890 research outputs found
Coexistence of Ferromagnetism and Superconductivity in Noncentrosymmetric Materials with Cubic Symmetry
This is a model study for the emergence of superconductivity in
ferromagnetically ordered phases of cubic materials whose crystal structure
lacks inversion symmetry. A Ginzburg-Landau-type theory is used to find the
ferromagnetic state and to determine the coupling of magnetic order to
superconductivity. It is found that noncentrosymmetricity evokes a helical
magnetic phase. If the wavelength of the magnetic order is long enough, it
gives rise to modulations of the order parameter of superconductivity, both in
modulus and complex phase. At magnetic domain walls the nucleation of
superconductivity is found to be suppressed as compared to the interior of
ferromagnetic domains.Comment: 5 pages, 2 figure
Thermodynamic Studies on Non Centrosymmetric Superconductors by AC Calorimetry under High Pressures
We investigated the non centrosymmetric superconductors CePtSi and UIr by
the ac heat capacity measurement under pressures. We determined the pressure
phase diagrams of these compounds. In CePtSi, the N\'{e}el temperature
= 2.2 K decreases with increasing pressure and becomes zero at the
critical pressure 0.6 GPa. On the other hand, the
superconducting phase exists in a wider pressure region from ambient pressure
to 1.5 GPa. The phase diagram of CePtSi is very
unique and has never been reported before for other heavy fermion
superconductors. In UIr, the heat capacity shows an anomaly at the Curie
temperature = 46 K at ambient pressure, and the heat capacity
anomaly shifts to lower temperatures with increasing pressure. The present
pressure dependence of was consistent with the previous studies by
the resistivity and magnetization measurements. Previous ac magnetic
susceptibility and resistivity measurements suggested the existence of three
ferromagnetic phases, FM1-3. shows a bending structure at 1.98,
2.21, and 2.40 GPa .The temperatures where these anomalies are observed are
close to the phase boundary of the FM3 phase.Comment: This paper was presented at the international workshop ``Novel
Pressure-induced Phenomena in Condensed Matter Systems(NP2CMS)" August 26-29
2006, Fukuoka Japa
Discovery of Li2(Pd,Pt)3B superconductors
Critical temperature Tc of the Li2(Pd1-xPtx)3B was reported to be 7-8K for
x=0 and 2.2-2.8K for x=1. In this article we present our preliminary results on
behavior of magnetization-temperature curves with starting composition of Pd-B
precursor, y-Li concentration in LiyPd3B and post-annealing of the Pd-end
compound. Results suggest that to maximize Tc ratio Pd:B should be close to
3:1, while y-Li has to be optimum. The lowest Tc for LiyPd3B was 4.4-4.6K,
while post-annealings at 560 deg. C allowed enhancement of Tc up to 8.2-8.4K.
Compositions Li2Z3B with Z=Ni, Ru, Rh, Re, Ag are not superconducting down to
1.8K. Exception is composition with Re showing superconductivity due to Re3B
compound. All samples were prepared by arc melting.Comment: 6 pages, 5 figs. presented at M2S, 200
Nonuniform Spin Triplet Superconductivity due to Antisymmetric Spin-Orbit Coupling in Noncentrosymmetric Superconductor CePtSi
We show that the nonuniform state (Fulde-Ferrel-Larkin-Ovchinnikov (FFLO)
state) of the spin triplet superconductivity in noncentrosymmetric systems is
stabilized by antisymmetric spin-orbit coupling even if the magnetic field is
absent. The transition temperature of the spin triplet superconductivity is
reduced by the antisymmetric spin-orbit coupling in general. This pair breaking
effect is shown to be similar to the Pauli pair breaking effect due to magnetic
field for the spin singlet superconductivity, in which FFLO state is stabilized
near the Pauli limit (or Chandrasekhar-Clogston limit) of external magnetic
field. Since there are gapless excitations in nonuniform superconducting state,
some physical quantities such as specific heat and penetration depth should
obey the power low temperature-dependences. We discuss the possibility of the
realization of nonuniform state in CePtSi.Comment: 8 pages, 6 figure
Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies
Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)
studies were performed in the recently discovered UCoGe, in which the
ferromagnetic and superconducting (SC) transitions were reported to occur at
K and K (N. T. Huy {\it et al.}, Phys.
Rev. Lett. {\bf 99} (2007) 067006), in order to investigate the coexistence of
ferromagnetism and superconductivity as well as the normal-state and SC
properties from a microscopic point of view. From the nuclear spin-lattice
relaxation rate and Knight-shift measurements, we confirmed that
ferromagnetic fluctuations which possess a quantum critical character are
present above and the occurrence of ferromagnetic transition at
2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal
state show that UCoGe is an itinerant ferromagnet similar to ZrZn and
YCo. The onset SC transition was identified at K, below
which of 30 % of the volume fraction starts to decrease due to the
opening of the SC gap. This component of , which follows a
dependence in the temperature range of K, coexists with the
magnetic components of showing a dependence below .
From the NQR measurements in the SC state, we suggest that the self-induced
vortex state is realized in UCoGe.Comment: 5 pages, 7 figures. submitted to J. Phys. Soc. Jpn. To appear in J.
Phys. Soc. Jp
Double-Exchange Ferromagnetism and Orbital-Fluctuation-Induced Superconductivity in Cubic Uranium Compounds
A double-exchange mechanism for the emergence of ferromagnetism in cubic
uranium compounds is proposed on the basis of a - coupling scheme. The
idea is {\it orbital-dependent duality} of electrons concerning itinerant
and localized states in the cubic structure. Since
orbital degree of freedom is still active in the ferromagnetic phase,
orbital-related quantum critical phenomenon is expected to appear. In fact,
odd-parity p-wave pairing compatible with ferromagnetism is found in the
vicinity of an orbital ordered phase. Furthermore, even-parity d-wave pairing
with significant odd-frequency components is obtained. A possibility to observe
such exotic superconductivity in manganites is also discussed briefly.Comment: 4 pages, 4 figures. To appear in J. Phys. Soc. Jp
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