Intrinsic tunneling spectra of Bi2(Sr(2-x)Lax)CuO(6+delta)

We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped Bi2(Sr(2-x)Lax)CuO(6+delta) (Bi2201-Lax). Despite a difference of a factor of 3 in the optimal superconducting critical temperatures for Bi2201-La0.4 and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature T* approximately 230-300 K for both compounds. We find also that, in Bi2201-Lax, pseudogap humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.     

Field-induced thermal metal-to-insulator transition in underdoped La(2-x)Sr(x)CuO(4+delta)

The transport of heat and charge in cuprates was measured in single crystals of La(2-x)Sr(x)CuO(4+delta) (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T-->0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied magnetic field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.     

Two- to three-dimensional crossover in the electronic structure of (Bi, Pb)2(Sr, La)2CuO(6+delta) from angle-resolved photoemission spectroscopy

The hole-concentration (x) dependence of the three-dimensional energy-momentum dispersion in (Bi, Pb)2(Sr, La)2CuO(6+delta) has been investigated by angle-resolved photoemission spectroscopy. For a heavily overdoped sample of T(c) < or = 0.5 K, an energy dispersion of approximately 10 meV in width is observed in the vicinity of the (pi, 0) point with varying momentum along the c axis (k(z)). This k(z) dispersion is zero for underdoped, optimally doped, and slightly overdoped samples up to a doping level corresponding to T(c) = 22 k. At higher doping levels we observe significant dispersion of the order of 10 meV (sample with T(c) < or = 0.5 K). This is clear evidence that at a doping value corresponding to T(c) = 22 K, a crossover from two- to three-dimensional electronic structure occurs.     

Coulomb-Gas scaling law for a superconducting Bi(2+y)Sr(2-x-y)La(x)CuO(6+delta) thin films in magnetic fields

The electrical transport properties of epitaxial superconducting Bi(2+y)Sr(2-x-y)La(x)CuO(6+delta) thin films have been studied in magnetic fields. Using a modified Coulomb-gas scaling law, we can fit all the magnetic field dependent low resistance data with a universal scaling curve, which allows us to determine a relation between the activation energy of the thermally activated flux flow resistance and the characteristic temperature scaling parameters.     

Evidence for two energy scales in the superconducting state of optimally doped (Bi,Pb)2(Sr,La)2CuO6+delta

We use angle-resolved photoemission spectroscopy to investigate the energy gap(s) in (Bi,Pb)2(Sr,La)2CuO6+delta. We find that the spectral gap has two components in the superconducting state: a superconducting gap and pseudogap. Differences in their momentum and temperature dependence suggest that they represent two separate energy scales. Spectra near the node reveal a sharp peak with a small gap below T(c) that closes at T(c). Near the antinode, spectra are broad with a large energy gap of approximately 40 meV above and below T(c). The latter spectral shape and gap magnitude are almost constant across T(c), indicating that the pseudogap state coexists with the superconducting state below T(c), and it dominates spectra around the antinode. We speculate that the pseudogap state competes with the superconductivity by diminishing spectral weight in antinodal regions, where the superconducting gap is largest.     

Normal state properties of La2−x Sr x CuO4

The frequency dependence of the reflectivity spectra of doped La_2–x Sr _x CuO₄ in a high frequency region is analyzed based on a picture of correlated hole hopping within a lower Hubbard subband. It is shown that experimental trends are fairly well accounted for by this model, suggesting that the normal state of this compound is quite different from the metallic state.     

Percolative phase separation in La2CuO4+δ and La2−x Sr x CuO4

We report on the formation of conducting phases in slightly doped La₂CuO₄ samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.     

Interplay of externally doped and thermally activated holes in La(2-x)Sr(x)CuO4 and their impact on the pseudogap crossover

We presented the recent Hall effect data for a number of carriers in La(2-x)Sr(x)CuO4 as the sum of two components: the temperature independent term n0(x), which is due to external doping, and the thermally activated contribution. Their balance determines the crossover temperature T*(x) from the marginal Fermi liquid to pseudogap regime. The activation energy Delta(x) for thermally excited carriers equals the energy between the Fermi surface "arc" and the band bottom, as seen in angle-resolved photoemission spectroscopy experiments. Other implications for the (T, x)-phase diagram of cuprates are also discussed.     

Superconducting coherence peak in the electronic excitations of a single-layer Bi2Sr1.6La0.4CuO6+delta cuprate superconductor

Angle resolved photoemission spectroscopy study is reported on a high quality optimally doped Bi2Sr1.6La0.4CuO6+delta high-Tc superconductor. In the antinodal region with a maximal d-wave gap, the symbolic superconducting coherence peak, which has been widely observed in multi-CuO2-layer cuprate superconductors, is unambiguously observed in a single-layer system. The associated peak-dip separation is just about 19 meV, which is much smaller than its counterparts in multilayered compounds, but correlates with the energy scales of spin excitations in single-layer cuprates.     

Phason and amplitudon in the charge-density-wave phase of one-dimensional charge stripes in La(2-x)Sr(x)CuO4

The present systematic Raman scattering experiments reveal the phason and amplitudon of the charge density wave (CDW) mode in the charge stripes of La(2-x)Sr(x)CuO4. Only about 15% of the electronic density of states condenses into the CDW state. The symmetries of the CDW modes change by the rotation of the stripes at the insulator-metal transition. The energy of the phason is finite at 0.06 < or = x < or = 0.1 and zero at x = 0.035 and 0.115 < or = x < or = 0.135, which suggests that the CDW is commensurate at 0.06 < or = x < or = 0.1 and incommensurate otherwise. The zero-energy phason seems to reduce T(c) at x = 1/8.     

Electrostatic lattice coefficients and binding energy of orthorhombic La2-x Sr x CuO4

Three valency models for orthorhombic La_2-x Sr _x CuO₄ were investigated for increasing Sr concentrationsx (0x0.21): 1. Cu²⁺Cu³⁺, 2. apex O^2–O^– and 3. in-plane O^2–O^–. All calculations were done by using structural parameters valid for the temperature range from 10 to 22 K. We thereby calculated the electrostatic interaction energy which, next to ionization potentials and electron affinities, comprises a major of the binding energyE _B of crystals. Second-order effects were accounted for by calculating the strength of ionic dipole moments induced by crystal electric fields at relevant lattice sites. Their largest strengths are comparable to the dipole moment of the water molecule. Three out of five dipoles in La_2-x Sr _x CuO₄ vanish during the transition from the orthorhombic to the tetragonal phase. The binding energy differences between the different models suggest that the system is in a state of model 1. However, the differences are very small, being in the order of 0.3 to 0.76 eV atx=0.13.     

139La NMR-NQR study of the oxygen diffusion in La2−x Sr x CuO4+δ

Il Nuovo Cimento D - High-temperature (T&gt;200 K) 139La NMR relaxation rates in single crystals of La2CuO4+δ (δ∼0.015 and δ∼0.002) at two measuring frequencies are...     

Quasiparticles in the superconducting state of Bi(2)Sr(2)CaCu(2)O(8+delta)

Recent improvements in momentum resolution lead to qualitatively new angle-resolved photoemission spectroscopy results on the spectra of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) along the (pi,pi) direction, where there is a node in the superconducting gap. We now see the intrinsic line shape, which indicates the presence of true quasiparticles at all Fermi momenta in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity.