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We studied the partial substitution of Ag in the noncentrosymetric superconducting compounds Li$_2$Pd$_{3-x}$Ag$_x$B with x=0.0, 0.1 and 0.3. Magnetization, resistivity and specific heat measurements were performed in this system. From electrical measurements we obtained the critical temperature $T_c$. The upper critical field at zero temperature was determined with Werthamer-Helfand equation, $\mu_0H^{WHH}_{c2}(0)$, and $\mu_0H^{linear}_{c2}(0)$ and the coherence length, $\xi(0)$. The Value of $\mu_0H^{linear}_{c2}(0)$ and $\mu_0H^{WHH}_{c2}(0)$ is lower than the calculated paramagnetic Pauli limit suggesting that the spin-triplet pairing is weak in the system. From specific heat measurements we obtained the parameters of the normal state; the Sommerfeld coefficient, $\gamma$, the electronic density of states at the Fermi level, $N(E_F)$ and the Debye temperature, $\theta_D$. In the superconducting state the obtained values were; $\Delta C/\gamma T_c$, the electron phonon coupling, $\lambda_{e-ph}$, the superconducting energy gap, $2\Delta_0$, and the ratio $2\Delta_0 /k_BT_c$. The electronic specific heat is well described by the BCS theory, suggesting that the energy gap is isotropic and in a strong coupling state., Comment: 7 pages

A reentrant superconducting effect was observed in YBCO particles as a consequence of their size and microstructure. It rises from the coexistence between bulk superconductivity and surface ferromagnetism, where the enhanced magneto-electric coupling induced by broken spatial-inversion symmetry in layers near structural defects plays a very important role on surface ferromagnetism. The study assumes grain boundaries inside the particles act as additional surface regions that enhance the ferromagnetic properties. Magnetic and superconducting properties as a function of particle size were studied. Particularly, the 385 nm particles which were the ones that exhibited the reentrant superconducting effect. The Ginzburg-Landau parameter and the critical temperatures are $k \sim 1.29$, T$_{c1}$ = 92 $\pm$ 5 K, and T$_{c2}$ = 10 $\pm$ 3 K., Comment: 8 pages, 6 figures. Journal of Materials Science: Materials in Electronics (2020)

In this study, a thenoyltriflouroacetone ligand (TTA) with a Tb3+ MOF was synthesized (Tb=10 and 50% mol) and its structural and luminescent properties were analyzed. The metalorganic compound was generated in a simple one-pot reaction from terbium nitrate and 2-thenoyltrifluoroacetone precursors at room temperature. By means of FTIR, it was confirmed the presence of carbon groups, which made possible the terbium ion chelation, and also the Tb-O bonds vibration modes. HNMR results confirm that the complex with 10% mol of Tb3+ contains three coordinates molecules of TTA and two waters molecules. The powders exhibit rod-like morphology with size about 170 nm of diameter and a length about 2 {\mu}m, the rod-like nature of powders was confirmed by SEM and TEM analyses. By XRD it was concluded that at higher terbium concentration (TTA-50Tb sample) higher the crystallite size and crystallinity, in fact the TTA-10Tb sample shows a partial-amorphous nature. By photoluminescence analyses, the 5D4-7FJ (J=3, 4, 5 and 6) emissions were recorded for both synthesized samples ({\lambda}exc=376 nm). Furthermore, it was observed that the emission intensity was enhanced in a factor of 3.5 for the TTA-50Tb. The energy transfer efficiency from TTA to Tb3+ (antenna effect) was 0.984 for TTA-10Tb and 0.993 for TTA-50Tb. Decay time analyses indicate effective lifetime of 1.45 and 1.60ms for the samples doped at 10 and 50%, respectively, indicating that the forbidden transition rules are stronger at higher crystallinity. The integrated intensities of the 5D4 - 7F5 (green at 541 nm) and 5D4 - 7F6 (blue at 486 nm) emissions and their intensity ratios IG/IB upon 376 nm excitation have been evaluated for TTA-10Tb andTTA-50Tb samples., Comment: Optical Materials (2018)

Crystal structure, thermal and magnetic properties were systematically studied in the Y$_{1-x}$Pr$_x$CrO$_3$ with $0 \leq x \leq 0.3$ compositions. Magnetic susceptibility and specific heat measurements show an increase of the antiferromagnetic transition temperature ($T_N$) as Pr is substituted in the Y sites and notable magnetic features are observed below $T_N$. Strong coupling between magnetic and crystalline parameters is observed in a small range of Pr compositions. A small perturbation in the lattice parameters by Pr ion is sufficient to induce a spin reorientation transition followed by magnetization reversal, to finally induce exchange bias effect. The spin reorientation temperature ($T_{SR}$) is increased from 35 K to 149 K for $0.025 \leq x \leq 0.1$ compositions. It is found that the Cr spins sublattice rotates continuously from $T_{SR}$ to a new spin configuration a lower temperature. In addition, magnetization reversal is observed at $T^* \sim 35$ K for x= 0.05 up to $T^* \sim 63$ K for x = 0.20 composition. The $M-H$ curves show negative exchange bias effect induced by Pr ions, which are observed below of 100 K and being more intense at 5 K. At 10 K, the magnetic contribution of the specific heat, as well as the ZFC magnetization, show the rise of a peak with increasing Pr content. The magnetic anomaly could be associated with the freezing of the Pr magnetic moment randomly distributed at the 4c crystallographic site. A clear correspondence between spin reorientation, magnetization reversal and exchange bias anisotropy with the tilting and octahedral distortion is also discussed., Comment: 14 pages, 12 figures, to be published in Physical Review Materials

We present a study of reentrant behavior in a composite formed by a Hight-T$ _{c} $ superconductor, YBa$_2$Cu$_3$O$ _{7-\delta}$ and Ammonium Terbium Oxalate, Tb(H$_2$O)(C$ _2$O$_4$)$_2$ $\cdot $NH$_4$. The composite has a transition temperature about 92 K, and it presents a reentrant behaviour resulting of the coexistence between superconductivity and magnetism. According to this study the values and shape of the critical magnetic fields were dramatically reduced in a similar form as in other known reentrant superconductors., Comment: Submitted to a journal

We present an investigation on the intermetallic compound formed by HoCuBi which has a monoclinic crystalline P2 structure with lattice parameters; a = 9:8012(26)A, b = 6:0647(6)A, c = 6:1663(13)A. In this report we performed an extensive analysis of the magnetic characteristics, specific heat at low temperature mainly close to the region of the observed antiferromagnetic transition with Neel temperature about 7 K, and to the field induced spin-crossover; the metamagnetic transition typically observed in antiferromagnetic systems by measurements of M-H, (Magnetization - Magnetic Field) at low temperatures., Comment: Accepted in Journal of Superconductivity and Novel Magnetism

ZrSe2 is a band semiconductor studied long time ago. It has interesting electronic properties, and because its layers structure can be intercalated with different atoms to change some of the physical properties. In this investigation we found that Zr deficiencies alter the semiconducting behavior and the compound can be turned into a superconductor. In this paper we report our studies related to this discovery. The decreasing of the number of Zr atoms in small proportion according to the formula ZrxSe2, where x is varied from about 8.1 to 8.6 K, changing the semiconducting behavior to a superconductor with transition temperatures ranging between 7.8 to 8.5 K, it depending of the deficiencies. Outside of those ranges the compound behaves as semiconducting with the properties already known. In our experiments we found that this new superconductor has only a very small fraction of superconducting material determined by magnetic measurements with applied magnetic field of 10 Oe. Our conclusions is that superconductivity is filamentary. However, in one studied sample the fraction was about 10.2 %, whereas in others is only about 1 % or less. We determined the superconducting characteristics; the critical fields that indicate a type two superonductor with Ginzburg-Landau ? parameter of the order about 2.7. The synthesis procedure is quite normal fol- lowing the conventional solid state reaction. In this paper are included, the electronic characteristics, transition temperature, and evolution with temperature of the critical fields., Comment: Accepted in J of superconductivity and novel magnetism

According to theoretical predictions the quasicrystalline (QC) electronic density of states (DOS) must have a rich and fine spiky structure which actually has resulted elusive. The problem with its absence may be related to poor structural characteristics of the studied specimens, and/or to the non-existence of this spike characteristic. Recent calculations have shown that the fine structure indeed exists, but only for two dimensional approximants phases. The aim of the present study is to show our recent experimental studies with point contacts tunnel junction spectroscopy performed in samples of very high quality. The studies were performed in icosahedral QC alloys with composition Tb$_9$Mg$_{35}$Zn$_{56}$. We found the presence of a pseudogap feature at the Fermi level, small as compared to the pseudogap of other icosahedral materials. This study made in different spots on the QC shows quite different spectroscopic features, where the observed DOS was a fine non-spiky structure, distinct to theoretical predictions. In some regions of the specimens the spectroscopic features could be related to Kondo characteristics due to Tb magnetic atoms acting as impurities. Additionally, we observed that the spectroscopic features vanished under magnetic field., Comment: 9 pages, 8 figures, accepted in Journal of Non-Crystalline Solids

Some topological insulators become superconducting when doped with Cu and Pd. Superconductivity in a non-superconductor may be induced by proximity effect: i.e. Contacting a non-superconductor with a superconductor. The superconducting macroscopic wave function will induce electronic pairing into the normal compound. In the simplest topological insulator, Bi$_2$Se$_3$, superconductivity may be induced with Pb. We studied with point contact junctions formed by contacting Bi$_2$Se$_3$ crystals and Pb, glued and pressed with silver paste and/or applying low heat to the Pb to improve the contact. Junctions were formed with a thin tungsten plated gold W(Au) wire as one electrode, and the other $Bi_2Se_3$ and Pb. We study the characteristics of the electron coupling; the transition temperature,$T_C$, evolution with temperature of the energy gap, $\Delta$, and 2$\Delta$/K$_B$T$_C$ ratio. The superconductor Bi$_2$Se$_3$-Pb behaves different as explained in the classical BCS theory. In BCS a superconductor is only weak or strong coupled, depending on the electronic interaction that form the electronic condensate. This differentiation is given by the size of the mentioned ratio. BCS Typical values are 3.53 to about 4.3 for weak or strong coupling limits respectively. In this study performed in Bi$_2$Se$_3$-Pb we found different values to the normal ones from 10 to 23, indicating very strong limit. Those values never have been observed in other superconductor. The transition temperatures found varies from 2.7 to 7 K. This information and other results will be presented in this paper., Comment: These are our new superconducting studies in single crystal of a topological insulator observed by proximity effect induced with Pb

We present a study of the superconducting energy gap performed with point contact spectroscopy in a single crystal of the superconductor Zr0:97V0:03B2. We determined the size and evolution with temperature of the energy gap, the transition temperature in the compound occurs at the onset of Tc = 9.3 K. The point contact junctions were measured from 1.7 to 20 K. The differential conductance as a function of bias voltage was analyzed with the Blonder-Tinkham-Klapwijk model, the evolution of the gap with temperature was compared with the BCS theory, the ratio 2{\Delta}(0)/KBTC was of the order of 8.95 indicative of a very strong coupling limit., Comment: Accepted in Journal of Superconductivity and Novel Magnetism

URu2Si2 presents superconductivity at temperatures below 1.5 K, and a hidden order (HO) at about 17.5 K, both electronic phenomena are in uenced by Fano and Kondo resonances. At 17.5 K the HO was related in the past to a Peierls distortion that produces an energy gap deformed by the resonances. This order has been studied by more than 20 years, still without a clear understanding. In this work we studied the electronic characteristics of URu2Si2 in a single crystal, with tunneling and metallic point contact spectroscopies. In the superconducting state, we determined the energy gap, which shows the in uence of the Fano and Kondo resonances. At temperatures where HO is observed, the tunnel junctions spectra show the in uence of the two resonances. Tunnel junctions characteristics show that the Fermi surface nesting depends on the crystallographic direction., Comment: Accepted in Journal of Physics: Condensed Matter

Magnetic properties of DyAl$_{0.926}$Cr$_{0.074}$O$_3$ and DyAlO$_3$ were studied. We found that both compounds are antiferromagnetic with a low N\'eel transition temperature. At higher temperatures the magnetic characteristics show a Curie-Weiss dependence. The N\'eel temperature disappears when a field of about 2 T is applied, the system changes from an antiferromagnetic to a weak ferromagnetic behavior due to a metamagnetic transition. Furthermore, AC magnetic measurements in the Cr doped compound, at different frequencies, show a spin glass-like behavior. These transitions were studied and corroborated by specific heat measurements. We found the presence of metamagnetism and spin glass in the compound doped with chromium, determining that the small addition of chromium atoms modifies the magnetic properties of the compound DyAlO$_3$, resulting in new features such as the spin glass-like behavior., Comment: 7 pages, 8 figures, accepted in Journal of Materials Science

The temperature dependence of magnetic susceptibility of the Bi$_{2-y}$Sr$_{y}$Ir$_{2}$O$_{7}$ system was studied from 2 to 300 K. According to these measurements, the solid solution $(0\leq y\leq 0.9)$ does not show any magnetic transition; however, a noticeable deviation from the Curie-Weiss law occurs and the magnetic behavior of this system can be ascribed to short-range magnetic order that is also present in several geometrically frustrated systems. The Pauli magnetic susceptibility decreases as the Sr content increases, this can be associated with a shift towards Ir$^{5+}$ in samples; also the Ir effective magnetic moment decreases. The estimated Wilson-ratio indicates the presence of strong electron-electron correlations., Comment: Accepetd in: Journal of Solid State Chemistry

Microcrystals of orthorhombic nickel (II) oxalate dihydrate were synthesized through a precipitation reaction of aqueous solutions of nickel chloride and oxalic acid. Magnetic susceptibility exhibits a sharp peak at 3.3 K and a broadrounded maximum near 43 K. We associated the lower maximum with a metamagnetic transition that occurs when the magnetic field is about \geq 3.5 T. The maximum at 43 K is the typical of 1D antiferromagnets, whereas weak ferromagnetism behavior was observed in the range of 3.3 to 43 K., Comment: Accepted in Journal of Physics: Condensed Matter

The effects of Ca substitutions on the structure, magnetism and electrical properties of YCrO3 ceramics are investigated by X-ray diffraction, magnetic susceptibility and electrical conductivity measurements. The cell volume decrease occurs through the change from Cr(III) to Cr(IV) as a result of the charge compensation of the Ca doping. No changes are observed in the antiferromagnetic transition temperature while strong changes are observed in the transport measurements due to Ca content. The increase of the electrical conductivity as well as the decrease of the activation energy are caused by the formation of the small-polarons localized in the O-Cr-O lattice distortion. The origin of small-polarons in the undoped sample is different in nature from the calcium doped. "Local non-centrosymetry" is the source of the small-polaron formation in undoped sample, while the change from Cr(III) to Cr(IV) through the charge compensation of Ca(II) in the Y(III) site is the source of small-polarons formations. The decrease of the average bond length Cr-O as well as effective moments in the paramagnetic state and the increase of the electrical conductivity are clear evidence that the Ca doping induces localized polarons, which in turn, these quasiparticles move from site to site by a thermally activated process in the doped YCrO3 compound. Here, we also discuss a possible mechanism of small-polaron injections in YCrO3 matrix., Comment: 17 pages, 6 figures, acepted in Mater. Chem. Phys

Microcrystals of diaquocobalt(II) oxalate have been synthesized by the coprecipitation reaction of aqueous solutions of Cobalt (II) bromide and oxalic acid. Chemical analysis and thermal experiments revealed that there is only one phase present. X-ray powder diffraction studies show that this compound is orthorhombic with space group Cccm. Molar susceptibility versus temperature measurements show the existence of an antiferromagnetic ordering, however, the hysteresis measured in magnetization measurements as a function of magnetic field reveals a weak ferromagnetic behavior., Comment: 7 pages, 11 figures

ZnO nanoparticles with Wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. Nanoparticles are in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization vs temperature, M(T) and isothermal measurements M(H) were determined. The coercive field clearly shows ferromagnetism above room temperature. The chemical synthesis, structural defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior. PACS numbers: 61.46.Hk Nanocrystals, 75.50.Pp Magnetic semiconductors, 81.05.Dz II-VI semiconductors

High-pressure electrical resistivity, r(T), measurements on intermetallic Pr(Co, Ni, Pt)2B2C compounds were performed down to 2K. At room pressure the r(T) in a-b direction curves for the non superconducting Pr(Co, Ni)2B2C compounds exhibit magnetic correlations at about 10 and 4 K, respectively. At low temperatures, PrCo2B2C shows a large spin-dependent electron scattering in comparison to PrNi2B2C. Under applied pressure the magnetic scattering tends to be suppressed more effectively in PrCo2B2C than in PrNi2 B2C. The low temperature behavior of r(T,P) for PrNi2B2C and PrCo2B2C suggests a spin fluctuations mechanism. In the other hand PrPt2B2C compound shows superconductivity at about 6 K and under pressure its superconducting transition temperature tends to be degraded at a rate dTc/dP = -0.34 K/GPa, as expected in compounds with transition metals. The experimental results in Co, Ni and Pt based compounds are analyzed from the point of view of the external and chemical internal pressure effects.

Synthesis, electrical and magnetic characterization of superconducting FeSe0.85 compound is reported. An anomaly in the magnetization against temperature around 90K is observed. Magnetic characterization of a commercial compound with nominal FeSe stoichiometry is also presented. The overall magnetic behaviors as well as the magnetic anomaly in both compounds are discussed in terms of magnetic impurities and secondary phases. Keyword: A. Superconductors, Comment: Submitted to Solid State Communications. 10 pages, 4 figures.

Polycrystalline samples of FeSe1-xTex (x = 0.00, 0.25, 0.50, 0.75 and 1.00) were synthesized by solid-state reaction to study the effects of substituting Se with Te in the system. The magnetization properties of the resulting compounds were investigated and the crystallographic structures of the samples analyzed through X-ray diffraction. Moessbauer spectroscopy was used to determine the ionic state of the Fe ions and the hyperfine fields. The magnetic susceptibility curves of the samples with x = 0.25, 0.50 and 0.75 show superconducting behavior. The lattice parameters and the cell volume increase monotonically with increasing Te concentration and the Moessbauer spectra reveal the absence of internal magnetic hyperfine fields., Comment: Eighteen pages, six figures, two tables

Two Single crystals of Nb3Sn presenting the martensitic anomaly at different temperature and shape, as observed with specific heat measurements, were used to study structural features in the electronic density of states with point contact spectroscopy. At high temperature below the martensitic anomaly, we observed different spectroscopic characteristics. One sample displaying a well marked specific heat peak, shows a clear defined structure in the differential conductance that evolves with temperature and may be associated with changes on the density of states due to the opening of a charge density wave gap. Those features are very depending on the crystallographics characteristics of the single crystal examined., Comment: 13 pages 6 figures. accepted in Solid State Communication

Specific heat measurements performed on high purity vapor-grown Nb$_3$Sn crystals show clear features related to both the martensitic and superconducting transitions. Our measurements indicate that the martensitic anomaly does not display hysteresis, meaning that the martensitic transition could be a weak first or a second order thermodynamic transition. Careful measurements of the two transition temperatures display an inverse correlation between both temperatures. At low temperature specific heat measurements show the existence of a single superconducting energy gap feature., Comment: Accepted in Journal of Physics: Condensed Matter

We have measured the tunnelling current in Nb/Nb$_x$O$_y$/Ni planar tunnel junctions at different temperatures. The junctions are in the intermediate transparency regime. We have extracted the current polarization of the metal/ferromagnet junction without applying a magnetic field. We have used a simple theoretical model, that provides consistent fitting parameters for the whole range of temperatures analyzed. We have also been able to gain insight into the microscopic structure of the oxide barriers of our junctions.%, Comment: 12 pages, 6 figures

The effects of Co substitution on structural and superconducting properties of RuSr2GdCu2O8 compound have been studied. Rietveld refinements of the X-ray diffraction patterns indicate that the cobalt ion progressively replaces ruthenium sites. This replacement induces significant changes on the crystal structure and on the magnetic and superconducting properties. The effects Co substitution on the superconducting behaviour, and more particulary on the changes induce by the hole doping mechanism, were investigated in (Ru1-xCox)Sr2GdCu2O8 by a "bond valence sum" analysis with Co content from x= 0.0 to x = 0.2. The weak ferromagnetic transition at Tm= 138.2 K is shifted to lower temperature, and suppressed at higher Co content. From the crystallographic point of view the Ru-O(1)-Cu bond angle, associated to the rotation of the RuO6 octahedra, around the c-axis remain essetially constant when Ru is substituted by Co. Furthermore, increasing Co content has the effect to increase the weak ferromagnetic moment, which may be interpreted as the main responsible for breaking the delicate balance between magnetic and superconducting ordering., Comment: 21 pages, 8 figures

We have measured and modeled, using three different approaches, the tunneling current in a Nb/NbxOy/Ni planar tunnel junction. The experimental data could be fitted and the correct current polarization could be extracted using a simple quasiclassical model, even in the absence of an applied magnetic field. We also discuss the microscopic structure of the barrier.

Single-walled carbon nanotubes (SWNTs) have many interesting properties; they may be metallic or semiconducting depending on their diameter and helicity of the graphene sheet. Hydrostatic or quasi-hydrostatic high pressures can probe many electronic features. Resistance - temperature measurements in SWNTs from normal condition and under 0.4 GPa of quasi-hydrostatic pressures reveal a semiconducting-like behavior. From 0.5 to about 2.0 GPa the resistance changes to a Kondo-like feature due to magnetic impurities used to catalyse the nanotube formation. Above 2.0 GPa, they become metallic and at about 2.4 GPa the resistance decreases dramatically around 3 K suggesting a superconducting transition., Comment: 14 pages, 4 figures, accepted for publication in Solid State Communications

We demonstrate that at the usual 1E-7 torr range of base pressures in the sputtering chamber, X-ray photoelectron spectroscopy shows the existence of a thin AlOx layer at the Nb/Al interface in both Nb/Al-AlOx/Pb tunnel junctions and Nb/Al bilayers. This is due to the time elapsed between the deposition of the Nb and Al bottom layers, even at times as short as 100 s. We also give some direct evidence of the oxidation of the top Pb electrode on the Nb electrode surface. Such oxidation probably occurs at the pinholes of the intermediate Al-AlOx layer of the junctions, as a consequence of the oxidation state at the Nb/Al interface. We therefore suggest that both the base pressure and the time lapse between layer depositions should be carefully controlled in magnetic tunnel junctions., Comment: 4 pages and 2 figures, submitted to APL

Interest in magnetic-tunnel junctions has prompted a re-examination of tunneling measurements through thin insulating films. In any study of metal-insulator-metal trilayers, one tries to eliminate the possibility of pinholes (small areas over which the thickness of the insulator goes to zero so that the upper and lower metals of the trilayer make direct contact). Recently, we have presented experimental evidence that ferromagnet-insulator-normal trilayers that appear from current-voltage plots to be pinhole-free may nonetheless in some cases harbor pinholes. Here, we show how pinholes may arise in a simple but realistic model of film deposition and that purely classical conduction through pinholes may mimic one aspect of tunneling, the exponential decay in current with insulating thickness., Comment: 9 pages, 3 figures, plain TeX; submitted to Journal of Applied Physics