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New Geometric Phase Effects in Mn12 Variants

This study investigates the geometric phase effects in Mn12 variants, specifically the effect of a transverse field on the tunneling rate and the interference between tunneling paths. The results show steps and plateaus in the relaxation rate as a function of the transverse field, both on and off resonance. The study also predicts a pressure-induced interference effect.

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New Geometric Phase Effects in Mn12 Variants

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  1. Some New Geometric Phase Effects in Mn12 Variants Jonathan FriedmanEduardo H. da Silva Neto Michael Foss-FeigAmherst College Christos LampropoulosGeorge Christou UFL - Chemistry Nurit AvrahamYuri MyaesoedovHadas ShtrikmanEli ZeldovWeizmann Institute of Science Funding: NSF, Research Corporation and Amherst College Dean of Faculty

  2. Mn12 Ac

  3. The Effect of a Transverse Field The tunneling rate for a particular pair of resonant levels depends on the transverse field (H┴). H┴ increases the tunneling rate and reduces the barrier. Thermal Activation New Barrier -9 9 -10 10 D. A. Garanin and E. M. Chudnovsky, Phys. Rev. B 56, 11 102 (1997) J. R. Friedman, Phys. Rev. B, 57, 10291 (1998)

  4. Easy axis Z A Solid Angle W j H X B Hard axis Interference between Tunneling Paths in Fe8 • Theoretical Prediction: • A. Garg., EPL, 1993. W. Wernsdorfer and R. Sessoli, Science, 1999.

  5. Predicted Interference Effect for Mn12 Park and Garg, PRB, 2002

  6. Solvent Disorder in Mn12Ac Acetic acid of solvation Angle-selected relaxation rate. (del Barco, et al., PRL, 2003) Two positions of C deduced from X-ray diffraction → induced 2nd order anisotropy ESR spectrum as a function of azimuthal angle. (Edwards, et al., PRL, 2003) Cornia, et al., PRL, 2002

  7. A new SMM - Mn12 tBuAc Mn12-Ac Mn12-tBuAc • Mn12-tBuAc has less solvent disorder and weaker dipole interactions. • S. Hill et al., J. Appl. Phys. 97 (2005). • W. Wernsdorfer et al. Phys. Rev. Lett. 96, 057208 (2006). • A.-L. Barra et al., JACS, 129, 10754 (2007).

  8. Controlling the Relaxation Rate of Mn12-tBuAc with a Transverse Field Applied Magnetic Field position 1 position 2 θ Sample is rotated to position 1 T = 2.98(3) K HT = 4200 Oe HL=-500 Oe Large component of magnetic field along easy axis Lower well populated Sample is rotated to position 2 Small component of magnetic field along easy axis Large component of magnetic field transverse to easy axis Quasi-exponential decay in magnetization.

  9. Longitudinal Field Rate Distribution T = 3.21 (3) K HT = 4800 Oe ON Resonance Relaxation Rate OFF Resonance Relaxation Rate

  10. T = 3.21 (3) K Thermal Activation -9 9 -10 10

  11. Rates on and off resonance Off-resonance On-resonance Plateaus indicate dominant tunneling resonance • Plateaus (much flatter!) in the off-resonance relaxation. • Steps and plateaus occur at different fields on and off resonance

  12. Predicted Dependence of Relaxation Rate on Transverse Field On resonance Off resonance D. Garanin, arXiv:0805.0391

  13. Now Add Transverse Anisotropy (Fourth Order!) HT along hard axis. S. Hill et al., J. Appl. Phys. 97, 10M510(2005) A.-L. Barra et al., JACS, 129, 10754 (2007)

  14. Calculated Tunnel Splittings Park and Garg, PRB, 2002

  15. Full Data vs Simulations T = 3.21(3) K

  16. Predicted Pressure-Induced Interference Effect M. S. Foss-Feig and JRF, ArXiv:0809.2289

  17. Conclusions • Measured relaxation rate as a function of transverse field in highly symmetric Mn12-tBuAc. • Steps and plateaus in the relaxation rate as a function of transverse field both on and off resonance. • Off-resonance results appear to be a remnant of a geometric-phase interference effect. • Predicted new interference effect induced by uniaxial pressure.

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