Optimally-doped Samples Research Articles

High external quantum yield in near-infrared phosphor Bi2Ga4O9:Cr3+ excited by near-ultraviolet or blue light

The proliferation of portable and intelligent devices featuring Near-Infrared (NIR) light sources as a central component has resulted in a significant need for compact NIR light sources. The technique of NIR phosphor-converted light-emitting diodes (NIR-pc-LEDs) could offer a fresh viewpoint on NIR light sources. The performance of NIR-pc-LEDs is directly influenced by NIR phosphors. However, the development of NIR phosphors with high external quantum yield (EQY) NIR phosphors has been hampered. In this work, we report a mullite-structure NIR phosphor, Bi2Ga4O9: Cr3+ (BGO), which exhibits excellent NIR luminescence properties. The emission spectrum of the optimal doped sample BGO:0.06Cr3+ has two emission peaks at 711 and 763 nm. All samples can be efficiently excited by commercial 365 nm near-ultraviolet LED chips and 450 nm blue LED chips. The internal quantum yield (IQY) reaches an astonishing value of 89.13 % and an EQY of about 65.36 % at 365 nm excitation. Furthermore, BGO:0.06Cr3+ shows a high IQY (∼87.78 %) and EQY (∼41.64 %) upon 450 nm excitation. Moreover, concerning thermal stability, the integrated photoluminescence intensity remains at 82 % and 42 % of that at room temperature when measured at 373 K and 423 K, respectively. The capsuled NIR-pc-LED device can be utilized for blood veins imaging, food safety analysis, and information encryption. These results highlight the promising commercial prospects of BGO:0.06Cr3+ in the fields of bio-imaging and non-destructive testing, among others.

On the dilemma between percolation processes and fluctuating pairs as the origin of the enhanced conductivity above the superconducting transition in cuprates

The confrontation between percolation processes and superconducting fluctuations to account for the observed enhanced in-plane electrical conductivity above but near T c in cuprates is revisited. This dilemma is currently an open and debated question, whose solution would contribute to the phenomenological understanding of the emergence of superconductivity in these compounds. The cuprates studied here, La1.85Sr0.15CuO4, Bi2Sr2CaCu2O, and Tl2Ba2Ca2Cu3O10, have a different number of superconducting CuO2 (ab)-layers per unit-cell length and different Josephson coupling between them, and are optimally-doped to minimize T c -inhomogeneities. The excellent chemical and structural quality of these optimally-doped samples also contribute to minimize the effect of extrinsic -inhomogeneities, a crucial aspect when analyzing the possible presence of intrinsic percolative processes. Our analyses also cover the so-called high reduced-temperature region, up to the resistivity rounding onset ɛ onset. By using the simplest form of the effective-medium theory, we show that possible emergent percolation processes alone cannot account for the measured enhanced conductivity. In contrast, these measurements can be quantitatively explained using the Gaussian–Ginzburg–Landau (GGL) approach for the effect of superconducting fluctuations in layered superconductors, extended to ɛ onset by including a total energy cutoff, which takes into account the limits imposed by the Heisenberg uncertainty principle to the shrinkage of the superconducting wavefunction. Our present analysis confirms the adequacy of this cutoff, which was introduced heuristically, and that the effective periodicity length is controlled by the relative Josephson coupling between superconducting layers, two long standing debated aspects of the GGL approaches for multilayered superconductors. These conclusions are reinforced by analyzing, as an example, one of the recent works that allegedly discards the superconducting fluctuations scenario while supporting a percolative scenario for the enhanced conductivity above T c in cuprates.

Enhancement of maximum superconducting temperature by applying pressure and reducing the charge transfer gap

Recent Scanning Tunneling Spectra(STS) measurements on underdoped cuprates have found that the maximum superconducting transition temperature Tc increases when the size of charge transfer gap (CTG) is reduced. Applying pressure is another well-known method to increase the maximum Tc. However, these pressure experiments also found that Tc is enhanced in underdoped and optimally-doped samples but suppressed in overdoped ones. Here we present a possible mechanism based on the charge fluctuation to explain both these observed effects. Starting from the 3-band Hubbard model, we retrieve the charge fluctuation (CF) between the oxygen 2p6 band and copper 3d10 band, which is ignored in the t−J model. This model is investigated using the variational Monte Carlo method(VMC).

Phonon spectrum of underdoped HgBa2CuO4+δ investigated by neutron scattering

The cuprates exhibit a prominent charge-density-wave (CDW) instability with wavevector along [100], i.e., the Cu-O bond direction. Whereas CDW order is most prominent at moderate doping and low temperature, there exists increasing evidence for dynamic charge correlations throughout a large portion of the temperature-doping phase diagram. In particular, signatures of incipient charge order have been observed as phonon softening and/or broadening near the CDW wavevector approximately half-way through the Brillouin zone. Most of this work has focused on moderately-doped cuprates, for which the CDW order is robust, or on optimally-doped samples, for which the superconducting transition temperature ($T_c$) attains its maximum. Here we present a time-of-flight neutron scattering study of phonons in simple-tetragonal $\text{HgBa}_2\text{CuO}_{4+\delta}$ ($T_c = 55$ K) at a low doping level where prior work showed the CDW order to be weak. We employ and showcase a new software-based technique that mines the large number of measured Brillouin zones for useful data in order to improve accuracy and counting statistics. Density-functional theory has not provided an accurate description of phonons in $\text{HgBa}_2\text{CuO}_{4+\delta}$, yet we find the right set of parameters to qualitatively reproduce the data. The notable exception is a dispersion minimum in the longitudinal Cu-O bond-stretching branch along [100]. This discrepancy suggests that, while CDW order is weak, there exist significant dynamic charge correlations in the optic phonon range at low doping, near the edge of the superconducting dome.

Copper-doped ZrO2 nanoparticles as high-performance catalysts for efficient removal of toxic organic pollutants and stable solar water oxidation

Doping effect on the photoelectrochemical (PEC) water splitting efficiency and photocatalytic activities of ZrO2 under visible light are reported. The XRD analysis revealed that pure, 0.1 and 0.3 mol% doped samples showed mixed crystal phases (tetragonal and monoclinic) and 0.5 mol% doped sample showed a pure tetragonal phase. Under visible light, 90% of methyl orange dye degradation was achieved with in 100 min. Moreover, the optimal doped sample showed a significant degradation rate constant over other samples. The doped photoelectrodes display a better PEC water oxidation performance over pure photoelectrode. Furthermore, the optimal doped (0.3 mol %) electrode shows 0.644 mAcm−2 photocurrent density, corresponding to an approximate 50-fold enhancement over pure electrode (0.013 mAcm−2). The optimized doped sample achieved 98% degradation of methyl orange within 100 min of light irradiation. The superior PEC water oxidation and photocatalytic activity of optimal doped samples under visible light are credited to suitable doping content, crystalline size, greater surface area, suitable bandgap, a lower charge carrying resistance, surface properties and the ability for decreasing the charge carrier's recombination rate.

Unravelling the mechanism of the semiconducting-like behavior and its relation to superconductivity in (CaFe1−xPtxAs)10Pt3As8

The temperature-dependence of the in-plane optical properties of (CaFe$_{1-x}$Pt$_{x}$As)$_{10}$Pt$_{3}$As$_{8}$ have been investigated for the undoped ($x=$0) parent compound, and the optimally-doped ($x=$0.1) superconducting material ($T_{c}\simeq$ 12 K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and magnetic transitions at $\simeq$ 96 and 83 K, respectively, strength is transferred from the free-carrier components into a bound excitation at $\simeq$ 1000 cm$^{-1}$, and the material exhibits semiconducting-like behavior. In the optimally-doped sample, at room temperature the optical properties are again described by narrow and broad Drude responses comparable to the parent compound; however, below $T^\ast \simeq$ 100 K, strength from the narrow Drude is transferred into a newly-emergent low-energy peak at $\simeq$ 120 cm$^{-1}$, which arises from a localization process, resulting in semiconducting-like behavior. Interestingly, below $T_{c}$, this peak also contributes to the superfluid weight, indicating that some localized electrons condense into Cooper pairs; this observation may provide insight into the pairing mechanism in iron-based superconductors.

Transverse resistance in HoBa2Cu3O7−δ single crystals

The transverse electrical resistance of HoBa2Cu3O[Formula: see text] single crystals is investigated in the temperature range [Formula: see text] for optimally-doped [Formula: see text] and oxygen-poor [Formula: see text] samples. With decreasing temperature, the resistivity of the optimally-doped samples has been found to transit from the regime of scattering on phonons and defects to the regime of “semiconductor” character and, near [Formula: see text], of the fluctuation conductivity. The oxygen-poor samples have been revealed to exhibit only a variable range hopping conductivity of “semiconductor” character, which near [Formula: see text] transits into the fluctuation conductivity. A significant anisotropy of the residual resistivity and characteristics of the fluctuation conductivity is observed for samples of both types.

In-plane paraconductivity of optimally doped and slightly overdoped cuprates: implication and origin of the pseudogap

In conventional superconductors the magnitude of the pairing fluctuation is primarily determined by Tc and the superconducting coherence length ξ. In low-dimensional systems with strong structural and electronic anisotropies, the interlayer separation s plays a significant role. In cuprates, the pseudogap (PG) correlation induces a downturn in the temperature-dependent resistivity. As Tc is approached from above, this downturn in the resistivity is supposed to either (i) add independently to, or (ii) join smoothly to that due to paraconductivity caused by short-lived Cooper pairs. It is important to differentiate between these two possibilities, since they are closely linked to the nature of the PG. It would be reasonable to assume that if the first scenario is correct, then the PG has a non-superconducting origin, while the second scenario, if found to hold, would relate precursor pairing to the PG correlations. In this paper we have studied the in-plane fluctuation conductivity of two c-axis-oriented thin films of Y0.95Ca0.05Ba2Cu3O7−δ with similar hole contents (p), p = 0.165 (optimally doped (OPD)) and p = 0.184 (slightly overdoped (SOD)). The hole contents are fixed at these values so that the PG affects the slope of the resistivity data only at temperatures close to Tc. Analysis of paraconductivity Δσab(T) within the mean-field Gaussian Ginzburg–Landau (MFGGL) framework reveals different qualitative and quantitative features for the OPD and the SOD compounds. The excess conductivity due to Cooper pair fluctuations of the SOD sample can be described reasonably well by the MFGGL formalism. The excess conductivity of the OPD compound, on the other hand, cannot be accounted for by the MFGGL formalism with a reasonable set of parameters. There is a significant added contribution to Δσab(T) for the OPD sample that appears to come from the presence of a PG in the quasiparticle (QP) spectral density. These findings point towards a non-pairing origin of the PG.

Non-ohmic Electrical Transport Properties Above the Critical Temperature in Optimally and Underdoped Superconducting YBa2Cu3O6+x

The resistivity and the Hall effect as a function of current density and temperature in the normal state of very thin films of YBa2Cu3O6+x are measured with a fast pulsed-current technique. The in-plane longitudinal and transverse (Hall) conductivities are reduced in intense current densities up to several $\mathrm{MA\,cm}^{-2}$ . In optimally-doped samples, this non-ohmic effect is limited to temperatures below 100 K. In a moderately underdoped sample, however, the non-linearity extends to the temperature range from T c =53 K to ∼150 K. The non-linear part of the Hall conductivity does not scale with a critical exponent, thus excluding classical amplitude fluctuations of the superconducting order parameter as possible origin.

75As Nuclear Magnetic Resonance Studies on Ba(Fe1−xNix)2As2 Single Crystals under High Pressure

75As nuclear magnetic resonance (NMR) was measured for Ba(Fe1−xNix)2As2 single crystals with x = 0.05 and x = 0.1 under 0 and 1.5 GPa, respectively. For the optimal doped sample with x = 0.05, the superconducting transition temperature, Tc, is strongly suppressed from 18 to 5 K, while for the over-doped sample with x = 0.1, it is turned from a superconducting ground state to a disordered paramagnetic state under 1.5 GPa. Our experimental results show that the antiferromagnetic spin fluctuations, as well as Tc, are suppressed. The experimental results can be explained with the two-band model. As a result, the electronic band is shifted downwards with an increase in pressure, and the electrons become the dominant carriers in the system.

Evidence for line nodes in the energy gap of the overdoped Ba(Fe1−xCox)2As2from low-temperature specific heat measurements

Low-temperature specific heat (SH) is measured on Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in a wide doping region under different magnetic fields. For the overdoped sample, we find the clear evidence for the presence of $T^2$ term in the data, which is absent both for the underdoped and optimal doped samples, suggesting the presence of line nodes in the energy gap of the overdoped samples. Moreover, the field induced electron specific heat coefficient $\Delta\gamma(H)$ increases more quickly with the field for the overdoped sample than the underdoped and optimal doped ones, giving another support to our arguments. Our results suggest that the superconducting gap(s) in the present system may have different structures strongly depending on the doping regions.

Structural and superconducting properties of LaFeAs1−x Sb x O1−y F y

We report the antimony (Sb) doping effect in a prototype system of iron-based superconductors LaFeAsO1−y F y (y=0, 0.1, 0.15). X-ray powder diffraction indicates that the lattice parameters increase with Sb content within the doping limit. Rietveld structural refinements show that, with the partial substitution of Sb for As, the thickness of the Fe2As2 layers increases significantly, whereas that of the La2O2 layers shrinks simultaneously. So a negative chemical pressure is indeed “applied” to the superconducting-active Fe2As2 layers, in contrast to the effect of positive chemical pressure by the phosphorus doping. Electrical resistance and magnetic susceptibility measurements indicate that, while the Sb doping hardly influences the SDW anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of y=0.1. In the meantime, the superconducting transition temperature can be raised up to 30 K in LaFeAs1−x Sb x O1−y F y with x=0.1 and y=0.15. The Sb doping effects are discussed in term of both J 1–J 2 model and Fermi Surface (FS) nesting scenario.

Phase-sensitive harmonic measurements of microwave nonlinearities in cuprate thin films

Investigations of the intrinsic electromagnetic nonlinearity of superconductors give insight into the fundamental physics of these materials. Phase-sensitive third-order harmonic voltage data $\tilde{u}_{3f}=|u_{3f}|exp(i\phi_{3f})$ are acquired with a near-field microwave microscope on homogeneous YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) thin films in a temperature range close to the critical temperature T$_c$. As temperature is increased from below T$_c$, the harmonic magnitude exhibits a maximum, while the phase, $\pi/2$ in the superconducting state, goes through a minimum. It is found that samples with doping ranges from near optimal ($\delta=0.16$) to underdoped ($\delta=0.47$) exhibit different behavior in terms of both the harmonic magnitude and phase. In optimally-doped samples, the harmonic magnitude reaches its maximum at a temperature $T_M$ slightly lower than that associated with the minimum of phase $T_m$ and drops into the noisefloor as soon as $T_m$ is exceeded. In underdoped samples $T_M$ is shifted toward lower temperatures with respect to $T_m$ and the harmonic voltage magnitude decreases slower with temperature than in the case of optimally-doped samples. A field-based analytical model of $\tilde{u}_{3f}$ is presented, where the nonlinear behavior is introduced as corrections to the low-field, linear-response complex conductivity. The model reproduces the low-temperature regime where the $\sigma_2$ nonlinearity dominates, in agreement with published theoretical and experimental results. Additionally the model identifies $T_m$ as the temperature where the order parameter relaxation time becomes comparable to the microwave probing period and reproduces semi-quantitatively the experimental data.

Superconductivity induced by Ni doping inSmFe1−xNixAsO

Superconductivity with a Tc of about 10 K is observed in the Ni-dopedSmFe1−xNixAsO system. The measurements of resistivity and magnetic susceptibility show that the spin-density wave(SDW) order is quickly suppressed with increasing Ni content, and superconductivity emerges asx≥0.04.Tcmid shows a maximum of 10.8 Kat x = 0.06, and it drops to lowerthan 2 K as x>0.12. Meanwhile,the upper critical field (Hc2(0)) is estimated to be about 40 T for the optimally-doped sample (x = 0.06). The normal state thermopower is negative for all the Ni-doped samples,indicating that an electron-type charge carrier dominates in the transportproperties. Moreover, the magnitude of the room-temperature thermopowerincreases with increasing Ni content, and then shows a broad peak aroundx = 0.06.We found that there is an obvious correlation between the anomalously enhanced thermopower andsuperconductivity. A phase diagram is derived based on the transport measurements and a dome-likeTc(x) curve is established.

Effect of ${\hbox{Eu}}^{+3}$ on Flux Pinning Characteristics of Grain Aligned High-${T}_{c}$ Superconductor $\hbox{Sm}_{1-{\rm x}}\hbox{Eu}_{\rm x}\hbox{Ba}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$

Flux pinning characteristics of grain aligned high- Tc superconductor Sm1-xEuxBa2Cu3O7-s (where x=0.0, 0.3, 0.5, 0.7, 0.9) were studied by magnetization data. Samples were prepared by solid state reaction and aligned under high magnetic field of 6 T. Results show that the transition temperature Tc and critical current density Jc was found to enhanced in optimal doped samples (x = 0.5-0.7). Peak positions of pinning force density Fp,max were shifted towards higher fields in optimal doped samples. Furthermore, the normalized pinning force density showed peaks at lower reduced fields. Results are interpreted on the basis of presence of ionic radius difference between Sm+3 and Eu+3 and secondary phases which act effectively at high temperature and low fields.

Low-energy spectroscopic mapping studies in optimally-doped Ca 2− xNa xCuO 2Cl 2

We performed high-resolution scanning tunneling microscopy/spectroscopy on an optimally-doped Ca 2− x Na x CuO 2Cl 2 crystal with T c ∼ 25 K. The so-called “checkerboard” local-density-of-state modulation previously found in heavily underdoped regime also manifests in the spectroscopic map of the optimally-doped sample. In addition, spatially-inhomogeneous energy gap with peaks at the gap edges is observed below about 10 meV. The gap tends to be buried at elevated temperatures and correlates with the checkerboard modulation. These results suggest that the gap is related to superconductivity which coexists with the checkerboard modulation.

Photo-Deformation Effect in HTSC Copper Oxides

HTSC copper oxides demonstrate phase separation with typical size of coherent areas from few to hundreds unit cells. Radiation leads to photo-doping of these materials, the domain structure and doping state of the main fracture change at the same time. As charge carrier concentration in CuO2 planes controls their degree of distortion (“buckling”), so radiation causes photo-deformation effect in considered materials. Photo-deformation effect has been investigated for Bi2Sr2Ca2Cu3O x with different oxygen content. It has been shown that radiation of both under-doped and optimally-doped samples leads to the increase of material fraction with optimally doped CuO2-planes due to charge exchange with reservoir block BiO x .

The Influence of Oxygen Partial Pressure on Growth of the (Hg,Re)-1223 Intergrain Junction

Hg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> Re <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+delta</sub> polycrystalline samples were successfully obtained by using different oxygen partial pressure in the annealing treatment of the precursor ceramic. The doping state was confirmed by X-ray powder diffraction pattern analysis and by observing distinct thermopower values at room temperature. Also, the intergrain regions have shown an improvement in the critical current density when using the precursor preparation with 10% O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and 90% Ar (optimal doped). The optimal doped sample has presented the highest alpha exponent of the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> prop[1-(T/T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> ] <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">alpha</sup> dependence. For the case of (Hg,Re)-1223 polycrystalline superconductor applications, the alpha exponent can be used as a junction quality parameter

Distortion of ReO6 octahedron in the Hg0.82Re0.18Ba2Ca2Cu3O8+d superconductor

Rhenium (Re) LIII edge X-ray absorption spectroscopy was used in order to determine rhenium valence and the local oxygen coordination in Hg0.82Re0.18Ba2Ca2Cu3O8+d polycrystalline samples prepared with three different oxygen content. Thermoelectric power measurements confirmed small increments of charge carrier number as a function of oxygen content. The X-ray absorption near-edge spectroscopy (XANES) analysis showed a valence variation for Re (+6.8, +6.9 and +7.0) in these samples. The extended X-ray absorption fine structure (EXAFS) analysis indicated that the oxygen local order around Re atoms in these Hg0.82Re0.18Ba2Ca2Cu3O8+d samples can be described as a distorted ReO6 octahedron with two different Re–O bound lengths. The valence evaluated by the bound length in the distorted ReO6 octahedron is in agreement with the XANES measurements. Moreover, the distorted ReO6 octahedron and the Cu–Opl angle built a scenario which can justify the high intrinsic term value found in the optimal doped sample under external hydrostatic pressure.

Resonant inelastic x-ray scattering study of overdopedLa2−xSrxCuO4

Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge has been performed for heavily overdoped samples of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x= 0.25$ and 0.30. We have observed the charge transfer and molecular-orbital excitations which exhibit resonances at incident energies of $E_i= 8.992$ and 8.998 keV, respectively. From a comparison with previous results on undoped and optimally-doped samples, we determine that the charge-transfer excitation energy increases monotonically as doping increases. In addition, the $E_i$-dependences of the RIXS spectral weight and absorption spectrum exhibit no clear peak at $E_i = 8.998$ keV in contrast to results in the underdoped samples. The low-energy ($\leq 3$ eV) continuum excitation intensity has been studied utilizing the high energy resolution of 0.13 eV (FWHM). A comparison of the RIXS profiles at $(\pi ~0)$ and $(\pi ~\pi)$ indicates that the continuum intensity exists even at $(\pi ~\pi)$ in the overdoped samples, whereas it has been reported only at $(0 ~0)$ and $(\pi ~0)$ for the $x=0.17$ sample. Furthermore, we also found an additional excitation on top of the continuum intensity at the $(\pi ~\pi)$ and $(\pi ~0)$ positions.