698 research outputs found
Metallic Ferromagnetism in the Kondo Lattice
Metallic magnetism is both ancient and modern, occurring in such familiar
settings as the lodestone in compass needles and the hard drive in computers.
Surprisingly, a rigorous theoretical basis for metallic ferromagnetism is still
largely missing. The Stoner approach perturbatively treates Coulomb
interactions when the latter need to be large, while the Nagaoka approach
incorporates thermodynamically negligible electrons into a half-filled band.
Here, we show that the ferromagnetic order of the Kondo lattice is amenable to
an asymptotically exact analysis over a range of interaction parameters. In
this ferromagnetic phase, the conduction electrons and local moments are
strongly coupled but the Fermi surface does not enclose the latter (i.e. it is
small). Moreover, non-Fermi liquid behavior appears over a range of frequencies
and temperatures. Our results provide the basis to understand some
long-standing puzzles in the ferromagnetic heavy fermion metals, and raises the
prospect for a new class of ferromagnetic quantum phase transitions.Comment: 21 pages, 9 figures, including Supporting Informatio
Quantum Criticality and Novel Phases: A panel discussion
Physicists gathered in august at Dresden for a conference about "Quantum
Criticality and Novel Phases". As one part of the meeting, nine panelists
hosted an open and free-wheeling discussion on the topic of the meeting. This
article outlines the discussions that took place during at this panel-meeting
on the afternoon of August 3rd, 2009.Comment: Report on the panel discussion at the 2009 Dresden Meeting on Quantum
Criticality and Novel Phase
Charge and spin density wave ordering transitions in strongly correlated metals
We study the quantum transition from a strongly correlated metal, with heavy
fermionic quasiparticles, to a metal with commensurate charge or spin density
wave order. To this end, we introduce and numerically analyze a large
dimensionality model of Ising spins in a transverse field, coupled to two
species of fermions; the analysis borrows heavily from recent progress in the
solution of the Hubbard model in large dimensions. At low energies, the Ising
order parameter fluctuations are characterized by the critical exponent , while above an energy scale, , there is a crossover to criticality. We show that is of the order of the width of the
heavy quasiparticle band, and can be made arbitrarily small for a correlated
metal close to a Mott-Hubbard insulator. Therefore, such a correlated metal has
a significant intermediate energy range of behavior, a single
particle spectrum with a narrow quasiparticle band, and well-formed analogs of
the lower and upper Hubbard bands; we suggest that these features are
intimately related in general.Comment: 14 pages, REVTEX 3.0, 2 postscript figure
Non-Fermi Liquids in the Extended Hubbard Model
I summarize recent work on non-Fermi liquids within certain generalized
Anderson impurity model as well as in the large dimensionality () limit of
the two-band extended Hubbard model. The competition between local charge and
spin fluctuations leads either to a Fermi liquid with renormalized
quasiparticle excitations, or to non-Fermi liquids with spin-charge separation.
These results provide new insights into the phenomenological similarities and
differences between different correlated metals. While presenting these
results, I outline a general strategy of local approach to non-Fermi liquids in
correlated electron systems.Comment: 30 pages, REVTEX, 14 figures included. To appear in ``Non Fermi
Liquid Physics'', J. Phys: Cond. Matt. (1997
Mixed-valent regime of the two-channel Anderson impurity as a model for UBe_13
We investigate the mixed-valent regime of a two-configuration Anderson
impurity model for uranium ions, with separate quadrupolar and magnetic
doublets. With a new Monte Carlo approach and the non-crossing approximation we
find: (i) A non-Fermi-liquid fixed point with two-channel Kondo model critical
behavior; (ii) Distinct energy scales for screening the low-lying and excited
doublets; (iii) A semi-quantitative explanation of magnetic-susceptibility data
for UThBe assuming 60-70% quadrupolar doublet ground-state
weight, supporting the quadrupolar-Kondo interpretation.Comment: 4 Pages, 3 eps figures; submitted to Phys. Rev. Let
Frustration and the Kondo effect in heavy fermion materials
The observation of a separation between the antiferromagnetic phase boundary
and the small-large Fermi surface transition in recent experiments has led to
the proposal that frustration is an important additional tuning parameter in
the Kondo lattice model of heavy fermion materials. The introduction of a Kondo
(K) and a frustration (Q) axis into the phase diagram permits us to discuss the
physics of heavy fermion materials in a broader perspective. The current
experimental situation is analysed in the context of this combined "QK" phase
diagram. We discuss various theoretical models for the frustrated Kondo
lattice, using general arguments to characterize the nature of the -electron
localization transition that occurs between the spin liquid and heavy Fermi
liquid ground-states. We concentrate in particular on the Shastry--Sutherland
Kondo lattice model, for which we establish the qualitative phase diagram using
strong coupling arguments and the large- expansion. The paper closes with
some brief remarks on promising future theoretical directions.Comment: To appear in a special issue of JLT
Weak magnetism and non-Fermi liquids near heavy-fermion critical points
This paper is concerned with the weak-moment magnetism in heavy-fermion
materials and its relation to the non-Fermi liquid physics observed near the
transition to the Fermi liquid. We explore the hypothesis that the primary
fluctuations responsible for the non-Fermi liquid physics are those associated
with the destruction of the large Fermi surface of the Fermi liquid. Magnetism
is suggested to be a low-energy instability of the resulting small Fermi
surface state. A concrete realization of this picture is provided by a
fractionalized Fermi liquid state which has a small Fermi surface of conduction
electrons, but also has other exotic excitations with interactions described by
a gauge theory in its deconfined phase. Of particular interest is a
three-dimensional fractionalized Fermi liquid with a spinon Fermi surface and a
U(1) gauge structure. A direct second-order transition from this state to the
conventional Fermi liquid is possible and involves a jump in the electron Fermi
surface volume. The critical point displays non-Fermi liquid behavior. A
magnetic phase may develop from a spin density wave instability of the spinon
Fermi surface. This exotic magnetic metal may have a weak ordered moment
although the local moments do not participate in the Fermi surface.
Experimental signatures of this phase and implications for heavy-fermion
systems are discussed.Comment: 20 pages, 8 figures; (v2) includes expanded discussion and solution
of quantum Boltzmann equatio
Universality class of non-Fermi liquid behavior in mixed valence systems
A generalized Anderson single-impurity model with off-site Coulomb
interactions is derived from the extended three-band Hubbard model, originally
proposed to describe the physics of the copper-oxides. Using the abelian
bosonization technique and canonical transformations, an effective Hamiltonian
is derived in the strong coupling limit, which is essentially analogous to the
Toulouse limit of the ordinary Kondo problem. In this limit, the effective
Hamiltonian can be exactly solved, with a mixed valence quantum critical point
separating two different Fermi liquid phases, {\it i.e.} the Kondo phase and
the empty orbital phase. In the mixed valence quantum critical regime, the
local moment is only partially quenched and X-ray edge singularities are
generated. Around the quantum critical point, a new type of non-Fermi liquid
behavior is predicted with an extra specific heat and a
singular spin-susceptibility . At the same time, the
effective Hamiltonian under single occupancy is transformed into a
resonant-level model, from which the correct Kondo physical properties
(specific heat, spin susceptibility, and an enhanced Wilson ratio) are easily
rederived. Finally, a brief discussion is given to relate these theoretical
results to observations in () alloys, which show
single-impurity critical behavior consistent with our predictions.Comment: 26 pages, revtex, no figure. Some corrections have been made, but the
basic results are kept. To be published in Physical Review
Non-Fermi liquid behavior in an extended Anderson model
An extended Anderson model, including screening channels (non-hybridizing,
but interacting with the local orbit), is studied within the Anderson-Yuval
approach, originally devised for the single-channel Kondo problem. By comparing
the perturbation expansions of this model and a generalized resonant level
model, the spin-spin correlation functions are calculated which show non-Fermi
liquid exponent depending on the strength of the scattering potential. The
relevance of this result to experiments in some heavy fermion systems is
briefly discussed.Comment: REVTEX, 17 pages, no figures, to be published in Phys. Rev.
Mott transition at large orbital degeneracy: dynamical mean-field theory
We study analytically the Mott transition of the N-orbital Hubbard model
using dynamical mean-field theory and a low-energy projection onto an effective
Kondo model. It is demonstrated that the critical interaction at which the
insulator appears (Uc1) and the one at which the metal becomes unstable (Uc2)
have different dependence on the number of orbitals as the latter becomes
large: Uc1 ~ \sqrt{N} while Uc2 ~ N. An exact analytical determination of the
critical coupling Uc2/N is obtained in the large-N limit. The metallic solution
close to this critical coupling has many similarities at low-energy with the
results of slave boson approximations, to which a comparison is made. We also
discuss how the critical temperature associated with the Mott critical endpoint
depends on the number of orbitals.Comment: 13 pages. Minor changes in V
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