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Ettore Vicari

Publications and source records attributed to Ettore Vicari.

At least 163 records · Page 9Linked to original sources
arXiv2008

Nodal quasiparticles and the onset of spin density wave order in the cuprates

We present a theory for the onset of spin density wave order in the superconducting ground state of the cuprates. We compute the scaling dimensions of allowed perturbations of a `relativistic' fixed point with O(4)xO(3) symmetry, including those associated with the fermionic nodal Bogoliubov quasiparticles. Analyses of up to six loops show that all perturbations with square lattice symmetry are likely irrelevant. We demonstrate that the fermion spectral functions are primarily damped by the coupling to fluctuations of a composite field with Ising nematic order. We also discuss the influence of quenched disorder.

cond-mat.str-el↗
arXiv2007

High-order perturbative expansions of multi-parameter Phi^4 quantum field theories

We present high-order pertubative expansions of multi-parameter Phi^4 quantum field theories with an N-component fundamental field, containing up to 4th-order polynomials of the field. Multi-parameter Phi^4 theories generalize the simplest O(N)-symmetric Phi^4 theories, and describe more complicated symmetry breaking patterns. These notes collect several high-order perturbative series of physically interesting multi-parameter Phi^4 theories, to five or six loops. We consider the O(M)XO(N)-symmetric Phi^4 model, the so-called MN model, and a spin-density-wave Phi^4 model containing five quartic terms.

hep-th↗
arXiv2007

The uniformly frustrated two-dimensional XY model in the limit of weak frustration

We consider the two-dimensional uniformly frustrated XY model in the limit of small frustration, which is equivalent to an XY system, for instance a Josephson junction array, in a weak uniform magnetic field applied along a direction orthogonal to the lattice. We show that the uniform frustration (equivalently, the magnetic field) destabilizes the line of fixed points which characterize the critical behaviour of the XY model for T <= T_{KT}, where T_{KT} is the Kosterlitz-Thouless transition temperature: the system is paramagnetic at any temperature for sufficiently small frustration. We predict the critical behaviour of the correlation length and of gauge-invariant magnetic susceptibilities as the frustration goes to zero. These predictions are fully confirmed by the numerical simulations.

cond-mat.stat-mech↗
arXiv2007

Relaxational dynamics in 3D randomly diluted Ising models

We study the purely relaxational dynamics (model A) at criticality in three-dimensional disordered Ising systems whose static critical behaviour belongs to the randomly diluted Ising universality class. We consider the site-diluted and bond-diluted Ising models, and the +- J Ising model along the paramagnetic-ferromagnetic transition line. We perform Monte Carlo simulations at the critical point using the Metropolis algorithm and study the dynamic behaviour in equilibrium at various values of the disorder parameter. The results provide a robust evidence of the existence of a unique model-A dynamic universality class which describes the relaxational critical dynamics in all considered models. In particular, the analysis of the size-dependence of suitably defined autocorrelation times at the critical point provides the estimate z=2.35(2) for the universal dynamic critical exponent. We also study the off-equilibrium relaxational dynamics following a quench from T=\infty to T=T_c. In agreement with the field-theory scenario, the analysis of the off-equilibrium dynamic critical behavior gives an estimate of z that is perfectly consistent with the equilibrium estimate z=2.35(2).

cond-mat.dis-nn↗
arXiv2007

Critical phenomena and renormalization-group flow of multi-parameter Φ^4 field theories

In the framework of the renormalization-group (RG) approach, critical phenomena can be investigated by studying the RG flow of multi-parameter $Φ^4$ field theories with an $N$-component fundamental field, containing up to 4th-order polynomials of the field. Some physically interesting systems require $Φ^4$ field theories with several quadratic and quartic parameters, depending essentially on their symmetry and symmetry-breaking pattern at the transition. Results for their RG flow apply to disorder and/or frustrated systems, anisotropic magnetic systems, density wave models, competing orderings giving rise to multicritical behaviors. The general properties of the RG flow in multi-parameter $Φ^4$ field theories are discussed. An overview of field-theoretical results for some physically interesting cases is presented, and compared with other theoretical approaches and experiments. Finally, this RG approach is applied to investigate the nature of the finite-temperature transition of QCD with $N_f$ light quarks.

hep-lat↗
arXiv2007

Zero-temperature behavior of the random-anisotropy model in the strong-anisotropy limit

We consider the random-anisotropy model on the square and on the cubic lattice in the strong-anisotropy limit. We compute exact ground-state configurations, and we use them to determine the stiffness exponent at zero temperature; we find $θ= -0.275(5)$ and $θ\approx 0.2$ respectively in two and three dimensions. These results show that the low-temperature phase of the model is the same as that of the usual Ising spin-glass model. We also show that no magnetic order occurs in two dimensions, since the expectation value of the magnetization is zero and spatial correlation functions decay exponentially. In three dimensions our data strongly support the absence of spontaneous magnetization in the infinite-volume limit.

cond-mat.dis-nn↗
arXiv2007

SU(N) gauge theories in the presence of a topological term

We review recent results on the theta dependence of the ground-state energy and spectrum of four-dimensional SU(N) gauge theories, where theta is the coefficient of the CP-violating topological term F-Fdual in the Lagrangian. In particular, we discuss the results obtained by Monte Carlo simulations of the lattice formulation of QCD, which allow the investigation of theta dependence around theta=0 by determining the moments of the topological charge distribution, and their correlations with other observables. The results for N=3 and larger values of N support the scenario obtained by general large-N scaling arguments.

hep-th↗
arXiv2007

The 3D +-J Ising model at the ferromagnetic transition line

We study the critical behavior of the three-dimensional $\pm J$ Ising model [with a random-exchange probability $P(J_{xy}) = p δ(J_{xy} - J) + (1-p) δ(J_{xy} + J)$] at the transition line between the paramagnetic and ferromagnetic phase, which extends from $p=1$ to a multicritical (Nishimori) point at $p=p_N\approx 0.767$. By a finite-size scaling analysis of Monte Carlo simulations at various values of $p$ in the region $p_N<p<1$, we provide strong numerical evidence that the critical behavior along the ferromagnetic transition line belongs to the same universality class as the three-dimensional randomly-dilute Ising model. We obtain the results $ν=0.682(3)$ and $η=0.036(2)$ for the critical exponents, which are consistent with the estimates $ν=0.683(2)$ and $η=0.036(1)$ at the transition of randomly-dilute Ising models.

cond-mat.dis-nn↗
arXiv2007

Renormalised four-point coupling constant in the three-dimensional O(N) model with N=0

We simulate self-avoiding walks on a cubic lattice and determine the second virial coefficient for walks of different lengths. This allows us to determine the critical value of the renormalized four-point coupling constant in the three-dimensional N-vector universality class for N=0. We obtain g* = 1.4005(5), where g is normalized so that the three-dimensional field-theoretical beta-function behaves as β(g) = - g + g^2 for small g. As a byproduct, we also obtain precise estimates of the interpenetration ratio Psi*, Psi* = 0.24685(11), and of the exponent ν, ν= 0.5876(2).

cond-mat.stat-mech↗
arXiv2007

Multicritical behavior of two-dimensional anisotropic antiferromagnets in a magnetic field

We study the phase diagram and multicritical behavior of anisotropic Heisenberg antiferromagnets on a square lattice in the presence of a magnetic field along the easy axis. We argue that, beside the Ising and XY critical lines, the phase diagram presents a first-order spin-flop line starting from T=0, as in the three-dimensional case. By using field theory we show that the multicritical point where these transition lines meet cannot be O(3) symmetric and occurs at finite temperature. We also predict how the critical temperature of the transition lines varies with the magnetic field and the uniaxial anisotropy in the limit of weak anisotropy.

cond-mat.stat-mech↗
arXiv2007

Universality class of 3D site-diluted and bond-diluted Ising systems

We present a finite-size scaling analysis of high-statistics Monte Carlo simulations of the three-dimensional randomly site-diluted and bond-diluted Ising model. The critical behavior of these systems is affected by slowly-decaying scaling corrections which make the accurate determination of their universal asymptotic behavior quite hard, requiring an effective control of the scaling corrections. For this purpose we exploit improved Hamiltonians, for which the leading scaling corrections are suppressed for any thermodynamic quantity, and improved observables, for which the leading scaling corrections are suppressed for any model belonging to the same universality class. The results of the finite-size scaling analysis provide strong numerical evidence that phase transitions in three-dimensional randomly site-diluted and bond-diluted Ising models belong to the same randomly dilute Ising universality class. We obtain accurate estimates of the critical exponents, $ν=0.683(2)$, $η=0.036(1)$, $α=-0.049(6)$, $γ=1.341(4)$, $β=0.354(1)$, $δ=4.792(6)$, and of the leading and next-to-leading correction-to-scaling exponents, $ω=0.33(3)$ and $ω_2=0.82(8)$.

cond-mat.dis-nn↗
arXiv2007

22nd order high-temperature expansion of nearest-neighbor models with O(2) symmetry on a simple cubic lattice

We present the high-temperature series for a nearest-neighbor model with O(2) symmetry on a simple cubic lattice with the most general single-site potential. In particular, the magnetic susceptibility and the second-moment correlation length are computed to 22nd order. The series specialized to some particular improved Hamiltonians have been already analyzed in the paper M. Campostrini, M. Hasenbusch, A. Pelissetto, and E. Vicari, Phys. Rev. B 74, 144506 (2006) [cond-mat/0605083], to determine the critical exponents and other universal quantities of the three-dimensional XY universality class.

cond-mat.stat-mech↗
arXiv2006

Fixed point stability and decay of correlations

In the framework of the renormalization-group theory of critical phenomena, a quantitative description of many continuous phase transitions can be obtained by considering an effective $Φ^4$ theories, having an N-component fundamental field $Φ_i$ and containing up to fourth-order powers of the field components. Their renormalization-group flow is usually characterized by several fixed points. We give here strong arguments in favour of the following conjecture: the stable fixed point corresponds to the fastest decay of correlations, that is, is the one with the largest values of the critical exponent $η$ describing the power-law decay of the two-point function at criticality. We prove this conjecture in the framework of the $ε$-expansion. Then, we discuss its validity beyond the $ε$-expansion. We present several lower-dimensional cases, mostly three-dimensional, which support the conjecture. We have been unable to find a counterexample.

cond-mat.stat-mech↗
arXiv2006

The spectrum of SU(N) gauge theories at nonzero theta

We study the theta dependence of the spectrum of four-dimensional SU(N) gauge theories, where theta is the coefficient of the topological term in the Lagrangian, for N>=3 and in the large-N limit. We compute the O(theta^2) terms of the expansions around theta=0 of the string tension and the lowest glueball mass, respectively sigma(theta) = sigma (1 + s_2 theta^2 + ...) and M(theta) = M (1 + g_2 theta^2 + ...), where sigma and M are the values at theta=0. For this purpose we use numerical simulations of the Wilson lattice formulation of SU(N) gauge theories for N=3,4,6. The O(theta^2) coefficients turn out to be very small for all N>=3. For example, s_2=-0.08(1) and g_2=-0.06(2) for N=3. Their absolute values decrease with increasing N. Our results are suggestive of a scenario in which the theta dependence in the string and glueball spectrum vanishes in the large-N limit, at least for sufficiently small values of |theta|. They support the general large-N scaling arguments that indicate (theta/N) as the relevant Lagrangian parameter in the large-N expansion.

hep-th↗
arXiv2006

Comment on "Spurious fixed points in frustrated magnets," cond-mat/0609285

We critically discuss the arguments reported in cond-mat/0609285 by B. Delamotte, Yu. Holovatch, D. Ivaneyko, D. Mouhanna, and M. Tissier. We show that their conclusions are not theoretically founded. They are contradicted by theoretical arguments and numerical results. On the contrary, perturbative field theory provides a robust evidence for the existence of chiral fixed points in O(2) X O(N) systems with N>=2. The three-dimensional perturbative results are consistent with theory and with all available experimental and Monte Carlo results. They provide a consistent scenario for the critical behavior of chiral systems.

cond-mat.stat-mech↗
arXiv2006

The random-anisotropy model in the strong-anisotropy limit

We investigate the nature of the critical behaviour of the random-anisotropy Heisenberg model (RAM), which describes a magnetic system with random uniaxial single-site anisotropy, such as some amorphous alloys of rare earths and transition metals. In particular, we consider the strong-anisotropy limit (SRAM), in which the Hamiltonian can be rewritten as the one of an Ising spin-glass model with correlated bond disorder: H = - J \sum_{< xy >} j_{xy} σ_x σ_y, where j_{xy} = \vec{u}_x \cdot \vec{u}_y and \vec{u}_x is a random three-component unit vector. We performed Monte Carlo simulations of the SRAM on simple cubic L^3 lattices, up to L=30, measuring correlation functions of the replica-replica overlap, which is the order parameter at a glass transition. The corresponding results show critical behaviour and finite-size scaling. They provide evidence of a finite-temperature continuous transition with critical exponents η_o=-0.24(4) and ν_o=2.4(6). These results are close to the corresponding estimates that have been obtained in the usual Ising spin-glass model with uncorrelated bond disorder, suggesting that the two models belong to the same universality class. This is consistent with arguments that suggest that the disorder correlations present in the SRAM are irrelevant.

cond-mat.dis-nn↗
arXiv2006

Theta-dependence of the spectrum of SU(N) gauge theories

We study the theta dependence of the spectrum of four-dimensional SU(N) gauge theories, where theta is the coefficient of the topological term in the Lagrangian, for N>=3 and in the large-N limit. We compute the O(theta^2) terms of the expansions around theta=0 of the string tension and the lowest glueball mass, respectively sigma(theta) = sigma (1 + s_2 theta^2 + ...) and M(theta) = M (1 + g_2 theta^2 + ...), where sigma and M are the values at theta=0. For this purpose we use numerical simulations of the Wilson lattice formulation of SU(N) gauge theories for N=3,4,6. The O(theta^2) coefficients turn out to be very small for all N>=3. For example, s_2=-0.08(1) and g_2=-0.06(2) for N=3. Their absolute values decrease with increasing N. Our results are suggestive of a scenario in which the theta dependence in the string and glueball spectrum vanishes in the large-N limit, at least for sufficiently small values of |theta|. They support the general large-N scaling arguments that indicate (theta/N) as the relevant Lagrangian parameter in the large-N expansion.

hep-th↗
arXiv2006

Critical behavior of the random-anisotropy model in the strong-anisotropy limit

We investigate the nature of the critical behavior of the random-anisotropy Heisenberg model (RAM), which describes a magnetic system with random uniaxial single-site anisotropy, such as some amorphous alloys of rare earths and transition metals. In particular, we consider the strong-anisotropy limit (SRAM), in which the Hamiltonian can be rewritten as the one of an Ising spin-glass model with correlated bond disorder. We perform Monte Carlo simulations of the SRAM on simple cubic L^3 lattices, up to L=30, measuring correlation functions of the replica-replica overlap, which is the order parameter at a glass transition. The corresponding results show critical behavior and finite-size scaling. They provide evidence of a finite-temperature continuous transition with critical exponents $η_o=-0.24(4)$ and $ν_o=2.4(6)$. These results are close to the corresponding estimates that have been obtained in the usual Ising spin-glass model with uncorrelated bond disorder, suggesting that the two models belong to the same universality class. We also determine the leading correction-to-scaling exponent finding $ω= 1.0(4)$.

cond-mat.dis-nn↗