Publications (239)537.53 Total impact
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ABSTRACT: We use the coupled cluster method to high orders of approximation in order to calculate the groundstate energy, the groundstate magnetic order parameter, and the spin gap of the spin1/2 J_1J_2 model on the square lattice. We obtain values for the transition points to the magnetically disordered quantum paramagnetic phase of J_2^{c1}=0.454J_1 and J_2^{c2}= 0.588 J_1. The spin gap is zero in the entire parameter region accessible by our approach, i.e. for J_2 \le 0.49J_1 and J_2 > 0.58J_1. This finding is in favor of a gapless spinliquid or a nearcritical quantum paramagnetic ground state in this parameter regime.08/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate ground states of $s$=1/2 Heisenberg antiferromagnets on the eleven twodimensional (2D) Archimedian lattices by using the coupled cluster method. Magnetic interactions and quantum fluctuations play against each other subtly in 2D quantum magnets and the magnetic ordering is thus sensitive to the features of lattice topology. Archimedean lattices are those lattices that have 2D arrangements of regular polygons and they often build the underlying magnetic lattices of insulating quasitwodimensional quantum magnetic materials. Hence they allow a systematic study of the relationship between lattice topology and magnetic ordering. We find that the Archimedian lattices fall into three groups: those with semiclassical magnetic groundstate longrange order, those with a magnetically disordered (cooperative quantum paramagnetic) ground state, and those with a fragile magnetic order. The most relevant parameters affecting the magnetic ordering are the coordination number and the degree of frustration present.Physical Review B 05/2014; 89:184407. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate a mechanism to establish groundstate ferromagnetism in flatband Hubbard systems based on a kind of {\it orderfromdisorder} effect driven by dispersion. As a paradigm we consider a frustrated diamond chain, where for ideal diamond geometry the lowest oneelectron band is flat, but the ground state remains paramagnetic for arbitrary onsite repulsion $U$. We focus on half filling of the flat band. By using numerical and analytical arguments we present the groundstate phase diagram for a distorted diamond chain, i.e., the former flat band becomes dispersive. Driven by the interplay of dispersion and interaction the ground state is ferromagnetic if the interaction exceeds a critical value $U_c$.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the spin1/2 Heisenberg model on the delta chain (sawtooth chain) with ferromagnetic nearestneighbor and antiferromagnetic nextneighbor interactions. For a special ratio between these interactions there is a class of exact ground states formed by localized magnons and the ground state is macroscopically degenerate with a large residual entropy per spin $s_0=\frac{1}{2}\ln 2$. An important feature of this model is a sharp decrease of the gaps for excited states with an increase of the number of magnons. These excitations give an essential contribution to the lowtemperature thermodynamics. The behavior of the considered model is compared with that of the delta chain with both antiferromagnetic interactions.02/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The spin1/2 alternating Heisenberg chain system Na$_3$Cu$_2$SbO$_6$ features two relevant exchange couplings: $J_{1a}$ within the structural Cu$_2$O$_6$ dimers and $J_{1b}$ between the dimers. Motivated by the controversially discussed nature of $J_{1a}$, we perform extensive densityfunctionaltheory (DFT) calculations, including DFT+$U$ and hybrid functionals. Fits to the experimental magnetic susceptibility using hightemperature series expansions and quantum Monte Carlo simulations yield the optimal parameters $J_{1a}\!=\!217$ K and $J_{1b}\!=\!174$ K with the alternation ratio $\alpha=J_{1a}/J_{1b}\simeq1.25$. For the closely related system Na$_2$Cu$_2$TeO$_6$, DFT yields substantially enhanced $J_{1b}$, but weaker $J_{1a}$. The comparative analysis renders the buckling of the chains as the key parameter altering the magnetic coupling regime. By simulating the dispersion relations of the alternating chain model and comparing them to the inelastic neutron scattering data $[$Y. Miura et al., J. Phys. Soc. Jpn. 77, 104709 (2008)$]$, we obtain an unequivocal evidence for a ferromagnetic $J_{1a}$ in Na$_3$Cu$_2$SbO$_6$.02/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experiments on lowdimensional frustrated quantum magnets with competing nearestneighbor exchange coupling J1 and next nearestneighbor exchange coupling J2 we investigate the magnetic susceptibility of twodimensional J1J2 Heisenberg models with arbitrary spin quantum number s. We use exact diagonalization and hightemperature expansion up to order 10 to analyze the influence of the frustration strength J2/J1 and the spin quantum number s on the position and the height of the maximum of the susceptibility. The derived theoretical data can be used to get information on the ratio J2/J1 by comparing with susceptibility measurements on corresponding magnetic compounds.Journal of Physics Conference Series 01/2014; 529(1).  [Show abstract] [Hide abstract]
ABSTRACT: We present the hightemperature expansion (HTE) up to 10th order of the specific heat C and the uniform susceptibility χ for Heisenberg models with arbitrary exchange patterns and arbitrary spin quantum number s. We encode the algorithm in a C++ program which allows to get explicitly the HTE series for concrete Heisenberg models. We apply our algorithm to pyrochlore ferromagnets and kagome antiferromagnets using several Padé approximants for the HTE series. For the pyrochlore ferromagnet we use the HTE data for χ to estimate the Curie temperature Tc as a function of the spin quantum number s. We find that Tc is smaller than that for the simple cubic lattice, although both lattices have the same coordination number. For the kagome antiferromagnet the influence of the spin quantum number s on the susceptibility as a function of renormalized temperature T /s(s + 1) is rather weak for temperatures down to T /s(s + 1) ∼ 0.3. On the other hand, the specific heat as a function of T /s(s + 1) noticeably depends on s. The characteristic maximum in C(T) is monotonously shifted to lower values of T /s(s + 1) when increasing s.Physical Review B 01/2014; 89:014415. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present the hightemperature expansion (HTE) up to tenth order of the specific heat C and the uniform susceptibility χ for Heisenberg models with arbitrary exchange patterns and arbitrary spin quantum number s. We encode the algorithm in a c++ program provided in the Supplemental Material [http://link.aps.org/supplemental/10.1103/PhysRevB.89.014415] which allows to explicitly get the HTE series for concrete Heisenberg models. We apply our algorithm to pyrochlore ferromagnets and kagome antiferromagnets using several Padé approximants for the HTE series. For the pyrochlore ferromagnet, we use the HTE data for χ to estimate the Curie temperature Tc as a function of the spin quantum number s. We find that Tc is smaller than that for the simplecubic lattice, although both lattices have the same coordination number. For the kagome antiferromagnet, the influence of the spin quantum number s on the susceptibility as a function of renormalized temperature T /s(s+1) is rather weak for temperatures down to T /s(s+1)̃0.3. On the other hand, the specific heat as a function of T /s(s+1) noticeably depends on s. The characteristic maximum in C (T) is monotonously shifted to lower values of T /s(s+1) when increasing s.12/2013; 89(1).  [Show abstract] [Hide abstract]
ABSTRACT: We consider the spin1/2 antiferromagnetic Heisenberg model on the twodimensional squarekagome lattice with almost dispersionless lowest magnon band. For a general exchange coupling geometry we elaborate lowenergy effective Hamiltonians which emerge at high magnetic fields. The effective model to describe the lowenergy degrees of freedom of the initial frustrated quantum spin model is the (unfrustrated) squarelattice spin1/2 $XXZ$ model in a $z$aligned magnetic field. For the effective model we perform quantum Monte Carlo simulations to discuss the lowtemperature properties of the squarekagome quantum Heisenberg antiferromagnet at high magnetic fields. We pay special attention to a magneticfield driven BerezinskiiKosterlitzThouless phase transition which occurs at low temperatures.12/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We consider a lattice of antiferromagnetically interacting equal spins that have a ferrimagnetic ground state. We show that a special arrangement of S=5/2 Fe$^{3+}$ ions in double perovskites AFe$_{1/2}$M$_{1/2}$O$_{3}$ exhibits the ferrimagnetic ordering below T_{fe} ~ 5.6J_1 (J_1/k_B ~ 50 K), which is close to room temperature. Small clusters of the same structure exhibit a superparamagnetic behavior at T < T_{fe}. The possibility of formation of such clusters explains the roomtemperature (superpara)magnetism in 3dmetal based oxides.10/2013; 
Article: Numerical study of magnetization plateaux in the spin1/2 kagome Heisenberg antiferromagnet
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ABSTRACT: We clarify the existence of several magnetization plateaux for the kagome $S=1/2$ antiferromagnetic Heisenberg model in a magnetic field. Using approximate or exact localized magnon eigenstates, we are able to describe in a similar manner the plateau states that occur for magnetization per site $m=1/3$, 5/9, and 7/9 of the saturation value. These results are confirmed using largescale Exact Diagonalization on lattices up to 63 sites.Physical Review B 10/2013; 88(14):144416. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a comprehensive macroscopic thermodynamic study of the quasionedimensional (1D) $s = \tfrac{1}{2}$ frustrated spinchain system linarite. Susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermalexpansion measurements were performed to characterize the magnetic phase diagram. In particular, for magnetic fields along the b axis five different magnetic regions have been detected, some of them exhibiting shortrangeorder effects. The experimental magnetic entropy and magnetization are compared to a theoretical modelling of these quantities using DMRG and TMRG approaches. Within the framework of a purely 1D isotropic model Hamiltonian, only a qualitative agreement between theory and the experimental data can be achieved. Instead, it is demonstrated that a significant symmetric anisotropic exchange of about 10% is necessary to account for the basic experimental observations, including the 3D saturation field, and which in turn might stabilize a triatic (threemagnon) multipolar phase.Physical Review B 05/2013; 88(18). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider the spin1/2 antiferromagnetic Heisenberg model on three frustrated lattices (the diamond chain, the dimerplaquette chain and the twodimensional squarekagome lattice) with almost dispersionless lowest magnon band. Eliminating highenergy degrees of freedom at high magnetic fields, we construct lowenergy effective Hamiltonians which are much simpler than the initial ones. These effective Hamiltonians allow a more extended analytical and numerical analysis. In addition to the standard strongcoupling perturbation theory we also use a localizedmagnon based approach leading to a substantial improvement of the strongcoupling approximation. We perform extensive exact diagonalization calculations to check the quality of different effective Hamiltonians by comparison with the initial models. Based on the effectivemodel description we examine the lowtemperature properties of the considered frustrated quantum Heisenberg antiferromagnets in the highfield regime. We also apply our approach to explore thermodynamic properties for a generalized diamond spin chain model suitable to describe azurite at high magnetic fields. Interesting features of these highly frustrated spin models consist in a steep increase of the entropy at very small temperatures and a characteristic extra lowtemperature peak in the specific heat. The most prominent effect is the existence of a magneticfield driven BerezinskiiKosterlitzThouless phase transition occurring in the twodimensional model.Physical review. B, Condensed matter 04/2013; 88(9). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We quantify the instability towards the formation of multipolar states in coupled spin1/2 chain systems with a frustrating J1J2 exchange, in parameter regimes that are of directly relevance to edgeshared cuprate spinchain compounds. Three representative types of interchain coupling and the presence of uniaxial exchange anisotropy are considered. The magnetic phase diagrams are determined by Density Matrix Renormalization Group calculations and completed by exact analytic results for the nematic and dipolar phases. We establish that the residual couplings strongly affect the pitch of spiral states and their instability to multipolar phases. Our theoretical results bring to the fore novel candidate materials close to quantum nematic/triatic ordering.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Coupled s = 1/2 frustrated Heisenberg chains with ferromagnetic nearestneighbor and antiferromagnetic nextnearestneighbor exchange interactions in high magnetic field are studied by densitymatrix renormalization group (DMRG) and hardcore boson (HCB) approaches at T = 0. First, we propose an appropriate onedimensional array for the construction of a 3D system to be studied with the DMRG method and demonstrate the performance by comparing the groundstate energy to the exact solution. Next, the binding energy of multimagnon bound state is calculated as a function of interchain coupling. We find that the multimagnon bound state is easily destroyed by weak interchain coupling. In the 2magnon phase the DMRG results are supported by the HCB approach.Journal of Physics Conference Series 12/2012; 400(3):2069.  [Show abstract] [Hide abstract]
ABSTRACT: We apply the coupled cluster method and exact diagonalzation to study the uniform susceptibility and the groundstate magnetization curve of the triangularlattice spin1 Heisenberg antiferromagnet. Comparing our theoretical data for the magnetization curve with recent measurements on the s=1 triangular lattice antiferromagnet Ba3NiSb2O9 we find a very good agreement.Journal of the Physical Society of Japan 10/2012; 82:015002. · 2.09 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the temperature dependence of the correlation length xi and the uniform susceptibility chi_0 of the frustrated J1J2 squarelattice Heisenberg ferromagnet in the collinear stripe phase using Greenfunction technique. The height chi_{max} and the position T(chi_{max}) of the maximum in the chi_0(T) curve exhibit a characteristic dependence on the frustration parameter J2/J1, which is well described by power laws, chi_{max}=a(J2J2^c)^{nu} and T(chi_{max})=b(J_2J_2^c), where J2^c = 0.4 and nu is of the order of unity.The correlation length diverges at low temperatures as xi \propto e^{A/T}, where A increases with growing J2/J1. We also compare our results with recent measurements on layered vanadium phosphates and find reasonable agreement.Physical review. B, Condensed matter 10/2012; 87(5). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the location and nature of the paraferro transition of interacting electrons in dispersionless bands using the example of the Hubbard model on the Tasaki lattice. This case can be analyzed as a geometric sitepercolation problem where different configurations appear with nontrivial weights. We provide a complete exact solution for the 1D case and develop a numerical algorithm for the 2D case. In two dimensions the paramagnetic phase persists beyond the uncorrelated percolation point, and the grandcanonical transition is via a firstorder jump to an unsaturated ferromagnetic phase.Physical Review Letters 08/2012; 109(9):096404. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We use the coupled cluster method to study the zerotemperature properties of an extended twodimensional Heisenberg antiferromagnet formed from spin1/2 moments on an infinite spatially anisotropic kagome lattice of cornersharing isosceles triangles, with nearestneighbor bonds only. The bonds have exchange constants $J_{1}>0$ along two of the three lattice directions and $J_{2} \equiv \kappa J_{1} > 0$ along the third. In the classical limit the groundstate (GS) phase for $\kappa < 1/2$ has collinear ferrimagnetic (N\'{e}el$'$) order where the $J_2$coupled chain spins are ferromagnetically ordered in one direction with the remaining spins aligned in the opposite direction, while for $\kappa > 1/2$ there exists an infinite GS family of canted ferrimagnetic spin states, which are energetically degenerate. For the spin1/2 case we find that quantum analogs of both these classical states continue to exist as stable GS phases in some regions of the anisotropy parameter $\kappa$, namely for $0<\kappa<\kappa_{c_1}$ for the N\'{e}el$'$ state and for (at least part of) the region $\kappa>\kappa_{c_2}$ for the canted phase. However, they are now separated by a paramagnetic phase without either sort of magnetic order in the region $\kappa_{c_1} < \kappa < \kappa_{c_2}$, which includes the isotropic kagome point $\kappa = 1$ where the stable GS phase is now believed to be a topological ($\mathbb{Z}_2$) spin liquid. Our best numerical estimates are $\kappa_{c_1} = 0.515 \pm 0.015$ and $\kappa_{c_2} = 1.82 \pm 0.03$.Physical Review B 08/2012; 86:214403. · 3.66 Impact Factor 
Article: Semiquantitative theory for highfield lowtemperature properties of a distorted diamond spin chain
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ABSTRACT: We consider the antiferromagnetic Heisenberg model on a distorted diamond chain and use the localizedmagnon picture adapted to a distorted geometry to discuss some of its highfield lowtemperature properties. More specifically, in our study we assume that the partition function for a slightly distorted geometry has the same form as for ideal geometry, though with slightly dispersive onemagnon energies. We also discuss the relevance of such a description to azurite.06/2012;
Publication Stats
3k  Citations  
537.53  Total Impact Points  
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Institutions

1992–2014

OttovonGuerickeUniversität Magdeburg
 Institute of Theoretical Physics (ITP)
Magdeburg, SaxonyAnhalt, Germany


2011

GeorgAugustUniversität Göttingen
 Institute for Theoretical Physics
Göttingen, Lower Saxony, Germany 
Donostia International Physics Center
San Sebastián, Basque Country, Spain


2010

Technische Universität Braunschweig
 Institut für Theoretische Physik
Braunschweig, Lower Saxony, Germany


2009

The University of Manchester
 School of Physics and Astronomy
Manchester, ENG, United Kingdom


2008

Max Planck Institute for Chemical Physics of Solids
Dresden, Saxony, Germany


1999–2007

Technische Universität Dresden
 Institut für theoretische Physik
Dresden, Saxony, Germany


2006

Max Planck Institute of Physics
München, Bavaria, Germany


2004–2006

National Academy of Sciences of Ukraine
 Institute for Condensed Matter Physics
Kievo, Kyiv City, Ukraine 
Universität Augsburg
Augsberg, Bavaria, Germany


2001

Universität Osnabrück
Osnabrück, Lower Saxony, Germany


1991–1992

Technische Universität Dortmund
Dortmund, North RhineWestphalia, Germany
