arXiv subjects

Krzysztof Wohlfeld

Publications and source records attributed to Krzysztof Wohlfeld.

63 records · Page 4Linked to original sources

arXiv2009

Orbitally induced string formation in the spin-orbital polarons

We study the spectral function of a single hole doped into the (a,b) plane of the Mott insulator LaVO$_3$, with antiferromagnetic (AF) spin order of S=1 spins accompanied by alternating orbital (AO) order of active $\{d_{yz},d_{zx}\}$ orbitals. Starting from the respective t-J model, with spin-orbital superexchange and effective three-site hopping terms, we derive the polaron Hamiltonian and show that a hole couples simultaneously to the collective excitations of the AF/AO phase, magnons and orbitons. Next, we solve this polaron problem using the self-consistent Born approximation and find a stable quasiparticle solution -- a spin-orbital polaron. We show that the spin-orbital polaron resembles the orbital polaron found in $e_g$ systems, as e.g. in K$_2$CuF$_4$ or (to some extent) in LaMnO$_3$, and that the hole may be seen as confined in a string-like potential. However, the spins also play a crucial role in the formation of this polaron -- we explain how the orbital degrees of freedom: (i) confine the spin dynamics acting on the hole as the classical Ising spins, and (ii) generate the string potential which is of the joint spin-orbital character. Finally, we discuss the impact of the results presented here on the understanding of the phase diagrams of the lightly doped cubic vanadates.

cond-mat.str-el↗

arXiv2009

Polaron in the t-J Models with Three-Site Terms: the SU(2) and the Ising Cases

We compare the role of the three-site terms added to the t-J models in the SU(2) and the Ising cases in the extremely low doping regime, i.e. when a single hole added to the strongly interacting half-filled system becomes a polaron. We show that in the realistic Ising case the three-site terms play a vital role in the polaron movement and should never be neglected unlike in the SU(2) case.

cond-mat.str-el↗

arXiv2008

Reiter's polaron wavefunction applied to a $t_{2g}$ orbital t-J model

Using the self-consistent Born approximation we calculate Reiter's wavefunction for a single hole introduced into the undoped and orbitally ordered ground state of the t-J model with $t_{2g}$ orbital degrees of freedom. While the number of excitations is similar to the spin t-J model for a given J/t, a distinct structure of the calculated wavefunction and its momentum dependence is identified suggesting the formation of a novel type of mobile polarons.

cond-mat.str-el↗

arXiv2008

Spectral properties of orbital polarons in Mott insulators

We address the spectral properties of Mott insulators with orbital degrees of freedom, and investigate cases where the orbital symmetry leads to Ising-like superexchange in the orbital sector. The paradigm of a hole propagating by its coupling to quantum fluctuations, known from the spin t-J model, then no longer applies. We find instead that when one of the two orbital flavors is immobile, as in the Falicov-Kimball model, trapped orbital polarons coexist with free hole propagation emerging from the effective three-site hopping in the regime of large on-site Coulomb interaction U. The spectral functions are found analytically in this case within the retraceable path approximation in one and two dimensions. On the contrary, when both of the orbitals are active, as in the model for $t_{2g}$ electrons in two dimensions, we find propagating polarons with incoherent scattering dressing the moving hole and renormalizing the quasiparticle dispersion. Here, the spectral functions, calculated using the self-consistent Born approximation, are anisotropic and depend on the orbital flavor. Unbiased conclusions concerning the spectral properties are established by comparing the above results for the orbital t-J models with those obtained using the variational cluster approximation or exact diagonalization for the corresponding Hubbard models. The present work makes predictions concerning the essential features of photoemission spectra of certain fluorides and vanadates.

cond-mat.str-el↗

arXiv2007

Origin of charge density wave in the coupled spin ladders in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$

We formulate a d-p multiband charge transfer model for Cu$_2$O$_5$ coupled spin ladders, relevant for Cu$_2$O$_3$ plane of Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$, and solve it using Hartree-Fock approximation. The results explain that (i) the charge density wave (CDW) with its periodicity dependent on doping is stabilized by purely electronic many-body interactions in a single spin ladder and (ii) the inclusion of the interladder interactions favors (disfavors) the stability of the CDW with odd (even) periodicity, respectively. This stays in agreement with recent experimental results and suggests the structure of the minimal microscopic model which should be considered in future more sophisticated studies.

cond-mat.str-el↗

arXiv2007

Doped Spin Ladder: Zhang-Rice Singlets or Rung-centred Holes?

We formulate charge transfer model for a single doped spin ladder, relevant for the intrinsically doped plane of coupled spin ladders in Sr$_{3}$Ca$_{11}$Cu$_{24}$O$_{41}$. Assuming the presence of the experimentally observed charge order in the system we solve the model using Hartree-Fock approximation. Our results show the profound stability of the Zhang-Rice singlets with respect to other configurations of doped holes, in agreement with recent x-ray absorption measurements.

cond-mat.str-el↗

arXiv2006

Charge density wave in the spin ladder of Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$

We consider a multiband charge transfer model for a single spin ladder describing the holes in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$. Using Hartree-Fock approximation we show how the charge density wave, with its periodicity dependent on doping as recently observed in the experiment, can be stabilized by purely electronic many-body interactions.

cond-mat.str-el↗

arXiv2006

Double exchange model in cubic vanadates

We investigate the role of the double exchange mechanism for stability of the metallic C-type antiferromagnetic (C-AF) phase, which was observed experimentally in hole doped La$_{1-x}$Sr$_x$VO$_3$. The double exchange model treats localized electrons in xy orbitals as classical S=1/2 spins, which interact by Hund's exchange $J_H$ with yz/zx electrons in partly filled $t_{2g}$ orbitals. Including strong on-site Coulomb repulsion $U$ between $t_{2g}$ electrons, and using slave boson method we demonstrate that C-AF and metallic phase can be stabilized due to the specific features of the hopping in degenerate and partly filled $t_{2g}$ orbitals.

cond-mat.str-el↗

arXiv2006

Double exchange model for correlated electrons in systems with $t_{2g}$ orbital degeneracy

We formulate the double exchange (DE) model for systems with $t_{2g}$ orbital degeneracy, relevant for hole-doped cubic vanadates. In the relevant regime of strong on-site Coulomb repulsion $U$ we solve the model using two distinct mean-field approximations: Hartree-Fock, and mean-field applied to the formulation of the model in the Kotliar-Ruckenstein slave boson representation. We show how, due to the relative weakness of the DE mechanism via $t_{2g}$ degenerate orbitals, the anisotropic C-type antiferromagnetic metallic phase and the orbital liquid state can be stabilized. This is contrasted with the DE mechanism via $e_g$ degenerate orbitals which stabilizes the ferromagnetic order in the hole-doped regime of manganites. The obtained results are in striking agreement with the observed magnetic structures and the collapse of the orbital order in the doped La$_{1-x}$Sr$_x$VO$_3$, Pr$_{1-x}$Ca$_x$VO$_3$ and Nd$_{1-x}$Sr$_x$VO$_3$.

cond-mat.str-el↗