Narrow-band Noise Frequency Research Articles

Mode locking phenomena of the current-induced skyrmion-lattice motion in microfabricated MnSi

Using voltage fluctuation spectroscopy, we observe the mode locking phenomena of current-induced skyrmion-lattice motion in microfabricated MnSi. When only a dc electric current is applied, the frequency of the emergent narrow-band noise (NBN) monotonously increases with increasing dc current. By contrast, if an ac electric current is further added, the NBN frequency becomes much less dependent on the dc current when it satisfies a simple integer ratio to the ac current frequency, a hallmark of mode locking. In the mode locked state, the linewidth of the NBN is sharpened, indicating the enhanced coherence of the skyrmion-lattice motion. The overall features of the mode locking phenomena are qualitatively reproduced within the framework of the classical rigid-sphere model.

Slow steady flow of a skyrmion lattice in a confined geometry probed by narrow-band resistance noise

Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $\mu$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermally activated processes, which are most likely due to skyrmion creation and annihilation at the sample edges. This scenario is qualitatively supported by our numerical simulations considering boundary effects, which reveals that the edges limit the steady flow of skyrmions, especially at low temperatures. We discuss a mechanism that dramatically slows the skyrmion steady flow in a microfabricated specimen.

Topological defects in spin density waves

The rich order parameter of Spin Density Waves allows for unusual object of a complex topological nature: a half-integer dislocation combined with a semi-vortex of a staggered magnetization. It becomes energetically preferable to ordinary dislocation due to enhanced Coulomb interactions in the semiconducting regime. Generation of these objects changes e.g. the narrow band noise frequency.

Topological coupling of dislocations and magnetization vorticity in spin density waves

The rich order parameter of Spin Density Waves allows for an unusual object of complex topological nature: a half-integer dislocation combined with a semi-vortex of the staggered magnetization. It becomes energetically preferable to ordinary dislocation due to enhanced Coulomb interactions in the semiconducting regime. Generation of these objects changes the narrow band noise frequency.

Effect of an applied magnetic field on the charge-density-wave carrier concentration in NbSe3.

We report measurements of the resistance and narrow-band noise (NBN) in ${\mathrm{NbSe}}_{3}$ for magnetic fields from 0 to 9.6 T and temperatures from 24 to 55 K. We have observed that the magnetic field has a very small effect on the charge-density-wave (CDW) carrier concentration in the lower temperature range 24T38 K. Above T\ensuremath{\approxeq}40 K, the application of a magnetic field has essentially no discernible effect on ${\mathit{n}}_{\mathrm{CDW}}$. We have analyzed the data in such a way as to investigate any influence Joule heating might have on the data, especially in the lower-temperature range. We find that for T38 K an applied magnetic field of 9.6 T changes the CDW carrier concentration obtained from the slope of the CDW current versus NBN frequency curves by approximately \ensuremath{\le}5--6 %.

Charge-density-wave narrow-band noise in NbSe3 at T

We report observations and the first study of the properties of charge-density-wave (CDW) narrow-band noise, i.e., current oscillations, at T4 K in the CDW material ${\mathrm{NbSe}}_{3}$. The CDW narrow-band noise (NBN) appears to exist only in certain ranges of magnetic-field strength and the amplitude tends to die out at high electric fields. The observation of NBN, which is unique to CDW systems, confirms that the non-Ohmic resistance in a magnetic field that is found in ${\mathrm{NbSe}}_{3}$ at these temperatures, T4 K, is correlated with motion of a CDW. We also find that the CDW current versus NBN frequency relationship is nonlinear at T4 K, unlike that found at higher temperatures, T>25 K and B\ensuremath{\le}9.6 T. The nonlinear current at a given frequency is also very large compared to that at higher temperatures.

Scanning tunneling microscopy observation of sliding charge-density wave in blue bronze

We investigated the sliding motion of charge-density wave (CDW) in K0.3 MoO3 with scanning tunneling microscopy (STM). With fixed position of the tunneling tip, a sharp peak was found in the spectra of tunneling current for sample bias current exceeding the threshold for the CDW depinning. The peak frequency increased with the extra current carried by the CDW condensate. This peak is understood to be generated by the modulation of gap distance between the tunneling tip and sample atom due to the sliding motion of CDW. The velocity of CDW at the surface was obtained directly: vs =5.7×10−3 cm/s just above the threshold. The corresponding sliding velocity in the interior of the same sample was obtained as vb =6.3×10−4 cm/s from the narrow band noise frequency. It was confirmed that the CDW is easily depinned at the surface and begins to slide with much higher velocity than the interior of sample. Tunneling spectroscopy was also measured and the CDW gap structure was obtained as a preliminary result.

Noise and ac-dc interference phenomena in the charge-density-wave conductor K0.3MoO

We demonstrate that extremely thin (l1 \ensuremath{\mu}m thick) samples of the charge-density-wave (CDW) conductor ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ display high-quality narrow-band noise spectra when driven by a dc electric field. In the presence of combined dc and large-amplitude ac fields, the CDW becomes mode locked to the external signal, giving rise to Shapiro steps in the dc I-V characteristics. In the presence of combined dc and small-amplitude ac fields, distinct resonance features in the complex ac conductivity \ensuremath{\sigma}(\ensuremath{\omega}) are observed whenever the internal and external frequencies coincide. Analysis of the Shapiro-step interference suggests that the ac-dc interference arises from a direct coupling between the driving fields and the low-frequency dielectric relaxation mode in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$. Resonances in the ac conductivity are shown to be analogous to those present in a phase-shifted resonant harmonic oscillator. Both noise and interference experiments yield a ratio between the narrow-band noise frequency and the CDW current-density of ${f}_{\mathrm{NBN}/{J}_{\mathrm{CDW}}}$=12\ifmmode\pm\else\textpm\fi{}3 kHz ${\mathrm{cm}}^{2}$/A, suggesting that the intrinsic pinning potential in ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ has a periodicity equal to the CDW wavelength.

Linear ac response of a depinned charge-density wave

We calculate the linear ac response of a depinned charge-density wave (CDW) in samples with size on the order of a Fukuyama-Lee-Rice coherence length. At dc fields near threshold the ac conductivity displays strong resonant structure at the narrow-band noise frequency ${\ensuremath{\omega}}_{0}$ and its harmonics. At larger dc fields we also find a broad inductive dip in the real part of the ac conductivity, \ensuremath{\sigma}(\ensuremath{\omega}), near ${\ensuremath{\omega}}_{0}$. All these features in \ensuremath{\sigma}(\ensuremath{\omega}) vary substantially in width and shape as the dc bias is increased. The sharp resonances near threshold reflect the periodic potential in which the CDW moves, while the inductive dip is a result of frequency-dependent damping. Implications for experiments on larger samples are discussed.

Linear and non-linear ac response of the stochastic classical model for sliding charge density waves

A comprehensive study of the ac response properties of the classical stochastic model for sliding charge density waves (CDW) in quasi one-dimensional metals is made by numerically solving the associated Fokker-Planck equation. Above the conductivity threshold a noise mechanism is indispensable to give finite line widths to the resonances of the applied ac signal of frequency ω with the narrow band noise frequency ωOSC inherent in the model. In the present investigation a current noise of strengthTNproportional to the CDW current is used in the Fokker-Planck equation in order to model the broad band current noise frequently observed above threshold. The present model thus incorporates three characteristic frequency scales: ωOSC,TN,and a “crossover” frequency ωOSC. Results are evaluated for the ac conductivity σ(ω;E0,Eω) as function of frequency ω, dc bias electric fieldE0 and ac signal field strengthEω. ForEω→0 the linear ac response is obtained by a separate treatment of the Fokker-Planck equation. The resonances near ω=ωOSC are studied in detail. Strong ac signals reduce the response at the fundamental resonance and lead to a harmonic interference structure nearnω=ωOSC. The overall agreement of the present results with recent measurements of the linear ac response is not good. In reality our results seem to be superimposed on a background not reproduced by the classical model with one “cross over” frequency. However, the peak in Im σ(ω;E0,Eω=0) vs.E0, when the narrow band noise frequency is near ω, is well reproduced. The spectral width of this peak which corresponds to the inductive dip in the susceptibility is studied as function of current noise strengthTN.The results stress the need for a complete Fokker-Planck treatment sinceTNis not simply related to the line width.

Dielectric response enhancement in the current carrying CDW state

Abstract The dc electric field dependence of the low frequency dielectric constant was investigated in the linear chain compound orthorhombic TaS3. While the dielectric constant is barely effected by an applied dc field below the threshold field of the nonlinear conduction, in the current carrying state a large enhancement is observed. Inductive response at the narrow band noise frequency was also detected.