Underdoped Bi Research Articles

X-ray Optical Activity in Underdoped Bi-Based High-TcSuperconductor

We performed X-ray optical activity (XOA) measurements at Cu K absorption edge for the underdoped Bi2212 ( T c =80 K). X-ray natural circular dichroism (XNCD) appears below T * (∼200 K) and becomes larger with decreasing temperature, whereas nonreciprocal X-ray linear dichroism (NRXLD) is not seen. It is revealed that time reversal symmetry is preserved in the pseudogap phase of the underdoped Bi2212. The results demonstrate that XOA is a powerful tool to clarify electronic structure in terms of time reversal and/or chiral symmetry.

STM/STS Study on 4a×4aElectronic Charge Order of Superconducting Bi2Sr2CaCu2O8+δ

We performed low-bias STM measurements on underdoped Bi2212 crystals, and confirmed that a two-dimensional (2D) superstructure with a periodicity of four lattice constants (4 a ) is formed within the Cu–O plane at T < T c . This 4 a ×4 a superstructure, oriented along the Cu–O bonding direction, is nondispersive and more intense in lightly doped samples with a zero temperature pseudogap (ZTPG) than in samples with a d -wave gap. The nondispersive 4 a ×4 a superstructure was clearly observed within the ZTPG or d -wave gap, while it tended to fade out outside the gaps. The present results provide a useful test for various models proposed for an electronic order hidden in the underdoped region of high- T c cuprates.

Anisotropy dependence of Josephson-vortex flow resistance

Periodic oscillations of the Josephson-vortex flow resistance as a function of parallel magnetic field have been observed when Josephson vortices form a triangular lattice. Depending on the applied current and sample size, the period of oscillations has been found to double above a particular magnetic field H 2s. In order to investigate how H 2s depends on the anisotropy of the sample, we have measured the vortex-flow resistance as a function of parallel magnetic field for an underdoped Bi 2Sr 2CaCu 2O 8+ y (Bi-2212) single crystal whose anisotropy was controlled by annealing. We have found that H 2s increases systematically with decreasing anisotropy. This behavior is qualitatively consistent with the theoretical expectation that H 2s is inversely proportional to the square of the anisotropy parameter γ.

Inhomogeneous superconductivity in deeply underdoped Bi 2Sr 2CaCu 2O 8+ δ

To study electronic states in deeply underdoped cuprates, we have measured the interlayer tunneling spectra of Bi 2Sr 2CaCu 2O 8+ δ ( T c = 44 K) realized by partially substituting Sr with La. A mesa structure is manufactured which is composed of 10 or less stacked intrinsic tunnel Josephson junctions. The tunneling characteristics exhibits a totally unknown behavior that the differential conductance increases significantly with decreasing temperature below T c. This anomalous behavior observed only for deeply underdoped samples strongly suggest that inhomogeneous superconductivity or phase separation in CuO 2 planes is essential bulk property of deeply underdoped cuprates.

Bare electron dispersion from experiment: Self-consistent self-energy analysis of photoemission data

Performing an in-depth analysis of the photoemission spectra along the nodal direction of the high temperature superconductor Bi-2212 we have developed a procedure to determine the underlying electronic structure and established a precise relation of the measured quantities to the real and imaginary parts of the self-energy of electronic excitations. The self-consistency of the procedure with respect to the Kramers-Kronig transformation allows us to draw conclusions on the applicability of the spectral function analysis and on the existence of well defined quasiparticles along the nodal direction even for the underdoped Bi-2212 in the pseudogap state.

STM/STS STUDY ON LOCAL ELECTRONIC STATES OF UNDERDOPED Bi2Sr2CaCu2O8+δ

Spatial variation of low temperature tunneling spectrum on atomic scale was examined on the slightly underdoped Bi 2212 sample (Tc~74 K , p~0.12) by using a STM/STS technique. We observed a V-shaped gap with sharp coherence peaks, which is expected for d-wave superconductors, over the distance of ~18 nm and no other kind of gap. The spatial variation of gap size Δ0 is small, ranging from 41 to 53 meV . These results indicate that the superconductivity takes place rather homogeneously in the slightly underdoped sample.

Angle-resolved photoemission study of the lightly doped cuprates Bi2212

Insulating and underdoped Bi 2−z Pb z Sr 2 Ca 1−x R x Cu 2 O 8+y ( R= Pr, Er) (Bi2212) has been studied by photoemission spectroscopy and compared with La 2− x Sr x CuO 4 (LSCO). The lower-Hubbard band of the insulating Bi2212 shows a stronger dispersion along the underlying Fermi surface than that of undoped La 2CuO 4. The flat band at k∼( π,0) is found to be deeper in Bi2212 than in LSCO. These observations suggest that the magnitude of the next-nearest-neighbor hopping | t′| of the single-band model is larger in Bi2212 than in LSCO.

Vortex solid-liquid transition inBi2Sr2CaCu2O8+δwith a high density of strong pins

The introduction of a large density of columnar defects in %underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals does not, at sufficiently low vortex densities, increase the irreversibility line beyond the first order transition (FOT) field of pristine crystals. At such low fields, the flux line wandering length $r_{w}$ behaves as in pristine %Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. Next, vortex positional correlations along the $c$--axis in the vortex Bose glass at fields above the FOT are smaller than in the low--field vortex solid. Third, the Bose-glass-to-vortex liquid transition is signaled by a rapid decrease in c-axis phase correlations. These observations are understood in terms of the ``discrete superconductor'' model.

Pairing-induced kinetic energy lowering in doped antiferromagnets

We analyse lowering of the kinetic energy in doped antiferromagnets at the transition to thesuperconducting state. Measurements of optical conductivity indicate that suchunconventional behaviour takes place in underdoped Bi-2212. We argue thatthe definition of the operator representing the kinetic energy is determined byexperimental conditions. The thermodynamic average of that operator is related to theintegrated spectral weight of the optical conductivity and thus depends on the cut-offfrequency limiting that integral. If the upper limit of the integral lies below thecharge transfer gap the spectral weight represents the average of the hoppingterm in the space restricted to the energy range below the gap. We show thatthe kinetic energy is indeed lowered at the superconducting transition in thet–J model(tJM),which is an effective model defined in the restricted space. That result is in agreement withexperimental observations and may be attributed to the formation of spin polarons and thechange of roles which are played by the kinetic and the potential energy in thetJM and in some effective model for spin polarons. The total spectral weight represents thekinetic energy in a model defined in a broader space if the upper limit in the integral of theoptical conductivity is set above the gap. We demonstrate that the kinetic energy in theHubbard model is also lowered in the superconducting state. That result doesnot agree with experimental observations, indicating that the spectral weight isconserved for all temperatures if the upper limit of the integral is set above thecharge transfer gap. This discrepancy suggests that a single band model is notcapable of describing in some respects the physics of excitations across the gap.

Elementary excitations investigated by femtosecond Raman spectroscopy in underdoped cuprates

Magnetic excitations have been investigated in heavily underdoped Bi2212 by femtosecond Raman spectroscopy. From time-resolved Raman spectra of the B 1g two-magnon peak, we observed a fast relaxation component of 0.4 ps and a slow component with a lifetime of τ ≈70 ps. The fast response was analyzed in terms of a rate equation considering non-radiative CT relaxation process, magnon emission, and magnon decay process. On the other hand, the A 1g + B 2g component showed time constant of τ ≈15 ps. The different time response clearly indicates that the origin of the peak observed in x ′ x ′ configuration is different from the two-magnon peak observed in B 1g symmetry.

Magnetic excitations investigated by ultrashort pulse excitation in high-Tc superconductors

We have investigated the relaxation of elementary excitations in underdoped Bi2212 by ultrafast Raman spectroscopy. In x́ý symmetry, a broad feature has been observed around 2500 cm−1 in the Stokes side and assigned to B1g two-magnon peak. In investigating the power dependence of the broad feature, we observed a super-linear behavior in B1g symmetry, which is ascribed to the excess magnon population generated through the non-radiative relaxation across the CT gap. We observed a fast relaxation component around 0.4 ps and a long lifetime component greater than 20 ps for the B1g two-magnon peak.

Coexistence of superconducting gap and pseudogap in underdoped Bi2212

We have carefully studied the temperature dependence of tunneling spectra in underdoped and slightly overdoped Bi2212 single crystals. The underdoped spectra clearly show a pseudogap above Tc, while this gap features get very weak in slightly overdoped crystals. Without the pseudogap depression, the states in slightly overdoped spectra are always conserved. However, underdoped spectra do not conserve states even at temperatures when the material is in its superconducting state. We found that the conservation of states can be recovered by normalizing the superconducting spectra with the normal state pseudogap spectra. This clearly indicates the coexistence of pseudogap and superconducting gap. As we will show in the analysis section, this coexistence can also explain the hump and dip features commonly observed in tunneling spectra.

Measuring the gap in angle-resolved photoemission experiments on cuprates

Angle-resolved photoemission spectroscopy (ARPES) is considered as the only experimental tool from which the momentum distribution of both the superconducting and pseudo-gap can be quantitatively derived. The binding energy of the leading edge of the photoemission spectrum, usually called the leading edge gap (LEG), is the model-independent quantity which can be measured in the modern ARPES experiments with the very high accuracy--better than 1 meV. This, however, may be useless as long as the relation between the LEG and the real gap is unknown. We present a systematic study of the LEG as a function of a number of physical and experimental parameters. The absolute gap values which have been derived from the numerical simulation prove, for example that the nodal direction in the underdoped Bi-2212 in superconducting state is really the node--the gap is zero. The other consequences of the simulations are discussed.

A Hidden Order in the Cuprate Superconductors: The d-Density-Wave Phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudo-gap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx2-y2 symmetry. Thus the electronic structure near impuritiescan provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.

Vortex phase diagram for extremely underdoped Bi 2.2Sr 1.72La 0.08CaCu 2O 8+ δ

Vortex phase diagram for H∥ c of extremely underdoped Bi 2.2Sr 1.72La 0.08CaCu 2O 8+ δ superconductor was determined from a magnetization measurement. In the magnetization curve, a second peak in the low temperature region and a sharp change in the high temperature region were observed. The second-peak field and the sharp-change field are explained based on the three-to-two dimensional crossover and the decoupling transition of the vortex lattice, respectively. For the most underdoped sample, which has the transition temperature T c=60.2 K, the anisotropy constant γ and the ab plane penetration length λ 0 have been deduced to be γ=320 and λ 0∼340 nm, respectively.

Resonant impurity states in the d-density-wave phase.

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudogap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx(2)(-y(2)) symmetry. Thus, the electronic structure near impurities can provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.

Picosecond Raman spectroscopy and relaxation study of elementary excitations in Bi2212

Using picosecond Raman spectroscopy we investigated the relaxation of elementary excitations in heavily underdoped Bi2212. We observed super-linear behavior in the power dependence of the anti-Stokes phonon in contrast to linear behavior on the Stokes side. A broad feature has been observed in the region 2000–3000 cm −1 in a sample with T c=62–65 K. Non-linear behavior has been observed in the power dependence of the broad feature in two different polarizations. With time resolved Raman measurement we could observe a time response <0.5 ps for the B 1g two-magnon peak. We propose to understand the non-linear behavior in terms of non-equilibrium phonons, and excess magnon population.

Electronic excitations near the Brillouin zone boundary ofBi2Sr2CaCu2O8+δ

Based on angle resolved photoemission spectra measured on different systems at different dopings, momenta and photon energies, we show that the anomalously large spectral linewidth in the $(\pi,0)$ region of optimal doped and underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ has significant contributions from the bilayer splitting, and that the scattering rate in this region is considerably smaller than previously estimated. This new picture of the electronic excitation near $(\pi,0)$ puts additional experimental constraints on various microscopic theories and data analysis.

Vortex quasiordered–disordered solid transition in strongly underdoped Bi 2Sr 2CaCu 2O 8+ x single crystals

The dynamic behavior of the magnetization around the second peak was studied for strongly underdoped Bi 2Sr 2CaCu 2O 8+ x single crystals by the local Hall probe technique. The strong material anisotropy results in a very low second peak field, as low as 65 G . The second peak in the magnetization curves is identified as the sudden change of the flux creep energy barrier versus screening current. This indicates the different electrodynamics of the low-field quasiordered versus the high-field disordered phase.

Resonance Raman and quasi-particle dynamics investigated in underdoped Bi2212

In this work, we present Raman scattering study of anomalous features and magnetic excitations in underdoped Bi2212. We carried out systematic resonance Raman study and observed resonance profiles for different vibrational and electronic excitations. We clarified the origin of the anomalous feature at 600 cm −1 excitations. We have extended our study to investigate quasi-particle dynamics and elementary excitations by picosecond Raman spectroscopy. We observed super-linear behaviour in the power dependence of the anti-Stokes side in contrast to the Stokes side. As for magnetic excitations we also observed non-linear behaviour in the power dependence of the two-magnon peak. We propose to understand the non-linear behaviour in terms of non-equilibrium phonons and excess magnon population.