Scanning tunneling microscopy and spectroscopy have been performed on superconducting La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.06, 0.12, 0.16$, and 0.21) at 4.2 K. Atomically resolved surface topography has been observed on the $ab$ surface, which exhibits a one-dimensional structure corresponding to the buckling pattern of the orthorhombic phase. The tunneling spectra of all samples exhibit kinks at about $\pm 5$ meV in addition to the energy gap, indicating the existence of two energy scales. While the gap magnitude varies on the length scale of a few nm and steadily increases upon underdoping, the kink energy is spatially uniform on the surface and almost independent of the doping level.
We determined the optical conductivity of Bi(2)Sr(2-x)La(x)CuO(6) at dopings covering the phase diagram from the underdoped to the overdoped regimes. The frequency dependent scattering rate shows a pseudogap extending into the overdoped regime. We found that the effective mass enhancement calculated from the optical conductivity is constant throughout the phase diagram. Conversely, the effective optical charge density varies almost linearly with doping. Our results suggest that the low frequency electrodynamics of Bi(2)Sr(2-x)La(x)CuO(6) is not strongly affected by the long range Mott transiti…
Doping dependence of the vortex glass and sublimation transitions in the high-$T_{c}$ superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ as determined from macroscopic measurements
磁化强度和交流磁化率测量用于表征高温铜酸盐超导体La $_{2-x}$ Sr $_{x}$ CuO $_{4}$在大范围的掺杂（0.075 $\leq x\leq$ 0.20）的混合相。研究了高磁场（垂直于$CuO_2$面施加8特斯拉）下的一级涡旋晶格相变线$H_{FOT}(T)$、上临界场$H_{c2}(T)$和第二峰$H_{sp}(T)$。我们的研究结果揭示了一个强大的掺杂依赖的磁相图，这主要可以解释与欠掺杂的各向异性增加。在我们的解释，第一阶涡晶格相变是由于升华（而不是熔化）的涡晶格成气体的煎饼涡，而第二个峰值是有关的过渡到一个更无序的涡玻璃态。
Transport properties of Nd$_{2-x}$Ce$_x$CuO$_{4+δ}$ single crystal films are investigated in magnetic fields $B$ up to 9T at $T$=(0.4-4.2)K. An analysis of normal state (at $B>B_{c2}$) Hall coefficient $R_H$$^n$ dependence on Ce doping takes us to a conclusion about the existence both of electron-like and hole-like contributions to transport in nominally electron-doped system. In accordance with $R_H$$^n$(x) analysis an anomalous sign reversal of Hall effect in mixed state at $B<B_{c2}$ may be ascribed to a flux-flow regime for two types of carriers with opposite charges.
We demonstrate that tunneling spectra in deeply underdoped Bi$_{2}$Sr$_{2}$CuO$_{6+δ}$ (Bi2201) and Ca$_2$CuO$_2$Cl$_2$ (CCOC) provide clear evidence for nanoscale phase separation (NPS), causing the gap to fill with doping rather than close. The phase separation extends over a doping range from half filling to approximately $x\sim 0.09$. Assuming the NPS is in the form of stripes, then the nodal gap -- which we model as a Coulomb gap -- arises from impurity pinning of the charged stripes, ultimately driving a metal-insulator transition.
First principles band-structure calculations for large supercells of Ba$_{2}$CuO$_{4-δ}$ and La$_{2}$CuO$_{4-δ}$ with different distributions and concentrations of oxygen vacancies show that the effective doping on copper sites strongly depends on where the vacancy is located. A vacancy within the Cu layer produces a weak doping effect while a vacancy located at an apical oxygen site acts as a stronger electron dopant on the copper layers and gradually brings the electronic structure close to that of La$_{2-x}$Sr$_x$CuO$_{4}$. These effects are robust and only depend marginally on lattice dist…
The pseudogap (PG) state and its related intra-unit-cell symmetry breaking remain the focus in the research of cuprate superconductors. Although the nematicity has been studied in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, especially underdoped samples, its behavior in other cuprates and different doping regions is still unclear. Here we apply a scanning tunneling microscope to explore an overdoped (Bi, Pb)$_2$Sr$_2$CuO$_{6+δ}$ with a large Fermi surface (FS). The establishment of a nematic order and its real-space distribution is visualized as the energy scale approaches the PG.
X-ray absorption in $\rm Sr_{2}CuO_{4-δ}-La_{2}CuO_4$ (SCO-LCO) superlattices shows a variable occupation with doping of a hole state different from holes doped for $x \lesssim x_{optimal}$ in bulk $\rm La_{2-x}Sr_{x}CuO_4$ and suggests that this hole state is on apical oxygen atoms and polarized in the $a-b$ plane. Considering the surface reflectivity gives a good qualitative description of the line shapes of resonant soft X-ray scattering. The interference between superlattice and surface reflections was used to distinguish between scatterers in the SCO and the LCO layers, with the two hole …
We performed measurements of switching current distribution in a submicron La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) intrinsic Josephson junction (IJJ) stack in a wide temperature range. The escape rate saturates below approximately 2\,K, indicating that the escape event is dominated by a macroscopic quantum tunneling (MQT) process with a crossover temperature $T^{*}\approx2\,$K. We applied the theory of MQT for IJJ stacks, taking into account dissipation and the phase re-trapping effect in the LSCO IJJ stack. The theory is in good agreement with the experiment both in the MQT and in the thermal activati…
We study magnetotransport properties of the electron-doped superconductor Pr$_{2-x}$Ce$_x$CuO$_{4\pmδ}$ with $x$ = 0.14 in magnetic fields up to 92~T, and observe Shubnikov de-Haas magnetic quantum oscillations. The oscillations display a single frequency $F$=255$\pm$10~T, indicating a small Fermi pocket that is $\sim$~1\% of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large hole-like cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consist…
Electron correlations play a dominant role in the charge dynamics of the cuprates. We use resonant inelastic x-ray scattering (RIXS) to track the doping dependence of the collective charge excitations in electron doped La$_{2-x}$Ce$_{x}$CuO$_{4}$(LCCO). From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional (3D) plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long length scales. With increasing electron doping, the plasmon excitations show monotonic hardening…
The in-plane penetration depth of Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films at various doping obtained from oxygen reduction has been measured, using AC susceptibility measurements. For the higher doping samples, the superfluid density deviates strongly from the s-wave behavior, suggesting, in analogy with other electron-doped cuprates, a contribution from a nodal hole pocket, or a small gap on the Fermi surface such as an anisotropic s-wave order parameter. The low value of the superfluid densities, likely due to a strong doping-induced disorder, places the superconducting transition of ou…
Novel nanoscale probes are opening new venues for understanding unconventional electronic and magnetic functionalities driven by multiscale lattice complexity in doped high temperature superconducting perovskites. In this work we focus on the multiscale texture at supramolecular level of atomic oxygen interstitials (O-i) stripes in HgBa$_2$CuO$_{4+y}$ at optimal doping for the highest superconducting critical temperature $T_C$=94K. We report compelling evidence for the nematic phase of oxygen-interstitial O-i atomic wires with fractal-like spatial distribution over multiple scales by using sca…
Angle resolved photoemission spectroscopy (ARPES) studies of the overdoped cuprate superconductor La$_{2-x}$Sr$_x$CuO$_4$ find only small changes in the near nodal electron self energy over a spectral range of several hundred meV as the doping increases from x=0.2 to x=0.3 and the superconducting transition temperature T_c decreases from 32K to 0K. These measurements put constraints on the structure of the electron-electron interaction. Here we show that a spin-fluctuation interaction leads to behavior which is consistent with these experimental results.
We investigated Ce-substitution and reduction annealing effects on the electronic states at copper sites by Cu ${K}$-edge x-ray absorption near-edge structure measurements in Pr$_{2-x}$Ce$_x$CuO$_{4+α-δ}$ (PCCO) with varying $x$ and $δ$ (the amount of oxygen loss during annealing) values. Absorption near-edge spectra were modified by Ce-substitution and reduction annealing in a similar manner with increasing $x$ and $δ$. Considering electron doping by Ce-substitution, this similarity indicates an increase of electron number at the copper sites due to annealing $n_{\rm AN}$. Thus, the total num…
The possible scenario of what happens in the plane during the doping of cuprates with oxygen holes is subject of this brief communication. The simultaneously inserted oxygen holes and electrons create two components plasma. The oxygen holes driven by the repelling Coulomb force create a quasi-static lattice, the dimension of which depends on the doping level. The stability of such a lattice, according to the equipartition theorem, depends on the temperature. On this basis one can define the temperature at which the Coulomb repelling energy is balanced by thermal energy of the displacements of …
We report a detailed spectroscopic study of the electron doped cuprate superconductor Pr(2-x)Ce(x)CuO(4-δ) using point contact junctions for x=0.125(underdoped), x=0.15(optimally doped) and x=0.17(overdoped). From our conductance measurements we are able to identify bosonic resonances for each doping. These excitations disappear above the critical temperature, and above the critical magnetic field. We find that the energy of the bosonic excitations decreases with doping, which excludes lattice vibrations as the paring glue. We conclude that the bosonic mediator for these cuprates is more likel…
This article presents the results of several magnetic phases of doped La$_{2-x}$Sr$_x$CuO$_4$ using density-functional theory with an added Hubbard term (DFT+U). Doping factors from $x=0$ to 0.25 were examined. We found that a bond centered stripe is the magnetic ground state for $x=1/8$ and $x=1/4$. No stable stripe order was found for $x=1/6$. Analysis of the electron density revealed that apical oxygen atoms, those located above and below the copper atoms in the CuO$_2$ planes, hold a non negligible part of the holes at large doping and present a small spin polarization. Finally, the charge…
Quasi-two dimensional quantum magnetism is clearly highly correlated with superconducting ground states in cuprate-based High T$_c$ superconductivity. Three dimensional, commensurate long range magnetic order in La$_2$CuO$_4$ quickly evolves to quasi-two dimensional, incommensurate correlations on doping with mobile holes, and superconducting ground states follow for x as small as 0.05 in the La$_{2-x}$Sr$_x$/Ba$_x$CuO$_4$ family of superconductors. It has long been known that the onset of superconducting ground states in these systems is coincident with a remarkable rotation of the incommensu…
Finding more efficient ways of exploring the doping phase diagrams of high temperature superconductors as well as probing the fundamental properties of these materials are essential ingredients for driving the discovery of new materials. We use a doping technique involving gating with ionic liquids to systematically and continuously tune the T$_{c}$ of superconducting La$_{2}$CuO$_{4+x}$ thin films. We probe both the transport properties and the penetration depth of these samples and find that Homes scaling $λ^{-2}\proptoσT_{c}$ is obeyed, consistent with these materials being in the dirty lim…
The structural and electronic properties of La$_{2-x}$Ba$_x$CuO$_4$ were investigated as a function of Ba-concentration $0 \leq x \leq 1.2$, within the virtual crystal approximation (VCA) by means of first-principles total-energy calculations. We found Ba doping induces a quasi-rigid displacement of Cu d$_{x^{2}-y^{2}}$ and d$_{z^{2}}$ bands toward higher energies. This effect generates important changes in the Fermi surface topology, which manage to imitate the ARPES momentum distribution map (MDM) measurements and tight-binding calculations recently reported for doped La-based cuprates. The …
Exploring whether a spin density wave (SDW) is responsible for the charge excitations gap in the high-temperature superconducting cuprates is difficult, since the region of the phase diagram where the magnetic properties are clearly exposed is different from the region where the band dispersion is visible. On the one hand, long range magnetic order disappears as doping approaches 2% from below, hindering our ability to perform elastic neutron scattering (ENS). On the other hand, cuprates become insulating at low temperature when the doping approaches 2% from above, thus restricting angle-resol…
Using resonant X-ray diffraction and Raman spectroscopy, we study charge correlations and lattice dynamics in two model cuprates, HgBa$_{2}$CuO$_{4+δ}$ and HgBa$_{2}$CaCu$_{2}$O$_{6+δ}$. We observe a maximum of the characteristic charge order temperature around the same hole concentration ($p \approx 0.09$) in both compounds, and concomitant pronounced anomalies in the lattice dynamics that involve the motion of atoms in and/or adjacent to the CuO$_2$ layers. These anomalies are already present at room temperature, and therefore precede the formation of the static charge correlations, and we a…
Doping evolution of the electronic structure in the single-layer cuprates Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$: Comparison with other single-layer cuprates
我们已经进行了角分辨光电子能谱和芯层X射线光电子能谱研究的单层铜酸盐Bi $_2$ Sr $_{2-x}$ La $_x$ CuO $_{6+δ}$（Bi 2201），并揭示了掺杂的电子结构的演变从轻掺杂到最佳掺杂区域。我们已经观察到的色散准粒子带的形成，费米“弧”的费米面的演变和化学势的移动与空穴掺杂在其他铜酸盐。Bi 2201中的掺杂演变类似于Ca $_{2-x}$ Na $_{x}$ CuO $_{2}$ Cl $_2$（Na-CCOC）中的掺杂演变，其中已经观察到朝向母绝缘体的较低哈伯德带的快速化学势偏移，但是与La $_{2-x}$ Sr $_{x}$ CuO $_{4}$（LSCO）中的掺杂演变完全不同，其中化学势不偏移，而色散带和费米弧/表面已经在轻掺杂区中的费米能级周围形成。利用紧束缚拟合定量分析了费米面形状和能带色散，推导出的次近邻跳跃积分$t'$也证实了与Na-CCOC的相似性和与LSCO的不同.
A central issue for copper oxides is the nature of the insulating ground state at low carrier densities and the emergence of high-temperature superconductivity from that state with doping. Even though this superconductor-insulator transition (SIT) is a zero-temperature transition, measurements are not usually carried out at low temperatures. Here we use magnetoresistance to probe both the insulating state at very low temperatures and the presence of superconducting fluctuations in La_{2-x}Sr_{x}CuO_{4}(LSCO) films, for doping levels that range from the insulator to the superconductor (x=0.03-0…
Point contact spectroscopy of the electron-doped cuprate superconductor Pr{2-x}Ce{x}CuO4: The dependence of conductance-voltage spectra on cerium doping, barrier strength and magnetic field
Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic studies of electron-doped cuprate superconductor $Sr_{0.9}La_{0.1}CuO_2$
Here we calculate the strength of the $d$-wave pairing and the $k$ dependence of the gap function associated with the nematic fluctuations of a CuO$_2$ model as the doping $p$ approaches a quantum critical point. Higher order $d$-wave harmonics contribute to the $k$ dependence of the resulting superconducting gap function reflecting the longer range nature of the nematic pairing interaction.
While recent experiments have found that at optimum doping for the highest critical temperature in HgBa$_2$CuO$_{4+y}$ (Hg1201) the oxygen interstitials (O-i) are not homogeneously distributed but form one-dimensional atomic wires, there are no available information of its electronic structure considering self-organized O-i atomic wires. Here we report the calculated electronic structure of HgBa$_2$CuO$_{4+y}$ where oxygen interstitials form atomic wires along (1,0,0) crystal direction in the Hg layer. We find that at optimum doping for superconductivity the chemical potential is tuned near an…
