Surface Currents Found in Unusual Material TaSb

Researchers are increasingly focused on topological semimetals due to their unique electronic properties and potential for novel device applications. Susmita Changdar, Heike Schlörb, and Oleksandr Suvorov, working at the Leibniz Institute for Solid State and Materials Research (IFW Dresden) in collaboration with Alexander Yaresko from the Max Planck Institute for Solid State Research, have investigated the electronic structure of the dual topological semimetal TaSb₂. Their work, alongside contributions from Dimitry Efremov, Rui Lou, Alexander Fedorov, Bernd Büchner, Andy Thomas, and Sergey Borisenko, all at IFW Dresden, details the identification of open-orbit topological surface states on the weakly conducting plane of this material. This research is significant because it distinguishes between bulk and surface states, revealing a near-perfect carrier compensation and, critically, demonstrating surface-originating open-orbit Fermi surfaces with spin-momentum locking, positioning TaSb₂ as a promising candidate for spin-polarized transport technologies.

Scientists are unlocking new routes to more efficient electronics by exploring unusual materials with unique surface properties. This work details the discovery of exceptionally well-behaved electrons flowing on the surface of a compound called tantalum antimonide, which could underpin future devices that consume less power and offer enhanced performance.

Tantalum antimonide (TaSb₂) exhibits a rare combination of insulating and crystalline states depending on its crystallographic orientation, positioning it as a promising material for advanced spintronic devices. Recent work has distinguished the interplay between bulk and surface electronic states within TaSb₂, revealing unexpectedly strong spin-polarized transport characteristics.

Researchers have now identified open-orbit topological surface states, electronic states confined to the material’s surface with unique spin properties, that are entirely separate from the bulk electronic structure. These surface states display spin-momentum locking, where the direction of an electron’s spin is directly tied to its momentum, enhancing the potential for low-dissipation electronics.

This study employed a combination of angle-resolved photoemission spectroscopy, a technique used to map the energy and momentum of electrons, alongside theoretical calculations and precise transport measurements. The investigations focused on the (20 1) plane of TaSb₂, a weakly topological surface where the interplay between different topological phases is most pronounced.

By carefully disentangling the contributions from bulk and surface states, the team observed multiple electron- and hole-like bands within the material, achieving near-perfect balance between charge carriers. Crucially, the observed surface states form closed orbits within the material’s electronic structure, a characteristic that facilitates robust spin-polarized conduction.

Circular-dichroism ARPES measurements confirmed the spin-momentum locking within these surface states, while magnetotransport measurements demonstrated weak antilocalization, a quantum mechanical effect that further supports the presence of coherent, spin-polarized transport. These findings establish TaSb₂ as a versatile platform for exploring the complex behaviour of topological materials and developing novel spintronic technologies that exploit the intrinsic spin of electrons for information storage and processing. The ability to directly probe and manipulate these surface states opens avenues for designing low-energy devices with enhanced performance and functionality.

Iodine flux growth establishes TaSb₂ single crystals via controlled thermal gradients

Single crystals of TaSb₂ were grown utilising a chemical vapor transport method, beginning with the preparation of a polycrystalline sample. Stoichiometric amounts of tantalum and antimony were thoroughly mixed and sealed within a quartz ampule under vacuum before undergoing a two-stage heat treatment in a muffle furnace at 650°C for 12 hours, followed by 750°C for 72 hours, ensuring the formation of the desired polycrystalline compound.

Subsequently, the polycrystalline material was resealed in a quartz ampule containing iodine as a transport agent at a concentration of 10 gm/cc. The team maintained a pressure of 10⁻⁴ mbar. The ampule was then positioned within a gradient tube furnace for 14 days, with precisely controlled temperature gradients established between the source end at 1050°C and the sink end at 950°C. This temperature differential drives the transport of material, facilitating crystal growth, and yielding several single crystals, typically measuring 2×3 mm². Angle-resolved photoemission spectroscopy (ARPES) measurements were then performed on the (20 1) plane of these crystals at the BESSY-II synchrotron radiation centre, employing an angular resolution of 0.2° and maintaining a sample temperature of 1.5 K.

Photon energy varied between 30-100 eV, achieving an overall energy resolution of 10-15 meV, while a chamber pressure of 9 × 10⁻¹¹ mBar was maintained to preserve sample integrity and data quality. Complementary magnetotransport measurements were conducted using an Oxford Instruments cryostat equipped with a variable temperature insert and a 7 T superconducting split coil magnet, allowing for precise control of the magnetic field.

A stepper motor rotation platform enabled sample rotation within the magnetic field, while current was supplied via a Keithley 2450 sourcemeter and voltages were measured using Keithley 2182A nanovoltmeters. To support the experimental findings, density functional theory (DFT) calculations were performed, employing the full relativistic generalised gradient approximation with the Perdew-Burke-Ernzerhof exchange correlation potential within the full potential local orbital (FPLO) package.

A 12×12×12 mesh was used for Brillouin zone integration via the tetrahedron method, and the resulting states were used in semi-slab geometry calculations, constructing a tight-binding model Hamiltonian. Bulk band structure calculations were also performed using the PY LMTO implementation of the Linear Muffin Tin Orbitals method, incorporating spin-orbit coupling for increased accuracy.

Detailed Fermi surface mapping of TaSb₂ using angle-resolved photoemission spectroscopy

TaSb₂ single crystals exhibit a well-faceted morphology as confirmed by scanning electron microscopy, displaying a cleaved (20 1) plane consistent with X-ray diffraction data. Angle-resolved photoemission spectroscopy (ARPES) measurements, combined with first-principles calculations, reveal detailed in-plane Fermi surface (FS) maps at a binding energy of −0.2 eV and −0.3 eV, demonstrating excellent agreement between experimental data and calculated bulk band structures near the L and Y high symmetry points.

A 50 meV energy shift was applied to calculated bands to align them with the experimental Fermi level, facilitating direct comparison. Focusing on the FS centred at kz = π near L, ARPES spectra reveal a ripple-like, open-orbit structure near X, a feature not present at the Γ point. Constant energy contours at EF −0.2 eV and EF −0.3 eV further delineate the FS topology, showing distinct features around the L and Y points.

Detailed analysis of energy distribution maps (EDMs) along the L, X direction identifies multiple spectral features, providing a fine-grained view of the electronic band structure. Measurements taken with a photon energy of 80 eV confirm the persistence of these features, with EDMs again highlighting the spectral weight distribution along the L, X cut.

The observed open-orbit FSs are particularly noteworthy, indicating a unique electronic structure within TaSb₂. These findings establish TaSb₂ as a promising material for exploring the interplay between topological phases and transport phenomena, offering a platform for investigating spin-polarized transport on weakly-surfaced materials.

Tantalum antimonide presents simplified spin physics for advanced electronic investigation

The persistent quest to manipulate spin for next-generation electronics has often been hampered by the difficulty of finding materials where electron behaviour is simultaneously predictable and amenable to control. Tantalum antimonide, or TaSb, may represent a subtle but significant step forward in this endeavour, not through a dramatic performance breakthrough, but by identifying a material platform where fundamental spin-related phenomena are unusually clear and accessible.

The ability to disentangle bulk and surface electronic states, and to demonstrate spin-momentum locking on a weakly conducting surface, offers a rare opportunity to study and potentially engineer spin transport. For years, the field has been dominated by the search for topological materials with robust, symmetry-protected surface states, but many such materials suffer from complexities that obscure the underlying physics.

TaSb, with its near-perfect carrier compensation and distinct dual-phase behaviour, appears to offer a comparatively clean system. The observation of surface-originating Fermi surfaces, coupled with weak antilocalization in magnetotransport, suggests a pathway towards spin-polarized currents with reduced scattering. Limitations remain, as the weak conductivity of the surface states will necessitate innovative device architectures to fully exploit this behaviour.

Moreover, understanding the interplay between the bulk and surface states across varying temperatures and material compositions requires further investigation. Future work might focus on heterostructures incorporating TaSb, or on exploring similar materials within the transition metal dipnictide family, potentially unlocking even more refined control over spin-based phenomena and bringing genuinely novel electronic devices closer to reality.