Finding real roots of cubic polynomial

I want to find the only one real roots of the equation and wrote the following code:

e = 0.3; gama = 0.8; damp = 0.0031; A = 12; M = 2*10^-4; mu = 0.05/M; St = 0.2; %parameters
Ur = 4:0.1:6;
for i = 1:size(Ur,2)
    del(i) = 1/(St*Ur(i));
    p(i,:)=roots([1   (-(1+2*del(i)^2-(2*damp*del(i)+(gama/mu))^2)+A*M)   (-(-2*del(i)^2+(2*damp*del(i)+(gama/mu))^2-del(i)^4)-A*M*del(i)^2)   (-del(i)^4)]);
    r = p;
    r = r(imag(r) == 0 );
end

Only one real root corresponding to each 'del(i)' is required but in some iteration, 'del(i)' leads to all three real roots of 'p'. (If all the three roots are real, max value of roots out of three would be considered)

How can I improve my code?

Answer 1

[Andrei Bobrov]

e = 0.3;
gama = 0.8;
damp = 0.0031;
A = 12;
M = 2*10^-4;
mu = 0.05/M;
St = 0.2;
AM = A*M; 
%parameters
Ur = (4:0.1:6)';
n = numel(Ur);
del = 1./(St*Ur);
del2 = del.^2;
B = 2*del2-(2*damp*del+gama/mu).^2;
X = [ones(n,1) ,  AM-1-B ,  B+del2.^2-AM*del2   ,-del2.^2];
r = zeros(n,1);
for i = 1:n
    p = roots(X(i,:));
    r(i) = max(p(imag(p) == 0 ));
end

[S. R. S.]

Thank you so much Andrei Bobrov for your brilliant work. The code served my purpose.