Superconductivity in LaNiPO is disrupted by small (~5%) amounts of non-stoichiometry on the lanthanum site, even though the electronic contribution to the heat capacity increases with increasing non-stoichiometry. All samples also exhibit specific heat anomalies consistent with the presence of ferromagnetic spin fluctuations (Tsf ~ 14 K). Comparison of layered nickel phosphide and nickel borocarbide superconductors reveals different structure-property correlations in the two families.
The recently discovered iron arsenide superconductors, which display superconducting transition temperatures as high as 55 K, appear to share a number of general features with high-Tc cuprates, including proximity to a magnetically ordered state and robustness of the superconductivity in the presence of disorder. Here we show that superconductivity in Fe1+dSe, the parent compound of the superconducting arsenide family, is destroyed by very small changes in stoichiometry. Further, we show that non-superconducting Fe1+dSe is not magnetically ordered down to low temperatures. These results sugges…
The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high quality nickelate films using molecular beam epitaxy (MBE). We found that stoichiometry is crucial in optimizing the crystalline structure and realizing…
We report synthesis of high quality Fe1+y(Te1-xSx)z single crystals and a comprehensive study of structural, magnetic and transport properties. There is high sensitivity to material stoichiometry which includes vacancies on the Te(S) site. Our results reveal competition and coexistence of magnetic order and percolative superconductivity for x >= 0.03, while zero resistivity is acheived for x >= 0.1.
Reversible tuning of magnetic order and intrinsic superconductivity in strained FeTe films via stoichiometry control
Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Renmin University of China（中国人民大学）；Shanxi Normal University（山西师范大学）；Tsinghua University（清华大学）
Synthesis of monolayer FeSe on SrTiO3, with greatly enhanced superconductivity compared to bulk FeSe, remains difficult. Lengthy annealing within a certain temperature window is always required to achieve superconducting samples as reported by different groups around the world, but the mechanism of annealing in inducing superconductivity has not been elucidated. We grow FeSe films on SrTiO3 by molecular beam epitaxy and adjust the stoichiometry by depositing additional small amounts of Fe atoms. The monolayer films become superconducting after the Fe deposition without any annealing, and show …
Bulk crystals of electron-doped cuprates with the T'-type structure require both Ce substitutions and reduction annealing for the emergence of superconductivity while the reduction annealing alone can induce superconductivity in thin films of the T'-type cuprates. In order to reveal low-energy electronic states which are responsible for the superconductivity, we have conducted angle-resolved photoemission spectroscopy measurements on thin films of the superconducting Ce-free T'-type cuprate Pr$_2$CuO$_4$. The results indicate that the overall band structure and the Fermi surface area of the su…
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Department of Physics, Stanford University（斯坦福大学物理系）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；School of Applied and Engineering Physics, Cornell University（康奈尔大学应用与工程物理学院）；Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory（斯坦福同步辐射光源，SLAC国家加速器实验室）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Kavli Institute at Cornell for Nanoscale Technology, Cornell University（康奈尔大学卡弗里纳米技术研究所）；Geballe Laboratory for Advanced Materials, Stanford University（斯坦福大学盖巴尔先进材料实验室）；State Key Laboratory of Surface Physics and Department of Physics, Fudan University（复旦大学表面物理国家重点实验室和物理系）
In this letter, we reported the results of NMR study on LiFeAs single crystals. We find a strong evidence of the low temperature spin fluctuations; by changing sample preparation conditions, the system can be tuned toward an spin-density-wave (SDW) quantum-critical point. The detection of an interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests that the off-stoichiometry and/or lattice defect can probably account for the absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a strongly correlated system and the superconductivity is likely originated from t…
The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range ant…
Antisite defects play a critical role in Nb$_3$Sn superconducting radio frequency (SRF) cavity physics. Such defects are the primary form of disorder in Nb$_3$Sn, and are responsible for stoichiometry variations, including experimentally observed tin-depleted regions within grains and tin-rich regions around grain boundaries. But why they cluster to form regions of different stoichiometries and how they affect the SRF properties of Nb$_3$Sn cavities are not fully understood. Using $\textit{ab initio}$ techniques, we calculate the A15 region of the Nb-Sn phase diagram, discuss a possible modifi…
We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a monotonous decrease of the lattice parameters and of the c/a ratio, but no structural transition down to parameters smaller than those of AlB2. The transition superconducting temperature also decreases with temperature in a sample dependent way. The results are explained by an increase of the filling of the 2D pxy bands with pressure, the Mg stoichiometry determining the starting position of the Fermi level. Our measurements indicate that these hole bands are the relevant ones for superconductivity.
Nanoscale Spatial Tuning of Superconductivity in Cuprate Thin Films via Direct Laser Writing
Politecnico di Milano（米兰理工大学）；Institut de Ciència de Materials de Barcelona, ICMAB-CSIC（巴塞罗那材料科学研究所）；Universitat Autònoma de Barcelona（巴塞罗那自治大学）；Université de Liège（列日大学）
We report fully momentum dependent, self-consistent calculations of the gap symmetry, Fermi surface (FS) anisotropy and Tc of superconducting (SC) LiFeAs using the experimental band structure and a realistic small-q electron phonon interaction within the framework of Migdal-Eliashberg theory. In the stoichiometric regime, we find the exact s++ gap as reported by ARPES. For slight deviations from stoichiometry towards electron doping, we find that a chiral triplet p_x+ip_y state stabilizes near Tc and that at lower temperatures a transition from the triplet to singlet s+- SC takes place. Furthe…
The iron-based superconductors had a significant impact on condensed matter physics. They have a common structural motif of a two-dimensional square iron lattice and exhibit fruitful physical properties as a strongly correlated electron system. During the extensive investigations, quasi-one-dimensional iron-based spin-ladder compounds attracted much attention as a platform for studying the interplay between magnetic and orbital ordering. In these compounds, BaFe2S3 and BaFe2Se3 were found to exhibit superconductivity under high pressure, having a different crystal and magnetic structure at low…
In many cases, unconventional superconductivity are realized by suppressing another order parameter, such as charge density wave (CDW) or spin density wave (SDW). This suggests that the fluctuations of these order parameters play an important role in producing superconductivity. LaPt$_2$Ge$_2$ undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at $T_{\rm s}$ = 394 K, accompanying a double period modulation in the $a$-axis direction, and superconducts at $T_{\rm c}$ = 0.41 K. We performed band calculations and found 2D (two dimensional)-like Fermi surfaces with partial nesti…
A systematic study of the superconducting properties in a series of arc-melted Nb-B samples close to the 1:1 composition was carried out. Powder X-ray diffraction (XRD) shows that all samples are both non-stoichiometric, and comprising of two crystal phases: a majority orthorhombic NbB-type phase, and traces of a minor body-centered cubic Nb-rich phase Nb$_{ss}$ with stoichiometry close to Nb$_{0.98}$B$_{0.02}$. The emergence of superconductivity near T$_c \sim$ 9.0 K was inferred from magnetization data in chunk and powder samples. However, the very small superconducting volume fractions are …
Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Department of Physics, Stanford University（斯坦福大学物理系）；National Synchrotron Light Source II, Brookhaven National Laboratory（美国国家同步辐射光源II，布鲁克海文国家实验室）；Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory（斯坦福同步辐射光源，SLAC国家加速器实验室）；Advanced Light Source, Lawrence Berkeley National Laboratory（先进光源，劳伦斯伯克利国家实验室）；Canadian Light Source（加拿大光源）；Department of Physics, University of Florida（佛罗里达大学物理系）；Department of Materials Science and Engineering, Stanford University（斯坦福大学材料科学与工程系）
Charge distribution and magnetism in bilayer La$_3$Ni$_2$O$_7$: a hybrid functional study
University of Notre Dame（圣母大学）；Stavropoulos Center for Complex Quantum Matter, University of Notre Dame（圣母大学斯塔夫罗普洛斯复杂量子物质中心）；Department of Physics & Astronomy, University of British Columbia（不列颠哥伦比亚大学物理与天文学系）；Stewart Blusson Quantum Matter Institute, University of British Columbia（不列颠哥伦比亚大学斯图尔特·布鲁森量子物质研究所）
