Coevolution of superconductivity and Hall coefficient with anisotropic lattice shrinkage in compressed KCa_(2)Fe_(4)As_(24F_(2)

The stability of superconductivity in superconductors is widely recognized to be determined by various factors,including charge,spin,orbit,lattice,and other related degrees of freedom.Here,we report our findings on the pressure-induced coevolution of superconductivity and Hall coefficient in KCa_(2)Fe_(4)As_(24F_(2),an iron-based superconductor possessing a hybrid crystal structure combining KFe_(2)As_(2) and CaFeAsF.Our investigation,involving high-pressure resistance,Hall effect and x-ray diffraction(XRD) measurements,allows us to observe the connection of the superconductivity and Hall coefficient with the anisotropic lattice shrinkage.We find that its ambient-pressure tetragonal(T) phase presents a collapse starting at around 18 GPa,where the sign of the Hall coefficient(R_(H)) changes from positive to negative.Upon further compression,both superconducting transition temperature(T_(c)) and R_(H) exhibit a monotonous decrease.At around 41 GPa,the superconductivity is completely suppressed(T_(c)=0),where the parameter a begins to decline again and the Hall coefficient remains nearly unchanged.Our experiment results clearly demonstrate that the pressure-induced anisotropic lattice collapse plays a crucial role in tuning the interplay among multiple degrees of freedom in the superconducting system and,correspondingly,the stability of the superconductivity.

The c-axis complex permittivity and electrical impedance in BaFe_2As_2: Experimental examination on transformation validity

Complex permittivity and electrical impedance have been measured along the c-axis in single crystals BaFe_2 As_2,which are the conductors known as the parent compound of 122-type iron superconductor. The resultant relative errors defined in the study indicate the existence of the transformation between complex permittivity and electrical impedance in the conductors, and these two physics quantities possibly reveal different aspects of the consistent superconductivityrelevant physics picture.

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 √2×√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 √2×√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.

Excess-iron driven spin glass phase in Fe_(1+y)Te_(1-x)S_(ex)

The iron-chalcogenide superconductor FeTe_(1-x)Se_(x) displays a variety of exotic features distinct from iron pnictides.Although much effort has been devoted to understanding the interplay between magnetism and superconductivity near x=0.5,the existence of a spin glass phase with short-range magnetic order in the doping range(x~0.1-0.3)has rarely been studied.Here,we use DC/AC magnetization and(quasi)elastic neutron scattering to confirm the spin-glass nature of the short-range magnetic order in a Fe_(1.07)Te_(0.8)Se_(0.2) sample.The AC-frequency dependent spin-freezing temperature T_(f) generates a frequency sensitivityΔT_(f)(ω)/[T_(f)(ω)Δlog_(10)ω]≈0.028 and the description of the critical slowing down withτ=τ0(T_(f)/T_(SG-1))^(-zv) gives T_(SG)≈22 K and zv≈10,comparable to that of a classical spin-glass system.We have also extended the frequency-dependent T_(f) to the smaller time scale using energy-resolution-dependent neutron diffraction measurements,in which the T_(N) of the short-range magnetic order increases systematically with increasing energy resolution.By removing the excess iron through annealing in oxygen,the spin-freezing behavior disappears,and bulk superconductivity is realized.Thus,the excess Fe is the driving force for the formation of the spin-glass phase detrimental to bulk superconductivity.

Magnetic Phase Diagram of Cu4-xZnx(OH)6FBr Studied by Neutron-Diffraction and μSR Techniques

We systematically investigate the magnetic properties of Cu4-xZnx(OH)6FBr using the neutron diffraction and muon spin rotation and relaxation(μSR) techniques.Neutron-diffraction measurements suggest that the longrange magnetic order and the orthorhombic nuclear structure in the x=0 sample can persist up to x=0.23 and 0.43,respectively.The temperature dependence of the zero-field μSR spectra provides two characteristic temperatures,TA0 and Tλ,which are associated with the initial drop close to zero time and the long-time exponential decay of the muon relaxation,respectively.Comparison between TA0 and TM from previously reported magnetic-susceptibility measurements suggest that the former comes from the short-range interlayer-spin clusters that persist up to x=0.82.On the other hand,the doping level where Tλ becomes zero is about 0.66,which is much higher than threshold of the long-range order,i.e.,~0.4.Our results suggest that the change in the nuclear structure may alter the spin dynamics of the kagome layers and a gapped quantum-spin-liquid state may exist above x=0.66 with the perfect kagome planes.

Optimization for epitaxial fabrication of infinite-layer nickelate superconductors

The discovery of nickelates superconductor creates exciting opportunities to unconventional superconductivity. However, its synthesis is challenging and only a few groups worldwide can obtain samples with zero-resistance. This problem becomes the major barrier for this field. From plume dynamics perspective, we found the synthesis of superconducting nickelates is a complex process and the challenge is twofold, i.e., how to stabilize an ideal infinite-layer structure Nd_(0.8)Sr_(0.2)NiO_(2), and then how to make Nd_(0.8)Sr_(0.2)NiO_(2) superconducting? The competition between perovskite Nd_(0.8)Sr_(0.2)NiO_(3) and Ruddlesden−Popper defect phase is crucial for obtaining infinite-layer structure. Due to inequivalent angular distributions of condensate during laser ablation, the laser energy density is critical to obtain phase-pure Nd_(0.8)Sr_(0.2)NiO_(3). However, for obtaining superconductivity, both laser energy density and substrate temperature are very important. We also demonstrate the superconducting Nd_(0.8)Sr_(0.2)NiO_(2) epitaxial film is very stable in ambient conditions up to 512 days. Our results provide important insights for fabrication of superconducting infinite-layer nickelates towards future device applications.

Protonation induced high-T_c phases in iron-based superconductors evidenced by NMR and magnetization measurements

Chemical substitution during growth is a well-established method to manipulate electronic states of quantum materials, and leads to rich spectra of phase diagrams in cuprate and iron-based superconductors. Here we report a novel and generic strategy to achieve nonvolatile electron doping in series of(i.e.11 and 122 structures) Fe-based superconductors by ionic liquid gating induced protonation at room temperature. Accumulation of protons in bulk compounds induces superconductivity in the parent compounds, and enhances the Tclargely in some superconducting ones. Furthermore, the existence of proton in the lattice enables the first proton nuclear magnetic resonance(NMR) study to probe directly superconductivity. Using Fe S as a model system, our NMR study reveals an emergent high-Tcphase with no coherence peak which is hard to measure by NMR with other isotopes. This novel electric-fieldinduced proton evolution opens up an avenue for manipulation of competing electronic states(e.g.Mott insulators), and may provide an innovative way for a broad perspective of NMR measurements with greatly enhanced detecting resolution.

Surface morphology and electronic structure in stoichiometric superconductor CaKFe_(4)As_(4) robed by scanning tunneling microscopy/spectroscopy

CaKFe_(4)As_(4) is a new-type superconductor with a relatively high transition temperature of 35 K among stoichiometric iron-based superconductors. Based on scanning tunneling microscopy/spectroscopy, the surface morphology and electronic structure of CaKFe_(4)As_(4) single crystal were systematically investigated. The cleaved CaKFe_(4)As_(4) showed various morphologies, such as atomically resolved 1×1, 1×2, and √2×√2 lattices. By analyzing the geometrical correlations of these morphologies, the 1×1 and 1×2 lattices were identified as the original and reconstructed As layers, respectively, whereas the √2×√2 lattice was distinguished as the reconstructed alkaline-earth-metal or alkali-metal layer. The superconducting energy gap of 7.3 me V and bosonic mode of 12.7 me V were resolved in the scanning tunneling spectra. In addition, the superconducting energy gaps measured on different terminations were identical and consistent with the values obtained by bulk-sensitive techniques, indicating that the electronic structures of CaKFe_(4)As_(4) were insensitive to the surface reconstructions. Our study clarifies the relationships between complex surface reconstructions and surface terminations and preliminarily presents that there is no obvious effect of surface reconstructions on electronic states.

Stability of superconducting Nd_(0.8)Sr_(0.2)NiO_(2) thin films

The discovery of superconducting states in the nickelate thin film with infinite-layer structure has paved a new way for studying unconventional superconductivity.So far,research in this field is still very limited due to difficulties in sample preparation.Here we report the successful preparation of the superconducting state of Nd_(0.8)Sr_(0.2)NiO_(2)thin film(T_(c)=8.0-11.1 K)and study the stability of such films in the ambient environment,water,and under electrochemical conditions.Our work demonstrates that the superconducting state of Nd_(0.8)Sr_(0.2)NiO_(2)is remarkably stable,which can last for at least 47-day continuous exposure to air at 20°C and 35%relative humidity.We also show that the superconductivity disappears after being immersed in de-ionized water at room temperature for 5 h.Surprisingly,it can also survive under ionic liquid gating conditions with an applied voltage of about 4 V,which is even more stable than conventional perovskite complex oxides.

Anisotropic magnetoelastic response in the magnetic Weyl semimetal Co_(3)Sn_(2)S_(2)

Co_(3)Sn_(2)S_(2) is a recently identified magnetic Weyl semimetal in Shandite compounds. Upon cooling, Co_(3)Sn_(2)S_(2) undergoes a ferromagnetic transition with c-axis polarized moments(0.3 μ_(B)/Co) around T_(C)= 175 K, followed by another magnetic anomaly around T_(A)≈ 140 K. A large intrinsic anomalous Hall effect is observed in the magnetic state below TC with a maximum of anomalous Hall angle near T_(A). Here, we report an elastic neutron scattering on the crystalline lattice of Co_(3)Sn_(2)S_(2) in a magnetic field up to 10 T. A strongly anisotropic magnetoelastic response is observed, while only a slight enhancement of the Bragg peaks is observed when B//c. The in-plane magnetic field(B//ab) dramatically suppresses the Bragg peak intensity probably by tilting the moments and lattice toward the external field direction. The in-plane magnetoelastic response commences from T_(C), and as it is further strengthened below T_(A), it becomes nonmonotonic against the field between T_(A) and T_(C) because of the competition from another in-plane magnetic order. These results suggest that a magnetic field can be employed to tune the Co_(3)Sn_(2)S_(2) lattice and its related topological states.

Spin excitations and spin wave gap in the ferromagnetic Weyl semimetal Co3Sn2S2

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co3Sn2S2 with a quasi-two-dimensional structure.Both in-plane and out-of-plane dispersions of the spin waves were revealed in the ferromagnetic state.Similarly,dispersive but damped spin excitations were found in the paramagnetic state.The effective exchange interactions were estimated using a semi-classical Heisenberg model to consistently reproduce the experimental TCand spin stiffness.However,a full spin wave gap below Eg=2.3 meV was observed at T=4 K.This value was considerably larger than the estimated magnetic anisotropy energy(~0.6 meV),and its temperature dependence indicated a significant contribution from the Weyl fermions.These results suggest that Co3Sn2S2 is a three-dimensional correlated system with a large spin stiffness,and the low-energy spin dynamics can interplay with the topological electron states.

Planckian dissipation and non-Ginzburg-Landau type upper critical field in Bi2201

Resistivity and Hall effect measurements have been carried out on a micro-fabricated bridge of Bi2201 single crystal at low temperatures down to 0.4 K under high magnetic fields.When superconductivity is crashed by a high magnetic field,the recovered "normal state" resistivity still shows a linear temperature dependence in the low temperature region.Combining with the effective mass and the charge carrier density,we get a linear scattering rate 1/τ=αkBT/h with 0.77 <α <1.16,which gives a strong evidence of the Planckian dissipation.Furthermore,our results reveal a new type of temperature dependence of the upper critical field,H_(c2)(T)=H*■,which is totally different from the expectation of the Ginzburg-Landau theory,and suggests the existence of uncondensed Cooper pairs above H_(c2)(T) line.