Control of Structural and Magnetic Properties of Polycrystalline Co2FeGe Films via Deposition and Annealing Temperatures

: Thin polycrystalline Co2FeGe films with composition close to stoichiometry have been fabricated using magnetron co-sputtering technique. Effects of substrate temperature (TS) and postdeposition annealing (Ta) on structure, static and dynamic magnetic properties were systematically studied. It is shown that elevated TS (Ta) promote formation of ordered L21 crystal structure. Variation of TS (Ta) allow modification of magnetic properties in a broad range. Saturation magnetization ~920 emu/cm3 and low magnetization damping parameter α ~ 0.004 were achieved for TS = 573 K. This in combination with soft ferromagnetic properties (coercivity below 6 Oe) makes the films attractive candidates for spin-transfer torque and magnonic devices

Evolution of over-conductivity of YВа₂Сu₃О₇-δ single crystals under the exposure of irradiation by high-energy electrons

The evolution of the excess conductivity of YВа₂Сu₃О₇-δ single crystals upon electron irradiation is investigated. It was shown that electron irradiation leads to a significant expansion of the temperature range for the existence of excess conductivity, thereby narrowing the region of the linear dependence of p(Т) in the ab plane. It was found that the excess conductivity Δσ(Т) of YВа₂Сu₃О₇-δ single crystals in a wide temperature range Tf<Т<T⚹ (Тf – the transition temperature from the PG to the FP mode) is subject to exponential temperature dependence. Moreover, the description of excess conductivity using the relation Δσ ⁓ (1-Т/Т⚹)exp(Δ⚹ab/T) (Δ⚹ab – the pseudogap in ab-plane) can be interpreted in terms of the mean-field theory, where T⚹ is presented as the mean-field temperature of the transition to the PG state, and the temperature dependence of the pseudogap is satisfactorily described in the framework of the BCS-BEC crossover theory. In this case, the value of the transverse coherence length ξс(0) increases 1.4 times and the 2D-3D crossover point shifts in temperature.Досліджено еволюцію надлишкової провідності монокристалів YВа₂Сu₃О₇-δ при опроміненні електронами. Показано, що опромінення електронами призводить до значного розширення температурного інтервалу існування надлишкової провідності, тим самим, звужуючи область лінійної залежності p(Т) в ab площині. Встановлено, що надлишкова провідність Δσ(Т) монокристалів YВа₂Сu₃О₇-δ в широкому інтервалі температур Tf<Т<T⚹ підпорядковується експоненційній температурній залежності. При цьому опис надлишкової провідності за допомогою співвідношення Δσ⁓(1–Т/Т⚹)exp(Δ⚹ab/T) може бути інтерпретовано в термінах теорії середнього поля, де Т⚹ представлена, як середньополева температура переходу в ПЩ-стан, а температурна залежність псевдощілини задовільно описується в рамках теорії кросовера БКШ-БЕК. При цьому величина поперечної довжини когерентності ξс(0) збільшується в 1,4 рази, і зміщується по температурі точка 2D-3D кросовера.Исследована эволюция избыточной проводимости монокристаллов YВа₂Сu₃О₇-δ при облучении электронами. Показано, что облучение электронами приводит к значительному расширению температурного интервала существования избыточной проводимости, тем самым, сужая область линейной зависимости p(Т) в ab-плоскости. Установлено, что избыточная проводимость Δσ(Т) монокристаллов YВа₂Сu₃О₇-δ в широком интервале температур Tf<Т<T⚹ подчиняется экспоненциальной температурной зависимости. При этом описание избыточной проводимости с помощью соотношения Δσ⁓(1–Т/Т⚹)exp(Δ⚹ab/T) может быть интерпретировано в терминах теории среднего поля, где Т⚹ представлена, как среднеполевая температура перехода в ПЩ-состояние, а температурная зависимость псевдощели удовлетворительно описывается в рамках теории кроссовера БКШ-БЭК. При этом величина поперечной длины когерентности ξс(0) увеличивается в 1,4, раза и смещается по температуре точка 2D-3D кроссовера

Observation of inverse Compton emission from a long γ-ray burst.

Long-duration γ-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterized by an initial phase of bright and highly variable radiation in the kiloelectronvolt-to-megaelectronvolt band, which is probably produced within the jet and lasts from milliseconds to minutes, known as the prompt emission1,2. Subsequently, the interaction of the jet with the surrounding medium generates shock waves that are responsible for the afterglow emission, which lasts from days to months and occurs over a broad energy range from the radio to the gigaelectronvolt bands1-6. The afterglow emission is generally well explained as synchrotron radiation emitted by electrons accelerated by the external shock7-9. Recently, intense long-lasting emission between 0.2 and 1 teraelectronvolts was observed from GRB 190114C10,11. Here we report multi-frequency observations of GRB 190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from 5 × 10-6 to 1012 electronvolts. We find that the broadband spectral energy distribution is double-peaked, with the teraelectronvolt emission constituting a distinct spectral component with power comparable to the synchrotron component. This component is associated with the afterglow and is satisfactorily explained by inverse Compton up-scattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed teraelectronvolt component are typical for GRBs, supporting the possibility that inverse Compton emission is commonly produced in GRBs

Effect of praseodymium doping on electroresistivity along c-axis in Y₁₋xPrxBa₂Cu₃O₇₋δ single crystals

In the present study influence of praseodymium doping on conductivity across (transverse) the basal plane of high-temperature superconducting Y₁₋xPrxBa₂Cu₃O₇₋δ single crystals is investigated. It is determined that increase of praseodymium doping leads to increased localization effects and implementation of the metal – insulator transition Y₁₋xPrxBa₂Cu₃O₇₋δ, which always precedes the superconducting transition. The praseodymium concentration increase also leads to significant displacement of the point of the metal – insulator transition to the low temperature region

Scattering of electrons in oxygen underdoped YBa₂Cu₃O₇₋x single crystals

The electrical resistivity in the range of Tc-300 K in the layer planes of YBa₂Cu₃O₇₋x single crystals with a range of oxygen deficiency, which is characterized by the Tc in the range 78 + 92 K was studied. The experimental data on the resistance in normal state are approximated by an expression that takes into account the scattering of electrons on phonons, as well as on defects and the fluctuation conductivity in 3D-model of the Aslamazov-Larkin theory. According to this approximation, depending upon the oxygen deficiency, the Debye temperature changes from 245 to 400 K, coherence length ξc(0) ≈ 0.5 Å

Effect of high pressure on conductivity in the basal plane of Y₁₋xPrxBa₂Cu₃O₇₋δ single crystals lightly doped of praseodymium

Effect of high hydrostatic pressure up to 17 kbar on conductivity of lightly Pr-doped Y₁₋xPrxBa₂Cu₃O₇₋δ (x~0.05) single crystals is investigated. We show that in contrast to non-doped YBa2Cu3O7-δ samples, application of the high pressure leads to a substantial increase of the pressure derivative of the coherence length dξc/dP and temperature shift of 2D-3D crossover point. Possible mechanisms of the influence of the high pressure on the critical temperature and the coherence length are discussed within the frames of a model assuming the presence of singularities in the charge carriers electron spectrum typical for lattices with strong coupling. The excess conductivity Δ σ(T) inY₀.₉₅xPr₀Ba₂Cu₃O₇₋δ has beenY0.95Pr0.05Ba2Cu3O7-δ revealed to obey an exponential dependence in the wide temperature range Tf < T < T*. At this, description of the excess conductivity by the expression Δ σ ~ (1 - T/T*)exp(Δ*ab/T) can be interpreted in terms of the mean-field theory, where T* is the mean-field superconducting transition temperature and pseudogap temperature dependence is satisfactory described within the framework of the BCS-BEC crossover theory. An increase of the applied pressure leads to narrowing of the temperature range of realization of the pseudogap regime, thereby expanding the linear temperature dependence of the basal-plane resistivity ρab(T)

Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode

International audienceThe nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high energy (VHE) gamma-rays, as well as details of the individual observations and light curves. We also conduct phenomenological modelling to investigate the basic source properties. We present the first VHE gamma-ray flare from M87 detected since 2010. The flux above 350 GeV has more than doubled within a period of about 36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE gamma-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and emphasises the need for combined image and spectral modelling

Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode

International audienceThe nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high energy (VHE) gamma-rays, as well as details of the individual observations and light curves. We also conduct phenomenological modelling to investigate the basic source properties. We present the first VHE gamma-ray flare from M87 detected since 2010. The flux above 350 GeV has more than doubled within a period of about 36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE gamma-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and emphasises the need for combined image and spectral modelling