Undoped System Research Articles

Doped bilayer antiferromagnets: Hole dynamics on both sides of a magnetic ordering transition

The two-layer square lattice quantum antiferromagnet with spins $\frac{1}{2}$ shows a magnetic order-disorder transition at a critical ratio of the interplane to intraplane couplings. We investigate the dynamics of a single hole in a bilayer antiferromagnet described by a $t\ensuremath{-}J$ Hamiltonian. To model the spin background we propose a ground-state wave function for the undoped system that covers both magnetic phases and includes transverse as well as longitudinal spin fluctuations. The photoemission spectrum is calculated using the spin-polaron picture for the whole range of the ratio of the magnetic couplings. This allows for the study of the hole dynamics on both sides of the magnetic order-disorder transition. For small interplane coupling we find a quasiparticle with properties known from the single-layer antiferromagnet, e.g., the dispersion minimum is at $(\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2).$ For large interplane coupling the hole dispersion is similar to that of a free fermion (with reduced bandwidth). The crossover between these two scenarios occurs inside the antiferromagnetic phase, which indicates that the hole dynamics is governed by the local environment of the hole.

Wet-chemical synthesis of doped nanoparticles: Blue-colored colloids of n-doped SnO2:Sb

Nanocrystallites of antimony-doped tin dioxide have been prepared hydrothermally by treating colloids of tin antimony oxide in an autoclave. At a synthesis temperature of 270 °C stable colloidal solutions of blue-colored SnO2:Sb nanocrystallites have been obtained. High resolution transmission electron microscopy (TEM) images show highly crystalline particles in the 4 to 9 nm size regime. X-ray powder diffraction patterns of the nanocrystals indicate the same rutile lattice structure as known from bulk SnO2. Powders of the antimony-doped nanocrystals exhibit an up to 105 fold increase in electrical conductivity as compared to the corresponding undoped systems. The blue color of the doped colloids corresponds to a broad absorption peak in the red and the IR region. The IR-absorption spectrum of doped nanoparticles deposited onto sapphire substrates has been fit to a model based on the Drude theory of a free electron gas in SnO2:Sb and an effective medium approximation. The model indicates that the IR absorption corresponds to a plasmon polariton excitation of weakly interacting n-doped nanoparticles.

On the Magnetic State of Monoclinic-Cu1-xAxNb2O6(A=Zn, Co and Ni)

Effects of Cu-site substitution with Zn, Co and Ni on the spin-gapped state of CuNb 2 O 6 , which has ferromagnetic-antiferromagnetic alternating spin ( S =1/2) chains of Cu 2+ ions, have been studied by measuring the magnetic susceptibilities and the specific heats. The results indicate that impurity atoms induce an “isolated” magnetic moment mainly at one of the neighboring Cu sites originally connected with the antiferromagnetic exchange interaction, with the other part of Cu sites being essentially kept in the spin-gapped states. For Co 2+ (Ni 2+ ) impurities, localized spins with S =2 (3/2) are formed by the ferromagnetic coupling between the impurity spins and the induced ones (note that the coupling is now ferromagnetic even though it is antiferromagnetic in the undoped system).

Systematic Studies of the Square-Hexagonal Flux Line Lattice Transition inLu(Ni1−xCox)2B2C: The Role of Nonlocality

We have studied, using small angle neutron scattering, the flux line lattice square to hexagonal symmetry transition in single crystal $\mathrm{Lu}({\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{B}}_{2}\mathrm{C}$. Low Co concentrations $(x<0.1)$, which reduce the mean free path and increase the coherence length, also move the structural transition to higher fields than in the undoped system. These data, quantitatively understood within the framework of a theory that includes nonlocal corrections to the London model due to the Fermi surface anisotropy, can be modeled using a simple ratio of the nonlocality range to the intervortex spacing.

Surface chemistry effects on the processing and superplastic properties of nanocrystalline oxide ceramics

The unusual bulk behavior of nanoparticle and nanograined systems often originates in surface chemistry effects. Three examples are used to illustrate this point. In the first, newly precipitated nanocrystalline titania is washed with ethanol, and the mixture of these two supposedly inert substances causes the titania to lose its anatase crystal structure and become amorphous. This phenomenon is attributed to a reverse hydrolysis reaction at the particle surface. In the second example, nanocrystalline ZrO 2-3mol%Y 2O 3 is observed to partially dissolve on exposure to pH-adjusted water, due to the formation of soluble hydroxides at the particle surface. A major consequence of the dissolution is the formation of large, hard, multiparticle agglomerates on subsequent drying. In the final example, ZrO 2-3mol%Y 2O 3 particles are intentionally surface-doped with submonolayer levels of Cu-containing ions from ammoniacal solutions. The ceramics fabricated from such powders exhibit superplastic strain rates 100 or so times faster than in comparable undoped systems, due to the dopant’s role in lowering of the activation energy for diffusion along grain boundaries.

Thermal processing and properties of BaTi4O9 and Ba2Ti9O20 dielectric resonators

Sn-doped and undoped barium titanates pressed powder pellets were sintered at 1360 and 1390°C for 5 h. Though batched to form Ba2Ti9O20, a two-phase microstructure of BaTi4O9 and TiO2 formed from the undoped system. The dielectric constant at 6 GHz was 40 without SnO2 additions. Doping with 1.64 mol % SnO2 stabilized Ba2Ti9O20 and formed a single-phase microstructure but resulted in higher porosity. Dilatometry studies implied that SnO2 additions facilitated a greater fraction of reaction to occur in the solid state, causing a lower quantity of pore-filling fluid to form above 1317°C. As a result of the higher porosity, the dielectric constants and quality factors were also reduced.

Pulse radiolysis of polypropylene

Polypropylene is a widely used material for medicaldisposables and food packing where sterilization isvery important. The protection e•ect o•ered by aro-matic additives has been observed. However, the rela-tive contribution of charge and energy transferprocesses has not been estimated up to now.A pulse radiolysis study of isotactic polypropylene(PP) film has been carried out with the main aim of in-vestigating ionic and excited species generated in pureand doped PP over a wide temperature range from ca30 K up to 295 K. Full experimental and other detailscan be found elsewhere (Mayer et al., 1998).In PP, electron (6–8 MeV) pulse irradiation gener-ates electron-positive hole pairs. The electron can bestabilized in the undoped system as a trapped electron,e

Quantum Transport in Two-Dimensional Graphite System

In a self-consistent Born approximation, the density of states and the conductivity are calculated in a two-dimensional graphite sheet in magnetic fields. Two different cases of scatterers are considered, the short-range case where the range is smaller than the lattice constant and the long-range case where it is comparable or slightly larger. The quantum theory provides results quite different from the results of Boltzmann transport theory even in the absence of a magnetic field. In high magnetic fields, the conductivity exhibits a series of peaks, whose values depend only on the natural constants and the Landau level index. The conductivity of undoped systems is always given by a universal conductivity \(e^{2}/\pi^{2}\hbar\) independent of a magnetic field.

Doped antiferromagnets in high dimension

The ground-state properties of the t-J model on a d-dimensional hypercubic lattice are examined in the limit of large d. It is found that the undoped system is an ordered antiferromagnet, and that the doped system phase separates into a hole-free antiferromagnetic phase and a hole-rich phase. The latter is electron free if J > 4t and is weakly metallic (and typically superconducting) if J < 4t. The resulting phase diagram is qualitatively similar to the one previously derived for d=2 by a combination of analytic and numerical methods. Domain wall (or stripe) phases form in the presence of weak Coulomb interactions, with periodicity determined by the hole concentration and the relative strength of the exchange and Coulomb interactions. These phases reflect the properties of the hole-rich phase in the absence of Coulomb interactions, and, depending on the value of J/t, may be either insulating or metallic (i.e. an ``electron smectic'').

Ionic and excited intermediates in pulse-irradiated polypropylene doped with aromatics

A pulse radiolysis study of isotactic polypropylene (PP) film has been carried out with the main aims of investigating charge trapping in an undoped system and solute radical ion generation in an pyrene (Py) doped matrix. In PP, pulse radiolysis gives electron–positive hole pairs. The electron can be stabilized in the undoped system as a trapped electron, e. The transient absorption spectrum of e in the near-IR (up to 1800 nm) was observed in the temperature range 30–100 K. This IR absorption was not detected in the case of oxidized PP. In such a matrix electrons can be scavenged by oxidation products generating respective radical-anions (absorption in the UV RANGE, λ < 350 nm). In a doped matrix transient absorption bands centered at 450 and 500 nm were observed which can be assigned to the Py radical cation and anion, respectively. The recombination of these ionic species leads to monomer excited-state formation observed during and after the 17 ns pulse. Contrary to the Py-doped polyethylene no excimer emission was detected at room temperature even if Py content in PP was close to 0.02 mol dm−3. The rate of Py radical-ion decay was found to be temperature dependent. Two linear parts of the Arrhenius plot were observed which intersected at ca. 240 K, the glass transition temperature, Tg, for PP. The activation energies calculated for two parts of Arrhenius plot were equal to 111 and ca. 0.78 kJ mol−1 for T > Tg and T < Tg, respectively. Some preliminary results concerning the ionic processes in PP containing two solutes (Py, 3,3′-dimethyldiphenyl) were presented. The mechanism of ionic recombination in PP will be proposed and discussed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1217–1226, 1998

Impact of Pr on structural, superconducting and magnetic properties of Y 1− xPr xBaSrCu 3O 7

We study the structural, superconducting and magnetic properties of the Y 1− x Pr x BaSrCu 3O 7 system with x=0.0, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.75 and 1.0, from X-ray diffraction, AC and DC magnetic susceptibility measurements. X-ray diffraction results reveal that while Pr substitutes isostructurally until x=0.50 in Y 1− x Pr x BaSrCu 3O 7, for x=0.75 and 1.0 few small intensity un-reacted lines are observed in the spectrum. The orthorhombic distortion decreases with an increase in x. Both x=0.75 and 1.0 samples are tetragonal. The c lattice parameter of the substituted samples increases with x, indicating probable substitution of comparatively bigger ionic radii Pr 3+/4+ ion at Y 3+ in Y 1− x Pr x BaSrCu 3O 7 system. The AC susceptibility results showed that the transition temperature T c of the Y 1− x Pr x BaSrCu 3O 7 system decreases monotonically with an increase in x. Both x=0.75 and 1.0 samples do not show a superconducting transition down to 4.2 K. The critical concentration of Pr to completely suppress superconductivity in Y 1− x Pr x BaSrCu 3O 7 is higher (0.75) than that reported (0.55) for Y 1− x Pr x Ba 2Cu 3O 7. DC magnetic susceptibility measurements done on PrBaSrCu 3O 7 and PrBa 2Cu 3O 7 samples at 0.5 T revealed that while the Pr ordering temperature seems to be 12 K for the former, the same is 17 K for the latter. These results indicate that the ordering temperature of Pr moments in RE 1− x Pr x : 123-type systems, which decides the Pr 4f hybridisation with the Cu–O conduction band has a direct connection to the suppression of superconductivity. The lower T N of Pr explains the less destructive effect of the same on the superconductivity of the parent undoped system.

The influence of doping on the operation of lithium manganese oxide spinel

LiMn 2O 4 and LiMn 2 − x Co x O 4 (0.1 ≤ x ≤ 0.4) samples were prepared by a precipitation technique followed by an appropriate heat treatment in the 600–800 °C temperature range. The spinel materials were characterized by powder X-ray diffraction and electrochemical measurements. The differential capacity versus voltage plots indicate that the presence of cobalt strongly influences the order/disorder phase transition of the inserted lithium ions in the upper potential region of the spinel phase. This can be proven by X-ray diffraction measurements of delithiated spinels, showing two coexisting spinel phases in the undoped system whereas a single phase behavior in the cobalt-doped spinel system is observed. These materials show excellent capacity retention during extended cycling.

Frequency dependent electrical conductivity and dielectric relaxation behavior in amorphous (90V 2O 5–10Bi 2O 3) oxide semiconductors doped with SrTiO 3

We report on the experimental results of frequency dependent a.c. conductivity and dielectric constant of SrTiO 3 doped 90V 2O 5–10Bi 2O 3 semiconducting oxide glasses for wide ranges of frequency (500–10 4 Hz) and temperature (80–400 K). These glasses show very large dielectric constants (10 2–10 4) compared with that of the pure base glass (≈10 2) without SrTiO 3 and exhibit Debye-type dielectric relaxation behavior. The increase in dielectric constant is considered to be due to the formation of microcrystals of SrTiO 3 and TiO 2 in the glass matrix. These glasses are n-type semiconductors as observed from the measurements of the thermoelectric power. Unlike many vanadate glasses, Long's overlapping large polaron tunnelling (OLPT) model is found to be most appropriate for fitting the experimental conductivity data, while for the undoped V 2O 5–Bi 2O 3 glasses, correlated barrier hopping conduction mechanism is valid. This is due to the change of glass network structure caused by doping base glass with SrTiO 3. The power law behavior ( σ ac= A( ω s) with s<1) is, however, followed by both the doped and undoped glassy systems. The model parameters calculated are reasonable and consistent with the change of concentrations ( x).

Carrier Concentration Dependence of Spectral Functions in Strongly Correlated System – Study by the Dynamical Mean Field Theory –

We study the formation of itinerant electronic states on carrier doping in a strongly correlated insulator by calculating one-particle-excitation spectral functions. We apply (1) the quantum Monte Carlo method and (2) the exact diagonalization method in the framework of the dynamical mean field theory to a two-band Hubbard model. The parameters of the model are chosen so that the undoped system is a charge-transfer-type insulator. We find that the itinerant states originate from local singlet states formed by hybridized d - and p -holes. We also find asymmetry between hole-doped and electron-doped cases: (1) in the electron-doped case a peak is not observed at the chemical potential in contrast to the hole-doped case; and (2) the mass-enhancement factor in the electron-doped case is smaller than that in the hole-doped case. The results of the one-particle-excitation spectrum and the mass-enhancement factor are discussed in the light of experimental and theoretical results for high- T c cuprates.

Structure and magnetic order in undoped lanthanum manganite

The undoped lanthanum manganite system of nominal composition ${\mathrm{LaMnO}}_{3}$ has been analyzed by neutron powder diffraction for different sample heat treatment methods. Four distinct crystallographic phases have been identified: (i) an orthorhombic phase of space group Pnma and lattice parameters (at 300 K) a=5.7385(3), b=7.7024(3), c=5.5378(2) \AA{}, produced by annealing in a reducing atmosphere. The system develops long-range antiferromagnetic order below ${\mathrm{T}}_{\mathrm{N}}$=140 K with the ${\mathrm{Mn}}^{3+}$ spins coupled ferromagnetically in the a-c plane and antiferromagnetically along b, with the spin direction along a. The volume of this phase increases monotonically with increasing temperature, but both the a and c lattice parameters exhibit negative thermal expansion in alternate temperature regimes. (ii) A second (previously unreported) orthorhombic phase that exhibits a smaller splitting, also of space group Pnma and lattice parameters (at 300 K) a=5.4954(3), b=7.7854(4), c=5.5355(3) \AA{}, produced by annealing in an oxygen (or air) atmosphere. This system orders with a simple ferromagnetic structure at ${\mathrm{T}}_{\mathrm{c}}$=140 K, with the spin direction along c. Phases (i) and (ii) can be transformed reversibly by suitable heat treatment of the same sample, and exist with a range of lattice parameters and compositions. The unit-cell volume for the antiferromagnetic phase is considerably larger than for the ferromagnetic phase, which agrees with the double-exchange model proposed for this system. (iii) A monoclinic phase of space group ${\mathrm{P}112}_{1}$/a and lattice parameters (at 200 K) a=5.4660(4), b=7.7616(7), c=5.5241(5) \AA{}, \ensuremath{\gamma}=90.909\ifmmode^\circ\else\textdegree\fi{}(5) that orders ferromagnetically below 140 K. (iv) A rhombohedral phase of space group R3-barc and hexagonal lattice parameters (at 300 K) a=5.5259(2), c=13.3240(4) \AA{}, that is observed only above room temperature. Occupancy refinements show that phase (i) ideally has the stoichiometric composition ${\mathrm{LaMnO}}_{3}$, while the results for the Mn-O bond distances suggest that phases (ii), (iii), and (iv) are progressively richer in oxygen (and thus ${\mathrm{Mn}}^{4+}$). The results of our study strongly suggest the progressive development of cation vacancies in equal numbers on the La and Mn sites as the oxygen content is increased by heat treatment. In the monoclinic phase the Mn ions occupy two crystallographically independent sites, but no evidence of ordering of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ was observed. The structures of the four phases are closely related to that of perovskite. The ${\mathrm{MnO}}_{6}$ octahedra are tilted from the undistorted configuration, the tilt system being ${\mathrm{a}}^{\mathrm{\ensuremath{-}}}$${\mathrm{a}}^{\mathrm{\ensuremath{-}}}$${\mathrm{a}}^{\mathrm{\ensuremath{-}}}$ in the rhombohedral structure and ${\mathrm{a}}^{+}$${\mathrm{b}}^{\mathrm{\ensuremath{-}}}$${\mathrm{b}}^{\mathrm{\ensuremath{-}}}$ in both orthorhombic modifications.

Local Enhancement of Antiferromagnetic Correlations by Nonmagnetic Impurities

The local enhancement of antiferromagnetic correlations near vacancies observed in a variety of spin systems is analyzed in a single framework. Variational calculations suggest that the resonating-valence-bond character of the spin correlations at short distances is responsible for the enhancement. Numerical results for uniform spin chains, with and without frustration, dimerized chains, ladders, and two dimensional clusters are in agreement with our conjecture. This short distance phenomenon occurs independently of the long distance behavior of the spin correlations in the undoped system. Experimental predictions for a variety of compounds are briefly discussed.

Scaling Properties of Antiferromagnetic Transition in Coupled Spin Ladder Systems Doped with Nonmagnetic Impurities

We study effects of interladder coupling on critical magnetic properties of spin ladder systems doped with small concentrations of nonmagnetic impurities, using the scaling theory together with quantum Monte Carlo (QMC) calculations. Scaling properties in a wide region in the parameter space of the impurity concentration x and the interladder coupling are governed by the quantum critical point (QCP) of the undoped system for the transition between antiferromagnetically ordered and spin-gapped phases. This multi-dimensional and strong-coupling region has characteristic power-law dependences on x for magnetic properties such as the N\'eel temperature. The relevance of this criticality for understanding experimental results of ladder compounds is stressed.

Doping-dependent quasiparticle band structure in cuprate superconductors

We present an exact diagonalization study of the single particle spectral function in the so-called t-t'-t''-J model in 2D. As a key result, we find that unlike the `pure' t-J model, hole doping leads to a major reconstruction of the quasiparticle band structure near (pi,0): whereas for the undoped system the quasiparticle states near (pi,0) are deep below the top of the band at (pi/2,pi/2), hole doping shifts these states up to E_F, resulting in extended flat band regions close to E_F and around (pi,0). This strong doping-induced deformation can be directly compared to angle resolved photoemission results on Sr_2 Cu Cl_2 O_2, underdoped Bi2212 and optimally doped Bi2212. We propose the interplay of long range hopping and decreasing spin correlations as the mechanism of this deformation.

Spectral weight transfer in a doped strongly correlated insulator.

We study the spectral weight transfer and the chemical potential shift in strongly correlated electron systems caused by carrier doping by applying the dynamical mean-field theory to a two-band Hubbard model. The parameters of the model are chosen so that the undoped system is a charge-transfer-type insulator. We find that in the insulating phase there are finite intensities corresponding to local singlet states formed by hybridized d and p electrons, and that, on doping a part of them, turn into itinerant states. The results are discussed in the light of experimental and theoretical results for high-${\mathit{T}}_{\mathit{c}}$ cuprates. \textcopyright{} 1996 The American Physical Society.

Effect of Electron Doping on Spectral Function of Cu-Oxide Cluster Model Including Full Multiplet: Shift of Valence Band Edge

The effect of electron doping on the wave vector averaged 3 d photoemission and inverse photoemission spectra is investigated for Cu-oxide finite clusters, including the intraatomic full multipole electrostatic interactions, by means of the recursion method. It is shown, by calculations for clusters containing up to four Cu atoms, that the doping leads to an upward shift of the upper edge of the main valence band relative to the d 9 → d 8 peak and a shift of the chemical potential (µ) to the lower edge of the conduction band of undoped systems. Also the upper valence band edge changes its character from “ d 9 → d 9 L ” to “ d 10 → d 9 ” with mainly x y , y z , z x and 3 z 2 - r 2 symmetries. The “mid-gap µ” in doped metals inferred from experimental results is interpreted as a consequence of the disregard of the shift of the valence band relative to the d 8 peak due to finite electron doping.