Temperature-dependent integral exciton absorption in semiconducting GaAs crystals

This Letter is devoted to an experimental investigation of the temperature dependence of the fundamental absorption edge of thin GaAs layers grown by molecular beam epitaxy. Critical temperature T _cr = 135 K, above which the integral absorption becomes constant, corresponding values of critical damping parameter ?? _c = 0.248 meV, and longitudinal-transverse splitting kh??_LT = 0.082 meV are determined experimentally. The temperature dependence of the true dissipative damping is separated.     

Possible Multigap Superconductivity in SmFeAsO0.8F0.2: A Point-contact Andreev-reflection Spectroscopy Study

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO_0.8F_0.2 state-of-the-art polycrystals, with critical temperature T _c≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB₂ and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to T _c, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ₁=6.15±0.45 meV and Δ₂=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same T _c. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.     

Point-Contact Spectroscopy of the Borocarbide Superconductor YNi2B2C in the Normal and Superconducting State

Point-contact (PC) spectroscopy measurements of YNi₂B₂C single crystals in the normal and superconducting (SC) state (T _c ≃ 15.4 K) for the main crystallographic directions are reported. The PC study reveals the electron–phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No “soft” modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in Δ_{[100] ≈ 1.5 meV for the a direction and Δ_{[001] ≈ 2.3 meV along the c axis; however the critical temperature T _c in PC in all cases is near to that in the bulk sample. The value 2Δ_[001]/k _B T _c ≈ 3.6 is close to the BCS value of 3.52, and the temperature dependence Δ_[001](T) is BCS-like, while the for small gap Δ_[100](T) is below BCS behavior at T > T _c /2 similarly as in the two-gap superconductor MgB₂. It is supposed that the directional variation Δ can be attributed to a multiband nature of the SC state in YNi₂B₂C.     

Activation Energy of the Photo Induced Q2 Oxygen Ordered Phase in the La2CuO4.08 Superconductor

The interstitial oxygen ordering in a La₂CuO_{4+ y} (y = 0.08) single crystal with a T _c = 38 K has been investigated. We have studied the growth dynamics of the three dimensional (3D) ordered phase, called Q2, at different temperatures under X-ray illumination. The critical temperature T ₀∼334 K of the order-disorder transition for the Q2 phase has been determined. We have found that the illumination of the sample with a continuous polarized synchrotron radiation X-ray flux stimulates the oxygen ordering kinetics. The photoinduced ordering process shows a threshold characteristic of cooperative phenomena and its kinetics shows a temperature dependence that is controlled by the activation energy E _A = 100 meV.     

Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl₂CaBa₂Cu₂O₈ (Tl-2122,T _c =107 K) and YBa₂Cu₃O₇ (Y-123,T _c =92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.     

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273–343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using α = α_max + (α_min − α_max)/[1 + exp 2(E − E_g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E_g(T) = E_g(273 K) − (8.8 × 10⁻⁴T²)/(483 + T) + 0.088 (Varshni empirical formula) and E_g(T) = E_g(273 K)−0.231447/[exp(362/T)−1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor ∼0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.     

Increase in the exactness of measurement of physical parameters of high-temperature superconductor materials

A procedure for estimating the critical temperature T _c and width ΔT _c for the superconducting transition of high-temperature superconductor materials based on plotting the first derivative of the temperature correlation dependence of the electrical resistance of a sample is developed. The critical point of the derivative corresponds to T _c , and its width at half-height corresponds to ΔT _c . The procedure provides a decrease in the relative error of estimation of T _c and ΔT _c for qualitative samples by a factor of 1.7–2.     

Polarization Effect in ESR of Co Doped CuGeO<Subscript>3</Subscript>

Electron spin resonance of a single crystal of CuGeO₃ doped with 2% of Co has been studied at f = 99 GHz in temperature range 1.8–50 K. Contributions to ESR absorption from Cu²⁺ chains and from Co²⁺ ions were derived. It is found that functions obtained for ESR integrated intensities: Curie-Weiss for Cu²⁺ (χCu ∼ C _Cu/(T + Θ), with Θ = 92 K) and Curie for Co²⁺ (χCo ∼ C _Co/T) are well consistent with temperature dependence of static magnetic susceptibility. Strong dependence of ESR absorption on polarization of oscillating magnetic field was discovered for Co²⁺ contribution. Polarization effect was studied for magnetic field applied along a, b and c directions. Values of g-factors of resonance lines are presented.     

Photoinduced Quasiparticle Relaxation Dynamics in Near-optimally Doped SmFeAsO0.8F0.2 Single Crystals

We investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor with T _c=49.5 K by means of femtosecond spectroscopy. Multiple relaxation processes are evident, with distinct superconducting state dynamics and a clear “pseudogap”-like feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude Δ _PG=61±9 meV above and below T _c. The fluence and temperature dependence of the superconducting state dynamics shows similar behaviour to the cuprate superconductors with the superconducting-condensate vaporization energy of E _va/k _B≈1.5 K per Fe ion.     

Short-term synthesis of poly- and single-crystalline iron monochalcogenides

Poly- and single-crystalline samples of the Fe(Se,Te)_{1 − δ} system have been synthesized for short times by annealing bulk iron in chalcogen vapors and by reacting powdered iron with chalcogen melts. The lattice unit cell parameters and critical temperatures (T _c) of the superconducting transition in the synthesized compounds agree with published data for the materials obtained by long-term synthesis using other methods. For Fe(Se_0.8Te_0.2)_0.82 block crystals, the temperature dependence of the second critical field in the vicinity of T _c is linear with a slope of −4.9 T/K.     

Temperature Dependence of the Upper Critical Field in bcc Solid 3He

The temperature dependence of the upper critical field for the antiferromagnetic high-field phase of bcc solid ³He has been calculated by Iwahashi and Masuda utilizing the Green-function method with Tyablikov decoupling. In the low temperature limit the upper critical field, H _c2(T), is found to decrease from H _c2(0) with increasing temperature as a power law with exponent 3/2. Interestingly, the same power law dependence has been predicted for a system of dilute magnons undergoing Bose–Einstein condensation and has been observed in the spin-gap antiferromagnets TlCuCl₃, BaCuSi₂O₆, and NiCl₂–4SC(NH₂)₂. An experiment has been attempted to determine precisely the temperature dependence of the upper critical field in bcc solid ³He by measuring the pressure of the solid during adiabatic demagnetization through the critical field. The theoretical framework motivating the experimental study is presented along with details of the experimental setup.     

Double-dimensional crossover in layered superconductor

The critical magnetic fieldsH _c2 of superconducting layered structures V/Cu were investigated. The double-dimensional crossover 3D-2D-3D was observed on the temperature (H′ _c2(θ)) dependences of critical magnetic fields. The field crossover 3D-2D is caused by strong temperature dependence of superconducting coherence length ξ _s nearT _c . The second crossover 2D-3D is provided by temperature dependence of normal metal coherence length ξ _N and reflects the 3D isotropization of layered structure V/Cu at low temperature.     

Extraction of Critical Current Density and Flux Creep Exponent in the Magnetic Superconductor Ru-1212 Using the ac Magnetic Susceptibility Measurements

The ac susceptibility data was employed to extract the temperature dependence of the critical current density, J _c(T), as well as the variation of flux-creep exponent n(T,H _ac) with temperature and ac field amplitude in bulk samples of polycrystalline magnetic superconductor RuSr₂GdCu₂O₈ (Ru-1212). The critical state models and the collective flux-creep approximation model were successfully accounted to describe such behavior below the transition temperature. The calculated values of n(T,H) are well fitted to a power law of the following form: n(T,H)=n ₀(H)T ^−s(H), where s is field dependent exponent whose values varied from −2.4, −1.01 for field amplitudes ranging from 0.5 G and 3.8 G. The power law describing the frequency dependence of χ′ is found to be consistent with the results of the current-dependent effective activation energy of the form U(J)=U ₀ln (J _c/J). Additionally, the dependence of the current density is found to scale according as: J _c(T)=J _c0(1−T/T _c) ⁿ , where the exponent n values varied from 1.05 to 1.25. Such dependence is an indication of intergrain coupling that could be ascribed in terms of superconductor–insulator–superconductor junctions. The derived temperature dependence of J _c(T) is in good agreement with the data obtained from the measurements using the traditional “loss-maximum” approach. Furthermore, the flux-creep effect increased with increasing both ac fields and temperatures except at about 15–25 K below the onset of T _c, where a slowing down of the flux creep was observed.     

Revealing the Anisotropy in MgB<Subscript>2</Subscript> Superconductor and Investigation of Critical Current near Critical Temperature by Using Hall Probe Susceptibility Technique

We report the results of an investigation on critical current measurements in bulk MgB₂ superconductor by using Hall probe ac susceptibility. The temperature versus ac susceptibility has been measured as a function of external ac magnetic field with no dc part and magnitude in the range 240–1200 A/m by using the Hall probe method. The real part of the Hall probe susceptibility showed two-step transitions near critical temperature. We attributed this behavior to the anisotropic nature of the MgB₂ compound. Close to the critical temperature the estimated anisotropy parameter is γ≈2.5, a value that is in fair agreement to the ones reported in the literature for polycrystalline and single crystals MgB₂. We have also analyzed the susceptibility data within the framework of a critical state model (using the real part of the susceptibility) and an anisotropic nature of MgB₂ compound. The inter-grain critical current is found to be 4×10⁶ A/m² at 39.59 K by using the critical state model. The temperature dependence of the critical current density was found to be in the form of J _c (T)∝(1−T/T _c ) ^β , where β was calculated as 2.30 by using critical state model, respectively. Scanning electron microscopy was used for grain size estimation.     

Thermodynamic Properties of Methanol in the Critical and Supercritical Regions

The isochoric heat capacity of pure methanol in the temperature range from 482 to 533 K, at near-critical densities between 274.87 and 331.59 kg· m⁻³, has been measured by using a high-temperature and high-pressure nearly constant volume adiabatic calorimeter. The measurements were performed in the single- and two-phase regions including along the coexistence curve. Uncertainties of the isochoric heat capacity measurements are estimated to be within 2%. The single- and two-phase isochoric heat capacities, temperatures, and densities at saturation were extracted from experimental data for each measured isochore. The critical temperature (T_c = 512.78±0.02K) and the critical density (ρ_c = 277.49±2 kg · m⁻³) for pure methanol were derived from the isochoric heat-capacity measurements by using the well-established method of quasi-static thermograms. The results of the C_VVT measurements together with recent new experimental PVT data for pure methanol were used to develop a thermodynamically self-consistent Helmholtz free-energy parametric crossover model, CREOS97-04. The accuracy of the crossover model was confirmed by a comprehensive comparison with available experimental data for pure methanol and values calculated with various multiparameter equations of state and correlations. In the critical and supercritical regions at 0.98T_c≤ T ≤ 1.5T_c and in the density range 0.35ρ_c ≤ ρ leq 1.65 ρ_c, CREOS97-04 represents all available experimental thermodynamic data for pure methanol to within their experimental uncertainties.     

Optimum Synthesis Temperature of (Cu1?xTlx)Ba2Ca3Cu4O12?δ Superconductor

The effect of synthesis temperature on the superconducting properties of (Cu_1−x Tl _x )Ba₂Ca₃Cu₄O_12−δ (CuTl-1234) samples has been explored. Almost all the superconducting parameters studied in this research work are observed to be suppressed with the increase of synthesis temperature beyond 880 °C, which may be due to impurities caused by the volatility of some constituents such as thallium and oxygen deficiencies as well in the final compound. The Fluctuation Induced Conductivity (FIC) analysis has shown a decrease in the cross-over temperature (T ₀) and the shift of three-dimensional (3D) Aslamasov–Larkin (AL) regions to the lower temperature with the increase of synthesis temperature beyond 880 °C. A direct correlation between the cross-over temperature (T ₀), the zero temperature coherence length {ξ _c (0)}, the zero resistivity critical temperature {T _c (R=0)} as well as carrier concentration has also been observed.     

Comparison of Superconductivity Parameters of (Cu1?x Tl x )Ba2Ca4Cu5O14?δ Superconductor Synthesized at Different Temperatures

The superconductivity parameters of (Cu_1−x Tl _x )Ba₂Ca₄Cu₅O_14−δ (CuTl-1245) samples synthesized at different temperatures has been compared. Almost all the superconductivity parameters studied in this research work are suppressed with the increase of synthesis temperature beyond 890 ^○C, which may be due to defects induced by the volatility of some constituents such as thallium, oxygen deficiencies, and multiphase in the final compound. The fluctuation induced conductivity (FIC) analysis has shown a decrease in the cross-over temperature (T _o) as well as the mean-field critical temperature (T_c^mf)T_{\mathrm{c}}^{\mathrm{mf}}) and the shift of three- dimensional (3D) Aslamasov–Larkin (AL) regions to the lower temperature with the increase of synthesis temperature beyond 890 ^○C. A direct correlation between (T _o), the zero temperature coherence length {ξ _c (0)}, the interlayer coupling strength (J), mean-field critical temperature (T_c^mf)T_{\mathrm{c}}^{\mathrm{mf}}) as well as carrier concentration has also been observed.     

Analysis of observations on solid-solution hardening in KBr–KCl single crystals

Available data on the temperature and concentration dependences of the critical resolved shear stress (CRSS) of KBr–KCl solid-solution crystals containing 8–41 mol% KCl in the temperature range 77–230 K have been analysed within the frame work of the kink-pair nucleation model of plastic flow in solid-solution crystals. It is found that CRSS τ decreases with the increase in temperature T in accord with the model relation ln τ = A−BT, where A and B are positive constants. The CRSS τ at a given temperature depends on solute concentration c as τ ∝ c ^p , where exponent p has a value between 0.5 and 1 depending on the temperature at which deformation is carried out. The model parameter W _o, i.e. binding energy between the edge-dislocation segment involved in the unit activation process and the solute atoms close to it (T→0 K), which is inversely proportional to B, increases with solute concentration c monotonically in accord with the model prediction W _o ∝ c ^1/4 upto a critical value c _m = 35 mol% KCl. However, W _o decreases with increase in c beyond c _m , which indicates somewhat ordered distribution of solute in the host lattice of concentrated solid-solutions with c > c _m .     

Flux dynamics of YBCO/Ag composite

DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andH _max, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJ _cgm on low-field M-H loop. The estimation of intergranular critical current densityJ _cjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJ _c(T) with silver content under low AC fields only, consistent with the transportJ _c data; beyond thatJ _c(T) decreases. This improvement inJ _c(T) and transportJ _c with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (H _max>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionH _c2j and lower critical field of intragrain regionH _c1g also decrease with silver content.