Field-modulated Microwave Research Articles

Probing FeSi, a d-electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T, magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a "topological Kondo insulator" (TKI). The overall behavior of the magnetoresistanceof FeSi at temperatures above and below the onset temperature TS = 19 K of the CSS is similar to that of SmB6. The two energy gaps, inferred from the ρ(T) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of TS. Several studies of ρ(T) under pressure on SmB6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which TS vanishes, although the energy gaps in SmB6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at TS ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at TS ≈ 4.5 K for SmB6.

Field induced effect of relaxor ferroelectrics at high electric field

Relaxor ferroelectrics have been widely used in dielectric, pyroelectric, electric field modulation microwave devices and electrostriction device. These applications refer to effect of electric field, increase in strength of polarization from ferroelectric phase stretching to paraelectric phase successively, to give rise to an effect similar to field induced effect. A result of the peak temperature of dielectric permittivity changing with the electric field 2/3 power was obtained by using approximation method, which is coincident with the experimental result of dielectric peak. The numerical simulation results found that there exist a thermal dynamics related polarization-temperature coefficient. Increasing the coefficient will decrease shift effect of dielectric peak, or will decrease effect of field induced phase transition, but, will increase pyroelectric effect. On the contrary, the stronger shift effect of dielectric peak will increase dielectric tunability and its temperature stability for electric field modulated microwave devices. The above theoretic results have certain reference implications for dielectric tunability of microwave device and pyroelectric property by doping modification method.

Strengthening of pinning by magnetic particles in high temperature superconductors

Strengthening of flux-line pinning in high- T c compounds by ferromagnetic particles created by implantation of iron ions has been observed. Measurements were carried out at single crystals of Bi 2Sr 2CaCu 2O 8 and thin films of YBa 2Cu 3O 7 by means of field-modulated microwave absorption technique. In case of YBa 2Cu 3O 7, experiments were performed on composite structure consisting of the superconducting film and non-conducting SiO 2 film containing magnetic particles.

Microwave absorption in micron- and nano-size YBaCuO powders

Size-dependent effects of microwave absorption are investigated in micron and submicron size YBaCuO powders. It is shown that the hysteresis and step-like change of field-modulated microwave absorption (FMMA) decreases when the particle size decreases. Two contributions to the relaxation of the FMMA signals were found.

Field-modulated microwave absorption in thin YBaCuO films

Hysteretic behaviour of field-modulated microwave absorption is investigated in thin YBaCuO films with thickness d≈50 or 400 nm, by scanning the external magnetic field in opposite directions, and in constant magnetic fields when the specimen is rotated. A negative hysteresis loop of field-modulated microwave absorption is detected in the films with d≈50 nm . An addition signal corresponding to decrease of the absorption when the field is parallel to the film surface is found.

New memory effects detected in field-modulated microwave absorption of magnetized YBa 2Cu 3O 7−δ powders

We describe two new memory effects detected as additional changes in signals of field-modulated microwave absorption of YBa 2Cu 3O 7−δ powders. They appear when the scanning direction of the field is reversed or when the sample is cooled. The observed effects can be attributed to vortex distribution in superconducting YBa 2Cu 3O 7−δ particles.

Field modulated microwave absorption and irreversibility line in [formula omitted] multilayers

We describe field modulated microwave absorption measurements in two series of YBa 2Cu 3O 7 PrBa 2Cu 3O 7 multilayers: those with PrBa 2Cu 3O 7 thickness in the range ∼2.5 to ∼10 nm and with a fixed YBa 2Cu 3O 7 thickness of ∼10 nm and those with different resistivities of the PrBa 2Cu 3O 7 barrier with the same thickness of ∼2.5 nm for YBa 2Cu 3O 7 and PrBa 2Cu 3O 7 layers. The Fourier spectrum of the reflected microwave power reveals one fundamental frequency which appears at T c and even harmonics of the modulation field frequency f at lower temperatures. The determination of the irreversibility line near T c inferred from the vanishing of the nonlinear response signal at 2 f in the presence of superimposed d.c. and modulation fields is reported. A comparison with results obtained on YBa 2Cu 3O 7 thin films is also given.

Microwave studies of the superconducting state in Rb 3C 60

Methods of electron-spin resonance (ESR) and direct, non-linear field-modulated microwave absorption (FMMA) were applied for the measurements in low- and high-purity samples of rubidium-doped fullerene, Rb 3C 60. The coexistence of the normal strong ESR signals and regular series of weak absorption lines similar to those seen in systems of Josephson junctions was observed in the high-purity sample. The possible influence of the vortex lattice on the ESR signals was also studied. We determined from FMMA investigations using the Portis model the critical field μ 0H ∗ = 40 μ T , the depinning current density J c ∗(μ 0H 0 = 1 mT) ∼ 4 × 10 8 A/m 2 in low magnetic field and J c ∗(μ 0H 0 > 100 mT) ∼ 1.6 × 10 8 A/m 2 in higher fields. These values were generally one order of magnitude higher than the highest corresponding values previously observed in high-temperature superconductors (HTS's).

Microwave absorption in non- and hydrogenated La 1.85Sr 0.15CuO 4 high-T c superconductor

The field-modulated microwave (MW) absorption in non- and hydrogenated La 1.85Sr 0.15CuO 4 high-temperature superconductors was measured. The relationship between the microwave hysteresis at high magnetic field ( H = 3 kG) and the irreversible magnetization has been investigated. It has been established that the temperature dependence of the MW hysteresis is not in a line with magnetic hysteresis data. The value of the pinning energy of 50 meV at 15 K in magnetic field of 3.0 kG for the non- and hydrogenated samples has been estimated.

Flux-pinning effects in field-modulated microwave absorption of YBa 2Cu 3O 7−δ thin films

It is shown that the shape of the field-modulated microwave-absorption signal in the mixed state of type-II superconductors reveals the field dependence of the flux-pinning parameters. Experimental data are presented for thin films of YBa 2Cu 3O 7−δ grown by the pulsed laser deposition (PLD) technique.

Magnetically modulated microwave absorption and harmonic generation in YBa 2Cu 3O 7 thin films

We have performed a comparative study of field-modulated microwave absorption of YBa 2Cu 3O 7 textured and epitaxial thin films using an EPR spectrometer. The Fourier analysis of the signal obtained from microwave absorption of the films has been investigated as a function of temperature and AC magnetic field amplitude. The response is found to be essentially non-linear. We show that the model of Portis et al. based on the existence of a critical state, with pinning and depinning of vortices during a modulation cycle, can account for the observed non-linear absorption signals. The response at the excitation frequency observed close to T c is discussed in terms of flux diffusion.

Surface resistance of grain-aligned YBa2Cu3Ox bulk: Evidence for two kinds of weak link.

We have measured the magnetic-field and temperature dependence of the surface resistance ${\mathit{R}}_{\mathit{s}}$ of grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$. Experiments were conducted on ab-plane- and ac-plane-oriented samples, with the dc magnetic field parallel and perpendicular to the sample surface. We explain the nonlinear behavior of ${\mathit{R}}_{\mathit{s}}$ at low field using a model with an array of Josephson weak links and the linear dependence on magnetic field at high field using an oscillatory-vortex-motion-loss model. The model allows a quantitative fit to the experimental data using the lower critical field of the grain, the entry field ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ of weak links, the junction decoupling parameter ${\mathit{H}}_{\mathit{d}}$, and the vortex viscosity as fitting parameters. Two different sets of parameters ${\mathit{H}}_{\mathit{d}}$ and ${\mathit{H}}_{\mathit{c}1\mathit{J}}$ were used to fit the data, suggesting that in grain-aligned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ bulk material at least two kinds of weak link are responsible for low-field losses. An estimation of the Josephson-junction area was obtained from field-modulated microwave absorption. In addition, thermal-excitation and radiation-activation mechanisms for rf losses are compared.

Measurements of surface resistance of grain-aligned bulk material as a function of DC magnetic field; weak link study

The temperature and DC magnetic field dependences of the surface resistance of grain-aligned YBa/sub 2/Cu/sub 3/O/sub x/ bulk material were measured at 9.5 and 75 GHz. Experiments were conducted on a-b-plane- and a-c-plane-oriented sample surfaces. A nonlinear increase of R/sub s/(H/sub dc/) at low fields, which turns into a linear dependence on H at fields larger than H/sub c1/, was observed and explained. A model that allows a quantitative fit using as parameters the lower critical field of the weak links or the grain, the decoupling parameter, and the slope alpha was employed. Two different sets of these parameters must be used to fit all of the data, indicating the existence of two kinds of weak links. The parameter values indicate that there exist inter- and intragrain weak links that contain Josephson fluxons, and a bulk crystal region that contains Abrikosov fluxons. The field-modulated microwave absorption method was used to identify intergrain weak links with lower critical fields <10/sup -4/ T. The correlation between microstructure of the samples and the angular dependence of the field modulated microwave absorption lines is discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of the magnetic field-induced microwave absorption across Tc in single crystals of YBa 2Cu 3O 7−δ and BiSrCaCuO : Anisotropy and magnetic field dependence

The magnetic field-modulated microwave absorption near T c has been studied in high-quality single crystals of YBa 2Cu 3O 7− x ( T c ≈ 90 K) and in BiSrCaCuO ( T c ≈ 85 K) as a function of magnetic field B < 1.3 T and orientation. Sharp microwave absorption peaks were observed when sweeping the temperature through T c from above. The analysis of the peaks using Tinkham's model for fluxon losses gives values for the pinning strength of A ≈ 1000 T and A ≈ 3 T for the YBaCuO and BSCCO crystals, respectively. The anisotropy values obtained from the orientation dependence of the peaks were ϵ = 5 and ϵ = 40−60 for these two compounds, respectively. The results are also discussed with respect to the B-T vortex phase diagram. A marked difference between the YBaCuO and BSCCO compounds is the presence of a low-temperature absorption signal with a different orientation dependence, observed only in the BSCCO compound, in addition to the signals just below T c observed in both compounds.

Field-modulated microwave surface resistance in a single-crystal Tl2Ca2BaCu2O8 superconductor.

We report magnetic-field-modulated microwave surface resistance in a single-crystal ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{BaCu}}_{2}$${\mathrm{O}}_{8}$ superconductor. At a fixed temperature, as the applied magnetic field is varied, the observed signal has two features: an intense sharp peak at low field and a broad maximum at high field. The peak field defined for the broad maximum does not have the same temperature dependence as the irreversiblity line determined from dc magnetization measurement. The results are analyzed on the basis of recent theoretical models. It is found that the flux-flow resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{f}}$\ensuremath{\propto}B for B${\mathit{B}}_{\mathrm{peak}}$ and ${\mathrm{\ensuremath{\rho}}}_{\mathit{f}}$=const for B&gt;${\mathit{B}}_{\mathrm{peak}}$. Scaling of the data at different T and B gives a universal function for the surface resistance.

Field-modulated microwave absorption in granular superconductors: First and second harmonic signals

Abstract Field-modulated microwave absorption in granular superconductors is analyzed within the weak link model. The signals are calculated for various modulation amplitudes, both for the first and the second harmonic detection. The calculation signal shapes are in good agreement with experimental results.

Field-modulated microwave absorption study of Pb-modified Bi Ca Sr Cu O

Abstract High-resolution transmission-electron-microscopy (HRTEM) has established that samples of nominal composition Bi 2 Sr 2 Ca 2 Cu 4 O y prepared by solid-state reaction are of mixed phase. These samples are mostly composed of the 115 K polytypoid structure with the 80 K polytypoid structure found at grain-boundaries and at planar defects within grains. The partial replacement of Bi by Pb leads to a single phase of the 115 K polytypoid as determined from HRTEM. Low-magnetic-field modulated microwave absorption in the absence of Pb yields absorption onsets associated with transitions around 115 K and around 80 K. The addition of Pb eliminates the lower temperature transition. This observation is consistent with the HRTEM observations and supports the general view that microwave absorption arises primarily from grain boundaries and planar defects.

Microwave absorption and intermittency in textured Y 1Ba 2Cu 3O 7−x thin films

We report simultaneous measurements of direct and field-modulated microwave absorption in textured Y 1Ba 2Cu 3O 7−x thin film and ceramic samples. The two techniques are shown to monitor two different characteristic onset temperatures in the superconducting system. Below the critical transition temperature T c, the modulated signal shows noise-like fluctuations whose onset temperature T ∗ decreases logarithmically with the amplitude of the modulation field. These results are ascribed to a distribution of weak Josephson links. Intermittent very low frequency microwave absorption jumps of constant amplitude are observed at fixed temperatures, and are similar to the predicted behaviour of an ideal Josephson junction shunted with a capacitor and a resistor.

Analysis of superconducting transitions in c ≈ 31 and c ≈ 37Å BSCCO systems

Abstract The relation between the 110K transition and the c≈37 A structure in BSCCO system is examined. The superconducting properties of several samples showing the c ≈ 31 and the c ≈ 37 A structures are studied by means of electrical resistivity and field-modulated microwave absorption.

Composition effects on the formation and superconducting character of c≈31 Å and c≈37 Å phases in the Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O systems. X-ray and ESR analysis

In this paper we examine the effect of starting composition on the formation of the two crystallographic phases characterized by c≈31 Å and c≈37 Å in the Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O systems. The superconducting properties have been studied by field-modulated microwave non-resonant absorption. The c≈37 Å phase was completely isolated in the Pb-containing system and shows a quite sharp superconductive transition at 105 K, well evidenced by the microwave absorption profile as a function of temperature. For the c≈31 Å phase, obtained in a wide range of composition, the ESR analysis points out a more complex behaviour, with a superconductive transition taking place in the 75–110 K temperature range. Strong contribution to the 110 K transition has been observed in samples showing no evidence in the X-ray spectra of the typical c≈37 Å reflections, nor of peak broadening due to stacking faults. A possible correlation of this phenomenon with the partial substitution of Bi in the Bi 2O 2 double layers by lighter Ca, Sr or Cu ions is discussed.