718 research outputs found
Critical phenomena near the antiferromagnetic quantum critical point of Heavy-Fermions
We present a study of the critical phenomena around the quantum critical
point in heavy-fermion systems. In the framework of the S=1/2 Kondo lattice
model, we introduce an extended decoupling scheme of the Kondo interaction
which allows one to treat the spin fluctuations and the Kondo effect on an
equal footing. The calculations, developed in a self-consistent one-loop
approximation, lead to the formation of a damped collective mode with a dynamic
exponent z=2 in the case of an antiferromagnetic instability. The system
displays a quantum-classical crossover at finite temperature depending how the
energy of the mode, on the scale of the magnetic correlation length, compares
to k_B T. The low temperature behavior, in the different regimes separated by
the crossover temperatures, is then discussed for both 2- and 3-dimensional
systems.Comment: 24 pages, 5 figures, added reference
Vortices and charge order in high-T_c superconductors
We theoretically investigate the vortex state of the cuprate high-temperature
superconductors in the presence of magnetic fields. Assuming the recently
derived nonlinear -model for fluctuations in the pseudogap phase, we
find that the vortex cores consist of two crossed regions of elliptic shape, in
which a static charge order emerges. Charge density wave order manifests itself
as satellites to the ordinary Bragg peaks directed along the axes of the
reciprocal copper lattice. Quadrupole density wave (bond order) satellites, if
seen, are predicted to be along the diagonals. The intensity of the satellites
should grow linearly with the magnetic field, in agreement with the result of
recent experiments
Kondo Breakdown and Hybridization Fluctuations in the Kondo-Heisenberg Lattice
We study the deconfined quantum critical point of the Kondo-Heisenberg
lattice in three dimensions using a fermionic representation for the localized
spins. The mean-field phase diagram exhibits a zero temperature quantum
critical point separating a spin liquid phase where the hybridization vanishes
and a Kondo phase where it does not. Two solutions can be stabilized in the
Kondo phase, namely a uniform hybridization when the band masses of the
conduction electrons and the spinons have the same sign, and a modulated one
when they have opposite sign. For the uniform case, we show that above a very
small temperature scale, the critical fluctuations associated with the
vanishing hybridization have dynamical exponent z=3, giving rise to a
resistivity that has a T log T behavior. We also find that the specific heat
coefficient diverges logarithmically in temperature, as observed in a number of
heavy fermion metals.Comment: new Figure 2, new results on spin susceptibility, some minor changes
to tex
Multi-scale fluctuations near a Kondo Breakdown Quantum Critical Point
We study the Kondo-Heisenberg model using a fermionic representation for the
localized spins. The mean-field phase diagram exhibits a zero temperature
quantum critical point separating a spin liquid phase where the f-conduction
hybridization vanishes, and a Kondo phase where it does not. Two solutions can
be stabilized in the Kondo phase, namely a uniform hybridization when the band
masses of the conduction electrons and the f spinons have the same sign, and a
modulated one when they have opposite sign. For the uniform case, we show that
above a very small Fermi liquid temperature scale (~1 mK), the critical
fluctuations associated with the vanishing hybridization have dynamical
exponent z=3, giving rise to a specific heat coefficient that diverges
logarithmically in temperature, as well as a conduction electron inverse
lifetime that has a T log T behavior. Because the f spinons do not carry
current, but act as an effective bath for the relaxation of the current carried
by the conduction electrons, the latter result also gives rise to a T log T
behavior in the resistivity. This behavior is consistent with observations in a
number of heavy fermion metals.Comment: 17 pages, 10 figure
The modulated spin liquid: a new paradigm for URuSi
We argue that near a Kondo breakdown critical point, a spin liquid with
spatial modulations can form. Unlike its uniform counterpart, we find that this
occurs via a second order phase transition. The amount of entropy quenched when
ordering is of the same magnitude as for an antiferromagnet. Moreover, the two
states are competitive, and at low temperatures are separated by a first order
phase transition. The modulated spin liquid we find breaks symmetry, as
recently seen in the hidden order phase of URuSi. Based on this, we
suggest that the modulated spin liquid is a viable candidate for this unique
phase of matter.Comment: 4 pages, 2 figure
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