Electron-doped High-temperature Superconductors Research Articles

Point-Contact Spectroscopy in Bulk Samples of Electron-Doped Cuprate Superconductors.

Point-contact spectroscopy was performed on bulk samples of electron-doped high temperature superconductor Nd2-xCexCuO4-δ. The samples were characterized using X-ray diffraction and scanning electron microscopy equipped with a wavelength-dispersive spectrometer and an electron backscatter diffraction detector. Samples with Ce content x = 0.15 showed the absence of spurious phases and randomly oriented grains, most of which had dimensions of approximately 220 µm2. The low-bias spectra in the tunneling regime, i.e., high-transparency interface, exhibited a gap feature at about ±5 meV and no zero-bias conductance, despite the random oriented grains investigated within our bulk samples, consistent with most of the literature data on oriented samples. High-bias conductance was also measured in order to obtain information on the properties of the barrier. A V-shape was observed in some cases, instead of the parabolic behavior expected for tunnel junctions.

Vortex motion in tilted magnetic fields in highly layered electron-doped superconductor Nd2-xCexCuO4

The carrier transport and the motion of a vortex system in a mixed state of an electron-doped high-temperature superconductors Nd2-xCexCuO4 were investigated. To study the anisotropy of galvanomagnetic effects of highly layered NdCeCuO system we have synthesized Nd2-xCexCuO4/SrTiO3 epitaxial films with non-standard orientations of the c-axis and conductive CuO2 layers relative to the substrate. The variation of the angle of inclination of the magnetic field, B, relative to the current, j, reveals that the behavior of both the in-plane ρxx(B) and the out-of-plane ρxy(B) resistivities in the mixed state is mainly determined by B⊥, the perpendicular to j component of B, that indicates the crucial role of the Lorentz force, FL∼[j×B] and defines the motion of Josephson vortices across the CuO2 layers.

Vortex bound states and Majorana zero mode in electron-doped FeSe-based high-temperature superconductor

As a novel quantum state in condensed matter physics, Majorana zero mode has become a popular research topic at present because of its potential value in topological quantum computing. Theory predicts that Majorana zero mode appears in the vortex core of the topological superconductor as a unique bound state. However, due to various factors such as the existence of conventional low energy bound states or impurity states, it is difficult to identify the Majorana zero mode and to put it into the specific applications. Nowadays, it is still urgent to find a suitable topological superconducting system and identify the clean Majorana zero mode in experiment. In this paper, we study the vortex states of electron-doped iron-selenium-based superconductors (Li, Fe)OHFeSe and single-layer FeSe/SrTiO<sub>3</sub> with extremely high energy resolution STM. There exists a robust and clean Majorana zero mode in the free vortex core of (Li, Fe)OHFeSe, which has the quantized conductance. As for single-layer FeSe/SrTiO<sub>3</sub> film, it has only conventional Caroli-de Gennes-Matricon (CdGM) bound states without zero energy mode. These experimental results provide a suitable platform for further studying the physical properties of Majorana zero mode, and also shed light on the source of topological superconductivity in iron-based superconductors.

Lateral vortex motion in highly layered electron-doped superconductor Nd[formula omitted]CexCuO4

The carrier transport and the motion of a vortex system in the electron-doped high-temperature superconductors Nd2−xCexCuO4 in underdoped and optimally doped (x = 0.135, 0.145, 0.15) regions, in the area of the evolution from antiferromagnetic to superconducting order were investigated. To study the anisotropy of the transport properties of highly layered NdCeCuO system we have synthesized Nd2−xCexCuO4+δ/SrTiO3 epitaxial films of three types with different orientations of the c-axis and conductive CuO2 layers relative to the substrate. Such a set of samples allowed us to study the processes of both standard (in the CuO2 layers) and lateral (across the CuO2 layers) carrier transfer in the normal and the mixed states of a superconductor. In a flux-flow regime, in magnetic field B, the dynamics of Abrikosov (B ∥ c-axis) and Josephson (B ∥ ab-plane) vortices are thoroughly investigated and analyzed which is perspective for scientific purposes and for practical applications in measurement technology.

Ionic Liquid Gating Induced Protonation of Electron-Doped Cuprate Superconductors.

Ion injection controlled by electric field has attracted growing attention due to its tunability over bulk-like materials. Here, we achieve protonation of an electron-doped high-temperature superconductor, La2-xCexCuO4, by gating in the electrochemical regime of the ionic liquid. Such a process induces a superconductor-insulator transition together with the crossing of the Fermi surface reconstruction point. Applying negative voltages not only can reverse the protonation process but also recovers superconductivity in samples deteriorated by moisture in the ambient. Our work extends the application of electric-field-induced protonation into high-temperature cuprate superconductors.

Hole pocket-driven superconductivity and its universal features in the electron-doped cuprates.

After three decades of intensive research attention, the emergence of superconductivity in cuprates remains an unsolved puzzle. One major challenge has been to arrive at a satisfactory understanding of the unusual metallic "normal state" from which the superconducting state emerges upon cooling. A second challenge has been to achieve a unified understanding of hole- and electron-doped compounds. Here, we report detailed magnetoresistance measurements for the archetypal electron-doped cuprate Nd2-x Ce x CuO4+δ that, in combination with previous data, provide crucial links between the normal and superconducting states and between the electron- and hole-doped parts of the phase diagram. The characteristics of the normal state (magnetoresistance, quantum oscillations, and Hall coefficient) and those of the superconducting state (superfluid density and upper critical field) consistently indicate two-band (electron and hole) features and point to hole pocket-driven superconductivity in these nominally electron-doped materials. We show that the approximate Uemura scaling between the superconducting transition temperature and the superfluid density found for hole-doped cuprates also holds for the small hole component of the superfluid density in electron-doped cuprates.

Electrical transport properties of sputtered Nd2−xCexCuO4±δ thin films

Thin films of the electron-doped high-temperature superconductor Nd2−xCexCuO4±δ have been deposited by dc sputtering technique on (100) SrTiO3 substrates. A tuning of the oxygen content in the as-grown non-superconducting samples has been achieved by changing the oxygen partial pressure during the growth in the Argon sputtering atmosphere. All samples show the superconducting transition after a suitable two-step thermal treatment in an oxygen-reducing environment. Structural and electrical transport properties on the as-grown as well as on the superconducting samples have been investigated. We find that the structural properties are consistent with a deficiency of the oxygen content with respect to optimally annealed samples, and that the transition to the superconducting phase is always accompanied by an increase of the c-axis lattice parameter. Measurements of the Hall coefficient RH as a function of temperature and in the normal state of our epitaxial films are presented and discussed. RH results negative for all the films regardless of the oxygen content and it decreases with the temperature. In particular, the Hall coefficient is only about 10% lower than the value measured in the as-grown oxygen-deficient phase, in contrast to the results reported in literature. The removal of the excess oxygen in as-grown samples seems not to be the only requirement for triggering the superconducting transition in electron-doped compounds. The microstructural change associated with the increase of the c-axis parameter in our deoxygenated samples could help in understanding the microscopic mechanism underlying the reduction process of n-type superconductors, which is still under debate.

Spectral properties near the Mott transition in the two-dimensional t-J model with next-nearest-neighbor hopping

The single-particle spectral properties of the two-dimensional t-J model with next-nearest-neighbor hopping are investigated near the Mott transition by using cluster perturbation theory. The spectral features are interpreted by considering the effects of the next-nearest-neighbor hopping on the shift of the spectral-weight distribution of the two-dimensional t-J model. Various anomalous features observed in hole-doped and electron-doped high-temperature cuprate superconductors are collectively explained in the two-dimensional t-J model with next-nearest-neighbor hopping near the Mott transition.

BaySr1 − yTiO3 buffer layers for strain tuning of infinite-layer Sr1 − xLaxCuO2 thin films

We report on the precise tuning of lattice strain in an infinite-layer electron-doped high temperature superconductor Sr1−xLaxCuO2 (SLCO; aSLCO=0.3949nm for x=0.1), which is a perovskite-related oxide, using perovskite BaTiO3–SrTiO3 (BSTO; BaySr1−yTiO3) buffer layers. The BSTO buffer layers formed on (001) (La0.18Sr0.82)(Al0.59Ta0.41)O3 substrates by magnetron sputtering were fully relaxed with high crystalline quality due to high oxygen partial pressure deposition and post annealing at 950°C. The lattice constants of the BSTO buffer layers could be controlled in the range of 0.3926–0.3973nm by changing the Ba content (y=0.2–0.7). These BSTO buffer layers allow coherent growth of SLCO thin films, and a clear dependence of the superconducting transition temperature on the lattice strain was observed. The fabrication of these BSTO/superconductor heterostructures may provide novel devices composed of functional perovskite thin films, in addition to a general approach for the precise control of lattice strain in functional perovskite thin films.

Non-BCS Temperature Dependence of Energy Gap in Thin Film Electron-Doped Cuprates

We investigate the dependence of the energy gap ($G$) on the temperature ($T$) for the electron-doped high-temperature superconductors. The following compounds, in the form of the thin films, have been taken into consideration: $\rm La_{2-x}Ce_xCuO_{4}$ (LCCO), $\rm Pr_{2-x}Ce_xCuO_{4}$ (PCCO), and $\rm Nd_{2-x}Ce_xCuO_4$ (NCCO). It was found that $G\left(T\right)$ deviates from the BCS prediction more, if a concentration of cerium assumes the lower values. For the lowest concentration (in the case of LCCO and NCCO), the function $G\left(T\right)$ is not quite like the BCS curve, which is connected with the existence of the residual Nernst region. Next, it has been pointed out that the NCCO superconductor becomes structurally unstable for the maximum concentration of cerium, which is leading to the anomalous dependence of the energy gap on the temperature and the induction of the wide Nernst region.

Suppression of the antiferromagnetic pseudogap in the electron-doped high-temperature superconductor by protect annealing.

In the hole-doped cuprates, a small number of carriers suppresses antiferromagnetism and induces superconductivity. In the electron-doped cuprates, on the other hand, superconductivity appears only in a narrow window of high-doped Ce concentration after reduction annealing, and strong antiferromagnetic correlation persists in the superconducting phase. Recently, Pr1.3−xLa0.7CexCuO4 (PLCCO) bulk single crystals annealed by a protect annealing method showed a high critical temperature of around 27 K for small Ce content down to 0.05. Here, by angle-resolved photoemission spectroscopy measurements of PLCCO crystals, we observed a sharp quasi-particle peak on the entire Fermi surface without signature of an antiferromagnetic pseudogap unlike all the previous work, indicating a dramatic reduction of antiferromagnetic correlation length and/or of magnetic moments. The superconducting state was found to extend over a wide electron concentration range. The present results fundamentally challenge the long-standing picture on the electronic structure in the electron-doped regime.

Anomalous Temperature Dependence of the Upper Critical Magnetic Field in Electron-Doped High-Temperature Superconductor

We present resistivity measurements of the upper critical field as a function of temperature for Nd2-xCexCuO4+δ/SrTiO3single crystal films near antiferromagnetic-superconducting phase boundary (x= 0.14) in magnetic fields up to 90kOe at temperatures down to 0.7K. The data are well described by a two-band/two-gap model for a superconductor in the dirty limit.

Existence of Pseudogap at the Transition Temperature of an Electron-Doped Copper–Oxide Superconductor

We present the scanning tunnelling spectroscopy (STS) studies of the energy gap (Δ) at the Fermi surface of electron-doped high-temperature (high-Tc) cuprate superconductor Pr0.88LaCe0.12CuO4−δ, which is commonly known as PLCCO. We found that the energy gap does not vanish at the transition temperature (Tc). Moreover, it was observed that gap-like features are present at some locations of the sample at temperatures above Tc. The energy gap with magnitude is found consistent with d-wave symmetry. We also found the value of the ratio 2Δ/KBTc higher than 7, which puts electron-doped cuprates in strong coupling regime.

Oxygen stoichiometry of low-temperature synthesized metastable T′-La2CuO4

We have investigated the oxygen stoichiometry of low-temperature synthesized metastable T′-La2CuO4 bulk samples by neutron scattering. T′-La2CuO4 is the parent compound of electron-doped high-temperature superconductors that is chemically closest to the parent compound of the hole-doped superconductor T-La2CuO4. While usually all electron-doped cuprates require strong reduction to approach stoichiometry, the low-temperature synthesis directly yields fully stoichiometric compounds. This result makes low-temperature grown T′-La2CuO4 the ideal material to perform a comparison of the two parent compounds of hole- and electron-doped cuprates.

Pseudogap behavior in the emery model for the electron-doped superconductor Nd2 − x Ce x CuO4: Multiband LDA + DMFT + Σk approach

We propose a generalization of the LDA + DMFT + Σk approach to the multiband case, in which correlated and uncorrelated states are present in the model simultaneously. Using the multiband version of the LDA + DMFT + Σk approach, we calculate the density of states and spectral functions for the Emery model in a wide energy interval around the Fermi level. We also obtain the Fermi surfaces for the electron-doped high-temperature superconductor Nd2 − xCexCuO4 in the pseudogap phase. The self-energy part Σk introduced additionally to take into account pseudogap fluctuations describes the nonlocal interaction of correlated electrons with collective Heisenberg short-range spin fluctuations. To solve the effective impurity model, the numerical renorm-group (NRG) method is used for the DMFT equations. Good qualitative agreement of the Fermi surfaces calculated using the LDA + DMFT + Σk approach and experimental angle-resolved photoemission spectroscopic data is attained. The stability of the dielectric solution with charge transfer in the Emery model with correction for double counting is analyzed in the Appendix.

Interaction of itinerant electrons and spin fluctuations in electron-doped cuprates

We performed angle-resolved photoemission and optical studies on electron-doped high-temperature superconductors (HTSCs), and compared the results with various theoretical models. Based on the fit to the experimental data, we conclude that itinerant electrons in the nonmagnetic phase predominantly couple to the phase fluctuations of the remnant antiferromagnetic (AF) order, rather than to the spin excitations derived from particle-hole pairs. Our observation naturally accounts for the pseudogap phenomenon and other experimental facts in electron-doped HTSCs in terms of the size of the remnant moment and the AF-correlation length. We propose a microscopic model based on the phase fluctuation scenario which leads to a $d$-wave pairing gap.

Role of the perovskite-like lattice in the high-temperature superconductor mechanism: EXAFS data analysis

The temperature-dependent extended X-ray absorption fine structure (EXAFS) spectra of hole- and electron-doped high-temperature superconductors (La2 − xSrxCuO4 and Nd2 − xCexCuO4 − δ, respectively) are investigated above the CuK absorption edge. It is demonstrated that, for superconducting compounds (x = 0.15), the part of the oxygen ions in the CuO2 plane oscillates in the double-well potential, and their oscillations are correlated with the charge-carrier transport. The presented data indicate the direct relation between the experimentally observed local dynamic structural inhomogeneity and the local dynamic charge ordering. A phenomenological description of the correlation between the local electronic and local crystalline structures, which enables us to reveal the role of collective oxygen-ion oscillations inherent to the perovskite-like lattice in the establishment of the superconducting state in doped La2CuO4 and Nd2CuO4, is proposed to explain the experimental results.

Magnetic breakdown and quantum oscillations in electron-doped high-temperature superconductor Nd2−xCexCuO4

Recent more precise experiments have revealed both a slow and a fast quantum oscillation in the c-axis resistivity of nearly optimal to overdoped electron-doped high temperature superconductor $\mathrm{Nd_{2-x}Ce_{x}CuO_{4}}$. Here we study this problem from the perspective of Fermi surface reconstruction using an exact transfer matrix method and the Pichard-Landauer formula. In this method, neither quasiclassical approximations for magnetic breakdown, nor {\em ad ho}c broadening of Landau levels, are necessary to study the high field quantum oscillations. The underlying Hamiltonian is a mean field Hamiltonian that incorporates a two-fold commensurate Fermi surface reconsruction. While the specific mean field considered is the $d$-density wave, similar results can also be obtained by a model of a spin density wave, as was explicitly demonstrated earlier. The results are consistent with an interplay of magnetic breakdown across small gaps in the reconstructed Fermi surface and Shubnikov-de Haas oscillations.

Quantum oscillations in electron-doped high-temperature superconductors

Quantum oscillations in hole doped high temperature superconductors are difficult to understand within the prevailing views. An emerging idea is that of a putative normal ground state, which appears to be a Fermi liquid with a reconstructed Fermi surface. The oscillations are due to formation of Landau levels. Recently the same oscillations were found in the electron doped cuprate, $\mathrm{Nd_{2-x}Ce_{x}CuO_{4}}$, in the optimal to overdoped regime. Although these electron doped non-stoichiometric materials are naturally more disordered, they strikingly complement the hole doped cuprates. Here we provide an explanation of these observations from the perspective of density waves using a powerful transfer matrix method to compute the conductance as a function of the magnetic field.

Local dynamic deformation of the superconducting CuO2 plane in the Nd2 − x Ce x CuO4 + δ compound

The local structure of the electron-doped high-temperature superconductor Nd2 − x Ce x CuO4 + δ has been investigated by EXAFS spectroscopy using synchrotron radiation in the temperature range 5–300 K. The radii, coordination numbers, and Debye-Waller factors of the nearest coordination spheres of the copper, neodymium, and cerium local environment have been obtained. The Einstein temperatures, characterizing the strength of Cu-O, Nd-O, and Ce-O interatomic bonds have been determined. The dynamic local deformation of the superconducting CuO2 plane in the form of vibrations of some part of oxygen ions in a double-well potential due to the difference in the electronic filling of neighboring CuO4 complexes has been established.