35 research outputs found
Half-skyrmion picture of single hole doped CuO_2 plane
Based on the Zhang-Rice singlet picture, it is argued that the half-skyrmion
is created by the doped hole in the single hole doped high-T_c cuprates with
N'eel ordering. The spin configuration around the Zhang-Rice singlet, which has
the form of superposition of the two different d-orbital hole spin states, is
studied within the non-linear \sigma model and the CP^1 model. The spin
configurations associated with each hole spin state are obtained, and we find
that the superposition of these spin configuration turns out to be the
half-skyrmion that is characterized by a half of the topological charge. The
excitation spectrum of the half-skyrmion is obtained by making use of Lorentz
invariance of the effective theory and is qualitatively in good agreement with
angle resolved photoemission spectroscopy on the parent compunds. Estimated
values of the parameters contained in the excitation spectrum are in good
agreement with experimentally obtained values. The half-skyrmion theory
suggests a picture for the difference between the hole doped compounds and the
electron doped compounds.Comment: 13 pages, 2 figures, to be published in Phys. Rev.
Tilted-Cone Induced Cusps and Nonmonotonic Structures in Dynamical Polarization Function of Massless Dirac Fermions
The polarization function of electrons with the tilted Dirac cone found in
organic conductors is studied using the tilted Weyl equation. The dynamical
property is explored based on the analytical treatment of the particle-hole
excitation. It is shown that the polarization function as the function of both
the frequency and the momentum exhibits cusps and nonmonotonic structures. The
polarization function depends not only on the magnitude but also the direction
of the external momentum. These properties are characteristic of the tilted
Dirac cone, and are contrast to the isotropic case of grapheme. Further, the
results are applied to calculate the optical conductivity, the plasma frequency
and the screening of Coulomb interaction, which are also strongly influenced by
the tilted cone.Comment: 28 pages, 12 figures, to be published in Journal of the Physical
Society of Japan Vol. 79 (2010) No. 1
Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition
Nematicity, defined as broken rotational symmetry, has recently been observed
in competing phases proximate to the superconducting phase in the cuprate high
temperature superconductors. Similarly, the new iron-based high temperature
superconductors exhibit a tetragonal to orthorhombic structural transition
(i.e. a broken C4 symmetry) that either precedes or is coincident with a
collinear spin density wave (SDW) transition in undoped parent compounds, and
superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state in this family of
compounds has been reported by neutron scattering, scanning tunneling
microscopy, and transport measurements. Here we present an angle resolved
photoemission spectroscopy study of detwinned single crystals of a
representative family of electron-doped iron-arsenide superconductors,
Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via
application of in-plane uniaxial stress, enabling measurements of single domain
electronic structure in the orthorhombic state. At low temperatures, our
results clearly demonstrate an in-plane electronic anisotropy characterized by
a large energy splitting of two orthogonal bands with dominant dxz and dyz
character, which is consistent with anisotropy observed by other probes. For
compositions x>0, for which the structural transition (TS) precedes the
magnetic transition (TSDW), an anisotropic splitting is observed to develop
above TSDW, indicating that it is specifically associated with TS. For
unstressed crystals, the band splitting is observed close to TS, whereas for
stressed crystals the splitting is observed to considerably higher
temperatures, revealing the presence of a surprisingly large in-plane nematic
susceptibility in the electronic structure.Comment: final version published in PNAS, including supplementary informatio
Evolution of Bilayer Quantum Hall Ferromagnet
The natures of the ground state in a bilayer quantum Hall
system at a variety of layer spacing are investigated. At small layer
separations the system exhibits spontaneous interlayer phase coherence. It is
claimed that the Halperin's (1,1,1) state is not relevant in the incompressible
regime near the incompressible to compressible transition point in which the
Josephson-like effect was observed. The two-particle correlation function shows
the deflated correlation hole at this regime. An effective model that can give
a good approximation to the ground state is proposed. A connection to the
modified composite fermion theory is discussed
Strong quasi-particle tunneling study in the paired quantum Hall states
The quasi-particle tunneling phenomena in the paired fractional quantum Hall
states are studied. A single point-contact system is first considered. Because
of relevancy of the quasi-particle tunneling term, the strong tunneling regime
should be investigated.
Using the instanton method it is shown that the strong quasi-particle
tunneling regime is described as the weak electron tunneling regime
effectively.
Expanding to the network model the paired quantum Hall liquid to insulator
transition is discussed
Origin of In-Plane Anisotropy in Optical Conductivity for Antiferromagnetic Metallic Phase of Iron Pnictides
We examine the optical conductivity in antiferromagnetic (AFM) iron pnictides
by mean-field calculation in a five-band Hubbard model. The calculated spectra
are well consistent with the in-plane anisotropy observed in the measurements,
where the optical conductivity along the direction with the AFM alignment of
neighboring spins is larger than that along the ferromagnetic (FM) direction in
the low-energy region; however, that along the FM direction becomes larger in
the higher-energy region. The difference between the two directions is
explained by taking account of orbital characters in both occupied and
unoccupied states as well as of the nature of Dirac-type linear dispersions
near the Fermi level.Comment: 4pages, 3 figures, 1 tabl
Strongly coupled quantum criticality with a Fermi surface in two dimensions: fractionalization of spin and charge collective modes
We describe two dimensional models with a metallic Fermi surface which
display quantum phase transitions controlled by strongly interacting critical
field theories below their upper critical dimension. The primary examples
involve transitions with a topological order parameter associated with
dislocations in collinear spin density wave ("stripe") correlations: the
gapping of the order parameter fluctuations leads to a fractionalization of
spin and charge collective modes, and this transition has been proposed as a
candidate for the cuprates near optimal doping. The coupling between the order
parameter and long-wavelength volume and shape deformations of the Fermi
surface is analyzed by the renormalization group, and a runaway flow to a
non-perturbative regime is found in most cases. A phenomenological scaling
analysis of simple observable properties of possible second order quantum
critical points is presented, with results quite similar to those near quantum
spin glass transitions and to phenomenological forms proposed by Schroeder et
al. (cond-mat/0011002).Comment: 16 pages, 4 figures; (v2) additional clarifying remark
Fluctuation Effect in the \pi-flux State for Undoped High-Temperature Superconductors
The effect of fluctuations about the pi-flux mean field state for the undoped
high-temperature superconductors is investigated. It is shown that fluctuations
of the mean fields lead to a self-energy correction that doubles the band width
of the fermion dispersion in the lowest order. The dynamical mass generation is
associated with the self-energy effect due to the interaction mediated by the
Lagrange multiplier field, which is introduced to impose the constraint on the
fermions. A self-consistent picture about the mass generation and the prop-
agation of the Lagrange multiplier field without damping is proposed. The
antiferromagnetic long-range ordering is described without introducing an
additional repulsive interaction. The theory suggests a natural framework to
study spin disordered systems in which fermionic excitations are low-lying
excitations.Comment: 20 pages, 7 figures, title changed, added an appendix and figure
Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2
Raman selection rules for electronic and magnetic excitations in BaFe2As2
were theoretically investigated and applied them to the separate detection of
the nodal and anti-nodal gap excitations at the spin density wave (SDW)
transition and the separate detection of the nearest and the next nearest
neighbor exchange interaction energies. The SDW gap has Dirac nodes, because
many orbitals participate in the electronic states near the Fermi energy. Using
a two-orbital band model the electronic excitations near the Dirac node and the
anti-node are found to have different symmetries. Applying the symmetry
difference to Raman scattering the nodal and anti-nodal electronic excitations
are separately obtained. The low-energy spectra from the anti-nodal region have
critical fluctuation just above TSDW and change into the gap structure by the
first order transition at TSDW, while those from the nodal region gradually
change into the SDW state. The selection rule for two-magnon scattering from
the stripe spin structure was obtained. Applying it to the two-magnon Raman
spectra it is found that the magnetic exchange interaction energies are not
presented by the short-range superexchange model, but the second derivative of
the total energy of the stripe spin structure with respect to the moment
directions. The selection rule and the peak energy are expressed by the
two-magnon scattering process in an insulator, but the large spectral weight
above twice the maximum spin wave energy is difficult to explain by the decayed
spin wave. It may be explained by the electronic scattering of itinerant
carriers with the magnetic self-energy in the localized spin picture or the
particle-hole excitation model in the itinerant spin picture. The magnetic
scattering spectra are compared to the insulating and metallic cuprate
superconductors whose spins are believed to be localized.Comment: 38 pages, 11 figure