arXiv subjects

T. Plefka

Publications and source records attributed to T. Plefka.

9 recordsLinked to original sources

arXiv2020

The Marginal Stability of the Meta-stable Thouless-Anderson-Palmer States

The existing investigations on the complexity are extended. In addition to the Edward-Anderson Parameter q_2 the fourth moment q_4 of the magnetizations m_i is included to the set of constrained variables and the constrained complexity is numerically determined. The maximum of the constrained complexity (representing the total complexity) sticks at the boundary for temperatures at and below a new critical temperature. This implies marginal stability. The temperature dependence of lowest value of the Gibbs potential consistent with all requirements is presented.

cond-mat.dis-nn↗

arXiv2005

Expansion of the Gibbs potential for quantum many-body systems: General formalism with applications to the spin glass and the weakly non-ideal Bose gas

For general quantum systems the power expansion of the Gibbs potential and consequently the power expansion of the self energy is derived in terms of the interaction strength. Employing a generalization of the projector technique a compact representation of the general terms of the expansion results. The general aspects of the approach are discussed with special emphasis on the effects characteristic for quantum systems. The expansion is systematic and leads directly to contributions beyond mean-field of all thermodynamic quantities. These features are explicitly demonstrated and illustrated for two non-trivial systems, the infinite range quantum spin glass and the weakly interacting Bose gas. The Onsager terms of both systems are calculated, which represent the first beyond mean-field contributions. For the spin glass new TAP-like equations are presented and discussed in the paramagnetic region. The investigation of the Bose gas leads to a beyond mean-field thermodynamic description. At the Bose-Einstein condensation temperature complete agreement is found with the results presented recently by alternative techniques.

cond-mat.stat-mech↗

arXiv2003

Evidence for a Vanishing Complexity of the Sherrington-Kirkpatrick model

Based on the modified Thouless-Anderson-Palmer equations a detailed numerical investigation for the complexity of the Sherrington-Kirkpatrick spin glass is worked out. The data suggest a scaling law which leads to a vanishing of the complexity in the thermodynamic limit as proposed recently. The results for the total number of stable states agree with existing approaches.

cond-mat.dis-nn↗

arXiv2003

Dynamic Domains in Strongly Driven Ferromagnetic Films

The spatiotemporal structure formation problem is investigated in the region far above the transverse ferromagnetic resonance instability. The investigations are based on the dissipative Landau-Lifshitz equation and have been performed on a model which takes external fields, isotropic exchange fields, anisotropy fields and the demagnetizing part of the dipolar field into consideration. The numerical simulations for these models exhibit stationary domain structure in the rotating frame. Employing analytical methods and simplifying the model, certain features, such as the magnetization within the domains and the proportion of the system in each domain, are described analytically.

nlin.PS↗

arXiv2003

Dynamics of Ising models coupled microscopically to bath systems

Based on the Robertson theory the nonlinear dynamics of general Ising systems coupled microscopically to bath systems is investigated leading to two complimentary approaches. Within the master equation approach microscopically founded transition rates are presented which essentially differ from the usual phenological rates. The second approach leads to coupled equations of motion for the local magnetizations and the exchange energy. Simple examples are discussed and the general results are applied to the Sherrington-Kirkpatrick spin glass model.

cond-mat.stat-mech↗

arXiv2002

Dynamic linear response of the SK spin glass coupled microscopically to a bath

The dynamic linear response theory of a general Ising model weakly coupled to a heat bath is derived employing the quantum statistical theory of Mori, treating the Hamiltonian of the spin bath coupling as a perturbation, and applying the Markovian approximation. Both the dynamic susceptibility and the relaxation function are expressed in terms of the static susceptibility and the static internal field distribution function. For the special case of the SK spin glass this internal field distribution can be related to the solutions of the TAP equations in the entire temperature region. Application of this new relation and the use of numerical solutions of the modified TAP equations leads for finite but large systems to explicit results for the distribution function and for dynamic linear response functions. A detailed discussion is presented which includes finite size effects. Due to the derived temperature dependence of the Onsager-Casimir coefficients a frequency-dependent shift of the cusp temperature of the real part of the dynamic susceptibility is found.

cond-mat.dis-nn↗

arXiv2002

Modified Thouless-Anderson-Palmer equations for the Sherrington-Kirkpatrick spin glass: Numerical solutions

For large but finite systems the static properties of the infinite ranged Sherrington-Kirkpatrick model are numerically investigated in the entire the glass regime. The approach is based on the modified Thouless-Anderson-Palmer equations in combination with a phenomenological relaxational dynamics used as a numerical tool. For all temperatures and all bond configurations stable and meta stable states are found. Following a discussion of the finite size effects, the static properties of the state of lowest free energy are presented in the presence of a homogeneous magnetic field for all temperatures below the spin glass temperature. Moreover some characteristic features of the meta stable states are presented. These states exist in finite temperature intervals and disappear via local saddle node bifurcations. Numerical evidence is found that the excess free energy of the meta stable states remains finite in the thermodynamic limit. This implies a the `multi-valley' structure of the free energy on a sub-extensive scale.

cond-mat.dis-nn↗

arXiv2002

Modified TAP equations for the SK spin glass

The stability of the TAP mean field equations is reanalyzed with the conclusion that the exclusive reason for the breakdown at the spin glass instability is an inconsistency for the value of the local susceptibility. A new alternative approach leads to modified equations which are in complete agreement with the original ones above the instability. Essentially altered results below the instability are presented and the consequences for the dynamical mean field equations are discussed.

cond-mat.dis-nn↗

arXiv1993

Dynamic Domains Above the Ferromagnetic Resonance Instability

A simplified model is introduced and analysed to show, that for the Landau-Lifshitz equation stable, steady state solutions of domain type exist in ferromagnetic systems, strongly driven by external transverse fields. These dynamic domain states are able to describe the drastic reduction of the power absorption found experimentally above the instability of the homogeneous magnetisation. The excitations of the domain states are presented and the relevance of the model for real ferromagnets is discussed.

cond-mat↗