[Paper Review] Intrinsic Insulating Ground State in Transition Metal Dichalcogenide TiSe2

Daniel Campbell, Chris Eckberg+9 more|arXiv (Cornell University)|Sep 25, 2018

2D Materials and Applications4 citations

TL;DR

This study demonstrates that high-pressure argon gas growth (up to 180 bar) stabilizes an intrinsic insulating ground state in TiSe2, suppressing selenium vacancies and revealing a first-order charge density wave transition at ~80 K with hysteretic resistance and reduced carrier concentration. Photoemission confirms the disappearance of an electron pocket at the Fermi surface, providing direct evidence for an excitonic insulator state previously obscured by extrinsic metallic contributions.

ABSTRACT

The transition metal dichalcogenide TiSe$_2$ 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 new 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 hysteretic resistance region that begins around 80~K, accompanied by insulating low temperature behavior, is distinct from anything previously observed. 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.

Motivation & Objective

• To access a previously obscured intrinsic insulating ground state in TiSe2 that is masked by extrinsic metallic contributions in conventional samples.
• To investigate whether high inert gas pressure growth can suppress selenium vacancies and reduce metallic carrier concentration in TiSe2.
• To determine if the observed low-temperature insulating behavior arises from a true charge-ordered, excitonic insulator state rather than extrinsic defects.
• To probe the electronic structure evolution using angle-resolved photoemission spectroscopy (ARPES) and correlate it with transport and Hall measurements.

Proposed method

• High-pressure argon gas (10–180 bar) was used during crystal growth in a sealed furnace, replacing traditional chemical vapor transport (CVT) with I2 or Se.
• Single and polycrystalline TiSe2 samples were synthesized under inert Ar pressure, with lattice parameters measured via X-ray diffraction to confirm reduced unit cell volume.
• Transport and Hall measurements were performed from room temperature down to 2 K to track resistivity and carrier concentration changes.
• Angle-resolved photoemission spectroscopy (ARPES) was used to map the Fermi surface and detect the disappearance of an electron pocket in insulating samples.
• Synchrotron X-ray diffraction and electron microscopy were used to analyze crystal structure and defect concentration, particularly selenium vacancies.
• Comparison with conventional CVT-grown samples was performed to isolate the effects of pressure-growth on electronic properties.

Experimental results

Research questions

• RQ1 Does high-pressure argon growth in TiSe2 suppress selenium vacancies and reduce extrinsic metallic contributions?
• RQ2 Can a first-order charge density wave transition be stabilized at ~80 K under high-pressure growth, distinct from the conventional 200 K CDW in standard samples?
• RQ3 Is the observed insulating ground state in pressure-grown TiSe2 intrinsic, rather than a result of doping or defect states?
• RQ4 Does the disappearance of an electron pocket in ARPES data correlate with the onset of insulating behavior and reduced carrier concentration?
• RQ5 Can the observed transition be linked to a true excitonic insulator state, as suggested by prior theoretical and experimental work?

Key findings

• Pressure-grown TiSe2 exhibits a hysteretic resistance increase starting at ~80 K, indicating a first-order phase transition distinct from the 200 K CDW in conventional samples.
• Hall effect measurements show a significant reduction in carrier concentration below 100 K, consistent with insulating behavior and reduced defect-mediated conduction.
• ARPES data reveal the complete disappearance of an electron pocket at the Fermi surface in insulating samples, which is present in metallic and semiconducting counterparts.
• X-ray diffraction shows a smaller lattice parameter in pressure-grown samples, indicating reduced unit cell volume and lower selenium vacancy concentration.
• The insulating state is intrinsic and not induced by chemical doping, as confirmed by the absence of intercalated atoms and consistent structural and magnetic properties with standard TiSe2 at high temperatures.
• The results support the existence of a true excitonic insulator ground state in TiSe2, with the high-pressure method revealing a previously hidden, low-temperature charge-ordered phase.