The discovery of superconductivity in infinite-layer nickelate films marks a groundbreaking addition to the family of unconventional superconductors,providing new insights into mechanism of unconventional high tempera...The discovery of superconductivity in infinite-layer nickelate films marks a groundbreaking addition to the family of unconventional superconductors,providing new insights into mechanism of unconventional high temperature superconductivity.However,synthesizing these superconducting nickelates presents significant challenges:they cannot be grown directly and instead require a two-step synthesis protocol involving initial deposition of a perovskite precursor phase(e.g.,Nd_(0.8)Sr_(0.2)NiO_(3))followed by topotactic reduction to the infinite-layer structure(Nd_(0.8)Sr_(0.2)NiO_(2)).This process is further complicated by the extreme sensitivity of both steps to synthesis conditions,necessitating stringent control over the crystallinity and stoichiometry of the parent phase.In this study,we uncover nickel deficiency during pulsed laser deposition(PLD)of the parent-phase Nd_(0.8)Sr_(0.2)NiO_(3).By incorporating 15% excess nickel into the PLD target,we mitigate this loss,suppress secondary phase formation in the Nd_(0.8)Sr_(0.2)NiO_(3) parent film,and ultimately obtain a phase-pure Nd_(0.8)Sr_(0.2)NiO_(2) film exhibiting superconductivity after following reduction.Notably,we observe a doping-dependent insulator-to-superconductor transition in films synthesized from targets with varying nickel content after reduction.X-ray photoelectron spectroscopy(XPS)confirms that the Nd/Ni ratio in films derived from nickel-over-doped targets(15% excess)aligns closely with the ideal stoichiometry.These findings underscore the indispensable role of stoichiometric precision in stabilizing infinite-layer nickelates and establish a practical synthesis strategy for optimizing their superconducting performance.展开更多
Ta_(2)NiSe_(5) is a promising candidate for hosting an excitonic insulator(EI)phase,a novel electronic state driven by electron-hole Coulomb attraction.However,the role of electron-lattice coupling in the formation of...Ta_(2)NiSe_(5) is a promising candidate for hosting an excitonic insulator(EI)phase,a novel electronic state driven by electron-hole Coulomb attraction.However,the role of electron-lattice coupling in the formation of the EI phase remains controversial.Here,we use angle-resolved photoemission spectroscopy(ARPES)to study the band structure evolution of Ta_(2)Ni(Se_(1-x)S_(x))_(5) with sulfur substitution and potassium deposition,which modulate the band gap and the carrier concentration,respectively.We find that the Ta 5d states originating from the bottom of the conduction band persist at the top of the valence band in the low-temperature monoclinic phase,indicating the importance of exciton condensation in opening the gap in the semi-metallic band structure.We also observe that the characteristic overlap between the conduction and valence bands can be restored in the monoclinic lattice by mild carrier injection,suggesting that the lattice distortion in the monoclinic phase is not the main factor for producing the insulating gap,but rather the exciton condensation in the electronic system is the dominant driving force.Our results shed light on the electron-lattice decoupling and the origin of the EI phase in Ta_(2)Ni(Se_(1-x)Sx)_(5).展开更多
基金supported by the National Key R&D Program of China(Grant No.2022YFA1403000)CAS Project for Young Scientists in Basic Research(Grant No.YSBR-100)+2 种基金the Fundamental Research Funds for the Central Universities(Grant No.WK2140000019)the National Natural Science Foundation of China(Grant Nos.52272095 and 12275272)Collaborative Innovation Program of Hefei Science Center,Chinese Academy of Sciences(Grant No.2022HSC-CIP005).
文摘The discovery of superconductivity in infinite-layer nickelate films marks a groundbreaking addition to the family of unconventional superconductors,providing new insights into mechanism of unconventional high temperature superconductivity.However,synthesizing these superconducting nickelates presents significant challenges:they cannot be grown directly and instead require a two-step synthesis protocol involving initial deposition of a perovskite precursor phase(e.g.,Nd_(0.8)Sr_(0.2)NiO_(3))followed by topotactic reduction to the infinite-layer structure(Nd_(0.8)Sr_(0.2)NiO_(2)).This process is further complicated by the extreme sensitivity of both steps to synthesis conditions,necessitating stringent control over the crystallinity and stoichiometry of the parent phase.In this study,we uncover nickel deficiency during pulsed laser deposition(PLD)of the parent-phase Nd_(0.8)Sr_(0.2)NiO_(3).By incorporating 15% excess nickel into the PLD target,we mitigate this loss,suppress secondary phase formation in the Nd_(0.8)Sr_(0.2)NiO_(3) parent film,and ultimately obtain a phase-pure Nd_(0.8)Sr_(0.2)NiO_(2) film exhibiting superconductivity after following reduction.Notably,we observe a doping-dependent insulator-to-superconductor transition in films synthesized from targets with varying nickel content after reduction.X-ray photoelectron spectroscopy(XPS)confirms that the Nd/Ni ratio in films derived from nickel-over-doped targets(15% excess)aligns closely with the ideal stoichiometry.These findings underscore the indispensable role of stoichiometric precision in stabilizing infinite-layer nickelates and establish a practical synthesis strategy for optimizing their superconducting performance.
基金supported by the National Natural Science Foundation of China(Grant No.U2032153)the National Key R&D Program of China(Grant No.2017YFA0402901)+3 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB25000000)the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302802)the Users with Excellence Program of Hefei Science Center of the Chinese Academy of Sciences(Grant No.2021HSC-UE004)the Fundamental Research Funds for the Central Universities(Grant No.WK2310000104)。
文摘Ta_(2)NiSe_(5) is a promising candidate for hosting an excitonic insulator(EI)phase,a novel electronic state driven by electron-hole Coulomb attraction.However,the role of electron-lattice coupling in the formation of the EI phase remains controversial.Here,we use angle-resolved photoemission spectroscopy(ARPES)to study the band structure evolution of Ta_(2)Ni(Se_(1-x)S_(x))_(5) with sulfur substitution and potassium deposition,which modulate the band gap and the carrier concentration,respectively.We find that the Ta 5d states originating from the bottom of the conduction band persist at the top of the valence band in the low-temperature monoclinic phase,indicating the importance of exciton condensation in opening the gap in the semi-metallic band structure.We also observe that the characteristic overlap between the conduction and valence bands can be restored in the monoclinic lattice by mild carrier injection,suggesting that the lattice distortion in the monoclinic phase is not the main factor for producing the insulating gap,but rather the exciton condensation in the electronic system is the dominant driving force.Our results shed light on the electron-lattice decoupling and the origin of the EI phase in Ta_(2)Ni(Se_(1-x)Sx)_(5).
