7,770 research outputs found
Exceeding the Manley-Rowe quantum efficiency limit in an optically pumped THz amplifier
Using a microscopic theory based on the Maxwell-semiconductor Bloch
equations, we investigate the possibility of an optically-assisted
electrically-driven THz quantum cascade laser. Whereas in optical conversion
schemes the power conversion efficiency is limited by the Manley-Rowe relation,
the proposed optically-assisted scheme can achieve higher efficiency by
coherently recovering the optical pump energy. Furthermore, due to quantum
coherence effects the detrimental effects of scattering are mitigated
Mott physics, sign structure, ground state wavefunction, and high-Tc superconductivity
In this article I give a pedagogical illustration of why the essential
problem of high-Tc superconductivity in the cuprates is about how an
antiferromagnetically ordered state can be turned into a short-range state by
doping. I will start with half-filling where the antiferromagnetic ground state
is accurately described by the Liang-Doucot-Anderson (LDA) wavefunction. Here
the effect of the Fermi statistics becomes completely irrelevant due to the no
double occupancy constraint. Upon doping, the statistical signs reemerge,
albeit much reduced as compared to the original Fermi statistical signs. By
precisely incorporating this altered statistical sign structure at finite
doping, the LDA ground state can be recast into a short-range antiferromagnetic
state. Superconducting phase coherence arises after the spin correlations
become short-ranged, and the superconducting phase transition is controlled by
spin excitations. I will stress that the pseudogap phenomenon naturally emerges
as a crossover between the antiferromagnetic and superconducting phases. As a
characteristic of non Fermi liquid, the mutual statistical interaction between
the spin and charge degrees of freedom will reach a maximum in a
high-temperature "strange metal phase" of the doped Mott insulator.Comment: 12 pages, 12 figure
Stability of antiphase line defects in nanometer-sized boron-nitride cones
We investigate the stability of boron nitride conical sheets of nanometer
size, using first-principles calculations. Our results indicate that cones with
an antiphase boundary (a line defect that contains either B-B or N-N bonds) can
be more stable than those without one. We also find that doping the antiphase
boundaries with carbon can enhance their stability, leading also to the
appearance of localized states in the bandgap. Among the structures we
considered, the one with the smallest formation energy is a cone with a
carbon-modified antiphase boundary that presents a spin splitting of about 0.5
eV at the Fermi level.Comment: 5 two-column pages with 2 figures Accepted for publication in
Physical Review B (vol 70, 15 Nov.
Spin-charge separation in the single hole doped Mott antiferromagnet
The motion of a single hole in a Mott antiferromagnet is investigated based
on the t-J model. An exact expression of the energy spectrum is obtained, in
which the irreparable phase string effect [Phys. Rev. Lett. 77, 5102 (1996)] is
explicitly present. By identifying the phase string effect with spin backflow,
we point out that spin-charge separation must exist in such a system: the doped
hole has to decay into a neutral spinon and a spinless holon, together with the
phase string. We show that while the spinon remains coherent, the holon motion
is deterred by the phase string, resulting in its localization in space. We
calculate the electron spectral function which explains the line shape of the
spectral function as well as the ``quasiparticle'' spectrum observed in
angle-resolved photoemission experiments. Other analytic and numerical
approaches are discussed based on the present framework.Comment: 16 pages, 9 figures; references updated; to appear in Phys. Rev.
Mean-Field Description of Phase String Effect in the Model
A mean-field treatment of the phase string effect in the model is
presented. Such a theory is able to unite the antiferromagnetic (AF) phase at
half-filling and metallic phase at finite doping within a single theoretical
framework. We find that the low-temperature occurrence of the AF long range
ordering (AFLRO) at half-filling and superconducting condensation in metallic
phase are all due to Bose condensations of spinons and holons, respectively, on
the top of a spin background described by bosonic resonating-valence-bond (RVB)
pairing. The fact that both spinon and holon here are bosonic objects, as the
result of the phase string effect, represents a crucial difference from the
conventional slave-boson and slave-fermion approaches. This theory also allows
an underdoped metallic regime where the Bose condensation of spinons can still
exist. Even though the AFLRO is gone here, such a regime corresponds to a
microscopic charge inhomogeneity with short-ranged spin ordering. We discuss
some characteristic experimental consequences for those different metallic
regimes. A perspective on broader issues based on the phase string theory is
also discussed.Comment: 18 pages, five figure
Effect of Ce on stainless steel performance during electroslag remelting (ESR)
Three electroslag remelting heats were carried out by using a 1-ton argon atmosphere ESR furnace under three kinds of slag containing different Ce2O3 content. Specimens were taken at electrode and each ingot for analyzing the inclusions by scanning electron microscope - energy dispersive spectrometer (SEM-EDS). After heat treatment, the tensile and impact of each steel product was measured to study the effect of Ce content on steel performance.The results show that the non-metallic inclusions content was largely reduced in each ingot compared with that in electrode, and the ingot containing 0,05 % Ce has the best steel cleanliness and performance, while the ingot containing 0,13 % Ce has the worst steel cleanliness and performance
Effect of Ce on stainless steel performance during electroslag remelting (ESR)
Three electroslag remelting heats were carried out by using a 1-ton argon atmosphere ESR furnace under three kinds of slag containing different Ce2O3 content. Specimens were taken at electrode and each ingot for analyzing the inclusions by scanning electron microscope - energy dispersive spectrometer (SEM-EDS). After heat treatment, the tensile and impact of each steel product was measured to study the effect of Ce content on steel performance.The results show that the non-metallic inclusions content was largely reduced in each ingot compared with that in electrode, and the ingot containing 0,05 % Ce has the best steel cleanliness and performance, while the ingot containing 0,13 % Ce has the worst steel cleanliness and performance
Magnetic Incommensurability in Doped Mott Insulator
In this paper we explore the incommensurate spatial modulation of spin-spin
correlations as the intrinsic property of the doped Mott insulator, described
by the model. We show that such an incommensurability is a direct
manifestation of the phase string effect introduced by doped holes in both one-
and two-dimensional cases. The magnetic incommensurate peaks of dynamic spin
susceptibility in momentum space are in agreement with the neutron-scattering
measurement of cuprate superconductors in both position and doping dependence.
In particular, this incommensurate structure can naturally reconcile the
neutron-scattering and NMR experiments of cuprates.Comment: 12 pages (RevTex), five postscript figure
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