Semiclassical Theory Research Articles

Nuclear level density in the statistical semiclassical micro-macroscopic approach

Level density ρ is derived for a finite system with strongly interacting nucleons at a given energy E, neutron N, and proton Z particle numbers, projection of the angular momentum M, and other integrals of motion, within the semiclassical periodic-orbit theory (POT) beyond the standard Fermi-gas saddle-point method. For large particle numbers, one obtains an analytical expression for the level density which is extended to low excitation energies U in the statistical micro-macroscopic approach (MMA). The interparticle interaction averaged over particle numbers is taken into account in terms of the extended Thomas - Fermi component of the POT. The shell structure of spherical and deformed nuclei is taken into account in the level density by the Strutinsky shell correction method through the mean-field approach used near the Fermi energy surface. The MMA expressions for the level density ρ reaches the well-known macroscopic Fermi-gas asymptote for large excitation energies U and the finite combinatoric power-expansion limit for low energies U. We compare our MMA results for the averaged level density with the experimental data obtained from the known excitation energy spectra by using the sample method under statistical and plateau conditions. Fitting the MMA ρ to these experimental data on the averaged level density by using only one free physical parameter - inverse level density parameter K - for several nuclei and their long isotope chain at low excitation energies U one obtains the results for K. These values of K might be much larger than those deduced from neutron resonances. The shell, isotopic asymmetry, and pairing effects are significant for low excitation energies.

Ultralow lattice thermal conductivity and thermoelectric performance of twisted Graphene/Boron Nitride heterostructure through strain engineering

We designed and investigated the electronic, mechanical, and thermoelectric properties of Graphene/hexagonal Boron Nitride (Gr/h-BN) heterostructure at various twisting angles based on the Ab-initio simulation. The structural stability was studied at optimized rotation angles (ϕ) = 0∘, 16.10∘, 21.79∘, 38.21∘, 43.90∘ and 60∘. The heterostructure shows semiconducting nature at ϕ=0∘, 21.79∘ and 38.21∘. These twisted heterostructures have demonstrated extraordinary mechanical properties such as Young’s modulus and bulk modulus. Using the semiclassical Boltzmann transport theory, it is observed that the Seebeck coefficient, electric conductivity, and power factor at ϕ=0∘, 21.79∘, 38.21∘, and 60∘ are much higher than the values measured at ϕ=16.10∘ and 43.90∘. Moreover, at ϕ=60∘, the Power Factor for the n-type dopants can reach 1.37 × 1011 W/msK2. The lattice thermal conductivity at room temperature is found to be very low for ϕ=16.10∘, 21.79∘, 43.90∘ and 38.21∘ rotation angles. An ultralow lattice thermal conductivity with a value of 0.095 W/mK at 300K has been observed for 21.79∘ rotation angle, which is lower than other rotation angles because of very low group velocity (22.1 km/s) and short phonon lifetime (∼0.12 ps). The high thermoelectric performance results from an ultralow thermal conductivity arising due to the strong lattice anharmonicity. The present observations can offer significant impact on the design of high performance thermoelectric materials based on twisted van der Waals heterostructure (vdWH).

DFT modeling techniques to understand the potential applicability of novel double perovskite semiconductor Ba2LuNbO6 for sustainable thermoelectrics and optoelectronic perspectives

Searching for novel functional materials represents an important direction in the research and development of renewable energy. Herein, experimentally synthesised Ba2LuNbO6 (BLNO) host double perovskite has been theoretically computed by first principles techniques to understand its several physical properties like electronic-structure, mechanical behaviour, transport along with optical and thermal properties upon the basis of full potential linearized augmented plane wave (FP-LAPW) followed by density functional theory (DFT). First and foremost, the attempt was significant made to relax the molecular crystal structure of BLNO at an experimental lattice constant to evaluate its total ground state and cohesive energy (Ecoh) for descripting its required structural stability. After that, we have overseen its mechanical stability from the computation of second order elastic constants (SOEC's). Later on, the electronic properties of this alloy have been executed within the two potential schemes like Perdew-Burke Generalized gradient approximation (PBE-GGA) and Tran-Blaha modified Becke-Johnson (TB-mBJ) which alternatively certifies the retention of the semiconducting nature respectively. Besides this, the essential use of semi-classical Boltzmann theory within the sophisticated scheme of BoltzTraP has been inducted to explore its transport coefficients. And finally, the optical has been summarised to see the applicability of this particular alloy towards optoelectronic application purposes. Therefore, the overall tendency of this particular alloy can favor their potential multiple applications in sustainable thermoelectrics, optoelectronic features etc.

Comprehensive investigation of electronic structure, phonon spectrum and thermoelectric performance of LuMSb (M = Ni, Pd, Pt) half Heusler compounds from first principles.

We studied the structural, electronic, phonon spectrum and thermoelectric properties of ternary LuMSb (M = Ni, Pd, Pt) half Heusler compounds by using first principles method. The electronic properties are calculated via energy band structure and density of states by using GGA + U approximation. The calculations reveal that the replacement of Ni with Pd and Pt, energy gap decreases and LuNiSb, LuPdSb are found to have narrow indirect band gaps and exhibit semiconducting nature, while LuPtSb is found to be a gapless semiconductor. Phonon band structure calculations give only positive values of phonon frequency indicating the dynamically stability of these compounds. The thermoelectric properties have been computed using semi-classical Boltzmann transport theory. We found high Seebeck coefficient (S) and high power factor (PF) for LuNiSb and LuPdSb compounds in the whole temperature range. The ZT values of LuNiSb and LuPdSb are high in general and reach a maximum of 0.67 and 0.69 at 450 K, respectively, whereas 0.39 is the maximum ZT value for LuPtSb at the same temperature. These findings propose LuNiSb and LuPdSb compounds as promising materials for thermoelectric applications at room temperature.

Exact large charge in mathcal{N} = 4 SYM and semiclassical string theory

In four-dimensional mathcal{N} = 4 super Yang-Mills theory with gauge group SU(N), we present a closed-form solution for a family of integrated four-point functions involving stress tensor multiplet composites of arbitrary R-charge. These integrated correlators are shown to be equivalent to a one-dimensional semi-infinite lattice of harmonic oscillators with nearest-neighbor interactions, evolving over the fundamental domain of SL(2, ℤ). The solution, exceptionally simple in an SL(2, ℤ)-invariant eigenbasis, is exact in the R-charge p, rank N, and complexified gauge coupling τ. This permits a systematic and non-perturbative large charge expansion for any N and τ. Especially novel is a double-scaled “gravity regime” in which p ~ N2 ≫ 1, holographically dual to a large charge regime of semiclassical type IIB string theory in AdS5 × S5. Our results in this limit provide a holographic computation of integrated semiclassical string amplitudes at arbitrary string coupling, including an emergent string scale with a large charge dressing factor. We compare to extremal correlators in superconformal QCD, for which we predict new genus expansions at large charge scaling with N.

Semiclassical theory for plasmons in two-dimensional inhomogeneous media

Semiclassical theory for plasmons in two-dimensional inhomogeneous media

Photoproduct formation in coenzyme B12-dependent CarH photoreceptor via a triplet pathway.

CarH is a cobalamin-based photoreceptor which has attracted significant interest due to its complex mechanism involving its organometallic coenzyme-B12 chromophore. While several experimental and computational studies have sought to understand CarH's mechanism of action, there are still many aspects of the mechanism which remain unclear. While light is needed to activate the Co-C5' bond, it is not entirely clear whether reaction pathway involves singlet or triplet diradical states. A recent experimental study implicated triplet pathway and importance of intersystem crossing (ISC) as a viable mechanistic route for photoproduct formation in CarH. Herein, a combined quantum mechanics/molecular mechanics approach (QM/MM) was used to explore the involvement of triplet states in CarH. Two possibilities were explored. The first possibility involved photo-induced homolytic cleavage of the Co-C5' where the radical pair (RP) would deactivate to a triplet state (T0) on the ground state potential energy surface (PES). However, a pathway for the formation of the photoproduct, 4',5'-anhydroadenosine (anhAdo), on the triplet ground state PES was not energetically feasible. The second possibility involved exploring a manifold of low-lying triplet excited states computed using TD-DFT within the QM/MM framework. Viable crossings of triplet excited states with singlet excited states were identified using semiclassical Landau-Zener theory and the effectiveness of spin-orbit coupling by El-Sayed rules. Several candidates along both the Co-NIm potential energy curve (PEC) and Co-C5'/Co-NIm PES were identified, which appear to corroborate experimental findings and implicate the possible role of triplet states in CarH.

Stretch effects on structural, electronic, optical, and thermoelectric features of LiGeBr3: A DFT calculations

Strain engineering is generally employed as an efficient means of tuning the physical properties of a target compound. In this research paper, we investigated the effect of three axial stretches on the structural, electronic, optical, and thermoelectric properties of inorganic Ge-based halide perovskites LiGeBr3. Computations were carried out using Density Functional Theory (DFT) and the semi-classical Boltzmann transport theories. The electronic investigation indicated that LiGeBr3 perovskite exhibits a direct bandgap of 0.480[Formula: see text]eV. The findings suggest that the bandgap is highly responsive to strain, which improves the absorption ability in the visible light range (300–500[Formula: see text]nm). The relative stability of the strained compounds and the feasibility of their synthesis were robustly demonstrated by the negative formation energies. The transport features were assessed as a function of temperature and yielded interesting results. The electrical conductivity was considerably enhanced under strain, and the highest figure of merit was found at low temperatures (approximately 0.741). Our theoretical discovery proved that strain is an enormously exciting technique for extending semiconductor applications and boosting thermoelectric and solar cell devices.

Epitaxial Kagome Thin Films as a Platform for Topological Flat Bands.

Systems with flat bands are ideal for studying strongly correlated electronic states and related phenomena. Among them, kagome-structured metals such as CoSn have been recognized as promising candidates due to the proximity between the flat bands and the Fermi level. A key next step will be to realize epitaxial kagome thin films with flat bands to enable tuning of the flat bands across the Fermi level via electrostatic gating or strain. Here, we report the band structures of epitaxial CoSn thin films grown directly on the insulating substrates. Flat bands are observed by using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). The band structure is consistent with density functional theory (DFT) calculations, and the transport properties are quantitatively explained by the band structure and semiclassical transport theory. Our work paves the way to realize flat band-induced phenomena through fine-tuning of flat bands in kagome materials.

Stochastic semiclassical theory for nonequilibrium electron-phonon coupled systems

Stochastic semiclassical theory for nonequilibrium electron-phonon coupled systems

Tuning the electronic and thermoelectric properties of selenium monolayers through atomic impurities: A DFT study

The structural, electronic, and thermoelectric properties of selenium monolayer with impurity adsorption and substitutional atoms have been studied using density functional theory (DFT) combined with semiclassical Boltzmann theory. The adsorption energy of the impurity adatoms in their stable position ranged from −1.46 eV for Te to −4 .44 eV for Pt. Regarding the atomic substitution, Pt impurities exhibited the highest stability in the intermediate position, with a formation energy of −150.5 meV, followed by Sn with a formation energy of −90.40 meV. We further investigated the impact of atomic impurities on the electronic properties of the material and found that the bandgap energy was significantly reduced, resulting in a semiconductor-to-metal transition. These results highlight the effectiveness of defect engineering for electronic band tuning. Furthermore, we discovered that a selenium monolayer with substitution of an Sn or Te atom in the intermediate layer exhibits a maximum value of the dimensionless figure of merit (including electronic and phononic contributions) of 0.51 and 0.52, respectively, while the clean selenium monolayer has a maximum value of ZT=0.43. These finding suggest that selenium monolayer with Sn defects could be a promising thermoelectric materials that offer an alternative for recovering waste heat and transforming it into electricity.

Semiclassical kinetic theory for systems with non-trivial quantum geometry and the expectation value of physical quantities

Starting from the Keldysh theory, for a general low-energy N-band Hamiltonian in the clean limit, we perform a manifestly gauge-invariant semiclassical expansion. A generalized Berry curvature tensor is shown to control a redistribution of spectral weights. New expressions for certain physical quantities ensue, establishing the limits of a previously proposed correction to the density of states. In the two-band case, we derive a completely general semiclassical kinetic theory including all quantum corrections. As an application, we show how one can recover, out of a single simple calculation, the chiral anomaly, intrinsic anomalous Hall conductivity and chiral magnetic effect, in all generality. The demonstrated flexibility and efficiency of our formalism derives from the insulation it provides from the underlying complexity of the quantum kinetics, notwithstanding its rigorous connection to this deeper level.

DFT study of structural optoelectronic and thermoelectric properties of CuNiO ferromagnetic alloys

In this study, the structural, electronic, and thermoelectric properties of pure NiO and Cu-doped NiO were investigated. The structural and optoelectronic properties of nickel oxide and in the supercell containing atoms CuNi15O16 were obtained by density functional theory (DFT) using the generalized gradient approximation (GGA) with the Hubbard correction potential U. The theoretical results obtained using the quantum espresso code show that the ferromagnetic character of the supercell structure is more stable than the antiferromagnetic configuration, and incorporating copper in the NiO matrix reduces the spin-up band structure. The value of the band gap channel is approximately 1.7 eV. The spin-down states cross the Fermi level and indicate the half-metal character of the CuNiO alloys. These results suggest that Cu-doped NiO is a potential candidate for infrared light and spintronic applications. The transport coefficients of the nickel oxide and copper doped NiO were calculated using the semi-classical Boltzmann theory implemented in BoltzTraP code. The thermoelectric properties, such as the Seebeck coefficient, power factor, and figure of merit, are calculated with respect to the chemical potential and carrier concentration in the temperature range of 300 K–800 K. We found an increase in the factor of merit and switching of NiO oxide from n-type to p-type after copper incorporation.

Enhanced thermoelectric properties of SWCNT by new nitrogen chains doping

Single walled carbon nanotubes (SWCNT) are attractive materials for thermoelectric (TE) devices because of their unique one-dimensional structure and provide us for developing a flexible TE devices due to their mechanical properties. In this paper, the TE properties of pristine small SWCNT (3,3) and three new nitrogen (N) doping models namely CN, C2N2 (doping by two N-chains) and C4N2 (doping by three N-chains) are calculated using the density functional theory (DFT) combined with Boltzmann's semi-classical transport theory. Dynamical and energetic stability of the compounds are confirmed from the phonon density of states curve and formation energy calculations. The values of Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (κ), power factor (PF) and figure of merit (ZT) are reported at different temperature and discussed in detail. The results show that N-doping significantly improves the TE properties of the pristine nanotube and show a reasonable agreement with previous experiments and ab-initio calculations. It was found that the C2N2 configuration presents the largest Seebeck coefficient of 70.53 μV/K and the figure of merit ZT of 0.25 compared to other doping models. N-chains doping is therefore a new suitable way to improve the TE properties of small SWCNT in order to making them a good candidate for flexible TE devices.

Temperature and density dependence of line profiles of sodium perturbed by helium

Context. Ultracool stellar atmospheres and hot exoplanets show absorption by alkali resonance lines severely broadened by collisions with neutral perturbers. In the coolest and densest atmospheres, such as those of T dwarfs, Na I and K I broadened by molecular hydrogen and helium can come to dominate the entire optical spectrum. The effects of Na–He collision broadening are also central to understanding the opacity of cool DZ white dwarf stars. Aims. In order to be able to construct synthetic spectra of brown dwarfs and cool DZ white dwarfs, where helium density can reach several 1021 cm−3, Na–He line profiles of the resonance lines have been computed over a wide range of densities and temperatures. Methods. Unified line profiles that are valid from the core to the far wings at high densities are calculated in the semiclassical approach using up-to-date molecular data including electronic spin-orbit coupling from the sodium atom. Far wings are extended to more than 4000 cm−1 from the line center when the helium density can reach 1021 cm−3 at 5000 K. Results. We present a comprehensive study of Na–He collisional profiles at high density, and temperatures from 5000 K, which is the temperature prevailing in the atmosphere of ultra-cool DZ white dwarf stars, down to 1 K, which is the temperature in liquid helium clusters. Collision broadening and shift parameters within the impact approximation obtained in the semiclassical and quantum theory using our new accurate molecular data are presented.

Lessons and complications from gravitationally induced entanglement

We critically review arguments depicting entanglement generated between exclusively gravitationally interacting particles as evidence for the necessity of a quantized gravitational field. For proposed experiments which are supposed to witness this gravitationally induced entanglement, we discuss the implications of a positive result and defend the possibility of a semiclassical theory of quantum matter on a classical spacetime which cannot be conclusively ruled out by witnessing entanglement. For thought experiments that resolve apparent causality issues with quantum systems entangled via the Newtonian gravitational interaction by considering a quantized gravitational field, we demonstrate that quantization of gravitational degrees of freedom is not only unnecessary but may result in remaining paradoxical behavior, unless a previously disregarded decoherence channel is taken into consideration.

DMRG analysis of magnetic order in the zigzag edges of hexagonal CrN nanoribbons

We investigate the finite-temperature magnetic order at the edges of hexagonal CrN nanoribbons by using density functional theory combined with the density matrix renormalization group (DMRG) method. Moreover, the spin-dependent transport in nanoribbons is calculated within the semiclassical Boltzmann transport theory. We find that the zigzag edges have lower energy with respect to armchair edges. The zigzag edges of CrN nanoribbon show half-metallic electronic character, which is the same as for the two-dimensional (2D) monolayer. The localized electronic states on the zigzag edges reduce the electronic band gap energy for spin-down electrons. The ab initio electronic results are mapped into an effective 1D Heisenberg spin model up to the next-nearest-neighbor exchange interaction term. For zigzag ribbons, the nearest-neighbor and next-nearest-neighbor magnetic exchange are around 10--12 and $\ensuremath{-}2$ to 0 $\text{meV}/\text{Cr}\phantom{\rule{4.pt}{0ex}}\text{atom}$, respectively. The finite spin correlation length in 1D nanoribbons drops sharply to zero with temperature. The absence of long-range spin correlations at the edges is a practical drawback for future room temperature 2D spintronic devices. The maximally localized Wannier functions are used for band interpolation and spin-dependent transport calculations by using the semiclassical Boltzmann equation. We show that the zigzag edges of CrN are a perfect spin filter under both electron and hole doping.

Unusual dynamics of spin-[formula omitted] antiferromagnets on the triangular lattice in magnetic field

We theoretically discuss dynamical properties of spin-12 Heisenberg antiferromagnet on the triangular lattice in magnetic field H. We use the recently proposed bond-operator theory which operates with quantum states of the whole magnetic unit cell containing three spins. This technique describes accurately short-range spin correlations and provides a quantitative description of elementary excitations which appear in other approaches as bound states of conventional low-energy quasiparticles (e.g., magnons). In quantitative agreement with previous numerical and analytical findings, we observe four phases with coplanar spin arrangements upon the field increasing: the three-sublattice Y-phase, the collinear “up-up-down” (UUD) state, the non-collinear V-phase, and the collinear fully polarized (FP) state. We demonstrate that apart from magnons (spin-1 quasiparticles) there are spin-0 elementary excitations in the UUD state one of which is long lived and its spectrum lies below magnon branches. This mode originates from a high-energy quasiparticle at H=0 and it produces anomalies only in the longitudinal spin correlator because longitudinal and transverse channels are separated in collinear states. All other spin-0 excitations have finite lifetime and produce visibly anomalies in the dynamical structure factor. In the V-phase, we obtain multiple short-wavelength spin excitations which have no counterparts in the semiclassical spin-wave theory. Besides, we demonstrate a highly nontrivial field evolution of quasiparticles spectra on the way from one collinear state (UUD) to another one (FP) via the non-collinear V-phase (in which the longitudinal and the transverse channels are mixed). In particular, some parts of the spin-0 branch in the UUD state become parts of the spin-1 (magnon) branch in the FP phase whereas some parts of one magnon branch turn into parts of spin-2 branch. Such evolution would be very difficult to find by any conventional analytical approach. Our results are in good agreement with neutron experimental data obtained recently in Ba3CoSb2O9, KYbSe2, and CsYbSe2.

Structural arrangement and improved thermoelectric figure of merit in hexagonal SiX (X = N, P, As, Sb, Bi) monolayers: understanding from first-principles calculations

Materials with thermoelectric (TE) efficiencies comparable to Carnot efficiency are desirable for applications in devices that use TE effects to produce electricity from heat. The figure of merit of a TE material is a measure of its efficiency. This quantity is determined by the intricate interplay of electronic and thermal transport parameters. Layered materials at low dimensions, such as 2D monolayers, have been in focus with regard to the quest for new TE materials. Group III–VI semiconductors in their 2D monolayers have exhibited fascinating TE properties. Naturally, it is expected that isoelectronic IV–V semiconductors will possess similar novelty. In this work, we have investigated the electronic, dynamical and TE properties of IV–V monolayer SiN, SiP, SiAs, SiSb and SiBi using first-principles electronic structure methods in conjunction with the semiclassical Boltzmann transport theory. Unlike previous studies, here the focus was to compute and analyse the effects of structural arrangements on TE properties. We found that upon consideration of an alternative stacking arrangement in the systems, significant changes occur in the transport phenomena, leading to a large figure of merit and thus TE efficiency. A comprehensive comparative analysis is presented to interpret the results. This work bears significance in the fact that it demonstrates a very important structure–property relationship aspect for these compounds that has been overlooked so far, and this approach may lead to the discovery of new TE materials with desirable efficiencies.

Effect of Sr substitution on the structural, electronic and thermoelectric properties of the Zintl-phase compound BaZn 2 Sb 2

The structural, electronic, and thermoelectric properties of Ba1−x Sr x Zn2Sb2 (x = 0, 0.25, 0.5, and 0.75) are studied by the full-potential linearized augmented plane wave (FP-LAPW) method in the density functional theory (DFT) framework. The band structure and density of states of the studied system are calculated using PBE functional and Tran-Blaha modifed Becke-Johnson (mBJ) exchange potential. Our generalized gradient approximation (GGA) results show that the system exhibits a phase transition from the orthorhombic phase to the trigonal one at x = 60%. With the TB-mBJ potential these compounds are narrow-gap semiconductors with a direct band gap for x = 0 and 0.25, whereas an indirect band gap semiconductor for x = 0.5 and 0.75. The thermoelectric properties are calculated using the semi-classical Boltzmann transport theory. The temperature dependence of thermoelectric transport properties of these compounds is discussed and compared with available experimental data. Substitution of Ba by Sr leads to an increase in the figure of merit (ZT). The Seebeck coefficients and ZT of Ba0.75Sr0.25Zn2Sb2 and Ba0.5Sr0.5Zn2Sb2 are even larger than those of BaZn2Sb2. The predicted highest ZT values are found for x = 0.5 at higher temperature.