544 research outputs found
Role of defects and disorder in the half-metallic full-Heusler compounds
Half-metallic ferromagnets and especially the full-Heusler alloys containing
Co are at the center of scientific research due to their potential applications
in spintronics. For realistic devices it is important to control accurately the
creation of defects in these alloys. We review some of our late results on the
role of defects and impurities in these compounds. More precisely we present
results for the following cases (i) doping and disorder in CoCr(Mn)Al(Si)
alloys, (ii) half-metallic ferrimagnetism appeared due to the creation of
Cr(Mn) antisites in these alloys, (iii) Co-doping in MnVAl(Si) alloys
leading to half-metallic antiferromagnetism, and finally (iv) the occurrence of
vacancies in the full-Heusler alloys containing Co and Mn. These results are
susceptible of encouraging further theoretical and experimental research in the
properties of these compounds.Comment: Chapter intended for a book with contributions of the invited
speakers of the International Conference on Nanoscale Magnetism 2007. Revised
version contains new figure
A science-based sector in the making: the formation of the biotechnology sector in two regions
This paper analyses two case studies, Skåne-Blekinge in Sweden and the Southern-Eastern region in Ireland, to examine different current development paths for the biotechnology sector. The aim is to codify the process, identifying actions and priorities towards these paths. The national innovation systems theory provides the theoretical framework that guided a series of interviews in the two regions. The findings demonstrate that the sustainable development of a science-based sector does not depend on the original priorities or directions, but rather on the level of consistency of those policies and their continuous evolution towards a complete systemic value generation system
Quantum Criticality and Incipient Phase Separation in the Thermodynamic Properties of the Hubbard Model
Transport measurements on the cuprates suggest the presence of a quantum
critical point hiding underneath the superconducting dome near optimal hole
doping. We provide numerical evidence in support of this scenario via a
dynamical cluster quantum Monte Carlo study of the extended two-dimensional
Hubbard model. Single particle quantities, such as the spectral function, the
quasiparticle weight and the entropy, display a crossover between two distinct
ground states: a Fermi liquid at low filling and a non-Fermi liquid with a
pseudogap at high filling. Both states are found to cross over to a marginal
Fermi-liquid state at higher temperatures. For finite next-nearest-neighbor
hopping t' we find a classical critical point at temperature T_c. This
classical critical point is found to be associated with a phase separation
transition between a compressible Mott gas and an incompressible Mott liquid
corresponding to the Fermi liquid and the pseudogap state, respectively. Since
the critical temperature T_c extrapolates to zero as t' vanishes, we conclude
that a quantum critical point connects the Fermi-liquid to the pseudogap
region, and that the marginal-Fermi-liquid behavior in its vicinity is the
analogous of the supercritical region in the liquid-gas transition.Comment: 18 pages, 9 figure
Thermodynamics of the Quantum Critical Point at Finite Doping in the 2D Hubbard Model: A Dynamical Cluster Approximation Study
We study the thermodynamics of the two-dimensional Hubbard model within the
dynamical cluster approximation. We use continuous time quantum Monte Carlo as
a cluster solver to avoid the systematic error which complicates the
calculation of the entropy and potential energy (double occupancy). We find
that at a critical filling, there is a pronounced peak in the entropy divided
by temperature, S/T, and in the normalized double occupancy as a function of
doping. At this filling, we find that specific heat divided by temperature,
C/T, increases strongly with decreasing temperature and kinetic and potential
energies vary like T^2 ln(T). These are all characteristics of quantum critical
behavior.Comment: 4 pages, 4 figures. Submitted to Phys. Rev. B Rapid Communications on
June 27, 200
First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors
Employing first principles electronic structure calculations in conjunction
with the frozen-magnon method we calculate exchange interactions, spin-wave
dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin
gapless semiconductor (SGS) compounds MnCoAl, TiMnAl, CrZnSi,
TiCoSi and TiVAs. We find that their magnetic behavior is similar to
the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice
exchange interactions play an essential role in the formation of the magnetic
ground state and in determining the Curie temperature, . All
compounds, except TiCoSi possess a ferrimagnetic ground state. Due to the
finite energy gap in one spin channel, the exchange interactions decay sharply
with the distance, and hence magnetism of these SGSs can be described
considering only nearest and next-nearest neighbor exchange interactions. The
calculated spin-wave dispersion curves are typical for ferrimagnets and
ferromagnets. The spin-wave stiffness constants turn out to be larger than
those of the elementary 3-ferromagnets. Calculated exchange parameters are
used as input to determine the temperature dependence of the magnetization and
of the SGSs. We find that the of all compounds is
much above the room temperature. The calculated magnetization curve for
MnCoAl as well as the Curie temperature are in very good agreement with
available experimental data. The present study is expected to pave the way for
a deeper understanding of the magnetic properties of the inverse-Heusler-based
SGSs and enhance the interest in these materials for application in spintronic
and magnetoelectronic devices.Comment: Accepted for publ;ication in Physical Review
Towards New Half-Metallic Systems: Zinc-Blende Compounds of Transition Elements with N, P, As, Sb, S, Se, and Te
We report systematic first-principles calculations for ordered zinc-blende
compounds of the transition metal elements V, Cr, Mn with the sp elements N, P,
As, Sb, S, Se, Te, motivated by recent fabrication of zinc-blende CrAs, CrSb,
and MnAs. They show ferromagnetic half-metallic behavior for a wide range of
lattice constants. We discuss the origin and trends of half-metallicity,
present the calculated equilibrium lattice constants, and examine the
half-metallic behavior of their transition element terminated (001) surfaces.Comment: 2nd Version: lattice constants calculations added, text revise
Surface Properties of the Half- and Full-Heusler Alloys
Using a full-potential \textit{ab-initio} technique I study the electronic
and magnetic properties of the (001) surfaces of the half-Heusler alloys,
NiMnSb, CoMnSb and PtMnSb and of the full-Heusler alloys CoMnGe, CoMnSi
and CoCrAl. The MnSb terminated surfaces of the half-Heusler compounds
present properties similar to the bulk compounds and, although the
half-metallicity is lost, an important spin-polarisation at the Fermi level. In
contrast to this the Ni terminated surface shows an almost zero net
spin-polarisation. While the bulk CoMnGe and CoMnSi are almost
half-ferromagnetic, their surfaces lose the half-metallic character and the net
spin-polarisation at the Fermi level is close to zero. Contrary to these
compounds the CrAl terminated (001) surface of CoCrAl shows a spin
polarisation of about 84%.Comment: 14 pages, 6 figure
Breakdown of Strong-Coupling Perturbation Theory in Doped Mott Insulators
We show that doped Mott insulators, such as the copper-oxide superconductors,
are asymptotically slaved in that the quasiparticle weight, , near
half-filling depends critically on the existence of the high energy scale set
by the upper Hubbard band. In particular, near half filling, the following
dichotomy arises: when the high energy scale is integrated out but Z=0
in the thermodynamic limit when it is retained. Slavery to the high energy
scale arises from quantum interference between electronic excitations across
the Mott gap. Broad spectral features seen in photoemission in the normal state
of the cuprates are argued to arise from high energy slavery.Comment: Published versio
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