We report a Raman scattering study of low-temperature, pressure-induced melting of the CDW phase of 1T-TiSe2. Our Raman scattering measurements reveal that the collapse of the CDW state occurs in three stages: (i) For P<5 kbar, the pressure dependence of the CDW amplitude mode energies and intensities are indicative of a "crystalline" CDW regime; (ii) for 5 < P < 25 kbar, there is a decrease in the CDW amplitude mode energies and intensities with increasing pressure that suggests a regime in which the CDW softens, and may decouple from the lattice; and (iii) for P>25 kbar, the absence of amplitude modes reveals a melted CDW regime. PACS numbers: 71.30.+h; 71.45.Lr; There has been a great deal of interest in the relationship between various diverse and exotic low temperature phases of strongly correlated systems, including the antiferromagnetic insulating and unconventional superconducting phases of the high T c cuprates,[1] the chargeordered insulating and ferromagnetic metal phases of the manganese perovskites,[2] the orbital-ordered and ferromagnetic metal phases of the ruthenates, [3,4,5] and the charge-density-wave (CDW) and superconducting phases of layered dichalcogenides such as 2H-NbSe 2 .[6] Of particular interest is the exotic phase behavior that is expected between fully-ordered (crystalline) and disordered (isotropic) phases as one tunes the interactions in these systems using some control parameter other than temperature. These include, for example, electronically phase-separated regimes, [2] and "quantum liquid crystal" smectic and nematic phases, which are expected to be observed between charge-ordered insulating and 'disordered' metallic or superconducting phases as one increases the interactions between the charge stripes. [7] Clearly, therefore, it is of great interest to carefully explore the manner in which quantum ordered phases collapse, or 'melt', into quantum disordered phases as a function of some control parameter that tunes the competing interactions in the material at low temperatures.In this paper, we report a pressure-dependent low temperature Raman scattering study of the CDW system 1T-TiSe 2 , in which we are able to explore the manner in which the CDW state 'melts' with increasing pressure near T∼0 K. Because of its layered structure and simple commensurate CDW phase, 1T-TiSe 2 is an ideal system for such an investigation. 1T-TiSe 2 is also of interest because the CDW transition is not driven by conventional Fermi surface nesting, but rather by an unconventional mechanism involving electron-hole coupling and an "indirect" Jahn-Teller effect.[8] Our low-temperature, pressure-dependent light scattering approach allows us to explore unique details associated with quantum mechanical melting of the CDW in 1T-TiSe 2 . In particular, this study reveals that the CDW state evolves with increasing pressure in a manner reminiscent of classical 2D melting, with 'crystalline' and 'disordered' regimes, as well as an intermediate 'soft' CDW regime in which the CDW may be incommensurate with the...

Temperature-and x-dependent Raman scattering studies of the charge density wave (CDW) amplitude modes in CuxTiSe2 show that the amplitude mode frequency ωo exhibits identical powerlaw scaling with the reduced temperature, T/TCDW, and the reduced Cu content, x/xc, i.e., ωo ∼ (1 -p) 0.15 for p = T/TCDW or x/xc, suggesting that mode softening is independent of the control parameter used to approach the CDW transition. We provide evidence that x-dependent mode softening in CuxTiSe2 is caused by the reduction of the electron-phonon coupling constant λ due to expansion of the lattice, and that x-dependent 'quantum' (T ∼ 0) mode softening reveals a quantum critical point within the superconductor phase of CuxTiSe2. 1T -TiSe 2 is a semimetal or small-gap semiconductor in the normal state, [6,7,8,9] which develops a commensurate CDW with a 2a o ×2a o ×2c o superlattice structure at temperatures below a second-order phase transition at T CDW ∼ 200 K. [6,10] Increasing Cu intercalation in TiSe 2 (increasing x in Cu x TiSe 2 ) results in (i) an expansion of the a-and c-axis lattice parameters, [5] (ii) increased electronic density of states near the L point, [7,8] (iii) a suppression of the CDW transition temperature, [5] and (iv) the emergence near x = 0.04 of a SC phase having a maximum T c of 4.15 K at x = 0.08. [5] The Cu x TiSe 2 system provides an ideal opportunity to investigate the microscopic details of quantum (T ∼ 0) phase transitions between CDW order and SC. It is of particular interest to clarify the nature of the "soft mode" in CDW/SC transitions: the behavior of the soft mode -i.e., the phonon mode whose eigenvector mimics the CDW lattice distortion, and hence whose frequency tends towards zero at the second-order phase transition -is one of the most fundamental and well-studied phenomena associated with classical (thermally driven) displacive phase transitions;[11] on the other hand, soft mode behavior associated with quantum phase transitions is not well understood. In this investigation, we use Raman scattering to study the temperature-and dopingdependent evolution of the CDW 'amplitude' modes in Cu x TiSe 2 . The CDW amplitude mode [12] -which is associated with collective transverse fluctuations of the CDW order parameter -offers detailed information regarding the evolution and stability of the CDW state and the CDW soft mode. In this study, we show that the amplitude mode frequency in Cu x TiSe 2 exhibits identical power-law scaling with the reduced temperature, T/T CDW , and the reduced Cu content, x/x c , indicating that mode softening in Cu x TiSe 2 is independent of the control parameter used to approach the CDW transition. Further, we show that 'quantum' (T ∼ 0) softening of the CDW amplitude mode is consistent with a quantum critical point hidden in the superconductor phase of Cu x TiSe 2 , suggesting a possible connection between quantum criticality and superconductivity.Raman scattering measurements were performed on high quality single-crystal and pressed-pellet samples of Cu x TiSe 2 for x = ...

Magnetic-field- and temperature-dependent Raman scattering studies of Ca3Ru2O7 reveal dramatic field-dependent properties arising from transitions between various complex orbital and magnetic phases, including a field-induced orbital-ordered to orbital-disordered transition (H(o) // hard axis), and a reentrant orbital-ordered to orbital-disordered to orbital-ordered transition (H(o) // easy axis). We find that the dramatic magnetic-field properties are most prevalent in a "mixed"-magnetic and -orbital phase regime, providing evidence for a strong connection between orbital phase inhomogeneity and "colossal" field effects in the ruthenates.

We present a magnetic-field-and pressure-dependent Raman scattering study of the complex orbital, magnetic, and conducting phases of Ca 3 Ru 2 O 7 , which result from a rich interplay between the orbital, spin, and electronic degrees of freedom. The Raman-active phonon and magnon excitations in Ca 3 Ru 2 O 7 convey sufficient information to map out the orbital, magnetic, and conducting ͑H , T͒ and ͑P , T͒ phase diagrams of this material. We find that quasihydrostatic pressure causes a linear suppression of the orbital-ordering temperature ͑T OO =48 K at P =0͒, up to a T = 0 critical point near P * ϳ 55 kbar, above which the material is in a metallic, orbital-degenerate phase. We associate this pressure-induced collapse of the antiferromagnetic orbital-ordered phase with a suppression of the RuO 6 octahedral distortions that are responsible for orbital-ordering. We also find that an applied magnetic field at low temperatures induces a change from an orbital-ordered to orbitaldegenerate phase for fields aligned along the in-plane b-axis ͑H ʈ hard axis͒, but induces a reentrant orbitalordered to orbital-disordered to orbital-ordered phase change for fields aligned along the in-plane a-axis ͑H ʈ easy axis͒. This complex magnetic field dependence betrays the importance of spin-orbit coupling in this system, which makes the field-induced phase behavior highly sensitive to both the applied magnetic-field magnitude and direction. It is further shown that rapid field-induced changes in the structure and orbital populations are responsible for the highly field-tunable conducting properties of Ca 3 Ru 2 O 7 , and that the most dramatic magnetoconductivities are associated with an "orbital disordered" phase regime in which there is a random mixture of a-and b-axis oriented Ru moments and d-orbital populations on the Ru ions.

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