Thermally Activated Resonant Magnetization Tunneling in Molecular Magnets: Mn_12Ac and others

The dynamical theory of thermally activated resonant magnetization tunneling in uniaxially anisotropic magnetic molecules such as Mn_12Ac (S=10) is developed.The observed slow dynamics of the system is described by master equations for the populations of spin levels.The latter are obtained by the adiabatic elimination of fast degrees of freedom from the density matrix equation with the help of the perturbation theory developed earlier for the tunneling level splitting [D. A. Garanin, J. Phys. A, 24, L61 (1991)]. There exists a temperature range (thermally activated tunneling) where the escape rate follows the Arrhenius law, but has a nonmonotonic dependence on the bias field due to tunneling at the top of the barrier. At lower temperatures this regime crosses over to the non-Arrhenius law (thermally assisted tunneling). The transition between the two regimes can be first or second order, depending on the transverse field, which can be tested in experiments. In both regimes the resonant maxima of the rate occur when spin levels in the two potential wells match at certain field values. In the thermally activated regime at low dissipation each resonance has a multitower self-similar structure with progressively narrowing peaks mounting on top of each other.Comment: 18 pages, 8 figure

Experimental evidence of the role of quasilocalized phonons in the thermal conductivity of simple alcohols in orientationally ordered crystalline phases

The thermal conductivity к(T) of crystalline alcohols (methyl, ethyl and 1-propyl) within their thermodynamic equilibrium phases for T ≥ 2 K and under the equilibrium vapor pressures has been measured and analyzed. While such compounds usually exhibit a rich polymorphism including amorphous and partially ordered crystals, the phases here explored correspond to crystals showing complete orientational order. The results show that the temperature dependence of к(T) above its maximum deviates from the expected decrease following a 1/T law with increasing temperature arising from anharmonic interactions involving acoustic excitations. Such a deviation is here attributed to the presence of a component кII(T) corresponding to the shortest-lifetime phonons (Cahill–Pohl model) additional to that кI(T) related to propagating phonons and thus: к(T) = кI(T) + кII(T). Above T = 40 K кI(T) does follow the law 1/T and кII(T) is basically temperature independent. The component кI(T) is well described by the Debye–Peierls model taking into account the phonon–phonon Umklapp processes and phonon scattering by dislocations. In turn, the contribution кII(T) is attributed to the effects of higher lying excitations which get thermally populated above some 40 K. Finally, a systematic trend is found concerning the strength of phonon–phonon scattering which is seen to diminish as the number of carbon atoms in the alcohol molecule increases

Insight into the physics of the 5f -band antiferromagnet U₂Ni₂Sn from the pressure dependence of crystal structure and electrical resistivity

A resistivity study of a single crystal of U2Ni2Sn has been performed at ambient pressure and under hydrostatic pressure up to p=3.3GPa. It revealed Fermi-liquid behavior accompanied by spin excitations with an energy gap Δ=30-55K in the whole pressure range. The Néel temperature varies with pressure in a nonmonotonous way. It increases at the rate dTN/dp=+0.6K/GPa, and later, after passing through the maximum at ≈3 GPa, it starts to decrease quickly. High-pressure x-ray diffraction indicated that an orthorhombic distortion of the tetragonal structure takes place around the pressure of this TN maximum. The computational study based on the density functional theory illustrates that the loss of magnetism in U2Ni2Sn with pressure is primarily due to 5f-band broadening, which results from the collapse of the U spacing within the U-U dimers. © 2021 American Physical Society