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...
Pressure-dependent, low-temperature inelastic light (Raman) scattering measurements of KCuF(3) show that applied pressure above P* ~ 7 kbar suppresses a previously observed structural phase transition temperature to zero temperature in KCuF(3), resulting in the development of a fluctuational (quasielastic) response near T ~ 0 K. This pressure-induced fluctuational response--which we associate with slow fluctuations of the CuF(6) octahedral orientation--is temperature independent and exhibits a characteristic fluctuation rate that is much larger than the temperature, consistent with quantum fluctuations of the CuF(6) octahedra. A model of pseudospin-phonon coupling provides a qualitative description of both the temperature- and pressure-dependent evolution of the Raman spectra of KCuF(3).
The hybrid perovskite copper(ii) guanidinium formate, C(NH2)3[Cu(HCO2)3], is a close analog of the inorganic material KCuF3, a well-studied exemplar of the Heisenberg spin-12 antiferromagnetic chain. However, the polymorphism known in KCuF3, based on different orbital ordering of the d9 Cu2+ ions, has never been observed in C(NH2)3[Cu(HCO2)3]. We use density-functional theory calculation to construct a hypothetical polymorph of the hybrid perovskite analogous to the known structure of KCuF3. This polymorph is very similar in geometric and magnetic structures to the known phase of C(NH2)3[Cu(HCO2)3], but it is no longer polar. It has only a marginally higher internal energy than the known phase but also a lower vibrational entropy and, hence, higher free energy. Furthermore, stacking faults are far more energetically costly in C(NH2)3[Cu(HCO2)3] than in KCuF3. These results together help to explain why KCuF3 is experimentally polymorphic but C(NH2)3[Cu(HCO2)3] is not. The detailed comparison between analogous inorganic and hybrid materials illustrates the subtle differences that can arise from replacing atomic ions with molecular ions and that may be exploited in crystal engineering of these materials.
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