Conclusion

1
Insulator-metal transition under high pressure
in BiFeO3 from first-principles calculations
S.Li (??) and Z.Q.Yang (???) Department of
Physics, Fudan University, Shanghai 200433, China
Calculation Methods and Models
Calculation method Vienna ab initio simulation
package (VASP). The plane wave cutoff energy was
set to 420 eV. To improve the convergence in the
eigenstates at the Fermi level, a Gaussian
smearing of sigma0.01 eV has been applied.We
used the primitive unit cell with 8x8x8 k-point
grids.The criteria for terminating the electronic
and ionic iterations are energy differences of
10-5 and 10-4 eV, respectively.We use the slope
of E-V curve to confirm the insulator-metal-transi
tion with different Hubbard U ranges from 0 to 7
eV, which gives a self-consistent result.
Electron distribution for d5 configuration in
high- and low-spin states
Results and Discussion
Other theoretical model assumes that (i) all
intraatomic Coulomb matrix elements are
independent of the orbit number,(ii) the eg and
t2g electrons possess the energies 6Dq and
-4Dq,respectively, and (iii) each pair of
parallel spins provides an energy gain of J(Jgt0
is the Hund exchange parameter). Ueff depends on
the crystal field D10Dq, which increases with
the pressure.For d5 configuration
EHS(d5)Ec(d5)-10JELS(d5)Ec(d5)-20Dq-4JSo the
high-spin-low-spin (HS-LS) crossover takes place
at Dgt3J As the pressure increases, the
interatomic distance decreases and the crystal
field parameter increases. There is an assumption
that the growth of field parameter can be
described by a linear relation as
D(P)D0aDPWith Mott-Hubbard transition
WcaUeff(D), we can confirm the pressure where
IMT takes place.
We have calculated the electronic structure of
BiFeO3. The left fig shows the ground state of
BiFeO3 and Fe DOS without Hubbard U taken into
consideration. The right fig shows BiFeO3 with
Ueff2.0eV under high pressure where IMT takes
place. It is clear that BiFeO3 transits from
insulator to metal under specific condition.
The left graph shows that with larger Hubbard U
we need more pressure to affect IMT because W and
D is consistent with pressure. It is clear that
our calculation fit well the with the model since
we can control Ueff to affect on the pressure
where IMT happens. The HS-LS crossover is shown
explicitly in these graph. We also find he
magnetic moment is smaller than the theory number
of HS and LS states as 5mB because of the finite
bandwdth of 3d states.Furthermore, BiMnO3(d4)
and polarization calculation is still in
progress.
For BiFeO3 d-electron bandwidth is large, W1eV
IMT will be enabled in this criterion
W/UHSlt1?W/ULSgt1
IMT and HS-LS crossover under U1.5eV and
U2.5eV.(Red circle emphasize the pressure of IMT)

• Conclusion
• We studied the electronic structure of BiFeO3
  from first principles calculations. Ground states
  (AFM insulator) are given. With different sets of
  Hubbard U, BiFeO3 can change from insulator to
  metal .
• We have given the magnetic moment with different
  Hubbard U as 3.7mB(U2.5eV) , 3.4mB(U1.5eV) and
  the pressure where IMT takes place as 68GPa
  (55GPa for experiment) with Hubbard U1.5eV.
• References
• 1 Alexander G.Gavriliuk, Viktor V. Struzhkin
  et al. Another mechanism for the insulator-metal
  transition observed in Mott insulators Phys.Rev.B
  77,155112(2008)
• 2 S.G.Ovchinnikov Effect of Spin Crossovers on
  the Mott-Hubbard Transition at High Pressures
  Journal of Experimental and Theoretical Physics
  107 (2008) 140-146
• 3 J.B.Neaton, C.Ederer, U.V.Waghmare,
  N.A.Spaldn, and K.M.Rabe First-principles study
  of spontaneous polarization in multiferroic
  BiFeO3Phys.Rev.B 71,014113(2005)4C.Ederer and
  N.A.Spaldin Weak ferromagnetism and
  magnetoelectric coupling in bismuth ferrite
  Phys.Rev.B 71, 060401(2005)