SpiDME meeting, Nijmegen, May 2007
Stefano Sanvito and Nadjib BaadjiComputational Spintronics Group
School of Physics and CRANN, Trinity College
People Dr. Nadjib Baadji (Uni. Strasbourg), April ‘07
Mr. Sankar Kesanakurthi (U. Hiderabad), April ‘07
VisitsSanvito to Hamburg (Feb. 2007)
Outlook
A simple model for transport
Salen on Cu
SP-STM for molecules
Ab initio transport theory
L RL R
V 0
L
R VL
R
V ≠ 0
In equilibriumOut of equilibrium
2
L
R VL
R
V ≠ 02|EF-|
- 00 fNU
L
R VL
R
V ≠ 0
LL R
R
E +eV/2F
E -eV/2F
T(E)
E
I
H1
H0 H0
H1
HM (n)
H0
HRM
H0
H1
H0
H1
H0
HLM
RL
H= HM+H0 +H0 +H0 +….
HM+L (E)+R(E)
Lead’s Self-energy
Density Matrix
Current
Molecule Green function
A. R. Rocha and S. Sanvito, PRB 70, 094406 (2004)
D. Sánchez-Portal, P. Ordejón, E. Artacho, and J.M. Soler, Int. J. Quant. Chem. 65, 453 (1997)
KS-DFT Hamiltonian
We implemented NEGF in Siesta Localized multiple-z Pseudo-atomic orbitals (non-orthogonal) Optimized Pseudopotential Super-cells with up to 2,000 atoms
http://www.smeagol.tcd.ie/
A. R. Rocha et al., Phys. Rev. B 73, 085414 (2006); Nature Materials 4, 335 (2005)
Mailing list http://lists.tchpc.tcd.ie/listinfo/smeagol-discuss
Molecular Spin valvesNature Mat. 4, 335
(2005)
Fe/MgO TMR junctionI. Rungger et al
DNA transportA.R.Rocha et al., in
preparation
Ni point contactsA.R.Rocha et al.,
cond-mat/0701512
Problems with molecular transport
C. Toher et al., PRL 95, 146402 (2005)
Spin TorqueM. Stamenova et al., in preparation
0nA
40nA
80nA
0nA
50nA
100nA
Au on AuV=250mVd=0.4nm
Ni on NiV=250mVd=0.4nm
0%
10%
20%
500 mV
250 mV
-30%
-10%
10%
-70%
-55%
-40%
-250 mV-60%
-45%
-30%
-500 mV
P=I II +I
I to tip
I from tip
Does the GMR mirror the polarization ?
-30%
-10%
10%
P=I II +I
-20%
-15%
-10%
R=IP IAP
IAP
250 mV
500 mV
250 mV
-250 mV
-500 mV
20%
10%
-45%
-50%
V=0
V=0
TIP
M+S
S
S
tiptip V
V=0
TIP
M+S
V=400mV
TIP
M+S
Current to the tip
V=0
TIP
M+S
V=-400mV
Current to the S+M
TIP
M+S
Direct calculations of the tunneling currents are possible and include:
Some prospects of investigating the bonding of molecules on magnetic surfaces
Electronic Structure of the tip Tip to sample interaction Charging of the moleculae Accurate determination of the spin-polarization Non-collinear spin Spin-orbit
Molecule
N,N'-BIS(SALICYLIDENE)ETHYLENEDIAMINO-TMWhere TM could be : Cu, Zn, Ni or Co
C1
C2C3
C1
C2
C3
Comparison between the DOS of the Salen
molecule and the hypothetical small molecule
E (eV)
DOS
(arb
. uni
ts)
Big
Small
E (eV)
Big
Small
Molecule on Cu surfaces (un-relaxed)
Cu-salen on Cu(001)
Cu-salen on Cu(111)
DOS
(arb
. uni
ts)
E (eV)
Relaxation on Cu(001) surface
Unrelaxed structure Relaxed structure
DOS for different TM-salen
Cu
E (eV)
DOS
(arb
. uni
ts)
4s13d10
E (eV)
DOS
(arb
. uni
ts)
Zn 4s23d10
E (eV)
DOS
(arb
. uni
ts)
Co 4s23d7
E (eV)
DOS
(arb
. uni
ts)
Ni 4s23d8
Simulation STM imagesFree Cu-Salen
EF-0.2eV < E < EF
EF < E < EF +0.2eVI molecule to tip I tip to molecule
Constant current STM imagesCu-Salen un-relaxed
EF-0.2eV < E < EF
EF < E < EF +0.2eVI molecule to tip I tip to molecule
(a) (b)
(c) (d)
EF-0.2eV < E < EF
EF < E < EF +0.2eVI molecule to tip I tip to molecule
Cu
Zn
This is very much work in progress
First find the right atomic configuration Then simulate the current Compare the images for different TM Hopefully they will compare with experiments
integral of the DOS near Ef (pos. & neg. bias L-resolved DOS for Cu atom in the small molecule
L-resolved DOS for Zn atom in the small moleculeCu DOS in free mole. and in mole. on Cu (001)