Spin Density Correlation Research Articles

Kriticheskie yavleniya v dinamicheskoy teorii spinovykh fluktuatsiy

Paramagnetic susceptibility and spin-density correlation function near the Curie temperature TC are studied using the dynamic spin fluctuation theory. The calculated critical indices of the susceptibility and correlation radius for Fe, Co, and Ni are found in reasonable agreement with bulk susceptibility measurements and neutron scattering experiments. It is shown that the critical power-law behavior holds at temperatures up to 1.10–1.15TC, which gives an estimate of the critical temperature region in ferromagnetic metals.

Spin relaxation rate for heavy quarks in weakly coupled QCD plasma

We compute the relaxation rate of the spin density of heavy quarks in a perturbative QCD plasma to leading-log order in the coupling constant g. The spin relaxation rate Γs in spin hydrodynamics is shown to be Γs ~ g4 log(1/g)T(T/M)2 in the heavy-quark limit T/M ≪ 1, which is smaller than the relaxation rate of other non- hydrodynamic modes by additional powers of T/M. We demonstrate three different methods to evaluate the spin relaxation rate: 1) the Green-Kubo formula in the spin hydrodynamic regime, 2) the spin density correlation function in the strict hydrodynamic limit, and 3) quantum kinetic theory of the spin distribution function in momentum space. We highlight the interesting differences between these methods, while they are ultimately connected to each other by the underlying Ward-Takahashi identity for the non-conserved spin density.

FFLO state driven by quasiperiodic Zeeman field and its transition to localized states

We numerically study the ground-state properties of a one-dimensional lattice model in the presence of a quasiperiodic Zeeman field, by means of the density matrix renormalization group algorithm. We map out a global phase diagram, which encloses a Bardeen-Cooper-Schrieffer (BCS) phase, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase where the Cooper pairs obtain a finite center-of-mass momentum, and a localized phase. The FFLO phase is promoted by the quasiperiodic Zeeman field, which demonstrates a power-law pairing correlation with a critical exponent ${\ensuremath{\eta}}_{\mathrm{FFLO}}<1$ decaying much slower than the charge density and spin density correlations. By tuning the interaction and the filling factor, we find the FFLO phase is suppressed in the strong interaction and low filling regions. A large Zeeman field destroys the FFLO state and drives a superconductor-insulator transition. Furthermore, we propose this FFLO phase and the superconductor-insulator transition could be observed in the optical lattice experiment.

Short-range order in metals above the Curie temperature

We use the dynamic spin-fluctuation theory to study spin-density correlations in ferromagnetic metals above the Curie temperature. We calculate the spatial spin-correlation function, dynamic susceptibility and local magnetic moment. The dynamic susceptibility is related to the energy-integrated scattering cross-section (effective moment). Calculated effective and local moments of Fe and Ni are found in good agreement with results of polarized neutron scattering experiment over a wide temperature range. We find that short-range order remains up to distances of about 6 Å in Fe and 8 Å in Ni at TTC = 1.1 and slowly decreases with temperature.

Spin-Density Correlations and Short-Range Order in Metals above the Curie Temperature

In the dynamic spin-fluctuation theory, we calculate the effective and local magnetic moments and spatial spin-density correlator. Our theoretical results are demonstrated by the example of bcc Fe. The effective and local moments are found in good agrement with results of polarized neutron scattering experiment over a wide temperature range. The calculated short-range order is small (up to 5 Å) and slowly decreases with temperature

Finite-temperature phases of two-dimensional spin-orbit-coupled bosons

We determine the finite temperature phase diagram of two dimensional bosons with two hyperfine (pseudo-spin) states coupled via Rashba-Dresselhaus spin-orbit interaction using classical field Monte Carlo calculations. For anisotropic spin-orbit coupling, we find a transition to a Berenzinskii-Kosterlitz-Thouless superfluid phase with quasi-long range order. We show that the spin-order of the quasi-condensate is driven by the anisotropy of interparticle interaction, favoring either a homogeneous plane wave state or stripe phase with broken translational symmetry. Both phases show characteristic behavior in the algebraically decaying spin density correlation function. For fully isotropic interparticle interaction, our calculations indicate a fractionalized quasi-condensate where the mean-field degeneracy of plane wave and stripe phase remains robust against critical fluctuations. In the case of fully isotropic spin-orbit coupling, the circular degeneracy of the single particle ground state destroys the algebraic ordered phase in the thermodynamic limit, but a cross-over remains for finite size systems.

Spin–density correlations and magnetic neutron scattering in ferromagnetic metals

We obtain expressions for the spatial spin-density correlator and for effective and local magnetic moments in the dynamic spin-fluctuation theory. We derive formulas for the magnetic scattering cross section in the theory of itinerant electron magnets. We calculate magnetic characteristics of bcc Fe in the paramagnetic state and compare our numerical results with the polarized neutron scattering experiment. We show that the short-range order in bcc Fe persists up to a temperature much higher than the Curie temperature but at rather small distances (up to 5A).

Spin-density correlations and magnetic neutron scattering in ferromagnetic metals

Получены выражения для пространственного коррелятора спиновой плотности, эффективного и локального магнитных моментов в динамической теории спиновых флуктуаций. Выведены формулы для сечения магнитного рассеяния в теории магнетиков с коллективизированными электронами. Рассчитаны магнитные характеристики ОЦК-железа в парамагнитном состоянии и проведено сравнение численных результатов с экспериментом по поляризованному рассеянию нейтронов. Показано, что ближний порядок в ОЦК-железе сохраняется при температурах значительно выше температуры Кюри, но лишь на небольших расстояниях (не более 5 A).

Spin-density correlations in the dynamic spin-fluctuation theory: Comparison with polarized neutron scattering experiments

To study the spin-density correlations in the ferromagnetic metals above the Curie temperature, we relate the spin correlator and neutron scattering cross-section. In the dynamic spin-fluctuation theory, we obtain explicit expressions for the effective and local magnetic moments and spatial spin-density correlator. Our theoretical results are demonstrated by the example of bcc Fe. The effective and local moments are found in good agreement with results of polarized neutron scattering experiment over a wide temperature range. The calculated short-range order is small (up to 4Å) and slowly decreases with temperature.

Short-range order above the Curie temperature in the dynamic spin-fluctuation theory

Based on the dynamic spin-fluctuation theory, we study the spin-density correlations in the ferromagnetic metals. We obtain computational formulae for the correlation function and correlation radius in different approximations of the theory. Using these formulae, we calculate the magnetic short-range order above the Curie temperature in bcc Fe. Results of the calculation confirm our theoretical prediction that the inverse correlation radius increases linearly with temperature for T sufficiently large. The calculated short-range order is small but sufficient to correctly describe neutron scattering experiments. A considerable amount of the short-range order is shown to persist up to temperatures much higher than the Curie temperature.

Short-range Order Above TC in Ferromagnetic Metals

We study the spin-density correlations in ferromagnetic metals using the dynamic spin-fluctuation theory. We derive computational formulae for the spatial correlation function and use them to calculate the short-range order above TC in bcc Fe. Results of the calculation confirm our theoretical prediction that the inverse correlation radius increases linearly with temperature for T sufficiently large. The calculated short-range order is small but sufficient to correctly describe neutron scattering experiments. We show that considerable amount of the short-range order persists up to temperatures much higher than TC.

Spin diffusion in anisotropic Heisenberg chains: S≥1/2

Abstract In this paper, we investigate spin diffusion in Heisenberg chains with uniaxial nearest-neighbor interactions. The approach followed is based on an analysis of the infinite-temperature longitudinal spin density and spin current correlation functions. For S =1/2, exact results are presented for the time-dependent correlation functions in the XY limit. Away from this limit, the second and fourth moments of the Fourier transform of the spin density correlation function provide information about spin dynamics for arbitrary values of the spin. The moments are used in an assessment of the accuracy of the Gaussian approximation for the spin diffusion constant for S =1/2. The general behavior of the Gaussian approximation when S >1/2 is discussed, and numerical results for the spin diffusion constant are presented for S =1/2, 1, 3/2, 2 and in the classical limit. A moment-based criterion for the boundary in reciprocal space between diffusive and non-diffusive dynamics that applies to arbitrary values of the spin is presented.

Magnetic phase transition in CoO under high pressure: A challenge forLSDA+U

The high-spin low-spin transition under pressure in CoO is investigated by the local spin-density approximation plus onsite Coulomb correlation $(\text{LSDA}+U)$ approach. The magnetic moment collapse is found to be caused by a competition between the ligand field and the intra-atomic exchange. The interplay of the interactions leads to a reduction in the symmetry below that of the antiferromagnetic order. This symmetry breaking causes a considerable reduction in the predicted moment-collapse transition pressure. For the multivalley functional of the $\text{LSDA}+U$ approach it is crucial to consider initial conditions of self-consistency of sufficiently low symmetry. In particular, if an imposed symmetry would enforce a metallic solution for $U$ of the order of the bandwidth, the commonly used symmetry breaking just by initial spin polarization may not be sufficient.

Ultrashort lifetime expansion for indirect resonant inelastic x-ray scattering

In indirect resonant inelastic X-ray scattering (RIXS) an intermediate state is created with a core-hole that has a ultrashort lifetime. The core-hole potential therefore acts as a femtosecond pulse on the valence electrons. We show that this fact can be exploited to integrate out the intermediate states from the expressions for the scattering cross section. By this we obtain an effective scattering cross section that only contains the initial and final scattering states. We derive in detail the effective cross section which turns out to be a resonant scattering factor times a linear combination of the charge response function $S({\bf q},\omega)$ and the dynamic longitudinal spin density correlation function. This result is asymptotically exact for both strong and weak local core-hole potentials and ultrashort lifetimes. The resonant scattering pre-factor is shown to be weakly temperature dependent. We also derive a sum-rule for the total scattering intensity and generalize the results to multi-band systems. One of the remarkable outcomes is that one can change the relative charge and spin contribution to the inelastic spectral weight by varying the incident photon energy.

Rotation of Cosmic Voids and Void Spin Statistics

We present a theoretical study of void spins and their correlation properties. The concept of the spin angular momentum for an unbound void is introduced to quantify the effect of the tidal field on the distribution of matter that make up the void. Both the analytical and numerical approaches are used for our study. Analytically, we adopt the linear tidal torque model to evaluate the void spin-spin and spin-density correlations, assuming that a void forms in the initial region where the inertia momentum and the tidal shear tensors are maximally uncorrelated with each other. Numerically, we use the Millennium run galaxy catalog to find voids and calculate their spin statistics. The numerical results turn out to be in excellent agreement with the analytic predictions, both of which consistently show that there are strong spatial alignments between the spin axes of neighbor voids and strong anti-alignments between the void spin axes and the directions to the nearest voids. We expect that our work will provide a deeper insight into the origin and properties of voids and the large scale structure.

Metal–insulator transition in 2D: the role of interactions and disorder

We present a model for the metal–insulator transition in 2D, observed in the recent years. Our starting point consists of two ingredients only, which are ubiquitous in the experiments: Coulomb interactions and weak disorder spin scattering (coming from the interfaces of the heterostructures in question). In a diagramatic approach, we predict the existence of a characteristic temperature T 0 = T 0 ( n , ω H ) , n being the density of carriers, and ω H the Zeeman energy, below which these systems become metallic, due to the onset of strong spin–density correlations. This is in very good agreement with experiments, and corroborates the fact that varying n and ω H are equivalent ways into/out of the metallic regime. The conductivity, calculated as a function of temperature and ω H in the metallic state, compares favorably to experiment. Moreover, we give an explicit expression for the conventional weak disorder contributions to the conductivity in the frame of our model. We comment on the nature of the transition, we calculate the specific heat of the system and we discuss the fate of the metallic state in the limit of zero temperature.

Theory of indirect resonant inelastic X-ray scattering

We express the cross section for indirect resonant inelastic X-ray scattering in terms of an intrinsic dynamic correlation function of the system that is studied with this technique. The cross section is a linear combination of the charge response function and the dynamic longitudinal spin density correlation function. This result is asymptotically exact for both strong and weak local core–hole potentials. We show that one can change the relative charge and spin contribution to the inelastic spectral weight by varying the incident photon energy.

Renormalization group for two-dimensional fermions with a flat Fermi surface

We present a renormalization-group analysis of two-dimensional interacting fermion systems with a closed and partially flat Fermi surface. Numerical solutions of the one-loop flow equations show that for a bare local repulsion, the system evolves through three different regimes as the typical energy is lowered: a high-energy transient with a strong competition between particle-particle and particle-hole channels, an intermediate regime with dominant spin-density wave correlations, and finally a ``hot spot'' regime exhibiting d-wave superconductivity. We study, mostly by analytical methods, how this flow pattern depends on the number N of Fermi-surface patches used in the numerical solution. This clearly indicates that the final regime requires vanishingly small microscopic interactions, for the one-loop approximation to be valid, as N is going to infinity.

The phase diagram and inhomogenous superconductivity in the one-dimensional attractive Hubbard model

The thermodynamic phase diagram of the attractive Hubbard model in the presence of magnetic field is calculated in wide range of temperature, electron concentration and coupling strength within the generalized self-consistent field (GSCF) approach, including the spin density correlations. The theory provides a smooth transition driven by applied magnetic field h and temperature T from the homogoneous superconducting state into a spatially inhomogeneous state with the order parameter Δ Q (−) ≠ 0.

QUANTUM MONTE CARLO SIMULATION STUDY OF ONE-DIMENSIONAL PERIODIC ANDERSON MODEL

Many rare earth and actinide intermetallics, known as Heavy Fermion Systems, have recently been successfully described by the Periodic Anderson Model. We investigate, in this work, various electronic and magnetic properties of the one-dimensional Periodic Anderson model using path integral formulation along with the quantum Monte Carlo simulation technique. We have studied the singlet and triplet pairing correlation functions, nearest neighbor charge-density correlations, spin density correlations, local squared magnetic moment and probability of double occupancy of f-electrons, as a function of intra-atomic Coulomb interaction for various values of hybridization parameter and the temperatures.