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Register a new userUnusual magnetoresistance oscillations in preferentially oriented p-type polycrystalline ZrTe5
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Recently H. Wang et al. (arXiv 1704.00995) have reported quantum oscillation in magnetoresistance with the periodicity in logarithmic of magnetic field (B) for the p-type ZrTe5. They have ascribed this type of behavior to the discrete scale invariance, resulting from Effimov bound states. We have prepared high quality stoichiometric (p-type) ZrTe5 polycrystals and observed magnetoresistance (MR) oscillations, which are periodic in B. These oscillations are in contrast to usual SdH oscillations or log B dependent oscillations as observed for tellurium deficient and stoichiometric ZrTe5 respectively. The MR follows the three dimensional Weyl semimetal like behavior, and Kohlers rule is obeyed at low temperatures. We obtained small cyclotron effective mass (m* = 0.05 m_e), very high mobility of 2.2 X 10^4 cm^2/V.s and the signature of topological protected surface states in the compound. The magnetic data shows zero cusp paramagnetic susceptibility which supports the existence of topological surface states in ZrTe5.
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Thermoelectric properties of polycrystalline p-type ZrTe5 are reported in temperature (T) range 2 - 340 K. Thermoelectric power (S) is positive and reaches up to 458 uV/K at 340 K on increasing T. The value of Fermi energy 16 meV, suggests low carrier density of ~ 9.5 X 10^18 cm-3. A sharp anomaly in S data is observed at 38 K, which seems intrinsic to p-type ZrTe5. The thermal conductivity value is low (2 W/m-K at T = 300 K) with major contribution from lattice part. Electrical resistivity data shows metal to semiconductor transition at T ~ 150 K and non-Arrhenius behavior in the semiconducting region. The figure of merit zT (0.026 at T = 300 K) is ~ 63% higher than HfTe5 (0.016), and better than the conventional SnTe, p-type PbTe and bipolar pristine ZrTe5 compounds.
Novel phases of matter with unique properties that emerge from quantum and topological protection present an important thrust of modern research. Of particular interest is to engineer these phases on demand using ultrafast external stimuli, such as photoexcitation, which offers prospects of their integration into future devices compatible with optical communication and information technology. Here, we use MeV Ultrafast Electron Diffraction (UED) to show how a transient three-dimensional (3D) Dirac semimetal state can be induced by a femtosecond laser pulse in a topological insulator ZrTe$_5$. We observe marked changes in Bragg diffraction, which are characteristic of bond distortions in the photoinduced state. Using the atomic positions refined from the UED, we perform density functional theory (DFT) analysis of the electronic band structure. Our results reveal that the equilibrium state of ZrTe$_5$ is a topological insulator with a small band gap of $sim$25 meV, consistent with angle-resolved photoemission (ARPES) experiments. However, the gap is closed in the presence of strong spin-orbit coupling (SOC) in the photoinduced transient state, where massless Dirac fermions emerge in the chiral band structure. The time scale of the relaxation dynamics to the transient Dirac semimetal state is remarkably long, $tau sim$160 ps, which is two orders of magnitude longer than the conventional phonon-driven structural relaxation. The long relaxation is consistent with the vanishing density of states in Dirac spectrum and slow spin-repolarization of the SOC-controlled band structure accompanying the emergence of Dirac fermions.
We report measurements of the c-axis angular-dependent magnetoresistance (AMR) for a Na$_{0.48}$CoO$_{2}$ single crystal, with a magnetic field of 10 T rotating within Co-O planes. Below the metal-insulator transition temperature induced by the charge ordering, the oscillation of the AMR is dominated by a two-fold rotational symmetry. The amplitudes of the oscillation corresponding to the four- and six-fold rotational symmetries are distinctive in low temperatures, but they merge into the background simultaneously at about 25 K. The six-fold oscillation originates naturally from the lattice symmetry. The observation of the four-fold rotational symmetry is consistent with the picture proposed by Choy, et al., that the Co lattice in the charge ordered state will split into two orthorhombic sublattice with one occupied by Co$^{3+}$ ions and the other by Co$^{4+}$ ions. We have also measured the c-axis AMR for Na$_{0.35}$CoO$_{2}$ and Na$_{0.85}$CoO$_{2}$ single crystals, and found no evidence for the existence of two- and four-fold symmetries.
We present low-temperature and high-field magnetotransport data on SrTiO3-LaAlO3 interfaces. The resistance shows hysteresis in magnetic field and a logarithmic relaxation as a function of time. Oscillations in the magnetoresistance are observed, showing a square root periodicity in the applied magnetic field, both in large-area unstructured samples as well as in a structured sample. An explanation in terms of a commensurability condition of edge states in a highly mobile two-dimensional electron gas between substrate step edges is suggested.
We performed single electron tunneling measurements on bilayer ruthenate Sr$_3$Ru$_2$O$_7$. We observe an unusual oscillation in tunneling magnetoresistance near the metamagnetic quantum phase transition at temperatures below 7 K. The characteristic features of this oscillation suggest that it is unrelated to traditional quantum oscillations caused by orbit quantization. In addition, tunneling spectra are found to change sharply in the low bias voltage range of $V$ $<$ 2mV near the transition field. These observations reveal that the Fermi surface of Sr$_3$Ru$_2$O$_7$ changes in a surprising way as the system undergoes strong critical fluctuations.
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