Intrinsic constraint on Tc for unconventional superconductivity

Chow S.L., Ariando A.

AbstractUnconventional high‐temperature superconductivity has long been a captivating puzzle in condensed matter physics. The 1987 Nobel Prize in Physics celebrated the discovery of high‐temperature superconductivity in copper oxide ceramics. Nearly four decades later, a broad class of high‐temperature superconducting oxides has yet to be demonstrated, and the fundamental understanding of the pairing mechanism remains inconclusive. Recently, nickel oxides have emerged as a new class of high‐temperature superconductors, beyond copper, where correlated phases can be controlled by doping, pressure, strain, and dimensionality. In this article, we provide our perspective on the recent developments and prospects of the nickel age of high‐temperature superconductivity.

Chen X., Choi J., Jiang Z., Mei J., Jiang K., Li J., Agrestini S., Garcia-Fernandez M., Sun H., Huang X., Shen D., Wang M., Hu J., Lu Y., Zhou K., et. al.

High-temperature superconductivity was discovered in the pressurized nickelate La3Ni2O7 which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3 $${d}_{{x}^{2}-{y}^{2}}$$ , Ni 3 $${d}_{{z}^{2}}$$ , and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3 $${d}_{{z}^{2}}$$ orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La3Ni2O7 superconductor. It was recently found that a certain nickelate compound, La3Ni2O7, at moderately high pressures has a superconducting phase that persists to above liquid nitrogen temperatures. Here, by studying the parent phase at ambient pressure, Chen et al uncover rich magnetic properties and show the vital role of the strong bonding of the inter-layer Ni orbitals in the magnetic and electronic excitations.

Xie T., Huo M., Ni X., Shen F., Huang X., Sun H., Walker H.C., Adroja D., Yu D., Shen B., He L., Cao K., Wang M.

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La

Wu Y., Wang Y.

We present a theory for charge-4e superconductivity as a leading low-temperature instability with a nontrivial d-wave symmetry. We show that in several microscopic models for the pair-density-wave (PDW) state, when the PDW wave vectors connect special parts of the Fermi surface, the predominant interaction is in the bosonic pairing channel mediated by exchanging low-energy fermions. This bosonic pairing interaction is repulsive in the s-wave channel but attractive in the d-wave one, leading to a d-wave charge-4e superconductor. By analyzing the Ginzburg-Landau free energy including higher-order fluctuation effects of PDW, we find that the charge-4e superconductivity emerges as a vestigial order of PDW, and sets in via a first-order transition. Both the gap amplitude and the transition temperature decay monotonically with increasing superfluid stiffness of the PDW order. Our work provides a microscopic mechanism of higher-charge condensates with unconventional ordering symmetry in strongly-correlated materials.

Luo Z., Lv B., Wang M., Wú W., Yao D.

AbstractThe recently discovered high-Tc superconductor La3Ni2O7 has sparked renewed interest in unconventional superconductivity. Here we study superconductivity in pressurized La3Ni2O7 based on a bilayer two-orbital t−J model, using the renormalized mean-field theory. Our results reveal a robust s±-wave pairing driven by the inter-layer $${d}_{{z}^{2}}$$ d z 2 magnetic coupling, which exhibits a transition temperature within the same order of magnitude as the experimentally observed Tc ~ 80 K. We establish a comprehensive superconducting phase diagram in the doping plane. Notably, the La3Ni2O7 under pressure is found to be situated roughly in the optimal doping regime of the phase diagram. When the $${d}_{{x}^{2}-{y}^{2}}$$ d x 2 − y 2 orbital becomes close to half-filling, d-wave and d + is pairing can emerge from the system. We discuss the interplay between Fermi surface topology and different pairing symmetries. The stability of the s±-wave pairing against Hund’s coupling and other magnetic exchange couplings is discussed.

Zhu Y., Peng D., Zhang E., Pan B., Chen X., Chen L., Ren H., Liu F., Hao Y., Li N., Xing Z., Lan F., Han J., Wang J., Jia D., et. al.

The pursuit of discovering new high-temperature superconductors that diverge from the copper-based model1–3 has profound implications for explaining mechanisms behind superconductivity and may also enable new applications4–8. Here our investigation shows that the application of pressure effectively suppresses the spin–charge order in trilayer nickelate La4Ni3O10−δ single crystals, leading to the emergence of superconductivity with a maximum critical temperature (Tc) of around 30 K at 69.0 GPa. The d.c. susceptibility measurements confirm a substantial diamagnetic response below Tc, indicating the presence of bulk superconductivity with a volume fraction exceeding 80%. In the normal state, we observe a strange metal behaviour, characterized by a linear temperature-dependent resistance extending up to 300 K. Furthermore, the layer-dependent superconductivity observed hints at a unique interlayer coupling mechanism specific to nickelates, setting them apart from cuprates in this regard. Our findings provide crucial insights into the fundamental mechanisms underpinning superconductivity, while also introducing a new material platform to explore the intricate interplay between the spin–charge order, flat band structures, interlayer coupling, strange metal behaviour and high-temperature superconductivity. The application of pressure effectively suppresses the spin–charge order in trilayer nickelate La4Ni3O10−δ single crystals, leading to the emergence of superconductivity.

Profe J.B., Beck S., Kennes D.M., Georges A., Gingras O.

AbstractThe pairing symmetry of Sr2RuO4 is a long-standing fundamental question in the physics of superconducting materials with strong electronic correlations. We use the functional renormalization group to investigate the behavior of superconductivity under uniaxial strain in a two-dimensional realistic model of Sr2RuO4 obtained with density functional theory and incorporating the effect of spin-orbit coupling. We find a dominant $${d}_{{{{{\rm{x}}}}}^{2}-{{{{\rm{y}}}}}^{2}}$$ d x 2 − y 2 superconductor mostly hosted by the dxy-orbital, with no other closely competing superconducting state. Within this framework, we reproduce the experimentally observed enhancement of the critical temperature under strain and propose a simple mechanism driven by the density of states to explain our findings. We also investigate the competition between superconductivity and spin-density wave ordering as a function of interaction strength. By comparing theory and experiment, we discuss constraints on a possible degenerate partner of the $${d}_{{{{{\rm{x}}}}}^{2}-{{{{\rm{y}}}}}^{2}}$$ d x 2 − y 2 superconducting state.

Gao Q., Fan S., Wang Q., Li J., Ren X., Biało I., Drewanowski A., Rothenbühler P., Choi J., Sutarto R., Wang Y., Xiang T., Hu J., Zhou K., Bisogni V., et. al.

AbstractStrongly correlated materials respond sensitively to external perturbations such as strain, pressure, and doping. In the recently discovered superconducting infinite-layer nickelates, the superconducting transition temperature can be enhanced via only ~ 1% compressive strain-tuning with the root of such enhancement still being elusive. Using resonant inelastic x-ray scattering (RIXS), we investigate the magnetic excitations in infinite-layer PrNiO2 thin films grown on two different substrates, namely SrTiO3 (STO) and (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) enforcing different strain on the nickelates films. The magnon bandwidth of PrNiO2 shows only marginal response to strain-tuning, in sharp contrast to the enhancement of the superconducting transition temperature Tc in the doped superconducting samples. These results suggest the bandwidth of spin excitations of the parent compounds is similar under strain while Tc in the doped ones is not, and thus provide important empirics for the understanding of superconductivity in infinite-layer nickelates.

Chen Q., Wang Z., Boyack R., Yang S., Levin K.

Di Cataldo S., Worm P., Tomczak J.M., Si L., Held K.

AbstractHigh-temperature unconventional superconductivity quite generically emerges from doping a strongly correlated parent compound, often (close to) an antiferromagnetic insulator. The recently developed dynamical vertex approximation is a state-of-the-art technique that has quantitatively predicted the superconducting dome of nickelates. Here, we apply it to study the effect of pressure in the infinite-layer nickelate SrxPr1−xNiO2. We reproduce the increase of the critical temperature (Tc) under pressure found in experiment up to 12 GPa. According to our results, Tc can be further increased with higher pressures. Even without Sr-doping the parent compound, PrNiO2, will become a high-temperature superconductor thanks to a strongly enhanced self-doping of the Ni $${d}_{{x}^{2}-{y}^{2}}$$ d x 2 − y 2 orbital under pressure. With a maximal Tc of 100 K around 100 GPa, nickelate superconductors can reach that of the best cuprates.

Rossi M., Lu H., Lee K., Goodge B.H., Choi J., Osada M., Lee Y., Li D., Wang B.Y., Jost D., Agrestini S., Garcia-Fernandez M., Shen Z.X., Zhou K., Been E., et. al.

We conducted a comparative study of the rare-earth infinite-layer nickelate films, $R{\mathrm{NiO}}_{2}$ ($R$ = La, Pr, and Nd), using resonant inelastic x-ray scattering (RIXS). We found that the gross features of the orbital configurations are essentially the same, with minor variations in the detailed hybridization. For low-energy excitations, we unambiguously confirm the presence of damped magnetic excitations in all three compounds. By fitting to a linear spin-wave theory, comparable spin exchange coupling strengths and damping coefficients are extracted, indicating a universal magnetic structure in the infinite-layer nickelates. Interestingly, while signatures of a charge order are observed in ${\mathrm{LaNiO}}_{2}$ in the quasielastic region of the RIXS spectrum, it is absent in ${\mathrm{NdNiO}}_{2}$ and ${\mathrm{PrNiO}}_{2}$. This prompts further investigation into the universality and the origins of charge order within the infinite-layer nickelates.

Chen J., Wang J., Yang Y.

Motivated by the recent discovery of an intermediate quantum critical phase between the antiferromagnetic order and the Fermi liquid in the frustrated Kondo lattice CePdAl, we study here a Kondo-Heisenberg chain with frustrated ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ XXZ interactions among local spins using the density matrix renormalization group method. Our simulations reveal a global phase diagram with rich ground states including the antiferromagnetic order, the valence-bond-solid and bond-order-wave orders, the pair density wave state, the uniform superconducting state, and the Luttinger liquid state. We show that both the pair density wave and uniform superconductivity belong to the family of Luther-Emery liquids and may arise from pair instability of an intermediate quantum critical phase with medium Fermi volume in the presence of strong quantum fluctuations, while the Luttinger liquid has a large Fermi volume. Our work provides a comprehensive picture of the frustrated Kondo lattice physics at one dimension, and suggests a deep connection between the pair density wave, the unconventional superconductivity, and the non-Fermi liquid quantum critical phase.

Yang Y., Zhang G., Zhang F.

The recent discovery of high-${T}_{c}$ superconductivity in bilayer nickelate ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ under high pressure has stimulated great interest concerning its pairing mechanism. We argue that the weak coupling model from the almost fully filled ${d}_{{z}^{2}}$ bonding band cannot give rise to its high ${T}_{c}$, and thus propose a strong coupling model based on local interlayer spin singlets of Ni-${d}_{{z}^{2}}$ electrons due to their strong on-site Coulomb repulsion. This leads to a minimal effective model that contains local pairing of ${d}_{{z}^{2}}$ electrons and a considerable hybridization with near quarter-filled itinerant ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ electrons on nearest-neighbor sites. Their strong coupling provides a unique two-component scenario to achieve high-${T}_{c}$ superconductivity. Our theory highlights the importance of the bilayer structure of superconducting ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ and points out a potential route for the exploration of more high-${T}_{c}$ superconductors.

Ji H., Liu Y., Li Y., Ding X., Xie Z., Ji C., Qi S., Gao X., Xu M., Gao P., Qiao L., Yang Y., Zhang G., Wang J.

AbstractThe infinite-layer nickelates, isostructural to the high-Tc cuprate superconductors, have emerged as a promising platform to host unconventional superconductivity and stimulated growing interest in the condensed matter community. Despite considerable attention, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still under debate. Moreover, the strong electronic correlation in the nickelates may give rise to a rich phase diagram, where the underlying interplay between the superconductivity and other emerging quantum states with broken symmetry is awaiting exploration. Here, we study the angular dependence of the transport properties of the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films with Corbino-disk configuration. The azimuthal angular dependence of the magnetoresistance (R(φ)) manifests the rotational symmetry breaking from isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a symmetry-breaking phase transition. Approaching the low-temperature and large-magnetic-field regime, an additional two-fold (C2) symmetric component in the R(φ) curves and an anomalous upturn of the temperature-dependent critical field are observed simultaneously, suggesting the emergence of an exotic electronic phase. Our work uncovers the evolution of the quantum states with different rotational symmetries in nickelate superconductors and provides deep insight into their global phase diagram.

Shipulin I., Stegani N., Maccari I., Kihou K., Lee C., Hu Q., Zheng Y., Yang F., Li Y., Yim C., Hühne R., Klauss H., Putti M., Caglieris F., Babaev E., et. al.

AbstractMaterials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (Tc). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z2 time-reversal symmetry was reported in Ba1−xKxFe2As2. A phase transition to the new state of matter should be accompanied by a specific heat anomaly at the critical temperature where Z2 time-reversal symmetry is broken ($${T}_{{{{{{{{\rm{c}}}}}}}}}^{{{{{{{{\rm{Z2}}}}}}}}} \, > \, {T}_{{{{{{{{\rm{c}}}}}}}}}$$ T c Z2 > T c ). Here, we report on detecting two anomalies in the specific heat of Ba1−xKxFe2As2 at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating the breakdown of Z2 symmetry. The second anomaly at the lower temperature coincides with the transition to a zero-resistance state, indicating the onset of superconductivity. Our data provide the first example of the appearance of a specific heat anomaly above the superconducting phase transition associated with the broken time-reversal symmetry due to the formation of the novel fermion order.