988 research outputs found
Uniform and staggered magnetizations induced by Dzyaloshinskii-Moriya interactions in isolated and coupled spin 1/2 dimers in a magnetic field
We investigate the interplay of Dzyaloshinskii-Moriya interactions and an
external field in spin 1/2 dimers. For isolated dimers and at low field, we
derive simple expressions for the staggered and uniform magnetizations which
show that the orientation of the uniform magnetization can deviate
significantly from that of the external field. In fact, in the limit where the
vector of the Dzyaloshinskii-Moriya interaction is parallel to the
external field, the uniform magnetization actually becomes {\it perpendicular}
to the field. For larger fields, we show that the staggered magnetization of an
isolated dimer has a maximum close to one-half the polarization, with a large
maximal value of in the limit of very small Dzyaloshinskii-Moriya
interaction. We investigate the effect of inter-dimer coupling in the context
of ladders with Density Matrix Renormalization Group (DMRG) calculations and
show that, as long as the values of the Dzyaloshinskii-Moriya and of the
exchange interaction are compatible with respect to the development of a
staggered magnetization, the simple picture that emerges for isolated dimers is
also valid for weakly coupled dimers with minor modifications. The results are
compared with torque measurements on
Cu(CHN)Cl.Comment: 8 pages, 9 figure
Frustrated three-leg spin tubes: from spin 1/2 with chirality to spin 3/2
Motivated by the recent discovery of the spin tube
[(CuCltachH)Cl]Cl, we investigate the properties of a frustrated
three-leg spin tube with antiferromagnetic intra-ring and inter-ring couplings.
We pay special attention to the evolution of the properties from weak to strong
inter-ring coupling and show on the basis of extensive density matrix
renormalization group and exact diagonalization calculations that the system
undergoes a first-order phase transition between a dimerized gapped phase at
weak coupling that can be described by the usual spin-chirality model and a
gapless critical phase at strong coupling that can be described by an effective
spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in
the gapped phase and slightly beyond. The implications for
[(CuCltachH)Cl]Cl are discussed, with the conclusion that this
system behaves essentially as a spin-3/2 chain.Comment: 8 pages, 9 figures, revised versio
Charge order induced by electron-lattice interaction in NaV2O5
We present Density Matrix Renormalization Group calculations of the
ground-state properties of quarter-filled ladders including static
electron-lattice coupling. Isolated ladders and two coupled ladders are
considered, with model parameters obtained from band-structure calculations for
-NaVO. The relevant Holstein coupling to the lattice
causes static out-of-plane lattice distortions, which appear concurrently with
a charge-ordered state and which exhibit the same zigzag pattern observed in
experiments. The inclusion of electron-lattice coupling drastically reduces the
critical nearest-neighbor Coulomb repulsion needed to obtain the
charge-ordered state. No spin gap is present in the ordered phase. The charge
ordering is driven by the Coulomb repulsion and the electron-lattice
interaction. With electron-lattice interaction, coupling two ladders has
virtually no effect on or on the characteristics of the charge-ordered
phase. At V=0.46\eV, a value consistent with previous estimates, the lattice
distortion, charge gap, charge order parameter, and the effective spin coupling
are in good agreement with experimental data for NaVO_5$.Comment: 7 pages, 9 figure
Influence of Turbulence Modeling On Aftbody Surface Heating Prediction For A Hypersonic Entry Capsule
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106463/1/AIAA2013-2774.pd
Condensation of magnons and spinons in a frustrated ladder
Motivated by the ever-increasing experimental effort devoted to the
properties of frustrated quantum magnets in a magnetic field, we present a
careful and detailed theoretical analysis of a one-dimensional version of this
problem, a frustrated ladder with a magnetization plateau at m=1/2. We show
that even for purely isotropic Heisenberg interactions, the magnetization curve
exhibits a rather complex behavior that can be fully accounted for in terms of
simple elementary excitations. The introduction of anisotropic interactions
(e.g., Dzyaloshinskii-Moriya interactions) modifies significantly the picture
and reveals an essential difference between integer and fractional plateaux. In
particular, anisotropic interactions generically open a gap in the region
between the plateaux, but we show that this gap closes upon entering fractional
plateaux. All of these conclusions, based on analytical arguments, are
supported by extensive Density Matrix Renormalization Group calculations.Comment: 15 pages, 15 figures. minor changes in tex
Multi-frequency aerodynamic control of a yawed bluff body optimized with a genetic algorithm
This experimental work aims to investigate the manipulation of a bluff body flow with a yaw angle of 10\ub0 based on a genetic algorithm optimization. Two loudspeakers are used to generate zero-net mass-flux jets through streamwise slots, which span a large portion of the rounded A-pillars of the bluff body. The actuations produce a maximum drag reduction of 17% and 2% for the leeward and windward side control, respectively. The genetic algorithm has found two typical frequencies to separately drive the actuators on the windward and leeward sides. The drag reduction is 20% under the optimal control law, 3% larger than the 17% attained from the reference single frequency control. In addition, a beneficial effect is observed when considering energy efficiency, which increases by 30% in the optimal control compared to the single frequency control. The drag spectra and velocity mapping in the wake are measured with and without control, and, based on the measurement, the underlying flow mechanism behind the control is proposed
Phase Diagram of the 1D Anderson Lattice
We map out the phase diagram of the one--dimensional Anderson lattice by
studying the ground state magnetization as a function of band--filling using
the density matrix renormalization group technique. For strong coupling, we
find that the quarter--filled system has an S=0 ground state with strong
antiferromagnetic correlations. As additional electrons are put in, we find
first a ferromagnetic phase, as reported by M\"{o}ller and W\"{o}lfle, and then
a phase in which the ground state has total spin . Within this
phase, we find RKKY oscillations in the spin--spin correlation functions.Comment: REVTEX manuscript with 5 Postcript figures included in uu file.
Submitted to Phys. Rev.
Transition between Two Oscillation Modes
A model for the symmetric coupling of two self-oscillators is presented. The
nonlinearities cause the system to vibrate in two modes of different
symmetries. The transition between these two regimes of oscillation can occur
by two different scenarios. This might model the release of vortices behind
circular cylinders with a possible transition from a symmetric to an
antisymmetric Benard-von Karman vortex street.Comment: 12 pages, 0 figure
- …