  The recent discovery of superconductivity in the quasi-one-dimensional
compound K$_2$Cr$_3$As$_3$, which consists of double-walled tubes of
[(Cr$_3$As$_3$)$^{2-}]^\infty$ that run along the c axis, has attracted
immediate attention as a potential system for studying superconductors with
reduced dimensionality. Here we report clear experimental evidence for the
unconventional nature of the superconducting order parameter in
K$_2$Cr$_3$As$_3$, by precisely measuring the temperature dependence of the
change in the penetration depth $\Delta\lambda(T)$ using a tunnel diode
oscillator. Linear behavior of $\Delta\lambda(T)$ is observed for $T\ll T_c$,
instead of the exponential behavior of conventional superconductors, indicating
that there are line nodes in the superconducting gap. This is strong evidence
for unconventional behavior and may provide key information for identifying the
pairing state of this novel superconductor.
