Critical Doping Research Articles

Cuprate diamagnetism in the presence of a pseudogap: Beyond the standard fluctuation formalism

It is often claimed that among the strongest evidence for preformed-pair physics in the cuprates are the experimentally observed large values for the diamagnetic susceptibility and Nernst coefficient. These findings are most apparent in the underdoped regime, where a pseudogap is also evident. While the conventional (Gaussian) fluctuation picture has been applied to address these results, this preformed-pair approach omits the crucial effects of a pseudogap. In this paper we remedy this omission by computing the diamagnetic susceptibility and Nernst coefficient in the presence of a normal state gap. We find a large diamagnetic response for a range of temperatures much higher than the transition temperature. In particular, we report semi-quantitative agreement with the measured diamagnetic susceptibility onset temperatures, over the entire range of hole dopings. Notable is the fact that at the lower critical doping of the superconducting dome, where the transition temperature vanishes and the pseudogap onset temperature remains large, the onset temperature for both diamagnetic and transverse thermoelectric transport coefficients tends to zero. Due to the importance attributed to the cuprate diamagnetic susceptibility and Nernst coefficient, this work helps to clarify the extent to which pairing fluctuations are a component of the cuprate pseudogap.

Topological, chemical and electro-optical characteristics of riboflavin-doped artificial and natural DNA thin films.

DNA is considered as a useful building bio-material, and it serves as an efficient template to align functionalized nanomaterials. Riboflavin (RF)-doped synthetic double-crossover DNA (DX-DNA) lattices and natural salmon DNA (SDNA) thin films were constructed using substrate-assisted growth and drop-casting methods, respectively, and their topological, chemical and electro-optical characteristics were evaluated. The critical doping concentrations of RF ([RF]C, approx. 5 mM) at given concentrations of DX-DNA and SDNA were obtained by observing the phase transition (from crystalline to amorphous structures) of DX-DNA and precipitation of SDNA in solution above [RF]C. [RF]C are verified by analysing the atomic force microscopy images for DX-DNA and current, absorbance and photoluminescence (PL) for SDNA. We study the physical characteristics of RF-embedded SDNA thin films, using the Fourier transform infrared spectrum to understand the interaction between the RF and DNA molecules, current to evaluate the conductance, absorption to understand the RF binding to the DNA and PL to analyse the energy transfer between the RF and DNA. The current and UV absorption band of SDNA thin films decrease up to [RF]C followed by an increase above [RF]C. By contrast, the PL intensity illustrates the reverse trend, as compared to the current and UV absorption behaviour as a function of the varying [RF]. Owing to the intense PL characteristic of RF, the DNA lattices and thin films with RF might offer immense potential to develop efficient bio-sensors and useful bio-photonic devices.

Charge-density-wave order takes over antiferromagnetism in Bi2Sr2\u2212xLaxCuO6 superconductors

Superconductivity appears in the cuprates when a spin order is destroyed, while the role of charge is less known. Recently, charge density wave (CDW) was found below the superconducting dome in YBa2Cu3Oy when a high magnetic field is applied perpendicular to the CuO2 plane, which was suggested to arise from incipient CDW in the vortex cores that becomes overlapped. Here by 63Cu-nuclear magnetic resonance, we report the discovery of CDW induced by an in-plane field, setting in above the dome in single-layered Bi2Sr2−xLaxCuO6. The onset temperature TCDW takes over the antiferromagnetic order temperature TN beyond a critical doping level at which superconductivity starts to emerge, and scales with the pseudogap temperature T*. These results provide important insights into the relationship between spin order, CDW and the pseudogap, and their connections to high-temperature superconductivity.

Fermi surface reconstruction and anomalous low-temperature resistivity in electron-doped La2−xCexCuO4

We report $ab$-plane Hall-effect and magnetoresistivity measurements on $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{C}{\mathrm{e}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ thin films as a function of doping for magnetic fields up to 14 T and temperatures down to 1.8 K. A dramatic change in the low-temperature (1.8 K) normal-state Hall coefficient is found near a doping $\mathrm{Ce}=0.14$. This, along with a nonlinear Hall resistance as a function of magnetic field, suggests that the Fermi surface reconstructs at a critical doping of $\mathrm{Ce}=0.14$. A competing antiferromagnetic phase is the likely cause of this Fermi surface reconstruction. Low-temperature linear-in-$T$ resistivity is found at $\mathrm{Ce}=0.14$, but anomalously, also at higher doping. We compare our data with similar behavior found in hole-doped cuprates at a doping where the pseudogap ends.

Evolution of Hall resistivity and spectral function with doping in the SU(2) theory of cuprates

Recent transport experiments in the cuprate superconductors linked the opening of the pseudogap to a change in electronic dispersion [S. Badoux et al., Nature 531, 210 (2015)]. Transport measurements showed that the carrier density sharply changes from $x$ to $1+x$ at the pseudogap critical doping, in accordance with the change from Fermi arcs at low doping to a large hole Fermi surface at high doping. The SU(2) theory of cuprates shows that antiferromagnetic short range interactions cause the arising of both charge and superconducting orders, which are related by an SU(2) symmetry. The fluctuations associated with this symmetry form a pseudogap phase. Here we derive the renormalised electronic propagator under the SU(2) dome, and calculate the spectral functions and transport quantities of the renormalised bands. We show that their evolution with doping matches both spectral and transport measurements.

Appearance of Fröhlich-like phonon mode and defect dynamics in La3+-doped ceria

The underlying physics which explains the role of cationic and anionic defect dynamics in determining the catalytic activity and ionic conductivity of aliovalent ion-doped ceria is complex and not yet fully understood. To address this issue, we have investigated the structural evolution of LaxCe1-xO2–δ, where x = 0.1, 0.2, 0.3, 0.4, and 0.5. Rietveld refinements of the X-ray diffraction patterns reveal that the crystal structure belongs to a disordered fluorite phase for all samples. We find a systematic increase in the lattice parameter along with a gradual decrease in oxygen site occupancy with the increase in La3+ doping concentration. In Raman spectra of doped compounds, other than well-known crystalline F2g and defect related modes, we observe the presence of an additional Raman mode. The Raman line-shape analysis indicates the non-resonant Fröhlich character of this mode. Temperature dependent Raman measurements demonstrate unique characteristics of this Raman peak. In addition, from the change in relative intensity ratio of the Raman modes related to different types of defect states, we show that vacancies cannot be considered as isolated defects beyond a critical doping level. We propose that the evolution of Raman intensities of defect related modes and Fröhlich mode with the doping level can be used as a marker to determine the role of electron-phonon coupling and anion vacancies in the catalytic activity of doped ceria. Furthermore, by studying the photocatalytic measurements for La3+ doped ceria compounds, we demonstrate that the anion vacancies do not always play a direct role in controlling functional properties.

Phenomenological theories of the low-temperature pseudogap: Hall number, specific heat, and Seebeck coefficient

Since its experimental discovery, many phenomenological theories successfully reproduced the rapid rise from $p$ to $1+p$ found in the Hall number $n_H$ at the critical doping $p^*$ of the pseudogap in superconducting cuprates. Further comparison with experiments is now needed in order to narrow down candidates. In this paper, we consider three previously successful phenomenological theories in a unified formalism---an antiferromagnetic mean field (AF), a spiral incommensurate antiferromagnetic mean field (sAF), and the Yang-Rice-Zhang (YRZ) theory. We find a rapid rise in the specific heat and a rapid drop in the Seebeck coefficient for increasing doping across the transition in each of those models. The predicted rises and drops are locked, not to~$p^*$, but to the doping where anti-nodal electron pockets, characteristic of each model, appear at the Fermi surface shortly before~$p^*$. While such electron pockets are still to be found in experiments, we discuss how they could provide distinctive signatures for each model. We also show that the range of doping where those electron pockets would be found is strongly affected by the position of the van~Hove singularity.

Synthesis and luminescent properties of Tb3Al5O12:Ce3+ phosphors for warm white light emitting diodes

A series of Tb3Al5O12:Ce3+ phosphors were successfully synthesized by a precipitation method. The pure Tb3Al5O12 phase was obtained in the synthesized Tb3Al5O12:Ce3+ phosphors after heat treatments at 500 °C in air for 3 h. The excitation spectra of Tb3Al5O12:Ce3+ phosphors include excitation bands corresponding to Tb3+ and Ce3+ ions. Under the excitation at 455 nm, Tb3Al5O12:Ce3+ phosphors show emission band at around 553 nm. The critical doping concentration of Ce3+ in Tb3Al5O12 is 6mol%, which shows the highest emission intensity. White light-emitting diodes were fabricated by combining InGaN-based blue light-emitting diodes with Tb3Al5O12:Ce3+ and Y3Al5O12:Ce3+ phosphors. The Tb3Al5O12:Ce3+ based white light-emitting diode shows a lower color temperature than that of Y3Al5O12:Ce3+ based white light-emitting diode. The experimental results clearly indicate that the prepared Tb3Al5O12:Ce3+ has potential applications in white light emitting diodes.

Magnetic and electronic crossovers in graphene nanoflakes

Manipulation of intrinsic magnetic and electronic structures of graphene nanoflakes is of technological importance. Here we carry out systematic study of the magnetic and electronic phases, and its manipulation in graphene nanoflakes employing first-principles calculation. We illustrate the intricate shape and size dependence on the magnetic and electronic properties, and further investigate the effects of carrier doping, which could be tuned by gate voltage. A transition from nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance, which we identify to be originated from the armchair defect at the junctions of two sublattices on the edge. Electron, or hole doping simultaneously influences the magnetic and electronic structures, and triggers phase changes. Beyond a critical doping, crossover from antiferromagnetic to ferromagnetic phase is observed for the flakes without sublattice imbalance, while suppression of magnetism, and a possible crossover from magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous to magnetic phase changes, a semiconductor to (half) metal transition is observed, upon carrier doping.

Structural, morphological, photoluminescence and photocatalytic properties of Gd-doped ZnO films

Nanostructured Gd-doped ZnO films were deposited at 600°C using RF magnetron sputtering technique. The effects of Gd concentration varying from 0 to 2.0at.% on structure as well as morphology and subsequent changes in optical and photocatalytic properties were studied using X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), optical absorption spectra and photoluminescence (PL) spectroscopy. XRD and selected area diffraction results indicated that all Gd-doped ZnO films were of highly (002) preferred orientation and possessed the hexagonal wurtzite structure. SEM images revealed that ZnO nanowires/film structures were grown with a lower Gd content or pure ZnO, and the film crystalline sizes were decreased with Gd doping. XPS results confirmed that the Gd ion was +3 valence states. PL spectroscopy demonstrated an immense enhanced deep-level emission which attributes to the increase in defects concentration due to Gd doping. With a critical doping concentration of 1.6at.% Gd, the ratio of the deep-level emission and the near band edge emission increases by a factor of almost five as compared with pure ZnO. The photocatalytic activities of the films were evaluated by the degradation of methylene blue in aqueous solutions under UV light. The photocatalytic results indicated that the 0.7at.% Gd-doped ZnO nanowires/film structure performed the hightest photocatalytic activity.

Mottness at finite doping and charge-instabilities in cuprates.

The influence of the Mott physics on the doping-temperature phase diagram of copper oxides represents a major issue that is subject of intense theoretical and experimental effort. Here, we investigate the ultrafast electron dynamics in prototypical single-layer Bi-based cuprates at the energy scale of the O-2p→Cu-3d charge-transfer (CT) process. We demonstrate a clear evolution of the CT excitations from incoherent and localized, as in a Mott insulator, to coherent and delocalized, as in a conventional metal. This reorganization of the high-energy degrees of freedom occurs at the critical doping pcr ≈0.16 irrespective of the temperature, and it can be well described by dynamical mean field theory calculations. We argue that the onset of the low-temperature charge instabilities is the low-energy manifestation of the underlying Mottness that characterizes the p < pcr region of the phase diagram. This discovery sets a new framework for theories of charge order and low-temperature phases in underdoped copper oxides.

Metal-Organic Framework-Derived Atomic Iron-Dispersed Carbon Electrocatalysts for Oxygen Reduction in Acidic Polymer Electrolyte Fuel Cells

Low cost and high performance cathode catalysts for oxygen reduction reaction (ORR) in acidic Nafion® based polymer electrolyte fuels still remain a grand challenge in the field.1-5 Here, we present an atomic iron-dispersed carbon catalyst with homogeneous microstructure. The iron atoms were found atomically embedded into partially graphitized carbon. Due to the significantly improved uniformity and increased density of active sites, the ORR activity of this new catalyst reached to a new milestone, showing a half-wave potential of 0.84 V vs RHE in 0.5 M H2SO4 by rotating disk electrode along with fuel cell performance (0. 044 A/cm2 at 0.87 ViR-free in a H2-O2 cell) and stability at a practical operation voltage of 0.7 V. The high-performance platinum group metal (PGM)-free catalyst is derived from a well-defined iron doped metal-organic framework (e.g., zeolitic imidazolate framework, ZIF-8) precursor with ordered crystal structure through a single carbonization step in N2 atmosphere. Obtaining the optimal doping content of Fe into the ZIF-8 during the precursor synthesis played a key role in achieving the atomically dispersed iron morphology associated with the improvement of activity and stability. Unlike previous studied Fe-based catalysts, the particle size and shape of this catalyst were well-controlled and highly homogeneous and can be directly transferred from the morphology of Fe-doped ZIF precursors. Notably, higher Fe doping contents yielded larger crystal sizes in the precursors. Furthermore, optimal iron doping content is crucial for activity and stability enhancement, which correlated to iron distribution, carbon structures, surface areas/porosity, and nitrogen doping. Doping high Fe content is desirable to provide more active sites. However, atomic iron tends to agglomerate and form clusters when Fe content is above 5 at%. Higher Fe content yields highly graphitized carbon, but mitigates the total pore volumes, which inhibits active formation in micropores and mass transfer through meso/macro pores. Clearly, doped Fe content affects the critical nitrogen doping including the total content and doping position. In principle, high Fe content in catalyst should coordinate more nitrogen. Oppositely, instead of coordinating with nitrogen, Fe just tends to form inactive metallic iron and iron carbides. Therefore, one future focus is to develop effective solution synthesis to chemically doped Fe into MOFs capable of forming atomic iron distribution in carbon, rather than aggregates. This work provides evidence that superior ORR activity arises from atomically and homogenously dispersed FeNx active sites located in the carbon fringes. The elucidation of structure-property correlations revealed that Fe content in the precursor is the foremost variable in controlling the synthetic chemistry of these catalysts, and affects various properties including particle size, porosity, graphitization, and nitrogen-doping. Most importantly, this work strongly supports the theoretical prediction that iron-based catalysts are able to achieve comparable catalytic activity of Pt in the highly challenging environment of acid media.

Doping-induced perturbation and percolation in the two-dimensional Anderson lattice

We examine the doping effects in the two-dimensional periodic Anderson model using the determinant Quantum Monte Carlo (DQMC) method. We observe bound states around the Kondo hole site and find that the heavy electron states are destroyed at the nearest-neighbor sites. Our results show no clear sign of hybridization oscillation predicted in previous mean-field calculations. We further study the electron transport with increasing doping and as a function of temperature and obtain a critical doping xc ≈ 0.6 that marks a transition from the Kondo insulator regime to the single-ion Kondo regime. The value of xc is in good agreement with the predicted threshold for the site percolation. Our results confirm the percolative nature of the insulator-metal transition widely observed in doped Kondo insulators.

Correlation between battery material performance and cooperative electron-phonon interaction in LiCoyMn2-yO4

Understanding the basic physics related to archetypal lithium battery material (such as LiCoyMn2-yO4) is of considerable interest and is expected to aid the designing of cathodes of high capacity. The relationship between electrochemical performance, activated-transport parameters, thermal expansion, and cooperativity of electron-phonon-interaction distortions in LiCoyMn2-yO4 is investigated. The first order cooperative-normal-mode transition, detected through coefficient of thermal expansion, is found to disappear at a critical doping (y∼0.16); interestingly, for y≳0.16, the resistivity does not change much with doping, and the electrochemical capacity becomes constant over repeated cycling. The critical doping y∼0.16 results in breakdown of the network of cooperative/coherent normal-mode distortions; this leads to vanishing of the first-order transition, establishment of hopping channels with lower resistance, and enhancing lithiation and delithiation of the battery, thereby minimizing electrochemical capacity fading.

A Novel Warm White Light Phosphor Ca2Al2SiO7:Dy3+ for White Light-Emitting Diodes

The white light emitting phosphor Ca2Al2SiO7:Dy3+ was synthesized by a conventional high-temperature solid-state method. X-ray diffraction and photoluminescence spectra were used to characterize the as-synthesized phosphor. Strong excitation peaks of Ca2Al2SiO7: Dy3+ were found around 350-370 nm, which matched well with the UV LED chip. Under 350 nm excitation, Ca2Al2SiO7: Dy3+ phosphor can exhibit an intense warm white emission by combining the two emission peaks locating at blue region (485 nm) and yellow region (575 nm), attributed to the characteristic F-4(9/2)-H-6(15/2) and F-4(9/2)-H-6(13/2) transition of Dy3+. The critical doping content of Dy3+ is 7%, and the intensity of the emission increased proportionally as the Dy3+ concentration increased until it was reached at 7%. When the doping concentration occurred from 7% to 9%, the luminescence intensity gradually reduced. Moreover, The CIE coordinates and the correlated color temperature (CCT) are also investigated in this work. The result shows that the optimum phosphor Ca1.93Al2SiO7:0.07Dy(3+) exhibits excellent color purity with chromaticity coordinates of (0.375, 0.391) and the CCT of 4231 K and could be considered as potential research value on white-LEDs.

Effect of doping on the surface modification of nebulizer sprayed BaxZn1−xO nanocrystalline thin films

The influence of Ba doped zinc oxide films were investigated by nebulizer spray pyrolysis technique at 673 K. X-ray diffraction reveals the polycrystalline hexagonal (wurtzite) crystal structure with (0 0 2) preferential orientation. Energy dispersive spectroscopy confirms the presence of Ba, Zn and O elements in the films. Field emission scanning electron microscopy shows that the surface morphology of the nanocrystalline films were changed from spherical shape structure to flake net-like shape and sprout like spherical structure with average grain size is ~100 nm due to the critical doping concentration. PL spectra prominent peaks corresponding to near band edge UV emission and intrinsic defect of the visible blue light region and defect related deep level green emission regions were discussed. The films are highly transparent in the visible region with a transmittance higher than 74%, and have an optical band gap energy values are increased from 3.22 eV to 4.02 eV depending on the Ba doping concentration. Interparticle like grains, grain boundary effect of deposited films is studied by complex impedance spectroscopy.

Normal state pair nematicity and hidden magnetic order and metal-insulator (fermionboson) crossover origin of pseudogap phase of cuprates II

In the present paper II, we will gain an understanding of the nematicity, insulating ground state (IGS), nematicity to stripe phase transition, Fermi pockets evolution, and resistivity temperature upturn, as to be metal - insulator (fermion-boson)- crossover (MIC) phenomena for the pseudogap (PG) region of cuprates. While in the paper I [1], we obtained an understanding of the observed heat conductivity downturn, anomalous Lorentz ratio, insulator resistivity boundary, nonlinear entropy as manifestations of the same MIC. The recently observed nematicity and hidden magnetic order are related to the PG pair intra charge and spin fluctuations. We will try to obtain an answer on the question: why ground state of YBCO is Fermi liquid oscillating and of Bi-2212 is insulating? We will also clarify the physics of the recently observed MIC results of Lalibert\'e et al. [2] and explain the long-discussed transition of the electric charge density from doping to doping+1 dependence at the critical doping. We predict that at the upturns this density should have the temperature dependence $n\sim T^3n_2$ for $T\rightarrow 0$, where $n_2$ is density for dopings close to the critical value. We understood that the upturns before and after the first critical doping have the same nature. We will find understanding of all above mentioned phenomena within the PG pair physics.

Unconventional superconductivity in generalized Hubbard model: role of electron–hole symmetry breaking terms

We investigate the effect of the electron–hole (e–h) symmetry breaking on d-wave superconductivity induced by non-local effects of correlations in the generalized Hubbard model. The symmetry breaking is introduced in a two-fold manner: by the next-to-nearest neighbor hopping of electrons and by the charge-bond interaction—the off-diagonal term of the Coulomb potential. Both terms lead to a pronounced asymmetry of the superconducting order parameter. The next-to-nearest neighbor hopping enhances superconductivity for h-doping, while diminishes it for e-doping. The charge-bond interaction alone leads to the opposite effect and, additionally, to the kinetic-energy gain upon condensation in the underdoped regime. With both terms included, with similar amplitudes, the height of the superconducting dome and the critical doping remain in favor of h-doping. The influence of the charge-bond interaction on deviations from symmetry of the shape of the gap at the Fermi surface in the momentum space is briefly discussed.

Effect of Ni doping on optical and magnetic properties of ZnO

Nowadays, the experimental results of absorption spectrum distribution of Ni doped ZnO suffer controversy when the mole fraction of impurity is in a range from 2.78% to 6.25%. However, there is still lack of a reasonable theoretical explanation. To solve this problem, the geometry optimizations and energies of different Ni-doped ZnO systems are calculated at a state of electron spin polarization by adopting plane-wave ultra-soft pseudo potential technique based on the density function theory. Calculation results show that the volume parameter and lattice parameter of the doping system are smaller than those of the pure ZnO, and they decrease with the increase of the concentration of Ni. The formation energy in the O-rich condition is lower than that in the Zn-rich condition for the same doping system, and the system is more stable in the O-rich condition. With the same doping concentration of Ni, the formation energies of the systems with interstitial Ni and Ni replacing Zn cannot be very different. The formation energy of the system with Ni replacing Zn increases with the increase of the concentration of Ni, the doping becomes difficult, the stability of the doping system decreases, the band gap becomes narrow and the absorption spectrum is obviously red shifted. The Mulliken atomic population method is used to calculate the orbital average charges of doping systems. The results show that the sum of the charge transitions between the s state orbital and d state orbital of Ni2+ ions in the doping systems Zn0.9722Ni0.0278O, Zn0.9583Ni0.0417O and Zn0.9375Ni0.0625O supercells are all closed to +2. Thus, it is considered that the valence of Ni doped in ZnO is +2, and the Ni is present as a Ni2+ ion in the doping system. The ionized impurity concentrations of all the doping systems exceed the critical doping concentration for the Mott phase change of semiconductor ZnO, which extremely matches the condition of degeneration, and the doping systems are degenerate semiconductors. Ni-doped ZnO has a conductive hole polarization rate of up to nearly 100%. Then the band gaps are corrected via the LDA (local density approximation)+U method. The calculation results show that the doping system possesses high Curie temperature and can achieve room temperature ferromagnetism. The magnetic moment is derived from the hybrid coupling effect of p-d exchange action. Meanwhile, the magnetic moment of the doping system becomes weak with the increase of the concentration of Ni. In addition, the absorption spectrum of Ni-interstitial ZnO is blue-shifted in the ultraviolet and visible light bands.

Destruction of Magnetic Long-Range Order by Hole-Induced Skyrmions in Two-Dimensional Heisenberg Model

Motivated by the rapid destruction of antiferromagnetic long-range order in hole-doped cuprate high-temperature superconductors, we study the effect of skyrmions on the magnetic long-range order (MLRO). Here we assume that either a skyrmion or antiskyrmion is introduced by a doped hole. Our numerical simulation indicates that in the case of isolated skyrmions, there is an abrupt disappearance of MLRO for doping concentration $x < 1.0\times 10^{-4}$. In the case of skyrmion-antiskyrmion pairs, the critical doping concentration $x_c$ for the suppression of MLRO is given as a function of the separation of the pairs. For a moderate separation of $3 - 4$ lattice constants, we find that the critical doping is consistent with the experimental value.