Effects Of Impurity Doping Research Articles

Quantal phases, disorder effects, and superconductivity in spin-Peierls systems.

In view of recent developments in the investigation on cuprate high- T(c) superconductors and the spin-Peierls compound CuGeO3, we study the effect of dilute impurity doping on the spin-Peierls state in quasi-one-dimensional systems. We identify a common origin for the emergence of antiferromagnetic order upon the introduction of static vacancies and superconductivity for mobile holes.

Concentration of point defects in growing CZ silicon crystal under the internal stresses: effects of impurity doping and thermal stress

Behavior of point defects under the internal stress in a growing Czochralski (CZ) silicon is theoretically examined. The changes of point defect concentration by the impurity doping and by the thermal stress are quantitatively estimated. Contributions of them to the formation of secondary defects are discussed.

Influence of Composition and Dopant on Photostriction in Lanthanum-Modified Lead Zirconate Titanate Ceramics

The anomalous photovoltaic effect, i.e., substantial voltage generation exceeding band gap energies, by homogeneous illumination in the non-centrosymmetric materials is a wellknown phenomenon. This mechanism is fundamentally different from those observed in heterojunctions/homojunctions (i.e., solar cells) based on semiconductor materials with a symmetric crystal structure. Recently, light induced strain termed as Photostriction, has been demonstrated in noncentrosymmetric perovskite ferroelectrics as lanthanum-modified lead zirconate titanate (PLZT) ceramics. PLZT ceramics have gained considerable attention due to their large photostrictive responses. When a light is illuminated on a polarized photostrictive bimorph, a large photovoltage of the order of kV/mm is generated across its length in the spontaneous polarization direction which results in strain generation by converse-piezoelectric effect and caused the bimorph to deflect in the direction away from the illumination. The photoactuation has been demonstrated in PLZT ceramic wafers as two different kinds of actuators, (i) photo-driven relay and (ii) photo-driven micro walking machine. These materials are also promising for the photo-acoustic device (e.g., Photophone) of the optical communication system. However, photostriction being a recently observed phenomenon, is far from optimum condition. It is expected that the optimization with respect to materials and microstructural characteristics will lead to a substantial enhancement in the photoactuation efficiency. Increasing the photoactuation efficiency will eliminate the need for electrical energy, and will open up numerous possibilities of contactless actuators. In this paper the influence of composition dependence and impurity doping effects on microstructure and photostrictive responses of PLZT ceramics have been investigated.

Effect of multiple impurity doping on the photovoltaic properties of lead zirconate-titanate ceramics

Pb(Zr0.5, Ti0.5)O3 (PZT) ceramics singly doped with 0-3 mol% of Ta and those multiply doped with 1 mol% of Na and 0-4 mol% of Ta have been sintered at 1050-1300 °C for 2h to investigate the change in photovoltaic properties with changes in the concentration of Pb vacancies, and also grain size, upon ultraviolet irradiation. Both photovoltaic current and photovoltage were markedly increased by the doping of Ta in the case of single doping, whereas in the multiply doped samples they showed no significant change up to 1 mol% of doped Ta. Beyond this amount, however, they were found to significantly increase. The average grain size in the singly doped samples decreased with increasing amounts of doped Ta, while that in the multiply doped samples revealed a maximum at around 1 mol% Ta, below which grain size was smaller than that of the nondoped sample. It is concluded from these results that the formation of Pb vacancies onto the perovskite lattice, rather than the existence of doped Ta, has a fundamental significance on the photovoltaic properties in PZT ceramics

Anisotropic pair-breaking by impurity doping in high-T-c cuprates: Modification of interlayer coupling

Impurity (Zn/Ni) doping effects on the superconducting coherence length xi and the penetration depth lambda have been studied for the in-plane and the out-of-plane directions of YBa2Cu3Oy single crystals. We determined xi from diamagnetic susceptibility i

Dopant and fluence effects on the ablation of silicon surfaces under pulsed CO 2 laser irradiation

Effects of impurity doping and laser-pulse fluence on the laser ablation of silicon wafers are studied by using a TEA CO 2 laser. Silicon ions with charge from 1 to 4 a.u. were observed in the ablation species. The average ion charge increased with dopant concentration and the laser fluence. The velocity of the ions depends on the fluence and slightly on the dopant concentration. The electric current flowing from a target into the ground level was also measured and showed dopant and fluence effects. The correlation among these experiments is examined and the ablation mechanism is discussed.

Impurity Scattering in a d-Wave Superconductor

The influence of (non-magnetic and magnetic) impurities on the transition temperature of a d-wave superconductor is studied anew within the framework of BCS theory. Pairing interaction decreases linearly with the impurity concentration. Accordingly T c suppression is proportional to the (potential or exchange) scattering rate, 1/τ, due to impurities. The initial slope versus 1/τ is found to depend on the superconductor contrary to Abrikosov–Gor'kov type theory. Near the critical impurity concentration T c drops abruptly to zero. Because the potential scattering rate is generally much larger than the exchange scattering rate, magnetic impurities will also act as non-magnetic impurities as far as the T c decrease is concerned. The implication for the impurity doping effect in high T c superconductors is also discussed.

On the effect of impurities on resistivity recovery, short-range ordering, and defect migration in electron-irradiated concentrated Fe - Cr alloys

The resistivity recovery of pure and impurity-doped (0.2 - 1.5% Si, 0.15% C+N) concentrated ferritic Fe - Cr alloys after electron irradiation at 50 - 60 K has been investigated over the temperature range 110 - 390 K. A fine recovery spectrum structure consisting of five peaks has been observed over the range 135 - 230 K. Short-range ordering starts with the onset of vacancy long-range migration. Doping with both types of impurity suppresses the recovery spectrum structure over the range 160 - 220 K in a similar manner. Additional effects of impurity doping on the resistivity recovery caused by deviation from Matthiessen's rule have been analysed. A stage III peak is found at 210 K, and two peaks - at 175 K and 195 K - are interpreted as being due to the vacancy short-range migration. It is supposed that such a manifestation of short-range vacancy migration is due to a strong immobilization of self-interstitial atoms, and suppression of short-range and long-range defect annealing processes in stage I, according to a configuration-trapping mechanism similar to that of Ag - Zn alloys.

Spin gap and antiferromagnetic fluctuation in spin ladder system doped with nonmagnetic impurities

Effects of nonmagnetic impurity doping in a spin ladder system with a spin gap are investigated by the exact diagonalization and the variational Monte Carlo calculations. Small amount of impurity doping generates a nearly free spin bound to each impurity. These nearly free spins interact each other only weakly giving rise to gapless spin excitations. Another important aspect is that short-ranged antiferromagnetic correlation is enhanced especially around the impurity sites. We propose a possible scenario for this drastic change in the spin gapped ladder system. We also compare with the experimental results of Zn-doped SrCu2O3.

Effects of Nonmagnetic Impurity Doping on Spin Ladder System

Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder system are studied by the QMC method. A single nonmagnetic impurity induces a localized spin-1/2 moment accompanied by "static" and enhanced AF correlations around it. Small and finite concentration of impurities induces a remarkable change of magnetic and thermodynamic properties with gapless excitations. It also shows rather sharp but continuous crossover around the concentration of about 4%. Above the crossover concentration, all the spins are strongly coupled participating in the enhanced and rather uniform power-law decay of the antiferromagnetic correlation. Below the crossover, each impurity forms an antiferromagnetic cluster only weakly coupled each other. For random distribution of impurities, large Curie-like susceptibility accompanied with small residual entropy is obtained at low temperatures in agreement with recent experimental observation in Zn-doped $SrCu_{2}O_{3}$. Temperature dependence of AF susceptibility shows power-law-like but weaker divergence than the single chain AFH in the temperature range studied.

Impurity Effect on Spin Ladder System

Effects of nonmagnetic impurity doping in a spin ladder system with a spin gap are investigated by the exact diagonalization as well as by the variational Monte Carlo calculations. Substantial changes in macroscopic properties such as enhancements in spin correlations and magnetic susceptibilities are observed in the low impurity concentration region, which are caused by the increase of low-energy states. These results suggest that small but finite amount of nonmagnetic impurity doping relevantly causes the reduction or the vanishment of the spin gap. This qualitatively explains the experimental result of Zn-doped SrCu$_{2}$O$_{3}$ where small doping induces gapless nature. We propose a possible scenario for this drastic change as a quantum phase transition in a spin gapped ladder system due to spinon doping effects.

Impurity Doping Effect on Piezoelectricity and Strain Induced by Electric Field in Piezoelectric Ceramics

Impurity Doping Effect on Piezoelectricity and Strain Induced by Electric Field in Piezoelectric Ceramics

The Effect of Impurity Doping of The Magnetic Layer on the Magnetoresistance and Saturation Field of FeCr/Cr and CoCu/Cu Multilayers

ABSTRACTThe effect of doping the magnetic layer on the magnetoresistance and saturation field of epitaxial, sputter-deposited FeCr/Cr(001) and CoCu/Cu(001) multilayers was examined. FeCr/Cr films with a composition of [14 Å Fe1−xCrx(001)/8 Å Cr(001)]50 where x was varied from 0 to 0.5 and CoCu/Cu films with a composition of [8 Å Co1−xCux(001)/21 Å Cu(001)]40 where x was varied from 0 to 0.2 were deposited onto single crystal MgO(001). The bilayer period and epitaxial orientation of these films were determined by x-ray diffraction. In the FeCr/Cr system the room temperature magnetoresistance was constant at approximately 31% for x≤0.2 and decreased for larger Cr concentrations. The spin-dependent scattering (Δp) is increased over the x=0 value for x=0.1 and x=0.2. The magnetic field required to saturate these multilayers decreases linearly with increasing Cr concentration. The net result is that the sensitivity of these films is increased by Cr doping of the Fe layer. In the CoCu/Cu system the room temperature magnetoresistance, saturation field, and saturation magnetization decrease with the addition of Cu to the Co layer. In contrast to the FeCr/Cr system Δp does not increase with doping of the ferromagnetic layer. However, as in the FeCr/Cr films the sensitivity of these multilayers is increased with respect to that of the x=0 CoCu/Cu multilayer.

Crack-impurity interactions and their role in the embrittlement of Fe alloy crystals charged with light elements

The effect of mobile impurity doping on fracture behaviour has been investigated using plasma charging of light elements for Fe-2.5 wt.%Si alloy crystals with particular attention to the role of crack-impurity elastic interactions. FeSi crystals are markedly embrittled by plasma charging of helium as well as hydrogen at around room temperature, this being accompanied by slow crack growth. Neon charging contributes little to the embrittlement, but argon charging does not contribute. The crystals are also embrittled by nitrogen charging in the tests at 450 K and exhibit slow crack growth during the tests. Elastic analyses indicate that crack-impurity interactions are induced not only by the applied K I field but also by the stress modification due to ambient impurities in the presence of a crack. The interactions serve effectively to concentrate mobile impurities ahead of a crack tip, leading to the increase in the local stress intensity k I. The effect of interstitial impurities on crack extension is discussed in connection with the modification of stress states due to impurities around a crack tip.

Doping effect on spin-Peierls instability.

We study the effects of doping on spin-Peierls (SP) systems using the unimodular mean-field theory. The impurity spins affect the singlet valence bond field and renormalize the magnetic excitations. The SP transition temperature and the energy gap of magnetic excitations are reduced by factors ~ n« and ~ nf, respectively, with rii as the impurity density. At certain value of rij, a gapless SP phase occurs, and the interaction between impurities becomes RKKY-like. The recently observed reduction of SP transition temperature upon doping and occurrence of a spin glass phase is interpreted using the proposed theory. MIRAMARE TRIESTE January 1994 'Present address: International School for Advanced Studies (SISSA), Via Beirut No.24, 34013 Trieste, Italy. The theoretically predicted spin-Peierls (SP) state with alternating bond length has been observed, so far only in a few quasi-one-dimensional (ID) organic compounds with antiferromagnetic (AF) interactions [1]. Very recently, it was found in an inorganic compound CuGeO:, [2]. Moreover, a drastic reduction of SP transition temperature Tr was observed upon Zn-doping and a spin glass (SG) phue appeared in the doping range 0.02 < n, < 0.08 [3]. In this Letter we propose a theory of SP transition in doped systems explaining these findings and making further predictions on a gapleat SP state to be checked by experiments. The SP transition is driven by the interaction between the (1-D) spin-1/2 chains and the three-dimensional (3-D) lattice,which makes a mean-field (MF) approach available due to its suppression of fluctuations. Below the SP transition an uniform AF chain is deformed into an alternating AF chain with a singlet ground state and a magnetic gap [4]. Up to now there are two successful theories of SP transition, i.e. that of Pytte [S] and of Cross and Fisher [6]. In these theories, a fermion representation via the Jordan-Wigner transformation (JWT) is used to describe the spin-1/2 chain, and the fermion-phonon interactions are taken into account in the random phase approximation. However, it is difficult to investigate effects of impurity doping upon the SP system) within this approach due to nonlocal features of JWT. P.W. Anderson [7] has proposed the Resonant-Valence-Bond model to describe twodimensional spin-1/2 AF systems. Later, Arovas and Girvin [8] put forward an unimodulr as the impurity density, whereas the decrease of the energy gap is proportional to nf and it collapses at some value of n,, but the system is still in the SP phase, i.e. the spin-lattice dimeriz&tion remains. Thus we predict the existence of a gnpiess SP state to be checked by direct experiments. This is rather similar to superconductors doped with paramagnetic impurities which reduce the energy gap and eventually give rise to gapless superconductivity [13]. The Hamiltonian describing an impurity-doped SP system is [12,14] = #„ #1 = Zi H = E An, = ff(a + 1)5, • S,+, + AnI • Si, 5TM J(a,l)Sn, Ei 2A-u, (1)

Effects of impurity doping on terrace width stability

A simple model is proposed to describe the effect of impurity doping on terrace width ordering during epitaxial growth. A single, unspecified parameter of smallness is used to describe the strength of the impurity effect and we find the conditions for which the terrace widths become ordered or disordered following a perturbing fluctuation. The results found are meant to be interpretive rather than predictive, i.e. they allow experimental outcomes to be explained in terms of the impurity effects at the steps but they cannot predict what these outcomes will be without a better understanding of the impurity-mediated adatom-step kinetics.

Impurity doping effect on photostriction in PLZT ceramics

Photostriction is the superposition of a photovoltaic and piezoelectric effects. In this study, photostrictive effects in the perovskite (Pb, La) (Zr, Ti)O3 were investigated as a function of B-site impurity doping. Donor doping was found to reduce both the grain size and room-temperature dielectric constant, influencing photovoltaic response. WO3 and Ta2O5 doping increase the photovoltaic response, but do not significantly affect the piezoelectric effect in this material. Maximum photostriction is obtained for samples with 0.4 at% WO3 doped or 1 at% Ta2O5 doped PLZT

Effects of the impurity potential on the band structure of doped trans-polyacetylene

In order to understand the drastic effect of impurity doping on the transport properties of polyacetylene, the authors investigate in detail how the impurity ion potential alone affects the band structure of electrons and the order parameter in doped polyacetylene by a self-consistent numerical calculation based upon the Su-Schrieffer-Heeger Hamiltonian plus the impurity potential. Their results show that the potential has only a small effect on gap closure, which is mainly determined by the larger overlap between the electronic wavefunctions in the nearest soliton states. They also find that the order parameter does not go to zero in the heavily doped region where there is no gap, and it is almost unaffected by the impurity potential.

Effect of Impurity Doping on Tc of 123 Superconductors

We have investigated the effect of substitution of magnetic impurities for copper in YBa2Cu3-xMxO7 (M=Fe, Co) superconductors by developing a Fourier transformed effective potential (FTEP), which involves the influence of two dimensional (2D) acoustic phonons and plasmons. We have demonstrated that 2D acoustic plasmons have introduced 20–25% enhancement over the phonon contribution to the transition temperature (TC) at various compositions (x). Further, we have found that the increase in the values of the plasmon coupling parameter (λpl=0.0 to 0.5) enhances TC from 70 to 300 K. These achievements clearly indicates that the inclusion of the effect of 2D acoustic plasmons is essential for proper description of the mechanism of high TC superconductivity.

Theoretical study of the fracture of brittle materials: atomistic calculations

Using the lattice Green's function approach and linear combination of atomic orbitals (LCAO) electronic theory, we investigate the fracture behavior of brittle materials such as semiconductors and ceramic materials. The cracked-lattice Green's functions are derived by applying the Fourier transform technique and solving the Dyson equation for a three-dimensional simple cubic lattice (cracks with kink pairs) as well as for the diamond cubic lattice. The total energy calculation scheme based on the LCAO electronic theory is used to obtain the nonlinear bond-breaking forces, taking into account the effects of impurity doping and/or environment effects of the external molecules. The calculated crack properties and the related fracture behavior of the brittle materials are discussed in detail.