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Some New Geometric Phase Effects in Mn 12 Variants Jonathan Friedman Eduardo H. da Silva Neto Michael Foss-Feig Amherst College Funding: NSF, Research.

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Presentation on theme: "Some New Geometric Phase Effects in Mn 12 Variants Jonathan Friedman Eduardo H. da Silva Neto Michael Foss-Feig Amherst College Funding: NSF, Research."— Presentation transcript:

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 Thermal Activation New Barrier D. A. Garanin and E. M. Chudnovsky, Phys. Rev. B 56, (1997) J. R. Friedman, Phys. Rev. B, 57, (1998)

4 Interference between Tunneling Paths in Fe 8 W. Wernsdorfer and R. Sessoli, Science, 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, (2006). A.-L. Barra et al., JACS, 129, (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 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: 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, (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:

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


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