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.

/pdf/nickelate-superconductors-a-renaissance-of-the-one-band-cihsc4wzwc.pdf

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

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.

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

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 CsV$_3$Sb$_5$ with in-plane rotating magnetic field

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

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

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.

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

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.

/pdf/two-superconducting-components-with-different-symmetries-in-3csw9xtu3g.pdf

Two superconducting components with different symmetries in Nd1-xSrxNiO2 films

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.

/pdf/single-particle-tunneling-spectrum-of-superconducting-nd1-258khggse9.pdf

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

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.

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

In cuprate superconductors, due to strong electronic correlations, there are multiple intertwined orders which either coexist or compete with superconductivity. Among them the antiferromagnetic (AF) order is the most prominent one. In the region where superconductivity sets in, the long-range AF order is destroyed. Yet the residual short-range AF fluctuations are present up to a much higher doping and their role in the emergence of the superconducting phase is still highly debated. Here, by using a spin polarized scanning tunneling microscope, for the first time, we directly visualize an emergent incommensurate AF order in the nearby region of Fe impurities embedded in the optimally doped Bi2Sr2CaCu2O8+{\delta} (Bi2212). Remarkably the Fe impurities suppress the superconducting coherence peaks with the gapped feature intact, but pin down the ubiquitous short-range incommensurate AF order. Our work shows an intimate relation between antiferromagnetism and superconductivity.

Direct Visualization of a Static Incommensurate Antiferromagnetic Order by Suppressing the Superconducting Phase Coherence in Fe-doped Bi2Sr2CaCu2O8+delta

Compounds with kagome lattice usually host many exotic quantum states, including the quantum spin liquid, non-trivial topological Dirac bands and a strongly renormalized flat band, etc. Recently an interesting vanadium based kagome family $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, or Cs) was discovered, and these materials exhibit multiple interesting properties, including unconventional saddle-point driven charge density wave (CDW) state, superconductivity, etc. Furthermore, some experiments show anomalous Hall effect which inspires that there might be some chiral flux current states. Here we report scanning tunneling measurements by using spin polarized tips. Although we have observed clearly the $2\times2$ and $1\times4$ CDW orders, the well-designed experiments with refined spin polarized tips do not reveal any trace of the chiral flux current phase in CsV$_3$Sb$_5$. Thus it remains debatable whether this state really exists in CsV$_3$Sb$_5$ and we may need additional scenario to explain the anomalous Hall effect.

/pdf/no-indication-of-chiral-flux-current-in-the-topological-sd651m83zk.pdf

Huazhou Li

Papers

Two superconducting components with different symmetries in Nd1-xSrxNiO2 films

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

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

Direct Visualization of a Static Incommensurate Antiferromagnetic Order by Suppressing the Superconducting Phase Coherence in Fe-doped Bi2Sr2CaCu2O8+delta

No indication of chiral flux current in the topological kagome metal CsV$_{3}$Sb$_{5}$