Values Of Chemical Potential Research Articles

Thermoelectric properties and thermal stability of ferromagnetic half metallic CoVTe alloy, first principles study

ABSTRACT Thermoelectric properties and the thermal stability of ternary half-Heusler compound CoVTe are investigated for the first time, using the first principles computations. It is found that CoVTe alloy is an indirect half metallic ferromagnetic alloy. The integral value of the total magnetic moment obeys the Slater-Pauling rule which is in accordance with the half-metallic nature of this compound. The negative value of the formation energy EF and the positive value of the cohesive energy Ec show that this alloy can be easily produced experimentally with exothermic formation processes. The thermoelectric properties are computed at different temperatures from 0 to 900 K by step of 50 K and at different chemical potential values from −2 to 2 eV. Moreover, the lattice dynamical results regarding the phonon dispersion curves and phonon density of states are obtained and discussed. The investigated thermodynamic properties are in line with the results obtained by the Debye theory applied to several materials. The attractive magnetic half metallicity as well as the interesting thermoelectric properties make this alloy a good candidate for spintronic and thermoelectric applications.

The surface energy phase diagrams of CO adsorption on the low index iridium surfaces and the morphology of iridium nanoparticles

The stability of molecular CO over the unreconstructed low-index surfaces was studied using ab initio atomistic thermodynamics. Results for Ir(100) in a diluted CO environment indicated that the 0.5 ML-phase was the most stable structure, whereas, in a concentrated CO medium, a 0.75 ML-phase exhibited the highest stability. Results of CO/Ir(110) systems indicated the presence of three stable configurations at 0.5, 0.75, and 1.0 ML, which are in agreement with reported experimental findings. Additional results revealed that among all CO/Ir(111) systems, the most stable structures were at 0.38, 0.44, and 0.81 ML coverages of CO. Shapes of Ir nanoparticles were predicted at different values of CO chemical potential using the Wulff construction. A truncated-tetrahedron shape was obtained for nanoparticles at low CO chemical potential values due to contributions of (100) and (111) facets. On the other hand, a cubic shape was obtained for nanoparticles at large CO chemical potential values. This was attributed to the sole contributions of the (100) surface to the Wulff construction.

A reconfigurable beam sweeping patch antenna utilizing parasitic graphene elements for terahertz applications

A terahertz beam steering microstrip patch antenna is designed in this manuscript. The metallic radiating patch is surrounded by six parasitic graphene ribbons which are located at lateral, upper, and lower sides of main radiating patch. The embedded graphene ribbons are employed to change the antenna boresight in θ-constant plane. The states of chemical potential values for graphene elements are switched between 0 and 1 eV to enhance beam steering. Two lateral ribbons with chemical potential of 1 eV act as director elements whereas four upper and lower parasitic elements have reflector role resulting in a deflected beam into opposite direction. As a result, an overall 120° beam steering (30°~150°) is obtained. It is notable that proposed antenna benefits from a fixed operational frequency of 1.47 THz in all considered configurations.

Bismuth-containing semiconductors GaAs1−xBix for energy conversion: Thermoelectric properties

The electronic transport properties of GaAs1−xBix alloys, are obtained using the semi-classical Boltzmann theory as incorporated in BoltzTraP code. The thermoelectric properties as a function of temperature at a constant value of chemical potential (μ) were calculated for the GaAs1−xBix alloys at x = 0.0–1.0 with stepsize of 0.25. GaAs1−xBix alloys are modeled using special quasi-random structures’ (SQS) Zunger approach. The GaAs1−xBix alloys have a direct band gap of 1.42 eV, 0.449 eV, 0.052 eV, 0.016 eV and −0.0056 eV for x = 0.0, 0.25, 0.50, 0.75 and 1.0, respectively. It is clearly seen that the energy gap decreases with increasing Bi concentration. The electronic band structures of GaAs1−xBix alloys show that the bands are less dispersive for all high symmetry directions, suggesting that these alloys possess large effective mass for the carriers and hence a high thermopower. Calculation show that GaAs0.75Bi0.25 exhibits high carrier concentration and hence high electronic conductivity and power factor, whereas GaAs shows the highest value for Seebeck coefficient. GaBi exhibits the lowest values for most of the transport properties. The GaAs1-xBix (x = 0.0, 0.25, 0.5, 0.75) alloys represents the p-type carrier except GaBi represent n-type at low temperate till 300 K then above this temperature it represents the p-type carrier. We should mention here the fluctuation in the values of σ/τ with changing the content of Bi atoms is attributed to the mobility and the concentration of the charge carrier. Finally substituting all As atoms by Bi atoms (GaBi) leads to reduce ke/τ to lower than that of GaAs. GaAs1−xBix alloys could be promising materials for thermoelectric applications due to the decrease in thermal conductivity with increasing x because bismuth is a heavy atom (better phonon scattering).

Computer simulation investigation of the adsorption of acetamide on low density amorphous ice. An astrochemical perspective.

The adsorption of acetamide on low density amorphous (LDA) ice is investigated by grand canonical Monte Carlo computer simulations at the temperatures 50, 100, and 200K, characteristic of certain domains of the interstellar medium (ISM). We found that the relative importance of the acetamide-acetamide H-bonds with respect to the acetamide-water ones increases with decreasing temperature. Thus, with decreasing temperature, the existence of the stable monolayer, characterizing the adsorption at 200K, is gradually replaced by the occurrence of marked multilayer adsorption, preceding even the saturation of the first layer at 50K. While isolated acetamide molecules prefer to lay parallel to the ice surface to maximize their H-bonding with the surface water molecules, this orientational preference undergoes a marked change upon saturation of the first layer due to increasing competition of the adsorbed molecules for H-bonds with water and to the possibility of their H-bond formation with each other. As a result, molecules stay preferentially perpendicular to the ice surface in the saturated monolayer. The chemical potential value corresponding to the point of condensation is found to decrease linearly with increasing temperature. We provide, in analogy with the Clausius-Clapeyron equation, a thermodynamic explanation of this behavior and estimate the molar entropy of condensed phase acetamide to be 34.0 J/mol K. For the surface concentration of the saturated monolayer, we obtain the value 9.1 ± 0.8 µmol/m2, while the heat of adsorption at infinitely low surface coverage is estimated to be -67.8 ± 3.0 kJ/mol. Our results indicate that the interstellar formation of peptide chains through acetamide molecules, occurring at the surface of LDA ice, might well be a plausible process in the cold (i.e., below 50K) domains of the ISM; however, it is a rather unlikely scenario in its higher temperature (i.e., 100-200K) domains.

Graphene-based ultra-wideband absorber for terahertz applications using hexagonal split ring resonators

In this paper, a hexagonal split ring ultra-wideband absorber is proposed at THz frequency. The proposed structure consists of four graphene based hexagonal split rings, a dielectric substrate and graphene layer at the bottom. The proposed absorber achieves an ultra-wideband absorption characteristics from 0.95 THz to 2.96 THz with percentage bandwidth of 102.8% and bandwidth of 2.1THz with absorptivity beyond 90%. Also, 100% absorption is achieved from 2.07 to 2.33 THz making this unique feature for THz applications. Ultra-wideband response is mainly resulted because of overlapping of strong Electromagnetic (EM) resonance of the four split rings. The modes generated within the graphene split rings are merged for achieving ultra wide band response. Furthermore, the proposed absorber is polarization insensitive because of symmetry geometry and also exhibits absorption greater than 90% for incidence angle up to 75° for both TE and TM waves. In addition, tunability is achieved by varying the graphene chemical potential. These features make the proposed metal free absorber useful for terahertz applications and future nanoscale systems. The proposed absorber shows narrow absorption characteristics for higher graphene chemical potential values which can also be utilized for sensor applications.

Dilepton production from chirally asymmetric matter

We evaluate the dilepton production rate (DPR) from hot and dense chirally asymmetric quark matter. The presence of a finite chiral chemical potential (CCP) in the electromagnetic spectral function results in the appearance of new cut structures signifying additional scattering processes in the medium which leads to a significant enhancement in the DPR at lower values of invariant mass. The constituent quark mass evaluated using a 3-flavour Nambu--Jona-Lasinio model is also non-trivially affected by the CCP. These are found to result in a continuous dilepton production rate as a function of the invariant mass for higher values of temperature and baryonic chemical potential.

Mobility and thermoelectric properties of semiconducting diamanes

In this paper, mobility and thermoelectric properties of diamanes C2X (X = H, F, Cl) are studied using quantum espresso and Boltztrap computational package based on density functional theory. In all structures of C2X (X = H, F, Cl), the mobility of holes is smaller than the mobility of the electrons, which is due to the shape of the band structure of each structure. Maximum of Seebeck coefficient is 2733, 2811, 2201 µV/K for n-type C2H, C2Fand C2Cl and it is -2767, -2696 and -2269 µV/K for p-type C2H, C2Fand C2Cl, respectively. In all structures, thermoelectric parameters such as electrical conductivity, electrical thermal conductivity and power factor are maximum in positive values of chemical potential. As a result, these materials can be more suitable thermoelectric materials with n-type doping. Also, all three structures have a maximum power factor in the temperature range of 200-500 Kelvin.

Taylor expansions and Padé approximants for cumulants of conserved charge fluctuations at nonvanishing chemical potentials

Using high statistics datasets generated in ($2+1$)-flavor QCD calculations at finite temperature, we present results for low-order cumulants of net-baryon-number fluctuations at nonzero values of the baryon chemical potential. We calculate Taylor expansions for the pressure (zeroth-order cumulant), the net-baryon-number density (first-order cumulant), and the variance of the distribution on net-baryon-number fluctuations (second-order cumulant). We obtain series expansions from an eighth-order expansion of the pressure and compare these to diagonal Pad\'e approximants. This allows us to estimate the range of values for the baryon chemical potential in which these expansions are reliable. We find ${\ensuremath{\mu}}_{B}/T\ensuremath{\le}2.5$, 2.0, and 1.5 for the zeroth-, first-, and second-order cumulants, respectively. We, furthermore, construct estimators for the radius of convergence of the Taylor series of the pressure. In the vicinity of the pseudocritical temperature ${T}_{pc}\ensuremath{\simeq}156.5\text{ }\text{ }\mathrm{MeV}$, we find ${\ensuremath{\mu}}_{B}/T\ensuremath{\gtrsim}2.9$ at vanishing strangeness chemical potential and somewhat larger values for strangeness neutral matter. These estimates are temperature dependent and range from ${\ensuremath{\mu}}_{B}/T\ensuremath{\gtrsim}2.2$ at $T=135\text{ }\text{ }\mathrm{MeV}$ to ${\ensuremath{\mu}}_{B}/T\ensuremath{\gtrsim}3.2$ at $T=165\text{ }\text{ }\mathrm{MeV}$. The estimated radius of convergences is the same for any higher-order cumulant.

Quark matter properties and fluctuations of conserved charges in (2+1)-flavored quark model * *Supported by the DST-SERB, Government of India for funding of research project (CRG/2019/000096)

In this study, the susceptibilities of conserved charges, baryon number, charge number, and strangeness number at zero and low values of chemical potential are presented. Taylor series expansion was used to obtain results for the three-flavor Polyakov quark meson (PQM) model and the Polyakov loop extended chiral quark mean-field (PCQMF) model. Mean-field approximation was used to study quark matter with the inclusion of the isospin chemical potential, as well as the vector interactions. The effects of isospin chemical potential and vector-interactions on phase diagrams were analyzed. A comparative analysis of the two models was completed. Fluctuations of the conserved charges were enhanced in the transition temperature regime and hence provided information about the critical end point (CEP). Susceptibilities of conserved quantities were calculated by using the Taylor series method. Enhancement of fluctuations in the transition temperature neighborhood provided a clear signature of a quantum chromodynamics (QCD) critical-point.

An equilibrium ab initio atomistic thermodynamics investigation of hydrogen adsorption on the low index iridium surfaces and the morphology of iridium nanoparticles

The ab initio atomistic thermodynamics technique was used to calculate the surface free energy diagrams. These diagrams were obtained for hydrogen over the unreconstructed low-index iridium surfaces. Results indicated that the Ir(110) surface acquired the highest hydrogen intake among the low-index surfaces with a coverage of up to 2.33 ML. However, high-pressure hydrogen conditions were essential to maintain the stability of this phase. The Ir(100) surface required −0.81 eV of hydrogen chemical potential to produce a stable H/Ir phase with a 1.0 ML coverage. This potential represented the lowest value among the low-index surfaces. Phase diagram calculations of H/Ir(111) illustrated that the 0.75 ML phase has the highest stability at low to moderate hydrogen pressures. On the other hand, the 1.25 ML-H/Ir(111) phase became the most stable under high-pressure conditions. Finally, Wulff construction was implemented to predict the shapes of iridium nanoparticles in a hydrogen environment. Predictions indicated a truncated-octahedron shape for the nanoparticles irrespective of the value of the hydrogen chemical potential.

Gate tunable light–matter interaction in natural biaxial hyperbolic van der Waals heterostructures

Abstract The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, has opened up new avenues for mid-IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α-MoO3 for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid-infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography-free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub-wavelength thick naturally biaxial hyperbolic material.

Removal of Heavy Metal Ions from Water Using Functionalized Carbon Nanosheet: A Density-Functional Theory Study

Background: Currently, a clean environment and human health have been regarded as the most important challenges of humankind; therefore, the ability of carbon-based nanomaterials as a new type of adsorbents in the removal of various pollutants from aqueous solutions has caused the widespread attention of research groups. Objectives: Carbon nanosheet (CNS) and its amine-functionalized derivatives were considered to remove heavy metal ions (HMIs) (zinc(II) ion [Zn2+]/cadmium(II) ion [Cd2+]/mercury(II) ion [Hg2+]) from water. Methods: This theoretical study in the framework of density-functional theory (DFT) was performed to gain insight into the HMIs (Zn2+/Cd2+/Hg2+) adsorption and removal by CNS and {(NH2)n-CNS (n = 1 - 2)}. The calculation level was hybrid DFT methods, such as wB97XD/lanl2dz. Results: Based on DFT calculations, more negative chemical potential value (-1.827 eV), high global softness (0.964 eV), low highest occupied molecular orbital (NH2)2-CNS lowest unoccupied molecular orbital-HMI gap, and more negative adsorption energy (range: -75 to -93 kJ mol-1) in (NH2)2-CNS demonstrated that this compound as a suitable adsorbent removed HMIs (Zn2+/Cd2+/Hg2+) from water than other nanosheets. The HMIs adsorption was confirmed by the natural bond orbital and quantum theory of atoms in molecules. Conclusions: The CNS and amine-functionalized structures have a good ability for HMIs (Zn2+/Cd2+/Hg2+) removal due to high chemical potential, electrophilicity, and adsorption energy.

Temperature-dependent effect of modulation in graphene-supported metamaterials

We report on a novel effect of temperature-dependent modulation in graphene-supported metamaterials. The effect was observed during the theoretical analysis of a model graphene-supported electro-optical modulator having silicon dioxide (SiO2) or hafnium dioxide (HfO2) as a buffer dielectric layer. Comparative analysis of the two materials showed that they provide approximately the same maximum values for transmission and reflection modulation depths. However, in the case of a HfO2 buffer layer, a lower chemical potential of the graphene is required to achieve the maximum value. Moreover, theoretical calculations revealed that a lower gate voltage (up to 6.4 times) is required to be applied in the case of a HfO2 layer to achieve the same graphene chemical potential. The graphene layer was found to possesses high absorption (due to the additional resonance excitation) for some values of chemical potential and this effect is extremely temperature dependent. The discovered modulation effect was demonstrated to further increase the transmission modulation depth for the simple model structure up to 2.7 times (from 18.4% to 50.1%), while for the reflection modulation depth, this enhancement was equal to 2.2 times (from 24.4% to 52.8%). The novel modulation effect could easily be adopted and applied over a wide range of metadevices which would serve as a quick booster for the development of related research areas.

Adsorption of acetamide on crystalline and amorphous ice under atmospheric conditions. A grand canonical Monte Carlo simulation study

The adsorption of acetamide at the surface of crystalline (Ih) and low density amorphous (LDA) ices under tropospheric conditions is investigated by performing a set of grand canonical Monte Carlo simulations at the temperature of 200 K. Our results show that the adsorption is governed by the possibility of new H-bond formation. Further, acetamide-water and acetamide-acetamide hydrogen bonds are found to be of rather similar strengths, and hence these H-bonds are exchangeable with each other around an adsorbed molecule for practically no energy cost. In the case of monolayers being well below the saturation surface concentration, acetamide molecules prefer to stay parallel with the ice surface and form 3H-bonds with the surface waters. The energy of the adsorption at infinitely low surface concentration is estimated to be –64.5 kJ/mol on LDA and –63.7 kJ/mol on Ih ice. Increasing saturation of the surface triggers a marked change in the orientational preference of the adsorbed molecules, thus, once the interaction between the adsorbed molecules is no longer negligible (which occurs already at mildly unsaturated coverages) they prefer to stay perpendicular to the surface, forming 2 H-bonds with the ice phase and another 2 with each other. As a consequence, the ice and adsorbed phases give similar contributions to the adsorption energy at this coverage, which results in a remarkable stability of the adsorption monolayer. Indeed, after their initial rise the adsorption isotherms exhibit a plateau, corresponding to the more or less saturated adsorption monolayer, in a rather broad range of chemical potential values. The surface concentration of the saturated adsorption monolayer is estimated to be 10.2 μmol/m2 on LDA and 8.1 μmol/m2 on Ih ice. Besides adsorption, non-negligible dissolution of the acetamide molecules is found in both ice phases; however, the concentration of the dissolved molecules is still low enough that the solution exhibits ideal behaviour.

Semi-analytical Equations for Designing Terahertz Graphene Dipole Antennas on Glass Substrate

Semi-analytical equations are developed for aiding the process of designing terahertz graphene-based rectangular dipole antennas lying on glass substrates. It directly provides the dipole length required for obtaining resonance at a desired frequency since antenna width and graphene chemical potential are known. By using the finite-difference time-domain (FDTD) method, a large number of computational simulations were performed considering several combinations of antenna dimensions and chemical potential values. The simulation results were used along with graphene electrostatic scaling law combined with the least squares method to optimize the formulation coefficients. With the optimized coefficients, we obtain very satisfying accuracy levels. In the frequency range from 0.5 THz to 3.0 THz, the average relative absolute error is 1.50%, with maximum relative absolute error of 6.77%.

A reconfigurable narrow and wide band multi bias graphene based THz absorber

A three layers graphene-based THz absorber composed of multi-bias graphene ribbons is presented. The structure is described using equivalent circuit model and design methodology is developed via impedance matching concept. Also, relatively heavy optimization is utilized to obtain two sets of chemical potential values. Two operational behaviors are expressed corresponding to each bias set. Leveraging both equivalent circuit representation and impedance matching concept, the proposed structure is able to show both narrow multi band and wide band absorption over 0.1 THz to 5 THz frequency range. The first mode of operation can perfectly absorb THz incident waves in 0.5 THz, 1.4 THz, 2.35 THz, 3.23 THz and 4.11 THz while the second mode of operation absorbs THz incident waves between 0.5 THz to 2.5 THz as wide band response. Additionally, to investigate device dependency against geometrical parameters, electron relaxation time, chemical potentials and incident angle, ample simulations are reported for both modes of operations. Achieving both multi band and wide band absorption via a unique structure, makes the proposed device an ideal candidate to be used in optical systems and sensors for medical imaging, indoor communications and security.

Hydrogen adsorption on Ni doped carbon nanocone

Hydrogen adsorption was investigated on Ni doped carbon nanocone (Ni-CNC) 180° by Density Functional Theory (DFT). The WB97XD method was used with the 6-31G(d,p) and LanL2DZ basis sets. The electronegativity, HOMO and LUMO energies, chemical hardness, chemical potential, adsorption enthalpy and adsorption energy values have been calculated for hydrogen adsorption on Ni-CNC structure. The adsorption enthalpy value of hydrogen molecule was calculated as −27.8 kJ/mol. According to the storage capacity study for the Ni-CNC structure, it was found that this structure adsorbed eight hydrogen molecules and the hydrogen storage capacity was calculated to be ~4.3 wt%. After hydrogen adsorption, there was no significant decrease in HOMO-LUMO gap value. In addition, this situation indicates that the electrical conductivity of the Ni-CNC structure does not increase. These results show that the Ni-CNC structure cannot be used as a sensor for the hydrogen molecule, but it is a promising candidate material for hydrogen storage at room temperature.

Adsorption-active polydisperse brush with tunable molecular mass distribution.

Recently, a novel class of responsive uncharged polymer brushes has been proposed [Klushin et al., J. Chem. Phys. 154(7), 074904 (2021)] where the brush-forming chains have an affinity to the substrate. For sufficiently strong surface interactions, a fraction of chains condenses into a near-surface layer, while the remaining ones form the outer brush with a reduced grafting density. The dense layer and the more tenuous outer brush can be seen as coexisting microphases. The effective grafting density of the outer brush is controlled by the adsorption strength and can be changed reversibly as a response to changes in environmental parameters. In this paper, we use numerical self-consistent field calculations to study this phenomenon in polydisperse brushes. Our results reveal an unexpected effect: Although all chains are chemically identical, shorter chains are adsorbed preferentially. Hence, with the increase in the surface affinity parameter, a reduction in the surface grafting density of the residual brush is accompanied by a change in the shape of its molecular mass distribution (MMD). In particular, an originally bidisperse brush can be effectively transformed into a nearly monodisperse one containing only the longer chain fraction. We introduce a method of assigning different chain conformations to one or the other microphase, based on analyzing tail length distributions. In a polydisperse brush with a uniform MMD, short chains are relegated to the adsorbed phase, leading to a narrower effective MMD in the residual brush. Preferential adsorption is not absolute, and longer chains are also partially involved in adsorption. As a result, not only the width of the distribution decreases but also its shape evolves away from the initial uniform distribution. We believe that the effect of preferential adsorption stems from a fundamental property of a polydisperse brush, which is characterized by a spectrum of chemical potential values for monomers belonging to chains of different lengths. Hence, preferential adsorption is also expected in polyelectrolyte brushes; moreover, brush polydispersity would affect coexistence with any other condensed phase, not necessarily related to adsorption.

Modeling structural and energy characteristics of atoms in a GaS 2D-crystal with point defects

The properties of hexagonal gallium monosulphide (GaS) were modeled within the framework of the density functionality theory (DFT) taking into account the impact of vacancies related to a short range ordered structure. It is shown that electron irradiation of a GaS monolayer causes a decrease in conductivity due to formation of point defects. The band structure, density of states and energy properties of GaS supercells with 36 and 48 atoms with monovacancies were calculated. DFT calculations were performed to obtain values of the formation energy of GaS and Ga and S atom vacancies, as well as to determine the degree of vacancies' impact on the properties. Impact of GaS compound composition on the chemical potential value was studied taking into account the Ga-S phase diagram. Key words: modeling, DFT calculation, GaS supercells, point defects, energy structure, density of states, formation energy, chemical potential.