1. Some New Geometric Phase Effects in Mn 12 Variants Jonathan Friedman Eduardo H. da Silva Neto Michael Foss-Feig Amherst College Funding: NSF, Research Corporation and Amherst College Dean of Faculty Christos Lampropoulos George Christou UFL - Chemistry Nurit Avraham Yuri Myaesoedov Hadas Shtrikman Eli Zeldov Weizmann Institute of Science

2. Mn 12 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. -10 -9 9 10 Thermal Activation New Barrier D. A. Garanin and E. M. Chudnovsky, Phys. Rev. B 56, 11 102 (1997) J. R. Friedman, Phys. Rev. B, 57, 10291 (1998)

4. Interference between Tunneling Paths in Fe 8 W. Wernsdorfer and R. Sessoli, Science, 1999. Theoretical Prediction: A. Garg., EPL, 1993.

5. Predicted Interference Effect for Mn 12 Park and Garg, PRB, 2002

6. Two positions of C deduced from X-ray diffraction → induced 2 nd order anisotropy ESR spectrum as a function of azimuthal angle. (Edwards, et al., PRL, 2003) Solvent Disorder in Mn 12 Ac Acetic acid of solvation Cornia, et al., PRL, 2002 Angle-selected relaxation rate. (del Barco, et al., PRL, 2003)

7. A new SMM - Mn 12 tBuAc Mn 12 -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). Mn 12 -tBuAc Mn 12 -Ac

8. Sample is rotated to position 1 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 Quasi-exponential decay in magnetization. Controlling the Relaxation Rate of Mn 12 -tBuAc with a Transverse Field Large component of magnetic field transverse to easy axis position 1 position 2 Applied Magnetic Field θ T = 2.98(3) K H T = 4200 Oe HL=-500 Oe

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

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

11. Rates on and off 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 On-resonance Off-resonance

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

13. Now Add Transverse Anisotropy (Fourth Order!) H T 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. Measured relaxation rate as a function of transverse field in highly symmetric Mn 12 -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. Conclusions