Hole Doping Level Research Articles

NONADIABATIC EFFECTS AND THE ROLE OF SMALL FERMI ENERGY IN MgB2

There is nowadays a general agreement on a key role of the σ bands in the superconducting properties of MgB 2. We show that peculiar characteristics of the σ bands give rise to nonadiabatic and anharmonic effects which break the conventional Migdal-Eliashberg framework. Both these features are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the σ bands. In this context we discuss how the nonadiabatic theory leads to a coherent interpretation of the superconducting properties of MgB 2 without invoking very large couplings and it naturally explains the role of the disorder on T c . It also leads to various specific predictions for the properties of MgB 2 and for the material optimization of these type of compounds. Anharmonicity is also investigated by means of LDA calculations. We find that the anharmonic character of the E2g phonon is essentially driven by the small Fermi energy of the σ holes. We present a simple analytic model which allows us to understand in microscopic terms the role of the small Fermi energy and of the electronic structure. The relation between anharmonicity and nonadiabaticity is pointed out and discussed in relation to various materials.

Vertical and diagonal stripes in the extended Hubbard model

AbstractWe extend previous real‐space Hartree–Fock studies of static stripe stability to determine the phase diagram of the Hubbard model with anisotropic nearest‐neighbor hopping t, by varying the on‐site Coulomb repulsion U and investigating locally stable structures for representative hole doping levels x = 1/8 and x = 1/6. We also report the changes in stability of these stripes in the extended Hubbard model due to next‐neighbor hopping t′ and to a nearest‐neighbor Coulomb interaction V.

Thermal transport of Cr-doped double-layered LaSr 2Mn 2O 7

We have carried out thermoelectric power ( S), magnetization and magnetoresistance measurements on LaSr 2Mn 2− y Cr y O 7 compounds ( y=0.1, 0.2 and 0.4). In the bilayer manganite LaSr 2Mn 2O 7 (hole doping level x= 1 2 ), it is well known that the coexistence of metallic A-type antiferromagnetic (AF) phase and CE-type charge/orbital order (CO/OO) phase is essential to understand physical properties. The Cr-doping strongly suppressed the AF transition temperature and its associated peak in S down to lower temperatures. The Cr impurities on the Mn site in the bilayer manganite system did not cause such a dramatic influence as the insulator to metal transition in cubic manganites. This finding is discussed on the basis of phase separation model concerning the A-type majority and CE-type minority phases.

Isovalent cation substitutions to control the intrinsic Hirr characteristics of superconductive copper oxides

The crucial role of the overall hole-doping level in controlling the magnetic irreversibility field ( H irr) values has been well documented for a number of copper-oxide superconductors. Here we show that not only the high overall hole-doping level but also the homogeneity of the hole distribution over the layered superconductive M m A 2 Q n−1 Cu n O m+2+2 n± δ or M– m ( A) 2 ( Q) ( n−1) n crystal contributes to enhance the intrinsic H irr characteristics. To control the homogeneity of the hole distribution, i.e. the distribution of holes (i) between the superconductive CuO 2–( Q-CuO 2) n−1 block and the non-superconductive AO– M m O m± δ – AO blocking block and (ii) within the superconductive CuO 2–( Q-CuO 2) n−1 block, isovalent cation substitutions are found effective. To demonstrate this, we summarize our studies on the effects of isovalent substitutions at Q and A sites on the H irr characteristics of polycrystalline Cu(Ba,Sr) 2RECu 2O 7− δ [Cu-1 (Ba,Sr)2 (RE)12] samples containing various “heavy rare earths” up to RE=Yb 0.6Lu 0.4 and Sr at the Ba site up to 40%, and also for single-crystal (Hg,Pb)(Ba,Sr) 2Ca 2Cu 3O 8+ δ [(Hg,Pb)-1 (Ba,Sr)2 (Ca)23] samples with a Sr-for-Ba substitution level up to 75%.

Structural effects of grinding on La 0.7Sr 0.3MnO 3 ceramic studied by neutron diffraction

The bulk and powdered La 0.7Sr 0.3MnO 3 ceramic sample was investigated by neutron diffraction in the temperature range from 80 to 280 K and in magnetic field up to 4.5 T. The crystallographic and magnetic structures were analyzed by the Rietveld method. An increased contribution of the orthorhombic Pbnm phase was observed after powdering the bulk ceramic sample with rhombohedral structure described by the R3̄c space group. The appearance of the orthorhombic part (20–30%) was explained by Sr or/and O escape out of the grain surface. The refined magnetic moment, μ=3.7 μ B, was consistent with the theoretical value calculated from the hole doping level. The temperature dependence of the unit-cell parameters of the dominating rhombohedral phase was also determined.

High T(c) superconductivity in MgB2 by nonadiabatic pairing.

The evidence for the key role of the sigma bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the sigma bands. We show that the nonadiabatic theory leads to a coherent interpretation of T(c) = 39 K and the boron isotope coefficient alphaB = 0.30 without invoking very large couplings and it naturally explains the role of the disorder on T(c). It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.

Hole-doping effect on the Verwey-type transition and magnetoresistivity of Ba(Sm,Ca)Fe 2O 5+ δ

Two series of samples of the oxygen-deficient double-perovskite phase, Ba(Sm,Ca)Fe 2O 5+ δ were synthesized employing a sample encapsulation technique that utilizes Fe metal as an oxygen getter. The hole-doping level on the FeO 2 plane was controlled by varying the amount of excess oxygen or the Ca concentration at the Sm site. Earlier it had been found that at room temperature five-coordinated Fe 2+ and Fe 3+ species form pairs by sharing a d electron, leading to the formation of a Fe 2.5+ fluctuating valence state [Phys. Rev. B 60 (1999) 15,251]. At T V, a Verwey-type transition occurred, signifying the charge separation of the Fe 2.5+ fluctuating valence state into high-spin Fe 2+ and Fe 3+. Moreover, related to the Verwey-type transition a negative magnetoresistance peak with a magnitude of a few percent was observed [Appl. Phys. Lett. 77 (2000) 1683]. The position of the peak was found to correspond to the jumps seen in the susceptibility and resistivity vs. temperature curves. For the present samples, it is found that the value of T V is severely decreased upon increasing the Ca concentration, as seen by susceptibility and magnetoresistivity measurements. Introduction of excess oxygen leads to a less severe decrease of the transition temperature. The observed behavior of T V was reproduced by a simple model based on combinatorial-entropy calculations.

Dependence of carrier doping level on the photo control of (La, Sr)MnO3/SrTiO3 functional heterojunction

In a junction system which consists of ferromagnetic- (La1−xSrx)MnO3 and band insulating-SrTiO3, the double exchange ferromagnetism in the (La, Sr)MnO3 layer was controlled by light irradiation through photocarrier injection form SrTiO3. We investigated the dependence of hole doping level (Curie temperature) on the photo control of the (La, Sr)MnO3/SrTiO3 heterojunction. Whereas magnetization and conductivity of the heterojunction with TC of 240 K is strongly reduced, including from metal–insulator transition, by light irradiation, that with TC of 280 K cannot be modulated. This result indicates that phase boundary, between the metallic and insulating region on the (La1−xSrx)MnO3 phase diagram, plays an important role in photo carrier control of the functional heterojunction.

RETRACTED: Field-induced superconductivity in a spin-ladder cuprate.

We report on the modulation of the transport properties of thin films, grown by molecular beam epitaxy, of the spin-ladder compound [CaCu2O3]4, using the field effect in a gated structure. At high hole-doping levels, superconductivity is induced in the nominally insulating ladder material without the use of high-pressure or chemical substitution. The observation of superconductivity is in agreement with the theoretical prediction that holes doped into spin ladders could pair and possibly superconduct.

The Charge Ordering Behavior of Colossal Magnetoresistive (CMR) Layered Compounds La2-2xSr1+2xMn2O7 (x = 0.5 ∼ 0.6)

In colossal magnetoresistive (CMR) materials, large changes in physical properties stem from the interplay between charge ordering (CO) coupled with orbital and magnetic orderings. Thus detailed microstructural studies are important in understanding these changes in physical properties. In-situTEM observation is a very useful approach to study CO transitions in these CMR compounds. We have recently studied the CO behavior of the three-dimensional perovskite Nd1/2Sr1+2xMnO3. in this work, we report the CO of two-dimensional naturally layered compounds La2-2xSr1+2xMn2O7 (327 phase), where x is the hole doping level and varies from 0.5 to 0.6 in this work.Thin foil TEM specimens were prepared from polycrystalline bulk samples by conventional thinning and ion milling method at liquid N2 temperature. The in-situ TEM work was carried out in the Electron Microscopy Center at Argonne National Laboratory using a Philips CM30 with a liquid N2 cold stage, and a Hitachi H-9000 with liquid He cold stage.

Magnetic-field irreversibility in superconductive Cu- m212 ( m=1, 2)

The magnetic-field irreversibility ( H irr) characteristics of the CuBa 2YCu 2O 7− δ (Cu-1212 or so-called “123”) and Cu 2Ba 2YCu 2O 8− δ (Cu-2212 or so-called “124”) phases are studied with respect to the hole-doping level and the isovalent-substitution effect. For both the phases the H irr characteristics are enhanced with increasing overall hole-doping level. When comparing fully oxygenated samples, the H irr line of the Cu-2212 phase is located at magnetic fields lower than that of the Cu-1212 phase. The Sr(II)-for-Ba(II) substitution is not effective in enhancing the H irr characteristics of the Cu- m212 phases. It rather lowers the H irr line in the case of the Cu-1212 phase. On the other hand, the Yb(III)-for-Y(III) substitution enhances the H irr characteristics of the Cu-1212 phase. Both types of isovalent substitution, i.e. Sr-for-Ba and Yb-for-Y, result in contraction of the lattice dimensions, but work oppositely in terms of the effect on the H irr characteristics. Therefore it is considered that the control over the H irr characteristics is not simply achieved by the lattice-dimension change but rather by the change in the homogeneity of the hole-density distribution over the crystal.

Crystal growth and transport properties of spin ladder compounds La 4+4 nCu 8+2 nO 14+8 n ( n=2, 3)

Here we summarize recent progress on the study of spin ladder compounds, La 2Cu 2O 5 (four-leg) and La 8Cu 7O 19 (five-leg), n=2 and 3 members of the homologous series La 4+4 n Cu 8+2 n O 14+8 n . Following the successful crystal growth of La 2Cu 2O 5, we grew a pure-phase five-leg system La 8Cu 7O 19 by a modified slow cooling method. The room-temperature resistivity of the as-grown La 8Cu 7O 19 crystal was 1.9 Ω cm, which is much lower than that of the La 2Cu 2O 5 crystals (≈1 .7×10 3 Ω cm). A significant drop in the resistivity (11.0 m Ω cm at 300 K) was observed upon hole doping by means of high oxygen-pressure annealing, although no transition was observed from the semiconducting behavior. In addition, a new crystal growth method to enhance hole doping level of La 2Cu 2O 5 is shown.

Structure and charge-ordering transition in Bi-based bilayeredBi0.44Ca2.56Mn2O7manganite

The normal and charge-ordered phases in ${\mathrm{Bi}}_{2\ensuremath{-}2x}{\mathrm{Ca}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ with a hole-doping level of $x=0.78$ have been investigated by transmission electron microscopy. ${\mathrm{Bi}}_{0.44}{\mathrm{Ca}}_{2.56}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7},$ the first identified member of the 327-type Bi-Ca-Mn-O family, was obtained under high oxygen pressure, and presents an orthorhombic structure with Amam symmetry at room temperature. A structural modulation is observed in this unusual layered manganite below 210 K, characterized by the one-dimensional wave vector $\mathbf{q}=0.22{\mathbf{b}}^{*}$ and a single transverse component. At low temperature the superspace group of the charge ordered state is ${P}_{s\ensuremath{-}11}^{\mathit{Amam}}.$

Anisotropic scattering rates and antiferromagnetic precursor effects in the t-t'-U Hubbard model

We have investigated the evolution of the electronic properties of the t-t'-U Hubbard model with hole doping and temperature. Due to the shape of the Fermi surface, scattering from short wavelength spin fluctuations leads to strongly anisotropic quasi-particle scattering rates at low temperatures near half-filling. As a consequence, significant variations with momenta near the Fermi surface emerge for the spectral functions and the corresponding ARPES signals. At low doping the inverse lifetime of quasiparticles on the Fermi surface is of order varying linearly in temperature from energies of order t down to a very low energy scale set by the spin fluctuation frequency while at intermediate doping a sub-linear T-dependence is observed. This behavior is possibly relevant for the interpretation of photoemission spectra in cuprate superconductors at different hole doping levels.

Magnetic-field irreversibility in superconductiveCu2Ba2(Y, Ca)Cu2O8±δ phase with Cu2O2 double-chaincharge reservoir

The magnetic irreversibility-field lines (Hirr(T)) for bothnon-substituted (x = 0) and Ca-substituted (x = 0.1) Cu2Ba2(Y1-xCax)Cu2O8±δ superconductorsof the Cu-2212 structure (so-called `124') with a Cu2O2±δ double chain as the charge-reservoir constituent have beenestablished. The obtained Hirr(T) lines are compared to data measured forCuBa2(Y1-xCax)Cu2O7-δ samples ofthe Cu-1212 structure (so-called `123') with a CuO1-δsingle-chain charge reservoir. The Ca-free Cu-2212 sample with Tc = 80 K is assumed to be underdoped in terms of the hole-doping level. Increasingthe hole-doping level through the Ca(II)-for-Y(III) substitution not onlyincreases the value of Tc, but also shifts the Hirr line towards thehigher magnetic field regime. Nevertheless, the Hirr(T) line for theCa-substituted Cu-2212 sample (Tc = 87 K) is clearly located lower thanthat for the Ca-free sample, which is of the optimally-doped Cu-1212(Tc = 91 K) phase. This is attributed to the yet higher overallhole-doping level of the Cu-1212-phase sample. It is interesting to note thatfor the Cu-1212 phase, the Hirr lines are nearly identical for both theCa-free and the Ca-substituted samples, even though the Tc decreases withCa substitution. This is believed to be due to a slightly lower oxygen contentin the Ca-substituted Cu-1212 sample.

Resonance as a measure of pairing correlations in the high-Tc superconductor YBa2Cu3O6.6

One of the most striking properties of the high-transition-temperature (high-Tc) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-Tc superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy. For YBa2Cu3O(6+x), where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6-11). Here we show that for underdoped YBa2Cu3O6.6, where x and Tc are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-Tc superconductivity. The persistence of a field effect above Tc favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors, awaiting transition to the superconducting state.

A comparison of the structural evolution induced by changing thehole-doping level in three series (Hg,Tl)-22(n-1)n (n = 2,3)samples

Three series of (Hg,Tl)-22(n-1)n (n = 2,3) sampleswere synthesized by a high-pressure technique. One series of samples wasprepared by substituting Y for the Ca site in (Hg0.7,Tl0.3)2Ba2(Yx,Ca1-x)Cu2O8-2δ(x = 0.4, 0.6, 0.8 and 0.9), while two other series were prepared by annealing(Hg0.7,Tl0.3)2Ba2(Y0.8,Ca0.2)Cu2O8-2δ and (Hg0.6,Tl0.4)2Ba2Ca2Cu3O10-2δ samples in Ar. The crystal structures ofthe samples were determined by x-ray Rietveld refinement. The variationtendencies of their lattice parameters as well as several particularinter-planar distances with the change of the hole doping level in these threeseries were investigated and compared with one another. Based on thecomparison, a qualitative explanation for the structural evolution tendencyunder hole doping for these series was proposed.

Dielectric Breakdown of the Insulating Charge-Ordered State inLa2−xSrxNiO4

Dielectic breakdown associated with characteristic delay time and subsequent negative differential resistance (NDR) have been observed in the charge-stripe phase of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ crystal, whose hole doping level ${n}_{h}$ ( $\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}x+2\ensuremath{\delta}$) is around 1/3. Threshold voltage ( ${E}_{\mathrm{th}}$), above which the NDR emerges, varies with ${n}_{h}$ and reaches a maximum at the commensurate doping of ${n}_{h}\ensuremath{\sim}1/3$. The ${E}_{\mathrm{th}}$ increases exponentially with decreasing $T$, but tends to saturate at low temperatures perhaps due to the quantum fluctuation of the stripes. The results indicate that the dielectric breakdown is related to depinning and collective transport of the ordered holes on the stripe.

Oxygen content in high-pressure synthesized CuBa 2Ca 3Cu 4O 10+ δ (Cu-1234:P) superconductor

Superconducting CuBa 2Ca 3Cu 4O 10+ δ (Cu-1234:P) samples were synthesized by a high-pressure technique at 5 GPa and 1100–1150°C utilizing highly oxidized BaCuO 2.13 precursor material as the only excess oxygen source, i.e., as an internal oxidizing agent, in the synthesis. Owing to the absence of undesired decomposition products from the commonly utilized external oxygen generators, e.g., AgO, and the essentially single-phase character of the obtained samples, the oxygen content in the Cu-1234:P phase could be established for the first time by means of wet-chemical analysis. In the as-synthesized sample the amount of excess oxygen was found small ( δ≈0.09). Several different attempts to tune the oxygen stoichiometry by means of various post-annealings were done. Upon post-annealing the as-synthesized Cu-1234:P sample in Ar and O 2 atmospheres up to 500°C the oxygen content remained unchanged, but by utilizing a special low-temperature high-pressure (500°C, 5–6 GPa) O 2-annealing technique excess oxygen could be loaded into the Cu-1234:P structure up to δ≈0.35. However, the increase in the oxygen content was reflected as a decrease of the c-axis length only; both the a-axis length (hole-doping level in the CuO 2 planes) and the T c were found quite insensitive to the changes in the oxygen stoichiometry, at least in the range of excess oxygen achieved (0.09≤ δ≤0.35). The T c was 116–117 K for all the samples obtained.

Successive structural transitions coupled with magnetotransport properties inLaSr2Mn2O7

Temperature variations of the lattice structure and the magnetotransport properties have been investigated for a bilayered manganite crystal ${\mathrm{LaSr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ (hole-doping level of $x=0.5$) which undergoes the charge-ordering below $\ensuremath{\sim}210 \mathrm{K}.$ The charge-ordered state associated with the ${d}_{{3x}^{2}\ensuremath{-}{r}^{2}}{/d}_{{3y}^{2}\ensuremath{-}{r}^{2}}$ orbital ordering of ${\mathrm{Mn}}^{3+}$ appears to collapse again with decreasing temperature below 50--100 K, which was proved by both measurements of electron and x-ray diffraction. The structural change or the switching of the orbital state in this bilayered manganite manifests itself in temperature profiles of the charge-transport and magnetic properties.