Maximum-entropy TFμSR study of vortex states in 1/2/3 cuprates

A maximum-entropy (ME) method has been applied to TFSR vortex data of RBa₂Cu₃O₇ (R1237; R=Er, Gd and Eu). The obtained information on the field distribution is of much better quality than that resulting from Fourier transformation and curve-fitting of the same data. ME application to R1237 TFSR data taken below T_c at LAMPF show asymmetrical, nearly triangular, field distributions, which broaden with decreasing temperature. Significant deviations from Abrikosov predictions have been observed for the estimated R1237-vortex field distributions. Below 10 K, for Er1237 and Gd1237, the non-Abrikosov features appear to be influenced by the rare-earth magnetism. The low-field tail in the field distribution and the ME-intensity anomaly near T_c suggest glassy features in the 1/2/3 cuprate vortex states.     

Projected-wave-function study of the spin-1/2 Heisenberg model on the Kagomé lattice

We perform a Gutzwiller projected-wave-function study for the spin-1/2 Heisenberg model on the Kagomé lattice to compare energies of several spin-liquid states. The result indicates that a U(1)-Dirac spin-liquid state has the lowest energy. Furthermore, even without variational parameters, the energy turns out to be very close to that found by exact diagonalization. We show that such a U(1)-Dirac state represents a quantum phase whose low-energy physics is governed by four flavors of two-component Dirac fermions coupled to a U(1) gauge field. These results are discussed in the context of recent experiments on ZnCu(3)(OH)(6)Cl(2).     

μSR study of UTr2Si2, Tr=Co,Rh OR Pt

We have studied bySR spectroscopy the intermetallicsUTr ₂Si₂ where Tr=Co, Rh or Pt. Whereas for the Co compound we only get information on the muon localisation site, from the other two materials we obtain information on their electronic properties. We compare the characteristics of theirSR response.     

The spin-1/2 J1–J2 Heisenberg antiferromagnet on the square lattice: Exact diagonalization for N=40 spins

We present numerical exact results for the ground state and the low-lying excitations for the spin-1/2 J₁-J₂ Heisenberg antiferromagnet on finite square lattices of up to N=40 sites. Using finite-size extrapolation we determine the ground-state energy, the magnetic order parameters, the spin gap, the uniform susceptibility, as well as the spin-wave velocity and the spin stiffness as functions of the frustration parameter J₂/J₁. In agreement with the generally excepted scenario we find semiclassical magnetically ordered phases for J₂ < $J_2^{c_1}$ and J₂ > $J_2^{c_2}$ separated by a gapful quantum paramagnetic phase. We estimate $J_2^{c_1}$ ≈0.35J₁ and $J_2^{c_2}$ ≈0.66J₁.     

RVB description of the low-energy singlets of the spin 1/2 kagomé antiferromagnet

Extensive calculations in the short-range RVB (Resonating valence bond) subspace on both the trimerized and the regular (non-trimerized) Heisenberg model on the kagomé lattice show that short-range dimer singlets capture the specific low-energy features of both models. In the trimerized case the singlet spectrum splits into bands in which the average number of dimers lying on one type of bonds is fixed. These results are in good agreement with the mean field solution of an effective model recently introduced. For the regular model one gets a continuous, gapless spectrum, in qualitative agreement with exact diagonalization results. Received 7 March 2000     

Magnetic ordering of two dimensional antiferromagnets on the triangular lattice: Mössbauer study

Mössbauer study has been performed on⁵⁷Fe in a single crystal of YbFe₂O₄ in order to obtain the information about the magnetic ordering. The essential difference between zerofield-cooling- and field-cooling- spectra has been decisive for settling the problem. A model of Ising spin system in 2d-antiferromagnet on the triangular lattice is proposed to interpret the spin behavior of Fe³⁺ in this system. It is very different from ordinary antiferromagnets that the microscopic change of 3d-state is induced by the external magnetic field even at low enough temperatures.     

An improvement of the lattice theory of dislocation for a two-dimensional triangular crystal

The structure of dislocation in a two-dimensional triangular crystal has been studied theoretically on the basis of atomic interaction and lattice statics. The theory presented in this paper is an improvement to that published previously.Within a reasonable interaction approximation, a new dislocation equation is obtained, which remedies a fault existing in the lattice theory of dislocation. A better simplification of non-diagonal terms of the kernel is given. The solution of the new dislocation equation asymptotically becomes the same as that obtained in the elastic theory, and agrees with experimental data. It is found that the solution is formally identical with that proposed phenomenologically by Foreman et al, where the parameter can be chosen freely, but cannot uniquely determined from theory. Indeed, if the parameter in the expression of the solution is selected suitably, the expression can be well applied to describe the fine structure of the dislocation.     

muSR study of the quantum dynamics in the frustrated S=3/2 kagomé bilayers

We present muSR experiments in the S=3/2 kagomé bilayer compound Ba(2)Sn(2)ZnGa(10-7p)Cr(7p)O22 [BSZCGO(p)] and compare it to the isostructural SrCr(9p)Ga(12-9p)O19 [SCGO(p)], including for the latter new results for p > or =0.89. Quantum-dynamical low energy magnetic excitations are evidenced in this novel compound. We study the evolution of the muon relaxation rate with p, T, and field. A phenomenological model for the muon relaxation based on sporadic dynamics due to spin excitations in a singlet sea proposed by Uemura et al. is extended to all fields and T range. Its connection to the RVB picture is discussed, and we argue that such coherent states might mediate the interactions between "impurities" which induce the spin glass freezing.     

μSR study of the properties of Fe3O4-based nanostructured magnetic systems

A ferrofluid based on Fe₃O₄ nanoparticles dispersed in heavy water D₂O is studied using the μSR method. The experiment has been carried out at temperatures 26–300 K. It is found that the diamagnetic (muon) fraction is formed in the ferrofluid in about the same amount as in D₂O, but the muon-spin relaxation rate in the ferrofluid is much higher than in D₂O. A significant shift of the muon-spin precession frequency in the ferrofluid is observed. It is shown that the shift of the muon precession frequency as a function of the external magnetic field is described by the Langevin function typical of paramagnetic magnetization. The mean magnetic field in the medium due to magnetic-nanoparticle polarization in an external field is experimentally determined. The nanoparticle sizes are estimated.     

μ+SR study of two-dimensional antiferromagnets,delafossite-type compounds

⁺SR experiments were performed on delafossite-type compounds, CuCrO₂, AgCrO₂, CuFeO₂, which are model compounds of triangular lattice antiferromagnets. The initial asymmetries are much smaller than the expected value, implying muonium formation. The time spectra are composed of slow andfast relaxation components. We attributed the components to signals from ⁺ stopped at the center of O^2– ions andmuonium far from nuclear dipole moments, respectively. The asymmetries decrease belowT _N but no precession spectra were observed. Relaxation rates of slow andfast relaxation components show maxima atT _N.     

Ground state of the kagomé-like S=1/2 antiferromagnet volborthite Cu3V2O7(OH)2 x 2H2O

The volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagomé-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2 K which is further confirmed by a peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4 K +/- 0.2 K. Through 51V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% corresponds to a well defined short-range order, possibly of the sqrt(3) x sqrt(3) type. While the freezing involves all the Cu2+ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.     

Applications of μSR to the study of fullerenes

A review is given of the current status of SR results on muonium states in the pure fullerenes C₆₀ and C₇₀ as well as on the alkali doped series of C₆₀. In particular the results of studies of the hyperfine interaction of the muonated radicals are discussed in order to determine their electronic distribution and the rotational ordering and dynamics of the fullerenes. Unique information on the alkali doped C₆₀ species is obtained from studies of the endohedral muonium species.     

Single crystal study of the heavy-fermion antiferromagnet CePt?In?

We report the synthesis, structure, and physical properties of single crystals of CePt(2)In(7). Single crystal x-ray diffraction analysis confirms the tetragonal I4/mmm structure of CePt(2)In(7) with unit cell parameters a = 4.5886(6) ?, c = 21.530(6) ? and V = 453.32(14) ?(3). The magnetic susceptibility, heat capacity, Hall effect and electrical resistivity measurements are all consistent with CePt(2)In(7) undergoing an antiferromagnetic order transition at T(N) = 5.5 K, which is field independent up to 9 T. Above T(N), the Sommerfeld coefficient of specific heat is γ ≈ 300 mJ mol(-1) K(-2), which is characteristic of an enhanced effective mass of itinerant charge carriers. The electrical resistivity is typical of heavy-fermion behavior and gives a residual resistivity ρ(0) ～ 0.2 μΩ cm, indicating good crystal quality. CePt(2)In(7) also shows moderate anisotropy of the physical properties that is comparable to structurally related CeMIn(5) (M = Co, Rh, Ir) heavy-fermion superconductors.     

Full lattice QCD study of the κ scalar meson

We studied the κ light scalar meson in 2+1 flavor full QCD with sufficiently light u and d quarks. Via lattice simulation we measured the correlators for the κ channel in the “Asqtad” improved staggered fermion formulation. After chiral extrapolation we obtained the mass of the κ meson with 826±119 MeV, which is within recent experimental values of 800-900 MeV. The simulations were carried out with the MILC 2+1 flavor gauge configurations at lattice spacing a ≈ 0.15 fm.     

RF-μSR study of muonium charge states and dynamics in Si

The radio frequencySR technique developed at TRIUMF was used to measure the temperature dependence of the diamagnetic muon, Mu, and Mu^* amplitudes in silicon between 10 K and 500 K. Six samples doped with phosphorus (n-type) and boron (p-type) in the concentration range 10¹¹ to 10¹⁵ cm^–3 were studied. In pure Si a very good fit over the whole temperature range is obtained from a model that includes the ionization of Mu^* and Mu to a bond centered ⁺ followed at high temperature by charge exchange involving Mu.     

μSR studies of YFeMnO4

Muon spin relaxation (SR) studies were carried out on YFeMnO₄. Two interesting phenomena were found in measurements of longitudinal relaxation time spectra atT>T _cusp for a single crystal: (1) difference of the relaxation rates between the directions parallel with and perpendicular to thec-axis, and (2) suppression of local field fluctuations in a longitudinal external field. The spectra atT >T _cusp were reproduced with the function exp[-(₀ t)]. The results are discussed under a picture of the magnetic spin fluctuations of triangular antiferromagnets.     

μSR study of a mixed compound NiC2O4 2⋅(2-methylimidazole)x(H2O)1-x

We examined the muon spin relaxation (μSR) of mixed compounds NiC₂O₄ 2⋅(2-methylimidazole)_x(H₂O)_1-x with x=1.0 and 0.49. Although the macroscopic magnetic properties are obviously different from each other, both systems exhibit similar behavior in the muon spin relaxation. In addition, in the x=0.49 (SG) sample, a critical slowing down of spin dynamics was not observed in this μSR measurement, though the spin-glass like freezing was observed in the susceptibility measurements. Qualitative explanation of these anomalous observations is given. This revised version was published online in August 2006 with corrections to the Cover Date.