9 research outputs found
Specific heat study of single crystalline Pr Ca MnO in presence of a magnetic field
We present the results of a study of specific heat on a single crystal of
PrCaMnO performed over a temperature range 3K-300K in
presence of 0 and 8T magnetic fields. An estimate of the entropy and latent
heat in a magnetic field at the first order charge ordering (CO) transition is
presented. The total entropy change at the CO transition which is 1.8
J/mol K at 0T, decreases to 1.5 J/mol K in presence of 8T magnetic
field. Our measurements enable us to estimate the latent heat
235 J/mol involved in the CO transition. Since the entropy of the
ferromagnetic metallic (FMM) state is comparable to that of the charge-ordered
insulating (COI) state, a subtle change in entropy stabilises either of these
two states. Our low temperature specific heat measurements reveal that the
linear term is absent in 0T and surprisingly not seen even in the metallic FMM
state.Comment: 8 pages (in RevTEX format), 12 figures (in postscript format)
Submitted to Phys. Rev.
Specific heat and magnetic measurements in Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3 samples
We studied the magnetization as a function of temperature and magnetic field
in the compounds Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3. It allowed
us to identify the ferromagnetic, antiferromagnetic and charge ordering phases
in each case. The intrinsic magnetic moments of Nd3+ and Ho3+ ions experienced
a short range order at low temperatures. We also did specific heat measurements
with applied magnetic fields between 0 and 9 T and temperatures between 2 and
300 K in all three samples. Close to the charge ordering and ferromagnetic
transition temperatures the specific heat curves showed peaks superposed to the
characteristic response of the lattice oscillations. Below 10 K the specific
heat measurements evidenced a Schottky-like anomaly for all samples. However,
we could not successfully fit the curves to either a two level nor a
distribution of two-level Schottky anomaly. Our results indicated that the peak
temperature of the Schottky anomaly was higher in the compounds with narrower
conduction band.Comment: submitted to PR
Dynamical Mean-Field Theory of Electron-Phonon Interactions in Correlated Systems: Application to Isotope Effects on Electronic Properties
We use a recently developed formalism (combining an adiabatic expansion and
dynamical mean-field theory) to obtain expressions for isotope effects on
electronic properties in correlated systems. As an example we calculate the
isotope effect on electron effective mass for the Holstein model as a function
of electron-phonon interaction strength and doping. Our systematic expansion
generates diagrams neglected in previous studies, which turn out to give the
dominant contributions. The isotope effect is small unless the system is near a
lattice instability. We compare this to experiment.Comment: 6 pages, 4 figures; added discussion of isotope effect away from half
fillin
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
Non-linear electrical response in a charge/orbital ordered CaMnO crystal : the charge density wave analogy
Non-linear conduction in a charge-ordered manganese oxide
PrCaMnO is reported. To interpret such a feature, it is
usually proposed that a breakdown of the charge or orbitally ordered state is
induced by the current. The system behaves in such a way that the bias current
may generate metallic paths giving rise to resistivity drop. One can describe
this feature by considering the coexistence of localized and delocalized
electron states with independent paths of conduction. This situation is
reminiscent of what occurs in charge density wave systems where a similar
non-linear conduction is also observed. In the light of recent experimental
results suggesting the development of charge density waves in charge and
orbitally ordered manganese oxides, a phenomenological model for charge density
waves motion is used to describe the non-linear conduction in
PrCaMnO. In such a framework, the non-linear conduction
arises from the motion of the charge density waves condensate which carries a
net electrical current.Comment: 13 pages, 6 figure
Specific heat and magnetic order in LaMnO_{3+\delta}
Magnetic and specific-heat measurements are performed in three different
samples of LaMnO_{3+\delta}, with \delta=0.11, 0.15 and 0.26, presenting
important disorder effects, such as carrier localization, due to high amounts
of La and Mn vacancies. For the samples with \delta =0.11 and 0.15, magnetic
measurements show signatures of a two-step transition: as the temperature is
lowered, the system enters a ferromagnetic phase followed by a disorder-induced
cluster-glass state. Spin-wave-like contributions and an unexpected large
linear term are observed in the specific heat as a function of temperature. In
the sample with the highest vacancy content, \delta=0.26, the disorder is
sufficient to suppress even short-range ferromagnetic order and yield a
spin-glass-like state.Comment: RevTeX 2-col, 8 pages, 5 ps figures included, submitted to PR