Preparation of Superconducting Thin Films of Infinite-Layer Nickelate Nd_(0.8)Sr_(0.2)NiO_(2)

The recent observation of superconductivity in thin films of infinite-layer nickelate Nd_(0.8)Sr_(0.2)NiO_(2) has received considerable attention.Despite the many efforts to understand the superconductivity in infinite-layer nickelates,a consensus on the underlying mechanism for the superconductivity has yet to be reached,partly owing to the challenges with the material synthesis.Here,we report the successful growth of superconducting infinite-layer Nd_(0.8)Sr_(0.2)NiO_(2) films by pulsed laser deposition and soft chemical reduction.The details on the growth process are discussed.

Evidence for Multiple Underlying Fermi Surface and Isotropic Energy Gap in the Cuprate Parent Compound Ca2CuO2Cl2

The parent compounds of the high-temperature cuprate superconductors are Mott insulators.It has been generally agreed that understanding the physics of the doped Mott insulators is essential to understanding the mechanism of high temperature superconductivity.A natural starting point is to elucidate the basic electronic structure of the parent compound.Here we report comprehensive high resolution angle-resolved photoemission measurements on Ca_2CuO_2Cl_2,a Mott insulator and a prototypical parent compound of the cuprates.Multiple underl.ying Fermi surface sheets are revealed for the first time.The high energy waterfall-like band dispersions exhibit different behaviors near the nodal and antinodal regions.Two distinct energy scales are identified：a d-wave-like low energy peak dispersion and a nearly isotropic lower Hubbard band gap.These observations provide new information of the electronic structure of the cuprate parent compound,which is important for understanding the anomalous physical properties and superconductivity mechanism of the high temperature cuprate superconductors.

Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi2Sr2CaCu2O8+δ

High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers.A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced.Here,by in situ vacuum annealing and Rb deposition on the Bi2Sr2Ca0.6Dy0.4Cu2O8+δ(Bi2212)sample surface to push its doping level continuously from deeply underdoped(Tc=25K,doping level p^0.066)to the near-zero doping parent Mott insulator,angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time.Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping,which is quite close to the upper Hubbard band.With increasing hole doping,the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior.When the chemical potential approaches the charge transfer band at a doping level of^0.05,the nodal spectral weight near the Fermi level increases,followed by the emergence of the coherent quasiparticle peak and the insulator-superconductor transition.Our observations provide key insights in understanding the insulator-superconductor transition in doping the parent cuprate compound and for establishing related theories.

Distinct Superconducting Gap on Two Bilayer-Split Fermi Surface Sheets in Bi_2Sr_2CaCu_2O_(8+δ) Superconductor

High resolution laser-based angle-resolved photoemission measurements are carried out on an overdoped superconductor Bi_2Sr_2CaCu_2O_(8+)with a_(c )of 75 K.Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels:the bonding sheet corresponds to a doping level of 0.14,which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped,giving an overall doping level of 0.20 for the sample.Different superconducting gap sizes on the two Fermi surface sheets are revealed.The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet.The maximum gap difference between the two Fermi surface sheets near the antinodal region is～2 meV.These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity,and the layer-dependent superconductivity in high temperature cuprate superconductors.

Evolution of incommensurate superstructure and electronic structure with Pb substitution in(Bi2-xPbx)Sr2CaCu2O8+δ superconductors

High-quality Bi2-xPbxSr2 CaCu2O8+δ(Bi2212) single crystals have been successfully grown by the traveling solvent floating zone technique with a wide range of Pb substitution(x = 0–0.8).The samples are characterized by transmission electron microscope(TEM) and measured by high resolution laser-based angle-resolved photoemission spectroscopy(ARPES) with different photon energies.A systematic evolution of the electronic structure and superstructure with Pb substitution has been revealed for the first time.The superstructure shows a significant change with Pb substitution and the incommensurate modulation vector(Q) decreases with increasing Pb substitution.In the meantime, the superstructure intensity from ARPES measurements also decreases dramatically with increasing Pb concentration.The superstructure in Bi2212 can be effectively suppressed by Pb substitution and it nearly disappears with a Pb substitution of x = 0.8.We also find that the superstructure bands in ARPES measurements depend sensitively on the photon energy of lasers used;they can become even stronger than the main band when using a laser photon energy of 10.897 eV.These results provide important information on the origin of the incommensurate superstructure and its control and suppression in bismuth-based high temperature superconductors.

Detailed electronic structure of three-dimensional Fermi surface and its sensitivity to charge density wave transition in ZrTe3 revealed by high resolution laser-based angle-resolved photoemission spectroscopy

The detailed information of the electronic structure is the key to understanding the nature of charge density wave （CDW） order and its relationship with superconducting order in the microscopic level. In this paper, we present a high resolution laser-based angle-resolved photoemission spectroscopy （ARPES） study on the three-dimensional （3D） hole-like Fermi surface around the Brillouin zone center in a prototypical quasi-one-dimensional CDW and superconducting system ZrTe3. Double Fermi surface sheets are clearly resolved for the 3D hole-like Fermi surface around the zone center. The 3D Fermi surface shows a pronounced shrinking with increasing temperature. In particular, the quasiparticle scattering rate along the 3D Fermi surface experiences an anomaly near the charge density wave transition temperature of ZrTe3 - 63 K）. The signature of electron-phonon coupling is observed with a dispersion kink at -20 meV; the strength of the electron-phonon coupling around the 3D Fermi surface is rather weak. These results indicate that the 3D Fermi surface is also closely connected to the charge-density-wave transition and suggest a more global impact on the entire electronic structure induced by the CDW phase transition in ZrTe3.

Evidence of Electron-Hole Imbalance in WTe2 from High-Resolution Angle-Resolved Photoemission Spectroscopy

WTe2 has attracted a great deal of attention because it exhibits extremely large and non-saturating magnetore- sistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe2. This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concen- tration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range,and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe2. We identify a fiat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a fiat band can play an important role in dictating the transport properties of WTe2. Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe2.

Temperature Evolution of Energy Gap and Band Structure in the Superconducting and Pseudogap States of Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

We carry out detailed momentum-dependent and temperature-dependent measurements on Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi2212)superconductor in the superconducting and pseudogap states by super-high resolution laser-based angleresolved photoemission spectroscopy.The precise determination of the superconducting gap for the nearly optimally doped Bi2212(T_(c)=91 K)at low temperature indicates that the momentum-dependence of the superconducting gap deviates from the standard d-wave form(cos(2Φ)).It can be alternatively fitted by including a high-order term(cos(6Φ))in which the next nearest-neighbor interaction is considered.We find that the band structure near the antinodal region smoothly evolves across the pseudogap temperature without a signature of band reorganization which is distinct from that found in Bi_(2)Sr_(2)CuO_(6+δ)superconductors.This indicates that the band reorganization across the pseudogap temperature is not a universal behavior in cuprate superconductors.These results provide new insights in understanding the nature of the superconducting gap and pseudogap in high-temperature cuprate superconductors.

Advances in deep ultraviolet laser based high-resolution photoemission spectroscopy

We briefly review recent results on photoemission spectroscopy based on the deep and vacuum ultraviolet diode pumped solid-state lasers which we have developed.Cascaded second harmonic generation with the nonlinear crystal KBe2BO3F2(KBBF)is used to generate deep ultraviolet and vacuum ultraviolet laser radiation,which complements traditional incoherent light sources such as gas discharge lamps and synchrotron radiation,and has greatly improved resolution with respect to energy,momentum,and spin of photoemission spectroscopy.Many new functions have been developed with the advantages of high photon energy,narrow linewidth,high photon flux density,and so on.These have led to the observation of various new phenomena and the amassment of new data in the fields of high temperature superconductivity,topological electronics,Fermi semi-metals,and so forth.These laser systems have revived the field of photoemission spectroscopy and provided a new platform in this frontier research field.

An efficient route to prepare suspended monolayer for feasible optical and electronic characterizations of two-dimensional materials

Two-dimensional(2D)materials are highly sensitive to substrates,interfaces,and the surrounding environments.Suspended 2D materials are free from substrate-induced effects,thus an ideal approach to study their intrinsic properties.However,it is very challenging to prepare large-area suspended 2D materials with high efficiency.Here we report a universal method,based on pretreatments of densely patterned hole array substrates with either oxygenplasma or gold film deposition,to prepare large-area suspended mono-and few-layer 2D materials.Multiple structural,optical,and electrical characterization tools were used to fully evaluate the improved performance of various suspended 2D layers.Some of these observations reported in this study are:(1)Observation of a new Raman low frequency mode for the suspended MoS_(2);(2)Significantly stronger photoluminescence(PL)and second harmonic generation(SHG)signals of suspended WSe_(2),which enables the study of new optical transition processes;(3)The low energy electron diffraction pattern on suspended MoS_(2) also exhibits much sharper spots than that on the supported area;and(4)The mobility of suspended graphene device approaches 300000 cm^(2) V^(-1) s^(-1),which is desirable to explore the intrinsic properties of graphene.This work provides an innovative and efficient route for fabricating suspended 2D materials,and we expect that it can be broadly used for studying intrinsic properties of 2D materials and in applications of hybrid active nanophotonic and electronic devices.

Superconductivity in the cuprates： Deduction of mechanism for d-wave pairing through analysis of ARPES

In the Eliashberg integral equations for d-wave superconductivity, two different functions （α2F）n（ω, θ） and （α2F）p,d（ω） determine, respectively, the ＂normal＂ self-energy and the ＂pairing＂ self-energy, w is the frequency of fluctuations scattering the fermions whose momentum is near the Fermi-surface and makes an angle θ to a chosen axis. We present a quantitative analysis of the high-resolution laser based Angle Resolved Photoemission Spectroscopy （ARPES） data on a slightly under doped cuprate compound Bi2212 and use the Eliashberg equations to deduce the w and dependence of （α2F）n（ω, θ） for T just above Tc and below Tc. Besides its detailed w dependence, we find the remarkable result that this function is nearly independent of θ between the （π; π）-direction and 25 degrees from it, except for the dependence of the cut-off energy on 8. Assuming that the same fluctuations determine both the normal and the pairing self-energy, we ask what theories give the function （α2F）p,d（ω） required for the d-wave pairing instability at high temperatures as well as the deduced （α2F）n（θ, w）. We show that the deduced （α2F）n（θ, w） can only be obtained from antiferromagnetic （AFM） fluctuations if their correlation length is smaller than a lattice constant. Using （α2F）p,d（W） consistent with such a correlation length and the symmetry of matrix-elements scatter- ing fermions by AFM fluctuations, we calculate Tc and show that AFM fluctuations are excluded as the pairing mechanism for d-wave superconductivity in cuprates. We also consider the quantumcritical fluctuations derived microscopically as the fluctuations of the observed loop-current order discovered in the under-doped cuprates, and which lead to the marginal Fermi-liquid properties in the normal state. We show that their frequency dependence and the momentum dependence of their matrix-elements to scatter fermions are consistent with the θ and w dependence of the deduced （α2F）n（ω, θ）. The pairing kernel （α2F）p,d（W） calculated using the experimental values in the Eliashberg equation gives d-wave instability at Tc comparable to the experiments.