共查询到20条相似文献,搜索用时 15 毫秒
1.
S.G. Magalhães A. Theumann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):5-13
We have examined the role of the BCS pairing mechanism in the formation of the magnetic moment and henceforth a spin glass
(SG) phase by studying a fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the fermions. This model
is obtained by using perturbation theory to trace out the conduction electrons degrees of freedom in conventional superconducting
alloys. The model is formulated in the path integral formalism where the spin operators are represented by bilinear combinations
of Grassmann fields and it reduces to a single site problem that can be solved within the static approximation with a replica
symmetric ansatz. We argue that this is a valid procedure for values of temperature above the de Almeida-Thouless instability
line. The phase diagram in the T-g plane, where g is the strength of the pairing interaction, for fixed variance J
2
/N of the random couplings Jij, exhibits three regions: a normal paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase where there
is formation of local pairs.The NP and PAIR phases are separated by a second order transition line g=g
c
(T) that ends at a tricritical point T
3
=0.9807J, g
3
=5,8843J, from where it becomes a first order transition line that meets the line of second order transitions at T
c
=0.9570J that separates the NP and the SG phases. For T<T
c
the SG phase is separated from the PAIR phase by a line of first order transitions. These results agree qualitatively with
experimental data in .
Received 14 May 1998 相似文献
2.
We study a fermionic infinited-ranged Ising spin glass with a real space BCS interaction
in the presence of an applied transverse field. The problem is formulated in the integral
functional formalism where the SU(2) spins are given in terms of bilinear combinations of Grassmann fields.
The problem is solved within static approximation and the replica symmetry ansatz combined with previous approaches
used to study the critical behavior of the quantum Ising spin glass in a transverse field and the spin glass
Heisenberg model with BCS pairing.
Our results show that the transverse field has strong effect in the phase boundary of the spin glass phase and the
PAIR phase in which there is a long range order corresponding to formations of pairs. The location of the tricritical
point in the PAIR phase transition line is also affected. 相似文献
3.
M. S. Long L. B. Hu W. LiMing 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,75(2):205-208
The spin density wave and its temperature dependence in oxypnictide are studied in a
three-band model. The spin susceptibilities with various interactions are calculated in
the random phase approximation (PPA). It is found that the spin susceptibility peaks
around the M point show a spin density wave (SDW) with momentum (0, π) and a clear
stripe-like spin configuration. The intra-band Coulomb repulsion enhances remarkably the
SDW but the Hund’s coupling weakens it. It is shown that a new resonance appears at
higher temperatures at the Γ point indicating the formation of a paramagnetic
phase. There is a clear transition from the SDW phase to the paramagnetic phase. 相似文献
4.
R. Mélin S. Peysson 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(1):169-176
A Bethe-Peierls treatment to dilution in frustrated magnets and spin liquids is given. A spin glass phase is present at low
temperatures and close to the percolation point as soon as frustration takes a finite value in the dilute magnet model; the
spin glass phase is reentrant inside the ferromagnetic phase. An extension of the model is given, in which the spin glass/ferromagnet
phase boundary is shown not to reenter inside the ferromagnetic phase asymptotically close to the tricritical point whereas
it has a turning point at lower temperatures. We conjecture similar phase diagrams to exist in finite dimensional models not
constraint by a Nishimori's line. We increase frustration to study the effect of dilution in a spin liquid state. This provides
a “minimal” ordering by disorder from an Ising paramagnet to an Ising spin glass.
Received 9 April 1999 and Received in final form 27 September 1999 相似文献
5.
H. Feldmann R. Oppermann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):429-445
We analyse the competition between spin glass (SG) order and local pairing superconductivity (SC) in the fermionic Ising spin
glass with frustrated fermionic spin interaction and nonrandom attractive interaction. The phase diagram is presented for
all temperatures T and chemical potentials μ. SC-SG transitions are derived for the relevant ratios between attractive and frustrated-magnetic
interaction. Characteristic features of pairbreaking caused by random magnetic interaction and/or by spin glass proximity
are found. The existence of low-energy excitations, arising from replica permutation symmetry breaking (RPSB) in the Quantum
Parisi Phase, is shown to be relevant for the SC-SG phase boundary. Complete 1-step RPSB-calculations for the SG-phase are
presented together with a few results for -step breaking. Suppression of reentrant SG-SC-SG transitions due to RPSB is found and discussed in context of ferromagnet-SG
boundaries. The relative positioning of the SC and SG phases presents a theoretical landmark for comparison with experiments
in heavy fermion systems and high superconductors. We find a crossover line traversing the SG-phase with as its quantum critical (end)point in complete RPSB, and scaling is proposed for its vicinity. We argue that this line indicates
a random field instability and suggest Dotsenko-Mézard vector replica symmetry breaking to occur at low temperatures beyond.
Received 26 November 1998 and Received in final form 25
January 1999 相似文献
6.
A. Joknys 《Journal of magnetism and magnetic materials》2009,321(3):137-143
We performed Monte Carlo simulation of phase transitions from isotropic stripe phase with short-range order to long-range stripe phase in a model with competing ferromagnetic exchange and antiferromagnetic dipolar interactions on triangular lattice. We calculated phase diagram for different values of exchange and dipolar interaction constants ratio, η. We also determined the order of the transitions to stripe phases AFh of different stripe widths h: first-order phase transition was found to transitions into AF1 and AF2 phases, while transitions to AF3 and AF4 phases were of the second order. In the phase diagram the tricritical point was determined at the AF2 and AF3 phase boundary. We observed the peak of nematic phase at the transition region to the AF1 phase, but found it metastable at low values of η. We have also found that in AF1 phase spin relaxation corresponds to the Ising model dynamics. In phases AF3 and AF4 the dynamics slows down, and stripe domain growth with time is proportional to logt. 相似文献
7.
Yu. F. Markov E. M. Roginskii A. S. Yurkov 《Journal of Experimental and Theoretical Physics》2012,114(2):288-295
Information is obtained about the temperature behavior of the order parameter of a phase transition by theoretical and experimental
investigation of odd (acoustic and IR-active) phonons that appear in the Raman scattering spectra from the X points of the Brillouin zone (BZ) boundary in the paraphase of Hg2Cl2 crystals and are induced by the phase transition, unit-cell doubling, and the X → Γ folding in the BZ. The relevant critical exponents are determined, whose values are in agreement with the results of
X-ray diffraction measurements and, within the Landau phenomenological theory of phase transitions, indicate that the phase
transition in these crystals is close to the tricritical point. 相似文献
8.
Marcia C. Barbosa 《Il Nuovo Cimento D》1994,16(7):795-802
Summary We consider a spin system with competing interactions isotropic with respect to the axes of a cubic lattice in the presence
of an external field. We show that for small values of the external fieldH, the paramagnetic to modulated phase transition is fluctuation-induced first order, while for larger fields, such transition
changes to continuous at a tricritical point. Applications for fluids systems are proposed.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献
9.
In 1964 Davis and Adams established that the large increase of the thermal expansion and compressibility in the critical region
of the γ-to α-Ce phase transition occurs predominantly in the α phase. This provides strong evidence that a tricritical point
is realized in Ce. This also means that the aforementioned transition is not isomorphic and that α-Ce should have a distorted
fcc structure. A careful examination of Jayaraman’s data (1965) shows that a second-order transition line continues beyond
the tricritical point to the vicinity of a triple point on the melting curve. The phase boundary with the tricritical point
and the minimum of the melting curve are reconstructed within the framework of Landau theory.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 111–117 (25 January 1998)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
10.
Jeckelmann E 《Physical review letters》2002,89(23):236401
The density-matrix renormalization group is used to study the phase diagram of the one-dimensional half-filled Hubbard model with on-site (U) and nearest-neighbor (V) repulsion and hopping t. A critical line V(c)(U) approximately U/2 separates a Mott insulating phase from a charge-density-wave phase. The formation of bound charge excitations for V>2t changes the phase transition from continuous to first-order at a tricritical point U(t) approximately 3.7t, V(t)=2t. A frustrating effective antiferromagnetic spin coupling induces a bond-order-wave phase on the critical line V(c)(U) for U(t)相似文献
11.
Low-temperature specific-heat measurements on YbRh2Si2 at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at TN=72 mK. The corresponding critical exponent α turns out to be α=0.38, which differs significantly from that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where α?0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives α?1/2. We demonstrate that this value of α is in good agreement with the specific-heat measurements. 相似文献
12.
We present a mean-field solution for a quantum, short-range interacting, disordered, SO(3) Heisenberg spin model, in which the Gaussian distribution of couplings is centered in an antiferromagnetic (AF) coupling J[over ]>0, and which, for weak disorder, can be treated as a perturbation of the pure AF Heisenberg system. The phase diagram contains, apart from a Néel phase at T=0, spin-glass and paramagnetic phases whose thermodynamic stability is demonstrated by an analysis of the Hessian matrix of the free-energy. The magnetic susceptibilities exhibit the typical cusp of a spin-glass transition. 相似文献
13.
Ji-Xuan Hou Xu-Chen Yu Jing-Min Hou 《International Journal of Theoretical Physics》2016,55(9):3923-3926
We study a long-range interacting spin chain placed in a staggered magnetic field using microcanonical approach and obtain the global phase diagram. We find that this model exhibits both first order phase transition and second order phase transition separated by a tricritical point, and temperature jump can be observed in the first order phase transition. 相似文献
14.
Marius Costeniuc Richard S. Ellis Peter Tak-Hun Otto 《Journal of statistical physics》2007,127(3):495-552
We derive probabilistic limit theorems that reveal the intricate structure of the phase transitions in a mean-field version
of the Blume–Emery–Griffiths model [Phys. Rev. A
4 (1971) 1071–1077]. These probabilistic limit theorems consist of scaling limits for the total spin and moderate deviation
principles (MDPs) for the total spin. The model under study is defined by a probability distribution that depends on the parameters
n, β, and K, which represent, respectively, the number of spins, the inverse temperature, and the interaction strength. The intricate
structure of the phase transitions is revealed by the existence of 18 scaling limits and 18 MDPs for the total spin. These
limit results are obtained as (β,K) converges along appropriate sequences (βn, kn) to points belonging to various subsets of the phase diagram, which include a curve of second-order points and a tricritical
point. The forms of the limiting densities in the scaling limits and of the rate functions in the MDPs reflect the influence
of one or more sets that lie in neighborhoods of the critical points and the tricritical point. Of all the scaling limits,
the structure of those near the tricritical point is by far the most complex, exhibiting new types of critical behavior when
observed in a limit-theorem phase diagram in the space of the two parameters that parametrize the scaling limits.
American Mathematical Society 2000 Subject Classifications. Primary 60F10, 60F05, Secondary 82B20 相似文献
15.
The competition between spin glass (SG), antiferromagnetism (AF) and Kondo effect is studied here in a model which consists of two Kondo sublattices with a Gaussian random interaction between spins in different sublattices with an antiferromagnetic mean J
0 and standard deviation J. In the present approach there is no hopping of the conduction electrons between the sublattices and only spins in different sublattices can interact. The problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields which can be solved at mean field level within the static approximation and the replica symmetry ansatz. The obtained phase diagram shows the sequence of phases SG, AF and Kondo state for increasing Kondo coupling. This sequence agrees qualitatively with experimental data of the Ce2Au1-x
Co
x
Si3 compound.Received: 9 April 2003, Published online: 9 September 2003PACS:
05.50.+q Lattice theory and statistics; Ising problems - 64.60.Cn Order disorder transformations; statistical mechanics of model systems 相似文献
16.
We compute the isotherms and phase diagram in the constant coupling approximation for an Ising metamagnet with various values for the ratio of the ferromagnetic to the antiferromagnetic coupling parameters. The constant coupling method is set up entirely with the internal fields as the variation parameter. The search for the tricritical point, both directly and indirectly via the hessian of the internal fields led to the conclusion that this model has no tricritical point, but a critical end point. Comparing our computation with the experimental result for FeBr2, we find that the value for the critical end point lies closer to the experiment than either the molecular field theory or the random phase approximation. 相似文献
17.
Induced modification of geometric phase of a qubit coupled to an XY spin chain by the Dzyaloshinsky–Moriya interaction
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We consider a qubit symmetrically and transversely coupled to an XY spin chain with Dzyaloshinsky-Moriya (DM) interaction in the presence of a transverse magnetic field. An analytical expression for the geometric phase of the qubit is obtained in the weak coupling limit. We find that the modification of the geometrical phase induced by the spin chain environment is greatly enhanced by the DM interaction in the weak coupling limit around the quantum phase transition point of the spin chain. 相似文献
18.
A transverse Ising spin system, in the presence of time-dependentlongitudinal field, is studied by the effective-field theory (EFT). Theeffective-field equations of motion of the average magnetization are givenfor the simple cubic lattice (Z = 6) and the honeycomb lattice (Z = 3).The Liapunov exponent λ is calculated for discussing the stability of the magnetization and it is used to determine the phase boundary. Thedynamic phase transition diagrams in h0/ ZJ -Γ/ZJ plane and in h0/ZJ-T/ZJ plane have been drawn, and there is no dynamical tricritical point on the dynamic phase transition boundary. The effect of the thermal fluctuations upon the dynamic phase boundary has been discussed. 相似文献
19.
The spin-one Ising ferromagnet on a simple cubic lattice is treated in the mean-spherical approximation (MSA) for an exchange
potentialJ(r) parametrized by a Kac-Baker inverse-range parameter γ. The mean-field result is recovered when γ→ 0; in this limit the result is exact. For γ≠ 0, a detailed analysis is given of the phase separation associated with the tricritical point that occurs. The analysis is
made through the relation that gives the internal energy viaJ(r). It shows that the MSA result satisfactorily captures the important thermodynamic features of the tricritical point as long
as γ is not too large. The case of CoulombicJ(r) is also considered; hereJ(r) is antiferromagnetic. An argument is given in support of the expectation that on the simple cubic and body-centered cubic
lattices the CoulombicJ(r) will give rise to a tricritical point at which a λ-line of Néel points meets a paramagnetic-antiferromagnetic coexistence
boundary. 相似文献
20.
R. Heid 《Zeitschrift für Physik B Condensed Matter》1995,99(1):15-18
The thermodynamic stability of odd-frequency pairing states is investigated within an Eliashberg-type framework. We find the
rigorous result that in the weak coupling limit a continuous transition from the normal state to a spatially homogeneous odd-in-ω
superconducting state is forbidden, irrespective of details of the pairing interaction and of the spin symmetry of the gap
function. For isotropic systems, it is shown that the inclusion of strong coupling corrections does not invalidate this result.
We discuss a few scenarios that might escape these thermodynamic constraints and permit stable odd-frequency pairing states. 相似文献