arXiv subjects

Frederic Mila

Publications and source records attributed to Frederic Mila.

At least 73 records · Page 4Linked to original sources

arXiv1998

Comparison of tunneling through molecules with Mott-Hubbard and with dimerization gaps

In order to study the tunneling of electrons through an interacting, 1D, dimerized molecule connected to leads, we consider the persistent current in a ring embedding this molecule. We find numerically that, for spinless fermions, a molecule with a gap mostly due to interactions, i.e. a Mott-Hubbard gap, gives rise to a larger persistent current than a molecule with the same gap, but due only to the dimerization. In both cases, the tunneling current decreases exponentially with the size of the molecule, but more slowly in the interacting case. Implications for molecular electronic are briefly discussed.

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arXiv1997

Persistent currents in a Moebius ladder: A test of interchain coherence of interacting electrons

Persistent currents in a Moebius ladder are shown to be very sensitive to the effects of intrachain interactions on the hopping of electrons between chains. Their periodicity as a function of flux is doubled for strong enough repulsive interactions because electrons cannot hop coherently between the chains and have to travel along the full edge of the Moebius ladder, thus encircling the flux twice. Mesoscopic devices that should enable one to observe these effects are proposed.

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arXiv1997

Spectral Function and Self-Energy of the One-Dimensional Hubbard Model in the U --> infinity Limit

The interpretation of the k dependent spectral functions of the one-dimensional, infinite U Hubbard model obtained by using the factorized wave-function of Ogata and Shiba is revisited. The well defined feature which appears in addition to low energy features typical of Luttinger liquids, and which, close to the Fermi energy, can be interpreted as the shadow band resulting from $2k_F$ spin fluctuations, is further investigated. A calculation of the self-energy shows that, not too close to the Fermi energy, this feature corresponds to a band, i.e. to a solution of the Dyson equation $ω- ε(k) - Re Σ(k,ω) =0$.

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arXiv1997

Integrability and coherence of hopping between 1D correlated electron systems (Reply Comment)

The main point of Clarke and Strong's comment, namely that our results concerning the effect of integrability on the hopping of electrons between chains are probably not related to the notion of coherence proposed in \cite{clarke}, is important and well taken. In fact, we have already stressed in our work that integrability affects the intermediate-time behaviour but not the short-time one, while in \cite{clarke} the authors make predictions about the long-time dynamics on the basis of a calculation at short times. The logical conclusion is then that these two effects are in fact different phenomena. In more recent papers we have numerically investigated other physical quantities (spectral function, transverse conductivity in relation to the notion of coherence developed in \cite{clarke}.

cond-mat↗

arXiv1997

Spectral functions of the 1D Hubbard model in the U -> \infty limit: How to use the factorized wave-function

We give the details of the calculation of the spectral functions of the 1D Hubbard model using the spin-charge factorized wave-function for several versions of the U -> +\infty limit. The spectral functions are expressed as a convolution of charge and spin dynamical correlation functions. A procedure to evaluate these correlation functions very accurately for large systems is developed, and analytical results are presented for the low energy region. These results are fully consistent with the conformal field theory. We also propose a direct method of extracting the exponents from the matrix elements in more general cases.

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arXiv1996

Spectral Functions of One-dimensional Models of Correlated Electrons

Using the Ogata-Shiba wave function, the spectral functions of the one-dimensional infinite U Hubbard model are calculated for various concentrations. It is shown that the ``shadow band'' feature due to 2k_F fluctuations becomes more intense close to half-filling. Comparing these results with exact diagonalization data obtained on finite clusters for the finite U Hubbard model and for the t-J model, it is also shown that this feature remains well-defined for physically reasonable values of the parameters (U/t\simeq 10, J/t\simeq 0.4). The ``shadow'' structure in the spectral functions should thus be observable in angle-resolved photoemission experiments for a variety of quasi-one dimensional compounds.

cond-mat.str-el↗

arXiv1996

Spin and Charge Texture around In-Plane Charge Centers in the CuO_2 planes

Recent experiments on La_2Cu_{1-x}Li_xO_4 show that although the doped holes remain localized near the substitutional Li impurities, magnetic order is rapidly suppressed. An examination of the spin texture around a bound hole in a CuO_2 plane shows that the formation of a skyrmion is favored in a wide range of parameters, as was previously proposed in the context of Sr doping. The spin texture may be observable by elastic diffuse neutron scattering, and may also have a considerable effect on NMR lineshapes.

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arXiv1996

On the presence of mid-gap states in CaV4O9

Using exact diagonalizations of finite clusters with up to 32 sites, we study the $J_1-J_2$ model on the 1/5 depleted square lattice. Spin-spin correlation functions are consistent with plaquette order in the spin gap phase which exists for intermediate values of $J_2/J_1$. Besides, we show that singlet states will be present in the singlet-triplet gap if $J_2/J_1$ is not too small ($J_2/J_1 \gtrsim 0.47$). We argue that this property should play a central role in determining the exchange integrals in ${\rm CaV}_4{\rm O}_9$

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arXiv1996

Magnetic properties of NaV2O5, a one-dimensional spin 1/2 antiferromagnet with finite chains

We have performed measurements of the magnetic susceptibility of NaV$_2$O$_5$ between 2 and 400 K. The high temperature part is typical of spin 1/2 chains with a nearest--neighbour antiferromagnetic exchange integral $J$ of 529 K. We develop a model for the susceptibility of a system with finite chains to account for the low temperature part of the data, which cannot be fitted by a standard Curie-Weiss term. These results suggest that the next nearest--neighbour exchange integral $J_2$ in CaV$_4$O$_9$ should be of the order of 500 K because, like $J$ in NaV$_2$O$_5$, it corresponds to corner sharing VO$_5$ square pyramids.

cond-mat↗

arXiv1995

Shadow band in the one-dimensional large $U$ Hubbard model

We show that the factorized wave-function of Ogata and Shiba can be used to calculate the $k$ dependent spectral functions of the one-dimensional, infinite $U$ Hubbard model, and of some extensions to finite $U$. The resulting spectral function is remarkably rich: In addition to low energy features typical of Luttinger liquids, there is a well defined band, which we identify as the shadow band resulting from $2k_F$ spin fluctuations. This band should be detectable experimentally because its intensity is comparable to that of the main band for a large range of momenta.

cond-mat↗

arXiv1995

Spin Gap in CaV4O9: a Large S Appraoch

We study the stability of the 4-site plaquette order in a model recently introduced by Katoh and Imada to explain the spin gap in CaV4O9. We show that all relevant types of order can be described within the framework of Schwinger boson mean fiel theory, and we make predictions regarding their relative stability. The results are compared to exact diagonalisation of finite cluster.

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arXiv1995

Integrability and coherence of hopping between 1D correlated electrons systems

We present numerical evidence that the hopping of electrons between chains described by the $t-J$ model is coherent in the integrable cases ($J=0$ and $J=2$) and essentially incoherent otherwise. This effect is {\it not} related to the value of the exponent $α$, (which is restricted to the interval [0,1/8] when $0\le J\le 2$), and we propose that enhanced coherence is characteristic of integrable systems.

cond-mat↗

arXiv1995

First order transition between magnetic order and valence-bond order in a 2D frustrated Heisenberg model

We study the competition between magnetic order and valence bond order in a two dimensional (2D) frustrated Heisenberg model introduced some time ago by Shastry and Sutherland ({\sc B. Sriram Shastry} and {\sc Bill Sutherland}, {\em Physica} 108{\bf B},1069 (1981) ) for which a configuration of dimers is known to be the ground state in a certain range of parameters. Using exact diagonalisation of small clusters, linear spin wave theory and Schwinger boson mean field theory, we show that the transition between the two types of order is first-order, and that it takes place inside the domain where magnetic long-range order is stable with respect to quantum fluctuations.

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arXiv1995

Susceptibility of the one-dimensional, dimerized Hubbard model

We show that the zero temperature susceptibility of the one-dimensional, dimerized Hubbard model at quarter-filling can be accurately determined on the basis of exact diagonalization of small clusters. The best procedure is to perform a finite-size scaling of the spin velocity $u_σ$, and to calculate the susceptibility from the Luttinger liquid relation $χ=2/πu_σ$. We show that these results are reliable by comparing them with the analytical results that can be obtained in the weak and strong coupling limits. We have also used quantum Monte Carlo simulations to calculate the temperature dependence of the susceptibility for parameters that should be relevant to the Bechgaard salts. This shows that, used together, these numerical techniques are able to give precise estimates of the low temperature susceptibility of realistic one-dimensional models of correlated electrons.

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arXiv1995

Deducing correlation parameters from optical conductivity in the Bechgaard salts

Numerical calculations of the kinetic energy of various extensions of the one-dimensional Hubbard model including dimerization and repulsion between nearest neighbours are reported. Using the sum rule that relates the kinetic energy to the integral of the optical conductivity, one can determine which parameters are consistent with the reduction of the infrared oscillator strength that has been observed in the Bechgaard salts. This leads to improved estimates of the correlation parameters for both the TMTSF and TMTTF series.

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arXiv1994

Exact result on the Mott transition in a two-dimensional model of strongly correlated electrons

We study the properties of a quarter-filled system of electrons on a square lattice interacting through a local repulsion $U$ and a nearest-neighbour repulsion $V$ in the limit $V=+\infty$. We identify the ground-state for $U$ large enough and show that domain walls appear below a critical value $U_c=4t/π$. We argue that this corresponds to a metal-insulator transition due to Mott localization.

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arXiv1993

Phase diagram of the one-dimensional extended Hubbard model with attractive and/or repulsive interactions at quarter filling

We study the phase diagram of the one dimensional (1D) $U-V$ model at quarter filling in the most general case where the on-site and first-neighbour interactions $U$ and $V$ can be both attractive and repulsive. The results have been obtained using exact diagonalization of small clusters and variational techniques, as well as exact results in various limits. We have analyzed four properties of the groundstate: i)~whether it is insulating or metallic; \hbox{ii)~whether} it is homogenous or phase separated; iii)~whether it has a spin gap; iv)~whether it has dominant superconducting fluctuations. With eight phases, the resulting phase diagram is unexpectedly rich. The four phases not found in the weak coupling limit are: i) an insulating phase when $U$ and $V$ are large enough; ii) a region of phase separation when $V$ is attractive; iii) another region of phase separation when $V$ is large enough and $U$ small; iv) a region with dominant superconducting fluctuations when $V$ is intermediate and $U$ small. The actual nature of this last phase, which has pairs but no spin gap, is not fully clear yet.

cond-mat↗

arXiv1993

Phase diagram of the one-dimensional extended Hubbard model at quarter filling

We used exact diagonalization of small clusters and exact results in various limits to determine the phase diagram and the critical exponents of the one dimensional (1D) $U-V$ model at quarter-filling. We found an instability of the Luttinger liquid to a charge-density wave (CDW) insulator across a boundary going from ($U$,$V$) =($+\infty$,$2t$) to ($4t$,$+\infty$). In the metallic phase, the dominant fluctuations are superconducting if $V$ is large enough and spin-density wave (SDW) otherwise. In the latter case, the critical exponent $α$ of the momentum distribution does not exceed 9/16. The relevance of these results to 1D organic conductors is discussed.

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