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Register a new userNon-contact dissipation on the surface of charge density wave materials
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Bulk electrical dissipation caused by charge-density-wave (CDW) depinning and sliding is a classic subject. We present a novel local, nanoscale mechanism describing the occurrence of mechanical dissipation peaks in the dynamics of an atomic force microscope tip oscillating above the surface of a CDW material. Local surface 2$pi$ slips of the CDW phase are predicted to take place giving rise to mechanical hysteresis and large dissipation at discrete tip surface distances. The results of our static and dynamic numerical simulations are believed to be relevant to recent experiments on NbSe$_2$; other candidate systems in which similar effects should be observable are also discussed.
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Bulk electrical dissipation caused by charge-density-wave (CDW) depinning and sliding is a classic subject. We present a novel local, nanoscale mechanism describing the occurrence of mechanical dissipation peaks in the dynamics of an atomic force microscope tip oscillating above the surface of a CDW material. Local surface 2$pi$ slips of the CDW phase are predicted to take place giving rise to mechanical hysteresis and large dissipation at discrete tip surface distances. The results of our static and dynamic numerical simulations are believed to be relevant to recent experiments on NbSe$_2$; other candidate systems in which similar effects should be observable are also discussed.
A mechanism is proposed to describe the occurrence of distance-dependent dissipation peaks in the dynamics of an atomic force microscope tip oscillating over a surface characterized by a charge density wave state. The dissipation has its origin in the hysteretic behavior of the tip oscillations occurring at positions compatible with a localized phase slip of the charge density wave. This model is supported through static and dynamic numerical simulations of the tip surface interaction and is in good qualitative agreement with recently performed experiments on a NbSe$_2$ sample.
The two charge-density wave (CDW) transitions in NbSe$_3$ %at wave numbers at $bm{q_1}$ and $bm{q_2}$, occurring at the surface were investigated by scanning tunneling microscopy (STM) on emph{in situ} cleaved $(bm{b},bm{c})$ plane. The temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, was measured between 5-140 K to extract the surface critical temperatures (T$_s$). The low T CDW transition occurs at T$_{2s}$=70-75 K, more than 15 K above the bulk T$_{2b}=59$K while at exactly the same wave number. %determined by x-ray diffraction experiments. Plausible mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation.% The large interval of the 2D regime allows to speculate on % %the special Berezinskii-Kosterlitz-Thouless type of the surface transition expected for this incommensurate CDW. This scenario is checked by extracting the temperature dependence of the order % %parameter correlation functions. The regime of 2D fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of surface transition, expected for this incommensurate 2D CDW, by extracting the temperature dependence of the order parameter correlation functions.
We investigate the charge density wave transport in a quasi-one-dimensional conductor, orthorhombic tantalum trisulfide ($o$-TaS$_3$), by applying a radio-frequency ac voltage. We find a new ac-dc interference spectrum in the differential conductance, which appear on both sides of the zero-bias peak. The frequency and amplitude dependences of the new spectrum do not correspond to those of any usual ac-dc interference spectrum (Shapiro steps). The results suggest that CDW phase dynamics has a hidden degree of freedom. We propose a model in which $2pi$ phase solitons behave as liquid. The origin of the new spectrum is that the solitons are depinned from impurity potentials assisted by an ac field when small dc field is applied. Our results provide a new insight as regards our understanding of an elementary process in CDW dynamics.
Via spin-polarized scanning tunneling microscopy, we revealed a long-range ordered spin density wave (SDW) for the first time on a Cr (001) surface, corresponding to the well-known incommensurate SDW of bulk Cr. It displays a (~ 6.0 nm) long-period spin modulation in each (001) plane and an anti-phase behavior between adjacent planes, which are confirmed by changing the magnetization of the tip. Meanwhile, we simultaneously observed the coexisting charge density wave (CDW) with half the period of the SDW. Taking advantage of real-space measurement, we found the charge and spin modulations are in-phase, and their domain structures are highly correlated. Surprisingly, the phase of CDW in dI/dV map displays a {pi} shift around a density-of-states dip at about -22 meV, indicating an anomalous CDW gap opened below EF. These observations support that the CDW is a secondary order driven by SDW. Therefore, our work is not only the first real space characterization of incommensurate SDW, but also provide new insights on how SDW and CDW coexist.
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