3,556 research outputs found
Defect propagation in one-, two-, and three-dimensional compounds doped by magnetic atoms
Inelastic neutron scattering experiments were performed to study
manganese(II) dimer excitations in the diluted one-, two-, and
three-dimensional compounds CsMn(x)Mg(1-x)Br(3), K(2)Mn(x)Zn(1-x)F(4), and
KMn(x)Zn(1-x)F(3) (x<0.10), respectively. The transitions from the ground-state
singlet to the excited triplet, split into a doublet and a singlet due to the
single-ion anisotropy, exhibit remarkable fine structures. These unusual
features are attributed to local structural inhomogeneities induced by the
dopant Mn atoms which act like lattice defects. Statistical models support the
theoretically predicted decay of atomic displacements according to 1/r**2, 1/r,
and constant (for three-, two-, and one-dimensional compounds, respectively)
where r denotes the distance of the displaced atoms from the defect. The
observed fine structures allow a direct determination of the local exchange
interactions J, and the local intradimer distances R can be derived through the
linear law dJ/dR.Comment: 22 pages, 5 figures, 2 table
Hysteresis effects in rotating Bose-Einstein condensates
We study the formation of vortices in a dilute Bose-Einstein condensate
confined in a rotating anisotropic trap. We find that the number of vortices
and angular momentum attained by the condensate depends upon the rotation
history of the trap and on the number of vortices present in the condensate
initially. A simplified model based on hydrodynamic equations is developed, and
used to explain this effect in terms of a shift in the resonance frequency of
the quadrupole mode of the condensate in the presence of a vortex lattice.
Differences between the spin-up and spin-down response of the condensate are
found, demonstrating hysteresis phenomena in this system.Comment: 16 pages, 7 figures; revised after referees' report
Dimensional reduction by pressure in the magnetic framework material CuF(DO)pyz: from spin-wave to spinon excitations
Metal organic magnets have enormous potential to host a variety of electronic
and magnetic phases that originate from a strong interplay between the spin,
orbital and lattice degrees of freedom. We control this interplay in the
quantum magnet CuF(DO)pyz by using high pressure to drive the
system through a structural and magnetic phase transition. Using neutron
scattering, we show that the low pressure state, which hosts a two-dimensional
square lattice with spin-wave excitations and a dominant exchange coupling of
0.89 meV, transforms at high pressure into a one-dimensional spin-chain
hallmarked by a spinon continuum and a reduced exchange interaction of 0.43
meV. This direct microscopic observation of a magnetic dimensional crossover as
a function of pressure opens up new possibilities for studying the evolution of
fractionalised excitations in low dimensional quantum magnets and eventually
pressure-controlled metal--insulator transitions
The role of Yb2+ as a scintillation sensitiser in the near-infrared scintillator CsBa2I5:Sm2+
The feasiblity of using Yb2+ as a scintillation sensitiser for CsBa2I5:Sm2+ near-infrared scintillators has been assessed. CsBa2I5 samples with concentrations ranging from 0.3% to 2% Yb2+ and 0–1% Sm2+ have been studied. The scintillation properties have been determined and the dynamics of the scintillation mechanism have been studied through photoluminescence measurements. Radiationless energy transfer between Yb2+ ions plays a key role in increasing the ratio between the spinforbidden and spin-allowed emission with increasing Yb2+ concentration in samples where Yb2+ is the only dopant. In samples co-doped with Sm2+, the Yb2+ 4f13[2F7/2]5d1[LS] and 4f13[2F7/2]5d1[HS] states both serve as donor states for radiationless energy transfer to Sm2+ with a rate of energy transfer that is inversely proportional to the luminescence lifetime the respective donor states. At a Sm2+ concentration of 1%, 85% of the Yb2+ excitations are transferred to Sm2+ through radiationless energy transfer. Almost all of the remaining Yb2+ emission is reabsorbed by Sm2+, resulting in nearly complete energy transfer
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