391 research outputs found
Strange magnetic multipoles and neutron diffraction by an iridate perovskite (Sr2IrO4)
A theoretical investigation of a plausible construct for electronic structure
in iridate perovskites demonstrates the existence of magnetic multipoles
hitherto not identified. The strange multipoles, which are parity-even,
time-odd and even rank tensors, are absent from the so-called jeff = 1/2 model.
We prove that the strange multipoles contribute to magnetic neutron
diffraction, and we estimate their contribution to intensities of Bragg spots
for Sr2IrO4. The construct encompasses the jeff = 1/2 model, and it is
consistent with the known magnetic structure, ordered magnetic moment, and
published resonant x-ray Bragg diffraction data. Over and above time-odd
quadrupoles and hexadecapoles, whose contribution changes neutron Bragg
intensities by an order of magnitude, according to our estimates, are
relatively small triakontadipoles recently proposed as the primary magnetic
order-parameter of Sr2IrO4
Anapole Correlations in Sr2IrO4 Defy the jeff = 1/2 Model
Zel'dovich (spin) anapole correlations in Sr2IrO4 unveiled by magnetic
neutron diffraction contravene the spin-orbit coupled ground state used by the
jeff = 1/2 (pseudo-spin) model. Specifically, spin and space know inextricable
knots which bind each to the other in the iridate. The diffraction property
studied in the Letter is enforced by strict requirements from quantum mechanics
and magnetic symmetry. It has not been exploited in the past, whereas neutron
diffraction by anapole moments is established. Entanglement of the electronic
degrees of freedom is captured by binary correlations of the anapole and
position operators, and hallmarked in the diffraction amplitude by axial atomic
multipoles with an even rank
A possible chiral spin-liquid phase in non-centrosymmetric BaCoO
Based on a symmetry approach, we propose a possible explanation of the weak
ferromagnetic component recently observed in YBaCoFeO (Valldor et al.
Phys Rev B, 224426 (2011)) and other isostructural compounds in the
high-temperature spin-liquid phase. Due to the polar nature of their crystal
structure, a coupling between time-odd scalar spin chirality which we suggest
as the primary order parameter and macroscopic magnetization is possible as
follows from the general form of the appropriate free-energy invariant. The
deduced pseudoproper coupling between both physical quantities provides a
unique possibility to study the critical behaviour of the chiral order
parameter
Ordered state of magnetic charge in the pseudo-gap phase of a cuprate superconductor (HgBa2CuO4)
A symmetry-based interpretation of published experimental results
demonstrates that the pseudo-gap phase of underdoped HgBa2CuO4(Hg1201)
possesses an ordered state of magnetic charge epitomized by Cu magnetic
monopoles. Magnetic properties of one-layer Hg1201 and two-layer YBa2Cu3O6 + x
(YBCO) cuprates have much in common, because their pseudo-gap phases possess
the same magnetic space-group, e.g., both underdoped cuprates allow the
magneto-electric (Kerr) effect. Differences in their properties stem from
different Cu site symmetries, leaving Cu magnetic monopoles forbidden in YBCO.
Resonant x-ray Bragg diffraction experiments can complement the wealth of
information available from neutron diffraction experiments on five Hg1201
samples on which our findings are based. In the case of Hg1201 emergence of the
pseudo-gap phase, with time-reversal violation, is accompanied by a reduction
of Cu site symmetry that includes loss of a centre of inversion symmetry. In
consequence, parity-odd x-ray absorption events herald the onset of the
enigmatic phase, and we predict dependence of corresponding Bragg spots on
magneto-electric multipoles, including the monopole, and the azimuthal angle
(crystal rotation about the Bragg wavevector)
X-ray diffraction by magnetic charges (monopoles)
Magnetic charges, or magnetic monopoles, may form in the electronic structure
of magnetic materials where ions are deprived of symmetry with respect to
spatial inversion. Predicted in 2009, the strange magnetic, pseudoscalars have
recently been found different from zero in simulations of electronic structures
of some magnetically ordered, orthorhombic, lithium orthophosphates (LiMPO4).
We prove that magnetic charges in lithium orthophosphates diffract x-rays tuned
in energy to an atomic resonance, and to guide future experiments we calculate
appropriate unit-cell structure factors for monoclinic LiCoPO4 and orthorhombic
LiNiPO4
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