Transverse Spin Fluctuations Research Articles

Pseudogap and the Fermi surface in the t– J model

We calculate spectral functions within the t– J model as relevant to cuprates in the regime from low to optimum doping. On the basis of equations of motion for projected operators an effective spin–fermion coupling is derived. The self-energy due to short-wavelength transverse spin fluctuations is shown to lead to a modified self-consistent Born approximation, which can explain strong asymmetry between hole and electron quasiparticles. The coupling to long-wavelength longitudinal spin fluctuations governs the low-frequency behavior and results in a pseudogap behavior, which at low doping effectively truncates the Fermi surface.

Stability of the doped antiferromagnetic state of thet−t′Hubbard model

The next-nearest-neighbour hopping term t' is shown to stabilize the AF state of the doped Hubbard model with respect to transverse perturbations in the order- parameter by strongly suppressing the intraband particle-hole processes. For a fixed sign of t', this stabilization is found to be significantly different for electron and hole doping, which qualitatively explains the observed difference in the degree of robustness of the AF state in the electron-doped (Nd_{2-x}Ce_{x}CuO_{4}) and hole-doped (La_{2-x}Sr_{x}CuO_{4}) cuprates. The t'-U phase diagram is obtained for both signs of the t' term, showing the different regions of stability and instability of the doped antiferromagnet. Doping is shown to suppress the t'-induced frustration due to the competing interaction J'. A study of transverse spin fluctuations in the metallic AF state reveals that the decay of magnons into particle-hole excitations yields an interesting low-energy result \Gamma \sim \omega for magnon damping.

Study of Magnetic Excitation in Singlet-Ground-State Magnets CsFeCl3and RbFeCl3by Nuclear Magnetic Relaxation

The temperature dependences of spin-lattice relaxation time $T_1$ of $^{133}$Cs in CsFeCl$_3$ and $^{87}$Rb in RbFeCl$_3$ were measured in the temperature range between 1.5 K and 22 K, at various fields up to 7 T applied parallel (or perpendicular) to the c-axis, and the analysis was made on the basis of the DCEFA. The mechanism of the nuclear magnetic relaxation is interpreted in terms of the magnetic fluctuations which are characterized by the singlet ground state system. In the field region where the phase transition occurs, $T_1^{-1}$ exhibited the tendency of divergence near $T_{\rm N}$, and this feature was ascribed to the transverse spin fluctuation associated with the mode softening at the $K$-point. It was found that the damping constant of the soft mode is remarkably affected by the occurrence of the magnetic ordering at lower temperature, and increases largely in the field region where the phase transition occurs.

Effective interactions in the stripe state of the two-dimensional Hubbard model

We investigate the structure of the pairing potential in the stripe phase of the two-dimensional Hubbard model. Based on the random phase approximation we discuss in detail the interactions in the charge- and spin channel and compare our calculations with related considerations in commensurate antiferromagnets. Our main finding is that due to the incommensurate charge-density wave formation the exchange of collective modes in the charge channel is significantly enhanced with respect to the spin bag approach whereas due to the inhomogeneous charge distribution the coupling to transverse spin fluctuations tends to be suppressed.

Quantum spin fluctuation theory of the magnetic equation of state of weak itinerant-electron ferromagnets

On the basis of the spin fluctuation mechanism, magnetic properties of itinerant-electron weak ferromagnets are discussed, for the wide temperature range from the ground state to the paramagnetic state, explicitly taking into account the effects of zero-point quantum spin fluctuations. Particular attention is focused on properties of the ordered phase. The temperature dependence of the spontaneous magnetic moment, for instance, is quantitatively analysed in close comparison with experiments. It is also shown that the fourth-order expansion coefficient of the free energy in powers of the static magnetic moment is temperature dependent, and therefore magnetic isotherms are not so simple as was originally anticipated in the Stoner–Wohlfarth theory.We explicitly examine the effect of the spin-wave mode on the transverse spin fluctuation amplitude, and show that this effect is crucial for the proper theoretical description of magnetic behaviours in the ordered state.

Hole motion in an anisotropic antiferromagnet studied using an extendedt−Jmodel

In this paper, we have found an alternative operator transform to study hole motion in an anisotropic antiferromagnet background described by an extended t-J model. Using this transform, the systems with any spin magnitude can be treated, and the no-double occupancy constraint is satisfied automatically. In the extended t-J model, we take into account not only the long-range hopping but also the nearest-anisotropic and the long-range-isotropic exchange. The single-hole spectrum function is evaluated with the self-consistent Born approximation. The dependencies of the hole-spectrum function on both long-range exchange and long-range hopping are discussed. The dispersion relation of quasiparticle states is also studied. We find that: (1) both the long-range hopping and transverse-spin fluctuation enhance the coherent and suppress the incoherent spectrum weight; (2) the contribution of the long-range exchange to the spectral function cancels that of the nearest-neighbor one; (3) the second-nearest-neighbor hopping favors the formation of the large Fermi surface, while the third favors that of hole pockets; and (4) the third-nearest-neighbor hopping is more important than the second in accounting for the spectral behavior near the momentum ({pi},0), especially the shoulderlike structure.

Spectral functions and the pseudogap in thet−Jmodel

We calculate spectral functions within the $t\ensuremath{-}J$ model as relevant to cuprates in the regime from low to optimum doping. On the basis of equations of motion for projected operators an effective spin-fermion coupling is derived. The self-energy due to short-wavelength transverse spin fluctuations is shown to lead to a modified self-consistent Born approximation, which can explain strong asymmetry between hole and electron quasiparticles. The coupling to long-wavelength longitudinal spin fluctuations governs the low-frequency behavior and results in a pseudogap, which at low doping effectively truncates the Fermi surface, in particular near the $(\ensuremath{\pi},0)$ point in the Brillouin zone.

Nuclear Magnetic Relaxation of1H in the Organic Radical Saltsm-MPYNN+·ClO4-andm-MPYNN+·BF4-

We have measured nuclear spin-lattice relaxation times T 1 of 1 H in the organic salts m -MPYNN + ·ClO 4 - and m -MPYNN + ·BF 4 - in the temperature range between 2 K and 100 K under external fields up to 70 kOe. In these salts, two cationic (spin-1/2) radicals constitute a dimer due to the ferromagnetic interaction J 1 . We interpreted the experimental results of T 1 -1 below about 10 K (∼ J 1 / k B ) in terms of two relaxation processes based on the energy levels of the ferromagnetic dimer. The flip-flop and the direct processes which are due to longitudinal and transverse spin fluctuations become dominant at higher and lower fields, respectively, the crossover field being around 10 kOe. For the field dependence of T 1 -1 at higher temperatures, the spin diffusive model of spin-1/2 in a two-dimensional system is satisfactorily applied.

Local stiffness and stability of the ferromagnetic state in metals

The concept of local stiffness associated with transverse local spin fluctuations is introduced and used to investigate the coupling between the magnetization of atomic planes in the ferromagnetic state of a metallic material. It is shown that the coupling strength in the surface region may differ significantly from that in the bulk, and the possibility of an antiferromagnetic alignment between surface and bulk magnetizations is pointed out. A comparison between calculated values of local and spin-wave stiffnesses is presented, and the stability of the ferromagnetic state against both short- and long-wavelength spin fluctuations is discussed.

A local moment approach to magnetic impurities in gapless Fermi systems

A local moment approach is developed for single-particle excitations of a symmetric Anderson impurity model (AIM) with a soft-gap hybridization vanishing at the Fermi level: I||r, with r>0. Local moments are introduced explicitly from the outset, and a two-self-energy description is employed in which single-particle excitations are coupled dynamically to low-energy transverse spin fluctuations. The resultant theory is applicable on all energy scales, and captures both the spin-fluctuation regime of strong coupling (large U), as well as the weak-coupling regime where it is perturbatively exact for those r-domains in which perturbation theory in U is non-singular. While the primary emphasis is on single-particle dynamics, the quantum phase transition between strong-coupling (SC) and local moment (LM) phases can also be addressed directly; for the spin-fluctuation regime in particular a number of asymptotically exact results are thereby obtained, notably for the behaviour of the critical Uc(r) separating SC/LM states and the Kondo scale m(r) characteristic of the SC phase. Results for both single-particle spectra and SC/LM phase boundaries are found to agree well with recent numerical renormalization group (NRG) studies; and a number of further testable predictions are made. Single-particle spectra are examined systematically for both SC and LM states; in particular, for all 0 r<½, spectra characteristic of the SC state are predicted to exhibit an r-dependent universal scaling form as the SC/LM phase boundary is approached and the Kondo scale vanishes. Results for the `normal' r = 0 AIM are moreover recovered smoothly from the limit r0, where the resultant description of single-particle dynamics includes recovery of Doniach-Sunjic tails in the wings of the Kondo resonance, as well as characteristic low-energy Fermi liquid behaviour and the exponential diminution with U of the Kondo scale itself. The normal AIM is found to represent a particular case of more generic behaviour characteristic of the r>0 SC phase which, in agreement with conclusions drawn from recent NRG work, may be viewed as a non-trivial but natural generalization of Fermi liquid physics.

Noncollinear itinerant-electron magnets: ground- and excited states simulations

The Heisenberg model cannot uncritically be applied to itinerant-electron magnets, including those that show noncollinear order. Density functional theory is, therefore, generalized to apply to noncollinear itinerant-electron magnets. The appropriate Kohn-Sham equations are discussed and the total energy of Spiral Magnetic Order (SMO) is found to describe the ground states of those magnets that possess a high-moment to low-moment transition such as, e.g., INVAR. Concerning excited-states properties, the energy spectrum of transverse and longitudinal spin fluctuations can be estimated with the total energy of SMO and allows a determination of thermal properties of itinerant-electron magnets. As an example calculated results for bcc-Fe are discussed in detail and compared with experimental results. Furthermore, results for Co, Ni and FeCo are collected.

Non-Collinear Magnetism in Itinerant-Electron Systems

show non-collinear order. Density func- tional theory is therefore generalized to apply to non-collinear itinerant-elec- tron magnets. The appropriate Kohn—Sham equations are discussed and the total energy of spiral magnetic order is used to determine the magnon spec- trum in the adiabatic approximation. The energy spectrum of transverse spin fluctuations can also be estimated with the total energy of spiral magnetic order and allows a determination of thermal properties of itinerant-electron magnets. Calculated results for Fe, Co, Ni, and FeCo are discussed and com- pared with experimental results.

On evaluation of transverse spin fluctuations in quantum magnets

A numerical method is described for evaluating transverse spin correlations in the random phase approximation. Quantum, spin-fluctuation corrections to sublattice magnetization are evaluated for the half-filled Hubbard antiferromagnet in two and three dimensions in the whole U/t range. Extension to the case of defects in the Hubbard AF is discussed, and the vacancy-induced enhancement in transverse spin fluctuation is obtained in the infinite-U limit for the spin vacancy problem in two dimensions, for vacancy concentration up to the percolation threshold. For low-U impurities, the overall spin fluctuation correction is found to be strongly suppressed, although surprisingly spin fluctuations are locally enhanced at the low-U sites.

Softening Effect of Magnetic Excitation on Nuclear Spin-Lattice Relaxation of133Cs in Singlet-Ground-State Magnet CsFeCl3

The temperature dependence of the spin-lattice relaxation time T 1 of 133 Cs in CsFeCl 3 was measured from 1.5 to 20 K with external fields up to 7 T applied along the c -axis, and the analysis was performed on the basis of the dynamical correlated-effective-field approximation theory. T 1 -1 below 2 T, which decreases markedly with decreasing temperature, was well interpreted in terms of the longitudinal spin fluctuation characterized by a large energy gap. For higher fields however, T 1 -1 exhibited an almost temperature-independent behavior accompanied by an significant increase below 5 K. The latter feature was ascribed to the transverse spin fluctuation associated with the softening of the lower branch of the excited doublet around the K -point.

Holes in thet−Jzmodel: A diagrammatic study

The t-J_z model is the strongly anisotropic limit of the t-J model which captures some general properties of the doped antiferromagnets (AF). The absence of spin fluctuations simplifies the analytical treatment of hole motion in an AF background and allows us to calculate the single- and two-hole spectra with high accuracy using regular diagram technique combined with real-space approach. At the same time, numerical studies of this model via exact diagonalization (ED) on small clusters show negligible finite size effects for a number of quantities, thus allowing a direct comparison between analytical and numerical results. Both approaches demonstrate that the holes have tendency to pair in the p- and d-wave channels at realistic values of t/J. The interactions leading to pairing and effects selecting p and d waves are thoroughly investigated. The role of transverse spin fluctuations is considered using perturbation theory. Based on the results of the present study, we discuss the pairing problem in the realistic t-J-like model. Possible implications for preformed pairs formation and phase separation are drawn.

Polarization analysis of low-energy exitations in single-domain Cr

The longitudinal and transverse spin fluctuations in single-domain Cr have been investigated in the transverse spin-density-wave phase using constant energy scans with 4⩽ E⩽25 meV and polarization analysis. The transverse excitations show well-defined peaks centered at the incommensurate satellite positions (1± δ 0 0) and a broad peak at the commensurate (1 0 0) position. The longitudinal fluctuations exhibit a much more complicated structure due to the additional appearance of magnetic excitations below 9 meV (Fincher–Burke modes). The fluctuations with longitudinal polarization show a pronounced renormalization between 136 and 230 K raising the possibility that the Fincher–Burke modes are responsible for the phase transition at T N. Above E≃18 meV, the longitudinal fluctuations decrease with increasing E at T=230 K in contrast to the transverse fluctuations. Our results indicate that unpolarized beam data is not really useful for making comparisons with existing theories. There is an urgent need to extend polarization analysis to higher neutron energies.

Competition between SDW and SC states in two-Hubbard chains

Two coupled chains of Hubbard model with spin anisotropic backward scattering have been studied by applying the renormalization group method to the bosonized Hamiltonian. From two kinds of fixed points for transverse spin fluctuations, which exist in the presence of anisotropic interactions, it has been shown that the dominant state moves from a singlet superconducting (SC) state with interchain pairing into a longitudinal spin density wave (SDW) state with intrachain pairing when the repulsive interaction increases. Phase diagram at finite temperatures is also obtained.

Exchange interactions in itinerant electron ferromagnetic systems

The exchange coupling between local moments in itinerant electron systems is investigated. A local stiffness constant D l associated with transverse local spin fluctuations is introduced and used to study the stability of the ferromagnetic state. Results of model calculations show that the ferromagnetic state is stable only in those ranges of parameters in which both D l and the spin-wave stiffness D sw are positive.

Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet

The interplay of disorder and spin-fluctuation effects in a disordered antiferromagnet is studied. In the weak-disorder regime (W \le U), while the energy gap decreases rapidly with disorder, the sublattice magnetization, including quantum corrections, is found to remain essentially unchanged in the strong correlation limit. Magnon energies and Neel temperature are enhanced by disorder in this limit. A single paradigm of disorder-enhanced delocalization qualitatively accounts for all these weak disorder effects. Vertex corrections and magnon damping, which appear only at order (W/U)^4, are also studied. With increasing disorder a crossover is found at W \sim U, characterized by a rapid decrease in sublattice magnetization due to quenching of local moments, and formation of spin vacancies. The latter suggests a spin-dilution behavior, which is indeed observed in softened magnon modes, lowering of Neel temperature, and enhanced transverse spin fluctuations.

Theory of isotropic itinerant-electron spin glasses in the vicinity of amorphous Fe

Finite-temperature magnetism in the vicinity of amorphous Fe has been investigated by developing the theory of amorphous metallic magnetism which takes into account transverse spin fluctuations. The calculated magnetic phase diagram as a function of temperature and d electron number shows up the existence of isotropic spin glass around amorphous Fe and non-collinear ferromagnetism near the ferromagnetic boundary.