291 research outputs found
Studies of superconducting materials with muon spin rotation
The muon spin rotation/relaxation technique was found to be an exceptionally effective means of measuring the magnetic properties of superconductors, including the new high temperature superconductor materials, at the microscopic level. The technique directly measures the magnetic penetration depth (type II superconductors (SC's)) and detects the presence of magnetic ordering (antiferromagnetism or spin-glass ordering were observed in some high temperature superconductor (HTSC's) and in many closely related compounds). Extensive studies of HTSC materials were conducted by the Virginia State University - College of William and Mary - Columbia University collaboration at Brookhaven National Laboratory and TRIUMF (Vancouver). A survey of LaSrCuO and YBaCaCuO systems shows an essentially linear relationship between the transition temperature T(sub c) and the relaxation rate. This appears to be a manifestation of the proportionality between T(sub c) and the Fermi energy, which suggests a high energy scale for the SC coupling, and which is not consistent with the weak coupling of phonon-mediated SC. Studies of LaCuO and YBaCuO parent compounds show clear evidence of antiferromagnetism. YBa2Cu(3-x)CO(x)O7 shows the simultaneous presence of spin-glass magnetic ordering and superconductivity. Three-dimensional SC, (Ba, K) BiO3, unlike the layered CuO-based compounds, shows no suggestion of magnetic ordering. Experimental techniques and theoretical implications are discussed
Pressure Effects in Manganites with Layered Perovskite Structure
Pressure effects on the charge and spin dynamics in the bilayer manganite
compounds are studied theoretically by taking into
account the orbital degrees of freedom. The orbital degrees are active in the
layered crystal structure, and applied hydrostatic pressure stabilizes the
orbital in comparison with . The change of the
orbital states weakens the interlayer charge and spin couplings, and suppresses
the three dimensional ferromagnetic transition. Numerical results, based on an
effective Hamiltonian which includes the energy level difference of the
orbitals, show that the applied pressure controls the dimensionality of the
spin and charge dynamics through changes of the orbital states.Comment: 5 pages, 2 figure
Magnetic pair breaking in HoNi2B2C
Neutron-diffraction techniques have been used to study the interplay between superconductivity and magnetism in HoNi2B2C (Tc=8 K). The experimental results, obtained on single crystals, show that below approximately 4.7 K, this compound is in a simple antiferromagnetic state that coexists with superconductivity. Between approximately 4.7 and 6 K, an incommensurate modulated magnetic structure has been found. This observation strongly suggests that pair breaking associated with this incommensurate magnetic structure is responsible for the deep minimum in Hc2 and the near-reentrant behavior observed in this compound at approximately 5 K
Antiferromagnetic Excitations and Van Hove Singularities in YBaCuO
We show that in quasi-two-dimensional -wave superconductors Van Hove
singularities close to the Fermi surface lead to novel magnetic quasi-particle
excitations. We calculate the temperature and doping dependence of dynamical
magnetic susceptibility for YBCO and show that the proposed excitations are in
agreement with inelastic neutron scattering experiments. In addition, the
values of the gap parameter and in-plane antiferromagnetic coupling are much
smaller than usually believed.Comment: REVTeX, 4 pages + 3 PostScript (compressed) figures; to appear in
Phys. Rev. B (Rap. Comm.
Anomalous Spin Dynamics in Doped Quantum Antiferromagnets
Finite-temperature spin dynamics in planar t-J model is studied using the
method based on the Lanczos diagonalization of small systems. Dynamical spin
structure factor at moderate dopings shows the coexistence of free-fermion-like
and spin-fluctuation timescales. At T<J, the low-frequency and static
susceptibility show pronounced T dependence, supporting a scenario, related to
the marginal Fermi-liquid one, for the explanation of neutron-scattering and
NMR-relaxation experiments in cuprates. Calculated NMR relaxation rates
reasonably reproduce experimental ones.Comment: 10 pages + 4 figures, Postscript in uuencoded compressed tar file,
IJS-TP-94/2
Interplay of the CE-type charge ordering and the A-type spin ordering in a half-doped bilayer manganite La{1}Sr{2}Mn{2}O{7}
We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7}
exhibits CE-type charge-ordered and spin-ordered states below K and below K, respectively. However, the volume
fraction of the CE-type ordering is relatively small, and the system is
dominated by the A-type spin ordering. The coexistence of the two types of
ordering is essential to understand its transport properties, and we argue that
it can be viewed as an effective phase separation between the metallic
orbital ordering and the charge-localized
orbital ordering.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
On the Magnetic Excitation Spectra of High Tc Cu Oxides up to the Energies far above the Resonance Energy
Magnetic excitation spectra c"(q,w) of YBa2Cu3Oy and La214 systems have been
studied. For La1.88Sr0.12CuO4, c"(q,w) have been measured up to ~30 meV and
existing data have been analyzed up to the energy w~150 meV by using the
phenomenological expression of the generalized magnetic susceptibility
c(q,w)=c0(q,w)/{1+J(q)c0(q,w)}, where c0(q,w) is the susceptibility of the
electrons without the exchange coupling J(q) among them. In the relatively low
energy region up to slightly above the resonance energy Er, it has been
reported by the authors' group that the expression can explain characteristics
of the q- and w-dependence of the spectra of YBa2Cu3Oy (YBCO or YBCOy). Here,
it is also pointed out that the expression can reproduce the rotation of four
incommensurate peaks of c"(q,w) within the a*-b* plane about (p/a, p/a) {or
so-called (p, p)} point by 45 degree, which occurs as w goes to the energy
region far above Er from E below Er. For La2-xSrxCuO4 and La2-xBaxCuO4,
agreements between the observed results and the calculations are less
satisfactory than for YBCO, indicating that we have to take account of the
existence of the "stripes" to consistently explain the observed c"(q,w) of
La214 system especially near x=1/8.Comment: 14 pages, 5 figure
Stripe order, depinning, and fluctuations in LaBaCuO and LaBaSrCuO
We present a neutron scattering study of stripe correlations measured on a
single crystal of LaBaCuO. Within the
low-temperature-tetragonal (LTT) phase, superlattice peaks indicative of spin
and charge stripe order are observed below 50 K. For excitation energies
meV, we have characterized the magnetic excitations that
emerge from the incommensurate magnetic superlattice peaks. In the ordered
state, these excitations are similar to spin waves. Following these excitations
as a function of temperature, we find that there is relatively little change in
the {\bf Q}-integrated dynamical spin susceptibility for
meV as stripe order disappears and then as the structure transforms from LTT to
the low-temperature-orthorhombic (LTO) phase. The {\bf Q}-integrated signal at
lower energies changes more dramatically through these transitions, as it must
in a transformation from an ordered to a disordered state. We argue that the
continuous evolution through the transitions provides direct evidence that the
incommensurate spin excitations in the disordered state are an indicator of
dynamical charge stripes. An interesting feature of the thermal evolution is a
variation in the incommensurability of the magnetic scattering. Similar
behavior is observed in measurements on a single crystal of
LaBaSrCuO; maps of the scattered intensity
in a region centered on the antiferromagnetic wave vector and measured at
meV are well reproduced by a model of disordered stripes with a
temperature-dependent mixture of stripe spacings. We discuss the relevance of
our results to understanding the magnetic excitations in cuprate
superconductors.Comment: 12 pages, 11 figures, 1 tabl
Herbivore-mediated negative frequency-dependent selection underlies a trichome dimorphism in nature
Authors are grateful for funding provided by an NSF GRFP to J.K.G. (2015195769) and DEB‐1353970 to L.F.D.Negative frequency‐dependent selection (NFDS) has been shown to maintain polymorphism in a diverse array of traits. The action of NFDS has been confirmed through modeling, experimental approaches, and genetic analyses. In this study, we investigated NFDS in the wild using morph‐frequency changes spanning a 20‐year period from over 30 dimorphic populations of Datura wrightii. In these populations, plants either possess glandular (sticky) or non‐glandular (velvety) trichomes, and the ratio of these morphs varies substantially among populations. Our method provided evidence that NFDS, rather than drift or migration, is the primary force maintaining this dimorphism. Most populations that were initially dimorphic remained dimorphic, and the overall mean and variance in morph frequency did not change over time. Furthermore, morph‐frequency differences were not related to geographic distances. Together, these results indicate that neither directional selection, drift, or migration played a substantial role in determining morph frequencies. However, as predicted by negative frequency‐dependent selection, we found that the rare morph tended to increase in frequency, leading to a negative relationship between the change in the frequency of the sticky morph and its initial frequency. In addition, we found that morph‐frequency change over time was significantly correlated with the damage inflicted by two herbivores: Lema daturaphila and Tupiochoris notatus. The latter is a specialist on the sticky morph and damage by this herbivore was greatest when the sticky morph was common. The reverse was true for L. daturaphila, such that damage increased with the frequency of the velvety morph. These findings suggest that these herbivores contribute to balancing selection on the observed trichome dimorphism.Publisher PDFPeer reviewe
Independent freezing of charge and spin dynamics in La1.5Sr0.5CoO4
We present elastic and quasielastic neutron scattering measurements
characterizing peculiar short-range charge-orbital and spin order in the
layered perovskite material La1.5Sr0.5CoO4. We find that below Tc~750 K holes
introduced by Sr doping lose mobility and enter a statically ordered {\it
charge glass} phase with loosely correlated checkerboard arrangement of empty
and occupied d{3z2-r2} orbitals (Co3+ and Co2+). The dynamics of the resultant
mixed spin system is governed by the anisotropic nature of the crystal-field
Hamiltonian and the peculiar exchange pattern produced by the orbital order. It
undergoes a {\it spin freezing} transition at much a lower temperature, Ts~30
K.Comment: 4 pages, 3 figures, Latex. Submitted to PR
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