25 research outputs found

### Reply to a Comment on ``Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory''

In our reply, we show that the objections put forward in cond-mat/0508763
concerning our paper, Phys. Rev. Lett. 93, 136405 (2004), are not valid:
(i) There is no orthogonality catastrophe (OC) for our calculations, and it
is also generally not ``unpractical'' to avoid it.
(ii) The OC does not affect our results.Comment: 1 page, 1 figure, Phys. Rev. Lett. in print; also note
cond-mat/050944

### Comment on "Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory"

A comment about importance of Anderson's orthogonality catastrophe for
projective Quantum Monte Carlo methods.Comment: Replaced by final versio

### Pressure-induced metal-insulator transition in LaMnO3 is not of Mott-Hubbard type

Calculations employing the local density approximation combined with static
and dynamical mean-field theories (LDA+U and LDA+DMFT) indicate that the
metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room
temperature is not a Mott-Hubbard transition, but is caused by orbital
splitting of the majority-spin eg bands. For LaMnO3 to be insulating at
pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller
distortion are needed.Comment: 4 pages, 3 figure

### Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory

We develop a projective quantum Monte Carlo algorithm of the Hirsch-Fye type
for obtaining ground state properties of the Anderson impurity model. This
method is employed to solve the self-consistency equations of dynamical mean
field theory. It is shown that the approach converges rapidly to the ground
state so that reliable zero-temperature results are obtained. As a first
application, we study the Mott-Hubbard metal-insulator transition of the
one-band Hubbard model, reconfirming the numerical renormalization group
results.Comment: 4 pages, 4 figure

### Single-hole dynamics in the half-filled two-dimensional Kondo-Hubbard model

We consider the Kondo lattice model in two dimensions at half filling. In
addition to the fermionic hopping integral $t$ and the superexchange coupling
$J$ the role of a Coulomb repulsion $U$ in the conduction band is investigated.
We find the model to display a magnetic order-disorder transition in the U-J
plane with a critical value of J_c which is decreasing as a function of U. The
single particle spectral function A(k,w) is computed across this transition.
For all values of J > 0, and apart from shadow features present in the ordered
state, A(k,w) remains insensitive to the magnetic phase transition with the
first low-energy hole states residing at momenta k = (\pm \pi, \pm \pi). As J
-> 0 the model maps onto the Hubbard Hamiltonian. Only in this limit, the
low-energy spectral weight at k = (\pm \pi, \pm \pi) vanishes with first
electron removal-states emerging at wave vectors on the magnetic Brillouin zone
boundary. Thus, we conclude that (i) the local screening of impurity spins
determines the low energy behavior of the spectral function and (ii) one cannot
deform continuously the spectral function of the Mott-Hubbard insulator at J=0
to that of the Kondo insulator at J > J_c. Our results are based on both, T=0
Quantum Monte-Carlo simulations and a bond-operator mean-field theory.Comment: 8 pages, 7 figures. Submitted to PR

### Efficient calculation of imaginary time displaced correlation functions in the projector auxiliary field quantum Monte-Carlo algorithm

The calculation of imaginary time displaced correlation functions with the
auxiliary field projector quantum Monte-Carlo algorithm provides valuable
insight (such as spin and charge gaps) in the model under consideration. One of
the authors and M. Imada [F.F. Assaad and M. Imada, J. Phys. Soc. Jpn. 65 189
(1996).] have proposed a numerically stable method to compute those quantities.
Although precise this method is expensive in CPU time. Here, we present an
alternative approach which is an order of magnitude quicker, just as precise,
and very simple to implement. The method is based on the observation that for a
given auxiliary field the equal time Green function matrix, $G$, is a
projector: $G^2 = G$.Comment: 4 papes, 1 figure in eps forma

### Coexistence of s-wave Superconductivity and Antiferromagnetism

We study the phase diagram of a new model that exhibits a first order
transition between s-wave superconducting and antiferromagnetic phases. The
model, a generalized Hubbard model augmented with competing spin-spin and
pair-pair interactions, was investigated using the projector Quantum Monte
Carlo method. Upon varying the Hubbard $U$ from attractive to repulsive we find
a first order phase transition between superconducting and antiferromagnetic
states.Comment: 4 page

### Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model

We present a detailed numerical study of spin and charge dynamics of the
two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J
> 0, the competition between the RKKY interaction and Kondo effect triggers a
quantum phase transition between magnetically ordered and disordered
insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega),
evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi)
to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation
following that of hybridized bands. For J < J_c this feature is supplemented by
shadows thus pointing to a coexistence of Kondo screening and magnetism. For J
< 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a
staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For
weak to intermediate couplings, T_S marks the onset of antiferromagnetic
fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C
scales as J both at weak and strong couplings. At and slightly below T_C we
observe i) a rise in the resistivity as a function of decreasing temperature,
ii) a dip in the integrated density of states at the Fermi energy and iii) the
occurrence of hybridized bands in A(k,\omega). It is shown that in the weak
coupling limit, the charge gap of order J is of magnetic origin. The specific
heat shows a two peak structure, the low temperature peak being of magnetic
origin. Our results are compared to various mean-field theories.Comment: 30 pages, 24 figure