Hysteresis, spin transitions, and magnetic ordering in the fractional quantum Hall effect

We have studied the development of metastable properties associated with a nearly spin-degenerate two-dimensional electron system. Application of large hydrostatic pressure significantly reduces the g-factor experienced by electrons in GaAs/AlGaAs heterostructure, and various fractional quantum Hall effect (FQHE) states are found to undergo transition to a spin-unpolarized ground state. In case of even numerator FQHE states, the spin transitions are accompanied by hysteresis and nonlinearity in the magnetotransport. These results strongly support a recent theory of quantum Hall magnetism in which competition between spin-polarized and spin-unpolarized ground states leads to an ordered phase that exhibits ferromagnetic correlation.     

The spin-dependent transport properties of endohedral transition-metal-fullerene X@C66H4 (X=Fe,Co, Mn,Ni)

Inspired by recent experiments on the successful synthesis of hydrofullerene C₆₆H₄ in Tian et al. (2019) [12] with two negatively curved heptagons. Based on the density functional theory and nonequilibrium Green's function method, we report the spin-dependent transport through transition-metal-atom-encapsulated C₆₆H₄ hydrofullerene, i.e., X@C₆₆H₄(X=Fe, Co, Mn, Ni), contacted by single gold atoms via semi-infinite non-magnetic Au electrodes. It is found that, Mn- and Fe-doped systems show highly spin-polarized transmission as well as considerable magnetic moments whereas Ni-doped systems show completely spin-unpolarized transmission and nonmagnetic. Interestingly, Co-doped systems show two spin states, i.e., spin-polarized and spin-unpolarized ones. Further analysis shows that, for Mn-, Fe- and Ni-doped systems, the spin-polarized/unpolarized state is caused by the finite/(nearly-)zero magnetism of the encapsulated metal atom. While the magnetism in Co-doped systems is quenched for the top hexagonal doping case, but not for the side heptagonal doping one, which induces the spin-unpolarized/spin-polarized state. And the screening effect of electrodes on the magnetism of Co is the underlying physical mechanism. Our findings would be beneficial to the design of spintronics devices.     

Effective electron-electron interactions and magnetic phase transition in a two-dimensional electron liquid

We investigate the spin-dependent effective electron-electron interactions in a uniform system of two-dimensional electrons to understand the spontaneous magnetization expected to occur at very low density. For this purpose, we adopt the Kukkonen-Overhauser form for the effective interactions which are built by accurately determined local-field factors describing the charge and spin fluctuations. The critical behavior of the effective interaction for parallel spin electrons allows us to quantitatively locate the transition to the ferromagnetic state at r_s≈27. When the finite width effects are approximately taken into account the transition occurs at r_s≈30 in agreement with recent quantum Monte Carlo calculations.     

Theoretical study of magnetic ordering and electronic properties of AgxAl1−x N compounds

Ferromagnetic ordering of silver impurities in the AlN semiconductor is predicted by plane-wave ultrasoft pseudopotential and spin-polarized calculations based on density functional theory (DFT). It was found that an Ag impurity atom led to a ferromagnetic ground state in Ag_0.0625Al_0.9375N, with a net magnetic moment of 1.95 μ_B per supercell. The nitrogen neighbors at the basal plane in the AgN₄ tetrahedron are found to be the main contributors to the magnetization. This magnetic behavior is different from the ones previously reported on transition metal (TM) based dilute magnetic semiconductor (DMS), where the magnetic moment of the TM atom impurity is higher than those of the anions bonded to it. The calculated electronic structure band reveals that the Ag-doped AlN is p-type ferromagnetic semiconductor with a spin-polarized impurity band in the AlN band gap. In addition, the calculated density of states reveals that the ferromagnetic ground state originates from the strong hybridization between 4d-Ag and 2p-N states. This study shows that 4d transition metals such as silver may also be considered as candidates for ferromagnetic dopants in semiconductors.     

Spatial correlations in the electron gas: Path integral Monte Carlo simulation

Thermally excited states of the three-dimensional electron gas in a neutralizing background are computed by path integral Monte Carlo simulation for values of the Wigner-Seitz radius within the interval 5 < r _s < 15. Coulomb and exchange interactions, permutation symmetry, and spin state are treated explicitly. Variation of electron correlation functions with density and temperature is analyzed. Quantum effects suppress and enhance spatial correlation at low and high densities, respectively. Transition between the electron-gas states characterized by these opposite trends corresponds to a density of approximately 2.5 × 10²¹ cm^?3. A transition line between liquid-like and gaslike phases is determined in the temperature-density diagram. Weak anisotropy of many-body correlations in the liquid-like state stimulates excitation of spherically symmetric collective rotational modes. The effective short-range pseudopotential exhibits strong temperature dependence due to exchange effects. For strongly correlated systems, the characteristic screening length deviates from that predicted by the Thomas-Fermi screening model ( $ \sim \sqrt {r_s } $ ), approaching a linear function of r _s. The effective short-range interaction substantially differs from the Yukawa potential in mean field theory. Coulomb interaction shifts the Fermi level up by an order of magnitude or higher, and this effect becomes stronger with decreasing density.     

Electronic structure of adsorbed alkali atoms for comparison with UPS data

We have generalised our earlier work calculating the induced density of states in alkali chemisorption to include the valence np level in addition to the s level. This allows the theory to take account of the possibility of sp hybridization. Hybridization is said to be weak is the sp level separation ΔE_sP exceeds twice the width of the s or p_z resonance. In the converse case hybridization is strong and the induced density of states no longer reflects distinguishable s and p_z resonances. We argue that ΔE_sP for the chemisorbed alkali exceeds considerably that for the free alkali atom. In the case of Na on r_s = 2 jellium (considered typical of the smaller alkali on r_s = 2?3 substrates) hybridization is strong for the atomic value of ΔE_sp but weak when a somewhat larger value is taken, making the conclusions a little indefinite. For Cs on Cu(111), we find a clear situation of weak hybridization with the atomic ΔE_sP and a fortiori with a more realistic value. There should thus be a well defined 6s resonance in this system. Our estimate of the width of the 6s resonance is in good agreement with photoemission data.     

Insulating state of Na clusters and their metallic transition in low-silica X zeolite

Optical and magnetic properties are investigated for low-silica X (LSX) zeolite loaded with Na metals at various loading densities. In LSX, β-cages with the inside diameter of ∼7 Å are arrayed in a diamond structure and the supercages with that of ∼13 Å are formed among them. When the average number of guest Na atoms per β cage, n, is smaller than ∼2, optical reflectance shows a peak at ∼2.5 eV. This corresponds to the 1s-1p transition of the cluster formed in β cage. At n>2, this peak disappears and several strong peaks are seen at 1-3 eV. The oscillator strength increases with n. They can be attributed to surface-plasmon-like excitations of the Na clusters formed in supercages. At n≤10, clear absorption tails with energy gaps are observed at near IR region indicating insulating states. However, when n is increased up to ∼12, a clear Drude reflection suddenly appears in the IR region, indicating that an insulator-to-metal transition abruptly occurs. This dramatic change of the electronic state may be caused by the successive connection between adjacent clusters in supercages due to some special arrangement of Na⁺ ions as well as the delocalized wave function of electrons at high Na-loading density.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

Correlation energy of 2-D electrons

The correlation energy and the Fermi momentum of an electron gas in 2-D are evaluated explicitly as functions of density. The ring diagram and first- and second-order exchange contributions are treated. In comparison with the 3-D case, the kinetic energy for the same r_s is approximately one-half and the exchange and correlation energies are somewhat larger. The ground state energy plotted against r_s shows a minimum at around r_s = 1.65 with a minimum value of ?0.9858 Ryd. If the third-order ring contribution is added, the curve is shifted upward. The correlation energy is ?0.6258 to order e⁴. The third-order ringw contribution increases this value almost linearly with r_s. The Fermi momentum decreases with r_s due to the contribution. Different from the 3-D case, no ln r_s term appears in the correlation energy within the approximation.     

Kondo effect and spin filtering in triangular artificial atoms

We studied, strongly correlated states in triangular artificial atoms. Symmetry-driven orbital degeneracy of the single particle states can give rise to an SU(4) Kondo state with entangled orbital and spin degrees of freedom, and a characteristic phase shift δ=π/4. Upon application of a Zeeman field, a purely orbital Kondo state is formed with somewhat smaller Kondo temperature and a fully polarized current through the device. The Kondo temperatures are in the measurable range. The triangular atom also provides a tool to systematically study the singlet-triplet transitions observed in recent experiments [Phys. Rev. Lett., 88 (2002) 126803, cond-mat/0208268 (2002)].     

Strange-quark matter within the Nambu-Jona-Lasinio model

The equation of state of baryon-rich quark matter is studied within the SU(3) Nambu-Jona-Lasinio model with flavor-mixing interaction. Possible bound states (strangelets) and chiral phase transitions in this matter are investigated at various values of the strangeness fraction r _s. Model predictions are very sensitive to the ratio of the vector and scalar coupling constants, ξ=G _V/G _S. At ξ=0.5 and zero temperature, the binding energy takes a maximum value of about 15 MeV per baryon at r _s?0.4. Such strangelets are negatively charged and have typical lifetimes of about 10^?7s. Calculations are performed at finite temperatures as well. According to these calculations, bound states exist up to temperatures of about 15 MeV. The model predicts a first-order chiral phase transition at finite baryon densities. The parameters of this phase transition are calculated as functions of r _s.     

Metamorphosis of the quantum Hall ferromagnet at nu=2/5

We report on the dramatic evolution of the quantum Hall ferromagnet in the fractional quantum Hall regime at nu=2/5 filling. A large enhancement in the characteristic time scale gives rise to a dynamical transition into a novel quantized Hall state. The observed Hall state is determined to be a zero-temperature phase distinct from the spin-polarized and spin-unpolarized nu=2/5 fractional quantum Hall states. It is characterized by a strong temperature dependence and puzzling correlation between temperature and time.     

Quasi-Debye response of ion-hopping conduction at lowest frequencies

On the basis of the relaxation mode theory, quasi-Debye response in the dynamic conductivity by hopping ions is theoretically investigated in three-dimensional random lattices with a uniform distribution of activation energies. It is shown at lowest frequencies that the dynamic conductivity is approximately given by σ(ω)∼σ(0)+A″ω^s″, s″=2−n, where n is a non-integer exponent originated from the mode diffusion length and density of states. The numerical analyses give rise to some values of s″<2, for example s″∼1.5, which behaves as s″→2 with increasing temperature.     

Ground-state properties of jellium metals with low electron densities

The ground state of a three-dimensional electron gas is theoretically investigated within the framework of the local spin density approximation with the Perdew–Zunger exchange-correlation energy. The system has been found to be in a one- or two-dimensional crystal state, when the Wigner sphere radius r_s has an intermediate value. At r_s=60, a triangular lattice with the lattice spacing 96.10 is the lowest energy state among fluids, 1D, 2D, and 3D crystals.     

Magnetic phase diagram in a quasi-two-dimensional electron gas

Using spin density functional theory within the framework of the local spin density approximation with Perdew-Zunger type exchange-correlation energy, ferromagnetism in a quasi-two-dimensional electron gas (Q-2DEG) is studied. The electronic and magnetic structures of a thin film are calculated as a function of film thickness and electron density. Ferromagnetism in the Q-2DEG is found to appear at a higher electron density than in the three-dimensional electron gas. Unless a film is very thin, with decreasing electron density, a magnetic phase transition occurs from a spin-unpolarized fluid to a Wigner film with surface magnetism, in which the spin polarization localizes only in the neighborhood of surfaces. Further decreasing density induces another transition to a fully spin-polarized ferromagnetic Wigner film.     

Passage from spin-polarized surface states to unpolarized quantum well states in topologically nontrivial Sb films

Topological materials have unusual surface spin properties including a net surface spin current protected by the bulk symmetry properties. When such materials are reduced to thin films, their gapless spin-polarized surface states must connect, by analytic continuation, to bulk-derived quantum-well states, which are spin-unpolarized in centrosymmetric systems. The nature of this passage in a model system, Sb films, is investigated. Angle-resolved photoemission shows a smooth transition, while calculations elucidate the correlated evolution of the spin and charge distributions in real space.     

Spin excitations in granular structures with ferromagnetic nanoparticles

Spin excitations and relaxation in a granular structure which contains metallic ferromagnetic nanoparticles in an insulating amorphous matrix are studied in the framework of the s-d exchange model. As the d system, we consider the granule spins, and the s system is represented by localized electrons in the amorphous matrix. In the one-loop approximation with respect to the s-d exchange interaction for a diagram expansion of the spin Green’s function, the spin excitation spectrum is found, which consists of spin-wave excitations in the granules and of polarized spin excitations. In polarized spin excitations, a change in the granule spin direction is accompanied by an electron transition with a spin flip between two sublevels of a split localized state in the matrix. We considered polarized spin relaxation (relaxation of the granule spins occurring by means of polarized spin excitations) determined by localized deep energy states in the matrix and the thermally activated electronic cloud of the granule. It is found that polarized spin relaxation is efficient over a wide frequency range. Estimates made for structures with cobalt granules showed that this relaxation could be observed in centimetric, millimetric, and submillimetric wavelength ranges.     

Motion of a 3D exciton in a magnetic field: Exciton-magnetoexciton “phase” transition

A rearrangement of the ground state of a Wannier-Mott exciton upon an increase in its momentum is considered. The phase diagram of the electron and the hole experiencing the Coulomb interaction on the magnetic momentum-external magnetic field plane is investigated. A jumplike exciton-magnetoexciton “phase” transition is observed upon an increase in the momentum in fields B weaker than a certain value B<B _tr1. As momentum P increases above a certain critical value P _tr(B), the ground state of the system changes from the hydrogen-like state polarized by the Lorentz force to the magnetoexciton state in which the average distance 〈 r〉 between the electron and the hole increases jumpwise in the transverse direction relative to the field. As the exciton momentum increases, its wave function is extended along the magnetic field, acquiring the shape of a strongly prolate ellipsoid. It is interesting that the momentum of the transition tends to a finite value P ₀>0 even for B→0. At the point of transition, the exciton energy-momentum relation changes jumpwise from a quadratic law to a relation virtually independent of the momentum. For B<B _tr1, the exciton-magnetoexciton transition becomes blurred.     

Nonuniform van der Waals theory

The liquid-vapor interface of a confined fluid at the condensation phase transition is studied in a combined hydrostatic/mean-field limit of classical statistical mechanics. Rigorous and numerical results are presented. The limit accounts for strongly repulsive short-range forces in terms of local thermodynamics. Weak attractive longer-range ones, like gravitational or van der Waals forces, contribute a self-consistent mean potential. Although the limit is fluctuationfree, the interface is not a sharp Gibbs interface, but its structure is resolved over the range of the attractive potential. For a fluid of hard balls with –r ^–6 interactions the traditional condensation phase transition with critical point is exhibited in the grand ensemble: A vapor state coexists with a liquid state. Both states are quasiuniform well inside the container, but wall-induced inhomogeneities show up close to the boundary of the container. The condensation phase transition of the grand ensemble bridges a region of negative total compressibility in the canonical ensemble which contains canonically stable proper liquid-vapor interface solutions. Embedded in this region is a new, strictly canonical phase transition between a quasiuniform vapor state and a small droplet with extended vapor atmosphere. This canonical transition, in turn, bridges a region of negative total specific heat in the microanonical ensemble. That region contains subcooled vapor states as well as superheated very small droplets which are microcanonically stable.