874 research outputs found
Spin - Phonon Coupling in Nickel Oxide Determined from Ultraviolet Raman Spectroscopy
Nickel oxide (NiO) has been studied extensively for various applications
ranging from electrochemistry to solar cells [1,2]. In recent years, NiO
attracted much attention as an antiferromagnetic (AF) insulator material for
spintronic devices [3-10]. Understanding the spin - phonon coupling in NiO is a
key to its functionalization, and enabling AF spintronics' promise of
ultra-high-speed and low-power dissipation [11,12]. However, despite its status
as an exemplary AF insulator and a benchmark material for the study of
correlated electron systems, little is known about the spin - phonon
interaction, and the associated energy dissipation channel, in NiO. In
addition, there is a long-standing controversy over the large discrepancies
between the experimental and theoretical values for the electron, phonon, and
magnon energies in NiO [13-23]. This gap in knowledge is explained by NiO
optical selection rules, high Neel temperature and dominance of the magnon band
in the visible Raman spectrum, which precludes a conventional approach for
investigating such interaction. Here we show that by using ultraviolet (UV)
Raman spectroscopy one can extract the spin - phonon coupling coefficients in
NiO. We established that unlike in other materials, the spins of Ni atoms
interact more strongly with the longitudinal optical (LO) phonons than with the
transverse optical (TO) phonons, and produce opposite effects on the phonon
energies. The peculiarities of the spin - phonon coupling are consistent with
the trends given by density functional theory calculations. The obtained
results shed light on the nature of the spin - phonon coupling in AF insulators
and may help in developing innovative spintronic devices.Comment: 16 pages; 2 figure
Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene
The non-interacting energy spectrum of graphene and its bilayer counterpart
consists of multiple degeneracies owing to the inherent spin, valley and layer
symmetries. Interactions among charge carriers are expected to spontaneously
break these symmetries, leading to gapped ordered states. In the quantum Hall
regime these states are predicted to be ferromagnetic in nature whereby the
system becomes spin polarized, layer polarized or both. In bilayer graphene,
due to its parabolic dispersion, interaction-induced symmetry breaking is
already expected at zero magnetic field. In this work, the underlying order of
the various broken-symmetry states is investigated in bilayer graphene that is
suspended between top and bottom gate electrodes. By controllably breaking the
spin and sublattice symmetries we are able to deduce the order parameter of the
various quantum Hall ferromagnetic states. At small carrier densities, we
identify for the first time three distinct broken symmetry states, one of which
is consistent with either spontaneously broken time-reversal symmetry or
spontaneously broken rotational symmetry
A correction method for large deflections of cantilever beams with a modal approach
Modal-based reduced-order models are preferred for modeling structures due to their computational efficiency in engineering problems. One of the important limitations of the classic modal approaches is that they are geometrically linear. This study proposes a fast correction method to account for geometric nonlinearities which stem from large deflections in cantilever beams. The method relies on pre-computed correction terms and thus adds negligibly small extra computational efforts during the time domain response analyses. The accuracy of the method is examined on a straight-beam model and International Energy Agency (IEA) 15 MW wind turbine blade model. The results show that the proposed method increases the accuracy of modal approaches significantly in secondary deflections due to nonlinearities such as axial and torsional motions for the two studied cases.</p
Energy gaps at neutrality point in bilayer graphene in a magnetic field
Utilizing the Baym-Kadanoff formalism with the polarization function
calculated in the random phase approximation, the dynamics of the
quantum Hall state in bilayer graphene is analyzed. Two phases with nonzero
energy gap, the ferromagnetic and layer asymmetric ones, are found. The phase
diagram in the plane , where is a
top-bottom gates voltage imbalance, is described. It is shown that the energy
gap scales linearly, $\Delta E\sim 14 B[T]K, with magnetic field.Comment: 5 pages, 3 figures, title changed, references added, JETP Letters
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Quantum Hall Effects in Graphene-Based Two-Dimensional Electron Systems
In this article we review the quantum Hall physics of graphene based
two-dimensional electron systems, with a special focus on recent experimental
and theoretical developments. We explain why graphene and bilayer graphene can
be viewed respectively as J=1 and J=2 chiral two-dimensional electron gases
(C2DEGs), and why this property frames their quantum Hall physics. The current
status of experimental and theoretical work on the role of electron-electron
interactions is reviewed at length with an emphasis on unresolved issues in the
field, including assessing the role of disorder in current experimental
results. Special attention is given to the interesting low magnetic field limit
and to the relationship between quantum Hall effects and the spontaneous
anomalous Hall effects that might occur in bilayer graphene systems in the
absence of a magnetic field
The Role of Qualitative Research in Clinical Trial Development: The EASE Back Study
This article outlines the rationale for adopting a mixed methods approach within randomized controlled trials (RCTs) and explores challenges associated in doing so. Taking the example of the EASE Back feasibility and pilot study (Evaluating Acupuncture and Standard care for pregnant womEn with BACK pain: ISRCTN49955124), we detail why and how we operationalized a concurrent-sequential mixed methods research design. We present key findings from the exploratory research (focus groups and interviews) and explain how these were integrated with descriptive findings (a national survey of physical therapists) in order to inform and refine the design of the explanatory phase (the pilot RCT). We conclude with a discussion of lessons learned and implications for future research design and conduct
Broken symmetry states and divergent resistance in suspended bilayer graphene
Graphene [1] and its bilayer have generated tremendous excitement in the
physics community due to their unique electronic properties [2]. The intrinsic
physics of these materials, however, is partially masked by disorder, which can
arise from various sources such as ripples [3] or charged impurities [4].
Recent improvements in quality have been achieved by suspending graphene flakes
[5,6], yielding samples with very high mobilities and little charge
inhomogeneity. Here we report the fabrication of suspended bilayer graphene
devices with very little disorder. We observe fully developed quantized Hall
states at magnetic fields of 0.2 T, as well as broken symmetry states at
intermediate filling factors , , and . The
devices exhibit extremely high resistance in the state that grows
with magnetic field and scales as magnetic field divided by temperature. This
resistance is predominantly affected by the perpendicular component of the
applied field, indicating that the broken symmetry states arise from many-body
interactions.Comment: 23 pages, including 4 figures and supplementary information; accepted
to Nature Physic
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