16 research outputs found
Powerlaw optical conductivity with a constant phase angle in high Tc superconductors
In certain materials with strong electron correlations a quantum phase
transition (QPT) at zero temperature can occur, in the proximity of which a
quantum critical state of matter has been anticipated. This possibility has
recently attracted much attention because the response of such a state of
matter is expected to follow universal patterns defined by the quantum
mechanical nature of the fluctuations. Forementioned universality manifests
itself through power-law behaviours of the response functions. Candidates are
found both in heavy fermion systems and in the cuprate high Tc superconductors.
Although there are indications for quantum criticality in the cuprate
superconductors, the reality and the physical nature of such a QPT are still
under debate. Here we identify a universal behaviour of the phase angle of the
frequency dependent conductivity that is characteristic of the quantum critical
region. We demonstrate that the experimentally measured phase angle agrees
precisely with the exponent of the optical conductivity. This points towards a
QPT in the cuprates close to optimal doping, although of an unconventional
kind.Comment: pdf format, 9 pages, 4 color figures include
Soft wall model for a holographic superconductor
We apply the soft wall holographic model from hadron physics to a description
of the high- superconductivity. In comparison with the existing bottom-up
holographic superconductors, the proposed approach is more phenomenological. On
the other hand, it is much simpler and has more freedom for fitting the
conductivity properties of the real high- materials. We demonstrate some
examples of emerging models and discuss a possible origin of the approach.Comment: 17 pages, 16 figure
Functional MRI and Diffusion Tensor Imaging of Brain Reorganization After Experimental Stroke
The potential of the adult brain to reorganize after ischemic injury is critical for functional recovery and provides a significant target for therapeutic strategies to promote brain repair. Despite the accumulating evidence of brain plasticity, the interaction and significance of morphological and physiological modifications in post-stroke brain tissue remain mostly unclear. Neuroimaging techniques such as functional MRI (fMRI) and diffusion tensor imaging (DTI) enable in vivo assessment of the spatial and temporal pattern of functional and structural changes inside and outside ischemic lesion areas. This can contribute to the elucidation of critical aspects in post-stroke brain remodeling. Task/stimulus-related fMRI, resting-state fMRI, or pharmacological MRI enables direct or indirect measurement of neuronal activation, functional connectivity, or neurotransmitter system responses, respectively. DTI allows estimation of the structural integrity and connectivity of white matter tracts. Together, these MRI methods provide an unprecedented means to (a) measure longitudinal changes in tissue structure and function close by and remote from ischemic lesion areas, (b) evaluate the organizational profile of neural networks after stroke, and (c) identify degenerative and restorative processes that affect post-stroke functional outcome. Besides, the availability of MRI in clinical institutions as well as research laboratories provides an optimal basis for translational research on stroke recovery. This review gives an overview of the current status and perspectives of fMRI and DTI applications to study brain reorganization in experimental stroke models
Intermediate scalings in holographic RG flows and conductivities
We construct numerically finite density domain-wall solutions which interpolate between two AdS 4 fixed points and exhibit an intermediate regime of hyperscaling violation, with or without Lifshitz scaling. Such RG flows can be realized in gravitational models containing a dilatonic scalar and a massive vector field with appropriate choices of the scalar potential and couplings. The infrared AdS 4 fixed point describes a new ground state for strongly coupled quantum systems realizing such scalings, thus avoiding the well-known extensive zero temperature entropy associated with AdS2ĂR2. We also examine the zero temperature behavior of the optical conductivity in these backgrounds and identify two scaling regimes before the UV CFT scaling is reached. The scaling of the conductivity is controlled by the emergent IR conformal symmetry at very low frequencies, and by the intermediate scaling regime at higher frequencies
Dark Matter in the Milky Way's Dwarf Spheroidal Satellites
The Milky Way's dwarf spheroidal satellites include the nearest, smallest and
least luminous galaxies known. They also exhibit the largest discrepancies
between dynamical and luminous masses. This article reviews the development of
empirical constraints on the structure and kinematics of dSph stellar
populations and discusses how this phenomenology translates into constraints on
the amount and distribution of dark matter within dSphs. Some implications for
cosmology and the particle nature of dark matter are discussed, and some
topics/questions for future study are identified.Comment: A version with full-resolution figures is available at
http://www.cfa.harvard.edu/~mwalker/mwdsph_review.pdf; 70 pages, 22 figures;
invited review article to be published in Vol. 5 of the book "Planets, Stars,
and Stellar Systems", published by Springe
Relativistic Dynamics and Extreme Mass Ratio Inspirals
It is now well-established that a dark, compact object (DCO), very likely a
massive black hole (MBH) of around four million solar masses is lurking at the
centre of the Milky Way. While a consensus is emerging about the origin and
growth of supermassive black holes (with masses larger than a billion solar
masses), MBHs with smaller masses, such as the one in our galactic centre,
remain understudied and enigmatic. The key to understanding these holes - how
some of them grow by orders of magnitude in mass - lies in understanding the
dynamics of the stars in the galactic neighbourhood. Stars interact with the
central MBH primarily through their gradual inspiral due to the emission of
gravitational radiation. Also stars produce gases which will subsequently be
accreted by the MBH through collisions and disruptions brought about by the
strong central tidal field. Such processes can contribute significantly to the
mass of the MBH and progress in understanding them requires theoretical work in
preparation for future gravitational radiation millihertz missions and X-ray
observatories. In particular, a unique probe of these regions is the
gravitational radiation that is emitted by some compact stars very close to the
black holes and which could be surveyed by a millihertz gravitational wave
interferometer scrutinizing the range of masses fundamental to understanding
the origin and growth of supermassive black holes. By extracting the
information carried by the gravitational radiation, we can determine the mass
and spin of the central MBH with unprecedented precision and we can determine
how the holes "eat" stars that happen to be near them.Comment: Update from the first version, 151 pages, accepted for publication @
Living Reviews in Relativit