277 research outputs found
Entropy per particle spikes in the transition metal dichalcogenides
We derive a general expression for the entropy per particle as a function of
chemical potential, temperature and gap magnitude for the single layer
transition metal dichalcogenides. The electronic excitations in these materials
can be approximately regarded as two species of the massive or gapped Dirac
fermions. Inside the smaller gap there is a region with zero density of states
where the dependence of the entropy per particle on the chemical potential
exhibits a huge dip-and-peak structure. The edge of the larger gap is
accompanied by the discontinuity of the density of states that results in the
peak in the dependence of the entropy per particle on the chemical potential.
The specificity of the transition metal dichalcogenides makes possible the
observation of these features at rather high temperatures order of 100 K. The
influence of the uniaxial strain on the entropy per particle is discussed.Comment: 6 pages, 4 figures; Special Issue to the 90th birthday of A.A.
Abrikoso
Spherical functions on the de Sitter group
Matrix elements and spherical functions of irreducible representations of the
de Sitter group are studied on the various homogeneous spaces of this group. It
is shown that a universal covering of the de Sitter group gives rise to
quaternion Euler angles. An explicit form of Casimir and Laplace-Beltrami
operators on the homogeneous spaces is given. Different expressions of the
matrix elements and spherical functions are given in terms of multiple
hypergeometric functions both for finite-dimensional and unitary
representations of the principal series of the de Sitter group.Comment: 40 page
The Ferromagnetism in the Vicinity of Lifshitz Topological Transitions
We show that the critical temperature of a ferromagnetic phase transition in
a quasi-two-dimensional hole gas confined in a diluted magnetic semiconductor
quantum well strongly depends on the hole chemical potential and hole density.
The significant variations of the the Curie temperature occur close to the
Lifshitz topological transition points where the hole Fermi surface acquires
additional components of topological connectivity due to the filling of excited
size-quantization subbands. The model calculations demonstrate that the Curie
temperature can be doubled by a small variation of the gate voltage for the
CdMnTe/CdMgTe quantum well based device
Detection of topological phase transitions through entropy measurements: the case of germanene
We propose a characterization tool for studies of the band structure of new
materials promising for the observation of topological phase transitions. We
show that a specific resonant feature in the entropy per electron dependence on
the chemical potential may be considered as a fingerprint of the transition
between topological and trivial insulator phases. The entropy per electron in a
honeycomb two-dimensional crystal of germanene subjected to the external
electric field is obtained from the first principle calculation of the density
of electronic states and the Maxwell relation. We demonstrate that, in
agreement to the recent prediction of the analytical model, strong spikes in
the entropy per particle dependence on the chemical potential appear at low
temperatures. They are observed at the values of the applied bias both below
and above the critical value that corresponds to the transition between the
topological insulator and trivial insulator phases, while the giant resonant
feature in the vicinity of zero chemical potential is strongly suppressed at
the topological transition point, in the low temperature limit. In a wide
energy range, the van Hove singularities in the electronic density of states
manifest themselves as zeros in the entropy per particle dependence on the
chemical potential.Comment: 8 pages, 5 figures; final version published in PR
New experimental limits on neutron - mirror neutron oscillations in the presence of mirror magnetic field
Present probes do not exclude that the neutron () oscillation into mirror
neutron (), a sterile state exactly degenerate in mass with the neutron,
can be a very fast process, in fact faster than the neutron decay itself. This
process is sensitive to the magnetic field. Namely, if the mirror magnetic
field exists at the Earth, oscillation probability can be
suppressed or resonantly amplified by the applied magnetic field ,
depending on its strength and on the angle between and
. We present the results of ultra-cold neutron storage measurements
aiming to check the anomalies observed in previous experiments which could be a
signal for oscillation in the presence of mirror magnetic field ~G. Analyzing the experimental data on neutron loses, we obtain a new lower
limit on oscillation time s (95 % C.L.) for any
between 0.08 and 0.17 G, and s (95 % C.L.)
for any in the interval () G
Self-consistent calculation of nuclear photoabsorption cross section: Finite amplitude method with Skyrme functionals in the three-dimensional real space
The finite amplitude method (FAM), which we have recently proposed (T.
Nakatsukasa, T. Inakura, and K. Yabana, Phys. Rev. C 76, 024318 (2007)),
simplifies significantly the fully self-consistent RPA calculation. Employing
the FAM, we are conducting systematic, fully self-consistent response
calculations for a wide mass region. This paper is intended to present a
computational scheme to be used in the systematic investigation and to show the
performance of the FAM for a realistic Skyrme energy functional. We implemented
the method in the mixed representation in which the forward and backward RPA
amplitudes are represented by indices of single-particle orbitals for occupied
states and the spatial grid points for unoccupied states. We solve the linear
response equation for a given frequency. The equation is a linear algebraic
problem with a sparse non-hermitian matrix, which is solved with an iterative
method. We show results of the dipole response for selected spherical and
deformed nuclei. The peak energies of the giant dipole resonance agree well
with measurements for heavy nuclei, while they are systematically
underestimated for light nuclei. We also discuss the width of the giant dipole
resonance in the fully self-consistent RPA calculation.Comment: 11 pages, 10 figure
UCN anomalous losses and the UCN capture cross-section on material defects
Experimental data shows anomalously large Ultra Cold Neutrons (UCN)
reflection losses and that the process of UCN reflection is not completely
coherent. UCN anomalous losses under reflection cannot be explained in the
context of neutron optics calculations. UCN losses by means of incoherent
scattering on material defects are considered and cross-section values
calculated. The UCN capture cross-section on material defects is enhanced by a
factor of 10^4 due to localization of UCN around defects. This phenomenon can
explain anomalous losses of UCN.Comment: 13 pages, 4 figure
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