Strain-induced topological modulation and phonon hourglass like nodal surfaces in square-net Dirac semimetal CsCdSb

We present a first-principles investigation of the electronic and phononic topological
properties of the ternary compound CsCdSb, with particular emphasis on the role
of crystalline symmetry and external strain. In the absence of spin-orbit coupling
(SOC), the electronic band structure hosts symmetry-protected nodal lines located in
the mirror-invariant planes at kz = 0 and kz = π, forming distinct ring- and petal-like
configurations in momentum space. The system hosts nodal lines in the absence of
spin-orbit coupling, which acquire finite energy gaps once SOC is included. Under
compressive strain, corresponding to an experimentally accessible pressure of about
1.93 GPa at -5% deformation, the system enters a topologically nontrivial phase
confirmed by the calculated Z2 invariant. The phonon spectrum exhibits symmetry-
enforced hourglass-type band connectivity arising from the interplay of nonsymmorphic
and mixed symmorphic and nonsymmorphic crystal symmetries. These symmetry
constraints generate hourglass like nodal loops and extended degeneracies that
collectively form a nodal manifold in momentum space. Along generic momentum
paths, symmetry-driven partner switching produces hourglass like crossings whose
neck points define a hybrid nodal surface bounded by a quadratic nodal line.
Our results demonstrate that CsCdSb provides a suitable platform for studying
symmetry-protected topological phenomena in both electronic and phononic sectors,
with strain-induced tunability confined to the electronic topology. These findings
offering promising prospects for fundamental studies and potential multifunctional
applications.