Type Cuprates Research Articles

139La-NMR Study of Spin Dynamics Coupled with Hole Mobility in T*-type La0.86Eu0.86Sr0.28CuO4−δ

In T*-type cuprate oxides with five-oxygen coordination, the relationship between spin correlations and doped carriers has not been well understood. To clarify this relationship, and the magnetic and superconducting (SC) properties of T*-type cuprate oxides, we performed 139La-nuclear magnetic resonance (NMR) and electrical resistivity measurements on as-sintered (AS) and oxidation-annealed (OA) polycrystalline T*-type La0.86Eu0.86S0.28CuO4 (LESCO) to investigate its magnetic and SC properties. Upon cooling, the NMR spectrum of AS LESCO broadened below 3 K, where the nuclear spin–lattice relaxation rate 1/T1 with respect to the temperature exhibited a maximum, indicating the appearance of static magnetism. The temperature dependence of 1/T1 between 3 and 20 K was similar to that of resistivity, which displays semiconducting behavior. Furthermore, it was found that the energy scales of the transport gap and spin dynamics were comparable. These results suggest a close connection between the mobility of the doped carriers and low-energy spin dynamics, as reported for lightly doped T-type La2−xSrxCuO4. In the OA SC sample, we confirmed the absence of a magnetic order and the linear relation between 1/T1 and T above 10 K. These results suggest that the magnetic state coupled with holes markedly changes to the weakly correlated metallic state by oxidation annealing in the T*-type LESCO with x = 0.28.

A Structural Investigation of T’-Pr2-xCexCuO4 nanocrystals with x = 0.13

This report is briefly investigating the structural defect information of T’-Pr 2- x Ce x CuO 4 (T’-PCCO) nanocrystals with x = 0.13 at three different calcination temperatures. The as-synthesized powder of Pr 2- x Ce x CuO 4 (PCCO) nanocrystals with x = 0.13 were synthesized by using the chemically dissolved method within three different calcination temperatures, namely 900 o C, 950 o C, and 1000 o C for 15 h in air. X-ray diffraction (XRD) technique is used to characterize the phase, structural, and crystalline defect information on the nanocrystal powder. It is found that all the indexed peaks are confirmed as PCCO phase with space group of I4/mmm and a minor impurity of Fm-3m space group. The lattice parameters are confirmed to be fluctuated signifying the stability of the Coulomb force at those temperatures. The post-Rietveld analysis using Bond Valance Sum (BVS) calculation shows the Ce-dependent of T’-PCCO nanocrystals while the Fourier difference suggests that the created crystalline defect as a part of the tetravalent doping effect which binds stronger apical oxygen. Moreover, at Pr- and Cu-site, the scattering distribution are fluctuating with the stable structure at 1000 o C. This initial structural information is worth complementing the fundamental feature for understanding the T’-type cuprates.

Synthesis method comparison of compositionally complex rare earth‐based Ruddlesden–Popper n = 1 T′‐type cuprates

AbstractThe multicomponent approach has been successfully expanded to the Ruddlesden–Popper structure with the synthesis of two different high‐entropy cuprate compositions: (La0.2Nd0.2Gd0.2Tb0.2Dy0.2)2CuO4 and (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4. The effect of synthesis method is explored using both solid‐state reaction and polymeric steric entrapment (PSE) methods. It is found that PSE leads to more randomly distributed cation species, providing an advantageous method of synthesis for the growing field of high entropy oxides. In situ high‐temperature x‐ray diffraction tracks the amorphous to crystalline phase transformation in (La0.2Nd0.2Gd0.2Tb0.2Dy0.2)2CuO4 powder, synthesized using the PSE method. Using the High‐Temperature XRD data, a method for gaining information on the kinetic behavior is also applied. Magnetometry of both compositions indicates ferrimagnetic behavior at low temperatures.

Angle-resolved photoemission spectroscopy of the low-energy electronic structure of superconducting Pr2CuO4 driven by oxygen nonstoichiometry

Bulk crystals of electron-doped cuprates with the T'-type structure require both Ce substitutions and reduction annealing for the emergence of superconductivity while the reduction annealing alone can induce superconductivity in thin films of the T'-type cuprates. In order to reveal low-energy electronic states which are responsible for the superconductivity, we have conducted angle-resolved photoemission spectroscopy measurements on thin films of the superconducting Ce-free T'-type cuprate Pr$_2$CuO$_4$. The results indicate that the overall band structure and the Fermi surface area of the superconducting Pr$_2$CuO$_4$ are similar to those of superconducting Ce-doped bulk single crystals, highlighting the importance of the actual electron concentration rather than the Ce concentration when discussing the physical properties of the T'-type cuprates.

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 order to realize superconductivity in cuprates with the T^{'}-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T^{'}-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T^{'}-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd_{2-x}Ce_{x}CuO_{4} (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO_{2} plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd_{2}CuO_{4} film is doped with electrons despite the absence of the Ce substitution.

Incommensurate Phonon Anomaly and the Nature of Charge Density Waves in Cuprates

While charge density wave (CDW) instabilities are ubiquitous to superconducting cuprates, the different ordering wavevectors in various cuprate families have hampered a unified description of the CDW formation mechanism. Here we investigate the temperature dependence of the low energy phonons in the canonical CDW ordered cuprate La$_{1.875}$Ba$_{0.125}$CuO$_{4}$. We discover that the phonon softening wavevector associated with CDW correlations becomes temperature dependent in the high-temperature precursor phase and changes from a wavevector of 0.238 reciprocal space units (r.l.u.) below the ordering transition temperature up to 0.3~r.l.u. at 300~K. This high-temperature behavior shows that "214"-type cuprates can host CDW correlations at a similar wavevector to previously reported CDW correlations in non-"214"-type cuprates such as YBa$_{2}$Cu$_{3}$O$_{6+\delta}$. This indicates that cuprate CDWs may arise from the same underlying instability despite their apparently different low temperature ordering wavevectors.

An effective Hamiltonian and its phase diagram for T’-structure cuprates

We study the superconducting transition near the half-filling of electron-doped cuprates with the so-called T’-structure based on an effective two-band model using fluctuation-exchange approximation and solving the linearized Eliashberg equation. We find d-wave superconductivity even at the half-filling of this model when the energy difference between the upper Hubbard level of Cu orbitals and O 2p levels is less than about 1.5eV, which is plausible for the T’-type cuprates.

Two‐particle self‐consistent analysis for the electron‐hole asymmetry of superconductivity in cuprate superconductors

In the hole‐doped type cuprate superconductors, it is well‐known that the superconducting transition temperature Tc exhibits a dome‐like structure against doping. On the other hand, recent experiments unveil that Tc in the electron‐doped compounds shows a monotonic increase with decreasing the doping, at least down to a very small doping rate. Our recent study for the three‐band d–p model has unveiled that this asymmetric behavior can be explained as a combined effect of the intrinsic electron‐hole asymmetry in systems comprising Cu3 d and O2 p orbitals and the band‐filling‐dependent vertex correction . In the present study, we study another compound Tl2 Ba2 CuO6 to show that this explanation can be applied to other cuprate superconductors with the small orbital mixture. By varying the d–p offset, we also study how the strength of the d–p hybridization controls the spin fluctuation and hence the pairing interaction.

μSR Studies on Magnetism in High-Tc Cuprates

Since the discovery of high-Tc superconductivity in cuprates, muon spin relaxation (μSR) measurements have greatly contributed to the understanding of high-Tc superconductivity. In this paper, μSR studies on the magnetism in high-Tc cuprates obtained these past three decades are reviewed. Antiferromagnetic long-range order, 1/8 anomaly, stripes of Cu spins and holes, impurity-induced magnetism, magnetic-field-induced magnetism, pseudogap, ferromagnetism in the heavily overdoped regime, and undoped superconductivity in T′-type cuprates are discussed. Moreover, the fundamentals of μSR measurements for the study of magnetism are described for μSR beginners.

Bulk superconductivity in undopedT′-La1.9Y0.1CuO4probed by muon spin rotation

The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_c\simeq21$ K) in its {\sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $\sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.

An Analysis of the Fermi Surface of Hole-Doped Type Cuprate Superconductor

In order to investigate the doping dependence of Fermi surface (DDFS) of cuprate, we have constructed a mean field model for the electronic structure of cuprate as an effective theory of strongly correlated system. Empirical parameters involved there were determined to reproduce the experimental data of the Hall coefficient. Obtained result for the DDFS is that the Fermi surface forms hole pockets in underdoped regime, which become larger in the optimum-doped regime, and the outer part in the Brillouin zone centered at X-point approaches a large circle in the heavily overdoped regime. However, the inner structure of Fermi surface always remains. The change of Fermi surface by doping amount is a continuous deformation throughout the whole regime. It is suggested that the experimental study of the higher vacant band relevant to the photo-absorbed excited state is important to select from among scenarios on the Fermi surface of cuprate.

Spectroscopic Evidence for Competing Order-Induced Pseudogap Phenomena and Unconventional Low-Energy Excitations in High-T c Cuprate Superconductors

The low-energy excitations of cuprate superconductors exhibit various characteristics that differ from those of simple Bogoliubov quasiparticles for pure d_{x^2-y^2}-wave superconductors. Here we report experimental studies of spatially resolved quasiparticle tunnelling spectra of hole- and electron-type cuprate superconductors that manifest direct evidences for the presence of competing orders (COs) in the cuprates. In contrast to conventional type-II superconductors that exhibit enhanced local density of states (LDOS) peaking at zero energy near the centre of field-induced vortices, the vortex-state LDOS of YBa_2Cu_3O_{7-\delta} (Y-123) and La_{0.1}Sr_{0.9}CuO_2 (La-112) remains suppressed inside the vortex core, with pseudogap (PG)-like features at an energy larger (smaller) than the superconducting (SC) gap \Delta_{SC} in Y-123 (La-112). Energy histograms of the SC and PG features reveal steady spectral shifts from SC to PG with increasing magnetic field H. These findings may be explained by coexisting COs and SC: For hole-type cuprates with PG above T_c, the primary CO gap (V_{CO}) is larger than \Delta_{SC} and the corresponding COs are charge/pair-density waves with wave-vectors parallel to (\pi,0)/(0,\pi). For electron-type cuprates without PG above T_c, V_{CO} is smaller than \Delta_{SC} and the CO wave-vector is along (\pi,\pi). This CO scenario may be extended to the ARPES data to consistently account for the presence (absence) of Fermi arcs in hole- (electron)-type cuprates. Fourier transformation of the vortex-state LDOS in Y-123 further reveals multiple sets of energy-independent wave-vectors due to field-enhanced pair- and spin-density waves. These results imply important interplay of SC with low-energy collective excitations.

Origin of the anomalous Hall effect in the overdopedn-type superconductorPr2−xCexCuo4: Current-vertex corrections due to antiferromagnetic fluctuations

The anomalous magneto-transport properties in electron doped (n-type) cuprates were investigated using Hall measurements at THz frequencies. The complex Hall angle was measured in overdoped Pr$_{\rm 2-x}$Ce$_{\rm x}$CuO$_{\rm 4}$ samples (x=0.17 and 0.18) as a continuous function of temperature above $T_c$ at excitation energies 5.24 and 10.5 meV. The results, extrapolated to low temperatures, show that inelastic scattering introduces electron-like contributions to the Hall response. First principle calculations of the Hall angle that include current vertex corrections (CVC) induced by electron interactions mediated by magnetic fluctuations in the Hall conductivity reproduce the temperature, frequency, and doping dependence of the experimental data. These results show that CVC effects are the source of the anomalous Hall transport properties in overdoped n$\text{-}$type cuprates.

Crystal growth and characterization of T∗ cuprate superconductor [formula omitted

One of the simplest structural and “low”-Tc cuprates, T∗-type cuprates, SmLa1-xSrxCuO4-δ(SLSCO,Tc(max)∼32K) and Nd1.6-ySr0.4CeyCuO4(NSCCO,Tc(max)∼25K), can be a key material to considerate the mechanism for high-Tc cuprate superconductor. Contrary to SLSCO whose as-grown sample does not show superconductivity at all, due to heavy deficiency of apical oxygen, NSCCO shows superconductivity even in as-grown crystal. We grew some compositions of wide range of Ce concentration and carefully investigated the change of Tc and show that NSCCO becomes sufficient bulk superconductivity, even without an extreme oxygen annealing.

Correlated Electrons in Fe-As Compounds: A Quantum Chemical Perspective

State-of-the-art quantum chemical methods are applied to the study of the multiorbital correlated electronic structure of a Fe-As compound, the recently discovered LiFeAs. Our calculations predict a high-spin, S=2, ground-state configuration for the Fe ions, which shows that the on-site Coulomb interactions are substantial. Also, orbital degeneracy in the (xz, yz) sector and a three-quarter filling of these levels suggest the presence of strong fluctuations and are compatible with a low metallic conductivity in the normal state. The lowest electron-removal states have As 4p character, in analogy with the ligand hole states in p-type cuprate superconductors.

Local structure study of T′ cuprate superconductors

Undoped T′-type cuprates are believed to be charge transfer (Mott–Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La 3+,Y 3+) 2CuO 4 and doped T′-(La 3+,Ce 4+) 2CuO 4 thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane ( E// ab) polarized Cu K-EXAFS data shows a remarkable Cu–O bond distance displacement. The broadened Cu–O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.

Effects of competing orders and quantum phase fluctuations on the low-energy excitations and pseudogap phenomena of cuprate superconductors

We investigate the low-energy quasiparticle excitation spectra of cuprate superconductors by incorporating both superconductivity (SC) and competing orders (CO) in the bare Green’s function and quantum phase fluctuations in the proper self-energy. Our approach provides consistent explanations for various empirical observations, including the excess subgap quasiparticle density of states, “dichotomy” in the momentum-dependent quasiparticle coherence and the temperature-dependent gap evolution, and the presence (absence) of the low-energy pseudogap in hole- (electron-) type cuprates depending on the relative scale of the CO and SC energy gaps.

Electronic structure of electron-dopedSm1.86Ce0.14CuO4: Strong pseudogap effects, nodeless gap, and signatures of short-range order

Angle-resolved photoemission spectroscopy (ARPES) data from the electron-doped cuprate superconductor ${\mathrm{Sm}}_{1.86}{\mathrm{Ce}}_{0.14}\mathrm{Cu}{\mathrm{O}}_{4}$ show a much stronger pseudogap or hot-spot effect than that observed in other optimally doped $n$-type cuprates. Importantly, these effects are strong enough to drive the zone-diagonal states below the chemical potential, implying that $d$-wave superconductivity in this compound would be of the nodeless gap variety. The gross features of the Fermi surface topology and low-energy electronic structure are found to be well described by reconstruction of bands by a $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ order. Comparison of the ARPES and optical data from the same sample shows that the pseudogap energy observed in optical data is consistent with the interband transition energy of the model, allowing us to have a unified picture of pseudogap effects. However, the high-energy electronic structure is found to be inconsistent with such a scenario. We show that a number of these model inconsistencies can be resolved by considering a short-range ordering or inhomogeneous state.

Pseudogap in electron-doped superconductingSm2−xCexCuO4−δby interlayer magnetotransport

$c$-axis interlayer magnetoresistivity is measured for an electron-doped ($n$-type) superconducting cuprate ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ with $x=0.14--0.16$ using $30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick small mesa structures. A systematic doping dependence is observed in the negative interlayer magnetoresistance (MR) component, from which the pseudogap onset temperature ${T}^{*}$ is determined as the negative MR appearance temperature. For a doping level close to the phase boundary between superconductivity and antiferromagnetism, a ${T}^{*}$ of $48\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is observed. It is also found that ${T}^{*}$ decreases systematically with increasing $x$ but is still higher than ${T}_{c}$. For all the doping levels, the result represents features characteristic of hole-doped ($p$-type) cuprates in the overdoped region, suggesting that the phase diagrams for the pseudogap are primarily similar for both the $n$- and $p$-type cuprates.

Single-crystal growth and anisotropy of the tetragonal 1-2-3 type superconductor Ca0.5La1.25Ba1.25Cu3Oy

Single crystals of the tetragonal 1-2-3 type cuprate Ca 0.5 La 1.25 Ba 1.25 Cu 3 O y have been grown by the travelling-solvent floating-zone method. The crystal structure has been determined to belong to the tetragonal P4/mmm by the single-crystal x-ray diffraction for several crystals with different oxygen-contents. The superconducting transition temperature T c has increased up to 75 K by the annealing in 1000 bar of O 2 . Temperature dependences of the resistivities ρ ab, ρ c and also the angular dependence of r ab in various constant magnetic fields have been measured. The anisotropic parameter γ has been estimated as 6.2, which is similar to that of YBa 2 Cu 3 O 7 .