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Intrinsic insulating ground state in transition metal dichalcogenide TiSe2

Daniel J. Campbell, Chris Eckberg, Peter Y. Zavalij, Hsiang-Hsi Kung, Elia Razzoli, Matteo Michiardi, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Andrea Damascelli, and Johnpierre Paglione
Phys. Rev. Materials 3, 053402 – Published 14 May 2019
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  • INTRODUCTION
  • EXPERIMENTAL METHODS
  • PHYSICAL PROPERTIES
  • DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The transition metal dichalcogenide TiSe2 has received significant research attention over the past four decades. Different studies have presented ways to suppress the 200 K charge-density-wave transition, vary low-temperature resistivity by several orders of magnitude, and stabilize magnetism or superconductivity. Here we give the results of a synthesis technique whereby samples were grown in a high-pressure environment with up to 180 bar of argon gas. Above 100 K, properties are nearly unchanged from previous reports, but a distinct hysteretic resistance region begins around 80 K, accompanied by insulating low-temperature behavior. An accompanying decrease in carrier concentration is seen in Hall effect measurements, and photoemission data show a removal of an electron pocket from the Fermi surface in an insulating sample. We conclude that high inert gas pressure synthesis accesses an underlying nonmetallic ground state in a material long speculated to be an excitonic insulator.

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    • Received 25 September 2018

    DOI:https://doi.org/10.1103/PhysRevMaterials.3.053402

    ©2019 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Charge density wavesCompositionCrystal growth
    1. Physical Systems
    Single crystal materials
    1. Techniques
    CrystallographyResistivity measurements
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Daniel J. Campbell1,*, Chris Eckberg1, Peter Y. Zavalij2, Hsiang-Hsi Kung3,4, Elia Razzoli3,4, Matteo Michiardi3,4,5, Chris Jozwiak6, Aaron Bostwick6, Eli Rotenberg6, Andrea Damascelli3,4, and Johnpierre Paglione1,7,†

    • 1Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
    • 2Department of Chemistry, University of Maryland, College Park, Maryland 20742, USA
    • 3Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
    • 4Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
    • 5Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
    • 6Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
    • 7Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada

    • *Corresponding author: djcampbe@umd.edu
    • †Corresponding author: paglione@umd.edu

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    Issue

    Vol. 3, Iss. 5 — May 2019

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    Images

    • Figure 1
      Figure 1

      (a) The Morris HPS-3210 furnace used for growth. A quartz ampule, open at one end, was inserted with its closed end on the right-hand side near the heating element. The entire chamber was then sealed and the furnace lid closed. Tilting the furnace is suspected to help nucleate crystals. (b) A typical ampule after growth. The dark area on the closed right end is polycrystalline TiSe2, while the region in the middle is elemental Se. (c) A small, fragile, pressure-grown TiSe2 crystal. (d) Larger pressure-grown TiSe2 single crystals (on 1×1mm2 scale paper).

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    • Figure 2
      Figure 2

      TiSe2 structural data obtained from refinement of powder synchrotron data of CVT (squares, “0 bar”), Se flux (triangles, “1 bar”), and pressure (circles) growths. (a) shows the a (left, blue) and c (right, red) axis lengths and (b) the volumes, with error bars magnified ten times from those given by GSAS-II. Note that data points for two of the CVT growths nearly completely overlap. (c) is the refined Se occupancy of the same data, with error bars, where a CVT and Se flux growth overlap at 0.98.

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    • Figure 3
      Figure 3

      Resistance (scaled to 300 K value) as a function of temperature for pressure-grown TiSe2 single crystals listed in Table 1, with maximum growth pressures noted. Note that even the two samples from the same batch exhibit very different behavior. Hysteresis in the 30–80 K region is marked by arrows for the black curve to show the difference in warming and cooling, which is the same for unmarked samples. In some cases, there is also hysteresis around the CDW-related upturn from 150–200 K.

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    • Figure 4
      Figure 4

      ΔR≡Rwarming−Rcooling, scaled to room temperature resistance, as a function of temperature for seven crystals from the same 101 bar growth. Hysteresis is most evident around 80 K but also manifests at the higher temperature CDW resistance peak. ΔR/R reaches a maximum of 5.0, 6.7, and 6.9 for the red, purple, and blue curves, respectively. Inset: ρ(T) of each sample, with matching colors. Stars indicate samples for which data are also presented in other figures: 101B (Table 1 and Fig. 3, here green), 101C (Fig. 5, pink), and 101D (Fig. 7, purple).

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    • Figure 5
      Figure 5

      (a) Temperature-dependent resistivity for TiSe2 single crystals grown by I2 chemical vapor transport, excess Se flux, and 101 bar of Ar gas pressure. At high temperatures, the behavior of all three samples is similar, but differences emerge below 150 K. No hysteresis is observed in the CVT or flux crystals. (b) The derivative of the cooling data from (a) in a narrower temperature region. The inset is the second derivative of the same data, where the peak is identified as the onset of the CDW.

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    • Figure 6
      Figure 6

      (a) The temperature dependence of the Hall coefficient of TiSe2 single crystals. Note that the CVT sample's temperature dependence, including a zero-crossing, is too small to see on this scale. The inset shows the zero-crossing region for selected samples from the main plot. (b) Specific heat of a polycrystalline TiSe2 sample with a maximum growth pressure of 160 bar as a function of temperature. Inset: Low temperature data for the same sample and two other polycrystals, with maximum growth pressures noted. Lines are fits to the Debye low-temperature specific heat model. (c) Paramagnetic susceptibility of polycrystalline TiSe2 chunks taken in constant applied fields of 20 (116 bar), 50 (80, 127, and 156 bar), or 70 kOe (CVT and 114 bar) due to the small intrinsic moment. (d) The hysteretic area (see Discussion for details) versus resistance increase for pressure-grown single crystals, on a log-log scale. Inset: The base temperature Hall coefficient as a function of the same quantity. More insulating samples generally showed a larger |RH|. Error bars come from uncertainty in the measurement of sample thickness and in some cases are smaller than the symbol size.

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    • Figure 7
      Figure 7

      (a) Resistivity as a function of temperature for the two pressure-grown ARPES samples, with a schematic drawing of the Brillouin zone. 114B shows a small amount of hysteresis that is less obvious with the logarithmic scale. (b) Comparison of the constant energy contour measured at kz=π/c (the A–L–H plane) at EF for 114B (left) and the L valence band maximum (60 meV below EF) for 101D (right) at T=20 K, plotted in the kx<0 and kx>0 half-plane, respectively. (c) and (d) ARPES map measured along the A–L cut [the dashed red line in (a)] for 114B and 101D, respectively, with the same photon energy of 122 eV for both. To the right of each cut are the EDCs at the L point (vertical red solid line), which were fit by Voigt functions multiplied by the Fermi-Dirac distribution (blue curves).

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