Scattering rates and conductivties

Chapter VI: Adatom doped graphene

6.3 Scattering rates and conductivties

Figure 6.2: (a) Elastic scattering length of graphene with 25% estimation by Drude model (Green), compared to the scattering length of intrinsic disorder source (blue) used in the later MC simulation to ensure the diffusiveness. The black line is the system length in simulationL. (b) Elastic scattering time of graphene with randomly distributed 1% and 10% Tl atoms. The coherence and elastic time are extracted from the experiment[16]. In both simulation and experiment, we can see that the disorder from adatoms is much weaker than other disorder.

them for simplicity in our analytical calculations below. In the following sections, we instead explore how the higher-order Rashba-like SOC’s modify magnetotransport in graphene.

and so do adatoms particularly if randomly distributed. However, we should be more careful in treating different types of scattering process within δH_R, since they are from the same adatom and thus highly correlated with each other. In this section, we will derive the scattering times and conductivities for adatom-decorated graphene with and without background disorder H_dis. Then we show that the correlation between different scattering process within adatoms is negligible in some specific situations.

After averaging over disorder, the propagation of an electron on adatom-doped graphene can be described by retarded and advanced Green’s functions ¯G^r/a,

h

E − H_g,0−Σ^r/a±iηi

G¯^r/a =I. (6.17)

Pristine graphene, without any disorder, is described byH_g,0=−i~vF ∂xσ^xτ^z+∂yσ^y , and all the scattering processes caused by either intrinsic disorder or adatoms are encoded in the self-energy Σ^r/a. We firstly consider graphene with weak adatom potentialδHRonly and expand ¯Garound the bare Green’s funcitonG₀to the leading order in(δH )²,

= + +

with the leading order correction of self energyΣ₀, Σ^r,a₀ (k, ω)=

∫ d²k⁰

(2π)²hδH (k,k⁰)G^r,a₀ (k⁰, ω)δH (k⁰,k)i. (6.18) Hereh· · · irepresents averaging over disorder realizations. Since an adatom scatterer contains multiple scattering types, the self-energy matrix has non-zero off-diagonal elements reflecting the high correlation properties between different scattering types.

Therefore, the conventional Matthiessen rule for total scattering is no longer appli- cable, and we also need to re-define the average scattering time of an electron. Now we will derive the average scattering time for bothd−and p−orbital adatoms.

d-orbital adatoms

We consider pristine graphene with randomly distributed d−orbital adatoms with coverage nI, and derive the scattering time matrix τ = − ^~

2ImΣ^R from the leading order self-energyΣ0. Due to the high correlation, τis a non-diagonal matrix with

eigenvalues

τ₊ = ~ Γa−Γc

, τ₀ = ~

Γa

, τ− = ~

Γa+Γc

, (6.19)

where

Γa= πνnIA7(V₀²+λ²_KM+2λ²_R+V_i²+λ²_i),

Γ_c = 2πνnIA7λ_R(V₀+λ_KM). (6.20) There is no unique life time for correlated disorder, and three of the scattering times together determine the transport properties, e.g. conductivity and magneto- conductance. The self-energy componentsΓ_adescribes the self-energy contribution from uncorrelated part, whileΓ_c reflects the correlation between different types of disorder within δH. Density of states per valley per spin at Fermi energy F is ν(F) = _2π^F

~²v²_F. If different scattering types induced by adatoms are uncorrelated, then Γ_c will vanish and three life times are identical τ± = τ₀ which is simply the addition of scattering rate for each scattering type. If we further include the background disorderH_disin the system, the corresponding scattering times are the same as Eq. (6.19), except that Γ_a → Γ_a + Γ_bg where Γ_bg denotes the imaginary self-energy resulted by background disorder.

The classical diagonal conductivityσx x(ω)for a system of volumeΩ at zero tem- perature is given by Kubo-Greenwood formula,

σx x = ~

πΩReTr[jxG^rjxG^a] (6.21) where jx =−evFτ^zσ^x is the current operator. After disorder averaging, the leading three scattering mechanisms are Drude-Boltzmann(bare), diffuson, and Cooperon scattering (which corresponding to the maximally crossed diagram),

....

The former two describe classical conductivity and the last one shows the quantum interference correction which we will discuss in the next section. By plugging the Green’s function in Eq. (6.17) with the approximated self energy in Eq. (6.18) into the first diagram, we derive the bare conductivity

σ₀(ω)=e²ν(F)v²_F

~

Γ_a+Γ_c−i~ω + ~ Γ_a−Γ_c−i~ω

(6.22) which can be reduced to

σ₀(ω= 0)=e²ν(F)v_F²(τ₊+τ−) (6.23)

for DC conductivity. By comparing to the conventional Drude conductivity for Dirac fermion, we identify the “elastic scattering rate" τe = ^τ++₂^τ−. Again, we can simply replaceΓ_awithΓ_a+Γ_bgif background disorder is included in the system.

We obtain some insights for this system by considering the following limits: un- correlated limit Γ_a + Γ_bg Γ_c and correlated limit Γ_a + Γ_bg Γ_c. When the background disorder Γ_bg or, more precisely, the uncorrelated part of self-enegy—

Γ_a + Γ_bg—is much stronger than adatom disorder, this equation will recover the conventional uncorrelated results,

τ₊+τ− =2τ₀

1+ε²+O(ε⁴)

(6.24) withε= _Γa^Γ₊^c_Γbg, despite the none-zero correlation between scattering types induced by adatoms. The background disorderΓ_bgcan be estimated as~/τ_e⁰with the elastic scattering τ_e⁰ caused by background disorder. Therefore, in a strong disordered graphene sample, the correlation between different scattering types can be neglected.

On the other hand, in a clean system or for some specific adatoms whereΓ_a+Γ_bg Γ_c, the correlation becomes important and should be considered in the conductivity and MC.

The total classical conductivity includes both the bare conductivity and diffuson scattering, which can be renormalized into the current operator as

. The normalized current operator can be written as

j˜x =(1− (δH_I ⊗δH_I)(G^r ⊗G^r))⁻¹jx, (6.25) and the explicit index contraction and derivation can be found in Appendix. ??.

Though there is no nice general analytical form for this normalized factor, the above equation can be simplified to ˜jx ∼ 2jx in the uncorrelated limit. Therefore, total classical conductivity, i.e. Drude conductivity, of the uncorrelated disorder system is simply, and widely known as

σ_cl =4e²ν(F)D (6.26)

with diffusion coefficientD = v²_Fτtr/2 = v_F²τe. If the correlation between different scattering types in adatoms is not negligible, this factor will change accordingly and can be estimated numerically for the given details of adatoms.

p-orbital adatoms

The same derivation applies to p-orbital adatoms as well, with the corresponding self-energy, up to first order in momentum,determined by

Γ_a =πνnI

V₀²+V_i²+λ²_i +λ²_KM+O(k_F²) A7 Γ_b=2πνnIkFaλ⁽¹⁾_R

V0−Vi

(6.27)

and other correlation-related Γs listed in the Appendix.??. The inclusion of back- ground disorder, again, only modifies Γa to beΓa+Γ_bg. The corresponding bare conductivity reads

σ₀(ω)=e²ν(F)v²_F×

~

Γ_a+Γ_b(F) −i~ω + ~

Γ_a−Γ_b(F) −i~ω

. (6.28)

At Dirac points kF = 0, the correlated term Γb vanishes and the system can be viewed as independent.

We estimated that the conductivity is dominated by background disorder in adatom- decorated graphene with In and Tl, based on the parameter values given by Ref. [32].

Figure 6.2(a) and (b) show the comparison of elastic time/length of adatoms and background disorder in both the experiment [16] and our simulations. In Fig. 6.2(a), we estimate the elastic length(green line) with le/v_F = τe ∼ ~/Γ_a for 25% In- decorated graphene and find that it is shorter than the system size in all the studied range. This is consistent with our quantum transport simulation results: the system is always ballistic. In the later analysis and simulation, we will include the intrinsic disorder (blue line) to ensure our system diffusive and having strong short range disorder to study the quantum interference effect. In Fig. 6.2(b), the intrinsic disorder (e.g. ripple, charge puddles) in adatom-doped experiments induces strong scattering compared to the low coverage adatoms. Again,τe(red lines) are roughly estimated by ∼ ~/Γ_a. However, we should clarify that, in an experiment, extra

“disorder" might be induced to graphene in the process of doping adatoms, but this type of disorder comes from, for example, charge-puddle or ripple enhancement instead of the adatom itself. To sum up, we find that the correlation property of the disorder induced by adatoms has little effect on conductivity and elastic time, when the background disorder is strong. On the contrary, in the case of clean graphene strongly coupled to heavy adatoms, conductivity may increase due to correlation.