Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q8: Quantum Spin Dynamics and Relaxation in Molecular Magnets |
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Sponsoring Units: GMAG Chair: Andrew Kent, New York University Room: 414/415 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q8.00001: Coherent Manipulation and Decoherence of S=10 Single-Molecule Magnets Invited Speaker: A single crystal of high-spin single-molecule magnets (SMMs) is an attractive testbed for quantum science and technologies. High-spin SMMs are suitable for applications to dense quantum memory and computing devices. Because SMM clusters are identical and interact weakly, the ensemble properties of single crystals of SMMs reflect the properties of a single cluster. However coherent manipulation of high-spin SMM crystals has never been demonstrated due to strong spin decoherence. For spins in the solid state, an interaction with fluctuations of surrounding spin bath is a major source of spin decoherence. One approach to reduce spin bath fluctuations is to bring the spin bath into a well-known quantum state that exhibits little or no fluctuations. A prime example is the case of a fully polarized spin bath. In diamond, spin decoherence has been quenched using high-frequency pulsed electron paramagnetic resonance (EPR) [1]. We present coherent manipulation and decoherence of a single-crystal of S=10 Fe8 SMMs. Through polarizing a spin bath in Fe8 single-molecule magnets at 4.6 T and 1.3 K, we demonstrate that spin decoherence is significantly suppressed to extend the spin decoherence time ($T_{2})$ up to 700 ns [2]. Investigation of temperature dependence of spin relaxation times reveals the nature of spin decoherence. This work is collaboration with J. van Tol, C. C. Beedle, D. N. Hendrickson, L.-C. Brunel, and M. S. Sherwin.\\[4pt] [1] S. Takahashi, R. Hanson, J. van Tol, M. S. Sherwin, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{101}, 047601 (2008).\\[0pt] [2] S. Takahashi, J. van Tol, C. C. Beedle, D. N. Hendrickson, L.-C. Brunel, and M. S. Sherwin, arXiv: 0810.1254. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q8.00002: The Role of Antisymmetric Exchange on the Quantum Interference between States of Different Spin Length in a dimeric Molecular Nanomagnet. Invited Speaker: We report direct evidence of quantum oscillations of the \textit{total spin length} of a dimeric molecular nanomagnet through the observation of quantum interference associated with tunneling trajectories between states having different spin quantum numbers. As we outline, this is a consequence of the unique characteristics of a molecular Mn$_{12}$ wheel which behaves as a (weak) ferromagnetic exchange-coupled molecular dimer: each half of the molecule acts as a single-molecule magnet (SMM), while the weak coupling between the two halves gives rise to an additional internal spin degree of freedom within the molecule, namely that its total spin may fluctuate. This extra degree of freedom accounts for several magnetization tunneling resonances that cannot be explained within the usual giant spin approximation. More importantly, the observation of quantum interference provides unambiguous evidence for the quantum mechanical superposition involving entangled states of both halves of the wheel. Magnetization results obtained in two other versions of this compound, in which the ligands have been modified, show that slight variations of the relative distance between the Mn ions determine whether the molecule behaves as a rigid magnetic unit of spin $S$~=~7 or as two exchange-coupled halves of spin $S$~=~7/2. We analyze the effect of the Dzyaloshinskii-Moriya antisymmetric exchange interaction in a molecule with a centre of inversion symmetry and propose a formal model to account for the observed broken degeneracy that preserves the molecular inversion symmetry. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q8.00003: Spin dynamics in the single molecule magnet Ni$_4$ under microwave irradiation Invited Speaker: Quantum mechanical effects such as quantum tunneling of magnetization (QTM) and quantum phase interference have been intensively studied in single molecule magnets (SMMs). These materials have also been suggested as candidates for qubits and are promising for molecular spintronics. Understanding decoherence and energy relaxation mechanisms in SMMs is then both of fundamental interest and important for the use of SMMs in applications. Interestingly, the single-spin relaxation rate due to direct process of a SMM embedded in an elastic medium can be derived without any unknown coupling constant [1]. Moreover, nontrivial relaxation mechanisms are expected from collective effects in SMM single crystals, such as phonon superradiance or phonon bottleneck. In order to investigate the spin relaxation between the two lowest lying spin-states of the $S=4$ single molecule magnet Ni$_4$, we have developed an integrated sensor that combines a microstrip resonator and micro-Hall effect magnetometer on a chip [2]. This sensor enables both real time studies of magnetization dynamics under pulse irradiation as well as simultaneous measurements of the absorbed power and magnetization changes under continuous microwave irradiation. The latter technique permits the study of small deviations from equilibrium under steady state conditions, i.e. small amplitude cw microwave irradiation. This has been used to determine the energy relaxation rate of a Ni$_4$ single crystal as a function of temperature at two frequencies, 10 and 27.8 GHz. A strong temperature dependence is observed below 1.5 K, which is not consistent with a direct spin-phonon relaxation process. The data instead suggest that the spin relaxation is dominated by a phonon bottleneck at low temperatures and occurs by an Orbach process involving excited spin-levels at higher temperatures [3]. Experimental results will be compared with detailed calculations of the relaxation rate using the density matrix equation with the relaxation terms in the universal form.\\ 1. E. M. Chudnovsky, D. A. Garanin and R. Schilling, Phys. Rev. B \textbf{72}, 094426 (2005)\\ 2. G. de Loubens \textit{et al.}, J. Appl. Phys. \textbf{101}, 09E104 (2007)\\ 3. G. de Loubens, D. A. Garanin, C. C. Beedle, D. N. Hendrickson and A. D. Kent, Europhys. Lett. \textbf{83}, 37006 (2008)\\ [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q8.00004: Multiphoton Coherent Manipulation in Large Spin Qubits Invited Speaker: Manipulation of quantum information allows certain algorithms to be performed at unparalleled speeds. Photons are an ideal choice to manipulate qubits as they interact with quantum systems in predictable ways. They are a versatile tool for manipulating, reading/coupling qubits and for encoding/transferring quantum information over long distances. Spin-based qubits have well known behavior under photon driving and can be potentially operated up to room temperature. When diluted enough to avoid uncontrolled spin-spin interactions, a variety of spin qubits show long coherence times, $e.g.$ the nitrogen vacancies in pure diamonds (1,2), nitrogen atoms trapped in a C60 cage (3), Ho3+ and Cr5+ ions (4,5) and molecular magnets (6,7). We have used large spin Mn2+ ions (S=5/2) to realize a six level system that can be operated by means of single as well as multi-photon coherent Rabi oscillations (8). This spin system has a very small anisotropy whose effect can be tuned \textit{in-situ} to turn the system into a multi-level harmonic system. This offer new ways of manipulating, reading and resetting a spin qubit. Decoherence effects are strongly reduced by the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. \\[0pt] 1. R. Hanson \textit{et al.,} \textit{Science} \textbf{320}, 352 (2008). \\[0pt] 2. M.V. Gurudev Dutt \textit{et al.,} \textit{Science} \textbf{316}, 1312 (2007). \\[0pt] 3. G.W. Morley \textit{et al., Phys. Rev. Lett.} \textbf{98}, 220501 (2007). \\[0pt] 4. S. Bertaina \textit{et al.}, \textit{Nat. Nanotech.} \textbf{2}, 39 (2007). \\[0pt] 5. S. Nellutla \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{99}, 137601 (2007). \\[0pt] 6. A. Ardavan \textit{et al.,} \textit{Phys. Rev. Lett.} \textbf{98}, 057201 (2007). \\[0pt] 7. S. Bertaina \textit{et al.}\textbf{, }\textit{Nature} \textbf{453}, 203,(2008). \\[0pt] 8. S. Bertaina \textit{et al., submitted.} [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q8.00005: Understanding electron and nuclear spin dynamics in Cr$^{5+}$ doped K$_{3}$NbO$_{8}$ Invited Speaker: Chromium(V) doped in the diamagnetic host potassium niobate, a simple spin $S=\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, $I$ = 0 system, has been proposed as an alternative standard for field calibration and g-standard for high-field EPR [1]. This system constitutes a dilute two-level model relevant for use as a electron spin qubit [2] and as such coherent electron spin manipulation at X-band ($\sim $9.5 GHz) was observed over a wide range temperature. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At 4 K, a Rabi frequency $\Omega _{R}$ of 20 MHz together with the phase coherence relaxation (spin-spin relaxation) time, $T_{2}$ of $\sim $10 $\mu $s results in the single qubit figure of merit $Q_{M}$ (=$\Omega _{R}T_{2}$/$\pi )$ as about 500, showing that a diluted ensemble of Cr(V)$ ($S = 1/2) doped K$_{3}$NbO$_{8}$ is a potential candidate for solid-state quantum information processing. Also, the field and temperature dependence of the $T_{1}$ (spin-lattice relaxation) and $T_{2}$ times was investigated [3] for a further understanding of the relaxation mechanisms governing the phase decoherence in this system. These studies show that the coupling of the electron spin with the neighboring $^{39}$K nuclei ($I$ = 3/2) is one of the prominent $T_{2}$ mechanisms. The hyperfine and quadrupole interactions with $^{39}$K nuclei was resolved by using the high-frequency (240 GHz) pulsed electron nuclear double resonance (ENDOR). \\[3pt] [1]. B. Cage, A. Weekley, L. -C. Brunel and N. S. Dalal, \textit{Anal. Chem.} \textbf{71}, 1951 \textbf{(}1999). \\[0pt] [2]. S. Nellutla, K.-Y. Choi, M. Pati, J. van Tol, I. Chiroescu and N. S. Dalal, \textit{Phys. Rev. Lett.} \textbf{99}, 137601 (2007). \\[0pt] [3]. S. Nellutla, G. W. Morley, M. Pati, N. S. Dalal and J. van Tol, \textit{Phys. Rev. B.} \textbf{78}, 054426 (2008). [Preview Abstract] |
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