Presence Of Short-range Correlations Research Articles

Effect of Short-Range Correlations on Spectral Properties of Doped Mott Insulators

In the framework of cluster perturbation theory for the 2D Hubbard and Hubbard-Holstein models at low hole doping we have studied the effect of local and short-range correlations in strongly correlated systems on the anomalous features in the electronic spectrum by investigating the fine structure of quasiparticle bands. Different anomalous features of spectrum are obtained as the result of intrinsic properties of strongly correlated electron and polaron bands in the presence of short-range correlations. Particularly, features similar to the electron-like Fermi-pockets of cuprates at hole doping $p\sim0.1$ are obtained without ad hoc introducing a charge density wave order parameter within the Hubbard model in a unified manner with other known peculiarities of the pseudogap phase like Fermi-arcs, pockets, waterfalls, and kink-like features. The Fermi surface is mainly formed by dispersive quasiparticle bands with large spectral weight, formed by coherent low-energy exications. Within the Hubbard-Holstein model at moderate phonon frequencies we show that modest values of local electron-phonon interaction are capable of introducing low-energy kink-like features and affecting the Fermi surface by hybridization of the fermionic quasiparticle bands with the Franck-Condon resonances.

Three-dimensional magnetic interactions in quasi-two-dimensional PdAs2O6

Millimeter-sized PdAs2O6 single crystals are grown using the vapor transport technique. The magnetic order at K is studied by measuring magnetic properties, specific heat, and neutron single crystal diffraction. The anisotropic magnetic susceptibility and a metamagnetic transition observed in magnetic fields above 20 kOe suggest that the magnetic moment lies in the ab plane, consistent with the magnetic structure determined by neutron single crystal diffraction. Below 140 K, Pd2+ ions order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is refined to be 2.09(2) /Pd2+ using the fitted magnetic form factor of Pd2+ . A weak λ-type anomaly around TN was observed in specific heat and the magnetic entropy change across TN is 1.72 J mol−1 K.This small entropy change and the temperature dependence of the magnetic susceptibility support the presence of short range correlations in a wide temperature range 250 K. The comparison with SrRu2O6 suggests that the magnetic interactions in PdAs2O6 are dominated by Pd-(O--O)-Pd super-superexchange and three dimensional despite the quasi-two-dimensional arrangement of magnetic ions. The comparison with NiAs2O6 suggests that increasing covalency of isostructural compounds is an effective approach to design and to discover new materials with higher magnetic order temperatures in the localized regime.

Periodically driven interacting electrons in one dimension: Many-body Floquet approach

We propose a method to study the time evolution of correlated electrons driven by an harmonic perturbation. Combining Floquet formalism to include the time-dependent field and Cluster Perturbation Theory to solve the many-body problem in the presence of short-range correlations, we treat the electron double dressing - by photons and by e-e interaction - on the same footing. We apply the method to an extended Hubbard chain at half occupation and we show that in the regime of small field frequency and for given values of field strength the zero-mode Floquet band is no more gapped and the system recovers a metallic state. Our results are indicative of an omnipresent mechanism for insulator-to-metal transition in 1D systems.

Investigations of the singlet ground state system: PrIrSi3

We report our comprehensive study of physical properties of a ternary intermetallic compound PrIrSi3 investigated by dc magnetic susceptibility χ(T), isothermal magnetization M(H), thermo-remnant magnetization M(t), ac magnetic susceptibility χac(T), specific heat Cp(T), electrical resistivity ρ(T), muon spin relaxation (µSR) and inelastic neutron scattering (INS) measurements. A magnetic phase transition is marked by a sharp anomaly at Ttr = 12.2 K in χ(T) measured at low applied fields which is also reflected in the Cp(T) data through a weak anomaly at 12 K. An irreversibility between the zero field cooled and field cooled χ(T) data below 12.2 K and a very large relaxation time of M(t) indicates the presence of ferromagnetic correlation. The magnetic part of specific heat shows a broad Schottky-type anomaly near 40 K due to the crystal electric field (CEF) effect. An extremely small value of magnetic entropy below 12 K suggests a CEF-split singlet ground state which is confirmed from our analysis of INS data. The INS spectra show two prominent inelastic excitations at 8.5 meV and 18.5 meV that could be well accounted by a CEF model. The CEF splitting energy between the ground state singlet and the first excited doublet is found to be 92 K. Our µSR data reveal a possible magnetic ordering below 30 K, which is much higher than that found from the specific heat and magnetic susceptibility measurements. This could be due to the presence of short range correlations well above the long range magnetic ordering or due to the electronic changes induced by muons. The induced moment magnetism in the singlet ground state system PrIrSi3 with such a large splitting energy of 92 K is quite surprising.

Delocalization of Acoustic Waves in a One-Dimensional Random Dimer Media

The propagation of classical waves in one-dimensional random media is examined in presence of short-range correlation in disorder. A classical analogous of the Kronig-Penney model is proposed by means a chain of repeated sub-systems, each of them constituted by a mass connected to a rigid foundation by a spring. The masses are related to each other by a string submitted to uniform tension. The nature of the modes is investigated by using different transfer matrix formalisms. It is shown that in presence of short-range correlation in the medium which corresponds to the RD model- the localization-delocalization transition occurs at a resonance frequency . The divergence of near is studied, and the critical exponent that characterizes the power-law behavior of near is estimated. Moreover an exact analytical study is carried out for the delocalization properties of the waves in the RD media. In particular, we predict the resonance frequency at which the waves can propagate in the entire chain. The transmission properties of the system are numerically studied using a statistical procedure yielding various physical magnitudes such the transmission coefficient, the localization length and critical exponents. In particular, it is shown that the presence of correlation in disorder restores a large number of extended Bloch-like modes in contradiction with the general conclusion of the localization phenomenon in one-dimensional systems with correlated disorder.

Experimental observation of superdiffusive transport in random dimer lattices

We experimentally observe anomalous wavepacket evolution in a realization of a one-dimensional finite binary Anderson model in the presence of short-range correlations. To this end, we employ weakly-coupled optical waveguides with propagation constants ε1 and ε2. The correlations enforce the creation of dimers, i.e. two adjacent waveguides with the same ε, randomly placed along the lattice. A transition from a ballistic to a superdiffusive wavepacket expansion and, eventually, to localization is observed as the contrast between the two propagation constants increases.

Thermal and transport behavior of single-crystallineR2CoGa8(R=Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y) compounds

The anisotropy in electrical transport and thermal behavior of single crystalline R$_{2}$CoGa$_{8}$ series of compounds is presented. These compounds crystallize in a tetragonal structure with space gropup P4/mmm. The nonmagnetic counterparts of the series namely Y$_{2}$CoGa$_{8}$ and Lu$_{2}$CoGa$_{8}$show a behavior consistent with the low density of states at the fermi level. In Y$_{2}$CoGa$_{8}$, a possibility of charge density wave transition is observed at $\approx$ 30 K. Gd$_{2}$CoGa$_{8}$ and Er$_{2}$CoGa$_{8}$ show a presence of short range correlation above the magnetic ordering temperature of the compound. In case of Gd$_{2}$CoGa$_{8}$, the magnetoresistance exhibits a significant anisotropy for current parallel to {[}100{]} and {[}001{]} directions. Compounds with other magnetic rare earths (R = Tb, Dy, Ho and Tm) show the normal expected magnetic behavior whereas Dy$_{2}$CoGa$_{8}$ exhibits the possibility of charge density wave (CDW) transition at approximately same temperature as that of Y$_{2}$CoGa$_{8}$. The thermal property of these compounds is analysed on the basis of crystalline electric field (CEF) calculations.

Enhanced magnetocaloric effect in single crystalline Nd0.5Sr0.5MnO3

The magnetocaloric effect in single crystalline Nd0.5Sr0.5MnO3 (NSMO 0.5) is investigated by computing the field dependent entropy change (ΔS) and adiabatic temperature change (ΔTad). At the charge ordering temperature (TCO), the value of ΔSmax is found to be much higher than ΔSmax reported in polycrystalline samples. This “giant” entropy change is attributed to interplay (stronger in single crystals) among spin, charge, lattice, and orbital degrees of freedom resulting in a field induced transition at TCO. In contrast, the change in entropy associated with Curie temperature (TC) is very low. The direct measurements of the field induced temperature change (ΔT) are in agreement with the computed value of ΔS. The presence of short-range correlations with charge/orbital order (COO) above and below TC may be responsible for the suppression of the negative MCE at TC. A critical exponent analysis of the paramagnetic (PM) to ferromagnetic (FM) transition using magnetization data yields mean-field-like values, which is likely to be operative in inhomogeneous systems such as NSMO 0.5 with correlated COO clusters larger than lattice parameter.

The phase diagram of a bilayer Ising model

The phase diagram of a quasi-2D two-layer Ising lattice, with a variable inter-layer interaction parameter, is determined by phenomenological order-disorder, mean-field (MF), and simplified renormalization group (RG) approaches. Bethe approximation is used to determine phase diagrams of bond-diluted bilayer lattices, exhibiting non-trivial behavior in the presence of short-range correlations between the bonds. The critical behavior is discussed and the problem is shown to belong to the 2D-Ising universality class, for any given value of the inter-layer interaction. The phase diagram is verified by comparison to earlier results, obtained by computer simulations, perturbation series expansion, Bethe-tree approximation and other methods.

Lévy flights search patterns of biological organisms

We discuss recent findings suggesting that an inverse square probability density distribution P(ℓ)∼ℓ −2 of step lengths ℓ leads to an optimal random search strategy for organisms that can search efficiently for randomly located objects that can only be detected in the limited vicinity of the searcher and can be revisited any number of times. We explore the extent to which these findings may be dependent on the dimensionality of the search space and the presence of short-range correlations in the step lengths and directions.

Frustration and short range magnetic correlations in the pyrochlore Y2Mn2O7 (abstract)

The bulk magnetic susceptibility of Y2Mn2O7 is suggestive of ferromagnetic order, θc=+40 K, apparent Tc=25 K. However, both dc and ac susceptibility data show a broad maximum near 6 K which is frequency and field dependent. Heat capacity studies to 2 K show no evidence for long range magnetic order. Also, neutron scattering data indicate the presence of short range correlations. These results are compared to mean field theory predictions for the highly frustrated 16c sites in the pyrochlore lattice.

Pion condensation and medium polarization effects on the spin-isopspin-dependent particle-hole interaction

In the context of possible precursor phenomena of a phase transition in nuclear matter we have considered the effects of the dressing of π and p ρ mesons on the relevant p-h spin-isospin-dependent residual interactiong’. Our model takes into account the contributions tog’ due to the direct exchange of π and ρ mesons in the presence of short-range cor relations. Since the exact self-energies are expressed effectively throughg’, one is faced with a self-consistent problem which we have solved numerically at increasing densities. The results show a remarkable dependence of the residual interaction on the density as well as on the momentum, together with a rather different behaviour in the longitudinal (π0 channel as com pared to the transverse ( ρ) one. The general trend is an increase ing’ which, depending on the model parameters such as ρ coupling strength, cut-offs and background term, may prevent the onset of the precritical situation even at very high densities. When the precritical condition is attained, the mechanism of self-consistency shifts the critical momentum up to a value depending only on the range of the repulsive core and narrows considerably the momentum region for the amplification of the pion field.

On high-energy scattering by nuclei—II

A coupled equation description of the scattering of fast particles is formulated. For elastic scattering two coupled channels are employed. One wavefunction describes the elastic channel. The other is an “average” wavefunction which takes into account the collisions which remove the incident particles from the elastic channel. Explicit formulas are given for the coupling and diagonal potentials as obtained from the Kerman, McManus, and Thaler multiple scattering theory which includes the effects of single and double scattering in their nonlocal optical potential. The properties of the target nucleus involved are its density and pair correlation function. These are described in terms of intrinsic coordinates, the relation to model wavefunctions being briefly discussed. Three kinds of correlations are considered; the center-of-mass correlation required of the target nucleons in order that the target recoil as a unit, the exclusion principle or Pauli correlation and the short-range dynamical correlations. The coupling potential is directly related to correlation function vanishing if the correlation funciton vanishes. Numerical calculations are performed for protons of 1.69 Gev/c momentum incident on 4He and 16O. It is found that the Pauli correlation has little effect, that the center-of-mass correlation is the dominant correlation effect for the 4He target but not important for 16O. The effect of the short-range correlation is measured by the parameter ( r cR )/[1 + 2 r v 2r c 2 ] where r c is the correlation length, R the nuclear radius, r v the nuclear “force range.” In the heavier nuclei, the effect of short-range correlations is proportional to this parameter; in 4He, because of interference with the center-of-mass correlation terms, the effect is proportional to its square root. Short-range correlations are thus more difficult to observe for heavier nuclei and when their range is small compared to the ranges of nuclear forces. Absence of required nucleon-nucleon data prevents a definitive statement with respect to the presence of short range correlations. However the present nucleon-nucleon and proton- 4He data are not inconsistent with the presence of a repulsive short-range correlation.

Glauber's multiple scattering series for nuclei in the presence of short range correlations

Using the eikonal expansion of Sugar and Blankenbecker, we investigate the conditions under which Glauber's multiple scattering formalism may be applied to nuclei. If no information about nuclear pair correlations is to be extracted from the experiment, the momentum k of the projectile is restricted by kR ⪢ 1, where R is the nuclear radius. The angular range is found to include the domain of single and double scattering and thus is considerably larger than the usually stated limit of θ 2 ⪡ ( kR) −1. Pair correlations of dimensions ν c can be determined only if kν c ⪢ 1. Furthermore, only those correlations can be extracted reliably for which ν c larger than the range of the projectile-nucleon interaction.

Monte Carlo calculations of the density dependence of sound velocities in solid neon

A calculation of the density dependence of the instantaneous contributions to sound velocities in solid neon demonstrates the use of the Monte Carlo method in studying the influence of dispersive (e.g., odd-derivative) anharmonicities in the presence of short-range correlations.