Susceptibility Chi Research Articles

Langmuir-Blodgett films and optical second-harmonic generation of a crowned [60]fulleropyrrolidine

The Langmuir and Langmuir-Blodgett (LB) films of a novel crowned [60]fullero-pyrrolidine (CFP) were produced in different conditions. Macroscopic second-harmonic generation of the LB film was investigated by means of AM1 Hamiltonian as well as experiments. The monolayer LB film displayed a periodic fringe pattern. A linear dependence of second-harmonic intensity on the number of layers was observed. The second-order molecular susceptibility chi((2)) and hyperpolarizability beta were evaluated to be 3.2 x 10(-8) and 8.3 x 10(-29) esu.

Real-time monitoring of all-optical poling of azo-dye polymer thin film

Simultaneous writing and probing of photoinduced second-order susceptibility chi((2)) in an azo-dye polymer thin film is demonstrated. The method is based on the fact that tensor properties of chi((2)) provide different planes of polarization for seeding and signal second harmonics. Our technique allows the chi((2)) value to be monitored in real time, completely eliminating the distortion of optical poling that can occur in traditional probing configurations. The possibility of attaining high values of chi((2)) in a DR1-PMMA guest¿host system under low seeding intensities is shown.

Thermodynamics of the Spin-1/2Antiferromagnetic Uniform Heisenberg Chain

We present a new application of the traditional thermodynamic Bethe ansatz to the spin-1/2 antiferromagnetic uniform Heisenberg chain and derive exact nonlinear integral equations for just two functions describing the elementary excitations. By using this approach, the magnetic susceptibility chi and specific heat C versus temperature T are calculated to high accuracy for 5x10(-25)</=T/J</=5. The chi(T) data agree very well at low T with the asymptotically exact theoretical low- T prediction of Lukyanov [Nucl. Phys. B522, 533 (1998)]. The unknown coefficients of the second and third lowest-order logarithmic correction terms in Lukyanov's theory for C(T) are estimated from the C(T) data.

Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures

We discuss the magnetic response of clean Ag coated Nb proximity cylinders in the temperature range 150 &mgr;K<T<9 K. In the mesoscopic temperature regime, the normal-metal-superconductor system shows the yet unexplained paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C. Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full Meissner screening. The logarithmic slope of the reentrant paramagnetic susceptibility chi(para)(T) approximately exp(-L/xi(N)) is limited by the condition xi(N) = n L, with xi(N) = Planck's over 2piv(F)/2pik(B)T, the thermal coherence length and n = 1,2,4. At the lowest temperatures, chi(para) compensates the diamagnetic susceptibility of the whole AgNb structure.

Pi conjugation across the tetrathiafulvalene core: synthesis of extended tetrathiafulvalene derivatives and theoretical analysis of their unusual electrochemical properties

A series of extended tetrathiafulvalene (TTF) derivatives bearing one or two 1,4-dithiafulven-6-yl substitutents has been prepared. The new compounds present remarkable electrochemical singularities compared with other TTF derivatives, which are strongly affected by the nature of the substitution on the lateral heterocycle(s). This unusual electrochemical behaviour follows a square-scheme sequence and is attributed to structural changes upon oxidation of the pi-donating molecules. Digital simulations of the electrochemical data have been used to reach the values of the kinetic and thermodynamic constants involved in the square scheme. Theoretical calculations establish an important contribution of a highly delocalised resonant form involving a tetravalent sulphur in oxidised species, which could justify the occurrence of an electrochemical behaviour distinct from that of TTF. Finally, third-order susceptibilities chi 3 of two of these systems, for which electron-donating and electron-withdrawing substituents coexist and are conjugated through the TTF pi system, are given.

Thermodynamics of spinS=1/2antiferromagnetic uniform and alternating-exchange Heisenberg chains

The magnetic susceptibility chi and specific heat C versus temperature T of the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper and Johnston are made with the asymptotically exact low-T field theory predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T) are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above numerical data are obtained. We examine in detail the theoretical predictions of Bulaevskii for chi(alpha,T) and compare them with our results. Our experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are not in agreement with the prediction for the uniform Heisenberg chain, but can be explained if there is a moderate ferromagnetic interchain coupling and/or if J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2) K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are derived from the data. A spin pseudogap with a large magnitude \approx 0.4 Delta(0) is consistently found just above Tc, which decreases with increasing T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy change due to the transition at Tc and associated order parameter fluctuations arise from the lattice and/or charge degrees of freedom and less than 23% from the spin degrees of freedom.

Kerr effect investigations in a liquid crystal

Measurements are reported for the d.c. Kerr response and the pretransitional behaviour of a new liquid crystal, 4-cyanophenyl 4-pentylbenzoate at and above the nematic-isotropic phase transition temperature. The real parts of the third order nonlinear susceptibilities chi(3) at and above the nematic-isotropic transition temperatures are determined for the new liquid crystal from the electro-optic Kerr effect (EOKE) experiments. The variation of the Kerr constant with temperature is discussed on the basis of the Landau-de Gennes model of the nematic-isotropic phase transition. The susceptibility values are also compared with that of the well known liquid crystal 4'-pentyl-4-cyanobiphenyl (5CB). The effect of the linking group of the liquid crystal on the suceptibility value is also discussed.

Re-entrant spin susceptibility of a superconducting grain

We study the spin susceptibility chi of a small, isolated superconducting grain. Because of the interplay between parity effects and pairing correlations, the dependence of chi on temperature T is qualitatively different from the standard BCS result valid in the bulk limit. If the number of electrons on the grain is odd, chi shows a re-entrant behavior as a function of temperature. This behavior persists even in the case of ultrasmall grains where the mean level spacing is much larger than the BCS gap. If the number of electrons is even, chi(T) is exponentially small at low temperatures.

Enhanced susceptibility in LNiO3 perovskites ( L = La,Pr,Nd,Nd0.5Sm0. 5)

The temperature dependence of the resistivity rho(T) and of the dc magnetic susceptibility chi(T) were measured on high-quality LNiO3 (L = La,Pr,Nd,Nd0.5Sm0.5) samples synthesized under high oxygen pressure. Subtraction of the rare-earth contribution to chi(T) allows the presentation of the evolution of the susceptibility of the NiO3 array from Pauli to Curie-Weiss paramagnetism with decreasing bandwidth. A metal-insulator transition occurring at a temperature T(t) = T(N) is first order for L = Pr and Nd; it becomes second order and produces no anomaly in chi(-1)(T) at a T(t)>T(N) for L = Nd0.5Sm0.5. In the antiferromagnetic domain T<T(N), the chi(T) curve for the NiO3 array resembles that of a paramagnet.

Solvable kinetic gaussian model in an external field

In this paper, the single-spin transition dynamics is used to investigate the kinetic Gaussian model in a periodic external field. We first derive the fundamental dynamic equations, and then treat an isotropic d-dimensional hypercubic lattice Gaussian spin system with Fourier's transformation method. We obtain exactly the local magnetization and the equal-time pair-correlation function. The critical characteristics of the dynamical relaxation tau(q), the complex susceptibility chi(omega,q), and the dynamical response are discussed. The results show that the time evolution of the dynamical quantities and the dynamical responses of the system strongly depend on the frequency and the wave vector of the external field.

Computer simulations of the two-dimensional melting transition using hard disks

We present detailed Monte Carlo results for the two-dimensional melting transition of various systems up to N=65536 hard disks. The simulations are performed in the NVT ensemble, using a new updating scheme. In the isotropic phase the bond orientational correlation length xi_6 and the susceptibility chi_6 are measured and compared with the predictions of the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. From the scaling relation of xi_6 and chi_6 we calculate the critical exponent eta_6. In the phase transition region we use finite-size scaling methods to locate the disclination binding transition point and compare the results with the values obtained from the behaviour in the isotropic phase. Additionally, we measure the topological defect density, the pressure and the distribution of the second moment of the local bond orientational order parameter. All results are in good agreement with the KTHNY theory, while a first-order phase transition with small correlation length and a one-stage continuous transition can be ruled out.

All-optical induction of noncentrosymmetry in a transparent nonlinear polymer rod

What is believed to be the first evidence of an all-optical orientation of a highly transparent polymer is presented. The samples are bulk rods of poly(methyl methacrylate) grafted with paranitroaniline (PNA) obtained by use of copolymerization of methyl methacrylate (MMA) monomers mixed with grafted MMA-PNA monomers. Bulk samples were obtained after molding and polishing of the copolymer with standard techniques. Experiments show that the photoinduced second-order(1) susceptibility chi((2)) may occur through molecular reorientation following selective polar excitation of the nonlinear chromophores by simultaneous two- and three-photon absorption on the same electronic level.

ΣcandΛcmagnetic moments from QCD spectral sum rules

We use QCD spectral sum rules to extract the Sigma_c and Lambda_c magnetic moments. The elctromagnetic field is treated as the external field. We adopt the value of the susceptibility chi=-4.5 GeV^{-2}, which is determined from an overall analysis of the octet baryon magnetic moments. Our result for Sigma_c is in agreement with the quark model prediction, while slight deviation exists from the Lambda_c.

Na0.5La0.5RuO3: Structure and electronic properties

A compound Na0.5La0.5RuO3 has been synthesized by the solid-state reaction method. It crystallizes in the perovskite structure with a lattice parameter a = 3.8874(1) Angstrom and space group Pm3m. The large cations Na+ and La+ are randomly distributed at the A sites, and no structural ordering has been found. All samples, prepared at different post-treatment conditions, exhibit good conduction over the temperature range from 4.2 K to room temperature, with a resistivity similar to 2-7 m Omega cm. A crossover from metallic transport to a semiconductinglike transport with decreasing temperatures, characterized by a minimum resistivity rho(min) in the rho-T curves, was observed for some samples. The temperature dependence of resistivity at T > 50 K is very weak, \d rho/dT < 5 x 10(-3) m Omega cm/K, regardless of the metallic or the semiconductinglike transport. At low temperatures, the temperature dependence of resistivity for all samples can be described by a three-dimensional variable-range-hopping conduction due to the hopping between localized states near the Fermi energy in an inhomogeneous medium. The measurement of the magnetization indicates that there is no magnetic order in Na0.5La0.5RuO3 from 250 K down to 5 K. Instead, the magnetic susceptibilities obey well the Curie-Weiss law. The fitted effective moment and Weiss temperature are p(eff) = 293(1)mu(B) and Theta = -243.5(1) K. The Pauli paramagnetic susceptibility chi(pauli) is estimated to be similar to 1.43 x 10(-4) emu/mole Ru. Moreover, we found that at low temperatures the susceptibility still obeys the Curie-Weiss law, but with a reduced Curie constant. The observed transport and magnetic data can be understood based on a qualitative density-of-states model, combined with the Anderson localization.

Ultrafast response of third-order optical nonlinearity in glasses containing Bi_2O_3

We prepared nonresonant-type homogeneous glasses containing Bi(2)O(3) by the melting method. The thirdorder optical nonlinear susceptibility chi((3)) was found by third-order harmonic generation measurement to be 9.3 x 10(-12) esu. We attribute the nonlinearity of these glasses to the large molar refraction of glasses containing Bi(2)O(3). These glasses also exhibited an ultrafast response time below 200 fs by a degenerate fourwave mixing method using a femtosecond laser, indicating that the nonlinearity of these glasses originated mainly from pure electronic polarization.

Concentration and wavelength dependence of the effective third-order susceptibility and optical limiting of C60 in toluene solution

The optical limiting action of C60 in toluene solution is mainly due to reverse saturable absorption (RSA). It is shown that the formalism describing nonlinear optical response due to instantaneous two-photon absorption can be used in the case of sequential two-photon absorption, yielding effective values of the relevant parameters of optical nonlinearity due to RSA. The effective two-photon absorption parameter beta eff and the effective nonlinear refractive index parameter gamma eff', which are related respectively to the imaginary and real parts of the effective third-order susceptibility chi eff(3) were measured by the z-scan technique as a function of fullerene concentration and of incident laser intensity and wavelength over the 420-640 nm region. The concentration dependence of these parameters indicates that the solution is optically thin as far as the ground state of C60 is concerned, whereas the wavelength dependence confirms the applicability of the formalism used in the sequential two-photon absorption model. Comparisons are made with other z-scan results on C60.

Series approach to the bond bending model

The bond bending model is studied using the series expansion method on a honeycomb lattice. The elastic splay susceptibility chi SR and the elastic compressional susceptibility chi el are calculated up to 18th order. The elastic splay crossover exponent, zeta SP, is found to be zeta SP=1.31+or-0.02 which is very close to the conductivity exponent, zeta Re, of the resistor network. From the scaling relation fB=d nu + zeta SP, we found that the bulk modulus exponent fB=3.98+or-0.02 which is in excellent agreement with the result fB=3.96+or-0.04, obtained by Zabolitzky et al. (1986) using a transfer matrix technique on the same lattice.

Kondo-like effect in antiferromagnetic gamma -Fe-Mn-Ge alloys at low temperature

The temperature T dependence of the magnetic susceptibility chi and electrical resistivity rho of gamma -Fe-31 at.% Mn alloys containing 2-8.7 at.% Ge and 5.2 at.% Cr were measured in detail over the temperature range 5-500 K. The Neel transition of these alloys accompanied by an anomalous change in rho has been observed at a temperature of 300-430 K. In lowering the temperature to about 5 K, no second resistivity anomaly for gamma -Fe-31 at.% Mn or gamma -Fe-31 at.% Mn-5.2 at.% Cr alloys was found, but a secondary resistivity minimum was observed below 50 K in the gamma -Fe-31 at.% Mn-Ge alloys. The resistivity minimum temperature Tmin increases in proportion to the Ge concentration, and the resistivity measured from Tmin to about 5 K of the Fe-Mn-Ge alloys can be plotted as a linear function of log T, T2 and square root T. These results suggest that the resistivity minimum in antiferromagnetic gamma -Fe-Mn-Ge alloys is due to a Kondo-like effect caused by localized magnetic moments on Fe atom sites which are induced or enhanced by the solution of Ge in gamma -Fe-Mn alloys.

Giant forced-volume and saturation magnetostrictions of amorphous La(FexAl1-x)13 alloys composed of icosahedral clusters

The dependences of saturation magnetostriction lambda s and forced-volume magnetostriction delta omega / delta H on the concentration and temperature have been investigated for amorphous La(FexAl1-x)13 alloys prepared by high-rate DC sputtering. The magnetic phase diagram obtained from the differential magnetic susceptibility dM/dH has been correlated with these magnetostrictions. The spin freezing temperature Tf is drastically decreased but the Curie temperature TC is slightly increased on application of a magnetic field. The temperature dependences of lambda s and delta omega / delta H exhibit a broad peak at the spin freezing temperature Tf. The temperature dependence of lambda s for the ferromagnetic alloys is explained by the two-ion model, reflecting the peculiar amorphous structure consisting of icosahedral clusters. The peak of delta omega / delta H at the Curie temperature becomes indistinct with increasing ferromagnetic state in contrast with the peak at the spin freezing temperature. The giant values of delta omega / delta H and its divergent behaviour at the spin freezing temperature are accounted for by the variable amplitude of the local magnetic moment in the itinerant spin glasses. The significant large delta omega / delta H is connected with the pronounced large magnetovolume and magnetoelastic effects such as the spontaneous volume magnetostriction omega s, compressibility kappa and high-field susceptibility chi hf which bring about various Invar anomalies.

Diamagnetic susceptibility of amorphous III-V compound semiconductors

We analyse, for the first time, the effect of bond angle disorder on the diamagnetic susceptibility ( chi ) of a model amorphous III-V compound semiconductor by using a linear combination of hybrids formalism. We introduce distortion in the bond angles and construct an orthonormal basis set for the disorder network to derive an expression for chi of amorphous semiconductors in terms of the bond-angle distortion parameter Delta . We obtain chi ( Delta )= chi c+ chi v( Delta )+ chi p( Delta ), where chi c is the core diamagnetic term, independent of Delta . chi v( Delta ) is the Langevin-like diamagnetic term due to valence electrons, which varies very slowly with increasing Delta . However, chi p( Delta ), which is the Van Vleck-like paramagnetic contribution, decreases with increasing Delta . For covalent semiconductors (Si, Ge) chi v(O) and chi p(O) ( Delta =0.0 corresponds to crystalline value) are individually large and nearly cancel each other. Therefore, reduction in chi p( Delta ) with increasing Delta drastically increases the total susceptibility chi ( Delta ), giving rise to large diamagnetic enhancement. However, for Ill-V compound semiconductors, the magnitude of chi v(O) is large compared to chi p(O). Therefore, the decrease in chi p( Delta ) hardly affects the total susceptibility, leading to a small diamagnetic enhancement in the amorphous phase.