Femtosecond Spin Dynamics Mechanism In Graphenes: The Bloch Nmr-Schrödinger Probe (SAO/NASA ADS Indexed) (original) (raw)
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Femtosecond Spin Dynamics Mechanism In Graphenes: The Bloch NMR-Schrödinger Probe
The mechanism of the femtosecond spin dynamics is still not properly understood. The remodeled Bloch-Schrödinger equation was incorporated into the Hamiltonian. The mechanism of the femtosecond dynamics was investigated under three quantum states. The spin relaxation mechanism operated in a single continuous time scale (>70ps) which was in variance with known postulate. The transient reflectivity was measured to be within an angular range of 18.6 o to 90.0 o at a pulse range of 1MHz to 6.5 MHz. Beyond the pulse intensity of-2.5, the system elapsed into a quasi-equilibrium state which explains the independence of the magnetic moment on the pulse intensity. Different possibilities of the femtosecond spin dynamics were worked out for future study.
Virtual Observation of Femtosecond Spin Dynamics Mechanism in Graphene
—The mechanism of the femtosecond spin dynamics is still not properly understood. The remodeled Bloch – Schrödinger equation was incorporated into the Hamiltonian. The mechanism of the femtosecond dynamics was investigated under three quantum states. The spin relaxation mechanism operated in a single continuous time scale (>70ps) which was in variance with knownpostulate. The transient reflectivity was measured to be within an angular range of 18.6o to 90.0o at a pulse range of 1MHz to 6.5 MHz. Beyond the pulse intensity of-2.5, the system elapsed into a quasi-equilibrium state which explains the independence of the magnetic moment on the pulse intensity. Different possibilities of the femtosecond spin dynamics were worked out for future study.
NMR relaxation rate and static spin susceptibility in graphene
The NMR relaxation rate and the static spin susceptibility in graphene are studied within a tight-binding description. At half filling, the NMR relaxation rate follows a power law as T 2 on the particle-hole symmetric side, while with a finite chemical potential µ and next-nearest neighbor t ′ , the (µ + 3t ′ ) 2 terms dominate at low excess charge δ. The static spin susceptibility is linearly dependent on temperature T at half filling when t ′ = 0, while with a finite µ and t ′ , it should be dominated by (µ + 3t ′ ) terms in low energy regime. These unusual phenomena are direct results of the low energy excitations of graphene, which behave as massless Dirac fermions. Furthermore, when δ is high enough, there is a pronounced crossover which divides the temperature dependence of the NMR relaxation rate and the static spin susceptibility into two temperature regimes: the NMR relaxation rate and the static spin susceptibility increase dramatically as temperature increases in the low temperature regime, and after the crossover, both decrease as temperature increases at high temperatures. This crossover is due to the well-known logarithmic Van Hove singularity in the density of states, and its position dependence of temperature is sensitive to δ.
Electron spin dynamics and electron spin resonance in graphene
2010
A theory of spin relaxation in graphene including intrinsic, Bychkov-Rashba, and ripple spin-orbit coupling is presented. We find from spin relaxation data by Tombros et al. [Nature 448, 571 (2007).] that intrinsic spin-orbit coupling dominates over other contributions with a coupling constant of 3.7 meV. Although it is 1-3 orders of magnitude larger than those obtained from first principles, we show that comparable values are found for other honeycomb systems, MgB2 and LiC6; the latter is studied herein by electron spin resonance (ESR). We predict that spin coherence is longer preserved for spins perpendicular to the graphene plane, which is beneficial for spintronics. We identify experimental conditions when bulk ESR is realizable on graphene.
Simple Model of Nonlinear Spin Waves in Graphene Structures
A series of theoretical and experimental works is known which investigated the magnetic properties of graphene structures. This is due, among other things, to the prospects of using graphene as a material for the needs of the future nanoelectronics and spintronics. In particular, it is known about the presence of ferromagnetic properties at temperatures up to 200 C and above in a single-layer graphene films that are free from impurities. Previously there was proposed a quantum field theoretical model describing the possible mechanism of ferromagnetism in graphene as a result of spontaneous breaking of spin symmetry of the surface density of valence electrons. The possible spatial configurations of the localized spin density were described. In this paper we investigate such spatially localized nonlinear spin configurations of the valence electron density on the graphene surface such as kinks, and their interactions, as well as quasibound metastable states of the interacting kinks and antikinks, that are breathers. The spectrum of such breathers is investigated. It is shown that under certain conditions, this spectrum has a discrete sector, which, in turn, allows us to speak about the possibility of coherent quantum generation of spin waves in graphene structures, which is important in terms of practical applications in nanoelectronics and spintronics.
Spin relaxation times in disordered graphene
The European Physical Journal Special Topics, 2007
We consider two mechanisms of spin relaxation in disordered graphene. i) Spin relaxation due to curvature spin orbit coupling caused by ripples. ii) Spin relaxation due to the interaction of the electronic spin with localized magnetic moments at the edges. We obtain analytical expressions for the spin relaxation times τSO and τJ due to both mechanisms and estimate their values for realistic parameters of graphene samples. We obtain that spin relaxation originating from these mechanisms is very weak and spin coherence is expected in disordered graphene up to samples of length L ∼ 1µm.
Spin-Orbit-Mediated Spin Relaxation in Graphene
Physical Review Letters, 2009
We investigate how spins relax in intrinsic graphene. The spin-orbit coupling arises from the band structure and is enhanced by ripples. The orbital motion is influenced by scattering centers and ripple-induced gauge fields. Spin relaxation due to Elliot-Yafet and Dyakonov-Perel mechanisms and gauge fields in combination with spin-orbit coupling are discussed. In intrinsic graphene, the Dyakonov-Perel mechanism and spin flip due to gauge fields dominate and the spin-flip relaxation time is inversely proportional to the elastic scattering time. The spin-relaxation anisotropy depends on an intricate competition between these mechanisms. Experimental consequences are discussed.
Journal of Physics: Condensed Matter, 2018
We analyze the magnetic oscillations (MO) due to the de Haas-van Alphen effect, in pristine graphene under a perpendicular magnetic field, taking into account the Zeeman effect. We consider a constant Fermi energy, such that the valence band is always full and only the conduction band is available. At zero temperature the MO consist of two sawtooth peaks, one for each spin. Both peaks have the same frequency, but different amplitude and phase. We show that, in order to observe the spin splitting in the MO, Fermi energy of about 0.1 eV is required. At low temperatures we obtain that the MO can be expressed as the MO at zero temperature, plus small Fermi-Dirac like functions, each centered around the MO peaks. Using this expression, we show that the spin splitting is observable in the MO only when the thermal energy is smaller than the Zeeman energy. We also analyze the shift of the MO extrema as the temperature increases. We show that it depends on the magnetic field, which implies a broken periodicity at nonzero temperature. Finally, we obtain an analytical expression for the MO envelope. The results obtained could be used to infer temperature changes from the MO extrema shift and vice versa.
Theory of electron Zitterbewegung in graphene probed by femtosecond laser pulses
Physical Review B, 2009
We propose an experiment allowing an observation of Zitterbewegung (ZB, trembling motion) of electrons in graphene in the presence of a magnetic field. In contrast to the existing theoretical work we make no assumptions concerning shape of the electron wave packet. A femtosecond Gaussian laser pulse excites electrons from the valence n = −1 Landau level into three other levels, creating an oscillating electron wave packet with interband and intraband frequencies. Oscillations of an average position of the packet are directly related to the induced dipole moment and oscillations of the average packet's acceleration determine emitted electric field. Both quantities can be measured experimentally. A broadening of Landau levels is included to make the description of ZB as realistic as possible. Criteria of realization of a ZB experiment are discussed.
NMR parameters in gapped graphene systems
The European Physical Journal B, 2016
We calculate the nuclear spin-lattice relaxation time and the Knight shift for the case of gapped graphene systems. Our calculations consider both the massive and massless gap scenarios. Both the spin-lattice relaxation time and the Knight shift depend on temperature, chemical potential, and the value of the electronic energy gap. In particular, at the Dirac point, the electronic energy gap has stronger effects on the system nuclear magnetic resonance parameters in the case of the massless gap scenario. Differently, at large values of the chemical potential, both gap scenarios behave in a similar way and the gapped graphene system approaches a Fermi gas from the nuclear magnetic resonance parameters point of view. Our results are important for nuclear magnetic resonance measurements that target the 13 C active nuclei in graphene samples.