16 research outputs found

### Quantum phase transition in a two-channel-Kondo quantum dot device

We develop a theory of electron transport in a double quantum dot device
recently proposed for the observation of the two-channel Kondo effect. Our
theory provides a strategy for tuning the device to the non-Fermi-liquid fixed
point, which is a quantum critical point in the space of device parameters. We
explore the corresponding quantum phase transition, and make explicit
predictions for behavior of the differential conductance in the vicinity of the
quantum critical point

### Functional Integral Approach to the Single Impurity Anderson Model

Recently, a functional integral representation was proposed by Weller
(Weller, W.: phys.~stat.~sol.~(b) {\bf 162}, 251 (1990)), in which the
fermionic fields strictly satisfy the constraint of no double occupancy at each
lattice site. This is achieved by introducing spin dependent Bose fields. The
functional integral method is applied to the single impurity Anderson model
both in the Kondo and mixed-valence regime. The f-electron Green's function and
susceptibility are calculated using an Ising-like representation for the Bose
fields. We discuss the difficulty to extract a spectral function from the
knowledge of the imaginary time Green's function. The results are compared with
NCA calculations.Comment: 11 pages, LaTeX, figures upon request, preprint No. 93/10/

### Fusion of the $q$-Vertex Operators and its Application to Solvable Vertex Models

We diagonalize the transfer matrix of the inhomogeneous vertex models of the
6-vertex type in the anti-ferroelectric regime intoducing new types of q-vertex
operators. The special cases of those models were used to diagonalize the s-d
exchange model\cite{W,A,FW1}. New vertex operators are constructed from the
level one vertex operators by the fusion procedure and have the description by
bosons. In order to clarify the particle structure we estabish new isomorphisms
of crystals. The results are very simple and figure out representation
theoretically the ground state degenerations.Comment: 35 page

### Flow equation analysis of the anisotropic Kondo model

We use the new method of infinitesimal unitary transformations to calculate
zero temperature correlation functions in the strong-coupling phase of the
anisotropic Kondo model. We find the dynamics on all energy scales including
the crossover behaviour from weak to strong coupling. The integrable structure
of the Hamiltonian is not used in our approach. Our method should also be
useful in other strong-coupling models since few other analytical methods allow
the evaluation of their correlation functions on all energy scales.Comment: 4 pages RevTeX, 2 eps figures include

### Exact perturbative solution of the Kondo problem

We explicitly evaluate the infinite series of integrals that appears in the
"Anderson-Yuval" reformulation of the anisotropic Kondo problem in terms of a
one-dimensional Coulomb gas. We do this by developing a general approach
relating the anisotropic Kondo problem of arbitrary spin with the boundary
sine-Gordon model, which describes impurity tunneling in a Luttinger liquid and
in the fractional quantum Hall effect. The Kondo solution then follows from the
exact perturbative solution of the latter model in terms of Jack polynomials.Comment: 4 pages in revtex two-colum

### Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics

A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally.Comment: 16 pages, RevTeX, 4 Postscript figures. Extended versio

### Entanglement between a qubit and the environment in the spin-boson model

The quantitative description of the quantum entanglement between a qubit and
its environment is considered. Specifically, for the ground state of the
spin-boson model, the entropy of entanglement of the spin is calculated as a
function of $\alpha$, the strength of the ohmic coupling to the environment,
and $\epsilon$, the level asymmetry. This is done by a numerical
renormalization group treatment of the related anisotropic Kondo model. For
$\epsilon=0$, the entanglement increases monotonically with $\alpha$, until it
becomes maximal for $\alpha \lim 1^-$. For fixed $\epsilon>0$, the entanglement
is a maximum as a function of $\alpha$ for a value, $\alpha = \alpha_M < 1$.Comment: 4 pages, 3 figures. Shortened version restricted to groundstate
entanglemen

### Low-Temperature Thermodynamics of $A^{(2)}_2$ and su(3)-invariant Spin Chains

We formulate the thermodynamic Bethe Ansatz (TBA) equations for the closed
(periodic boundary conditions) $A^{(2)}_2$ quantum spin chain in an external
magnetic field, in the (noncritical) regime where the anisotropy parameter
$\eta$ is real. In the limit $\eta \to 0$, we recover the TBA equations of the
antiferromagnetic su(3)-invariant chain in the fundamental representation. We
solve these equations for low temperature and small field, and calculate the
specific heat and magnetic susceptibility.Comment: 31 pages, UMTG-16

### Multi-Channel Kondo Necklace

A multi--channel generalization of Doniach's Kondo necklace model is
formulated, and its phase diagram studied in the mean--field approximation. Our
intention is to introduce the possible simplest model which displays some of
the features expected from the overscreened Kondo lattice. The $N$ conduction
electron channels are represented by $N$ sets of pseudospins \vt_{j}, $j=1,
... , N$, which are all antiferromagnetically coupled to a periodic array of
|\vs|=1/2 spins. Exploiting permutation symmetry in the channel index $j$
allows us to write down the self--consistency equation for general $N$. For
$N>2$, we find that the critical temperature is rising with increasing Kondo
interaction; we interpret this effect by pointing out that the Kondo coupling
creates the composite pseudospin objects which undergo an ordering transition.
The relevance of our findings to the underlying fermionic multi--channel
problem is discussed.Comment: 29 pages (2 figures upon request from [email protected]), LATEX,
submitted for publicatio

### Spinless impurities and Kondo-like behavior in strongly correlated electron systems

We investigate magnetic properties induced by a spinless impurity in strongly
correlated electron systems, i.e. the Hubbard model in the spatial dimension
$D=1,2,$ and 3. For the 1D system exploiting the Bethe ansatz exact solution we
find that the spin susceptibility and the local density of states in the
vicinity of a spinless impurity show divergent behaviors. The results imply
that the induced local moment is not completely quenched at any finite
temperatures. On the other hand, the spin lattice relaxation rate obtained by
bosonization and boundary conformal field theory satisfies a relation analogous
to the Korringa law, $1/T_1T \sim \chi^2$. In the 2D and 3D systems, the
analysis based upon the antiferromagnetically correlated Fermi liquid theory
reveals that the antiferromagnetic spin fluctuation developed in the bulk is
much suppressed in the vicinity of a spinless impurity, and thus magnetic
properties are governed by the induced local moment, which leads to the
Korringa law of $1/T_1$.Comment: 9pages,1figure, final version accepted for publication in
Phys.Rev.B(Jan2001