11,484 research outputs found
High-temperature ferroelectric order and magnetic field-cooled effect driven magnetoelectric coupling in R2BaCuO5 (R= Er, Dy, Sm)
The high-temperature ferroelectric order and a remarkable magnetoelectric
effect driven by the magnetic field cooling are reported in R2BaCuO5 (R = Er,
Dy, Sm) series. The ferroelectric (FE) orders are observed at much higher
temperatures than their magnetic orders for all three members. The value of FE
Curie temperature (TFE) is considerably high as ~ 235 K with the polarization
value (P) of ~ 1410 {\mu}C/m2 for a 4 kV/cm poling field in case of Er2BaCuO5,
whereas the values of TFE and P are also promising as ~ 232 K and ~ 992
{\mu}C/m2 for Dy2BaCuO5, and ~ 184 K and ~ 980 {\mu}C/m2 for Sm2BaCuO5. The
synchrotron diffraction studies of Dy2BaCuO5 confirm a structural transition at
TFE to a polar Pna21 structure, which correlates the FE order. An unusual
magnetoelectric coupling is observed below the R order for Er and Dy compounds
and below the Cu order for Sm compound, when the pyroelectric current is
recorded only with the magnetic field both in heating and cooling cycles i.e.
typical magnetic field cooled effect. The magnetic field cooled effect driven
emergence of polarization is ferroelectric in nature, as it reverses due to the
opposite poling field. The unexplored R2BaCuO5 series attracts the community
for large TFE, high P value, and strange magnetoelectric consequences.Comment: 9 figures and 2 supporting figure
Preferential antiferromagnetic coupling of vacancies in graphene on SiO_2: Electron spin resonance and scanning tunneling spectroscopy
Monolayer graphene grown by chemical vapor deposition and transferred to
SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic
energy of 50 eV. The density of defects visible in scanning tunneling
microscopy (STM) is considerably lower than the ion fluence implying that most
of the defects are single vacancies. The vacancies are characterized by
scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak
close to the Fermi level. The peak persists after air exposure up to 180 min,
albeit getting broader. After air exposure for less than 60 min, electron spin
resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a
signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2
spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian
of equal weight exhibiting a width down to 0.17 mT, which, however, depends on
details of the sample preparation. The g-factor anisotropy is about 0.02%.
Temperature dependent measurements reveal antiferromagnetic correlations with a
Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of
graphene is significantly reduced by ion bombardment, the spin resonance
induced change in conductivity is below 10^{-5}.Comment: 10 pages, 5 figures, discussion on STM images in the literature of
defects in graphene adde
Evidence for coordinated induction and repression of ecto-5'-nucleotidase (CD73) and the A2a adenosine receptor in a human B cell line
In the human B cell line P493-6 two mitogenic signals, the EpsteinBarr virus nuclear antigen 2 (EBNA2) and myc, can be independently regulated by means of an estrogen receptor fusion construct or an inducible expression vector, respectively. Shut off of EBNA2, either in the presence or absence of myc, leads to a significant increase in enzymatic activity and surface expression of ecto-5nucleotidase (CD73) as well as an increased adenosine receptor response in cyclic AMP formation. Shut off of myc expression has a small additional positive effect on CD73 activity. Among the four different subtypes of adenosine receptors, the A2a receptor exclusively is subject to regulation in this system, which is substantiated by pharmacologic data (specific agonists and inhibitors), as well as on the mRNA level. With upregulated CD73 and A2a, cells also respond to 5AMP with increased cyclic AMP formation. Turn on of EBNA2 has the reverse effect of repression of CD73 and A2a expression. The time course of both induction and repression of CD73 and A2a is rather slow
Analysis of the exciton-exciton interaction in semiconductor quantum wells
The exciton-exciton interaction is investigated for quasi-two-dimensional
quantum structures. A bosonization scheme is applied including the full spin
structure. For generating the effective interaction potentials, the
Hartree-Fock and Heitler-London approaches are improved by a full two-exciton
calculation which includes the van der Waals effect. With these potentials the
biexciton formation in bilayer systems is investigated. For coupled quantum
wells the two-body scattering matrix is calculated and employed to give a
modified relation between exciton density and blue shift. Such a relation is of
central importance for gauging exciton densities in experiments which pave the
way toward Bose-Einstein condensation of excitons
Spin dynamics and magnetic interactions of Mn dopants in the topological insulator BiTe
The magnetic and electronic properties of the magnetically doped topological
insulator BiMnTe were studied using electron spin
resonance (ESR) and measurements of static magnetization and electrical
transport. The investigated high quality single crystals of BiMnTe show a ferromagnetic phase transition for
at K. The Hall measurements reveal a p-type finite
charge-carrier density. Measurements of the temperature dependence of the ESR
signal of Mn dopants for different orientations of the external magnetic field
give evidence that the localized Mn moments interact with the mobile charge
carriers leading to a Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling
between the Mn spins of order 2-3 meV. Furthermore, ESR reveals a
low-dimensional character of magnetic correlations that persist far above the
ferromagnetic ordering temperature
Anomalous Phase Transition in Strained SrTiO Thin Films
We have studied the cubic to tetragonal phase transition in epitaxial
SrTiO films under various biaxial strain conditions using synchrotron X-ray
diffraction. Measuring the superlattice peak associated with TiO octahedra
rotation in the low temperature tetragonal phase indicates the presence of a
phase transition whose critical temperature is a strong function of strain,
with T as much as 50K above the corresponding bulk temperature.
Surprisingly, the lattice constants evolve smoothly through the transition with
no indication of a phase change. This signals an important change in the nature
of the phase transition due to the epitaxy strain and substrate clamping
effect. The internal degrees of freedom (TiO rotations) have become
uncoupled from the overall lattice shape.Comment: 4 pages, 3 figures, REVTeX
Enhanced charge stripe order of superconducting La(2-x)Ba(x)CuO(4) in a magnetic field
The effect of a magnetic field on the charge stripe order in
La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray
diffraction for charge carrier concentrations ranging from strongly underdoped
to optimally doped. We find that charge stripe order can be significantly
enhanced by a magnetic field applied along the c-axis, but only at temperatures
and dopings where it coexists with bulk superconductivity at zero field. The
field also increases stripe correlations between the planes, which can result
in an enhanced frustration of the interlayer Josephson coupling. Close to the
famous x=1/8 compound, where zero field stripe order is pronounced and bulk
superconductivity is suppressed, charge stripe order is independent of a
magnetic field. The results imply that static stripe order and
three-dimensionally coherent superconductivity are competing ground states.Comment: 6 pages, 4 figure
Excitonic BCS-BEC crossover at finite temperature: Effects of repulsion and electron-hole mass difference
The BCS to Bose-Einstein condensation (BEC) crossover of electron-hole (e-h)
pairs in optically excited semiconductors is studied using the two-band Hubbard
model with both repulsive and attractive interactions. Applying the
self-consistent t-matrix approximation combined with a local approximation, we
examine the properties of a normal phase and an excitonic instability. The
transition temperature from the normal phase to an e-h pair condensed one is
studied to clarify the crossover from an e-h BCS-like state to an excitonic
Bose-Einstein condensation, which takes place on increasing the e-h attraction
strength. To investigate effects of the repulsive interaction and the e-h mass
difference, we calculate the transition temperature for various parameters of
the interaction strengths, the e-h particle density, and the mass difference.
While the transition temperature in the e-h BCS regime is sufficiently
suppressed by the repulsive interaction, that of the excitonic BEC is largely
insensitive to it. We also show quantitatively that in the whole regime the
mass difference leads to large suppression of the transition temperature.Comment: 8 pages, 7 figures, to be published in Phys. Rev.
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