect to each other, the influence of interactions is strongly reduced. In molecular magnets the main source of static disorder is the coupling to nuclear spins. Our calculations using the actual shape of the Fe8{\mathrm{Fe}}_{8}Fe8​ crystal studied in the Landau-Zener experiments [Wernsdorfer et al., Europhys. Lett. 50, 552 (2000)] yield a value of I0{I}_{0}I0​ that is in good agreement with the value extracted from the experimental data.
We study the Landau-Zener-Stueckelberg (LZS) effect for a two-level system with a time-dependent nonlinear bias field (the sweep function) W(t).W(t).W(t). Our main concern is to investigate the influence of the nonlinearity of W(t)W(t)W(t) on the probability P to remain in the initial state. The dimensionless quantity $\ensuremath{\varepsilon}=\ensuremath{\pi}{\ensuremath{\Delta}}^{2}/(2\ensuremath{\Elzxh}v)$ depends on the coupling $\ensuremath{\Delta}$ of both levels and on the sweep rate v.v.v. For fast sweep rates, i.e., \ensuremath{\varepsilon}\ensuremath{\ll}1, and monotonic, analytic sweep functions linearizable in the vicinity of the resonance we find the transition probability $1\ensuremath{-}P\ensuremath{\cong}\ensuremath{\varepsilon}(1+a),$ where a>0a>0a>0 is the correction to the LSZ result due to the nonlinearity of the sweep. Further increase of the sweep rate with nonlinearity fixed brings the system into the nonlinear-sweep regime characterized by $1\ensuremath{-}P\ensuremath{\cong}{\ensuremath{\varepsilon}}^{\ensuremath{\gamma}}$ with \ensuremath{\gamma}\ensuremath{\ne}1, depending on the type of sweep function. In the case of slow sweep rates, i.e., \ensuremath{\varepsilon}\ensuremath{\gg}1, an interesting interference phenomenon occurs. For analytic W(t)W(t)W(t) the probability ${P=P}_{0}{e}^{\ensuremath{-}\ensuremath{\eta}}$ is determined by the singularities of $\sqrt{{\ensuremath{\Delta}}^{2}{+W}^{2}(t)}$ in the upper complex plane of t. If W(t)W(t)W(t) is close to linear, there is only one singularity, which leads to the LZS result ${P=e}^{\ensuremath{-}\ensuremath{\varepsilon}}$ with important corrections to the exponent due to nonlinearity. However, for, e.g., $W(t)\ensuremath{\propto}{t}^{3}$ there is a pair of singularities in the upper complex plane. Interference of their contributions leads to oscillations of the prefactor P0{P}_{0}P0​ that depends on the sweep rate through \ensuremath{\varepsilon} and turns to zero at some \ensuremath{\varepsilon}. Measurements of the oscillation period and of the exponential factor would allow one to determine \ensuremath{\Delta}, independently.
Landau-Zener (LZ) transitions of a two-level system (e.g., electronic spin in molecular magnets) coupled to one or many environmental spins (e.g., nuclear spins) are studied. For rather general interactions the LZ problem is reduced to that of a Landau-Zener grid. It is shown analytically that environmental spins initially in their ground state do not influence the staying probability PPP. This changes if they are prepared in a statistical ensemble. For a more specific model with environmental spins in a transverse field, LZ transitions are studied in the case of well-separated resonances in the LZ grid. The full evolution of the system is described as a succession of elementary transitions at avoided crossings and free evolution between them. If the environmental spins are strongly coupled to the central spin, their effect on PPP is weak. In other cases LZ transitions are strongly suppressed and PPP is decreasing very slowly with the sweep-rate parameter $\ensuremath{\epsilon}\ensuremath{\propto}1/v$, vvv being the energy sweep rate.
By considering a quantum critical Lipkin-Meshkov-Glick model we analyze a new type of Landau-Zener transitions where the population transfer is mediated by interaction rather than from a direct diabatic coupling. For this scenario, at a mean-field level the dynamics is greatly influenced by quantum interferences. In particular, regardless of how slow the Landau-Zener sweep is, for certain parameters almost no population transfer occurs, which is in stark contrast to the regular Landau-Zener model. For moderate system sizes, this counterintuitive mean-field behaviour is not duplicated in the quantum case. This can be attributed quantum fluctuations and the fact that multi-level Landau-Zener-Stückelberg interferences have a `dephasing' effect on the above mentioned phenomenon. We also find a discrepancy between the quantum and mean-field models in terms of how the transfer probabilities scale with the sweep velocity.