Samvel Gevorgyan

Modeling of tunnel diode oscillators and their use for some low temperature investigations

The simple method of modeling of tunnel diode (TD) oscillators’ work is described, and may be applied also to the other types of negative-differential conductance oscillators. In particular, the calculation of the harmonic oscillation amplitude is performed by the energy balance method, based on TD measured the current–voltage characteristic’ (I–V curve), using only one fitting parameter—the tunnel diode’s measured bias at which oscillations are quenched by means of which the oscillator’s effective total losses are determined also. The formula for calculation of the TD generator’s frequency is derived; it agrees with experiment and permits determination of its dependence on the radiotechnical scheme’s elements and TD I–V curve’s shape. The results of the calculation in comparison with the experiment are reported for the oscillators of various constructions operating at different conditions. For this, more attention is given to the constructive solutions applied in the low temperature experiments. Examples...

Highly sensitive open-flat coil magnetometer for the λ(H,T) measurements in plate-like high-Tc cuprates

The “LC resonator” method was greatly improved for high resolution measurements of magnetic-field penetration depth, λ, of radio frequency into thin films or plate-like high-Tc superconductors by replacing a solenoid testing coil by an open-flat coil (OFC) driven by a tunnel diode oscillator of a low power and a highly stable frequency. The assembled OFC magnetometer showed new technical advantages such as simplicity, convenience, and high resolution of Δλ∼1–3 A (Δλ/λ∼10−6), ability of reliable operation in wide ranges of temperature (4.2–300 K) and magnetic field (up to 12 T), etc. This enables us to use it for studying peculiarities of the vortex dynamics with weak signal in small sample volume high-Tc cuprates. As a first test of the assembled 23 MHz measuring setup, the temperature dependence of the upper critical field, HC2(T), is determined from the magnetic transition curves at different temperatures for Y1Ba2Cu3O7−δ thin-film ring. The HC2(T) data obey the formula: [1−(T/Tc)2]β with β=1.22±0.03 at...

The possibility of detection of small absorption in HTS thin films by means of the highly sensitive OFC magnetometer

Abstract We have essentially widened the capabilities of the open-flat coil-based tunnel diode oscillator technique (OFC magnetometer) for simultaneous tests of magnetic penetration depth, λ, of rf magnetic field and for detection of small changes of absorption inside the thin plate-like HTS materials. The updated method is based on simultaneous detection of the inductance and Q-factor value changes of the unusual flat (single-layer open-faced geometry) coil-based MHz-range resonator, which finally leads to the resonant frequency and amplitude changes of the oscillator. Our technique enables detecting the changes ∼10−12 H of HTS films magnetic inductance (the changes ∼1–3 A of λ). It detects also the changes ∼10−9 W of testing fields power absorption by the film, originated from the variation of its internal flux distribution (due to vortex flow during the suppression of superconductivity by external magnetic field). It can operate in wide ranges of temperature and magnetic field and measure in wide dynamic ranges of the testing parameters. We present the test results of the 0.2 μm thick and a few millimeter in diameter disk- and ring-shaped YBaCuO films illustrating wide capabilities of the updated OFC magnetometer to study weakly expressed peculiarities of superconductive state related, in particular, with the insignificantly small absorption at vortex motion.

Peculiarities of the magnetic phase diagram in small-size untwinned YBa2Cu3Oy crystal constructed by highly sensitive OFC-magnetometer

Abstract We applied OFC-magnetometer (open-flat coil-based tunnel diode oscillator) to study fine peculiarities of vortex flow and magnetic phase (MP) transitions between different phases of the vortex-matter in a high quality YBa 2 Cu 3 O y crystal. The method is based on a detection of the inductance and Q -factor value changes of a single-layer flat coil, which is placed at a face of flat sample. These finally lead to frequency and/or amplitude changes of the oscillator. Simultaneous frequency and amplitude tests enabled to construct the vortex MP-diagram for untwinned YBa 2 Cu 3 O y crystal with (0.25×0.6) mm 2 area and ∼50 μm-thick in the temperature range of 65 K to T c and the field of 0–12 T. In outlines, the plotted diagram is similar to the one defined by several other methods such as SQUID and cantilever magnetometers, etc. for the same crystal. However, our technique revealed clear separation of the lines on a diagram below the upper critical point H ucp i.e., no kink point was observed in our method. The amplitude tests revealed also some new lines on a diagram below the lower critical point H lcp (close to T c ). The obtained results may help us to understand the behavior of the vortex-mater and its dynamics in HTS materials. The problem how to improve resolution of the method for very small-size HTS objects is also discussed.

Imaging of the Local Dissipation in YBCO Coated Tapes

Current transport properties in YBCO coated tapes have been studied by the low temperature scanning laser microscopy (LTSLM) as well as the four‐probe method. The LTSLM measurement allows us to visualize the local resistive transition with a spatial resolution of a few μm. It has been shown that the localized resistive region percolates into the sample as the bias current density is increased. From the LTSLM measurements, we can estimate the spatial distribution of the inhomogeneous Jc. Relationship between the local Jc distribution and the global E‐J characteristics has also been investigated. The percolation probability obtained based on the LTSLM measurement agreed quantitatively with that of the four‐probe measurement. Namely, those two measurements support the percolation model.

Weakly expressed effects in HTS detected by a flat coil-based TD oscillator, demonstrating its wide possibilities for high-resolution detection

A flat coil-based tunnel diode (TD) oscillator is a fine instrument for high-resolution measurements of the penetration depth, λ, of a MHz-range field and for the detection of small power absorption inside flat high-Tc superconductors (HTS). The method is based on detection of changes in the inductance and Q-factor value of a single-layer flat coil, which leads to frequency and/or amplitude changes of the oscillator. It enables detection of ∼1 pH changes in film inductance (changes ∼1 A of λ) with ∼10−6 relative resolution. It also detects ∼1 nW changes in the power absorbed by a film, due to vortex motion. It operates in magnetic fields, measures in wide dynamic ranges of testing parameters, and is sensitive especially near Tc. These allow fine details of the normal-to-superconductive (N/S) transition and flux flow in a flat HTS at start of the formation of Cooper pairs to be revealed. A new paramagnetic peculiarity of the N/S transition in YBaCuO films, which precedes the well-known diamagnetic repulsio...

Construction of the magnetic phase diagram in small-volume HTS by an OFC magnetometer

We applied the OFC magnetometer for studies of vortex dynamics in small-volume HTS. The substitution of the solenoid coil by a flat one in a tunnel diode oscillator made the coils filling factor maximal for flat specimens enabling to reach 1-3 /spl Aring/ absolute and 10/sup -6/ relative changes of the penetration depth, /spl lambda/, of RF magnetic field into HTS films. Our technique operates in high magnetic fields and measures linear changes of /spl lambda/ in 1 mm range with high resolution. It is also an excellent Q-meter for detection of changes /spl sim/10/sup -9/ W of the absorption in HTS samples. Such abilities allow it to be applied in many fields of science and technology. In particular, detection of the magnetic field-dependent changes of the frequency and amplitude of a 23 MHz oscillator enabled lines on the magnetic phase diagram for Y-Ba-Cu-O film to be constructed. It seems to have many similarities as compared to the diagrams defined for bulk HTS by other methods. However, how close the relation is in fact, may be revealed after additional tests on the same quality specimens.

Study on the Magnetization of HTSC Films by a New Method Based on the Tunnel Diode Oscillator with an Open-Flat Coil

A new method for the study of the magnetization of thin films of magnetic materials is described. It is based on detecting of the inductance change of an Open-Flat Coil (OFC) which leads to the frequency changes of tunnel diode oscillator. The method is sensitive to small changes of distances with the 1A resolution. It can detect the changes ∼10−12H of HTSC film’s magnetic inductance originated from the variation of its internal flux distribution. We present the basic principle and describe the initial version of OFC magnetometer. The measurement results on the YBCO thin film magnetization and the advantages of the new test method are discussed also.

The Possibility of Determination of the Material Parameters in HTS by Means of the Improved “LC Resonator” Method

We have essentially improved the “LC resonator” method for high-resolution measurements of penetration depth λ of radio frequency magnetic field into thin films and small-size plate-like high-Tc superconductors by replacing the solenoid testing coil by the flat one driven by a highly stable-frequency and low-power tunnel diode oscillator. The advantages of the new technique such as high resolution-Δλ~ 1–3A (Δλ/λ ~ 2–10-6), ability of reliable operation in wide ranges of temperature and magnetic field, etc. enable to use it, in particular, for many-sided studies of the peculiarities of vortex dynamics in thin-film high-Tc superconductors with high resolution. We used it for determination of some material parameters in high-Tc cuprates. Namely, the measurements of the field-dependent radio frequency (~23MHz) penetration depth λ(H,T) allowed to determine the temperature dependencies of the upper critical field, Hc2(T), the Labusch parameter, α(T), and the depinning field value in YBaCuO thin-film ring specimen.

Implementation of the Tunnel Diode Oscillator with the Open-Flat Measuring HTSC Coil for Creation of New Type Detectors of Particles and Radiation

We consider the feasibility of use of high-Tc superconductors for the development of completely new type of high-sensitive and less-costly detectors of particles and radiation. At this, the use of the tunnel diode oscillator technique with the open-flat measuring coil (HTSC made, in particular) is very promising. This technique may be used also in a number of other applications in different areas of the science and technology. The feasibility of new detectors is evaluated and compared with existent ones. Some opportunities of sufficient improvement of limits of new detectors are suggested. Two designs of these detectors are proposed suitable for various applications. The sensitivities of the detectors achieved up-to-date are: about l0−11W/cm2/Hz>−1/2 for the visible spectrum radiation; about 0.1mOe/cm2/Hz−1/2 for the constant magnetic fields and about 0.1μA/cm2/Hz−1/2 for the DC passing through the HTSC thin-film structures. The fast operating time ~10−5÷10−6 s was achieved during registration of changes of magnetic fields or penetrating of currents through the HTSC films.