arXiv subjects

P. Horsch

Publications and source records attributed to P. Horsch.

At least 19 recordsLinked to original sources

arXiv2012

Magnetic properties of nanoscale compass-Heisenberg planar clusters

We study a model of spins 1/2 on a square lattice, generalizing the quantum compass model via the addition of perturbing Heisenberg interactions between nearest neighbors, and investigate its phase diagram and magnetic excitations. This model has motivations both from the field of strongly correlated systems with orbital degeneracy and from that of solid-state based devices proposed for quantum computing. We find that the high degeneracy of ground states of the compass model is fragile and changes into twofold degenerate ground states for any finite amplitude of Heisenberg coupling. By computing the spin structure factors of finite clusters with Lanczos diagonalization, we evidence a rich variety of phases characterized by Z2 symmetry, that are either ferromagnetic, C-type antiferromagnetic, or of Neel type, and analyze the effects of quantum fluctuations on phase boundaries. In the ordered phases the anisotropy of compass interactions leads to a finite excitation gap to spin waves. We show that for small nanoscale clusters with large anisotropy gap the lowest excitations are column-flip excitations that emerge due to Heisenberg perturbations from the manifold of degenerate ground states of the compass model. We derive an effective one-dimensional XYZ model which faithfully reproduces the exact structure of these excited states and elucidates their microscopic origin. The low energy column-flip or compass-type excitations are robust against decoherence processes and are therefore well designed for storing information in quantum computing. We also point out that the dipolar interactions between nitrogen-vacancy centers forming a rectangular lattice in a diamond matrix may permit a solid-state realization of the anisotropic compass-Heisenberg model.

cond-mat.str-el↗

arXiv2011

Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2

We report the results of a comprehensive spectroscopic ellipsometry study of NaCu2O2, a compound composed of chains of edge-sharing Cu2+O4 plaquettes and planes of Cu1+ ions in a O-Cu1+-O dumbbell configuration, in the spectral range 0.75-6.5 eV at temperatures 7 -300 K. The spectra of the dielectric function for light polarized parallel to the Cu1+ planes reveal a strong in-plane anisotropy of the interband excitations. Strong and sharp absorption bands peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along (perpendicular) to the Cu2+O2 chains. They are superimposed on flat and featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on density-functional calculations, the anomalous absorption peaks can be assigned to transitions between bands formed by Cu1+ 3dxz(dyz) and Cu2+ 3dxy orbitals, strongly hybridized with O pstates. The major contribution to the background response comes from transitions between Cu1+ 3dz2 and 4px(py) bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the Cu2+O2 chains develops a weak two-peak structure centered at 2.1 and 2.65 eV upon cooling below 100 K, along with the appearance of spin correlations along the Cu2+O2 chains. These features bear a striking resemblance to those observed in the single-valent Cu2+O2 chain compound LiCuVO4, which were identified as an exciton doublet associated with transitions to the upper Hubbard band that emerges as a consequence of the long-range Coulomb interaction between electrons on neighboring Cu2+ sites along the chains. An analysis of the spectral weights of these features yields the parameters characterizing the on-site and long-range Coulomb interactions.

cond-mat.str-el↗

arXiv2011

The dimerized ferromagnetic Heisenberg chain

Ferromagnetic, in contrast to antiferromagnetic, Heisenberg chains can undergo a Spin-Peierls dimerization only at finite temperatures. They show reentrant behavior as a function of temperature, which might play a role for systems with small effective elastic constants as, for example, monatomic chains on surfaces. We investigate the physical properties of the dimerized ferromagnetic Heisenberg chain using a modified spin-wave theory. We calculate the exponentially decaying spin and dimer correlation functions, analyze the temperature dependence of the corresponding coherence lengths, the susceptibility, as well as the static and dynamic spin structure factor. By comparing with numerical data obtained by the density-matrix renormalization group applied to transfer matrices, we find that the modified spin wave theory yields excellent results for all these quantities for a wide range of dimerizations and temperatures.

cond-mat.str-el↗

arXiv2011

Spin exchange dominated by charge fluctuations of the Wigner lattice in the newly synthesized chain cuprate Na5Cu3O6

Na5Cu3O6, a new member of one dimensional charge ordered chain cuprates, was synthesized via the azide/nitrate route by reacting NaN3, NaNO3 and CuO. According to single crystal X-ray analysis, one dimensional CuO2 chains built up from planar, edge-sharing CuO4 squares are a dominant feature of the crystal structure. From the analysis of the Cu-O bond lengths we find that the system forms a Wigner lattice. The commensurate charge order allows to explicitly assign the valence states of either +2 or +3 to each copper atom resulting in a repetition according to Cu(2+)-Cu(3+)-Cu(2+)-Cu(2+)-Cu(3+)-Cu(2+). Following the theoretical analysis of the previously synthesized compounds Na3Cu2O4 and Na8Cu5O10, the magnetic susceptibility was expected to show a large dimer gap. Surprisingly, this is not the case. To resolve this puzzle, we show that the magnetic couplings in this compound are strongly affected by excitations across the Wigner charge gap. By including these contributions, which are distinct from conventional superexchange in Mott-insulators, we obtain a quantitative satisfying theoretical description of the magnetic susceptibility data.

cond-mat.str-el↗

arXiv2011

Magnetic excitations in one-dimensional spin-orbital models

We study the dynamics and thermodynamics of one-dimensional spin-orbital models relevant for transition metal oxides. We show that collective spin, orbital, and combined spin-orbital excitations with infinite lifetime can exist, if the ground state of both sectors is ferromagnetic. Our main focus is the case of effectively ferromagnetic (antiferromagnetic) exchange for the spin (orbital) sector, respectively, and we investigate the renormalization of spin excitations via spin-orbital fluctuations using a boson-fermion representation. We contrast a mean-field decoupling approach with results obtained by treating the spin-orbital coupling perturbatively. Within the latter self-consistent approach we find a significant increase of the linewidth and additional structures in the dynamical spin structure factor as well as Kohn anomalies in the spin-wave dispersion caused by the scattering of spin excitations from orbital fluctuations. Finally, we analyze the specific heat c(T) by comparing a numerical solution of the model obtained by the density-matrix renormalization group with perturbative results. At low temperatures T we find numerically c(T) T pointing to a low-energy effective theory with dynamical critical exponent z=1.

cond-mat.str-el↗

arXiv2009

Exciton doublet in the Mott-Hubbard LiCuVO$_4$ insulator identified by spectral ellipsometry

Spectroscopic ellipsometry was used to study the dielectric function of LiCuVO$_{4}$, a compound comprised of chains of edge-sharing CuO$_4$ plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K. For photon polarization along the chains, the data reveal a weak but well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral weight is strongly enhanced upon cooling near the magnetic ordering temperature. We identify these features as an exciton doublet in the Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction between electrons on nearest and next-nearest neighbor sites along the chains. Our results and methodology can be used to address the role of the long-range Coulomb repulsion for compounds with doped copper-oxide chains and planes.

cond-mat.str-el↗

arXiv2008

Magnetism of one-dimensional Wigner lattices and its impact on charge order

The magnetic phase diagram of the quarter-filled generalized Wigner lattice with nearest- and next-nearest-neighbor hopping t_1 and t_2 is explored. We find a region at negative t_2 with fully saturated ferromagnetic ground states that we attribute to kinetic exchange. Such interaction disfavors antiferromagnetism at t_2 <0 and stems from virtual excitations across the charge gap of the Wigner lattice, which is much smaller than the Mott-Hubbard gap proportional to U. Remarkably, we find a strong dependence of the charge structure factor on magnetism even in the limit U to infinity, in contrast to the expectation that charge ordering in the Wigner lattice regime should be well described by spinless fermions. Our results, obtained using the density-matrix renormalization group and exact diagonalization, can be transparently explained by means of an effective low-energy Hamiltonian.

cond-mat.str-el↗

arXiv2008

Thermally activated Peierls dimerization in ferromagnetic spin chains

We demonstrate that a Peierls dimerization can occur in ferromagnetic spin chains activated by thermal fluctuations. The dimer order parameter and entanglement measures are studied as functions of the modulation of the magnetic exchange interaction and temperature, using a spin-wave theory and the density-matrix renormalization group. We discuss the case where a periodic modulation is caused by spin-phonon coupling and the case where electronic states effectively induce such a modulation. The importance of the latter for a number of transition metal oxides is highlighted.

cond-mat.str-el↗

arXiv2008

Absence of Hole Confinement in Transition Metal Oxides with Orbital Degeneracy

We investigate the spectral properties of a hole moving in a two-dimensional Hubbard model for strongly correlated t_2g electrons. Although superexchange interactions are Ising-like, a quasi-one-dimensional coherent hole motion arises due to effective three-site terms. This mechanism is fundamentally different from the hole motion via quantum fluctuations in the conventional spin model with SU(2) symmetry. The orbital model describes also propagation of a hole in some e_g compounds, and we argue that orbital degeneracy alone does not lead to hole self-localization.

cond-mat.str-el↗

arXiv2007

Fractional charges and spin-charge separation in one-dimensional Wigner lattices

We study density response N(k,omega) and one-particle spectra A(k,omega) for a Wigner lattice model at quarter filling using exact diagonalization. We investigate these observables for models with short and long-range electron-electron interaction and show that truncation of the electron repulsion can lead to very different results. The spectra show clear signatures of charge fractionalization into pairs of domain walls, whose interaction can be attractive or repulsive and is controlled by the formal fractional charges. In striking contrast to a bound exciton in N(k,omega), we find an antibound quasi-particle in A(k,omega), which undergoes spin-charge separation. We present a case of extreme particle-hole asymmetry, where photoemission shows spin-charge separation, while inverse photoemission exhibits an uncorrelated one-particle band.

cond-mat.str-el↗

arXiv2006

Magnetic properties of spin-orbital polarons in lightly doped cobaltates

We present a numerical treatment of a spin-orbital polaron model for Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intra-layer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.

cond-mat.str-el↗

arXiv2005

Wigner crystallization in Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10) chain compounds

We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10), which form insulating states with commensurate charge order. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by long-range Coulomb interaction and the number of holes in the d-shell of Cu. Our interpretation is supported by X-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for a distinction between the (classical) Wigner lattice and the 4k_F charge-density wave of quantum mechanical origin.

cond-mat.str-el↗

arXiv2004

Helicoidal magnetic order in a clean copper oxide spin chain compound

We report susceptibility, specific heat, and neutron diffraction measurements on NaCu$_2$O$_2$, a spin-1/2 chain compound isostructural to LiCu$_2$O$_2$, which has been extensively investigated. Below 13 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu$_2$O$_2$. In contrast to the Li analogue, substitutional disorder is negligible in NaCu$_2$O$_2$. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu$_2$O$_2$. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.

cond-mat.str-el↗

arXiv2003

Charge dynamics of t-J model and anomalous bond-stretching phonons in cuprates

The density response of a doped Mott-Hubbard insulator is discussed starting from the t-J model in a slave boson 1/N representation. In leading order O(1) the density fluctuation spectra $N({\bf q},ω)$ are determined by an undamped collective mode at large momentum transfer, in striking disagreement with results obtained by exact diagonalization, which reveal a very broad dispersive peak, reminescent of strong spin-charge coupling. The 1/N corrections introduce the polaron character of the bosonic holes moving in a uniform RVB background. The resulting $N({\bf q},ω)$ captures all features observed in diagonalization studies, fulfills the appropriate sum rules, and apart from the broadening of the collective mode shows a new low energy feature at the energy $χJ+δt$ related to the polaron motion in the spinon background. It is further shown that the low energy structure, which is particularly pronounced in $(π,0)$ direction, describes the strong renormalization and anomalous damping of the highest bond-stretching phonons in La$_{2-x}$Sr$_x$CuO$_4$.

cond-mat.str-el↗

arXiv2003

Planar Cu and O hole densities in high-Tc cuprates determined with NMR

The electric hyperfine interaction observable in atomic spectroscopy for O and Cu ions in various configurations is used to analyze the quadrupole splitting of O and Cu nuclear magnetic resonance (NMR) in La2-xSrxCuO4 and YBa2Cu3O6+y and to determine the hole densities at both sites as a function of doping. It is found that in La2-xSrxCuO4 all doped holes (x) reside in the Cu-O plane but almost exclusively at O. For YBa2Cu3O6+y and y<0.6 doped holes are found at planar Cu as well as O. For y>0.6 further doping increases the hole content only for planar O. The phase diagram based on NMR data is presented. Further implications from the Cu A and B site in La2-xSrxCuO4 and the two planar O sites in YBa2Cu3O6+y and consequences for the phase diagram are discussed.

cond-mat.supr-con↗

arXiv2001

Density response of the t-J model and renormalization of breathing and half-breathing phonon modes: A slave-fermion calculation

The density fluctuation spectrum $N({\bf k},ω)$ is calculated for the t-J model in the low-doping regime using a slave-fermion method for the constrained fermions. The obtained results for $N({\bf k},ω)$ are in good agreement with diagonalization results. The density response is characterized by incoherent, momentum dependent spectral functions reaching up to energies $\sim 8t$ and a low-energy structure at energy $\sim J$ due to transitions in the quasiparticle band. $N({\bf k},ω)$ is shown to lead to a strong renormalization of planar bond-streching and breathing phonon modes with a large phonon linewidth at intermediate momenta caused by the low-energy response. Our results are consistent with recent neutron scattering data, showing the peculiar behavior of these modes.

cond-mat.str-el↗

arXiv2001

Density response of cuprates and renormalization of breathing phonons

We analyse the dynamical density fluctuation spectra for cuprates starting from the $t-J$ model in a slave-boson 1/N representation. The results obtained are consistent with diagonalization studies and show novel low-energy structure on the energy scale $J+δt$ due to the correlated motion of holes in a RVB spin liquid. The low-energy response implies an anomalous renormalization of several phonon modes. Here we discuss the renormalization of the highest breathing phonons in La$_{2-x}$Sr$_x$CuO$_4$.

cond-mat.str-el↗

arXiv2001

Spin Order due to Orbital Fluctuations: Cubic Vanadates

We investigate the highly frustrated spin and orbital superexchange interactions in cubic vanadates. The fluctuations of $t_{2g}$ orbitals trigger a {\it novel mechanism of ferromagnetic interactions} between spins S=1 of V$^{3+}$ ions along one of the cubic directions which operates already in the absence of Hund's rule exchange $J_H$, and leads to the C-type antiferromagnetic phase in LaVO$_3$. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due to orbital fluctuations favors again the C-phase at higher temperatures, as observed in YVO$_3$.

cond-mat.str-el↗