Transcript Document

Insulator-metal transition under high pressure in
BiFeO3 from first-principles calculations
S.Li (李晟) and Z.Q.Yang (杨中芹)
Department of Physics, Fudan University, Shanghai 200433, China
Motivation
• Recently experiments showed the insulator-metal transition
(IMT) in the transition metal oxide BiFeO3, which was
believed to be caused by crossover of high-spin (HS) and
low-spin (LS) states in 3d shell of Fe3+ with d5 configuration
under high pressure. The trend results in the decrease of the
Mott-Hubbard energy.
• We apply LSDA+U methods to study the transition pressures
and the behavior of spin crossover in BiFeO3. Furthermore,
we also plan to explore those properties in BiMnO 3 and
BiCrO3 to find the general rules of IMT in those kind of
systems.
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
sigma=0.01 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-transition
with different Hubbard U ranges from 0 to 7 eV, which gives a selfconsistent result.
Perovskite structure with R3c is Gtype antiferromagnetic.
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 e g and
t2g electrons possess the energies +6Dq and -4Dq,respectively, and
(iii) each pair of parallel spins provides an energy gain of –J(J>0 is
the Hund exchange parameter). Ueff depends on the crystal field
D=10Dq, which increases with the pressure.
For d5 configuration:
EHS(d5)=Ec(d5)-10J;ELS(d5)=Ec(d5)-20Dq-4J
So the high-spin-low-spin (HS-LS) crossover takes place at D>3J
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)=D0+aDP
With Mott-Hubbard transition: Wc=aUeff(D), we can confirm the
pressure where IMT takes place.
We have calculated the electronic structure of BiFeO 3. The left fig
shows the ground state of BiFeO3 and Fe DOS without Hubbard U
taken into consideration. The right fig shows BiFeO3 with
Ueff=2.0eV 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,
W~1eV IMT will be
enabled in this
criterion:
W/UHS<1W/ULS>1
T0+U
EF
T0
IMT and HS-LS crossover under U=1.5eV and U=2.5eV.(Red circle
emphasize the pressure of IMT)
1.15
W/U
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(U=2.5eV) , 3.4mB(U=1.5eV) and the
pressure where IMT takes place as 68GPa (55GPa for experiment) with Hubbard U=1.5eV.
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)
[4]C.Ederer and N.A.Spaldin Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite Phys.Rev.B 71, 060401(2005)