Effect of Cu intercalation and pressure on excitonic interaction in 1T−TiSe2

Abstract

1T-TiSe 2 has a semimetallic band structure at room temperature and undergoes phase transition to a triple-q charge density wave (CDW) state with a commensurate superlattice structure (2a × 2a × 2c) below T c ≈ 200 K at ambient pressure. This phase transition is caused by cooperative phenomena involving electron-phonon and electron-hole (excitonic) interactions, and cannot be described by a standard CDW framework. By Cu intercalation or the application of pressure, this phase transition temperature is suppressed and superconductivity (SC) appears. However, it is not clear what kind of order parameters are affected by these two procedures. We investigated the crystal structure of Cu x TiSe 2 and pressurized 1T-TiSe 2 around the SC state by synchrotron x-ray diffraction on single crystals. In the high-temperature phase, the variation of structural parameters for the case of Cu intercalation and application of pressure are considerably different. Moreover, the relationship between the critical points of the CDW phase transition and the SC dome are also different for the two cases. The excitonic interaction appears to play an important role in the P−T phase diagram of 1T-TiSe 2 , but not in the x−T phase diagram.

Attractive Coulomb interaction, temperature-dependent hybridization, and natural circular dichroism in 1T−TiSe2

“…This indicates that the Ti 3d-Se 4p hybridization responsible for the ∼3 eV feature becomes stronger in the low-T CDW phase. This is consistent with the decrease in Ti-Se bond distances in the CDW phase, as is known from structural studies [38,60]. The T -dependent changes for the CW and CCW spectra are plotted as difference curves in the same figure.…”

The maximal superconductivity in proximity to charge density wave quantum critical point in Cu$_x$TiSe$_2$

“…As for the Cu intercalation, T c becomes maximum of 3.79 − 4.15 K at x opt ≈ 0.077 − 0.08 in close proximity to the QCP of x c ≈ 0.06 − 0.07 at which the CDW is completely suppressed. [7,8] Also, Raman scattering experiments showed the frequency softening and divergent linewidth of the CDW amplitude mode corresponding to the L point as x approaches x c . [9,10] These seem in accord with the quantum critical superconductivity alluded above which suggests that the very interaction underlying the CDW formation may also induce the superconductivity in Cu x TiSe 2 .…”

“…[11] As to the pressure, Kusmartseva et al observed by transport measurements that superconductivity appears in the range of P ≈ 2 − 4 GPa and the maximum T c ≈ 1.8 K occurs around P opt ≈ 3 GPa close to the CDW suppression. [12] However, Joe et al [13] and Kitou et al [14] observed with the synchrotron X-ray diffraction on single crystals that the P induced suppression is at P c ≈ 5.1 GPa which is more than 1 GPa beyond the end of the SC region. Joe et al also observed a reentrant incommensurate CDW phase appeared near the P opt above the SC dome which seemed to indicate that the pressure induced superconductivity in TiSe 2 may not be connected to the CDW suppression but to the CDW domain walls.…”

“…[17] Kogar et al performed X-ray diffraction experiments on Cu x TiSe 2 and proposed that the incommensuration and domain walls of the CDW may play a crucial role in the formation of the SC state as in the pressure induced SC mentioned above. [18] On the other hand, Kitou et al [14] and Maschek et al [19] stressed the dissimilarity between the doping induced and pressure induced cases. For instance, Maschek et al argued that the doping induced SC can be understood by the phonon mediated pairing mechanism alone while a hybridization of phonon and exciton modes is necessary for the pressure induced SC.…”

Investigação ab initio de fases supercondutoras e topológicas em dicalcogenetos de metais de transição

“…Notavelmente, o ZrTe 2 possui uma estrutura eletrônica praticamente idêntica ao TiSe 2 (ZUNGER; FREEMAN, 1978;FANG;GROOT;HAAS, 1997;BIANCO;CALANDRA;MAURI, 2015). O TiSe 2 é composto por hole-pockets Se-4p em Γ e electron-pockets Ti-3d situados no ponto de alta simetria L. Esse composto é amplamente estudado, sendo o sistema de estado sólido protótipo para o estudo da relação entre materiais de baixa dimensionalidade, transições de fase CDW (charge density waves) e supercondutividade (MOROSAN et al, 2006;KUSMARTSEVA et al, 2009;MOROSAN et al, 2010;ROHWER et al, 2011;MÖHR-VOROBEVA et al, 2011;LI et al, 2016;KITOU et al, 2019). Em T c = 202 K é possível observar uma anomalia na resistividade elétrica do TiSe 2 , caracterizando a transição do estado fundamental do sistema para uma fase CDW (SALVO; MONCTON;WASZCZAK, 1976).…”