323 research outputs found
Controlled enhancement or suppression of exchange biasing using impurity -layers
The effects of inserting impurity -layers of various elements into a
Co/IrMn exchange biased bilayer, at both the interface, and at given points
within the IrMn layer a distance from the interface, has been investigated.
Depending on the chemical species of dopant, and its position, we found that
the exchange biasing can be either strongly enhanced or suppressed. We show
that biasing is enhanced with a dusting of certain magnetic impurities, present
at either at the interface or sufficiently far away from the Co/IrMn interface.
This illustrates that the final spin structure at the Co/IrMn interface is not
only governed by interface structure/roughness but is also mediated by local
exchange or anisotropy variations within the bulk of the IrMn
Systematic study of d-wave superconductivity in the 2D repulsive Hubbard model
The cluster size dependence of superconductivity in the conventional
two-dimensional Hubbard model, commonly believed to describe high-temperature
superconductors, is systematically studied using the Dynamical Cluster
Approximation and Quantum Monte Carlo simulations as cluster solver. Due to the
non-locality of the d-wave superconducting order parameter, the results on
small clusters show large size and geometry effects. In large enough clusters,
the results are independent of the cluster size and display a finite
temperature instability to d-wave superconductivity.Comment: 4 pages, 3 figures; updated with version published in PRL; added
values of Tc obtained from fit
Combined density-functional and dynamical cluster quantum Monte Carlo calculations for three-band Hubbard models for hole-doped cuprate superconductors
Using a combined local density functional theory (LDA-DFT) and quantum Monte
Carlo (QMC) dynamic cluster approximation approach, the parameter dependence of
the superconducting transition temperature Tc of several single-layer
hole-doped cuprate superconductors with experimentally very different Tcmax is
investigated. The parameters of two different three-band Hubbard models are
obtained using the LDA and the downfolding Nth-order muffin-tin orbital
technique with N=0 and 1 respectively. QMC calculations on 4-site clusters show
that the d-wave transition temperature Tc depends sensitively on the
parameters. While the N=1 MTO basis set which reproduces all three
bands leads to a d-wave transition, the N=0 set which merely reproduces the LDA
Fermi surface and velocities does not
Hidden zero-temperature bicritical point in the two-dimensional anisotropic Heisenberg model: Monte Carlo simulations and proper finite-size scaling
By considering the appropriate finite-size effect, we explain the connection
between Monte Carlo simulations of two-dimensional anisotropic Heisenberg
antiferromagnet in a field and the early renormalization group calculation for
the bicritical point in dimensions. We found that the long length
scale physics of the Monte Carlo simulations is indeed captured by the
anisotropic nonlinear model. Our Monte Carlo data and analysis confirm
that the bicritical point in two dimensions is Heisenberg-like and occurs at
T=0, therefore the uncertainty in the phase diagram of this model is removed.Comment: 10 pages, 11 figure
Monte Carlo simulations of , a classical Heisenberg antiferromagnet in two-dimensions with dipolar interaction
We study the phase diagram of a quasi-two dimensional magnetic system with Monte Carlo simulations of a classical Heisenberg spin
Hamiltonian which includes the dipolar interactions between
spins. Our simulations reveal an Ising-like antiferromagnetic phase at low
magnetic fields and an XY phase at high magnetic fields. The boundary between
Ising and XY phases is analyzed with a recently proposed finite size scaling
technique and found to be consistent with a bicritical point at T=0. We discuss
the computational techniques used to handle the weak dipolar interaction and
the difference between our phase diagram and the experimental results.Comment: 13 pages 18 figure
Study of the One- and Two-Band Models for Colossal Magnetoresistive Manganites Using the Truncated Polynomial Expansion Method
Considerable progress has been recently made in the theoretical understanding
of the colossal magnetoresistance (CMR) effect in manganites. The analysis of
simple models with two competing states and a resistor network approximation to
calculate conductances has confirmed that CMR effects can be theoretically
reproduced using non-uniform clustered states. In this paper, the recently
proposed Truncated Polynomial Expansion method (TPEM) for spin-fermion systems
is tested using the double-exchange one-band, with finite Hund coupling , and two-band, with infinite , models. Two dimensional lattices
as large as 4848 are studied, far larger than those that can be handled
with standard exact diagonalization (DIAG) techniques for the fermionic sector.
The clean limit (i.e. without quenched disorder) is here analyzed in detail.
Phase diagrams are obtained, showing first-order transitions separating
ferromagnetic metallic from insulating states. A huge magnetoresistance is
found at low temperatures by including small magnetic fields, in excellent
agreement with experiments. However, at temperatures above the Curie transition
the effect is much smaller confirming that the standard finite-temperature CMR
phenomenon cannot be understood using homogeneous states. By comparing results
between the two methods, TPEM and DIAG, on small lattices, and by analyzing the
systematic behavior with increasing cluster sizes, it is concluded that the
TPEM is accurate to handle realistic manganite models on large systems. Our
results pave the way to a frontal computational attack of the colossal
magnetoresistance phenomenon using double-exchange like models, on large
clusters, and including quenched disorder.Comment: 14 pages, 17 figure
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