S.Yu. Shabanov

Nonlinear conductivity of orthorhombic TaS3 under uniaxial strain: Kink versus charge-density-wave transport

Abstract The nonlinear conductivity in a Pierls state of orthorhombic (o-)TaS 3 is found to be depressed by small uniaxial strain and almost disappears at some temperature dependent value of a strain while being restored at higher deformations. The phenomena observed are hard to interpret in terms of sliding charge-density-wave (CDW) transport. A model attributing the nonlinear conductivity at high fields to the kink movement, accompanied by free carrier drag, is discussed. The model does account for strain dependence of the linear and nonlinear parts of conductivity, and some other properties of unstrained o-TaS 3 as well.

Thermal conductivity of high-T c crystals: Dependence on magnitude and orientation of magnetic field, and effect of Zn doping

We present precise measurements of in-plane thermal conductivity for superconducting single crystals of YBa2Cu3O7−x (YBCO) withTc=92 and 60 K, Bi2Sr2CaCu2O8 (BSCCO), and of Zn-doped YBCO. Magnetization and thermal conductivity data obtained with the same 90-K YBCO crystal demonstrate a close relationship between the magnetic thermal resistivity and the internal magnetic fieldB in a superconductor in the mixed state. For all superconductors studied here, the magnetic thermal resistivity is a sublinear function of magnetic field. The origins of the nonlinearity are discussed.Angular dependences of the magnetic thermal resistivity have been shown to depart from the anisotropic 3D superconductor model and are in quantitative agreement with a quasi-2D model. Implications for spatial modulation of the order parameter are made.

Time domain study of charge density wave motion in Q1D conductors

We report the experimental results on temporal variation of CDW velocity in mesoscopic samples of ortho-TaS 3 . In this study we use specific property inherent to wavelet transform to localise the phase slip in time-varying signal produced by moving CDW and the well proved relation between CDW velocity and the frequency of narrow-band signal.

The fine structure of a signal produced by moving charge density wave

Abstract The signal produced by moving charge density wave (CDW) in mesoscopic (cross-dimensions less, then CDW coherence length) o-TaS 3 samples —including so called narrow-band “noise” and broadband noise —has been captured in real time domain. It is analyzed by means of wavelet transforms and several related methods. The following types of disorder have been found to be characteristic for moving CDW in mesoscopic samples: i) the intermittent-like behavior of narrow-band signal; ii) the splitting of narrow-band line into several components with strongly fluctuating amplitude; iii) the poor correlation between “harmonics” of this signal.

Dynamics of charge density waves in Q1D conductors : an approach based on broad-band noise analysis

The most direct manifestation of complexity of CDW dynamics and a possible clue to its understanding is a signal produced when CDW is brought to motion in electric fields. The signal produced by moving CDW in mesoscopic samples of ortho-TaS 3 studied by means of wavelet based methods with a special emphasis to a broad-band noise and its relation to narrow-band signal. It has been shown that the singularity points in the noise signal can be identified using wavelet transform modulus maxima method. These points are believed to correspond to moments when a strong scattering of CDW occurs.

Non-linear dynamics of nearly-commensurate CDW in uniaxially stretched mesoscopic o-TaS3 samples

Abstract The dynamics of CDW has been studied in mesoscopically thin (transversal dimensions of order of Lee-Rice coherence length) samples of orthorhombic TaS 3 under uniaxial stretch deformation. The current-voltage (I-V) characteristics and CDW related noise have been measured in a vicinity of a region (T=77 - 100 K, S>S c , S c - critical value of uniaxial strain deformation), where a negative differential resistance has been recently observed. The phenomena revealed are discussed within a model providing a transfer to a normal carrier screening current dissipative mechanism for CDW motion at temperatures less than 80–95 K and CDW velocity less than the critical one.

Transition to a highly conducting state in mesoscopic o-TaS3 samples

The non-linear dynamics of charge density waves (CDW) has been studied in mesoscopically thin (transversal dimensions of order of the Lee-Rice coherence length) samples of o-TaS3 under uniaxial stretch deformation. Electric field induced transition to a highly conducting state has been observed in the fields exceeding several times the threshold one for CDW motion. In a broad transitional region a negative differential resistance is observed in a current driven mode, while a bistability of I-V characteristics is found for voltage driven conditions. The transition is seen at T < 95 K under a reversible uniaxial stretch deformation in excess of some critical value ∼ 0.6%. The phenomena revealed are discussed within a model providing a transfer from a normal carrier screening current dissipative mechanism for CDW motion, dominating at low temperatures and low CDW velocities, to the phonon/phason one at high temperatures and high velocities. The most general result of this study is the recognition of the essential role of mesoscopic level processes for description of the dynamical behavior of CDW.

Phase Relationships in Synchronous Magnetic and Electric Alpha Activity in the Parieto-Occipital Area of the Human Brain

Previous combined EEG-MEG measurements of the human alpha-rhythm showed clear difference between magnetic and electric signals, so large, that their Fourier-spectra were different (Hughes, 1976, Carelli, 1983). This can be due in part to positioning of electrodes and coil, so that MEG and EEG were sensitive to different sources. In order to establish relationships between magnetic and electric fields generated by presumably the same process we tried to find signals least different in EEG and MEG. We chose experimental setup which was expected to suit best our aim.

A portable biomagnetic measuring system

SummaryThe instrumentation for biomagnetic applications is described. The small size and weight of the SQUID gradiometer permit easy transport and positioning of the instrument, thus making biomagnetic measurements possible in any noise-free location. The equipment is provided with a battery power supply, and, when filled with 60 cm3 of liquid helium, will operate continuously for 6 h. The device is suitable for magnetocardiography.RiassuntoSi descrive la strumentazione per le applicazioni biomagnetiche. Le piccole dimensioni e la leggerezza del gradiometro SQUID permettono il facile trasporto e posizionamento dello strumento, rendendo cosí possibile le misurazioni biomagnetiche in una qualsiasi locazione in cui non ci siano rumori. L’apparecchio è provvisto di un alimentatore a batteria e, se riempito con 60 cm3 di elio liquido, funzionerà initerrottamente per 6 ore. L’apparecchio è adatto per la magnetocardiografia.