The molecular compound K6[V IV 15 As III 6 O42(H2O)] · 8H2O, in short V15, has shown important quantum effects such as coherent spin oscillations. The details of the spin quantum dynamics depend on the exact form of the spin Hamiltonian. In this study, we present a precise analysis of the intramolecular interactions in V15. To that purpose, we performed high-field electron spin resonance measurements at 120 GHz and extracted the resonance fields as a function of crystal orientation and temperature. The data are compared against simulations using exact diagonalization to obtain the parameters of the molecular spin Hamiltonian.
We performed quantum manipulations of the multi-level spin system S=5/2 of a Mn 2+ ion, by means of a two-tone pulse drive. The detuning between the excitation and readout radio frequency pulses allows one to select the number of photons involved in a Rabi oscillation as well as increase the frequency of this nutation. Thus detuning can lead to a resonant multi-photon process. Our analytical model for a two-photon process as well as a numerical generalization fit well the experimental findings, with implications in the use of multi-level spin systems as tunable solid state qubits.
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 K6[V15As6O42(D2O)] · 8D2O, 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. INTRODUCTIONIn solid-state systems, interactions between electronic spins and their environment are the limiting factor of spin phase lifetime, or decoherence time. Important advances have been recently realized in demonstrating long-lived spin coherence via spin dilution [1][2][3][4][5][6] and isolating a spin in non-magnetic cages [7], for instance. The presence of a lattice can be felt by spins through orbital symmetries and spin-orbit coupling. An isolated free electron has a spin angular momentum associated with a g-factor g e = 2.00232 but in general, spin-orbit coupling changes the g-factor by the admixture of excited orbital states [8] into the ground state. In this Letter, we demonstrate that fluctuations in the spin-orbit interaction can be a significant source of spin decoherence. We present a general theoretical framework to obtain noise spectrum. The method is applied to fluctuations of the long-range dipolar interactions and we observe how the spin-orbit term is modulating the induced decoherence. The model describes spin dilution and thermal excitations effects as well. Experimentally, we analyze shape and orientation anisotropy of ESR linewidths of the molecular compound. This system has shown spin coherence at low temperatures [5,9] and interesting out of equilibrium spin dynamics due to phonon bottlenecking [10,11]. However, the details of the spin decoherence are still not fully understood. In the case of diluted or molecular spins, little evidence has been brought up to now on the role of spin-orbit coupling on spin coherence time. This study elucidates this decoherence mechanism and how to mitigate its effect. FLUCTUATIONS IN SPIN HAMILTONIANThe V 15 cluster anions form a lattice with trigonal symmetry containing two clusters per unit cell [12]. Individual molecules have fifteen V IV s = 1/2 ions arranged into three layers, two non-planar hexagons sandwiching a triangle (see Fig. 1a). Exchange couplings between the spins in the triangle and hexagons exceed 100 K [13,14] and at low temperatures this spin system can be modeled as a triangle of spins 1/2. The spin Hamiltonian is, as discussed in Supplemental Material [15] (SM) Section I:where H 0 describes the Zeeman splitting in an external field B 0 , H J is the symmetric exchange term, and H DM is the anti-symmetric Dzyaloshinsky-Moriya (DM) term (see [16] for a detailed formulation). H st eigenvalues are shown in Fig. 1(b) and are used to calculate resonant field positions B res of the E...
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