Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W3: Quantum Spin Dynamics in Single-Molecule Magnets |
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Sponsoring Units: DCMP GMAG Chair: Stephen Hill, University of Florida Room: LACC 515B |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W3.00001: Photon-Induced Magnetization Reversal in Single Molecule Magnets Invited Speaker: Single-molecule magnets (SMM) have been the subject of intensive research for more than a decade now because of their unique properties such as macroscopic quantum tunneling. Recent work in this area is focused on whether SMM are potential qubits, as proposed theoretically [1]. We use continuous millimeter wave radiation to manipulate the populations of the energy levels of a single crystal molecular magnet Fe$_{8}$ [2]. When radiation is in resonance with the transitions between energy levels, the steady state magnetization exhibits dips. As expected, the magnetic field locations of these dips vary linearly with the radiation frequency. We will describe our experimental results, which provide a lower bound of 0.17 ns for transverse relaxation time. Transitions between excited states are found even though these states have negligible population at the experimental temperature. We find evidence that the sample heating is significant when the resonance condition is satisfied. Recent experiments are concentrated on the spin dynamics of Fe$_{8}$ induced by pulsed radiation and results of these studies will also be presented. [1] Leuenberger, M. N. and Loss, D., Nature 410, 789 (2001). [2] M. Bal et al., Phys. Rev. B 70, 100408(R) (2004). [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:42PM |
W3.00002: Decoherence on Quantum Spin Systems Invited Speaker: The main problem blocking development of solid-state qubits is decoherence, caused in insulators by nuclear spins, paramagnetic impurities, phonons, and flux noise. Decoherence rates are calculated for several real spin systems, including $Fe_8$ molecules and the $LiHo_xY_{1-x}F_4$ rare earth magnets. A generic result is a ``coherence window''- over a small range of transverse applied fields the decoherence rate will drop by 4-6 orders of magnitude, allowing qubit operation. ``Disentanglement rates'' for {\it pairs} of coupled magnetic qubits also show coherence windows. Predictions for linewidths and lineshapes in ESR and microwave experiments on the $Fe_8$ and $LiHo_xY_{1-x}F_4$ systems are given, both in low applied fields (where the dynamics is incoherent) and in the coherence window. Dipolar interactions between many spins are also included. Since the qubits strongly entangle with the nuclear spin environment, the effect of qubit dynamics on the nuclear $T_1$ and $T_2$ is evaluated. Conduction electrons can be introduced into the environment of many quantum nanomagnets, creating tunable Kondo and RKKY couplings with the qubits (in addition to the phonons, photons, and nuclear spins). The disentanglement rate for 2 qubits is evaluated, along with their contribution to the electronic noise spectrum. \newline \newline Stamp, P.C.E., Tupitsyn, I.S., ``Coherence window in the dynamics of quantum nanomagnets,'' Phys Rev {\bf B69}, 014401 (2004) \newline Stamp, P.C.E., ``Phase Dynamics of Solid-State Qubits: Magnets and Superconductors,'' J. Quantum Computers and Computing {\bf 4}, 20-62 (2003) \newline I. Adagideli, M. Schechter, P.C.E. Stamp (2005) [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 4:18PM |
W3.00003: Electron spin resonance and muon spin relaxation studies of single molecule magnets Invited Speaker: We use a combination of electron spin resonance, muon-spin relaxation and SQUID magnetometry to study polycrystalline and single crystal samples of various novel single molecule magnets (SMMs). We also describe a theoretical framework which can be used to analyse the results from each technique. Electron spin resonance measurements are performed using a millimetre vector network analyser and data are presented on several SMM systems using microwave frequencies from 40-300 GHz. Muon-spin relaxation measurements have been performed on several SMM systems in applied longitudinal magnetic field and in temperatures down to 20 mK. The results suggest that dynamic local magnetic field fluctuations are responsible for the relaxation of the muon spin ensemble. We discuss what can be learned from these experiments concerning SMMs and suggest experiments which can probe the quantum nature of SMMs. (Work in collaboration with S Sharmin, T Lancaster, A Ardavan, F L Pratt, E J L McInnes and R E P Winpenny) References: S. J. Blundell and F. L. Pratt, J. Phys.: Condens. Matter 16, R771 (2004); T. Lancaster et al., J. Phys.: Condens. Matter 16, S4563 (2004); S. Sharmin et al., Appl. Phys. Lett. in press. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:54PM |
W3.00004: Frequency Domain Magnetic Resonance Spectroscopy on Molecular Magnets Invited Speaker: We have advanced a novel technique of frequency domain magnetic resonance spectroscopy to investigate molecular magnets. It allows the extremely accurate determination of zero-field splitting spin Hamiltonian parameters without application of an external field. Extensive resonance lineshape studies give quantitative information on distributions within the sample. The resonance lineshapes in magnetized single-crystalline samples of Mn$_{12}$Ac in the blocked regime are very sensitive to the orientation of the sample with respect to the radiation propagation direction (Faraday and Voigt geometry). The asymmetric and double peak resonance lines found, could be explained by the magneto-optical properties of the material and could be simulated quantitatively. Circularly polarized radiation allows the selective excitation of $\Delta m$ = +1 and $\Delta m$ = -1 transitions. Finally we were able to spectroscopically observe the relaxation of the magnetization as well as quantum tunnelling of the magnetization directly. These relaxation studies using spectroscopic techniques can act as a much more local probe than magnetization measurements. [Preview Abstract] |
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