Investigation of a magnetohyperthermia system efficacy

This study reports on the successful use of magnetic albumin nanosphere (MAN) with in vivo magnetohyperthermia (MHT) in a mouse Ehrlich tumor. Maghemite nanoparticles (8.9 nm average diameter) were encapsulated within MAN (73.0 nm average diameter). Ehrlich tumor obtained after implantation of tumor cells in the subcutaneous tissue of mice was used as a model throughout this study. MHT was performed with MAN (40 mu L) containing 1.2 x 10(15) particle/mL and 40 Oe amplitude ac magnetic field oscillating at 1 MHz. Animals not treated, treated with MAN, or exposed to the ac field were used as controls. Histopathological analysis was carried out after 2, 5, or 11 days of tumor implantation. We found that the MHT most efficient condition was obtained while applying the ac field protocol twice a day during three consecutive days. Further, in this ac field-treated group no proliferation cells were detected. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3559498]

Low-amplitude variations detected by CoRoT in the B8IIIe star HD 175869

Context. The origin of the short-term variability in Be stars remains a matter of controversy. Pulsations and rotational modulation are the components of the favored hypothesis. Aims. We present our analysis of CoRoT data of the B8IIIe star HD 175869 observed during the first short run in the center direction (SRC1). Methods. We review both the instrumental effects visible in the CoRoT light curve and the analysis methods used by the CoRoT Be team. We applied these methods to the CoRoT light curve of the star HD 175869. A search for line-profile variations in the spectroscopic data was also performed. We also searched for a magnetic field, by applying the LSD technique to spectropolarimetric data. Results. The light curve exhibits low-amplitude variations of the order of 300 mu mag with a double wave shape. A frequency within the range determined for the rotational frequency and 6 of its harmonics are detected. The main frequency and its first harmonic exhibit amplitude variations of a few days. Other significant frequencies of low-amplitude from 25 to a few mu mag are also found. The analysis of line profiles from ground-based spectroscopic data does not detect any variation. In addition, no Zeeman signature was found. Conclusions. Inhomogeneities caused by stellar activity in or just above the photosphere are proposed to produce the photometric variability detected by CoRoT in the Be star HD 175869. The hypothesis that non-radial pulsations are the origin of these variations cannot be excluded.

The pulsations of the B5IVe star HD 181231 observed with CoRoT and ground-based spectroscopy

Context. HD 181231 is a B5IVe star, which has been observed with the CoRoT satellite during similar to 5 consecutive months and simultaneously from the ground in spectroscopy and spectropolarimetry. Aims. By analysing these data, we aim to detect and characterize as many pulsation frequencies as possible, to search for the presence of beating effects possibly at the origin of the Be phenomenon. Our results will also provide a basis for seismic modelling. Methods. The fundamental parameters of the star are determined from spectral fitting and from the study of the circumstellar emission. The CoRoT photometric data and ground-based spectroscopy are analysed using several Fourier techniques: CLEAN-NG, PASPER, and TISAFT, as well as a time-frequency technique. A search for a magnetic field is performed by applying the LSD technique to the spectropolarimetric data. Results. We find that HD 181231 is a B5IVe star seen with an inclination of similar to 45 degrees. No magnetic field is detected in its photosphere. We detect at least 10 independent significant frequencies of variations among the 54 detected frequencies, interpreted in terms of non-radial pulsation modes and rotation. Two longer-term variations are also detected: one at similar to 14 days resulting from a beating effect between the two main frequencies of short-term variations, the other at similar to 116 days due either to a beating of frequencies or to a zonal pulsation mode. Conclusions. Our analysis of the CoRoT light curve and ground-based spectroscopic data of HD 181231 has led to the determination of the fundamental and pulsational parameters of the star, including beating effects. This will allow a precise seismic modelling of this star.

Proper motions of thermally emitting isolated neutron stars measured with Chandra

The remarkable astrometric capabilities of the Chandra Observatory offer the possibility to measure proper motions of X-ray sources with an unprecedented accuracy in this wavelength range. We recently completed a proper motion survey of three of the seven thermally emitting radio-quiet isolated neutron stars (INSs) discovered in the ROSAT all-sky survey. These INSs (RXJ0420.0-5022, RXJ0806.4-4123 and RXJ1308.6+2127) either lack an optical counterpart or have one so faint that ground based or space born optical observations push the current possibilities of the instrumentation to the limit. Pairs of ACIS observations were acquired 3 to 5 years apart to measure the displacement of the sources on the X-ray sky using as a reference the background of extragalactic or remote Galactic X-ray sources. We derive 2 sigma upper limits of 123 mas yr(-1) and 86 mas yr(-1) on the proper motion of RXJ0420.0-5022 and RXJ0806.4-4123, respectively. RXJ1308.6+2127 exhibits a very significant displacement (similar to 9 sigma) yielding mu = 220 +/- 25 mas yr(-1), the second fastest measured among all ROSAT-discovered INSs. The source is probably moving away rapidly from the Galactic plane at a speed which precludes any significant accretion of matter from the interstellar medium. Its transverse velocity of similar to 740 (d/700 pc) km s(-1) might be the largest of all ROSAT INSs and its corresponding spatial velocity lies among the fastest recorded for neutron stars. RXJ1308.6+2127 is thus a middle-aged (age similar to 1 My) high velocity cooling neutron star. We investigate its possible origin in nearby OB associations or from a field OB star. In most cases, the flight time from birth place appears significantly shorter than the characteristic age derived from spin down rate. Overall, the distribution in transverse velocity of the ROSAT INSs is not statistically different from that of normal radio pulsars.

Transport properties in nontwist area-preserving maps

Nontwist systems, common in the dynamical descriptions of fluids and plasmas, possess a shearless curve with a concomitant transport barrier that eliminates or reduces chaotic transport, even after its breakdown. In order to investigate the transport properties of nontwist systems, we analyze the barrier escape time and barrier transmissivity for the standard nontwist map, a paradigm of such systems. We interpret the sensitive dependence of these quantities upon map parameters by investigating chaotic orbit stickiness and the associated role played by the dominant crossing of stable and unstable manifolds. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3247349]

Electron dephasing scattering rate in two-dimensional GaAs/InGaAs heterostructures with embedded InAs quantum dots

We report a comprehensive study of weak-localization and electron-electron interaction effects in a GaAs/InGaAs two-dimensional electron system with nearby InAs quantum dots, using measurements of the electrical conductivity with and without magnetic field. Although both the effects introduce temperature dependent corrections to the zero magnetic field conductivity at low temperatures, the magnetic field dependence of conductivity is dominated by the weak-localization correction. We observed that the electron dephasing scattering rate tau(-1)(phi), obtained from the magnetoconductivity data, is enhanced by introducing quantum dots in the structure, as expected, and obeys a linear dependence on the temperature and elastic mean free path, which is against the Fermi-liquid model. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.2996034]

Magnetoelectric effects in an organometallic quantum magnet

Metal-organic materials constitute a new field in which to search for ferroelectricity and coupling between electricity and magnetism. We observe a magnetic field-induced change in the electric polarization, Delta P(H), that reaches 50 mu C/m(2) in single crystals of NiCl(2)-4SC(NH(2))(2) (DTN). DTN forms a tetragonal structure that breaks inversion symmetry with the electrically polar thiourea molecules [SC(NH(2))] all tilted in the same direction along the c axis. The field H induces canted antiferromagnetism of the Ni S = 1 spins between 2 and 12 T and our measurements show that the electric polarization increases monotonically in this range, saturating above 12 T. By modeling the microscopic origin of this magnetoelectric effect, we find that the leading contribution to Delta P comes from the change in the crystal electric field, with a smaller contribution from magnetic exchange striction. The finite value of Delta P induced by magnetostriction results from the polar nature of the thiourea molecules bonded to the Ni atoms, and it is amplified by the softness of these organic molecules.

Metal-insulator transition in Nd(1-x) Eu(x) NiO(3) probed by specific heat and anelastic measurements

Oxides RNiO(3) (R - rare-earth, R not equal La) exhibit a metal-insulator (MI) transition at a temperature T(MI) and an antiferromagnetic (AF) transition at T(N). Specific heat (C(P)) and anelastic spectroscopy measurements were performed in samples of Nd(1-x)Eu(x)NiO(3), 0 <= x <= 0.35. For x - 0, a peak in C(P) is observed upon cooling and warming at essentially the same temperature T(MI) - T(N) similar to 195 K, although the cooling peak is much smaller. For x >= 0.25, differences between the cooling and warming curves are negligible, and two well defined peaks are clearly observed: one at lower temperatures that define T(N), and the other one at T(MI). An external magnetic field of 9 T had no significant effect on these results. The elastic compliance (s) and the reciprocal of the mechanical quality factor (Q(-1)) of NdNiO(3), measured upon warming, showed a very sharp peak at essentially the same temperature obtained from C(P), and no peak is observed upon cooling. The elastic modulus hardens below T(MI) much more sharply upon warming, while the cooling and warming curves are reproducible above T(MI). Conversely, for the sample with x - 0.35, s and Q(-1) curves are very similar upon warming and cooling. The results presented here give credence to the proposition that the MI phase transition changes from first to second order with increasing Eu doping. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3549615]

Thermal Transport and Strong Mass Renormalization in NiCl(2)-4SC(NH(2))(2)

Several quantum paramagnets exhibit magnetic-field-induced quantum phase transitions to an anti-ferromagnetic state that exists for H(c1) <= H <= H(c2). For some of these compounds, there is a significant asymmetry between the low-and high-field transitions. We present specific heat and thermal conductivity measurements in NiCl(2)-4SC(NH(2))(2), together with calculations which show that the asymmetry is caused by a strong mass renormalization due to quantum fluctuations for H <= H(c1) that are absent for H >= H(c2). We argue that the enigmatic lack of asymmetry in thermal conductivity is due to a concomitant renormalization of the impurity scattering.

Quantum Hall Effect near the Charge Neutrality Point in a Two-Dimensional Electron-Hole System

We study the transport properties of HgTe-based quantum wells containing simultaneously electrons and holes in a magnetic field B. At the charge neutrality point (CNP) with nearly equal electron and hole densities, the resistance is found to increase very strongly with B while the Hall resistivity turns to zero. This behavior results in a wide plateau in the Hall conductivity sigma(xy) approximate to 0 and in a minimum of diagonal conductivity sigma(xx) at nu = nu(p) - nu(n) = 0, where nu(n) and nu(p) are the electron and hole Landau level filling factors. We suggest that the transport at the CNP point is determined by electron-hole ""snake states'' propagating along the nu = 0 lines. Our observations are qualitatively similar to the quantum Hall effect in graphene as well as to the transport in a random magnetic field with a zero mean value.

Transport in disordered two-dimensional topological insulators

The transport properties of the ""inverted"" semiconductor HgTe-based quantum well, recently shown to be a two-dimensional topological insulator, are studied experimentally in the diffusive regime. Nonlocal transport measurements are performed in the absence of magnetic field, and a large signal due to the edge states is observed. This shows that the edge states can propagate over a long distance, similar to 1 mm, and therefore, there is no difference between local and nonlocal electrical measurements in a topological insulator. In the presence of an in-plane magnetic field a strong decrease of the local resistance and complete suppression of the nonlocal resistance is observed. We attribute this behavior to an in-plane magnetic-field-induced transition from the topological insulator state to a conventional bulk metal state.

Magnetoresistance oscillations in multilayer systems: Triple quantum wells

Magnetoresistance of two-dimensional electron systems with several occupied subbands oscillates owing to periodic modulation of the probability of intersubband transitions by the quantizing magnetic field. In addition to previous investigations of these magnetointersubband (MIS) oscillations in two-subband systems, we report on both experimental and theoretical studies of such a phenomenon in three-subband systems realized in triple quantum wells. We show that the presence of more than two subbands leads to a qualitatively different MIS oscillation picture, described as a superposition of several oscillating contributions. Under a continuous microwave irradiation, the magnetoresistance of triple-well systems exhibits an interference of MIS oscillations and microwave-induced resistance oscillations. The theory explaining these phenomena is presented in the general form, valid for an arbitrary number of subbands. A comparison of theory and experiment allows us to extract temperature dependence of quantum lifetime of electrons and to confirm the applicability of the inelastic mechanism of microwave photoresistance for the description of magnetotransport in multilayer systems.

Logarithmic contribution to the electrical resistivity in (Ru(1-x)Ir(x))Sr(2)GdCu(2)O(8) compounds

A systematic study of magnetoresistance and dc magnetization was conducted in polycrystalline (Ru(1-x)Ir(x))Sr(2)GdCu(2)O(8) [(Ru,Ir)-1212] compounds, for 0 <= x <= 0.15. We found that a deviation from linearity in the normal-state electrical resistivity (rho) curves for temperatures below the magnetic transition temperature T(M) < 130 K can be properly described by a logarithmic term. The prefactor C(x, H) of this anomalous ln T contribution to rho(T) increases linearly with the Ir concentration, and diminishes rapidly with increasing applied magnetic field up to H approximate to 4 T, merging with the C(0,H) curve at higher magnetic fields. Correlation with magnetic susceptibility measurements supports a scenario of local perturbations in the orientation of Ru moments induced in the neighborhood of the Ir ions, therefore acting as scattering centers. The linear dependence of the prefactor C(x,H=0) and the superconducting transition temperature T(SC) on x points to a common source for the resistivity anomaly and the reduction in T(SC), suggesting that the CuO(2) and RuO(2) layers are not decoupled.

Classical and quantum magnetoresistance in a two-subband electron system

We observe a large positive magnetoresistance in a bilayer electron system (double quantum well) as the latter is driven by the external gate from double to single layer configuration. Both classical and quantum contributions to magnetotransport are found to be important for explanation of this effect. We demonstrate that these contributions can be separated experimentally by studying the magnetic-field dependence of the resistance at different gate voltages. The experimental results are analyzed and described by using the theory of low-field magnetotransport in the systems with two occupied subbands.

Linear and nonlinear transport in a small charge-tunable open quantum ring

We experimentally study the Aharonov-Bohm-conductance oscillations under external gate voltage in a semiconductor quantum ring with a radius of 80 nm. We find that, in the linear regime, the resistance-oscillation plot in the voltage-magnetic-field plane corresponds to the quantum ring energy spectra. The chessboard pattern assembled by resistance diamonds, while loading the ring, is attributed to a short electron lifetime in the open configuration, which agrees with calculations within the single-particle model. Remarkably, the application of a small dc current allows observing strong deviations in the oscillation plot from this pattern accompanied by a magnetic-field symmetry break. We relate such behavior to the higher-order-conductance coefficients determined by electron-electron interactions in the nonlinear regime.