91 research outputs found
Magnetic aspects of QCD and compact stars
Magnetic properties of quark matter are discussed. The possibility of
ferromagnetic transition is studied by using the one-gluon-exchange
interaction. Magnetic susceptibility is evaluated within Landau Fermi liquid
theory, and the important roles of the screening for the gluon propagation are
elucidated. Static screening for the longitudinal gluons improves the infrared
singularities, while the transverse gluons receive only dynamic screening. The
latter property gives rise to a novel non-Fermi-liquid behaviour in the
magnetic susceptibility. The critical density is estimated to be around the
nuclear density and the Curie temperature several tens MeV. The spin density
wave is also discussed at moderate densities, where chiral transition becomes
important. Pseudoscalar condensate as well as scalar one takes a spatially
non-uniform form in a chirally invariant way. Accordingly magnetization
oscillates like spin density wave. These results should have some implications
on compact star phenomena.Comment: 6 pages, 6 figures, Proc. of NFQCD10 to appear in Prog. Theor. Phys.
Supp
Effect of magnetic and non-magnetic impurities on highly anisotropic superconductivity
We generalize Abrikosov-Gor'kov solution of the problem of weakly coupled
superconductor with impurities on the case of a multiband superconductor with
arbitrary interband order parameter anisotropy, including interband sign
reversal of the order parameter. The solution is given in terms of the
effective (renormalized) coupling matrix and describes not only
suppression but also renormalization of the superconducting gap basically at
all temperatures. In many limiting cases we find analytical solutions for the
critical temperature suppression. We illustrate our results by numerical
calculations for two-band model systems.Comment: 18 pages (12pt) RevTeX, 4 postscript figure
Quasiparticle spectrum in the vortex state of d-wave superconductors
Quasiparticle spectrum associated with the nodal structure in d-wave
superconductors is of great interest. We study theoretically the quasi-particle
spectrum in a planar magnetic field, where the effect of the magnetic field is
treated in terms of the Doppler shift. We obtain the angular dependent specific
heat in the presence of a planar magnetic field and impurities, both in the
superclean limit (Gamma/Delta << H/H_{c2}<< 1) and in the clean limit (H/H_{c2}
<< Gamma/Delta <<1). Also a similar analysis is used for the thermal
conductivity tensor within the a-b plane. In particular, in contrast to the
earlier works, we find a fourfold symmetry term in \kappa_{\parallel} and
\kappa_{\perp} \sim -H \sin(2\theta) where kappa_{perp} are the diagonal- and
the off-diagonal components of the thermal conductivity tensor and \theta is
the angle between the heat current and the magnetic field.Comment: RevTex, 5 pages, 5 postscript figure
What is the fate of the heavy electron at a quantum critical point?
A growing body of evidence suggests that the quantum critical behavior at the
onset of magnetism in heavy fermion systems can not be understood in terms of a
simple quantum spin density wave. This talk will discuss the consequences of
this conclusion, touching on its possible implications in the realm of two
dimensional systems and outlining current theoretical and experimental efforts
to characterize the nature of the critical point in heavy fermion materials.Comment: To appear in SCES 200
Plane - Chain coupling in YBa_{2}Cu_{3}O_{7} : temperature dependence of the penetration depth
We have studied the penetration depth for a model of
involving pairing both in the planes and in the CuO chains. In this
model pairing in the planes is due to an attractive interaction, while Coulomb
repulsion induces in the chains an order parameter with opposite sign. Due to
the anticrossing produced by hybridization between planes and chains, one
obtains a d-wave like order parameter which changes sign on a single sheet of
the Fermi surface and has nodes in the gap. We find that our model accounts
quite well for the anisotropy of the penetration depth and for the absolute
values. We reproduce fairly well the whole temperature dependence for both the
a and the b directions, including the linear dependence at low temperature. We
use a set of parameters which are all quite reasonable physically. Our results
for the c direction are also satisfactory, although the situation is less clear
both experimentally and theoretically.Comment: 11 pages, revtex, 6 figure
Universality in Heavy Fermions Revisited
A previous scaling analysis of pressure experiments in heavy fermion is
reviewed and enlarged. We show that the critical exponents obtained from this
analysis indicate that a one-parameter scaling describes these experiments. We
obtain explicitly the enhancemente factors showing that these systems are
indeed near criticality and that the scaling approach is appropriate. The
physics responsible for the one-parameter scaling and breakdown of hyperscaling
is clarified. We discuss a microsocopic theory that is in agreement with the
experiments. The scaling theory is generalized for the case the shift and
crossover exponents are different. The exponents governing the physical
behavior along the non-Fermi liquid trajectory are obtained for this case.Comment: 7 pages, Latex, 3 Postscript figures, to be published in Physical
Review
Nonanalytic behavior of the spin susceptibility in clean Fermi systems
The wavevector and temperature dependent static spin susceptibility,
\chi_s(Q,T), of clean interacting Fermi systems is considered in dimensions
1\leq d \leq 3. We show that at zero temperature \chi_s is a nonanalytic
function of |Q|, with the leading nonanalyticity being |Q|^{d-1} for 1<d<3, and
Q^2\ln|Q| for d=3. For the homogeneous spin susceptibility we find a
nonanalytic temperature dependence T^{d-1} for 1<d<3. We give qualitative
mode-mode coupling arguments to that effect, and corroborate these arguments by
a perturbative calculation to second order in the electron-electron interaction
amplitude. The implications of this, in particular for itinerant
ferromagnetism, are discussed. We also point out the relation between our
findings and established perturbative results for 1-d systems, as well as for
the temperature dependence of \chi_s(Q=0) in d=3.Comment: 12pp., REVTeX, 5 eps figures, final version as publishe
Coexistence of Singlet and Triplet Attractive Channels in the Pairing Interactions Mediated by Antiferromagnetic Fluctuations
We propose a phase diagram of quasi-low-dimensional type II superconductors
in parallel magnetic fields, when antiferromagnetic fluctuations contribute to
the pairing interactions. We point out that pairing interactions mediated by
antiferromagnetic fluctuations necessarily include both singlet channels and
triplet channels as attractive interactions. Usually, a singlet pairing is
favored at zero field, but a triplet pairing occurs at high fields where the
singlet pairing is suppressed by the Pauli paramagnetic pair-breaking effect.
As a result, the critical field increases divergently at low temperatures. A
possible relation to experimental phase diagrams of a quasi-one-dimensional
organic superconductor is briefly discussed. We also discuss a possibility that
a triplet superconductivity is observed even at zero field.Comment: 4 pages, 1 figure (Latex, revtex.sty, epsf.sty
Nonmonotonic d_{x^2-y^2} Superconducting Order Parameter in Nd_{2-x}Ce_xCuO_4
Low energy polarized electronic Raman scattering of the electron doped
superconductor Nd_1.85Ce_0.15CuO_4 (T_c=22 K) has revealed a nonmonotonic
d_{x^2-y^2} superconducting order parameter. It has a maximum gap of 4.4 k_BT_c
at Fermi surface intersections with antiferromagnetic Brillouin zone (the ``hot
spots'') and a smaller gap of 3.3 k_BT_c at fermionic Brillouin zone
boundaries. The gap enhancement in the vicinity of the ``hot spots'' emphasizes
role of antiferromagnetic fluctuations and similarity in the origin of
superconductivity for electron- and hole-doped cuprates.Comment: 4 pages, 4 figure
Indirect Exchange Interaction between two Quantum Dots in an Aharonov-Bohm Ring
We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between
two spins located at two quantum dots embedded in an Aharonov-Bohm (AB) ring.
In such a system the RKKY interaction, which oscillates as a function of the
distance between two local spins, is affected by the flux. For the case of the
ferromagnetic RKKY interaction, we find that the amplitude of AB oscillations
is enhanced by the Kondo correlations and an additional maximum appears at half
flux, where the interaction is switched off. For the case of the
antiferromagnetic RKKY interaction, we find that the phase of AB oscillations
is shifted by pi, which is attributed to the formation of a singlet state
between two spins for the flux value close to integer value of flux.Comment: 10 pages, 5 figure
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