>> TGS_phase_analysis('V_single_crystal-2019-06-30-06.40um-15degrees#2-POS-1.txt','V_single_crystal-2019-06-30-06.40um-15degrees#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 =     0.05137  (0.05102, 0.05172)
       k =     2.1e-05  (2.072e-05, 2.127e-05)
       beta =   5.034e-13  (fixed at bound)
       B =    0.004856  (0.004677, 0.005035)
       p =     -0.1412  (-0.1666, -0.1158)
       t =   6.027e-08  (5.704e-08, 6.349e-08)
       D =    0.001643  (0.001577, 0.00171)
     Problem parameters:
       q =   9.817e+05
       start_time =     2.2e-09
       peak_freq =   3.958e+08
Bad fit for:V_single_crystal-2019-06-30-06.40um-15degrees#2-POS-1.txt~re: beta

ans =

   3.9584e+08