Researchers have uncovered fascinating insights into the complex phase transitions occurring in the transition metal dichalcogenide (TMD) material IrTe2. Using advanced techniques like second harmonic generation (SHG) and electrical resistance measurements, the team has shed light on the intricate structural changes and symmetry transformations that take place in this unique material as it is cooled. Their findings not only resolve long-standing discrepancies in the reported phase transition temperatures but also reveal the emergence of a remarkable low-temperature trigonal phase, providing a deeper understanding of the interplay between surface and bulk properties in IrTe2. This research opens up new avenues for exploring the rich physics and potential applications of this intriguing TMD system. Transition metal dichalcogenides, Superconductivity, Symmetry, Phase transitions
Unraveling the Complexity of IrTe2
Transition metal dichalcogenides (TMDs) like IrTe2 have long captivated the attention of condensed matter physicists due to their unique structural and electronic properties. These layered materials, composed of a transition metal (T) sandwiched between chalcogen (Ch) atoms, exhibit a diverse range of behaviors, from insulating to superconducting, making them promising candidates for a variety of applications in nanoelectronics, nanophotonics, and beyond.
IrTe2, in particular, has been the subject of extensive research, as it undergoes a series of intriguing phase transitions upon cooling. At room temperature, IrTe2 crystallizes in a trigonal structure, but as the temperature is lowered, it experiences multiple structural phase transitions, accompanied by the formation of complex stripe phases and the emergence of superconductivity below around 2.5 Kelvin.
Despite the wealth of information available on IrTe2, many questions have remained unanswered regarding the exact nature of these phase transitions and the underlying symmetry changes. Researchers from the University of South Carolina have now shed new light on this enigmatic material by combining powerful experimental techniques, including second harmonic generation (SHG) and electrical resistance measurements.
Probing the Surface Symmetry with SHG
The researchers utilized SHG, a highly sensitive technique that can probe the surface symmetry of materials, to investigate the phase transitions in IrTe2 over a wide temperature range, from 4 Kelvin to 300 Kelvin. By performing SHG polarimetry measurements, they were able to identify distinct symmetry regions corresponding to the different phases of IrTe2.
Fig. 2
Their findings reveal that above a certain transition temperature, Ts1, the material exhibits a trigonal symmetry, which is consistent with the known high-temperature structure of IrTe2. However, between Ts1 and another transition temperature, Ts2, the symmetry is reduced to a triclinic structure, corresponding to the formation of the previously reported 5 × 1 stripe phase.
Interestingly, the researchers observed that the values of Ts1 and Ts2 shifted significantly as the sample was subjected to multiple thermal cycles. Specifically, Ts1 decreased from the commonly reported value of around 280 Kelvin to as low as 235 Kelvin, while Ts2 increased from 155 Kelvin to 190 Kelvin.
Reconciling Surface and Bulk Behavior
To understand the origin of these remarkable changes in the transition temperatures, the team conducted electrical resistance measurements on the same IrTe2 sample. Surprisingly, they found that the bulk transition temperatures remained unchanged, indicating that the observed shifts were a surface-specific phenomenon.
Fig. 3
The researchers suggest that the surface strain in IrTe2 is likely responsible for the observed changes in the transition temperatures. As the sample is subjected to thermal cycling, the surface strain is gradually relieved, leading to the shifting of the phase transition temperatures. This highlights the importance of considering the interplay between surface and bulk properties in understanding the complex behavior of materials like IrTe2.
Emergence of a Remarkable Low-Temperature Phase
The most intriguing finding from this study is the observation of a reemergence of a trigonal phase at temperatures below 10 Kelvin. The researchers detected a sharp increase in the SHG intensity, accompanied by a six-fold symmetric polarimetry pattern, suggesting the formation of a new phase with trigonal symmetry.
This low-temperature trigonal phase is particularly noteworthy, as it is likely responsible for the superconductivity observed in IrTe2 at around 2.5 Kelvin. The researchers hypothesize that the formation of these trigonal domains within the complex stripe phases at low temperatures is the key to understanding the interplay between the different phases and the emergence of superconductivity in this material.
Implications and Future Directions
The comprehensive investigation by the University of South Carolina team has not only resolved the long-standing discrepancies in the reported phase transition temperatures of IrTe2 but has also uncovered the remarkable low-temperature trigonal phase. This research highlights the power of combining complementary techniques, such as SHG and electrical resistance measurements, to unravel the intricate surface-bulk interactions in complex materials.
The findings from this study open up new avenues for exploring the rich physics of IrTe2 and other TMD systems. Understanding the delicate balance between the various phases and the role of surface strain in driving the phase transitions could pave the way for the design and engineering of novel functional materials with tailored properties. Additionally, the insights gained from this work may have broader implications for the study of phase transitions and the interplay between surface and bulk properties in a wide range of condensed matter systems.
Author credit: This article is based on research by Govinda Kharal, Bryan L. Chavez, Silu Huang, Rongying Jin, Yanwen Wu.
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