1,165 research outputs found
Universal behavior at discontinuous quantum phase transitions
Discontinuous quantum phase transitions besides their general interest are
clearly relevant to the study of heavy fermions and magnetic transition metal
compounds. Recent results show that in many systems belonging to these classes
of materials, the magnetic transition changes from second order to first order
as they approach the quantum critical point (QCP). We investigate here some
mechanisms that may be responsible for this change. Specifically the coupling
of the order parameter to soft modes and the competition between different
types of order near the QCP. For weak first order quantum phase transitions
general results are obtained. In particular we describe the thermodynamic
behavior at this transition when it is approached from finite temperatures.
This is the discontinuous equivalent of the non-Fermi liquid trajectory close
to a conventional QCP in a heavy fermion material.Comment: 7 pages, 3 figure
Orbital degeneracy as a source of frustration in LiNiO
Motivated by the absence of cooperative Jahn-Teller effect and of magnetic
ordering in LiNiO, a layered oxide with triangular planes, we study a
general spin-orbital model on the triangular lattice. A mean-field approach
reveals the presence of several singlet phases between the SU(4) symmetric
point and a ferromagnetic phase, a conclusion supported by exact
diagonalizations of finite clusters. We argue that one of the phases,
characterized by a large number of low-lying singlets associated to dimer
coverings of the triangular lattice, could explain the properties of LiNiO,
while a ferro-orbital phase that lies nearby in parameter space leads to a new
prediction for the magnetic properties of NaNiO.Comment: 18 pages, 17 figure
Emergent Phases of Nodeless and Nodal Superconductivity Separated by Antiferromagnetic Order in Iron-based Superconductor (Ca4Al2O6)Fe2(As1-xPx)2: 75As- and 31P-NMR Studies
We report P- and As-NMR studies on
(CaAlO)Fe(AsP) with an isovalent substitution
of P for As. We present the novel evolution of emergent phases that the
nodeless superconductivity (SC) in 00.4 and the nodal one around
=1 are intimately separated by the onset of a commensurate stripe-type
antiferromagnetic (AFM) order in 0.5 0.95, as an isovalent
substitution of P for As decreases a pnictogen height measured from
the Fe plane. It is demonstrated that the AFM order takes place under a
condition of 1.32\AA1.42\AA, which is also the case for other
Fe-pnictides with the Fe state in (Fe) layers. This novel
phase evolution with the variation in points to the importance of
electron correlation for the emergence of SC as well as AFM order.Comment: 5pages, 4figures; accepted for publication as a Rapid Communication
in Phys. Rev.
High-Tc Nodeless s_\pm-wave Superconductivity in (Y,La)FeAsO_{1-y} with Tc=50 K: 75As-NMR Study
We report 75As-NMR study on the Fe-pnictide high-Tc superconductor
Y0.95La0.05FeAsO_{1-y} (Y0.95La0.051111) with Tc=50 K that includes no magnetic
rare-earth elements. The measurement of the nuclear-spin lattice-relaxation
rate 75(1/T1) has revealed that the nodeless bulk superconductivity takes place
at Tc=50 K while antiferromagnetic spin fluctuations (AFSFs) develop moderately
in the normal state. These features are consistently described by the multiple
fully-gapped s_\pm-wave model based on the Fermi-surface (FS) nesting.
Incorporating the theory based on band calculations, we propose that the reason
that Tc=50 K in Y0.95La0.051111 is larger than Tc=28 K in La1111 is that the FS
multiplicity is maximized, and hence the FS nesting condition is better than
that in La1111.Comment: 4 pages, 3 figures, accepted for publication in Phys Rev. Let
Realization of odd-frequency p-wave spin-singlet superconductivity coexisting with antiferromagnetic order near quantum critical point
A possibility of the realization of the p-wave spin-singlet superconductivity
(SS), whose gap function is odd both in momentum and in frequency, is
investigated by solving the gap equation with the phenomenological interaction
mediated by the antiferromagnetic spin fluctuation. The SS is realized
prevailing over the d-wave singlet superconductivity (SS) in the vicinity of
antiferromagnetic quantum critical pint (QCP) both on the paramagnetic and on
the antiferromagnetic sides. Off the QCP in the paramagnetic phase, however,
the SS with line-nodes is realized as \textit{conventional} anisotropic
superconductivity. For the present SS state, there is no gap in the
quasiparticle spectrum everywhere on the Fermi surface due to its odd
frequency. These features can give a qualitative understanding of the anomalous
behaviors of NQR relaxation rate on CeCuSi or CeRhIn where the
antiferromagnetism and superconductivity coexist on a microscopic level.Comment: 20 pages with 12 figures. To appear in J. Phys. Soc. Jpn. Vol. 72,
No. 1
Microscopic Evidence for Evolution of Superconductivity by Effective Carrier Doping in Boron-doped Diamond:11B-NMR study
We have investigated the superconductivity discovered in boron (B)-doped
diamonds by means of 11B-NMR on heteroepitaxially grown (111) and (100) films.
11B-NMR spectra for all of the films are identified to arise from the
substitutional B(1) site as single occupation and lower symmetric B(2) site
substituted as boron+hydrogen(B+H) complex, respectively. A clear evidence is
presented that the effective carriers introduced by B(1) substitution are
responsible for the superconductivity, whereas the charge neutral B(2) sites
does not offer the carriers effectively. The result is also corroborated by the
density of states deduced by 1/T1T measurement, indicating that the evolution
of superconductivity is driven by the effective carrier introduced by
substitution at B(1) site.Comment: 4 pages, 6 figures, to be published in Phys. Rev. B (Brief report
Enhancement of Superconducting Transition Temperature due to the strong Antiferromagnetic Spin Fluctuations in Non-centrosymmetric Heavy-fermion Superconductor CeIrSi3 :A 29Si-NMR Study under Pressure
We report a 29Si-NMR study on the pressure-induced superconductivity (SC) in
an antiferromagnetic (AFM) heavy-fermion compound CeIrSi3 without inversion
symmetry. In the SC state at P=2.7-2.8 GPa, the temperature dependence of the
nuclear-spin lattice relaxation rate 1/T_1 below Tc exhibits a T^3 behavior
without any coherence peak just below Tc, revealing the presence of line nodes
in the SC gap. In the normal state, 1/T_1 follows a \sqrt{T}-like behavior,
suggesting that the SC emerges under the non-Fermi liquid state dominated by
AFM spin fluctuations enhanced around quantum critical point (QCP). The reason
why the maximum Tc in CeIrSi3 is relatively high among the Ce-based
heavy-fermion superconductors may be the existence of the strong AFM spin
fluctuations. We discuss the comparison with the other Ce-based heavy-fermion
superconductors.Comment: 4 pages, 5 figures, To be published in Phys. Rev. Let
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