Light-induced dimension crossover dictated by excitonic correlations

Abstract

In low-dimensional systems with strong electronic correlations, the application of an ultrashort laser pulse often yields novel phases that are otherwise inaccessible. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations together govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe2, whose three-dimensional charge-density-wave (3D CDW) state also features exciton condensation due to strong electron-hole interactions. We find that photoexcitation suppresses the equilibrium 3D CDW while creating a nonequilibrium 2D CDW. Remarkably, the dimension reduction does not occur unless bound electron-hole pairs are broken. This relation suggests that excitonic correlations maintain the out-of-plane CDW coherence, settling a long-standing debate over their role in the CDW transition. Our findings demonstrate how optical manipulation of electronic interaction enables one to control the dimensionality of a broken-symmetry order, paving the way for realizing other emergent states in strongly correlated systems.

Electron correlations rule phonon-driven instability in single layer TiSe$_2$

“…Accordingly, invoking the presence of purely electronic soft modes in the context of CDW melt or formation is not crucial in single layer TiSe 2 . The latter is in line with the recent time-resolved ultrafast electron diffraction study of bulk TiSe 2 indicating that excitonic correlations are in charge only for the out-of-plane CDW order [5].…”

“…Despite almost five decades of extensive research [1,2], microscopic origins of charge-density-wave (CDW) formation in bulk and two-dimensional (2D) 1T-TiSe 2 are still a matter of active debate [3][4][5][6], where either electronphonon [7][8][9][10][11][12][13] or electron-electron [14][15][16][17][18][19][20][21] interactions are discussed as a main driving force.…”

VO2 Does Not Host a Photoinduced Long-lived Monoclinic Metallic Phase

“…Ultrafast photoexcitation by the femtosecond (fs) laser opens up new opportunities not only in understanding but also in manipulating the interaction among electronic, magnetic and structural degrees of freedom [1][2][3][4][5]. Varies quantum materials, including charge-density-wave materials [6][7][8][9][10][11], superconductors [12][13][14][15], ferromagnets [16], phase transition materials [17][18][19][20], have been explored by ultrafast photoexication techniques. Among those systems, the strongly correlated oxide VO 2 has attracted extensive interests.…”

Light-induced hexatic state in a layered quantum material

“…These phases may exhibit drastically different macroscopic properties, but only subtle changes of the periodic lattice distortion (PLD) coupled to the CDW [26][27][28] . As demonstrated in a wide range of ultrafast experiments, optical pulses can be used to transiently suppress CDW phases and drive transitions between them 26,27,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] , induce transient CDW order 45 or open up paths into thermodynamically inaccessible hidden states 30,[46][47][48] . The high degree of in-plane structural disorder induced by the optical excitation not only influences switching behaviour 40,43,49 , but also the final phase texture 42,46,47 .…”

“…Hence, a phase transition that modifies interlayer correlations may lead to the occurrence of a dimensional crossover 50,51 . On ultrafast timescales, CDW stacking dynamics have recently been studied by means of ultrafast electron 39,44 and x-ray diffraction 43,52 . In particular, an optically induced breakdown of excitonic correlations was shown to quench out-of-plane order 44 , and careful tuning of the photoexcitation density was used to realise interlayer phase boundaries with two-dimensional characteristics 53 -raising the question if optical control may serve as a gateway to fundamentally low-dimensional phenomena such as the KTHNY transition.…”