547 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 "Quantum Monte Carlo Evidence for Superconductivity in the Three-Band Hubbard Model in Two Dimensions"
In a recent Letter, Kuroki and Aoki [Phys. Rev. Lett. 76, 440 (1996)]
presented quantum Monte-Carlo (QMC) results for pairing correlations in the
three-band Hubbard model, which describes the Cu-d_{x^2-y^2} and O-p_{x,y}
orbitals present in the CuO_2 planes of high-T_c materials. In this comment we
argue that (i) the used parameter set is not appropriate for the description of
high-T_c materials since it does not satisfy the minimal requirement of a
charge-transfer gap at half-filling, and (ii) the observed increase in the
d_{x^2-y^2} channel is dominantly produced by the pair-field correlations
without the vertex part. Hence, the claim of evidence of ODLRO is not
justified.Comment: 1 page latex and 2 eps-figures, uses epsfig, submitted to PR
Finite-temperature properties of hard-core bosons confined on one-dimensional optical lattices
We present an exact study of the finite-temperature properties of hard-core
bosons (HCB's) confined on one-dimensional optical lattices. Our solution of
the HCB problem is based on the Jordan-Wigner transformation and properties of
Slater determinants. We analyze the effects of the temperature on the behavior
of the one-particle correlations, the momentum distribution function, and the
lowest natural orbitals. In addition, we compare results obtained using the
grand-canonical and canonical descriptions for systems like the ones recently
achieved experimentally. We show that even for such small systems, as small as
10 HCB's in 50 lattice sites, there are only minor differences between the
energies and momentum distributions obtained within both ensembles.Comment: RevTex file, 12 pages, 16 figures, published versio
Critical Exponents of the Metal-Insulator Transition in the Two-Dimensional Hubbard Model
We study the filling-controlled metal-insulator transition in the
two-dimensional Hubbard model near half-filling with the use of zero
temperature quantum Monte Carlo methods. In the metallic phase, the
compressibility behaves as where
is the critical chemical potential. In the insulating phase, the
localization length follows with . Under the assumption of hyperscaling, the compressibility
data leads to a correlation length exponent . Our
results show that the exponents and agree within
statistical uncertainty. This confirms the assumption of hyperscaling with
correlation length exponent and dynamical exponent . In
contrast the metal-insulator transition in the generic band insulators in all
dimensions as well as in the one-dimensional Hubbard model satisfy the
hyperscaling assumption with exponents and .Comment: Two references added. The DVI file and PS figure files are also
available at http://www.issp.u-tokyo.ac.jp/labs/riron/imada/furukawa/; to
appear in J. Phys. Soc. Jpn 65 (1996) No.
Dynamic Exponent of t-J and t-J-W Model
Drude weight of optical conductivity is calculated at zero temperature by
exact diagonalization for the two-dimensional t-J model with the two-particle
term, . For the ordinary t-J model with =0, the scaling of the Drude
weight for small doping concentration is
obtained, which indicates anomalous dynamic exponent =4 of the Mott
transition. When is switched on, the dynamic exponent recovers its
conventional value =2. This corresponds to an incoherent-to-coherent
transition associated with the switching of the two-particle transfer.Comment: LaTeX, JPSJ-style, 4 pages, 5 eps files, to appear in J. Phys. Soc.
Jpn. vol.67, No.6 (1998
Identifiability of direct effects from summary causal graphs
Dynamic structural causal models (SCMs) are a powerful framework for
reasoning in dynamic systems about direct effects which measure how a change in
one variable affects another variable while holding all other variables
constant. The causal relations in a dynamic structural causal model can be
qualitatively represented with a full-time causal graph. Assuming linearity and
causal sufficiency and given the full-time causal graph, the direct causal
effect is always identifiable and can be estimated from data by adjusting on
any set of variables given by the so-called single-door criterion. However, in
many application such a graph is not available for various reasons but
nevertheless experts have access to an abstraction of the full-time causal
graph which represents causal relations between time series while omitting
temporal information. This paper presents a complete identifiability result
which characterizes all cases for which the direct effect is graphically
identifiable from summary causal graphs and gives two sound finite adjustment
sets that can be used to estimate the direct effect whenever it is
identifiable
Doping induced metal-insulator transition in two-dimensional Hubbard, , and extended Hubbard, , models
We show numerically that the nature of the doping induced metal-insulator
transition in the two-dimensional Hubbard model is radically altered by the
inclusion of a term, , which depends upon a square of a single-particle
nearest-neighbor hopping. This result is reached by computing the localization
length, , in the insulating state. At finite values of we find
results consistent with where is
the critical chemical potential. In contrast, for the Hubbard model. At finite values of , the presented
numerical results imply that doping the antiferromagnetic Mott insulator leads
to a superconductor.Comment: 19 pages (latex) including 7 figures in encapsulated postscript
format. Submitted for publication in Phys. Rev.
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