Fully gapped d-wave superconductivity in CeCu2Si2

Solid Electrolyte Interphase Architecture for a Stable Li-electrolyte Interface

Li metal anode has attracted extensive attention as the state-of-the-art anode material for rechargeable batteries. It is defined as the ultimate anode material for the high theoretical specific capacity (3860 mAh g-1 ) and the lowest negative electrochemical potential (-3.04 V vs. Standard Hydrogen Electrode). However, the uncontrolled Li dendrites and the spontaneous side reactions between Li and electrolytes hinder its commercialization. To overcome these obstacles, the optimized solid electrolyte interphase (SEI) with excellent performance was proposed by the artificial method. The improved performance includes high stability, ionic conductivity, compactness, and flexibility. In this review, the strategies for artificial SEI engineering in liquid and solid electrolytes are summarized. To fabricate an ideal artificial SEI, the component, distribution, and structure should be fully and reasonably considered. This review will also provide perspectives for the SEI design and lay a foundation for the future research and development of Li metal batteries.

An emerging global picture of heavy fermion physics

Recent progresses using state-of-the-art experimental techniques have motivated a number of new insights on heavy fermion physics. This article gives a brief summary of the author's research along this direction. We discuss five major topics including: (1) development of phase coherence and two-stage hybridization; (2) two-fluid behavior and hidden universal scaling; (3) quantum phase transitions and fractionalized heavy fermion liquid; (4) quantum critical superconductivity; (5) material-specific properties. These cover the most essential parts of heavy fermion physics and lead to an emerging global picture beyond conventional theories based on mean-field or local approximations.

Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe2

Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature T_c (ref. ¹). Electron Cooper pairs in most superconductors form anti-parallel spin singlets with total spin S = 0 (ref. ²), although they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction³. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for quantum computation^4,5. Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations³, uranium-based materials near an FM instability are considered to be ideal candidates for realizing spin-triplet superconductivity⁶. Indeed, UTe₂, which has a T_c ≈ 1.6 K (refs. ^7,8), has been identified as a candidate for a chiral spin-triplet topological superconductor near an FM instability^7-14, although it also has antiferromagnetic (AF) spin fluctuations^15,16. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe₂ is coupled to a sharp magnetic excitation, termed resonance^17-23, at the Brillouin zone boundary near AF order. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability^17-23, its observation in UTe₂ suggests that AF spin fluctuations may also induce spin-triplet pairing²⁴ or that electron pairing in UTe₂ has a spin-singlet component.

Regulating Interfacial Lithium Ion by Artificial Protective Overlayers for High-Performance Lithium Metal Anodes

The main limitation of lithium (Li) metal anodes lies in their severe dendrite growth due to nonuniform Li ion flux and sluggish Li ion transportation at the anode surface. Fabricating artificial protective overlayer with tunable surficial properties on Li metal is a precise and effective strategy to relieve this problem. In this Concept article, we focus on the basic principles of regulating interfacial Li ion through artificial protective overlayers and summarize the material preparation as well as structural design of these overlayers. The remaining challenges and promising directions of artificial protective overlayers are then highlighted to provide clues for the practical application of Li metal anodes.

Revealing the Heavy Quasiparticles in the Heavy-Fermion Superconductor CeCu_{2}Si_{2}

The superconducting order parameter of the first heavy-fermion superconductor CeCu_{2}Si_{2} is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.

Stamping Flexible Li Alloy Anodes

Li metal holds great promise to be the ultimate anode choice owing to its high specific capacity and low redox potential. However, processing Li metal into thin-film anode with high electrochemical performance and good safety to match commercial cathodes remains challenging. Herein, a new method is reported to prepare ultrathin, flexible, and high-performance Li-Sn alloy anodes with various shapes on a number of substrates by directly stamping a molten metal solution. The printed anode is as thin as 15 µm, corresponding to an areal capacity of ≈3 mAh cm^-2 that matches most commercial cathode materials. The incorporation of Sn provides the nucleation center for Li, thereby mitigating Li dendrites as well as decreasing the overpotential during Li stripping/plating (e.g., <10 mV at 0.25 mA cm^-2 ). As a proof-of-concept, a flexible Li-ion battery using the ultrathin Li-Sn alloy anode and a commercial NMC cathode demonstrates good electrochemical performance and reliable cell operation even after repetitive deformation. The approach can be extended to other metal/alloy anodes such as Na, K, and Mg. This study opens a new door toward the future development of high-performance ultrathin alloy-based anodes for next-generation batteries.

Review on Li Deposition in Working Batteries: From Nucleation to Early Growth

Lithium (Li) metal is one of the most promising alternative anode materials of next-generation high-energy-density batteries demanded for advanced energy storage in the coming fourth industrial revolution. Nevertheless, disordered Li deposition easily causes short lifespan and safety concerns and thus severely hinders the practical applications of Li metal batteries. Tremendous efforts are devoted to understanding the mechanism for Li deposition, while the final deposition morphology tightly relies on the Li nucleation and early growth. Here, the recent progress in insightful and influential models proposed to understand the process of Li deposition from nucleation to early growth, including the heterogeneous model, surface diffusion model, crystallography model, space charge model, and Li-SEI model, are highlighted. Inspired by the abovementioned understanding on Li nucleation and early growth, diverse anode-design strategies, which contribute to better batteries with superior electrochemical performance and dendrite-free deposition behavior, are also summarized. This work broadens the horizon for practical Li metal batteries and also sheds light on more understanding of other important metal-based batteries involving the metal deposition process.

High-energy magnetic excitations from heavy quasiparticles in CeCu2Si2

Magnetic fluctuations is the leading candidate for pairing in cuprate, iron-based, and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in C e C u 2 S i 2 , and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in superconducting (S-type) C e C u 2 S i 2 using inelastic neutron scattering, finding a strongly asymmetric dispersion for E ≲ 1.5 m e V , which at higher energies evolves into broad columnar magnetic excitations that extend to E ≳ 5 m e V . While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in C e C u 2 S i 2 are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in C e C u 2 S i 2 arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in C e C u 2 S i 2 , and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals.

Symmetry of Order Parameters in Multiband Superconductors LaRu_{4}As_{12} and PrOs_{4}Sb_{12} Probed by Local Magnetization Measurements

The temperature dependencies of the lower critical field H_{c1}(T) of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs_{4}As_{12} and PrRu_{4}As_{12} exhibit the H_{c1}(T) dependencies consistent with the single-band BCS prediction, for LaRu_{4}As_{12} (the superconducting temperature T_{c}=10.4  K) with a similar three-dimensional Fermi surface, we observe a sudden increase in H_{c1}(T) deep in a superconducting state below about 0.32T_{c}. Remarkably, a rapid rise of H_{c1}(T) at approximately the same reduced temperature 0.27T_{c} is also found for the heavy-fermion compound PrOs_{4}Sb_{12} (T_{c}≃1.78  K), in fair accord with the earlier macroscopic study. We attribute the unusual H_{c1}(T) dependencies of LaRu_{4}As_{12} and PrOs_{4}Sb_{12} to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas LaRu_{4}As_{12} is established as a two-band isotropic s-wave superconductor, nonsaturating behavior of H_{c1}(T) is observed for PrOs_{4}Sb_{12}, indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters.

Topological phase transition in superconductors with mirror symmetry

We provide analytical and numerical evidence that the attractive two-dimensional Kitaev model on a lattice with mirror symmetry demonstrates an unusual 'intrinsic' phase at half filling. This phase emerges in the phase diagram at the boundary separating two topological superconductors with opposite Chern numbers and exists due to the condensation of non-zero momentum Cooper pairs. Unlike Fulde-Ferrell-Larkin-Ovchinnikov superconductivity, the Cooper pairs momenta are lying along two lines in the Brillouin zone meaning simultaneous condensation of a continuum of Cooper pairs.

Classifying superconductivity in Moiré graphene superlattices

Several research groups have reported on the observation of superconductivity in bilayer graphene structures where single atomic layers of graphene are stacked and then twisted at angles θ forming Moiré superlattices. The characterization of the superconducting state in these 2D materials is an ongoing task. Here we investigate the pairing symmetry of bilayer graphene Moiré superlattices twisted at θ = 1.05°, 1.10° and 1.16° for carrier doping states varied in the range of n = (0.5 - 1.5) · 1012 cm-2 (where superconductivity can be realized) by analyzing the temperature dependence of the upper critical field Bc2(T) and the self-field critical current Jc(sf,T) within currently available models - all of which start from phonon-mediated BCS theory - for single- and two-band s-, d-, p- and d + id-wave gap symmetries. Extracted superconducting parameters show that only s-wave and a specific kind of p-wave symmetries are likely to be dominant in bilayer graphene Moiré superlattices. More experimental data is required to distinguish between the s- and remaining p-wave symmetries as well as the suspected two-band superconductivity in these 2D superlattices.

Nature of the spin resonance mode in CeCoIn5

Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.

Dual-Phase Single-Ion Pathway Interfaces for Robust Lithium Metal in Working Batteries

The lithium (Li) metal anode is confronted by severe interfacial issues that strongly hinder its practical deployment. The unstable interfaces directly induce unfavorable low cycling efficiency, dendritic Li deposition, and even strong safety concerns. An advanced artificial protective layer with single-ion pathways holds great promise for enabling a spatially homogeneous ionic and electric field distribution over Li metal surface, therefore well protecting the Li metal anode during long-term working conditions. Herein, a robust dual-phase artificial interface is constructed, where not only the single-ion-conducting nature, but also high mechanical rigidity and considerable deformability can be fulfilled simultaneously by the rational integration of a garnet Al-doped Li_6.75 La₃ Zr_1.75 Ta_0.25 O₁₂ -based bottom layer and a lithiated Nafion top layer. The as-constructed artificial solid electrolyte interphase is demonstrated to significantly stabilize the repeated cell charging/discharging process via regulating a facile Li-ion transport and a compact Li plating behavior, hence contributing to a higher coulombic efficiency and a considerably enhanced cyclability of lithium metal batteries. This work highlights the significance of rational manipulation of the interfacial properties of a working Li metal anode and affords fresh insights into achieving dendrite-free Li deposition behavior in a working battery.

Stable Conversion Chemistry-Based Lithium Metal Batteries Enabled by Hierarchical Multifunctional Polymer Electrolytes with Near-Single Ion Conduction

The low Coulombic efficiency and serious safety issues resulting from uncontrollable dendrite growth have severely impeded the practical applications of lithium (Li) metal anodes. Herein we report a stable quasi-solid-state Li metal battery by employing a hierarchical multifunctional polymer electrolyte (HMPE). This hybrid electrolyte was fabricated via in situ copolymerizing lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethanesulfonyl)imide (LiMTFSI) and pentaerythritol tetraacrylate (PETEA) monomers in traditional liquid electrolyte, which is absorbed in a poly(3,3-dimethylacrylic acid lithium) (PDAALi)-coated glass fiber membrane. The well-designed HMPE simultaneously exhibits high ionic conductivity (2.24×10^-3  S cm^-1 at 25 °C), near-single ion conducting behavior (Li ion transference number of 0.75), good mechanical strength and remarkable suppression for Li dendrite growth. More intriguingly, the cation permselective HMPE efficiently prevents the migration of negatively charged iodine (I) species, which provides the as-developed Li-I batteries with high capacity and long cycling stability.

Two-dimensional molecular brush-functionalized porous bilayer composite separators toward ultrastable high-current density lithium metal anodes

Lithium metal batteries have been considerably limited by the problems of uncontrolled dendritic lithium formation and the highly reactive nature of lithium with electrolytes. Herein, we have developed functional porous bilayer composite separators by simply blade-coating polyacrylamide-grafted graphene oxide molecular brushes onto commercial polypropylene separators. Our functional porous bilayer composite separators integrate the lithiophilic feature of hairy polyacrylamide chains and fast electrolyte diffusion pathways with the excellent mechanical strength of graphene oxide nanosheets and thus enable molecular-level homogeneous and fast lithium ionic flux on the surfaces of electrodes. As a result, dendrite-free uniform lithium deposition with a high Coulombic efficiency (98%) and ultralong-term reversible lithium plating/stripping (over 2600 h) at a high current density (2 mA cm-2) are achieved for lithium metal anodes. Remarkably, lithium metal anodes with an unprecedented stability of more than 1900 h cycling at an ultrahigh current density of 20 mA cm-2 are demonstrated.

Evidence for the Presence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCu_{2}Si_{2} Revealed Using ^{63}Cu NMR

Nuclear magnetic resonance measurements were performed on CeCu_{2}Si_{2} in the presence of a magnetic field close to the upper critical field μ_{0}H_{c2} in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature 1/T_{1}T abruptly decreased below the SC transition temperature T_{c}(H), a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, 1/T_{1}T was enhanced just below T_{c}(H) and exhibited a broad maximum when magnetic fields close to μ_{0}H_{c2}(0) were applied parallel or perpendicular to the c axis; although the Knight shift decreased just below T_{c}(H). This enhancement of 1/T_{1}T, which was recently observed in the organic superconductor κ-(BEDT-TTF)_{2}Cu(NCS)_{2}, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.