Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Substitution effects on the electronic anomaly and superconductivity in La2−xBaxCuO4 and La2−xSrxCuO4 with x ∼ 1/8

The effects of substitution of Y or Bi on the low-temperature structural phase transition, electronic anomaly and superconductivity have been studied in La_2-y-xY_yBa_xCuO La_2-y-xBi_yBa_xCuO₄ and La_1.9-y-xTb_0.1Bi_ySr_xCuO₄ with x = 1/8. Moreover, an electronic anomaly, which is analogous to those observed in La_2–xBa_xCuO₄ and La_1.6-xNd_0.4Sr_xCuO₄ with x 1/8, has been found below about 60 K in the thermoelectric power measurements for the Zn-substituted La_2–xSr_xCu_1–yZn_yO₄ with x 0.115 and y = 0.01 – 0.02. These experimental results support the conclusions by Tranquada et al. that static order of the stripe correlations of holes and spins, owing to pinning by the low-temperature structure or impurities (Zn in this case), is the origin of the electronic anomaly, leading to the suppression of superconductivity.     

Photoinduced Superconductivity in La2?x Sr x CuO4

Dependences of the superconducting transition temperature (T _c) and chemical potential shift (????) on the hole concentration (n _h ) for La_2?x Sr _x CuO₄ cuprate is obtained taking into account of canonical two-band BCS model containing Fermi surfaces of p and d holes. The shift of chemical potential (??) leads to the curve T _c(n _h ) with a maximum. The dependences of T _c(n _h ) for our system compared with available experimental results. Downward shift (????) of the electron chemical potential (??) with the hole concentration (n _h ) have been found. Self-consistent equations for superconducting order parameters ( $\bar{\Delta}_{p}$ and $\bar{\Delta}_{d}$ ) for both p and d holes are derived using Green??s function and equation of motion method. The temperature dependences of superconducting gaps and specific heat based on this model are also calculated. The enhancement of T _c due to doping is observed.     

Optical Properties of the Cu–O Plane in the Bi2Sr2−x La x CuO6 Family

The ab-plane optical conductivity of seven single crystals, belonging to the family Bi₂Sr_2?x La _x CuO₆, has been measured for hole concentrations per Cu site 0.03≤p≤0.18, and for 6 K≤T≤300 K (500 K for p=0.16). At low doping, ten phonon lines are detected, which are due to the removal of the degeneracy of five E _u modes (out of the predicted six). They are superimposed to a far-infrared band, which as doping increases, closes the insulating gap thus building up the Drude term. The insulator-to-metal transition occurs between p=0.7 and p=0.10 consistently with a Mott mechanism. In the metallic phase, a multiband analysis identifies a Drude term plus a mid-infrared band, which weakly depends on temperature and softens as p increases, like in other cuprates. The optical response of the crystal at optimum doping has been analyzed also in the superconducting phase. The Ferrel–Glover–Tinkham sum rule requires an integration up to Ω?6Δ and the penetration depth is 290 nm. The bosonic spectral function includes a strong peak around 50 meV, which survives up to 500 K and, therefore, might be assigned to an electron–phonon interaction.     

Real Space Imaging of the Electronic States in Underdoped Ca2−x Na x CuO2Cl2 Single Crystals

Scanning tunneling microscopy (STM) has been performed on underdoped Ca_2–x Na _x CuO₂Cl₂ (Na-CCOC) single crystals to investigate the electronic states of doped Mott insulators near the metal-insulator transition. STM images taken at 7 K show patch-like or river-like irregular features superposed on the atomic corrugations. The irregular structure has a characteristic length scale of 20 Å, which is unchanged in different samples and at different doping levels studied (0.08<x<0.12). Bias voltage dependence of the STM image suggests that Na-CCOC consists of two distinct phases with different electronic states.     

139La-NMR study of spin-flop and spin structure in La2−x Sr x CuO4(x∼1/8)

The external field dependence of the hyperfine field at La-site has been studied on La _2–x Sr _x CuO ₄ (x=0.115) and La _2–x Ba _x CuO ₄ (x=0.125), where the magnetic ordering in Cu-3d spins exists at low temperature. In La _1.885 Sr _0.115 CuO ₄, a significant decrease in the resonance line width was observed above a field of H _c 7.5T, which is the direct evidence of the spin-flop in the canted-spin system with Dzyaloshinski-Moriya interaction. The spin-flop field H _c of 7.5T is slightly lower than that in La ₂ CuO ₄ (H _c 10T), reflecting the decrease of the tilting angle of the CuO ₆ octahedra by the substitution of Sr ²⁺ ion for La-site.     

Systematic Evolution of the Magnetotransport Properties of Bi2Sr2−x La x CuO6 with Carrier Concentration

We report that it is possible to obtain a series of high-quality crystals of Bi ₂ Sr _2–x La _x CuO ₆ , of which the transport properties have been believed to be dirtier than those of other cuprates. In our crystals, the normal-state transport properties display behaviors which are in good accord with other cuprates; for example, in the underdoped region the in-plane resistivity _ab shows the pseudogap feature and in the overdoped region the T dependence of _ab changes to T ⁿ with n>1. The characteristic temperatures of the pseudo-gap deduced from the resistivity and the Hall coefficient data are presented.     

Theory of Plane Copper and Oxygen Nuclear Spin–Lattice Relaxation Rates in Lightly Doped La2− x Sr x CuO4

Using a Mori-Zwanzig projection operator procedure the relaxation function theory of doped two-dimensional Heisenberg antiferromagnetic (AF) system in the paramagnetic state is presented taking into account the hole subsystem as well as both the electron and AF correlations. At low temperatures the main contribution to the nuclear spin–lattice relaxation rate, ⁶³(1/T ₁), of plane ⁶³Cu, arises from the AF fluctuations, and ¹⁷(1/T ₁), of plane ¹⁷O, has the contributions from the wave vectors in the vicinity of (π,π) and small q ∼ 0. The effects of thermal spin-wave damping Γ _q on ¹⁷(1/T ₁) in lightly doped regime are investigated, suggesting either a polynomial of up to third order (not simply (T/J)³) or exponential temperature dependence of Γ _q at low temperatures. It is shown that the theory is able to explain the main features of experimental data on temperature and doping dependence of ^17,63(1/T ₁) in the paramagnetic state of La_{2− x} Sr _x CuO₄ compounds.     

Magnetic Phase Transition and Electronic Anomalies in La1.96−x Y0.04Sr x CuO4 around x∼1/8

La-NMR, Hall coefficient and sound velocity have been measured in order to investigate the correlation between the magnetic ordering, the prominent change of transport properties, the suppression of T _c and the structural transformation around x1/8 in La _1.96–x Y _0.04 Sr _x CuO ₄. By the substitution of 0.04/2 Y for La-sites, the structural transformation to the low temperature tetragonal phase (LTT) is caused at 57±14 K for samples around x0.115. The. prominent decrease of the Hall coefficient followed by the sign reversal and the magnetic ordering are observed around x0.115 below temperatures T _s and T _N, respectively. T _s vs x shows a bell shaped curve with the maximum value of 65 K at x00115 where most prominent suppression of T _c appears. T _N shows similar x dependence to T _s with the maximum value of 40 K at x0.115. The change of the electronic state below T _s and the suppression of T _c become more prominent and the magnetic ordering is observed more wide range of x under the LTT phase.     

Theoretical Study on the Spin-State Transition of Electron-Doped Sr2−x La x CoO4 Compounds

The spin-state transition is an interesting and unresolved problem in layered perovskite Sr_2?x La _x CoO₄ under doping. The unrestricted Hartree-Fock approximation on a realistic multiband lattice model has been applied to study the various spin states of electron-doped Sr_2?x La _x CoO₄ compounds. In the doping range 0.0≤x≤0.9, various spin states in an enlarged double cell are investigated and four of them are the ground states at different doping concentration. The magnetic ground state of the doped system takes a low-spin–intermediate-spin ferromagnetically ordered state for x<0.26, a high-spin–intermediate-spin ferromagnetically ordered state for 0.26≤x<0.47, an intermediate-spin state for 0.47≤x<0.65, and followed by an high-spin antiferromagnetically ordered state for 0.65≤x≤0.9. The densities of states of all magnetic ground states are computed and their electronic and magnetic properties are discussed.