arXiv subjects

G. S. Uhrig

Publications and source records attributed to G. S. Uhrig.

66 records · Page 4Linked to original sources

arXiv2000

Magnetic properties of (VO)_2P_2O_7: two-plane structure and spin-phonon interactions

Detailed experiments on single-crystal (VO)_2P_2O_7 continue to reveal new and unexpected features. We show that a model composed of two, independent planes of spin chains with frustrated magnetic coupling is consistent with nuclear magnetic resonance and inelastic neutron scattering measurements. The pivotal role of PO_4 groups in mediating intrachain exchange interactions explains both the presence of two chain types and their extreme sensitivity to certain lattice vibrations, which results in the strong magnetoelastic coupling observed by light scattering. We compute the respective modifications of the spin and phonon dynamics due to this coupling, and illustrate their observable consequences on the phonon frequencies, magnon dispersions, static susceptibility and specific heat.

cond-mat.str-el↗

arXiv1998

Magnetic Properties of (VO)_2P_2O_7 from Frustrated Interchain Coupling

Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon dispersion and an unexpected, low-lying second mode. The proximity and intensity of these modes suggest a frustrated coupling between the alternating spin chains. We deduce the minimal model containing such a frustration, and show that it gives an excellent account of the magnon dispersion, static susceptibility and electron spin resonance absorption. We consider two-magnon states which bind due to frustration, and demonstrate that these may provide a consistent explanation for the second mode.

cond-mat.str-el↗

arXiv1998

Comment on "Robustness of a Local Fermi Liquid against Ferromagnetism and Phase Separation"

In this paper we comment on the "Local Fermi Liquid Theory", proposed by Engelbrecht and Bedell [1]. We emphasize several important properties of the limit of infinite dimensions, in particular the k-dependence of the irreducible vertex function, the anisotropy of the Fermi surface and the necessary conditions for phase separation. It appears that these issues have not been fully appreciated in Ref. [1].

cond-mat.str-el↗

arXiv1998

Dopant-Bound Spinons in Cu_(1-x)Zn_xGeO_3

Polarized inelastic light scattering experiments on Cu_(1-x)Zn_xGeO_3 (0<= x <= 0.045) single crystals show for x<>0 a new distinct mode at nearly half the energy of the singlet response below the spin-Peierls transition. The temperature, magnetic field, polarization, and doping dependencies of this mode are similar to those of the singlet bound state. The data are interpreted in terms of a spinon-assisted light scattering process. Position and form of the peak provide strong evidence for the presence of dopant-bound spinons in Cu_(1-x)Zn_xGeO_3.

cond-mat.str-el↗

arXiv1998

A magnetic model for the incommensurate I phase of spin-Peierls systems

A magnetic model is proposed for describing the incommensurate I phase of spin-Peierls systems. Based on the harmonicity of the lattice distortion, its main ingredient is that the distortion of the lattice adjusts to the average magnetization such that the system is always gapful. The presence of dynamical incommensurabilities in the fluctuation spectra is also predicted. Recent experimental results for CuGeO_3 obtained by NMR, ESR and light scattering absorption are well understood within this model.

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↗

arXiv1997

Observation of three-magnon light scattering in CuGeO_3

Temperature and magnetic field dependent polarized Brillouin spectra of CuGeO_3 are reported. In addition to a bound singlet state at 30 cm^{-1}, a new feature has been observed at 18 cm^{-1}. This feature is interpreted in terms of a novel three-magnon light scattering process between excited triplet states.

cond-mat.str-el↗

arXiv1997

Berry's phase for large spins in external fields

It is shown that even for large spins $J$ the fundamental difference between integer and half-integer spins persists. In a quasi-classical description this difference enters via Berry's connection. This general phenomenon is derived and illustrated for large spins confined to a plane by crystalline electric fields. Physical realizations are rare-earth Nickel Borocarbides. Magnetic moments for half-integer spin (Dy$^{3+}$, $J=15/2$) and magnetic susceptibilities for integer spin (Ho$^{3+}$, $J=8$) are calculated. Experiments are proposed to furnish evidence for the predicted fundamental difference.

quant-ph↗

arXiv1995

Zero and Finite Temperature Phase Diagram of the Spinless Fermion Model in Infinite Dimensions

The phase diagram of the model of spinless fermions with repulsive nearest neighbour interaction is calculated analytically on a hypercubic lattice in infinite dimensions $(d\to \infty)$. In spite of its simplicity the model displays a rich phase diagram depending on the doping $δ$, the interaction $U$ and the temperature $T$. The system can be in the homogeneous phase (HOM), the nonsegregated AB charge density wave (AB-CDW), the AB phase separation region (PS-AB/HOM; coexistence of AB-CDW and HOM), the incommensurate phase (IP) or the IP phase separation region (PS-AB/IP; coexistence of AB-CDW and IP). We identify three important values of the interaction $U_{IPL}=0.572< U_{IPH}=1.914<U_{IP/PS}=4.212$ which distinguish four intervals of $U$. These imply four different types of phase diagrams. In all the three phase diagrams with $U$ below $U_{IP/PS}$ the IP appears. We propose a {\em new} general ansatz for the order parameter of this phase. A competition between the IP, the PS-AB/IP and the PS-AB/HOM is found. The relevance of our findings for the phase scenario of the Hubbard model is shown.

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↗

arXiv1994

Existence of an incommensurate ground state of interacting spinless fermions in infinite dimensions

We focus on the ground state phase diagram of a system of spinless fermions with repulsion on a hypercubic lattice in the limit of infinite dimensions. It occurs spontaneous symmetry breaking into a charge density wave (CDW). Using an ansatz for the order parameter which includes the homogeneous, the AB- and a large class of incommensurate phases we are able to calculate the phase diagram.

cond-mat↗

arXiv1994

Conductivity of interacting spinless fermion systems via the high dimensional approach

Spinless fermions with repulsion are treated non-perturbatively by classifying the diagrams of the generating functional $Φ$ in powers of the inverse lattice dimension $1/d$. The equations derived from the first two orders are evaluated on the one- and on the two-particle level. The order parameter of the AB-charge density wave (AB-CDW) occurring at larger interaction is calculated in $d=3$. The Bethe-Salpeter equation is evaluated for the conductivity $σ(\om)$ which is found to have two peaks within the energy gap $2Δ$ in the AB-CDW: a remnant of the Drude peak and an excitonic resonance. Unexpectedly, $σ_{\rm\scriptscriptstyle DC}$ does not vanish for $T\to 0$

cond-mat↗