arXiv subjects

J. Lopez

Publications and source records attributed to J. Lopez.

At least 37 records · Page 2Linked to original sources

arXiv2003

High temperature specific heat and magnetic measurements in Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd, Sm, Dy and Ho) samples

We have made a magnetic characterization of Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3, Sm0.5Ca0.5MnO3, Dy0.5Ca0.5MnO3 and Ho0.5Ca0.5MnO3 polycrystalline samples. Ferromagnetic, antiferromagnetic and charge ordering transitions in our samples agree with previous reports. We also report specific heat measurements with applied magnetic fields between 0 and 9 T and temperatures between 2 and 300 K in all cases. Each curve was successfully fitted at high temperatures by an Einstein model with three optical phonon modes. Close to the charge ordering and ferromagnetic transition temperatures the specific heat curves showed peaks superposed to the characteristic response of the lattice oscillations. The entropy variation corresponding to the charge ordering transition was higher than the one corresponding to the ferromagnetic transition. The external magnetic field seems to have no effect in specific heat of the CO phase transition.

cond-mat.str-el↗

arXiv2002

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.

cond-mat↗

arXiv2001

Study of magnetic and specific heat measurements at low temperatures in Nd0.5Sr0.5MnO3 and Nd0.5Ca0.5MnO3

The magnetization at low temperatures for Nd0.5Sr0.5MnO3 and Nd0.5Ca0.5MnO3 samples showed a rapid increase with decreasing temperatures, contrary to a La0.5Ca0.5MnO3 sample. Specific heat measurement at low temperatures showed a Schottky-like anomaly for the first two samples. However, there is not a straight forward correlation between the intrinsic magnetic moment of the Nd3+ ions and the Schottky-like anomaly.

cond-mat.str-el↗

arXiv2001

Unusual magnetic relaxation behavior in La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3

We have carried out a systematic magnetic relaxation study, measured after applying and switching off a 5 T magnetic field to polycrystalline samples of La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3. The long time logarithmic relaxation rate (LTLRR), decreased from 10 K to 150 K and increased from 150 K to 195 K in La0.5Ca0.5MnO3. This change in behavior was found to be related to the complete suppression of the antiferromagnetic phase above 150 K and in the presence of a 5 T magnetic field. At 195 K, the magnetization first decreased, and after a few minutes increased slowly as a function of time. Moreover, between 200 K and 245 K, the magnetization increased throughout the measured time span. The change in the slope of the curves, from negative to positive at about 200 K was found to be related to the suppression of antiferromagnetic fluctuations in small magnetic fields. A similar temperature dependence of the LTLRR was found for the Nd0.5Sr0.5MnO3 sample. However, the temperature where the LTLRR reached the minimum in Nd0.5Sr0.5MnO3 was lower than that of La0.5Ca0.5MnO3. This result agrees with the stronger ferromagnetic interactions that exist in Nd0.5Sr0.5MnO3 in comparison to La0.5Ca0.5MnO3. The above measurements suggested that the general temperature dependence of the LTLRR and the underlying physics were mainly independent of the particular charge ordering system considered. All relaxation curves could be fitted using a logarithmic law at long times. This slow relaxation was attributed to the coexistence of ferromagnetic and antiferromagnetic interactions between Mn ions, which produced a distribution of energy barriers.

cond-mat.str-el↗

arXiv2000

Magnetic relaxation and magnetization field dependence measurements in La0.5Ca0.5MnO3

We reported a systematic change in the average magnetic relaxation rate, after the application and removal of a 5 T magnetic field, in a polycrystalline sample of La0.5Ca0.5MnO3. Magnetic relaxation measurements and magnetization versus field curves were taken from 10 K to 160 K. The long time behavior of the relaxation curves was approximately logarithmic in all cases. Keywords: Charge Ordering, Relaxation, Magnetic measurements

cond-mat.str-el↗

arXiv2000

Unusual magnetic relaxation behavior in La0.5Ca0.5MnO3

We present an extensive study of the time dependence of the magnetization in a polycrystalline and low temperature charge ordered La0.5Ca0.5MnO3 sample. After application and removal of a 5 T magnetic field, a systematic variation of the magnetic relaxation rate from 10 K to 245 K was found. At 195 K, the magnetization decreases in a very short time and after that it increases slowly as a function of time. Moreover, between 200 and 245 K, an increase in magnetization, above the corresponding value just after removing the 5 T magnetic field, was measured. This unusual behavior was tested in several other relaxation procedures. PACS: 70, 74.25 Ha, 75.60.-d, 76.60.Es

cond-mat.str-el↗

arXiv1999

Characterisation, Raman, Magnetic and Resistivity measurements in polycrystalline samples of LaMnO3 doped with Cd

We report a study of polycrystalline samples of the family La1-xCdxMnO3+dwith different percentage of Mn4+ ions. X-rays diffraction, Iodometric titration, Raman, Magnetic and Electrical Resistivity measurements provide a general characterisation of the physical properties. Results are qualitatively similar to the ones found in Ca doped manganese perovskites. Key words: Cd, manganese perovskites, magnetoresistance, Raman, magnetic, resistivity

cond-mat.str-el↗

arXiv1997

New (g-2)_μ Constraints on Light-Gravitino Models

We show that the gravitino contribution to (g-2)_μ is finite in many popular supergravity models, including no-scale supergravity and string and M-theory models. This contribution is greatly enhanced for very light gravitino masses, and leads to new upper bounds on the ratio of the smuon to the gravitino mass.

hep-ph↗

arXiv1997

Compactifications of M-theory and their Phenomenological Consequences

We compactify the M-theory proposed by Horava and Witten on a Calabi-Yau manifold with boundary $S_1/Z_2$. A no-scale-like Kähler potential, the superpotential, and the gauge kinetic function are obtained in this 4-dimensional $E_6\times E_8$ model. We also study the general phenomenological consequences of the resulting M-theory-inspired model, which may include very light gravitinos, axions and axinos.

hep-ph↗

arXiv1997

M-theory Inspired No-scale Supergravity

We propose a supergravity model that contains elements recently shown to arise in the strongly-coupled limit of the $E_8\times E_8$ heterotic string (M-theory), including a no-scale--like Kähler potential, the identification of the string scale with the gauge coupling unification scale, and the onset of supersymmetry breaking at an intermediate scale determined by the size of the eleventh dimension of M-theory. We also study the phenomenological consequences of such scenario, which include a rather constrained sparticle spectrum within the reach of present-generation particle accelerators.

hep-ph↗

arXiv1997

Flipped No-scale Supergravity: a Synopsis

We discuss the highlights of recent developments in no-scale supergravity and flipped SU(5), especially in the context of a very light gravitino scenario that may explain the puzzling CDF e+e+gamma+gamma+E_T,miss event. We update the status of both subjects and discuss the impact of the latest experimental constraints from LEP and the Tevatron. We also comment on a new form of cold dark matter that may solve this phenomenological `problem' of light gravitino models. [Lecture presented by D. V. Nanopoulos at the 34th International School of Subnuclear Physics "Effective theories and fundamental interactions", Erice, July 3-12, 1996.]

hep-ph↗

arXiv1996

Supersymmetry, Supergravity and R_b revisited in the light of LEP 2

A previous study of supersymmetric models has indicated that they are unlikely to make a large contribution to R_b. We revisit this analysis, taking into account the improved lower limits on sparticle masses provided recently by LEP 2 and the Tevatron, finding that a generic supersymmetric model cannot contribute more than about one-and-a-half current experimental standard deviations to R_b. We then specialize this analysis to minimal supergravity models with universal high-energy boundary conditions, and find a much more stringent upper bound R^{susy}_b < 0.0003. We discuss in detail why such models can only attain values of R^{susy}_b that are considerably smaller even than those obtainable in more general supersymmetric models.

hep-ph↗

arXiv1996

Single-photon signals at LEP in supersymmetric models with a light gravitino

We study the single-photon signals expected at LEP in models with a very light gravitino. The dominant process is neutralino-gravitino production (e+e- -> chi+ G) with subsequent neutralino decay via chi->gamma+G, giving a gamma+E_miss signal. We first calculate the cross section at arbitrary center-of-mass energies and provide new analytic expressions for the differential cross section valid for general neutralino compositions. We then consider the constraints on the gravitino mass from LEP 1 and LEP161 single-photon searches, and possible such searches at the Tevatron. We show that it is possible to evade the stringent LEP 1 limits and still obtain an observable rate at LEP 2, in particular in the region of parameter space that may explain the CDF e+e+gamma+gamma+E_T,miss event. As diphoton events from neutralino pair-production would not be kinematically accessible in this scenario, the observation of whichever photonic signal will discriminate among the various light-gravitino scenarios in the literature. We also perform a Monte Carlo simulation of the expected energy and angular distributions of the emitted photon, and of the missing invariant mass expected in the events. Finally we specialize the results to the case of a recently proposed one-parameter no-scale supergravity model.

hep-ph↗

arXiv1996

Supersymmetric photonic signals at LEP

We explore and contrast the single-photon and diphoton signals expected at LEP 2, that arise from neutralino-gravitino (e^+ e^- -> chi + gravitino -> gamma + E_miss) and neutralino-neutralino (e^+ e^- -> chi + chi -> gamma + gamma + E_miss) production in supersymmetric models with a light gravitino. LEP 1 limits imply that one may observe either one, but not both, of these signals at LEP 2, depending on the values of the neutralino and gravitino masses: single-photons for m_chi > Mz and m_gravitino < 3 x 10^-5 eV; diphotons for m_chi < Mz and all allowed values of m_gravitino.

hep-ph↗

arXiv1996

Analysis of LEP Constraints on Supersymmetric Models with a Light Gravitino

We propose an analysis of LEP constraints on radiative neutralino decays into a light gravitino, based on the plane of the Higgs mixing parameter mu and the SU(2) gaugino mass M_2. The preliminary LEP 2W constraints in the (mu, M_2) plane are considerably stronger than for supersymmetric models in which the lightest neutralino is stable. A significant portion of the parameter space in which chargino or selectron decay into a final state containing a light gravitino could provide an interpretation of the CDF ee gamma gamma + E_T,miss event can now excluded by the preliminary LEP 2W data.

hep-ph↗

arXiv1996

No-scale supergravity confronts LEP diphoton events

We examine the possibility that some anomalous acoplanar diphoton events observed at LEP may be consistent with the predictions of our previously proposed one-parameter no-scale supergravity model with a light gravitino, via the process e^+e^- -> chi chi -> gamma gamma+E_miss. We find that one such event may indeed be consistent with the model predictions for m_chi \approx (60-70) GeV. This region of parameter space is also consistent with the selectron and chargino interpretations of the CDF ee gamma gamma + E_T,miss event.

hep-ph↗

arXiv1996

Leptophobic $Z'$ in stringy flipped SU(5)

We show that leptophobic $Z'$ gauge bosons occur naturally in flipped SU(5) and may shift $R_b$ in an interesting way without upsetting the good values of $Γ_{had}$ and $R_c$. Within a string-derived version of the model, we study three possible scenarios and the constraints imposed on model building that would allow the new symmetry to remain unbroken down to low energies. Such $Z'$ gauge boson has generation non-universal couplings to quarks that violate parity maximally in the up-quark sector, and may contribute significantly to spin asymmetries in polarized $pp$ scattering experiments now being prepared for RHIC.

hep-ph↗

arXiv1996

Experimental consequences of no-scale supergravity in light of the CDF $eeγγ$ event

We explore two possible interpretations for the CDF $eeγγ+E_T\hskip-13pt/\quad$ event in the context of a recently proposed one-parameter no-scale supergravity model with a light gravitino. We delineate the region in parameter space consistent with the kinematics of the event interpreted either as selectron pair-production ($p\bar p\to\tilde e^+\tilde e^-\,X$, $\tilde e=\tilde e_R,\tilde e_L$) or as chargino pair-production ($p\bar p\toχ^+_1χ^-_1\,X$). In the context of this model, the selectron interpretation requires $\tilde e=\tilde e_R$ and predicts comparable rates for $\ell^\pmγγ+E_T\hskip-13pt/\quad$, whereas the chargino interpretation predicts comparable rates for $(\ell^\pm\ell^{'+}\ell^{'-},\ell^+\ell^-,\ell^+\ell^-jj)\,γγ+E_T\hskip-13pt/\quad$. We also consider the constraints from LEP~1.5 and the expectations for LEP~2. We point out that one of the acoplanar photon pairs observed by the OPAL collaboration at LEP~1.5 may be attributable to supersymmetry in the present model via $e^+e^-\toχ^0_1χ^0_1\to γγ+E_T\hskip-13pt/\quad$. We also show how similar future events at LEP~2 may be used to deduce the neutralino mass.

hep-ph↗