938 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
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
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
On the origin of interface states at oxide/III-nitride heterojunction interfaces
The energy spectrum of interface state density, D-it(E), was determined at oxide/III-N heterojunction interfaces in the entire band gap, using two complementary photo-electric methods: (i) photo-assisted capacitance-voltage technique for the states distributed near the midgap and the conduction band (CB) and (ii) light intensity dependent photo-capacitance method for the states close to the valence band (VB). In addition, the Auger electron spectroscopy profiling was applied for the characterization of chemical composition of the interface region with the emphasis on carbon impurities, which can be responsible for the interface state creation. The studies were performed for the AlGaN/GaN metal-insulator-semiconductor heterostructures (MISH) with Al2O3 and SiO2 dielectric films and AlxGa1-x layers with x varying from 0.15 to 0.4 as well as for an Al2O3/InAlN/GaN MISH structure. For all structures, it was found that: (i) D-it(E) is an U-shaped continuum increasing from the midgap towards the CB and VB edges and (ii) interface states near the VB exhibit donor-like character. Furthermore, D-it(E) for SiO2/AlxGa1-x/GaN structures increased with rising x. It was also revealed that carbon impurities are not present in the oxide/III-N interface region, which indicates that probably the interface states are not related to carbon, as previously reported. Finally, it was proven that the obtained D-it(E) spectrum can be well fitted using a formula predicted by the disorder induced gap state model. This is an indication that the interface states at oxide/III-N interfaces can originate from the structural disorder of the interfacial region. Furthermore, at the oxide/barrier interface we revealed the presence of the positive fixed charge (Q(F)) which is not related to D-it(E) and which almost compensates the negative polarization charge (Q(pol)(-))
Strong-Coupling Superconductivity of CeIrSi with the Non-centrosymmetric Crystal Structure
We studied the pressure-induced superconductor CeIrSi with the
non-centrosymmetric tetragonal structure under high pressure. The electrical
resistivity and ac heat capacity were measured in the same run for the same
sample. The critical pressure was determined to be = 2.25 GPa,
where the antiferromagnetic state disappears. The heat capacity
shows both antiferromagnetic and superconducting transitions at pressures close
to . On the other hand, the superconducting region is extended to
high pressures of up to about 3.5 GPa, with the maximum transition temperature
= 1.6 K around GPa. At 2.58 GPa, a large heat capacity
anomaly was observed at = 1.59 K. The jump of the heat capacity in
the form of is 5.7 0.1.
This is the largest observed value among previously reported superconductors,
indicating the strong-coupling superconductivity. The electronic specific heat
coefficient at is, however, approximately unchanged as a function
of pressure, even at .Comment: This paper will be published in J. Phys. Soc. Jpn. on the August
issue of 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
Nodal Structure of Unconventional Superconductors Probed by the Angle Resolved Thermal Transport Measurements
Over the past two decades, unconventional superconductivity with gap symmetry
other than s-wave has been found in several classes of materials, including
heavy fermion (HF), high-T_c, and organic superconductors. Unconventional
superconductivity is characterized by anisotropic superconducting gap
functions, which may have zeros (nodes) along certain directions in the
Brillouin zone. The nodal structure is closely related to the pairing
interaction, and it is widely believed that the presence of nodes is a
signature of magnetic or some other exotic, rather than conventional
phonon-mediated, pairing mechanism. Therefore experimental determination of the
gap function is of fundamental importance. However, the detailed gap structure,
especially the direction of the nodes, is an unresolved issue in most
unconventional superconductors. Recently it has been demonstrated that the
thermal conductivity and specific heat measurements under magnetic field
rotated relative to the crystal axes are a powerful method for determining the
shape of the gap and the nodal directions in the bulk. Here we review the
theoretical underpinnings of the method and the results for the nodal structure
of several unconventional superconductors, including borocarbide YNiBC,
heavy fermions UPdAl, CeCoIn, and PrOsSb, organic
superconductor, -(BEDT-TTF)Cu(NCS), and ruthenate
SrRuO, determined by angular variation of the thermal conductivity and
heat capacity.Comment: topical review, 55 pages, 35 figures. Figure quality has been reduced
for submission to cond-mat, higher quality figures available from the authors
or from the publishe
Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe
Unambiguous evidence for the microscopic coexistence of ferromagnetism and
superconductivity in UCoGe ( K and
0.6 K) is reported from Co nuclear quadrupole resonance (NQR). The
Co-NQR signal below 1 K indicates ferromagnetism throughout the sample
volume, while nuclear spin-lattice relaxation rate in the ferromagnetic
(FM) phase decreases below due to the opening of the
superconducting(SC) gap. The SC state was found to be inhomogeneous, suggestive
of a self-induced vortex state, potentially realizable in a FM superconductor.
In addition, the Co-NQR spectrum around show that the FM
transition in UCoGe possesses a first-order character, which is consistent with
the theoretical prediction that the low-temperature FM transition in itinerant
magnets is generically of first-order.Comment: 5 pages, 5 figure
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