arXiv subjects

I. Felner

Publications and source records attributed to I. Felner.

At least 19 recordsLinked to original sources

arXiv2024

Anisotropy in Electronic and Magneto-transport of 2D superconductor NbSe$_2$

This article reports the successful synthesis of single crystalline two-dimensional thin flakes of NbSe$_2$. The XRD pattern of the grown crystal ensured its crystallization in a single phase with a hexagonal structure. The EDAX endorsed the stoichiometry of the as grown sample. To study the vibrational modes, the Raman spectra were recorded, which exhibited the expected four Raman active modes. The resistance vs temperature measurement showed a well-established superconducting transition (Tc) at 7.3 K. The ZFC (Zero-Field Cooled) & FC (Field Cooled) magnetization curves, as well as the isothermal MH (Magnetization vs. field) measurements, have been performed for both in plane and out-of-plane H directions. Distinct anisotropy is observed in both magnetization and magneto-transport measurements with field direction, leading to different critical fields (Hc). Out-of-plane magneto-transport data hints towards the existence of a filamentary state. The density functional theory (DFT) has been used to study the band structure of NbSe$_2$. Although the bulk band structure confirmed metallic behavior, the same for mono-layers of NbSe$_2$ within the GGA+U framework showed a band gap of 1.17eV. The article addresses the anisotropy in the electronic and magneto-transport of 2D superconductor NbSe$_2$.

cond-mat.supr-con↗

arXiv2021

Effect of magnetic (Nd) doping on electrical and magnetic properties of topological Sb2Te3 single crystal

Here, we report the growth and characterization of single crystals of NdxSb2-xTe3, by solid state reaction route via self-flux method. The phase and layered growth are confirmed through x-ray diffraction and Scanning electron microscopy respectively. A slight contraction in lattice parameters is seen after Nd doping. Also a minute shift in vibrational modes of recorded Raman spectra has been observed by doping of Nd in Sb2Te3. The magneto-resistance values under magnetic field of 5Tesla for Sb2Te3 are 75 percent at 2.5K and 60 percent at 20K, but only 40 percent at 5K for Nd0.1Sb1.9Te3. DC magnetic measurements exhibit expected diamagnetic and paramagnetic behaviors for pure and Nd doped crystals respectively. A cusp-like behavior is observed in magneto conductivity of both pure and Nd doped crystals at low magnetic fields below 1 Tesla which is analyzed using Hikami Larkin Nagaoka (HLN) model. For Sb2Te3 the fitted parameters alpha values are -1.02 and -0.58 and the phase coherence lengths are 50.8(6)nm & 34.9(8)nm at temperatures 2.5K and 20K respectively. For Nd0.1Sb1.9Te3, alpha is -0.29 and coherence length is 27.2(1) nm at 5K. The {\alpha} values clearly show the presence of weak anti localization effect in both, pure and Nd doped samples. Also with Nd doping, the contribution of bulk states increases in addition to conducting surface states in overall conduction mechanism.

cond-mat.mtrl-sci↗

arXiv2021

AC losses in macroscopic thin-walled superconducting cylinders

Measurements of the ac response represent a valuable method for probing the properties of superconductors. In the surface superconducting state (SSS), a current exceeding the surface critical current $I_\mathrm{c}$ leads to breakdown of SSS and penetration of external magnetic field into the sample bulk. An interesting free-of-bulk system in SSS is offered by thin-walled cylinders. According to the full penetration of magnetic flux (FPMF) model, each time the instant value of an ac field is equal to a certain critical value, the ac susceptibility $\chi$ will exhibit \emph{jumps} as a function of the ac field amplitude $H_\mathrm{ac}$ because of the periodic destruction and restoration of SSS in the cylinder wall. Here we study the low-frequency (128-8192\,Hz) ac response of thin-walled niobium cylinders under superimposed dc and ac magnetic fields applied parallel to the cylinder axis. In contrast to the FPMF model predictions, experiments reveal a \emph{smooth} $\chi(H_\mathrm{ac})$ dependence. To explain the experimental observations, we propose a phenomenological \emph{partial} penetration magnetic flux (PPMF) model, which assumes that after restoration of the superconducting state, the magnetic fields inside and outside the cylinder are unequal and the magnitude of the penetrating flux is random for every penetration. This model fits very well the experimental data on the temperature dependence of the first harmonic $\chi_1$ for any dc field and ac amplitude.

cond-mat.supr-con↗

arXiv2020

Crystal Growth and characterization of possible New Magnetic Topological Insulators FeBi2Te4

Here we report successful single crystal growth of new possible magnetic topological insulator (MTI) FeBi2Te4 by self-flux method via vacuum encapsulation process. The detailed Rietveld analysis of Powder XRD data shows the as grown MTI crystal to be mainly dominated by FeBi2Te4 phase along with minority phases of Bi2Te3 and FeTe. Scanning electron microscope (SEM) image shows the morphology of as grown MTI single crystal to be of layered type laminar structure. Raman spectroscopy of the crystal exhibited three distinct phonon modes at 65, 110, and 132 cm-1 along with two split secondary modes at 90, and 144cm-1. The secondary split modes are result of FeTe intercalation in Bi2Te3 unit cell. Magneto-resistance measurement has been performed at different temperatures i.e. 200K, 20K and 2K in applied magnetic fields up to 12 Tesla, which showed very low MR in comparison to pure Bi2Te3 crystal. Temperature dependence of DC magnetization measurements show the FeBi2Te4 crystal to be mainly of ferromagnetic (FM) or ferri-magnetic nature above 295 K, albeit a secondary weak magnetic transition is seen at 54-46K as well. Detailed isothermal magnetization (MH) results showed that FM saturation moment at 295K is 0.00213emu/g, which is nearly invariant till 400 K. Summary, we had grown an MTI FeBi2Te4 single crystal, which may be a possible entrant for Quantum Anomalous Hall (QAH) effect at room temperature or above.

cond-mat.mtrl-sci↗

arXiv2019

On the magnetic phase diagram of a sigma-phase Fe47Re53 compound in the H-T plane

Sigma-phase Fe47Re53 compound was investigated by means of in-field DC and AC magnetization measurements. Field cooled, MFC, and zero-field cooled, MZFC, DC magnetization curves were recorded in the magnetic field, H, up to 1000 Oe. AC magnetization measurements were performed at the constant frequency of 1465 Hz in the field up to H=500 Oe. Both types of measurements gave evidence in favor of a re-entrant character of magnetism in the studied sample. Based on characteristic temperatures determined from the measurements magnetic phase diagrams have been outlined in the H-T plane. The diagrams are similar but not identical. The main difference is that in the diagram constructed based on the DC data there is an intermediate antiferromagnetic-like phase which is absent in the AC diagram. Furthermore, the border lines (irreversibility, cross-over and Curie temperature) follow the power law but with correspondingly different exponents. The studied sample was also characterized by determining the effect of H on the degree of irreversibility and that of frustration.

cond-mat.str-el↗

arXiv2018

Testing the influence of acceleration on time dilation using a rotating M\"Ossbauer absorber

The aim of the experiment series was to test the influence of acceleration on time dilation by measuring the relative spectral shift between the resonance spectra of a rotating Mossbauer absorber with acceleration anti-parallel and parallel to the direction of the incident beam. Based on the experiences and know-how acquired in our previous experiments, We collected data for rotation frequencies up to 510Hz in both directions of rotation and also used different slits. For each run with high rotation, we observed a stable statistically significant relative shift between the spectra of the two states with opposite acceleration. This indicates the influence of acceleration on time dilation. However, we found that this shift also depends on the choice of the slit, and on the direction of rotation. These new unexpected findings, resulting from the loss of symmetry in obtaining the resonant lines in the two states, could overshadow the relative shift due to acceleration. This loss of the symmetry is caused by the deflection of the radiative decay due to the Nuclear Lighthouse effect from the rotating Mossbauer absorber. We also found that it is impossible to keep the alignment (between the optical and the dynamical rotor systems) with accuracy needed for such experiment, for long runs, which resulted in the reduction of the accuracy of the observed relative shift. These prevent us to claim with certainty the influence of acceleration on time dilation using the currently available technology. An improved KB optics with focal spot of less than 1 micron to avoid the use of a slit and a more rigid mounting of the rotor system, are necessary for the success of such experiment. Hopefully, these findings together with the indispensable plan for a conclusive experiment presented in the paper, will prove useful to future experimentalists wishing to pursue such an experiment.

physics.acc-ph↗

arXiv2016

Nearly ferromagnetic metal state in the collapsed tetragonal phase of YFe$_2$(Ge,Si)$_2$

The surprising discovery of tripling the superconducting critical temperature of KFe$_2$As$_2$ at high pressures issued an intriguing question of how the superconductivity in the collapsed tetragonal phase differs from that in the non-collapsed phases of Fe-based superconductors. Here we report $^{89}$Y nuclear magnetic resonance study of YFe$_2$Ge$_{x}$Si$_{2-x}$ compounds whose electronic structure is similar to that of iron-pnictide collapsed tetragonal phases already at ambient pressure. Fe(Ge,Si) layers show strong ferromagnetic spin fluctuations whereas layers are coupled antiferromagnetically -- both positioning the studied family close to a quantum critical point. Next, localized moments attributed either to Fe interstitial or antisite defects may account for magnetic impurity pair-breaking effects thus explaining the substantial variation of superconductivity among different YFe$_2$Ge$_2$ samples.

cond-mat.str-el↗

arXiv2016

Giant flux jumps through a thin superconducting Nb film

The dynamics of magnetic field penetration into thin-walled superconducting niobium cylinders is experimentally investigated. It is shown that magnetic field penetrates through the wall of a cylinder in a series of giant jumps with amplitude 10 - 20 Oe and duration of a few $\mu$s. The jumps take place when the total current in the wall, not the current density, exceeds some critical value. In addition there are small jumps and/or smooth penetration, and their contribution can reach 20% of the total penetrating flux. It is demonstrated that the magnetic field inside the cylinder exhibits several oscillations. The number of giant jumps reduces with temperature.

cond-mat.supr-con↗

arXiv2015

dc and ac magnetic properties of thin-walled Nb cylinders with and without a row of antidots

dc and ac magnetic properties of two thin-walled superconducting Nb cylinders with a rectangular cross-section are reported. Magnetization curves and the ac response were studied on as-prepared and patterned samples in magnetic fields parallel to the cylinder axis. A row of micron-sized antidots (holes) was made in the film along the cylinder axis. Avalanche-like jumps of the magnetization are observed for both samples at low temperatures for magnetic fields not only above $H_{c1}$, but in fields lower than $H_{c1}$ in the vortex-free region. The positions of the jumps are not reproducible and they change from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than $H_{c1}$. At temperatures above $0.66T_c$ and $0.78T_c$ the magnetization curves become smooth for the patterned and the as-prepared samples, respectively. The magnetization curve of a reference planar Nb film in the parallel field geometry does not exhibit jumps in the entire range of accessible temperatures. The ac response was measured in constant and swept dc magnetic field modes. Experiment shows that ac losses at low magnetic fields in a swept field mode are smaller for the patterned sample. For both samples the shapes of the field dependences of losses and the amplitude of the third harmonic are the same in constant and swept field near $H_{c3}$. This similarity does not exist at low fields in a swept mode.

cond-mat.supr-con↗

arXiv2015

On the magnetism of sigma-Fe54Cr46 alloy: AC and DC susceptibility studies

Sigma-phase intermetallic compound of Fe54Cr46 was investigated using DC and AC magnetic susceptibility techniques. A clear-cut evidence was found that the sample orders magnetically at Tc=23.5 K and its ground magnetic state is constituted by a spin glass. The temperature at which the zero-field cooled magnetization has its maximum decreases with an external magnetic field in line with the Gabay-Toulouse prediction. The temperature at which the AC magnetic susceptibility has its maximum does not depend on frequency which, in the light of the mean-field theory, testifies to very long magnetic interactions.

cond-mat.mtrl-sci↗

arXiv2014

Anomalous Eu Valence State and Superconductivity in Undoped Eu3Bi2S4F4

We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampoules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) A, c = 32.4330(6) A, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers stack alternately along the crystallographic c axis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond-valence-sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, whilst Eu(2) has an average valence of +2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mossbauer and magnetization measurements. The Eu3+ components donate electrons into the conduction bands that are mainly composed of Bi- 6px and 6py states. Consequently, the material itself shows metallic conduction, and superconducts at 1.5 K without extrinsic chemical doping.

cond-mat.supr-con↗

arXiv2014

Charge-density wave, superconductivity and $f$-electron valence instability in EuBiS$_2$F

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS$_2$F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6$p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ~ 50 times, owing to the significant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verified by band-structure calculations. Thus, EuBiS$_2$F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and $f$-electron valence instability.

cond-mat.supr-con↗

arXiv2013

Magnetic Properties of RFe2Si2 and R(Fe1-xMx)2Si2 Systems (R=La, Y and Lu, M= Ni, Mn and Cu)

Due to the similarity to BaFe2As2 and SrFe2As2 the RFe2Si2 (R=La, Y and Lu) system has been proposed as a potential candidate for a new superconducting family containing Fe-Si layers as a structural unit. Various R(Fe1-xMx)2Si2 M=Ni, Mn and Cu) materials were synthesized and measured for their magnetic properties. None of these materials is superconducting down to 5 K. Fe in RFe2Si2 is paramagnetic. A pronounced peak at 232 K was observed in the magnetization curve of YFe2Si2. 57Fe M\"ossbauer studies confirm the absence of any magnetic ordering at low temperatures. Similar peaks at various temperatures also appear in R(Fe1-xMx)2Si2 samples. Four independent factors affect the peak position and shift it to lower temperatures: (i) the lattice parameters, (ii) the concentration of x, (iii) the applied magnetic field, and (iv) the magnetic nature of M. The peak position is dramatically affected by the magnetic Mn dopants. It is propose that the magnetic peaks observed in RFe2Si2 and in R(Fe1-xMx)2Si2 represent a new nearly ferromagnetic Fermi liquid (NFFL) system and their nature is yet to be determined.

cond-mat.supr-con↗

arXiv2013

Superconductivity in Sulfur-Doped Amorphous Carbon Films

Following our previous investigations on superconductivity in amorphous carbon (aC) based systems; we have prepared thin composite aC-W films using electron-beam induced deposition. The films did not show any sign for superconductivity above 5 K. However, local, non-percolative, superconductivity emerged at Tc = 34.4 K after treatment with sulfur at 250 C for 24 hours. The superconducting features in the magnetization curves were by far sharper compared to our previous results, and the shielding fraction increased by about an order of magnitude. Our data suggest that pairing and localized superconductivity take place in the aC-S regions, whereas phase coherence, assisted by the W inclusions, was enhanced compared to our previous samples, yet still not to the degree of achieving global phase-coherence and percolating superconductivity.

cond-mat.supr-con↗

arXiv2012

Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single Crystals

Optimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured. The H-T phase diagram is presented. Close to TC =35 K, for the c-crystal, {\gamma}c = / = 1.80(2), whereas for the a-crystal the {\gamma}a = / =4.0(2) obtained, is much higher than the theoretical value 1.69. At low applied dc fields, positive field-cooled branches known as the "paramagnetic Meissner effect" (PME) are observed, their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high {\gamma}a and the PME are connected to each other.

cond-mat.supr-con↗

arXiv2012

Role of Carbon in Enhancing the Performance of MgB2 superconductor

The enhancement of the critical current density (Jc(H)) of carbon and nano-SiC doped MgB2 is presented and compared. The upper critical field (Hc2) being determined from resistivity under magnetic field experiments is though improved for both C substitution and nano-SiC addition the same is more pronounced for the former. In MgB2-xCx carbon is substituted for boron that induces disorder in the boron network and acts as internal pinning centres. The optimal Jc(H) values are obtained for x = 0.1 sample . In case of nano-SiC doped in MgB2, the Jc(H) improves more profoundly and two simultaneous mechanisms seems responsible to this enhancement. Highly reactive nano-SiC releases free carbon atom, which gets easily incorporated into the MgB2 lattice to act as intrinsic pinning centres. Further enhancement is observed for higher nano-SiC concentrations, where the un-reacted components serve as additional extrinsic pinning centres.

cond-mat.supr-con↗

arXiv2011

Superconducting critical fields of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal

We report the results of an experimental study of dc and low frequencies magnetic properties of K$_{0.8}$Fe$_{2}$Se$_2$ single crystal when the dc magnetic field is applied parallel to the $\bf{ab}$ plane. From the data obtained, we deduce the full H-T phase diagram which consists of all three H$_{c1}$(T), H$_{c2}(T)$ and H$_{c3}(T)$ critical magnetic field plots. The two H$_{c1}$(T) and H$_{c2}$(T) curves were obtained from dc magnetic measurements, whereas the surface critical field H$_{c3}$(T) line was extracted by ac susceptibility studies. It appears that near T$_c$, the H$_{c3}$(T)/H$_{c2}$(T) ratio is $\approx 4.4$ which is much larger than expected.

cond-mat.supr-con↗

arXiv2010

Magnetic ordering and dense Kondo behaviour in EuFe$_{2}$P$_{2}$

Ternary iron phosphide EuFe$_2$P$_2$ with ThCr$_2$Si$_2$-type structure has been systematically studied by the measurements of crystal structure, magnetization, Mössbauer effect, transport properties and specific heat. The structural refinement result confirms no direct P-P covalent bonding. The Mössbauer spectra indicate no magnetic moment for the Fe atoms and, that the Eu ions are divalent in the whole temperatures, carrying local moments of $S$=7/2. The Eu$^{2+}$ spins order ferromagnetically at $T_C$=29.5 K, followed by a possible helimagnetic ordering below $T_{HM}$=26 K, where the Eu$^{2+}$ moments tilt a little from the c-axis. External magnetic field increases $T_C$ gradually, but suppresses $T_{HM}$ rapidly. (Magneto)resistivity data indicate characteristic dense Kondo behaviour above the Curie temperature. The result is discussed in terms of the interplay between intersite RKKY and intrasite Kondo interactions.

cond-mat.str-el↗