arXiv subjects

E. Müller-Hartmann

Publications and source records attributed to E. Müller-Hartmann.

25 records · Page 2Linked to original sources

arXiv2001

Orbital Correlations in the Ferromagnetic Half-Metal $CrO_{2}

We deduce a model relevant for the description of the ferromagnetic half-metal Chromium dioxide ($CrO_{2}$), widely used in magnetic recording technology. The model describes the effect of dynamical, local orbital correlations arising from local quantum chemistry of the material. A finite temperature solution of the model in $d=\infty$ provides a natural explanation of the optical response, photoemission, resistivity and the large Woods-Saxon ratio observed in experiments. Our study confirms the important role of many body dynamical correlation effects for a proper understanding of the metallic phase of $CrO_{2}$.

cond-mat.str-el↗

arXiv2000

Role of Orbital Degeneracy in Double Exchange Systems

We investigate the role of orbital degeneracy in the double exchange (DE) model. In the $J_{H}\to\infty$ limit, an effective generalized ``Hubbard'' model incorporating orbital pseudospin degrees of freedom is derived. The model possesses an exact solution in one- and in infinite dimensions. In 1D, the metallic phase off ``half-filling'' is a Luttinger liquid with pseudospin-charge separation. Using the $d=\infty$ solution for our effective model, we show how many experimental observations for the well-doped ($x\simeq 0.3$) three-dimensional manganites $La_{1-x}Sr_{x}MnO_{3}$ can be qualitatively explained by invoking the role of orbital degeneracy in the DE model.

cond-mat.str-el↗

arXiv1999

Disordered Hubbard model in $d=\infty$

We investigate the effects of static, diagonal disorder in the $d=\infty$ Hubbard model by treating the dynamical effects of local Hubbard correlations and disorder on an equal footing. This is achieved by a proper combination of the iterated perturbation theory and the coherent potential approximation. Within the paramagnetic phase, we find that the renormalized Fermi liquid metal phase of the pure Hubbard model is stable against disorder for small disorder strengths. With increasing disorder, strong resonant scattering effects destroy low-energy Fermi liquid coherence, leading to an {\it incoherent} metallic non-FL state off half-filling. Finally, for large enough disorder, a {\it continuous} transition to the Mott-Anderson insulating phase occurs. The nature of the non-FL metallic phase, as well as the effects of the low energy coherence (incoherence) on optical conductivity and electronic Raman spectra, are considered in detail.

cond-mat.str-el↗

arXiv1999

Triplet superconductivity in a one-dimensional ferromagnetic t-J model

In this paper we study the ground state phase diagram of a one-dimensional $t-U-J$ model, at half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy, a phase with gapless charge and massive spin excitations, characterized by the coexistence of triplet superconducting ($TS$) and spin density wave ($SDW^{z}$) instabilities is realized in the ground state. With reduction of the bandwidth, a transition into an insulating phase showing properties of the spin-1/2 XY model takes place. In the case of weakly anisotropic antiferromagnetic exchange the system shows a long range dimerized (Peierls) ordering in the ground state. The complete weak-coupling phase diagram of the model, including effects of the on-site Hubbard interaction, is obtained.

cond-mat.str-el↗

arXiv1999

Optical and Magneto-optical Response of a Doped Mott Insulator

We study the optical, Raman, and ac Hall response of the doped Mott insulator within the dynamical mean-field theory ($d=\infty$) for strongly correlated electron systems. The occurence of the {\it isosbectic} point in the optical conductivity is shown to be associated with the frequency dependence of the generalized charge susceptibility. We compute the Raman response, which probes the fluctuations of the "stress tensor", and show that the scattering is characterized by appreciable incoherent contributions. The calculated ac Hall constant and Hall angle also exhibit the isosbectic points. These results are also compared with those obtained for a {\it non-FL} metal in $d=\infty$. The role of low-energy coherence (FL) or incoherence (non-FL) in determining the finite frequency response of strongly correlated metals in $d=\infty$ is discussed in detail.

cond-mat.str-el↗

arXiv1998

On the Incommensurate Phase in Modulated Heisenberg Chains

Using the density matrix renormalization group method (DMRG) we calculate the magnetization of frustrated S=1/2 Heisenberg chains for various modulation patterns of the nearest neighbour coupling: commensurate, incommensurate with sinusoidal modulation and incommensurate with solitonic modulation. We focus on the order of the phase transition from the commensurate dimerized phase (D) to the incommensurate phase (I). It is shown that the order of the phase transition depends sensitively on the model. For the solitonic model in particular, a $k$-dependent elastic energy modifies the order of the transition. Furthermore, we calculate gaps in the incommensurate phase in adiabatic approximation.

cond-mat↗

arXiv1995

Spin Waves in Quantum Antiferromagnets

Using a self-consistent mean-field theory for the $S=1/2$ Heisenberg antiferromagnet Krüger and Schuck recently derived an analytic expression for the dispersion. It is exact in one dimension ($d=1$) and agrees well with numerical results in $d=2$. With an expansion in powers of the inverse coordination number $1/Z$ ($Z=2d$) we investigate if this expression can be {\em exact} for all $d$. The projection method of Mori-Zwanzig is used for the {\em dynamical} spin susceptibility. We find that the expression of Krüger and Schuck deviates in order $1/Z^2$ from our rigorous result. Our method is generalised to arbitrary spin $S$ and to models with easy-axis anisotropy $\D$. It can be systematically improved to higher orders in $1/Z$. We clarify its relation to the $1/S$ expansion.

cond-mat↗