Spin-Polarized States of Nuclear Matter

The equations of state of spin-polarized nuclear matter and pure neutron matter are studied in theframework of the Brueckner-Hartree-Fock theory including a three-body force. The energy per nucleon E A (δ) calculatedin the full range of spin polarization δ = (ρ↑ - ρ↓)/ρ for symmetric nuclear matter and pure neutron matter fulfills aparabolic law. In both the cases the spin-symmetry energy is calculated as a function of the baryonic density alongwith the related quantities such as the magnetic susceptibility and the Landau parameter Go. The main effect of thethree-body force is to strongly reduce the degenerate Fermi gas magnetic susceptibility even more than the value withonly two-body force. The equation of state is monotonically increasing with the density for all spin-aligned configurationsstudied here so that no any signature is found for a spontaneous transition to a ferromagnetic state.     

Microscopic calculations of spin polarized neutron matter at finite temperature

The properties of spin polarized neutron matter are studied both at zero and finite temperature within the framework of the Brueckner–Hartree–Fock formalism, using the Argonne v18 nucleon–nucleon interaction. The free energy, energy and entropy per particle are calculated for several values of the spin polarization, densities and temperatures together with the magnetic susceptibility of the system. The results show no indication of a ferromagnetic transition at any density and temperature.     

Equation of State of Spin-Polarized Neutron Metier

Within the spin-dependent Brueckner-Hatree-Fock framework, the equation of state of the spin-polarized neutron matter has been investigated by adopting the realistic nucleon-nucleon interaction AV₁₈ supplemented with a microscopic three-body force. The related physical quantities such as spin-symmetry energy, magnetic susceptibility and the Landau parameter G₀ in spin channel, have been extracted. The three-body force effects have been studied and discussed with a special attention. It is shown that in the whole range of spin-polarization, the energy per particle of spin-polarized neutron matter fulfills a quadratic relation versus the spin-polarization parameter δ= (p-p )/p . The predicted spin-symmetry energy is positive in the density region up to ρ = 0.8fm^-3 and increase monotonically as increasing density so that no any evidence is found for a spontaneous transition to a ferromagnetic state in neutron matter. The three-body force effect is to strongly increase the spin-symmetry energy and reduce the magnetic susceptibility at high densities, as a consequence , to make neutron matter become more stable against spin fluctuation. The obtained Landau parameter G₀ and its density dependence may serve as a constraint on the spin-spin parts of the phenomenological Skyrme and Skyrme-like interactions .     

Current-induced torques in magnetic metals: Beyond spin-transfer

Current-induced torques (CITs) on ferromagnetic (FM) nanoparticles and on domain walls in FM nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles, we have discussed a microscopic picture of CITs in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that CITs can also act on the order parameter of an antiferromagnetic (AFM) metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.     

Magnetized hot neutron matter: Lowest order constrained variational calculations

We have studied the spin polarized hot neutron matter in the presence of strong magnetic field. In this work, using the lowest order constrained variational method at finite temperature and employing _AV18${\mathrm{AV}}_{18}$ nuclear potential, some thermodynamic properties of spin polarized neutron matter such as spin polarization parameter, free energy, equation of state and effective mass have been calculated. It has been shown that the strong magnetic field breaks the symmetry of the free energy, leading to a magnetized equilibrium state. We have found that the equation of state becomes stiffer by increasing both magnetic field and temperature. The magnetic field dependence of effective mass for the spin-up and spin-down neutrons has been investigated.     

Relativistic equation of state of nuclear matter for supernova and neutron star

We construct the equation of state (EOS) of nuclear matter using the relativistic mean field (RMF) theory in the wide density, temperature range with various proton fractions for the use of supernova simulation and the neutron star calculations. We first construct the EOS of homogeneous nuclear matter. We use then the Thomas-Fermi approximation to describe inhomogeneous matter, where heavy nuclei are formed together with free nucleon gas. We discuss the results on free energy, pressure and entropy in the wide range of astrophysical interest. As an example, we apply the resulting EOS on the neutron star properties by using the Oppenheimer-Volkoff equation.     

Ab initio calculation and analysis of the properties of digital magnetic heterostructures and diluted magnetic semiconductors of IV and III-V groups

We present the first-principles calculations of digital magnetic heterostructures Si/M, Ge/M. GaAs/M, GaSb/M, GaN/M and GaN/M (50%) with M=Cr, Mn, Fe, and Co. The interaction between magnetic dopants results in a wide spin-polarized two-dimensional band inside the gap. It is found that beginning occupation of the minority-spin band greatly increases the energy of the ferromagnetic (FM) state and leads, as a rule, to the antiferromagnetic (AFM) spin ordering. This mechanism causes transition to the AFM state, when interaction between magnetic atoms is too strong, and defines the optimum of Curie temperature as a function of transition element concentration in magnetic layers.     

Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La(1-x)Ba(x)CoO3

The nature of the magnetic ground state near the insulator-metal transition (IMT) in La(1-x)Ba(x)CoO3 was investigated via neutron scattering. Below the critical concentration, x(c)～0.22, a commensurate antiferromagnetic (AFM) phase appears initially. Upon approaching x(c), the AFM component weakens and a ferromagnetic (FM) ordered phase sets in while in the rhombohedral lattice. At x(c), a spin flip to a new FM structure occurs at the same time as the crystal symmetry transforms to orthorhombic (Pnma). The Pnma phase may be the driving force for the IMT.     

The theory of nuclear orientation via electron spin locking (NOVEL)

Nuclear orientation via electron spin locking (NOVEL) is a technique to orient nuclear spins embedded in a solid. Like other methods of dynamic nuclear polarization (DNP) it employs a small amount of unpaired electron spins and uses a microwave field to transfer the polarization of these unpaired electron spins to the nuclear spins. Traditional DNP uses CW microwave fields, but NOVEL uses pulsed electron spin resonance (ESR) techniques: a 90 degree pulse–90 degree phase shift–locking pulse sequence is applied and during the locking pulse the polarization transfer is assured by satisfying the Hartmann–Hahn condition. The transfer is coherent and similar to coherence transfer between nuclear spins. However, NOVEL requires an extension of the existing theory to many, inequivalent nuclear spins and to arbitrary, i.e. high electron and nuclear spin polarization. In this paper both extensions are presented. The theory is applied to the system naphthalene doped with pentacene, where the proton spins are polarized using the photo-excited triplet states of the pentacene molecules and found to show excellent agreement with the experimentally observed evolution of the polarization transfer during the locking pulse.     

Nonequilibrium nuclear-electron spin dynamics in semiconductor quantum dots

We study the spin dynamics in charged quantum dots in the situation where the resident electron is coupled to only about 200 nuclear spins and where the electron spin splitting induced by the Overhauser field does not exceed markedly the spectral broadening. The formation of a dynamical nuclear polarization as well as its subsequent decay by the dipole-dipole interaction is directly resolved in time. Because not limited by intrinsic nonlinearities, almost complete nuclear polarization is achieved, even at elevated temperatures. The data suggest a nonequilibrium mode of nuclear polarization, distinctly different from the spin temperature concept exploited on bulk semiconductors.     

Nuclear spin relaxation induced by a mechanical resonator

We report on measurements of the spin lifetime of nuclear spins strongly coupled to a micromechanical cantilever as used in magnetic resonance force microscopy. We find that the rotating-frame correlation time of the statistical nuclear polarization is set by the magnetomechanical noise originating from the thermal motion of the cantilever. Evidence is based on the effect of three parameters: (1) the magnetic field gradient (the coupling strength), (2) the Rabi frequency of the spins (the transition energy), and (3) the temperature of the low-frequency mechanical modes. Experimental results are compared to relaxation rates calculated from the spectral density of the magnetomechanical noise.     

Pion condensation at finite temperature: (I). Mean field theory

The pion-condensed state of neutron-rich matter at finite temperature is calculated within the framework of a simple σ-model, treating the pion field as a mean field. At high densities the matter is condensed with a spatially non-uniform condensate. However, we find the unexpected result that as the density is lowered, at any finite temperature, pure neutron matter always makes a transition to a state with a spatially uniform condensate. Pure neutron matter, within mean field theory, is condensed at all non-zero temperatures and densities. Matter with a small proton fraction at zero temperature has a qualitatively similar phase diagram, except that it is normal when both the temperature and density are sufficiently low.     

Thermally driven spin transport through a transverse-biased zigzag-edge graphene nanoribbon

In the framework of the Landauer-Büttiker formalism, we investigate coherent spin transport through a transverse-biased magnetic zigzag-edge graphene nanoribbon, with a temperature difference applied between the source and the drain. It is shown that a critical source temperature is needed to generate a spin-polarized current due to the presence of a forbidden transport gap. The magnitude of the obtained spin polarization exceeds 90% in a wide range of source temperatures, and its polarization direction could be changed by reversing the transverse electric field. We also find that, at fixed temperature difference, the spin-polarized current undergoes a transition from increasing to decreasing as the source temperature rises, which is attributed to the competition between the excited energy of electrons and the relative temperature difference. Moreover, by modulating the transverse electric field, the source temperature and the width of the ribbon, we can control the device to work well for generating a highly spin-polarized current.     

The Landau-Migdal parameters from the Brueckner theory

The zero-order Landau-Migdal parameters are discussed in the framework of the Brueckner-Hartree-Fock approximation with realistic two- and three-body forces. Their usefulness is proved in two applications. First, the structure functions in high-density nuclear matter are calculated within the linear response theory to weak probes and the neutrino mean free path is predicted for neutron stars. Second, the screening of the low-density neutron matter to the neutron pairing is calculated in the RPA without and with induced interaction. The extension of the calculation to symmetric nuclear matter reveals the anti-screening effect of the proton medium polarization.     

Spin polarised nuclear matter and its application to neutron stars

An equation of state (EOS) of nuclear matter with explicit inclusion of a spin-isospin dependent force is constructed from a finite range, momentum and density dependent effective interaction. This EOS is found to be in good agreement with those obtained from more sophisticated models for unpolarised nuclear matter. Introducing spin degrees of freedom, it is found that it is possible for neutron matter to undergo a ferromagnetic transition at densities realisable in the core of neutron stars. The maximum mass and the surface magnetic field of the neutron star can be fairly explained in this model. Since finding quark matter rather than hadronic matter at the core of neutron stars is a possibility, the proposed EOS is also applied to the study of hybrid stars. It is found using the bag model picture that one can in principle describe both the mass as well as the surface magnetic field of hybrid stars satisfactorily.     

Steady state and transient Overhauser effect in systems of three spins

The rate equations describing spin polarization in a system of three spins are derived and solved for the case of a free radical dissolved in a solvent containing two nuclear spins. Triple irradiation experiments indicate that a nuclear spin A can be effectively coupled to an electron spin C via a second nuclear spin B and measurements of both the steady state and transient Overhauser effects are in accord with the theoretical predictions for a three-spin system. The ‘three spin effect’ is found to operate only in dilute solutions of free radicals in which case the probabilities for transitions between different nuclear or electronic energy levels can be determined. It was found to be effective for fluorine nuclei—in the presence of both protons and a free radical and for carbon [13] nuclei in the presence of either protons or fluorine nuclei and a free radical. Detailed measurements have been performed for CHFCl₂, para-difluorobenzene, and meta-fluorotoluene containing the tritertiary butyl phenoxyl radical.     

Relaxation of polarized nuclei in superconducting rhodium

Nuclear spin lattice relaxation rates were measured in normal and superconducting (sc) rhodium with nuclear polarizations up to p = 0. 55. This was sufficient to influence the sc state of Rh, whose T(c) and B(c) are exceptionally low. Because B(c)相似文献   

Tunable electronic structure and magnetic coupling in strained two-dimensional semiconductor MnPSe<Subscript>3</Subscript>

The electronic structures and magnetic properties of strained monolayer MnPSe₃ are investigated systematically via first-principles calculations. It is found that the magnetic ground state of monolayer MnPSe₃ can be significantly affected by biaxial strain engineering, while the semiconducting characteristics are well-preserved. Owing to the sensitivity of the magnetic coupling towards structural deformation, a biaxial tensile strain of approximately 13% can lead to an antiferromagnetic (AFM)- ferromagnetic (FM) transition. The strain-dependent magnetic stability is mainly attributed to the competition of the direct AFM interaction and indirect FM superexchange interaction between the two nearest-neighbor Mn atoms. In addition, we find that FM MnPSe₃ is an intrinsic half semiconductor with large spin exchange splitting in the conduction bands, which is crucial for the spin-polarized carrier injection and detection. The sensitive interdependence among the external stimuli, electronic structure, and magnetic coupling makes monolayer MnPSe₃ a promising candidate for spintronics.     

Half-metallic exchange bias ferromagnetic/antiferromagnetic interfaces in transition-metal chalcogenides

To investigate half-metallic exchange bias interfaces, magnetic structures at ferromagnetic (FM)/antiferromagnetic (AFM) interfaces in the zinc blende transition-metal chalcogenides, and with compensated and uncompensated AFM interfaces, were determined by the full-potential linearized augmented plane-wave method. With the uncompensated AFM interface, an antiparallel alignment of the Cr and Mn moments induces an excellent half-metallicity. More striking still, in the compensated AFM interface the Cr moments in the FM layer lie perpendicular to the Mn moments in the AFM layer but the Mn moments strongly cant to induce a net moment so as to retain the half-metallicity. These findings may offer a key ingredient for exchange biased spintronic devices with 100% spin polarization, having a unidirectional anisotropy to control and manipulate spins at the nanoscale.     

Nuclear spin nanomagnet in an optically excited quantum dot

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins-the nuclear spin nanomagnet.