Spin-wave renormalization by mobile holes in a two-dimensional quantum antiferromagnet

The coupling of antiferromagnetic spin excitations and propagating holes has been studied theoretically on a square lattice in order to investigate the dependence of antiferromagnetic order on hole doping, being of relevance, e.g., for the Cu–3 d⁹ system in antiferromagnetic CuO₂-planes of high-T_c superconductors. An effective Hamiltonian has been used, which results from a 2D Hubbard model (hopping integral t) with holes and with strong on-site Coulomb repulsion U. Bare antiferromagnetic excitations and holes with energies of the same order of magnitude t²/U are interacting via a coupling term being proportional to t and allowing holes to hop by emitting and absorbing spinwaves. In terms of a self-consistent one-loop approximation the renormalization of the spectral function both of holes and antiferromagnetic spin excitations are calculated.     

High temperature superconductivity; The spin polaron theory

An outline is given of one of the many models which have been advanced to explain high temperature superconductivity in the quasi-two-dimensional oxides. In these the compensation ensures that there are free positive holes, formed in the oxygen 2p states, which form ‘spin polarons’ by orienting neighbouring spins in Cu 3d⁹ ions. Two spin polarons then combine to form a spinless pair, which obeys Einstein–Bose statistics. It is shown that a degenerate gas of such entities is a superconductor. If this is a correct model, such pairs (bosons) must exist above the critical temperature (T _c). Particular attention is given to the properties of these materials at temperatures above T _c. An unresolved problem is whether all, or only some, of the carriers form bosons.     

Relaxation times and line widths of isotopically-substituted nitroxides in aqueous solution at X-band

Optimization of nitroxides as probes for EPR imaging requires detailed understanding of spectral properties. Spin lattice relaxation times, spin packet line widths, nuclear hyperfine splitting, and overall lineshapes were characterized for six low molecular weight nitroxides in dilute deoxygenated aqueous solution at X-band. The nitroxides included 6-member, unsaturated 5-member, or saturated 5-member rings, most of which were isotopically labeled. The spectra are near the fast tumbling limit with T₁∼T₂ in the range of 0.50–1.1 μs at ambient temperature. Both spin–lattice relaxation T₁ and spin–spin relaxation T₂ are longer for ¹⁵N- than for ¹⁴N-nitroxides. The dominant contributions to T₁ are modulation of nitrogen hyperfine anisotropy and spin rotation. Dependence of T₁ on nitrogen nuclear spin state m_I was observed for both ¹⁴N and ¹⁵N. Unresolved hydrogen/deuterium hyperfine couplings dominate overall line widths. Lineshapes were simulated by including all nuclear hyperfine couplings and spin packet line widths that agreed with values obtained by electron spin echo. Line widths and relaxation times are predicted to be about the same at 250 MHz as at X-band.     

Phonon-assisted spin diffusion in solids

The spin flip-flop transition rate is calculated for the case of spectral spin diffusion within a system of dipolarly coupled spins in a solid where the lattice vibrations are present. Long-wavelength acoustic phonons time-modulate the interspin distance r_ij and enhance the transition rate via the change of the 1/r³_ij term in the coupling dipolar Hamiltonian. The phonon-assisted spin diffusion rate is calculated by the golden rule in the Debye approximation of the phonon density of states. The coupling of the spins to the phonons introduces temperature dependence into the transition rate, in contrast to the spin diffusion in a rigid lattice, where the rate is temperature-independent. The direct (one-phonon absorption or emission) processes introduce a linear temperature dependence into the rate at temperatures not too close to T = 0. Two-phonon processes introduce a more complicated temperature dependence that again becomes simple analytical for temperatures higher than the Debye temperature, where the rate is proportional to T², and in the limit T → 0, where the rate varies as T⁷. Raman processes (one-phonon absorption and another phonon emission) dominate by far the phonon-assisted spin flip-flop transitions.     

Solutions for the T = 0 quantum spin glass transition in a metallic model with spin-charge coupling

We solve several low temperature problems of an infinite range metallic spin glass model. A compensation problem of T 0 divergencies is solved for the free energy which helped to extract the quantum critical behaviour of the spin glass order parameters as a function of δJ = J – J_c (T = 0). The critical value J_c(T = 0) = 3/16p_F^?1 of the frustrated spin coupling J, which separates spin glass from nonmagnetic (spin liquid) phase, is determined exactly in the static saddle point solution for a semielliptic metallic band model in terms of the density of states at the Fermi level. In addition to the replica-overlap order parameter 〈Q^ab〉, a ≠ b, the diagonal 〈Q^aa〉 is confirmed as order parameter by the result 〈Q^aa〉 _SP ～ (δJ)^β, β = 1, and its susceptibility χ^aaaa ～(-δJ)^?γ with γ = 1/2 at T = 0. The value for γ agrees with the one for the transverse field Ising spin glass. The low γ decay of 〈Q^aa〉, ～ T is obtained exactly in the whole quantum disordered phase including the critical value.     

第一性原理研究Mg，Si和Mn共掺GaN

采用基于密度泛函理论(DFT)的第一性原理平面波赝势法(PWP)计算Mg,Si和Mn共掺GaN电子结构和光学性质,分析比较计算结果.计算表明：掺杂后体系均在能隙深处产生自旋极化杂质带,具有半金属性,能产生自旋注入.与Mn掺杂GaN比较,Mg共掺后能使居里温度(T_C)升高,并在1.0eV出现源于Mn⁴⁺离子基态⁴T₁(F)到⁴T_{2关键词： Mg Si和Mn共掺GaN 电子结构 TC)')" href="#">居里温度(TC) 光学性质}     

Hyperfine coupling of12B in noble metals and Al; enhancement of the conduction-electron spin susceptibility by e-e exchange

We have remeasured the Korringa contributionT _1e to the nuclear spin lattice relaxation time of the interstitial¹²B in nobel metals and in Al. TheT _1e T times the squared Knight shiftK of¹²B taken from the literature, that is the Korringa product of¹²B, shows that the conduction-electron spin susceptibility is exchange enhanced similarly to that sensed by the matrix nucleus. The enhancement factors are equal within the experimental errors for both interstitial and matrix positions in Cu and Ag. The enhancement factors at the boron and at the matrix position are different in Au and Al.     

Effective exponents for de almeida-thouless line

The experimental irreversibility line at the magnetic phase diagram H-T of spin glasses is often approximated using a power law Hα (T_G-T)^φ/2. The mean field theory of de Almeida and Thouless predicts φ=3 for temperatures T close to the spin-glass transition temperature T_G . For a range of reduced temperature involved in an experiment one should use, however, effective exponents (φ>3) obtained by fitting the power law to the exact expression for de Almeida-Thouless line. These compare favorably with experimental values of φ obtained for different spin glasses. The increase of effective exponent up to φ=7 on approaching the spin-glass-ferromagnetism multicritical point is also considered.     

Power Reflection,Transmission and Absorption Coefficients for a Moving Plasma Slab

Expressions for power reflection (R), transmission (T) and absorption (A) coefficients for p-polarized wave for a warm, collisional, magnetized and moving plasma slab (with sharp boundaries and thickness d₀) are investigated. The effects of plasma slab velocity (β=v/c), electron density (ωp/ω)² and plasma temperature (K_BT) on reflection (R), transmission (T) and absorption (A) coefficients are discussed numerically. It is observed that for the value β=?0.6, reflection coefficient (R) becomes more than unity, whereas absorption coefficient (A) becomes quite negligible while transmission coefficient (T) shows oscillatory behaviour. The variation with plasma frequency (ω_p/ω)² shows that at lower plasma frequency (ω_p/ω)²=0.2 transmission (T) and absorption (A) coefficients are minimum while reflection coefficient (R) is maximum.     

Quasi-one-dimensional spin chains in CuSiO3: an EPR study

Temperature-dependent electron paramagnetic resonance (EPR) studies were performed on CuSiO₃. This recently discovered compound is isostructural with the spin-Peierls compound CuGeO₃. The EPR signals show characteristics different from those of CuGeO₃ and are due to Cu²⁺ spins located along quasi one-dimensional chains. ForT>8.2 K the spin susceptibility closely follows the predictions of anS=1/2 one-dimensional Heisenberg antiferromagnet withJ/k _B=21 K. BelowT=8.2 K the spin susceptibility immediately drops to zero indicating long-range magnetic order.     

μ+SR Study of Ordering Phenomena in the Heavy-Fermion State of Ce3Pd20Ge6

Magnetic and quadrupolar ordering phenomena in a Ce₃Pd₂₀Ge₆ single crystal have been investigated by muon spin rotation/relaxation (μ⁺SR) spectroscopy. We have observed spontaneous precession of muons in zero-field below T _N =0.7 K in the antiferromagnetic state. The precession frequency follows the power law: ν(T)=ν(0)(1−T/T _N ) ⁿ . The exponent n=0.43(2) is close to the mean-field value of 0.5. The muon longitudinal spin relaxation rate 1/T ₁ is found to be nearly independent of temperature in the range of 0.3 to 2 K, i.e., across either T _N or T _Q =1.2 K, the quadrupolar ordering temperature. Two likely mechanisms for the temperature independent behavior of 1/T ₁ are suggested. This revised version was published online in September 2006 with corrections to the Cover Date.     

Photoinduced dynamics to photoluminescence in Ln3+ (Ln = Ce,Pr) doped β-NaYF4 nanocrystals computed in basis of non-collinear spin DFT with spin-orbit coupling

ABSTRACT

In this work, non-collinear spin DFT + U approaches with spin-orbit coupling (SOC) are applied to Ln³⁺ doped β-NaYF₄ (Ln = Ce, Pr) nanocrystals in Vienna ab initio Simulation Package taking into account unpaired spin configurations using the Perdew–Burke–Ernzerhof functional in a plane wave basis set. The calculated absorption spectra from non-collinear spin DFT + U approaches are compared with that from spin-polarised DFT + U approaches. The spectral difference indicates the importance of spin–flip transitions of Ln³⁺ ions. Suite of codes for nonadiabatic dynamics has been developed for 2-component spinor orbitals. On-the-fly nonadiabatic coupling calculations provide transition probabilities facilitated by nuclear motion. Relaxation rates of electrons and holes are calculated using Redfield theory in the reduced density matrix formalism cast in the basis of non-collinear spin DFT + U with SOC. The emission spectra are calculated using the time-integrated method along the excited state trajectories based on nonadiabatic couplings.     

Temperature dependence of muonium spin exchange with O2 in the range 88 K to 478 K

Our group at TRIUMF reported earlier a study of the spin exchange reactions of Mu with O₂ and NO in the range 295 K to 478 K. We have extended the measurement with O₂ to a low temperature region down to 88 K. From 135 K to 296 K, the spin depolarization rate constantk _d(T) was found to vary according to the relative velocity of the colliding species,T ^1/2, which indicates that the spin exchange cross section of Mu-O₂ is temperature independent in this temperature range. The value ofk _d(T) at 296 K is in good agreement with our earlier study. However, it was found that below 105 K and above 400 K,k _d(T) tends to have stronger temperature dependences (T ⁿ , withn>1/2). This deviation fromT ^1/2-behavior can be attributed to the velocity (energy) dependence of the spin exchange cross section.     

On the possibility of binding of two holes in an antiferromagnetic state

The energy of a pair of holes in a half-filled 2—d Hubbard model is studied assuming an antiferromagnetic spin configuration. By comparing with the energy of a single hole, we discuss the possibility of pairing of holes in connection with the recently discovered highT _c superconductivity.     

Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation

Muon spin relaxation has been observed in both the normal and superconducting states of Rb₃C₆₀ (T _c=29.3K). The field dependence of theT ₁ spin relaxation rate is due to muonium undergoing spin-exchange scattering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence ofT ₁ ^–1 shows a Hebel-Slichter coherence peak just belowT _c which is not seen in¹³C spin relaxation. The peak can be fit assuming spin relaxation due to interaction with the quasiparticle excitations of a BCS superconductor provided the density of states is broadened relative to that of BCS. Such fits yield a value for the zero temperature energy gap, ₀/k _B , of 53(4)K, consistent with weak-coupling BCS.     

Exact results for the dynamics of the classical one dimensional easy plane ferromagnet at low temperatures

The in-plane and out-of-plane dynamical correlation functions for the classical one dimensional easy plane ferromagnet are calculated asymptotically exactly at low temperatures. The results are restricted to temperatures much below the crossover temperature at which spins begin aligning in the plane. The long wavelength behavior of the in-plane fluctuations is consistent with dynamical scaling, in contrast to the isotropic case, and agrees with the results of Villain and of Nelson and Fisher. The linewidths for the in-plane fluctuations at short wavelengths are calculated exactly, and approach those of the isotropic model for small anisotropy. The theory of Villain, the theory of Cieplak and Sjolander, and the simulations of Loveluck, Jauslin, Schneider and Stoll all give incorrect results for these linewidths. The out of plane linewidths show an anomalous temperature dependence due to a singularity in the three spin wave density of states that is characteristic of one dimensional systems. The linewidth is proportional toT ² lnT except at the wavevector for which the second derivative of the spin wave frequency with respect to wavevector vanishes (/2 for CsNiF₃) where the linewidth is proportional toT ^5/3. The linewidth has a strong discontinuity as the wavevector increases as a result of a catastrophe occurring in the calculation of the three spin wave density of states. The position and strength of the discontinuity are temperature dependent. The diffusion coefficient is logarithmically dependent on the anisotropy, and diverges as (T ² lnD)^–1, which is consistent with the (lnT)^–1 behavior predicted for the isotropic ferromagnet in earlier work. The results are derived for the case of single ion anisotropy, using a spin wave theory for static correlations and the spin current damping function, and can be readily extended to the case of anisotropic exchange.     

Carrier spin polarization in ferromagnet-semiconductor (Ga,Mn)As/GaAs structures

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet-semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.     

Extrinsic curvature for the two-black-hole problem

The solutions of the momentum constraints on the Einstein-Rosen manifold with two bridges, representing two black holes, are analyzed. These solutions are in the form of an infinite series. Their higher order terms are shown to fall off asr ^–6. These terms add multipole moments and gravitational radiation to the initial data and do not contribute to the linear and (spin) angular momenta of the black holes.