86 research outputs found
Spin wave dispersion based on the quasiparticle self-consistent method: NiO, MnO and -MnAs
We present spin wave dispersions in MnO, NiO, and -MnAs based on the
quasiparticle self-consistent method (\qsgw), which determines an optimum
quasiparticle picture. For MnO and NiO, \qsgw results are in rather good
agreement with experiments, in contrast to the LDA and LDA+U description. For
-MnAs, we find a collinear ferromagnetic ground state in \qsgw, while
this phase is unstable in the LDA.Comment: V2: add another figure for SW life time. Formalism is detaile
Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2
The recent discovery that superconductivity occurs in several clean itinerant
ferromagnets close to low temperature magnetic instabilities naturally invites
an interpretation based on a proximity to quantum criticality. Here we report
measurements of the pressure dependence of the low temperature magnetisation in
one of these materials, UGe_2. Our results show that both of the magnetic
transitions observed in this material as a function of pressure are first order
transitions and do not therefore correspond to quantum critical points. Further
we find that the known pressure dependence of the superconducting transition is
not reflected in the pressure dependence of the static susceptibility. This
demonstrates that the spectrum of excitations giving superconductivity is not
that normally associated with a proximity to quantum criticality in weak
itinerant ferromagnets. In contrast our data suggest that instead the pairing
spectrum might be related to a sharp spike in the electronic density of states
that also drives one of the magnetic transitions.Comment: to appear in Phys. Rev. Let
Simultaneous suppression of ferromagnetism and superconductivity in UCoGe by Si substitution
We investigate the effect of substituting Si for Ge in the ferromagnetic
superconductor UCoGe. Dc-magnetization, ac-susceptibility and electrical
resistivity measurements on polycrystalline UCoGeSi samples show
that ferromagnetic order and superconductivity are progressively depressed with
increasing Si content and simultaneously vanish at a critical concentration
. The non-Fermi liquid temperature variation in the
electrical resistivity near and the smooth depression of the ordered
moment point to a continuous ferromagnetic quantum phase transition.
Superconductivity is confined to the ferromagnetic phase, which provides
further evidence for magnetically mediated superconductivity.Comment: 5 pages, 4 figures, accepted for publication in PR
Spin-fluctuation theory beyond Gaussian approximation
A characteristic feature of the Gaussian approximation in the
functional-integral approach to the spin-fluctuation theory is the jump phase
transition to the paramagnetic state. We eliminate the jump and obtain a
continuous second-order phase transition by taking into account high-order
terms in the expansion of the free energy in powers of the fluctuating exchange
field. The third-order term of the free energy renormalizes the mean field, and
fourth-order term, responsible for the interaction of the fluctuations,
renormalizes the spin susceptibility. The extended theory is applied to the
calculation of magnetic properties of Fe-Ni Invar.Comment: 20 pages, 4 figure
c-axis magnetotransport in CeCoIn
We present the results of out-of-plane electrical transport measurements on
the heavy fermion superconductor CeCoIn at temperatures from 40 mK to 400
K and in magnetic field up to 9 T. For 10 K transport measurements show
that the zero-field resistivity changes linearly with temperature
and extrapolates nearly to zero at 0 K, indicative of non-Fermi-liquid (nFL)
behavior associated with a quantum critical point (QCP). The longitudinal
magnetoresistance (LMR) of CeCoIn for fields applied parallel to the
c-axis is negative and scales as between 50 and 100 K, revealing
the presence of a single-impurity Kondo energy scale K.
Beginning at 16 K a small positive LMR feature is evident for fields less than
3 tesla that grows in magnitude with decreasing temperature. For higher fields
the LMR is negative and increases in magnitude with decreasing temperature.
This sizable negative magnetoresistance scales as from 2.6 K to
roughly 8 K, and it arises from an extrapolated residual resistivity that
becomes negative and grows quadratically with field in the nFL temperature
regime. Applying a magnetic field along the c-axis with B B restores
Fermi-liquid behavior in at less than 130 mK. Analysis of the
resistivity coefficient's field-dependence suggests that the QCP in
CeCoIn is located \emph{below} the upper critical field, inside the
superconducting phase. These data indicate that while high- c-axis transport
of CeCoIn exhibits features typical for a heavy fermion system, low-
transport is governed both by spin fluctuations associated with the QCP and
Kondo interactions that are influenced by the underlying complex electronic
structure intrinsic to the anisotropic CeCoIn crystal structure
Magnon-Paramagnon Effective Theory of Itinerant Ferromagnets
The present work is devoted to the derivation of an effective
magnon-paramagnon theory starting from a microscopic lattice model of
ferromagnetic metals. For some values of the microscopic parameters it
reproduces the Heisenberg theory of localized spins. For small magnetization
the effective model describes the physics of weak ferromagnets in accordance
with the experimental results. It is written in a way which keeps O(3) symmetry
manifest,and describes both the order and disordered phases of the system.
Analytical expression for the Curie temperature,which takes the magnon
fluctuations into account exactly, is obtained. For weak ferromagnets is
well below the Stoner's critical temperature and the critical temperature
obtained within Moriya's theory.Comment: 14 pages, changed content,new result
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
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