Simultaneous Spin Flips Research Articles

Strain-promoted reversible spin transfer in rhombic graphene nanoflakes

The strain-promoted reversible spin flip and spin transfer on double-magnetic-center graphene nanoflakes (Ni2&GNFs) are investigated with first-principles quantum chemical computations. We find that applying strain along the two diagonals of the rhombic graphene nanoflakes can significantly lead to the redistribution of spin density, and can therefore dominate the spin-flip and spin-transfer processes. The applied strain effectively changes the spin-density localization of the system, thus opening a channel for successful spin-transfer processes in all distorted structures. Especially, simultaneous spin-flip and transfer processes are achieved when the tensile strain is applied along the short diagonal, and some of the spin-transfer and spin-flip-transfer scenarios achieved on the stretched Ni2&GNF are reversible. This reversible strain-assisted spin dynamics provides a feasible yet efficient way to control the spin magnetism of GNFs, which could increase the functionality and the flexibility of the integrated spin-logic in real straintronic devices.

Carrier Transport Engineering in Carbon Nanotubes by Chirality-Induced Spin Polarization.

Carbon nanotubes (CNTs), helically wrapped with single-stranded DNA, have recently emerged as a spin-filtering material. The inversion asymmetric helical potential of DNA creates a spin-filtering effect (commonly known as "chirality-induced spin selectivity" or CISS), which polarizes carrier spins in the nanotube. Thus, tuning of the DNA-CNT interaction is expected to affect carrier spins in nanotubes. The CISS effect induces spin polarization, which is coupled with the carrier's momentum direction, and therefore, in one-dimensional systems, such as nanotubes, momentum flip must be accompanied by a simultaneous spin flip. This spin momentum locking can have a profound impact on charge transport in nanotubes as backscattering due to phonons and disorder will be suppressed as these mechanisms are spin-independent. Here, we report DNA-CNT spin filters in which CNTs have been functionalized with two different classes of sequences, exhibiting different degrees of interaction with the CNT. They induce different degrees of spin polarization in the channel, with significant impact on temperature-dependent charge transport and interference phenomena arising from carrier backscattering. This work raises the intriguing possibility of engineering charge transport in nanotubes via CISS-induced spin polarization by tailor-made DNA sequences.

Ab initio study of ultrafast laser-induced spin flip, spin-flip transfer, and spin crossover in ComBzn+/0 clusters ( m,n=1,2)

We present a fully ab initio study of the laser-induced ultrafast magnetization dynamics in cobalt-benzene clusters ${\mathrm{Co}}_{m}{\mathrm{Bz}}_{n}^{+/0}$ ($m,n=1,2$). It is found that different geometric configurations and $d\ensuremath{-}\ensuremath{\pi}$ interactions of the four structures cause their high-level electronic structures to exhibit distinct characteristics. By using well-tailored laser pulses to propagate the systems via a spin-orbit-coupling enabled $\mathrm{\ensuremath{\Lambda}}$ process, various local/global spin-flip scenarios are achieved within subpicosecond timescales. Besides, a spin-flip-transfer process within around 300 fs is predicted in ${\mathrm{Co}}_{2}\mathrm{Bz}$. Governed by the Goodenough-Kanamori rules, the simultaneous spin flip and transfer is favored when the two magnetic centers are antiferromagnetically coupled, reflected in the required lowest laser energy and intensity. Most interestingly, we obtain a novel, $W$-process facilitated spin-crossover scenario in the cluster ${\mathrm{CoBz}}^{+}$, which offers additional flexibility of designing relevant all spin-based functions such as molecular switches and sensors. The transferability of the latter two scenarios is also analyzed from the optical spectra of the initial and final states. These results provide the precursory guidance for further study of ultrafast magnetization control in more extended metal-aromatic polymers, and also show their great potential in future nanoscale spintronic applications and quantum information processing.

Classical Simulated Annealing Using Quantum Analogues

In this paper we consider the use of certain classical analogues to quantum tunneling behavior to improve the performance of simulated annealing on a discrete spin system of the general Ising form. Specifically, we consider the use of multiple simultaneous spin flips at each annealing step as an analogue to quantum spin coherence as well as modifications of the Boltzmann acceptance probability to mimic quantum tunneling. We find that the use of multiple spin flips can indeed be advantageous under certain annealing schedules, but only for long anneal times.

Laser-induced ultrafast spin dynamics in di-, tri- and tetranuclear nickel clusters, and the M process

In this manuscript we present an ab initio picture of ultrafast magneto-optical dynamics in clusters containing 2, 3, and 4 Ni atoms. The presence of the magnetic centers in the clusters renders our systems of choice highly interesting for studying ultrafast spin dynamics. Here we systematically study functional cooperativity by increasing both the number of active centers and the spin multiplicities included in our Hilbert space (singlets $\ensuremath{\rightarrow}$ triplets $\ensuremath{\rightarrow}$ quintets), and deriving several ultrafast, laser-driven, spin-manipulation scenarios. Our results indicate various cooperative effects like spin flip by the M process, and simultaneous spin flip and spin transfer, as well as reversible and irreversible demagnetization scenarios. As it turns out the functional cooperativity of the clusters strongly benefits from the delicate interplay of the spin multiplicity and the number of active centers.

Optical signals of spin switching using the optical Stark effect in a Mn-doped quantum dot

The optically induced spin dynamics of a single Mn atom embedded into a single semiconductor quantum dot can be strongly influenced by using the optical Stark effect. The exchange interaction gives rise to simultaneous spin flips between the quantum dot electron and Mn. In the time domain, these flips correspond to exchange induced Rabi oscillations, which are typically off-resonant. By applying a detuned laser pulse, the states involved in the flipping can be brought into resonance by means of the optical Stark effect increasing the amplitude of the Rabi oscillations to one. In this paper, we study theoretically how this spin dynamics can be monitored in time-resolved spectroscopy. In the spectrum, the exchange interaction leads to a splitting of the exciton line into six lines, each corresponding to one of the six Mn spin states. The dynamical behavior of the Mn spin is reflected by the strength of the individual lines as a function of time. When an off-resonant optical pulse is applied, the spectral positions of the lines shift, but still the flipping dynamics is visible.

Electron Cotunneling into a Kondo Lattice

Motivated by recent experimental interest in tunneling into heavy-electron materials, we present a theory for electron tunneling into a Kondo lattice. The passage of an electron into a Kondo lattice is accompanied by a simultaneous spin flip of the localized moments via cotunneling mechanism. We compute the tunneling current with the large-N mean field theory. In the absence of disorder, differential tunneling conductance exhibits two peaks separated by the hybridization gap. Disorder effects lead to the smearing of the gap resulting in a Fano line shape.

Optical control of the spin state in a semimagnetic quantum dot

AbstractIn a Mn doped quantum dot the spin state of the Mn atom can be controlled via a well chosen sequence of laser pulses, which create or annihilate excitons in the quantum dot. The change of the state consists of a flip of the Mn spin accompanied by a simultaneous spin flip of an electron or light hole induced by the exchange interaction between Mn spin and electron or hole spin. All flipping processes can be interpreted in terms of exchange‐induced Rabi oscillations. The period and amplitude of the oscillations are determined by the coupling strength of the exchange interactions, which also strongly influences the energy splitting between the involved states. External magnetic fields can be used to tune the energies of the exciton states and thereby minimize the energy splitting between the states involved in the spin flip process. Thus the switching dynamics can be optimized by applying a magnetic field. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin Texture and Magnetoroton Excitations atν=1/3

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

Mn spin domains in highly photoexcited (Cd,Mn)Te/(Cd,Mg)Te quantum wells

Direct spin and energy transfer between a dense two-dimensional electron-hole ( e-h) magnetoplasma and a Mn spin subsystem via spd exchange has been studied. Time-resolved photoluminescence spectra of the magnetoplasma excited by a powerful 10-ns laser pulse in a single Cd0.97Mn0.03Te/Cd0.75Mg0.25Te quantum well have been investigated in magnetic fields B<14 T at helium temperature. A giant asymmetry of the plasma spin splitting has been observed in the spectra of dense homogeneously photoexcited plasma at high magnetic fields. This asymmetry has been explained in terms of formation of a domain structure within the magnetic subsystem, i.e., spatial stratification into regions with different Mn-spin temperatures. The Mn spin domains result from the Mn spin heating due to the direct spin-spin sd exchange between hot carriers and Mn via effective spin-flip scattering of electrons with simultaneous spin-flip of Mn. During the Mn spin heating under quasistationary excitation, first the Mn spin fluctuations grow and spin domains form, then the spin domain’s volume increases. The spin domain’s formation occurs due to resonant spin-flip scattering with a characteristic time ;1 ns. The nonresonant Mn spin heating has been observed also with a characteristic time of .10 ns. @S0163-1829~98!09039-0#

Stimulated far-infrared emission from combined cyclotron resonances in germanium

We have measured the spectrum of stimulated far-infrared emission from Be- and Zn-doped germanium single crystals in crossed electric and magnetic fields. The spectrum consists of broad radiation bands in the frequency range of $40--130{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ that are due to intervalence band transitions and new narrow emission bands with their center frequencies rising linearly with magnetic field. We demonstrate that the latter emission is due to cyclotron resonance hole transitions between Landau levels of the light-hole subband accompanied by simultaneous spin flip, ``combined resonances.''

Microwave-induced optical nuclear polarization (MI ONP) of benzophenone in dibromodiphenylether

Abstract In this paper we report the results of microwave-induced optical nuclear polarization experiments on photoexcited mixed crystals of dibromodiphenylether doped with benzophenone. The highest polarization obtained is an enhancement with respect to the Boltzmann polarization of = 820. This corresponds to an absolute nuclear spin polarization of 11%. In addition, we have measured the dependence of the optically detected magnetic resonance spectrum and of the nuclear spin polarization on the microwave power. The results are in quantitative agreement with a theory which is based on the induction of forbidden simultaneous spin flips of the triplet electrons with one or more protons by one microwave quantum.

Nonlinear-response theory of Ising-like systems

The nonequilibrium response of a two-dimensional, paramagnetic system to a rapid and arbitrarily large change (quench) in a symmetry-breaking field is investigated. The complete time dependence of the conjugate order parameter is calculated with a real-space renormalization-group (RG) method. The nonlinear relaxation "rate" is found to be time dependent. Its approach to the corresponding prediction of linear-response theory is characterized. The nonlinear response is found to go over to linear response via an early, exponential regime followed by a crossover to an asymptotic, algebraic approach to linear response at long times. The initial transient decay is quench-strength dependent in contrast with the final behavior, which is largely "universal" and quench insensitive. We study a nearest-neighbor Ising model in a magnetic field in the paramagnetic, "disordered" phase. The dynamics are controlled by a kinetic Ising model with simultaneous spin-flip (adsorption-desorption) and spin-exchange (diffusion) kinetics. The model is applicable to non-equilibrium "dosing" experiments on adsorbed systems. Diffusion is shown not to affect the growth rate of the total order parameter (magnetization or coverage). General recursion relations for the temperature and magnetic field are derived for the entire thermodynamic plane. The RG analysis reproduces linear-response theory for arbitrary magnetic field and is consistent with known scaling laws concerning nonlinear relaxation. The time-dependent magnetization in the critical region has a power-law regime before crossing over into its final exponential decay.

The 2 1A g–1 1B u energy gap in the polyenes: An extended configuration interaction study

For a correct account of the ordering of excited states in the polyenes, in particular the low-lying 2 1Ag and 1 1Bu states, single as well as double excited configurations must be included in a CI expansion. However, for longer π systems such expansion shows, in contrast to spectroscopic observations, a divergence of excitation energies and a reversal of the 2 1Ag and 1 1Bu state ordering. We have therefore extended the CI expansion to include all triple and quadruple excitations for the polyene series CnHn+2’ n=4,6,8,10,12 and n=4,6,8,10, respectively. In our calculations we employed a PPP model Hamiltonian. The extended CI expansions correct the faults of previous treatments and predict an increase of the 2 1Ag–11Bu energy gap with increasing polyene length in agreement with recent spectroscopic observations. The effect of higher excitations is mainly due to triple excitations involving three simultaneous spin flips of ground state electrons.

Theory of Spin Pumping and Relaxation in Systems with a Low Concentration of Electron Spin Resonance Centers

A model is developed to describe nuclear relaxation and spin pumping caused by sparsely distributed electronic spins. It is assumed that the nuclei which are outside the interaction sphere of the paramagnetic centers are influenced indirectly via a fast diffusion process. Nuclei close to the electron spins are assumed to combine with these to form spin pairs of finite lifetime $\ensuremath{\tau}$. Spin pumping and relaxation in these pairs is described phenomenologically. The coupling with a radio-frequency power source is described in terms of a spectral distribution function with Lorentzian shape. Simultaneous spin flips, made possible by the static part of the spin-spin interaction, are included. The equations for two-spin relaxation are generalized to account for the fluid motion and electron spin relaxation as two independent sources of randomness. The results for relaxation and spin pumping are adapted to various special cases and compared with experiments. In particular, qualitative changes in the pumping process are shown to take place when the constant external magnetic field is changed from low to high values.

Experimental Study of Spin-Lattice Relaxation Times in Arsenic-Doped Silicon

Measurements of the spin-lattice relaxation times of arsenic donors in a doped silicon crystal at 8500 gauss and 1.3\ifmmode^\circ\else\textdegree\fi{}K are reported. The relaxation times for the stable ${\mathrm{As}}^{75}$ were measured by using adiabatic fast-passage techniques to observe the relative amplitudes of the electron resonance signals as a function of time. The relaxation times for the radioactive ${\mathrm{As}}^{76}$ were measured by observing the formation and decay of the nuclear alignment. For the ${\mathrm{As}}^{75}$, of which there are 2.8\ifmmode\times\else\texttimes\fi{}${10}^{16}$ atoms/${\mathrm{cm}}^{3}$, the direct electron relaxation time is 6.5\ifmmode\pm\else\textpm\fi{}0.5 minutes and the relaxation time for simultaneous spin-flips of the nucleus and the electron is 4.7\ifmmode\pm\else\textpm\fi{}1 minutes. For the ${\mathrm{As}}^{76}$, of which there are 3\ifmmode\times\else\texttimes\fi{}${10}^{10}$ atoms/${\mathrm{cm}}^{3}$, the direct electron relaxation time is 4\ifmmode\pm\else\textpm\fi{}2 minutes and the relaxation time for simultaneous spin-flips of the nucleus and the electron is \ensuremath{\ge}75 minutes.