2,312 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
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
Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHoErF
We present a study of the model spin-glass LiHoErF using
simultaneous AC susceptibility, magnetization and magnetocaloric effect
measurements along with small angle neutron scattering (SANS) at sub-Kelvin
temperatures. All measured bulk quantities reveal hysteretic behavior when the
field is applied along the crystallographic c axis. Furthermore avalanche-like
relaxation is observed in a static field after ramping from the
zero-field-cooled state up to Oe. SANS measurements are employed to
track the microscopic spin reconfiguration throughout both the hysteresis loop
and the related relaxation. Comparing the SANS data to inhomogeneous mean-field
calculations performed on a box of one million unit cells provides a real-space
picture of the spin configuration. We discover that the avalanche is being
driven by released Zeeman energy, which heats the sample and creates positive
feedback, continuing the avalanche. The combination of SANS and mean-field
simulations reveal that the conventional distribution of cluster sizes is
replaced by one with a depletion of intermediate cluster sizes for much of the
hysteresis loop.Comment: 6 pages, 4 figure
Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice
Quantum spin liquids are materials that feature quantum entangled spin
correlations and avoid magnetic long-range order at T = 0 K. Particularly
interesting are two-dimensional honeycomb spin lattices where a plethora of
exotic quantum spin liquids have been predicted. Here, we experimentally study
an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and
next-nearest neighbor interactions. We demonstrate that YbBr avoids order
down to at least T=100 mK and features a dynamic spin-spin correlation function
with broad continuum scattering typical of quantum spin liquids near a quantum
critical point. The continuum in the spin spectrum is consistent with plaquette
type fluctuations predicted by theory. Our study is the experimental
demonstration that strong quantum fluctuations can exist on the honeycomb
lattice even in the absence of Kitaev-type interactions, and opens a new
perspective on quantum spin liquids.Comment: 32 pages, 7 Figure
Quantum Statistics of Interacting Dimer Spin Systems
The compound TlCuCl3 represents a model system of dimerized quantum spins
with strong interdimer interactions. We investigate the triplet dispersion as a
function of temperature by inelastic neutron scattering experiments on single
crystals. By comparison with a number of theoretical approaches we demonstrate
that the description of Troyer, Tsunetsugu, and Wuertz [Phys. Rev. B 50, 13515
(1994)] provides an appropriate quantum statistical model for dimer spin
systems at finite temperatures, where many-body correlations become
particularly important.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Heavy Quark Production and PDF's Subgroup Report
We present a status report of a variety of projects related to heavy quark
production and parton distributions for the Tevatron Run II.Comment: Latex. 8 pages, 7 eps figures. Contribution to the Physics at Run II
Workshops: QCD and Weak Boson Physic
Spinon localization in the heat transport of the spin-1/2 ladder compound (CHN)CuBr
We present experiments on the magnetic field-dependent thermal transport in
the spin-1/2 ladder system (CHN)CuBr. The thermal
conductivity is only weakly affected by the field-induced
transitions between the gapless Luttinger-liquid state realized for and the gapped states, suggesting the absence of a direct
contribution of the spin excitations to the heat transport. We observe,
however, that the thermal conductivity is strongly suppressed by the magnetic
field deeply within the Luttinger-liquid state. These surprising observations
are discussed in terms of localization of spinons within finite ladder segments
and spinon-phonon umklapp scattering of the predominantly phononic heat
transport.Comment: 4 pages, 3 figure
Excited-state absorption in Er:BaY2F8 and Cs3Er2Br9 and comparison with Er:LiYF4
The influence of Excited-State Absorption (ESA) on the green laser transition and the overlap of Ground-State Absorption (GSA) and ESA for 970 nm upconversion pumping in erbium is investigated in Er3+:BaY2F8 and Cs3Er2Br9. Results are compared to Er3+:LiYF4. In Er3+:BaY2F8, a good overlap between GSA and ESA is found at 969 nm in one polarization direction. The emission cross section at 550 nm is a factor of two smaller than in LiYF4. In Cs3Er2Br9, the smaller Stark splitting of the levels shifts the wavelengths of the green emission and ESA from 4I13/2 off resonance. It enhances, however, ground-state reabsorption. The emission cross section at 550 nm is comparable to LiYF4. Upconversion leads to significant green fluorescence from 2H9/2. A significant population of the 4111/2 level and ESA at 970 nm are not present under 800 nm pumping
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