arXiv subjects

C. Martin

Publications and source records attributed to C. Martin.

At least 145 records · Page 8Linked to original sources

arXiv2003

Critical field and Shubnikov-de Haas oscillations of k-(ET)2Cu(NCS)2 under pressure

A tuned tank circuit in combination with a nonmetallic diamond anvil cell has been successfully used to measure the change in critical field with angle in k-(ET)2Cu(NCS)2 at pressures up to 1.75 kbar and at temperatures down to 70 mK. The critical field has been found to decrease by more than 90% within less than 2 kbar and at a much higher rate for the field applied parallel to the conducting planes. For this orientation, at 1.75 kbar, we have seen a clear change from the ambient pressure behavior of the critical field with temperature at low temperatures. Up to P = 1.75 kbar, the H_c2($θ$) phase diagram is in good agreement with the theoretical prediction for weakly coupled layered superconductors. We have also succeeded in measuring oscillations in the resistivity of the normal state at higher magnetic field. The $α$-orbit Shubnikov-de Haas frequency was found to increase at a rate of 44 T/kbar. Our experiment opens the possibility for further investigations of the effective mass with pressure, especially because the setup is suitable for pulse fields as well.

cond-mat.supr-con↗

arXiv2003

Field-Induced Magnetization Steps in Intermetallic Compounds and Manganese Oxides: The Martensitic Scenario

Field-induced magnetization jumps with similar characteristics are observed at low temperature for the intermetallic germanide Gd5Ge4and the mixed-valent manganite Pr0.6Ca0.4Mn0.96Ga0.04O3. We report that the field location -and even the existence- of these jumps depends critically on the magnetic field sweep rate used to record the data. It is proposed that, for both compounds, the martensitic character of their antiferromagnetic-to-ferromagnetic transitions is at the origin of the magnetization steps.

cond-mat.mtrl-sci↗

arXiv2003

Staircase-like metamagnetic transitions in phase-separated manganites: influence of thermal and mechanical treatments

Substitutions in the Mn-sublattice of antiferromagnetic, charge and orbitally ordered manganites was recently found to produce intriguing metamagnetic transitions, consisting of a succession of sharp magnetization steps separated by plateaus. The compounds exhibiting such features can be divided in two categories, depending on whether they are sensitive to thermal cycling effects or not. One compound of each category has been considered in the present study. The paper reports on the influence of two treatments: high-temperature annealing and grinding. It is shown that both of these treatments can drastically affect the phenomenon of magnetization steps. These results provide us with new information about the origin of these jumps in magnetization.

cond-mat.mtrl-sci↗

arXiv2003

Optical Pulse-Phased Photopolarimetry of PSR B0656+14

We have observed the optical pulse profile of PSR B0656+14 in 10 phase bins at a high signal-to-noise ratio, and have measured the linear polarization profile over 30% of the pulsar period with some significance. The pulse profile is double-peaked, with a bridge of emission between the two peaks, similar to gamma-ray profiles observed in other pulsars. There is no detectable unpulsed flux, to a 1-sigma limit of 16% of the pulse-averaged flux. The emission in the bridge is highly (~ 100%) polarized, with a position angle sweep in excellent agreement with the prediction of the Rotating Vector Model as determined from radio polarization observations. We are able to account for the gross features of the optical light curve (i.e., the phase separation of the peaks) using both polar cap and outer gap models. Using the polar cap model, we are also able to estimate the height of the optical emission regions.

astro-ph↗

arXiv2003

Specific heat investigation of the magnetic ordering in two frustrated spin-chain oxides: Ca3Co2O6 and Ca3CoRhO6

Specific heat measurements were carried out on the closely related spin-chain oxides Ca3Co2O6 and Ca3CoRhO6. Both compounds consist of Ising magnetic chains that are arranged on a triangular lattice. The spin coupling along and between the chains are ferromagnetic and antiferromagnetic, respectively. Geometrical frustration is expected from the combination of these magnetic features. The present study reports that the specific heat data of these compounds exhibit similarities in the spin-freezing process at low T, whereas striking differences exist in the antiferromagnetic interchain ordering at higher T. These results are discussed in connection with previous magnetization and neutron diffraction data.

cond-mat.str-el↗

arXiv2003

Large positive magnetocaloric effect in a perovskite manganite

We investigate the magnetothermodynamic properties of the perovskite manganite Pr0.46Sr0.54MnO3. Our data imply that the paramagnetic to antiferromagnetic transition at TN = 210 K is first order with an abrupt decrease of volume (~ 0.14 % in zero magnetic field) and is accompanied by a sharp anomaly in specific heat. Upon application of a sufficiently large magnetic field, the antiferromagnetic phase transforms into a ferromagnetic phase with a sharp increase in sample volume. This field induced transition results in a large positive magnetocaloric effect just below TN (∆Sm = 10.4 Jkg-1K-1 at H = 5.5 T), which is associated with the increasing stability of the antiferromagnetic state with decreasing temperature.

cond-mat.mtrl-sci↗

arXiv2003

Staircase effect in metamagnetic transitions of charge and orbitally ordered manganites

This paper reports on peculiar metamagnetic transitions which take place in antiferromagnetic, charge and orbitally ordered (AFCOO) manganites. At very low temperatures, the virgin magnetization curves of some of these compounds exhibit several, sharp steps giving rise to a staircase-like shape. This staircase effect is shown to be sensitive to various experimental parameters. Several potential interpretations of the staircase effect are discussed in relation to this set of results. A martensitic-like scenario, involving a leading role of the structural distortions associated to the collapse of the orbital ordering, is found to be the most plausible interpretation.

cond-mat.str-el↗

arXiv2003

Relaxor ferromagnetic behavior below the antiferromagnetic transition in La0.5Ca0.5MnO3

A detailed reinvestigation of magnetization in La0.5Ca0.5MnO3 reveals that although a field of H = 7 T applied at T = 5 K after zero field cooling is insufficient to convert the low temperature charge ordered antiferromagnetic phase into ferromagnetic, annealing in a magnetic field as small as H = 1 T induces ferromagnetic clusters in the charge ordered matrix. The volume phase fraction of the ferromagnetic clusters increases nearly linearly with the annealing field until Han = 4 T and then changes dramatically from about 11 % at 5 T to 60 % at Han = 7 T. This is analogous to the field-induced micro to macro polar domains in relaxor ferroelectrics. It is suggested that charge ordered clusters coexist with ferromagnetic phase above the Neel temperature (TN) and these clusters which transform into ferromagnetic by external magnetic fields are supercooled below TN. Our results will be relevant to the observation of many orders of decrease in the resistivity found for H << HC, where HC is the critical field for metamagnetic transition.

cond-mat.mtrl-sci↗

arXiv2002

Models for Evolution of Dusty Galaxies and E/S0s Seen in Multiband Surveys

Phenomenological models for evolution of dusty galaxies and E/S0 galaxies, respectively, are developed to address two major questions concenring galaxy populations in deep infrared (IR) surveys: (1) Do normal late-type galaxies or starburst galaxies (including galaxies with obscured AGNs) dominate among sources in deep IR surveys? (2) How much do E/S0 galaxies contribute to the counts in deep mid-infrared (MIR: 3 -- 20$μm$) surveys? Among three new models for evolution of dusty galaxies, it is assumed in Model S1 that starburst galaxies are the dominant population, and in Model S2 that normal galaxies dominate. Model S3 is an intermediate model. Comparing the model predictions with a wide range of observational data collected from the literature, we find that none of these models can be ruled out, given the uncertainties of the data. We show that the most direct method to distinguish these models is to compare the predicted color distributions of IR galaxies with observations, which will soon be available from the SWIRE survey. The models for E/S0 galaxies follow a simple passive evolution approach. Among the three E/S0 models (E1, E2 and E3) investigated in this paper, Model E2 which is specified by a peak formation redshift $z_{peak}=2$, and an e-folding formation time scale $ω=2$ Gyr, fits the data best. This suggests a synchronization between the evolution of E/S0 galaxies and of starburst galaxies, in the sense that the peak of the formation function of E/S0s ($z_{peak}=2$) is close to the peak of the evolution functions of starburst galaxies ($z_{peak} = 1.4$). New predictions for counts and confusion limits in the SIRTF bands are presented.

astro-ph↗

arXiv2002

Ultra-sharp magnetization steps in perovskite manganites

We report a detailed study of step-like metamagnetic transitions in the magnetization and resistivity of polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset below a critical temperature, are extremely sharp (width < 0.2 mT), and occur at critical fields which are linearly dependent on the absolute value of the cooling field in which the sample is prepared. Similar transitions are also observed at low temperature in non-Co doped manganites, including single crystal samples. These data show that the steps are an intrinsic property, qualitatively different from either previously observed higher temperature metamagnetic transitions in the manganites or metamagnetic transitions observed in other materials.

cond-mat.mtrl-sci↗

arXiv2002

Ordering phenomena and transport properties of Bi1/2Sr1/2MnO3 single crystals

Measurements of the electrical and thermal conductivities, thermopower and paramagnetic susceptibility have been performed on single crystal samples of Bi1/2Sr1/2MnO3 and complemented with the X-ray powder diffraction data. A pronounced hysteretic behavior, observed below the cubic-to- orthorhombic transition at Tcrit = 535 K, is related to the onset of long-range charge order at TCO = 450 K and its further evolution down to about 330 K. The diffraction data suggest that the charge ordered state is formed by Zener pairs, represented by Mn4+ dimers linked by one extra eg electron, and is possibly stabilized by cooperative Bi,Sr displacements. An extremely low thermal conductivity is observed down to the lowest temperatures, without any recovery at the anti- ferromagnetic ordering temperature TN = 150 K. Such be- havior points to a presence of strong scatterers of phonons. Their possible origin can be linked to "optical-like" oscilla- tions which are associated with fluctuating charges within the Zener pairs.

cond-mat.str-el↗

arXiv2002

Microphase separation in Pr0.67Ca0.33MnO3 by small angle neutron scattering

We have evidenced by small angle neutron scattering at low temperature the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in Pr0.67Ca0.33MnO3. The results are compared to those obtained in Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF respectively. Quantitative analysis shows that the small angle scattering is not due to a mesoscopic mixing but to a nanoscopic electronic and magnetic ''red cabbage'' structure, in which the ferromagnetic phase exists in form of thin layers in the AF matrix (stripes or 2D ''sheets'').

cond-mat.str-el↗

arXiv2002

Current-driven magnetization decrease in single crystalline ferromagnetic manganese oxide

The electrical and magnetic response to a bias current has been investigated in a singlecrystalline ferromagnetic manganese oxide $\Pr_{0.8}$Ca$_{0.2}$MnO$_3$ . A significant decrease of the magnetization is observed at the same threshold current where a non-linearity of V-I characteristics appears. Such a behavior cannot be understood in the framework of the filamentary picture usually invoked for the non linearity of the other manganese oxides. Instead, an analogy with spintronic features might be useful and experimental signatures seem to be in agreement with excitations of spin waves by an electric current. This provides an example of a bulk system in which the spin polarized current induces a macroscopic change in the magnetization.

cond-mat.str-el↗

arXiv2002

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator: Pr0.8Ca0.2MnO3

The Pr(1-x}CaxMnO3 system exhibits a ferromagnetic insulating state for the composition range x<0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr0.8Ca0.2MnO3 single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr{3+} ions.

cond-mat↗

arXiv2002

Non-linear electrical response in a non-charge-ordered manganite: Pr0.8Ca0.2MnO3

Up to now, electric field induced non-linear conduction in the Pr(1-x)CaxMnO3 system has been ascribed to a current-induced destabilization of the charge ordered phase. However, for x<0.25, a ferromagnetic insulator state is observed and charge-ordering is absent whatever the temperature. A systematic investigation of the non-linear transport in the ferromagnetic insulator Pr0.8Ca0.2MnO3 shows rather similar results to those obtained in charge ordered systems. However, the experimental features observed in Pr0.8Ca0.2MnO3 are distinct in that the collapse of the CO energy gap can not be invoked as usually done in the other members of the PCMO system. We propose interpretations in which the effectiveness of the DE is restored upon application of electric field.

cond-mat↗

arXiv2001

The Interacting Dwarf Galaxy NGC 3077: The Interplay of Atomic and Molecular Gas with Violent Star Formation

We present a comprehensive multi-wavelength study of the nearby interacting dwarf galaxy NGC3077 (member of the M81 triplet). High resolution VLA HI observations show that most of the atomic gas (~90%) around NGC3077 is situated in a prominent tidal arm with a complex velocity structure. Little HI (~5 x 10^7 M_sun) is associated with NGC3077 itself. High resolution OVRO observations of the molecular component (CO) reveal the presence of 16 molecular complexes near the center of NGC3077 (total mass: ~1.6 x 10^6 M_sun). A virial mass analysis of the individual complexes yields a lower CO-to-H_2 conversion factor in NGC3077 than the Galactic value - a surprising result for a dwarf galaxy. The total (atomic and molecular) gas content in the centre of NGC3077 is displaced from the stellar component of NGC3077 -- this implies that not only the gas at large galactocentric radii is affected by the interaction within the triplet but also the center. We speculate that the starburst activity of NGC3077 was triggered by this redistribution of gas in the center. Some of these supershells are surrounded by neutral hydrogen. In a few cases, the rims of the ionized supershells are associated with dust absorption. The most prominent star forming region in NGC3077 as probed by Pa-alpha observations is hidden behind a dust cloud which is traced by the molecular complexes. Correcting for extinction we derive a star forming rate of 0.05 M_sun year^-1, i.e. given the reservoir in atomic and molecular gas in NGC3077, star formation may proceed at a similar rate for a few 10^8 years. The efficiency to form stars out of molecular gas in NGC3077 is similar to that in M82.

astro-ph↗

arXiv2001

Lattice coupled first order magnetoresistance transition in an A-type antiferromagnet: Pr$_{0.46}$Sr$_{0.54}$MnO$_3$

We investigated magnetic, magnetotransport and magnetostriction properties of the A-type antiferromagnet Pr$_{0.46}$Sr$_{0.54}$MnO$_3$ which undergoes a first order paramagnetic-antiferromagnetic transition below T$_N$ = 210 K while cooling and T$_N$ = 215 K while warming. The zero field ($μ_0$H = 0 T) resistivity shows a sudden jump at T$_N$ and a small bump around T$_{max}$ = 220 K (\TEXTsymbol{>} T$_N$). T$_N$ shifts down and T$_{max}$ shifts up with increasing $μ_0$H. Magnetoresistance as high as -45-57 % at 7 T is found in the temperature range 180 K-230 K. Isothermal measurements indicate that the field induced antiferromagnetic to ferromagnetic transition below T$_N$ is accompanied by a rapid decrease of the resistivity but increase of volume ($Δ$V/V = +0.25 % at 180 K and 13.7 T). This lattice coupled magnetoresistance transition is suggested due to the field induced structural transition from the low volume orthorhombic to the high volume tetragonal structure.

cond-mat.mtrl-sci↗

arXiv2001

Distinct origins of magnetic -field -induced resistivity irreversibility in two manganites with similar ground states : Pr$_{0.5}$Sr$_{0.41}$Ca$_{0.09}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$

Our investigation of the magnetotransport in two charge ordered manganites with similar magnetic ground states reveals that the origin of magnetoresistance can not be concluded from the isofield resistivity, $ρ$(T, constant H), measurements alone. Both Pr$_{0.5}$Sr$_{0.41}$Ca$_{0.09}$MnO$_{3}$ (PrSrCa) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (LaCa) show a ferromagnetic transition (T$_{C}$ = 260 K for PrSrCa, 230 K for LaCa) followed by an antiferromagnetic transition (T$_{N}$ = 170 K for PrSrCa, 140 K for LaCa). These compounds show qualitatively similar magnetotransport : Below the irreversibility temperature T$_{IR}$, field cooled (FC) resistivity is lower than zero field cooled (ZFC) and decreases continuously with T, whereas the ZFC $ρ$(T, H) resembles $\ $the behavior of $ρ$(T, H = 0 T). The value of $ρ$(ZFC)/$ρ$(FC) is $\approx $ 10$^{4}$ at 5 K and $μ_{0}$H = 7 T in both compounds. However, isothermal magnetic measurements suggest distinct origins of magnetoresistance : Field cooling enhances ferromagnetic phase fraction in LaCa whereas it drives PrSrCa into a metastable state with high magnetization. The distinct origins of magnetotransport is also reflected in other magnetic history dependent properties.

cond-mat.mtrl-sci↗