interplay between spin, charge, lattice and orbital degrees of freedom
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Interplay between spin, charge, lattice and orbital degrees of freedom. Lecture notes Les Houches June 2006 lecture 3 George Sawatzky. Need multiband models to describe TM compounds. - PowerPoint PPT PresentationTRANSCRIPT
Interplay between spin, charge, lattice and orbital degrees of
freedom
Lecture notes Les Houches June 2006 lecture 3
George Sawatzky
Need multiband models to describe TM compounds
However numerous studies have shown that this can sometimes be reduced to an effective
single band Hubbard model at least for highTc’s BUT ONLY FOR LOW ENERGY
EXCITATIONS E<0.5eV
Macridin et al Phys. Rev. B 71, 134527 (2005)
Tpd(eg) = 2x Tpd(t2g)
Crystal and ligand field splittings
Often about 0.5 eVIn Oh symmetry
Angular integrals Are different for t23g and eg
Eg-O2p hoping is 2 times as large as T2g-O-2p hoping
Often about 1-2eVIn Oxides
High Spin – Low Spin transition very common inCo(3+)(d6), as in LaCoO3, not so common in Fe(2+)(d6)Because of the smaller hybridization with O(2p)
Mixed valent system could lead to strange effects Such as spin blockade for charge transport and high thermoelectric powers
What would happen if 2Jh <10Dq<3JhIf we remove one electron from d6 we would go fromS=0 in d6 to S=5/2 in d5. The “hole “ would carry a spinOf 5/2 as it moves in the d6 lattice.
If the charge transfer energy gets small we have to Modify the superexchange theory
Anderson 1961
New term
tij = t cos (Oij/2)
Oij = angle between neighbouring spins
Khomskii et al S S Comm.102,87, 1997
dxy
dxz
dyz
Pen et al PRL 78,1323
This orbital ordering yields a large internal Antiferromagnetic exchange and a weak external ferromagnetic exchange .
Orbital ordering removes frustration
YVO3 Pervoskite structureV(3+) 2 electrons in T2g Orbitals S=1.Note the tilted and Rotated octahedra
Tsvetkov et alPRB 69, 075110 (2004)
YVO3 PerovskiteV(d2 S=1) O not In inversion symmetryDM canting competingWith staggered magneticAnisotropy . See Aharoni’s lectures
After applying a high fieldJust above above transOn the downward tragectory
Without the high fieldApplied in the downwardtrajectroy
All V have one electron in a dxy orbital
O between the V ions are not in inversion centerTilted Octahedra D.SxS interactions compete With local staggered anisotropy
Three ways to get ferromagnets with High Tc without using 3d’s or 4f’s
1. Use electronic reconstruction of polar surfaces
2. Use defects and topology and symmetry
3. Use doping and large Hund’s rule coupling of O,N
Electronic reconstruction at surfaces and interfaces
By moving 1 electr. Per O From 2- side to 2+ side the Potential becomes flat.
LSDA Band Structure of CaO (111) Slab
-10
-5
0
5
10
Γ K M Γ A L H A
Ener
gy (e
V)
-10
-5
0
5
10
Γ K M Γ A L H A
Spin Up Spin Down
12
-4
-2
0
2
4
6
8
10
L X W L K
Ener
gy (e
V)Note:
Bulk material(no surface)
is an insulator
But surface is metallic!
Ca 4s
O 2p
Example of two particles in U= limit
t t
t
1 1
2 2 2 1
00
0
tttttt
H
),(),( 2121 ss mmxx
212
1
),( 21 ss mmTriplet
Singlet
“+” for singlet; “-” for triplet
Energy level diagram for holes (t>0)
-2t-t
t2t
Triplet
Singlet
Balla et al
One electron spectral function in a magnetically and orbitally Ordered system
Hopping :
abplane c axis
Interactions :
( Jab> 0 and Jc>0 )c
b
a
Resonant soft x ray scattering
Doped holes in cuprate
C. T. Chen et al. PRL 66, 104 (1991)
Cu2+ d9 S=1/2
O 2- full shell
La2-xSrxCuO4 Sr ---doped holes
Nature 431, 1078 (2004)
Chains with model of spin singlets
CDW with Q=0.2 is 5 cl modulation along the ladder
Rushdy et al PRL in press
Models mentioned in White , Affleck and Scalapino PRB 65 165122For ¼ modulation
RXS shows that ¼ does not exist but 1/3 and 1/5 do and more recent Results show the model with paired holes along the rungs is most likely Correct. This could be of great importance for the understanding of High Tc’s
NOTE WE DON’T SEE A 4 FOLD MODULATION
Pr1-xCaxMnO3
Pr0.6Ca0.4MnO3 CE type charge, orbital and magnetic order
Goodenough (1955)
• Charge ordering below TCO ~ 240K
• Cooperative orbital ordering + oxygen distortion at TOO = TCO
• Magnetic ordering below TN ~ 170K
K.J. Thomas et al NSLS/BNL
Phys. Rev. Lett. 92, 237204 (2004)