Electron-doped Superconductor Research Articles

Reduction and oxidation annealing effects on Cu K-edge XAFS for electron-doped cuprate superconductors

Reduction and oxidation annealing effects on the electronic states around the copper sites for Pr2-xCexCuO4 and Nd2-xCexCuO4 with x = 0 and x = 0.15 were investigated by Cu K-edge x-ray absorption measurements. Cu K near-edge spectra were changed by the reduction annealing in a manner similar to the case of Ce substitution for both x = 0 and x = 0.15. This means an increase of electron density at the copper sites, indicating the aspect of electron doping in the reduction annealing. This reduction annealing effect on the near-edge spectra are reverted by the additional oxidation annealing. The amount of electron density around the copper sites is varied by the reduction and oxidation annealing, reversibly, corresponding to the reversible variation of the physical property from insulating to superconductivity.

Nernst effect in the electron-doped cuprate superconductor La2−xCexCuO4

We report a systematic study of the Nernst effect in films of the electron-doped cuprate superconductor $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{C}{\mathrm{e}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ as a function of temperature and magnetic field (up to 14 T) over a range of doping from underdoped ($x=0.08$) to overdoped ($x=0.16$). We have determined the characteristic field scale ${H}_{C2}^{*}$ of superconducting fluctuation which is found to track the domelike dependence of superconductivity (${T}_{C}$). The fall of ${H}_{C2}^{*}$ and ${T}_{C}$ with underdoping is most likely due to the onset of long-range antiferromagnetic order. We also report the temperature onset, ${T}_{\mathrm{onset}}$, of superconducting fluctuations above ${T}_{C}$. For optimally doped $x=0.11{T}_{\mathrm{onset}}$ ($\ensuremath{\cong}39\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is high compared to ${T}_{C}$ (26 K). For higher doping ${T}_{\mathrm{onset}}$ decreases and tends to zero along with the critical temperature at the end of the superconducting dome. The superconducting gap closely tracks ${H}_{C2}^{*}$ measured from the temperature- and field-dependent Nernst signal.

Superconductivity in Perovskite Ba1-xLnx(Bi0.20Pb0.80)O3-δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).

Solid solutions Ba1-xLnx(Bi0.20Pb0.80)O3-δ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; 0.00 ≤ x ≲ 0.15) have been prepared under 850 °C. They all crystallize in space group P1 at room temperature. XPS data indicate that the valences are 5+ and 3+ for bismuth, 4+ and 2+ for lead, and 3+ or 4+ for lanthanide. Some of them are superconductors. The superconductive transition temperature Tczero decreases or remains constant with an increase of Ln in the sample when Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. However, Tczero first decreases, then increases, and finally decreases when Ln = Ce, Pr, which is due to the corresponding sample changes from hole-doped to electron-doped superconductors with an increase of Ce or Pr in the sample.

The nematicity induced d-symmetry charge density wave in electron-doped iron-pnictide superconductors

The interplay among the nematicity, the stripe spin-density-wave (SDW) order and superconductivity in iron-pnictides is studied in a self-consistent Bogoliubov-de Gennes equations. Our calculations have shown that the nematic-order breaks the degeneracy of dxz and dyz orbitals and causes the elliptic Fermi surface near the Γ point in the normal state. In addition, the appearance of the orthorhombic magnetic fluctuations generates two uneven pairs of peaks at ( ± π, 0) and (0, ± π) in its Fourier transformation. All these are comparing favorably with experimental measurements. In the nematic phase, our results indicate that the charge density and its spatial image in the local density of states exhibit a dx2−y2-like symmetry. Finally, the complete phase diagram is obtained and the nematic phase is found to be in a narrow region close to the SDW transition in the electron-doped iron-pnictide superconductors.

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.

Doping dependence of charge order in electron-doped cuprate superconductors

In the recent studies of the unconventional physics in cuprate superconductors, one of the central issues is the interplay between charge order and superconductivity. Here the mechanism of the charge-order formation in the electron-doped cuprate superconductors is investigated based on the t-J model. The experimentally observed momentum dependence of the electron quasiparticle scattering rate is qualitatively reproduced, where the scattering rate is highly anisotropic in momentum space, and is intriguingly related to the charge-order gap. Although the scattering strength appears to be weakest at the hot spots, the scattering in the antinodal region is stronger than that in the nodal region, which leads to the original electron Fermi surface is broken up into the Fermi pockets and their coexistence with the Fermi arcs located around the nodal region. In particular, this electron Fermi surface instability drives the charge-order correlation, with the charge-order wave vector that matches well with the wave vector connecting the hot spots, as the charge-order correlation in the hole-doped counterparts. However, in a striking contrast to the hole-doped case, the charge-order wave vector in the electron-doped side increases in magnitude with the electron doping. The theory also shows the existence of a quantitative link between the single-electron fermiology and the collective response of the electron density.

Neutron scattering studies on the magnetic interaction in iron-based superconductor BaFe<sub>2</sub><sub>−</sub><sub><italic>x</italic></sub>Ni<sub><italic>x</italic></sub>As<sub>2</sub>

Iron-based superconductivity emerges from the suppression of the long-ranged antiferromagnetic order in the parent compounds. To solve the microscopic mechanism of the superconductivity, the key issue is fully understanding about the evolution of magnetic interaction and its relationship between superconductivity. Here, we take the electron-doped iron-based superconductor BaFe2- x Ni x As2 as atypical example, and summarize the neutron scattering results on the electronic phase diagram, magnetic ordered state, low-energy spin excitations and high-energy spin fluctuations, particularly for their systematic evolution versus electron dopings and the recent progress on the spin nematicity. Firstly, we will introduce about the crystal structure and magnetic structure, detailed procedure of the single crystal growth is also presented. By focusing on the structural transition temperature ( T s) and magnetic transition temperature ( T N) near the optimal doping level, we have found both T s and T N vanish above the superconducting transition temperature ( T c) as a first order manner, due to the lattice distortion and magnetically ordered moment decrease beyond the lower limit of the instrument resolution, giving an avoid quantum critical point at the optimal doping. In addition, the magnetic order becomes incommensurate and short-ranged magnetic cluster and competes with superconducting order, suggesting strong interplay between magnetism and superconductivity. Secondly, we will discuss about the itinerant magnetism and spin resonance mode at low energy for spin dynamics. The spin resonance mode in iron pnictides is explained as the collective quasiparticle excitations from the Fermi surface nesting between the hole pocket at the Γ point and electron pocket at the M point. This requires itinerant magnetism from the electrons near Fermi surfaces, which is further confirmed by neutron scattering by discovering the longitudinal mode and line shape change of low-energy spin excitations upon doping. Thirdly, we will present the doping evolution of spin fluctuations throughout the whole phase diagram. Although the spin waves in the parent compound can be described by an effective Heisenberg mode with anisotropic exchange coupling in FeAs plane, the damping features and unexpected low total fluctuation moments reveal both local moments and itinerant electrons contribute to the magnetism. Upon Ni doping, the high energy spin fluctuations is very robust, but the low energy spin excitations are suppressed quickly to zero when cross the boundary of the overdoped superconducting regime. The analysis on the change of the magnetic coupling energy associated with the superconducting transition gives evidence that the magnetic fluctuations are strong enough to drive the condensation of the Cooper pairs. Fourthly, we will further introduce the recent progress on the spin anisotropy by polarized neutron scattering experiments, and spin nematicity measured on detwinned samples. Anisotropic spin excitations at low energy are discovered by polarized neutron scattering, which is caused by the in-plane orbital ordering and persists away up to the tetragonal paramagnetic state. Further experiments on the detwinned sample under uniaxial pressure give direct evidence of the spin nematicity shown as breaking C4 rotation symmetry spin excitations in the tetragonal phase, similar to the anisotropic resistivity and electronic nematic phase discovered in other probes. Such novel electron state in these materials suggests the iron pnictides are unconventional. Finally, we summarize the results about the magnetic phase transition and spin dynamics and discuss its physical origin. Then a possible picture about the spin driven unconventional superconductivity in these materials is proposed. Further perspectives on the research of magnetism in iron-based superconductivity are also given.

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.

Spin and charge excitations in artificial hole- and electron-doped infinite layer cuprate superconductors

The asymmetry between electron and hole doping in high critical-temperature superconducting (HTS) cuprates is key information for the understanding of Cooper pair formation mechanisms. Despite intensive studies on different cuprates, a comprehensive description of related magnetic and charge excitations is still fragmentary. In the present work, artificial cuprates were used to cover the entire phase diagram within the same HTS family. In particular, Cu ${L}_{3}$-edge resonant inelastic x-ray scattering (RIXS) measurements were performed on artificial $n$- and $p$-type infinite layer (IL) epitaxial films. Beside several similarities, RIXS spectra show noticeable differences in the evolution, with doping level, of magnetic and charge intensity and damping. Compatible trends can be found in spectra measured on bulk cuprates, as well as in theoretical calculations of the spin dynamical structure factor $S(\mathbf{q},\ensuremath{\omega})$. The findings give a deeper insight into the evolution of collective excitations across the cuprate phase diagram, and on underlying general features, only connected to the doping type. Moreover, they pave the way to the exploration of general properties of HTS physics over a broad range of conditions, by means of artificial compounds not constrained by the thermodynamic limitations governing the chemical stability of bulk materials.

Extreme anisotropy and anomalous transport properties of heavily electron doped Lix(NH3)yFe2Se2 single crystals

The missing hole packets near the Brillouin zone center render unique electronic structures to the heavily electron doped FeSe-based superconductors with Tc above 40 K. It challenges the existing scenario accounting for the nature of superconductivity in the iron-based family. Yet, one hurdle that has to be overcome is the materials complexity in the rather limited number of compounds. Here we report the growth of heavily electron doped Li-NH3 intercalated FeSe single crystals that are free of such complexities and allow access to the intrinsic superconducting properties. Our results show extremely large electronic anisotropy in both normal and superconducting states. Moreover, the anomalous transport properties appear in normal state, which are believed related to the anisotropy of relaxation time and/or temperature dependent electron carrier concentration. In view of the great chemical flexibility of intercalants, our findings provide novel platform to understanding of superconductivity origin of FeSe-related superconductors.

Nature of the effective interaction in electron-doped cuprate superconductors: A sign-problem-free quantum Monte Carlo study

Superconductivity is an emergent phenomena in the sense that the energy scale associated with Cooper pairing is generically much lower than the typical kinetic energy of electrons. Addressing the mechanism of Cooper pairing amounts to determine the effective interaction that operates at low energies. Deriving such an interaction from a bottom-up approach has not been possible for any superconductor, especially strongly correlated ones. Top-down approaches, where one assumes an effective interaction, is plagued with the difficulty of extracting the implied electronic instabilities without uncontrolled approximations. These facts severely hinder our ability to determine the pairing mechanism for high temperature superconductors. Here we perform large-scale sign-problem-free quantum Monte-Carlo simulations on an effective theory, featured with antiferromagnetic and nematic fluctuations, to study the intertwined antiferromagnetic, superconducting, and charge density wave instabilities of the cuprates. Our results suggest the inclusion of nematic fluctuations is essential in order to produce the observed type of charge density wave ordering. Interestingly we find that the d-wave Cooper pairing is enhanced by nematic fluctuations.

Hall Resistivity Correlations in Disordered Electron-Doped $$\hbox {Nd}_{2-x}\hbox {Ce}_x\hbox {CuO}_{4+\delta }$$ Nd 2 - x Ce x CuO 4 + δ Films

The resistivity tensor correlations $\rho_{xy}$($B$)$\sim$ $[\rho_{xx}(B)]^\beta$ for the mixed state magnetic field dependencies of the resistivity tensor of electron-doped Nd$_{2-x}$Ce$_x$CuO$_{4+\delta}$/SrTiO$_3$ single crystal films near the antiferromagnetic (AF) - superconducting (SC) phase transition and with varying degree of disorder ($\delta$) were studied. The decrease of $\beta$ from 1.2$\pm$0.2 at $x$ = 0.14 to 0.6$\pm$0.1 at $x$ = 0.15 points out on the evidence of the change from the anisotropic $s$ - wave to the $d$ - wave pairing symmetry in the external magnetic field at the transition from underdoped to optimally doped region. Peculiarities of the power law dependence of the vortex motion in the mixed state can be connected with some features of the nonstoichiometric disorder in layered electron-doped superconductors.

Topological phase transition from nodal to nodeless d-wave superconductivity in electron-doped cuprate superconductors

Unlike the hole-doped cuprates, both nodal and nodeless superconductivity (SC) are observed in the electron-doped cuprates. To understand these two types of SC states, we propose a unified theory by considering the two-dimensional model in proximity to an antiferromagnetic (AF) long-range ordering state. Within the slave-boson mean-field approximation, the d-wave pairing symmetry is still the most energetically favorable even in the presence of the external AF field. In the nodal phase, it is found that the nodes carry vorticity and are protected by the adjoint symmetry of time-reversal and one unit lattice translation. Robust edge modes are obtained, suggesting the nodal d-wave SC being a topological weak-pairing phase. As decreasing the doping concentration or increasing the AF field, the nodes with opposite vorticity annihilate and the nodeless strong-pairing phase emerges. The topological phase transition is characterized by a critical point with anisotropic Bogoliubov quasiparticles, and a universal understanding is thus established for all electron-doped cuprates.

Energy dependence of the electron-boson coupling strength in the electron-doped cuprate superconductor Pr1.85Ce0.15CuO4−δ

We studied the superconducting (SC) state depletion process in an electron doped cuprate Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ by pumping with near-infrared (NIR) and narrow-band THz pulses. When pumping with THz pulses tuned just above the SC gap, we find the absorbed energy density required to deplete superconductivity, $A_{dep}$, matches the thermodynamic condensation energy. Contrary, by NIR pumping $A_{dep}$ is an order of magnitude higher, despite the fact that the SC gap is much smaller than the energy of relevant bosonic excitations. The result implies that only a small subset of bosons contribute to pairing.

Momentum and Doping Dependence of Spin Excitations in Electron-Doped Cuprate Superconductors

Superconductivity in copper oxides emerges on doping holes or electrons into their Mott insulating parent compounds. The spin excitations are thought to be the mediating glue for the pairing in superconductivity. Here the momentum and doping dependence of the dynamical spin response in the electron-doped cuprate superconductors is studied based on the kinetic-energy-driven superconducting mechanism. It is shown that the dispersion of the low-energy spin excitations changes strongly upon electron doping, however, the hour-glass-shaped dispersion of the low-energy spin excitations appeared in the hole-doped side is absent in the electron-doped case due to the electron-hole asymmetry. In particular, the commensurate resonance appears in the superconducting-state with the resonance energy that correlates with the dome-shaped doping dependence of the superconducting gap. Moreover, the spectral weight and dispersion of the high-energy spin excitations in the superconducting-state are comparable with those in the corresponding normal-state, indicating that the high-energy spin excitations do not play an important part in the pair formation.

P-wave triggered superconductivity in single-layer graphene on an electron-doped oxide superconductor

Electron pairing in the vast majority of superconductors follows the Bardeen–Cooper–Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

Impurity effect in electron-doped high-Tc superconductors

The quasiparticle states around a nonmagnetic impurity in electron-doped high-Tc superconductors are studied systematically based on the Bogoliubov–de Gennes equations. In the antiferromagnetic state, one in-gap impurity resonance state is revealed. As the impurity scattering potential increases, the resonance peak shifts to the gap edge and finally disappears for a strong impurity. The antiferromagnetic order and superconducting order coexist when the doping density increases. In this coexisting state, the in-gap resonance peaks are rather robust and appear in pairs that are lying symmetric with the Fermi energy. The peak positions and intensities strongly depend on the impurity potentials and the next-nearest-neighbour hopping constants. For a rather strong impurity, the resonance peaks shift to near the gap edge. When further increasing the doping density, the system is in the pure superconducting state. The resonance peaks still appear in pairs, with the peak intensities being weaker compared to those in the coexisting state. The two resonance peaks may occasionally merge into one zero-energy peak for both the coexisting state and the pure superconducting state. The spin-resolved LDOS are also investigated and may be used to detect possible antiferromagnetic order.

Hidden Fermi-liquid Charge Transport in the Antiferromagnetic Phase of the Electron-Doped Cuprate Superconductors.

Systematic analysis of the planar resistivity, Hall effect, and cotangent of the Hall angle for the electron-doped cuprates reveals underlying Fermi-liquid behavior even deep in the antiferromagnetic part of the phase diagram. The transport scattering rate exhibits a quadratic temperature dependence, and is nearly independent of doping and compound and carrier type (electrons versus holes), and hence is universal. Our analysis moreover indicates that the material-specific resistivity upturn at low temperatures and low doping has the same origin in both electron- and hole-doped cuprates.

Dielectric functions of La-based cuprate superconductors for visible and near-infrared wavelengths

The highly c-axis oriented La1.9Ce0.1CuO4 (LCCO) film was prepared on SrTiO3 (STO) substrate by pulsed-laser deposition (PLD). The identification and characterization of the LCCO film was determined by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Spectroscopic ellipsometry (SE) measurement was performed on the LCCO film in 0.5–4.13eV range. The complex pseudodielectric function of the LCCO film was reported by Drude and Lorentz oscillators in this work. Critical point analysis on third-derivative spectra of the Lorentz contributions was accomplished to obtain the interband transition energies. The results indicate that the interband transitions are mainly related to the copper-oxygen planes. The metallic behavior of the pseudodielectric function, which is a common feature in all known electron-doped superconductors, can be obviously observed in the LCCO film.

Electrical and thermal transport properties of the electron-doped cuprate Sm2−xCexCuO4−y system

Electrical and thermal transport measurements were performed on thin films of the electron-doped superconductor Sm2−xCexCuO4−y (x = 0.13 − 0.19) in order to study the evolving nature of the charge carriers from the under-doped to over-doped regime. A temperature versus cerium content (T − x) phase diagram has been constructed from the electrical transport measurements, yielding a superconducting region similar to that found for other electron-doped superconductors. Thermopower measurements show a dramatic change from the underdoped region (x < 0.15) to the overdoped region (x > 0.15). Application of the Fisher–Fisher–Huse (FFH) vortex glass scaling model to the magnetoresistance data was found to be insufficient to describe the data in the region of the vortex-solid to vortex-liquid transition. It was found instead that the modified vortex glass scaling model of Rydh, Rapp, and Anderson provided a good description of the data, indicating the importance of the applied field on the pinning landscape. A magnetic field versus temperature (H − T) phase diagram has also been constructed for the films with , displaying the evolution of the vortex glass melting lines Hg(T) across the superconducting regime.