87 research outputs found
The necessity of accommodation in size constancy
The necessity of accommodation in size constanc
Site dilution of quantum spins in the honeycomb lattice
We discuss the effect of site dilution on both the magnetization and the
density of states of quantum spins in the honeycomb lattice, described by the
antiferromagnetic Heisenberg spin-S model. For this purpose a real-space
Bogoliubov-Valatin transformation is used. In this work we show that for the
S>1/2 the system can be analyzed in terms of linear spin wave theory. For spin
S=1/2, however, the linear spin wave approximation breaks down. In this case,
we have studied the effect of dilution on the staggered magnetization using the
Stochastic Series Expansion Monte Carlo method. Two main results are to be
stressed from the Monte Carlo method: (i) a better value for the staggered
magnetization of the undiluted system, m=0.2677(6); (ii) a finite value of the
staggered magnetization of the percolating cluster at the classical percolation
threshold, showing that there is no quantum critical transition driven by
dilution in the Heisenberg model. In the solution of the problem using linear
the spin wave method we pay special attention to the presence of zero energy
modes. Using a combination of linear spin wave analysis and the recursion
method we were able to obtain the thermodynamic limit behavior of the density
of states for both the square and the honeycomb lattices. We have used both the
staggered magnetization and the density of states to analyze neutron scattering
experiments and Neel temperature measurements on quasi-two- -dimensional
honeycomb systems. Our results are in quantitative agreement with experimental
results on Mn_pZn_{1-p}PS_3 and on the Ba(Ni_pMg_{1-p})_2V_2O_8.Comment: 21 pages (REVTEX), 16 figure
Orbital occupancies and the putative jeff = 1/2 groundstate in Ba2IrO4: a combined oxygen K edge XAS and RIXS study
The nature of the electronic groundstate of Ba2IrO4 has been addressed using
soft X-ray absorption and inelastic scattering techniques in the vicinity of
the oxygen K edge. From the polarization and angular dependence of XAS we
deduce an approximately equal superposition of xy, yz and zx Ir4+ 5d orbitals.
By combining the measured orbital occupancies, with the value of the spin-orbit
coupling provided by RIXS, we estimate the crystal field splitting associated
with the tetragonal distortion of the IrO6 octahedra to be small, \Delta=50(50)
meV. We thus conclude definitively that Ba2IrO4 is a close realization of a
spin-orbit Mott insulator with a jeff = 1/2 groundstate, thereby overcoming
ambiguities in this assignment associated with the interpretation of X-ray
resonant scattering experiments.Comment: 5 pages, 5 figure
Laser-Induced Skyrmion Writing and Erasing in an Ultrafast Cryo-Lorentz Transmission Electron Microscopy
We demonstrate that light-induced heat pulses of different duration and
energy can write skyrmions in a broad range of temperatures and magnetic field
in FeGe. Using a combination of camera-rate and pump-probe cryo-Lorentz
Transmission Electron Microscopy, we directly resolve the spatio-temporal
evolution of the magnetization ensuing optical excitation. The skyrmion lattice
was found to maintain its structural properties during the laser-induced
demagnetization, and its recovery to the initial state happened in the
sub-{\mu}s to {\mu}s range, depending on the cooling rate of the system
Spin gap evolution upon Ca doping in the spin ladder series by inelastic neutron scattering
The spin gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically
investigated using inelastic neutron scattering. We discover that the
singlet-triplet spin gap excitation survives in this series with x up to 13,
indicating the singlet dimer ground state in these compounds. This observation
corrects the previous speculation that the spin gap collapses at x~13 by the
NMR technique. The strong intensity modulation along QH in x=0 gradually
evolves into a Q-independent feature in x>11. This could be attributed to the
localized Cu moment magnetism developing into an itinerant magnetism with
increasing x. It is a surprise that the spin gap persists in the normal state
of this spin ladder system with metallic behaviour, which evidences the
possibility of magnetically-mediated carrier pairing mechanism in a two-leg
spin ladder lattice.Comment: 17 pages, 6 figure
Magnetic-field-induced spin excitations and renormalized spin gap of the underdoped superconductor LaSrCuO
High-resolution neutron inelastic scattering experiments in applied magnetic
fields have been performed on LaSrCuO (LSCO). In zero
field, the temperature dependence of the low-energy peak intensity at the
incommensurate momentum-transfer $\mathbf{Q}^{\
}_{\mathrm{IC}}=(0.5,0.5\pm\delta,0),(0.5\pm\delta,0.5,0)T^{\}_{c}$ which broadens and shifts to lower
temperature upon the application of a magnetic field along the c-axis. A
field-induced enhancement of the spectral weight is observed, but only at
finite energy transfers and in an intermediate temperature range. These
observations establish the opening of a strongly downward renormalized spin gap
in the underdoped regime of LSCO. This behavior contrasts with the observed
doping dependence of most electronic energy features.Comment: accepted for publication in Phys. Rev. Let
Pressure induced evolution of superconductivity and magnetic hourglass dispersion in Fe1.02Te0.7Se0.3
Iron based high temperature superconductors have several common features with superconducting cuprates, including the square lattice and the proximity to an antiferromagnetic phase. The magnetic
excitation spectrumbelowTc of Fe1.02Te0.7Se0.3 shows an hourglass-shaped dispersion with a resonance around the commensurate point . In a previous inelastic neutron scattering study, we showed that the hourglass-shaped dispersion is most likely a prerequisite for superconductivity, while
the consequences are the opening of a gap and a shift of spectral weight. In this paper we follow the evolution of the hourglass shaped dispersion under applied pressure up to 12 kbar. Our results show
that that the pressure-induced 37% increase of Tc is concomitant with a change in the magnetic excitation spectrum, with an increase of the hourglass energy by 38%
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