In our science classes at school, we learnt about the types of semiconductors as either n-type (electron-dominant) or p-type (hole-dominant), a fundamental property that dictates how they conduct electricity. This classification has been a cornerstone of semiconductor technology, from the tiny transistors in our smartphones to the large power converters in our grids. However, a rare and intriguing class of materials challenges this simple division, exhibiting what scientists call Direction-Dependent Conduction Polarity (DDCP). These unique substances can simultaneously conduct electricity predominantly via electrons in one direction and via holes (the absence of electrons, which act as positive charge carriers) in the other. This unusual ability holds immense potential for creating more versatile and efficient electronic devices, particularly in emerging fields such as thermoelectrics, which directly convert heat into electricity.
Recently, researchers have turned their attention to the material chromium antimonide, or CrSb, a compound that has also gained significant interest as a candidate for a newly discovered state of matter known as an altermagnet. Unlike traditional magnets that have a clear north and south pole, or antiferromagnets where magnetic moments cancel out, altermagnets possess a unique spin-split energy band structure without any net magnetism. This combination of properties makes CrSb an exceptionally promising candidate for advanced spintronic applications, which aim to use the intrinsic spin of electrons, not just their charge, to store and process information.
In a new study, researchers from S. N. Bose National Centre for Basic Sciences, Kolkata and Indian Association for the Cultivation of Science (IACS), Kolkata, have now experimentally confirmed that CrSb indeed exhibits DDCP, a discovery that could pave the way for a new generation of electronic devices.
To understand CrSb's electrical personality, the research team employed two key experimental techniques: Hall effect measurements and Seebeck thermopower measurements. These were complemented by theoretical calculations using Density Functional Theory (DFT). The Hall effect involves applying a magnetic field perpendicular to the flow of electric current and measuring the resulting voltage that builds up across the material. The direction of this Hall voltage reveals the type of dominant charge carrier. In CrSb, when the magnetic field was applied within the flat, hexagonal plane of the crystal, the Hall measurements consistently showed a positive voltage, indicating that holes were the primary charge carriers. However, when the magnetic field was applied perpendicular to the plane, the Hall voltage flipped sign, becoming negative, which indicated that electrons were the dominant carriers. This initial finding was a strong hint of DDCP.
To further confirm this dual behaviour, the researchers turned to the Seebeck effect, a thermoelectric phenomenon that doesn't involve magnetic fields. By applying a temperature difference across the CrSb crystal and measuring the generated voltage, they could directly determine the type of charge carrier that was present. A positive Seebeck coefficient indicates hole-dominant conduction, while a negative one signifies electron-dominant conduction. The results were consistent with the Hall effect measurements: the Seebeck coefficient measured along the plane was negative, confirming electron-dominated conduction, while along the perpendicular axis, it was positive, indicating hole-dominated conduction. This dual signature, observed consistently across both experimental methods and various temperatures, firmly established CrSb as a material with true direction-dependent conduction polarity.
To answer to CrSb’s unique ability lies within its atomic structure and the quantum behavior of its electrons. Using Density Functional Theory (DFT), a computational method, the researcher studied the material's electronic band structure and its Fermi surface – an imaginary boundary in energy space that dictates how electrons move. Their calculations revealed that CrSb's DDCP arises from a multicarrier mechanism.
Did You Know? A Fermi surface is not a physical surface you can touch! It's a concept in quantum physics that describes the boundary between occupied and unoccupied electron energy states in a material. Its shape tells scientists a lot about how a material conducts electricity. |
Unlike some other DDCP materials where a single type of carrier behaves differently in different directions, CrSb has distinct pockets of electrons and holes that reside in different energy bands. The shapes and orientations of these pockets are highly anisotropic, meaning they are elongated or flattened in specific directions. When all these pockets are considered together, the overall effective mass becomes electron-like in the plane and hole-like along the perpendicular axis. This intricate interplay of electron and hole pockets, each with its own directional preference, leads to the observed DDCP. The alternagnetic nature of CrSb, with its unique spin-split energy bands, subtly influences these electronic structures, contributing to the conditions necessary for DDCP.
The theoretical calculations also yielded a crucial prediction: DDCP in CrSb is very sensitive and exists only within a narrow energy range near the Fermi level. This implied that even a slight change in the number of electrons or holes, by a process known as doping, could switch off this unique dual behaviour. To test this prediction, the researchers introduced a tiny amount of vanadium (2%) into CrSb, creating Cr0.98V0.02Sb. This small doping effectively added a few extra holes to the material. As predicted, both the Hall and Seebeck measurements on the doped sample showed a dramatic change: the material lost its DDCP. It became entirely p-type, with holes dominating conduction in all directions. This experimental validation of the theoretical prediction underscores the delicate balance required for DDCP in CrSb and highlights its potential for tunable electronic behaviour.
While a few such materials have been identified previously, many are either not air-stable or composed of rare and expensive elements, which limits their practical applications. CrSb, on the other hand, is made of earth-abundant and non-toxic elements, making it an ideal candidate for widespread use. Furthermore, previous studies on DDCP materials have sometimes shown conflicting results between different experimental methods, or only one method has clearly demonstrated the direction-dependent signs. This study on CrSb provides consistent and robust evidence from both Hall and Seebeck measurements, addressing some of these past inconsistencies.
The ability to have both p-type and n-type functionalities within a single material, and to even tune this behaviour through doping, opens up exciting possibilities for device design. This could lead to breakthroughs in thermoelectric devices, making them more effective at converting waste heat into usable electricity or vice versa for cooling purposes. Moreover, combining DDCP with CrSb's alternating magnetic properties could lead to novel spintronic devices that leverage both charge and spin for unprecedented performance.
This article was written with the help of generative AI and edited by an editor at Research Matters.