Thermal properties from first-principles DFT calculations

Thermal transport from

first-principles DFT calculations

Keivan Esfarjani

Department of Mechanical Engineering

5/23/2012 Phonon School @ UWM 1

Classical MD simulations …

… use an empirical potential fitted to

reproduce lattice constant, cohesive

energy, elastic properties, some

phonons…

But are not supposed to produce correct

third derivatives of the potential energy,

determining phonon lifetimes

They can be used to explain trends in

thermal transport, but can not predict

quantitatively the thermal conductivity. 5/23/2012 Phonon School @ UWM 2

To develop a general methodology to get

accurate estimates of thermal properties

using available density functional theory

(DFT) tools.

To make accurate / reliable predictions

without fitting any parameters to

experimental data.

Outline

New force field based on FP-DFT

Approaches for thermal conductivity

calculation + illustrations

◦ Lattice dynamics model

◦ MD

◦ Green’s functions for coherent transport

Conclusions

Force field model

Derivatives are calculated at or near equilibrium

i i i ij j ijk j k

j, jk,

ijkl j k l

i ij ijk

i i j i j k

i j k l

1F E u u u

1u u u

E E E; ; ;

u u u u u u

u u u u

Force displacements

Exact for small displacements! 5/23/2012 Phonon School @ UWM 5

They need to be reduced

As the FCs are 2nd or higher rank tensors,

there are a huge number of terms in the

Taylor expansion!

9 for each pair * number of pairs

27 for each triplet * number of triplets

81 for each quadruplet * number of quadruplets

Method to extract anharmonic force constants from first principles calculations

Keivan Esfarjani and Harold T. Stokes

Phys. Rev. B 77, 144112 (2008)

Symmetry constraints

The FCs are related by the following

symmetry constraints:

Permutation of the order of derivatives

Global Translational invariance

ij ijk

Keivan Esfarjani and Harold T. Stokes, Phys. Rev. B 77, 144112 (2008)

Symmetry constraints

Global Rotational invariance Links FCs to higher-order ones

Group symmetry operation invariance

These symmetries must be enforced for any

FC model to be physically correct

ij ' ' S(i)S( j)

S S ...

Methodology : FP-DFT calculations

in a supercell (real space)

Supercell size should ideally be larger than the

range of the force constants

0Invariance constraints

FP-DFT Force-displacement data

Extraction of FCs: • Constraints are linear in FCs • Force-displacement relations also linear in FCs

• Over-complete system of linear equations

• FCs are deduced from a SVD algorithm

i i ij j ijk j k

j, jk,

1F u u u ...

Keivan Esfarjani and Harold T. Stokes, Phys. Rev. B 77, 144112 (2008)

Move one Si atom along [111]

How accurate are the obtained FCs?

More validation

Use the FCs to calculate forces for

arbitrary atomic displacements

Denominator s(Poly) s(SW)

DISP=0.1 Ang 205.67 0.046941 0.34490

DISP=0.2 Ang 1166.9 0.082234 0.28324

Error is due to choosing (5,1,1) neighbors in the model 5/23/2012 Phonon School @ UWM 12

Now what ?

Use second derivatives to get phonon

dispersion

Use the third derivatives to get phonon

lifetimes

Use all to do MD

Alternative approach to determining FCs uses

(See talk by Derek Stewart)

5/23/2012 14

FP-DFT in a supercell

Potential

Thermal

Properties

MD-GK Latt

Summary of the approach

Phonon School @ UWM

ZrCoSb Half-heusler; By J. Shiomi, KE, G. Chen, PRB 84 (2011)

Phonon Dispersion

For Si, Graphene, ZrCoSb

d ln V

N. Mingo, K. Esfarjani, D. A. Broido, and D. A. Stewart

Phys. Rev. B 81, 045408 (2010)

K. Esfarjani, G. Chen, and H. T. Stokes

Phys. Rev. B 84, 085204 (2011)

RuleGolden Fermi)(||||2

fi EEfVifiW

Normal process

Umklapp process

lkjiijkl

kjiijk uuuuuuuV

Phonon creation and annihilation operators

a 4-phonon process

Lifetimes due to 3-phonon processes

Thermal conductivity

Use of the relaxation time approximation

Cv per mode Relaxation time

ZrCoSb lifetimes: LD-FGR versus MD

0 2 4 6 8 10 12 14 16

Frequency (Thz)

Velocity autocorrelation (5x5x5 supercell=1000 Si atoms)

5/23/2012 Phonon School @ UWM 18 Shiomi, Esfarjani, Chen, Phys. Rev. B 84, 104302 (2011)

Thermal conductivity of ZrCoSb

Simulations vs Experiments

40/1/1 B

Impurity scattering

Y. Xia et al., J. Appl. Phys. 88, 1952 (2000).

T. Sekimoto et al., Jpn. J. Appl. Phys. 46, L673 (2007).

Shiomi, Esfarjani, Chen, Phys. Rev. B 84, 104302 (2011)

0 0.2 0.4 0

Hf x Zr

1-x CoSb

Hf 1-x

Ti x CoSb

Zr x Ti

1-x CoSb

Alloying effect on half-Heusler thermal conductivity

Experiment

Hf0.5Zr0.5CoSb0.8Sn0.2

(Yan, Nano Lett, 2011)

Transferability of force constants

Only changing mass

16 Wm-1K-1

22 Wm-1K-1

Green Kubo Calculation (300 K)

ZrCoSb

HfCoSb

Shiomi, Esfarjani, Chen, Phys. Rev. B 84, 104302 (2011)

Thermal conductivity of PbTe

Takuma Shiga

Why is thermal conductivity

of PbTe so low

(~2 Wm-1K-1 @300K)?

Rocksalt PbTe

This workW. Cochran, et al., (1966) Freq

Microscopic mechanism of low thermal conductivity in lead telluride

T Shiga, J Shiomi, J Ma, O Delaire, T Radzynski, A Lusakowski, K E, and G Chen

Phys. Rev. B 85, 155203 (2012)

Phonons in lead telluride

Takuma Shiga

Microscopic mechanism of low thermal conductivity in lead telluride

T Shiga, J Shiomi, J Ma, O Delaire, T Radzynski, A Lusakowski, K E, and G Chen

Phys. Rev. B 85, 155203 (2012)

Summary of LD: MFP distribution

MD using the polynomial force field

◦ Of accuracy similar to DFT

◦ Includes anharmonic effects to higher order

◦ Can not describe structural changes

◦ Hard to include surfaces and defects

Ideal for thermal conductivity using GK

Equilibrium Green-Kubo (GK)

Half-Heuslers

0 10 20 30 40 50 600

t (ps)

Silicon

Wave effects in thermal transport

Quantization of thermal conductance

Coherent (ballistic) phonon contribution

Coherent phonon transport

Landauer formula

Transmission formula = 1

d n(T, )G D ( ) v ( )

Experimental samples

12 nm GaAs

12 nm AlAs

7 periods

3 periods

~100 nm Al

350μm GaAs

12 nm GaAs 12 nm AlAs

1 period

5 periods

9 periods

GaAs-[(AlAS)22/(GaAs)22]N=1,…,9 -Al

AlAs AlAs Al GaAs GaAs GaAs

1 unit= 22 layers AlAs+22 layers GaAs

Period length=2*(22*5.4 Ang)=23.76 nm

N-periods (N=1,3,5,7,9); 3x3 cross section

for a total of 1584,…,up to 14256 atoms

(a) (b)

Transmission and Conductivity

Conclusions

The developed force field can be calculated for an arbitrary

crystal structure

It can be used to:

◦ Do LD, get phonon spectra, lifetimes, k (fast)

◦ Do MD, get lifetimes and k including alloy scattering and anharmonic

effects nonperturbatively

◦ Set up the GFs to calculate transmission & conductance

LD faster and more accurate than GK-MD, but valid at low T

In nanostructured materials:

◦ Wave effects are important,

◦ Heat transport can have a large ballistic component

◦ Boundary scattering can be dominant

◦ Optical modes contribution is increased

Acknowledgements

Gang Chen (MIT)

Junichiro Shiomi (U Tokyo) ZrCoSb, PbTe

Takuma Shiga (U Tokyo) PbTe

Maria Luckyanova (MIT) GaAs/AlAs SLs

J. Garg (MIT) GaAs/AlAs phonon lifetimes

TF Luo (Notre Dame) GaAs

Thermal properties from first-principles DFT calculations

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