Cuprate La1 Research Articles

Electronic Band Structure of a Superconducting Nickelate Probed by the Seebeck Coefficient in the Disordered Limit

Superconducting nickelates are a new family of strongly correlated electron materials with a phase diagram closely resembling that of superconducting cuprates. While analogy with the cuprates is natural, very little is known about the metallic state of the nickelates, making these comparisons difficult. We probe the electronic dispersion of thin-film superconducting five-layer (n=5) and metallic three-layer (n=3) nickelates by measuring the Seebeck coefficient S. We find a temperature-independent and negative S/T for both n=5 and n=3 nickelates. These results are in stark contrast to the strongly temperature-dependent S/T measured at similar electron filling in the cuprate La1.36Nd0.4Sr0.24CuO4. The electronic structure calculated from density-functional theory can reproduce the temperature dependence, sign, and amplitude of S/T in the nickelates using Boltzmann transport theory. This demonstrates that the electronic structure obtained from first-principles calculations provides a reliable description of the fermiology of superconducting nickelates and suggests that, despite indications of strong electronic correlations, there are well-defined quasiparticles in the metallic state. Finally, we explain the differences in the Seebeck coefficient between nickelates and cuprates as originating in strong dissimilarities in impurity concentrations. Our study demonstrates that the high elastic scattering limit of the Seebeck coefficient reflects only the underlying band structure of a metal, analogous to the high magnetic field limit of the Hall coefficient. This opens a new avenue for Seebeck measurements to probe the electronic band structures of relatively disordered quantum materials. Published by the American Physical Society 2024

Tuning of charge order by uniaxial stress in a cuprate superconductor

Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct regions. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La2−xBaxCuO4, around x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at σ3D ≈ 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to σ3D. Upon the increase of stress beyond this point, no further changes were observed. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller regions. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place.

Enhanced charge density wave with mobile superconducting vortices in La1.885Sr0.115CuO4

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 La1.885Sr0.115CuO4, 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 with longer-range CDW coexisting with static spin density wave (SDW). With increasing magnetic field, the CDW first grows smoothly in a manner similar to the SDW. However, at high fields we discover a sudden increase in the CDW amplitude upon entering the vortex-liquid state. Our results signify strong coupling of the CDW to mobile superconducting vortices and link enhanced CDW amplitude with local superconducting pairing across the H − T phase diagram.

Pseudogap in underdoped cuprate seen in longitudinal magnetoresistance

We report the results of in-plane magnetotransport study of slightly underdoped cuprate La1.85Sr0.15CuO4 (LSCO15) with Ni impurity. Increasing Ni content y causes a sharp drop in longitudinal magnetoresistance (LMR) in LSCO15 to broaden and move towards higher temperatures. Temperature of this local maximum in LMR coincides with temperature , below which ideal resistivity from the parallel-resistor model deviates from its T 2-dependence. A direct comparison with the hole doping evolution of pseudogap (PG) in LaSr x CuO4 (LSCO), possible through the mobile-carrier concentration extracted from the thermopower measurements, allows to equate both characteristic temperatures with PG opening temperature . The rate of PG closing by magnetic field parallel to the CuO2 plane, in measurements up to 9 T, is consistent with spin-paramagnetic effect in this configuration and yields PG closing field B pc close to the second critical field predicted for superconducting gap with the help of Werthamer–Helfand–Hohenberg theory. The field anisotropy of B pc suggests that orbital degrees of freedom also play a role in PG formation.

Antinodal kink in the band dispersion of electron-doped cuprate La2\u2212xCexCuO4\xb1\u03b4

Angle-resolved photoemission spectroscopy (ARPES) measurements have established the phenomenon of kink in band dispersion of high-Tc cuprate superconductors. However, systematic studies of the kink in electron-doped cuprates are still lacking experimentally. We performed in situ ARPES measurements on La2−xCexCuO4±δ (LCCO) thin films over a wide electron doping (n) range from 0.05 to 0.23. While the nodal kink is nearly invisible, an antinodal kink around 45 meV, surviving above 200 K, is observed for n ~ 0.05–0.19, whose position is roughly independent of doping. The fact that the antinodal kink observed at high temperatures and in the highly overdoped region favors the phonon mechanism with contributions from the Cu–O bond-stretching mode and the out-of-plane oxygen buckling mode.

Emergence of quasiparticles in a doped Mott insulator

How a Mott insulator develops into a weakly coupled metal upon doping is a central question to understanding various emergent correlated phenomena. To analyze this evolution and its connection to the high-Tc cuprates, we study the single-particle spectrum for the doped Hubbard model using cluster perturbation theory on superclusters. Starting from extremely low doping, we identify a heavily renormalized quasiparticle dispersion that immediately develops across the Fermi level, and a weakening polaronic side band at higher binding energy. The quasiparticle spectral weight roughly grows at twice the rate of doping in the low doping regime, but this rate is halved at optimal doping. In the heavily doped regime, we find both strong electron-hole asymmetry and a persistent presence of Mott spectral features. Finally, we discuss the applicability of the single-band Hubbard model to describe the evolution of nodal spectra measured by angle-resolved photoemission spectroscopy (ARPES) on the single-layer cuprate La2−xSrxCuO4 (0 ≤ x ≤ 0.15). This work benchmarks the predictive power of the Hubbard model for electronic properties of high-Tc cuprates.

Investigations on synthesis and electrical properties of the La1.75Nd0.1Ce0.15CuO3.5 prepared by topotactic reduction with CaH2

This paper reports the reaction of a cuprate La1.75Nd0.1Ce0.15CuO4 with CaH2 which yields La1.75Nd0.1Ce0.15CuO3.5, a new oxygen-vacancy-ordered arrangement of cooperatively-distorted Cu2O3 planes containing four-coordinate Cu+ sites. This new compound has been characterized by X-ray diffraction, electron paramagnetic resonance (EPR), differential scanning calorimetry (DSC) and impedance spectroscopy. The powder X-ray diffraction data have shown that La1.75Nd0.1Ce0.15CuO3.5 adopts a face-centered monoclinic crystal structure (A2/m, a = 8.6216(6), b = 3.8452(3) Å, c = 13.0157(12) and β = 109.691(2)°) with anion vacancies located within the CuO2 (CuO1.5□0.5, where □ represents an anion vacancy) layer of the phase. The EPR measurements have shown the presence of Cu2+ and a hyperfine structure suggesting a pronounced hybridization of the copper-oxygen bond. The impedance spectra were analyzed in terms of equivalent circuits involving resistors, capacitors and constant phase elements (CPE). The re-oxidization under mild conditions at 420 °C in oxygen atmosphere transforms La1.75Nd0.1Ce0.15CuO3.5 phase to T’-La1.75Nd0.1Ce0.15CuO4. This phase transition is confirmed by DSC and impedance spectroscopy. The significance of this reaction relates to its mechanism, as it implies low temperature oxygen mobility realized in a solid oxide, which has technological importance for oxygen membranes of SOFCs.

Increasing the Transition Temperature of High-TC Superconductor Thin Films by Organic Linking of Gold Nanoparticles

The transition temperature, TC, of two types of optimally doped cuprate high-TC superconductor films, YBa2Cu3O7-δ and La1.85Sr0.15CuO4, is found to increase upon linking Au nanoparticles to their surface via organic molecules. At the same time, and quite surprisingly, the critical current is reduced. We attribute these results to screening of the Coulomb interactions and smearing the pinning potential landscape. The transition temperature was increased also when the Au NPs were not chemically linked to the surface, consistent with the screening scenario. The linking molecules, however, enable selective adsorption of the gold nanoparticles and their separation from the surface, thus eliminating the conventional proximity effect.

Unidirectional spin density wave state in metallic (Sr1\u2212xLax)2IrO4

Materials that exhibit both strong spin–orbit coupling and electron correlation effects are predicted to host numerous new electronic states. One prominent example is the Jeff = 1/2 Mott state in Sr2IrO4, where introducing carriers is predicted to manifest high temperature superconductivity analogous to the S = 1/2 Mott state of La2CuO4. While bulk superconductivity currently remains elusive, anomalous quasiparticle behaviors paralleling those in the cuprates such as pseudogap formation and the formation of a d-wave gap are observed upon electron-doping Sr2IrO4. Here we establish a magnetic parallel between electron-doped Sr2IrO4 and hole-doped La2CuO4 by unveiling a spin density wave state in electron-doped Sr2IrO4. Our magnetic resonant X-ray scattering data reveal the presence of an incommensurate magnetic state reminiscent of the diagonal spin density wave state observed in the monolayer cuprate (La1−xSrx)2CuO4. This link supports the conjecture that the quenched Mott phases in electron-doped Sr2IrO4 and hole-doped La2CuO4 support common competing electronic phases.

High-temperature charge density wave correlations in La1.875Ba0.125CuO4 without spin–charge locking

Although all superconducting cuprates display charge-ordering tendencies, their low-temperature properties are distinct, impeding efforts to understand the phenomena within a single conceptual framework. While some systems exhibit stripes of charge and spin, with a locked periodicity, others host charge density waves (CDWs) without any obviously related spin order. Here we use resonant inelastic X-ray scattering to follow the evolution of charge correlations in the canonical stripe-ordered cuprate La1.875Ba0.125CuO4 across its ordering transition. We find that high-temperature charge correlations are unlocked from the wavevector of the spin correlations, signaling analogies to CDW phases in various other cuprates. This indicates that stripe order at low temperatures is stabilized by the coupling of otherwise independent charge and spin density waves, with important implications for the relation between charge and spin correlations in the cuprates.

Anomalous in-plane magnetoresistance of electron-doped cuprate La2−x Ce x CuO4±δ

We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2−x Ce x CuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14 T. A crossover from negative to positive magnetoresistance occurs between the doping level x = 0.07 and 0.08. Above x = 0.08, the positive magnetoresistance effect appears, and is almost indiscernible at x = 0.15. By tuning the oxygen content, the as-grown samples show negative magnetoresistance effect, whereas the optimally annealed ones display positive magnetoresistance effect at the doping level x = 0.15. Intriguingly, a linear-field dependence of in-plane magnetoresistance is observed at the underdoping level x = 0.06, the optimal doping level x = 0.1 and slightly overdoping level x = 0.11. These anomalies of in-plane magnetoresistance may be related to the intrinsic inhomogeneity in the cuprates, which is discussed in the framework of network model.

Unconventional charge order in a co-doped high-Tc superconductor.

Charge-stripe order has recently been established as an important aspect of cuprate high-Tc superconductors. However, owing to the complex interplay between competing phases and the influence of disorder, it is unclear how it emerges from the parent high-temperature state. Here we report on the discovery of an unconventional ordered phase between charge-stripe order and (pseudogapped) metal in the cuprate La1.8−xEu0.2SrxCuO4. We use three complementary experiments—nuclear quadrupole resonance, nonlinear conductivity and specific heat—to demonstrate that the order appears through a sharp phase transition and exists in a dome-shaped region of the phase diagram. Our results imply that the new phase is a state, which preserves translational symmetry: a charge nematic. We thus resolve the process of charge-stripe development in cuprates, show that this nematic phase is distinct from high-temperature pseudogap and establish a link with other strongly correlated electronic materials with prominent nematic order.

Electrotransport properties of SOFC cathode materials based on lanthanum cuprate doped with praseodymium and strontium oxides

A complex study of thermal, conducting, and electrocatalytic properties of cuprates La1.8‒x Pr x Sr0.2CuO4–δ (х = 0.2; 0.4) with the K2NiF4 structure is carried out in order to assess their prospects as the cathode materials for solid-oxide fuel cells. The thermal analysis reveals stability of samples heated up to 950°С in air. The conductivity of cuprates measured in the temperature range of 100–900°С and the partial oxygen pressure from 10–3 to 1 atm is of the metallic nature and varies from 70 to 40 S/cm in the temperature interval of 500–900°С in air. The studies of chemical stability of cuprates with respect to solid electrolytes demonstrate the absence of their chemical interaction with Ce0.9Gd0.1O1.95 (GDC) at 900°С and with La0.8Sr0.2Ga0.85Mg0.15O3–δ (LSGM) at 1000°C after 25 h annealing. For La1.6Pr0.2Sr0.2CuO4–δ electrodes deposited on the surface of GDC or LSGM solid electrolytes, the studies of electrocatalytic activity in the oxygen reduction reaction demonstrate that the smallest polarization resistance is typical of electrodes deposited on the GDC surface.

Fluorination of La2−xSrxCuO4 (x = 0, 0.15, 0.3) and study on the crystal structures, magnetic properties of their fluorinated products

Here we report using the transition metal difluoride ZnF2 to fluorinate K2NiF4-type cuprates La2−xSrxCuO4 (x=0, 1.5, 0.3). Unlike other fluorinating agents, the technique is nontoxic, easy to handle and the byproduct ZnO can be removed. After fluorination, the fluorinated product of La2CuO4 suffers a phase transformation and unit cell expansion. While La1.85Sr0.15CuO4 and La1.7Sr0.3CuO4 indicate no change in structure after fluorination, their space groups still are I/4mmm, however, their lattices become larger, too. We emphasis the structural characterizations for fluorinated product of La1.7Sr0.3CuO4 by high-resolution transmission electron microscopy (HRTEM) images and electron diffraction (ED) patterns. Moreover, we determine the chemical formula to be La1.54Sr0.46CuO3.1F0.9 and the fluorine ions are prone to be located in the apical sites of the Cu(O, F)6 octahedron in the structure of post-treated fluorinated product of La1.7Sr0.3CuO4. Magnetization investigations demonstrate that partial replacement of the lanthanum by strontium changes the magnetism of post-treated fluorinated products of La2−xSrxCuO4 (x=0, 0.15, 0.3) and they exhibit a paramagnetic behavior.

Neutron-scattering evidence for a periodically modulated superconducting phase in the underdoped cuprate La1.905Ba0.095CuO4.

The role of antiferromagnetic spin correlations in high-temperature superconductors remains a matter of debate. We present inelastic neutron-scattering evidence that gapless spin fluctuations coexist with superconductivity in La1.905Ba0.095CuO4. Furthermore, we observe that both the low-energy magnetic spectral weight and the spin incommensurability are enhanced with the onset of superconducting correlations. We propose that the coexistence occurs through intertwining of spatial modulations of the pair wave function and the antiferromagnetic correlations. This proposal is also directly relevant to sufficiently underdoped La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x).

Topotactic reduction and phase transitions in (T,T′) La1.8Pr0.2CuO4

The purpose of this paper is to present the reaction of a cuprate La1.8Pr0.2CuO4 with CaH2 which yields La1.8Pr0.2CuO3.5, a new oxygen-vacancy-ordered arrangement of cooperatively-distorted Cu2O3 planes containing 4-coordinate Cu+sites. This new compound has been characterized by X-ray diffraction, electron paramagnetic resonance (EPR) and DSC.The X-ray powder diffraction data have revealed that this cuprate is crystallized with the so-called “pseudo-S” type structure, showing a monoclinic symmetry and space group A2/m. Concerning the EPR measurements, they have shown the presence of Cu2+ cation and a hyperfine structure suggesting a pronounced hybridization of the copper–oxygen bond. Finally, X-ray diffraction has demonstrated that the obtained La1.8Pr0.2CuO3.5, heated in oxygen at 420°C, turns topotactically into La1.8Pr0.2CuO4 with a T′-type structure (I4/mmm).

Low-temperature nonlinear effects in the conductivity of lightly doped cuprates La2−xSrxCuO4 in antiferromagnetic state

Low-temperature conductivity of antiferromagnetic cuprates La2−xSrxCuO4 prepared by solid-phase synthesis was investigated. The concentration of strontium in the samples was 0.01, 0.005 and 0.001. In the temperature range T < 100 K for all the samples the conduction mechanism corresponded to 3D variable-range hopping. For T > TN (where TN is the Neel temperature) a transition to the metallic type of conductivity was observed. Nonlinear effects in low-temperature conductivity, magnetoresistivity, as well as current-controlled negative differential resistance were found. It was established that the nonlinear behavior of conductivity intensified with decreasing the strontium concentration. For temperatures T < 10 K, the effect of positive magnetoresistance was observed. It is suggested that this effect can be attributed to the presence of a new low-temperature magnetic phase (spin density waves).

Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2−xSrxCuO4

High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki-Wood's relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

A model with electron–phonon interaction for high-Tc d-wave cuprates

Within the BCS framework a model with d-wave symmetry is introduced to describe high-Tc superconductor cuprates. The electron–phonon interaction is considered as the dominant cause of superconductivity. In order to increase the electronic density of states at the Fermi level, an anomalous occupation, has been proposed. The phonon available energy is introduced by the half-breathing phonons. Numerical results for the coupling parameter λ, the energy gap Δ0, and parameters of the model, as a function of doping, in the (0.105, 0.22) range, are obtained for the cuprate La2−xSrx CuO4. The λ values obtained are in the intermediate and the weak coupling region and the experimental results for the gap Δ0 are reproduced with our model.

63Cu-NMR study on High-Tc Superconductor La2−xBaxCuO4

In order to investigate the local structure in the CuO2 plane of the La-based high-Tc cuprate La2−xBaxCuO4, we have carried out measurements of 63Cu-NMR on single crystals with x = 0.08 and 0.10 at low temperatures down to 4 K. In both samples, the local structure deduced from observed NMR spectra agreed with the averaged structure. In high-temperature region around 300K, detailed investigation of NMR line width has shown the existence of the local structure different from the averaged structure; that is, CuO6 octahedra tilt from the c-axis toward the random direction by approximately 1.4 deg.