1,388 research outputs found
D3/D7 holographic Gauge theory and Chemical potential
N=2 supersymmetric Yang-Mills theory with flavor hypermultiplets at finite
temperature and in the dS are studied for finite quark number density
() by a dual supergravity background with non-trivial dilaton and axion.
The quarks and its number density are introduced by embedding a probe D7
brane. We find a critical value of the chemical potential at the limit of
, and it coincides with the effective quark mass given in each theory
for . At this point, a transition of the D7 embedding configurations
occurs between their two typical ones. The phase diagrams of this transition
are shown in the plane of chemical potential versus temperature and
cosmological constant for YM theory at finite temperature and in dS
respectively. In this phase transition, the order parameter is considered as
. % and the critical value of the chemical potential This result seems to
be reasonable since both theories are in the quark deconfinement phase.Comment: 17 pages, 8 figure
A first-principles study of tunneling magnetoresistance in Fe/MgAl2O4/Fe(001) magnetic tunnel junctions
We investigated the spin-dependent transport properties of Fe/MgAl2O4/Fe(001)
magnetic tunneling junctions (MTJs) on the basis of first-principles
calculations of the electronic structures and the ballistic conductance. The
calculated tunneling magnetoresistance (TMR) ratio of a Fe/MgAl2O4/Fe(001) MTJ
was about 160%, which was much smaller than that of a Fe/MgO/Fe(001) MTJ
(1600%) for the same barrier thickness. However, there was an evanescent state
with delta 1 symmetry in the energy gap around the Fermi level of normal spinel
MgAl2O4, indicating the possibility of a large TMR in Fe/MgAl2O4/Fe(001) MTJs.
The small TMR ratio of the Fe/MgAl2O4/Fe(001) MTJ was due to new conductive
channels in the minority spin states resulting from a band-folding effect in
the two-dimensional (2-D) Brillouin zone of the in-plane wave vector (k//) of
the Fe electrode. Since the in-plane cell size of MgAl2O4 is twice that of the
primitive in-plane cell size of bcc Fe, the bands in the boundary edges are
folded, and minority-spin states coupled with the delta 1 evanescent state in
the MgAl2O4 barrier appear at k//=0, which reduces the TMR ratio of the MTJs
significantly.Comment: 5 pages, 6 figures, 1 tabl
Quantum melting of charge ice and non-Fermi-liquid behavior: An exact solution for the extended Falicov-Kimball model in the ice-rule limit
An exact solution is obtained for a model of itinerant electrons coupled to
ice-rule variables on the tetrahedron Husimi cactus, an analogue of the Bethe
lattice of corner-sharing tetrahedra. It reveals a quantum critical point with
the emergence of non-Fermi-liquid behavior in melting of the "charge ice"
insulator. The electronic structure is compared with the numerical results for
the pyrochlore-lattice model to elucidate the physics of electron systems
interacting with the tetrahedron ice rule.Comment: 5 pages, 4 figure
Holographic Confining Gauge theory and Response to Electric Field
We study the response of confining gauge theory to the external electric
field by using holographic Yang-Mills theories in the large limit.
Although the theories are in the confinement phase, we find a transition from
the insulator to the conductor phase when the electric field exceeds its
critical value. Then, the baryon number current is generated in the conductor
phase. At the same time, in this phase, the meson melting is observed through
the quasi-normal modes of meson spectrum. Possible ideas are given for the
string state corresponding to the melted mesons, and they lead to the idea that
the source of this current may be identified with the quarks and anti-quarks
supplied by the melted mesons. We also discuss about other possible carriers.
Furthermore, from the analysis of the massless quark, chiral symmetry
restoration is observed at the insulator-conductor transition point by studying
a confining theory in which the chiral symmetry is broken.Comment: 27 pages, 14 figure
Spectral Evolution of the Universe
We derive the evolution equations for the spectra of the Universe.
Here "spectra" means the eigenvalues of the Laplacian defined on a space,
which contain the geometrical information on the space.
These equations are expected to be useful to analyze the evolution of the
geometrical structures of the Universe.
As an application, we investigate the time evolution of the spectral distance
between two Universes that are very close to each other; it is the first
necessary step for the detailed analysis of the model-fitting problem in
cosmology with the spectral scheme.
We find out a universal formula for the spectral distance between two very
close Universes, which turns out to be independent of the detailed form of the
distance nor the gravity theory. Then we investigate its time evolution with
the help of the evolution equations we derive.
We also formulate the criteria for a good cosmological model in terms of the
spectral distance.Comment: To appear in Phys. Rev.
Leptonic CP Violation Search and the Ambiguity of dm^2_31
We consider a search for the CP-violating angle deltaCP in long baseline
neutrino oscillation experiments. We show that the subleading deltaCP-dependent
terms in the nu_mu -> nu_e oscillation probability can be easily obscured by
the ambiguity of the leading term which depends on |dm^2_31|. It is thus
necessary to determine the value of dm^2_31 with a sufficient accuracy. The
nu_mu survival events, which can be accumulated simultaneously with the nu_e
appearance events, can serve for this purpose owing to its large statistics.
Therefore, the combined analysis of nu_e appearance and nu_mu survival events
is crucial to provide a restrictive constraint on deltaCP. Taking a test
experimental setup, we demonstrate in the deltaCP-dm^2_31 plane that the
analysis of nu_e appearance events leads to less restrictive constraints on the
value of deltaCP due to the ambiguity of dm^2_31 and that the combined analysis
efficiently improves the constraints.Comment: ReVTeX file, 9 pages, 7 figures. Discussions added in Sections 1, 2,
and 4; Reference expande
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Advances in Kriging-Based Autonomous X-Ray Scattering Experiments.
Autonomous experimentation is an emerging paradigm for scientific discovery, wherein measurement instruments are augmented with decision-making algorithms, allowing them to autonomously explore parameter spaces of interest. We have recently demonstrated a generalized approach to autonomous experimental control, based on generating a surrogate model to interpolate experimental data, and a corresponding uncertainty model, which are computed using a Gaussian process regression known as ordinary Kriging (OK). We demonstrated the successful application of this method to exploring materials science problems using x-ray scattering measurements at a synchrotron beamline. Here, we report several improvements to this methodology that overcome limitations of traditional Kriging methods. The variogram underlying OK is global and thus insensitive to local data variation. We augment the Kriging variance with model-based measures, for instance providing local sensitivity by including the gradient of the surrogate model. As with most statistical regression methods, OK minimizes the number of measurements required to achieve a particular model quality. However, in practice this may not be the most stringent experimental constraint; e.g. the goal may instead be to minimize experiment duration or material usage. We define an adaptive cost function, allowing the autonomous method to balance information gain against measured experimental cost. We provide synthetic and experimental demonstrations, validating that this improved algorithm yields more efficient autonomous data collection
Cluster dynamical mean-field study of the Hubbard model on a 3D frustrated hyperkagome lattice
We study the Hubbard model on a geometrically-frustrated hyperkagome lattice
by a cluster extension of the dynamical mean field theory. We calculate the
temperature () dependences of the specific heat () and the spin-lattice
relaxation time () in correlated metallic region. shows a peak at
and rapidly decreases as . On the other hand, has a
peak at a higher temperature than , and largely decreases
below , followed by the Korringa law as . Both
peak temperatures are suppressed and the peaks become sharper as electron
correlation is increased. These behaviors originate from strong renormalization
of the energy scales in the peculiar electronic structure in this frustrated
system; a pseudo-gap like feature, the van-Hove singularity, and the flat band.
The results are discussed in comparison with the experimental data in the
hyperkagome material, NaIrO.Comment: 4 pages, 4 figures, Conference proceedings for Highly Frustrated
Magnetism 200
Carrier doping to a partially disordered state in the periodic Anderson model on a triangular lattice
We investigate the effect of hole and electron doping to half-filling in the
periodic Anderson model on a triangular lattice by the Hartree-Fock
approximation at zero temperature. At half-filling, the system exhibits a
partially disordered insulating state, in which a collinear antiferromagnetic
order on an unfrustrated honeycomb subnetwork coexists with nonmagnetic state
at the remaining sites. We find that the carrier doping destabilizes the
partially disordered state, resulting in a phase separation to a doped metallic
state with different magnetic order. The partially disordered state is
restricted to the half-filled insulating case, while its metallic counterpart
is obtained as a metastable state in a narrow electron doped region.Comment: 4 pages, 2 figure
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