Temperature Dependence Of Gap Research Articles

Temperature-dependent indirect gaps for two-dimensional bismuth oxychalcogenides probed by spectroscopic ellipsometry

In-plane optical properties of two-dimensional bismuth oxychalcogenides Bi2O2X (X = S, Se, and Te) are reported for a wide spectral range of 0.73–6.42 eV and at temperatures of 4.5–500 K by spectroscopic ellipsometry. At room temperature, Bi2O2S, Bi2O2Se, and Bi2O2Te exhibit an indirect band gap of 1.18 ± 0.02, 0.95 ± 0.01, and 0.60 ± 0.01 eV, respectively. As the temperature decreases, the indirect absorption edge of Bi2O2S undergoes a blueshift, while the indirect band gap of Bi2O2Se shows a redshift, and Bi2O2Te remains independent of temperature. The chalcogenide-dependent behavior as a function of temperature may be relevant to electron–phonon interactions in Bi2O2X materials. The observed pseudo-isotropic complex dielectric function and optical absorption coefficient by spectroscopic ellipsometry are directly compared with the first-principles calculations with a hybrid functional approach.

Emergence of two distinct phase transitions in monolayer CoSe2 on graphene

Dimensional modifications play a crucial role in various applications, especially in the context of device miniaturization, giving rise to novel quantum phenomena. The many-body dynamics induced by dimensional modifications, including electron-electron, electron-phonon, electron-magnon and electron-plasmon coupling, are known to significantly affect the atomic and electronic properties of the materials. By reducing the dimensionality of orthorhombic CoSe2 and forming heterostructure with bilayer graphene using molecular beam epitaxy, we unveil the emergence of two types of phase transitions through angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements. We disclose that the 2 × 1 superstructure is associated with charge density wave induced by Fermi surface nesting, characterized by a transition temperature of 340 K. Additionally, another phase transition at temperature of 160 K based on temperature dependent gap evolution are observed with renormalized electronic structure induced by electron-boson coupling. These discoveries of the electronic and atomic modifications, influenced by electron-electron and electron-boson interactions, underscore that many-body physics play significant roles in understanding low-dimensional properties of non-van der Waals Co-chalcogenides and related heterostructures.Graphical

Pseudo-Goldstone Modes and Dynamical Gap Generation from Order by Thermal Disorder.

Accidental ground state degeneracies-those not a consequence of global symmetries of the Hamiltonian-are inevitably lifted by fluctuations, often leading to long-range order, a phenomenon known as "order-by-disorder" (ObD). The detection and characterization of ObD in real materials currently lacks clear, qualitative signatures that distinguish ObD from conventional energetic selection. We show that for order by thermal disorder (ObTD) such a signature exists: a characteristic temperature dependence of the fluctuation-induced pseudo-Goldstone gap. We demonstrate this in a minimal two-dimensional model that exhibits ObTD, the ferromagnetic Heisenberg-compass model on a square lattice. Using spin-dynamics simulations and self-consistent mean-field calculations, we determine the pseudo-Goldstone gap, Δ, and show that at low temperatures it scales as the square root of temperature, sqrt[T]. We establish that a power-law temperature dependence of the gap is a general consequence of ObTD, showing that all key features of this physics can be captured in a simple model of a particle moving in an effective potential generated by the fluctuation-induced free energy.

Alloy Disordering Effects on the Thermal Conductivity and Energy Gap Temperature Dependence of Cd1-xZnxSe Ternary Crystals Grown by the Bridgman Method.

Investigated in this work, Cd1-xZnxSe-mixed ternary compounds were grown by the Bridgman method. Several compounds with zinc content varying in the range 0 < x < 1 were produced between two binary parents, CdSe and ZnSe crystals. Using the SEM/EDS technique, the accurate composition of formed crystals was determined along the growth axis. Thanks to that, the grown crystals' axial and radial uniformity were determined. Characterization of the optical and thermal properties was undertaken. The energy gap was measured using photoluminescence spectroscopy for different compositions and temperatures. The bowing parameter describing the behavior of the fundamental gap with composition for this compound was found to be 0.416 ± 0.06. The thermal characteristics of grown Cd1-xZnxSe alloys were systematically studied. The thermal diffusivity and effusivity of the crystals under investigation were experimentally determined, allowing the calculation of the thermal conductivity. We applied the semi-empirical model that Sadao Adachi developed to analyze the results. Thanks to that, it was possible to estimate the contribution arising from chemical disorder to the crystal's total resistivity.

Angle-resolved photoemission spectroscopy study of charge density wave order in the layered semiconductor EuTe4

Layered tellurides have been extensively studied as a platform for investigating the Fermi surface (FS) nesting-driven charge density wave (CDW) states. EuTe4, one of quasi-two-dimensional (quasi-2D) binary rare-earth tetratellurides CDW compounds, with unconventional hysteretic transition, is currently receiving much attention. Here, the CDW modulation vector, momentum and temperature dependence of CDW gaps in EuTe4 are investigated using angle-resolved photoemission spectroscopy. Our results reveal that (i) a FS nesting vector q ~ 0.67 b* drives the formation of CDW state, (ii) a large anisotropic CDW gap is fully open in the whole FS, and maintains a considerable size even at 300 K, leading to appearance of semiconductor properties, (iii) an abnormal non-monotonic increase of CDW gap in magnitude as a function of temperature, (iv) an extra, larger gap opens at a higher binding energy due to the interaction between the different orbits of the main bands.

Fully-gapped superconductivity in single crystalline NbC and TaC probed by point-contact spectroscopy

We report our point-contact spectroscopy (PCS) study on the single-crystalline transition metal carbide NbC and TaC with a superconducting transition temperature = 12.0 and 10.2 K, respectively. The differential conductance curves from both soft-PCS and mechanical-PCS (MPCS) on NbC at 1.9 K can be well fitted by a single-gap s-wave Blonder–Tinkham–Klapwijk model, and the temperature dependent gap follows a standard Bardeen–Cooper–Schrieffer (BCS) behavior, yielding = 2.0 ± 0.3 meV and 2/ = 3.9 ± 0.3 in the intermediate coupling regime. Similarly, our MPCS data on TaC crystals also show its superconducting gap as a function of temperature follows the BCS behavior, yielding = 1.7 ± 0.1 meV and 2/ = 3.9 ± 0.1. Our results thus support that both NbC and TaC are fully gapped s-wave superconductors.

Spectroscopic ellipsometry from 10 to 700 K

Abstract The temperature dependence of the optical constants of materials (refractive index, absorption and extinction coefficients, and dielectric function) can be determined with spectroscopic ellipsometry over a broad range of temperatures and photon energies or wavelengths. Such results have practical value, for example for applications of optical materials at cryogenic or elevated temperatures. The temperature dependence of optical gaps and their broadenings also provides insight into the scattering of electrons and holes with other quasiparticles, such as phonons or magnons. This review presents a detailed discussion of the experimental considerations for temperature-dependent ellipsometry and selected results for insulators, semiconductors, and metals in the infrared to ultraviolet spectral regions.

Thermal processes contributions to the temperature dependence of the energy gap in dilute bismuth III-V alloys

This paper presents a methodological analysis of the gap energy temperature dependence of GaAsBi, GaSbBi, InSbBi, and InAsBi. The temperature dependence of the gap energy in these alloys has been analyzed by considering separately the contributions due to thermal expansion and electron-phonon interaction based on Varshni, Viňa, and Pässler models. For the thermal expansion contribution, we use a term that explicitly takes into account the temperature variation of the thermal expansion coefficient and the pressure variation of the bandgap. The thermal expansion coefficient of semi-metallic compounds X–Bi (X = Ga or In) and III-V-Bi alloys is estimated on the basis of the physical properties of material atomic constitution. The overview of different results indicates the competition between the two processes in the temperature sensitivity of bandgap energy referring to a critical temperature of about100K. The coexistence of Bi and Sb has a particular influence on the fitting physical parameters issuing from different fits.

Alloy Disordering Effects on the Thermal Conductivity and Energy Gap Temperature Dependence of Cdznse Mixed Crystals Grown by the Bridgman Technique

Alloy Disordering Effects on the Thermal Conductivity and Energy Gap Temperature Dependence of Cdznse Mixed Crystals Grown by the Bridgman Technique

Antibonding-Induced Anomalous Temperature Dependence of the Band Gap in Crystalline Ge2Sb2Te5

Antibonding-Induced Anomalous Temperature Dependence of the Band Gap in Crystalline Ge₂Sb₂Te₅

Full superconducting gap and type-I to type-II superconductivity transition in single crystalline NbGe2

We report a mechanical point-contact spectroscopy study on the single crystalline NbGe$_2$ with a superconducting transition temperature $T\rm_c$ = 2.0 - 2.1 K. The differential conductance curves at 0.3 K can be well fitted by a single gap s-wave Blonder-Tinkham-Klapwijk model and the temperature dependent gap follows a standard Bardeen-Cooper-Schrieffer behavior, yielding $\Delta_0 \sim$ 0.32 meV and 2$\Delta_0$/$k\rm_{B}$$T\rm_{c}$ = 3.62 in the weak coupling limit. In magnetic field, the superconducting gap at 0.3 K keeps constant up to $H_{c1}\sim$150 Oe and gradually decreases until $H_{c2}\sim$350 Oe, indicating NbGe$_2$ going through a transition from type-I to type-II (possible type-II/1) superconductor at low temperature.

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.

As-related stability of the band gap temperature dependence in N-rich GaNAs

GaNAs layers with a low As concentration (As ≤ 0.6%) have been grown by molecular beam epitaxy and studied by structural and optical methods. It has been observed that the incorporation of a small amount of As atoms into the GaN host leads to a significant reduction of the bandgap due to the formation of an As-related band above the valence band of the GaN host. The position of this band does not change with temperature, and therefore, a reduced temperature dependence of the bandgap is observed for As-diluted GaN compared to the pure GaN host, which is ∼40 meV vs ∼70 meV in the 10–295 K temperature range. The observed effect is explained within the band anticrossing model. It is expected that the reduced temperature dependence of the bandgap in As-diluted GaN can be utilized in lasers with improved thermal stability.

Temperature dependence of resistivity at the transition to a charge density wave state in rare-earth tritellurides

About a half of the Fermi surface in rare-earth tritellurides, RTe3 becomes gapped below the transition to a charge-density-wave (CDW) state, as revealed by ARPES data. However, the observed jump in resistivity during the CDW transition is less than 20%. Previously this phenomenon was explained by hypothesizing a very slow evolution of CDW energy gap below transition temperature in RTe3 compounds, which contradicts the X-ray measurements. Here we show that this weak change in resistivity can be explained in the framework of standard mean-field temperature dependence of the CDW energy gap in agreement with X-ray data. The change of resistivity caused by CDW is weak because the decrease in conducting electron density at the Fermi level is almost compensated by the decrease in their scattering rate. We calculate resistivity in RTe3 compounds using the Boltzmann transport equation and the mean-field description of the CDW state, and obtain a good agreement with experimental data.

Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps.

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high-pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase, which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts.

Ultrafast hot carrier dynamics of ZrTe5 from time-resolved optical reflectivity

We investigate the hot carrier dynamics of ZrTe$_5$ by ultrafast time-resolved optical reflectivity. Our results reveal a phonon-mediated across-gap recombination, consistent with its temperature-dependent gap nature as observed previously by photoemission. In addition, two distinct relaxations with a kink feature right after initial photoexcitation are well resolved, suggesting the complexity of electron thermalization process. Our findings indicate that correlated many-body effects play important role for the transient dynamics of ZrTe$_5$.

Time-resolved reflectivity and Raman studies of the interplay of electronic orders in Mo8O23

Monoclinic semi-metallic Mo$_8$O$_{23}$ belongs to a multifunctional series of compounds showing multiple ordering phenomena that have not achieved much attention till now. Previous X-rays studies of this compound have revealed an incommensurate ordering transition at $T_{\mathrm{IC}}\sim350$ K, followed by a structural transition to commensurate order at $T_{\mathrm{IC-C}}=285$ K. In addition, an enigmatic resistance maximum is observed at $T_{\mathrm{el}}\sim150$ K, whose origin has so far proved elusive. Aiming to disentangle these multiple orders we use the polarized transient optical spectroscopy supplemented by Raman spectroscopy to study the electronic relaxation dynamics and lattice vibrational modes in Mo$_8$O$_{23}$ single crystals. Remarkably, both the coherent vibrational mode response and single particle response display extrema of damping/relaxation times close to $T_{\mathrm{el}}$ with the concurrent appearance of new coherent vibrational modes and a characteristic polarization asymmetry which saturates below $T_{\mathrm{el}}$. The single-particle relaxation data analysis shows the appearance of a temperature-independent gap in the electronic excitation spectrum below $T_{\mathrm{IC}}$ and additional temperature-dependent gap opening near $T_{\mathrm{el}}$. Concurrently, a low frequency vibrational mode shows anomalous softening around $T_{\mathrm{m}}\sim200$ K, far below $T_{\mathrm{IC-C}}$ and $T_{\mathrm{IC}}$. The observations are interpreted in terms of the appearance of a hidden gapped state below $T_{\mathrm{el}}$ that has so far eluded detection by structural analyses.

Theoretical study of anisotropic tunnelling conductance in iron-based orbitally ordered superconductors

We address here the role of Jahn-Tellar (JT) distortion on the superconducting (SC) gap in iron-based superconductors taking into account of the first and second nearest-neighbour (NN) electron hoppings in the square lattice within one band model approach. The Green's functions are calculated by using Zubarev's Green's function technique. The temperature dependent superconducting gap and the lattice strain are calculated from the correlation functions of the corresponding Green's functions and are computed self-consistently. The temperature dependent gap equations show that the superconducting gap is enhanced with the decrease of second nearest-neighbour hopping accompanied by the suppression of lattice strain near the superconducting transition temperature. The second nearest hopping introduces asymmetry in the tunnelling conductance.

Neutron scattering investigation of rhenium orbital ordering in the 3d−5d double perovskite Ca2FeReO6

We have carried out inelastic neutron scattering experiments to study magnetic excitations in ordered double perovskite Ca$_2$FeReO$_6$. We found a well-defined magnon mode with a bandwidth of $\sim$50meV below the ferri-magnetic ordering temperature ($T_c\sim$520K), similar to previously studied Ba$_2$FeReO$_6$. The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of $\sim$10meV opens up below $\sim$150K, which is well below the magnetic ordering temperature but coincides with a previously reported metal-insulator transition and onset of structural distortion. The observed temperature dependence of spin gap provides strong evidence for ordering of Re orbitals at $\sim$150~K, in accordance with earlier proposal put forward by Oikawa $\it{et.\,al}$ based on neutron diffraction [J. Phys. Soc. Jpn., $\bf{72}$, 1411 (2003)] as well as recent theoretical work by Lee and Marianetti [Phys. Rev. B, $\bf{97}$, 045102 (2018)]. The presence of separate orbital and magnetic ordering in Ca$_2$FeReO$_6$ suggests weak coupling between spin and orbital degrees of freedom and hints towards a sub-dominant role played by spin orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near magnetic zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments, but suggests a strong damping of Re magnon mode.

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.