The underlying mechanism behind the metal-to-insulator transition in VO2 is still a topic of intense debate. The two leading theoretical interpretations associate the transition with either electron-lattice or electron-electron correlations. Novel experimental results are required to converge towards one of the two scenarios. Here we report on a temperature-dependent angle-resolved photoelectron study of VO2 thin films across the metal-to-insulator transition. The obtained experimental results are compared to density functional theory calculations. We find an overall energy shift and compression of the electronic band structure across the transition while the overall band topology is conserved. The results demonstrate the importance of electron-electron correlations in establishing the insulating state.

The K 2 Cr 8 O 16 compound belongs to a series of quasi-1D compounds with intriguing magnetic properties that are stabilized through a high-pressure synthesis technique. In this study, a muon spin rotation, relaxation and resonance (μ + SR) technique is used to investigate the pressure dependent magnetic properties up to 25 kbar. μ + SR allows for measurements in true zero applied field and hereby access the true intrinsic material properties. As a result, a refined temperature/pressure phase diagram is presented revealing a novel low temperature/high pressure (p C1 = 21 kbar) transition from a ferromagnetic insulating to a high-pressure antiferromagnetic insulator. Finally, the current study also indicates the possible presence of a quantum critical point at p C2 ~ 33 kbar where the magnetic order in K 2 Cr 8 O 16 is expected to be fully suppressed even at T = 0 K.

In this study, the magnetic ground state of the hollandite type material K2Cr8O16 was tuned by externally applied pressure and investigated using µ+SR method in Zero-field (ZF) and weak-transversefield (wTF) configurations. As a result, the obtained magnetic transition temperature for the measuredpressures differs notably from magnetization measurements. Moreover, both wTF and ZF data reveala transition between two different magnetically ordered states at low temperatures for higher pressures. Further theoretical and experimental studies are currently being planned in order to elucidatethe detailed nature of the magnetically ordered phase. 

Since the advent of topological insulators hosting Dirac surface states, efforts have been made to gap these states in a controllable way. A new route to accomplish this was opened up by the discovery of topological crystalline insulators where the topological states are protected by crystal symmetries and thus prone to gap formation by structural changes of the lattice. Here we show a temperature-driven gap opening in Dirac surface states within the topological crystalline insulator phase in (Pb,Sn) Se. By using angle-resolved photoelectron spectroscopy, the gap formation and mass acquisition is studied as a function of composition and temperature. The resulting observations lead to the addition of a temperature-and composition-dependent boundary between massless and massive Dirac states in the topological phase diagram for (Pb,Sn) Se (001). Overall, our results experimentally establish the possibility to tune between massless and massive topological states on the surface of a topological system.