Showing papers in "European Physical Journal B in 1996"

Electronic structure of divalent hexaborides

Abstract: Recent experiments suggest that divalent hexaborides like SrB6 and CaB6, traditionally considered small-gap semiconductors, can actually be semimetals. We calculate the structural and electronic properties of SrB6, CaB6 and of ferromagnetic EuB6, using the full-potential linearized augmented plane wave (FLAPW) method, within the local (spin) density approximation. The lattice constants and internal parameters are in very good agreement with the measured ones. Because of a small band overlap at the X point, all the materials are semimetals. The calculated Hall coefficient for SrB6 changes sign around zero doping, and has the freeelectron value for doping beyond ≈ 1.5%. The plasma frequency has a minimum at zero doping. We interpret the high-temperature transport properties of SrB6 and CaB6 in terms of a thermal gap deduced from the shape of the density of states around the Fermi energy. We also calculate the imaginary part of the dielectric function for SrB6, which can be compared to recent experiments.

Optical constants of gold and silver clusters in the spectral range between 1.5 eV and 4.5 eV

Abstract: Optical constants of gold and silver clusters of various sizes are determined from measurements of their optical extinction in the range from 1.5 eV to 4.5 eV photon energy. For this purpose, it is shown that the optical extinction by spherical clusters satisfies a Kramers-Kronig relation, yielding the second quantity needed for the determination of the complex dielectric constant e of the cluster material. The dielectric constant is then obtained applying a generalized Newton-Raphson iteration method on the measured extinction data and the Kramers-Kronig data. The results show a clear dependence on cluster size, and also deviations from bulk dielectric constants in the range of the interband transitions. From the various e-data, a “bulk” dielectric constant of gold and silver is obtained, which fits the positions of the cluster plasmons more precisely than bulk dielectric constants from the literature.

Non-Fermi-liquid behavior and d-wave superconductivity near the charge-density-wave quantum critical point

Abstract: A scenario is presented, in which the presence of a quantum critical point due to formation of incommensurate charge density waves accounts for the basic features of the high temperature superconducting cuprates, both in the normal and in the superconducting states. Specifically, the singular interaction arising close to this charge-driven quantum critical point gives rise to the non-Fermi liquid behavior universally found at optimal doping. This interaction is also responsible for d-wave Cooper pair formation with a superconducting critical temperature strongly dependent on doping in the overdoped region and with a plateau in the optimally doped region. In the underdoped region a temperature dependent pairing potential favors local pair formation without superconducting coherence, with a peculiar temperature dependence of the pseudogap and a non-trivial relation between the pairing temperature and the gap itself. This last property is in good qualitative agreement with so far unexplained features of the experiments.

Elastic properties of several amorphous solids and disordered crystals below 100 K

Abstract: We have measured the internal friction and speed of sound variation at temperatures between 60 mK and room temperature for amorphous CdGeAs2, Polystyrene, and Stycast 2850FT epoxy, and the disordered crystals (ZrO2)0.89(CaO)0.11 and (CaF2)0.74(LaF3)0.26. A comparison of our results with an extensive review of previously published data shows a remarkable similarity in the internal friction of disordered solids below ∼5 K. The low temperature elastic behavior of these solids is adequately described by the standard tunneling model, from which one finds a nearly universal density of tunneling states for glasses. Internal friction above ∼10 K for different materials, however, displays a wide range of magnitudes and temperature dependence that is far from universal. Attempts to directly link the tunneling states observed by internal friction at low temperatures to configurational states of localized oscillators existing at high temperatures must take into account this striking variation among disordered solids above 10 K.

Intercalation and staging behavior in super-oxygenated La2CuO4 + δ

Abstract: A high temperature electrochemical oxidation process has been used to produce large single crystals of La2CuO4 + δ suitable for neutron scattering experiments. Below room temperature the oxygen-rich phases have structural superlattice scattering peaks which indicate new periodicities ranging from 2 to 6.6 layers perpendicular to the copper oxide planes. A model structure originally proposed for La2NiO4 + δ can account for the superlattice peaks as a result of anti-phase domain boundaries between different tilt directions of the CuO6 octahedra. Within this model, the changes in CuO6 tilt directions are induced by segregated layers of interstitial oxygen which order in a manner similar to intercalants in graphite. This structural model thus clarifies previous work and establishes La2CuO4 + δ as a unique lamellar superconducting system with annealed disorder.

Nearly antiferromagnetic Fermi liquids: a progress report

Abstract: I describe recent theoretical and experimental progress in understanding the physical properties of the two dimensional nearly antiferromagnetic Fermi liquids (NAFL's) found in the normal state of the cuprate superconductors. In such NAFL's, the magnetic interaction between planar quasiparticles is strong and peaked at or near the commensurate wave vector, Q ≡ (fy fy). For the optimally doped and underdoped systems, the resulting strong antiferromagnetic correlations produce three distinct magnetic phases in the normal state: mean field above Tcr, pseudoscaling between Tcr and T*, and pseudogap below T*. I present arguments which suggest that the physical origin of the pseudogap found in the quasiparticle spectrum below Tcr is the formation of a precursor to a spin-densitywave- state, describe the calculations based on this scenario of the dynamical spin susceptibility, Fermi surface evolution, transport, and Hall effect, and summarize the experimental evidence in its support.

The cooperative spin transition in [FexZn1 − x(ptz)6](BF4)2: II. Structural properties and calculation of the elastic interaction

Abstract: The cooperativity of the thermal spin transition in the Fe(II) spincrossover compound [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) and in isomorphous mixed crystals with the isostructural zinc complex is investigated. From powder X-ray measurements the lattice deformation (tensor e) accompanying the spin transition is determined. For diluted mixed crystals with x 0.44 a first order crystallographic phase transition (R3i → P1i) is observed on cooling, which is triggered by the spin transition and can be suppressed by cooling rapidly. The thermal spin transition is measured with UV/VIS optical absorption spectroscopy on mixed single crystals in the R3i structure. From this metal dilution experiment an interaction constant of 169 cm − 1 for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is calculated on the grounds of elasticity theory: The lattice deformation due to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fields and indirectly via the surface of the elastic crystal by an image pressure. The elastic properties of the crystalline matrix are taken in the isotropic approximation. They are derived from the complete sets of anisotropic elastic constants of the pure iron and zinc compounds, which have been measured previously by Brillouin spectroscopy. The contribution of elastic energy calculated this way is ≃ 80% of the experimental value of the interaction constant, i.e. the cooperativity in crystalline spincrossover compounds is quantitatively of elastic nature.

Positivity violation and initial slips in open systems

Abstract: Master equations without Lindblad form can in principle violate the positivity of the density operator. However, when such a master equation arises through systematic adiabatic elimination of fast variables from underlying microscopic dynamics, the violation can at worst arise on time scales one has coarsegrained over when performing the adiabatic elimination and can thus simply be ignored. Examples of non-Lindblad master equations are shown to arise for the Ornstein Uhlenbeck process both at high and low temperatures and for a single-mode laser operated near threshold.

Atomic resolution in dynamic force microscopy across steps on Si(1 1 1)7×7

Abstract: In this note we report the first observation of salient features of the Si(1 1 1)7×7 reconstructed surface across monatomic steps by dynamic atomic force microscopy (AFM) in ultrahigh vacuum (UHV). Simultaneous measurements of the resonance frequency shift Δf of the Si-cantilever and of the mean tunneling current Ī i from the cleaned Si tip indicate a restricted range for stable imaging with true atomic resolution. The corresponding characteristics vs. distance reveal why feedback control via Δf is problematic, whereas it is as successful as in conventional STM via Ī i .

Dimer and cluster formation in C60 photoreaction

Abstract: The process of phototransformation in C60 was analysed by means of Raman spectroscopy for single crystals irradiated at various temperatures between 80 K and 450 K. The activation window for the transformation process was found to be between the temperature of the first order phase transition of 260 K and an upper temperature of about 400 K. Detailed features of the resulting spectra were found to depend on the transformation temperature. From a comparison with ab initio calculations of Porezag et al. the material irradiated at high temperatures could be assigned to a C60 dimer whereas the material phototransformed at room temperature could not be identified with a simple cluster.

A simple analytical EAM model for bcc metals including Cr and its application

Abstract: An analytical embedded atom method (EAM) model, which can treat bcc transition metal Chromium, has been developed. In this model, a new potential was presented, and a modified term has been introduced to fit the negative Cauchy pressure P-c=(C-12-C-44)/2 for element Cr. The new model was applied to calculating the thermodynamic properties of binary alloys of all bcc transition metals V, Nb, Ta, Cr, Mo, W and Fe. The calculated dilute-solution enthalpies and formation enthalpies of random alloys are in good agreement with the experimental data available, the results from the first-principles calculations, and the results of thermodynamic calculations.

Mott-Hubbard transition and antiferromagnetism on the honeycomb lattice

Abstract: The Hubbard model is investigated for a halffilled honeycomb lattice, using a variational method. Two trial wave functions are introduced, the Gutzwiller wave function, well suited for describing the “metallic” phase at small U and a complementary wave function for the insulating regime at large values of U. The comparison of the two variational ground states at the mean-field level yields a Mott transition at U c /t ≈ 5:3. In addition, a variational Monte Carlo calculation is performed in order to locate the instability of the “metallic” wave function with respect to antiferromagnetism. The critical value U m/t ≈ 3:7 obtained in this way is considered to be a lower bound for the true critical point for antiferromagnetism, whereas there are good arguments that the mean-field value U c/t ≈ 5:3 represents an upper bound for the Mott transition. Therefore the “metal”- insulator transition for the honeycomb lattice may indeed be simultaneously driven by the antiferromagnetic instability and the Mott phenomenon.

Anomalous magnetic and superconducting properties in a Ru-based double perovskite

Abstract: We have studied the double perovskite [1] structure Sr2Y(Ru1-xCux)O6 system. The parent compound is an antiferromagnetic insulator with Neel temperature ∼ 26 K. Partially substituted the Ru ion by Cu the compounds increase their conductivity drastically and eventually become superconducting. More intriguingly is the observation of the coexistence of superconductivity and magnetic ordering. The superconducting transition temperature Tc and the magnetic ordering temperature Tm are of the same order. The observed magnetic structure and superconductivity of these compounds can be understood in terms of a plausible theoretical model based on the double exchange idea.

The cooperative spin transition in [FexZn1 − x(ptz)6](BF4)2: I. Elastic properties — an oriented sample rotation study by Brillouin spectroscopy

Abstract: The complete set of seven elastic constants of the Fe(II) spincrossover compound [Fe(ptz)6] (BF4)2 (ptz = 1-propyltetrazole) with crystal symmetry R3i was measured by Brillouin spectroscopy at 300K. The measurement on the plate-like single crystals were performed using an oriented sample rotation technique and the combination of two scattering geometries. For comparison the elastic constants of two isostructural compounds were also measured at 300K: the analogous perchlorate spin-crossover compound [Fe(ptz)6] (ClO4)2 and the zinc complex [Zn(ptz)6] (BF4)2. The knowledge of the elastic constants is neccessary for the calculation of the elastic interaction between the spincrossover molecules, and this is the first time that the complete set of elastic constants of a spincrossover compound has been measured directly.

Non-perturbative approaches to magnetism in strongly correlated electron systems

Abstract: The microscopic basis for the stability of itinerant ferromagnetism in correlated electron systems is examined. To this end several routes to ferromagnetism are explored, using both rigorous methods valid in arbitrary spatial dimensions, as well as Quantum Monte Carlo investigations in the limit of infinite dimensions (dynamical mean-field theory). In particular we discuss the qualitative and quantitative importance of (i) the direct Heisenberg exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a high spectral density near the band edges caused by an appropriate lattice structure and/or kinetic energy of the electrons. We furnish evidence of the stability of itinerant ferromagnetism in the pure Hubbard model for appropriate lattices at electronic densities not too close to half-filling and large enough U. Already a weak direct exchange interaction, as well as band degeneracy, is found to reduce the critical value of U above which ferromagnetism becomes stable considerably. Using similar numerical techniques the Hubbard model with an easy axis is studied to explain metamagnetism in strongly anisotropic antiferromagnets from a unifying microscopic point of view.

On the solitary waves in the sine-Gordon model of the two-dimensional Josephson junction

Abstract: The properties of the possible solitary electromagnetic waves, propagating in two-dimensional SIS Josephson junction without dissipative losses are investigated on the basis of the local theory of the junction. A classification of the waves in the junction with respect to the Swihart velocity is made. It is shown that allowed and forbidden areas for the wave numbers, wave frequency and wave amplitude exist. The cut-off frequency for the solitary waves which velocity is greater than the Swihart velocity can be smaller than the Josephson plasma frequency and moreover these waves can propagate only in a junction that is large in the direction perpendicular to the propagation direction. On the contrary the solitary waves which velocity is smaller than the Swihart velocity request junction size in the above direction to be smaller than a critical one. The investigated two-dimensional solitary waves can be connected with one or two quanta of the magnetic flux.

Substitution of manganese by trivalent and tetravalent elements in the CMR perovskites Pr1-x(Ca, Sr)xMnO3

Abstract: The substitution of trivalent (M = In, Ga) and tetravalent elements (M = Sn, Ti) for Mn(III) and Mn(IV) respectively has been studied in the colossal magnetoresistant (CMR) perovskites Pr0.7Ca0.2Sr0.1MnO3 (type I) and Pr0.5Sr0.5MnO3 (type II). For the former compound, whatever the element, the temperature transition (Tmax or Tc) separating the ferromagnetic metallic (FM) state and the paramagnetic semiconductinc (PSC) state decreases dramatically when the substituted element content is increased, the saturated magnetic moment at low temperature being slightly decreased. For these type I perovskites the maximum magnetoresistance is achieved for Pr0.7Ca0.2Sr0.1Mn0.99Ga0.01O3, reaching a resistance ratio of 600 at 127 K against 275 at 151 K for the pristine sample. These results show that by decreasing or increasing the hole concentration via M(IV) or M(III) substitutions on the manganese site the Curie temperature Tc is always decreased. In contrast to the type I perovskites, two different effects are evidenced for the Pr0.5Sr0.5Mn1-xMxO3 substituted type II phases depending on the M valence. On one hand for M = Ga, In when x increases the antiferromagnetic semiconducting (AFSC) state and the PSC state are favoured at the expense of ferromagnetism. On the other hand for M = Sn, Ti the low temperature AFSC state tends to disappear and for x > 0.04 only a FM to PSC transition still exits similarly to that observed in type I perovskites. Correspondingly the magnetization versus temperature curves evolve from the bell shape curve typical of the charge ordering state to that of a ferromagnetic compound. The effect of valence and d0, d10 electronic configurations in both type I an II CMR perovskites is discussed.

Biquadratic exchange interactions in the Europium monochalcogenides

Abstract: We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetization data of the paramagnetic phase and elaborate the consequences this additional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility χ3 obeys a Curie-Weiss law at suffciently high temperatures and that the associated Curie-Weiss temperature θ3 is a measure for the biquadratic interaction strength. For all these materials the biquadratic interaction is ferromagnetic (θ3 > 0). This leads to a conflicting situation in the case of EuTe for which θ1 < 0. We attribute the peculiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that moment expected for perfect magnetic order, to the presence of biquadratic interactions. The critical field Bc follows a T2 law in the spin-wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility χ3 diverges at a temperature T* which is 2.5 K and 1.2 K above the ordering temperature, respectively. In the temperature range Tc < T < T* the magnetization curves exhibit some weak but definite anomaly which might be interpreted as a field-induced transition into the ferromagnetic state. A new multicritical point has been identified along the critical field curve Bc of EuSe.

Solution of a one-dimensional diffusion-reaction model with spatial asymmetry

Abstract: We study classical particles on the sites of an open chain which diffuse, coagulate and decoagulate preferentially in one direction. The master equation is expressed in terms of a spin one-half Hamiltonian H and the model is shown to be completely solvable if all processes have the same asymmetry. The relaxational spectrum is obtained directly from H and via the equations of motion for strings of empty sites. The structure and the solvability of these equations are investigated in the general case. Two phases are shown to exist for small and large asymmetry, respectively, which differ in their stationary properties.

Synchronization in networks of limit cycle oscillators

Abstract: We investigate the synchronization behaviour of three different networks of nonlinearly coupled oscillators. Each network consists of several clusters of oscillators, and the clusters themselves consist of any number of oscillators. In each cluster the eigenfrequencies scatter around the cluster frequency (mean frequency). The coupling strength varies in each cluster, too. We analyze the synchronized states by means of the center manifold theorem. This enables us to calculate these states explicitly, and to prove their stability. Moreover we are able to determine frequency shifts caused by different coupling mechanisms. In a number of cases we calculate the synchronisation threshold explicitely. Numerical simulations illustrate our analytical results. In one of the three networks we have additionally analyzed a single cluster consisting of infinitely many oscillators, that is an oscillatory field. Again, the center manifold theorem enabled us to calculate the synchronized state explicitly and to prove its stability. Our results concerning the oscillatory field are in contradiction to Ermentrout’s analysis [6].

Quantum theory of the interaction of Josephson junctions with non-classical microwaves

Abstract: We present a study of the interaction between Josephson junctions in circular superconducting rings and non-classical microwaves, treating both quantum mechanically. A Hamiltonian that describes both inductive and capacitive coupling between the two systems is derived within the external field approximation. Other Hamiltonians which go beyond the external field approximation, and describe explicitly the interaction of the quantum circuit that produces the non-classical microwaves with the Josephson junction circuit, are also presented. A comparison between current experiments which use classical electromagnetic fields and the proposed experiments that use non-classical microwaves, is made.

PAC-studies of Sn-doped In2O3: electronic defect relaxation following the 111In(EC)111Cd-decay

Abstract: Perturbed γ-angular correlation measurements as function of the measuring temperature and the Sn-content have been performed in Sn-doped cubic In2O3 using 111In(EC)111Cd and 111m Cd probes. By comparing the spectra of both isotopes, electronic relaxation phenomena, socalled decay after-effects, following the electron capture radioactive decay of 111In, were established. A time independent loss of the static quadrupole interaction amplitudes was found to be characteristic for the electronic relaxation, and its temperature and doping dependence were measured. The high statistical accuracy of the PAC-data allowed a separation between structural and dynamic effects and the observation of each lattice site’s behaviour. Relaxation rates were extracted from numerical simulations based on Blume’s theory and related to the predominant electron transport mechanisms in In2O3, especially, with rising Sn-content, to the transition to metallic conduction.

t - J ladders and cuprate ladder compounds

Abstract: The properties of strongly correlated electrons confined to move on coupled chains, or ladders, are reviewed. Theoretical interest focuses on the crossover between one and two dimensions which can be achieved by assembling chains to form ladders. The properties at and near half filling are sensitive to the ladder width - ladders with even and odd numbers of legs belong to different families. Cuprates can be synthesized with weakly coupled ladders as key structural units. The recent discovery of superconductivity under pressure in one such cuprate holds promise but many issues remain to be resolved.

Hyperfine field and ordering in bcc CoFe bulk alloys studied by 59Co NMR and Monte-Carlo simulation

Abstract: Bcc CoFe alloys and ordered compounds with Fe concentrations of 50 at.%, 40 at.% and 30 at.% have been studied by 59Co NMR. Spectra with regularly spaced satellites are observed and interpreted as resulting from a step increase of the resonance frequency with each additional Fe impurity in the first coordination shell. The influence of the second-neighbour shell is also demonstrated. Satellite position analysis leads to the relationship between the hyperfine field and the first- and secondneighbour shell composition, which is found valid for the three compositions both in ordered compounds and disordered alloys. This relationship is used to determine the chemical short range order ruling the compound structure. The analysis reveals that the probability for Co to be found in perfectly ordered stoichiometric regions is higher than expected from a random distribution of the excess Co in off-stoichiometric compounds. Monte-Carlo simulations indicate that such short range order would result not only from first-neighbour interactions but also from rather large second- and/or third-neighbour interactions. However, the observations made during isothermal annealing show that the phase transformation takes place through a nucleation and growth process and not by way of homogeneous ordering. Therefore, the final short range order is also discussed in terms of kinetics of the phase transformation.

Heavy fermions in high magnetic fields

Abstract: A qualitative picture of the metamagnetic transition in the Anderson lattice model of heavy fermion Ce compounds is described and a strong coupling spin fluctuation theory of the high field state is presented. The field dependence of the minority spin quasiparticle mass is calculated and the onset of the metamagnetic transition with decreasing field is discussed. The theory of the high field state is extended to include Landau levels and the oscillatory behaviour of the spin self-energy as a function of the inverse applied field is investigated. For the heavy fermion model considered such oscillations of the self-energy lead to significant modifications in the standard theory of the de Haas - van Alphen effect. The possible relevance to anomalous experimental results on CeRu2Si2 is discussed.

Semiclassics of rotation and torsion

Abstract: We discuss semiclassical approximations of the spectrum of the periodically kicked top, both by diagonalizing the semiclassically approximated Floquet matrix F and by employing periodic-orbit theory. In the regular case when F accounts only for a linear rotation periodic-orbit theory yields the exact spectrum. In the chaotic case the first method yields the quasienergies with an accuracy of better than 3% of the mean spacing. By working in the representation where the torsional part of the Floquet matrix is diagonal our semi-classical work is mostly an application of the asymptotics of the rotation matrix, i. e. of Wigner’s so-called d-functions.

Anomalous dependence of the critical current of 45° grain boundaries in YBa2Cu3O7−x on an applied magnetic field

Abstract: Grain boundaries grown by the biepitaxial technique do not show Fraunhofer-type critical current Ic vs. magnetic field Ha dependences, which are a hallmark of standard Josephson junctions. To clarify the reason for this unusual behavior, we have fabricated asymmetric 45° grain boundaries using the bicrystal technique and analyzed their Ic (Ha) characteristics. These characteristics crisply show a number of remarkable features, which suggest that due to the particular orientation of these junctions, their Ic (Ha) dependence is intrinsically non-Fraunhofer. Conventional models, relying on standard tunneling and a superconducting order parameter with s-wave symmetry, do not account for the Ic (Ha) characteristics observed.

Are heavy-fermion metals Fermi liquids?

Abstract: We present the results of specific-heat and resistivity measurements as a function of temperature, magnetic field and hydrostatic pressure on the Kondo lattice CeNi2Ge2, the heavy-fermion superconductors CeCu2Si2 and UBe13 as well as the low-carrier-density system Yb4As3. “Non-Fermi-liquid” effects in the low-temperature normalstate properties of the three former systems are consistent with the existence of a “nearby” quantum critical point, presumably of antiferromagnetic type. Yb4As3, though showing the outward appearance of a Landau-type heavy-fermion metal, behaves very differently, i.e. as an extreme two-fluid system.

Equilibrium ensemble approach to disordered systems I: general theory, exact results

Abstract: An outline of Moritaś equilibrium ensemble approach to disordered systems is given, and hitherto unnoticed relations to other, more conventional approaches in the theory of disordered systems are pointed out. It is demonstrated to constitute a generalization of the idea of grand ensembles and to be intimately related also to conventional low-concentration expansions as well as to perturbation expansions about ordered reference systems. Moreover, we draw attention to the variational content of the equilibrium ensemble formulation. A number of exact results are presented, among them general solutions for site- and bond- diluted systems in one dimension, both for uncorelated, and for correlated disorder.

Functional bosonization of interacting fermions in arbitrary dimensions

Abstract: We bosonize the long-wavelength excitations of interacting fermions in arbitrary dimension by directly applying a suitable Hubbard-Stratonowich transformation to the Grassmannian generating functional of the fermionic correlation functions. With this technique we derive a surprisingly simple expression for the singleparticle Greens-function, which is valid for arbitrary interaction strength and can describe Fermi- as well as Luttinger liquids. Our approach sheds further light on the relation between bosonization and the random-phase approximation, and enables us to study screening in a nonperturbative way.