Quantum Materials

Research in the Neupane group primarily focuses on understanding the electronic properties of quantum materials. The term quantum materials has a broad meaning in condensed matter physics, generally referring to materials whose properties cannot be fully explained within a non-interacting framework—particularly when strong Coulomb interactions among individual constituents are neglected.

In strongly correlated electron systems, exotic electronic structures often defy explanation from a purely semiclassical perspective, as various quantum effects play a critical role. Over the past few decades, intensive research efforts have led to the discovery of a wide range of new quantum materials and phenomena in both strongly correlated systems and beyond, including topological quantum materials, topological superconductors, and excitonic states. The revolutionary promise of quantum materials lies in their potential applications in areas such as spintronics, quantum computing, and high-temperature superconductivity.

Topological Semimetals

Topological semimetals are a class of quantum materials that exhibit gapless, linearly dispersive, and topologically stable band crossings, which can mimic the behavior of elementary particles through their quasiparticle excitations. Unlike the novel surface states found in topological insulators, topological semimetals display exotic phenomena in both their bulk and surface states.

They can be classified in several ways—by the degeneracy of their band crossings, the dimensionality of the bands, and the symmetries derived from the 230 crystallographic space groups. Among these, Dirac, Weyl, and nodal-line (or nodal-loop) semimetals are the most well-known and extensively studied. Notably, topological semimetals with intrinsic superconductivity offer a promising platform for realizing Majorana states.

These materials exhibit remarkable and unconventional properties, such as giant magnetoresistance, ultrahigh carrier mobility, anisotropic Hall effects, and the quantum spin Hall effect. Such phenomena are not only of great interest for advancing our understanding of fundamental physics but also hold significant promise for future technological applications.

Our research aims to discover and deepen the understanding of topological quantum semimetals. Some of our recent publications in this area include:

M. Mofazzel Hosen, Klauss Dimitri, Alex Aperis, Pablo Maldonado, Ilya Belopolski, Gyanendra Dhakal, Firoza Kabir, Christopher Sims, M Zahid Hasan, Dariusz Kaczorowski, Tomasz Durakiewicz, Peter M. Oppeneer, and Madhab Neupane; Observation of Gapless Dirac Surface States in ZrGeTe. Phys. Rev. B 97, 121103 (2018). https://doi.org/ 10.1103/PhysRevB.97.121103

M. Mofazzel Hosen, Klauss Dimitri, Ilya Belopolski, Raman Sankar, Maria Szlawska, Su-Yang Xu, Nagendra Dhakal, Pablo Maldonado, Peter M. Oppeneer, Dariusz Kaczorowski, Fangcheng Chou, M. Zahid Hasan, Tomasz Durakiewicz, Madhab Neupane; Tunability of the topological nodal line semimetal phase in ZrSiX-type materials. Phys. Rev. B 95, 161101 (R) (2017). https://doi.org/ 10.1103/PhysRevB.95.161101

Madhab Neupane, Ilya Belopolski, M. Mofazzel Hosen, Daniel S. Sanchez, Raman Sankar, Maria Szlawska, Su-Yang Xu, Klauss Dimitri, Nagendra Dhakal, Pablo Maldonado, Peter M. Oppeneer, Dariusz Kaczorowski, Fangcheng Chou, M. Zahid Hasan, Tomasz Durakiewicz; Observation of Topological Nodal Fermion Semimetal Phase in ZrSiS; Phys. Rev. B 93, 201104 (R) (2016) [Editors’ Suggestion]. https://doi.org/10.1103/PhysRevB.93.201104

Multi-Fermionic States

Crystallographic space group symmetries play a crucial role in protecting topological states. For example, surface states in trivial semimetals can be easily modified by small perturbations, whereas the surface states in topological insulators are robust against nonmagnetic perturbations due to the protection provided by time-reversal symmetry. Furthermore, inversion symmetry, time-reversal symmetry, and other crystallographic symmetries are essential in stabilizing various types of topological semimetals.

The coexistence of multiple topological states within a single material is extremely rare, yet it is particularly important for advancing our understanding of the interactions between these states. Our recent discovery of multiple distinct topological phases in Hf₂Te₂P offers a unique opportunity to explore such interactions. In this material, we report the coexistence of both weak and strong topological insulating states, along with a one-dimensional Dirac-node-arc—a combination that could serve as a key platform for studying the interplay of diverse quantum phases.

Our recent result: M. Mofazzel Hosen, Klauss Dimitri, Ashis K. Nandy, Alex Aperis, Raman Sankar, Gyanendra Dhakal, Pablo Maldonado, Firoza Kabir, Christopher Sims, Fangcheng Chou, Dariusz Kaczorowski, Tomasz Durakiewicz, Peter M. Oppeneer, Madhab Neupane; Distinct multiple fermionic states in a single topological metal. Nat. Commun. 9, 3002(2018). https://doi-org/10.1038/s41467-018-05233-1

Magnetic Topological Semimetals

Topological Dirac semimetals, characterized by accidental band touchings between conduction and valence bands protected by both time-reversal and inversion symmetries, are at the forefront of modern condensed matter research. When either time-reversal or inversion symmetry is broken, Dirac semimetals can evolve into Weyl or nodal-line semimetals, depending on the dimensionality of the node and the associated symmetry protections.

Magnetic, or time-reversal-symmetry-broken, Dirac semimetals have also been theoretically predicted, where a combination of broken time-reversal symmetry and specific crystallographic symmetries can still protect the topological invariants. Our group is actively engaged in the discovery of magnetic topological materials. Recently, we identified a magnetic topological state in the nodal-line semimetal GdSbTe, providing a promising platform for exploring the interplay between magnetism and topological phases.

Our recent result on magnetic topological material:

M. Mofazzel Hosen, Gyanendra Dhakal, Klauss Dimitri, Pablo Maldonado, Alex Aperis, Firoza Kabir, Christopher Sims, Peter Riseborough, Peter M. Oppeneer, Dariusz Kaczorowski, Tomasz Durakiewicz, Madhab Neupane; Discovery of topological nodalline fermionic phase in a magnetic material GdSbTe. Sci. Rep. 8, 13283 (2018).

Time-Resolved Phenomena of Quantum Materials

Probing the electronic structure of quantum materials is essential for understanding their electrical, magnetic, and optical properties. Among the available techniques, angle-resolved photoemission spectroscopy (ARPES) has become a powerful tool due to its ability to directly map electronic band structures. However, conventional ARPES is limited to equilibrium states and cannot capture nonequilibrium processes or track electron dynamics in real time.

To overcome this limitation, pump–probe techniques are employed. In this approach, an ultrafast infrared pump pulse excites electrons, and a subsequent probe pulse snapshots the transient electronic structure. This method reveals hidden nonequilibrium dynamics that are inaccessible by static measurements. Advances in ultrafast optics now enable time resolutions from the picosecond scale down to the attosecond regime, greatly expanding the scope of such studies.

Time- and angle-resolved photoemission spectroscopy (trARPES)—which combines time-resolved spectroscopy with ARPES—has emerged as a leading method for investigating ultrafast phenomena in quantum materials. By varying the time delay between the pump and probe pulses, trARPES allows direct observation of pump-induced nonequilibrium dynamics with femtosecond precision. This capability provides unique insight into the fundamental processes governing material behavior. To date, trARPES has been successfully applied to study topological insulators, superconductors, and charge-density-wave materials. However, the field is still in its early stages, with many promising applications yet to be explored.

Publication based on tr-ARPES

Madhab Neupane, Yukiaki Ishida, Raman Sankar, Jian-Xin Zhu, Daniel S. Sanchez, Ilya Belopolski, Su-Yang Xu, Nasser Alidoust, M. Mofazzel Hosen, Shik Shin, Fangcheng Chou, M. Zahid Hasan and Tomasz Durakiewicz; Electronic structure and relaxation dynamics in a superconducting topological material. Sci. Rep. 6, 22557 (2016).
https://doi.org/10.1038/srep22557

Superconducting Topological Materials

The quest for a quantum computer is often regarded as the holy grail of modern science due to its transformative potential across a wide range of fields. Quantum computers promise unprecedented computational power, enabling highly accurate and ultrafast processing far beyond the reach of classical computers. Unlike conventional systems based on binary bits, quantum systems operate on qubits, which can exist in superposition states and enable entirely new computing paradigms.

One particularly promising route toward realizing quantum computing involves Majorana zero modes—non-Abelian quasiparticles predicted to appear at the interface between a topological insulator and an s-wave superconductor. Majorana zero modes are considered ideal building blocks for fault-tolerant quantum computation due to their topological protection from decoherence. Several theoretical proposals have outlined strategies for engineering these modes and realizing topological superconductivity, but experimental progress has so far been limited.

Our group is dedicated to discovering and understanding topological superconducting materials, with the goal of identifying platforms capable of hosting Majorana states. Some of our recent results in this area include:

Klauss Dimitri, M. Mofazzel Hosen, Gyanendra Dhakal, Hongchul Choi, Firoza Kabir, Dariusz Kaczorowski, Tomasz Durakiewicz, Jian-Xin Zhu, Madhab Neupane; Dirac State in a Centrosymmetric Superconductor alpha-PdBi2. Phys. Rev. B 97, 144514 (2018).
https://doi.org/ 10.1103/PhysRevB.97.1445141

Madhab Neupane, Nasser Alidoust, M. Mofazzel Hosen, Jian-Xin Zhu, Klauss Dimitri, Su-Yang Xu, Nagendra Dhakal, Raman Sankar, Ilya Belopolski, Daniel S. Sanchez, Tay-Rong Chang, Horng-Tay Jeng, Koji Miyamoto, Taichi Okuda, Hsin Lin, Arun Bansil, Dariusz Kaczorowski, Fangcheng Chou, M. Zahid Hasan, Tomasz Durakiewicz; Observation of the spin-polarized surface state in a noncentrosymmetric superconductor BiPd. Nat. Commun. 7, 13315 (2016). https://doi.org/10.1038/ncomms13315

5f-Electron Systems

Research on the electronic structure of 5f-electron materials has been driven by their complex and remarkable physical properties, including metal–insulator transitions, valence fluctuations, the Kondo effect, heavy-fermion behavior, and superconductivity—all of which stem from strong electron correlation effects. From an experimental standpoint, angle-resolved photoemission spectroscopy (ARPES) is one of the most direct tools for probing the band structures and Fermi surfaces of 5f-electron systems. While ARPES has been extensively applied to study a wide range of strongly correlated materials, its use in exploring the electronic structure of nuclear fuel materials has remained largely unexplored. Our vision is to employ ARPES to uncover the electronic structure and phonon dynamics of metallic fuels, with a particular focus on 5f-electron-based systems.