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J. Schnack

Publications and source records attributed to J. Schnack.

At least 55 records · Page 3Linked to original sources
arXiv2015

Thermodynamic observables of Mn12-acetate calculated for the full spin-Hamiltonian

35 years after its synthesis magnetic observables are calculated for the first time for the molecular nanomagnet Mn12-acetate using a spin-Hamiltonian that contains all spins. Starting from a very advanced DFT parameterization [Phys. Rev. B 89, 214422] we evaluate magnetization and specific heat for this anisotropic system of 12 manganese ions with a staggering Hilbert space dimension of 100,000,000 using the Finite-Temperature Lanczos Method.

cond-mat.str-el↗
arXiv2014

Phase diagram of the alternating-spin Heisenberg chain with extra isotropic three-body exchange interactions

For the time being isotropic three-body exchange interactions are scarcely explored and mostly used as a tool for constructing various exactly solvable one-dimensional models, although, generally speaking, such competing terms in generic Heisenberg spin systems can be expected to support specific quantum effects and phases. The Heisenberg chain constructed from alternating S=1 and sigma=1/2 site spins defines a realistic prototype model admitting extra three-body exchange terms. Based on numerical density-matrix renormalization group (DMRG) and exact diagonalization (ED) calculations, we demonstrate that the additional isotropic three-body terms stabilize a variety of partially-polarized states as well as two specific non-magnetic states including a critical spin-liquid phase controlled by two Gaussinal conformal theories as well as a critical nematic-like phase characterized by dominant quadrupolar S-spin fluctuations. Most of the established effects are related to some specific features of the three-body interaction such as the promotion of local collinear spin configurations and the enhanced tendency towards nearest-neighbor clustering of the spins. It may be expected that most of the predicted effects of the isotropic three-body interaction persist in higher space dimensions.

cond-mat.str-el↗
arXiv2014

Advanced Finite-Temperature Lanczos Method for anisotropic spin systems

It is virtually impossible to evaluate the magnetic properties of large anisotropic magnetic molecules numerically exactly due to the huge Hilbert space dimensions as well as due to the absence of symmetries. Here we propose to advance the Finite-Temperature Lanczos Method (FTLM) to the case of single-ion anisotropy. The main obstacle, namely the loss of the spin rotational symmetry about the field axis, can be overcome by choosing symmetry related random vectors for the approximate evaluation of the partition function. We demonstrate that now thermodynamic functions for anisotropic magnetic molecules of unprecedented size can be evaluated.

cond-mat.str-el↗
arXiv2013

Classical molecular dynamics investigations of biphenyl-based carbon nano membranes

Free-standing carbon nanomembranes (CNM) with molecular thickness and macroscopic size are fascinating objects both for fundamental reasons and for applications in nanotechnology. Although being made from simple and identical precursors their internal structure is not fully known and hard to simulate due to the large system size that is necessary to draw definite conclusions. We performed large-scale classical molecular dynamics investigations of biphenyl-based carbon nanomembranes. We show that one-dimensional graphene-like stripes constitute a highly symmetric quasi one-dimensional ground state. This state does not crosslink. Instead crosslinked structures are formed from highly excited precursors with a sufficient amount of broken phenyls. The internal structure of CNM is very likely a disordered metastable state which is formed in the process of cooling.

cond-mat.mtrl-sci↗
arXiv2013

Exchange randomness and spin dynamics in the frustrated magnetic Keplerate {W72V30}

The magnetic properties and spin dynamics of the spin frustrated polyoxometalate {W72V30}, where 30 V^{4+} ions (s = 1/2) occupy the sites of an icosidodecahedron, have been investigated by low temperature magnetization, magnetic susceptibility, proton and vanadium nuclear magnetic resonance, and theoretical studies. The field-dependent magnetization at 0.5 K increases monotonically up to 50 T without any sign of staircase behavior. This low-temperature behavior cannot be explained by a Heisenberg model based on a single value of the nearest-neighbor exchange coupling. We analyze this behavior upon assuming a rather broad distribution of nearest-neighbor exchange interactions. Slow spin dynamics of {W72V30} at low temperatures is observed from the magnetic field and temperature dependence of nuclear spin-lattice relaxation rate 1/T_1 measurements.

cond-mat.str-el↗
arXiv2012

Numerical Renormalization Group calculations of the magnetization of isotropic and anisotropic Kondo impurities

We study a Kondo impurity model with additional uniaxial anisotropy D in a non-zero magnetic field B using the Numerical Renormalization Group (NRG). The ratio g_e/g_S of electron and impurity g-factor is regarded as a free parameter and, in particular, the special cases of a "local" (g_e=0) and "bulk" (g_e=g_S) field are considered. For a bulk field, the relationship between the impurity magnetization M and the impurity contribution to the magnetization M_imp is investigated. Furthermore, we study how the value of g_e affects the impurity magnetization curves. In case of an isotropic impurity with g_e=g_S, it is demonstrated that at zero temperature M(B), unlike M_imp(B), does not display universal behavior. With additional "easy axis" anisotropy, the impurity magnetization is well described by a shifted and rescaled Brillouin function on energy scales that are small compared to |D|. In case of "hard axis" anisotropy, the magnetization curves can feature steps which are due to field-induced pseudo-spin-1/2 Kondo effects. For large anisotropy and a local field, these screening effects are described by an anisotropic spin-1/2 Kondo model with an additional scattering term that is spin-dependent (in contrast to ordinary potential scattering). Our study is motivated by the question how the magnetic properties of a deposited magnetic molecule are modified by the interaction with a non-magnetic metallic surface.

cond-mat.str-el↗
arXiv2012

Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules

We discuss the magnetocaloric properties of gadolinium containing magnetic molecules which potentially could be used for sub-Kelvin cooling. We show that a degeneracy of a singlet ground state could be advantageous in order to support adiabatic processes to low temperatures and simultaneously minimize disturbing dipolar interactions. Since the Hilbert spaces of such spin systems assume very large dimensions we evaluate the necessary thermodynamic observables by means of the Finite-Temperature Lanczos Method.

cond-mat.str-el↗
arXiv2012

Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18

The low-temperature elementary spin excitations in the AFM molecular wheel Fe18 were studied experimentally by inelastic neutron scattering and theoretically by modern numerical methods, such as dynamical density matrix renormalization group or quantum Monte Carlo techniques, and analytical spin-wave theory calculations. Fe18 involves eighteen spin-5/2 Fe(III) ions with a Hilbert space dimension of 10^14, constituting a physical system that is situated in a region between microscopic and macroscopic. The combined experimental and theoretical approach allowed us to characterize and discuss the magnetic properties of Fe18 in great detail. It is demonstrated that physical concepts such as the rotational-band or L&E-band concepts developed for smaller rings are still applicable. In particular, the higher-lying low-temperature elementary spin excitations in Fe18 or AFM wheels in general are of discrete antiferromagnetic spin-wave character.

cond-mat.str-el↗
arXiv2011

Thermodynamics of the one-dimensional frustrated Heisenberg ferromagnet with arbitrary spin

The thermodynamic quantities (spin-spin correlation functions <{\bf S}_0{\bf S}_n>, correlation length ξ, spin susceptibility χ, and specific heat C_V) of the frustrated one-dimensional J1-J2 Heisenberg ferromagnet with arbitrary spin quantum number S below the quantum critical point, i.e. for J2< |J1|/4, are calculated using a rotation-invariant Green-function formalism and full diagonalization as well as a finite-temperature Lanczos technique for finite chains of up to N=18 sites. The low-temperature behavior of the susceptibility χ and the correlation length ξ is well described by χ= (2/3)S^4 (|J1|-4J2) T^{-2} + A S^{5/2} (|J1|-4J2)^{1/2} T^{-3/2} and ξ= S^2 (|J1|-4J2) T^{-1} + B S^{1/2} (|J1|-4J2)^{1/2} T^{-1/2} with A \approx 1.1 ... 1.2 and B \approx 0.84 ... 0.89. The vanishing of the factors in front of the temperature at J2=|J1|/4 indicates a change of the critical behavior of χ and ξ at T \to 0. The specific heat may exhibit an additional frustration-induced low-temperature maximum when approaching the quantum critical point. This maximum appears for S=1/2 and S=1, but was not found for S>1.

cond-mat.str-el↗
arXiv2010

Quasi-exact evaluation of the magnetic properties of a giant Keplerate molecule

In this Letter we report how thermodynamic properties of a giant frustrated magnetic Keplerate molecule of N=30 spins s=1/2 can be evaluated with the help of the highly accurate finite-temperature Lanczos method. The comparison to experimental data shows excellent agreement. Since this molecule is structurally related to the archetypical kagome lattice antiferromagnet we expect new detailed insight into properties of this important class of frustrated materials.

cond-mat.str-el↗
arXiv2010

Properties of highly frustrated magnetic molecules studied by the finite-temperature Lanczos method

The very interesting magnetic properties of frustrated magnetic molecules are often hardly accessible due to the prohibitive size of the related Hilbert spaces. The finite-temperature Lanczos method is able to treat spin systems for Hilbert space sizes up to 10^9. Here we first demonstrate for exactly solvable systems that the method is indeed accurate. Then we discuss the thermal properties of one of the biggest magnetic molecules synthesized to date, the icosidodecahedron with antiferromagnetically coupled spins of s=1/2. We show how genuine quantum features such as the magnetization plateau behave as a function of temperature.

cond-mat.str-el↗
arXiv2010

Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni4Mo12

The tetrameric Ni(II) spin cluster Ni4Mo12 has been studied by INS. The data were analyzed extensively in terms of a very general spin Hamiltonian, which includes antiferromagnetic Heisenberg interactions, biquadratic 2-spin and 3-spin interactions, a single-ion magnetic anisotropy, and Dzyaloshinsky-Moriya interactions. Some of the experimentally observed features in the INS spectra could be reproduced, however, one feature at 1.65 meV resisted all efforts. This supports the conclusion that the spin Hamiltonian approach is not adequate to describe the magnetism in Ni4Mo12. The isotropic terms in the spin Hamiltonian can be obtained in a strong-coupling expansion of the Hubbard model at half-filling. Therefore detailed theoretical studies of the Hubbard model were undertaken, using analytical as well as numerical techniques. We carefully analyzed its abilities and restrictions in applications to molecular spin clusters. As a main result it was found that the Hubbard model is also unable to appropriately explain the magnetism in Ni4Mo12. Extensions of the model are also discussed.

cond-mat.mes-hall↗
arXiv2010

Heat capacity uncovers physics of a frustrated spin tube

We report on refined experimental results concerning the low-temperature specific heat of the frustrated spin tube material [(CuCl2tachH)3Cl]Cl2. This substance turns out to be an unusually perfect spin tube system which allows to study the physics of quasi-one dimensional antiferromagnetic structures in rather general terms. An analysis of the specific heat data demonstrates that at low enough temperatures the system exhibits a Tomonaga-Luttinger liquid behavior corresponding to an effective spin-3/2 antiferromagnetic Heisenberg chain with short-range exchange interactions. On the other hand, at somewhat elevated temperatures the composite spin structure of the chain is revealed through a Schottky-type peak in the specific heat located around 2 K. We argue that the dominating contribution to the peak originates from gapped magnon-type excitations related to the internal degrees of freedom of the rung spins.

cond-mat.str-el↗
arXiv2010

Calculating the energy spectra of magnetic molecules: application of real- and spin-space symmetries

The determination of the energy spectra of small spin systems as for instance given by magnetic molecules is a demanding numerical problem. In this work we review numerical approaches to diagonalize the Heisenberg Hamiltonian that employ symmetries; in particular we focus on the spin-rotational symmetry SU(2) in combination with point-group symmetries. With these methods one is able to block-diagonalize the Hamiltonian and thus to treat spin systems of unprecedented size. In addition it provides a spectroscopic labeling by irreducible representations that is helpful when interpreting transitions induced by Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance (NMR) or Inelastic Neutron Scattering (INS). It is our aim to provide the reader with detailed knowledge on how to set up such a diagonalization scheme.

cond-mat.str-el↗
arXiv2009

Effects of frustration on magnetic molecules: a survey from Olivier Kahn till today

In magnetism, of which molecular magnetism is a part, the term frustration is used rather sloppily. Sometimes one gains the impression that if the reason for some phenomenon is not quite clear then it is attributed to frustration. In this paper I discuss the effects of frustration that I feel are relevant for the field of molecular magnetism. As will become clear later they indeed lead to a variety of unusual magnetic properties.

cond-mat.str-el↗
arXiv2009

Magnetic response of magnetic molecules with non-collinear local d-tensors

Investigations of molecular magnets are driven both by prospective applications in future storage technology or quantum computing as well as by fundamental questions. Nowadays numerical simulation techniques and computer capabilities make it possible to investigate spin Hamiltonians with realistic arrangements of local anisotropy tensors. In this contribution I will discuss the magnetic response of a small spin system with special emphasis on non-collinear alignments of the local anisotropy axes.

cond-mat.str-el↗
arXiv2009

Approximate eigenvalue determination of geometrically frustrated magnetic molecules

Geometrically frustrated magnetic molecules have attracted a lot of interest in the field of molecular magnetism as well as frustrated Heisenberg antiferromagnets. In this article we demonstrate how an approximate diagonalization scheme can be used in order to obtain thermodynamic and spectroscopic information about frustrated magnetic molecules. To this end we theoretically investigate an antiferromagnetically coupled spin system with cuboctahedral structure modeled by an isotropic Heisenberg Hamiltonian.

cond-mat.str-el↗