170 research outputs found
A fractional-spin phase in the power-law Kondo model
We consider a Kondo impurity coupled to a fermionic host with a power-law
density of states near the Fermi level, rho(epsilon) ~ |epsilon|^r, with
exponent r<0. Using both perturbative renormalization group (poor man's
scaling) and numerical renormalization group methods, we analyze the phase
diagram of this model for ferromagnetic and antiferromagnetic Kondo coupling.
Both sectors display non-trivial behavior with several stable phases separated
by second-order transitions. In particular, on the ferromagnetic side there is
a stable intermediate-coupling fixed point with universal properties
corresponding to a fractional ground-state spin.Comment: 5 pages, 4 figs; (v2) extended discussion and added refs; final
version as publishe
Critical quasiparticles in single-impurity and lattice Kondo models
Quantum criticality in systems of local moments interacting with itinerant
electrons has become an important and diverse field of research. Here we review
recent results which concern (a) quantum phase transitions in single-impurity
Kondo and Anderson models and (b) quantum phase transitions in heavy-fermion
lattice models which involve critical quasiparticles. For (a) the focus will be
on impurity models with a pseudogapped host density of states and their
applications, e.g., in graphene and other Dirac materials, while (b) is devoted
to strong-coupling behavior near antiferromagnetic quantum phase transitions,
with potential applications in a variety of heavy-fermion metals.Comment: 18 pages, 4 figs, mini-review. arXiv admin note: text overlap with
arXiv:1208.311
Gate-controlled Kondo screening in graphene: Quantum criticality and electron-hole asymmetry
Magnetic impurities in neutral graphene provide a realization of the
pseudogap Kondo model, which displays a quantum phase transition between phases
with screened and unscreened impurity moment. Here, we present a detailed study
of the pseudogap Kondo model with finite chemical potential mu. While carrier
doping restores conventional Kondo screening at lowest energies, properties of
the quantum critical fixed point turn out to influence the behavior over a
large parameter range. Most importantly, the Kondo temperature T_K shows an
extreme asymmetry between electron and hole doping. At criticality, depending
on the sign of mu, T_K follows either the scaling prediction T_K ~ |mu| with a
universal prefactor, or T_K ~ |mu|^x with x = 2.6. This asymmetry between
electron and hole doping extends well outside the quantum critical regime and
also implies a qualitative difference in the shape of the tunneling spectra for
both signs of mu.Comment: 6 pages, 6 figs; (v2) extended discussion of RG flow, final version
as publishe
Equilibrium and non-equilibrium dynamics of the sub-ohmic spin-boson model
Employing the non-perturbative numerical renormalization group method, we
study the dynamics of the spin-boson model, which describes a two-level system
coupled to a bosonic bath with spectral density J(omega) propto omega^s. We
show that, in contrast to the case of ohmic damping, the delocalized phase of
the sub-ohmic model cannot be characterized by a single energy scale only, due
to the presence of a non-trivial quantum phase transition. In the strongly
sub-ohmic regime, s<<1, weakly damped coherent oscillations on short time
scales are possible even in the localized phase - this is of crucial relevance,
e.g., for qubits subject to electromagnetic noise.Comment: 4 pages, 6 figures; final version, as publishe
Real-space renormalization group flow in quantum impurity systems: local moment formation and the Kondo screening cloud
The existence of a length-scale (with the Kondo
temperature) has long been predicted in quantum impurity systems. At low
temperatures , the standard interpretation is that a
spin- impurity is screened by a surrounding `Kondo cloud' of
spatial extent . We argue that renormalization group (RG) flow between
any two fixed points (FPs) results in a characteristic length-scale, observed
in real-space as a crossover between physical behaviour typical of each FP. In
the simplest example of the Anderson impurity model, three FPs arise; and we
show that `free orbital', `local moment' and `strong coupling' regions of space
can be identified at zero temperature. These regions are separated by two
crossover length-scales and , with the latter
diverging as the Kondo effect is destroyed on increasing temperature through
. One implication is that moment formation occurs inside the `Kondo
cloud', while the screening process itself occurs on flowing to the strong
coupling FP at distances . Generic aspects of the real-space
physics are exemplified by the two-channel Kondo model, where now
separates `local moment' and `overscreening' clouds.Comment: 6 pages; 5 figure
Charge ordering and phase separation in the infinite dimensional extended Hubbard model
We study the extended Hubbard model with both on-site (U) and nearest
neighbor (V) Coulomb repulsion using the exact diagonalization method within
the dynamical mean field theory. For a fixed U (U=2.0), the T-n phase-diagrams
are obtained for V=1.4 and V=1.2, at which the ground states of n=1/2 system is
charge-ordered and charge-disordered, respectively. In both cases, robust
charge order is found at finite temperature and in an extended filling regime
around n=1/2. The order parameter changes non-monotonously with temperature.
For V=1.4, phase separation between charge-ordered and charge-disordered phases
is observed in the low temperature and n < 0.5 regime. It is described by an
"S"-shaped structure of the n-/mu curve. For V=1.2, the ground state is
charge-disordered, and a reentrant charge-ordering transition is observed for
0.42 < n < 0.68. Relevance of our results to experiments for doped manganites
is discussed.Comment: 9 pages, 7 figures, submitted to Phys. Rev.
Quantum phase transitions and thermodynamics of the power-law Kondo model
We revisit the physics of a Kondo impurity coupled to a fermionic host with a
diverging power-law density of states near the Fermi level, , with exponent . Using the analytical understanding of
several fixed points, based partially on powerful mappings between models with
bath exponents and , combined with accurate numerical renormalization
group calculations, we determine thermodynamic quantities within the stable
phases, and also near the various quantum phase transitions. Antiferromagnetic
Kondo coupling leads to strong screening with a negative zero-temperature
impurity entropy, while ferromagnetic Kondo coupling can induce a stable
fractional spin moment. We formulate the quantum field theories for all
critical fixed points of the problem, which are fermionic in nature and allow
for a perturbative renormalization-group treatment.Comment: 13 pages, 11 figure
Numerical Renormalization Group for Impurity Quantum Phase Transitions: Structure of Critical Fixed Points
The numerical renormalization group method is used to investigate zero
temperature phase transitions in quantum impurity systems, in particular in the
particle-hole symmetric soft-gap Anderson model. The model displays two stable
phases whose fixed points can be built up of non-interacting single-particle
states. In contrast, the quantum phase transitions turn out to be described by
interacting fixed points, and their excitations cannot be described in terms of
free particles. We show that the structure of the many-body spectrum of these
critical fixed points can be understood using renormalized perturbation theory
close to certain values of the bath exponents which play the role of critical
dimensions. Contact is made with perturbative renormalization group
calculations for the soft-gap Anderson and Kondo models. A complete description
of the quantum critical many-particle spectra is achieved using suitable
marginal operators; technically this can be understood as epsilon-expansion for
full many-body spectra.Comment: 14 pages, 12 figure
Numerical Renormalization Group for Bosonic Systems and Application to the Subohmic Spin-Boson Model
We describe the generalization of Wilson's Numerical Renormalization Group
method to quantum impurity models with a bosonic bath, providing a general
non-perturbative approach to bosonic impurity models which can access
exponentially small energies and temperatures. As an application, we consider
the spin-boson model, describing a two-level system coupled to a bosonic bath
with power-law spectral density, J(omega) ~ omega^s. We find clear evidence for
a line of continuous quantum phase transitions for subohmic bath exponents
0<s<1; the line terminates in the well-known Kosterlitz-Thouless transition at
s=1. Contact is made with results from perturbative renormalization group, and
various other applications are outlined.Comment: 4 pages, 5 figs, (v2) final version as publishe
Phenomenological Modeling of Photoemission Spectra in Strongly Correlated Electron Systems
A phenomenological approach is presented that allows one to model, and
thereby interpret, photoemission spectra of strongly correlated electron
systems. A simple analytical formula for the self-energy is proposed. This
self-energy describes both coherent and incoherent parts of the spectrum
(quasiparticle and Hubbard peaks, respectively). Free parameters in the
expression are determined by fitting the density of states to experimental
photoemission data. An explicit fitting is presented for the
LaSrTiO system with . In general, our
phenomenological approach provides information on the effective mass, the
Hubbard interaction, and the spectral weight distribution in different parts of
the spectrum. Limitations of this approach are also discussed.Comment: 13 pages, 4 figures, IJMPB style (included
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