Pseudogap behavior in single-crystal<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Bi</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CaCu</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn><mml:mo>+</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>probed by Cu NMR

We report that the nuclear spin-lattice relaxation rate divided by temperature, ${1/T}_{1}T,$ has a broad maximum around ${T}^{*}\ensuremath{\sim}210\mathrm{K}$ and \ensuremath{\sim}100 K in underdoped and overdoped single crystals of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) with ${T}_{c}=79$ and 77.3 K, respectively, showing the normal-state spin-gap behavior. One result is that the steep decrease in the ${}^{63}$Cu Knight shift, $K(T),$ has been observed below ${T}_{K}^{*}\ensuremath{\sim}200\mathrm{K}$ and \ensuremath{\sim}100 K, suggesting the normal-state pseudogap behavior in quasiparticle density of states (DOS), which is consistent with the pseudogap behavior below ${T}_{\mathrm{ARPES}}^{*}\ensuremath{\sim}170\mathrm{K}$ revealed on underdoped Bi2212 from studies of angle-resolved photoemission spectroscopy. We propose that the spin-gap behavior in ${1/T}_{1}T$ has the same origin as the pseudogap in the quasiparticle DOS.

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