482 research outputs found
One-loop counterterms for the dimensional regularization of arbitrary Lagrangians
We present master formulas for the divergent part of the one-loop effective
action for an arbitrary (both minimal and nonminimal) operators of any order in
the 4-dimensional curved space. They can be considered as computer algorithms,
because the one-loop calculations are then reduced to the simplest algebraic
operations. Some test applications are considered by REDUCE analytical
calculation system.Comment: 39 pages, Latex, 3 PS figures, replaced with published versio
Pressure-dependent optical investigations of -(BEDT-TTF)I: tuning charge order and narrow gap towards a Dirac semimetal
Infrared optical investigations of -(BEDT-TTF)I have been
performed in the spectral range from 80 to 8000~cm down to temperatures
as low as 10~K by applying hydrostatic pressure. In the metallic state, ~K, we observe a 50\% increase in the Drude contribution as well as the
mid-infrared band due to the growing intermolecular orbital overlap with
pressure up to 11~kbar. In the ordered state, , we extract how
the electronic charge per molecule varies with temperature and pressure:
Transport and optical studies demonstrate that charge order and metal-insulator
transition coincide and consistently yield a linear decrease of the transition
temperature by ~K/kbar. The charge disproportionation
diminishes by /kbar and the optical gap between
the bands decreases with pressure by -47~cm/kbar. In our high-pressure
and low-temperature experiments, we do observe contributions from the massive
charge carriers as well as from massless Dirac electrons to the low-frequency
optical conductivity, however, without being able to disentangle them
unambiguously.Comment: 13 pages, 17 figures, submitted to Phys. Rev.
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
Flavor Symmetry Breaking in Strongly Coupled N=1 Supersymmetric SO(N_c) Gauge Theory with N_f=N_c-2
In the SO(N_c) gauge theory with N_f quarks for N_f=N_c-2, its instanton
effects indicate the signal of dynamical flavor symmetry breakdown of SU(N_f)
to SO(N_f), which is not described by the conventional "magnetic" degree's of
freedom. It is argued that this breaking is well described by our effective
superpotential consisting of "electric" quarks and gluons instead of monopoles
of SO(N_c). The low-energy particles include the Nambu-Goldstone superfields
associated with this breakdown. The proposed superpotential is found to exhibit
the holomorphic decoupling property and the anomaly-matching property on a
residual chiral U(1) symmetry.Comment: 6 pages, version to appear in Europhysics Letter
Optical study of superconducting Ga-rich layers in silicon
We performed phase-sensitive terahertz (0.12 - 1.2 THz) transmission
measurements of Ga-enriched layers in silicon. Below the superconducting
transition, T_{c} = 6.7 K, we find clear signatures of the formation of a
superconducting condensate and of the opening of an energy gap in the optical
spectra. The London penetration depth, \lambda(T), and the condensate density,
n_{s} = \lambda^{2} 0)/\lambda^{2}(T), as functions of temperature demonstrate
behavior, typical for conventional superconductors with \lambda(0) = 1.8 \mu m.
The terahertz spectra can be well described within the framework of Eliashberg
theory with strong electron-phonon coupling: the zero-temperature energy gap is
2\Delta(0) = 2.64 meV and 2\Delta(0)/k_{B}T_{c} = 4.6 \pm 0.1, consistent with
the amorphous state of Ga. At temperatures just above T_{c}, the optical
spectra demonstrate Drude behavior.Comment: 5 pages, 4 figure
Optical conductivity and penetration depth in MgB2
The complex conductivity of a MgB2 film has been investigated in the
frequency range 4 cm^{-1}< nu < 30 cm^{-1} and for temperatures 2.7 K < T <300
K. The overall temperature dependence of both components of the complex
conductivity is reminiscent of BCS-type behavior, although a detailed analysis
reveals a number of discrepancies. No characteristic feature of the isotropic
BCS gap temperature evolution is observed in the conductivity spectra in the
superconducting state. A peak in the temperature dependence of the real part of
the conductivity is detected for frequencies below 9 cm^{-1}. The
superconducting penetration depth follows a T^2 behavior at low temperatures.Comment: 4 pages, 4 figure
Characterization of nonlinear effects in edge filters
A specially designed and produced edge filter with pronounced nonlinear
effects is carefully characterized. The nonlinear effects are estimated at the
intensities close to the laser-induced damage.Comment: 3 page
B-T phase diagram of CoCr2O4 in magnetic fields up to 14 T
We have measured the magnetization and specific heat of multiferroic CoCr2O4
in magnetic fields up to 14 T. The high-field magnetization measurements
indicate a new phase transition at T* = 5 - 6 K. The phase between T* and the
lock-in transition at 15 K is characterized by magnetic irreversibility. At
higher magnetic fields, the irreversibility increases. Specific-heat
measurements confirm the transition at T*, and also show irreversible behavior.
We construct a field-temperature phase diagram of CoCr2O4.Comment: 4 page
Gauge and parametrization dependence in higher derivative quantum gravity
The structure of counterterms in higher derivative quantum gravity is
reexamined. Nontrivial dependence of charges on the gauge and parametrization
is established. Explicit calculations of two-loop contributions are carried out
with the help of the generalized renormgroup method demonstrating consistency
of the results obtained.Comment: 22 pages, Latex, no figure
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