78 research outputs found
Pressure dependence of the Curie temperature in Ni2MnSn Heusler alloy: A first-principles study
The pressure dependence of electronic structure, exchange interactions and
Curie temperature in ferromagnetic Heusler alloy Ni2MnSn has been studied
theoretically within the framework of the density-functional theory. The
calculation of the exchange parameters is based on the frozen--magnon approach.
The Curie temperature, Tc, is calculated within the mean-field approximation by
solving the matrix equation for a multi-sublattice system. In agrement with
experiment the Curie temperature increased from 362K at ambient pressure to 396
at 12 GPa. Extending the variation of the lattice parameter beyond the range
studied experimentally we obtained non-monotonous pressure dependence of the
Curie temperature and metamagnetic transition. We relate the theoretical
dependence of Tc on the lattice constant to the corresponding dependence
predicted by the empirical interaction curve. The Mn-Ni atomic interchange
observed experimentally is simulated to study its influence on the Curie
temperature.Comment: 8 pages, 8 figure
Magnetoelectric Effect and Spontaneous Polarization in HoFe(BO) and HoNdFe(BO)
The thermodynamic, magnetic, dielectric, and magnetoelectric properties of
HoFe(BO) and HoNdFe(BO) are
investigated. Both compounds show a second order Ne\'{e}l transition above 30 K
and a first order spin reorientation transition below 10 K.
HoFe(BO) develops a spontaneous electrical polarization below the
Ne\'{e}l temperature (T) which is diminished in external magnetic fields.
No magnetoelectric effect could be observed in HoFe(BO). In
contrast, the solid solution HoNdFe(BO) exhibits
both, a spontaneous polarization below T and a magnetoelectric effect at
higher fields that extends to high temperatures. The superposition of
spontaneous polarization, induced by the internal magnetic field in the ordered
state, and the magnetoelectric polarizations due to the external field results
in a complex behavior of the total polarization measured as a function of
temperature and field.Comment: 12 pages, 15 figure
Role of the conduction electrons in mediating exchange interactions in Heusler alloys
Because of large spatial separation of the Mn atoms in Heusler alloys the Mn
3d states belonging to different atoms do not overlap considerably. Therefore
an indirect exchange interaction between Mn atoms should play a crucial role in
the ferromagnetism of the systems. To study the nature of the ferromagnetism of
various Mn-based semi- and full-Heusler alloys we perform a systematic
first-principles calculation of the exchange interactions in these materials.
The calculation of the exchange parameters is based on the frozen-magnon
approach. The calculations show that the magnetism of the Mn-based Heusler
alloys depends strongly on the number of conduction electrons, their spin
polarization and the position of the unoccupied Mn 3d states with respect to
the Fermi level. Various magnetic phases are obtained depending on the
combination of these characteristics. The Anderson's s-d model is used to
perform a qualitative analysis of the obtained results. The conditions leading
to diverse magnetic behavior are identified. If the spin polarization of the
conduction electrons at the Fermi energy is large and the unoccupied Mn 3d
states lie well above the Fermi level, an RKKY-type ferromagnetic interaction
is dominating. On the other hand, the contribution of the antiferromagnetic
superexchange becomes important if unoccupied Mn 3d states lie close to the
Fermi energy. The resulting magnetic behavior depends on the competition of
these two exchange mechanisms. The calculational results are in good
correlation with the conclusions made on the basis of the Anderson s-d model
which provides useful framework for the analysis of the results of
first-principles calculations and helps to formulate the conditions for high
Curie temperature.Comment: 16 pages, 9 figures, 2 table
Material-Specific Investigations of Correlated Electron Systems
We present the results of numerical studies for selected materials with
strongly correlated electrons using a combination of the local-density
approximation and dynamical mean-field theory (DMFT). For the solution of the
DMFT equations a continuous-time quantum Monte-Carlo algorithm was employed.
All simulations were performed on the supercomputer HLRB II at the Leibniz
Rechenzentrum in Munich. Specifically we have analyzed the pressure induced
metal-insulator transitions in Fe2O3 and NiS2, the charge susceptibility of the
fluctuating-valence elemental metal Yb, and the spectral properties of a
covalent band-insulator model which includes local electronic correlations.Comment: 14 pages, 7 figures, to appear in "High Performance Computing in
Science and Engineering, Garching 2009" (Springer
Recommended from our members
Pressure-Induced Electronic Spin Transition of Iron in Magnesiow?stite-(Mg,Fe)O
An electronic transition of iron in magnesiowuestite has been studied with synchrotron Moessbauer and X-ray emission spectroscopies under high pressures. Synchrotron Moessbauer studies show that the quadrupole splitting disappears and the isomer shift drops significantly across the spin-paring transition of iron in (Mg{sub 0.75},Fe{sub 0.25})O between 62 and 70 GPa, whereas X-ray emission spectroscopy of the Fe-K{sub {beta}} fluorescence lines in dilute (Mg{sub 0.95},Fe{sub 0.05})O also confirms that a high-spin to low-spin transition occurs between 46 GPa and 55 GPa. Based upon current results and percolation theory, we reexamine the high-pressure phase diagram of (Mg,Fe)O and find that iron-iron exchange interaction plays an important role in stabilizing the high-spin state of iron in FeO-rich (Mg,Fe)O
Anomalous high-temperature superconductivity in YH
Pressure-stabilized hydrides are a new rapidly growing class of
high-temperature superconductors which is believed to be described within the
conventional phonon-mediated mechanism of coupling. Here we report the
synthesis of yttrium hexahydride Im3m-YH that demonstrates the
superconducting transition with T = 224 K at 166 GPa, much lower than the
theoretically predicted (>270 K). The measured upper critical magnetic field
B(0) of YH was found to be 116-158 T, which is 2-2.5 times larger
than the calculated value. A pronounced shift of T in yttrium deuteride
YD with the isotope coefficient 0.4 supports the phonon-assisted
superconductivity. Current-voltage measurements showed that the critical
current I and its density J may exceed 1.75 A and 3500 A/mm at 0 K,
respectively, which is comparable with the parameters of commercial
superconductors, such as NbTi and YBCO. The superconducting density functional
theory (SCDFT) and anharmonic calculations suggest unusually large impact of
the Coulomb repulsion in this compound. The results indicate notable departures
of the superconducting properties of the discovered YH from the
conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.Comment: arXiv admin note: text overlap with arXiv:1902.1020
Dynamical mean-field approach to materials with strong electronic correlations
We review recent results on the properties of materials with correlated
electrons obtained within the LDA+DMFT approach, a combination of a
conventional band structure approach based on the local density approximation
(LDA) and the dynamical mean-field theory (DMFT). The application to four
outstanding problems in this field is discussed: (i) we compute the full
valence band structure of the charge-transfer insulator NiO by explicitly
including the p-d hybridization, (ii) we explain the origin for the
simultaneously occuring metal-insulator transition and collapse of the magnetic
moment in MnO and Fe2O3, (iii) we describe a novel GGA+DMFT scheme in terms of
plane-wave pseudopotentials which allows us to compute the orbital order and
cooperative Jahn-Teller distortion in KCuF3 and LaMnO3, and (iv) we provide a
general explanation for the appearance of kinks in the effective dispersion of
correlated electrons in systems with a pronounced three-peak spectral function
without having to resort to the coupling of electrons to bosonic excitations.
These results provide a considerable progress in the fully microscopic
investigations of correlated electron materials.Comment: 24 pages, 14 figures, final version, submitted to Eur. Phys. J. for
publication in the Special Topics volume "Cooperative Phenomena in Solids:
Metal-Insulator Transitions and Ordering of Microscopic Degrees of Freedom
- …