Carreras Oropesa, W. G., S. Encina, P. Pedrazzini, V. F. Correa, J. G. Sereni, V. Vildosola, García D.J., and P. S. Cornaglia. "Minimal model for the magnetic phase diagram of CeTi1-xScxGe, GdFe1-xCoxSi, and related materials." Journal of Magnetism and Magnetic Materials 503 (2020): 166614.
Abstract: We present a theoretical analysis of the magnetic phase diagram of CeTi1-xScxGe and GdFe1-xCoxSi as a function
of the temperature and the Sc and Co concentration x, respectively. CeScGe and GdCoSi, as many other RTX
(R=rare earth, T=transition metal, X=p-block element) compounds, present a tetragonal crystal structure
where bilayers of R are separated by layers of T and X. While GdFeSi and CeTi0.75Sc0.25Ge are ferromagnetic,
CeScGe and GdCoSi order antiferromagnetically with the R 4f magnetic moments on the same bilayer aligned
ferromagnetically and magnetic moments in nearest neighbouring bilayers aligned antiferromagnetically. The
antiferromagnetic transition temperature TN decreases with decreasing concentration x in both compounds and
for low enough values of x the compounds show a ferromagnetic behavior. Based on these observations we
construct a simplified model Hamiltonian that we solve numerically for the specific heat and the magnetization.
We find a good qualitative agreement between the model and the experimental data. Our results show that the
main magnetic effect of the Sc→Ti and Co→Fe substitution in these compounds is consistent with a change in
the sign of the exchange coupling between magnetic moments in neighbouring bilayers. We expect a similar
phenomenology for other magnetic RTX compounds with the same type of crystal structure.
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Haberkorn, N., S. Suárez, S. L. Bud'ko, and P. C. Canfield. "Strong pinning and slow flux creep relaxation in Co-doped CaFe2As2 single crystals." Solid State Communications 318 (2020): 113963.
Abstract: We report on measurements of critical current densities Jc and flux creep rates S of freestanding Ca(Fe1−xCox)2As2 (x ≈ 0.033) single crystals with Tc ≈ 15.7 K by performing magnetization measurements. The magnetic field dependences of Jc at low temperature display features related to strong pinning. In addition, we find that the system displays small flux creep rates. The characteristic glassy exponent, μ, and the pinning energy, U0, display exceptional high values for pristine crystals. We find that for magnetic fields between 0.3 T and 1 T, μ decreases from ≈ 2.8 to ≈ 2 and U0 remains ≈ 300 K. Analysis of the pinning force indicates that the mechanism is similar to the observed in polycrystalline systems in which grain boundaries and random disorder produce the vortex pinning. Considering the large U0 observed in the single crystal, we attempt to improve the pinning by adding random point disorder by 3 MeV proton irradiation with a fluence of 2 × 1016 proton/cm2. The results show that, unlike other iron-based superconductors, the superconducting fraction is sharply reduced by irradiation. This fact indicates that the superconductivity in the system is extremely fragile to an increment in the disorder. The superconducting volume fraction in the irradiated crystal systematically recovers after removal disorder by thermal annealing, which evidences as to the observation of critical state in curves of magnetization versus magnetic field. No features related to a reentrant antiferromagnetic transition are observed for the irradiated sample.
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Haberkorn, N., M. Xu, W. R. Meier, S. Suárez, S. L. Bud’ko, and P. C. Canfield. "Enhancement of critical current density in CaKFe4As4 single crystals through 3 MeV proton irradiation." Superconductor Science and Technology 33, no. 2 (2020): 025008.
Abstract: We study the influence of random point disorder on the vortex dynamics and critical current densities Jc of CaKFe4As4 single crystals by performing magnetization measurements. Different samples were irradiated with a proton (p) beam at constant energy of 3 MeV to fluencies from 2 × 1015 p cm−2 to 4 × 1016 p cm−2. The results show the addition of extrinsic random point disorder enhances the Jc values at low and intermediate temperatures over the entire range of magnetic fields applied. The optimum pinning enhancement is achieved with a proton fluence of 3 × 1016 p cm−2, increasing Jc at 5 K by factors ≈5 and 14 at self-field and μ0H = 3 T, respectively. We analyze the vortex dynamics using the collective creep theory. The enhancement in Jc matches with a systematic reduction in the flux creep relaxation rates as a consequence of a gradual increase in the collective pinning energy U0. The substantial increment in Jc produced by random point disorder, reaching values of 9 MA cm−2 at 5 K and self-field, makes CaKFe4As4 a promising material for applications based on current carrying capacity at high magnetic fields.
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Hofer, J. A., S. Bengio, G. Rozas, P. D. Pérez, M. Sirena, S. Suárez, and N. Haberkorn. "Compositional effects on the electrical properties of extremely disordered molybdenum oxynitrides thin films." Materials Chemistry and Physics 242 (2020): 122075.
Abstract: Molybdenum oxynitride (MoNxOy) thin films were grown by reactive sputtering on Si (100) substrates at room temperature. The partial pressure of Ar was fixed at 90%, and the remaining 10% was adjusted with mixtures N2:O2 (varying from pure N2 to pure O2). The electrical properties of the films depend on the chemical composition. Thin films grown using mixtures up to 2% O2 have γ-Mo2N phase and display superconductivity. The superconducting critical temperature Tc reduces from ∼6.8 K to below 3.0 K as the oxygen increases. On the other hand, the films are mostly amorphous for gas mixtures above 2% O2. The electrical conductivity shows a semiconductor-like behavior well described by variable-range hopping conduction. The analysis of the optical properties reveals that the samples do not have a defined semiconductor bandgap, indicating that the high structural disorder produces electron excitation for a wide range of energies.
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Lipovetzky, J., F. Alcalde Bessia, J. Guimpel, M. Perez, and M. Gomez Berisso. "Characterization of a low-power CMOS operational amplifier from 12.5K to 273K for low temperature experiments." In 2020 Argentine Conference on Electronics (CAE), 73–76. IEEE, 2020.
Abstract: In this work, we present the design and first characterization of an operational amplifier for use at cryogenic temperatures. We show the functionality of the amplifier in a range of temperatures from 12.5K to 273K. Drain current to gate voltage curves of n-channel and p-channel MOS transistors, resistors and the amplifier response were measured. The circuit allows the amplification of signals up to 100kHz with a power consumption of 48μW.
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Lipovetzky, J., A. Cicuttin, M. L. Crespo, M. Sofo Haro, F. Alcalde Bessia, M. Pérez, and M. Gómez Berisso. "Multi-spectral X-ray transmission imaging using a BSI CMOS Image Sensor." Radiation Physics and Chemistry 167 (2020): 108244.
Abstract: In this work we study the performance to obtain X-ray images of a Back Side Illuminated CMOS Image Sensor, the Omnivision OV5647, empoying X-rays from tube with a palladium anode and voltages from 7.5 keV to 50 keV. The performance is compared with the Timepix detector operating in the Time Over Threshold mode. False color images are obtained using data from different energies and brightnesses, to fussion different information on the same picture. The different attenuations are analyzed and discussed in terms of the charge detection efficiency of the CMOS sensor, measured using Fluorescence X-rays and gamma rays from calibrated sources.
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Llorens, J. B., L. Embon, A. Correa, J. D. González, E. Herrera, I. Guillamón, R. F. Luccas, J. Azpeitia, F. J. Mompeán, M. García-Hernández et al. "Observation of a gel of quantum vortices in a superconductor at very low magnetic fields." Physical Review Research 2 (2020): 013329.
Abstract: A gel consists of a network of particles or molecules formed for example using the sol-gel process, by which a solution transforms into a porous solid. Particles or molecules in a gel are mainly organized on a scaffold that makes up a porous system. Quantized vortices in type-II superconductors mostly form spatially homogeneous ordered or amorphous solids. Here we present high-resolution imaging of the vortex lattice displaying dense vortex clusters separated by sparse or entirely vortex-free regions in
β
−
Bi
2
Pd
superconductor. We find that the intervortex distance diverges upon decreasing the magnetic field and that vortex lattice images follow a multifractal behavior. These properties, characteristic of gels, establish the presence of a novel vortex distribution, distinctly different from the well-studied disordered and glassy phases observed in high-temperature and conventional superconductors. The observed behavior is caused by a scaffold of one-dimensional structural defects with enhanced stress close to the defects. The vortex gel might often occur in type-II superconductors at low magnetic fields. Such vortex distributions should allow to considerably simplify control over vortex positions and manipulation of quantum vortex states.
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Navarro, H., M. Sirena, and N. Haberkorn. "Improving the Josephson energy in high-Tc superconducting junctions for ultra-fast electronics." Nanotechnology 31 (2020): 105701.
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Navarro, H., M. Sirena, J. Kim, and N. Haberkorn. "Josephson coupling in high-Tc superconducting junctions using ultra-thin BaTiO3 barriers." Materials Science and Engineering: B 262 (2020): 114714.
Abstract: We study the electrical transport of vertically-stacked Josephson tunnel junctions using GdBa2Cu3O7−δ electrodes and a BaTiO3 barrier with thicknesses between 1 nm and 3 nm. Current-voltage measurements at low temperatures show a Josephson coupling for junctions with BaTiO3 barriers of 1 nm and 2 nm. Reducing the barrier thickness bellow a critical thickness seems to suppress the ferroelectric nature of the BaTiO3. The Josephson coupling temperature reduces as the barrier thicknesses increases. The Josephson energies at 12 K are of ≈ 1.5 mV and ≈ 7.5 mV for BaTiO3 barriers of 1 nm and 2 nm, respectively. Fraunhofer patterns are consistent with fluctuations in the critical current due to structural inhomogeneities in the barriers. Our results are promising for the development of Josephson junctions using high-Tc electrodes with energy gaps much higher than those usually present in conventional low-temperature superconductors.
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Nikitin, S. E., D. G. Franco, J. Kwon, R. Bewley, A. Podlesnyak, A. Hoser, M. M. Koza, C. Geibel, and O. Stockert. "Gradual pressure-induced enhancement of magnon excitations in CeCoSi." Physical Review B 101 (2020): 214426.
Abstract: CeCoSi is an intermetallic antiferromagnet with a very unusual temperature-pressure phase diagram: at ambient pressure it orders below
TN=8.8K, while application of hydrostatic pressure induces a new magnetically ordered phase with exceptionally high transition temperature of ∼40K
at 1.5 GPa. We studied the magnetic properties and the pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic neutron scattering (INS) and heat capacity measurements. At ambient pressure CeCoSi orders into a simple commensurate AFM structure with a reduced ordered moment of only
mCe=0.37(6)μB.Specific heat and low-energy INS indicate a significant gap in the low-energy magnon excitation spectrum in the antiferromagnetic phase, with the CEF excitations located above 10 meV. Hydrostatic pressure gradually shifts the energy of the magnon band towards higher energies and the temperature dependence of the magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the CEF excitations are also drastically modified under pressure.
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Pérez, M., M. S. Haro, J. Lipovetzky, A. Cicuttin, M. L. Crespo, F. Alcade Bessia, M. Gómez Berisso, and J. J. Blostein. "Evaluation of a Commercial Off The Shelf CMOS Image Sensor for X-ray spectroscopy up to 24.9 keV." Radiation Physics and Chemistry 177 (2020): 109062.
Abstract: We studied the X-ray spectroscopy capability and the detection efficiency of a low cost Commercial Off The Shelf CMOS Image Sensor (CIS) in the energy range from 6.4 to 24.9 keV using the fluorescence spectra emitted by FeNi, Cu, Zr, Pb, and Ag. The obtained results are compared with that obtained using a Silicon Drift Detector (SDD). We conclude that CIS is able to resolve fluorescence lines up to 17.7 keV but with a reduced detection efficiency. At lower energies, the energy resolution of the CIS is comparable to that obtained with the SDD. By the comparison of both detectors we also estimate the detection efficiency of the proposed method and the effective thickness of the CIS for all the measured X-ray lines.
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Sanchez, J. A., G. Rumi, R. C. Maldonado, N. R. C. Bolecek, J. Puig, P. Pedrazzini, G. Nieva, M. I. Dolz, M. Konczykowski, C. J. van der Beek et al. "Non-Gaussian tail in the force distribution: a hallmark of correlated disorder in the host media of elastic objects." Scientific Reports 10, no. 1 (2020): 19452.
Abstract: Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial importance for many applications but remains a challenging basic-science problem. Here we propose a method to discern whether weak-point or strong-correlated disorder dominates based on characterizing the distribution of the interaction forces between objects mapped in large fields-of-view. We illustrate our proposal with the case-study system of vortex structures nucleated in type-II superconductors with different pinning landscapes. Interaction force distributions are computed from individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude-wide vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian distributions of the interaction force components. In contrast, if the media have dilute and randomly-distributed correlated disorder, these distributions present non-Gaussian algebraically-decaying tails for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning centers.
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Sereni, J. G. "Thermomagnetic properties of very heavy fermions suitable for adiabatic demagnetisation refrigeration at low temperature." Philosophical Magazine 100, no. 10 (2020): 1211.
Abstract: With the aim to improve the performance of classical paramagnetic salts for adiabatic refrigeration processes at the sub-Kelvin range, relevant thermodynamic parameters of some new Yb-based intermetallic compounds are analysed and compared. Two main applications are considered: (i) those requiring temperature fixed-points to be reached applying magnetic fields of moderate intensity for satellite applications, and (ii) those which can be used for controlled thermal drifts in laboratory applications. Different thermomagnetic trajectories of the entropy are identified depending on respective specific heat behaviours and the constraints imposed by the Nernst postulate at the sub-Kelvin range. Some simple relationships are proposed to compare the diverse magnetocaloric characteristics of different systems. This procedure allows to include the classical Cerium-Magnesium-Nitride (CMN) salt in that comparison.
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Sidelnik, I., H. Asorey, N. Guarín, M. S. Durán, M. G. Berisso, J. Lipovetzky, and J. J. Blostein. "Simulation of 500 MeV neutrons by using NaCl doped Water Cherenkov detector." Advances in Space Research 65, no. 9 (2020): 2216.
Abstract: In this work we show the capabilities of a water Cherenkov detector (WCD) to detect high energy neutrons. We present the simulation of the response of a doped WCD to 500 MeV monochromatic neutrons using Geant4. To do this, a detailed model of the WCD has been implemented. The active volume of the detector is composed of pure water and different concentrations of a Cl based additive. The addition of this dopant shows an enhancement in the detection of high energetic neutrons. The sensitivity of this detector to neutrons achieved in our simulations is a relevant result for cosmic rays and space weather studies.
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Sidelnik, I., H. Asorey, N. Guarin, M. S. Durán, F. A. Bessia, L. H. Arnaldi, M. G. Berisso, J. Lipovetzky, M. Pérez, M. S. Haro et al. "Neutron detection capabilities of Water Cherenkov Detectors." Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 952 (2020): 161962.
Abstract: In this work we show the neutron detection capabilities of a water Cherenkov detector (WCD). The experiments presented here were performed using a simple WCD with a single photomultiplier tube (PMT) and a 252Cf neutron source. We compared the use of pure water and water with non contaminant additive as the detection volume. We show that fast neutrons from the 252Cf source can be detected over the flux of atmospheric particles background. Our first estimation for the neutron detection efficiency is at the level of (19)% for pure water and (44)% for the water with the additive. We also present the simulation of the response of the WCD to neutrons using a simulated 252Cf source. We implemented a detailed model of the WCD and of the neutron source spectra using Geant 4. The results of our simulations show the detailed mechanism for the detection of neutrons using WCD and support the experimental evidences presented. Since both active volumes studied, H2O pure and with additive, are cheap, non-toxic and easily accessible materials, the results obtained are of interest for the development of large neutron detectors for different applications. Of special importance are those related with space weather phenomena as well as those for the detection of special nuclear materials. We conclude that WCD used as neutron detectors can be a complementary tool for standard neutron monitors based on 3He.
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Sofo Haro, M., F. Alcalde Bessia, M. Pérez, J. J. Blostein, D. F. Balmaceda, M. Gómez Berisso, and J. Lipovetzky. "Soft X-rays spectroscopy with a commercial CMOS image sensor at room temperature." Radiation Physics and Chemistry 167 (2020): 108354.
Abstract: Besides their application in point and shoot cameras, webcams, and cell phones, it has been shown that CMOS image sensors (CIS) can be used for dosimetry, X-ray and neutron imaging applications. In this work we will discuss the application of an ON Semiconductor MT9M001 CIS, in low energy X-ray spectroscopy. The device is a monochromatic front-side illuminated sensor, very popular in consumer electronics. In this work we introduce the configuration selected for the mentioned sensor, the image processing techniques and event selection criteria, implemented in order to measure the X-ray energy in the range from 1-10 keV. Several fluorescence lines of different samples have been resolved, and for first time the line resolution have been measured and analyzed. We achieved a FWHM of 232 eV at 6.4 keV, and we concluded that incomplete charge collection (ICC) of the charge produced by the X-ray contributes to the resolution, being this effect more important at higher X-ray energies. The results analyzed in this work indicate that the mentioned CIS are specially suitable for X-ray applications in which energy and spatial resolutions are simultaneously required.
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Sutter, J. G., A. S. Chávez, S. Soria, M. Granada, L. Neñer, S. Bengió, P. Granell, F. Golmar, N. Haberkorn, A. G. Leyva et al. "Tuning the magneto-electrical properties of multiferroic multilayers through interface strain and disorder." Journal of Alloys and Compounds (2020): 157820.
Abstract: Artificially engineered superlattices were designed and fabricated to induce different growth mechanisms and structural characteristics. DC sputtering was used to grow ferromagnetic (La0.8Ba0.2MnO3)/ferroelectric (Ba0.25Sr0.75TiO3 or BaTiO3) superlattices. We systematically modified the thickness of the ferromagnetic layer to analyze dimensional and structural disorder effects on the superlattices with different structural characteristics. The crystalline structure was characterized by X-ray diffraction and transmission electron microscopy. The magnetic and electronic properties were investigated by SQUID magnetometry and resistance measurements. The results show that both strain and structural disorder can significantly affect the physical properties of the systems. Ba0.25Sr0.75TiO3 based superlattices with a low thickness of the ferromagnetic layers (4 nm.) present compressive strain that decreases the ferromagnetic transition temperature from 250 K corresponding to the unstressed samples to 230 K. In these samples, the localization energy of the charge carrier through the electron-phonon interaction decreases at low temperatures (∼100 meV). Ba0.25Sr0.75TiO3 based superlattices with thicknesses of the ferromagnetic layers higher than 12 nm present tensile strain that reduces the charge carrier localization energy at low temperatures (∼1 meV), increasing the ferromagnetic transition temperature (Tc∼265K). Structural defects in BaTiO3 based superlattices have a stronger influence on the magnetic properties than on the transport properties. Nevertheless, disorder blocks the ferromagnetic transition for highly disordered samples (thickness of the ferromagnetic layer < 3 nm). These results help to further understand the role of strain and interface effects in the magnetic and transport properties of manganite based multiferroic systems.
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