Fractional diffusion equations in time and frequency domain: models and applications Juan Bisquert...
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Fractional diffusion equations in time and frequency domain: models and applications
Juan BisquertDepartament de Ciències ExperimentalsUniversitat Jaume I, Spain
València, 2 february 2005
Outline
•Fractional diffusion equation with decaying probability density
•Interpretation of the FDE in the time domain: meaning of the decay law in multiple trapping transport
•Transient diffusion photocurrent
•The frequency domain: anomalous diffusion
The fractional diffusion equation FDE
We discuss the physical interpretation of a FDE based on the replacement of the time derivative in the diffusion equation with a derivative of noninteger order
(1)
is the fractional Riemann-Liouville derivative operator. The fractional time derivative can be written
and its Laplace transform is
2
2
0),(
),(x
txfCtxfDt
tD0
t
t tdtt
tf
dt
dtfD
00
)(
)(
)1(
1)(
)0()()(0 fufutfDL t
Initial condition of the FDE
Eq. (1) was discussed as a possible generalized diffusion equation describing anomalous diffusion process, but the feasibility of this application was doubted because the f is not a normalized function. Indeed, Eq. (1) requires an initial condition for the Green function of the form
(2)Where
is the Riemann-Liouville fractional integral operator. This is a nonlocal initial condition. It implies the divergence of f as t-> 0 [R. Hilfer, J. Phys. Chem. B 104 (2000) 3914 ].
)()0,( ,01
0 xfxfI t
x
axa dyyfyxxfI )()(
)(
1)( 1
Decay of the probability
The expression of the FDE in Laplace-Fourier space (u, q) is
Note that for q =0
Hence the time decay of the probability in spatially homogeneous conditions (without diffusion) is given by
Here we aim to provide a physical interpretation of this decay law and the implications for an anomalous diffusion process.
20
1,0),(
qKu
fquf
ufuf 1,0)0,(
11
,0
)()(
tf
tf
Background
The mathematical properties of Eq. (1) have been amply studied in the literature:
the solutions in different dimensions of space, the behaviour of the initial conditions the moments of the distribution
W.R. Schneider and W. Wyss, Journal of Mathematical Physics 30 (1989) 134.
M.M. Meerschaert, D.A. Benson, H.-P. Scheffler and B. Baeumer, Physical Review E 65 (2002) 041103.
M.M. Meerschaert, D.A. Benson, H.-P. Scheffler and P. Becker-Kern, Physical Review E 66 (2002) 060102(R).
J.-S. Duan, Journal of Mathematical Physics 46 (2005) 013504. E.K. Lenzi, R.S. Mendes, K.S. Fa, L.R. da Silva and L.S. Lucena, Journal of Mathematical Physics 45 (2004) 3444.
A.A. Kilbas, T. Pierantozzi, J.J. Trujillo and L. Vázquez, J. Phys. A: Math. Gen. 37 (2004) 3271.
Background
Hilfer described Eq. (2) as a fractional stationarity condition related to a dissipative dynamics. Feldman et al. considered Eq. (1) as the fractional generalization of the Liouville equation for dissipative systems. Tarasov provided a fractional analog for the normalization conditions for distribution functions. Eq. (1) was used by Nigmatullin to define a non-exponential relaxation process
R. Hilfer, Fractals 3 (1995) 549 Y. Feldman, A. Puzenko and Y. Ryabov, Chem. Phys. 284 (2002) 139 V.E. Tarasov, Chaos 14 (2004) 123 R.R. Nigmatullin and Y.E. Ryabov, Phys. Solid State 39 (1997) 87
CTRW model
Another FDE has been amply studied in the literature of anomalous diffusion
(3)
Space-time Fourier-Laplace
Eq. (3) is rigurously related to the Continuous Time Random Walk formalism of Scher and Montroll. Compare
(1)
Eq. (3) has the initial condition and the probability is conserved
Eqs. (1) and (3) are not equivalent
),(),(2
210 tx
x
fDKtxf
t
20
1
1,0),(
qKu
ufquf
2
2
0),(
),(x
txfCtxfDt
20
1,0),(
qKu
fquf
)()0,( 0 xfxf
The time domain
Recently it was shown J. Bisquert, Physical Review Letters 91 (2003) 010602 that the FDE in Eq. (1) describes the diffusion of free carriers in multiple trapping (MT) with an exponential distribution of gap states.
The present work develops the previous one with the goals:
•To present a concrete instance of a physical system with an intrinsic dissipative dynamics leading to a shrinking phase space.
•To study the physical behaviour in experimental techniques: Transient photocurrents, frequency techniques.
Multiple trapping
Multiple trapping model has been widely used to describe carrier transport in amorphous semiconductors. In 1970s MT model explained long tails of the electrical current observed in time-of-flight experiments and the time dependence of transient photocurrents.
Multiple trapping
Propagation of a gaussian packet by drift transport and the result current
Multiple trapping
Propagation of a packet by drift transport in CTRW, the predicted current and the current measured in amorphous semiconductor
H. Scher and E.W. Montroll, Phys. Rev. B 12 (1975) 2455.
Multiple trapping
More recently, starting in 1995, MT has become very popular for explaining different kinds of experimental results in nanostructured semiconductors such as networks of TiO2 nanoparticles used in dye-sensitized solar cells.
Multiple trapping
MT applies to a semiconductor with a band of extended states where transport is fast and a tail of localized states, which usually takes the exponential form
cBccB
L TkEETk
NEg /)(exp)(
Multiple trapping
The probability of trapping is similar for all the bandgap states, but the probability of release of a trapped electron is exponentially decreasing with the depth of the trap (detailed balance).
TkEEE Bc /exp)( 0
Transient photocurrent spectroscopy
A fast laser pulse excites electrons to the conduction bandThe carrier density is homogeneous and will be detected by measuring the photocurrent.Initially the electrons will be trappedThen equilibrium will be established at states above a demarcation energy level that depends on time:
TktEEt Bdc /)(exp10
Multiple trapping
Evolution of trapped electrons:Thermal distribution above Ed
Increasing occupancy, with the same shape as the exponential distribution below Ed.
The carrier concentration sinks deeper in the bandgap.The free carrier concentration is given by the Boltzmann tails of the peak, and decreases with time
Experimental results
transient photocurrent in a-As2Se3
Only free electron density is measured
The decay law is given by
The fractional exponent depends on temperature as
110)( ttn
1/ cTT
Interpretation of the decaying probability density
The measurements of transient photocurrent showed the decay law that is predicted by the FDE
The decay of the probability is associated to the removal of some degrees of freedom (trap levels) in the model.The disappearance of the probability takes on a perfectly valid physical meaning, in correspondendence with the requirements of the experimental technique.
11
,0
)()(
tf
tf
Interpretation of the decaying probability density
It has been remarked in the literaure the divergence of the solution of the FDE as t -> 0
In the system described above the decay law cannot be extrapolated to t = 0, because the decay makes no sense without a minimal time for initial thermalisation. This time is typically in the ps range. In normal experiments there is an initial time associated with the injection or photogeneration pulse, which is much longer than this, in the ns range. Transport experiments resolve the evolution of a large ensemble of electrons and not individual electronic transitions. Therefore the divergence of the initial condition is not an impediment for the application of the FDE of eq. (1) in the description of experiments.
11
,0
)()(
tf
tf
The frequency domain
Impedance spectroscopy is a common experimental technique that measures the impedance in a material system, i.e. the relation between ac voltage and ac current
Equivalent circuit of physical processes
Charge storage is represented by capacitorsIn general capacitor is a reversible energy storage
Interfacial charge-transfer, transport, recombination is represented by resistancesIn general resistance is an irreversible process
Diffusion impedance
Equivalent circuit
Boundary conditionsBlocked diffusion
J. Bisquert, J. Phys. Chem. B 106, 325-333 (2002)
Impedance of diffusion is obtained by solving the diffusion equation in Laplace domain
Diffusion impedance
2/ LDd
chCiZ
1
2/1)( iAZ
Impedance of diffusion is obtained by solving the diffusion equation in Laplace domain
Two state model
Impedance of nanostructured TiO2
Z' ()
0 1000 2000 3000 4000
Z'' (
)
0
1000
2000
Z' ()0 200 400
Z'' (
)
0
200
400
substra te so lu tion
TiO 2
W E R E F
lifetime
Mg2+ and Li+ ion insertions into Mo6S8
Nyquist plots for Mg-ion insertion into the Chevrel phase, covering the whole frequency domain
M. D. Levi, H. Gizbar, E. Lancry, Y. Gofer, E. Levi and D. AurbachJ. Electroanal. Chem. 569, 211-223 (2004)
Anomalous diffusion model
Fractional time diffusion
2/)( iAZ
)(
1
iQZ
J. Bisquert and A. CompteJ. Electroanal. Chem. 499, 112-120 (2001).
Anomalous diffusion impedance
J. Bisquert, G. Garcia-Belmonte, A. PitarchChemPhysChem, 4, 287-292 (2003).
Model of 8 traps following an exponential distribution in energy
lithium transport through vanadium pentoxide film
The Nyquist plots of the ac-impedance spectra measured on the V2O5 xerogel film electrode in a 1 M LiClO4–PC solution at the lithium content, , 0.75, 1.25 and 1.55 which corresponds to the electrode potential, E, 2.8, 2.5 and 2.2 VLi/Li+, respectively.
Kyu-Nam Jung, Su-Il Pyun and Jong-Won Lee, Electrochim. Acta 49, 4371-4378 (2004)
lithium transport through vanadium pentoxide film
The anodic current transients theoretically calculated for lithium transport by using random walk simulation in consideration of the residence time distribution with =1.3, 1.5 and 1.8. For comparison, the current transient simulated without considering the residence time distribution ( =0) is also presented.
Kyu-Nam Jung, Su-Il Pyun and Jong-Won Lee, Electrochim. Acta 49, 4371-4378 (2004)
Conclusions
• The FDE equation with decaying probability density has been interpreted in terms of multiple trapping transport.
• The solutions of the FDE appear as a new tool for the analysis of important experimental problems, both in time and frequency domain
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