Timir Datta

Electrical conductivity of p‐type CuInSe2 thin films

The dark ohmic conductivity σ(T) of several p‐type CuInSe2 thin films of different compositions was measured in vacuum in the temperature range 100–350 K. The room‐temperature resistivity of Cu‐rich, highly conducting films and the Cu‐poor, highly resistive films varied by seven orders of magnitude. For the most conducting films (Cu:In>1, ρ∼0.1 Ω cm), σ(T) was pseudometallic, i.e., dσ/dT≲0. The Cu‐poor films exhibited a thermally activated conductivity, i.e., σ(T)=σ0 exp(−E/kT), over a considerable temperature range. The data of post‐deposition anneal in air and subsequently in hydrogen are in excellent agreement with Meyer–Neldel type of conduction, i.e., σ(T)=σ00 exp(−E/kT0)exp(E/kT). We attribute this behavior to the compensation of the films by oxygen.

BaCo2Si2O7 : a new one-dimensional antiferromagnet based on chains of oxide bridged CoO4 tetrahedra

Abstract The new compound BaCo 2 Si 2 O 7 was obtained by heating the mixture BaCO 3 , CoCO 3 and SiO 2 to 1300°C for 48 h. The material was characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. Each cobalt ion is surrounded by four oxygen atoms arranged in a distorted tetrahedral shape. These tetrahedral are linked into chains through the sharing of one of the oxygen atoms. The CoO 4 chains are cross-linked by bridging Si 2 O 7 groups. The barium ions occupy channels that run parallel to the crystallographic c -axis. The effective magnetic moment, μ eff = 4.70 B.M., at 25°C is consistent with Co 2+ in a tetrahedral environment having three unpaired electrons. Anti-ferromagnetic coupling of the electrons occurs between the nearest neighbour cobalt ions, as indicated by the sign of the Weiss constant, θ p = −46 K, and by the presence of a broad maximum in the susceptibility curve. At 26 K it undergoes a three-dimensional magnetic transition into a field-induced weak ferromagnet that is characteristic of a one-dimensional metamagnet.

Crystal structure analysis of Y2Cu2O5

Single crystals of Y2Cu2O5 were obtained in the flux growth process by controlled heating of a mixture of Y2O3, BaO, and CuO in a molar ratio of 1∶8∶20. These crystals were analyzed by a single-crystal X-ray diffraction analysis. The crystal contains polymeric chains of Cu2O5 interspersed by yttrium ions surrounded by octahedral arrangements of oxygen atoms. Crystal data: space group=Pna21,a=10.799(2) Å,b=3.4990(5) Å,c=12.459(2) Å,Z=4, 380 reflections,R=0.026,Rw=0.030.

Pressure study of Tc and Josephson absorption in YBa2Cu3O7−λ and Tl-Ca-Ba-Cu-O superconductors

Abstract The effect of pressure on high- T c superconductors was studied by Josephson absorption as a new and very accurate method for determination of the critical temperature. The uncertainty in T c was less than 0.06 K. We propose our method as a new standard of T c measurements. The pressure derivative d T c /d p equals 1.06 K GPa -1 for YBa 2 Cu 3 O 7−λ and 2.20 K GPa -1 for Tl-Ca-Ba-Cu-O ceramics. Absence of thermal hysteresis was observed. The change of microwave absorption under hydrostatic pressure was also investigated.

Structural commonalities of high-temperature superconductors

It is shown that all of the presently known high-temperature superconductors belong to one structural type with a basic cell of approximate 4/mmm,D4h symmetry in tetragonal compounds andmmm,D2h symmetry in orthorhombic compounds. In the Ba(Pb1−xBix)O3 and YBa2Cu3O7−δ compounds the basic cell is the unit cell and in the (La1−xSrx)2CuO4, BiSrCaCuO and TIBaCaCuO compounds it is half the unit cell. The features of more than one copper oxide layer per basic cell, oxygen-free cation layers (e.g., Ca, Y) between CuO2 layers, and slightly puckered CuO2 planes seem to favor higher transition temperatures. The feasibility of several proposed or potentially superconducting structures are analyzed by this scheme.

Superconductivity in nanostructured lead

Abstract Three-dimensional nanoscale structures of lead were fabricated by electrodeposition of pure lead into artificial porous opal. The size of the metallic regions was comparable to the superconducting coherence length of bulk lead. T c as high as 7.36 K was observed, also d T c /d H was 2.7 times smaller than in bulk lead. Many of the characteristics of these differ from bulk lead, a type I superconductor. Irreversibility line and magnetic relaxation rates ( S ) were also studied. S ( T ) displayed two maxima, with a peak value about 10 times smaller than that of typical high- T c superconductors.

Transport critical current and magnetization measurements of melt‐processed YBa2Cu3O7−x

We report magnetic field dependence of the transport critical current and dc magnetic susceptibility measurements on YBa2Cu3O7−x superconductors formed by melt‐solid reactions at 950 °C between Ba‐Cu‐O (or Tb‐Ba‐Cu‐O) and solid nonstoichiometric Y‐Ba‐Cu‐oxide. Four‐probe dc critical current measurements at 77, 64, and 4.2 K show strong depression of the critical current density with increasing magnetic field in agreement with a model of weakly linked superconducting regions. Diamagnetic shielding and Meissner flux expulsion measurements in the temperature range 10–300 K show about one third volume fraction of perfect superconductivity. Both shielding and flux expulsion were observed to be approximately temperature independent below 60 K indicating strong coupling between the grains throughout the entire volume below this temperature.

Geometric dependence of transport and universal behavior in three dimensional carbon nanostructures

Carbon nanostructures with the spherical voids exhibit interesting temperature and magnetic field dependent transport properties. By increasing the void size, the structures are tuned from metallic to insulating; in addition, the magnetoresistance (MR) is enhanced. Our investigation in the magnetic fields (B) up to 18 T at temperatures (T) from 250 mK to 20 K shows that at high temperatures (T > 2 K), the MR crosses over from quadratic to a non-saturating linear dependence with increasing magnetic field. Furthermore, all MR data in this temperature regime collapse onto a single curve as a universal function of B/T, following Kohlers rule. Remarkably, the MR also exhibits orientation insensitivity, i.e., it displays a response independent of the direction on the magnetic field.The optical absorption spectrums of nanomotors made from double-wall carbon nanotubes have been calculated with the time-dependent density functional based tight binding method. When the outer short tube of the nanomotor moves along or rotates around the inner long tube, the peaks in the spectrum will gradually evolve and may shift periodically, the amplitude of which can be as large as hundreds of meV. We show that the features and behaviors of the optical absorption spectrum could be used to monitor the mechanical motions of the double-wall carbon nanotube based nanomotor.

Linear magnetoresistance in three-dimensional carbon nanostructure with periodic spherical voids

We report electrical transport properties of carbon nanostructures with close-packed spherical voids. Under zero magnetic field, a non-metallic behavior is observed. With increasing magnetic field, magnetoresistance (MR) crosses over from quadratic to linear dependence. Longitudinal response, typically negligible in most materials, exhibits the same value and field-temperature dependence as transverse MR. At intermediate angles (0°–90°) MR is also found to be independent of the direction of magnetic field. It is reasoned that orientation-insensitive, linear MR is due to distorted current flow in the 3-dimensional porous structures of this system.

Magnetic penetration depth in high-Tc superconducting Tl2Ca1Ba2Cu2O8−δ single crystals

The temperature dependence of Cu-O plane magnetic penetration depth,λab(T), for T1-2122 single crystals has been determined by measuring the low-field d.c. magnetization using a SQUID susceptometer. Almost full (97%) diamagnetism at low temperature was reached by carefully minimizing the background field, which is an essential condition for this method.λab(0) was obtained uniquely by a self-consistent condition to be 182 nm. Results of our measurements show a deviation from the single-band BCSs-wave prediction. The exponential behavior in very limited temperature regions may be reconciled with the BCS-like prediction. However, ourλ(T) data follow the theoretical prediction very accurately for ap-wave polar state pairing.