5,517 research outputs found
Resonant tunneling and the multichannel Kondo problem: the quantum Brownian motion description
We study mesoscopic resonant tunneling as well as multichannel Kondo problems
by mapping them to a first-quantized quantum mechanical model of a particle
moving in a multi-dimensional periodic potential with Ohmic dissipation. From a
renormalization group analysis, we obtain phase diagrams of the quantum
Brownian motion model with various lattice symmetries. For a symmorphic
lattice, there are two phases at T=0: a localized phase in which the particle
is trapped in a potential minimum, and a free phase in which the particle is
unaffected by the periodic potential. For a non-symmorphic lattice, however,
there may be an additional intermediate phase in which the particle is neither
localized nor completely free. The fixed point governing the intermediate phase
is shown to be identical to the well-known multichannel Kondo fixed point in
the Toulouse limit as well as the resonance fixed point of a quantum dot model
and a double-barrier Luttinger liquid model. The mapping allows us to compute
the fixed-poing mobility of the quantum Brownian motion model exactly,
using known conformal-field-theory results of the Kondo problem. From the
mobility, we find that the peak value of the conductance resonance of a
spin-1/2 quantum dot problem is given by . The scaling form of the
resonance line shape is predicted
Cubic symmetry and magnetic frustration on the spin lattice in KIrCl
Cubic crystal structure and regular octahedral environment of Ir
render antifluorite-type KIrCl a model fcc antiferromagnet with a
combination of Heisenberg and Kitaev exchange interactions. High-resolution
synchrotron powder diffraction confirms cubic symmetry down to at least 20 K,
with a low-energy rotary mode gradually suppressed upon cooling. Using
thermodynamic and transport measurements, we estimate the activation energy of
eV for charge transport, the antiferromagnetic Curie-Weiss
temperature of K, and the extrapolated saturation
field of T. All these parameters are well reproduced \textit{ab
initio} using eV as the effective Coulomb repulsion
parameter. The antiferromagnetic Kitaev exchange term of K is about
one half of the Heisenberg term K. While this combination removes
a large part of the classical ground-state degeneracy, the selection of the
unique magnetic ground state additionally requires a weak second-neighbor
exchange coupling K. Our results suggest that KIrCl may
offer the best possible cubic conditions for Ir and demonstrates the
interplay of geometrical and exchange frustration in a high-symmetry setting.Comment: 9 page
The two-point correlation function of three-dimensional O(N) models: critical limit and anisotropy
In three-dimensional O(N) models, we investigate the low-momentum behavior of
the two-point Green's function G(x) in the critical region of the symmetric
phase. We consider physical systems whose criticality is characterized by a
rotational-invariant fixed point. Several approaches are exploited, such as
strong-coupling expansion of lattice non-linear O(N) sigma models,
1/N-expansion, field-theoretical methods within the phi^4 continuum
formulation. In non-rotational invariant physical systems with O(N)-invariant
interactions, the vanishing of space-anisotropy approaching the
rotational-invariant fixed point is described by a critical exponent rho, which
is universal and is related to the leading irrelevant operator breaking
rotational invariance. At N=\infty one finds rho=2. We show that, for all
values of , . Non-Gaussian corrections to the universal
low-momentum behavior of G(x) are evaluated, and found to be very small.Comment: 65 pages, revte
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