[Paper Review] Correlation of Crystal Quality and Extreme Magnetoresistance of WTe$_2$

Mazhar N. Ali, Leslie M. Schoop+6 more|arXiv (Cornell University)|Jun 16, 2015

Magnetic and transport properties of perovskites and related materialsMaterials Science11 references18 citations

TL;DR

This study develops a reproducible flux-growth method with self-vapor transport cleaning to produce high-quality WTe₂ single crystals, achieving a record 1.75 million percent magnetoresistance at 9 T and 2 K—nearly an order of magnitude higher than prior reports—due to a residual resistivity ratio (RRR) of 1250 and an average carrier mobility of 167,000 cm²/Vs, with magnetoresistance scaling nearly as B² and strongly correlated to crystal quality via cooling rate control.

ABSTRACT

High quality single crystals of WTe$_2$ were grown using a Te flux followed by a cleaning step involving self-vapor transport. The method is reproducible and yields consistently higher quality single crystals than are typically obtained via halide assisted vapor transport methods. Magnetoresistance (MR)values at 9 Tesla and 2 Kelvin as high as 1.75 million \%, nearly an order of magnitude higher than previously reported for this material, were obtained on crystals with residual resistivity ratio (RRR) of approximately 1250. The MR follows a near B$^2$ law (B = 1.95(1)) and, assuming a semiclassical model, the average carrier mobility for the highest quality crystal was found to be ~167,000 cm$^2$/Vs at 2 K. A correlation of RRR, MR ratio and average carrier mobility ($μ_{avg}$) is found with the cooling rate during the flux growth.

Motivation & Objective

• To develop a reproducible, high-yield method for growing high-quality WTe₂ single crystals with minimal defects.
• To establish a direct correlation between crystal quality (measured by RRR), magnetoresistance (MR), and carrier mobility.
• To demonstrate that slower cooling rates during flux growth significantly enhance crystal quality and extreme magnetoresistance.
• To provide a consistent, high-quality crystal source for reliable, reproducible studies of WTe₂'s electronic properties.
• To resolve conflicting reports in the literature by linking extreme magnetoresistance values to sample quality and growth methodology.

Proposed method

• Grew WTe₂ single crystals using a Te flux method with a pre-reacted WTe₂ precursor to improve consistency.
• Applied a secondary self-vapor transport cleaning step in a sealed quartz tube at 415 °C to remove residual Te and defects.
• Controlled crystal quality by varying the cooling rate (2–3 °C/h) during the flux growth process.
• Measured residual resistivity ratio (RRR) as a quantitative metric of crystal quality, with higher RRR indicating fewer defects.
• Fitted field-dependent resistivity data to the Lorentz law (MR = 1 + (μB)²) to extract average carrier mobility (μ_avg).
• Correlated RRR, MR ratio at 9 T and 2 K, and μ_avg across multiple growth batches and methods (flux vs. vapor transport).

Experimental results

Research questions

• RQ1 Can a reproducible, high-yield crystal growth method be developed for WTe₂ that consistently produces high-quality single crystals?
• RQ2 How does the cooling rate during flux growth affect the residual resistivity ratio (RRR) and, consequently, the magnetoresistance (MR) in WTe₂?
• RQ3 What is the relationship between RRR, carrier mobility (μ_avg), and extreme magnetoresistance (XMR) in WTe₂?
• RQ4 Can the observed XMR values be explained by a semiclassical model, and does the MR follow a B² dependence?
• RQ5 To what extent do crystal defects and impurities limit the magnitude of magnetoresistance in WTe₂?

Key findings

• The flux-growth method with self-vapor transport cleaning produced WTe₂ crystals with a residual resistivity ratio (RRR) of up to 1250, significantly higher than typical values from halide-assisted vapor transport.
• A record magnetoresistance of 1,750,000% was achieved at 9 T and 2 K in the highest-quality crystal, nearly an order of magnitude higher than previously reported values.
• The magnetoresistance followed a near-B² dependence (B = 1.95(1)), consistent with the Lorentz law, indicating a semiclassical origin.
• The average carrier mobility (μ_avg) for the highest-quality crystal was 167,000 cm²/Vs at 2 K, unusually high for a non-Dirac/Weyl semimetal.
• A strong, non-linear correlation was found between RRR and magnetoresistance, with slower cooling rates yielding higher RRR and higher MR.
• The method showed high batch-to-batch consistency for flux-grown crystals, unlike vapor transport, which exhibited greater sample-to-sample variation.