Interlayer Coupling Parameter Research Articles

Infrared Spectroscopy and Excess Conductivity Analysis for (Y3Fe5O12)x/Cu0.5Tl0.5Ba2Ca2Cu3O10-δ Composites

In this study, the influence of adding Yttrium iron garnet (Y3Fe5O12) nanoparticles (NPs) on the microstructure and fluctuation-induced conductivity (FIC) of Cu0.5Tl0.5Ba2Ca2Cu3O10-δ (CuTl-1223) superconductor was studied. Y3Fe5O12 NPs were produced by the co-precipitation technique. By solid state route, (Y3Fe5O12)x/Cu0.5Tl0.5Ba2Ca2Cu3O10-δ composites, with x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10 wt. % were prepared. The tetragonal unit cell parameters of (Y3Fe5O12)x/Cu0.5Tl0.5Ba2Ca2Cu3O10-δ composites were found to be invariable with Y3Fe5O12 content. The volume fraction of the host phase was increased with Y3Fe5O12 addition till x = 0.04 wt. %. The different vibrational modes of the samples were identified through Fourier transform infrared spectroscopy (FTIR). The transition from normal to the superconducting state, for the prepared composites, was done through d.c resistivity measurements from room temperature down to zero critical temperature (T0). The Aslamazov–Larkin (AL) model was used to examine fluctuation regions in resistivity-temperature curves. At high temperatures, short wave fluctuation was observed. A cross-over between short wave fluctuation and the mean-field region was spotted at lower temperatures. The mean field region for the examined composites was composed of two-dimensional fluctuations along with one-dimensional fluctuation. The coherence length along the c-axis ζc(0), interlayer coupling (J), and anisotropy parameter (γ) were estimated from the Lawrence–Doniach (LD) model as a function of Y3Fe5O12 content.

Synchronization of excitation waves in a two-layer network of FitzHugh–Nagumo neurons with noise modulation of interlayer coupling parameters

The purpose of this work is to study the possibility of synchronization of wave processes in distributed excitable systems by means of noise modulation of the coupling strength between them. Methods. A simple model of a neural network, which consists of two coupled layers of excitable FitzHugh–Nagumo oscillators with a ring topology, is studied by numerical simulation methods. The connection between the layers has a random component, which is set for each pair of coupled oscillators by independent sources of colored Gaussian noise. Results. The possibility to obtain a regime close to full (in-phase) synchronization of traveling waves in the case of identical interacting layers and a regime of synchronization of wave propagation velocities in the case of non-identical layers differing in the values of the coefficients of intra-layer coupling is shown for certain values of parameters of coupling noise (intensity and correlation time). Conclusion. It is shown that the effects of synchronization of phases and propagation velocities of excitation waves in ensembles of neurons can be controlled using random processes of interaction of excitable oscillators set by statistically independent noise sources. In this case, both the noise intensity and its correlation time can serve as control parameters. The results obtained on a simple model can be quite general.

Influence of pseudo-gap and interlayer coupling on isotope effect in bilayer cuprate superconductors

The present investigations deal with the study of the isotope effect in bilayer cuprate superconductors. We have considered the BCS-effective Hamiltonian that includes the intra and inter-layer kinetic energies and in plane attractive interaction responsible for pairing in these systems. In order to look for pseudo-gap having momentum dependence of the form of d-wave superconducting order parameter, a pseudo-gap order parameter is also introduced in the in-plane quasi-particle energy. Such a model calculation can be applicable for the high Tc cuprates like Bi2Sr2CaCu2O8+x having two CuO2 planes per unit cell and also for several other cuprate systems with a possibility to extend for the multi-layer cuprates. Employing Green's function formalism, we obtain the expressions for the superconducting order parameter and hence the isotope effect coefficient as a function of interlayer coupling, transition temperature and pseudo-gap order parameter. On the basis of numerical computation, we have pointed out that isotope effect increases due to coupling between the planes and decreases with the transition temperature. The presence of phenomenological pseudo-gap parameter in the quasi-particle spectrum enhances the isotope effect. These results are viewed in terms of recent experimental works on isotope effect in layered cuprate superconductors.

Optimizing state change detection in functional temporal networks through dynamic community detection

Abstract Dynamic community detection provides a coherent description of network clusters over time, allowing one to track the growth and death of communities as the network evolves. However, modularity maximization, a popular method for performing multilayer community detection, requires the specification of an appropriate null network as well as resolution and interlayer coupling parameters. Importantly, the ability of the algorithm to accurately detect community evolution is dependent on the choice of these parameters. In functional temporal networks, where evolving communities reflect changing functional relationships between network nodes, it is especially important that the detected communities reflect any state changes of the system. Here, we present analytical work suggesting that a uniform null network provides improved sensitivity to the detection of small evolving communities in temporal networks with positive edge weights bounded above by 1, such as certain types of correlation networks. We then propose a method for increasing the sensitivity of modularity maximization to state changes in nodal dynamics by modelling self-identity links between layers based on the self-similarity of the network nodes between layers. This method is more appropriate for functional temporal networks from both a modelling and mathematical perspective, as it incorporates the dynamic nature of network nodes. We motivate our method based on applications in neuroscience where network nodes represent neurons and functional edges represent similarity of firing patterns in time. We show that in simulated data sets of neuronal spike trains, updating interlayer links based on the firing properties of the neurons provides superior community detection of evolving network structure when groups of neurons change their firing properties over time. Finally, we apply our method to experimental calcium imaging data that monitors the spiking activity of hundreds of neurons to track the evolution of neuronal communities during a state change from the awake to anaesthetized state.

High energy shift in the optical conductivity spectrum of the bilayer graphene

We calculate theoretically the optical conductivity in the bilayer graphene by considering Kubo-Green-Matsubara formalism. Different regimes of the interlayer coupling parameter have been considered in the paper. We show that the excitonic effects substantially affect the optical conductivity spectrum at the high-frequency regime when considering the full interaction bandwidth, leading to a total suppression of the usual Drude intraband optical transition channels and by creating a new type of optical gap. We discuss the role of the interlayer coupling parameter and the Fermi level on the conductivity spectrum, going far beyond the usual tight-binding approximation scheme for the extrinsic bilayer graphene.Graphical abstract

A polynomial eigenvalue approach for multiplex networks

We explore the block nature of the matrix representation of multiplex networks, introducing a new formalism to deal with its spectral properties as a function of the inter-layer coupling parameter. This approach allows us to derive interesting results based on an interpretation of the traditional eigenvalue problem. Specifically, our formalism is based on the reduction of the dimensionality of a matrix of interest but increasing the power of the characteristic polynomial, i.e, a polynomial eigenvalue problem. This approach may sound counterintuitive at first, but it enable us to relate the quadratic eigenvalue problem for a 2-Layer multiplex network with the spectra of its respective aggregated network. Additionally, it also allows us to derive bounds for the spectra, among many other interesting analytical insights. Furthermore, it also permits us to directly obtain analytical and numerical insights on the eigenvalue behavior as a function of the coupling between layers. Our study includes the supra-adjacency, supra-Laplacian and the probability transition matrices, which enables us to put our results under the perspective of structural phases in multiplex networks. We believe that this formalism and the results reported will make it possible to derive new results for multiplex networks in the future.

Modified synthesis route to achieve Sr substituted Cu0.5Tl0.5-1234 superconductor phase

A modified synthesis route to prepare Cu0.5Tl0.5Ba2Ca3Cu4O12-y superconductor is proposed. In this modified approach a pure Cu0.5Tl0.5Ba2Ca3Cu4O12-y superconducting phase is achieved at normal pressure. The charge reservoir layer of Cu0.5Tl0.5-1234 superconductor is also modified by substituting Sr at Ba site in 0, 0.15, 0.25 and 0.35 percentages. The substitution of smaller sized Sr is resulted in the decrease of c-axis lattice parameter, which can be resulted in enhanced interlayer coupling strength. The fourier transform infrared spectroscopy also confirms the Sr substitution at Ba site. The superconductivity critical temperature is increased with the increase of Sr content upto x = 0.25, which is then decreased for x = 0.35. The analysis of excess conductivity in the light of Aslamazov-Larkin and Lawrance-Donaich shows that the crossover temperatures such as critical region-3D, 3D-2D and 2D-SWF are shifted to lower temperature with the increase of Sr content; similar is the case with c-axis coherence length. However, the interlayer coupling parameter is increased with the increase of Sr content upto x = 0.35, which shows the enhancement of interlayer coupling due to smaller sized Sr at Ba site. An inverse correlation between critical temperature and c-axis coherence length is also observed from the analysis of fluctuations induced conductivity.

Highly Anisotropic Magnon Dispersion in Ca_{2}RuO_{4}: Evidence for Strong Spin Orbit Coupling.

The magnon dispersion in Ca_{2}RuO_{4} has been determined by inelastic neutron scattering on single crytals containing 1% of Ti. The dispersion is well described by a conventional Heisenberg model suggesting a local moment model with nearest neighbor interaction of J=8 meV. Nearest and next-nearest neighbor interaction as well as interlayer coupling parameters are required to properly describe the entire dispersion. Spin-orbit coupling induces a very large anisotropy gap in the magnetic excitations in apparent contrast with a simple planar magnetic model. Orbital ordering breaking tetragonal symmetry, and strong spin-orbit coupling can thus be identified as important factors in this system.

Effect of temperature on magnetization reversal characteristics of ferromagnetic 3d metal layers within exchange-coupled FeMn-based structures

Multilayer films Fe20Ni80/FeMn/FM (with FM = Ni, Fe11Ni89, Fe20Ni80, Co30Ni70, Fe, Co) have been prepared and their structure has been estimated. Their hysteretic properties and interlayer coupling parameters have been studied in a temperature range of 5–350 K. Regularities of the magnetization reversal of ferromagnetic layers under magnetic exchange-bias conditions have been interpreted.

Density of states and excitonic condensation in the double layer correlated systems

We consider the single-particle density of states (DOS) in the strongly correlated double layer (DL) system, without applied external fields. We demonstrate an unusual collapse effect in the spectrum of the normal single-particle spectral function at the particular high-symmetry point corresponding to the specific bunching-point solution of the chemical potential in the Frenkel channel. We show that at the low-temperature limit the anomalous spectral function obeys a concave like structure, which is directly related to the interlayer pair formation and condensation. We calculate the normal DOS functions, and we find their temperature dependence for different values of the interlayer Coulomb interaction parameter. We show that the normal electron and hole DOS functions demonstrate typical condensates double peak structures on the background of the excitonic pair formation quasiparticle spectra and we have found the evidence of the hybridization gap in the case of high-temperature limit, and small interlayer coupling parameter. Meanwhile, we show a possible crossover from the excitonic condensate regime into the band insulator state. The structure of the normal DOS spectra, in the Frenkel channel and for the strong interlayer coupling regime, is found gapless for all temperature limits, which clearly indicates the strong coherence effects in the DL structure, and the excitonic condensates therein. We have shown that the excitonic pair formation and pair condensation occur simultaneously in the DL system, in contrast with the purely three-dimensional (3D) or two-dimensional cases (2D), discussed previously.

Electronic spectrum of trilayer graphene

Present work deals with the analysis of the single particle electronic spectral function in trilayer (ABC-, ABA- and AAA-stacked) graphene. Tight binding Hamiltonian containing intralayer nearest-neighbor and next-nearest neighbor hopping along-with the interlayer coupling parameter within two triangular sub-lattice approach for trilayer graphene has been employed. The expression of single particle spectral functions A(kw) is obtained within mean-field Green’s function equations of motion approach. Spectral function at Γ, M and K points of the Brillouin zone has been numerically computed. It is pointed out that the nature of electronic states at different points of Brillouin zone is found to be influenced by stacking order and Coulomb interactions. At Γ and M points, a trilayer splitting is predicted while at K point a bilayer splitting effect is observed due to crossing of two bands (at K point). Interlayer coupling ( $$ t_{ \bot } $$ ) is found to be responsible for the splitting of quasi-particle peaks at each point of Brillouin zone. The influence of $$ t_{ \bot } $$ in trilayer graphene is prominent for AAA-stacking compared to ABC- and ABA-stacking. On the other hand, onsite Coulomb interaction reduces the trilayer splitting effect into bilayer splitting at Γ and M points of Brillouin zone and bilayer splitting into single peak spectral function at K point with a shifting of the peak away from Fermi level.

Quasi-particle spectrum and density of electronic states in AA- and AB-stacked bilayer graphene

The present work deals with the analysis of the quasi-particle spectrum and the density of states of monolayer and bilayer (AB- and AA-stacked) graphene. The tight binding Hamiltonian containing nearest-neighbor and next-nearest neighbor hopping and onsite Coulomb interaction within two triangular sub-lattice approach for monolayer graphene, along-with the interlayer coupling parameter for bilayer graphene has been employed. The expressions of quasi-particle energies and the density of states (DOS) are obtained within mean-field Green’s function equations of motion approach. It is found that next-nearest-neighbour intralayer hopping introduce asymmetry in the electronic states above and below the zero point energy in monolayer and bilayer (AA- and AB-stacked) graphene. The behavior of electronic states in monolayer and bilayer graphene is different and highly influenced by interlayer coupling and Coulomb interaction. It has been pointed out that the interlayer coupling splits the quasi-particle peak in density of states while the Coulomb interaction suppresses the bilayer splitting and generates a gap at Fermi level in both AA- and AB-stacked bilayer graphene. The theoretically obtained quasi-particle energies and density of states in monolayer and bilayer (AA- and AB-stacked) graphene has been viewed in terms of recent ARPES and STM data on these systems.

Para-conductivity and critical regime of (Tl1−xCx)Ba2Ca3Cu4O12−δ superconductors

We have synthesized (Tl1−yCy)Ba2Ca3Cu4O12−δ (Tl1−yCy-1234) (y = 0, 0.25, 0.5, and 0.75) samples at normal pressure and studied the role of carbon doping on the intrinsic superconductivity parameters via excess conductivity analysis (FIC) of resistivity data. The FIC analysis in the transition region is carried out by using Ginzburg-Landau equations, whereas beyond the critical regimes, we have employed Lawrence and Doniach (LD) and Maki-Thompson models. Through such analyses, it is observed that the values of Bc(0), Bc1(0), and Jc(0) are marginally decreased whereas vF, τφ, ξc(0), J (interlayer-coupling parameter), and width of three dimensional regimes are increased with carbon doping in (Tl1−yCy)Ba2Ca3Cu4O12−δ (Tl1−yCy-1234) (y = 0, 0.25, 0.5, and 0.75) samples. Since the absolute values of vF, τφ, and ξc(0) crucially depend on the coherence length of the carriers, which in turn depends on the density of the carriers. Increase in the values of these parameters strongly suggests that the numbers of the carriers get homogenized in the inner and outer CuO2 planes, which suppress the pseudo-gap in the final compound. The suppression in the pseudo-gap can be witnessed in the form of an increase in the width of 3D LD regime with the doping of carbon in the final compound.

Collective excitations of strongly coupled bilayer charged Bose liquids in the third-frequency-moment sum rule

We calculate the collective excitation modes of strongly coupled bilayer charged Bose systems. We employ the dielectric matrix formulation to study the correlation effects within the random-phase approximation (RPA), the self consistent field approximation Singwi, Tosi, Land, and Sj\"olander (STLS), and the quasilocalized charge approximation (QLCA), which satisfies the third-frequency-moment $(⟨{\ensuremath{\omega}}^{3}⟩)$ sum rule. We find that the QLCA predicts a long-wavelength correlation-induced energy gap in the out-of-phase plasmon mode, similar to the situation in electronic bilayer systems. The energy gap and the plasmon density of states are studied as a function of interlayer separation and coupling parameter ${r}_{s}$. The results should be helpful for experimental investigations.

Characterization of graphene through anisotropy of constant-energy maps in angle-resolved photoemission

We theoretically show how constant-energy maps of the angle-resolved photoemission intensity can be used to test wave function symmetry in graphene. For monolayer graphene, we demonstrate that the observed anisotropy of angle-resolved photoelectron spectroscopy spectra is a manifestation of what has been recently branded as an electronic chirality. For bilayer graphene, we show that the anisotropy of the constant-energy maps may be used to extract information about the magnitude and sign of interlayer coupling parameters and about symmetry breaking inflicted on a bilayer by the underlying substrate.

Magnetic interlayer coupling across parabolic quantum-well

A qualitatively different mechanism of magnetic oscillations within non-square quantum wells has been found compared to the conventional magnetic superlattices which are formed of quantum wells with rectangular potential profiles. In our contribution we will present theory of interlayer coupling in the case of parabolic quantum-well system. Contrary to the conventional models based on perturbative or total energy approaches we exploit the similarities of the quasi-2D electron gas oscillations within parabolic quantum well to the de Haas-van Alphen (dHvA) effect. We derive formula for the interlayer coupling parameters as the function of potential barrier heights and the nonmagnetic layer thickness. Applications of the results obtained to the description of real systems will be widely discussed.

Electronic spectra of doped bilayer high Tc cuprates within t–t′–t″–t⊥–U model

We present the calculation of the electronic spectral function and density of states (DOS) in the underdoped and optimal doped bilayer high Tc cuprates (like Bi2Sr2CaCu2O8+x) in normal state. We have used the tight binding extended Hubbard Hamiltonian containing next-nearest-neighbour hopping (t′), second next-nearest-neighbour hopping (t″) along with the momentum dependent interlayer coupling parameter for two CuO2 planes per unit cell system. To obtain the expressions of single particle spectral function A(k,ω) and DOS, we have used Green’s function equations of motion approach within semi-classical approximation due to Kishore [R. Kishore, Phys. Rev. B 35 (1987) 6854]. It is pointed out through our numerical analysis that t′ shifts the A(k,ω) away from the Fermi level with increasing sharpness and provides favourable conditions for the bilayer splitting effects in the electronic spectra of bilayer cuprates. On the other hand the effect of second next-nearest-neighbour hopping (t″) is just opposite to that of t′ and opposes the bilayer splitting effects. The Coulomb energy U also plays an important role in the shape of spectral function at Fermi level. The DOS is suppressed at the Fermi level in underdoped region with two peak structure and in optimal doped region, a finite DOS emerges at the Fermi level with the evolution of third peak structure. The variation of A(k,ω) and DOS are found to be in qualitative agreement with the recent ARPES measurements in bilayer cuprates.

Electronic spectra of optimal doped bilayer and trilayer high-Tc cuprate superconductors

We present the calculation of the electronic spectral function and density of states in the optimal and overdoped bilayer and trilayer cuprates (like Bi2Sr2CaCu2O8+x and Bi2Sr2Ca2Cu3O10+x) as a function of interlayer coupling in d-wave superconducting state. We have used the multilayer tight binding attractive Hubbard Hamiltonian containing the momentum dependent interlayer coupling parameter to obtain the expressions of single particle spectral function and density of states within BCS-mean-field Green’s function equations of motion approach for bilayer and trilayer superconducting cuprates. On the basis of numerical computation, it has been found that the coupling between the cuprates plane within the unit cell is responsible for the splitting of bonding and antibonding bands in the electronic spectral function at (π,0) point of Brillouin zone for optimal doped bilayer cuprates, while a trilayer splitting in the spectral function at (π,0) point is pointed out in the form of bonding, non-bonding and antibonding bands in optimal doped trilayer cuprates in superconducting state. These findings are in qualitative agreement with the recent photoemission spectroscopic measurements in superconducting bilayer and trilayer cuprates. We have also computed numerically the density of states in optimal and overdoped bilayer and trilayer cuprates in the superconducting state and predicted the splitting of coherence peak in the density of states as a manifestation of bilayer and trilayer couplings. The variation of density of states found to be in agreement with the recent tunneling measurements in multilayered cuprates in superconducting state.

The effect of interfacial roughness on the magnetic characteristics of bilayer system

The interlayer coupling parameter between two ferromagnetic layers separated by a nonmagnetic spacer is calculated for the case of rough interface. Using the Heisenberg model for the spin system it is shown, how spin wave resonance spectra and the Curie temperature of a system of two ferromagnetic layers, exchange-coupled through Cu interface, depend on the roughness parameter.

Characterization of spin valves fabricated on opaque substrates by optical ferromagnetic resonance

We have used a transmission line deposited on a transparent substrate to deliver an optically triggered magnetic field pulse to a spin valve structure deposited upon an opaque substrate. The ensuing ferromagnetic resonance oscillations have been studied in optical pump-probe experiments in which the probe passes through the transmission line substrate. The resonance frequencies have been modeled by solving the Landau–Lifshitz equation and are used in determining the anisotropy, exchange bias, and interlayer coupling parameters of the sample.