Analytic perturbation theory for bound states in the transfer matrix spectrum of weakly correlated lattice ferromagnetic spin systems

We consider general d-dimensional lattice ferromagnetic spin systems with nearest neighbor interactions in the high temperature region ('beta' << 1). Each model is characterized by a single site apriori spin distribution taken to be even. We also take the parameter 'alfa' = ('S POT.4') - 3 '(S POT.2') POT.2' > 0, i.e. in the region which we call Gaussian subjugation, where ('S POT.K') denotes the kth moment of the apriori distribution. Associated with the model is a lattice quantum field theory known to contain a particle of asymptotic mass -ln 'beta' and a bound state below the two-particle threshold. We develop a 'beta' analytic perturbation theory for the binding energy of this bound state. As a key ingredient in obtaining our result we show that the Fourier transform of the two-point function is a meromorphic function, with a simple pole, in a suitable complex spectral parameter and the coefficients of its Laurent expansion are analytic in 'beta'.

The Ground State Energy per Site of the Quantum and Classical Edwards-Anderson Spin Glass in the Thermodynamic Limit

We derive general rigorous lower bounds for the average ground state energy per site e ((d)) of the quantum and classical Edwards-Anderson spin-glass model in dimensions d=2 and d=3 in the thermodynamic limit. For the classical model they imply that e ((2))a parts per thousand yena'3/2 and e ((3))a parts per thousand yena'2.204a <-.

Inhomogeneous Fermi and quantum spin systems on lattices

We study the thermodynamic properties of a certain type of space-inhomogeneous Fermi and quantum spin systems on lattices. We are particularly interested in the case where the space scale of the inhomogeneities stays macroscopic, but very small as compared to the side-length of the box containing fermions or spins. The present study is however not restricted to "macroscopic inhomogeneities" and also includes the (periodic) microscopic and mesoscopic cases. We prove that - as in the homogeneous case - the pressure is, up to a minus sign, the conservative value of a two-person zero-sum game, named here thermodynamic game. Because of the absence of space symmetries in such inhomogeneous systems, it is not clear from the beginning what kind of object equilibrium states should be in the thermodynamic limit. However, we give rigorous statements on correlations functions for large boxes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4763465]

Generalized correlation functions for conductance fluctuations and the mesoscopic spin Hall effect

We study the spin Hall conductance fluctuations in ballistic mesoscopic systems. We obtain universal expressions for the spin and charge current fluctuations, cast in terms of current-current autocorrelation functions. We show that the latter are conveniently parametrized as deformed Lorentzian shape lines, functions of an external applied magnetic field and the Fermi energy. We find that the charge current fluctuations show quite unique statistical features at the symplectic-unitary crossover regime. Our findings are based on an evaluation of the generalized transmission coefficients correlation functions within the stub model and are amenable to experimental test. DOI: 10.1103/PhysRevB.86.235112

Quantum oscillations of spin polarization in a GaAs/AlGaAs double quantum well

5 We employ the circular-polarization-resolved magnetophotoluminescence technique to probe the spin character of electron and hole states in a GaAs/AlGaAs strongly coupled double-quantum-well system. The photoluminescence (PL) intensities of the lines associated with symmetric and antisymmetric electron states present clear out-of-phase oscillations between integer values of the filling factor. and are caused by magnetic-field-induced changes in the population of occupied Landau levels near to the Fermi level of the system. Moreover, the degree of circular polarization of these emissions also exhibits the oscillatory behavior with increasing magnetic field. Both quantum oscillations observed in the PL intensities and in the degree of polarizations may be understood in terms of a simple single-particle approach model. The k . p method was used to calculate the photoluminescence peak energies and the degree of circular polarizations in the double-quantum-well structure as a function of the magnetic field. These calculations prove that the character of valence band states plays an important role in the determination of the degree of circular polarization and, thus, resulting in a magnetic-field-induced change of the polarization sign.

Supersymmetric extension of the quantum spherical model

In this work, we present a supersymmetric extension of the quantum spherical model, both in components and also in the superspace formalisms. We find the solution for short- and long-range interactions through the imaginary time formalism path integral approach. The existence of critical points (classical and quantum) is analyzed and the corresponding critical dimensions are determined.

Longitudinal and transverse spin asymmetries for inclusive jet production at mid-rapidity in polarized p+p collisions at root s=200 GeV

We report STAR measurements of the longitudinal double-spin asymmetry A(LL), the transverse singlespin asymmetry A(N), and the transverse double-spin asymmetries A(Sigma) and A(TT) for inclusive jet production at mid-rapidity in polarized p + p collisions at a center-of-mass energy of root s = 200 GeV. The data represent integrated luminosities of 7.6 pb(-1) with longitudinal polarization and 1.8 pb(-1) with transverse polarization, with 50%-55% beam polarization, and were recorded in 2005 and 2006. No evidence is found for the existence of statistically significant jet A(N), A(Sigma), or A(TT) at mid-rapidity. Recent model calculations indicate the A(N) results may provide new limits on the gluon Sivers distribution in the proton. The asymmetry A(LL) significantly improves the knowledge of gluon polarization in the nucleon.

Spin accumulation encoded in electronic noise for mesoscopic billiards with finite tunneling rates

We study the effects of spin accumulation (inside reservoirs) on electronic transport with tunneling and reflections at the gates of a quantum dot. Within the stub model, the calculations focus on the current-current correlation function for the flux of electrons injected into the quantum dot. The linear response theory used allows us to obtain the noise power in the regime of thermal crossover as a function of parameters that reveal the spin polarization at the reservoirs. The calculation is performed employing diagrammatic integration within the universal groups (ensembles of Dyson) for a nonideal, nonequilibrium chaotic quantum dot. We show that changes in the spin distribution determine significant alterations in noise behavior at values of the tunneling rates close to zero, in the regime of strong reflection at the gates.

Signatures of quantum phase transitions in parallel quantum dots: Crossover from local moment to underscreened spin-1 Kondo physics

We study a strongly interacting "quantum dot 1" and a weakly interacting "dot 2" connected in parallel to metallic leads. Gate voltages can drive the system between Kondo-quenched and non-Kondo free-moment phases separated by Kosterlitz-Thouless quantum phase transitions. Away from the immediate vicinity of the quantum phase transitions, the physical properties retain signatures of first-order transitions found previously to arise when dot 2 is strictly noninteracting. As interactions in dot 2 become stronger relative to the dot-lead coupling, the free moment in the non-Kondo phase evolves smoothly from an isolated spin-one-half in dot 1 to a many-body doublet arising from the incomplete Kondo compensation by the leads of a combined dot spin-one. These limits, which feature very different spin correlations between dot and lead electrons, can be distinguished by weak-bias conductance measurements performed at finite temperatures.

Generalized Metropolis dynamics with a generalized master equation: An approach for time-independent and time-dependent Monte Carlo simulations of generalized spin systems

The extension of Boltzmann-Gibbs thermostatistics, proposed by Tsallis, introduces an additional parameter q to the inverse temperature beta. Here, we show that a previously introduced generalized Metropolis dynamics to evolve spin models is not local and does not obey the detailed energy balance. In this dynamics, locality is only retrieved for q = 1, which corresponds to the standard Metropolis algorithm. Nonlocality implies very time-consuming computer calculations, since the energy of the whole system must be reevaluated when a single spin is flipped. To circumvent this costly calculation, we propose a generalized master equation, which gives rise to a local generalized Metropolis dynamics that obeys the detailed energy balance. To compare the different critical values obtained with other generalized dynamics, we perform Monte Carlo simulations in equilibrium for the Ising model. By using short-time nonequilibrium numerical simulations, we also calculate for this model the critical temperature and the static and dynamical critical exponents as functions of q. Even for q not equal 1, we show that suitable time-evolving power laws can be found for each initial condition. Our numerical experiments corroborate the literature results when we use nonlocal dynamics, showing that short-time parameter determination works also in this case. However, the dynamics governed by the new master equation leads to different results for critical temperatures and also the critical exponents affecting universality classes. We further propose a simple algorithm to optimize modeling the time evolution with a power law, considering in a log-log plot two successive refinements.

Magnetic moments of the low-lying J(P)=1/2(-), 3/2(-) Lambda resonances within the framework of the chiral quark model

The magnetic moments of the low-lying spin-parity J(P) = 1/2(-), 3/2(-) Lambda resonances, like, for example, Lambda(1405) 1/2(-), Lambda(1520) 3/2(-), as well as their transition magnetic moments, are calculated using the chiral quark model. The results found are compared with those obtained from the nonrelativistic quark model and those of unitary chiral theories, where some of these states are generated through the dynamics of two hadron coupled channels and their unitarization.

Spin-glass behaviour on random lattices

The ground-state phase diagram of an Ising spin-glass model on a random graph with an arbitrary fraction w of ferromagnetic interactions is analysed in the presence of an external field. Using the replica method, and performing an analysis of stability of the replica-symmetric solution, it is shown that w = 1/2, corresponding to an unbiased spin glass, is a singular point in the phase diagram, separating a region with a spin-glass phase (w < 1/2) from a region with spin-glass, ferromagnetic, mixed and paramagnetic phases (w > 1/2).

Annealed Ising model on hierarchical structures: a transfer matrix approach.

Spin systems in the presence of disorder are described by two sets of degrees of freedom, associated with orientational (spin) and disorder variables, which may be characterized by two distinct relaxation times. Disordered spin models have been mostly investigated in the quenched regime, which is the usual situation in solid state physics, and in which the relaxation time of the disorder variables is much larger than the typical measurement times. In this quenched regime, disorder variables are fixed, and only the orientational variables are duly thermalized. Recent studies in the context of lattice statistical models for the phase diagrams of nematic liquid-crystalline systems have stimulated the interest of going beyond the quenched regime. The phase diagrams predicted by these calculations for a simple Maier-Saupe model turn out to be qualitative different from the quenched case if the two sets of degrees of freedom are allowed to reach thermal equilibrium during the experimental time, which is known as the fully annealed regime. In this work, we develop a transfer matrix formalism to investigate annealed disordered Ising models on two hierarchical structures, the diamond hierarchical lattice (DHL) and the Apollonian network (AN). The calculations follow the same steps used for the analysis of simple uniform systems, which amounts to deriving proper recurrence maps for the thermodynamic and magnetic variables in terms of the generations of the construction of the hierarchical structures. In this context, we may consider different kinds of disorder, and different types of ferromagnetic and anti-ferromagnetic interactions. In the present work, we analyze the effects of dilution, which are produced by the removal of some magnetic ions. The system is treated in a “grand canonical" ensemble. The introduction of two extra fields, related to the concentration of two different types of particles, leads to higher-rank transfer matrices as compared with the formalism for the usual uniform models. Preliminary calculations on a DHL indicate that there is a phase transition for a wide range of dilution concentrations. Ising spin systems on the AN are known to be ferromagnetically ordered at all temperatures; in the presence of dilution, however, there are indications of a disordered (paramagnetic) phase at low concentrations of magnetic ions.

Probing the interaction of brain fatty acid binding protein (B-FABP) with model membranes

Brain fatty acid-binding protein (B-FABP) interacts with biological membranes and delivers polyunsaturated fatty acids (FAs) via a collisional mechanism. The binding of FAs in the protein and the interaction with membranes involve a motif called "portal region", formed by two small α-helices, A1 and A2, connected by a loop. We used a combination of site-directed mutagenesis and electron spin resonance to probe the changes in the protein and in the membrane model induced by their interaction. Spin labeled B-FABP mutants and lipidic spin probes incorporated into a membrane model confirmed that BFABP interacts with micelles through the portal region and led to structural changes in the protein as well in the micelles. These changes were greater in the presence of LPG when compared to the LPC models. ESR spectra of B-FABP labeled mutants showed the presence of two groups of residues that responded to the presence of micelles in opposite ways. In the presence of lysophospholipids, group I of residues, whose side chains point outwards from the contact region between the helices, had their mobility decreased in an environment of lower polarity when compared to the same residues in solution. The second group, composed by residues with side chains situated at the interface between the α-helices, experienced an increase in mobility in the presence of the model membranes. These modifications in the ESR spectra of B-FABP mutants are compatible with a less ordered structure of the portal region inner residues (group II) that is likely to facilitate the delivery of FAs to target membranes. On the other hand, residues in group I and micelle components have their mobilities decreased probably as a result of the formation of a collisional complex. Our results bring new insights for the understanding of the gating and delivery mechanisms of FABPs.

Spin polarised Heusler compounds

In der vorliegenden Dissertation dient ein einfaches Konzept zur Systematisierung der Suche nach neuen Materialien mit hoher Spinpolarisation. Dieses Konzept basiert auf zwei semi-empirischen Modellen. Zum einen kann die Slater-Pauling Regel zur Abschätzung magnetischer Momente verwendet werden. Dieses Modell wird dabei durch Rechnungen zur elektronischen Struktur unterstützt. Das zweites Modell kann insbesondere für die Co2YZ Heusler Verbindungen beim Vergleich ihrer magnetischen Eigenschaften gefunden werden. Für diese Verbindungen ergibt sich eine scheinbare lineare Abhängigkeit der Curie-Temperatur beim Auftragen als Funktion des magnetischen Momentes. Angeregt durch diese Modelle wurde die Heusler Verbindung Co2FeSi nochmals detailliert im Hinblick auf ihre geometrische und magnetische Struktur hin untersucht. Als Methoden dienten dabei die Pulver-Röntgenbeugung, die EXAFS Spektroskopie, Röntgen Absorptions- and Mößbauer Spektroskopie sowie Hoch- und Tieftemperatur Magnetometrie, XMCD and DSC. Die Messungen zeigten, dass es sich bei Co2FeSi um das Material mit dem höchsten magnetischen Moment (6 B) und der höchsten Curie Temperatur (1100 K) sowohl in der Klasse der Heusler Verbindungen als auch in der Klasse der halbmetallischen Ferromagnete handelt. Zusätzlich werden alle experimentellen Ergebnisse durch detaillierte Rechnungen zur elektronischen Struktur unterstützt. Die gleichen Konzepte wurden verwendet, um die Eigenschaften der Heusler Verbindung Co2Cr1-xFexAl vorherzusagen. Die elektronische Struktur und die spektroskopischen Eigenschaften wurden mit der voll-relativistischen Korringa-Kohn-Rostocker Methode berechnet, unter Verwendung kohärenter Potentialnäherungen um der zufälligen Verteilung von Cr und Fe Atomen sowie zufälliger Unordnung Rechnung zu tragen. Magnetische Effekte wurden durch die Verwendung Spin-abhängiger Potentiale im Rahmen der lokalen Spin-Dichte-Näherung mit eingeschlossen. Die strukturellen und chemischen Eigenschaften der quaternären Heusler Verbindung Co2Cr1-xFexAl wurden an Pulver und Bulkproben gemessen. Die Fernordnung wurde mit der Pulver Röntgenbeugung und Neutronenbeugung untersucht, während die Nahordnung mit der EXAFS Spektroskopie aufgeklärt wurde. Die magnetische Struktur von Pulver und Bulkproben wurde mitttels 57Fe-Mößbauer Spektroskopie gemessen. Die chemische Zusammensetzung wurde durch XPS analysiert. Die Ergebnisse dieser Methoden wurden verglichen, um eine Einsicht in die Unterschiede zwischen Oberflächen und Volumeneigenschaften zu erlangen sowie in das Auftreten von Fehlordnung in solchen Verbindungen. Zusätzlich wurde XMCD an den L3,2 Kanten von Co, Fe, and Cr gemessen, um die elementspezifischen magnetischen Momente zu bestimmen. Rechnungen und Messungen zeigen dabei eine Zunahme des magnetischen Momentes bei steigendem Fe-Anteil. Resonante Photoemission mit weicher Röntgenstrahlung sowie Hochenergie Photoemission mit harter Röntgenstrahlung wurden verwendet, um die Zustandsdichte der besetzten Zustände in Co2Cr0.6Fe0.4Al zu untersuchen. Diese Arbeit stellt außerdem eine weitere, neue Verbindung aus der Klasse der Heusler Verbindungen vor. Co2CrIn ist L21 geordnet, wie Messungen mittels Pulver Röntgenbeugung zeigen. Die magnetischen Eigenschaften wurden mit magnetometrisch bestimmt. Co2CrIn ist weichmagnetisch mit einer Sättigungsmagnetisierung von 1.2B bei 5 K. Im Gegensatz zu den bereits oben erwähnten Co2YZ Heusler Verbindungen ist Co2CrIn kein halbmetallischer Ferromagnet. Im Rahmen dieser Arbeit wird weiterhin eine Regel zur Vorhersage von halbmetallischen komplett kompensierten Ferrimagneten in der Klasse der Heusler Verbindungen vorgestellt. Dieses Konzept resultiert aus der Kombination der Slater-Pauling Regel mit der Kübler-Regel. Die Kübler Regel besagt, dass Mn auf der Y Position zu einem hoch lokalisierten magnetischen Moment tendiert. Unter Verwendung dieses neuen Konzeptes werden für einige Kandidaten in der Klasse der Heusler Verbindungen die Eigenschaft des halbmetallischen komplett kompensierten Ferrimagnetismus vorhergesagt. Die Anwendung dieses Konzeptes wird anhand von Rechnungen zur elektronischen Struktur bestätigt.