The combined effect of temperature of operation and external optical feedback on the turn-on time delay of semiconductor laser diodes

In this paper, the combined effect of temperature of operation (T) and external optical feedback (EOF) on the turn-on time delay (t_on) of semiconductor laser (SLD) is investigated theoretically. General and exact analytical model of t_on in term nonradiative (A), radiative (B) and Auger (C) recombination coefficients, carrier density, injection current (I_inj) and T is derived for the first time. In addition, EOF parameters, i.e. external reflectivity (r_ext) and phase of reflected light (?) is also included. The combined effect of T and EOF on t_on is calculated according to its effect on threshold carrier density (N_th). We show that when SLD operates under the effect of coherent (incoherent) EOF, t_on decreases (increases) as r_ext increases due to decreasing (increasing) of N_th at coherent (incoherent) EOF. Moreover, t_on increases as T increases due to increasing of N_th. Moreover, we identified in operable regions of SLD where the latter fails to operate due to very long t_on.     

Temperature dependence of turn-on time delay of semiconductor laser diode: Theoretical analysis

Temperature dependence of the turn-on time delay (t_on) of uncooled semiconductor laser diodes biased below and above threshold is analyzed in presence of data pattern effect. We show that even when the laser is biased at or slightly above threshold, the increase in temperature of operation will lead to increase in the threshold carrier (N_th) and consequently the laser diode will be biased below the threshold again and a significant value of t_on will be produced. Thus, knowledge about a value of dc-bias current required to achieve zero t_on within wide range of temperature degrees is important when considering uncooled laser diode in high-speed optical communication systems. The temperature dependence of t_on is calculated according to the temperature dependence of N_th and Auger recombination coefficient (C) and not by the well-know exponentional relationship of threshold current with temperature. The temperature dependence of N_th is calculated according to the temperature dependence of laser cavity parameters. Advanced analytical model is derived in term of carrier density, recombination coefficients and the injection current (I_inj). The validity of proposed model is confirmed by a numerical method. In addition, approximated models are included where under specified assumptions the proposed model reduces to the well-known approximate models of t_on. According to our typical values and at a specified value of modulation current, the dc-bias one (I_ib) should be increased from I_ib = I_th to I_ib ≈ 1.25 and 1.5I_th in order to achieve approximately zero t_on when the temperature increases from 25°C to 55°C and 85°C, respectively.     

Exciton polariton in an organic–inorganic multiple-quantum-well crystal (C4H9NH3)2PbBr4

Exciton polariton in an organic–inorganic multiple-quantum-well (MQW) single crystal (C₄H₉NH₃)₂PbBr₄ at low temperature has been investigated by photoluminescence excitation (PLE), reflection, and time-resolved photoluminescence measurements. Since (C₄H₉NH₃)₂PbBr₄ has ideal two-dimensional excitons with an extremely large oscillator strength and forms self-organized MQW with a very short well-period (d～10 Å), polaritonic coupling among excitons is strong and extends over a large number of wells. Therefore, observed MQW polariton features were the same as those of bulk polaritons. We have also investigated relaxation dynamics of the MQW polariton in the same framework as discussions on bulk polaritons.     

Study of temperature dependence of density, bulk modulus, shear modulus and thermal pressure for mantle minerals

The temperature dependence of different parameters i.e. density ρ(T), bulk modulus K_T(T), shear modulus G_T(T) and thermal pressure ΔP_th for mantle minerals i.e. X₂SiO₄ (X=Mg, Fe, Co, Mn) have been studied in high temperature range on the basis of semi-phenomenological isobaric equation of state. The calculated values of these parameters are showing good agreements with experimental value in case of each mantle minerals.     

Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La_0.67Ba_0.33MnO₃ bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (T_IM) and the change in MR around the ferromagnetic transition temperature (T_C). The decrease in MR around T_C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T_IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.     

Wetting transitions at triple points; A study based upon the analysis of stepwise adsorption isotherms

This adsorption isotherm study finds that ethylene adsorption on PbI₂ is limited to only a few layers at T<T_3t (the triple point temperature of bulk ethylene) but that almost unlimited adsorption can take place as T_3t, is approached. This behaviour is qualitatively identical to the behaviour of the ethylene/graphite system previously reported. In both cases wetting by ethylene is incomplete below T_3t since the adlayers formed have an orientational order which is in compatible with that which is found in the bulk solid ethylene. However, C₂H₄ shows a strong tendency to wetting somewhat below its bulk triple point temperature because it can form a liquid or at least an orientationally disordered solid film of limited thickness. The similarities and differences between these two systems are discussed. In addition, we discuss in general systems for which wetting is temperature dependent. The first part of this paper is devoted to outlining in detail how stepwise adsorption isotherms on powdered samples can be used to give information about wetting of the exposed basal face. The merits and limitations of this method are discussed in the context of the ethylene/PbI₂ example.     

Anomalous temperature dependence of magnetic relaxation in YBa<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>7 − δ</Subscript> oxygen-deficient single crystals

The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa₂Cu₃O_{7 ? δ} samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δ_th = 0.37. For δ < δ_th (T _c > 60 K, where T _c is the superconducting transition temperature), function S(T) has the well-known peak at T/T _c = 0.4. For δ > δ_th (at T _c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T _c = 0.1. The threshold value δ_th of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition.     

Atomistic simulation of temperature dependent thermal transport across nanoconfined liquid

Non-equilibrium classical molecular dynamics simulation was carried out to study the temperature dependence of thermal transport across nanogap confined liquid. The research was focused on the temperature response behavior of nanoconfined liquid subjected to different temperature environment. The simulation was carried out with simplified molecular model. It was found that the molecular mobility and thermal conductance of liquid reduces significantly due to the confinement in a nanometer scale cavity. The simulation also reveals that thermal resistance responses oppositely to the change in temperature for the nanoconfined liquid compared to the bulk liquid; thermal resistance of the nanoconfined liquid was observed to decrease with the increase of temperature following a power law relation (R_t/R_{t_inital}∼0.9837(T/T_initial)^−0.617) whereas it increases for the bulk liquid. Thermal resistance of the confined liquid and its dependence on temperature was also observed to strongly depend on the gap thickness.     

The effect of codeposited nitrogen on the amorphous-polycrystalline resistivity transformation temperature of thin iron films

Results are presented for the dependence of the amorphous-polycrystalline transformation temperature, T_t, on the concentration of codeposited gaseous nitrogen in thin iron films. These are compared with the effect of other impurities on T_t for the case of amorphous iron films. It is found that there is an approximate logarithmic dependence of T_t on impurity concentration.     

Temperature dependence of the radiation-stimulated conductivity of CsI crystals upon picosecond excitation by electron beams

The temperature dependence of the pulse conductivity for CsI crystals upon excitation with an electron beam (0.2 MeV, 50 ps, 400 A/cm²) at a time resolution of 150 ps is investigated. Under experimental conditions, the time of bimolecular recombination of electrons and holes (V _k centers) is directly measured in the temperature range 100–300 K. This made it possible to calculate the temperature dependence of the effective recombination cross section S(T)=7.9×10^?8 T² cm². The temperature dependence of the conductivity σ(T) is interpreted within the model of the separation of genetically bound electron-hole pairs. The activation energy of this process is found to be E _G =0.07 eV.     

Components of the low-temperature heat capacity of rare-earth hexaborides

The temperature dependence of heat capacity C _p(T) was studied for nine rare-earth hexaborides MB₆(M=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) at temperatures of 5–300 K. Using the correspondence principle for lattice heat capacities of isostructural compounds, the lattice contribution C ₁(T) and the excess contribution ΔC(T) to the heat capacity of the hexaborides were determined. The lattice heat capacity C ₁(T) is represented as the sum of the Debye contributions of the metal and boron sublattices: C ₁(T)=C _M (T)+6C _B(T). The Debye temperatures π_M and π_B of the metal and boron sublattices were determined. The anomalies in the excess heat capacity ΔC(T)=C _p (T)?C ₁(T) are related to the magnetic ordering effects, the Schottky contribution, and the Jahn-Teller effect.     

Manifestation of quantum statistics in vibrational dynamics of poly(ethylene) crystals

The temperature dependences of the transverse expansion ?_⊥(T) and the longitudinal contraction ?_‖(T) (with respect to the axes of chain molecules) in large-sized poly(ethylene) (PE) crystal grains (100×60×60 nm) are measured using x-ray diffraction in the temperature range 5–380 K. The temperature dependence of the elongation of the molecular skeleton ?_C(T) is obtained by Raman spectroscopy. It is found that the dependences ?_⊥(T), ?_‖(T), and ?_C(T) exhibit a similar specific nonlinear behavior. Analysis of these dependences indicates that the nonlinearity is associated with the quantum statistics of transverse vibrations. The energies and amplitudes of zero-point (at T=0) transverse (torsional and bending) vibrations and the relevant zero-point components ?_‖(0) and ?_C(0) are estimated. It is revealed that the zero-point components make a considerable contribution to the dynamics of the PE crystal up to the melting temperature (～400 K).     

Effect of surface orientation and thickness on the magnetization of anisotropic FCC ferromagnetic films

The dependence on temperature of the layer magnetization of a Heisenberg ferromagnetic ultrathin film in presence of magnetocrystalline single-ion anisotropy was theoretically investigated in the framework of a Green's function approach using the random phase approximation (RPA). The effect of surface orientation and of film thickness N on the Curie temperature T_C was carefully investigated in the case of face centered cubic (FCC) films: the steepest increase of T_C(N) was found in the case of the FCC(1 1 1) orientation and the smoothest in the FCC(1 1 0) one. Our results for T_C(N) were successfully fitted by a finite-size scaling relation [T_C(∞)−T_C(N)]/T_C(N)=(N/N₀)^−λ, giving a shift exponent λ≃1.5, irrespectively of the surface orientation. Finally, the temperature evolution of the magnetization profile was analyzed, as well as its limiting shape at T_C.     

Expansion of monomer units in poly(ethylene terephthalate) molecules under conditions of thermal and zero-point atomic vibrations

The temperature dependence of the expansion ?_C of trans conformers of the carbon skeleton of macromolecules in crystallized and amorphous poly(ethylene terephthalate) (PET) due to zero-point and thermal atomic vibrations is investigated by IR spectroscopy. It is found that the thermal expansion coefficient β_C jumpwise increases at the characteristic temperatures T _t and T _b. This increase is associated with the crossover from the quantum to classical statistics of torsional and bending vibration modes. The quantum and classical contributions to the expansion ?_C are determined for each mode. The quantum and classical contributions of the torsional vibrational mode in the amorphous polymer are approximately 1.5 times larger than those in the crystallized polymer. This effect is caused by an increase in the anharmonicity of torsional vibrations in the amorphous polymer.     

Temperature derivatives at constant volume of the elastic constants of the alkali halides

Values of dC/dT)_V have been computed from experimental dC/dT)_P and dC/dP)_T for the three elastic constants (expressed as $\text{B}{}_{\mathrm{T}}\text{, C}{}_{44}\text{and}\text{(C}{}_{11}\text{?C}{}_{12}\text{)}\text{2)}$ of each of the 16 Li, Na, K, and Rb halides. The dC/dT)_V measure the explicit dependence of C on T, the effect of thermal expansion having been removed. The dC/dT)_V are all small (compared with dC/dT)_P), are all negative, and vary quite systematically and smoothly with anion, with cation and with the three elastic constants. Negative dB_T/dT)_V is accounted for both thermodynamically and by available lattice dynamical calculations. dC/dT)_V for shear constant is expressed in terms of two vibrational mode parameters, whose values can then be estimated from the observed value, and the trend of dC/dT)_V with Debye temperature.     

Pressure dependence of the Grüneisen parameter of sodium and potassium

Temperature changes associated with adiabatic pressure changes (?T/?P)_s have been measured for sodium and potassium at room temperature (297 K) under hydrostatic pressures up to 32 kbar. For both metals a large decrease in the value of (?T/?P)_s is observed as the pressure increases. The Grüneisen parameter γ is related to (?T/?P)_s by γ = B_s(?T/?T)_s/T where B_s is the adiabatic bulk modulus. Available data for the pressure dependence of B_s and the isothermal bulk modulus B_T are used to predict the pressure and volume dependence of γ.     

Nonlinear interaction of an rf field with a bulk superconductor

Using a very sensitive intermodulation technique, we have measured the temperature (T) and magnetic fielf (H) dependence of the nonlinear rf interaction with pure bulk indium. A signal which varies extremely rapidly with both T and H is seen near T_c.     

Study of ferromagnetism–superconductivity interactions in Co/Nb multilayers

We investigate Co/Nb multilayers to explore the spontaneous π-phase shift between the superconducting (SC) layers, which is attributed for causing the non-monotonic change of the SC transition temperature (T_c) with the ferromagnetic (FM) layer thickness (t_FM) in several FM/SC multilayered systems. The issue of interfacial roughness is also explored by growing Co/Nb multilayers at various sputtering pressures. Transport measurements show a non-monotonic dependence of T_c on t_FM, and this dependence is insensitive to the structural variation present in the samples, as measured by X-ray scattering.     

Effect of hydrostatic pressure on the magnetic and lattice properties of the ferromagnet La(Fe0.86Si0.14)13

The temperature dependence of the lattice constant of the ferromagnet La(Fe_0.86Si_0.14)₁₃ has been measured at different pressures and temperatures. The numerical value of the bulk modulus at room temperature has been determined. The measurements of the temperature dependence of the lattice constant of the ferromagnet La(Fe_0.86Si_0.14)₁₃ at zero pressure and at 11.5 kbar have demonstrated that, at zero pressure, the lattice constant sharply decreases in the range from 160 to 210 K, whereas at a pressure of 11.5 kbar, the lattice constant decreases in the range from 110 to 180 K. This indicates that the Curie temperature T _C changes from 210 to ～170 K under pressure. The experimental results have been analyzed using the equations of states for the magnetic and elastic subsystems of the ferromagnet.     

Dielectric properties of C60 molecular complexes

The permittivity ? of the molecular complexes (BTX)C₆₀CS₂, (DAN)C₆₀(C₆H₆)₃, and (S₄N₄)_1.2C₆₀(C₆H₆)_0.8 has been studied at 100 MHz. A maximum has been observed to appear in the temperature dependence ?(T) at T≈90 K, which can be assigned to freezing of the orientational disorder. A region of anomalous growth of ? with decreasing temperature has been found in the low-temperature range T≤25 K, this feature being apparently sensitive to the actual packing pattern of C₆₀ spheres in the molecular complex.