  We present a detailed investigation of the Doppler shift oscillations in a
hot loop during an M7.1 flare on 2014 October 27 observed by the Interface
Region Imaging Spectrograph. The periodic oscillations are observed in the
Doppler shift of Fe XXI 1354.09 A (logT~7.05), and the dominant period is about
3.1 minutes. However, such 3.1-min oscillations are not found in the
line-integrated intensity of Fe XXI 1354.09 A, AIA EUV fluxes, or microwave
emissions. SDO/AIA and Hinode/XRT imaging observations indicate that the
Doppler shift oscillations locate at the hot loop-top region (> 11MK).
Moreover, the differential emission measure (DEM) results show that the
temperature is increasing rapidly when the Doppler shift oscillates, but the
number density does not exhibit the corresponding increases nor oscillations,
implying that the flare loop is likely to oscillate in an incompressible mode.
All these facts suggest that the Doppler shift oscillations at the shorter
period are most likely the standing kink oscillations in a flare loop.
Meanwhile, a longer period of about 10 minutes is identified in the time series
of Doppler shift and line-integrated intensity, GOES SXR fluxes and AIA EUV
light curves, indicating the periodic energy release in this flare, which may
be caused by a slow mode wave.
