Three-dimensional collective charge excitations in electron-doped copper oxide superconductors.

High-temperature copper oxide superconductors consist of stacked CuO2 planes, with electronic band structures and magnetic excitations that are primarily two-dimensional1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state3,4 within the limited range of momenta accessible by optics. Here we use resonant inelastic X-ray scattering to explore the charge dynamics across all three dimensions of the Brillouin zone. Polarization analysis of recently discovered collective excitations (modes) in electron-doped copper oxides5-7 reveals their charge origin, that is, without mixing with magnetic components5-7. The excitations disperse along both the in-plane and out-of-plane directions, revealing its three-dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the distance between neighbouring CuO2 planes rather than to the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought 'acoustic plasmon', which is a branch of distinct charge collective modes predicted for layered systems8-12 and argued to play a substantial part in mediating high-temperature superconductivity10-12.

Link between spin fluctuations and electron pairing in copper oxide superconductors.

Although it is generally accepted that superconductivity is unconventional in the high-transition-temperature copper oxides, the relative importance of phenomena such as spin and charge (stripe) order, superconductivity fluctuations, proximity to a Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate. In electron-doped copper oxides, the absence of an anomalous pseudogap phase in the underdoped region of the phase diagram and weaker electron correlations suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic spin fluctuations are the dominant feature. Here we report a study of magnetotransport in thin films of the electron-doped copper oxide La(2 - x)Ce(x)CuO(4). We show that a scattering rate that is linearly dependent on temperature--a key feature of the anomalous normal state properties of the copper oxides--is correlated with the electron pairing. We also show that an envelope of such scattering surrounds the superconducting phase, surviving to zero temperature when superconductivity is suppressed by magnetic fields. Comparison with similar behaviour found in organic superconductors strongly suggests that the linear dependence on temperature of the resistivity in the electron-doped copper oxides is caused by spin-fluctuation scattering.

Two-Fermi-surface superconducting state and a nodal d-wave energy gap of the electron-doped Sm1.85Ce0.15CuO(4-δ) cuprate superconductor.

We report on laser-excited angle-resolved photoemission spectroscopy in the electron-doped cuprate Sm1.85Ce0.15CuO(4-δ). The data show the existence of a nodal hole-pocket Fermi surface both in the normal and superconducting states. We prove that its origin is long-range antiferromagnetism by an analysis of the coherence factors in the main and folded bands. This coexistence of long-range antiferrmagnetism and superconductivity implies that electron-doped cuprates are two-Fermi-surface superconductors. The measured superconducting gap in the nodal hole pocket is compatible with a d-wave symmetry.

Conducting organic materials.

Some of the current directions of scientific research on electrically conducting organic solids are reviewed. Both molecular charge transfer salts and polymers are included, with emphasis on the present level of understanding of the novel solid-state properties of these materials in terms of their chemistry and structure. For the charge transfer salts the various types of metal-to-insulator phase transitions which dominate the properties of most of these materials are discussed. Also described are the superconducting and magnetic states which have been found recently. In the case of the polymers the chemistry and physics of the conduction mechanism is examined and contrasted with that of their classical inorganic counterparts.

Chemical signature of the superconducting phase in the nd-ce-cu-o system.

The electron-doped material Nd2-xCexCuO(4) becomes superconducting with a Ce(4+) composition around 0.16, but only after removal of a minuscule amount of extraneous oxygen. This enigmatic behavior was addressed here. A small fraction of copper in the CuO(2) planes of Nd2-xCexCuO(4) was substituted by cobalt-57, which serves as a microprobe of the chemical environment. Deoxygenation brought about little change in the Mössbauer spectra both above and below the optimal superconducting concentration; however, for x = 0.16 a change was observed. In the latter, a major fraction of the magnetically split, five-coordinate species showed itself as a paramagnetically relaxed doublet upon deoxygenation. The abundance of the paramagnetically relaxed species corresponds closely to the diamagnetic volume fraction and thus provides a microscopic signature of the superconducting phase.

Direct Observation of Microscopic Inhomogeneities with Energy-Dispersive Diffraction of Synchrotron-Produced X-rays.

Evidence of structural inhomogeneities in two high-transition-temperature superconductors, YBa(2)Cu(3)O(7-delta) and Nd2-xCexCuO4-y, is presented. When samples were illuminated by highly collimated x-rays produced on a synchrotron wiggler, small changes in the lattice were detected over a spatial scale of 10 micrometers. These changes are interpreted as evidence of variations in the oxygen content in one case and in the cerium content in the other; both affect the superconducting properties. The existence of such structural inhomogeneities brings into question whether exotic experimental results obtained from superconducting materials with high transition temperatures actually reflect intrinsic properties.

High-field Hall resistivity and magnetoresistance of electron-doped Pr2-xCexCuO4-delta.

We report resistivity and Hall effect measurements in electron-doped Pr2-xCexCuO4-delta films in magnetic field up to 58 T. In contrast to hole-doped cuprates, we find a surprising nonlinear magnetic field dependence of Hall resistivity at high field in the optimally doped and overdoped films. We also observe a crossover from quadratic to linear field dependence of the positive magnetoresistance in the overdoped films. A spin density wave induced Fermi surface reconstruction model can be used to qualitatively explain both the Hall effect and magnetoresistance.

The phase diagram of electron-doped La(2-x)Ce(x)CuO(4-δ).

Superconductivity is a striking example of a quantum phenomenon in which electrons move coherently over macroscopic distances without scattering. The high-temperature superconducting oxides (cuprates) are the most studied class of superconductors, composed of two-dimensional CuO2 planes separated by other layers that control the electron concentration in the planes. A key unresolved issue in cuprates is the relationship between superconductivity and magnetism. Here we report a sharp phase boundary of static three-dimensional magnetic order in the electron-doped superconductor La(2-x)Ce(x)CuO(4-δ), where small changes in doping or depth from the surface switch the material from superconducting to magnetic. Using low-energy spin-polarized muons, we find that static magnetism disappears close to where superconductivity begins and well below the doping level at which dramatic changes in the transport properties are reported. These results indicate a higher degree of symmetry between the electron and hole-doped cuprates than previously thought.

Anomalous magnetoresistance in the spinel superconductor LiTi2O4.

LiTi2O4 is a unique compound in that it is the only known spinel oxide superconductor. The lack of high quality single crystals has thus far prevented systematic investigations of its transport properties. Here we report a careful study of transport and tunnelling spectroscopy in epitaxial LiTi2O4 thin films. An unusual magnetoresistance is observed which changes from nearly isotropic negative to prominently anisotropic positive as the temperature is decreased. We present evidence that shows that the negative magnetoresistance likely stems from the suppression of local spin fluctuations or spin-orbit scattering centres. The positive magnetoresistance suggests the presence of an orbital-related state, also supported by the fact that the superconducting energy gap decreases as a quadratic function of magnetic field. These observations indicate that the spin-orbital fluctuations play an important role in LiTi2O4 in a manner similar to high-temperature superconductors.

Clinical depression and implicit memory.

We investigated performance on implicit and explicit memory tasks in subjects diagnosed with major depression and matched controls. Depressed subjects showed impaired performance on both the explicit and implicit tasks in comparison with controls. These findings are in contrast to groups such as amnesic patients and older adults, who show preserved abilities on implicit tasks and deficits on explicit tasks.

Immediate serial recall of mixed-modality lists.

Seven experiments are reported in which subjects were tested for immediate serial recall of mixed-modality lists. On mixed auditory-visual lists, there was an advantage for auditory items at all serial positions. This was due to both a facilitation of auditory items and an inhibition of visual items on mixed lists, as compared with single-modality lists. When presented on a list containing items read silently, recall of items that were silently mouthed by the subject demonstrated patterns similar to those found with auditory items. When presented on a list containing items read aloud, recall of mouthed items showed patterns similar to those found with silently read items. The auditory advantage on mixed lists was found even when the list items were acoustically similar or identical and was not reduced by midlist auditory suffixes. The results suggest that modality differences in recall of mixed-modality lists are based on information different from that responsible for modality differences in recall of single-modality lists.

Recency and suffix effects in serial recall of musical stimuli.

Auditory presentation of verbal items leads to larger recency effects in recall than visual presentation. This enhanced recency can be eliminated if a stimulus suffix (an irrelevant sound) follows the last item. Four experiments tested the hypothesis that recency and suffix effects in serial recall result from a speech-specific process. It was demonstrated that serial recall of musical notes played on a piano exhibited substantial recency effects. These recency effects were reduced when the list items were followed by either a piano chord or the word start. However, a white-noise suffix had no effect on recency. This pattern of data is consistent with current work on auditory perception and places constraints on theories of recency and suffix effects.

Category-order effects in memory span.

When Ss are required to recall lists containing both words and digits, memory span is higher when the digits precede the words than when the words precede the digits. In Experiment 1, both forward and backward recall were tested; it was demonstrated that this category-order effect reflects the input position, and not the output position, of the items. Experiment 2 revealed that this effect was not eliminated by a filled retention interval. Experiment 3 showed that the effect was eliminated when lists were presented at a fast presentation rate. In Experiment 4, the effect was eliminated when Ss engaged in articulatory suppression. A 5th experiment extended the findings of Experiment 4 to the case in which lists are composed of semantically related or unrelated words. These results suggest that category-order effects reflect mnemonic activity that Ss engage in during list presentation and do not arise from structural characteristics of the memory system.

Mirror effect in frequency discrimination.

In recognition, types of stimuli that are relatively easy to classify as old when old are also relatively easy to classify as new when new. The experiments reported here extend this mirror effect to discriminations among above-zero situational frequencies. Frequency discrimination exhibits a mirror effect when words are compared with nonwords or when low-linguistic-frequency words are compared with high-linguistic-frequency words. Accurate knowledge concerning the relative memorability of test items is neither necessary nor sufficient for the presence of a mirror effect.

On the generality of the revelation effect.

Seven experiments demonstrate the robustness of the revelation effect, which is the tendency to call recognition test items old if they are distorted when they initially appear and if they are revealed before the recognition judgment. With anagrams as the distortion, a revelation effect was found in within- and between-subjects designs, in a frequency-judgment task, in a list-discrimination task, when new items were used as targets, when the study list and the test were presented in different modalities, and when the word that was revealed did not match the word that was recognized. These results challenge accounts that attribute the revelation effect either to an increase in the familiarity of the revealed test word or to a positive response bias.

The revelation that the revelation effect is not due to revelation.

The revelation effect is the tendency to call an item on a recognition test "old" if it is preceded by a different task interpolated between study and test. Seven experiments explored the generality of the revelation effect across a number of interpolated tasks. A revelation effect emerged when a variety of tasks preceded recognition test items; the effect was found for test items that followed a memory-span task, a synonym-generation task, and a letter-counting task. The compatibility between the test stimuli and the stimuli that composed the interpolated task was found to be a critical factor. With words as stimuli on a recognition test, a revelation effect was found when the stimuli in the interpolated task were words and letters. However, when numbers were the stimuli in the interpolated task, no revelation effect was found.

Effects of level of processing on implicit and explicit tasks.

The series of experiments presented in this article replicate the interaction that B. H. Challis and D. R. Brodbeck (1992) reported between list design (blocked or mixed) and level of processing for word fragment completion: The advantage for semantically processed words over shallowly processed words was greater when the conditions were blocked than when they were mixed on the study list. A similar interaction was found for perceptual identification (a data-driven implicit task) and priming in general knowledge questions (a conceptually driven implicit task). However, both data-driven and conceptually driven explicit tasks failed to reveal such a pattern.

Recognition without identification.

The ability of people to recognize words that they could not identify was examined. After studying a list of 15 words, participants completed a word fragment test consisting of 4-letter fragments of both studied and nonstudied words. Whether they were able to solve a particular fragment or not, participants then made an episodic recognition judgment. Even when participants were unable to solve a fragment, their recognition accuracy was significantly higher than chance. This effect was significant when list length was increased, when 2-letter fragments were used, when first letters were excluded from fragments, and when the letter casing and the presentation modality were changed from study to test. It also occurred when participants attempted to identify fragments at study and rated words at test. Recognition without identification is attributed to the use of orthographic information when determining the familiarity of a test item.

The role of familiarity in recognition.

The role of familiarity in recognition was investigated by having subjects study a list of stimuli, some of which had been presented earlier in the experiment. The number of positive responses, both to targets and distractors, increased as a result of this familiarization process. This familiarization process had a greater effect on false alarms than on hits, so that recognition accuracy was lower for familiar stimuli than for relatively novel stimuli. This pattern of results differs from that found in most experiments studying the effects of linguistic word frequency on recognition.