Finite size effects in layered superconductors

We investigate the transition temperature of layered superconductors by considering a stack ofL _z superconducting layers, separated by insulating material. We adopt a pairing Hamiltonian, invoke the variational principle and solve the resulting gap equations numerically. Our results confirm previous weak coupling and Ginzburg-Landau treatments and reveal a rise ofT _c withL _z and saturation at the bulk transition temperature. Thus, the rise ofT _c is traced back to a finite size effect, corresponding to a crossover from 2-d to 3-d superconductivity. The results also reveal a sizeable variation of the gap along the stack with pronounced variation at the ends.     

电子型掺杂高温超导体La2-xCexCuO4飞秒时间分辨动力学研究

利用飞秒时间分辨光抽运探测技术研究了电子型掺杂La_2-xCe_xCuO₄(LCCO)高温超导材料的准粒子超快动力学过程.得到低温(T<0.7T_c)、转变温度附近(0.7T_c≤T≤T_c)和高温(T>T_c)三个温区内的动力学行为.研究发 关键词： 电子型掺杂高温超导体 飞秒时间分辨 准粒子 声子瓶颈     

Electroluminescent characteristics of InGaAsSb/GaAlAsSb heterostructure Mid-IR LEDs at high temperatures

The electroluminescent characteristics of an InGaAsSb/GaAlAsSb heterostructure LED emitting at 1.85 μm are studied in the temperature range 20–200°C. It is shown that the emission power exponentially drops as P ≅ 0.4exp(2.05 × 10³/T) with a rise in temperature primarily because of an increase in the Auger recombination rate. It is found that band-to-band radiative recombination goes in parallel with recombination through acceptor levels, the latter causing the emission spectrum to broaden. With a rise in temperature, the activation energy of the acceptor levels decreases by the law ΔE≅ 32.9 − 0.075T and the maximum of the LED’s emission spectrum shifts toward the long-wavelength range (hν _max = 0.693 − 4.497 × 10⁻⁴ T). Based on the dependence E _g = hν _max − 0.5kT and experimental data, an expression is derived for the temperature variation of the bandgap in the In_0.055Ga_0.945AsSb active area, E _g ≅ 0.817 − 4.951 × 10⁻⁴ T, in the range 290 K < T < 495 K. The resistance of the heterostructure decreases exponentially with rising temperature as R ₀ ≅ 5.52 × 10⁻²exp(0.672/2kT), while cutoff voltage U _cut characterizing the barrier height of a p−n junction decreases linearly with increasing temperature (U _cut = −1.59T + 534). It is found that the current through the heterostructure is due to the generation-recombination mechanism throughout the temperature interval.     

Temperature‐dependent Raman study of PrFeO3thin film

Low‐temperature Raman study of (001)‐oriented PrFeO₃ thin film of around 200 nm thickness deposited on a LaAlO₃ (001) substrate by using the pulsed‐laser deposition technique is presented. X‐ray diffraction analysis of this film shows an orthorhombic structure with Pbnm space group. The observed substrate‐induced strain is found to be small. In the room temperature Raman spectra, different Raman modes were observed that were classified according to the orthorhombic structure. All the observed modes show a decrease in wavenumber with rise in temperature, except the B_1g mode (624 cm⁻¹) which shows some anomalous behavior. We tried to correlate the variations in linewidth and position with temperature for the observed modes with the octahedral disorder of FeO₆. Many possibilities are presented to explain the observed results. Copyright © 2010 John Wiley & Sons, Ltd.     

Temperature Effects on the Performance of On-Chip Spiral Inductors Used in RF(MM)ICs

Based on the measured S-parameters and proposed circuit model for on-chip spiral inductors, the overall effects of temperature rise on the inductor performance are examined in this paper. For circular spiral inductors on silicon substrates, it is shown that when the temperature increases from 25°C to 85°C, the peak values of Q-factor (Q _max) of these inductors, corresponding to the turn number N = 3 to 7, decrease about 8.8% to 19%, respectively. The metal trace and silicon substrate resistances both increase linearly with temperature, while the capacitance of silicon substrate has a negative temperature coefficient. For a square spiral inductor on GaAs substrate, its Q _max decreases about 37.2% as temperature increases from 25°C to 185°C. The corresponding frequency f _max tends to shift from 9.44GHz to 7.73GHz, and it is reduced about 18.1%.     

Effects of low-level La doping and ageing on TlPb-1223 high Tc superconductors

Bulk samples of the nominal composition of (Tl_0.6Pb_0.5)(Sr_0.8Ba_0.2)₂Ca₂Cu₃O_8+δ−xLaO_1.5 (x = 0–0.1) were prepared by using two-step process and their microstructure, T _c values, and magnetization were studied. The samples consist of the Tl-1223 dominant phase with small Tl-1212 admixture, which increases with a rise of La content. Five years ageing and following oxygen annealing at 450°C and subsequently at 750°C have only a modest effect on T _c values of the studied samples. Low-level La doping (x = 0:04) leads to an increase of T _c values by about 2 K in comparison with undoped samples. Oxygen annealing at 750°C results in an increase in the volume magnetization hysteresis in low applied magnetic fields and rise of critical current density at zero magnetic field and 77 K. This effect is most pronounced for the low La doped sample with x = 0.04. Changes of the induced voltage, U originating in the Meissner effect and of its harmonics in dependence on temperature were measured and used for characterization of the temperature distribution of inter-grain junctions.     

Magnetocaloric effect in antiferromagnetic Dy3Co compound

Magnetic properties and magnetocaloric effects (MCEs) of the Dy₃Co compound are studied. Two successive magnetic transitions: the antiferromagnetic (AFM)-to-AFM transition at T _AF =29 K and the AFM-to-paramagnetic (PM) transition with increasing temperature at the Néel temperature T _N =44 K are observed. Dy₃Co undergoes a field-induced metamagnetic transition from the AFM to the ferromagnetic (FM) state below T _N , giving rise to a large MCE. The maximal value of magnetic entropy change ΔS _m is −13.9 J/kg K with a refrigerant capacity (RC) of 498 J/kg around T _N for a field change of 0–5 T. A sign change of MCE in Dy₃Co with magnetic field and temperature is observed near the critical field where the metamagnetic transition occurs.     

Gradient-stress on polarization in Ba1-xSrxTiO3 films

A group of position-thickness-dependent stresses are used to modified Landau-Devonshire theory to investigate the second-order phase transition in Ba_1−x Sr _x TiO₃ films. The result shows that the short-range interaction between the unit cells of the film and the substrate induces the phase transition dispersion and the rise of the transition temperature in the films. The dependence of the effective dielectric constant on the temperature and the average spontaneous polarization on the film thickness are computed, which qualitatively agree with the experiments.      

Electric field gradients of111Cd in the hexagonal (A-phase) La2O3 and Nd2O3 sesquioxides

Perturbed angular correlation measurements of¹¹¹In in hexagonal La₂O₃ and Nd₂O₃ yield well defined symmetric electric field gradients with coupling constantv _Q =280 MHz (extrapolated) in La₂O₃ andv _Q =269 MHz for Nd₂O₃ at room temperature. The coupling constant in La₂O₃ linearly decreases with temperature. The spectra of La₂O₃, strongly damped at RT, rise to full amplitude at 870 K indicating dynamical interactions.     

Appearance of a Hα Minimumat the Onset of Net Recombination in Hydrogen Plasmas

Hydrogen plasmas out of ionization equilibrium are either ionizing or recombining depending on the electron temperature T_e . Within the transition region between these two opposite states a minimum of the H_α emission is often experimentally observed. Simple cases were previously analyzed which could be interpreted assuming only a temperature variation, i.e the electron density was constant in the transition region. Here we discuss two examples in which both the density and the temperature vary at the transition. In the linear plasma generator PSI‐II a hydrogen plasma is cooled down by puffing additional gas. We find a minimum at T_min ≈ 1.1 eV. A second example is the effect of an ELM(edge localized mode) pulse propagating through a recombining divertor plasma in the tokamak ASDEX Upgrade. The H_α response shows a double peak which can be interpreted as a local minimum assuming a simultaneous rise of density and temperature during an ELM. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aggregate color center formation processes in lithium fluoride crystals after irradiation

Lithium fluoride crystals were irradiated by different doses of gamma photons at a temperature of 77 K. We measured the aggregation kinetics for the color centers with different annealing temperatures above the temperature of anion vacancy mobility. We show that the lifetimes of the vacancies decrease while the lifetimes of the F₂⁺ F_2^{+} centers increase as the irradiation dose increases. We explain these types of dependences based on the aggregation processes for color centers in the post-radiation period. We determine the time constants and energies (analogous to activation energies in the Arrhenius equation) for the various processes involving rise and fall in the concentration of aggregate color centers. Based on the experimental data obtained, we have established the processes forming F ₂ and F₃⁺ F_3^{+} centers in the post-radiation period. The F ₂ centers are formed when vacancies ν_a add to F₁^- F_1^{-} centers. Vacancies arising during irradiation of the crystal participate in their creation in the first fast stage. In the long final stage, vacancies are used which appear in the post-radiation period on occurrence of the reaction F₂⁺ F_2^{+} + H → ν_a + fluoride ion at the lattice site, where H is an interstitial fluorine atom. The F₃⁺ F_3^{+} centers are formed both by merging F₂⁺ F_2^{+} and F ₁ centers and as a result of addition of vacancies to F ₂ centers. In this case, vacancies are used that are generated not only during irradiation of the crystal but also in the post-radiation period. The rise in the concentration of F₃⁺ F_3^{+} centers occurs faster than the rise in the concentration of F ₂ centers.     

镁含量和热处理对Zn1-xMgxO薄膜结构和发光性能的影响

采用溶胶-凝胶工艺在玻璃衬底上制备了Zn_1-xMg_xO(x=0.1,0.2,0.3, 0.4,0.5,0.6,0.7)薄膜.X射线衍射谱(XRD)测试结果发现,在 0.1<x<0.3 范围内,薄膜仍然保持氧化锌六角纤锌矿结构,(002)面衍射峰位向大角度方向移动,超过0.3时出现氧化镁立方相.对镁含量为0.1,0.2,0.3薄膜的光致发光谱研究表明：紫外发光峰随镁含量的增加向短波方向移动.对于Zn_0.9Mg_0.1O薄膜,在5,5.5和6℃/min的升温速率下,升温速率越快结晶程度越好.在相同升温速率下,随着退火温度从500 ℃升高到560 ℃,样品的结晶程度变好,当退火温度达到590 ℃时,结晶质量下降. 关键词： 氧化锌 结构 禁带宽度 光致发光谱     

Magnetic properties of magnetite thin films close to the Verwey transition

Temperature dependence of the magnetic properties of magnetite thin film across the Verwey transition has been investigated. As the temperature is decreased, the magnetization of the film in a fixed field showed a sharp decrease close to the Verwey transition temperature (T_v). The M–H loops of the film have been recorded at various temperatures below and above T_v. It is found that film does not saturate at any temperature and saturation becomes more difficult below T_v. While cooling through T_v, the extrapolated value of magnetization to infinite field (Q), calculated from the numerical fit 4πM=Q [1−(H*/H)^1/2], does not show a drop, but the coefficient indicating difficulty in saturation (H*) shows a sharp rise as does the coercivity.     

Influence of the soft phonon mode on charge carrier hopping transfer in bismuth-containing layered ferroelectric ceramic oxides

Formulas and calculations explaining the experimental data on maxima frequency displacement for imaginary dielectric components in a low frequency range with a rise in temperature within the limits of the ferroelectric phase were obtained for the ceramic structure SrBi₂(W _x/2Ti _x/2Nb_{1 − x} )₂O₉. At the heart of our consideration lies our assumption of the interaction of jumping charge carriers with a soft phonon mode.     

ESR of Gd-impurities in the heavy fermion compounds CeCu2Si2 and CeAl3

The heavy fermion systems CeCu₂Si₂ and CeAl₃ are characterized by a huge quasiparticle density of states responsible for the large electronic specific heat. The observation of a Gd³⁺ electron spin resonance (ESR) in single crystals CeCu₂Si₂ and in polycrystalline CeAl₃ clearly demonstrates the local character of these quasiparticles. Nevertheless, the Gd-spin relaxation shows remarkable anomalies with respect to the isostructural compounds LaCu₂Si₂ and LaAl₃: Probably via RKKY coupling, Ce 4f-spin fluctuations give rise to an enhanced Gd-spin relaxation resulting in an unusual non-linear thermal broadening around the Kondo temperature. From this we obtain information about the temperature dependence of the Ce 4f-spin correlation time.     

Microstructural evolution of pure copper during friction-stir welding

The microstructural evolution of pure copper during friction-stir welding was found to be principally influenced by welding temperature. At temperatures below ~0.5 T_m (where T_m is melting point), the microstructure was shown to be essentially determined by continuous recrystallization, leading to significant grain refinement and related material strengthening in the stir zone. In contrast, grain structure development at temperatures above ~0.5 T_m was dominated by discontinuous recrystallization producing a relatively coarse grain structure in the stir zone and giving rise to material softening.     

Effect of mechanical deformation on the critical current in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">b</Emphasis></Subscript> superconducting films

It is shown that local mechanical bending of YBa₂Cu₃O_7−b Ag-coated superconducting films deposited on flexible metal substrates in the temperature interval 77–300 K may increase the critical density J _c of the transport current to values as high as 10⁶ A/cm² or even higher at 77 K. Also, bending decreases the voltage criterion. This means a rise in the intergranular conductivity and, accordingly, a reduction of Joule losses.     

The thermal properties of a single-heterostructure laser diode supplied with short current pulses

The analysis of the heat spreading in the single-heterostructure GaAs-Ga₁-x Al _x As laser diode supplied with short current pulses (in the case, however, when the adiabatic approximation is no longer valid) at room temperature is presented in this paper. Relations are derived, describing the time-dependent temperature rise within the volume of the laser diode. The calculations are carried out for a typical SH laser diode. It turns out that in the duration of the short current pulses (t _I=200 ns,j=1.5 × 10⁴A cm^–2) the increase in junction temperature of the typical SH laser diode amounts to about 6.1 K. This increase leads to an increase of about 9% in the threshold current, to a decrease of about 18% in the laser radiation intensity, and to a shift of the spontaneous radiation band and of the stimulated radiation modes of about 1.9 nm and 0.22 nm, respectively, during each current pulse.     

Magnetic and structural properties of Pd–Mn–Sn intermetallics compounds

Magnetic and structural properties of Heusler Pd_0.5Mn_{0.5- x} Sn _x with x = 0.05, 0.10, 0.17, 0.20 and 0.25, have been studied by magnetisation and X-ray diffraction measurements at room and low temperatures. The crystal structure at room temperature is L2₁ cubic phase for x = 0.17, 0.20, 0.25 and B2 cubic phase for x = 0.10. Martensite structure 10M, was observed at room temperature for x = 0.05. X-ray measurements at low temperatures revealed a structural transformation from B2 to 14M for the x = 0.10 case. The lattice parameter of the L2₁ phase decreases linearly with the concentration, x. A ferromagnetic behaviour has been detected for L2₁ compounds, but the ferromagnetic exchange characteristic of each composition is of different strength. This gives rise to different Curie temperatures.     

Screened potential of a test charge in a two-dimensional Bose condensate

We calculate the screening properties of a boson superlattice with distanced between the planes at temperature zero. The screened Coulomb potentialV _s (r,d) of a test charge located in one plane is calculated in the neighboring plane. It is found that this interplane potential is attractive with a maximal atraction at the intraplane distancer _m:V _s (r _m ,d)<0.V _s (r _m ,d) versusd shows a minimum atd _m , indicating a maximal attraction for anoptimal superlattice period d_m. The attraction could give rise topairing of bosons between different planes.