High-Temperature Superconductivity in La_(3)Ni_(2)O_(7)

Motivated by the recent discovery of high-temperature superconductivity in bilayer La_(3)Ni_(2)O_(7) under pressure,we study its electronic properties and superconductivity due to strong electron correlation.Using the inversion symmetry,we decouple the low-energy electronic structure into block-diagonal symmetric and antisymmetric sectors.It is found that the antisymmetric sector can be reduced to a one-band system near half filling,while the symmetric bands occupied by about two electrons are heavily overdoped individually.Using the strong coupling mean field theory,we obtain strong superconducting pairing with B_(1g)symmetry in the antisymmetric sector.We propose that due to the spin-orbital exchange coupling between the two sectors,B_(1g)pairing is induced in the symmetric bands,which in turn boosts the pairing gap in the antisymmetric band and enhances the high-temperature superconductivity with a congruent d-wave symmetry in pressurized La_(3)Ni_(2)O_(7).

Signature of Superconductivity in Pressurized La_(4)Ni_(3)O_(10)

The discovery of high-temperature superconductivity near 80K in bilayer nickelate La_(3)Ni_(2)O_(7)under high pressures has renewed the exploration of superconducting nickelate in bulk materials.The extension of superconductivity in other nickelates in a broader family is also essential.Here,we report the experimental observation of superconducting signature in trilayer nickelate La_(4)Ni_(3)O_(10)under high pressures.By using a modified solgel method and post-annealing treatment under high oxygen pressure,we successfully obtained polycrystalline La_(4)Ni_(3)O_(10)samples with different transport behaviors at ambient pressure.Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus.Surprisingly,the signature of possible superconducting transition with a maximum transition temperature(T_(c))of about 20K under high pressures is observed,as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields.Although the resistance drop is sample-dependent and relatively small,it appears in all of our measured samples.We argue that the observed superconducting signal is most likely to originate from the main phase of La_(4)Ni_(3)O_(10).Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-T_(c) superconductivity in nickelates.

Decoupling of Rattling Mode and Superconductivity in Filled-Skutterudite Ba_(x)Ir_(4)Sb_(12)

The rattling mode,an anharmonic vibrational phonon,is widely recognized as a critical factor in the emergence of superconductivity in caged materials.Here,we present a counterexample in a filled-skutterudite superconductor Ba_(x)Ir_(4)Sb_(12)(x=0.8,0.9,1.0),synthesized via a high-pressure route.Transport measurements down to liquid 3He temperatures reveal a transition temperature(T_(c))of 1.2 K and an upper critical field(H_(c2))of 1.3 T.Unlike other superconductors with caged structures,the Ba_(x)Ir_(4)Sb_(12)(X=P,As,Sb)family exhibits a monotonic decreasing T_(c) with the enhancement of the rattling mode,as indicated by fitting the Bloch–Grüneisen formula.Theoretical analysis suggests that electron doping from Ba transforms the direct bandgap IrSb3 into a metal,with the Fermi surface dominated by the hybridization of Ir 5d and Sb 5p orbitals.Our findings of decoupled rattling modes and superconductivity distinguish the Ba_(x)Ir_(4)Sb_(12) family from other caged superconductors,warranting further exploration into the underlying mechanism.

Unveiling a novel metal-to-metal transition in LuH_(2):Critically challenging superconductivity claims in lutetium hydrides

Following the recent report by Dasenbrock-Gammon et al.[Nature 615,244–250(2023)]of near-ambient superconductivity in nitrogendoped lutetium trihydride(LuH_(3-δ)N_(ε)),significant debate has emerged surrounding the composition and interpretation of the observed sharp resistance drop.Here,we meticulously revisit these claims through comprehensive characterization and investigations.We definitively identify the reported material as lutetium dihydride(LuH_(2)),resolving the ambiguity surrounding its composition.Under similar conditions(270–295 K and 1–2 GPa),we replicate the reported sharp decrease in electrical resistance with a 30%success rate,aligning with the observations by Dasenbrock-Gammon et al.However,our extensive investigations reveal this phenomenon to be a novel pressure-induced metal-to-metal transition intrinsic to LuH_(2),distinct from superconductivity.Intriguingly,nitrogen doping exerts minimal impact on this transition.Our work not only elucidates the fundamental properties of LuH_(2)andLuH_(3),but also critically challenges the notion of superconductivity in these lutetium hydride systems.These findings pave the way for future research on lutetium hydride systems,while emphasizing the crucial importance of rigorous verification in claims of ambient-temperature superconductivity.

Superconductivity in kagome metal ThRu_(3)Si_(2)

We report the physical properties of ThRu_(3)Si_(2)featured with distorted Ru kagome lattice.The combined experiments of resistivity,magnetization and specific heat reveal bulk superconductivity with T_(c)=3.8 K.The specific heat jump and calculated electron–phonon coupling indicate a moderate coupled BCS superconductor.In comparison with LaRu_(3)Si_(2),the calculated electronic structure in ThRu_(3)Si_(2)shows an electron-doping effect with electron filling lifted from 100 meV below flat bands to 300 meV above it.This explains the lower superconducting transition temperature and weaker electron correlations observed in ThRu_(3)Si_(2).Our work suggests the Tc and electronic correlations in the kagome superconductor could have an intimate connection with the flat bands.

First-principles study on stability and superconductivity of ternary hydride LaYH_(x)(x=2,3,6 and 8)

Recent studies have shown that the La-and Y-hydrides can exhibit significant superconducting properties under high pressures.In this paper,we investigate the stability,electronic and superconducting properties of LaYH_(x)(x=2,3,6 and 8)under 0-200 GPa.It is found that LaYH_(2) stabilizes in the C2/m phase at ambient pressure,and transforms to the Pmmn phase at 67 GPa.LaYH_(3) stabilizes in the C2/m phase at ambient pressure,and undergoes phase transitions of C2/m→P2_(1)/m→R3m at 12 GPa and 87 GPa,respectively.LaYH_(6) stabilizes in the P4_32_12 phase at ambient pressure,and undergoes phase transitions of P4_(3)2_(1)2→P4/mmm→Cmcm at 28 GPa and 79 GPa,respectively.LaYH_(8) stabilizes in the Imma phase at 60 GPa and transforms to the P4/mmm phase at 117 GPa.Calculations of the electronic band structures show that the P4/mmm-LaYH_(8) and all phases of LaYH_(2) and LaYH_(3) exhibit metallic character.For the metallic phases,we then study their superconducting properties.The calculated superconducting transition temperatures(T_c)are 0.47 K for C2/m-LaYH_(2) at 0 GPa,0 K for C2/m-LaYH_(3) at 0 GPa,and 55.51 K for P4/mmm-LaYH_(8) at 50 GPa.

Coexistence of antiferromagnetism and unconventional superconductivity in a quasi-one-dimensional flat-band system:Creutz lattice

We study the coexistence of antiferromagnetism and unconventional superconductivity on the Creutz lattice which shows strictly flat bands in the noninteracting regime.The famous renormalized mean-field theory is used to deal with strong electron-electron repulsive Hubbard interaction in the effective low-energy t-J model,the superfluid weight of the unconventional superconducting state has been calculated via the linear response theory.An unconventional superconducting state with both spin-singlet and staggered spin-triplet pairs emerges beyond a critical antiferromagnetic coupling interaction,while antiferromagnetism accompanies this state.The superconducting state with only spin-singlet pairs is dominant with paramagnetic phase.The A phase is analogous to the pseudogap phase,which shows that electrons go to form pairs but do not cause a supercurrent.We also show the superfluid behavior of the unconventional superconducting state and its critical temperature.It is proven directly that the flat band can effectively raise the critical temperature of superconductivity.It is implementable to simulate and control strongly-correlated electrons'behavior on the Creutz lattice in the ultracold atoms experiment or other artificial structures.Our results may help the understanding of the interplay between unconventional superconductivity and magnetism.

Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO_(3)

We report high transition temperature superconductivity in one unit-cell(UC)thick FeSe films grown on a Seetched SrTiO_(3)(001)substrate by molecular beam epitaxy(MBE).A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy(STM)suggest that the superconductivity of the 1 UC FeSe films could occur around 77K.The control transport measurement shows that the onset superconductivity temperature is well above 50K.Our work not only demonstrates a powerful way for finding new superconductors and for raising Tc,but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.

Direct evidence of high temperature superconductivity in one-unit-cell FeSe films on SrTiO_3 substrate by transport and magnetization measurements

Zero resistance and Meissner effect are two crucial experimental evidences of superconductivity in determining a new kind of superconductor, which can be detected by transport and diamagnetic measurements. In this paper, we briefly review the main transport and magnetization results on the one unit cell （1-UC） FeSe films grown on SrTiO3 （STO） substrates from our team in recent years, which identify the high temperature superconductivity in 1-UC FeSe films.

The near-room-temperature upsurge of electrical resistivity in Lu-H-N is not superconductivity,but a metal-to-poor-conductor transition

The recent report of superconductivity in nitrogen-doped lutetium hydride(Lu-H-N)at 294 K and 1 GPa brought hope for long-sought-after ambient-condition superconductors.However,the failure of scientists worldwide to independently reproduce these results has cast intense skepticism on this exciting claim.In this work,using a reliable experimental protocol,we synthesized Lu-H-N while minimizing extrinsic influences and reproduced the sudden change in resistance near room temperature.With quantitative comparison of the temperaturedependent resistance between Lu-H-N and the pure lutetium before reaction,we were able to clarify that the drastic resistance change is most likely caused by a metal-to-poor-conductor transition rather than by superconductivity.Herein,we also briefly discuss other issues recently raised in relation to the Lu-H-N system.

No evidence of superconductivity in a compressed sample prepared from lutetium foil and H_(2)/N_(2) gas mixture

A material described as lutetium–hydrogen–nitrogen(Lu-H-N in short)was recently claimed to have“near-ambient superconductivity”[Dasenbrock-Gammon et al.,Nature 615,244–250(2023)].If this result could be reproduced by other teams,it would be a major scientific breakthrough.Here,we report our results of transport and structure measurements on a material prepared using the same method as reported by Dasenbrock-Gammon et al.Our x-ray diffraction measurements indicate that the obtained sample contains three substances:the facecentered-cubic(FCC)-1 phase(Fm-3m)with lattice parameter a=5.03Å,the FCC-2 phase(Fm-3m)with a lattice parameter a=4.755Å,and Lu metal.The two FCC phases are identical to the those reported in the so-called near-ambient superconductor.However,we find from our resistance measurements in the temperature range from 300 K down to 4 K and the pressure range 0.9–3.4 GPa and our magnetic susceptibility measurements in the pressure range 0.8–3.3 GPa and the temperature range down to 100 K that the samples show no evidence of superconductivity.We also use a laser heating technique to heat a sample to 1800 XC and find no superconductivity in the produced dark blue material below 6.5 GPa.In addition,both samples remain dark blue in color in the pressure range investigated.

Superconductivity Observed in Tantalum Polyhydride at High Pressure

We report experimental discovery of tantalum polyhydride superconductor.It was synthesized under highpressure and high-temperature conditions using diamond anvil cell combined with in situ high-pressure laser heating techniques.The superconductivity was investigated via resistance measurements at pressures.The highest superconducting transition temperature T_(c)was found to be~30 K at 197 GPa in the sample that was synthesized at the same pressure with~2000 K heating.The transitions are shifted to low temperature upon applying magnetic fields that support the superconductivity nature.The upper critical field at zero temperatureμ_0H_(c2)(0)of the superconducting phase is estimated to be~20 T that corresponds to Ginzburg-Landau coherent length~40 A.Our results suggest that the superconductivity may arise from 143d phase of TaH_(3).It is,for the first time to our best knowledge,experimental realization of superconducting hydrides for the VB group of transition metals.

Giant enhancement of superconductivity in few layers MoTe2

Recently, the layered transition metal dichalcogenide 1 T MoTe2 has attracted considerable attention due to its non-saturating magnetoresistance, type-II Weyl semimetal properties, superconductivity, and potential candidate for twodimensional（2 D） topological insulator in the single-and few-layer limit. Here in this work, we perform systematic transport measurements on thin flakes of MoTe2 prepared by mechanical exfoliation. We find that MoTe2 flakes are superconducting and have an onset superconducting transition temperature Tc up to 5.3 K, which significantly exceeds that of its bulk counterpart. The in-plane upper critical field(Hc2||) is much higher than the Pauli paramagnetic limit, implying that the MoTe2 flakes have Zeeman-protected Ising superconductivity. Furthermore, the Tc and Hc2|| can be tuned by up to 320 mK and 400 mT by applying a gate voltage. Our result indicates that MoTe2 flake is a good candidate for studying exotic superconductivity with nontrivial topological properties.

Tunable superconductivity in parent cuprate Pr_2CuO_(4±δ) thin films

We studied the role of oxygen in Pr2 CuO_(4±δ) thin films fabricated by the polymer assisted deposition method. The magnetoresistance and Hall resistivity of Pr2 CuO_(4±δ) samples were systematically investigated. It was found that with decreasing oxygen content, the low-temperature Hall coefficient(RH) and magnetoresistance changed from negative to positive, similar to those with the increase of Ce-doped concentration in R_(2-x)Ce_x CuO_4 (R = La, Nd, Pr, Sm, Eu). In addition, we observed that the dependence of the superconducting critical temperature Tc with RH for the Pr_(2-x) Ce_x CuO_4 perfectly overlapped with that of Pr_2 CuO_(4±δ) . These findings point to the fact that the doped electrons induced by the oxygen removal are responsible for the superconductivity of the T-phase parent compounds.

Superconductivity of bilayer phosphorene under interlayer compression

According to first-principles calculations, it is our prediction that bilayer phosphorene（BLP） will become a quasitwo-dimensional superconductor under a certain degree of interlayer compression. A decreasing interlayer distance may realize the transition in the BLP from a semiconducting phase to a metallic phase. On the other hand, a severe vertical compression may make the BLP lattice become dynamically unstable. It is found that in the stable metallic phase of the BLP, interlayer phonon modes dominate the electron-phonon coupling λ. The obtained λ can be greater than 1 and the superconducting temperature T_c can be higher than 10 K.

Enhancing superconductivity of ultrathin YBa2Cu3O7-δ films by capping non-superconducting oxides

In this study, we have explored the ways to fabricate and optimize high-quality ultrathin YBa2 Cu3 O7-δ(YBCO) films grown on single-crystal(001) SrTiO3 substrates. Nearly atomic-flat YBCO films are obtained by pulsed laser deposition.Our result shows that the termination of SrTiO3 has only a negligible effect on the properties of YBCO. In contrast, we found that capping a non-superconducting oxide layer can generally enhance the superconductivity of YBCO. PrBa2 Cu3 O7,La2 CuO4, LaMnO3, SrTiO3, and LaAlO3 have been examined as capping layers, and the minimum thickness of superconducting YBCO with capping is ~ 2 unit cells–3 unit cells. This result might be useful in constructing good-performance YBCO-based field effect devices.

Superconductivity with peculiar upper critical fields in quasi-one-dimensional Cr-based pnictides

The discovery of superconductivity in quasi-one-dimensional Cr-based pnictides A2Cr3As3（A = alkali metals） has generated considerable research interest, primarily owing to their reduced dimensionality, significant electron correlations,and possible unconventional superconductivity. The upper critical field（Hc2） provides important information on the superconducting pairing. In this paper, we first briefly overview the latest research progress on the Cr-based superconductors.Then, we introduce typical Hc2（T） behaviors of type-II superconductors in relation with the pair-breaking mechanisms.After a description of the measurement method for Hc2, we focus on the analysis of Hc2 data, especially for the temperature and angle dependence, in K2Cr3As3 crystals. The result indicates（i） an absence of Pauli-paramagnetic pair breaking for field perpendicular to the Cr3As3 chains, and（ii） a unique threefold modulation for the in-plane Hc2Φ profile. Finally we conclude with remarks on the possible unconventional superconducting pairing symmetry.

First-principles study on the conventional superconductivity of N-doped fcc-LuH_(3)

Recently,room-temperature superconductivity has been reported in a nitrogen-doped lutetium hydride at near-ambient pressure[Dasenbrock-Gammon et al.,Nature 615,244(2023)].The superconducting properties might arise from Fm3m-LuH_(3)−δNε.Here,we systematically study the phase diagram of Lu–N–H at 1 GPa using first-principles calculations,and we do not find any thermodynamically stable ternary compounds.In addition,we calculate the dynamic stability and superconducting properties of N-doped Fm3m-LuH_(3) using the virtual crystal approximation(VCA)and the supercell method.The R3m-Lu_(2)H_(5)N predicted using the supercell method could be dynamically stable at 50 GPa,with a T_(c) of 27 K.According to the VCA method,the highest T_(c) is 22 K,obtained with 1%N-doping at 30 GPa.Moreover,the doping of nitrogen atoms into Fm3m-LuH_(3) slightly enhances T_(c),but raises the dynamically stable pressure.Our theoretical results show that the T_(c) values of N-doped LuH_(3) estimated using the Allen–Dynes-modified McMillan equation are much lower than room temperature.

Synthesis and superconductivity in yttrium superhydrides under high pressure

Flourishing rare earth superhydrides are a class of recently discovered materials that exhibit near-room-temperature superconductivity at high pressures,ushering in a new era of superconductivity research at high pressures.Yttrium superhydrides drew the most attention among these superhydrides due to their abundance of stoichiometries and excellent superconductivities.Here,we carried out a comprehensive study of yttrium superhydrides in a wide pressure range of 140 GPa-300 GPa.We successfully synthesized a series of superhydrides with the compositions of YH_(4),YH_(6),YH_(7),and YH_(9),and reported superconducting transition temperatures of 82 K at 167 GPa,218 K at 165 GPa,29 K at 162 GPa,and230 K at 300 GPa,respectively,as evidenced by sharp drops in resistance.The structure and superconductivity of YH_(4) were taken as a representative example and were also examined using x-ray diffraction measurements and the superconductivity suppression under external magnetic fields,respectively.Clathrate YH_(10),a candidate for room-temperature superconductor,was not synthesized within the study pressure and temperature ranges of up to 300 GPa and 2000 K.The current study established a detailed foundation for future research into room-temperature superconductors in polynary yttrium-based superhydrides.

Superconductivity and Fermi Surface Anisotropy in Transition Metal Dichalcogenide NbTe_2

Transition metal dichalcogenides, featuring layered structures, have aroused enormous interest as a platform for novel physical phenomena and a wide range of potential applications. Among them, special interest has been placed upon WTe_2 and MoTe_2, which exhibit non-trivial topology both in single layer and bulk as well as pressure induced or enhanced superconductivity. We study another distorted IT material NbTe_2 through systematic electrical transport measurements. Intrinsic superconductivity with onset transition temperature(T_c^(onset)) up to 0.72 K is detected where the upper critical field(H_c) shows unconventional quasi-linear behavior,indicating spin-orbit coupling induced p-wave paring. Furthermore, a general model is proposed to fit the angledependent magnetoresistance, which reveals the Fermi surface anisotropy of NbTe_2. Finally, non-saturating linear magnetoresistance up to 50 T is observed and attributed to the quantum limit transport.