The spin relaxation in the molecular magnet Mn12ac is investigated by a novel type of highfrequency magnetic spectroscopy. By reversing the external magnetic field the intensities of the | ± 10 → | ± 9 transitions within the ground multiplet redistribute; below T ≈ 2 K resonant quantum tunneling is directly observed as magnetic hole burning in the absorption spectra of a polycrystal. We quantitatively describe our observations by taking into account inhomogeneous line broadening, the time dependence of the level population during the relaxation process, and the resonance behavior of the relaxation rate due to quantum tunneling near the avoided level crossing.PACS numbers: 75.50.Xx, 78.20.Ls, 75.45.+j, In the last years molecular magnets comprising large clusters of coupled magnetic ions have attracted considerable interest as mesoscopic systems which exhibit new properties such as molecular magnetic bistability, macroscopic quantum tunneling of magnetization, quantum phase interference, etc. [1][2][3][4]. Among them clusters with a high-spin ground state and large anisotropy, such as Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]2CH 3 COOH·4H 2 O (abbreviated as Mn 12 ac) are of a paramount interest.Mn 12 ac cluster can be considered as nanoparticle with an effective magnetic moment of 20µ B (µ B is the Bohr magneton), corresponding to a collective spin S = 10 of the exchange coupled twelve Mn ions within the molecule. Due to the strong anisotropy, the magnetic moment freezes along one of the two easy directions at low temperatures. This is governed by a crystal field (CF) which splits the ground S = 10 multiplet and provides a quasidoublet structure | ± m of the lowest energy levels. The energy barrier ∆E ≈ 60 K between the two lowest lying states | ± 10 determines the thermally activated relaxation of the magnetization in Mn 12 ac [3,4]. Below a blocking temperature of about 3 K resonant quantum tunneling was discovered in the form of peaks in the relaxation rate and steps in the hysteresis loops at regular intervals of magnetic field corresponding to a coincidence of the CF energy levels [1,2]. Since the rate of the tunneling for a ground state is small in Mn 12 ac the tunneling occurs via appropriate thermally excited CF states for temperatures above approximately 2 K resulting in a shortcut at the top of the barrier which accelerates the relaxation; at even lower temperatures pure quantum tunneling is dominant [3,4].Various spectroscopic investigations have been applied to Mn 12 ac, including electron-spin resonance (ESR) [5,6] and inelastic neutron scattering [7]. In particular, a new type of high-frequency ESR method based on a quasioptical spectroscopy [8] made it possible to directly observe the CF transitions in an equilibrium state and to study in details their characteristics, including the line-shapes [9]. In this Letter we report on the high-frequency magnetic spectroscopy of non-equilibrium phenomena in Mn 12 ac which allows to directly and independently determine the full set of magnetic characteristics.Mn ...

We study a Bose-condensed gas at finite temperature, in which the particles of the condensate and of the thermal cloud are constrained to move in a plane under radial harmonic confinement and interact via strictly two-dimensional collisions. The coupling parameters are obtained from a calculation of the many-body T-matrix and decreases as temperature increases through a dependence on the chemical potential and on the occupancy of excited states. We discuss the consequences on the condensate fraction and on the density profiles of the condensed and thermal components as functions of temperature, within a simplified form of the two-fluid model.

No abstract

An optical potential method to study the positron-hydrogen atom scattering system within the close-coupling framework that includes both the positron-hydrogen and positroniumproton channels has been implemented. Ionization, positronium formation and total cross sections are reported and are compared to other available theoretical and experimental data.

No abstract