Temperature Dependence Of Energy Gap Research Articles

Deuteration Effects on the Transport Properties of (TMTTF)2X Salts

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge transport in deuterated and protonated (TMTTF)2X salts with the anions X = Br, PF6, SbF6, and ClO4, ranging from Mott and Efros–Shklovskii variable-range hopping to activated band transport with a temperature dependent energy gap. The strong dependence of charge localization and ordering on the anion size and deuteration confirms the subtle structural involvement of the anions in the charge transport along the TMTTF stack.

Optical and Tunneling Studies of Energy Gap in Superconducting Niobium Nitride Films

We have prepared an epitaxial niobium nitride (NbN) film and NbN/AlN/NbN tunnel junctions to investigate the energy gaps. By measuring the optical conductivity spectrum of a 41-nm-thick film with the critical temperature (TC) of about 14 K by terahertz time-domain spectroscopy, we obtained the gap frequency of about 1.2 THz. An ill-defined gap was suggested due to the broadening of the onset of absorption as the temperature increases. On the other hand, the current–voltage curve measurement of the tunnel junctions showed the current rise at the gap voltage of 5.6 mV (corresponding to 1.4 THz) with a smeared shape as the temperature increases. We found that both gap broadenings can be explained by introducing a temperature-dependent imaginary energy gap part into the superconducting energy gap, corresponding to a finite quasi-particle lifetime in the NbN films.

Pressure and Temperature Dependence of Energy Gap in SiC and Si1-xGex

The effect of pressure on energy gap for IV-IV compound SiC and Si1-xGex alloy have been investigated and evaluated by using Birch-Murnaghan equation of state (EOS) and Bardeen equation of state. Ambiguity in the effect of pressure and temperature on Eg of different SiC polytypes (3C, 4H, 6H) have been investigated and attributed. Variation of Eg in Si1-x Gex evaluated and an interpretation, for it, has been suggested.

Magnetocaloric effect in Fe2P : Magnetic and phonon degrees of freedom

Devices based on magnetocaloric materials provide great hope for environmentally friendly and energy efficient cooling that does not rely on the use of harmful gasses. Fe2P based compounds are alloys that have shown great potential for magnetocaloric devices. The magnetic behavior in Fe2P is characterized by a strong magnetocaloric effect that coexists with a first-order magnetic transition (FOMT). Neutron diffraction and inelastic scattering, Mossbauer spectroscopy, and first-principles calculations have been used to determine the structural and magnetic state of Fe2P around the FOMT. The results reveal that ferromagnetic moments in the ordered phase are perturbed at the FOMT such that the moments cant away from the principle direction within a small temperature region. The acoustic-phonon modes reveal a temperature-dependent nonzero energy gap in the magnetically ordered phase that falls to zero at the FOMT. The interplay between the FOMT and the phonon energy gap indicates hybridization between magnetic modes strongly affected by spin-orbit coupling and phonon modes leading to magnon-phonon quasiparticles that drive the magnetocaloric effect.

Temperature dependence of the energy band gap of CuSi2P3 semiconductor using PSOPW method

Abstract Theoretical formalism based on the orthogonalized plane wave method supplemented by a potential scaling scheme was used to predict the temperature dependence of energy gap of CuSi2P3 semiconductor. A computer code in Pascal was used to perform the variation of fundamental energy gap with temperature in the range of 150 K to 800 K. The dependence of energy gap on temperature for lattice dilation contribution, lattice vibration contribution and total temperature effect were performed separately. The results revealed that, as temperature increases, the top of the valence band and the bottom of the conduction band increase, while the energy band gap decreases. Generally, at low temperatures, the energy gap varies slowly and exhibits a nonlinear dependence and approaches linearity as temperature increases. The calculated energy gap of CuSi2P3 at T = 300 K is 0.4155 eV. The temperature coefficients in the linear region due to lattice dilation contribution, lattice vibration contribution and total temperature effect were calculated as –1.101 × 10−5 eV/K, –1.637 × 10−4 eV/K and –1.7523 × 10−4 eV/K, respectively. Also, the ratio of temperature coefficient of the energy gap due to LV contribution to its value and LD contribution in the linear region is equal to 14.868. That ratio is compared to those of CuGe2P3 and III-V compounds, where those of the latter show a systematic change with Eg. Moreover, the Eg of all the compounds shows a quadratic dependence on the inverse of mean bond length.

Combining the roles of bose correlations and pairing associations to condensation and its influence on optical conductivity; special superconductors

Bose correlations (BC) and pairing associations (PA) are known as two distinct origins for condensation. For most known condensates, condensation is only attributed to one of them. Recent developments call for research on combining the roles of the two origins. Therefore, we investigate the pairing and condensation in a boson-like system through a quantum microscopic approach based on correlation functions and order parameter equation (OPE). For the first time, we obtain general expressions for complex optical conductivity of such condensates. We focus on an interesting self-consistent solution of the OPE, i.e. a condensate with spherically symmetric order parameter growing linearly up to a radius in the momentum-space, and extract interesting predictions for it. The appearance of a temperature dependent energy gap in the middle of excitation spectrum and the appearance of a pseudogap in the absorption spectrum (without a counterpart in the excitation spectrum) in addition to the possibility of condensation at room temperature are among our predictions. Also, the paper provides several coherent pieces of evidence for the interest: the roles of BC and PA combine for creating a novel superconductor similar to MgB2.

Temperature dependent energy gap shifts of single color center in diamond based on modified Varshni equation

Although the diamond has the similar structure (diamond cubic) and symmetry (Oh) as many other conventional semiconductor materials, the extraordinary large Debye temperature and small lattice expansion coefficient make the temperature dependence of energy gaps in diamond to be totally different from those in other semiconductors. Here we propose a modified Varshni formula to describe the temperature dependent zero field splitting and zero phonon line of the nitrogen vacancy center in diamond. The shift of energy gaps shows a T4 scaling at low temperature and a T2 scaling at high temperature. The crossover between these two regimes is determined by two positive parameters in the modified formula. This empirical formula can give an excellent prediction of the temperature dependent energy gaps up to 700K. We experimentally verify this temperature dependent behavior with single negatively charged nitrogen vacancy centers in diamond. These observations demonstrate the leading role of electron-acoustic phonon interaction in the temperature dependent shift of energy gaps in diamond. This empirical equation can benefit the diamond-based applications such as quantum sensing and computing.

Temperature shift of intraband absorption peak in tunnel-coupled QW structure

An experimental study of the intersubband light absorption by the 100-period GaAs/Al0.25Ga0.75As double quantum well heterostructure doped with silicon is reported and interpreted. Small temperature redshift of the 1–3 intersubband absorption peak is detected. Numerical calculations of the absorption coefficient including self-consistent Hartree calculations of the bottom of the conduction band show good agreement with the observed phenomena. The temperature dependence of energy gap of the material and the depolarization shift should be accounted for to explain the shift.

Quantum confinement effect in low temperature grown homo-epitaxial GaN nanowall network by laser assisted molecular beam epitaxy

Vertically well-aligned homo-epitaxial GaN nanowall network (NWN) was grown on metal organic chemical vapor deposited 3.5 μm thick GaN (0001) on c-sapphire by laser assisted molecular beam epitaxy (LMBE). The honeycomb GaN NWN with wall width of 8–12 nm and pore size of ∼100–200 nm were obtained at a low growth temperature of 600 °C. The optical properties measured by photoluminescence (PL) spectroscopy showed a blue-shift of ∼100 meV near band edge (NBE) emission for GaN NWN. In addition, temperature dependent (90–300 K) PL measurements of GaN NWN were investigated which follow the Varshni's temperature dependent energy gap relation. The structural and optical properties of pristine and wet-etched LMBE grown homo-epitaxial GaN thin film at 500 °C were also investigated and compared with the GaN NWN structures. X-ray photoelectron spectroscopy study confirms Ga rich nature of GaN nanowalls along with significant decrease in surface band bending in NWN compared to film. The KOH wet etched of GaN NWN showed that the wall width decreased to 6–10 nm and exhibited a blue-shift of 120 meV in the NBE in PL emission confirming the realization of quantum size effect in the GaN NWN structure. The low temperature grown GaN NWN structures can be integrated with light emitting diodes and solar cells for the enhancement of light extraction and device efficiency.

The importance of temperature dependent energy gap in the understanding of high temperature thermoelectric properties

In this work, we show the importance of temperature dependent energy band gap, Eg(T), in understanding the high temperature thermoelectric (TE) properties of material by considering LaCoO3 (LCO) and ZnV2O4 (ZVO) compounds as a case study. For the fix value of band gap, Eg, deviation in the values of α has been observed above 360 K and 400 K for LCO and ZVO compounds, respectively. These deviation can be overcomed by consideration of temperature dependent band gap. The change in used value of Eg with respect to temperature is ∼4 times larger than that of In As. This large temperature dependence variation in Eg can be attributed to decrement in the effective on-site Coulomb interaction due to lattice expansion. At 600 K, the value of ZT for n and p-doped, LCO is ∼0.35 which suggest that it can be used as a potential material for TE device. This work clearly suggest that one should consider the temperature dependent band gap in predicting the high temperature TE properties of insulating materials.

Temperature dependence of energy gap of Ge1−xSnx alloys with x < 0.11 studied by photoreflectance

Compressively strained Ge1−xSnx layers of various Sn content grown on Ge/Si templates have been studied by photoreflectance in 15–295 K temperature range. It has been confirmed that the direct optical transition between heavy-hole band and conduction band shifts to lower energy upon incorporation of Sn atoms. The shift of the transition energy in that temperature range has been found to be 68–91 meV which is slightly higher than the energy shift of direct transitions in Ge over the same temperature range. Varshni and Bose–Einstein coefficients for compressively strained Ge1−xSnx have been extracted from the analysis of the direct transitions and compared with parameters from literature. In addition the heavy- and light-hole related direct optical transitions in tensile strained Ge buffer layers have been measured and analyzed.

Non-BCS Temperature Dependence of Energy Gap in Thin Film Electron-Doped Cuprates

We investigate the dependence of the energy gap ($G$) on the temperature ($T$) for the electron-doped high-temperature superconductors. The following compounds, in the form of the thin films, have been taken into consideration: $\rm La_{2-x}Ce_xCuO_{4}$ (LCCO), $\rm Pr_{2-x}Ce_xCuO_{4}$ (PCCO), and $\rm Nd_{2-x}Ce_xCuO_4$ (NCCO). It was found that $G\left(T\right)$ deviates from the BCS prediction more, if a concentration of cerium assumes the lower values. For the lowest concentration (in the case of LCCO and NCCO), the function $G\left(T\right)$ is not quite like the BCS curve, which is connected with the existence of the residual Nernst region. Next, it has been pointed out that the NCCO superconductor becomes structurally unstable for the maximum concentration of cerium, which is leading to the anomalous dependence of the energy gap on the temperature and the induction of the wide Nernst region.

Temperature-Dependent Energy Gap Shift and Thermally Activated Transition in Multilayer CdTe/ZnTe Quantum Dots.

We investigated the influence of growth conditions on carrier dynamics in multilayer CdTe/ZnTe quantum dots (QDs) by monitoring the temperature dependence of the photoluminescence emission energy. The results were analyzed using the empirical Varshni and O'Donnell relations for temperature variation of the energy gap shift. Best fit values showed that the thermally activated transition between two different states occurs due to band low-temperature quenching with values separated by 5.0-6.5 meV. The addition of stack periods in multilayer CdTe/ZnTe QDs plays an important role in the energy gap shift, where the exciton binding energy is enhanced, and, conversely, the exciton-phonon coupling strength is suppressed with an average energy of 19.3-19.8 meV.

Signatures of Unconventional Superconductivity in Granular Aluminum

We present measurements of the complex transmission of superconducting granular aluminum thin films in the THz optical range. We discuss the dynamical conductivity and temperature-dependent energy gap of two films of different granular coupling. While the film of well-coupled grains follows the predictions for a conventional BCS superconductor, signatures of enhanced absorption are found at sub-gap frequencies when coupling is decreased. We discuss possible explanations which might account for the observed deviations from BCS theory.

Temperature dependence of photoluminescence from InNAsSb layers: The role of localized and free carrier emission in determination of temperature dependence of energy gap

The temperature dependence of energy gap-related emission from InNAsSb layers was studied by Fourier transform infrared photoluminescence (PL) spectroscopy. The shape of PL peak was analyzed using a theoretical expression, which takes into account both the localized and free carrier emission. Proper accounting for those two effects is very important for an accurate determination of the Varshni and Bose-Einstein parameters from PL data. It is shown that nitrogen incorporation has a very week effect on the temperature induced bandgap reduction in InNAsSb alloys and that the Varshni and Bose-Einstein parameters are very close to those observed in InAs and InSb.

Temperature Dependence of Energy Gap in the Superconducting State in URu2Si2

The excitation spectrum of URu 2 Si 2 in the superconducting state has been investigated by a precise neutron-scattering experiment. A weak but clear positive energy shift of ∼40 µeV has been detected in the superconducting phase (at T =400 mK) for the resonance at Q 0 = (1,0,0). For the resonance at Q 1 = (0.6,0,0), a two times smaller shift is observed but with an incertitude of the same order than this shift. This result is in agreement with our previous results indicating that the wave-vector Q 0 is connected with the order parameters of the hidden order and of superconductivity.

Temperature-dependent energy gap variation in InAs/GaAs quantum dots

In this letter, we report a photoluminescence (PL) study of the temperature-dependent energy gap variation in InAs/GaAs quantum dots (QD). Energy gaps E(T) of different InAs/GaAs QD samples with various numbers of QD stacking layers were measured from the ground state PL emissions at various sample temperatures. For each of the QD samples, linear dependences between [E(T)−E0](β+T) and T (where E0=0.42 eV and β=−550 K) is obtained in low and high temperature regions. The transition temperatures between the two temperature regions are found to be related to the numbers of QD stacking layers. A linear relation between the number of the QDs and the phonon densities at the corresponding transition temperatures is obtained.

Temperature-Dependent Energy Gap of the Primary Charge Separation in Photosystem I: Study of Delayed Fluorescence at 77−268 K

The dynamics of fluorescence decay and charge recombination were studied in the ether-extracted photosystem I reaction center isolated from spinach with picosecond resolution over a wide time range up to 100 ns. At all temperatures from 268 to 77 K, a slow fluorescence decay component with a 30-40 ns lifetime was detected. This component was interpreted as a delayed fluorescence emitted from the singlet excited state of the primary donor P700*, which is repopulated through charge recombination that was increased by the lack of secondary acceptor phylloquinone in the sample. Analysis of the fluorescence kinetics allowed estimation of the standard free-energy difference -DeltaG between P700* and the primary radical pair (P700(+)A0(-)) state over a wide temperature range. The values of -DeltaG were estimated to be 160/36 meV at 268/77 K, indicating its high sensitivity to temperature. A temperature-dependent -DeltaG value was also estimated in the delayed fluorescence of the isolated photosystem I in which the secondary acceptor quinone was partially prereduced by preillumination in the presence of dithionite. The results revealed that the temperature-dependent -DeltaG is a universal phenomenon common with the purple bacterial reaction centers, photosystem II and photosystem I reaction centers.

Magnetic and electrical transport properties in (Cr 99V 1) 95Fe 5 and (Cr 99V 1) 75Fe 25 alloys

DC magnetization M ( T ) and electrical resistivity ρ ( T ) have been studied in the temperature range of 4–300 K for (Cr 99V 1) 95Fe 5 and (Cr 99V 1) 75Fe 25 alloys. A minimum is observed near T N in ρ ( T ) curve for (Cr 99V 1) 95Fe 5, which indicates the spin-density wave (SDW) antiferromagnetic transition. The progressive opening up of the SDW antiferromagnetic energy gap can be well described by a temperature-dependent energy gap function as 2 Δ ∝ T N − T . A paramagnetic–ferromagnetic transition is confirmed near T C in M ( T ) curve for (Cr 99V 1) 75Fe 25, but no anomaly is observed around T C in ρ ( T ) curve that would be covered up by magnetic cluster-dependent scattering. The analyses indicated the magnetic clusters start to form at relatively high temperature above the transition temperature. T N and the SDW antiferromagnetism are rapidly depressed for Fe-poor (Cr 99V 1) 95Fe 5 while T C and the ferromagnetism are rapidly increased for Fe-rich (Cr 99V 1) 75Fe 25 by the substitution of light doping V for Cr.

Dipolon theory of energy gap parameters at finite temperature and transition temperatures Tc and T∗ in high-temperature superconductors

First temperature dependent regular and pseudo-energy gap parameters and regular and pseudo-transition temperatures arising from the same physical origin have been calculated in the strong coupling formalism. Temperature dependent many-body field-theoretic techniques have been developed, as an extension of our previous zero-temperature formalism, to derive temperature dependent general expressions for the renormalized energy gap parameter Δ ( k → , ω ) , the gap renormalization parameter Z ( k → , ω ) and energy band renormalization parameter χ ( k → , ω ) for momentum k → and frequency ω making use of dipolon propagator and electron Green’s function taking into account explicitly the dressed dipolons as mediators of superconductivity, the screened Coulomb repulsion and nonrigid electron energy bands considering retardation and damping effects and electron–hole asymmetry. The theory takes into account all necessary and important correlations. Our self-consistent calculations utilize the previously symmetry predicted two energy gap parameters for superconducting cuprates, one being antisymmetric (“as”) with respect to the exchange of the k x and k y components of vector k → and the other being symmetric (“s”) with respect to the exchange of k x and k y . Our present temperature dependent self-consistent solutions of the real and imaginary parts of the Δ ( k → , ω ) , Z ( k → , ω ) and χ ( k → , ω ) confirm the existence of these two (different) solutions and conclude that the antisymmetric solution of the gap parameter corresponds to the observed regular (“reg”) superconducting energy gap whereas the symmetric solution corresponds to the observed pseudo-(“pse-”) energy gap. Explicit temperature dependent self-consistent calculations have been performed here for Bi 2Sr 2CaCu 2O 8+ δ as well as Bi 2Sr 2CaCu 2O 8 giving temperature dependent energy gap parameters and corresponding transition temperatures. The calculated results are consistent with the available experimental data. Our calculated results take into account different appropriate sets of electron–electron Coulomb interaction energy and screening constant with Pauling’s as well as TKS polarizabilities for various ions in these systems. The uncertainties in the calculated values of the energy gap parameters and transition temperatures are mainly due to uncertainties in the polarizabilities of the ions, Coulomb energy and the screening constant. Migdal vertex correction has been estimated. The calculated results have been discussed with respect to the available experiments. In particular, our calculations show that T ∗ is greater than the corresponding T c and they ( T c and T ∗) have the same physical origin, in agreement with experiments. Our results have been discussed and directions for further investigations have been pointed out.