Dielectric response of cylindrical nanostructures in a magnetic field

We study the magnetic field dependence of the dielectric response of large cylindrical molecules such as nanotubes. When a field-induced level crossing takes place, an applied electric field has two effects: it may cause a linear instead of the usual quadratic Stark effect or the difference in the quadratic Stark coefficient of the two levels leads to a discontinuity in the polarization. Explicit calculations are performed for doped nanotubes and a rich structure in the real part of the low-frequency dielectric function is found when a magnetic field is applied along the cylinder axis. It is suggested that studies of can serve as a spectroscopic tool for the investigation of large ring-shaped or cylindrical molecules. Received 11 January 2000 and Received in final form 19 May 2000     

Magnetoquantum de Haas-van Alphen oscillations in spin-split two-dimensional Fermi liquid

Theory of magnetoquantum oscillations with spin-split structure in strongly anisotropic (two-dimensional (2D)) metal is developed in the formalism of level approach. Parametric method for exact calculation of oscillations wave forms and amplitudes, developed earlier for spin degenerate levels is generalized on a 2D electron system with spin-split levels. General results are proved: 1) proportionality relation between magnetization and chemical potential oscillations accounting for spin-split energy levels and magnetic field unperturbed levels (states of reservoir), 2) basic equation for chemical potential oscillations invariant to various models of 2D and 1D energy bands (intersecting or overlapping) and localized states. Equilibrium transfer of carriers between overlapping 2D and 1D bands, characterizing the band structure of organic quasi 2D metals, is considered. Transfer parameter, calculated in this model to be of the order of unity, confirms the fact that the wave form of oscillations in organic metals should be quasisymmetric up to ultralow temperature. Presented theory accounts for spin-split magnetization oscillations at magnetic field directions tilted relative to the anisotropic axis of a metal. Theoretical results are compared with available experimental data on organic quasi-2D metal α-(BEDT-TTF)₂KHg(SNC)₄ explaining the appearance of clear split structure under the kink magnetic field and absence above by the corresponding change in the electron g-factor rather than cyclotron mass. Received 20 December 2000 and Received in final form 13 July 2001     

Inclusions and inhomogeneities in electroelastic media with hexagonal symmetry

For a long time, the absence of explicit Green's functions (fundamental solutions) for electroelastic media has hindered progress in the modelling of the properties of piezoelectric materials. Michelitsch's recently derived explicit electroelastic Green's function for the infinite medium with hexagonal symmetry (transversely isotropic medium) [4] is used here to obtain compact closed-form expressions for the electroelastic analogue of the Eshelby tensor for spheroidal inclusions. This represents a key quantity for the material properties of piezoelectric solids and analysis of the related electroelastic fields in inclusions. For the limiting case of continuous fibers our results coincide with Levin's expressions [8]. The derived method is useful for an extension to non-spheroidal inclusions or inhomogeneities having an axis of rotational symmetry parallel to the hexagonal c-axis. Received 14 September 1999     

Coherent transport in disordered metals: zero dimensional limit

We consider non-equilibrium transport in disordered conductors. We calculate the interaction correction to the current for a short wire connected to electron reservoirs by resistive interfaces. In the absence of charging effects we find a universal current-voltage-characteristics. The relevance of our calculation for existing experiments is discussed as well as the connection with alternative theoretical approaches. Received 2 September 2002 Published online 29 October 2002     

Magnetic properties of the cyclic spincluster Fe6:bicine

The magnetic properties of the cyclic compound [Fe₆(bicine)₆] LiClO₄ ^. 2MeOH are reported. The cluster Fe₆(bicine)₆ forms an antiferromagnetically coupled ring structure of Fe ^III ions. The magnetic susceptibility is measured between 2 and 300 K and yields the exchange coupling of J/k _B = - 27.5±0.5 K. The field dependence of the magnetic moment is studied at 3 and 6 K in magnetic fields up to 5 T. The zero-field splitting of the first excited spin states with S = 2 and 3 are determined by ESR at 94 GHz. The intra-molecular interactions of the Fe ^III ions are analyzed and the on-site anisotropy of the Fe ^III due to the ligand-configuration is determined to d /k _B = - 0.633±0.008K. Received 28 October 2002 / Received in final form 22 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: bernd@piobelix.physik.uni-karlsruhe.de     

Spin-dependent tunneling in nanostructures consisting of magnetic barriers

We study the spin-dependent transport properties of the nanostructures consisting of realistic magnetic barriers produced by the deposition of ferromagnetic stripes on heterostructures. It is shown that, only in the nanostructures with symmetric magnetic field with respect to the magnetic-modulation direction, electrons exhibit a considerable spin-polarization. It is also shown that the degree of the electron spin polarization is greatly dependent on the ferromagnetic stripe and its position relative to the 2DEG. A much larger electron-spin polarization can be obtained by properly fabricating the ferromagnetic stripe and by adjusting its distance above the 2DEG. Received 27 December 2001 and Received in final form 13 March 2002 Published online 25 June 2002     

Aging,rejuvenation, and memory phenomena in a lead-based relaxor ferroelectric

Isothermal aging and temperature cycle experiments were done on the relaxor ferroelectric lead magnesium niobate mixed with 10% lead titanate (PMN-10PT) around and below the diffuse maximum of the dielectric loss. With increasing aging time t_w the isothermal evolution of the linear susceptibility follows a power law and does not show frequency scaling. The non-linear susceptibility, however, obeys nearly perfect ωt _w-scaling. After aging the sample at a single temperature we observed both rejuvenation and memory effects in temperature cycle experiments. This observation indicates symmetric behavior in the sense that it shows up irrespective of whether cooling with subsequent re-heating or heating with subsequent re-cooling was performed. The memory effect is absent if subsequent to aging the temperature is increased significantly above that corresponding to the maximum in the dielectric loss. The symmetric behavior within negative and positive temperature cycles under these conditions can be rationalized by the notion of movable domain walls. These become fixed in their configuration on a large spatial scale while more flexible wall segments still show re-conformation processes when cooling or heating the sample after aging. Received 18 December 2001 and Received in final form 28 January 2002     

An enhanced polarization mechanism for the metal cations modified amorphous TiO<Subscript>2</Subscript> based electrorheological materials

In the present work, we developed a new kind of electrorheological (ER) materials, metal cations modified amorphous TiO₂ gels. The static yield stress of Sn⁴⁺ modified amorphous TiO₂ gel based ER fluid with a volume fraction = 38% reaches 26.2 kPa at E = 3.5 kV/mm. The result shows that metal cations can significantly enhance the ER activity of amorphous TiO₂ gels. We then proposed a novel ER effect mechanism (metal cations enhanced polarization mechanism) to clarify the experimental results. We believe that it is the metal cations that enhanced the polarization of the polar groups (-OH) which results in the corresponding enhancement of the interfacial polarization.     

Distribution of the delay time and the dwell time for wave reflection from a long random potential

We re-examine and correct an earlier derivation of the distribution of the Wigner phase delay time for wave reflection from a long one-dimensional disordered conductor treated in the continuum limit. We then numerically compare the distributions of the Wigner phase delay time and the dwell time, the latter being obtained by the use of an infinitesimal imaginary potential as a clock, and investigate the effects of strong disorder and a periodic (discrete) lattice background. We find that the two distributions coincide even for strong disorder, but only for energies well away from the band-edges. Received 11 June 2001 and Received in final form 30 July 2001     

Electron transport in a mesoscopic superconducting / ferromagnetic hybrid conductor

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors. Received 9 July 2002 / Received in final form 22 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: herve.courtois@grenoble.cnrs.fr RID="b" ID="b"associated to Université Joseph Fourier     

Optical properties of 4 Å single-walled carbon nanotubes inside the zeolite channels studied from first principles calculations

The structural, electronic, and optical properties of 4 ? single-walled carbon nanotubes (SWNTs) contained inside the zeolite channels have been studied based upon the density-functional theory in the local-density approximation (LDA). Our calculated results indicate that the relaxed geometrical structures for the smallest SWNTs in the zeolite channels are much different from those of the ideal isolated SWNTs, producing a great effect on their physical properties. It is found that all three kinds of 4 ? SWNTs can possibly exist inside the Zeolite channels. Especially, as an example, we have also studied the coupling effect between the ALPO₄-5 zeolite and the tube (5,0) inside it, and found that the zeolite has real effects on the electronic structure and optical properties of the inside (5,0) tube. Received 26 January 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: yxptl@hotmail.com     

Non ergodic aging in potassium niobo-tantalate crystals

Slow dynamics has been studied in various potassium niobo-tantalate crystals (KTN) by recording the complex dielectric constant after several thermal histories: isothermal evolution following controlled cooling or rapid quenching, positive or negative temperature cycles. The results reveal most of the behaviours of aging already found in lithium-potassium tantalate (KLT): effective ergodicity breaking (the asymptotic value of the dielectric constant varies as the logarithm of the cooling rate R), quasi-independence of the isothermal evolution with respect to a sojourn at lower temperature. But some differences between KTN and KLT are noticeable: coefficients of with opposite signs, role of the quenching temperature on the subsequent evolution, no overshot after a temperature jump. In order to explain the results we propose to extend the model initially developped for KLT taking into account the different nature of the low temperature phase, paraelectric for KLT and ferroelectric for KTN. In this model the variations of the dielectric constant are attributed to the slow movements of polarization domain walls hindered by static random fields. By measuring the dielectric constant during cooling and immediate heating at the same rate, an illustrative comparison is provided, showing that the evolution of the domain size is reversible in KLT and not in KTN. Received 17 November 1999     

Memory against temperature or electric field sweeps in potassium niobo-tantalate crystals

Aging, memories after temperature sweeps (double ramp and double jump) and memories after electric field sweeps (double ramp and double jump) were studied as a function of frequency. The experiments were performed at low temperatures in the ferroelectric phase of two potassium niobo-tantalate crystals K Ta_1-xNb_xO₃ with the niobium concentration x close to 0.02. Five complex quantities are defined, which respectively characterize these five phenomena. The main feature is that isothermal aging and memories after temperature sweeps have exactly the same frequency dependence while after electric field sweeps the frequency dependence is clearly different. Additionally, the role of the characteristics of the sweeps (amplitudes, rates of changes, durations) on these memories were measured. The observed behaviours are discussed in term of a model which attributes the time dependent effects to growth and reconformation of ferroelectric domains and takes into account that the domain wall motion is hindered by pinning sites. The difference in the frequency dependences against the nature of the swept parameters shows that the distribution of the reconformation time is sensitive to the biasing electric field. Received 28 May 2001 and Received in final form 10 October 2001     

On the competition between magnetic order and local Kondo effect in Kondo lattice

We investigate the competition between magnetic order and local Kondo effect in a Kondo lattice model (i.e. the Coqblin-Schrieffer Hamiltonian extended to a lattice) in a mean-field approximation, taking account of the spin-orbit degeneracy of each localized f level. This leads to the definition of a dependent Kondo temperature. We study the Kondo phase and compare its energy with the energies of magnetic phases, when the number of the conduction band electron per site is near one. We present a phase diagram which shows the occurrence of three phases: Kondo, antiferromagnetic and paramagnetic phases. Our model in the mean-field approximation also shows a somewhat flat Kondo temperature, for large values of , as a function of the exchange coupling J between conduction and localized f electrons. Finally we show some scaling effects between and J and we define a corresponding Kondo temperature. Received 21 September 1998 and Received in final form 8 February 1999     

Structure of quantum disordered wave functions: weak localization, far tails, and mesoscopic transport

We report on the comprehensive numerical study of the fluctuation and correlation properties of wave functions in three-dimensional mesoscopic diffusive conductors. Several large sets of nanoscale samples with finite metallic conductance, modeled by an Anderson model with different strengths of diagonal box disorder, have been generated in order to investigate both small and large deviations (as well as the connection between them) of the distribution function of eigenstate amplitudes from the universal prediction of random matrix theory. We find that small, weak localization-type, deviations contain both diffusive contributions (determined by the bulk and boundary conditions dependent terms) and ballistic ones which are generated by electron dynamics below the length scale set by the mean free path ℓ. By relating the extracted parameters of the functional form of nonperturbative deviations (“far tails”) to the exactly calculated transport properties of mesoscopic conductors, we compare our findings based on the full solution of the Schr?dinger equation to different approximative analytical treatments. We find that statistics in the far tail can be explained by the exp-log-cube asymptotics (convincingly refuting the log-normal alternative), but with parameters whose dependence on ℓ is linear and, therefore, expected to be dominated by ballistic effects. It is demonstrated that both small deviations and far tails depend explicitly on the sample size--the remaining puzzle then is the evolution of the far tail parameters with the size of the conductor since short-scale physics is supposedly insensitive to the sample boundaries. Received 19 August 2002 Published online 19 November 2002     

Tunneling time in magnetic barrier structures

We adopt the group velocity approach to the issue of tunneling time in two configurations of magnetic barrier structures, which are arranged with identical or unidentical building blocks. The effects of an external electric field are also taken into account. The tunneling time in magnetic barrier structures is found to be strongly dependent on the magnetic configuration, the applied bias, the incident energy as well as the longitudinal wave vector. The results indicate that for electrons with equal energy but different incident angles, the tunneling processes are significantly separated in time within the same magnetic barrier structure. In the configuration arranged with unidentical building blocks, there exists obvious asymmetry of tunneling time in two opposite tunneling directions. Such a discrepancy of the tunneling time varies distinctly with the longitudinal wave vector and the applied bias. Received 4 March 2002 / Received in final form 22 May 2002 Published online 17 September 2002     

Frequency dependence of aging, rejuvenation and memory in a disordered ferroelectric

We characterize in details the aging properties of the ferroelectric phase of KTa_1-xNb_x O₃ (KTN), where both rejuvenation and (partial) memory are observed. In particular, we carefully examine the frequency dependence of several quantities that characterize aging, rejuvenation and memory. We find a marked subaging behaviour, with an a.c. dielectric susceptiblity scaling as ω, where t _w is the waiting time. We suggest an interpretation in terms of pinned domain walls, much along the lines proposed for aging in a disordered ferromagnet, where both domain wall reconformations and overall (cumulative) domain growth are needed to rationalize the experimental findings. Received 10 November 2000 and Received in final form 20 February 2001     

Characteristics of transmission of electrons in a quantum wire tangentially attached to a superconductor ring threaded by magnetic flux

We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached to a superconductor ring threaded by magnetic flux. A point scatterer with a δ -function potential is placed at node to model scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially modified. Based on the condition of the formation of the standing wave functions in the ring and the broken of the time-reserve symmetry of Schr?dinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure can be well understood. Received 6 November 2001     

Shapes of nearly cylindrical, axisymmetric bilayer membranes

Shapes of nearly cylindrical sections of axisymmetric phospholipid membranes are studied theoretically. Describing the shape of such sections by their deviation from a reference cylinder, the well-established shape equation for axisymmetric bilayer membranes is expanded in terms of this deviation, and it is then solved analytically. The phase diagram shows the resulting stationary shapes as functions of system parameters and external conditions, i.e., the pressure difference across the membrane, the membrane tension, the difference between the tensions of the two monolayers, and the axial force acting on the vesicle. The accuracy of the approximate analytical solution is demonstrated by comparison with numerical results. The obtained analytical solution allows to extend the analysis to include shapes where numerical methods have failed. Received 27 September 2000 and Received in final form 26 March 2001     

Structural and electronic properties of gold nanowires

We have used high resolution transmission electron microscopy to determine the structure of gold nanowires generated by mechanical stretching. Just before rupture, the contacts adopt only three possible atomic configurations, whose occurrence probabilities and quantized conductance were subsequently estimated. These predictions have shown a remarkable agreement with conductance measurements from a break junction operating in ultra-high-vacuum, corroborating the derived correlation between nanowire atomic structure and conductance behavior. Received 28 November 2000