Interface Ferromagnetism in a SrMnO3/LaMnO3 Superlattice

Resonant soft x-ray absorption measurements at the O K edge on a SrMnO3/LaMnO3 superlattice show a shoulder at the energy of doped holes, which corresponds to the main peak of resonant scattering from the modulation in the doped hole density. Scattering line shape at the Mn L3,2 edges has a strong variation below the ferromagnetic transition temperature. This variation has a period equal to half the superlattice superperiod and follows the development of the ferromagnetic moment, pointing to a ferromagnetic phase developing at the interfaces. It occurs at the resonant energies for Mn3+ and Mn4+ valences. A model for these observations is presented, which includes a double-exchange two-site orbital and the variation with temperature of the hopping frequency tij between the two sites.Comment: 8.1 pages, 6 figure

Charge modulations vs. strain waves in resonant x-ray scattering

A method is described for using resonant x-ray scattering to separately quantify the charge (valence) modulation and the strain wave associated with a charge density wave. The essence of the method is a separation of the atomic form factor into a "raw" amplitude, fR(w), and a valence-dependent amplitude, fD(w), which in many cases may be determined independently from absorption measurements. The advantage of this separation is that the strain wave follows the quantity |fR(w) + fD(w)|^2 whereas the charge modulation follows only |fD(w)|^2. This allows the two distinct modulations to be quantified separately. A scheme for characterizing a given CDW as Peierls-like or Wigner-like naturally follows. The method is illustrated for an idealized model of a one-dimensional chain.Comment: 6 pages, 4 figure

Mapping the magneto-structural quantum phases of Mn3O4

We present temperature-dependent x-ray diffraction and temperature- and field-dependent Raman scattering studies of single crystal Mn3O4, which reveal the novel magnetostructural phases that evolve in the spinels due to the interplay between strong spin-orbital coupling, geometric frustration, and applied magnetic field. We observe a structural transition from tetragonal to monoclinic structures at the commensurate magnetic transition at T2=33K, show that the onset and nature of this structural transition can be controlled with an applied magnetic field, and find evidence for a field-tuned quantum phase transition to a tetragonal incommensurate or spin glass phase.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Lett; typos correcte

Enhancement of Wigner crystallization in quasi low-dimensional solids

The crystallization of electrons in quasi low-dimensional solids is studied in a model which retains the full three-dimensional nature of the Coulomb interactions. We show that restricting the electron motion to layers (or chains) gives rise to a rich sequence of structural transitions upon varying the particle density. In addition, the concurrence of low-dimensional electron motion and isotropic Coulomb interactions leads to a sizeable stabilization of the Wigner crystal, which could be one of the mechanisms at the origin of the charge ordered phases frequently observed in such compounds

Pressure- and Field-Tuning the Magnetostructural Phases of Mn3O4: Raman Scattering and X-Ray Diffraction Studies

We present temperature-, magnetic-field-, and pressure-dependent Raman scattering studies of single crystal Mn3O4, combined with temperature- and field-dependent x-ray diffraction studies, revealing the novel magnetostructural phases in Mn3O4. Our temperature-dependent studies showed that the commensurate magnetic transition at T2=33K in the binary spinel Mn3O4 is associated with a structural transition from tetragonal to orthorhombic structures. Field-dependent studies showed that the onset and nature of this structural transition can be controlled with an applied magnetic field, and revealed evidence for a field-tuned quantum phase transition to a tetragonal spin-disordered phase for H||[1-10]. Pressure-dependent Raman measurements showed that the magnetic easy axis direction in Mn3O4 can be controlled---and the ferrimagnetic transition temperature increased---with applied pressure. Finally, combined pressure- and magnetic-field-tuned Raman measurements revealed a rich magnetostructural phase diagram---including a pressure- and field-induced magnetically frustrated tetragonal phase in the PH phase diagram---that can be generated in Mn3O4 with applied pressure and magnetic field.Comment: 12 pages, 13 figures, to be published in Phys. Rev.

Raman scattering studies of temperature- and field-induced melting of charge order in (La,Pr,Ca)MnO3_{3}

We present Raman scattering studies of the structural and magnetic phases that accompany temperature- and field-dependent melting of charge- and orbital-order (COO) in La0.5Ca0.5MnO3 and La0.25Pr0.375Ca0.375MnO3. Our results show that thermal and field-induced COO melting in La0.5Ca0.5MnO3 exhibits three stages in a heterogeneous melting process associated with a structural change: a long-range, strongly JT distorted/COO regime; a coexistence regime; and weakly JT distorted/PM or FM phase. We provide a complete structural phase diagram of La0.5Ca0.5MnO3 for the temperature and field ranges 6<=T<=170 K and 0<=H<=9 T. We also investigate thermal and field-induced melting in La0.25Pr0.375Ca0.375MnO3 to elucidate the role of disorder in melting of COO. We find that while thermal melting of COO in La0.25Pr0.375Ca0.375MnO3 is quite similar to that in La0.5Ca0.5MnO3, the field-induced transition from the COO phase to the weakly JT-distorted/FM phase in La0.25Pr0.375Ca0.375MnO3 is very abrupt, and occurs at significantly lower fields (H~2 T at T~0 K) than in La0.5Ca0.5MnO3 (H~30 T at T=0 K). Moreover, the critical field H_c increases with increasing temperature in La0.25Pr0.375Ca0.375MnO3 in contrast to La0.5Ca0.5MnO3. To explain these differences, we propose that field-induced melting of COO in La0.25Pr0.375Ca0.375MnO3 is best described as the field-induced percolation of FM domains, and we suggest that Griffiths phase physics may be an appropriate theoretical model for describing the unusual temperature- and field- dependent transitions observed in La0.25Pr0.375Ca0.375MnO3.Comment: 14 pages, 8 figures, to be published in PR

Graded Orbital Occupation near Interfaces in a La2NiO4 - La2CuO4 Superlattice

X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a non-uniform distribution of oxygen holes in a La2NiO4 - La2CuO4 (LNO-LCO) superlattice, with excess holes concentrated in the LNO layers. Weak ferromagnetism with Tc = 160 K suggests a coordinated tilting of NiO6 octahedra, similar to that of bulk LNO. Ni d3z2-r2 orbitals within the LNO layers have a spatially variable occupation. This variation of the Ni valence near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at the Ni L edge, at a reflection suppressed by the symmetry of the structure, and is possible through graded doping with holes, due to oxygen interstitials taken up preferentially by inner LNO layers. Since the density of oxygen atoms in the structure can be smoothly varied with standard procedures, this orbital occupation, robust up to at least 280 K, is tunable.Comment: 11 pages, 8 figure