# INVARIANTS Help

Harold T. Stokes and Dorian M. Hatch, Department of Physics and Astronomy, Brigham Young University, Provo, Utah, 84602, USA, stokesh@byu.edu

INVARIANTS produces a list of invariant polynomials of the components of order parameters associated with one or more irreducible representations of a space group.

## Background

In a crystalline phase transition, the space-group symmetry of the high- and low-symmetry phases often exhibit a group-subgroup relationship. In the Landau theory of phase transitions, the distortion which accompanies the transition is described by a primary order parameter (OP). Such an OP is an n-dimensional vector in the space defined by an irreducible representation (irrep) of the space group H of the high-symmetry phase. In addition to the primary OP, there are also secondary OPs belonging to other irreps of H and corresponding to addtional distortions that accompany the transition.

The free energy of the crystal (often called the Landau potential) in both the high- and low-symmetry phases is written as a linear combination of sets of invariant polynomials in the components of the order parameters. Each polynomial is invariant under the action of any operator in H. Some polynomials contain components of both the primary and secondary OPs and describe the coupling between those OPs. The purpose of this program is to generate the invariant polynomials for a given set of OPs.

A more complete discussion of the role of invariants, including example polynomial forms, can be found in:

(1) H. T. Stokes and D. M. Hatch, Isotropy Subgroups of the 230 Crystallographic Space Groups, (World Scientific, Singapore, 1988).

(2) A. Saxena, G. R. Barsch, and D. M. Hatch, Phase Transitions 46, 89 (1994).

(3) H. T. Stokes and D. M. Hatch, Phase Transitions 34, 53 (1991).

## Input and Output

Step 1

Choose a space group from a list. We use the notation of International Tables of Crystallography.

Step 2

Choose a point in the first Brillouin zone from a list.

Points in the first Brillouin zone and irreducible representations are given using the notation of Miller and Love (S. C. Miller, and W. F. Love, Tables of Irreducible Representations of Space Groups and Co-Representations of Magnetic Space Groups, Pruett, Boulder, 1967; see also A. P. Cracknell, B. L. Davies, S. C. Miller, and W. F. Love, Kronecker Product Tables,, Vol. 1, Plenum, New York, 1979) followed by the notation of Kovalev (O. V. Kovalev, Representations of the Crystallographic Space Groups: Irreducible Representations, Induced Representations and Corepresentations, Gordon-Breach, Amsterdam, 1993). The Kovalev notation is given only for points of symmetry (no parameters in the coordinates of the point). We have not yet worked out the correspondance between the Miller-Love and Kovalev notations for the other points in the first Brillouin zone.

The coordinates for the points in the first Brillouin zone are given in terms of "conventional" vectors of the reciprocal lattice, as defined by Miller and Love. Such vectors are derived from the conventional unit cell in real space, as given in the International Tables for Crystallography. For example, the conventional unit cell for cubic space groups is a cube of side a, and therefore the conventional vectors of the reciprocal lattice are mutually orthogonal with lengths 2pi/a.

Some points require one or more additional parameters to specify it. For example, the delta point in the simple cubic lattice is (0,a,0) which means anywhere along the line from the gamma point (0,0,0) to the X point (0,1/2,0). If you want to specify the point half-way between, then enter 1/4 for the variable a. All points must be specified using rational numbers, i.e., 1/4 instead of 0.25.

Step 3

Choose an irreducible representation (irrep) from a list. See Step 2 above for an explanation about the notation used.

Step 4

Choose a direction of the order parameter from a list. The variables a,b,c,... have arbitrary values. (These variables should not be confused with the variables a,b,c in Step 2 used for specifying a point in the first Brillouin zone.)

Each direction of the order parameter listed determines a structure of different symmetry. The space-group symmetry (called the isotropy subgroup) determined by the order parameter is listed following the direction, along with the basis vectors of its lattice (in terms of the basis vectors of the parent space group) and the origin of the standard setting of the subgroup (relative to the origin of the parent). The invariant polynomials will be evaluated for the direction selected. If you want complete invariant polynomials as a function of every component of the order parameter, then choose the general direction of the order parameter (the last item on the list).

The labeling (P1, P2, C1, etc.) for the direction of order parameters is from Stokes and Hatch (H. T. Stokes and D. M. Hatch, Isotropy Subgroups of the 230 Crystallographic Space Groups, World Scientific, Singapore, 1988).

The directions of order parameters for the case of points of symmetry in the first Brillouin zone are obtained from a data base. For other points in the first Brillouin zone, these directions are obtained by calculating the isotropy subgroups associated with the irreducible represenation chosen. In some cases this calculation may require a few minutes. You just have to be patient and wait for the results, even if the browser claims to be "stalled". In some rare cases, this calculation could require hours. However, once the calculation has been completed, the results are stored in a file which the program accesses the next time you want results for the same irreducible representation.

Step 5

Choose one of the equivalent directions of the order parameter.

Each direction of the order parameter has associated with it a number of other equivalent directions. Equivalent directions are obtained by applying space-group operators to an order parameter. (If the order parameter represents a phase transition, then these equivalent order parameters belong to different domains that arise from the same phase transition.) The invariant polynomials for equivalent order parameters are identical, and it makes no difference which one you select (except for the case of coupled order parameters, which are discussed in the next step). We list only equivalent directions which are distinct, i.e., are not obtained by just redefining the variables a,b,c,... The numbers in parentheses refer to the numbering of domains by ISOTROPY.

Step 6

At this step, you may choose to add an order parameter associated with another irrep. These are called coupled order parameters. It will result in some invariant polynomials which contain components of order parameters from more than one irrep. (In phase transitions, these coupling polynomials are responsible for the activation of secondary order parameters.) You will be returned to Step 2 so that you can choose the irrep and direction of the additional order parameter.

At this step, you may also choose to finish and generate the invariant polynomials for the order parameter(s) chosen thus far. Here, you may also choose the degree(s) of polynomials you would like to generate.

Invariant Results

The invariant polynomials are calculated for each request and are not stored in files for future use. The calculation may require a few minutes for some cases with many degrees of freedom. Some cases may even require hours. For this reason, we have restricted the list of order parameter directions to those with no more than eight degrees of freedom. If you really need invariant polynomials with more than eight degrees of freedom, you should use the ISOTROPY program itself.

## Examples

### Single order parameter for an irrep at a k point of symmetry

Space Group: 221 Pm-3m Oh-1
Point in first Brillouin zone: GM, k12 (0,0,0)
Irreducible Representation: GM4-, k12t10
Order Parameter: S1(1) (a,b,c)

Display invariant polynomials with degree 1 through degree 4

Invariant Polynomials

Degree 2

a2 + b2 + c2

Degree 4

a4 + 2 a2 b2 + 2 a2 c2 + b4 + 2 b2 c2 + c4

a4 + b4 + c4

### Single order parameter for an irrep on a k line of symmetry

Space Group: 148 R-3 C3i-2
Point in first Brillouin zone: LD (0,0,3a)
Parameters: a = 1/3
Irreducible Representation: LD2LD3
Order Parameter: 4D1(1) (a,b,c,d)

Display invariant polynomials with degree 1 through degree 4

Invariant Polynomials

Degree 2

a2 + b2 + c2 + d2

Degree 3

a3 - 3 a c2 + b3 - 3 b d2

3 a2 c + 3 b2 d - c3 - d3

Degree 4

a4 + 2 a2 b2 + 2 a2 c2 + 2 a2 d2 + b4 + 2 b2 c2 + 2 b2 d2 + c4 + 2 c2 d2 + d4

a4 + 2 a2 c2 + b4 + 2 b2 d2 + c4 + d4

### Subspace of a single order parameter

Space Group: 228 Fd-3c Oh-8
Point in first Brillouin zone: W, k8 (1/2,1,0)
Irreducible Representation: W1W2, k8t1t2
Order Parameter: C2(1) (a,0,a,0,0,0,0,0,0,0,0,0,b,0,b,0,0,0,0,0,0,0,0,0)

Display invariant polynomials with degree 1 through degree 4

Invariant Polynomials

Degree 2

a2 + b2

Degree 4

a4 + 2 a2 b2 + b4

a4 + b4

a3 b - a b3

### Coupled ordered parameters

Space Group: 221 Pm-3m Oh-1

Irrep 1: Point in first Brillouin zone: M, k11 (1/2,1/2,0)
Irreducible Representation: M3+, k11t3
Order Parameter: S1(1) (a,b,c)

Irrep 2: Point in first Brillouin zone: R, k13 (1/2,1/2,1/2)
Irreducible Representation: R4+, k13t9
Order Parameter: S1(1) (d,e,f)

Display invariant polynomials with degree 1 through degree 4

Invariant Polynomials

Degree 2

a2 + b2 + c2

d2 + e2 + f2

Degree 4

a4 + 2 a2 b2 + 2 a2 c2 + b4 + 2 b2 c2 + c4

a2 d2 + a2 e2 + a2 f2 + b2 d2 + b2 e2 + b2 f2 + c2 d2 + c2 e2 + c2 f2

d4 + 2 d2 e2 + 2 d2 f2 + e4 + 2 e2 f2 + f4

a4 + b4 + c4

a2 d2 + b2 e2 + c2 f2

d4 + e4 + f4