Room-temperature oxidation of La2−x Sr x CuO4

A series of oxidized La_2?x Sr _x CuO_4+y $(0.05 \preccurlyeq x \preccurlyeq 0.3)$ compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content $0.05 \preccurlyeq x \preccurlyeq 0.15$ and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x?0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

Pressure effects of resistivity along thec axis in La2−x Sr x CuO4

Out-of-plane resistivity p_c(T) of optimally-doped high-T_c superconductor La_2–xSr_xCuO₄ has been investigated under hydrostatic pressure up to 8GPa. We found a drastic decrease in _c which reaches 50% at 8GPa. The large change of _c is understood as due to a small variation of the lattice parameter c. In comparison with a change of p_c(T) estimated from a change of c due to thermal contraction, we conclude that the well known temperature-linear dependence of _c is apparent metallic one; essentially _c is nonmetallic over the whole temperature range, and can be described in terms of tunneling or quantum hopping conduction between CuO₂ layers. Such tunneling conduction is enhanced by the scattering in the CuO₂ plane at the oxygen defects which affect the in-plane resistivity only very weakly.     

Role of Stripes in Spin and Charge Dynamics of Underdoped La2−x Sr x CuO4

We investigate the electronic states of underdoped La _2–x Sr _x CuO ₄ (LSCO) by using a microscopic model, i.e., t-t-t-J model, containing charge stripes. The numerically exact diagonalization calculation is employed on small clusters. The physical properties observed in the angle-resolved photoemission and optical conductivity experiments are consistently explained in the model with vertical stripes, but not diagonal ones. These results demonstrate a crucial role of the vertical stripes in underdoped LSCO.     

Effect of Nonstoichiometric Disorder on the Upper Critical Field in Electron Doped Nd2−x Ce x CuO4+δ Single Crystals

Temperature dependences of the electrical resistivity measured in different magnetic fields (B ∥ c, J ∥ ab) of electron doped Nd_2−x Ce _x CuO_4+δ single crystals with different cerium concentration (x=0.15, 0.17, and 0.20) and oxygen content (δ) were studied in a temperature range of 1.8–40 K. It was found that the slope of the upper critical field decreases with the increase of the disorder in the system. So, we can distinguish experimentally d-wave and anisotropic s-wave superconductors. Moreover, the slope of the upper critical field and critical temperature decrease with increase of the disorder parameter like in d-wave superconductors with anisotropic scattering.     

Transport Properties and Electronic Specific Heat of La2−x Ba x Cu1−y Zn y O4+d Bulk

The effect of the partial substitution of Cu by Zn and La by Ba in La: 214 prepared by conventional solid state reaction method has been investigated by using Hall coefficient, Seebeck coefficient and electronic specific heat measurements, over the wide temperature range between 4.2 K and 300 K, in magnetic fields up to 5 T. The samples with y=0 and d=0, showed superconductivity for x between 0.055 and 0.30. The critical temperature, the Hall and Seebeck coefficients strongly depend on Zn content. The phonon specific heat C _ph and electronic specific heat C _el have been extracted from the total specific heat C. Zn-doping effect on the (T)=C _el/T behavior suggests that the large range stripe order and the fluctuating stripe suppress the singlet formation and pseudo gap.     

Optical conductivity of a superconductor withd x 2− y 2 symmetry

Using a simplified Eliashberg formulation in which we keep only that part of the susceptibility which is separable and of $d_{x^2 - y^2 } $ symmetry in the pairing channel and keep the isotropic part in the renormalization channel, we have calculated the ac conductivity in the superconducting state. At zero temperature, in the clean limit, we find a finite amount of absorption at any frequency, however small, although it is substantially reduced over its normal-state value. Besides this boson-assisted absorption, when impurities are added a new channel is opened up in which momentum is given up to the impurity system and absorption proceeds directly through the creation of a hole-particle pair. Born and unitary impurity scattering are considered.     

Impurity scattering effect on the anisotropic resistivity for La2−x Sr x CuO4−δ with a wide range of x

We have investigated the effect of the impurity scattering due to a small amount of oxygen defect, ( < 0.01) on the anisotropic resistivity of La _2–x Sr _x CuO _4– in a wide x range up to x = 0.26. In the superconducting region with 0.06 x 0.22, we found that impurity scattering by the oxygen defect remarkably decreases the c-axis resistivity c over the whole temperature range. The finding is interpreted in terms of the nonmetallic conduction along the c axis which is enhanced by scattering the confined carriers in the CuO ₂ plane. On the other hand, the enhancement of the c-axis conduction diminishes in the nonsuperconducting region. This is consistent with the two-dimensional nature of the electronic state which stabilizes the superconducting ground state in the cuprates.     

Transport Properties of Heavy Fermion Compounds: YbIn1−x Cu4+x and YbIn1−y Ag y Cu4

The intermetallic compounds of Yb with In and Cu (YbIn_1–x Cu_4+x ) and Yb with In, Ag, and Cu (YbIn_1–y Ag _y Cu₄) exhibit interesting magnetic and transport properties. Of the compounds of Yb with In and Cu the compound with x=0, namely YbInCu₄, has attracted particular attention, because—while being a Curie–Weiss paramagnet—it undergoes a first-order isostructural phase transition at T _v =approx. 40 to 80 K and atmospheric pressure. Below T _v the ytterbium in this compound is in a mixed-valence state and the compound as a whole is sometimes called a light heavy-fermion system. Above T _v , the compound is known as a Curie–Weiss paramagnet of localized magnetic moments and, below T _v , a Pauli paramagnet in a nonmagnetic Fermi-liquid state. In the present paper the results of measurements of the thermal conductivity of polycrystalline samples, YbIn_1–x Cu_4+x with x=0,0.015, 0.095, and 0.17 and YbIn_1–y Ag _y Cu₄ with y=0, 0.3, 0.7, and 1.0, are reported. The thermal conductivity is separated into the phonon thermal conductivity ( _ph ) (i.e., related to the heat carried by phonons) and into the electronic thermal conductivity ( _e ) (related to the heat carried by electrons). The electrical resistivity of the compounds was measured to determine the temperature dependence of the Lorenz number. The results show that in the temperature interval 4.2 to 300 K the latter quantity behavior follows the theoretical predictions for heavy fermion materials.     

T c enhancement in La2−x Sr x CuO4 under anisotropic pressure

We have measured _c (T) for La _2–x Sr _x CuO ₄ (LSCO) with x=0.15 under anisotropic pressures. The c-axis compression, which decreases _c (T), reduces T _c drastically, whereas the c-axis stretch, which increases _c (T), enhances T _c from 38K at the ambient pressure to 51.6K at 8GPa. The T _c = 51.6K is the highest record so far reported for LSCO. The variation of T _c is well scaled with variations of _c . We deduce that the enhancement of T _c in LSCO is mainly caused by the suppression of interlayer coupling, though two dimensionality is generally known as a destructive factor for conventional superconductivity.     

Effects of Non-Magnetic Impurities on Cu-Spin Dynamics and Superconductivity in La2−x Sr x Cu1−y Zn y O4 Around x = 0.115

Muon-spin-relaxation (SR) and magnetic susceptibility measurements have been carried out in La_2–x Sr _x Cu_1–y Zn _y O₄ with x=0.13 changing y finely up to 0.10, with the aim at clarifying effects of the non-magnetic impurity Zn on the Cu-spin dynamics and superconductivity. It has been found that the y dependence of the volume fraction of the superconducting region estimated from susceptibility measurements highly correlates with that of the magnetically ordered region estimated from the SR results. The rapid decrease in the superconducting region and the rapid increase in the magnetically ordered region by the doping of a small amount of Zn can be interpreted as follows; Zn pins the dynamical spin correlation or the dynamical stripe correlations and hence the superconductivity is destroyed around itself.     

μSR Study in La2−x Sr x Cu1−y Zn y O4 with x≃l/8

In order to investigate the magnetic state around hole density 1/8 in LaSr-214, SR experiments were carried out for the samples of Zn-free La _2–x Sr _x CuO ₄ and Zn-doped La _2–x Sr _x Cu _1–y Zn _y O ₄. For Zn-free samples, -spin rotation was observed in x=0.115 below about T _r =10K, implying magnetic order, but not observed in x=0.125. On the other hand, Zn-doping brought about -spin rotation in x=0.125 sample below T _r =7K, implying growth of magnetic order but decreased T _r in x=0.115, meaning destruction of magnetic order. This indicates that Zn-doping pins dynamical spin modulation, resulting in static magnetic order in x=0.125 sample.     

Superconductivity of YBa2−x Nd x Cu3O y Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2?x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x≤0.30. The onset transition temperature $T_{\text{c}}^{{\text{on}}}$ is insensitive to the Nd content x in the region of x≤0.40. All are higher than 95 K. The zero resistance transition temperatures $T_{\text{c}}^{{\text{zero}}}$ , however, exhibits two-step variation with the increase of x. For x≤0.25, $T_{\text{c}}^{{\text{zero}}}$ are all above 92 K. The highest $T_{\text{c}}^{{\text{zero}}}$ of 94 K was obtained for x=0.25. For x≥0.3 $T_{\text{c}}^{{\text{zero}}}$ drops sharply to about 84 K. Finally $T_{\text{c}}^{{\text{zero}}}$ falls to 30 K and $T_{\text{c}}^{{\text{zero}}}$ is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Independent Control of Low-Energy Resonant States and Polaron States by the Zn-Doping and the Structural Transition in La2−x Sr x CuO4 and La2−x Ba x CuO4 (x=0.11)

The superconductivity depression mechanisms at x≈1/8 in La_2−x Ba _x CuO₄ and Zn substitution in La_2−x Sr _x CuO₄ were investigated by Raman scattering. About 80% of low-energy electronic states are two-dimensional at x≈1/8 and form the Fermi arc around (π/2,π/2). The low-energy states are composed of the resonant peak relating to the insulator–metal transition and the polaron states of the B _3u phonons. Zn substitution depresses the resonant peak, while the LTT structure depresses the polaron states. The superconductivity is suppressed if one of them is reduced.     

Magnetization curves of (La1−x Sr x )2CuO4−δ single crystals as functions ofx and δ

The second peak effect in magnetization curves for overdoped (La_1–xSr_x)CuO_4– single crystals was examined by means of systematic variations of Sr content x and oxygen deficiency . Oxygen defect concentration was found to sensitively affect the critical temperature T_c and the macroscopic pinning force F_p, resulting in significant changes in magnetization hysteresis loops. Observations of dependence of M and F_p at the same reduced temperature T/T_c yielded an insight into the role of oxygen defects: increasing results in increasing the pinning center density N. The second peak field B_2pk seemed to be determined by the development of a percolating network of magnetically reversible regions from the observation that the temperature dependence of B_2pk showed similar behavior to the irreversibility field B_irr.     

Microscopic Scenario for x = 1/8 Anomaly in La2 − x Sr x CuO4

We adopt a t ₁-t ₂-t ₃-J-G model for explanation of x = 1/8 anomaly in La_{2 ? x} Sr _x CuO₄ family compound. The calculated charge susceptibility shows a maximum near Q = (π, π) at intermediate temperatures and near (π, π/2) as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion G between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band t ^eff ₁, t ^eff ₂, t ^eff ₃ < G. For physically realistic hopping values we obtain the CDW amplitude e _Q = x. The in-phase domain structure as a candidate for “stripe” picture is proposed.     

Vortex Lattice Melting into Pinned Liquid State in La2−x Sr x CuO4

Magnetization measurements in La_2–x Sr _x CuO₄ (LSCO) crystals indicate vortex order-disorder transition manifested by a sharp kink in the second magnetization peak. The transition field exhibits unique temperature dependence, namely a strong decrease with temperature in the entire measured range. This behavior rules out the conventional interpretation of a disorder-driven transition into an entangled vortex solid phase. We argue that the transition in LSCO is driven by both thermally- and disorder-induced fluctuations. The resulting pinned liquid disordered state is characterized by large thermal fluctuations and irreversible magnetic behavior. We extend these results and postulate that melting, solid-solid, and solid to pinned liquid transitions, are special cases of a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of transition lines. In order to test our numerical results we present studies on a series of irradiated LSCO crystals, and show that increase of pinning, causes a depression of the transition line as well as a decrease in the curvature as the transition line, in agreement with our calculations.     

In-Plane Anisotropy of the Upper Critical Field of La2−x Sr x CuO4 in the Underdoped and Overdoped Regimes

For La_2–x Sr _x CuO₄ single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, _c , under constant magnetic fields, changing the field direction in the ab-plane. At x=0.08, 0.12 and 0.16 in the underdoped and almost optimally-doped regimes, clear anisotropy of _c with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H _c2, and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d_x ^{2-y 2} pairing. The magnitude of the fourfold component of H _c2, H _c2/H _c2, is largest in x=0.08 among the three crystals. At x=0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.     

Two-site Behavior of Chlorine Atoms in Ca2−x Na x CuO2Cl2 Single Crystals

The local structural properties of Ca_2?x Na _x CuO₂Cl₂ (x=0.0 and 0.18) single crystals were investigated by using X-ray absorption fine structure (XAFS) measurements at the Cu K edge at the temperature range 10–300 K. XAFS showed that the chemical valence state of the Cu atoms was 2+ and that there are two different bonding lengths between the apical Cl atoms and Cu atoms. The long and short bonding lengths differed by approximately 0.25 Å and their ratio was 1:1 regardless of the temperature for both specimens. The two-site behavior of the apical Cl atoms of the CuO₄Cl₂ octahedra in the Ca_2?x Na _x CuO₂Cl₂ crystals resulted from the removal of degeneracy of the Cu 3d ⁹ electron ground state, and might be related to two-dimensional charge ordering in the Ca_2?x Na _x CuO₂Cl₂ crystals.     

On the space group of La2CuO4−δ

The space groups of orthorhombic La₂CuO_4−δ and related superconducting perovskites have variously been reported as Fmmm, Abma, Bmab, Cmca, Fmm2, Cmmm, Pccm and Pccn. The lack of a universal convention for the selection of the a, b and c axes in orthorhombic structures has added to the confusion. The presence of transformation twins in the microstructure is a possible cause of some of the erroneous determinations. In this paper we review the different space groups suggested in the literature along with the choice of axes. We also present electron diffraction data obtained from single-crystal La₂CuO_4−δ and a discussion of the conditions limiting possible reflections which help to distinguish between the various possible space groups. We conclude that the space group is Bmab based on a unit cell with lattice parameters a₀<b₀<c₀.