2,303 research outputs found
S-wave pairing in a two-orbital t-J model on triangular lattice: possible application to PbCu(PO)O
Recently room temperature superconductor was claimed in
PbCu(PO)O (also known as LK-99) with .
Density functional theory (DFT) calculations suggest that the conduction
electrons are from the doped Cu atoms with valence close to . Motivated
by this picture, we build a two-orbital Hubbard model on a triangular lattice
formed by the and orbitals with total hole density (summed
over spin and orbital) . When , the system is in a Mott insulator
within this model. When , we derive a model and perform a
self-consistent slave boson mean field calculation. Interestingly we find a
s-wave pairing in contrast to the one-orbital t-J model which favors
pairing. S wave pairing should be more robust to disorder and may lead to high
Tc superconductor with sufficiently large values of and . However, the
DFT calculations predict a very small value of and then the is
expected to be small. If LK99 is really a high Tc superconductor, ingredients
beyond the current model are needed. We conjecture that the doped Cu atoms may
distort the original lattice and form local clusters with smaller Cu -Cu
distance and thus larger values of and . Within these clusters, we may
locally apply our t-J model calculation and expect high Tc s-wave
superconductor. Then the superconducting islands couple together, which may
eventually become a global superconductor, an insulator or even an anomalous
metal depending on sample details.Comment: 6 pages, 2 figure
Type II t-J model and shared antiferromagnetic spin coupling from Hund's rule in superconducting LaNiO
Recently a 80 K superconductor was observed in LaNiO under high
pressure. Density function theory (DFT) calculations identify and
as two active orbitals and a bilayer square lattice structure. The
averange valence of Ni is with per site. Naively one may
expect a description in terms of a two-orbital t-J model. However, there should
be significant inter-orbital repulsion and Hund's coupling larger
than the bare value of and . Especially the Hund's coupling can share
the inter-layer super-exchange of to , an
effect beyond any perturbative and mean field treatment. In the limit that
is Mott localized, we integrate it out and deal with a bialyer t-J
model for only. We find strong inter-layer pairing due to the
transmitted which can survive to hole doping relevant to the
experiment. In real system we expect that orbital will also be
slightly hole doped and can not be simply ignored. To deal with this situation,
we take the limit and propose a type II t-J model with
four singlon () states and three spin-triplet doublon () states.
Through a parton mean field treatment of the constrained Hilbert space, we
derive the bilayer one-orbital t-J model for an emergent `'
orbital with significant , justifying our phenomenological treatment.
The type II t-J model can also describe the regime where the orbital
is also slightly hole doped through tuning an orbital energy splitting
. From our calculation the pairing strength decreases with the hole
doping and is likely larger than the optimal doping. We propose
future experiments to electron dope the system to further enhance .Comment: 5 pages, 2 figures, 1 tabl
Strong pairing from small Fermi surface beyond weak coupling: Application to LaNiO
The studies of high-temperature superconductors raise a fundamental question:
Can a small Fermi surface phase, which violates the Luttinger theorem, exist
and give rise to superconductivity? Our work provides a positive answer through
a controlled theory based on a bilayer model with strong inter-layer spin-spin
coupling () but no inter-layer hopping (). Then small hole
doping of the rung-singlet insulator with two electrons per rung naturally
leads to small hole pockets with Fermi surface volume per flavor smaller than
the free fermion result by of the Brillouin zone(BZ). We construct a new
t-J model on a bilayer square lattice, so called ESD t-J model and employ a
generalized slave boson theory, which captures this small Fermi surface phase
at small hole doping . This metallic state is an intrinsically strongly
correlated Fermi liquid beyond weak coupling theory, violating the perturbative
Luttinger theorem but consistent with the Oshikawa's non-perturbative proof. We
further show that it transitions into an inter-layer paired -wave
superconductor at lower temperature through Feshbach resonance with a virtual
Cooper pair, with a surprising doping-induced crossover from
Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) at higher
hole doping levels. This leads to a superconducting dome centered around
, with the normal state changing from the conventional Fermi liquid in
the to the unusual small Fermi surface state in the side. Our
theoretical findings including phase diagrams are also confirmed by density
matrix renormalization group (DMRG) simulation in quasi one dimension. Applying
our theoretical framework, we provide a plausible scenario for the recently
found nickelate LaNiO materials.Comment: 11+13 pages, 8+13 figur
Extended states in 1D lattices: application to quasiperiodic copper-mean chain
The question of the conditions under which 1D systems support extended
electronic eigenstates is addressed in a very general context. Using real space
renormalisation group arguments we discuss the precise criteria for determining
the entire spertrum of extended eigenstates and the corresponding
eigenfunctions in disordered as well as quasiperiodic systems. For purposes of
illustration we calculate a few selected eigenvalues and the corresponding
extended eigenfunctions for the quasiperiodic copper-mean chain. So far, for
the infinite copper-mean chain, only a single energy has been numerically shown
to support an extended eigenstate [ You et al. (1991)] : we show analytically
that there is in fact an infinite number of extended eigenstates in this
lattice which form fragmented minibands.Comment: 10 pages + 2 figures available on request; LaTeX version 2.0
Duality and Enhanced Gauge Symmetry in 2+1 Dimensions
We investigate the enlarged CP(N) model in 2+1 dimensions. This is a hybrid
of two CP(N) models coupled with each other in a dual symmetric fashion, and it
exhibits the gauge symmetry enhancement and radiative induction of the finite
off-diagonal gauge boson mass as in the 1+1 dimensional case. We solve the mass
gap equations and study the fixed point structure in the large-N limit. We find
an interacting ultraviolet fixed point which is in contrast with the 1+1
dimensional case. We also compute the large-N effective gauge action
explicitly.Comment: 26 pags, latex, 5 .eps figures, typos corrected. To appear in J.
Phys.
Prospects for terahertz imaging the human skin cancer with the help of gold-nanoparticles-based terahertz-to-infrared converter
The design is suggested, and possible operation parameters are discussed, of
an instrument to inspect a skin cancer tumour in the terahertz (THz) range,
transferring the image into the infrared (IR) and making it visible with the
help of standard IR camera. The central element of the device is the THz-to-IR
converter, a Teflon or silicon film matrix with embedded 8.5 nm diameter gold
nanoparticles. The use of external THz source for irradiating the biological
tissue sample is presumed. The converter's temporal characteristics enable its
performance in a real-time scale. The details of design suited for the
operation in transmission mode (in vitro) or on the human skin in reflection
mode {in vivo) are specified.Comment: To be published in the proceedings of the FANEM2018 workshop - Minsk,
3-5 June 201
Effects of the field modulation on the Hofstadter's spectrum
We study the effect of spatially modulated magnetic fields on the energy
spectrum of a two-dimensional (2D) Bloch electron. Taking into account four
kinds of modulated fields and using the method of direct diagonalization of the
Hamiltonian matrix, we calculate energy spectra with varying system parameters
(i.e., the kind of the modulation, the relative strength of the modulated field
to the uniform background field, and the period of the modulation) to elucidate
that the energy band structure sensitively depends on such parameters:
Inclusion of spatially modulated fields into a uniform field leads occurrence
of gap opening, gap closing, band crossing, and band broadening, resulting
distinctive energy band structure from the Hofstadter's spectrum. We also
discuss the effect of the field modulation on the symmetries appeared in the
Hofstadter's spectrum in detail.Comment: 7 pages (in two-column), 10 figures (including 2 tables
Gauge Symmetry Enhancement and Radiatively Induced Mass in the Large N Nonlinear Sigma Model
We consider a hybrid of nonlinear sigma models in which two complex
projective spaces are coupled with each other under a duality. We study the
large N effective action in 1+1 dimensions. We find that some of the
dynamically generated gauge bosons acquire radiatively induced masses which,
however, vanish along the self-dual points where the two couplings
characterizing each complex projective space coincide. These points correspond
to the target space of the Grassmann manifold along which the gauge symmetry is
enhanced, and the theory favors the non-Abelian ultraviolet fixed point.Comment: 11 pages, REVTEX, typos are corrected, version to appear in Phys.
Rev.
Persistent Current From the Competition Between Zeeman Coupling and Spin-Orbit Interaction
Applying the non-adiabatic Aharonov-Anandan phase approach to a mesoscopic
ring with non-interacting many electrons in the presence of the spin-orbit
interaction, Zeeman coupling and magnetic flux, we show that the time-reversal
symmetry breaking due to Zeeman coupling is intrinsically different from that
due to magnetic flux. We find that the direction of the persistent currents
induced by the Zeeman coupling changes periodically with the particle number,
while the magnetic flux determines the direction of the induced currents by its
sign alone.Comment: 5 pages, ReVTeX, including 3 figures on request,Submitted to
Phys.Rev.Let
Astrometry of Water Maser Sources in Nearby Molecular Clouds with VERA - II. SVS 13 in NGC 1333
We report on the results of multi-epoch VLBI observations with VERA (VLBI
Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the
young stellar object SVS 13 in the NGC 1333 region. We have carried out
phase-referencing VLBI astrometry and measured an annual parallax of the maser
features in SVS 13 of 4.25+/-0.32 mas, corresponding to the distance of
235+/-18 pc from the Sun. Our result is consistent with a photometric distance
of 220 pc previously reported. Even though the maser features were detectable
only for 6 months, the present results provide the distance to NGC 1333 with
much higher accuracy than photometric methods. The absolute positions and
proper motions have been derived, revealing that the H2O masers with the LSR
(local standard of rest) velocities of 7-8 km s-1 are most likely associated
with VLA4A, which is a radio counterpart of SVS 13. The origin of the observed
proper motions of the maser features are currently difficult to attribute to
either the jet or the rotating circumstellar disk associated with VLA4A, which
should be investigated through future high-resolution astrometric observations
of VLA4A and other radio sources in NGC 1333.Comment: 9 pages, 5 figures. PASJ, in press (2008, Vol. 60, No. 1
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