Temperature-dependent Green Function Research Articles

Phase Transition Behavior in Antiferroelectric Copper Formate Tetrahydrate Crystal

Antiferroelectric properties of copper formate tetrahydrate (CFT) crystal having phase transition temperature at were investigated theoretically. CFT have a layer-type structure in which layers of are sandwiched between the layers of copper formate in the lattice plane. By considering modified two sublattice pseudospin lattice coupled mode model Hamiltonian and double-time temperature-dependent Green function method for antiferroelectric copper formate tetrahydrate (CFT) crystal, the phase transition properties of crystal have been studied. Expressions are derived for soft mode frequency, dielectric constant and loss tangent for considered crystal. Model values are fitted in the derived expressions and the dependence of the above properties on temperature have been calculated. Theoretically obtained results are compared with experimental data and show similarities between them. Dielectric constant as a function of temperature shows a behaviour of the first-order transition with a large discontinuity at the transition point. These properties can be employed in a wide range of applications including memory devices, solar cells, biosensors and photovoltaic devices.

The Study of Interplay of Singlet Superconductivity and Ferromagnetism in Superconducting HoMo6Se8

This article reports the effect of magnetic ordering and superconducting order parameter on superconducting and magnetic transition temperatures of interacting singlet superconductivity and ferromagnetism in HoMo6Se8 material. The basic superconducting parameters were calculated starting with the BCS type model Hamiltonian and using the double time temperature dependent Green function formalisms. Results showed that the superconducting critical temperature decreases with enhancement of superconducting order parameter and vice versa. This is perhaps due to the weakening of the binding energy as the temperature approaches its critical value. On the other hand, the superconducting critical temperature demonstrates attenuation with increasing magnetic order parameters . It was also observed that the enhancement of magnetic transition temperature with the increase of the magnetic order parameter demonstrates variation with the alteration of the superconducting order parameter. Moreover, the calculations revealed that there is a temperature region where both superconductivity and magnetic order coexist. Furthermore, the present theoretical analysis is broadly in agreement with existing experimental findings.

The effect of anisotropic field on magnon dispersion of antiferromagnetic metal fluoride materials

This article investigates the dynamics of spin waves in two sublattice antiferromagnetic (AFM) system spins’ interaction and magnon dispersion relation. The analysis is done starting with a standard model that includes a uniaxial magnetic anisotropy field. Quantum field theory is employed in formulating the problem, and double-time temperature-dependent Green function technique is used to obtain magnon dispersion, whereby random phase approximation is considered to decouple and diagonalize the higher order terms. At low temperatures and long wavelength approximation, dispersion of uniaxial symmetric AFM crystal lattice of metallic fluoride materials, such as MnF2, FeF2, and RbMnF3, is analyzed for illustration. Our findings indicate that as the anisotropic field increases, the magnon dispersion vs wave vector k gradually varies, attempts linearity from being curved, and attains a sinusoidal structure when extended further in the first Brillouin zone. Perhaps, these results are useful to understand the feature of magnon dispersion of stable similar AFM materials for practical application.

Electron-phonon interaction and elastic anomaly in heavy fermions (HF) systems with finite wave vector ‘q’ and finite ‘T’ analysis

In this paper, we have used the Periodic Anderson Model (PAM) for the half-filled band and introduced electron-phonon interaction in the normal state of the HF system. Using Zubarev type double time temperature dependent Green function the sound velocity and the reduced value of the elastic constant Ĉ = C/C0 is evaluated in presence of electron–Phonon interaction to explain the experimental anomalies seen at various temperature. The calculated renormalized elastic constants are analyzed for different system parameters like effective coupling strength ‘g’, the position of f-level ‘d’ and the onsite coulomb interaction ‘U’ and the wave vector ‘q’. The results obtained are found to give good agreement with the general features observed experimentally for CeAl3 HF system.

Explanation of Onset Ferroelectric Transition and Anomalous Tangent Delta in HBonded Rubidium Dihydrogen Phosphate Crystal

The H-bonded ferroelectric and ordered-disordered, type of Rubidium Dihydrogen Phosphate crystal is theoretically investigated. It undergoes a ferroelectric phase transition at 146K. Modified earlier author’s (Ganguli et al, 1980) simple Pseudo Spin Lattice Coupled mode (PLCM) model Hamiltonian by adding some extra interactions term into it, like third-order and fourth-order, phonon anharmonic interactions, direct spin-spin terms, extra spin-lattice terms and four spin coupling terms are added. By using Zuberav’s statistical approach, two times temperature dependent Green function method, and Dyson’s equation in the mean-field approximation (MFA), to obtain the formulae of various quantities like (ε), Ω̃, and tan(δ), Curie’s temperature, and response function (shift and width) as a function of temperature. Thermal variations of above quantities are obtained for Rubidium Dihydrogen Phosphate (RbH2PO4) crystal. A comparison with theoretical findings has been made with the experimental findings reported by (Peercy et al, 1974) which shows good agreement.

Effect of magnon-exciton coupling on magnetic phase transition of diluted magnetic semiconductors

This article reports effects of magnon-exciton interaction on magnetic ordering in diluted magnetic semiconductors (DMS). Quantum field theory is employed using the double time temperature dependent Green function technique to obtain dispersion. It is understood that interaction of the two quasi particles take place in the exciton cloud and consequently the spontaneously ordered localized electrons might be partly trapped and subjected to different angular precision resulting in increase ofthe number of magnons. According to our analysis the exciton-magnon coupling phenomena may be the reason for the attenuation of spontaneous magnetization and ferromagnetic transition temperature T c . Further observations indicate that there is a significant departure of magnetic impurity concentration, x m , vs. T c relation from the linearity as suggested by electronic calculations and experimental estimations near absolute zero temperature.

Possible coexistence of superconductivity and antiferromagnetism in Cept2in7 superconductor

This work focuses on the theoretical investigation of the possible coexistence of superconductivity and antiferromagnetism in CePt2In7 superconductor. By developing a model Hamiltonian for the system under consideration, by employing double time temperature dependent Green's function formalism and by applying a suitable decoupling approximation technique, the possible coexistence of superconductivity and antiferromagnetism in CePt2In7 superconductor has been shown to be a very distinct possibility. The phase diagrams of superconducting gap parameter (Δ) versus temperature (T), the superconducting transition temperature (TC) and antiferromagnetic order temperature (TN) versus antiferromagnetic order parameter (η) have been plotted. Finally, by combining the two phase diagrams, the possible coexistence of superconductivity and antiferromagnetism in CePt2In7 superconductor has been demonstrated. Our finding is in agreement with the experimental observations.

Low temperature anomaly of light stimulated magnetization and heat capacity of the 1D diluted magnetic semiconductors

This article reports magnetization and specific heat capacity anomalies in one dimensional diluted magnetic semiconductors observed at very low temperatures. Based on quantum field theory double time temperature dependent Green function technique is employed to evaluate magnon dispersion and the time correlation function. It is understood that magnon-photon coupling and magnetic impurity concentration controls both, such that near absolute temperature magnetization is nearly zero and abruptly increase to saturation level with decreasing magnon-photon coupling strength. We also found out dropping of magnetic specific heat capacity as a result of increase in magnetic impurity concentration x, perhaps because of inter-band disorder that would suppress the enhancement of density of spin waves.

Study of Coexistence of Superconductivity and Antiferromagnetism in Superconducting Ce2PdIn8

Coexistence of Superconductivity and magnetism has always been the appealing area of interest for studying condensed-matter physics. Various current research efforts in strongly correlated systems focus on the interplay between magnetism and superconductivity. We report a study on the theoretical investigation of the coexistence of superconductivity and antiferromagnetism in Ce2PdIn8 superconductor. Hamiltonian model, double time temperature dependent Green's function formalism and a suitable decoupling approximation technique has been applying to show the coexistence of superconductivity and antiferromagnetism in Ce2PdIn8 superconductor. The phase diagrams of superconducting gap parameter (Δ) versus temperature (T), the superconducting transition temperature (TC) and antiferromagnetism order temperature (TN) versus antiferromagnetic order parameter (·) have been plotted. Finally, by combining the two phase diagrams, coexistence of superconductivity and antiferromagnetism in Ce2PdIn8 superconductor has been demonstrated. The model employed in this work, shows that, there is a common region where both superconductivity and antiferromagnetism can possibly coexist in superconducting Ce2PdIn8.

Phonon response of some heavy Fermion systems in dynamic limit

The phonon excitation spectrum of some Heavy Fermion (HF) systems in the presence of electron–phonon interaction is studied in the dynamic limit ([Formula: see text]). The renormalized excitation phonon frequencies ([Formula: see text]) are evaluated through Periodic Anderson Model (PAM) in the presence of electron–phonon interaction using Zubarev-type double time temperature-dependent Green function. The calculated renormalized phonon energy is analyzed through the plots of ([Formula: see text]) against temperature for different system parameters like effective coupling strength ‘[Formula: see text]’ and the position of [Formula: see text]-level ‘[Formula: see text]’. The observed behavior is analyzed and found to agree with the general features of HF systems found in experiments. Further, it is observed that in finite but small [Formula: see text]-values the propagating phonons harden and change to localized peaks.

Irreducible Green Functions Method Applied to Nanoscopic Systems

The equation of motion method for the Green functions is one of the tools used in the analysis of quantum dot system coupled with the metallic and superconducting leads. We investigate modified equation of motion, based on dierentiation of double-time temperature dependent Green functions both over the primary time t and secondary time t 0 . Our equation of motion approach allows us to obtain the Abrikosov‐Suhl resonance in the particle‐hole symmetric case and also in the asymmetric cases. We will apply the irreducible Green functions technique to analyses of the equation of motion applied to the dot system. This method gives a workable decoupling scheme breaking the infinite set of the Green function equations. We apply this technique to calculate the density of the states and the dierential conductance of single-level quantum dot with the Coulomb repulsion attached to one metallic and one superconducting leads (N‐QD‐SC). Our results are compared with the previous calculations. DOI: 10.12693/APhysPolA.130.551 PACS/topics: 73.63.Kv, 73.23.Hk, 74.45.+c

Efficient Temperature-Dependent Green's Function Methods for Realistic Systems: Using Cubic Spline Interpolation to Approximate Matsubara Green's Functions.

The popular, stable, robust, and computationally inexpensive cubic spline interpolation algorithm is adopted and used for finite temperature Green's function calculations of realistic systems. We demonstrate that with appropriate modifications the temperature dependence can be preserved while the Green's function grid size can be reduced by about 2 orders of magnitude by replacing the standard Matsubara frequency grid with a sparser grid and a set of interpolation coefficients. We benchmarked the accuracy of our algorithm as a function of a single parameter sensitive to the shape of the Green's function. Through numerous examples, we confirmed that our algorithm can be utilized in a systematically improvable, controlled, and black-box manner and highly accurate one- and two-body energies and one-particle density matrices can be obtained using only around 5% of the original grid points. Additionally, we established that to improve accuracy by an order of magnitude, the number of grid points needs to be doubled, whereas for the Matsubara frequency grid, an order of magnitude more grid points must be used. This suggests that realistic calculations with large basis sets that were previously out of reach because they required enormous grid sizes may now become feasible.

Effects of electric field on magnetic properties of $$Mn_{x}Ge_{1-x}$$ M n x G e 1 - x diluted magnetic semiconductors

We report the effect of external electric field (EEF) on the magnetic properties of \(\small {Mn_{x}Ge_{1-x}}\), diluted magnetic semiconductor. We present a Kondo Lattice Model type Hamiltonian with exchange coupling between localized spins, itinerant holes and the EEF. The magnetization, the dispersion and critical temperature (\(T_{c}\)) are calculated for different values of EEF parameters (\(\alpha\)) using double time temperature-dependent Green function formalism. The enhancement of the (\(T_{c}\)) with the EEF is shown to be very distinct and is in agreement with recent experimental observation and much required for spintronics applications and devices.

Influence of interlayer coupling and intra-layer Coulomb interaction on electronic transport in bilayer graphene

Calculations of renormalized perpendicular conductivity within Kubo formula employing single particle temperature dependent Green's function formalism for bilayer graphene has been attempted. On the basis of numerical analysis, perpendicular conductivity as a function of temperature, interlayer coupling, onsite Coulomb interaction and carrier concentration per site has been analyzed for both AA- and AB-stacked bilayer graphene. It is found that perpendicular conductivity increases with interlayer coupling and also with temperature at low temperatures while at higher temperatures, there is saturation in perpendicular conductivity. Influences of onsite Coulomb interaction and carrier concentration per site on perpendicular conductivity is just opposite to each other while onsite Coulomb energy suppresses the rate of increase of σ⊥/σ⊥0 with temperature, on the other hand increase in carrier density per site enhance this rate significantly. Finally, theoretically obtained results on temperature dependent perpendicular conductivity are viewed in terms of electronic transport data as well as recent theoretical works available in bilayer graphene.

Correlation functions and momentum distribution of one-dimensional hard-core anyons in optical lattices

We address the problem of calculating the correlation functions in a system of one-dimensional hard-core anyons that can be experimentally realized in optical lattices. Using the summation of form factors we have obtained Fredholm determinant representations for the time-, space-, and temperature-dependent Green's functions which are particularly suited to numerical investigations. In the static case we have also derived the large distance asymptotic behavior of the correlators and computed the momentum distribution function at zero and finite temperature. We present extensive numerical results highlighting the characteristic features of one-dimensional systems with fractional statistics.

Role of Interactions in Size-Dependent Curie Temperature of Magnetic Ultrathin Films

An arbitrary quantum spin Heisenberg model containing nearest neighbor antiferromagnetic and long-range dipole-dipole interactions, and single ion anisotropy is used to investigate the frustration effect in ultrathin magnetic films. Calculations using functional integral method and Gaussian spin fluctuation approximations showed that thicker films and larger spin S enhance the frustration critical ratio ε = |J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> |/J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> where J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> are antiferromagnetic exchange at surfaces and dipolar interaction strengths. The perpendicular positive (negative) anisotropy increases (reduces) this critical value. Results for spin S = 1/2 case are agreed with one of the temperature-dependent Green function method applied for quantum Heisenberg model in random phase approximation.

Green's function approach of an anisotropic Heisenberg ferrimagnetic system

We have investigated the influence of the exchange anisotropy parameter on the magnetization, critical and compensation temperatures and susceptibility of the anisotropic Heisenberg ferrimagnetic system with the single-ion anisotropy under an external magnetic field using the double-time temperature-dependent Green's function theory. In order to decouple the higher order Green's functions, Anderson-Callen's decoupling and random phase approximations have been used. This model is useful for understanding the temperature dependence of total magnetization of Lithium-chromium ferrites Li0.5Fe1.25Cr1.25O4 for which negative magnetization is characteristic. We observe that the critical temperature increases when the exchange anisotropy increases. When the system is under an external magnetic field, one obtains the first-order phase transition where the magnetization jumps for all the values of the exchange anisotropy parameters.

PERMITTIVITY IN PERTURBED MOLECULAR NANOFILMS

A microscopic theory of dielectric properties of thin molecular films, i.e., quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green's functions. It has been shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Conditions for the appearance of localized or unoccupied exciton states have been especially analyzed.

Green's function study of a mixed spin-1 and spin-3/2 Heisenberg ferrimagnetic system

The magnetic properties of a mixed spin-1 and spin-3/2 Heisenberg ferrimagnetic system on a square lattice are investigated by using the double-time temperature-dependent Green's function technique. In order to decouple the higher order Green's functions, Anderson and Callen's decoupling and random phase approximations have been used. The nearest- and next-nearest-neighbor interactions and the single-ion anisotropies are considered and their effects on compensation and critical temperature are studied.

THEORY OF THERMAL CONDUCTIVITY OF HIGH TEMPERATURE SUPERCONDUCTORS: A NEW APPROACH

An ab initio formulation of relaxation times of various contributing processes have been observed with newer understanding in terms of electron and phonon line widths. This is dealt with the help of double time temperature-dependent Green's function via a non-perturbative approach using a crystal Hamiltonian which comprises of the effects of electrons, phonons, impurities, anharmonicities and interactions thereof. The frequency line widths is observed as an extremely sensitive quantity in the transport phenomena of high temperature superconductors (HTS) as a collection of a large number of scattering processes, namely: boundary scattering, impurity scattering, multi-phonon scattering, interference scattering, electron–phonon processes and resonance scattering. The behavior of electrons and phonons is then investigated to describe the thermal conductivity of a variety of HTS samples specially in the vicinity of transition temperature to successfully explain the spectacular dip region of thermal conductivity curve which was lacking in explanation earlier with a sound physical justification.