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AC-driven double quantum dots as spin pumps and spin filters. R. Sánchez 1 , E. Cota 2 , R. Aguado 1 and G. Platero 1 1 Instituto de Ciencia de Materiales de Madrid – CSIC, Cantoblanco, Madrid 28049, Spain 2 Centro de Ciencias de la Materia Condensada-UNAM, Ensenada, México.
R. Sánchez1, E. Cota2 , R. Aguado1 and G. Platero1
1Instituto de Ciencia de Materiales de Madrid – CSIC, Cantoblanco, Madrid 28049, Spain
2Centro de Ciencias de la Materia Condensada-UNAM, Ensenada, México
“Nanoscale Dynamics and Quantum Coherence”
Catania october 2005
.One of the most important requirements for any spin-based electronics devices is the ability to generate spin polarized currents.Control of spin dynamics of electrons spintronicsProposals for generating spin-polarized currents: spin injection by ferromagnetic metals or DMS.Semiconducting quantum dots can be used as spin filters or spin pumps: We propose a system for realizing both spin filtering and spin pumping with unpolarized leads, by using an ac-driven double QD connectedin series in thepresence of a uniform magnetic field.
Current Rectification by Pauli Exclusion in a Weakly Coupled Double Quantum Dot System
K. Ono , D.G. Austing, Y. Tokura and S. Tarucha, Science, vol 297 (02)
J.Iñarrea et al.
two main peaks I/V in a double QD. The spin blockade region
is the plateau between the peaks.
Austing, Y. Tokura
and S. Tarucha,
Science, 297 (02)
and helium atoms:
Electrical pump and probe experiments.
T. Fujisawa et al.,
Average number of tunneling
electrons per pulse:
one level in left dot
one level in right dot
Tunnelling between dots
Blum K., 1981,
Theory and Applications,
From Liouville equation
: Occupation probability of the level s
:Coherence and phase of superpositions of dot levels s,s’
its real part is responsible for the time decay of the off-diagonal DM elements (coherences)
Electrons coming onto left dot from the left lead
Electrons leaving from left dot onto the left lead
We consider that the AC does not affect the transition probability through the contact barriers
We include spin-flip processes:
Electronic dot states are affected by intrinsic degrees of freedom: hyperfine coupling, S-O interaction: spin relaxation and decoherence.
: Spin relaxation time:includes processes where the electron spin is flipped
The spin relaxation rates
At low T:
:Spin decoherence time(decay of the off-diagonal elements of the DM) it destroys the information about the relative phase in a superposition of and
it accounts as well for the intrinsic spin decoherence which is present even with no coupling to the leads
Coherent manipulations of electron spins:
gate operations for quantum computation
must be performed faster than T2
Where s states are such that the right dot is occupied and
s’ are states with one electron less
Appearance of interdot triplet
blocks pumping: spin blockade
Too high in
Spin down polarized contacts
= < mR
Spin pump follows the next sequence
= U2 >mR
E (=U2) -
E (=U2) -
Spintronics in quantum dots
An AC gate voltage acts as spin pump in
a DQD with unpolarized leads
Spin-polarized pumping in a double quantum dot,
E. Cota, R. Aguado, C. E. Creffield and G. Platero, Nanotechnology 14, 152 (2003)
two levels in the right dot:
As a consequence of Hund’s rule
the intradot exchange J is
current peaks also at
Absorption of N photons
An external B breaks the degeneracy for one and two-electron states, but there is a degeneracy in the three electron sector for
spin up current
spin down current
The degeneracy is broken as
as a function of
The width of
the peaks is
it is determined
Spin-up current sensitive to
relaxation processes if
Pumped current near resonance: =0.6 for different relaxation rates. Inset: FWHM of I as a function of the relaxation rate for strong ( black dots) and weak (red squares) field intensity.
E. Cota et al., PRL, 107202 (2005)
Black dots: low VAC Red squares: high VAC
Measuring the width of the current peak as a function of one gets direct information on the spin decoherence time:
for FW is linear with : FW=2 . Measuring FW at
we obtain the decoherence timefor the closed system:
singlet S0 in the
Spin down current
Spin up current through PAT at the leads
Comparison of I versus VAC including (non including) PAT through the contacts.
(not charge current) could be achieved
Cotunnel including PAT in the contacts
Analysis of the relaxation and
decoherence times T1 and T2 at finite T.
EARLY STAGE RESEARCHER MC RTN POSITION
Applications are encouraged for a two-year position as an “early stage researcher” in the Madrid node of this Marie Curie Research and Training Network (RTNNANO). The successful candidate will initiate his/her contract shortly before completion of Ph. D., afterwards continuing in Madrid as a postdoc.
Starting date: between January 1 and (ideally) April 1, 2006.
Research interests in the Madrid node include electron entanglement and decoherence, spintronics, electron and heat AC transport, and NS transport.
Interested applicants should contact one of the following:
Francisco Guinea (ICMM-CSIC)
Gloria Platero (ICMM-CSIC)
Fernando Sols (Universidad Complutense de Madrid) (contractor)
Closing date: November 30, 2005.