Electron-doped Cuprate Superconductors Research Articles

Enhanced two-dimensional behavior of metastableT′-La2CuO4, the parent compound of electron-doped cuprate superconductors

We synthesized crystalline bulk samples of lanthanum cuprate in the metastable ${T}^{\ensuremath{'}}$ phase using cesium hydroxide flux. Its crystal structure was determined as space group $I4/mmm$, no. 139, $a=401.02\text{ }\text{pm}$, $c=1252.66\text{ }\text{pm}$. Muon spin rotation reveals a gradual slowing down of magnetic fluctuations below ${T}_{\text{N}1}\ensuremath{\approx}200\text{ }\text{K}$, and static magnetic order below ${T}_{\text{N}2}=115\text{ }\text{K}$, in sharp contrast to ${\text{La}}_{2}{\text{CuO}}_{4}$ in the $T$ structure where ${T}_{\text{N}1}\ensuremath{\approx}{T}_{\text{N}2}\ensuremath{\approx}300\text{ }\text{K}$. Our result shows that the strikingly different magnetic behavior of the two parent compounds has its origin in the two crystal structure modifications. In addition, we find that ${T}^{\ensuremath{'}}{\text{-La}}_{2}{\text{CuO}}_{4}$ has strongly reduced magnetic interactions compared to the other ${T}^{\ensuremath{'}}$ materials ${\text{Nd}}_{2}{\text{CuO}}_{4}$ and ${\text{Pr}}_{2}{\text{CuO}}_{4}$, where ${\text{Nd}}^{3+}$ and ${\text{Pr}}^{3+}$ are magnetic ions in contrast to the nonmagnetic ${\text{La}}^{3+}$.

Resistivity at low temperatures in electron-doped cuprate superconductors

We measured the magnetoresistance as a function of temperature down to 20 mK and magnetic field for a set of underdoped ${\text{Pr}}_{1.88}{\text{Ce}}_{0.12}{\text{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ thin films with controlled oxygen content. This allows us to access the edge of the superconducting dome on the underdoped side. The sheet resistance increases with increasing oxygen content whereas the superconducting transition temperature is steadily decreasing down to zero. Upon applying various magnetic fields to suppress superconductivity we found that the sheet resistance increases when the temperature is lowered. It saturates at very low temperatures. These results, along with the magnetoresistance, cannot be described in the context of zero-temperature two-dimensional superconductor-to-insulator transition nor as a simple Kondo effect due to scattering off spins in the copper-oxide planes. We conjecture that due to the proximity to an antiferromagnetic phase magnetic droplets are induced. This results in negative magnetoresistance and in an upturn in the resistivity.

Doping and temperature dependence of the superconducting energy gap in the electron-doped cuprate Pr2−xCexCuO4−δ

In hole-doped cuprate superconductors at low carrier concentrations two energy scales are identified: the superconducting energy gap and the pseudogap. The relation between these energy scales is still a puzzle. In these compounds a measurement of the energy gap is not necessarily a probe of the order parameter. In the electron-doped cuprates the pseudogap does not obscure the superconducting state. Consequently, the superconducting gap can be studied directly in a tunneling experiment. Here we show that by studying superconductor/insulator/superconductor planar tunnel junctions we are able to map the behavior of the gap amplitude for the entire (doping-temperature) phase diagram of the electron-doped cuprate superconductor Pr2-xCexCuO4-δ. The superconducting gap, Δ, shows a BCS-like temperature dependence even for extremely low carrier concentrations. Moreover, Δ follows the doping dependence of Tc. We can therefore conclude that there is a single superconducting energy scale in the electron-doped cuprates.

Nernst effect in the electron-doped cuprate superconductors

We calculate the normal-state Nernst signal in the cuprates resulting from a reconstruction of the Fermi surface due to spin-density wave order. An order parameter consistent with the reconstruction of the Fermi surface detected in electron-doped materials is shown to sharply enhance the Nernst signal close to optimal doping. Within a semiclassical treatment, the obtained magnitude and position of the enhanced Nernst signal agrees with Nernst measurements in electron-doped cuprates. Our result is mainly caused by the role of Fermi-surface geometry under influence of a spin-density wave gap. We discuss also possible roles of short-ranged magnetic order in the normal-state Nernst effect and the Fermi-surface reconstruction observed by photoemission spectroscopy.

Spin-resolved impurity resonance states in electron-doped cuprate superconductors

With the aim at understanding the non-monotonic $d_{x^{2}-y^{2}}$-wave gap, we analyze the local electronic structure near impurities in the electron-doped cuprate superconductors. We find that the local density of states near a non-magnetic impurity in the scenario of $d_{x^{2}-y^{2}}$-wave superconductivity with higher harmonics is qualitatively different from that obtained from the $d_{x^{2}-y^{2}}$-wave superconductivity coexisting with antiferromagnetic spin density wave order. We propose that spin-polarized scanning tunneling microscopy measurements can distinguish the two scenarios and shed light on the real physical origin of a non-monotonic $d_{x^{2}-y^{2}}$-wave gap.

Effect of spin excitations on the property of quasiparticles in electron-doped cuprate superconductors

It is proposed that the 50\char21{}70 meV dispersion anomaly (kink) in electron-doped cuprates revealed by recent angle-resolved photoemission spectroscopy experiments is caused by coupling with the spin fluctuation. We elaborate that the kink exists both along nodal and antinodal directions, and both in the superconducting and normal state. The renormalized effect for the density of states is also studied and the hump feature outside the superconducting coherent peak is established, consistent with recent scanning tunneling microscopy experiments.

Interplay between antiferromagnetic order and spin polarization in ferromagnetic metal/electron-doped cuprate superconductor junctions

Recently we proposed a theory of point-contact spectroscopy and argued that the splitting of zero-bias conductance peak in electron-doped cuprate superconductor point-contact spectroscopy is due to the coexistence of antiferromagnetic (AF) and $d$-wave superconducting orders [Phys. Rev. B 76, 220504(R) (2007)]. Here we extend the theory to study the tunneling in the ferromagnetic metal/electron-doped cuprate superconductor (FM/EDSC) junctions. In addition to the AF order, the effects of spin polarization, Fermi-wave vector mismatch between the FM and EDSC regions, and effective barrier are investigated. It is shown that there exists midgap surface-state contribution to the conductance to which Andreev reflections are largely modified due to the interplay between the exchange field of ferromagnetic metal and the AF order in EDSC. Low-energy anomalous conductance enhancement can occur which could further test the existence of AF order in EDSC. Finally, we propose a more accurate formula in determining the spin-polarization value in combination with the point-contact conductance data.

Intrinsic electron and hole bands in electron-doped cuprate superconductors

Based on the analysis of the measurement data of angle-resolved photoemission spectroscopy (ARPES) and optics, we show that the charge transfer gap is significantly smaller than the optical one and is reduced by doping in electron doped cuprate superconductors. This leads to a strong charge fluctuation between the Zhang-Rice singlet and the upper Hubbard bands. The basic model for describing this system is a hybridized two-band $t$-$J$ model. In the symmetric limit where the corresponding intra- and inter-band hopping integrals are equal to each other, this two-band model is equivalent to the Hubbard model with an antiferromagnetic exchange interaction (i.e. the $t$-$U$-$J$ model). The mean-field result of the $t$-$U$-$J$ model gives a good account for the doping evolution of the Fermi surface and the staggered magnetization.

Monotonicd-wave superconducting gap of the optimally dopedBi2Sr1.6La0.4CuO6superconductor by laser-based angle-resolved photoemission spectroscopy

We propose that the upper Hubbard band (electronlike) and the Zhang-Rice singlet band (holelike) are two essential components in describing low-energy excitations of electron-doped cuprate superconductors. We find that the gap between these two bands is significantly smaller than the charge-transfer gap measured by optics and is further reduced upon doping. This indicates that the charge fluctuation is strong and the system is in the intermediate correlation regime. A two-band model is derived. In the limit that the intraband and interband hopping integrals are equal to each other, this model is equivalent to the unconstrained t-J model with on-site Coulomb repulsions.

Polaronic signatures in the optical properties of the electron-doped cuprate superconductorNd2−xCexCuO4

We investigate the temperature and doping dependences of the midinfrared band in the optical conductivity of electron-doped cuprates in terms of magnetic/lattice polaron formation. We employ dynamical mean-field theory in the context of the Holstein-$t\text{\ensuremath{-}}J$ model for the specific case of ${\text{Nd}}_{2\ensuremath{-}x}{\text{Ce}}_{x}{\text{CuO}}_{4}$. We find that the onset of short-range magnetic correlations at the pseudogap temperature triggers the formation of the lattice polaron responsible for the midinfrared band.

Interband Model Scenario of Cuprate Superconductivity

A multiband approach for hole- and electron-doped cuprate superconductors is developed. The model electron spectrum includes nodal and antinodal defect (polaronic) subbands created by doping. Bare gaps between itinerant and defect subbands close with extended doping. The overlap conditions determine phase diagram special points. There are analogies for both types of doping despite the activation of different sublattices. Illustrative calculations have given self-consistent results reproducing qualitatively doping dependences of T c , superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, etc. Interband pairing scenario is suggested to be an essential aspect of cuprate superconductivity.

Rare earth ion effects on the pseudo-gap in electron-doped superconductors and possible nodeless d-wave gap

We report angle resolved photoemission (ARPES) studies on electron-doped cuprate superconductor Sm 2 - x Ce x CuO 4 ( x = 0.14 and 0.18). A wide energy range scan shows clear “waterfall” effect at an energy scale close to 500 meV which is consistent with the value found in Nd 2 - x Ce x CuO 4 (NCCO) but larger than that from hole-doped superconductors. High resolution results from both dopings show pseudo-gap effects that were observed in NCCO. However, the effects are found to be stronger than that observed in optimally doped NCCO. The overall electronic structure is well understood within a simple model in which a 2 × 2 static order is assumed. Both ARPES and optical measurements give the coupling strengths to the Q =( π / 2 , π / 2 ) (due to the 2 × 2 order) to be about 0.1 eV, compatible with each other. The effect is strong enough to push the band near the nodal region below the Fermi energy, resulting in possible nodeless d-wave superconductivity where zero energy quasi-particle excitation is inhibited.

Evolution of spin dynamics in electron-doped cuprate superconductors

Within the kinetic energy driven superconducting mechanism, the evolution of the magnetic excitations of the electron-doped cuprates in the superconducting state is studied. It is shown that there is a broad commensurate low energy magnetic scattering peak, while the magnetic resonance energy is located among this broad commensurate low energy scattering range. This broad commensurate low energy magnetic scattering disperses outward into a continuous ring-like incommensurate magnetic scattering at high energy.

Nodeless superconductivity in the infinite-layer electron-doped cuprate superconductorSr0.9La0.1CuO2

We report on measurements of the in-plane magnetic penetration depth ${\ensuremath{\lambda}}_{ab}$ in the infinite-layer electron-doped high-temperature cuprate superconductor ${\text{Sr}}_{0.9}{\text{La}}_{0.1}{\text{CuO}}_{2}$ by means of muon-spin rotation. The observed temperature and magnetic field dependences of ${\ensuremath{\lambda}}_{ab}$ are consistent with the presence of a substantial $s$-wave component in the superconducting order parameter, in good agreement with the results of tunneling, specific heat, and small-angle neutron scattering experiments.

Electromagnetic response of high-Tcsuperconductors: Slave-boson and doped-carrier theories

We evaluate the doping dependence of the quasiparticle current and low-temperature superfluid density in two slave-particle theories of the $t{t}^{\ensuremath{'}}{t}^{\ensuremath{''}}J$ model---the slave-boson theory and doped-carrier theory. In the slave-boson theory, the nodal quasiparticle current renormalization factor $\ensuremath{\alpha}$ proportionally vanishes to the zero temperature superfluid density ${\ensuremath{\rho}}_{S}(0)$; however, we find that away from the ${\ensuremath{\rho}}_{S}(0)\ensuremath{\rightarrow}0$ limit, $\ensuremath{\alpha}$ displays a much weaker doping dependence than ${\ensuremath{\rho}}_{S}(0)$. A similar conclusion applies to the doped-carrier theory, which differentiates the nodal and antinodal regions of momentum space. Due to its momentum space anisotropy, the doped-carrier theory enhances the value of $\ensuremath{\alpha}$ in the hole doped regime, bringing it to quantitative agreement with experiments, and reproduces the asymmetry between hole and electron doped cuprate superconductors. Finally, we use the doped-carrier theory to predict a specific experimental signature of local staggered spin correlations in doped Mott insulator superconductors, which, we propose, should be observed in scanning tunneling microscopy measurements of underdoped high-${T}_{c}$ compounds. This experimental signature distinguishes the doped-carrier theory from other candidate mean-field theories of high-${T}_{c}$ superconductors, such as the slave-boson theory and the conventional BCS theory.

Doping and energy evolution of spin dynamics in the electron-doped cuprate superconductorPr0.88LaCe0.12CuO4−δ

The doping and energy evolution of the magnetic excitations of the electron-doped cuprate superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ in the superconducting state is studied based on the kinetic energy driven superconducting mechanism. It is shown that there is a broad commensurate scattering peak at low energy, then the resonance energy is located among this low energy commensurate scattering range. This low energy commensurate scattering disperses outward into a continuous ring-like incommensurate scattering at high energy. The theory also predicts a dome shaped doping dependent resonance energy.

Theory of point-contact spectroscopy in electron-doped cuprate superconductors

In the hole-doped d(x2-y2)-wave cuprate superconductors, due to the midgap surface state (MSS), a zero-bias conductance peak (ZBCP) is widely observed in [110] interface point-contact spectroscopy (PCS). However, a ZBCP of this geometry is rarely observed in the electron-doped cuprates, even though their pairing symmetry is still likely of the d(x2-y2) wave type. We argue that this is due to the coexistence of antiferromagnetic (AF) and superconducting orders. Generalizing the Blonder-Tinkham-Klapwijk formula to include an AF coupling, it is shown explicitly that the MSS is destroyed by the AF order. The calculated PCS is in good agreement with the experiments.

Behaviour of superconductivity energetic characteristics in electron-doped cuprates. A simple model

A simple model to describe the energetic phase diagram of electron-doped cuprate superconductor is developed. Interband pairing operates between the UHB and the defect states created by doping and supplied by both extincting HB-s. Two defect subbands correspond to the ( π , 0 ) and ( π / 2 , π / 2 ) momentum regions. Extended doping quenches the bare normal state gaps (pseudogaps). Maximal transition temperature corresponds to overlapping bands ensemble intersected by the chemical potential. Illustrative results for T c , pseudo- and superconducting gaps are calculated on the whole doping scale. Major characteristic features on the phase diagram are reproduced. Anticipated manifestation of gaps doping dynamics is discussed.

Magnetic Resonance in the Spin Excitation Spectrum of Electron-Doped Cuprate Superconductors

We study the emergence of a magnetic resonance in the superconducting state of the electron-doped cuprate superconductors. We show that the recently observed resonance peak in the electron-doped superconductor Pr0.88LaCe0.12CuO4-delta is consistent with an overdamped spin exciton located near the particle-hole continuum. We present predictions for the magnetic-field dependence of the resonance mode as well as its temperature evolution in those parts of the phase diagram where dx2-y2-wave superconductivity may coexist with an antiferromagnetic spin-density wave.

Gap-function symmetry and spin dynamics in electron-doped cuprate superconductors

An antiferromagnetic (AF) spin fluctuation induced pairing model is proposed for the electron-doped cuprate superconductors. It suggests that, similar to the hole-doped side, the superconducting gap function is monotonic ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ wave and explains why the observed gap function has a nonmonotonic ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave behavior when an AF order is taken into account. Dynamical spin susceptibility is calculated and shown to be in good agreement with the experiment. This gives a strong support to the proposed model.