Siyuan Wan

Bio: Siyuan Wan is an academic researcher from Nanjing University. The author has contributed to research in topics: Superconductivity & Quasiparticle. The author has an hindex of 6, co-authored 8 publications receiving 116 citations.

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.

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.

Directly visualizing the sign change of d -wave superconducting gap in Bi 2 Sr 2 CaCu 2 O 8+δ by phase-referenced quasiparticle interference

Abstract: The superconducting state is formed by the condensation of Cooper pairs and protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the gap function. Thus, it is pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the gap may have a d-wave function. This gap function has an alternative sign change in the momentum space. It is however hard to visualize this sign change. Here we report the measurements of scanning tunneling spectroscopy in Bi2Sr2CaCu2O8+δ and conduct the analysis of phase-referenced quasiparticle interference (QPI). We see the seven basic scattering vectors that connect the octet ends of the banana-shaped contour of Fermi surface. The phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our results illustrate an effective way for determining the sign change of unconventional superconductors.

Sign-reversal superconducting gaps revealed by phase-referenced quasiparticle interference of impurity-induced bound states in ( Li 1 − x Fe x ) OHFe 1 − y Zn y Se

Abstract: It is very important, but not easy, to determine the superconducting gap function in unconventional superconductors. The key issue concerns whether there is a sign change in the gap function. In iron-based superconductors, this becomes even more difficult due to the complexity of multiple Fe 3$d$ orbitals. Here, the authors adopt a recently proposed method -- phase-referenced quasiparticle interference (QPI) of defect-bound states (DBS) via scanning tunneling microscope/spectroscopy -- and visualize directly the gap sign change of the superconducting gaps in the iron-based superconductor (Li${}_{1\ensuremath{-}x}$Fe${}_{x}$)OHFe${}_{1\ensuremath{-}y}$Zn${}_{y}$Se. This work provides an easy and feasible way for detecting the gap function of unconventional superconductors.

Twofold symmetry of proximity-induced superconductivity in Bi 2 Te 3 / Bi 2 Sr 2 CaCu 2 O 8 + δ heterostructures revealed by scanning tunneling microscopy

Abstract: We observe proximity-induced superconductivity in in situ prepared heterostructures constructed by topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films and high-temperature cuprate superconductors ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. The superconducting gap maximum is about 7.6 meV on the surface of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. This gap form is consistent with the ${\mathrm{\ensuremath{\Delta}}}_{4y}$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information on the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.

Nickelate superconductors—a renaissance of the one-band Hubbard model

Abstract: The recently discovered nickelate superconductors appear, at first glance, to be even more complicated multi-orbital systems than cuprates. To identify the simplest model describing the nickelates, we analyse the multi-orbital system and find that it is instead the nickelates which can be described by a one-band Hubbard model, albeit with an additional electron reservoir and only around the superconducting regime. Our calculations of the critical temperature TC are in good agreement with experiment, and show that optimal doping is slightly below 20% Sr-doping. Even more promising than 3d nickelates are 4d palladates.

Nickelate Superconductivity without Rare-Earth Magnetism: (La,Sr)NiO2.

Abstract: The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO$_{2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO$_{2}$ [16,17], but not previously in (La,Sr)NiO$_{2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La$_{1-x}$Sr$_{x}$NiO$_{2}$ phase diagram can enter the regime of coherent low-temperature transport ($x$ = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ~ 9 K at $x$ = 0.20. Additionally, we observe the unexpected indication of a superconducting ground state in undoped LaNiO$_{2}$, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)$_{1-x}$Sr$_{x}$NiO$_{2}$ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

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$_5$.

Hund's metal physics: From SrNiO 2 to LaNiO 2

Abstract: Using SrNiO${}_{2}$ as a reference system, the authors find multiorbital Hund's metal behavior in the new Ni-based superconductor Nd${}_{1\ensuremath{-}x}$ SrxNiO${}_{2}$ by many-body $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations They show that both the weights of Ni 3$d$ multiplets and total Ni 3$d$ occupancy are barely changed from the typical Hund's metal SrNiO${}_{2}$ to LaNiO${}_{2}$ in spite of the large charge-carrier doping The calculations are well justified by existing experiments with important implications on the pairing mechanism