Possible nodeless s~±-wave superconductivity in twisted bilayer graphene

The recent discovery of superconductivity in the twisted bilayer graphene has stimulated numerous theoretical proposals concerning its exact gap symmetry.Among them, the d + id or p + ip-wave was believed to be the most plausible solution.Here, considering that the superconductivity emerges near a correlated insulating state and may be induced by antiferromagnetic spin fluctuations, we apply the strong-coupling Eliashberg theory with both inter-and intraband quantum critical pairing interactions and discuss the possible gap symmetry in an effective low-energy four-orbital model.Our calculations reveal a nodeless s~±-wave as the most probable candidate for the superconducting gap symmetry in the experimentally relevant parameter range.This solution is distinctly different from previous theoretical proposals.It highlights the multi-gap nature of the superconductivity and puts the twisted bilayer graphene in the same class as the iron-pnictide,electron-doped cuprate, and some heavy fermion superconductors.     

Superconducting Properties and Absence of Time Reversal Symmetry Breaking in the Noncentrosymmetric Superconducting Compounds TaxRe1-x(0.1≤x≤0.25)

Unconventional superconductivity,in particular,in noncentrosymmetric systems,has been a long-sought topic in condensed matter physics.Recently,Re-based superconductors have attracted great attention owing to the potential time-reversal symmetry breaking in their superconducting states.We report the superconducting properties of noncentrosymmetric compounds Ta_xRe_1-x with 0.1 ≤x≤0.25,and find that the superconducting transition temperature reaches a maximum of ~8 K at the optimal level x=0.15.Nevertheless,muon-spin rotation and relaxation measurements reveal no time-reversal symmetry breaking existing in its superconducting state,which is in sharp contrast to both centrosymmetric Re metal and many other noncentrosymmetric Re-based superconductors.     

Nodeless superconductivity in a vanadium kagome metal

正The pairing symmetry of the superconducting order parameter contains key information about the pairing mechanism of the superconductor. For conventional superconductors, whose pairing is usually induced by the electron-phonon interaction, the pairing symmetry is s-wave, and the superconducting gap is fully open everywhere on the Fermi surface. For unconventional superconductors such as the cuprate and heavy fermion superconductors, other pairing symmetries may occur, leading to symmetry-mandated nodes in the superconducting order parameter.     

Nodal superconducting gap in LiFeP revealed by NMR:Contrast with LiFeAs

Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides.Here,we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms.The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth λ_L.We find that λ_L saturates below T～0.2 T_C in LiFeAs,where T_C is the superconducting transition temperature,indicating nodeless superconducting gaps.Furthermore,by using a two-gaps model,we simulate the temperature dependence of λ_L and obtain the superconducting gaps of LiFeAs,as Δ_1=1.2 k_B T_C and Δ_2=2.8 k_BT_C,in agreement with previous result from spin-lattice relaxation.For LiFeP,in contrast,λ_L does not show any saturation down to T～0.03 T_C,indicating nodes in the superconducting gap function.Finally,we demonstrate that strong spin fluctuations with diffusive characteristics exist in LiFeP,as in some cuprate high temperature superconductors.     

Broken Time-Reversal Symmetry in Superconducting Partially Filled Skutterudite Pr_(1-δ)Pt_4Ge_(12)

Time reversal symmetry(TRS)is a key symmetry for classification of unconventional superconductors,and the violation of TRS often results in a wealth of novel properties.Here we report the synthesis and superconducting properties of the partially filled skutterudite Pr_(1-δ)Pt_4Ge_(12).The results from x-ray diffraction and magnetization measurements show that the[Pt_4 Ge_(12)]cage-forming structure survives and bulk superconductivity is preserved below the superconducting transition temperature T_c = 7.80 K.The temperature dependence of both the upper critical field and the electronic specific heat can be described in terms of a two-gap model,providing strong evidence of multi-band superconductivity.TRS breaking is observed using zero Held muon-spin relaxation experiments,and the magnitude of the spontaneous field is nearly half of that in PrPt_4Ge_(12).     

Competitions of magnetism and superconductivity in FeAs-based materials

Using the numerical unrestricted Hartree--Fock approach, we study the ground state of a two-orbital model describing newly discovered FeAs-based superconductors. We observe the competition of a (0, π) mode spin-density wave and the superconductivity as the doping concentration changes. There might be a small region in the electron-doping side where the magnetism and superconductivity coexist. The superconducting pairing is found to be spin singlet, orbital even, and coexisting s_xy + d_x²-y² wave (even parity).     

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

Pressure-Dependent Point-Contact Spectroscopy of Superconducting PbTaSe_2 Single Crystals

We develop an experimental tool to investigate the order parameter of superconductors by combining pointcontact spectroscopy measurement with high-pressure technique.It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe_2 single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar.Under such a high pressure,the normal-state contact resistance is sensitive to the applied pressure,reflecting the underlying variation of contact transparency upon pressures.In a superconducting state,the pressure dependence of the energy gap △_0 and the critical temperature T_c for gap opening/closing are extracted based on a generalized Blond-Tinkham-Klapwijk model.The gap ratio 2△_0/k_B T_c indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe_2.Our experimental results show the accessibility and validity of high-pressure pointcontact spectroscopy,offering rich information about high-pressure superconductivity.     

Angle-resolved photoemission spectroscopy study on iron-based superconductors

Angle-resolved photoemission spectroscopy （ARPES） has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.     

Common Electronic Features and Electronic Nematicity in Parent Compounds of Iron-Based Superconductors and FeSe/SrTiO_3 Films Revealed by Angle-Resolved Photoemission Spectroscopy

We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO,BaFe_2As_2,NaFeAs,FeSe and undoped FeSe/SrTiO_3 films with 1,2 and 20 layers.While the electronic structure near the Brillouin zone center Γ varies dramatically among different materials,the electronic structure near the Brillouin zone corners(M points),as well as their temperature dependence,are rather similar.The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the d_(xz) and d_(yz) bands below the nematic transition temperature.A clear relation is observed between the band splitting magnitude and the onset temperature of nematicity.Our results may shed light on the origin of nematicity,its effect on the electronic structures,and its relation with superconductivity in the iron-based superconductors.     

Physics picture from neutron scattering study on Fe-based superconductors

Neutron scattering, with its ability to measure the crystal structure, the magnetic order, and the structural and magnetic excitations, plays an active role in investigating various families of Fe-based high-T c superconductors. Three different types of antiferromagnetic orders have been discovered in the Fe plane, but two of them cannot be explained by the spin-density-wave (SDW) mechanism of nesting Fermi surfaces. Noticing the close relation between antiferromagnetic order and lattice distortion in orbital ordering from previous studies on manganites and other oxides, we have advocated orbital ordering as the underlying common mechanism for the structural and antiferromagnetic transitions in the 1111, 122, and 11 parent compounds. We observe the coexistence of antiferromagnetic order and superconductivity in the (Ba,K)Fe 2 As 2 system, when its phase separation is generally accepted. Optimal T c is proposed to be controlled by the local FeAs 4 tetrahedron from our investigation on the 1111 materials. The Bloch phase coherence of the Fermi liquid is found crucial to the occurrence of bulk superconductivity in iron chalcogenides of both the 11 and the 245 families. Iron chalcogenides carry a larger staggered magnetic moment ( 2 μ B /Fe) than that in iron pnictides ( 1 μ B /Fe) in the antiferromagnetic order. Normal state magnetic excitations in the 11 superconductor are of the itinerant nature while in the 245 superconductor the spin-waves of localized moments. The observation of superconducting resonance peak provides a crucial piece of information in current deliberation of the pairing symmetry in Fe-based superconductors.     

Spin fluctuations and unconventional superconducting pairing in iron-based superconductors

In this article, we review the recent theoretical works on the spin fluctuations and superconductivity in iron-based superconductors. Using the fluctuation exchange approximation and multi-orbital tight-binding models, we study the char- acteristics of the spin fluctuations and the symmetries of the superconducting gaps for different iron-based superconductors. We explore the systems with both electron-like and hole-like Fermi surfaces （FS） and the systems with only the electron-like FS. We argue that the spin-fluctuation theories are successful in explaining at least the essential part of the problems, indicating that the spin fluctuation is the common origin of superconductivity in iron-based superconductors.     

Effect of transparency on Josephson junction between s+g-wave superconductors

In this paper, a dc Josephson junction between borocarbide superconductors has been studied theoretically. The s+g-wave pairing symmetry which is observed in rare earth complex of borocarbides has a huge anisotropy and is an interesting form of unconventional superconductivity. We calculate the Josephson current in a superconductor--insulator--superconductor (SIS) Josephson junction with s+g-wave superconducting pairing symmetry. In our planar junction c-axis is parallel to an interface with finite transparency but ab-planes of two tetragonal superconductors are misorientated by angle α. We obtain that the Josephson current is strongly dependent on mis-orientation between the left and the right ab-planes. An insulator sandwiched between two superconductors which acts as a potential barrier is demonstrated by a transparency coefficient. The effects of the potential barrier and the mis-orientation on the current are studied analytically and numerically. Occurrence of 0--π transition in this s+g-wave junction is investigated in this paper. A comparison between d-wave Josephson junction and s+g-wave one is also made in the present paper.     

Nodeless superconductivity in the kagome metal CsV_3Sb_5

The recently discovered kagome metal series AV_3Sb_5(A=K, Rb, Cs) exhibits topologically nontrivial band structures, chiral charge order and superconductivity, presenting a unique platform for realizing exotic electronic states. The nature of the superconducting state and the corresponding pairing symmetry are key questions that demand experimental clarification. Here, using a technique based on the tunneling diode oscillator, the magnetic penetration depth ?λ(T) of CsV_3Sb_5 was measured down to 0.07 K. A clear exponential behavior in ?λ(T) with marked deviations from a T or T2 temperature dependence was observed at low temperatures, indicating an absence of nodal quasiparticles. Temperature dependence of the superfiuid density and electronic specific heat can be described by two-gap s-wave superconductivity, consistent with the presence of multiple Fermi surfaces in CsV_3Sb_5. These results evidence nodeless superconductivity in CsV_3Sb_5 under ambient pressure, and constrain the allowed pairing symmetry.     

Observation of a Ubiquitous(π,π)-Type Nematic Superconducting Order in the Whole Superconducting Dome of Ultra-Thin BaFe_(2-x)Ni_xAs_2 Single Crystals

In iron-based superconductors,the(0,π) or(π,0) nematicity,which describes an electronic anisotropy with a fourfold symmetry breaking,is well established and believed to be important for understanding the superconducting mechanism.However,how exactly such a nematic order observed in the normal state can be related to the superconducting pairing is still elusive.Here,by performing angular-dependent in-plane magnetoresistivity using ultra-thin flakes in the steep superconducting transition region,we unveil a nematic superconducting order along the(π,π) direction in electron-doped BaFe_(2-x)Ni_xAs_2 from under-doped to heavily overdoped regimes with x=0.065-0.18.It shows superconducting gap maxima along the(π,π) direction rotated by 45° from the nematicity along(0,π) or(π,0) direction observed in the normal state.A similar(π,π)-type nematicity is also observed in the under-doped and optimally doped hole-type Ba_(1-y)K_yFe_2 As_2,with y=0.2-0.5.These results suggest that the(π,π) nematic superconducting order is a universal feature that needs to be taken into account in the superconducting pairing mechanism in iron-based superconductors.     

Common (π,π) Band Folding and Surface Reconstruction in Fe As-Based Superconductors

High resolution angle-resolved photoemission spectroscopy(ARPES) measurements are carried out on CaKFe_4 As_4,KCa_2 Fe_4 As_4 F_2 and(Ba_(0.6)K_(0.4))Fe_2 As_2 superconductors.Clear evidence of band folding between the Brillouin zone center and corners with a(π,π) wave vector has been found from the measured Fermi surface and band structures in all the three kinds of superconductors.A dominant ~(1/2)×~(1/2) surface reconstruction is observed on the cleaved surface of CaKFe_4As_4 by scanning tunneling microscopy(STM) measurements.We propose that the commonly observed ~(1/2)×~(1/2) reconstruction in the FeAs-based superconductors provides a general scenario to understand the origin of the(π,π) band folding.Our observations provide new insights in understanding the electronic structure and superconductivity mechanism in iron-based superconductors.     

Superconducting gap ratio from strange metal phase in the absence of quasiparticles

A lttice model for strongly interacting electrons motivated by a rank-3 tensor model provides a tool for understanding the pairing mechanism of high-temperature superconductivity.This Sachdev Ye Kitaev-like model describes the strange metal phase in the cuprate high temperature superconductors.Our calculation indicates that the superconducting gap ratio in this model is higher than the ratio in the BCS theory due to the coupling term and the spin operator.Under certain conditions,the ratio also agrees with the BCS theory.Our results relate to the case of strong coupling,so it may pave the way to gaining insight into the cuprate high temperature superconductors.     

Superconductivity in octagraphene

This article reviews the basic theoretical aspects of octagraphene, an one-atom-thick allotrope of carbon, with unusual two-dimensional(2 D) Fermi nesting, hoping to contribute to the new family of quantum materials. Octagraphene has an almost strongest sp²hybrid bond similar to graphene, and has the similar electronic band structure as iron-based superconductors, which makes it possible to realize high-temperature superconductivity. We have compared various possible mechanisms of superconductivity, including the unconventional s;superconductivity driven by spin fluctuation and conventional superconductivity based on electron–phonon coupling. Theoretical studies have shown that octagraphene has relatively high structural stability. Although many 2 D carbon materials with C;carbon ring and C;carbon ring structures have been reported, it is still challenging to realize the octagraphene with pure square-octagon structure experimentally.This material holds hope to realize new 2 D high-temperature superconductivity.     

Mesoscopic Persistent Currents,Aharonov—Bohm Magnetic Flux and Time Reversal Symmetry

We discuss the effect of Aharonov-Bohm magnetic flux on the time reversal symmetric properties of mesoscopic metallic ring systems.It is usually believed that AB flux causes time reversal symmetry breaking.We analyse the case of mesoscopic persistent currents and find out that AB flux does not break time reversal symmetry.our arguments are supported by the general theory of mesoscopic persistent currents.