50 research outputs found
Symmetry breaking and unconventional charge ordering in single crystal NaRuO
The interplay of charge, spin, and lattice degrees of freedom in matter leads
to various forms of ordered states through phase transitions. An important
subclass of these phenomena of complex materials is charge ordering (CO),
mainly driven by mixed-valence states. We discovered by combining the results
of electrical resistivity (), specific heat, susceptibility
(\textit{T}), and single crystal x-ray diffraction (SC-XRD) that
NaRuO with the monoclinic tunnel type lattice (space group
2/) exhibits an unconventional CO at room temperature while retaining
metallicity. The temperature-dependent SC-XRD results show successive phase
transitions with super-lattice reflections at \textbf{q}=(0, ,
0) and \textbf{q}=(0, , ) below
(365 K) and only at \textbf{q}=(0, , 0) between
and (630 K). We interpreted these as an
evidence for the formation of an unconventional CO. It reveals a strong
first-order phase transition in the electrical resistivity at
(cooling) = 345 K and (heating) = 365 K. We argue that the
origin of the phase transition is due to the localized 4 Ru-electrons. The
results of our finding reveal an unique example of Ru/Ru mixed
valance heavy \textit{d} ions.Comment: 10 pages, 9 figure
Hexagonal RMnO3: a model system for two-dimensional triangular lattice antiferromagnets
The hexagonal RMnO3(h-RMnO3) are multiferroic materials, which exhibit the coexistence of a magnetic order and ferroelectricity. Their distinction is in their geometry that both results in an unusual mechanism to break inversion symmetry and also produces a two-dimensional triangular lattice of Mn spins, which is subject to geometrical magnetic frustration due to the antiferromagnetic interactions between nearest-neighbor Mn ions. This unique combination makes the h-RMnO3 a model system to test ideas of spin-lattice coupling, particularly when both the improper ferroelectricity and the Mn trimerization that appears to determine the symmetry of the magnetic structure arise from the same structure distortion. In this review we demonstrate how the use of both neutron and X-ray diffraction and inelastic neutron scattering techniques have been essential to paint this comprehensive and coherent picture of h-RMnO3. (c) 2016 International Union of Crystallography110111scopu
Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling
Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been
studied via inelastic neutron scattering. A simple Heisenberg model with a
single-ion anisotropy describes most features of the spin-wave dispersion
curves. However, there is shown to be a renormalization of the magnon energies
located at around 11 meV. Since both the magnon-magnon interaction and
magnon-phonon coupling can affect the renormalization in a noncollinear magnet,
we have accounted for both of these couplings by using a Heisenberg XXZ model
with 1=S expansions [1] and the Einstein site phonon model [13], respectively.
This quantitative analysis leads to the conclusion that the renormalization
effect primarily originates from the magnon-phonon coupling, while the
spontaneous magnon decay due to the magnon-magnon interaction is suppressed by
strong two-ion anisotropy.Comment: 5 pages, 4 figure
Thermal Hall effects due to topological spin fluctuations in YMnO_3
The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO3 produces a highly unusual thermal Hall effect under an applied magnetic field. Our theoretical calculations demonstrate that the thermal Hall conductivity is related to the splitting of the otherwise degenerate two chiralities of its 120˚ magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken Z2 symmetry of the chirality in the supposedly paramagnetic state of YMnO3, due to strong topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction
Properties of spin 1/2 triangular lattice antiferromagnets: CuRE2Ge2O8 (RE=Y, La)
We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems:
CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray
and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La)
exhibits a 2D triangular lattice linked via weak bonds along the perpendicular
b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a
long-range order at 0.51(1) and 1.09(4) K for the Y and La systems,
respectively. Interestingly, they also exhibit field induced phase transitions.
For theoretical understanding, we carried out the density functional theory
(DFT) band calculations to find that they are typical charge-transfer-type
insulators with a gap of Eg = 2 eV. Taken together, our observations make
CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin
triangular antiferromagnets with the low-temperature magnetic ordering.Comment: 15 pages, 6 figures, and 1 tabl
Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3
Novel effects induced by nonmagnetic impurities in frustrated magnets and
quantum spin liquid represent a highly nontrivial and interesting problem. A
theoretical proposal of extended modulated spin structures induced by doping of
such magnets, distinct from the well-known skyrmions has attracted significant
interest. Here, we demonstrate that nonmagnetic impurities can produce such
extended spin structures in h-YMnO3, a triangular antiferromagnet with
noncollinear magnetic order. Using inelastic neutron scattering (INS), we
measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed
the presence of magnon damping with a clear momentum dependence. Our
theoretical calculations can reproduce the key features of the INS data,
supporting the formation of the proposed spin textures. As such, our study
provides the first experimental confirmation of the impurity-induced spin
textures. It offers new insights and understanding of the impurity effects in a
broad class of noncollinear magnetic systems.Comment: 18 pages, 4 figures and supplementary Information. Accepted for
publication in Nature Communication
Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of LiRuO
When an electronic system has strong correlations and a large spin-orbit
interaction, it often exhibits a plethora of mutually competing quantum phases.
How a particular quantum ground state is selected out of several possibilities
is a very interesting question. However, equally fascinating is how such a
quantum entangled state breaks up due to perturbation. This important question
has relevance in very diverse fields of science from strongly correlated
electron physics to quantum information. Here we report that a quantum
entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows
nontrivial doping dependence as we perturb the Ru honeycomb lattice by
replacing Ru with Li. Through extensive experimental studies, we demonstrate
that the VBC phase melts into a valence bond liquid phase of the RVB
(resonating valence bond) type. This system offers an interesting playground
where one can test and refine our current understanding of the quantum
competing phases in a single compound.Comment: Scientific Reports (in press