Ion-channeling studies of lattice anomalies well aboveT c in YBa2CU3O7−y

Backscattering yields in the <001> axial channeling mode using D⁺ ions have been measured for YBa₂Cu₃O_7–y single crystals with T_c=59K. 68K and 91K at temperatures between 40K and 295K. In 60 K-class YBa₂Cu₃O_7–y with significant anomalies associated with the spin gap, it is found that the channeling anomalies are observed at 130–140 K(T_pa) in addition to anomalies at T_c. The channeling anomalies at T_c follow to the shift of T_c, suggesting the phonon anomalies induced by the superconducting-gap opening. On the other hand, T_pa is found to be almost unchanged for varying T_c, although T_pa appears near the temperature where the spin gap is opened. This leads a question whether the channeling anomalies at T_pa is directly related to the phonon anomalies induced by the spin-gap opening. The existence of lattice instability is suggested as one of possible explanations for the anomalies at T_pa.     

Investigations on Tl-2223 thin films fabricated through ultrasonic spray pyrolysis under oxygen deficient conditions

In this paper we have shown that polycrystalline films corresponding to Tl-2223 phase can be grown by employing high thalliation temperatures and short thalliation times. Ultrasonically deposited precursor films corresponding to Ba₂Ca_2.2Cu_3.3O_x(Ag_y) have been thalliated under high vacuum (∼ 10^-5 torr) at 890°C to obtain single phase Tl-2223 films. An off-stoichiometric and unreacted pellet of composition Tl_2.05Ba₂Ca₂Cu₃O_z has been used as source of Tl. We have shown that oxygen ambient is not necessary for the growth of Tl-2223 phase. The as-thalliated films have T_c’s in the range 123 K ±0.70 K. TheT _c has been found to be independent of the addition of AgNO₃ to the precursor. The zero field transportJ _c has been observed to be > 1.2 X 10⁵ A/cm² at 77 K. NearT _c (110 K-122 K),J _c has been observed to follow the power lawJ _c ∞ (1-T/T _c )_p,p 2. A power law withp tt 1.4 has been observed for the temperature range 70 K-110 K. An optimum doping of Ag has been observed to induce about 25% increase inJ _c and it also leads to uniform and enlarged grain growth. The surface morphology of Ag free samples contains plate like grains having arbitrary shapes. In contrast to this 0.35 Ag doped sample exhibits nearly rectangular plate like grains     

Phonon anomalies in ZrRuP?

The specific heat of a new orthorhombic structure modification of ZrRuP has been measured. The superconducting transition temperatureT _c of this new phase, 3.76 K, is concluded to be much lower than that of the hexagonal modification (T _c =13.0 K) due to the 30% stiffer lattice of the orthorhombic phase as determined by the Debye temperature. The high- and low-T _c phases of ZrRuP are found to have nearly identical densities of states at the Fermi energy.     

Effect of negative pressure on superconducting transition temperature of MgB2

Atomistic simulation has been performed with interatomic potentials to investigate the effect of negative hydrostatic pressure on the superconducting transition temperature (T_c) of MgB₂. The calculation reveals that T_c can be greatly enhanced by applying negative pressure and it can reach up to 52.2 K at ?13 GPa, about 13 K higher than that at ambient pressure. The mechanism for the enhancement of T_c is attributed to that negative pressure reduces high-frequency phonon vibrations of B and thus dramatically enhances the electron–phonon coupling. Our result seems to open up a possible way for the enhancement of T_c in MgB₂.     

Influence de divers types de substitution sur les températures de transition des oxyfluorures ferroélectriques de type chiolite

A great number of oxyfluorides with a chiolite related structure have been prepared. They show all a ferroelastic, ferroelectric-prototype, paraelectric transition. The highest Curie temperature (T_c = 800 K) has been obtained for the tungsten compound Na₅W₃O₉F₅. The value of T_c is found to vary as a function of the substitutions from Na₅W₃O₉F₅. The largest decrease is observed for the composition Na₄Lu (WNb₂)O₉F₅ with T_c = 88K. Pyroelectric and piezoelectric applications are expected. The influence of the composition on other transitions observed in these materials has been determined.     

Investigation of the properties of superconducting niobium nitride films

Niobium nitride films with a value for T_c of up to 17.3 K have been prepared by reactive magnetron and diode sputtering in Ar and N₂ gas mixture. Alteration of T_c and N₂ ion implantation into NbN films was studied. It is shown that small doses of implanted ions cause an abrupt decrease of T_c to 12.8 K. Annealing at 900°C restores the high T_c By the tunnel effect, the magnitude of the energy gap is 3.05 Me V at 1.56 K for NbN with T_c = 17.1 K and $\text{22/kT}{}_{\mathrm{c}}\text{= 4.14}$. The existence of an undersurface layer has been discovered, whose width is of the order of the coherence length, with a lower T_c, of 12.0–12.8 K. The superconductivity of this layer above T_c is due to the proximity effect. NbN tunnel junctions are made with a density of the Josephson current i=(1.7-2.87) · 10³ A cm^?2.     

Etude optique et couplage ferroélectrique-ferroélastique de cristaux de composition Li0, 02Na0, 98NbO3

Crystal of composition Li_0.02Na_0.98NbO₃ is ferroelectric at room temperature (space group Pc2m, T_c = 640 K). Two kinds of domain walls parallel to (101) or (141) planes have been observed at 300 K. The birefringence n_a-n_c and the domain pattern have been investigated from 300 to 950 K. Four transitions have been detected at T₂ = 540 K, T₃ = T_c = 640 K, T₄ = 825 K and T₅ = T′_c = 940 K. The transition at T′_c is of ferroelastic-prototype type. The switching of the polarization associated with the disappearing of the domain walls is in good agreement with a ferroelectric-ferroelastic coupling.     

Research and Prospects of Iron‐Based Superconductors

The discovery of a new superconductor, LaFeAsO_1?xF_x with a superconducting critical temperatureT,_c, of 26 K in 2008, has quickly renewed interest in the exploration of iron‐based superconductors. More than 70 new superconductors have been discovered within several months, with the highest T_c of up to 55 K being observed in the SmFeAsO_1?x compound. High T_cs have previously only been observed in cuprates; these new iron‐based superconductors have been added as second members of the high‐T_c family. The crystal structure of these compounds contains an almost 2D Fe–As layer formed by FeAs₄ tetrahedrons, which can be separated by an oxide or metal layer that provides extra electrons to the Fe–As layer, and the itinerant iron 3d electrons form an antiferromagnetic (AFM) order state in the undoped parent compounds at around 100–200 K. Superconductivity can be induced by carrier doping, which destroys the AFM ground state. In this Review, the most recent findings on and basic experimental facts about this class of high‐T_c materials will be presented, including the various superconducting structures, the synthesis methods, the physical properties of the parent compounds, the doping methods that could produce superconductivity, pressure effects, and the prospects for this new iron‐based high‐T_c family.     

H/F‐Substitution‐Induced Homochirality for Designing High‐Tc Molecular Perovskite Ferroelectrics

A ferroelectric with a high phase‐transition temperature (T_c) is an indispensable condition for practical applications. Over the past decades, both strain engineering and the isotope effect have been found to effectively improve the T_c within ferroelectric material systems. However, the former strategy seems to prefer working in inorganic ferroelectric thin films, while the latter is also limited to some certain systems, such as hydrogen‐bonded ferroelectrics. It is noted that a mono‐fluorinated molecule is geometrically very similar to its parent molecule and the substitution of H by an F atom can introduce a chiral center on the molecule to template or stabilize polar structures. Significantly, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in T_c. Herein, by applying the molecular design strategy of H/F substitution to the organic–inorganic perovskite ferroelectric (pyrrolidinium)CdCl₃ with a low T_c of 240 K, two high‐T_c chiral perovskite ferroelectrics, (R)‐ and (S)‐3‐F‐(pyrrolidinium)CdCl₃ are successfully synthesized, for which the T_c reaches 303 K. The significant enhancement of 63 K in T_c extends the ferroelectric working temperature range to room temperature. This finding provides a new effective way to regulate the T_c in ferroelectrics and to design high‐T_c molecular ferroelectrics.     

Preparation and degradation behaviour of Bi-cuprate superconductors

The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theT _c,J _c and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi₂O₃ after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highT _c phase (2223) exhibiting a maximum transition temperatureT _c=110 K and ΔT _c=2 K. The addition of lead and grain orientation together result in an increase inJ _c by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inT _c,J _c and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withT _c=97 K even after 250 days. The highT _c phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.     

Certification of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Single Crystals

A method for evaluation of the critical temperature T _c and the width of the superconducting transition ΔT _c in HTSC single crystals has been developed. By this method, the first derivative of the temperature dependence of the resistivity, \(\frac{\partial \rho (T)}{\partial T}\), is constructed. A technique for synthesis of YBa₂Cu₃O_7?x single crystals with highly reproducible physical-mechanical properties has been described. A standard sample with T _c=94 K and ΔT _c=0.25 K has been synthesized and certified.     

Thermopower of ion-irradiated YBa2Cu3O7−δ-films

Experimental data of the temperature dependence of the thermoelectric power S(T) for differently doped HTSC-samples recently found renewed interest in the context of the van Hove scenario as well as the spin gap behavior. Furthermore, a universal relation between S(290K) and the hole concentration p has been reported. On the other hand, ion irradiation of HTSC- films with ions of medium energy (typically 350 ke V) is known to produce defects leading to a strong increase in normal-state resistivity and a depression of the superconducting transition temperature T_c. The idea of the present work is to combine both techniques to test whether the ion-induced defects are predominantly oxygen vacancies within the CuO-chains, reducing p as monitored by S(290K). For this purpose, c-axis oriented YBa₂Cu₃O₇-films on (100)-SrTiO₃ substrates were irradiated with 350 ke V He⁺-ions. After each irradiation step an XRD analysis and an S(T)-measurement for T_c < T < 295 K is performed. It is shown that for small fluences the ioninduced S(T)-changes can be attributed to oxygen displacements destroying the CuO-chains, while for higher fluences additional defects within the CuO₂-planes have to be taken into account. The corresponding S(T) behavior is comparable to what has been reported for Zn-substituted underdoped HTSC-samples.     

Preparation and analysis of high-T c Nb-Ge films

The dependence of T_c on sputtering conditions, chemical composition, and phase structure for Nb-Ge films has been studied. It was found that T_c varied with composition, but was not dependent on exact stoichiometry of the film; the Nb/Ge ratio was <3 for very high T_c 23 K films and 4 for films with T_c 20 K. X-ray results showed that the films with T_c 23 K contained a certain amount of tetragonal Nb₅Ge₃ with a lattice parameter of A15 phase 5.14 Å. The depth profile of very high T_c films showed no increase of oxygen concentration at the film-substrate interface; no correlation of high T_c and impurities was found.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedT _c in calcium-free Tl compounds of the series Tl _m Ba₂Ca _n−1Cu _n O _x (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestT _c(onset) ranges from 103 K to ∼ 115 K andT _c (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentT _cs.     

Crystal Chemical Estimation of Possible Exceeding of a Tc of 77 K in Diborides

We establish the empirical correlation for diborides (AB₂) between T _c and crystal chemical parameters of a simpler structural fragment—sandwich A₂(B₂) where the superconductivity appears—like that found in high-T _c cuprate superconductors. Our results suggest that in the absence of vacacies in B₂ plane, T _c of diborides can be higher, that is, 77 K, the evaporation temperature of liquid nitrogen. We discuss critical crystal chemistry parameters controlling T _c and ways to achieve higher transition temperatures in diborides, based on this correlation.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1−x Nb x )Sr2(GdCe1.8Sr0.2)Cu2O z System

The Ru-1232 compounds have been synthesized in the (Ru_1?xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Peculiarities of phase transitions in polytypes of monoclinic TlInS<Subscript>2</Subscript>

The existence of two polytypes at room temperatures, C-TlInS₂ and 2C-TlInS₂, with different monoclinic cell parameters, c and 2c, has been revealed. Significant differences in crystal lattice dynamics of these polytypes have been found. In particular, two phase transitions (PTs) have been detected for the polytype C-TlInS₂ as the temperature varies: a second-order PT from paraphase to incommensurate phase at T _i = 215 K and a first-order ferroelectric PT accompanied by a quadrupling of the parameter c at T _c = 197 K. No PT accompanied by an increase in unit cell parameter c has been found in the polytype 2C-TlInS₂, but a global temperature hysteresis characteristic of crystals with an incommensurately modulated structure has been detected at T = 180–230 K.     

Sample Preparation and Specific Heat Measurements of Hg x Ba2CuO4+δ with Different Transition Temperatures

Various ceramic samples of Hg _x Ba₂CuO_4+δ have been synthesised by the quartz ampoule method with previously prepared Ba₂CuO_3+y as a precursor. By tuning the mercury content x < 1 and controlling the synthesis of Ba₂CuO_3+y , we have prepared samples which show sharp superconducting transitions with different T _c-values for different samples. The temperature of the onset of diamagnetism varies between 80 K and 96 K. For two samples, each with T _c = 95 K, the specific heat c _p(T) has been measured between 60 and 110 K with a continuous heating method. An O₂-annealed sample A reveals a broad magnetic superconducting transition, whereas the transition of the as-synthesised sample B is narrow. The specific heat anomaly of the superconducting transition can clearly be identified for sample B and is broadened in applied magnetic fields. It cannot be resolved in the O₂-annealed sample A, most likely because of a broad distribution of T _c-values as indicated by the broad transition.     

Isomorphs of the superconducting/magnetic ternary stannides

Single crystals of some new ternary stannides in the systems (M)CoSn, (M)RuSn and (M)OsSn, where M = rare earth, Y. Sc, Ca, Sr, or Th have been grown from a tin solvent in vacuo. We find the new compounds to be isomorphous to the published rare earth rhodium tin compounds. The ternary, TmOs_xSn_y, has been found to be a reentrant superconductor with T_c = 1.1K and T_m = 0.6K. Superconducting and/or magnetic transition temperatures and lattice parameters are given. A tabular summary of identified phases and their lattice constants for all the ternary stannides prepared so far is included for comparative purposes.