Charge-density waves in rubidium blue bronze ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$ observed by scanning tunneling microscopy

Using low-temperature scanning tunneling microscopy under ultrahigh vacuum, high-resolution topographical images were obtained on a cleaved $\left(\overline{2}01\right)$ surface of a rubidium blue bronze $\left({\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3\right)$ single crystal. Only molecular lattice was observed at room temperature. At temperatures of 63 and $78\mathrm{K}$, well below the charge density wave (CDW) transition temperature in ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$, underlying molecular lattice and CDW superlattice were observed simultaneously in topographical constant current images. The amplitude of the CDW modulation is about $0.1\mathrm{\AA }$ along $\mathbf{b}$. On the Fourier transform of these images, both main Bragg spots (molecular lattice related) and satellite spots (CDW superlattice related) coexist unambiguously. The average projection of the CDW wave vector on the $\left(\overline{2}01\right)$ surface deduced by Fourier transform is consistent with the bulk value obtained previously by other techniques. Our results show clearly that the Peierls phase of the quasi-one-dimensional blue bronze can be studied by STM.

Figure 1

(Color online) (a) Constant current mode topographical image of $6.2\times 7.0{\mathrm{nm}}^2$ on a $\left(\overline{2}01\right)$ plane of ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$ at $63\mathrm{K}$ (raw data image). The applied bias voltage is $420\mathrm{mV}$ and the setup tunneling current is $110\mathrm{pA}$. Surface lattice vectors $b$ and $a+2c$ are indicated by two arrows and the corresponding centered rectangular unit cell is represented by a rectangle. Three pairs of arrows indicate respectively the observed type I and II $\mathrm{Mo}{\mathrm{O}}_6$ octahedra and the expected position of type III $\mathrm{Mo}{\mathrm{O}}_6$ octahedra. (b) Idealized surface structure of blue bronze in $\left(\overline{2}01\right)$ cleavage plane. The filled and dashed square units are $\mathrm{Mo}{\mathrm{O}}_6$ octahedra in the first molecular layer. The centers of the filled octahedra (left arrow indicates that they correspond to type-I octahedra) are $0.6\mathrm{\AA }$ higher than the centers of the dark dashed ones (corresponding, respectively, to type-II ones) while the centers of the light dashed ones (corresponding to type-III ones) are $1.1\mathrm{\AA }$ lower than those of the type-II ones [after Walter et al. (Ref. 19)]. (c) Profile along the type-I $\mathrm{Mo}{\mathrm{O}}_6$ octahedra indicated by left arrows in (a). Different height of the peak represents different local integrated electronic density of states. The periodic variation of height is due to the formation of CDW in blue bronze at low temperature. The amplitude of the modulation indicated by the double arrow is about $0.1\mathrm{\AA }$.

Figure 2

(Color online) (a) Constant current mode topographical STM image on the $\left(\overline{2}01\right)$ plane of ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$ at $63\mathrm{K}$ (raw data image). The image area is $28.2\times 25.9{\mathrm{nm}}^2$. Molecular lattice and CDW superlattice coexist in the image. The applied bias voltage is $420\mathrm{mV}$ and setup tunneling current is $110\mathrm{pA}$. (b) 2D Fourier transform of (a). The brightest Bragg spots correspond to the centered rectangular surface unit cell on the $\left(\overline{2}01\right)$ surface of ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$. A reciprocal unit cell in Fourier space consists of five Bragg spots with unit vectors $2{\mathit{b}}^{\star }$ and $2{(\mathit{a}+2\mathit{c})}^{\star }$, as indicated by two arrows at the right of the image. Each Bragg spot is surrounded by four satellite spots. This demonstrates the existence of a CDW superlattice with observed projected wave vectors ${\mathit{q}}_{\mathrm{CDW}}=\pm 0.25{\mathit{b}}^{\star }+{(\mathit{a}+2\mathit{c})}^{\star }$, giving experimental evidence of CDW condensation on a cleaved surface of ${\mathrm{Rb}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$. (c) Diagram showing the respective positions of the atomic lattice Bragg spots (closed circles) and of the CDW superlattice spots (open circles) in the $\left(\overline{2}01\right)$ plane, as expected from x-ray and neutron diffraction results at $T<100\mathrm{K}$.

Figure 3

(Color online) Idealized side view of a single unit cell of blue bronze projected onto the plane perpendicular to the $\mathit{b}$ axis. In the STM experiment, the sample was cleaved in situ along $\mathit{a}+2\mathit{c}$ to obtain a $\left(\overline{2}01\right)$ surface. Closed circles are the Rb atoms in the uppermost positions. Open circles are the Rb atoms $1.2\mathrm{\AA }$ below the “surface.” Each square represents a $\mathrm{Mo}{\mathrm{O}}_6$ octahedron with Mo atom located at the center of oxygen octahedron. The three highest octahedra with respect to the “surface” are the dashed squares indicated by the arrows as type I, II, and III ones. Their centers lie at levels 1.8, 2.4, and $3.5\mathrm{\AA }$ below the “surface.” Type-I $\mathrm{Mo}{\mathrm{O}}_6$ octahedra are the uppermost octahedra.