686 research outputs found
Exact ground states for a class of one-dimensional frustrated quantum spin models
We have found the exact ground state for two frustrated quantum spin-1/2
models on a linear chain. The first model describes ferromagnet-
antiferromagnet transition point. The singlet state at this point has
double-spiral ordering. The second model is equivalent to special case of the
spin-1/2 ladder. It has non-degenerate singlet ground state with exponentially
decaying spin correlations and there is an energy gap. The exact ground state
wave function of these models is presented in a special recurrent form and
recurrence technics of expectation value calculations is developed.Comment: 16 pages, 3 figures, RevTe
Valence bond solids for SU(n) spin chains: exact models, spinon confinement, and the Haldane gap
To begin with, we introduce several exact models for SU(3) spin chains: (1) a
translationally invariant parent Hamiltonian involving four-site interactions
for the trimer chain, with a three-fold degenerate ground state. We provide
numerical evidence that the elementary excitations of this model transform
under representation 3bar of SU(3) if the original spins of the model transform
under rep. 3. (2) a family of parent Hamiltonians for valence bond solids of
SU(3) chains with spin reps. 6, 10, and 8 on each lattice site. We argue that
of these three models, only the latter two exhibit spinon confinement and hence
a Haldane gap in the excitation spectrum. We generalize some of our models to
SU(n). Finally, we use the emerging rules for the construction of VBS states to
argue that models of antiferromagnetic chains of SU(n) spins in general possess
a Haldane gap if the spins transform under a representation corresponding to a
Young tableau consisting of a number of boxes \lambda which is divisible by n.
If \lambda and n have no common divisor, the spin chain will support deconfined
spinons and not exhibit a Haldane gap. If \lambda and n have a common divisor
different from n, it will depend on the specifics of the model including the
range of the interaction.Comment: 26 pages two-column style, 15 figures, revtex4; references adde
Adiabatic description of nonspherical quantum dot models
Within the effective mass approximation an adiabatic description of
spheroidal and dumbbell quantum dot models in the regime of strong dimensional
quantization is presented using the expansion of the wave function in
appropriate sets of single-parameter basis functions. The comparison is given
and the peculiarities are considered for spectral and optical characteristics
of the models with axially symmetric confining potentials depending on their
geometric size making use of the total sets of exact and adiabatic quantum
numbers in appropriate analytic approximations
Driven diffusion in a periodically compartmentalized tube: homogeneity versus intermittency of particle motion
We study the effect of a driving force F on drift and diffusion of a point Brownian particle in a tube formed by identical ylindrical compartments, which create periodic entropy barriers for the particle motion along the tube axis. The particle transport exhibits striking features: the effective mobility monotonically decreases with increasing F, and the effective diffusivity diverges as F → ∞, which indicates that the entropic effects in diffusive transport are enhanced by the driving force. Our consideration is based on two different scenarios of the particle motion at small and large F, homogeneous and intermittent, respectively. The scenarios are deduced from the careful analysis of statistics of the particle transition times between neighboring openings. From this qualitative picture, the limiting small-F and large-F behaviors of the effective mobility and diffusivity are derived analytically. Brownian dynamics simulations are used to find these quantities at intermediate values of the driving force for various compartment lengths and opening radii. This work shows that the driving force may lead to qualitatively different anomalous transport features, depending on the geometry design
Valence bond solid states with symplectic symmetry
We introduce a one-dimensional valence bond solid (VBS) state with symplectic
symmetry SP(n) and construct the corresponding parent Hamiltonian. We argue
that there is a gap in the spectrum. We calculate exactly the static
correlation functions, which fall off exponentially. Hence the model introduced
here shares all properties of the Haldane scenario for integer-spin quantum
antiferromagnets. We further show that the VBS state possesses string order and
discuss its generalization to higher dimensions.Comment: 10 pages, 5 figure
One and Two Dimensional Spin Systems in the Regime Close to Deconfinement of Spinons
Based on the Majumdar-Ghosh chain we construct several spin models which
allow us to investigate spinon dynamics in the regime close to deconfinement of
spinons. We consider the J_1 - J_2 - \delta model, two coupled J_1 - J_2 chains
(ladder), and a 2D array of coupled J_1 - J_2 chains. Using the picture of two
spinons interacting with a string confining potential we calculate the singlet-
triplet splitting, magnetic structure factor, tunneling amplitude of two
spinons and the excitation spectra for the ladder and the array.Comment: 12 pages, 16 figures (2 eps figures
Spin-Gap Phases in Tomonaga-Luttinger Liquids
We give the details of the analysis for critical properties of spin-gap
phases in one-dimensional lattice electron models. In the Tomonaga-Luttinger
(TL) liquid theory, the spin-gap instability occurs when the backward
scattering changes from repulsive to attractive.
This transition point is shown to be equivalent to that of the level-crossing
of the singlet and the triplet excitation spectra, using the c=1 conformal
field theory and the renormalization group. Based on this notion, the
transition point between the TL liquid and the spin-gap phases can be
determined with high-accuracy from the numerical data of finite-size clusters.
We also discuss the boundary conditions and discrete symmetries to extract
these excitation spectra. This technique is applied to the extended Hubbard
model, the t-J model, and the t-J-J' model, and their phase diagrams are
obtained. We also discuss the relation between our results and analytical
solutions in weak-coupling and low-density limits.Comment: 14 pages(REVTeX), 9 figures(EPS), 1 table, To appear in PRB, Detailed
paper of PRL 79 (1997) 3214 and JPSJ 67 (1998) 71
Personalized neck irradiation guided by sentinel lymph node biopsy in patients with squamous cell carcinoma of the oropharynx, larynx or hypopharynx with a clinically negative neck:(Chemo)radiotherapy to the PRIMary tumor only. Protocol of the PRIMO study
Background: Elective neck irradiation (ENI) is performed in head and neck cancer patients treated with definitive (chemo)radiotherapy. The aim is to eradicate nodal metastases that are not detectable by pretreatment imaging techniques. It is conceivable that personalized neck irradiation can be performed guided by the results of sentinel lymph node biopsy (SLNB). It is expected that ENI can be omitted to one or both sides of the neck in 9 out of 10 patients, resulting in less radiation side effects with better quality of life. Methods/design: This is a multicenter randomized controlled trial aiming to compare safety and efficacy of treatment with SLNB guided neck irradiation versus standard bilateral ENI in 242 patients with cN0 squamous cell carcinoma of the oropharynx, larynx or hypopharynx for whom bilateral ENI is indicated. Patients randomized to the experimental-arm will undergo SLNB. Based on the histopathologic status of the SLNs, patients will receive no ENI (if all SLNs are negative), unilateral neck irradiation only (if a SLN is positive at one side of the neck) or bilateral neck irradiation (if SLNs are positive at both sides of the neck). Patients randomized to the control arm will not undergo SLNB but will receive standard bilateral ENI. The primary safety endpoint is the number of patients with recurrence in regional lymph nodes within 2 years after treatment. The primary efficacy endpoint is patient reported xerostomia-related quality of life at 6 months after treatment. Discussion: If this trial demonstrates that the experimental treatment is non-inferior to the standard treatment in terms of regional recurrence and is superior in terms of xerostomia-related quality of life, this will become the new standard of care.</p
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