John Robertson's research while affiliated with University of Cambridge and other places

... Others have noted the absence of deep level defects. 13,14 Wuttig et al. 15 recently noted that the p-p transition between an unusual B-X multi-centered bonding contributes 92% of their optical absorption below 4 eV while the s-p transition contributes only 8%. They also showed that the p-p transition is mainly responsible for small effective masses of electrons and holes at the R point based on a simple tight-binding model. ...

... This is consistent with the reported total energy of the odd layers of a GeTe block in a GeTe/Sb 2 Te 3 SL structure. [28] These observations strongly indicated that previously proposed switching models for iPCM should be revised with consideration to the Teterminated GeTe block and Ge/Sb intermixing. ...

... When studying time-resolved PCM crystallization and amorphization there is a limit of the fluence, known as threshold fluence [44], over which the processed spot shows Reflective pixel [38] an irreversible change after the preceding pump pulse, thus, each measurement needs to be performed on a fresh region of sample or the sample has to be moved between single measurements and has to be homogeneous over the investigated area [45]. Non optical methods such as time-resolved X-ray absorption [46,47], time-resolved X-ray diffraction [48,49] and time-resolved electron diffraction [44] has been applied to the study of PCM. Similarly, time resolved optical methods have been applied to study both phenomena, the ones happening above (single shot approach) and the ones below the fluence threshold in the reversible state of the system. ...

... Recently, transmission electron microscopy (TEM) experiments and density functional theory (DFT) calculations demonstrated that the atomic structures of the iPCM differed from those of the existing models in explaining low-(LRS) and high-resistance (HRS) states. [11,[17][18][19][20][21][22] iPCM determined that Ge and Sb atoms inevitably intermixed [11,17] and that Ge atoms were predominantly located near the GeSbTe (GST) block core in the GeTe/ Sb 2 Te 3 SL. [11,18,19] Additionally, randomly distributed vacancies in GST building units were transformed into ordered vacancy layers. ...

... Because these four phases exhibit distinct abc-atomic site occupancies and exclude tetrahedral local environments, the reported phase-change mechanisms in their structure models inevitably include the gliding of the atomic plane rather than a transition in the local environment of elements. 31,37,38 However, because gliding is a collective motion of atoms considered as the origin of the second-order transition, it contradicts the superior cyclability (~10 9 ) and ...