Qing Li

Bio: Qing Li is an academic researcher from Nanjing University. The author has contributed to research in topics: Superconductivity & Physics. The author has an hindex of 9, co-authored 23 publications receiving 293 citations.

Absence of superconductivity in bulk Nd 1−x Sr x NiO 2

Abstract: Superconductivity at 9–15 K was recently discovered in Nd0.8Sr0.2NiO2 films. Since the Ni1+ ionic state in NdNiO2 may have the same 3d9 outer-shell electronic orbital as in cuprate superconductors, it is interesting to know whether superconductivity has a similar mechanism in these two systems. Here we synthesize bulk samples of Nd1−xSrxNiO2 (x = 0, 0.2, 0.4) with inhomogeneous Sr distribution and Ni deficiency. Resistivity measurements show insulating behavior without the presence of superconductivity, different to the previously reported films. Although applying pressure up to about 50.2 GPa significantly suppresses the insulating behavior, superconductivity remains absent. The magnetization behavior exhibits a Curie–Weiss law with a paramagnetic moment of about 2 μB/f.u. Since the lattice constants derived from our diffraction data are very close to the previously reported superconducting Nd0.8Sr0.2NiO2 films, we suggest that superconductivity in that system may have arisen from interface or stress-related effects, or nickel deficiency in our bulk samples that might prevent the emergence of superconductivity. Superconductivity was recently observed in Nd0.8Sr0.2NiO2 films. Here, superconductivity is found to be absent in bulk polycrystalline samples of Sr-doped NdNiO2, raising questions regarding the origin of superconductivity in this material.

Two superconducting components with different symmetries in Nd1-xSrxNiO2 films

Abstract: The pairing mechanism in cuprates remains as one of the most challenging issues in the field of condensed matter physics. The unique 3d9 electron orbital of the Cu2+ ionic states in cuprates is supposed to be the major player for the occurrence of superconductivity. Recently, superconductivity at about 9-15 K was discovered in infinite layer thin films of nickelate Nd1-xSrxNiO2 (x=0.1-0.2) which is believed to have the similar 3d9 orbital electrons. The key issue concerned here is about the superconducting gap function. Here we report the first set data of single particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a V-shape feature which can be fitted very well by a d-wave gap function with gap maximum of about 3.9 meV, another one exhibits a full gap of about 2.35 meV. Some spectra demonstrate mixed contributions of these two components. Our results suggest that the newly found Ni-based superconductors play as close analogs to cuprates, and thus demonstrate the commonality of unconventional superconductivity.

Twofold symmetry of $c$-axis resistivity in topological kagome superconductor CsV$_3$Sb$_5$ with in-plane rotating magnetic field

Abstract: In transition metal compounds, due to the interplay of charge, spin, lattice and orbital degrees of freedom, many intertwined orders exist with close energies. One of the commonly observed states is the so-called nematic electron state, which breaks the in-plane rotational symmetry. This nematic state appears in cuprates, iron-based superconductor, etc. Nematicity may coexist, affect, cooperate or compete with other orders. Here we show the anisotropic in-plane electronic state and superconductivity in a recently discovered kagome metal CsV$_3$Sb$_5$ by measuring $c$-axis resistivity with the in-plane rotation of magnetic field. We observe a twofold symmetry of superconductivity in the superconducting state and a unique in-plane nematic electronic state in normal state when rotating the in-plane magnetic field. Interestingly these two orders are orthogonal to each other in terms of the field direction of the minimum resistivity. Our results shed new light in understanding non-trivial physical properties of CsV$_3$Sb$_5$.

Twofold symmetry of c-axis resistivity in topological kagome superconductor CsV3Sb5 with in-plane rotating magnetic field.

Abstract: In transition metal compounds, due to the interplay of charge, spin, lattice and orbital degrees of freedom, many intertwined orders exist with close energies. One of the commonly observed states is the so-called nematic electron state, which breaks the in-plane rotational symmetry. This nematic state appears in cuprates, iron-based superconductor, etc. Nematicity may coexist, affect, cooperate or compete with other orders. Here we show the anisotropic in-plane electronic state and superconductivity in a recently discovered kagome metal CsV3Sb5 by measuring c-axis resistivity with the in-plane rotation of magnetic field. We observe a twofold symmetry of superconductivity in the superconducting state and a unique in-plane nematic electronic state in normal state when rotating the in-plane magnetic field. Interestingly these two orders are orthogonal to each other in terms of the field direction of the minimum resistivity. Our results shed new light in understanding non-trivial physical properties of CsV3Sb5.

Single particle tunneling spectrum of superconducting Nd1-xSrxNiO2 thin films.

Abstract: The pairing mechanism in cuprates remains as one of the most challenging issues in condensed matter physics. Recently, superconductivity was discovered in thin films of the infinite-layer nickelate Nd1-xSrxNiO2 (x = 0.12–0.25) which is believed to have the similar 3d9 orbital electrons as that in cuprates. Here we report single-particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a V-shape feature which can be fitted well by a d-wave gap function with gap maximum of about 3.9 meV, another one exhibits a full gap of about 2.35 meV. Some spectra demonstrate mixed contributions of these two components. Combining with theoretical calculations, we attribute the d-wave gap to the pairing potential of the $${\mathrm{Ni - }}3d_{x^2 - y^2}$$ orbital. Several possible reasons are given for explaining the full gap feature. Our results indicate both similarities and distinctions between the newly found Ni-based superconductors and cuprates. The recent observation of superconductivity in nickelate thin films has attracted a lot of attentions. Here, authors report single particle tunneling spectra on the superconducting nickelate thin films revealing two types of gap feature with one V-shape and the other a full gap.

Gamma Titanium Aluminide Alloys

Abstract: Extensive progress and improvements have been made in the science and technology of gamma titanium aluminide alloys within the last decade. In particular, our understanding of their microstructural characteristics and property/microstructurc relationships has been substantially deepened. Based on these achievements, various engineering two-phase gamma alloys have been developed and their mechanical and chemical properties have been assessed. Aircraft and automotive industries arc pursuing their introduction for various structural components. At the same time, recent basic studies on the mechanical properties of two-phase gamma alloys, in particular with a controlled lamellar structure have provided a considerable amount of fundamental information on the deformation and fracture mechanisms of the two-phase gamma alloys. The results of such basic studies are incorporated in the recent alloy and microstructure design of two-phase gamma alloys. In this paper, such recent advances in the research and development of the two-phase gamma alloys and industrial involvement are summarized.

Phase Diagram and Superconducting Dome of Infinite-Layer Nd 1 -x Sr x NiO 2 Thin Films

Abstract: Infinite-layer ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{2}$ thin films with Sr doping level $x$ from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome $x$ between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For $x\ensuremath{\ge}0.18$, the normal state Hall coefficient (${R}_{H}$) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing $x$ from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome.

Critical Nature of the Ni Spin State in Doped NdNiO 2

Abstract: Superconductivity with ${T}_{c}\ensuremath{\approx}15\text{ }\text{ }\mathrm{K}$ was recently found in doped ${\mathrm{NdNiO}}_{2}$. The ${\mathrm{Ni}}^{1+}{\mathrm{O}}_{2}$ layers are expected to be Mott insulators, so hole doping should produce ${\mathrm{Ni}}^{2+}$ with $S=1$, incompatible with robust superconductivity. We show that the ${\mathrm{NiO}}_{2}$ layers fall inside a critical region where the large $pd$ hybridization favors a singlet $^{1}{A}_{1}$ hole-doped state like in ${\mathrm{CuO}}_{2}$. However, we find that the superexchange is about one order smaller than in cuprates, thus a magnon ``glue'' is very unlikely and another mechanism needs to be found.

Cascade of correlated electron states in the kagome superconductor CsV3Sb5.

Abstract: The kagome lattice of transition metal atoms provides an exciting platform to study electronic correlations in the presence of geometric frustration and nontrivial band topology1–18, which continues to bear surprises. Here, using spectroscopic imaging scanning tunnelling microscopy, we discover a temperature-dependent cascade of different symmetry-broken electronic states in a new kagome superconductor, CsV3Sb5. We reveal, at a temperature far above the superconducting transition temperature Tc ~ 2.5 K, a tri-directional charge order with a 2a0 period that breaks the translation symmetry of the lattice. As the system is cooled down towards Tc, we observe a prominent V-shaped spectral gap opening at the Fermi level and an additional breaking of the six-fold rotational symmetry, which persists through the superconducting transition. This rotational symmetry breaking is observed as the emergence of an additional 4a0 unidirectional charge order and strongly anisotropic scattering in differential conductance maps. The latter can be directly attributed to the orbital-selective renormalization of the vanadium kagome bands. Our experiments reveal a complex landscape of electronic states that can coexist on a kagome lattice, and highlight intriguing parallels to high-Tc superconductors and twisted bilayer graphene. A study reveals a temperature-dependent cascade of different symmetry-broken electronic states in the kagome superconductor CsV3Sb5, and highlights intriguing parallels between vanadium-based kagome metals and materials exhibiting similar electronic phases.