Knight shifts in solid noble metals

The direct contribution to the Knight shift in solid noble metals is evaluated. The entire calculation is accomplished in the framework of the pseudopotential formalism and the effective shape of the Fermi surface is taken into account by using the simple “eight-cone” model by Ziman. The theoretical results are discussed and compared with the available experimental data.     

Knight shifts of metals in the liquid state

A single orthogonalized plane wave calculation has been made, to zero order in the pseudopotential, of the direct contact Knight shift for 30 liquid metals. The average deviation from experimental values is approximately 15%. Larger unexplained divergences occur for Cu and Hg. In addition it is shown that to zero order in the pseudopotential the factorΩP _F in the Knight shift describing the electron density at the nucleus is almost unaffected by thermal expansion.     

Topological and dynamical excitations in a classical 2D easy-axis Heisenberg model

The properties of dynamical solitons (magnon droplets) in the classical, two-dimensional anisotropic Heisenberg model with easy-axis exchange anisotropy are studied. The solution of the Landau-Lifshitz equation in the continuum limit for the soliton with topological charge q = 1 is obtained numerically using a shooting method. We analized a wide range of the anisotropy parameter and our results are in good agreement with results obtained from spin dynamics simulations. The dependence of an internal precession frequency of the soliton on both the anisotropy parameter and the radius of the soliton is also investigated. Finally, the limits of applicability of the continuum approach are discussed. Received 22 August 2000     

Haldane's instanton in 2D Heisenberg model revisited: Along the avenue of topology

Deconfined quantum phase transition from Néel phase to valence bond crystal state in 2D Heisenberg model is under debate nowadays. One crucial issue is the suppression of Haldane's instanton on quantum critical point which drives the spinon deconfined. In this Letter, by making use of the ?-mapping topological current theory, we reexamine the Haldane's instanton in an alternative way along the direction of topology. We find that the monopole events are space-time singularities of Néel field , the corresponding topological charges are the wrapping number of around the singularities which can be expressed in terms of the Hopf indices and Brouwer degrees of ?-mapping. The suppression of the monopole events can only be guaranteed when the ?-field possesses no zero points. Moreover, the quadrapolarity of monopole events in the Heisenberg model due to the Berry phase is also reproduced in this topological argument.     

Knight shifts and relaxation in gadolinium above the curie temperature

We have studied the transverse μSR signal in Gd metal in the temperature region 1–2 T_C, and it is demonstrated that measurements in the paramagnetic range can be used to obtain the hyperfine field at the muon site, as well as to give information on the fluctuations of the Gd spins. The critical exponent for slowing down of Gd spin fluctuations is 0.56(5) above 1.05 T_C but decreases to 0.15(5) below 1.05 T_C. Comparisons with results from PAC and Mössbauer measurements are discussed.     

Knight shifts in binary liquid alloys of equi-valent metals

The N.M.R. Knight shift depends upon the conduction electron density at the nucleus under observation. An interpretation of its absolute magnitude in liquid metals cannot be undertaken in terms of a pseudopotential formulation, but relative changes of shift with some parameter such as temperature, volume or alloy composition can be investigated. Knight shift changes with concentration have been measured for a series of binary liquid alloys. Since application of a pseudopotential treatment is simplest when the two constituents have the same valency, the alloys chosen have mostly been of this type, e.g. In-Ga, In-Tl, Sn-Pb, Sb-Bi. Although theory predicts the correct qualitative behaviour of the shift changes, the precise theoretical predictions are very sensitive to details of both the pseudopotentials and the liquid structure factor. In the case of In-Sn and In-Pb, where the shifts of both constituents decrease with increasing indium content for In-Sn, but increase for In-Pb, the difference between the pseudopotentials of tin and lead does not appear to be sufficient to explain the observed differences in changes of shift.     

An investigation of the Knight shifts in LiMg alloys

The lithium Knight shift has been measured throughout the full range of LiMg alloys and the magnesium Knight shift has been measured for magnesium rich LiMg alloys. Calculations of the Knight shifts have been performed using the coherent potential approximation to find the partially average Green's functions and taking some account of the Bloch character of the electron wavefunctions. The results are in good agreement with the experiment.     

Quantum Monte Carlo investigation of the 2D Heisenberg model with S=1/2

The two-dimensional (2D) Heisenberg model with anisotropic exchange (Δ = 1−J _x /J _z ) and S=1/2 is investigated by the quantum Monte Carlo method. The energy, susceptibility, specific heat, spin-spin correlation functions, and correlation radius are calculated. The sublattice magnetization (σ) and the Néel temperature of the anisotropic antiferromagnet are logarithmic functions of the exchange anisotropy: 1/σ+1+0.13(1)ln(1/Δ). Crossover of the static magnetic structural factor as a function of temperature from power-law to exponential occurs for T _c /J≈0.4. The correlation radius can be approximated by 1/ξ=2.05T ^1.0(6)/exp(1.0(4)/T). For La₂CuO₄ the sublattice magnetization is calculated as σ=0.45, the exchange is J=(1125–1305) K; for Er₂CuO₄ J∼625 K and the exchange anisotropy Δ∼0.003. The temperature dependence of the static structural magnetic factor and the correlation radius above the Néel temperature in these compounds can be explained by the formation of topological excitations (spinons). Fiz. Tverd. Tela (St. Petersburg) 41, 116–121 (January 1999)     

Phase transitions in the 2D J 1-J 2 Heisenberg model with arbitrary signs of exchange interactions

The ground state of the J ₁-J ₂ Heisenberg model with arbitrary signs of exchange is studied for spin S = 1/2 in the case of the two-dimensional (2D) square lattice. The states with different types of spin long-range order (antiferromagnetic checkerboard, stripe, collinear ferromagnetic) as well as the disordered spin liquid states are described in the framework of one analytical approach. In particular, it is shown that the phase transition between the ferromagnetic spin liquid and the ferromagnet with long-range order is of the second order. In the vicinity of such transition, we have found the ferromagnetic state with a rapidly varying condensate function.     

Temperature dependence of Knight shifts for 12 B in Pt

The Knight shift of a short lived β-emitting nucleus ¹²B (I = 1, T _1/2 = 20 ms) implanted into Pt has been measured as a function of temperature (140?600 K) by means of the β-NMR method. The relation between the Knight shift and the susceptibility for Pt was deduced, which shows the similar tendency to that for the case of ¹²B in Pd.     

Magnetic state around cation vacancies in II-VI semiconductors

The magnetic properties of neutral cation vacancies in II-VI semiconductors are examined using first principles calculations and group theory. A molecular cluster model of a single vacancy in II-VI semiconductors is developed to explain the observed chemical trend. We show that a single Zn vacancy in ZnO yields a total spin ST=1 in agreement with experiments. But for the other less ionic Zn-based II-VI semiconductors ZnA (A=S, Se, Te) this spin triplet state is nearly degenerate with a nonmagnetic spin singlet state.     

Spin Stiffness of 2D Isotropic Heisenberg Antiferrornagnet

The spin stiffness of spin-1/2 two-dimensional isotropic Heisenberg antiferromagnet is calculated by a perturbation method based on the extended Schwinger-Boson mean-field theory. We arrive at a value of the spin stiffness, p_s = 0.176J, which is in quite well agreement with experiments and the numerical results.