54 research outputs found

### Effect of long-range hopping on Tc in a two-dimensional Hubbard-Holstein model of the cuprates

We study the effect of long-range hoppings on Tc for the two-dimensional (2D)
Hubbard model with and without Holstein phonons using parameters evaluated from
band-structure calculations for cuprates. Employing the dynamical cluster
approximation (DCA) with a quantum Monte Carlo (QMC) cluster solver for a
4-site cluster, we observe that without phonons, the long-range hoppings, t'
and t'', generally suppress Tc. We argue that this trend remains valid for
larger clusters. In the presence of the Holstein phonons, a finite t' enhances
Tc in the under-doped region for the hole-doped system, consistent with
local-density approximation (LDA) calculations and experiment. This is
interpreted through the suppression of antiferromagnetic (AF) correlations and
the interplay between polaronic effects and the antiferromagnetism.Comment: 5 pages, 4 figure

### Quantum Criticality and Incipient Phase Separation in the Thermodynamic Properties of the Hubbard Model

Transport measurements on the cuprates suggest the presence of a quantum
critical point hiding underneath the superconducting dome near optimal hole
doping. We provide numerical evidence in support of this scenario via a
dynamical cluster quantum Monte Carlo study of the extended two-dimensional
Hubbard model. Single particle quantities, such as the spectral function, the
quasiparticle weight and the entropy, display a crossover between two distinct
ground states: a Fermi liquid at low filling and a non-Fermi liquid with a
pseudogap at high filling. Both states are found to cross over to a marginal
Fermi-liquid state at higher temperatures. For finite next-nearest-neighbor
hopping t' we find a classical critical point at temperature T_c. This
classical critical point is found to be associated with a phase separation
transition between a compressible Mott gas and an incompressible Mott liquid
corresponding to the Fermi liquid and the pseudogap state, respectively. Since
the critical temperature T_c extrapolates to zero as t' vanishes, we conclude
that a quantum critical point connects the Fermi-liquid to the pseudogap
region, and that the marginal-Fermi-liquid behavior in its vicinity is the
analogous of the supercritical region in the liquid-gas transition.Comment: 18 pages, 9 figure

### Systematic analysis of a spin-susceptibility representation of the pairing interaction in the 2D Hubbard model

A dynamic cluster quantum Monte Carlo algorithm is used to study a spin
susceptibility representation of the pairing interaction for the
two-dimensional Hubbard model with an on-site Coulomb interaction equal to the
bandwidth for various doping levels. We find that the pairing interaction is
well approximated by {3/2}\Ub(T)^2\chi(K-K') with an effective temperature
and doping dependent coupling \Ub(T) and the numerically calculated spin
susceptibility $\chi(K-K')$. We show that at low temperatures, \Ub may be
accurately determined from a corresponding spin susceptibility based
calculation of the single-particle self-energy. We conclude that the strength
of the d-wave pairing interaction, characterized by the mean-field transition
temperature, can be determined from a knowledge of the dressed spin
susceptibility and the nodal quasiparticle spectral weight. This has important
implications with respect to the questions of whether spin fluctuations are
responsible for pairing in the high-T$_c$ cuprates.Comment: 5 pages, 5 figure

### Thermodynamics of the Quantum Critical Point at Finite Doping in the 2D Hubbard Model: A Dynamical Cluster Approximation Study

We study the thermodynamics of the two-dimensional Hubbard model within the
dynamical cluster approximation. We use continuous time quantum Monte Carlo as
a cluster solver to avoid the systematic error which complicates the
calculation of the entropy and potential energy (double occupancy). We find
that at a critical filling, there is a pronounced peak in the entropy divided
by temperature, S/T, and in the normalized double occupancy as a function of
doping. At this filling, we find that specific heat divided by temperature,
C/T, increases strongly with decreasing temperature and kinetic and potential
energies vary like T^2 ln(T). These are all characteristics of quantum critical
behavior.Comment: 4 pages, 4 figures. Submitted to Phys. Rev. B Rapid Communications on
June 27, 200

### Charge density wave in the spin ladder of Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$

We consider a multiband charge transfer model for a single spin ladder
describing the holes in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$. Using Hartree-Fock
approximation we show how the charge density wave, with its periodicity
dependent on doping as recently observed in the experiment, can be stabilized
by purely electronic many-body interactions.Comment: 4 pages, 2 figures, accepted for publication in Physica C as the
proceedings of the M2S-HTSC VIII Conference, Dresden 200

### Combined density-functional and dynamical cluster quantum Monte Carlo calculations for three-band Hubbard models for hole-doped cuprate superconductors

Using a combined local density functional theory (LDA-DFT) and quantum Monte
Carlo (QMC) dynamic cluster approximation approach, the parameter dependence of
the superconducting transition temperature Tc of several single-layer
hole-doped cuprate superconductors with experimentally very different Tcmax is
investigated. The parameters of two different three-band Hubbard models are
obtained using the LDA and the downfolding Nth-order muffin-tin orbital
technique with N=0 and 1 respectively. QMC calculations on 4-site clusters show
that the d-wave transition temperature Tc depends sensitively on the
parameters. While the N=1 MTO basis set which reproduces all three $pd\sigma$
bands leads to a d-wave transition, the N=0 set which merely reproduces the LDA
Fermi surface and velocities does not

### Physics of cuprates with the two-band Hubbard model - The validity of the one-band Hubbard model

We calculate the properties of the two-band Hubbard model using the Dynamical
Cluster Approximation. The phase diagram resembles the generic phase diagram of
the cuprates, showing a strong asymmetry with respect to electron and hole
doped regimes, in agreement with experiment. Asymmetric features are also seen
in one-particle spectral functions and in the charge, spin and d-wave pairing
susceptibility functions. We address the possible reduction of the two-band
model to a low-energy single-band one, as it was suggested by Zhang and Rice.
Comparing the two-band Hubbard model properties with the single-band Hubbard
model ones, we have found similar low-energy physics provided that the
next-nearest-neighbor hopping term t' has a significant value ($t'/t \approx
0.3$). The parameter t' is the main culprit for the electron-hole asymmetry.
However, a significant value of t' cannot be provided in a strict Zhang and
Rice picture where the extra holes added into the system bind to the existing
Cu holes forming local singlets. We notice that by considering approximate
singlet states, such as plaquette ones, reasonable values of t', which capture
qualitatively the physics of the two-band model can be obtained. We conclude
that a single-band t-t'-U Hubbard model captures the basic physics of the
cuprates concerning superconductivity, antiferromagnetism, pseudogap and
electron-hole asymmetry, but is not suitable for a quantitative analysis or to
describe physical properties involving energy scales larger than about 0.5 eV.Comment: 14 pages, 16 figure

### Two-dimensional Hubbard-Holstein bipolaron

We present a diagrammatic Monte Carlo study of the properties of the
Hubbard-Holstein bipolaron on a two-dimensional square lattice. With a small
Coulomb repulsion, U, and with increasing electron-phonon interaction, and when
reaching a value about two times smaller than the one corresponding to the
transition of light polaron to heavy polaron, the system suffers a sharp
transition from a state formed by two weakly bound light polarons to a heavy,
strongly bound on-site bipolaron. Aside from this rather conventional bipolaron
a new bipolaron state is found for large U at intermediate and large
electron-phonon coupling, corresponding to two polarons bound on
nearest-neighbor sites. We discuss both the properties of the different
bipolaron states and the transition from one state to another. We present a
phase diagram in parameter space defined by the electron-phonon coupling and U.
Our numerical method does not use any artificial approximation and can be
easily modified to other bipolaron models with longer range electron-phonon
and/or electron-electron interaction.Comment: 14 pages, 12 figure

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