Average Structure Of Quasicrystals

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Average Structure Of Quasicrystals. José Luis Aragón Vera Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México . Gerardo G. Naumis Instituto de Física , Universidad Nacional Autónoma de México. Rafael A. Barrio

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Average Structure Of Quasicrystals

José Luis Aragón Vera

Centro de Física Aplicada yTecnología Avanzada, Universidad Nacional Autónoma de México.

Gerardo G. Naumis

Instituto de Física, Universidad Nacional Autónoma de México.

Rafael A. Barrio

Instituto de Física, UNAM, México D.F., México

Manuel Torres

Instituto Superior de Investigaciones Científicas

Michael Thorpe

Arizona State University, Tempe, Arizona, USA.

Summary
• The main problem: since quasicrystals lack periodicity, conventional Bloch theory does not apply (electronic and phonon propagation)
• Average structure in 1D and 2D
• Conclusions.

Quasicrystal

A material with sharp diffraction peaks with a forbidden symmetry by crystallography.

They have long-range positional order without periodic translational symmetry

Quasicrystals as projections

5

1

4

x

2

3

( )

1

1

0

-1

1

Since the star is eutactic, there exists an orthonormal basis {e1,e2,...,e5} in R5and a projector P such that P(ei)=ai , i=1,..,5.

The eigenstates  of the one-electron Hamiltonian

H=~2r2/2m + U(r) , where U(r+R)=U(r) for all R in a Bravais lattice, can be chosen to have the form of a plane wave times a function with the periodicity of the lattice:

where

for all R in the Bravais lattice.

Bloch’s theorem

E(k)

k

G

Since quasicrystals lack periodicity, conventional Bloch theory is not useful.

• In a crystal: a Bragg spot in the diffraction pattern can open a gap in the electronic density of states since the wave is diffracted (with such a wave-length, it has the same “periodicity” of the lattice and becomes a standing wave).
• The reciprocal space of a quasicrystal is filled in a dense way with Bragg peaks
• Thus, the density of states is full of singularities (1D), (2D and 3D??) Van Hove singularities

There are however indications that Bloch theory may be applicable in quasiperiodic systems:

Albeit the reciprocal space of quasicrystal is a countable dense set, it has been shown that only very few of the reciprocal-lattice vectors are of importance in altering the overall electronic structure. A.P. Smith and N.W. Ashcroft, PRL59 (1987) 1365.

• To a given quasiperiodic structure we can associate an average structure whose reciprocal is discrete and contains a significant fraction of the scattered intensity of the quasiperiodic structure. J.L. Aragón, Gerardo G. Naumis and M. Torres, Acta Cryst. A 58 (2002) 352.
• Through angle-resolved photoemission on decagonal Al71.8Ni14.8Co13.4 it was found that s-p and d states exhibit band-like behavior with the rotational symmetry of the quasiperiodic lattice. E. Rotenberg et al. NATURE 406 (2000) 602.
A classical experiment

Liquid: Fluorinert FC75

Tiling edge length 8 mmNumber of vertices (wells) 121 Radius of cylindrical wells 1.75 mmDepth of cylindrical wells 2 mmLiquid depth 0.4 mmFrequency 35 Hz

0.00 s

0.04 s

Wave pulse is launched along this direction

0.08 s

Snapshots of transverse waves

0.24 s

S

L

L

S

L

L

L

S

L

L

The quasiperiodic grid

The above quasiperiodic sequence (silver or octonacci) can be generated starting from two steps L and S by iteration of substitution rules:

L!LSLS!L.

L

S

L

L

L

S

L

L

S

L

L

Testing the Bloch-like nature

The quasiperiodically spaced standing waves can be consi- dered quasiperiodic Bloch-like waves if they are generated by discrete Bragg resonances.

1. The quasiperiodic sequence:

G-X

Average structure of a quasicrystal

For phonons: what is the sound velocity? Dynamical structure factor?

where the Green´s function is given by,

S

For a Fibonacci chain the positions are given by:

but:

S

L

L

L

3

1

2

Sound velocity:

G.G. Naumis, Ch. Wang, M.F. Thorpe, R.A. Barrio, Phys. Rev. B59, 14302 (1999)

4

• Select a star-vector:

3

4

3

2

2

1

1

0

0

-1

-1

-2

-2

-3

-3

• The equations of the grid arewhere nj is an integer, x2 R2 and j are shifts of the grid with respect to the origin.

3

0

-4

-3

-2

-1

0

1

2

3

4

2

-1

2

3

The generalized dual method (GDM)

Each region can be indexed by N integers defined by its ordinal position in the grid.

(2,2,-1)

For the direction el, the ordinal coordinates are:

where

ej? is perpendicular to ej and ajk is the area of the rhombus generated by ej and ek.

• Finally, the dual transformation associates to each region the pointt is then a vertex of the tiling.

By considering the pairs (jk), we obtain a formula for the vertex coordinates of a quasiperiodic lattice:

where , and .

A formula for the quasilattice

Gerardo G. Naumis and J.L. Aragón, Z. Kristallogr.218 (2003) 397.

By using the identity , the equation for the quasilattice can be written as

defines an average structure which consists of a superposition of lattices.

is a fluctuation part that we expect to have zero average in the sense that

The average structure

Multiplicity=4

Multiplicity=2

Properties of the average lattice

The reciprocal of the average structure contains a significant fraction of the scattered intensity of the quasiperiodic structure.

The average structure dominates the response for long-wave modes of incident radiation.

The average structure then can be useful to determine the main terms that contribute to define a physically relevant Brillouin zone.

J.L. Aragón, Gerardo G. Naumis and M. Torres, Acta Crystallogr. A58 (2002) 352.