343 research outputs found
A seamless trespass: germ cell migration across the seminiferous epithelium during spermatogenesis
During spermatogenesis, preleptotene spermatocytes traverse the bloodâtestis barrier (BTB) in the seminiferous epithelium, which is reminiscent of viral pathogens breaking through the tight junctions of host epithelial cells. The process also closely resembles the migration of leukocytes across endothelial tight junctions to reach inflammation sites. Cell adhesion molecules of the immunoglobulin superfamily (e.g., JAM/CAR/nectin) participate in germ cell migration by conferring transient adhesion between Sertoli and germ cells through homophilic and heterophilic interactions. The same molecules also comprise the junctional complexes at the BTB. Interestingly, JAM/CAR/nectin molecules mediate virus uptake and leukocyte transmigration in strikingly similar manners. It is likely that the strategy used by viruses and leukocytes to break through junctional barriers is used by germ cells to open up the interâSertoli cell junctions. In associating these diverse cellular events, we highlight the âguidingâ role of JAM/CAR/nectin molecules for germ cell passage. Knowledge on viral invasion and leukocyte transmigration has also shed insights into germ cell movement during spermatogenesis
Quantization of adiabatic pumped charge in the presence of superconducting lead
We investigate the parametric electron pumping of a double barrier structure
in the presence of a superconducting lead. The parametric pumping is
facilitated by cyclic variation of the barrier heights and of the
barriers. In the weak coupling regime, there exists a resonance line in the
parameter space so that the energy of the quasi-bound state is in
line with the incoming Fermi energy. Levinson et al found recently that the
pumped charge for each pumping cycle is quantized with for normal
structure when the pumping contour encircles the resonance line. In the
presence of a superconducting lead, we find that the pumped charge is quantized
with the value
Optimal quantum pump in the presence of a superconducting lead
We investigate the parametric pumping of a hybrid structure consisting of a
normal quantum dot, a normal lead and a superconducting lead. Using the time
dependent scattering matrix theory, we have derived a general expression for
the pumped electric current and heat current. We have also derived the
relationship among the instantaneous pumped heat current, electric current, and
shot noise. This gives a lower bound for the pumped heat current in the hybrid
system similar to that of the normal case obtained by Avron et al
Magnetocaloric effect and magnetostructural coupling in Mn0.92Fe0.08CoGe compound
The structural properties of Mn0.92Fe0.08CoGe have been investigated in detail using synchrotron x-ray diffraction in zero and applied pressure (p = 0-10 GPa). A ferromagnetic transition occurs around TC = 300 K and a large magnetic-entropy change -ÎSM = 17.3 J/kg K detected at TC for a field change of ÎB = 5 T. The field dependence of -ÎSM max can be expressed as -ÎSM max â B. At ambient temperature and pressure, Mn0.92Fe0.08CoGe exhibits a co-existence of the orthorhombic TiNiSi-type structure (space group Pnma) and hexagonal Ni2In-type structure (space group P63/mmc). Application of external pressure drives a structure change from the orthorhombic TiNiSi-type structure to the hexagonal Ni2In-type structure. A large anomaly in heat capacity around TC is detected and the Debye temperature ΞD (=319(±10) K) has been derived from analyses of the low temperature heat capacity, T âČ 10 K
Entangled quantum heat engines based on two two-spin systems with Dzyaloshinski-Moriya anisotropic antisymmetric interaction
We construct an entangled quantum heat engine (EQHE) based on two two-spin
systems with Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction.
By applying the explanations of heat transferred and work performed at the
quantum level in Kieu's work [PRL, 93, 140403 (2004)], the basic thermodynamic
quantities, i.e., heat transferred, net work done in a cycle and efficiency of
EQHE are investigated in terms of DM interaction and concurrence. The validity
of the second law of thermodynamics is confirmed in the entangled system. It is
found that there is a same efficiency for both antiferromagnetic and
ferromagnetic cases, and the efficiency can be controlled in two manners: 1.
only by spin-spin interaction J and DM interaction D; 2. only by the
temperature T and concurrence C. In order to obtain a positive net work, we
need not entangle all qubits in two two-spin systems and we only require the
entanglement between qubits in a two-spin system not be zero. As the ratio of
entanglement between qubits in two two-spin systems increases, the efficiency
will approach infinitely the classical Carnot one. An interesting phenomenon is
an abrupt transition of the efficiency when the entanglements between qubits in
two two-spin systems are equal
Measurements of J/psi Decays into 2(pi+pi-)eta and 3(pi+pi-)eta
Based on a sample of 5.8X 10^7 J/psi events taken with the BESII detector,
the branching fractions of J/psi--> 2(pi+pi-)eta and J/psi-->3(pi+pi-)eta are
measured for the first time to be (2.26+-0.08+-0.27)X10^{-3} and
(7.24+-0.96+-1.11)X10^{-4}, respectively.Comment: 11 pages, 6 figure
Non-adiabatic charge pump: an exact solution
We derived a general and exact expression of current for quantum parametric
charge pumps in the non-adiabatic regime at finite pumping frequency and finite
driving amplitude. The non-perturbative theory predicts a remarkable plateau
structure in the pumped current due to multi-photon assisted processes in a
double-barrier quantum well pump involving only a {\it single} pumping
potential. It also predicts a current reversal as the resonant level of the
pump crosses the Fermi energy of the leads
Search for the Rare Decays J/Psi --> Ds- e+ nu_e, J/Psi --> D- e+ nu_e, and J/Psi --> D0bar e+ e-
We report on a search for the decays J/Psi --> Ds- e+ nu_e + c.c., J/Psi -->
D- e+ nu_e + c.c., and J/Psi --> D0bar e+ e- + c.c. in a sample of 5.8 * 10^7
J/Psi events collected with the BESII detector at the BEPC. No excess of signal
above background is observed, and 90% confidence level upper limits on the
branching fractions are set: B(J/Psi --> Ds- e+ nu_e + c.c.)<4.8*10^-5, B(J/Psi
--> D- e+ nu_e + c.c.) D0bar e+ e- + c.c.)<1.1*10^-5Comment: 10 pages, 4 figure
Measurements of the observed cross sections for exclusive light hadron production in e^+e^- annihilation at \sqrt{s}= 3.773 and 3.650 GeV
By analyzing the data sets of 17.3 pb taken at GeV
and 6.5 pb taken at GeV with the BESII detector at the
BEPC collider, we have measured the observed cross sections for 12 exclusive
light hadron final states produced in annihilation at the two energy
points. We have also set the upper limits on the observed cross sections and
the branching fractions for decay to these final states at 90%
C.L.Comment: 8 pages, 5 figur
BESII Detector Simulation
A Monte Carlo program based on Geant3 has been developed for BESII detector
simulation. The organization of the program is outlined, and the digitization
procedure for simulating the response of various sub-detectors is described.
Comparisons with data show that the performance of the program is generally
satisfactory.Comment: 17 pages, 14 figures, uses elsart.cls, to be submitted to NIM
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