44 research outputs found
Elastic neutron scattering in Quantum Critical Antiferromagnet CrV
We have performed elastic neutron scattering studies of the quantum critical
antiferromagnet CrV. We have found that unlike pure Cr,
which orders at two incommensurate wavevectors, CrV orders
at four incommensurate and one commensurate wavevectors. We have found strong
temperature dependent scattering at the commensurate and incommensurate
wavevectors below 250 K. Results indicate that the primary effect of V doping
on Cr is the modification of the nesting conditions of the Fermi surface and
not the decreasing of the Neel temperature.Comment: 2 pages, 2 figures, submitted to SCES07 (to be published in Physica
B), typos correcte
Structure of the exotic spin-flop states in BaCu2Si2O7
The unusual 2-stage spin flop transition in BaCu2Si2O7 is studied by
single-crystal neutron diffraction. The magnetic structures of the various
spin-flop phases are determined. The results appear to be inconsistent with the
previously proposed theoretical explanation of the 2-stage transition.Comment: 6 pages 5 figure
Critical Susceptibility Exponent Measured from Fe/W(110) Bilayers
The critical phase transition in ferromagnetic ultrathin Fe/W(110) films has
been studied using the magnetic ac susceptibility. A statistically objective,
unconstrained fitting of the susceptibility is used to extract values for the
critical exponent (gamma), the critical temperature Tc, the critical amplitude
(chi_o) and the range of temperature that exhibits power-law behaviour. A
fitting algorithm was used to simultaneously minimize the statistical variance
of a power law fit to individual experimental measurements of chi(T). This
avoids systematic errors and generates objective fitting results. An ensemble
of 25 measurements on many different films are analyzed. Those which permit an
extended fitting range in reduced temperature lower than approximately .00475
give an average value gamma=1.76+-0.01. Bilayer films give a weighted average
value of gamma = 1.75+-0.02. These results are in agreement with the
-dimensional Ising exponent gamma= 7/4. Measurements that do not exhibit
power-law scaling as close to Tc (especially films of thickness 1.75ML) show a
value of gamma higher than the Ising value. Several possibilities are
considered to account for this behaviour.Comment: -Submitted to Phys. Rev. B -Revtex4 Format -6 postscript figure
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Tomonaga–Luttinger liquid behavior and spinon confinement in YbAlO 3
Low dimensional quantum magnets are interesting because of the emerging collective behavior arising from strong quantum fluctuations. The one-dimensional (1D) S = 1/2 Heisenberg antiferromagnet is a paradigmatic example, whose low-energy excitations, known as spinons, carry fractional spin S = 1/2. These fractional modes can be reconfined by the application of a staggered magnetic field. Even though considerable progress has been made in the theoretical understanding of such magnets, experimental realizations of this low-dimensional physics are relatively rare. This is particularly true for rare-earth-based magnets because of the large effective spin anisotropy induced by the combination of strong spin–orbit coupling and crystal field splitting. Here, we demonstrate that the rare-earth perovskite YbAlO3 provides a realization of a quantum spin S = 1/2 chain material exhibiting both quantum critical Tomonaga–Luttinger liquid behavior and spinon confinement–deconfinement transitions in different regions of magnetic field–temperature phase diagram
High Magnetic Field NMR Studies of LiVGeO, a quasi 1-D Spin System
We report Li pulsed NMR measurements in polycrystalline and single
crystal samples of the quasi one-dimensional S=1 antiferromagnet
LiVGeO, whose AF transition temperature is K.
The field () and temperature () ranges covered were 9-44.5 T and
1.7-300 K respectively. The measurements included NMR spectra, the spin-lattice
relaxation rate (), and the spin-phase relaxation rate (),
often as a function of the orientation of the field relative to the crystal
axes. The spectra indicate an AF magnetic structure consistent with that
obtained from neutron diffraction measurements, but with the moments aligned
parallel to the c-axis. The spectra also provide the -dependence of the AF
order parameter and show that the transition is either second order or weakly
first order. Both the spectra and the data show that has at
most a small effect on the alignment of the AF moment. There is no spin-flop
transition up to 44.5 T. These features indicate a very large magnetic
anisotropy energy in LiVGeO with orbital degrees of freedom playing an
important role. Below 8 K, varies substantially with the orientation
of in the plane perpendicular to the c-axis, suggesting a small energy
gap for magnetic fluctuations that is very anisotropic.Comment: submitted to Phys. Rev.
Spin-orbit-driven magnetic structure and excitation in the 5d pyrochlore Cd2Os2O7
Much consideration has been given to the role of spin-orbit coupling (SOC) in 5d oxides,
particularly on the formation of novel electronic states and manifested metal-insulator
transitions (MITs). SOC plays a dominant role in 5d5 iridates (Ir4þ), undergoing MITs both
concurrent (pyrochlores) and separated (perovskites) from the onset of magnetic order.
However, the role of SOC for other 5d configurations is less clear. For example, 5d3
(Os5þ) systems are expected to have an orbital singlet with reduced effective SOC. The
pyrochlore Cd2Os2O7 nonetheless exhibits a MIT entwined with magnetic order phenomenologically
similar to pyrochlore iridates. Here, we resolve the magnetic structure in
Cd2Os2O7 with neutron diffraction and then via resonant inelastic X-ray scattering determine
the salient electronic and magnetic energy scales controlling the MIT. In particular, SOC plays
a subtle role in creating the electronic ground state but drives the magnetic order and
emergence of a multiple spin-flip magnetic excitation
Significant suppression of weak ferromagnetism in (LaEu)CuO
The magnetic structure of (LaEu)CuO has been
studied by magnetization measurements of single crystals, which show
antiferromagnetic long-range order below = 265 K and a structural phase
transition at = 130 K. At , the Cu spin susceptibility
exhibits almost the same behavior as that of LaCuO in the
low-temperature orthorhombic phase, which indicates the existence of finite
spin canting out of the CuO plane. At , the magnitude of the
weak-ferromagnetic moment induced by the spin canting is suppressed
approximately by 70{%}. This significant suppression of the weak-ferromagnetic
moment is carefully compared with the theoretical analysis of weak
ferromagnetism by Stein {\it et al.} (Phys. Rev. B {\bf 53}, 775 (1996)), in
which the magnitude of weak-ferromagnetic moments strongly depend on the
crystallographic symmetry. Based on such comparison, below
(LaEu)CuO is in the low-temperature less-orthorhombic
phase with a space group of . We also discuss the possible magnetic
structure of the pure low-temperature tetragonal phase with space group
, which is relevant for rare-earth and alkaline-earth ions co-doped
LaCuO.Comment: 16 pages including 5 figures, submitted to Phys. Rev. B. Fig. 4 is
newly adde
Cosmology with clusters of galaxies
In this Chapter I review the role that galaxy clusters play as tools to
constrain cosmological parameters. I will concentrate mostly on the application
of the mass function of galaxy clusters, while other methods, such as that
based on the baryon fraction, are covered by other Chapters of the book. Since
most of the cosmological applications of galaxy clusters rely on precise
measurements of their masses, a substantial part of my Lectures concentrates on
the different methods that have been applied so far to weight galaxy clusters.
I provide in Section 2 a short introduction to the basics of cosmic structure
formation. In Section 3 I describe the Press--Schechter (PS) formalism to
derive the cosmological mass function, then discussing extensions of the PS
approach and the most recent calibrations from N--body simulations. In Section
4 I review the methods to build samples of galaxy clusters at different
wavelengths. Section 5 is devoted to the discussion of different methods to
derive cluster masses. In Section 6 I describe the cosmological constraints,
which have been obtained so far by tracing the cluster mass function with a
variety of methods. Finally, I describe in Section 7 the future perspectives
for cosmology with galaxy clusters and the challenges for clusters to keep
playing an important role in the era of precision cosmology.Comment: 49 pages, 19 figures, Lectures for 2005 Guillermo Haro Summer School
on Clusters, to appear in "Lecture notes in Physics" (Springer