Presence Of Magnetic Impurities Research Articles

Cooper Pair Excitation Mediated by a Molecular Quantum Spin on a Superconducting Proximitized Gold Film.

Breaking a correlated pair in a superconductor requires an even number of fermions providing at least twice the pairing energy Δ. Here, we show that a single tunneling electron can also excite a pair breaking excitation in a proximitized gold film in the presence of magnetic impurities. Combining scanning tunneling spectroscopy with theoretical modeling, we map the excitation spectrum of an Fe-porphyrin molecule on the Au/V(100) proximitized surface into a manifold of entangled Yu-Shiba-Rusinov and spin excitations. Pair excitations emerge in the tunneling spectra as peaks outside the spectral gap only in the strong coupling regime, where the presence of a bound quasiparticle in the ground state ensures the even fermion parity of the excitation. Our results unravel the quantum nature of magnetic impurities on superconductors and demonstrate that pair excitations unequivocally reveal the parity of the ground state.

Kondo resonance effects in emergent flat band materials

Macroscopic degrees of freedom that are involved in the transport of carriers through mesoscopic electronic devices are susceptible to the effects of strong many-body correlations. The presence of magnetic impurities in dilute magnetic alloys typically allow for insights into Kondo effect from the scattering of free carriers by localized electron states of the magnetic impurities but this effect is not well understood when there are no d-band electron states. Herein, the signatures of Kondo resonance effect are elucidated in quantum dots derived from a carbon-nanoline embedded monolayer hexagonal boron nitride whose electron states host flat band ferromagnetism as distinct broken symmetry states. Quantum transport state of mesoscopic devices modelled as quantum dots tunnel coupled to metallic leads is computed by direct diagonalization of the Hamiltonian. The possibility of realizing quantum dots with highly tunable electron states in energy interconversion devices is discussed to show the importance of screening effects on single-electron energy levels. The quantum master equation is solved within different formalisms to determine the stationary-state particle and energy currents. Stability diagrams are calculated to show the dependence of the conductance on experimental control variables of the quantum dot device. The computed responses of the stationary-state transport signatures are used to characterize Kondo resonance effects from flat band states of embedded carbon nanoline-based quantum dots. It is found that the local network structure of the hexagonal ring carbon cluster-based quantum dot has a broken particle-hole symmetry in the transport state. This signals the formation of the quasiparticle states expected in second order scattering when the macroscopic “charge” pseudospin symmetry of the tunnelling electron state is broken dynamically due to charging. The results are discussed to show the implications of a vanishing particle-hole symmetry in the carrier transport state of quantum dots for energy conversion applications.

Physico-Chemical Features of Undoped and Fe/Cu-Doped Na0.67MnO2-Layered Cathodes for Sodium Batteries

Na0.67MnO2 (NMO) stands out among the layered cathode materials used for sodium batteries due to its high-capacity values, low cost, and environmental friendliness. Unfortunately, many drawbacks arise during cycling, but nanostructure tailoring and doping can help to mitigate them. Our aim was to synthesize undoped and Cu- or Fe-doped NMO samples via the sol-gel route, with a different cooling step to room temperature, i.e., in a natural way or via quenching. The formation of a mixture of polymorphs was observed, as well as differences in the external morphology of the powders’ grains. The use of spectroscopic techniques, Mössbauer spectroscopy for the Fe-doped samples and Electron paramagnetic resonance, allowed us to gain insights into the oxidation states of transition metals and to make suggestions about the magnetic ordering, as well as on the possible presence of magnetic impurities. Cyclic voltammetry and galvanostatic cycling results were interpreted on the basis of the spectroscopic data: the introduction of substituents, in general, worsens the capacity values, due to the decrease in the P2 amount and the introduction of structural distortions. The structural stability of the samples in air as a function of time was also analyzed via X-ray diffraction, demonstrating the positive effect of Cu presence.

Large suppression of spin-relaxation rate in graphene nanoribbons in the presence of magnetic impurities

Achievement of low spin-relaxation rate is an important goal for spintronics development. We study the spin-relaxation rate arising from a low concentration of magnetic impurities in armchair graphene nanoribbons (AGNR). Large suppression in the spin-relaxation rate, exhibited as dip structures, is found when the Fermi energy approaches a subband band edge. This suppression originated from the quasi-one-dimensional density of states and is manifested via the singular features in the AGNR same-site Green's function ${G}_{\mathcal{II}}$, where $\mathcal{I}$ denotes site locations of magnetic impurities. Analytic analysis of the spin-relaxation rate in the close vicinity of a subband band edge is performed to further reveal the physical nature of the suppression. The robustness of the suppression feature in the spin-relaxation rate is demonstrated by systematically increasing the number of magnetic impurities involved in a coherent multiple scattering with the electrons. Major peaks in the spin-relaxation rate are analyzed in light of their connection with spin-flipped resonances. Competition between magnetic impurities with similar resonance energies is found to lead to suppression in the spin-relaxation rate. Our calculations have taken into account the hydrogen-passivation effects at the AGNR edges when the hopping constant between edge carbons is modified.

Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles

We have implemented the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn- Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals including substitutional impurities. In this report we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3d magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in sup-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of the first principles based description which captures the quantitative details making direct comparisons possible with experimental findings.

Spin-dependent transport through helical Aharonov-Bohm interferometer

We discuss spin-dependent transport via tunneling Aharonov-Bohm interferometer formed by helical edge states tunnel-coupled to helical leads. We focus on the experimentally relevant high-temperature case as compared to the level spacing and obtain the full 4×4 matrix of transmission coefficients in the presence of magnetic impurities. We show that spin conserving and spin-flip transmission coefficients of the setup can be effectively tuned by the magnetic flux. These features are attractive due to possible applications for spintronics, magnetic field detection, and quantum computing.

Quantized spin Hall conductance in a magnetically doped two dimensional topological insulator

Soon after the discovery of the quantum spin Hall effect, it has been predicted that a magnetic impurity in the presence of strong Coulomb interactions will destroy the quantum spin Hall effect. However, the fate of the quantum spin Hall effect in the presence of magnetic impurities has not yet been experimentally investigated. Here, we report the successful experimental demonstration of a quantized spin Hall resistance in HgTe quantum wells dilutely alloyed with magnetic Mn atoms. These quantum wells exhibit an inverted band structure that is very similar to that of the undoped material. Micron sized devices of (Hg,Mn)Te quantum well (in the topological phase) show a quantized spin Hall resistance of h/2e2 at low temperatures and zero magnetic field. At finite temperatures, we observe signatures of the Kondo effect due to interaction between the helical edge channels and magnetic impurities. Our work lays the foundation for future investigations of magnetically doped quantum spin Hall materials towards the realization of chiral Majorana fermions.

Effect of Zeeman coupling on the Majorana vortex modes in iron-based topological superconductors

In the superconducting regime of FeTe$_{(1-x)}$Se$_x$, there exist two types of vortices which are distinct by the presence or absence of zero energy states in their core. To understand their origin, we examine the interplay of Zeeman coupling and superconducting pairings in three-dimensional metals with band inversion. Weak Zeeman fields are found to suppress the intra-orbital spin-singlet pairing, known to localize the states at the ends of the vortices on the surface. On the other hand, an orbital-triplet pairing is shown to be stable against Zeeman interactions, but leads to delocalized zero-energy Majorana modes which extend through the vortex. In contrast, the finite-energy vortex modes remain localized at the vortex ends even when the pairing is of orbital-triplet form. Phenomenologically, this manifests as an observed disappearance of zero-bias peaks within the cores of topological vortices upon increase of the applied magnetic field. The presence of magnetic impurities in FeTe$_{(1-x)}$Se$_x$, which are attracted to the vortices, would lead to such Zeeman-induced delocalization of Majorana modes in a fraction of vortices that capture a large enough number of magnetic impurities. Our results provide an explanation to the dichotomy between topological and non-topological vortices recently observed in FeTe$_{(1-x)}$Se$_x$.

Emergence of long-range magnetic order stabilized by magnetic impurities in pnictides

We examine the temperature dependence of the magnetic ordering in the frustrated Heisenberg $J_1-J_2$ model in presence of two different kind of dopants: vacancies or magnetic impurities. We demonstrate that, irrespective to their magnetic ratio, the introduction of impurities quenches the order by disorder selection mechanism associated with an Ising-like phase transition at low temperatures and gives way to a $90^\circ$ (anticollinear) order . The presence of dopants triggers a non trivial competition between entropically selected states (collinear) and energetically favoured ones (anticollinear) in dependence of both dilution and temperature. While in case of magnetic impurity, the interesting magnetic phases are observed for full range of temperature and doping, in case of nonmagnetic impurities every magnetic order is destroyed at all temperatures above $12\%$ dilution. At fixed low temperature and tuning the doping we show a first order phase transition leading to the re-entrance of the Ising-like order with percolation of islands of $90^\circ$ order. At fixed doping and varying the temperature we observe a transition from the anticollinear to the collinear phase assisted by a new emerging magnetic phase in the presence of magnetic impurities, whilst in case of vacancies this transition is characterised by a coexistent region of both. Furthermore, tuning the magnetic moment of the impurities, a complete collapse of the Ising-like order is attained. This is in agreement with observations of Ir dopant atoms in superconducting Ba(Fe$_{1-x}$Ir$_x$)$_2$As$_2$ with $x<0.047$.

Tunneling in presence of magnetic impurities in graphene

The tunneling rate of a particle between two sites situated in different sublattices of a single-layer graphene sheet which can interact with the conduction electrons has been calculated. The interaction between the impurity sites is decisive in determining the tunneling rate. Furthermore, the rate also depends on the energy of the impurities along with value of the chemical potential of the system.

Interplay between Carrier Polarization, Spin-Orbit Coupling and Exchange Field on Anomalous Hall Conductivity in the Presence of Magnetic Impurity in Mn Doped GaAs

We develop a model Hamiltonian to treat anomalous Hall conductivity in dilute magnetic semiconductor (DMS) of type (III, Mn, V) considering the impurity potentials (potential due to interaction of spin of carriers with localized spin of dopant (Mn) and coulomb like potential). Using equation of motion in Green function together with Quantum Kubo-formula of conductivity, the anomalous Hall conductivity is calculated as function of spin-orbit coupling, exchange field and carrier polarization. The calculated result shows that at low impurity concentration, the interplay between spin polarization of carriers, spin-orbit coupling and exchange fields is crucial for existence of anomalous Hall conductivity. The monotonic increment of anomalous Hall conductivity with exchange field is observed for strong spin-orbit coupling limit. In weak spin-orbit coupling limit, the magnitude of anomalous Hall conductivity increases parabolically with the spin-orbit coupling. Our results provide an important basis for understanding the interplay between the spin polarization, spin-orbit coupling, and exchange field on anomalous Hall conductivity at low impurity concentration. The findings are also a key step to realize dissipationless quantum transport without external magnetic field.

Tunneling Aharonov-Bohm interferometer on helical edge states

We discuss transport through interferometer formed by helical edge states tunnel-coupled to metallic leads. We focus on the experimentally relevant case of relatively high temperature as compared to the level spacing and discuss a response of the setup to the external magnetic flux $\phi$ (measured in units of flux quantum) piercing the area encompassed by edge states. We demonstrate that tunneling conductance of the interferometer is structureless in ballistic case but shows a sharp antiresonances, as a function of magnetic flux $\phi$ - with the period 1/2 - in the presence of magnetic impurity. We interpret the resonance behavior as a coherent enhancement of backward scattering off magnetic impurity at integer and half-integer values of flux, which is accompanied by suppression of the effective scattering at other values of flux. Both enhancement and suppression are due to the interference of processes with multiple returns to magnetic impurity after a number of clockwise and counterclockwise revolutions around setup. This phenomenon is similar to the well-known weak-localization-induced enhancement of backscattering in disordered systems. The quantum correction to the tunneling conductance is shown to be proportional to flux-dependent "ballistic Cooperon". The obtained results can be used for flux-tunable control of the magnetic disorder in Aharonov-Bohm interferometers built on helical edge states.

Emergence of Kondo Resonance in Graphene Intercalated with Cerium.

The interaction between a magnetic impurity, such as cerium (Ce) atom, and surrounding electrons has been one of the core problems in understanding many-body interaction in solid and its relation to magnetism. Kondo effect, the formation of a new resonant ground state with quenched magnetic moment, provides a general framework to describe many-body interaction in the presence of magnetic impurity. In this Letter, a combined study of angle-resolved photoemission (ARPES) and dynamic mean-field theory (DMFT) on Ce-intercalated graphene shows that Ce-induced localized states near Fermi energy, EF, hybridized with the graphene π-band, exhibit gradual increase in spectral weight upon decreasing temperature. The observed temperature dependence follows the expectations from the Kondo picture in the weak coupling limit. Our results provide a novel insight how Kondo physics emerges in the sea of two-dimensional Dirac electrons.

Calculation of magnetic devices cleaning coolants in the agro–industrial complex

Traditional ways of cleaning of cooling lubricants in agrarian–industrial complex are considered in the article. Experimental data on the presence of magnetic impurities and its fractional composition as a result of parts machining on machines with the use of cooling lubricants are cited. Combined devices based on the combination of traditional methods for cleaning technical fluids have been used to intensify the cleaning process. The use of such combined cleaning methods is grounded, the forces effecting on the particles in the process of separation – coagulation forces, magnetic deposition, and medium resistance – are considered. The mathematical model of the coagulation process of two particles having similar size in a homogeneous magnetic field is made up. However, it is impossible to solve the obtained system even by a numerical method with unknown initial conditions. In this connection, the inverse problem is proposed. The physical significance of the effect of forces is as follows: a floccule consisting of two particles is destroyed under the effect of coagulation forces; the resistance of the medium prevents it. Its numerical solution allows finding the main dynamic characteristics of particle coagulation: the speed and distance from which they interact.

Yu-Shiba-Rusinov bands in superconductors in contact with a magnetic insulator

Superconductor-Ferromagnet (SF) heterostructures are of interest due to numerous phenomena related to the spin-dependent interaction of Cooper pairs with the magnetization. Here we address the effects of a magnetic insulator on the density of states of a superconductor based on a recently developed boundary condition for strongly spin-dependent interfaces. We show that the boundary to a magnetic insulator has a similar effect like the presence of magnetic impurities. In particular we find that the impurity effects of strongly scattering localized spins leading to the formation of Shiba bands can be mapped onto the boundary problem.

Semiclassical Boltzmann theory of spin Hall effects in giant Rashba systems

For the spin Hall effect arising from strong band-structure spin-orbit coupling, a semiclassical Boltzmann theory reasonably addressing the intriguing disorder effect called side-jump is still absent. In this paper we describe such a theory of which the key ingredient is the spin-current-counterpart of the semiclassical side-jump velocity (introduced in the context of the anomalous Hall effect). Applying this theory to spin Hall effects in a two-dimensional electron gas with giant Rashba spin-orbit coupling, we find largely enhanced spin Hall angle in the presence of magnetic impurities when only the lower Rashba band is partially occupied.

Anomalous room temperature magnetorheological behavior of colloidal graphene nanogels

Weak magnetism is known to exist in graphene systems, yet, as of now, it remains unharnessed for bulk scale applications. Weak ferromagnetic response from chemically exfoliated graphene samples has been employed to harness appreciable magnetorheological effects via the use of novel graphene nanogels. Chemically exfoliated graphene samples have been synthesized and ferromagnetic response has been registered (in tune with scarce reports in literature). Characterizations reveal that presence of magnetic impurities would be unable to yield magnetic moments as obtained and hence the response is innate to the vacancy and defects in the graphenic structure. Polymer based nanogels with infused graphene nanoplatelets has been synthesized and the colloidal gel phase enables harnessing of magnetorheology from such weak moment systems. Strong magnetorheological response is obtained from the colloidal gel phase due to the compact texture which aids in fibrillation of the nanoplatelets. Largely enhanced magnetoviscosity and yield stress have been observed from the gels in similitude to conventional magnetorheological nanocolloids. The presence of dispersed phase with platelet morphology as well as surface lubrication behavior of graphene has been found responsible for certain anomalous behavior such as magnetic shear thickening in the gels which has been explained based on order to disorder transitions under field influence. Transient response of the gels show good modulation caliber in field actuated viscous control as well as minute magnetoviscous hysteresis. The present colloids show novel promise in use of graphene as potential magnetic material in magnetic field governed actuation, control and tuning MEMS/NEMS and allied devices.

Dynamics of vortices with magnetic impurities

We investigate the dynamics of Bogomolny-Prasad-Sommerfield (BPS) vortices in the presence of magnetic impurities taking the form of axially symmetric localised lumps and delta-functions. We present numerical results for vortices on flat space, as well as exact results for vortices on hyperbolic space in the presence of delta-function impurities. In fact, delta-function impurities of appropriate strength can be captured within the moduli space approximation by keeping one or more of the vortices fixed. We also show that previous work on vortices on the 2-sphere extends naturally to the inclusion of delta-function impurities.

Homogeneous versus composite Cd1−x−yMnxZnySnAs2 crystals: Magnetic interactions and transport properties

We present the studies of structural, magnetic, and magnetotransport properties of ${\mathrm{Cd}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mn}}_{x}{\mathrm{Zn}}_{y}{\mathrm{SnAs}}_{2}$ crystals with the average Mn contents $x$ changing from 0.013 to 0.170 and Zn contents $y$ varying from 0.002 to 0.051. Homogenous distribution of Mn ions is observed for the samples with $x\ensuremath{\le}0.025$. The presence of MnAs clusters in the studied alloy for $x&gt;0.025$ induces room-temperature ferromagnetism with the Curie temperature ${T}_{C}$, with values around $325\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. High crystal quality leads to high carrier mobility values observed for all of our samples, as high as $7100\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/(\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s})$ for $x=0.025$ and $y=0.028$. The Shubnikov--de Haas oscillations are observed at $T\ensuremath{\le}50$ K for all of our samples. The oscillations allowed the calculation of the effective mass ${m}^{*}$, giving values of about $0.11--0.12{m}_{e}$. The presence of magnetic impurities has a strong influence on the magnetoresistance of the alloy. For the samples with $x\ensuremath{\ge}0.076$, the Shubnikov--de Haas oscillations are observed on the background of a strong linear positive magnetoresistance, present up to room temperature. The maximum values of the linear positive magnetoresistance are close to 200% for the sample with $x=0.170$ and $y=0.002$, at $T=1.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. This positive magnetoresistance is related to the presence of MnAs clusters in the semiconductor lattice.

Time-reversal breaking and spin transport induced by magnetic impurities in a 2D topological insulator

We employed the formalism of bond currents, expressed in terms of non-equilibrium Green’s function to obtain the local currents and transport features of zigzag silicene ribbon in the presence of magnetic impurity. When only intrinsic and Rashba spin–orbit interactions are present, silicene behaves as a two-dimensional topological insulator with gapless edge states. But in the presence of finite intrinsic spin–orbit interaction, the edge states start to penetrate into the bulk of the sample by increasing Rashba interaction strength. The exchange interaction induced by local impurities breaks the time-reversal symmetry of the gapless edge states and influences the topological properties strongly. Subsequently, the singularity of partial Berry curvature disappears and the silicene nanoribbon becomes a trivial insulator. On the other hand, when the concentration of the magnetic impurities is low, the edge currents are not affected significantly. In this case, when the exchange field lies in the x–y plane, the spin mixing around magnetic impurity is more profound rather than the case in which the exchange field is directed along the z-axis. Nevertheless, when the exchange field of magnetic impurities is placed in the x–y plane, a spin-polarized conductance is observed. The resulting conductance polarization can be tuned by the concentration of the impurities and even completely polarized spin transport is achievable.