7 research outputs found
Neel to spin-Peierls transition in a quasi-1D Heisenberg model coupled to bond phonons
The zero and finite temperature spin-Peierls transitions in a
quasi-one-dimensional spin-1/2 Heisenberg model coupled to adiabatic bond
phonons is investigated using the Stochastic Series Expansion (SSE) Quantum
Monte Carlo (QMC) method. The quantum phase transition from a gapless Neel
state to a spin-gapped Peierls state is studied in the parameter space spanned
by spatial anisotropy, inter-chain coupling strength and spin-lattice coupling
strength. It is found that for any finite inter-chain coupling, the transition
to a dimerized Peierls ground state only occurs when the spin-lattice coupling
exceeds a finite, non-zero critical value. This is in contrast to the pure 1D
model (zero inter-chain coupling), where adiabatic/classical phonons lead to a
dimerized ground state for any non-zero spin-phonon interaction. The phase
diagram in the parameter space shows that for a strong inter-chain coupling,
the relation between the inter-chain coupling and the critical value of the
spin-phonon interaction is linear whereas for weak inter-chain coupling, this
behavior is found to have a natural logarithm-like relation. No region was
found to have a long range magnetic order and dimerization occurring
simultaneously. Instead, the Neel state order vanishes simultaneously with the
setting in of the spin-Peierls state. For the thermal phase transition, a
continuous heat capacity with a peak at the critical temperature, ,
shows a second order phase transition. The variation of the equilibrium bond
length distortion, , with temperature showed a power law relation
which decayed to zero as the temperature was increased to , indicating a
continuous transition from the dimerized phase to a paramagnetic phase with
uniform bond length and zero antiferromagnetic susceptibility.Comment: 7 pages, 5 figures, updated and extended version of
arXiv:cond-mat/030774
Measurement of a topological edge invariant in a microwave network
We report on the measurement of topological invariants in an electromagnetic
topological insulator analog formed by a microwave network, consisting of the
winding numbers of scattering matrix eigenvalues. The experiment can be
regarded as a variant of a topological pump, with non-zero winding implying the
existence of topological edge states. In microwave networks, unlike most other
systems exhibiting topological insulator physics, the winding can be directly
observed. The effects of loss on the experimental results, and on the
topological edge states, is discussed.Comment: 10 pages, 10 figure
Matrix product state study of strongly-interacting systems and quantum phase transitions
Neel to spin-Peierls transition in the ground and excited state of a quasi-1D Heisenberg model coupled to bond phonons.
The spin-Peierls transtion in the ground state (quantum phase transition) and excited state(thermal phase transition) of a quasi-1D Heisenberg model is investigated using the stochastic series expansion quantum monte carlo method. The transition from a gapless Neel state to a spin-gapped Peierls state is studied in the parameter space spanned by spatial anisotropy, inter-chain coupling and spin-lattice coupling. It is found that for any inter-chain coupling, the transition to a dimerized Peierls state only occurs when the spin-lattice coupling exceeds a finite, non-zero critical value.Bachelor of Science in Physic
Generalized sub–Schawlow-Townes laser linewidths via material dispersion
A recent S-matrix-based theory of the quantum-limited linewidth, which is applicable to general lasers, including spatially nonuniform laser cavities operating above threshold, is analyzed in various limits. For broadband gain, a simple interpretation of the Petermann and bad-cavity factors is presented in terms of geometric relations between the zeros and poles of the S matrix. When there is substantial dispersion, on the frequency scale of the cavity lifetime, the theory yields a generalization of the bad-cavity factor, which was previously derived for spatially uniform one-dimensional lasers. This effect can lead to sub-Schawlow-Townes linewidths in lasers with very narrow gain widths. We derive a formula for the linewidth in terms of the lasing mode functions, which has accuracy comparable to the previous formula involving the residue of the lasing pole. These results for the quantum-limited linewidth are valid even in the regime of strong line pulling and spatial hole burning, where the linewidth cannot be factorized into independent Petermann and bad-cavity factors
NĂ©el to spin-Peierls transition in a quasi-one-dimensional Heisenberg model coupled to bond phonons
Generalized sub–Schawlow-Townes laser linewidths via material dispersion
A recent S matrix-based theory of the quantum-limited linewidth, which is
applicable to general lasers, including spatially non-uniform laser cavities
operating above threshold, is analyzed in various limits. For broadband gain, a
simple interpretation of the Petermann and bad-cavity factors is presented in
terms of geometric relations between the zeros and poles of the S matrix. When
there is substantial dispersion, on the frequency scale of the cavity lifetime,
the theory yields a generalization of the bad-cavity factor, which was
previously derived for spatially uniform one-dimensional lasers. This effect
can lead to sub-Schawlow-Townes linewidths in lasers with very narrow gain
widths. We derive a formula for the linewidth in terms of the lasing mode
functions, which has accuracy comparable to the previous formula involving the
residue of the lasing pole. These results for the quantum-limited linewidth are
valid even in the regime of strong line-pulling and spatial hole-burning, where
the linewidth cannot be factorized into independent Petermann and bad-cavity
factors