arXiv subjects

R. Oppermann

Publications and source records attributed to R. Oppermann.

32 records · Page 2Linked to original sources

arXiv1999

Selforganized 3-band structure of the doped fermionic Ising spin glass

The fermionic Ising spin glass is analyzed for arbitrary filling and for all temperatures. A selforganized 3-band structure of the model is obtained in the magnetically ordered phase. Deviation from half filling generates a central nonmagnetic band, which becomes sharply separated at T=0 by (pseudo)gaps from upper and lower magnetic bands. Replica symmetry breaking effects are derived for several observables and correlations. They determine the shape of the 3-band DoS, and, for given chemical potential, influence the fermion filling strongly in the low temperature regime.

cond-mat.dis-nn↗

arXiv1999

Pseudogaps and Charge Band in the Parisi Solution of Insulating and Superconducting Electronic Spin Glasses at Arbitrary Fillings

We report progress in understanding the fermionic Ising spin glass with arbitrary filling. A crossover from a magnetically disordered single band phase via two intermediate bands just below the freezing temperature to a 3-band structure at still lower temperatures - beyond an almost random field instability - is shown to emerge in the magnetic phase. An attempt is made to explain the exact solution in terms of a quantum Parisi phase. A central nonmagnetic band is found and seen to become sharply separated at T=0 by gaps from upper and lower magnetic bands. The gap sizes tend towards zero as the number of replica symmetry breaking steps increases towards infinity. In an extended model, the competition between local pairing superconductivity and spin glass order is discussed.

cond-mat.str-el↗

arXiv1999

Replica symmetry breaking solution for the fermionic Ising spin glass and the Ghatak-Sherrington model

We solve the fermionic version of the Ising spin glass for arbitrary filling μand temperature T taking into account replica symmetry breaking. Using a simple exact mapping from μto the anisotropy parameter D, we also obtain the solution of the S=1 Sherrington-Kirkpatrick model. An analytic expression for T=0 gives an improved critical value for the first-order phase transition. We revisit the question of stability against replica-diagonal fluctuations and find that the appearance of complex eigenvalues of the Almeida-Thouless matrix is not an artifact of the replica-symmetric approximation.

cond-mat.stat-mech↗

arXiv1999

Random Magnetic Interactions and Spin Glass Order Competing with Superconductivity: Interference of the Quantum Parisi Phase

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 infinity-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 T_c superconductors. We find a crossover line traversing the SG-phase with (μ=0,T=0) 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-Mezard vector replica symmetry breaking to occur at low temperatures beyond.

cond-mat↗

arXiv1998

Metal-Insulator Transition in Randomly Interacting Systems

We discuss a metal-insulator transition caused by random couplings of magnetic moments in itinerant systems. An analytic solution for the single particle Green function is derived from dynamical self consistency equations, the corresponding density of states is characterized by the opening of a gap. The scaling behavior of observables is analyzed in the framework of a scaling theory and different crossover lines are identified. A fluctuation expansion around the mean field solution accounts for both interaction and localization effects in a consistent manner and is argued to be relevant for the description of the recently discovered metal-insulator transition in 2d electronic systems.

cond-mat.str-el↗

arXiv1998

Low-energy excitations in fermionic spin glasses: A quantum-dynamical image of Parisi symmetry breaking

We report large effects of Parisi replica permutation symmetry breaking (RPSB) on elementary excitations of fermionic systems with frustrated magnetic interactions. The electronic density of states is obtained exactly in the zero temperature limit for (K=1)-step RPSB together with exact relations for arbitrary breaking K, which lead to a new fermionic and dynamical Parisi solution at K=\infty. The Ward identity for charge conservation indicates RPSB-effects on the conductivity in metallic quantum spin glasses. This implies that RPSB is essential for any fermionic system showing spin glass sections within its phase diagram. An astonishing similarity with a neural network problem is also observed.

cond-mat.dis-nn↗

arXiv1998

Competition between spin glass order and strong coupling superconductivity in a single-species fermion model

The phase diagram of a single species fermion model allowing for local pairing superconductivity (SC) and spin glass order (SG) is derived as a function of chemical potential μand ratio r=v/J between attractive coupling v and frustrated magnetic interaction J. For ratios larger than a characteristic r_c(μ), superconductivity does not allow for SG order, while for smaller values a very detailed phase diagram arises with entangled spin glass and superconducting transitions. Our results for the Green's functions show that superconductivity occurring in the magnetic interaction band is of gapless type with a crossover from strongly gapless, within a certain range below T_c, to very weakly gapless in a wide low temperature regime, and hardgapped at T=0.

cond-mat↗

arXiv1998

Studies of the phase diagram of randomly interacting fermionic systems

We present details of the phase diagrams of fermionic systems with random and frustrated interactions, emphasizing the important role of the chemical potential. The insulating fermionic Ising spin glass model is shown to reveal different entangled magnetic instabilities and phase transitions. We review tricritical phenomena related to the strong correspondence between charge and spin fluctuations, being controlled by quantum statistics. We compare the spin density diluted Sherrington-Kirkpatrick spin glass with classical spin 1 models such as the BEG model. We analyse in detail the infinite range model and show that spin glass order must decay discontinuously as the chemical potential exceeds a critical value, provided the temperature is below the tricritical one, and that the T=0 transition is of classical type. Parisi replica permutation symmetry breaking (RPSB) governs the thermal spin glass transitions and fermionic modifications of the SK-models AT-line emerge. RPSB takes place everywhere within the fermionic spin glass phase. Although the critical field theory of the quantum paramagnet to spin glass transition in metallic systems remains replica--symmetric at T=0, with only small corrections at low T from RPSB, the phase diagram is affected at O(T^0) by RPSB. Generalizing our results for the fermionic Ising spin glass we consider aspects of models with additional spin and charge quantum--dynamics such as metallic spin glasses.

cond-mat.dis-nn↗

arXiv1998

Parisi-Symmetry of the Many-Body Quantum Theory of randomly interacting fermionic systems

We show that fermion systems with random interactions lead to strong coupling of glassy order and fermionic correlations, which culminates in the implementation of Parisi replica permutation symmetry breaking (RPSB) in their T=0 quantum field theories. Precursor effects below fermionic AT-lines become stronger as the temperature decreases and play a crucial role within the entire low T regime. The Parisi ultrametric structure is shown to determine low energy excitations and the dynamic behaviour of fermionic correlations for large times, which is predicted to affect transport properties in metallic (and superconducting) spin glasses. Thus we reveal quantum dynamical fingerprints of the Parisi scheme. These effects, being strongest as T->0, are contrasted with quantum spin glass transitions at T=0 displaying only small RPSB corrections at low T. RPSB-effects moreover appear to influence the loci of the ground state transitions at O(T^0) and hence the phase diagrams. We derive a new representation of the T=0 Green's function which leads to a map of the fermionic (insulating) spin glass solution to the local limit solution of a Hubbard model with a random repulsive interaction. We obtain the distribution of the Hubbard interaction fluctuation and its dependence on the order of RPSB. A generalized mapping between metallic spin glass and random U Hubbard model is conjectured. The new representation of the Green's function at T=0 is suggested to be useful for generalizations to superconductors with spin glass phases.

cond-mat.dis-nn↗

arXiv1998

Magnetic Gaps related to Spin Glass Order in Fermionic Systems

We provide evidence for spin glass related magnetic gaps in the fermionic density of states below the freezing temperature. Model calculations are presented and proposed to be relevant for explaining resistivity measurements which observe a crossover from variable-range- to activated behavior. The magnetic field dependence of a hardgap and the low temperature decay of the density of states are given. In models with fermion transport a new metal-insulator transition is predicted to occur due to the spin-glass gap, anteceding the spin glass to quantum paramagnet transition at smaller spin density. Important fluctuation effects due to finite range frustrated interactions are estimated and discussed.

cond-mat.dis-nn↗

arXiv1997

Effect of spin-glass order on magnetic polarons in semimagnetic semiconductors

A theory accounting for the specific features of magnetic polarons (MP) in the presence of spin glass order is presented. We derive and solve selfconsistent equations for i) the polaron magnetisation, ii) the thermodynamically averaged carrier--spin, and iii) for the spin glass order parameter. The temperature dependence of these quantities is analysed in detail. The modification of the spin glass phase due to the presence of the exchange field of the carrier inside the magnetic polaron volume is investigated. The onset of spin glass order leads to a plateau--like flattening in the temperature dependence of the MP energy at low temperatures. It is found that solutions of spin glass equations are needed to optimally fit the experimental data of the temperature dependence of the exciton magnetic polaron (EMP) energy in (Cd,Mn)Te. Moreover, the dynamical aspects of the MP formation are discussed. Our model predicts qualitatively different temperature dependences of the MP formation time in different dynamical scenarios.

cond-mat.dis-nn↗

arXiv1996

Fermionic Quantum Spin Glass Transitions

This article reviews recent progress of the analytical theory of quantum spin glasses (QSG). Exact results for infinite range and one loop renormalisation group calculations for finite range models of either insulating or metallic type are presented. We describe characteristics of fermionic spin glass transitions and of fermionic correlations which are affected by these transitions and by spin glass order. Connections between tricritical thermal-- and $T=0$ QSG transitions are described. A general phase diagram with tricritical QSG transitions caused either by random chemical potential or by elastic electron scattering, and implying discontinuous $T=0$--transitions in weak and in strong filling regimes, is also derived.

cond-mat.dis-nn↗

arXiv1996

Tricritical behaviour of Ising spin glasses with charge fluctuations

We show that tricritical points displaying unusal behaviour exist in phase diagrams of fermionic Ising spin glasses as the chemical potential or the filling assumes characteristic values. Exact results for infinite range interaction and a one loop renormalization group analysis of thermal tricritical fluctuations for finite range models are presented. Surprising similarities with zero temperature transitions and a new $T=0$ tricritical point of metallic quantum spin glasses are derived.

cond-mat↗

arXiv1995

Quantum field theory of metallic spin glasses

We introduce an effective field theory for the vicinity of a zero temperature quantum transition between a metallic spin glass (``spin density glass'') and a metallic quantum paramagnet. Following a mean field analysis, we perform a perturbative renormalization-group study and find that the critical properties are dominated by static disorder-induced fluctuations, and that dynamic quantum-mechanical effects are dangerously irrelevant. A Gaussian fixed point is stable for a finite range of couplings for spatial dimensionality $d > 8$, but disorder effects always lead to runaway flows to strong coupling for $d \leq 8$. Scaling hypotheses for a {\em static\/} strong-coupling critical field theory are proposed. The non-linear susceptibility has an anomalously weak singularity at such a critical point. Although motivated by a perturbative study of metallic spin glasses, the scaling hypotheses are more general, and could apply to other quantum spin glass to paramagnet transitions.

cond-mat↗