add a function to calculate berry curvature for each band and k along…

… a kpath

examples/Haldane_model/Berrycurvature_line.gnu0

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+# requirement: gnuplot version>5.4
+# Please open the data file to check the data:   Berrycurvature_line.dat
+set terminal pdf enhanced color font ",24"
+set palette defined ( 0  "green", 5 "yellow", 10 "red" )
+set output 'Berrycurvature_line.pdf' 
+set style data linespoints
+unset key
+set pointsize 0.8
+#set xtics font ",24"
+#set ytics font ",24"
+#set ylabel font ",24"
+set ylabel offset 0.5,0
+set border lw 2
+set xrange [0:    7.30156]
+emin=   -0.110000
+emax=    0.110000
+set ylabel "Energy (eV)"
+set yrange [ emin : emax ]
+set xtics ("M  "    0.00000,"K' "    3.05921,"G  "    4.75615,"K  "    6.45309,"M  "    7.30156)
+set arrow from    3.05921, emin to    3.05921, emax nohead lw 2
+set arrow from    4.75615, emin to    4.75615, emax nohead lw 2
+set arrow from    6.45309, emin to    6.45309, emax nohead lw 2
+# please comment the following lines to plot the fatband 
+# uncomment the following lines to plot the fatband 
+plot 'Berrycurvature_line.dat' u 1:2:5  w lp lw 2 pt 7  ps 0.2 lc palette, 0 w l lw 2 dt 2
+# uncomment the following lines to plot the spin if necessary
+#plot 'Berrycurvature_line.dat u 1:2 w lp lw 2 pt 7  ps 0.2, \
+#plot 'Berrycurvature_line.dat u 1:2:($3/6):($4/6) w vec