11 research outputs found
The Ising phase in the J1-J2 Heisenberg Model
The two dimensional Heisenberg antiferromagnet on the square lattice with
nearest (J1) and next-nearest (J2) neighbor couplings is investigated in the
strong frustration regime (J2/J1>1/2). A new effective field theory describing
the long wavelength physics of the model is derived from the quantum
hamiltonian. The structure of the resulting non linear sigma model allows to
recover the known spin wave results in the collinear regime, supports the
presence of an Ising phase transition at finite temperature and suggests the
possible occurrence of a non-magnetic ground state breaking rotational
symmetry. By means of Lanczos diagonalizations we investigate the spin system
at T=0, focusing our attention on the region where the collinear order
parameter is strongly suppressed by quantum fluctuations and a transition to a
non-magnetic state occurs. Correlation functions display a remarkable size
independence and allow to identify the transition between the magnetic and
non-magnetic region of the phase diagram. The numerical results support the
presence of a non-magnetic phase with orientational ordering.Comment: 6 pages, 4 figures, to be published in PR
Spin-charge decoupling and the photoemission line-shape in one dimensional insulators
The recent advances in angle resolved photoemission techniques allowed the
unambiguous experimental confirmation of spin charge decoupling in quasi one
dimensional (1D) Mott insulators. This opportunity stimulates a quantitative
analysis of the spectral function of prototypical one dimensional
correlated models. Here we combine Bethe Ansatz results, Lanczos
diagonalizations and field theoretical approaches to obtain for
the 1D Hubbard model as a function of the interaction strength. By introducing
a {\it single spinon approximation}, an analytic expression is obtained, which
shows the location of the singularities and allows, when supplemented by
numerical calculations, to obtain an accurate estimate of the spectral weight
distribution in the plane. Several experimental puzzles on the
observed intensities and line-shapes in quasi 1D compounds, like , find a natural explanation in this theoretical framework.Comment: 8 pages, submitted to Physical Review
Magneto-elastic effects and magnetization plateaus in two dimensional systems
We show the importance of both strong frustration and spin-lattice coupling
for the stabilization of magnetization plateaus in translationally invariant
two-dimensional systems. We consider a frustrated spin-1/2 Heisenberg model
coupled to adiabatic phonons under an external magnetic field. At zero
magnetization, simple structures with two or at most four spins per unit cell
are stabilized, forming dimers or plaquettes, respectively. A much
richer scenario is found in the case of magnetization , where larger
unit cells are formed with non-trivial spin textures and an analogy with the
corresponding classical Ising model is detectable. Specific predictions on
lattice distortions and local spin values can be directly measured by X-rays
and Nuclear Magnetic Resonance experiments.Comment: 4 pages and 4 figure
Spin-Peierls instabilities of antiferromagnetic rings in a magnetic field
Motivated by the intriguing properties of magnetic molecular wheels at field-induced level crossings, we investigate the spin-Peierls instability of antiferromagnetic rings in a field by exact diagonalizations of a microscopic spin model coupled to the lattice via a distortion-dependent Dzyaloshinskii-Moriya interaction. We show that, beyond the unconditional instability at level crossings for infinitesimal magnetoelastic coupling, the model is characterized by a stronger tendency to distort at higher level crossings and by a dramatic angular dependence with very sharp torque anomalies when the field is almost in the plane of the ring. These predictions are shown to compare remarkably well with available torque and nuclear magnetic resonance data on CsFe8. © 2009 The American Physical Society
The FOOT FragmentatiOn Of Target Experiment
International audienceIn proton-therapy clinical practice a constant RBE equal to 1.1 is adopted, regardless of the demonstrated RBE variations, which depends on physical and biological parameters. Among other mechanisms, nuclear interactions might influence the proton-RBE due to secondary heavier particles produced by target fragmentation that can significantly contribute to the total dose: an un-wanted and undetermined increase of normal tissues complications probability may occur. The FOOT experiment is designed to study these processes. Target (O,C) fragmentation induced by 150 − 250 M eV proton beam will be studied via inverse kinematic approach, where O and C therapeutic beams, with the same kinetic energy per nucleon of the proton, collide on graphite and hydrocarbons target to provide the cross section on Hydrogen (to explore also the projectile fragmentation). The detector design, the performances and expected resolution results obtained form Monte Carlo study, based on the FLUKA code will be presented
Development and characterization of a Δ E-TOF detector prototype for the FOOT experiment
This paper describes the development and characterization of a ΔE-TOF detector composed of a plastic scintillator bar coupled at both ends to silicon photomultipliers. This detector is a prototype of a larger version which will be used in the FOOT (FragmentatiOn Of Target) experiment to identify the fragments produced by ion beams accelerated onto a hydrogen-enriched target. The final ΔE-TOF detector will be composed of two layers of plastic scintillator bars with orthogonal orientation and will measure, for each crossing fragment, the energy deposited in the plastic scintillator (ΔE), the time of flight (TOF), and the coordinates of the interaction position in the scintillator. To meet the FOOT experimental requirements, the detector should have energy resolution of a few percents and time resolution of 70 ps, and it should allow to discriminate multiple fragments belonging to the same event. To evaluate the achievable performances, the detector prototype was irradiated with protons of kinetic energy in the 70–230 MeV range and interacting at several positions along the bar. The measured energy resolution σΔE∕ΔE was 6–14%, after subtracting the fluctuations of the deposited energy. A time resolution σ between 120 and 180 ps was obtained with respect to a trigger detector. A spatial resolution σ of 1.9 cm was obtained for protons interacting at the center of the bar
The Drift Chamber detector of the FOOT experiment: Performance analysis and external calibration
The study that we present is part of the preparation work for the setup of the FOOT (FragmentatiOn Of Target) experiment whose main goal is the measurement of the double differential cross sections of fragments produced in nuclear interactions of particles with energies relevant for particle therapy. The present work is focused on the characterization of the gas-filled drift chamber detector composed of 36 sensitive cells, distributed over two perpendicular views. Each view consists of six consecutive and staggered layers with three cells per layer. We investigated the detector efficiency and we performed an external calibration of the space–time relations at the level of single cells. This information was then used to evaluate the drift chamber resolution. An external tracking system realized with microstrip silicon detectors was adopted to have a track measurement independent on the drift chamber. The characterization was performed with a proton beam at the energies of 228 and 80 MeV. The overall hit detection efficiency of the drift chamber has been found to be 0.929±0.008 , independent on the proton beam energy. The spatial resolution in the central part of the cell is about 150±10 μ m and 300±10 μ m and the corresponding detector angular resolution has been measured to be 1.62±0.16 mrad and 2.1±0.4 mrad for the higher and lower beam energies, respectively. In addition, the best value on the intrinsic drift chamber resolution has been evaluated to be in the range 60−100 μ m. In the framework of the FOOT experiment, the drift chamber will be adopted in the pre-target region, and will be exploited to measure the projectile direction and position, as well as for the identification of pre-target fragmentation events