Thermal transport studies on charge density wave materials LaPt2Si2 and PrPt2Si2.

We measured the thermal properties of polycrystalline samples of LaPt2Si2 and PrPt2Si2 using thermopower (S) along with thermal conductivity (κ) in the temperature range 10 K-300 K. Significant anomalies related to charge density waves (CDW) around 112 K and 88 K respectively have been observed in [Formula: see text] in both systems. Analysis of thermopower by a two band model suggests that the observations are consistent with a reduction of electron charge density. A change in slope accompanied by a drop in the value of thermal conductivity has been observed around T CDW in case of LaPt2Si2. Analysis of thermal conductivity of this material suggests that the CDW mainly affects electronic contribution to thermal transport. Only a slight change of slope has been detected in temperature dependent thermal conductivity in the case of PrPt2Si2 around T CDW, while its resistivity shows a clear anomaly which shows that electronic part of thermal conductivity is mainly influenced by the CDW in this case also. It is interesting to note that the lattice contribution to thermal conductivity remains unaffected by the CDWs in both materials.

Superconducting and charge density wave transition in single crystalline LaPt2Si2.

We present results of our comprehensive studies on single crystalline LaPt2Si2. Pronounced anomaly in electrical resistivity and heat capacity confirms the bulk nature of superconductivity (SC) and charge density wave (CDW) transition in the single crystals. While the charge density wave transition temperature is lower, the superconducting transition temperature is higher in single crystal compared to the polycrystalline sample. This result confirms the competing nature of CDW and SC. Another important finding is the anomalous temperature dependence of upper critical field H C2(T). We also report the anisotropy in the transport and magnetic measurements of the single crystal.

Coexistence of superconductivity and a charge density wave in LaPt2(Si1-x Ge x )2 (0 ⩽ x ⩽ 0.5).

Interplay between a charge density wave (CDW) and superconductivity in LaPt2(Si1-x Ge x )2 has been studied by electrical transport and magnetic measurements. LaPt2Si2 crystallizes in CaBe2Ge2 type structure which shows a first order structural phase transition from tetragonal to orthorhombic accompanied by a CDW transition at 112 K and superconducting transition at around 1.22 K as confirmed by temperature dependence of resistivity and magnetic measurements. For 2[Formula: see text] doping of germanium, while the CDW temperature T CDW decreases, the superconducting transition temperature T C shows an increase. T CDW increases for 5[Formula: see text] doping of germanium and the superconducting transition decreases. These findings demonstrate the competing nature of a CDW and superconductivity.

Evidence for topological surface states in metallic single crystals of Bi2Te3.

Bi2Te3 is a member of a new class of materials known as topological insulators which are supposed to be insulating in the interior and conducting on the surface. However, experimental verification of the conductive qualities of the surface states has been hindered by parallel bulk conductions. We report low temperature magnetotransport measurements on single crystal samples of Bi2Te3. We observe metallic character in our samples and large and linear magnetoresistance from 1.5 K to 290 K with prominent Shubnikov-de Haas (SdH) oscillations whose traces persist up to 20 K. Even though our samples are metallic, we are able to obtain a Berry phase close to the value of π, which is expected for Dirac fermions of the topological surface states. This indicates that we have obtained evidence for the topological surface states in metallic single crystals of Bi2Te3. Other physical measurements obtained from the analysis of the SdH oscillations are also in close agreement with those reported for the topological surface states. The linear magnetoresistance observed in our sample, which is considered as a signature of the Dirac fermions of the surface states, lends further credence to the existence of topological surface states.

Memory and aging effects in NiO nanoparticles.

We report studies on magnetization dynamics in NiO nanoparticles of average size 5 nm. Temperature and time dependence of dc magnetization, wait time dependence of magnetic relaxation (aging) and memory phenomena in dc magnetization are studied with various temperature and field protocols. We observe that the system shows memory and aging in field-cooled and zero-field-cooled magnetization measurements. These experiments show that the magnetic behavior of NiO nanoparticles is similar to spin glasses. We argue that the spin glass behavior originates from the freezing of spins at the surface of the individual particles.

Slow dynamics in hard condensed matter: a case study of the phase separating system NdNiO(3).

We report the time dependent response of electrical resistivity in the non-magnetic perovskite oxide NdNiO(3) in its phase separated state and provide a physical explanation of the observations. We also model the system and make an accurate Monte Carlo simulation of the observed behavior. While cooling, a phase separation takes place in the system below its metal-insulator transition temperature and in this state the material exhibits various dynamical phenomena such as relaxation of resistivity, dependence of resistivity on cooling rate and rejuvenation of the material after ageing. These phenomena signal that the phase separated state of NdNiO(3) is not in thermodynamic equilibrium, and we conjecture that it consists of supercooled paramagnetic metallic and antiferromagnetic insulating phases. The supercooled phases are metastable and they switch over to the insulating equilibrium state stochastically, and this can account for the slow dynamics observed in our system. We also verify the predictive power of our model by simulating the result of a new experiment and confirming it by actual measurements.

Evidence of kinetically arrested supercooled phases in the perovskite oxide NdNiO(3).

We report the time and temperature dependent response of thermopower in the non-magnetic perovskite oxide NdNiO(3). We find that on cooling below the metal-insulator transition temperature the system evolves into a phase separated state which consists of supercooled metallic and insulating phases. This phase separated state exhibits out of equilibrium features, such as cooling rate dependence and time dependence. The existence of these dynamical features is attributed to the transformation of supercooled metallic phases to the insulating state. On cooling, a small fraction of the supercooled phases get kinetically arrested in a glassy state and these supercooled phases remain in that state down to low temperature. In the heating cycle the arrested states de-arrest above 150 K and this results in the reappearance of time dependent features.

Structural, magnetic, and electron transport studies on nanocrystalline layered manganite La1.2Ba1,8Mn2O7 system.

The structure property relationship for Ru doped La1.2Ba1.8Mn2-RuxO7 system has been studied systematically. The system crystallizes in the single-phase tetragonal structure with space group of 14/mmm. The unit cell volume is found to increase with Ru doping. The sheet type microstructure could be seen in this system, which is important for anisotropic nature of layered structure. The crystallite size is found to be 25 nm indicating nanocrystalline nature of the system. Ferromagnetic to paramagnetic (Tc) transition above the room temperature is observed in all except the highest doped Ru (x = 1.0) where the Tc is 254 K using a.c. susceptibility measurement. The large values of magnetoresistance for the x = 0.0 sample at 10 K is found to be 57% and 64% at applied fields of 5 and 10 T, respectively.