Width Of Conduction Band Research Articles

Antiadiabatic Phonons and Superconductivity in Eliashberg–McMillan Theory

The standard Eliashberg - McMillan theory of superconductivity is essentially based on the adiabatic approximation. Here we present some simple estimates of electron - phonon interaction within Eliashberg - McMillan approach in non - adiabatic and even antiadiabatic situation, when characteristic phonon frequency $\Omega_0$ becomes large enough, i.e. comparable or exceeding the Fermi energy $E_F$. We discuss the general definition of Eliashberg - McMillan (pairing) electron - phonon coupling constant $\lambda$, taking into account the finite value of phonon frequencies. We show that the mass renormalization of electrons is in general determined by different coupling constant $\tilde\lambda$, which takes into account the finite width of conduction band, and describes the smooth transition from the adiabatic regime to the region of strong nonadiabaticity. In antiadiabatic limit, when $\Omega_0\gg E_F$, the new small parameter of perturbation theory is $\lambda\frac{E_F}{\Omega_0}\sim\lambda\frac{D}{\Omega_0}\ll 1$ ($D$ is conduction band half -- width), and corrections to electronic spectrum (mass renormalization) become irrelevant. However, the temperature of superconducting transition $T_c$ in antiadiabatic limit is still determined by Eliashberg - McMillan coupling constant $\lambda$. We consider in detail the model with discrete set of (optical) phonon frequencies. A general expression for superconducting transition temperature $T_c$ is derived, which is valid in situation, when one (or several) of such phonons becomes antiadiabatic. We also analyze the contribution of such phonons into the Coulomb pseudopotential $\mu^{\star}$ and show, that antiadiabatic phonons do not contribute to Tolmachev's logarithm and its value is determined by partial contributions from adiabatic phonons only.

Influence of the symmetry of hybridization on the critical temperature of multiband superconductors

In this paper, we study a two-band model of a superconductor in a square lattice. One band is narrow in energy and includes local Coulomb correlations between its quasiparticles. Pairing occurs in this band due to nearest-neighbor attractive interactions. Extended $s$-wave as well as $d$-wave symmetries of the superconducting order parameter are considered. The correlated electrons hybridize with those in another wide conduction band through a $k$-dependent mixing with even or odd parity depending on the nature of the orbitals. The many-body problem is treated within a slave-boson approach that has proved adequate to deal with the strong electronic correlations that are assumed here. Since applied pressure changes mostly the ratio between hybridization and bandwidths, we can use this ratio as a control parameter to obtain the phase diagrams of the model. We find that, for a wide range of parameters, the critical temperature increases as a function of hybridization (pressure) with a region of first-order transitions. When frustration is introduced, it gives rise to a stable superconducting phase. We find that superconductivity can be suppressed for specific values of band filling due to the Coulomb repulsion. We show how pressure, composition, and strength of correlations affect the superconductivity for different symmetries of the order parameter and the hybridization.

Effect of isotropic pressure on structural and electronic properties of silicon system with Fd-3m space group

We study on the effect of isotropic pressure on structural and electronic properties of silicon system with Fd-3m space group which are calculated using generalized gradient approximation method with Perdew-Burke-Erzenhof-type exchange-correlation functional energy (PBE-GGA method). As the results, the calculated lattice parameter is 5.475 Â at room pressure and decreased to 5.287 Å at the pressure of 11.3 GPa, at which the space group is transformed to I41/amd. The fit bulk modulus and derivative pressure from the relation between lattice parameter and pressure are 89 GPa and 3.5, respectively. Besides, the calculated indirect bandgap is 0.645 eV (1.170 eV after correction) at room pressure and linearly decreased to 0.482 eV (1.007 eV after correction) at 11.3 GPa, when the space group is transformed to I41/amd. The calculated pressure coefficient (α = dEg/dP) of silicon system is -14.4 meV/GPa using the linear relation. On the other hand, the valence and conduction bandwidths are increased by the increase of pressure, mainly promoted by Si 3p states. Overall, the result shows that the reasonable pressure-dependence of structural and electronic properties of silicon can be obtained by using PBE-GGA method. This study can be used as a guide for silicon-based photonic applications.

Electron–Phonon Coupling in Eliashberg–McMillan Theory Beyond Adiabatic Approximation

Eliashberg - McMillan theory of superconductivity is essentially based on the adiabatic approximation. Small parameter of perturbation theory is given by $\lambda\frac{\Omega_0}{E_F}\ll 1$, where $\lambda$ is the dimensionless electron - phonon coupling constant, $\Omega_0$ is characteristic phonon frequency, while $E_F$ is Fermi energy of electrons. Here we present an attempt to describe electron - phonon interaction within Eliashberg - McMillan approach in situation, when characteristic phonon frequency $\Omega_0$ becomes large enough (comparable or exceeding the Fermi energy $E_F$). We consider the general definition of electron - phonon pairing coupling constant $\lambda$, taking into account the finite value of phonon frequency. Also we obtain the simple expression for the generalized coupling constant $\tilde\lambda$, which determines the mass renormalization, with the account of finite width of conduction band, and describing the smooth transition from the adiabatic regime to the region of strong nonadiabaticity. In the case of strong nonadiabaticity, when $\Omega_0\gg E_F$, the new small parameter appears $\lambda\frac{E_F}{\Omega_0}\sim\lambda\frac{D}{\Omega_0}\ll 1$ ($D$ is conduction band half - width), and corrections to electronic spectrum become irrelevant. At the same time, the temperature of superconducting transition $T_c$ in antiadiabatic limit is still determined by Eliashberg - McMillan coupling constant $\lambda$, while the preexponential factor in the expression for $T_c$, conserving the form typical of weak - coupling theory, is determined by the bandwidth (Fermi energy). For the case of interaction with a single optical phonon we derive the single expression for $T_c$, valid both in adiabatic and antiadiabatic regimes and describing the continuous transition between these two limiting cases.

CT-X: An efficient continuous-time quantum Monte Carlo impurity solver in the Kondo regime

In the present paper, we present an efficient continuous-time quantum Monte Carlo impurity solver termed CT-X with high acceptance rate at low temperature for multi-orbital quantum impurity models with general interaction. In this hybridization expansion impurity solver, the imaginary time evolution operator for the high energy multiplets, which decays very rapidly with the imaginary time, is approximated by a probability normalized δ-function. As the result, the virtual charge fluctuations of fn→fn±1 are well included on the same footing without applying Schrieffer–Wolff transformation (SWT) explicitly. CT-X is proven to be equivalent to SWT on the leading order of W/U, where W is half the conduction band width and U is the local interaction strength. As benchmarks, our algorithm perfectly reproduces the results for both Coqblin–Schriffeer model and Kondo lattice model obtained by CT-J method developed by Otsuki et al.. Furthermore, it allows capturing low energy physics of heavy-fermion materials directly without fitting the exchange coupling J in the Kondo model. Finally, we reformulate the dynamical mean-field theory loops using only the quasi-particle part of the f-electron Green’s function measured in CT-X. Our benchmark calculations on CeIrIn5 at low temperature demonstrate a very reasonable low-energy f-electron density of states, which is in good agreement with the recent ARPES experiment.

Concentration-Mediated Band Gap Reduction of Bi2MoO6 Photoanodes Prepared by Bi3+ Cation Insertions into Anodized MoO3 Thin Films: Structural, Optical, and Photoelectrochemical Properties

A secondary cation insertion technique to fabricate ternary Bi2MoO6 thin films with reduced optical band gaps and shallow valence bands by the controllable insertion of Bi3+ cations into anodized MoO3 thin films has been established. Near-complete conversion of the MoO3 thin film to a low-temperature-phase γ(L)-Bi2MoO6 thin film was achieved when the MoO3 thin films were subject to hydrothermal treatment in a low Bi(NO3)3·5H2O solution concentration. In contrast, a bilayered Bi2MoO6/MoO3 thin film photoelectrode comprising predominantly a high-temperature-phase γ(H)-Bi2MoO6 oxide–electrolyte interface top region and a MoO3 oxide–collector interface bottom region was formed when a high Bi(NO3)3·5H2O solution concentration was utilized. UV–vis spectroscopy shows both the γ(L)-Bi2MoO6 (Eg = 2.7 eV) and γ(H)-Bi2MoO6 (Eg = 3.05 eV) thin films exhibit smaller band gaps than MoO3 (Eg = 3.4 eV). For γ(L)-Bi2MoO6, the reduction in optical band gap was attributed to the formation of a higher-lying O 2p valence band maximum while, for the γ(H)-Bi2MoO6 thin film, hybridization of the Bi 6s orbitals with the O 2p valence orbitals lowers the potential of the valence band maximum, leading to the reduced band gap. Overall, the Bi2MoO6 thin films with the highest γ(L)-Bi2MoO6 concentration exhibited the highest photocurrent density. The photocurrent enhancement can be attributed to two main reasons: first, the trilayer Bi2MoO6/MoO3 heterostructure obtained from the direct thin film assembly enables a smooth percolation of photoexcited charges from the surface generation sites to the charge collection sites at the Mo substrate, minimizing charge recombination losses; second, the MoO6 octahedra-coordinated γ(L)-Bi2MoO6 possesses a wide conduction band enabling fast separation and migration of delocalized charges. The secondary cation insertion technique has potential as a universal method to prepare complex oxides with narrow band gaps and shallow valence bands from insertion-type oxides for solar energy applications.

New Type of 2D Perovskites with Alternating Cations in the Interlayer Space, (C(NH2)3)(CH3NH3)nPbnI3n+1: Structure, Properties, and Photovoltaic Performance.

We present the new homologous series (C(NH2)3)(CH3NH3)nPbnI3n+1 (n = 1, 2, 3) of layered 2D perovskites. Structural characterization by single-crystal X-ray diffraction reveals that these compounds adopt an unprecedented structure type, which is stabilized by the alternating ordering of the guanidinium and methylammonium cations in the interlayer space (ACI). Compared to the more common Ruddlesden-Popper (RP) 2D perovskites, the ACI perovskites have a different stacking motif and adopt a higher crystal symmetry. The higher symmetry of the ACI perovskites is expressed in their physical properties, which show a characteristic decrease of the bandgap with respect to their RP perovskite counterparts with the same perovskite layer thickness (n). The compounds show a monotonic decrease in the optical gap as n increases: Eg = 2.27 eV for n = 1 to Eg = 1.99 eV for n = 2 and Eg = 1.73 eV for n = 3, which show slightly narrower gaps compared to the corresponding RP perovskites. First-principles theoretical electronic structure calculations confirm the experimental optical gap trends suggesting that the ACI perovskites are direct bandgap semiconductors with wide valence and conduction bandwidths. To assess the potential of the ACI perovskites toward solar cell applications, we studied the (C(NH2)3)(CH3NH3)3Pb3I10 (n = 3) compound. Compact thin films from the (C(NH2)3)(CH3NH3)3Pb3I10 compound with excellent surface coverage can be obtained from the antisolvent dripping method. Planar photovoltaic devices from optimized ACI perovskite films yield a power-conversion-efficiency of 7.26% with a high open-circuit voltage of ∼1 V and a striking fill factor of ∼80%.

Evolution of magnetic and transport properties in pyrochlore iridates A2Ir2O7 (A=Y, Eu, Bi)

We study the structure, magnetization, electrical transport properties and heat capacity of the pyrochlore iridates A2Ir2O7 (A=Y, Eu, Bi). It is found that magnetic frustration and long-range magnetic order coexist in Y2Ir2O7 and Eu2Ir2O7, while Bi2Ir2O7 shows magnetic frustration or short-range magnetic order without long-range magnetic order. The former two compounds show insulating behavior. However, the latter one shows metallic behavior. The variations of the radius of A-site ion affect the conduction bandwidth, correlation and Ir-Ir exchange energy, which in turn determine transport properties and magnetic properties. We also find that the antiferromagnetic ordering of pyrochlore iridates is weakly frustrated. The all-in/all-out type of anisotropy based on spin-orbit coupling plays a key role in releasing the frustration.

Impact of Interface Trap Charges on Performance of Electrically Doped Tunnel FET With Heterogeneous Gate Dielectric

In this paper, we investigate for the first time effect of positive (donor) and negative (acceptor) interface trap charges on the performance of proposed heterogeneous gate dielectric (HD) electrically doped tunnel field-effect transistor (EDTFET) in terms of dc, analog/RF, and linearity distortion parameters, where the HD layer is considered as a gate dielectric to improve the ON-state current and device performance. For this, a comparative analysis has been performed between conventional and proposed EDTFET with identical dimensions in the presence of interface trap charges. ATLAS device simulation of both devices is performed for different performance metrics such as transfer characteristics, parasitic capacitances, device efficiency, output conductance (gds), cut-off frequency (ft), and gain bandwidth product. However, linearity distortion parameters of the proposed device such as third-order transconductance coefficient (gm3), VIP2, VIP3, IIP3, and IMD3 are also investigated. The device simulations show that HD-EDTFET is more immune in terms of performance variation than conventional EDTFET with different interface trap charges available at the Si/SiO2 interface. Thus, it can be utilized as a suitable candidate for low power analog/RF applications.

Band offsets and trap-related electron transitions at interfaces of (100)InAs with atomic-layer deposited Al2O3

Spectral analysis of optically excited currents in single-crystal (100)InAs/amorphous (a-)Al2O3/metal structures allows one to separate contributions stemming from the internal photoemission (IPE) of electrons into alumina and from the trapping-related displacement currents. IPE spectra suggest that the out-diffusion of In and, possibly, its incorporation in a-Al2O3 lead to the development of ≈0.4 eV wide conduction band (CB) tail states. The top of the InAs valence band is found at 3.45 ± 0.10 eV below the alumina CB bottom, i.e., at the same energy as at the GaAs/a-Al2O3 interface. This corresponds to the CB and the valence band offsets at the InAs/a-Al2O3 interface of 3.1 ± 0.1 eV and 2.5 ± 0.1 eV, respectively. However, atomic-layer deposition of alumina on InAs results in additional low-energy electron transitions with spectral thresholds in the range of 2.0–2.2 eV, which is close to the bandgap of AlAs. The latter suggests the interaction of As with Al, leading to an interlayer containing Al-As bonds providing a lower barrier for electron injection.

Analog/RF performance of four different Tunneling FETs with the recessed channels

In this paper, the performance comparisons of analog and radio frequency (RF) in the four different tunneling field effect transistors (TFETs) with the recessed channels are performed. The L-shaped channel TFET (LTFET), U-shaped channel TFET (UTFET), U-shaped channel with L-shaped gate TFET (LGUTFET) and U-shaped channel with dual sources TFET (DUTFET) are investigated by using Silvaco-Atalas simulation tool. The transconductance (gm), output conductance (gds), gate capacitance (Cgg), cut-off frequency (fT) and gain bandwidth product (GBW) are the parameters by analyzed. Among all the considered devices, the DUTFET has the maximum gm and gds due to the improved on-state current by dual sources, and the LTFET has the minimum Cgg because of the minimum gate-to-drain capacitance (Cgd). Since analog/RF characteristics of a device are proportional to gm and inversely proportional to Cgg, the LTFET and DUTFET have better analog/RF performance compared to the UTFET and LGUTFET. The extracted largest fT is 3.02 GHz in the LTFET and the largest GBW is 1.02 GHz in the DUTFET. The simulation results in this paper can be used as a reference to choose the TFET among these four TFETs for analog/RF applications.

Efficient quantum transport in disordered interacting many-body networks.

The coherent transport of n fermions in disordered networks of l single-particle states connected by k-body interactions is studied. These networks are modeled by embedded Gaussian random matrix ensemble (EGE). The conductance bandwidth and the ensemble-averaged total current attain their maximal values if the system is highly filled n∼l-1 and k∼n/2. For the cases k=1 and k=n the bandwidth is minimal. We show that for all parameters the transport is enhanced significantly whenever centrosymmetric embedded Gaussian ensemble (csEGE) are considered. In this case the transmission shows numerous resonances of perfect transport. Analyzing the transmission by spectral decomposition, we find that centrosymmetry induces strong correlations and enhances the extrema of the distributions. This suppresses destructive interference effects in the system and thus causes backscattering-free transmission resonances that enhance the overall transport. The distribution of the total current for the csEGE has a very large dominating peak for n=l-1, close to the highest observed currents.

Electronic structure and optical properties of α-RDX crystal under pressure from first-principles calculations

ABSTRACTIn this study, we investigate the electronic structure and optical properties of α-RDX crystal under pressure from 0 to 4 GPa using first-principles calculations based on the density functional theory. When pressure increases, the calculations figure out that the band gap linearly decreases, the peaks of the total density of states are lowered, the valence bandwidths broadened and the conduction bandwidths compressed. The results also show that the optical properties including dielectric function, absorption spectrum, refractive index, reflectivity and energy-loss function respond to compression, that is, the intensity of the maximal peak of absorption coefficient increases from 7.07 × 104 to 7.98 × 104 cm−1 with corresponding photon energy from 5.10 to 5.15 eV, the static refractive index increases from 1.23 to 1.30, the intensity of the maximal peak of reflectivity function increases from 0.24 to 0.32 accompanied by its photon energy from 5.74 to 5.97 eV, and the plasma resonance energy and its intensity increases from 5.92 to 6.20 eV and 3.81 to 5.13, respectively.

Piezoelectric polarization and quantum size effects on the vertical transport in AlGaN/GaN resonant tunneling diodes**Project supported by the Deanship of Scientific Research of University of Dammam (Grant No. 2014137).

In this work, the electronic properties of resonant tunneling diodes (RTDs) based on GaN-AlxGa(1−x)N double barriers are investigated by using the non-equilibrium Green functions formalism (NEG). These materials each present a wide conduction band discontinuity and a strong internal piezoelectric field, which greatly affect the electronic transport properties. The electronic density, the transmission coefficient, and the current–voltage characteristics are computed with considering the spontaneous and piezoelectric polarizations. The influence of the quantum size on the transmission coefficient is analyzed by varying GaN quantum well thickness, AlxGa(1−x)N width, and the aluminum concentration xAl. The results show that the transmission coefficient more strongly depends on the thickness of the quantum well than the barrier; it exhibits a series of resonant peaks and valleys as the quantum well width increases. In addition, it is found that the negative differential resistance (NDR) in the current–voltage (I–V) characteristic strongly depends on aluminum concentration xAl. It is shown that the peak-to-valley ratio (PVR) increases with xAl value decreasing. These findings open the door for developing vertical transport nitrides-based ISB devices such as THz lasers and detectors.

Electrooxidation of Ethanol and Formic Acid on Core-Shell Nanoparticles with Different Platinum Shell Thickness

Core-shell nanoparticles with platinum as the shell and less expensive metal as a core allow for important reduction of the amount of expensive and rare platinum used while maintaining similar platinum active surface area. More efficient use of platinum is not the only benefit of core-shell nanoparticles. Platinum monolayer deposited on the core made from another metal exhibits modified catalytic and electronic properties. Partially responsible for this behavior are: i) charge transfer from the core metal and ii) changes induced by modified lattice parameter. The monolayer shell forms a so called “pseudomorphic monolayer” with lattice parameter equal to that of metal forming the core, resulting in altered conduction band width and center of d-band energy in relation to Fermi edge. In the case of platinum shell on a palladium core, contracted Pt lattice leads to broadening of the conduction band and shifting d-band center closer to the Fermi edge. Core-shell nanoparticles with platinum shell have been widely investigated as the catalyst in the process of oxygen reduction, but data available on their properties as the catalysts for oxidation of small organic molecules is scarce. Experimental investigation of such properties of core-shell nanoparticles with different platinum coverage and different platinum shell lattice parameter due to changes in shell thickness is an important step in determination of relation between electronic properties of the material used and reaction mechanism. Understanding this relation is crucial in constructing more effective direct ethanol fuel cell anode catalysts.

Real Time Relaxation Dynamics of Macroscopically Photo-Excited Electrons toward the Fermi Degeneracy Formation in the Conduction Band of Semiconductors

Concerning with the recent experiment of time-resolved two-photon photo-emission spectral measurements on semiconductors (GaAs, InP), we theoretically study real time relaxation dynamics of macroscopically photo-excited electrons, toward the Fermi degeneracy formation in an originally vacant conduction band of these semiconductors. Very soon after the photo-excitation, the whole electrons are shown to exhibit a quite rapid relaxation, like an avalanching phenomenon, mainly due to successive multi-(optical and acoustic) phonon emission from them. Repeating this multi-phonon process, the whole energy distribution of the electrons is shown to become a multi-peaked structure largely elongated over the lower part of the wide conduction band. However, after around 1 ps from the excitation, this relaxation critically slows down, since the emission of a long-wave acoustic phonon from electrons around the Fermi level becomes prohibitively difficult. By using the electron temperature approximation, we show that this slow relaxation is inversely proportional to time. Thus, the formation of the complete Fermi degeneracy takes an infinite time. These theoretical results are quite consistent to the aforementioned recent experiment.

Influence of Grinding Particle Size on Structure and Electrical Properties of La0.67Sr0.33MnO3−δ Compound

In this article, the influences of grinding particle size on the structure and electrical transport properties of La0.67Sr0.33MnO3−δ compound have been investigated. The compound was prepared by gel combustion process followed by grinding for different duration time. It was found that the lattice parameters, unit-cell volume, strain, oxygen deficiency, and temperature dependence of resistivity ρ–T increase with decreasing particle size. The ρ–T growth can be explained by the increasing contribution of the grain boundary effect, strain, and even oxygen deficiency. The electrical transport mechanism over the whole temperature range of 100–400 K was thoroughly described by applying the phase separation model around the phase transition temperature TP. This model was subsequently employed to calculate the ρ–T at high temperature up to 1000 K. The result showed there was a second phase transition at a typical temperature T*>TP, which could be attributed to the relaxation of small polarons to large ones. This relaxation occurred at lower temperatures when the particle size decreased. It might be due to the decrease in conduction bandwidth caused by the defects arisen during the grinding.

Electronic Properties of Mixed-Stack Organic Charge-Transfer Crystals

The electronic structures of a series of donor–acceptor mixed-stack crystals have been investigated by means of density functional theory calculations. The results highlight that a number of the donor–acceptor crystals under consideration are characterized by wide valence and conduction bands, large hole and electron electronic couplings, and as a result very low hole and electron effective masses. The fact that the effective masses and electronic couplings for holes and electrons are nearly equal along the stacking directions implies that the hole and electron mobilities in these systems are also similar. In addition, in several of these crystals, charge transport has a two-dimensional character. The impact on the charge transport properties of the electronic couplings between donor and acceptor frontier orbitals and of the related energy gaps is also discussed.

Selenium in Diketopyrrolopyrrole‐based Polymers: Influence on Electronic Properties and Charge Carrier Mobilities

AbstractDiketopyrrolopyrrole (DPP)‐based π‐conjugated copolymers with thiophene have exceptionally high electron mobilities. This paper investigates electronic properties and charge carrier mobilities of selenophene containing analogues. Two new copolymers, with alternating thiophene DPP (TDPP) and selenophene DPP (SeDPP) units, were synthesized. Two side‐chains, hexyl (Hex) and triethylene glycol (TEG) were employed, yielding polymers designated as PTDPPSeDPP‐Hex and PTDPPSeDPP‐TEG. Selenophene systems have smaller band gaps, with concomitant enhancement of the stability of the reduced state. For both polymers, ambipolar mobilities were observed in organic field‐effect transistors (OFET). Grazing incidence X‐ray diffraction (GIXD) data indicates preferential edge‐on orientation of PTDPPSeDPP‐TEG, which leads to superior charge transport properties of the TEG substituted polymer, as compared to its Hex analogue. Time‐dependent‐density functional theory (TDDFT) calculations corroborate the decrease in the optical band gap with the inclusion of selenophene. Ambipolar charge transport is rationalized by exceptionally wide conduction bands. ΔSCF calculations confirm the larger electron affinity, and therefore the greater stability, of the reduced form of the selenophene‐containing DPP polymer in presence of chloroform.

Conducción hopping en películas nanocristalinas del compuesto CZTSe usado como capa absorbente en celdas solares

Here, we present electronic and transport properties of quaternary Cu2ZnSnSe4 (CZTSe) nanocrystalline films fabricated by physical co-evaporation. The samples were grown on soda-lime glass substrates and synthesis parameter ranges, Cu mass and substrate temperature were varied. Using thermopower at room temperature and spectral transmittance we found that the material is characterized by n-type conductivity and forbidden energy bandwidth of 1.7 eV, respectively. Electrical conductivity means (low temperature region; 90-200 K) showed that conductivity processes occur via variable range hopping between extended states. We obtained the parameters characterizing this mechanism, activation energy (Whopp), and range hopping (Rhopp), by employing the percolation theory and diffusion model. The density of defect states near the Fermi level of the material, N (EF) of the CZTSe samples is about 3,403x1018 cm-3 eV-1. We found a correlation between deposition parameters and electrical properties and observed a parameter influence on the formation of additional phases in the compound.