  We report a new design of temperature-modulated dilatometer, which obtains
the linear thermal expansion coefficient by measuring the oscillating changes
of the sample's length and temperature by a piezobender and a thermocouple,
respectively. Using an iron-based superconductor KFe$_2$As$_2$ as an example,
we show that this device is able to measure thin samples with high resolutions
at low temperatures and high magnetic fields. Despite its incapability of
giving absolute values, the new dilatometer provides a high-resolution method
to study many important physical properties in condensed matter physics, such
as thermal and quantum phase transitions, and vortex dynamics in the
superconducting state. The prototype design of this device can be further
improved in many aspects to meet particular requirements.
