37 research outputs found
Quantum FFLO state in clean layered superconductors
We investigate the influence of Landau quantization on the superconducting
instability for a pure layered superconductor in the magnetic field directed
perpendicular to the layers. We demonstrate that the quantization corrections
to the Cooper-pairing kernel with finite Zeeman spin splitting promote the
formation of the nonuniform state in which the order parameter is periodically
modulated along the magnetic field, i.e., between the layers
(Fulde-Ferrell-Larkin-Ovchinnikov [FFLO] state). The conventional uniform state
experiences such a quantization-induced FFLO instability at low temperatures
even in a common case of predominantly orbital suppression of superconductivity
when the Zeeman spin splitting is expected to have a relatively weak effect.
The maximum relative FFLO temperature is given by the ratio of the
superconducting transition temperature and the Fermi energy. This maximum is
realized when the ratio of the spin-spitting energy and the Landau-level
separation is half-integer. These results imply that the FFLO states may exist
not only in the Pauli-limited superconductors but also in very clean materials
with small Zeeman spin-splitting energy. We expect that the described
quantization-promoted FFLO instability is a general phenomenon, which may be
found in materials with different electronic spectra and order-parameter
symmetries.Comment: 18 pages, 10 figures, minor correction
Interplay between orbital-quantization effects and the Fulde-Ferrell-Larkin-Ovchinnikov instability in multiple-band layered superconductors
We explore superconducting instability for a clean two-band layered
superconductor with deep and shallow bands in the magnetic field applied
perpendicular to the layers. In the shallow band, the quasiclassical
approximation is not applicable, and Landau quantization has to be accounted
for exactly. The electronic spectrum of this band in the magnetic field is
composed of the one-dimensional Landau-level minibands. With increasing
magnetic field the system experiences series of Lifshitz transitions when the
chemical potential enters and exits the minibands. These transitions profoundly
influence the shape of the upper critical field at low temperatures. In
addition, the Zeeman spin splitting may cause the nonuniform state with
interlayer modulation of the superconducting order parameter
(Fulde-Ferrell-Larkin-Ovchinnikov state). Typically, the quantization effects
in the shallow band strongly promote the formation of this state. The uniform
state remains favorable only in the exceptional resonance cases when the
spin-splitting energy exactly matches the Landau-level spacing. Furthermore,
for specific relations between electronic spectrum parameters, the alternating
FFLO state may realize, in which the order parameter changes sign between the
neighboring layers. For all above cases, the reentrant high-field
superconducting states may emerge at low temperatures if the shallow band has
significant contribution to the Cooper pairing.Comment: 25 pages, 14 figures, minor revisions and more references adde
Strong Landau-quantization effects in high-magnetic-field superconductivity of a two-dimensional multiple-band metal near the Lifshitz transition
We investigate the onset of superconductivity in magnetic field for a clean
two-dimensional multiple-band superconductor in the vicinity of the Lifshitz
transition when one of the bands is very shallow. Due to small number of
carriers in this band, the quasiclassical Werthamer-Helfand approximation
breaks down and Landau quantization has to be taken into account. We found that
the transition temperature TC2(H) has giant oscillations and is resonantly
enhanced at the magnetic fields corresponding to full occupancy of the Landau
levels in the shallow band. This enhancement is especially pronounced for the
lowest Landau level. As a consequence, the reentrant superconducting regions in
the temperature-field phase diagram emerge at low temperatures near the
magnetic fields at which the chemical potential matches the Landau levels. The
specific behavior depends on the relative strength of the intraband and
interband pairing interactions and the reentrance is most pronounced in the
purely interband coupling scenario. The reentrant behavior is suppressed by the
Zeeman spin splitting in the shallow band, the separated regions disappear
already for very small spin-splitting factors. On the other hand, the
reentrance is restored in the resonance cases when the spin-splitting energy
exactly matches the separation between the Landau levels. The predicted
behavior may realize in the gate-tuned FeSe monolayer.Comment: 23 pages, 9 figures, more references added and one figure adde
The light pseudoscalar Higgs boson in NMSSM
We study the associated production of a very light pseudoscalar Higgs boson
with a pair of charginos. The novel signature involves a pair of charged
leptons from chargino decays and a pair of photons from the pseudoscalar Higgs
boson decay, plus large missing energy at the LHC and ILC. The signal may help
us to distinguish the NMSSM from MSSM, provided that the experiment can resolve
the two photons from the decay of the pseudoscalar Higgs boson.Comment: pages, 2 figures; to appear in Proceedings of SUSY06, the 14th
International Conference on Supersymmetry and the Unification of Fundamental
Interactions, UC Irvine, California, 12-17 June 200
Excitonic Instabilities and Insulating States in Bilayer Graphene
The competing ground states of bilayer graphene are studied by applying
renormalization group techniques to a bilayer honeycomb lattice with nearest
neighbor hopping. In the absence of interactions, the Fermi surface of this
model at half-filling consists of two nodal points with momenta ,
, where the conduction band and valence band touch each other,
yielding a semi-metal. Since near these two points the energy dispersion is
quadratic with perfect particle-hole symmetry, excitonic instabilities are
inevitable if inter-band interactions are present. Using a perturbative
renormalization group analysis up to the one-loop level, we find different
competing ordered ground states, including ferromagnetism, superconductivity,
spin and charge density wave states with ordering vector
, and excitonic insulator states. In
addition, two states with valley symmetry breaking are found in the excitonic
insulating and ferromagnetic phases. This analysis strongly suggests that the
ground state of bilayer graphene should be gapped, and with the exception of
superconductivity, all other possible ground states are insulating.Comment: 17 pages, 6 figures, 2 Tables, Added reference
Possible Nematic Order Driven by Magnetic Fluctuations in Iron Pnictides
In this paper, instabilities of the isotropic metallic phase in iron
pnictides are investigated. The relevant quartic fermionic interaction terms in
the model are identified using phase space arguments. Using the functional
integral formalism, a Hubbard-Stratonovich transformation is used to decouple
these quartic terms. This procedure introduces several bosonic fields which
describe the low-energy collective modes of the system. By studying the
behavior of these collective modes, a possible instability is found in the
forward scattering channel of the isotropic phase driven by magnetic
fluctuations. Using mean field analysis, we obtain a static and homogeneous
ground state. This ground state is metallic, but the electron Fermi pockets are
distorted unequally at different pockets in momentum space. This results in a
desirable nematic ordering which breaks the lattice C4 symmetry but preserves
translational symmetry and may explain several experimental observations.Comment: 9 pages, 2 figure
Attractive trion-polariton nonlinearity due to Coulomb scattering
We theoretically investigate the nonlinearity of trion-polaritons in a
two-dimensional material that arises from Coulomb interaction between
quasiparticles. To evaluate the interaction constant, we solve a three-body
Wannier equation precisely by expanding trion wavefunctions into a Gaussian
basis. Using these wavefunctions, we calculate the trion-polariton interaction
energies for the exchange processes, resolving the outstanding question of
trion-trion scattering. We find that the nonlinearity is the result of the
competition between different scattering channels. Such a cancellation effect
is sensitive to wavefunction overlaps and depends on material parameters. Most
importantly, our result shows that the nonlinear interaction between
trion-polaritons is attractive, and is fivefold stronger than exciton-polariton
interaction. Our work thus describes the regime where trion-polaritons offer
the prospects for attractive fluids of light in monolayers of transition metal
dichalcogenides.Comment: 5 pages, 3 figures; the accompanying paper with all derivations will
be posted soo
Crossover from weakly indirect to direct excitons in atomically thin films of InSe
We perform a theory analysis of the spectra of the
lowest energy and excited states of the excitons in few-layer atomically thin
films of InSe taking into account in-plane electric polarizability of the film
and the influence of the encapsulation environment. For the thinner films, the
lowest-energy state of the exciton is weakly indirect in momentum space, with
its dispersion showing minima at a layer-number-dependent wave number, due to
an inverted edge of a relatively flat topmost valence band branch of the InSe
film spectrum and we compute the activation energy from the momentum dark
exciton ground state into the bright state. For the films with more than seven
InSe layers, the exciton dispersion minimum shifts to -point.Comment: 12 pages, 7 figure
Iron Biofortification of Rice: Progress and Prospects
Biofortification is the process of improving the bioavailability of essential nutrients in food crops either through conventional breeding or modern biotechnology techniques. Rice is one of the most demanding staple foods worldwide. Most global population live on a diet based on rice as the main carbohydrate source that serve as suitable target for biofortification. In general, polished grain or white rice contains nutritionally insufficient concentration of iron (Fe) to meet the daily requirements in diets. Therefore, iron biofortification in rice offers an inexpensive and sustainable solution to mitigate iron deficiency. However, understanding on the mechanism and genes involved in iron uptake in rice is a prerequisite for successful iron biofortification. In this chapter, the overview of iron uptake strategies in plants and as well as different iron-biofortified approaches used in rice will be outlined. Then, the challenges and future prospects of rice iron biofortification to improve global human health will also be discussed
Nanocalorimetric Evidence for Nematic Superconductivity in the Doped Topological Insulator SrBiSe
Spontaneous rotational-symmetry breaking in the superconducting state of
doped has attracted significant attention as an
indicator for topological superconductivity. In this paper, high-resolution
calorimetry of the single-crystal
provides unequivocal evidence of a two-fold rotational symmetry in the
superconducting gap by a \emph{bulk thermodynamic} probe, a fingerprint of
nematic superconductivity. The extremely small specific heat anomaly resolved
with our high-sensitivity technique is consistent with the material's low
carrier concentration proving bulk superconductivity. The large basal-plane
anisotropy of is attributed to a nematic phase of a two-component
topological gap structure and caused by a
symmetry-breaking energy term .
A quantitative analysis of our data excludes more conventional sources of this
two-fold anisotropy and provides the first estimate for the symmetry-breaking
strength , a value that points to an onset transition of
the second order parameter component below 2K