102,392 research outputs found
k-dependent SU(4) model of high-temperature superconductivity and its coherent-state solutions
We extend the SU(4) model [1-5] for high-Tc superconductivity to an SU(4)k
model that permits explicit momentum (k) dependence in predicted observables.
We derive and solve gap equations that depend on k, temperature, and doping
from the SU(4)k coherent states, and show that the new SU(4)k model reduces to
the original SU(4) model for observables that do not depend explicitly on
momentum. The results of the SU(4)k model are relevant for experiments such as
ARPES that detect explicitly k-dependent properties. The present SU(4)k model
describes quantitatively the pseudogap temperature scale and may explain why
the ARPES-measured T* along the anti-nodal direction is larger than other
measurements that do not resolve momentum. It also provides an immediate
microscopic explanation for Fermi arcs observed in the pseudogap region. In
addition, the model leads to a prediction that even in the underdoped regime,
there exist doping-dependent windows around nodal points in the k-space, where
antiferromagnetism may be completely suppressed for all doping fractions,
permitting pure superconducting states to exist.Comment: 10 pages, 7 figure
Oscillation of spin polarization in a two-dimensional hole gas under a perpendicular magnetic field
Spin-charge coupling is studied for a strongly confined two-dimensional hole
gas subject to a perpendicular magnetic field. The study is based on
spin-charge coupled drift-diffusion equations derived from quantum-kinetic
equations in an exact manner. The spin-orbit interaction induces an extra
out-of-plane spin polarization. This contribution exhibits a persistent
oscillatory pattern in the strong-coupling regime.Comment: 11 pages and 1 figur
Temperature-dependent gap equations and their solutions in the SU(4) model of high-temperature superconductivity
Temperature-dependent gap equations in the SU(4) model of high-Tc
superconductivity are derived and analytical solutions are obtained. Based on
these solutions, a generic gap diagram describing the features of energy gaps
as functions of doping P is presented and a phase diagram illustrating the
phase structure as a function of temperature T and doping P is sketched. A
special doping point P_q occurs naturally in the solutions that separates two
phases at temperature T = 0: a pure superconducting phase on one side (P > P_q)
and a phase with superconductivity strongly suppressed by antiferromagnetism on
the other (P < P_q). We interpret P_q as a quantum phase transition point.
Moreover, the pairing gap is found to have two solutions for P < P_q: a small
gap that is associated with competition between superconductivity and
antiferromagnetism and is responsible for the ground state superconductivity,
and a large gap without antiferromagnetic suppression that corresponds to a
collective excited state. A pseudogap appears in the solutions that terminates
at P_q and originates from the competition between d-wave superconductivity and
antiferromagnetism. Nevertheless, this conclusion does not contradict the
preformed pair picture conceptually if the preformed pairs are generally
defined as any pairs formed before pairing condensation.Comment: 23 pages, 5 color figure
Nature of magnetism in CaCoO
We find using LSDA+U band structure calculations that the novel
one-dimensional cobaltate CaCoO is not a ferromagnetic half-metal
but a Mott insulator. Both the octahedral and the trigonal Co ions are formally
trivalent, with the octahedral being in the low-spin and the trigonal in the
high-spin state. The inclusion of the spin-orbit coupling leads to the
occupation of the minority-spin orbital for the unusually coordinated
trigonal Co, producing a giant orbital moment (1.57 ). It also results
in an anomalously large magnetocrystalline anisotropy (of order 70 meV),
elucidating why the magnetism is highly Ising-like. The role of the oxygen
holes, carrying an induced magnetic moment of 0.13 per oxygen, for
the exchange interactions is discussed.Comment: 5 pages, 4 figures, and 1 tabl
Existence of vertical spin stiffness in Landau-Lifshitz-Gilbert equation in ferromagnetic semiconductors
We calculate the magnetization torque due to the spin polarization of the
itinerant electrons by deriving the kinetic spin Bloch equations based on the
- model. We find that the first-order gradient of the magnetization
inhomogeneity gives rise to the current-induced torques, which are consistent
to the previous works. At the second-order gradient, we find an effective
magnetic field perpendicular to the spin stiffness filed. This field is
proportional to the nonadiabatic parameter . We show that this vertical
spin stiffness term can significantly modify the domain-wall structure in
ferromagnetic semiconductors and hence should be included in the
Landau-Lifshitz-Gilbert equation in studying the magnetization dynamics.Comment: 7 pages, 4 figure
Complete d-Band Dispersion and the Mobile Fermion Scale in NaxCoO2
We utilize fine-tuned polarization selection coupled with excitation-energy
variation of photoelectron signal to image the \textit{complete d}-band
dispersion relation in sodium cobaltates. A hybridization gap anticrossing is
observed along the Brillouin zone corner and the full quasiparticle band is
found to emerge as a many-body entity lacking a pure orbital polarization. At
low dopings, the quasiparticle bandwidth (Fermion scale, many-body
\textit{E} 0.25 eV) is found to be smaller than most known oxide
metals. The low-lying density of states is found to be in agreement with
bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D
Luttinger theorem is also observed to be satisfied.Comment: 4+ pages, 5 Fig
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