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31.
Magana D Perera SC Harter AG Dalal NS Strouse GF 《Journal of the American Chemical Society》2006,128(9):2931-2939
Mn ion doping of CdSe and other semimagnetic quantum dot (QDs) alloys has been an area of active speculation for over a decade. We report evidence of Mn(II) doping of CdSe grown from a cubic single source precursor that is superparamagnetic (SPM) with a blocking temperature of 40 K following thermal annealing. Prior to thermal annealing the 4 nm Mn/CdSe (1% Mn) QDs exhibit mainly paramagnetic behavior between 300 and 2 K, with a weak antiferromagnetic exchange. Following thermal annealing of the sample, high-temperature ferromagnetic exchange is observed in the magnetization data with the onset of an SPM phase at 40 K that exhibits a coercivity of 0.1 T at 2 K. The switching-on of SPM behavior is believed to be linked to ion migration with formation of (Se-Mn-Se-Mn-Se-Mn)n centers within the nanocrystal that exhibit coupled magnetic moments. Electron paramagnetic resonance (EPR) provides evidence of two distorted T(d) Mn core sites, a clustered site (dipolar broadened), and a localized Mn site (hyperfine-split). The ratio of the EPR signature for the dipolar broadened site increases following annealing and shows a hysteretic response around the blocking temperature. These observations suggest that thermal annealing results in enhanced cluster formation explaining the onset of the SPM phase in these nanoscale materials. Evidence of SPM behavior is evident in the field-dependent non-Langevin magnetization with a tangential loss in the ac-magnetic susceptibility and the Mydosh parameter (phi = 0.16). 相似文献
32.
Chakov NE Lee SC Harter AG Kuhns PL Reyes AP Hill SO Dalal NS Wernsdorfer W Abboud KA Christou G 《Journal of the American Chemical Society》2006,128(21):6975-6989
Detailed studies are reported of a Mn(12) single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn(12)O(12)(O(2)CCH(2)Br)(16)(H(2)O)(4)].4CH(2)Cl(2) (2.4CH(2)Cl(2) or Mn(12)-BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn(12) family, and displays frequency-dependent out-of-phase AC susceptibility signals and hysteresis in single-crystal magnetization vs applied DC field sweeps. Single-crystal high-frequency EPR spectra in frequencies up to 360 GHz exhibit narrow signals that are not overlapping multiplets, in contrast to [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)].2MeCO(2)H.4H(2)O (1 or Mn(12)-Ac), which also crystallizes in an axial (tetragonal) space group but which now is recognized to consist of a mixture of six hydrogen-bonded isomers in the crystal and thus gives multiple, inhomogeneously broadened EPR signals. Similarly, single-crystal (55)Mn NMR spectra on Mn(12)-BrAc display much sharper signals than a single crystal of Mn(12)-Ac, and this allows one Mn(III) signal to show an almost baseline-resolved quintet from quadrupolar splitting ((55)Mn, I = 5/2, 100%), allowing quadrupole coupling parameters (e(2)qQ) to be determined. In addition, it was found that crushing crystals of Mn(12)-BrAc into a microcrystalline powder causes severe broadening and shifts of the NMR resonances, emphasizing the superiority of single-crystal studies. The combined results establish that Mn(12)-BrAc is far superior to Mn(12)-Ac for the study of the intrinsic properties of the Mn(12) family of SMMs in axial symmetry, and for the search for new phenomena such as quantum interference effects caused by higher-order (>2nd-order) transverse terms in the spin Hamiltonian. 相似文献
33.
M.E. Fermann A. Galvanauskas G. Sucha D. Harter 《Applied physics. B, Lasers and optics》1997,65(2):259-275
Received: 7 March 1997/Revised version: 16 April 1997 相似文献
34.
J. Theuerkauf H. Harter P. von Brentano R. F. Casten 《Zeitschrift für Physik A Hadrons and Nuclei》1987,326(1):65-68
It is shown that the principal features of theN p Nn scheme can be reproduced with an IBA-2 Hamiltonian with constant parameters. 相似文献
35.
The general formula for the number of diagrammatic terms occurring in the Tn equation within a particular coupled cluster model is derived. Both the antisymmetrized and Goldstone diagrams are considered. In addition to the full coupled cluster equation approximate approaches are discussed, and for each the general formula for the number of terms is given. Analogous expressions are presented for the number of diagrammatic terms contributing to the elements of the transformed Hamiltonian [Hbar] = e?T HeT . 相似文献
36.
Chris W. Patterson Harold W. Galbraith Burton J. Krohn William G. Harter 《Journal of Molecular Spectroscopy》1979,77(3):457-473
Clustering of tetrahedral levels is shown to occur in ν2 + ν3 type combination bands of spherical top molecules due to Coriolis and vibrational anharmonic interactions. Cluster correlation diagrams are shown for cases where the vibrational splitting is both large and small compared to the Coriolis splitting, and cluster approximations are given for both cases. 相似文献
37.
Fermann ME Galvanauskas A Stock ML Wong KK Harter D Goldberg L 《Optics letters》1999,24(20):1428-1430
A Raman-shifted and frequency-doubled high-power Er-fiber soliton laser for seeding an efficient high-power Yb fiber femtosecond amplifier is demonstrated. The Raman-shifted and frequency-doubled Er-soliton laser is tunable from 1.00 to 1.070microm and produces bandwidth-limited 24-pJ pulses at a repetition rate of 50 MHz with a FWHM pulse width of 170 fs at 1.040microm . The Yb(3+) amplifier has a slope efficiency of 52% and generates 3-ps linearly chirped pulses with an average power of 0.8 W at 1.05microm . After pulse compression, 74-fs bandwidth-limited pulses with an average power of 0.4 W and a pulse energy of 8 nJ are generated. 相似文献
38.
Marian L. Harter Ira C. Felkner ‡ William W. Mantulin‡ Dyna L. McInturff‡ John N. Marx‡ Pill-Soon Song 《Photochemistry and photobiology》1974,20(5):407-413
Abstract —Potential DNA cross-linking agents, benzo{1,2-b:4,5-b'}dipyran-2,8-dione ( cis -benzodipyrone) and benzo{1,2-b:4,5-b'}dipyran-2,7-dione ( trans -benzodipyrone), have been synthesized. The excited triplet states of these reagents resemble those of other coumaryl derivatives in their spectroscopic assignments and characteristics and thus their reactivites. In particular, trans -benzodipyrone is significantly more reactive toward pyrimidine bases in solution than the other coumaryl derivatives examined, by virtue of its possessing two reactive pyrone C=C bonds within the molecular framework. The unique reactivity of trans -benzodipyrone, with its two reactive pyrone bonds across the long molecular axis, appears to be reflected in its ability to photoinduce mutation in certain Bacillus subtilis strains. This preliminary study provides data that will be useful in designing experiments to elucidate the molecular mechanisms for the biological activities of these compounds. 相似文献
39.
We present the first experimental evidence that the heat capacity of superfluid 4He, at temperatures very close to the lambda point T(lambda), is enhanced by a constant heat flux Q. The heat capacity at constant Q, C(Q), is predicted to diverge at a temperature T(c)(Q)相似文献
40.