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RESEARCH HIGHLIGHTS
Giant Magneto-Elastic Coupling in Iron-Pnictides:
In this study, we showed that Fe-spin state in iron-pnictides has an unprecedented control
on the structural and other physical properties of these systems.
We discovered surprisingly strong interactions between arsenic ions,
the strength of which is controlled by the Fe-spin state in an unprecedented way.
Reducing the Fe-magnetic moment, weakens the Fe-As bonding, and in turn,
increases both intra-and inter-plane As-As interactions, causing giant reduction in the c-axis.
For CaFe2As2 system, this reduction of c-axis with the loss of the Fe-moment is as large as 1.4 Å,
an unheard of giant coupling of local spin-state of an ion to its lattice.
Since the calculated large c-reduction has been recently observed only under high-pressure,
we suggest that the iron magnetic moment should be present in Fe-Pnictides at all times at ambient pressure.
The giant coupling of the on-site Fe-magnetic moment with the As-As bonding
that we have discovered in this study
may provide a mechanism for the superconductivity.
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Contour plot of the As-As Pz-hybridization orbital for
cT-phase and T-phase, respectively.
Note that the As-As hybridization present in both phases is
much more significant in the cT-phase. There
is also clear hybridization between intra-As atoms
below and above the Fe-plane in the T-phase (right).
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Further Readings:
Competing Magnetic Interactions,Frustration,
and Structural Phase Transition in Iron-Pnictide Superconductors
The recent discovery of superconductivity at TC's up to 55 K in layered rare-earth (RE) iron-pnictide quaternary compounds
REOFeAs (R=La, Ce, Sm, etc) has sparked enormous interest in this class of materials.
These are the first non-copper based materials that exhibit superconductivity at relatively high
temperatures upon electron (O1-xF) and hole doping (La1-xSrx)
of their non-superconducting parent compounds, just like high-TC cuprates. Clearly,
the understanding of electronic, magnetic, and structural properties of the parent compound
LaOFeAs is the key to determining the underlying mechanism that makes these materials
superconduct upon electron/hole doping.
We show that there are strong competing antiferromagnetic interactions in FeAs plane,
suggesting magnetism and superconductivity in doped LaOFeAs may be strongly coupled,
much like in the high-Tc cuprates[T. Yildirim, Phys. Rev. Lett. 101, 057010 (2008)].
Accurate all-electron fixed-spin-moment calculations clearly indicated that the ferromagnetic
and checkerboard antiferromagnetic ordering in LaOFeAs were not stable and the stripe
antiferromagnetic Fe-spin configuration (i.e. SDW) was the only stable ground state.
We further showed that the main exchange interaction between Fe ions are large,
antiferromagnetic, and frustrated. We find that only the magnetic stripe SDW phase
breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion.
We even predicted several fine-details of the magnetic structure such as the ferromagnetically
alinged spin direction in SDW state is along the short axis.
This prediction has been now confirmed in several 122 systems (i.e. SrFe2As2)
from single crystal neutron data.
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The total energy per cell versus the angle gamma for NM, F, AF1 and AF2 spin-configurations.
Note that only the AF2 spin configuration yields structural distortion.
The inset shows that as gamma increases, the ferromagnetic aligned Fe
ions (i.e., Fe1-Fe2) get closer while the antiferomagnetically aligned ions (i.e., Fe1-Fe3) move apart.
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Further Readings:
Open metal sites found to play an important role for CH4
storage in MOFs
We found that MOF compounds M 2(dhtp) (also known as MOF-74
analogues) possess exceptionally large densities of open
metal sites. By adsorbing one CH 4 molecule per
open metal, these sites alone can generate very large
methane storage capacities, approaching the DOE target for
material-based methane storage at room temperature. Our adsorption isotherm
measurements at 298 K and 35 bar for the M 2(dhtp)
compounds yield excess methane adsorption capacities roughly
equal to the predicted, maximal adsorption capacities of the
open metal sites. Our neutron diffraction experiments
clearly reveal that the primary CH 4 adsorption
occurs right on the open metals. DFT calculations show that
the binding energies of CH 4 on the open metal
sites are significantly higher than those on typical
adsorption sites in classical MOFs.
Further Readings:
H2 binding strength found to depend strongly on open metal
species in MOFs
MOFs with open metal sites exhibit much stronger
H 2 binding strength than classical MOFs.
Yet, how the binding strength varies
with different open metal species was previously unknown. We conducted a
systematic study of the H 2 adsorption on a series of
isostructural MOFs, M 2(dhtp) (M=Mg, Mn, Co, Ni, Zn). The
experimental Qst for H 2 of these MOFs
range from 8.5 to 12.9 KJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni. The H 2 binding energies derived from DFT calculations follow the same
trend. We also found a strong correlation between the metal ion
radius, the M-H 2 distance and the H 2 binding
strength, which provides a viable, empirical method to predict the
relative H 2 binding strength of different open metals.
Further Readings:
Metal-Organic Molecule Complexes as a high-capacity hydrogen storage medium
 In our recent studies
[ Phys. Rev. B 76, 085434 (2007),
Phys. Rev. Lett. 97, 226102 (2006)],
we suggest that co-deposition of metals (i.e. Ti/Li) with small organic molecules such
as very cheap ethylene molecule (a ho-hum material that is the building block of the
most common plastic) into nanopores of low-density high surface materials could be
a very promising direction for discovering new materials with better storage properties.
We found to our surprise that the interaction of Ti with the C=C double bond of
ethylene molecule (i.e. C2H4 ) mimics what we found in C60.
Detailed first-principles calculations show that the complex resulting
from attaching a Ti atom to each ends of C2H4 (see figure) will reversibly
adsorb ten H2 molecules. The equivalent material gravimetric capacity of 14%,
if realized in practice, would readily exceed the 2015 DOE system goal.
Another advantage is that, unlike our previously predicted structures
involving fullerenes and nanotubes, the metal-ethylene complexes
have actually been synthesized and actively studied as catalytic
systems for several decades. Their potential for hydrogen storage
was first revealed by our theory and modeling work.
Extending this study to include a large number of other metals and different isomeric structures
yielded interesting results for light metals such as Li. The ethylene molecule is able to
complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to
two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %,
a record high value reported so far. The stability of the proposed metal-ethylene complexes
was tested by extensive calculations such as normal-mode analysis, finite temperature
first-principles molecular dynamics (MD) simulations, and reaction path calculations.
The phonon and MD simulations indicate that the proposed structures are stable up to 500 K.
The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene
complex to transform into a possible lower energy configuration where the ethylene molecule
is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex
possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable
binding energy for room temperature storage. These results suggest that co-deposition
of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host
materials may be a very promising route to discovering new materials with high-capacity
hydrogen absorption properties.
Interestingly, an independent group at University of
Virginia have recently confirmed our predictions by demonstrating about 14 wt% H2 absorption
on a thin-film of Ti-C2H4 complex at room temperature (see Phys. Rev. Lett. 100, 105505 (2008)).
Further Readings:
Press Coverage:
Packing Hydrogen into Three Dimensional Networks of Nano-Clusters
 The success of future hydrogen and fuel-cell technologies is critically dependent upon
the discovery of new materials that can store large amount of hydrogen at ambient conditions.
Metal-organic framework (MOF) compounds, which consist of metal-oxide clusters connected by
organic linkers, are a relatively new class of nano-porous material that show promise for
hydrogen storage applications because of their tunable pore size and functionality.
In this letter (PRL Nov.23 issue 2005), using difference-Fourier analysis of neutron powder
diffraction data along with first-principles total-energy calculations, for the first time,
we directly determine the available adsorption sites in MOF5. Surprisingly, the MOF5 host
lattice has enough space available to hold many hydrogen molecules, up to ~10 wt-% at low temperatures.
We determined the nature of the MOF-hydrogen interaction and the manner in which hydrogen
molecules are adsorbed onto the structure. These results hold the key to
engineering MOF materials for practical hydrogen storage applications. Equally important,
we find that at high-concentration loadings hydrogen molecules form unique 3 dimensional
networks of H2 nano-clusters with short intermolecular distances. These findings suggest
that MOF materials can also be used as templates to create artificial, interlinked hydrogen nano-cages.
Such materials could exhibit very unexpected properties due to quantum confinement
effects and the small intermolecular distances, such as metallic behavior.
Further Readings:
Ti-decorated Nanostructures as a Potential High Capacity Hydrogen Storage Medium
There have been a great number of reports on the search of new routes to engineer nanomaterials
so that (1) they dissociate H2 molecules into H atoms and (2) reversibly adsorb hydrogen
molecules at ambient conditions. Much effort has been focused on the engineering of
carbon-based materials such as nanotubes and transition metal hydrides without success.
It is found that while hydrogen-carbon interaction is too weak, the metal-hdyrogen interaction
is too strong for room temperature reversible storage. Here we show a novel way to overcome this
difficulty by forming artificial metal-carbides on nanotubes/C60 molecules!
From accurate first-principles calculations, we show that a single Ti-atom coated on a
SWNT/C60 can strongly adsorb up to four hydrogen molecules.
The hydrogen-metal bonding is an unusual combination of chemi- and physi-sorption,
an essential ingredient needed for reversible hydrogen storage medium near room temperature.
Remarkably, this adsorption occurs with no energy barrier.
At large Ti coverage we show that a (8,0) SWNT and C60 can store hydrogen molecules
up to 8-wt%, exceeding the minimum requirement of 6-wt% for practical applications. Finally,
we present high temperature quantum molecular dynamics simulations showing that these systems
are stable and indeed exhibit associative desorption of H2 upon heating, another
requirement for reversible storage.These results are quite remarkable, unanticipated, and
to the best of our knowledge, the first-demonstration of hydrogen-metal complex formation
containing FOUR H2 molecules.
SEE FULL STORY..
Magnetically Driven Ferroelectric Order in Ni3V2O8
In recent years there has been marked interest in investigating systems
exhibiting simultaneous long range magnetic and ferroelectric orders.
The magnetoelectric effects in these multiferroic materials offer insight
into the role of spin-charge-lattice couplings in producing complex long-range orders.
Developing insulating materials in which charge and spin are strongly coupled
may allow the fabrication of new devices incorporating the best of both worlds
such as voltage-driven fast switching magnetic memory. However, to date,
there has been little understanding of how this multiferroic behavior arises.
In this work (PRL 95, 87205 and PRL 93 247201), we discuss our measurements
on Ni3V2O8 and present a model which fully accounts for the observed multiferroic
behavior of this material. Our investigations of Ni3V2O8, a compound with both
charge and spin ordering, are used to develop a model explaining in detail
how this spin-charge coupling arises. Understanding this coupling is crucial
for developing the materials which will form the basis for future electronic devices.
For the first time we presents the discussion of
how a ferroelectric phase transition can be driven solely by magnetic order.
By carefully investigating the magnetic and ferroelectric
properties of Ni3V2O8, we have identified a mechanism through which
two distinct magnetic order parameters can combine to break the spatial
inversion symmetry, which promotes ferroelectric ordering.
We believe that Ni3V2O8 exhibits an experimental realization
of this effect, and are successfully able to explain its novel
magnetoelectric couplings. In particular, using solely the
symmetries of the magnetically ordered phases, we are able to
understand how ferroelectric order in the system can be
reversibly switched on and off by applying a magnetic field.
The general model we propose is expected to be relevant to
understanding a wide range of magnetoelectric multiferroics,
and more generally, how complex order parameters can be combined to
break symmetries which in turn allows new phases to arise.
Further Readings:
Hidden Symmetries and Their Consequences in the t2g Cubic Perovskites
In these studies (Phys. Rev. Lett, 91, 087206 (2003) and Phys. Rev. B 69, 035107 (2004))
we report crucial new results concerning the possible ordered phases of transition metal perovskites,
and their possible theoretical origins. The transition metal oxides are very important materials,
because they have been a rich source of novel and intriguing physical phenomena such as high
Tc superconductivity, colossal magnetoresistance (CMR), and most recently,
the fascinating combined spin-orbital ordering phenomena. Much of the theoretical
literature on these systems has used the standard Hubbard model, or its low temperature
version derived by Kugel and Khomskii (KK). In this work, for the first time,
we show that for the t2g orthogonal perovskites, these models possess several hidden
novel symmetries, upon which rigorous conclusions can be based. In particular,
we use these symmetries to prove that – contrary to the general belief in the
literature - these basic models forbid any long range spin order at nonzero temperatures.
Inclusion of spin-orbit coupling allows such ordering, but even then the excitation
spectrum is gapless due to a continuous symmetry. Experimental systems, such as LaTiO3,
do exhibit long-range order and an excitation spectrum with a gap. The important point
of our work is that we show that a consistent theoretical explanation of such experimental
facts must include additional suitable perturbations to the Hubbard or to the KK Hamiltonian.
We also show that the uncovered symmetries of the Hubbard model are very useful in
exact numerical studies of finite clusters. For example, finding the ground
state of a cube of eight Ti ions requires the diagonalization of a matrix having
about ½ million rows and columns. However, using the symmetries discovered
in this Letter, this problem is reduced to a 16x16 dimensional matrix diagonalization!
It is surprising that, although the crystal field theory of cubic perovskites and
the corresponding Hubbard model have been the subjects of many studies and even textbooks,
the novel hidden symmetries uncovered in this work were still missed.
Hence, we expect that this work will make a big impact in the physics
community and that these results will inspire experimentalists to
synthesize new t2g transition metal oxides with tetragonal or higher
symmetry. Such systems would have quite striking and anomalous properties.
Theory of Superconducivity in MgB2
The surprising discovery of superconductivity in magnesium diboride (MgB2)
at 39 K raised the question of whether the origin of the high Tc is due to
electron-phonon coupling as is the case for “conventional” superconductors
or to a more exotic mechanism.
By combining first-principles calculations and inelastic neutron scattering measurements
of the phonons in MgB2, we offered the first direct answer to this fundamental question.
We demonstrated that the in-plane boron phonons (with E2g symmetry) near the zone-center are very anharmonic.
Moreover our calculations revealed that these modes strongly couple
to the partially occupied planar B s electronic bands near the Fermi level.
We showed that this giant anharmonicity and non-linear electron-phonon
coupling is the key to quantitatively explaining not only the observed high
Tc and boron isotope effect in MgB2 but also the pressure dependence of the Tc.
Further details of this research can be found at Dr. Yildirim's
MgB2 Website:
Our work (Phys. Rev. Lett. 87, 37001 (2001)) has revived a great deal of interest
within the physics community as evidenced by recent articles about our work:
Physical Review Focus News, July 2001 (http://focus.aps.org/story/v8/st2 )
Materials Today, Research News, p. 10, September/October 2001
NIST News Brief, p. 612, Vol. 106, No. 3, May-June 2001
TODAY’s CHEMIST at WORK, p. 15, September 2003 ( www.tcawonine.org )
Interlinking, Band Gap Engineering, Tunable Adsorption and Functionalization
of Carbon Nanotubes
 Carbon nanotubes, originally discovered as a by-product of fullerenes synthesis,
are one of the most promising nanomaterials. We have used first principles calculations
to elucidate details of the electronic and chemical properties of these systems.
Our first work in this area resulted in the prediction that by applying pressure,
carbon nanotubes can be covalently joined to form one and two-dimensional networks
of interlinked nanotubes (Phys. Rev. B 62, 12648 (2000)). Several months after
our prediction was published, the results were independently confirmed
experimentally (see Phys. Rev. Lett. 86, 3056 and Phys. Rev. B 64,153401).
The second major accomplishment in nanotube research was the discovery that the
band gap of an insulating nanotube can be engineered by elliptical distortion
(Phys. Rev. B 62, R16345 (2000)). This work has significant implications for
designing electronic nanodevices based on nanotubes. Finally, we have very
recently shown (see Phys. Rev. Lett. 87, 116802 (2001)) that the chemical
reactivity of nanotubes can be tuned by elliptical deformation, which may
provide a way to attach various atoms such as metals to a specific location on a nanotube.
In summary, our studies suggested promising new avenues for nanotube research.
We showed how changing the shape of tiny single-walled tubes of carbon may open
a potential mother lode of technologically useful properties, such as engineering
nanotubes to be insulating or semiconducting in a controlled manner,
reversible metal-insulator junctions, and tunable absorption.
Our investigations are pointing out productive paths for other researchers
to follow in experiments that pursue opportunities to make new materials and technologies with nanotubes.
Our nanotube reserach resulted about sixteen separate
publications which had significant impact in nanotube field.
Further details of this research can be found at
Dr. Yildirim's Nanotube Website.
Our work has revived a great deal of interest
within the physics community as evidenced by following media coverages:
Materials Today, Research News, p. 10, December 2002
Materials Research Society (MRS) Bulletin, p. 944, December 2002.
NIST News Release, Sept. 20 2002
Nano Today, Research News, p. 6, December 2003.
Solid Cubane
The cubane molecule (C8H8) is aesthetically appealing due to the highly symmetric cubic shape.
Moreover it is the most highly strained, kinetically stable ring system available in quantity,
and therefore offers promise in many diverse and interesting practical applications as
explosives, high-refractive index lenses, specialty polymers, and fuel additives.
While cubane was first synthesized and the room temperature crystal structure
was determined in the early 1960's, the high temperature, orientationally
disordered structure remained unknown due to experimental difficulties.
In 1997, we decided to determine the phase diagram of this interesting compound theoretically.
We were successfully determine the complicated phase diagram and discovered the orientatioally
disorded phase for the first time. We also confirmed our calculations by
X-ray and neutron scattering measurements (see Phys. Rev. Lett. 78, 4938, 1997).
The calculated phase diagram and image of cubane molecule is shown in the figure.
Further Readings:
This work received enormous interest as evidenced by following media coverage of our work:
Press Coverage:
- The New York Times, July 15,1997.
- Science News 52, p. 34, 1997.
- Physics Today News Updates, p. 9 August 1997.
- Physics News Update, Number 326, June 18, 1997.
- NIST Tech Beat, September 1997.
The Synthesis and Characterization of NaxC60
The discovery of a simple method of making large quantities of fullerenes
initiated a great deal of research into the properties of fullerenes and
compounds based on fullerenes. Several phases of C60 with alkali metals
such as K, Rb, and Cs were reported. However, due to reactivity of smaller
alkali metals and several þechnical difficulties, the phase diagram of
C60 with Na and Li were not known. In 1992 we developed a new synthesis
technique using copper tubing and alkali-metal azide to remedy these
difficulties and reported the first solid-state synthesis and x-ray
characterization of the saturated phase of the Na-C60 system ( T. Yildirim et al., Nature 350, 568 (1992)).
This work suggested that by using smaller alkali metals (i.e., Li),
it would be possible to intercalate even more atoms into the host lattice,
a finding of great interest for Li batteries.
The Role of Carrier Concentration in Superconductivity of C60-based Compounds.
 The discovery of superconductivity in A3C60 compounds where A is K, Rb, or
Cs sparked a great deal of interest in the condensed matter physics community.
It was quickly realized that the superconducting transition temperature depends
strongly on the lattice constant which could be easily controlled by changing
the alkali metal or by applying external pressure. This dependence
was attributed to the changing density of electronic states at the Fermi level.
To explore this idea further, one would like to adjust the carrier
concentration in these systems. Unfortunately this is fixed at three
electrons per C60 molecule due the narrow solubility range in the phase
diagram of these compounds. We overcame this problem by synthesizing
two novel families of fullerides, Na2CsxC60 and M3-xBax C60, thereby
allowing the first studies of the correlation between the superconducting
transition temperature and the carrier concentration.
(Phys. Rev. Lett. 77, 167 (1996)). This work clearly demonstrated that
the mechanism of superconductivity in these systems is unique
and quite different than that in the high Tc cuprates.
Furher reading:
Phys. Rev. Lett. 77, 167 (1996)
Theory of Orientational Ordering in C60 and C60-based Compounds.
C 60 molecules adopt several different orientations in the solid state depending on various factors
including temperature and intercalated species. In 1993, we combined synthesis
and characterization of new C60 based materials with theoretical calculations to provide
a unified view of orientational ordering of C60 in C60-based solids
(Phys. Rev. Lett. 71, 1383 (1993)). This work resulted in the widespread realization
of the importance of molecular orientational order/disorder phenomena
on properties such as superconductivity.
Furher reading:
Phys. Rev. Lett. 71, 1383 (1993)
Theory of Spin Orbit Interactions and Magnetic Anisotropy in Magnetic Insulators
A longstanding problem concerns the mechanism whereby spin-orbit interactions give
rise to magnetic anisotropy in magnetic insulators. Until recently the discussions
of the origins of anisotropy found in cuprates were confined to the orthorhombic
structure (i.e., La2CuO4) However, more recently a family of copper oxide materials
of similar structure, but which are actually tetragonal, have been studied and
found to have roughly the same out-of-plane anisotropy as La2CuO4.
The earlier studied did not predict any anisotropy in the tetragonal limit.
Explaining this discrepancy was the main purpose of our work. We developed
a theory which for the first time corrects, extends, and clarifies results
concerning the spin Hamiltonian used to describe the ground manifold of Hubbard
models for magnetic insulators in the presence of spin-orbit interactions.
(Phys. Rev. Lett. 73, 2919 (1994)). The biggest impact of this work is the
demonstration that the orthorhombic distortion is not responsible for the
observed anisotropy of the spin structure in the cuprates. Rather our theory
shows that this anisotropy is due to spin-orbit and Coulomb exchange interactions.
Furher reading:
Phys. Rev. Lett. 73, 2919 (1994)
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