Search Results (139 total)

We have investigated the dispersion renormalization Z_disp in La_2−xSr_xCuO₄ over the wide doping range of x=0.03–0.30, for binding energies extending to several hundred meV’s. Strong correlation effects conspire in such a way that the system exhibits a local-density-approximation-like dispersion which essentially “undresses” (Z_disp→1) as the Mott insulator is approached. Our finding that the Mott insulator contains “nascent” or “preformed” metallic states with a vanishing spectral weight offers a challenge to existing theoretical scenarios for cuprates.

We report that in YBa₂Cu₃O_y and La_2-xSr_xCuO₄ there is a spatially inhomogeneous response to the magnetic field for temperatures T extending well above the bulk-superconducting transition temperature T_c. An inhomogeneous magnetic response is observed above T_c even in ortho-II YBa₂Cu₃O_6.50, which has highly ordered doping. The degree of the field inhomogeneity above T_c tracks the hole-doping dependences of both T_c and the density of the superconducting carriers below T_c, and therefore is apparently coupled to superconductivity.

We present infrared magneto-optical measurements of the c-axis conductivity of YBa₂Cu₃O_y in both the underdoped (y=6.67 and 6.75) and optimally doped (y=6.95) regimes. We show that modest c-axis magnetic fields radically modify the condensate formation and restore conventional BCS-like energetics. Additionally, we demonstrate the pivotal role of interplane coherence in the anomalous high-energy contribution to the superfluid density.

We have performed both Josephson and quasiparticle tunneling in vacuum tunnel junctions formed between a conventional superconducting scanning tunneling microscope tip and overdoped Bi₂Sr₂CaCu₂O_8+δ single crystals. A Josephson current is observed with a peak centered at a small finite voltage due to the thermal-fluctuation-dominated superconducting phase dynamics. Josephson measurements at different surface locations yield local values for the Josephson I_CR_N product. Corresponding energy gap measurements were also performed and a surprising inverse correlation was observed between the local I_CR_N product and the local energy gap.

Single crystal synchrotron x-ray diffraction study has been performed on the layered perovskites RBaCo₂O_∼5.5 (R=Tb,Nd) over a wide temperature range 100<T<375 K. By means of reciprocal space mapping and symmetry analysis, it is shown that the first-order structural transition, accompanied by a metal-insulator transition, turns out to be a Pmmm(a_c×2a_c×2a_c)↔Pmma(2a_c×2a_c×2a_c) transformation. The detailed structural analysis shows that all four different Co³⁺ ions possible within the space-group Pmma have different oxygen polyhedron distortions, thus, indicating ordering of different spin states.

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi₂Sr_2−xLa_xCuO_6+δ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly doped to optimally doped regions. We have observed the formation of the dispersive quasiparticle band, evolution of the Fermi “arc” into the Fermi surface, and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca_2−xNa_xCuO₂Cl₂ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La_2−xSr_xCuO₄ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc and/or surface are formed around the Fermi level already in the lightly doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral t^′ also confirms the similarity to Na-CCOC and the difference from LSCO.

The temperature and magnetic-field (H) dependences of thermal conductivity (κ) of Bi₂Sr₂CaCu₂O_8+δ (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi₂Sr₂Ca_1−xDy_xCu₂O_8+δ (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10  K is observed, which means Dy³⁺ ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of κ at temperature above 5  K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of κ, the Dy-doped samples present an additional “dip-like” term of κ(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy³⁺ ions. For nonsuperconducting Dy-Bi2212 samples with x≃0.50, an interesting “plateau” feature shows up in the low-temperature κ(H) isotherms with characteristic field at 1–2  T, for which we discuss the possible revlevance of magnon excitations.

Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La_2-xSr_xCuO₄ and Nd₂CuO₄ at 500 meV. The mode is peaked around the (π, 0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with x=0, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr content and is absent at x=0.17, where it is replaced by a continuum of excitations. This mode is only observed when the incident x-ray polarization is normal to the CuO planes.

We investigate the effects of electronic correlations in the full-Heusler Co₂MnSi, by combining a theoretical analysis of the spin-resolved density of states with tunneling-conductance spectroscopy measurements using Co₂MnSi as electrode. Both experimental and theoretical results confirm the existence of so-called nonquasiparticle states and their crucial contribution to the finite-temperature spin polarization in this material.

To study the effects of paramagnetic spins on phonons, both the in-plane and the c-axis heat transport of GdBaCo₂O_5+x single crystals are measured at low temperature down to 0.36  K and in magnetic field up to 16  T. It is found that the phonon heat transport is very strongly affected by the magnetic field and nearly five times increase of the thermal conductivity in several tesla field is observed at 0.36  K. It appears that phonons are resonantly scattered by paramagnetic spins in zero field and the application of magnetic field removes such strong scattering, but the detailed mechanism is to be elucidated.

We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa₂Cu₃O_6.45 (T_c=48  K). In contrast to earlier work on YBa₂Cu₃O_6.5 (T_c=58  K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed “resonance” and a large (ℏω≈15  meV) superconducting spin gap, we find that the spin excitations in YBa₂Cu₃O_6.45 are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the a^*∕b^*-axis directions at different energies reveals that spin excitations are unisotropic and consistent with the “hourglasslike” dispersion along the a^*-axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa₂Cu₃O_6+y when superconductivity is suppressed takes place at y∼0.5 with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulatinglike phase, our results suggest a clear connection between the large change in spin excitations and the MIC. The resonance therefore is a fundamental feature of metallic ground state superconductors and a consequence of high-T_c superconductivity.

A Comment on the Letter by Nicolas Doiron-Leyraud et al. [Phys. Rev. Lett. 97, 207001 (2006)]. The authors of the Letter offer a Reply.

We report on the c-axis magneto-optical response of YBa₂Cu₃O_y (y=6.65 and 6.75) single crystals, with magnetic fields oriented both parallel and perpendicular to the CuO₂ planes. The dominant characteristic of the c-axis electrodynamics in the superconducting state, the Josephson plasma resonance (JPR), is remarkably sensitive to fairly modest magnetic fields below 8  T. Fields oriented perpendicular to the CuO₂ planes are shown to shift the edge of the JPR and also reduce the weight of the so-called “400-cm⁻¹ mode,” shedding light on this enigmatic feature. In the H‖CuO₂ geometry, where the magnetic field initiates Josephson vortices, we observed a strong mode in the far-infrared which hardens with increasing field. The field dependence of the low-frequency resonance behavior is contrasted to that of two other cuprate materials: La_2−xSrCuO₄ compounds that we have investigated earlier, and Bi₂Sr₂CaCu₂O_8−δ. Specifically, there exist disparities in the number and field dependence of longitudinal modes measured for each system. Many of these differences can be explained through a new numerical solution of the interlayer phase equations which includes effects of both in-plane and c-axis dissipation parameters. Support for this approach is given by calculations of the Josephson vortex lattice ground state configuration, and further insight is gained through the phenomenological framework of the transverse JPR model, as well as a classical model of vortex dynamics.

Highly disordered magnetism confined to individual weakly interacting vortices is detected by muon spin rotation in two different families of high-transition-temperature (T_c) superconductors but only in samples on the low-doping side of the low-temperature normal-state metal-to-insulator crossover (MIC). The results support an extended quantum phase transition (QPT) theory of competing magnetic and superconducting orders that incorporates the coupling between CuO₂ planes. Contrary to what has been inferred from previous experiments, the static magnetism that coexists with superconductivity near the field-induced QPT is not ordered. Our findings unravel the mystery of the MIC and establish that the normal state of high-T_c superconductors is ubiquitously governed by a magnetic quantum critical point in the superconducting phase.

The magnetic properties of GdBaMn₂O_5.0, which exhibits charge ordering, are studied from 2 to 400 K using single crystals. In a small magnetic field applied along the easy axis, the magnetization M shows a temperature-induced reversal which is sometimes found in ferrimagnets. In a large magnetic field, on the other hand, a sharp change in the slope of M(T) coming from an unusual turnabout of the magnetization of the Mn sublattices is observed. Those observations are essentially explained by a molecular field theory which highlights the role of delicate magnetic interactions between Gd³⁺ ions and the antiferromagnetically coupled Mn²⁺/Mn³⁺ sublattices.

To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher-energy scale, we present improved experimental data from four families of high-T_c superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low-energy anomaly (“kink”) of 0.03–0.09 eV, a high-energy anomaly of 0.3–0.5 eV, and an anomalous enhancement of the width of the local-density-approximation-based CuO₂ band extending to energies of ≈2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.

We have performed transverse-field (TF) and zero-field (ZF) μSR measurements of Bi₂Sr_2−xLa_xCuO_6+δ (Bi2201) systems with x=0.2, 0.4, 0.6, and 1.0, using ceramic specimens with modest c-axis alignment and single-crystal specimens. The absence of static magnetic order has been confirmed in underdoped (x=0.6) and optimally doped (x=0.4) systems at T=2 K, while only a very weak signature towards static magnetism has been found at T=2 K in the x=1.0 system, which is a lightly hole-doped nonsuperconducting insulator. In the superconducting (x=0.6, 0.4, and 0.2) systems, the relaxation rate σ in TF-μSR, proportional to n_s∕m^* (superconducting carrier density and effective mass), followed a general trend found in other cuprate systems in a plot of T_c vs n_s∕m^*(T→0). Assuming the in-plane effective mass m^* for Bi2201 to be comparable to three to four times the bare electron mass m_e as found in La_2−xSr_xCuO₄ (LSCO) and YBa₂Cu₃O_7−δ (YBCO) systems, we obtain n_s∼0.15–0.2 per Cu for the x=0.4 Bi2201 system. This carrier density is much smaller than the Hall number n_Hall∼10 per Cu obtained at T<1.6 K in high magnetic fields (40–60 T) along the c axis applied to suppress superconductivity. The present results of the superfluid density (n_s∕m^*) in Bi2201 are compared with those from other cuprate systems, including YBCO systems with very much reduced T_c<20 K studied by microwave, H_c1, and inductance methods. Additional muon-spin-relaxation (μSR) measurements have been performed on a single-crystal specimen of Bi2201 (x=0.4) in a high transverse magnetic field of 5 T parallel to the c axis, in order to search for the field-induced muon spin relaxation recently found in LSCO and some other high-temperature superconducting cuprate (HTSC) systems well above T_c. The nearly temperature-independent and very small relaxation rate observed in Bi2201 above T_c rules out a hypothesis that the field-induced relaxation is directly proportional to the magnitude of the Nernst coefficient, which is a measure of the strength of dynamic superconductivity. We also describe a procedure for angular averaging of σ in μSR measurements using ceramic specimens with modest alignment of c-axis orientations, together with the neutron-scattering results obtained for determining the orientation distribution of microcrystallites in the present ceramic specimens.

By measuring the Hall coefficient R_H up to 1000 K in La₂CuO₄, Pr_1.3La_0.7CuO₄, and La_2−xSr_xCuO₄ (LSCO), we found that the temperature (T) dependence of R_H in LSCO for x=0–0.05 at high temperature undoubtedly signifies a gap over which the charge carriers are thermally activated, which in turn indicates that in lightly doped cuprates strong charge fluctuations are present at high temperature and the carrier number is not a constant. At higher doping (x=0.08–0.21), the high-temperature R_H(T) behavior is found to be qualitatively the same, albeit with a weakened temperature dependence, and we attempt to understand its behavior in terms of a phenomenological two-carrier model where the thermal activation is considered for one of the two species. Despite the crude nature of the model, our analysis gives a reasonable account of R_H both at high temperature and at 0 K for a wide range of doping, suggesting that charge fluctuations over a gap remain important at high temperature in LSCO deep into the superconducting doping regime. Moreover, our model gives a perspective to understand the seemingly contradicting high-temperature behavior of R_H and the in-plane resistivity near optimum doping in a consistent manner. Finally, we discuss possible implications of our results for such issues as the scattering-time separation and the large pseudogap.

We have performed a systematic doping-dependent study of La_2−xSr_xCuO₄ (LSCO) (0.03⩽x⩽0.3) by angle-resolved photoemission spectroscopy. Over this entire doping range, the underlying “Fermi surface” determined from the low-energy spectral weight approximately satisfies Luttinger’s theorem, even down to the lightly doped region. This is in strong contrast to the results on Ca_2−xNa_xCuO₂Cl₂ (Na-CCOC), which show a clear deviation from Luttinger’s theorem. We correlate these differences between LSCO and Na-CCOC with differences in the behavior of chemical potential shift and spectral weight transfer induced by hole doping.

X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) techniques are utilized to explore the ferromagnetic/barrier interface in Co₂MnSi full Heusler alloy magnetic tunnel junctions. Structural and magnetic properties of the interface region are studied as a function of the degree of site disorder in the alloy and for different degrees of barrier oxidation. Photoelectron scattering features that depend upon the degree of L2₁ ordering are observed in the XAS spectra. Additionally, the moments per 3d hole for Co and Mn atoms are found to be a sensitive function of both the degree of L2₁ ordering and the barrier oxidation state. Significantly, a multiplet structure is observed in the XMCD spectra that indicates a degree of localization of the moments and may result from the half-metallic ferromagnetism (HMF) in the alloy. The magnitude of this multiplet structure appears to vary with preparation conditions and could be utilized to ascertain the role of the constituent atoms in producing the HMF, and to examine methods for preserving the half-metallic state after barrier preparation. The changes in the magnetic structure caused by barrier oxidation could be reversed by inserting a thin Mg interface layer in order to suppress the oxidation of Mn in the Co₂MnSi layer.

We use inelastic neutron scattering to explore the evolution of the low energy spin dynamics in the electron-doped cuprate Pr_0.88LaCe_0.12CuO_4−δ (PLCCO) as the system is tuned from its nonsuperconducting, as-grown antiferromagnetic (AF) state into an optimally doped superconductor (T_c≈24 K) without static AF order. The low-temperature, low-energy response of the spin excitations in underdoped samples is coupled to the presence of the AF phase, whereas the low-energy magnetic response for samples near optimal T_c exhibits spin fluctuations surprisingly insensitive to the sample temperature. This evolution of the low-energy excitations is consistent with the influence of a quantum critical point in the phase diagram of PLCCO associated with the suppression of the static AF order. We carried out scaling analysis of the data and discuss the influence of quantum critical dynamics in the observed excitation spectrum.

Thus far, there is no cuprate system where both n-type and p-type charge carriers can be doped without changing the crystallographic structure. For studying the electron-hole symmetry in an identical structure, we try to dope n-type carriers to YBa₂Cu₃O_y system by reducing oxygen content and substituting La³⁺ ions for Ba²⁺. Single crystals of La-doped YBa₂Cu₃O_y are grown by a flux method with Y₂O₃ crucibles and it is confirmed that La actually substitutes ∼13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by annealing the crystals in an atmosphere with controlled oxygen partial pressure. The in-plane resistivity ρ_ab at room temperature was found to increase with decreasing oxygen content y down to 6.32, but interestingly further decrease in y results in a decrease in ρ_ab. The most reduced samples with y=6.21 show ρ_ab of ∼30 mΩ cm at room temperature, which is as much as seven orders of magnitude smaller than the maximum value at y=6.32. Furthermore, both the Hall coefficient and the Seebeck coefficient of the y=6.21 samples are found to be negative at room temperatures. The present results demonstrate that the nondoped Mott-insulating state has been crossed upon reducing y and n-type carriers are successfully doped in this material.

We compare the theoretical predictions of the previous paper on the field dependence of the magnetic spectrum in anisotropic two-dimensional and Dzyaloshinskii-Moriya layered antiferromagnets [L. Benfatto and M. B. Silva Neto, Phys. Rev. B 74, 024415 (2006)], with Raman spectroscopy experiments in Sr₂CuO₂Cl₂ and untwinned La₂CuO₄ single crystals. We start by discussing the crystal structure and constructing the magnetic point group for the magnetically ordered phase of the two compounds, Sr₂CuO₂Cl₂ and La₂CuO₄. We find that the magnetic point group in the ordered phase is the m̱mm̱ orthorhombic group, in both cases. Furthermore, we classify all the Raman active one-magnon excitations according to the irreducible co-representations for the associated magnetic point group. We find that the in-plane (or Dzyaloshinskii-Moriya) mode belongs to the DA_g co-representation while the out-of-plane (XY) mode belongs to the DB_g co-representation. We then measure and fully characterize the evolution of the one-magnon Raman energies and intensities for low and moderate magnetic fields along the three crystallographic directions. In the case of La₂CuO₄, a weak-ferromagnetic transition is observed for a magnetic field perpendicular to the CuO₂ layers. We demonstrate that from the jump of the Dzyaloshinskii-Moriya gap at the critical magnetic field H_c≃6.6 T one can determine the value of the interlayer coupling J_⊥∕J≃3.2×10⁻⁵, in good agreement with previous estimates. We furthermore determine the components of the anisotropic gyromagnetic tensor as g_s^a=2.0, g_s^b=2.08, and the upper bound g_s^c=2.65, also in very good agreement with earlier estimates from magnetic susceptibility. For the case of Sr₂CuO₂Cl₂, we compare the Raman data obtained in an in-plane magnetic field with previous magnon-gap measurements done by electron spin resonance (ESR). Using the very low magnon gap estimated by ESR (∼0.05 meV), the data for the one-magnon Raman energies agree reasonably well with the theoretical predictions for the case of a transverse field (only hardening of the gap). On the other hand, an independent fit of the Raman data provides an estimate for g_s≃1.98 and gives a value for the in-plane gap larger than the one measured by ESR. Finally, because of the absence of the Dzyaloshinskii-Moriya interaction in Sr₂CuO₂Cl₂, no field-induced modes are observed for magnetic fields parallel to the CuO₂ layers in the Raman geometries used, in contrast to the situation in La₂CuO₄.

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in detail the formation of the quasiparticle spectrum along the nodal direction of high-T_c cuprates. The heavily discussed “70 meV kink” on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects such as bilayer splitting, diffraction replicas, etc. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below some doping dependent temperature and sharpening below T_c, and is mainly responsible for the formation of the kink in question.