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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Bonding states underpinning structural transitions in IrTe2 observed with micro-ARPES C. W. Nicholson , M. D. Watson , A. Pulkkinen , M. Rumo , G. Kremer , K. Y. Ma , F. O. Von Rohr , C. Cacho , C. Monney Physical Review B 110 DOI: 10.1103/PhysRevB.110.205123 Diamond Proposal Number(s): [25906] Open Access Show Abstract ▼ Abstract: Competing interactions in low-dimensional materials can produce nearly degenerate electronic and structural phases. We investigate structural phase transitions in layered IrTe2 for which a number of potential transition mechanisms have been postulated. The spatial coexistence of multiple phases on the micron scale has prevented a detailed analysis of the electronic structure. By exploiting micro-angle-resolved photoemission spectroscopy obtained with synchrotron radiation we extract the electronic structure of the multiple structural phases in IrTe2 in order to address the mechanism underlying the phase transitions. We find direct evidence of lowered energy states that appear in the low-temperature phases, states previously predicted by ab initio calculations and extended here. Our results validate a proposed scenario of bonding and antibonding states as the driver of the phase transitions.	Nov 2024
	Generation and evolution of spin-, valley-, and layer-polarized excited carriers in inversion-symmetric R. Bertoni , C. W. Nicholson , L. Waldecker , H. Hübener , C. Monney , U. De Giovannini , M. Puppin , M. Hoesch , E. Springate , R. T. Chapman , C. Cacho , M. Wolf , A. Rubio , R. Ernstorfer Physical Review Letters 117 DOI: 10.1103/PhysRevLett.117.277201 Show Abstract ▼ Abstract: We report the spin-selective optical excitation of carriers in inversion-symmetric bulk samples of the transition metal dichalcogenide (TMDC) WSe 2 . Employing time- and angle-resolved photoelectron spectroscopy (trARPES) and complementary time-dependent density functional theory (TDDFT), we observe spin-, valley-, and layer-polarized excited state populations upon excitation with circularly polarized pump pulses, followed by ultrafast (<100 fs ) scattering of carriers towards the global minimum of the conduction band. TDDFT reveals the character of the conduction band, into which electrons are initially excited, to be two-dimensional and localized within individual layers, whereas at the minimum of the conduction band, states have a three-dimensional character, facilitating interlayer charge transfer. These results establish the optical control of coupled spin-, valley-, and layer-polarized states in centrosymmetric materials with locally broken symmetries and suggest the suitability of TMDC multilayer and heterostructure materials for valleytronic and spintronic device concepts.	Dec 2016
	Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1T − TiSe2 C. Monney , M. Puppin , C. W. Nicholson , M. Hoesch , R. T. Chapman , E. Springate , H. Berger , A. Magrez , C. Cacho , R. Ernstorfer , M. Wolf Physical Review B 94, 165165 DOI: 10.1103/PhysRevB.94.165165 Show Abstract ▼ Abstract: Using time- and angle-resolved photoemission spectroscopy with selective near- and midinfrared photon excitations, we investigate the femtosecond dynamics of the charge density wave (CDW) phase in 1T−TiSe 2 as well as the dynamics of CDW fluctuations at 240 K. In the CDW phase, we observe the coherent oscillation of the CDW amplitude mode. At 240 K, we single out an ultrafast component in the recovery of the CDW correlations, which we explain as the manifestation of electron-hole correlations. Our momentum-resolved study of femtosecond electron dynamics supports a mechanism for the CDW phase resulting from the cooperation between the interband Coulomb interaction, the mechanism of excitonic insulator phase formation, and electron-phonon coupling.	Oct 2016

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