cold melting of solid electron phases in quantum dots m. rontani, g. goldoni infm-s3, modena, italy...
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Cold Melting of Solid Electron Phases in Quantum Dots
Cold Melting of Solid Electron Phases in Quantum Dots
M. Rontani, G. GoldoniINFM-S3, Modena, Italy
phase diagram
correlation in quantum dots
configuration interaction
spin polarization
Wignermolecule
Fermi liquid -
like
high density low density
Why quantum dots?
potential for new devicessingle-electron transistor, laser, single-photon emitter
quantum control of charge and spin degrees of freedom
laboratory to explore fundamentals of few-body physics
easy access to different correlation regimes
Energy scales in artificial atoms
experimental control: N, density, / e2/(l)
low density n high B fieldnT
Tuning electron phases à la Wigner
H = T + Vkineticenergy
e-einteraction
2/1nV T quenched
rs = l / aB n = 1 / l2
= lQD / aB
2DEG:
QD:2/1*
QD )/( ml
Open questions in correlated regimes
crystal
liquid
ferromagnet
Tanatar and Ceperley 1989
2D:spin-polarized phase?disorder favors crystal
0D:crystallization?spin polarization?melting?
controversy for N = 6
QMC: R. Egger et al., PRL 82, 3320 (1999)
CI: S. M. Reimann et al., PRB 62, 8108 (2000)
ji jir
ii
N
i rr
er
m
mH
||
1
22*2
22
20
*2
1
2
envelope function approximation, envelope function approximation, semiconductor effective parameters semiconductor effective parameters
dcbaabcd
abcdbaa
ab ccccVccH '†
'†
'
†
, second quantization formalismsecond quantization formalism
1) Compute 1) Compute HH parameters from the chosen single-particle basis parameters from the chosen single-particle basis
rdrrHr baab
)()()( 0
* 'dd)()'('
)'()(2
** rrrrrr
errV dc
rbaabcd
2) Compute the wavefunction as a superposition of Slater 2) Compute the wavefunction as a superposition of Slater
determinantsdeterminants ij
iii HHc 0|| †
'† mli cc
Configuration interaction
sp
d
Monitoring crystallization
example:N = 5
tota
l den
sity
con
ditio
nal p
rob
ab
ility
Rontani et al., Computer Phys. Commun. 2005
Classical geometrical phases
•crystallization around (agreement with QMC)•N = 6 ?
con
dit
ion
al pro
bab
ility
No spin polarization!N = 6
•single-particle basis: 36 orbitals•maximum linear matrix size ≈ 1.1 106 for S = 1•about 600 hours of CPU time on IBM-SP4 with 40 CPUs, for each value of and M
= 2 = 3.5
= 6
Fine structure of transition
conditional probability= fixed electron
N = 6
“Normal modes” at low density
N = 6 = 8
(mod 5) - replicas
rotational bands
cf. Koskinen et al. PRB 2001
Monitoring crystallization
= 2
Monitoring crystallization
= 2.5
Monitoring crystallization
= 3
Monitoring crystallization
= 3.5
Monitoring crystallization
= 4
Monitoring crystallization
= 5
Monitoring crystallization
= 6
The six-electron double-dot system
top view top-dot electron
bottom-dot electron
phase I phase II phase III
t
Numerical results
t tRontani et al., EPL 2002
Cold meltingI and III classical configurations
II novel quantum phase, liquid-like
I III
(rad)
same dot
different dots
Ronta
ni et
al.,
EPL
2002
Conclusion
phase diagram of low-density quantum dots
spin-unpolarized N = 6 ground state
classically metastable phase close to melting
How to measure?inelastic light scattering
[EPL 58, 555 (2002); cond-mat/0506143]
tunneling spectroscopies
[cond-mat/0408454]
FIRB, COFIN-2003, MAE, INFM I.T. Calcolo Parallelohttp://www.s3.infm.it