Emergence of CDW domain wall discommensurations above the
superconducting dome in TiSe2
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. In this
talk I will describe our use of high-pressure x-ray scattering to
directly study the CDW order in the layered dichalcogenide 1T-TiSe
2,
which was previously shown to exhibit SC when the CDW is suppressed
by pressure or intercalation of Cu atoms. 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
dome. 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 amplitude QCP itself, but to the formation of CDW domain
walls.