Observation of three-dimensional Dirac semimetal state in topological insulator Bi2Se3

The three dimensional (3D) topological insulators are predicted to exhibit a 3D Dirac semimetal state in critical regime of topological to trivial phase transition. Here we demonstrate the first experimental evidence of 3D Dirac semimetal state in topological insulator Bi2Se3 with bulk carrier concentration of ~ 10^19 cm^{-3}, using magneto-transport measurements. At low temperatures, the resistivity of our Bi2Se3 crystal exhibits clear Shubnikov-de Haas (SdH) oscillations above 6T. The analysis of these oscillations through Lifshitz-Onsanger and Lifshitz-Kosevich theory reveals a non-trivial pi Berry phase coming from 3D bands, which is a decisive signature of 3D Dirac semimetal state. The large value of Dingle temperature and natural selenium vacancies in our crystal suggest that the observed 3D Dirac semimetal state is an outcome of enhanced strain field and weaker effective spin-orbit coupling.

Experimental evidence for stochastic switching of supercooled phases

First-order phase transition in a highly correlated electron system can manifest as a dynamic phenomenon. The presence of multiple domains of the coexisting phases average out the dynamical effects making it nearly impossible to predict the exact nature of phase transition dynamics. Here we report the metal-insulator transition in samples of sub-micrometer size NdNiO3 where the effect of averaging is minimized by restricting the number of domains under study. We observe the presence of supercooled metallic phases with supercooling of 40 K or more. The transformation from supercooled metallic to insulating state is a stochastic process that happens at different temperature and time in different experimental runs. The experimental results are understood without incorporating material specific properties suggesting their universal nature. The size of the sample needed to observe individual switching of supercooled domains, the degree of supercooling, and the time-temperature window of switching is expected to depend on the parameters such as quenched disorder, strain, magnetic field etc.

A critical examination of magnetic states of La$_{0.5}$Ba$_{0.5}$CoO$_3$: non-Griffiths phase and interacting ferromagnetic-clusters

We report detailed dc magnetization, linear and non-linear ac susceptibility measurements on the hole doped disordered cobaltite La$_{0.5}$Ba$_{0.5}$CoO$_3$. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic-clusters. The percolating ferromagnetic-clusters possibly undergo a 3D Hisenberg like magnetic ordering at the Curie temperature of 202(3) K. In between 202 and 220 K, the linear and non-linear ac susceptibility measurements show the presence of magnetic correlations even when the spontaneous magnetization is zero which indicates the presence of preformed short range magnetic-clusters. The characteristics of these short range magnetic-clusters that exist above Curie temperature are quite distinct than that of Griffiths phase e.g the inverse dc susceptibility exhibits an field independent upward deviation, and the second harmonic of ac susceptibility is non-negative. Below Curie temperature the system exhibit spin-glass like features such as irreversibility in the field cooled and zero field cooled magnetization and frequency dependence in the peak of ac susceptibility. The presence of a spin or cluster -glass like state is ruled out by the absence of field divergence in third harmonic of ac susceptibility and zero field cooled memory. This indicates that the observed spin-glass like features are possibility due to progressive thermal blocking of ferromagnetic-clusters which is further confirmed by the Wohlfarths model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with $tau_0approx 10^{-9}$ s. This together with the existence of an AT line in H-T space indicates the existence of significant inter-cluster interaction among these ferromagnetic-clusters.

Tuning the phase transition dynamics by variation of cooling field and metastable phase fraction in Al doped Pr$_{0.5}$Ca$_{0.5}$MnO$_3$

We report the effect of field, temperature and thermal history on the time dependence in resistivity and magnetization in the phase separated state of Al doped Pr$_{0.5}$Ca$_{0.5}$MnO$_3$. The rate of time dependence in resistivity is much higher than that of magnetization and it exhibits a different cooling field dependence due to percolation effects. Our analysis show that the time dependence in physical properties depends on the phase transition dynamics which can be effectively tuned by variation of temperature, cooling field and metastable phase fraction. The phase transition dynamics can be broadly divided into the arrested and un-arrested regimes, and in the arrested regime, this dynamics is mainly determined by time taken in the growth of critical nuclei. An increase in cooling field and/or temperature shifts this dynamics from arrested to un-arrested regime, and in this regime, this dynamics is determined by thermodynamically allowed rate of formation of critical nuclei which in turn depends on the cooling field and available metastable phase fraction. At a given temperature, a decrease in metastable phase fraction shifts the crossover from arrested to un-arrested regimes towards lower cooling field. It is rather significant that inspite of the metastable phase fraction calculated from resistivity being somewhat off from that of magnetization, their cooling field dependence exhibits a striking similarity which indicate that the dynamics in arrested and un-arrested regimes are so different that it comes out vividly provided that the measurements are done around percolation threshold.

Spin canted magnetism, decoupling of charge and spin ordering in NdNiO$_3$

We report detailed magnetization measurements on the perovskite oxide NdNiO$_3$. This system has a first order metal-insulator (M-I) transition at about 200 K which is associated with charge ordering. There is also a concurrent paramagnetic to antiferromagnetic spin ordering transition in the system. We show that the antiferromagnetic state of the nickel sublattice is spin canted. We also show that the concurrency of the charge ordering and spin ordering transitions is seen only while warming up the system from low temperature. The transitions are not concurrent while cooling the system through the M-I transition temperature. This is explained based on the fact that the charge ordering transition is first order while the spin ordering transition is continuous. In the magnetically ordered state the system exhibits ZFC-FC irreversibilities, as well as history-dependent magnetization and aging. Our analysis rules out the possibility of spin-glass or superparamagnetism and suggests that the irreversibilities originate from magnetocrystalline anisotropy and domain wall pinning.

Heterogeneous nucleation and metal-insulator transition in epitaxial films of NdNiO$_3$

We have investigated the temperature driven first order metal-insulator (M-I) transition in thin films of NdNiO$_3$ and have compared it with the bulk behavior. The M-I transition of thin films is sensitive to epitaxial strain and a partial relaxation of epitaxial strain creates an inhomogeneous strain field in the films which broadens the M-I transition. Both the thin film and the bulk samples exhibit non equilibrium features in the transition regime which are attributed to the presence of high temperature metallic phases in their supercooled state. The degree of supercooling in the thin films is found to be much smaller than in the bulk which suggests that the metal insulator transition in the thin film occurs through heterogeneous nucleation.