87 research outputs found
Field-Induced Staggered Magnetic Order in La(2)NiO(4.133)
At low temperature the holes doped into the NiO(2) planes of La(2)NiO(4.133)
by the excess oxygen collect in diagonal stripes that separate narrow
antiferromagnetic domains. The magnetic order drops abruptly to zero at T_m =
110.5 K, but charge order remains with a period of 3a/2. We show that
application of a magnetic field in the regime T > T_m induces staggered
magnetic order of period 3a due to the net magnetic moment of the
high-temperature bond-centered stripes, together with the odd number of Ni
spins across an antiferromagnetic domain.Comment: 4 pages, Revtex, 4 epsf figs included with psfig
Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet
The Schwinger-boson mean-field theory is used to study the three-dimensional
antiferromagnetic ordering and excitations in compounds , a large
family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate
magnetic properties of these compounds, we introduce a three-dimensional
mixed-spin antiferromagnetic Heisenberg model based on experimental results for
the crystal structure of . This model can explain the experimental
discovery of coexistence of Haldane gap and antiferromagnetic long-range order
below N\'{e}el temperature. Properties such as the low-lying excitations,
magnetizations of and rare-earth ions, N\'{e}el temperatures of different
compounds, and the behavior of Haldane gap below the N\'{e}el temperature are
investigated within this model, and the results are in good agreement with
neutron scattering experiments.Comment: 12 pages, 6 figure
Energy Storage in Electrochemical Cells with Molten Sb Electrodes
An energy-storage concept is proposed using molten Sb as the fuel in a reversible solid-oxide electrochemical cell (SOEC). Because both Sb and Sb2O3 are liquids at typical SOEC operating temperatures, it is possible to flow Sb from an external tank and use it as the fuel under fuel-cell conditions and then electrolyze Sb2O3 during recharging. This concept was tested using a button cell with a Sc-stabilized zirconia electrolyte at 973 K by measuring the impedances under fuel-cell and electrolyzer conditions for a range of stirred Sb-Sb2O3 compositions. The Sb-Sb2O3 electrode impedances were found to be on the order of 0.15 Ωcm2 for both fuel-cell and electrolyzer conditions, for compositions up to 30% Sb and 70% Sb2O3. The open circuit voltages (OCV) were 0.75 V, independent of oxygen composition. Some features of using molten Sb as an energy-storage medium are discussed
Effect of the Orbital Level Difference in Doped Spin-1 Chains
Doping of a two-orbital chain with mobile S=1/2 Fermions and strong Hund's
rule couplings stabilizing the S=1 spins strongly depends on the presence of a
level difference among these orbitals. By DMRG methods we find a finite spin
gap upon doping and dominant pairing correlations without level-difference,
whereas the presence of a level difference leads to dominant charge density
wave (CDW) correlations with gapless spin-excitations. The string correlation
function also shows qualitative differences between the two models.Comment: 4 pages, 4 figure
Magnetic transitions in Pr2NiO4 single crystal
The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first‐order character and involves both the out‐of‐plane and the in‐plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first‐order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out‐of‐phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field‐induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high‐field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4
Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes
The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in
various ways, but so far the proposed interpretations have neglected the fact
that the holes doped into the NiO(2) planes order in diagonal stripes, as
established by neutron and X-ray scattering. Here we present a study of optical
conductivity in La(2)NiO(4+d) with d=2/15, a material in which the charge
stripes order three-dimensionally. We show that the conductivity can be
decomposed into two components, a mid-infrared peak that we attribute to
transitions from the filled valence band into empty mid-gap states associated
with the stripes, and a Drude peak that appears at higher temperatures as
carriers are thermally excited into the mid-gap states. The shift of the mid-IR
peak to lower energy with increasing temperature is explained in terms of the
Franck-Condon effect. The relevance of these results to understanding the
optical conductivity in the cuprates is discussed.Comment: final version of paper (minor changes from previous version
Random interactions and spin-glass thermodynamic transition in the hole-doped Haldane system YCaBaNiO
Magnetization, DC and AC bulk susceptibility of the =1 Haldane chain
system doped with electronic holes, YCaBaNiO
(0x0.20), have been measured and analyzed. The most striking
results are (i) a sub-Curie power law behavior of the linear susceptibility,
, for temperature lower than the Haldane gap
of the undoped compound (x=0) (ii) the existence of a spin-glass thermodynamic
transition at = 2-3 K. These findings are consistent with (i) random
couplings within the chains between the spin degrees of freedom induced by hole
doping, (ii) the existence of ferromagnetic bonds that induce magnetic
frustration when interchain interactions come into play at low temperature.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Doped two orbital chains with strong Hund's rule couplings - ferromagnetism, spin gap, singlet and triplet pairings
Different models for doping of two-orbital chains with mobile
fermions and strong, ferromagnetic (FM) Hund's rule couplings stabilizing the
S=1 spins are investigated by density matrix renormalization group (DMRG)
methods. The competition between antiferromagnetic (AF) and FM order leads to a
rich phase diagram with a narrow FM region for weak AF couplings and strongly
enhanced triplet pairing correlations. Without a level difference between the
orbitals, the spin gap persists upon doping, whereas gapless spin excitations
are generated by interactions among itinerant polarons in the presence of a
level difference. In the charge sector we find dominant singlet pairing
correlations without a level difference, whereas upon the inclusion of a
Coulomb repulsion between the orbitals or with a level difference, charge
density wave (CDW) correlations decay slowest. The string correlation functions
remain finite upon doping for all models.Comment: 9pages, 9figure
Structure of end states for a Haldane Spin Chain
Inelastic neutron scattering was used to probe edge states in a quantum spin
liquid. The experiment was performed on finite length antiferromagnetic spin-1
chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance
below the Haldane gap. The wave vector dependence of its intensity provides
direct evidence for staggered magnetization at chain ends, which decays
exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions
to the chain end spectrum indicate inter-chain-segment interactions. We also
observe a finite size blue shift of the Haldane gap.Comment: 4 pages RevTex, 3 figure
Coexistence of Haldane gap excitations and long range antiferromagnetic order in mixed-spin nickelates R_2 Ba Ni O_5
The spin dynamics of the S=1 Ni-chains in mixed-spin antiferromagnets Pr_2 Ba
Ni O_5 and Nd_x Y_2-x Ba Ni O_5 is described in terms of a simple
Ginzburg-Landau Lagrangian coupled to the sublattice of rare-earth ions. Within
this framework we obtain a theoretical explanation for the experimentally
observed coexistence of Haldane gap excitations and long-range magnetic order,
as well as for the increase of the Haldane gap energy below the Neel point. We
also predict that the degeneracy of the Haldane triplet is lifted in the
magnetically ordered phase. The theoretical results are consistent with the
available experimental data.Comment: 4 pages, 1 figure, submitted to PRL An alternative derivation of main
results and new references adde
- …