Evidence for a universal length scale of dynamic charge inhomogeneity in cuprate superconductors

Time-resolved optical experiments can give unique information on the characteristic length scales of dynamic charge inhomogeneity on femtosecond timescales. From data on the effective quasiparticle relaxation time r in La2-xSrxCuO4 and Nd2-xCexCuO4 we derive the temperature- and doping- dependence of the intrinsic phonon escape length le, which, under certain circumstances, can be shown to be a direct measure of charge inhomogeneity. Remarkably, a common feature of both p and n-type cuprates - which has important consequences for superconductivity - is that as T  Tc from above, the escape length approaches the zero-temperature superconducting coherence length, le  s(0). In close vicinity of Tc, le appears to follow the critical behaviour of the Ginsburg-Landau coherence length, GL(T). In the normal state le is found to be in excellent agreement with the mean free path lm obtained from the resistivity data. The data on le also agree well with the data on structural coherence lengths ls obtained from neutron scattering experiments, implying the existence of complex intrinsic textures on different length scales which may have a profound effect on the functional properties of these materials.Comment: To appear in Physical Review Letter

E-assessment: past, present and future

This review of e-assessment takes a broad definition, including any use of a computer in assessment, whilst focusing on computer-marked assessment. Drivers include increased variety of assessed tasks and the provision of instantaneous feedback, as well as increased objectivity and resource saving. From the early use of multiple-choice questions and machine-readable forms, computer-marked assessment has developed to encompass sophisticated online systems, which may incorporate interoperability and be used in students’ own homes. Systems have been developed by universities, companies and as part of virtual learning environments. Some of the disadvantages of selected-response question types can be alleviated by techniques such as confidence-based marking. The use of electronic response systems (‘clickers’) in classrooms can be effective, especially when coupled with peer discussion. Student authoring of questions can also encourage dialogue around learning. More sophisticated computer-marked assessment systems have enabled mathematical questions to be broken down into steps and have provided targeted and increasing feedback. Systems that use computer algebra and provide answer matching for short-answer questions are discussed. Computer-adaptive tests use a student’s response to previous questions to alter the subsequent form of the test. More generally, e-assessment includes the use of peer-assessment and assessed e-portfolios, blogs, wikis and forums. Predictions for the future include the use of e-assessment in MOOCs (massive open online courses); the use of learning analytics; a blurring of the boundaries between teaching, assessment and learning; and the use of e-assessment to free human markers to assess what they can assess more authentically