Emergence of charge density wave domain walls above the superconducting dome in TiSe2 (1309.4051v1)

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 TiSe2, 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 Pc = 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 TiSe2 may not be connected to the QCP itself, but to the formation of CDW domain walls.

• The paper demonstrates that suppressing CDW order under pressure reveals a quantum critical point at 5.1 GPa, decoupled from the superconducting dome.
• It employs a unique two-energy x-ray scattering method to precisely measure CDW commensurability and identify a reentrant, weakly first-order incommensurate phase at 3 GPa.
• The findings suggest that domain walls in the CDW order, rather than amplitude fluctuations alone, may play a critical role in facilitating superconductivity.

Emergence of Charge Density Wave Domain Walls Above the Superconducting Dome in TiSe

This paper presents an investigation into the interplay between charge density wave (CDW) order and superconductivity (SC) in the layered dichalcogenide TiSe$_2$. It provides compelling insights into the quantum critical behavior of CDW under the influence of high pressure and its potential implications on the emergence of superconductivity.

The authors systematically applied hydrostatic pressure to suppress the CDW order in TiSe$_2$ and characterize its behavior near the phase boundary. Previous studies had established that superconductivity in these systems tends to manifest near competing order phases, making the paper of quantum critical points (QCP) vital. By leveraging x-ray scattering techniques at low temperatures, they observed the complete suppression of the CDW to zero temperature, thereby identifying a QCP at $P_c = 5.1 \pm 0.2$ GPa, which notably falls outside the superconducting dome's range of $2 \text{ GPa} < P < 4 \text{ GPa}$.

A significant observation was the reentrant, weakly first-order, incommensurate phase detected at $P = 3$ GPa. This behavior suggests the presence of a Lifshitz tricritical point above the superconducting dome. The emergence of weak, residual CDW correlations even above the QCP indicates that superconductivity in TiSe$_2$ might not directly correlate with the CDW suppression but could be linked instead to the quantum dynamics of domain walls that appear as phase slips within the CDW order.

One particularly novel aspect of this investigation is the precise measurement of the CDW's commensurability utilizing a unique two-energy x-ray scattering technique. The authors detail how pressure influences CDW commensurability and discover a weak incommensurate behavior at higher pressures. This finding underscores a shift in the natural wave vector of the CDW and suggests that the incommensurate phase could play a critical role in facilitating superconductivity through the formation of domain walls.

Numerically, the pressure dependence of the CDW's T$_{CDW}$ adhered to a power-law form $T_{CDW}(P) \propto |P-P_c|^{\beta}$, with $\beta = 0.87 \pm 0.08$, aligning closely with theoretical expectations. In parallel, the coupling constant $g$ increased with pressure up to the QCP, extending crucial insights into the underlying quantum mechanics. The phase diagram synthesized in the paper highlights the intricate relationships between pressure, temperature, and electronic ordering in these systems, offering a window into the profound effects intrinsic stresses and domain wall dynamics may have on the CDW and SC states.

The implications of this paper extend to the broader understanding of unconventional superconductors. It challenges the straightforward model of superconductivity emerging at the QCP, suggesting instead that the formation of domain walls within CDW order is a vital contributing factor. For theorists exploring superconductivity in these materials, the paper provides a compelling avenue to consider domain wall dynamics beyond amplitude fluctuations in mediating superconductivity.

This work further emphasizes the importance of precise experimental methods and theoretical frameworks to unravel the complex interplay between different order parameters in low-dimensional systems. Future research may continue to explore similar phenomena across diverse materials, with particular attention to the role of domain walls and incommensurate phase transitions in governing both the emergence of superconductivity and its underlying mechanisms.