Karlo Penc

Hungarian Academy of Sciences, Budapeŝto, Budapest, Hungary

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Publications (121)378.98 Total impact

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    ABSTRACT: We revisit the SU(6) Heisenberg model on the honeycomb lattice, which has been predicted to be a chiral spin liquid by mean-field theory [G. Szirmai et al., Phys. Rev. A 84, 011611 (2011)]. Using exact diagonalizations of finite clusters, infinite projected entangled pair states simulations, and variational Monte Carlo simulations based on Gutzwiller projected wave functions, we provide strong evidence in favour of the competing plaquette state, which was reported to be higher but close by in energy according to mean-field theory. This is further confirmed by the investigation of the model with a ring exchange term, which shows that there is a transition between the plaquette state and the chiral state at a finite value of the ring exchange term.
    Preview · Article · Jan 2016
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    ABSTRACT: We show that, in the presence of a $\pi/2$ artificial gauge field per plaquette, Mott insulating phases of ultra-cold fermions with $SU(N)$ symmetry and one particle per site generically possess an extended chiral phase with intrinsic topological order characterized by a multiplet of $N$ low-lying singlet excitations for periodic boundary conditions, and by chiral edge states described by the $SU(N)_1$ Wess-Zumino-Novikov-Witten conformal field theory for open boundary conditions. This has been achieved by extensive exact diagonalizations for $N$ between $3$ and $9$, and by a parton construction based on a set of $N$ Gutzwiller projected fermionic wave-functions with flux $\pi/N$ per triangular plaquette. Experimental implications are briefly discussed.
    Preview · Article · Jan 2016
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    ABSTRACT: Recent experiments on the spin-ice material Dy2Ti2O7 suggest that the Pauling "ice entropy," characteristic of its classical Coulombic spin-liquid state, may be lost at low temperatures [Pomaranski, Nat. Phys. 9, 353 (2013)1745-247310.1038/nphys2591]. However, despite nearly two decades of intensive study, the nature of the equilibrium ground state of spin ice remains uncertain. Here we explore how long-range dipolar interactions D, short-range exchange interactions, and quantum fluctuations combine to determine the ground state of dipolar spin ice. We identify the organizational principle that ordered ground states are selected from a set of "chain states" in which dipolar interactions are exponentially screened. Using both quantum and classical Monte Carlo simulation, we establish phase diagrams as a function of quantum tunneling g and temperature T, and find that only a very small gcaD is needed to stabilize a quantum spin liquid ground state. We discuss the implications of these results for Dy2Ti2O7.
    No preview · Article · Sep 2015 · Physical Review B
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    Judit Romhányi · Karlo Penc · R Ganesh
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    ABSTRACT: SrCu2(BO3)2 is the archetypal quantum magnet with a gapped dimer-singlet ground state and triplon excitations. It serves as an excellent realization of the Shastry-Sutherland model, up to small anisotropies arising from Dzyaloshinskii-Moriya interactions. Here we demonstrate that these anisotropies, in fact, give rise to topological character in the triplon band structure. The triplons form a new kind of Dirac cone with three bands touching at a single point, a spin-1 generalization of graphene. An applied magnetic field opens band gaps resulting in topological bands with Chern numbers ±2. SrCu2(BO3)2 thus provides a magnetic analogue of the integer quantum Hall effect and supports topologically protected edge modes. At a threshold value of the magnetic field set by the Dzyaloshinskii-Moriya interactions, the three triplon bands touch once again in a spin-1 Dirac cone, and lose their topological character. We predict a strong thermal Hall signature in the topological regime.
    Full-text · Article · Apr 2015 · Nature Communications
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    ABSTRACT: Recent experiments on the spin ice Dy$_2$Ti$_2$O$_7$, suggest that the Pauling "ice entropy", characteristic of its Coulombic spin-liquid state, may be lost at low temperatures [D. Pomaranski et al., Nature Phys. 9, 353 (2013)]. However, despite nearly two decades of intensive study, the nature of the equilibrium ground state of spin-ice remains uncertain. Here we explore how long-range dipolar interactions $D$, short-range exchange interactions and quantum fluctuations combine to determine the ground state of dipolar spin ice. We find that ordered ground states are selected from a set of "chain states" in which dipolar interactions are exponentially screened. Using both quantum and classical Monte Carlo simulation, we establish phase diagrams as a function of quantum tunneling $g$, and temperature $T$, and find that only a very small $g_c \ll D$ is needed to stabilize a quantum spin-liquid ground state. We discuss the implications of these results for Dy$_2$Ti$_2$O$_7$, and for the "quantum spin ice" materials Yb$_2$Ti$_2$O$_7$, Tb$_2$Ti$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$.
    Preview · Article · Oct 2014
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    Judit Romhányi · Karlo Penc · R. Ganesh
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    ABSTRACT: The celebrated Shastry Sutherland model has a gapped dimer singlet ground state. The material SrCu$_2$(BO$_3$)$_2$ serves as a good realization of this model, upto small anisotropies arising from Dzyaloshinskii Moriya (DM) interactions. The DM interactions admix a triplet component into the singlet ground state and give rise to weakly dispersing triplon bands. We show that an applied magnetic field splits the triplon modes and opens band gaps. Surprisingly, we are left with topological bands with Chern numbers $\pm 2$. SrCu$_2$(BO$_3$)$_2$ thus supports topologically protected triplonic edge modes and is a magnetic analogue of the integer quantum Hall effect. At a critical value of the magnetic field set by the strength of DM interactions, the three triplon bands touch once again in a spin-1 generalization of a Dirac cone, and lose their topological character. We predict a strong thermal Hall signature in the topological regime.
    Full-text · Article · Jun 2014
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    ABSTRACT: We study electrons hopping on a kagome lattice at third filling described by an extended Hubbard Hamiltonian with on-site and nearest-neighbour repulsions in the strongly correlated limit. As a consequence of the commensurate filling and the large interactions, each triangle has precisely two electrons in the effective low energy description, and these electrons form chains of different lengths. The effective Hamiltonian includes the ring exchange around the hexagons as well as the nearest- neighbor Heisenberg interaction. Using large scale exact diagonalization, we find that the effective model exhibits two different phases: If the charge fluctuations are small, the magnetic fluctuations confine the charges to short loops around hexagons, yielding a gapped charge ordered phase. When the charge fluctuations dominate, the system undergoes a quantum phase transition to a resonating plaquette phase with ordered spins and gapless spin excitations. We find that a peculiar conservation law is fulfilled: the electron in the chains can be divided into two sublattices, and this division is conserved by the ring exchange term.
    Full-text · Article · Feb 2014 · Physical Review B
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    Shijie Hu · Ari M. Turner · Karlo Penc · Frank Pollmann
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    ABSTRACT: We investigate the effect of the Berry phase on quadrupoles that occur for example in the low-energy description of spin models. Specifically we study here the one-dimensional bilinear-biquadratic spin-one model. An open question for many years about this model is whether it has a non-dimerized fluctuating nematic phase. The dimerization has recently been proposed to be related to Berry phases of the quantum fluctuations. We use an effective low-energy description to calculate the scaling of the dimerization according to this theory, and then verify the predictions using large scale density-matrix renormalization group (DMRG) simulations, giving good evidence that the state is dimerized all the way up to its transition into the ferromagnetic phase. We furthermore discuss the multiplet structure found in the entanglement spectrum of the ground state wave functions.
    Preview · Article · Jan 2014 · Physical Review Letters
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    ABSTRACT: We investigate the ground-state properties of the highly degenerate non-coplanar phase of the classical bilinear-biquadratic 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 long-range 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 large-scale simulations also demonstrate that the scalar chirality exhibits long-range order at zero temperature, implying that the system has to undergo a finite-temperature 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.
    Preview · Article · May 2013 · Physical review. B, Condensed matter
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    Miklos Lajko · Karlo Penc
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    ABSTRACT: The SU(4) Heisenberg model can serve as a low energy model of the Mott insulating state in materials where the spins and orbitals are highly symmetric, or in systems of alkaline-earth atoms on optical lattice. Recently, it has been argued that on the honeycomb lattice the model exhibits a unique spin-orbital liquid phase with an algebraic decay of correlations [P. Corboz et al., Phys. Rev. X 2, 041013 (2012)]. Here we study the instability of the algebraic spin-orbital liquid toward spontaneous formation of SU(4) singlet plaquettes (tetramerization). Using a variational Monte Carlo approach to evaluate the projected wave-function of fermions with $\pi$-flux state, we find that the algebraic liquid is robust, and that a finite value of the next nearest exchange is needed to induce tetramerization. We also studied the phase diagram of a model which interpolates between the nearest neighbor Heisenberg model and a Hamiltonian for which the singlet-plaquette product state is an exact ground state.
    Preview · Article · Mar 2013 · Physical review. B, Condensed matter
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    ABSTRACT: Ba2CoGe2O7 is a multiferroic material where spin waves exhibit giant directional dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric effect [S. Bordacs et al., Nature Physics 8, 734 (2012)]. We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba2CoGe2O7 in high magnetic fields up to 33 T [Penc et al., Phys. Rev. Lett. 108, 257203 (2012)]. In the ESR and THz absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. A multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active.
    No preview · Article · Mar 2013
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    ABSTRACT: Dynamical properties of the lattice structure were studied by optical spectroscopy in ACr2O4 chromium spinel oxide magnetic semiconductors over a broad temperature region of T=10–335 K. The systematic change of the A-site ions (A= Mn, Fe, Co, Ni and Cu) showed that the occupancy of 3d orbitals on the A site has strong impact on the lattice dynamics. For compounds with orbital degeneracy (FeCr2O4, NiCr2O4, and CuCr2O4), clear splitting of infrared-active phonon modes and/or activation of silent vibrational modes have been observed upon the Jahn-Teller transition and at the onset of the subsequent long-range magnetic order. Although MnCr2O4 and CoCr2O4 show multiferroic and magnetoelectric character, no considerable magnetoelasticity was found in spinel compounds without orbital degeneracy as they closely preserve the high-temperature cubic spinel structure even in their magnetic ground state. Aside from lattice vibrations, intra-atomic 3d-3d transitions of the A2+ ions were also investigated to determine the crystal field and Racah parameters and the strength of the spin-orbit coupling.
    Full-text · Article · Feb 2013 · Physical review. B, Condensed matter
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    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 flavor-wave 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 projected-entangled pair states (iPEPS), for which the accuracy can be systematically controlled by the so-called 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 non-chiral flux states.
    Preview · Article · Feb 2013 · Physical review. B, Condensed matter
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    ABSTRACT: Faraday rotation and magneto-optical absorption spectral measurements were conducted on a spinel oxide, ZnCr2O4, a prototype of three-dimensional geometrically frustrated magnet. The measurements were carried out at temperatures down to 4.6 K under ultra-high magnetic fields of up to 600 T. The ultra-high magnetic fields were generated by the electro-magnetic flux compression method. We obtained a precise magnetization curve up to a fully polarized phase, where the phase transition takes place above 400 T. The experimental magnetization curves were compared with those obtained by Monte Carlo calculations with an effective spin model including spin--lattice coupling up to fully saturated magnetization. The absorption spectral peaks of the intra-d-band transitions in Cr3+ ions as well as the exciton--magnon--phonon transition were used for monitoring the crystal and magnetic structures subjected to a strong external magnetic field. A novel magnetic phase was found prior to the fully polarized phase, which was clarified by the change in magneto-absorption intensity around 350 T. An umbrella-like magnetic structure was proposed to be the most plausible candidate for the novel phase. A physical analogy between the magnetic structures of ZnCr2O4 and the quantum phases of 4He was discussed based on the similarity of symmetry breaking.
    No preview · Article · Nov 2012 · Journal of the Physical Society of Japan
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    ABSTRACT: We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba_{2}CoGe_{2}O_{7} in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S>1/2 spin systems with strong single-ion anisotropy and/or noncentrosymmetric lattice structure.
    No preview · Article · Sep 2012 · Physical Review Letters
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    ABSTRACT: In addition to low-energy spin fluctuations, which distinguish them from band insulators, Mott insulators often possess orbital degrees of freedom when crystal-field levels are partially filled. While in most situations spins and orbitals develop long-range order, the possibility for the ground state to be a quantum liquid opens new perspectives. In this paper, we provide clear evidence that the SU(4) symmetric Kugel-Khomskii model on the honeycomb lattice is a quantum spin-orbital liquid. The absence of any form of symmetry breaking - lattice or SU(N) - is supported by a combination of semiclassical and numerical approaches: flavor-wave theory, tensor network algorithm, and exact diagonalizations. In addition, all properties revealed by these methods are very accurately accounted for by a projected variational wave-function based on the \pi-flux state of fermions on the honeycomb lattice at 1/4-filling. In that state, correlations are algebraic because of the presence of a Dirac point at the Fermi level, suggesting that the symmetric Kugel-Khomskii model on the honeycomb lattice is an algebraic quantum spin-orbital liquid. This model provides a good starting point to understand the recently discovered spin-orbital liquid behavior of Ba_3CuSb_2O_9. The present results also suggest to choose optical lattices with honeycomb geometry in the search for quantum liquids in ultra-cold four-color fermionic atoms.
    Preview · Article · Jul 2012 · Physical Review X
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    Judit Romhányi · Karlo Penc
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    ABSTRACT: We consider the square-lattice antiferromagnetic Heisenberg Hamiltonian extended with a single-ion axial anisotropy term as a minimal model for the multiferroic Ba2CoGe2O7. Developing a multiboson spin-wave theory, we investigate the dispersion of the spin excitations in this spin-3/2 system. As a consequence of a strong single-ion anisotropy, a stretching (longitudinal) spin-mode appears in the spectrum. The inelastic neutron scattering spectra of Zheludev et al. [Phys. Rev. B 68, 024428 (2003)] are successfully reproduced by the low energy modes in the multiboson spin-wave theory, and we anticipate the appearance of the spin stretching modes at 4meV that can be identified using the calculated dynamical spin structure factors. We expect the appearance of spin stretching modes for any S>1/2 compound where the single-ion anisotropy is significant.
    Full-text · Article · May 2012 · Physical review. B, Condensed matter
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    ABSTRACT: We present a numerical study of the SU(N) Heisenberg model with the fundamental representation at each site for the kagome lattice (for N=3) and the checkerboard lattice (for N=4), which are the line graphs of the honeycomb and square lattices and thus belong to the class of bisimplex lattices. Using infinite projected entangled-pair states (iPEPS) and exact diagonalizations, we show that in both cases the ground state is a simplex solid state with a two-fold ground state degeneracy, in which the N spins belonging to a simplex (i.e. a complete graph) form a singlet. Theses states can be seen as generalizations of valence bond solid states known to be stabilized in certain SU(2) spin models.
    Preview · Article · Apr 2012 · Physical review. B, Condensed matter
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    ABSTRACT: We studied spin excitations of the multiferroic Ba2CoGe2O7 in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multi-boson spin-wave theory can capture these unconventional modes, that include spin-stretching modes associated with an oscillating magnetic dipole (or only quadrupole) moment. The lack of the inversion symmetry allows these modes to become electric dipole active. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S > 1/2 spin systems with strong single-ion anisotropy and/or non-centrosymmetric lattice structure.
    Full-text · Article · Feb 2012 · Physical Review Letters
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    S. E. Korshunov · F. Mila · K. Penc
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    ABSTRACT: We investigate the zero-temperature behavior of the classical Heisenberg model on the triangular lattice in which the competition between exchange interactions of different orders favors a relative angle between neighboring spins in the interval (0,2pi/3). In this situation, the ground states are noncoplanar and have an infinite discrete degeneracy. In the generic case, the set of the ground states is in one to one correspondence (up to a global rotation) with the non-crossing loop coverings of the three equivalent honeycomb sublattices into which the bonds of the triangular lattice can be partitioned. This allows one to identify the order parameter space as an infinite Cayley tree with coordination number 3. Building on the duality between a similar loop model and the ferromagnetic O(3) model on the honeycomb lattice, we argue that a typical ground state should have long-range order in terms of spin orientation. This conclusion is further supported by the comparison with the four-state antiferromagnetic Potts model [describing the case when the angle between neighboring spins is equal to arccos(-1/3)], which at zero temperature is critical and in terms of the solid-on-solid representation is located exactly at the point of roughening transition. At other values of the angle between neighboring spins an additional constraint appears, whose presence drives the system into an ordered phase (unless this angle is equal to pi/2, when another constraint is removed and the model becomes trivially exactly solvable).
    Preview · Article · Feb 2012 · Physical review. B, Condensed matter

Publication Stats

2k Citations
378.98 Total Impact Points

Institutions

  • 2012-2015
    • Hungarian Academy of Sciences
      • Institute for Solid State Physics and Optics
      Budapeŝto, Budapest, Hungary
  • 2012-2013
    • Budapest University of Technology and Economics
      • Department of Physics
      Budapeŝto, Budapest, Hungary
  • 2006
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2005
    • University of Minho
      Bracara Augusta, Braga, Portugal
  • 2004
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
  • 1997-2002
    • Max Planck Institute for Dynamics of Complex Technical Systems
      Magdeburg, Saxony-Anhalt, Germany
  • 1999
    • University of Manitoba
      • Department of Physics and Astronomy
      Winnipeg, Manitoba, Canada
  • 1995
    • Tokyo Institute of Technology
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1993-1994
    • Université de Neuchâtel
      • Institut de physique (IPH)
      Neuenburg, Neuchâtel, Switzerland