Publications (3)43.65 Total impact
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Article: Decoherence Window and Electron-Nuclear Cross Relaxation in the Molecular Magnet V15
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ABSTRACT: Rabi oscillations in the V 15 single molecule magnet embedded in the surfactant ðCH 3 Þ 2 Â ½CH 3 ðCH 2 Þ 16 CH 2 2 N þ have been studied at different microwave powers. An intense damping peak is observed when the Rabi frequency R falls in the vicinity of the Larmor frequency of protons ! N . The experiments are interpreted by a model showing that the damping (or Rabi) time R is directly associated with decoherence caused by electron-nuclear cross relaxation in the rotating reference frame. This decoherence induces energy dissipation in the range ! N À e < R < ! N , where e is the mean superhyperfine field induced by protons at V 15 . Weaker decoherence without dissipation takes place outside this window. Specific estimations suggest that this rapid cross relaxation in a resonant microwave field, observed for the first time in V 15 , should also take place, e.g., in Fe 8 and Mn 12 . Single molecule magnets (SMMs) are composed of a lattice of molecules or clusters whose collective ground-state spin S is associated with strong intramolecular inter-actions. SMMs are considered as promising systems for quantum information processing because they can be self-organized in 2D or 3D networks, while their relatively small size ($ 1 nm) makes them good quantum objects with weak self-decoherence (which is generally noncon-trolled, as in, e.g., superconducting qubits). Predictions announce phase coherence times 2 ¼ 100–500 s in V 15 [1] and Fe 8 [2], provided that the nuclear spin bath is absent and the temperature and magnetic field are opti-mized. However, this takes place in the absence of micro-waves or under a short pulse sequence (as in the Hahn spin-echo method). A continuous resonant microwave field produces nutations of the spin magnetization (Rabi oscil-lations [3]). The relaxation dynamics in this transient regime changes drastically. For a dipolar-coupled system of electronic spins, R can become much shorter than 2 [4–6]. As opposed to common knowledge [7], under the condition R < 2 , the utmost time of coherent spin ma-nipulations in quantum computation will be limited by R rather than 2 , and oscillations disappear if R is less than the oscillation period. The first evidence of Rabi oscillations in a SMM was obtained in the cluster ½V IV 15 As III 6 O 42 ðH 2 OÞ 6À (V 15) em-bedded in the surfactant ðCH 3 Þ 2 ½CH 3 ðCH 2 Þ 16 CH 2 2 N þ (DODA) to reduce dipolar interactions [1,8]. In a later study, Rabi oscillations were detected in an Fe 4 SMM [9]. In both cases, the coherence times 2 reached several hundred nanoseconds. Similar 2 were obtained in Cr 7 -based [10] and Fe 8 [2] SMMs, but Rabi oscillations could not be detected. In this Letter, we show that Rabi oscillations of V 15 -DODA are subjected to a very efficient and apparently general decoherence mechanism accompa-nied by energy dissipation into the proton spin bath. In a certain range of Rabi frequencies where this new phenome-non takes place, the condition R < 2= R is nearly fulfilled and oscillations almost disappear. The V 15 cluster has a layer structure: three V IV form a central triangle sandwiched by two smaller V IV 6 hexagons [11,12] (Fig. 4 in [12]). The 15 spins 1=2 are coupled by strong antiferromagnetic superexchange interactions in the external hexagons and by a relatively weak exchange through the bridges in the central triangle [11,13,14]. The lowest energy levels are a pair of doublets (with a gap of Á 1=2 ¼ 200 mK caused by the Dzyaloshinskii-Moriya in-teractions [11,14]) and a quartet (with small zero-field splitting Á 3=2 ¼ 12 mK [1,11]) isolated from the above quasicontinuous spectrum by a gap of Á ¼ 250 K [11]. A series of V 15 -DODA samples with V 15 concentration c ¼ ð1–5Þc 0 (c 0 ¼ 4:3 Â 10 17 cm À3) [15] was prepared following Ref. [1]. Variations of both the phase and spin-lattice relaxation times were measured vs c (Fig. 6 in [12]). The observed linear slope of d 2 =dc is apparently related to the decoherence via V 15 intercluster dipolar interactions. The phase coherence time measured at 4.2 K is of 0:5 s for c ¼ c 0 and decreases down to 0:25 s for c ¼ 4c 0 . The spin-lattice relaxation can be neglected below 10 K, where the spin-lattice relaxation time 1 > 10 s [12].Physical Review Letters 08/2012; 109:050401. · 7.37 Impact Factor -
Article: Quantum oscillations in a molecular magnet.
Nature 08/2010; 466(7309):1006. · 36.28 Impact Factor -
Article: Driven spin-bath decoherence in the molecular magnet V15
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ABSTRACT: The measured Rabi oscillations of an ensemble of spins (here the spins 3/2 of the V15 single molecular magnet) are strongly damped by the excitation ac-field. The corresponding decoherence mechanism is attributed to an electromagnetic noise created by the dipolar spin-bath, inhomogeneously driven by the microwave field. Decoherence by electronic and nuclear spin-baths express themselves very differently vs microwave power. This decoherence mechanism should apply to all types of weakly interacting ensembles of qubits, beyond magnetism.
