Quantum materials

The research at Neupane’s group mostly focuses on understanding the electronic properties of quantum materials. Quantum material itself has a broader meaning in condensed matter material system and any material which is not explainable via non-interacting nature especially neglecting strong Coulomb interaction of individual constituents are known as quantum materials. Specifically, in strongly correlated electron systems exotic electronic structures cannot be explained from a pure semiclassical standpoint where various quantum effect plays a crucial role. The tremendous burst of research activity in the last few decades discovered a pool of new quantum materials and quantum properties in strongly correlated systems and beyond such as topological quantum materials, topological superconductor, excitons, etc. The most revolutionary advantages of quantum materials are the potential application in spintronics, quantum computer, high T_c superconductor, etc.

Topological semimetals

Topological semimetals are new quantum materials, which exhibit gapless linearly dispersive topologically stable band crossings which can mimic the properties of elementary particles in term of quasiparticle excitations. In contrast to the novel surface states in topological insulators, topological semimetals show such exotic phenomenon in both bulk and surface states. There are several types of topological semimetals based on their degeneracy, dimensionality of the bands as well as from the combinations of 230 space groups symmetries. Among them Dirac, Weyl and nodal-line/loop semimetals are most well-known and widely studied topological semimetals. Topological semimetals with intrinsic superconductivity hold the potential for realizing Majorana states. The novel and exotic properties from topological semimetals such as giant magnetoresistance, high mobility of carriers, anisotropic Hall effect, quantum spin Hall effect not only important for fundamental physics, but also hold key for future technologies.

Our research focuses to discover and understand the topological quantum semimetals. Some of our recent publications are as follows:

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

The various crystallographic space group symmetry plays a crucial role in protecting the topological states. For instance, surface state in trivial semimetals can easily be manipulate via various small perturbations but the surface state in topological insulator is robust against nonmagnetic perturbations due to the protection of time reversal symmetry. Furthermore, inversion symmetry, time reveal symmetry and other crystallographic symmetry play crucial role in corresponding topological semimetals. Therefore, coexistence of multiple topological states in a single material is extremely rare but particularly important and required for gaining deeper understanding about the interaction between them. Our recent discovery of multiple distinct topological phases in Hf2Te2P could therefore be a key to gain a deeper understanding of the interactions between various quantum phases. Here, we report both topological weak and strong insulating state along with a one-dimensional Dirac-node-arc.

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 with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. The Dirac semimetals evolve into a Weyl or nodal line semimetals when either time reversal or inversion symmetry is broken depending on the dimensionality of the node and corresponding symmetry protections. Magnetic or time reversal symmetry broken Dirac semimetals also have been predicted where a combination of broken time reversal symmetry and crystallographic symmetry protect the topological invariant. Our group is extensively working to discover magnetic topological materials. Recently we discovered magnetic topological state in a nodal-line semimetal in GdSbTe.

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

To study the quantum materials, measuring the electronic structure is very useful since it can give us insights into the electrical, magnetic and optical properties. In this case, angle-resolved photoemission spectroscopy is widely used to study the quantum materials due to its ability of directly detecting the electronic structure. However, the conventional ARPES fails to access the nonequilibrium states and track the electron dynamics.

To overcome this drawback, the pump-probe technique is required. In pump-probe scheme, an infrared pump pulse is employed to excite the electrons and a subsequent probe pulse is used to snapshot the transient electronic structures. Using this technique, one can bring the hidden nonequilibrium dynamics to light. Also, the prosperity of the ultrafast optics enables spectroscopy to access to the study of ultrafast dynamics with time resolution ranging from picoseconds down to even attoseconds. Therefore, the development of time- and angle- resolved photoemission spectroscopy (trARPES), which is achieved by combining time-resolved spectroscopy with ARPES, has becoming a trend.

In trARPES, one can observe the pump-induced nonequilibrium dynamics in real time with femtosecond time resolution by measuring the photoelectron spectrum at different time delays between the pump and probe pulses. This time-resolved property will provide a unique perspective for studying quantum materials. To date, trARPES has shown its wide applications in studying topological insulators, superconductors and charge density materials. While, trARPES is still not mature. There are still many new applications undiscovered.

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

Quantum computer serves as a holy grail among the scientific community due to its promising applications in multidimensional sectors. The discovery of quantum computer will be a milestone extending computational realms further with accuracy and fast processing, which cannot be accessed through the traditional computers. Unlike the classical computers based on binary bit system, quantum systems are based on qubit. Majorana zero modes, non-abelian quasiparticles that are anticipated to exist at the interface between topological insulator and a s-wave superconductor, may be used as the building block of the quantum computers. A number of schemes have been proposed to engineer the Majorana zero modes and the topological superconductors. However, a little success has been achieved so far. Our group focus on the discovery and understanding of the topological superconducting materials. Our recent results on topological superconducting materials are:

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

Scientific works on the electronic structure of 5f-electron materials have been inspired by the complex and striking physical properties including metal-insulator transitions, valence fluctuations, Kondo effect, heavy-fermion behavior and superconductivity, which arise from strong electron correlation effects of these materials. From an experimental perspective, angle-resolved photoemission spectroscopy (ARPES) is one of the most direct methods to provide information on the band structures and Fermi surfaces of 5f-electron materials. While this technique has been applied to investigate notable behavior of a variety of strongly correlated systems, exploration of the electronic structure of nuclear fuel materials using ARPES has been mostly unexamined. Our vision is to utilize ARPES to discover the electronic structure and phonon behavior of metallic fuels, specifically on 5f-electron-based systems.