Magnetostrictive Neel ordering of the spin-5/2 ladder compound BaMn2O3: distortion-induced lifting of geometrical frustration

The crystal structure and the magnetism of BaMn$_2$O$_3$ have been studied by thermodynamic and by diffraction techniques using large single crystals and powders. BaMn$_2$O$_3$ is a realization of a $S = 5/2$ spin ladder as the magnetic interaction is dominant along 180$^\circ$ Mn-O-Mn bonds forming the legs and the rungs of a ladder. The temperature dependence of the magnetic susceptibility exhibits well-defined maxima for all directions proving the low-dimensional magnetic character in BaMn$_2$O$_3$. The susceptibility and powder neutron diffraction data, however, show that BaMn$_2$O$_3$ exhibits a transition to antiferromagnetic order at 184 K, in spite of a full frustration of the nearest-neighbor inter-ladder coupling in the orthorhombic high-temperature phase. This frustration is lifted by a remarkably strong monoclinic distortion which accompanies the magnetic transition.Comment: 9 pages, 8 figures, 2 tables; in V1 fig. 2 was included twice and fig. 4 was missing; this has been corrected in V

Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling

High-resolution and high-flux neutron as well as X-ray powder-diffraction experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with 0<x<0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for ReOFeAs (with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F-doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x=0.06. In the undoped sample, several interatomic distances and FeAs4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magneto-elastic coupling in this material.Comment: 8 pages, 7 figures, regular articl

Vibrational edge modes in intrinsically heterogeneous doped transition metal oxides

By applying an unrestricted Hartree-Fock and a Random Phase approximations to a multiband Peierls-Hubbard Hamiltonian, we study the phonon mode structure in models of transition metal oxides in the presence of intrinsic nanoscale inhomogeneities induced by hole doping. We identify low frequency $local$ vibrational modes pinned to the sharp interfaces between regions of distinct electronic structure (doped and undoped) and separated in frequency from the band of extended phonons. A characteristic of these ``edge'' modes is that their energy is essentially insensitive to the doping level. We discuss the experimental manifestations of these modes in inelastic neutron scattering, and also in spin and charge excitation spectra.Comment: 5 pages, 4 figure

Renormalization group trajectories from resonance factorized S-matrices

We propose and investigate a large class of models possessing resonance factorized S-matrices. The associated Casimir energy describes a rich pattern of renormalization group trajectories related to flows in the coset models based on the simply laced Lie Algebras. From a simplest resonance S-matrix, satisfying the ``$\phi^3$-property'', we predict new flows in non-unitary minimal models.Comment: (7 pages) (no figures included

Social Competence Treatment after Traumatic Brain Injury: A Multicenter, Randomized, Controlled Trial of Interactive Group Treatment versus Non-Interactive Treatment

Objective To evaluate the effectiveness of a replicable group treatment program for improving social competence after traumatic brain injury (TBI). Design Multicenter randomized controlled trial comparing two methods of conducting a social competency skills program, an interactive group format versus a classroom lecture. Setting Community and Veteran rehabilitation centers. Participants 179 civilian, military, and veteran adults with TBI and social competence difficulties, at least 6 months post-injury. Experimental Intervention Thirteen weekly group interactive sessions (1.5 hours) with structured and facilitated group interactions to improve social competence. Alternative (Control) Intervention Thirteen traditional classroom sessions using the same curriculum with brief supplemental individual sessions but without structured group interaction. Primary Outcome Measure Profile of Pragmatic Impairment in Communication (PPIC), an objective behavioral rating of social communication impairments following TBI. Secondary Outcomes LaTrobe Communication Questionnaire (LCQ), Goal Attainment Scale (GAS), Satisfaction with Life Scale (SWLS), Post-Traumatic Stress Disorder Checklist – (PCL-C), Brief Symptom Inventory 18 (BSI-18), Scale of Perceived Social Self Efficacy (PSSE). Results Social competence goals (GAS) were achieved and maintained for most participants regardless of treatment method. Significant improvements in the primary outcome (PPIC) and two of the secondary outcomes (LCQ and BSI) were seen immediately post-treatment and at 3 months post-treatment in the AT arm only, however these improvements were not significantly different between the GIST and AT arms. Similar trends were observed for PSSE and PCL-C. Conclusions Social competence skills improved for persons with TBI in both treatment conditions. The group interactive format was not found to be a superior method of treatment delivery in this study

The microscopic spin-phonon coupling constants in CuGeO_3

Using RPA results, mean field theory, and refined data for the polarization vectors we determine the coupling constants of the four Peierls-active phonon modes to the spin chains of CuGeO_3. We then derive the values of the coupling of the spin system to the linear ionic displacements, the bond lengths and the angles between bonds. Our values are consistent with microscopic theories and various experimental results. We discuss the applicability of static approaches to the spin-phonon coupling. The c-axis anomaly of the thermal expansion is explained. We give the values of the coupling constants in an effective one-dimensional Hamiltonian.Comment: 11 pages, two figures, 13 tables, PRB 59 (in press

Evidence for incommensurate spin fluctuations in Sr_2RuO_4

We report first inelastic neutron scattering measurements in the normal state of Sr_2RuO_4 that reveal the existence of incommensurate magnetic spin fluctuations located at ${\bf q}_0=(\pm 0.6\pi/a, \pm 0.6\pi/a, 0)$. This finding confirms recent band structure calculations that have predicted incommensurate magnetic responses related to dynamical nesting properties of its Fermi surface

High resolution X-ray scattering studies of structural phase transitions in underdoped La2−x_{2-x}Bax_xCuO4_4

We have studied structural phase transitions in high quality underdoped La$_{2-x}$Ba$_x$CuO$_4$ single crystals using high resolution x-ray scattering techniques. Critical properties associated with the continuous High Temperature Tetragonal (HTT, $I4/mmm$) to Middle Temperature Orthorhombic (MTO, $Cmca$) phase transition were investigated in single crystal samples with x=0.125, 0.095, and 0.08 and we find that all behavior is consistent with three dimensional XY criticality, as expected from theory. Power law behavior in the orthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably wide temperature range, spanning most of the MTO regime in the phase diagram. Low temperature measurements investigating the Low Temperature Tetragonal (LTT, $P4_{2}/ncm$) phase, below the strongly discontinuous MTO$\to$LTT phase transition, in x=0.125 and x=0.095 samples show that the LTT phase is characterized by relatively broad Bragg scattering, compared with that observed at related wavevectors in the HTT phase. This shows that the LTT phase is either an admixture of tetragonal and orthorhombic phases, or that it is orthorhombic with very small orthorhombic strain, consistent with the ``less orthorhombic" low temperature structure previously reported in mixed La$_{2-x}$Sr$_{x-y}$Ba$_y$CuO$_4$ single crystals. We compare the complex temperature-composition phase diagram for the location of structural and superconducting phase transitions in underdoped La$_{2-x}$Ba$_x$CuO$_4$ and find good agreement with results obtained on polycrystalline samples.Comment: 8 pages, 7 figures, 1 tabl

Soft phonons and structural phase transitions in La1.875_{1.875}Ba0.125_{0.125}CuO4_{4}

Soft phonon behavior associated with a structural phase transition from the low-temperature-orthorhombic (LTO) phase ($Bmab$ symmetry) to the low-temperature-tetragonal (LTT) phase ($P4_{2}/ncm$ symmetry) was investigated in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ using neutron scattering. As temperature decreases, the TO-mode at $Z$-point softens and approaches to zero energy around $T_{\rm d2}=62$ K, where the LTO -- LTT transition occurs. Below $T_{\rm d2}$, the phonon hardens quite rapidly and it's energy almost saturates below 50 K. At $T_{\rm d2}$, the energy dispersion of the soft phonon along in-plane direction significantly changes while the dispersion along out-of-plane direction is almost temperature independent. Coexistence between the LTO phase and the LTT phase, seen in both the soft phonon spectra and the peak profiles of Bragg reflection, is discussed in context of the order of structural phase transitions.Comment: 6 pages, 8 figure