List of poster contributions


Andraschko, FelixMany-body localization in the XXZ spin chain with binary disorderAbstract
Chung, MingeeEvidences for attractive Tomonaga-Luttinger liquid in a quantum spin ladderAbstract
Herbrych, JacekMagnetic moment distribution within the random exchange Heisenberg chainAbstract
Karnaukhov, Igor Quantum phase transitions in 2D topological Kondo latticeAbstract
Laad, MukulExactly Solvable Models for Geometrically Frustrated S=1/2 Systems in D=2,3Abstract
Maryasin, VladimirOrder from structural disorder in frustrated magnetsAbstract
Morampudi, SiddhardhNumerical study of a transition between Z2 topologically ordered phasesAbstract
Novotný, TomášExactly solvable model for energy transport at quantum criticalityAbstract
Rachel, StephanSpiral order in the honeycomb iridate Li2IrO3Abstract
Richter, JohannesThermodynamics of frustrated quantum magnets: High temperature expansion revisitedAbstract
Romhanyi, JuditMulti-boson theory for the magnetoelectric helimagnet Cu2OSeO3Abstract
Schuler, MichaelExact Diagonalization results for Spin Dynamics in the Kitaev-Heisenberg modelAbstract
Singh, RajeevMany-body localization and entanglement in disordered quantum spin modelsAbstract
Smerald, AndrewSpin and orbital physics in Ba3CuSb2O9Abstract
Songvilay, ManilaRandom dilution effects in 1D spin system CaCr2-xScxO4Abstract
Sorokin, AleksandrQuantum phase transitions in ring-type networks of Lipkin--Meshkov--Glick modelsAbstract
Many-body localization in the XXZ spin chain with binary disorder
Andraschko, Felix (University of Manitoba, Department of Physics and Astronomy, Winnipeg MB, Canada) 
We investigate the spin-1/2 XXZ chain with a binary magnetic field disorder. We show that the non-equilibrium dynamics show clear signatures of many-body localization, including a logarithmic increase of the entanglement entropy in time, as well as a lack of thermalization. Our numerical density-matrix renormalization group calculations for infinite system size are based on a purification approach which allows to perform the disorder average exactly.
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Evidences for attractive Tomonaga-Luttinger liquid in a quantum spin ladder
Chung, Mingee (École Polytechnique Fédérale de Lausanne, Institute for Condensed Matter Physics, Physics, Lausanne, Switzerland) 
A spin-$frac{1}{2}$ Heisenberg antiferromagnetic ladder can be described as a Tomonaga-Luttinger liquid (TLL) when a sufficiently strong magnetic field closes the gap. An interesting observation is that the relative strength of the couplings along the leg and rung determines the sign of the interaction in the TLL phase: a strong-rung for repulsive interactions while a strong-leg for attractive interactions. Here we present direct evidences through NMR relaxation measurements for the realization of attractive TLL in a strong-leg ladder compound $(C_7H_{10}N)_2CuBr_4$, called DIMPY, in magnetic fields above 2.5 T. Through the splitting of NMR spectral lines, we also show development of staggered magnetic order at low temperatures below 300 mK, which may correspond to Bose-Einstein condensate of magnons. 
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Magnetic moment distribution within the random exchange Heisenberg chain
Herbrych, Jacek (Crete Center for Quantum Complexity and Nanotechnology, Department of Physics, University of Crete, Heraklion, Greece) 
The ordering of weakly coupled random antiferromagnetic $S=1/2$ chains is considered theoretically. The isotropic spin chain model with random exchange interactions is solved numerically within density-matrix renormalization group approach, while the interchain exchange interaction is treated analytically within the mean-filed approximation. Results for the ordering temperature $T_N$ as well as the ordered moment, both induced by interchain coupling $J_{perp}$, are presented and are both
reduced by increasing disorder with the (possible) exception of an extreme disorder or very small $J_{perp}$. The pronounced effect of the random singlet concept is the large span of local ordered moments, becoming dominant for small $J_{perp}$.
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Quantum phase transitions in 2D topological Kondo lattice
Karnaukhov, Igor (The National Academy of Sciences of Ukraine, G. V. Kurdyumov Institute of Metal Physics, Department of theory of nonideal crystals, Kyiv , Ukraine) 
 We have proposed an exact solvable quantum model of a spin-$frac{1}{2}$
topological Kondo lattice on a square decorated lattice. A theory of topological Kondo lattice involving
the interaction of itinerant spinless fermions with spins located at lattice sites is developed.
Itinerant fermions defined in the framework of the Haldane model interact via the Kitaev interaction with
spin-$frac{1}{2}$ Kitaev sublattice. The model presented is the first exact solvable model of topological Kondo lattice
whose exact ground state is a nontrivial topological phase, topological phase transitions are accompanied with both the change and unchange topological numbers. We provide a detailed analysis
of the ground-state phase diagram of the model and demonstrate how quantum
phase transitions between topological states arise. We have found that the states with both the same and different topological numbers
are separated by the quantum phase transition without gap closing at the point of the phase transition. The phase transition between topological phases accompanied by a rearrangement of the spectrum of spin subsystem from band to flat-band states. 

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Exactly Solvable Models for Geometrically Frustrated S=1/2 Systems in D=2,3
Laad, Mukul (The Institute of Mathematical Sciences (IMSc), Theoretical Physics, Chennai, India) 
 We present an exactly solvable $S=1/2$ Heisenberg model on a $D=2$ tetrahedron-based lattice.  The model exhibits rigorous
valence-bond solid (VBS) and partial spin-liquid phases, the quantum phase
transition between which turns out to be of a novel ``Bose-Lifshitz'' type.
In a finite magnetic field, we analytically find a bosonic realisation of the
$nu=1/3$ Laughlin (or Tao-Thouless) state.  At the maximally frustrated point,
 a dynamical large-$N$ analysis uncovers a critical quantum spin liquid with
singular spin {it and} dimer responses.  Finally, we also present the first
analytic demonstration of a rigorous VBS-AF transition in a {it three-dimensional} generalization of the above.
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Order from structural disorder in frustrated magnets
Maryasin, Vladimir (CEA Grenoble, Institut Nanosciences et Cryogénie (INAC), SPSMS, Grenoble, France) 
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Numerical study of a transition between Z2 topologically ordered phases
Morampudi, Siddhardh (Max Planck Institute for Physics of Complex Systems, Dresden, Germany) 
Distinguishing different topologically ordered phases and characterizing phase transitions between them is a difficult task due to the absence of local order parameters. In this work, we use a combination of analytical and numerical approaches to distinguish two such phases and characterize a phase transition between them. The "toric code" and "double semion" models are simple lattice models exhibiting Z2 topological order. Although both models express the same topological ground state degeneracies and entanglement entropies, they are distinct phases of matter because their emergent quasi-particles obey different statistics. For a 1D model, we tune a phase transition between these two phases and obtain an exact solution to the entire phase diagram, finding a second-order Ising x Ising transition. We then use exact diagonalization to study the 2D case and find indications of a first-order transition. We show that the quasi-particle statistics provides a robust indicator of the distinct topological orders throughout the whole phase diagram. 
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Exactly solvable model for energy transport at quantum criticality
Novotný, Tomáš (Charles University of Prague, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Praha 2, Czech Republic) 
We study an exactly solvable variant of the transverse field Ising chain used recently in several publications [e.g., PRL 109, 240402 (2012) or PRL 101, 105701 (2008)] for the analysis of the heat transport at the quantum critical point. Our work enables to critically examine the role of the quantum Markov approximation employed in those studies and properly incorporates the effects of finite coherent coupling to the heat reservoirs. 
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Spiral order in the honeycomb iridate Li$_2$IrO$_3$
Rachel, Stephan (TU Dresden, Institute for Theoretical Physics, Dresden, Germany) 
The honeycomb iridates A$_2$IrO$_3$ (A=Na, Li) constitute promising candidate materials to realize the Heisenberg-Kitaev model (HKM) in nature, hosting unconventional magnetic as well as spin liquid phases.
Recent experiments suggest, however, that Li$_2$IrO$_3$ exhibits a magnetically ordered state of incommensurate spiral type which has not been identified in the HKM. We show that these findings can be understood in the context of an extended Heisenberg-Kitaev scenario [1] satisfying all tentative experimental evidence: (i) the maximum of the magnetic susceptibility is located inside the first Brillouin zone, (ii) the Curie-Weiss temperature is negative relating to dominant antiferromagnetic fluctuations, and (iii) significant second-neighbor spin-exchange is involved.

[1] J.Reuther, R.Thomale, S.Rachel, arXiv:1404.5818; to appear in PRB Rapid Comm.
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Thermodynamics of frustrated quantum magnets: High temperature expansion revisited
Richter, Johannes (Otto-von-Guericke Universität Magdeburg, Institut für Theoretische Physik, Magdeburg, Germany) 
We present the high-temperature expansion (HTE) up to 10th order of the
specific heat C and the
uniform susceptibility $chi$  for Heisenberg models with arbitrary exchange
patterns and arbitrary
spin quantum number s. We encode the algorithm in a C++ program provided at
http://www.unimagdeburg.de/jschulen/HTE10/
which allows to get explicitly the HTE series for
concrete Heisenberg models. We use our HTE scheme to study the specific heat
and the
susceptibility of frustrated quantum magnets. In particular, we consider
magnetic systems with
highly degenerate classical ground states, such as the kagome,
the J1-J2, as well as the pyrochlore antiferromagnets. We investigate to
what extent
strong frustration is evident at moderate and high temperatures. Moreover,
we discuss the
influence of the spin quantum number s on the thermodynamic properties and
compare this way
quantum and classical systems.
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Multi-boson theory for the magnetoelectric helimagnet Cu2OSeO3
Romhanyi, Judit (Leibniz Institute for Solid State and Materials Research Dresden (IFW-Dresden), Leibniz Institute for Solid State and Materials Research Dresden, Institute for Theoretical Solid State Physics, Dresden, Germany) 
The Cu2OSeO3 is the first insulating system showing a skyrmion-lattice phase, in addition it is a magnetoelectric material, enabling technological applications. In Cu2OSeO3 the Cu2+ ions form a distorted pyrochlore lattice of corner sharing tetrahedra. 

Recent ab initio density functional calculations show that there are two well separated exchange energy scales, dividing the system into strong and weak tetrahedra. 
Building on this fact, we perform a microscopic multi-boson theory which treats the strong tetrahedra fully quantum mechanically, and the weak couplings between them on a mean field level. This theory captures the experimentally observed spin reduction and provides the excitation spectrum, as well as the dynamical structure factors, pertinent to inelastic neutron scattering, Raman, and other spectroscopic probes. 
We compare our theory with recent ESR measurements, and show that the multi-boson spectrum is in excellent agreement with the experiment.
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Exact Diagonalization results for Spin Dynamics in the Kitaev-Heisenberg model
Schuler, Michael (University of Innsbruck, Institut für Theoretische Physik, Innsbruck, Austria) 
We present a study of dynamical spin-correlation functions for the Kitaev-Heisenberg model on the honeycomb lattice obtained by exact diagonalization techniques on finite lattices. Results are presented for the entire parameter space of the model, comprising both, Kitaev Spin Liquid phases and different magnetically ordered phases.
Furthermore, as a limiting case, spin dynamics of the antiferromagnetic Heisenberg model in a magnetic field is investigated.
Our results can be directly compared with inelastic neutron scattering experiments of corresponding materials.
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Many-body localization and entanglement in disordered quantum spin models
Singh, Rajeev (Max Planck Institute for the Physics of Complex Systems, Max Planck Institute for the Physics of Complex Systems, Condensed Matter Physics, Dresden, Germany) 
The presence of disorder in a non-interacting system can localize all the energy eigenstates, a phenomena dubbed Anderson localization. Many-body localization is a generalization of this phenomena to include interactions. The dynamics of disordered interacting quantum systems shows a logarithmic growth (associated with glassy systems) in the entanglement entropy after a global quench [1]. For finite systems, this growth saturates and the saturation value obeys a volume law. A volume law leads to a constant entanglement entropy per site which might be related to thermal entropy and imply partial thermalization of the system. In this work, we study further the dynamics of disordered quantum spin systems and parameter dependence of the long time saturation.

[1] J. H. Bardarson, F. Pollmann and J. E. Moore, Phys. Rev. Lett. 109, 107202 (2012).
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Spin and orbital physics in Ba$_3$CuSb$_2$O$_9$
Smerald, Andrew (Ecole Polytechnique Federale de Lausanne (EPFL), Theoretical physics, Lausanne, Switzerland) 
Recent experiments have shown that the material Ba$_3$CuSb$_2$O$_9$ fails to order magnetically at temperatures as low as 20mK. Furthermore, no Jahn-Teller distortion is observed, leading to speculation that the ground state may be a spin-orbital liquid, as has been shown to be the case for the SU(4) symmetric Khugel-Khomskii model on the honeycomb lattice.
Starting from a Hubbard model for degenerate Cu $e_g$-orbitals, we derive a spin-orbital Hamiltonian in the limit of strong on-site Coulomb repulsion. We solve this Hamiltonian for small clusters by decoupling spin and orbital degrees of freedom, and also by exact diagonalisation. We find that the inclusion of out-of-plane Cu′-sites, which decorate the honeycomb lattice, are crucial to understand the low-energy physics. The resulting ground state involves nearest-neighbour spin singlets coexisting with orbital order, and can be understood in terms of dimer coverings of an emergent square lattice. While the experimental picture is complicated by structural disorder, we find qualitative agreement between our theory and NMR experiments.
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Random dilution effects in 1D spin system CaCr_{2-x}Sc_{x}O_{4}
Songvilay, Manila (CEA Saclay, Laboratoire Léon Brillouin, France) 
{beta}-CaCr_{2}O_{4} is a compound in which magnetic chromium ions (S = 3/2) form parallel triangular ladders. This topology allows to combine both magnetic frustration and low dimensionality. Long-range magnetic order with a propagation vector mathbf{k} = (0,0,q) (q simeq 0.47 at 1.5 K) sets in at approximately T_{N} = 21 K [2] but for T < 150 K, 1D like magnetic excitations are revealed by inelastic neutron scattering measurements [1].
A series of substituted compounds CaCr_{2-x}Sc_{x}O_{4} (0 leq x leq 1) has been investigated by neutron scattering and magnetization measurements in order to study the effects of random dilution on the magnetic properties and the one-dimensional character of this system. With increasing x up to 0.3, long-range magnetic order still persists with a decreasing ordering temperature and correlation length. Non-classical 1D excitations are still observable, with a characteristic frequency increasing both with temperature and substitution, as reported for other one-dimensional systems [3]. These features are replaced by a quasi-elastic signal extending up to 8 meV for x = 0.5.
The results are interpreted in terms of a progressive confinement of the 1D excitations within shorter chains as x increases, until the average chain length reaches a threshold for which the system breaks apart into disconnected finite 3D clusters. The experimental correlation length is close to the theoretically calculated one for a J1-J2 chain and shows the crucial role of J2 in propagating the 1D excitations.

[1] F. Damay, C. Martin, V. Hardy, A. Maignan, C. Stock and S. Petit, Phys. Rev. B, 84 (2011).
[2] F. Damay, C. Martin, V. Hardy, A. Maignan, G. André, K. Knight, S.R. Giblin and L. Chapon, Phys. Rev. B, 81 (2010).
[3]  I.A. Zaliznyak, L.-P. Regnault, D. Petitgrand, Phys. Rev. B, 50 (1994)

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Quantum phase transitions in ring-type networks of Lipkin--Meshkov--Glick models
Sorokin, Aleksandr (Technische Universität Berlin, Institut für Theoretische Physik, Berlin, Germany) 
The Lipkin--Meshkov--Glick (LMG) model was proposed to describe phase transitions in nuclei and was later adopted for other many-spin-$frac12$-particle systems such as interacting spin systems, Bose--Einstein condensates, magnetic molecules and like.  This model, being on the one hand sufficiently simple to be integrable in certain cases, shows on the other hand sufficient complexity.

By creating a chain of coupled LMG models and imposing cyclic boundary conditions thereupon, yet another dimension is added to the system, allowing for the possibility of creation of additional quantum phases.  In this work, we explore such additional phases that arise from adding nearest-neighbour exchange interaction between $y$-components of total angular momenta to anisotropic LMG sites in the ring.  Thus the Hamiltonian of the whole system takes form $mathcal{H}=gsum_{i=1}^{N}{J_i^z}-frac{gamma}{2j}sum_{i=1}^{N}{J_i^x J_i^x}-frac{kappa}{2j}sum_{i=1}^{N}{J_i^y J_{i+1}^y}$.

We performed calculations in the thermodynamic limit using bosonisation of total angular momentum operators by Holstein--Primakoff transformations.  The Hamiltonian was then expanded in series up to $O(j^2)$, which allowed us to get higher-order quantum corrections to the classical energy.  According to the number and positions of the ground state energy minima, four distinct phases were found, and it was shown that in the ground state all the angular momenta are identical.  These results were confirmed by direct diagonalisation of the Hamiltonian for a finite number of nodes (and also finite number of particles within one node) as well as by semi-classical modelling.

Quantum corrections to the ground state energy let us calculate dispersion relations of excitation energy depending on parameters $kappa$ and $gamma$ of the system.  It was shown that away from the phase boundaries the dispersion is quadratic in the neighbourhood of $k=0$ and gapped, while at the phase boundaries it becomes linear and gapless.

Finally, in order to classify the phases with respect to the long-range ordering, we calculated correlation functions for different angular momenta components.  The results show that there is no long-range correlations between any of the components in $g$-dominated phase, while $x$- and $y$- components are correlated in $gamma$- and $kappa$-dominated phases, respectively.
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