Effect of strain-induced electronic topological transitions on the superconducting properties of La 2 - x Sr x CuO 4 thin films

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-T _c cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of T _c in most cuprates. In particular, a nonmonotonic dependence of T _c on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed. Received 1st October 2001 and Received in final form 5 December 2001     

Thermal evolution of hole dynamics in the large-dimensional model

The dynamics of a single hole in the t - J model is solved exactly for all temperature, T, in the limit of large spatial dimensions, , using the Feenberg renormalized perturbation series. We focus in particular on single-particle spectra, together with optical and static hole conductivities. Explicit results are illustrated for a Bethe lattice, and exemplify the continuous thermal evolution of the underlying string picture from the T =0 string-pinned limit through to the paramagnetic phase. Quenched site-disorder is also readily incorporated, exact results thereby being obtained for the interplay between disorder and thermally-induced hole dynamics. Received 16 September 1998     

Properties of doubly excited states of H- and He associated with the manifolds from N = 6 up to N = 25

This paper discusses theory and results on ¹P⁰ doubly excited states (DES) in He and in H^- of very high excitation, up to the N = 25 manifold. Our calculations employed full configuration interaction (CI) with large hydrogenic basis sets and produced correlated wavefunctions for the four lowest roots at each hydrogenic manifold by excluding open channels and the small contribution of series belonging to lower thresholds. The suitability of the hydrogenic basis sets for such calculations is justified, apart from their practicality, by the fact that, by computing from them natural orbitals, the results were shown to be the same with those of earlier multiconfigurational Hartree-Fock (MCHF) calculations on low-lying DES. In total, 160 states were computed, most of them for the first time. Their energy spectrum should be of use to possible future photoabsorption experiments. For certain low-lying DES up to N = 13, for which previous reliable results are available, comparison of the calculated energies shows good agreement. The correlated wavefunctions contain systematically chosen single and double excitations from each hydrogenic manifold of interest. From their analysis, we determined the “goodness" of different quantum numbers and the geometry (average angles and radii) as a function of excitation. For the Sinano lu-Herrick ( K , T ) classification scheme, whose basis is a restricted CI with hydrogenic functions and which has thus far been tested only on low-lying DES, we established that, whereas T remains a good index as energy increases, K does not. Consequently, a more flexible than K quantum number is needed in order to account for most of the additional correlation. This number, represented by F = N - K - 1, where N and K are not good numbers anymore, produces consistently a much higher degree of purity than the ( K , T ) scheme does, especially as N increases and as the relative significance of various virtual excitations due to electron correlation increases. Among the four states of each manifold, in all cases in H^- and in most cases in He, the three are of the intrashell type and one is of the intershell type with ( F , T ) = (0, 0). The lowest intrashell states and the lowest intershell states exhibit a wide angle geometry tending to 180 ^° as N ↦∞. Received 10 September 2001 and Received in final form 12 November 2001     

Brillouin scattering study of transverse elastic waves in the α, incommensurate and β phases of AlPO4

Transverse and pseudo-transverse elastic waves have been studied in several scattering geometries in order to investigate the temperature dependences of C E ₆₆ and C E ₁₄ over the range 300-1100 K, including the transitions near 860 K. These results complete those on C E ₄₄ we have obtained in a previous work. All these constants display discontinuities at the lock-in transition. In the phase, the results are analysed in term of lowest order couplings between strains (e) and the order parameter (Q). The main features are described by the lowest order biquadratic e2Q2 coupling, in particular for C E ₄₄ in a large temperature range. However, it appears that a contribution of the next coupling term arises for C E ₆₆ below K and that the first two lowest order terms have to be taken into account even just below the lock-in transition in the case of C E ₁₄ . The temperature dependence of Q has been deduced and it can be well described in the framework of Landau's theory. Received: 2 October 1997 / Received in final form: 3 December 1997 / Accepted: 29 January 1998     

Magnetic phase transition and magnetic structure of ground state in CsDyW O

Magnetic phase transition in the CsDyW₂O₈ magnet has been studied by means of low temperature specific heat C ( T ) measurements. The magnetic ordering temperature of the Dy³⁺ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy³⁺ ions. The behavior of the C ( T ) dependencies above and below T _N is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state. Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002     

Electronic and geometric properties of alkali-C60 molecules

First-principle electronic structure calculations are carried out for M_xC ₆₀ ^q , where M = Li, Na, K and ( x , q ) = (1, 0),(1,±1),(2, 0),(3, - 1),(6, 0),(6, - 1),(12, 0) using the local density functional. The electric dipole moment for MC₆₀ agrees with the experimental results. The calculated Mulliken charge indicates that the bonding of the alkali atom with C₆₀ is mostly ionic except for lithium. The alkali atom prefers to make many bonds with the carbon atoms rather than a single bond with the neighbor carbon atom. The calculated adsorption energy suggest that the metal-metal bonding of sodiums and potassiums on C₆₀ arises for more than the six valence electrons in the alkali atoms. The lithium-lithium bond is, on the other hand, not appeared for x ? 12. The difference in the most stable geometry between lithiums and the other alkali atoms on C₆₀ comes from the covalent character of the lithium-carbon bond.     

Sisyphus cooling of rubidium atoms on the line: The role of the neighbouring transitions

We study one-dimensional Sisyphus cooling on the transition of ⁸⁷ Rb atoms in the electric field created by two counter-propagating linearly polarized laser beams with an angle of between the polarization directions. The neighbouring F '=0 and F '=2 excited states are found to play an important role in the cooling mechanism, e.g., by inhibiting a significant population of the velocity-selective dark state. Our experimental data, such as temperatures and probe absorption coefficients, agree well with the results of quantum Monte-Carlo wavefunction simulations. Received 26 November 1998 and Received in final form 20 April 1999     

69,71Ga NMR studies of a superconducting Ga 84 cluster compound

We present ^69,71Ga-NMR experiments on microcrystalline samples of the recently discovered supramolecular compound Ga ₈₄ [ N ( SiMe ₃ ) ₂ ] ₂₀ Li ₆ Br ₂ ( thf ) ₂₀ ^. 2 toluene, which is composed of ligand-coordinated Ga₈₄ metal clusters, packed together in a fully ordered crystalline matrix. The compound is highly conducting and even shows superconductivity below T _c ~ 7.2 K. Our preliminary results between 10-300 K show a metallic-like behavior: the nuclear spin-lattice relaxation rate T ₁ ^-1 follows the Korringa law ⁶⁹ ( T ₁ T ) ^-1 = 0.36 s ^-1 K ^-1 , but with a relaxation rate approximately three times smaller than in bulk -Ga metal. No quantum-size effects are observed, the Korringa law being followed down to 10 K, whereas the quantum-gaps for individual clusters should amount to ~ 10 ³ K. These results therefore suggest a transport process based on intermolecular charge transfer, similar as in alkali-doped fullerenes and silicon-clathrates.     

An investigation of the quantum J 1 - J 2 - J 3 model on the honeycomb lattice

We have investigated the quantum J ₁ - J ₂ - J ₃ model on the honeycomb lattice with exact diagonalizations and linear spin-wave calculations for selected values of J ₂ / J ₁ , J ₃ / J ₁ and antiferromagnetic (J ₁ > 0) or ferromagnetic (J ₁ < 0) nearest neighbor interactions. We found a variety of quantum effects: “order by disorder" selection of a Néel ordered ground-state, good candidates for non-classical ground-states with dimer long range order or spin-liquid like. The purely antiferromagnetic Heisenberg model is confirmed to be Néel ordered. Comparing these results with those observed on the square and triangular lattices, we enumerate some conjectures on the nature of the quantum phases in the isotropic models. Received 17 November 2000 and Received in final form 21 January 2001     

Hyperfine interactions at 181Hf(→181Ta) impurities implanted in Er2O3 and Gd2O3: structural and electronic dependence of the EFG in bixbyite sesquioxides

The time-differential-perturbed γ-γ angular-correlation technique (TDPAC) with ion-implanted ¹⁸¹Hf tracers has been applied to study the hyperfine interactions of ¹⁸¹Ta impurities in the cubic bixbyite structure of Er ₂ O ₃ and Gd ₂ O ₃ . The TDPAC experiments were performed in air in the temperature range 300-1073 K (in the case of Er ₂ O ₃ ) and 300-1173 K (in the case of Gd ₂ O ₃ ). Three electrical-quadrupole interactions were found in each oxide in the whole studied temperature range. Two of them were attributed to the electric-field gradients (EFG) acting on ¹⁸¹Ta probes substitutionally located on the two nonequivalent free-of-defects cation sites of the bixbyite structure. The EFG results are compared with predictions of the point-charge model and discussed together with previous results obtained with the probes ¹¹¹Cd and ¹⁸¹Ta in other isomorphous sesquioxides. The temperature dependence of the hyperfine parameters for both oxides is also discussed in terms of dilatometric expansion data. Received 29 December 2000 and Received in final form 8 March 2001     

Exact ground-state for the periodic Anderson model in dimensions at finite value of the interaction and absence of the direct hopping in the correlated f-band

We report for the first time exact ground-states deduced for the D = 2 dimensional generic periodic Anderson model at finite U, the Hamiltonian of the model not containing direct hopping terms for f-electrons ( t ^f = 0). The deduced itinerant phase presents non-Fermi liquid properties in the normal phase, emerges for real hybridization matrix elements, and not requires anisotropic unit cell. In order to deduce these results, the plaquette operator procedure has been generalised to a block operator technique which uses blocks higher than an unit cell and contains f-operator contributions acting only on a single central site of the block. Received 1st July 2002 / Received in final form 16 September 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: gulacsi@heavy-ion.atomki.hu     

Possibility of the levitation of droplets in the atmosphere when they are charged by induction in an electric field under nonuniform evaporation conditions

The behavior of droplets in the atmosphere in an electric field has important bearing on the theory of thunderstorms. One of the possible mechanisms by which droplets become charged—the induction mechanism in the presence of nonuniform evaporation—is investigated in greater detail on the basis of recent experimental results (V. A. Saranin, Zh. Tekh. Fiz. 65(6), 21 (1995) [Tech. Phys. 40, 332 (1995)]). Zh. Tekh. Fiz. 68, 16–21 (February 1998)     

The properties of some derivative autocorrelation functions computed with the atom-atom potential

High-resolution quasi-elastic neutron-scattering measurements have been made on two nematogens: DMBCA with a nematic range 108 to 119°C, and 5CB and a tail-deuteriated sample (D5CB), having a nematic range 22·6 to 35·1°C.

Results on 5CB in the crystal phase at ～18°C showed no significant quasielastic broadening, which means that any random motions of the alkyl chain are slower than about 5 × 10⁹ rad s^-1. Measurements were made at a single temperature in the nematic phases on specimens aligned in a magnetic field of 0·25T; for DMBCA with scattering vector Q⊥n (n is the nematic director) and for 5CB and D5CB with Q⊥n and Q∥n and also on the isotropic liquid phase of D5CB at 45°C. Analysis of the coherent scattering from nematic D5CB at Q = 1·2 Å^-1 and 25°C gave an order parameter <P ₂>=0·55, close to the simple mean field value for this temperature. The coherent scattering from DMBCA is too weak to allow this experiment to be performed.

The most remarkable qualitative feature of the results is the close similarity of the scattering law S(Q, ω) for D5CB (and 5CB) with Q⊥n and Q∥n. Analysis of the results in all cases was made using values for the translational diffusion constants measured previously. Corrections for multiple scattering are shown to be important and a single simple model has been devised which fits the line shapes of all the results for D5CB in nematic (Q⊥n and Q∥n) and isotropic liquid phases and DMBCA. The model involves uniaxial rotational diffusion about the long molecular axis m coupled to a displacement along the rotation axis giving a net rotation in a plane whose normal makes an angle ∝ relative to the direction m. Values for the rotational diffusion constant D _rd ns^-1 are as follows: D5CB, 25°C, 6 (∝ ～ 50°); 45°C, 10. DMBCA, 112°C, 16, (all ±10–15 per cent).

The results for D5CB and 5CB are so similar that no additional detailed model fitting was attempted for the fully hydrogenous sample and it is concluded that while the motion of the alkyl tails is freer, the time scale of the motions is not more than about a factor of 2 faster than that of the molecular cores.     

Pattern formation in the reaction with anisotropic lateral adsorbate-adsorbate interactions

We present Monte Carlo simulations of the formation of (1×2) islands in the case of the 2 A + B ₂ → 2 AB reaction occurring via the Langmuir-Hinshelwood mechanism on a square lattice under steady-state conditions. The model employed takes into account the effect of anisotropic lateral B-B interactions on the rates of B diffusion and elementary reaction events. The results obtained with qualitatively realistic ratio of the rate of elementary reaction steps indicate that the average island size depends on the details of diffusion and reaction dynamics in a similar way as in the earlier studied case of the simplest A + B reaction running via the Eley-Rideal mechanism. Received 4 January 2002 and Received in final form 2 April 2002 Published online 25 June 2002     

Comparison of density-functional approaches and Monte Carlo simulations for free planar films of liquid

Density functionals proposed in the literature for describing the behaviour of liquid helium at T =0 K are examined. In so doing, several properties of the ground states of free films of superfluid ⁴ He are calculated by using zero- and finite-range density functional theories and these results are compared to that computed with Monte Carlo simulations. We mainly focus the attention on the energy per particle of the slabs, the surface tension and the width of the liquid-vacuum interfaces, all as a function of the inverse of coverage. The largest differences are found in the case of the surface widths. Received 26 July 1999     

DFT studies of the molecular nanomagnet Fe 8 and the V 15 spin system

Based on first-principles all-electron density-functional calculations we report the electronic structure and magnetic ordering of the molecular magnet Fe₈ and the V₁₅ spin system. The ferrimagnetic ordering with total spin S = 10 of the eight iron atoms in the Fe₈ cluster agrees well with experimental results from polarized neutron data. In comparison the low spin system V₁₅ shows a spin S = 1/2 ground state which is also found from our calculations. Received 30 October 2000     

Soft X-ray magnetic circular dichroism in Fe and Fe 0.50 Co 0.48 V 0.02 films: quantitative analysis of transmission

Magnetic circular dichroism (MCD) in X-ray absorption has been measured at the L _{2, 3} edges of Fe in ex-situ grown Fe and Fe _0.50 Co _0.48 V _0.02 films by means of the transmission method. A new approach is developed for fitting the observed transmittance, which describes the resonance lineshapes as (generalized) Fano profiles. Analytical integration of each single resonance allows a more reliable determination of the orbital and spin magnetic moments based on the MCD sum rules. The results are consistent with an increase of the Fe spin and orbital magnetic moment in Fe-Co alloys as obtained by other experiments and band structure calculations. Received 15 August 2000 and Received in final form 11 June 2001     

Structure determination, valence, and superexchange in the dimerized low temperature phase of α ′ - NaV 2 O 5

We report results of a new analysis for the low-temperature structure of α ^′ -NaV₂O₅ from synchrotron X-ray diffraction experiments. We confirm the existence of two inequivalent ladder structures in each vanadium layer. Based on our structural data we perform a bond-valence calculation for the vanadium sites in the low temperature state. Due to an asymmetric charge ordering we obtain only two different vanadium valences despite the three inequivalent sites. This explains the ⁵¹V-NMR observation of only two resonant peaks in the charge ordered phase. By use of a Slater-Koster method to obtain hopping matrix elements and cluster calculations we obtain effective vanadium-vanadium hoppings which compare well to LDA results. Using these in a cluster calculation we obtain a superexchange of 0.047 eV between electrons on neighbouring rungs of the same ladder for the undistorted phase. For the distorted phase we find a significant alternation in the shifts of the oxygen atoms along the legs of one of the two ladder types which leads to a significant exchange dimerisation δ _J ≈ 0.25. Received 24 November 2000     

On the 240 K anomaly in the magnetic properties of LiNiO2

In this work, results of X-band ESR spectroscopy, ac-magnetic susceptibility and X-ray powder diffraction measurements on Li_1-xNi _1+x O₂ (x = 0.02 and x = 0.07) are presented and discussed. While the susceptibility of the compound with x = 0.02 is shown to follow a Curie-Weiss law, with a Weiss temperature of the order of 30 K, the compound with x = 0.07 is found to order ferromagnetically below K. However, an additional anomaly is observed in the magnetic properties of this latter compound at around 240 K. We attribute this anomaly to the presence of macroscopic Ni-rich regions which order ferrimagnetically below this temperature. This phenomenon is different from the bulk ferromagnetism that occurs at much lower temperatures, and allows us to discard earlier suggestions proposed in the literature in which the 240 K anomaly has been considered as denoting an intrinsic phenomenon. Received 14 May 1999 and Received in final form 5 August 1999