I need to list files in alphabetical-group and each such group listed in increasing order of timestamp.

Example : If I list files in a directory ( ls -lrt ) it shows like below.

c3
b4
b3
a4
a3
c2
b2
c1
b1
a2
a1

What I need is

c3
c2
c1
b4
b3
b2
b1
a4
a3
a2
a1

How can this be achieved using a command. ? Please help ?