Spinless Defects Research Articles

Phase diagrams of a classical two-dimensional Heisenberg antiferromagnet with single-ion anisotropy

A classical variant of the two-dimensional anisotropic Heisenberg model reproducing inelastic neutron scattering experiments on La_5 Ca_9 Cu_24 O_41 [M. Matsuda et al., Phys.Rev. B 68, 060406(R) (2003)] is analysed using mostly Monte Carlo techniques. Phase diagrams with external fields parallel and perpendicular to the easy axis of the anisotropic interactions are determined, including antiferromagnetic and spin-flop phases. Mobile spinless defects, or holes, are found to form stripes which bunch, debunch and break up at a phase transition. A parallel field can lead to a spin-flop phase.

Universal behavior of spinless defects in the Haldane chain Y2Ba(Ni,Zn)O5: a study via bulk (squid) and local (NMR) probes

Abstract Spinless-impurity induced magnetic effects in Y 2 Ba(Ni,Zn)O 5 have been probed via susceptibility and 89 Y NMR. 89 Y NMR spectra of these samples contain multiple peaks, as a consequence of impurity-induced excitations. Here, the T -dependence of the impurity-induced magnetization is consistent with a net “effective” spin S ≈0.4 on either side of the Zn-impurity in the one-dimensional chain. Further, 89 Y NMR experiments with (Y 1− y Ca y ) 2 BaNi 1− x Zn x O 5 , indicate a non-interacting behavior of the hole (introduced due to Ca doping) and S =0 impurity.

SPINLESS IMPURITIES IN CUPRATES: LOCAL MAGNETISM AND KONDO EFFECT IN THE NORMAL AND SUPERCONDUCTING STATES

Spinless point defects in the CuO 2 planes of the cuprates have been shown for long to induce a magnetic response on the neighboring copper sites which behaves as a nearly free local moment in the under-doped case.1,2 In the case of Li + substitution on the Cu site, accurate 7 Li NMR shift data allow to evidence that the local moment susceptibility displays a ( T + T K ) -1 dependence, with a Kondo like temperature T K which increases abruptly near optimal doping.3 While a Curie paramagnetic response can be understood on general theoretical grounds in the case of undoped quantum spin systems, the Kondo screening of the moment might directly reflect the influence of charge carriers. This Kondo like behavior is also corroborated by measurements of the fluctuation time of the local moment deduced from 7 Li NMR spin lattice relaxation time.4 The local moment is found to survive in the superconducting state although a marked reduction of the screening is observed when T K > T c (see Ref. [5]), as might be expected from theoretical considerations. Correlatively it will be shown that the scattering of charge carriers by spinless defects can be studied accurately in single crystals in which such defects are created by electron irradiation. The low T upturns of the resistivity can be associated with a Kondo-like T dependence of the scattering in the under-doped case. In the over-doped case, T K being very high, the scattering becomes T-independent and 2D weak localization effects dominate.6

INFLUENCE OF ELECTRON IRRADIATION DEFECTS ON THE TRANSPORT PROPERTIES OF CUPRATES

We use electron irradiation at low temperature to introduce defects in a controlled manner in the cuprates. We have shown in YBCO 7 that the main influence on transport properties is due to stable defects in the CuO 2 plane. They are found analogous to Zn substitution on the copper planar site.1 Compared to impurity substitutions, the great advantage of electron irradiation is to allow to study the physical properties of a single sample with an increasing defect content. Tc and electrical resistivity measurements have been performed on electron irradiated single crystals of different cuprates (YBCO, Bi-2212, Hg-1223, T1-2201) for a wide range of hole dopings nh. A simple correlation between the decrease of Tc and the increase of residual resistivity ΔR2D is found, as expected for d ware superconductivity. The ratio ΔTc/ΔR2D, which is expected to be proportional to nh/m* is found to reproduce the variation of nh from the under-doped to the over-doped regime.2 This demonstrates that the hole content is the relevant parameter to describe the transport properties all over the phase diagram. For large defect contents, low T upturns of the resistivity are observed in under-doped YBCO 6.6 and over-doped T1-2201 (see Ref. [3]). In the highly over-doped T1 compound the decrease of conductivity scales as expected from weak localization theory. For YBCO 6.6 the large low T contribution to the resistivity is shown to be initially proportional to the defect content. It might therefore be associated to Kondo like spin flip scattering term. This would be consistent with the results on the magnetic properties induced by spinless defects such as Li (see Ref. [4]).

Disorder and transport in cuprates: weak localization and magnetic contributions.

We report resistivity measurements in underdoped YBa 2Cu3O6.6 and overdoped Tl 2Ba2CuO6+x single crystals in which the concentration of defects in the CuO (2) planes is controlled by electron irradiation. Low T upturns of the resistivity are observed in both cases for large defect content. In the Tl compound the decrease of conductivity scales as expected from weak localization theory. On the contrary, in YBa 2Cu3O6.6 the much larger low T contribution to the resistivity is proportional to the defect content and might then be associated with a Kondo-like spin flip scattering term. This would be consistent with the results on the magnetic properties induced by spinless defects.

Charge storage states in polyenes

Changes in the geometric and electronic structure of polyenes upon sodium doping have been studied in detail. A short polyene containing three double bonds, hexatriene, forms a charge transfer complex with a maximum of one sodium ion. The interaction results in the formation of a polaron (radical anion) on the polyene chain. This type of charge storage state is shown to be unstable relative to the formation of a hexatriene dimer when interaction between two polyenes is allowed for; the two polyenes are then bound to each other via a carbon–carbon single bond. The charge storage states in this case correspond to two singly charged, spinless defects, one on each polyene segment of the dimer molecule. Decapentaene, which contains five carbon–carbon double bonds, is shown to be able to accept two electrons from a pair of sodium atoms. Similar to the case of doped trans-polyacetylene, the charges are stored as a soliton–antisoliton pair. However, when there is an excess of the number of polyene molecules relative to the number of sodium atoms, i.e., below saturation doping, this type of charge storage state is also unstable towards the formation of a decapentaene dimer of the same type as in hexatriene.

Spin dilution in a ferromagnetic chain.

We have systematically introduced spinless defects in the quasi-one-dimensional ferromagnetic system decamethylferrocenium tetracyanoethanide, (FeCp{sub 2}{sup *})(TCNE). The three-dimensional ordering temperature {ital T}{sub {ital c}} decreases rapidly with the concentration of spinless sites. The rate of decrease of {ital T}{sub {ital c}} with concentration is in the order of the exchange spatial anisotropy (i.e., {ital J}{sub interchain}/{ital J}{sub intrachain}) of the parent sysem in accordance with long-standing theoretical predictions.

Systematic study of light-induced effects in hydrogenated amorphous silicon.

A systematic investigation of the origin of the photoinduced changes in the defect density of states in the pseudogap of undoped hydrogenated amorphous silicon (a-Si:H) has been carried out on films deposited by glow-discharge decomposition of silane under different conditions. We used several complementary techniques, such as junction capacitance versus temperature, electrical conductivity, electron-spin resonance, and photothermal deflection spectroscopy, to determine the shifts in the Fermi-level (${\mathit{E}}_{\mathit{F}}$) position as well as the variation of the defect density of states, within the forbidden gap, after annealing (state A) and light-soaking (state B) treatments. The total hydrogen concentration and the hydrogen bonding configurations of each sample were analyzed by elastic-recoil-detection analysis (ERDA) and infrared (ir) transmission measurements, respectively, both for state A and state B. The experimental results show in all cases an increase in the bulk density of states accompanied by a shift of ${\mathit{E}}_{\mathit{F}}$ towards the valence band in the light-soaked state B with respect to the annealed state A, but these changes depend considerably on the deposition conditions of the films. On the other hand, no change is detected either in the ERDA or in the ir spectra. Moreover, it is found that when the saturated light-soaked samples are partially annealed at temperatures ranging from 110 \ifmmode^\circ\else\textdegree\fi{}C to 130 \ifmmode^\circ\else\textdegree\fi{}C and for times less than 15 min, and then left in the dark at room temperature for 24 h and more, a long-time process, characterized by a variation of the density of states as well as by a motion of ${\mathit{E}}_{\mathit{F}}$ within the gap, occurs in these films. The observed behavior is completely different from one sample to another. From the analysis of the results, it is suggested that in some samples the photodegradation is mainly due to a trapping of the excess free carriers at charged spinless defects, which must be present in the sample at equilibrium in addition to the neutral dangling bonds.

Phenyl ring rotations, structural order and electronic states in polyaniline

Much progress has recently been made in the physics and chemistry of the polyanilines with respect to several central issues, such as the nature of the electron-phonon coupling in these materials, the stability of polarons versus bipolarons, and the role of disorder. The three primary oxidation states of polyaniline (leucoemeraldine, emeraldine and pernigraniline) have been synthesized. Photoinduced absorption studies on these materials reveal long-lived weak induced infrared activity and strong induced electronic transitions, demonstrating the importance of massive polarons (and other related defects) involving distortions of the ground state ring angles in these ring-containing polymers. The properties of recently synthesized ring derivatives of polyaniline, such as poly( o-toluidine), can also be understood by changes in ring-rotational properties. Localization effects are enhanced through chain derivatization, which increases the chain separation and thus reduces interchain interactions. The isolation of partially crystalline forms, fibers and oriented films of the emeraldine oxidation state of polyaniline has allowed not only the determination of structural information via X-ray diffraction, but also a correlation between the microscopic structure and the magnetic properties of the charged defect states of these materials. While a metallic polaron lattice is formed in protonated crystalline regions of emeraldine base, spinless defects form upon protonation of the amorphous regions.

Origin of the photo-induced changes in hydrogenated amorphous silicon

The electronic properties of hydrogenated amorphous silicon films are discussed in detail. Particular attention is paid to the changes induced by photogeneration of excess free carriers. Previous models which have been proposed to account for such effects are classified and criticized. An alternative explanation, which is based on the unique electronic structure of hydrogenated amorphous silicon, is proposed and analyzed. In this model, no new defects are created by the light, but rather the photo-induced effects follow from a metastable trapping of the excess free carriers at charged spinless defects which are present at equilibrium.

A physicochemical interpretation of the photoconductivity of a-Si : H

Total hydrogen content and its distribution among SiH, SiH2 and other H' sites of sputtered a-Si : H films vary after posthydrogenation but the dangling bond concentrations remain constant. This can be explained by preferential H diffusion in the tissue zone where most of the hydrogen is located, while the residual dangling bonds are in the islands. The photoconductivity is controlled by spinless defects of the tissue zone originating from back defects of H on their various sites. It decreases as the relative SiH 2 content increases.