608 research outputs found
Monte Carlo Study of the S=1/2 and S=1 Heisenberg Antiferromagnet on a Spatially Anisotropic Square Lattice
We present a quantum Monte Carlo study of a Heisenberg antiferromagnet on a
spatially anisotropic square lattice, where the coupling strength in the
x-direction () is different from that in the y-direction (). By
varying the anisotropy from 0 to 1, we interpolate between the
one-dimensional chain and the two-dimensional isotropic square lattice. Both
and S=1 systems are considered separately in order to facilitate
comparison. The temperature dependence of the uniform susceptibility and the
spin-spin correlation length are computed down to very low temperatures for
various values of . For S=1, the existence of a quantum critical point
at as well as the scaling of the spin gap is
confirmed. Universal quantities predicted from the nonlinear
model agree with our results at without any adjustable
parameters. On the other hand, the results are consistent with
, as discussed by a number of previous theoretical studies.
Experimental implications for compounds such as SrCuO are also
discussed.Comment: 8 pages, 7 figures, to be published in Phys. Rev.
Monte Carlo Study of Correlations in Quantum Spin Chains at Non-Zero Temperature
Antiferromagnetic Heisenberg spin chains with various spin values
() are studied numerically with the quantum Monte Carlo
method. Effective spin chains are realized by ferromagnetically coupling
antiferromagnetic spin chains with . The temperature dependence
of the uniform susceptibility, the staggered susceptibility, and the static
structure factor peak intensity are computed down to very low temperatures,
. The correlation length at each temperature is deduced from
numerical measurements of the instantaneous spin-spin correlation function. At
high temperatures, very good agreement with exact results for the classical
spin chain is obtained independent of the value of . For =2 chains which
have a gap , the correlation length and the uniform susceptibility in
the temperature range are well predicted by a semi-classical
theory due to Damle and Sachdev.Comment: LaTeX EPJ macr
First and second order magnetic and structural transitions in BaFeCoAs
We present here high resolution magnetization measurements on high-quality
BaFeCoAs, 0x0.046 as-grown single crystals.
The results confirm the existence of a magnetic tricritical point in the
(,) plane at x0.022 and reveal the emergence of the
heat capacity anomaly associated with the onset of the structural transition at
x0.0064. We show that the samples with doping near x
do not show superconductivity, but rather superconductivity emerges at a
slightly higher cobalt doping, x0.0315Comment: 4 pages, 5 figure
<i>d</i>-wave superconductivity from electron-phonon interactions
I examine electron-phonon mediated superconductivity in the intermediate coupling and phonon frequency regime of the quasi-two-dimensional Holstein model. I use an extended Migdal-Eliashberg theory that includes vertex corrections and spatial fluctuations. I find a d-wave superconducting state that is unique close to half filling. The order parameter undergoes a transition to s-wave superconductivity on increasing filling. I explain how the inclusion of both vertex corrections and spatial fluctuations is essential for the prediction of a d-wave order parameter. I then discuss the effects of a large Coulomb pseudopotential on the superconductivity (such as is found in contemporary superconducting materials like the cuprates), which results in the destruction of the s-wave states, while leaving the d-wave states unmodified
Specific heat of BaKFeAs, and a new method for identifying the electron contribution: two electron bands with different energy gaps in the superconducting state
We report measurements of the specific heat of
BaKFeAs, an Fe-pnictide superconductor with
= 36.9 K, for which there are suggestions of an unusual electron pairing
mechanism. We use a new method of analysis of the data to derive the parameters
characteristic of the electron contribution. It is based on comparisons of
-model expressions for the electron contribution with the total
measured specific heat, which give the electron contribution directly. It
obviates the need in the conventional analyses for an independent, necessarily
approximate, determination of the lattice contribution, which is subtracted
from the total specific heat to obtain the electron contribution. It eliminates
the uncertainties and errors in the electron contribution that follow from the
approximations in the determination of the lattice contribution. Our values of
the parameters characteristic of the electron contribution differ significantly
from those obtained in conventional analyses of specific-heat data for five
similar hole-doped BaFeAs superconductors, which also differ
significantly among themselves. They show that the electron density of states
is comprised of contributions from two electron bands with
superconducting-state energy gaps that differ by a factor 3.8, with 77
coming from the band with the larger gap. The variation of the specific heat
with magnetic field is consistent with extended -wave pairing, one of the
theoretical predictions. The relation between the densities of states and the
energy gaps in the two bands is not consistent with a theoretical model based
on interband interactions alone. Comparison of the normal-state density of
states with band-structure calculations shows an extraordinarily large
effective mass enhancement, for which there is no precedent in similar
materials and no theoretical explanation.Comment: 30 pages, 7 figures, submitte
Neutron Scattering and magnetization studies of BaCuCoOCl: A decorated two-dimensional antiferromagnet
BaCuOCl has two inter-penetrating square Cu sublattices, one
with square root 2 times the in-plane spacing of the other. Isotropic magnetic
interactions between the two sublattices are completely frustrated. Quantum
fluctuations resolve the intrinsic degeneracy in the ordering direction of the
more weakly coupled sublattice in favor of collinear ordering. We present
neutron scattering and magnetization studies of the magnetic structure when the
Cu ions are substituted with Co. The Co spins create new magnetic interactions
between the two sublattices. The ordering behavior of both Cu sublattices is
retained largely unmodified. Between the phase transitions of the two
sublattices spin-glass behavior is observed. Magnetization results show a
strong enhancement to the ferromagnetic aspect of the magnetic structure. The
combination of glassy behavior and large moments strongly suggest that the Co
moments induce the formation of local canted states.Comment: 4 figure
Strain-activated structural anisotropy in BaFe2As2
High-resolution single crystal neutron diffraction measurements are presented
probing the magnetostructural response to uniaxial pressure in the iron
pnictide parent system BaFe2As2. Scattering data reveal a strain-activated,
anisotropic broadening of nuclear Bragg reflections, which increases upon
cooling below the resolvable onset of global orthorhombicity. This anisotropy
in lattice coherence continues to diverge until a lower temperature scale---the
first-order onset of antiferromagnetism---is reached. Our data suggest that
antiferromagnetism and strong magnetoelastic coupling drive the
strain-activated lattice response in this material and that the development of
anisotropic lattice coherence under strain is the physical origin for the
anomalous nematic anisotropy in this compound.Comment: 5 pages, 4 figure
Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of KFeSe
The recently discovered K-Fe-Se high temperature superconductor has caused
heated debate regarding the nature of its parent compound. Transport,
angle-resolved photoemission spectroscopy, and STM measurements have suggested
that its parent compound could be insulating, semiconducting or even metallic
[M. H. Fang, H.-D. Wang, C.-H. Dong, Z.-J. Li, C.-M. Feng, J. Chen, and H. Q.
Yuan, Europhys. Lett. 94, 27009 (2011); F. Chen et al. Phys. Rev. X 1, 021020
(2011); and W. Li et al.,Phys. Rev. Lett. 109, 057003 (2012)]. Because the
magnetic ground states associated with these different phases have not yet been
identified and the relationship between magnetism and superconductivity is not
fully understood, the real parent compound of this system remains elusive.
Here, we report neutron-diffraction experiments that reveal a semiconducting
antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic
order of the semiconducting phase is the same as the stripe AFM order of the
iron pnictide parent compounds. Moreover, while the root5*root5 block AFM phase
coexists with superconductivity, the stripe AFM order is suppressed by it. This
leads us to conjecture that the new semiconducting magnetic ordered phase is
the true parent phase of this superconductor.Comment: 1 table, 4 figures,5 page
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