Publications (271)918.44 Total impact
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ABSTRACT: Quantum antiferromagnets have proven to be some of the cleanest realizations available for theoretical, numerical, and experimental studies of quantum fluctuation effects. At finite temperatures, however, the additional effects of thermal fluctuations in the restricted phase space of a lowdimensional system have received much less attention, particularly the situation in frustrated quantum magnets, where the excitations may be complex collective (bound or even fractionalized) modes. We investigate this problem by studying the thermodynamic properties of the frustrated twoleg S=1/2 spin ladder, with particular emphasis on the fully frustrated case. We present numerical results for the magnetic specific heat and susceptibility, obtained from exact diagonalization and quantum Monte Carlo studies, which we show can be rendered free of the sign problem even in a strongly frustrated system and which allow us to reach unprecedented sizes of L=200 ladder rungs. We find that frustration effects cause an unconventional evolution of the thermodynamic response across the full parameter regime of the model. However, close to the firstorder transition they cause a highly anomalous reduction in temperature scales with no concomitant changes in the gap; the specific heat shows a very narrow peak at very low energies and the susceptibility rises abruptly at extremely low temperatures. Unusually, the two quantities have different gaps over an extended region of the parameter space. We demonstrate that these results reflect the presence of large numbers of multiparticle boundstate excitations, whose energies fall below the onetriplon gap in the transition region. 
Article: Magnons in tetragonal CuO
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ABSTRACT: We investigate by resonant inelastic xray scattering the magnetic excitations in thin films of tetragonal CuO. We identify a spin wave excitation, dispersing on two cupratelike antiferromagnetic sublattices. Its energy at the boundary of the Brillouin zone (220 meV), is significantly lower than typical values (E∼300 meV) found in twodimensional cuprates. A spin wave expansion starting from an extended Hubbard model suggests two possible scenarios for this energy lowering. © 2015 American Physical Society.Physical Review B 10/2015; 92(14). DOI:10.1103/PhysRevB.92.140404 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recent experiments on the Ba3XSb2O9 family have revealed materials that potentially realize spin and spinorbital liquid physics. However, the lattice structure of these materials is complicated due to the presence of charged X2+Sb5+ dumbbells, with two possible orientations. To model the lattice structure, we consider a frustrated model of charged dumbbells on the triangular lattice, with longrange Coulomb interactions. We study this model using Monte Carlo simulation, and find a freezing temperature, Tfrz, at which the simulated structure factor matches well to lowtemperature xray diffraction data for Ba3CuSb2O9. At T=Tfrz we find a complicated "branching" structure of superexchangelinked X2+ clusters, which form a fractal pattern with fractal dimension df=1.90. We show that this gives a natural explanation for the presence of orphan spins. Finally we provide a plausible mechanism by which such dumbbell disorder can promote a spinorbital resonant state with delocalized orphan spins.Physical Review Letters 10/2015; 115(14). DOI:10.1103/PhysRevLett.115.147202 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Using quantum Monte Carlo simulations along with higherorder spinwave theory, bondoperator and strongcoupling expansions, we analyse the dynamical spin structure factor of the spinhalf Heisenberg model on the squarelattice bilayer. We identify distinct contributions from the lowenergy Goldstone modes in the magnetically ordered phase and the gapped triplon modes in the quantum disordered phase. In the antisymmetric (with respect to layer inversion) channel, the dynamical spin structure factor exhibits a continuous evolution of spectral features across the quantum phase transition, connecting the two types of modes. Instead, in the symmetric channel we find a depletion of the spectral weight when moving from the ordered to the disordered phase. While the dynamical spin structure factor does not exhibit a welldefined distinct contribution from the amplitude (or Higgs) mode in the ordered phase, we identify an only marginallydamped amplitude mode in the dynamical singlet structure factor, obtained from interlayer bond correlations, in the vicinity of the quantum critical point. These findings provide quantitative information in direct relation to possible neutron or light scattering experiments in a fundamental twodimensional quantumcritical spin system.  [Show abstract] [Hide abstract]
ABSTRACT: The magnetic excitation spectrum in the bilayer iridate Sr$_3$Ir$_2$O$_7$ has been investigated using highresolution resonant inelastic xray scattering (RIXS) performed at the iridium L$_3$ edge and theoretical techniques. A study of the systematic dependence of the RIXS spectrum on the orientation of the wavevector transfer, $\mathbf{Q}$, with respect to the iridiumoxide bilayer has revealed that the magnon dispersion is comprised of two branches well separated in energy and gapped across the entire Brillouin zone. Our results contrast with those of an earlier study which reported the existence of a single dominant branch. While these earlier results were interpreted as two overlapping modes within a spinwave model of weakly coupled iridiumoxide planes, our results are more reminiscent of those expected for a system of weakly coupled dimers. In this latter approach the lower and higher energy modes find a natural explanation as those corresponding to transverse and longitudinal fluctuations, respectively. We have therefore developed a bondoperator theory which describes the magnetic dispersion in Sr$_3$Ir$_2$O$_7$ in terms of quantum dimer excitations. In our model dimerisation is produced by the leading Heisenberg exchange, $J_c$, which couples iridium ions in adjacent planes of the bilayer. The Hamiltonian also includes in plane exchange, $J$, as well as further neighbour couplings and relevant anisotropies. The bondoperator theory provides an excellent account of the dispersion of both modes, while the measured $\mathbf{Q}$ dependence of the RIXS intensities is in reasonable qualitative accord with the spinspin correlation function calculated from the theory. We discuss our results in the context of the quantum criticality of bilayer dimer systems in the presence of anisotropic interactions derived from strong spinorbit coupling.Physical Review B 07/2015; 92:024405. DOI:10.1103/PhysRevB.92.024405 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recent experiments on the Ba$_3$XSb$_2$O$_9$ family have revealed materials that potentially realise spin and spinorbital liquid physics. However, the lattice structure of these materials is complicated due to the presence of charged X$^{2+}$Sb$^{5+}$ dumbbells, with two possible orientations. To model the lattice structure, we consider a frustrated model of charged dumbbells on the triangular lattice, with longrange Coulomb interactions. We study this model using Monte Carlo simulation, and find a freezing temperature, $T_{\sf frz}$, at which the simulated structure factor matches well to lowtemperature xray diffraction data for Ba$_3$CuSb$_2$O$_9$. At $T=T_{\sf frz}$ we find a complicated "branching" structure of superexchangelinked X$^{2+}$ clusters, and show that this gives a natural explanation for the presence of orphan spins. Finally we provide a plausible mechanism by which such dumbbell disorder can promote a spinorbital resonant state with delocalised orphan spins.  [Show abstract] [Hide abstract]
ABSTRACT: We report on the real space profile of spin polarons in the quasitwodimensional frustrated dimer spin system SrCu_{2}(BO_{3})_{2} doped with 0.16% of Zn. The ^{11}B 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 SrCu_{2}(BO_{3})_{2}.Physical Review Letters 02/2015; 114(5):056402. · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experimental progress in the context of ultracold multicolor fermionic atoms in optical lattices, we have investigated the properties of the SU($N$) Heisenberg chain with totally antisymmetric irreducible representations, the effective model of Mott phases with $m < N$ particles per site. These models have been studied for arbitrary $N$ and $m$ with nonabelian bosonization [I. Affleck, Nuclear Physics B 265, 409 (1986); 305, 582 (1988)], leading to predictions about the nature of the ground state (gapped or critical) in most but not all cases. Using exact diagonalization and variational MonteCarlo based on Gutzwiller projected fermionic wave functions, we have been able to verify these predictions for a representative number of cases with $N \leq 10$ and $m \leq N/2$, and we have shown that the opening of a gap is associated to a spontaneous dimerization or trimerization depending on the value of m and N. We have also investigated the marginal cases where abelian bosonization did not lead to any prediction. In these cases, variational MonteCarlo predicts that the ground state is critical with exponents consistent with conformal field theory.Physical Review B 02/2015; 91(17). DOI:10.1103/PhysRevB.91.174427 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The magnetic excitation spectrum in the bilayer iridate Sr3Ir2O7 has been investigated using highresolution resonant inelastic xray scattering (RIXS) performed at the iridium L3 edge and theoretical techniques. A study of the systematic dependence of the RIXS spectrum on the orientation of the wavevector transfer Q, with respect to the iridiumoxide bilayer, has revealed that the magnon dispersion is comprised of two branches well separated in energy and gapped across the entire Brillouin zone. Our results contrast with those of an earlier study which reported the existence of a single dominant branch. While these earlier results were interpreted as two overlapping modes within a spinwave model of weakly coupled iridiumoxide planes, our results are more reminiscent of those expected for a system of weakly coupled dimers. In this latter approach, the lowerand higherenergy modes find a natural explanation as those corresponding to transverse and longitudinal fluctuations, respectively. We have therefore developed a bondoperator theory which describes the magnetic dispersion in Sr3Ir2O7 in terms of quantum dimer excitations. In our model, dimerization is produced by the leading Heisenberg exchange J(c), which couples iridium ions in adjacent planes of the bilayer. The Hamiltonian also includes inplane exchange J, as well as further neighbor couplings and relevant anisotropies. The bondoperator theory provides an excellent account of the dispersion of both modes, while the measured Q dependence of the RIXS intensities is in reasonable qualitative accord with the spinspin correlation function calculated from the theory. We discuss our results in the context of the quantum criticality of bilayer dimer systems in the presence of anisotropic interactions derived from strong spinorbit coupling.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate by angleresolved photoemission the electronic structure of in situ grown tetragonal CuO, a synthetic quasitwodimensional edgesharing cuprate. We show that, in spite of the very different nature of the copper oxide layers, with twice as many Cu in the CuO layers of tetragonal CuO as compared to the CuO_{2} layers of the highT_{c} cuprates, the lowenergy electronic excitations are surprisingly similar, with a ZhangRice singlet dispersing on weakly coupled cupratelike sublattices. This system should thus be considered as a member of the highT_{c} cuprate family, with, however, interesting differences due to the intralayer coupling between the cupratelike sublattices.Physical Review Letters 10/2014; 113(18):187001. DOI:10.1103/PhysRevLett.113.187001 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Building on advanced results on permutations, we show that it is possible to construct, for each irreducible representation of SU(N), an orthonormal basis labelled by the set of {\it standard Young tableaux} in which the matrix of the Heisenberg SU(N) model (the quantum permutation of Ncolor objects) takes an explicit and extremely simple form. Since the relative dimension of the full Hilbert space to that of the singlet space on $n$ sites increases very fast with N, this formulation allows to extend exact diagonalizations of finite clusters to much larger values of N than accessible so far. Using this method, we show that, on the square lattice, there is longrange color order for SU(5), spontaneous dimerization for SU(8), and evidence in favor of a quantum liquid for SU(10).Physical Review Letters 08/2014; 113(12). DOI:10.1103/PhysRevLett.113.127204 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
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.Physical Review B 07/2014; 90(13). DOI:10.1103/PhysRevB.90.134413 · 3.74 Impact Factor  [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.Physical Review Letters 07/2014; 114(5). DOI:10.1103/PhysRevLett.114.056402 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the absence of both spin freezing and a cooperative JahnTeller effect at the lowest measured temperatures, we study the ground state of Ba3CuSb2O9. We solve a general spinorbital model on both the honeycomb and the decorated honeycomb lattice, revealing rich phase diagrams. The spinorbital model on the honeycomb lattice contains an SU(4) point, where previous studies have shown the existence of a spinorbital liquid with algebraically decaying correlations. For realistic parameters on the decorated honeycomb lattice, we find a phase that consists of clusters of nearestneighbour spin singlets, which can be understood in terms of dimer coverings of an emergent square lattice. While the experimental situation is complicated by structural disorder, we show qualitative agreement between our theory and a range of experiments.Physical Review B 06/2014; 90(9). DOI:10.1103/PhysRevB.90.094422 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Using infinite projected entangledpair states (iPEPS), 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 {\it 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 SrCu$_2$(BO$_3$)$_2$, 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 {\it localized} bound state with $S_z=2$. 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. DOI:10.1103/PhysRevLett.112.147203 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the structure of the spectrum of antiferromagnetically coupled spin1 bosons on a square lattice using degenerate perturbation theory and exact diagonalizations of finite clusters. We show that the superfluid phase develops an Anderson tower of states typical of nematic longrange order with broken SU(2) symmetry, by contrast to the case of spin1 bosons in a trap where a genuine polar superfluid ground state only appears in the presence of an external perturbation that breaks the SU(2) symmetry. We further show that this order persists into the Mott insulating phase down to zero hopping for one boson per site, and down to a critical hopping for two bosons per site, in agreement with meanfield and Quantum Monte Carlo results.Physical Review Letters 03/2014; 113(20). DOI:10.1103/PhysRevLett.113.200402 · 7.51 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. DOI:10.1103/PhysRevLett.111.137204 · 7.51 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.Physical Review B 08/2013; 90(10). DOI:10.1103/PhysRevB.90.100406 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
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). DOI:10.1103/PhysRevB.88.094435 · 3.74 Impact Factor
Publication Stats
6k  Citations  
918.44  Total Impact Points  
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Institutions

20042015

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


20002012

University of Lausanne
 Institut de géophysique
Lausanne, Vaud, Switzerland


2007

Université de Neuchâtel
 Institut de physique (IPH)
Neuchâtel, NE, Switzerland


19932007

Paul Sabatier University  Toulouse III
 Laboratoire de Physique Théorique  UMR 5152  LPT
Tolosa de Llenguadoc, MidiPyrénées, France


19902007

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


2001

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