Temperature and field dependence of the phase separation, structure, and magnetic ordering in La1−x_{1-x}Cax_xMnO3_3, (x=0.47x=0.47, 0.50, and 0.53)

Neutron powder diffraction measurements, combined with magnetization and resistivity data, have been carried out in the doped perovskite La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.47$, 0.50, and 0.53) to elucidate the structural, magnetic, and electronic properties of the system around the composition corresponding to an equal number of Mn3+ and Mn4+. At room temperature all three samples are paramagnetic and single phase, with crystallographic symmetry Pnma. The samples then all become ferromagnetic (FM) at $T_C\approx 265$ K. At $\sim 230$ K, however, a second distinct crystallographic phase (denoted A-II) begins to form. Initially the intrinsic widths of the peaks are quite large, but they narrow as the temperature decreases and the phase fraction increases, indicating microscopic coexistence. The fraction of the sample that exhibits the A-II phase increases with decreasing temperature and also increases with increasing Ca doping, but the transition never goes to completion to the lowest temperatures measured (5 K) and the two phases therefore coexist in this temperature-composition regime. Phase A-II orders antiferromagnetically (AFM) below a N\'{e}el temperature $T_N \approx 160$ K, with the CE-type magnetic structure. Resistivity measurements show that this phase is a conductor, while the CE phase is insulating. Application of magnetic fields up to 9 T progressively inhibits the formation of the A-II phase, but this suppression is path dependent, being much stronger for example if the sample is field-cooled compared to zero-field cooling and then applying the field. The H-T phase diagram obtained from the diffraction measurements is in good agreement with the results of magnetization and resistivity.Comment: 12 pages, 3 tables, 11 figure

Magnetic light

In this paper we report on the observation of novel and highly unusual magnetic state of light. It appears that in small holes light quanta behave as small magnets so that light propagation through such holes may be affected by magnetic field. When arrays of such holes are made, magnetic light of the individual holes forms novel and highly unusual two-dimensional magnetic light material. Magnetic light may soon become a great new tool for quantum communication and computing.Comment: Submitted to Phys.Rev.Lett., 3 figure

Anomalous field-dependent specific heat in charge-ordered Pr1−x_{1-x}Cax_xMnO3_3 and La0.5_{0.5}Ca0.5_{0.5}MnO3_3

We report low temperature specific heat measurements of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ ($0.3\leq x \leq 0.5$) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ with and without applied magnetic field. An excess specific heat, $C^{\prime}(T)$, of non-magnetic origin associated with charge ordering is found for all the samples. A magnetic field sufficient to induce the transition from the charge-ordered state to the ferromagnetic metallic state does not completely remove the $C^{\prime}$ contribution. This suggests that the charge ordering is not completely destroyed by a "melting" magnetic field. In addition, the specific heat of the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ compounds exhibit a large contribution linear in temperature ($\gamma T$) originating from magnetic and charge disorder.Comment: submitted to PRL 5 pages, 3 figures include

Electrical transport and magnetic properties of a possible electron-doped layered manganese oxide

We report on the structural, transport, and magnetic properties of La0.67Sr0.33MnOx thin films grown in vacuum by pulsed-laser deposition. The as-grown thin films have both the matrix La1.34Sr0.66MnO4 phase with K2NiF4 structure and an embedded MnO phase. The electrical transport and magnetic properties of the films are determined mainly by those of the matrix phase. By annealing, the as-grown thin films can be transformed into the normal La0.67Sr0.33MnO3 single phase, which shows the expected colossal magnetoresistance effect. Based on the composition of the matrix phase, and the structural, electrical, and magnetic properties of the films, we propose that the matrix phase is possibly electron doped with a mixed valence of Mn2+/Mn3+ instead of the Mn3+/Mn4+ as in the hole-doped case

Phase Separation and the Low-Field Bulk Magnetic Properties of Pr0.7Ca0.3MnO3

We present a detailed magnetic study of the perovskite manganite Pr0.7Ca0.3MnO3 at low temperatures including magnetization and a.c. susceptibility measurements. The data appear to exclude a conventional spin glass phase at low fields, suggesting instead the presence of correlated ferromagnetic clusters embedded in a charge-ordered matrix. We examine the growth of the ferromagnetic clusters with increasing magnetic field as they expand to occupy almost the entire sample at H ~ 0.5 T. Since this is well below the field required to induce a metallic state, our results point to the existence of a field-induced ferromagnetic insulating state in this material.Comment: 15 pages with figures, submitted to Physical Review

Magnetic Field Dependence of Electronic Specific Heat in Pr_{1.85} Ce_{0.15} CuO_4

The specific heat of electron-doped Pr_{1.85} Ce_{0.15} CuO_4 single crystals is reported for the temperature range 2 - 10 K and magnetic field range 0 - 10 T. A non-linear magnetic field dependence is observed for the field range 0 - 2 T. Our data supports a model with lines of nodes in the gap function of these superconductors. Theoretical calculations of the electronic specific heat for dirty d-wave, clean d-wave, and s-wave symmetries are compared to our data.Comment: 10 pages Latex and 4 eps figures, submitted to Phys. Rev.

The interplay between double exchange, super-exchange, and Lifshitz localization in doped manganites

Considering the disorder caused in manganites by the substitution of Mn by Fe or Ga, we accomplish a systematic study of doped manganites begun in previous papers. To this end, a disordered model is formulated and solved using the Variational Mean Field technique. The subtle interplay between double exchange, super-exchange, and disorder causes similar effects on the dependence of T_C on the percentage of Mn substitution in the cases considered. Yet, in La$_{2/3}$Ca$_{1/3}$Mn$_{1-y}$Ga$_y$O$_3$ our results suggest a quantum critical point (QCP) for $y\approx 0.1-0.2$, associated to the localization of the electronic states of the conduction band. In the case of La$_x$Ca$_x$Mn$_{1-y}$Fe$_y$O$_3$ (with $x=1/3,3/8$) no such QCP is expected.Comment: 6 pages + 3 postscript figures. Largely extended discussio

Composite Spin Waves, Quasi-Particles and Low Temperature resistivity in Double Exchange Systems

We make a quantum description of the electron low temperature properties of double exchange materials. In these systems there is a strong coupling between the core spin and the carriers spin. This large coupling makes the low energy spin waves to be a combination of ion and electron density spin waves. We study the form and dispersion of these composite spin wave excitations. We also analyze the spin up and down spectral functions of the temperature dependent quasi-particles of this system. Finally we obtain that the thermally activated composite spin waves renormalize the carriers effective mass and this gives rise to a low temperature resistivity scaling as T ^{5/2}.Comment: 4 pages, REVTE

Evidence of a d to s-wave pairing symmetry transition in the electron-doped cuprate superconductor Pr2−x_{2-x}Cex_xCuO4_4

We present point contact spectroscopy (PCS) data for junctions between a normal metal and the electron doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO). For the underdoped compositions of this cuprate ($x \sim 0.13$) we observe a peak in the conductance-voltage characteristics of the point contact junctions. The shape and magnitude of this peak suggests the presence of Andreev bound states at the surface of underdoped PCCO which is evidence for a d-wave pairing symmetry. For overdoped PCCO ($x \sim 0.17$) the PCS data does not show any evidence of Andreev bound states at the surface suggesting an s-wave pairing symmetry.Comment: 4 pages Latex, 4 eps figures included. Submitted to Phys. Rev. Let

Specific heat and magnetization study on single crystals of a frustrated, quasi one-dimensional oxide: Ca3Co2O6

Specific heat and magnetization measurements have been carried out under a range of magnetic fields on single crystals of Ca3Co2O6. This compound is composed of Ising magnetic chains that are arranged on a triangular lattice. The intrachain and interchain couplings are ferromagnetic and antiferromagnetic, respectively. This situation gives rise to geometrical frustration, that bears some similarity to the classical problem of a two-dimensional Ising triangular antiferromagnet. This paper reports on the ordering process at low-T and the possibility of one-dimensional features at high-T.Comment: 7 pages, 6 figures, accepted for publication in PR