Superconducting Pr$_2$CuO$_x$, $x\simeq 4$ (PCO) films with $T^\prime$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency (7 - 55000 cm$^{-1}$) and temperature (2 to 300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about 300 cm$^{-1}$ persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for $d$-wave superconductors. The absolute value of the penetration depth for zero temperature is 1.6 $μ$m, indicating a rather low density of the superconducting condensate.
The specific heat $C$ of the single-layer cuprate superconductor HgBa$_2$CuO$_{4 + δ}$ was measured in an underdoped crystal with $T_{\rm c} = 72$ K at temperatures down to $2$ K in magnetic fields up to $35$ T, a field large enough to suppress superconductivity at that doping ($p \simeq 0.09$). In the normal state at $H = 35$ T, a residual linear term of magnitude $γ= 12 \pm 2$ mJ/K$^2$mol is observed in $C/T$ as $T \to 0$, a direct measure of the electronic density of states. This high value of $γ$ has two major implications. First, it is significantly larger than the value measured in overd…
Conventional proximity effect in bilayers of superconducting underdoped $La_{1.88}Sr_{0.12}CuO_4$ islands coated with non superconducting overdoped $La_{1.65}Sr_{0.35}CuO_4$
Electronic Structure of Ce-Doped and -Undoped Nd$_2$CuO$_4$ Superconducting Thin Films Studied by Hard X-ray Photoemission and Soft X-ray Absorption Spectroscopy
In this work zero-temperature phase diagrams of cuprates with Ba$_2$CuO$_3$-type CuO chain structure is investigated. The projective symmetry group analysis is employed in the strong coupling limit, and renormalization group with bosonization analysis is employed in the weak coupling limit. We find that in both of these two limits, large areas of the phase diagrams are filled with nodeless $s$-wave superconducting phases (with weak $d$-wave components), instead of pure $d$-wave phase mostly found in cuprates. This implies that nodeless $s$-wave phase is the dominant superconducting phase in cu…
For electron-doped cuprates, the strong suppression of antiferromagnetic spin correlation by efficient reduction annealing by the "protect-annealing" method leads to superconductivity not only with lower Ce concentrations but also with higher transition temperatures. To reveal the nature of this superconducting state, we have performed angle-resolved photoemission spectroscopy measurements of protect-annealed electron-doped superconductors Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ and directly investigated the superconducting gap. The gap was found to be consistent with $d$-wave symmetry, sugges…
Effects of out-of-plane disorder on the nodal quasiparticle and superconducting gap in single-layer Bi$_2$Sr$_{1.6}Ln_{0.4}$CuO$_{6+δ}$ ($Ln$ = La, Nd, Gd)
用角分辨光电子能谱研究了单层铜氧化物Bi $_2$ Sr $_{1.6}$ $Ln$ $_{0.4}$ CuO $_{6+δ}$（$Ln$ = La，Nd，Gd）的面外无序对电子结构的影响.我们观察到，随着无序度的增加，在费米面形状和能带色散不受影响的情况下，准粒子宽度增加，反节能隙增大，节区的超导能隙减小。结果表明，面外无序通过增大结区的反节能隙和降低结区的超导能隙而显著降低超导性.
The discovery of charge- and spin-density-wave (CDW/SDW) orders in superconducting cuprates has altered our perspective on the nature of high-temperature superconductivity (SC). However, it has proven difficult to fully elucidate the relationship between the density wave orders and SC. Here using resonant soft X-ray scattering we study the archetypal cuprate, La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over a broad doping range. We reveal the existence of two types of CDW orders in LSCO, namely CDW stripe order and CDW short-range order (SRO). While the CDW-SRO is suppressed by SC, it is partially transfor…
Monotonic d-wave Superconducting Gap in Optimally-Doped Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_6$ Superconductor by Laser-Based Angle-Resolved Photoemission Spectroscopy
利用超高分辨激光角分辨光电子能谱研究了最佳掺杂Bi $_2$ Sr $_{1.6}$ La $_{0.4}$ CuO $_6$超导体的超导能隙和赝能隙随动量和温度的变化关系。在超导状态下测得的能隙表现出标准的{ \it d}-波形。赝能隙在T $_c$以上的费米面的很大一部分打开，在结区附近形成“费米弧”。在“费米弧”以外的区域，赝能隙具有与超导态类似的能隙大小和动量依赖性，能隙随温度变化很小，在T_PDMATHABMR上没有突变。这些观测结果表明赝能隙和超导能隙是密切相关的，并支持赝能隙是超导能隙的先驱的观点。
Nonmonotonic $d_{x^2-y^2}$ superconducting gap in electron-doped Pr$_{0.89}$LaCe$_{0.11}$CuO$_4$: Evidence of coexisting antiferromagnetism and superconductivity?
Recent angle-resolved $c$-axis resistivity measurements of the stripe-ordered La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) with $x=1/8$ revealed an unexpected dependence on the direction of the in-plane magnetic field. We argue that these and other available data for the $c$-axis transport point to the existence of superconducting pairs of two different types in the $x=1/8$ LBCO below the stripe ordering temperature. The pairs of one type carry finite momentum and are confined to the Cu-O planes; the pairs of other type (probably the conventional $d$-wave with zero momentum) propagate along narrow conductin…
In high-temperature cuprate superconductors, the anti-ferromagnetic spin fluctuations are thought to have a very important role in naturally producing an attractive interaction between the electrons in the $d$-wave channel. The connection between superconductivity and spin fluctuations is expected to be especially consequential at the overdoped end point of the superconducting dome. In some materials, that point seems to coincide with a Lifshitz transition, where the Fermi surface changes from the hole-like centered at ($π, π$) to the electron-like, centered at the $Γ$ point causing a loss of …
We investigate the interplay between the lattice, charge, and spin dynamics in charge ordered high $T_c$ cuprate La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ ($T_{c} =4$~K) based on the inverse Laplace transform (ILT) analysis of the $^{139}$La nuclear spin-lattice relaxation rate $1/T_1$ (dubbed ILTT$_{1}$ analysis here after). A major thrust of the ILTT$_{1}$ analysis is that one can deduce the probability density function $P(1/T_1)$ of distributed $1/T_1$. We demonstrate that $1/T_{1}^{lm}$, defined as the log-mean (i.e. the center of gravity on a logarithmic scale) of $P(1/T_1)$, can be well approxim…
Superconductivity in the cuprates is found to be intertwined with charge and spin density waves. Determining the interactions between the different types of order is crucial for understanding these important materials. Here, we elucidate the role of the charge density wave (CDW) in the prototypical cuprate La$_{1.885}$Sr$_{0.115}$CuO$_4$, by studying the effects of large magnetic fields ($H$) up to 24 Tesla. At low temperatures ($T$), the observed CDW peaks reveal two distinct regions in the material: a majority phase with short-range CDW coexisting with superconductivity, and a minority phase…
Low-temperature electronic heat transport in La_{2-x}Sr_{x}CuO_{4} single crystals: Unusual low-energy physics in the normal and superconducting states
Charge order is commonly believed to compete with superconducting order. An intertwined form of superconducting wave function, known as pair-density-wave (PDW) order, has been proposed; however, there has been no direct evidence, theoretical or experimental, that it forms the ground state of any cuprate superconductor. As a test case, we consider \lbco\ with $x=1/8$, where charge and spin stripe orders within the CuO$_2$ planes compete with three-dimensional superconducting order. We report measurements of the superconducting critical current perpendicular to the planes in the presence of an i…
Growth, characterization and physical properties of high-quality large single crystals of Bi$_2$Sr$_{2-x}$La$_{x}$CuO$_6$ high-temperature superconductors
采用移动溶剂浮区法生长了高质量的大尺寸BiPDMATHABMM Sr $_{2-x}$ La $_{x}$ CuO $_6$（La-Bi 2201）单晶。样品的组成和结构的分析，其特征在于和它们的物理性质进行了研究，通过磁化率和电阻率测量。获得了具有尖锐超导转变的超导样品，其掺杂范围从过掺杂（x<0.40）、最佳掺杂（x~0.40）、欠掺杂（0.40<x<0.84）到严重欠掺杂（x>0.84）。对于x=0.73，获得大至~40 * 2.0 * 1 mm 3的晶体。当x=0.40时，获得了宽度小于2K的尖锐超导转变和近乎完美的超导电性迈斯纳信号。La-Bi_（2201）单晶的获得将为研究高温超导的物理性质、电子结构和机理提供一个理想的体系
We report a Cu K- and L$_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd$_{2-x}$Ce$_x$CuO$_4$, with focus on post-growth annealing effects. For the parent compound Nd$_2$CuO$_4$ ($x = 0$), a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of RE$_2$CuO$_4$ (RE = rare earth) can become metallic and superconducting at sufficiently high electron concentratio…
We study superconducting properties in multilayer thin films consisting of superconducting La$_{1.85}$Sr$_{0.15}$CuO$_4$ (LSCO) and Mott insulator Sr$_2$IrO$_4$ (SIO) and report enhanced superconductivity in optimized sample. These multilayer heterostructures show an increase in superconducting transition temperature ($T_C$) as compared to the single layer LSCO films. The temperature dependence of SIO single layer is also investigated under thermal activation, Arrhenius-type behaviour, and variable-range hopping mechanisms for different temperature regimes. The decrease in $T_C$ beyond an opti…
We measure the dispersion of the Cu-O bond-stretching phonon mode in the high-temperature superconducting parent compound Ca$_2$CuO$_2$Cl$_2$. Our density functional theory calculations predict a cosine-shaped bending of the dispersion along both the ($ξ$00) and ($ξξ$0) directions, while comparison with previous results on Ca$_{1.84}$CuO$_2$Cl$_2$ show it only along ($ξ$00), suggesting an anisotropic effect which is not reproduced in calculation at optimal doping. Comparison with isostructural La$_{2-x}$Sr$_x$CuO$_4$ suggests that these calculations reproduce well the overdoped regime, however…
We report the dynamics of the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_4$ ($x = 0.14$) after intense photoexcitation utilizing near-infrared (800 nm) optical pump-terahertz probe spectroscopy. In the superconducting state at 5 K, we observed a redshift of the Josephson plasma resonance that sustains for hundreds of picoseconds after the photoexcitation, indicating the destruction of the $c$-axis superconducting coherence. We show that the metastable spectral features can be described by the photoinduced surface heating of the sample. We also demonstrate that the conventional analysis used…
