7,130 research outputs found
Novel electric field effects on Landau levels in Graphene
A single graphene layer exhibits an anomalous Landau level spectrum. A
massless Dirac like low energy electronic spectrum underlies this anomaly. We
study, analytically and numerically, the effect of a uniform electric field
on the anomalous Landau levels. We solve the problem exactly within the
Dirac cone approximation and find an interesting scaling of the spectrum,
leading to the collapse of the Landau levels at a critical , for a
given magnetic field . We offer a physical interpretation of our result,
which uses `graphene relativity' and the boost operation. Electric fields,
non-uniform at nanoscopic (, magnetic) length scales, produce local
collapse at . We expect an anomalous breakdown of quantum Hall states
in real graphene, induced by large Hall currents.Comment: 4 pages, 3 figure
Spin-S Kitaev model: Classical Ground States, Order by Disorder and Exact Correlation Functions
In the first part of this paper, we study the spin-S Kitaev model using spin
wave theory. We discover a remarkable geometry of the minimum energy surface in
the N-spin space. The classical ground states, called Cartesian or CN-ground
states, whose number grows exponentially with the number of spins N, form a set
of points in the N-spin space. These points are connected by a network of flat
valleys in the N-spin space, giving rise to a continuous family of classical
ground states. Further, the CN-ground states have a correspondence with dimer
coverings and with self avoiding walks on a honeycomb lattice. The zero point
energy of our spin wave theory picks out a subset from a continuous family of
classically degenerate states as the quantum ground states; the number of these
states also grows exponentially with N. In the second part, we present some
exact results. For arbitrary spin-S, we show that localized Z_2 flux
excitations are present by constructing plaquette operators with eigenvalues
\pm 1 which commute with the Hamiltonian. This set of commuting plaquette
operators leads to an exact vanishing of the spin-spin correlation functions,
beyond nearest neighbor separation, found earlier for the spin-1/2 model [G.
Baskaran, S. Mandal and R. Shankar, Phys. Rev. Lett. 98, 247201 (2007)]. We
introduce a generalized Jordan-Wigner transformation for the case of general
spin-S, and find a complete set of commuting link operators, similar to the
spin-1/2 model, thereby making the Z_2 gauge structure more manifest. The
Jordan-Wigner construction also leads, in a natural fashion, to Majorana
fermion operators for half-integer spin cases and hard-core boson operators for
integer spin cases, strongly suggesting the presence of Majorana fermion and
boson excitations in the respective low energy sectors.Comment: 9 pages including 4 figures; added a section on an exactly solvable
higher spin version of the Kitaev model; this is the published versio
Comparing Nonparametric Bayesian Tree Priors for Clonal Reconstruction of Tumors
Statistical machine learning methods, especially nonparametric Bayesian
methods, have become increasingly popular to infer clonal population structure
of tumors. Here we describe the treeCRP, an extension of the Chinese restaurant
process (CRP), a popular construction used in nonparametric mixture models, to
infer the phylogeny and genotype of major subclonal lineages represented in the
population of cancer cells. We also propose new split-merge updates tailored to
the subclonal reconstruction problem that improve the mixing time of Markov
chains. In comparisons with the tree-structured stick breaking prior used in
PhyloSub, we demonstrate superior mixing and running time using the treeCRP
with our new split-merge procedures. We also show that given the same number of
samples, TSSB and treeCRP have similar ability to recover the subclonal
structure of a tumor.Comment: Preprint of an article submitted for consideration in the Pacific
Symposium on Biocomputing \c{opyright} 2015; World Scientific Publishing Co.,
Singapore, 2015; http://psb.stanford.edu
Pulsed Ultrasound Does Not Affect Recovery From Delayed Onset Muscle Soreness
Aim: To investigate the effects of pulsed Ultrasound (US) in recovery from Delayed Onset Muscle Soreness (DOMS).
Methods: Twelve healthy male athletes (mean age 23.83±1.697 year) performed an eccentric exercise protocol of non-dominant elbow flexors to induce muscle soreness on 2 occasions separated by 3 weeks. Subjects in experimental group received pulsed US (1 MHz, intensity 0.8 W/cm2, mark space ratio 1:10), whereas control group received sham US after 24 h, 48 h and 72 h. Perception of muscle soreness, active ROM and muscle strength were the parameters measured at 0 h, 24 h, 48 h and 72 h with the help of VAS, manual goniometer and JONEX muscles master instrument respectively.
Results: Post hoc t test analysis revealed significant differences (p <0.05) between 0 h and 72 h in the parameter of ROM (t = 6.18) and muscle power (t = 2.54) as well as between 24 h and 48 h in the parameter of muscle soreness (t = 3.13) in control group. Similar differences were also observed in the experimental group. No significant inter-group differences at α level of 0.05 was observed in any parameter at any level.
Conclusion: The pattern of recovery from DOMS was not influenced by the application of pulsed Ultrasound at the parameters discussed here
RVB gauge theory and the Topological degeneracy in the Honeycomb Kitaev model
We relate the Z gauge theory formalism of the Kitaev model to the SU(2)
gauge theory of the resonating valence bond (RVB) physics. Further, we
reformulate a known Jordan-Wigner transformation of Kitaev model on a torus in
a general way that shows that it can be thought of as a Z gauge fixing
procedure. The conserved quantities simplify in terms of the gauge invariant
Jordan-Wigner fermions, enabling us to construct exact eigen states and
calculate physical quantities. We calculate the fermionic spectrum for flux
free sector for different gauge field configurations and show that the ground
state is four-fold degenerate on a torus in thermodynamic limit. Further on a
torus we construct four mutually anti-commuting operators which enable us to
prove that all eigenstates of this model are four fold degenerate in
thermodynamic limit.Comment: 12 pages, 3 figures. Added affiliation and a new section,
'Acknowledgements'.Typos correcte
Evidence against strong correlation in 4d transition metal oxides, CaRuO3 and SrRuO3
We investigate the electronic structure of 4d transition metal oxides, CaRuO3
and SrRuO3. The analysis of the photoemission spectra reveals significantly
weak electron correlation strength (U/W ~ 0.2) as expected in 4d systems and
resolves the long standing issue that arose due to the prediction of large U/W
similar to 3d-systems. It is shown that the bulk spectra, thermodynamic
parameters and optical properties in these systems can consistently be
described using first principle approaches. The observation of different
surface and bulk electronic structures in these weakly correlated 4d systems is
unusual.Comment: 4 pages, 4 figure
Super-massive Black Hole Demography: the Match between the Local and Accreted Mass Functions
We have performed a detailed analysis of the local super-massive black-hole
(SMBH) mass function based on both kinematic and photometric data and derived
an accurate analytical fit in the range 10^6 <= (M_BH/M_sun) <= 5*10^9. We find
a total SMBH mass density of (4.2+/-1.1)*10^5 M_sun/Mpc^3, about 25% of which
is contributed by SMBHs residing in bulges of late type galaxies. Exploiting
up-to-date luminosity functions of hard X-ray and optically selected AGNs, we
have studied the accretion history of the SMBH population. If most of the
accretion happens at constant \dot{M_BH}/M_BH the local SMBH mass function is
fully accounted for by mass accreted by X-ray selected AGNs, with bolometric
corrections indicated by current observations and a standard mass-to-light
conversion efficiency \epsilon ~10%. The analysis of the accretion history
highlights that the most massive BHs accreted their mass faster and at higher
redshifts (z>1.5), while the lower mass BHs responsible for most of the hard
X-ray background have mostly grown at z<1.5. The accreted mass function matches
the local SMBH mass function if \epsilon ~0.09(+0.04,-0.03) and the Eddington
ratio \lambda=L/L_Edd \~0.3(+0.3,-0.1) (68% confidence errors). The visibility
time, during which AGNs are luminous enough to be detected by the currently
available X-ray surveys, ranges from ~0.1 Gyr for present day BH masses
M_BH(z=0) ~10^6 M_sun to ~0.3 Gyr for M_BH(z=0) >= 10^9 M_sun. The mass
accreted during luminous phases is >= 25-30% even if we assume extreme values
of \epsilon (\epsilon \~0.3-0.4). An unlikely fine tuning of the parameters
would be required to account for the local SMBH mass function accomodating a
dominant contribution from 'dark' BH growth (due, e.g., to BH coalescence).Comment: 12 pages, 14 figures, accepted for publication in MNRAS, minor
changes following referee's comment
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