Publications (174)404.66 Total impact
 [Show abstract] [Hide abstract] ABSTRACT: Motivated by recent experiments on volborthite single crystals showing a wide 1/3magnetization plateau, we perform microscopic modeling by means of density functional theory (DFT) with the singlecrystal structural data as a starting point. Using DFT+U, we find four leading magnetic exchanges: antiferromagnetic J and J2, as well as ferromagnetic J' and J1. Simulations of the derived spin Hamiltonian show good agreement with the experiment. The 1/3plateau phase pertains to polarized magnetic trimers formed by strong J bonds. An effective J$\rightarrow\infty$ model shows a tendency towards condensation of magnon bound states preceding the plateau phase.
 [Show abstract] [Hide abstract] ABSTRACT: We apply the coupled cluster method to high orders of approximation and exact diagonalizations to study the groundstate properties of the triangularlattice spin$s$ Heisenberg antiferromagnet. We calculate the fundamental groundstate quantities, namely, the energy $e_0$, the sublattice magnetization $M_{\rm sub}$, the inplane spin stiffness $\rho_s$ and the inplane magnetic susceptibility $\chi$ for spin quantum numbers $s=1/2, 1, \ldots, s_{\rm max}$, where $s_{\rm max}=9/2$ for $e_0$ and $M_{\rm sub}$, $s_{\rm max}=4$ for $\rho_s$ and $s_{\rm max}=3$ for $\chi$. We use the data for $s \ge 3/2$ to estimate the leading quantum corrections to the classical values of $e_0$, $M_{\rm sub}$, $\rho_s$, and $\chi$. In addition, we study the magnetization process, the width of the 1/3 plateau as well as the sublattice magnetizations in the plateau state as a function of the spin quantum number $s$.

Article: Frustrated Heisenberg antiferromagnet on the honeycomb lattice: Spin gap and lowenergy parameters
[Show abstract] [Hide abstract] ABSTRACT: We use the coupled cluster method implemented to high orders of approximation to investigate the frustrated spin$\frac{1}{2}$ $J_{1}$$J_{2}$$J_{3}$ antiferromagnet on the honeycomb lattice with isotropic Heisenberg interactions of strength $J_{1} > 0$ between nearestneighbor pairs, $J_{2}>0$ between nextnearestneighbor pairs, and $J_{3}>0$ between nextnextneareastneighbor pairs of spins. In particular, we study both the groundstate (GS) and lowestlying triplet excitedstate properties in the case $J_{3}=J_{2} \equiv \kappa J_{1}$, in the window $0 \leq \kappa \leq 1$ of the frustration parameter, which includes the (tricritical) point of maximum classical frustration at $\kappa_{{\rm cl}} = \frac{1}{2}$. We present GS results for the spin stiffness, $\rho_{s}$, and the zerofield uniform magnetic susceptibility, $\chi$, which complement our earlier results for the GS energy per spin, $E/N$, and staggered magnetization, $M$, to yield a complete set of accurate lowenergy parameters for the model. Our results all point towards a phase diagram containing two quasiclassical antiferromagnetic phases, one with N\'{e}el order for $\kappa < \kappa_{c_{1}}$, and the other with collinear striped order for $\kappa > \kappa_{c_{2}}$. The results for both $\chi$ and the spin gap $\Delta$ provide compelling evidence for a quantum paramagnetic phase that is gapped over a considerable portion of the intermediate region $\kappa_{c_{1}} < \kappa < \kappa_{c_{2}}$, especially close to the two quantum critical points at $\kappa_{c_{1}}$ and $\kappa_{c_{2}}$. Each of our fully independent sets of results for the lowenergy parameters is consistent with the values $\kappa_{c_{1}} = 0.45 \pm 0.02$ and $\kappa_{c_{2}} = 0.60 \pm 0.02$, and with the transition at $\kappa_{c_{1}}$ being of continuous (and probably of the deconfined) type and that at $\kappa_{c_{2}}$ being of firstorder type. 
Article: Groundstate phase diagram of the XXZ spin s kagome antiferromagnet: A coupledcluster study
[Show abstract] [Hide abstract] ABSTRACT: We use the coupled cluster method to high orders of approximation in order to calculate the groundstate phase diagram of the XXZ spin$s$ kagome antiferromagnet with easyplane anisotropy, i.e. the anisotropy parameter $\Delta$ varies between $\Delta=1$ (isotropic Heisenberg model) and $\Delta=0$ ($XY$ model). We find that for the extreme quantum case $s=1/2$ the ground state is magnetically disordered in the entire region $0 \le \Delta \le 1$. For $s=1$ the ground state is disordered for $0.818 < \Delta \le 1$, it exhibits $\sqrt{3}\times\sqrt{3}$ magnetic longrange order for $0.281 < \Delta <0.818$, and $q=0$ magnetic longrange order for $0 \le \Delta < 0.281$. We confirm the recent result of Chernyshev and Zhitomirsky (Phys. Rev. Lett. 113, 237202 (2014)) that the selection of the ground state by quantum fluctuations is different for small $\Delta$ ($XY$ limit) and for $\Delta$ close to one (Heisenberg limit), i.e., $q=0$ magnetic order is favored over $\sqrt{3}\times\sqrt{3}$ for $0\le \Delta <\Delta_c$ and vice versa for $\Delta_c < \Delta \le 1$. We calculate $\Delta_c$ as a function of the spin quantum number $s$.  [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.
 [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.
 [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$_6$.
 [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.
 [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.
 [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.
 [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.
 [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.
 [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.
 [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.
 [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 $\kappa_{c_2} = 1.82 \pm 0.03$.

Article: Groundstate phase diagram of the spin1/2 squarelattice J1J2 model with plaquette structure
[Show abstract] [Hide abstract] ABSTRACT: Using the coupled cluster method for high orders of approximation and Lanczos exact diagonalization we study the groundstate phase diagram of a quantum spin1/2 J1J2 model on the square lattice with plaquette structure. We consider antiferromagnetic (J1>0) as well as ferromagnetic (J1<0) nearestneighbor interactions together with frustrating antiferromagnetic nextnearestneighbor interaction J2>0. The strength of interplaquette interaction lambda varies between lambda=1 (that corresponds to the uniform J1J2 model) and lambda=0 (that corresponds to isolated frustrated 4spin plaquettes). While on the classical level (s \to \infty) both versions of models (i.e., with ferro and antiferromagnetic J1) exhibit the same groundstate behavior, the groundstate phase diagram differs basically for the quantum case s=1/2. For the antiferromagnetic case (J1 > 0) Neel antiferromagnetic longrange order at small J2/J1 and lambda \gtrsim 0.47 as well as collinear striped antiferromagnetic longrange order at large J2/J1 and lambda \gtrsim 0.30 appear which correspond to their classical counterparts. Both semiclassical magnetic phases are separated by a nonmagnetic quantum paramagnetic phase. The parameter region, where this nonmagnetic phase exists, increases with decreasing of lambda. For the ferromagnetic case (J1 < 0) we have the trivial ferromagnetic ground state at small J2/J1. By increasing of J2 this classical phase gives way for a semiclassical plaquette phase, where the plaquette block spins of length s=2 are antiferromagnetically longrange ordered. Further increasing of J2 then yields collinear striped antiferromagnetic longrange order for lambda \gtrsim 0.38, but a nonmagnetic quantum paramagnetic phase lambda \lesssim 0.38.  [Show abstract] [Hide abstract] ABSTRACT: We study the groundstate (gs) phases of the spinhalf anisotropic planar pyrochlore (or crossed chain) model using the coupled cluster method (CCM). The model is a frustrated antiferromagnetic (AFM) $J_{1}$$J_{2}$ system on the checkerboard lattice, with nearestneighbor exchange bonds $J_{1}>0$ and nextnearestneighbor bonds $J_{2} \equiv \kappa J_{1} > 0$. Using various AFM classical ground states as CCM model states we present results for their gs energy, average onsite magnetization, and susceptibilities to plaquette valencebond crystal (PVBC) and crosseddimer valencebond crystal (CDVBC) ordering. We show that the state with Neel ordering is the gs phase for $\kappa < \kappa_{c_1} \approx 0.80 \pm 0.01$, but that none of the fourfold set of AFM states selected by quantum fluctuations at $O(1/s)$ in a large$s$ analysis (where $s$ is the spin quantum number) from the infinitely degenerate set of AFM states that form the gs phase for the classical version of the model (for $\kappa>1$) survives the quantum fluctuations to form a stable magneticallyordered gs phase for the spinhalf case. The Neel state becomes susceptible to PVBC ordering at or very near to $\kappa = \kappa_{c_1}$, and the fourfold AFM states become infinitely susceptible to PVBC ordering at $\kappa = \kappa_{c_2} \approx 1.22 \pm 0.02$. In turn, we find that these states become infinitely susceptible to CDVBC ordering for all values of $\kappa$ above a certain critical value at or very near to $\kappa = \kappa_{c_2}$. We thus find a Neelordered gs phase for $\kappa<\kappa_{c_1}$, a PVBCordered phase for $\kappa_{c_1} < \kappa < \kappa_{c_2}$, and a CDVBCordered phase for $\kappa > \kappa_{c_2}$. Both transitions are probably direct ones, although we cannot exclude very narrow coexistence regions confined to $0.79 \lesssim \kappa \lesssim 0.81$ and $1.20 \lesssim \kappa \lesssim 1.22$ respectively.
 [Show abstract] [Hide abstract] ABSTRACT: We report the microscopic magnetic model for the spin1/2 Heisenberg system CdCu2(BO3)2, one of the few quantum magnets showing the 1/2magnetization plateau. Recent neutron diffraction experiments on this compound [ M. Hase et al. Phys. Rev. B 80 104405 (2009)] evidenced longrange magnetic order, inconsistent with the previously suggested phenomenological magnetic model of isolated dimers and spin chains. Based on extensive density functional theory band structure calculations, exact diagonalizations, quantum Monte Carlo simulations, thirdorder perturbation theory as well as highfield magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2 are accounted for by a frustrated quasi2D magnetic model featuring four inequivalent exchange couplings: the leading antiferromagnetic coupling Jd within the structural Cu2O6 dimers, two interdimer couplings Jt1 and Jt2, forming magnetic tetramers, and a ferromagnetic coupling Jit between the tetramers. Based on comparison to the experimental data, we evaluate the ratios of the leading couplings Jd : Jt1 : Jt2 : Jit = 1 : 0.20 : 0.45 : −0.30, with Jd of about 178 K. The inequivalence of Jt1 and Jt2 largely lifts the frustration and triggers longrange antiferromagnetic ordering. The proposed model accounts correctly for the different magnetic moments localized on structurally inequivalent Cu atoms in the groundstate magnetic configuration. We extensively analyze the magnetic properties of this model, including a detailed description of the magnetically ordered ground state and its evolution in magnetic field with particular emphasis on the 1/2magnetization plateau. Our results establish remarkable analogies to the ShastrySutherland model of SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material realization for the spin1/2 decorated anisotropic ShastrySutherland lattice.
 [Show abstract] [Hide abstract] ABSTRACT: Starting with the sqrt{3} x sqrt{3} and the q=0 states as reference states we use the coupled cluster method to high orders of approximation to investigate the ground state of the Heisenberg antiferromagnet on the kagome lattice for spin quantum numbers s=1/2,1,3/2,2,5/2, and 3. Our data for the groundstate energy for s=1/2 are in good agreement with recent largescale densitymatrix renormalization group and exact diagonalization data. We find that the groundstate selection depends on the spin quantum number s. While for the extreme quantum case, s=1/2, the q=0 state is energetically favored by quantum fluctuations, for any s>1/2 the sqrt{3} x sqrt{3} state is selected. For both the sqrt{3} x sqrt{3} and the q=0 states the magnetic order is strongly suppressed by quantum fluctuations. Within our coupled cluster method we get vanishing values for the order parameter (sublattice magnetization) M for s=1/2 and s=1, but (small) nonzero values for M for s>1. Using the data for the groundstate energy and the order parameter for s=3/2,2,5/2, and 3 we also estimate the leading quantum corrections to the classical values.
 [Show abstract] [Hide abstract] ABSTRACT: We study the groundstate (gs) properties of the frustrated spin1/2 $J_{1}$$J_{2}$$J_{3}$ Heisenberg model on a honeycomb lattice with ferromagnetic (FM) nearestneighbor ($J_{1}=1$) exchange and frustrating antiferromagnetic (AFM) nextnearestneighbor ($J_{2}>0$) and nextnextnearestneighbor ($J_{3}>0$) exchanges, for the case $J_{3}=J_{2}$. We use the coupled cluster method in high orders of approximation, complemented by the exact diagonalization of a lattice with 32 sites, and calculate the gs energy, magnetic order parameter, and spinspin correlation functions. We find a quantum phase transition between regions characterized by FM order and a form of AFM ("striped") collinear order at $J^{c}_{2} \approx 0.1095 \pm 0.0005$. We compare results for the FM case (with $J_{1}=1$) to previous results for the corresponding AFM case (with $J_{1}=+1$). While the magnetic order parameters behave similarly for the FM and the AFM models for large values of the frustration parameter $J_{2}$, there are considerable differences between them for $J_{2}/J_{1} \lesssim 0.6$. For example, the quasiclassical collinear magnetic longrange order for the AFM model (with $J_{1}=+1$) breaks down at $J^{c_{2}}_{2} \approx 0.60$, whereas the "equivalent" point for the FM model (with $J_{1}=1$) occurs at $J^{c}_{2} \approx 0.11$. Unlike in the AFM model (with $J_{1}=+1$), where a plaquette valencebond crystal phase intrudes between the two corresponding quasiclassical AFM phases (with N\'eel and striped order) for $J^{c_{1}}_{2} < J_{2} < J^{c_{2}}_{2}$, with $J^{c_{1}}_{2} \approx 0.47$, we find no clear indications in the FM model for an intermediate magnetically disordered phase between the phases exhibiting FM and striped order. Instead, the evidence points strongly to a direct firstorder transition between the two ordered phases of the FM model.
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3k  Citations  
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Institutions

19912015

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


2010

Technische Universität Dortmund
 Chair of Theoretical Physics I
Dortmund, North RhineWestphalia, Germany


19992007

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


2004

Universität Augsburg
Augsberg, Bavaria, Germany


2001

Universität Osnabrück
Osnabrück, Lower Saxony, Germany
