1. Polythiophenes Containing In-Chain Cobalt Carborane centers: Experimental and computational
explorations of cobalt carborane complexes that are covalently linked to polythiophenes were performed
to investigate their physical properties. Polythiophenes are polymerized thiophenes, a sulfur heterocycle,
that can become conducting with doping of their conjugated -orbitals. The goal of the work here was to
assess the expected improvements to the electrical conductivity of these metallopolymers with the
inclusion of the boron containing clusters that are known to contain delocalized electrons. J. Garno (LSU)
performed atomic force microscopy (AFM) surface studies and conducting probe measurements of charge
transport of these novel systems. Her conducting probe AFM characterizations indicate that polymers
with bithienyl and terthienyl behave like heavily doped semiconductors rather than pure semiconductors,
while the current-voltage (I-V) profile for poly- thienyl exhibits no measurable current consistent with the
insulating character of the film (submitted). Related simulations of these structures by P. Derosa, and N.
Ranjitkar (LaTech) employing Gaussian09[1] determined the spin
state of these systems. By using a variety of density functionals in
their simulations, they concluded that the system is in a spin singlet
state. In addition, they calculated the conductivity of these structures
using Green’s functions on a density functional theory (DFT)
Hamiltonian[2] which compared well to the conducting probe AFM
measurements [3] as shown in Fig. 1.
2. Magnetic and Multiferroic Materials
LaSigma supports several projects investigating multiferroic
materials, materials that exhibit both ferromagnetic and ferroelectric
properties, where there has been intense recent interest. S.
Whittenburg (UNO) has expanded his micromagnetics code to
Figure 1: Current vs. Voltage
include ferroelectric materials through the use of a Landaufor Polythiophenes Containing
Devonshire potential and correctly predicted the ferroelectric phase
an In-Chain
transition temperatures of BaTiO3. In addition, he has extended the
Cobaltabisdicarbollide . a)
code so that it can model the elastic properties of materials so that
DFT/Green functions
stress-induced changes to the morphology can be simulated. Thus,
simualtions. b) experiment[3].
the electric or magnetic field induced stress, and the resulting shape
1T, 2T, 3T=one, two, and
changes, can be calculated and directly compared to the experimental
three thiophene.
results of several multiferroic systems being investigated with
support provided by LaSigma. G. Caruntu (UNO), for example, has developed a novel experimental
methodology for the local measurement of the strain-mediated magneto-elastic coupling in
nanocomposite films. Here he employs an AFM tip to monitor the piezoresponse of a perovskite layer
caused by the magnetostriction of a ferrite layer[4-8]. LaSigma support to L. Malkinski (UNO) has also
assisted the development of new technologies to form multiwall microtubes of magnetic or magnetic and
piezoelectric materials where the magnetic properties have been found to depend on the curvature of the
films. He is also involved in the investigation of thin Fe1-xNix alloy films whose composition varies across
its thickness and displaying unusual hysteresis curves. In addition, he has explored liquid
crystal/ferromagnetic nanoparticles composites where switching characteristics of the liquid crystal
devices were found to depend on the applied magnetic field.
The understanding and control of the magnetic properties of nano- and microscopic materials is important
for a large range of applications from pharmaceuticals to improvements in magnetic storage densities. A.
Burin (Tulane) has used ORCA quantum chemistry software[9]
(BPW91/LanL2DZ level) to model nanoscopic iron oxide clusters finding
a high spin (S=12) ground state (Fig. 2)[10,11]. He plans on extending
these calculations to include relativistic corrections[12] to rule out other,
nearly degenerate, high spin states of this molecular magnet. Other
calculations include the study of DNA base pair radical cations[13,14] and
the modeling of electronic glasses exposed to electric fields[15]. Related
experimental work comes from the team of Kucheryavy, Goloverda, and
Kolesnichenko (Xavier U.) which has produced ultrasmall
superparamagnetic iron oxide nanoparticles in a surfactant-free colloidal
form with sizes ranging from 4 to 8 nm. This was accomplished by
varying the nucleation and growth conditions, and using a sequential
Figure 2: Optimized structure
growth technique. Since the T1 relaxivity for magnetite and its oxidized
of an iron cluster of the spineltype moiety
form, estimated by NMR, was found to be similar, they concluded that
oxidized magnetite would be preferred as a more stable and potentially
less toxic MRI contrast agent.
Several experimental investigations supported by LaSigma have both fundamental exploratory goals and
offer a forum for direct comparison to computations. R. Kurtz and P. Sprunger (LSU) investigated the
magnetic properties of FeAl where DFT calculations[16-23] predict a ferromagnetic ground state at odds
with experiment[24-25]. However, DFT+U methods find a paramagnetic state when U, the correlation
energy, is sufficiently strong[26]. More interestingly, DFT indicates that the bulk terminated and
incommensurate FeAl(110) surfaces may exhibit ferromagnetic ordering of the Fe atoms, with moments
enhanced compared to the bulk[23]. In addition to magnetometry showing that the bulk is paramagnetic,
their synchrotron X-ray magnetic circular dichroism (XMCD) measurements carried out at CAMD
indicates no ferromagnetism at either the commensurate FeAl(110) or the surface reconstructed
incommensurate FeAl2 surface[28-29].
Other examples include the investigation of the magnetism of transition
metal silicides, germanides and gallium compounds to explore their
interesting magnetic and electrical transport properties by J. DiTusa
(LSU). These materials are interesting and important because they are
relatively simply grown, have crystal structures that lack inversion
symmetry, and range from good metals, to magnetic semiconductors and
small band gap insulators. They have explored bulk crystals and crystalline
nanowires demonstrating the accurate control of Co dopants in FeSi at the
0.5% substitution level, the ability to measure the conductance of 20 nm
wide nanowires to temperatures below 300 mK (Fig. 3), and the
Figure 3: Fe1-xCoxSi nanowire
discovery of interesting behavior in the Hall effect of, Fe3Ga4. These
device. Successive
results were presented by students at the 2012 March meeting of the
magnifications of a device
APS and at a poster session at a Gordon Research conference on
designed to measure the
Strongly Correlated Electron Systems.
electrical conductivity of a
single crystalline nanowire.
3. Iron-based superconductors and related materials. Since their discovery in 2008, iron-based
superconductors have generated intense scientific interest because they seem to have an unusual
underlying mechanism and because they may provide a next generation of high-temperature
superconductors. The complex interplay between magnetism and superconductivity in these materials
suggests that the attraction needed to form bound electron pairs could be provided by spin fluctuations
[30-34]. The typical crystal structure is layered tetragonal, with layers of positive iron ions separated by
layers of negative ions. Most of the iron-based superconductors are pnictides, such as BaFe 2 As 2 . An
exception is the binary iron chalcogenide material Fe 1 y (Te 1 x Se x ), with the excess Fe occupying
interstitial sites of the chalcogen layers.
A systematic investigation of transport, magnetic, and superconducting properties of the phase
diagram of the chalcogenide material using resistivity, Hall coefficient, magnetic susceptibility, specific
heat, and neutron scattering was reported in 2010 [35]. Leonard Spinu is completing this picture by
measuring the London penetration depth in single crystals at ultra-low temperatures as a function of
temperature and Se concentration (25% to 45%). The penetration depth of a magnetic field is one of the
most important characteristic parameters in a type II superconductor, because it can give information
about the pairing mechanism. Its zero-temperature value is directly related to the density of
superconductive electrons in the crystal, and its low-temperature behavior can give insight into the pairing
symmetry and pairing energy gaps. The measurements employ a tunnel diode oscillator set up in a
dilution refrigerator that can reach temperatures as low as 40mK. Results were presented at several
conferences [36,37].
The Zhiqiang Mao group has synthesized a new layered iron pnictide CuFeSb [38]. In contrast with
the metallic antiferromagnetic or superconducting states of other iron pnictides and chalcogenides, this
material exhibits a metallic ferromagnetic state with a Curie temperature of 375 K. This finding suggests
that a competition between antiferromagnetic and ferromagnetic coupling may exist in iron-based
superconductors. It also supports theoretical predictions [39,40] that the nature of the magnetic coupling
within the iron plane depends on the height of the anion plane above the iron plane (~1.8 Å for the Sb
plane in CuFeSb vs. ~1.4 in FeAs compounds).
In strongly-correlated materials such as those discussed above, there is typically a close coupling
between structure, charge, and spin, leading to a competition among several phases at low temperature.
Structural changes in the topmost layers that occur at surfaces or crystal-vacuum interfaces are interesting
in themselves and can also drive changes in material functionality. Ward Plummer and Von Braun
Nascimento are measuring surface structure in complex materials via low energy electron diffraction
(LEED) [41], since the electron beam penetrates only into the surface region. But the analysis of LEED
data is an inverse problem: one must search for the surface structure that yields a given diffraction pattern.
The Plummer group has developed novel LEED codes that use global search algorithms [42] and can also
tackle the structural determination of multiple terminated crystallographic surfaces. A LA-SiGMA
supported graduate student, Diogo D. dos Reis, is participating in this work. The codes have been tested
successfully for BaTiO 3 ultra-thin films [42], and will next be applied to multi-phase (001) surfaces of the
BaFe 2 As 2 and Ba(Fe 1 x Co x ) 2 As 2 iron pnictide superconducting materials.
4. Broad-impact computational methodologies. The computation and theoretical prediction of materials
properties must confront the electron-electron interaction, which ties the electrons together into a
correlated whole. Correlated wavefunction methods, including Quantum Monte Carlo (QMC) [43], are
computationally inefficient for systems of many electrons. One way to deal with this problem is to
perform the wavefunction calculations for small systems, to introduce multiscale corrections, and to then
extrapolate calculated properties to larger or infinite systems. The remaining problems are then to do the
wavefunction calculations efficiently for small systems and to find effective multiscale corrections. The
other and more common way is to use Kohn-Sham density functional theory (DFT) [44], an orbital-based
approach in which the electron exchange-correlation energy is provided by a functional of the electron
density that must be approximated in practical calculations. The remaining problems are then to improve
the accuracy of the available approximations, to understand long-range correlations including van der
Waals interactions, and to deal with the fact that even the exact Kohn-Sham band structure can
underestimate the fundamental energy gap of a solid [45].
Mark Jarrell and Juana Moreno, with C.E. Ekuma, Z. Meng, S. Feng, and C. Moore, are developing
multiscale methods for disordered and interacting systems. They have found that graphics processing
units (GPU’s) greatly accelerate materials simulations, including simulations of Ising-model glasses
[46,47] and QMC calculations. They are developing QMC codes tuned [48,49] for the next generation of
Kepler GPU’s. To incorporate nonlocal correlations systematically, they have proposed a Cluster Typical
Medium Theory (CTMT) that opens a new avenue to the study of Anderson localization in both model
and real materials, unlike the coherent phase approximation [50] and its cluster extensions, including the
DCA [51]. The idea is to extend the Typical Medium Theory [52], which replaces average quantities with
typical values, to its cluster version. They have also shown that size extrapolations of calculated
properties over multiple scales can converge much better if three length scales are invoked: the shortest
one for explicit correlation, an intermediate one treated perturbatively, and a longest one treated for the
first time via mean field correlations [53]. Finally, they have used density functional theory to generate a
band structure for (Ga,Mn)As and (Ga,Mn)N, then applied a Wannier-based downfolding method [54] to
get effective interacting Hamiltonians.
The van der Waals interaction is a weak long-range attraction between two objects due to
correlations among their fluctuating multipole moments. It is most important when the objects are not
otherwise strongly bonded, as for two biological molecules or nanostructures. The accurate calculation of
this interaction via many-electron wavefunctions or DFT is feasible only for a pair of atoms or small
molecules. Thus standard intermolecular interactions are often based on an atom pair potential picture.
Jianmin Tao, John P. Perdew, and Adrienn Ruzsinszky have shown [55-57] how to evaluate this
interaction between two quasispherical objects accurately and efficiently, using just the electron densities
and static dipole polarizabilities. They have found that the atom pair potential picture is correct at best for
the interaction between two solid spheres, but not when one or both objects are spherical shells (e.g.,
fullerenes). Other work from the Perdew group [58-62] concerns improvements to semilocal and nonlocal
DFT approximations. The fundamental gaps in the Kohn-Sham band structure are maybe 20-100% too
small compared to experiment. Diola Bagayoko and C.E. Ekuma have shown how to find accurate gaps
and other properties by using a basis set of atom-centered orbitals which is extended only so far as it must
be to predict accurate occupied orbitals [63-69]. This method has been applied to ZnO [65], ScN [66], YN
[66], SrTiO 3 [67], Ge [68], and InP [69].
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