arXiv subjects

Y. Paltiel

Publications and source records attributed to Y. Paltiel.

13 recordsLinked to original sources

arXiv2022

Cooperative effect of electrons spin polarization in a hybrid nanostructure of a magnetic thin film with adsorbed chiral molecules studied with non-spin-polarized scanning tunneling microscopy

Polyalanine molecules (PA) with an α-helix conformation gathered recently a lot of interest as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy at ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al2O3/Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the was observed. The highest spin polarization values, P, were found for well-ordered self-assembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the helical molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap is resulting in a higher electron mobility yielding a higher P value. By switching the magnetization direction of the Co-layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a sub-molecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change of the magnetoresistance of such hybrid structures.

cond-mat.mes-hall↗

arXiv2016

Unconventional superconductivity induced in Nb films by adsorbed chiral molecules

Motivated by recent observations of chiral-induced magnetization and spin-selective transport we studied the effect of chiral molecules on conventional BCS superconductors. By applying scanning tunneling spectroscopy, we demonstrate that the singlet-pairing s-wave order parameter of Nb is significantly altered upon adsorption of chiral polyalanine alpha-helix molecules on its surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of unconventional triplet-pairing components with either d-wave or p-wave symmetry, as corroborated by simulations. These results may open a way for realizing simple superconducting spintroinics devices.

cond-mat.supr-con↗

arXiv2009

Collective effects in charge transfer within a hybrid organic-inorganic system

A collective electron transfer (ET) process was discovered by studying the current noise in a field effect transistor with light-sensitive gate formed by nanocrystals linked by organic molecules to its surface. Fluctuations in the ET through the organic linker are reflected in the fluctuations of the transistor conductivity. The current noise has an avalanche character. Critical exponents obtained from the noise power spectra, avalanche distributions, and the dependence of the average avalanche size on avalanche duration are consistent with each other. A plausible model is proposed for this phenomenon

cond-mat.mes-hall↗

arXiv2002

Vortex avalanches and self organized criticality in superconducting niobium

In 1993 Tang proposed [1] that vortex avalanches should produce a self organized critical state in superconductors, but conclusive evidence for this has heretofore been lacking. In the present paper, we report extensive micro-Hall probe data from the vortex dynamics in superconducting niobium, where a broad distribution of avalanche sizes scaling as a power-law for more than two decades is found. The measurements are combined with magneto-optical imaging, and show that over a widely varying magnetic landscape the scaling behaviour does not change, hence establishing that the dynamics of superconducting vortices is a SOC phenomenon.

cond-mat.supr-con↗

arXiv2002

V-I characteristics in the vicinity of order-disorder transition in vortex matter

The shape of the V-I characteristics leading to a peak in the differential resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2 is investigated. r_d is large when measured by dc current. However, for a small Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and displays a large out-of-phase signal. In contrast, the ac response increases with frequency in the absence of dc bias. These surprisingly opposite phenomena and the peak in r_d are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning.

cond-mat.supr-con↗

arXiv2002

Dynamic Creation and Annihilation of Metastable Vortex Phase as a Source of Excess Noise

The large increase in voltage noise, commonly observed in the vicinity of the peak-effect in superconductors, is ascribed to a novel noise mechanism. A strongly pinned metastable disordered vortex phase, which is randomly generated at the edges and annealed into ordered phase in the bulk, causes large fluctuations in the integrated critical current of the sample. The excess noise due to this dynamic admixture of two distinct phases is found to display pronounced reentrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise disappears

cond-mat.supr-con↗

arXiv2001

Edge and Bulk Transport in the Mixed State of a Type-II Superconductor

By comparing the voltage-current (V-I) curves obtained before and after cutting a sample of 2H-NbSe2, we separate the bulk and edge contributions to the transport current at various dissipation levels and derive their respective V- I curves and critical currents. We find that the edge contribution is thermally activated across a current dependent surface barrier. By contrast the bulk V-I curves are linear, as expected from the free flux flow model. The relative importance of bulk and edge contributions is found to depend on dissipation level and sample dimensions. We further show that the peak effect is a sharp bulk phenomenon and that it is broadened by the edge contribution.

cond-mat.supr-con↗

arXiv2001

On Ground State Tunneling in Mn$_{12}$-Acetate

We report Hall sensor measurements of the magnetic relaxation of Mn$_{12}$-acetate as a function of magnetic field applied along the easy axis of magnetization for a series of closely-spaced temperatures between 0.24 K and 1.9 K. We map out a region in the $H-T$ plane where ground state tunneling is observed, a region where tunneling proceeds via thermally-assisted tunneling, and a crossover region where both participate in the relaxation. We observe the occasional absence of ground-state tunneling under conditions where one would expect it to be present, and suggest a resolution to the enigma.

cond-mat↗

arXiv2001

A Distribution of Tunnel Splittings in Mn$_{12}$-Acetate

In magnetic fields applied parallel to the anisotropy axis, the relaxation of the magnetization of Mn$_{12}$ measured for different sweep rates is shown to collapse onto a single scaled curve. The form of the scaling implies that the dominant symmetry-breaking process that gives rise to tunneling is a locally varying second-order anisotropy, forbidden by tetragonal symmetry in the perfect crystal, which gives rise to a broad distribution of tunnel splittings in a real crystal of Mn$_{12}$-acetate. Different forms applied to even and odd-numbered steps provide a distinction between even step resonances (associated with crystal anisotropy) and odd resonances (which require a transverse component of magnetic field).

cond-mat↗

arXiv2000

Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet

We report Hall sensor measurements of the magnetic relaxation of Mn$_{12}$ acetate as a function of magnetic field applied along the easy axis of magnetization. Data taken at a series of closely-spaced temperatures between 0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order'' transition between thermally-assisted relaxation and magnetic decay via quantum tunneling.

cond-mat.mes-hall↗

arXiv2000

Instabilities and disorder-driven first-order transition of the vortex lattice

Transport studies in a Corbino disk geometry suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows a sharp reentrant behavior at low fields. In contrast, in the conventional strip configuration, the phase transition is obscured by the injection of the disordered vortices through the sample edges, which results in the commonly observed vortex instabilities and smearing of the peak effect in NbSe2 crystals. These features are found to be absent in the Corbino geometry, in which the circulating vortices do not cross the sample edges.

cond-mat.supr-con↗

arXiv1999

Dynamic instabilities and memory effects in vortex matter

Recent studies have shown a number of surprising vortex dynamics phenomena both in low and high temperature superconductors, which include: low frequency noise, slow voltage oscillations, history dependent dynamic response, memory of the direction, amplitude, duration, and frequency of the previously applied current, suppression of a large ac vortex response by a very small dc bias, and a strong frequency dependence. Taken together, these phenomena are incompatible with the current understanding of bulk vortex dynamics. We propose a generic mechanism to account for these observations in terms of the competition between the injection of a disordered vortex phase through the surface barriers at the sample edges, and the annealing of this metastable disorder by the transport current. The model is confirmed by investigating the current distribution across NbSe2 single crystals using arrays of Hall sensors. For an ac current only narrow regions near the edges are in the pinned disordered phase resulting in a large response. In the presence of a dc bias a wide region of the sample is filled by the disordered phase preventing vortex motion. The resulting spatial variation of the disorder across the sample acts as an active memory of the previously applied current sequence.

cond-mat.supr-con↗

arXiv1998

Surface barrier dominated transport in NbSe$_2$

Transport current distribution in clean 2$H$-NbSe$_{2}$ crystals is studied by measuring the self-induced magnetic field across the sample. Below $T_c$ most of the current flows at the edges of the crystals due to strong surface barriers, which are found to dominate the transport properties and the resistive transition. The measured critical current is determined by the critical current for vortex penetration through the surface barrier rather than by bulk pinning.

cond-mat.supr-con↗