Vibrational Properties of Monophosphate Tungsten Bronzes (PO₂)₄(WO₃)_2m (m = 4, 6)

Abstract

We report the vibrational properties of quasi-two-dimensional charge density wave (CDW) materials (PO2)4(WO3)2 m (m = 4, 6). The CDW transitions occur at 80 and 54 K for m = 4 and 120 and 60 K for m = 6. The polarized middle infrared response clearly demonstrates the two-dimensional character of both compounds, with strong vibrational modes and low reflectance along the interlayer direction (c) but screened vibrational modes and high reflectance in the conducting (ab) plane. The temperature dependence of the vibrational modes is weak for the m = 4 compound, whereas the spectrum of the m = 6 material displays mode splitting and additional fine structure in the CDW states. The results are discussed in terms of the CDW formation and structural changes at low temperature.

Dimensionality Effects on the Optical Properties of (PO2)4(WO3)2m (m = 2, 4, 6, 7)

“…Note that the m ) 7 sample has a superconducting transition at ∼0.3 K after two successive CDW transitions 34 and is thus a member of the interesting class of "density wave superconductors". 2,3 The temperature dependence of the vibrational properties of the m ) 6 compound suggests that the CDW transition below 120 K is associated with the central portion of the WO 6 layer, 32 in agreement with structural results on the m ) 4 compound. 31 For the m ) 2 sample, the temperature dependence of the resistivity along the highest-conducting c direction shows semiconducting behavior down to ∼50 K. 24 The m ) 2 compound also displays antiferromagnetic ordering at low temperature.…”

“…A large number of infrared-active phonon modes are expected in MPTBs because of their complicated unit cells and low site symmetry, according to a group theory analysis. 32 For instance, 134 normal modes are anticipated for the m ) 4 material. As shown in Figure 3, the vibrational features are strong along the c axis, whereas the phonons are heavily screened (but not completely absent) in the conducting ab plane, confirming the Q2D character of the MPTBs.…”

“…The Q2D physical properties and low-temperature CDW instabilities in the m ) 4, 6, and 7 tungsten bronzes have been characterized by transport, X-ray diffraction, thermopower, and infrared spectroscopy experiments. [23][24][25][26][27][28][29][30][31][32][33] In all of the MPTBs, the ab-plane conductivity is at least 1 order of magnitude larger than the conductivity along the interlayer direction. Note that the m ) 7 sample has a superconducting transition at ∼0.3 K after two successive CDW transitions 34 and is thus a member of the interesting class of "density wave superconductors".…”

“…The Q2D physical properties and low-temperature CDW instabilities in the m = 4, 6, and 7 tungsten bronzes have been characterized by transport, X-ray diffraction, thermopower, and infrared spectroscopy experiments. − In all of the MPTBs, the ab -plane conductivity is at least 1 order of magnitude larger than the conductivity along the interlayer direction. Note that the m = 7 sample has a superconducting transition at ∼0.3 K after two successive CDW transitions and is thus a member of the interesting class of “density wave superconductors”. , The temperature dependence of the vibrational properties of the m = 6 compound suggests that the CDW transition below 120 K is associated with the central portion of the WO 6 layer, in agreement with structural results on the m = 4 compound . For the m = 2 sample, the temperature dependence of the resistivity along the highest-conducting c direction shows semiconducting behavior down to ∼50 K .…”

Raman spectroscopy of optical phonon and charge density wave modes in 1T-TiSe2 exfoliated flakes

“…However, many experiments show that  has different values for different materials. 28,29 Here we choose 0.175 for  to match the T ICDW =122 K of the bulk sample reported in the literature (details in the SI). 3 Extracted T ICDW values are plotted as a function of sample thickness in Fig.…”