A new approach to analyse H⊗(2h⊕g) Jahn-Teller system for C60

It is now well-known that electron (hole)-vibron coupling and hence Jahn-Teller (JT) effect is important understanding the properties of C₆₀ and related molecules. In this paper, we study H(2) coupling case to find the potential energy surfaces for the positively charged C₆₀ molecule due to distortion. The H(2) Jahn-Teller system is of particular importance as this will be the JT effect displayed by C₆₀ molecules removed with an electron. C₆₀ ⁺ is obtained by removing one electron from fivefold degenerate H_u highest occupied molecular orbital (HOMO) and a hole in HOMO interacts with the vibrational modes of C₆₀ and symmetry is broken. We apply the method of symmetry breaking mechanism to obtain expressions for the potential energy surface. Received 27 December 1999 and Received in final form 15 May 2000     

Precise - and -factor measurements of Ba+ isotopes

Laser-microwave double and triple resonance experiments were performed on clouds of Ba⁺ ions confined in a Penning ion trap to induce and detect electronic and nuclear spin flip transitions. Collisions with buffer gas molecules in the trap was used to reduce the lifetime of a long lived metastable state of the ions, in which population trapping might occur, and to cool the ions to the ambient temperature. Loss of ions from the trap by collisions were prevented by coupling the magnetron and reduced cyclotron motions by an additional r.f. field at the sum frequency of the two motions. Electronic Zeeman transitions in ¹³⁸Ba⁺ and ¹³⁵Ba⁺ were observed at a full width of about 3 kHz at a transition frequency of 80 GHz. The uncertainty of the line center was . From the magnetic field calibration by the cyclotron resonance of electrons stored in the same trap the g_J-factor for both isotopes could be determined to . From radiofrequency induced transitions of ¹³⁵Ba⁺ the nuclear g-factor could be determined . Both measurements improve earlier results by about one order of magnitude. Received: 9 July 1998 / Accepted: 14 July 1998     

Precise measurement of hyperfine structure in the state of Rb

We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the 5 P _3/2 state of ⁸⁵Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant A = 25.038(5) MHz and the electric-quadrupole coupling constant B = 26.011(22) MHz. These values are a significant improvement over previous results. Received 6 March 2003 Published online 15 April 2003     

Soft modes and phonon interactions in studied by neutron scattering

The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn₂P₂S₆ is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a^*-c^* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (P_x), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn₂P₂Se₆, a strong coupling between the TO(P_x) and TA(u_xz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(P_x) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(P_x) dispersion in the a^*-b^* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(u_yx) and LA(u_xx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed. Received: 11 May 1998 / Revised and Accepted: 15 June 1998     

Resonance Raman excitation and electronic structure of the single bonded dimers (C ¯60)2 and (C 59N)2

Raman spectra are presented for the single bonded dimeric fullerene (C ₆₀ ^- ) ₂ and compared to optical spectra and Raman spectra of the isostructural and isoelectronic heterofullerene (C₅₉N)₂. The spectra of both materials exhibit strong correlations with respect to splitting, line position, and line intensity. This holds for non resonant excitation with blue and green lasers as well as for the strong resonances observed with red lasers. The latter observation is consistent with a downshift for the electronic transition energies as compared to C₆₀. The absorption edge of thin films of (C₅₉N)₂ was found at 1.4 eV. The three intercage modes were observed at 82, 103, and 111, and at 88, 98, and 106 cm^-1 for (C₅₉N)₂ and (C ₆₀ ^- ) ₂ , respectively. A surprising difference was found for the position of the pentagonal pinch modes in the two materials as they were observed at 1461 and at 1451 cm^-1, for (C₅₉N)₂ and (C ₆₀ ^- ) ₂ , respectively. This is interpreted as a consequence of some characteristic differences in the electronic structure of the two compounds. Received 25 January 2000 and Received in final form 10 April 2000     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000     

Hydrogenated carbon clusters produced by highly charged ion impact on solid

The emission of small (hydrogenated) carbon cluster ions C_nHm ⁺ (n =2-22) upon highly charged Xe^q+ (q =20-44) impact on C₈₄ surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation of a certain carbon cluster ion Cn ⁺ is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted as an entity in agreement with “equilibrium” and “shock wave” models. Received 4 February 2000     

Stabilization of A-type layered antiferromagnetic phase in LaMnO3 by cooperative Jahn-Teller deformations

It is shown that the layered antiferromagnetic order in stoechiometric cannot be understood purely from electronic interactions. On the contrary, it mainly results from strong cooperative Jahn-Teller deformation. Those involve a compression of the Mn-O octahedron along the c-axis (mode Q ₃ < 0), while alternate Jahn-Teller deformations occur in the ab-plane (mode Q₂). These deformations stabilize a certain type of orbital ordering. The resulting superexchange couplings are calculated by exact diagonalization, taking into account both e_g and t_2g orbitals. The main result is that antiferromagnetic (ferromagnetic) coupling along the c-direction (ab-planes) can be understood only if the Jahn-Teller energy is much larger than the superexchange couplings, which is consistent with experiments. This mechanism contrasts with that based on weak Jahn-Teller coupling which instead predicts elongation along the c-axis (Q ₃ > 0). The crucial role of the deformation anisotropy is also emphasized. Received 24 January 2000     

Ab initio determination of an extended Heisenberg Hamiltonian in CuO 2 layers

Accurate ab initio calculations on embedded Cu₄O₁₂ square clusters, fragments of the La₂CuO₄ lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J = 124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J' = 6.5 meV) and four-spin cyclic exchange (K = 14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic coupling with the Cu-O distance is Δ J /Δ d _{Cu - O} ∼ 1700 cm^-1?^-1. Received 20 November 2000     

Antiferromagnetic resonance and high magnetic field properties of NaNiO2

Magnetisation measurements up to 23 T and submillimeter wave ESR in the frequency region 48-380 GHz have been performed on NaNiO₂ powders at low temperature. Also typical spectra above the Néel temperature are shown. At 4 K the magnetisation shows a spin-flop transition at 1.8 T and saturation at 10 T. /Ni confirms the low spin state of the Ni³⁺ ions. The susceptibility exhibits a maximum at K with and K. NaNiO₂ is an A-type antiferromagnet: we derive K and K for the interactions between Ni ions within and between adjacent layers, respectively. The AFMR spectra yield an energy gap of 52.5 GHz, in agreement with the spin-flop value derived from the magnetisation. The anisotropy of the g factor observed at 100 K with can be attributed to the Jahn-Teller effect for Ni³⁺ ions in the low spin state, which stabilises the occupation. We finally comment on the isomorphic controversial Li_1-xNi_1+xO₂ compound. Received 9 March 2000 and Received in final form 13 July 2000.     

Binding energies and scattering observables in the 4He3 atomic system

The ⁴He₃ system is investigated using a hard-core version of the Faddeev differential equations. Realistic ⁴He-⁴He interactions are employed, among them the LM2M2 potential by Aziz and Slaman and the recent TTY potential by Tang, Toennies and Yiu. We calculate the binding energies of the ⁴He trimer, but concentrate in particular on scattering observables. The scattering lengths and the atom-diatom phase shifts are calculated for center of mass energies up to 2.45 mK. It is found that the LM2M2 and TTY potentials, although of quite different structure, give practically the same bound-state and scattering results. Received 19 June 2000     

A chiral axial next nearest neighbour xy-model for antiferroelectric liquid crystals

We describe a chiral axial next nearest neighbour xy-model to account for the various subphases exhibited by antiferroelectric liquid crystals made of chiral rod-like molecules. The assumed form of the interlayer interactions is based on physical processes which are discussed. Using a discrete model, the predicted sequence of transitions is SmA-SmC-SmC-FI_H-FI_I-FI_L-SmC ^* A, where FI stands for a ferriphase, as seen in many compounds. The ferri and SmC phases are characterized by relatively large angles between the c-vectors of successive layers and occur only when the compounds have high optical purity. The calculated field induced structures exhibit a plateau of the apparent tilt angle at , where is the tilt angle of the molecules in the ferriphase. The conoscopic figures in the presence of a field and ellipsometric parameters in the absence of a field have also been generated, which agree extremely well with the experimental results. Recent anomalous X-ray scattering studies prove the xy-character of the configurations, though the commensurate structures that are found in the ferriphase require an extension of the model to include lock-in terms. Received 23 August 1999     

Magnetic properties of the S = 1/2 one-dimensional antiferromagnet MgVO

The magnetic susceptibility and the electron spin resonance in the X-band of the transition metal oxide compound MgVO₃ are reported. We show that this compound, made of weakly coupled infinite chains of VO₅ pyramids, behaves as a S =1/2 one-dimensional Heisenberg antiferromagnet. From the ESR and magnetic experiments we deduce the Néel temperature K, the in-chain coupling constant K and the g-factor values g x = g z =1.972(2), g y =1.946(1) for V⁴⁺ ions in MgVO₃. Received 14 July 1999     

Electron phonon interactions and superconductivity in α ′ -TTF[Pd(dmit)2]2

A simple model is developed to understand superconductivity in α ^′ -TTF[Pd(dmit)₂]₂. We include electron-intra molecular and intermolecular phonon interactions as the mechanism of superconductivity. Intramolecular vibrations included are the eight symmetric A_g modes of the Pd(dmit)₂ molecule. Intermolecular vibrations included are the longitudinal acoustic and transverse acoustic (LA and TA) modes of the Pd(dmit)₂ column. All the electron-phonon coupling constants are calculated from first principles. We find that largest el-intramolecular vibration coupling is to the A_g mode with the highest frequency (1449 cm^-1). The el-intermolecular coupling to the LA mode is found to be larger than the total el-intramolecular couplings. We also find el-(TA)phonon coupling to be at least an order of magnitude smaller than el-(LA)phonon coupling. Estimate of superconducting transition temperature is comparable to experimental result. We also provide a detailed discussion, employing the results of recent numerical calculations on two-chain Hubbard model and the specific material parameters, on the relative importance of el-ph and Coulomb-origin mechanisms of superconductivity in α ^′ -TTF[Pd(dmit)₂]₂ and TTF[Ni(dmit) ₂ ] ₂ . Received 29 March 2001 and Received in final form 7 August 2001     

Flow equations for electron-phonon interactions: phonon damping

We report Nuclear Magnetic Resonance studies of CeAuAl₃ and LaAuAl₃ at frequencies between 1.30 and 76.91 MHz and at temperatures from up to . CeAuAl₃ is a new heavy-electron compound which orders antiferromagnetically at . For the two inequivalent Al sites of CeAuAl₃, the transferred hyperfine couplings and are relatively small compared to those in other Ce compounds. The nuclear quadrupolar coupling constants e2qQ / h are 2.9 MHz and 7.8 MHz, respectively. We suggest that the magnetic structure of the ordered state below is of a simple spiral type with the ordered Ce moments arranged ferromagnetically within the ( ab ) planes of the tetragonal crystal lattice. Based on this magnetic structure we estimate a 25% reduction of the Ce moments, most likely due to Kondo screening. At high temperatures, in the paramagnetic state of CeAuAl₃, the spin-lattice relaxation rate T1 ^-1 is dominated by the fluctuations of the localised Ce moments. At , T1 ^-1 is , more than two orders of magnitude larger than for the reference compound LaAuAl₃. For temperatures lower than T1 ^-1 decreases as a function of ( T / H ) and below , displays a linear-in-T behaviour, strongly enhanced compared to the reference compound LaAuAl₃. Received: 5 May 1998 / Received in final form: 16 July 1998 / Accepted: 20 July 1998     

Chemiluminescence of alkaline earth monohalides in the collisions of ground state Ca, Sr, and Ba atoms with Cl2, Br2, I2, ICl,, ICl, and IBr

Formation of electronically excited alkaline earth monohalides MX^* in the reactions of ground state Ca, Sr, and Ba atoms with Cl₂, Br₂, I₂, ICl, and IBr was studied in a beam-gas arrangement. The MX^* spectra were observed for all the reactive systems with the exception of Ca, . The energy balance indicates that MX^* can be formed in a single-collision exchange reaction; also the dependence of MX^*-chemiluminescence intensity on halogen gas pressure is typical of a bimolecular process. The MX^*-chemiluminescence cross sections and lower limits of photon yields are estimated. Received 17 April 2000 and Received in final form 18 July 2000     

Structural phase transition in the 2D spin dimer compound SrCu 2 ( BO 3 ) 2

A displacive, 2nd order structural phase transition at T _s = 395 K from space group I 2 m below T _s to I 4/m c m above T _s has been discovered in the two-dimensional spin dimer compound SrCu₂(BO₃)₂. The temperature evolution of the structure in both phases has been studied by X-ray diffraction and Raman scattering, supplemented by differential scanning calorimetry and SQUID magnetometry. The implications of this transition and of the observed phonon anomalies in Raman scattering for spin-phonon and interlayer coupling in this quantum spin system will be discussed. Received 24 July 2000 and Received in final form 2 November 2000     

Magnetostructural transition in NdCu2

The magnetic (H , T)-phase diagram of the orthorhombic compound NdCu₂ was investigated for external magnetic fields up to 15 T parallel to the crystallographic c-direction. Magnetization and magnetostriction measurements reveal an anomalous change of the magnetic properties as well as giant magnetostriction (GMS) and large hysteretic effects. This behaviour is similar to that observed in some other RCu₂ compounds where it has been interpreted as a conversion of the magnetic Ising axis. In contrast to these other RCu₂ compounds, however, the easy axis of magnetization in NdCu₂ is the b-axis. The macroscopic measurements are compared with neutron diffraction experiments which reveal GMS along the b-axis and a new magnetic phase with propagation vector in the converted crystal. Received 27 March 2000 and Received in final form 11 September 2000     

de Haas-van Alphen effect and the Fermi surface of PrNi5

The Fermi surface of PrNi₅ has been studied by the measurements of the de Haas-van Alphen (dHvA) effect at temperatures between 0.3 and in magnetic fields up to 12 T. Two dHvA frequencies have been obtained. The electronic structure of PrNi₅ was calculated using the full potential linearized augmented plane wave method. Five sheets of the Fermi surface and the multiple extremal cross sections were determined. First and second sheet have a hole-like structure. The agreement between theory and experiment is obtained by a small downward shift ( 0.1 eV) of the Fermi energy which is probably due to an underestimation of the role of 4 f electrons. Received 9 May 2000 and Received in final form 20 September 2000     

Spectrum of the parametric down converted radiation calculated in the Wigner function formalism

We continue the study of parametric down conversion within the framework of the Wigner representation, by using a Maxwellian approach developed in a recent paper [A. Casado et al., Eur. Phys. J. D 11, 465 (2000)]. This gives a mechanism, inside the crystal, for the production of the down-converted radiation. We obtain the electric field to second order in the coupling constant by using the Green's function method, and compare our treatment with the standard Hamiltonian approach. The spectrum of the down-converted radiation is calculated as a function of the parameters of the nonlinear crystal (in particular the length) and the radius of the pumping beam. Received 15 May 2000