362 research outputs found
Moving glass phase of driven lattices
We study periodic lattices, such as vortex lattices, driven by an external
force in a random pinning potential. We show that effects of static disorder
persist even at large velocity. It results in a novel moving glass state with
topological order analogous to the static Bragg glass. The lattice flows
through well-defined, elastically coupled, {\it % static} channels. We predict
barriers to transverse motion resulting in finite transverse critical current.
Experimental tests of the theory are proposed.Comment: Revised version, shortened, 8 pages, REVTeX, no figure
Instanton Corrections to Quark Form Factor at Large Momentum Transfer
Within the Wilson integral formalism, we discuss the structure of
nonperturbative corrections to the quark form factor at large momentum transfer
analyzing the infrared renormalon and instanton effects. We show that the
nonperturbative effects determine the initial value for the perturbative
evolution of the quark form factor and attribute their general structure to the
renormalon ambiguities of the perturbative series. It is demonstrated that the
instanton contributions result in the finite renormalization of the
next-to-leading perturbative result and numerically are characterized by a
small factor reflecting the diluteness of the QCD vacuum within the instanton
liquid model.Comment: Version coincident with the journal publication, 9 pages; REVTe
History effects and pinning regimes in solid vortex matter
We propose a phenomenological model that accounts for the history effects
observed in ac susceptibility measurements in YBa2Cu3O7 single crystals [Phys.
Rev. Lett. 84, 4200 (2000) and Phys. Rev. Lett. 86, 504 (2001)]. Central to the
model is the assumption that the penetrating ac magnetic field modifies the
vortex lattice mobility, trapping different robust dynamical states in
different regions of the sample. We discuss in detail on the response of the
superconductor to an ac magnetic field when the vortex lattice mobility is not
uniform inside the sample. We begin with an analytical description for a simple
geometry (slab) and then we perform numerical calculations for a strip in a
transverse magnetic field which include relaxation effects. In calculations,
the vortex system is assumed to coexist in different pinning regimes. The
vortex behavior in the regions where the induced current density j has been
always below a given threshold (j_c^>) is described by an elastic Campbell-like
regime (or a critical state regime with local high critical current density,
j_c^>). When the VS is shaken by symmetrical (e.g. sinusoidal) ac fields, the
critical current density is modified to j_c^) at
regions where vortices have been forced to oscillate by a current density
larger than j_c^>. Experimentally, an initial state with high critical current
density (j_c^>) can be obtained by zero field cooling, field cooling (with no
applied ac field) or by shaking the vortex lattice with an asymmetrical (e.g.
sawtooth) field. We compare our calculations with experimental ac
susceptibility results in YBa2Cu3O7 single crystals.Comment: 11 pages, 7 figures. To be published in PR
Slow relaxations and history dependence of the transport properties of layered superconductors
We study numerically the time evolution of the transport properties of
layered superconductors after different preparations. We show that, in
accordance with recent experiments in BSCCO performed in the second peak region
of the phase diagram (Portier et al, 2001), the relaxation strongly depends on
the initial conditions and is extremely slow. We investigate the dependence on
the pinning center density and the perturbation applied. We compare the
measurements to recent findings in tapped granular matter and we interpret our
results with a rather simple picture.Comment: 4 pages, 4 fig
Instanton effects in quark form factor and quark-quark scattering at high energy
The nonperturbative effects in the high-energy processes involving strongly
interacting particles are studied within the instanton liquid model of the QCD
vacuum (ILM) by using the Wilson integral framework. The detailed analysis of
nonperturbative contributions to the electromagnetic quark form factor is
presented considering the structure of the instanton induced effects in the
evolution equation describing the high energy behaviour of the form factor. It
is shown that the instantons yield in high energy limit the logarithmic
corrections to the amplitudes which are exponentiated in small instanton
density parameter. By using the Gaussian interpolation of the constrained
instanton solution, we show that the all-order multi-instanton contribution is
well approximated by the weak field limit result. The role of the instantons in
high energy diffractive quark-quark scattering, in particular, in formation of
the soft Pomeron, is also considered. We show that within the ILM the C-odd
diffractive amplitude is suppressed as 1/s compared to the C-even one. The
further applications of the developed approach in studying the nonperturbative
effects in high energy hadronic processes are briefly discussed.Comment: 37 pages + 9 figures; JHEP styl
Spin- and charge-density oscillations in spin chains and quantum wires
We analyze the spin- and charge-density oscillations near impurities in spin
chains and quantum wires. These so-called Friedel oscillations give detailed
information about the impurity and also about the interactions in the system.
The temperature dependence of these oscillations explicitly shows the
renormalization of backscattering and conductivity, which we analyze for a
number of different impurity models. We are also able to analyze screening
effects in one dimension. The relation to the Kondo effect and experimental
consequences are discussed.Comment: Final published version. 15 pages in revtex format including 22
epsf-embedded figures. The latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/density-osc.pd
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
Velocity-force characteristics of an interface driven through a periodic potential
We study the creep dynamics of a two-dimensional interface driven through a
periodic potential using dynamical renormalization group methods. We find that
the nature of weak-drive transport depends qualitatively on whether the
temperature is above or below the equilibrium roughening transition
temperature . Above , the velocity-force characteristics is Ohmic,
with linear mobility exhibiting a jump discontinuity across the transition. For
, the transport is highly nonlinear, exhibiting an interesting
crossover in temperature and weak external force . For intermediate drive,
, we find near a power-law velocity-force characteristics
, with , and well-below ,
, with . In the limit
of vanishing drive () the velocity-force characteristics crosses over
to , and is controlled by soliton nucleation.Comment: 18 pages, submitted to Phys. Rev.
Qualitative Analysis of the Factors Associated with Whistleblowing Intentions among Athletes from Six European Countries
Although whistleblowing is thought to represent an effective mechanism for detecting and uncovering doping in sport, it has yet to become a widely adopted practice. Understanding the factors that encourage or discourage whistleblowing is of vital importance for the promotion of this practice and the development of pedagogical material to enhance the likelihood of whistleblowing. The current study employed a qualitative methodology to explore the personal and organisational factors that underpin intentions to blow the whistle or that may lead to engagement in whistleblowing behaviours in sport. Thirty-three competitive athletes across a range of sports took part in a semi-structured interview which sought to explore what they would do should they encounter a doping scenario. Content analysis revealed that whistleblowing is a dynamic process characterised by the interaction of a range of personal and organisational factors in determining the intention to report PED use. These factors included moral reasoning, a desire to keep the matter "in-house", perceived personal costs, institutional attitudes to doping, and social support. Analysis revealed a number of 'intervening events' (Ajzen, 1991), including a perceived lack of organisational protection (e.g., ethical leadership) within some sporting sub-cultures, which present an important obstacle to whistleblowing. The intention to report doping was underpinned by a "fairness loyalty trade-off" which involved athletes choosing to adhere to either fairness norms (which relate to a sense that all people and groups are treated equally) or loyalty norms (which reflect preferential treatment towards an in-group) when deciding whether they would blow the whistle. The promotion of fairness norms that emphasise a group's collective interests might encourage athletes to view whistleblowing as a means of increasing group cohesiveness and effectiveness and thereby increase the likelihood of this practice
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