THE TEAM: Dr. Víctor Pardo Alberto Piñeiro, PhD Student Prof. Daniel Baldomir
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Ab initio studies at high pressure on TiOCl: electronic structure of the Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic orthorhombic and monoclinic phases and monoclinic phases V. Pardo THE TEAM: Dr. Víctor Pardo Alberto Piñeiro, PhD Student Prof. Daniel Baldomir Instituto de Investigacións Tecnolóxicas Universidade Santiago de Compostela, Spain Santiago Blanco-Canosa, PhD Student Dr. Francisco Rivadulla Departamento de Química-Física Universidade Santiago de Compostela, Spain Prof. José Rivas Departamento de Física Aplicada Universidade Santiago de Compostela, Spain Prof. D.I. Khosmkii Prof. M. Abd-Elmeguid II. Physikalisches Institut Universität zu Köln, Germany
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Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic and monoclinic phases V. Pardo. THE TEAM: Dr. Víctor Pardo Alberto Piñeiro, PhD Student Prof. Daniel Baldomir Instituto de Investigacións Tecnolóxicas Universidade Santiago de Compostela, Spain - PowerPoint PPT Presentation
Transcript of THE TEAM: Dr. Víctor Pardo Alberto Piñeiro, PhD Student Prof. Daniel Baldomir
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
THE TEAM:
Dr. Víctor PardoAlberto Piñeiro, PhD StudentProf. Daniel BaldomirInstituto de Investigacións TecnolóxicasUniversidade Santiago de Compostela, Spain
Santiago Blanco-Canosa, PhD StudentDr. Francisco RivadullaDepartamento de Química-FísicaUniversidade Santiago de Compostela, Spain
Prof. José RivasDepartamento de Física AplicadaUniversidade Santiago de Compostela, Spain
Prof. D.I. KhosmkiiProf. M. Abd-ElmeguidII. Physikalisches InstitutUniversität zu Köln, Germany
Oxyhalides TiOCl and TiOBr presents spin-Peierls transition at low temperature
Low dimensional spin-1/2 compound (Ti3+)
Presents 2 consecutive phase transitions
Incommensurate spin-Peierls Temperature (~91K)
Commensurate spin-Peierls Temperature (~66K)
Coupling of a one dimensional antiferromagnetic S=1/2 chain with the lattice results in a spin-Peierls transition with a dimerized ground state
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
Methodology
LDA+U method: WIEN2k software:
- All electron, full-potential scheme
U= 0.35 Ry (~ 5 eV)
• Reasonable value for a d1 electron system• Reproduces reasonably the gap of the structure measured al ambient pressure in orthorhombic structure (LDA+U gap proportional to U)• Reproduces very well the commensurate spin-Peierls transition temperature
Geometry optimization: GGA (Perdew-Burke-Ernzerhof)
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
High pressureMonoclinic structure (P21/m space group)
Low pressureOrthorhombic structure(Pmmn space group)
X-ray diffraction under pressure
S. Blanco-Canosa et al., ArXiv:0806.0230
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
(a)
(b)
d 1 d1
d 2 d3
Structure of the TiOCl compound (a) on the ambient pressure orthorhombic phase and (b) on the high-pressure monoclinic phase. Values of the distances are d1=3.36Å, d2=3.25Å and d3=3.39Å.
Atomic positions relaxed at every pressure
Position Atomic Position
Ti 2 (b) (0.0000, 0.5000, 0.1220)
Cl 2 (a) (0.0000, 0.0000, 0.3273)
O 2 (a) (0.0000, 0.0000, 0.9350)
Position Atomic Position
Ti 2 (b) (0.0000, 0.5000, 0.1133)
Cl 2 (a) (0.0000, 0.0000, 0.3200)
O 2 (a) (0.0000, 0.0000, 0.9435)
Structural optimization utilizing GGA scheme
Orthorhombic cell parameters
P (GPa) a (Å) b (Å) (x 2) c (Å)
0 3.79 6.74 8.06
4 3.72 6.67 7.77
7 3.67 6.66 7.56
10 3.62 6.65 7.36
15 3.54 6.63 7.02
20 3.45 6.61 6.69
Monoclinic cell parameters
P (GPa) a (Å) b (Å) c (Å)
0 3.79 3.37 8.06
4 3.68 3.20 7.36
7 3.66 3.15 7.23
9 3.65 3.12 7.10
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
Electron spin density plot of TiOCl in the orthorhombic
structure
Ti3+ (d1) ions have one t2g orbital occupied (dyz) with a large hopping integral along the b direction of the crystal
Highly one-dimensional electronic structure.
• Interactions along the a-axis are very small• Along the c axis, interactions are negligible due to the large Ti-Ti distance
TiOCl is structurally quasi-2-Dimensional, but electronicaly it is quasi-1-Dimensional
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
Heissenberg type Hamiltonian
Ground state
P (GPa) 0 4 7 9
Js (K) -12 -10 0 10
Jd (K) 300 1100 1400 1650
Jl (K) -1 -4 -4 -4
Tc (K) 50 180 230 275
We need 3 diferent magnetic configurations to obtain the 3 magnetic couplings constants
1D-mean fieldapproximation
J>0 AF couplingJ<0 FM coupling
4 inequivalent Ti atoms
In agreement with experimental commensurate TSP
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
J’s in the orthorhombic structure
0 2 4 6 8 101.0
1.1
1.2
1.3
60 70 80 90 100 110 120
0.0
1.5x10-5
3.0x10-5
9.71 kbar
0 kbar
TISP
TSP
d/d
T (
emu/
mol
Oe)
Temperature (K)
TISP
TSP
T(P
)/T
(P=
0)
P (kbar)
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
crossing withroom T expected
between 10 and 17 GPa
consistent with ourtheoretical findings
S. Blanco-Canosa et al., ArXiv:0806.0230
Gap evolution
Decrease of the gap ~ 30%In orthorhombic structure
In concordance with experimental data (resistivity measurements):
M.K. Forthaus et al., Phys. Rev. B 77, 165121 (2008)
Value near experiment
Drastic reduction of the gap:Structural transition
In concordance with experiment
We would need more than 30 GPa to get an insulator-to-metal transition in orthorhombic structure!!
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
$
-8 -6 -4 -2 0 2 4 6
-20
-10
0
10
20
30
DO
S (st
ates
/(eV
spi
n))
ENERGY (eV)
P= 0 GPa
-8 -6 -4 -2 0 2 4 6
-20
-10
0
10
20
30
DO
S (st
ates
/(eV
spi
n))
ENERGY (eV)
P= 15 GPa
orthorhombic
monoclinic
Drastic reduction of the gap(from 1.5 eV to 0.3 eV)
States of d1 electron of Ti are good localized in a ~ 1 eV band near Fermi energy
Mot-Hubbard type gap (d-d gap)
Increase ~20% of the bandwith
Oxygen bands are in energies down than -2 eV
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
P=0 (orthorhombic) : GGA (U= 0) LDA+U (U= 5 eV)
- Nonmagnetic solution E> 0 E> 0 metal metal
- Magnetic solution E=0 (minimum) E=0 (minimum) metal insulator
P=15 GPa (monoclinic): GGA (U= 0) LDA+U (U= 5 eV)
- Nonmagnetic solution E=0 (minimum) E> 0 metal metal
- Magnetic solution does not converge E= 0 (minimum) insulator
Possible solutions vs. U value
‘’dimerized’’ 3%Monoclinic space group: 3.25 Å 3.43 Å2 types of dimerized structures
‘’long dimerized’’ 11% Ti atom 2.95 Å 3.69 Å
b axis
Close to metal Ti bond length!!!Limit for electron-itinerancySpin-Peierls distortion is expected to be supported by a conventional Peierls distortion of the 1D chain
0 5 10 15 20
20
40
60
80
100
120
Long dimerization
dT
i-T
i/b (x1
0-3)
P (GPa)
Short dimerization
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
6,61 6,62 6,63 6,64 6,65 6,66 6,67 6,68
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
60
P = 20 GPa
Ene
rgy
[sho
rt/lo
ng-v
.sho
rt/v
.long
] (m
eV/T
i)
b (A)
P=0 GPa
Energy analysis: long-dimerized expected more stable above 15 GPa.
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
P
long-dimerizedshort-dimerized
monoclinic
orthorhombic
TTSP
Conclusions:
•The quasi-one-dimensional electronic structure of TiOCl was shown and also the one-dimensionality of its magnetic properties
•TiOCl presents a double structural transition above 10 GPa and 15 GPa (first one from undimerized phase to dimerized phase and the second one from a short dimerized structure to a long dimerized structure)
• Evolution with pressure of TSP is consistent with high pressure dimerization and drastic change in the band gap
• The system TiOCl does not present an insulator-to-metal transition, neither from ab initio calculations nor from experimental measurements
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
Acknowledgments:
“Study of phase separationin magnetic oxides combining
theory and experiment”
Project MAT 2006-10027
Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic Ab initio studies at high pressure on TiOCl: electronic structure of the orthorhombic
and monoclinic phasesand monoclinic phases V. Pardo
Supercomputing Centreof Galicia
Ramón y Cajal Program
F.P.U. Program
“Electronic phase separation andSuperstructures in transition metal
Oxides: a dual theoretical/experimental approach”
Joint Project Spain/Germany HA2006-0119
Project PXIB-20919-PR
