arXiv subjects

F. Alet

Publications and source records attributed to F. Alet.

At least 19 recordsLinked to original sources

arXiv2015

Magnetization plateaus of an easy-axis Kagom\'e antiferromagnet with extended interactions

We investigate the properties in finite magnetic field of an extended anisotropic XXZ spin-1/2 model on the Kagome lattice, originally introduced by Balents, Fisher, and Girvin [Phys. Rev. B, 65, 224412 (2002)]. The magnetization curve displays plateaus at magnetization m=1/6 and 1/3 when the anisotropy is large. Using low-energy effective constrained models (quantum loop and quantum dimer models), we discuss the nature of the plateau phases, found to be crystals that break discrete rotation and/or translation symmetries. Large-scale quantum Monte-Carlo simulations were carried out in particular for the m=1/6 plateau. We first map out the phase diagram of the effective quantum loop model with an additional loop-loop interaction to find stripe order around the point relevant for the original model as well as a topological Z2 spin liquid. The existence of a stripe crystalline phase is further evidenced by measuring both standard structure factor and entanglement entropy of the original microscopic model.

cond-mat.str-el↗

arXiv2014

Analytical and numerical studies of disordered spin-1 Heisenberg chains with aperiodic couplings

We investigate the low-temperature properties of the one-dimensional spin-1 Heisenberg model with geometric fluctuations induced by aperiodic but deterministic coupling distributions, involving two parameters. We focus on two aperiodic sequences, the Fibonacci sequence and the 6-3 sequence. Our goal is to understand how these geometric fluctuations modify the physics of the (gapped) Haldane phase, which corresponds to the ground state of the uniform spin-1 chain. We make use of different adaptations of the strong-disorder renormalization-group (SDRG) scheme of Ma, Dasgupta and Hu, widely employed in the study of random spin chains, supplemented by quantum Monte Carlo and density-matrix renormalization-group numerical calculations, to study the nature of the ground state as the coupling modulation is increased. We find no phase transition for the Fibonacci chain, while we show that the 6-3 chain exhibits a phase transition to a gapless, aperiodicity-dominated phase similar to the one found for the aperiodic spin-1/2 XXZ chain. Contrary to what is verified for random spin-1 chains, we show that different adaptations of the SDRG scheme may lead to different qualitative conclusions about the nature of the ground state in the presence of aperiodic coupling modulations.

cond-mat.str-el↗

arXiv2014

Hindered magnetic order from mixed dimensionalities in CuP$_2$O$_6$

We present a combined experimental and theoretical study of the spin-1/2 compound CuP$_2$O$_6$ that features a network of two-dimensional (2D) antiferromagnetic (AFM) square planes, interconnected via one-dimensional (1D) AFM spin chains. Magnetic susceptibility, high-field magnetization, and electron spin resonance (ESR) data, as well as microscopic density-functional band-structure calculations and subsequent quantum Monte-Carlo simulations, show that the coupling $J_{2D}\sim$ 40 K in the layers is an order of magnitude larger than $J_{1D}\sim$ 4 K in the chains. Below $T_N\sim$ 8 K, CuP$_2$O$_6$ develops long-range order (LRO), as evidenced by a weak net moment on the 2D planes induced by anisotropic magnetic interactions of Dzyaloshinsky-Moriya type. A striking feature of this 3D ordering transition is that the 1D moments grow significantly slower than the ones on the 2D layers, which is evidenced by the persistent paramagnetic ESR signal below $T_N$. Compared to typical quasi-2D magnets, the ordering temperature of CuP$_2$O$_6$ $T_N/J_{2D}\sim$ 0.2 is unusually low, showing that weakly coupled spins sandwiched between 2D magnetic units effectively decouple these units and impede the long-range ordering.

cond-mat.mtrl-sci↗

arXiv2010

Entanglement of quantum spin systems: a valence-bond approach

In order to quantify entanglement between two parts of a quantum system, one of the most used estimator is the Von Neumann entropy. Unfortunately, computing this quantity for large interacting quantum spin systems remains an open issue. Faced with this difficulty, other estimators have been proposed to measure entanglement efficiently, mostly by using simulations in the valence-bond basis. We review the different proposals and try to clarify the connections between their geometric definitions and proper observables. We illustrate this analysis with new results of entanglement properties of spin 1 chains.

cond-mat.str-el↗

arXiv2010

Quantum criticality in the SO(5) bilinear-biquadratic Heisenberg chain

The zero-temperature properties of the SO(5) bilinear-biquadratic Heisenberg spin chain are investigated by means of a low-energy approach and large scale numerical calculations. In sharp contrast to the spin-1 SO(3) Heisenberg chain, we show that the SO(5) Heisenberg spin chain is dimerized with a two-fold degenerate ground state. On top of this gapful phase, we find the emergence of a non-degenerate gapped phase with hidden (Z$_2$ $\times$ Z$_2$)$^2$ symmetry and spin-3/2 edge states that can be understood from a SO(5) AKLT wave function. We derive a low-energy theory describing the quantum critical point which separates these two gapped phases. It is shown and confirmed numerically that this quantum critical point belongs to the SO(5)$_1$ universality class.

cond-mat.str-el↗

arXiv2010

Impurity effects in coupled-ladder BiCu2PO6 studied by NMR and quantum Monte Carlo simulations

We present a 31P NMR study of the coupled spin 1/2 ladder compound BiCu2PO6. In the pure material, intrinsic susceptibility and dynamics show a spin gap of about . Substitution of non magnetic Zn or magnetic Ni impurity at Cu site induces a staggered magnetization which results in a broadening of the 31P NMR line, while susceptibility far from the defects is unaffected. The effect of Ni on the NMR line broadening is twice that of Zn, which is consistent with Quantum Monte Carlo (QMC) calculations assuming that Ni couples ferromagnetically to its adjacent Cu. The induced moment follows a 1/T temperature dependence due to the Curie-like development of the moment amplitude while its extension saturates and does not depend on impurity content or nature. This allow us to verify the generically expected scenario for impurity doping and to extend it to magnetic impurity case: in an antiferromagnetically correlated low dimensional spin system with antiferromagnetic correlations, any type of impurity induces a staggered moment at low temperature, whose extension is not linked to the impurity nature but to the intrinsic physics at play in the undoped pure system, from 1D to 2D systems.

cond-mat.str-el↗

arXiv2010

Asymmetric spin-1/2 two-leg ladders

We consider asymmetric spin-1/2 two-leg ladders with non-equal antiferromagnetic (AF) couplings J_|| and κJ_|| along legs (κ<= 1) and ferromagnetic rung coupling, J_\perp. This model is characterized by a gap Δin the spectrum of spin excitations. We show that in the large J_\perp limit this gap is equivalent to the Haldane gap for the AF spin-1 chain, irrespective of the asymmetry of the ladder. The behavior of the gap at small rung coupling falls in two different universality classes. The first class, which is best understood from the case of the conventional symmetric ladder at κ=1, admits a linear scaling for the spin gap Δ~ J_\perp. The second class appears for a strong asymmetry of the coupling along legs, κJ_|| << J_\perp << J_|| and is characterized by two energy scales: the exponentially small spin gap Δ~ J_\perp \exp(-J_|| / J_\perp), and the bandwidth of the low-lying excitations induced by a Suhl-Nakamura indirect exchange ~ J_\perp^2 /J_|| . We report numerical results obtained by exact diagonalization, density matrix renormalization group and quantum Monte Carlo simulations for the spin gap and various spin correlation functions. Our data indicate that the behavior of the string order parameter, characterizing the hidden AF order in Haldane phase, is different in the limiting cases of weak and strong asymmetry. On the basis of the numerical data, we propose a low-energy theory of effective spin-1 variables, pertaining to large blocks on a decimated lattice.

cond-mat.str-el↗

arXiv2010

Phase diagram of an extended classical dimer model

We present an extensive numerical study of the critical behavior of dimer models in three dimensions, focusing on the phase transition between Coulomb and crystalline columnar phases. The case of attractive interactions between parallel dimers on a plaquette was shown to undergo a continuous phase transition with critical exponents close to those of the O(N) tricritical universality class, a situation which is not easily captured by conventional field theories. That the dimer model is exactly fine-tuned to a highly symmetric point is a non trivial statement which needs careful numerical investigation. In this paper, we perform an extensive Monte Carlo study of a generalized dimer model with plaquette and cubic interactions and determine its extended phase diagram. We find that when both interactions favor alignment of the dimers, the phase transition is first order, in almost all cases. On the opposite, when interactions compete, the transition becomes continuous, with a critical exponent η~ 0.2. The existence of a tricritical point between the two regimes is confirmed by simulations on very large size systems and a flowgram method. In addition, we find a highly-degenerate crystalline phase at very low temperature in the frustrated regime which is separated from the columnar phase by a first order transition.

cond-mat.str-el↗

arXiv2008

Gauge theory picture of an ordering transition in a dimer model

We study a phase transition in a 3D lattice gauge theory, a "coarse-grained" version of a classical dimer model. Duality arguments indicate that the dimer lattice theory should be dual to a XY model coupled to a gauge field with geometric frustration. The transition between a Coulomb phase with dipolar correlations and a long range ordered columnar phase is understood in terms of a Higgs mechanism. Monte Carlo simulations of the dual model indicate a continuous transition with exponents close but apparently different from those of the 3d XY model. The continuous nature of the transition is confirmed by a flowgram analysis.

cond-mat.str-el↗

arXiv2008

The ALPS project release 1.3: open source software for strongly correlated systems

We present release 1.3 of the ALPS (Algorithms and Libraries for Physics Simulations) project, an international open source software project to develop libraries and application programs for the simulation of strongly correlated quantum lattice models such as quantum magnets, lattice bosons, and strongly correlated fermion systems. Development is centered on common XML and binary data formats, on libraries to simplify and speed up code development, and on full-featured simulation programs. The programs enable non-experts to start carrying out numerical simulations by providing basic implementations of the important algorithms for quantum lattice models: classical and quantum Monte Carlo (QMC) using non-local updates, extended ensemble simulations, exact and full diagonalization (ED), as well as the density matrix renormalization group (DMRG). Changes in the new release include a DMRG program for interacting models, support for translation symmetries in the diagonalization programs, the ability to define custom measurement operators, and support for inhomogeneous systems, such as lattice models with traps. The software is available from our web server at http://alps.comp-phys.org/ .

cond-mat.str-el↗

arXiv2007

Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder: a quantum Monte Carlo study

We consider a spin-1/2 ladder with a ferromagnetic rung coupling J_\perp and inequivalent chains. This model is obtained by a twist (θ) deformation of the ladder and interpolates between the isotropic ladder (θ=0) and the SU(2) ferromagnetic Kondo necklace model (θ=π). We show that the ground state in the (θ,J_\perp) plane has a finite string order parameter characterising the Haldane phase. Twisting the chain introduces a new energy scale, which we interpret in terms of a Suhl-Nakamura interaction. As a consequence we observe a crossover in the scaling of the spin gap at weak coupling from Δ/J_\| \propto J_\perp/J_\| for θ< θ_c \simeq 8π/9 to Δ/J_\| \propto (J_\perp/J_\|)^2 for θ> θ_c. Those results are obtained on the basis of large scale Quantum Monte Carlo calculations.

cond-mat.str-el↗

arXiv2007

Criticality of a classical dimer model on the triangular lattice

We consider a classical interacting dimer model which interpolates between the square lattice case and the triangular lattice case by tuning a chemical potential in the diagonal bonds. The interaction energy simply corresponds to the number of plaquettes with parallel dimers. Using transfer matrix calculations, we find in the anisotropic triangular case a succession of different physical phases as the interaction strength is increased: a short range disordered liquid dimer phase at low interactions, then a critical phase similar to the one found for the square lattice, and finally a transition to an ordered columnar phase for large interactions. The existence of the critical phase is in contrast with the belief that criticality for dimer models is ascribed to bipartiteness. For the isotropic triangular case, we have indications that the system undergoes a first order phase transition to an ordered phase, without appearance of an intermediate critical phase.

cond-mat.stat-mech↗

arXiv2005

Neel Temperature of Quasi-Low-Dimensional Heisenberg Antiferromagnets

The Néel temperature, $T_{\rm N}$, of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of inter-chain (inter-layer) to intra-chain (intra-layer) coupling $J'/J$ down to $J'/J\simeq 10^{-3}$. We find that $T_{\rm N}$ obeys a modified random-phase approximation-like relation for small $J'/J$ with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulae describing $T_{\rm N}$ for a wide range of $J'$ and useful for the analysis of experimental measurements are presented.

cond-mat.str-el↗

arXiv2004

Bosons in optical lattices - from the Mott transition to the Tonks-Girardeau gas

We present results from quantum Monte Carlo simulations of trapped bosons in optical lattices, focusing on the crossover from a gas of softcore bosons to a Tonks-Girardeau gas in a one-dimensional optical lattice. We find that depending on the quantity being measured, the behavior found in the Tonks-Girardeau regime is observed already at relatively small values of the interaction strength. A finite critical value for entering the Tonks-Girardeau regime does not exist. Furthermore, we discuss the computational efficiency of two quantum Monte Carlo methods to simulate large scale trapped bosonic systems: directed loops in stochastic series expansions and the worm algorithm.

cond-mat.stat-mech↗

arXiv2004

The ALPS project: open source software for strongly correlated systems

We present the ALPS (Algorithms and Libraries for Physics Simulations) project, an international open source software project to develop libraries and application programs for the simulation of strongly correlated quantum lattice models such as quantum magnets, lattice bosons, and strongly correlated fermion systems. Development is centered on common XML and binary data formats, on libraries to simplify and speed up code development, and on full-featured simulation programs. The programs enable non-experts to start carrying out numerical simulations by providing basic implementations of the important algorithms for quantum lattice models: classical and quantum Monte Carlo (QMC) using non-local updates, extended ensemble simulations, exact and full diagonalization (ED), as well as the density matrix renormalization group (DMRG). The software is available from our web server at http://alps.comp-phys.org.

cond-mat.str-el↗

arXiv2004

A cluster algorithm for Lattice Gauge Theories

A new algorithm for simulating compact U(1) lattice gauge theory in three dimensions is presented which is based on global changes in the configuration space. We show that this algorithm provides an effective way to extract partition functions at given external flux. As an application, we study numerically the finite temperature deconfinement phase transition.

hep-lat↗

arXiv2004

Local Simulation Algorithms for Coulombic Interactions

We consider dynamically constrained Monte-Carlo dynamics and show that this leads to the generation of long ranged effective interactions. This allows us to construct a local algorithm for the simulation of charged systems without ever having to evaluate pair potentials or solve the Poisson equation. We discuss a simple implementation of a charged lattice gas as well as more elaborate off-lattice versions of the algorithm. There are analogies between our formulation of electrostatics and the bosonic Hubbard model in the phase approximation. Cluster methods developed for this model further improve the efficiency of the electrostatics algorithm.

cond-mat.stat-mech↗

arXiv2003

Comparison of S=0 and S=1/2 Impurities in Haldane Chain Compound, $Y_{2}BaNiO_{5}$

We present the effect of Zn (S=0) and Cu (S=1/2) substitution at the Ni site of S=1 Haldane chain compound $Y_{2}BaNiO_{5}$. $^{89}$Y NMR allows us to measure the local magnetic susceptibility at different distances from the defects. The $^{89}$Y NMR spectrum consists of one central peak and several less intense satellite peaks. The shift of the central peak measures the uniform susceptibility, which displays a Haldane gap $Delta$$equiv$100 K and it corresponds to an AF coupling J$equiv$260 K between the near-neighbor Ni spins. Zn or Cu substitution does not affect the Haldane gap. The satellites, which are evenly distributed on the two sides of the central peak, probe the antiferromagnetic staggered magnetization near the substituted site, which decays exponentially. Its extension is found identical for both impurities and corresponds accurately to the correlation length $xi$(T) determined by Monte Carlo (QMC) simulations for the pure compound. In the case of non-magnetic Zn, the temperature dependence of the induced magnetization is consistent with a Curie law with an "effective" spin S=0.4 on each side of Zn, which is well accounted by Quantum Monte Carlo computations of the spinless-defect-induced magnetism. In the case of magnetic Cu, the similarity of the induced magnetism to the Zn case implies a weak coupling of the Cu spin to the nearest- neighbor Ni spins. The slight reductionin the induced polarization with respect to Zn is reproduced by QMC computations by considering an antiferromagnetic coupling of strength J'=0.1-0.2 J between the S=1/2 Cu-spin and nearest-neighbor Ni-spin.

cond-mat.str-el↗