( ( BACKGROUND Superconductors ) ( ( are ( materials ( ( in which ) ( ( the ( electrical resistivity ) ) ( is ( zero ( when ( ( ( the temperature ) ( of ( the material ) ) ) ( is ( less ( than ( ( ( a ( critical temperature ) ) , ) Tc ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP BACKGROUND) (NNS Superconductors)) (VP (VBP are) (NP (NP (NNS materials)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (DT the) (JJ electrical) (NN resistivity)) (VP (VBZ is) (NP (NP (CD zero)) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN temperature)) (PP (IN of) (NP (DT the) (NN material)))) (VP (VBZ is) (ADJP (ADJP (JJR less)) (PP (IN than) (NP (NP (DT a) (JJ critical) (NN temperature)) (, ,) (NP (NNP Tc)))))))))))))) (. .)))
BACKGROUND Superconductors are materials in which the electrical resistivity is zero when the temperature  of the material is less than a critical temperature, Tc.
(ROOT (S (NP (NNS Superconductors)) (VP (VBP are) (NP (NP (NNS materials)) (PP (IN with) (NP (NP (DT a) (NN resistivity)) (PP (IN of) (NP (CD zero))))))) (. .)))