Reflection of electrons from a domain wall in magnetic nanojunctions

Electronic transport through thin and laterally constrained domain walls in ferromagnetic nanojunctions is analyzed theoretically. The description is formulated in the basis of scattering states. The resistance of the domain wall is calculated in the regime of strong electron reflection from the wall. It is shown that the corresponding magnetoresistance can be large, which is in a qualitative agreement with recent experimental observations. We also calculate the spin current flowing through the wall and the spin polarization of electron gas due to reflections from the domain wall.Comment: 7 pages, 4 figure

Angular dependence of domain wall resistivity in SrRuO3_{{\bf 3}} films

${\rm SrRuO_3}$ is a 4d itinerant ferromagnet (T$_{c}$ $\sim$150 K) with stripe domain structure. Using high-quality thin films of SrRuO$_{3}$ we study the resistivity induced by its very narrow ($\sim 3$ nm) Bloch domain walls, $\rho_{DW}$ (DWR), at temperatures between 2 K and T$_{c}$ as a function of the angle, $\theta$, between the electric current and the ferromagnetic domains walls. We find that $\rho_{DW}(T,\theta)=\sin^2\theta \rho_{DW}(T,90)+B(\theta)\rho_{DW}(T,0)$ which provides the first experimental indication that the angular dependence of spin accumulation contribution to DWR is $\sin^2\theta$. We expect magnetic multilayers to exhibit a similar behavior.Comment: 5 pages, 5 figure

An analysis framework for the integration of broadband NIRS and EEG to assess neurovascular and neurometabolic coupling

With the rapid growth of optical-based neuroimaging to explore human brain functioning, our research group has been developing broadband Near Infrared Spectroscopy (bNIRS) instruments, a technological extension to functional Near Infrared Spectroscopy (fNIRS). bNIRS has the unique capacity of monitoring brain haemodynamics/oxygenation (measuring oxygenated and deoxygenated haemoglobin), and metabolism (measuring the changes in the redox state of cytochrome-c-oxidase). When combined with electroencephalography (EEG), bNIRS provides a unique neuromonitoring platform to explore neurovascular coupling mechanisms. In this paper, we present a novel pipeline for the integrated analysis of bNIRS and EEG signals, and demonstrate its use on multi-channel bNIRS data recorded with concurrent EEG on healthy adults during a visual stimulation task. We introduce the use of the Finite Impulse Response functions within the General Linear Model for bNIRS and show its feasibility to statistically localize the haemodynamic and metabolic activity in the occipital cortex. Moreover, our results suggest that the fusion of haemodynamic and metabolic measures unveils additional information on brain functioning over haemodynamic imaging alone. The cross-correlation-based analysis of interrelationships between electrical (EEG) and haemodynamic/metabolic (bNIRS) activity revealed that the bNIRS metabolic signal offers a unique marker of brain activity, being more closely coupled to the neuronal EEG response

Response function analysis of excited-state kinetic energy functional constructed by splitting k-space

Over the past decade, fundamentals of time independent density functional theory for excited state have been established. However, construction of the corresponding energy functionals for excited states remains a challenging problem. We have developed a method for constructing functionals for excited states by splitting k-space according to the occupation of orbitals. In this paper we first show the accuracy of kinetic energy functional thus obtained. We then perform a response function analysis of the kinetic energy functional proposed by us and show why method of splitting the k-space could be the method of choice for construction of energy functionals for excited states.Comment: 11 page

Comparative study of density functional theories of the exchange-correlation hole and energy in silicon

We present a detailed study of the exchange-correlation hole and exchange-correlation energy per particle in the Si crystal as calculated by the Variational Monte Carlo method and predicted by various density functional models. Nonlocal density averaging methods prove to be successful in correcting severe errors in the local density approximation (LDA) at low densities where the density changes dramatically over the correlation length of the LDA hole, but fail to provide systematic improvements at higher densities where the effects of density inhomogeneity are more subtle. Exchange and correlation considered separately show a sensitivity to the nonlocal semiconductor crystal environment, particularly within the Si bond, which is not predicted by the nonlocal approaches based on density averaging. The exchange hole is well described by a bonding orbital picture, while the correlation hole has a significant component due to the polarization of the nearby bonds, which partially screens out the anisotropy in the exchange hole.Comment: 16 pages, 5 figures, RevTeX, added conten

Thermal Density Functional Theory in Context

This chapter introduces thermal density functional theory, starting from the ground-state theory and assuming a background in quantum mechanics and statistical mechanics. We review the foundations of density functional theory (DFT) by illustrating some of its key reformulations. The basics of DFT for thermal ensembles are explained in this context, as are tools useful for analysis and development of approximations. We close by discussing some key ideas relating thermal DFT and the ground state. This review emphasizes thermal DFT's strengths as a consistent and general framework.Comment: Submitted to Spring Verlag as chapter in "Computational Challenges in Warm Dense Matter", F. Graziani et al. ed

Suppression of matching field effects by splay and pinning energy dispersion in YBa_2Cu_3O_7 with columnar defects

We report measurements of the irreversible magnetization M_i of a large number of YBa_2Cu_3O_7 single crystals with columnar defects (CD). Some of them exhibit a maximum in M_i when the density of vortices equals the density of tracks, at temperatures above 40K. We show that the observation of these matching field effects is constrained to those crystals where the orientational and pinning energy dispersion of the CD system lies below a certain threshold. The amount of such dispersion is determined by the mass and energy of the irradiation ions, and by the crystal thickness. Time relaxation measurements show that the matching effects are associated with a reduction of the creep rate, and occur deep into the collective pinning regime.Comment: 7 pages, 5 figures, submitted to Phys. Rev.

Ballistic electron transport through magnetic domain walls

Electron transport limited by the rotating exchange-potential of domain walls is calculated in the ballistic limit for the itinerant ferromagnets Fe, Co, and Ni. When realistic band structures are used, the domain wall magnetoresistance is enhanced by orders of magnitude compared to the results for previously studied two-band models. Increasing the pitch of a domain wall by confinement in a nano-structured point contact is predicted to give rise to a strongly enhanced magnetoresistance.Comment: 4 pages, 2 figures; to appear in PRB as a brief repor

Magneto-transport in periodic and quasiperiodic arrays of mesoscopic rings

We study theoretically the transmission properties of serially connected mesoscopic rings threaded by a magnetic flux. Within a tight-binding formalism we derive exact analytical results for the transmission through periodic and quasiperiodic Fibonacci arrays of rings of two different sizes. The role played by the number of scatterers in each arm of the ring is analyzed in some detail. The behavior of the transmission coefficient at a particular value of the energy of the incident electron is studied as a function of the magnetic flux (and vice versa) for both the periodic and quasiperiodic arrays of rings having different number of atoms in the arms. We find interesting resonance properties at specific values of the flux, as well as a power-law decay in the transmission coefficient as the number of rings increases, when the magnetic field is switched off. For the quasiperiodic Fibonacci sequence we discuss various features of the transmission characteristics as functions of energy and flux, including one special case where, at a special value of the energy and in the absence of any magnetic field, the transmittivity changes periodically as a function of the system size.Comment: 9 pages with 7 .eps figures included, submitted to PR

Dijet Rapidity Gaps in Photoproduction from Perturbative QCD

By defining dijet rapidity gap events according to interjet energy flow, we treat the photoproduction cross section of two high transverse momentum jets with a large intermediate rapidity region as a factorizable quantity in perturbative QCD. We show that logarithms of soft gluon energy in the interjet region can be resummed to all orders in perturbation theory. The resummed cross section depends on the eigenvalues of a set of soft anomalous dimension matrices, specific to each underlying partonic process, and on the decomposition of the scattering according to the possible patterns of hard color flow. We present a detailed discussion of both. Finally, we evaluate numerically the gap cross section and gap fraction and compare the results with ZEUS data. In the limit of low gap energy, good agreement with experiment is obtained.Comment: 37 pages, Latex, 17 figure