266 research outputs found
Superconducting Properties of MgB2 Bulk Materials Prepared by High Pressure Sintering
High-density bulk materials of a newly discovered 40K intermetallic MgB2
superconductor were prepared by high pressure sintering. Superconducting
transition with the onset temperature of 39K was confirmed by both magnetic and
resistive measurements. Magnetization versus field (M-H) curve shows the
behavior of a typical Type II superconductor and the lower critical field
Hc1(0) estimated from M-H curve is 0.032T. The bulk sample shows good
connection between grains and critical current density Jc estimated from the
magnetization hysteresis using sample size was 2x104A/cm2 at 20K and 1T. Upper
critical field Hc2(0) determined by extrapolating the onset of resistive
transition and assuming a dirty limit is 18T.Comment: 3Pages PD
Gigantic terahertz magnetochromism via electromagnons in hexaferrite magnet BaMgFeO
Effects of temperature (6--225 K) and magnetic field (0--7 T) on the
low-energy (1.2--5 meV) electrodynamics of the electromagnon, the magnetic
resonance driven by the light electric field, have been investigated for a
hexaferrite magnet BaMgFeO by using terahertz time-domain
spectroscopy. We find the gigantic terahertz magnetochromism via
electromagnons; the magnetochromic change, as defined by the difference of the
absorption intensity with and without magnetic field, exceeds 500% even at 0.6
T. The results arise from the fact that the spectral intensity of the
electromagnon critically depends on the magnetic structure. With changing the
conical spin structures in terms of the conical angle from the proper
screw () to the ferrimagnetic () through the
conical spin-ordered phases () by external magnetic
fields, we identify the maximal magnetochromism around .
On the contrary, there is no remarkable signature of the electromagnon in the
proper screw and spin-collinear (ferrimagnetic) phases, clearly indicating the
important role of the conical spin order to produce the
magnetically-controllable electromagnons. The possible origin of this
electromagnon is argued in terms of the exchange-striction mechanism.Comment: 19 pages including 7 figures; Accepted for publication in Phys. Rev.
Hydrogen-assisted laser-induced resonant transitions between metastable states of antiprotonic helium atoms
Laser resonance transitions between normally metastable states of antiprotonic helium atoms were observed making use of state dependent quenching effects caused by small admixtures of \htwo\ molecules. By selectively shortening the lifetimes of states with higher principal quantum number as compared to those of lower , this method for the first time provides access to all initially populated metastable states of \pbar\hep\ atoms. This was demonstrated by observing the transitions and
Superconductivity in Cu_xTiSe_2
Charge density waves (CDWs) are periodic modulations of the conduction
electron density in solids. They are collective states that arise from
intrinsic instabilities often present in low dimensional electronic systems.
The layered dichalcogenides are the most well-studied examples, with TiSe_2 one
of the first CDW-bearing materials known. The competition between CDW and
superconducting collective electronic states at low temperatures has long been
held and explored, and yet no chemical system has been previously reported
where finely controlled chemical tuning allows this competition to be studied
in detail. Here we report how, upon controlled intercalation of TiSe_2 with Cu
to yield Cu_xTiSe_2, the CDW transition is continuously suppressed, and a new
superconducting state emerges near x = 0.04, with a maximum T_c of 4.15 K found
at x = 0.08. Cu_xTiSe_2 thus provides the first opportunity to study the CDW to
Superconductivity transition in detail through an easily-controllable chemical
parameter, and will provide new insights into the behavior of correlated
electron systems.Comment: Accepted to Nature Physic
Flat-band ferromagnetism in quantum dot superlattices
Possibility of flat-band ferromagnetism in quantum dot arrays is
theoretically discussed. By using a quantum dot as a building block, quantum
dot superlattices are possible. We consider dot arrays on Lieb and kagome
lattices known to exhibit flat band ferromagnetism. By performing an exact
diagonalization of the Hubbard Hamiltonian, we calculate the energy difference
between the ferromagnetic ground state and the paramagnetic excited state, and
discuss the stability of the ferromagnetism against the second nearest neighbor
transfer. We calculate the dot-size dependence of the energy difference in a
dot model and estimate the transition temperature of the
ferromagnetic-paramagnetic transition which is found to be accessible within
the present fabrication technology. We point out advantages of semiconductor
ferromagnets and suggest other interesting possibilities of electronic
properties in quantum dot superlattices.Comment: 15 pages, 7 figures (low resolution). High-resolution figures are
available at
http://www.brl.ntt.co.jp/people/tamura/Research/PublicationPapers.htm
Quenching of metastable states of antiprotonic helium atoms by collisions with H molecules
Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of \mbox{H}\ to the target helium gas. With this method of ``\mbox{H}-assisted inverse resonances'' the decay rates of the states and of \mbox{}\mbox{He}\ were determined as a function of the \mbox{H}\ admixture. The quenching cross sections at 30~K deduced therefrom for the states with were found to be of the order of the geometrical cross section for \mbox{}\mbox{He}-\mbox{H}\ collisions (~cm), with a moderate decrease with increasing . Within a given cascade with constant , the quenching cross sections for states with are smaller by a factor of 4--6 than those for states with
Laser-induced resonant transitions in the v=n-l-1=2 and 3 metastable cascades of antiprotonic $^{3}He atoms
Laser-induced resonant transitions in metastable antiprotonic ^3He atoms have been found. The observed transitions at wavelengths 593.388 \pm 0.001~nm and at 463.947 \pm 0.002~nm have been respectively ascribed to the (n,l)~=~(38,34)\,\rightarrow\,(37,33) and the (36 33)\,\rightarrow\,(35,32) transitions
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