2,230 research outputs found
A general approximation of quantum graph vertex couplings by scaled Schroedinger operators on thin branched manifolds
We demonstrate that any self-adjoint coupling in a quantum graph vertex can
be approximated by a family of magnetic Schroedinger operators on a tubular
network built over the graph. If such a manifold has a boundary, Neumann
conditions are imposed at it. The procedure involves a local change of graph
topology in the vicinity of the vertex; the approximation scheme constructed on
the graph is subsequently `lifted' to the manifold. For the corresponding
operator a norm-resolvent convergence is proved, with the natural
identification map, as the tube diameters tend to zero.Comment: 19 pages, one figure; introduction amended and some references added,
to appear in CM
Duality between N=5 and N=6 Chern-Simons matter theory
We provide evidences for the duality between Chern-Simons matter theory and theory for a suitable by working out the
superconformal index, which shows perfect matching. For theories,
we show that supersymmetry is enhanced to by explicitly
constructing monopole operators filling in -currents. Finally we
work out the large index of and show that
it exactly matches with the gravity index on , which
further provides additional evidence for the duality between the
and theory for Comment: 15 pages; references adde
Interwoven magnetic and flux line structures in single crystal (Tm,Er)Ni2B2C
We review studies of the interactions between magnetic order and the flux line lattice (FLL) in the (RE)Ni2B2Cintermetallic borocarbides for (RE)=Tm and Er using small angle neutron scattering (SANS) and magneto-transport. For (RE)=Tm the magnetic order and the FLL assume a common symmetry, sharing a phase transition at ∼2 kOe, despite an order of magnitude difference in periodicity. For (RE)=Er, the penetration depth λ and the coherence length ξ, both of which are derived from the FLL form factor, are modified near TN=6 K by a theoretically predicted weakly divergent pairbreaking. Finally, below 2.3 K, (RE)=Er shows a coexistence of weak ferromagnetism and superconductivity. This state reveals a highly disordered FLL and a striking increase in the critical current, both arising from the strong ferromagnetic pairbreaking
Nature of 45 degree vortex lattice reorientation in tetragonal superconductors
The transformation of the vortex lattice in a tetragonal superconductor which
consists of its 45 degree reorientation relative to the crystal axes is studied
using the nonlocal London model. It is shown that the reorientation occurs as
two successive second order (continuous) phase transitions. The transition
magnetic fields are calculated for a range of parameters relevant for
borocarbide superconductors in which the reorientation has been observed
Boron Isotope Effect in Superconducting MgB
We report the preparation method of, and boron isotope effect for MgB, a
new binary intermetallic superconductor with a remarkably high superconducting
transition temperature (B) = 40.2 K. Measurements of both
temperature dependent magnetization and specific heat reveal a 1.0 K shift in
between MgB and MgB. Whereas such a high transition
temperature might imply exotic coupling mechanisms, the boron isotope effect in
MgB is consistent with the material being a phonon-mediated BCS
superconductor.Comment: One figure and related discussion adde
Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNiBC
We have investigated the temperature dependence of the H || c flux line
lattice structural phase transition from square to hexagonal symmetry, in the
tetragonal superconductor LuNi_2B_2C (T_c = 16.6 K). At temperatures below 10 K
the transition onset field, H_2(T), is only weakly temperature dependent. Above
10 K, H_2(T) rises sharply, bending away from the upper critical field. This
contradicts theoretical predictions of H_2(T) merging with the upper critical
field, and suggests that just below the H_c2(T)-curve the flux line lattice
might be hexagonal.Comment: 4 pages, 3 figure
A layering model for superconductivity in the borocarbides
We propose a superlattice model to describe superconductivity in layered
materials, such as the borocarbide families with the chemical formul\ae\
BC and BC, with being (essentially) a rare earth, and a
transition metal. We assume a single band in which electrons feel a local
attractive interaction (negative Hubbard-) on sites representing the B
layers, while U=0 on sites representing the C layers; the multi-band
structure is taken into account minimally through a band offset . The
one-dimensional model is studied numerically through the calculation of the
charge gap, the Drude weight, and of the pairing correlation function. A
comparison with the available information on the nature of the electronic
ground state (metallic or superconducting) indicates that the model provides a
systematic parametrization of the whole borocarbide family.Comment: 4 figure
Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C
We present optical conductivity data for Lu(NiCo)BC over
a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials
show evidence of being good Drude metals with the infrared data in reasonable
agreement with dc resistivity measurements at low frequencies. An absorption
threshold is seen at approximately 700 cm-1. In the cobalt-doped material we
see a superconducting gap in the conductivity spectrum with an absorption onset
at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong
coupling. The pure material is in the clean limit and no gap can be seen. We
discuss the data in terms of the electron-phonon interaction and find that it
can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an
electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega)
spectrum fit to the resistivity.Comment: 10 pages with 10 embedded figures, submitted to PR
Towards the F-Theorem: N=2 Field Theories on the Three-Sphere
For 3-dimensional field theories with {\cal N}=2 supersymmetry the Euclidean
path integrals on the three-sphere can be calculated using the method of
localization; they reduce to certain matrix integrals that depend on the
R-charges of the matter fields. We solve a number of such large N matrix models
and calculate the free energy F as a function of the trial R-charges consistent
with the marginality of the superpotential. In all our {\cal N}=2
superconformal examples, the local maximization of F yields answers that scale
as N^{3/2} and agree with the dual M-theory backgrounds AdS_4 x Y, where Y are
7-dimensional Sasaki-Einstein spaces. We also find in toric examples that local
F-maximization is equivalent to the minimization of the volume of Y over the
space of Sasakian metrics, a procedure also referred to as Z-minimization.
Moreover, we find that the functions F and Z are related for any trial
R-charges. In the models we study F is positive and decreases along RG flows.
We therefore propose the "F-theorem" that we hope applies to all 3-d field
theories: the finite part of the free energy on the three-sphere decreases
along RG trajectories and is stationary at RG fixed points. We also show that
in an infinite class of Chern-Simons-matter gauge theories where the
Chern-Simons levels do not sum to zero, the free energy grows as N^{5/3} at
large N. This non-trivial scaling matches that of the free energy of the
gravity duals in type IIA string theory with Romans mass.Comment: 66 pages, 10 figures; v2: refs. added, minor improvement
Operator Counting for N=2 Chern-Simons Gauge Theories with Chiral-like Matter Fields
The localization formula of Chern-Simons quiver gauge theory on nicely
reproduces the geometric data such as volume of Sasaki-Einstein manifolds in
the large- limit, at least for vector-like models. The validity of
chiral-like models is not established yet, due to technical problems in both
analytic and numerical approaches. Recently Gulotta, Herzog and Pufu suggested
that the counting of chiral operators can be used to find the eigenvalue
distribution of quiver matrix models. In this paper we apply this method to
some vector-like or chiral-like quiver theories, including the triangular
quivers with generic Chern-Simons levels which are dual to in-homogeneous
Sasaki-Einstein manifolds . The result is consistent
with AdS/CFT and the volume formula. We discuss the implication of our
analysis.Comment: 23 pages; v2. revised version; v3. corrected typos and clarified
argument
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