Search results for "electrons"
showing 10 items of 1325 documents
Hund's Rules and Spin Density Waves in Quantum Dots
1997
Spin density functional theory is used to calculate the ground state electronic structures of circular parabolic quantum dots. We find that such dots either have a spin configuration determined by Hund's rule or make a spin-density-wave-like state with zero total spin. The dependence of the spin-density-wave amplitudes on the density of the two-dimensional electron gas is studied.
Exchange and correlation energy functionals for two-dimensional open-shell systems
2009
We consider density functionals for exchange and correlation energies in two-dimensional systems. The functionals are constructed by making use of exact constraints for the angular averages of the corresponding exchange and correlation holes, respectively, and assuming proportionality between their characteristic sizes. The electron current and spin are explicitly taken into account, so that the resulting functionals are suitable to deal with systems exhibiting orbital currents and/or spin polarization. Our numerical results show that in finite systems the proposed functionals outperform the standard two-dimensional local spin-density approximation, still performing well also in the importa…
Spin pumping in noncollinear antiferromagnets
2021
The ac spin pumping of noncollinear antiferromagnets is theoretically investigated. Starting from an effective action description of the spin system, we derive the Onsager coefficients connecting the spin pumping and spin-transfer torque associated with the dynamics of the SO(3)-valued antiferromagnetic order parameter. Our theory is applied to a kagome antiferromagnet resonantly driven by a uniform external magnetic field. We demonstrate that the reactive (dissipative) spin-transfer torque parameter can be extracted from the pumped ac spin current in-phase (in quadrature) with the driving field. Furthermore, we find that the three spin-wave bands of the kagome AF generate spin currents wit…
Minimal Model of Spin-Transfer Torque and Spin Pumping Caused by the Spin Hall Effect
2015
In the normal metal/ferromagnetic insulator bilayer (such as Pt/Y$_{3}$Fe$_{5}$O$_{12}$) and the normal metal/ferromagnetic metal/oxide trilayer (such as Pt/Co/AlO$_{x}$) where spin injection and ejection are achieved by the spin Hall effect in the normal metal, we propose a minimal model based on quantum tunneling of spins to explain the spin-transfer torque and spin pumping caused by the spin Hall effect. The ratio of their damping-like to field-like component depends on the tunneling wave function that is strongly influenced by generic material properties such as interface $s-d$ coupling, insulating gap, and layer thickness, yet the spin relaxation plays a minor role. The quantified resu…
Spin states, vibrations and spin relaxation in molecular nanomagnets and spin qubits: a critical perspective
2018
Spin–vibration coupling has been proven to be crucial for spin dynamics; theoretical studies are now addressing this experimental challenge.
Quantum Spin-Tunneling:A Path Integral Approach
1995
We investigate the quantum tunneling of a large spin in a crystal field and an external magnetic field. The twofold degeneracy of the corresponding classical ground state is removed due to tunneling. The tunnel splitting ΔE o of the ground state is calculated by use of a path integral formalism. It is shown that coherent spin state path integrals do not yield a reasonable result. However a “bosonlzation” of the spin system yields excellent results in the semiclassical limit. This result follows from the coherent spin state approach from replacing the spin quantum number s by s + 1/2 which causes a renormalization of the preexponential factor of ΔE o .
Exact dynamics of XX central spin models
2009
The dynamical behavior of a star network of spins, wherein each of N decoupled spins interact with a central spin through non uniform Heisenberg XX interaction is exactly studied. The time-dependent Schrodinger equation of the spin system model is solved starting from an arbitrary initial state. The resulting solution is analyzed and briefly discussed.
Excitation of singlet–triplet coherences in pairs of nearly-equivalent spins
2019
We present approaches for an efficient excitation of singlet–triplet coherences in pairs of nearly-equivalent spins. Standard Nuclear Magnetic Resonance (NMR) pulse sequences do not excite these coherences at all or with very low efficiency. The single quantum singlet–triplet coherences, here termed the outer singlet–triplet coherences, correspond to lines of low intensity in the NMR spectrum of a strongly-coupled spin pair (they are sometimes referred to as “forbidden transitions”), whereas the zero-quantum coherences, here termed the inner singlet–triplet coherences, do not have a direct spectral manifestation. In the present study, we investigated singlet–triplet coherences in a pair of …
Electron-driven spin diffusion supports crossing the diffusion barrier in MAS DNP
2018
Dynamic nuclear polarization (DNP) can be applied to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude due to microwave-driven transfer of spin polarization from unpaired electrons to nuclei. While the underlying quantum mechanical aspects are sufficiently well understood on a microscopic level, the exact description of the large-scale spin dynamics, usually involving hundreds to thousands of nuclear spins per electron, is still lacking consensus. Generally, it is assumed that nuclear hyperpolarization can only be observed on nuclei which do not experience strong influence of the unpaired electrons and thus being significantly removed from the paramagneti…
Antiferromagnetic Topological Insulator with Nonsymmorphic Protection in Two Dimensions
2019
The recent demonstration of topological states in antiferromagnets (AFMs) provides an exciting platform for exploring prominent physical phenomena and applications of antiferromagnetic spintronics. A famous example is the AFM topological insulator (TI) state, which, however, was still not observed in two dimensions. Using a tight-binding model and first-principles calculations, we show that, in contrast to previously observed AFM topological insulators in three dimensions, an AFM TI can emerge in two dimensions as a result of a nonsymmorphic symmetry that combines the twofold rotation symmetry and half-lattice translation. Based on the spin Chern number, Wannier charge centers, and gapless …