high-Tc Phase Research Articles

Investigation of morphotropic phase boundaries in PIN–PSN–PT relaxor ferroelectric ternary systems with high Tr-t and Tc phase transition temperatures

New morphotropic phase boundary (MPB) compositions with relatively high Tcs were projected in Pb(In1/2Nb1/2)O3–Pb(Sc1/2Nb1/2)O3–PbTiO3 (PIN–PSN–PT) solid solution based on the perovskite tolerance factor, and were experimentally confirmed. The phase structure, dielectric, pyroelectric, piezoelectric and ferroelectric properties of PIN–PSN–PT ceramics were investigated. The rhombohedral–tetragonal phase transition temperatures Tr-t on the order of 189–210°C, Curie temperatures Tc on the order of 274–285°C and piezoelectric coefficients d33 in the range of 310–360pC/N, were achieved in xPIN– (1−x)PSN–0.37PT (x=0.15–0.23) ceramics, demonstrating a promising relaxor–PbTiO3 system with high phase transition temperatures. In addition, the maps of Tc, Tr-t, d33 and εr with respect to composition were established, based on which, a clear direction for composition screening in future crystal growth of the PIN–PSN–PT system was given.

Synthesis and characterization of PbO–CdO nanocomposite and its effect on (Bi,Pb)-2223 superconductor

A PbO–CdO nanocomposite-added Bi1.6Pb0.4Sr2Ca2Cu3O10 ((Bi,Pb)-2223) superconductor has been prepared. The effect of the PbO–CdO nanocomposite addition on the transport critical current density (Jc) of (Bi,Pb)-2223 superconductor was investigated. The transition temperature (Tc−onset), zero electrical resistance temperature (Tc−R=0), and Jc of the samples were measured by the four-probe method. Phase formation, structure, and microstructure of samples were investigated. The distribution of nanoparticle size was determined. The results indicated that the PbO–CdO-added samples showed larger grain size and an increased volume fraction of high-Tc phase (Bi-2223) compared to the non-added sample. A slight increase in Tc−R=0 of x = 0.15 wt% was observed. Jc of the PbO–CdO nanocomposite-added samples was significantly higher than for the non-added sample. That could be explained by the possibility that the PbO–CdO nanocomposite acts as an effective flux pinning center in (Bi,Pb)-2223. At 77 K, Jc of x = 0.15 wt% added sample was more than 20 times larger than Jc of the non-added sample (x = 0 wt%). A combined effect of enhanced flux pinning, increased fraction of high-Tc phase and improved grain size, which led to increase in the Jc of added samples, is discussed.

Effect of re-pelletization on structural, mechanical and superconducting properties of BSCCO superconductors

We have examined the effect of re-pelletization on structural, mechanical and superconducting properties of high temperature superconducting (HTS) bulk samples using Bi1.85Pb0.35Sr2Ca2Cu3Oy stoichiometry. Conventional solid state reaction method is used for preparation of three different bulk samples. XRD measurement of the samples shows that a high percentage of high-Tc phase is determined. Superconducting critical transition temperature of the samples have been obtained from the curves of ρ-T and χ-T. Vickers micro-hardness are also calculated for all samples. Re-pelletization has a positive effect on structural, mechanical and superconducting properties of bulk samples.

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe.

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (Tc) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of Tc has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ∼15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ∼6 GPa the sudden enhancement of superconductivity (Tc≤38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-Tc phase above 6 GPa. The obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-Tc cuprates.

Pressure and high-Tc superconductivity in sulfur hydrides.

The paper discusses fundamentals of record-TC superconductivity discovered under high pressure in sulfur hydride. The rapid increase of TC with pressure in the vicinity of Pcr ≈ 123GPa is interpreted as the fingerprint of a first-order structural transition. Based on the cubic symmetry of the high-TC phase, it is argued that the lower-TC phase has a different periodicity, possibly related to an instability with a commensurate structural vector. In addition to the acoustic branches, the phonon spectrum of H3S contains hydrogen modes with much higher frequencies. Because of the complex spectrum, usual methods of calculating TC are here inapplicable. A modified approach is formulated and shown to provide realistic values for TC and to determine the relative contributions of optical and acoustic branches. The isotope effect (change of TC upon Deuterium for Hydrogen substitution) originates from high frequency phonons and differs in the two phases. The decrease of TC following its maximum in the high-TC phase is a sign of intermixing with pairing at hole-like pockets which arise in the energy spectrum of the cubic phase at the structural transition. On-pockets pairing leads to the appearance of a second gap and is remarkable for its non-adiabatic regime: hydrogen mode frequencies are comparable to the Fermi energy.

Heat-Treatment-Induced Switching of Magnetic States in the Doped Polar Semiconductor Ge1-xMnxTe.

Cross-control of a material property - manipulation of a physical quantity (e.g., magnetisation) by a nonconjugate field (e.g., electrical field) – is a challenge in fundamental science and also important for technological device applications. It has been demonstrated that magnetic properties can be controlled by electrical and optical stimuli in various magnets. Here we find that heat-treatment allows the control over two competing magnetic phases in the Mn-doped polar semiconductor GeTe. The onset temperatures Tc of ferromagnetism vary at low Mn concentrations by a factor of five to six with a maximum Tc ≈ 180 K, depending on the selected phase. Analyses in terms of synchrotron x-ray diffraction and energy dispersive x-ray spectroscopy indicate a possible segregation of the Mn ions, which is responsible for the high-Tc phase. More importantly, we demonstrate that the two states can be switched back and forth repeatedly from either phase by changing the heat-treatment of a sample, thereby confirming magnetic phase-change-memory functionality.

Role of preparation conditions of Bi-2223 ceramic materials and optimization of Bi-2223 phase in bulk materials with experimental and statistical approaches

This study deals with the determination of optimum preparation conditions (press load, annealing temperature and time) for the Bi-2223 superconducting compound with the aid of both the experimental methods regarding dc resistivity, transport critical current density and powder X-ray diffraction measurements and statistical approaches including response surface explorer based on Box–Behnken designs for the first time. It is found that the Bi-2223 polycrystalline compound prepared under 300 MPa pressure load at 840 °C for 48 h presents the highest formation of Bi-2223 phase due to the considerable elimination of the impurity scattering and lattice strain in the crystal structure. On the other hand, the sample prepared at 860 °C under 350 MPa for 48 h exhibits the worst characteristic features of high Tc-phase as a consequence of the enhancement in local structural distortions, dislocations, defects and disorders in the Cu–O2 consecutively stacked layers. Thus, the latter condition causes to both the degradation in amplitude of pair wave function (decrease in the overlapping of Cu 3d and O 2p functions) and metastability due to the reduction of hole trap energy. In this respect, the best material displays the largest Tconset of 110.63 K and Tcoffset of 108.46 K values while the worst material obtains the smallest values (Tconset of 95.45 K and Tcoffset of 45.32 K). Similarly, the maximum Jc value of 818 A/cm2 is experimentally observed for the best compound whereas the worst sample obtains the smallest value of 112 A/cm2. The decrement in the Jc parameter is attributed to the regression of intergrain coupling and flux pinning vortices in the Bi-2223 crystal structure due to the increment of misorientations and especially grain boundary weak-interactions in the crystal system. The XRD results also reveal that the combination of 300 MPa press load, 840 °C annealing temperature and 48 h annealing time for the preparation condition promotes seriously the high Tc-phase as a consequence of the enhancement in the average crystallite size and lattice parameter c or decrement in the a-axis length. Accordingly, the best material obtains the largest c-axis length of about 37.22 Å and average grain size of 71.4 nm but the smallest lattice parameter a of about 5.29 Å. At the same time, the surface response designs of statistical analyses show that the optimum preparation conditions are defined to be 306.5657 MPa press load, 840 °C annealing temperature and 48 h annealing time to maximize the high Tc-phase (with the maximum Tconset,TcoffsetandJc parameters.

Superconducting H5S2 phase in sulfur-hydrogen system under high-pressure.

Recently, hydrogen sulfide was experimentally found to show the high superconducting critical temperature (Tc) under high-pressure. The superconducting Tc shows 30–70 K in pressure range of 100–170 GPa (low-Tc phase) and increases to 203 K, which sets a record for the highest Tc in all materials, for the samples annealed by heating it to room temperature at pressures above 150 GPa (high-Tc phase). Here we present a solid H5S2 phase predicted as the low-Tc phase by the application of the genetic algorithm technique for crystal structure searching and first-principles calculations to sulfur-hydrogen system under high-pressure. The H5S2 phase is thermodynamically stabilized at 110 GPa, in which asymmetric hydrogen bonds are formed between H2S and H3S molecules. Calculated Tc values show 50–70 K in pressure range of 100–150 GPa within the harmonic approximation, which can reproduce the experimentally observed low-Tc phase. These findings give a new aspect of the excellent superconductivity in compressed sulfur-hydrogen system.

Evolution of High-Temperature Superconductivity from a Low-T_{c} Phase Tuned by Carrier Concentration in FeSe Thin Flakes.

We report the evolution of superconductivity in an FeSe thin flake with systematically regulated carrier concentrations by the liquid-gating technique. With electron doping tuned by the gate voltage, high-temperature superconductivity with an onset at 48K can be achieved in an FeSe thin flake with T_{c} less than 10K. This is the first time such high temperature superconductivity in FeSe is achieved without either an epitaxial interface or external pressure, and it definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with T_{c}^{onset} as high as 48K in bulk FeSe. Intriguingly, our data also indicate that the superconductivity is suddenly changed from a low-T_{c} phase to a high-T_{c} phase with a Lifshitz transition at a certain carrier concentration. These results help to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on the further pursuit of a higher T_{c} in these materials.

Search for spontaneous magnetization in noncentrosymmetric superconductors

We have measured the magnetization of two noncentrosymmetric superconductors: Ir2Ga9 and CePt3Si using a SQUID magnetometer, neither of which has been tested for the time-reversal symmetry in the superconducting state. For Ir2Ga9, the magnetization change ΔM below the superconducting transition temperature Tc ∼ 2.2 K was smaller than 10-5 G in zero magnetic field, suggesting that ΔM originates in the Meissner effect induced by the possible residual field. When CePt3Si was cooled in zero magnetic field, ΔM along the c axis first decreased below Tc+ ∼ 0.75 K (high-Tc phase) and then increased below Tc– ∼ 0.45 K (bulk superconducting phase). Since this behavior cannot be explained by the Meissner effect, it may be a sign of a spontaneous magnetization.

Observation of a Pressure-Induced Phase Transition for Single Crystalline LaO0.5F0.5BiSeS Using a Diamond Anvil Cell

A pressure-induced phase transition in LaO0.5F0.5BiSeS is demonstrated by performing resistance measurements with a diamond anvil cell. The superconducting transition temperature (Tc) of LaO0.5F0.5BiSeS changes from a low-Tc phase (4.9 K) to a high-Tc phase (7 K) owing to the application of pressure up to 5.1 GPa.

Emergence of Multiple Superconducting Phases in (NH3)yMxFeSe (M: Na and Li).

We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH3)yNaxFeSe. To clarify the origin of the multiple superconducting phases, the variation of Tc was fully investigated as a function of x in (NH3)yNaxFeSe. The 32 K superconducting phase is mainly produced in the low-x region below 0.4, while only a single phase is observed at x = 1.1, with Tc = 45 K, showing that the Tc depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH3)yNaxFeSe does not change as x increases up to 1.1, i.e., the space group of I4/mmm. The lattice constants, a and c, of the low-Tc phase (Tc = 32.5 K) are 3.9120(9) and 14.145(8) Å, respectively, while a = 3.8266(7) Å and c = 17.565(9) Å for the high-Tc phase (~46 K). The c increases in the high Tc phase, implying that the Tc is directly related to c. In (NH3)yLixFeSe material, the Tc varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of Tc is different from that of (NH3)yNaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.

The Influence of Sintering Time in High and Low Density Bi-2223 Superconductor

The effects of sintering time in high and low density Bi-2223 phase formation have been investigated. The samples were prepared by the solid-state reaction method at various sintering times ranging from 24, 48, 72 and 96 hours. Sucrose was added during palletization and after heated at 400°C for two hours the sucrose was removed and hence low density sample was created. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and critical current density. The transition temperature varies between 102 K and 96 K with increasing of sintering times. The optimal sintering time of the samples Bi-2223 system was found at 850°C for 72 hours. The critical current density, JC of high density and low density Bi-2223 was measured to be 7.547 A/cm2 and 8.333 A/cm2 respectively at 77 K under zero magnetic field. The critical current density, JC and superconductivity transition temperature, TC of low density were found to be higher than the pure samples. The critical transition temperature increased with a short gap between TConset and TC zero. The most intense peak in the XRD pattern of sample at sintering time 72 hours belong to the high-TC phase which also indicates an increase in the volume fraction of the high-TC phase with optimum sintering time.

Electrical, Structural and Morphological Properties of Sb-Doped Bi-Based Superconductors

In this paper, samples of antimony doped Bi-based superconductor with stoichiometric composition Bi1.7Pb0.2Sb0.1Sr2Ca2Ca3O10 were prepared by a solid state reaction method. The effect of sintering time on the superconducting properties was studied; all samples were sintered in air at 850°C for different sintering time (80, 100, 120, 140, and 160) h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements were performed for determination of the crystal structure and surface morphology of samples, respectively. All samples showed an orthorhombic structure with two phases, high-Tc phase (2223) and low-Tc phase (2212) in addition to an impure phase. It has been observed that the critical temperature and the high-Tc phase increases and appears to be the dominant phase when the sintering time is increased to 140 h, while with increasing sintering time to 160 h, both Tc and the high phase started to decrease. (SEM) results show that increasing sintering time enhances the growing of superconducting phase unidirectional and suppresses the high phase intrusion which leads to the production of nearly single Bi-2223 phase with higher Tc.

A prominent dielectric material with extremely high-temperature and reversible phase transition in the high thermally stable perovskite-like architecture

A perovskite-like dielectric material (C3N2H5)·[Mn(HCOO)3] (1), exhibiting remarkably highTc(phase transition temperature) and pronounced dielectric anomaly, can be considered as a model of novel extremely high-temperature dielectric materials.

Investigating the Preparation Conditions on Superconducting Properties of Bi<SUB>2-x</SUB> Li<SUB>x</SUB>Pb<SUB>0.3</SUB>Sr<SUB>2</SUB>Ca<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>10+δ</SUB>

One and multi-step solid state reaction methods were used to prepare a high temperature superconductor with a nominal compositionBi2-x LixPb0.3Sr2Ca2Cu3O10+δ for ( 0 ≤ x ≤ 0 .5). T he effect o f preparation conditions and substituting Li on Bi sites had been investigated by the use of X-ray diffraction, resistance measurements and oxygen content to obtain the optimum conditions for formation and stabilization of the 2223-phase. It has been found that intermediate grinding will force to convert and accelerate the formation rate of the 2223-phase. The morphological analyses were carried out by SEM. The results showed that the multi-step technique was appropriate to prepare the compositio Bi2-x LixPb0.3Sr2Ca2Cu3O10+δ . X-ray diffraction analysis showed two phases: high-TC phase 2223 and low-TC phase 2212 with orthorhombic structure for all samples. However, the optimum concentration was found for 0.3 which improved the microstructure and had the highest TC value 130 K for the highest value of oxygen content.

Pressure-induced phase transition for single-crystalline LaO0.5F0.5BiSe2

We have demonstrated a pressure-induced phase transition from a low-Tc phase to a high-Tc phase in a single crystal of the superconductor LaO0.5F0.5BiSe2. The high-Tc phase appears at 2.16 GPa and the maximum superconducting transition temperature is observed at 6.7 K under 2.44 GPa. Although the anisotropy for the low-Tc phase is estimated to be 20, it is reduced by around half (9.3) in the high-Tc phase. This tendency is the same for the BiS2 system. The Tc of LaO0.5F0.5BiSe2 has continued to increase up to the maximum pressure of this study (2.44 GPa). Therefore, applying further pressure has the potential to induce a much higher Tc in this system.

Phase diagram and weak-link behavior in Nd-doped CaFe2As2

The transport properties, phase diagram, and dopant distribution are investigated in systematically Nd-doped CaFe2As2 single crystals. Coexistence of two superconducting (SC) phases with different critical transition temperature (Tc) is observed. The low-Tc phase emerges as x ≥ 0.031, and the Tc value increases to its maximum value of about 20 K at x = 0.083, the maximum doping level in our study. As x ≥ 0.060, the high-Tc phase with a Tc value of about 40 K is observed. The structure transition (STr) from tetragonal to orthorhombic phase vanishes suddenly around x = 0.060, where a new STr from tetragonal to collapsed tetragonal phase begins to turn up. Compared to the low-Tc phase, the end point of SC transition of the high-Tc phase is more sensitive to the magnetic field, showing a characteristic of Josephson weak-link behavior. Possible scenarios about this system are discussed based on our observations. We also find that the non-uniform SC properties cannot be attributed to the heterogeneous Nd distribution on the micro scale, as revealed by the detailed energy dispersive x-ray spectroscopy measurements.

BISCCO Thin Films: Superconducting Phases, Irreversibility Line and Critical Current

The transport properties such as irreversibility line (IL), critical current density (Jc), and activation energy (Ea) were studied in highly oriented BISCCO thin films with different superconducting phase relations H-2223:L-2212 by ZFC-FC method in the low magnetic field region (less than 0.1 T). The c-axis oriented BISCCO thin films with different superconducting phases H-2223:L-2212, high Tc phase (110 K), and low Tc phase (80 K), respectively, as determined by x-ray diffraction (XRD), were produced by r.f. magnetron sputtering on MgO (100) substrates and different post-annealing duration times. Tirr(H) data appear to be dominated by the intergrain Josephson flux. They exhibit the behavior of the grain coupling systems (similar to that displayed by spin glasses) and can be well described by a power law behavior like Almeida-Thoules. The irreversibility line (IL), however, was shifted to lower temperatures by decreasing the phase relation. The IL shifting to lower temperatures increases the liquid vortex region and would decrease the critical current density. The critical current density (Jc) and the activation energy (Ea) were determined using the 10 μVolt criterion and from Arrhenius plots of resistance data, respectively. Both decreased by decreasing the phase relation, i.e., by increasing the low Tc phase, which can be related to an increase in the granularity of films. The film granularity has been assumed as any crystal lattice defect, which creates a local depression of the superconducting order parameter.

High-Tc Phase of PrO0.5F0.5BiS2 single crystal induced by uniaxial pressure

We demonstrated a pressure-induced phase transition from a low-Tc phase (3.5 K) to a high-Tc phase (8.7 K) in single crystalline PrO0.5F0.5BiS2. The high-Tc phase is easily observed even at 0.7 GPa by uniaxial pressure obtained from a geometric effect of a single crystal with an extremely thin thickness. For this phase transition, superconducting anisotropy and the coherence length along the c axis change from γ = 20 to 9.3 and from ξc = 0.56 to 0.71 nm, respectively. It is suggested that a shrinkage in the c lattice constant by applied uniaxial pressure enhances the three dimensionality in the superconducting state.