BSCCO Superconductors: Hole-Like Fermi Surface and Doping Dependence of the Gap Function

We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at (, ). Then, the superconducting gap along the FS is precisely determined from ARPES measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k _x) – cos(k _y) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.     

Electronic specific heat of YBa2Cu3O6+x from 1.8 to 300 K

We have determined for the first time the electronic specific heat(x, T) of YBa₂Cu₃O_6+x for 0.16x0.97 from 1.8 to 300 K. Weakly superconducting behavior betweenx=0.4 and 0.8 progresses rapidly to strong coupling BCS-like superconducting and metallic normal state behavior forx> 0.9. However, the continuous development of the entropyS(x, T) withx andT across the entire series suggests a progressive modification of the low-energy spin spectrum with hole doping rather than a simple band model. Fermi statistics andk-space pairing for allx is indicated by the magnitude andT-dependence ofS(x, T). Pseudogap behavior inS(x, T) is observed over a temperature region aboveT _c , which increases rapidly with oxygen depletion to around 200 K forx0.7. This loss of entropy reflects normal-state correlations apparently unrelated to the superconducting pairing.     

Optical Properties, Electronic Structure and Magnetism of α′-NaxV2O5

The optical properties of sodium-deficient -Na _x V₂O₅ (0.85 x 1.00) single crystals are analyzed using ellipsometry, and infrared reflectivity techniques. In sodium deficient samples, the optical absorption peak associated to the fundamental electronic gap develops in the middle of the pure -NaV₂O₅ gap at 0.44 eV, and the material remains insulating up to the maximal achieved hole concentration of about 15%. Nonmetallic behavior under hole doping provoked reinterpretation of the -NaV₂O₅ optical spectra. We argue that the absorption peak at about 0.9 eV corresponds to the photoionization energy of a large polaron.     

Metamagnetic Transition in the Quasi-Two-Dimensional Mott Transition System Ca2−xSrxRuO4

Ca _2–x Sr _x RuO ₄ is the new type of quasi-two-dimensional Mott transition system that connects the Mott insulator Ca ₂ RuO ₄ with the spin-triplet superconductor Sr ₂ RuO ₄ . In the metallic region near the metal-insulator boundary (0.2x0.5), we found an unusual magnetic state with a broad peak in the susceptibility at low temperatures. In this region, a clear metamagnetic transition was observed at the field of several tesla.     

Critical Behavior in the Low-Energy Spin Fluctuations in High-Tc Superconductors

We study the spin fluctuations of pure and Zn-substituted La_{2 – x} Sr _x CuO₄ using the muon spin relaxation technique. Superconductivity is found to coexist with low-frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by SR decreases with increasing x and vanishes above a critical doping x _c 0.19. This value of x _c coincides with the doping at which the normal state pseudogap extrapolates to zero. These results are discussed in terms of a quantum transition that separates the superconducting phase diagram of high-temperature superconductors into two distinct ground states.     

Heat conduction in Ba1?xKxBiO3

We have investigated heat conduction of single crystal Ba_1–xK_xBiO₃ in the temperature range of 2–300 K and in a magnetic field of up to 6 Tesla. Temperature dependence of thermal conductivity(T) reveals the participation of both electrons and phonons with their relative contributions that depend critically on the potassium doping concentration. Crystals underdoped with potassium (samples with higherT _c) exhibit a strong suppression of and a glass-like temperature dependence. In contrast, those with a higher potassium content (lowerT _c) show an increase as temperature decreases with a peak near 23 K. Field dependence of(H) is also very sensitive to the level of potassium doping. Crystals exhibiting a large phonon contribution show an initial drop in(H) at low fields followed by a minimum and then a slow rise to saturation as the field increases. The initial drop is due to the additional phonon scattering by magnetic vortices as the sample enters a mixed state. The high field behavior of(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in this material.     

Possible Superconductivity in Ba-Rich Pr123

Polycrystalline (Pr_1–xBa_x)Ba₂Cu₃O_7– samples with 0.0 x 0.6 are prepared by the standard solid state reaction technique. X-ray diffraction and resistivity measurements are performed. For x 0.3, the 123 phase is formed, but for x > 0.3, due to the solubility limit, the 123 phase is not the dominant phase. The normalized resistivity increases with the increase of Ba doping to about x = 0.08, and then decreases in the 123 phase up to x = 0.3. The existence of an extremum in the resistivity is explained as the competition between scattering by impurities and increase of the number of carriers by Ba doping. For x > 0.3, due to a large increase in the non-123 phase formation, the resistivity increases drastically. The normalized resistivity results suggest possible observation of superconductivity in Pr123 system by Ba doping. It also indicates that the reported superconductivity in Pr123 could be as the result of extra Ba concentration positioning at Pr site.     

Theory of Cyclotron Resonance and Magneto-Optics in n- and p-Type InMnAs in Ultra-high Magnetic Fields

We present a theory for the electronic and optical properties of n- and p-type In_1–x Mn _x As in ultra-high magnetic fields. An eight-band effective mass model based on the Pidgeon–Brown model and including the wavevector dependence of the electronic states as well as the s–d and p–d exchange interactions with Mn d-electrons is used to determine the electronic states. The optical properties such as cyclotron resonance are computed using Fermi's golden rule. Comparison of the theory with ultra-high magnetic field (>50 T) cyclotron resonance experiments shows that the electron cyclotron resonance peak shifts with Mn doping and that the shift allows one to extract the Mn-electron/hole exchange parameters, and . The hole cyclotron resonance shows multiple resonance peaks, which we attribute a heavy to heavy and light to light hole transitions.     

Optical absorption in the two-dimensional two-band model

The optical properties are studied within the two-dimensional two-band model with the antiferromagnetic exchange interaction between nearest-neighbord andp spins in the Cu-O sheet. The optical-absorption spectra are computed within the mean-field theory with the periodic boundary condition. An insulating gap of about 1.7 eV appears for nondoping. A Drude peak appears for a singlep hole. The Drude peak and the in-gap state appear for twop holes. The intensity of the charge-transfer band decreases, while those of the Drude peak and the in-gap state increase with doping. It turns out that the strong Coulomb repulsion and the orbital are important such as thep _x orp _y orbital in the electronic properties of high-T _c copper oxides.     

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.     

Doping-Dependent Superconducting Symmetry in Hubbard Systems

The d-wave superconducting state is studied by using a generalized Hubbard model, in which a next-nearest-neighbor correlated-hopping interaction (t ₃) is included. The results obtained within the BCS framework suggest the existence of a critical hole concentration, below which a d_x ^{2-y 2}-wave superconducting gap is observed. However, above this critical concentration the maxima of the single-particle excitation energy gap are rotated by /4 and no real nodes exist, as observed in tunneling experiments. In this study, the parameters estimated by first-principle calculations for cuprate superconductors are used.     

Charge dynamics in (La,Sr)2CuO4: from underdoping to overdoping

The normal state transport properties (resistivity, Hall effect) of La_2–xSr_xCuO⁴ have been studied over wide ranges of Sr doping and temperature. The familiar T-linear resistivity and the strongly T dependent Hall effect R_H(T) are found only near the optimal hole concentration (x 0.15–0.18). The charge dynamics changes significantly in the non-superconducting overdoped range with R_H(T) becoming constant above a characteristic temperature T*, and p(T) Tⁿ (n > 1; n 1.5±0.1). The temperature scale T*, decreasing with increasing hole concentration, provides a link between transport and magnetic properties.     

Phonon Vibration and Conduction of La2 ? xSrxCuO4 ? δ and La1.85 ? xSr0.15 + xCu1 ? xCoxO4 ? δ

Polycrystalline samples La_{2 – x} Sr _x CuO_{4 –} (0.06 x 0.5) and La_{1.85 – x} Sr_{0.15 + x} Cu_{1 – x} Co _x O_{4 –} (0 x 1) were synthesized by a conventional solid state reaction method. The structure, phonon vibration, and conduction were investigated by means of XRD, infrared (IR) spectra, and resistance. It is found that the increase of itinerant hole carriers could mask the in-plane stretching vibration mode (689 cm^–1). The softening of the phonon vibration mode gives an index of the weakening of hybridization. The distortion of CuO₆ octahedron and the microstructural inhomogeneity induced by Co doping lead to the widening of the IR absorption peaks.     

ESR Studies of Doped La2CuO4

ESR of Gd spin probes in La _2–x–y Eu _y Sr _x CuO ₄ has been employed to study the Cu spin dynamics and its relation to stripe order of spins and charges in the low temperature tetragonal phase of this compound. In the magnetically ordered hole doped samples (0 < x 0.02) a strong enhancement of magnetic fluctuations on the ESR frequency scale is observed in this structural phase. At larger hole dopings (0.08 x 0.17) a drastic decrease of the Cu spin fluctuation frequency, competing with bulk superconductivity, is apparent. For the elevated hole concentrations the obtained results are consistent with the stripe model, whereas in the low hole doping regime the data give no evidence for the stripe phase formation.     

Superconductivity of barium-doped Bi2Sr2CaCu2Oy

Sintered ceramic samples of Bi₂Sr_2–x Ba _x CaCu₂O_y with nominal barium fraction 0x0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

New features of the Mott-Hubbard insulator gap of parent HTS compounds found by resonance Raman scattering and UV-excited ordinary Raman scattering

Optical absorption at the insulating gap in the parent phase of cuprate superconductors shows a broad exciton-like peak near 1.7 eV, followed by a gradual decrease in absorption persisting 1 eV above the gap. By using ultraviolet laser lines to excite Raman spectra, we have found a Raman peak 0.2 eV below the first absorption peak in insulating cuprates. The Raman peak is much narrower than the absorption peak and hasA _2g symmetry. We assign it to an exciton consisting of a hole transition from Cu to a linear combination of Cud _xy and nearest neighbor Op orbitals. We have also studied the resonance Raman profile for two-magnon Raman scattering in the same samples. We find a sharp resonance feature at about 2.7 eV, and little Raman intensity for photon energies at the 1.7 eV absorption peak. The state created at the peak must therefore be an inappropriate intermediate state for the double spin-flip Raman process.     

Comparison of Superconductivity and Structure for Lanthanum Replacing for Barium and Copper in YBa2Cu3Od

X-ray diffraction studies and the resistivity measurements are used to characterize the structure and the superconductivity of the nominal composition of YBa₂Cu_{3 – x} La _x O _d (YBCLO) cuprates with x 0.30. There was a BaCuO₂ impurity phase detected with x 0.10. The structure of the main phase (123) has the orthorhombic form with Pmmm symmetry in the whole doping range. With increasing content of lanthanum, x, the lattice constants increase for x < 0.04, and decrease for x 0.04. Rietveld refinements for X-ray diffraction show that the dopant of lanthanum substitutes for copper in the lower doping level, and replaces for both barium and copper in the high doping level. The zero-resistance temperature T _c0 first increases with the increase of the content of lanthanum in YBCLO as x 0.04 and then decreases with x as x 0.04. We compared the results with those of La-doped YBa_{2 – z} La _z Cu₃O _y cuprates. The different relationship in superconductivity dependence of lanthanum content may result from the strains due to the different occupancy of lanthanum in the unit cell of YBa₂Cu₃O _d .     

Electron tunneling in oxide superconductors

Electron tunneling experiments in very well-characterized polycrystalline samples of Ba_1–xK_xBiO₃ and Bi₂Sr₂Ca₂Cu₃O_10– show very clear features of the energy gap. The energy gap 2 and the ratio 2/K _B T _c were determined. In the Ba_1–xK_xBiO₃ compound we also study the behavior of the tunneling conductance in the normal state.     

On the high-temperature superconductivity of SrxLa2?xCuO4?δ

The superconducting properties of Sr_xLa_2–xCuO_4– are analyzed by employing the model of interaction between charge carriers and the oxygen-displacive modes of the lattice. The role played by the hole concentration, as related to the Sr content x and oxygen deficiency, is thoroughly emphasized. The critical temperature, the ratio of twice the superconducting gap to the critical temperature, and the isotope shift are estimated as functions of the hole concentration and compared with the experimental data.     

Antiferromagnetic Ordering in the One-Dimensional Edge-Sharing CuO2 Chain System Ca2+x Y2−x Cu5O10

We have investigated the spin state of the one-dimensional edge-sharing chain system Ca_2+x Y_2–x Cu₅O₁₀ with 0x1.667 by means of the magnetic susceptibility and specific heat measurements, using the large-size single-crystals. It has been found that the magnetic ground state is an antiferromagnetically ordered state for 0x1.0, while a spin-gap state seems to be formed for x1.5. Accordingly, it appears that the quantum critical point is located between x=1.0 and 1.5 in Ca_2+x Y_2–x Cu₅O₁₀.