We investigate the dispersive paramagnetic excitons on the honeycomb lattice that originate from the crystalline-electric field (CEF) split localized f-electron states in the paramagnetic state due to intersite exchange. We start with a symmetry analysis of possible Ising-type singlet-singlet and xy-type singlet-doublet models. The former supports only symmetric intersite-exchange while the latter additionally allows for antisymmetric Dzyaloshinski-Moriya (DM) exchange interactions. We calculate the closed expressions for magnetic exciton dispersion using both response function formalism and the bosonic Bogoliubov approach. We do this for the most general model that shows inversion symmetry breaking on the honeycomb lattice but also discuss interesting special cases. By calculating Berry curvatures and Chern numbers of paramagnetic excitons we show that the xy model supports nontrivial topological states in a wide range of parameters. This leads to the existence of excitonic topological edge states with Dirac dispersion lying in the zone boundary gap without the presence of magnetic order.

The quantum magnetism in a three-singlet model (TSM) with singlet crystalline electric field (CEF) states interacting on a lattice is investigated, motivated by its appearance in compounds with 4f^2 and 5f^2 electronic structure. Contrary to conventional (semi-classical) magnetism there are no preformed moments above the ordering temperature Tm. They appear spontaneously as induced or excitonic moments due to singlet-singlet mixing at Tm. In most cases the transition is of second order, however for large matrix elements between the excited states it turns into a first order transition at a critical point. Furthermore we derive the excitonic mode spectrum and its quantum critical soft mode behaviour which leads to the criticality condition for induced order as expressed in terms of the control parameters of the TSM and discuss the distinctions to the previously known two-singlet case. We also derive the temperature dependence of order parameters for second and first order transitions and the exciton spectrum in the induced magnetic phase.

A study of the magnetic susceptibility under variable hydrostatic (He gas) pressure on single crystals of Cs2CuCl4−xBrx is presented. This work includes the border compounds x = 0 and 4, known as good realizations of the distorted triangular‐lattice spin‐1/2 Heisenberg antiferromagnet, as well as the recently discovered well‐ordered isostructural systems Cs2CuCl3Br1 and Cs2CuCl2Br2. For the determination of the exchange coupling constants J and J′ of their anisotropic triangular lattice, the susceptibility data are fitted by the recently proposed J–J′ model [Schmidt and Thalmeier, New J. Phys. 2015, 17, 073025]. Its application on magnetic susceptibility data, validated for the border compounds, yields a degree of frustration J′/J = 0.47 for Cs2CuCl3Br1 and J′/J ≃ 0.63–0.78 for Cs2CuCl2Br2, making these systems particular interesting representatives of this family. From the evolution of the magnetic susceptibility under pressure up to about 0.4 GPa, the maximum pressure applied, two observations were made for all the compounds investigated here. First, it has been found that the overall energy scale, given by Jc = (J2 + J′2)1/2, increases under pressure, whereas the ratio J′/J remains unchanged in this pressure range. These experimental observations are in accordance with the results of DFT calculations performed for these materials. Secondly, for the magnetoelastic coupling constants, extraordinarily small values are obtained, about two orders of magnitude smaller compared to other Cu‐based quantum magnets. These observations have been assigned to a structural peculiarity of this class of materials, consisting of well‐isolated magnetic units not sharing any common coordination element.

In quasi-2D quantum magnets the ratio of N\'eel temperature $T_\text N$ to Curie-Weiss temperature $\Theta_\text{CW}$ is frequently used as an empirical criterion to judge the strength of frustration. In this work we investigate how these quantities are related in the canonical quasi-2D frustrated square or triangular $J_1$-$J_2$ model. Using the self-consistent Tyablikov approach for calculating $T_\text N$ we show their dependence on the frustration control parameter $J_2/J_1$ in the whole N\'eel and columnar antiferromagnetic phase region. We also discuss approximate analytical results. In addition the field dependence of $T_\text N(H)$ and the associated possible reentrance behavior of the ordered moment due to quantum fluctuations is investigated. These results are directly applicable to a class of quasi-2D oxovanadate antiferromagnets. We give clear criteria to judge under which conditions the empirical frustration ratio $f=\Theta_\text{CW}/T_\text N$ may be used as measure of frustration strength in the quasi-2D quantum magnets.

We investigate thermodynamic properties like specific heat cV and susceptibility χ in anisotropic J1–J2 triangular quantum spin systems ( S = 1 / 2 ?> ). As a universal tool we apply the finite temperature Lanczos method (FTLM) based on exact diagonalization of finite clusters with periodic boundary conditions. We use clusters up to N = 28 sites where the thermodynamic limit behavior is already stably reproduced. As a reference we also present the full diagonalization of a small eight-site cluster. After introducing model and method we discuss our main results on cV(T) and χ ( T ) ?> . We show the variation of peak position and peak height of these quantities as function of control parameter J 2 / J 1 ?> . We demonstrate that maximum peak positions and heights in Néel phase and spiral phases are strongly asymmetric, much more than in the square lattice J1–J2 model. Our results also suggest a tendency to a second side maximum or shoulder formation at lower temperature for certain ranges of the control parameter. We finally explicitly determine the exchange model of the prominent triangular magnets Cs2CuCl4 and Cs2CuBr4 from our FTLM results.

The technique of Bogoliubov quasiparticle interference (QPI) has been successfully used to investigate the symmetry of unconventional superconducting gaps, also in heavy fermion compounds. It was demonstrated that QPI can distinguish between the d-wave singlet candidates in CeCoIn5. In URu2Si2 presumably a chiral d-wave singlet superconducting (SC) state exists inside a multipolar hidden order (HO) phase. We show that hidden order leaves an imprint on the symmetry of QPI pattern that may be used to determine the essential question whether HO in URu2Si2 breaks the in-plane rotational symmetry or not. We also demonstrate that the chiral d-wave SC gap leads to a crossover to a quasi-2D QPI spectrum below Tc which sharpens the HO features. Furthermore we investigate the QPI image of chiral p-wave multigap superconductor Sr2RuO4.

The Heisenberg model on a triangular lattice is a prime example of a geometrically frustrated spin system. However most experimentally accessible compounds have spatially anisotropic exchange interactions. As a function of this anisotropy, ground states with different magnetic properties can be realized. Motivated by recent experimental findings on Cs2CuCl4-xBrx, we discuss the full phase diagram of the anisotropic model with two exchange constants J(1) and J(2), including possible ferromagnetic exchange. Furthermore a comparison with the related square lattice model is carried out. We discuss the zero-temperature phase diagram, ordering vector, ground-state energy, and ordered moment on a classical level and investigate the effect of quantum fluctuations within the framework of spin-wave theory. The field dependence of the ordered moment is shown to be nonmonotonic with field and control parameter.

We have investigated the electronic structure of CaCu3Ru4O12 and LaCu3Ru4O12 using soft x-ray photoelectron and absorption spectroscopy together with band structure and cluster configuration interaction calculations. We found the Cu to be in a robust divalent ionic state while the Ru is more itinerant in character and stabilizes the metallic state. Substitution of Ca by La predominantly affects the Ru states. We observed strong correlation effects in the Cu 3d states affecting the valence band line shape considerably. Using resonant photoelectron spectroscopy at the Cu L3 edge we were able to unveil the position of the Zhang-Rice singlet states in the one-electron removal spectrum of the Cu with respect to the Ru-derived metallic bands in the vicinity of the chemical potential.

For low-dimensional frustrated quantum magnets, the dependence of the staggered moment on a magnetic field is nonmonotonic: For small and intermediate fields, quantum fluctuations are gradually suppressed, leading to an increase of the staggered moment as a function of the field strength. For large applied magnetic fields, the classically expected field dependence is recovered, namely a monotonous decrease with increasing field strength. The staggered moment is eventually suppressed when reaching the fully polarized state at the saturation field. The quantitative analysis of this behavior is an excellent tool to determine the frustration parameter of a magnetic compound. We have developed a general finite-size scaling scheme for numerical exact-diagonalization data of low-dimensional frustrated magnets, which we apply to the recently measured field dependence of the magnetic neutron scattering intensity of Cu(pz) 2 (ClO 4 ) 2 in the framework of the S = 1/2 two-dimensional (2D) J 1 – J 2 Heisenberg model. We also apply linear spin-wave theory to complement our numerical findings. Our results show that Cu(pz) 2 (ClO 4 ) 2 is a quasi-2D antiferromagnet with intermediate frustration J 2 / J 1 = 0.2.

The local moments, magnetic correlations and susceptibility in the 2D half-filled correlated Kondo lattice model are studied. We calculate their dependence on the control parameters given by local exchange coupling JK and Coulomb repulsion U. Exact diagonalization (ED) approach for ground state properties as well as finite temperature Lanczos method (FTLM) for the uniform susceptibility are employed for small tiles on the square lattice. The competition of on-site screening and induced inter-site correlations leads to non-monotonic local moment dependence on U for weak Kondo coupling JK. In the large U limit the numerical results are compared to those of the analytical bond operator method in mean field treatment. The variation of the Kondo temperature scale with U is obtained from the temperature dependence of the susceptibility. A monotonic increase with U is found.

We study the ternary clathrate Pr3Pd20Si6 in specific heat and ac susceptibility measurements on a high-quality single crystal, distinguishing antiferromagnetic and antiferroquadrupolar ordering, as well as a hitherto unknown magnetic low-temperature transition. The specific heat shows the direct involvement of nuclear spin degrees of freedom in the antiferromagnetic ordering, which is well supported by our calculation of the hyperfine level scheme without adjustable parameters. Pr3Pd20Si6 is, therefore, one of the rare materials where the nuclear moments are involved in the formation of the magnetic ground state.

Magnetic ions such as Fe, Mn, and Co with localized spins may be adsorbed on the surface of topological insulators such as Bi2Se3. They form scattering centers for the helical surface states which have a Dirac cone dispersion as long as the local spins are disordered. However, the local density of states (LDOS) may be severely modified by the formation of bound states. Commonly, only elastic scattering due to normal and exchange potentials of the adatom is assumed. Magnetization measurements show, however, that considerable magnetic single-ion anisotropies exist which lead to a splitting of the local impurity spin states, resulting in a singlet ground state. Therefore inelastic scattering processes of helical Dirac electrons become possible, as described by a dynamical local self-energy of second order in the exchange interaction. The self energy influences bound-state formation and leads to significant new anomalies in the LDOS at low energies and low temperatures, which we calculate within the T-matrix approach. We propose that they may be used for spectroscopy of local impurity spin states by appropriate tuning of the chemical potential and magnetic field. DOI: 10.1103/PhysRevB.86.245426

We investigate the spin-exciton modes in the superconducting d_{x^2-y^2} state of CeMIn_5 heavy fermion compounds found at the antiferromagnetic wave vector by inelastic neutron scattering. We present a theoretical model that explains the field dependence for both field directions. We show that the recently observed splitting of the spin exciton doublet in CeCoIn_5 into two non-degenerate modes for in-plane field appears naturally in this model. This is due to the spin anisotropy of g- factors and quasiparticle interactions which lead to different resonant conditions for the dynamic susceptibility components. We predict that the splitting of the spin resonance doublet becomes strongly nonlinear for larger fields when the energy of both split components decreases. For field along the tetragonal axis no splitting but only a broadening of the resonance is found in agreement with experiment.

For low-dimensional frustrated quantum magnets, the dependence of the staggered moment ms on a magnetic field is nonmonotonic: For small and intermediate fields, quantum fluctuations are gradually suppressed, leading to an increase of ms (H). For large applied magnetic fields however, the classically expected monotonous decrease is recovered. For the same reasons, the Néel ordering temperature TN of such compounds first increases and then exhibits a reentrant behavior as a function of the field strength. The quantitative analysis of this behavior is an excellent tool to determine the frustration parameter of a given compound. We have derived a general linear spin-wave (LSW) theory in the presence of a magnetic field. Based on our LSW theory, including a small interlayer coupling, we use a self-consistent approach determining TN by the condition of a vanishing total moment. We apply our findings to the recently measured field dependence of the magnetic ordering temperature TN of Cu(pz)2 (ClO4)2 in the framework of the S = 1/2 two-dimensional J1-J2 Heisenberg model. The observed increase with increasing field strength can be understood naturally using an intermediate frustration ratio J2/J1 ≈ 0.2, which is in accordance with the field dependence of the staggered moment.

We study theoretically the distributions of charge and spin polarization of a topological insulator ribbon, with a realistic rectangular cross section. Due to constriction in two lateral directions, the surface states discretize into a series of subbands inside of the bulk band gap. The charge and spin distribution show interesting characters which are different from an ideal topological surface state. The effect of merging of four different surface states into the new one as an entity are analyzed. Optical conductivity and dynamical spin susceptibility of the ribbon are studied. Different from a single ideal surface, the optical response and dynamical spin susceptibility of a ribbon do not have a clear correspondence. The dynamical spin susceptibility could be used to identify the more adequate model for Bi$_2$Se$_3$.

We discuss the conclusions on the symmetry of hidden order (HO) in URu2Si2 that may be drawn from recent torque experiments in rotating magnetic field by Okazaki et al. [1]. They are very sensitive to changes in the magnetic susceptibility induced by HO. We show that the observed twofold angular torque oscillations give evidence that hidden order has degenerate E- type (yz,zx) symmetry where both components are realised. The oscillations have the wrong characteristics or are absent for the 1D nontrivial representations like quadrupolar B1 (x^2-y^2) and B2 (xy) type HO or hexadecapolar A2(xy(x^2-y^2)) type HO. Therefore they may be excluded as candidates for hidden order. We also predict the field-angular variation of possible field-induced Bragg peaks based on underlying E-type order parameter and discuss the expected elastic constant anomalies.

To investigate correlation effects on antiferromagnetic order in Fe pnictides, we apply a variational Monte Carlo method to a two-orbital model. We obtain a small ordered moment consistent with experimental observations even for a Coulomb interaction comparable to the band width. Studies of estimation of the Coulomb interaction for Fe pnictides suggest values comparable to or slightly smaller than the band width, and much larger ordered moments have been obtained by the Hartree-Fock approximation for such a large Coulomb interaction. Thus, the correlation effect is important for Fe pnictides at least quantitatively.

A magnetic torque method is proposed that probes the warping and mass gap of Dirac cone surface states in topological insulators like Bi2X3 (X=Se,Te). A rotating field parallel to the surface induces a paramagnetic moment in the helical surface states for nonzero warping. It is non-collinear with the applied field and therefore produces torque oscillations as function of the field angle which are a direct signature of the surface states. The torque dependence on field strength and angle, the chemical potential and the Dirac cone parameters like warping strength and mass gap is calculated. It is shown that the latter leads to a symmetry reduction in the fourfold torque oscillations.

Using the multiband model we analyze the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the magnetic impurities in layered ferropnictide superconductors. In the normal state the interaction is spin isotropic and is dominated by the nesting features of the electron and hole bands separated by the antiferromagnetic momentum, Q_AF. In the AF state the RKKY interaction maps into an effective anisotropic XXZ-type Heisenberg exchange model. The anisotropy originates from the breaking of the spin-rotational symmetry induced by the AF order and its strength depends on the size of the AF gap and the structure of the folded Fermi surface. We discuss our results in connection to the recent experiments.

We propose an efficient method to identify the degree of frustration in quasi-2D antiferromagnets described by the J_1-J_2 Heisenberg model. The frustration ratio J_2/J_1 is usually obtained from analysis of susceptibility, specific heat and saturation field. We show that the non-monotonic field dependence of the staggered moment caused by the suppression of quantum fluctuations in a field depends strongly on the frustration ratio. This gives a powerful criterion to determine J_2/J_1 using a combination of exact diagonalization (ED) method for finite clusters and spin wave analysis. We apply this method to the quasi-2D compound Cu(pz)_2(ClO_4)_2 and show that it leads to an intermediate ratio J_2/J_1 ~ 0.2 for the frustration. We also explain the observed anomalous increase of transition temperature in applied fields as an effect of reduced quantum fluctuations.