374 research outputs found
1/f Noise and Extreme Value Statistics
We study the finite-size scaling of the roughness of signals in systems
displaying Gaussian 1/f power spectra. It is found that one of the extreme
value distributions (Gumbel distribution) emerges as the scaling function when
the boundary conditions are periodic. We provide a realistic example of
periodic 1/f noise, and demonstrate by simulations that the Gumbel distribution
is a good approximation for the case of nonperiodic boundary conditions as
well. Experiments on voltage fluctuations in GaAs films are analyzed and
excellent agreement is found with the theory.Comment: 4 pages, 4 postscript figures, RevTe
Spin and orbital frustration in MnSc_2S_4 and FeSc_2S_4
Crystal structure, magnetic susceptibility, and specific heat were measured
in the normal cubic spinel compounds MnSc_2S_4 and FeSc_2S_4. Down to the
lowest temperatures, both compounds remain cubic and reveal strong magnetic
frustration. Specifically the Fe compound is characterized by a Curie-Weiss
temperature \Theta_{CW}= -45 K and does not show any indications of order down
to 50 mK. In addition, the Jahn-Teller ion Fe^{2+} is orbitally frustrated.
Hence, FeSc_2S_4 belongs to the rare class of spin-orbital liquids. MnSc_2S_4
is a spin liquid for temperatures T > T_N \approx 2 K.Comment: 4 pages, to be published in Physical Review Letter
Hamiltonian Dynamics and the Phase Transition of the XY Model
A Hamiltonian dynamics is defined for the XY model by adding a kinetic energy
term. Thermodynamical properties (total energy, magnetization, vorticity)
derived from microcanonical simulations of this model are found to be in
agreement with canonical Monte-Carlo results in the explored temperature
region. The behavior of the magnetization and the energy as functions of the
temperature are thoroughly investigated, taking into account finite size
effects. By representing the spin field as a superposition of random phased
waves, we derive a nonlinear dispersion relation whose solutions allow the
computation of thermodynamical quantities, which agree quantitatively with
those obtained in numerical experiments, up to temperatures close to the
transition. At low temperatures the propagation of phonons is the dominant
phenomenon, while above the phase transition the system splits into ordered
domains separated by interfaces populated by topological defects. In the high
temperature phase, spins rotate, and an analogy with an Ising-like system can
be established, leading to a theoretical prediction of the critical temperature
.Comment: 10 figures, Revte
High pressure route to generate magnetic monopole dimers in spin ice
The gas of magnetic monopoles in spin ice is governed by one key parameter: the monopole chemical potential. A significant variation of this parameter could access hitherto undiscovered magnetic phenomena arising from monopole correlations, as observed in the analogous electrical Coulomb gas, like monopole dimerization, critical phase separation, or charge ordering. However, all known spin ices have values of chemical potential imposed by their structure and chemistry that place them deeply within the weakly correlated regime, where none of these interesting phenomena occur. Here we use high-pressure synthesis to create a new monopole host, Dy2Ge2O7, with a radically altered chemical potential that stabilizes a large fraction of monopole dimers. The system is found to be ideally described by the classic Debye–Huckel–Bjerrum theory of charge correlations. We thus show how to tune the monopole chemical potential in spin ice and how to access the diverse collective properties of magnetic monopoles
Models with short and long-range interactions: phase diagram and reentrant phase
We study the phase diagram of two different Hamiltonians with competiting
local, nearest-neighbour, and mean-field couplings. The first example
corresponds to the HMF Hamiltonian with an additional short-range interaction.
The second example is a reduced Hamiltonian for dipolar layered spin
structures, with a new feature with respect to the first example, the presence
of anisotropies. The two examples are solved in both the canonical and the
microcanonical ensemble using a combination of the min-max method with the
transfer operator method. The phase diagrams present typical features of
systems with long-range interactions: ensemble inequivalence, negative specific
heat and temperature jumps. Moreover, in a given range of parameters, we report
the signature of phase reentrance. This can also be interpreted as the presence
of azeotropy with the creation of two first order phase transitions with
ensemble inequivalence, as one parameter is varied continuously
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