8,325 research outputs found
Two non-commutative parameters and regular cosmological phase transition in the semi-classical dilaton cosmology
We study cosmological phase transitions from modified equations of motion by
introducing two non-commutative parameters in the Poisson brackets, which
describes the initial- and future-singularity-free phase transition in the
soluble semi-classical dilaton gravity with a non-vanishing cosmological
constant. Accelerated expansion and decelerated expansion corresponding to the
FRW phase appear alternatively, and then it ends up with the second accelerated
expansion. The final stage of the universe approaches the flat spacetime
independent of the initial state of the curvature scalar as long as the product
of the two non-commutative parameters is less than one. Finally, we show that
the initial-singularity-free condition is related to the second accelerated
expansion of the universe.Comment: 13 pages, 4 figures; v2. to appear in Mod. Phys. Lett.
State-constraint static Hamilton-Jacobi equations in nested domains
We study state-constraint static Hamilton-Jacobi equations in a sequence of
domains in such that for all . We obtain rates of convergence
of , the solution to the state-constraint problem in , to ,
the solution to the corresponding problem in . In many cases, the rates obtained are proven to be
optimal. Various new examples and discussions are provided at the end of the
paper.Comment: 23 pages, 1 figur
A note on conductivity and charge diffusion in holographic flavour systems
We analyze the charge diffusion and conductivity in a Dp/Dq holographic setup
that is dual to a supersymmetric Yang-Mills theory in p+1 dimensions with N_f<<
N_c flavour degrees of freedom at finite temperature and nonvanishing U(1)
baryon number chemical potential. We provide a new derivation of the results
that generalize the membrane paradigm to the present context. We perform a
numerical analysis in the particular case of the D3/D7 flavor system. The
results obtained support the validity of the Einstein relation at finite
chemical potential.Comment: 15 pages, 3 figures, v2 with minor correction
Holographic DC conductivities from the open string metric
We study the DC conductivities of various holographic models using the open
string metric (OSM), which is an effective metric geometrizing density and
electromagnetic field effect. We propose a new way to compute the nonlinear
conductivity using OSM. As far as the final conductivity formula is concerned,
it is equivalent to the Karch-O'Bannon's real-action method. However, it yields
a geometrical insight and technical simplifications. Especially, a real-action
condition is interpreted as a regular geometry condition of OSM. As
applications of the OSM method, we study several holographic models on the
quantum Hall effect and strange metal. By comparing a Lifshitz background and
the Light-Cone AdS, we show how an extra parameter can change the temperature
scaling behavior of conductivity. Finally we discuss how OSM can be used to
study other transport coefficients, such as diffusion constant, and effective
temperature induced by the effective world volume horizon.Comment: 33 page
Exact 4-point Scattering Amplitude of the Superconformal Schrodinger Chern-Simons Theory
We consider the non-relativistic superconformal U(N) X U(N) Chern-Simons
theory with level (k,-k) possessing fourteen supersymmetries. We obtain an
exact four-point scattering amplitude of the theory to all orders in 1/N and
1/k and prove that the scattering amplitude becomes trivial when k=1 and 2. We
confirm this amplitude to one-loop order by using an explicit field theoretic
computation and show that the beta function for the contact interaction
vanishes to the one-loop order, which is consistent with the quantum conformal
invariance of the underlying theory.Comment: 16 page
RG flow of transport quantities
The RG flow equation of various transport quantities are studied in arbitrary
spacetime dimensions, in the fixed as well as fluctuating background geometry
both for the Maxwellian and DBI type of actions. The regularity condition on
the flow equation of the conductivity at the horizon for the DBI action
reproduces naturally the leading order result of {\it Hartnoll et al.}, [{\it
JHEP}, {\bf 04}, 120 (2010)]. Motivated by the result of {\it van der Marel et
al.}, [{\it science}, {\bf 425}, 271 (2003], we studied, analytically, the
conductivity versus frequency plane by dividing it into three distinct parts:
and . In order to compare, we choose 3+1
dimensional bulk spacetime for the computation of the conductivity. In the
range, the conductivity does not show up the Drude like form in any
spacetime dimensions. In the range and staying away from the
horizon, for the DBI action with unit dynamical exponent, non-zero magnetic
field and charge density, the conductivity goes as , whereas the
phase of the conductivity, goes as,
and . There exists a universal
quantity at the horizon that is the phase angle of conductivity, which either
vanishes or an integral multiple of . Furthermore, we calculate the
temperature dependence to the thermoelectric and the thermal conductivity at
the horizon. The charge diffusion constant for the DBI action is studied.Comment: 1+68 pages, 12 figures and 4 appendices; V2: The charge diffusion
constant is calculated for arbitrary spacetime dimensions and related
references added; v3: Connection with the RG flow of 1010.4036 is made; v4:
Several corrections, typos fixed and a ref. adde
Electronic and Magnetic Properties of Partially-Open Carbon Nanotubes
On the basis of the spin-polarized density functional theory calculations, we
demonstrate that partially-open carbon nanotubes (CNTs) observed in recent
experiments have rich electronic and magnetic properties which depend on the
degree of the opening. A partially-open armchair CNT is converted from a metal
to a semiconductor, and then to a spin-polarized semiconductor by increasing
the length of the opening on the wall. Spin-polarized states become
increasingly more stable than nonmagnetic states as the length of the opening
is further increased. In addition, external electric fields or chemical
modifications are usable to control the electronic and magnetic properties of
the system. We show that half-metallicity may be achieved and the spin current
may be controlled by external electric fields or by asymmetric
functionalization of the edges of the opening. Our findings suggest that
partially-open CNTs may offer unique opportunities for the future development
of nanoscale electronics and spintronics.Comment: 6 figures, to appear in J. Am. Chem. So
Holographic zero sound at finite temperature in the Sakai-Sugimoto model
In this paper, we study the fate of the holographic zero sound mode at finite
temperature and non-zero baryon density in the deconfined phase of the
Sakai-Sugimoto model of holographic QCD. We establish the existence of such a
mode for a wide range of temperatures and investigate the dispersion relation,
quasi-normal modes, and spectral functions of the collective excitations in
four different regimes, namely, the collisionless quantum, collisionless
thermal, and two distinct hydrodynamic regimes. For sufficiently high
temperatures, the zero sound completely disappears, and the low energy physics
is dominated by an emergent diffusive mode. We compare our findings to
Landau-Fermi liquid theory and to other holographic models.Comment: 1+24 pages, 19 figures, PDFTeX, v2: some comments and references
added, v3: some clarifications relating to the different regimes added,
matches version accepted for publication in JHEP, v4: corrected typo in eq.
(3.18
Bulk properties of the van der Waals hard ferromagnet VI3
We present comprehensive measurements of the structural, magnetic, and electronic properties of layered van der Waals ferromagnet VI3 down to low temperatures. Despite belonging to a well-studied family of transition-metal trihalides, this material has received very little attention. We outline, from high-resolution powder x-ray diffraction measurements, a corrected room-temperature crystal structure to that previously proposed and uncover a structural transition at 79 K, also seen in the heat capacity. Magnetization measurements confirm VI3 to be a hard ferromagnet (9.1 kOe coercive field at 2 K) with a high degree of anisotropy, and the pressure dependence of the magnetic properties provide evidence for the two-dimensional nature of the magnetic order. Optical and electrical transport measurements show this material to be an insulator with an optical band gap of 0.67 eV - the previous theoretical predictions of d-band metallicity then lead us to believe VI3 to be a correlated Mott insulator. Our latest band-structure calculations support this picture and show good agreement with the experimental data. We suggest VI3 to host great potential in the thriving field of low-dimensional magnetism and functional materials, together with opportunities to study and make use of low-dimensional Mott physics
Anomalies and the chiral magnetic effect in the Sakai-Sugimoto model
In the chiral magnetic effect an imbalance in the number of left- and
right-handed quarks gives rise to an electromagnetic current parallel to the
magnetic field produced in noncentral heavy-ion collisions. The chiral
imbalance may be induced by topologically nontrivial gluon configurations via
the QCD axial anomaly, while the resulting electromagnetic current itself is a
consequence of the QED anomaly. In the Sakai-Sugimoto model, which in a certain
limit is dual to large-N_c QCD, we discuss the proper implementation of the QED
axial anomaly, the (ambiguous) definition of chiral currents, and the
calculation of the chiral magnetic effect. We show that this model correctly
contains the so-called consistent anomaly, but requires the introduction of a
(holographic) finite counterterm to yield the correct covariant anomaly.
Introducing net chirality through an axial chemical potential, we find a
nonvanishing vector current only before including this counterterm. This seems
to imply the absence of the chiral magnetic effect in this model. On the other
hand, for a conventional quark chemical potential and large magnetic field,
which is of interest in the physics of compact stars, we obtain a nontrivial
result for the axial current that is in agreement with previous calculations
and known exact results for QCD.Comment: 35 pages, 4 figures, v2: added comments about frequency-dependent
conductivity at the end of section 4; references added; version to appear in
JHE
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