arXiv subjects

Valeri S. Stepanyuk

Publications and source records attributed to Valeri S. Stepanyuk.

9 recordsLinked to original sources

arXiv2018

Fast spin dynamics in hexagonal arrays of Fe atoms on metallic surface

Understanding the nature of magnetic interactions in ultra-small magnetic ensembles and their intrinsic properties is vital to uncover the dynamics therein. In this study we reveal the spin dynamics of hexagonally arranged Fe atoms on metallic surface that are triggered by magnetic pulse. The switching process among various spin configurations and their relative magnetic order is tuned by the amplitude and duration of the magnetic pulse. Even more we observe a parity effect in the switching time as the size of the cluster varies in which even number of Fe atoms shows faster dynamics. The changes in the multistable magnetic states and switching times are explained by using the relaxation of the exchange and anisotropy energies in time.

cond-mat.mes-hall↗

arXiv2017

Gate control of spin-polarized conductance in alloyed transition metal nano-contacts

To date, endeavors in nanoscale spintronics are dominated by the use of single-electron or single-spin transistors having at their heart a semiconductor, metallic or molecular quantum dot who's localized states are non-spin-degenerate and can be controlled by an external bias applied via a gate electrode. Adjusting the bias of the gate one can realign those states with respect to the chemical potentials of the leads and thus tailor the spin-polarized transmission properties of the device. Here we show that similar functionality can be achieved in a purely metallic junction comprised of a metallic magnetic chains attached to metallic paramagnetic leads and biased by a gate electrode. Our ab initio calculations of electron transport through mixed Pt-Fe (Fe-Pd and Fe-Rh) atomic chains suspended between Pt (Pd and Rh) electrodes show that spin-polarized confined states of the chain can be shifted by the gate bias causing a change in the relative contributions of majority and minority channels to the nano-contact's conductance. As a result, we observe strong dependence of conductance spin-polarization on the applied gate potential. In some cases the spin-polarization of conductance can even be reversed in sign upon gate potential application, which is a remarkable and promising trait for spintronic applications.

cond-mat.mes-hall↗

arXiv2016

Potential Energy Driven Spin Manipulation via a Controllable Hydrogen Ligand

Spin-bearing molecules can be stabilized on surfaces and in junctions with desirable properties such as a net spin that can be adjusted by external stimuli. Using scanning probes, initial and final spin states can be deduced from topographic or spectroscopic data, but how the system transitioned between these states is largely unknown. Here we address this question by manipulating the total spin of magnetic cobalt hydride complexes on a corrugated boron nitride surface with a hydrogen- functionalized scanning probe tip by simultaneously tracking force and conductance. When the additional hydrogen ligand is brought close to the cobalt monohydride, switching between a corre- lated S = 1 /2 Kondo state, where host electrons screen the magnetic moment, and a S = 1 state with magnetocrystalline anisotropy is observed. We show that the total spin changes when the system is transferred onto a new potential energy surface defined by the position of the hydrogen in the junction. These results show how and why chemically functionalized tips are an effective tool to manipulate adatoms and molecules, and a promising new method to selectively tune spin systems.

cond-mat.mes-hall↗

arXiv2015

Direct surface charging and alkali-metal doping for tuning the interlayer magnetic order in planar nanostructures

The continuous reduction of magnetic units to ultra small length scales inspires efforts to look for a suitable means of controlling magnetic states. In this study we show two surface charge alteration techniques for tuning the interlayer exchange coupling (IEC) of ferromagnetic layers separated by paramagnetic spacers. Our study reveals that already a modest amount of extra charge can switch the mutual alignment of the magnetization from anti-ferromagnetic to ferromagnetic or vice verse. We also propose adsorption of alkali metals as an alternative way of varying the electronic and chemical properties of magnetic surfaces. Clear evidence is found that the interlayer magnetic order can be reversed by adsorbing alkali metals on the magnetic layer. Moreover, alkali metal overlayers strongly enhance the perpendicular magnetic anisotropy in FePt thin films. These findings combined with atomistic spin model calculations suggest that electronic or ionic way of surface charging can have a crucial role for magnetic hardening and spin state control.

cond-mat.mes-hall↗

arXiv2015

Ultrafast quantum spin-state switching in the Co-octaethylporphyrin molecular magnet with a terahertz pulsed magnetic field

Molecular spin crossover switches are the objects of intense theoretical and experimental studies in recent years. This interest is due to the fact that these systems allow one to control their spin state by applying an external photo-, thermo-, piezo-, or magnetic stimuli. The greatest amount of research is currently devoted to the study the effect of the photoexcitation on the bi-stable states of spin crossover single molecular magnets (SMMs). The main limitation of photo-induced bi-stable states is their short lifetime. In this paper we present the results of a study of the spin dynamics of the CoOEP molecule in the Low Spin (LS) state and the High Spin (HS) state induced by applying the magnetic pulse of 36.8 T. We show that the spin switching in case of the HS state of the CoOEP molecule is characterized by a long lifetime and is dependent on the magnitude and duration of the applied field. Thus, after applying an external stimuli the system in the LS state after the spin switching reverts to its ground state, whereas the system in the HS state remains in the excited state for a long time. We found that the temperature dependency of magnetic susceptibility shows an abrupt thermal spin transition between two spin states at 40K. The proposed here theoretical approach opens the way to create modern devices for spintronics with the controllable spin switching process.

cond-mat.mes-hall↗

arXiv2012

Relativistic peculiarities at stepped surfaces: surprising energetics and unexpected diffusion patterns

We revive intriguing, yet still unexplained, experimental results of Ehrlich and co-workers [ Phys. Rev. Lett. 77 1334 (1996); Phys. Rev. Lett. 67 2509 (1991)] who have observed, that 5d adatoms distributed on (111) surface islands of 5d metals favor the adsorption at the cluster's edge rather than at the cluster's interior, which lies in contrast with the behavior of 4d and 3d elements. Our state of the art ab initio calculations demonstrate that such behavior is a direct consequence of the relativity of 5d metals.

cond-mat.mes-hall↗

arXiv2012

Confined bulk states as a long-range sensor for impurities and a transfer channel for quantum information

We show that confinement of bulk electrons can be observed at low-dimensional surface structures and can serve as a long-range sensor for the magnetism and electronic properties of single impurities or as a quantum information transfer channel with large coherence lengths. Our ab initio calculations reveal oscillations of electron density in magnetic chains on metallic surfaces and help to unambiguously identify the electrons involved as bulk electrons. We furthermore discuss the possibility of utilizing bulk state confinement to transfer quantum information, encoded in an atom's species or spin, across distances of several nanometers with high efficiency.

cond-mat.mes-hall↗

arXiv2012

Quantum Spin Holography with Surface State Electrons

In a recent paper Moon and coworkers [C.R. Moon et al., Nature Nanotechnology 4, 167 (2009)] have shown that the single-atom limit for information storage density can be overcome by using the coherence of electrons in a two-dimensional electron gas to produce quantum holograms comprised of individually manipulated molecules projecting an electronic pattern onto a portion of a surface. We propose to further extend the concept by introducing quantum spin holography - a version of quantum holographic encoding allowing to store the information in two spin channels independently.

cond-mat.mes-hall↗

arXiv2009

Hopf algebras and the logarithm of the S-transform in free probability

Let k be a positive integer and let G_k denote the set of non-commutative k-variable distributions μsuch that μ(X_1) = ... = μ(X_k) = 1. G_k is a group under the operation of free multiplicative convolution. We identify G_k as the group of characters of a certain Hopf algebra Y_k. Then, by using the log map from characters to infinitesimal characters of Y_k, we introduce a transform LS_μ for distributions μin G_k. The main property of the LS-transform is that it linearizes commuting products in G_k. For μin G_k, the transform LS_μ is a power series in k non-commuting indeterminates; its coefficients can be computed from the coefficients of the R-transform of μby using summations over chains in the lattices NC(n) of non-crossing partitions. In the particular case k=1 one has that Y_1 is naturally isomorphic to the Hopf algebra Sym of symmetric functions, and that the LS-transform is very closely related to the logarithm of the S-transform of Voiculescu, by the formula LS(z) = - z log S(z). In this case the group G_1 can be identified as the group of characters of Sym, in such a way that the S-transform, its reciprocal 1/S and its logarithm log S relate in a natural sense to the sequences of complete, elementary and respectively power sum symmetric functions.

math.OA↗