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LOWER CRITICAL MAGNETIC FIELDS OF Y-Ba-Cu-O SUPERCONDUCTOR IN HIGH TEMPERATURE REGION
Authors: SHIYING DING , JIALIE YAN , KEXING SHI , ZHENG YU , JUN CHENG , LI QIU , HONGWU TONG
Published: November 2004

We have measured the magnetizations and determined the dependence of lower critical magnetic fields on temperature for high Tc Y-Ba-Cu-O Superconductor in liquid nitrogen temperature region. The structure examinations and the measurements of virgin magnetization in small fields show that our samples are made up of granules coupled to each other. Therefore the measured lower critical fields are attributed to the values at which flux enters the granulas.
FTIR SPECTRA OF YBa2Cu3O7−δ AND RELATED OXIDES
Authors: FERNANDES J.R. , VARMA VIJAY , GANGULY P. , RAO C.N.R.
Published: November 2004

Fourier-transform infrared spectra of superconducting YBa 2 Cu 3 O 7 and semiconducting YBa 2 Cu 3 O 6 have been investigated along with those of other related oxides of this family.
STRUCTURAL STUDIES ON THE SUPERCONDUCTING PHASE Bi4Sr4Ca2Cu4O2−δ
Authors: TANG YOUQI , LIN BINGXIONG , ZHENG XIANGMIAO , ZHU WENJIE , ZHANG YUFEN , LIU ZHENGYI , ZHANG WANJING
Published: November 2004

Very recently H. Maeda et al.1 reported a high Tc superconducting material of the composition BiSrCaCu2O χ that makes a feature of containing no rare earth metal. We are interested in the structural and chemical aspects of this new class of materials.2 We have prepared a homogeneous sample for the superconducting phase in the Bi-Sr-Ca-Cu-O system, and have determined its crystal structure by powder XRD method. At a very late stage we noticed the structural work by a Bellcore group.3 Fortunately our work has quite a number of distinguishable features.
ANALYTICAL ELECTRON MICROSCOPY STUDY ON HIGH Tc SUPERCONDUCTOR Th(Y)-Ba-Cu-O
Authors: ZHAO J.G. , CAI L.Y. , ZHANG Y.L. , CHENG X.R. , Li F.H.
Published: November 2004

Electron diffraction and X-ray energy dispersive analysis are used to determine the phases and chemical composition of the compound Th(Y)-Ba-Cu-O. The main phase is Th(Y)Sa2Cu3O 7−x which has the similar crystal structure as YBa2Cu3O 7−x and shows the superconductivity with Tc=67 K.
THE PRELIMINARY EXPLORATION OF THE REASON OF LOW Jc FOR OXIDE SUPERCONDUCTOR
Authors: YU-HUA HUO , BAO-RUI LIU , FENG-TONG SUN , ER-LI CHEN , JIE YAN
Published: November 2004

Using the experimental methods about weak current and magnetic field, we make the experimental observation for oxide superconductor. The result obtained shows that there is Josephson weak link between superconducting grain. We consider that the weak link is the main reason of low Jc for oxide superconductor.
THE POSSIBILITY OF THE LIBRATIONAL MECHANISM OF HIGH-TEMPERATURE SUPERCONDUCTIVITY IN THE YBa2Cu3O7−x TYPE TERNARY OXIDE
Authors: HANUZA J. , KLAMUT J. , HORYŃ R. , JEŹOWSKA-TRZEBIATOWSKA B.
Published: November 2004

IR absorption and reflectance spectra as well as Raman scattering spectra of the YBa2Cu3O 7−x superconductor in the orthorhombic phase have been measured at temperatures below and above the critical point. The role of librational modes in the superconductive transition is discussed. The mechanism of the high-temperature superconductivity of YBa2Cu3O7− x is proposed based on the electron coupling by libron.
ON THE 110K SUPERCONDUCTING PHASE IN THE Bi-Sr-Ca-Cu OXIDE SYSTEM
Authors: RAN Z.Y. , CHU X. , WANG J.H. , ZHENG D.N. , JIA S.L. , MAI Z.H. , ZHAO Z.X.
Published: November 2004

We have prepared material with high content of 110K superconducting phase by a novel two-step synthetic method. The samples have been characterized by both resistive and a.c. magnetic measurements, secondary electron microscopy, EDX micro-analysis and X-ray diffraction. The 110K superconducting phase has been found to be the Bi 2 Sr 2 Ca 2 Cu 3 O y compound with a layered orthorhombic structure, with a=5.42, b=5.44 and c=36.8 Å respectively.
NUCLEAR MAGNETIC RESONANCE IN YBa2Cu3O7
Authors: TUNSTALL D.P. , SOLANKI-MOSER M.E.
Published: November 2004

This is a preliminary report of a structural nature on the properties of 1–2–3 YBaCuO, the 93K superconductor. The data to be presented constitute field swept NMR spectra taken at 111 and 141 MHz, using spin echo techniques, and using magnetic fields in the range 8 to 12.7 T. In addition, experiments are reported on the temperature dependence in the liquid helium range of the field position of a sharp feature in the spectrum. The particular sample on which these NMR results were taken was supplied in powder form, by Imperial College, with magnetic measurements indicating a superconducting transition at 90K.
EVIDENCE OF HIGH Tc SUPERCONDUCTIVITY IN THE SERIES OF Y1−xPbxBa2Cu3O7−δ, (0<x<1) AND YBa2Cu3O7−δNy, (0<y<2.0)
Authors: NIARCHOS DIMITRIS
Published: November 2004

The high Tc superconductor of the type YBa 2 Cu 3 O 7 is characterized with transition temperatures around 92K and very large critical fields, in excess of a 100T. Although many substitutions for the Y with other rare earths have been tried and reported, no data have been reported to our knowledge on the substitution of Y with Pb and very few controversial attempts on partial replacement of O with N. Here we report data on the system Y 1−x Pb x Ba 2 Cu 3 O 7, where 0<x<.7, which show that the system has transition temperatures around liquid nitrogen and for the YBa 2 Cu 3 O 7−δ N y, (0<y<2.0), where the transition temperature is only slightly affected with the addition of N.
TUNNELLING JUNCTIONS OF SINGLE CRYSTAL BCSCO/SN FILM
Authors: CHEN YINGFEI , TAO HONGJIE , ZHAO SHIPING , YAN YIFENG , YANG QIANSHENG
Published: November 2004

The single particle tunnelling junctions consisting of single crystal BCSCO and Sn film counter electrode have been prepared. The I-V and dI/dV-V characteristics have been measured. Observed value of the energy gap is 23 meV which corresponds to 2Δ/kT c of 6.0, much larger than the BCS weak-coupling limit of 3.52.
RESEARCH ACTIVITIES ON HIGH-Tc SUPERCONDUCTORS IN JAPAN
Authors: SUGAWARA K.
Published: November 2004

Newly discovered high-T c superconductor, Bi-Sr-Ca-Cu-O (BSCCO) systems are intensively investigated in Japan. Most works are experimental, but not theoretical. Some recent papers on (1) crystal structure, (2) impurity substitution effects and (3) thin film fabrication technologies are briefly reviewed.
IS THE MAGNETIC SUSPENSION OF HIGH-TEMPERATURE SUPERCONDUCTORS A GENERAL PHENOMENON?
Authors: POLITIS C. , STUBHAN F.
Published: November 2004

We have observed that all high-T c superconductors like RE Ba 2 Cu 3 O 7, Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O can be suspended below a strong permanent magnet. The magnetic suspension in high-temperature superconductors is obviously a general phenomenon, i.e., it is not restricted to samples of YBa 2 Cu 3 O 7 doped with silver oxide as suggested in literature.
PASSIVATION PROCESS OF HIGH Tc SUPERCONDUCTORS YBa2Cu3O7−δ BY FLUORINE GAS TREATMENT
Authors: TRESSAUD A. , CHEVALIER B. , LEPINE B. , DANCE J.M. , LOZANO L. , GRANNEC J. , ETOURNEAU J. , TOURNIER R. , SULPICE A. , LEJAY P.
Published: November 2004

The atmospheric degradation of high T c superconductors YBa 2 Cu 3 O 7−δ can be prevented using fluorine gas processings around room temperature. A thin layer of amorphous fluorides (and/or oxyfluorides) is formed at the surface of the particles, which passivates the material against hydrolysis and gas-exchange processes. A fluorination mechanism has been proposed, which can account for both fluorine analysis and weight uptake. The superconductivity onset does not seem to be affected by fluorination. Furthermore, an improvement of the shielding effect-Meissner effect cycle to 91% reversibility is achieved when F 2-treated oxides are heated at 350°C under vacuum.
CLOSE-PACKING OF GROWING DISCS
Authors: BURSILL L.A. , XUDONG FAN
Published: November 2004

Spiral lattices are derived by allowing growing discs to aggregate under a close-packing rule. Both Fibonacci and Lucas numbers of visible spirals arise naturally, dependent only on the choice of growth centre. Both the rate of convergence towards an ideal spiral, and chirality, are determined by the initial placement of the first few discs (initial conditions). Thus the appearance of spiral packings is no more or less mysterious than the appearance of hexagonal packed arrays of equal discs.
SMALL BIPOLARONS AND SOME ANOMALOUS PROPERTIES OF HIGH-Tc SUPERCONDUCTORS
Authors: ALEXANDROV A.S.
Published: November 2004

Some peculiar properties of the high temperature superconductor Y 1 Ba 2 Cu 3 O 7−x, namely a surprising oxygen isotope effect, a strong upward curvature of H c2 (T) and an enormously high heat capacity jump per one carrier ΔC/p≃kB, where p is the hole concentration, and kB is the Boltzmann constant, are explained in the framework of the small bipolaron theory.
EFFECT OF A STEP AT AN INTERFACE ON TUNNELLING BARRIER PARAMETERS
Authors: MORALES F. , HEIRAS J.
Published: November 2004

The problem of penetration by a conducting slab in tunnel junction metal-oxide interfaces is treated using a step model for the barrier. The dynamical conductance of a junction having a composed barrier (two sections with different widths but the same barrier heights) is calculated using a trapezoidal barrier model for each section. An unexpected increment of the barrier height is found when a very thin conducting slab penetrates the barrier of a junction.
A NEW LOWER CRITICAL FIELD AND ANOMALY OF MAGNETIZATION CURVE IN HIGH TcY1Ba2Cu3O7−δ BULK SUPERCONDUCTORS
Authors: XIAOWEN CAO , GUCHANG HAN , TINGYU ZHANG
Published: November 2004

The magnetization behaviors of single phase Y1Ba2Cu3O7−δ bulk superconductors have been studied by measurement of the DC susceptibility. It is found that there are two lower critical field. One which is near 10 Oe at 77K is defined as a weak lower critical field and denoted by HclJ. dHc1J/dT is about 0.2 Oe/K near Tc. Another is still called as usual lower critical field, Hcl. It is 60 Oe at 77K and dHc1/dT is about 4.6 Oe/K. The detailed measurement of DC magnetization shows also that an anomaly of the magnetization curve appears between HclJ and Hc1. The amplitudes of anomalous peak and valley have some difference depending on how samples are prepared. The mechanism of Hc1J appearance and the anomaly of the magnetization has been discussed in “Soft Sponge Model” which will be published in Chin. J. Low Temperature Physics.
STUDYING IN-SITU ON THE SUPERCONDUCTING TRANSITION AND INTERNAL FRICTION OF A HIGH Tc SUPERCONDUCTOR YBa2Cu307−δ
Authors: DU JIA-JU , JIANG JIAN-YI , WANG XIANG , YIN HUA-QING
Published: November 2004

The resistence, internal friction and modulus of a high Tc superconductor YBa2Cu3O7−δ were measured in-situ in 80–300 K. Two obvious peaks of internal friction were observed about Tc. On account of character of peaks, it is suggested that these two peaks are associated with the electronic relaxation between Cu3+ and Cu2+. The peak and anomalies of Young’s modulus observed in the range between 160 K and 280 K may be related to the adjustment of deficiency of oxygen.
DEFECT STRUCTURES AND PHASE RELATIONSHIPS IN THE Y-Ba-Cu-O SUPERCONDUCTOR
Authors: He A.Q. , Ye H.Q.
Published: November 2004

Powder and foil specimens of the high Tc Y-Ba-Cu-O oxide superconductor have been observed by Transmission Electron Microscopy (TEM). The results showed that the main phase in all studied samples has an orthorhombic perovoskite-like structure, Y1Ba2Cu3O7−δ, with lattice parameters a=0.386, b=0.389 and c=1.159 nm (phase I). The minor phase has also an orthorhombic structure with lattice constants a=1.23, b=0.74 and c=0.57 nm (phase II). Several defect structures have been observed, such as rotation twins, lattice distortions in phase I, and stacking faults in phase II. At the interface between phase I and phase II an amorphous area was also observed.
TRANSMISSION ELECTRON MICROSCOPY STUDY OF TWIN DOMAIN STRUCTURES IN YBa2Cu3O7−x SUPERCONDUCTOR
Authors: CAI LIYING , ZHAO GANGKAI , ZHAO MEIJUAN , ZHANG MEIZHEN , ZHANG XIAOWEN
Published: November 2004

The study of the twin domain structure of YBa2Cu3O7−x superconductor synthesized by coprecipitation method has been carried out by using the transmission electron microscopy. It was found that the twin domain boundary was parallel to the (110) lattice planes. The twin domain is designated as a 90° one. The same twin structure exhibited different characteristics of spot splitting in its selected area eletron diffraction patterns (SADP) when it was irradiated in different incidence directions. The experimental results were satisfactorily consistent with the theoretical calculation. The mechanism of the formation of 90° twin domain has also been discussed. The phase transformation from tetragonal to orthorhombic taking place in YBa2Cu3O7−x ceramics during cooling is considered to be of the “Martensite”.
MAGNETIZATION MEASUREMENTS ON Bi2Sr2CaCu2Oy SINGLE-CRYSTAL
Authors: CAO Z.S. , ZHENG D.N. , CHEN G.H. , WANG J.H. , YAN Y.F. , NI Y.M. , JIA S.L. , ZHAO Z.X.
Published: November 2004

Diamagnetic susceptibility and the magnetization curves at 1.5 K and 77 K were measured on the single crystal of Bi 2 Sr 2 Ca 1 Cu 2 O y. The results show that the Meissner fraction is anisotropic and the susceptibility reduces slightly with temperature decreasing below 20 K. The magnetization curves exhibit the hysteresis at 1.5 K but no hysteresis is observed at 77 K. Critical current J c at 1.5 K and 77 K are estimated using critical state model.
SCANNING TUNNELING SPECTROSCOPY OF THIN FILMS OF HIGH TEMPERATURE SUPERCONDUCTORS
Authors: DE LOZANNE ALEX
Published: November 2004

In this paper recent results in the synthesis of thin films of Y-Ba-Cu-O and their characterization by scanning tunneling microscopy/spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy and wavelength dispersive spectroscopy, resistivity and critical currents will be briefly reviewed.
MODULATED STRUCTURES OF THE MAIN PHASE IN Bi-Sr-Ca-Cu-O SUPERCONDUCTOR
Authors: ZHU J. , YE H.Q. , LIN X.W. , HE A.Q. , FENG S.Q. , ZHU X. , GAN Z.Z.
Published: November 2004

An important structure characteristic of the main phase in Bi-Sr-Ca-Cu-O superconductor is the modulated structure. In this paper, two types of one dimensional modulation, one type of two-dimensional modulation as well as other modulations have been observed by using TEM and discussed.
CRITICAL CURRENT IN Tl2Ba2Ca2Cu3Oy SUPERCONDUCTOR
Authors: WEI C.D. , LIN C. , SHEN Z.H. , LIU Z.X. , LAN J. , WU K. , XIE L.X. , DAI Y.D. , GAN Z.Z.
Published: November 2004

Magnetic critical current J c of Tl 2 Ba 2 Ca 2 Cu 3 O y was measured. It is comparable with that of YBa 2 Cu 3 O 7−x in a field H up to 60 kOe at T≤15 K. For higher temperature, the J c of Tl 2 Ba 2 Ca 2 Cu 3 O y decreases more rapidly than that of YBa 2 Cu 3 O 7−x. Possible origin is discussed.
FLUCTUATION CONDUCTIVITY EFFECTS ON THERMOELECTRIC POWER OF GRANULAR Bi1.75Pb0.25Ca2Sr2Cu3O10 SUPERCONDUCTOR
Authors: LAURENT CH. , PATAPIS S.K. , GREEN S.M. , LUO H.L. , POLITIS C. , DURCZEWSKI K. , AUSLOOS M.
Published: November 2004

We report precise measurements of the thermoelectric power (TEP) of granular superconducting Bi 1.75 Pb 0.25 Ca 2 Sr 2 Cu 3 O 10. The TEP is strictly linear at high temperature. Superconductivity fluctuations set in at about 140 K. From the temperature derivative of the excess TEP (with respect to a straight line at “high temperature”), the critical behavior is obtained in the mean field regime, and is found identical to that of the temperature derivative of the excess electrical resistivity.
INTERBAND OPTICAL CONDUCTIVITY OF Bi2CaSr2Cu2O8
Authors: CHING W.Y. , ZHAO G.L. , XU Y.N. , WONG K.W.
Published: November 2004

The band structure and interband optical conductivity of Bi 2 CaSr 2 Cu 2 O 8 superconductor in the body-centered tetragonal sub-unit is calculated self-consistently using orthogonalized linear combination of atomic orbitals method. The result shows Bi 2 CaSr 2 Cu 2 O 8 has both semi-metal-like and semiconductor-like features with small band gap and band overlap. The interband optical conductivity shows strong anisotropy between the in-plane and z-direction components. A plasmon frequency at 6 eV is predicted.
MICROWAVE INDUCED DC VOLTAGE ACROSS THE BRIDGE OF YBaCuO SUPERCONDUCTING BULK
Authors: HUO Y.H. , SUN F.T. , FEN H.H. , YAN J.
Published: November 2004

For the bridge of YBaCuO superconducting bulk, there is an induced dc voltage across the bridge when it is exposed to microwave radiation alone and left unbiased. The magnitude and the polarity of induced dc voltage are sensitive to microwave power and frequency. For some bridges, we carved the change of its induced dc voltage with time has better repeativety.
HIGH Tc SUPERCONDUCTING FILMS AND POWDERS SYNTHESIZED BY SOL-GEL PROCESSES
Authors: CHEN F. , LUO S. , LI N. , LI C. , JIE B. , LI G. , YIN D.
Published: November 2004

A sol-gel method producing slice-form fine powder of high T c Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O by organic amine-precipitation is presented. The average grain size of such powders is much smaller than that by the conventional solid-reaction. Superconducting films of Y-Ba-Cu-O with zero resistance T co =82.5 K prepared by this method manifest strong (001) texture. The reason of preferential orientation on polycrystal substrate is discussed.
SYNTHESIS OF SUPERCONDUCTING Bi-Sr-Ca-Cu-O OXIDES VIA MECHANICAL ALLOYING
Authors: WHANG S.H. , LI Z.X.
Published: November 2004

Bi-Sr-Ca-Cu-O superconductors have been prepared from the constituent pure metals by mechanical alloying in air and succeeding sintering at high temperatures in flowing oxygen. X-ray patterns from the milled powders indicate that the dissolution of excess Cu and Bi in the Bi-Sr-Ca-Cu matrix occurs progressively with milling. However, a prolonged milling resulted in amorphous-like diffraction patterns with the first peak centered around 30°. The particle size rapidly asymptotes to a fixed value near 30 h milling. The X-ray patterns of the sintered materials are in good agreement with those of the orthorhombic structure reported in the superconducting Bi-base oxides. The low Tc phase in the Bi-base oxides was identified from the resistivity and susceptibility measurements. The superconducting temperature appears to be sensitive to the milling conditions.
ANOMALIES OF CELL PARAMETERS BEFORE SUPERCONDUCTING TRANSITION FOR BISMUTH BASED OXIDE SUPERCONDUCTOR
Authors: YUAN S.L. , GUAN W.Y. , CHEN Z.J. , HUANG Y.L. , JIA Y.B. , ZHENG J.Q. , ZHU B. , WANG W. , ZHENG G.G.
Published: November 2004

The results of X-ray diffraction analysis at low temperatures for bismuth-based high-Tc oxide superconductors are reported in this letter. There is no significant structural phase transition during superconducting transition. However, it appears that the cell parameters change anomalously before superconducting transition. This anomalous change in cell parameters is repeated for different bismuth-based high-Tc oxide superconductors.
SUPERCONDUCTIVITY IN Y-Ba-Cu-Pb-O SYSTEM
Authors: NOWAKOWSKI T. , SYSŁO W.A. , ZUROWSKA B. , MROZIŃSKI J.
Published: November 2004

Superconductivity in YBa 2( Cu 1−x Pb x)3O7−δ system for x=0~0.4 at LN 2 temperature was observed. For this range of x, the zero resistance temperature is nearly independent of x.
NEUTRON DIFFRACTION STUDY ON Nd2(Fe0.95Si0.05)14B
Authors: DING Y.F. , YANG J.L. , ZHANG B.S. , JIN L. , ZHOU H.M. , YE C.T. , YANG Y.C. , XING F. , KONG L.S.
Published: November 2004

In Nd2Fe14B permanent magnet, substitution of a small amount of Si for Fe would increase the value of Curie temperature. Powder neutron diffraction study has been carried out on Nd2(Fe0.95Si0.05)14B to probe the origin of this effect. Refined results show that the Si atom prefers c site and j1 site. This site preference of Si atoms results in the increase of the proportion of positive exchange interactions to total exchange interactions and accounts for the increase of Curie temperature.
RESISTIVE TRANSITIONS OF SOME SUPERCONDUCTING 123-AgO COMPOSITES IN HIGH MAGNETIC FIELDS
Authors: HUANG C.Y. , WU M.K.
Published: November 2004

We have measured the resistance of superconducting n RBa 2 Cu 3 O x: AgO (n=2 to 5, R=Y, Eu and Gd) composites near the superconducting transition temperatures in fields up to 20 T. The transition width is proportional to H2/3, and the superconducting critical current density at 0 K is estimated to be ~107 A/cm 2.
BOUND STATES BETWEEN TWO PARTICLES IN A TWO- OR THREE-DIMENSIONAL INFINITE LATTICE WITH ATTRACTIVE KRONECKER δ-FUNCTION INTERACTION
Authors: DONG SHAO-JING , YANG CHEN NING
Published: November 2004

The bound state energy and wave function for two particles in a 2- or 3-dimensional infinite lattice with attractive Kronecker δ-function interaction are discussed.
STUDY OF Cu DOPING ON PbS THIN FILMS
Authors: SHARMA T.P. , KUMAR R. , JAIN G. , SHARMA S.K.
Published: November 2004

Lead sulphide thin films have been deposited by conventional chemical bath technique. The effect of Cu doping is studied on the resistivity and the band gap of these films. An anamolous behavior is obtained in the case of variation of band gap with impurity (mol%) which shows that certain values of impurity have prominent features. While the resistivity decreases first with the increase in impurity, after a particular value the resistivity adopts a rising trend, and on still higher doping the decreased resistivity again starts to take place.
Nd, Ce(fπ)-O(pπ) HYBRIDIZATION IN Nd2−xCexCuO4 AND DYNAMIC JAHN-TELLER PAIRING IN HIGH-Tc SUPERCONDUCTORS
Authors: JOHNSON K.H. , CLOUGHERTY D.P. , McHENRY M.E.
Published: November 2004

Hybridization of Nd (fπ) and Ce (fπ) orbitals with composite O (pπ)- O (pπ) bonding/ Cu(dπ*)-O(pπ) antibonding orbitals at the Fermi energy (EF) is shown to promote high-Tc superconductivity in Nd 2−x Ce x CuO 4. Dynamic Jahn-Teller coupling of these hybrid molecular orbitals to the lattice leads to Cooper pairing as it does in other high-Tc superconductors, such as La 2−x Sr x CuO 4, where O(pπ) character at E F is dominant.
ON THE LINEAR RESISTIVITY LAW ρ∝TOF HIGH Tc SUPERCONDUCTORS
Authors: ENZ CHARLES P.
Published: November 2004

It is shown that at low temperatures, T≪TF, a general form of short-range s-d type interaction between two species of fermions leads to a resistivity law ρ∝Td−1 where d=2, 3 is the dimension of the system. In energy units T F is of the order of the bandwidths of the charge carriers which are assumed to be quasi-free. The calculation is based on a Boltzmann transition rate deduced from the two-body s-d type scattering matrix element. The consequences of making one of the fermions very heavy and of replacing it by a boson are also discussed.
EFFECT OF SUBSTITUTION OF Na, Cd, Bi for Ca in Bi-Sr-Ca-Cu OXIDE SUPERCONDUCTORS
Authors: MEI YU , GREEN S.M. , JIANG C. , LUO H.L. , POLITIS C.
Published: November 2004

Samples with compositions Bi 1.8 Pb 0.2 Sr 2( Ca 2−x Na x) Cu 3 O y, BiSr(Ca 1−x Cd x) Cu 2 O y and BiSr ( Ca 1−x Bi x) Cu 2 O y were prepared through solid state reaction. The X-ray diffraction results were analyzed and their resistivity behavior was studied. In general, all such substitution adversely affected their superconductive properties. All samples are of multiphase. The normal state resistivity behaves quite differently with the substitution of electropositive Na as compared with the replacement of electronegative Bi.
COLD FUSION: EFFECTS OF POSSIBLE NARROW NUCLEAR RESONANCE
Authors: SHIHAB-ELDIN A.A. , RASMUSSEN J.O. , JUSTICE M. , STOYER M.A.
Published: November 2004

The influence of a possible and as yet undiscovered narrow resonance in 4 He on the d-d fusion rate near threshold is examined. A qualitative discussion of the structure of the lowest four 0+ states and its impact on the partial widths of decay channels is presented.
TRANSPORT PROPERTIES OF Tl–BASED SUPERCONDUCTORS PREPARED BY LIQUID-GAS-SOLIDIFICATION PROCESS
Authors: CHEN H. S. , GYORGY E. M. , KIMERLING L. C. , KORTAN A. R. , THIEL F. A.
Published: November 2004

Tl-based superconducting oxides have been fabricated by a novel liquid-gas solidification (LGS) process. The resultant oxides exhibit onset temperature of 120 K and zero-resistance above 113 K. Critical current densities J c obtained from dynamic hysteresis loops at a sweep rate 15.4 kOe/min and H a = 9 kOe are ~ 106 A/cm 2 at 10 K and ~ 5 × 104 A/cm 2 at 60 K. Appreciable time effects on magnetic hysteresis loops are seen at 77 K.
WEAK FIELD MAGNETIZATION AND SUSCEPTIBILITY FLUCTUATION IN Bi2Sr2CaCu2Oy SINGLE CRYSTALS
Authors: CHEN G. H. , YANG G. , YAN Y. F. , JIA S. L. , NI Y. M. , ZHENG D. N. , YANG Q. S. , ZHAO Z. X.
Published: November 2004

Some magnetization curves of single crystal of Bi 2 Sr 2 CaCu 2 O y in low magnetic field H ‖ c axis are given. The magnetization loop is getting smaller with the temperature increase and then disappears at temperature even higher than T c . The experimental measurements of the susceptibility in zero-field limit show that the results can be treated with two-dimensional thermo-fluctuation theory.
ON TOPOLOGICAL STATISTICS TRANSMUTATION
Authors: DZYALOSHINSKII I.
Published: November 2004

PT parity violation, 2d "angular" spin of holons and spinions and a vague possibility of topological statistics transmutation in 3d are discussed.
SQUEEZING IN COLLECTIVE RESONANCE FLUORESCENCE: EFFECT OF A THERMAL FIELD
Authors: LAN LE HONG , SHUMOVSKY A. S. , QUANG TRAN
Published: November 2004

The influence of a thermal field on the squeezing for the mixture of two sidebands of Mollow's triplet is investigated.
X-RAY INVESTIGATION ON TEMPERATURE DEPENDENCE OF THE TILT ANGLE IN FERROELECTRIC LIQUID CRYSTALS
Authors: ZHANG XIANG , SUN ZHENGMIN , FENG DUAN , LI GUOZHEN
Published: November 2004

The temperature dependence of the molecular tilt angle in a series of ferroelectric liquid crystals is deduced from X-ray diffraction measurement. The critical exponent β has been estimated from the experimental data. The non-classical value of β for the compounds lies in the range 0.36–0.41.
SCALING LAWS FOR SPIRAL AGGREGATION OF CONTACT DISCS
Authors: BURSILL L. A. , FAN XUDONG
Published: November 2004

The relationship between disc and aggregate radial growth laws has been investigated for spiral structures. Scaling laws for power law and logarithmic functions were obtained. Packing efficiencies and fractal dimensions were also defined and determined. The latter allow close-packed structures to be found.
GROWTH OF SUPERCONDUCTING THICK FILMS OF YBaCuO BY SPRAY PYROLYSIS TECHNIQUE AND STUDY OF QUANTUM INTERFERENCE EFFECTS
Authors: REDDY G. S. N. , GUPTA A. K. , OJHA V. N. , WALIA D. K. , KATARIA N. D. , KHARE NEERAJ , TOMAR V. S.
Published: November 2004

Superconducting thick films of YBaCuO are made by spray pyrolysis technique on single crystal SrTiO 3 and MgO substrates. The thickness of the film is about 10 µm. The best film shows T c (R = 0) of the order of 86 K. No marked degradation in superconducting transition temperature is observed when the films are thermally cycled between room temperature and 4.2 K inside a closed liquid helium container. Temperature dependence of the critical current of microbridges fabricated on this film indicates the presence of SNS type weak links. dc SQUID configuration has been made in this film by scribing two bridges in parallel. V - Φ curves show periodic behavior superposed with additional structures.
INTERPRETATION OF MAGNETIC ANOMALIES IN MÖSSBAUER SPECTRUM INTENSITY
Authors: YUKALOV V. I.
Published: November 2004

The interpretation of the anomalous increase of the area under the Mössbauer spectrum below the transition temperature from the paramagnetic to magnetic state is suggested. It is shown that such a strong increase as has been observed cannot be due to a change in phonon characteristics. This anomaly can be caused by a new collective effect in the system of radiating nuclei. The effect in the formation consists of a coherent state of Mössbauer nuclei and in the appearance of coherent gamma-radiation induced by a hyperfine magnetic field. The possibility of using this effect for constructing gamma lasers is discussed.
DISLOCATION DISSOCIATION IN HIGH Tc BiSrCaCuO
Authors: LIU FANG , GU HUI , LIN TIAN XIAO , ZHANG JIN LONG , XIONG GUANG CHENG , YIN DAO LE , SONG CHENG YI
Published: November 2004

Transmission electron microscopy (TEM) shows that the dislocation on basal plane (001) of high T c BiSrCaCuO can dissociate into 2 partials of the type [Formula: see text]. The related stacking fault energy is estimated of the order about µb/365 similar to that observed in Cu and implication of this finding has been discussed.
POSSIBILITY OF n-TYPE AND p-TYPE CARRIER COUPLING IN THE EXCITONIC ENHANCEMENT MODEL OF SUPERCONDUCTING MECHANISM
Authors: WONG K. W. , CHING W. Y.
Published: November 2004

We discuss the possibility of both n-type and p-type carrier coupling in the excitonic enhancement model (EEM) for high temperature superconductivity. It is argued that p-type quasi-particles will generally lead to a higher T c than the n-type. The differences between EEM model and other exciton-related models are illuminated by the corresponding Feynman diagrams for the carrier coupling.
HIERARCHICAL ASSOCIATIVE MEMORY IN NEURAL NETWORKS WITH LOW ACTIVITY LEVEL
Authors: TSODYKS M. V.
Published: November 2004

A Hopfield-like neural network that can store hierarchically correlated patterns with low level of activity is studied. Three learning rules are proposed which enable to obtain nearly optimal storage capacity. These learning rules have different rate of biological relevancy and the restrictions they put upon the structure of hierarchical tree. By varying the value of the neural threshold, it is possible to climb up and down the hierarchical tree.
TETRAGONAL-ORTHORHOMBIC UNIT CELL TRANSFORMATION IN THE HYDROGEN-DOPED Y–Ba–Cu–OXIDE
Authors: DRULIS H. , KLAMUT J. , BUKOWSKI Z.
Published: November 2004

A hydride sample with nominal composition H 0.7 YBa 2 Cu 3 O 6.1 was synthesized. An X-ray diffraction study showed a phase transition from the tetragonal to orthorhombic cell. It is the first example of such transition among Y–Ba–Cu–O ceramics induced by non-oxygen atoms.
ELECTRON-PHONON INTERACTION WITHIN THE FRAMEWORK OF THE FLUCTUATING VALENCE OF COPPER ATOMS — A THEORETICAL MODEL FOR HIGH TEMPERATURE SUPERCONDUCTIVITY
Authors: VETTOOR SURESH V. , NANDAKUMARAN V. M.
Published: November 2004

Electron-phonon interaction is considered within the framework of the fluctuating valence of Cu atoms. Anderson's lattice Hamiltonian is suitably modified to take this into account. Using Green's function technique the possible quasiparticle excitations are determined. The quantity 2∆k(0)/k B T c is calculated for T c = 40 K . The calculated values are in good agreement with the experimental results.
INELASTIC NEUTRON SCATTERING IN THE ANHARMONIC MODEL OF HIGH-Tc SUPERCONDUCTORS
Authors: PLAKIDA N. M. , AKSENOV V. I. , DRECHSLER S. L. , GALBAATAR T. , STAMENKOVIC S.
Published: November 2004

The inelastic neutron scattering on highly anharmonic vibrations of oxygen ions in high-T c superconductors is studied in the framework of a pseudo-spin model. By evaluating the coherent and incoherent cross-sections, it is found that the scattering intensity exhibits an anomalous temperature dependence.
LOW TEMPERATURE RESISTANCE BEHAVIOR AND SUPERCONDUCTIVITY OF La–Sr–Cu–O SYSTEM: ROLE OF ALTERED Cu–VALENCE AND O–CONTENT
Authors: RAO C. V. NARASIMHA , AGARWAL S. K. , NARLIKAR A. V. , DAS M. P.
Published: November 2004

In this note we are communicating the results obtained on various samples of lanthanum-based cuprate in which Sr and Zn have been partially substituted for La and Cu, respectively over a range where the superconductivity is gradually destroyed. The decrease in T c is accompanied by a changeover from metallic to semiconducting-like resistance behavior occurring at lower temperatures. The observed behavior is related to the effective copper valence and the change in the oxygen content of the samples and the results are discussed in terms of a tentative phase diagram which would be necessary to examine the mechanism of superconductivity.
GROWTH OF ORIENTED SUPERCONDUCTING FILMS OF Bi–Sr–Ca–Cu–O BY SPRAY PYROLYSIS TECHNIQUE
Authors: WALIA D. K. , GUPTA A. K. , REDDY G. S. N. , KATARIA N. D. , KHARE NEERAJ , OJHA V. N. , TOMAR V. S.
Published: November 2004

Highly oriented superconducting thick films of Bi 2 Sr 2 CaCu 2 O 8+x have been prepared by low cost spray pyrolysis technique. Aqueous solution containing nitrates of Bi, Sr, Ca, and Cu in the atomic ratio of 2:2:1:2 is sprayed on MgO (100) substrates which are kept at elevated temperature. After annealing in air near the melting temperature, highly oriented films with c-axis perpendicular to the substrate are obtained. The films show a sharp superconducting transition with zero resistance at 85 K. The transport critical current density of the film at 77 K and zero field is of the order of 4000 A/cm 2.
TEMPERATURE DEPENDENCE OF THE MICROSTRUCTURE IN YBa2Cu3Oy: TRANSMISSION ELECTRON MICROSCOPY STUDY
Authors: LI J. Q. , ZHAO Z. X. , LI F. H. , YANG D. Y. , SHI G. S.
Published: November 2004

A transmission electron microscopy study has confirmed some new features of thermal variations of the microstructure in the high-T c superconducting compound YBa 2 Cu 3 O y. The changes of domain structures and the nucleation process of the tetragonal phase with the increase of temperature are demonstrated by series of images taken under in-situ heating observations. At temperature near 250°C, two new sets of planar defects have been observed along the directions of a and b-axes, respectively. It has been identified that a continuous structural phase transition, from orthorhombic to tetragonal phase, occurs at about 480 °C. The structural properties of the tetragonal phase have been studied at 700°C.
VALENCE STATES OF BISMUTH AND THALLIUM IN SUPERCONDUCTING CUPRATES: A Bi–L3 AND Tl–L3 X-RAY ABSORPTION SPECTROSCOPIC STUDY
Authors: VIJAYAKRISHNAN V. , KULKARNI G. U. , RAO C. N. R.
Published: November 2004

Based on a Bi – L 3 edge X-ray absorption spectroscopic study of superconducting bismuth cuprates and model Bi compounds, it is shown that Bi is essentially in the 3 + state in the cuprates. Tl – L 3 absorption edge studies a number of thallium cuprate superconductors along with those of model compounds show some evidence for the occupancy of the Tl -6s band, but Tl 3+ is the prepondarant species. Cu K-edge studies of the thallium cuprates show that Cu is essentially in the 2+ state.
ANNEALING EFFECTS OF TlCaBaCuO THIN FILMS ON YSZ SUBSTRATES
Authors: YAN S. L. , CAO H. L. , SONG Q. X. , YANG X. M. , YAN J. , ZHOU X. D. , ZHANG Q. M.
Published: November 2004

Superconducting thin films of Tl 2 Ca 2 Ba 2 Cu 3 O x have been prepared by dc magnetron sputtering from a pair of stoichiometric Tl-based targets onto YSZ substrates. The superconducting transition properties and the morphology of the films depend strongly on the post-annealing conditions. The zero resistance temperature for the best films we got is 117 K. X-ray diffraction analysis indicates that the annealed films with needle-like structure and terraced configuration are both highly c-axis oriented normal to the film surface.
SOME NEW ASPECTS OF SUPER-HIGH TEMPERATURE SUPERCONDUCTORS
Authors: HUANG C. Y. , RABINOWITZ M.
Published: November 2004

We review some salient features of the recently observed experimental evidence of ceramic super-high temperature superconductivity (T c > 200 K ). Two theoretical estimates which are capable of accounting for super-high T c will be presented. For comparison, several reports on non-ceramic oxide super-high temperature superconductors will also be discussed.
CLUSTER-IMPACT FUSION: NEW PHYSICS OR EXPERIMENTAL ERROR
Authors: RABINOWITZ MARIO
Published: November 2004

Deuteron-deuteron cluster-impact fusion at ~100 eV was demonstrated by Beuhler et al. Their results are analyzed to reduce a discrepancy which they note to be "more than 10 orders of magnitude". By their method of calculation, it is found to be 25 orders of magnitude, of which 10 orders of magnitude can be accounted for by compression and electron screening. Analysis is presented to show that the remaining 15 orders of magnitude discrepancy cannot reasonably be resolved by electron screening and proximity (degree of compression), or experimental errors in cluster energy and/or size.
HIGH CRITICAL CURRENT DENSITY IN NEUTRON-IRRADIATED BULK YBa2Cu3O7
Authors: HOR P.H. , HUANG Z.J. , GAO L. , MENG R.L. , XUE Y.Y. , CHU C.W. , JEAN Y.C. , FARMER J.
Published: November 2004

Bulk melt-textured YBa 2 Cu 3 O 7 irradiated by fast neutrons has been investigated. We observed: (1) a critical current density (Jc) determined magnetically at 77 K exceeds 5×105 A/cm 2 in 0.1 T, 2×105 A/cm 2 in 1 T, and 1.2×103 A/cm 2 in 5.0 T; (2) at 5 K, Jc is >2.1×107 A/cm 2 in 0.5 T, ~1.8×107 A/cm 2 at 1 T and >8×106 A/cm 2 in 5.0 T and (3) the Jc-anisotropy decreases rapidly with the external field (H), e.g. at 77 K, it becomes 1 at ~4.8 T with Jc ~3×103 A/cm 2. We also found that (1) such a Jc-enhancement is most effective in low H but less so in high H, (2) the enhancement is temperature independent at low H, (3) the pinning potential remains unchanged to within ~10% as Jc varies by a factor of 15, and (4) thermal annealing up to 600°C recovers the un-irradiated state. These results and their implications are presented and discussed.
SPIN-SPLIT STATES IN AROMATIC MOLECULES
Authors: HIRSCH J.E.
Published: November 2004

A state where spin currents exist in the absence of external fields has recently been proposed to describe the low-temperature phase of Chromium. It is proposed here that such a state may also describe the ground of aromatic molecules. It is argued that this point of view provides a more natural explanation for the large diamagnetic susceptibilities and NMR shifts observed in these molecules than the conventional viewpoint. Our model suggests a new memory mechanism.
ORDER PHASE TRANSITION NEAR THE INVERSION POINT OF MAIN ANISOTROPY
Authors: BARBERI R. , BARBERO G. , EVANGELISTA L.R.
Published: November 2004

The order phase transition induced by an external field near the inversion point of the main anisotropy is analysed. It is shown that if the medium has the symmetry of the nematic liquid crystal or of the ferromagnetic material, the phase transition near this point can be of first order. The tricritical field and the hysteresis loop of this first order phase transition are evaluated. An evaluation of the tricritical point is also reported.
DOUBLE-MICROBRIDGE DC SQUIDS MADE FROM YBCO SUPERCONDUCTING FILMS
Authors: CHEN Y.F. , TAO H.J. , LI J.D. , YANG Q.S. , ZHANG Y.Z. , ZHAO B.R. , LI H.C. , ZHOU P. , DONG Z.W. , LIU G.R. , LI L.
Published: November 2004

Double-microbridge DC SQUIDs have been fabricated from YBCO superconducting films by lithographically patterning technique. The voltage versus the applied magnetic field dependence of a DC SQUID has been obtained in the region of temperature from 4.2 K to 23 K, The largest output voltage of the devices used in our experiments is 3.4 uV /Φ0(Φ0 is h/2e the flux quantum). The smallest measured periodicity of the flux-voltage characteristics of the DC SQUID, 12.7 mG, is consistent with the designed value. In addition, some properties of the flux-voltage characteristics of the DC SQUIDs have been discussed.
DIELECTRIC CONSTANT MEASUREMENTS ON POLY (IRON-NICKEL) ACRYLATE
Authors: CHOHAN M.H. , KHWAJA F.A. , BUKHARI A.H.S.
Published: November 2004

Measurements of the impedance (z) and the dielectric constant (ε) on poly acrylates have been made as a function of temperature and frequency by employing a parallel RLC circuit arrangement and using standard formula. The values of ε have been determined after identifying the resonance frequencies of the circuit at different temperatures. Variation of ‘z’ with frequency and ε as a function of temperature are presented in the graphs.
STRUCTURE AND EXCITATIONS OF QUANTUM LIQUID CLUSTERS
Authors: RAMA KRISHNA M.V. , WHALEY K.B.
Published: November 2004

We review recent theoretical progress in the evaluation of collective excitation spectra of quantum liquid clusters, with specific application to 4 He N. An excitation operator theory together with the variational Monte Carlo method has provided a powerful means of calculating the excited state wavefunctions and energies. The compressional excitations of 4 He N clusters calculated from the excitation operator theory agree well with the results of a quantum liquid drop theory based on accurate knowledge of microscopic density correlations in the ground state. The compressional excitation spectra evolve toward the spectrum of bulk He II as N increases, with the roton structure first appearing at N≈70. The implication for the finite size scaling of superfluid behavior in these clusters is discussed. These techniques now allow microscopic study of clusters containing foreign species, and can also be extended to quantum solid clusters.
THE EFFECT OF THE LAYERED TO PHONONS ON CARRIER DISTRIBUTION IN SUPERCONDUCTING OXIDES
Authors: WANG HANTING , WANG WENZHEN , XIONG SHIJIE
Published: November 2004

The effect of layered TO phonons on the carrier concentration of superconducting oxides with layered structure is theoretically investigated. The origin of this effect is the removal of the degeneracy of the carrier subbands for the equivalent conducting sheets. The degeneracy of two neighbouring conducting sheets can be removed by the lattices distortions due to the z-polarized vibrations of the TO phonons in the mediating insulating plane, the carrier distribution is then shifted. A statistical estimate shows a linear temperature dependence of carrier concentration. The relevance of this result to the unusual temperature dependence of the Hall effect is discussed.
BEHAVIOUR OF ORIENTED POLYCRYSTALLINE YBaCuO SUPERCONDUCTING THIN FILMS IN A WEAK MAGNETIC FIELD
Authors: WEN H.H. , CAO X.W. , SHI J.Z. , SHI W.Z. , YAO W.G. , QI Z.Z.
Published: November 2004

We determined a few characteristics of granular superconductivity through magnetoresistance measurements made on oriented polycrystalline YBaCuO superconducting thin films, and found that, at 77.3 K, the critical field for breaking a major portion of the superconducting weak links between grains was about 80 G, while the average lower critical field of the grains was about 175 G. Detail analyses on flux flow resistance show that a maximum of the increasing of flux flow resistance induced by a unit field will exist at a specified temperature, which we think is related to the dependence of the pinning energy on the temperature.
GEOMETRIC PHASE IN THE CLASSICAL CONTINUOUS ANTIFERROMAGNETIC HEISENBERG SPIN CHAIN
Authors: BALAKRISHNAN RADHA , BISHOP ALAN R. , DANDOLOFF ROSSEN
Published: November 2004

We show that the time evolution of a space curve is associated with a geometric phase. This phase arises from the path dependence of the rotation of the natural Frenet-Serret triad with respect to a nonrotating (Fermi-Walker) frame.
QUANTUM CONFINEMENT AND COULOMB EFFECTS IN SEMICONDUCTOR QUANTUM DOTS
Authors: HU Y.Z. , KOCH S.W. , THOAI D.B. TRAN
Published: November 2004

Coulomb and quantum confinement effects in small semiconductor microcrystallites are analyzed. Energies and wavefunctions for one- and two-electron-hole-pair states are computed and optical absorption spectra are evaluated.
EFFECTIVE LATTICE MODELS FOR TWO-DIMENSIONAL QUANTUM ANTIFERROMAGNETS
Authors: SACHDEV SUBIR , JALABERT R.
Published: November 2004

We introduce a 2+1 dimensional lattice model, S0, of N complex scalars coupled to a compact U(1) gauge field as a description of quantum fluctuations in SU(N) antiferromagnets. Duality maps are used to obtain a single effective action for the Néel and spin-Peierls order parameters. We examine the phases of S0 as a function of N: the N→∞ limit can be deduced from previous work. At N=1, S0 describes monopoles and their Berry phases, spin-Peierls order, but not the Néel field: Monte-Carlo simulations show a second-order transition from a spin-Peierls phase to a Higgs phase which is the remnant of the Néel phase.
ELECTRON DIFFRACTION AND LATTICE IMAGE STUDY OF HIGH Jc YBCO AND GBCO THIN FILMS
Authors: LIU W. , WANG Y.G. , LI F.H. , LI H.C. , ZHANG Y.Z. , LI L.
Published: November 2004

High resolution electron microscopy observations of YBa 2 Cu 3 O 7 thin films deposited on LaAlO 3 substrate and GdBa 2 Cu 3 O 7 thin films deposited on SrTiO 3 substrate are reported. The microstructure of films with different critical current densities Jc’s are compared. It was found that Jc becomes very high >106 A/cm 2 when the film is epitaxially single crystalline with c-axis perpendicular to the film surface. The effect of grain boundaries and defects in the film on Jc was discussed. The effect of the smoothness, orientation and lattice constants of the substrate on the quality of the deposited film was also discussed.
AN UPPER BOUND FOR THE GROUND STATE ENERGY OF BIPOLARON: A PATH INTEGRAL TREATMENT
Authors: KHANDEKAR D.C.
Published: November 2004

A path integral formulation to study the properties of bipolaron is presented. The formulation is subsequently used to derive an upper bound for the ground state energy of the bipolaron. The estimate is used to discuss the stability of bipolaron.
STUDY OF Y-Ba-Cu-O SUPERCONDUCTORS PREPARED WITH THE OXALATE PRECIPITATION/EVAPORATION METHOD
Authors: LEVENTOURI TH. , LIAROKAPIS E. , MARTINEZ L. , MEDINA F.D. , MORENO M. , LANDRETH B.D. , WALLACE W.J. , CLARK R.J. , ANDRAKA B. , KIM J.S. , STEWART G.R.
Published: November 2004

A study is presented on a series of Y-Ba-Cu-O superconductors prepared with the Oxalate Precipitation/Evaporation method. The effect of the preparation conditions on the properties of these superconductors has been studied with X-rays, scanning electron microscopy, Raman spectroscopy, and magnetic measurements. The experiments show that good quality superconductors can be fabricated with this preparation procedure and also indicate that the properties of these materials are very sensitive to the preparation conditions.
A SCENARIO FOR THE ABSENCE OF LOCALISATION IN 1-DIMENSION
Authors: PHILLIPS PHILIP , WU H-L. , DUNLAP DAVID H.
Published: November 2004

We review here the random dimer model of a binary alloy which we have recently demonstrated exhibits an absence of localisation. Specifically we show that if one (or both) of the site energies is assigned at random to pairs of lattice sites (that is, two sites in succession), an initially localised particle can become delocalised. We show explicitly that transport obtains in this model because [Formula: see text] of the electronic states are extended over the whole sample. The relevance of this model to the insulator-metal transition in polyaniline is discussed. The dual of the random dimer model is also shown to exhibit an absence of localisation and is shown to be relevant to transmission resonances in Fibonacci lattices.
LOCALIZED ELECTRONS IN A MAGNETIC FIELD
Authors: SHAPIR Y. , WANG X.R.
Published: November 2004

We briefly review recent developments on the response of localized electrons to an external magnetic field. They originate from subtle interference effects between forward-scattered paths which, in the localized regime, make the dominant contribution to the transition amplitude. On a lattice the correlations between the paths produce self-similar (quasiperiodic) spatial interference patterns for commensurate (incommensurate) ratios of the applied magnetic flux per plaquette with the flux quantum. If localization is due to strong disorder (as in lightly doped semiconductors), this is augmented by a logarithmic averaging. They give rise to striking quantum interference phenomena observed in mesoscopic and macroscopic insulators: Aharonov-Bohm oscillations with periods of one and one half flux-quantum, the Hall effect, pronounced conductance fluctuations, and anomalous negative (and positive in presence of spin-orbit scattering) magnetoresistance. The theoretical works based on the “directed paths” approach are reviewed. Experimental results which motivated these works, are summarized.