Separate Spin-up Research Articles

Modulation instability of ion-acoustic waves and exchange interaction in magnetized quantum plasma with relative density effects of spin-up and spin-down electrons

The separate-spin evolution quantum hydrodynamic (SSE-QHD) model is used to study Ion-Acoustic Waves (IAWs) in a magnetized quantum plasma having electrons with different spin populations in the presence of the spin particle exchange interactions (Coulomb type) in spin-down electrons only. The plasma consists of inertia-less separated spin-up and spin-down electrons states, while ions are inertial and classical. Besides, a uniform magnetic field is assumed to be directed in the Z-direction i.e. . An interesting interplay between exchange interaction and density polarization ratio κ has been observed on dispersive properties, effective Debye screening length and effective ion-acoustic speed. The non-linear Schrödinger equation is derived through a reductive perturbation approach. Moreover, the modulation instability condition and resulting envelope waves are analyzed. The parametric effects of density polarization ratio κ and Coulomb exchange interaction on dispersion, effective screening length, effective ion-acoustic speed, instability growth rate, and on the solitons profile have been discussed in detail. This model can be helpful to study properties of the dilute magnetic semiconductors and degenerate 2D & 3D Fermi electron gas, where exchange interactions are significant.

Skyrmion–electron interaction in the separated spin-up and spin-down quantum hydrodynamics approach

Abstract The Berry gauge field associated with adiabatic quantum evolution represents a very interesting area of research, especially for the description of non-relativistic fermions coupled to a skyrmion texture. We construct a quantum hydrodynamics model that describes the electron–skyrmion interactions, with separate descriptions of spin-up and spin-down electrons as two different fluids. The quantum effects, represented by the quantum Bohm potential, are taken into account. We investigate the electron gas moving in a smooth magnetic texture, where the magnetic moments adjust to the local magnetization direction of the skyrmion, and derive the equations of motion and evolution of spin density for two fluids of electrons in the emergent gauge fields. Applying the separated spin evolution quantum hydrodynamics to the 2D electron gas, traveling on the background of the Berry gauge field in plane samples, located in an external magnetic field, we predict a wave of a new kind in the electron gas. This novel wave appears as an influence of emergent fields and quantum effects.

Spin- and valley-polarized transport in a monolayer ofMoS2

We investigate dc and ac transport in a monolayer of ${\mathrm{MoS}}_{2}$ in the presence of an effective mass asymmetry, a momentum-dependent term, and an out-of plane or in-plane Zeeman term. Analytical results are presented for both dc and ac conductivities in the framework of linear response theory. We show that the spin-Hall conductivity exhibits a strong dependence on the strength of the spin-orbit interaction while both spin- and valley-Hall conductivities show a very weak dependence on the temperature. The sum of the well separated spin-up and spin-down components of the diffusive dc longitudinal conductivity is linear in the electron concentration though the Fermi level is a nonmonotonic function of it. Further, we evaluate the power absorption spectrum and assess its dependence on the spin and valley degrees of freedom as well as on the scattering which is essential at low frequencies.

Spin-electron acoustic waves: The Landau damping and ion contribution in the spectrum

Separated spin-up and spin-down quantum kinetics is derived for more detailed research of the spin-electron acoustic waves (SEAWs). This kinetic theory allows us to obtain the spectrum of the SEAWs including the effects of occupation of quantum states more accurately than the quantum hydrodynamic theory. We derive and apply the quantum kinetic theory to calculate the Landau damping of the SEAWs. We consider the contribution of ions dynamics into the SEAW spectrum. We obtain the contribution of ions in the Landau damping in the temperature regime of classic ions. Kinetic analysis for the ion-acoustic, zero sound, and Langmuir waves at the separated spin-up and spin-down electron dynamics is presented as well.

Separated spin-up and spin-down evolution of degenerated electrons in two-dimensional systems: Dispersion of longitudinal collective excitations in plane and nanotube geometry

Applying the separated spin evolution quantum hydrodynamics to the two-dimensional electron gas in plane samples and nanotubes located in external magnetic fields we have found a novel type of waves in the electron gas which is called spin-electron acoustic wave. A separate spin-up and spin-down electrons' evolution reveals the replacement of the Langmuir wave by a pair of hybrid waves. One of the two hybrid waves is a modified Langmuir wave. Another hybrid wave is a spin-electron acoustic wave. We studied the dispersion of these waves in two-dimensional structures of electrons. We also considered the dependence of dispersion properties on spin polarization of electrons in an external magnetic field.

Spin current evolution in the separated spin-up and spin-down quantum hydrodynamics

We have developed a method of quantum hydrodynamics (QHD) that describes particles with spin-up and with spin-down in separate. We have derived the equation of the spin current evolution as a part of the set of the quantum hydrodynamics equations that treat particles with different projection of spin on the preferable direction as two different species. We have studied orthogonal propagation of waves in the external magnetic field and determined the contribution of quantum corrections due to the Bohm potential and to magnetization energy of particles with different projections of spin in the spin-current wave dispersion. We have analyzed the limits of weak and strong magnetic fields.

Direct observation of the spin-dependent Peltier effect

The Peltier coefficient describes the amount of heat that is carried by an electrical current when it passes through a material. When two materials with different Peltier coefficients are placed in contact with one another, the Peltier effect causes a net flow of heat either towards or away from the interface between them. Spintronics describes the transport of electric charge and spin angular momentum by separate spin-up and spin-down channels in a device. The observation that spin-up and spin-down charge transport channels are able to transport heat independently of each other has raised the possibility that spin currents could be used to heat or cool the interface between materials with different spin-dependent Peltier coefficients. Here, we report the direct observation of the heating and cooling of such an interface by a spin current. We demonstrate this spin-dependent Peltier effect in a spin-valve pillar structure that consists of two ferromagnetic layers separated by a non-ferromagnetic metal. Using a three-dimensional finite-element model, we extract spin-dependent Peltier coefficients in the range -0.9 to -1.3 mV for permalloy. The magnetic control of heat flow could prove useful for the cooling of nanoscale electronic components or devices.

Spin Manipulation Using Magnetic II–VI Semiconductors

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.

Construction and application of an accurate local spin-polarized Kohn-Sham potential with integer discontinuity: Exchange-only theory.

An accurate spin-polarized exchange-only Kohn-Sham (KS) potential is constructed from a consideration of the optimized-effective-potential (OEP) method. A detailed analysis of the OEP integral equation for the exchange-only case results in a set of conditions which are manifestly satisfied by the exact OEP; these conditions are employed to construct an approximate OEP, ${\mathit{V}}_{\mathit{x}\mathrm{\ensuremath{\sigma}}}$, and therefore an approximate KS exchange-only potential as a functional of KS orbitals. Further, it is shown that this ${\mathit{V}}_{\mathit{x}\mathrm{\ensuremath{\sigma}}}$ can be derived analytically based on a simple approximation of the Green's functions in the OEP integral equation. The constructed potential, although approximate, contains many of the key analytic features of the exact KS potential: it reduces to the exact KS result in the homogeneous-electron-gas limit, approaches -1/r as r\ensuremath{\rightarrow}\ensuremath{\infty}, yields highest occupied-orbital energy eigenvalues ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{m}\mathrm{\ensuremath{\sigma}}}$ that satisfy Koopmans's theorem, and exhibits an integer discontinuity when considered as a function of fractional occupancy of the highest-energy occupied single-particle state of a given spin projection \ensuremath{\sigma}. In addition ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{m}\mathrm{\ensuremath{\sigma}}}$ nearly exactly satisfies Janak's theorem. The approximate OEP is a simple but remarkably accurate representation of the exact, numerically derived exchange-only OEP.Detailed numerical results obtained by employing ${\mathit{V}}_{\mathit{x}\mathrm{\ensuremath{\sigma}}}$ as the exchange-only potential for ten atoms with closed subshells yield total energies, Hartree potentials, single-particle expectation values, and ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{m}}$ which are in excellent agreement with both exact OEP and Hartree-Fock (HF) results and represent a significant improvement over the results obtained by employing other exchange-only potentials. Similarly, the properties of alkali-metal atoms are calculated including the separate spin-up and spin-down densities to obtain results in excellent agreement with those of spin-unrestricted OEP and HF methods. Finally, we demonstrate the accuracy of ${\mathit{V}}_{\mathit{x}\mathrm{\ensuremath{\sigma}}}$ by calculating the total energy, ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{m}\mathrm{\ensuremath{\uparrow}}}$, and ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{m}\mathrm{\ensuremath{\downarrow}}}$ as a function of fractional filling f, of the highest occupied single-particle orbital for the magnesium atom (Z=12) from N=9--12 electrons and find excellent agreement with both spin-unrestricted OEP and HF results even when ${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{m}\mathrm{\ensuremath{\sigma}}}$ is strongly dependent on f. In addition we display the integer discontinuity in ${\mathit{V}}_{\mathit{x}\mathrm{\ensuremath{\sigma}}}$ when the highest-energy spin subshell begins to be filled.

Nonlinear ionic pseudopotentials in spin-density-functional calculations

A new method for generating and using first-principles pseudopotentials is developed to treat explicitly the nonlinear exchange and correlation interaction between the core and the valence charge densities. Compared to existing potentials, the new scheme leads to significant improvement in the transferability of the potential. In particular, the spin-polarized configurations are well described with a single potential. The need for separate spin-up and spin-down ionic pesudopotentials is, thus, eliminated. The method can easily be implemented with minimal increase in computational effort. Results for both atoms and solids are demonstrated.