100 research outputs found
Pseudospin excitations in coaxial nanotubes
In a 2DEG confined to two coaxial tubes the `tube degree of freedom' can be
described in terms of pseudospin-1/2 dynamics. The presence of tunneling
between the two tubes leads to a collective oscillation known as pseudospin
resonance. We employ perturbation theory to examine the dependence of the
frequency of this mode with respect to a coaxial magnetic field for the case of
small intertube distances. Coulomb interaction leads to a shift of the
resonance frequency and to a finite lifetime of the pseudospin excitations. The
presence of the coaxial magnetic field gives rise to pronounced peaks in the
shift of the resonance frequency. For large magnetic fields this shift vanishes
due to the effects of Zeeman splitting. Finally, an expression for the
linewidth of the resonance is derived. Numerical analysis of this expression
suggests that the linewidth strongly depends on the coaxial magnetic field,
which leads to several peaks of the linewidth as well as regions where damping
is almost completely suppressed.Comment: 11 pages, 7 figure
Probing topological transitions in HgTe/CdTe quantum wells by magneto-optical measurements
In two-dimensional topological insulators, such as inverted HgTe/CdTe quantum
wells, helical quantum spin Hall (QSH) states persist even at finite magnetic
fields below a critical magnetic field , above which only quantum
Hall (QH) states can be found. Using linear-response theory, we theoretically
investigate the magneto-optical properties of inverted HgTe/CdTe quantum wells,
both for infinite two-dimensional and finite-strip geometries, and possible
signatures of the transition between the QSH and QH regimes. In the absorption
spectrum, several peaks arise due to non-equidistant Landau levels in both
regimes. However, in the QSH regime, we find an additional absorption peak at
low energies in the finite-strip geometry. This peak arises due to the presence
of edge states in this geometry and persists for any Fermi level in the QSH
regime, while in the QH regime the peak vanishes if the Fermi level is situated
in the bulk gap. Thus, by sweeping the gate voltage, it is possible to
experimentally distinguish between the QSH and QH regimes due to this
signature. Moreover, we investigate the effect of spin-orbit coupling and
finite temperature on this measurement scheme.Comment: 14 pages, 13 figure
Excitonic Stark effect in MoS monolayers
We theoretically investigate excitons in MoS monolayers in an applied
in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations
predict a quadratic Stark shift, of the order of a few meV for fields of 10
V/m, in the linear absorption spectra. The spectral weight of the main
exciton peaks decreases by a few percent with an increasing electric field due
to the exciton field ionization into free carriers as reflected in the exciton
wave functions. Subpicosecond exciton decay lifetimes at fields of a few tens
of V/m could be utilized in solar energy harvesting and photodetection. We
find simple scaling relations of the exciton binding, radius, and oscillator
strength with the dielectric environment and an electric field, which provides
a path to engineering the MoS electro-optical response.Comment: 9 pages, 7 figure
Monte Carlo simulations of the inside-intron recombination
Biological genomes are divided into coding and non-coding regions. Introns
are non-coding parts within genes, while the remaining non-coding parts are
intergenic sequences. To study the evolutionary significance of recombination
inside introns we have used two models based on the Monte Carlo method. In our
computer simulations we have implemented the internal structure of genes by
declaring the probability of recombination between exons. One situation when
inside-intron recombination is advantageous is recovering functional genes by
combining proper exons dispersed in the genetic pool of the population after a
long period without selection for the function of the gene. Populations have to
pass through the bottleneck, then. These events are rather rare and we have
expected that there should be other phenomena giving profits from the
inside-intron recombination. In fact we have found that inside-intron
recombination is advantageous only in the case when after recombination,
besides the recombinant forms, parental haplotypes are available and selection
is set already on gametes.Comment: 12 pages inc. 5 Figs., for Int. J. Mod. Phys. C 17, issue 4 (2006
Theory of thermal spin-charge coupling in electronic systems
The interplay between spin transport and thermoelectricity offers several
novel ways of generating, manipulating, and detecting nonequilibrium spin in a
wide range of materials. Here we formulate a phenomenological model in the
spirit of the standard model of electrical spin injection to describe the
electronic mechanism coupling charge, spin, and heat transport and employ the
model to analyze several different geometries containing ferromagnetic (F) and
nonmagnetic (N) regions: F, F/N, and F/N/F junctions which are subject to
thermal gradients. We present analytical formulas for the spin accumulation and
spin current profiles in those junctions that are valid for both tunnel and
transparent (as well as intermediate) contacts. For F/N junctions we calculate
the thermal spin injection efficiency and the spin accumulation induced
nonequilibrium thermopower. We find conditions for countering thermal spin
effects in the N region with electrical spin injection. This compensating
effect should be particularly useful for distinguishing electronic from other
mechanisms of spin injection by thermal gradients. For F/N/F junctions we
analyze the differences in the nonequilibrium thermopower (and chemical
potentials) for parallel and antiparallel orientations of the F magnetizations,
as evidence and a quantitative measure of the spin accumulation in N.
Furthermore, we study the Peltier and spin Peltier effects in F/N and F/N/F
junctions and present analytical formulas for the heat evolution at the
interfaces of isothermal junctions.Comment: to be published in PRB (in press), 19 pages, 19 figure
The inverse-Compton ghost HDF 130 and the giant radio galaxy 6C 0905+3955: matching an analytic model for double radio source evolution
We present new GMRT observations of HDF 130, an inverse-Compton (IC) ghost of
a giant radio source that is no longer being powered by jets. We compare the
properties of HDF 130 with the new and important constraint of the upper limit
of the radio flux density at 240 MHz to an analytic model. We learn what values
of physical parameters in the model for the dynamics and evolution of the radio
luminosity and X-ray luminosity (due to IC scattering of the cosmic microwave
background (CMB)) of a Fanaroff-Riley II (FR II) source are able to describe a
source with features (lobe length, axial ratio, X-ray luminosity, photon index
and upper limit of radio luminosity) similar to the observations. HDF 130 is
found to agree with the interpretation that it is an IC ghost of a powerful
double-lobed radio source, and we are observing it at least a few Myr after jet
activity (which lasted 5--100 Myr) has ceased. The minimum Lorentz factor of
injected particles into the lobes from the hotspot is preferred to be
for the model to describe the observed quantities well,
assuming that the magnetic energy density, electron energy density, and lobe
pressure at time of injection into the lobe are linked by constant factors
according to a minimum energy argument, so that the minimum Lorentz factor is
constrained by the lobe pressure. We also apply the model to match the features
of 6C 0905+3955, a classical double FR II galaxy thought to have a low-energy
cutoff of in the hotspot due to a lack of hotspot
inverse-Compton X-ray emission. The models suggest that the low-energy cutoff
in the hotspots of 6C 0905+3955 is , just slightly above
the particles required for X-ray emission.Comment: 9 pages, 3 figure
The Galaxy Cluster Luminosity-Temperature Relationship and Iron Abundances - A Measure of Formation History ?
Both the X-ray luminosity-temperature (L-T) relationship and the iron
abundance distribution of galaxy clusters show intrinsic dispersion. Using a
large set of galaxy clusters with measured iron abundances we find a
correlation between abundance and the relative deviation of a cluster from the
mean L-T relationship. We argue that these observations can be explained by
taking into account the range of cluster formation epochs expected within a
hierarchical universe. The known relationship of cooling flow mass deposition
rate to luminosity and temperature is also consistent with this explanation.
From the observed cluster population we estimate that the oldest clusters
formed at z>~2. We propose that the iron abundance of a galaxy cluster can
provide a parameterization of its age and dynamical history.Comment: 13 pages Latex, 2 figures, postscript. Accepted for publication in
ApJ Letter
An XMM-Newton observation of the massive, relaxed galaxy cluster ClJ1226.9+3332 at z=0.89
A detailed X-ray analysis of an XMM-Newton observation of the high-redshift
(z=0.89) galaxy cluster ClJ1226.9+3332 is presented. The X-ray temperature is
found to be 11.5{+1.1}{-0.9}keV, the highest X-ray temperature of any cluster
at z>0.6. In contrast to MS1054-0321, the only other very hot cluster currently
known at z>0.8, ClJ1226.9+3332 features a relaxed X-ray morphology, and its
high overall gas temperature is not caused by one or several hot spots. The
system thus constitutes a unique example of a high redshift, high temperature,
relaxed cluster, for which the usual hydrostatic equilibrium assumption, and
the X-ray mass is most reliable. A temperature profile is constructed (for the
first time at this redshift) and is consistent with the cluster being
isothermal out to 45% of the virial radius. Within the virial radius
(corresponding to a measured overdensity of a factor of 200), a total mass of
(1.4+/-0.5)*10^15 M_solar is derived, with a gas mass fraction of 12+/-5%. The
bolometric X-ray luminosity is (5.3+/-0.2)*10^45 erg/s. The probabilities of
finding a cluster of this mass within the volume of the discovery X-ray survey
are 8*10^{-5} for Omega_M=1 and 0.64 for Omega_M=0.3, making Omega_M=1 highly
unlikely. The entropy profile suggests that entropy evolution is being
observed. The metal abundance (of Z=0.33{+0.14}{-0.10} Z_solar), gas mass
fraction, and gas distribution are consistent with those of local clusters;
thus the bulk of the metals were in place by z=0.89.Comment: 13 pages, 8 figures. Accepted for publication in MNRA
Common nonlinear features and spin-orbit coupling effects in the Zeeman splitting of novel wurtzite materials
The response of semiconductor materials to external magnetic fields is a reliable approach to probe intrinsic electronic and spin-dependent properties. In this study, we investigate the common Zeeman splitting features of novel wurtzite materials, namely, InP, InAs, and GaAs. We present values for the effective g factors of different energy bands and show that spin-orbit coupling effects, responsible for the spin splittings, also have noticeable contributions to the g factors. Within the Landau level picture, we show that the nonlinear Zeeman splitting recently explained in magnetophotoluminescence experiments for InP nanowires by D. Tedeschi et al. [Phys. Rev. B 99, 161204 (2019)] is also present in InAs, GaAs, and even the conventional GaN. Such nonlinear features stem from the peculiar coupling of the A and B valence bands as a consequence of the interplay between the wurtzite crystal symmetry and the breaking of time-reversal symmetry by the external magnetic field. Moreover, we develop an analytical model to describe the experimental nonlinear Zeeman splitting and apply it to InP and GaAs data. Extrapolating our fitted results, we found that the Zeeman splitting of InP reaches a maximum value, which is a prediction that could be probed at higher magnetic fields
Hydrodynamic simulations of merging clusters of galaxies
We present the results of high-resolution AP3M+SPH simulations of merging clusters of galaxies. We find that the compression and shocking of the core gas during a merger can lead to large increases in bolometric X-ray luminosities and emission-weighted temperatures of clusters. Cooling flows are completely disrupted during equal-mass mergers, with the mass deposition rate dropping to zero as the cores of the clusters collide. The large increase in the cooling time of the core gas strongly suggests that cooling flows will not recover from such a merger within a Hubble time. Mergers with subclumps having one eighth of the mass of the main cluster are also found to disrupt a cooling flow if the merger is head-on. However, in this case the entropy injected into the core gas is rapidly radiated away and the cooling flow restarts within a few Gyr of the merger. Mergers in which the subcluster has an impact parameter of 500 kpc do not disrupt the cooling flow, although the mass deposition rate is reduced by ∼30 per cent. Finally, we find that equal mass, off-centre mergers can effectively mix gas in the cores of clusters, while head on mergers lead to very little mixing. Gas stripped from the outer layers of subclumps results in parts of the outer layers of the main cluster being well mixed, although they have little effect on the gas in the core of the cluster. None of the mergers examined here resulted in the intracluster medium being well mixed globally
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