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R. Ramesh

Publications and source records attributed to R. Ramesh.

81 records · Page 5Linked to original sources
arXiv2004

Co-occurrence of Superparamagnetism and Anomalous Hall Effect in Highly Reduced Cobalt Doped Rutile TiO2 Films

We report a detailed magnetic and structural analysis of highly reduced Co doped rutile TiO2 films displaying an anomalous Hall effect (AHE). The temperature and field dependence of magnetization, and transmission electron microscopy clearly establish the presence of nano-sized superparamagnetic cobalt clusters of 8-10 nm size in the films at the interface. The co-occurrence of superparamagnetism and AHE raises questions regarding the use of the AHE as a test of the intrinsic nature of ferromagnetism in diluted magnetic semiconductors.

cond-mat.mtrl-sci↗
arXiv2003

Ferroelectric Nanotubes

We report the independent invention of ferroelectric nanotubes from groups in several countries. Devices have been made with three different materials: lead zirconate-titanate PbZr1-xTixO3 (PZT); barium titanate BaTiO3; and strontium bismuth tantalate SrBi2Ta2O9 (SBT). Several different deposition techniques have been used successfully, including misted CSD (chemical solution deposition) and pore wetting. Ferroelectric hysteresis and high optical nonlinearity have been demonstrated. The structures are analyzed via SEM, TEM, XRD, AFM (piezo-mode), and SHG. Applications to trenching in Si dynamic random access memories, ink-jet printers, and photonic devices are discussed. Ferroelectric filled pores as small as 20 nm in diameter have been studied.

cond-mat.mtrl-sci↗
arXiv2000

Quantitative imaging of dielectric permittivity and tunability with a near-field scanning microwave microscope

We describe the use of a near-field scanning microwave microscope to image the permittivity and tunability of bulk and thin film dielectric samples on a length scale of about 1 micron. The microscope is sensitive to the linear permittivity, as well as to nonlinear dielectric terms, which can be measured as a function of an applied electric field. We introduce a versatile finite element model for the system, which allows quantitative results to be obtained. We demonstrate use of the microscope at 7.2 GHz with a 370 nm thick barium strontium titanate thin film on a lanthanum aluminate substrate. This technique is nondestructive and has broadband (0.1-50 GHz) capability. The sensitivity of the microscope to changes in relative permittivity is 2 at permittivity = 500, while the nonlinear dielectric tunability sensitivity is 10^-3 cm/kV.

cond-mat.mtrl-sci↗
arXiv2000

Colossal electroresistance and electronic phase separation in mixed-valent manganites

The sensitivity of transport in CMR manganite to external fields is examined using a PZT-based field effect configuration with a La0.7Ca0.3MnO3 channel subjected to electric and magnetic fields, separately and in conjugation. We not only find a colossal electroresistance (CER) of ~76% at 4X10^5 V/cm, but also a remarkable complimentarity of this CER with the widely studied CMR. Specifically, the ERmax (MRmax) of the system is largely independent of the magnetic (electric) field biasing, and for equal conductivity values, the ER (MR) is lower (higher) for T Tp. The size and systematics of the effect strongly favor a percolative phase separation picture.

cond-mat.str-el↗
arXiv1999

Apertureless Near-Field Second Harmonic Microscopy with Bare Tapered Optical Fiber Tips

We present a near-field optical technique for second harmonic imaging using bare tapered optical fiber tip illuminated with femtosecond laser pulses. Enhancement of electric field at the tip of the fiber results in enhanced second harmonic generation (SHG) from the sample region near the tip. This SH emission is collected by the same tapered fiber. Spatial distribution of SHG from thin ferroelectric Pb(Zr_xTi_1-x)O_3 films and model metal-ferroelectric devices has been studied. Spatial resolution on the order of 80 nm has been achieved. This is the first time diffraction limit is surpassed in SH microscopic measurements. Electric field induced changes in SHG from individual grains and/or domains of ferroelectric thin films have been observed.

cond-mat.mtrl-sci↗
arXiv1999

Imaging of Microwave Permittivity, Tunability, and Damage Recovery in (Ba,Sr)TiO3 Thin Films

We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 micron. We demonstrate this technique with permittivity images and local hysteresis loops of a 370 nm thick barium strontium titanate thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity as small as 2 at 500, and changes in dielectric tunability as small as 0.03 V$^{-1}$.

cond-mat.mtrl-sci↗
arXiv1998

Optical Conductivity of Manganites: Crossover from Jahn-Teller Small Polaron to Coherent Transport in the Ferromagnetic State

We report on the optical properties of the hole-doped manganites Nd_{0.7}Sr _{0.3}MnO_{3}, La_{0.7}Ca_{0.3}MnO_{3}, and La_{0.7}Sr_{0.3}MnO_{3}. The low-energy optical conductivity in the paramagnetic-insulating state of these materials is characterized by a broad maximum near 1 eV. This feature shifts to lower energy and grows in optical oscillator strength as the temperature is lowered into the ferromagnetic state. It remains identifiable well below Tc and transforms eventually into a Drude-like response. This optical behavior and the activated transport in the paramagnetic state of these materials are consistent with a Jahn-Teller small polaron. The optical spectra and oscillator strength changes compare well with models that include both double exchange and the dynamic Jahn-Teller effect in the description of the electronic structure.

cond-mat.str-el↗
arXiv1996

Giant Magneto-Resistance in Nd$_{0.7}$Sr$_{0.3}$MnO$_3$ at Optical Frequencies

The optical properties of Nd$_{0.7}$Sr$_{0.3}$MnO$_3$ thin films have been studied from 5 meV to 25 meV and from 0.25 eV to 3 eV, at temperatures from 15 K to 300 K and magnetic fields up to 8.9 T. A large transfer of spectral weight from high energy to low energy occurs as the temperature is decreased below 180 K, where the dc resistivity peaks, or as the magnetic field is increased. The optical data are found to be consistent with models that include both the double exchange interaction and the dynamic Jahn-Teller effect on the Mn$^{3+}$ e$_g$ levels.

cond-mat↗