>> TGS_phase_analysis('Cr_single_crystal-2019-06-30-06.40um-35degrees#2-POS-1.txt','Cr_single_crystal-2019-06-30-06.40um-35degrees#2-NEG-1.txt',6.4,1)

f2 = 

     General model:
     f2(x) = A.*(erfc(q*sqrt(k*(x+start_time)))-beta*exp(-q^2*k*(x+start_time)
                    )./sqrt((x+start_time)))+B.*sin(2*pi*(peak_freq)*(x+
                    start_time)+p)*exp(-(x+start_time)/t)+D
     Coefficients (with 95% confidence bounds):
       A =     0.01861  (0.01844, 0.01878)
       k =   2.987e-05  (2.94e-05, 3.035e-05)
       beta =    7.22e-11  (fixed at bound)
       B =     0.00855  (0.008432, 0.008669)
       p =       3.113  (3.103, 3.123)
       t =   2.745e-08  (2.689e-08, 2.802e-08)
       D =   -0.000228  (-0.0002522, -0.0002038)
     Problem parameters:
       q =   9.817e+05
       start_time =       9e-10
       peak_freq =   5.475e+08
Bad fit for:Cr_single_crystal-2019-06-30-06.40um-35degrees#2-POS-1.txt~re: beta

ans =

   5.4748e+08