Publications (251)825.54 Total impact
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ABSTRACT: Exact ground states of a spin1/2 IsingHeisenberg model on the ShastrySutherland lattice with Heisenberg intradimer and Ising interdimer couplings are found by two independent rigorous procedures. The first method uses a unitary transformation to establish a mapping correspondence with an effective classical spin model, while the second method relies on the derivation of an effective hardcore boson model by continuous unitary transformations. Both methods lead to equivalent effective Hamiltonians providing a convincing proof that the spin1/2 IsingHeisenberg model on the ShastrySutherland lattice exhibits a zerotemperature magnetization curve with just two intermediate plateaus at onethird and onehalf of the saturation magnetization, which correspond to stripe and checkerboard orderings of singlets and polarized triplets, respectively. The nature of the remarkable stripe order relevant to the onethird plateau is thoroughly investigated with the help of the corresponding exact eigenvector. The rigorous results for the spin1/2 IsingHeisenberg model on the ShastrySutherland lattice are compared with the analogous results for the purely classical Ising and fully quantum Heisenberg models. Finally, we discuss to what extent the critical fields of SrCu2(BO3)2 and (CuCl)Ca2Nb3O10 can be described within the suggested IsingHeisenberg model.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We report on the real space profile of spin polarons in the quasi twodimensional frustrated dimer spin system SrCu2(BO3)2 doped with 0.16% of Zn. The 11B nuclear magnetic resonance spectrum exhibits 15 additional boron sites near nonmagnetic Zn impurities. With the help of exact diagonalizations of finite clusters, we have deduced from the boron spectrum the distribution of local magnetizations at the Cu sites with fine spatial resolution, providing direct evidence for an extended spin polaron. The results are confronted with those of other experiments performed on doped and undoped samples of SrCu2(BO3)2.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Using infinite projected entangledpair states, we show that the ShastrySutherland model in an external magnetic field has lowmagnetization plateaus which, in contrast to previous predictions, correspond to crystals of bound states of triplets, and not to crystals of triplets. The first sizable plateaus appear at magnetization 1/8, 2/15 and 1/6, in agreement with experiments on the orthogonaldimer antiferromagnet SrCu2(BO3)2, and they can be naturally understood as regular patterns of bound states, including the intriguing 2/15 one. We also show that, even in a confined geometry, two triplets bind into a localized bound state with Sz=2. Finally, we discuss the role of competing domainwall and supersolid phases, as well as that of additional anisotropic interactions.Physical Review Letters 04/2014; 112(14):147203. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The magnetization process of the orthogonaldimer antiferromagnet SrCu_{2}(BO_{3})_{2} is investigated in high magnetic fields of up to 118 T. A 1/2 plateau is clearly observed in the field range 84 to 108 T in addition to 1/8, 1/4, and 1/3 plateaus at lower fields. Using a combination of stateoftheart numerical simulations, the main features of the highfield magnetization, a 1/2 plateau of width 24 T, a 1/3 plateau of width 34 T, and no 2/5 plateau, are shown to agree quantitatively with the ShastrySutherland model if the ratio of inter to intradimer exchange interactions J^{'}/J=0.63. It is further predicted that the intermediate phase between the 1/3 and 1/2 plateaus is not uniform but consists of a 1/3 supersolid followed by a 2/5 supersolid and possibly a domainwall phase, with a reentrance into the 1/3 supersolid above the 1/2 plateau.Physical Review Letters 09/2013; 111(13):137204. · 7.73 Impact Factor 
Dataset: Magnetization of SrCu2(BO3)2 in ultrahigh magnetic fields up to 118 T: supplemental material
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ABSTRACT: We revisit the old problem of the spin1/2 Heisenberg antiferromagnet on kagome, a major candidate for a quantum spin liquid, focusing on the renormalization of nearestneighbor valencebond (NNVB) tunneling amplitudes by longrange singlet fluctuations outside the NNVB basis. We find that such virtual singlets have a dramatic influence on the dominant quantum dimer model (QDM) parameters, leading to qualitatively different physics within the NNVB framework. Based on recent numerical studies on the unconstrained QDM, the renormalized parameters favor a Z2 spin liquid, thus providing a plausible way to reconcile the NNVB description with density matrix renormalization group (DMRG) studies.08/2013; 
Article: Phase diagram of the spin1 Heisenberg model with threesite interactions on the square lattice
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ABSTRACT: We study the spin S=1 antiferromagnetic Heisenberg model on the square lattice with, in addition to the nearestneighbor interaction, a threesite interaction of the form (S_i S_j)*(S_j S_k) + h.c.. This interaction appears naturally in a strong coupling exansion of the twoorbital, halffilled Hubbard model. For spin 1/2, this model reduces to a Heisenberg model with bilinear interactions up to third neighbors, with a secondneighbor interaction twice as large the thirdneighbor one, a very frustrated model with an infinite family of helical classical ground states in a large parameter range. Using a variety of analytical and numerical methods, we show that the spin1 case is also very frustrated, and that its phase diagram is even richer, with possibly the succession of seven different phases as a function of the ratio of the threesite interaction to the bilinear one. The phases are either purely magnetic phases with collinear order, or of mixed magnetic and quadrupolar character with helical order.Physical Review B 07/2013; 88(9). · 3.66 Impact Factor 
Article: Exact Ground States of Frustrated Spin1 IsingHeisenberg and Heisenberg Ladders in a Magnetic Field
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ABSTRACT: Ground states of the frustrated spin1 IsingHeisenberg twoleg ladder with Heisenberg intrarung coupling and only Ising interaction along legs and diagonals are rigorously found by taking advantage of local conservation of the total spin on each rung. The constructed groundstate phase diagram of the frustrated spin1 IsingHeisenberg ladder is then compared with the analogous phase diagram of the fully quantum spin1 Heisenberg twoleg ladder obtained by density matrix renormalization group (DMRG) calculations. It is demonstrated that both investigated spin models exhibit quite similar magnetization scenarios, which involve intermediate plateaux at onequarter, onehalf and threequarters of the saturation magnetization.05/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the groundstate properties of the highly degenerate noncoplanar phase of the classical bilinearbiquadratic Heisenberg model on the triangular lattice with Monte Carlo simulations. For that purpose, we introduce an Ising pseudospin representation of the ground states, and we use a simple Metropolis algorithm with local updates, as well as a powerful cluster algorithm. At sizes that can be sampled with local updates, the presence of longrange order is surprisingly combined with an algebraic decay of correlations and the complete disordering of the chirality. It is only thanks to the investigation of unusually large systems (containing $\sim 10^8$ spins) with cluster updates that the true asymptotic regime can be reached and that the system can be proven to consist of equivalent (i.e., equally ordered) sublattices. These largescale simulations also demonstrate that the scalar chirality exhibits longrange order at zero temperature, implying that the system has to undergo a finitetemperature phase transition. Finally, we show that the average distance in the order parameter space, which has the structure of an infinite Cayley tree, remains remarkably small between any pair of points, even in the limit when the real space distance between them tends to infinity.Physical review. B, Condensed matter 05/2013; 88(9). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study a quantum version of the threestate Potts model that includes as special cases the effective models of bosons and fermions on the square lattice in the Mott insulating limit. It can be viewed as a model of quantum permutations with amplitudes J_parallel and J_perp for identical and different colors, respectively. For J_parallel=J_perp>0, it is equivalent to the SU(3) Heisenberg model, which describes the Mott insulating phase of 3color fermions, while the parameter range J_perp<min(0,J_parallel) can be realized in the Mott insulating phase of 3color bosonic atoms. Using linear flavor wave theory, infinite projected entangledpair states (iPEPS), and continuoustime quantum MonteCarlo simulations, we construct the full T=0 phase diagram, and we explore the T>0 properties for J_perp<0. For dominant antiferromagnetic J_parallel interactions, a threesublattice longrange ordered stripe state is selected out of the ground state manifold of the antiferromagnetic Potts model by quantum fluctuations. Upon increasing J_perp, this state is replaced by a uniform superfluid for J_perp<0, and by an exotic threesublattice superfluid followed by a twosublattice superfluid for J_perp>0. The transition out of the uniform superfluid (that can be realized with bosons) is shown to be a peculiar type of KosterlitzThouless transition with three types of elementary vortices.Physical Review B 04/2013; 88(15). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the properties of antiferromagnetic spinS ladders with the help of local Berry phases defined by imposing a twist on one or a few local bonds. In gapped systems with time reversal symmetry, these Berry phases are quantized, hence able in principle to characterize different phases. In the case of a fully frustrated ladder where the total spin on a rung is a conserved quantity that changes abruptly upon increasing the rung coupling, we show that two Berry phases are relevant to detect such phase transitions: the rung Berry phase defined by imposing a twist on one rung coupling, and the twist Berry phase defined by twisting the boundary conditions along the legs. In the case of nonfrustrated ladders, we have followed the fate of both Berry phases when interpolating between standard ladders and dimerized spin chains. A careful investigation of the spin gap and of edge states shows that a change of twist Berry phase is associated to a quantum phase transition at which the bulk gap closes, and at which, with appropriate boundary conditions, edge states appear or disappear, while a change of rung Berry phase is not necessarily associated to a quantum phase transition. The difference is particularly acute for regular ladders, in which the twist Berry phase does not change at all upon increasing the rung coupling from zero to infinity while the rung Berry phase changes 2S times. By analogy with the fully frustrated ladder, these changes are interpreted as crossovers between domains in which the rungs are in different states of total spin from 0 in the strong rung limit to 2S in the weak rung limit. This interpretation is further supported by the observation that these crossovers turn into real phase transitions as a function of rung coupling if one rung is strongly ferromagnetic, or equivalently if one rung is replaced by a spin 2S impurity.Physical Review B 04/2013; 88(18). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that spinS ladders undergo, for antiferromagnetic rung coupling, a series of 4S topological phase transitions when the ratio of the leg to rung coupling changes from strongly antiferromagnetic to strongly ferromagnetic. These phase transitions are characterized by a change of Berry phase on the rungs, and, with appropriate boundary conditions, by the appearance and disappearance of edge states without the bulk gap ever closing. The various phases can be distinguished by the number of valence bond singlets on the rungs, which decreases from 2S for strong rung coupling to 0 for dominant leg coupling. In particular, the isotropic spin3/2 ladder with equal leg and rung coupling is explicitly shown to be in the valence bond solid phase with one singlet per bond.04/2013; 
Article: Phase diagram of quantum square ice
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ABSTRACT: We have investigated the groundstate and finitetemperature phase diagram of quantum square ice  realized by the transversefield Ising model on a checkerboard lattice  using both linear spinwave (LSW) theory and quantum Monte Carlo (QMC). We generalize the model with different couplings between nearest (J1) and nexttonearest (J2) neighbors on the checkerboard lattice. Our QMC approach generalizes the loop algorithm  very efficient in the study of constrained classical systems  to a ``brane algorithm'' for quantum systems. At the LSW level the vast degeneracy of the groundstate for J1=J2 and J2>J1 remains intact; moreover LSW theory breaks down in extended regions of the phase diagram, pointing at nonclassical states [1]. Our QMC study goes beyond perturbative schemes and addresses directly the nature of the lowtemperature phases. We have critically examined the possibility of a resonatingplaquette state for J1=J2, suggested by degenerate perturbation theory on the icerule manifold for weak fields. Our QMC results for finite fields confirm the absence of Néel or collinear order, but they do not confirm the presence of resonatingplaquette order, pointing at a possibly more complex nonclassical state.[4pt] [1] L.P. Henry et al., PRB 85, 134427 (2012).03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the intriguing report, in some frustrated quantum antiferromagnets, of magnetization plateaus whose simple collinear structure is not stabilized by an external magnetic field in the classical limit, we develop a semiclassical method to estimate the zeropoint energy of collinear configurations even when they do not correspond to a local minimum of the classical energy. For the spin1/2 frustrated squarelattice antiferromagnet, this approach leads to the stabilization of a large 1/2 plateau with "upupupdown" structure for J_2/J_1>1/2, in agreement with exact diagonalization results, while for the spin1/2 anisotropic triangular antiferromagnet, it predicts that the 1/3 plateau with "upupdown" structure is stable far from the isotropic point, in agreement with the properties of Cs2CuBr4.Physical Review B 02/2013; 87(6):060407. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Conflicting predictions have been made for the ground state of the SU(3) Heisenberg model on the honeycomb lattice: Tensor network simulations found a plaquette order [Zhao et al, Phys. Rev. B 85, 134416 (2012)], where singlets are formed on hexagons, while linear flavorwave theory (LFWT) suggested a dimerized, color ordered state [Lee and Yang, Phys. Rev. B 85, 100402 (2012)]. In this work we show that the former state is the true ground state by a systematic study with infinite projectedentangled pair states (iPEPS), for which the accuracy can be systematically controlled by the socalled bond dimension $D$. Both competing states can be reproduced with iPEPS by using different unit cell sizes. For small $D$ the dimer state has a lower variational energy than the plaquette state, however, for large $D$ it is the latter which becomes energetically favorable. The plaquette formation is also confirmed by exact diagonalizations and variational Monte Carlo studies, according to which both the dimerized and plaquette states are nonchiral flux states.Physical review. B, Condensed matter 02/2013; 87(19). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Building on the generalization of the exactly dimerized MajumdarGhosh ground state to arbitrary spin S for the Heisenberg chain with a threesite term (S_{i1} S_i)(S_i S_{i+1}) + H.c., we use densitymatrix renormalization group simulations and exact diagonalizations to determine the nature of the dimerization transition for S=1, 3/2 and 2. The resulting central charge and critical exponent are in good agreement with the SU(2)_{k=2S} WessZuminoWitten values c=3k/(2+k) and \eta=3/(2+k). Since the 3site term that induces dimerization appears naturally if exchange interactions are calculated beyond second order, these results suggest that SU(2)_{k>1} WessZuminoWitten models might finally be realized in actual spin chains.Physical review. B, Condensed matter 01/2013; 87(14). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The ground state and zerotemperature magnetization process of the spin1/2 IsingHeisenberg model on twodimensional trianglesintriangles lattices is exactly calculated using eigenstates of the smallest commuting spin clusters. Our groundstate analysis of the investigated classicalquantum spin model reveals three unconventional dimerized or trimerized quantum ground states besides two classical ground states. It is demonstrated that the spin frustration is responsible for a variety of magnetization scenarios with up to three or four intermediate magnetization plateaus of either quantum or classical nature. The exact analytical results for the IsingHeisenberg model are confronted with the corresponding results for the purely quantum Heisenberg model, which were obtained by numerical exact diagonalizations based on the Lanczos algorithm for finitesize spin clusters of 24 and 21 sites, respectively. It is shown that the zerotemperature magnetization process of both models is quite reminiscent and hence, one may obtain some insight into the ground states of the quantum Heisenberg model from the rigorous results for the IsingHeisenberg model even though exact ground states for the IsingHeisenberg model do not represent true ground states for the pure quantum Heisenberg model.Physical review. B, Condensed matter 01/2013; 87(5). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In order to understand the nature of the twodimensional BoseEinstein condensed (BEC) phase in BaCuSi2O6, we performed detailed 63Cu and 29Si NMR above the critical magnetic field, Hc1= 23.4 T. The two different alternating layers present in the system have very different local magnetizations close to Hc1; one is very weak, and its size and field dependence are highly sensitive to the nature of interlayer coupling. Its precise value could only be determined by "onsite" 63Cu NMR, and the data are fully reproduced by a model of interacting hardcore bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed.Physical review. B, Condensed matter 12/2012; 87(18). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We simulate the ShastrySutherland model in two dimensions by means of infinite projected entangledpair states (iPEPS)  a variational tensor network method where the accuracy can be systematically controlled by the socalled bond dimension. Besides the well established dimer and N\'eel phase iPEPS confirms the presence of an intermediate phase with plaquette longrange order, and we determine its phase boundaries with high accuracy. The first order phase transition for $J=0.675(2)$ between dimer and plaquette phase is compatible with previous series expansion results. iPEPS predicts a weak firstorder phase transition between plaquette and N\'eel phase occurring for $J=0.765(15)$. We do not find a stable intermediate columnardimer phase, even when we bias the state towards this order.Physical review. B, Condensed matter 12/2012; 87(11). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the possibility to load multicolor fermionic atoms in optical lattices, we study the entropy dependence of the properties of the onedimensional antiferromagnetic SU(N) Heisenberg model, the effective model of the SU(N) Hubbard model with one particle per site (filling 1/N) in the large U/t limit. Using continuoustime worldline Monte Carlo simulations for N=25, we show that characteristic shortrange correlations develop at low temperature as a precursor of the ground state algebraic correlations. We also calculate the entropy as a function of temperature, and we show that the first sign of shortrange order appears at an entropy per particle that increases with N and already reaches 0.8k_{B} at N=4, in the range of experimentally accessible values.Physical Review Letters 11/2012; 109(20):205306. · 7.73 Impact Factor
Publication Stats
4k  Citations  
825.54  Total Impact Points  
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Institutions

2005–2014

École Polytechnique Fédérale de Lausanne
 Institute of Theoretical Physics
Lausanne, Vaud, Switzerland


2002–2013

The University of Tokyo
 Institute for Solid State Physics
Edo, Tōkyō, Japan


2001–2012

French National Centre for Scientific Research
 Institut Néel
Lutetia Parisorum, ÎledeFrance, France


2009–2011

Université ParisSud 11
 Laboratoire de Physique des Solides
Paris, IledeFrance, France


1989–2011

ETH Zurich
 • Institute for Theoretical Physics
 • Laboratory for Solid State Physics
Zürich, ZH, Switzerland


1993–2007

Paul Sabatier University  Toulouse III
 Laboratoire de Physique Théorique  UMR 5152  LPT
Toulouse, MidiPyrenees, France


1990–2007

Rutgers, The State University of New Jersey
 Department of Physics
New Brunswick, New Jersey, United States


2004

University Joseph Fourier  Grenoble 1
Grenoble, RhôneAlpes, France


2000–2004

University of Lausanne
 Institut de géophysique
Lausanne, VD, Switzerland
