We report the experimental observation of variable range hopping conduction in focused-ion-beam (... more We report the experimental observation of variable range hopping conduction in focused-ion-beam (FIB) fabricated ultra-narrow nanowires of topological insulator (Bi 2 Se 3). The value of the exponent (d + 1) −1 in the hopping equation was extracted as ~1 2 for different widths of nanowires, which is the proof of the presence of Efros-Shklovskii hopping transport mechanism in a strongly disordered system. High localization lengths (0.5 nm, 20 nm) were calculated for the devices. A careful analysis of the temperature dependent fluctuations present in the magnetoresistance curves, using the standard Universal Conductance Fluctuation theory, indicates the presence of 2D topological surface states. Also, the surface state contribution to the conductance was found very close to one conductance quantum. We believe that our experimental findings shed light on the understanding of quantum transport in disordered topological insulator based nanostructures.
Measurements of magnetization and magnetoresistance have been carried out for crystals of (La \W ... more Measurements of magnetization and magnetoresistance have been carried out for crystals of (La \W Nd W) Sr MnO (y"0, 0.5, 0.75 and 1) in pulsed magnetic "elds up to 40 T. The enormous changes of resistivity, which are more than three orders of magnitude at ¹(100 K, enable us to clarify the functional formula of resistivity vs. magnetization. We will compare these experimental results with various models which have been proposed to explain the colossal magnetoresistance in manganites up to the present.
We report the effect of replacing Cu by Pd in RCu5 (R=Pr, Nd, Sm and Eu). The parent RCu5 compoun... more We report the effect of replacing Cu by Pd in RCu5 (R=Pr, Nd, Sm and Eu). The parent RCu5 compounds crystallize in the hexagonal CaCu5-type structure. The hexagonal symmetry is retained in PrCu4Pd and EuCu5-xPdx (x=1 and 2) but the crystal structure changes to cubic AuBe5-type in PrCu3Pd2, NdCu5-xPdx (x=1 and 2) and SmCu4Pd. Substitution with Pd leads to lattice expansion and modifies the magnetic behavior. While PrCu5 is known to be a van-Vleck paramagnet with a singlet ground state, PrCu4Pd and PrCu3Pd2 show ferromagnetic-like behavior at low temperatures. SmCu4Pd orders ferromagnetically near 28 K in contrast to the antiferromagnetic nature of the parent SmCu5. The divalent nature of the Eu ions in EuCu5 is retained in the ternary alloys, but the Curie temperature is reduced from 57 to 24.5 and 14.5 K in EuCu4Pd and EuCu3Pd2, respectively, inferred from the location of peak in the heat capacity of these two compounds. The magnetic hyperfine field at the Eu nucleus measured with 151Eu Mössbauer spectroscopy in the ternary Eu-alloys is comparable to that in EuCu5. The magnetic behavior of NdCu4Pd is similar to that reported in NdCu5. The zero-field-cooled, low-field magnetization of NdCu3Pd2 shows a region of diamagnetic behavior roughly between 21 and 4 K, but the field-cooled response is positive.
Using the techniques of magnetization, resistivity, and specific heat, we show evidence of strong... more Using the techniques of magnetization, resistivity, and specific heat, we show evidence of strong electron correlations in orthorhombic R2Ru3Ga9 (R=Ce and U). U2Ru3Ga9 orders antiferromagnetically with TN=16.5 K, while the Ce analog shows Pauli paramagnetic, mixed valence behavior with a linear coefficient of the electronic heat capacity ~250 mJ/mol K2. Their resistivity increases with decreasing temperature below 300 K, showing
Based on magneto-resistance studies performed over a wide range of magnetic fields as well as tem... more Based on magneto-resistance studies performed over a wide range of magnetic fields as well as temperatures, we establish that correction to conductivity in heavily nitrogen doped ultrananocrystalline diamond (UNCD) films is dominated by anisotropic weak localization (WL) in three dimensions associated with a propagative Fermi surface. Also, low temperature electrical conductivity can show weakly localized transport in 3D combined with the effect of electronelectron interactions in these materials, which is remarkably different from the conductivity in 2DWL or strong localization regime. The corresponding dephasing time of electronic wavefunctions in these systems described as ~ T -p with p < 1, follows a relatively weak temperature dependence compared to the generally expected nature for bulk dirty metals having p 1. The temperature dependence of Hall (electron) mobility together with an enhanced electron density has been used to interpret the unusual magneto-transport features and show delocalized electronic transport in these n-type UNCD films, which can be described as low-dimensional superlattice structures. _____________________________________________________________________________________________
Physica C-superconductivity and Its Applications, 2001
The degradation behavior of polycrystalline MgB2 superconductor, exposed to water and ambient atm... more The degradation behavior of polycrystalline MgB2 superconductor, exposed to water and ambient atmosphere, has been studied. We have found that prolonged exposure to water completely destroys superconductivity. Optical microscopic and X-ray diffraction analyses indicate that, on exposure to water, the material becomes amorphous like. X-ray photoelectron spectroscopic measurements reveal the strongly hygroscopic character of MgB2 and we have found that, even at room temperature, water and air change it into Mg(OH)2, MgCO3 and B2O3.
We report the experimental observation of variable range hopping conduction in focused-ion-beam (... more We report the experimental observation of variable range hopping conduction in focused-ion-beam (FIB) fabricated ultra-narrow nanowires of topological insulator (Bi 2 Se 3). The value of the exponent (d + 1) −1 in the hopping equation was extracted as ~1 2 for different widths of nanowires, which is the proof of the presence of Efros-Shklovskii hopping transport mechanism in a strongly disordered system. High localization lengths (0.5 nm, 20 nm) were calculated for the devices. A careful analysis of the temperature dependent fluctuations present in the magnetoresistance curves, using the standard Universal Conductance Fluctuation theory, indicates the presence of 2D topological surface states. Also, the surface state contribution to the conductance was found very close to one conductance quantum. We believe that our experimental findings shed light on the understanding of quantum transport in disordered topological insulator based nanostructures.
Measurements of magnetization and magnetoresistance have been carried out for crystals of (La \W ... more Measurements of magnetization and magnetoresistance have been carried out for crystals of (La \W Nd W) Sr MnO (y"0, 0.5, 0.75 and 1) in pulsed magnetic "elds up to 40 T. The enormous changes of resistivity, which are more than three orders of magnitude at ¹(100 K, enable us to clarify the functional formula of resistivity vs. magnetization. We will compare these experimental results with various models which have been proposed to explain the colossal magnetoresistance in manganites up to the present.
We report the effect of replacing Cu by Pd in RCu5 (R=Pr, Nd, Sm and Eu). The parent RCu5 compoun... more We report the effect of replacing Cu by Pd in RCu5 (R=Pr, Nd, Sm and Eu). The parent RCu5 compounds crystallize in the hexagonal CaCu5-type structure. The hexagonal symmetry is retained in PrCu4Pd and EuCu5-xPdx (x=1 and 2) but the crystal structure changes to cubic AuBe5-type in PrCu3Pd2, NdCu5-xPdx (x=1 and 2) and SmCu4Pd. Substitution with Pd leads to lattice expansion and modifies the magnetic behavior. While PrCu5 is known to be a van-Vleck paramagnet with a singlet ground state, PrCu4Pd and PrCu3Pd2 show ferromagnetic-like behavior at low temperatures. SmCu4Pd orders ferromagnetically near 28 K in contrast to the antiferromagnetic nature of the parent SmCu5. The divalent nature of the Eu ions in EuCu5 is retained in the ternary alloys, but the Curie temperature is reduced from 57 to 24.5 and 14.5 K in EuCu4Pd and EuCu3Pd2, respectively, inferred from the location of peak in the heat capacity of these two compounds. The magnetic hyperfine field at the Eu nucleus measured with 151Eu Mössbauer spectroscopy in the ternary Eu-alloys is comparable to that in EuCu5. The magnetic behavior of NdCu4Pd is similar to that reported in NdCu5. The zero-field-cooled, low-field magnetization of NdCu3Pd2 shows a region of diamagnetic behavior roughly between 21 and 4 K, but the field-cooled response is positive.
Using the techniques of magnetization, resistivity, and specific heat, we show evidence of strong... more Using the techniques of magnetization, resistivity, and specific heat, we show evidence of strong electron correlations in orthorhombic R2Ru3Ga9 (R=Ce and U). U2Ru3Ga9 orders antiferromagnetically with TN=16.5 K, while the Ce analog shows Pauli paramagnetic, mixed valence behavior with a linear coefficient of the electronic heat capacity ~250 mJ/mol K2. Their resistivity increases with decreasing temperature below 300 K, showing
Based on magneto-resistance studies performed over a wide range of magnetic fields as well as tem... more Based on magneto-resistance studies performed over a wide range of magnetic fields as well as temperatures, we establish that correction to conductivity in heavily nitrogen doped ultrananocrystalline diamond (UNCD) films is dominated by anisotropic weak localization (WL) in three dimensions associated with a propagative Fermi surface. Also, low temperature electrical conductivity can show weakly localized transport in 3D combined with the effect of electronelectron interactions in these materials, which is remarkably different from the conductivity in 2DWL or strong localization regime. The corresponding dephasing time of electronic wavefunctions in these systems described as ~ T -p with p < 1, follows a relatively weak temperature dependence compared to the generally expected nature for bulk dirty metals having p 1. The temperature dependence of Hall (electron) mobility together with an enhanced electron density has been used to interpret the unusual magneto-transport features and show delocalized electronic transport in these n-type UNCD films, which can be described as low-dimensional superlattice structures. _____________________________________________________________________________________________
Physica C-superconductivity and Its Applications, 2001
The degradation behavior of polycrystalline MgB2 superconductor, exposed to water and ambient atm... more The degradation behavior of polycrystalline MgB2 superconductor, exposed to water and ambient atmosphere, has been studied. We have found that prolonged exposure to water completely destroys superconductivity. Optical microscopic and X-ray diffraction analyses indicate that, on exposure to water, the material becomes amorphous like. X-ray photoelectron spectroscopic measurements reveal the strongly hygroscopic character of MgB2 and we have found that, even at room temperature, water and air change it into Mg(OH)2, MgCO3 and B2O3.
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Papers by Kunjal Shah