650 research outputs found
Failure mechanisms of graphene under tension
Recent experiments established pure graphene as the strongest material known
to mankind, further invigorating the question of how graphene fails. Using
density functional theory, we reveal the mechanisms of mechanical failure of
pure graphene under a generic state of tension. One failure mechanism is a
novel soft-mode phonon instability of the -mode, whereby the graphene
sheet undergoes a phase transition and is driven towards isolated benzene rings
resulting in a reduction of strength. The other is the usual elastic
instability corresponding to a maximum in the stress-strain curve. Our results
indicate that finite wave vector soft modes can be the key factor in limiting
the strength of monolayer materials
High resolution X-ray scattering studies of structural phase transitions in underdoped LaBaCuO
We have studied structural phase transitions in high quality underdoped
LaBaCuO single crystals using high resolution x-ray scattering
techniques. Critical properties associated with the continuous High Temperature
Tetragonal (HTT, ) to Middle Temperature Orthorhombic (MTO, )
phase transition were investigated in single crystal samples with x=0.125,
0.095, and 0.08 and we find that all behavior is consistent with three
dimensional XY criticality, as expected from theory. Power law behavior in the
orthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably wide
temperature range, spanning most of the MTO regime in the phase diagram. Low
temperature measurements investigating the Low Temperature Tetragonal (LTT,
) phase, below the strongly discontinuous MTOLTT phase
transition, in x=0.125 and x=0.095 samples show that the LTT phase is
characterized by relatively broad Bragg scattering, compared with that observed
at related wavevectors in the HTT phase. This shows that the LTT phase is
either an admixture of tetragonal and orthorhombic phases, or that it is
orthorhombic with very small orthorhombic strain, consistent with the ``less
orthorhombic" low temperature structure previously reported in mixed
LaSrBaCuO single crystals. We compare the complex
temperature-composition phase diagram for the location of structural and
superconducting phase transitions in underdoped LaBaCuO and
find good agreement with results obtained on polycrystalline samples.Comment: 8 pages, 7 figures, 1 tabl
Drought-stressed, irrigated, and additive-treated corn silages for growing cattle
Cattle fed drought-stressed corn silage gained about 10% slower but were just as efficient as cattle fed irrigated corn silage. Because the irrigated corn out yielded the drought corn (17.4 VS. 8.2 tons per acre), the irrigated silage gave a much higher cattle gain per acre (1928 VS. 940 1b). Silo Guard II®-treated silage had an advantage in DM recovery and feed conversion over its control and produced 4.6 more pounds of cattle gain per ton of crop ensiled. Cattle fed H/M Inoculant®- treated silage gained significantly faster than cattle fed the control, however, the treated silage gave only slight improvements in DM recovery and gain per ton of crop ensiled
Inelastic neutron and x-ray scattering as probes of the sign structure of the Fe-pnictide superconducting gap
Neutron spin-flip scattering observations of a resonance in the
superconducting state is often taken as evidence of an unconventional
superconducting state in which the gap changes sign
for momentum transfers which play an important role in the pairing.
Recently questions regarding this identification for the Fe-pnictide
superconductors have been raised and it has been suggested that
. Here we propose that inelastic neutron or x-ray
scattering measurements of the spectral weight of a phonon of momentum can
distinguish between these two pairing scenarios.Comment: 4 pages, 4 figure
Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal
state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal
pronounced changes of the Cu magnetism at the structural transition from the
orthorhombic to the low temperature tetragonal phase for all
non-superconducting compositions. Both a jumplike decrease of \chi as well as
the ESR data show an increase of the in-plane magnetic correlation length in
the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and
Sr concentrations in the LTT phase the correlation length increases up to more
than 100 lattice constants and the fluctuation frequency of the CuO_2 spin
system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static
order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4
with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the
International Conference "Stripes, Lattice Instabilities and High Tc
Superconductivity", (Rome, Dec. 1996
Coexistence of Band Jahn Teller Distortion and superconductivity in correlated systems
The co-existence of band Jahn-Teller (BJT) effect with superconductivity (SC)
is studied for correlated systems, with orbitally degenerate bands using a
simple model. The Hubbard model for a doubly degenerate orbital with the
on-site intraorbital Coulomb repulsion treated in the slave boson formalism and
the interorbital Coulomb repulsion treated in the Hartree-Fock mean field
approximation, describes the correlated system. The model further incorporates
the BJT interaction and a pairing term to account for the lattice distortion
and superconductivity respectively. It is found that structural distortion
tends to suppress superconductivity and when SC sets in at low temperatures,
the growth of the lattice distortion is arrested. The phase diagram comprising
of the SC and structural transition temperatures and versus the
dopant concentration reveals that the highest obtainable for an
optimum doping is limited by structural transition. The dependence of the
occupation probabilities of the different bands as well as the density of
states (DOS) in the distorted-superconducting phase, on electron correlation
has been discussed.Comment: RevTex, 4 pages, 4 figuers (postscript files attached) Journal
Reference : Phys. Rev. B (accepted for publication
Stripes and spin-incommensurabilities are favored by lattice anisotropies
Structural distortions in cuprate materials give a natural origin for
anisotropies in electron properties. We study a modified one-band t-J model in
which we allow for different hoppings and antiferromagnetic couplings in the
two spatial directions ( and ). Incommensurate peaks
in the spin structure factor show up only in the presence of a lattice
anisotropy, whereas charge correlations, indicating enhanced fluctuations at
incommensurate wave vectors, are almost unaffected with respect to the
isotropic case.Comment: accepted for publication on Physical Review Letters, one color figur
Soft phonons and structural phase transitions in LaBaCuO
Soft phonon behavior associated with a structural phase transition from the
low-temperature-orthorhombic (LTO) phase ( symmetry) to the
low-temperature-tetragonal (LTT) phase ( symmetry) was investigated
in LaBaCuO using neutron scattering. As temperature
decreases, the TO-mode at -point softens and approaches to zero energy
around K, where the LTO -- LTT transition occurs. Below , the phonon hardens quite rapidly and it's energy almost saturates below
50 K. At , the energy dispersion of the soft phonon along in-plane
direction significantly changes while the dispersion along out-of-plane
direction is almost temperature independent. Coexistence between the LTO phase
and the LTT phase, seen in both the soft phonon spectra and the peak profiles
of Bragg reflection, is discussed in context of the order of structural phase
transitions.Comment: 6 pages, 8 figure
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
Two-Photon Spectroscopy Between States of Opposite Parities
Magnetic- and electric-dipole two-photon absorption (MED-TPA), recently
introduced as a new spectroscopic technique for studying transitions between
states of opposite parities, is investigated from a theoretical point of view.
A new approximation, referred to as {\it weak quasi-closure approximation}, is
used together with symmetry adaptation techniques to calculate the transition
amplitude between states having well-defined symmetry properties. Selection
rules for MED-TPA are derived and compared to selection rules for
parity-forbidden electric-dipole two-photon absorption (ED-TPA).Comment: 7 pages, Revtex File, to be published in Physical Review
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