63 research outputs found
Separation of Spin and Charge Quantum Numbers in Strongly Correlated Systems
In this paper we reexamine the problem of the separation of spin and charge
degrees of freedom in two dimensional strongly correlated systems. We establish
a set of sufficient conditions for the occurence of spin and charge separation.
Specifically, we discuss this issue in the context of the Heisenberg model for
spin-1/2 on a square lattice with nearest () and next-nearest ()
neighbor antiferromagnetic couplings. Our formulation makes explicit the
existence of a local SU(2) gauge symmetry once the spin-1/2 operators are
replaced by bound states of spinons. The mean-field theory for the spinons is
solved numerically as a function of the ratio for the so-called s-RVB
Ansatz. A second order phase transition exists into a novel flux state for
. We identify the range as the s-RVB phase. It is characterized by the existence of a finite gap
to the elementary excitations (spinons) and the breakdown of all the continuous
gauge symmetries. An effective continuum theory for the spinons and the gauge
degrees of freedom is constructed just below the onset of the flux phase. We
argue that this effective theory is consistent with the deconfinement of the
spinons carrying the fundamental charge of the gauge group. We contrast this
result with the study of the one dimensional quantum antiferromagnet within the
same approach. We show that in the one dimensional model, the spinons of the
gauge picture are always confined and thus cannot be identified with the
gapless spin-1/2 excitations of the quantum antiferromagnet Heisenberg model.Comment: 56 pages, RevteX 3.
The mechanism of spin and charge separation in one dimensional quantum antiferromagnets
We reconsider the problem of separation of spin and charge in one dimensional
quantum antiferromagnets. We show that spin and charge separation in one
dimensional strongly correlated systems cannot be described by the slave boson
or fermion representation within any perturbative treatment of the interactions
between the slave holons and slave spinons. The constraint of single occupancy
must be implemented exactly. As a result the slave fermions and bosons are not
part of the physical spectrum. Instead, the excitations which carry the
separate spin and charge quantum numbers are solitons. To prove this {\it
no-go} result, it is sufficient to study the pure spinon sector in the slave
boson representation. We start with a short-range RVB spin liquid mean-field
theory for the frustrated antiferromagnetic spin- chain. We derive
an effective theory for the fluctuations of the Affleck-Marston and Anderson
order parameters. We show how to recover the phase diagram as a function of the
frustration by treating the fluctuations non-perturbatively.Comment: 53 pages; Revtex 3.
Dispersion of the second-order nonlinear susceptibility in ZnTe, ZnSe, and ZnS
We have measured the absolute values of the second-harmonic generation (SHG) coefficient |d| for the zinc-blende II-VI semiconductors ZnTe, ZnSe, and ZnS at room temperature. The investigated spectral region of the fundamental radiation λF ranges from 520 to 1321 nm using various pulsed laser sources. In the transparent region of the II-VI semiconductors, the SHG coefficient exceeds the values of birefringent materials as ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP) by one or two orders of magnitudes. Above the E0 band gap a strong dispersion of |d| is observed, showing a maximum for a second-harmonic frequency close to the E1 gap. The experimental results are compared to calculated values using a simple three-band model including spin-orbit splitting. Substantial agreement is found to the experimentally observed dispersion of the second-order nonlinear susceptibility
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