arXiv subjects

M. E. Raikh

Publications and source records attributed to M. E. Raikh.

At least 55 records · Page 3Linked to original sources

arXiv2015

Effective spin Hall properties of a mixture of materials with and without spin-orbit coupling: Tailoring the effective spin-diffusion length

We study theoretically the effective spin Hall properties of a composite consisting of two materials with and without spin-orbit (SO) coupling. In particular, we assume that SO material represents a system of grains in a matrix with no SO. We calculate the effective spin Hall angle and the effective spin diffusion length of the mixture. Our main qualitative finding is that, when the bare spin diffusion length is much smaller than the radius of the grain, the effective spin diffusion length is strongly enhanced, well beyond the "geometrical" factor. The physical origin of this additional enhancement is that, with small diffusion length, the spin current mostly flows around the grain without suffering much loss. We also demonstrate that the voltage, created by a spin current, is sensitive to a very weak magnetic field directed along the spin current, and even reverses sign in a certain domain of fields. The origin of this sensitivity is that the spin precession, caused by magnetic field, takes place outside the grains where SO is absent.

cond-mat.mes-hall↗

arXiv2015

Plasmon spectrum and plasmon-mediated energy transfer in a multi-connected geometry

Surface plasmon spectrum of a metallic hyperbola can be found analytically with the separation of variables in elliptic coordinates. The spectrum consists of two branches: symmetric, low-frequency branch, $ω<ω_0/\sqrt{2}$, and antisymmetric high-frequency branch, $ω>ω_0/\sqrt{2}$, where $ω_0$ is the bulk plasmon frequency. The frequency width of the plasmon band increases with decreasing the angle between the asymptotes of the hyperbola. For the simplest multi-connected geometry of two hyperbolas separated by an air spacer the plasmon spectrum contains two low-frequency branches and two high-frequency branches. Most remarkably, the lower of two low-frequency branches exists at $ω\rightarrow 0$, i.e., unlike a single hyperbola, it is "thresholdless." We study how the complex structure of the plasmon spectrum affects the energy transfer between two emitters located on the surface of the same hyperbola and on the surfaces of different hyperbolas.

cond-mat.mes-hall↗

arXiv2015

Spin pumping from a ferromagnet into a hopping insulator: the role of resonant absorption of magnons

Motivated by recent experiments on spin pumping from a ferromagnet into organic materials in which the charge transport is due to hopping, we study theoretically the generation and propagation of spin current in a hopping insulator. Unlike metals, the spin polarization at the boundary with ferromagnet is created as a result of magnon absorption within pairs of localized states and it spreads following the current-currying resistor network (although the charge current is absent). We consider a classic resonant mechanism of the ac absorption in insulators and adapt it to the absorption of magnons. A strong enhancement of pumping efficiency is predicted when the Zeeman splitting of the localized states in external magnetic field is equal to the frequency of ferromagnetic resonance. Under this condition the absorption of a magnon takes place within individual sites.

cond-mat.mes-hall↗

arXiv2015

Resonant magneto-tunneling between normal and ferromagnetic electrodes in relation to the three-terminal spin transport

The recently suggested mechanism [Y. Song and H. Dery, Phys. Rev. Lett. 113, 047205 (2014)] of the three-terminal spin transport is based on the resonant tunneling of electrons between ferromagnetic and normal electrodes via an impurity. The sensitivity of current to a weak external magnetic field stems from a spin blockade, which, in turn, is enabled by strong on-site repulsion. We demonstrate that this sensitivity exists even in the absence of repulsion when a single-particle description applies. Within this description, we calculate exactly the resonant-tunneling current between the electrodes. The mechanism of magnetoresistance, completely different from the spin blocking, has its origin in the interference of virtual tunneling amplitudes. Spin imbalance in ferromagnetic electrode is responsible for this interference and the resulting coupling of the Zeeman levels. This coupling also affects the current in the correlated regime.

cond-mat.mes-hall↗

arXiv2015

Evolution of the inhomogeneously-broadened spin noise spectrum with ac drive

In the presence of random hyperfine fields, the noise spectrum, δs_ω^2, of a spin ensemble represents a narrow peak centered at ω=0 and a broad "wing" reflecting the distribution of the hyperfine fields. In the presence of an ac drive, the dynamics of a single spin acquires additional harmonics at frequencies determined by both, the drive frequency and the local field. These harmonics are reflected as additional peaks in the noise spectrum. We study how the ensemble-averaged δs_ω^2 evolves with the drive amplitude, ω_dr (in the frequency units). Our main finding is that additional peaks in the spectrum, caused by the drive, remain sharp even when ω_dr is much smaller than the typical hyperfine field. The reason is that the drive affects only the spins for which the local Larmour frequency is close to the drive frequency. The shape of the low-frequency "Rabi"-peak in δs_ω^2 is universal with both, the position and the width, being of the order of ω_dr. When the drive amplitude exceeds the width of the hyperfine field distribution, the noise spectrum transforms into a set of sharp peaks centered at harmonics of the drive frequency.

cond-mat.mes-hall↗

arXiv2014

Spin transport with traps: dramatic narrowing of the Hanle curve

We study theoretically the spin transport in a device in which the active layer is an organic film with numerous deep in-gap levels serving as traps. A carrier, diffusing between magnetized injector and detector, spends a considerable portion of time on the traps. This new feature of transport does not affect the giant magnetoresistance, which is sensitive only to the mutual orientation of magnetizations of the injector and detector. By contrast, the presence of traps strongly affects the sensitivity of the spin transport to external magnetic field perpendicular to the magnetizations of the electrodes (the Hanle effect). Namely, the Hanle curve narrows dramatically. The origin of such a narrowing is that the spin precession takes place during the entire time of the carrier motion between the electrodes, while the spin relaxation takes place only during diffusive motion between the subsequent traps. If the resulting width of the Hanle curve is smaller than the measurement resolution, observation of the Hanle peak becomes impossible.

cond-mat.mes-hall↗

arXiv2014

Manifestation of two-channel nonlocal spin transport in the shapes of the Hanle curves

Dynamics of charge-density fluctuations in a system of two tunnel-coupled wires contains two diffusion modes with dispersion iw=Dq^2 and iw =Dq^2+2/tau_t, where D is the diffusion coefficient and tau_t is the tunneling time between the wires. The dispersion of corresponding spin-density modes depends on magnetic field as a result of spin precession with Larmour frequency, w_L. The presence of two modes affects the shape of the Hanle curve describing the spin-dependent resistance, R, between ferromagnetic strips covering the non-magnetic wires. We demonstrate that the relative shapes of the R(w_L)-curves, one measured within the same wire and the other measured between the wires, depends on the ratio tau_t/tau_s, where tau_s is the spin-diffusion time. If the coupling between the wires is local, i.e. only at the point x=0, then the difference of the shapes of intra-wire and inter-wire Hanle curves reflects the difference in statistics of diffusive trajectories which "switch" or do not switch near x=0. When one of the coupled wires is bent into a loop with a radius, a, the shape of the Hanle curve reflects the statistics of random walks on the loop. This statistics is governed by the dimensionless parameter, a/(D tau_s)^(1/2).

cond-mat.mes-hall↗

arXiv2014

The shape of the Hanle curve in spin-transport structures in the presence of the ac drive

Resistance between two ferromagnetic electrodes coupled to a normal channel depends on their relative magnetizations. The spin-dependent component, R, of the resistance changes with magnetic field, B, normal to the directions of magnetizations. In the field of spin transport, this change, R(B), originating from the Larmour spin precession, is called the Hanle curve. We demonstrate that the shape of the Hanle curve evolves upon application of an ac drive and study this evolution theoretically as a function of the amplitude, B_1, and frequency, w, of the drive. If the distance between the electrodes, L, is smaller than the spin-diffusion length, l_s, the prime effect of a weak circular-polarized drive is the shift of the center of the curve to the value of B for which the Larmour frequency, w_L, is ~B_1^2 w. Magnetic resonance at w_L~w manifests itself in the derivative, dR/dB. For large L >> l_s the ac drive affects the Hanle curve if the drive amplitude exceeds the spin relaxation rate, 1/t_s, i.e. at B_1 t_s > 1. The prime effect of the drive is the elimination of a minimum in R(B). Linearly polarized drive has a fundamentally different effect on the Hanle curve, affecting not its shape, but rather its width.

cond-mat.mes-hall↗

arXiv2014

Spin injection from a ferromagnet into a semiconductor in the case of a rough interface

The effect of the interface roughness on the spin injection from a ferromagnet into a semiconductor is studied theoretically. Even a small interface irregularity can lead to a significant enhancement of the injection efficiency. When a typical size of the irregularity, a, is within a domain lambda_F << a << lambda_N, where lambda_F and lambda_N are the spin-diffusion lengths in the ferromagnet and semiconductor, respectively, the geometrical enhancement factor is ~lambda_N. The origin of the enhancement is the modification of the local electric field on small scales ~a near the interface. We demonstrate the effect of enhancement by considering a number of analytically solvable examples of injection through curved ferromagnet-semiconductor interfaces. For a generic curved interface the enhancement factor is ~lambda_N / R, where R is the local radius of curvature.

cond-mat.mes-hall↗

arXiv2014

Spin relaxation of a diffusively moving carrier in a random hyperfine field

Relaxation, , of the average spin of a carrier in course of hops over sites hosting random hyperfine fields is studied theoretically. In low dimensions, d = 1, 2, the decay of average spin with time is non-exponential at all times. The origin of the effect is that for d = 1, 2 a typical random-walk trajectory exhibits numerous self-intersections. Multiple visits of the carrier to the same site accelerates the relaxation since the corresponding partial rotations of spin during these visits add up. Another consequence of self-intersections of the random-walk trajectories is that, in all dimensions, the average, , becomes sensitive to a weak magnetic field directed along z. Our analytical predictions are complemented by the numerical simulations of .

cond-mat.mes-hall↗

arXiv2013

Giant fluctuations of local magnetoresistance of organic spin valves and non-hermitian 1D Anderson model

Motivated by recent experiments, where the tunnel magnetoresitance (TMR) of a spin valve was measured locally, we theoretically study the distribution of TMR along the surface of magnetized electrodes. We show that, even in the absence of interfacial effects (like hybridization due to donor and acceptor molecules), this distribution is very broad, and the portion of area with negative TMR is appreciable even if on average the TMR is positive. The origin of the local sign reversal is quantum interference of subsequent spin-rotation amplitudes in course of incoherent transport of carriers between the source and the drain. We find the distribution of local TMR exactly by drawing upon formal similarity between evolution of spinors in time and of reflection coefficient along a 1D chain in the Anderson model. The results obtained are confirmed by the numerical simulations.

cond-mat.mes-hall↗

arXiv2013

Organic magnetoresistance near saturation: mesoscopic effects in small devices

In organic light emitting diodes with small area the current may be dominated by a finite number, N of sites in which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, b_h, that are generated in these sites by the environment nuclei is incomplete. This creates a random (mesoscopic) current component, ΔI(B), at field B having relative magnitude ~ N^(-1/2). To quantify the statistical properties of ΔI(B) we calculate the correlator K(B, ΔB)= <δI(B - ΔB/2)δI(B + ΔB/2)> for parallel and perpendicular orientations of ΔB. We demonstrate that mesoscopic fluctuations develop at fields B>>b_h, where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between S and T_0 states of the recombining e-h spin pair-partners. We identify the most relevant processes responsible for the current fluctuations as due to anomalously slow beatings that develop in sparse e-h polaron pairs at sites for which the b_h projections on the external field direction almost coincide.

cond-mat.mes-hall↗

arXiv2013

Tunnel magnetoresistance in organic spin valves in the regime of multi-step tunneling

A model of a spin valve in which electron transport between the magnetized electrodes is due to multistep tunneling is analyzed. Motivated by recent experiments on organic spin valves, we assume that spin memory loss in the course of transport is due to random hyperfine fields acting on electron while it waits for the next tunneling step. Amazingly, we identify the three-step configurations of sites, for which the tunnel magnetoresistance (TMR) is negative, suggesting that the resistance for antiparallel magnetizations of the electrodes is smaller than for parallel magnetizations. We analyze the phase volume of these configurations with respect to magnitudes and relative orientations of the on-site hyperfine fields. The effect of sign reversal of TMR is exclusively due to interference of the spin-flip amplitudes on each site, it does not emerge within commonly accepted probabilistic description of spin transport. Another feature specific to multistep inelastic tunneling is bouncing of electron between nearest neighbors while awaiting a "hard" hop. We demonstrate that this bouncing, being absolutely insignificant for conduction of current, can strongly affect the spin memory loss. This effect is also of interference origin.

cond-mat.mes-hall↗

arXiv2013

Organic magnetoresistance under resonant ac drive

We study the spin dynamics of an electron-hole pair in a random hyperfine magnetic field and an external field, B0, under a resonant drive with frequency omega0 = gamma B0. The fact that the pair decays by recombination exclusively from a singlet configuration, S, in which the spins of the pair-partners are entangled, makes this dynamics highly nontivial. Namely, as the amplitude, B1, of the driving field grows, mixing all of the triplet components, the long-living modes do not disappear, but evolve from T+, T- into (T+ +/- sqrt(2) T0 + T-)/2. Upon further increase of B1, the lifetime of the S-mode is cut in half, while the T0-mode transforms into an antisymmetric combination (T- - T-)/sqrt(2) and acquires a long lifetime, in full analogy to the superradiant and subradiant modes in the Dicke effect. Peculiar spin dynamics translates into a peculiar dependence on B1. In particular, at small B1, the radiation-induced correction to the current is linear in B1.

cond-mat.mes-hall↗

arXiv2013

Slow dynamics of spin pairs in random hyperfine field: Role of inequivalence of electrons and holes in organic magnetoresistance

In an external magnetic field B, the spins of the electron and hole will precess in effective fields b_e + B and b_h + B, where b_e and b_h are random hyperfine fields acting on the electron and hole, respectively. For sparse "soft" pairs the magnitudes of these effective fields coincide. The dynamics of precession for these pairs acquires a slow component, which leads to a slowing down of recombination. We study the effect of soft pairs on organic magnetoresistance, where slow recombination translates into blocking of the passage of current. It appears that when b_e and b_h have identical gaussian distributions the contribution of soft pairs to the current does not depend on B. Amazingly, small inequivalence in the rms values of b_e and b_h gives rise to a magnetic field response, and it becomes progressively stronger as the inequivalence increases. We find the expression for this response by performing the averaging over b_e, b_h analytically. Another source of magnetic field response in the regime when current is dominated by soft pairs is inequivalence of the g-factors of the pair partners. Our analytical calculation indicates that for this mechanism the response has an opposite sign.

cond-mat.mes-hall↗

arXiv2012

Magnetic resonance in slowly modulated longitudinal field: Modified shape of the Rabi oscillations

The sensitivity of the Rabi oscillations of a resonantly driven spin-1/2 system to a weak and slow modulation of the static longitudinal magnetic field, B_0, is studied theoretically. We establish the mapping of a weakly driven two-level system with modulation onto a strongly driven system without modulation. The mapping suggests that different regimes of spin dynamics, known for a strongly driven system, can be realized under common experimental conditions of weak driving (driving field B_ 1 << B_ 0) upon proper choice of the domains of modulation frequency, ω_m, and amplitude, B_2. Fast modulation ω_m >> Ω_R, where Ω_R is the Rabi frequency, emulates the regime of driving frequency much bigger than the resonant frequency. Strong modulation, B_2 >> B_1, emulates the regime B_1 >> B_0. Resonant modulation, ω_m \approx Ω_R, gives rise to an envelope of the Rabi oscillations. The shape of this envelope is highly sensitive to the detuning of the driving frequency from the resonance. Theoretical predictions for different domains of B_2 and ω_m were tested experimentally using NMR of protons in water, where, without modulation, the pattern of Rabi oscillations could be observed over many periods. We present experimental results which reproduce the three predicted modulation regimes, and agree with theory quantitatively.

cond-mat.mes-hall↗

arXiv2012

Analytical study of spin-dependent transition rates within pairs of dipolar and strongly exchange coupled spins with (S = 1/2) during magnetic resonant excitation

We study theoretically the spectrum, F(s), of spin-dependent transition rates within dipolar D and exchange J coupled pairs of two spins with S=1/2 undergoing Rabi oscillations due to a coherent magnetic resonant excitation. We show that the Rabi oscillation controlled rates exhibit a spectrum with three frequency components. When exchange is stronger than the Rabi frequency (J>>Ω_R), the frequency components of the Rabi oscillation do not depend on J, rather they are determined by the relation between Ω_R and D. We derive analytical expressions for the frequencies and the intensities of all three Rabi oscillation components as functions of Ω_R/D and δ/D, where δis detuning of the driving ac field from the Larmor frequency. When Ω_R>>D, the two lower frequencies approach s=Ω_R, while the upper line approaches s=2Ω_R. Disorder of the local Larmor frequencies leads to a Gaussian broadening of the spectral lines. We calculate corresponding widths for different Ω_R/D and δ/D. Unexpectedly, we find that one of the frequency components exhibits an unusual evolution with Ω_R: its frequency decreases with Ω_R at Ω_R<D. Upon further increase of Ω_R this frequency then passes through a minimum and, eventually, approaches s=Ω_R. Nonmonotonic behavior of the frequencies is accompanied by nonmonotonic behavior of the respective oscillation intensity.

cond-mat.mtrl-sci↗

arXiv2012

Interplay of spin-orbit coupling and Zeeman splitting in the absorption lineshape of 2D fermions

We suggest that electron spin resonance (ESR) experiment can be used as a probe of spinon excitations of hypothetical spin-liquid state of frustrated antiferromagnet in the presence of asymmetric Dzyaloshinskii-Moriya (DM) interaction. We describe assumptions under which the ESR response is reduced to the response of 2D electron gas with Rashba spin-orbit coupling. Unlike previous treatments, the spin-orbit coupling, Δ_{SO}, is not assumed small compared to the Zeeman splitting, Δ_Z. We demonstrate that ESR response diverges at the edges of the absorption spectrum for ac magnetic field perpendicular to the static field. At the compensation point, Δ_{SO}\approx Δ_Z, the broad absorption spectrum exhibits features that evolve with temperature, T, even when T is comparable to the Fermi energy.

cond-mat.str-el↗