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1.
The LiHoxY1-xF4 magnetic material in a transverse magnetic field Bx x perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in a random disordered system. We show that the Bx-induced magnetization along the x direction, combined with the local random dilution-induced destruction of crystalline symmetries, generates, via the predominant dipolar interactions between Ho3+ ions, random fields along the Ising z direction. This identifies LiHoxY1-xF4 in Bx as a new random field Ising system. The random fields explain the rapid decrease of the critical temperature in the diluted ferromagnetic regime and the smearing of the nonlinear susceptibility at the spin-glass transition with increasing Bx and render the Bx-induced quantum criticality in LiHoxY1-xF4 likely inaccessible.  相似文献   

2.
ac susceptibility measurements are presented on the dilute, dipolar coupled, Ising magnet LiHoxY1-xF4 for a concentration x=0.045. The frequency and temperature dependences of the susceptibility show characteristic glassy relaxation. The absorption spectrum is found to broaden with decreasing temperature suggesting that the material is behaving as a spin glass and not as an exotic spin liquid as was previously observed. A dynamical scaling analysis suggests a spin glass transition temperature of 43+/-2 mK with an exponent znu=7.8+/-0.2.  相似文献   

3.
When performed in the proper low-field, low-frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in a transverse field LiHo(x)Y(1-x)F(4) prove the existence of a spin-glass transition for x=0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.  相似文献   

4.
We consider the quantum magnet at LiHo(x)Y(1-x)F(4) at x = 0.167. Experimentally the spin glass to paramagnet transition in this system was studied as a function of the transverse magnetic field and temperature, showing peculiar features: for example, (i) the spin glass order is destroyed much faster by thermal fluctuations than by the transverse field; and (ii) the cusp in the nonlinear susceptibility signaling the glass state decreases in size at lower temperature. Here we show that the hyperfine interactions of the Ho atom must dominate in this system, and that along with the transverse inter-Ho dipolar interactions they dictate the structure of the phase diagram. The experimental observations are shown to be natural consequences of this.  相似文献   

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The change in the specific heat and spin susceptibility of a metal of valence Z due to the introduction of a small number of impurity atoms with valence (Z + q) is calculated by the Thomas-Fermi method. It is shown that in the case of copper an exact Thomas-Fermi calculation accounts approximately for the seemingly linear behaviour of the change in Cv and X from q = 1 to q = 3 observed in recent experiments by Rayne. It does not, however, account for absolute values. In an appendix a check on the validity of the Thomas-Fermi theory is made for q < 1 by recalculating the specific heat and spin susceptibility by means of the techniques of many-body perturbation theory. The different methods give essentially the same results.  相似文献   

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We investigate the kinetics of domain growth in the three-dimensional Ising model with quenched random site dilution, using Monte Carlo simulation technique. A crossover from the power law growth regime to a much slower growth observed in our simulation is interpreted through the roughening of the interfaces by the quenched impurities. The results are also compared with the corresponding results in two dimensions.  相似文献   

11.
The dependence of Curie point, θ c, on magnetic concentration, p, is investigated, for the randomly dilute ferromagnetic Heisenberg spin 1/2 problem, by considering successive approximations to 1/χ=0, where χ is the susceptibility. This extends to the next approximation work previously reported by the authors. The simple cubic and body-centred cubic lattices are dealt with in detail. It is concluded that the behaviour of θ c(p) for small p is not yet fully established: it is possible that the critical concentration p c, below which there is no ferromagnetic behaviour, is greater for the Heisenberg model than for the corresponding Ising model (where it is the analogous site percolation problem critical probability).  相似文献   

12.
The effect of randomly dilute impurities in the percolation limit on the relaxation of magnetization M(t) and on the way the time-dependent correlation function M(t)M(0) decays in a one dimensional Ising ferromagnet is considered, as is the behaviour of the dynamic susceptibility. It is found that the decay of the magnetization is hardly altered except for small faster decaying boundary effects. However, the correlation function is drastically altered, decaying in general faster asymptotically than the pure system. The dynamical susceptibility also changes, showing a breaking in the simple relaxational scaling form of the pure system. Crossover behaviour with non-critical slowing down (apart from the critical slowing down asTT c=0) is also observed in the transition from the pure ferromagnetic to randomly dilute behaviour.  相似文献   

13.
Maurizio Serva 《Physica A》2011,390(13):2443-2451
The ground state energy and entropy of the dilute mean field Ising model is computed exactly by a single order parameter as a function of the dilution coefficient. An analogous exact solution is obtained in the presence of a magnetic field with random locations. Results lead to a complete understanding of the geography of the associated random graph. In particular, we give the size of the giant component (continent) and the number of isolated clusters of connected spins of all given size (islands). We also compute the average number of bonds per spin in the continent and in the islands. Then, we tackle the problem of solving the dilute Ising model at strictly positive temperature. In order to obtain the free energy as a function of the dilution coefficient and the temperature, it is necessary to introduce a second order parameter. We are able to find out the exact solution in the paramagnetic region and exactly determine the phase transition line. In the ferromagnetic region we provide a solution in terms of an expansion with respect to the second parameter which can be made as accurate as necessary. All results are reached in the replica frame by a strategy which is not based on multi-overlaps.  相似文献   

14.
Autocorrelation times for thermodynamic quantities atT C are calculated from Monte Carlo simulations of the site-diluted simple cubic Ising model, using the Swendsen-Wang and Wolff cluster algorithms. Our results show that for these algorithms the autocorrelation timesdecrease when reducing the concentration of magnetic sites from 100% down to 40%. This is of crucial importance when estimating static properties of the model, since the variances of these estimators increase with autocorrelation time. The dynamical critical exponents are calculated for both algorithms, observing pronounced finite-size effects in the energy autocorrelation data for the algorithm of Wolff. We conclude that, when applied to the dilute Ising model, cluster algorithms become even more effective than local algorithms, for whichincreasing autocorrelation times are expected.  相似文献   

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A multispin coding program for site-diluted Ising models on large simple cubic lattices is described in detail. The spontaneous magnetization is computed as a function of temperature, and the critical temperature as a function of concentration is found to agree well with the data of Marro et al.(4) and Landau(3) for smaller systems.The first successful epsilon expansion seems to be by D. E. Khmelnitskii,ZhETF 68:1960 (1975), English translationSov. Phys. JETP 41:981 (1975); for numerical estimates see K. E. Newman and E. K. Riedel,Phys. Rev. H25:264 (1982), for experiments see R. J. Birgenau, R. A. Cowley, G. Shirane and H. Yoshizawa,J. Stat. Phys. 34:817 (1984).  相似文献   

17.
The dilute Ising ferromagnet with the three-atom interaction is studied by a new effective field theory based on Callen's identity. The initial slope of the Curie temperature with dilution is shown to be strongly affected by the presence of this interaction.  相似文献   

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《Physica B+C》1977,86(3):369-372
Specific-heat measurements on EuSO4 samples show a phase transition at (0.38 ± 0.02) K. Susceptibility measurements show the phase transition to be antiferromagnetic. The high-temperature tail of the specific heat, the Curie-Weiss constant and the total ordering energy have been calculated assuming that the magnetic interactions between the Eu2+ ions are purely dipolar. Comparisons of experimental and calculated results indicate the iteractions to be mainly dipolar. Some preliminary results on EuCO3 are included.  相似文献   

20.
The properties of Ising square lattices with nearest neighbor ferromagnetic exchange confined in a corner geometry, are studied by means of Monte Carlo simulations. Free boundary conditions at which boundary magnetic fields ±h are applied, i.e., at the two boundary rows ending at the lower left corner a field +h acts, while at the two boundary rows ending at the upper right corner a field −h acts. For temperatures T less than the critical temperature Tc of the bulk, this boundary condition leads to the formation of two domains with opposite orientation of the magnetization direction, separated by an interface which for T larger than the filling transition temperature Tf(h) runs from the upper left corner to the lower right corner, while for T<Tf(h) this interface is localized either close to the lower left corner or close to the upper right corner. It is shown that for T=Tf(h) the magnetization profile m(z) in the z-direction normal to the interface simply is linear and the interfacial width scales as wL, while for T>Tf(h) it scales as . The distribution P(?) of the interface position ? (measured along the z-direction from the corners) decays exponentially for T<Tf(h) from either corner, is essentially flat for T=Tf(h), and is a Gaussian centered at the middle of the diagonal for T>Tf(h). Unlike the findings for critical wetting in the thin film geometry of the Ising model, the Monte Carlo results for corner wetting are in very good agreement with the theoretical predictions.  相似文献   

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