Superconducting State of Small Nanoclusters
Transcript of Superconducting State of Small Nanoclusters
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Superconducting State of Small Nanoclusters
Vladimir Kresin* and Yurii Ovchinnikov**
* Lawrence Berkeley National Laboratory
** L. Landau Institute for Theoretical Physics
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H. Kamerling - Onnes (1911)
R=0
“The experiment left no doubt that the resistance disappeared. Thus the mercury at 4.2K has entered a new state which can be called the state of superconductivity”.
H.Kamerling-Onnes
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At present:
-25 pure elements (highest transition temperature:
Nb (Tc=9.2K))
- ≈ 6000 alloys and compounds
Superconducting state is a peculiar state of matter. Superconducting materials possess anomalous magnetic ,thermal and other properties
e.g., electronic heat capacity:
Celn =γT;Cel
s =ae−b
T
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Meissner effect(anomalous diamagnetism)
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Nature of superconductivity
A.Einstein (1922)
F.Bloch and L.Landau (1932)
W.Heisenberg (1947)
M.Born (1948)
1957
J.Bardeen, L.Cooper, J.Schrieffer (BCS)
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BCS
Electron-electron attraction
F=e1e2
εr2 ;ε<0;
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BCS
Electron-electron attraction
Pairing ( the attraction between the pair (p,s=1/2;-p,s=-1/2) is the strongest
Electronic system:
set of bound pairs (bound energy: 2∆ (energy gap))
excitation of the electronic system breakup of such a pair∆
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Applied superconductivity
(electronics, large scale magnets,etc.)
Restriction:
superconductivity -low temperature phenomenon
Direction:
search for a new class of materials with high value of the transition temperature (Tc).
Goal:
Room temperature superconductor
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“The magnet floats freely above the sheet supported entirely by its magnetic field. The field provides a cushion on which the magnet rests. It is easy to imagine hovecraft of the future utilizing this principle to carry passengers and cargo above roadways of superconducting sheet,moving like flying carpet without friction and without material wear or tear . We can even imagine riding on magnetic skis down superconducting slopes and ski jumps-many fantastic things would become possible”
W.Little, Scientific American (1965)
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Superconducting state of small metallic nanoparticles
N ≈ 102 - 103
(N is a number of delocalized electrons)
New family of high Tc superconductors
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Superconducting state of metallic nanoparticles
Granular filmsTunneling (Harvard U.)
N ≈ 104 – 105
N ≈ 102 - 103
Small metallic nanoparticles (clusters)
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Clusters – small aggregates of atoms or molecules
An (e.g., Nan , Znn, Aln)
e.g., Al56 : N = 56 x 3 = 168 (each Al atom has 3 valence electrons)
Zn66 : N = 66 x 2 = 132
Metallic nanoclusters
Discrete energy spectrum
Energy spacing depends on the particle size
Nanoclusters
EF δE
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Nanoparticles
Discrete energy spectrum
The pairing is not essential if δE > ∆(Anderson, 1959)
The pairing is important if δE < ∆Usual superconductors : ∆≈10ΚEstimation:
δΕ ~EF
N;EF ≈105K
if N ≈103 ; δE ≈102K >> ∆
Condition: N 104 (δE≈10Κ)Assumption : the energy levels are equidistant (?)
shell structure
˜ >
δE
δ E
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Metallic clusters contain delocalized electrons whose states form shellssimilar to those in atoms or nuclei
Shell structure
W. Knight et al. ( Berkeley, 1984)
Metallic nanoclusters
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Metallic clusters contain delocalized electrons whose states form shells similar to those in atoms or nuclei
Shell Structure
Clusters
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Mass spectra of metallic clusters display magic numbers
Metallic clusters
Magic numbers (Nm=8,20,40,…,168,…,192,…)correspond to filled electronic shells (similar to inert atoms)
Cluster shapesClusters with closed electronic shells are spherical
Number of electrons shape energy spectrum
There is a strong correlation:
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spherical shape (quantum numbers: n,L)
degeneracy : g = 2(2L+1)
e.g. Nm = 168 ; L = 7
g = 30 (!)
“Magic” clusters
Lowestunoccupied shell
HighestOccupied shell
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Incomplete shelle.g., N =166 (Nm = 168)
shape deformation
sphere ellipsoid
splitting
LUS
HOS
Energy levels are not equidistant
Highest occupied shell (HOS)
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spherical shape (quantum numbers: n,L)
degeneracy : g = 2(2L+1)
e.g. Nm = 168 ; L = 7
g = 30 (!)
Electrons at HOS (EF) can form the pairs
Superconducting state
“Magic” clusters
Lowestunoccupied shell
HighestOccupied shell
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Pairing is similar to that in nuclei
A.Bohr , B.Mottelson, D.Pines(1958)S.Beljaev (1959)A.Migdal (1959)
The pair is formed by two nucleonsmj,1/2; -mj,-1/2
Metallic clusters- Coulomb forces- electrons and ions- electronic and vibrational energy levels
Mechanism: electron- vibrational interaction
-increase in size bulk metal
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high degeneracy
peak in density of states(similar to van Hove)
increase in TC
e.g., L = 7 (N=168; e.g., Al56
degeneracy G = 2(2L +1) = 30
(30 electrons at EF)
EFHOS
λ = VνF
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Arbitrary strength of the electron – vibrational coupling
∆ ωn( )Z = λ EF
NT gi
i
∑˜ Ω 2
ωn −ωn '( )2
+ ˜ Ω 2ωn '
∑ •∆ ω
n '( )ω
n '2 + ε j − µ( )2 + ∆2 ω
n '( )
ωn = 2n +1( )πT
λ =I
2νF
M ˜ Ω 2
νF dε∫ gjj
∑
µ = εFµ ≡ µ T( )
Bulk Clusters
(W. McMillan (1968))
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Critical temperature
T =TC
∆ ωn( )Z = λEF
NT gi
j
∑˜ Ω 2
ωn −ωn'( )2
+ ˜ Ω 2ωn'
∑ •∆ω
n'( )ω
n'2 + εj − µ( )2 TC
N = gi
1+ exp ε j − µ( )/T[ ]j
∑
ωn = 2n +1( )πTC
∆n = Knm ' ∆m '
m
∑
1− Knn ' = 0
C. Owen and D. Scalapino (1971)V. Kresin (1987)
Matrix method:
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Parameters: N, ∆εLH, gL, gH; EF, , λb
Examples:
N = 168; ∆εLH = 70meV,
gHOS = 30; gLUS = 18; = 25meV; λb = 0.5; EF=105K(L = 7)
TC=150K
˜ Ω
˜ Ω (L = 4)
Ga56 (N = 168) TC=160K
Zn190(N=380) TC=105K
(Tcb =1.1K)
(Tcb =0.9K)
LUS (lowest unoccupied shell)
HOS (highest occupied shell)µ
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Simple metallic clusters (Al, Ga, Zn, Cd)
Change in the parameters ( , ∆E, etc)
Room temperature
˜ Ω
Tc ≈ 150K
Conditions:- small HOS-LUS energy spacing- large degeneracy of H and L shells- small splitting for slightly unoccupied shells
e.g., N=168, 340; N=166
EF δE
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( manifestation, observables )
Energy spectrum
experimentally measured excitation spectrum
(e.g. HOS – LUS internal (∆E)) is temperature
dependent
- clusters at various temperatures
e.g., Cd83 (N=166)1)T<<Tc ; 2)T>Tc
hωmin.≈ 34 meV; hωmin.≈ 6 meV
- photoemission spectroscopy
odd-even effect
the spectrum strongly depends on the number of electrons being odd or even
Superconducting state of nanoclusters :
∆EIT ≈ OK >> ∆EIT >TC( )
HOSLUS
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Bulk superconductivity (R=0)
Tunneling--> Josephson tunneling(tunneling of the pairs)
dissipationless macroscopic current (R=0)
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Clusters with pair correlation are promising
building blocks for tunneling networks.
Macroscopic superconducting current at high temperatures
Depositing clusters on a surface without strong disturbance of the shell structure
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Proposed Experiments
Energy spectrum
experimentally measured excitation spectrum
(e.g. HOS – LUS internal (∆E)) is temperature
dependent
- clusters at various temperatures1)T<<Tc ; 2)T>Tc
hωmin.(T<<Tc) >> hωmin.(T>Tc)- photoemission spectroscopy
odd-even effect
the spectrum strongly depends on the number of electrons being
odd or even
Superconducting state of isolated nanoclusters :
∆EIT ≈ OK >> ∆EIT >TC( )
HOSLUS
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Superconducting state of an isolated cluster
Experiments:
- Selection ( mass spectroscopy;e.g., Ga56)
- Cluster beams at different temperatures
(T>Tc and T<Tc)
- Spectroscopy ( photoemission)
- Magnetic properties
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Experiments:
- Selection ( mass spectroscopy;e.g.,
Ga56)
- Cluster beams at different temperatures
(T>Tc and T<Tc)
- Spectroscopy ( photoemission)
-Magnetic properties
-UVA : 1), 2), 3), 4) (!)
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SummaryThe presence of shell structure and the accompanying high level of degeneracy in small metallic nanoclusters leads to large increase in the value of the critical temperature
e.g., Ga56 (N=168) : Tc ≈ 150KMain factors:- large degeneracy of the highest occupied shell (HOS); small HOS- LUS space
incomplete shellsmall shape deformation
Manifestations of the pairing:-temperature dependence of the spectrum:
- odd-even effect
- clusters with superconducting pair correlation are promising blocks for tunneling network.
∆EIT ≈ 0K ≠ ∆EIT >TC
Europhys J. B45, 5 (2005) ; B47, 333 (2005)
Phys.Rev.B 74,024514 (2006)
Small nanoclusters form a new family of high Tc superconductors
HOSLUS
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Fluctuations
T ~ Tc
Ginzburg - Landau functional
δTc (broadening of the transition)
δTc
Tc
≈ 5%
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Tc ≈ ˜ Ω e−1/λ
is the characteristic vibrational frequency (~ΩD)
λ is the coupling constant (λ<<1)
The expression for Tc can not be obtained with use of the perturbation theory
can not be expanded ina Taylor series for small x
f = e−1/x
˜ Ω