Emergence of charge density wave domain walls above the superconducting dome in 1T-TiSe2

Abstract

Superconductivity (SC) in so-called "unconventional superconductors" is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. To better understand this connection, we used high-pressure x-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe 2 , which was previously shown to exhibit SC when the CDW is suppressed by pressure [1] or intercalation of Cu atoms [2]. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at P c = 5.1 ± 0.2 GPa, which is more than 1 GPa beyond the end of the SC region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in TiSe 2 may not be connected to the QCP itself, but to the formation of CDW domain walls. *The term "unconventional superconductor" once referred to materials whose phenomenology does not conform to the Bardeen-Cooper-Schrieffer (BCS) paradigm for superconductivity. It is now evident that, by this definition, the vast majority of known superconductors are unconventional, notable examples being the copper-oxide, iron-arsenide, and iron-selenide high temperature superconductors, heavy Fermion materials such as CeIn 3 and CeCoIn 5 , ruthenium oxides, organic superconductors such as ϰ-(BEDT-TTF)2X, filled skutterudites, etc.Despite their diversity in structure and phenomenology, the phase diagrams of these materials all exhibit the common trait that superconductivity (SC) resides near the boundary of an ordered phase with broken translational or spin rotation symmetry. For example, SC resides near antiferromagnetism in CeIn 3 [3], near a spin density wave in iron arsenides [4], orbital order in ruthenates [5], and stripe and nematic order in the copper-oxides [6]. The pervasiveness of this "universal phase diagram" suggests that there exists a unifying framework -more general than BCS -in which superconductivity can be understood as coexisting with some ordered phase, and potentially emerging from its fluctuations.A classic example is the transition metal dichalcogenide family, MX 2 , where M=Nb, Ti, Ta, and X=Se, S, which exhibit a rich competition between superconductivity and Peierls-like charge density wave (CDW) order [7]. A recent, prominent case is 1T-TiSe 2 , which under ambient pressure exhibits CDW order below a transition temperature T CDW = 202 K [8]. This CDW phase can be suppressed either with intercalation of Cu atoms [2,9], or through the application of hydrostatic pressure [1,10], causing SC to emerge. These studies suggest that the emergence of SC coincides with a quantum critical point (QCP) at which T CDW goes to zero, suggesting that TiSe 2 exemplifies the universal phenomenon of superconductivity em...

Superconductivity and hybrid soft modes inTiSe2

“…Going beyond the reported critical pressure of 5.1 GPa (Ref. [5]), we find a still slightly enhanced 0.3 at 6.3 GPa, which is further reduced to the ambient pressure value of around 0.1 at 9.6 GPa [ Fig. 5(e)].…”

“…X-ray diffraction [5] revealed that the superconducting phase under pressure is completely surrounded by the CDW ordered phase rather than being centered on the critical pressure [4]. In this work we use inelastic x-ray scattering to investigate the superconducting pairing mechanisms in Cu X TiSe 2 and 1T-TiSe 2 under pressure.…”

Stripe and Short Range Order in the Charge Density Wave of 1T−CuxTiSe2

“…Instead, as already proposed in high-pressure [16] and carrier injection [17] experiments, superconductivity may arise from the inhomogeneities of the CDW pattern. Moreover, spectroscopy shows Cu intercalation to shift the chemical potential to higher energies and increase the LDOS at the Fermi level providing a favorable electronic configuration for the appearance of superconductivity above x ¼ 0.04.…”

“…These domains are predominantly surrounded by Cu-rich regions, and tend to be π-phase shifted with a few atomically sharp boundaries separating adjacent domains. Each domain is locally commensurate, and the slight incommensurability reported in some bulk experiments [16,17] might result from the stacking along the c axis of such phase-shifted domains. A similar break up of the CDW into phase-shifted domains has been reported recently in Ti intercalated crystals [21] where the effect is observed already for 2% Ti content.…”

“…Transport measurements [14,15] indicate the CDW is suppressed upon increasing the Cu content which would suggest a competition with superconductivity. A more recent report of an incommensurate CDW above the superconducting dome in pristine crystals under pressure [16] suggests a more complex scenario, where CDW fluctuations promote superconductivity. Traces of incommensurate CDW patches have also been found in gated TiSe 2 thin films [17].…”